Co²⁺-exchange mechanism of birnessite and its application for the removal of Pb²⁺ and As(III)
2011
Yin, Hui | Liu, Fan | Feng, Xionghan | Liu, Mingming | Tan, Wenfeng | Qiu, Guohong
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 33.35m²g⁻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 2538mmolkg⁻¹ for the unmodified birnessite to 1500mmolkg⁻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.
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