Insight into the Fenton Stage for DOC Removal in the Photo-Fenton Process for Treating Synthetic Textile Wastewater

The photo-Fenton process combines both Fenton and H2O2/UV processes within a single reactor for dissolved organic carbon (DOC) removal. However, the dominant mechanism—iron coagulation or OH• oxidation—at the Fenton stage remains unclear. Synthetic textile wastewater containing reactive dye and polyvinyl alcohol was treated using a batch UV photoreactor. The first 10 min was designated as the Fenton stage, while DOC removal during the initial 30 min of the photo-Fenton process was evaluated through re-dissolution experiments to quantify the relative contributions of iron coagulation and OH• oxidation. The results show that at ferrous dosages of 10–40 mg/L, the Fenton stage contributed 55.6–91.9% of the overall DOC removal achieved by the photo-Fenton process. DOC re-dissolution experiments further demonstrated that DOC removal during this stage was predominantly governed by iron coagulation, which accounted for 68.6–87.2% of the overall DOC removal. In contrast, within the photo-Fenton process, when residual H2O2 was present, DOC removal was predominantly driven by OH• oxidation, accounting for 64.9–86.3% of total DOC removal. In addition, the specific H2O2 consumption per mg of DOC removed during the Fenton stage was approximately 2–6 times higher than that in the photo-Fenton process, demonstrating that the Fenton stage consumes H2O2 inefficiently and fails to effectively remove DOC through OH• oxidation. To reduce H2O2 dosage, UV irradiation demand, and overall oxidation time, separation of the Fenton and H2O2/UV processes into two distinct reactors is recommended.