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

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.