SYNERGIC EXTRACTION OF RARE EARTH

ELEMENTS USING PICROLONIC ACID AND

OTHER NEUTRAL

OXO-DONORS

HABIB-UR-REHMAN

INSTITUTE OF CHEMISTRY

UNIVERSITY OF THE PUNJAB, LAHORE

PAKISTAN

2009

SYNERGIC EXTRACTION OF RARE

EARTH ELEMENTS USING PICROLONIC

ACID AND OTHER NEUTRAL

OXO-DONORS

HABIB-UR-REHMAN

A THESIS SUBMITTED TO THE UNIVERSITY OF THE PUNJAB

IN FULFILLMENT OF THE REQUIREMENT FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

IN

CHEMISTRY

INSTITUTE OF CHEMISTRY

UNIVERSITY OF THE PUNJAB, LAHORE PAKISTAN

DECEMBER 2009

Dedicated to my Parents

&

Family

i

CERTIFICATE

This is to certify that the thesis entitled “Synergic Extraction of Rare Earth Elements

Using Picrolonic acid and other neutral oxo-donors” submitted by Mr. Habib-ur-Rehman is

based on the original research done under our direct supervision in fulfillment of the

requirement for the degree of the Doctor of Philosophy in Chemistry. We have personally

gone through all the data / results / materials reported in the manuscript and certify their

correctness / authenticity. We further certify that, the material included in this thesis has not

been used in part or full in a manuscript already submitted or in the process of submission in

partial / complete fulfillment of the award of any other degree from any other institution. We

also certify that the thesis has been prepared under our supervision according to the

prescribed format and we endorse its evaluation for the award of Ph.D. degree through the

official procedures of the university.

(Prof. Dr.Muhammad Jamil Anwar) Research Supervisor Institute of Chemistry University of the Punjab, Lahore Pakistan

(Dr. Akbar Ali.) Deputy Chief Scientist, Research Co-Supervisor PINSTECH, Nilore, Islamabad Pakistan

Dated:________________

ii

Declaration

Except where specific reference has been made to other sources, the work presented

in this thesis is the original work of the author. It has not been submitted, in whole or in part,

for any other degree.

(HABIB-UR-REHMAN) PINSTECH, Nilore, Islamabad Pakistan

iii

Abstract

Extensive work is being carried on the extraction of rare earth elements due to their

special chemical, metallurgical, optical, magnetic and nuclear properties and their use in

advanced technologies as well as in nuclear industry. Different chemical processes are being

applied for the extraction of rare earth elements from their ores and their mutual separation

on laboratory scale as well as on commercial basis. However, these processes are facing

problems such as large number of stages due to low separation factor, low efficiency and

waste management. Keeping in view of these problems, in the present research work, a

synergic extraction system comprising of picrolonic acid as an acidic chelating agent and

oxygen based neutral donors, for the extraction / separation of rare earth elements has been

studied.

Synergic extraction of Ce(III), Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III) as

representative of trivalent lanthanides, using picrlonic acid (1-p-nitrophenyl-3-methyl-4-

nitro-5-pyrazolone, HPA, pKa = 2.52) as acidic chelating agent with crown ether such as 18-

crown-6 (18C6), Benzo-15crown-5 (B15C5), 12crown4 (12C4) as neutral oxo-donors in

chloroform from aqueous buffer solution of pH 1-2 having ionic strength 0.1 mol L-1 (K+/H+,

Cl-) has been studied. Radiotracer technique using their appropriate radio-isotopes prepared

in the research reactor of PINSTECH such as Ce141, Nd147, Sm151, Eu152/154, Tb160, Tm170,

Lu177, Hg 203, Fe59 etc., were used for the quantification of metal ions in the aqueous and

organic phases. Quantitative extraction (>98%) of these metal ions was observed only using

HPA and B15C5 synergic mixture at pH 2 within five minutes and the extraction was

increased with the increase in ionic radii of lanthanide ions. Composition of the extracted

species was determined by slope analysis method and found to be Ln(PA)3.nS, where Ln

represent lanthanide ion, PA conjugate base of HPA molecule and S as neutral oxo-donor.

The value of n is 1 and 2. Among the various cations and anions tested for their influence on

iv

the extraction these lanthanides only Fluoride, oxalate, Cu(II) , Fe(II) and Zn(II) had some

deleterious effect. The proposed synergic system presented clean separation of lanthanide

ions from mono, and various divalent metal ions especially alkali and alkaline earth metal

ions.

The effect of other neutral donors such as trioctylphosphineoxide (TOPO),

triphenylphosphineoxide (TPPO), tributylphosphate (TBP) and triphenylphosphate (TPP) was

also studied on the extraction of Eu(III). Quantitative extraction of Eu(III) was observed with

TOPO, TPPO and TBP from aqueous phase of pH2. Synergic adduct composition was found

to be Eu(PA)3TBP, Eu(PA)3.2TOPO and Eu(PA)3.2TPPO by slope analysis method. On the

basis of the estimated values of the synergic coefficient, and extraction constants (log Kex),

the oxo-donor effect was found in the order of TOPO>TPPO>TBP.

The effect of various diluents such as 1-octanol (ONL), 1-hexanol (HNL), 1-butanol

(nBNL), 2-butanol (2-BNL), n-butylether (BE), dichloroethylether (DCEE), acetylacetone

(ACAC), diisobutylketone (DIBK), cyclohexanone (CHN), benzene, toluene on the

extraction of Eu(III) from aqueous solution of pH 1-2 using HPA as extractant has been

studied. The extraction of Eu(III) using benzene and toluene was found to be negligible, with

1 & 2-butanol it was low (< 50%), where as with the other diluents studied, the extraction

was quantitative at pH 2. On the basis of log Kex, the solvents can be arranged with respect to

their extractability in the order ACAC > DIBK > BE > DCEE > ONL > HNL > CHN.

To find the trend of lanthanide extraction within the series, three solvents CHN, ONL

and DCEE as representative of ketones, alcohols and ethers, respectively, were selected for

the extraction of Ce(III), Tb(III) and Lu(III) using HPA as chelating agent from aqueous

solutions of pH 1-2, quantitative extraction was observed at pH2 and their extraction order

was found to be Ce(III)>Tb(III) >Lu(III).The composition of the extracted adduct was found

to be M(PA)3 in CHN, ONL and M(PA)3.HPA in DCEE by slope analysis method.

v

The synergic mixture comprising HPA and B15C5 in benzene and toluene separately

were studied for the extraction of Eu(III) from aqueous solution of pH 1-2 and quantitative

extraction was observed at pH 1 with both the solvents. On the basis of their estimated values

of synergic coefficient and log Kex, benzene was found to be better solvent than toluene. The

composition of the synergic adduct was found to be Eu(PA)3.2B15C5 and proposed to be a

sandwich type complex having one crown ether molecule on either side of the metal chelate

bound to the central metal only through three oxygen atoms.

vi

Acknowledgement

All praises are for Almighty Allah who blessed me with the wealth of knowledge and

enabled me to complete this task. I feel privileged and pleasure to express my profound and

cordial gratitude to acknowledge the worthy guidance, inspiration and encouragement of my

learned supervisors, Prof. Dr. Muhammad Jamil Anwar, Pro-Voice Chancellor, University

of the Punjab, Lahore and Dr. Akbar Ali, Deputy Chief Scientist, Chemistry Division,

Pakistan Institute of Nuclear Science and Technology (PINSTECH), Islamabad, Pakistan. I

wish to thank Prof. Dr. Saeed Ahmad Nagra, Director, Institute of Chemistry, University of

the Punjab, for his support.

My special thanks are due to Mrs. Wasim Yawar, Head Central Analytical Facility

Division, PINSTECH, for her valuable guidance and constant support during research work. I

am also thankful to Dr. Ishrat Rehana, Head Spectroscopy group, CAFD, PINSTECH, for her

continuous support. I would like to thank Mr. Shafaat Ahmad, Director ACL, former Head

Central Analytical Facility Division, PINSTECH, for his kind permission and sincere

suggestions to carry out this work within and outside the division, without which I would not

have been able to complete this study.

I am thankful to all the Heads of Chemistry Division, PINSTECH, especially Dr.

Muhammad Mufazzal Saeed, Dr. Shujaat Ahmad, Dr. Jamshed Hussain Zaidi and Dr. Riaz

Ahmad, for providing me the opportunity and necessary facilities to carry out this research

work.

I wish to express my gratitude to all those who contributed towards the completion of

this work. I would like to thank my friends Dr. Muhammad Daud, Dr. Shahid Parvez, Dr.

Muhammad Wasim and Dr. Munir Ahmed for their valuable suggestions and technical help.

Special thanks are due to all the staff members of Radiochemical Separation Group,

of Chemistry Division at PINSTECH, especially Mr. Nizakat Hussain and Mr. M. A. Rizvi,

vii

for their continuous support during this research work. Reactor Operation Group of this

institute deserves special thanks for the provision of irradiation facility during the course of

studies. I am thankful to Mr. Ghulam Murtaza, Scientific Assistant of our Laboratory, for his

help. In the end, I would like to thank Mr. Ishfaq Aziz, Scientific Assistant, who helped me

considerably in the preparation of this dissertation by typing the manuscript.

Lastly, my profound gratitude goes to my wife for her moral support, generosity

and sharing burden of my responsibilities that enabled me to complete this work. My

affections are for my late parents and for my children who gave me the spirit and

determination throughout this period. Finally, I am grateful to Pakistan Atomic Energy

Commission and PINSTECH, for granting me the permission and providing the necessary

facilities to complete this research work.

HABIB-UR-REHMAN

viii

Symbols and abbreviations 12C4 12-crown-4 15C5 15-crown-5 18C6 18-crown-6 21C7 21-crown-7 24C8 24-crown-8 AAS atomic absorption spectrometry ACAC acetylacetone B15C5 Benzo-15-crown-5 C6H6 Benzene CCl4 Carbon tetra chloride CHCl3 Chloroform CHN cyclohexanone D Distribution Ratio D2EHIBA Di-(2-ethylhexyl) isobutyricamide D2EHPA 2-ethylhexylphosphoric acid DB18C6 dibenzo-18-Crown-6 DBSO dibutylsulfoxide DCEE dichloroethyl ether DCH18C6 dicylohexano-18-crown-6 DCHSO dicyclohexyl-sulfoxide DEHP di(2-ethyl-hexyl) phosphoric acid DHA Dihexylamide DHHA dihexylhexaneamide DHSO dihexyl sulfoxide DOSO Dioctylsulfoxide DPSO Diphenyl sulfoxide E Percent Extraction ETA-AAS Electro-thermal atomization-atomic absorption spectrometer H3C(CH2)5COOH Heptanoic acid HBTFA 4, 4, 4-tri-fluoro-1-phenyl-1, 3-butan-dione HFAA Hexafluoroacetylaceton HOX 8-hydroxy-quinoline HPA Picrolonic acid HPBI 3-methyl-4-benzoyl-5-isoxazolone HPMAP Phenyl-3-methyl-4-acyl-5-pyrazolone HPMPP 1-phenyl-3-methyl-4-pivaloyl-5-pyrazolone HPMTFP 1-Phenyl-3-methyl-4-trifuoroacetyl pyrazolone-5 HTTA Thenoyltrifluoroacetone Ibnl 2-butanol ICP-AES Inductively coupled plasma atomic emission spectrometry IR Infra Red MIBK Methylisobutylketone NAA Neutron activation analysis nBE n-butyl ether nBNL 1- butanol nHNL hexanol NMR Nuclear Megnatic Resonance ONL 1-octanol

ix

PINSTECH Pakistan Institute of Nuclear Science and Technology PMR Proton Megnatic Resonance PSO Petroleum sulfoxides REEs Rare earth elements S.F Synergistic factor SCA Salicylic acid TBP Tributylphosphate TBPO Tributylphosphineoxide TOA Trioctylamine TOPO Trioctylphodsphineoxide TPP Triphenylylphosphate TPPO Triphenylylphosphineoxide TRAMEX Tertiary Amine Extraction XRF X-ray fluorescence Γ Separation Factor

x

Table of Contents CHAPTER -1 1. INTRODUCTION 1

CHAPTER-2 2. SOLVENT EXTRACTION 12

Distribution Ratio (D) 13

Percent Extraction (E) 13

Separation Factor (γ) 14

2.1. Distribution Law 14

2.2. Process of Extraction 16

2.2.1. Chemical Interactions in the Aqueous Phase 16

2.2.2. Distribution of Extractable Species 17

2.2.3. Chemical Interaction in Organic Phase 20

2.3. Extraction Systems 20

2.3.1. Types of Inorganic Extractable Complexes 20

2.3.1.1. Coordination Complexes 21

2.3.1.1.1. Simple or Monodentate Complexes 21

2.3.1.1.2. Chelate or Polydentate Complexes 21

2.3.1.2. Ion association complexes 24

2.4. Extraction Equilibria 26

2.4.1. Extraction of Metal Chelates 26

2.4.1.1. Effect of the Reagent 28

2.4.1.2. Effect of Reagent Concentration and pH 28

2.4.1.3. Effect of Metal Ion Concentration 29

xi

2.4.1.4. Effect of the Organic Solvent 29

2.4.1.5: Selectivity in Chelate Extractions 30

2.5. Kinetic Factors in Extraction 32

2.6. Methods of Extraction 33

2.6.1. Batch Extraction 34

2.6.2. Continuous Extraction 35

2.6.3. Countercurrent Extractions 36

2.7. Factors influencing the extraction efficiency 36

2.7.1. Choice of Solvent 37

2.7.2. Acidity of the Aqueous Phase 37

2.7.3. Salting-out Agents 38

2.7.4. Oxidation State 39

2.7.5. pH 40

2.7.6. Masking 40

2.7.7. Backwashing 41

2.8. Synergic extraction 41

2.8.1. Methods used for the study of synergistic Extraction 43

2.8.1.1. Slope Analysis Method 44

2.8.1.1.1. Extraction with acidic ligand 44 2.8.1.1.2, Synergistic Extraction 45

2.8.1.2, Job’s Method 46

xii

CHAPTER-3

3. LITERATURE REVIEW 48

3.1. Use of Picrolonic acid (HPA) in Copmlexation / Extraction 52

3.2. Use of Crown Ethers in Extraction of REEs 55

CHAPTER-4

4. EXPERIMENTAL 64

4.1. Apparatus 64

4.2. Materials 64

4.3. Buffer Solutions 65

4.4. Chemicals/reagents used to study anions and cations effects 65

4.5. Preparation of Radionuclides 66

4.5.1. Calculation of The Activity of Radiotracer 67

4.6 Experimental Procedures 69

4.6.1. Extraction procedure 69

4.6.2. Effect of Shaking Time on Extraction of REEs 70

4.6.3. Synergistic Extraction with Mixture of HPA and B15C5 70

4.6.4. Effect of REEs Concentration 71

4.6.5. Composition of Extractable Organometallic Complex 71

4.6.6. Effect of Neutral Ligands on the Extraction of REEs 72

4.6.7. Effect of Anions on the Extraction of REEs 72

4.6.8. Effect of Cations on the Extraction of REEs 73

4.6.9. Effect of Solvents on the Extraction of REEs 73

4.6.10. Extraction of other Matel ions with HPA + B15C5 74

4.6.11. Back Extraction of REEs 74

xiii

4.7. Acid dissociation equilibria of HPA 74

CHAPTER-5

RESULTS AND DISCUSSION 76

5.1 Extraction of REEs with HPA and crown ethers 76 5.1.1 Effect of pH of Aqueous Phase 76

5.1.2. Effect of Equilibration Time 77

5.1.3. Effect of Metal Ion Concentration 77 5.1.4. Composition of Synergic Adduct 78 5.1.4.1. Effect of pH Variation 78 5.1.4.2. Effect of HPA Concentration Variation 78

5.1.4.3. Effect of Crown Ether Concentration Variation 78

5.1.5. The anions effect 82 5.1.6. The Cations Effect 83

5.1.7. The Selectivity of Extraction System 83

5.1.8. Acid dissociation constant 84

5.2. Solvent effect 105

5.2.1. Composition of the Extracted adduct 107

5.2.1.1. Effect of HPA Concentration 108

5.3. Extraction of Rare Earth Elements in Different Solvents 114

5.3.1. Extraction of Ce(III), Tb(III) and Lu(III) in Octanol 114

5.3.2. Extraction of Ce, Tb and Lu in Cyclohexanone 117

5.3.3. Extraction of Tb and Lu in DCEE 120

5.4. Synergistic Extraction of Eu(III) in Benzene and Toluene 124

5.4.1. Effect of pH of Aqueous Phase 124

5.4.2. Composition of Synergic Adduct 124

5.4.2.1. Effect of pH Variation 125

5.4.2.2. Effect of HPA Concentration Variation 125

5.4.2.3. Effect of Crown Ethers Concentration Variation 125

xiv

5.5. Effect of Neutral donors 131

5.5.1 Composition of the Synergistic Adducts 131

5.5.1.1. Effect of pH 132

5.5.1.2. Effect of HPA Concentration 132

5.5.1.3. Effect of Concentration of Neutral Donors 132

5.6 Conclusion 141

CHAPTER-6

REFERENCES 143

xv

APPENDIX-A

List of Publications

1. Habib-ur-Rehman, Akbar Ali, Jamil Anwar and Wasim Yawar. Synergistic

extraction of Ce(III), Eu(III) and Tm(III) with a mixture of picrolonic acid and

benzo-15-crown 5 in chlofororm. J. Radioanal. Nucl. Chem. 267(2) 421-425

(2006).

2. Habib-ur-Rehman, Akbar Ali, Jamil Anwar and Shafaat Ahmed: Synergistic

extraction of Nd(III), Tb(III) and Lu (III) with a mixture of picrolonic acid and

benzo-15-crown 5 in chlofororm Radiochim. Acta. 94, 475-480 (2006).

xvi

List of Tables

Table 2. 1 Metal Extraction Systems 23

Table 2.2 Ion Association Systems 25

Table 4.1 Radioisotopes prepared in PARR-1 68

Table 5.1 Equilibrium constants of the synergistic extraction of lanthanide (III)

ns with (HPA + B15C5)/CHCl3

80

Table 5.2 Effect of various anions on the extraction of lanthanide ions with 0.01

mol dm-3 (HPA + B15C5)/CHCl3 from aqueous solution at pH 2

85

Table 5.3 Extraction of lanthanide (III) ions in the presence of various cations

with 0.01 mol dm-3 (HPA + B15C5)/CHCL3 from pH 2 aqueous

solution

86

Table 5.4 Extraction of various metal ions with 0.01 moldm-3 (HPA + B15C5) /

CHCl3 from pH 2.0 aqueous solution

87

Table 5.5 Slope with correlation coefficients, for the extractin of Eu(III) from

different solvents from Fig. 20

107

Table 5.6 Slope with correlation coefficients, for the extractin of Eu(III) from

different solvents from Fig. 21

108

Table 5.7 Extractin constants for Eu(III) extraction in different solvents.

110

xvii

List of Figures

Fig. 1 Extraction of Ce(III) with B15C5, HPA and HPA+B15C5 42 Fig. 2 Extraction of Ce(III) and Nd(III) with B15C5, HPA and HPA+B15C5

in chloroform

88

Fig. 3 Extraction of Eu(III) and Tb(III) with B15C5, HPA and HPA+B15C5

in chloroform

89

Fig. 4 Extraction of Tm(III) and Lu(III) with B15C5, HPA and HPA+B15C5

in chloroform

90

Fig. 5 Dependence of metal ion extraction on its concentration by (HPA+B15C5) from

pH 2 buffer solution in chloroform

91

Fig. 6 log D as a function of pH for Ce(III) and Nd(III) with (HPA+B15C5)/CHCl3 92

Fig. 7 log D as a function of pH for Eu(III) and Tb(III) with (HPA+B15C5)/CHCl3 93

Fig. 8 log D as a function of pH for TmIII) and Lu(III) with (HPA+B15C5)/CHCl3 94

Fig. 9 log –log plot of (D-DCE) related to Ce(III) and Nd(III) vs.[HPA];

[B15C5] = 0.005 mol dm-3, pH=2.0

95

Fig. 10 log –log plot of (D-DCE) related to Eu(III) and Tb(III) vs.[HPA];

[B15C5] = 0.005 mol dm-3, pH=2.0

96

Fig. 11 log –log plot of (D-DCE) related to Tm(III) and Lu(III) vs.[HPA];

[B15C5] = 0.005 mol dm-3 , pH=2.0

97

Fig. 12 log-log plot of (D-DHPA) related to Ce(III) and Nd(III) vs. [B15C5];

[HPA] = 0.005 mol , dm-3 , pH=2.0

98

Fig. 13 log-log plot of (D-DHPA) related to Eu(III) and Tb(III) vs. [B15C5];

[HPA] = 0.005 mol dm-3 , pH=2.0

99

Fig. 14 log-log plot of (D-DHPA) related to Tm(III) and Lu(III) vs. [B15C5];

[HPA] = 0.005 mol dm-3 , pH=2.0

100

Fig. 15 D-DHPA)/[B15C5] vs [B15C5] at [HPA] =0.005 mol dm-3, pH2.0

101

Fig. 16 (D-DHPA)/[B15C5] vs [B15C5] at [HPA] =0.005 mol dm-3,, pH2.0 102

Fig. 17 (D-DHPA)/[B15C5] vs [B15C5] at [HPA] =0.005 mol dm-3, pH2.0 103

xviii

Fig. 18 Dependence of pKa of HPA on the concentration (v/v) of 1,4-dioxane in water 104

Fig. 19 Extraction of Eu(III) as a function of pH with HPA in different solvents 106

Fig. 20 log D as a function of pH for Eu(III) with (HPA) in ACAC, DIBK, ONL, Nbe,

CHN, nHNL and DCEE

112

Fig. 21 log – log plot of D related to Eu(III) vs. HPA concentration in ACAC, DIBK,

ONL, Nbe, CHN, nHNL and DCEE

113

Fig. 22 Extraction of Ce, Tb and Lu as a function of pH with HPA in octanol 116

Fig. 23 log D as a function of pH for Ce, Tb and Lu (HPA) in octanol 116

Fig. 24 log – log plot of D related to Ce, Tb and Lu vs. HPA concentration in octanol 117

Fig. 25 Extraction of Ce, Tb and Lu as a function of pH with HPA in cyclohexanone 118

Fig. 26 log D as a function of pH for Ce, Tb and Lu with (HPA) in cyclohexanone 119

Fig. 27 log – log plot of D related to Ce, Tb and Lu vs. HPA concentration in

cyclohexanone

120

Fig. 28 Extraction of Tb and Lu as a function of pH with HPA in DCEE 121

Fig. 29 log D as a function of pH for Eu, Tb and Lu with (HPA) in DCEE 122

Fig. 30 log – log plot of D related to Eu, Tb and Lu vs. HPA concentration in DCEE 123

Fig. 31 Extraction of Eu (III) with HPA, B15C5 and HPA+ B15C5 in Benzene 128

Fig. 32 Extraction of Eu (III) with HPA, B15C5 and HPA+ B15C5 in Toluene 128

Fig. 33 Effect of pH on the extraction of Eu (III) with HPA+B15C5 in Benzene

and Toluene

129

Fig. 34 log – log plot of (D-DCE) related to Eu(III) vs. HPA concentration in

Benzene and Tolueneat constant concentration (0.01 mol dm-3 ) B15C5

129

Fig. 35 log – log plot of (D-DHPA) related to Eu(III) vs. B15C5 concentration in benzene

and toluene at constant concentration (0.01 mol dm-3 ) HPA

130

Fig. 36 Extraction of Eu (III) with HPA, TOPO and HPA+TOPO in chloroform 136

xix

Fig. 37 Extraction of Eu (III) with HPA, TPPO and HPA+TPPO in chloroform 137

Fig. 38 Extraction of Eu (III) with HPA, TBP and HPA+TBP in chloroform 137

Fig. 39 Extraction of Eu (III) with (0.01mol dm-3 ) HPA, TPP and HPA+TPP in chloroform

138

Fig. 40 Effect of pH on the extraction of Eu (III) with (HPA+S) (S= TBP, TOPO and

TPPO) in chloroform

138

Fig. 41 log – log plot of (D-DS) related to Eu(III) vs. HPA concentration in chloroform

at constant concentration TOPO, TPPO and TBP

139

Fig. 42 log –log plot of (D-DHPA) related to Eu(III) vs. [S] Neutral donors(TOPO, TPPO

and TBP) concentration into chloroform at constant concentration HPA

139

Fig. 43 Extraction of Eu(III) with isomolar mixture of HPA and TPPO 140

CHAPTER – 1 INTRODUCTION

1

1. INTRODUCTION

The rare earths are special group of elements placed within the periodic table. This

is a group of seventeen elements with atomic numbers 21, 39, and 57-71. The name

lanthanide is reserved for the elements 58-71. The name rare earths is misnomer, because

they are neither rare nor earths. The early Greeks believed that every thing in the world

was made of four elements: air, earth, fire and water. The earths were substances, which

could not be changed by the temperature then available to the scientists. The first rare

earth was discovered in the early part of the nineteenth century, and resembled the

common earths, which were oxides of magnesium, calcium and aluminum. Since the rare

earths were found to be very rare minerals, they were thus called rare earths. They are not

rare; since cerium is reported to be more abundant in the earth crust than tin, while

yttrium is more abundant than lead and some other elements of this family are also more

abundant than the platinum group elements. All these elements form trivalent bonds and

when their salts are dissolved in water, they ionize to form trivalent ions and the solutions

exhibit very similar chemical properties. The elements scandium, yttrium, lanthanum, and

actinium in the III column of the extended periodic table show similar properties in

aqueous solution. Yttrium and lanthanum are always found associated with the rare earths

in nature.

Rare earths are widely distributed in the earth crust and exhibit a great diversity in

the geological type of deposits. These occur as important constituents of more than 100

minerals and in trace quantity in many others [1]. The rare earth elements can be broadly

placed in two groups i.e., ‘light’ (cerium group) and ‘heavy’ (yttrium group).

The cerium group consists of La, Ce, Pr, Nd, Pm Sm, Eu and Gd while, the

yttrium group comprises of of Y, Tb, Dy, Ho, Er, Tm, Yb and Lu. Monazite, bastnasite,

and zenotime are major ores of rare earth elements. Monazite is orthophosphate of

2

essentially the cerium group elements. Bastnasite is a fluorocarbonate of the same group

and zenotime is the orthophosphate of the heavy group of rare earths but less abundant

than monazite.

Modern placer deposits, particularly the beach deposits in the form of heavy

mineral sands are the major source of monazite in the world and occur in Australia,

Brazil, India, South Africa and USA. Rare earth reserves are widely distributed in the

world but major reserves are found in China, USA and India. China holds almost 80% of

rare earth reserves, USA 11% and India 5% [1].

About 76500 tons of rare earth metals calculated as oxides are currently consumed

in the world per year [1]. This quantity is divided among a dazzling variety of

applications with reference to their special chemical, metallurgical, optical, magnetic and

nuclear properties. Most of the traditional applications of rare earths in industry are based

on their similar chemical properties due to essentially identical outer electronic

configuration of their atoms. For majority of the purposes, mixed compounds of these

elements are sufficient. Some of the major uses of this category are rare earth chloride for

mish metal production and cracking catalyst, oxides for glass polishing, fluorides for

manufacture of arc carbon and metallurgy [2-5]. Metallurgical applications of rare earth

metals and alloys form a sizable outlet [6-10]. The most common uses of rare earth

elements are as phosphors, permanent magnet, batteries and in petroleum industries.

Rare earth phosphors have been extensively used in color TV screens, computer

monitors, fluorescent lights and medical X-ray photography [11-13]. Recent innovations

in the area of phosphors are trichromatic and superdeluxe lamps. Both of them employ

rare earths (RE) ions such as Eu3+, Ce3+, and Tb3+ as activators in an oxide, aluminate or

borate lattice.

3

Permanent magnets are being used extensively in industrial and commercial

appliances. They are used in micro motors, capacitors in computers, audiovisual,

automobiles and household electronics. Permanent magnets are also used in limiting

motors in industrial robots, military and space technology. The Sm-Co magnets are used

in miniature earphones. Nd2Fe14B magnet currently constitute over 25% of world wide

market used in high technology, notably in stepper motors for computers peripherals and

consumers electronic industry [14].

The presence of the alloying elements in aluminum brings down its electrical

conductivity sharply. Rare earth elements (REEs) have low solid solubility and similar

electronic structure to that of aluminum. In addition, REEs form inter-metallic bonds with

some of the alloying elements present in aluminum matrix. This results in strengthening

of Al matrix without affecting ductility and little decrease in electrical conductivity [15-

16].

Similarly, addition of REEs to steel has strong influence on sulfur and / or sulfides

[17]. This results in cleaner steels with the alteration of the shape and distribution of

sulfides and oxysulfides [18]. High-purity individual lanthanides are being used as major

components in lasers, phosphors, magnetic bubble memory films, refractive index lenses,

fiber optics, and superconductors [19, 20].

Although RE-based superconductors are well known, their importance became

greatly enhanced after the discovery of high-temperature superconductors with rare earth

and cupric oxide as major constituents [21]. The role of trace REE in environment is not

clear, as there are conflicting reports such as reports from China indicating that REs are

used as nutrients for getting higher crop yield [22]. Toxicity of Nd salts in mice increased

in the order: chloride < propionate < acetate < 3-sulfonicotinate < sulfate < nitrate [23]

4

and terbium group elements have lesser toxicity than other REE above or below them in

the periodic table [24].

The abundance of REEs in geological materials like rocks and minerals for

elucidating pathogenesis and in the evaluation of the coefficients of partitioning of REEs

between various minerals and melts received major impetus with the availability of

various powerful analytical techniques [25, 26].

Rare earths are of great significance in nuclear technology. Discovery,

exploration, and utilization of nuclear fission is one of the mankind’s greatest intellectual

achievements of 20th century. At present nuclear energy is most conveniently supplied in

the form of electrical energy. Nuclear energy is obtained from fission of uranium. A device

in which fission energy is produced in controlled manner from nuclear fuel is called

nuclear reactor. RE elements have high capture cross-section for thermal neutrons. Due to

this property, these are important in nuclear technology in two ways. Bearing high thermal

neutron cross-section, they may be used as reactor control rods. Due to the great capacity

of europium for neutron absorption, it may be used as europium bearing control rods.

Cerium and yttrium hydrides were successfully tried as neutron moderators because of

their thermal stability. In this way, these may be important component in reactor materials.

On the other hand, due to their high absorption cross-section for thermal neutrons,

these elements if present in nuclear fuel will absorb the neutrons and slow down the

reaction. In this way they are poison for nuclear fuel. If their concentration is too high,

these affect the nuclear fuel by decreasing its thermal conductivity, increasing internal

temperature and changing melting point. As, fission products migrate up and down during

fission, due to which temperature gradient change, which leads to fuel cracking.

At present the nuclear energy is based almost on thermal fission of uranium.

Separation of trans plutonium actinides from the rare earth fission products, which are

5

produced about 40% of all thermal- neutron induced fission of U235 and Pu239 is very

important for any nuclear fuel cycle designed either for actinide production or burn-up for

electricity generation due to their high thermal neutron absorption. After some time, it is

necessary to reprocess the burnt fuel for the separation of fission products. The

technology of spent fuel reprocessing started in 1944 and has been continuously evolving

in the last six decades. A major credit for the success of reprocessing must go to the Oak

Ridge National Laboratory and the Knolls Atomic Power Laboratory who were pioneer in

the development of reprocessing.

The role of analyst in the preparation of high purity nuclear fuel is extremely

important. The neutron economy of uranium fueled nuclear reactors may be significantly

impaired by the presence of rare earth impurities at the level of parts per million (ppm)

which have high absorption cross section for thermal neutrons [27]. Trace impurities

specially those which have high neutron cross section not only hinder in fission reaction

but can also alter the metallurgical characteristics of uranium metal.

Determination of rare earths at ppm level in uranium is very difficult task [28]. A

number of techniques are needed to determine over 40 metallic impurities in uranium,

used as fuel. The techniques include carrier distillation, solvent extraction and

precipitation as separation methods followed by the estimation of these impurities by DC-

arc emission spectrometry, atomic absorption spectrometry (AAS), X-ray fluorescence

(XRF), neutron activation analysis (NAA) and inductively coupled plasma atomic

emission spectrometry (ICP-AES). These instrumental methods suffer from lack of

sensitivity for the determination of REEs. Therefore, direct determination of impurities in

the uranium is often hampered by matrix and spectral interferences [29]. In the last

decade, several approaches have been developed for trace metal analysis using different

instrumental techniques.

6

An electro-thermal atomization-atomic absorption spectrometric (ETA-AAS)

method [30] was used for trace metal analysis by preparing matching matrix standards for

accurate analysis. This technique is less attractive due to single element analysis.

For determination of rare earths in uranium by NAA, prior separation of uranium

is required [28]. This technique is very sensitive for determination of rare earths but

procedure requires a lot of manipulations, which are time consuming.

In XRF, various uranium lines with high mass absorption coefficient affect the

estimation of REEs. Precision and accuracy are probably suffered from interfering lines

of other elements [33,34].

ICP-AES has grown into a major technique to analyze trace metals in uranium. In

this technique uranium solution having REEs impurities is directly aspirated. Multiple

standard addition method may also be used in this technique. In ICP-AES analysis, the

high uranium concentration (up to 35mg/mL) leads to enormous amount of more or less

strong emission lines, completely burying the trace element emission. ICP-AES is an

excellent technique but do not provide detailed results to achieve the conclusion [31, 32].

Therefore, separation and pre-concentration is required. Usually column or extraction is

applied. Comparing mass spectrometer (MS) detection with respect to AES detection,

spectral interferences are reduced and better detection limits are obtained. The high mass

range is not expected to give any problem. However, the mass range 16-80 suffers

polyatomic ions interference. Due to compromised operating conditions and space effect,

the lowest mass exhibit low sensitivities. Ionization suppression effects on analytes are

large in the presence of uranium. Therefore, uranium separation is still required with

liquid-liquid extraction, with which excellent detection limits are obtained.

Despite the availability of the more selective, modern methods of measurement,

successful solution to many analytical problems depends heavily on separation process.

7

Many separation processes of vital interest to the analytical chemists have been

successfully translated to plant scale operation. A review of the literature reveals the

valuable assistance of the solvent extraction technique to many fields of analysis

especially to radiochemistry, colorimetry and for preconcentration in conjunction with

many of the more sophisticated instrumental techniques because of its ease, simplicity,

selectivity, speed and wide scope [35-37].

In solvent extraction, the metal or metals of interest are selectively complexed by

the ligand from feed solution with an organic extractant and after that, these metal ions

are striped from the organic phase to aqueous phase. Separation of rare earths by

extraction is not only time saving but also enhance resolution and reproducibility.

Solvent extraction continued to attract the immense interest of analytical chemists

with the broad variety of topics, from large scale separations and purification processes

for the preparation of carrier free trace level isotopes and new extraction procedures using

familiar reagents to developing and testing new reagents and study of the kinetics as well

as thermodynamic aspects of chemical reactions.

Prior to trace metal analysis in any matrix, liquid-liquid extraction with selective

organic complexing ligand i.e. Tributylphosphate (TBP), Dihexylamide (DHA) and

thenoyltrifluoroacetone (HTTA) is applied for the separation of metal of interest from the

matrix in organic solvents, i. e. carbon tetrachloride, toluene, benzene and chloroform.

Repeated extractions are required for trace metal analysis in uranium. It is also desirable

to avoid excessive pretreatment of sample before analysis [38]. Selective extraction is

possible for small amount of metal ions, which can be separated from large amount metal

ions.

8

For the separation of rare earths, four extraction processes namely,

Tributylphosphate (TBP) process, Versatic acid process, Di-2-ethylhexylphosphoric acid

(D2EHPA) process and Rhone-Poulenc process are of commercial importance [39].

In TBP process, 50% TBP in trimethylbenzene is used as extractant and the

separation factor (SF) for the neighboring rare earths from La – Sm appeared to be 2 – 1.5

and rare earths heavier than Sm can not be separated. Versatic acid process uses 50 %

versatic acid in trimethylbenzene and SF from La – Pr varies from 3 -1.8 and for heavier

REEs is similar to TBP system. D2EHPA gives high separation factors from Sm-Lu and

widely used for preparing Eu concentrates. For Rhone-Poulenc Separation process is not

known but they have outlined a comprehensive manufacturing procedure that uses variety

of extractant for the extraction of REEs. This brief introduction of commercial processes

shows that no simple and efficient process is available for the separation of REEs. A

typical extraction plant may contain 75-80 stages for (1) solvent loading section (2)

countercurrent separation section, (3) stripping section and (4) purification of extractant

and among which ~50 stages are required for the extraction section only due to the low

separation factor for these extractants [39].

The extraction procedures that have been developed at the laboratory stage use

mainly neutral, anionic and acidic types of extractant.

Organo-phosphorus compounds such as trioctylphosphineoxide (TOPO),

tributylphosphineoxide (TBPO) and TBP etc. [40-42], alkylsulfoxides, dioctylsulfoxide

(DOSO), Di-n-butylsulfoxide (DBSO) etc. [43, 44] and N, N, dialkylamides such as

Dihexylhexaneamide (DHHA), Di-(2-ethylhexyl) isobutyricamide (D2EHIBA) are

classified as neutral extractants [45-47]. The main feature of this class of extractants is

their use in the separation of light actinides such as U and Pu which can exist in IV, V and

VI oxidation state in aqueous medium. As, most of the rare earths exist in III oxidation

9

state, so these extractants have little application for the separation of rare earths. Some

macromolecules having cyclic structures such as crown ethers and calixarenes have also

been used for rare earth separation [48, 49]. The methods based on neutral extractants for

rare earth separation uses high acid and salt concentration which causes waste problems

in nuclear industry.

REEs are hydrolysable metals, amines are the extractants that have the potential

for their extraction from basic media [50, 51]. The extraction of lanthanide/actinides by

amine extractants suffers from many of the same limitations as the neutral

organophosphorus extractants.

Roelandt has discussed briefly the application of the TRAMEX (Tertiary Amine

Extraction) process for the purification of Cm242. Primary and quaternary amines are

indicated to be useful for REEs extraction in alkaline medium [38].

A variety of diluents have been used but their nature seems to have little effect on the

extraction efficiency or separation factor. β-diketones [thenoyltrifluoroacetone (HTTA),

Hexafluoroacetylaceton (HFAA)], 4-acyl pryazolones, salicylic acid, and organo-

phosphoric acid or their thio-derivatives [52-55] are the extractants that belong to this

category. Some work has been done evaluating the extraction of the subject metal ions by

sulfonic acids, but this class of liquid cation exchanger exhibit little selectivity, and they

have not proven useful for REEs separations. The separation factors for the REEs by

HTTA / benzene system varies from 1.18 to 9.1 [56]

The phenomenon in which two extractants taken together extract a metal ion

species with a much higher efficiency as compared to the normal additive effect of these

extractants (separately) is called synergism. From the first observation of this

phenomenon of synergism by Blake in 1958, extensive work has been carried out on the

synergistic extraction of 5f elements [57].

10

In the nuclear fuel industry, synergistic extraction has been recommended for the

recovery of the REE. In the extraction of trivalent lanthanides by various mixtures of

extractants, it has been observed that the synergistic enhancement (S.E.= log [KD (1,2)

/KD(1) + KD(2)) in many cases is very high, of the order of 105. This is one of the reasons

for the great interest shown in the synergistic extraction of REEs.

Synergic extraction systems comprising of various acidic extractants HTTA, 1-

phenyl-3-methyl-4-acyl-5-pyrazolone (HPMAP), salicylic acid (SCA), oxine, HDEHP

etc. and various neutral donors TBP, TOPO, DOSO, trioctylamine (TOA),

methylisobutylketone (MIBK), etc. are mostly used for the extraction of REEs. Mathur

has published a review on the synergic extraction of trivalent actinides and lanthanides

with 160 references [58]. The author has suggested that in synergic systems adducts or

mixed ligand complexes are formed that can be used for the optimum separation of

lanthanides from each other. It is clear from this literature overview that inspite of having

so much research work done in this field, no simple, efficient and economical method for

rare earth separation is available and need of such a method still exist.

The class of chelating extractants which have received the most attention in the

recent years have the basic structure of 4-acylpyrazolone [59-61]. Because of their

increased acidity (relative to β-diketones) and various synthetic modifications which can

be made to their basic structure, these extractants present some possibilities for the

improved separation procedures for the f-block elements [54]. This is of great interest for

the treatment and recycling of industrial wastes and particularly nuclear one.

Picrolonic acid (1-p-nitrophenyl-3-methyl-4-nitro-5-pyrazolone, HPA, pKa =

2.52) belongs to pyrazolone family extractants with strong acidity, having capability for

the extraction / mutual separation of lanthanides from acidic aqueous solutions. Literature

11

shows that very little work [62-65] has been done for the separation of lanthanides by

using this reagent.

The f-elements cations are “hard acid” – that is, their bonding in complexes is

rather well described by an electrostatic model and they show strong preferences for oxo-

donor atoms [66].

In view of the importance of the rare earth elements and the synergic extraction

systems comprising picrolonic acid as acidic chelating agent and oxygen based neutral

donors (oxo-donor), this study was proposed for the present research program.

CHAPTER – 2 SOLVENT EXTRACTION

12

2. SOLVENT EXTRACTION

Although solvent extraction as a method of separation has long been known to the

chemists, only in recent years it has achieved recognition among analysts as a powerful

separation technique. Liquid-liquid extraction, mostly used in analysis, is a technique in

which a solution is brought into contact with a second solvent, essentially immiscible

with the first, in order to bring the transfer of one or more solutes into the second solvent.

The separations that can be achieved by this method are simple, convenient and rapid to

perform; they are clean as much as the small interfacial area certainly precludes any

phenomena analogous to the undesirable co-precipitation encountered in precipitation

separations.

Solvent extraction is one of the most extensively studied and most widely used

techniques for the separation and pre-concentration of elements [67-69]. The technique

has become more useful in recent years due to the development of selective chelating

agents [70-73] for trace metal determination. With proper choice of extracting agents, this

technique can achieve group separation or selective separation of trace elements with high

efficiencies. In analytical applications solvent extraction may serve the following three

purposes:

i) Preconcentration of trace elements

ii) Elimination of matrix interference

iii) Differentiation of chemical species.

The procedure is applicable to both, trace and macro levels. A further advantage

of solvent extraction method lies in the convenience of subsequent analysis of the

extracted species. If the extracted species are coloured, as is the case with many chelates,

spectrophotometric methods can be employed. Alternatively, the solution may be

13

aspirated for atomic absorption or ICP-emission spectrometric analysis. If radiotracers are

used, radioactive counting techniques can be employed.

Before going in detailed discussion of fundamental principles of extraction, the

three mostly used terms for expressing the effectiveness of extraction processes are being

defined below. These terms are basic for understanding of theoretical as well as practical

considerations of the subject.

Distribution Ratio (D)

The distribution of a solute between two immiscible solvents can be described by

the distribution ratio “D”.

2

1

][][

AAD = (2.1)

Where [A] represents the stoichiometric or formal concentration of a substance A and the

subscripts 1 and 2 refer to the two phases. Since in most cases, two-phase system is of

analytical interest, an organic solvent and aqueous are involved, D will be understood to

be;

Aq

Org

AA

D][][

= (2.2)

The subscripts org and Aq refer to the organic and aqueous phases respectively.

Percent Extraction (E)

The more commonly used term for expressing the extraction efficiency by

analytical chemist is the percent extraction “E”, which is related to “D” as

OrgAq VVD

D+

=+

=100

V[A]V[A]V100[A]

(E)Extraction %AqAqOrgOrg

OrgOrg (2.3)

14

Where V represent solvent volume and the other quantities remain as previously defined.

The percent extraction may be seen to vary with the volume ratio of the two phases as

well as with D.

It may also be seen from equation (2.3) that at extreme values of “D”, “E”

becomes less sensitive to changes in “D”. For example, at a phase volume ratio of unity,

for any value of D below 0.001, the solute may be considered quantitatively retained in

the aqueous phase whereas for D values from 500 to 1000, the value of “E” changes only

from 99.5 to 99.9%.

Separation Factor (γ)

Since solvent extraction is used for the separation of different elements and

species from each other, it becomes necessary to introduce a term to describe the

effectiveness of separation of two solutes. The separation factor γ is related to the

individual distribution ratios as follows:

γB

A

AqOrg

AqOrg

AqAq

OrgOrg

DD

BBAA

BABA

===][][][][

][][][][

(2.4)

where A and B represent the respective solutes.

In those systems where one of the distribution ratios is very small and the other

relatively large, complete separations can be quickly and easily achieved. If the separation

factor is large but the smaller distribution ratio is sufficiently large then less separation of

both components occurs. It is then necessary to apply various techniques to suppress the

extraction of the undesired component.

2.1 Distribution Law

In the simplest extraction case, the distribution ratio is constant in accordance with

the classical Nernst distribution law, a solute will distribute itself between two essentially

15

immiscible solvents so that at equilibrium the ratio of the concentrations of the solute in

the two phases at a particular temperature will be constant, provided the solute is not

involved in chemical interactions in either phase [74]. For such a solute, then

DAA

KAq

Orgd ==

][][

(2.5)

where “Kd” is termed as the distribution coefficient.

Deviations from the distribution law arise from two sources: (a) neglect of activity

corrections and (b) participations of the distributing solute in chemical interactions in

either or both of the two solvent phases. Although the distribution law, as described in

equation (2.5) is not thermodynamically rigorous, variation in Kd due to variation in

activity coefficients is likely to be under one order of magnitude for most extraction

systems of interest to analysts. Far more important are the changes in extraction

characteristics of solute because of chemical changes, which occur. Such changes do not

represent failure of the law. Rather, they add complexity to the distribution expressions,

which can be properly accounted for by using appropriate equilibrium expressions.

The distribution of acetic acid between benzene and water may serve as an

illustration of the effects of chemical interactions of the solute. The distribution of acetic

acid itself may be described as follows:

OrgAq COOHCHCOOHCH )()( 33 ⇔

Aq

OrgD COOHCH

COOHCHK

][][

3

3= (2.6)

However, acetic acid dissociates in aqueous phase

+− +↔ HCOOCHCOOHCH 33

WA COOHCH

COOCHHK

][]][[

3

3−+

= (2.7)

16

and forms a dimmer in benzene

233 )(2 COOHCHCOOHCH ↔

23

23

][])[(

O

OP COOHCH

COOHCHK = (2.8)

The overall distribution of acetic acid is described by “D”, which is

][][])[(][

][][

33

2233

3

3−+

+==

COOCHCOOHCHCOOHCHCOOHCH

COOHCHCOOHCH

DW

OO

W

O (2.9)

Upon incorporation of the equilibrium expression in Eq. (2.6), (2.7) and (2.8) in Eq. (2.9)

there results

][1]][21[ 3

+++

=HK

COOHCHKKD

A

OPD (2.10)

This shows how the distribution of acetic acid varies as a function of pH and acetic acid

concentration.

2.2 Process of Extraction

From the above equations, it is clear that three essential aspects are involved in the

extraction of acetic acid:

A: Chemical interaction in the aqueous phase.

B: Distribution of extractable species.

C: Chemical interactions in the organic phase.

These three aspects are shared by almost all extraction systems and serve as the basis of a

useful organizational pattern.

2.2.1 Chemical interactions in the aqueous phase

A major point of differentiation between extraction of organic and inorganic

materials is the extent to which the formation of an uncharged extractable species

depends on chemical interactions in the aqueous phase. Most organic compounds are

17

already uncharged and extractable. Such aqueous phase reactions if do occur might well

transform these to charged non-extractable species, e.g.

+− +↔+ OHRCOOOHRCOOH 32

−+

+↔+ OHHNROHRNH 322

In contrast, most of the inorganic compounds are dissociated, so that in order to

extract a species of interest into an organic solvent, reactions in the aqueous phase leading

to the formation of an uncharged, extractable complex must be utilized. For example, in

order to extract aluminum-III from an aqueous solution of aluminum nitrate, one must

bring about the reaction of the aluminum-III cation with a reagent such as 8-quinolinol to

form aluminum-8-quinolinate, which may be extracted into a variety of organic solvents

such as chloroform or benzene [75].

Therefore, the formation of an uncharged complex is very important in the

extraction of metals and other inorganic species that makes it convenient to classify such

extractions according to the nature of the complexes.

2.2.2 Distribution of extractable species

Although the ratio of solubilities of a solute in each of two solvents may not be

critically equated to the distribution coefficient of the solute between the two solvents

[76], the underlying factors affecting relative solubility and distribution are undoubtedly

similar. It is, therefore, useful to discuss solubility characteristics of various types of

substances and to note structural effects in both solvent and solute on the solubility.

In solutions where specific chemical forces are not active, the classical principle

of “like dissolves like” is of great help in predicting solubility. This principle may be

expressed in modern terms as Hildebrand’s theory of regular solutions from which, the

solubility is seen to increase as values of the solubility parameter “δ” of solute and

18

solvent approach each other [76]. The solubility parameter, defined as the square root of

the heat of vaporization per milliliter, is a measure of cohesive energy density.

Comparison of solubility parameters should be of maximum assistance of dealing with

those organic extraction systems in which specific chemical or associative forces are

inoperative. Burrell has successfully used solubility parameters to rationalize the

solubility behavior of various polymers [77].

In systems, where hydrogen bonding may be present, particularly those involving

an aqueous phase, the solubility parameter is inadequate in predicting solubility. This

might be expected in as much as this concept is, strictly speaking, applicable only in

regular solutions. Collander has been able to observe regularities in distribution

characteristics in systems involving hydrogen bonding. On the basis of the determination

of Kd values for two hundred organic compounds in the ethyl ether-water system, he

noted that low Kd values were obtained for compounds having groups capable of

hydrogen bonding, such as alcohol, amines, carboxylic acids, and acid amides [78].

Increasing the molecular weight of the organic portion of the molecule would increase the

Kd value about two to four times for each additional methylene group in the homologous

series. The effect to the oxygen in the molecule seemed to be about the same for alcohols,

ketones, aldehydes and carboxylic acids. Increase in Kd resulting in replacing alcoholic or

carboxylic hydrogen with methyl group seemed to be little more than would be expected

upon the increase in molecular weight. Increase in Kd were observed with the introduction

of a halogen atom.

Pasquinelli has been able to correlate the mutual solubilities of a pair of liquids

with the electric and magnetic properties of the pure components [79]. The relation, that

has been used to predict solubilities with probable absolute error of about ± 3% for 100

pairs of liquids, involves the dipole moments, dielectric constant, specific magnetic

19

susceptibility and molar volume. In as much as the prediction may be made for systems

involving hydrogen bonding, the relation may be more generally applicable than the

comparison of solubility parameters.

Solubility of metal salts in aqueous media can be explained on the basis of two

special properties of water. First, its high dielectric constant permits dissociation of ionic

species relatively easily. Even more important, the high basic character of water results in

the solvation of cations (and anions), which gives these ions a solvent sheath serving to

reduce electrostatic interaction and to make the ions more “solvent-like”. The role of the

complex forming extraction agent is largely to replace the coordinated water from around

the metal ion to give a species that is more likely to be soluble in organic solvents. The

solubility characteristics of metal chelates in organic solvents in general terms are not at

all unlike those of conventional organic compounds. For example, hydrocarbon

substituents will increase the solubility of chelates in organic solvents. Although the

neodymium chelate of cupferron-(I) is not soluble in chloroform where as the

corresponding neocupferron (II) is soluble in chloroform.

Polar substituents will of course reduce solubility in organic solvents. The chelates

of 8-quinolinol-5-sulphonic acid are not at all soluble in organic solvents but are quite

soluble in water.

N

N = O

O N

O

O

= N

(I) (II)

20

Among the ion association complexes, the oxonium type is noteworthy, since in

most cases the solvent participates directly in complex formation. The ability of the

oxonium solvent to replace water from the coordination sphere of the metal would depend

upon the basicity of solvent, which in turn would reflect the electron density and steric

availability of the electron pair in the oxygen of the solvent molecule. Many ion

association extractions are aided by the use of salting-out agents, electrolytes used in high

concentrations to;

(a) Produce a mass action effect by adding a common ion,

(b) Reduce water activity greatly,

(c) Lower the dielectric constant so as to favor ion-pair formation.

The use of salting-out agents in organic extractions is also well known.

2.2.3 Chemical interactions in organic phase

Chemical interactions of the extracted species in the organic phase would

naturally lower its concentration in this phase and hence, improve extractability. If, in the

case of a carboxylic acid extraction, the organic solvents is one in which the acid

dimerizes, this would result in a higher D value than if the reaction does not occur. Ion

association complexes, being dipoles, tend to form higher aggregates in organic solvents

at higher concentrations. Where there is a polymerization reaction of any type, the value

of D will be found to vary with the concentration of the extracted material.

2.3 Extraction Systems

2.3.1 Types of inorganic extractable complexes

Most salts are strong electrolytes whose solubility in water can be attributed to the

high dielectric constant of water which greatly reduced the work of dissociation and

21

solvating tendency of water since hydrated ions experience less inter ionic attraction and

resemble more closely the medium in which they are dispersed. In fact, for a metal to

form an extractable complex, it is necessary to remove some or all of the water molecules

associated with the metal ion.

Complexing of metal ions leading to the formation of uncharged species falls into

two main categories, one involving coordination and the other ion association.

2.3.1.1 Coordination complexes

A coordination complex, as the term implies, is formed by coordinate bonds in

which a previously unshared pair of electrons on donor atom or ion is now shared with an

acceptor atom or ion [80]. Three types of coordination complexes are of interest here:

2.3.1.1.1 Simple or monodentate complexes

In simple or monodentate complexes, central metal ion acting as acceptor having a

coordination number “n”, accepts ‘n’ pairs of electrons from ‘n’ individual donor groups,

e.g.

44 :4 GeClClGe →++

−+ →+ 43 :4 FeClClFe

++ →+ 2433

2 )(:4 NHCuNHCu

From the above examples, only the first one give the neutral, extractable complex.

2.3.1.1.2 Chelate or polydentate complexes

Chelate or polydentate complexes [81] with the central metal atom or ion having

coordination number n, combines with no more than n/2 molecules of a specie having at

least two donor atoms per molecule; these being so located as to permit the formation of a

relatively strain-free (i.e. , 5-6 membered) ring, e.g.

22

1/3 Fe2+ +N N

N N

Fe/3

dipyridyl cationic

2+

1/2 Cu 2++ CH 3 C CCH

O HO

CH 3 3CH

OO

HC

CC3CH

Cu /2

acetylacetone uncharged

+ H +

0

Chelates have relatively large stability constants, so their formation greatly lowers the

concentration of hydrated metal ion. Those chelating agents such as acetylacetone,

cupferron, dithizone, and 8-quinolinol form uncharged, essentially covalent compounds,

which are readily soluble in organic solvents. Chelating agents such as dipyridyl or

ethylene diamine tetra acid (EDTA) which form charged chelates are useful as metal

masking agents.

23

Table-2.1 Metal Extraction Systems Chelate Systems A. 4-Membered ring systems Reactive Grouping

1. Dialkyl dithiocarbamates (-)

N C S

2. Xanthates (-)

S C S B. 5-Membered ring systems

1. Benzoylphenylhydroxylamine (-)

O C N O

2. Cupferron (-)

O N N O

3. a-Dioximes (-)

N C C N

4. Dithizone (-)

N N C S

5. 8-Quinolinols (-)

N C C O

6. Toluene-3:4-dithiol (-) (-)

S C C S

7. Catechol (-) (-)

O C C O C. 6-Membered ring systems

1. β-Diketones and Hydroxycarbonyls

a) Acetylacetone b) Thenoyltrifluoracetone c) Morin d) Quinalizarine

(-)

O C C C O

2. Nitrosonaphthols (-)

O N C C O

3. Salicylaldoxime (-)

N C C C O

1. Pyridyl-azo-naphthol (PAN) (-)

N C N N C C O

24

2.3.1.2 Ion association complexes

Ion association complexes are uncharged species formed by the association of

ions because of purely electrostatic attraction. The extent of such association increases

sharply as the dielectric constant of the solvent decreases below 40 to 50 [82]. This

condition not only exists in all of the commonly used organic solvents but also in highly

concentrated aqueous solutions of strong electrolytes [83]. Ion-pairs, which preferentially

dissolve in the organic phase, are those, which resemble the solvent. Ion association

complexes are capable of forming clusters larger than just pairs with increasing

concentration, particularly in organic solvents. In some cases, aggregates large enough to

be described as micelles are encountered.

Two categories of ion association complexes may be recognized. The first

includes those ion-pair formed from a reagent having large organic ion such as

tetraphenylarsonium ion, tribenzylammonium ion or perfluorobutyrate ion. These

reagents combine with a suitable metal-containing ion to give a large organic solvent-like

ion-pair. The second type of ion-pair is essentially like that of the first with the exception

that solvent molecules are directly involved in its formation. Thus in the extraction of

uranyl nitarate with isobutyl alcohol, the extractable complex is probably

UO2(BuOH)6.(NO3)2 in which the coordinated solvent molecules contribute both to the

size of cation and the resemblance of the complex to the solvent [84]. Most of the

solvents which participate directly in the formation of ion association complexes are

containing oxygen. The term oxonium complex is used here to describe such a complex,

since the solvent molecules from coordinate linkages to the metal atoms through their

oxygen atoms.

25

Table-2.2 Ion Association Systems

A. Metals contained in cationic number of ion-pair

1. Alkylphosphoric Acids

2. Carboxylic Acids

3. Cationic chelates

a. Phenanthrolines

b. Polypyridyls

4. Nitrate

5. Trialkylphosphine oxide

B. Metal contained in anionic number of ion-pair

1. Halides (GaCl4-)

2. Thiocynate [Co (CNS)42-]

3. Oxyanions (MnO4- )

4. Anionic Chelates [Co (Nitroso R salt)33

In view of the important role played by complex formation in inorganic extraction

systems, it is appropriate to classify these systems in terms of the type of complex formed

[85]. Two broad categories are utilized in Table 2.1 and 2.2.

Chelate extraction systems include only those involving neutral chelates, since

charged chelates must pair with oppositely charged ions to form extractable species. It

will be noted that the chelate systems are ordered with respect to the size of the chelate

ring.

Differentiation of ion association extraction systems is based on the sign of the

charge of the metal-containing ion. In those systems in which the metal is part of the

26

anion, a further classification on the basis of the nature of the cation is helpful. These

cations are usually varieties of “onium” ions such as oxonium, ammonium, arsonium,

etc., as mentioned in the Table 2.1.

Organic extractions may be classified in a manner similar to that utilized for metal

extractions that is on the basis of chemical interactions. However, aside from

conventional acid-base reactions which permit control of the extraction via pH, the

utilization of chemical reactions in organic extractions are not frequently encountered. In

view of the vast number of organic compounds and biological material that extract, a

simple method of classification of organic extractions based on the class of compounds to

which a particular solute belongs is often used [86, 87] and any general organic text may

be consulted for these classes.

2.4 Extraction Equilibria

A consideration of the extraction equilibria from a quantitative standpoint is

helpful in pointing out which experimental parameters play an important role in the

completeness as well as the selectivity of the extraction. Apart from the utility in predicting the course of extractions, such a quantitative

treatment opens the way for understanding the applicability of extractable substance.

Treatment of the extraction equilibria is illustrated below. Although a chelate extraction

was chosen to represent the inorganic type, the same general approach may be used for

ion association extractions [88].

2.4.1 Extraction of metal chelates

The equation describing the extraction of metal chelates may be derived by

considering the reactions occurring when an aqueous phase containing a metal ion is

contacted with an organic phase containing a chelate extractant. The steps leading to the

27

extraction may be conveniently visualized as follows. The chelating agent distributes

between the two phases. Since the majority of chelating extractants exhibit an acid

dissociation, the symbol HR will serve as a general formula for the reagent.

orgaq HRHR ↔

aq

OrgD HR

HRK

R ][][

= (2.11)

The regent will dissociate in the aqueous phase

−+ +↔ RHHR

][]][[

HRRHK a

−+

= (2.12)

To give a chelating anion R-, which reacts with the metal ion and forms the extractable

chelate

nn MRnRM ↔+ −+

nnn

f RMMR

K]][[

][−+= (2.13)

Which, in turn distributes between the phases

)()( orgnaqn MRMR ↔

aqn

OrgnD MR

MRK

X ][][

= (2.14)

The distribution ratio “D” can be evaluated from these equilibrium expressions if the

metal chelate MRn, may be assumed to be the only metal-containing species in the organic

28

phase and the metal ion, M+, essentially the only metal-containing species in the aqueous

phase. Thus

nAq

nOrg

nD

Dn

af

Aqn

Orgn

Aq

Org

HHR

K

KKKMMR

MM

DR

X

][][

][][

][][

++ ==≡ (2.15)

The validity of this equation was first verified by Kolthoff and Sandell [89] for

dithizone extractions and later by Furman et al. for cupferron extraction [90], extends to

many chelate extraction systems.

2.4.1.1 Effect of the reagent

Eq. (2.15) clearly indicates the importance of chelate stability (Kf) and the relative

solubility of the chelate in the organic phase (KDx). It is also seen, that an acidic reagent

having high Ka and relatively good solubility in water favours good extraction. Since

chelate stability increases as reagent acidity decreases, these effects must be considered

together [91]. Thus, if Ka values of a family of reagents increase faster than do the

corresponding Kf values, the Kf Ka value would be larger for the reagent forming less

stable chelates. This seems to be the case in the β-diketones, with TTA possessing this

type of advantage over acetylacetone.

2.4.1.2 Effect of reagent concentration and pH

It may be noted from equation (2.15) that the extractability of a metal with a given

reagent and organic solvent depends equally upon the organic phase concentration of the

reagent and upon the hydrogen ion concentration in the aqueous phase. A tenfold increase

in the reagent concentration will increase D as much as would a rise of one unit in pH. In

case, where metal hydrolysis is significant, i.e., where the reaction

+−−

+ +↔ HOHOHMOHM nx

nx

1122 )()()(

29

is important, the assumption made in the derivation of equation (2.15) that the only metal-

containing species in the aqueous phase is +nM must be modified to include the hydrolysis

products. This results in an expression for D in which the average value of the exponent,

and hence the importance of the hydrogen ion concentration decreases, while the reagent

concentration factor remains unchanged [92].

In early extraction work, the reagent concentrations employed were little more

than needed to form the metal chelate [93]. An increase in the pH range of good

extraction was achieved by using somewhat higher reagent concentrations [94], with

employment of very high reagent concentrations a substantial reduction in the pH of

extraction can be achieved, permitting extraction from highly acidic solutions [95]. An

advantage in using high reagent concentrations that is not evident from Eq. (2.15) arises

when the metal involved commonly forms a hydrated, non-extractable chelate, which

with high reagent concentrations is transformed to one in which the coordinated water

molecules are replaced by those of the reagent [96].

2.4.1.3 Effect of metal ion concentration

As indicated by the absence of any metal ion concentration term in Eq.(2.15), the

distribution ratio is independent of initial metal concentration. Thus, both tracer and

macro amounts of metal may be expected to extract to the same extent under similar

equilibrium conditions, provided that the solubility of the chelate in the organic phase is

not exceeded.

2.4.1.4 Effect of the organic solvent

Quite a variety of organic solvents have been employed in metal chelate

extractions and by and large, the nature of the solvent has not been too critical factor in

determining the success of an extraction. The choice of benzene or chloroform for

example, seems to be dictated more by the desire to have the organic extract lighter or

30

heavier than the aqueous phase than by the relative extraction efficiency of the solvents.

However, a closer examination reveals differences, which, may be put to practical use. As

may be seen from Eq. (2.15), the solvent affects two quantities, the distribution

coefficients of the reagent and of the chelate. A change in solvent would bring about the

following relative change in the value of D:

nDD

nDD

RX

RX

KK

KKDD

.

.*

**

= (2.16)

If it is assumed that the change in Kd value for a reagent and chelate are about the

same, then Eq. (2.16) predicts that a change to a solvent in which the reagent is more

soluble will result in lower “D” values for polyvalent metals (n >1). An interesting

confirmation of this may be found in the comparison of the use of carbon tetrachloride

and chloroform for dithizone extractions. Dithizone and its chelates are more soluble in

chloroform than in carbon tetrachloride and extractions with the former solvent require a

higher pH region than when the latter is used [97].

2.4.1.5 Selectivity in chelate extractions

The separation of two metals with a particular reagent-solvent system may be

evaluated with Eq. (2.15). The separation factor “γ” defined as the ratio of D values of the

metals in question, is seen to be

22

11

.

.

2

1

X

X

Df

Df

KK

KK

DD

==γ (17)

The ease of separation of two metals is seen to be depending not only on the

difference in the stability of their chelates but also on the relative solubility of these

chelates in the organic solvent. A sufficiently great difference in solubility may result in

an extraction sequence that differs from the stability sequence. For instance, although

31

nickel-II and cobalt-II form more stable acetylacetonates than does zinc II [98], the latter

is extractable whereas the former two are not [95].

With regard to chelate stability, the order of stability of a number of metal ions

has been shown to be fairly independent of the nature of the chelating reagent employed.

Mellor and Maley [99] list the following stability sequence for bivalent metal ions:

Pd > Cu > Ni > Pb > Co > Zn > Cd > Fe > Mn > Mg

Despite the adherence of the behavior of many reagents to the “natural stability

sequence for metals” a number of interesting exceptions are noteworthy. One such

example involves the exceptionally high stability of the tris-phenanthroline-iron-II

complex [100,101]. The fact that, this complex is more stable than of the corresponding

nickel or cobalt chelats has been attributed to resonance stabilization.

Steric hindrance in a chelating agent can result increased selectivity. For example,

2,9-dimethylphenanthroline (neocuproine) (1)

N

CH 3

N

CH 3(1)

no longer gives the typical phenanthroline like complex with iron-II since the methyl

groups greatly hinder the attachment of three reagent molecules around the iron-II ion.

This hindrance is minimum in the tetrahedral geomety of two reagent molecules about a

univalent tetracoordinated ion such as copper. Steric hindrance of the 2-methyl groups to

chelate formation is the reason for the non-reactivity of 2-methyl-8-quinolinol with the

small aluminium ion [102]. This reagent offers a distinct advantage over 8-quinolinol in

the determination of many metals in the presence of aluminium [103]. It is also possible

32

that reagents containing the mercapto functionality may exhibit a different metal stability

sequence than do those containing oxygen [104].

A more generally applicable approach to increasing selectivity in chelate

extractions than that of depending on “unusual” reagents may be based on the use of

competing complexing agents, called masking agents. These masking agents, illustrated

by cyanide ion or EDTA, form water-soluble complexes with some metals and thus alter

the extraction characteristics of these metals. The use of two competing reagents will

tend, in favorable cases, to exaggerate even small differences in the stability order to the

point where dramatic changes may be observed. For example, copper-II gives more stable

chelates with both 8-quinolinol and EDTA than does uranyl ion and hence in the presence

of EDTA only uranyl ion may be extracted with 8-quimolinol [105].

2.5 Kinetic Factors in Extraction

The previous discussion has been based on the assumption that the system had

achieved equilibrium. Although, in most cases, optimum extraction is obtained under

equilibrium conditions, occasionally the slow extraction rate of one or more components

may serve to improve selectivity. The rate of extraction depends on factors affecting (a)

the rate of formation of the extraction species and (b) the rate of transfer of the extractable

species.

As expected the rate of formation of ion association complexes which involve

essentially electrostatic forces is very rapid. Formation of metal chelates on the other

hand may sometimes take place at measurable rates. Slow extraction has been observed

with some of the chelates of dithizone [106] and thenoyltrifluoroacetone (HTTA) [107].

The presence of EDTA has been found to reduce the rate of extraction in a number of

instances [105,108]. In cases where the formation of metal chelate is the rate-determining

step, the extraction may be speeded by increasing the concentration of the reagent [109].

33

The rate of transfer of the extractable species from one phase to the other is

relatively rapid when reasonable agitation is employed. Barry et al. have shown that

simple repeated inversion of the two phases is sufficient to give equilibrium in a relatively

short time even if the species concerned possessed a relatively high molecular weight

[110]. Unless the liquids are viscous, it may be expected that transfer rates are sufficiently

high to permit equilibration with a shaking time of several minutes.

2.6 Methods of Extraction

Three basic methods of liquid-liquid extraction are generally utilized in the

analytical laboratory. Batch extraction, the simplest and mostly used method, consists of

extracting the solute from one immiscible layer by shaking the two layers until

equilibrium is attained, after which the layers are allowed to settle before sampling. The

second type, continuous extraction, makes use of a continuous flow of immiscible solvent

through the solution or a continuous countercurrent flow of both phases. In the former

case the spent solvent may be stripped and recycled by distillation or fresh solvent may be

added continuously from a reservoir. Extraction by discontinuous countercurrent

distribution is the third general type and is used primarily for fractionation purposes. A

series of separatory funnels or contacting vessels are employed to achieve many

individual extractions rapidly and in sequence. The choice of method to be employed will

depend primarily upon the value of the distribution ratio of the solute of interest, as well

as on the separation factors of the interfering materials.

The basic principles in designing an extraction for laboratory use are relatively

simple and just a few basic types of apparatus are more than adequate for most needs. An

infinite number of modifications of these basic designs, however, have resulted from the

chemist’s desire to improve a particular apparatus for the specific problem.

34

2.6.1 Batch extraction

The simplest extraction procedure possible and the technique most employed in

the laboratory for analytical separations involves the bringing of a given volume of

solution into contact with a given volume of solvent until equilibrium has been attained,

followed by separation of the liquid layers. If necessary, the procedure may be repeated

after the addition of fresh solvent. This batch extraction process provides rapid, simple,

and clean separations, and is more beneficial when the distribution ratio of the solute of

interest is larg. In such instances, a few extractions will effect quantitative separation.

Various methods for increasing the distribution ratio as well as the selectivity of an

extraction are discussed later.

The most commonly employed apparatus for performing a batch extraction is a

separatory funnel, since it is a relatively simple matter to add and withdraw the respective

liquid phases. When extracting from a heavier liquid to a lighter solvent, it is necessary to

remove the lower phase from the funnel after each extraction before removing the

extracting solvent as in the case of ethyl ether extractions from aqueous solutions.

When performing a batch extraction, it is important to follow a few simple steps

to separate the phases for sampling for subsequent processing or estimation. Most batch

extractors are separatory funnels taper off into a narrow bottom with a sealed stopcock.

Thus, it is a relatively easy task to separate the two phases on withdrawal for further

processing. It is, of course, essential to wait until the phases have completely settled after

agitation. Usually this will occur in a matter of minutes.

If only aliquots of the phases are to be used, it is necessary to notice any volume

changes of the phases due to mutual solubility of the solvents. The extraction and

sampling must be performed at a constant temperature, since both the distribution ratio

and the volumes of the solvents are influenced by temperature changes. A useful method

35

of withdrawing the phases for sampling involves the use of three graduates. Most of the

heavier phase is withdrawn into the first graduate and then the remainder of the heavier

phase and a little of the lighter phase are withdrawn into the second graduate. The

remaining portion of the lighter phase is run into the third graduate, and the volumes of

the three are noted. The second graduate can now be discarded and aliquots of the other

two taken without danger of contamination of one by the other.

If droplets of aqueous phase are entrained in the organic extract, it is possible to

remove them by filtering the extract through a dry filter paper. The aqueous droplets will

be absorbed by the paper, which should be washed several times with fresh organic

solvent. Another method commonly used in extractions is the addition of a drying agent,

such as sodium sulphate, to the organic extract. Perhaps, the simplest technique for

removal of slight traces of the aqueous phase which may contain certain impurities is the

use of the backwash technique, described later.

2.6.2 Continuous extraction

Continuous extractions are particularly applicable when the distribution ratio is

relatively small, so that a large number of batch extractions would normally be necessary

for quantitative separation. Most continuous extraction devices operate on the same

general principle, which consists of distilling the extracting solvent from a boiler flask

and condensing it and passing it continuously through the solution being extracted. The

extracting liquid separates out and flows back into the receiving flask, where it is again

evaporated and recycled while the extracted solute remains in the receiving flask. When

the solvent cannot easily be distilled, a continuous supply of fresh solvent may be added

from a reservoir.

High efficiency in continuous extraction depends on the viscosity of the phases

and other factors affecting the rate of attaining equilibrium, the value of the distribution

36

ratio and the relative volumes of the two phases and other factors. One practical method

of improving the efficiency is to ensure as high an area of contact as possible between the

two liquids. As the extracting solvent passes through the solution, fritted-glass discs,

small orifices, baffles and stirrers may be used to bring the two immiscible layers in

closer contact.

2.6.3 Countercurrent extractions

The separation through continuous countercurrent method is achieved by virtue of

the density difference between the fluids in contact. In vertical columns, the more dense

phase enters at the top and flows downwards while the less dense phase enters at the

bottom and flows upwards. Only one of the phases can be pumped through the column at

any desired flow rate, the maximum rate of the second will be limited by that of the

former and the physical properties of both. The method has the advantage for separating

materials for purification purposes and is extensively used in engineering problems.

2.7 Factors Influencing the Extraction Efficiency

Primary requirement of solvent extraction for separation /removal purposes is a

high distribution ratio of the solute of interest between the two liquid phases. Though,

continuous and countercurrent distribution techniques may be used for the cases where

low distribution ratios are present, it is generally desirable to attain as high a value as

possible for the development of simple analytical procedures. It is useful to employ a

number of different techniques for enhancing the distribution ratio. These depend on the

nature of the species being extracted and extraction system.

The attainment of selectivity in an extraction procedure is also very important.

Some of the factors, which affect the distribution of solute of intrest are given below.

37

2.7.1 Choice of solvent

Use of a suitable solvent for effective separation is very important. Metal chelates

and many organic molecules, being essentially covalent compounds do not impose many

restrictions on the solvent and the general rules of solubility are of great use. In ion

association systems and particularly in oxonium type ions, the role of solvents is very

important. This is due to involvement of solvent in the formation of extractable species.

In addition to the consideration of the distribution of the solute in a particular

solvent system, the ease of recovery of the solute from the solvent is important for

subsequent analytical processing. Thus, the boiling point of the solvent or the ease of

stripping by chemical reagents is considered in the selection of a solvent where the choice

exists. Similarly, the degree of miscibility of the two phases, the relative specific

gravities, viscosity and tendency to form emulsions should be considered. With regard to

safety, the toxicity and flammability of the organic solvents must be considered.

Some times it is possible to achieve the desired characteristics of a solvent by

employing a mixed-solvent system. An example of this is the use of mixtures of alcohols

and ethers for the extraction of the thiocyanate complexes of metals. Another method of

varying the properties of the extracting solvent is to use organic diluents. Various organic

compounds such as kerosene and other hydrocarbons are employed to dilute tributyl

phosphate for extraction purposes.

2.7.2 Acidity of the aqueous phase

The extractability of metal complexes is greatly influenced by the acidity of the

aqueous phase, so it is necessary to assure optimum concentration of H+ ions for

maximum extraction.

In case of chelate extraction, it can be seen from Eq.(2.15) that provided the

chelating reagent concentration is maintained constant, the distribution of the metal in a

38

given system is a function of pH. For this reason, curves of extractability versus pH at

constant reagent concentration are of great analytical significance.

Acidity greatly affects many of the oxonium type of extraction systems,

particularly when the metal is extracted as a complex acid. In many of the extractions

involving metal halide complexes, e.g., the distribution increases within certain limits

with increasing acid concentration. Thus, maximum extraction of iron as the chloride is

observed at 6M hydrochloric acid using ethyl ether as the solvent [111]. A decrease in

extraction at higher acid concentrations has been attributed to the high solubility of ethyl

ether in highly concentrated hydrochloric acid and extraction does not occur until there is

a much higher acid concentration with the less soluble isopropyl ether. No decrease is

observed even at 12 M hydrochloric acid when β: β’ – dichloroehtyl ether is used [112].

The addition of high concentrations of acid also enhances the distribution of metal

complexes as a result of the common ion effect resulting from the anion of the acid.

As the removal of the acidic or basic solute will tend to change the pH value, even

more important in multiple extractions (countercurrent distribution), the use of buffer

mixture in the aqueous phase aids in the attainment of reproducible constant distribution

ratios. Buffers should be chosen which do not interfere in the subsequent analysis [113].

2.7.3 Salting-out agents

A technique that has resulted in marked enhancement of extraction of metals,

particularly in the oxonium type of extraction systems, is the use of salting-out agents.

The addition of high concentrations of inorganic salts to the aqueous phase greatly

increases the distribution ratio of many metal complexes to the organic phase. This

salting-out effect may be explained in part by the pronounced effect of the added salt on

the activity of the distributing species, the common ion effect, as well as the strong ability

of these ions to bind water around them, thereby depleting the aqueous phase of water

39

molecules for use as a solvent. High concentrations of inorganic salts are usually required

to produce the desired effect, the aqueous phase often being saturated with the added salt.

It is essential that the added salt` is not extracted to an appreciable extent with the desired

species in order to maintain the optimum effect and to permit direct use of the organic

extract without further separation. Sometimes the aqueous phase after extraction may be

of interest so that the presence of large amounts of added cations prevent further use of

this phase in the subsequent analytical steps unless the added salt can be easily removed

or destroyed, like ammonium salts.

In addition to enhancement of the extraction of the metal of interest using salting-

out agents, it is also possible to increase the extraction of impurities in the system. Thus,

it is necessary to choose an agent that produces a favorable separation factor between the

element of interest and the impurities. The magnitude of enhancement of extraction by the

added salt depends on the charge as well as the ionic size of the added cation for a given

anion. Thus, polyvalent cations provide better salting-out agents, and for a given charge,

the smaller the cationic size, the greater the effect on extraction. However, it must be

remembered that anomalies sometimes result from specific interaction effects. Aluminum

or calcium salts are strong salting-out agents, whereas ammonium salts are much weaker

but analytically more convenient.

Among the metal extraction systems that have benefitted through the use of

salting-out agents are the nitrate, halide, and thiocyanate systems. Other oxonium

extraction systems should be investigated so that useful analytical separations may result

where now only limited extractability of a substance occurs.

2.7.4 Oxidation state

A useful method of increasing the selectivity of metal extractions involves

modification of the oxidation states of the interfering ions present in solution, in order to

40

prevent the formation of their extractable metal complexes. For example, the extraction of

iron from chloride solutions can be prevented by reduction to iron-II, which is not

extractble. Similarly, antimony-V may be reduced to the tervalent state to suppress its

extraction. Conversely, it is important in the preparation of a solution for extraction to

adjust the proper valence state of metal ion required for formation of the complex in order

to insure complete extraction of that element. Selectivity can also be achieved by

variation of the oxidation state of the co-extracted interfering ions during the stripping

operation.

2.7.5 pH

The attainment of selectivity in metal chelate extractions is greatly dependent

upon proper pH control. As has been mentioned earlier, the distribution of chelates in a

given system is a function of pH alone, provided the reagent concentration is maintained

constant. Increased selectivity can be achieved in the extraction of acidic or basic organic

substances by the addition of buffer salts to the aqueous phase to control the pH [113].

2.7.6 Masking

In the extraction procedures for metal pairs that are difficult to separate, masking

or sequestering agents are introduced to improve the separation factor. The masking agent

forms water-soluble complexes with the metals in competition with the extracting agent.

Masking agents form sufficiently strong complexes with interfering metals to prevent

their reactions with the extraction agents, either altogether or at least until the pH is much

higher than the value needed for quantitative extraction of the metal of interest. Very

often the metal of interest also forms a complex with masking agent, with the result that a

somewhat higher pH range is needed for the extraction The application of masking

agents, which include cyanide, tartarate, citrate, fluoride, and EDTA, is restricted largely

to metal chelate extraction systems, since in the highly acidic solutions encountered in

41

many inorganic extraction systems most masking agents, being weak bases, do not

function effectively. Ethylenediaminetetraacetic acid, which is proving a most useful

masking agent, has been applied to dithizone, 8-quinolinol, carboxylic acids, acetyl

acetone, and diethyldithiocarbamate extractions.

2.7.7 Backwashing

An auxiliary technique used with batch extractions to effect quantitative

separations of elements is backwashing. The combined organic phases from several

extractions of the original aqueous phase contain practically all the desired elements and

possibly some of the impurities that have been extracted to a much smaller extent. This

combined organic phase when shaken with one or more small portions of a fresh aqueous

phase containing the optimum reagent /salting agent concentration, acidity, etc., will

result in a redistribution of the impurities in favour of the aqueous phase since their

distribution ratios are low. Under optimum conditions, most of the elements of interest

will remain in the organic layer, since their distribution ratio is high. This technique is

analogous in many respects to the re-precipitation step in a gravimetric precipitation

procedure. With the proper conditions, most of the impurities can be removed by this

backwashing operation, with negligible loss of the main component, thereby attaining a

selective separation.

2.8 Synergic Extraction

Synergism is defined as the combined action of two complexing reagents, which

is greater than the sum of the actions of the individual reagents used alone. A typical

example of the synergic extraction of Ce(III) with picrolonic acid (HPA) and benzo-15-

crown-5 (B15C5) system [114] is shown in Fig. 1, which reveals that no extraction was

42

observed by the B15C5 while little extraction with HPA was observed but quantitative

extraction exhibits with the mixture.

Ce(III)

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extra

ctio

n (%

)

Fig. 1 Extraction of Ce(III) B15C5(▲), HPA (■) and HPA+B15C5(♦)

In general, the enhancement of the extraction may be attributed to either

thermodynamical changes in the activities of the extractants or the composition of the

metal-bearing species in the organic phase, which is not the same as in the cases of

individual extractant systems. Synergic systems are usually mixture of cation exchange

extractant and coordination extractant and the synergistic effect is thought to operate by

an enhancement of the ease with which the coordination sphere of the metal ion can be

satisfied.

Two methods of accomplishing this have been proposed. In the first, the synergist,

“S” replaces coordinated water in an extracted metal complex, thus making the resulting

complex more organophilic. In the second, the original extractable complex is

coordinately unsaturated and the synergist “S” adds to the complex, thereby enhancing its

stability.

Synergism was first reported in the literature for the extraction of uranium (VI)

with various dialkylphosphoric acid and neutral phosphorus alkylesters [115]. Now the

synergism appears to be a common phenomenon in many mixed extractant systems. In

43

addition to organophosphorus compounds, hetrocyclic bases, sulfoxides, carboxylic acids,

phenols and amines are also common synergists. The synergistic effect is extremely

useful in solvent extraction practices and has been used for the extraction of various alkali

and alkaline earth metals [116-119], transition metals [120-123] and rare earths [124-129]

using different combinations of reagents.

2.8.1 Methods used for the study of synergistic extraction

The study of the nature of species formed and the equilibrium constants involved

with the synergistic extraction of metal ions have mostly been evaluated by the slope

analysis method [52]. This method basically deals with the determination of distribution

coefficient values (Kd) of the metal ions by varying one of the parameters ie., pH,

concentration of one extractant (chelating agent) or the other (neutral donor), while

keeping the other parameters constant. Sekine and Dyrssen [130] have used a curve fitting

method for the calculation of synergistic equilibrium constants of different metal ions.

Taketatsu et. al., [131,132] have evaluated the equilibrium constant values of the rare

earths using HTTA – TOPO system in the presence of different anions by a

spectrophotometric method. Desreux et.al., [133] have calculated the stability constants

of the adduct formed between Eu(TTA)3 and 4-methyl-2-pantanone, TPPO, or 2-

methylpyridine from the analysis of the concentration dependence on the induced shift

yields by a proton nuclear magnetic resonance study. Due to the simplicity of the “slope

analysis method” it has been widely used to get a clear picture of the stoichiometry and

extraction constant of adducts formed in the synergistic extraction of metal ion, while the

other methods mentioned above have been used only rarely.

44

2.8.1.1 Slope analysis method

Slope analysis method is applied to determine the number of ligand molecules in

simple extraction using single extractant as well as in synergistic extraction where a

neutral donor is also added to the extraction system.

2.8.1.1.1 Extraction with acidic ligand

As an example we can consider the equilibria of a metal ion, Mn+ with chelating

agent (e.g. HA), the extraction of Mn+ by HA alone can be represented by the following

reaction.

++ +→←+ nHMAnHAMorg

An

Korg

n

for which the equilibrium constant KA is

nOrg

n

norgn

A HAMHMA

K]][[

][][+

+

= (2.18)

Anorgn D

MMA

=+ ][][

norg

n

AA HAHDK

][][ +

= (2.19)

or

n

norg

AA HHA

KD][

][+= (2.20)

norg

nAA HAHDK ]log[]log[loglog −+= + (2.21)

orgA HAnHnD ]log[]log[log −+= +

]log[ +−= HpH

45

orgA HAnnpHD ]log[log −−= (2.22)

orgAA HAnnpHKD ]log[loglog ++= (2.23)

At constant concentration of HA, this equation represents the equation of a line. If log DA is poltted a

of intercept of this line. At constant pH, the plot of log DA vs log [HA] will give a straight

line, having slope equal to total number of HA molecules participating in the complex

formation.

2.8.1.1.2 Synergistic Extraction

As an example we can consider the equilibria of a metal ion, Mn+ with chelating

agent (e.g. HA) and a neutral oxo-donor, S (TBP, TOPO, TPPO and DOSO etc.).

Considering the synergistic extraction

++ + →←++ aqorgnK

orgorgnaq nHmSAMmSnHAM nsyn .)( (2.24)

Where m=1 or 2 and nsynK is the mixed equilibrium constant.

It can be shown that

naq

Orgnorg

synsyn HSHA

KD][

][][ 2

11= (2.25)

naq

orgnorg

synsyn HSHA

KD][

][][ 2

22 += (2.26)

The equilibrium constants of such reactions refer only to concentration quotients

whose calculations are based on the assumption that the activity coefficient of the species

involved do not change significantly under the experimental conditions applied. If it is

assumed that M(A)n.2S are the only synergistic specie present in the organic phase, the

overall distribution coefficient D is given by

21 synsynA DDDD ++= (2.27)

46

It can be shown by Eqs.2.20, 2.25, 2.26 and 2.27) that

orgnaq

norg

synnaq

norg

synorg

A SHHA

KHHA

KS

DD][

][][

][][

][ 21 ++ +=− (2.28)

It is clear from Eq. (2.28) that if orgHA][ and aqH ][ + are maintained constant, plot of

vsSDD orgA ][)( −− [-S]org would be straight line with intercept and slope equal to

)]/[}.([1

naq

norgsyn HHAK + and )]/[}.([

2

naq

norgsyn HHAK + respectively. The

organic phase equilibrium constants β1, β2 and K2 of the synergistic reactions

SAMSAM nn .)()( 1→←+ β (2.29)

)(2.)(2)( 2 SAMSAM nn →←+ β (2.30)

)(2.)(.)( 2 SAMSSAM nK

n →←+ (2.31)

can be easily obtained from the slope and intercept mentioned above. Further details of

these calculations can be seen from references [134] and [135].

In earlier publications [136,137,138,139,140] on the synergistic extraction of

metal ions (trivalent lanthanides), several authors have reported the formation of only

M(A)3.(S)2 type species in the organic phase. The conclusions have been drawn from the

observed second power dependence of the neutral donor (S) in the plots of log D vs log

[S]org (keeping other parameters constant). With the knowledge of the stepwise formation

of all the complex species, the above supposition would be highly misleading. However,

in most of the publications of seventies and later, the formation of the stepwise first and

second synergistic species have been reported.

2.8.1.2 Job’s Method

Job’s method or method of continuous variation is also applied to determine the

composition of extracted species in synergistic extraction system [62]. In this method,

47

overall concentration of both the ligands used in the synergistic extraction is maintained

constant (e.g., 0.01 mol dm-3 ) while changing concentration of both the ligands. log D is

plotted vs. mol fraction of the ligands. Number of ligands of both the extractants is

determined from the mol fraction of the ligands where maximum extraction is achieved.

CHAPTER – 3 LITERATURE REVIEW

48

3. LITERATURE REVIEW

Chemical analysis has become an important need of modern age. Analytical

measurements play key role in many areas of every day life i.e. controlling the quality of

products and processes, monitoring health, enforcement of regulations and the protection

of environment etc. Analysis is frequently an essential ingredient in research and

development and innovation. Billions of analytical measurements are undertaken daily in

the world. Many analytical techniques and instrumental methods are used for estimating

trace and ultra-trace elements in different materials. After gravimetric and volumetric

analysis, colourimetric analysis was introduced for the measurements of trace metals. For

measurement or to detect metals at trace level, some organic reagents are required.

Organic compounds have a great role in inorganic analysis. During the past 60-70

years, several hundreds organic reagents described in the analytical chemistry, have been

used in the metals analyses. Many of those reported earlier have been completely replaced

by the new ones. In analytical practice, much more efficient reagents are being used now

adays for metals analyses. Dialkyldithiocarbamate, xanthates, dithizone and toluene-3, 4-

dithiol are the sulphur containing organic reagents and are being used in metals analyses

[141].

Yoe has given a list of organic compounds, which were used in more than twenty

different ways covering a great variety of purposes [142]. Welcher has surveyed the

analytical work published before 1947, listing over seven hundred organic reagents [143].

Organic reagents contain an acidic or basic group. Most of these organic reagents react

with metals by the formation of covalent or co-ordinate bonds or both. The extraction

procedures that have been developed at the laboratory scale use mainly the neutral,

anionic and acidic types of extractant.

49

Organo-phosphorus compounds (TOPO, TBPO, TBP etc.) [40-42], alkyl

sulfoxides (DOSO, DBSO, etc.) [43, 44] and N, N, dialkylamides (DHHA, D2EHIBA)

[45-47] are classified as neutral extractants. The main feature of this class of extractants is

their use in the separation of light actinides such as U and Pu, which can exist in IV, V

and VI oxidation state in aqueous medium. As, most of the rare earths exist in + III state,

so these extractants have little application for the separation of rare earths. Some

macromolecules having cyclic structures such as crown ethers and calixarenes have also

been used for rare earth separation [48, 49]. The methods based on neutral extractants for

rare earth separation use high acid and salt concentration which causes waste problems in

nuclear industry.

The REEs are hydrolysable metals and amines have the potential to extract them

from basic media [50,51]. The extraction of lanthanides/actinides by amine extractants

suffers from many of the same limitations as the neutral organophosphorus extractants.

Roelandt has discussed briefly the application of the TRAMEX process (based on

trialkylamine or tetraalkylammonium salts) for the purification of 242Cm [38]. Primary

and quaternary amines are indicated to be useful for REEs extraction in alkaline medium.

Another class of extractants is termed as acidic extractants. β-diketones (HTTA,

HFAA etc.), 4-acyl pyrazolones, salicylic acid and organo-phosphoric acid or their thio-

derivatives are the extractants that belong to this category [52-55]. Some work has been

cited in the literature evaluating the extraction of the metal ions by sulfonic acids, but this

class of liquid cation exchangers exhibit little selectivity and they have not proven useful

for REEs separations. The separation factors for the REEs by HTTA/benzene system

varies from 1.18 to 9.1 [56].

50

The solvent extraction of the lanthanide elements is a very broad subject and has

been reviewed by several authors. Interest has been shown in their group separation from

trivalent actinide elements, and in their mutual separation.

Sekine and Hasegawa have reviewed the work of several authors [144]. Extraction

of rare earths with various alcohols, with cyclohexanone, from nitrate solution with TBP,

with trioctylphosphine oxide (TOPO), with other alkyl phosphates and phosphine oxides

with mixture of TBP and TOPO, under various conditions have been investigated. The

extraction with TBP is not very effective. The distribution ratios of the heavy lanthanides

between undiluted TBP and 12.3M nitric acid were reported to be from 10 (Terbium) to

7.2 (Lutetium), and those of the lighter lanthanides even lower The higher the

concentration of nitric acid and the larger the atomic number, the better the extraction,

although it reaches a maximum in some systems and then falls off with elements of larger

atomic number. The extraction of trivalent lanthanides from hydrochloric acid with TBP

is also poor. Trivalent lanthanides can be effectively extracted from thioctyanate solution

and with TBP. In the case of TBP, the extraction equilibria for europium (and americium)

were studied in detail. These ions in perchlorate solutions are also extractable with

trialkylphophine oxide. The extraction of perchlorates decreases with decreasing ionic

radius, which is just opposite of the extraction of nitrates or thiocyanates. Extractions of

trivalent lanthanides in various solutions with primary to tertiary amines and with

quaternary ammonium salts have been reported. In general, the extraction with amines is

effective only from sulfuric or sulfate solution. Into xylene, with 5% triisooctylamine,

10% Amberlite LA-1, or 10% Primene JM-T, it is 1% or less when the aqueous phase is

nitric acid or hydrochloric acid. From sulfuric acid with 5% triisooctylamine or 10%

Amberlite LA-1 it is not effective either, but with Primene JM-T it is quantitative if the

aqueous phase contains less than 0.1 M sulfuric acid and the amine concentration is

51

higher than 0.1%. Extractions of organic ligand complexes of lanthanides with amines are

sometimes useful. Extractions of lanthanides as ion-pairs with a dye or other reagents

were also reported. Extractions of trivalent lanthanides with dibutylphosphoric acid and

other monoalkyl and dialkylphosphoric acids and relatives have been frequently studied.

However, the extraction with di(2-ethyl-hexyl) phosphoric acid (DEHP) has been

investigated most systematically . The separation factor of adjacent lanthanides in the

trivalent state by extraction with DEHP is around 2.5, and the extraction is the highest at

pH 1 to 4 when the concentration of the reagent in the diluent is not very low. Extractions

of lanthanides with butyric naphthenic and other carboxylic acids are also effective under

certain conditions. Chelating extractions are another important group of reagents for the

trivalent lanthanides. β-Diketones such as acetylacetone which does not extract these ions

very well by a single extraction and TTA have been used very often, but there have also

been reports on the extraction with other β-diketones. Synergism in the extraction with

TTA and other β-diketones and the extraction of mixed chelates and ternary complexes

with β-diketones has been investigated. Studies on the extraction of some of the

lanthanides with β-isopropyltropolone, with oxine and its homologues and with PAN

have been reported

The phenomenon in which two extractants taken together extract a metal ion

species with a much higher efficiency as compared to the normal additive effect of these

extractants (separately) is called synergism. From the first observation of this

phenomenon of synergism by Blake et al. in 1958 [57], extensive work has been carried

out on the synergistic extraction of 4f block elements.

In the nuclear industry, synergistic extraction has been recommended for the

recovery of the precious metal ions. In the extraction of trivalent lanthanides by using the

various mixtures of extractants, it has been observed that the synergistic enhancement

52

S. E. (S. E. = log [KD (1,2)/KD(1) + KD(2)) in many cases is very high. This is one of the

reasons for the great interest shown in the synergistic extraction of REEs.

Synergic extraction systems comprising of various acidic extractants HTTA, 1-

Phenyl-3-methyl-4-acyl-5-pyrazolone (HPMAP), Salicylic acid (SCA), 8-

Hydroxyquinoline (oxine), HDEHP etc.) and various neutral donors (TBP, TOPO,

DOSO, TOA, MIBK, etc) are mostly used for the extraction of REEs. Mathur has

published a review on the synergic extraction of trivalent actinides and lanthanides and

suggested that synergic systems in which adducts or mixed ligand complexes are formed

can be used for the optimum separation of lanthanides from each other [58].

It is evident from this literature that inspite of having so much research work done

in this field, no simple, efficient and economical method for rare earth separation is

available and need of such a method still exist.

The class of chelating extractants which have received the most attention in the

recent years have the basic structure of 4-acylpyrazolone [59-61]. Because of their

increased acidity (relative to β-diketones) and various synthetic modifications, which can

be made to the basic structure, these extractants possess some possibilities for the

improved separation for the f-block elements [54].

3.1 Use of Picrolonic acid (HPA) in Copmlexation / Extraction

Picrolonic acid (HPA) belongs to pyrazolone family of extractants with strong

acidity, having capability for the extraction/mutual separation of lanthanides from acidic

aqueous solutions. Literature shows that very little work has been done for the extraction

of lanthanides by using this reagent.

Aleksandrov and Aleksandrova have carried out the photometric deterimanation

of Sn(II) with picrolonic acid. Stannous ions are precipitated with picrolonic acid.

53

Deterimanation of Sn(II) up to 125 mg L of solution can be carried out with an average

relative error of 1.58%. Myasoedov et al. carried out the extraction studies of americium

in different oxidation states from nitric acid solutions using picrolonic acid in MIBK as a

function of acidity of solution, concentration of extractant and time of extraction.

Composition of extracted chelate compounds of americium was determined. They

concluded that picrolonic acid and also its mixtures with sulfoxides can be used for the

extractive separation of trans-plutonium elements from nitric acid solutions. During

extraction from nitric acid solutions by a mixture of picrolonic acid with petroleum

sulfoxides (PSO) and dihexyl sulfoxide (DHSO) in xylene, one can isolate Am(V) from

mixture of actinides as it is not extracted appreciably in these conditions [63].

Nikolaev et al. studied the synergistic extraction of trans-plutonium elements

using a solution of picrolonic acid in sulfoxides. The compositions have been determined

for the compounds of americium formed in a mixture of picrolonic acid with dihexyl

sulfoxide (DHSO) in xylene, as AmA3.2DHSO, where A is picrolonic acid ion. Effect of

diluent was also studied. Maximum extraction constant was observed in cyclohexane

solution, but the use of cyclohexane was restricted due to low solubilities of the reagents

in it. Therefore, use of xylene, benzene or toluene was recommended [64].

The synergistic extraction of uranium was carried out by. Kuvatov et al. using a

mixture of sulfoxides and picrolonic acid. Composition of the extractable complex was

established as UO2A2.2S. A general mechanism of extraction was proposed. The

individual sulfoxides i.e., dihexyl sulfoxide (DHSO), diphenyl sulfoxide (DPSO) and

dicyclohexyl-sulfoxide (DCHSO) as well as petroleum sulfoxides (PSO) representing a

mixture of sulfoxides of natural origin, chiefly of cyclic structure were used. The

extraction constants were calculated. These constants exhibit an increasing trend with

54

increasing basicity of the neutral ligand. The extraction involved the formation of a

sulfoxide-picrolonic acid adduct in the organic phase [62].

Osman et al. prepared the complexes of picrolonic acid with the divalent metals

(Mn, Fe, Co, Ni, Cu, and Zn) and trivalent lanthanides (La, Nd, Eu, Gd, and Er). They

characterized the compounds by the chemical / thermal analysis and IR. The electronic

spectra and magnetic spectra suggest the octahedral structure for the metal (II)

complexes. Neutral complexes of general formula M(PA)2.2H2O and M(PA)3.2H2O were

isolated for metal (II) and metal (III) ions respectively (PA = Picrolonic ion).

Coordination of the ligand with the metal ions occurs through the carbonyl and adjacent

nitro-group [145].

Lorenzotti et al. prepared and studied the complexes of Ni(II) with picrolonic

acid. The complexes were characterized by elemental analysis, magnetic susceptibility,

electronic, IR and NMR spectral methods. These complexes were generally insoluble due

to a polymeric structure probably involving a bridging of picrolonic anion [146].

Metal complexes of Mn, Fe, Co, Ni, Cu, and Zn were prepared and studied by

Lorenzotti et al. These were characterized as ML2.H2O, where M is metal and L is

picrolonic acid. The compounds were insoluble in common solvents except DMSO,

where solvation takes place. Some of the compounds were remarkably stable thermally

but some also deflagrate. Picrolonic acid acts as a possibly bidentate ligand. The

complexes were probably polymeric except in dimethyl sulfoxide DMSO [147].

Komarek et al. studied the estimation of Ca using atomic spectrophotometry. They

have extracted Ca using picrolonic acid in order to remove the interference of NH4+, PO4

3-

Cr, Ba, Mn, Li , Al and Ni. The picrolonic acid extraction procedure gave sensitivities of

0.07 and 0.05 ppm in C2H2-air and N2O-C2H2 flames respectively [148].

55

Nina and Valerica have studied the complexes of Zn with picrolonic acid. Zn

reacts with picrolonic acid in the presence of 2, 2 Dipyridyl or 1-10 phenanthroline

forming ternary complexes with a component ratio of 1:2:2. The ternary complexes were

extracted in chloroform. Characterization of the complexes was investigated

spectrophotometrically. The molar absorptivities and the optimal concentration were

determined [149].

Ali has studied the extraction of Eu (III) and Tm (III) using picrolonic acid in

methyl isobutyl ketone. Composition of the adduct has been determined as M (PA)3 (M =

(Eu (III), Tm (III)) [65]. He also studied the extraction of Nd (III), Tb (III) and Lu (III)

with picrolonic acid in MIBK. Composition of complexes was found to be M(PA)3 [150].

Synergistic extraction of Ce (III), Eu (III) and Tm (III) was studied by Ali with a

mixture of picrolonic acid and tributyl phosphine oxide in chloroform. Composition of

synergistic adduct has been determined to be M (PA)3. 2TBPO (M = Ce (III)), Eu (III)

and Tm (III)) [151].

The literature cited indicates limited use of picrolonic acid for the determination

of a few divalent metals and for the complexation of trivalent lanthanide whereas only a

few references have been cited for the extraction studies of uranium and americium in

combination with sulfoxides and dipyridyl as neutral donors, as well as, as single

extrtactant in MIBK, which itself acts as neutral donor. The use of picrolonic acid in

combination with crown ethers as neutral donor has not been reported in literature so far.

3.2 Use of Crown Ethers in Extraction of REEs

Macrocyclic polyethers generally called as “Crown ethers” have gained attention

due to their special selectivity arising presumably from their ring-size comparable with

the ionic radii of certain alkali metals [152-154]. The crown ethers can be used as neutral

oxodonors for the synergistic extraction of various metal ions with chelating, acidic or

56

neutral extractants and the information regarding such extraction systems have been

reviewed [6]. The synergistic extraction systems mostly used for the separation of alkali,

alkaline earth and divalent transition metals; comprises crown ethers and

thenoyltrifouoroacetone [158-163] or 4-acyl-pyrazolones [163-166]. The same extraction

system [167-175] as well as combinations of β-diketone (4, 4, 4-trifluoro-1-phenyl-1, 3-

butanedione) [176] and 4-acyl isoxazolone [177,178] with crown ethers have been used

for the extraction of lanthanides and actinides. Mixtures of crown ethers and

alkylcarboxylic acids [179,180] and organophosphoric acid [181] have also been

investigated for the extraction of various metal ions.

Aly et al. have studied the extraction of Eu3+, Tm3+ and Yb3+ using a mixture of

theonyltrifluoroacetone (HTTA) and 15-crown-5 (15C5) in chloroform from 0.1M ionic

strength acetate buffer. It was found that the extraction increases to a large extent when

the mixture is used. This enhancement is due to formation of adduct in the organic phase.

The complexes were characterized as Ln(TTA)3.2CE where CE is crown ether. No

enhancement in extraction was observed when 12-crown-4 (12C4) was used. This was

explained due to small cavity size of the 12C4 as compared to 15C5. The same system

(HTTA+ 15C5) was applied for the extraction of Pu4+, Am3+, Nd3+ and Er3+. Moderate

enhancement was observed in the extraction of Am3+ while no enhancement was

observed in the case of Pu4+. Effect of metal ion concentration was also studied and it was

found that concentration affect the synergistic factor (S.F) for Nd3+and Er3+[167].

The synergistic extraction studies of trivalent Am, Cm, Cf and Eu were carried

out by Mathur and Khopkar using a mixtures of 1-Phenyl-3-methyl-4-trifuoroacetyl

pyrazolone-5 (HPMTFP) and a crown ether dicylohexano-18-crown-6 (DCH18C6) or

monobenzo-15-crown-5 (B15C5) in chloroform. With (DCH18C6) the synergistic species

extracted were M (PMTFP)3 .(HPMTFP).(DCH18C6) where M = Am, Cm and Eu, and

57

Cf (PMTFP)3.(DCH18C6), where as with B15C5 the species are M(PMTFP)3.n(B15C5),

n being 1 or 2 for all these metal ions. They have studied the effect of pH, HPMTFP

concentration and concentration of crown ethers. They have calculated synergistic

constants for the extraction of all the metal ions mentioned above [175]..

Pavithran et al. investigated the extraction behaviour of Nd(III), Eu (III) and

Tm(III) from perchlorate solution into chloroform with 1-phenyl-3-methyl-4-pivaloyl-5-

pyrazolone(HPMPP) in the presence and absence of various crown ethers i. e.,18-crown-6

(18C6), (DCH18C6) and dibenzo-18-Crown-6 (DB18C6). The complexes were

characterized as Ln(PMPP)3 with HPMPP alone and in the presence of CE as

Ln(PMPP)3.CE. They calculated the equilibrium constants of the extraction of complexes

and found to increase with decreasing ionic radii of these metal ions. The addition of CE

to metal chelate system not only enhances the extraction efficiency but also improves the

selectivity among these trivalent lanthanides [182].

Mathur and Choppin carried out the extraction studies of UO22+, Eu3+, La3+ and

Th4+ complexes of TTA with 12C4, 15C5, 18C6, DCH18C6 and DB18C6 in benzene and

chloroform. They studied the thermodynamic parameters, NMR spectra and the nature of

complexes. The complexes were characterized as Ln(TTA)3.CE, UO2(TTA)2.CE and

Th(TTA)4.CE. They have reported slope 1 for all the crown ethers except that of Eu-

TTA-B15C5 which is reported as 1.5±0.1 showing the equal amounts of extracted species

with 1 and 2 molecules of B15C5 [169].

Reddy et al. studied the synergistic extraction of trivalent lanthanides ( Nd, Eu and

Tm) using mixtures of 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) and 18C6, 15C5,

B15C5 or DB18C6. The trivalent metal ions were extracted into chloroform as Ln(PBI)3

with HPBI alone and as Ln (PBI)3.CE in the presence of crown ethers . The equilibrium

58

constants of the above species were found to increase monotonically with the decreasing

ionic radii of these trivalent lanthanides [183].

Dukov studied the synergistic solvent extraction of Pr, Gd, and Yb with mixtures

of 1-phenyl-3-methyl-4-benzoyl-pyrazol-5-one (HP) and benzo-15-crown-5 in CCl4, C6H6

and CHCl3. The composition of the extracted species were determined as LnP3.nS where

Ln =Pr, Gd, Yb and n = 1 or 2. Mixed complexes Ln P3.S have been found when C6H6

and CHCl3 were used as diluents and both LnP3.S and LnP3.2S when CCl4 was used[171].

Georgiev and Zakharieva carried out the extraction studies of Pr, Gd and Yb with

mixtures of heptanoic acid H3C(CH2)5COOH (HA) and crown ethers B15C5, DCH18C6

and 18C6 in CCl4 as a solvent from aqueous chloride medium at constant ionic strength

0.1mol dm-3. A synergistic effect in the extraction of Pr, Gd and Yb with B15C5 and

antisynergistic effects in the system containing the other two crown ethers DCH18C6 and

18C6 were observed. For Pr, formation of a mixed complex Pr(HA)3.B15C5 is found and

the corresponding equilibrium constants were reported, while Gd and Yb form the

complexes Gd(HA)3.nB15C5 and Yb(HA)3.nB15C5 respectively, where n < 1 [180].

The synergistic extraction of Co(II) and Ni(II) with 1-phenyl-3-methyl-4-benzoyl

pyrazol-5-one (HPMBP) in the presence of crown ethers 18C6 or DCH18C6 in toluene

from 1.0 mol dm-3 chloride medium was investigated by Lakkis et al. [184]. Slope

analysis of the distribution curves showed that the composition of extracted species

depends on the cations found in the aqueous solution. From a LiCl or (CH3)4NCl

solutions the extracted species were M (PMBP)2.CE (M = CO, Ni).

Meguro et al. have studied the extraction of Am3+ in benzene with HTTA and

crown ethers such as 15C5, 18C6, DCH18C6, 24C8 and DB24C8. Synergistic effect by

the CE was observed regardless of the kind of CE used. The extracted species was found

to be Am(TTA)3.CE in the organic phase [185].

59

The synergic extraction of lanthanide ions with HTTA and crown ethers in 1, 2-

dichloroethane was studied by Yoshihiro et al. [186]. Characteristic ion pair extraction of

the lighter Ln (III) was observed with 1, 2-dichloroethane containing HTTA and 18C6 or

DCH18C6 in which the cationic complexes, Ln(TTA)2.CE+ was formed and extracted.

Remarkable increase of extractability and selectivity were attained in the synergistic ion

pair extraction of lighter Ln ions which could be elucidated on the basis of size fitting

effect in complex formation of the lighter Ln ion with CE.

Khalifa et al. investigated the synergistic extraction of Co(II) with HTTA and its

mixture with DB18C6 at different temperatures in nitrobenzene, toluene or their mixture

from perchlorate aqueous media of constant ionic strength(0.1 M; H+,NaClO4) buffered

with acetic acid- sodium acetate solutions [161]. Composition of the adduct extracted was

deduced as Co(TTA)2 and Co(TTA)2.DB18C6. The extraction constants of the chelate

(K2,0), the mixed species (K2,1) and the formation constant of the adduct (β2,1) were

evaluated for different diluents used at different temperatures. It was found that logK2,0

and logK2,1 were increased with the increase in the dielectric constant (E) of the diluents

whereas logβ2,1 was decreased with the increase in E. The thermodynamic constant of the

system were calculated.

Ensor and Shah studied the synergistic extraction of Ce, Pm, Eu, Tm, Am, Cm, Bk

and Cf using HTTA, a nitrogen containing cryptand (222BB) and crown ether (15C5) in

chloroform from an aqueous media containing 0.05M NaNO3 and 0.01M acetate buffer to

control the pH. Neither the 15C5 nor the 222BB showed any ability to extract the

trivalent metal ions by themselves alone under the experimental conditions used.

However, both the compounds showed synergistic activity when combined with HTTA.

222BB showed more enhancement in the extraction than 15C5 [168].

60

Sachleben et al. carried out the solvent extraction studies of Li, Na, K, Rb and Cs

nitrate salts by solutions of crown ethers in 1,2-dichloroethane and 1-octanol. The crown

ethers used were 18C6, 21C7 and 24C8 bearing cyclohexano, benzo, t-alkylbenzo and for

no substituents. The extraction selectivities expressed as separation factor for Cs over Na

were examined in relation to crown ether structure. They reported that cyclohexano

substituted crown ethers extract cations more strongly than the corresponding benzo

substituted crwon ethers [156].

The synergistic extraction of trivalent actinides and lanthanides was studied by

Dale et al. using HTTA and an aza-crown ether, 4,13-didecyl-1, 7, 10, 16-tetraoxa-4, 13-

diaza cyclooctadecane (k22DD) [187]. The extraction of Am(III), Cm(III), Eu(III),

Ce(III) and Pm(III) from an aqueous acetate buffer system (pH 4.8) into

HTTA/K22DD/chloroform phase was studied at 25°C

. Distribution coefficients were measured as a function of pH and HTTA and K22DD

concentration. The synergistic adduct was characterized as M(TTA)3.K22DD. The results

showed that K22DD synergizes the extraction of each metal studied by a factor of 104-105

approximately. Slightly large stability constant were found for the trivalent actinides

relative to trivalent lanthanides.

Richard et al. have studied the extraction of cesium nitrate from a mixture of

alkali metal nitrates by calix(4)arene crown-6 ethers in 1,2-dichloroethane. Results

showed that smaller substituents (but larger than C2) at the phenolic position of calixarene

opposite the crown ether increases both the extraction efficiency and cesium selectivity.

Benzo subsitituents on the crown ethers tend to decrease the extraction, while increase the

cesium to sodium selectivity [188].

Dessouky et al. carried out the synergistic extraction studies of octahedral Co (II)

from nitrate medium by 8-hydroxy-quinoline (HOX) and DB18C6 in chloroform. The

61

effect of stirring speed, temperature, specific interfacial area and interfacial tension on the

extraction showed, that the extraction process is bulk aqueous phase rather than at

interface [189].

Mathur studied the extraction of Am3+ with pyrazolones (HA = HPMBP or

HPMTFP) alone as well as their mixtures with B15C5 or DCH18C6 in chloroform. The

extracted complexes were stoichiometrically identified as Am(A)3.HA,

Am(A)3.HA.DCH18C6, Am(A)3HA.B15C5 and Am(A)3.B15C5 or Am(A)3.2B15C5

[190].

Lin-Mie et al. carried out the extraction of rare earth metals using crown ethers

such as 15C5, 12C4 and DB18C6 from aqueous solution containing picrate ion into

nitrobenzene solution. The rare earth metal ion Eu(III) was extracted as 2:1 crown-ion

sandwich complex with 12C4 but as 1:1 complex with both 15C5 and DB18C6. The

effect of picric acid concentration was also studied. The extracted species of Eu(NO3)3

with 15C5 and Db18C6 were characterized as Eu[(15C5)-(picrate)2.(NO3)] and

Eu[(DB18C6)-(picrate)2.(NO3)], respectively, but Eu[(12C4)2.(picrate)3)] were found

with 12C4. The extraction of the other rare earth ions showed that Tb3+, Eu3+, Gd3+, Nd3+

and Yb3+ can be easily extracted using 15C5, however, the extraction of Ce4+, Sm3+,

Dy3+and Lu3+ was difficult [191].

Dale et al. have studied the extraction of Ce(III), Pm(III), Eu(III) and Tm(III) by a

mixture of didodecylnaphthalensulfonic acid and 15C5. The extraction was carried out

from aqueous solution of pH 2 (0.5M, NaClO4+ HClO4) into toluene and it was found that

extraction of Ce(III), Pm(III) and Eu(III) was enhanced while extraction of Tm(III) was

unaffected [192].

Mohapatra and Manchanda studied the ion pair extraction behaviour of uranyl ion

from aqueous solution of pH 3 using B15C5, 18C6, DB18C6 and DB24C8 and picric acid

62

in chloroform. The extracted species were characterized corresponding to

[UO2(CE)n]2+[pic-] where n = 1.5 for B15C5 and 1 for 18C6 as well as DB18C6. Adduct

of DB24C8 was not observed, as no extraction was found practically using this reagent.

Separation of lanthanides (Nd, Ce, La and Gd) from uranium was studied and separation

factors were calculated using B15C5, 18C6 and DB18C6. Highest separation factors for

these elements were found using B15C5 [193].

Shehata et al. have studied the synergistic extraction of trivalent Gd, Eu and Am

using 15C5 or 18C6 with HTTA in chloroform from perchlorate medium at pH 3.45. The

slope analyses results indicated a general formula of M(TTA)3.(CE)2 for the extracted

species. The extraction constants of the extracted species were also determined [194].

Vanura et al. have studied the extraction of Cs by nitrobenzene solution of

hydrogen bis-1, 2-dicarbolylcobaltate in the presence of 15C5 from a water- HNO3

system [195].

Yonezawa and Choppin invstigated the extraction of UO22+, Am3+ and Th4+ using

1-phenyl-3-methyl-4-benzoyl-5-pyrazolone and 12C4, 15C5, 18C6, DB18C6 and

DCH18C6 [196]. The extraction was carried out from 0.1M (NaClO4) solution into

toluene. The synergic equilibrium constants were calculated.

Pavithran and Reddy carried out the synergistic extraction of trivalent lanthanides

Nd(III), Eu(III) and Tm(III) from nitrate solution into chloroform with 3-phenyl-4(4-

fluorobenzoyl)-5-isoxazolone (HFBPI) in the presence and absence of various crown

ethers 18C6, DCH18C6, B18C6 and DB18C6. They reported that these lanthanide ions

were extracted into chloroform as Ln(FBPI)3 with HFBPI alone and as Ln(FBPI)3.CE in

the presence of crown ethers. The equilibrium constants for these extracted complexes

were found to increase monotonically with the decrease in ionic radii of these metal ions.

The addition of crown ether to metal chelate system significantly improves the extraction

63

efficiency of these metal ions. The strength of trivalent lanthanides complexes with

various CEs follows the order DCH18C6>18C6>B18C6> DB18C6 [178].

Reddy et al. have investigated the synergistic extraction of the trivalent

lanthanides Nd, Eu and Tm with mixtures of 4, 4, 4-tri-fluoro-1-phenyl-1, 3-butan-dione

(HBTFA) and 18C6, DCH18C6 or DB18C6 in 1,2-dichloroethane from perchlorate

solution. The extracted species were characterized as Ln(BTFA)3.CE and heavier

lanthanide Tm(III) was extracted as Tm(BTFA)3.CE. The addition of crown ethers to

metal chelate system not only improves the extraction efficiency of these trivalent metal

ions but also improves the selectivities significantly among the lighter and middle

lanthanides [176].

Sahu et al. carried out investigation on the extraction of thorium (IV) and uranium

(VI) from nitric acid solution into chloroform using a mixture of 3-phenyl-4-benzoyl-5-

isoxazolone (FPBI) and DCH18C6, B18C6, DB18C6, or B15C5 [177]. These complexes

were extracted as Th(PBI)4 and UO2(PBI)2 with HPBI alone and as Th(PBI)4.CE and

UO2(PBI)2.CE in the presence of crown ethers. The equilibrium constants of the above

species were deduced by non-liear regression analysis. The addition of a CE to the metal

chelate system enhanced the extraction efficiency and also improved the selectivities

between thorium and uranium.

From the literature cited above and survey made from a lot of other published

work, it appears that crown ethers are being used extensively for the extraction of metal

ions as single extractant as well as, as neutral donors with many other complexing /

extracting agents. However, no reference has been cited showing the use of crown ethers

as neutral donor in combination with picrolonic acid for the extraction of lanthanides.

CHAPTER – 4 EXPERIMENTAL

64

4. EXPERIMENTAL

4.1 Apparatus

A pH meter model 605 from METROHM Ltd, Switzerland was used for the

measurement of pH of buffer solutions. Electric hot plate (George & Griffin) was used for

dissolution of samples. Nal(T1) scintillation detector coupled with the counting assembly

from Tennelec Inc., USA was used for gross gamma counting. The purity of

radionuclides was checked by p-type coaxial high purity germanium (HPGe) detector

(Eurisys Mesures) with 60% relative efficiency and 1.95 FWHM at 1332 keV γ-ray of

60Co. The detector is connected to an Ortec-570 amplifier and Trump PCI 8k ADC/MCA

card with Gamma Vision-32 ver.6 software. An electrical wrist action shaker from

George and Griffin, UK, was used for mixing aqueous and organic phases. A centrifuge

machine (Gallenkamp) was used for separating the aqueous and organic phases.

4.2 Materials

Picrolonic acid (HPA) was procured from Eastman Organic Chemicals, USA and

benzo-15-crown-5 (B15C5), 12-crown-4 (12C4) and 18-crown-6(18C6) were procured

from E. Merk, Germany. All other chemical used in this study were of Analar grade.

Solvents, acids and chemical reagents used in this study were acetylacetone, benzene,

chloroform, 1-butanol, 2-butanol, n-hexanol, 1-octanol, n-butylether, toluene,

dichloroethylether, cyclohexanone, di-isobutylketone, hydrochloric acid (HCl), nitric acid

(HNO3), perchloric acid (HClO4), sulfuric acid (H2SO4), sodium hydroxide (NaOH),

acetic acid (CH3 COOH), boric acid (H3BO3), potassium chloride (KCl) ammonium

hydroxide (NH4OH), sodium acetate (CH3COONa), tributylphosphate,

tributylphosphineoxide, triphenylphosphate, triphenylphosphineoxide, and were used as

received.

65

For target preparation quartz ampoules (0.5×1.5 cm), polyethylene capsules

(1.25×2.5 cm) and aluminum capsules (2.2×4.6 cm) were used. For extraction of REEs

pyrex glass culture tubes (16x120 mm) with screw caps were used.

4.3 Buffer Solutions

The buffer solutions of pH 1.0 to 3.0 were prepared by mixing appropriate

quantities of potassium chloride (0.1 mol dm-3) and hydrochloric acid (0.1 mol dm-3)

using pH meter. The solutions of pH 4-6 were prepared by mixing 0.1 mol dm-3 solution

of acetic acid with 0.1 mol dm-3 sodium acetate solution. The solutions of pH 7-10 were

prepared by mixing 0.1 mol dm-3 boric acid solution with 0.1 mol dm-3 sodium hydroxide

solution. The stability of the buffer solutions was checked on alternate days.

4.4 Chemicals/reagents used to study anions and cations

effects

Sodium citrate(Na3C6H5O7.3H2O), ascorbic acid (C6H8O6), sodium thiosulphate

(Na2S2O3.5H2O), sodium oxalate (Na2C2O4), sodium tartrate (Na2C6H4O6.2H2O), sodium

acetate (CH3OONa.3H2O), sodium fluoride (NaF), potassium chloride (KCl), sodium

bromide (NaBr), sodium iodide (NaI), potassium thiocyanate (KSCN), potassium cyanide

(KCN), sodium carbonate, and sodium phosphate (Na3PO4) were used to observe the

anions effects. Cobalt chloride (CoCl2), copper sulphate (CuSO4), manganese chloride

(MnCl2), ferric nitrate [Fe(NO3)3], barium chloride (BaCl2), cadmium chloride (CdCl2),

strontium chloride (SrCl2), zirconium chloride (ZrCl2), lead nitrate [Pb(NO3)2], nikal

nitrate [Ni(NO3)2], chromium chloride (CrCl3), zinc chloride (ZnCl2) and magnesium

floride (MgCl2) were used to study effect of cations on the extraction of REEs using this

synergic system.

66

4.5 Preparation of Radionuclides

All the radionuclides, i.e.141Ce, 147 Nd, 154Eu, 160Tb, 170Tm, 177Lu, 56Mn, 65Ni,

60Co, 59Fe, 64Cu, and 75Se were prepared by irradiating a known amount (~ 10 mg) of the

respective spec-pure metal or metal oxides (Johnson Matthey Chemical Ltd., England)

separately, in the Pakistan Research Reactor-1 (PARR-1) of Pakistan Institute of Nuclear

Science & Technology (PINSTECH) having an average thermal neutron flux of 5×1013 n

cm2 s-1. The irradiated material was given enough cooling time to allow the decay of the

short-lived radioisotopes formed (if any) during the thermal neutron irradiation process.

The time for thermal neutron irradiation and cooling was selected in such a way that the

major radioactivity was due to the radionuclide of interest. Each irradiated metal/metal

oxide was dissolved in 10 mL concentrated nitric acid in a 100 mL Pyrex glass beaker on

an electric hot plate, separately. The contents of the beaker were heated to near dryness

and then re-dissolved in 5.0 mL of 0.01 mol dm-3 nitric acid and the volume was made up

to 25 mL with de-ionized water and stored in pre-cleaned vials as stock solution. The

radioactivity of 50 µL of each stock solution was measured by using well type NaI(T1)

scintillation detector coupled with the counting assembly from Tennelec Inc., USA., and

further dilution was done using de-ionized water in such a way that the 50 µL of final

solution would have radioactivity of 25000-30000 counts per minute. This solution was

marked as radiotracer solution of particular radionuclide. The metal ion concentration of

appropriate amount of carrier solution (non-active solution of respective metal/metal

oxide was added to the radiotracer solution so that the 50 µL of each radiotracer in 2 mL

aqueous phase give constant metal ion concentration (~ 1.5×10-5 mol dm-3). 137Cs+ and

203Hg2+ solution were provided by isotope production division of this institute. Different

isotopes used in this study along with their half lives are given in Table 4.1.

67

4.5.1 Calculation of the Activity of Radiotracer

Radioactivity of the radiotracer was calculated by the following equation.

Tt eeNA λλσφ −−−= )1( (4.1)

..wtAtfWA

N××

= o (4.2)

Where

λ = 0.693/t1/2

A = Disintegration of radiotracer in one second (dps).

σ = Thermal neutron absorption cross section of isotopes in barn (b=10-24cm).

f = % Abundance.

Ao = Avogadro number = 6.023×1023

φ = Neutrons flux of reactor = 5×1013 n cm-2 s-1

W = Weight of target material in gms

Wt. = Atomic wt. of target material

t = Time of irradiation

t1/2 = Half life of desired isotopes

Curi = 3.7×1010 dps

T= Cooling time

68

Table 4.1 Radioisotopes prepared in PARR-1

Isotope Half life

Ce141 32.5d

Nd147 10.9h

Sm151 93 y

Eu154 8.8 y

Tb160 72.1d

Tm170 128.6d

Lu177 160.1d

56Mn 2.58h

59Fe 44.6d

60Co 5.27y

65Ni 2.52h

64Cu 12.7h

75Se 120d

137Cs

203Hg 46.4d

69

4.6 Experimental Procedures

4.6.1 Extraction procedure A 2 mL portion of aqueous phase of known pH was taken in a glass culture tube

having a screw cap and added 50 µL solution of radiotracer solution of particular

radionuclide to it. Organic phase (2.0 mL) containing a known amount of picrolonic acid

or/and B15C5 in chloroform was added to the culture tube and mixed together using wrist

action electrical shaker for five minutes. After centrifugation for 3 minutes, one mL of

each phase was pippeted out in glass counting vials and assayed radiometrically using

well type Nal(Ti) scintillation counter. All the experiments were carried out in duplicate

and the average results were taken.

The extraction (%) and distribution ratio (D) of 154Eu+3 was deduced from counts

per minute (CPM) of the organic and aqueous phases by the following relationships:

backgroundphaseaqueous

backgroundphaseorganic

CPMCPMCPMCPM

D−

−= (4.3)

)/(

100%phaseorganicphaseaqueous VVD

DExtraction+

×= (4.4)

The same extraction procedure was applied using 141Ce+3, 147Nd+3, 160Tb3+,

170Tm+3 and 177Lu3+ radiotracers for the determination of their distribution ratios. The

estimated error in the distribution coefficient was about ± 4%.

70

4.6.2 Effect of shaking time on extraction of REEs The equilibration time required for the extraction of lanthanide ions with a

mixture of HPA and B15C5 in chloroform was studied by shaking the aqueous and

organic phase. Two mL aqueous buffer solution of known pH containing ~1.5x10-5 mole

dm-3 solution of radiotracer of REEs(III) was taken in pyrex glass culture tubes. An equal

volume of chloroform containing 0.01mol dm-3 HPA and B15C5 was added, equilibrated

for a specific time and centrifuged for three minutes. One mL of aliquot of each phase

was taken and assayed radiometrically on Nuclear Chicago Model 8725 well type NaI(Tl)

scintillation counter for gross gamma counting. All the experiments were carried out in

duplicate and average results were taken for further use. The estimated error in the

distribution coefficient was about ± 4%.

4.6.3 Synergistic extraction with mixture of HPA and B15C5

The extraction of rare earth ions Ce(III) , Nd(III), Eu(III), Tb(III), Tm(III),

Lu(III), as representatives of the lanthanide series was studied using their radiotracers

141Ce3+, 147Nd3+, 154Eu3+, 160Tb3+, 170Tm3+ and 177Lu3+, respectively as per “Extraction

Procedure” (section 4.6.1). Equimolar (0.01 molL-1) solutions of HPA, B15C5 and their

mixture in chloroform were used for the extraction of these lanthanide ions from aqueous

solutions of known pH, separately. The synergism (Dsyn) produced was calculated using

the correlation:

Dsyn = Dmix/ (DHPA + DB15C5) (4.5)

DHPA, DB15C5 and Dmix represent the distribution coefficients using organic phase

containing HPA, B15C5 alone, and mixture of HPA and B15C5, respectively.

71

4.6.4 Effect of REEs concentration

The procedure was similar to that of extraction. Only carrier of REEs having

different concentrations in the range of 6.5×10-5 to 3.3×10-3 mol L-1 was added before the

addition of radiotracer of desired rare earth metal. Rest of the procedure is same as given

under extraction in section 4.6.1.

4.6.5 Composition of extractable organometallic complex

Composition of the synergistic adduct responsible for the extraction of lanthanide

ions was investigated using the slope analysis and Job’s methods.

Slope Analysis method

Three types of experiments were performed with all the rare earths studied

separately. Number of conjugate base molecules of HPA, participating in extractable

complex formation to satisfy the primary valency of the central metal ion was determined

by studying the extraction of rare earth metal ions at different pH of aqueous phase. For

this purpose 2.0 mL aqueous buffer solution of known pH containing a fixed amount of

radiotracer of REE was taken in pyrex glass culture tubes and shaken for five minutes

with equal volume of chloroform having an equi-molar (0.01 mol dm-3) mixture of HPA

and B15C5. Centrifuged for three minutes and separated the layers. One mL from each

layer was taken and assayed radiometrically. Slope of the graph of distribution coefficient

(log D) vs. pH of aqueous phase gives the number of conjugate base (PA-) of HPA

participating in reaction to satisfy the primary valency of the rare earth metal ion.

To determine the total number of HPA molecules participating in the formation of

extractable species, extraction of REEs was studied at a fixed pH by varying the

concentration of HPA. For this purpose 2.0 mL aqueous buffer solution of known pH

containing a fixed amount of radiotracer of REE was taken in pyrex glass culture tubes

and shaken for five minutes with equal volume of chloroform having different

72

concentration of HPA (e.g. 0.0001 to 0.005 mol dm-3) and a constant concentration (0.005

mol dm-3) of B15C5. Centrifuged for three minutes and separated the layers. One mL

from each layer was taken and assayed radiometrically. Distribution coefficients (log D)

were plotted against HPA concentration. Slope of this graph gives the total number of

molecules of HPA attached to central metal ion.

In order to find the number of molecules of B15C5, the same procedure was

repeated keeping the metal ion and HPA concentration constant and varying the

concentration of B15C5 (e.g 0.0001 to 0.01 mol dm-3). Distribution coefficients (log D)

were plotted against concentration of B15C5. Slope of this grap gives the number of

molecules of B15C5 attached to metal ion.

Job’s Method

In this method, overall concentration of both the ligands used in the synergistic

extraction is maintained constant (e.g., 0.01 mol dm-3 ) while changing concentration of

both the ligands. log D is plotted vs. mol fraction of the ligands. Number of ligands of

both the extractants is determined from the mol fraction of the ligands where maximum

extraction is achieved.

4.6.6 Effect of neutral ligands on the extraction of REEs

In order to study the effect of other neutral donors on the extraction of REEs using

HPA, extraction of Eu(III) was studied using TOPO, TPPO, TBP and TPP as neutral

donors. All the experiments were performed as per extraction procedure given in sections

4.6.1, 4.6.3 and 4.6.5.

4.6.7 Effect of Anions on the Extraction of REEs

Two mL aqueous buffer solution of pH 2 having a known concentration of

radiotracer of REE and anions as their sodium or potassium salts (as per list given in

73

section 4.4) at a 100 fold higher concentration than the concentration of REE was shaken

with mixture of 0.01 mol dm-3 of HPA and B15C5 in chloroform for five minute.

Centrifuged for three minutes, separated the layers, one mL of liquid from each layer was

taken and was assayed radiometrically as per extraction procedure given in section 4.6.1.

4.6.8 Effect of cations on the extraction of REEs

Similar to anions effect, two mL aqueous buffer solution of pH 2 having a known

concentration of radio tracer of REE and cation at a ~100 fold higher concentration than

the concentration of RE metal ion was shaken with mixture of 0.01 mol dm-3 of HPA and

B15C5 in chloroform for five minute. Centrifuged for three minutes, separated the layers,

one mL of liquid from each layer was taken and was assayed radiometrically and rest of

the procedure was as per section 4.6.1.

4.6.9 Effect of solvents on the extraction of REEs

In order to study the effect of solvents on the extraction of REEs using HPA as

extractant, extraction of Eu(III) was studied in acetylacetone, benzene, cyclohexanone,

dichloroethylether, n-butylether, di-isobutylketone, 1-octanol, n-hexanol and toluene. All

the experiments were performed as per extraction procedure given in sections 4.6.1 and

4.6.5.

Composition of the synergistic adduct responsible for the extraction of lanthanide

ions was investigated using the slope analysis method. Two types of experiments were

performed with all the rare earths studied, separately.

74

4.6.10 Extraction of other metal ions with HPA+B15C5

(Decontamination studies)

The selectivity of this synergic extraction system has been checked by the

extraction of various metal ions with (HPA+B15C5)/CHCl3 (0.01mol dm-3) at the

optimum conditions of extraction using the radiotracers of Cs+1, Hg+2, Fe+3, Mn+2, Co+2,

Ni+2, Se+4and Cu64.

Similar to anions and cations effect, two mL aqueous solution of buffer of pH 2

was taken, radiotracer of different metal (Cs+1, Hg+2, Fe+3, Mn+2, Co+2, Ni+2, Se+4, Cu64)

were added separately. An equal volume (2.0 mL) of 0.01 mol dm-3 (HPA+B15C5)

solution in chloroform was added. After shaking for five minutes, centrifuged for three

minutes, phases were separated and one mL was taken from each layer and assayed

radiometrically. Further procedure was carried as per section 4.6.1.

4.6.11 Back extraction of REEs

Twenty five mL buffer solution of pH 2 containing radiotracer of REEs was

shaken for five minutes with equal volume (25 mL) of 0.01 mol dm-3 (HPA+B15C5) in

chloroform and separated both the phases. Two mL of organic layer loaded with REEs

was taken and it was shaken for five minutes with equal volume of water and nitric acid

of different concentration (0.2-1.0 mol dm-3) separately and centrifuged for three minutes.

One mL from each layer was taken and assayed radiometrically. Similar experiments

were performed with hydrochloric acid and perchloric acid.

4.7 Acid dissociation equilibria of HPA

The acid dissociation equilibrium of HPA is given by the Eq. (4.6).

−+ +↔ PAHHPA

75

][][][

HPAPAHKa

−+

= 4.6

Ka is the acid dissociation constant. This was determined by titrating HPA (0.01

mol dm-3) in a mixture of 1, 4-dioxane and water against sodium hydroxide in the same

solvent mixture. For this purpose, HPA solution was prepared by dissolving a known

amount of HPA in 1, 4-dioxane and diluting it with water. A series of solutions having

0.01 mol dm-3 HPA in 20%, 30%, 40% and 50% 1,4-dioxane in water wrer prepared.

Similarly, 0.01 mol dm-3 sodium hydroxide solution was prepared by dissolving a known

amount of NaOH in 20-50% 1,4-dioxane solutions separately.. 30 ml of HPA in 20% 1,4-

dioxane solution was taken in a conical flask and titrated with NaOH solution in 20%

dioxane solution, while measuring pH. Same titration was carried out using HPA

solutions in 30%, 40% and 50% dioxane, with corresponding NaOH solutions. pH of the

resulting solution after each addition of NaOH solution was measured , until the pH

became more than 12. pH was plotted against volume of NaOH solution used for each

concentration of dioxane solution and apparent pKa values were calculated for all the

solutions separately. The pKa (-log Ka) of HPA was determined by extrapolating the

linear plot of apparent pKa values obtained in 20%-50% υ/υ 1, 4-dioxane/ water solutions

against the 1, 4-dioxane concentration to the intercept.

CHAPTER-6 RESULTS AND DISCUSSION

76

5. RESULTS AND DISCUSSION

5.1 Extraction of REEs with HPA and Crown Ethers

In the preliminary studies, the synergistic extraction of Eu(III) (~ 1.5×10-5 mol

dm-3) was studied using equimolar solutions of crown ethers (0.01 mol dm-3 ) i.e. (12C4,

B15C5 and 18C6) as neutral donor and HPA in chloroform separately as well as in

combined from aqueous solutions of pH 1-2 having ionic strength 0.1 mol dm-3 (H+/K+,

Cl). Yellow colour of HPA appeared in aqueous phase beyond pH 2 indicating the

solubility of HPA in aqueous phase beyond pH 2 thus reducing its concentration in

organic phase. Therefore, extraction was not studied beyond pH 2. The results showed no

significant synergism in the extraction of metal ions by HPA with 12C4 and 18C6

contrary to B15C5 in the pH range studied. Therefore, further studies were carried out

using only B15C5 with HPA in chloroform. The various parameters of extraction such as

pH of the aqueous phase, effect of equilibration time etc. were optimized and results are

discussed below.

5.1.1 Effect of pH of aqueous phase

The extraction of Ce(III), Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III) (~ 1.5×10-5

mol dm-3) separately with equimolar (0.01 mol dm-3) solutions of HPA, B15C5 and their

mixture in chloroform from buffer solutions of pH (1.0 - 2.0) having ionic strength of 0.1

mol dm-3 (H+/K+, Cl-) has been studied and results are shown in Figs. 2-4. The extraction

of these metal ions with HPA and B15C5 alone was negligible in this pH range, whereas

with the equimolar mixture of HPA and B15C5, extraction was quite high even at pH 1

and continued increasing with increase in pH and became quantitative (≥ 99%) at pH 2

showing a pronounced synergism

Dsyn = Dmix/(DHPA+DB15C5) 5.1.1

77

in the range of 102 to 103. As the extraction of all the metal ions studied became

quantitative at pH 2, it was selected for all the further experimental work.

5.1.2 Effect of equilibration time

To optimize the equilibration time sufficient for the extraction of lanthanide ions,

extraction of Eu(III) (1.5×10-5 mol dm-3), as a representative of REEs, with 0.01 mol dm-3

mixture of HPA and B15C5 in chloroform was studied by shaking the aqueous and

organic phases for one minute to 10 minutes. The result showed that the equilibration was

achieved with in ≤ 3 minutes. However, five minutes was selected as optimum shaking

time for further studies.

5.1.3 Effect of metal ion concentration

The effect of metal ion concentration on the extraction of Nd(III), Eu(III) and

Tm(III) with 0.01 mol dm-3 HPA and B15C5 in chloroform. Results are shown in Fig. 5.

Extraction of Nd(III) in the range of 1.5×10-5 to 1.38×10-3 mol dm-3 was studied and

results showed that the extraction of Nd(III) was almost quantitative up to 9.7×10-4 mol

dm-3 after that it started decreasing and became 90.6% at 1.38×10-3 mol dm-3 . Extraction

of Eu(III) was studied in the range of 6.5×10-5 to 3.3×10-3 mol dm-3 and it was found that

the extraction of Eu(III) was almost quantitative up to 9.2×10-4 mol dm-3 after that it

started decreasing and became 92.5% at 3.3×10-3 mol dm-3. Extraction of Tm(III) in the

range of 1.5×10-5 to 5.92×10-3 mol dm-3 was studied and results showed that the

extraction of Tm(III) was almost quantitative up to 4.4×10-4 mol dm-3 after that it started

decreasing and became 94% at 5.92×10-4 mol dm-3 (highest concentration studied). This

can be attributed to the insufficient quantity of the extractants in the organic phase.

78

5.1.4 Composition of synergic adduct

The composition of the synergistic adduct responsible for the extraction of Ce(III),

Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III) into organic phase was investigated using the

slope analysis method as per section 4.6.5 and the results are presented in the Figs. 6-14

and 18.

5.1.4.1 Effect of pH variation

The Fig. 6-8 demonstrate the results of the plots of log D vs pH of the aqueous

solution for these six lanthanide ions, which gave the slope equal to three with coefficient

of correlation ≥ 0.995. The organic phase used was equimolar mixture of HPA and

B15C5 (0.005 mol dm-3) in chloroform. This suggests the presence of three conjugate

base ions (PA-) per adduct for each of the lanthanide ions investigated, which are required

to neutralize the charge of the central metal ion.

5.1.4.2 Effect of HPA concentration variation

The plots of log (D-DCE) vs log [HPA] at fixed concentration of B15C5 (0.005

mol dm-3) are given in Fig. 9-11. DCE represents the distribution coefficient using organic

phase containing 0.005 mol dm-3 of B15C5 alone. The plots gave the slope of three (i.e.

Ce: 2.95 ±0.08; Nd: 3.02±0.03; Eu: 2.97 ± 0.1; Tb: 3.02±0.02; Tm: 3.01 ± 0.08 and Lu:

2.99±0.04) with coefficients of correlation ≥ 0.998, thus indicating that only three HPA

molecule are present per adduct for each of these lanthanide ions and no HPA molecule

takes part as a neutral donor

5.1.4.3 Effect of crown ether concentration variation

Fig. 12-14 show the plots of log (D-DHPA) vs log[B15C5] at constant HPA

concentration (0.005 mol dm-3). DHPA represent the distribution coefficients using organic

phase containing 0.005 mol dm-3 of HPA alone. The plots of Fig. 12-14 present the

79

straight lines of slopes, 1.49 ± 0.04, 1.52 ±0.04, 1.52 ± 0.03, 1.48±0.03, 1.61 ± 0.06 and

1.52±0.04 for Ce (III), Nd(III) , Eu (III), Tb(III), Tm (III) and Lu(III) respectively.

From the Fig. 6 to 14, the extraction reaction can be deduced as follow:

++ + →←++ HCnBPAMK

CnBHPAM nsyn 3515.)(5153 3,3 (5.1.2)

M and the expression under bar ( ) represents the rare earth metal ion and the

species in the organic phase, respectively. The value of n may be 1 or 2.

The values of the corresponding equilibrium constants, i.e. log Ksyn,1 and

log.Ksyn,2, for n one or two, respectively, can be deduced from the intercept and the slope

of the plots of (D-DHPA)/[B15C5] vs [B15C5] as shown in Figs. 15-17 . The plots of Figs.

15-17 are based on the Eq. 5.3 and the derivation of this equation can be seen on page 44.

]515[][][][][]515)[( 332.

331.

1CBHHPAKHHPAKCBDD synsynHPA

−+−+−+=− (5.1.3)

The values of log Ksyn,1 and log.Ksyn,2 are related with intercept and slope of the

plots of Figs.15-17 in simplified Eqs. 5.4 and 5.5, respectively which have been used for

the computation of these constants.

log Ksyn,1 = log (intercept) –3 log [HPA] –3pH (5.1.4)

log Ksyn,2 = log (slope) –3 log [HPA] –3pH (5.1.5)

80

The computed values of logKsyn,1 and log Ksyn,2 using Eq. 5.1.4 and 5.1.5 are

presented in Table 5.1. The equilibrium constants thus calculated refer only to the

concentration quotients, calculated on the assumption that the activity coefficients of the

species involved do not change significantly under the experimental conditions. The

existence of two kinds of adducts (having one or two molecule of crown ether per adduct)

have been reported in the literature [167,170,172,175,180,183]. Synergistic adducts

containing one molecule of B15C5 with HPBI [183] and HA [180] while two molecules

of 15C5 with HTTA [167,177] for the extraction of various lanthanide ions in chloroform

has been reported. Mathur et al. have reported the synergistic adduct of the type Eu

(PMTFP)3.2B15C5 during the extraction of various lanthanides and actinides with

mixture of HPMTFP and crown ethers in chloroform [175]. We have observed a

synergistic adduct of M(PA)3.2TBPO with a synergistic mixture containing HPA and

TBPO in chloroform for the extraction of various lanthanide (III) ions from pH 2 aqueous

solution.

Table 1 Equilibrium constants of the synergistic extraction of lanthanide (III) ions with

(HPA + B15C5)/CHCl3

Metal ion Equilibrium constant Log Ksyn, 1 log Ksyn, 2

Ce (III) 4.12 ± 0.04 7.48 ± 0.03

Nd (III) 3.97 ± 0.07 7.59 ± 0.03

Eu (III) 4.47 ± 0.04 7.75 ± 0.03

Tb (III) 4.44 ± 0.01 7.86 ± 0.01

Tm (III) 3.81 ± 0.05 7.17 ± 0.04

Lu (III) 3.65 ± 0.04 7.00 ± 0.03

81

It was assumed that M(PA)3.2B15C5 may be a sandwich type complex having one

crown ether molecule on either side of the metal chelate bound to the central metal only

through three oxygen atoms as suggested by Mathur on the basis of thermodynamic

studies [175].

The formation of an aqua complex for the rare earth metals with HPA in aqueous

phase has been cited in the literature [151]. Therefore, the formation of synergistic adduct

by the replacement of water molecules can be suggested and the reaction may be written

as:

++ +→←++ HOHnPAMOHnHPAM 3.)(3 2323 (5.1.6)

OHnCnBPAMCnBOHnPAM 2323 515.)(515.)( +←→+ (5.1.7)

The assumption of the formation of an aqua complex (M(PA)3 .nH2O) seems to be

supported by the fact that little extraction of these metal ions is observed with HPA alone

in chloroform at pH 2.0. Furthermore, the net total count rate [net total count rate =

(count rate of aqueous phase - background) + (count rate of organic phase - background)]

only amounted to 85 % and 80 % of 154Eu and 170Tm, respectively, at pH 2.0 suggesting

formation of aqua complex as shown in Eq. 5.6, which is not extractable in chloroform

and resides at the interface of aqueous-organic layer [114]. The formation of aqua

complexes for the rare earth metals (M(PA)3.nH2O) is also cited in the literature [151]. It

has been noticed that the adduct formation in the organic phase is a stepwise process [58].

So, it could be assumed that an adduct containing one crown ether molecule is formed by

replacement of water of hydration and, after that, depending on the concentration of

crown ether, the addition of second molecule of the crown ether is possible.

On the other hand, it can be suggested that in the organic phase HPA and B15C5

molecules are linked together forming a mixed adduct which reacts with metal ions, thus

82

forming an extractable organic chelate. Kuvatov et al. have pointed out that HPA and

dihexylsufoxide (neutral donor) in organic phase are in a bound state and the degree of

interaction increases with decreasing polarity and dielectric permeability of the solvent

(o-xylene, chlorobenzene, benzene, and carbon tetrachloride) [62].

The values of the logKsyn,2 for Ce(III), Eu(III) and Tm(III) as shown in Table-1,

are somewhat higher than the values previously reported (6.53, 6.98 and 7.07) for the

extraction of these metal ions with HPA+TBPO/CHCl3 [35].

The importance of the cavity size of the crown ether relative to the cation radius is

emphasized by our finding that no significant synergism was observed for the extraction

of Eu(III) from pH 2 aqueous solution when 12C4 and 18C6 were used as neutral donor

with HPA in chloroform as the former has a relatively small cavity size ( 0.13nm) [168]

and the latter has too large size (0.26nm) [171].

5.1.5 The anions effect

The effect of various anions on the synergistic extraction of Ce(III), Nd(III,

Eu(III), Tb(III), Tm(III) and Lu(III) with (HPA+B15C5)/CHCl3 (0.01 mol dm-3) from pH

2 buffer solution having ionic strength 0.1 mol dm-3 (H+/K+, Cl-) at a ~100 fold higher

concentration than the concentration of the metal of interest, was studied and the results

are presented in Table 5.2. The anions were taken as their sodium salts except where

stated otherwise. The data show that among the anions tested oxalate has reduced the

extraction of Ce(III) and Tm(III) to ~92 %, Nd(III) and Lu(III) to 82%, fluoride and

thiosulphate that of Lu(III) to 93 and 94% respectively, carbonate that of Lu(III) to 96%,

cyanide that of Ce(III) to 85 % and tartrate and ascorbate that of Tm(III) to 95%.

However, all the other anions have not affected the extraction of these lanthanide ions. In

our earlier study, fluoride, oxalate, cyanide and citrate have masked the extraction of

these lanthanide (III) ions using (HPA+TBPO)/CHCl3. The present proposed synergistic

83

mixture proved to be much less sensitive to the interference of these anions in the

extraction of these lanthanide ions.

5.1.6 The Cations Effect

Among the cations tested, Fe(II) has reduced the extraction of Tm(III) to 88 %

and Pb(II) to 95 %. Zn(II) has reduced the extraction of Ce(III), Nd(III), Eu(III), Tm(III)

and Lu(III) to 79 %, 93 %, 94%, 82 % and 91% respectively. Cu(II) has reduced the

extraction of Nd(III), Tb(III) and Lu(III) to 88%, 93% and 78% respectively. Fe(III) has

reduced the extraction ofTb(III) and Lu(III) to 96% and to 83% respectively. The

extraction of Nd(III) and Lu(III) is reduced to 96% and 93% respectively due to the

presence of Ni(II) ions. However, Zn(II) has reduced the extaction of Nd(II) and Lu(III)

to 93% and 91% respectively.The rest of the cations have no deleterious effect on the

extraction of the metal ions under study. The reason for the interference of these

transition metals may lie in the formation of organo-metallic complexes, which are also

co-extracted, with these rare earth metal ions [38]. As the concentrations of transition

metal ions are ~100 fold higher than Ce(III), Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III),

the extraction of these rare earth metal ions may presumably be affected due to an

insufficient quantity of the extractant.

5.1.7 The Selectivity of Extraction System

Selectivity of this proposed synergic extraction system has been checked by

extraction of various metal ions with (HPA+B15C5)/CHCl3 (0.01 mol dm-3) at the

optimized conditions of extraction using the radiotracers such as 137Cs+(2.2 × 10-5 mol

dm-3), 56Mn2+(4.5×10-6 mol dm-3), 65Ni2+(2.2×10-4 mol dm-3), 60Co2+(3.4×10-5 mol dm-3),

59Fe3+(1.9×10-4) mol dm-3), 64Cu2+(2.4×10-4 mol dm-3), 75Se4+(3.3×10-4 mol dm-3) and

calculated the separation factor (Kd R.E / Kd M) for rare earth metal ions and results are

84

presented in Table 4 . The data showed that Fe(III), Cu(II) and Se(IV) can be extracted

upto 65%, 70% and 95%, respectively. All other metals have low Kd values. The

separation factors for most of these metal ions lie in the range of 102-104 except Se(IV),

showing clean separation of rare earth metal ions under study from these metal ions

having the same concentration level.

Acid dissociation equilibria of HPA

The pKa of picrolonic acid was determined by using the procedure given in section

4.7. and results are plotted in Fig. 18. pKa (-log Ka) of HPA was determined by

extrapolating the linear plot of apparent pKa values obtained in 20%-50%υ/υ 1, 4-

dioxane/ water solutions against the 1, 4-dioxane concentration to the intercept, as shown

in Fig. 18. The pKa value thus obtained was 2.52 ± 0.01.

85

Table 5.2 Effect of various anions on the extraction of lanthanide ions with 0.01 mol

dm-3 (HPA + B15C5)/CHCl3 from aqueous solution at pH 2

Extraction (%)

Aniona Ce(III) Nd(III) Eu(III) Tb (III) Tm (III) Lu(III)

Citrate 99 98 >99 99 99 98

Fluoride 98 97 >99 99 99 93

Oxalate 92 82 99 95 93 82

Bromide 99 99 >99 99 99 98

Phosphate 99 98 >99 99 99 99

Ascorbate 99 99 >99 98 95 98

Thiosulfate 99 99 >99 99 99 94

Acetate 99 99 >99 99 99 98

Thiocyanate 99 99 >99 99 99 98

Cyanide* 85 98 >99 99 99 97

Tartrate NS >99 >99 >99 95 98

Carbonate 99 98 >99 99 >99 96

a: Salt Concentration = 10µg/mL * Potassium salt used.

NS: Not studie

86

Table 5.3 Extraction of lanthanide (III) ions in the presence of various cations with 0.01

mol L-1 (HPA + B15C5)/CHCL3 from pH 2 aqueous solution.

Extraction (%)

Cationa Ce(III)

Nd(III) Eu(III) Tb (III) Tm (III) Lu(III)

Mg (II) NS 98 98 99 99 97

Co (II) 99 98 99 99 99 97

Cu (II)* 98 88 99 93 99 78

Mn (II) 99 98 99 99 99 98

Fe (III) 98 91 98 97 98 83

Fe (II) 98 N.S. 96 N.S. N.S. N.S.

Ba (II) 99 98 99 99 99 99

Sr (II) 99 97 99 98 99 97

Pb (II) 92 98 N.S. 99 95 98

Ni (II) 98 96 99 97 98 93

Cr (III) 99 97 99 99 99 98

Zn(II) 79 93 94 98 82 91

a: Salt concentration=10µg/mL *: Sulphate salt used N. S. : Not Studied

87

Tab

le 5

.4 E

xtra

ctio

n of

var

ious

met

al io

ns w

ith 0

.01

mol

L-1

(HPA

+ B

15C

5) /

CH

Cl 3

from

pH

2.0

aqu

eous

solu

tion.

Elem

ent

Con

c.

(mol

l-1)

Kd

Extra

ctio

(%

)

Se

para

tion

fact

ora

Ce3+

K

d = 6

54

Se

para

tion

fa

ctor

a N

d3+

Kd =

585

Se

para

tion

fa

ctor

a Eu

3+

Kd=

644

Se

para

tion

fa

ctor

a Tb

3+

Kd =

585

Se

para

tion

fa

ctor

a Tm

3+

Kd =

315

Se

para

tion

fact

ora

Lu3+

K

d=72

C

s1+

2.2 ×

10-5

0.01

3 1.

3 1.

2 ×

104

2.1 ×

104

5.0 ×

104

4.5 ×

104

2.4×

104

5.5 ×

103

Mn2+

4.

5 ×

10-6

0.01

6 1.

6 1.

1 ×

104

1.7 ×

104

4.1 ×

104

3.6 ×

104

2.01

04 4.

5 ×

104

Hg2+

3.

9 ×

1

0.05

5 5.

2 3.

2 ×

103

4.9 ×

104

1.2 ×

104

1.0 ×

104

5.7×

103

1.3 ×

103

Ni2+

2.

2 ×

10-4

0.06

5.

6

2.9 ×

103

4.5 ×

103

1.1 ×

104

9.7 ×

103

5.2×

103

1.2 ×

103

Co2+

3.

4 ×

10-5

0.09

8.

4 1.

9 ×

103

2.9 ×

103

7.1 ×

103

6.3 ×

103

3.4×

103

7.8 ×

102

Fe3+

1.

9 ×

10-4

1.87

65

.1

94.6

1.

4 ×

102

3.5×

102

3.1 ×

102

1.7×

102

38.5

Cu2+

2.

4 ×

10-4

2.3

69.7

76

.5

1.2 ×

102

2.8×

102

2.5 ×

102

1.4×

102

31.3

Se4+

3.

3 ×

10-4

23.5

95

.5

7.5

11.6

27

.8

24.9

13

.4

3.1

a: S

epar

tion

fact

or =

Kd R

.E /

Kd M

, R.E

= ra

re e

arth

ele

men

t

88

Ce(III)

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extra

ctio

n (%

)

Nd(III)

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extra

ctio

n (%

)

Fig. 2 Extraction of Ce(III) and Nd(III) (1.5×10-5 mol dm-3) with 0.01 mol dm-3 B15C5(▲), HPA (■)and HPA+B15C5(♦) in chloroform.

89

Eu(III)

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extra

ctio

n (%

)

Tb(III)

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extra

ctio

n (%

)

Fig. 3 Extraction of Eu(III) and Tb(III) (1.5×10-5 mol dm-3) with 0.01 mol dm-3

B15C5(▲), HPA (■)and HPA+B15C5(♦) in chloroform.

90

Tm(III)

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extra

ctio

n (%

)

Lu(III)

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extra

ctio

n (%

)

Fig. 4 Extraction of Tm(III) and Lu(III) (1.5×10-5 mol dm-3) with 0.01 mol dm-3

B15C5(▲), HPA (■) and HPA+B15C5(♦) in chlororfor

91

Nd(III)

90

92

94

96

98

100

-4 -3.5 -3 -2.5log[metal ion] (M)

Ext

ract

ion

(%)

Eu(III)

92

94

96

98

100

-4.5 -4 -3.5 -3 -2.5log[metal ion] (M)

Extra

ctio

n (%

)

Tm(III)

92

94

96

98

100

-4.5 -4 -3.5 -3log[metal ion] (M)

Ext

ract

ion

(%)

Fig. 5 Dependence of metal ion extraction on its concentration by (HPA+B15C5) from pH 2 buffer solution

92

dy/dx(Ce)=2.99

-1.5

-0.5

0.5

1.5

2.5

1 1.2 1.4 1.6 1.8 2

pH

log

D

dy/dx(Nd)=3.02

-1.5

-0.5

0.5

1.5

2.5

1 1.2 1.4 1.6 1.8 2

pH

log

D

Fig. 6 log D as a function of pH for Ce(III) and Nd(III) (1.5×10-5 mol dm-3 ) with 0.005 mol dm-3 (HPA+B15C5)/CHCl3

93

dy/dx(Eu)=2.92

-1.5

-0.5

0.5

1.5

2.5

1 1.2 1.4 1.6 1.8 2

pH

log

D

dy/dx(Tb)=2.98

-1.5

-0.5

0.5

1.5

2.5

1 1.2 1.4 1.6 1.8 2

pH

log

D

Fig. 7 log D as a function of pH for Eu(III) and Tb(III) (1.5×10-5 mol dm-3) with 0.005 mol dm-3 (HPA+B15C5)/CHCl3

94

dy/dx(Tm)=3.02

-1.5

-0.5

0.5

1.5

2.5

1 1.2 1.4 1.6 1.8 2

pH

log

D

dy/dx(Lu)=2.94

-1.5

-0.5

0.5

1.5

2.5

1 1.2 1.4 1.6 1.8 2

pH

log

D

Fig. 8 log D as a function of pH for Tm(III) and Lu(III) (1.5×10-5 mol dm-3) with 0.005 mol dm-3 (HPA+B15C5)/CHCl3

95

dy/dx(Ce)=2.99

-2

-1

0

1

2

-3.5 -3 -2.5 -2log [HPA]

log

(D-D

CE)

dy/dx(Nd)=3.02

-2

-1

0

1

2

-3.5 -3 -2.5 -2

log [HPA]

log

(D-D

CE)

Fig. 9 log-log plot of (D-DCE) related to Ce(III) and Nd(III) (1.5×10-5 mol dm-3 vs. [HPA]; [B15C5]= 0.005 mol dm-3, pH=2.0

96

dy/dx(Eu)=2.98

-2

-1

0

1

2

-3.5 -3 -2.5 -2

log [HPA]

log

(D-D

CE)

dy/dx(Tb)=3.02

-2

-1

0

1

2

-3.5 -3 -2.5 -2

log [HPA]

log

(D-D

CE)

Fig. 10 log-log plot of (D-DCE) related to Eu(III) and Tb(III) (1.5×10-5 mol dm-3) vs. [HPA]; [B15C5]= 0.005 mol dm-3, pH=2.0

97

dy/dx(Tm)=3.02

-2

-1

0

1

2

-3.5 -3 -2.5 -2log [HPA]

log

(D-D

CE)

dy/dx(Lu)=2.94

-2

-1

0

1

2

-3.5 -3 -2.5 -2log [HPA]

log

(D-D

CE)

Fig. 11 log-log plot of (D-DCE) related to Tm(III) and Lu(III) (1.5×10-5 mol dm-3) vs. [HPA]; [B15C5]= 0.005 mol dm-3, pH=2.0

98

dy/dx(Ce) = 1.49

-1

0

1

2

-4 -3.5 -3 -2.5 -2log[B15C5]

log

(D-D

HPA

)

dy/dx(Nd)=1.52

-1.5

-0.5

0.5

1.5

2.5

-4 -3.5 -3 -2.5 -2log[B15C5]

log(

D-D

HPA

)

Fig.12 log-log plot of (D-DHPA) related to Ce(III) and Nd(III) (1.5×10-5 mol dm-3)

vs. [B15C5]; [HPA]= 0.005 mol dm-3, pH=2.0

99

dy/d(Eu) = 1.52

-1

-0.5

0

0.5

1

1.5

2

-4 -3.5 -3 -2.5

log[B15C5]

log

(D-D

HP

A)

dy/dx(Tb)=1.48

-1

-0.5

0

0.5

1

1.5

2

-4 -3.5 -3 -2.5log[B15C5]

log(

D-D

HP

A)

Fig. 13 log-log plot of (D-DHPA) related to Eu(III) and Tb(III) (1.5×10-5 mol dm-3)

vs. [B15C5]; [HPA]= 0.005 mol dm-3, pH=2.0

100

dy/dx(Tm) = 1.61

-1.5

-0.5

0.5

1.5

2.5

-4 -3.5 -3 -2.5 -2

log[B15C5]

log

(D-D

HP

A)

dy/dx(Lu)=1.52

-1.5

-0.5

0.5

1.5

2.5

-4 -3.5 -3 -2.5 -2

log[B15C5]

log(

D-D

HP

A)

Fig. 14 log-log plot of (D-DHPA) related to Tm(III) and Lu(III) (1.5×10-5 mol dm-3) vs. [B15C5]; [HPA]= 0.005 mol dm-3, pH=2.0

101

y(Ce) = 4E+06x + 1680.1

0

1000

2000

3000

4000

5000

6000

0 0.0002 0.0004 0.0006 0.0008 0.001

[B15C5] (mol dm-3)

(D-D

HPA

)/[B

15C

5]

y(Nd) = 5E+06x + 1185.2

0

1000

2000

3000

4000

5000

6000

7000

0 0.0002 0.0004 0.0006 0.0008 0.001

[B15C5] (mol dm-3)

(D-D

HPA

)/[B

15C

5]

Fig. 15 (D-DHPA)/[B15C5] vs [B15C5] at [HPA]=0.005 mol dm-3; pH2.0

102

y(Eu) = 7E+06x + 3789.8

0

2000

4000

6000

8000

10000

12000

0 0.0002 0.0004 0.0006 0.0008 0.001

[B15C5] (mol dm-3)

(D-D

HPA

)/[B

15C

5]

y(Tb) = 9E+06x + 3440.5

0

2000

4000

6000

8000

10000

12000

14000

0 0.0002 0.0004 0.0006 0.0008 0.001

[B15C5] (mol dm-3)

(D-D

HPA

)/[B

15C

5]

Fig. 16 (D-DHPA)/[B15C5] vs [B15C5] at [HPA]=0.005 mol dm-3; pH2.0

103

y(Tm) = 2E+06x + 825.26

0

500

1000

1500

2000

2500

3000

0 0.0002 0.0004 0.0006 0.0008 0.001

[B15C5] (mol dm-3)

(D-D

HPA

)/[B

15C

5]

y(Lu)= 1E+06x + 565.13

0

400

800

1200

1600

2000

0 0.0002 0.0004 0.0006 0.0008 0.001

[B15C5] (mol dm-3)

(D-D

HPA

)/[B

15C

5]

Fig. 17 (D-DHPA)/[B15C5] vs [B15C5] at [HPA]=0.005 mol dm-3; pH2.0

104

Fig. 18 Dependence of pKa of HPA on the concentration (v/v) of 1,4-dioxane in water.

y = 0.004x + 2.5246R2 = 0.9986

2.5

2.6

2.7

2.8

0 10 20 30 40 50

1,4-DIOXANE (%)

pKa

105

5.2 Solvent Effect

In order to study the effect of solvents on the extraction of rare earth elements

using picrolonic acid as extractant, extraction of europium (Eu) as a representative of

REEs was studied in acetylacetone (ACAC), 1-octanol (ONL), hexanol (nHNL), 1-

butanol (nBNL), 2-butanol (iBNL), cyclohexanone (CHN), n-butyl ether (nBE) ,

dichloroethyl ether (DCEE), benzene, toluene, and diisobutylketone (DIBK). Extraction

of Eu(III) (~1.5×10-5 mol dm-3) was studied using picrolonic acid (0.01 mol dm-3) in the

above mentioned solvents as a function of pH from pH 1-2, separately. The results are

shown in Fig 19, where extraction of Eu(III) is plotted against the pH of aqueous phase.

This graph shows that, extraction of Eu (III) using picrolonic acid alone, increases with

increase in pH and becomes quantitative in ACAC, ONL, nHNL, CHN, nBE, DIBK, and

DCEE at pH 2 where as it is low in 1-BNL (<10%) and 2-BNL (<50%) and negligible in

benzene and toluene.

As the picrolonic acid alone can extract rare earths quantitatively in ACAC, ONL,

nHNL, CHN, nBE, DIBK, and DCEE, it can be concluded that synergism will be very

small in these solvents. Therefore, the effect of solvents was studied in order to

understand the nature and mechanism of extraction using picrolonic acid alone in all these

solvents.

Extraction of Eu(III) was very low in nBNL and iBNL and negligible in benzene,

and toluene using picrolonic acid alone, synergistic extraction of Eu(III) was studied in

these solvents using B15C5 as neutral donor except nBNL and iBNL and results of this

study are discussed separately in section 5.4

106

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extra

ctio

n (%

)

DCEEACACDIBKONLnBEnHNLCHN2-BNLnBNL

Fig. 19, Extraction of Eu(III) as a function of pH with HPA (0.01mol dm-3) in different solvents.

107

5.2.1 Composition of the extracted adduct

In order to find the stoichiometric composition of the extracted adduct during the

extraction of Eu(III) in ACAC, ONL, nHNL, CHN, nBE, DIBK, and DCEE using HPA

as extractant, slope analysis technique was applied using Eq. 2.23, where log D is plotted

against pH and log[HPA] concentration, separately as per extraction procedure in sections

4.6.1 and 4.6.5

In order to find the stoichiometric ratio of metal ion and HPA from the pH studies,

extraction of Eu (III) at constant concentration (~ 1.5 × 10-5 mol. dm-3) was studied using

HPA (0.01 mol dm-3) in ACAC, ONL, nHNL, CHN, nBE, DIBK, and DCEE as a

function of pH of aqueous phase from pH 1-2 using the extraction procedure as given in

sections 4.6.1 and 4.6.5. The plots of log D vs pH are drawn and the results are shown in

Fig. 20. The slopes of these plots with the correlation coefficients for all the solvents are

shown in Table 5.5. These slopes are three or very close to three. This shows that, three

conjugate base molecules i.e. PA- of HPA are attached with each metal ion in the

extracted adduct.

Table 5.5 Slope with correlation coefficients, for the extractin of Eu(III) from different

solvents from Fig. 20

S.No. Solvent Slope Correlation Coefficient

1

2

3

4

5

6

7

Acetylacetone

Di-isobutyl ketone

n-butylether

Octanol

Dichloroethylether

n-hexanol

Cyclohexanone

2.93 ± 0.12

2.95 ± 0.07

2.93 ± 0.17

2.96 ± 0.06

2.78 ± 0.18

2.94 ± 0.08

2.93 ± 0.06

0.999 ± 0.103

0.999 ± 0.061

0.999 ± 0.148

0.992 ± 0.053

0.99 ± 0.151

0.999 ± 0.068

0.999 ± 0.055

108

5.2.1.1 Effect of HPA concentration

Composition of adduct responsible for extraction was also studied by slope

analysis technique by varying the concentration of HPA (10-3 to 10-2 mol dm-3 ) in all the

solvents separately and studying the extraction of Eu(III) (1.5×10-5 mol dm-3) from

aqueous solution of pH-2. The plots of these experiments are depicted in Fig. 21, where

log D is plotted against log[HPA]. The slopes along with correlation coefficients for all

the solvents used are shown in Table 5.6. These values are also very close to three except

in DCEE which is close to four. It indicates the presence of three molecules of HPA in the

extracted adducts in all the solvents except DCEE where four molecules of HPA are

present. In DCEE one molecule of HPA is present as neutral donor.

Table 5.6 Slope with correlation coefficients, for the extractin of Eu(III) from different

solvents from Fig. 21

S.No. Solvent Slope Correlation Coefficient

1

2

3

4

5

6

7

Acetyl acetone,

Di-isobutyl ketone

n-butylether

Octanol

Dichloroethylether

n-hexanol

Cyclohexanone

2.94 ± 0.05

2.939 ± 0.07

2.78 ± 0.7

2.97 ± 0.08

4.06±0.06

2.95 ± 0.09

2.94 ± 0.09

0.996 ± 0.058

0.998 ± 0.037

0.997 ± 0.063

0.996 ± 0.069

0.995 ± 0.084

0.997 ± 0.075

0.999 ± 0.065

From the results of above experiments, the extraction mechanism can be deduced

as:

+−+ +→←++ mHnHPAPAEuHPAnmEu mm

Kex .)()( 33 5.2.1

109

nm

mmm

ex HPAEuHnHPAPAEu

K ++

+−

=]][[

][])([3

3

5.2.2

m=3 and n=0 / 1 as the case may be.

The bar represents the organic phase.

In all the solvents studied as diluents, the extracted species is characterized as

Eu(PA)3 while in case of DCEE, the composition of the species is suggested as

Eu(PA)3.HPA.

Two types of extraction mechanisms may be proposed

1- Water molecules present in the aqua complex “Eu(PA)3.nH2O” may be replaced

by oxygenated solvent molecules directly.

2- The interaction of HPA with solvent molecules results in a mixed adduct which

further reacts with Eu(III) in aqueous phase to extract it in organic phase.

As indicated by Osman, that in aqueous medium Eu(III) forms an aqua complex

with HPA like M(PA)3.nH2O [45]. In our studies, formation of aqua complexes of Eu of

type Eu(PA)3.nH2O which are not extractable in organic solvents has already been

discussed in section 5.4.3. The formation of aqua complexes of Eu of type M(PA)3.nH2O

and role of organic solvent as neutral donor has been discussed by Ali in his study on the

extraction of Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III) using HPA from an aqueous

solution of pH 2 in MIBK. He suggested that, in MIBK, water molecules of aqua

complex may be replaced by MIBK thus making adduct extractable into organic phase

[150, 151].

It has been reported by Kuvatov et al. that in the organic phase, HPA and di-

hexylsulfoxide (DHSO), a neutral donor, are in a bound state and form a mixed adduct

which further reacts with metal ion [62].

110

On the basis of the results of these experiments, we can suggest that Eu(III) is

extracted as Eu(PA)3 in ACAC, ONL, nHNL, CHN, nBE and DIBK and HPA is not

acting as neutral donor. Water molecules of Eu(PA)3.nH2O complex are replaced by

solvent making it extractable. However, from the slope of Fig.21 for DCEE, the extracted

species may be as Eu(PA)3.HPA. In this case one molecule of HPA is present as neutral

donor. This may be due to less electron donating ability of O in DCEE due to the

presence of two chlorine atoms in the molecule which have more electron withdrawing

effect and thus reducing the electron density on the O atom of DCEE. Sadanobu and

Qiangbin have studied the extraction of lanthanides using BPHA (HL) in various solvents

and they have reported the formation of LnL3.(HL) in chloroform [199]. Their findings

support the composition of our proposed extracted species Eu(PA)3.HPA in DCEE

Extraction constant were also calculated. These are given below in Table-5.7.

Table 5.7 Extractin constants for Eu(III) extraction in different solvents.

S. No Solvent Log kex

1

2

3

4

5

6

7

Acetyl acetone

Di-isobutylketone

n-butylether

Dichloroethylether

Octanol

n-hexanol

Cyclohexanone

3.14±0.13

2.96±0.11

2.94±0.09

2.23±0.09

2.22±0.07

1.95±0.08

1.91±0.06

On the basis of log Kex, the solvents can be arranged with respect to thsir

extractability in the order ACAC > DIBK > Nbe > DCEE > ONL > nHNL > CHN.

111

ACAC and DIBK are better extractants as compared to cyclohexanone. Extraction

constants (log kex) for Eu in ACAC is greater than that of DIBK and CHN which may be

due to the presence of two oxygen atoms in the ACAC molecule. Extraction constant for

Eu is higher in DIBK as compared to CHN. This may be due to cyclic structure of CHN,

since the aliphatic solvents are considered better extractant [201].

Similarly, in case of alcohols, log kex for Eu(III) is higher in ONL than nHNL.

This may be due to longer carbon chain in ONL than in nHNL and hence more electron

donating ability of O in ONL as compared to nHNL.

In case of ethers used as diluents, log kex is higher in nBE than DCEE and hence

nBE seems to be a better solvent for extraction of Eu(III) using HPA as extractant. This

may be due to less electron donating ability of O in DCEE due to the presence of two

chlorine atoms in the molecule which have more electron withdrawing effect and thus

reducing the electron density on the O atom of DCEE.

The role of solvent in the extraction of rare earth elements using a single

extractant has not been discussed to a greater extant in the literature. However, some

workers have studied the role of solvents in the extraction of REEs and various other

metal ions and can be discussed shortly to compare the extraction mechanism proposed in

our studies.

Healy et al. have studied the extraction of Am(III) and Pm(III) using HTTA as

extractant and TOPO, TPPO, TBP, TPP, ethyl hexyl alcohol (EHA) and MIBK as neutral

donors in cyclohexane and benzene as solvents. They reported the formation of

Am(TTA)3.(EHA)2, Pm(TTA)3.(EHA)2, Am(TTA)3.(MIBK)2 and Pm(TTA)3.(MIBK)2

They have concluded that, this synergistic system increases the partition coefficient up to

400 times greater than for either HTTA or the neutral donor (EHA, MIBK) alone [200].

112

This report also supports the proposed composition of the extracted species in the present

work.

Low extraction of Eu(III) in CHN as compared to DIBK can be supported to some

extant by the conclusion drawn by Akiba, who pointed out that aliphatic diluents are

better solvents for the extraction of metal ions [201].

It is very difficult to correlate extraction constants (log kex) given in Table-5.7

with any single physical property of the solvents. Yang et al. have studied the extraction

of U(VI) with petroleum sulfooxide (PSO) in seven solvents and concluded that no

correlation can be drawn with any single physical parameter of the diluents [202] which is

in accordance with the findings of the present study.

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

1 1.2 1.4 1.6 1.8 2

pH

log

D

ACACDIBKnBEDCEEONLnHNLCHN

Fig. 20 log D as a function of pH for Eu(III) (~1.5×10-5 mol dm-3)with 0.01mol dm-3 (HPA) ACAC, DIBK, ONL, Nbe, CHN, nHNL and DCEE

113

-3

-2

-1

0

1

2

3

-3 -2.8 -2.6 -2.4 -2.2 -2log [HPA]

log

D

ACACDIBKnBEDCEEONLnHNLCHN

Fig. 21 log – log plot of D related to Eu(III) (~1.5×10-5 mol dm-3) vs. HPA concentration in ACAC, DIBK, ONL, nBE, CHN, nHNL and DCEE

114

5.3 Extraction of Rare Earth Elements in Different Solvents

During our study on role of various diluents on the extraction of Eu(III) using

HPA as extractant, it was found that alcohols, ketones and ethers were good solvents.

Among the three categories of solvents, namely, alcohols, ketones and ethers, ONL, CHN

and DCEE were chosen as representative diluents of each category, respectively. Further

to investigate the extraction behaviour of these solvents along the lanthanide series,

extraction studies of Ce, Tb and Lu were carried in ONL and CHN while that of Tb and

Lu in DCEE.

5.3.1 Extraction of Ce(III), Tb(III) and Lu(III) in Octanol

To study the extraction of REEs with in the series, extraction of Ce(III), Tb(III)

and Lu(III) as a representative of REEs was carried out in ONL. Extraction of Ce(III)

(~1.52×105 mol dm3), Tb(III) (~1.48×105 mol dm3) and Lu(III) (~1.54×105 mol

dm3) was studied from aqueous solutions of pH 1-2 using HPA (0.01 mol dm-3) in ONL.

The results are plotted in Fig. 22 which shows that the extraction increases with the

increase in pH and becomes quantitative (>98%) at pH 2 for Ce(III) and Tb(III) where

as it is low for Lu(III) (78%). The extraction trend among these elements appeared to be

Ce(III) >Tb(III) >Lu(III). This shows that extraction of rare earth elements using HPA in

octanol decreases with the decrease in ionic radii.

Composition of the extracted species was determined using slope analysis. For

this purpose, extraction of Ce(III), Tb(III) and Lu(III) was studied using HPA (0.01 mol

dm-3) from aqueous solutions of pH 1-2 as per extraction procedure ginen in section 4.6.1

and 4.6.5. Log D was plotted against pH of aqueous phase for all the three elements and

the results are shown in Fig. 23. The slope of these graphs gives the number of conjugate

base (PA-) of HPA molecules attached to RE metal ion. These plots show slopes 2.97,

2.93 and 2.92 with coefficients of correlation having values 0.99, 0.995 and 0.992 for Ce,

115

Tb and Lu, respectively. This shows that three molecules of conjugate base of HPA are

present per molecule of extracted species.

Presence of three molecules of HPA was further verified by studying slope

analysis by varying the HPA from 0.001 to 0.01 mol dm-3 concentration at pH 2 as per

extraction procedure ginen in section 4.6.9. Log D is plotted against log [HPA] and the

results are presented in Fig.24. The slope of these plots gives the number of molecules of

HPA attached to metal ion and from the plots slopes of 2.96, 2.94 and 2.97 with

coefficients of correlation having values 0.997, 0.997 and 0.996 were observed for Ce, Tb

and Lu, respectively. This also confirms the presence of three conjugate base molecules

of HPA per extracted adduct. On the basis of these results, composition of the extracted

adduct can be suggested as M(PA)3 where M = Ce(III), Tb(III) and Lu(III).

Extraction constants (log Kex) were also calculated. These were found to be2.20,

1.77 and 0.86 for Ce(III), Tb(III) and Lu(III), respectively. This shows that extraction

decreases as the ionic radii of the REEs decrease. Similar trend has been reported by Ali

in his studies on the extraction of Nd(III), Tb(III) and Lu(III) [150] and Eu(III), Tm(III)

[151] using HPA in MIBK. As far as the mechanism of extraction is concerned, similar

extraction mechanism can be proposed as has already been discussed in section 5.2.1.1 on

the studies of solvent effect.

116

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extr

actio

n (%

)

Fig.22 Extraction of Ce(♦), Tb(■) and Lu(▲) as a function of pH with HPA 0.01 mol dm-3 in octanol

dy/dx(Ce) = 2.97

dy/dx(Lu) = 2.92

dy/dx(Tb) = 2.93

-3

-2

-1

0

1

2

3

1 1.2 1.4 1.6 1.8 2

pH

log

D

Fig.23 log D as a function of pH for Ce(♦), Tb(■) and Lu(▲) with (HPA) 0.01 mol dm-3 in octanol

117

dy/dx(Ce) = 2.96

dy/dx(Lu) = 2.97

dy/dx(Tb) = 2.94

-3

-2

-1

0

1

2

-3 -2.8 -2.6 -2.4 -2.2 -2

log[HPA]

log

D

Fig.24 log – log plot of D related to Ce(♦), Tb(■) and Lu(▲) vs. HPA concentration in octanol

5.3.2. Extraction of Ce, Tb and Lu in Cyclohexanone

Among the ketones such as ACAC, DIBK and CHN, studied earlier in section

5.2 for the extraction of Eu(III) using HPA as chelating agent , CHN has been chosen

for further studies about the extraction trend along the REEs series. Extraction of Ce(III)

(~1.52×105 mol dm3), Tb(III) (~1.48×105 mol dm3) and Lu(III) (~1.54×105 mol

dm3) was studied from aqueous solution of pH 1-2 using HPA (0.01 mol dm-3) in CHN

as per extraction procedure given in section 4.6.1 and 4.6.5 and the results are plotted in

Fig.25. This Fig. shows that extraction increased with increase in pH and became

quantitative (>98%) at pH 2 for Ce and Tb where as, it was low for Lu (46%).

Extraction was not studied beyond pH 2 due to the increased solubility of HPA in

118

aqueous phase above this pH, as indicated by the presence of yellow colour of HPA in

aqueous phase. Highest extraction was observed for Ce(III), less for Tb(III) and least

for Lu(III). This shows that extraction of REEs using HPA in CHN decreases with the

decrease in ionic radii.

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extr

actio

n ( %

)

Fig.25 Extraction of Ce(♦), Tb(■) and Lu(▲) as a function of pH with HPA (0.01mol dm-3) in cyclohexanone

Composition of the extracted species was determined using slope analysis. For

this purpose, extraction of Ce(III), Tb(III) and Lu(III) was studied from aqueous solutions

of pH 1-2 separately using HPA (0.01 mol dm-3) as per extraction procedure given in

section 4.6.5 to study the effect of pH. log D was plotted as a function of pH of aqueous

phase for all the three elements. The results are shown in Fig.26. These plots show slope

2.98, 2.93 and 2.91 with coefficients of correlation having values 0.996, 0.996 and 0.998

for Ce(III), Tb(III), and Lu(III), respectively. This shows that, three molecules of

conjugate base of HPA are attached per molecule of extracted species.

119

dy/dx(Ce)= 2.98dy/dx (Tb)= 2.93dy/dx(Lu) = 2.91

-3

-2

-1

0

1

2

1 1.2 1.4 1.6 1.8 2

pH

log

D

Fig.26 log D as a function of pH for Ce(♦), Tb(■) and Lu(▲) with (HPA) 0.01mol dm-3

in cyclohexanone Presence of three molecules of HPA in extracted species for each metal ions was

further verified by varying the HPA concentration from 0.001 mol dm-3 to 0.01 mol dm-3 at

pH 2. The extraction was carried as per extraction procedure given in section 4.6.9 to study

the effect of HPA concentration. Log D is plotted against log [HPA] and the results are

shown in Fig.27. These plots also show slope 3.22, 3.12 and 3.03 with coefficients of

correlation having values 0.996, 0.99 and 0.997 for Ce(III), Tb(III) and Lu(III). This also

confirms the presence of three conjugate base molecules of HPA per extracted adduct. On

the basis of the results of these experiments, composition of the extracted species can be

suggested as M(PA)3.

120

dy/dx(Tb) = 3.13

dy/dx(Lu) = 3.03

dy/dx(Ce) = 3.23

-4

-3

-2

-1

0

1

2

-3 -2.8 -2.6 -2.4 -2.2 -2

log[HPA]

log

D

Fig.27 log – log plot of D related to Ce(♦), Tb(■) and Lu(▲) vs. varying concentration of HPA in cyclohexanone

Extraction constants (log Kex) were also calculated. These were found to be1.78,

1.32 and 0.41 for Ce, Tb and Lu, respectively. This shows that rate of extraction

decreased with the dexrease in ionic radii of the REE, which is the same trend as

observed earlier wih HPA/ONL and in accordance with the literature [150,151]. As far as

the mechanism of extraction is concerned, similar extraction mechanism can be proposed

as has already been discussed in section 5.2.1.1 on the studies of solvent effect.

5.3.3 Extraction of Tb and Lu in DCEE

During our study on the effect of solvents on the extraction of rare earth elements,

ethers (nBE and DCEE) were found as good solvents for the extraction Eu(III) using HPA

as extractant. Further to study the trend of extraction of REEs with in the period,

extraction of Tb(III) (~1.48×105 mol dm3) and Lu(III) (~1.54×105 mol dm3) was

121

carried out in DCEE. Extraction of these elements was studied from aqueous solutions of

pH 1-2 using HPA (0.01 mol dm-3). The extraction was carried as per extraction

procedure given in section 4.6.1 and 4.6.5. The results are plotted in Fig.28 where

extraction is plotted against pH. This Fig. shows that extraction increases with increase in

pH and becomes quantitative at pH 2 for Tb(III) where as it is low for Lu(III) (60%). The

order of extraction is Tb(III) >Lu(III). This shows that extraction using HPA in DCEE

decreases with the decrease in ionic radii.

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Ext

ract

ion

( % )

Fig. 28 Extraction of Tb(■) and Lu(▲) as a function of varying pH with HPA

(0.01mol L-1) in DCEE

Composition of the extracted species was determined using slope analysis. For

this purpose, extraction of Tb(III) and Lu(III) was studied separately using HPA (0.01

mol dm-3) from aqueous solutions of pH 1-2 as per extraction procedure given in section

4.6.1 and 4.6.5. The results are shown in Fig.29. These plots show as slope of 2.80 and

2.87 for Tb and Lu with coefficients of correlation having values 0.999 and 0.998 for Tb

122

and Lu, respectively. This shows that three molecules of conjugate base of HPA are

attached per molecule of extracted species.

dy/dx (Tb)= 2.80

dy/dx (Lu)= 2.87

-2.5

-1.5

-0.5

0.5

1.5

2.5

1 1.2 1.4 1.6 1.8 2pH

log

D

Fig. 29 log D as a function of pH for Tb(■) and Lu(▲) with 0.01mol L-1 (HPA) in DCEE

To determine the number of HPA molecules taking part in the complex formation,

extraction of Tb(III) and Lu(III) was studied by varying the HPA concentration from

0.001 mol dm-3 to 0.01 mol dm-3 at pH 2. The extraction was carried out as per extraction

procedure given in section 4.6.1 and 4.6.5 to study the effect of HPA concentration. Log

D is plotted against log [HPA]. The results are shown in Fig.30. These plots show slope

3.86 and 3.89 for Tb and Lu with coefficients of correlation having values 0.995 and

0.999. This slope is close to four which indicates involvement of four molecules of HPA

per extracted adduct. In this case one molecule of HPA is present as a neutral donor. This

may be due to less electron donating ability of oxygen in DCEE due to the presence of

two chlorine atoms in the molecule which have more electron withdrawing effect and

thus reducing the electron density on the oxygen atom of DCEE. On the basis of these

experiments, composition of the extracted adduct can be suggested as M(PA)3.HPA.

123

Sadanobu and Qiangbin have studied the extraction of lanthanides using BPHA (HL) in

various solvents and they have reported the formation of LnL3.(HL) in chloroform [199].

Their findings support the composition of our proposed extracted species M(PA)3.HPA in

DCEE.

dy/dx(Tb) = 3.86 dy/dx(Lu) = 3.89

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

-3.5 -3 -2.5 -2

log[HPA]

log

D

Fig. 30 log – log plot of D related to Tb(■) and Lu(▲) vs. HPA concentration in DCEE Extraction constants (log Kex) were also calculated. These were found to be 2.16

and 1.11 for Tb and Lu, respectively. This shows that extraction of REEs decrease as the

ionic radii of the REEs decreases. Similar trend has been reported by Ali in his studies on

the extraction of lanthanides using HPA in MIBK [151]. As far as the mechanism of

extraction is concerned, similar extraction mechanism can be proposed as has already

been discussed in section 5.2.1.1on solvent effect.

124

5.4 Synergistic Extraction of Eu(III) in Benzene and

Toluene

During our study on the role of solvent in the extraction of Eu(III) using

picrolonic acid alone as extractant, no significant extraction of Eu(III) was observed in

benzene and toluene. Therefore, synergistic extraction of Eu(III) (~1.5×10-5 mol dm-3)

using picrolonic acid and B15C5 as neutral donor was studied in these solvents. All the

experiments were carried in the same way as described earlier in sections 4.6.1, and 4.6.5

in chloroform as solvent.

5.4.1 Effect of pH of aqueous phase

The extraction of Eu(III) (~1.5×10-5 mol dm-3) with equimolar (0.01 mol dm-3)

solutions of HPA & B15C5 separately and with their mixture in benzene or toluene from

pH buffer solutions (1.0 – 2.0) having ionic strength of 0.1 mol dm-3 (H+ / K+ , Cl-) has

been studied and results are shown in Figs. 31 and 32. The extraction of Eu(III) with

B15C5 and HPA alone was negligible in this pH range. Whereas, with the mixture of

HPA and B15C5, extraction was quantitative ( ≥ 98 %) at pH 1 in benzene and quite high

( ≥ 95 %) in toluene showing a pronounced synergism [Dsyn = Dmix / (DHPA + DB15C5) ]

of 6.3×103 and 1.4×103 in benzene and toluene respectively. As the extraction of Eu(III)

became quantitative at pH 1 in both the solvents, therefore pH 1 was selected for all the

further experimental work.

5.4.2 Composition of synergic adduct

The composition of the synergistic adduct responsible for the extraction of Eu(III)

into organic phase was investigated using the slope analysis method and the results are

presented in the Fig. 33-35

125

5.4.2.1 Effect of pH variation

The Fig. 33 demonstrates the results of the plots of log D vs. pH of the aqueous

solution of Eu(III) which gave the slopes as 3.27, 3.25 with coefficient of correlation

0.997 and 0.998 for benzene and toluene respectively. The organic phase used was equi

molar mixture of HPA and B15C5 (0.005 mol dm-3) in both the solvents. These slopes

which are close to three indicate the presence of three conjugate base molecules ( PA-)

per adduct for each Eu(III) ion under investigation.

5.4.2.2 Effect of HPA concentration variation

The plots of log [D-DB15C5] vs. log [HPA] (0.001 -0.01 mol dm-3) at fixed

concentration of B15C5 in (0.01 mol dm-3) in benzene and toluene are given in Fig 34.

DB15C5 is distribution coefficient of Eu(III) using B15C5 alone. The plots gave the slope

of three (3.14 and 3.00), with coefficients of correlation 0.992 and 0.995 for benzene and

toluene, respectively, indicating that only three HPA molecule are involved in the

extraction of Eu(III).

5.4.2.3 Effect of CE concentration variation

Fig. 35 shows the plots of log [D-DHPA] vs. log [B15C5] at constant HPA

concentration (0.005 mol dm3) for toluene and (0.01 mol dm3) for benzene. DHPA is

distribution coefficient of Eu(III) using HPA alone The plots of Fig. 35 present the

straight lines having slopes 2.12 and 1.87 with correlation coefficient 0.999 and 0.99, for

benzene and toluene, respectively.

From the Fig. 34 and 35, the extraction reaction can be deduced as follow:

++ +→←++ HOHPAEuOHHPAEu 32.)(23 2323 (5.4.1)

OHCBPAEuCBOHPAEu 2323 25152.)(51522.)( +→←+ (5.4.2)

126

Where M and the expression under bar ( ) represent the rare earth metal ion and the

species in the organic phase respectively.

The formation of aqua complex (Eu (PA)3.2H2O is supported by the fact that low

extraction of Eu(III) is observed in chloroform at pH 2 and has already been discussed in

section 5.4.3 .

The overall extraction reaction by the mixture of HPA and B15C5 in both the

solvents can be suggested by the following reaction

++ +→←++ HCBPAEuCBHPAEu 35152.)(51523 33 (5.4.3)

233 ]515[][][ CBHPAHKK d+= (5.4.4)

On the basis of the results of these experiments, we can propose the extracted

species as Eu(PA)3.2B15C5.

The values of the corresponding extraction constants i.e. logKex for the extraction

of Eu(III) for both the solvents were found to be 8.85 and 8.31 using Eq.(4) for benzene

and toluene respectively. These extraction constants show that extraction of Eu(III) was

more favourable in benzene than in toluene under these experimental conditions

Synergistic adducts containing two molecules of 15C5 with HTTA for the

extraction of various lanthanide ions in chloroform has been reported [167, 177] and

support our proposed composition. Mathur et al. have reported the synergistic adduct of

the type Eu(PMTFP)3.2B15C5 during the extraction of various lanthanides and actinides

with a mixture of (HPMTFP) and crown ethers in chloroform [175]. Similarly the

synergistic adduct of M(PA)3.2TBPO with a mixture of HPA and TBPO in chloroform

127

for the extraction of various lanthanide (III) ions from pH 2 aqueous solution has been

reported in literature [150].

We assume that M(PA)3.2B15C5 may be a sandwich type complex having one

crown ether molecule on either side of the metal chelate bound to the central metal only

through three oxygen atoms as suggested by Mathur on the basis of thermodynamic

studies [175].

It has been noticed that the adduct formation in the organic phase is a stepwise

process [58]. So, it could be assumed that an adduct containing one crown ether molecule

is formed by replacement of water of hydration and after that, addition of second

molecule of the crown ether is possible.

On the other hand, it can be suggested that in the organic phase HPA and B15C5

molecule are linked together forming a mixed adduct which reacts with metal ions, thus

forming an extractable organic chelate. Kuvatov at al. have pointed out that HPA and

DHSO (neutral donor) in the organic phase are in a bound state [62]. All these reports

support the composition of the extracted species and mechanism of extraction proposed in

the present studies.

128

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8

pH

Extra

ctio

n (%

)

Fig. 31 Extraction of Eu (III) with (0.01mol dm-3) HPA(♦), B15C5 (■) and HPA+ B15C5 ( ▲) in Benzene

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8

pH

Extra

ctio

n (%

)

Fig. 32 Extraction of Eu (III) with (0.01mol dm-3) HPA(♦), B15C5 (■) and

HPA+ B15C5 ( ▲) in Toluene

129

dy/dx(B) = 3.27

dy/dx(T) = 3.25

-1

0

1

2

3

4

1 1.2 1.4 1.6 1.8 2

pH

log

D

Fig. 33 Effect of pH on the extraction of Eu (III) with HPA+B15C5 (0.01 mol dm-3) in Benzene (♦)and Toluene (■)

dy/dx(B) = 3.14

dy/dx(T) = 3.00

-2.5

-1.5

-0.5

0.5

1.5

-3.1 -2.9 -2.7 -2.5 -2.3 -2.1

log[HPA]

log

[D-D

CE]

Fig. 34 log – log plot of [D-DB15C5] related to Eu(III)(~1.5×10-5 mol dm-3) vs. HPA Concentration in Benzene (♦)and Toluene (■) at constant concentration of B15C5 (0.01 mol dm-3 )

130

dy/dx(B) = 2.127

dy/dx(T) = 1.87

-3.5

-2.5

-1.5

-0.5

0.5

1.5

-3.5 -3 -2.5 -2

log [B15C5]

log

[D-D

HPA

]

Fig. 35, log – log plot of [D-DHPA] related to Eu(III)(~1.5×10-5 mol dm-3) vs. B15C5 concentration in Benzene (♦)and Toluene (■) at constant concentration of HPA (0.01 mol dm-3)

131

5.5 Effect of Neutral donors In order to study the effect of neutral donors on the extraction of rare earth

elements, extraction of Eu(III) (~1.5×10-5 mol dm-3) was studied with HPA using TOPO,

TPPO, TBP and TPP, as neutral donors in chloroform. Extraction was studied using

0.01mol dm-3 of HPA, TBP, TPPO, TPP and TOPO as single extractant and equimolar

mixture of each neutral donor with HPA in chloroform separately from aqueous buffer

solutions of pH 1-2. The results are shown in figs. 36-39 where extraction is plotted

against pH of the aqueous phase. It is clear from these figures that HPA as well as all the

four neutral donors separately, do not extract Eu from aqueous solution. However, using

the mixture of HPA and neutral donors, extraction is quite high even at pH 1 and it

increases with increase in pH and becomes quantitative (≥98 %) at pH 2 with TBP, TPPO

and TOPO using as neutral donors. Extraction of Eu with the mixture of HPA and TPP

was (~86%) at pH 2 and yellow colour of HPA appeard in aqueous phase beyond pH 2.

Therefore, higher pH of aqueous phase was not studied and pH 2 was selected for further

studies related to the extraction of Eu(III) with these synergic extraction systems. It is

evident from Figs. 36-39, that quantitative extraction of Eu was observed at pH 1.2 for the

mixture of HPA with TOPO and TPPO and at pH 1.6 for HPA and TBP mixture.

Therefore, synergism was calculated at these pH values and was found to be 1.49×104,

1.12×104 and 1.35×104 for the extraction of Eu using TOPO, TPPO and TBP

respectively, as neutral donors with HPA. Synergism for HPA and TPP mixture was

calculated at pH 2 and was found to be 53.46.

5.5.1 Composition of the synergistic adducts

Composition of the extracted complexes of Eu with HPA and TOPO, TPPO &

TBP as neutral donors was studied using slope analysis method and Job’s method was

was also applied to Eu-HPA-TPPO system. The results are shown in Fig. 40-43.

132

5.5.1.1 Effect of pH

The effect of pH of the aqueous phase on the extraction of Eu(III) (~1.5×10-5 mol

dm-3 ) was studied using an equimolar concentration (0.01 mol. dm-3) of a mixture of

HPA with TBP. In case of synergic mixture of HPA with TOPO and TPPO, concentration

of HPA was 0.01mol dm-3 while that of TOPO and TPPO was 0.002 mol dm-3 in

chloroform. The results are shown in Fig. 40, where log D is plotted against pH. These

plots have a slope of 2.97, 3.08 and 2.88 with correlation coefficients 0.997, 0.996 and

0.999 for TOPO, TPPO and TBP respectively. This shows that three molecules of

conjugate base (PA-) of HPA are present per extracted adduct using all the three neutral

donors in chloroform.

5.5.1.2 Effect of HPA concentratiom

In order to study the effect of HPA concentration on the extraction of Eu(III),

studies were carried out using a mixture of HPA with TOPO, TPPO and TBP as neutral

donors separately at constant concentration of TOPO, TPPO (0.002 mol dm-3 ) and TBP

(0.01 mol dm-3 ) while varying the concentration of HPA (0.001- 0.01 mol dm-3 ) at pH 2.

The results of this study are plotted in Fig. 41 where log(D-Ds) is plotted against log

[HPA]. DS is the distribution coefficient for neutral donor alone used as extractant. All the

three plots have a slope of 2.95, 3.03 and 3.05 with the coefficient of correlation having

values of 0.992, 0.99 and 0.995 for TOPO, TPPO and TBP respectively. The values of

three slopes which are approaching to 3 also suggest the presence of three conjugate base

[PA-] of HPA molecume per adduct and no HPA molecule is acting as neutral donor.

5.5.1.3 Effect of concentration of neutral donors

In order to study the effect of concentration of neutral donors on the extraction of

Eu(III), extractions were carried out keeping the concentration of HPA constant (0.01

133

mol dm-3) and varying the concentration of TOPO, TPPO (0.0001- 0.001 mol dm-3 ) and

TBP (0.001- 0.01 mol dm-3) at pH 2. The results are plotted in Fig.42 where log[D-DHPA]

is plotted vs. log[S] (S=TOPO, TPPO and TBP). DHPA is the distribution coefficient for

HPA alone used as extractant. This graph shows a slope of 2.09, 2.05 and 0.95 for

TOPO, TPPO and TBP respectively. This shows that two molecules of TOPO and TPPO

are present per adduct extracted while in case of TBP only one molecule of TBP is

present. On the basis of the data of these experiments, composition of the extracted

species can be suggested as Eu(PA)3.2TOPO, Eu(PA)3.2TPPO and Eu(PA)3.TBP

Composition of adduct responsible for extraction of Eu(III) using HPA and TPPO

was further investigated using Job’s method ( method of continuous variation) [62, 197]

where overall concentration of HPA and TPPO is maintained constant at 0.01 mol dm-3

while changing concentration of both the ligands. The log D vs. mol fraction of HPA has

been plotted in Fig 43. The results showed that maximum extraction of Eu(III) was

observed at a ratio of HPA : TPPO as 0.006 : 0.004 mol dm-3, respectively, depicting the

composition of the complexes as M(PA)3.2TPPO which is in accordance with the earlier

observations using slope analysis method in the preceeding section.

The extraction mechanism which can be proposed on the basis of above

mentioned results is as follows

++ +↔++ HOnHPAMOnHHPAEu 3.)(3 2323 (5.5.1)

OnHnSPAEunSOnHPAEu 2323 .)(.)( +↔+ (5.5.2)

Where S is TOPO, TPPO and TBP and value of n is 2 for TOPO, TPPO and 1 for TBP.

Expression under bar ( ) represent the organic phase.

The formation of aqua complexes of rare earths is also cited in literature [145].

134

Kuvatov et al., studied the extraction of Am(III) using HPA and dihexylsulfoxide

(DHSO) (neutral donor) and pointed out that in the organic phase HPA and DHSO are

in a bound state [62]. Considering their finding about the mutual bound state of both the

ligands, it can be suggested that in the organic phase HPA and each neutral donor (TOPO,

TPPO or TBP) are linked together forming a mixed adduct which further reacts with

metal ion forming an extractable metal chelate. The extraction mechanism for the

extraction of Eu(III) by the mixture of HPA and all the three neutral donors (TOPO,

TPPO and TBP) separately in chloroform can be expressed by the following expression.

nSmHPAnSmHPA ..→+ (5.5.3)

++ + →←+ mHnSPAEumHPAnSEu mKMix .)(3 (5.5.4)

or simply it can be written as

++ + →←++ HnSPAMnSHPAM MixK 3.)(3 3

3 (5.5.5) and

ndMix SHPAHkK

][][][

3

3+

= (5.5.6)

where S is TOPO, TPPO and TBP and value of n may be 1 or 2.

On the basis of the results of all these experiments, composition of the extracted

adduct can be suggested as M(PA)3.2TOPO, M(PA)3.2TPPO and M(PA)3.TBP.

The equilibrium constants (logKMix) for Eu(III) were calculated to be 4.54, 5.69

and 6.17 using TBP, TPPO and TOPO respectively. These extraction constants show that

highest extraction rate is observed with TOPO, followed by TPPO and least with TBP.

Therefore, the order of extraction becomes as TOPO > TPPO > TBP.

The formation of two kinds of adducts having one or two molecules of neutral

donor per adduct have been reported in the literature. During the synergistic extraction

135

studies of trivalent Pm, Tm, Am and Cm from aqueous chloride medium with mixtures of

HTTA and TOPO, TBP, DBB, TPPO or TPP in benzene, Healy has reported very high

extraction of these metal ions with TOPO and TPPO. They reported that the rate of

extraction increases in the order from neutral alkyl phosphates, through phosphonates to

the phosphine oxides. The extraction follows the order of basicity of the neutral oxo-

donors used. During this study, the synergistic extracted species into the organic phase

were characterized as M(TTA)3.2S for Am(III), Cm(III) and Pm(III) and M(TTA)3.S and

M(TTA)3.2S for Tm(III) [119, 134]. These reports support extraction mechanism and

order of extraction (TOPO > TPPO > TBP) which has been observed in our studies on the

extraction of Eu(III) using picrolonic acid along with TOPO, OPPO and TBP in

chloroform.

Formation of the synergistic species Nd (TTA)3.(TOPO)2 has also been reported

by Healy and Ferraro. They have confirmed the formation of such complexes using

visible and I.R. spectra [204,205]. Newman has also studied the synergistic extraction of

trivalent Am, Cm, Pm and Tm with a mixture of HTTA and TBP (or TOPO) in benzene

and cyclohexane as the diluents and reported similar results [206].

Sekine and Dyressen have carried out the extraction studies of La, Eu, Lu and Am

in carbon tetrachloride using a mixtures of HTTA + S (S = TBP, TOPO, DBSO, Hexone

and several other neutral donors) and reported the formation of species M (TTA)3.(S) and

M(TTA)3.2S [207, 208]. Cary and Banks have reported the formation of the species Eu

(TTA)3.2S when S is TOPO [209]. Mathur et al. have studied the extraction of Eu(III)

using HTTA chelate with DPhSO, TBP and TOPO in benzene and have reported the

formation of Eu(TTA)3.S and Eu(TTA)3.2S adduct species of considerable stability.

Synergistic equilibrium constants follow the order DPhSO< TBP<TOPO [210]. Aly et al.

have studied extraction of Sm (III) using a mixtures of HTTA + TPPO or TOPO in

136

benzene and have characterized the species to be Sm(TTA)3.2S [211]. Synergistic

extraction of trivalent lanthanides (all except La and Eu) was carried out by Frabu et al.,

using a mixture of HTTA and TBP in carbon tetrachloride as diluent. The extraction

constants for the species M(TTA)3.S and M(TTA)3.2S have been calculated [212]. Akiba

et al., have extensively studied the extraction of Eu(III) – HTTA – TOPO system and

reported similar results [215].

All these studies support the formation of extracted species Eu(PA)3.nS and

mechanism proposed in our studies on the extraction of Eu(III) in chloroform.

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extra

ctio

n (%

)

Fig. 36 Extraction of Eu (III) with (0.01mol L-1) HPA (■), TOPO (♦) and HPA+TOPO (▲) in chloroform

137

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extra

ctio

n (%

)

Fig. 37 Extraction of Eu (III) with (0.01mol L-1) HPA (■), TPPO (♦) and HPA+TPPO (▲) in chloroform

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extra

ctio

n (%

)

Fig. 38 Extraction of Eu (III) with (0.01mol L-1) HPA (■), TBP (♦) an HPA+TBP(▲) in chloroform

138

0

20

40

60

80

100

1 1.2 1.4 1.6 1.8 2

pH

Extra

ctio

n (%

)

Fig. 39 Extraction of Eu (III) with (0.01mol L-1) HPA (■), TPP (♦) an HPA+TPP (▲) in chloroform

dy/dx(TOPO) = 2.97

dy/dx(TPPO) = 3.08

dy/dx(TBP) = 2.96

-2

-1

0

1

2

3

4

1 1.2 1.4 1.6 1.8 2

pH

log

D

Fig.40 Effect of pH on the extraction of Eu (III) with (HPA+S) in chloroform, HPA= 0.01 mol dm-3, S = 0.002 mol dm-3 (S= TOPO ♦ and TPPO ■) and (HPA+TBP ▲) = 0.01 mol dm-3

139

dy/dx(TPPO) = 3.02

dy/dx(TOPO) = 2.97

dy/dx (TBP) = 3.01

-1

0

1

2

3

-3 -2.8 -2.6 -2.4 -2.2 -2

log[HPA]

log

D

Fig. 41 log – log plot of [D-DS] related to Eu(III)(~1.5×10-5 mol dm-3) vs. HPA concentration in chloroform at constant concentration (0.002 mol dm-3) TOPO(♦), TPPO (■) and (0.01 mol dm-3) TBP (▲)

dy/dx(TOPO) = 2.05

d/dxy(TPPO) = 2.09

dy/dxy (TBP) = 0.95

-1.5

-1

-0.5

0

0.5

1

1.5

-4 -3.5 -3 -2.5 -2

log[S]

log[

D-D H

PA]

Fig. 42 log – log plot of [D-DHPA] related to Eu(III)(~1.5×10-5 mol dm-3) vs. [S], (S=TOPO ♦, TPPO ■ and TBP ▲) concentration into chloroform at a constant concentration of HPA (0.01 mol dm-3)

140

0

0.5

1

1.5

2

2.5

3

0 0.002 0.004 0.006 0.008 0.01

Conc. of HPA (mol dm-3)

D

Fig. 43 Extraction of Eu(III) with isomolar mixture of HPA and TPPO

141

5.6 Conclusion

During this study, synergic extraction of rare earth elements was carried out using

picrolonic acid with various neutral donors in different solvents. Extraction of rare earth

elements (Ce(III), Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III) ) was studied using

picrolonic acid and crown ethers (12C4, B15C5 and 18C6) in chloroform from aqueous

solution of pH 1-2. It was found that quantitative extraction can be achieved with in three

minutes from aqueous solution of pH 2 in chloroform using B15C5, as neutral oxo-donor

where as 12C4 and 18C6 did not show significant synergistic effect indicating that ring

size of these crown ethers also played a very important role in addition to their simple

neutral donor effect. This synergic extraction system showed a good tolerance to a

number of anions (citrate, ascorbate, thiosulphate, tartarate, acetate, fluoride, chloride,

bromide, iodide, thiocyanate, cyanide, carbonate, nitrate and phosphate) and cations

(cobalt, copper, manganese, ferric, barium, cadmium, strontium zirconium, lead, nickel,

chromium, zinc and magnesium) as the extraction of rare earth elements is not affected

by their presence in the aqueous phase. High metal concentration can be quantitatively

extracted up to 10-3 mol dm-3 showing its good loading capacity. However, the solubility

of picrolonic acid in aqueous phase became apparent at higher pH, therefore, it can not be

considered suitable for the extraction of metal ions from aqueous phase of pH >3.

During synergic extraction of Eu(III) using picrolonic acid and benzo-15-crown-5

as extraction system in benzene, butanol and toluene, no synergism was observed in

butanol and quantitative extraction was observed in benzene and toluene with in five

minutes, from aqueous solution of pH1. In this way benzene and toluene proved to be

better solvents for the extraction of rare earth elements using picrolonic acid and benzo-

15-crown-5 as synergic extraction system.

142

During the study on the use of other neutral donors such as TOPO, TPPO, TBP

and TPP with picrolonic acid, it was found that quantitative extraction of lanthanides can

be achieved with all the neutral donors except TPP from aqueous solution up to pH 2.

On the study of the effect of solvents, using picrolonic acid as extractant, it was

found that picrolonic acid alone can extract rare earth elements quantitatively from

aqueous solutions at pH2 in ACAC, DIBK, nBE, DCEE, ONL HNL and CHN with in

five minutes. By comparing the extraction of Eu(III) with HPA in these solvents, it

appeared that those solvents having the oxygen proved to be good diluents due to their

ability to coordinate as neutral donor in the formation of the metal complex responsible

for metal extraction. The order of their extraction ability follow the trend as

ketones>ethers>alcohols.

From this study, it is concluded that synergic extraction system comprising

picrolonic acid and any one of the neutral donors i.e., B15C5, TOPO, TPPO and TBP, in

any one of the diluents, chloroform, benzene and toluene; and HPA alone in any

oxygenated diluents can be used for the rapid and quantitative extraction of rare earth

elements from the aqueous phase of pH < 2 where these elements have least possibility of

hydrolysis..

CHAPTER – 6 REFERENCES

143

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