Modelling SO42- surface complexation on variable charge minerals. I. H+ and SO42- exchange under different solution conditions

Surface complexation of SO42- 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 SO42- and H+ using the diffuse layer model (DLM). Three surface group configurations for the SO42- adsorption were tested. A model with only one SO42- surface species and no H+ explicitly taking part in the reaction gave the overall best prediction of adsorption. The concomitant H+ exchange occurring during SO42- adsorption was attributed to decreased surface potential caused by charge neutralization in the inner layer. The H+/SO42- stoichiometric ratios (eta) were determined by back-titration under different pH, ionic strength (I) and SO42- concentrations. The experimentally determined eta 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 SO42- and H+ exchange at small I, but predictions were less accurate at large I. The surface complexation model give insight in the role of SO42- adsorption for the reversibility of soil acidification but remain to be evaluated in experiments more closely related to field conditions.