(702d) Electro-Assisted Ion Exchange Pretreatment Process for Enhancing Reverse Osmosis Water Recovery during Impaired Water Reuse

Inorganic membrane scaling is a major cause of process failure for reverse osmosis (RO), which can recover potable water from municipal wastewater. Pretreatment using selective ion exchangers can reduce membrane scaling by selectively removing scale-forming ions such as calcium and phosphate. Calcium- and phosphate-selective ion exchangers also enable calcium phosphate recovery via precipitation from spent regenerants enriched with both ions. However, conventional regeneration approaches are prohibitively chemical-intensive. We investigated an electro-assisted regeneration approach for RO pretreatment using two pH-sensitive ion exchangers: a hybrid anion exchanger (HAIX) with confined ferric oxide nanoparticles (FeOnp), and a weak acid cation exchanger (WAC) with carboxylic functional groups. Using an HAIX-WAC column train and synthetic secondary wastewater effluent, HAIX selectively removed > 90% phosphate due to Lewis acid-base interactions between FeOnp and phosphate; WAC selectively and completely removed calcium due to both electrostatic interaction and bidentate calcium complexation. Calcium and phosphate removal reduced their saturation indices, minimizing membrane scaling risks and enabling RO-based nutrient recovery. Upon exhaustion, we leveraged the pH sensitivity of HAIX and WAC to enable regeneration using pH 11 and pH 3 regenerant for HAIX and WAC, respectively. pH-dependent surface charges of FeOnp resulted in phosphate desorption when the FeOnp surface was negatively charged at pH 11. High pKa (~5) of carboxylic groups in WAC enabled calcium desorption at pH 3 when carboxylic functional groups were protonated. Via electrochemical water splitting of effluent from the column treatment train, we demonstrated that pH 11 catholyte and pH 3 anolyte were produced within 60 minutes. 50% HAIX capacity and 80% WAC were maintained for multiple cycles in batch using pH 11 catholyte and pH 3 anolyte regenerant, respectively. Column tests exhibited the same 50% HAIX regeneration efficiency. Despite WAC exhibiting marked decline of regeneration efficiency in columns, > 80% calcium removal was achieved after electro-assisted regeneration. Phosphate recovery (> 95%) from spent HAIX regenerant was demonstrated by precipitating with calcium in spent regenerant of WAC. By probing intraparticle diffusivity and intraparticle diffusion path length of HAIX and WAC, we determined that intraparticle diffusion path length was the key parameter controlling HAIX and WAC regeneration. This study will inform ion exchange material design for improved regeneration efficiency using mild regenerants and facilitate chemical-free RO pretreatment to enhance water and nutrient recovery from wastewaters.