Microplastics influence the fate of antibiotics in freshwater environments: Biofilm formation and its effect on adsorption behavior

Highlights

• Biological senescence of different MPs under same culture conditions was compared.

• The community characteristics of MPs biofilms were studied.

• The attachment of biofilms increased the adsorption capacity of MPs to NOR.

• Different MPs adsorption mechanisms were explored.

Abstract

Microplastics (MPs) are substrates available for biofilms colonization in natural water environments. The biofilms formation may enhance the ability of MPs to adsorb harmful contaminants. Herein, we investigated the biofilms formation of three different MPs (PVC, PA and HDPE) in simulated natural environment, and observed the chemical structure, charge property, hydrophobicity and other properties of MPs affect microbial biomass and community composition. More importantly, potential pathogens were found in all three MPs biofilms. Furthermore, the adsorption capacities of original MPs and biological aging MPs for norfloxacin (NOR) was compared. HDPE has the largest adsorption capacity for NOR, while PA has the smallest adsorption capacity for NOR. It was concluded that the formation of biofilms enhanced the adsorption of NOR by 50.60 %, 24.17 % and 46.02 % for PVC, PA and HDPE, respectively. In addition, hydrogen-bond interaction, electrostatic interaction and hydrophobic interaction were found to dominate the adsorption of NOR by MPs. Our study contributed to improve the understanding of the interactions between aging MPs and contaminants in the natural water environments, and provided essential information for ecological risk assessment of MPs.

Introduction

The wide use of plastic and their durability have made them ubiquitous. Plastics have been found in soils, oceans, freshwater, and polar regions (Liu et al., 2019). Plastics are exposed to the natural environment and break down into smaller particles under physical, chemical and biological action. Microplastic (MPs) are generally defined as plastic fragments that are smaller than 5 mm in size (Thompson et al., 2004). For a long time, MPs research has focused on the ocean. Although most MPs can be removed by sewage treatment plants, the huge amount of sewage discharged makes the unremoved MPs not ignored (Martínez-Campos et al., 2021). Several studies have confirmed that inland lakes and reservoirs are sinks of MPs in freshwater environments (Qi et al., 2021, Wu et al., 2019). The characteristics of MPs include strong hydrophobicity, large specific surface area, high porosity and high mobility (He et al., 2022). These special properties allow MPs to adsorb pollutants from the surrounding environment and act as carriers to extend the migration of pollutants. Additionally, MPs not only cause direct damage to the organisms that ingest them, but also carry contaminants that accumulate through the food chain into more organisms (Guo and Wang, 2019, Liu et al., 2019, Wang et al., 2015). Therefore, as common environmental pollutants worldwide, MPs have received increasing attention due to their complexity and potential high ecological risk.

In fact, MPs, as a unique ecological environment different from the surrounding environment, can promote the formation of biofilms. Biofilms are dynamic microbial systems composed of complex and diverse microorganisms, which exist widely in aquatic ecosystems (Guan et al., 2020b, Pham et al., 2021). There are some pathogenic bacteria among them. The formation of biofilms may change the morphology and physicochemical properties of MPs, thereby enhancing the adsorption capacity of MPs for pollutants (Guo and Wang, 2019, Liu et al., 2021). Although some studies on the adsorption of pollutants by MPs have been published, there are limited studies on the adsorption of pollutants by biologically aged MPs. However, biological aging of MPs is the ultimate fate of MPs in the environment.

Antibiotics are widely used in the medicine and aquaculture industries. Over the past few decades, antibiotics have been overused (Tong et al., 2022, Wang et al., 2020). After ingestion of antibiotics by humans or animals, most of the antibiotics cannot be completely metabolized and enter the sewage treatment plant with excrement (Valdés et al., 2021, Wang et al., 2021). At the same time, most sewage treatment plants cannot complete the removal of antibiotics, so some antibiotics are still released and accumulated in natural water (Li et al., 2022). Although antibiotics are present in low concentrations in natural waters, they could promote the development of antibiotic resistance in natural bacteria (Niu et al., 2016). Natural bacteria may then transfer resistance to other bacteria, including human pathogens (Liu et al., 2012). In summary, they have been considered as a new type of pollutant. Fluoroquinolones are among the most commonly used antibiotics and are used to treat respiratory and bacterial infections. Of these, norfloxacin (NOR) is a widely used fluoroquinolone antibiotic, with ng/L to μg/L levels detected in various environments (Amorim et al., 2014, Jing et al., 2022, Li et al., 2022, Zhang et al., 2022). Therefore, MPs and NOR coexist in the same environment in a natural water body. MPs inevitably adsorbs NOR. It is of great significance to explore the adsorption of NOR by MPs, particularly biologically aged MPs.

In this study, we chosen polyvinyl chloride (PVC), polyamide (PA) and high-density polyethylene (HDPE), which occur frequently in life, as representative MPs, and NOR as representative antibiotics. The adsorption behavior of original MPs and biological aging MPs to antibiotic organic pollutants was investigated. Herein, the specific purposes of this paper are as follows: (1) compare the biological aging of three different MPs under the same incubation time and conditions; (2) study the community characteristics on biofilms of MPs and the environmental risks posed by potential pathogenic bacteria; (3) explore the adsorption kinetics and adsorption isotherms of NOR on original MPs and biological aged MPs; (4) reveal the effects of pH and salinity on the adsorption of NOR by different MPs.