ABSTRACT
The deuoridation 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 uoride, necessitating its removal. Adsorption is a popular technique practiced in uoride endemic areas of the developing world. In this method, the adsorbent is used in xed columns in packed beds and uoride-laced water is cycled through it. The pollutant from a relatively bulk liquid volume gets concentrated and conned 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 eld. Electrocoagulation involves the use of aluminum electrodes that release Al3+ ions (by an anodic reaction) that react with uoride ions near the anode. In this process, the removal of uoride by precipitation is expected to occur at the electrodeelectrolyte interface. Membrane techniques generally include reverse osmosis (RO), nanoltration (NF), ultraltration (UF), electrodialysis, and Donnan dialysis. A combination of two or more of these membrane techniques for enhanced removal of uoride was also reported.1,5,6
In general, the removal mechanisms that are operative in coagulation include: (1) charge neutralization of negatively charged colloids by cationic hydrolysis products and (2) incorporation of impurities onto an amorphous precipitate of metal hydroxide. The relative importance of these two mechanisms
depends on many factors, which are mainly pH and coagulant dosage.7 Deuoridation processes by coagulation include (1) precipitation of uoride by a suitable reagent through chemical reactions; (2) co precipitation of uoride, which involves its simultaneous precipitation with a macro-component from the same solution through the formation of mixed crystals, by multiple mechanisms such as adsorption, occlusion, or mechanical entrapment.1
