PNNL

Abstract

A technical basis is required for optimizing ion exchange (IX) resin performance to meet evolving contaminant treatment objectives at the 200 West Area pump-and-treat (200W P&T) facility at Hanford, a legacy nuclear site in Washington State (USA). The 200W P&T facility relies on two strong base anion styrene-divinylbenzene gel exchange resins in a six-IX vessel configuration to treat contaminated groundwater. A blended influent stream of groundwater from different contaminated sites in the 200W P&T remediation area first passes through three treatment vessels containing DOWEX 21K (DOWEX), with trimethylammonium functional groups, to remove uranium (U) as uranyl carbonate complexes (e.g., UO2(CO3)34–). The effluent from the DOWEX treatment train then passes through the next three vessels containing Purolite® A532E (A532E), with bifunctional quaternary amine groups (triethylammonium and trihexylammonium), to remove technetium-99 (Tc-99) as the weakly hydrated pertechnetate anion (TcO4-). However, nitrate (NO3-) and sulfate (SO42-) anions present in the groundwater at concentrations orders of magnitude higher than U or Tc-99 can compete for IX sites on the resins. Other common groundwater anions, e.g., chloride (Cl-) and carbonate (CO32-), influence the speciation of hexavalent uranium (U), and can also compete for IX sites. A planned expansion of the 200W P&T facility to remediate groundwater contaminated with Tc-99 and U from beneath aging radioactive waste storage tanks will change the relative concentrations of U, Tc-99, NO3-, and SO42- in the influent stream, and has the potential to impact A532E and DOWEX IX performance. This work assesses the impact of a broader range of influent chemistries on IX resin performance to confirm that that they will continue to remove contaminants to meet groundwater treatment objectives after the planned expansion under current and future facility operational configurations. Thermodynamic models can be used to predict IX resin performance and reduce the uncertainty associated with these changes in influent composition. Development of these models requires A532E and DOWEX selectivity for different anions to be determined over the range of concentrations present in groundwater. This study is presented in two parts, with the performance of A532E and DOWEX evaluated in Part I and Part II, respectively. In Part I, aqueous batch tests were conducted to quantify the competing effects of different concentrations of NO3-, SO42-, Cl-, HCO3-/CO32-, and U(VI) carbonate species on the selectivity of A532E for TcO4-. The results demonstrate that TcO4- uptake is not significantly impacted by the high HCO3-/CO32-, Cl-, NO3, and SO42- concentrations. Despite uptake of U(VI) carbonate species by A532E, ranging from 40% – 96% in the presence of NO3- or SO42-, uptake of TcO4- remained at greater than 99%. This is likely due to a high selectivity of the trihexylammonium exchange site in A532E for the large, weakly hydrated TcO4- anion. Equilibrium constants obtained by modeling these batch sorption results provide conservative parameters for predicting that A532E will remove TcO4- from current and future influent streams of similar chemistries studied here, such that effluent concentrations will meet groundwater treatment objectives, even in the presence of high concentrations of competing anions, NO3-, SO42-, Cl-, and HCO3-/CO32 -, and U(VI) carbonate as a co-contaminant. While these results suggest that thermodynamic considerations predict little effect of influent chemistry on Tc-99 removal by A532E, kinetic effects can play an important role in Tc-99 uptake efficiency under the high-flow conditions in the IX vessels at the 200W P&T facility. Here, kinetic effects are initially assessed in batch tests and will be further evaluated under flow conditions in a future column study that builds upon work presented in Parts I and II.