Removal of excess fluoride from water using waste residue from alum manufacturing process

Abstract

The ability of waste residue, generated from alum manufacturing process, to remove fluoride ion from water has been investigated. Series of batch adsorption experiments were carried out to assess parameters that influence the adsorption process. The factors investigated include the effect of contact time, adsorbent dose, thermal pretreatment of the adsorbent, neutralization of the adsorbent, initial fluoride concentration, pH of the solution and effect of co-existing anions. Results showed that Adsorption of fluoride is fairly rapid in first 5 min and thereafter increases slowly to reach the equilibrium in about 1 h. The removal efficiency of fluoride was increased with adsorbent dosage. About 85% removal efficiency was obtained within 1 h at an optimum adsorbent dose of 16 g/L for initial fluoride concentration of 10 mg/L. Heat treatment and surface neutralization of the adsorbent did not improve the fluoride removal capacity and efficiency. The amount of fluoride adsorbed increased with increasing initial fluoride concentration. The percentage of fluoride removal remains nearly constant within the pH range of 3–8. The adsorption data at ambient pH were well fitted to the Dubinin–Radushkevick (D–R) isotherm model with a capacity of 332.5 mg/g of the adsorbent. The adsorption kinetic was found to follow a pseudo-second-order rate equation with an average rate constant of 2.25 g min −1 mg −1. The presence of bicarbonate at higher concentrations (100–500 mg/L) decreased the fluoride removal efficiency while other anions (chloride, sulfate, phosphate and nitrate) have no significant effect within the concentration range tested. The overall result shows that the waste residue is efficient defluoridating material.