Ion Transport Channels Created by Anion Exchange Resin in Four-Chamber Flow Electrode Capacitive Deionization Enable Efficient Phosphorus Removal

Abstract

Flow electrode capacitive deionization (FCDI) technology can achieve effective phosphorus (P) removal from wastewater. In this study, a four-chamber FCDI (noted as F-FCDI) system was employed to systematically investigate the effects of P concentrations and pH values of the influent on P removal. It was observed that low influent P concentrations (p ≤ 500 mg/L) and low pH (pH < 2.5) significantly reduced the average phosphorus removal rate (APRR) and charge efficiency (CE) of F-FCDI. To address this issue, a FAD-FCDI system was developed by incorporating anion exchange resin into the diluate chamber of the F-FCDI system. Compared to the F-FCDI system, the FAD-FCDI system showed a 97.3–45.8% increase in APRR and a 103.4–40.0% increase in CE at the influent P concentration of 50–500 mg/L and a 57.3–33.5% increase in APRR and a 51.2–17.3% increase in CE at a pH of 1.6–2.5. The ion transport channels created by the anion exchange resins in the FAD-FCDI system are pivotal for maintaining ion conductivity at low P concentrations. The H⁺ exclusion effect, along with rapid adsorption of H₂PO₄^– of resin, facilitates the conversion of nonionic H₃PO₄ to H₂PO₄^– with rapid transportation ability at low pH. The complete mechanisms of electron transfer and ion transport in the FAD-FCDI system were elucidated. This study provides an energy-efficient strategy for the continuous removal of P from wastewater by an FCDI system.