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Aging reconfigures physicochemical inhibition mechanisms of polyamide microplastics on antibiotic transport in saturated soils: Micro-CT based pore characterization coupled with transport modeling
Summary
This study compared how fresh and environmentally aged polyamide microplastics affect the movement of the antibiotic trimethoprim through water-saturated soil, using soil column experiments, CT scanning to visualize pore structure, and computer modeling. Aged microplastics were more effective at blocking antibiotic movement than fresh ones, but the two types acted through different mechanisms — fresh plastics repelled the drug electrostatically while aged plastics changed soil pore structure. As both microplastics and antibiotics are widespread in agricultural soils, understanding how aging microplastics alter antibiotic transport is important for groundwater safety and antibiotic resistance risk.
Microplastics (MPs) and antibiotics coexist ubiquitously in terrestrial system, yet the mechanistic interplay between MP aging and antibiotic transport remains poorly understood. In this study, the effects of pristine polyamide MPs (PPA-MPs) and aged polyamide MPs (APA-MPs) on trimethoprim (TMP) mobility were investigated through coupled saturated soil column experiments, computed tomography scanning, and mechanistic modeling. Results demonstrate concentration-dependent TMP transport inhibition by both types of MPs, with APA-MPs exhibiting greater inhibitory effect than PPA-MPs at equivalent concentrations. Notably, the two types of MPs elicit distinct influencing mechanisms. The PPA-MPs mitigated TMP adsorption by inducing a dilution effect and enhancing electrostatic repulsion; however, they paradoxically hindered TMP mobility by causing pore occlusion. Aging shifted MP effects from physical blockage to adsorption-occlusion synergy via the following pathways: (i) particle size reduction exacerbates pore clogging and (ii) increased surface roughness and oxygen functionality enhance TMP adsorption. The >10-fold variation in breakthrough times and order-of-magnitude changes in adsorption coefficients under pH modulation far exceeded impacts from flow velocity or ionic strength. These findings reveal the aging-induced transition from physical blockage induced by PPA-MPs to adsorption-hydraulic synergy driven by APA-MPs, providing critical insights for predicting antibiotic fate in MP-contaminated saturated soils under dynamic environmental conditions.