RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The use of adsorption data from single sorbate systems to model metal adsorption in SO₄-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 SO₄. This effect can only be accurately modeled using the diffuse layer model and surface complexation theory if ternary surface complexes, ≡FeOHCuSO₄ or ≡FeOHZnSO₄, 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 SO₄ is accurately predicted for a wide range of metal, ferrihydrite and SO₄ concentrations. Adsorption of Cu and Zn onto the SO₄-rich Fe oxyhydroxide, schwertmannite, could also be accurately predicted and is indistinguishable from adsorption onto ferrihydrite in the presence of high solution SO₄ concentrations (e.g. 0.01 mol kg⁻¹ SO₄).