Solar light-driven degradation of thiacloprid by polymer composites based on P-doped TiO2 as photoactive phase: Theoretical and experimental assessment of the reaction mechanism and degradation pathway

In this paper, visible light-activated phosphorus-doped TiO2 (P-TiO2) was used as a photoactive phase to prepare polymer composites for the degradation of the pesticide thiacloprid under direct sunlight irradiation. In particular, a monolithic composite aerogel, consisting of P-TiO2 embedded in syndiotactic polystyrene (PTsPS), and a composite polymer film, consisting of P-TiO2 immobilized on the surface of a Corona-pretreated polypropylene film (PT/PP), were prepared and characterized by XPS, TEM, XRD and N2 adsorption at-196 degrees C. The latter were then tested for the degradation of thiacloprid under solar irradiation. The best results were obtained using the PT/PP composite film, which allowed the total degradation of thiacloprid after 180 min of treatment. This performance remained almost unchanged even after several reuse cycles. The effect of pH and the concentration of bicarbonate (HCO3-), calcium (Ca2+), and chloride (Cl-) ions on the PT/PP film photocatalytic activity was also investigated. In addition, the photocatalytic activity of the PT/PP film remained high even in the presence of drinking water spiked with the target pollutant. Photocatalytic tests in the presence of scavenger molecules clarified that the hydroxyl radical is the main reactive oxygen species (ROS) responsible for the photodegradation mechanism of the target pollutant with P-TiO2, even if a possible role of superoxide cannot be excluded. Finally, DFT studies and ESI(+)-FT-ICR-MS analysis were conducted to formulate a hypothesis on the degradation pathway, identifying possible reaction intermediates.