Effect of particle size on the transport of polystyrene micro- and nanoplastic particles through quartz sand under unsaturated conditions

Micro- and nanoplastics (MNPs) are contaminants of emerging concern recently found in soil ecosystems. Their presence in terrestrial environments and their migration to aquatic environments may become a risk for the health of ecosystems and, through them, of humans. Understanding the interaction between particle properties and physicochemical and hydrodynamic factors is crucial to evaluate their fate and their potential infiltration towards groundwater. This study investigates the impact of particle size on MNPs transport through sand under unsaturated conditions. Infiltration column experiments with polystyrene MNPs ranging from 120 to 10,000 nm were conducted and supported by numerical modelling to derive reactive transport parameters. Results show a significant effect of particle size on the transport of MNPs, with higher recovery values observed for smaller particles (120 nm; 95.11%) compared to larger particles (1000 nm; 71.44%). No breakthrough was observed for 10,000 nm particles, indicating a complete retention within the quartz sand matrix. DLVO theory confirmed the dominance of electrostatic repulsive forces between MNPs and sand grains, suggesting an unfavourable environment for MNPs to adhere to quartz sand. Consequently, particle retention in the sand matrix occurs predominantly by physical processes. Equilibrium sorption modelling reveals that larger particles (1000 nm) tend to be immobilized in small pores throats due to straining, resulting in lower recoveries. When they are not trapped, particles tend to travel faster through preferential flows due to a size exclusion effect, evidenced by shorter arrival times at the column outlet compared to tracers. These findings highlight the influence of particle size on the transport and retention of MNPs in quartz sand under unsaturated conditions and contribute to a better understanding of their transport dynamics and environmental fate.