Weathering of nanoplastics reduces the effect of sand surface heterogeneity on their attachment in saturated porous media

The deposition of nanoplastics (NPs) in porous media is strongly influenced by natural weathering processes, such as UV exposure and adsorption of natural organic matter (NOM), but the deposition mechanisms of both, non-weathered and weathered NPs, remain poorly understood. In this study the effect of NOM on the transport of non-weathered polystyrene (PS) NPs and UV-weathered PS NPs was examined in saturated porous media under low ionic strength conditions. It revealed that the physical and chemical heterogeneity of the sand surface created favorable attachment sites, leading to site blocking and retardation of NPs. NPs followed a non-steady-state, two-stage transport dynamic: rapid, irreversible, and reversible or pseudo-equilibrium attachment on heterogeneous areas until site saturation, followed by slow, irreversible attachment on relatively homogeneous surfaces. Both UV weathering and NOM coating generated more negatively charged NPs, reducing the irreversible and reversible deposition of NPs. These weathering processes reshaped NP transport dynamics and masked the retardation effects induced by sand surface heterogeneity. Moreover, the impact of NOM on NP deposition varied depending on the extent of UV weathering and the molecular weight of the NOM.