Interactions between P and S anions and soil colloids are significant, mainly those rich in iron and aluminum oxides, whose electric surface charges vary according to the pH and ionic strength of the solution. The objective of this study was the evaluation of phosphate and sulfate adsorption on surface and subsoil samples of a clayey Anionic Acrudox, a medium-textured Anionic Acrudox and a clayey Rhodic Kandiudalf from the northern region of the State of São Paulo, Brazil. Maximum phosphate and sulfate adsorption was determined by the isotherm of Langmuir, and the four layer model used for adsorption simulation. Phosphate and sulfate adsorption were superior on the subsoil, where organic matter contents were lower. On LAw samples, which presented lowest iron and aluminum oxide concentrations, maximum adsorption was smaller. Phosphate adsorption decreased when pH values were elevated up to 5 or 6, but pH above 6 provided higher adsorption rates. Sulfate adsorption decreased with pH elevation. Values obtained for the electric surface potential by the four layer model were not realistic, although the model was efficient for the simulation of anion adsorption and provides a comprehensive understanding of pH, and phosphate and sulfate concentration effects on surface potential variations.
phosphate; sulfate; adsorption; pH; model
