Arsenic(III) adsorption and oxidation by a Pleistocene brown sediment from the Hetao Basin, China

The ability of As(III) adsorption and oxidation by the Pleistocene brown sediment from inland basins still remains unclear. To address this issue, we investigated As(III) adsorption and oxidation by a Pleistocene brown sediment from the Hetao Basin in the presence of competing anions (e.g., OH^-, PO₄, and HCO₃). The results show that the brown sediment contained around 4 mmol/kg extractable Mn(IV) oxides, and was capable of oxidizing dissolved As(III). However, percentage of dissolved As(V) reached a maximum of around 20% with increasing initial As(III) concentration up to 25 μmol/L, indicating its limited ability of oxidizing As(III). Therefore, adsorbed As was believed to be the sum of adsorbed As(III) and adsorbed As(V). Arsenic adsorption exhibited a nonlinear isotherm. Maximum As adsorption occurred at around pH 7.2. Oxidation of As(III) was likely enhanced by the increasing pH. Phosphate competed stronger with As(V) than with As(III), while effect of HCO₃ competition was greater on As(III) adsorption.