Cu and Zn ternary surface complex formation with SO 4 on ferrihydrite and schwertmannite

Abstract

The use of adsorption data from single sorbate systems to model metal adsorption in SO 4-rich waters, such as acid mine drainage, can lead to inaccurate predictions of metal speciation. The adsorption of Cu and Zn on ferrihydrite, for example, is enhanced at low pH values in the presence of SO 4. This effect can only be accurately modeled using the diffuse layer model and surface complexation theory if ternary surface complexes, ≡FeOHCuSO 4 or ≡FeOHZnSO 4, are taken into consideration. Intrinsic adsorption constants for the formation of these ternary complexes on ferrihydrite have been derived from experimental data. When included in the model, Cu and Zn adsorption in the presence of SO 4 is accurately predicted for a wide range of metal, ferrihydrite and SO 4 concentrations. Adsorption of Cu and Zn onto the SO 4-rich Fe oxyhydroxide, schwertmannite, could also be accurately predicted and is indistinguishable from adsorption onto ferrihydrite in the presence of high solution SO 4 concentrations (e.g. 0.01 mol kg −1 SO 4).