Review of Advanced Liquid−Liquid Extraction Systems for the

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REVIEWS Review of Advanced Liquid-Liquid Extraction Systems for the Separation of Metal Ions by a Combination of Conversion of the Metal Species with Chemical Reaction Syouhei Nishihama,† Takayuki Hirai,*,†,‡ and Isao Komasawa†,‡ Department of Chemical Science and Engineering, Graduate School of Engineering Science, and Research Center for Solar Energy Chemistry, Osaka University, Machikaneyama-cho 1-3, Toyonaka, Osaka 560-8531, Japan

Recent developments in an advanced liquid-liquid extraction system for the separation of metal ions by combining a modification or chemical reaction of the metal species in the aqueous and organic phases demonstrate considerable potential. Possible techniques for the chemical conversion of aqueous-phase metal species include redox reactions for the metal ions, masking effects through the addition of water-soluble complexing agents, and complexing reactions with salting-out agents. For the conversion of organic-phase metal species, a synergistic effects through the addition of additional extractants and redox reactions for the extracted species are also useful. The separation of metal ions is effectively improved by the introduction of such chemical reactions to the extraction system. 1. Introduction As a result of the recent high rate of progress in the high-technology industry, the demand for rare metals of high purity has become considerably greater. Among the available techniques for the separation of such rare metals on the industrial scale, liquid-liquid extraction is one of the most useful, and many different extraction systems have been investigated. The results of such investigations have been summarized in several reviews1-3 and also in several books.4,5 The liquid-liquid extraction technique, when employed for the separation of rare metals, is based on a difference in the ability to form a chemical complex between the extractant and the particular metal species, and thus, the extractability and separation ability obtained are highly dependent on the chemical structure of the extractant. Recent studies based on molecular modeling of the extracted species using molecular mechanics (MM) calculations have been carried out,6-9 and the knowledge obtained from these investigations is expected to assist in the development of new extractants having effective ligands for the separation of rare metals. In recent years, macrocyclic ligands having ion-size selectivity, such as crown ethers10 and calixarenes,11 have been studied as possible candidates for extractants. There are some metals, however, for which a separation by conventional extraction is difficult. In these cases, the conversion of both aqueous- and organic* Author to whom correspondence should be addressed. Tel.: +81-6-6850-6272. Fax: +81-6-6850-6273. E-mail: hirai@ cheng.es.osaka-u.ac.jp. † Graduate School of Engineering Science, Osaka University. ‡ Research Center for Solar Energy Chemistry, Osaka University.

phase species by functional chemical reaction during actual extraction has proved very effective in improving the separation obtained for such metal species. In the case of aqueous-phase species, (1) a redox reaction of the metal ion, (2) a complexing reaction with watersoluble complexing agents, and (3) a complexing reaction with salting-out agents are all possible feasible solutions. The redox reaction for metal ions has been widely applied in the separation of U/Pu in the Purex process, commonly employed in the nuclear industry.12 Such reactions are based on the principle that metal ions having different valences behave like different elements with respect to their extractabilities. In the case of complexing reactions for metal ions with a water-soluble complexing agent, there exist two possible states, equilibrium and nonequilibrium, for improving the separation. The separation abilities of the differing metal species are based on a difference in their abilities to form complexes with the agent in the equilibrium case and on a difference in the rates of the complexing reactions in the nonequilibrium situation. In the case of treatment in the organic phase, three differing reaction possibilities exist. These are (1) modification of the extractant or of the extracted species using diluents and modifiers, (2) a synergistic effect obtained by adding additional extractants, and (3) a redox reaction for the extracted species. A diluent is normally employed to decrease the viscosity of the extractant solution, to provide a suitable concentration for the extractant, to decrease the emulsion-forming tendencies of the extractant, and/or to improve the dispersion and coalescence properties of the solvent. A modifier can also be used to overcome the formation of a third phase, which is essentially a solubility problem of the diluent. Both the diluent and the modifier

10.1021/ie010022+ CCC: $20.00 © 2001 American Chemical Society Published on Web 06/09/2001

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affect the extractant and extracted species through molecular interactions, such that the extractability or extraction equilibrium formulation changes dramatically. Synergistic extraction systems have been investigated widely, especially in the nuclear field with studies involving D2EHPA [bis(2-ethylhexyl)phosphoric acid]/TBP (tri-n-butyl phosphate) and Htta (thenoyltrifluoroacetone)/TBP,5 and this work has been summarized in several reviews.13-15 In the present paper, the use of liquid-liquid extraction systems, combined with functional chemical reactions, is reviewed and discussed. 2. Effect of Chemical Reactions for Metal Ions in the Aqueous Phase 2.1. Redox Reaction of the Metal Ion. Several published reports concern differences in the extraction behaviors of metals with different valences, for example, Fe(II/III),16,17 Cu(I/II),18,19 Co(II/III),20 Eu(II/III),21 Ce(III/IV),22 Np(IV/VI),23 and Pu(III/IV/V).24 Selective extraction (stripping) can be carried out, in which the redox reaction progresses during the actual extraction (stripping) process. There are three possible ways for producing the redox reaction in an extraction system. These are (1) chemical reaction using a chemical reagent, (2) electrochemical reaction, and (3) photochemical reaction. In the case of chemical reaction, the reductive stripping of V(V) in a TOMAC (tri-n-octylmethylammonium chloride) system25 and the reductive extraction of Cu(II) in a tba+ (tetrabutylammonium ion) or TOPO (trin-octylphosphine oxide) system18,19 were investigated using ascorbic acid as the reductant. Gaunand et al. reported an oxidative extraction of Fe(III), using the in situ reduction of Cu(II) in the presence of O2 gas in the VA-10 (2-ethyl-2-methylheptanoic acid) system.26 In this system, the oxidation of Cu(I) with O2 occurs first, followed by the oxidation of Fe(II) using the resultant Cu(II), as shown by eqs 1 and 2.

4Cu(I) + 4H+ + O2 f 4Cu(II) + 2H2O

(1)

Fe(II) + Cu(II) T Fe(III) + Cu(I)

(2)

More powerful reducing reagents, such as reactive metals and metal amalgams, are needed for the reduction of rare earth metals. Preston et al. investigated the extractive separation of Eu from Gd following reduction using metallic reductants, in D4OPA [bis(4-octhyl)phosphoric acid] and VA-10 systems.27 The extractability of Eu(II) is much less than that of Gd(III). Thus, a separation of Eu/Gd from an aqueous solution containing equal amounts of the two metals (0.05 mol/L) was attained with a purity of Eu in the aqueous phase of 99.7% and a purity of Gd in the organic phase of 93.5%, using a single-stage extraction with VA-10. The extractive separation of Eu/Gd, following Eu reduction with metallic zinc fine powder and a low-melting-point liquid alloy of Ga-In-Zn, was also investigated in 15C5 (15-crown 5-ether), 18C6 (18-crown 6-ether), and DC18C6 (dicyclohexyl 18-crown 6-ether) systems.28 In the crown ether extractant system, Eu(II) was found to be more extractable than Gd(III), as a result of the agreement between the diameters of Eu(II) and the cavity of the crown ether, such that a selective extraction of Eu(II) with a separation factor of 98 was attained.

In the case of actinides, much attention has been paid to valence adjustment of the metals. Chitnis et al. investigated the oxidative extraction of Np in the TBP system and reported that the distribution ratio of Np increases with reaction time when VO2+ is used as the oxidizing agent.29 In an extraction system for radioactive wastes, however, salt-free reagents, such as HNO3 as the oxidant30 and H2O2 as the reductant,31 are preferred. Chemical reduction can also be effective in improving the extractability of some metals that are normally ineffective for extraction because of the inert nature of the original metal ion. Thus, Yamada et al. investigated the extraction of Cr(III), which is known to be ineffective for extraction in the Hacac (acetylacetone) system, using the chemical reduction of Cr(VI) with ascorbic acid.32 In this case, the extraction efficiency is much improved by the reductive extraction method, probably as a result of the extraction occurring before the reduced form of the Cr is hydrated. Redox reactions using chemical reagents, however, make extraction systems further contaminated, and thus, procedures other than redox reactions, such as electro- or photochemical reactions, are favored. Several reports concern extraction systems combined with in situ electrochemical redox reactions. These include systems for V(IV/V),33 Ce(III/IV),34,35 Eu(II/III),36 and Am(III/IV/VI).37,38 In most of these extraction systems, a two-compartment cell separated by a cation-exchange membrane was used as the laboratory reactor. Industrialscale electrochemical reactors have also been described.39,40 In the case of a photochemical redox reaction in the aqueous phase, a weak chemical redox reagent is sometimes needed. Ohki et al. investigated the photoreductive extraction of Cu(II) using a Xe lamp in a thioether extractant system and in the presence of TiO2 as a photosensitizer.41 Hirai et al. investigated the extraction and separation of Ce from a La/Ce/Nd mixture, following the aqueous-phase photochemical oxidation of Ce in the presence of KBrO3.42 In these systems, in the absence of the chemical reagent, the photochemical reaction barely progresses. Hirai et al. also investigated the separation of Eu from Sm/Gd by photoreductive stripping of the Eu in D2EHPA/xylene.43 The photochemical reduction of Eu in the organic phase does not progress in this case, as the diluent, xylene, shows a large absorption band in the UV region where the charge-transfer (CT) band of Eu appears. The use of a two-compartment cell was, therefore, required in order for the Eu to be photoreduced in the aqueous phase. A quantitative expression for the difference in the distribution ratio of the metal ions from the complex formation point of view has not yet been obtained. In recent years, however, a density functional study for Pu(II/IV/VI)/Htta was presented.44 Further investigations in this field might shed light on the difference in the extractabilities of such metal ions. 2.2. Masking Effect with Water-Soluble Complexing Agents. The most popular water-soluble complexing agents employed are amino poly acetates, such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA). There are several reports concerning this extraction system, especially for the rare earth metals, when combined with a masking reaction with amino poly acetates at the equilibrium state. In the case of acidic organophosphorus extractants, however, the combined effect is barely

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obtained with most complexing agents, because the orders of magnitude of both the stability constants for the agents and the extraction equilibrium constants increase with atomic number. Hirai et al. reported the separation of rare earth metals using TOMAN (tri-noctylmethylammonium nitrate) and TOMAN/LIX54 (Racetyl-m-dodecylacetophenone) in the presence of EDTA.45 In this case, the separation is improved by adding EDTA, as the extractability of the rare earth metals with TOMAN decreases with atomic number. The extraction behavior can be expressed using an extraction equilibrium formulation, determined on the assumptions that (1) all of the EDTA in the aqueous phase forms a 1:1 complex with the rare earth metals and (2) the complexed rare earth metal is inactive in the subsequent extraction. Nishihama et al. investigated the selective extraction of Y from a Ho/Y/Er ternary mixture using EHPNA in the presence of EDTA (H4L).46 In this system, the extraction behavior can be expressed by extraction equilibrium formulations, combined with complex formation between the metals and the dissociated ligand of EDTA (L4-). A separation process for the metals based on a mixer-settler cascade can be then designed using the extraction equilibrium formulations determined. Recently, crown ethers have been studied as the ionsize-selective masking reagents acting in the aqueous phase in the separation of alkaline earth and rare earth species.47,48 HPMBP (1-phenyl-3-methyl-4-benzoylpyrazol-5-one)/TOPO was used for the separation of alkaline earth metals and D2EHPA/TOPO for the separation of rare earth metals, in the presence of 18C6 or 15C5. A combined effect between the chelating extractants and the crown ethers can be obtained in the rare earth metal system, because the stability of the complex formed between the crown ethers and the metal ions decreases with increasing atomic number. Sasaki et al. synthesized sulfonated crown ethers, which are more hydrophilic than conventional crown ethers,49,50 as for conventional crown ethers such as 18C6, the distribution of the ether in the organic phase is not negligible. It is known that the rate of extraction is decreased by the addition of DTPA or EDTA to the extraction system.51 A separation process under a nonequilibrium state can therefore be constructed using this property. Minagawa et al. investigated a nonequilibrium separation process for Y from other rare earth metals using acidic organophosphorus compounds.52 Matsuyama et al. also investigated the effect of the addition of an organic acid, such as citric or lactic acid, on the nonequilibrium separation of Y/Er using Cyanex 272 [bis(2,4,4-trimethylpentyl)phosphinic acid] to improve the rate of extraction.53 Under these conditions, the rate of extraction is increased remarkably, although the selectivity (ratio between the extraction rates) is decreased from 5.8 to ca. 3.5. One of the problems encountered in an extraction system in the presence of a water-soluble complexing agent concerns the recovery of both the complexed metals and the agents from the raffinate aqueous solution. It is well-known that the metal-EDTA complex is dissociated in highly acidic solution, as EDTA exists in the H4L form in such a solution. The recovery of metals complexed with EDTA in the raffinate solution, following dissociation of the metal, can therefore be achieved using TBP (tri-n-butyl phosphate), which is active in such an acidic region.45 In addition, the

protonated EDTA (H4L) also tends to precipitate under such conditions, which thus enables recovery of the EDTA. Matsuyama et al. synthesized new hydrophilic chelating polymers functionalized with EDTA54 and applied them to the extractive separation of heavy rare earth metals using Cyanex 272.55 The chelating polymers synthesized are precipitated in the presence of rare earth metals, thus suggesting that the recovery of the agent from the extraction system is feasible. 2.3. Complexing Reaction with Salting-Out Agents. In extraction systems with amines and ammonium salts, salting-out agents are used for the metal ion to form anionic species in the aqueous phase. The extraction, therefore, does not progress if the anionic species are not formed. A typical extraction process consists of the separation of Co/Ni from chloride solution with tri-n-octylamine (TOA).56 In this system, the extraction of nickel with TOA does not progress, because nickel does not form an anionic chloro complex. The selective separation of Co/Ni, therefore, is achieved from an aqueous solution of high chloride concentration. 3. Effect of Chemical Reactions for Extracted Species in the Organic Phase 3.1. Effect of Diluents and Modifiers. The effects of diluents are categorized according to (1) interactions with the extractant, which affect the activity of the extractant and which bring about favorable changes in the extraction performance of the extractant, and (2) interactions with the extracted species, which change the compositions of the species via coordination and substitution of the diluent molecules, resulting in a change in the extraction equilibrium formulation. Komasawa et al. investigated the effect of the diluent on the extraction of Cu with LIX-65N (2-hydroxy-5-nonylbenzophenone oxime).57 The hydroxyoxime has an oxime-hydroxyl group that behaves as an electron acceptor (hydrogen donor). Studies showed that the copper distribution ratio was lower by 4-5 orders of magnitude for electron-donor diluents, such as alcohols and ketones, as compared to nonpolar hydrocarbons and that this decrease occurred without a change in the composition of the extracted species. Komasawa et al. also investigated the effect of xylene and 2-ethylhexyl alcohol diluents on the extraction of Co and Ni with acidic organophosphorus compounds, such as D2EHPA and EHPNA. In the case of Co, the extracted species forms a complex with tetrahedral symmetry with EHPNA in both diluents and with D2EHPA in xylene solution. The nature of the species occurring in the D2EHPA/2-ethylhexyl alcohol system is reported to be of octahedral symmetry, because of the interaction with the extracted species. In the case of Ni, an extracted species with octahedral symmetry forms for all systems, although the Ni/extractant ratio in the extracted species differs according to the diluent because the molecules of alcohol substitute the neutral extractant in the species. The effect of modifier on the extractants has also been investigated. Komasawa et al. investigated the effect of oxygen-containing diluents (modifiers), such as alcohols, ethers, and ketones, on the extraction of Cu with LIX65N/n-heptane and LIX-65N/xylene.57 In both cases, the distribution of Cu decreases with addition of the modifier, as a result of the formation of an intermolecular complex between extractant and modifier. Yoshizuka et al. investigated the formation of the intermolecular

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complex between LIX-860 (5-dodecylsalicylaldoxime)/1tridecanol analytically using IR and NMR spectroscopies and found that the complex forms via a hydrogen bond between the extractant and the alcohol.59 Bogacki investigated the effect of the formation constant for the intermolecular complex between the extractant and the modifier on the metal recovery for a theoretical extraction system with countercurrent and crosscurrent cascades.60 3.2. Synergistic Effect by Adding Extra Extractants. The composition of the extracted species is also changed by addition of an extra extractant, especially by the addition of a neutral extractant to a chelating extractant system. In such synergistic extraction systems, the solvated water molecules on the extracted species are substituted by the neutral extractant, making the species more hydrophobic and thus causing an increase in the distribution ratio. Kandil et al. investigated the effect of mutual water-diluent solubility on the synergistic extraction of Ce with Htta and TOPO.61 They found that the synergistic adduct formation constants, β1 and β2, shown in eqs 3 and 4 below, decrease with increasing mole fraction of water dissolved in the diluent. This is because a diluent that contains more water hinders the further release of water from the coordination sphere of the metal.

Ce(tta)3 + TOPO T Ce(tta)3(TOPO), Ce(tta)3 + 2TOPO T Ce(tta)3(TOPO)2,

β1

(3)

β2 (4)

Watarai et al. investigated the interfacial reaction in the synergistic extraction of Ni with HDz (dithizone) and phen (1,10-phenanthroline) or DPP (4,7-diphenyl-1,10phenanthroline) using a high-speed stirring method and reported an extraction mechanism in which the charged complex that forms at the interface is gradually converted to the extracted species.62,63 Crown ethers have been used as ion-size-selective synergists for the extraction system, and a relationship between the synergistic effect and size-fitting effect between the cavity radius of the crown ethers and the ionic radius of the metals has been reported.64-66 An adequate relationship between the complexation and extraction properties, especially with the stereoisomeric ligands, however, cannot be obtained using only the sizefitting effect. Recently, Dietz and co-workers reported a relationship between the synergistic adduct formation constant (Ks) and the ligand strain energy (∆Ureorg) and pointed out an inverse linear relationship between log Ks and ∆Ureorg.67,68 Other macrocyclic ligands, such as cryptand,69 aza crown ether,70 thia crown ether,71 thiaaza crown ether,72 and calixarene,73 have also been investigated as synergists. The effect of the formation of reverse micelles on the synergistic effect of metals has been investigated in the D2EHPA/DNNSA (dinonylnaphthalene sulfonic acid),74,75 D2EHPA/M2EHPA [mono(2-ethylhexyl)phosphoric acid],76 and HEHφP (2-ethylhexyl phenylphosphonic acid)/DNNSA77 systems. For the case of the D2EHPA/DNNSA system, when employed in the extraction of Al and Be, the distribution ratio is dependent on the relative magnitudes of the DNNSA concentration and the critical micelle concentration (CMC). When the DNNSA concentration is greater than the CMC, the distribution ratio increases with increasing DNNSA concentration, whereas the extraction mechanism when

the DNNSA concentration is less than the CMC is identical to that for the single D2EHPA system. In this system, a synergistic effect on the extraction rate is obtained, together with an increase in the distribution ratio.78 Oil-soluble complexing agents, such as EDTAN,N′-DOLA (ethylenediaminediacetic acid-N,N′-dioleylamide) and DTPA-N,N′-DOLA (diethylenetriaminetriacetic acid-N,N′-dioleylamide), which are polyaminocarboxylic acid alkyl derivatives and which have interfacial activities similar to those of surfactants, have been developed for rare earth separations.79,80 The extraction mechanism for rare earth metals with EHPNA in the presence of EDTA-N,N′-DOLA (H2L) has been investigated, and the metal extraction was found to progress via two routes, i.e., direct extraction with the extractant and extraction via an interaction with EDTA-N,N′-DOLA at the organic/aqueous interface. In the latter case, a Ln2+-HL complex forms at the interface, with which the extractant then reacts gradually. 3.3. Redox Reaction of the Extracted Species. A redox reaction for the extracted species in the organic phase is as effective as that for the metal ions in the aqueous phase. Galvanic stripping of Fe(III) from the D2EHPA system was investigated by Chang et al.81 There are several reports concerning the effect of the auto-oxidation of the extracted species on the distribution ratios for Co,82-84 Mg,85,86 and Ce87-89 with chelating extractants. Oxidation of Co(II) to Co(III) in the organic phase is especially important, as the oxidized Co(III) complex is inert for stripping by mineral acid solution, and so this process is known commonly as cobalt poisoning. Nishihama et al. investigated the oxidation of Co(II) and the stripping of Co(III) loaded on LIX-84I (2-hydroxy-5-nonylacetophenone oxime) and LIX-65N using photochemical reduction.90 Here, the extracted Co(II) complex adds to an O2 molecule and is oxidized to Co(III), thus forming a superoxo complex, as shown by eq 5.

Co(II) + O2 f Co(III)-O-O‚

(5)

This superoxo complex then seems to react further with a second Co(II) to form a µ-peroxo complex, as shown by eq 6.

Co(III)-O-O‚ + Co(II) f Co(III)-O-O-Co(III) (6) Photoreductive stripping of Co(III) can be carried out using visible light produced by a halogen lamp, and the successful stripping of Co, with a stripping efficiency of about 90%, can be obtained in both systems under conditions of 3 M HCl and photoirradiation for 3 h. There are no reports, however, of the oxidation reaction of the extracted species in the organic phase being applied to the separation process. The reduction reaction, especially photochemical reduction, is very effective for the separation of rare metals with high selectivities. Hirai et al. investigated the photoreductive stripping of Eu from a Sm/Eu/Gd mixture in the D2EHPA system.91 The photochemical reduction of Eu in the organic phase proceeds well when cyclohexane or hexane is used the diluent, but it does not proceed in the organic phase with xylene as the diluent.43 A highly selective separation with a Eu purity of about 99% can be achieved from a Sm/Eu/Gd aqueous mixture with equal metal concentrations by the extrac-

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tion-photoreductive stripping process. Hirai et al. also investigated the separation of V/Mo with photoreductive stripping of the V in a TOMAC system.92-94 The mechanism of such extraction or stripping systems, when combined with the photochemical reductions of Fe(III) and V(V), in acidic organophosphorus compounds has been investigated.95-97 The photochemical reduction of the extracted species proceeds after an initial induction period caused by the dissolved oxygen in the extraction system. The photochemical reduction of the extracted V(V) ions occurs both at the interface and in the bulk organic phase, whereas that of the extracted Fe(III) ions occurs mainly at the interface. The difference in capability for photochemical reduction is caused by the difference in the quantity of dissolved water in the organic phase. The photochemical reduction of the extracted species also progresses following photoabsorption at the absorption band of the extracted species. In the case of Fe(III), the extracted Fe(III) species is photoexcited in the organic phase and photoreduced to Fe(II) by electron donation from the water at the aqueous/organic interface, as shown by eqs 7-12.

(RH)2 T 2RH,

K1

(7)

FeR3(RH)3 T FeR3(RH)3*,

K2

(8)

FeR3(RH)3* T FeR3(RH)3*,

K3

(9)

FeR3(RH)3* + H+ T FeR2(RH)3+ + RH,

K4 (10)

FeR2(RH)3+ + H2O f FeR2(RH)3 + H+ + ‚OH,

FeR2(RH)3 f FeR2(RH)2 + RH

k1 (11) (12)

If the rate-determining step is the photochemical reduction of Fe(III) as shown by eq 11, the overall reaction rate equation can be expressed by eq 13, which is identical to the form found by experiment.

r))

d[Fe] ) k1[FeR2(RH)3+] dt

k1K2K3K4 [FeR3(RH)3][H+] K10.5

[(RH)2]0.5

[Fe][H+] )k (13) [(RH)2]0.5

Conclusion Recent developments in the advanced liquid-liquid extraction system for the separation of metal ions through combinations with chemical reactions in the aqueous and organic phases have been reviewed, with the following findings: (1) The metal ions in the aqueous phase can be modified or converted by chemical reactions that can lead to selective extraction or stripping and, thus, to high separation efficiencies. (2) Modification or conversion of the extracted species in the organic phase by an extra extractant (synergistic

effect) and by redox reaction is a very feasible technique for improving the extraction and separation of metals. Acknowledgment S.N. is grateful to the Research Fellowships of the Japan Society for Promotion of Science for Young Scientists and to the Morishita Zintan Scholarship Foundation. Literature Cited (1) Flett, D. S.; Spink, D. R. Solvent Extraction of Non-Ferrous Metals: A Review 1972-1974. Hydrometallurgy 1976, 1, 207. (2) Nakashio, F.; Inoue, K.; Kondo, K. Extraction of Metals by the Formation of Complexes. Kagaku Kogaku 1978, 42, 182 (in Japanese). (3) Gupta, B.; Singh, D.; Tandon, S. N. Extraction of Metals by Carboxylic AcidssA Review. J. Sci. Ind. Res. 1993, 52, 808. (4) Lo, T. C.; Baird, M. H. I.; Hanson, C. Handbook of Solvent Extraction; John Wiley & Sons: New York, 1983. (5) Ritcey, G. M.; Ashbrook, A. W. Solvent Extraction; Elsevier: Amsterdam, 1984; Part I. (6) Yoshizuka, K.; Kosaka, H.; Shinohara, T.; Ohto, K.; Inoue, K. Structural Effect of Phosphoric Esters Having Bulky Substituents on the Extraction of Rare Earth Elements. Bull. Chem. Soc. Jpn. 1996, 69, 589. (7) Comba, P.; Gloe, K.; Inoue, K.; Krueger, T.; Stephan, H.; Yoshizuka, K. Molecular Mechanics Calculations and the Metal Ion Selective Extraction of Lanthanoids. Inorg. Chem. 1998, 37, 3310. (8) Goto, M.; Matsumoto, S.; Uezu, K.; Nakashio, F.; Yoshizuka, K.; Inoue, K. Development and Computational Modeling of Novel Bifunctional Organophosphorus Extractants for Lanthanoid Separation. Sep. Sci. Technol. 1999, 34, 2125. (9) Yoshizuka, K.; Inoue, K.; Comba, P. Quantitative StructureProperty Relationship of Extraction Equilibria of Lanthanoid Series Using Molecular Mechanics Calculations. Kagaku Kogaku Ronbunshu 2000, 26, 517 (in Japanese). (10) Honjo, T. Solvent Extraction: Crown Ether. Bunseki 1997, 127 (in Japanese). (11) Alexandratos, S. D.; Natesan, S. Coordination Chemistry of Phosphorylated Calixarenes and Their Application to Separations Science. Ind. Eng. Chem. Res. 2000, 39, 3998. (12) Benedict, M.; Pigford, T.; Levi, H. Nuclear Chemical Engineering; McGraw-Hill Book Company: New York, 1981. (13) Hala, J. Some Aspects of Synergistic Extractions with Chelating Extractants. J. Radioanal. Chem. 1979, 51, 15. (14) Akaiwa, H.; Kawamoto, H. The Application of Synergistic Extraction to Analytical Chemistry. Rev. Anal. Chem. 1982, 6, 65. (15) Choppin, G. R. Studies of the Synergistic Effect. Sep. Sci. Technol. 1981, 16, 1113. (16) Mickler, W.; Reich, A.; Uhlemann, E. Extraction of Iron(II) and Iron(III) with 4-Acyl-5-pyrazolones in Comparison with Long-Chain 1-Phenyl-1,3-(cyclo)alkanediones. Sep. Sci. Technol. 1998, 33, 425. (17) Nasu, A.; Takagi, H.; Ohmiya, Y.; Sekine, T. Solvent Extraction of Iron(II) and Iron(III) as Anionic Thiocyanate Complexes with Tetrabutylammonium Ions into Chloroform. Anal. Sci. 1999, 15, 177. (18) Nasu, A.; Yamaguchi, S.; Sekine, T. Solvent Extraction of Copper(I) and (II) as Thiocyanate Complexes with Tetrabutylammonium Ions into Chloroform and with Trioctylphosphine Oxide into Hexane. Anal. Sci. 1997, 13, 903. (19) Nasu, A.; Kato, K.; Sekine, T. Solvent Ectraction of Copper(I) and Copper(II) from Aqueous Halide Solutions with Tetrabutylammonium Ions into Chloroform. Bull. Chem. Soc. Jpn. 1998, 71, 2141. (20) Przeszlakowski, S.; Wydra, H. Extraction of Nickel, Cobalt and Other Metals [Cu, Zn, Fe(III)] with a Commercial β-Diketone Extractant. Hydrometallurgy 1982, 8, 49. (21) Preston, J. S.; Preez, A. C. D. The Solvent Extraction of Europium(II) by Some Organophosphorus and Carboxylic Acids. Solvent Extr. Ion Exch. 1991, 9, 237. (22) Soldenhoff, K. H. Options for the Recovery of Cerium by Solvent Extraction. Proc. Intl. Solvent Extr. Conf. ‘96 1996, 469.

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Received for review January 4, 2001 Revised manuscript received April 24, 2001 Accepted May 3, 2001 IE010022+