Layered double hydroxides-engineered biochar for selective phosphate recovery by flow-electrode capacitive deionization

Abstract

Flow-electrode capacitive deionization (FCDI) shows great potential for efficient phosphorus (P) extraction from wastewater. In this study, we introduce Mg-Al layered double hydroxides-engineered biochar (Mg-Al LDHs/BC) as flow electrodes in FCDI systems, demonstrating enhanced selective P removal and recovery. A higher Mg/Al ratio (4:1) increased the interlayer spacing of Mg-Al LDHs, enhanced the specific surface area, and improved the P static adsorption capacity (132.32 mg P g⁻¹) of the electrode materials. Under various operating conditions, FCDI with Mg-Al LDHs/BC as flow electrodes exhibited superior selectivity for phosphate ions compared to BC flow electrodes. In a 20-hour continuous treatment of artificial wastewaters, FCDI removed 52.3 % of the P in the influent, with 96.6 % adsorbed onto Mg-Al LDHs/BC electrodes while maintaining a low energy consumption (15.4–18.5 kWh kg⁻¹ P). This shows that the energy consumption, P removal efficiency and selectivity in our system are comparable to those reported in earlier studies. The strong affinity of Mg-Al LDHs/BC for HPO₄^2- enhanced selective adsorption of P in FCDI, driven by electrostatic adsorption and ligand exchange mechanisms. Our study demonstrates that Mg-Al LDHs/BC, with its high selectivity for P, enhances the selective removal of P in the FCDI system and is effectively immobilized on the electrode material for P recovery.