Visible-Light-Driven Heterogeneous Photo-Fenton Degradation of TCPP Using a PPy- g -C3N4-MIL-88B Heterojunction: Kinetics, Transformation Pathways, Toxicity Assessment, and Process Optimization

A visible-light-driven heterogeneous photo-Fenton system combining LED irradiation, H2O2, and a polypyrrole–graphitic carbon nitride–MIL-88B composite (PPy-g-C3N4-MIL-88B) was evaluated for the degradation of tris(chloropropyl) phosphate (TCPP) in water. Electron microscopy with elemental mapping, X-ray photoelectron spectroscopy, UV–vis diffuse-reflectance spectroscopy, and photoluminescence supported incorporation of the three components. The estimated optical bandgap decreased from 3.2 eV for MIL-88B to 2.3 eV for the composite; the Z-scheme charge-transfer configuration is proposed rather than established. In a four-factor central composite design (30 runs), degradation increased with irradiation time and airflow rate and decreased with pH, whereas catalyst dosage (50–250 mg/L) had only a minor effect; 98.6% degradation was measured at 50 min, 150 mg/L catalyst, pH 5, and 15 L/min airflow. Isopropanol produced the strongest inhibition, supporting a dominant contribution from •OH. Three chromatographically resolved transformation products were tentatively assigned from accurate-mass LC–MS data, whereas two ions with the di- and monoester formulas coeluted with TCPP and could not be distinguished from in-source fragments. Toxicity estimation predicted lower fish toxicity and bioconcentration for the candidate products. TCPP removal in four surface waters ranged from 65.0 to 79.6%, indicating matrix-dependent performance.