Microplastics in aquatic environment: Challenges and perspectives
Graphical abstract
Introduction
The European Chemical Agency (ECA) defines microplastics as plastic pieces less than 5 mm in size (European Chemicals Agency., 2020). Microplastics are often categorized as primary microplastics, which are manufactured for a particular application (such as micro-beads), and secondary microplastics, formed by fragmentation and degradation of microplastics, including the fibers from synthetic fibers (Zeng, 2018). A common way for microplastics to enter into the environment is through all the stages in the life cycle of the plastics products (from producers to the waste management systems), which having the potential for trophic transfer and human health exposure. In 2015, a study reported that 6300 million tons of plastic waste were generated, out of which 9% was recycled, 12% was incinerated, and 79% ended in landfills or the environment (The Lancet Planetary Health, 2017). The atmosphere is an essential pathway for the transportation of many suspended materials to the surfaces of the water bodies and even the urban, rural, and remote areas by processes such as the wind speed and the direction, up/down drafts, convection lifts, and turbulence. Earlier studies have found pieces of plastics raining from the sky at an average daily rate of 365 microplastic particles per square meter (Microplastics, 2020). In 2017, an analysis of global tap water samples found 83% of the collected samples contaminated with microplastics. The researchers have analyzed samples from the storm and sea fog and reported that the microplastics in sea spray ranges between 5 μm and 140 μm.
Human activities have a huge impact and contribution to the aquatic environment. A study reported that from 4.8 to 12.7 million metric tons of untreated plastic waste enters the oceans from coastal countries annually (Jambeck et al., 2015). The transport of plastic debris in the aquatic environment has gained much attention. The process of microplastics entering the water bodies depends on the characteristics of the microplastics. The physical and chemical properties of the water bodies, such as hydrodynamics, attachment, and uptake of the aquatic environment, also affect the settlement, re-suspension, and transportation distance of the plastic particles, ultimately contributing negatively to the environment (Huang et al., 2021). Microplastics' concentration in the environment varies from 10 to 1000 μm (Pabortsava and Lampitt, 2020). The omnipresence of plastic contamination can no longer be rejected.
Several studies have highlighted the toxic effect of microplastics on the environment and human health. A research study established the toxic effect of polystyrene microplastics (PSMPs) on earthworms (Eisenia fetida). The study revealed that PSMPs could induce oxidative stress, histopathological changes, and significant DNA damage in the Eisenia fetida. The exposure to 1300 nm PSMPs caused substantial toxicity due to its higher bioaccumulation than 100 nm PSMPs (Jiang et al., 2020). Similarly, the overall toxic effects broadly vary from the interference in the nutrient productivity and cycling in the environment to building physiological stress, including behavioural alterations, immune responses, abnormal metabolism, and changes in energy budgets in several organisms resulting in a threat to the ecosystem composition and stability (Llorca et al., 2020; Ma et al., 2020). Human health effects due to microplastic exposure may lead to particle toxicity, oxidative stress, inflammatory lesions, and increased uptake or translocation. Additionally, secondary infection due to the release of microplastic associated adsorbed contaminants and microorganism, chronic inflammation, and increased risk of neoplasia may further worsen human health (Prata et al., 2020).
Elimination of microplastics from the environment is more challenging than eliminating large plastic debris. Microplastics are ubiquitous and slow degrading contaminants with properties such as long residence time, high stability, high potential of being fragmented, and the ability to absorb other contaminants (Padervand et al., 2020). Previous studies have also reported the presence of organic pollutants such as polyamide, polyester, polymerizing vinyl chloride, and acrylics in microplastics (Chelsea et al., 2018). Therefore, it is necessary to investigate and develop methods to eliminate microplastics from the aquatic environment. This review focuses on the properties of microplastics, their toxicity, and their health effects. The source, transport occurrence, and detection and removal methods with their advantages and disadvantages are also discussed in detail.
Section snippets
Source and transport of microplastics
As of 2019, according to a review published by the European Union's Scientific Advice Mechanism, microplastics are present in every part of the environment. The primary sources of microplastics include household discharge, such as polymeric plastic from cosmetics and cleaning products, feedstocks used to manufacture these products, and the plastic powder or pellets used for air blasting, accounting for almost 5000–80,000 tones (Jiang, 2018) (Fig. 1). Regular fragmentation of larger plastic
Occurrence of microplastics in aquatic environment
According to a 2017 IUCN report, microplastics contribute to 30% of the Great Pacific Garbage Patch, further polluting the world's oceans. In many developed countries, these are the largest marine plastics pollution source than the visible larger pieces of marine litter (Boucher and Friot, 2017). WWTPs are the major source of microplastics release in the water environment. While larger plastic particles are effectively removed by the multiple treatment processes carried out, often microplastics
Toxicity and human health effects
Chemical additives including plasticizers, heat stabilizers, antioxidants, and colorants are commonly used during the manufacturing of polymers to improve product performance. The presence of these compounds, when exposed to the environment, cause chronic hazards. The plasticizers are toxic to the animals and have adverse effects on plants (Rehse et al., 2016). Another possible area of concern with the presence of chemicals associated with the microplastics is monomers-unbound plastics
Global policy framework
Microplastics have drawn worldwide attention among scientists and policymakers due to their adverse effects on aquatic flora, fauna, and human health. Microplastics have been recognized as one of the major environmental concerns by the United Nations (UN), G20, G7, and Asia Pacific Economic Cooperation (APEC). According to the World Economic Forum (WEF), the world's oceans will have more microplastics than aquatic life by 2050. In a recent study (Pabortsava and Lampitt, 2020), the researchers
Analytical methods for microplastics detection
The analytical methods help to detect the microplastics in various biotic and abiotic environmental matrices, which provide essential information on their pollution status, the hot spots of concentration, and the possible exposure to organisms (Fig. 3). Microplastics monitoring require reliable sampling and analysis methods, including laboratory accumulation, toxicity, and understanding of weathering studies. However, being an emerging contaminant, analytical methods for detecting microplastics
Fate of microplastics in wastewater treatment plants
The sewerage systems transport the microplastics into the wastewater treatment systems, effectively preventing the plastic debris from entering the aquatic environments. China alone releases around 9.1 × 1010 microplastics through domestic wastewater every day (Tang et al., 2020). About 87%–99% of plastic debris is retained from the sewage at different treatment stages (preliminary, primary, secondary, and tertiary) of WWTP. The conventional sewage treatment plants are not ideally designed to
Challenges for micro-plastics analysis and treatment
Microplastics are a type of emerging contaminant that poses several toxic effects on human and animal health through multiple mechanisms, as discussed earlier. To eliminate these plastic debris, identifying the entire range of microplastic type, shape, and total morphology is necessary, this being the first challenge. Microplastics are the most diverse contaminant, unlike pesticides and PCBs, which can be analyzed using different microscopy and spectroscopy technologies (Hale, 2017). In
Control the release of micro-plastics to the water environment
The microplastics are released into the aquatic environment from multiple sources, commonly from the degradation of larger plastic items found in various environmental compartments due to poor plastic waste management. A hydrodynamic plastic tracking model has been developed recently based on the topography and the tidal and metrological forces, which provide for the realistic circulation of floating plastic debris in Singapore's coastal waters (Tong et al., 2021). This study's results can be
Conclusion and future directives
Plastic polymers are inseparable items from people's lives as plastics' demand increases in direct and indirect applications. The plastic is released into the environment whenever plastic is used, which breaks into smaller debris (microplastics) due to environmental factors, and transport to water bodies. It is essential to establish models with the help of data and practical verification to understand the transport and the risk assessments of microplastics. A better and comprehensive
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
Authors are thankful to Department of Biotechnology-GoI (Grant No. BT/RLF/Re-entry/12/2016) for financial support to this research.
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