Enhanced fluoride removal from drinking water using non-calcined synthetic hydroxyapatite

Elevated level of fluoride in groundwater is a concern for human health in developing countries. This study presents the advantage of synthetic un-calcined hydroxyapatite over calcined hydroxyapatite for fluoride removal. The synthesized hydroxyapatite (HAP) was characterised using XRD, FEG-SEM, and XPS. The effect of HAP's calcination temperature was evaluated on fluoride removal. It was observed that fluoride uptake decreases with increasing calcination temperature. The fluoride uptake capacity of un-calcined HAP was found to be 4.38 mg F-/g HAP, whereas it was 3.53 mg/g for HAP calcined at 300 degrees C and further reduced to 0.7 mg/g for HAP calcined at 800 degrees C. Equilibrium studies revealed that fluoride uptake by HAP is best described by Langmuir isotherm and fluoride removal follows pseudo-second-order reaction kinetics. The defluoridation capacity decreased with an increase in pH (pH 5-9). Further studies revealed that HAP dose of around 2.5 g/L can effectively reduce fluoride to below 1 mg/L from groundwater having an initial fluoride concentration of 10 mg/L. Continuous flow column studies were also performed as a packed bed reactor mode of operation is preferred in a real-life scenario. The obtained results follow a conventional bed depth service time (BDST) model, hence uncalcined HAP packed bed reactor can be designed based on the BDST model to provide safe drinking water to the people at risk.