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An imperative approach for fluorosis mitigation: Amending aqueous calcium to suppress hydroxyapatite dissolution in defluoridation

Sankannavar, Ravi, Chaudhari, Sanjeev
Journal of environmental management 2019 v.245 pp. 230-237
Fourier transform infrared spectroscopy, adsorption, calcium, defluoridation, drinking, drinking water, fluorides, fluorosis, hydroxyapatite, ion exchange, pH, water pollution, water quality
Drinking of fluoride (F⁻) contaminated water causes fluorosis and thus providing safe drinking water to the affected community remains a major challenge. Therefore, defluoridation without disturbing water quality is imperative. Hydroxyapatite (HAP) is proved to have a potential application for defluoridation; however, its dissolution during defluoridation is a concern for its implementation. Experiments conducted by suspending HAP in F⁻ solution and deionized water without F⁻ show that former had high residual pH and PO43− than the latter with Ca²⁺ being absent in the former. This indicates that Ca²⁺ had participated in defluoridation and promoted HAP dissolution when Ca²⁺ was unavailable. Hence, HAP dissolution seems to be a governing step for defluoridation. However, higher residual PO43− and pH affect drinking water quality, and its usage may pose additional health problems. Thus, Ca²⁺ deficient defluoridated water is unfit for drinking unless it is treated further. Hence, the present work proposes a novel method to overcome HAP dissolution by amending aqueous Ca²⁺ to F⁻ water. The results show that amending Ca²⁺ efficiently prevents HAP dissolution and enhances defluoridation capacity as an added feature. Furthermore, speciation using MINEQL+ and FTIR of fluoride-calcium treated HAPs suggest the possibility of defluoridation by aqueous CaF⁺ adsorption onto HAP besides F⁻ ion exchange with OH⁻.