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Anal. Chem. W @ l ,63,1844-1847
Selective Column Concentration of Alkali-Metal Cations with a Crown Ether Carboxylic Acid Resin Takashi Hayashita, Joung Hae Lee,1Shiping Chen, and Richard A. Bartsch* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Column concentration of alkaii-metai cations from dilute aqwow, toktknr wlth the -acrylk acid Wxchange resin Amberlite CaSO and wlth redns 1 and 2 obtained by condensatlon polymertzationwith symdibenzo-16trown-5oxyacetlc and sym-(propyi)dibenzo-l6-crownSoxyacetlc acld, reapectlvely, with formaldehyde Is reported. Selective cokmn conccmtrakn of alka"etai c a m was attained wtth resin 2, in whichthe pendent carboxylic add group is orlented over the crown ether cavity. Due to the strong interaction of Na+ with redn 2, the elution peak for Na' in the stripping solution was retarded relative to those of the other aikailmetal cations. With gradient stripping, the maximum concentration factor for Na' from an aqueous sample solution which was 6.0 X 10" M in each of the five alkaii-metai cations reached 1030 with an 84% purity.
INTRODUCTION During the past two decades, a variety of macrocyclic polyether compounds (crown ethers) has been synthesized. Applications of crown ethers in separation chemistry have utilized their superior binding abilities for alkali-metal and alkaline-earth-metal cations in solution (1-4). By polymerization or immobilization on support materials, these macrocyclic ligands may be adapted to continuous separation processes and their toxicity alleviated (5). In an analytical application, macrocyclic polyether polymers have been utilized as stationary phases for chromatographic separations of alkali-metal and alkaline-earth-metal cations (6-16). However, relatively little information is available concerning their use in the selective column concentration of these metal cations (17,18). Since a chromatographic separation is accomplished by a multistage equilibrium, even weakly interacting macrocyclic polyether units and cations can be utilized. On the other hand, column concentration is based on a single-stage sorption. Thus column concentration requires strong and selective interactions between the binding sites and metal cations. Interaction between the neutral crown ether binding sites and metal cations in aqueous media are relatively weak, which makes selective column concentration difficult. However, the attachment of an ion-exchange group to the crown ether ring would be expected to enhance the binding ability for metal cations in aqueous media. In previous work, we synthesized novel ion-exchange resins that possessed both ion-exchange and cyclic polyether binding sites by condensation polymerization of dibenzocrown ether carboxylic acids with formaldehyde in formic acid (19). Results of alkali-metal cation sorption experiments conducted with these resins demonstrated that preorganization of the binding site by conformational positioning of the pendent carboxylic acid groups over the crown ether cavity provided good sorption selectivity and efficiency. In addition, alkalimetal cation sorption from the alkaline aqueous solution and stripping of the resin-bound metal cations with aqueous hyPermanent address: Korea Standards Research Institute, Tae-
dok Science Town, Taejon 305-606, Republic of Korea.
0003-2700/91/0363-1844$02.50/0
drochloric acid were found to be rapid (19). Thus, this type of crown ether resin is a good candidate as a stationary phase for selective column concentration of alkali-metal cations from dilute aqueous solution. In the present study, we have examined the column concentration behavior of alkali-metal cations with a commercial polymethacrylic acid resin, Amberlite CG-50, and the dibenzo-16-crown-5-oxyacetic acid resins 1 and 2 (Figure 1). Crown ether carboxylic acid resin 2 was found to possess superior separation ability for alkali-metal cations in column concentration. EXPERIMENTAL SECTION Apparatus. Concentrations of alkali-metal cations in aqueous solution were determined with a Dionex Model 2000i ion chromatograph with a HPIC-CS3 column. pH measurements were carried out with a Fisher Scientific Accumet Model 825 MP pH meter and a Corning 7605 glass body combination electrode. For elution of the sample and stripping solutions, a Model 396 Mini-Pump (Milton Roy Co.) was utilized. To prevent metal contamination, all glassware was soaked in 5% HN03 solution for 24 h and rinsed with distilled, deionized water before use. Reagents. Resin 1 from sym-dibenzo-16-crown-5-oxyacetic acid and resin 2 from sym-(propyl)dibenzo-l6-crownd-oxyacetic acid were prepared by condensation polymerization of the corresponding dibenzocrown ether carboxylic acid monomers with formaldehyde in formic acid, as previously described (19). The resins were ground and used in powder form (fier than 100 mesh). Amberlite CGW ion-exchange resin (100-200 mesh) was obtained from Aldrich Chemical Co. 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 combinationcartridges in series. Procedure. An aqueous sample solution containing five alkali-metal chlorides and hydroxides was eluted through a 0.42 cm i.d. column of resin (0.020 g for CG-50,0.100 g for 1 and 2) at a flow rate of 3.0 mL/min. At the same flow rate, the column was washed with 100 mL of pure water to remove uncomplexed alkali-metal salts from the resin bed. The alkali-metal cations bound to the resin were then stripped with an aqueous HCl solution at a flow rate of 1.0 mL/min. The stripping solutions were collected by 12-drop fractions and analyzed by ion chromatography after appropriate dilution. After each set of the experiments, the volume of the 12-dropfractions (0.38-0.45 mL) was calibrated with a microsyringe. The column concentration behavior for alkali-metal cations was evaluated in terms of the concentration factor (CF): conc of M+ in the stripping solution (1) CF = initial conc of M+ in the sample solution RESULTS AND DISCUSSION Crown ether carboxylic acid resins 1 and 2 were prepared by condensation polymerization of the corresponding dibenzo-lBcrown-5oxyaceticacids with formaldehyde in formic acid. Amberlite CG-50 is a commercially available polymethacrylic acid resin. Column Concentration of Alkali-Metal Cations with CG-50 and Resins 1 a n d 2. In an earlier investigation of competitive alkali-metal cation sorption, the observed sorption selectivities from 0.10 M alkali-metal cation solutions (above pH 7) were found to be Li+ > Na+ > K+ > Cs+ > Rb+ for 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 17, SEPTEMBER 1, 1991
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xH2C02H 1500
a
CG-50 1
H
2
c3HI
Flgurr 1. Structures of the carboxylic acM resins.
Table I. Recovery of Alkali-Metal Cations in Column Concentration with CG-60 and Resins 1 and 2" recovery, 5%
Elution volume (mL)
resin
Li+
Na+
K+
Rb+
Cs+
CG-50
39 45 21
31 56 14
30 54 29
33 46 23
41 58 30
1 2
Flgurr 2. Concentration factors (CF) for alkall-metal catlons from sample solutions (1 L, pH 10.4, 6.0 X lo-' M In each akall-metal cation) vs the elution vohnne of 0.10 M HCI wtth columns of (e) -50, (b) 1, and (c) 2 (A)Ll+, (0)Na+, (0)K+, (A)Rb+ (0)Cs+.
OSample solution: aqueous solution (1.0 L, pH 10.4) containing M in each alkali-metal cation. Stripping solution: 0.10 6.0 X M HCl. Column: CG-50 (0.020 g), 1 and 2 (0.100 8).
CG-50, Na+ > Li+, K+ > Cs+ > Rb+ for 1, and Na+ >> Li+ > K+, Cs+ > Rb+ for 2 (19). Clearly, interaction of the crown ether units with alkali-metal cations influenced the sorption selectivity. The Na+ selectivity observed for crown ether carboxylic acid resins 1 and 2 is predicted from the relationship between the crown ether cavity and metal ion diameters (20). Of particular interest is the superior sorption selectivity observed with resin 2 in which one carbon of the crown ether ring bears both a propyl group and an oxyacetic acid group side arm. The enhanced Na+ selectivity of resin 2 over resin 1 is attributed to preorganization of the carboxylic acid group over the crown ether cavity, which enhances the metal-crown ether interaction (19). The rapid alkali-metal cation sorption and desorption observed for 1 and 2 (19) suggests that such crown ether carboxylic acid resins could be utilized as stationary phases for column concentration of alkali-metal cations from aqueous solutions. The effective ion-exchange capacities of CG-50 and resins 1 and 2 measured previously (19) were 8.71,1.81,and 1.44 mmol/g, respectively. Thus the ion-exchange capacity for CG-50 is 5-6 times higher than that for resins 1 and 2. To examine the three ion-exchange resins under conditions of similar net ion-exchange capacity, the amount of resin for the stationary phase was 0.020 g for CG-50 and 0.100 g for 1 and 2. For column concentration from an aqueous sample solution (1.0L, pH 10.4) that was 6.0 X 10" M in each of the five alkali-metal cations, the concentration factors (CF) vs elution volumes of 0.10M HC1 (strippingsolution) are shown in Figure 2a-c for CG-50 and resins 1 and 2, respectively. The recovery of each alkali-metal cation species from the dilute aqueous solution with CG-50 and resins 1 and 2 after
stripping with 0.10 M HC1 is given in Table I. It is evident that CG-50 and resin 1 showed poor selectivity in alkali-metal cation recovery from the sample solution. On the other hand, upon a single passage of the sample solution through the column of resin 2, the recovery of Na+ reached 74%, which was much higher than the 23-30% recoveries for the other alkali-metal cations. For elution with 0.10 M HC1 solution, most of the resinbound alkali-metal cations were eluted in the first 3.0 mL of stripping solution and the maximum CF values were in the range 250-500, except for Na+ elution from resin 2 (Figure 2c). It is noteworthy that selective elution of Na+ was observed only for resin 2. Apparently, a stronger binding ability of resin 2 for Na+ retards the stripping of this cation, which produces a separation and concentration of Na+ from the dilute aqueous solution of five alkali-metal cations. The CF value for Na+ reached a value of 674 with a 64% purity of Na+ in the third fraction (Figure 212). Effect of Sample Solution Concentrations upon Column Concentration of Alkali-Metal Cations by Resin 2. For column concentration of alkali-metal cations by resin 2, the influence of sample solution concentrations were investigated. In Table I1 are recorded the loading and average CF values for the first 4.0mL of 0.10 HC1 eluent. In entries 1-3 the concentration of each alkali-metal cation (2.0 X lo6 M) and sample volume (3.0 L) are held constant, but the concentration of hydroxide in the hydroxide plus chloride anion mixture is varied. A marked influence of the hydroxide ion concentration is noted. Thus when no hydroxide was present (entry 3), only Na+ and K+ were detected in the stripping solution and the loading was very low. The much higher loadings obtained when the hydroxide:chloride ratios are 3:2 (entry 2) and 4:l (entry 1) clearly demonstrate that proton removal from the carboxylic acid groups is required for efficient metal ion complexation. Increasing the alkali-metal
Table 11. Loadings and Average Concentration FactorO (CF) of Alkali-Metal Cations with a Column of Resin 2 for Different Sample Solution Concentrations entry 1 2 3 4c 5
10" concn, M [M+]* [OH-] 2.0 2.0 2.0 6.0 6.0
8.0 6.0 0.0 24.0 6.0
loadings, mmol/g (CP) pH
sample vol, L
Li+
Na+
K+
Rb+
9.9 9.8 5.8 10.4 9.8
3.0 3.0 3.0 1.0 1.0
0.09 (113) 0.01 (13) 0.00 (0) 0.16 (67) 0.04 (17)
0.38 (475) 0.16 (200) 0.03 (38) 0.44 (183) 0.20 (83)
0.13 (163) 0.04 (50) 0.01 (13) 0.18 (75) 0.06 (25)
0.10 (125) 0.03 (38) 0.00 (0) 0.14 (58) 0.05 (21)
CS+ 0.10 0.04 0.00 0.18 0.06
(125) (50) (0) (75) (25)
each of the five alkali-metal cations. "Average concentration factor observed in the first 4.0 mL of stripping solution (0.10 M HCl). 'This experiment was repeated four times and the standard deviations were f 6 % of the stated loading values.
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ANALYTICAL CHEMISTRY, VOL. 63,NO. 17, SEPTEMBER 1, 1991 15001
I
I
1
1000 LL 0
500
'
Elution volume ImL)
Flgwe 3. Concentration factors (CF) for alkali-metal cations from sample solutions (1 L, pH 10.4,6.0 X lod M In each alkalhnetal cation) with a column of resin 2 vs the elution volume of (a) 0.05 M HCI, (b) 0.30 M %I, and (c) 0.50 M HCI: (A)U+, (0)Ne+,( 0 )K+, (A) Rb', (0)Cs'. cation concentration by decreasing the sample volume while the hydroxide ion concentration is held constant (compare entries 2 and 5) enhanced the loading somewhat. As expected, the CF values are lower when the alkali-metal cation concentrations are increased in the sample solution (entry 5). When both the alkali-metal cation and hydroxide concentrations are increased by reducing the sample volume from 3.0 to 1.0 L (compare entries 1and 41, the loadings increase somewhat but some loss of Na+ selectivity is noted. Thus it is found that column concentration of alkali-metal cations from dilute aqueous solutions by crown ether carboxylic acid resin 2 is effective only when the sample solution is quite alkaline and becomes more selective as the alkali-metal cation concentration is lowered. To obtain high loading and recovery of alkali-metal cations in this type of column concentration, the counteranions of the alkali-metal cations in the sample solution should be converted to hydroxide. Pretreatment of the sample solution with a hydroxide-type, anion-exchange resin would be effective for this purpose. Effect of HCl Concentrationin the Stripping Solution upon Alkali-Metal Cation Elution Behavior. The influence of stripping conditions on elution of alkali-metal cations from a column of resin 2 was also examined. The column was loaded with 1.0 L of aqueous solution (pH 10.4) that was 6.0 X M in each alkali-metal cation. After washing with water, the column was eluted with HCl solutions of varying concentrations. Results for stripping solution concentrations of 0.050,0.10,0.30, and 0.50 M HCl are shown in Figures 3a, 2c, 3b, and 3c, respectively. Although the alkali-metal cation loadings remain quite similar as the stripping solution concentration is varied, the elution peak shapes show substantial differences. Compared with the 0.10 M HCl elution (Figure 2c), the elution of alkali-metal cations with 0.05 M HCl (Figure 3a) exhibits broadened peaks, which results in low CF values (below 300). As the HCl concentration is increased from 0.050 to 0.10 to 0.30 (Figure 3b) to 0.50 M (Figure 3c) more and more of the resin-bound alkali-metal cations are eluted in the first fraction. Thus while the separation of Na+ decreased as the HC1 concentration was enhanced, the maximum CF value for Na+ changed from 280 with 0.050 M HCl to 1370 with 0.50 M HCl. It should be noted that elution of the other alkali-metal cations from resin 2 is relatively rapid compared with more strongly bound Na+, even for elution with 0.050 M HC1. This implies that Na+ separation efficiency might be enhanced by varying the eluent strength in the single elution experiment. T o examine this possibility, the sample solution (1.0 L, pH 10.4, 6.0 X M in each alkali-metal cation) was passed
0 2 4 6 8 Elution volumehL)
Flgure 4. Concentration factors (CF) for alkali-metal catlons from sample solutions (1 L, pH 10.4, 6.0 X lo-' M In each alkali-metal cation) with a column of resin 2 vs the elution volume of (I) 0.05 M HCI, (11) H,O, and (111) 0.50 M HCl: (A)Li', (0) ,'aN (0)K+, (A)Rb+,
( 0 )cs+.
1
2
0
0
1
Metal Ion Diameter&)
Figure 5. Concentration factors (CF) for alkall-metal cations In the second peak obtalned by step gradient elution from a .column of resin 2 vs the alkali-metal cation diameters. through the column of resin 2 and unbound metal ions were removed by washing with water, as before. Elution waa performed with 1.14 mL of 0.050 M HC1, then 3.80 mL of water, and then 0.50 M HCl. The results are presented in Figure 4 and show marked improvement in the separation of Na+. The maximum CF value for Na+ in the second peak is 1030, and Na+ comprises 84% of the alkali-metal cations in that peak. Of the recovered Na+, 57% is in the second peak. Figure 5 is a plot of the maximum CF values in the second peak vs the alkali-metal cation diameters and shows excellent Na+ selectivity. The Na+:K+concentrationratio in the second peak is 17, which compares well with that observed for competitive solvent extraction of alkali-metal cations into chloroform by sym-(propy1)dibenze16-crownd-oxyacetic acid (20, 21), the crown ether monomer of resin 2. Losses of slightly soluble diluents and extractants into the sample solution generally render conventional solvent extraction systems inappropriate for metal ion recovery from large volumes of dilute aqueous solutions. The type of column concentration system described in this paper possesses excellent potential for the efficiency and selective recovery of metal ions from such solutions. CONCLUSION Selective column concentration of Na+ from dilute aqueous alkali-metal cation solutions has been achieved with crown ether carboxylic acid resin 2. The strong interaction of Na+ with resin 2 is attributed to conformationalpositioning of the
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.A M I . 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-
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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
