Phosphate Removal from Aqueous Solutions using Neutralised Bauxite Refinery Residues (Bauxsol™)

Environmental Context. Eutrophication of freshwater and marine ecosystems is a global problem, which is frequently linked to high phosphorus concentrations. The present study investigated the use of Bauxsol™, a modified bauxite refinery residue, to remove dissolved phosphate from water, and has shown that it can be used as a cost-effective adsorbent for treating phosphate-contaminated waters. The results provide water and environmental managers with a new technique for decreasing the phosphate loads in water and wastewater. Environmental benefits include improved water quality, minimisation of excessive plant growth, including potentially toxic blue green algae, and the utilisation of an industrial residue for environmental remediation.

Abstract. Phosphate (PO₄^3–) removal by Bauxsol™, 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 PO₄^3– by Bauxsol™ is based on a ligand-exchange mechanism, although the low reversibility pH-independent desorption observed in acid-treated Bauxsol™ indicates a dominance of chemisorption. It was shown that PO₄^3– adsorption onto both Bauxsol™ and acid-treated Bauxsol™ followed a Langmuir isotherm model, with adsorption capacities of 0.21 and 0.48 mmol g⁻¹ at pH 9.0 and 5.2 respectively. Adsorption of PO₄^3– by Bauxsol™ increased with decreasing pH, with maximum adsorption efficiencies obtained at pH 5.2 ± 0.1 (the lowest pH investigated), higher Bauxsol™ to initial phosphate concentration ratios, and increased time. Studies of the effects of competing ions on the adsorption of PO₄^3– by Bauxsol™ indicated that adsorption decreased in the presence of HCO₃⁻ ions, whereas SO₄^2–and Cl⁻ ions had little effect, and Ca²⁺ and Mg²⁺ ions increased adsorption. These findings suggest that Bauxsol™ could be used as an efficient low-cost adsorbent for treating phosphate-contaminated waters.