Competitive and synergistic sorption of norfloxacin and bisphenol A onto two different microplastics

To address the incomplete understanding of concurrent pollutants' sorption behavior on microplastics (MPs), this study systematically investigated the competitive and synergistic sorption behaviors of norfloxacin (NOR) and bisphenol A (BPA) on pristine polypropylene (PP) and polyamide (PA) MPs. Six isothermal models were employed to analyze sorption data, with the Freundlich model demonstrating the best fit for both single and binary solute systems. Sorption capacities followed the order: PA-BPA > PP-NOR > PP-BPA ≈ PA-NOR. On PA, synergistic sorption was observed, where BPA enhanced NOR's multilayer formation at low concentrations, even though the two pollutants likely occupied distinct preferred sites. In contrast, on PP, the chemically dissimilar NOR and BPA exhibited competitive sorption, with the more hydrophobic BPA being prioritized. Notably, multilayer sorption occurred at high concentrations in the presence of co-sorbates, suggesting initial monolayer competition at low concentrations and subsequent multilayer accumulation at higher levels. Salinity effects showed that low initial pollutant concentrations were inhibited by high salinity, while high BPA concentrations combined with salinity significantly enhanced NOR sorption on PP by 31.39%–109.30%, revealing competitors could act as sorption enhancers under specific conditions. These findings highlight that synergistic sorption and salinity-driven effects may concentrate low-concentration pollutants like antibiotics onto MPs in saline environments, drawing more attention to their ecological fate and risks.