Effect of Preferential Microplastics Leaching Through Macropores on Vertical Soil Particle Transport

Abstract

Microplastic particles are an emerging pollutant that can be transported through preferential pathways, such as macropores. Agricultural soils, prone to microplastic contamination, often have macroporosity due to biological activity, natural formation of soil aggregates, desiccation cracks, or no-till practices. However, little is known about the factors controlling microplastic leaching through soil macropores. In this study, artificial soil-like macroporosity (2.5% v/v) was developed in packed soil columns to investigate the effect of macropore size, that is, 2, 3 and 4 mm, on the leaching of microplastics of three sizes, that is, 53–63, 75–90 and 125–150 μm, and the influence of both factors on the vertical mobilisation of soil particles. Under artificial rainfall (30.6 mm h⁻¹ for 30 min), microplastics were preferentially leached, with over 50% of the 75–90 μm and 125–150 μm particles transported, regardless of macropore size. Macropores without microplastics leached a significant amount of soil particles. Macropore characteristics, including pore diameter, number, and surface area, were strongly correlated with vertical soil particle transport. Microplastics increased vertical soil particle transport through macropores by a factor of about 1.6. A conceptual framework was developed to explain the observed mechanisms, emphasising the collisional forces exerted by microplastic particles detaching soil particles. Our results suggest that microplastic flow induced additional stress on macropore walls, intensifying soil particle mobilisation. This study also highlights that macropores (i.e., biopores) might be an underestimated driver of microplastic transport in the vadose zone and their role in intensifying vertical displacement of surface soil to subsurface layers. Given the environmental implications, further research will be needed to study these processes under field conditions.