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.

Different techniques, viz., XRD, FTIR, TGA-DSC, SEM-EDS and XPS have been used for analysis of crushed limestone used in fixed bed column for acid enhanced defluoridation with acetic acid and citric acid. The used limestone remains fit for reuse in the fluoride removal.Figure optionsDownload as PowerPoint slideResearch highlights▶ The quality of limestone used in acid enhanced limestone defluoridation analyzed. ▶ The calcium salts of the acids formed can be removed to regenerate the limestone. ▶ The dissolution of limestone by the acid renews the limestone surface. ▶ There is a little effect on the morphology of the limestone. ▶ The remaining limestone after use retains its activity and can be reused.