Birnessite-type manganese oxide is a highly efficient oxidant that has been investigated widely for As(III) oxidation. Nevertheless As(III) oxidation rate is inevitably reduced due to favorable adsorption of coexisting ions and As(V) which passivate its surface. In this paper we explore a novel strategy to significantly improve As(III) oxidation performance by controlling birnessite morphology. The batch experiment results show that the nanoflower-like birnessite (Bir-NF) exhibits an incredible improvement in As(III) oxidation activity compared to nanowire-like (Bir-NW) and nanosheet-like (Bir-NS) birnessites. The morphology of birnessite varies from nanosheet to nanoflower not only promotes As(III) oxidation rate from 1.4 to 24.7 lmol g (1) min (1), but also reduces the adverse effect of adsorption of As(V) and coexisting ions on As(III) removal. The origin of morphology-dependent enhancement of As(III) removal was experimentally and theoretically studied by As(V) adsorption on birnessites, phosphate adsorption kinetics, detection of dissolved Mn2+ concentration, average Mn oxidation state, the point of zero charge, and density functional theory (DFT) calculations. The results reveal that significant enhancement of As(III) oxidation activity in Bir-NF as compared to Bir-NW and Bir-NS is attributed to its highly efficient contact between As(III) species and manganese oxide, as well as its fast charge transfer from As atom to Mn atom due to its highest oxygen vacancy defect concentration, thus significantly promoting As(III) oxidation activity. (C) 2017 Elsevier B.V. All rights reserved.