Continuous fixed-bed column assessment for defluoridation of water using HAp-coated-limestone

Abstract

A continuous flow fixed-bed column study was carried out using hydroxyapatite (HAp)-coated limestone to remove fluoride from water. The adsorbent was prepared by reacting limestone with sodium dihydrogen phosphate. The influencing conditions studied are bed mass (30, 40, 50 g), particle size (0.005-0.1, 0.1-0.2, 0.2-0.5 cm), influent flow rate (8, 16, 32 mL min−1), inlet fluoride concentration (5, 7, 10 mg L−1) and coexisting anions (Cl-, NO3-, SO42-). Results confirmed that the defluoridation capacity of the adsorbent increases with increase in bed mass but decreases with increasing flow rate and influent fluoride concentration. Among the coexisting anions, SO42- had the highest negative effect on defluoridation. It was found that the sorbent has a defluoridation capacity of 3.86 mg g−1 with bed mass of 40 g, particle size 0.1-0.2 cm, influent flow rate 8 mL min−1 and influent fluoride concentration of 5 mg L−1. The Thomas model, with better predictions for the breakthrough curves, was more capable in describing the dynamics of the fluoride adsorption process than the Adam-Bohart and Yoon-Nelson models. With a good adsorption capacity, the present adsorbent is competitive among many previously reported adsorbents. Finally, regeneration studies indicated that the fluoride-loaded sorbent can be effectively regenerated using NaOH solution.