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Solvent extraction as a well-established process in hydrometallurgy, waste treatment, and material preparation requires selective and effective comple...
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Preorganized Complexing Agents as a Tool for Selective Solvent Extraction Processes Christine Chartroux,† Kathrin Wichmann,† Gudrun Goretzki,† Torsten Rambusch,† Karsten Gloe,*,† Ute Mu 1 ller,‡ Walter Mu 1 ller,‡ and Fritz Vo1 gtle‡ Institute of Inorganic Chemistry, TU Dresden, D-01062 Dresden, Germany, and Kekule´ Institute of Organic Chemistry and Biochemistry, University of Bonn, D-53121 Bonn, Germany

Solvent extraction as a well-established process in hydrometallurgy, waste treatment, and material preparation requires selective and effective complexing agents with high lipophilicity. The extractant types used in industry reach from simple acids, ethers, and esters to more complex chelating agents. Nowadays the progress of supramolecular chemistry offers interesting possibilities to control the selectivity of metal extraction and to achieve a tailored ligand. In this paper, solvent extraction studies are presented for transition-metal ions with position isomers of alkylated 8-hydroxyquinolines from the chelating type as well as for amino and imino cage compounds and their acyclic counterparts from the supramolecular type. In all cases the extraction selectivity and efficiency strongly depend both on the extractant architecture and on the aqueous-phase composition. It is clearly shown that manipulation of the chemistry in the investigated systems leads to significant changes of the graduated extractabilities. Introduction Solvent extraction belongs to the more important processes in hydrometallurgy, waste treatment, and material preparation. Small differences in the chemical behavior of the species can be, namely, used to obtain exceptional separation and concentration effects. Generally, the complexing agents have a significant role in such processes. Depending on the special problem, the process is directed to a selective separation of one metal species or to an extraction of several metal ions together. Whereas the first case is represented mostly by metal winning processes, the second one is used especially for the purification of effluents or process solutions. An efficient phase transfer of a metal ion can be achieved by different binding modes, as is shown in Figure 1. Type 1 is related to simple electrostatic interactions and is used in extraction systems with acids, ketones, esters, or ethers, whereas type 2 as a chelating agent gives an additional stabilization based on the chelating effect. A lot of possibilities relating to these two general extractant types 1 and 2 are discussed in the literature1 and also used successfully in industrial plants.2 The search for novel extractants, which are more effective and selective than the conventional systems 1 and 2, results especially from the need to separate very low concentrations of toxic and/or valuable metals from large volumes of solution even if present in difficult matrixes. The use of supramolecular strategies, as shown with types 3 and 4, offers growing possibilities to solve such problems and to control the metal extraction selectivity.3-5 The goals are the mimicry of biochemical principles to tailor-make a ligand with specific * To whom correspondence should be addressed. E-mail: [email protected]. Tel: +49 351 463 4357. Fax: +49 351 463 7287. † TU Dresden. ‡ University of Bonn.

binding sites6 as well as the use of self-assembly of suitable building units.7 Besides the pure liquid-liquid extraction technique, also membrane, coalescence, and solid-phase methods are discussed in view of a technical application of supramolecular systems.8,9 These techniques allow, in part, a better control of extractant losses, which is important both from an environmental and from an economic point of view. The aim of supramolecular chemistry is to achieve a better conformity between ligand and metal ion properties. In that way, the selectivity of metal extraction can be controlled and ligands are tailored for the specific binding of a metal ion much better than before. The introduction of high lipophilic parts as well as of different binding modessweak and strongsallows a further modification in view of an effective extractant. The well-known examples for cyclic structures 3 are crown ethers, which have already been intensively investigated. In this case an extended metal ion-ligand complementarity is achieved in comparison to chelating agents 2 by the correspondence of size and donor sites of the host with the corresponding properties of the guest.10 In spherical structures 4, such as cryptands or cages, the architecturesshape, size, and coordination patternscan be more exactly adjusted for a certain guest.11 However, the inclusion process of any guest into a cage can be now and then kinetically hindered. As a consequence, the adjustment of the extraction equilibrium must be proven carefully. Nevertheless, existing differences in the extraction kinetics of various species can lead to novel separation possibilities. From left to right in Figure 1, the extractant structure becomes more complicated. In the same direction, the preorganization for a selected guest and the selectivity are growing. Furthermore, as a consequence of the increasing adaptation between host and guest, the solvation behavior in such complex systems can be strongly influenced, resulting in poorly solvated binding sites.12

10.1021/ie000308z CCC: $19.00 © 2000 American Chemical Society Published on Web 10/02/2000

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Figure 1. General binding modes for metal extractants.

Figure 2. Investigated extractant structures 5-10.

In this paper we will illustrate two selected topics: first, the tuning of 8-hydroxyquinolines to extract toxic heavy-metal ions together from a neutral chloride solution and, second, the possibilities of supramolecular nitrogen-containing cage compounds and their openchain counterparts to transfer selectively one transitionmetal ion from an aqueous phase into an organic phase. The investigated ligands are shown in Figure 2. The extraction behavior of these compounds will be discussed in detail depending on the experimental conditions and interpreted using molecular modeling calculations.

Results and Discussion 8-Hydroxyquinolines. A challenging separation problem today is the removal of toxic heavy metals from effluents in industry or environment. We have proven the possibility of separating the heavy-metal ions Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) from garbage dump percolating water. Under the commercially available chelating extractants, 7-alkyl-8-hydroxyquinoline (5), well-known as Kelex 100 (Witco) or LIX 26 (Henkel), is an interesting type, which is useful for the winning of copper1 and gallium.13 Also, its applicability for the separation of toxic heavy-metal ions from dilute solu-

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Figure 3. Extractability of heavy-metal ions with 8-hydroxyquinolines 5 (Kelex 100) and 6. [MCl2] ) 1 × 10-4 M; [NaCl] ) 1 × 10-1 M; pH ) 7.6 (Tris/HCl buffer); [ligand] ) 1 × 10-2 M in n-decane (5 and 6) or in n-decane/decanol-1 (v:v ) 9:1) (5* and 6*).

tions has been discussed.14,15 We have already shown that the simultaneous separation of the heavy-metal ions Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) with Kelex 100 from percolating water model solutions is still possible, but the extraction efficiency for Cd(II) and Hg(II) is only low.16 An improvement of the extraction power of extractant 5 (Kelex 100), especially toward Hg(II) and Cd(II), should be possible by two different ways: on the one hand, by use of synergistic effects with additives and, on the other hand, by structure modification via electronic and steric effects in the molecule. As is shown in Figure 3, in fact, a significantly higher extractability of Cd(II) and Hg(II) is observed if de-

canol-1 is added as a modifier to the system with 5.16 Above all, this result is due to a direct interaction of the alcohol with the extracted metal chelate, forming a mixed complex.17 The so-called assembly effect, which allows a preorganization of the potential ligand molecules decanol-1 and 8-hydroxyquinoline, could also play a role because it has already been described for comparable systems,7 because the interactions between this two components via hydrogen bonding has been proven. The second way to improve the extraction capability is to change the alkyl substituent from the 7 position to the 2 position of the quinoline molecule, as has been done in compound 6 with a nonyl substituent.18 Figure 3 gives the results of comperative solvent extraction experiments for 8-hydroxyquinolines 5 and 6. It is clearly shown that especially in the presence of decanol-1 the extraction of Cd(II) and Hg(II) with 6 is significantly higher, whereas the extractabilities of Cu(II) und Zn(II) stay as they are. At the same time, only the Ni(II) extraction decreases but lies in an order of magnitude comparable to that for Hg(II). The reason for the observed behavior is obviously related to both a change of the pyridine nitrogen basicity and structural features caused by the alkyl substituent in the 2-position. The first factor is illustrated in Figure 4. Here the electrostatic potentials mapped to the electron density for 5 and 6 are shown, which were obtained by density functional theory (DFT) molecular modeling calculations. The partial negative charge at the pyridine nitrogen of 6 is twice as high as that in the case of 5. According to this and by analogy with the 2-methyl-substituted hydroxyquinoline compound,19,20 6 should have a higher protonation constant and more weaker complex stabilities than 5. However, the extractabilities with 6 are

Figure 4. Electrostatic potential mapped to the electron density for the 8-hydroxyquinolines 5 (top) and 6 (bottom) calculated by DFT (the mapped electrostatic potential of the molecule is characterized by the color change related to the range on the left).

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3619 Table 2. Characteristic Mercury Concentrations of All Steps of the Continuous Hg(II) Extraction with the 8-Hydroxyquinolines 5 (Kelex 100) and 6

Figure 5. Competitive extraction of Na(I), Ca(II), Hg(II), and Zn(II) with 5 and 6. [HgCl2] ) 10 mg/L; [ZnCl2] ) 20 mg/L; [CaCl2] ) 1 g/L; [NaCl] ) 10 g/L; pH ) 7.6 (Tris/HCl buffer); [ligand] ) 0.1 M in n-decane/decanol-1 (v:v ) 9:1). Table 1. Parameters of the Continuous Extraction Experiments in Mixer-Settler Equipment feed solution organic phase strip solution

[HgCl2], [CuCl2] ) 10 mg/L; [NaCl] ) 1 × 10-1 M; pH ) 8.0 (Tris/HCl buffer); 1.5 L/h [5] and [6] ) 1 × 10-1 M in decane/decanol-1 (9:1); 0.5 L/h [H2SO4] ) 5 × 10-1 M; 0.25 L/h

lower only for Ni(II) but are significantly higher for Cd(II) and Hg(II). So, in case of Ni(II) as well as especially for Cd(II) and Hg(II), different steric effects of the 2-alkyl group in the quinoline molecule 6 should influence the coordination pattern and therefore the extraction behavior. Ni(II) prefers an octahedral coordination with the unsubstituted hydroxyquinoline,21 which could be particularly disformed by 6, whereas the larger Hg(II) and Cd(II) can accept, in the case of 6, the favored tetrahedral geometry. A further important point of the studies was to investigate a possible transfer of the main ions, sodium and calcium, of such percolating water into the organic phase. In Figure 5 the results of competitive extraction studies with 5 and 6 are summarized. The metal concentrations differ from each other similarly to real percolating water. In both cases, no phase transfer for Na(I) and Ca(II) could be detected. Summing up, 2-nonyl-8-hydroxyquinoline (6) gives a good basis for the simultaneous separation of the heavy-metal ions Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) from percolating water. To verify these results, tests for Cu(II) and Hg(II) were performed in a continuous mixer-settler laboratory installation with four extraction and two backextraction stages over a 48 h period. In both cases comparable parameters have been used as is shown in Table 1. Under these conditions, Cu(II) and Hg(II) have been quantitatively extracted, as was expected from the single-batch experiments. Some difficulties were observed in the case of the Cu(II) backextraction using sulfuric acid, because Cu(II) could not be removed quantitatively from the organic phase. The known high stability of the copper chelate requires obviously a higher acid concentration. For Hg(II), however, the mixer-settler runs gave a high efficiency, especially with 6. The results are summarized in Table 2. For the mixer-settler experiments in the laboratory equipment, the extractant losses of 5 and 6 have been

feed (mg/L)

extractant

raffinate (mg/L)

strip solution (mg/L)

organic solution (mg/L)

10 10

6 5

0.2-1.1 3.5-4.5

45-53 22-26

1-2 approximately 4

measured over the 48 h run. For this determination, aliquots of the raffinate and strip solutions were investigated. Whereas the results for the extraction part were satisfied, the organic content in the raffinate was always in a low ppm range, the losses during the backextraction with sulfuric acid were significantly higher. The organic concentrations in the strip solution increase by up to 35 ppm and some times even higher. First experiments with ethylenediamine as the stripping agent give some evidence for a significant lowering of these losses. So, also from this point of view, the modified quinoline extractant 6 shows a remarkable behavior, even if the losses under industrial conditions will be higher. Azacages. Since the first synthesis of cryptands by Lehn and his group,22,23 a lot of different cage compounds have been synthesized and investigated with regard to their complex formation properties.24-26 In recent years the attention was focused on the improvement of metal ion selectivity by changing the structure in different ways.27-29 In this connection, cages containing tris(2-aminoethyl)amine (tren) as the bridgehead unit in the molecule have been synthesized and characterized.11,30-32 This architecture has a great potential to provide a closely defined coordination cavity for both metal ions and anions. Our research interest was first directed to cages with aromatic subunits, giving a preference for weak metal-π interactions.33,34 Solvent extraction studies showed that the phase-transfer efficiency is relatively low despite a high selectivity toward Ag(I). A significant improvement of the silver extractability with cage compounds could be achieved by introducing pyridine subunits.35,36 In this case, the interaction of Ag(I) with pyridine nitrogen is dominating. The same behavior could be observed with homocalixpyridines.37,38 In comparison to these studies, we tested azacryptands, which are being intensively investigated by Nelson’s group in Belfast.11 Preliminary experiments gave some evidence for interesting results in view of solvent extraction of transition-metal ions.39 In this paper we will now discuss the solvent extraction results with azacage 7, the structure related Schiff base 8, and its open-chain counterparts 9 and 10. A general impression of the extraction properties of these compounds for the picrate system is given in Figures 6 and 7. It is clearly shown that there is no significant selectivity of the amino cage 7 and the imino cage 8 toward any transition-metal ion. In both cases Ag(I) is extracted with the highest extractability, but the other investigated ions can be also extracted. A noticeable result is the lower extractability for Ni(II) and Cu(I) with 8 in comparison to 7. Amazingly, the open-chain imino compound 9 shows a high selectivity toward Ag(I). The other transition-metal ions are only slightly extracted. In the case of the hydroxy-substituted openchain analogue 10, the extraction graduation is once more changed. Cu(I), Cu(II), and Zn(II) show high extractabilities. Hg(II) and Co(II) give lower values, whereas Ag(I) and Ni(II) have, surprisingly, the lowest of all. Furthermore, it should be noticed that alkali and

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Figure 6. Extractability of metal ions from the picrate system with compounds 7 and 8. [MCln] and [M(NO3)n] ) 1 × 10-4 M; [picric acid] ) 5 × 10-3 M; pH ) 6.2 (MES/NaOH buffer); [ligand] ) 1 × 10-3 M in CH2Cl2/decanol-1 (v:v ) 8:2); shaking time ) 30 min.

Figure 8. Extractability of metal ions from nitrate systems with compounds 7-10. [MCln] and [M(NO3)n] ) 1 × 10-4 M; [NaNO3] ) 1 × 10-1 M; pH ) 6.2 (MES/NaOH buffer); [ligand] ) 1 × 10-3 M in CH2Cl2/decanol-1 (v:v ) 8:2); shaking time ) 30 min.

Figure 7. Extractability of metal ions from the picrate system with compounds 9 and 10. [MCln] and [M(NO3)n] ) 1 × 10-4 M; [picric acid] ) 5 × 10-3 M; pH ) 6.2 (MES/NaOH buffer); [ligand] ) 1 × 10-3 M in CH2Cl2/decanol-1 (v:v ) 8:2); shaking time ) 30 min.

Figure 9. Extractability of Co(II) and Ni(II) with compound 7 depending on the extraction time. [MCln] and [M(NO3)n] ) 1 × 10-4 M; [picric acid] ) 5 × 10-3 M; pH ) 6.2 (MES/NaOH buffer); [7] ) 1 × 10-3 M in CH2Cl2/decanol (v:v ) 8:2).

alkaline earth metal ions as well as Eu(III) are not extracted under the selected conditions. Compared with this, an interesting change of the extraction properties of compounds 7-10 could be observed when going from the picrate to the nitrate aqueous system, as is shown in Figure 8. In this case the extractability decreases dramatically for the majority of metal ions. Only Ag(I) can be extracted by 7-9 with a marked selectivity in comparison to the other metal ions. A high selectivity for Cu(II) is observed with 10. Up to now our knowledge on the exact reasons for the presented extraction behavior of compounds 7-10 is limited. The performed investigations show that the systems are very sensitive to changes in experimental conditions, such as pH and the compositions of the aqueous and organic phases. One reason in this respect should be the complex protonation behavior of the amine nitrogens especially in compound 7 resulting in a varying distribution of the extractants between the two phases and in a complexation ability only at pH values higher than 5. In some cases the extractability increases if decanol-1 is added to the organic phase. As expected, in analogy to simple amine extraction systems, the alcohol im-

proves, furthermore, the solubility of the ligands and their complexes in the organic phase. Another important point represents the time needed to adjust the extraction equilibrium. Detailed studies lead to the conclusion that the equilibrium is reached after 30 min only for copper, zinc, silver, and mercurys this is the shaking time for the experiments of Figures 6-8. The time for the extraction equilibrium adjustment in the picrate system for Co(II) and Ni(II) lies between 3 and 6 h, as is shown in Figure 9. Amazingly, no shaking time influence has been observed in the nitrate system for the ligands 7-10 up to 3 h. This fact, together with the observation that a rising nitrate concentration leads to a lower extractability, is an indication of a strong anion influence on the extraction in the nitrate system. This result is also in agreement with the anion influence on the protonation and stability constants of amino cages.40 Both the formation of cascade complexes with two metal ions and one anion in the cavity11 and the direct interaction of the nitrate ion with the protonated cage compound41 could be possible. The pH influence on the extraction of Co(II), Ni(II), Ag(I), and Hg(II) with 7 in the picrate system is summarized in Figure 10. According to these results,

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Figure 10. Extractability of metal ions with 7 depending on pH. [MCln] and [M(NO3)n] ) 1 × 10-4 M; [picric acid] ) 5 × 10-3 M; MES/NaOH buffer; [7] ) 1 × 10-3 M in CH2Cl2 for Ag(I) and 5 × 10-4 M in CH2Cl2/decanol (v:v ) 9:1) for Ni(II), Co(II), and Hg(II); shaking time ) 30 min for Ag(I) and Hg(II), 3 h for Co(II), and 24 h for Ni(II).

Figure 11. Extraction of silver(I) and cobalt(II) depending on the metal concentration with 7 and 8 [metal salt]/[L] ) 1, 2, 3, 4, and 6; [picric acid] ) 5 × 10-3 M; pH ) 6.4 (MES/NaOH buffer); [ligand] ) 5 × 10-4 M in CH2Cl2.

there is a strong influence at a pH lower than 6 in the case of Ni(II) and Co(II), whereas the variation for Hg(II) is smaller. Ag(I) is extracted from a pH of 5 with nearly constant distribution ratios. Generally, at pH values lower than 4, the extractabilities of all metal ions decrease significantly because in this range the protonation of the azacage dominates.11,40 Backextraction experiments are in agreement with this finding. The ions can be easily stripped from the organic phase by dilute acid. Loading experiments should give some information about the maximum possible concentration of the metal ions in the organic phase and consequently about the composition of the extracted complexes. The results of such studies with 7 and 8 for Co(II) and Ag(I) in the picrate system are presented in Figure 11. From these data it follows that Co(II) forms only 1:1 species with 7 also at different metal concentrations. In contrast to this, Ag(I) gives with 7 and 8 mono- and dinuclear complexes and with 7, furthermore, trinuclear complexes in the organic phase. A trinuclear Ag(I) species with inclusion of the silver ions inside the cavity of a structure-related imino cage was already described.42 In the case of silver, the formation of mixtures of 1:1, 2:1, and 3:1 complexes (Ag/ligand) with 7 has been proven by electrospray ionization mass spectrometric

measurements both in solid state and in solution.43 Cyclovoltammetric measurements point to a high stability of the formed species and give a high probability for the arrangement of the metal ions inside the cage.44 In the same way, a 1:1 complex of Co(II) with 7 in solid state and in solution could be characterized.43 The evidence of Co(II) in this compound and during solvent extraction has been proven by electron paramagnetic resonance spectroscopy.44 The reason for the observed differences of the formed Ag(I) and Co(II) complexes with 7 should be related to structure differences. To understand these findings, molecular modeling studies on the basis of DFT calculations of the possible complex structures were performed. The calculated ligand structure of 7 is in good agreement with the crystal structure of corresponding amino cages.40 The calculated structures of the 1:1 Co(II) complexes and the 3:1 Ag(I) complexes with 7 are shown in Figure 12. For silver a stepwise inclusion from one to three ions is calculated, resulting in a very symmetric arrangement of the three ions in the molecule with two different coordination patterns: once trigonal planar with the pyridine nitrogen atoms in the center of the cage and twice tetragonal with the four amine nitrogen atoms of the tren unit on the top and at the bottom of the molecule. In contrast to this, in the case of Co(II), a

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Figure 12. DFT-based calculated structures of the 3:1 Ag(I) and 1:1 Co(II) complexes with compound 7.

strong distorted octahedral coordination geometry is favored by the calculation. The inclusion of further metal ions should not be possible in this case. Based on these findings, the extraction results and the different compositions and reactivities of the extracted species can be better understood. In contrast to Ag(I), the octahedral geometry of Co(II) needs a strong rearrangement of the ligand structure during complex formation, which hinders the inclusion of further metal ions and leads to a slow process. Conclusions Selective metal ion recognition and effective phase transfer of metal complexes require the fulfillment of a lot of different and partly opposite demands, which are controlled by chemistry and process parameters. The discussed studies on 8-hydroxyquinolines, amino and imino cage compounds, and their open-chain counterparts give some information about the extraction possibilities and the how the systems can be modified. It is clearly shown that not only the extractant architecture but also the composition of the two phases are very important in order to find optimum conditions for selective binding and transport of a group of ions or of a single ion. It is interesting to observe that the chosen extraction conditions have a great influence on the selectivity and efficiency not only for chelating structures but also for spherical supramolecular compounds, although the preorganization of cage compounds for the metal ion binding is much better. In contrast to cage compounds, their open-chain derivatives show a surprisingly higher selectivity, in the presence of picrate. This result should remind us that the potential for structurally simpler compounds should not be neglected. To understand all of the phenomena discussed above, a lot remains to be done. At present our attention is focused on the further clarification of the observed extraction behavior with the cages using detailed mass

spectroscopic and cyclovoltammetric measurements and molecular modeling calculations. The next steps will be the modification of the basic cage and open-chain structures by changing the building blocks and introducing hydrophobic and ligating substituents and the characterization of the complexation behavior of these new compounds. Finally the extraction strength of the amino compounds toward anions in the presence and absence of metal ions should be investigated. It can be expected that cage compounds and related open-chain derivatives will open new interesting possibilities in the field of separation science of both cationic and anionic species. Experimental Section Solvent Extraction. The liquid-liquid extraction experiments were performed at 25 ( 1 °C in microcentrifuge tubes (2 cm3) by means of mechanical shaking. The phase ratio Vorg:Vw was 1:1 (0.5 cm3 each). The shaking time was chosen between 30 min and 24 h. After extraction, all samples were centrifuged and the phases separated. The determination of the metal concentration in both phases was carried out radiometrically by γ-radiation measurement of the isotopes 110mAg, 64Cu, 203Hg, 22Na, 60Co, and 65Zn using a NaI(Tl) scintillation counter (Cobra II; Canberra Packard) and β-radiation measurements of 45Ca, 115mCd, and 63Ni in a liquid scintillation counter (Tricarb 2500; Canberra Packard). For Cu(I), hydroxylamine sulfate (0.05 M) was added to the aqueous phase. Countercurrent Extraction. They were done using laboratory mixer-settler equipment MSU-0.5 (MEAB Va¨stra Fro¨lunda/Sweden). The mercury content in the aqueous phase was determined by atomic absorption measurements, and the copper concentrations and the losses of 8-hydroxyquinolines were determined by UVVIS spectrometry.45

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Computational Details. The starting geometry for a comprehensive geometry optimization of investigated compounds was obtained using the semiempirical PM3 method implemented in the HyperChem program.46 All calculations were carried out using the DFT with the gradient-corrected Becke-Perdew functional BP86 in the ADF 2.347 and ADF 1999 program package.48 The standard basis sets IV(TZ+P) were used in all calculations. Acknowledgment Thanks are due to the Deutsche Forschungsgemeinschaft (Bonn, Germany), the Saxon Ministry of Science and Arts (Dresden, Germany), and the Fonds der Chemischen Industrie (Frankfurt, Germany). The authors also thank M. J. Schwuger and G. Subklew (Ju¨lich, Germany) as well as R. Neumann and E. Weber (Freiberg, Germany) for cooperation during the research program on heavy-metal extraction with 8-hydroxyquinolines. Literature Cited (1) Rydberg, J., Musikas, C., Choppin, G. R., Eds. Principles and Practices of Solvent Extraction; Marcel Dekker, Inc.: New York, 1992. (2) Lo, T. C.; Baird, M. H. I.; Hanson, C. Handbook of Solvent Extraction; Krieger Publishing Co.: Malabar, FL, 1991. (3) Moyer, B. A. Basic Principles of Complexation and Transport. In Comprehensive Supramolecular Chemistry; Lehn, J.-M., Atwood, J. L., Davies, J. E. O., McNicol, D. D., Vo¨gtle, F., Eds.; Pergamon Press: Oxford, U.K., 1996; Vol. 1, p 377. (4) Yordanov, A. T.; Roundhill, D. M. Solution Extraction of Transition and Post-transition Heavy and Precious Metals by Chelate and Macrocyclic Ligands. Coord. Chem. Rev. 1998, 170, 93. (5) Bradshaw, J. S.; Izatt, R. M. Crown Ethers: The Search for Selective Ion Ligating Agents. Acc. Chem. Res. 1997, 30, 338. (6) Stephan, H.; Gloe, K.; Schiessl, P.; Schmidtchen, F. P. Lipophilic Ditopic Guanidinium Receptors: Selective Extractants for Tetrahedral Oxoanions. Supramol. Chem. 1995, 5, 273. (7) Adam, K. R.; Atkinson, I. M.; Farquhar, S.; Leong, A. J.; Lindoy, L. F.; Mahinay, M. S.; Tasker, P. A.; Thorp, D. Tailoring Molecular Assemblies for Metal Ion Binding. Pure Appl. Chem. 1998, 70, 2345. (8) Izatt, R. M.; Bradshaw, J. S.; Bruening, R. L. Accomplishment of Difficult Chemical Separations Using Solid Phase Extraction. Pure Appl. Chem. 1996, 68, 1237. (9) Lamb, J. D.; Christenson, M. D. Macrocyclic Ligands in Separations. J. Inclusion Phenom. Mol. Recognit. Chem. 1998, 32, 107. (10) Gloe, K.; Mu¨hl, P.; Beger, J. Lipophilic Crown Compounds: Selective Extraction of Metal Ions. Z. Chem. 1988, 28, 1. (11) Nelson, J.; McKee, V.; Morgan G. Coordination Chemistry of Azacryptands. In Progress in Inorganic Chemistry; Karlin, K. D., Ed.; Wiley: New York, 1998; Vol. 47, p 167. (12) Berger, M.; Schmidtchen, F. P. The Binding of Sulfate Anions by Guanidinium Receptors is Entropy-Driven. Angew. Chem., Int. Ed. Engl. 1998, 37, 2694. (13) Puvvada, G. V. K. Liquid-liquid Extraction of Gallium from Bayer Process Liquor using Kelex 100 in the Presence of Surfactants. Hydrometallurgy 1999, 52, 9. (14) Brooks, C. S. Applications of Solvent Extraction in Treatment of Metal Finishing Wastes. Met. Finish. 1987, 85, 55. (15) Sze, Y. K. P.; Lam, J. K. L. A Study of a Solvent Extraction Method for the Treatment of Spent Electrolyte Solutions Generated in Nickel-Cadmium Battery Manufacturing. Environ. Technol. 1999, 20, 943. (16) Gloe, K.; Stephan, H.; Kru¨ger, T.; Mo¨ckel, A.; Woller, N.; Subklew, G.; Schwuger, M. J.; Neumann, R.; Weber, E. Solvent Extraction of Toxic Heavy Metal Ions with 8-Hydroxyquinoline Extractants from Effluents. Prog. Colloid Polym. Sci. 1996, 101, 145.

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Received for review March 1, 2000 Revised manuscript received August 7, 2000 Accepted August 8, 2000 IE000308Z