Speciation and mechanisms of defluoridation by an electrochemical method
Electrochemically generated aluminum can remove most contaminants present in water and wastewater by precipitation and adsorption. 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 (electrocoagulation/flotation process). The solubility of aluminium in equilibrium with solid phase Al(OH)3 depends on the surrounding pH. The chemical composition of the system at equilibrium is computed using MINEQL+ software from the analytical data and the thermodynamic database. The optimal pH was found to be between 6 and 8. When pH<6, dissolved aluminium species such as Al3+ , Al(OH)2+ and are predominant and aluminium hydroxide is tended soluble. However when pH>9, soluble species Al(OH) + 2)(OHAl 4 is found to be predominant species. The strong presence of the hydroxy-aluminium in the pH range of 6-8, maximizes the formation of aluminium fluoride hydroxide complexes [Al(OH)3-xFx] and it is considered to be the main reason for defluoridation by electrocoagulation. The X-ray diffraction (XRD) analysis of the composition of the dried sludge obtained by electrocoagulation process shows the formation of Al(OH)3-xFx and the main reason for fluoride removal was confirmed.