The interfacial reactivity of arsenic species with green rust sulfate (GR_SO4)

Arsenic (As) contamination in groundwater is a significant health and environmental concern worldwide because of its wide distribution and toxicity. The fate and mobility of As is greatly influenced by its interaction with redox-active mineral phases, among which green rust (GR), an Fe^II-Fe^III layered double hydroxide mineral, plays a crucial role. However, the controlling parameters of As uptake by GR are not yet fully understood. To fill this gap, we determined the interfacial reactions between GR sulfate (GR_SO4) and aqueous inorganic As(III) and As(V) through batch adsorption experiments, under environmentally-relevant groundwater conditions. Our data showed that, under anoxic conditions, GR_SO4 is a stable and effective mineral adsorbent for the removal of As(III) and As(V). At an initial concentration of 10 mg L^-1, As(III) removal was higher at alkaline pH conditions (~95% removal at pH 9) while As(V) was more efficiently removed at near-neutral conditions (>99% at pH 7). The calculated maximum As adsorption capacities on GR_SO4 were 160 mg g^-1 (pH 8-9) for As(III) and 105 mg g^-1 (pH 7) for As(V). The presence of other common groundwater ions such as Mg²⁺ and PO₄^3- reduces the efficiency of As removal, especially at high ionic strengths. Long-term batch adsorption experiments (up to 90 days) revealed that As-interacted GR_SO4 remained stable, with no mineral transformation or release of adsorbed As species. Overall, our work shows that GR_SO4 is one of the most effective As adsorbents among iron (oxyhydr)oxide phases.