Removal of phosphate ions from aqueous solutions using bauxite obtained from Mulanje, Malawi

Abstract

Studies on stream water and effluent from selected wastewater treatment plants in Blantyre, Malawi, have reported phosphate concentrations above recommended levels. High phosphate levels in the effluent and streams pose a threat to aquatic life through the stimulation of excessive growth of plants and toxic cyanobacteria. Phosphate removal by bauxite was investigated as a function of pH, contact time, dosage, competing ions and initial phosphate concentration by means of jar tests. Phosphate removal increased with decreasing pH with maximum removal (99.75%) recorded at pH 2.40 ± 0.10. Phosphate removal was attributed to ligand exchange reactions with gibbsite and goethite minerals that are chemically and electrostatically favoured at low pH. Bauxite indicated a high phosphate removal capacity with 98.42% removal recorded for a dosage of 15 g/l. This was attributed to the presence of goethite and gibbsite minerals for which phosphate has a strong affinity. Kinetics studies revealed a fast adsorption reaction with 61 and 65% phosphate removal achieved after 30 min of contact at 20 and 40°C respectively. Phosphate removal was enhanced in the individual presence of calcium and magnesium ions whereas carbonate and sulphate ions reduced it by competing for active sites.

Key words: Adsorption, bauxite, phosphate, gibbsite, goethite, eutrophication.