Optimization of fluoride adsorption onto natural and modified pumice using response surface methodology: Isotherm, kinetic and thermodynamic studies

Natural pumice (NP), FeCl3 center dot 6H(2)O modified pumice (FEMP) and hexadecyl trimethyl ammonium bromide (HDTM.Br) modified pumice (HMP) were used for fluoride adsorption. The effect of pH (3-11), initial concentration (2-15mg/L), and adsorbent dosage (0.2-0.8 g/L) on the defluoridation was optimized by using central composite design (CCD) in the response surface methodology (RSM). Results showed optimum condition in the pH=3, initial concentration=2mg/L, and adsorbent dosage=0.71, 0.75, 0.70 g/L with the maximum removal efficiency of 9.39, 76.45, and 95.09% for NP, FEMP, and HMP, respectively. The adsorption equilibrium and kinetic data was in good agreement with Freundlich and pseudo-second order reaction. Thermodynamic parameters indicated a non-spontaneous nature for NP and spontaneous nature for FEMP and HMP. Positive enthalpy illustrated the endothermic nature of the process. On the basis of results, modification of pumice led to an increase in the fluoride removal efficiency.