Synergistic Degradation of 2,4,4′-Trihydroxybenzophenone Using Carbon Quantum Dots, Ferrate, and Visible Light Irradiation: Insights into Electron Generation/Consumption Mechanism

In this study, we reported a unique single-step synthesis of photocatalytic carbon quantum dots (CQDs) with high electron density, efficient aggregation-induced emission, and improved surface chemistry properties. As-synthesized CQDs were applied with ferrate (a reactive and green oxidant) to synergistically degrade 2,4,4′-trihydroxybenzophenone (2,4,4′-HBP). The restriction of molecular rotation, vibration in the crystal structure, modification of the surface chemistry, and the aggregation/quenching of CQDs were highlighted by comprehensive characterization. Newly synthesized CQDs' generated electrons were consumed by ferrate, and this mechanism was highlighted by photoluminescence analyses. It was found that the photoluminescence intensity of CQDs was reduced by more than 96% when ferrate was introduced into the CQDs reaction system. The reaction kinetics of 2,4,4′-HBP degradation was inspected at different pH values (7.0, 8.0 and 9.0) using ferrate in a single dose and multiple-dose (sequential) additions. The CQDs + ferrate + visible light (Vis) was proven to be the best reaction system with a 99.9% degradation efficiency, as compared to CQDs alone (63%) and ferrate single dose (85%) and multiple-dose (96%) methods. In addition, 10 reaction intermediates were identified, implying that C-C cleavage, hydroxylation, self, cross, and end linkage were involved in the potential reaction mechanism. Besides, hydroxyl radicals were found to be the primary reactive species in electron paramagnetic resonance (EPR) analyses. The computational analyses including density functional theory and molecular orbital distribution elucidated the reaction sites of CQDs and 2,4,4′-HBP. These findings have important implications in understanding electron generation and consumption mechanisms with the synergetic contribution of CQDs and ferrate to degrade organic pollutants such as 2,4,4′-HBP.