Chromate and phosphate adsorption on schwertmannite: Competition, mobilization and mechanisms

Chromate competes with phosphate in nature for sorption sites on schwertmannite due to their similar properties to sulfate. The competition has implications for the environmental remediation for chromate and phosphate contaminated soil and water. In this study, the adsorption of chromate (CrO42-) and phosphate (PO43-) on schwertmannite was investigated individually and in combination, in a unique insight with various addition order. In individual adsorption experiments, a larger amount of phosphate (1.9 mmol g-1) was adsorbed on schwertmannite than chromate (0.4 mmol g-1), showing better adsorption property of schwertmannite for phosphate. Phosphate exhibited fast and slow adsorption stages with the boundary of 0.5 h. In systems with both anions, results showed obvious competitive adsorption between chromate and phosphate on schwertmannite, which was affected by addition order. Phosphate addition induced greater decrease of chromate adsorption than vice versa. Phosphate competed more effectively with chromate for sorption sites, and could easily mobilize and replace the pre-adsorbed chromate, but chromate addition could not cause release of phosphate. For chromate, adsorption on schwertmannite mainly occurred through SO42--exchange, while for phosphate, adsorption involved both SO42--exchange and ligand exchange with surface hydroxyl groups. These findings provided a better understanding in competitive adsorption of oxyanions on schwertmannite, and thereby improve the strategy to use schwertmannite as a potential adsorbent for the remediation of polluted oxyanions.