Abstract
Many marine species ingest microplastic (MP), exhibiting extended periods of 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 sorption kinetics and present adsorption isotherms for two PAHs (fluoranthene and phenanthrene) to MP in natural seawater at 10 and 20 °C. Spherical polyethylene (PE) and polystyrene (PS) microbeads of 10 µm and 100 µm were used, representing ingestible and non-ingestible particle sizes for copepod species used in the bioavailability studies. Linear, Freundlich, Langmuir, Dual Langmuir, Redlich-Peterson and Dubinin-Ashtakhov isotherms were fitted to the data. 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-100, linear isotherms fitted best, suggesting that sorption is influenced by absorption. For PE-10 at 20 °C, the Dubinin-Ashtakhov model fitted best, indicating higher temperatures facilitate PAH transitioning into micropores. Using a novel approach (Figure 1), the modular effect of PAH sorption to PE-10 and PE-100 on PAH lethality and accumulation was investigated using two marine copepod species (Acartia tonsa and Calanus finmarchicus). The freely dissolved fraction of PAHs (Cfree) was measured before and after exposure, and the chemical body burden (C. finmarchicus) and observed lethality (both species) were used to determine PAH bioavailability. PE-10 reduced Cfree by ~50 % (FLA and PHE), PE-100 reduced Cfree by 21±5 % (FLA) and 32±3 % (PHE). The difference in Cfree reduction was not reflected in a reduction of lethality and body burden (> 50 % for both MPs), indicating a negligible amount of MP-sorbed PAH is bioavailable.