Aggregation of Nanoplastics via Eco-corona Formation and Hetero-Aggregation in Soil Solution

Plastic particles present in soil are exposed to soil solutions containing a mixture of microbial metabolites, dissolved organic matter, mineral and organic colloids, as well as inorganic ions. These components can interact with plastic particles in different ways that may alter their surface properties and environmental behavior. In this study, 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). Without the soil solution, both PBAT and polyethylene nanoplastics aggregated more readily in CaCl2 than in NaCl, with critical coagulation concentrations of 344 and 284 mM in NaCl and 31 and 36 mM in CaCl2, respectively. The addition of the soil solution promoted the aggregation of both nanoplastics, as evidenced by the larger aggregate sizes, despite that the critical coagulation concentrations did not decrease correspondingly. Such an increase in aggregate sizes was induced not only by the formation of an eco-corona on nanoplastics, which enhanced aggregation through polymer bridging and attractive patch-charge interactions, but also by the heteroaggregation between nanoplastics and colloids present in the soil solution. These results suggest that interaction with soil solution can promote the aggregation of nanoplastics through eco-corona formation and heteroaggregation, underlining the role of the complex interactions between nanoplastics and their surrounding matrices on the environmental behavior of nanoplastics.