Effect of acid on morphology of calcite during acid enhanced defluoridation

Fluoride removal from water by lime materials is a promising defluoridation process. Acid enhanced limestone defluoridation (AELD) technique involves precipitation of CaF₂ as well as adsorption of fluoride on the surface of limestone which is capable of reducing fluoride concentration to below the WHO guideline value of 1.5 mg/L. Acids such as acetic acid and citric acid are added to the fluoride water before filtration through limestone column to enhance the Ca²⁺ activity in solution for precipitation of fluoride as CaF₂. This paper describes the effects of these acids on the quality of the limestone during the AELD process, which has been studied to evaluate the reusability of the limestone. The reaction products that formed during the AELD process have also been analyzed. The detail study of the morphology of the limestone before and after use have been done using various analytical techniques, viz., X-ray diffraction, infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy combined with energy dispersive X-ray spectroscopy. The study reveals that the limestone degrades to some extent in the process due to dissolution of calcium carbonate by the acids and adsorption of fluoride by the limestone. While appreciable quantity of the citrate salt of calcium was formed in the column, the acetate salt mostly remained dissolved in the water. Since mainly the surface of the limestone particles take part in the reaction, the limestone particles can be reused for the defluoridation process after cleaning the outer surface. The limestone after use remains also suitable as raw material for cement.