Fluoride removal by hybrid cation exchanger impregnated with hydrated Al(III) oxide nanoparticles (HCIX-Al) with novel closed-loop recyclable regeneration system

Abstract

A high concentrations of fluoride (F-) in groundwater limits drinking water supplies and results in fluorosis for hundreds of millions of people globally. A novel F- selective adsorbent (HCIX-Al) with a closed-loop regeneration system was developed, characterized, and assessed for its effectiveness in F- removal using both synthetic and natural groundwater. The system used an inexpensive polymeric macroporous cation exchanger doped with hydrous aluminum oxide (HAlO) nanoparticles. The HAlO nanoparticles were loaded into the matrix of the polymer phase by ion-exchange reactions between Al3+ and Ca2+ ions that bind with sulfonic acid functional groups of the cation exchangers. Hydrogen ions were generated in-situ during the HAlO formation process, creating acidic conditions inside the polymeric phase, which significantly increased F- sorption capacity (maximum sorption capacity = 5.75 mg/g at pH 7.5). While most other aluminum oxide-based F- selective sorbents suffer from low F- removal capacity at neutral to alkaline pH of typical groundwater, the HCIX-Al exhibited high F- sorption capacity even using alkaline (pH 8.5) groundwater. The treated water had a pH greater than 6.5 and non-detectable levels of aluminum (detection limit 0.002 mg/L), which complies with drinking water regulations. The HCIX-Al material can be reused by conditioning the exhausted material with a closed-loop regeneration chemical containing a recyclable 12% AlCl3 solution. The F- was precipitated from the regeneration system as CaF2 using lime at pH 11. This material, including a novel regeneration scheme, offered high F- removal capacity, cost-effectiveness, and provided a sustainable defluoridation process.