Accumulation mechanisms for contaminants on weak-base hybrid ion exchange resins

Mechanism of hexavalent chromium removal (Cr(VI) as CrO42-) by the weak-base ion exchange (IX) resin Res-inTech & REG; SIR-700-HP (SIR-700) from simulated groundwater is assessed in the presence of radioactive contam-inants iodine-129 (as IO3-), uranium (U as uranyl UO22+), and technetium-99 (as TcO4-), and common environmental anions sulfate (SO42-) and chloride (Cl-). Batch tests using the acid sulfate form of SIR-700 demonstrated Cr(VI) and U(VI) removal exceeded 97%, except in the presence of high SO42-concentrations (536 mg/L) where Cr(VI) and U(VI) removal decreased to & GE; 80%. However, Cr(VI) removal notably improved with co-mingled U(VI) that complexes with SO42-at the protonated amine sites. These U-SO42-complexes are integral to U(VI) removal, as confirmed by the decrease in U(VI) removal (<40%) when the acid chloride form of SIR-700 was used instead. Solid phase characterization revealed that CrO42-is removed by IX with SO42-complexes and/or reduced to amorphous Cr(III)(OH)3 at secondary alcohol sites. Tc(VII)O4- and I(V)O3- also undergo chemical reduction, following a similar removal mechanism. Oxyanion removal preference is determined by the anion reduction potential (CrO42->TcO4->IO3-), geometry, and charge density. For these reasons, 39% and 69% of TcO4-and 17% and 39% of IO3-are removed in the presence and absence of Cr(VI), respectively.