Enhanced capacitive deionization for Cr(VI) removal from electroplating wastewater: Efficacy, mechanisms, and high-voltage flow electrodes

In recent years, researchers have increasingly focused on flow electrode capacitive deionization (FCDI) for heavy metal removal. However, the practical application of FCDI has been limited because of its low removal efficiency at low voltages. To address this limitation, we investigated Cr(VI) removal using FCDI in high-voltage mode (≥1.23 V). We aimed to clarify the mechanism and capabilities of high-voltage FCDI, including the removal energy efficiency, ion valence changes, migration process, and practical applications. Our findings demonstrate that even at high voltages (≤1.6 V), FCDI maintains efficient Cr(VI) removal and charge efficiency. This success was attributed to the significant IR drop in the FCDI system. Additionally, Cr (VI) removal by high-voltage FCDI results from a synergistic effect combining electroporation and electroreduction. The higher the applied voltage, the stronger was the electroporation capacity and the reduction ratio. Under the voltage condition of 2.2 V, the maximum electroporation capacity reached 172.82 mg g−1. However, excessively high-voltage can also lead to a more pronounced side reaction, resulting in a slow improvement in the removal efficiency. We further optimized the process parameters of the FCDI using response surface methodology (RSM). The optimized conditions yielded impressive results, a 99.96 % removal efficiency and low energy consumption (0.104 kWh/m3) in actual Cr(VI)-containing wastewater. Additionally, even in the 19th cycle of continuous operation, Cr(VI) removal by FCDI exceeded 90 %. This study unveils the efficacy and mechanism of Cr(VI) removal by high-pressure FCDI, which is expected to advance industrial applications of FCDI for treating heavy metal-containing wastewater. [Display omitted] •Electro-absorption can effectively increase the adsorption capacity of activated carbon.•Reduction of Cr(VI) to Cr(III) on activated carbon during the FCDI process.•High-voltage FCDI can effectively treat actual chromium-containing electroplating wastewater.•RSM-optimised FCDI removes 99.96 % of Cr(VI) from real electroplating wastewater.•Study reveals new insights into FCDI-based treatment.