Dual-Substrate Synergistic Photocatalysis: Exogenous Reagent-Free Co-Removal of Phenol and Cr(VI) via Electron-Donor-Mediated Redox Coupling over Modified Carbon Nitride.

Traditional photocatalytic systems for pollutant removal depend on exogenous oxidants or sacrificial agents, hampering sustainability. Herein, we report an exogenous reagent-free dual-substrate cooperative strategy. Without adding external chemicals (e.g., methanol, sulfite), the target pollutant phenol is directly used as an endogenous electron donor to drive the reduction of Cr(VI) and its own degradation over a thermally exfoliated graphitic carbon nitride (CN-T) catalyst. Phenol fulfills two critical roles in this system: (i) it is oxidized and degraded by photogenerated holes (h+), and (ii) it transfers electrons to CN-T, enhancing Cr(VI) reduction by photogenerated electrons (e-). Kinetic studies, DFT calculations, and EPR spectroscopy reveal that phenol adsorbs on CN-T via π-π stacking and hydrogen bonding, facilitating electron transfer from its HOMO to the CN-T LUMO. This process suppresses e--h+ recombination and promotes Cr(VI) reduction. In the coexisting system, CN-T achieved 76.4% Cr(VI) reduction and 99.8% phenol degradation, with a 6.4-fold higher Cr(VI) reduction rate constant than in the single pollutant system. Substituted phenols (4-methylphenol, 4-chlorophenol, p-nitrophenol) strongly correlate with electronic descriptors (Hammett constant, EHOMO), establishing a robust quantitative structure-activity relationship (QSAR) for predictive synergistic co-removal. This work advances photocatalytic redox coupling and provides a "waste-to-waste" strategy for sustainable complex wastewater.