Phosphate and sulfate adsorption on goethite: Single anion and competitive adsorption

The adsorption of phosphate and sulfate on goethite is studied individually and in combination at solution concentrations of phosphate ranging from 10(-8) to 10(-4) M and of sulfate ranging from 10(-5) to 10(-3) M. For single anion adsorption the influence of pH, ionic strength, and anion:goethite ratio was determined. The anion adsorption data were described well with a model in which surface complexation and electrostatic interaction is taken into account. In systems with both anions, the influence of phosphate on sulfate adsorption was much stronger than vice versa, which reflects the higher affinity of phosphate for the goethite surface. In spite of the rather small competitive effect of sulfate on phosphate adsorption expressed per unit surface area of goethite, a considerable increase in the solution concentration of phosphate was observed at relatively low pH in the presence of sulfate. The relative increase in the phosphate solution concentration was larger at a higher ratio of total concentrations of sulfate and phosphate in the system and when lowering the pH. The data indicate that the competitive interaction of phosphate and sulfate for adsorption may have an important effect on the bioavailability of these anions. The competitive adsorption data were predicted well using model parameters derived for single anion adsorption. Copyright (C) 1997 Elsevier Science Ltd.