# The Microplastics Cycle: An In-Depth Look at a Complex Topic
## 2 The Microplastics Cycle
### Microplastics in Land
China is, according to statistics, the leader in microplastics research connected to landfills, followed by the United States of America. It is noteworthy to observe that the number of published research papers on microplastics in soil has risen nearly 20-fold during the last five years. The problem associated with plastics intensifies as land-based plastics undergo aging. Physical, chemical, and biological reactions generate microplastics from waste plastics. According to recent study by [PERSON], [PERSON], et al. [33], carbonyl groups are produced in preference to hydroxyl groups, resulting in an increase in the hydrophilicity of waste plastics as landfills age. Therefore, microplastics may attract microorganisms from landfills and sewage systems. Also, metals (e.g., Al, Fe, Ca, Ti, Ni) may be released from plastic additives in discarded plastics [33]. Microplastics are capable of absorbing hazardous compounds from the environment [34; 35; 36]; hence, microplastics may function as a vector for these toxic chemicals, and microplastics can introduce these toxic chemicals into the food chain [37]. In [PERSON]'s [38] study, an investigation was carried out to assess the presence of microplastics in a stream located in close proximity to a landfill that had been inactive for over three decades. The study exclusively focused on the detection of microplastics larger than 1 mm. The results indicated the presence of microplastics in the stream, confirming  
Figure 2: The graph shows the volume of research publications on microplastics in soil, sludge, compost, and landfills published throughout time. The number of publications published by various nations on the same topics as those mentioned above is shown on the global map. that even though the landfill had been closed for three decades, it continued to release microplastics into the environment. It is plausible that aged plastics within the landfill were undergoing degradation, transforming into microplastics that were subsequently leached into the river water. Given that the analysis was limited to particles larger than 1 mm, it is reasonable to assume that there may be an even greater quantity of smaller-sized microplastics present than what was initially determined. As a result, it is evident that the long-term impact of plastics and microplastics on the environment remains significant, even when strict monitoring and control measures are promptly implemented. Primarily, the introduction of plastics into the soil occurs through municipal landfills designated for solid waste disposal. These landfills serve as repositories for various plastic items, which can inadvertently release microplastics over time due to degradation and environmental weathering. To address this issue, landfill leachate treatment systems have been established. These systems play a crucial role in recovering microplastics and, potentially, repurposing the polymers into valuable products. However, it is essential to recognize that collecting microplastics from these treatment facilities is a formidable task due to the small size and diverse nature of microplastics. Additionally, the transformations that microplastics undergo at different stages within these systems are intricate. From their initial introduction into landfills to their potential release into the environment through leachate, microplastics can experience a series of physical, chemical, and biological changes. These transformations are influenced by factors such as environmental conditions, polymer types, and the duration of exposure within the landfill. Given the complexity of this process and the challenges associated with collecting microplastics from treatment plants, further exploration and comprehensive research are necessary to gain a more profound understanding of the fate and behavior of microplastics in landfill environments and beyond [39]. Microplastics, tiny plastic particles under 5 mm in size, present complex challenges in landfill environments [40].