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

Mechanism of hexavalent chromium removal (Cr(VI) as CrO) by the weak-base ion exchange (IX) resin ResinTech® SIR-700-HP (SIR-700) from simulated groundwater is assessed in the presence of radioactive contaminants iodine-129 (as IO), uranium (U as uranyl UO), and technetium-99 (as TcO), and common environmental anions sulfate (SO) 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 SO concentrations (536 mg/L) where Cr(VI) and U(VI) removal decreased to ≥ 80%. However, Cr(VI) removal notably improved with co-mingled U(VI) that complexes with SO at the protonated amine sites. These U-SO 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 CrO is removed by IX with SO complexes and/or reduced to amorphous Cr(III)(OH) at secondary alcohol sites. Tc(VII)O and I(V)O also undergo chemical reduction, following a similar removal mechanism. Oxyanion removal preference is determined by the anion reduction potential (CrO>TcO>IO), geometry, and charge density. For these reasons, 39% and 69% of TcO and 17% and 39% of IO are removed in the presence and absence of Cr(VI), respectively.