Mechanistic insight into different adsorption of norfloxacin on microplastics in simulated natural water and real surface water

Microplastics (MPs) as carriers of various contaminants have attracted more attentions in water environments. However, the interactions between typical MPs and norfloxacin (NOR) in natural water environments were still not systematically studied. In this study, the adsorption of NOR onto four typical types of MPs (polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC)) was investigated in simulated natural water and real surface water, and the adsorption mechanisms were deeply explored to provide fundamental understandings of the MPs-NOR complicated pollution. The results showed that the kinetics of NOR onto all MPs obeyed pseudo-second-order model, and was greatly slowed down at lower temperature or higher salinity. The intrinsic structure and surface area of MPs played important roles in the adsorption behaviors of NOR on these four types of MPs. The adsorption isotherm of NOR onto all MPs could be well described by linear model, with the Kd values following the order of PVC > PS > PE > PP (i.e. 6.229–11.901 L/μg) in simulated natural water. However, in surface water the adsorption isotherms of NOR on all MPs could be well fitted by Freundlich model. For all MPs, the adsorption of NOR was quite pH-dependent due to the electrostatic interactions. Furthermore, the salinity and the presence of dissolved organic matter (DOM) had significantly hindered the NOR adsorption. More importantly, compared with adsorption behaviors in simulated natural water, the competition of coexisting substances such as cations and NOM for adsorption sites and higher water pH dramatically reduced the adsorption of NOR onto all types of MPs in Jiang'an River, with the reduction rate of 19.7–41.2%. Finally, the mechanism studies indicated that the electrostatic attractions played a key role in the adsorption of NOR onto MPs, and π-π, H-bonding, polar-polar, and Van Der Waals interactions were also involved in adsorption processes.