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Quantifying Spatiotemporal Variability in Nanoplastics During Transport in Porous Media Using Low-Field Nuclear Magnetic Resonance
Summary
Scientists used a specialized imaging technique to track how tiny plastic particles (nanoplastics) move through sand and soil, mimicking how they'd travel through groundwater. They found that water chemistry—specifically salt content—dramatically affects whether these particles get trapped in the ground or keep moving toward water sources, with higher salt levels essentially stopping their spread entirely. This matters because it helps predict how nanoplastic pollution might travel through soil and potentially reach drinking water supplies, informing better environmental monitoring and filtration strategies.
Understanding the spatiotemporal variability of nanoplastics (NPs) in porous media is vital for environmental risk assessment, yet quantitative in-media analysis of NP distributions during transport remains limited. To address this, we innovatively applied low-field nuclear magnetic resonance (LF-NMR) as a non-invasive approach to dynamically monitor magnetic polystyrene nanoplastic (MPSNP) transport in saturated quartz sand. By establishing the relationship between LF-NMR transverse relaxation rate [1/T2,I − 1/T2,0] and MPSNP concentrations, we reconstructed spatiotemporal concentration profiles via T2 inversion. This methodology enabled systematic evaluation of the effects of ionic strength (IS), flow velocity, initial concentration, and flow direction. Three mathematical models were further applied to analyze MPSNP transport behavior. Results revealed IS as the dominant factor; increasing IS (0.001 to 1 mM) dropped mass recovery from 85.7% to 0%, the migration front no longer advanced at IS > 5 mM. Lower flow rates, higher initial concentrations, and horizontal flow also enhanced retention. The two types of two-site kinetic models provide a better fit for the features of the breakthrough curves. This novel use of LF-NMR demonstrates its robust capability to resolve spatial transport heterogeneity, underscoring that flow velocity, flow direction, and ionic strength are critical regulatory parameters that should be carefully accounted for when evaluating nanoplastic transport in porous media.