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The oxidation of arsenite (As(III)) by manganese oxide is an important reaction in both the natural cycling of As and the development of remediation technology for lowering the concentration of dissolved As(III) in drinking water. This study used both a conventional stirred reaction apparatus and extended X-ray absorption fine structure (EXAFS) spectroscopy to investigate the reactions of As(III) and As(V) with synthetic birnessite (MnO₂). Stirred reactor experiments indicate that As(III) is oxidized by MnO₂ followed by the adsorption of the As(V) reaction product on the MnO₂ solid phase. The As(V)−Mn interatomic distance determined by EXAFS analysis for both As(III)- and As(V)-treated MnO₂ was 3.22 Å, giving evidence for the formation of As(V) adsorption complexes on MnO₂ crystallite surfaces. The most likely As(V)−MnO₂ complex is a bidentate binuclear corner sharing (bridged) complex occurring at MnO₂ …