PNNL

Abstract

The 200 West pump-and-treat facility at the Department of Energy’s Hanford Site (Washington State, USA) currently treats contaminated groundwater from the Central Plateau area using DOWEX 21K and Purolite A532E ion exchange (IX) resins for targeted removal of uranium (U) and technetium-99 (Tc-99), respectively. However, the groundwater composition and sources of influent streams are expected to change over time, resulting in variations in both groundwater geochemical conditions and contaminant concentrations. These changes drive the identification of an IX resin that can treat co-located contaminants, e.g. hexavalent chromium (Cr(VI)), and iodine-129 (I-129), in addition to U and Tc-99, and in the presence of common groundwater anions that can potentially foul resins, such as sulfate (SO42-) and chloride (Cl-). Here, the removal of Cr(VI) from co-mingled contaminant plumes by the weak-base resin ResinTech SIR-700 is assessed, including its potential for accumulation of radioactive contaminants. Batch tests demonstrated Cr(VI) (0.25 – 5 ppm) removal and U (50 ppm) removal at = 97%, except in the presence of high concentrations of SO42- (536 ppm). At high SO42- concentrations, the overall Cr(VI) and U uptake was = 80%, but Cr(VI) uptake was notably improved when co-mingled with U, likely because U forms complexes with SO42- and reduces the competition for IX sites. Solid phase characterization revealed that Cr(VI) is removed by IX, followed by a reduction mechanism that converts it to a Cr(III)(OH)3·nH2O phase at secondary alcohol sites on the resin. X-ray photoelectron spectroscopy confirmed that reduction of TcO4- and IO3- also occurred, following a similar removal mechanism to CrO42-. Hence, there is competition between these oxyanions, and the preference for oxyanion removal is determined by the reduction potential (Cr(VI) > TcO4- > IO3-), geometry, and charge density of the ion. For these reasons, 39 % and 69% of TcO4- (0.3 – 3 ppm), and 17 % and 39% of IO3- (0.15 – 1 ppm) are removed by SIR-700 in the presence and absence of Cr(VI), respectively. With SIR-700 resin in its acid sulfate form, U complexes with SO42 at the protonated amine resin sites, which is integral to U removal. This was demonstrated by a decrease in U uptake ( 90 % due to the different removal mechanism of a reduction reaction with the secondary hydroxyl groups. Therefore, SIR-700 resin in either acid sulfate or acid chloride form can efficiently remove Cr(VI) under co-mingled, Hanford groundwater conditions. SIR-700 also has the potential to accumulate radioactive contaminants (U, Tc-99 and I-129), and IX treatment would require a suitable shielded facility, with spent resin requiring disposal as radioactive waste. Given that SIR-700 in acid sulfate form can effectively uptake U, its efficiency should be evaluated against the current IX resin for U removal (DOWEX 21K) to determine if SIR-700 can be deployed to remove multiple contaminants.