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Discussion of thePhysicochemical Effects of Modifiers on the ExtractionProperties of Hydroxyoximes.A ReviewAna M. Sastre a & Jan Szymanowski b

a Department of Chemical Engineering ,Universitat Politcnica de Catalunya, E.T.S.E.I.B. ,Barcelona, Spainb Institute of Chemical Technology andEngineering , Poznan University of Technology , Pl.Sklodowskiej Curie, 260 965, Poznan, PolandPublished online: 24 Aug 2007.

To cite this article: Ana M. Sastre & Jan Szymanowski (2004) Discussion of thePhysicochemical Effects of Modifiers on the Extraction Properties of Hydroxyoximes.A Review, Solvent Extraction and Ion Exchange, 22:5, 737-759, DOI: 10.1081/SEI-200035610

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Discussion of the Physicochemical Effects of Modiers on the Extraction Properties of

Hydroxyoximes. A Review

Ana M. Sastre 1 and Jan Szymanowski 2, *

1 Department of Chemical Engineering, Universitat Polite `cnica deCatalunya, E.T.S.E.I.B., Barcelona, Spain

2 Institute of Chemical Technology and Engineering, Poznan Universityof Technology, Poznan, Poland

ABSTRACT

The physicochemical aspects of hydroxyoxime modication with variousmodiers containing oxygen atoms are discussed. This kind of modicationis unique to hydroxyoxime reagents, which form weak intramolecular andintermolecular hydrogen bonds. These bonds can easily be destroyed not

only by modier molecules (alcohols, alkylphenols, and esters) but alsoby solvating diluents. Alcohols are stronger modiers than alkylphenols.The drawback of these modiers is their strong self-association, which iseliminated in esters and sterically hindered alcohols. The extraction proper-ties of strong hydroxyoxime extractants can also be modied by adding

737

DOI: 10.1081/SEI-200035610 0736-6299 (Print); 1532-2262 (Online)Copyright # 2004 by Marcel Dekker, Inc. www.dekker.com

*Correspondence: Jan Szymanowski, Institute of Chemical Technology and Engineer-ing, Poznan University of Technology, Pl. Sklodowskiej-Curie, 260-965 Poznan,

Poland; E-mail: [email protected].

SOLVENT EXTRACTION AND ION EXCHANGEVol. 22, No. 5, pp. 737759, 2004

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weak reagents (hydroxyoximes or b -diketones), which form co-associates

with strong extractants. The use of sterically hindered esters seem rationalfor two reasons: they inhibit hydroxyoxime adsorption at the hydrocarbon /water interface with oximino group penetrating the aqueous layers; and theyshow negligible adsorption. Due to these phenomena, hydroxyoximeextractants are more stable, and a retardation of the extraction is notobserved in the presence of hindered esters. There is currently no generalexplanation of the chemistry of modication. Many published worksfocus only on the interactions of modiers with extractants, neglectingthe interactions with metal complexes. In order to achieve a substantial pro-gress in our understanding of the phenomenon of modication, interactionsbetween complexes and modiers must be studied and explained.

Key Words: Hydroxyoximes; Extractants; Dimerization; Modier.

INTRODUCTION

Modiers are often used in extraction systems for two reasons. Firstly, they

are used to increase the solubility of extractants and their complexes and toavoid third-phase formation. Secondly, they are used to modify extraction prop-erties, i.e., metal extraction and stripping abilities. The rst application isusually found in systems containing various amines. [16] However, additionof modiers is not unique to the routine extraction of metals. For instance, modi-ers are also added to increase the polarity of supercritical CO 2 in supercriticaluid extraction [e.g., to increase the extraction of mercury(II) with sodiumdiethyldithiocarbamate]. [7] Polar modiers dissolved in supercritical carbondioxide improve the extraction of Hg(II). The improvement in extraction ef-

ciency depends on the polarity and acidity of the modier. Extraction improvesin the following order: none , hexane , toluene , dichloroethane ,methanol , acetic acid. Although the effect is the opposite of that observedin solvent extraction, the analogy can be seen clearly. The second applicationis found in connection with hydroxyoximes (Structure 1) and the formation of tailored blends where extraction and stripping properties are adjusted to theaqueous feed [i.e., acidity and concentration of copper(II)] in such a way thatthe maximum of the metal transfer is achieved in extraction stripping cycle.

Sastre and Szymanowski738

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The technical aspects of these kinds of blends were recently discussed by

Kordosky and received very positively.[810]

The blends included: ACORGA5100 and 5050 from the rst generation of reagents, modied with nonylphenoland hydrophobic alcohols; M 5640 and M 5774 modied with hydrophobicesters; LIX 622 modied with tridecanol; and LIX 664N modied with esters(Table 1). The study also included LIX 984N and LIX 973, which are mixturesof aldoxime and ketoxime. The problem of industrial application of tailoredblends is still actual. It is easier and cheaper to optimize the composition of the blend than to invent and produce a reagent with a new active substance.Future developments will involve new and improved synergistic mixtures of old reagents, rather than manufacturing new extractants.

The aim of this work is to discuss the physicochemical aspects of modify-ing the extraction abilities of hydroxyoximes by adding modiers which formhydrogen bonding with the hydroxyl and oximino groups of these extractants.

SELF-ASSOCIATION OF EXTRACTANTS

A characteristic feature of some extractants is their self-association,especially the formation of cyclic dimers. Acidic extractants, e.g., monofunc-tional organic acids, form stable dimers with dimerization constants in theorder 10 4 or even higher. [11 16] Such a high value in the dimerization constantis the result of the presence of strongly polarized groups (C 55 O or P55 O incarboxylic or phosphorus organic acids, respectively). These groups arestrong hydrogen-bond acceptors (i.e., strong electron donors) and form hydro-gen bonds with acid hydroxyl group.

Hydroxyoximes contain a hydroxyl group which is less acidic(pK a 8.99.9) than the hydroxyl group found in the classical acidic extrac-tants. [17,18] Actually, higher values, up to p K a 12 are reported in the originalliterature because they were determined in waterdioxane mixtures. Theacidity of the phenolic hydroxyl group depends on hydroxyoxime structureand changes as follows: 2-hydroxy-5-alkylbenzaldehyde oxime (activesubstance in ACORGA reagents and LIX 860) . 2-hydroxy-5-alkylacetophe-none oxime (active substance in LIX 84) . 2-hydroxy-5-alkylbenzophenoneoxime (active substance in LIX 65 and LIX 64N). Actually, the last two hydro-xyoximes form two different isomers (E and Z, or anti and syn) that show a

different acidity of the phenolic group. The reagents are not currently pro-duced and the problem of their isomerization and the association of Z-isomer will not be discussed.

The most important issue is that the oximino group ( 55 NOH) is a weak proton acceptor. [17,18] Due to steric limitations, only the nitrogen atom canform hydrogen bonds with the undissociated hydroxyl group of the same

Physicochemical Effects of Modiers 739

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T a

b l e 1

.

H y d r o x y o x i m e e x t r a c t a n t s u s e d f o r c o p p e r r e c o v e r y .

R e a g e n t

T y p e

R

Y

M o d i e r

L I X 6 5 N

K e t o x i m e

C 9 H 1

9

C 6 H 5

L I X 6 4 N

K e t o x i m e

C 9 H 1

9

C 6 H 5

L I X 6 3 t o

a c c e l e r a t e

e x t r a c t i o n

L I X 8 4

K e t o x i m e

C 9 H 1

9

C H 3

A C O R G A P 5 0

A l d o x i m e

C 9 H 1

9

H

A C O R G A 5 1 0 0

A l d o x i m e

C 9 H 1

9

H

N o n y l p h e n o l

A C O R G A 5 0 5 0

A l d o x i m e

C 9 H 1

9

H

T r i d e c a n o l

A V E C I A M 5 6 4 0

A l d o x i m e

C 9 H 1

9

H

E s t e r s

A V E C I A M 5 7 7 4

A l d o x i m e

C 9 H 1

9

H

E s t e r s

L I X 6 2 2

A l d o x i m e

C 1 2

H 2 5

H

T r i d e c a n o l

L I X 8 6 0 - I

A l d o x i m e

C 1 2

H 2 5

H

L I X 8 6 0 N - I

A l d o x i m e

C 9 H 1

9

H

L I X 8 6 4 ( L I X 6 4

L I X 8 6 0 )

K e t o x i m e / a l d o x i m e

C 9 H 1

9 , C 1 2

H 2 5

C 6 H 5

, H

L I X 8 6 5 ( L I X 6 5 N

L I X 8 6 0 )

K e t o x i m e / a l d o x i m e

C 9 H 1

9 , C 1 2

H 2 5

C 6 H 5

, H

L I X 9 8 4 N ( L I X 8 6 0

L I X 8 4 )

A l d o x i m e / k e t o x i m e

C 9 H 1

9

H , C

H 3

L I X 9 7 3 N ( L I X 8 6 0 N - 1

L I X 8 4 - 1

)

A l d o x i m e / k e t o x i m e

C 9 H 1

9

H , C

H 3


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hydroxyoxime molecule (intramolecular bond, see Structure 1) or of the

second hydroxyoxime molecule (intermolecular bond) with the formation of dimer (see Structure 2). The formation of intramolecular hydrogen bondbetween the hydrogen atom of the oximino group and the oxygen of phenolicgroup (see Structure 3) is only possible after a dissociation of the hydroxylgroup. For example, this could occur during contact with an alkalineaqueous phase at pH . 11. These kinds of conditions are not encounteredin industrial copper extraction processes with hydroxyoxime reagents.

The IR spectra of hydroxyoximes depend on the diluent, and are signi-cantly different in non-solvating (aliphatic hydrocarbons) and solvating(aromatic hydrocarbons) diluents. [19] In practical terms, the non-solvatingdiluents are not used alone. Aromatics are, to some extent, present in com-mercial solvents to improve the solubility of hydroxyoxime copper com-plexes. The IR spectra of alkyl derivatives of salicyl aldoximes, of 2-hydroxyacetophenone and 2-hydroxybenzophenone oximes (the activesubstances present in commercial reagents) are similar in aliphatic hydrocar-bons. They show a sharp band at 3580 cm

2 1 and a broad band at 3420 cm2 1 .

This is characteristic of the monomeric and associated NOH group. [20] Afurther broad band is observed at 3220 cm

2 1 . This is characteristic of thephenolic hydroxyl group which is bound to the oxime nitrogen atom in

both the monomeric and the associated forms (Structures 1 and 2). Often,these two bands are not separated and show-up as one broad bandbetween 3200 and 3400 cm

2 1 . The ratio of the integrated monomeric andassociated densities decreases with an increase in hydroxyoxime concen-tration. This indicates a shift of the equilibrium towards associatedspecies. These three bands disappear in solvating diluents, such as toluene,


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hydrogen bonds (hydroxyoximes, for example). This effect is probably

smaller in association with acidic extractants, which form very stabledimers.

On modeling extraction equilibria, Piotrowicz et al. [31] reports that thedimerization of hydroxyoximes can be neglected in wet extraction systems.The average value of the hydroxyoxime dimerization constant in keroseneis only 1.7. Bogacki, [32,33] who models an extractionstripping system forcopper extraction with 2-hydroxy-5-nonylbenzaldehyde oxime and2-hydroxy-5-nonylbenzophenone oxime reports the same phenomenon.When using the Pitzer treatment [34] and calculating the activity coefcientsin the aqueous phase, a strong deviation from ideality is observed. [31]However, the estimated dimerization constants are small, statistically equalto zero (0.6 + 0.2 or 2 0.7 + 3.3). Tanaka [35] also neglects the dimerizationof hydroxyoximes. However, he uses the modied reagent ACORGA P5100to extract copper(II) from HCl and HNO 3 solutions. The activities are calcu-lated according to the Bromley and the Pitzer models. The model agrees withthe experimental extraction data very well. The computed associationconstants are, again, near zero and equal to 0.8 and 1.2 for the hydroxyoximedimerization and hydroxyoxime co-association with nonylphenol, respect-ively. Recently, two works were published [36,37] in which the extraction equi-librium with hydroxyoximes was modeled, neglecting the dimerizationconstants. The differences observed are explained by strong deviations fromideality in the aqueous acidic sulfate phase.

Actually, it is impossible to prove that the dimerization disappears inwet solvents. It is more justied to assume that the equilibrium constant issmall and the dimerization still occurs, decreasing the content of monomericforms.

CO-ASSOCIATION OF HYDROXYOXIMES ANDMODIFIERS HAVING HYDROXYL GROUP

Hydroxyoximes with a 55 NOH group can form co-associates with variousalkylphenols, alcohols, ethers, ketones, and esters. Alkylphenols, alcohols,and esters are, in practice, exploited in modern hydroxyoxime blends. Theformation of co-associates between different hydroxyoximes is also possible

and is currently exploited.A hydroxyoxime molecule with intramolecular hydrogen bonding(Structure 1) has two oxygen atoms. Each oxygen atom has two free elec-tron pairs and one mobile hydrogen atom that is able to participate inhydrogen bonds. Thus, theoretically, up to ve molecules (alcohols or alkyl-phenols) can be bound into hydroxyoxime associates. Four of the molecules


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would act as hydrogen-bond donors to the oxygen atoms of the hydroxy-

oxime, and one molecule would act as an hydrogen-bond acceptor for thehydrogen from the oximino group. The nitrogen atom from the oximinogroup also acts as an hydrogen-bond acceptor. Although the nitrogenatom may bind an alcohol molecule, the formation of an intramolecularbond with the hydrogen from the phenolic hydroxyl group seems moreprobable.

A modier molecule may approach a hydroxyoxime molecule at varioussites, giving associates of somewhat different energies. Molecular modeling,carried out for isolated molecules of hydroxyoxime and a modier, suggeststhe formation of associates containing only one or two modier mol-ecules. [38,39] The formation of 1 : 1 and 1 : 2 hydroxyoximealcohol associatesis also deduced by Freiser [40] from the modeling of copper extractions with2-hydroxy-5-alkylbenzaldehyde oximes.

It is impossible to specically locate the modier molecule in associ-ates. This is because the differences between the PM3 formation heatscomputed for the various associates are small (only a few kJ / mol). Differentassociates are, therefore, probably present in an equilibrium mixture.However, some peculiarities are observed when bulky branched alcoholmolecules are considered. The steric effect is evident and a modier mole-cule with a highly branched alkyl chain in the close proximity to thehydroxyl group is shifted to the hydroxyoxime group in modierextractantassociate. The band that is characteristic of the hydroxyoxime monomerrapidly disappears in the IR spectrum after introducing a modier to ahydroxyoxime solution in a hydrocarbon. Instead, a broad and intenseband appears which is characteristic in hydroxyoxime associates with modi-er molecule(s). Similar changes are observed in the IR spectra whenalcohol is added to pyridinecarboxamide. This demonstrates the formationof an intermolecular hydrogen bonded amide and alcohol co-associate. [41]

Polar solvents that have a proton acceptor group (e.g., alcohols), even break carboxylic acids dimers and form intermolecular H-bond complexes. [42]

Aliphatic alcohols show a greater electron density on the oxygen atomthan alkylphenols, and they form associates with hydroxyoxime moleculesmore easily. This effect is manifested in the reduced amount of alcohol


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needed to obtain the same modication of hydroxyoxime extraction /stripping abilities as when using nonylphenol. Actually, half the amountof tridecanol is needed to obtain the same effect as when using nonylphenol,when it is added to 2-hydroxy-5-nonylbenzaldehyde oxime (P 50 reagent). [43,44]

Alcohols also behave differently to alkylphenols in terms of self-association. Alkylphenols form linear associates (see Structure 4) with theassociation constants in the range 50 80 L mol

2 1 .[45,46] Alcohols form differ-ent species such as linear dimers, trimers, and tetramers (Structure 5). Thecyclic tetramer (Structure 6) is probably dominant for linear and slightlybranched alcohols. [47]

Tetramer content increases with alcohol concentration. It also increases astemperature decreases. At 25 8C, tetramers of decanol are the predominantspecies in decane when the alcohol concentration is greater than 4%.

However, theassociation of an alcohol depends upon its structure, and decreaseswith increased branching of the alkyl chain near the hydroxyl group. Forexample, the monomer is the dominant species at a 10% solution of 4-propyl-heptan-4-ol in decane at 25 8C. [48] Highly branched alcohols should be usedeven with smaller amounts for hydroxyoxime modication, because theycould be used exclusively for hydrogen-bond formation with hydroxyoximes.


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When considering the effect of the modier on the extraction of copper,

the following equilibria should be taken into account:

Cu 2w 2HL o O CuL 2o 2Hw 2

2HL o O HL 2o 3

HL o nROH O HL nROH o 4

mROH o O ROH mo 5

where HL stands for hydroxyoxime; subscripts o and w denote the organic andaqueous phase respectively; n is equal to 1 or 2; and m varies from 1 to 4. Is nequal to 4 for alcohols used in commercial blends? Only extractant producerscan answer this question, as only they know the composition and structures of the modiers added. Reaction (3) consumes monomeric hydroxyoxime mole-cules, thus shifting reaction (2) to the left decreasing the extraction abilities of the hydroxyoximes. This effect would be negative for weak extractants suchas 2-hydroxy-5-nonylbenzophenone oxime (LIX 65N and LIX 64N from therst generation of hydroxyoximes produced by General Mills). The effectwould be positive for hydroxyoximes which form strong and stable complexessuch as 2-hydroxy-5-nonyl(or dodecyl)benzaldehyde oxime (ACORGA P 50and LIX 860), and also for 2-hydroxy-5-nonylacetophenone oxime (LIX 84).

A decrease in extraction ability is compensated by an increase in strippingability. [43,44] As a result, more copper is transferred from an aqueous feed tothe hydrocarbon phase and, after that, to an aqueous strip phase in modiedstrong hydroxyoximes. The optimization of technological processes concern-ing reagents with different extraction abilities were rst modeled and dis-cussed by Tanaka [49] and then developed for modied extractants byBogacki. [50] Their studies showed that tailored modied reagents can meanan increased production of copper, or that the process can be carried out ina smaller plant.

A computer simulation of a steady-state countercurrent multistage extrac-tionstripping process shows that there is a value of the extraction equilibriumconstant that yields the maximum metal recovery when the other operationalparameters are constant. [49] The extraction is insufcient for a low extractionconstant. The stripping is excellent, but the total transfer of metal from theaqueous feed to the strip is low. The extraction is excellent for a high extrac-tion constant but the stripping is inefcient. As a result, the overall transfer is

also low.The association phenomena (extractant association, modier association,and extractant-modier co-association) change the optimum strength of extractant which is needed to obtain an optimum metal transfer rate. [50] Thisoptimum transfer from the feed to the loaded electrolyte is obtained when anappropriate combination of the equilibrium constants of extraction, association


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and co-association is found. Extractant association and co-association have a

negative effect in weak extractants and a positive effect in strong extractants.The co-association of the modier with the extractant changes the optimumstrength of the extractant (or the extraction equilibrium constant) for both themultistage countercurrent and the crosscurrent processes.

CO-ASSOCIATION OF METAL COMPLEXESAND MODIFIERS

An association of the modiers with the extracted hydroxyoximecoppercomplex (Structure 7) should also be considered.

The importance of this phenomenon can be deduced from the work of Moyer et al. [51] . This work shows the relationship between the modiershydrogen-bond donor strength and the cesium extraction with calix[4]arene-bis -(tert -octylbenzo-crown-6) and suggests the solvation of the complex viahydrogen-bond interactions. The importance of hydrogen bonding in boththe extractant and the complex is also observed during the extraction of alkali metal nitrates with crown ethers in octanol. [52] As a result, the effectof an alcohol modication on extraction efciency cannot be predicted. Forinstance, the extraction abilities of benzo-crowns increase when changingthe diluent from 1,2-dichloroethane to octanol, but the extraction ability of the dicyclohexano-crowns decreases. The same phenomenon is observedwhen alcohol is added to pyridenecarboxamides dissolved in toluene andthe mixture is used for the extraction of copper from chloride solutions. [41]

Depending on the experimental conditions, extraction can increase or decreaseslightly in the presence of alcohol. Addition of decanol (20% vol / vol)

improves the extraction of copper(II) from concentrated chloride solutionswith dipentyl pyridine-3,5-dicarboxylate. [53] This indicates that the associ-ation of the alcohol with the complex has a stronger positive effect than theco-association with the extractant. This effect becomes dominant whenthere is a very high alcohol content (50100% vol / vol). In this situation,the efciency of copper(II) extraction decreases. An improved extraction of


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cadmium(II) is observed with 3,5-diisopropylsalicylic acid in the presence of

thiols. This is explained by the incorporation of the thiol molecules in theextracted complexes. [54] Further conrmation of the complex solvation isshown by Gloe et al., [55] who demonstrated that the extraction of U(VI) andV(V) with pyrazolone derivatives depends on the presence of alcohol. Theaddition of dodecanol results in an enhancement of the extraction of vana-dium(V) at high HCl concentrations, but reduces the extraction of U(VI)due to a change in the extraction mechanism. Alcohol molecules incorporateinto the extracted complexes. There is some analogy between solvation andhydration of metal complexes with ligands containing oxygen atoms. Theuranyl-isoxazolonate is stabilized by both inner- and outer-sphere hydrationin the aqueous phase (Structure 8).[56] The introduction of an electrolyteenhances the extraction due to dehydration of the complex.

The problem of inner- and outer-sphere hydration was broadly discussedin the works of Narbutt, [57 60] however, rather for hydrophilic extractants.The hydrophilic properties of ligand oxygens result in outer-sphere hydrationof the complex and affect the partition coefcient of the complex. The outer-sphere hydration depends on the coordination mode and on the distribution of the electron density in the complex because the molecular structure may affectthe access of water molecules to oxygen atoms in the ligands, while electrondensity on oxygen atoms, which depends on the central metal ion, may affectthe strength of hydrogen bonds to water molecules. Two water molecules canform hydrogen bonds with ligand oxygens in the square-planar b -diketonates,compared with four in the tetrahedral b -diketonates. [60]

Chloroform and halophenols (Lewis acids) form outer-sphere solvates

with metal acetylacetonates by means of hydrogen bonding to donor oxygenatoms [57,61] which enhances the extraction of metal complexes to theorganic phase. Each acetoacetonate ligand binds one molecule of halophenolsor chloroform.

An increase in the hydrophobic character of the extracted metal complex isobserved in the synergistic extraction of mixed complexes (adducts formed


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when the additional ligand has a pair of unbonded electrons at the nitrogen or

oxygen atom). Three different mechanisms are postulated.[62]

The rst oneinvolves the opening of one or more chelate rings and occupation by theadduct molecule(s) of the vacated metal coordination site(s). In the secondmechanism, the metal ion is not coordinately saturated by the ligand and,hence, it retains residual water in the coordination sphere, which can be replacedby adduct molecules. The third mechanism involves the expansion of the coordi-nation sphere of the metal ion to allow bonding of the adduct molecules. Tributylphosphate, trialkylphosphine oxides, trialkylamines and also pyridinecarboxy-late esters are used to form adducts and increase the extraction. [62,63]

Electron spin resonance studies of hydroxyoxime complexes withcopper(II) show the formation of ve-coordinated square pyramidal speciescontaining one additional molecule of pyridine or ammonia (Structure 9).[64 66]

A pseudo-octahedral adduct of bis (2-hydroxybenzaldehyde oxime)nickel(II)with two pyridine molecules occupying trans axial positions (Structure 10)was isolated by Basolo and Matoush. [67] However, there are no more dataavailable.

Why, then, do hydroxyoximes behave differently to other extractants?This phenomenon has not been studied. However, one can deduce that modi-er molecules solvate both hydroxyoxime and its copper complex. The hydro-gen bonds in the complex may be broken by modier molecules, leading to adecreased stability of the complex. However, this kind of associate of thecomplex with modier molecules becomes somewhat more hydrophobicand is thus transferred more easily to the hydrocarbon phase. The effectcannot be signicant because both the hydroxyoxime and complex are very

hydrophobic. Although the modeling shows that the solvation of hydroxyox-ime molecules is a crucial factor as to the extraction abilities of hydroxyoximeextractants, it is also probable that the effect is caused by a decreased complexstability due to the breaking of hydrogen bonds in the vemember rings of the complex (see Structure 7). Further studies on the phenomenon areneeded. Actually, the literature upon that problem is very scarce and poor.


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to modier can be altered so as to control the extractive strength of the blend.

This means that the extractant can be tailored in order to meet the specicneeds of a leach solution. [73]

DRAWBACKS OF MODIFICATION

Alcohols and alkylphenols exhibit some surface activity and adsorb at thehydrocarbon / water interfaces. [23 25,75,76] When used in the mixtures withextractants, they compete for the adsorption area at the interface. The inter-facial population of the extractant molecules is therefore decreased, [77] andthey act in the same way as aromatic diluents. [78] The same phenomenon isobserved in other extractants, e.g., dinonylnaphthalene sulfonic acid, bis(2-ethylhexyl)phosphoric acid, decylpyridine-carboxylates, and HOSTAREXA327 and their blends with alcohols and alkylphenols. [79 84] This adsorptionof modiers results in a decreased rate of extraction. [8386] Fortunately, thisretardation in the classical dispersive extraction process is not very large,because the interface is dynamic and quickly regenerated. The problem maybecome considerable in the future, when membrane processes with stableinterfaces in the pores of bers or ceramics will be used. Then, a considerabledecrease in mass transfer with modied extractants would be observed. Itwould thus be necessary to return to the previous generation of unmodiedextractants. However, the benets of using esters as modiers can still be pos-tulated. Although such studies of interfacial activity and kinetics of extractionin ester have not been described in the literature, one can predict that they donot adsorb at the hydrocarbon / water interfaces or that their surface activity islow. As a result, they cannot accumulate at the interface and retard extractionin this way.

A decrease in mass transfer in the presence of alcohols and alkylphe-nols is not a general phenomenon. When using dibutyldithiophosphoric acidfor the extraction of nickel from acidic sulfate solution, it is observed thatthe stripping is facilitated and accelerated in the presence of octanol. [87,88]

This could be due to a different chemistry in the Cu(II) and Ni(II) com-plexation. An exchange of water molecules in nickel(II) complexation isslower than it is with copper(II). The extraction is slow and kineticequations are usually different from than those derived for copper(II). [70,71]

Both the rate and the efciency of gallium(III) extraction from alkalinesolutions with 5-alkanoyl-8-hydroxyquinoline are increased when either2-hexanone as a modier or hexadecyltrimethylammonium chloride(CTAB) as a surfactant is added to the extraction system. [89] The reasonfor the positive modication observed in the presence of the modiercannot be predicted, but the effect of CTAB is probably connected to the


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formation of reverse micelles and microemulsions, as reported by Bauer [90]

and reviewed by Paatero.[91]

The effect cannot be attributed to the adsorp-tion of CTAB molecules at the hydrocarbon / water interface because, due toelectrostatic repulsion, a retardation of extraction would be observed. [92,93]

Hydroxyoximes modied with alkylphenol cannot be used for the extrac-tion of copper from ammoniacal solutions because they co-transfer too muchammonia. Nonylphenol has a deleterious effect on certain constructionmaterials such as natural rubber.

CONCLUSIONS

This physicochemical discussion of hydroxyoxime modication withvarious modiers containing oxygen atoms shows that this kind of modi-cation is unique to hydroxyoxime reagents, which form weak intramolecularand intermolecular hydrogen bonds. They can easily be destroyed, not only bymodier molecules (alcohols, alkylphenols, and esters), but also by solvatingdiluents and the water present in actual extraction systems. Alcohols arestronger modiers than alkylphenols. The drawback of these modiers istheir strong self-association, which is eliminated in esters and sterically hin-dered alcohols. The extraction properties of strong hydroxyoxime extractantscan be also modied by an addition of weak reagents (hydroxyoximes orb -diketones), which form co-associates with strong extractants. The use of sterically hindered esters seems promising due to two reasons: they inhibithydroxyoxime adsorption at the hydrocarbon / water interface with theoximino group penetrating the aqueous layers and show little to no adsorp-tion. Due to these phenomena, hydroxyoxime extractants are more stable,and a retardation of the extraction is not observed in the presence of hinderedesters. The chemistry of modication is not well explained because the workspublished in the past focused solely on the interactions of modiers withextractants, neglecting their interactions with metal complexes. In order toachieve considerable progress in understanding the modication phenom-enon, interactions between complexes and modiers must be studied.

ACKNOWLEDGMENTS

This work was supported by the V Comisio n Mixta Hispano-Polaca.Ministerio de Asuntos Exteriores. One author (J.S.) would like to thank DS032 / 044, Poznan, Poland for his support.


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Received January 7, 2004Accepted April 30, 2004


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