Thermodynamics and kinetics study of de-fluoridation in waste water using hydroxyapatite (Hap) as adsorbent: Optimization using response surface methodology

Abstract

The adsorption of fluoride from aqueous solution onto synthesized hydroxyapatite is investigated in the present study. Hydroxyapatite is the cost effective and most efficient adsorbent, which is synthesized by co-precipitation method. Structure is characterized by FTIR, XRD methods. The Langmuir adsorption isotherm, Freun-dlich adsorption isotherm were used to explain adsorption between the liquid and solid phases in batch studies. The mechanism of adsorption process was illustrated by calculating Gibbs free energy, enthalpy and entropy from thermodynamic studies. This experiment was conducted to investigate the interaction of process param-eters using response surface methodology(RSM). The two level three factor(2 3) central composite design(CCD)with the help of Design Expert Software was used to optimize process parameters. The higher correlation coefficient(R2) value of 0.9671 indicates the fitness of response surface quadratic model. By using multiple numerical optimization, desirability function is achieved, which is used to identify the optimum conditions. The optimum removal efficiency of HAP for fluoride adsorption is 89.6% at optimum condition.