Photocatalytic Performance of Ag3PO4/BiVO4 P-N Type Heterojunction for Treatment of Landfill Leachate Tailwater

A novel Ag3PO4/BiVO4 heterojunction was synthesized via a combined hydrothermal–in situ precipitation method. With an optimal Bi:Ag molar ratio of 1:2 and after calcination at 200 °C for 22 h, 0.9 g of this composite reduced the chemical oxygen demand (COD) of landfill leachate tailwater from 232 mg·L−1 to 142 mg·L−1 and its UV254 absorbance from 0.22 to 0.156 under visible light irradiation within 140 min. The material exhibited a bandgap of 2.56 eV, along with enhanced visible-light absorption and improved charge-carrier separation efficiency. In the Ag3PO4/BiVO4/peroxymonosulfate (PMS)/visible light system, using 0.5 g of catalyst and 2.0 g·L−1 of PMS at pH 11 reduced the COD from 242 mg·L−1 to 138 mg·L−1. A subsequent two-stage treatment process, integrating the Ag3PO4/BiVO4/PMS/vis and P25/UV process, achieved a final tailwater COD of 90 mg·L−1—meeting standard discharge limits—and a 69.5% removal of humic-like substances. The heterojunction catalyst retained its activity over four consecutive cycles. Radical quenching experiments and electron paramagnetic resonance (EPR) spectroscopy identified photogenerated holes (h+), hydroxyl radicals(·OH), and sulfate radicals (SO4−·) as the primary reactive species. Gas chromatography–mass spectrometry (GC–MS) analysis identified intermediate organic compounds and proposed plausible degradation pathways. These results support a reaction mechanism in which h+ oxidizes H2O to generate ·OH, while PMS accepts electrons to produce SO4−· and further ·OH radicals, leading to effective pollutant mineralization. Collectively, this solar-driven, sulfate radical-based advanced oxidation process offers an energy-efficient strategy with reduced chemical consumption for the sustainable treatment of refractory wastewater.