Background: Contamination of water bodies by phosphorus has caused ecological problems worldwide. Phosphate, an essential macro-nutrient for aquatic systems, causes eutrophication where industrial and household activities lead to excess amount of it to water bodies through wastewater. Modern and low-cost treatment technologies to remove phosphate from wastewater are sorely needed for developing countries. Objective: We investigated and evaluated design parameters to remove phosphate by sorption onto crushed volcanic rock common in Ethiopia where eutrophication of waters from waste streams is common. Methodology: Kinetic and equilibrium sorption experiments were conducted to evaluate removal of phosphate from aqueous solutions by crushed pumice and scoria volcanic rocks common in the main rift valley of Ethiopia. Washed and dried rocks was ground to 0.075 to 0.425mm particle size and variable amounts immersed in molal concentration of Phosphate for 240min to fully saturate the exchange sites at adjusted pH. We methodically evaluated contact time, solution pH, initial concentration of phosphate, ground rocks quantity, and particle size effects on the removal of phosphate. Phosphate was measured by molybdenum-blue ascorbic acid method using continuous flow analyzer. Zero point charges of the ground rocks were measured using solid addition method with KNO3 as solution maker and impact of co-existing ions on the adsorption was also employed. Results: The values of point zero charge were measured to be 7.2 and 9.3 for pumice and scoria respectively at fine particle size. The maximum removal capacities of phosphate 294.30mg/Kg and 168.95mg/Kg were obtained for pumice and scoria respectively for a phosphate concentration of 10mg/L, fine particle size (0.075mm to 0.425mm), pH 6.05, crushed rock doze 40g/L and contact time of 240min. The experimental data are well described by the pseudo-second- order equation with a correlation value of R2 > 0.99. Langmuir, Freundlich and Dubinin Radushkevich isotherm models were examined and fitted to the experimental data. Conclusion: kinetic and adsorption isotherm results showing it is practical to use volcanic pumice and scoria to remove phosphate from water, but additional testing in a flow-through set-up is required to reach efficiency of the approach for the practical application.
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