Use of response surface methodology for optimization of fluoride removal efficiency by adsorption on black mustard husk ash

Abstract

Fluoride removal techniques have been studied for several years. In recent years various fluoride removal techniques have been developed in order to curb the health hazards from fluoride. Adsorption is the most widely used and researched method due to its advantages like economy, highly efficient, simplicity of application etc., over all other available defluoridation methods. This paper deals with the study of adsorption of fluoride using black mustard husk ash as an adsorbing material. The process parameters like pH, adsorbent dose, contact time and stirring rate were optimized using a three- level, four-factor, Box–Behnken design. The regression analysis of the experimental data acquired from 27 runs aided to develop a second order mathematical model in order to predict the fluoride removal efficiency. Using the response optimization, the optimum fluoride removal efficiency was found to be around 87%. The surface response graphs for the different process parameters were plotted and analyzed for studying their effect on adsorption phenomenon. The p-value obtained from the Analysis of Variance model suggested that the model was significant (p-value less than 0.005). The optimum values of pH, contact time, stirring rate and adsorbent dose were found to be 5, 75 min, 200 rpm and 2 g respectively. The predicted and adjusted R2 values were found to be 0.9177 and 0.9264 respectively. Adsorption models like Freundlich and Langmuir were used to validate the results. Langmuir model was seen best fiiting with the results having a R2 value equal to 0.9029. Pseudo first order kinetics and pseudo second order kinetic modelling was studied. The kinetic models showed the influence of physisorption as well as some amount of chemisorption in the adsorption process. From the results obtained it was concluded that the black mustard husk can be efficiently utilized for adsorption of fluoride from water.