Modelling SO 2− 4 surface complexation on variable charge minerals: I. H + and SO 2− 4 exchange under different solution conditions

Summary Surface complexation of SO 2− 4 and H + exchange were compared between goethite, gibbsite and a soil material from a podzol B horizon. Intrinsic equilibrium constants were determined for the surface complexation of SO 2− 4 and H + using the diffuse layer model (DLM). Three surface group configurations for the SO 2− 4 adsorption were tested. A model with only one SO 2− 4 surface species and no H + explicitly taking part in the reaction gave the overall best prediction of adsorption. The concomitant H + exchange occurring during SO 2− 4 adsorption was attributed to decreased surface potential caused by charge neutralization in the inner layer. The H + /SO 2− 4 stoichiometric ratios (η) were determined by back‐titration under different pH, ionic strength ( I ) and SO 2− 4 concentrations. The experimentally determined η was close to 1 for all materials at I = 0·1 M but 1·5, 1·0 and 1·7 in I = 1 mM for goethite, gibbsite and the soil suspension, respectively. The DLM gave a quantitatively correct description of SO 2− 4 and H + exchange at small I, but predictions were less accurate at large I. The surface complexation model give insight in the role of SO 2− 4 adsorption for the reversibility of soil acidification but remain to be evaluated in experiments more closely related to field conditions.