Introduction

The significant increase in solid waste production as one of the consequences of globalization and urbanization is one of the main challenges of municipal management1. The management of municipal solid waste is a continuously, multi-stage, and chain process2. However, the different characteristics of solid wastes generated in different sources, such as medical waste, industrial waste, and municipal waste, are an important factor in choosing a waste management method and its hierarchy3. The selection of available options in each chain of this process can have different environmental consequences1,4. For example, landfilling or incineration, as municipal solid waste management options, have different environmental burdens5,6. The most important consequences of landfilling are the need for land, leachate production, and biogas emission7,8. In addition, air pollution and by product ash, are among the negative consequences of incineration9,10.

The advantages and limitations of each solid waste management method can be analyzed based on technical, economic, and especially environmental criteria to select the best option. One of the main criteria in selecting the available options for solid waste management is the secondary pollutants and by-products in each method that can potentially pose a health and environmental risk. The economic situation and technology are among the effective factors in choosing the best option from available methods for solid waste management4,11. In developing countries, especially in metropolitan cities, per capita production of solid waste has increased in recent decades due to economic development and changes in consumption patterns12. However, waste management structures are still not as efficient as in developed countries. In this situation, solid waste landfilling is a widely used method in developed part of the world13 which can be an environmental threat due to non-compliance with standards, lack of technologies to control leachate, and weak post-landfill care. Therefore, the need for reducing the volume of waste and energy production, has caused to considered incineration as an essential alternative option in many developing societies14.

Incinerators have several advantages, one of which is a significant reduction in waste volume and the need for landfill14, but incinerator by-products, especially ash, are a concern in using this method in municipal solid waste management15. The characteristics of ash and the concentration of its pollutants are influenced by the composition of loaded wastes15. Accordingly, its environmental consequences is related to the efficiency of waste management system16. The presence of pollutants such as potentially toxic elements in incinerators’ ash has been reported in many studies17,18. However, the difference in the concentration of potentially toxic elements in incinerator ash is caused by the difference in the weight ratio of the components of solid waste such as cigarette butts, battery waste, plastic, and other waste containing potentially toxic elements18. In recent years, attention to microplastics as an emerging pollutant has raised a debate on the possibility of remaining this pollutant in incinerator ash18. Several studies have been conducted to identify and investigate the origin of microplastics in the environment, which indicate environmental pollution, including water resources19,20,21. Also, various studies have been conducted on various pollutants, including potentially toxic elements, in incinerator ash as the most important by-product of this process, and its management strategies have been evaluated15,16. However, investigating the abundance and characteristics of microplastics in incinerator ash is a gap in studies. Therefore, this study, as an innovation, investigated the abundance and morphology of microplastics in incinerator ash. Increasing plastic production and its consumption pattern, besides the weakness in SWM, have caused the plastic composition in municipal solid waste increased, especially in developing countries11,12. The aim of this study was to identify the quantity and morphology of microplastics in incinerators ash in two cities in Iran, including type, shape, size and color in the composition of solid waste.

Method

Study area

This study was conducted on the solid waste, generated in Tehran, Nowshahr, and Chalus districts in Iran. Tehran is the capital of Iran and its population was about nine million people. The population of Nowshahr and Chalus were 100,752 and 131,317 respectively. These cities are located on the southern and northern sides of the Alborz Mountain range. Per capita production of solid waste in Tehran reported as 840 gr/day and in Nowshahr and Chalus were reported 760 g per day11. The composition of municipal solid waste in these cities is almost similar. The ratio of organic waste in Tehran and reported as 74.5% and in Nowshahr and Chalus 77.7%. However, the other components of solid waste have a bit deference between Tehran comparing to other cities of Nowshahr and Chalus. The ratio of metals, paper, and plastic waste in Tehran reported as 2.48, 5.04, and 6.28%, respectively, comparing to Nowshahr and Chalus which was 0.89, 8.43, and 7.61% respectively. The waste management method was similar in the studied cities. Source separation ratio in these districts estimated less than 15%. The main part of segregation is related to waste scavengers and separation at the transfer stations and landfill sites, which was in accordance with the reported pattern in the whole country.

Studied incinerators

Two active solid waste incinerators in Iran, located in Tehran and Nowshahr, with the same technology and capacity, were considered in this study. The loading rate of solid waste in Nowshahr incinerator (NI), receiving generated waste from Nowshahr, and Chalus districts, was 179 and in Tehran incinerator (TI) was 200 tons/day. Both incinerators were dual-chamber, where the combustion temperature in the first chamber and the second chamber were 850–1050 °C and 950–1200 °C, respectively. Residual ash of primary chamber, secondary chamber and baghouse filter of these incinerators was used for this study.

Sampling

As shown in Fig. 1, the samples were collected from three parts of the incinerators. The samples were collected monthly, during March 2021–2022. For this purpose, in the middle of each month, one kilogram of ash from each selected parts of the incinerators was collected and then separately mixed to have a homogenous sample from chambers and baghouse filter in each incinerator, using an industrial homogenizer. Then, the samples were poured into a glass bottle by a clean metal spatula. The glass bottle was immediately sealed and taken to the laboratory for further analysis. The samples were opened in the laboratory under the hood to follow safety principles. Then, by steel sieve with a pore diameter of 1.0 mm, the sample particles were separated in two different sizes, particles larger than one millimeter and particles smaller than one millimeter. Glass bottles were washed with distilled water, before sampling and dried in an oven at 130 ºC. The spatula and sieve were washed with distilled water between each sampling and separation of each sample and dried in the oven at 130 ºC.

Fig. 1
Fig. 1The alternative text for this image may have been generated using AI.
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Schematic of studied incinerators and sample collecting points (A, B, and C).

Sample processing

Microplastics were isolated from the samples by sieving and flotation apparatus which has been used in many studies18,22,23,24. Briefly, the procedure is as follows. First, the samples were dried at 65 ºC for 24 h and sieved using a sieve (Damavand, Tehran, Iran) with a pore diameter of 1.0 mm to separate particles larger and smaller than one millimeter into two groups. Then NaCl (Merck, Extra pure, Germany) saturated solution was prepared for extraction. The density of saturated solution was 1.2 gr ml− 1. Ten grams of each dried and sieved sample was transferred to a beaker and 100 ml of the extraction solution was added to it. The beaker containing the sample and the extraction solution was stirred for 60 min at room temperature at a speed rate of 250 rpm. By using this method, low-density materials, including microplastics, floated on the top of the solution. In the next step, the solution stabilized for 2 h to settle the heavy particles. Then the supernatant was filtered using a 5 μm cellulose filter (Whatman AE 98, Germany). This process was repeated three times until that suspended particles were not observed in the solution25. The filters were read under a laminar hood in order to avoid secondary contamination by air born particles.

Sample analysis

Optical microscope (Carl Zeiss (model 492177, Germany) microscope with a binocular viewing head with 10x/18 mm eyepieces, 4 position nosepiece and 4×, 10×, 40×, and 100× objectives) were used to count the number of microplastics as well as morphology, color and size determination of particles smaller than one millimeter. Stereomicroscope (Sairan, ZSM-1001 model with a binocular viewing head and 10× objectives) were used to count the particles larger than one millimeter. Microplastic separation from the studied samples was done by flotation method. For this purpose, the samples were dried at 65 ºC for 24 h. The dried samples were sieved to remove particles larger than 1 mm. Saturated sodium chloride was added to the sieved samples, and the separation of microplastics was done by rotating at 250 rpm for 60 min at room temperature. After a 2-h pause to settle large particles, the supernatant was filtered three times using a 5 μm cellulose filter (Whatman, Germany). The microplastics kept in a petri dish for subsequent identification by Raman micro-spectroscopy to determine their origin. Raman micro-spectroscopy analyses were performed using a Horiba XploRA PLUS dispersive Raman microscopes with 532 nm and 785 nm lasers. Several different objectives (5×, 20×, 50× and 100×) were used to optimize the analytical laser spot size for spectral analyses (Araujo et al. 2018). To prevent interference from materials similar to microplastics, hot needle test was used to determine the presence of microplastics26.

SEM/EDS analysis

After initial screening of microplastics by optical microscopes, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis were conducted using TESCAN MIRA3 SEM (Brno, Czech) with EDS System. SEM/EDS allowed many potential microplastics particles to be screened in a relatively short time. SEM screening utilized surface morphology and EDS used to elemental composition to determine the metals sticking on the surface of the microplastic samples. For SEM/EDS analysis, the samples collected on the filter, were fixed on the glass slide using glue. In order to minimize the Effects of interference factors, the slides were coated with a thin layer of gold25.

Results and discussion

The results of microplastic quantity analysis in three types of studied ash, including bottom ash of the first chamber (FBA), bottom ash of the second chamber (SBA), and fly ash (FA) are shown in Table 1. In total 3491 particles of microplastics were counted. The results showed that the abundance of microplastics in the studied ashes had irregular spatial and temporal variations. As shown in Fig. 2, the abundance of microplastics in studied ash in Tehran Incinerator (TI) was higher than Nowshahr Incinerator (NI). On average, the abundance of microplastics in the FBA, SBA, and the FA of Tehran Incinerator compared to Nowshahr Incinerator were higher by 27 (14.13%), 21 (14.26%), and 21 (15.81%) particles/100 grams. 40% of the abundance of microplastics was observed in the FBA. The SBA and FA constituted 31.84% and 27.23% of the counted microplastics, respectively. Considering the total number, 53.92% of microplastics were counted in TI samples and 46.08% in NI samples. The abundance of microplastics in the studied ashes was not regular in different seasons. The highest microplastics in TI samples was observed in the summer season (27.57%), while it was 23.26%, 25.48%, and 23.49% in the spring, autumn, and winter, respectively. In NI, the highest microplastics was observed in spring (26.48%), while it was 24.48%, 23.27%, and 25.67% in summer, autumn, and winter, respectively. The abundance of microplastics in FBA, SBA and FA in TI samples was 40.85%, 31.62% and 27.43%, respectively, while in NI, it was 41.14%, 31.78%, and 26.98%, respectively.

Table 1 Abundance of microplastics in studied ashes (particle/100 grams).
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Fig. 2
Fig. 2The alternative text for this image may have been generated using AI.
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The average abundance of microplastics in the studied ashes (particles/100 grams).

Microplastics are one of the pollutants of concern in recent years, which, in addition to solid waste, have also been identified and reported in various media, including water sources27. The results showed that microplastic was a pollution in the studied ash samples. The presence of microplastics in the studied ashes is directly affected by the plastic wastes in the municipal waste mass11 which loaded in incinerator reactor with other waste compounds. Therefore, the abundance of microplastics in incinerator ash is affected by the proportion of loaded plastic waste in the reactor, which is different in different cities and countries11. However, the difference in the abundance of microplastics in the ash of the studied incinerators in Tehran and Nowshahr was due to the difference in the quantity and type of plastic loaded into the incinerators, which was due to the difference in the composition of municipal solid waste in Tehran and Nowshahr, as well as the difference in per capita waste production in the two studied cities. The factors affecting the number of microplastics in incinerator ash include the amount of plastic consumption and the ratio of separation of plastic waste from municipal solid waste. Plastic is a widely used in packaging, transportation, construction, electrical appliances, and many household appliances28. The characteristics of plastic, such as resistance, low price, and lightness, have caused its use to increase significantly in the past decades29, which has caused the increasing production of solid plastic waste30. An increase in plastic production from 1.5 million tons in 1950 to 245 million tons in 2008 was reported and it is estimated that the amount of plastic production in the world will increase by 5% annually28. Finally, some of the used plastics are turned into plastic waste. However, by proper management, it is possible to recycle more of the plastic waste and prevent it from being loaded into the incinerator.

The average per capita production of solid waste in Iran is 650 g/day and on average 8.4% of municipal solid waste includes plastic31. But, only 12% of plastic waste is recycled in Iran31, which directly affected the abundance of microplastics in incinerator ash. In this situation, the presence of microplastics in incinerator ash is expected due to loaded macro-plastics, which was seen in the results of this study. Although the decomposition of macro-plastics into secondary microplastics during the combustion process can be considered as the main affecting factor in microplastic aggregation in incinerator ash, it should be noted that the loaded waste mass can contain primary microplastics. For example, abundances of 20,000 to 91,000 microplastics per kilogram of solid waste in landfill sites was reported32. Therefore, it is expected that the loaded waste in the studied reactors had a significant number of primary microplastics. So, part of the observed microplastics in the ash of the studied incinerators are derived from primary microplastics in the mass of municipal solid waste, that influenced by the general conditions of waste management and the consumption pattern11. The affecting factors in the composition of municipal solid waste as well as environmental pollution can also be considered as indirect factors affecting the abundance of microplastics in incinerator ash. For example, the management of littered plastic waste in public environments can be an important source of microplastics in the environment33. The effect of marine plastics as one of the most important littered wastes in the environment in increasing primary microplastics in seafood can be considered34. Therefore, waste management and citizens’ behavior in reducing waste littering can be effective in changing the abundance of microplastics in incinerator ash. A clear example of these sources is cigarette butts, which are produced from cellulose acetate reinforced with plastic, that known as source of microplastic35,36,37.

The results of Raman analysis showed that investigated microplastics in the studied ashes has originated from six types of plastic polymers. These polymers included polypropylene, high-density polyethylene, low-density polyethylene, polyvinyl chloride, polystyrene, and polyethylene terephthalate. As shown in Fig. 3, polyvinyl chloride has the highest ratio (35.51%) and then polypropylene (21.66%), high density polyethylene (8.23%), low density polyethylene (10.94%), polystyrene (16.53%) and polyethylene terephthalate (7.35%). The size distribution of detected microplastics in the studied ashes is shown in Table 2. In total, 65.32% of the detected microplastics were less than 10 micrometers in size. 21.21% were between 10 and 100 microns and 13.56% were larger than 100 microns. The abundance of microplastic sizes as shown in Fig. 4, indicated that the ratio of the abundance of different sizes of detected microplastics in all types of ash and also in studied incinerators had the same pattern.

Fig. 3
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Types and proportion of polymers in detected microplastics.

Table 2 Size distribution of detected microplastics in the studied samples (particle/100 grams).
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Fig. 4
Fig. 4The alternative text for this image may have been generated using AI.
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Size distribution (%) of microplastics in different ash samples: TI (A), NI (B).

As shown in Table 3 the color of microplastics in the studied ashes included blue (40.7%), red (32.8%), transparent (14.7%), green (6.3%) and others (1.5%). Also, the share of each form of microplastic was different in all types of ash, however, in all samples, the share of fiber-shaped microplastics was much higher than film-shaped and irregularly shaped microplastics. The total detected microplastics in TI samples included 996 fibers, 481 films, and 196 irregular shapes. While in NI samples, 1006 fibers, 518 films, and 407 irregular shapes were detected. Therefore, the highest number of detected microplastics were fiber (65.3%), film (31.6%), and irregular shapes (12.4%). As shown in Fig. 5, the detected microplastics had various morphologies and colors.

Table 3 Color and shape distribution of detected microplastics in the studied samples (particle/100 grams).
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Fig. 5
Fig. 5The alternative text for this image may have been generated using AI.
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Photos of some detected microplastics in the ash and SEM/EDS results.

Considering the ability of microplastics to carry pollutants, in the analysis carried out to find polluting particles on the surface of identified microplastics, EDX combined with SEM was used for elemental analysis. The results of the aforementioned analysis for a sample of microplastics are given in Fig. 5. The results showed that there was a high percentage of metals (iron, manganese, calcium, etc.), along with carbon, sodium and chlorine in the surface of the sample. The variety of plastic types in the composition of plastic waste as well as difference in primary microplastic sources in the municipal solid waste had the main effect on the observed variation of shape, size, and color of microplastic in the studied ashes. Plastic morphology is effective in its decomposition and fate in the environment and in waste management processes38. Referring to many studies, the difference in the type, shape and size of microplastics in the municipal solid waste, which is as a result of the variety of plastic consumption in the community, has a reason for the observed differences in this study11. Therefore, the variety and contribution of each of the plastics observed in the ash was directly affected by the quantity of the types of plastic in the loaded waste.

The difference in the shape of the microplastics observed in the studied ashes was directly influenced by the difference in the shape of the primary microplastics in the waste mass loaded into the reactor and indirectly by the combustion process on the loaded plastic waste. The presence of microplastics in solid wastes has been reported in various shapes of particles, fibers, plates, granules, and irregular shapes39, which was consistent with the results of this study. However, the proportion of the contribution of each of the microplastic shape in the solid waste and the subsequent in the incinerator ash depend on the proportion of the shape of initial microplastic and also the effect of the combustion process in changing the shape of the loaded plastic waste in the reactor that may be different in different samples. Based on this, the difference in the size of observed microplastics in the studied ashes can also be interpreted. In various studies, the size of primary microplastics in municipal solid waste has been reported in the range of 0.02 to 4.9 mm40, which can be observed in the same proportion in incinerator ash. Also, the effect of the morphology of loaded plastic waste in the incinerator reactor on their fate during the combustion process and the formation of different sizes microplastics was one of the reasons for the observed different size24.

Plastic is a widely used in packaging, transportation, construction, electrical appliances, and many household appliances28. The characteristics of plastic, such as resistance, low price, and lightness, have caused its use to increase significantly in the past decades29, which has caused the increasing production of solid plastic waste30. An increase in plastic production from 1.5 million tons in 1950 to 245 million tons in 2008 was reported and it is estimated that the amount of plastic production in the world will increase by 5% annually28. Finally, some of the used plastics are turned into plastic waste. However, by proper management, it is possible to recycle more of the plastic waste and prevent it from being loaded into the incinerator.

The average per capita production of solid waste in Iran is 650 g/day and on average 8.4% of municipal solid waste includes plastic31. But, only 12% of plastic waste is recycled in Iran31, which directly affected the abundance of microplastics in incinerator ash. In this situation, the presence of microplastics in incinerator ash is expected due to loaded macro-plastics, which was seen in the results of this study. Although the decomposition of macro-plastics into secondary microplastics during the combustion process can be considered as the main affecting factor in microplastic aggregation in incinerator ash, it should be noted that the loaded waste mass can contain primary microplastics. For example, abundances of 20,000 to 91,000 microplastics per kilogram of solid waste in landfill sites was reported32. Therefore, it is expected that the loaded waste in the studied reactors had a significant number of primary microplastics. So, part of the observed microplastics in the ash of the studied incinerators are derived from primary microplastics in the mass of municipal solid waste, that influenced by the general conditions of waste management and the consumption pattern11. The affecting factors in the composition of municipal solid waste as well as environmental pollution can also be considered as indirect factors affecting the abundance of microplastics in incinerator ash. For example, the management of littered plastic waste in public environments can be an important source of microplastics in the environment33. The effect of marine plastics as one of the most important littered wastes in the environment in increasing primary microplastics in seafood can be considered34. Therefore, waste management and citizens’ behavior in reducing waste littering can be effective in changing the abundance of microplastics in incinerator ash. A clear example of these sources is cigarette butts, which are produced from cellulose acetate reinforced with plastic, that known as source of microplastic35,36,37.

Conclusion

The abundance and characteristics of microplastics in the ash of active incinerators in Iran were studied. The results showed 108 to 192 microplastics per 100 g of studied incinerators ash. 35% of the detected microplastics consisted of PVC and 7% consisted of PET, which were the highest and lowest detected types, respectively. The results showed that the incineration process does not lead to complete removal of microplastics, and it is possible that in addition to ash, microplastics may also be present in the exhaust gas from the incinerator, which can be considered as a limitation of this study in future studies. The entry of microplastics into natural resources, such as water, soil and air, been clearly defined due to the natural or mechanical process on plastic materials. While these pollutant particles are not expected to be detected in the ashes of urban waste incinerators after burning the waste with a temperature above 900 degrees Celsius. On the other hand, considering the possibility of incomplete burning of wastes in cases where the power plant experiences thermal fluctuations, it is possible to imagine the presence of unburned plastic parts, including large microplastic particles (up to 5 mm) in the bottom ash of waste incinerators, but identification of particles close to 10 μm in the fly ash settled in the baghouse, after passing the suspended ash particles through the second stage of burning with a temperature of about 1200 degrees Celsius in the secondary chamber, was unexpected but has proved in this research. These findings showed that the incinerator cannot end the microplastic cycle and the management of incinerator ash, especially fly ash, needs special attention. On the other hand, despite the efforts made in international forums to end plastic pollution and reduce the production of single-use plastics, unfortunately, the production and consumption of these materials in developing countries is increasing, which one of its consequences is presence of microplastic in MSW and ash of solid waste incinerators. On this basis, it is necessary to give priority to public education to reduce plastic consumption and source separation in MSW management. More research needed to identify the reasons for durability or the possibility of turning plastic components into microplastic particles in the burning process in waste incineration power plants and also the management of ash containing microplastics and other pollutants, to provide a clear way in preventing these pollutants from entering to natural resources and the food chain.