Selective defluoridation by electrodeionization in a mixed bed in well water

Excessive fluoride in groundwater poses both environmental and public health risks. Electrodeionization (EDI) is a continuous, energy-efficient, and sustainable electrochemical method that removes ions by coupling ion exchange and electrodialysis. This work evaluates a mixed-bed EDI system using commercial resins (Amberlite IRA-67 and Dowex MAC-3) for fluoride removal under synthetic and real well-water conditions. Adsorption isotherms and breakthrough tests were used to characterize the resin saturation behavior, yielding an apparent maximum capacity of 5.6 mg/g and a saturation time of ~53 min. Polarization analysis revealed the typical activation, ohmic, and concentration-controlled regions, with optimal operation in the ohmic regime (specific conductivity: 1.6 S/cm; limiting current density: 0.016 A/cm). Fluoride transport was primarily governed by electromigration, assisted by resin–ion interactions and sustained internal conductivity within the mixed bed. Competitive effects were strongly dependent on anion electrophoretic mobility, hydration energy, and charge, with chloride and carbonate ions facilitating fluoride transport. Removal efficiencies of 78–84% were achieved in synthetic solutions and 71.8% in real well water. During operation, the dilute compartment was maintained near-neutral pH (7–8), while the anion concentrate reached pH 11. Conductivity in the dilute stream decreased from 43.8 to 8.48 µS/cm, confirming continuous ion extraction, with a maximum fluoride mass flux of 1.28 × 10⁻⁴ mg/cm²∙s, low energy consumption, and a critical overpotential of approximately 0.15 V. These results demonstrate that mixed-bed EDI enables effective and selective fluoride removal through electromigration-dominated transport, even under complex ionic competition, offering a energy-efficient approach for groundwater defluoridation.