Editorial Overview: Emissions of Microplastics and Their Control in the Environment

Microplastics (MPs) (particle size $<5\mathrm{mm}$) have been found in almost all environments on Earth, including urban centers, terrestrial areas, and freshwater environments as well as in remote uninhabited islands, deep seafloor, and polar regions (Liu et al. 2019; Ni et al. 2020; Yin et al. 2021). They are released to the environment directly from primary sources (such as cosmetic and cleansing commodities, and personal care and pharmaceutical products), or indirectly from secondary sources through the fragmentation and degradation of meso and macroplastics (Schernewski et al. 2020). In the foreseeable future, such release will continue and may increase. For instance, about 360 million tons of plastics were manufactured worldwide in 2018 (PlasticsEurope 2019). The global plastic production has been projected to reach 25 billion tons in total by 2050 following current trends, in which approximately 4.9 billion tons will be discarded as waste and discharged to the environment (Geyer et al. 2017). Eventually, most plastic waste will accumulate and persist in ecosystems as MPs or even nanoplastics through fragmentation processes. Thus, the effects of microplastic pollution on the environment and the associated risks for human health are of growing concern. Researchers from a variety of scientific disciplines have been involved in studies on MPs, from limnology and oceanography to toxicology, and from marine biology to analytical and polymer chemistry. This has led to the breakout of new research and resultant papers on the occurrence, fate, transport, and effects of MPs in various environmental media in recent years (Table 1). In 2020 alone, more than 40 review articles on MPs in the environment were published. In addition to the explosion of interest in these topics, researchers started to make efforts on the source apportionment of microplastic emissions and their control. Therefore, the objective of this editorial is to provide an overview of microplastic emissions and their control in the environment, and to discuss the directions for future endeavors.

On one hand, the identified primary sources of microplastic emission to the environment mainly include plastic pellets from industry, microfibers from clothing, microbeads from personal care products (PCPs) and paint, as well as MPs from washing wastewater, wastewater treatment plants (WWTPs), rubber road, artificial turf, and tire wear (An et al. 2020). For example, PCPs (e.g., makeup cosmetics, cleansing products) contain abundant microbeads (Nizzetto et al. 2016). Based on the microplastic contents in PCPs and their consumption levels, the global emission of PCP-derived MPs could reach $1.2\times {10}^4\mathrm{t}/\mathrm{year}$ (Sun et al. 2020). Generally, most PCP-derived MPs enter municipal sewage networks, along with runoff and other kinds of wastewater from domestic and industrial activities, all of which contain many kinds of MPs (Birch et al. 2020b). At present, conventional WWTPs are unable to completely remove MPs (Sun et al. 2019). Thus, emissions from WWTPs are considered as one of the main sources of MPs to the environment because plenty of effluent is directly discharged into surface water every year (Conley et al. 2019; Edo et al. 2020). In addition, activated sludge that accumulates most of the removed MPs (69%–80%) can be also an emission source if improperly managed (Li et al. 2018).

On the other hand, secondary sources of microplastic emissions are larger plastic products that are not properly disposed. Such sources mainly include farming film, fishing waste, and municipal debris from plastic bags, bottles, tableware, and packing products (Ng et al. 2018; An et al. 2020). Currently, secondary sources are estimated to emit the majority of MPs to the environment even though breaking large plastic waste into MPs under natural conditions takes years (An et al. 2020). For example, microplastic films and foams could be mainly sourced from the erosion of plastic bags and packing products that are essential items in humans’ daily lives (Zhou et al. 2020). Since the 1990s, they have been widely used because of their advantages of low cost, large capacity, light weight, and easy storage. Globally, up to $\sim 5\mathrm{trillion}$ plastic bags are consumed every year, and $\sim 39.7\%$ of the total plastic production is used for packing (UNEP 2016).

As microplastic pollution has been reported unceasingly, it is realized that this is an international environment problem that needs to be coped with. Currently, some international or national laws or regulations have been legislated to decrease microplastic emissions. In 2015, the United Nations Environment Programme (UNEP) added plastic waste to the list of environmental issues that are worth constant concern. Many regions and countries have also launched restrictions on the single use of plastic bags and the addition of microbeads in PCPs (Xanthos and Walker 2017). For example, the Australian Capital Territory introduced a ban of single-use plastic bags in 2011, which led to the reduction of about 2,600 t of conventional polyethylene bag consumption by 2018 (Macintosh et al. 2020). A plastic bag ban policy in Scotland also prevented approximately 650 million plastic bags from entering waste streams (Sharma and Chatterjee 2017). The federal administrations of Canada, Australia, Austria, Luxembourg, Belgium, Netherlands, Sweden, and Germany imposed an all-out ban for the application of microbeads in PCPs (Reed and Perschbacher 2016). More recently, the European Union (EU) has put forward a Europe-wide plastic strategy as a portion of the transition to the circular economy (Pico et al. 2019). Based on this strategy, the consumption of disposable plastics will be significantly decreased and all plastic materials for packing will be recyclable in EU markets by 2030.

In addition to regulatory and social measures, remediation technologies have also been investigated to control microplastic pollution. The primary applications of technologies such as sedimentation, coagulation, air flotation, activated sludge, sand filter, membrane separation, and membrane bioreactor for microplastic removal from wastewater have been summarized in several reviews (Bui et al. 2020; Cristaldi et al. 2020; Zhang and Chen 2020). The existing knowledge of MPs removal in drinking water through traditional treatment processes, electrocoagulation, magnetic extraction, and membrane separation has also been discussed (Krause et al. 2020; Shen et al. 2020). However, due to limitations of these technologies and the complex properties of MPs, there are many challenges associated with the development and evaluation of these technologies.

To date, hundreds of studies have documented microplastic occurrences, characteristics, and ecological effects in a wide range of environments (Table 1). However, these studies mainly focused on aquatic environments, such as marine, rivers, and lakes. Few studies have focused on soil environments and agroecosystems. There are gaps in understanding the occurrences and characteristics of atmospheric MPs, as well as limited knowledge on their fate and transport. Several reviews began to analyze the current status of knowledge on atmospheric MPs, and highlight future research needs in identifying their potential impacts on human health (Chen et al. 2020a, b; Huang et al. 2020; Mbachu et al. 2020; Zhang et al. 2020b). While still limited, research has been expanded to include specific environments, such as urban watershed, drinking water, and WWTP (Sun et al. 2019; Birch et al. 2020b; Danopoulos et al. 2020).

Another noteworthy research gap in the study of MPs in the environment is emission source apportionment, particularly for atmospheric MPs. For example, studies on microplastic pollution in land-based watersheds have identified many sources, such as discharge from treated wastewater and plastic waste from industrial, commercial, and agricultural activities (Law 2017; Su et al. 2020). However, the relative contributions of these sources to microplastic pollution remain controversial. Each type of microplastic may come from a variety of sources. Due to the differences in plastic production types, waste disposal processes, and environmental conditions in different regions, the efforts for identifying microplastic sources may suffer from various uncertainties. Additionally, global monitoring data of environmental MPs are far from sufficient. For instance, although microplastic pollution occurs globally, the monitoring results may not be enough to form a global distribution map. Such a knowledge gap limits the evaluation of global microplastic emissions to the environment. Thus, more research is needed to better understand the source apportionment of microplastic emissions.

Finally, crucial knowledge gaps exist in the development of microplastic removal technologies and environmental limits. For example, several technologies were studied for removing MPs from water and wastewater (Padervand et al. 2020; Zhang and Chen 2020). However, there have been only limited or no research about MPs in soil or atmosphere. In addition, no governmental legislative standard for MPs in the environment has been issued.

To improve the understanding of microplastic emissions and their control, more research is needed to fill the mentioned gaps. Several challenges exist in conducting related studies. First, standardized methodologies for microplastic sample collection, extraction, and identification are desired, though researchers have been making progress (Zarfl 2019; Birch et al. 2020a). These are greatly linked to the reliability for comparative studies of research findings, such that the meta-analyses of global microplastic emissions, the development of reliable monitoring strategies, and the implementation of appropriate mitigation measures can be accomplished.

Second, MPs are highly heterogeneous mixtures that contain multiple types of solid polymers with different densities, sizes, and shapes, leading to intensified complexities in the related methodologies and their implications. For instance, the available microplastic data from field sampling mainly focus on sizes larger than $300\mathrm{\mu m}$, while the information on smaller MPs is much less. The information of nanoplastics is even less. However, most of the existing technologies are incapable of characterizing small-size MPs (e.g., Fourier transform infrared spectrometry $>20\mathrm{\mu m}$, Raman spectroscopy $>1\mathrm{\mu m}$) (Granek et al. 2020). This leads to difficulties to adequately study MPs in such small sizes. Consequently, the emissions of MPs with all size classes can hardly be quantified. In addition, the shape and polymer type of MPs may also influence the effectiveness of relevant mitigation measures. Understanding the complexity of MPs (including nanoplastics) is a challenge for researchers in this field.

Third, MPs can transport and enter the environment through multiple pathways, such as surface runoff, atmospheric deposition, and drainage. The evaluation and management of microplastic pollution from non-point sources are more difficult than those for point sources because it is hard to regulate and locate the emission contributors. For example, WWTPs, plastic industries, and fishing activities have been identified as main point sources of microplastic pollution in aquatic environments (Estahbanati and Fahrenfeld 2016; Su et al. 2020; Zhou et al. 2021). Thus, efforts have been made to reduce the contents of microbeads in PCPs and control microplastic emissions from WWTPs (Lares et al. 2018; Ngo et al. 2019). However, microplastic pollution from non-point sources has rarely been studied, and the mechanisms for the fate and transport of MPs are relatively unclear.

In addition to the preceding challenges, issues of inevitably massive emission, inappropriate management, and improper disposal of plastic waste also exist. Since plastic was invented, a huge volume of plastic waste has entered the environment, while the use of plastic is still increasing (Geyer et al. 2017). Thus, the challenge of massive emissions may continue to exist, although efforts are being made to improve this situation.

For MPs that have been discharged to the environment, it is difficult to clean them up through conventional means. Efforts should be made on the reduction of microplastic emissions and the removal of MPs at sources. First, there is a great need to establish effective policies and regulations to reduce microplastic emission and enhance public awareness of microplastic pollution. Environmental conservation agencies are getting a better understanding of this issue and making efforts to develop and implement relevant strategies (Xanthos and Walker 2017). These efforts are focused on public participation, such as reduction of plastics consumption, before more ambitious recycling/recovery targets are established. Additionally, there are increasing efforts in developing standardized protocols and quality assurance/quality control (QA/QC) techniques to improve the reliability of investigations about microplastic pollution. For example, Elkhatib and Oyanedel-Craver (2020) developed a ranking system to evaluate the extraction and identification methods of MPs in wastewater and drinking water. The development and standardization for analytical methods of atmospheric microplastic pollution are particularly desired. Moreover, research endeavors in the following aspects should be highlighted.

The first is the development of biodegradable plastics that are environment friendly, so as to mitigate the microplastic emission and accumulation in the environment. With the promulgation and implementation of policies to limit the use of traditional plastics in various regions, the demand for degradable plastics will significantly increase. There has thus been growing research on the issue of plastic biodegradability (Lambert and Wagner 2017). For example, many kinds of bio-based plastics have been made from renewable materials, such as starch, cellulose, and lignin (RameshKumar et al. 2020). Microalgae and food waste are also applied as feedstocks of bio-based polymers (Zhang et al. 2019). However, the high price of biodegradable plastics and the unclear ecological and societal implications may limit their applications.

The second is the advancement of microbial technologies to facilitate plastic/microplastic degradation. Microorganisms that are capable of utilizing synthetic polymers can be identified and isolated to degrade plastics (Ganesh Kumar et al. 2020; Yuan et al. 2020). Several specialized bacteria have been found to have the ability to break down poly(ethylene terephthalate) plastics through enzymatic hydrolysis (Yoshida et al. 2016; Kawai et al. 2019). If superbacteria can be cultivated to degrade MPs, it will provide a promising approach for microplastic pollution control.

Additionally, effective plastic waste management and recycling should be promoted at community and jurisdictional levels. Mismanagement of plastic waste is considered a major cause of microplastic pollution (Ni et al. 2020). It was estimated that only 9% of waste plastics were recycled by 2015 (Lambert and Wagner 2017). In 2015 alone, up to 60–99 million tons of plastic waste were produced due to mismanagement (Ni et al. 2020). However, conventional facilities of solid waste management, such as landfills and incinerators, may also release MPs to the environment, though they can stock or eliminate most plastic waste. For example, landfilling is a potential source of abundant MPs in leachate (He et al. 2019); also, a considerable amount of MPs is found in the bottom ash of incinerators (Yang et al. 2020). The importance of upgrading relevant technologies to enhance plastic waste management has become evident.

In such ongong efforts, the involvement of environmental engineers is critical to intensify the identification of environmentally sustainable solutions to the prevention, minimization, and remediation of pollution due to microplastics. In particular, environmental engineers should play key roles in research and development (R&D) of innovative technologies for battling microplastic pollution in ambient atmosphere, surface water, and soil/aquifer systems, as well as their impacts on human health and ecosystems.

This research was supported by Environment and Climate Change Canada’s Increasing Knowledge on Plastic Pollution Initiative, and the University of Cincinnati through the Herman Schneider Professorship in the College of Engineering and Applied Sciences.