Photocatalytic Degradation of Microplastics Using Graphitic Carbon Nitride Doped with Lanthanides

 This paper presents a novel method for degrading polyethylene microplastics (size range 1–50 µm) under visible light using a lanthanide-doped graphitic carbon nitride (g-C₃N₄) photocatalyst. Conventional photocatalysts like TiO₂ require UV activation, limiting practical applications. We synthesized europium-doped g-C₃N₄ via a one-step thermal polycondensation of urea with europium nitrate. Characterization by X-ray diffraction confirmed retention of the heptazine-based structure with a shifted (002) peak, indicating interlayer doping. X-ray photoelectron spectroscopy revealed Eu existing in both +2 and +3 oxidation states, creating mid-gap states that reduce the bandgap from 2.7 eV to 2.1 eV. Under simulated solar illumination (AM 1.5G, 100 mW/cm²), our catalyst achieved 89% mass reduction of microplastics within 72 hours, compared to 12% for undoped g-C₃N₄. Scanning electron microscopy showed progressive surface pitting and fragmentation into oligomers. High-performance liquid chromatography–mass spectrometry identified the primary degradation products as succinic acid, formic acid, and carbon dioxide. The reaction follows pseudo-first-order kinetics with a rate constant of 0.032 h⁻¹. Electron spin resonance trapping experiments confirmed that hydroxyl radicals (•OH) and superoxide anions (O₂•⁻) are the dominant reactive species. The europium sites facilitate electron transfer from the conduction band to dissolved oxygen, enhancing radical generation. Catalyst recycling over five cycles showed less than 5% loss in activity, with minimal europium leaching (0.2 mg/L). Toxicity assessment using Vibrio fischeri bioluminescence inhibition indicated that the degradation intermediates are non-toxic at concentrations below 50 mg/L. Comparative trials with real-world microplastics from laundry wastewater showed 76% degradation, highlighting practical potential. We also propose a continuous-flow reactor design for scaled implementation. Limitations include reduced efficiency in turbid water due to light scattering. Overall, this work provides a sustainable route to mitigate microplastic pollution in aquatic environments using earth-abundant materials.