What really limits photocatalytic water treatment? Charge separation and ROS‑oxidation versus electron‑driven reduction on MoS₂, Fe₃O₄, TiO₂ and ZnO

Understanding and improving photocatalytic water treatment requires directly linking catalyst band structure and surface properties to the fate of organic pollutants in aqueous media. In this presentation, layered MoS₂, non‑stoichiometric magnetite (Fe₃O₄), and phase‑pure rutile TiO₂ and wurtzite ZnO are benchmarked as photocatalysts for the degradation of environmentally relevant model contaminants in water: bromophenol blue (BPB), 2,6‑dibromo‑4‑methylphenol (DBMP) and rhodamine B (RhB). Layered MoS₂ (2H phase, Eg ≈ 2.03 eV) operates under strictly reductive conditions (argon, tert‑butanol, absence of dissolved oxygen) and achieves complete removal of BPB with rapid debromination, highlighting the potential of advanced reduction processes (ARPs) for halogenated pollutants that are poorly addressed by classical oxidative treatments. Non‑stoichiometric Fe₃O₄ samples with tunable Fe²⁺/Fe³⁺ ratios catalyse DBMP degradation at pH 8 with high bromide yields, consistent with a mixed reduction–oxidation mechanism that couples interfacial electron transfer to solution‑phase radical chemistry. In parallel, rutile TiO₂ and wurtzite ZnO are systematically evaluated for RhB degradation in water under air, argon and H₂O₂, enabling us to disentangle the roles of dissolved O₂, added oxidants and pH‑dependent surface charge in governing reactive oxygen species generation, adsorption and apparent kinetics. Across all systems, we show that surface charging (PZC, zeta potential), band‑edge positions and the hierarchy of available electron sinks determine whether oxidative AOP‑type pathways or reductive ARP‑type pathways dominate, and thereby control degradation efficiency and the extent of dehalogenation/mineralisation. These insights provide mechanistic design rules for environmentally oriented photocatalysts and operating conditions tailored to the treatment of halogenated aromatics and dye‑laden wastewaters.