In wastewater treatment, heterogeneous advanced oxidation processes (AOPs) are often evaluated by the degradation rate of pollutants, but this may overlook a problem: pollutants may not be completely mineralized but instead transfer and accumulate on the surface of the catalyst as degradation intermediates. Recently, the organic carbon transfer process (OCTP) proposed by Xing et al. in Nature Water indicates that partial degradation intermediates can mask active sites and lead to catalyst deactivation, but this deactivation is reversible. Based on this, we introduce OCTP as a question of where the carbon of heterogeneous AOPs goes, pointing out that the properties of the oxidant and the choice of the reaction path will determine whether the pollutants in the system move towards deep mineralization or surface accumulation, thereby affecting stability and long-term effectiveness. Based on this logic, we propose the index of catalyst regeneration extent (CRE) to quantify the recoverable effective life after cleaning and advocate designing corresponding strategies according to different situations.

Fe₃O₄ is a promising transition metal oxide for ion removal owing to its high theoretical capacity, hydrophilicity, and non-toxicity, but its structural instability during ion insertion-extraction limits practical application. Here, Fe₃O₄ was integrated with mesoporous carbon derived from biogas slurry to enhance conductivity and sustainability, followed by alkaline etching to introduce abundant iron vacancies (VFO). The resulting VFO-C composite exhibits accelerated charge transfer, numerous intercalation-active sites, and superior electrochemical stability. At 1.6 V, the material achieved a desalination capacity of 126 mg g^-1 and retained 96.6% of its initial capacity after prolonged cycling. This performance surpasses conventional Fe₃O₄ electrodes, highlighting the synergistic benefits of defect engineering and waste-derived carbon. The strategy not only advances high-efficiency and durable capacitive deionization but also broadens potential applications in energy storage systems such as supercapacitors and batteries.

Capacitive deionization (CDI) technology has been considered a promising desalination technique, especially for brackish water, because of its relatively low energy consumption, facile operation, and easy regeneration of electrodes. However, the desalination capacity, cost, fabrication method, electrochemical stability, and environmental unfriendliness of the electrodes have restricted the practical application of the CDI technique. Herein, we reported the one-step in situ preparation of nitrogen-doped and carbon-decorated MXene-derived TiO₂ (termed N-TiO_2−x/C) through the confinement-growth strategy. The small particle size (~ 25 nm) and uniform distribution of a peanut-like N-TiO_2−x/C material could be ascribed to the confined growth space created by the nanoporous structure of melamine foam. The defects produced by N doping provide an enhanced electrical conductivity and more adsorption sites, while wrapping with a carbon shell layer increases the conductivity and offers protection for N-TiO_2−x to achieve an excellent electrochemical stability. The prepared N-TiO_2−x/C electrode is hydrophilic due to the abundant oxygen-containing functional groups (e.g., C-O, N-Ti-O, -NO_x, and -OH) and exhibits a high salt removal capacity (33.4 mg·g⁻¹), desalination rate (1.5 mg·g⁻¹·min⁻¹), and remarkable cycling stability (without declining after 100 cycles), which might be ascribed to the synergistic effects of the short ion diffusion path, more active adsorption sites, enhanced conductivity, pseudocapacitive behavior, and protection of the carbon shell layer. This work provides a confined-growth strategy to develop MXene-derived oxide electrodes for electrochemical desalination.