Removal of fluoride from aqueous solution by adsorption on hydroxyapatite (HAp) using response surface methodology

A study on the adsorption of fluoride onto hydroxyapatite was conducted and the process parameters were optimized using Response Surface Methodology (RSM). Hydroxyapatite has been characterized by using different physicochemical methods. In order to determine the effects of process parameters namely temperature (20–40 °C), initial solution pH (4–11), adsorbent dose (0.1–0.3 g) and initial fluoride concentration (10–20 mg L−1) on fluoride uptake from aqueous solution, a three-level, four-factor, Box–Behnken design has been employed. The second order mathematical model was developed by regression analysis of the experimental data obtained from 29 batch runs. The optimum pH, temperature, adsorbent dose and initial concentration were found by desirability function to be 4.16, 39.02 °C, 0.28 g and 20 mg L−1, respectively. Fluoride removal was 86.34% at the optimum combination of process parameters. Dynamic adsorption data were applied to pseudo-first-order and pseudo-second-order rate equations. The time data fitted well to pseudo second order kinetic model. According to the correlation coefficients, the adsorption of fluoride on the hydroxyapatite was correlated well with the Langmuir and Freundlich models.