Phosphate removal from aqueous solutions using kaolinite obtained from Linthipe, Malawi

Earlier work on stream water and effluent from wastewater treatment plants in Blantyre, Malawi, has revealed high phosphate levels ranging from 0.63 to 5.50 mg/L. These phosphate levels would stimulate excessive growth of plants and toxic cyanobacteria in stagnant receiving water bodies hence posing a threat to aquatic life and water quality. Phosphate removal by kaolinite obtained from Linthipe, Malawi, was investigated as a function of pH, contact time, clay dosage, competing ions and initial phosphate concentration by means of jar tests. Phosphate uptake was pH dependent with adsorption mechanisms on kaolinite and iron oxide surfaces dominant generally below pH 7 and precipitation by calcium ions dominant above pH 7. Maximum phosphate removal occurred at high pH values of 11.22 (97.1%) and 10.96 (100%) for raw and treated kaolinite, respectively. Acid treated kaolinite indicated higher phosphate removal efficiency than the raw one at the normal working pH of 9.0 ± 0.2. This was attributed to release of extra calcium ions from CaCO3 present in the kaolinite samples during acid treatment. At the maximum dosage of 80 g/L, percent phosphate removal was 69.7 ± 0.100% and 98.5 ± 0.0577% for raw and treated kaolinite, respectively. This indicates requirement for high dosages to effect phosphate removal from the aqueous solutions. Studies on the effects of competing ions indicate that phosphate uptake decreased in the presence of CO 3 2 - , was little affected by SO 4 2 - , and increased in the presence of Ca2+, and Mg2+ ions. Furthermore, phosphate uptake approached equilibrium slowly at 20 °C and was faster at 40 °C.