Enhanced defluoridation using reusable strong acid cation exchangers in Al3+ form (SAC-Al) containing hydrated Al(III) oxide nanoparticles

Abstract

Currently used metal oxide based fluoride selective sorbents exhibit low sorption capacity, especially at near neutral to mildly alkaline pH (6.5–8.5) of typical fluoride-contaminated groundwater. The present study demonstrates a novel reusable strong acid cation preloaded in Al3+ form containing hydrated Al(III) oxide (HAlO) nanoparticles (SAC-Al) that can treat the fluoride across the pH range of groundwater with a simple regeneration capability. Once the cation exchange takes place (i.e., hardness cations namely Ca2+ displaces the Al3+ ions from the SAC-Al, the aluminum ions undergo hydrolysis reaction and simultaneously form HAlO nanoparticles onto the matrix of SAC-Al. Fluoride is then specifically removed from aqueous phase through Lewis acid-base interactions with freshly precipitated HAlO nanoparticles. Moreover, protons (H+) produced during the aluminum hydrolysis can promote acidic condition in the resin phase resulting in higher sorption affinity between HAlO and fluoride ions. From the experimental column runs, the hybrid material can remove fluoride below WHO recommended levels (1.5mg/L) for more than 400 bed volumes (BV) of mildly alkaline influent (5mg F/L, pH 8.0) and was stable for more than five exhaustion-regeneration cycles, over 2x greater performance than activated alumina (200 BV). The exhausted material can be efficiently regenerated by passing only stoichiometric amounts of diluted 2% AlCl3 solution. The results from SEM-EDX, TEM, and XRD techniques confirm that the aluminum was dispersed throughout the SAC resin as an amorphous hydrated Al(III) oxide nanoparticles.