Engineered Synergistic Nickel–Cobalt Oxide Electrode: Boosting In Situ As(III) Oxidation for Low-Carbon Asymmetric Capacitive Deionization

Addressing arsenic contamination in groundwater is challenging due to the presence of highly toxic and mobile As(III), which often requires inefficient preoxidation to less toxic As(V) for effective removal. To overcome this, we developed a bimetallic nickel–cobalt (Ni–Co) oxide/activated carbon (AC) nanocomposite electrode via electrodeposition. Electrochemical analysis revealed that Co(III) directly oxidized As(III), while Ni(III) facilitated redox cycling to regenerate active Co sites, enabling continuous in situ electrooxidation. In batch-mode asymmetric capacitive deionization (CDI) at 1.2 V and pH 8, the Ni1Co1/AC electrode achieved an arsenic removal capacity of 0.73 mg g–1 and low energy consumption (0.968 kWh mol–1 As), outperforming monometallic systems. The As(III) removal improved at higher pH, reaching a conversion efficiency of 74.9% at pH 10. Mechanistic studies confirmed the role of dynamic Ni(II)/Ni(III) and Co(II)/Co(III) transitions in enabling this performance. Single-pass tests using simulated groundwater demonstrated stable performance and electrode reusability. These findings position the Ni–Co oxide-modified AC electrode as a compact, low-carbon, and chemical-free solution for point-of-use arsenic remediation in groundwater.