Mechanisms of Long-Term Nanoplastic Release in Soil and Their Interaction with Soil Colloids

The quantification of nanoplastic retention in soils and release mechanisms mediated by soil colloids remains challenging. This study used palladium-labeled nanoplastics (Pd-NPs) in column experiments to investigate their retention and potential long-term release, focusing on interactions with soil colloids. Pd-NPs were primarily immobilized through low energy barriers related to charge heterogeneity or nanoscale roughness, but also influenced by blocking, ripening, charge reversal, and cation bridging. These interactions limited particle release under steady-state conditions or flow interruptions. However, transient changes in solution chemistry (e.g., increased pH and reduced ionic strength, IS) lowered energy barriers to detachment on some convex nanoscale roughness locations, facilitating remobilization of reversibly retained Pd-NPs. Cation exchange and IS reduction further promoted particle release (5.2%-30.7%) by decreasing charge heterogeneity and weakening cation bridging. Approximately 75.9% of the released Pd-NPs were mainly associated with soil colloids in the 0.1-2 μm range under reduced IS conditions via cotransport or heteroaggregation, and the detachment of these soil colloids was the dominant mechanism of their long-term release. In contrast, 68.7% of the released Pd-NPs under increased pH occurred independently of soil colloid release. Additionally, Pd-NPs bound to stable solid surfaces in deep primary minima were irreversibly retained (≤63.8%), preventing detachment. This study highlights the importance of understanding the interaction between Pd-NPs and soil colloids for accurate assessment of long-term environmental risks, particularly groundwater contamination.