Fluoride mitigation by co-precipitation in calcium phosphate systems: Effect of process parameters and presence of synthetic non-calcined hydroxyapatite

Presence of fluoride in groundwater is a major environmental issue affecting millions of people in Indian subcontinent. Present study investigates the efficacy of fluoride removal using co-precipitation with calcium and phosphate. Results indicated that fluoride removal by co-precipitation was faster in presence of fluorapatite (FAP) seed crystals as compared to fluoride removal in absence of seed. Slower removal rate in absence of seed is due to longer induction period of FAP nuclei formation. However, total fluoride removal was same in both the cases which indicates that the addition of seed (FAP) did not alter the extent of reaction. The XRD and XPS analyses of the precipitated solids indicated the formation of fluorapatite. However, further investigation of molar Ca/P ratios of solids revealed that other calcium-phosphate phases were also precipitated. The effect of pH was evaluated in the pH range of 6.0-8.5. It was observed that the highest fluoride removal was at pH 7. Effect of bicarbonate concentration was evaluated in the concentration range of 50-200 mg/L. In the presence of bicarbonate (HCO3-) ions, fluoride removal was highest in the case of 100 mg/L (HCO3-) compared to 50 mg/L and 200 mg/L. Further experiments were conducted to observe the effect of presence of hydroxyapatite (HAP) on fluoride removal by co-precipitation. In these experiments, along with Ca2+ and PO43-, HAP powder was also added to solution containing Fluoride. Fluoride removal by co-precipitation improved significantly in the presence of HAP. Hence, co-precipitation in presence of HAP holds promise to provide safe drinking water.