A study of suitability of limestone for fluoride removal by phosphoric acid-crushed limestone treatment

Abstract

Limestone samples collected from different locations have been characterised using IR spectroscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and Brunauer-Emmett-Teller analysis and correlated with their fluoride removal abilities by phosphoric acid (PA)-crushed limestone treatment (PACLT). The effects of varying initial fluoride [F]0 and initial phosphoric acid [PA]0 concentrations, contact time and coexisting anions on fluoride removal and pH of treated water have been studied. PHREEQC was used to analyse the mechanism of fluoride removal and effects of impurities in limestone on the defluoridation process. Three of the limestone samples were found suitable for PACLT, with the treated water conforming to WHO guideline. With all limestone samples, defluoridation increased with increasing [PA]0 and decreasing [F]0. Fluoride removal and pH of treated water attained stability within 3 h. The influence of coexisting anions on fluoride removal was low for all limestone samples and followed the order sulfate > chloride > nitrate. The surface area and total pore volume above 2.55 m2/g and 0.0094 cm3/g respectively, and density below 2.68 g/cm3 are required for a limestone sample for defluoridation from 10 mg/L [F]0 to 1.5 mg/L with 0.68 mM [PA]0. PHREEQC analysis suggests the mechanism of defluoridation to be a combination of precipitation and adsorption, and that the presence of acidic metal oxides in limestone, viz., Fe2O3 and Al2O3 contributes in fluoride removal and lowers the pH of treated water from the predicted equilibrium pH of 7.93 in presence of PA. The results may be useful for predicting suitability of limestones for PACLT.