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Natural Weathering Reshapes Nanoplastic Aggregation and Deposition Dynamics: Implications for Nanoplastic Removal during Riverbank Filtration
Summary
Researchers examined how UV weathering and natural organic matter coatings alter nanoplastic aggregation and transport behavior through sand-based porous media relevant to riverbank filtration systems used for drinking water. The findings reveal that weathering shifts nanoplastic surface chemistry in ways that reduce removal efficiency, meaning environmentally aged nanoplastics may pass through natural filtration barriers and reach drinking water supplies.
Nanoplastics (NPs) are emerging colloidal contaminants whose environmental fate and associated risks are governed by aggregation in the aqueous phase and deposition within porous media, particularly during riverbank filtration (RBF), a widely applied natural treatment process for drinking water production. NPs undergo natural weathering processes including UV-induced weathering and eco-corona formation due to natural organic matter (NOM) adsorption. Porous media, such as sand grains, likewise experience natural weathering such as the development of spatially heterogeneous biofilms. In this thesis these weathering processes were assessed on the regulation of NP aggregation and transport in RBF-relevant systems. Our findings revealed that UV weathering modifies eco-corona structure and thereby alters NOM effects on NP aggregation. Using Suwannee River NOM as a model, NOM stabilized non-weathered NPs through steric repulsion, whereas it destabilized UV-weathered NPs via polymer bridging. In both cases, the magnitude of NOM effects increased with NOM concentration and molecular weight. Transport experiments in saturated porous media revealed a previously overlooked two-stage transport behavior governed by intrinsic sand surface heterogeneity. An initial stage was dominated by favorable attachment, leading to strong retention and delayed breakthrough, followed by a later stage characterized by unfavorable attachment. Both UV weathering and NOM coating reduced NP deposition in both stages, thereby masking the influence of surface heterogeneity. Furthermore, naturally developed biofilms on sand grains enhanced both favorable and unfavorable NP attachment. Eco-corona formation on NPs in river water suppresses initial favorable deposition while enhancing unfavorable deposition, resulting in enhanced long-term NP retention during RBF. Overall, this work demonstrated that natural weathering of both NPs and porous media exerts a decisive control on NP mobility in RBF systems, underscoring the necessity of incorporating environmentally aged particles and media into experimental designs for realistic risk assessment.