Analytica Chimica Acta 558 (2006) 254–260

Selective recoveries of Fe(III) and Cr(III) from atannery filtrate using Cyanex 923

Akash Deep, Paulo F.M. Correia, Jorge M.R. Carvalho∗Centre of Chemical Processes, Department of Chemical Engineering, Instituto Superior Tecnico, Av. Rovisco Pais, 1049-001 Lisbon, Portugal

Received 16 September 2005; received in revised form 8 November 2005; accepted 9 November 2005Available online 4 January 2006

Abstract

The separation of Fe(III) and Cr(III) has been proposed by liquid–liquid extraction with 0.50 M Cyanex 923. Fe(III) was selectively extractedfrom the mixed H2SO4 and NaCl medium leaving pure Cr(III) in the raffinate. The developed method has been used to separate and recover Fe(III)and Cr(III) from a tannery filtrate. The modeling experiments, simulating the concentration of Fe(III) and H2SO4 in the original tannery filtrate,have suggested the presence of the metal ion predominantly in the form of FeHSO4

2+. The addition of Cl− ions in the aqueous phase resulted intothe development of FeCl3, which was extractable in Cyanex 923 by solvation with two extractant molecules. The kinetics of the extraction wasr eatede©

K

1

towatAdtPtfi5cTthr

forulti-

as thefersatic-lids2

di-es)e of

t al.etal

trice(III)tiontage

tants

0d

easonably fast. The stripping of Fe(III) has been carried out by 0.25 M H2SO4. The extractant solution maintained its capacity even after repxtraction and stripping cycles.2005 Elsevier B.V. All rights reserved.

eywords: Liquid–liquid extraction; Recovery; Fe(III); Cr(III); Tannery filtrate; Cyanex 923

. Introduction

Tannery waste water is considered as a major water pollu-ant due to the presence of toxic Cr(III) and other inorganic andrganic impurities. Most of the effluent treatment plants dealith tannery waste water by converting it into a sludge, which,fter stabilization, is dumped in landfills. In a previous study[1],

he sludge from a tannery waste water treatment plant located atlcanena (Portugal) was leached with sulphuric acid and thenewatered by an integrated drying and filtration process. This

reatment produced filter cakes with <500 ppm Cr which, by theortuguese legislation, were safe enough to be used as agricul-

ure soil or could be used in ceramics or as a fuel. However, theltrate from the above process contained around 300 ppm Fe,00 ppm Cr and 6 ppm Zn. This filtrate requires further purifi-ation before it might be discharged in the open environment.he objective of the present work is to decontaminate the fil-

rate by selectively recovering Fe(III) and Cr(III) so that theseeavy metals do not pose any harm to the environment and theirecovery can benefit the investor.

Liquid–liquid extraction is a well-known approachaccomplishing the complex separations of the metals from melement solutions. Its use may be of particular advantagepartitioning of Fe(III) is easier than Cr(III)[2]. The extraction oFe(III) has been proposed by several extractants such as vacid [3], tri-n-butyl phosphate (TBP)[4], 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC-88A)[5], octylphenyacid phosphate (OPAP)[6], di-2-ethylhexyl phosphoric ac(DEHPA) [6,7] and synergists[8,9] whereas that of Cr(III) ireported only by a few, e.g. Aliquat 336[10,11]and Cyanex 27[12].

Some of our preliminary laboratory trial experiments incated Cyanex 923 (a mixture of four trialkyl phosphine oxidto be capable of selectively extracting Fe(III) in the presencCr(III). A similar idea can be drawn from the work of Gupta e[13] who proposed the extraction behaviours of several 3d mions with Cyanex 923 from hydrochloric, sulphuric and niacid media. We also noticed that the stripping of extracted Fcould be carried out with a reasonably dilute (0.25 M) soluof sulphuric acid. This can be pointed out as a major advanof Cyanex 923 over some of the previously reported extrac

∗ Corresponding author. Tel.: +351 21 8417311; fax: +351 21 8499242.E-mail address: jcarv@ist.utl.pt (J.M.R. Carvalho).

like OPAP, DEHPA and TBP, which invariably require the use ofa high concentration (3–6 M) of hydrochloric acid for the strip-ping of the loaded metal ion. The use of dilute sulphuric acid

003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved.

oi:10.1016/j.aca.2005.11.029

A. Deep et al. / Analytica Chimica Acta 558 (2006) 254–260 255

Nomenclature

AT constant in Debye–Huckel term (mol kg−1)1/2

Bij, Bi, Sij Bromley interaction parameters (kg mol−1)ci constant in equation correlatingBij with charge

and ionic radiuscS extractant concentration (M)D distribution ratio%E extraction percentageF term in the modified Bromley equationI ionic strength of the solution (mol kg−1)Kd dissociation constant of the acidKC

ext concentration based extraction constantKC

i apparent equilibrium constantK0

i thermodynamic equilibrium constantL ligands; HSO4

−, SO42−, Cl−

M cations in the solutionn sloperi ionic radius (A)X anions in the solutionzi ionic charge

Greek lettersγ i activity coefficientδi constant in the Bromley equation correlatingBij

with Bi andBj

as the stripping solution helps in minimizing the degradation ofCyanex 923 solution during the repeated extraction–strippingcycles. Cyanex 923 has earlier been cited for the extraction forCd(II) [14], lanthanides[15] and Nb(V) and Ta(V)[16].

2. Determination of Fe(III) species

The tannery filtrate, picked up for this study, containsaround 300 ppm Fe(III), 500 ppm Cr(III) and 5 ppm Zn(II) asheavy metal contaminants in a medium of about 0.5 M H2SO4.The selective extraction of Fe(III) with Cyanex 923 has beenachieved by maintaining a required concentration of NaCl inthe aqueous phase. Cr(III) remains completely in the aqueousphase. The speciation of different possible aqueous phase ferriccomplexes may be useful to explain the mechanism of the Fe(III)extraction process. The succeeding text describes a theory toexplain the speciation of Fe(III) in the aqueous phase under theconditions of selective extraction. The different notations usedare defined in a separate list given in the paper.

The presence of Cl− (Eq.(1)) along with HSO4− (Eq.(2)) and

SO42− (Eq.(3)) lead to formation of different ferric complexes

as proposed in Eqs.(4)–(6).

NaCl ⇒ Cl− + Na+ (complete dissociation) (1)

H − + )

H

Table 1List of Bromley parameters andK0

i

K0i Value Reference

K0FeHSO4

2+ 2.31× 103 [17]

K0FeSO4

+ 1.06× 104 [17]

K0Fe(SO4)2

− 1.29× 106 [18]

K0FeCl2+ 20.4 [19]

K0FeCl2

+ 95.5 [19]

K0FeCl3

15.5 [19]K0

FeCl4− 0.049 [19]

Ions or ion pairs Bi (kg mol−1) δI Reference

H+ 0.0875 0.103 [21]Fe3+ 0.0206 0.200a This studyNa+ 0 0.028 [21]HSO4

− 0.0463 0.200a This studySO4

2− 0 −0.4 [21]Cl− 0.0643 −0.064 [21]H+, HSO4

− 0.1544 n.r. This studyH+, SO4

2− 0.0463 n.r. This studyH+, Cl− 0.1433 n.r. [21]Na+, HSO4

− 0.0519 n.r. This studyNa+, SO4

2− −0.0192 n.r. [21]Na+, Cl− 0.0574 n.r. [21]Fe3+, HSO4

− 0.1069 0.200a This studyFeHSO4

2+, SO42− 0.0269 n.r. This study

Fe3+, SO42− −0.0594 0.200a This study

FeSO4+, SO4

2− −0.1394 n.r. This studyFe3+, Cl− −0.0016 n.r. [19]FeCl2+, Cl− −0.1469 n.r. [19]FeCl2+, Cl− −0.5248 n.r. [19]FeCl3, HCl −0.5779 n.r. [19]FeCl4−, H −0.6110 n.r. [19]

n.r.: not required.a Value assumed on the basis of earlier observations of Bromley[21].

Fe3+ + HSO4− ⇔ FeHSO4

2+, K01 = KC

1

γFeHSO42+

γFe3+γHSO4−

(4)

Fe3+ + nSO42− ⇔ Fe(SO4)3−2n

n ,

K0i = KC

i

γFe(SO4)3−2nn

γFe3+γn

SO2−4

(n = 1 or 2) (5)

Fe3+ + nCl− ⇔ FeCl3−nn ,

K0i = KC

i

γFeCl3−nn

γFe3+γnCl−

(n = 1, 2, 3 or 4) (6)

The determination ofKCi and thus the concentration of different

possible Fe(III) species requires the knowledge of the respec-tive K0

i and γ i. The values ofK0i , taken from the literature

[17–19], are given inTable 1. The individual activity coefficientsγ i have been estimated using the modified Bromley model[19]explained by the following equations:

logγFe3+ = −z2Fe3+D + FFe3+ (7)

2SO4 ⇒ HSO4 + H (complete dissociation) (2

SO4− ⇔ SO4

2− + H+, Kd = 0.0105 (3)

256 A. Deep et al. / Analytica Chimica Acta 558 (2006) 254–260

logγLi− = −z2Li−D + FLi− (8)

logγFeL3−ii

= −(3 − i)2D + FFeL3−ii

(9)

logγFeLi = FFeLi (10)

whereD = AT√

I

1 + √I, having AT = 0.5108 (mol kg−1)

1/2(11)

andF, e.g. for the cations, is given by

FM =∑

X

(0.06+ 0.6BMX ) |zMzX |(

1 + 1.5zMzX

I) + BMX

×(

(|zM | + |zX |)24

[X]

))(12)

∑X extends to all the possible cation–anion interactions. The

F term for the anions and the ion pairs is defined in the similarway. Its value for the neutral species is given as:

FFeLi = SFeLi,LI (13)

Some of the requiredB parameters have been reported ear-lier [19] and given inTable 2. The unknownBFe3+,SO4

2− werecomputed with the help of Eq.(14) (proposed by Belaustegi etar

TB

w

B

ef-fi hasw f

TE0 1,T

D

EESVE

mass and charge into the account. These speciation equationswere solved by the iterative ‘Solver’ add-in of Microsoft Excel2003.

3. Experimental

Cyanex 923 was received from Cytec (Netherlands). Thisproduct was a mixture of four trialkylphosphine oxides: R3P O(≈14%), R′R2P O (≈42%), R′

2RP O (≈32%), and R′3P O(≈8%), where R and R′ representedn-octyl andn-hexyl hydro-carbon chains, respectively. The main extracting entity ofCyanex 923 is then-octyl phosphine oxide and its generalextracting properties are similar to those of tri-n-octyl phosphineoxide (TOPO)[22]. Apparently the presence ofn-hexyl phos-phine oxide lowers the freezing point and, to some extent, theviscosity of Cyanex 923 thus making the extractant soluble inall the commonly used hydrocarbon diluents even at a low ambi-ent temperature. Escaid 100, Escaid 110, Varsol 80 and ExxsolD80, used for preparing the dilute extractant solutions, werereceived from ExxonMobil (Spain). Another diluent namelyShellSol D70 was received from Shell Chemicals (Spain). Allother chemicals were AR grade materials from MERCK. Thestock solutions of the metal ions were prepared by dissolvingtheir suitable salts in de-ionized water containing the minimumrequired acidity. These stock solutions were diluted to preparetC

fun-n O = 1)w con-s ret s thec minedb theo werec ica

ppmFm

4

4t

c .T ffer-e edt e sub-j xsolD II)a s-f tioni

l. [19]) using the available values ofcFe3+ , cSO42− , rFe3+ and

SO42− [19,20]:

(BMX + cM)zM

rM+ cX

= −1.39

(log

r2X

zX− log 1.392

)(log

r2M

zM− 1.392

)(14)

he estimation of unknownBFe3+ , BH+,HSO42− , BH+,SO4

− ,

Na+,HSO4− , BFe3+,HSO4

− , BFeHSO42+,SO4

2− andBFeSO4+,SO4

2−as done by Eq.(15)proposed by Bromley[21]:

MX = BM + BX + δMδX (15)

The obtained data are given inTable 2.After determining the values of all the required activity co

cients the fractions of different species in the aqueous pere computed by solving (Eqs.(4)–(6)) taking the balance o

able 2ffect of diluents on the extraction of 1.0× 10−2 M Fe(III) from the mixed.50 M H2SO4 + 0.75 M NaCl medium using 0.50 M Cyanex 923 (A/O == 298 K, t = 5 min)

iluent Aromatic content(%, w/w)

Fe(III)extraction (%)

Remarks (phaseseparation)

xxsol D80 0.04 91± 1 <25 sscaid 110 0.4 91± 1 2–3 minhellSol D70 0.03 90± 2 4–5 minarsol 80 25 84± 1 30 s–1 minscaid 100 18 84± 1 1–2 min

e

he working solutions of 1.0× 10−2 M Fe(III), 1.0× 10−2 Mr(III) and 1.0× 10−4 M Zn(II).For the solvent extraction experiments the separating

els containing the aqueous and the organic phases (A/ere placed in an orbital shaker (Aralab Agitorb 160 E) attant temperature (298± 1 K) and shaken for 5 min to ensuhe complete equilibrium. After separating the two phaseoncentration of metals in the aqueous phase was detery AAS (AAnalyst 200, Perkin-Elmer) and subsequently inrganic phase by mass balance. The stripping experimentsarried out at 298± 1 K by using a solution of 0.25 M sulphurcid (A/O = 1) as the stripping reagent.

The tannery filtrate was composed with around 300e(III), 500 ppm Cr(III), 5 ppm Zn(II) and 2000 ppm Cl− in aedium of about 0.5 M H2SO4.

. Results and discussion

.1. Effect of Cl− ions on the extraction of metal ions fromhe tannery filtrate

Some preliminary studies from SO42− and Cl− media indi-

ated that Cyanex 923 extracts Fe(III) only from Cl− mediumhis extraction was selective over Cr(III). Subsequently dint concentrations of Cl− ions (0.10–1.0 M NaCl) were add

o the tannery filtrate and the resulting aqueous phases werected to the extraction steps with 0.50 M Cyanex 923 (Ex80). The addition of Cl− ions favoured the extraction of Fe(Ind Zn(II) (Fig. 1). At 0.75 M Cl−, around 90% Fe(III) was tran

erred into the organic phase. Cr(III) did not show any extracn Cyanex 923 over the applied conditions.

A. Deep et al. / Analytica Chimica Acta 558 (2006) 254–260 257

Fig. 1. Extraction of Fe(III) from the tannery filtrate as a function of Cl− con-centration ([Cyanex 923]organic= 0.50 M,T = 298 K, A/O = 1,t = 5 min).

4.2. Effect of the extractant concentration on the extractionof metal ions from the tannery filtrate

The extraction of metal ions from the tannery filtrate (con-taining 0.75 M Cl−) as a function of extractant concentrationis shown inFig. 2. Around 90% Fe(III) and 80% Zn(II) wereextracted in a single stage experiment with 0.50 M Cyanex 923(Exxsol D80). The extraction of Cr(III) was negligible in any ofthe tested extractant concentrations.

4.3. Effect of the nature of diluent on the extraction ofFe(III)

Exxsol D80, Escaid 110 and ShellSol D70, the aliphatic dilu-ents, and Varsol 80 and Escaid 100, with 25 and 18% (w/w)aromatics, were tested to check their effect on the extractivecapacity of Cyanex 923 toward Fe(III) from a mixed mediumof 0.50 M H2SO4 and 0.75 M Cl−. The results of this studyare given inTable 2. The performance of Cyanex 923 remainedunchanged in all the three aliphatic diluents. However, its extrac-tive capacity reduced to some extent by the presence of aromaticsin the diluent. Interestingly, no such pattern was observed withrespect to the time taken in the phase separation, which wasquickest in Exxsol D80 followed by Varsol 80, Escaid 100,Escaid 110 and ShellSol D70. In all other studies Exxsol D80w

Fig. 2. Extraction of Fe(III) from the tannery filtrate as a function of extractantconcentration ([Cl−]aqueous= 0.75 M,T = 298 K, A/O = 1,t = 5 min).

4.4. Speciation of extracting Fe(III) species

The values ofB andK0i used to model the activity coefficients

and speciation equations are given inTable 1. The data on thespeciation of different possible ferric complexes are given inTable 3. Evidently, FeHSO42+ is the predominant species in themodeled tannery filtrate. The addition of Cl− ions leads to thegradual formation of FeCl3. At 0.75 M Cl−concentration, FeCl3becomes the predominant species.

Referring Cyanex 923 as S the extraction reaction can nowbe written as:

FeCl3 + nS ⇔ FeCl3·nS (16)

The concentration based equilibrium constantKCext and the

distribution ratioD for the above reaction can be given as:

KCext =

cFeCl3·nS

cFeCl3cnS

(17)

D = cFeCl3·nS

cFeCl3(18)

Rearranging Eqs.(17)and(18)gives:

logD = logKCext + n logcS (19)

Thus, a plot of logD against logcS would give the values ofKC of

TS in a

C

Fe

0 60 30 00 01 0

F

as used as diluent.

able 3peciation of different ferric species with respect to varying Cl− concentration

l− added (M) Contribution of different species (%)

Fe3+ FeHSO42+ FeSO4

+

0.50 89 4.0.25 0.49 76 2.4.50 0.33 34 0.84.75 0.23 12 0.23.0 0.04 4.1 0.07

ixed parameters: [Fe3+] = 0.01 M, [H2SO4] = 0.50 M.

ext (intercept) andn (slope).Fig. 3 depicts the treatment

medium of 0.50 M H2SO4

(SO4)2− FeCl2+ FeCl2+ FeCl3 FeCl4−

.5 0 0 0 0.3 0.64 2.8 14 0.37.95 0.68 5.7 55 2.5.22 0.42 5.5 78 3.4.06 0.23 4.5 87 4.0

258 A. Deep et al. / Analytica Chimica Acta 558 (2006) 254–260

Fig. 3. Variation in the distribution of Fe(III) at different Cl− concentrationsas a function of free extractant concentration ([Fe(III)]aqueous= 1.0× 10−2 M,[H2SO4]aqueous= 0.50 M,T = 298 K, A/O = 1,t = 5 min).

the related data at different [Cl−]. The free extractant concen-tration, cS, was calculated ascS = c(taken)− c(complexed), wherec(complexed)was determined according to Eq.(20). The values ofKC

ext at 0.25, 0.50, 0.75 and 1.0 M Cl− were found to be 1.1, 7.7,43 and 103. The increasing values ofKC

ext explain the observedeffect of Cl− ions on the extraction of Fe(III) by Cyanex 923.The values ofn (around 2 in each case) confirm the stoichiome-try of the extracted complex as 1:2 (metal:extractant). Based onthese findings the extraction of Fe(III) with Cyanex 923 can beproposed to follow the mechanism

FeCl3 + 2S ⇔ FeCl3·2S (20)

4.5. Effect of [H2SO4]aqueous on the extraction of Fe(III)

The tannery filtrate is basically the sulphuric acid leachateof the tannery sludge. The acid content in the final filtrate canvary depending upon the leaching conditions. Therefore, theextraction of Fe(III) in Cyanex 923 from Cl− medium wasinvestigated with respect to the varying concentration of H2SO4in the aqueous phase. The results of the study are depicted inFig. 4. The extraction of Fe(III) gradually increased with theincrease in the acid concentration. This behaviour has beenexplained by determining the change in the aqueous phase

Fig. 4. Extraction of Fe(III) as a function of H2SO4 concentration([Fe(III)] aqueous= 1.0× 10−2 M, [Cl−]aqueous= 0.75 M, [Cyanex 923]organic=0.50 M,T = 298 K, A/O = 1,t = 5 min).

species upon the addition of H2SO4. As per the calculationsby Bromley model the different ferric complexes have beenfound to be in proportions given inTable 4. Besides maintain-ing the predominance of FeCl3 the addition of H2SO4 in theaqueous phase causes the development of an anionic speciesFeCl4−, which is known to form an extractable HFeCl4·2Scomplex with Cyanex 923[23]. The simultaneous formationof FeCl3·2S and HFeCl4·2S led to an increased extraction ofFe(III).

4.6. Fe(III) extraction kinetics and isotherm

The extraction of 1.0× 10−2 M Fe(III) from the mixed0.50 M H2SO4 + 0.75 M NaCl medium with 0.50 M Cyanex 923(A/O = 1) was recorded at different temperatures (298, 303, 313,323 K) for different time intervals (1–20 min). No significantchange in the extraction of Fe(III) was observed with the changein the temperature. In all the cases a contact time of 3 min wassufficient for achieving the equilibrium transfer of Fe(III) to theorganic phase.

The extraction isotherms of Fe(III) at two different Cl− con-centrations (0.50 and 0.75 M) using 0.50 M Cyanex 923 areshown inFig. 5. The data suggest the dependence of the loadingcapacity of Cyanex 923 on the concentration of Cl− ions in the

Table 4S tion i

H

Fe

01 01 02 02 03 0

F

peciation of different ferric species with respect to varying H2SO4 concentra

2SO4 total (M) Contribution of different species (%)

Fe3+ FeHSO42+ FeSO4

+

.50 0.23 12 0.23

.0 0.04 12 0.11

.5 0.01 9.2 0.05

.0 0 5.5 0.02

.5 0 2.8 0.01

.0 0 0.90 0.01

ixed parameters: Fe3+ = 0.01 M, [NaCl] = 0.75 M.

n a medium of 0.75 M NaCl

(SO4)2− FeCl2+ FeCl2+ FeCl3 FeCl4−

0.22 0.42 5.5 78 3.4.15 0.20 2.5 79 6.0.08 0.16 1.1 82 7.4.04 0.04 0.40 84 10.02 0.01 0.16 83 14.01 0.01 0.07 80 19

A. Deep et al. / Analytica Chimica Acta 558 (2006) 254–260 259

Fig. 5. Extraction isotherm of Fe(III) ([H2SO4]aqueous= 0.50 M, [Cl−]aqueous=0.75 M, [Cyanex 923]organic= 0.50 M,t = 5 min).

aqueous phase. The curves can be used to simulate the stagingrequirements during the extraction process.

4.7. Stripping kinetics and isotherm

A solution of 0.25 M H2SO4 was found suitable enough tocompletely strip the extracted Fe(III) from the organic phase(0.50 M Cyanex 923 loaded with 1.0× 10−2 M Fe(III)). Thekinetics of the stripping was studied for different time inter-vals (1–25 min) at different temperatures (298, 303, 313, 323 K)keeping an A/O ratio of 1. At 298 K the complete strippingrequired 25 min. An increase in the temperature improved thekinetics of the stripping. At 323 K, a contact time to 3 minensured the complete stripping of Fe(III).

The stripping isotherm of Fe(III) is shown inFig. 6. An idealshape of the curve proves the efficiency of H2SO4 as a strippingreagent. Depending upon the desired level of concentration ofFe(III) in the final stripping solution the phase ratio and numberof stages can be simulated.

It is noteworthy to mention here that during the processingof tannery filtrate the organic phase would load with Fe(III) andZn(II). Therefore, prior to the stripping of Fe(III) it would benecessary to wash the organic phase with water to remove Zn(II).It was checked that single washing would led to the removal of>90% Zn(II) with 2–3% loss of Fe(III).

Fig. 6. Stripping isotherm of Fe(III) ([Cyanex 923]organic= 0.50 M,[H2SO4]aqueous= 0.25 M (stripping reagent),T = 298 K, t = 25 min).

4.8. Application of the liquid–liquid extraction steps to thetannery filtrate

In some preliminary tests the problem of crud formation wasobserved with the direct use of the tannery filtrate in Cyanex 923extraction circuit. Consequently the filtrate was agitated withactivated carbon to ensure the removal of any organic impurity.Around 5 g of activated carbon was consumed in the treatmentof 1 L of the filtrate. The spent activated carbon was regeneratedby washing with acetone followed by washing with warm water.

A known aliquot of the tannery filtrate was taken and broughtup to 0.75 M Cl− by adding NaCl. Twenty five millilitres por-tion of this solution was subjected to a counter-current extraction(two stages, A/O = 1) step using 0.50 M Cyanex 923. As a resultof this, Fe(III) and Zn(II) were transferred to the organic phaseleaving pure Cr(III) in the raffinate. The extracted portion ofZn(II) was scrubbed with water and Fe(III) was finally recoveredby stripping with 0.25 M H2SO4. The organic phase was regen-erated by washing with water. The results of five consecutiveextraction (using the regenerated organic phase) and strippingcycles are given inTable 5. Fe(III) and Cr(III) were recoveredas reasonably pure solutions. As observed the extractant solu-tion maintained its capacity even after repeated extraction andstripping cycles.

TS ,298 K

Cm

entraate afex 92

F

3 0.30.20.30.20.3

n

able 5eparation of iron and chromium from tannery filtrate (extraction: A/O = 1T =

oncentration (in ppm) ofetals in tannery filtrate

Counter-currentrun number

ConcraffinCyan

e Cr Zn Fe

05± 2 505± 2 5.0± 0.5 1 4.0±2 3.9±3 4.1±4 3.9±5 3.9±

.d.: negligibly detected.

, t = 5 min; stripping: A/O = 1,T = 298 K, t = 25 min)

tion (in ppm) of metals inter extraction with 0.50 M3

Concentration (in ppm) of metals inaqueous phase after stripping with0.25 M H2SO4

Cr Zn Fe Cr Zn

505± 2 1.0± 0.1 295± 2 n.d. 0.7± 0.1505± 2 0.9± 0.1 296± 2 n.d. 0.8± 0.1505± 2 0.8± 0.1 297± 2 n.d. 0.8± 0.1505± 2 0.8± 0.1 296± 2 n.d. 0.8± 0.1505± 2 0.8± 0.1 295± 2 n.d. 0.8± 0.1

260 A. Deep et al. / Analytica Chimica Acta 558 (2006) 254–260

5. Conclusions

The present work demonstrates the utility of Cyanex 923 forthe selective recoveries of Fe(III) and Cr(III) from the tanneryfiltrate. The speciation studies have suggested FeHSO4

+ as thepredominant species in the original tannery filtrate. The additionof Cl− ions changes the equilibria and FeCl3 emerges as thepredominant species, which gets extracted in Cyanex 923 byforming the complex FeCl3·2S. The partioning of Fe(III) in theorganic phase is selective over Cr(III) with its kinetics beingreasonably fast. Although Zn(II) is co-extracted it is easy toscrub this impurity with water. A low concentration of sulphuricacid is sufficient for the stripping of Fe(III). This might be animportant advantage over other extractants like DEHPA, TBPand PC-88A, which require a higher acid concentration for thestripping[24]. Cyanex 923 maintains its extraction capacity evenafter multiple extraction and stripping cycles.

The addition of salt in the suggested process may be seen as alimitation. Tannery industries may use the common salt wastedfrom their skin and hide preservation activities. The final processraffinate (containing Cl−) requires further treatment unless it ispossible to dispose it directly into the open sea.

Acknowledgements

C wl-e g ths obi( pleso

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The financial support to the work by Fundac¸ao para aiencia e Tecnologia (FCT, Portugal) is gratefully acknodged. Thanks are due to Cytec, Netherlands for providinample of Cyanex 923. We express our gratitude to ExxonMSpain) and Shell Chemicals (Spain) for providing the samf required diluents.

el

Madariaga, Fluid Phase Equilib. 121 (1) (1996) 99–109.21] L.A. Bromley, AIChE J. 19 (2) (1973) 313–320.22] Cyanex 923 Extractant, Solvent Extraction Reagent, Cytec Indu

Inc., New Jersey, 1991.23] J. Saji, T. Prasada Rao, C.S.P. Iyer, M.L.P. Reddy, Hydrometallurg

(3) (1998) 289–296.24] M.R.C. Ismael, J.M.R. Carvalho, Miner. Eng. 16 (2003) 31–39.