Insight into the Key Role of Cr Intermediates in the Efficient and Simultaneous Degradation of Organic Contaminants and Cr(VI) Reduction via g‑C3N4‑Assisted Photocatalysis

Photocatalysis provides an impetus for the synergetic removal of Cr­(VI) and organic contaminants, but the generation of Cr intermediates and their potential oxidizability may be overlooked in pollutant conversion. Herein, the Cr intermediates in the Cr­(VI) reduction process were emphasized in Cr­(VI)/bisphenol A (BPA) by using graphitic carbon nitride as a photocatalyst. The active species for BPA photodegradation in the BPA system and Cr­(VI)/BPA system suggested that the Cr­(VI) reduction process indeed promotes BPA photodegradation. Electron paramagnetic resonance (EPR) of Cr complexes and in situ variable-temperature EPR analysis demonstrated Cr­(V) intermediate (g = 1.978) generation in Cr­(VI) reduction and its oxidization for BPA degradation in photocatalysis. By adding the electron donor Na2SO3, BPA degradation was induced in Cr­(VI)/BPA solution, further confirming the positive effect of Cr­(V). Moreover, the difference in BPA degradation products in the BPA/air, Cr­(VI)/BPA/air, and Cr­(VI)/BPA/Ar systems indirectly explained why the Cr­(V) intermediate was involved in BPA degradation. Density functional theory calculations revealed that photogenerated electrons can reduce the free energy (0.98 eV) of converting Cr­(VI) into Cr­(V), which can facilitate the subsequent Cr­(V) oxidation step for BPA degradation. This work contributes to the exploration of the Cr­(VI) reduction process and the synergistic removal of organic pollutants in Cr­(VI)/organics systems.