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

Abstract

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 °C and further reduced to 0.7 mg/g for HAP calcined at 800 °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 un-calcined HAP packed bed reactor can be designed based on the BDST model to provide safe drinking water to the people at risk.