Combined Process for Sustainable Water Defluoridation Without External Acid or Base Inputs

Continuous adsorption for fluoride removal from water often requires external acids or bases to regenerate exhausted adsorbents, producing significant liquid waste and limiting its widespread application. Bipolar membrane electrodialysis (BMED) offers a potential solution by enabling in situ acid/base generation. However, its sensitivity to hardness ions (Ca2+/Mg2+) necessitates the use of (ultra)pure water, and liquid waste disposal remains a concern. This study introduces BMED-Def-Sof, an electricity-driven process integrated bipolar membrane electrodialysis (referred to as BMED), defluoridation (referred to as Def), and water softening (referred to as Sof) addressing these challenges. This system incorporates a nanocomposite adsorbent column for fluoride removal (Def) and a BMED unit requiring a minimal amount of NaCl as an additive. Approximately 3% of the defluoridated water is recirculated through a chelating resin column to remove excess Ca2+/Mg2+ (Sof), ensuring optimal BMED function. Spent regenerants from Def and Sof are mixed to precipitate fluoride and hardness ions as CaF2/MgF2, with the supernatant recirculated back to Def to achieve a zero liquid discharge system. A scaled-up defluoridation assay (0.4 t/d) was performed over two months, encompassing eight defluoridation-regeneration cycles. The results demonstrated exceptional operational stability and efficiency, consistently maintaining effluent fluoride concentrations below 1 mg/L. This study highlights the potential of the BMED-Def-Sof system to inspire the development of innovative and sustainable water treatment processes.