Prediction of adsorption of divalent heavy metals at the goethite/water interface by surface complexation modeling

The triple-layer model was used to model the adsorption behavior of Cu(II), Pb(II), and Zn(II) onto goethite as a function of pH, total dissolved metal (Me) concentration, surface area, and ionic strength. A single reaction involving adsorption of Me2+ onto negative surface sites as an inner-sphere complex proved successful in describing adsorption of all three metals under the experimental conditions. The calibrated model was validated by comparing model predictions to independent data reported in the literature. A linear-free-energy relationship between the intrinsic surface complexation and the first-hydrolysis equilibrium constants was determined and was used to predict the intrinsic surface complexation constants for Co(II) and Hg(II). The triple-layer model, using the predicted constants, quantitatively described the adsorption of Co(II), but significantly over-predicted the extent of Hg(II) adsorption.