A study on the exchange additivity of zinc in solid mixtures in seawater——Ⅱ.Stepwise exchange in δ-MnO₂/montmorrillonite mixture

In this study the zinc exchange process and the reason for the non-additive exchange of δ-MnO₂/montmorrillonite have been investigated. The results suggest that with pH increasing from 2 to 8, the stepwise exchange of zinc occurs on δ-MnO₂ and montmorrillonite in their mixture.At low pH zinc ions are bound on δ-MnO₂, while at high pH, the exchange on montmorrillonite dominates the surface interaction in the mixture. The study demonstrates that the stepwise exchange and the heterogeneity of the binding sites on δ-MnO₂ surface are the reason for the non-additive exchange. It is also shown that when δ-MnO₂ amounts to 5%, zinc exchange on sea sediments is controlled by δ-MnO₂.