Effect of calcination temperature on hydroxyapatite in fluoride removal from groundwater: Process optimization and kinetic study

The present study investigates the defluoridation capacity of calcium hydroxyapatite as an efficient and environmentally safe adsorbent material. Calcium hydroxyapatite was synthesized by co-precipitation route using Ca/P = 1.76 ratio followed by calcination at various temperature ranges (30–900 °C) and termed as CaHAP-T. The physical and chemical properties of CaHAP-T samples were analyzed by TGA, SEM, BET, XRD, TEM, UV-DRS, FTIR, and NMR techniques. The CaHAP-900 sample exhibited exceptionally excellent thermal stability, crystallinity and ion-exchange capacity. Furthermore, the parametric studies revealed 98.12 ± 1.03 % fluoride removal efficiency at 30 °C temperature and 30 min contact time at neutral pH using 1 g/50 mL of adsorbent dose. The well-fitted pseudo-second-order and intraparticle diffusion model indicates the chemisorption combined with diffusion control of fluoride ions sorption on CaHAP-900 adsorbent. This work offers a deep understanding of the removal of fluoride ions from contaminated groundwater in prevalent areas and the calcium hydroxyapatite (CaHAP-900) can be utilized as an efficient cost-effective adsorbent material.