Arsenite photo-oxidation and removal by ferrihydrite in the presence of oxalate: a pH dependence and surface-mediated process

Abstract

In this study, the kinetics of arsenite (As(III)) removal in a ferrihydrite/oxalate system under irradiation was investigated, and the effects of pH, ferrihydrite dosage, As(III) initial concentration, and oxalate concentration were evaluated. Ultraviolet A (UVA) irradiation significantly promoted As(III) removal in the presence of ferrihydrite and oxalate. The greatest removal efficiency of As(III) (92.5%) was observed under the following conditions: pH 3.0, 100.0 μg L⁻¹ As(III), 0.0125 g L⁻¹ ferrihydrite, 0.1 mmol L⁻¹ oxalate, and after 90 min under UVA irradiation. The removal of As(III) in the ferrihydrite/oxalate system was strongly pH-dependent. Mechanistic investigations indicated that HO˙ was the primary oxidant for As(III) photo-oxidation in the ferrihydrite/oxalate system. Surface adsorbed hydroxyl radical (HO_ads˙) was the dominant oxidant over the entire pH range in the presence of oxalate, and the hydroxyl radical in aqueous solution (HO_aq˙) made only a small contribution under acidic conditions. The sources of HO˙ were also strongly dependent on pH; HO₂˙/O₂˙⁻ was the precursor of HO˙ under acidic conditions, while photo-generated holes gradually became the main source of HO˙ as the pH increased. The photo-oxidation of As(III) mainly occurred on the surface of ferrihydrite. Although some ferrihydrite transformed into goethite during the removal process, the iron oxides showed good reusability for up to five regeneration cycles. The findings indicate that As(III) can be efficiently removed by the ferrihydrite/oxalate system and provide guidance for developing effective treatment strategies for As(III)-contaminated waters.