Insights in photocatalytic/Fenton-based degradation of microplastics using iron-modified titanium dioxide aerogel powders†

Microplastic (MP) pollution has become a serious environmental problem in the current decade. Unfortunately, wastewater treatment plants are not favorable for treating MPs. Therefore, it is necessary to develop methodologies to treat MPs in water efficiently. Photocatalytic (PC) and photo-Fenton (PF) processes are among the promising treatment methodologies that utilize reactive oxygen species (ROS) to degrade MPs. In this study, TiO₂ aerogel powders (TiAP) were prepared by lyophilization and subsequent annealing of peroxo-titanic acid gels, followed by modification with Fe at the surface for the PC/PF-based degradation of MPs. Fe-modification on TiAP boosts the PC activity and activates the PF-based process in the presence of H₂O₂. The degradation of polystyrene (PS) MPs was evaluated using attenuated total reflection infrared (ATR-IR) spectroscopy, total organic carbon (TOC) analysis, thermogravimetric analysis coupled with differential scanning calorimetry and mass spectrometry (TGA-DSC/MS), nuclear magnetic resonance (NMR) spectroscopy, and high-performance liquid chromatography with high-resolution mass spectrometry (HPLC-HRMS). Photo-induced degradation of the PS MPs was evaluated by monitoring the changes in the carbonyl/peroxyl index (CI/PI) recorded by ATR-IR spectroscopy and the mass loss measured by TGA-DSC/MS techniques. Interestingly, the samples with higher CI value changes affected the total mass residue, while samples with lower changes in the CI value did not alter the total mass residue after the photo-induced treatment. Further, NMR spectra confirmed the formation of new peaks due to the oxidative degradation of PS MPs, especially between 0.8 and 1.3 ppm. Additionally, by-products formed after the photo-induced treatment process analyzed by the HPLC-HRMS technique indicate the degradation of PS MPs. The indirect techniques of electron paramagnetic resonance (EPR) spectroscopy revealed the ROS contributing to the oxidation of PS MPs during the PC and PF treatment process using Fe-modified TiAP. This study's findings have the potential to significantly influence future research and environmental policies by providing better insights into preparing efficient nanostructures for photo-induced degradation of MPs.