Sorption and sorptive interaction of cadmium and zinc on hydrous manganese oxide
Adsorption of Zn, Cd, and Ca by delta - MnO2 was measured at pH 4, 6, and 8 by a batch technique at steady-state conditions. Fitting the data to Langmuir and Freundlich expressions suggested that there were two or more mechanisms or sites of sorption for both Zn and Ca. The "higher energy" site(s) (first to fill) showed preference for Cd over Zn, whereas the reverse was true for the lower energy site(s). Simultaneous addition of both Cd and Zn reduced adsorption of both ions. Cadmium and Zn adsorption diminished somewhat when Ca was present at higher Zn and Cd loadings but competition from Ca was minimal at low metal loadings. It is proposed that the high energy binding is an exchange sorption on the negatively charged oxide sites by displacement of surface and diffuse layer cations and protons, which binds Cd more strongly than Zn. On a relative basis at pH 6 and 8, Zn is more strongly bound than Cd. The higher surface charge density and greater proportion of Zn hydroxy solution phase species at higher pH may favor Zn sorption. Competition between Cd, Zn, and Ca was observed at pH 4, 6, and 8, suggesting that solution bulk precipitation was not controlling solution metal concentrations.
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