The production of plastic has grown exponentially since the 1950 s all over the word (Ostle et al., 2019), about 8 million tons of plastic eventually flows into the sea every year, most of which come from land, and plastic has become a global pollutant (Baeta et al., 2021). Plastics have been reported to be found in many sampling sites from high mountains to the seafloor, and bulk plastics in the environment can be decomposed into microplastics (MPs, 1 µm < MPs particle size < 5 mm), including PP, PA, PS, PVC, et al., by physical, chemical and biological actions. For example, the concentration of microplastics in rivers is about 1.6–41 mg/L (Moore et al., 2011). Part of microplastics can be further degraded into nano-plastics (1 nm < NPs particle size < 1 µm) (Vandermeersch et al., 2015; Andrady et al., 2011; Lambert et al., 2016) with smaller particle size under the action of photodegradation, mechanical wear and biodegradation. micro / nano plastics will age and produce functional groups, such as amino group and carboxyl group, on their surface. Moreover, the micro / nano plastics can adsorb various heavy metal ions and organic pollutants, like lead ions, cadmium ions, antibiotics, pesticides and so on (Yu et al., 2020; Xu et al., 2021; Bao et al., 2021; Wang et al., 2020; Wang et al., 2019; Dong et al., 2020). However, due to the difficulties of current sampling and analysis strategies of granular plastics, it is difficult to evaluate their real exposure to humans or biota at a given location and their harm to human and biological health (Vandermeersch et al., 2015; Bergmann et al., 2019; Bergmann et al., 2017).
With the development of pharmaceutical industry, more and more antibiotics are used to treat microbial infections in people (Lage et al., 2018). Fluoroquinolones have ranked first in the use of all kinds of antibiotics since 2002, and the consumption of fluoroquinolones in 2013 was estimated to be as high as 27300 tons (Zhang et al., 2015). Especially, the output and dosage of fluoroquinolone antibiotics such as pefloxacin, ciprofloxacin and norfloxacin reached the maximum (Yu et al., 2020). However, the treatment efficiency of fluoroquinolone antibiotics was only 56–75% (Carvalho et al., 2016), with the concentration of antibiotics in the wastewater at the effluent of pharmaceutical factories even reached 30 mg/L (Aus et al., 2016), and antibiotics were also detected in many water bodies, such as East Dongting Lake, Pearl River Estuary, etc. (Yu et al., 2019; Ma et al., 2016).
Various behaviors of micro / nano plastics and antibiotics, such as aggregation and dispersion, adsorption, migration and deposition, may affect their compound pollution toxicity and ecological effect. The previous studies on the adsorption behavior of micro or nano plastics to antibiotics mainly included the adsorption of amoxicillin, tetracycline, ciprofloxacin, sulfadiazine, pefloxacin and levofloxacin by polystyrene (PS), polyethylene (PP), polyvinyl chloride (PVC) and polyamide (PA), mostly focusing on the kinetics, isotherm and pH, temperature, salt ions, etc., on the adsorption (Yu et al., 2020; Li et al., 2018; Atugoda et al., 2020). The adsorption of antibiotics by microplastics was mainly physical or chemical adsorption, which was jointly controlled by external and intra-particle diffusion, and the main mechanisms included hydrogen bonding, electrostatic interaction, van der Waals forces (vdW), hydrophobic interaction and so on (Li et al., 2018; Atugoda et al., 2020; Chen et al., 2021; Zhang et al., 2020). For example, electrostatic interaction was the main mechanism of pH affecting the adsorption of polyethylene to antibiotics (Chen et al., 2021), and due to the competitive adsorption and ion exchange among pollutants, the increase of dissolved organic matter concentration and ion strength will reduce the adsorption amount (Yu et al., 2020).
Compared with micron plastics, nano-plastics had larger specific surface area, higher surface hydrophobicity and better adsorption effect for heavy metals, polychlorinated biphenyls, antibiotics and other pollutants (Li et al., 2018; Zhang et al., 2020; Da Costa et al., 2016). In particular, the saturated adsorption capacity of nano-plastics for antibiotics is higher than that of microplastics for antibiotics, the former 20-50mg/g, the latter concentrated within 10 mg/g or less. Interestingly, PS-COOH adsorbed antibiotics better than PS because of the polarity between them, and for norfloxacin and levofloxacin, the adsorption capacity of PS-COOH was 54.9% and 69.6% higher than PS, respectively (Zhang et al., 2020; Yilimulati et al., 2021). However, there is still a lack of data on the interaction between functionalized nano-plastics and antibiotics. Therefore, in this paper, 400 nm polystyrene microspheres (PS), carboxyl modified PS (PS-COOH) and amino modified PS (PS-NH2) were selected as the research objects to explore their adsorption mechanism for gatifloxacin (GAT), a typical fluoroquinolone antibiotic. The effects of physical and chemical conditions, including pH, temperature, ionic strength, heavy metal ions and dissolved organic matter, on the interaction were investigated, and the interaction mechanism and theory between nano-plastics and antibiotics were clarified, which will provide theoretical guidance for the transformation and removal of micro-nano plastics in practical environment.