Development of Bifunctional Anion-Exchange Resins with Improved

Jul 29, 2000 - Development of Bifunctional Anion-Exchange Resins with Improved Selectivity and Sorptive Kinetics for Pertechnetate: Batch-Equilibrium ...
0 downloads 0 Views 71KB Size
Download PDF
Environ. Sci. Technol. 2000, 34, 3761-3766

Development of Bifunctional Anion-Exchange Resins with Improved Selectivity and Sorptive Kinetics for Pertechnetate: Batch-Equilibrium Experiments PETER V. BONNESEN,† G I L B E R T M . B R O W N , * ,† S P I R O D . A L E X A N D R A T O S , * ,‡ LAURIE BATES BAVOUX,† DEREK J. PRESLEY,† VIJAY PATEL,‡ ROBERT OBER,‡ AND BRUCE A. MOYER† Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6119, and Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996

A novel class of strong-base anion-exchange resins, bearing two different types of exchange sites, is described. These bifunctional resins have anion-exchange sites composed of two separate and differently sized trialkylammonium groups. In pertechnetate (TcO4-) uptake experiments from a solution matrix containing anions commonly encountered in groundwater, bifunctional resins containing exchange sites derived from tri-n-hexylamine in combination with either triethylamine, tri-n-propylamine, or tri-n-butylamine were shown to possess superior 24-h distribution ratios (Kd) for pertechnetate over all monofunctional anion-exchange resins (derived from a single trialkylamine) examined. For example, although monofunctional resins containing a tri-n-hexylammonium exchange site possessed excellent selectivity for pertechnetate over other anions, the exchange kinetics were poor. The superior performance of these resins can be attributed to the combination of a large trialkylammonium site for enhanced selectivity for large anions and a smaller trialkylammonium site for the retention of good exchange kinetics.

Introduction The radionuclide technetium-99 (Tc-99) is a contaminant in groundwater at several United States Department of Energy (DOE) facilities, including sites at Oak Ridge, TN; Paducah, KY; and Portsmouth, OH. In the oxidizing conditions present in near-surface groundwaters, the principal form of Tc-99 is the highly soluble TcO4- (pertechnetate) anion (1). The mobility of pertechnetate in biogeochemical cycles coupled with a long half-life of 213 000 yr makes its presence in groundwater an environmental concern (2, 3). Although Tc99, a soft β-emitter with a maximum energy of 292 keV, is not considered to be a significant human health risk, the * Corresponding authors phone: (865)576-2756 (G.M.B.)/ (865)974-3399 (S.D.A.); e-mail: [email protected] (G.M.B.)/ [email protected] (S.D.A.). † Oak Ridge National Laboratory. ‡ University of Tennessee. 10.1021/es990858s CCC: $19.00 Published on Web 07/29/2000

 2000 American Chemical Society

potential for bioaccumulation and toxicological effects in both plants and mammals (4) has generated interest in developing practical methods for removing it from contaminated groundwater. A variety of solid-phase materials have been studied for the removal of pertechnetate from groundwater. Sorbent materials investigated have included iron filings (5) and activated carbon (6). Ion-exchange materials investigated for the removal of pertechnetate from (simulated) groundwater have included strong-base anion-exchange resins (containing cationic nitrogen centers) such as Dowex SRBOH and weak-base polyvinyl pyridine resins such as Reillex HP (7). Strong-base anion-exchange resins have also successfully been shown to remove pertechnetate from nitric acid solutions [Reillex HPQ (8), Dowex 1-X8 (9, 10)] and from alkaline nuclear high-level waste simulants [Reillex HPQ (11)]. Other solid-phase materials that can successfully remove pertechnetate from high ionic strength solutions (although not from low ionic strength solutions such as groundwater) include hydrophilic aqueous biphasic extraction chromatography (ABEC) resins prepared from grafting poly(ethylene glycols) to chloromethylated polystyrene (12). In resin ion exchange, factors that increase the affinity of an anion for the resin include small charge-to-size ratio and low-hydration energy (13). Since pertechnetate is larger and has a lower hydration energy than most other anions encountered in groundwater (such as nitrate, chloride, and sulfate) or in tank waste (13,14), there is a natural bias toward exchanging pertechnetate preferentially over the other anions (nitrate, chloride, sulfate, etc.) in the solution. In fact, pertechnetate may be extracted by resins such as Reillex HPQ (11) and Dowex SRB-OH (7) even when the ratio of the competing anion (e.g., nitrate) is more than 10 000 times the pertechnetate concentration. However, this natural bias can be enhanced by chemical modification of the resin. Such modifications include altering the size and shape of the cationic exchange sites and altering the polymer cross-link density. We describe here the results of our efforts to design, synthesize, and evaluate strong-base resins with enhanced bias (and hence selectivity) for pertechnetate over other anions commonly found in groundwater. Improving the selectivity for pertechnetate has the advantage that more bed volumes of groundwater can be processed before significant breakthrough occurs and that greater decontamination factors can be realized. However, as a part of this study, it was observed that improving the selectivity could come at the cost of reducing the rate of exchange. We thus also report here our solution to that problem by designing and synthesizing new resins containing two different quaternary ammonium exchange sites that possess both enhanced selectivity for pertechnetate and acceptable exchange kinetics.

Materials and Methods All chemicals were reagent grade or better and were used as received. Deionized water was obtained from a Barnstead Nanopure filtering system (minimum resistivity 18 MΩ). Technetium-99. NIST-traceable ammonium pertechnetate (NH499TcO4) in water was obtained from Isotope Products Laboratories, Burbank, CA, typically at an activity of 0.2 mCi/ mL. Working stock solutions with a pertechnetate concentration of 3.0 mM (5.0 µCi/mL) were prepared by appropriate dilution of the primary stock solution with deionized water and were used to prepare all groundwater test solutions (see below). VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

3761

Anion-Exchange Resins. The commercially available resins Amberlite IRA-904 and IRA-900 (Rohm and Haas) were purchased from Aldrich Chemical Co. Other resins were generously supplied by the respective manufacturer as follows: Reillex HPQ (Lot 40404AD) from Reilly Industries, Inc., Indianapolis, IN; Purolite A-520E and A-850 from The Purolite Company, Bala Cynwyd, PA; and Sybron Ionac SR-6 and SR-7 from Sybron Chemicals, Inc., Birmingham, NJ. All laboratory-prepared resins were synthesized in the chloride form at the University of Tennessee, Knoxville. Each resin supplied to Oak Ridge National Laboratory (ORNL) was characterized by exchange capacity and moisture content. All resins were of the strong-base anion-exchange type with a macroporous morphology, except for Purolite A-850, which has a gel-type morphology. For uniformity, all resins were initially in the chloride form prior to preequilibration with matrix solutions. The laboratory-prepared resins were used as received from the University of Tennessee. The commercial resins were treated by washing successively with 1.0 N NaOH (1 L/10 g of resin), deionized water until the pH of the eluate was neutral, 1.0 N HCl (1 L/10 g of resin), and again with deionized water until the pH of the eluate was neutral. The resins were then partially dried to a slightly moist consistency by filtration through a Bu ¨ chner funnel. A percent-solids determination was performed on each resin by drying a weighed sample (150-200 mg) of Bu ¨ chner-dried resin for 2 h at 110-120 °C and reweighing the oven-dried resin. The total anionexchange capacity (TAEC) for the laboratory resins was determined by performing a Mohr titration on the chloride ion displaced when the resin was treated with excess sodium nitrate. TAEC information for the commercial resins was obtained from the manufacturer’s literature. In all cases, the TAEC is reported as milli-equivalents per gram of the ovendried resin. Laboratory Resin Syntheses. What follows is a typical procedure for a bifunctional resin (15). About 15 g of 5% divinylbenzene-cross-linked poly(vinylbenzyl chloride) macroporous copolymer beads (40-60 U.S. Standard mesh) was swollen overnight in 100 mL of dioxane. For the first amination, a solution comprised of 76 mL of tri-n-hexylamine, 125 mL dioxane, and 5 mL water was added, and the mixture was refluxed for 17 h. After amination, the beads were washed successively with dioxane, ethanol, ethanol/water (1:1 vol/ vol), 4% HCl, and water and then eluted with more water, 4% NaOH, water, 4% HCl, and finally water until the pH of the eluate was near neutral. The second amination was then conducted as follows: 26.6 g of the above material (after filtering off the water) was refluxed 17 h in a solution comprised of 210 mL of dioxane, 90 mL of tri-n-propylamine, and 10 mL of water. The wash and elution sequence was as above. The resin beads had the following characteristics: total solids ) 41.8 ( 1.2%; total anion exchange capacity ) 2.13 mequiv/g; anion exchange capacity due to tri-n-hexylamine site ) 0.99 mequiv/g; anion exchange capacity due to trin-propylamine site ) 1.14 mequiv/g. All aminations and formation of bifunctional polymers followed the sequence outlined for this resin. Monofunctional resins were prepared from 10% divinylbenzene-cross-linked poly(vinylbenzyl chloride) macroporous copolymer beads following the procedure for the first amination, unless otherwise noted. Groundwater Test Solution. An analysis of contaminated groundwater from Well 66 at Paducah (7) revealed the concentration of technetium to be in the range of 0.3-2.2 nM, while the concentrations of the most abundant anions chloride, nitrate, and sulfate lay in the ranges of 0.56-0.90, 0.04-0.15, and 0.10-0.26 mM, respectively (average values are provided in Table 1). A “groundwater test solution” composed of ammonium pertechnetate (NH499TcO4, 6.0 mM) and the sodium salts of the anions chloride, nitrate, and 3762

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 17, 2000

TABLE 1. Concentrations of Sodium, Chloride, Sulfate, Nitrate, and Tc-99 in Both Groundwater Test Solution and Paducah Well 66a

species

molarity in groundwater test solution

mol ratio of species to Tc-99

molarity in Paducah Well 66

mol ratio of species to Tc-99

sodium chloride nitrate sulfate Tc-99

1.8 × 10-1 6.0 × 10-2 6.0 × 10-2 6.0 × 10-2 6.0 × 10-6

3.0 × 104 1.0 × 104 1.0 × 104 1.0 × 104 1

8.9 × 10-4 7.3 × 10-4 1.0 × 10-4 1.2 × 10-4 1.7 × 10-9

5.2 × 105 4.3 × 105 5.9 × 104 7.1 × 104 1

a Average values cited in ref 7. The ratio of each ion to Tc-99 is also given for both solutions.

sulfate (60 mM in each anion) was prepared for test purposes, and all pertechnetate sorption experiments reported here were obtained using this solution. The molar ratio of each of these other anions to pertechnetate in this solution was set at 104, which is within an order of magnitude of the ratio of approximately 105 found in the actual groundwater (see Table 1). Although the concentrations of these anions and pertechnetate are higher than those in the actual groundwater, they permit testing to take place with reasonable volumes of solution and allow the Kd values (defined below) to fall in a useful range for comparing one resin to another. Batch-Equilibrium Sorption Studies. For batch-equilibrium studies, testing employed a quantity of Bu¨chnerdried resin equivalent to ∼200 mg of oven-dried resin (calculated on the basis of the percent-solids of the particular Bu ¨ chner-dried sample) to provide test uniformity and to ensure that the resin sample was sufficiently large and homogeneous to be representative of the bulk material. The resin samples were accurately weighed into 250-mL Teflon FEP bottles and first “preequilibrated” with groundwater test solution containing no pertechnetate by shaking the resins four times each with fresh 15-mL portions of the test solution for 15-min intervals. The preequilibration was performed to afford a linear adsorption isotherm and to ensure that pertechnetate uptake was measured in an environment where the resin was already at equilibrium with the bulk anions in solution. Two methods were employed to change the test solution during the preequilibrations. In the first method, the solution was withdrawn each time from the resin using a polyethylene transfer pipet that had the end drawn out to a fine tip. For the second method, at the end of each 15-min treatment, the resin slurry was filtered through a 1.0 cm wide by 10.0 cm long Kontes Flex-Column, and the resin was then rinsed back into the FEP bottle using fresh matrix solution. The latter procedure was preferred when using very fine (>50 mesh) resins. Following the equilibration, the resins were then shaken with 100 mL of the pertechnetate-containing test solution for 24 h on an Eberbach reciprocating shaker at 22 ( 2 °C. Aliquots of 8 mL were withdrawn at the end of the 24-h equilibration period (except for kinetic studies, where typically 4.0-mL aliquots were withdrawn at various time intervals). The aliquots were filtered through a 13-mm Acrodisc (Gelman) containing a 0.45-µm PTFE membrane directly into 20-mL standard polyethylene scintillation vials containing 10.0 mL of Packard Ultima Gold XR scintillation cocktail. All samples to be counted, including the stock matrix solution, were filtered for uniformity. The relative standard deviation for volumes delivered in this manner was found to be 0.3%. Following a dark-adaption period of at least 30 min, the Tc-99 activity in the vials was determined using a Packard Tricarb model 2700TR liquid-scintillation counter using standard counting techniques. The amount of Tc sorbed onto the resin was determined by subtracting the Tc activity in the equilibrium solution from the total Tc activity in the starting solution. The distribution ratio Kd (in mL/g) was

TABLE 2. Tc-99 Distribution Coefficient Kd for Selected Commercial Strong-Base Resins from Groundwater Test Solution

resina

anionexchange group

Reillex HPQ Purolite A-850 Amberlite IRA-900 Amberlite IRA-904 Purolite A-520E Sybron Ionac SR-7 Sybron Ionac SR-6

Me-pyridyl (methyl)3N(methyl)3N(methyl)3N(ethyl)3N(propyl)3N(butyl)3N-

total anionmesh Kd exchange size capacity (mL/g) (U.S. (mequiv/g, (24 h) dry wt)b standard) ((5%) 3.30 3.90 3.6 2.5 2.80 2.20 1.80

30-60 16-50 16-50 20-50 16-50 16-50 16-50

4540 102 2460 7590 12800 6950 20700

a All resins are type 1 strong-base, macroporous, and have a polystyrene backbone, except for Reillex HPQ, which is 70% methylated poly(vinyl pyridine), and Purolite A-850, which is a gel resin with a polyacrylic backbone and trimethyl amino group attached to a propyl linker to the acrylic ester functionality. All resins were in the chloride form and treated as described in the Experimental Section. b From company-provided technical literature for each resin. The value of 3.3 mequiv/g for Reillex HPQ represents only the amount of methylated pyridine sites and does not include the additional 1.3 mequiv/g of free pyridine sites. The values for Purolite A-850 and for the Amberlite resins were calculated from data in units of equiv/wet L provided respectively by Purolite (1.25 equiv/L), and Rohm and Haas (1.0 equiv/L for IRA-900 and 0.70 equiv/L for IRA-904).

determined as shown in the equation below, in a manner similar to that described by Ashley et al. (8):

Kd )

[(Tc99 dpm)total - (Tc99 dpm)solution]/g of resin (Tc99 dpm)solution/mL of solution

)

mmol of TcO4-/1g of dry resin mmol of TcO4-/1 mL of solution An alternative ratio KdEq (in mL/mequiv) was calculated by dividing Kd by the TAEC of the particular resin. The Kd value provides a measure of the sorptive ability for technetium per gram of resin, and KdEq provides a measure of the relative affinity per exchange site for technetium and, therefore, a measure of relative selectivity.

Results and Discussion Pertechnetate Sorption by Commercial Resins. The pertechnetate distribution ratio Kd for a 24-h equilibration period was obtained for the commercially available resins Reillex HPQ, Amberlite IRA-900 and IRA-904, Purolite A-520E and A-850, and Sybron Ionac SR-6 and SR-7 (Table 2). This selection of resins has either been previously used for pertechnetate sorption [Reillex HPQ (8, 11) and Purolite A-520E (16)] or represents a variation of polymer backbone and size of the tertiary amine. All the resins except Reillex HPQ and Purolite A-850 are type I strong-base macroporous resins with a polystyrene backbone. As shown in Table 2, these resins differ in the size of the tertiary amine used for the amination: Amberlite IRA-900 and IRA-904 differ in crosslink density, with IRA-904 possessing a significantly higher cross-link density than IRA-900, according to the manufacturer. Reillex HPQ is 70% methylated polyvinyl pyridine, and Purolite A-850 is a gel-type polyacrylic resin with a pendant trimethylamine group attached by a propyl linker to the acrylic ester functionality. The highest Kd was obtained with Sybron Ionac SR-6, the resin possessing the bulkiest trialkylammonium group (tri-n-butyl). This was not unexpected, as it is well-known that large hydrophobic anions prefer to be associated with large hydrophobic cations (13). Interestingly however, Sybron Ionac SR-7, with tri-n-propylammonium

TABLE 3. Effect of Quaternary Ammonium Group Size and Type on Tc-99 Distribution Coefficient (Kd) and Kd per Exchange Site (KdEq) for Laboratory Resins Prepared from Amination of Poly(vinylbenzyl chloride)a amine functional group (methyl)3N (ethyl)3N (propyl)3N (butyl)3N (hexyl)3N (isooctyl)3N (ethanol)3N (methyl)2(dodecyl)N pyridine 1-methylimidazole

total anionKd (mL/g) KdEq exchange capacity (24 h) (mL/mequiv) (mequiv/g, dry wt) ((5%) (24 h) ((5%) 3.77 2.84 2.33 1.66 0.98 0.70 2.71 2.08 2.59 2.68

6 350 16 200 22 300 31 800 1 540 942 1 590 20 300 5 070 8 420

1 680 5 700 9 570 19 200 1 570 1 350 587 9 760 1 960 3 140

a All resins possess a 10% DVB cross-link density, are 40-60 mesh, and are in the chloride form.

exchange sites, had a lower 24-hour Kd than either of the two resins with smaller trialkylammonium groups, Purolite A-520E (triethyl) or Amberlite IRA-904 (trimethyl). Other factors besides the size of the quaternary ammonium group, such as the nature and extent of cross-linking, can also influence the selectivity for pertechnetate over the nitrate, chloride, and sulfate anions in the solution. It has been observed that higher interchain cross-linking (as afforded by the percentage of divinylbenzene, DVB, in the resin) can enhance the affinity for less hydrated anions (e.g., pertechnetate) over more hydrated anions (e.g., nitrate) (13). Amberlite IRA-904, with a stated higher DVB cross-link density than IRA-900, does in fact have a higher affinity for pertechnetate. Pertechnetate Sorption by Laboratory-Prepared Monofunctional Resins. Toward a goal of developing a resin with superior selectivity and affinity for pertechnetate, it was important to obtain in a controlled manner a fundamental understanding of the factors that contribute to increased pertechnetate selectivity. To that end, the effects that changes in the microenvironment of the exchange sites have on pertechnetate selectivity were investigated so that a scientific basis for understanding which parameters have the most influence on selectivity could be obtained. A known method for enhancing the selectivity for less-hydrated anions such as pertechnetate (and perchlorate and perrhenate) over morehydrated anions such as nitrate is to increase the hydrophobicity and steric bulk of the alkyl groups surrounding the anion-exchange site in the resin. Thus, a series of quaternary ammonium resins having an identical poly(vinylbenzyl chloride)-10% divinylbenzene copolymer backbone was prepared in which the architecture of the quaternary ammonium exchange site was systematically varied. The importance of the size of the trialkylammonium group was investigated for the homologous series methyl through n-hexyl (with the exclusion of n-pentyl). The pertechnetate distribution ratio Kd for a 24-h equilibration period for these five resins (Table 3) steadily increases with increasing size of the alkyl group up to n-butyl; n-hexyl gave a substantially lower value. Since the total anion-exchange capacity (TAEC) decreases with increasing bulk of the trialkylammonium group, a way to gauge changes in the affinity per exchange site on the resin, rather than by the weight of the resin, is to divide the Kd by the TAEC to afford the parameter KdEq. The KdEq increased as the size of the alkyl group increased up to butyl, but as previously noted, the value of KdEq for the trin-hexyl derivative was lower than expected based on the homologous series trimethyl to tri-n-butyl. VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

3763

TABLE 4. Effect of Divinylbenzene (DVB) Cross-Link Density (%) on Tc-99 Distribution Coefficient (Kd) and Kd per Exchange Site (KdEq) for Selected Monofunctional Laboratory Resinsa

amine functional group

% DVB cross-link density

total anionexchange capacity (mequiv/ g, dry wt)

(propyl)3N (propyl)3N (propyl)3N (propyl)3N (hexyl)3N (hexyl)3N

5 10 15 25 5 10

2.58 2.33 1.58 1.32 1.02 0.98

a

FIGURE 1. Pertechnetate distribution Kd per exchange site, KdEq, for selected monofunctional and commercial resins as a function of time. Uncertainity is (5%. Increasing the chain length (and hence the hydrophobicity) of the alkyl group of the trialkylammonium group results in an overall decrease in the rate of anion exchange. The tri-n-hexylammonium resin possesses a high degree of hydrophobicity, and accordingly it was observed that 24 h was insufficient time for the anion-exchange reaction to reach equilibrium. The value of KdEq as a function of equilibration time out to 336 h was determined for the tri-n-propyl, trin-butyl, and tri-n-hexyl resins along with commercial resins Purolite A-520E and Sybron Ionac SR-6. The results plotted in Figure 1 show that in fact the tri-n-hexylammonium resin is not at equilibrium at 24 h and that the apparent KdEq continues to climb with equilibration time, surpassing that of the tri-n-butylammonium resin, affording a KdEq value of over 25 000 mL/mequiv, the highest of all resins tested. The other resins appear to be at or within 10% of equilibrium at 24 h. Thus, superior pertechnetate selectivity can be achieved with the larger trialkylammonium group but at the expense of slower exchange kinetics. Three other trialkylamines, tri-isooctylamine, tris(2-hydroxyethyl)amine, and dimethyldodecylamine, were investigated as well as the cyclic amines pyridine and 1-methylimidazole. As with the tri-n-hexylammonium resin, the triisooctylammonium resin gives an initially low KdEq that increases with equilibration time. The alcohol functionality of the hydrophilic tris(2-hydroxyethyl)ammonium resin provides a site for hydrogen bonding to both water and anions. Accordingly, the relative affinity for less-hydrated anions such as pertechnetate is less than for more-hydrated anions such as nitrate. The amount of steric congestion surrounding the cationic site affects the degree of charge separation between the cationic nitrogen and the anion. With dimethyldodecylamine, pyridine, and 1-methylimidazole the geometry about the cationic nitrogen centers is not as sterically congested as it would be for a tripodal tertiary alkylamine such as the trin-propyl. The somewhat planar geometry about the cationic nitrogen center occurring for the pyridyl and 1-methylimidazolyl resins results in less charge separation than would 3764

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 17, 2000

Kd (mL/g) (24 h) ((5%)

KdEq (mL/mequiv) (24 h) (( 5%)

20 300 22 300 20 400 17 300 9 030 1 540

7 870 9 570 12 900 13 100 8 850 1 570

All resins are 40-60 mesh and are in the chloride form.

be expected with triethylamine, and despite having one very large alkyl group, the KdEq obtained with dimethyldodecylamine is essentially the same as that obtained with tri-npropylamine. Effect of Cross-Linking. It has been noted that increasing the DVB cross-link density, and thus the rigidity of the polymer matrix, reduces the ability of the functional groups (in this case quaternary ammonium groups) to congregate in hydrated domains, thus reducing the affinity for the more hydrated anions and enhancing the selectivity for the less hydrated anions (13). The effect of DVB cross-link density on the pertechnetate Kd was examined for the tri-n-propylammonium and tri-n-hexylammonium resins (Table 4). As expected, for the tri-n-propyl series the Kd per exchange site KdEq increases with increasing DVB cross-link density. However, the increased percentage of DVB in the resin lowers the TAEC. Thus, the sorptive capacity of the resin (Kd) is controlled by two opposing factors, and at higher cross-link densities, the lower TAEC outweighs the increased selectivity, leading to a lower Kd. Cross-linking can also impact the exchange kinetics, especially for the more kinetically slow hydrophobic resins containing the larger trialkylammonium groups. For example, the kinetics of the pertechnetate uptake reaction are significantly faster for the tri-n-hexylammonium resin at 5% cross-linking as compared to the rate of uptake for the same derivative at 10% cross-linking as shown by the 24-h Kd values in Table 4. Pertechnetate Sorption by Bifunctional Resins. From the studies with the monofunctional resins, it was apparent that what was required to produce a practical resin with superior affinity for pertechnetate was a resin possessing the selectivity of the hydrophobic tri-n-hexylammonium resins but also the fast exchange kinetics of the smaller trimethylammonium through tri-n-propylammonium resins. To balance and optimize the competing properties of increased pertechnetate selectivity with large, bulky tertiary amines with the decreased rate of exchange and total anion-exchange capacity, a class of resins was prepared that contained a mixture of both large trialkylammonium groups (to enhance selectivity) and small trialkylammonium groups (to enhance exchange kinetics). Specifically, a series of “bifunctional” resins was prepared by first reacting a more lightly crosslinked (5%) poly(vinylbenzyl chloride)-divinylbenzene copolymer with first a large tertiary amine (tri-n-hexylamine) and then reacting the unreacted chloromethyl sites with a smaller amine (trimethyl through tri-n-butylamine). One additional resin that combined triisooctylamine with tri-npropylamine was prepared for comparative purposes. Exchange capacity and pertechnetate sorption data for these resins are given in Table 5. With the exception of the tri-n-hexyl/trimethylamine combination, all of these bifunctional resins display not only larger values of Kd, indicating

TABLE 5. Tc-99 Distribution Coefficient (Kd) and Kd per Exchange Site (KdEq) for Selected Bifunctional Laboratory Resinsa first amine functional group

exchange capacity (mequiv/g, dry wt)

second amine functional group

exchange capacity (mequiv/g, dry wt)

total anionexchange capacity (mequiv/g, dry wt)

Kd (mL/g) (24 h) ((5%)

KdEq (mL/mequiv) (24 h) ((5%)

(hexyl)3N (hexyl)3N (hexyl)3N (hexyl)3N (isooctyl)3N

0.70 1.18 0.99 1.18 0.70

(methyl)3N (ethyl)3N (propyl)3N (butyl)3N (propyl)3N

2.50 1.18 1.14 0.58 1.24

3.20 2.36 2.13 1.76 1.94

13 200 37 300 43 300 39 200 37 800

4 120 15 800 20 300 22 300 19 500

a

All resins possess a 5% DVB cross-link density, are 40-60 mesh, and are in the chloride form.

cationic exchange sites have a bias for sorption of large, poorly hydrated anions such as pertechnetate. A consideration of the rate at which sorption takes place led us to prepare bifunctional resins in which tri-n-hexylammonium sites, providing enhanced bias for pertechnetate sorption, are paired up with a smaller trialkylammonium group for improved kinetics. These resins having anion-exchange sites composed of two separate and differently sized trialkylammonium groups possess higher sorptive capacities in a given period of time for pertechnetate over resins possessing exchange sites derived from only one tertiary amine. The new materials improve on the problem of poor exchange capacity and kinetics characteristic of the most selective resins containing only one type of large exchange site. The expectation that improved selectivity for pertechnetate sorption would lead to enhanced performance in processing contaminated groundwater has been experimentally realized (16, 17).

Acknowledgments

FIGURE 2. Pertechnetate distribution Kd per exchange site, KdEq, for selected bifunctional resins (combined exchange sites) as a function of time. Selected monofunctional and commercial resins included for comparison. Uncertainity is (5%. a given mass of resin is more effective in removing pertechnetate from solution, but also larger values of KdEq, indicating that these resins are more selective than the monofunctional resins. The 24-h Kd for the tri-n-hexylamine/tri-n-propylamine combination is the highest, being approximately twice the 24-h Kd of any commercial resin and 50% better than any of the laboratory-prepared monofunctional resins. The kinetic behavior of selected bifunctional resins as compared to selected monofunctional laboratory and commercial resins is shown in Figure 2. The KdEq for the tri-nhexyl/triethylammonium resin increases only slightly on increasing the equilibration time from 24 to 168 h, comparatively similar to the commercial resins Purolite A-520E and Sybron Ionac SR-6. The more hydrophobic tri-n-hexyl/ tri-n-propylammonium and tri-n-hexyl/tri-n-butylammonium resins not surprisingly do show some improvement with increasing equilibration time, but they display better uptake kinetics than does the monofunctional tri-n-hexylammonium resin. Even after 2 weeks, it appears that the Kd value for pertechnetate uptake for the monofunctional trin-hexylammonium resin did not plateau, indicating that this resin has not reached equilibrium. The trend in equilibrium distribution coefficients for pertechnetate sorption to a series of monofunctional resins confirms the expectation of an electrostatic model (13). Large

We thank Dr. Baohua Gu (ORNL) for many helpful suggestions. This research was sponsored by the Efficient Separations and Processing Cross-Cutting Program of the Office of Science and Technology, Office of Environmental Management, U.S. Department of Energy, under Contract DE-AC0500OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.

Literature Cited (1) Pourbaix, M. Atlas of Electrochemical Equilibria; Pergamon Press: Oxford, 1966; p 24. (2) Yoshihara, K. In Technetium and Rhenium: Their Chemistry and Its Applications; Yoshihara, K., Omori, T., Eds.; Topics in Current Chemistry Vol. 176; Springer-Verlag: Heidelberg, 1996. (3) Meyer, R. E.; Arnold, W. D.; Case, F. I.; O’Kelley, G. D. Radiochim. Acta 1991, 55, 11-18. (4) (a) Garten, C. T., Jr.; Lomax, R. D. Health Phys. 1989, 57, 299307. (b) Zeevaert, Th.; Vandecasteele, C. M.; Kirchmann, R. Health Phys. 1989, 57, 337-343. (c) Gerber, G. B.; Hegala, M.; Vankerkom, J.; Kirchmann, R.; Maisin, J. R.; Lambiet-Collier, M. Health Phys. 1989, 57, 345-350. (5) Liang, L.; Gu, B.; Yin, X. Sep. Technol. 1996, 6, 111-122. (6) Gu, B.; Dowlen, K. A.; Liang, L.; Clausen, J. L. Sep. Technol. 1996, 6, 123-132. (7) Del Cul, G. D.; Bostick, W. D.; Trotter, D. R.; Osborne, P. E. Sep. Sci. Technol. 1993, 28, 551-564. (8) Ashley, K. R.; Ball, J. R.; Pinkerton, A. B.; Abney, K. D.; Schroeder, N. C. Solvent Extr. Ion. Exch. 1994, 12, 239-259. (9) Kawasaki, M.; Omori, T.; Hasegawa, K. Radiochim. Acta 1993, 63, 53-56. (10) Ihsanullah. Sep. Sci. Technol. 1994, 29, 239-247. (11) Ashley, K. R.; Ball, J. R.; Abney, K. D.; Turner, R.; Schroeder, N. C. J. Radioanal. Nucl. Chem. 1995, 194, 71-79. (12) Rogers, R. D.; Bond, A. H.; Griffen, S. T.; Horwitz, E. P. Solvent Extr. Ion Exch. 1996, 14, 919-946. (13) (a) Moyer, B. A.; Bonnesen, P. V. In Supramolecular Chemistry of Anions; Bianchi, A., Bowman-James, K., Garcia-Espan ˜ a, E., Eds.; VCH: New York, 1997, Chapter 1, and references cited therein. (b) Diamond, R. M.; Whitney, D. C. In Ion Exchange, VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

3765

A Series of Advances; Marinsky, J., Ed.; Marcel Dekker: New York, 1966; Vol. 1, pp 277-302. (14) Bonnesen, P. V.; Moyer, B. A.; Presley, D. J.; Armstrong, V. S.; Haverlock, T. J.; Counce, R. M.; Sachleben, R. A. Alkaline-Side Extraction of Technetium from Tank Waste Using Crown Ethers and Other Extractants; Report ORNL/TM-13241; Oak Ridge National Laboratory: Oak Ridge, TN, June 1996. (15) Alexandratos, S. D.; Brown, G. M.; Bonnesen, P. V.; Moyer, B. A. Bifunctional Anion-Exchange Resins with Improved Selectivity and Exchange Kinetics. U.S. Patent 6,059,975, May 9, 2000. (16) Gu, B.; Liang, L.; Brown, G. M.; Bonnesen, P. V.; Moyer, B. A.; Alexandratos, S. D.; Ober, R. A Field Trial of Novel Bifunctional

3766

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 17, 2000

Resins for Removing Pertechnetate (TcO4-) from Contaminated Groundwater; Report ORNL/TM-13593; Oak Ridge National Laboratory: Oak Ridge, TN, March 1998. (17) Gu, B.; Brown, G. M.; Bonnesen, P. V.; Liang, L.; Moyer, B. A.; Ober, R.; Alexandratos, S. D. Environ. Sci. Technol. 2000, 34, 1075-1080.

Received for review July 28, 1999. Revised manuscript received May 23, 2000. Accepted May 26, 2000. ES990858S