Abundance and Characterization of Microplastics in Compost Produced in Mazandaran, Iran

doi:10.21203/rs.3.rs-4928162/v1

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Abundance and Characterization of Microplastics in Compost Produced in Mazandaran, Iran

https://doi.org/10.21203/rs.3.rs-4928162/v1

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One of the source of plastics and microplastics (MPs) entering the environment and the food chain is waste from various sources. Most plastic resins are utilized in packaging with relatively short lifespans; after consumption, these materials contribute to municipal solid waste. The composting process struggles to completely separate plastics, making microplastics a significant contaminant during composting. MPs are defined as plastic particles smaller than 5 mm. This study aims to examine the frequency, distribution, and characteristics of microplastics found in compost produced by Mazandaran compost factories.

MPs in the samples were extracted and separated based on density differences and digestion. Subsequently, a stereomicroscope was used to observe the MPs.

This cross-sectional study found an average microplastic content of 16,981 items/kg across all compost samples. Behshahr compost had the highest average at 1,818,622, while Babol had the lowest at 15,744 items/kg. The Kruskal-Wallis test indicated no statistically significant differences between the sampling stations (p > 0.05). The most common shapes were fibers (75%), with the predominant color being transparent-white (56%), and the most frequently observed size range was 500–1000 µm (40%). Additionally, polymers identified included polyurethane, nylon, low-density polypropylene, and polycarbonate.

This study reveals the presence of microplastics in compost from Mazandaran's fertilizer and compost factories. To address this issue, it is essential to develop strategies for reducing plastic waste entering composting facilities and to establish effective monitoring programs to detect and identify microplastic hotspots.

Compost

microplastics

abundance of microplastics

Mazandaran province

The composition of urban solid waste typically comprises organic materials. In Iran, an analysis of urban gardens has revealed that over 70% of the products are food crops with high perishability (1). Composting is a dynamic, biological, and aerobic process whereby organic materials go through a thermophilic phase before being stabilized through biodegradation with the help of microorganisms. The effectiveness of composting is directly related to the activity of the microorganisms involved in the process (2, 3). In recent years, a new term called "emerging pollutants" has been introduced in scientific and international forums. This group of pollutants includes both natural and artificial substances that are typically not detected in the environment, and their adverse effects on human health and ecosystems are not well understood (4).

Plastics and their small particles are one example of emerging pollutants. Plastics have become ubiquitous in modern society due to its numerous benefits to human health and the environment. For instance, plastic packaging helps to prevent food waste and contamination, ultimately saving resources. Additionally, plastics possess desirable qualities, such as light weight, transparency, and good mechanical properties, which have led to their widespread use in various industries. However, the accumulation of plastic waste and microplastics in the environment has become a growing concern because of their potential negative impact on ecosystems and human health (5, 6). Recent estimates suggest that 80–95% of marine litter comprises plastic, highlighting the significant impact of plastic waste on our oceans. In comparison, plastic products in municipal solid waste typically make up about 8–12% of the total waste stream (7).

Microplastics are generally defined as plastic particles with dimensions less than 5 mm. The term "microplastics" was first proposed by Thompson in 2004 to describe small plastic particles found in oceans and other aquatic environments (8, 9).

Since then, microplastics have become a growing concern due to their widespread presence in the environment and their potential negative impacts on ecosystems and human health (10, 11). Plastic waste that ends up in landfills is exposed to a range of environmental conditions, including pH changes (ranging from 4.5 to 9), high salinity (approximately 3–41 mS/cm), temperature fluctuations (approximately 30–60°C), and microbial decomposition. These conditions can result in the erosion and fragmentation of larger plastic items, leading to the formation of smaller particles such as microplastics, and nanoplastics. This transformation can further exacerbate the problem of plastic pollution, as these smaller particles are more easily dispersed in the environment and can potentially be ingested by a wider range of organisms (12, 13).

Microplastics are not standalone pollutants, but can serve as carriers of other harmful adsorbed contaminants, including heavy metals, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and perfluoroalkyl substances (PFAS). This is because of the hydrophobic and electrostatic interactions that occur between microplastics and these contaminants. As a result, microplastics can play a significant role in the transport and bioaccumulation of these harmful substances in the environment, potentially posing a risk to human health and ecosystems (14). Compost is often used as a fertilizer to improve soil structure and quality. However, if the compost is contaminated by microplastics, it can transfer these pollutants to the soil and plants, potentially exacerbating environmental pollution and posing a risk to human health and ecosystems (15). The mechanisms and extent to which microplastics enter soil systems are not yet fully understood, but it is known that they can enter the soil through industrially processed compost that is used on agricultural land. This can potentially lead to the accumulation of microplastics in the soil and their subsequent uptake by plants, with potential implications for both human health and the environment (16). Research on the concentration of microplastics in compost is limited, both in Iran and globally. Therefore, the purpose of this study was to investigate the presence, frequency, types, and temporal changes in microplastics in compost produced by several compost factories in Mazandaran between 2019 and 2021. The findings of this research can contribute to a better understanding of the extent of microplastic contamination in compost and help identify potential strategies for reducing the risk of microplastic pollution in the environment.

Sampling of Compost

This analytical cross-sectional study was conducted to investigate the presence of microplastics in compost. Three active compost factories located in Tankabon, Behshahr, and Babol in Mazandaran province in north of Iran were included in this study. Sampling was conducted once every three months from each factory, with three repetitions, resulting in a total of 27 samples. The compost was sampled using compost analysis and composting methods that employ a composite sampling approach to collect representative samples for analysis. These methods are commonly used in compost analysis to ensure the accuracy and reliability of results (17).

The samples used in this experiment were collected randomly from the final compost produced by the Babol, Behshahr, and Tonkabon Compost factories. The sampling method involved collecting several bags from the center of the compost. The best sampling locations were identified as the conveyor belt, compost piles, and stored piles at the end of the production process. The samples were then placed in a resistant nonplastic bag covered with aluminum foil and transported to the laboratory at the Faculty of Health at Mazandaran University of Medical Sciences for analysis. This sampling approach ensures that the samples are representative of the compost produced by each factory and helps minimize potential sources of bias.

Extraction of microplastics

In the laboratory, microplastics were isolated from the samples using an adapted protocol. The most common method for extracting microplastics from a solid sample is separation through density difference followed by filtration (18, 19).

The method used in this study for extracting microplastics from compost samples takes advantage of the difference in density between polymers and solid samples. Wet compost samples, stored in aluminum bags, were first transferred to a clean aluminum plate and dried in an oven at a temperature ranging between 40–60 ^oC for 48–72 hours until they reached a constant weight. In order to avoid any potential contamination of microplastics from the air, the samples were protected with aluminum paper while they were being dried. After drying, the samples were sieved through a 5 mm sieve to remove any large particles, including stones, wood, sand, and large plastic debris. Then, 250 grams of the sieved compost was transferred to a 1000 ml beaker, and 200 ml of hydrogen peroxide solution (30%, V/V) was added to remove the organic matter in the samples. The remaining material were dried in an oven in 40–60°C. Next, the rest of the sample was transferred into another 1000 ml beaker, and a zinc chloride solution with a density of 1.6 g/cm3 was added to it. The sample was stirred using a glass rod for 5 min and then left aside for 24h. Afterward, the supernatant solution on the sample was transferred to a 50 ml Falcon tube and centrifuged at 3000 rpm for 5 min. The supernatant solution in the Falcon tube was filtered using a vacuum pump and filter paper made of cellulose acetate with a diameter of 120 mm and a pore size of 2 µm (S & S grade 3.589 filters). This process allowed for the isolation of microplastics from the compost samples, which were subsequently examined under a microscope for further analysis.

Identification of microplastics

Identification of microplastics was done using tweezers, needles and color recognition (20, 21). Based on their morphology, the identified microplastics were classified into four types, including fiber, fragment, film, and pellet, and their color was also divided into four color groups based on their surface color including black-gray, blue-green, pink-red and yellow-orange (22, 23).

In addition, in terms of size, they were divided into groups smaller than 250 µm, between 250 and 500 µm, between 500 and 1000 µm, and between 1000 and 5000 µm (24–27).

The abundance of microplastics was determined based on the number of MPs per kilogram (items·kg^− 1) of dry compost, and Raman spectroscopic microscopy (µ-Raman-532-Ci, Avantes, Apeldoorn, Netherland) was used to determine the type of polymers that make up MPs. The surface composition and appearance of several isolated microplastic particles were determined using an electron microscope equipped with an EDAX device (SEM-EDAX, TESCAN Vega 3, Czech Republic) (28).

This research utilized median, mean, minimum, maximum, and percentiles to analyze the variables. The Shapiro-Wilk test was employed to assess the normality of the variable's distribution. The relationship between different variables was examined using the Kruskal-Wallis H test.

Quality Controls

To minimize contamination, all tools used in the sampling and extraction processes, such as glassware, stainless-steel containers, and sieves, were thoroughly washed with distilled (deionized) water.

In this method, stainless-steel sieves were used, and glass beakers and other containers were covered with aluminum foil during laboratory work. Before use, all liquid reagents were filtered with cellulose acetate filter, 120 mm diameter and 2-micron hole diameter (S & S grade 3.589 filters), and the use of plastic instruments was avoided as much as possible. Nitrile gloves and lab coat were worn at all times during the sampling and laboratory processes. To detect the possible introduction of microplastic contamination due to the dry fall of laboratory air, a control sample including a 500 ml glass beaker containing filtered distilled water, was placed in the laboratory. In addition, to detect the introduction of microplastics through the testing processes or laboratory materials used, all stages of the test were performed on a control sample without any solid samples (12).

Microplastics (MPs) abundance

The existence of contaminants in composts poses considerable challenges, especially in the case of their use as fertilizers and soil amendments. As a result, composts act as a major pathway for the uncontrolled entry of MPs (defined as plastic particles ranging from 1 to 5000 µm) into agricultural environments, which may lead to the contamination of food products from crops and the infiltration of these materials into groundwater via leachate. Additionally, the problem of larger plastic debris is unlikely to be addressed in the near future, raising concerns about the potential accumulation of such materials in soils, where they can remain for centuries due to the inherent resilience of most plastics (29, 30).

Plastics come to be in organic compost regarding inappropriate waste management and disposal (31). MPs in domestic composts originate from various sources, such as synthetic fibers found in textiles, the production of polymers, industrial processing, and personal care items (32).

In this study, based on the counts performed under a light microscope, 1851 MP particles were observed in 10g of dry compost samples. The compost sample from Behshahr City in the spring season (BH2) had the highest number of MPS, (249 particles/kg), while the compost sample from the Babol City (B1) had the lowest number in the winter (139 particles/kg). According to Table 1, the average concentration of microplastics in all the studied samples was 16,981 particles/kg, which the highest average concentration observed in the compost samples from Behshahr,18,622 and the cities of Tonekabon ,16577 and Babol 15744 particles/kg following closely behind. The results indicate that the concentration of MPs in different seasons varied across stations and differed from that in other seasons, so that during spring, the concentration of MPs was highest, while it was lowest in winter and summer. The average concentrations of microplastics in all samples during spring (May), summer (July), and winter (March) was found to be 24433, 21133, and 5377 particles/kg respectively. As shown in Fig. 1, the concentration of MPs in spring (May) was higher than in summer (July) and winter (March).

Based on the Kruskal-Wallis test, the amount of total MPs was significantly different in three sampling times, so that the amount of MPs in March was significantly lower than other months (p < 0.05).

Scopetani et al. (2022) documented a MP mean concentration of 6.6 ± 1.5 particles/kg in their analysis of compost samples (33). Similarly, Weithmann et al. (2018) examined the prevalence of MPs in organic fertilizers that are released into the environment. Their findings indicated that the MP quantities varied between 14 and 895 particles per kilogram (34).

Table 1

Statistical summary of microplastics frequency in sampling locations (particles/kg)
Location	Mean ± S.D.	Maximum	Minimum
Babol Behshahr Tonekabon Total	15744 ± 9917 18622 ± 10528 16577 ± 10270 16981 ± 8961	23700 24900 24700 24900	4633 6467 5033 4633

In the current study, the station in Behshahr was found to be the most polluted, with 18,622 particles per kilogram of dry weight. According to Gui et al.'s study in 2021, the average frequency of microplastics found in compost samples was 2400 ± 358 per kilogram of dry weight. They also reported that microplastics were released into the compost due to the fragmentation of the macroplastics' surface during the composting process (35). Chen et al. (2020) conducted a study on the biological degradation of microplastics in sewage sludge using hyperthermophilic compost technology. This study found that the amount of microplastics was around 7.4 x 10⁴ particles per kilogram of sewage sludge dry weight (36). According to a study by Schothorst et al. (2021), the number of MPs found in samples of municipal organic waste compost and organic material compost from green pruning (gardens and greenhouses) was 2800 ± 616 and 1253 ± 561 particles per kilogram of dry weight, respectively (37).

Furthermore, Iswahyudi et al. (2024) reported that MPs in commercial compost samples reached up to 160 particles per 200 grams, exhibiting a variety of colors (blue, black, red, yellow, and white) (38).

Surendran et al. (2024) conducted a study that in the compost from Kozhikode, the average concentration of MPs was recorded at 840 ± 30 items/kg, while Kochi exhibited a higher concentration of 1600 ± 111 items/kg, predominantly consisting of polyethylene (PE) films. Notably, PE was the most abundant resin, accounting for 58.3% in Kozhikode and 73.37% in Kochi (39).

Khan et al. (2023) conducted a study on MPs within the agro-ecosystem of Hainan Island, China, exploring their associations with plastic mulching, agricultural practices, and various social and environmental factors. Their findings indicated that the concentration of MPs in the region varied from 2800 to 82,500 particles per kilogram, with an average concentration of 15,461.52 particles per kilogram (40).

In their study, Zhang et al. (2023), examined the presence and characteristics of MPs (MPs) in 124 samples of organic compost, which included both single feedstock types-such as livestock manure, poultry manure, crop straw, and solid waste-and compound organic composts. The analysis revealed that solid waste compost exhibited the highest abundance of MPs at 6615 items per kilogram, while crop straw compost had the lowest at 1500 items per kilogram (41).

Massahi et al. (2024) undertook research aimed at collecting compost samples from the primary compost production site in Kermanshah city to assess the presence of MPs in compost derived from municipal solid waste. The results indicated that all compost samples were contaminated with MPs, exhibiting an abundance range of 1000 to 4300 MPs/kg, with an average of 2160 ± 968 MPs/kg (42).

The results of these investigations align with the current study regarding the quantity of MPs found in compost samples.

The shape of MPs

It is important to emphasize that the identification of specific MP types within compost samples is vital, as it can yield significant insights into their potential origins and the pathways they traverse during the composting process. Such insights can inform the development of strategies aimed at mitigating their occurrence in compost and reducing their potential adverse effects on both the environment and human health (23).

In this study, different shapes of MPs were found in compost samples (Fig. 2). According to this figure, MPs have been found in different shapes of plates or films, fibers and fragments in the samples.

Approximately 75% of the MPs observed in the samples were fiber, while 24% were pellet, and only 1% were fragment (Fig. 3). The predominant form of MPs in the compost samples from Behshahr, Babol, and Tonekabon were stringy. It is important to note that the highest filamentous MPs were observed in the Behshahr (BH2) compost sample during spring, whereas the lowest was observed in the Babol (B1) compost sample during winter. The highest pellet MPs were observed in the Behshahr (BH2) compost sample during spring, whereas the lowest was observed in the Tonekabon (T1) compost sample during winter. Finally, the highest fragmented MPs were observed in the Behshahr (BH2) compost sample during spring, whereas the lowest was observed in the Tonekabon (T2) compost sample during spring and the Babol (B1) compost sample during winter. Depending on the compost factory, season, and shape, the types of MPs found in the compost samples varied.

The number of MPs with plate shape was significantly different in different months, so that it was the lowest in March and the highest in May (p < 0.05).

In the current study, MPs were categorized as fibrous, fragment, and plate with fibrous MPs being the most prevalent. No spherical MPs were found. These results indicate that secondary MPs are the main source of microplastic fragments. Urban waste may be the cause of this issue due to the presence of textiles and plastic fibers. Gui et al. (2021) found out that fiber and film are the most common forms of MPs in compost products (35). Chen et al. (2020) and Massahi et al. (2024) found that the most common type of MPs is fiber, which is consistent with our study (36), (42).

According to Zhang et al. (2023) research, the shapes of MPs in compost samples were categorized as fibers (29.3–42.9%), fragments (26.6–37.9%), films (15.0–8.3%), and pellets (3.9–8.6%) (43).

In a study by Weithmann et al. (2018) and Khan et al. (2023) MPs were identified as dominant in the form of fragments (34), (40).

Iswahyudi et al. (2024) reported that MPs in commercial compost samples exhibiting a variety of colors (blue, black, red, yellow, and white). The identified MPs varied in size (0.1–1 mm) and shape, with 81.8% classified as fragments, 16.2% as fibers, and 2% as filaments (38).

The findings of Le et al. (2023) indicate that the concentration of MPs (MPs) in organic compost can reach thousands of items per kilogram. Among the various types of micropollutants, fibers, fragments, and films are the most prevalent, with smaller MPs exhibiting a greater capacity to absorb additional pollutants, thereby posing significant risks to living organisms (31).

Color and Size of MPs

According to Fig. 4 (a), white-transparent MPs accounted for 56% of all MPs found in the compost samples. Green-blue, red-pink, black-gray, and yellow-orange MPs accounted for approximately 23%, 14%, 4%, and 3% of the total MPs, respectively. According to the Kruskal Wallis test, the number of MPs with blue/green color was significantly different in different months, so that it was the lowest in March and the highest in May (p < 0.05).

Additionally, white-transparent was the predominant color of MPs found in compost samples from Behshahr, Babol, and Tunkabon cities (Fig. 4 (b)).

Probably the predominance of white and transparent waste such as disposable containers in municipal waste delivered to composting units can be the reason for this. In the study by Chen et al. (2020), they reported the dominant color of MPs as white. which is consistent with the present study (36). On the other hand, Khan et al. (2023) reported the black was the most frequently observed color in the compost samples (40).

According to Massahi et al. (2024) study, the color distribution of MPs revealed that transparent-white particles comprised 56%, followed by golden-brown and blue particles, each at 13% (42).

In Fig. 4 (b), MPs with a size range of 500 to 1000 µm had the highest frequency, accounting for 40% of the sample. On the other hand, particles with a size range of 1000 to 5000 µm had the lowest frequency, accounting for only 19% of the samples. Additionally, 21% of the microplastic particles were less than 250 µm, and 20% of the particles were between 200 and 250 µm in size. These findings suggest that the majority of MPs in the compost samples fell within the size range of 500 to 1000 µm.

The Kruskal-Wallis test found no statistically significant difference in the frequency of microplastic size among the investigated cities.

According to this study, MPs below 1 mm in size were the most prevalent MPs size. If composts manufactured in the Mazandaran compost factories are used, there is a risk of MPs being transported into the environment, particularly into surface and underground water. Additionally, smaller MPs have a greater potential for absorbing pollutants from the environment compared to larger MPs. Gui et al. (2021), reported the size of MPs in compost samples to be 50 to 5000 µm (35). Weithmann et al. (2018) also reported that MPs are predominantly smaller than 1000 micrometers. These findings are consistent with the results of the present study.

In the study conducted by Zhang et al. (2023), the predominant size of compost MPs was found to be between 0.5 and 1 mm, constituting 39.5% of the total (41).

In Khan et al. (2023), the most abundant size range of MPs was identified as being between 20 and 200 µm, which accounted for 57.57% of the total (40).

Micro Raman microscopic studies

Micro Raman microscopy is a highly effective technique that can help identify a wide range of MPs. In this study, almost all of the particles chosen for micro-Raman analysis were found to be made up of the polymers typically used in plastic manufacturing. Upon analyzing the Raman spectrum of some MPs in the compost samples, the presence of polyurethane, nylon, low-density polypropylene, and polycarbonate was discovered (Fig. 5).

The findings of Micro Raman test are consistent with previous studies that have identified these types of polymers as the main components of MPs found in various environmental samples including composts (31, 41, 44, 45).

The study by Gui et al. (2021) found that polyester, polypropylene, and polyethylene polymers accounted for 70–80% of MPs in compost products (35). In a study conducted by Chen et al. (2020), the researchers found that compost is mainly composed of polypropylene (34.7%), polyethylene (33.8%), polyester (6.8%), and polyethylene terephthalate (4.5%) (36).

In Khan et al. (2023) study, the primary polymer types identified were polyethylene (PE) at 71.04% and polypropylene (PP) at 19.83% (40).

Additionally, Zhang et al. (2023a) investigated the polymer composition of MPs in organic compost samples, identifying eleven distinct polymer types through the use of a micro-Fourier Transform Infrared Spectrometer. The polymers detected included polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), a PP/PE copolymer, polyacrylonitrile (PAN), polyester, polystyrene (PS), polyamide (PA), rayon, and polyvinyl chloride (PVC). Among these, PE, PET, and PP were the most prevalent, comprising average proportions of 24.0%, 20.4%, and 20.0%, respectively (43).

In the study conducted by Zhang et al. (2023b), the most common materials being colorful polypropylene and polyethylene fragments and films, along with polyethylene terephthalate fibers (41).

In the study conducted by Wiesner et al. (2023), compost samples sourced from the same composting facility, primarily derived from municipal biowaste, revealed polymer types including PE, PP, PS, and styrene-butadiene rubber (SBR), with PE being the predominant polymer, constituting 85% of the total plastic content (45).

In the study conducted by Edo et al. (2021), five types of polymers accounted for 94% of the identified plastic items, which included polyethylene, polystyrene, polyester, polypropylene, polyvinyl chloride, and acrylic polymers, listed in order of prevalence. Polyethylene was predominantly found in film form, while polystyrene was more common in fragmentary shapes, polypropylene was primarily present as filaments, and polyester fibers made up the majority of the fiber content (44).

According to Massahi et al. (2024) study, FTIR analysis confirmed the presence of polyethylene (PE) and polystyrene (PS) polymers within the compost samples (42).

The findings of Le et al. (2023) indicate that A range of synthetic polymers, such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), polyvinyl chloride (PVC), polyester (PES), and acrylic polymers (AP), are extensively utilized in plastic products (31).

Surendran et al. (2024) conducted a study that utilized Fourier Transform Infrared (FTIR) spectroscopy to identify resin types in compost derived from unsegregated municipal solid waste (MSW). Their findings confirmed the presence of PE, PP, PS, nylon, PET, and allyl alcohol copolymer (39).

The main aim of this study was to investigate the characteristics of MPs present in the compost produced by Mazandaran Compost Factories in Iran. The results of the study revealed the presence of MPs in all compost samples. This highlights the pressing need for effective waste management practices that can reduce plastic waste in incoming waste and minimize its impact on the environment and human health. Despite the importance of microplastic pollution in compost, studies in this area are still insufficient. This study represents the first investigation of its kind in Iran. Therefore, further research is needed to better understand the sources and pathways of MPs in compost and their potential impact on the environment and human health. Apart from research, it is recommended that educational programs be developed to raise awareness about the importance of reducing plastic waste and promoting proper recycling practices. This can help to reduce the amount of plastic waste entering the composting process and minimize the presence of MPs in compost products. Ultimately, this will contribute to a healthier and more sustainable environment.

Competing Interests

The authors declare that there are no known financial conflicts of interest or personal relationships that could have influenced the findings presented in this paper.

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Editorial decision: Major revisions
04 Oct, 2024
Reviewers agreed at journal
23 Aug, 2024
Reviewers invited by journal
23 Aug, 2024
Editor assigned by journal
22 Aug, 2024
First submitted to journal
19 Aug, 2024

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Abundance and Characterization of Microplastics in Compost Produced in Mazandaran, Iran

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Abstract

Figures

Introduction

Material and Methods

Sampling of Compost

Extraction of microplastics

Identification of microplastics

Quality Controls

Results and Discussion

Microplastics (MPs) abundance

The shape of MPs

Color and Size of MPs

Micro Raman microscopic studies

Conclusion

Declarations

Competing Interests

References

Supplementary Files

Status:

Version 1