Anal. Chem. 1003, 65, 3380-3395
Extraction of Cu(11) from Sulfuric Acid by Macrocycle-Synergized Cation Exchange: Comparing a Novel Impregnated Resin with Its Solvent-Extraction Analog Bruce A. Moyer*and G. N. Case Chemistry Division, Oak Ridge National Laboratory, P.O.Box 2008 Oak Ridge, Tennessee 37831-6119
Spiro D. Alexandratos' and A. Amanda Kriger Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600
A novel extractive resin employing the tetradentate macrocyclic ionophore tetrathia-14-crown-4 (TT14C4)has been demonstratedvia impregnation of strong-acid poly(styrene4ivinylbenzene)cation-exchangebeads. Following uptake of several weight percent TT14C4, the sulfonic acid resin exhibits a l(k100-fold enhancement in the cationexchange extraction of Cu(I1) as expressed by the ) . exCu(I1) distribution coefficient ( 0 ~ "Since traction of Cu(1I) by unfunctionalized poly(styrene-divinylbenzene) impregnated with TT14C4 was too low to be detectable, the enhancement in cation exchange may be attributedto a synergistic effect involving coordination of the Cu(I1) by the mobile macrocycle and cation exchange by the immobile sulfonic acid groups. This is the first demonstration of synergism in a solvent-impregnated resin involving a functionalized support. INTRODUCTION Macrocyclic ionophores have attracted considerable attention as a tool for achieving high selectivity in metal ion separations.'" To exploit the favorable binding properties of these ligands for selective cation separations in phasetransfer methods such as solvent extraction (SX)and ion exchange (M)the principle of charge neutrality inescapably introduces the question of supplying counteranions. In practice, one may either effect the equivalent coextractionof counteranions (ion-pair extraction) or effect the equivalent back-transfer of hydrogen ions (or other cations) from the receiving phase to the source phase (cation exchange). In the former, dehydration of the transferring anion opposes the overall extraction, consequentlyforcing either the use of high ionic strengths in the case of mineral-acid anions or the use of added hydrophobic anions such as picrate. To obviate the dependenceof the driving force on the type and concentration of aqueous anions, the cation-exchange principle may be employed, allowing control of the extractive free energy via pH swing. Although ionizable macrocycles afford an elegant means of solving this problem,' a simpler approach offering ,5p6
(1) Takeda, Y. Top. Curr. Chem. 1984,121, 1-38. (2) Takagi, M.; Ueno, K. Top. Curr. Chem. 1984,121,39-66. (3) Smid, J.; Sinta, R. Top. Curr. Chem. 1984, 121, 105-156. (4) McDowell, W. J. Sep. Sci. Technol. 1988,23, 1251-1268. (5) Marcus, Y.; Kertes, A. S. Zon Exchange and Solvent Extraction of Metal Complexes; Wiley Interscience: New York, 1969. (6) Sekine, T. Solvent Extraction chemistry: Fundamentals and Applications; Marcel Dekker: New York, 1977. (7) Bartach, R.A. Solvent Extr. Zon Exch. 1989,7, 829-854. 0003-2700/93/0365-3389$04.00/0
greater economy and flexibility is to employ combinationsof neutral macrocycles and independent cation exchangers.' In this approach, one strives thereby to extract a target metal ion with greater efficiency than could be achieved if both reagents behaved independently. Under the general appellation "synergism" attributed to Coleman? this effect has been extensively studied and applied in a great variety of SX systems. The most successful of these employ a neutral coordinating extractant (not necessarily macrocyclic) and lipophilic acids such as carboxylic, phosphoric, or sulfonic acids.1s4-6ps16 In such a manner, the synergistic approach represents an effective tool for the exploitation of the high specificity of neutral macrocyclic i o n ~ p h o r e s . ~ J "Con~~ versely, macrocycles employed as synergists represent a tool for dramatically modifying the properties of the cation exchangers, leading to selectively enhanced cation exchange. Chemically analogous to certain synergistic SX systems, recently synthesized bifunctional poly(styrendiviny1benzene) resins have been demonstrated for the first time to exhibit synergistically enhanced extraction of metal cations.16J7 These systems combine cation-exchange functionalities with neutral coordination functionalities on the same polymer backbone and yield favorable capacities, selectivities, and kinetics. Bifunctional resins thus have the potential to borrow some of the desirable properties of synergistic SX systems while gaining the long life of polymer-immobilized reagents. In beginning the work that will be described in this paper, it was our aim to demonstrate for the first time synergism in a monofunctionalized resin impregnated with an extractant. This novel type of solvent-impregnatedresin (SIR)represents a hybrid of SX and resin-based systems that may (a) serve as a convenient vehicle to examine potential synergistic combinations of extractive functionalities, (b) permit the modification of the great variety of existing extractive resins, and (c) generally offer new possibilities for developing improved separation systems. The concept of the SIR is now (8) Baes, C. F., Jr. Nucl. Sci. Eng. 1963, 16, 405-412. (9) Irving, H.M.N. H. In Solvent Extraction Chemistry;Dyrseen, D., Liljenzin, J.-O., Rydberg, J., Eds.; North-Holland Publishing Co.: Amsterdam, 1967; pp 91-110. (10) Hala, J. J. Radiochim. Acta 1979,51, 15-25. (11) Akaiwa, H.; Kawamoto, H. Rev. Anal. Chem. 1982,6,66-86. (12) McDowell, W. J.; Moyer, B. A.; Case, G. N.; Case, F. I. Solvent Extr. Zon. Exch. 1986,4, 217-236. (13) Moyer, B. A.; Westarfield, C. L.; McDowell, W. J.; Case, G.N. Sep. Sci. Technol. 1988,23, 1325-1344. (14) McDowell, W. J.; Case, G. N.; McDonough, J. A,; Bartach, R. A. Anal. Chem. 1992, 64, 3013-3017. (15) Moyer,B. A.;Delmau,L.H.;Lumetta, G. J.;Baes,C. F.,Jr. Solvent Extr. Zon. Exch., in press. (16) Alexandratos, S.D. Sep. Pllrif. Methods 1988, 17, 67-102. (17) Alexandratos, S.D.;Crick, D. W.; Quillen, D. R. Ind. Eng. Chem. Res. 1991,30,772-778. 0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
well developed and has a strong place in extraction chromatography.laZ6 Normally, nonfunctionalized polymer beads are impregnated with solvent-extraction reagents by one of several means, and the resulting SIRS then function essentially as solid-supported solvent-extraction systems. From the available literature, it may be concluded that essentially any SX reagent, including crown ether^,^^^^^ may be employed in this manner. Moreover, this generality extends to SIRs containing a combination of two SX reagents that behave synergistically." The extraction reagents in SIRS are freely mobile within the bead, and thus, the extraction behavior of SIRS more closely resemblesSX behavior than it does resin ion exchange. It was thus logical to ask whether a new class of synergistic SIR could be introduced in which one of the extractive functionalities was fixed to the polymer backbone as in traditional resins and in which the other extractive functionality was present in the form of a freely mobile extractant as in normal SIRs. Thus, one would possibly gain some of the advantages of resin extraction but retain some advantages of SX, including selectivity and flexibility in choosing among a great variety of available reagents. Some examples of SIRS having functionalized polymers have in fact been reported, but the presence or absence of a synergistic effect was not c o n ~ i d e r e d . ~ ~ , ~ ~ To test the notion of selective synergism in an SIR containing a functionalized resin support, we chose to sorb the thia macrocycle tetrathia-14-crown-4 (TT14C4)onto poly(styrene-divinylbenzene) (PS-DVB) strong-acid resin beads to obtain an SIR for extraction of Cu(I1) from sulfuric acid. Thia macrocycles in general appear to be good candidates for this type of test because of their promising extraction properties for soft metals (e.g., Cu(I), Ag(I), and Hg(I1)) in SX ~ y s t e m s ~and ~ 9 in ~ ~polymer-immobilized -~~ system^.^^*^^ In the present case, the corresponding SX system involving TT14C4 combined with didodecylnaphthalenesulfonicacid (HDDNS) in toluene had been previously shown to give high selectivity for Cu(I1) over other first-row transition metal cations, including Fe(III).13 The similarity between sulfonic acid solvent-extraction reagents like HDDNS and strongacid cation-exchange resins includes the following: sulfonic acid functionalities, strong acidity, strong hydration, relatively low selectivity, aggregated structure, and thermodynamic proper tie^.^^^^^ Thus, it was thought that the SIR containing TT14C4 sorbed ona sulfonicacid PS-DVB resin would exhibit (18) Small, H. J. Inorg. N u l . Chem. 1961,18, 232-244. (19) Braun T., Ghershini, G., Eds. Extraction Chromatography; Elsevier: Amsterdam, 1975. (20) Flett, D. S. Chem. Ind. 1977, 6, 641-646. (21) Warshawsky, A. In Ion Exchange and Solvent Extraction: A Series of Aduances; Marinsky, J. A., Marcus, Y., Eds.; Marcel Dekker: New York, 1981; Vol. 8, pp 229-310. (22) Loret, J. F.;Brunette, J. P.; Leroy, M. J. F.; Candau, S. J.; Prevost, M. Solvent Extr. Ion Exch. 1988,6, 585-603. (23) Horwitz, E. P.; Dietz, M. L.; Fisher, D. E. Anal. Chem. 1991,63, 522-525. (24) Belfer, S.; Korngold, E. Israel J. Chem. 1985, 26, 71-75. (25) H. Small Ind. Eng. Chem. Prod. Res. Deu. 1967,6, 147-150. (26) Warshawsky,A.; Kahana, N. J. Am. Chem. Soc. 1982,104,26632664. (27) Sevdic, D.; Meider, H. J.Inorg. Nucl. Chem. 1981,43, 153-157. (28) Ohki, A.; Takagi, M.; Veno, K. Anal. Chim. Acta 1984,159, 245253. (29) Sekido, E.; Chayama, K.; Muroi, M. Talanta 1985,32, 797-802. (30) Gloe, K.; Muhl, P.; Beyer, L.; Muhlstadt, M.; Hoyer, E. Solvent Extr. Ion Exch. 1986, 4, 907-925. (31) Saito, K.; Murakami, S.; Muromatsu, A. Anal. Chim. Acta 1990, 237,245-249. (32) Tomoi, M.; Abe, 0.;Takasu, N.; Kakiuchi, H. Makromol. Chem. 1983,184, 2431-2436. (33) Oue, M.; Ishigaki, A.; Kimura, K.; Mataui, Y.; Shono, T. J.Polym. Sci., Polym. Chem. Ed. 1985,23, 2033-2042. (34) Hbgfeldt, E.; Chiarizia, R.; Danesi, P. R.; Soldatov, V. S. Chem. Scr. 1981, 18, 13-43. (35)Kuvaeva, Z. I.; Popov, A. V.; Soldatov, V. S.; Hbgfeldt, E. Solvent Extr. Ion Exch. 1986, 4, 361-381.
behavior closely related to the SX system employing the combination of TT14C4 and HDDNS. Since the macrocycle TT14C4 is sparingly soluble in water and partitions strongly from water to toluene (Drn14~4 > 1 0 0 0 ) , 1 3 9 3 7 it is not expected to be appreciably desorbed from beads in contact with water. Although it has good binding properties toward Cu(II),= TT14C4 extracts Cu(I1) weakly from sulfuric acid in the absence of HDDNS.13s3' Thus, without addition of hydrophobicanions such as picrate to the aqueous phase, synergistic systems represent the only means to exploit the selective binding properties of the macrocycle for extracting Cu(1I) from such media. It was our objective, then, to test for the presence of synergism in the proposed SIR and to compare the observed behavior with the analogous SX system.
EXPERIMENTAL SECTION Materials. HDDNS was obtained from King Industries and was purified by anion-exchange chr~matography.~~ Stock solutions of HDDNS were standardized by acid-base titrations in ethanol-water. Toluene was of spectrophotometric grade (Burdick and Jackson). W u and 67Cu tracers (NEN Research Products) were received as the nitrate salts and converted to the sulfate form by adding the received solutions to dilute sulfuric acid and evaporating to dryness. TT14C4 was obtained from Aldrich Chemical Co. and recrystallized from mixtures of ethyl acetate and petroleum ether. Water was doubly distilled. Reagent-grade p-dioxane was distilled over sodium. All other chemicals were of reagent quality and were used as received. Strong-acid PS-DVB resin beads tested included the two commercialresins Dowex 50W-X2(resin A) and Dowex SOW-XS (resin B) plus a partially sulfonated 2% DVB-cross-linked polymer (resin C) prepared by reaction of PS-DVB beads with concentrated sulfuric acid at 60 "C for 17 h. Nonfunctionalized 2 % PS-DVBresin beads (resinD) used as controls were prepared by standard techniques using suspension polymerization. Before use, the resin beads were washed with water, allowed to stand in water for 1 h, filtered (10-min suction), and stored in capped vials. Sorption of TT14C4 on Beads. M e t h o d 1 (SIRSA - 1 , B-I, a n d C-1). Portions of hydrated resins A (2.13 g), B (2.21 g), and C (2.28g) were weighed (wetbasis) intoseparatevialsandagitated with 5 mL of p-dioxane for 15 min. Upon decantation of the p-dioxane, the beads were washed two additional times with fresh p-dioxane in like manner. The beads were then gently rocked for 64 h at 25 "C in a solution of 0.1 g of TT14C4 in 3 mL of p-dioxane (0.12 M TT14C4), filtered, quickly rinsed with ca. 10 mL of p-dioxane, dried by suction 10 min, and vacuum-dried 16 h. Water (10 mL) was added to each vial, and the vials were rocked for 1h. Evolution of heat from SIR B-1 was noted. The aqueous phases, now hazy, were decanted, and the beads were rinsed twice with 10-mL portions of water; the final rinse water was clear. The beads were then filtered, dried 10min by suction, and stored in capped vials. M e t h o d s 2a-c (SIRSC-20, C-2b, and C-2c). A 6-g portion of hydrated resin C was weighed (wet basis) into a vial and agitated with 20 mL of p-dioxane for 15 min. Upon decantation of the p-dioxane, the beads were washed three additional times with fresh p-dioxane in like manner, filtered, and dried by suction (dry-air stream) for 10min. Three 1.5-gportions of the resulting beads, each placed into a separate vial, gently came into contact with 5-mL solutions of, respectively,0.015 (C-2a),0.050 (C-2b), and 0.15 (C-2c) M TT14C4 in p-dioxane for 17 h at ambient temperature (23 "C). Portions of each solution were then transferred to a preweighed vial and allowed to evaporate; the (36) H6gfeldt, E. In Deuelopments in Soluent Extraction; Legret, S. A., Ed.; Masson, M. R., Translation Ed.; Ellis Horwood: Chichester, UK, 1988; Chapter 10. (37)Moyer, B. A.; Sachleben, R. A.; Case, G. N. Proceedings, International Soluent Extraction Conference (ISEC 93),York, England, September 9-15,1993; Society for Chemical Industry: London 1993. (38) Sokol, L. S. W. L.; Ochrymowycz, L. A.; Rorabacher,D. B. Inorg. Chem. 1981.20. 3189-3195. (39) Danesi, P. R.; Chiarizia, R.; Scibona, G. J. Inorg. Nucl. Chem. 1973,35, 3926-3928.
ANAL.YTICALCHEMISTRY, VOL. 65, NO. 23,DECEMBER 1, 1993
Table I. Analytical Data for Treated and Untreated PI-DVB Resins. capacityb resin code resin source % solids (mequiv/g of dry resin) A
B C D A- 1
B-1
c-1
C-2a C-2b c-2c D-3
Untreated Resins Dowex SOW-X2 23 Dowex 50W-XS 47 46 synthesis synthesis Treated Resins A method 1 19 B method 1 48 C method 1 63 C method 2a 80 C method 2b 72 C method 2c 69 D method 3 81
4.4
4.8 1.7
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Table 11. Sorption Properties of TTl4C4 on Synthetic Resin at 211 TT14C4/ resin ITT14C41 hlac % SOa Ddat code at equilibb (L/kg) TT14C4d* ratidf 0.1 M H a 0 4 C-2a C-2b C-2~ C-3 D-3
0.0133 0.0443 0.131 0.129 0.124
1.06 1.05 1.20 3.98 1.11
0.38 1.23 4.06 12.1
3.58
0.0077 0.025 0.093 0.30
5.0 X l@ 1.7 X 108 8.3 x 108
