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

Co-containing birnessites were obtained by ion exchange at different initial concentrations of Co2+. Ion exchange of Co2+ had little effect on birnessite crystal structure and micromorphology, but resulted in an increase in specific surface areas from 19.26 to 33.35 m(2) g(-1), and a decrease in both crystallinity and manganese average oxidation state. It was due to that Mn(IV) in the layer structure was reduced to Mn(III) during the oxidation process of Co2+ to Co(III). The hydroxyl groups on the surface of Co-containing birnessites gradually decreased with an increase of Co/Mn molar ratio owing to the occupance of Co(III) into vacancies and the location of large amounts of Co2+/3+ and Mn2+/3+ above/below the vacant sites. This greatly accounted for the monotonous reduction in Pb2+ adsorption capacity, from 2538 mmol kg(-1) for the unmodified birnessite to 1500 mmol kg(-1) for the Co2+ ion-exchanged birnessite with a Co/Mn molar ratio of 0.16. The amount of As(III) oxidized by birnessite was enhanced after ion exchange, but the apparent initial reaction rate was greatly decreased. The present work demonstrates that Co2+ ion exchange has great influence on the adsorption and oxidation behavior of inorganic toxic metal ions by birnessite in water envrionments. (C) 2011 Elsevier B.V. All rights reserved.