pH Dependence of Phosphorus Speciation and Transport in Flow-Electrode Capacitive Deionization

Electrochemical processes such as capacitive deionization have shown great promise for salt removal and nutrient recovery, but their effectiveness on phosphate removal was lower than other charged ions. This study hypothesized that the speciation and transport behaviors of phosphate ions are highly influenced by electrolyte pH, and it used experimental and modeling approaches to elucidate such impacts in flow-electrode capacitive deionization (FCDI) cells. Phosphate removal was investigated in either constant current (CC) or constant voltage (CV) charging mode with pH ranged from 5 to 9 in the feed solution. Results showed that the average P removal rate increased from 20.8 (CC mode) and 16.8 mg/min (CV mode) at pH 9 to 38.3 (CC mode) and 34.3 mg/min (CV mode) at pH 5 (84-104% in improvement), respectively. Correspondingly, the energy consumption reduced from 1.04 kWh/kg P at pH 9 to 0.59 kWh/kg P at pH 5 (42.9-56.1% in saving). Such benefits were attributed to the shift in dominant P-species from HPO₄^2- to H₂PO₄^-. Conversely, high-electrolyte pH (pH = 11) for flow-electrode led to ∼74.8% higher phosphate recovery during discharge compared with pH 5, which was associated with the higher distribution of phosphate ions in the electrolyte versus on the flow-electrodes due to surface charge change. These results improved our understanding in ion distribution and migration and indicate that solution pH is critical for operating FCDI reactors. It shed lights on the best practices on electrochemical phosphate removal and recovery.