Modelling SO42- surface complexation on variable charge minerals. II. Competition between SO42-, oxalate and fulvate

The adsorption of oxalate and fulvate and the competition for surface-adsorption sites between SO42--oxalate and SO42--fulvate on goethite and on a soil material from a podzol B horizon were studied in batch titration experiments. The adsorption and competition processes were evaluated with surface complexation modelling using the diffuse-layer model (DLM). Two different adsorption reactions were tested for oxalate and fulvate adsorption and the reaction giving the largest charge neutralization at the surface gave the best description of experimental data. In the soil solution an Al-oxalate complex induced a substantial decrease in oxalate adsorption when the pH fell below 4.5. No such effect was found in the soil-fulvate system. Adsorption constants obtained from binary systems (oxide-ligand) were used to model adsorption and competition in the two ternary systems oxide-SO42--oxalate and oxide-SO42--fulvate. Experimental data showed that oxalate and fulvate competed with SO42- for adsorption sites. The DLM produced acceptable descriptions of experimental data, although the model slightly underpredicted SO(4)(2-)adsorption in ternary systems. The competition between adsorbates was attributed mainly to charge neutralization at the surface induced by adsorption of the ligands.