The occurrence and transport of microplastics: The state of the science

Highlights

• Assessed the current state of knowledge regarding the occurrence and transport of microplastics (MP).

• Most MP transport processes, and environmental impacts remains poorly understood.

• Identified current knowledge gaps and 16 areas where future MP research efforts should be focused.

• The review constitutes a valuable stepping-stone and will aid in effectively directing future MP research efforts.

Abstract

Microplastic (MP) particles have been observed in most environments and concentrations are expected to increase over the coming decades given continued and increased production of synthetic polymer products. The expected increase in plastic pollution (including MPs) may elevate the risk posed by these synthetic particles to both environmental and human health. The purpose of this review is to provide a review of the state of knowledge regarding the occurrence and transport of MPs in and across three of the Earths subsystems, specifically, the lithosphere, atmosphere, and hydrosphere. Evidence is presented that shows the lithosphere includes substantial MP accumulation (e.g. approximately 25 particles L⁻¹ in landfill leachate), the impacts of which remain poorly understood. The atmosphere plays an important role in MP transport, with increased occurrence and higher transport concentrations noted in more densely populated areas (e.g. 175 to 313 particles m⁻² d⁻¹ in Dongguan China). In the hydrosphere, freshwater ecosystems alternate between MP transport (e.g. rivers) and deposition (e.g. lakes) with flow rate being identified as a key factor determining the movement and fate of MPs. Conversely, marine ecosystems act as a major sink for MP pollution (e.g. MP comprise 94%, approximately 1.69 trillion pieces, of plastic pieces in the Great Pacific Garbage Patch), driven by direct deposition or by transport via the atmosphere or freshwater conveyance systems (e.g. streams, rivers, or ice sheets). Once ingested by organisms, the trophic transfer and bioaccumulation of MPs has been confirmed with the polymer particles potentially accumulating in or impacting fauna, flora, microbes, and humans. Finally, 16 areas are identified in which future MP research efforts should be focused, with the goal of accurately identifying the scope and potential risks posed by synthetic polymer pollution. This review serves as a valuable steppingstone for future research and researchers wishing to address MP research gaps across various environmental settings in the coming decades.

Introduction

The use and production of synthetic polymer (plastic) based products have led to the identification of microplastics (MPs; defined as plastic particles <5 mm in size; Masura et al., 2015) in numerous terrestrial and aquatic ecosystems (Barnes et al., 2009; Rillig, 2012; Ruimin et al., 2019). MP pollution includes primary MPs (small manufactured plastics) (Arthur et al., 2009) and meso-and macro-plastics, which can degrade into secondary MPs (Barnes et al., 2009; Cole et al., 2011), and nanoplastics (NPs). Secondary MPs are defined as fragments from larger plastic products (Gesamp, 2015). Additionally, NPs can be defined as plastic particles exhibiting colloidal behavior in the size range of 1–1000 nm (Gigault et al., 2018), however differing definitions of the term NP is prevalent in the literature. MPs and NPs have been identified as a serious global pollutant problem and among the top environmental challenges identified by the United Nations Environment Program (UNEP, 2014). The scope of small particle synthetic polymer pollution is expected to increase in the coming years. For example, an order of magnitude increase is expected in annual input of MPs into marine environments from 2015 (approximately 10 million tons) to 2025, as demand for synthetic polymer products continues to increase (Jambeck et al., 2015; Wagner and Lambert, 2018).

The demand for plastic products stems from the durability, flexibility, versatility, ease/low cost of production, light weight and water resistance of synthetic polymers allowing for widespread use in many industries globally including (but not limited to): packaging (146 million tons annually), building and construction (65 million tons annually), textiles (59 million tons annually), transportation (27 million tons annually), electrical (18 million tons annually) and industrial machinery (3 million tons annually) (Geyer et al., 2017). The increasing demand for plastics from numerous industries has led to increased global plastic production from approximately 1.5 million tons in 1950 to approximately 348 million tons in 2017 (Liu et al., 2018; PlasticsEurope, 2019; Garside, 2019a). China dominates global plastic production, manufacturing approximately 25%–30% (107.7 million metric tons annually) of total global plastics, producing 6.7 million tons of plastic in May of 2020 alone (Wong, 2020). Following China, the North American Free Trade Agreement (NAFTA; now the United States–Mexico–Canada Agreement (USMCA)) is the largest plastic producing region in the world, producing approximately 18% (64.62 million metric tons annually) of the world's plastics (Garside, 2019b). Among the synthetic polymers produced globally approximately 90% are polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), or polyethylene terephthalate (PET) types (Andrady and Neal, 2009). The production and demand of these common polymers are approximated as follows: polypropylene (PP; 19.3%), low-density polyethylene (LDPE; 17.5%), high-density polyethylene (HDPE; 12.3%), polyvinyl chloride (PVC; 10.2%), polyurethane (PUR; 7.7%), polyethylene terephthalate (PET, also known as polyester; 7.4%), and polystyrene (PS; 6.6%) (PlasticsEurope, 2019). The production of plastic products is expected to increase in the coming decades with global cumulative plastic production expected to increase from 8.3 billion metric tons in 2017 to 34 billion metric tons by 2050 (Garside, 2019c). Lebreton and Andrady (2019) estimated that the amount of mismanaged plastic waste added to the environment may increase from 60–99 million tons per year in 2015 to 155–265 million tons per year by 2060. Therefore, the disposal of plastic products will comprise a growing anthropogenically mediated environmental problem for the foreseeable future given that an estimated 79% of plastics are discarded and accumulate either on terrestrial locations (e.g. landfills) or in the oceans, with only 9% being recycled and 12% being incinerated (Geyer et al., 2017).

The increasing scope of MP pollution, the occurrence of MPs across various ecological spheres (Ruimin et al., 2019), and the potential risk posed by polymer particles (including the sorption and leaching of organic and inorganic chemicals) (Kasirajan and Ngouajio, 2012; Steinmetz et al., 2016; Wang et al., 2016; Alimi et al., 2018) has led to increased global interest and scientific research (Fig. 1). The annual number of publications in Elsevier's abstract and citation database (Scopus) containing the keyword “microplastics” increased from two in 2009 to 939 in 2019 (Fig. 1). Obviously, investigations that address contemporary knowledge gaps regarding MPs, including MP occurrence and transport, risks posed by polymer particles in different ecological spheres, and development of mitigation practices are of increasing societal interest.

The transport and loading of MPs must be understood if the impacts of MPs on ecosystem processes are to be fully understood and mitigated (Pohl et al., 2020). Dris et al. (2016) reported that atmospheric transport of MP pollution alone, which has been shown to be global in extent, exemplifies the far-reaching implications of plastic particle pollution. MP transport can also lead to the preferential accumulation of these particles in certain areas, such as aquatic locations with decreased velocity, and thus increased particle settling (Tibbetts et al., 2018). Preferential MP accumulation can increase the negative consequences of these particles by increased risk of organism ingestion (McNeish et al., 2018), or toxicity (Pannetier et al., 2020). Unfortunately, despite recent increases in the numbers of investigations regarding the occurrence of MPs in environmental spheres and organisms, limited information is available regarding the toxicity and chemicals associated with synthetic polymers (Campanale et al., 2020). This is important since different polymers may impact organisms in unknown ways due to distinct chemical composition (Pannetier et al., 2020; Zimmerman et al., 2020). For example, Zimmerman et al. (2020) reported that polyvinyl chloride (PVC) had the greatest impact on the reproduction of Daphnia magna, whereas polylactic acid (PLA) microplastics reduced survival most effectively. Additionally, MP concentrations may not exhibit a linear relationship with toxicity (Rochman, 2015; Rochman et al., 2019; Pannetier et al., 2020). Further, combinations of MPs, as they are often found in environmental spheres, may leach a cocktail of toxic chemicals potentially altering (increasing or decreasing) their toxicity (Rochman, 2015). Finally, the toxicity of MPs may also depend on the endpoints of the synthetic polymer particles (Campanale et al., 2020; Zimmerman et al., 2020). Ultimately, the toxicity of MPs comprise numerous knowledge gaps that must be addressed if the risk of synthetic polymer particles are to be accurately determined.

Transport processes can also lead to MPs impacting communities and ecosystems geographically removed from where originally released (Kane and Clare, 2019; Pohl et al., 2020). The exposure and ingestion of MPs by organisms and subsequent integration with the food chain are also subject to transport processes, including MP flux, residence time and burial efficiency (Kane and Clare, 2019; Pohl et al., 2020). Consequently, transport processes not only impact the extent of MP pollution but can also impact the affect these particles will have on organisms. Ingestion of MPs can lead to mechanical injury, false satiation, low growth rate, increased immune response, energy depletion, blocked enzyme production, decreased fecundity, oxidative stress, and even morbidity and mortality (Wright et al., 2013; Sussarellu et al., 2016). Compounding these issues, MPs can sorb toxic chemicals (Wang et al., 2018a) leading to the concentration of toxic water or soil pollutants, and creating toxicological hazards once ingested by organisms (Wright et al., 2013; Li et al., 2017). Additionally, MPs can leach harmful chemicals, both sorbed toxic chemicals, or harmful constituent chemicals (Wang et al., 2018a; Lithner et al., 2009) directly into the environment, thereby adversely affecting organisms (Wagner and Lambert, 2018). Toxic leachate from MPs can contain antimicrobial agents and nanomaterials lethal to microbes, including microbial keystone species vital for nutrient cycling (Wagner and Lambert, 2018). Transport of MPs through human food chains (e.g. via trophic transfer) includes ingestion, though transport mechanisms and pathways leading to human consumption is poorly understood (Nelms et al., 2018; Ruimin et al., 2019). However, Smith et al. (2018) concluded that the consumption of food sources (e.g. shellfish) contaminated with MPs is a pathway of concern to human health. Additionally, it has been reported that humans consume an average of one credit card's worth (approximately 5 g) of MPs a week (De Wit and Bigaud, 2019) and that monomers (plastic constituents) were found present in 47 out of 47 analyzed human tissue samples collected from liver, lung, spleen, kidney and adipose tissues (ACS, 2020).

Given the expected increase of MPs entering the environment in the coming decades and the negative effects associated with plastic particle pollution it is a critical moment to organize what is currently known regarding the occurrence and transport of these particles in the environment. In this manner, researchers, managers, and policy makers are better equipped to make science-based decisions for future needs. Therefore, the primary objective of this literature review was to provide an overview of the current state of knowledge on MPs occurrence and transport in the environment (lithosphere, atmosphere, and hydrosphere) and in organisms inhabiting the environment. Outcomes of this review include the identification of current knowledge gaps regarding the occurrence and transport of MPs and recommendations regarding future research directions.