Selective chromium and copper recovery from wastewater using flow-electrode capacitance deionization: In-situ reduction mechanism regulating metal charging characteristics.

[Display omitted] • A new extraction solution was prepared for the selective recovery of Cr. • The charged state of metal ions was adjusted using an in-situ reduction mechanism. • Improved FCDI can recover both Cr and Cu from wastewater simultaneously. • Membrane performance can be restored through acid washing to ensure long-term operation. Recovering valuable metal resources from industrial wastewater has environmental and economic significance. Flow-electrode capacitive deionization (FCDI) has been successfully used to remove salt and heavy metals. However, the use of this technology in heavy metal treatment has several limitations, including low selectivity, poor long-term stability, and production of low-quality metal products. In this study, a new strategy for recovering chromium (Cr) from wastewater was proposed using an in-situ reduction method. A liquid membrane chamber (LMC) was introduced into a classic three-chamber FCDI and an extraction solution containing Na 2 SO 3 and NaCl was used to capture Cr in wastewater. During FCDI operation, the CrO 4 2− ions entered the LMC were immediately reduced to Cr3+ and retained in the extraction solution. Completing anions (i.e., Cl−) in the extraction solution maintained the concentration of SO 3 2− ions, and thereby reducing the consumption of reagents. The improved FCDI system exhibited satisfactory separation performance and high stability during semi-continuous operation. Freshwater, Cr, and copper were obtained in the separation chamber, LMC, and flow electrode chamber, respectively. The facile approach may open the doors for the separation and recovery of metal resources from water/wastewater with high scalability and universality. [ABSTRACT FROM AUTHOR]