Vertical transport of polystyrene nanoplastics in natural soils under unsaturated conditions: influence of particle size and texture

Abstract

Nanoplastics (NPs) in terrestrial environments have emerged as a growing concern due to their ability to migrate vertically through soil, threatening the quality of groundwater and posing risks to ecosystems and human health. This study investigates how particle size and soil texture control the transport of polystyrene NPs (PSNPs) under unsaturated conditions. Infiltration column experiments were conducted using two natural soils and carboxylated NP spheres of 120, 500 and 1000 nm. Numerical modelling was applied to derive reactive transport parameters. Additionally, sorption experiments were performed to provide complementary information about the interaction between NPs and soils. Results show that a two-site kinetic model successfully describes the transport behaviour of PSNPs across different sizes. Smaller particles exhibit an overall higher mobility (51% of recovery), whereas larger particles are more strongly retained (1% of recovery). Chemical interactions dominate the fate of 120 nm NPs, while physical straining is the primary mechanism governing the immobilization of larger particles (500, 1000 nm). Soil texture also exerts a major influence on PSNP dynamics: clay loam soils effectively retain even the smallest particles, while coarser soils with higher porosity and permeability promote vertical transport. This study advances current understanding of the mechanisms driving NP transport through unsaturated soils and contributes to developing mitigation strategies to abate the presence of these contaminants in the environment.