Environ. Sci. Technol. 2000, 34, 1075-1080
Development of Novel Bifunctional Anion-Exchange Resins with Improved Selectivity for Pertechnetate Sorption from Contaminated Groundwater B A O H U A G U , * ,† G I L B E R T M . B R O W N , ‡ PETER V. BONNESEN,‡ LIYUAN LIANG,† BRUCE A. MOYER,‡ ROBERT OBER,§ AND SPIRO D. ALEXANDRATOS§ Environmental Sciences Division and Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, and Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996
The present study evaluated a new class of bifunctional anion-exchange resins with improved selectivity and sorption kinetics for removing pertechnetate (TcO4-) from contaminated groundwater. Both laboratory-column and field flow-through experiments were performed, and results indicated that superior performance of the bifunctional resins has been achieved through the use of two quaternary ammonium groups, one having long chains (trihexylamine) for higher selectivity for poorly hydrated large anions and one having shorter chains (triethylamine) for enhanced kinetics and exchange capacity. Field results indicated that the bifunctional resin performed ∼5 times better than one of the best commercial monofunctional anionexchange resins (e.g., Purolite A-520E) with respect to the removal of TcO4- from contaminated groundwater. Less than 3% of TcO4- breakthrough was observed after ∼700 000 bed volumes (BV) of contaminated groundwater had been treated by the bifunctional resin column (at a flow rate of ∼6 BV/min) at the U.S. Department of Energy’s Paducah Gaseous Diffusion Plant site. The results also demonstrate that the new resin is particularly effective in removing low levels of TcO4- (e.g., at nmol/L range), and a cost saving may be realized by using the bifunctional resins for the treatment of large quantities of contaminated groundwater because of its increased selectivity, treatment efficiency, and longevity. The new resin may also be applied for the efficient treatment of other poorly hydrated large anions such as perchlorate (ClO4-) from contaminated groundwater or surface water.
Introduction A significant amount of technetium-99 (99Tc) has been released to the environment for last few decades through * Corresponding author phone: (865)574-7286; fax: (865)576-8543; e-mail:
[email protected]. † Environmental Sciences Division, Oak Ridge National Laboratory. ‡ Chemical and Analytical Sciences Division, Oak Ridge National Laboratory. § University of Tennessee. 10.1021/es990951g CCC: $19.00 Published on Web 02/12/2000
2000 American Chemical Society
improper discharges from nuclear processing plants, fallout from nuclear weapons tests, and other sources such as nuclear power plants and medicinal use of meta-stable 99mTc (1). It has become an increasingly important public concern because 99Tc is a long-lived radionuclide (β-emitter) with a half-life of 2.13 × 105 years. On the other hand, 99mTc is shortlived (with a half-life of 6.02 h) and rapidly decays to 99Tc (2). In oxygenated and suboxygenated environments this radionuclide is in the form of the pertechnetate anion (TcO4-), which is highly soluble and mobile in the subsurface aquifer (3-6). Groundwater contamination with TcO4- has been reported at many of the U.S. Department of Energy’s (DOE’s) complexes, including the Paducah Gaseous Diffusion Plant (PGDP), Oak Ridge Y-12 Plant, Portsmouth Gaseous Diffusion Plant, and the Hanford nuclear waste storage facilities (68). In 1988, Clausen et al. (8) reported a large 99Tc plume extending from the northwest corner of the PGDP site toward the Ohio River with 99Tc concentration in groundwater ranging from ∼50 to >14 000 pCi/L (∼0.03 to >8 nmol/L) at the source. The TcO4- anion has been shown to be strongly sorbed on some commercially available anion-exchange resins and activated carbon, particularly at relatively high concentrations (6, 9-13). However, most groundwater contains an extremely low concentration of TcO4- but ∼4-6 orders of magnitude higher concentrations of competing anions such as chloride (Cl-), sulfate (SO42-), nitrate (NO3-), and bicarbonate (HCO3-). Highly selective anion-exchange resins for TcO4- are therefore required and are exceedingly important to the economics of groundwater treatment (14-16). For example, previous studies (9, 11-13) indicated a relatively low efficiency of some existing anion-exchange resins (such as Dowex SRB-OH, Reillex 402, and Dowex 1-X8) for TcO4- removal because of competitive interactions by naturally occurring background anions (e.g., Cl-, SO42-, NO3-, and HCO3-). Additionally, DelCul et al. (11) reported relatively slow reaction kinetics between TcO4- and some of these resins in a batch experiment. They found that ∼90% of TcO4- from PGDP groundwater (1 L) was removed with 1 g of Dowex SRB-OH resin but that it required ∼4 days of equilibration. A new class of the anion-exchange resins has recently been developed based on a systematic study of the relationship between resin selectivity and alkyl chain length of the quaternary ammonium group on anion-exchange resins synthesized from polychloromethystyrene-divinylbenzene copolymer beads and aminated with a trialkylamine (16, 17). They are called the “bifunctional” anion-exchange resins because they have two quaternary ammonium groups, one having long chains for higher selectivity and one having shorter chains for improved reaction kinetics. Because TcO4is larger and has a lower hydration energy than most of the other anions encountered in groundwater (such as Cl-, HCO3-, SO42-, NO3-), there is a chemical bias toward exchanging TcO4- preferentially over the other anions in an aqueous solution (18). This bias can be enhanced by chemical modification of the resin, including altering the size and shape of the cationic exchange sites and the polymer cross-linking density. It was found that resin selectivity for TcO4- sorption increased with the radius of the immobilized alkyl chain length of the quaternary ammonium groups on resin beads; however, the increase in radius also resulted in an overall decrease in the exchange capacity and rate of anion exchange (16). Accordingly, it led us to prepare bifunctional resins that have exchange sites comprising large alkyl quaternary ammonium groups for high selectivity and small alkyl groups for improved exchange kinetics. The present study was VOL. 34, NO. 6, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. General Properties of Synthetic Resinsa
resin RO-02-119 RO-02-61 VP-02-217 Purolite A-520E
first amine functional group
second amine functional group
mesh size
TAECb (mequiv/g)
trihexylamine trihexylamine trihexylamine triethylamine
triethylamine tripropylamine tripropylamine none
40-60 40-60 60-200 40-60
2.53 2.15 2.06 2.80
aAll resins used 100% chloromethylstyrene backbone with 5% divinylbenzene (DVB) cross-linking. b TAEC ) total anion-exchange capacity in mequiv/g dry resin.
undertaken to determine (1) the relative effectiveness of the bifunctional resins with different chain lengths of the alkyl functional groups and (2) the ability and long-term performance of these bifunctional resins to remove low levels of TcO4- from contaminated groundwater in both laboratorycolumn and field flow-through experiments.
Materials and Methods Synthetic Resins. All synthetic resins investigated were anionexchange resins cross-linked with divinylbenzene (DVB), which contained chloromethyl reaction sites that were functionalized by reaction with various trialkylamine groups to create quaternary ammonium strong-base exchange sites. The bifunctional resins were synthesized in the Chemistry Department at the University of Tennessee, Knoxville (17). The general properties of the bifunctional resins are listed in Table 1, along with a monofunctional anion-exchange resin (Purolite A-520E), which was provided by Purolite International, Inc. The Purolite A-520E was selected as the baseline resin for comparison because it is currently in use for treating groundwater at the PGDP site. Additionally, our previous studies have shown that it is one of the best commercial monofunctional anion-exchange resins with respect to TcO4- removal. The three bifunctional resins used for the present study varied in one of the quaternary amine functional groups and mesh sizes. They were selected on the basis of our systematic studies of a series of bifunctional resins to determine their reaction kinetics and selectivity for TcO4- by varying the trialkyl functional groups and the percentages of DVB cross-linking. Results of these studies were published elsewhere (16, 19). Column Flow-Through Experiments. Laboratory flowthrough experiments were performed in borosilicate-glass columns (10 mm diameter × 38 mm long) at a constant flow rate of 30 mL/min by using a high-performance liquidchromatography pump (Alltech 426, Deerfield, IL). Each of these resins was initially in the chloride form and was loaded into the column as a slurry to a height of 38 mm to provide a resin bed volume (BV) of 3.0 mL. The flow rate is thus equivalent to 10 BV/min or ∼123 cm/min interstitial velocity, based on an effective void fraction of 0.31. The influent test solution consisted of 6 µmol/L TcO4- (or ∼10 µCi/L 99Tc as ammonium salt) and 60 mmol/L each of NaCl, NaNO3, and Na2SO4 at pH ∼7. The column was first preequilibrated with a background solution of 60 mmol/L of NaCl, NaNO3, and Na2SO4 (without TcO4-) for approximately of 1000 BV before the TcO4- test solution was introduced into the column. The effluent solution (10 mL) was then collected at a given time interval and was mixed with 10 mL of a scintillation cocktail (Ultima-Gold XR, Packard Instrument Co., CT) for 99Tc radioactivity analysis. The TcO4- concentration (or 99Tc radioactivity) in each vial was determined by means of a liquid scintillation analyzer (Tri-Carb, Model 2000, Packard), and the measured counts per minute were converted to the TcO4concentration by the external standard method (6). The detection limit is approximately 1 nCi/L 99Tc (∼0.6 nmol/L). 1076
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Based on laboratory column studies, two bifunctional resins (RO-02-119 and VP-02-217) were used for field performance evaluation under realistic environmental conditions. The first test was conducted in the Pits and Trenches area (monitoring well 106) at DOE’s Oak Ridge Reservation, Oak Ridge, TN, where the groundwater was pumped directly through a small column (10 mm diameter × 38 mm long) at a flow rate of ∼18 mL/min. The Purolite A-520E resin was also tested in parallel to provide a basis for comparison on the relative effectiveness of the bifunctional resins in removing TcO4- from contaminated groundwater. A scale-up field demonstration was then conducted at the Northwest Plume Pump-and-Treat facility at the PGDP site for a 3-month period in the summer of 1997. Two columns (25 mm diameter × 110 mm long), each with a bed volume of ∼54 mL, were set up to run in parallel at the treatment facility. One column contained the bifunctional resin (RO02-119), and the other column contained commercially available Purolite A-520E resin (sieved to +40/-60 mesh). The contaminated groundwater was diverted through a halfinch diameter pipe from the main treatment facility through the resin columns at an initial flow rate of ∼300 mL/min (or ∼6 BV/min) per column. The flow rate was maintained relatively constant for the bifunctional resin column but fluctuated between 50 and 300 mL/min and stopped due to clogging after 6 weeks for the Purolite A-520E resin column. The effluent flow was diverted back to the equalization tank for treatment by the main treatment plant at PGDP. Concentrations of TcO4- in the influent and the effluent samples were monitored periodically and were analyzed for 99Tc radioactivity. The groundwater contained ∼1.2 nCi/L or ∼0.7 nmol/L TcO4- on average so that the TcO4- concentration was ∼6-7 orders of magnitude lower than the concentrations of competing anions (such as Cl-, SO42-, NO3-, and HCO3- at ∼2.5 mmol/L) present in the groundwater (6). Because the groundwater contained a relatively low concentration of TcO4-, ∼1 L groundwater sample had to be collected and preconcentrated by using the Empore technetium extraction disk (3M, St. Paul, MN) for TcO4- analysis. The extraction disk adsorbed with TcO4- was then immersed in 18 mL of scintillation cocktail and was analyzed for 99Tc radioactivity as described previously. The detection limit by using the Empore technetium disk is ∼1 pCi/L (or ∼0.6 pmol/L TcO4-), and the extraction efficiency is >95% on average as measured by the standard addition method to the groundwater. TcO4- Adsorption from Groundwater. The adsorption or partitioning of TcO4- from a 99Tc test solution on bifunctional resins was determined and published elsewhere (16). This study evaluated the partitioning of TcO4- as influenced by competing anions using the PGDP contaminated groundwater (with low levels of TcO4-). The groundwater had to be spiked with the 95mTc tracer (as NH4TcO4 with a specific activity of 29.7 mCi/mg) so that an extremely low equilibrium TcO4- concentration in solution could be determined after adsorption. The 95mTc is a γ-emitter and has a half-life of 61 days. The added 95mTc concentration ranged from 0 to ∼1500 nCi/L (or from 0 to ∼0.26 pmol/L), which constituted
