Electrodialytic removal of fluoride from water: Effects of process parameters and accompanying anions

The removal of fluoride from water by using electrodialysis with SB-6407 anion exchange membrane was tested at different current densities. After the determination of the optimum pH of feed phase under Donnan dialysis condition which gave the maximum transport of fluoride, the effect of mono- and bi-valent ions, such as chloride and sulphate, in the feed phase on the removal was investigated by electrodialysis. The transport of fluoride was also correlated with the flux data and explained according to structure of membrane. It was found that the removal of fluoride was higher in the absence of mono- and bi-valent ions. The reducing effect of sulphate ions on the transport of fluoride was higher than that of chloride ions, which was explained by considering interaction of both chloride and sulphate ions with the ionic group in the membrane and activity coefficient of fluoride in the presence of mono- and bi-valent ion set. Moreover, transport of fluoride increased with increasing current density and feed phase concentration. As the driving ion, the chloride ion is more efficient than the sulfate ion. Application of electrodialysis was carried out on the real water containing 20.6mg/L of fluoride. Despite the presence of chloride and sulphate in real water sample, fluoride concentration could be reduced to 0.8mg/L (96% removal), which was lower than World Health Organization (WHO) standard (1.5mg/L).