This study investigated the removal of Mn(II) through homogeneous and heterogeneous oxidation by air. A series of kinetic experiments were conducted with stirred batch reactors equipped with air spargers. The reactions were initiated by adding Mn(II) stock solutions to pH-buffered batch amended with dissolved Fe(II), Fe oxides, Fe oxyhydroxides, or Mn oxides. Under experimental conditions, the homogeneous oxidation of Mn(II) itself was minimal over pH 6.5-8.5. However, when dissolved Fe(II) existed, the immobilization of Mn occurred, with the extent becoming greater at higher pH. By Mn-K edge XAS analysis, the Mn removal at pH 6.5 was due to the oxidation of labile Mn(II) into insoluble Mn(III) solids, which was catalyzed by the homogeneous oxidation of Fe(II). On the other hand, Mn(II) at pH 7.0-8.0 remained largely unoxidized but partially oxidized to Mn(III) and subsituted into second shell of greenrusts. At pH 8.5, Mn(II) was precipitated as outer-sphere surface complexation. Goethite, HFO, Mn2O3, and MnO2 were added to mediate the heterogeneous oxidation of Mn(II). In the presence of goethite and HFO, dissolved Mn was initially quickly decreased, and later gradually decreased. In both batches, the initial removal was due to the surface complexation of Mn(II) with Fe oxyhydroxides. Also, the later removal in goethite-amend batches resulted from the surface complexation, whereas the later removal in HFO-amended batches was due to the heterogeneous oxidation of Mn(II) into Mn3O4. When Mn2O3 and MnO2 were used as heterogeneous catalysts, XAS analysis did not provide mechanistic insight into Mn removal. Nonetheless, Mn2O3 was found to immobilize Mn(II) under oxic conditions. Notably, MnO2 was far more effectively immobilize Mn(II) under both oxic and anoxic conditions, pointing to its superior oxidative capability.