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

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 Na2SO3 and NaCl was used to capture Cr in wastewater. During FCDI operation, the CrO42− 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 SO32− 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.