# The Microplastics Cycle: An In-Depth Look at a Complex Topic
## 2 The Microplastics Cycle
### Microplastics in Freshwater  
Regardless of where they are used, plastic products used both on land and water can introduce microplastics into freshwater bodies [49; 50]. Even the primary microplastics designed for use in terrestrial systems, including medicine, industry, home, recreation, etc., have the potential to progressively break down into microplastics and reach freshwater sources. Plastics undergo degradation in the soil and end up in rivers and lakes as microplastics [51]. Or they enter aquatic bodies as macroplastics, and are later transformed into microplastics [52]. After use, many plastics designed for use in the water are typically discarded into water sources. As stated above, most of the microplastics originated from production, and use on land can ultimately transverse wastewater treatment plants (WWTPs) [53; 54]. Microplastics can invade freshwater bodies through treated waters, since there is currently no effective method or procedure for eliminating microplastics [55]. Current methods used to intercept microplastics within WWTPs typically involve several key strategies, including physical separation [56] and filtration [57]. Although these methods hold promise for mitigating microplastic contamination, there are obstacles to their practical application. To address these obstacles and improve the viability of microplastic interception, researchers are developing cost-effective and flexible advanced treatment processes, such as magnetic seed filtration [58], magnetic micro-submarines [58], and photocatalytic micromotors [59]. Membrane bioreactors coupled with activated carbon filters [60] hold promise for enhancing the efficacy of microplastic removal. Following processing from WWTPs, solid wastes, often contaminated with microplastics, separated by the WWTP (i.e., sludge), are sometimes utilized for agricultural fertilization [61; 62]. Precipitation (i.e., rain, snow) and wind, however, can transport the microplastics to freshwater bodies [62]. Microplastics have been identified in plastic landfill leachate [41]. With the help of the Web of Science Core Collection database, we went through all 2249 publications, using key words (freshwater, river, and lake) that had been published up to that year (2022) to identify research on microplastic in surface water (i.e., lakes, rivers, oceans). The location and publication years were obtained from the research. The data suggest that China is the global leader in research on microplastics on freshwater ecosystems, followed by the United States (Figure 3). Over the last five years, there has been a near ninefold increase in the number of published research publications on microplastics in water. Microplastics can constitute a hazard to freshwater ecosystems, inflicting comparable physical and biophysiological harm, as in marine habitats [63]. Other than field experiments, the evidence on the relative effects of microplastics is often inconsistent with the concentrations of microplastics in water bodies [64]. Studies often employ a greater concentration of microplastics than is typically present in aquatic environments and focus on the impact on live organisms [64; 65; 66]. However, research indicates that the present amounts of microplastics have the potential to inflict significant harm to species over time. Another major concern associated with microplastics is that they concentrate other pollutants in water on their surface [67; 68; 69]. Microplastics supporting other contaminants may readily  
Figure 3: The graph shows the volume of research publications on microplastics related to freshwater, rivers, lakes, ponds, etc., published from 2011 to 2022. The number of publications published by various nations on the same topics as those mentioned above is shown on the global map. harm organisms and serve as a vector to move these other contaminants into the food web [70; 71; 72]. Transport of microplastics and other associated contaminants may be facilitated though biofilm growth on microplastics, and the subsequently mistaken for food.