Selective ion exchange adsorbents enable phosphate removal to ultra-low levels (e.g., &lt; 0.2 mg P/L) for eutrophication control. However, chemical-intensive regeneration of ion exchangers undermines process sustainability and accessibility in remote locations. We probed an electro-assisted regeneration approach for chemical-free phosphate removal and recovery 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 effluent, HAIX selectively removed phosphate with &lt; 0.2 mg P/L in effluent due to Lewis acid-base interactions between FeOnp and phosphate; WAC selectively and completely removed calcium due to both electrostatic interaction and bidentate complexation of calcium with carboxylic functional groups. 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, &gt; 95% phosphate recovery from spent HAIX regenerant was achieved 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. We envision this study to facilitate decentralized unit process development for phosphate removal and recovery in remote locations and inform material design for improved regeneration efficiency using mild regenerants.

Chemical-free phosphate removal and recovery using electro-assisted regeneration of pH-sensitive ion exchangers

ACS Fall 2020 Virtual Meeting

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