Manganese (hydr)oxides are powerful oxidants mediating the transformation of As(III) to As(V) under natural conditions, however, the presence of Mn(II) on the oxidation of As(III) in the pH range of 7.0-9.0 has not been reported so far. In this study, abiotic oxidation of Mn(II) to amorphous Mn(III, IV) (hydr)oxides (MnOx) on magnetite and hematite was confirmed, and the impact of newly formed MnOx on the fate of As(III) was investigated. With the addition of Mn(II) into As(III)-preloaded systems, the dissolved and the adsorbed As(III) was oxidized to As(V) at high pH, and Mn(II) mobilized the adsorbed As(III) and As(V) in hematite system. High production of dissolved As(V) and significant mobilization of As(III) were even more significant in hematite suspension (total As was 18.96 mg L-1 after 60 h at pH 8.62) with simultaneous addition of Mn(II) and As(III), while magnetite showed a higher capacity for the retention of As(III) and As(V). It could therefore be deduced that the newly formed MnOx on iron oxides could oxidize the dissolved and the adsorbed As(III) to As(V). In addition, the MnOx formed at high pH would take up the sorption sites previously occupied by the adsorbed As(III), and then mobilized a fraction of the adsorbed As(III) into solution. The present study reveals that MnOx formed via abiotic oxidation on iron oxides plays an important role in the oxidation and mobilization of both dissolved and adsorbed As(III) in aquatic environment. (C) 2013 Elsevier B.V. All rights reserved.