Transport of Microplastics Through Porous Media: Influence of Porosity and Pore-Water Velocity

In recent years, microplastics (MPs), have infiltrated diverse environments, including oceans, rivers, lakes, wetlands, groundwater, soils, sediments, air, human tissues, food systems, and even the atmosphere. Among these, groundwater, a critical freshwater source for industrial, agricultural, and domestic applications faces increasing threats from MPs pollution. There is still a knowledge gap in understanding the mechanisms and the pathways governing the transport of MPs in complex groundwater systems, which remains a significant research challenge. Hence, to understand the transport of MPs through the porous media, we have conducted a series of one-dimensional (1D) column transport experiments.  To quantify the transport of MPs through the porous media, two types of MPs with different functional groups, polypropylene (PP, C­­₃H₆) and polyethylene terephthalate (PET- C­­₁₀H₈O₄) of size 100-200 µm are considered for the present study. The experiments are conducted using porous media of IS Grade I (2mm-1mm, d₅₀= 1.5mm) and Grade II (1mm- 0.5mm, d₅₀= 0.75mm) experimental sand. Various flow velocities were used to determine the most vulnerable pore-water velocity on the transport of these contaminants through the porous media.  For each case, the experiment is conducted for 10 pore volume and after 10 pore volumes, the sand samples from different depths of the column are taken to determine the number of particles attached to the sand grains. We have observed that, as pore volume increases, the MPs count rises steadily for most samples, indicating enhanced transport through the porous media. This suggests that MPs are progressively mobilized through the porous media as the pore volume expands, with certain volumes contributing more significantly to the overall transport dynamics. Coarser sands (Grade I) with more prominent pores facilitate higher MPs movement, while finer sands (Grade II) reduce transport due to greater retention. Additionally, higher pore-water velocity enhances MP mobility, suggesting that environmental conditions with coarser soil and increased water flow can lead to greater MPs dispersion, impacting its distribution in natural soil-water systems. The findings of this study can play a crucial role in applying indirect site interventions to avoid the spreading of MPs through porous media at polluted sites.