Microplastic polymer type impacts water infiltration and its own transport in soil

Microplastics (MPs) pose a substantial threat to humans and ecosystems. How MPs move in soils is controlled by a large number of coupled parameters, including MPs and soil properties as well as hydrological and geochemical conditions. We conduct laboratory experiments where two commonly MPs types found in soils-polyethylene terephthalate (PET) and polypropylene (PP)-are leached into an idealized soil analog (glass beads). We use time-lapse imaging to analyze the water flow pathways and spectroscopy to measure the MPs transport. We find that MPs impede water infiltration into preferential pathways, with a stronger effect for the more hydrophobic PP, and that PET is more mobile than PP. We explain this by the stronger impedance of PP on water flow that carries the MPs (the driving force), as well as PP surface charge enhancing its adsorption onto soil particles, and its lower density that limits downward transport. These findings advance our understanding the mechanisms underlying MP transport in soils.