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Microplastics as a Vector for HOC Bioaccumulation in Earthworm Eisenia fetida in soil: Importance of Chemical Diffusion and Particle Size.

The role of microplastics as a carrier on the bioaccumulation of hydrophobic organic contaminants in soil organisms is poorly understood. Two environmentally relevant scenarios, including earthworm Eisenia fetida exposed to contaminated soil amended with clean microplastics (S1) and clean soil introduced with pre-contaminated microplastics (S2), were simulated to estimate the importance of microplastics in bioaccumulation. Three polychlorinated biphenyls and polyethylene particles with different sizes were employed. The microplastics under S1 scenario significantly decreased the PCB bioaccumulation, whereas for S2 scenario, the PCBs on microplastics can be assimilated by earthworm. Compared to plastic-free treatments, the freely dissolved concentrations (Cfree) of PCBs were consistently lower in the soils amended with microplastics. Biodynamic model analysis suggested that the contribution of microplastic ingestion to bioaccumulation under S2 scenario (mean 26.1%) was higher than that under S1 (8.7%), likely due to slow chemical transfer between plastics and ambient media. These results indicated whether microplastics act as a source or a sink of HOCs depended strongly on the diffusion gradients between microplastics and soils. As ingestion of smallest particles (50 - 150 μm) showed the greatest contribution to bioaccumulation under S1 and S2 scenarios, the gradual breakdown of microplastics may increase their relative importance in contaminant transfer.