Hydrogen peroxide (H₂O₂) is a common and eco-friendly oxidant with an energy density comparable to that of compressed hydrogen, and it is easier to store and transport. It can be used in pulp, biotherapeutics, wastewater treatment, and chemical oxidation processes. (1−3) H₂O₂ is also considered an ideal liquid fuel due to its ability to serve as an energy carrier for the conversion of chemical energy into electrical energy. (4) The global demand for H₂O₂ has increased in recent years. The traditional industrial production of H₂O₂ is based on the anthraquinone (AQ) process, which involves process complexity, pollution problems, and energy-intensive consumption. (5) In recent years, photocatalytic H₂O₂ production has been intensively studied as a safer and more eco-friendly method and has the potential to substitute the AQ method. So far, various materials, such as TiO₂, (6,7) g-C₃N₄, (8−12) transition metal sulfides, (13−15) and covalent organic frameworks (COFs), (16−18) have been reported to drive the photocatalysis. (10)

Significant in-depth exploration is also underway in the field of photocatalysis. Ling et al. developed an octonary high-entropy oxide (HEO) (Ti/V/Cr/Nb/Mo/W/Al/Cu) that integrates eight earth-abundant metal elements into a single rutile phase. This material achieves full-spectrum visible-light absorption enabled by its high-density oxygen vacancies and chemical complexity, thus facilitating efficient H₂O₂ production in the absence of any cocatalysts or sacrificial agents. (19) Wang et al. constructed a resorcinol–formaldehyde resin/MgIn2S4 S-scheme heterojunction (RF/MIS). The optimized material achieved a H₂O₂ production rate of 892.6 μmol·L–1·h–1. This breakthrough provides a new strategy for developing organic photocatalysts with a broad-spectrum response. (20) Yang et al. synthesized heteroaromatic covalent organic frameworks (TTT-COFs), enhancing stability and extending the π-conjugated system by introducing sulfur atoms. The resulting material combines high chemical stability with a reinforced donor–acceptor structure. It achieved a production rate of 29.9 mmol·g–1·h–1 in a system containing 10 vol % benzyl alcohol as a sacrificial agent. (21)

Although light, water, and O₂ are adopted in this method, most of the reported systems still encounter the problems of dependence on organic proton–electron donors, pure O₂ gas, and freshwater to increase the H₂O₂ productivity, which increases process complexity and costs. Seawater is one of the most abundant resources on Earth. Photocatalytic H₂O₂ production from seawater can reduce costs, and the as-prepared H₂O₂ can potentially be used in desalination, disinfection, and marine ecosystem protection. There have been a certain number of reports on the photoproduction of H₂O₂ in seawater. (22−29) However, more efficient photocatalysts for H₂O₂ production are in urgent need of being developed to solve the issues of dependence on organic proton–electron donors and pure O₂, as well as the influence of competitive ions in seawater.

Radioactive nuclide wastewater contains uranium, cesium, strontium, barium, etc. Among them, radioactive ¹³⁷Cs has a high fission yield and is highly radioactive. (30) During the release of ¹³⁷Cs, a large amount of radiation is generated with a half-life of up to 30 years, which greatly affects the ecological environment and human health. Many technologies, such as ion exchange, adsorption, filtration, chemical precipitation, and solvent extraction, have been developed for the treatment of nuclear wastewater containing cesium. (31−34) Among them, the adsorption method has the advantages of feasibility, high efficiency, and low cost, and is widely used in removing Cs⁺ from wastewater. As a member of the absorbent materials, it is reported that a wide variety of nuclear element adsorbent materials have been documented, including oxides, (35) metal–organic frameworks (MOFs), (36) graphene, (37) zeolites, (38) etc. MXene Ti₃C₂ and its derivatives have been frequently reported in the elimination of radioactive nuclides (e.g., Sr²⁺, Cs⁺) from nuclear wastewater. (39−45) MXene Ti₃C₂ is one of the famous transition metal carbides, possessing a two-dimensional (2D) layered structure, a high specific surface area, and sufficient adsorption sites...

Figure 1. (a) Adsorption capacity of Ti₃C₂ for Cs⁺ at different initial concentrations (conditions: m = 0.07 g, m/V = 1.4 g·L^-1, pH = 7.0, T = 298 K), (b) pseudo-first-order kinetic model, (c) pseudo-second-order kinetic model, and (d) intraparticle diffusion model for Cs⁺ adsorption on Ti₃C₂ (conditions: m = 0.07g, m/V = 1.4 g·L^-1, pH = 7.0, T = 298 K), and (e) adsorption isotherms for Cs⁺ adsorption on Ti₃C₂ (conditions: m = 0.07 g, m/V = 1.4 g·L^-1, pH = 7.0, T = 298 K, t = 7 h).

Figure 3. SEM images of (a) Ti₃C₂ and (b, c) Cs⁺(100)/Ti₃C₂; TEM images of (d) Ti₃C₂ and (e, f) Cs⁺(100)/Ti₃C₂; TEM elemental mapping images of (g) overall distribution of (h) C, (i) Ti, and (j) Cs.

In this study, a sustainable system was successfully constructed based on recycling the MXene Ti₃C₂ adsorbent after Cs⁺ uptake as an efficient photocatalyst, and further used for the photoproduction of H₂O₂ in real seawater. The adsorptive performance and behavior of Ti₃C₂ were evaluated for model nuclear wastewater at different Cs⁺ concentrations. Ti₃C₂ achieved a maximum adsorption capacity of 31.16 mg· g^-1 for Cs⁺ with an initial concentration of 100 mg·L^-1. Isotherm and kinetic studies suggested that the adsorption process followed the monolayer chemical adsorption behavior. For the recycled materials, Cs⁺ was uniformly distributed on the layer surface of the Ti₃C₂ substrate and promoted the delamination of Ti₃C₂. This unique architecture substantially enhances the density of active sites, electron transfer capability, and electron–hole pair separation efficiency. More importantly, the bandgap of Ti₃C₂ was interestingly regulated by the adsorbed Cs⁺ from 1.51 eV to a semiconductor range of 2.00 eV. The Cs⁺(100)/Ti₃C₂ sample demonstrated exceptional photocatalytic performance in real seawater, achieving 892.43 μmol·g^-1 h ^-1 H₂O₂ productivity, which is 1.84 times higher than that of pristine Ti₃C₂.