Adsorption of ciprofloxacin to functionalized nano-sized polystyrene plastic: Kinetics, thermochemistry and toxicity

Highlights

• Electrostatic interactions, hydrogen bonding, and hydrophobic interactions are critical mechanisms of CIP adsorption on PS-COOH.

• Binding reaction between PS-COOH is an exothermic and thermodynamically favorable.

• Toxicity of PS-COOH with adsorbed CIP to C. elegans was significantly higher than that of either CIP or PS-COOH.

• Small size of PS-COOH caused more adverse effects on survival of C. elegans than that of larger size microplastics.

Abstract

Plastic debris is ubiquitous in aquatic systems and has been proven vehicles for the transport of various pollutants including trace organic compounds. Nanoplastics have large specific surface area and hydrophobic characteristics and therefore are capable of adsorbing other organic or inorganic chemicals from the environment. Antibiotics, as another class of emerging contaminants, have raised significant research concern in recent years as they pose threats to the ecosytems and human health. Nevertheless, little information is available on the adsorption behaviors of antibiotics onto nano-sized plastics. The toxicity of combined nanoplastics and antibiotics is also largely unknown. In this study, the physicochemical and thermodynamic interactions between representative nanoplastics, which containing a carboxyl functional group of polystyrene nanoplastics (PS-COOH), and typical antibiotic, i.e., ciprofloxacin (CIP) were investigated in a batch adsorption experiment. The specific thermodynamic correlation function of PS-COOH combined with CIP was obtained through isothermal titration microcalorimetry (ITC) analysis. The adsorption kinetics and isotherm of CIP on PS-COOH closely fit the pseudo-second-order kinetic model (r² = 0.99) and Freundlich isotherm (r² = 0.99). The ITC results showed that the adsorption reaction of PS-COOH with CIP was a spontaneous exothermic reaction. The adsorption of antibiotics on nanoplastics may aggravate the negative impacts of these two pollutants on aqueous ecosystems, and we hypothesized that would be reflected in the survival rate of model organism of Caenorhabditis elegans when exposed to this combination. This work used a mechanistic approach to unravel the adsorption behavior of antibiotics on nanoplastics and shed light on their potential impact on aquatic ecosystems.

Introduction

Owing to their ubiquitous distribution and small size, micro/nanoplastics (<5 mm diameter) are easily ingested by aquatic organisms (Lu et al., 2016; Wang et al., 2020), and can adversely impact the health of those organisms. The existence and impact of nanoplastics in natural systems cannot be ignored (Hernandez et al., 2017). Since plastic particles are polymerized by various monomers and additives (Alimi et al., 2018), they have a variety of polarities and functional groups (Wang et al., 2018), and can act as carriers that enable the adsorption of various types of contaminants (Yonkos et al., 2014). Previous studies have confirmed that micro-plastics can adsorb heavy metals (Brennecke et al., 2016), persistent organic pollutants (POPs) (Bakir et al., 2014) or hydrophobic organic chemicals (HOCs) (Lee et al., 2014), highly hydrophobic organic chemicals such as polychlorinated biphenyls (Velzeboer et al., 2014), polycyclic aromatic hydrocarbons (Fries and Zarfl, 2012), polybrominated diphenyl ethers (Chua et al., 2014), and perfluorinated surfactants (Wang et al., 2015a). Antibiotics, serving as an important group of pharmaceuticals, are widely used in human and veterinary medicine for microbial infection treatment (Almeida Lage et al., 2018; Xu et al., 2018a). Extensive use of antibiotics has inevitably resulted in their discharge into to aquatic system and the frequent detection of antibiotics in water has been attracting research concerns in recent years (Menz et al., 2018; Wang et al., 2016). Previous studies have investigated the adsorption of representative antibiotics, e.g., sulfamethoxazole and tetracycline by microplastics and evaluated how pH, salinity, and dissolved organic matter influenced adsorption process (Guo et al., 2019; Xu et al., 2018b). While enlightening and thorough, the above studies were limited to micro-sized plastics (Guo et al., 2018). To understand the potential risks of antibiotics adsorbed by nanoplastics in the aquatic environment, it is crucial to understand the adsorption mechanisms of antibiotics by nanoplastics.

Nanoplastics are extremely small (<1 μm), and the large specific surface area have been proven effective sorbents for contaminants such as organic pollutants (Besseling et al., 2014; Cole et al., 2011). The discarded plastics in aquatic environment can be degraded into microplastics with different charges by weathering processes (e.g., hydrolysis, UV, biodegradation) and favor the adsorption of ionic contaminants (Li et al., 2018a; Rochman et al., 2013). Recent studies have suggested that nanoplastics can act as a vector by which certain chemicals are transported into marine organisms (Chen et al., 2017; Cole and Galloway, 2015; Dawson et al., 2018). After the chemicals are carried into an organism by nanoparticles, they can be released via desorption and may pose toxic effects on the organism (Neves et al., 2015). A positive correlation has been observed between the amount of microplastics in the tissue of lugworms (Arenicola marina) and their PCB concentration (Besseling et al., 2012). Kim et al. have evaluated the toxicity of polystyrene (PS) nanoparticles with Ni on Daphnia magna, and have confirmed the higher toxicity of functionalized PS (i.e., PS-COOH) nanoparticles compared to conventional PS nanoparticles in the absence of functional groups (Kim et al., 2017). However, the toxicity of functionalized nanoplastics after sorbing antibiotics remains largely unknown.

Currently, only a little information was available on the concentration and types of microplastics in different environment. However, according to reported data, PS represent the second most dominant microplastics in fresh water, sediments, soil, coastal ecosystems (Fuller and Gautam, 2016; Rochman, 2018; Scheurer and Bigalke, 2018; Weithmann et al., 2018). Recent studies in China also suggested that spheroidal polystyrenes were dominant polymer types in the wastewater sludge (Li et al., 2018b). Ciprofloxacin (CIP) is a one of the most commonly used antibiotics, frequently detected in aquatic environment at levels up to μg/L (Guerra et al., 2014). In the aquatic ecosystem, even at low concentrations, CIP can lead to the propagation of antibiotic resistant bacteria and genes (Afzal et al., 2018).

In this study the adsorption of ciprofloxacin (CIP), a widely used antibiotic, by functionalized polystyrene nanoplastics (PS-COOH) was investigated. The adsorption kinetics at different pH values and sorption isotherms of PS-COOH and CIP were studied. Isothermal titration microcalorimetry (ITC) was employed to elucidate the thermodynamic behavior of PS-COOH and CIP. The effect of PS-COOH with CIP on the survival rate of Caenorhabditis elegans was studied in an acute toxicity test. The results of this study will provide a basis for a more comprehensive understanding of the interaction of these two types of pollutants in the aquatic environment.