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

Mechanism of hexavalent chromium removal (Cr(VI) as CrO₄^2-) 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 IO₃^-), uranium (U as uranyl UO₂²⁺), and technetium-99 (as TcO₄^-), and common environmental anions sulfate (SO₄^2-) 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 SO₄^2- 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 SO₄^2- at the protonated amine sites. These U-SO₄^2- 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 CrO₄^2- is removed by IX with SO₄^2- 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 (CrO₄^2->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.