Fluoride removal from water using crushed limestone

An examination of defluoridation by industrial grade limestone indicated that a combination of precipitation and adsorption of fluoride can be more effective for defluoridation of water. Pre-acidified fluoride water using two edible acids, viz., acetic acid (AA) and citric acid (CA) have been used for treatment by crushed limestone of diameter 3-4 mm to precipitate fluoride as CaF2 in addition to adsorption of fluoride on limestone. The study has been carried out in batches by varying the acid concentration and contact time. Addition of the acids to the water before treatment with the crushed limestone in batch tests significantly improved the fluoride removal and this increased with the increase in the concentrations of the acids. The concentration of CA and AA required bringing down the fluoride concentration from 10 to 1.5 mg/L, are 0.05 M and 0.033 M, respectively, when the crushed limestone chips sample was used for the first time with contact time of 12 h. The acids are neutralized by limestone during the defluoridation process and the resulting final pH of the treated water was found to be in the ranges of 6.2 and 7.0 for CA and 5.7 and 7.0 for AA.