Selective chromium removal and detoxification via capacitive deionization using nickel oxide-loaded oxidized MXene electrode: The critical role of Ti/Ni dual-redox centers

The selective removal and detoxification of hexavalent chromium (Cr(VI)) from industrial wastewater remains a pressing challenged due to the interference of coexisting ions. Herein, we present a redox-engineered material, nickel oxide nanoparticle-loaded oxidized MXene (NiO/O_x-MXene), as an innovative electrode for capacitive deionization (CDI) to enable efficient and selective Cr(VI) capture and transformation. By employing a facile potassium persulfate oxidation strategy at ambient conditions, titanium vacancies on MXene nanosheets were converted into multivalent titanium oxide domains, while NiO nanoparticles were simultaneously coated forming a dual redox-active interface (Ti³⁺/Ti⁴⁺ and Ni²⁺/Ni³⁺). This heterostructure significantly lowered the interfacial charge transfer resistance by 67 %, thereby, enhancing electrochemical performances and ion capture kinetics. The optimized NiO/O_x-MXene electrode delivered a Cr(VI) adsorption capacity of 18.54 mg g⁻¹ at 0.9 V, outperforming that of pristine MXene by 1.6-fold, while reducing Cr(VI) concentrations from 5 to 0.06 mg L⁻¹ with a low energy consumption of 4.85 kWh kg⁻¹ Cr(VI). Notably, the system maintained high selectivity even in the presence of competitive Cl⁻, NO₃⁻, SO₄²⁻ anions, with selective factors of 83.81, 83.10, and 44.79, respectively. It also retained 82.78 % Cr(VI) removal efficiency after five operational cycles. Mechanistic analysis revealed that Cr(VI) removal was governed through synergistic combination of electrostatic adsorption, ion exchange, and surface complexation, and redox-mediated electro-reduction to less toxic Cr(III) via interfacial electron transfer at Ti/Ni dual-redox centers. This work established a scalable and chemically robust CDI platform for targeted Cr (VI) remediation, providing a blueprint for rational electrode design towards selective heavy metal remediation from complex water matrices.