Microplastic Properties Govern the Photodegradation of Sorbed Anthracene in Aquatic Environments

Current plastic manufacturing trends have led to increases in microplastic pollution and corresponding adverse effects for aquatic ecosystems. Microplastics are important sorption sites for hydrophobic organic contaminants. Over the microplastic environmental lifetime, sorbed contaminants may undergo solar photodegradation, but little is known about the influence of microplastic properties. In this work, we explored the photodegradation of anthracene, a model organic contaminant, framing our discussions around the complexity of the contributing processes. The different microplastics studied here exhibit different anthracene photodegradation kinetics: polystyrene (PS) ≫ low-density polyethylene (LDPE) > high-density polyethylene (HDPE). We find that intraparticle diffusion governs anthracene loss kinetics, with opposing effects during the sorption and illumination phases. The photodegradation rate of microplastic-sorbed anthracene increases with increasing concentration, which likely reflects additional reactive loss pathways for self-associated species. Finally, indirect photodegradation contributes to anthracene loss on PS, but not on LDPE or HDPE. Overall, the photodegradation of anthracene on microplastics is governed by multiple complex processes and microplastic properties (intraparticle diffusion, light transmission characteristics, potential for indirect photodegradation, contaminant self-association). This highlights that different microplastic types will have different effects on the fate and environmental impact of associated organic contaminants.