Study of the sorption of pollutants onto microplastics particles

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Plastic pollution in aquatic ecosystems

Increased plastic production and anthropic activities taking place in coastal areas, coupled with poor waste management, has led to ever-greater quantities of plastics ending up in aquatic areas (rivers, lakes, oceans etc.). These can come from either land-based sources (plastic bags, food packaging, industries, cosmetics etc.) or ocean-based sources (transport of plastic materials via boats, accidental discharge of raw plastic). The most commonly found polymer in aquatic ecosystems (ocean and freshwater) is PE (in most of abiotic compartments; e.g. beach, epipelagic zone and sediments) followed by PP (Figure 4) (Cheang et al., 2018; Fossi et al., 2017; Hidalgo-Ruz et al., 2012; Karthik et al., 2018). Plastics are transported via wind action (from water or coastal areas to beaches), sedimentation (from water to sediments), resuspension (biological and mechanic movements), wash-up (dredging of sediments for beach nourishment) and in sediments. Plastics in freshwater are also transferred to oceanic ecosystems via riverine horizontal transportation (water and sediments).
Various types of plastic can be found in the environment, in varying shapes and sizes. The scientific community generally divides them up into categories. Size is one common variable, with plastics found in the environment being categorized as macro, micro, and nano.
A macroplastic is defined as anything larger than 5mm. This category includes products such as plastic bags, straws, and bottles.
In 2010, it was estimated that between 5 and 13 million tons of plastic were introduced into world’s oceans (Jambeck et al., 2015), and that approximately 5 trillion pieces of microplastic were present (Eriksen et al., 2014).
Over the years, plastic debris has built up in oceans because of dumping and accidental releases of plastic items. Based on their size, plastic debris can be divided up into four categories: megaplastics (> 50 cm), macroplastics (5-50 cm), mesoplastics (0.5-5 cm) and microplastics (< 0.5 cm) (Andrady, 2011; Eriksen et al., 2014; Jambeck et al., 2015; Koelmans et al., 2017; Lebreton et al., 2018). Plastic debris in the oceans may persist and accumulate (Law and Thompson, 2014). Macro- and mesoplastic refers to plastic particles including nets, bags, bottles etc. Beyond being an eyesore, they have been known to injure and kill a wide range of aquatic organisms that either eat or become entangled in them. Traces of plastics have been found in the stomachs of sea turtles, whales and albatross, among others (Mrosovsky et al., 2009). Ingestion or entanglement result in impaired movement and feeding, a reduction of reproductive output, lacerations, ulcers and deaths in invertebrates, turtles, fish, seabirds and mammals (Barnes et al., 2010; Derraik, 2002; Laist, 1997; Rochman, Browne, et al., 2013). One of the more visible impacts is entanglement of sea turtles in discarded and lost plastic netting or rope from commercial fishing (Laist, 1997). Ingestion of plastic occurs at sea with the confusion between jellyfish and plastic bags for examples (Laist, 1997; Robards et al., 1997). It has been estimated that more than 100,000 sea animals die annually due to the presence of plastic debris in aquatic ecosystems, by ingestion or entanglement (Wilks, 2006).

Microplastics as an emerging contaminant

Primary MPs particles are particles manufactured at millimetric scale: preproduction pellets and microbeads. Pellets are used in the plastic industry, first as raw resins and then rounded with a diameter between 2 and 5 mm (Costa et al., 2010). Microbeads are used in a range of everyday products: exfoliating agents in facial cleansers, facial scrubs, pesticides, paint stripper, etc. Secondary MPs particles, known as small plastic fragments, are derived from the breakdown of macroplastics via physical, chemical or biological degradation (Andrady et al., 2003; Cooper and Corcoran, 2010; Derraik, 2002; Thompson et al., 2004). Physical fragmentation of plastics generally occurs due to waves, wind or ultra violet (UV) light (Galgani et al., 2015), while biological degradation consists of enzymatic oxidation or hydrolysis of plastics by microorganisms (Shah et al., 2008). Plastic particles degrade more quickly when they are deposited on beaches or inland rather than in the open ocean due to a higher exposition to UV radiation and wind processes (Biesinger et al., 2011; Corcoran et al., 2009).
Synthetic fibers may also be part of secondary MPs, entering the environment at microscopic size due to the absence of effective filters in washing machines and wastewater treatment plants (WWTPs) (Browne et al., 2011). Figure 5 shows different sources of MPs in the aquatic environment are represented. In the marine environment, about 18% of MPs entered the aquatic system via fishing activities (Andrady, 2011; Cole et al., 2011; Derraik, 2002) and approximately 80% of marine plastic debris originate from terrestrial sources (Jambeck et al., 2015; Mani et al., 2015). Specific weather conditions as floods or hurricanes can increase the transfer of MPs from lands to aquatic systems (Barnes et al., 2009). While WWTPs filter some MPs from domestic effluents, they are not designed or equipped to remove all plastic particles. WWTPs are therefore a frequent source of contamination of the aquatic environment (Fendall and Sewell, 2009; Wolff et al., 2019). MPs of both types are considered to be a potential threat for the aquatic environment (Cole et al., 2011; Thompson et al., 2009).

Dispersion of microplastics in aquatic environment

MPs are present throughout our oceans, the Atlantic to the Pacific Ocean, and in the Caribbean and Mediterranean seas (Cózar et al., 2014; Law and Thompson, 2014; Ory et al., 2018; Welden and Lusher, 2017). MPs have been detected at up to 96% of all water samples in the Pacific Ocean (Moore et al., 2001). Cozar and collaborators studied surface water samples from different locations and demonstrated the extensive presence of MPs by showing that 88% samples contained MPs with high variations in concentrations (Cózar et al., 2014). The worldwide distribution of MPs in the open seas has been demonstrated by the presence of particles in ice cores in the remote Polar areas of the Arctic Ocean and in the deep oceans (Obbard et al., 2014), with a frequency of occurrence of 93-95% in Arctic waterbodies. Accumulation can also occur in gulfs, closed bays and areas close to areas of dense human population (Eriksen et al., 2014), as the Mediterranean (Cózar et al., 2014; Suaria et al., 2016).
The abundance and dispersion of MPs have increased over the last few decades (Barnes et al., 2009; Lebreton et al., 2018; Pham et al., 2014). However, concentration of MPs in surface water is not uniform, with significant variations, ranging from 0.34 particles/m3 in the Barents sea (Lusher et al., 2015) up to 102 000 particles/m3 in coastal waters of Sweden (Norén and Naustvoll, 2010). In 1997, Charles Moore discovered the Great Pacific Garbage Patch in the North Pacific Ocean gyre. Tidal rhythms and currents tend to focus on plastic debris inside ocean gyres, leading to accumulation zones (Matsumura and Nasu, 1997). The abundance of MPs is more evident in areas with converging currents (Eriksen et al., 2014; Law and Thompson 2014; Moore et al., 2001). Figure 6 shows the five most important gyres: the Indian Ocean gyre, the North Atlantic gyre, the North Pacific gyre, the South Atlantic gyre, and the South Pacific gyre. Figure 7 shows the predicted number of MPs (particles/km2) in surface waters, with a high accumulation (red color) in oceanic gyres (Eriksen et al., 2014). Consequently, trends in plastic accumulation in marine environment are not uniformly increasing while average size of plastic particles seems to be decreasing (Eriksen et al., 2014).
Figure 7: Model prediction of global count density (particles/km 2; see color bar) for each of four size classes (0.33–1.00 mm, 1.01–4.75 mm, 4.76–200 mm, and > 200 mm). (Eriksen et al., 2014) Plastic items are found in the water column and in sediments, depending on their densities. Plastics with a density > 1, such as PS, PET and PVC, have a tendency to sink to the bottom, whereas PE and PP tend to float to the surface of water. However, the density of plastic debris can be modified once in the marine environment, for example, low density plastic can also be found on the seafloor, due to the development of a microbial biofilm on its surface, which can be followed by colonization by invertebrates and algae, which increase the density (Andrady, 2011). Biofilm development on plastic bags has been demonstrated after 7 days of immersion and show a significant increase of density after 3 weeks of immersion (Lobelle and Cunliffe, 2011). The growth of the biofilm is not the only phenomena that can change the position of plastic items in the aquatic environment: ocean swells can re-suspend particles in the water column (Cole et al., 2011). Indeed, the distribution of particles is not only driven by the density of inert polymers, but also by the currents and movement of the water column.

Combination of pollutants and microplastics

As well as numerous additives used in plastic production, MPs offer a surface where many waterborne-pollutants can be adsorbed, including metals (Rochman, Hentschel, et al., 2014) and persistent organic pollutants (Rios et al., 2007; Rochman, Hoh, Hentschel, et al., 2013). MPs are believed to act as vectors and carriers for a wide range of pollutants (Andrady, 2011; Koelmans et al., 2014).
These MPs associated with persistent organic pollutants (POPs) can then act as reservoirs and/or vectors for those pollutants (Barnes et al., 2009; Thompson et al., 2004). The processes of sorption of organic pollutants onto MPs are very complex and poorly understood. The equilibrium kinetics of contaminants on MPs depends on the intrinsic properties of the chemicals, but also on the size, density and nature of the MPs. For example, the diffusion rate for PAHs into polymeric material correlates to a high degree with their molecular weight (Hong and Luthy, 2008) and has been shown to take up to a month. A higher diffusion coefficient can be derived for LDPE than for HDPE, indicating a greater sorption velocity for LDPE (Fries and Zarfl, 2012). In addition, adsorption of PAHs, PCBs, and dioxins to PE correlates with their octanol-water partition constants (logKow) (Adams et al., 2007).
MPs particles were shown to sorb significant amounts of POPs from the surrounding environment (Barnes et al., 2009; Koelmans et al., 2016; Mato et al., 2001; Rios et al., 2007). The weathering and fragmentation of MPs lead to smaller particles, and may increase reactive specific surface area, thus further facilitating the sorption of such pollutants (Lee et al., 2014; Teuten et al., 2007). The importance of the role of MPs in transport of POPs in biota is still controversial (Bakir et al., 2016; Koelmans et al., 2016; Lohmann, 2017). However, under both laboratory and field conditions, different classes of organic pollutants such as per- and polyfluorinated compounds, PAHs, PCBs, pesticides or pharmaceutical and personal care products adsorb to different plastic materials (Rochman, Hoh, Hentschel, et al., 2013; Ziccardi et al., 2016). In the present study, three different organic pollutants were selected from different chemical groups, known to cause specific toxic effects, but also due to their use in plastic production or their presence in the aquatic environment. These compounds were studied for their sorption properties and to investigate how they are transferred in the environment using MPs as vector.
The sorption of pollutants to MPs is still poorly understood. Sorption of contaminants by plastic has been reported, and some studies have shown easier sorption of contaminants to plastic rather than to natural sediments (Teuten et al., 2007; Wardrop et al., 2016). However, Koelmans (2015) described that while chemicals accumulate on their surface; the role of plastics in chemical bioaccumulation by organisms seems relatively low compared to the bioaccumulation from natural particles or direct uptake from the environment.

Regulations concerning pollutants

To monitor and protect aquatic environments, the European Union (EU) has adopted two distinct directives: the Water Framework Directive (WFD) and the Marine Strategy Framework Directive (MSFD).
The WFD (2000/60/CE) was validated on October 23, 2000 (EU, 2010) and improved on August 12, 2013 (European Parliament and of the Council). The present framework aims to introduce a global policy of achieving good qualitative and quantitative status of all water systems by 2015. The framework applies to all member states of the EU. Among all substances used, 45 have been selected as priorities, and among those, 21 are priority hazardous substances, including PAHs. The MSFD (Directive 2008/56/EC) is complementary to the WFD, and has the aim of achieving and maintaining good environmental status in European oceans, seas and coasts. Descriptor 10, relating to marine litter, includes pollution by plastics and MPs. The directive aims to eliminate certain single-use plastic products by 2020, as well as decreasing the use of plastic food and beverage containers.

Table of contents :

Chapter 1: General Context
Plastic: background information
Plastic pollution in aquatic ecosystems
Microplastics as an emerging contaminant
Dispersion of microplastics in aquatic environment
Combination of pollutants and microplastics
Regulations concerning pollutants
Selected compounds
Ecotoxicological effects of plastic pollution in aquatic environments
Physical impacts of plastic debris in ocean
Microplastics toxicity in marine organisms: mechanisms involved
Combination of microplastics with additives/contaminants
Toxicity of MPs related to fish
Bioassays and biomarkers using zebrafish (Danio rerio)
The “3Rs” principle
Biological model
Life stages
Ecotoxicological biomarkers using fish
EPHEMARE project
Chapter 2: Materials and methods
Microplastics & Chemicals
Selection of industrial microplastics
Environmental microplastics
Selection of chemicals
Sorption of pollutants
Chemical analysis
Study of the sorption of pollutants onto microplastics particles
Sorption kinetics of PFOS and BaP
Isotherm sorption PFOS and BaP
Particle digestion under simulated physiological conditions
Biological model, zebrafish (Danio rerio)
Experimental designs
Exposure of zebrafish embryo for acute toxicity test
Trophic exposure
Endpoints analyzed
Standardized FET
In vivo ethoxyresorufin-O-deethylase (EROD) assay
Cyp1a gene transcription analysis
Acetylcholinesterase activity (AChE)
Lipid peroxidation
Behavioral response: Larval photomotor response (LPMR)
Reprotoxicity
Chemical analysis
Chapter 3: Sorption processes
Article 1
Extra-study
Chapter 4: Toxicity of artificial microplastics
Article 2
Article 3
Chapter 5: Toxicity of environmental microplastics
Article 4
Article 5
Chapter 6: General discussion
Sorption of organic pollutants on plastics
Toxicity of microplastics using early life stages of zebrafish
Toxicity following ingestion of microplastics
Chapter 7: Conclusion and Perspectives
References

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