Anal. Chem. 1886, 58,3031-3035
(5) Forrester, R. L.; Wataji, L. J.; Siiverman, D. A.; Pierre, K. J. Clin.
Chem. (Winston-Salem, N.C.) 1978, 22, 243-245. (6) Forsseli, H.; Olbe. L. S a n d . J . Gastroenterol. 1985, 20, 767-774. (7) Isenberg, J. I.; Flemstrom, 0.;Johansson, C. Mechanisms of Mucosal Protection in the Upper Gastrohrtestinal Tract: Allen, A., Ed.; Raven: New York, 1984; pp 175-180. (8) Kiviiaakso, E.; Flemstrum, G. Mechanism of MucosalProtection in the Upper Gastrointestinal Tract; Ailen, A,, Ed.; Raven: New York, 1984; pp 227-232,
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(9) Tabata, K.; Jacobson, E. D.; Chen, M.; Murphy, R. F.: Joffe, S. N. Gastroenterology 1984, 87, 396-401. (10) Isenberg, J. I.; Wallis, E.; Johansson, C.; Smedfors, E.; Mutt, V.; Tatemoto, K.; Emas, S. Regul. fept. 1984 3, 315-320. (11) Smith, F. C., Jr.; Chang, R. C. The Practice of Ion Chromatography; Wiiey: New York, 1983; p 132.
RECEIVED for review May 15, 1986. Accepted August 11, 1986.
Vinyl Polymer Agglomerate Based Transition Metal Cation Chelating Ion-Exchange Resin Containing the 8-H yd roxyquino1ine FunctionaI Group William M. Landing,*’Conny Haraldsson, and Nicklas Paxgus Department of Analytical a n d Marine Chemistry, Chalmers University of Technology a n d University of Goteborg, 8-412 96 Goteborg, Sweden
A simple synthetic route has been developed for the ImmobHlzation of 8-hydroxyqulnollne onto Fractogel TSK, a highly porous, mechanlcally and chemically stable, hydrophlllc organic resin gel. The product exhiblts an exchange capacity comparable to the hlghest values reported for slllca-lmmobillzed l-hydroxyqulnollne but Is more stable at hlgh pH. The resin’s selectlvlty and efflciency of coilectlon of catlonlc metal specles from freshwater and seawater were Investlgated. The resin was used In a column sequence to obtain concentration and spedatlon data for AI, Mn, Fe, Co, Cu, Zn, and Cd In an organic-rlch freshwater sample.
The chelating qualities of 8-hydroxyquinoline (also known as 8-quinolinol or oxine, and hereafter referred to as 8HQ) and its preference for transition- and heavy-metal cations relative to alkali and alkaline-earth cations are well-known. These properties have led to a significant research effort in order to suitably immobilize this chelating agent onto various solid supports to utilize it in ion-exchange or chromatographic applications. A number of methods for the immobilization of 8HQ onto silica substrates (silica gel or glass beads) have been proposed (1-3), modified, and optimized (4-7), and the characteristics of the various products have been investigated (7-13). Silica supports offer the advantages of good mechanical strength, resistance to swelling, and rapid overall exchange kinetics in column applications (11). However, they are unstable at high pH, leading to cleavage of the immobilized 8HQ and potential trace-metal contamination from the newly exposed silica surface (2, 3, 11). The preparation of condensation resins of the resorcinolformaldehyde8HQ type has also been reviewed (14),and their properties have been further investigated (14-1 7). These resins offer higher exchange capacities, but low stability in acid solution and slower overall kinetic exchange rates. The immobilization of 8HQ onto polystyrene-divinylbenzene has also been investigated (14, 15, 17-19). These resins are apparently quite stable with respect to extremes of p H and can *Present address: Department of Oceanography, Florida State University, Tallahassee, FL 32306.
be produced with high total exchange capacity. Their overall kinetic exchange rates are also reported to be slow (15); however they have been successfully used in column applications at flow rates u p to 16 mL/min (19). The polymer substrate used in this investigation, Fractogel-TSK, consists of intertwined vinyl polymer agglomerates, which offer mechanical and chemical stability, high porosity, and high hydrophilicity due to the presence of ether linkages and hydroxyl groups. The hydroxyl groups (as secondary alcohols) also enable relatively simple chemical modification, and this feature is used in the immobilization of 8HQ via phenyl-azo linkages. The preparation time is relatively short (>20 h) and yields a chelating resin with exchange capacities comparable to the highest values reported for silica-immobilized 8HQ. The polymer itself exhibits no cation exchange capacity and does not concentrate dissolved organic species such as humic or fulvic acids. The following properties of the Fractogel-immobilized 8HQ product were investigated: stability with respect to extremes of pH; exchange capacity with respect to pH and flow rate; and trace metal cation collection efficiency from a seawater reference material. The modified resin was further utilized to investigate speciation and quantify concentrations of trace metals in prepared seawater solutions and in an organic-rich freshwater sample.
EXPERIMENTAL SECTION Reagents. All reagents were analytical grade and were used as received unless otherwise specified. Intermediate-purity water was prepared by distillation of deionized water by use of a glass still equipped with a quartz immersion heater. High-purity water was prepared in a “clean”laboratory (positive-pressureClass-100 filtered air supply) using a Milli-Q (Millipore)system. High-purity acids were prepared in the clean lab by subboiling point quartz distillation of analytical grade acetic acid, HCl, and HN03. Fractogel TSK HW-75(F) (32-63 pm bead diameter, -7.5 nm pore diameter) was obtained from E. Merck and prepared in the following manner. A 50-mL volume of the resin slurry was washed three times with 100 mL of water to rinse away the NaN3 present as a preservative. The supernatant from each rinse was discarded after allowing the resin beads to settle for 30 min. The resin was then vacuum filtered onto a Whatman GF/F glass microfiber filter and rinsed with the following sequence of reagents: two 50-mL portions of 1.0 M NaOH; three 50-mL portions of H20;two 50-mL
0003-2700/86/0358-3031$01.50/00 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
portions of 1.0 M HC1; three 50-mL portions of H20;two 50-mL portions of EtOH; two 50-mL portions of acetone; two 50-mL portions of CHzClz. The resin was then allowed to air-dry. Apparatus. All pH measurements were made with a Schott-Gerate CG-818 pH meter and an Orion RX combination electrode. Atomic absorption measurements were performed with a Perkin-Elmer 2380 AAS (flame) or a Perkin-Elmer 4000 GAS (furnace, HGA-400) using optimized conditions similar to those recommended by the manufacturer and matrix-matched standards. Immobilization Procedure. A 5-g amount of the rinsed and dried Fractogel was placed in a solution of 95 mL of CHzCI,, 5 mL of Et,N, and 2 g of p-nitrobenzoyl chloride a t 40 O C for 1 2 h. The benzoylated product was filtered and rinsed with CH,C12 and allowed to air-dry. Reduction of the nitro group was accomplished in 3 h at room temperature in a solution of 5 g of Na2S204in 100 mL of H,O. The light yellow amine product was filtered and rinsed with HzO. Diazotization was accomplished in 45 min at 0 "C using a 100-mLsolution of 5 g of NaNO, in 0.2 M acetic acid. The product was filtered and rinsed with ice-cold H 2 0 and then added to a 50-mL solution of 1 g of 8-hydroxyquinolinein 95% EtOH. After 45 min the red-orange diazo-coupled 8HQ product (TSK-8HQ) was filtered and rinsed sequentially with two 50-mL portions of 0.5 M NaOH, three 50-mL portions of H20, two 50-mL portions of 1.0 M HC1, and three 50-mL portions of HzO. The resin was then stored as a slurry in H20. Characterization. All of the characterization experiments were carried out using 1.0-mL (gravity packed) volumes of resin contained in polypropylene columns (0.87 cm i.d.) supported by porous polyethylene frits. Diluted test solutions were made in intermediate-purity H 2 0 from commercial atomic absorption metal solutions (lo00 pprn). The pK of these solutions was poised by adding an appropriate amount of concentrated acetic acid (to yield a 0.03 M HACconcentration) followed by adjustment with 25+L portions of 7.0 M ammonia solution immediately before use (to avoid metal oxyhydroxide precipitation). Breakthrough curves were obtained as a function of flow rate using a 50 ppm Cu(1I) solution a t pH 6.0. Solution volumes of 100 mL were passed through the column and collected in 20 5-mL test tubes. After the resin was rinsed with three 5-mL volumes of HzO, the column was eluted with five 1.0-mL volumes of a 2.0 M HC1/0.1 M HN03 acid mixture. Copper concentrations on all fractions and eluates were then measured by flame AAS. To study the effects of pH on breakthrough capacity, a column was attached directly to the aspirator tubing of the flame AAS. Test solutions of 50 ppm Cu(I1) or 10 ppm Ca(I1) were prepared at various pH values and passed through the column at a measured flow rate of 6.8-6.9 mL/min. Breakthrough curves were then constructed by taking absorbance values off the chart-recorder curve at appropriate intervals. The column was eluted directly into the flame using three 5.0-mL volumes of the HCl/HN03 acid mixture and thoroughly rinsed with H,O before the next solution was introduced. The stability of the TSK-8HQ resin was investigated by treating the resin as a slurry in either 0.5 M NaOH or the HCl/HNO, acid mixture for various lengths of time. After thorough H20 rinsing, breakthrough curves were again obtained using a 50 ppm Cu(1I) solution (pH 6.0) a t a flow rate of 5 mL/min. The overall extraction/elution efficiency of the TSK-8HQ resin for a number of metals was investigated by using the NASS-1 seawater reference material (21). This study was carried out in the clean laboratory using quartz-distilled reagents. A 400-mL sample of NASS-1 was adjusted to pH 8 using 0.03 M HAC/", and passed through a 1mL TSK-8HQ column at 5 mL/min. The (pH 4.7) column was then rinsed with 10 mL of 0.2 M HAC/", to displace Ca2+and Mg2+,followed by 10 mL of H20. The column and was eluted with five 1-mL volumes 2.0 M HC1/0.1 M "03, the metal concentrations were determined by GFAAS. The column blank was determined by elution before and after use with five 1-mL volumes of the acid mixture. In a limited study of the speciation of trace metals in the presence of dissolved organic substances, sequential 1.0 mL columns of Fractogel TSK-75(F).Fractogel DEAE-650(M),and TSK-8HQ were utilized. Mixed metal seawater solutions (100 mI,i a e r e prepared at pH 5 . 5 with and without 10 mg/L of a
standard fulvic acid (obtained from the International Humic Substances Society) and passed through the sequential columns at 5 mL/min. In addition, a 100-mL volume of an organic-rich freshwater water sample (buffered to pH 5.5) was passed through the sequential columns at 5 mL/min. After the resin was rinsed with three 5 mL volumes of H,O, the columns were separated and eluted (0.2 mL/min) with 10 1.0-mL volumes of HCI/HN03 acid mixture. Metal concentrations were determined on these eluates, and on the bulk untreated sample, by flame AAS.
RESULTS AND DISCUSSION Immobilization Scheme. The reaction sequence described above is adapted from the scheme originally utilized in the modification of silica gel (2). Due to the presence of hydroxyl groups in the Fractogel material, the initial reaction with p-nitrobenzoyl chloride is favorable. The choice of CH2C1, as the solvent was due to its less carcinogenic nature (relative to chloroform). The presence of Et3N is necessary to complex the HCI produced in the esterification. The reaction conditions reflect a 4:l molar ratio of the acid chloride relative to resin hydroxyl groups (estimated from literature available from the manufacturer). Use of a 20:l molar ratio resulted in a product with identical exchange capacity. Use of a reflux system and/or a n organic solvent with a higher boiling point (such as CHC1,) might increase the rate and extent of reaction in this step. Reduction of the phenyl nitro group was apparently complete after 3 h in dilute dithionite solution. The production of elemental sulfur during this reaction has been reported ( 4 , 10) but was not observed during our synthesis. The diazotization reaction is reportedly complete after 30 min, and longer reaction times (2.5 h) should yield no increase in the ultimate exchange capacity (4). Effervescence during this step is probably an indication of diazo decomposition and should be avoided. T o avoid any degradation of the bound diazonium salt, the resin was quickly added t o the ethanolic 8HQ solution. Phenyl-azo coupling occurs immediately, as evidenced by the dramatic color change on the resin (deep red-orange). The total reaction capacities of other commercially available chemically modified Fractogel products (Fractogel DEAE-650, for example) are generally 2-fold higher than the cation chelating capacity of the TSK-8HQ resin produced in this synthesis. This suggests that the number of chemically reactive hydroxyl groups present on the resin is about 500 pmol/g, of which about 60% have been modified in this synthesis. Our efforts are continuing to optimize the reaction conditions at each step in the synthesis. Breakthrough Capacity Studies. Breakthrough capacity studies were chosen for the characterization of the TSK-8HQ resin since they are more illustrative of the results one might expect in dynamic column operation (7). While the breakthrough capacity is a function of many parameters, it is a useful way t o demonstrate the effect achieved by changing one parameter while holding all others constant. Breakthrough curves were obtained at various flow rates (2-8 mL/min) for a 1-mL TSK-8HQ column using 50 ppm Cu(I1) in 0.03 M HAC/", at pH 6.0. These curves showed no significant differences indicating that film-diffusion control, or a combination of film- and particle-diffusion control, may have been rate limiting under these experimental conditions (20). The dynamic exchange capacity, determined by subsequent elution of the column, ranged from 39 to 43 Fmol of Cu(II)/mL of resin. Similar experiments were also performed by using a seawater matrix a t pH 6.0 with comparable results. Since it has been noted that continued metal extraction may occur after breakthrough (13,ZO), a more meaningful measure of capacity could be the amount of metal collected a t greater than 95% efficiency. While the 5-mL fractions collected in these experiments did not allow precise resolution, those breakthrough capacities were estimated directly from the
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
3033
, ."
0
0.5.. V
0.0s
io
io
30 Cu ( 11)
Cu ( I I 1
( umoles
io
50
60
70
io
50
60
70
80
( urnoles)
1
Figure 1. Copper(I1) breakthrough curves for a 1.0-mL column of TSK-8HQ as a function of pH. C/Cois the effluent/infiuent concentration. Test solution was 50 pprn Cu(II), 0.03 M HAC/",; flow rate was 6.9 mL/min.
curves where C/C, = 0.05. They ranged from 25 to 30 pmol of Cu(II)/mL of resin over the flow range investigated. The apparent total exchange capacity of a 1-mL TSK-8HQ column was determined by using a 100 ppm Cu(I1) solution (50 mL, pH 6.3) a t the natural column flow rate (-0.2 mL/min). The breakthrough capacity a t 95% removal efficiency was 40 pmol of Cu(II)/mL of resin (after passage of 25 mL of the solution). The apparent (dynamic) total exchange capacity (determined by column elution after passage of the entire 50 mL solution volume) was 50 pmol of Cu(II)/mL of resin. Since the dry weight of a 1-mL column of TSK-8HQ is approximately 0.17 g, this represents an apparent total capacity of 294 pmol of Cu(II)/g of resin, which is greater than the highest values reported recently for silica-immobilized 8-HQ (5, 7). This experiment is equivalent to a batchequilibration experiment since the time of exposure of the entire column to the 100 ppm Cu(I1) solution is over 2 h. Effects of pH. The effect of pH on the breakthrough capacity was determined for Cu(I1) and Ca(I1) via direct attachment of a 1-mL TSK-8HQ column to the aspirator tubing of the flame AAS. Figure 1 shows the Cu(I1) breakthrough curves obtained by using a 50 ppm Cu(I1) solution a t a flow rate of 6.9 mL/min from pH 3.0 to 7.0. The breakthrough capacities (at 95% removal efficiency) ranged from 13 pmol of Cu(II)/mL of resin at pH 3.0 to 27 pmol of Cu(II)/mL of resin at pH 7.0, illustrating the competition of protons for exchange sites at low pH. To illustrate the preference of the immobilized 8HQ for transition-metal cations over alkaline-earth cations, breakthrough curves were obtained by using a 10 ppm Ca(I1) solution at a flow rate of 6.8 mL/min from p H 3.0 to 8.0. The breakthrough capacities (at 95% removal efficiency) ranged from less than 0.25 pmol of Ca(II)/mL of resin at pH 3.0 to about 7 pmol/mL of resin at pH 8.0. The retention of Cu(I1) relative to Ca(I1) therefore ranges from a molar ratio of 52/1 a t pH 3.0 to 9 / 1 at pH 7.0. It is difficult to directly compare these pH-capacity studies with previous work due t o differences in resin particle size, total exchange capacity, column dimensions, and Cu(I1) solution concentrations. However, a flow rate of 6.9 mL/min (50 ppm Cu(II), pH 5.0) through a 1-mL TSK-8HQ column yields a solution residence time in the column of approximately 7 s, which is similar to a reported experiment using silica-immobilized 8HQ in a 0.16-mL column a t a flow rate of 1 mL/min (20 ppm Cu(II), pH 5.0) (7). At pH 5.0, the fraction of column utilization at 95% collection efficiency was approximately the same for these two independent experi-
0
0.5.,
00. 10
20
30
40
Cu (11 ) (urnoles)
Figure 2. Copper(I1) breakthrough curves for 1.O-mL TSK-8HQ calumns after exposure to (A) 2.0 M HCI/O.l M HNO, and (B) 0.5 M NaOH. C/Cois the effluent/influent concentration. Test solution was 50 ppm Cu(II), 0.03 M HAC/",, pH 6.0; flow rate was 5.0 mL/min.
ments (42% vs. 48%). This indicates that the various kinetic factors influencing the breakthrough capacity have acted in a compensatory manner between these two sets of conditions. Resin Stability. Breakthrough curves for Cu(I1) (50 ppm, pH 6.0, flow rate 5 mL/min) were also obtained on 1-mL TSK-8HQ columns after exposure of the resin to extremes of pH for various time intervals. The TSK-8HQ resin is quite stable in acid solution (2.0 M HCl/O.l M "OB) as illustrated in Figure 2A. After 2 h of acid treatment the breakthrough capacity (at 95% efficiency) was 34 pmol of Cu(II)/mL of resin. After 240 h the breakthrough capacity had decreased only slightly to 30 pmol of Cu(II)/mL of resin. Treatment of the TSK-8HQ resin with 0.5 M NaOH for 2 h had little effect on the breakthrough capacity (Figure 2B), despite the obvious reddish coloration of the basic solution indicating base-catalyzed hydrolysis of the resin-benzoyl ester linkages. After 24 h of exposure the breakthrough capacity had decreased to 29 pmol of Cu(II)/mL of resin and the curve exhibited a steeper slope in the region after initial breakthrough. After 48 h the breakthrough capacity had decreased by only 32% (relative to untreated TSK-8HQ) to 23 pmol of Cu(II)/mL of resin. The basic solution was, by this time, a deep ruby red color, due to continued hydrolysis of the bound groups. By comparison, a silica-immobilized 8HQ lost 38% of its total Cu(I1) exchange capacity after 24 h at pH 12 (equivalent to 0.01 M NaOH) due to extensive hydrolysis of the silica support ( 3 ) . From the change in the shape of the breakthrough curve after 24 h of exposure to 0.5 M NaOH, one may conclude that the overall rate of ion exchange had increased, possibly by the loss of less-accessable chelation sites (20). An alternative
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
Table I. Analysis of the Seawater Reference Material NASS-I Using a 1-mL Column of TSK-BHQ Resin
metal found, metal
column blank, ng
nhl
reported value,“ nM
Mn Ff3 Ni
2 12 f 6
0.38
Zn Cd
7f4
