The landfill method of simply covering soil to treat waste can be traced back to 3000~1000 BC. Since no environmental protection measures have been implemented, simple landfill has caused environmental health problems, and then sanitary landfill technology has been gradually developed^[16]Sanitary landfill is a method of anti-seepage, paving, compaction and covering for the treatment of solid waste and the treatment of gas, leachate, flies and insects. In 1930, the United States built the world's first sanitary landfill using a single layer of compacted clay for seepage prevention. Since 1982, the simple compacted clay liner has been upgraded to single-layer geomembrane liner, double-layer geomembrane liner, bentonite waterproof blanket, etc. China has adopted sanitary landfill technology since the early 1990s. In 1991, Hangzhou Tianziling completed the construction of the first sanitary landfill with natural clay liner; In 1997, the first sanitary landfill site with horizontal anti-seepage of high-density polyethylene (PE) geomembrane was built in Xiaping, Shenzhen. So far, sanitary landfill has become one of the main ways of waste disposal. By 2022, 197 large landfills have been built in 18 states of the United States^[17]California has the largest number of landfills (58), followed by Florida (19). This is mainly because California is one of the most populous and economically developed states in the United States, with a high output of waste and the need for more large landfills to handle waste^[18]In the same year, there were 444 domestic waste sanitary landfills in use in China, with an average daily treatment capacity of 215000 tons and an annual treatment capacity of 30.432 million tons^[19]Among them, there are more than 30 landfills in Guangdong (32) and Xinjiang Uygur Autonomous Region (31); There are more than 20 landfills in Heilongjiang (27), Shandong (24), Shaanxi (24), Inner Mongolia Autonomous Region (23), Liaoning (23), Jilin (23), Hunan (23) and Hubei (21). In terms of waste disposal capacity, Guangdong (2.898 million tons), Liaoning (2.651 million tons), Xinjiang Uygur Autonomous Region (2.091 million tons), Hunan (2.081 million tons) and Shaanxi (2.052 million tons) all exceeded 2million tons. The number of sanitary landfills and waste disposal capacity in Guangdong Province rank first in the country, which is related to the large population, large output of domestic waste and high demand for landfills in the region. Secondly, Xinjiang Uygur Autonomous Region, Heilongjiang and Liaoning are rich in land resources, which is convenient for the construction of large landfills. In the future, with the acceleration of urbanization, the application of sanitary landfill in the treatment of domestic waste will be reduced compared with incineration, but it will still play a role in the temporary storage and disclosure of waste. In particular, excess waste generated by emergencies (such as typhoons), fly ash generated by incineration power plants, and domestic waste during maintenance should be disposed of by landfill.

The life cycle of a landfill site includes three stages: construction, operation and closure repair, which can last for about 10 to 15 years. The construction phase mainly includes the construction of the main works and equipment of the landfill site, supporting works, production management and auxiliary facilities, living service facilities, etc. When selecting a suitable landfill site for domestic waste, it is necessary to consider factors such as the degree of local population aggregation, land resources, economic and technological development level, composition and nature of domestic waste, and ensure that the site selection is reasonable, the scale is appropriate, the technology is feasible, the equipment is reliable and meets the requirements of sustainable development. The design and construction of sanitary landfills in China should meet the requirements of relevant standards such as technical code for sanitary landfill of domestic waste (cjj1-2004), construction standard for sanitary landfill treatment projects of domestic waste, and standard for pollution control of domestic waste landfills (GB 16889-2008). The main works of the landfill site include the bottom liner system (used to isolate leachate), the landfill unit (used to store garbage), the drainage and exhaust system (used to collect rainwater and leachate and biogas generated by garbage decomposition) and the top cover system (used to close the top of the landfill site)^[20-21]For the garbage with greater harm, a double-layer liner system is required, in which the leachate collection layer is above the main impervious layer and the leakage detection layer is below the foundation soil. The operation phase includes daily operation management of the landfill, leachate treatment, resource utilization of landfill gas, etc. In the landfill operation, layered covering (covering with soil on each layer of compacted waste) and anti-seepage treatment are implemented to control the diffusion of pollution, while monitoring the gas emission and leachate treatment. The phase of site closure and restoration includes the greening of the original site, excavation and treatment, site clearance and evacuation, and ecological restoration, aiming to eliminate pollution sources and restore the land use function.

The plastic waste in the landfill mainly comes from all kinds of plastics discarded in daily life, industrial and agricultural production, construction and other processes. In most urban landfills, the most common is plastic packaging, including PE, polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyamide (PA), polylactic acid and other materials^[22]Because these man-made polymers account for a high proportion of plastics produced worldwide^[4]Of course, it is the main type of landfill plastics. For coastal cities, aquaculture buoys and marine plastic waste are the main sources of landfill plastics^[23]Garbage such as PS foam plastic blocks or particles, cigarette butts (including acetate fiber) and so on are common on beaches all over the world^[24-25]According to the data of the 2021 bulletin on the state of China's marine ecological environment, there are 4580, 154816 and 4770 plastic wastes (mainly plastic ropes, plastic debris and plastic bags) per square kilometer of sea surface, beach and seabed in China's offshore waters^[26]In response to such a serious problem of marine plastic pollution, relevant ministries and commissions have successively issued the "action plan on printing and distributing plastic pollution control during the 14th five year plan"^[27]And the action plan for marine garbage disposal in coastal cities^[28]It is clearly proposed to strengthen the standardized recycling and removal of plastic waste, and achieve the goals of "timely and effective removal of beach garbage in 65 bays, significant reduction of floating garbage density on the sea surface" and "significant reduction of marine garbage density in 65 bays, and normalization to a clean level" by 2025 and 2027, respectively. With the promotion of marine waste cleaning, the waste plastics that cannot be recycled will be buried in the landfills of coastal cities, resulting in an increase in the base of plastic waste.

Sludge from sewage treatment plants is one of the main sources of micro plastics in landfills. Many studies have confirmed that domestic sewage contains a large number of polymer particles from personal care products and cosmetics^[29-30]There are also many synthetic fibers and plastic particles produced by the production of various plastics in industrial wastewater^[8,31]In addition, plastic products (such as fabrics, cleaning sponges, food packaging, etc.) may suffer from mechanical wear or chemical degradation during use, resulting in and releasing a large number of micro plastics^{[32⇓⇓⇓⇓⇓-38]}These micro plastic containing domestic sewage and industrial wastewater are discharged into the sewage treatment plant. After treatment by different processes, more than 90.0% will be collected in the sludge^[39]Some of them will be landfilled (22.0% of sludge from sewage treatment plants in the United States will be landfilled^[40]54.0% of the sludge from 11 municipal sewage plants in key river basins in China was buried^[41]）. In addition, microplastics suspended in the atmosphere, including rubber particles from tire wear, debris from pavement or building exterior wall paint falling off, and so on^[13,42]It will also enter the landfill site during the landfill process through dry and wet settlement.

Plastic waste will go through a long process of change after being buried. The stabilization cycle of landfill waste mainly includes five stages: initialization and adjustment, transition, acidification, methane fermentation and maturation. From the time the waste is placed in the landfill, it enters the initial adjustment stage, and the easily degradable components are degraded into CO by aerobic microorganisms₂And H₂O. With o₂The waste decomposition is transformed from aerobic to facultative anaerobic, organic hydrolysis and anaerobic fermentation produce a large number of organic acids (acidification stage), while organic acids and alcohols are further decomposed into co₂And Ch₄Etc. (methane fermentation stage). The biodegradable components in waste are fully consumed and enter the mature stage. Although petroleum based plastics are not easy to be utilized by microorganisms due to their high molecular weight, hydrophobicity and chemical bond energy, field investigation has confirmed that plastic waste will be decomposed and broken into micro plastics in the long-term stabilization process. In 2022, Huang et al^[43]It was revealed that the abundance of microplastics in landfill waste increased exponentially with the landfill time (7-31 years), and after 30 years of landfill, the relative abundance of microorganisms with plastic degradation function in plastic waste increased by more than three times, which means that biodegradation after long-term natural domestication is one of the main driving forces of landfill plastic micronization. In addition, the degradation of plastics will release small molecular products and additives. For example, bisphenol A is the characteristic residual monomer and degradation product of polycarbonate (PC) plastics, while phthalate plasticizers will be dissolved or hydrolyzed from plastics^[44-45]In 2021, Zhang et al^[44]By analyzing the concentration ratio of polymer, monomer and plasticizer, the degradation degree of plastic waste at different landfill ages was characterized. It was found that the ratio of "bisphenol A/PC" and "plasticizer/microplastics" was positively correlated with the landfill time and negatively correlated with the landfill time, respectively, indicating that PC plastic degraded and released bisphenol A during the long-term landfill process, and the leaching of plasticizer was faster than the formation of microplastics. Therefore, by detecting the content of plastic additives in landfill waste, it can also be inferred that plastics have been degraded in the landfill process. It has been decades since the early landfill of plastic waste, and a considerable part must have been degraded into micro plastics.

The simulation experiment of environmental conditions of landfill site shows that plastic aging generates micro plastics. The dark environment conditions of landfill waste are extremely complex and in dynamic change. The waste accumulation and extrusion will form friction shear force, and aerobic degradation will release heat. Although a large number of studies have confirmed that mechanical force and heat are common factors leading to plastic aging^[46]However, how these two effects affect the transformation of plastics into micro plastics in landfills is still relatively unknown. In 2023, Huang et al^[47]A systematic study was conducted on the aging behavior of PE plastic film in the landfill leachate from Laogang, Shanghai, which was more than 10 years old (filtered and deoxidized by 0.45 μ m) under mechanical stirring (200 RGMin) and high temperature (80 ℃). The study found that the surface structure of plastics would be damaged under mechanical force, and depolymerization and degradation would occur in high temperature environment, because the hydroxyl radical generated by dissolved organic matter in leachate promoted the pyrolysis of plastics. The two factors led to more obvious surface damage, forming holes, cracks and scratches, which accelerated the aging and micronization of plastics and released (14.2 ± 0.5) μ g/L of microplastics.

It is worth noting that due to the weakening of the properties of the aged plastics in the leachate, under the continuous action of mechanical force and high temperature, the damage and depolymerization rates of the aged plastics are higher than those of the original plastics, and they are more likely to form micro plastics. Before being affected by complex landfill conditions, plastic waste has experienced aging. After being buried, under the erosion of leachate, the aging rate of plastic waste into micro plastics will be accelerated. For example, ultraviolet light will promote the degradation of PP non-woven fabric and film in the artificially prepared landfill leachate, releasing dissolved organic matter and nano plastics^[10]In addition, human intervention may also accelerate the aging and crushing of landfill plastics, such as the widely used in-situ aeration in landfills^[48]It aims to promote the stabilization of landfill waste through high-pressure or low-pressure aeration and leachate recycling, but these external forces will inevitably lead to the flow of microplastics in the leachate, the increase of friction and wear, and the refinement of microplastics. Under these physical and biochemical effects, the aged plastic waste in the marine environment has the potential to rapidly generate micro plastics after being buried. Then, the occurrence of microplastics in landfills in coastal cities may be higher than that in landfills in inland cities.

There are a lot of microplastics derived from petroleum based plastics in landfills. At present, many domestic scholars have investigated and analyzed the abundance and particle size distribution of microplastics in landfills. Representative research results are as follows:Figure 2A, bAs shown in. 2019, su et al^[7]The microplastics in Shanghai Laogang landfill site were investigated and analyzed. It was found that there were 62 ± 23 microplastics (average particle size was 1.0 mm) in each gram of waste, of which 59.8% were debris, and most were PE. In 2021, Wan et al^[14]The content of microplastics detected from a landfill in Southeast Guangdong ranged from 580 to 103080 pieces/kg. Since no effective anti-seepage measures have been taken here, the study also reported that the abundance of microplastics in the subsoil of garbage is 570-14200/kg, in which fiber is more than film and debris, and the size is generally in the range of 0.03-0.15 mm. Among these micro plastics, PE, PP and pet accounted for 29.8%, 19.4% and 7.9% respectively. In the same year, Zhang et al^[44]The microplastics floatated from mineralized waste in a landfill in Henan Province were analyzed. The results showed that the abundance of microplastics was 25-113/g, the particle size was mainly distributed in the range of 0.5-1.0 and>1.0 mm (30.1% and 31.5% respectively), and the main components were PE (21.4% -30.1%), PP (11.8% -15.1%) and pet (9.2% -12.9%); Through liquid chromatography-mass spectrometry tandem technology, pet and PC microplastics were detected in the waste residue after flotation, and the contents were 37.9~674 μ g/g and 0.07~1.0 μ g/g, respectively. In 2022, Li et al^[49]The abundance of microplastics in Chengdu (sanitary) and Yibin (non sanitary) landfills in Sichuan was reported to be 10~144 and 4~168/g, respectively. Among them, the micro plastics with the size of<0.1 mm (37.8%) and 0.1~0.5 mm (34.3%) are mostly composed of pet (19.1%), PS (18.1%), PE (16.8%) and PP (13.4%). In 2023, Lin et al^[50]After the detection of a sanitary landfill in Shenzhen, it was found that each gram of waste contained 81-133 micro plastics, with particle sizes ranging from 30 to 5000 μ m (average particle size 1.2 ± 0.1 mm). From these results, it can be seen that the composition of micro plastics contained in landfills in different regions of China is similar, but the quantity is different by three orders of magnitude.

The difference of landfill age will affect the occurrence characteristics of microplastics in waste dumps. 2019, su et al^[7]It was found that compared with the original PE microplastics, the carbonyl index of PE microplastics in landfilled waste for more than 20 years increased from 0.2 to 1.3, indicating that the microplastics were significantly aged during landfill. In 2022, Yu et al^[51]It was found that the carbonyl index of PP microplastics increased from 0.16 to 0.24 in the waste landfilled in Shanghai for 7 and 27 years, indicating that the aging degree of plastics increased with the increase of landfill age; The crystallinity of PE microplastics decreased from 55.6% to 20.8%, indicating that the mechanical properties of plastics decreased significantly with the increase of landfill age. In 2022, Huang et al^[43]The amount of microplastics in 7-31-year-old municipal solid waste in Shanghai was analyzed. It was found that the amount of microplastics increased exponentially with the extension of landfill time (from 71 ± 18 to 653 ± 191/g); The formation rate of PE microplastics was the highest (28/g plastics/year), while the formation rate of PP and PS microplastics was relatively low (15 and 10/g plastics/year, respectively). This is related to the aging degree of different polymers in the landfill process. The carbonyl index of PE microplastics increased by an order of magnitude (from 0.2 to 2.6), while the carbonyl index of PP and PS microplastics increased significantly (from 0.1 to 1.0 and 0.4, respectively). According to the generation rate of microplastics in waste dumps, it is estimated that about 17.1 billion microplastics may be produced after 50 years of landfilling per ton of plastic waste. In 2023, Lou et al^[52]The waste samples from the north and south areas of a landfill in Zhejiang Province were detected by pyrolysis gas chromatography-mass spectrometry. The results showed that the average particle size of micro plastics in the waste in the south area was slightly smaller than that in the north area (1.5 vs 2.0 mm), but the content was higher (7.6 vs 5.5 g/kg) and there were more fibers (28.4% vs 11.0%). This may be because the landfill time in the south area (12-16 years) is longer than that in the north area (5-12 years), and the fabric fibers undergo a long period of aging to produce more micro plastics. In 2023, sholokhova et al^[53]The content of microplastics in waste from lap ė s landfill in Kaunas County, Lithuania, was investigated. It was also found that the content of microplastics increased with the increase of landfill age and depth (3-55 pieces/g or 1.7-52.8 g/kg). These results show that the landfill time is the key factor restricting the generation of micro plastics from plastic waste, and also indicate that the longer the landfill age, the more micro plastics may be accumulated in the waste dump.

Micro plastics are commonly found in landfill leachate. At present, many international researchers have investigated and analyzed the abundance and particle size distribution of microplastics in landfill leachateFig. 2c, dAs shown in. In 2018, Praagh et al^[54-55]Leachate samples from landfills in Iceland, Norway and Finland were collected, and microplastics of 50~5000 μ m were detected, with an average abundance of 4/L. the main components were PE, PVC, PA and polyurethane. 2019, he et al^[22]Twelve landfill leachate samples were collected from four active landfills in Shanghai, Wuxi, Suzhou and Changzhou, and two closed landfills in Shanghai. The micro plastics were analyzed by Fourier transform infrared spectroscopy (FTIR). The results showed that the average abundance of microplastics in leachate was 0~24/L, more than half of which were fragmented (58.6%), mainly concentrated in 100~1000 μ m (74.9%). These micro plastics include 17 kinds of polymers, mainly PE and PP (69.9%), which may be related to the high production and use of PE and PP plastics. Compared with the micro plastics in the landfill, the particle size of the micro plastics in the leachate is smaller, indicating that these micro plastics are more likely to exude from the garbage. In 2020, Xu et al^[56]Using a customized sampler, 20-100 μ m microplastics were collected and analyzed from the leachate of a landfill in Shanghai. It was found that PP (40.0%), PA (36.0%) and man-made fiber (18.0%) microplastics were the main materials, and the abundance was 291 ± 91/L. This value is much higher than su et al^[7]The quantitative concentration of microplastics (70-3670 μ m) in the leachate of a landfill in Shanghai was reported (4-13/L). In 2022, Mohammadi et al^[57]The microplastics (1000~5000 μ m) in the leachate of Bushehr port landfill in Iran were analyzed. It was found that the abundance of microplastics fluctuated seasonally (63~92/L), which was the highest in summer and the lowest in winter; There were many fibers and fragments (44.4% and 39.2% respectively), and most (80.0%) were PA microplastics. In 2023, trihadiningrum et al^[58]The occurrence of 100~5000 μ m microplastics in the leachate of randegan landfill in mojokerto, Indonesia, was reported: the average abundance was 9/L, with the most fiber (64.4%), followed by debris (28.9%) and film (6.7%); The particle size was mainly distributed in the range of 350-1000 μ m (64.4%), followed by 100-350 μ m (31.1%) and 1000-5000 μ m (4.4%). In 2024, chamanee et al^[59]The microplastics (1000-5000 μ m) in the leachate of Sri Lanka open landfill site were detected. The average abundance was 3 ± 2/L. the particle size of more than 1/2 was between 1000-2000 μ M. PE was the most (54.7%), followed by PP (13.3%) and PS (11.6%). There are different amounts of large size (100-5000 μ m) microplastics in landfill leachate from different countries and regions, but the occurrence information of small size (<100 μ m) microplastics is relatively scarce. Studies have found that there are still<100 μ m microplastics residues (2/l) in the leachate after aeration and sedimentation treatment, which means that these treatment processes can not completely remove the small particle size microplastics in the leachate^[58]Therefore, it is very important to determine the content of small particle size microplastics in Landfill Leachate for predicting its environmental pollution risk.

Landfill leachate usually contains high concentrations of new antibiotics and heavy metals, which will change the occurrence characteristics of microplastics. In 2019, Wang et al^[60]A variety of antibiotics (tetracyclines, sulfonamides, fluoroquinolones, macrolides and β - lactams) and metal elements (copper, zinc, chromium, cadmium, lead, manganese, nickel, antimony and cobalt) were detected in leachate samples from Guiyang, Suzhou and Hangzhou landfills, with total concentrations of 4.0~4.5 μ g/L and 9.2~10.8 mg/L, respectively. 2021, he et al^[61]The leachate samples from four landfills in Italy were analyzed, and higher concentrations of sulfadiazine (2.1~22.1 μ g/L) and sulfamethoxazole were detected8 μ g/L), sulfamethazine (3. 9~8. 5 μ g/L), ciprofloxacin (0~434. 7 μ g/L), enrofloxacin (0~9. 1 μ g/L) and erythromycin (8. According to a large number of research results on the adsorption behavior of microplastics to organic substances or heavy metals^[62-63]The microplastics in landfill leachate will inevitably interact with coexisting pollutants to form compound pollution through a variety of mechanisms (such as hydrophobic interaction, hydrogen bonding, π - π interaction, electrostatic interaction and pore filling, etc.). However, it is still unclear what exogenous pollutants are contained in the micro plastics in landfill leachate, as well as the concentration and distribution of various pollutants. Due to the high fluctuation of leachate quality, the adsorption of pollutants by microplastics will be affected by many factors, including the structural properties of microplastics (such as particle size^[64]Aging degree^[65], crystallinity^[66]Surface functional group^[67]Hydrophobicity and electrification of organic pollutants^[68]And environmental factors (pH, temperature, natural organic matter and ionic strength^[69]The interface behavior between microplastics and other pollutants will be affected to varying degrees.

Micro plastics in landfills will spread to the surrounding soil, water and air, thus causing non-point source pollution. The sanitary landfill constructed and operated in accordance with the standard technical specifications should be able to effectively prevent and control the environmental pollution caused by micro plastics in the landfill. However, there are still open-air waste dumps around the world. Due to the lack of anti-seepage measures at the bottom and covering measures at the top, the micro plastics in the yard will inevitably spread to the surrounding environment under the action of wind and precipitation. The storage capacity of some old sanitary landfills tends to be saturated, the amount of waste is too much, and the heap is unstable, so the landfill operation is difficult. Especially after the landfill has exceeded its service life or reached the set storage capacity, if the operation and management are not standardized, it is very likely to lead to the leakage of microplastics and pollutants in the soil of the landfill. Several studies have confirmed the existence of micro plastics in the soil of landfill sites. In 2022, Fernandes et al^[70]The study of a landfill site in San Andrea, Brazil, found that the abundance of microplastics in the 20 cm surface soil of the landfill site was as high as 1790~2393/kg. In 2023, ghorbaninejad Fard Shirazi et al^[71]The study found that the abundance of microplastics in the soil of Kahrizak landfill site in the south of Tehran, Iran was between 225 ± 138 and 863 ± 681/kg, of which 90.0% were PE, PP and PS microplastics, and 42.8% were between 100 and 500 μ m in particle size. In 2024, Guo et al^[72]The detection of micro plastics in the surface soil of a landfill in Hangzhou showed that the abundance of micro plastics increased from 4 to 12/kg with the increase of soil depth, especially the micro plastics with particle size less than 10 μ m (65.6%). The micro plastics in the soil of these landfills may spread to the surrounding soil. In 2023, Kim et al^[15]The study found that there were microplastics in the soils around two landfills in South Korea, with an average abundance of 73 and 98/kg, respectively. The main shapes were debris, fiber and film, and the main components were PP and PE (62.5% and 65.3% respectively). In addition, microplastics entering the soil from garbage will inevitably interact with coexisting pollutants, which may change the distribution and migration behavior of pollutants. Chen et al^[73]The results showed that after adding PE, pan and pet microplastics (0.5 wt%) to the soil, the soil aggregate decreased and its stability decreased, and its adsorption of hydrophobic organic matter phenanthrene increased, but the adsorption of heavy metal Pb decreased, which meant that the microplastics would inhibit phenanthrene and promote the migration of Pb in the soil. Thorsten et al^[74]The distribution coefficient of sorbic acid between PE microplastics and soil was analyzed. It was found that the distribution coefficient of sorbic acid on microplastics was less than that on soil; When the content of PE microplastics in soil reached 10 wt%, the adsorption of sorbic acid in soil decreased. This indicates that microplastics may enhance the migration ability of pollutants by reducing the adsorption capacity of soil. It is worth noting that the microplastics in landfills have aging characteristics, and aging will promote the migration of pollutants by enhancing the migration of microplastics and their adsorption of pollutants. Liu et al^[75]The results showed that PS nano plastics after UV or ozone aging treatment had higher migration in porous media, and the irreversibility of adsorption of non-polar pollutant pyrene and polar pollutant 4-nonylphenol was enhanced, which further promoted the migration of pollutants. Therefore, when the micro plastics in the landfill diffuse to the environment, it will inevitably carry other pollutants to diffuse together, and its environmental risk may also increase.

In the process of waste transfer and landfill, the waste to be landfilled or excavated will be directly exposed to the air, and the micro plastics in it will migrate for a long distance under the action of wind. In 2024, kannankai et al^[13]It is reported that the northeast monsoon brought micro plastics from a landfill in India to the city, which is the main source of micro plastics in the urban air. This provides observation data for the hypothesis of micro plastics' transmission from the landfill to the atmosphere. The micro plastics suspended in the urban air can be inhaled into the human body and accumulated in the lungs and respiratory tract, causing respiratory system related health problems^[76]Microplastics may also damage human immunity (lead to abnormal immune response or reduce immune efficacy)^[77]And may cause harm to the health of human nervous system, digestive system and reproductive system^[78]Long term exposure to microplastics may increase the risk of cardiovascular disease, diabetes and other chronic diseases and cancer^[79]。

In addition to migration between landfill waste and soil, micro plastics in non sanitary landfills/storage yards (especially open-air sites built before 2005) will also pollute groundwater with landfill leachate. In 2021, manikanda et al^[80]Micro plastics such as PP and PS were detected in the groundwater of an open-air waste dump in India, with a abundance of 3-23/L. In 2022, Wan et al^[14]The abundance of microplastics in leachate and groundwater from a non-standard landfill in South China was reported to be 3-25 and 11-17/L, respectively. The pollution levels of micro plastics in the two media are similar, indicating that the micro plastics in the groundwater of the landfill site mainly come from landfill leachate. The particle size of about 85.0% of the micro plastics in the leachate and groundwater is in the range of 20~150 μ m, which is smaller than the particle size of the micro plastics in the landfill waste and its subsoil. This result shows that small particle size microplastics are more likely to migrate to leachate and groundwater, while large particle size microplastics will remain in garbage and soil. In 2023, Priya et al^[81]Microplastics were detected in the groundwater of Kollam landfill in India. The abundance of microplastics ranged from 4 to 27/L (with an average of 12/L). Fibers and flakes accounted for 48.0% and 22.0% respectively. The abundance of microplastics in groundwater of landfills reported in these studies is similar. However, it is not clear how these micro plastics enter the groundwater. They may be contaminated due to leakage or groundwater immersion. In addition, continuous rainfall will also lead to the overflow of micro plastics from landfills to surface water. According to nurhasanah et al^[82]It is estimated that there are 80640 ± 605 micro plastics discharged into the river every day from the open dump in garuga, Indonesia. In addition to the fluid mediated propagation of air, leachate and precipitation, the micro plastics in the landfill will also be "transported" and transferred by soil animals. For example, the feeding and excretion behavior of earthworms and their disturbance to the soil will cause the migration of microplastics in the soil^[83-84]However, Collembola and mites will abrade or chew the soil, thus promoting the migration of soil micro plastics in the horizontal and vertical directions^[85]It should not be ignored that the intake of microplastics by soil animals may have a negative impact on their health. For example, the exposure of microplastics to earthworms can produce growth inhibition, intestinal injury, increased mortality, immune response, reproductive damage and changes in intestinal microbial activity^[86]Microplastics released from landfills may also be toxic to aquatic organisms^[87-88]。

When the micro plastics in the landfill diffuse into the surrounding environment, they will pollute edible plants. When the landfill operation reaches the design elevation or is no longer used due to waste, it is necessary to close the site to prevent the outward migration of pollutants. After the closure, ornamental plants and fruit trees will be planted for ecological restoration. But before that, the way that the micro plastics in the landfill pollute plants through dry and wet deposition has not been blocked. Loppi et al^[89]The study found that the abundance of microplastics in lichens near a landfill site in Italy was the highest (79/g), while the pollution level of microplastics in lichens at 200 and 1500 m away from the landfill site decreased to 13 and 7/g, respectively. In the process of ectopic treatment of sanitary landfills, the excavation and screening of stale waste will inevitably produce dust, resulting in the diffusion of residual micro plastics and polluting the vegetation. The humus soil obtained from the screening of stale waste will be used as organic fertilizer to backfill the soil^[90]This provides the possibility for the transfer of residual microplastics in soil plant system. Although there is no direct evidence that micro plastics from landfills are absorbed by plants, studies have reported that micro plastics can enter plant tissues through roots or leaves. Yin et al^[91]Microplastics were detected in wetland sediments and Phragmites australis in the east of Dongting Lake, with the abundance of 511 ± 295/kg and 0~14/plant, respectively, indicating that Phragmites australis can absorb microplastics in sediments. Li et al^[92]It was found that submicron and micron PS plastics could enter the tissues of wheat and lettuce, especially under the action of transpiration tension, the microplastics could be transported to the edible parts through the catheter. Zhang et al^[93]It was suggested that the gap between root tip and main root lateral root junction was the key position for PS microplastics to enter strawberry roots, but the root absorption pathways of different particle sizes of microplastics were different: 100 and 200 nm plastics were easier to transfer from root tip to petiole, while 2 μ m plastics mainly entered the plant through the gap between main root lateral root junction. After spraying micro plastic on lettuce leaves, Wang et al^[12]It was observed that microplastics accumulated in lettuce leaves, suggesting that stomatal absorption may be the main way for microplastics to enter the leaves. Characterized by scanning electron microscope and X-ray energy spectrum, conti et al^[94]A variety of vegetables and fruits sold in Italy were investigated and analyzed. It was found that both leaves and fruits contained microplastics (1.5~2.5 μ m), but the pollution degree was different, which might be related to the pollution level of microplastics in the planting area and the absorption capacity of different species to microplastics. A large number of studies have shown that exposure to microplastics will cause harm to a variety of plants (reed, onion, rice, lettuce, Welsh onion, broad bean, Oenanthe javanica, cucumber, corn, wheat and ryegrass, etc.), mainly affecting plant growth and photosynthesis, and causing genetic toxicity and oxidative damage^[84]The accumulation of microplastics in seed coat will limit seed germination and root growth, and microplastics additives will affect seed germination by inhibiting amylase activity and inducing oxidative stress^[95]In view of the biological toxicity of microplastics, when people ingest contaminated edible plants such as vegetables and crops, microplastics may accumulate in the intestine and cause intestinal inflammation and other diseases^[96]However, small-size microplastics can be transported across the intestinal epithelial barrier to other tissues and organs, causing ectopic damage^[97]。