Microplastics Modulate Aqueous Photolysis of Phenolic Intermediate-Volatility Organic Compounds

Microplastics (MPs) are ubiquitous in the atmosphere, yet their influence on the photochemical fate of coexisting organic pollutants in atmospheric aqueous phases remains poorly understood. This study investigated the effects of polystyrene (PS), polypropylene (PP), and polyethylene (PE) on the aqueous photodegradation of two phenolic intermediate-volatility organic compounds (IVOCs), 4-ethylphenol (4-EP) and 2,6-dimethoxy-4-methylphenol (DMMP), under natural and simulated sunlight. 4-EP loss in solution was significantly inhibited by all three MPs (PE > PP ≈ PS), whereas DMMP showed no significant variation among MP treatments. Adsorption of 4-EP followed the same polymer-dependent trend (PE > PP > PS), while DMMP exhibited uniformly low adsorption across all MPs. Aging introduced oxygen-containing groups onto MP surfaces, with PE showing substantial C-O enrichment. Under simulated sunlight conditions, sorption correction did not substantially alleviate the MP-induced inhibition of 4-EP, accounting for only 2.9%-4.2% of the total inhibition, suggesting that the suppression stemmed primarily from non-adsorptive mechanisms, such as optical shielding or interfacial interactions. Phototransformation also led to changes in product absorbance and oxidative potential (OP), with the extent of these changes modulated by MP type. Product absorbance at 275 nm followed PP > PS > PE for 4-EP, whereas DMMP showed the highest absorbance in the PE system. These findings suggest that atmospheric MPs may alter the aqueous-phase phototransformation of phenolic IVOCs and modulate the optical properties and OP of mixtures in a compound- and polymer-dependent manner.