Hybrid Anion Exchanger with Dispersed Zirconium Oxide Nanoparticles: A Durable and Reusable Fluoride-Selective Sorbent

Abstract

Abstract

More than 300 million people across the world are currently at risk for skeletal and dental fluorosis due to drinking fluoride-contaminated groundwater. Current treatment solutions are not adequate in a number of manners: short adsorbent lifespan, high waste generation, poor treated water aesthetics, continuous dependence on electricity, poor recovery of extracted groundwater, and economic unsustainability. Hybrid anion exchange resin, referred to as HAIX-Zr, was synthesized in this investigation by impregnating strong base anion exchange resins with nanoscale zirconium oxide (ZrO₂) particles at a loading rate of >10% zirconium content (w/w). Nanoparticles of ZrO₂ were <50 nm and had a mean diameter of 15 nm. Polymeric anion exchange resins are durable hosts with an environment of fixed positive charges (e.g., quaternary ammonium functional groups, R₄N⁺) that create enhanced local anion concentrations via the Donnan membrane effect. Synergy of the robust polymer phase and high surface area of dispersed zirconium oxide nanoparticles resulted in high fluoride sorption capacity even in the presence of high concentrations of other competing anions. Most notably, high concentrations of sulfate (500 mg/L) showed practically no noticeable competing effect and demonstrated the high fluoride selectivity of HAIX-Zr. No physical fragmentation or loss in capacity occurred during cycles of exhaustion-regeneration; ZrO₂ is chemically stable from pH 3 to 12. Kinetics of the sorption process during a fixed-bed column run was controlled by intraparticle diffusion, and intraparticle fluoride diffusivity for HAIX-Zr was found comparable to other selective processes. HAIX-Zr was regenerated efficiently (>90%) over two cycles of exhaustion-regeneration with alkali (3% NaOH/3% NaCl) and acid (CO₂ sparged water) without any noticeable loss in capacity. HAIX-Zr is ready for field applications under representative conditions.