Abstract

The defluoridation techniques generally practiced include (1) coagulation, (2) adsorption (including ion exchange), (3) electrochemical methods, and (4) membrane processes. Coagulation processes mainly use chemical reagents such as lime, calcium, or magnesium salts, poly aluminum chloride, and alum to make precipitation (or co precipitation) with fluoride, necessitating its removal. Adsorption is a popular technique practiced in fluoride endemic areas of the developing world. In this method, the adsorbent is used in fixed columns in packed beds and fluoride-laced water is cycled through it. The pollutant from a relatively bulk liquid volume gets concentrated and confined onto a small adsorbent mass, which can invariably be regenerated, reused, or safely disposed under control.^1,2 Electrochemical techniques mainly include electrocoagulation and other electrosorptive processes.^3,4 Electrosorptive techniques basically involve activation of an adsorbent bed and enhanced removal by application on an electric field. Electrocoagulation involves the use of aluminum electrodes that release Al³⁺ ions (by an anodic reaction) that react with fluoride ions near the anode. In this process, the removal of fluoride by precipitation is expected to occur at the electrode–electrolyte interface. Membrane techniques generally include reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), electrodialysis, and Donnan dialysis. A combination of two or more of these membrane techniques for enhanced removal of fluoride was also reported.^1,5,6