Soil Solution Promotes Nanoplastic Aggregation via Eco-corona Formation and Hetero-aggregation

Nanoplastics in soil are exposed to soil solution, which is a mixture of microbial metabolites, dissolved organic matter, mineral and organic colloids, as well as inorganic ions. These components can interact with nanoplastics, thereby altering their surface properties and environmental behavior. Here, we examined how soil solution affects the aggregation kinetics and colloidal stability of nanoplastics made from a soil-biodegradable plastic (poly(butylene adipate-co-terephthalate), PBAT) and a conventional plastic (polyethylene). We found that both PBAT and polyethylene nanoplastics formed bigger aggregates in the presence of a soil solution extracted from a sandy loam soil, suggesting that the soil solution promoted the aggregation of both nanoplastics, thereby reducing their colloidal stability. Fluorescent excitation–emission spectroscopy revealed that microbial biomass in the soil solution dominantly adsorbed onto nanoplastics, followed by humic acid, forming an eco-corona that induced polymer bridging and attractive patch-charge interactions. Despite the observed bigger aggregates, the critical coagulation concentrations did not decrease correspondingly for either PBAT or polyethylene nanoplastics, which is likely due to the uncertainties of the critical coagulation concentrations as well as the hetero-aggregation between nanoplastics and colloids present in the soil solution. These results indicate that interactions with soil solution can decrease the colloidal stability of nanoplastics via eco-corona formation and hetero-aggregation, underlining the role of the complex interactions between nanoplastics and their surrounding matrices on the environmental behavior of nanoplastics.