Abundance and characteristics of microplastics in soil and leachate at different zones of unsanitary landfill
Article information
Abstract
Landfills are increasingly acknowledged as significant sources of microplastic contamination. Landfills received huge amounts of plastic waste daily, which can degrade into microplastics over time and subsequently accumulate in soil or leach into surrounding environments through leachate. This study investigates the abundance and characteristics of microplastics (MPs) present in soil and leachate across various zones within a landfill., focusing on their size, shape, and polymer composition in young, middle-aged, and old landfill zones. The comprehensive approach involved sample collection, and laboratory analysis. Fourier-transform infrared (FTIR) spectroscopy identified the dominant polymers, and Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) were used to explore factors influencing MP distribution and grouping patterns between soil and leachate samples. The results showed that the abundance of MPs in leachate was significantly lower in middle-aged landfills compared to young and old zones (P < 0.05). Fiber-shaped MPs were most common, with particle sizes ranging from 0.1 mm to 1.6 mm. FTIR spectroscopy identified polyethylene terephthalate (PET), polypropylene (PP), and polystyrene (PS) as dominant polymer types. PCA indicated that landfill aging and environmental degradation influenced MP distribution, with HCA showing distinct patterns between soil and leachate. Smaller MPs were more mobile and found more often in leachate, while larger MPs were retained in soil. This study highlights the critical role landfills play as sources of MP pollution, emphasizing the need for improved waste management to reduce contamination and mitigate ecological and health risks. Effective strategies are essential to addressing the environmental impact of MPs in landfills.
Introduction
The rapid economic expansion and urbanization have contributed to an exponential increase in solid waste generation, particularly in developing countries like Malaysia. With economic growth, business activity, and higher consumption rates, the volume of municipal solid waste is expected to rise [1]. Malaysia's solid waste management practices have evolved, incorporating recycling since the 1980s, yet landfilling remains the primary disposal method. The increasing reliance on plastic materials in various industries has led to a significant accumulation of plastic waste in landfills. Malaysia, as one of the largest importers of plastic waste, faces challenges in managing plastic pollution effectively [2]. Studies indicate that landfill sites serve as long-term reservoirs for microplastics, which can migrate into surrounding soil and water bodies through leaching and surface runoff [3,4].
The composition and abundance of microplastics in landfill environments are influenced by several factors, including landfill age, waste composition, and the presence of moisture. In unsanitary landfills, the lack of engineered barriers allows for uncontrolled leachate flow, increasing the likelihood of microplastic contamination in surrounding ecosystems. Furthermore, microplastics in soil may interact with other contaminants, such as heavy metals or persistent organic pollutants, potentially enhancing their mobility and toxicity. The presence of these particles in the terrestrial environment could pose long-term risks to human health through crop uptake, groundwater contamination, or direct inhalation of resuspended particles.
Despite the growing awareness of plastic pollution, research on microplastic contamination in terrestrial environments, particularly in landfill ecosystems, remains limited. Previous studies have primarily focused on marine and freshwater systems, overlooking the potential impact of microplastics on soil and leachate composition in landfills [5]. In Malaysia, most existing studies have centered on aquatic microplastics, with relatively few investigations into terrestrial contamination. As such, there is a pressing need to characterize the presence and of microplastics within landfill environments, particularly at unsanitary sites that may serve as unregulated sources of environmental contamination.
A detailed understanding of the abundance and characteristics of microplastics across different zones of a landfill can reveal important patterns of distribution, accumulation, and potential migration. Variations in particle size, shape, polymer type, and surface morphology across these zones can provide insights into degradation processes and environmental fate. Therefore, present study aims to address these knowledge gaps by investigating the abundance and characteristics of microplastics in soil and leachate collected from different zones of the unsanitary. By analyzing variations in microplastic concentrations, morphologies, and polymer types across the site, this research contributes to the broader understanding of microplastic behavior in terrestrial environments. The findings can also inform landfill management practices, pollution mitigation strategies, and future policy frameworks targeting plastic waste in Malaysia.
Materials and Methods
Study location and sample collection
The Penderas landfill was selected as the study location due to its long operational history of over 30 years and its categorization as an unsanitary landfill. Covering an area of 7.59 acres, this landfill received approximately 120-150 metric tonnes of municipal solid waste daily. The landfill was divided into three zones: a young landfill zone (< 3 years old), a medium-aged landfill zone (~10 years old), and an old landfill zone (> 20 years old). Sampling was conducted across these zones to assess spatial variation in microplastic distribution. Soil and leachate samples were collected using standardized methods. Soil samples were obtained from a depth of 0-20 cm, with 6-10 kg of refuse collected from each zone. The samples were placed in stainless box and transported to the laboratory, where they were oven-dried at 90°C for 24 hours. For leachate collection, 5 L of leachate was manually collected using a stainless-steel bucket and preserved in glass bottles. All samples were stored at 4°C before further analysis.
Microplastics Extraction from Soil and Leachate
Microplastics were extracted from soil samples using a density separation method, which involved sieving dried soil samples to remove larger particles before subjecting them to high-density zinc chloride (ZnCl2) solution (1.7 g/mL) (6. The soil samples were stirred in the solution for 15 minutes, allowed to settle for two hours, and then vacuum-filtered using cellulose nitrate membrane filters. The process was repeated three times to ensure thorough extraction, and the filters were subsequently rinsed with distilled water, dried at 60°C overnight, and analyzed under a stereo microscope for microplastic identification [7]. Chemical digestion was conducted using 35% hydrogen peroxide (H2O2) to remove organic matter before final filtration and drying. This standardized approach facilitated the quantification of microplastics based on size, color, and morphology [5].
For leachate samples, 3 liters of leachate were filtered through a 48 μm sieve, and the retained material was washed with distilled water before being transferred onto nylon membrane filters with a 20 μm pore size [8]. The filtered material underwent chemical digestion using 30% hydrogen peroxide for 72 hours to break down organic components. The solution was then re- filtered, rinsed with distilled water, and air-dried before microscopic examination. The extracted microplastics were further analyzed using Fourier Transform Infrared Spectroscopy (FTIR) to determine polymer composition [9]. To ensure quality control, procedural blanks were used to check for contamination, and recovery tests with known microplastic standards were conducted, yielding a detection efficiency of approximately 90% [10]. These methods ensured reliable and reproducible extraction and characterization of microplastics from both soil and leachate samples in the landfill environment.
Characterization and quantification of microplastics
In this research, microplastic characteristics in samples were observed in three categories: morphotype, size and color. Microplastic characteristics were categorized based on size, shape, color, and polymer composition, providing insights into their potential sources and environmental behavior. Size classification was divided into two major groups: 0.45 μm–25 μm and 25 μm–5 mm, with fibers, fragments, beads, foams, and films being the predominant shapes identified [11]. Color analysis revealed a range of hues, including red, blue, green, black, and white, with variations potentially indicating different degradation states or polymer origins [5]. Polymer composition analysis using FTIR confirmed that polyethylene (PE), polypropylene (PP), and polystyrene (PS) were the most common types found, reflecting their widespread use in consumer and industrial applications [9]. These characteristics influence microplastic interactions with contaminants and biological systems, highlighting their persistence and potential ecological risks.
FTIR analysis
Fourier Transform Infrared Spectroscopy (FTIR) was used to identify and classify the polymer composition of microplastics in landfill soil and leachate. The obtained spectra were compared with reference polymer libraries to determine the presence of common plastics such as polyethylene (PE), polypropylene (PP), and polystyrene (PS). The analysis confirmed that the majority of microplastics were secondary microplastics, formed through the degradation of larger plastic waste. This characterization is essential for identifying pollution sources and assessing the long-term environmental impact of plastic decomposition in landfill environments.
Data analysis
Statistical analysis was performed using SPSS software to evaluate differences in microplastic abundance across landfill zones. One-way ANOVA was conducted to determine whether there were statistically significant differences in microplastic counts between young, middle-aged, and old landfill zones. The results indicated a significant variation (P < 0.05), with younger landfill zones exhibiting higher microplastic concentrations in leachate and older zones showing more microplastics in soil. Additionally, Principal Component Analysis (PCA) was used to identify key variables influencing microplastic distribution, while Hierarchical Cluster Analysis (HCA) grouped landfill zones based on similarities in microplastic characteristics. These statistical analyses provide critical insights into microplastic behavior, supporting the need for enhanced waste management practices to mitigate contamination.
Results
Abundance of microplastics in soil and leachate landfill
MP presence was observed in both soil and leachate samples of landfill zones. The abundance of MPs in landfill soil varied from about 11 to 33 items/g, while for landfill leachates, this abundance varied from about 5 to 33 items/L. Young landfill soil expressed a relatively stable abundance of MPs from 18 to 22 items/g with an average value of 20 items/g in the soil samples. The highest abundance for the medium-aged landfill soil ranged from 18-33 items/g, with an average of 25 items/g. The old landfill soil has the lowest, ranging from 11 to 24 items/g, with an average value of 18 items/g. Less MP was detected in the soil obtained from the old zone than in the young and middle landfill zones. The abundance of MPs in young landfill leachates ranged from 12 to 35 items/L, with an average of 22 items/L. In the middle-aged landfill leachates, it was the lowest and ranged from 5 to 10 items/L, with an average of 10 items/L. For the old landfill leachates, the abundance of MPs ranged from 12 to 17 items/L, with an average of 15 items/L. Among these, MPs' abundance was 21.67 ± 8.65 items/L from young landfill leachate, 10.67 ± 3.28 items/L from medium landfill leachate, and 14.67 ± 2.06 items/L from old landfill leachate.
Morphotype of microplastic in shellfish shapes o microplastics
Stereomicrographs of randomly selected microplastics (MPs) from leachate and refuse are presented in Figure 1. The stereomicrographs reveal that the MPs generally exhibit irregular shapes and a rough, hackly structure. Films are typically generated from plastic bags and packaging materials. Due to their thin and transparent nature, plastic bags are highly susceptible to breaking when exposed to sunlight. Conversely, pellets primarily originate from plastic containers, water bottles, microbeads, or food storage containers [12]. Additionally, the shape of microplastics can provide insights into their source, distinguishing whether they are primary (originating directly from manufacturing) or secondary (resulting from the breakdown of larger plastic items) in nature [13].
The different morphology, colours, and sizes of microplastics (A–F) depict fibers and films identified in the leachate samples, while (G–L) shows pellets and fragments from the soil.
Figure 2A shows the distribution of microplastics (MPs) by shape in environmental samples. Fibres and fragments are the most dominant shapes, each accounting for 37% of the total MPs. Films comprise 23% of the distribution, while pellets constitute only 3%. A recent study shows that fibres accounted for the majority of microplastics detected in the Hamadan landfill, comprising 71% of the total. Fragments represented 16%, films made up 12%, and pellets were the least common form, constituting only 1%. In another study by [8] a diverse range of microplastic shapes was identified in refuse samples, including fragments, granules, fibres, films, and rods. Among these, fragments were the most dominant, representing 59.82% of all microplastics detected across the refuse samples. Irregular fragments were also identified as a primary microplastic morphology in sediment samples [7]. Furthermore, film-type microplastics accounted for 7.86% of the total in refuse samples, with household plastic packaging likely being the primary source of their presence [5].
Total distribution patterns of MPs with different morphology in soil (A) and leachate (B) samples.
The analysis of the leachate sample's microplastic morphology reveals distinct distribution patterns based on Figure 2B. Fibers are the most dominant type, accounting for 68% of the identified microplastics. This indicates that fibre-shaped microplastics primarily contribute to leachate contamination, likely from synthetic textiles, clothing, or industrial sources [14]. Films are the second most common, comprising 27% of the microplastics, which may be attributed to thin plastic sheets and packaging waste. Conversely, fragments are the least prevalent, representing only 5% of the microplastics detected. This distribution aligns with previous findings by [5], that MPs in leachate are predominantly in the form of fibres, followed by fragments and films, reported to be 22.87%, 14.81%, and 3.06%, respectively. It also noted that leachate zones have the highest abundance of microplastic with different shapes compared to other zones.
Colors of microplastics
The present study categorized microplastics based on colour into black, white, red, brown and other colours. As shown in Figure 3(A) and supported by the pie chart data, the most abundant microplastics in all soil samples from the landfill site are black and red, with black microplastics comprising 32% and red microplastics accounting for 29%. White microplastics followed, representing 14%, brown microplastics comprised 16%, and other colour microplastics accounted for 9%. These findings suggest that black and red are the most dominant microplastic colours, followed by different colours in much smaller proportions. The pie chart in Figure 3(B) illustrates the abundance of colours in the leachate, where white dominates at 50%, black at 31%, and red at 19%. This suggests that white is the most prevalent colour among leachate particles, potentially indicating a higher presence of materials such as microplastics or other light-coloured substances commonly found in waste decomposition processes.
The Percentage of MPS Colors for soil sample (A) and leachate sample (B).
Sizes of microplastics
In soil samples, MPs ranged from 0.23 to 4.97 mm, with an average size of 1.03 mm, more significant than the average size in leachate (0.83 mm). The average sizes of MPs in soil were 1.23 mm, 0.91 mm, and 0.71 mm for young, medium, and old landfills, respectively. It's showing a significant decrease in size with landfill age (P < 0.05). The abundance of MPs also declines as landfill age increases, with young landfills having the highest abundance and old landfills the lowest. The graphs reveal a shift in size distribution, where more minor and intermediate-sized MPs (Classes A and B) decrease with landfill age, while more significant MPs (Class C) become more dominant in refuse. These patterns suggest that smaller particles are more likely to migrate into leachate, leaving larger particles retained in refuse as landfills age.
In leachate samples, the abundance of MPs decreases as landfill age increases, with young landfills showing the highest abundance. MPs in leachate ranged in size from 0.07 to 3.67 mm, with average sizes of 0.89 mm, 0.85 mm, and 0.77 mm for young, medium, and old landfills, respectively, indicating no significant difference in size with landfill age (P > 0.05). For the further analysis of MP size characteristics with varied landfill age, based on a study from (12), the size fraction was conducted, and MPs were categorized into three classes: class A, < 0.5 mm; class B, 0.5-1 mm; and class C < 1mm as shown in Figure 4.
Abundance (mean ± standard deviation) of MPs in different size classes (class A: < 0.5 mm; class B: 0.5 -1 mm; class C: 1 - 5 mm) of soil and leachate samples.
Chemical composition of microplastics
Analysis using Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy is to validate the microplastic samples by confirming the acquired data through transmittance and wavenumber values. The identified polymers are divided into two types of synthetic: Polystyrene(PS), Polyethylene (PE), Polytetrafluoroethylene (PTFE), Polychlorotrifluoroethylene (PCTFE), Polyvinyl Chloride (PVC), and natural polymer; Cellulose acetate (CA) and Polyacrylate (PA). The presence of diverse polymers indicates widespread plastic contamination and highlights the need for improved waste management and leachate treatment to reduce environmental and human health risks from microplastic pollution in unsanitary landfill environments.
Microplastics occurrence patterns across landfill age
Figure 5 illustrates the principal component analysis (PCA) results, highlighting the relationships and variations among microplastic (MP) characteristics, such as size, shape, and colour, found in soil and leachate samples across different landfill age categories: young, middle, and old. PC1 and PC2, which explain 38.68% and 24.61% of the variance, respectively, show distinct clustering patterns. Microplastics in young and middle-aged landfills are associated more with specific properties like size and shape. In contrast, older landfills display broader dispersions, suggesting a diversification or degradation of MP characteristics over time.
Principal Component Analysis (PCA) Biplot of Microplastic Characteristics in Soil and Leachate Across Landfill Zones.
The dendrogram in Figure 6 visualizes the similarity between different variables related to microplastics (MP) in soil and leachate samples. The hierarchical clustering groups these variables according to their degree of similarity, measured along the y-axis. The closer the branches join on the x-axis, the more similar those variables are. The diagram shows that "MP Soil" and "Size MP Soil" show the highest similarity, forming the first cluster. Similarly, "Colour MP Soil" clusters closely with this group, indicating a strong relationship between the characteristics of soil microplastics. On the other hand, "Shape MP Leachate," "MP Leachate," "Colour MP Leachate," and "Size MP Leachate" form a distinct cluster, suggesting that the characteristics of microplastics in leachate samples differ pretty significantly from those in soil.
Hierarchical Cluster Analysis (HCA) Dendrogram of Microplastic Characteristics in Soil and Leachate.
Discussion
The abundance of microplastics (MPs) varied across different landfill zones, with higher concentrations observed in soil from older landfills and in leachate from younger landfills. The higher MP abundance in older soil samples suggests that plastic waste undergoes fragmentation over time, leading to an accumulation of secondary microplastics. This trend is consistent with previous studies indicating that landfill aging significantly influences MP distribution and transport [15]. Microplastic shape analysis revealed that fibers were the most dominant form across all landfill zones, followed by fragments and pellets. The prevalence of fiber-shaped MPs suggests potential sources such as synthetic textiles, fishing lines, and industrial plastic waste. Fibers are known to be more resistant to degradation and can be easily transported through leachate movement, contributing to their widespread presence. Fragments, which result from the breakdown of larger plastic items, were more abundant in older landfill zones, indicating ongoing plastic degradation processes [16]. Regarding the types of plastic shapes found in soil, secondary plastics originating from the fragmentation of larger plastic items were more prevalent than primary microplastics, which are produced during the initial manufacturing process. A study [17] reported that microplastics can affect soil aggregation and the ability of plant roots to absorb microplastics. Similarly, [18] confirmed that microplastics play a key role in regulating soil aggregation and the decomposition of organic matter, with the shape of microplastics being a critical factor.
Color analysis of MPs indicated a diverse range of colors, with black, blue, and transparent particles being the most common. The presence of black and blue MPs suggests sources such as plastic packaging, vehicle tire wear, and industrial waste. Transparent MPs, often derived from plastic bags and food packaging, were more prevalent in younger landfill leachate samples, suggesting recent waste disposal. The variation in MP colors highlights their diverse origins and potential for environmental contamination [19]. A previous study by [20] also confirmed that regarding the colour of microplastics (MPs), black was found to have the highest proportions, accounting for 29.4% and 28.2% in two related landfills. Black plastics are used daily for various purposes, including tyres, wires, textiles, vinyl, masks, and packaging containers. While black MPs may have multiple potential sources, tyre wear is likely a significant contributor to their presence in these two landfills, mainly because both are near roadways.
Microplastic size distribution showed a higher proportion of smaller MPs (0.1–1 mm) in older landfill soil, whereas larger MPs (1–5 mm) were more abundant in younger landfill leachate. The dominance of smaller MPs in older zones suggests that prolonged environmental exposure leads to fragmentation into finer particles. Larger MPs in younger landfill leachate indicate that plastics from newly deposited waste have not yet undergone significant degradation. This finding aligns with studies showing that plastic size is an essential factor influencing microplastic mobility and environmental impact [21,22].
Polymer composition analysis using FTIR confirmed the presence of polyethylene (PE), polypropylene (PP), and polystyrene (PS) as the dominant microplastic types. PE and PP, commonly used in plastic bags and food packaging, were detected in both soil and leachate samples. PS, often found in disposable containers and insulation materials, was more abundant in older landfill soil, suggesting long-term plastic persistence. The polymer composition of microplastics has much to do with modern applications of polymeric materials in human activities. Due to their low cost and unique properties, large-scale use in applications such as shopping bags (PE), water bottles (PET), and disposable drinking cups (PS) occurs in everyday life [23]. Due to its high flexibility, PVC is extensively used in construction, waterproofing, medical devices, clothes, toys, and sporting goods. The identification of these polymers reinforces the need for stricter regulations on plastic waste disposal to mitigate their environmental effects [24,25].
The results of Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) further supported the spatial distribution of microplastics in landfill zones. PCA revealed distinct clustering of MPs based on landfill age, with older zones showing higher associations with fragmented microplastics, while younger zones were linked to larger, newly deposited MPs. HCA grouped microplastic samples according to their similarities in shape, size, and polymer type, reinforcing the findings on microplastic degradation and transport patterns. These analyses demonstrate the importance of statistical approaches in understanding microplastic behavior and guiding effective waste management strategies [26]. Similarly, [10] found that the primary contributors to microplastic contamination in landfill leachate are solid waste, which contains significant amounts of plastic debris and residues from wastewater treatment processes. Using the dataset of PCA, negative coefficients were obtained for microplastic in soil from middle and old landfills. Over time, soil microplastics can be degraded due to physical, chemical and biological processes.
Conclusions
This study highlights the significant variations in microplastics (MP) distribution across different landfill zones, with younger zones exhibiting higher concentrations in leachate due to recent plastic waste deposition, while older zones show greater accumulation in soil due to prolonged degradation. The findings indicate that landfill aging, waste composition, and environmental factors play a crucial role in MP behavior. The presence of small-sized secondary MPs in older zones suggests ongoing fragmentation of plastic waste, emphasizing the long-term environmental risks associated with MP contamination. These results underscore the need for a deeper understanding of MP dynamics within landfills to mitigate their impact on ecosystems and human health.
To address MP pollution in landfills, it is recommended that landfill management strategies incorporate advanced leachate treatment technologies to prevent MP leakage into surrounding water systems. Additionally, stricter regulations on plastic waste disposal and recycling should be enforced to reduce the volume of plastics entering landfills. Public awareness campaigns and research on alternative biodegradable materials can further contribute to minimizing MP contamination and promoting sustainable waste management practices.
Notes
Acknowledgement
The authors would like to express our sincere gratitude to the municipal council involved in this study.
Conflict of interest
All authors declare no conflict of interest.
CRediT author statement
SRMY: Conceptualization, Methodology, Writing- Original draft; NSH: Investigation, Data collection; NASF: Visualization, Data curation; ARI: Data collection, Data curation, Resources; NWR: Writing- Editing; RA: Writing.