Microplastics in the Environment: Sources, Pathways, and Abundance

Introduction

The widespread use of plastic may be attributed to their numerous attractive properties. Plastics are affordable, flexible, durable, lightweight, waterproof, easy to clean and sterilize, and act as insulators, among other things (Bergmann et al., 2015; Lebreton & Andrady, 2019). Plastics are commonly used in packaging, automotive, household leisure, electrical and electronic parts, building and construction materials, sports products, and the agricultural sector (Elizalde-Velázquez & Gómez-Oliván, 2021). Plastic production has grown exponentially, with large-scale production starting in the 1950s around 1.5 million tons per year to 348 million tons in the 2017s (Plastics Europe, 2018; WHO, 2019). Historically the global production of plastic materials has increased by around 8 - 10% each year (Crawford & Quinn, 2016; Peixoto et al., 2019). Plastic production is expected to double by 2025 and more than triple by 2050, based on the projected global population growth rate and existing consumption and waste patterns (FAO, 2017). Due to improper plastic waste disposal, a significant number of plastics eventually end up in the marine ecosystem via household materials, industrial discharge, wastewater treatment plants (WWTPs), incineration, landfills, and rivers. This has resulted in increasing quantities of plastic pollution in oceans worldwide. (Aslam et al., 2020; Crawford & Quinn, 2016; Henry et al., 2019). The occurrence and distribution of large plastic litter in the marine environment have been extensively documented (Ajith et al., 2020; Du et al., 2020). However, with prolonged physical, chemical, and biological weathering, these larger plastic materials eventually fragment, resulting in macro, meso, micro, and nanoplastics (Golwala et al., 2021).

Microplastic enters the environment as primary and secondary sources. These microplastics are ubiquitous in the atmospheric, terrestrial, and aquatic environment, mainly due to inappropriate plastic disposal and insufficient waste management (Ajith et al., 2020). Landfills, the most prominent technique of solid waste management, are the main repository and disseminator of microplastics, including both primary and secondary microplastics (Golwala et al., 2021). Different environmental conditions (such as rain and wind) facilitate the transport of landfilled microplastics to the surrounding ecosystem. Furthermore, due to inadequate separation, microplastics from different consumer commodities and microfibers from synthetic clothes are potential sources of microplastics in WWTPs (Z. Zhang & Chen, 2020). The majority of microplastics enter aquatic streams after passing through WWTPs (De Falco et al., 2018; Talvitie et al., 2017). The retained microplastics accumulates in sludge or biosolids applied for agricultural activities may result in microplastic contamination in agricultural soil (Q. Li et al., 2019; Y. Zhou et al., 2020). The majority of plastic waste created in industrial operations and small workshops are discharged immediately into the environment without treatment (Du et al., 2020). Finally, microplastics reach into the sea from diverse sources and are dispersed by ocean currents. Widespread deposition and distribution of microplastics raise a concern about their interaction and potential consequences on marine ecosystems. The gradual increase of microplastic in the sea will lead to significant accumulation in coastal and marine environments (Bergmann et al., 2015).