Removal of Co²⁺ Ions in Aqueous Solution via Capacitive Deionization Using a TiO₂ Nanoparticle-Based Electrode

Cobalt contamination from industrial and radioactive sources poses serious environmental and health risks. Conventional removal methods suffer from drawbacks, such as secondary waste generation, high costs, and poor selectivity. Here, we present a capacitive deionization (CDI) system using a TiO₂ nanoparticle-based electrode to utilize their abundant surface hydroxyl groups and strong capability for redox processes. In particular, to enhance the adsorption ability of the electrode, anodized TiO₂ nanoparticles was utilized as an active material of the electrode. The CDI system exhibited a clear trend of enhanced removal performance at higher applied voltage, near-neutral pH (5-7), and lower initial Co^{2 +} concentrations, achieving an excellent maximum removal efficiency of 99.1% under the optimized conditions. This system showed superior removal efficiency from the commercial TiO₂-based system or the conventional activated carbon-based system, and the removal efficiency was up to 80% under the conditions of coexistence with Na⁺, Ni²⁺, and Zn²⁺. In addition, the system showed about 88% desorption rate with 1 M HCl solution, and the system maintained its removal performance up to 89% in the third cycle. To elucidate the mechanisms underlying this excellent adsorption performance, isotherm and kinetic analyses were conducted, which revealed that the heterogeneous multilayer adsorption governs the reaction, with physical adsorption dominating the initial stage and chemical adsorption prevailing at the later stage. Surface chemical and electrochemical analyses identified three main reactions: Ti-O-Co surface complexation, electrodeposition of Co⁰, and hydrogen evolution reaction-induced Co-(OH)₂ formation. This work establishes TiO₂ NP-based CDI as a highly effective strategy for Co^{2 +} removal, offering exceptional efficiency and broad applicability to critical domains, such as environmental remediation, human health safeguarding, and radioactive wastewater treatment.