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

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 PO₄^3- 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 PO₄^3- 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^-.