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Concentration-Dependent Surface Oxidation of Polystyrene Microplastics in TiO2-Coated Hollow Glass Microsphere Composites Under UV Radiation in Solid-State Conditions
Summary
This study investigated whether exposing polystyrene microplastics embedded in a titanium dioxide composite to UV light under dry conditions actually breaks down the plastic in bulk, or merely oxidizes its surface. The results showed that surface oxidation increased with higher photocatalyst concentrations, but this did not translate into meaningful breakdown of the plastic's internal structure. The finding is a caution against interpreting surface chemical changes as evidence of true microplastic degradation — an important distinction for evaluating environmental remediation technologies.
Background/Objective: Photocatalytic oxidation is often interpreted as evidence of microplastic degradation, yet whether surface chemical modification under dry conditions corresponds to meaningful bulk polymer breakdown remains unclear. To help fill that gap, this study investigates the concentration-dependent photocatalytic aging of polystyrene (PS) microplastics incorporated into Titanium dioxide-coated hollow glass microsphere (TiO2–HGM) composites under solid-state UV irradiation, with emphasis on distinguishing surface oxidation from bulk degradation. Methods: Thin-film composites containing 1 wt%, 5 wt%, and 10 wt% TiO2–HGMs were exposed to UV-A irradiation (365 nm) for 183.5 h under dry conditions. Chemical and structural changes were evaluated using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and UV–visible spectroscopy. The carbonyl index (CI) was calculated from baseline-corrected integrated absorbance areas relative to an invariant aromatic reference band. Results: CI values increased from 0.483 (1 wt%) to 0.702 (5 wt%) and slightly decreased to 0.645 (10 wt%), indicating non-linear oxidation behavior and partial saturation. XPS showed a corresponding rise in the O/C ratio from 0.42 to 0.51. In contrast, UV–visible spectra exhibited minimal changes in aromatic absorption. Conclusions: Increasing photocatalyst concentration enhances surface oxidation but does not induce proportional bulk polymer degradation under solid-state conditions.