Impact of aging on nanoplastic aggregation and release from low-density polyethylene (LDPE)

The pervasive use of plastics has led to significant environmental pollution, with nanoplastics (NPs) emerging as a critical yet less understood component that poses elevated risks due to high mobility, bioavailability, and toxicity. To address the limitations of using artificial pristine NP proxies, we investigated the aggregation of NPs released from low-density polyethylene (LDPE) using ultraviolet (UV) irradiation and heat treatment. We measured physicochemical property changes, conducted aggregation experiments in various electrolyte solutions, applied DLVO theory, and developed a mathematical model to simulate aggregation kinetics. Our findings reveal that aging processes, particularly heat treatment, significantly promote NP generation from LDPE and alter their physicochemical properties such as surface charge. NP aggregation is notably influenced by aging, pH, and cation valence. Heat-aged NPs exhibited enhanced stability due to more negative surface charges, while UV-aged NPs showed complex aggregation behavior in the presence of Ca, potentially due to bridging effects. The effect of aging and water chemistry on aggregation in most cases can be explained by the DLVO theory. The simple model we developed effectively simulated the observed aggregation kinetics. These results underscore the critical role of aging in determining NP generation and aggregation dynamics. This study highlights both the applicability and limitations of using artificial NPs as environmental proxies and emphasizes the need for further research on the properties and behavior of real-world, aged nanoplastics to improve predictions of their environmental fate and transport.