## 1\. Introduction Over 335 million tons of plastic are produced annually globally, making it an indispensable component of modern life \ [1\]. Plastics will be widely used despite initiatives to reduce their usage due to their portability, durability, and low cost. COVID-19 has boosted the usage of plastic in personal protective equipment (PPE), such as facemasks and gloves \[ [\[2\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib2), [\[3\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib3), [\[4\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib4)\]. Plastic garbage enters aquatic and terrestrial areas, threatening ecosystems and possibly endangering biota and humans \ [5, 6\]. In the environment, plastics degrade with time through physical, chemical, and biological processes (i.e., hydrolysis, photodegradation, thermal oxidation, mechanical abrasion, and biodegradation) \ [7\]. As plastics deteriorate, their size reduces, and microplastics (MPs) and nanoplastics (NPs) are generated, making the problem less evident but more harmful. Plastics <5 mm are defined as microplastics, and nanoplastics are plastics with particle sizes ranging in sub-micron scale. Nevertheless, the precise scientific definition of NPs is still under discussion, with the size being described as either 100 or 1000 nm in one dimension. The first detection of MPs was made in oceans \ [8\]. An estimated 5 trillion MP particles are weighing 243,000 tons floating in the water due to MP degradation and deposition, posing a lethal threat to marine life \ [9\]. Several investigations have also identified NPs in the marine environment \ [10\]. In addition to the ocean, the abundance of MPs has been identified in every other environmental media, including wastewater, drinking water, surface water, and landfill \ [1, 9, 11, 12\]. The toxic effects of MPs are well documented in the environment \ [6, 13, 14\]. Due to the increasing concerns about the global impacts of MPs, many studies are going on, and documents are available on the occurrence and treatment of MPs in different environmental systems. Landfills receive a large number of MPs from several primary and secondary sources. Many other pollutants are also present in landfills that can be adsorbed by the MPs. MPs from landfill might infiltrate leachate via rainwater. If not properly engineered, MPs may percolate to groundwater or other terrestrial locations. Landfills have also been identified as a source of MPs input to the marine environment \ [15\]. However, landfill as a contamination source and hotspot of MPs is less explored than other aquatic environments, as current research mostly focused on the fields of water (ocean, surface, and groundwater) and wastewater \[ [\[16\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib16), [\[17\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib17), [\[18\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib18), [\[19\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib19), [\[20\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib20), [\[21\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib21)\]. This study critically reviewed some available scientific literature on MPs in landfill leachate and provides state-of-the-science regarding the source, occurrence, and remediation of MPs in landfill leachate. ## 2\. Source of microplastics in landfill leachate The two principal sources of MPs in landfill leachate are solid waste and wastewater treatment plant residual, e.g., sludge and fat, oil, and grease (FOG), etc. \ [22\]. Plastic waste dumped in landfills can go through several abrasive activities and produce secondary microplastics. MPs can also enter into landfill from numerous primary sources. Wastewater is also a significant collection point for MPs \ [20\]. MPs can get entrapped in FOG and solid sludge during the treatment process. Upon disposal in landfills, sludge and FOG from sewage can augment the abundance of MPs in landfill leachate \ [23\]. Fig. 1 demonstrates the sources of microplastics in the landfill leachate microplastics from solid waste. !Fig. 1 1. Download: Download high-res image (448KB)") 2. Download: Download full-size image Fig. 1. Source and environmental pathways of microplastics via landfill leachate. At the end-of-life of plastic products, the best possible and most desired approach is recycling. However, only 15–20% of all plastic waste can be effectively recycled using conventional technologies worldwide, and approximately 21–42% is deposited in landfills \ [12, 24\]. In 2018, landfills in the US received 27 million tons of plastic waste \ [25\]. Though some plastic products are marked as “biodegradable” nowadays, their complete breakage is only possible when composted in industrial units specifically designed for polymer's molecular breakdown, the biodegradation of these products in landfills is limited \ [26\]. All the waste undergoes numerous treatment stages in landfills: initial aerobic biodegradation, a transition from aerobic to anaerobic condition, acid formation and hydrolysis, methanogenesis to form methane, and final maturation and stabilization \ [27\]. Each stage accelerates the rate of plastic breakage and produces secondary MPs \ [28\]. Furthermore, different anthropogenic activities associated with mass-produced MPs/NPs, such as microbeads from pharmaceuticals and personal care products (PPCPs), shampoos, shower gels, lipsticks, sunscreens, masks, eye shadows, or other intentionally produced microparticles for particular purposes, often end up in landfills. Waste generated from dedicated industries and facilities that handle these products can also be a source of primary microplastics in the landfill leachate. ### 2.1. Microplastics from the wastewater treatment plant Microplastic existence ranges up to 3160 particles L−1, 125 particles L−1, and 170.9 × 103 particles kg−1 of total solid dry weight (TS dw) in untreated wastewater treated wastewater and sludge, correspondingly \ [29\]. Most MPs got trapped in the FOG flocs during treatment stages and/or settle downs in sludge \[ [\[30\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib30), [\[31\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib31), [\[32\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib32), [\[33\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib33)\]. While low-density MPs get trapped in FOG during skimming, the high-density MPs settle down with sludge \ [28\]. 60–99% of MPs from wastewater sources are detained in the sludge from wastewater treatment plants (WWTPs) \[ [\[30\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib30), [\[31\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib31), [\[32\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib32), [\[33\]](https://www.sciencedirect.com/science/article/pii/S2666498423000212?utm_source=chatgpt.com#bib33)\]. From management and materials handling standpoint, landfilling is one of the most straightforward solutions for sludge disposal \ [34\]. Based on the biosolid program report of 2019, around 22% of the sewage sludge is disposed of in landfills \ [35\]. According to the same report, approximately 4.75 million dry metric tons of biosolids were produced in the US in 2019 \ [35\]. During wastewater treatment, a significant quantity of MPs is retained in sludge. Therefore, landfilling sludge from WWTPs can deliver a substantial number of MPs in landfill leachate. According to the average reported MP concentration in sludge from the US, approximately 2.5 × 1012 items per gram of MPs can be delivered to landfills via WWTP sludge each year (assuming the MP counts from Rolsky et al. \ [36\]). Thus, even though sludge is a sink of MPs for WWTPs, it is an enormous source of MPs to landfills leachate.