Co2+-exchange mechanism of birnessite and its application for the removal of Pb2+ and As(III)

► Mn(IV) on the surface of birnessite was the electron sink of Co2+ oxidation. ► Mn(III) was the main reduction product of Co2+ oxidation. ► The content of hydroxyl groups in Co-containing birnessites gradually decreased. ► Pb2+ adsorption capacity of birnessite was reduced after Co2+-ion exchange. ► As(III) oxidation capacity of birnessite was enhanced by Co2+-ion exchange.