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Aging and salinity jointly control the co-transport mechanisms of nanoplastics and 4-nonylphenol in estuarine soils: Multi-scale insights from molecular interaction to pore network evolution
Summary
Researchers combined soil column experiments, micro-CT imaging, and molecular dynamics simulations to trace how nanoplastic aging and salinity jointly control the co-transport of nanoplastics and the organic pollutant 4-nonylphenol through estuarine soils, finding that aging reversed nanoplastic behavior from a pollutant trap to a mobile carrier, that salinity amplified aggregation and pore blockage, and that hydrogen bonding between aged particle surfaces and the contaminant molecule drove the system's dynamics.
Coastal estuaries are key hotspots for the accumulation of nanoplastics and hydrophobic organic pollutants, yet the co-transport of these contaminants under coupled salinity fluctuations and plastic aging remains poorly understood. This study integrated microcosm soil column experiments, micro-computed tomography (Micro-CT), and molecular dynamics (MD) simulations to elucidate the co-transport of pristine/aged polymethyl methacrylate nanoplastics (PMN) and 4-nonylphenol (4-NP) in saturated estuarine soils along salinity gradients. Pristine PMN inhibited 4-NP migration through co-deposition, whereas aged PMN acted as a source of dynamic release, facilitating its continuous breakthrough migration. Notably, 4-NP exerted a bidirectional feedback effect on PMN migration. Although it enhanced the migration efficiency of pristine PMN by 3.8-22.9% through electrostatic repulsion and site competition, it acted as a molecular bridge to trigger heterogeneous aggregation, thereby impeding the transport of aged PMN and reducing its recovery to as low as 10.3% at 5 PSU. Salinity further amplified these aging-related interactions, increasing aggregate size by 6.3-14.8% and enhancing attachment efficiency by a factor of 1.6-3.2 within the range of 0.5 to 5 PSU. MD simulations indicated that oxygen-containing functional groups on the surface of aged PMN provided high affinity binding sites for 4-NP by enhancing hydrogen bonding. Micro-CT visualisation and pore network modelling further confirmed that high salinity narrowed pore size distribution and reduced connectivity, increasing the fraction of low-connectivity pores from 4.3% to 6.2%, thereby forming hydrodynamic dead zones that enhanced the retention of enlarged PMN/4-NP hetero-aggregates within them. These findings provide multi-scale insights into the fate of composite pollutants and inform strategies for assessing pollution risks in salinized farmlands and coastal groundwater systems.