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Co-transport behavior of aged polymeric methyl methacrylate nanoplastics and florfenicol antibiotic in porous media: Effects of electrolyte, pH, and aging duration
Summary
This study investigated how aged nanoplastics and the veterinary antibiotic florfenicol travel together through soil and groundwater. Sunlight aging changed the nanoplastics' surface properties, affecting how far they migrate and how they interact with antibiotics in the environment. The findings are relevant to human health because they show how nanoplastics could help transport antibiotics through soil into groundwater, potentially contributing to antibiotic resistance in drinking water sources.
This study investigates single and co-transport behavior of aged (14 and 30 days) poly(methyl methacrylate) nanoparticles (14dPMMANPs, 30dPMMANPs) and florfenicol (FF) in saturated porous media, under varying ionic strengths (IS) and pH values. The results indicate that during the aging process, the carbon-oxygen double bonds in the ester group of PMMANPs were the first to be degraded under simulated sunlight exposure. In single transport experiments, the 14dPMMANPs exhibited higher mass recovery percentage, which can be attributed to their smaller hydrodynamic diameter and higher oxygen-containing functional groups. Interestingly, the oxygenated functional groups exposed on the 14dPMMANPs may provide more cation binding sites, resulting in stronger migration inhibition under Ca conditions compared to Na conditions. In contrast, the 30dPMMANPs displayed more negative zeta potential and a lower rate of particle size increase, weakening the inhibitory effect of divalent cations. Under co-transport conditions, FF promoted the migration of 30dPMMANPs in low IS, neutral solutions. Overall, FF reached a new equilibrium between transport inhibition (reduced electrostatic repulsion, increased hydrodynamic diameter of PMMANPs, and additional deposition sites on quartz sand (QS)) and transport promotion (PMMANPs as a carrier and competition for deposition sites on the QS surface). Changes in pH disrupted this equilibrium. Furthermore, the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, considering surface roughness (SR), provides a good explanation for the breakthrough curves (BTC) of aged PMMANPs during single and co-transport. The surface collapse, inter-particle aggregation, higher SR, and surface inhomogeneity observed in 30dPMMANPs suggest significant chemical heterogeneity, resulting in a lower energy barrier for migration. This study reveals the dynamic relationship between the physicochemical properties and the migration capacity of PMMANPs at different aging stages, demonstrates the dynamic equilibrium of the competition-carrier effect in the co-transport system (FF and PMMANPs), and uncovers the synergistic effect between cation valence and the coordination ability of surface functional groups on nanoplastics, overcoming the limitation of traditional studies that focus only on ionic strength.