Recent Advances in the Photocatalytic Reduction of Hexavalent Chromium Using Mesoporous Materials: A Review

The hazards associated with hexavalent chromium, Cr(VI), have prompted extensive research into treatment technologies for chromium-containing industrial effluents. Selection of an appropriate process depends on wastewater composition, treatment objectives, cost-effectiveness, operational feasibility, and monitoring and maintenance requirements. Mesoporous materials, with pore diameters of 2–50 nm, offer accessible surfaces and pore networks that can support adsorption and photocatalytic reactions. Following the independent discovery of ordered mesoporous materials in the early 1990s by researchers at Mobil and other research groups, their tunable physicochemical properties have supported applications across environmental catalysis. In Cr(VI) photoreduction, accessible surface area, pore connectivity, and surface functionality influence chromium-oxyanion adsorption and reactant transport, while semiconductor composition and interfacial architecture govern light absorption and charge-carrier behaviour. This review examines recent advances in mesoporous photocatalysts for Cr(VI) reduction, focusing on the relationships between pore structure, chemical composition, and reported treatment performance. Metal oxides, carbon-based composites, metal–organic and covalent organic frameworks, and heterostructured systems are discussed alongside strategies involving doping, sensitisation, and interfacial engineering. Particular attention is given to distinguishing adsorption and direct photolysis from photocatalytic reduction and to interpreting performance under different irradiation, pH, and electron-donor conditions. The reviewed literature indicates that mesostructuring can enhance photoreduction when accessible pores are combined with suitable surface chemistry and efficient charge utilisation. However, differences in experimental conditions limit direct comparison between materials. Catalyst recovery, aqueous stability, metal leaching, realistic wastewater matrices, and reactor design remain central considerations in translating laboratory performance into sustainable treatment applications.