AMI. Chem. 1991, 63, 1847-1850
pendent ion-exchange group over the crown ether cavity to provide preorganization of the binding site. Stronger complexation of Na+ by resin 2 retards stripping with aqueous acid, which allows it to be separated from the other alkalimetal cations. Such molecular design of new resins with appropriately positioned ion-exchange and crown ether binding sites provides a potential for efficient concentration and highly selective separation of target metal ions from dilute solutions.
LITERATURE CITED (I) PdWS6fI, C. J. J . Am. chem.SOC. 1987. 88, 7017-7036. (2) Hkaoka, M. clown G"&.Thek Cheracte&ibs and Apprvcebbns; Else* New York, 1962. (3) I a t t , R. M.; BradShaw, J. S.; " I , S. A.; Lamb, J. D.; Chrktensen, J. J.; Ssn, D. chsm. RSV. 1985, 85, 271-339. (4) Cabbn BMbg by AIecrocyabs. Cmpkxatkn of Catbnlc Species by clown ELhSrs; hue, Y., Gokel, 0. W., E&.; Marcsl, Dekker: New York, 1990. (5) Khura, K.; Shono, T. In FmctlonelMonamers and pdymsrs; Takem oto, K., Inaki, Y., Ottenbrlte, R. M., Eds.; Marcel Dekker: New York, 1987; pp 349-421. (8) Blasius, E.; Janzen, K.-P.; Adrian, W.; Kiautake, G.; Lorscheider, R.; Mauer. P.0.: Ngyren-Tlen, T.; schdtM1,G.; Stockemer. J. Frosen/" 2.AMI. m.1977, 284, 337-360. (7) Blasius, E.; Janzen, K.P.; Keller, M.; Lander, H.; Nguyen-Tien, T.; Schoten, 0. talanta 1980, 27, 107-126. (6) Bhdw, E.: Janzen, K.P.; Kiein, W.; Klotz, H.; Nguyen, V. V.; NguyenTien, T.; Pfeiffer, R.; Scholten, 0.; Simon. H.; Stockemer, H.; Tous-
1847
Mint, A. J . chrometog. 1980, 201, 147-166. (9) Warshawsky, A.; Kailr, R.; Deshe, A.; Berkovitz, H.; Patchomik, A. J . Am. chem.Soc. 1979, 101, 4249-4258. (IO) Igawa, M.;Selto, K.; Twkemoto, J.; Tanaka, M. Anal. chem.1981, 53, 1942-1949. (11) QrosSman, P.; SlmOn, W. J . c h r o m e m .1982, 235, 351-363. (12) Frere, Y.; (Lamain, Ph. Maknwnd. chsm.1982, 183, 2163-2172. (13) La&, M.; (Lamain, Ph. J . &. chrometog*. 1985. 8, 2403-2415. (14) Nakajima, M.; Kimua, K.; shono,T. Anal. Chem. 1989, 55,463-467. (15) NakaJlma,M.; Kknua, K.; Shono. T. Bull. Chem. Soc. Jpn. 1983, 56, 3052-3056. (16) NakaJlma,M.; Kimura, K.; Hayata, E.; Shono, T. J . &. Chrometo(r. 1984, 7 , 21162125. (17) Warshawsky, A.; Kahana, N. polvmeric Seperation AWk; Anthony, R. C. Ed.; PhW: New York, 1982; pp 227-231. (18) Bruenlng, M. L.; MltcheH, D. M.; Bradshaw, J. S.; Izatt, R. M.; BruenIIIQ, R. L. Anal. chem.1991, 63, 21-24. (19) Hayashlta, T.; Goo, M.; Lee, J. C.; Kim, J. S.; Krzykawski, J.; Bartsch, R. A. Anal. Chem. 1990, 62, 2283-2287. (20) Walkowiak, W.; Charewicz, W. A.; Kang, S. 1.; Yang, I.; Pugia. M. J.; Bartsch, R. A. Anal. chem.1990, 82, 2018-2021. (21) BartsCh, R. A. sdvent.
[email protected] EX&. 1989, 7, 829-854.
RECEIVEDfor review January 25,1991. Accepted May 24,1991. This research was supported by the Division of Chemical Sciences of the Office of Basic Energy Sciences of the U.S. Department of Energy (Grant DE-FG05-88ER13832)and the Advanced Technology Program of the State of Texas. J.H.L. received a postdoctoral fellowship from the Korea Science and Engineering Foundation.
Competitive Sorption of Alkali-Metal and Alkaline-Earth-Metal Cations by Carboxylic Acid Resins Containing Acyclic or Cyclic Polyether Units Takashi Hayashita and Richard A. Bartsch* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Sorption ol Na', K+, Md', and Caw from aqueous roluwone by the polymahacryik acid re& Amberlite CG-50 and by three carboxylic acldr realm that contain acyclic or cyclk pdyethor unlb has tmn h v . r t l g a d . Compared wllh -0, for whkh tho compet~~~e smptbn s e b c t ~ t yIS Caw > >> Na+, K', the -ton selectivtty order remains the same with the acycik polyether carboxylic acid redn and one of the crown ether carboxylic acid redns, but the Ca2+/Mg2' r o r p l k n ~ I n c n ~ w k t c m t l d l ontheotherhend, y. for a crown ether carboxylic acid redn in which the lon-exchange dte Is posttloned over the polyether rfng, sorption of the monovalent catlon that bed fits the crown ether cavity becomes comparable with Ca* sorption. Thus the incorporation of polyether units Into carboxylic acid redns and the porltlon of the lowxchange dto with respect to a crown ether unit are shown to exert an appreciable influence upon the competitive sorption of alkalhetai and alkaline-earthmetal cations.
INTRODUCTION Cation-exchange resins generally prefer sorption of more highly charged metal ions from solutions. This is attributed to the stronger electrostatic interaction between the ion-exchange groups and metal ions with higher charge. Thus, 0003-2700/91/0383-1847$02.50/0
cation-exchange resins usually show selectivity for sorption of alkaline-earth-metal cations over alkali-metal cations (1). For carboxylic acid resins, chelate formation also enhances the sorption selectivity for the alkaline-earth-metal cations. For stable chelate formation, the divalent metal cation should interact with two different carboxylate groups in the resin. Therefore, the positioning of the carboxylic acid groups should play an important role in divalent metal cation separations. Efficient separation of Mg2+and Ca2+at trace levels from a concentrated solution of LiC1, NaCl, and NaOH by carboxylic acid resins has been reported (2). Recently, we reported the synthesis of novel acyclic and cyclic polyether carboxylic acid resins 1-3 (Figure l), their alkali-metal cation sorption properties, and the use of resin 3 for the selective column concentrationof alkali-metalcations from dilute aqueous solutions (3,4). The pendent carboxylic acid groups in resins 1 and 2 are flexible. In contrast, attachment of a propyl group to the same ring carbon that beam the side arm in resin 3 is proposed to orient the carboxylic acid group over the crown ether cavity (3). To elucidate the influence of introducing polyether units into carboxylic acid resins as well as the flexibility of the ion-exchange group upon alkali-metal and alkaline-earth-metal cation sorption behavior, competitive sorption of Na+, K+, Mg2+,and Ca2+by polyether carboxylic resins 1-3 has now been investigated. For comparison the sorption selectivity Q I991 Amerlcan Chemical Society
1848
ANALYTICAL CHEMISTRY, VOL. 63, NO. 17, SEPTEMBER 1, 1991 (a)
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Figure 2. Single species sorption of Na+ and of Ca2+ by polyether carboxylic acid resins (a) 1, (b) 2, and 3 vs the equilibrlum pH of the aqueous solution: (0)Na , (A)Ca2 .
("1
Figure 1. Structures of carboxylic acid resins.
of Amberlite CG-50, a polymethacrylic acid resin, was also evaluated. EXPERIMENTAL SECTION Apparatus. The apparatus was the same as that utilized in the previous study (3). To prevent metal contamination, all glassware was soaked in 5% HN03 solution for 24 h and rinsed with distilled, deionized water before use. Reagents. Resins 1-3 were available from the previous investigation (3). Sources of other reagents include Aldrich Chemical Co. (Amberlite CG-50 ion-exchange resin; MgCl2.6Hz0,CaCI,. 2H20),Fisher Scientific (NaCl, KC1, Ca(OH)2,0.10 M HCl solution), Mallinckrodt (1.00 M NaOH solution). Stock aqueous solutions of 0.50M alkali-metal and 0.25 M alkaline-earth-metal chlorides (NaCl, KC1, MgC12, and CaCld, 0.50 M NaOH, and 0.010 M Ca(OH)2were stored in polyethylene bottles. Sample solutions were prepared by mixing and diluting these stock solutions. Other inorganic and organic compounds were reagent-grade commercial products and were used as received. Purified water was prepared by passing distilled water through three Barnstead D8922 combination cartridges in series. Sorption of Na+ and of Ca2+by Resins 1-3. An 8.0-mL sample solution of 0.515 M NaCl or 0.258 M CaClz with the corresponding hydroxide for pH adjustment was shaken with 0.030 g of the resin for 3.0 h in a 30-mL separatory funnel at room temperature (21-23 "C). After fltration with a sintered gless filter funnel and measurement of the equilibrium pH of the filtrate, the resin was washed with 100 mL of water and dried. Of the dried resin, a 0.010-g sample was shaken with 5.0 mL of 0.10 M HC1 solution for 1.0 h to strip the Na+ or Caz+from the resin into an aqueous solution for analysis by atomic absorption with a Perkin-Elmer Model 500 atomic asorption spectrophotometer. Competitive Sorption of Na+, K+,Mgz+, and Ca2+with CG-80 and Resins 1-3. An aqueous or aqueous methanolic solution (5.0 mL) of the alkali-metal and alkaline-earth-metal chlorides with hydroxide for pH adjustment (0.100 M in Na+ and K+ and 0.050 M in Mg2+and Cas+) was shaken with 0.040 g of resin 1-3 (0.030 g for CG-50) for 3.0 h in a 30-mL separatory funnel at room temperature (21-23 "C). The mixture was filtered with a sintered glass funnel, and the equilibrium pH of the filtrate was determined. The resin was washed with 100 mL of water and dried. Of the dried resin, a 0.020-g sample (0.010 g for CG-50) was shaken with 5.0 mL of 0.10 M HCl for 1.0 h to strip the metal cations from the resin into an aqueous solution for analysis by ion chromatography with a Dionex Model 2000i ion chromatograph. Good reproducibility for competitive alkali-metal cation sorption by crown ether carboxylic acid resin 3 using this method has been demonstrated (3). RESULTS AND DISCUSSION
Mechanism for Divalent Metal Ion Sorption. Although a 1:1sorption mechanism for a monovalent metal ion and the
ion-exchange site in a carboxylic acid resin is anticipated, two different mechanisms are possible for divalent metal ion sorption (eqs 1and 2, where RH represents the ion-exchange
M2++ 2RH + R2M + 2H+ M2++ RH
+ X-=
RMX
+ H+
(1) (2)
site in the resin). When the divalent metal ion interacts with two ion-exchange sites to form a chelate structure (eq 1),2:1 sorption takes place. On the other hand, 1:l sorption accompanied by an anion from the solution (eq 2) is also possible. These two divalent metal ion sorption mechanisms may be differentiated by comparison of the single species sorption behavior of the resin for monovalent and divalent metal ions. For 1:l sorption (eq 2), the maximum divalent cation sorption should be the same as the ion-exchange capacity determined by monovalent metal ion sorption. For 2:l sorption (eq 11, the maximum divalent metal ion sorption would be only half the effective ion-exchange capacity measured with monovalent metal ions. To probe the mechanism of divalent metal ion sorption by polyether carboxylic acids 1-3, their sorption capacities for Nat and for Ca2+were investigated. Each resin was shaken with 0.515 M solutions of Na+ and 0.258 M solutions of Caz+ chlorides with hydroxides for pH adjustment. After filtration the resin was thoroughly washed with distilled,deionized water and then dried. A portion of the dried resin was shaken with 0.10 M HC1 to strip the metal ion species from the resin into an aqueous solution for analysis by atomic absorption spectroscopy. Sorptions of Na+ and of Ca2+by resins 1-3 (mmol/g of the dried H-form) as a function of the equilibrium pH of the aqueous phase are shown in Figure 2a-c, respectively. Maximum sorption of Na+ was obtained under highly W i n e conditions, and the maximum sorptions for resins 1-3 at pH = 11.5 were 2.03, 1.71, and 1.37 mmol/g, respectively. These values are very close to the effective ion-exchange capacities for resins 1-3 of 2.10, 1.81, and 1.44 mmol/g, respectively, which were determined previously in 80% methanol-20% water (3). On the other hand, the maximum sorptions of Caw by resins 1-3 were 1.28, 1.04, and 0.91 mmol/g, respectively. These values are approximately half the corresponding effective ion-exchange capacities of the resins, which demonstrates that 2:l sorption (eq 1)is the primary mechanism for divalent metal ion sorption. However, it should be noted that the maximum Ca2+sorptions are slightly larger than half the effective ion-exchange capacities. This indicates that 1:l sorption (eq 2) is a competing, albeit minor, sorption mechanism.
ANALYTICAL CHEMISTRY, VOL. 63,NO. 17, SEPTEMBER 1, 1991 1848
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Flgurr 3. Competitlve sorption of Na', K+, Mg2+,and Ca2+by Am berllte -50 r@n vs the equlllbrlum pH of the aqueous sokrtlon: (0) Ne+, (A)K+, (0)Mg2+, (A)Ca2+.
Figure 4. Competltlve sorption of Na', K+, Mg2+,and Ca" by polyether carboxylic acM resins (a) 1, (b) 2, and 8 vs the equilibrium pH of the aqueous solution: (0)Na+, (A) K , (0)Mg2+, (A)Ca2+.
For comparison with a commercially available carboxylic acid resin, sorption for Na+ and Ca2+by Amberlite CG-50, a polymethacrylic resin was also investigated. The maximum CaH sorption level observed for CG-50 was 4.50 mmol/g. This value is slightly more than half the effective ion-exchange capacity of 8.71 mmol/g obtained for Na+,which demonstraw similar sorption behavior for CG-50 and the polyether carboxylic acid resins 1-3. Competitive Sorption of Na+,K+,Mg2+,and Ca2+. To evaluate the competitive sorption behavior of polyether resins 1-3 for alkali-metal and alkaline-earth-metal cations,the series of Na+, K+, Mg2+,and Ca2+,which includes the most frequently encountered alkali-metal and alkaline-earth-metal cations, was utilized. For the competitive sorption experiments the analytical method was changed from atomic absorption spectroscopy to ion chromatography. For comparison, the competitive sorption characteristics of the polymethacrylic acid CG-50 were evaluated. Data for the competitive sorption of Na+, K+, Mg2+,and Ca2+from aqueous solutions (0.100 M in Na+ and K+ and 0.050 M in Mg2+and Ca2+)by CG-50 as a function of the equilibrium pH of the aqueous phase is presented in Figure 3. As expected for a weak-acid, ion-exchange resin (5), the low metal ion sorption from moderately acidic solutions increases markedly for neutral solutions and reaches a maximum value in moderately alkaline solutions. The observed sorption selectivity for CG-50 is Ca2+> Mg2+>> Na+, K+. Preferential sorption of the divalent metal ions results from enhanced electrostatic interaction with the carboxylate groups of the resin. Results for the competitive sorption of Na+, K+, M$+, and Ca2+by polyether carboxylic acid resins 1-3 are recorded in Figure 4a-c, respectively. The sorption profiles as a function of the equilibrium aqueous phase pH for the polyether carboxylic acid resins exhibit important differences from those found for CG-50 (Figure 3). For the acyclic polyether carboxylic acid resin 1 and crown ether carboxylic acid resin 2 at pH < 8.5, the sorption selectivity order was the same as that observed for CG-50, but the Ca2+/Mg2+selectivity was considerably improved over that achieved with CG-50. Thus, the presence of polyether units with flexible ion-exchange sites is found to produce resins with enhanced Ca2+selectivity. Compared with those observed for CG-50 (Figure 3) and polyether carboxylic acid resins 1 and 2 (Figure 4a,b, respectively), the overall K+, Mg2+,and Ca2+sorption profiles and selectivitities (Ca2+> Mg2+> K+) are quite similar for crown ether carboxylic acid resin 3 (Figure 4c). However, Na+ sorption by resin 3 is markedly enhanced. In resin 3, attachment of the propyl group to the same carbon atom of the polyether ring that bears the sidearm is proposed to orient
the ion-exchange group over the polyether cavity to produce the enhanced Na+ selectivity observed in competitive alkali-metal cation sorption by resin 3 compared with CG-50 and resins 1 and 2 (3). For competitive alkali-metal and alkaline-earth-metal sorption in the present study, it is apparent that the constraint of the ion-exchange group to a position which is highly favorable for Na+ complexation allow sorption of this alkali-metal cation to compete favorably with that of the alkaline-earth-metal cations. It should be noted that sorption of K+, a monovalent cation which is too large to fit within the dibenzo-16-crown-5cavity, is not enhanced. For polyether carboxylic acid resin 1, the extent of Mg2+ and Ca2+sorption increased markedly when the equilibrium pH of the aqueous phase was changed from 8.5 to 9.0. For polyether carboxylic acid resins 2 and 3, only the extent of Mg2+sorption exhibited marked enhancements for the same increase in pH. The Mg2+sorption at pH = 9.0 for resins 1-3 was found to be 0.78, 1.04, and 1.43, respectively. At pH = 9.0 the total metal ion sorptions calculated by using 2:l sorption for the divalent metal cations exceeded the effective ion-exchange capacities of the resins. The anomalous Mg2+ sorption appears to result from some type of physical phenomena such as a colloidal system formed by interaction of Mg(OH), particles with the polyether resins or Ca2+complexed in the resin. Extraction of Mg(OH), colloids from basic aqueous solutions (pH > 9) of alkaline-earth-metal cations into chloroform by crown ether carboxylic acids has been reported (6). It is interesting to note that the abnormal alkaline-earth-metal cation sorption is influenced by the resin structure. Thus the Mg2+sorption at pH 9 by the polyether carboxylic acid resin series was found to increase in the order 1 < 2 < 3. Since there was not anomalous Mg2+sorption by CG-50 (Figure 3), it appears that the polyether units in resins 1-3 are in some way related to the phenomenon. The concomitant increase in Ca2+ sorption at high pH for resin 1 appears to arise from an acyclic polyether structure which is absent in resins 2 and 3. Since metal ion-crown ether interactions are enhanced when the medium is changed from water to methanol (7, €0, the influence of structural variation within polyether carboxylic acid resins 1-3 upon the competitive sorption of alkali-metal and alkaline-earth-metal cations was examined in aqueous methanol. Results for competitive sorption of Na+, K+, Mg', and Ca2+from aqueous and aqueous methanol solutions (10, 20,40, and 60% methanol) at pH = 5-6 with polyether carboxylic acid resins 1-3 are shown in Figure 5a-c, respectively. The pronounced Ca2+sorption selectivity of polyether carboxylic acid resins 1 and 2 is readily evident (Ca2+>> Mg2+, Na+, K+). For acyclic polyether carboxylic acid resin 1, the
IC)
ANALYTICAL CHEMISTRY, VOL. 63,NO. 17, SEPTEMBER 1, 1991
1850
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20
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20 40 60 0 Methanol (volume o/o)
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Figure 5 . Competitive sorption of Na+, K+, Mg2+,and Ca2+by poly-
ether carboxylic acld resins (a) 1, (b) 2, and (c) 3 vs the methanol percentage In the aqueous methanol solvent at pH = 5-6 (0)Na', (A)K+, (0)w*+,(A)Caw.
Ca2+sorption selectivity is enhanced as the methanol content of the solvent is increased. On the other hand, sorption selectivity of crown ether carboxylic acid resin 2 is unaffected by variation of the methanol proportion. When the structure of crown ether resin 2 is modified by incorporation of the propyl group that orients the sidearm over the polyether cavity, the sorption selectivity changes to Na+ > Ca2+> Mg2+ > K+with an appreciable enhancement in Na+ sorption selectivity as the methanol content of the solvent is increased. Thus preorganization of the binding site to favor monovalent metal ion complexation is demonstrated to markedly alter the usual propensity of carboxylic acid resins for divalent metal ion complexation.
CONCLUSIONS This study demonstrates that the incorporation of polyether units into carboxylic acid resins produces important changes in the competitive sorption of alkali-metal and alkalineearth-metal cations. Compared with the commercially available polymethacrylic acid CG-50, polyether carboxylic acids 1 and 2, which have flexible ion-exchange sites, improve the Ca2+sorption selectivity. For crown ether carboxylic acid resin 3 in which the ion-exchange site is conformationally restrained to favor monovalent cation sorption, Na+ and Ca2+ sorption become comparable. These results suggest that by appropriate structural design it will be possible to prepare novel ion-exchange resins with markedly altered metal ion sorption selectivities from those which are currently available. LITERATURE CITED (1) lhfner, K. Ion €xdMnpm; Ann Arbor Science Publishers, Inc.: Ann Arbor, MlcMgan 1972; pp 44-48. (2) Inczedy, J. A n a w l AppwcemwKI of Ions ExdMngefs; Pergamon Press: New York. 1986; pp 348-352. (3) Heyashita, T.; Goo, M.Q.; Lee, J. C.; Klm, J. S.; Krzykawskl, J.; Bartsch, R. A. Anal. Chem. 1890, 62, 2283-2287. (4) Hayashita, T.; Lee, J. H.;Chen, S.; Bartsch, R. A. Anel. chsm.proceding artlck in this issue. (5) Helfferich, F. Ion EXchenge; MC&aw-Hill: New York, 1962 p 86. (8) Stndbkki, J.; Bartsch, R. A. A d . CtWm. 1081, 59, 2247-2250. (7) Izatt, R. M.; Wadshaw, J. S.; Nklsen, S. A.; Lamb, J. D.; Sen, D. CY". Rev. 198s. 85, 271-339. (8) Bruenkrg, M. L.; Mlchell, D. M.; Bradshew, J. S.; Izatt, R. M.; Bruening, R. L. Anal. Chem. 1991, 89. 21-24.
RECEIVED for review March 11,1991. Accepted May 24,1991. This research was supported by the Division of Chemical Sciences of the Office of Basic Energy Sciences of the U. S. Department of Energy (Grant DEFG05-88ER13832)and the Advanced Technology Program of the State of Texas.
