Influence of current density on fluoride removal using batch electrodialysis

Fluorine is a common chemical element in the environment. It can have both a beneficial and adverse effect on human health. The World Health Organization estimated the permissible limit of fluoride in drinking water to be 1.5 mg F-/dm(3). Electrodialysis (ED) could be a suitable technique for fluoride ion separation. During ED experiments, model solutions containing 5, 10, 100, or 200 mg F-/dm(3), and 0.5 g NaCl/dm(3) were used. Conventional cation-exchange and monovalent selective anion-exchange membranes were selected for ED defluoridation. The effect of current density (0.78, 1.72, and 2.34 mA/cm(2)) on process efficiency was evaluated. It was observed that with an increasing current density, the fluoride removal efficiency was diminished. In the case of a low initial fluoride content (5, 10, mg F-/dm(3)), the ED process allowed the fluoride content to be decreased below the permissible level. For elevated fluoride content, the elimination of fluoride ions was high, but the final fluoride content exceeded the acceptable level of 1.5 mg F-/dm(3). The calculated specific energy demand (0.14-0.42 kWh/m(3)) was dependent on the applied current density, as well as on the salt concentration of the treated solutions. The study also revealed noticeable fluoride deposition on ion-exchange membranes.