High concentration of ammonium (NH4+) often occurs in groundwater. In this study, the behavior of NH4+ catalytic oxidation by a manganese oxide (MnOx) filter was investigated systematically and the formation and evolution of the MnOx films were characterized extensively. It was found that the MnOx based filter could be successfully started up within 6 days with the NH4+ removal efficiency increasing from 1 to 96%. After start-up, the removal efficiency of NH4+ could keep almost constant under different operating conditions, implying the MnOx filter has a good stability towards NH4+ removal. Both X-ray diffraction (XRD) and Raman results suggested that MnOx was related to hexagonal birnessite which has low crystallinity and small crystallite sizes. The X-ray photoelectron spectroscopy (XPS) spectra of the Mn2p taken on the surface of MnOx, revealed Mn(II), Mn(III) and Mn(IV) species existed in MnOx materials and their atomic concentrations were 20.2%, 57.8% and 22%, respectively. More importantly, the catalysis of films towards NH4+ oxidation was expected to be closely related to its feature structure. Specifically, the ability of manganese to cycle between +2, +3 and +4 oxidation states during catalysis was considered to play important roles in the NH4+ conversion on MnOx surface. Based on the above discussion, three major steps involve in NH4+ catalytic oxidation by MnOx were proposed, which are NH4+ adsorption, catalytic oxidation, and desorption of reaction products. (C) 2017 Elsevier B.V. All rights reserved.