Abstract
Many marine species ingest microplastic (MP), exhibiting extended periods of MP gut retention in some cases. Organic pollutants associated with MP may present an alternative exposure route for these chemicals to marine species. However, the effect of PAH sorption to MP on PAH bioavailability remains poorly understood. Here, we investigate the sorption kinetics for two PAHs (fluoranthene and phenanthrene) to MP in natural seawater under semi-polar (10°C) and temperate (20°C) conditions. Spherical polyethylene (PE) microbeads with mean diameters ranging from 10-200 µm (PE-10, PE-50, PE-100 and PE-200) and polystyrene (PS) microbeads of 10 µm (PS-10) were used. Linear, Freundlich, Langmuir, Dual Langmuir, Redlich-Peterson and Dubinin-Ashtakhov adsorption isotherms were fitted to the data and the best fit identified depending on the MP type, size and temperature combination. For polymer mass and particle surface area, PAH sorption increased in the order PE-10>PS-10>PE-100. For PS-10 and PE-10 at 10 °C, the Redlich-Peterson isotherm model best described the sorption, indicating a combination of monolayer and multilayer adsorption. For PE-10 at 20 °C, the Dubinin-Ashtakhov model fitted best, indicating higher temperatures facilitate PAH transitioning into micropores. For PE-100, linear isotherms fitted best. These results indicate that adsorption processes dominate for smaller MPs (PE-10 and PS-10), while absorption into the polymer matrix becomes significant for larger particles (PE-100).