Highlight: Emerging Contaminants: Microplastics

H1. Introduction

Modern society is impacting the environment with emerging contaminants, the consequences of which are largely unknown. Emerging contaminants are contaminants that are not currently regulated by government agencies and scarcely monitored. These contaminants have emerged because of increasing urbanization, modernization, and technological advancements. Examples include plasticizers, flame retardants, pharmaceuticals, personal care products, hormones, food additives, detergents, microplastics, and perlfluoroalkyl and polyfluoroalkyl substances (PFAS). The concentration of these compounds in a variety of water bodies, the environmental fate (where and how they will disperse in the environment), and the effects on organisms and ecosystems are relatively unknown.  Only in recent years have scientists begun to collect environmental data and perform toxicity testing with these compounds. The scientific community and the public have initiated concerns about the ecological risk of these emerging contaminants (EuropeanCommission 2013; Koelmans et al. 2014). 

This review focuses on microplastics. Microplastics are small pieces of plastic that are accumulating in rivers and oceans worldwide. Animals can ingest microplastics, which may cause various health effects in the consumer and in aquatic ecosystems. 

The 2021 State of the St. Johns River Report will present a review of PFAS.  For over 60 years, PFAS have been used in a variety of industrial and commercial products, including stain and water repellents for textiles, paper products, and food packaging, and in pharmaceuticals, surfactants, and fire-fighting foams. Over the past two decades PFAS have been detected in a wide variety of wildlife, which has raised public concern. 

H2. Background and Sources: Microplastics  

Figure H-1. Environmental Plastic Pollution.
Figure H-1. Environmental Plastic Pollution.

Plastic debris is an emerging class of contaminants and one of the most abundant forms of global pollution (Rios et al. 2007; Andrady 2011). The technical definition of a plastic is synthetic water-insoluble polymer that can be molded on heating and manipulated into various shapes designed to be maintained during use (GESAMP 2015). The use of plastics has increased since the 1950s and now plastic production accounts for more than 200 million tons annually (Ivar do Sul and Costa 2014). Plastics are entering the oceans in massive quantities (Figure 1), estimated at 5 to 13 million tons annually (Jambeck et al. 2015), as a consequence of both increased production and improper waste management (Mason et al. 2017). Plastics are durable by nature and therefore can potentially stay in the environment for hundreds of years or more. 

Microplastics are small fragments of plastic that are ≤5 mm in size (GESAMP 2015). Ocean currents carry microplastics to the center of ocean convergences where they are concentrated (Carpenter and Smith 1972 Microplastic particles were first documented in these locations (Carpenter and Smith 1972), and now they are found worldwide in air, water, sediment, and biota (Browne et al. 2011; Wright et al. 2013; Ivar do Sul and Costa 2014). 

Microplastics in the environment are classified for technical purposes by their source. Primary microplastics are made in the ≤5mm size range and used in products such as hand soaps, exfoliants, deodorants and makeup, as well as particles for use in sandblasting and air-blasting to remove rust and paint off of machinery (Andrady 2011; Browne et al. 2007; Browne 2015; Syberg et al. 2015; GESAMP 2015; Figure H-2). Plastic pellets, called “nurdles,” are the form in which raw plastic is distributed. Nurdles are also primary microplastics. Secondary microplastics are formed as larger plastics such as plastic bags, packaging, and bottles undergo physical (e.g., from sunlight and wave action) and chemical degradation processes. The plastics become gradually smaller until they reach microscopic size (Browne et al. 2007; Cooper and Corcoran 2010Lassen et al. 2015; Weinstein et al. 2016; Figure H-2). Sources of secondary microplastics include the release of fibers from clothes or textiles and waste management (Lassen et al. 2015; Figure H-2). 

Figure H-2. Sources of Primary and Secondary Microplastics. Image Credit: EncounterEdu.
Figure H-2. Sources of Primary and Secondary Microplastics. Image Credit: EncounterEdu.

Some studies have reported the degradation of plastics down to nano-sized particles (Lambert and Wagner 2016).  This size range is even less studied than micro-sized plastics (Everaert et al. 2018). Furthermore, nanoplastics are presumed to exhibit different interactions than microplastics in the environment, as they may have the capability of crossing biological membranes (Besseling et al. 2013). 

H3. Microplastics in the Environment 

We do not know the quantity of microplastics that are released into the environment each year. Currently, there is no standardized method for this determination, and many of the methods used can underestimate the values. The attempts that have been made to estimate the emissions of primary and secondary microplastics have concluded that tire dust (secondary microplastic) was the predominant land-based source of microplastics emissions, while microplastics in cosmetic beads accounted for less than 1% of the total emissions (Sundt et al. 2014Lassen et al. 2015Sherrington 2016).   

Because the sales volumes and usage rates of down-the drain primary microplastics, like microbeads, can be estimated, those have been the primary focus, particularly for regulatory efforts. These primary microplastics are presumed to enter the aquatic environment through wastewater treatment plants (Sundt et al. 2014). Several studies have been performed to estimate the removal rate of microplastics in different types of wastewater treatment plants (Murphy et al. 2016Michielssen et al. 2016Carr et al. 2016). These studies concluded that primary wastewater treatment can remove an average of 65% of the total microplastic load and secondary and tertiary treatment can remove up to 94% of the total microplastic load (Burns and Boxall 2018). Therefore, only an estimated 3% of the total microplastic load is presumed to enter the aquatic environment in wastewater effluent (Volertsen and Hansen 2017). However, microplastics used in wastewater treatment plant biosolids (sewage sludge) for agriculture and land-based waste disposal may be significant sources in agricultural soils (Wagner et al. 2014), which could enter aquatic systems via nonpoint source runoff. 

Predictive models suggest that many microplastics will be retained in rivers prior to entering the ocean (Besseling et al. 2013). Therefore, microplastics from wastewater treatment effluent (particularly from treatment plants with only secondary treatment) and biosolids may be more problematic in river systems like the St. Johns River (SJR), especially given the number of surrounding wastewater treatment facilities and agricultural lands. A review by Burns and Boxall 2018 documented that the majority of microplastics monitoring studies were performed in the oceans and seas (58 studies), as compared to only 10 studies focusing on freshwater systems. However, some of the highest concentrations of microbeads in the environment were documented in the Great Lakes (Eriksen et al. 2013). In general, the heavily urbanized and industrialized coastal areas and rivers have had the highest reported concentration of microplastics, with secondary microplastics comprising the majority detected (Burns and Boxall 2018Song et al. 2015).  

The distribution of microplastic type in the water and sediments consisted of fibers (~50%) and fragments (~30%) with beads/spherules, films foams, and other forms comprising a smaller percentage (Burns and Boxall 2018). Polyethylene was the primary microplastic polymer type found in water and sediment samples, followed by polyethylene terephthalate, and polyacrylamide and polypropylene (Burns and Boxall 2018). Migwi 2020 reported the presence of predominantly fragments, fibers, and films mostly composed of polypropylene, polyethylene, and polyester in a freshwater lake in Kenya. Polypropylene and polyethylene are produced in mass quantities for consumer packaging and are therefore especially abundant in the environment.  For example, polyethylene is used for plastic bags and bottle caps are made from polypropylene.   

As with all emerging contaminants, developing accurate and precise sampling procedures and analytical methods for identifying and quantifying these chemicals in water, sediment, and biota samples takes a great deal of time and effort.  Basic techniques such as visual identification and microscopy have been used as well as more advanced spectroscopic methods (Loder and Gerdts 2015Song et al. 2015Duis and Coors 2016). Some of the earlier techniques for identification and quantification of microplastics have resulted in substantial errors in estimation because of misidentification of natural particles like coal ash, quartz or calcium carbonate, steric acid, cotton, and others (Burns and Boxall 2018). Method development is underway, and many advancements have been made. However, more efforts are needed in this area and in standardization of the methods. 

Microplastics sampling and analysis in the SJR have been performed by the Florida Microplastic Awareness Project, a citizen-science project which was created with funding from the National Oceanic and Atmospheric Administration (NOAA) in 2015. Unfortunately, they originally used a method called the “hot needle technique” which is now known to mis-identify cotton fibers as being plastic.  Therefore, the results for the SJR samples should not be quantitatively used for comparison to other studies (McGuire 2020). Microplastics analysis of water, sediment, and biota samples throughout the SJR is of paramount importance 

H4Biological Accumulation and Toxicity: Microplastics 

Figure H-3. Dolphin in the Lower St. Johns River.
Figure H-3. Dolphin in the Lower St. Johns River.

Because the characteristics of microplastics are so different (e.g., polymer type, size, shape), characterizing their biological effects is challenging. The environmental fate and toxicity of microplastics may vary considerably because they have a range of properties (e.g., some float and others sink in the water (Rochman and Brookson 2019) and additives. 

Ingestion of larger plastics by aquatic life (e.g., fish, turtles, cetaceans; Derraik 2002) has been well documented and is known to obstruct the gut of the consumer (Law and Thompson 2014). Accumulation of microplastics can also occur in a variety of species and food webs including invertebrates, fish, and birds (Lusher et al. 2015; Burns and Boxall 2018). Limited field studies have reported that internal microplastic concentration was correlated with the concentration of microplastics in the surrounding environment, rather than species-specific traits of the consumer (e.g., feeding habits; Courtene-Jones et al. 2017; Pazos et al. 2017Steer et al. 2017). Several fish species, including some commercially important species (e.g., cod, dab, flounder, mackerel) have been reported to commonly ingest microplastics (Rummel et al. 2016Battaglia et al. 2016Santillo et al. 2017).  

Fibers and fragments comprised the greatest proportion of microplastics found in biota samples (plankton and fish digestive tracts), with beads only accounting for a small proportio (Beer et al. 2018). ). The higher concentration of fibers, as compared to other types microplastics in tissue samples, may be due to the slower egestion (excretion) rate of fibers (Murray and Cowie 2011Au et al. 2017). However, research assessing both ingestion and egestion rates of various microplastics is needed. Differences in ingestion rates of microplastics with the presence or absence of food have been reported, particularly in invertebrate animals that consume plankton species which are similar in size to some microplastic particles (Ayukai 1987; Connors et al. 2017; Gray and Weinstein 2017; Scherer et al. 2017; Weber et al. 2018; De Orte et al. 2019). Egestion rates of some microplastics are similar to those of natural food sources, while others (e.g., fiber microplastic particles) have slower egestion rates (Burns and Boxall 2018). Further, some of the consumed microplastic particles may be transferred and accumulated in other biological tissues and compartments as has been documented in crabs, mussels, and oysters (Browne et al. 2008; Farrell and Nelson 2013; Sussarellu et al. 2016). Transfer of microplastics up the food chain is thought to occur by direct ingestion (Au et al. 2017).  However, the effects of ingested microplastics are less known. 

Some microplastics are inherently toxic due to certain chemical properties (i.e., polyvinylchloride) and some contain chemical additives (i.e., softeners like phthalates and bisphenol A [BPA]) that can leach into the environment and cause toxicity (Syberg et al. 2015). Some of these softeners have been linked to cancers and endocrine disruption in mammalian models like mice and rats (Marcilla et al. 2004vom Saal and Hughes 2005). Microplastics can also sorb a variety of contaminants (e.g., hydrophobic organic contaminants like PCBs) which may change the uptake, distribution, toxicity, and elimination of the microplastic particles (Besseling et al. 2013). The microplastics may also act as a vector for these toxic chemicals, causing additional toxicity to the consumer (Batel et al. 2016). However, there is only limited support for this occurrence currently (Burns and Boxall 2018). The extent of biological effects from plastic compounds directly as compared to the release of sorbed compounds from the plastics in unclear (Petroff et al. 2019). Laboratory studies have documented toxicological effects of microplastic ingestion including changes in behaviour, reduced reproductive capabilities, reduced feeding activity, decreased energy reserves, growth inhibition, mortalitygenotoxicity, and oxidative stress (Besseling et al. 2013Wright et al. 2013De Sa et al. 2018Burns and Boxall 2018). Questions remain, however, regarding the environmental relevance of the exposure scenarios in the laboratory and how the results compare with what is happening in the environment. 

The extent of the ecological impact of microplastics on aquatic ecosystems and on human health is relatively unknown.  Lung and liver damage through tissue fibrosis and cancer can result from the inhalation of PVC dust by humans (Wagoner 1983). More research is needed to help determine the myriad effects on human health and the environment caused by these increasingly abundant contaminants.   

H5Regulations: Microplastics   

Most of the regulatory focus to date has been on primary microplastics, specifically microbeads (Gouin et al. 2015Burns and Boxall 2018). Some major corporations have agreed to phase out the use of plastic microbeads. The Microbead Free Water Act of 2015 in the United States and the Environmental Protection (Microbeads) (England) Regulations 2017 in the United Kingdom have been implemented to ban the use of microbeads in all wash-off cosmetic products. In the U.S., manufacture of personal care products containing microbeads (specifically rinse-off cosmetics designed to exfoliate or cleanse) was prohibited in 2017 and sales of items containing microbeads was prohibited in 2018. 

More monitoring of microplastics in the environment and toxicity testing with a variety of microplastic types and exposure scenarios with standard methods are needed to adequately determine ecological risk and human health risk. Existing monitoring studies indicate that secondary microplastics are most prevalent in the environment, however, there are currently no regulations for secondary microplastics.  

H6. Conclusions 

Microplastics have been recognized as an emerging threat in aquatic environments (Wright et al. 2013). Microplastics are everywhere, and our knowledge of what they do is limited. This review highlights the existing research and the knowledge gaps.  Microplastics have been found in both saltwater and freshwater systems with certain types, such as fibers, more prevalent than others.  Secondary microplastics seem to be more abundant and more of an environmental concern than primary microplastics. However regulatory agencies have only focussed on primary microplastics to date. We also know that microplastics can be ingested and passed up the food chain by some animals.  However, the extent of this process is dependent on the biological species, the size, origin, and type of microplastic, and the environmental concentration of the microplastics. More data are needed to elucidate these interactions and to determine if biological effects are occurring. Microplastics may act as a transfer vessel for many other contaminants. The differences in toxicity of the diverse array of microplastics and the chemicals absorbed to plastics needs further investigation.   

It is well known that ecological health is coupled to human health, however, understanding the intricacies of this relationship is very complex (Petroff et al. 2019). Seafood contamination with microplastics is a possibility.  The better our analytical capabilities in identifying and quantifying microplastics and the more data collected, the better our conclusions and predictions of the environmental fate and effects of this emerging class of contaminants. Field monitoring and assessment (with modern analytical techniques) of microplastics in water, sediment, and biota of the LSJR is essential.  

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