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

Understanding the fate of microplastics (MPs) in soil is one of the most urgent environmental tasks we face. It is also a very challenging one, as there are numerous properties of both MPs and the soil, as well as hydrological and geochemical conditions, that interplay to affect MPs transport. In this study, we conduct laboratory experiments in which two of the most commonly used MPs types in agriculture (polyethylene terephthalate (PET) and polypropylene (PP)) are leached into an idealized soil analog (glass beads). We find that MPs inhibit water flow, delaying its passage through the sample and making it more tortuous, forcing the flow to occur through preferential pathways (fingers). These effects are more pronounced for PP, which is more hydrophobic, than for PET. The transport of PP is inhibited relative to that of PET, which is attributed to its both its impeding effect on water flow (the driving force), as well as its surface charge which increases its tendency to adsorb onto soil particles, and lower density which curbs downward transport.