New insights into cupric ion-mediated ligand-to-metal charge transfer between TiO2 with peroxydisulfate under visible light for bolstering benzophenone-3 degradation

Effective removal of hazardous and low biodegradable benzophenone-3 (BP-3) has been a challenging problem. In this study, a new Vis/TiO2/PDS/Cu2+ system was constructed to evaluate the catalytic performance of BP-3 during the co-induced ligand-to-metal charge transfer (LMCT) process by Cu2+, PDS and TiO2. Under visible light irradiation, trace Cu2+ efficiently triggering LMCT process from PDS to TiO2 via Cu–O bridging effect was firstly reported. In this LMCT-mediated photocatalysis process, Cu2+ was not only involved in the formation of visible-light responsive TiO2-Cu-PDS complexes but also inhibited the back electron transfer and promoted PDS activation by accelerating the electron transfer from the conductive band (CB) of TiO2 to PDS via Cu2+/Cu+ cycle. Compared with BP–3 removal in Cu2+/PDS system (18%) and Vis/TiO2 system (48%), that in Vis/TiO2/Cu2+ system (58%) and Vis/TiO2/PDS system (54%) exhibited a slight improvement. It was worth noting that BP-3 removal in Vis/Cu2+/TiO2/PDS system reached 91% within 30 min and the corresponding TOC removal was up to 76% in 120 min, which proved that the significantly enhancement of BP-3 degradation in Vis/Cu2+/TiO2/PDS system. Meanwhile, this system could maintain efficient catalytic performance in a wide pH range from 3 to 8. The formation of an interfacial charge transfer complex of Cu-PDS on TiO2, and the boosted generation of O2–, SO4− and HO radicals over the Vis/Cu2+/TiO2/PDS system were confirmed. Notably, visible light excited Cu-PDS complexes to distribute electrons to TiO2 (CB) via Cu–O bridge action, thereby inducing the enhanced visible-light activity of TiO2 and achieving superior catalytic performance for various pollutants in Vis/Cu2+/TiO2/PDS system. Furthermore, enhanced activation mechanisms of TiO2 and PDS by visible-light induced LMCT process were proposed in detail. Hydroxylation, demethylation and ring-opening dominated BP-3 degradation, and the weakened toxicity of BP-3 in Vis/TiO2/PDS/Cu2+ system was confirmed via the distinct increase of E.coli colonies numbers at the introduction of intermediates. This study offers new insights into LMCT activation approach and broadens the understanding of the ability of Cu2+ to be applied in photo-activated persulfate-based advanced oxidation processes for environmental remediation.