Interaction between calcium and phosphate adsorption on goethite

Quantitatively, little is known about the ion interaction processes that ar e responsible for the binding of phosphate in soil, water, and sediment, wh ich determine the bioavailability and mobility of phosphate. Studies have s hown that metal hydroxides are often responsible for the binding Of PO4 in soils and sediments, but the binding behavior Of PO4 in these systems often differs significantly from adsorption studies on metal hydroxides in labor atory. The interaction between PO4 and Ca adsorption was studied on goethit e because Ca can influence the PO4 adsorption equilibria. Since adsorption interactions are very difficult to discriminate from precipitation reaction s, conditions were chosen to prevent precipitation of Ca-PO4 solids. Adsorp tion experiments Of PO4 and Ca, individually and in combination, show a str ong interaction between adsorbed Ca and PO4 on goethite for conditions belo w the saturation index of apatite. It is shown that it is possible to predi ct the adsorption and interaction Of PO4 and Ca on electrostatic arguments using the model parameter values derived from the single-ion systems and wi thout invoking ternary complex formation or precipitation. The model enable s the prediction of the Ca-PO4 interaction for environmentally relevant cal cium and phosphate concentrations.