High current density and flowrate favour the selective removal of fluoride over chloride in electrodialysis

Elevated fluoride (F-) levels in groundwater pose severe health risks, in necessitating efficient treatment methods to reduce these levels. In comparison with the traditional membrane-based ion removal technologies which require high energy to drive bulk water molecules to pass through the membranes, electrodialysis (ED) enables the transport of target ions through ion-exchange membranes under an electric potential difference, consuming comparatively less energy. However, ED exhibits low efficiency in selective F- removal because of its low concentrations in groundwater compared to other co-existing ions, particularly chloride (Cl-). This study establishes an approach to enhance the F- removal efficiency of ED by elucidating the impacts of applied current density (0.25 I/I-lim -0.75 I/I-lim) and flow rate (3.89-15.55 cm/s) on the selectivity of F- over Cl- under different F- (3-12 mg/L) and Cl- (30-120 mg/L) concentrations. Marked and marginal increases in F- selectivity were observed, respectively, under increasing current density and flow rate in all considered combinations of ion concentrations. Synergistic effect of current density and flow rate to the improvement of F- selectivity was observed, with the highest F- selectivity of 1.96 +/- 0.6 was achieved for the solution containing 12 mg/L F- and 30 mg/L Cl-. An empirical model was developed to estimate the F- selectivity coefficient, P-Cl-(F-), by focusing on the effects of five parameters: applied current, flow rate, and concentration of F-, Cl-, and other anions, with R-2 values of 0.8843 and 0.9064 respectively, for synthetic solutions and some selected Sri Lankan groundwater samples, respectively. The applied current (26.0 %), flow rate (12.2 %), and F- concentration (30.9 %) exhibited positive effects on Fselectivity, while other inorganic ions (20.0 % for Cl- and 10.9 % for the other anions) exhibited negative effects. This is the first time study revealing the synergistic effects of current density and flow rate on the selective removal of F- using ED.