The adsorption of arsenic to iron oxyhydroxides strongly depends on water chemistry. Iron(III) oxyhydroxides can also participate in the oxidation of As(III) to As(V), which changes arsenic's toxicity and adsorption behavior. As(III) and As(V) adsorption to lepidocrocite (gamma-FeOOH) were examined in batch experiments that explored the effects of lepidocrocite dose, pH, availability of dissolved oxygen, and the presence of aqueous Fe(II) on adsorption. Lepidocrocite is an iron oxyhydroxide found in soils, and it is one of the major products of iron electrocoagulation for water treatment. A surface complexation model was able to describe the adsorption of both As(III) and As(V) to lepidocrocite over a broad range of conditions. The concentration and oxidation states of arsenic in solution were measured over the course of the reactions. At both oxic and anoxic conditions. As(III) was oxidized to As(V) in systems that contained lepidocrocite together with Fe(II): this oxidation led to overall enhanced arsenic adsorption at near neutral pH. With oxygen the pH-dependent generation of oxidants from the Fenton reaction drove the As(III) oxidation. In the absence of oxygen the As(III) was probably oxidized by Fe(III) in lepidocrocite that had become more reactive upon reaction with Fe(II). (C) 2015 Elsevier Inc. All rights reserved.