Degradation of oxytetracycline by the in situ hydrogen peroxide-producing photo-Fenton system g-C₃N₄/CQDs/Fe²⁺: Mechanism, degradation pathway and toxicity change analysis

Fenton reaction is an oxidation process that generates hydroxyl radicals (•OH) through the decomposition of H₂O₂ catalyzed by Fe²⁺, which is widely used for the degradation of organic pollutants in wastewater because of its fast •OH generation rate and simple operation. In this study, the g-C₃N₄/CQDs/Fe²⁺ photo-Fenton system was constructed by adding Fe²⁺ activated composite photocatalyst graphite phase carbon nitride/carbon quantum dots (g-C₃N₄/CQDs) to generate H₂O₂ in situ, which avoids the potential risk of H₂O₂ during storage and transportation, and broadens the pH range of the reaction. The system showed excellent activity for the degradation of oxytetracycline (OTC) with a high degradation efficiency of 97% at a g-C₃N₄/CQDs dosing of 120 mg, an initial OTC concentration of 20 mg·L⁻¹, an Fe²⁺ dosing of 0.36 mmol·L⁻¹, and an initial pH of 7 for OTC solution. The excellent activity of g-C₃N₄/CQDs/Fe²⁺ photo-Fenton degradation of OTC based on experiments such as radical capture, determination of changes in the generation of •OH and comparison with the H₂O₂ generation process was demonstrated to be mainly derived from the •OH active species generated by Fe²⁺ activation of in situ H₂O₂ production. Further, the intermediate products of OTC degradation were detected and analyzed by mass spectrometry, and the possible degradation paths of OTC were inferred. In addition, by measuring the optical density (OD₆₀₀) value to obtain the growth curve of bacteria, the results showed that the toxicity of the reaction solution gradually decreased with the degradation of OTC by g-C₃N₄/CQDs/Fe²⁺. Finally, compared with the conventional Fenton system with the addition of H₂O₂ to degrade OTC, the results showed that the photo-Fenton system can basically achieve the effect of conventional Fenton, and it is not limited by the pH range which provides a new idea for improving the application of Fenton reaction in actual wastewater treatment.