pH dependence of phosphate sorption on aluminum

The sorption of phosphate ions on aluminum was studied by an in situ radiotracer technique. The kinetics and reversibility of sorption and the influence of the solution pH on phosphate coverage were followed in 0.1 mol dm⁻³ NaCl solution. It was found that phosphate adsorption on aluminum is determined by the surface charge and the protonation state of phosphate in the bulk solution. Despite the high chloride concentration, significant phosphate sorption was identified, indicating strong phosphate binding to the surface. Phosphate sorption increased with pH up to pH=4.7. As the pH of the solution increased further, phosphate desorbed from aluminum. The exchange experiments indicated that the phosphate sorption on the passive aluminum surface is a largely irreversible process. It is assumed that, in near neutral solution, there is an ongoing surface transformation, i.e., oxide film growth, that entraps phosphate ions from the solution.