D-band center-tuned CoNi alloy@carbon composite for high-efficiency Cr(VI) removal via capacitive deionization.

Hexavalent chromium (Cr(VI)) is a highly toxic heavy metal with serious environmental and health risks. Under the guidance of density functional theory (DFT), a CoNi@C composite was developed for Cr(VI) removal from water via capacitive deionization (CDI) by optimizing d-band centers of transition metal surface atoms to enhance interactions with Cr(VI) compounds. The performance experiment results show that the removal rate of this material for low-concentration Cr(VI) (<20 ppm) exceeds 99.67 %, and the adsorption capacity is higher than 253.87 mg/g. This exhibits performance superior to most previously reported electrode materials. In addition, CoNi@C demonstrated an excellent Cr(VI) removal rate (268.31 mg g ⁻¹ h-1) and stability (working on solar power for 30 h). Furthermore, the analysis results of the energy consumption for treating Cr(VI) indicate that by appropriately adjusting the flow rate of the input Cr(VI) solution, the energy consumption for removing Cr(VI) can be reduced to below 2.99 w∙h mg-1. Mechanism analysis indicates that the d-band center of the alloy CoNi nanoparticles is closer to the Fermi level. It promotes the strong adsorption of Cr (VI) through stable Co/Ni-O bonds and effectively transfers electrons to reduce Cr (VI) to Cr (III). The solar-powered CDI further verified its sustainability and practicality. This work provides a new strategy for the design of transition metal-carbon composites and advances the understanding of the mechanism for removing Cr (VI), offering great potential for eco-friendly water treatment technologies.