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ELECTROLYTIC DE-FLUORIDATION

ELECTROLYTIC DE-FLUORIDATION (EDF) a promising process of removing fluoride from water by electro-coagulation, which is an electrochemical technique, in which a variety of unwanted dissolved particles and suspended matter including fluoride can be effectively removed from an aqueous solution by electrolysis. Through the process of electrolysis, coagulating agents such as metal hydroxides are produced.The aluminum species act as a coagulant by combining with the pollutants to form large size flocs and then can be removed by settling and flotation making Electrolytic De-Fluoridation an efficient and better alternative to conventional coagulation.

In the EDF process, when aluminium electrodes are used, the aluminium dissolves at the anode and hydrogen gas is released at the cathode. During the dissolution of Al at the anodes various aqueous aluminium species are produced, which depend on the solution chemistry. The aluminium species act as a coagulant by combining with the pollutants to form large size flocs. Interactions occurring within an electrolytic defluoridation reactor are shown in the adjacent figure.

REACTIONS IN EDF

The electrolytic dissolution of anodes of EDF electrolyser by oxidation in water produces aqueous Al3+ species and the electrode reactions are outlined below:

Anodes : Al(s) -> Al3+ + 3e− (1)

Cathodes : 2H2O + 2e− -> H2(g) +2OH− (2)

The H2 bubbles float and hence drive the flotation process. The Al3+ ions further react as shown to form a solid Al(OH)3 precipitate:

Al3+ +3H2O -> Al(OH)3(s) +3H+ (3)

The strong presence of the hydroxy-aluminium thus generated maximizes the formation of aluminium fluoride hydroxide complexes [Al(OH)3-xFx] which is the main reason for defluoridation by electrolysis.