| Abstract: |
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 (η) were determined by back-titration under different pH, ionic strength (I) and SO42− 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 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. |
