PNNL

Abstract

At Hanford, a legacy nuclear site in Washington State (USA), a technical basis is required for optimizing ion exchange (IX) resin performance to meet evolving contaminant treatment objectives. Strong base anion IX resins, Purolite® A532E (A532E) and DOWEX 21K (DOWEX), are used in the 200 West Area pump-and-treat (200W P&T) facility to remove heptavalent technetium-99 (Tc-99), as the pertechnetate anion (Tc(VII)O4-), and hexavalent uranium (U(VI)) as negatively charged uranyl carbonate complexes (e.g., UO2(CO3)34-), respectively. Groundwater from site extraction wells is blended into an influent stream that first passes through three vessels containing DOWEX to remove U(VI) species. Then the U(VI)-stripped groundwater is blended with two additional groundwater influent streams with high Tc-99/low U(VI) concentrations before passing through three vessels containing A532E to remove Tc-99. A planned expansion of the 200W P&T facility to remediate Tc-99 and U contaminated groundwater from beneath aging radioactive waste storage tanks, will change the relative concentrations of U, Tc-99, and other common anions, including NO3-, and SO42-, in the influent stream, and this has the potential to impact A532E and DOWEX IX performance. This work to confirm that that A532E and DOWEX will remove contaminants to meet groundwater treatment objectives, despite the broader range of influent chemistries after the planned expansion, is presented in two parts. A532E performance was evaluated in Part I, with batch sorption tests showing Tc-99 removal that meets groundwater treatment objectives, even in the presence of competing anions, NO3-, SO42-, Cl-, HCO3-/CO32-, and U(VI). The selectivity of DOWEX for U(VI) has been quantitatively evaluated in Part II, with aqueous batch tests used to assess the competitive effects of different concentrations of TcO4-, nitrate (NO3-), sulfate (SO42 ), chloride (Cl-), and bicarbonate/carbonate (HCO3-/CO32-, which is known to influence U(VI) speciation). The results demonstrate that U(VI) carbonate uptake by DOWEX is not significantly impacted by TcO4-, or by high SO4- and Cl- concentrations. NO3- concentrations up to 100 mM did decrease the removal of U to varying extents (85% – 99%) depending on the competing effects of other anions present in solution, suggesting that high concentrations of NO3- may decrease DOWEX performance. However, at the highest NO3- concentrations reported across the Hanford Site Central Plateau (25 mM), the effect on U uptake is minimal. Thus, the trimethylammonium functional groups on DOWEX are highly selective for U carbonate species, and uptake was 85% – 100% under all conditions tested. Equilibrium IX coefficients obtained by modeling the batch sorption results predict little effect of influent chemistry on U removal by DOWEX from a thermodynamic perspective, suggesting that DOWEX will remove both U(VI) carbonate species and TcO4- from current and future influent streams such that effluent concentrations will meet groundwater treatment objectives, even in the presence of high concentrations of NO3-, SO42-, Cl-, and HCO3-/CO32-. However, kinetic effects can play an important role in U uptake efficiency under the high-flow conditions in the IX vessels at the 200W P&T facility. Here, kinetic effects have been initially assessed in batch tests. In a future study, column tests will be conducted, firstly with a synthetic groundwater containing the anions included in the batch tests performed here, and then secondly with actual groundwater from the 200 Area, with the batch-derived K values optimized to account for kinetic effects and used in flow and transport modeling to fit U breakthrough data.