Phosphate removal from aqueous solutions using neutralised bauxite refinery residues (Bauxsol (TM))

Phosphate (PO43-) removal by Bauxsol (TM), a neutralised bauxite refinery residue, was investigated as a function of time, pH, ionic strength, adsorbent dosage, competing ions, and initial phosphate concentration. The results of adsorption and desorption studies indicate that adsorption of PO43- by Bauxsol (TM) is based on a ligand-exchange mechanism, although the low reversibility pH-independent desorption observed in acid-treated Bauxsol (TM) indicates a dominance of chemisorption. It was shown that PO43- adsorption onto both Bauxsol (TM) and acid-treated Bauxsol(TM) followed a Langmuir isotherm model, with adsorption capacities of 0.21 and 0.48 mmol gat pH 9.0 and 5.2 respectively. Adsorption of PO43- by Bauxsol (TM) increased with decreasing pH, with maximum adsorption efficiencies obtained at pH 5.2 +/- 0.1 (the lowest pH investigated), higher Bauxsol (TM) to initial phosphate concentration ratios, and increased time. Studies of the effects of competing ions on the adsorption of PO43- by Bauxsol (TM) indicated that adsorption decreased in the presence of HCO3- ions, whereas SO42- and Cl- ions had little effect, and Ca2+ and Mg2+ ions increased adsorption. These findings suggest that Bauxsol (TM) could be used as an efficient low-cost adsorbent for treating phosphate-contaminated waters.