Microplastics as a Vector for HOC Bioaccumulation in Earthworm <i>Eisenia fetida</i> in Soil: Importance of Chemical Diffusion and Particle Size

The role of microplastics as a carrier in the bioaccumulation of hydrophobic organic contaminants (HOCs) in soil organisms is poorly understood. Two environmentally relevant scenarios, including earthworm <i>Eisenia fetida</i> exposed to contaminated soil amended with clean microplastics (S1) and clean soil introduced with precontaminated 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 the S1 scenario significantly decreased the polychlorinated biphenyl congener (PCB) bioaccumulation, whereas for the S2 scenario, the PCBs on the microplastics can be assimilated by earthworms. Compared to plastic-free treatments, the freely dissolved concentrations (<i>C</i>_free) of PCBs were consistently lower in the soils amended with microplastics. Biodynamic model analysis suggested that the contribution of microplastic ingestion to bioaccumulation under the S2 scenario (mean 26.1%) was higher than that under S1 (8.7%), likely because of the 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.