Surface complexation modeling of divalent metal cation adsorption on birnessite

Birnessite is one of the most common manganese oxides in the environment and an important scavenger of trace metals. This study investigated the adsorption of seven divalent metal cations (Cd2+, Co2+, Cu2+, Mn2+, Ni2+, Pb2+ and Zn2+) on birnessite at different pH conditions, ionic strengths and initial concentrations. The adsorption data were used to formulate a self-consistent, two-site surface complexation model consisting of a triple-corner-sharing (TCS) complex at interlayer vacancies and a double-corner-sharing (DCS) complex at mineral edge sites. Unlike the other metals, Ni2+ had a relatively low adsorption capacity on birnessite, attributable to its tendency to incorporate within Mn vacancies (INC); hence a two-site model with INC and DCS complexes was used to describe Ni2+ adsorption on birnessite. The model well described all the adsorption data and was verified by data from the literature. In addition, a linear free energy relationship (LFER) was developed between the TCS binding constants and metal electronegativity and hydrous ionic radii, which can be used to estimate the binding constants of other trace metals. The proposed model contributes to a better understanding of the adsorption mechanisms of trace metals by manganese oxides. Furthermore, the data set can be included in geochemical assemblage models to predict the dissolution and mobility of metals in the environment.