Kinetic study of microplastic reactivity with hydroxyl radicals: Insights into photochemical fate and transformation products

Plastic pollution in the form of microplastics (MPs) poses a significant environmental threat, due to the widespread dispersion and persistence of these materials in aquatic ecosystems. This study investigates the reactivity of polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) MPs with hydroxyl radicals (HO•) through a kinetic competition method, using sodium benzoate (NaBz) as a reference compound with selective HO• reactivity. Photosensitization by NaNO3 under UV-B irradiation was employed as known HO• source. The first-order reactivity constants for PE and PP MPs were (μ ± σ) kPE = (6.58 ± 4.99) × 104 s−1 and kPP = (6.24 ± 2.98) × 104 s−1, respectively. On the other hand, the reactivity of PVC MPs was influenced by reactions with organic compounds leached from the plastic material. Long-term photodegradation experiments revealed some small morphological changes in PE and PP MPs as a result of photoaging. The process also produced low molecular weight organic compounds, including short-chain carboxylic acids, and indicates that the presence of anions such as acetate and (in the case of PVC) chloride is associated with the photodegradation processes of the investigated MPs. FTIR spectroscopy suggested the presence of carbonyl groups, associated with oxidation of the polymer chains and the possible presence of plastic additives. Transformation products (TPs) were elucidated, indicating dechlorination processes for PVC MPs and/or the organic compounds they carried, and hydrogen abstraction, primary oxidation, dehydrogenation, and oxidative cleavage for PE and PP MPs.