Introduction

In many developing countries, including Iran, waste disposal is still the primary method used for waste management. This is mainly due to a lack of sufficient knowledge and equipment for effectively managing waste (Dashtian et al. 2022). Recycling and reuse of waste materials are alternatives widely implemented in developed countries as key components of waste management. However, authorities in developing countries, such as Iran, often lack technologies and systems to implement these alternative options and are compelled to resort to landfill sites for waste management (Kim & Owens 2010). Due to the lack of engineering and environmental management procedures, dumpsites and unsanitary landfills have become environmental problems (Lavagnolo 2019). Furthermore, these disposal sites contain waste from a variety of sources including industrial, healthcare, construction, electronic, and biomass, with each having varying levels of risk. Unfortunately, approximately 33% of the municipal solid waste produced worldwide continues to be inadequately disposed in dumpsites (Kaza et al. 2018).

In Iran, as is the case in many other countries, the generation of municipal solid waste has increased in tandem with rapid growth in population, as well as the expansion of commercial, industrial, and urban areas in recent years. These recent major advancements have resulted in numerous environmental issues affecting the air, soil, surface and ground water (Rezapour et al. 2018; Shahivand & Rouhani 2021). Furthermore, municipal and industrial waste management systems are limited, and landfills which are deemed to be disposal sites are without any treatments and are located close to public areas, agricultural fields, and urban rivers. Therefore, contaminants are easily transferred from disposal sites to the nearby areas serving as risks to humans, animals and the ecosystem. Common hazardous substances which pose a threat to the environment identified at landfill sites include potentially toxic elements (PTEs) [Cd, Pb, As, Zn, etc.], persistent organic pollutants (POPs), and polycyclic aromatic hydrocarbons (PAHs) (Chrysikou et al. 2008).

Given their persistence, non-biodegradability, toxicity, and bioaccumulation, PTEs have proven to be one of the most problematic environmental issues when considering the various types of soil pollution (Alloway 2012). PTEs may be released into the environment as a result of leachate generation and its migration from landfills, which could negatively impact soil, groundwater, and, eventually, surface water. The amount of received rainfall also has a significant impact on leachate generation and the mobility of PTEs (Adamcová et al. 2016). The content of PTEs in soil varies depending on the source and type of solid waste disposed of in the landfill (Popego et al. 2019). Electronic waste is a significant component of solid waste, characterized by high contents of metals. These e-wastes are commonly found in the vicinity of informal recycling sites (Ohajinwa et al. 2019).

Numerous studies on landfill leachate composition have identified organic pollutants derived from both xenobiotic and biogenic sources (Christensen 1992; Ghosh et al. 2015). PAHs are among the most common organic pollutants found in landfill soil and the surrounding aquatic ecosystems (Gade et al. 1996). Some of these compounds are formed in urban landfill fires, while others are deposited from the air, or derived from petroleum hydrocarbon residues (Loehr et al. 1993; Melnyk et al. 2015; Nadal et al. 2016). In soil and sediments, PAHs tend to be adsorbed on organic matter particles owing to their hydrophobic characteristics (Mostafa et al. 2009; Tipmanee et al. 2012). They are normally detected in distant areas, including polar and tropical regions, that are located far from human settlements and industrial sites (Becker et al. 2006; Perra et al. 2011). Nevertheless, the rate of pollution is lower in areas with less industrial activity compared to those with more extensive industrial activities (Melnyk et al. 2015).

Landfills are also significant repositories of plastic waste, storing an estimated 21–42% of the total globally produced plastics (He et al. 2019). Plastics found within landfills have the potential to undergo intricate biochemical reactions and physical transformations, resulting in the formation of secondary microplastics (MPs) (Golwala et al. 2021). Earlier studies found irregular forms and hack structures in refuse MPs, indicating that plastic debris degradation was the main source of MPs, particularly secondary MPs in landfills (Su et al. 2019). Furthermore, it is important to note that landfills serve as direct recipients of primary MPs in various forms, such as those found in disposed sludge (Yang et al. 2015). The landfilled MPs can be transported to the surrounding areas by various natural processes (such as rain and wind) (Yadav et al. 2020). A significant amount of plastic waste produced by industrial processes, including non-ferrous industries and small workshops, is often released into the surrounding environment without undergoing any treatment (Du et al. 2020). Furthermore, due to the extensive utilization of plastics in food production and processing, MPs can be detected in a variety of foods such as sea salt, honey, beer, vegetables, fruits, and seafood. Consequently, the insufficient management of food waste contributes to the release of MPs into the environment (Rist et al. 2018).

Based on existing literatures, there is no published review paper assessing different soil pollutants around municipal solid waste landfills not only in Iran but also in worldwide scale. To identify the most frequent and recently identified soil contaminants and compare them to soil contaminants from other landfills, it is required to review the literature on soil contamination surrounding landfills in Iranian cities. The obtained information will assist to establish appropriate monitoring procedures for these contaminants, which will serve as the basis for developing suitable remediation plans for Iranian landfill sites. The focus of this review is on PAHs and PTEs, which are well investigated as common and high-priority pollutants around landfill sites globally. It also looks at MPs, an emerging area of research driven by the growing problem of plastic waste in landfills.

Production of municipal solid wastes in Iran

Iran exhibits varying levels of development and climate zones across its cities, therefore, the rate of urban waste generation per person varies greatly among different cities. Unfortunately, precise statistics regarding waste management in Iran are currently unavailable from municipalities or waste management organizations. Thus, according to Rupani et al. (2019) existing data on waste management in Iran is primarily derived from individual investigations conducted by researchers. There are several factors that contribute to varying levels of waste production in different areas. For instance, the northern provinces of Iran, which are coastal areas and popular tourist destinations, generate the most amount of waste compared to other parts of the country. According to a study conducted in 2014, there were 1100 g of waste produced per person per day in Mazandaran Province and 1700 g per person per day in Guilan (Rezazadeh et al. 2014). In contrast, provinces in central Iran generate significantly lower amounts of waste. The estimated daily per capita waste generation in Yazd Province was 293 g (Vahidi et al. 2017), while 638 g per person per day was estimated in Kashan (Moharamnejad et al. 2011). Furthermore, Najafi et al. (2017) reported that the amount of waste generated in Zahedan was 800 g per person per day, while Shahr-e-Kord generated 411 g of waste per person per day (Talaiekhozani et al. 2018). There are also differences in the per capita waste generated between major cities in Iran. Tehran has the highest per capita waste generation between Iranian cities. In addition to Tehran, which is the capital city of Iran, other cities such as Mashhad, Shiraz, Isfahan, and Tabriz have experienced significant growth in waste generation (Rezazadeh et al. 2014). A region's changing climate has an influence on waste output as well. When it comes to waste generation, the summer months usually yield higher volumes than the other seasons. This is mostly because summer's hot weather hastens the deterioration process (Zazouli et al. 2013).

Municipal waste landfills

Municipal landfills have been widely recognized as the best strategy to properly managing the growing problem of solid waste. Municipal waste landfills have been specially constructed with the intention for receiving household waste and other non-hazardous wastes (Krčmar et al. 2018). Landfills are widely used for waste disposal in Iran, leading to ongoing issues in numerous areas. One of the main problems with Iranian landfills is the massive amounts of leachate that are produced as a result of ineffective leachate management and inadequate landfill management. Thus, only a small amount of the leachate is collected, while the remaining quantity penetrates into the soil, increasing the risk of contaminating groundwater (Rupani et al. 2019). Other factors that account to the content or quality of leachates from landfills depends on the age of landfill, composition of waste, rainfall rates and patterns, and climate (Robinson 2007). The leachate from landfill sites can contain a range of substances, including different types of organic matter, mineral pollutants, PTEs, and hazardous chemical pollutants (Safari and Bidhendi 2007). It is worth noting that Iran has a relatively high amount of organic load, which can be attributed to the lack of waste separation practices. The presence of PTEs in leachate can be attributed to their initial concentration in the waste materials.

Landfills can be classified into two categories: Conventional landfills (Opened dumps) and Engineered (Sanitary) landfills. Management practices and environmental impacts associated with these landfills are vary (Elliotte et al. 2009).

Open dumps landfills

Developing countries continue to rely on traditional burial methods for waste disposal, such as open-dumping or unsanitary waste disposal, despite advancements in sanitary landfill design and the establishment of regulatory programs. These traditional methods pose significant eco-toxicological risks as they are the main source of contamination by PTEs (Kanmani and Gandhimathi 2013). This type of waste disposal contributes to environmental pollution through various means including deep slug penetrations (layered soils), increase in soil degradation, the expansion of waste on the surface, and the generation of leachate, etc. Leachate, in particular, plays a significant role in spreading pollutants with the potential of contaminating groundwater, reaching distances of several kilometers (El-Fadel et al. 1997). Open dumps are the most common waste disposal strategy seen in many urban areas in Iran, characterized by the irresponsible disposal of waste products. In addition to open-pit dumping, unsafe disposal and open-air waste burning practices are widely used in Iran. These procedures have the potential to cause significant and irreversible harm to the environment and public health (Yazdani et al. 2017). Numerous studies have demonstrated the detrimental impacts of waste incineration fires on soil quality, specifically in terms of alterations to soil chemistry and microbiological characteristics (Martínez-Murillo et al. 2016; Pereira et al. 2016). A number of researchers have undertaken comprehensive investigations on unsanitary landfills, resulting in the identification of detrimental effects associated with this issue. These negative effects include: excess noise problems and respiratory diseases (Heaney et al. 2011; De Feo et al. 2013), negative birth outcomes (Elliott et al. 2009), and rising numbers of cancer cases (Mattiello et al. 2013).

Figure 1 shows Saravan open dump landfill in Guilan province, Iran.

Fig. 1
figure 1

(Source: Abdulmannan Rouhani)

Saravan open dumps landfill in Guilan province, Iran

Full size image

Sanitary landfills

In the years prior to the 1950s, waste materials were commonly disposed of in open dumpsites. At that time, the potential pollution and effects on the environment and human health resulting from waste disposal in dumpsites were not taken into consideration. It was generally believed that the leachate from these waste sites would be naturally treated by the soil and groundwater, with no significant short-term or long-term effect on the surrounding environment (Meegoda et al. 2016). Thus, one of the most significant challenges in waste disposal at open dumpsites was the infiltration of leachate due to the lack of a leachate collection system (Aziz et al. 2014). Despite Europe expressing concerns about the requirement to improve waste disposal methods in the early 1930s, no substantial actions had been taken to address this issue by 1959 (Vallero & Blight 2019). The first recognized definition of sanitary landfills was developed by the American Society of Civil Engineers in 1959. Hazardous and non-hazardous wastes are typically the two categories into which wastes are divided. Sanitary landfills, which are disposal sites for non-hazardous waste, must conform to specified design and operational standards outlined in this regulation. These standards necessitate the implementation of engineering operations (Worrell & Vesilind 2012; Meegoda et al. 2016). The United States Environmental Protection Agency (EPA) established the Solid Waste Disposal Act in 1979 for solid waste disposal facilities to mitigate the adverse effects of these sites on the environment and human health (Govinfo.gov 2021). Landfills that do not meet guidelines and standards of the Act are classified as open dumps (Iravanian & Ravari 2020).

Sanitary landfills are required to implement various measures to prevent pollution penetration into the soil and groundwater. These measures include the establishment of leachate collection systems, the use of liners composed of impervious clay or synthetic materials, and the implementation of capping measures groundwater (Worrell & Vesilind 2012; Aziz et al. 2014). Additionally, waste which is primarily composed of organic matter and decomposes anaerobically, results in the generation of gasses such as methane and CO2.

Figure 2 depicts a schematic overview of a sanitary landfill designed for the disposal of municipal solid waste.

Fig. 2
figure 2

(Adopted from Millis, 2017)

An overview of a sanitary landfill designed for the disposal of municipal solid waste

Full size image

Soil pollution

Leachates and gases released by poorly managed, controlled, monitored and constructed landfills can contaminate soils both inside and outside of landfill sites due to their potential to infiltrate or migrate, negatively affecting the quality of the surrounding soils. This occurs when landfills lack a protective liner or clay material at their base and perimeters, or when the liner has deteriorated or been displaced from its original position due to accidents or natural processes.

Potentially toxic elements

Municipal waste disposal creates significant issues in countries worldwide, more particularly in developing countries (Cittadino et al. 2020). Leachate derived from landfills contains substantial quantities of dissolved organic matter, as well as minerals Fe, SO2−4, Cl−, and PTEs. It has the potential to discharge contaminants initially into the soil and then into the groundwater, posing a significant risk to the health of nearby ecosystems, particularly those that are vulnerable and susceptible (Xu et al. 2017; Cittadino et al. 2020). Elevated levels of PTEs in soil near landfills can pose long-term risks to humans (Ogunbanjo et al. 2016). Additionally, the mobilization of PTEs that occur as a result of their elevated levels in wastes may lead to the contamination of agricultural soils and subsequently the food chain, posing a significant risk to organisms residing within and in nearby to landfill sites (Adelopo et al. 2018; Ye et al. 2019). Leachate can have a substantial impact on soil characteristics once it penetrates the soil. In addition, the quantity of leachate generated at landfills is influenced by factors such as landfill cover, climate conditions, and waste compaction. Earlier investigations have shown that landfills without treatment systems have the potential for leachate to move on the soil surface which can lead to changes in the soil's physicochemical and biological processes, ultimately impacting the level of soil contamination (Samadder et al. 2017). In developing countries, the level of PTEs in leachate may be higher owing to the lack of appropriate solid waste separation before landfilling (Carvajal-Flórez & Cardona-Gallo 2019).

In some instances, waste is deposited improperly, without considering environmental consequences, and at some dump sites, waste is burned in the open air, leaving ashes behind. Waste incineration eliminates combustible materials and oxidizes metals, resulting in an ash residue that is enriched with PTEs. These elements undergo oxidation and corrosion, leading to their dissolution in rainwater and subsequent leaching into the soil. This allows them to be absorbed by growing plants, ultimately entering the food chain (Kanmani & Gandhimathi 2013). Table 1 shows the conducted studies assessing polluted soils by PTEs around landfill sites from Iranian cities, and the concentration of most pollutant element in these areas also given in Table 2.

Table 1 Conducted studies on soil pollution by PTEs around landfill sites in Iranian cities
Full size table
Table 2 Concentrations of some PTEs in soils around landfill sites from Iran
Full size table

Considerable accumulations of the available and total concentrations of Ni, Pb, Cd, Cu, and Zn were detected in the leachate-affected soils of a landfill site located near Miandoab City, north-western Iran. Nonetheless, the average concentrations of these PTEs (with the exception of Cd) in the soil and wheat grains cultivated in stated soil were found to be within the permissible thresholds established by international guidelines pertaining to environmental health and foodstuffs (Rezapour et al. 2018). The disposal of municipal solid waste at the Tonekabon city landfill site was linked to significant contamination of Zn, Pb, Ni, Mn, and Cu, resulting in a high level of eco-toxicological risk (Azizpour et al. 2020). Elevated levels of As, Cd, and Hg were identified in the soil near the Saravan municipal solid waste disposal site in Rasht city (Sadeghi Poor Sheijany et al. 2020). Over 75% of soils in the surrounding area of an abandoned landfill site in Gorgan exhibited significant concentrations of Cd and Pb, ranging from moderate to high levels due to decomposition of waste at landfill site (Pazhmaan et al. 2021). The soil in the Kahrizak landfill, which is under the authority of Tehran municipality, has been found to be highly polluted by Zn, Pb, and Cu. This contamination also extended to the nearby residential area. Consequently, it is imperative to reassess the existing policies pertaining to solid waste management in order to address this issue effectively (Karimian et al. 2021). Elevated levels of Co and Pb pollution have been observed in soils within the Khesht landfill in southern Iran. However, pollution levels appear to decrease at further distances from the landfill (Rouhani et al. 2022).

Significant levels of PTEs pollution have been identified in the soils of both the municipal and medical landfills in Sabzevar. The most severe pollution in this area was found in sampling stations located nearby landfills that had elevated levels of Hg and Cd (Kowsari et al. 2022). The presence of Cr, Co, and Ni originating from the landfill site in Ilam city has had a substantial impact on the soil within the Zagros Forest. The levels of Ni, Co, and Cr in the soils of this landfill were found to be 408.8, 43.87, and 531.6 mg/kg, respectively. These concentrations were observed to be 5–10 times greater than the standards recommended by the United States Environmental Protection Agency (USEPA). The detection of PTEs in soil samples from this area indicates that the pollution was caused by leachate migration from the open dumping site (Solgi & Beigmohammadi 2023). From this study area, the highest mean concentrations of Pb and Cd were also recorded at 440.19 and1.45 mg/kg, respectively (Rostaminya et al. 2023). PTEs have been identified at high levels in 84.16% of the soil samples taken from Shiraz's landfill, with Cd and Pb being the primary contributors (averaging 712.2 and 57 ecological risks, respectively) (Balali et al. 2023).

Similar results were also reported from other parts of the world. For example, the soil samples collected in nearby areas of the Benguerir open dumpsite in Morocco showed significant levels of pollution, primarily as a result of enrichment by Pb and Cd (El Fadili et al. 2022). Soils in dump site at Amakom and Kronum in Ghana contained moderate concentrations of PTEs including Zn, Ni, Hg, Pb, Cu, Cr, Cd, and As. It was found that only a small percentage of the metal concentrations in these soils exceeded the international soil quality guidelines (Akanchise et al. 2020). The soils surrounding Tibet landfill, China, revealed a moderate to high risk of pollution characterized by the presence of PTEs. In this area, the concentration of Hg was found to be the highest at 0.015 mg/kg, which is about 50% greater than the background concentration of 0.008 mg/kg (Wang et al 2020). A study conducted near an unmonitored municipal landfill in Prague, Czech Republic, revealed that the surrounding environment was contaminated with PTEs. The presence of metal-bearing calcite, which contains high concentrations of Ni, Zn, Pb, Ba, Sr, Mg, Mn, and Fe, was observed. These metals were found to bind to Fe (III) oxyhydroxides, such as Ni, Cu, Zn, and Pb, as well as preferentially bond to sediment organic matter, particularly Cu (Ettler et al. 2005). Conversely, soil surrounding the Łubna Landfill in Poland was found to contain lower levels of PTEs (Gworek et al. 2016). Similarly, there was no PTE accumulation in the soils surrounding the Cà Mascio landfill (central Italy) after three decades of waste management given that concentrations were within the natural geochemical background of this area (Nannoni et al. 2015).

Therefore, it is evident that PTEs concentrations and toxicity in soils of municipal waste landfills and surrounding areas are higher in countries in Africa and Asia that are comparable to Iran, while results in Europe are observed to be inconsistent, with some studies concluding that PTE concentrations in landfill sites and their environs are low, while few studies found the opposite. The kind of landfills being used and how they are managed may have an impact on the variations in PTEs concentrations. While open dump landfills are popular in developing nations in Asia and Africa, sanitary landfills are more common in Europe where they also have superior landfill and leachate management.

As shown in the Fig. 3a, Cd and Pb by twenty cases for each were the most analyzed elements in soil near to landfills in Iran. Following these two elements, Zn by fifteen cases is the third most analyzed elements, while Al and V by one case for each of them were the least analyzed elements. Based on Fig. 3b, Cd and Pb followed by Zn are the most pollutants PTEs which were detected in soil from landfills in Iran.

Fig. 3
figure 3

(a) frequency of most analyzed PTEs (b) most pollutant elements in soils around landfills

Full size image

PAHs

The emergence of pollutants, such as PAHs, is another significant issue associated with municipal solid waste landfills (Petrovic et al. 2018; Koshlaf et al. 2019). As organics mix with soil with limited oxygen availability, pyrolysis activities in landfills, as well as continued burning of organic waste, play an important role in the formation and accumulation of PAHs. A significant portion of PAHs are formed through the pyrolysis process of polyvinyl chloride, a compound that is commonly found in municipal solid waste (Rochman et al. 2013; Zhou et al. 2016). Inadequate treatment of biomass combustion ashes over an extended period can also result in the release of significant amounts of PAHs into the surrounding environment through various pathways (Krčmar et al. 2018). Numerous research studies have proven that waste biomass ash from landfill sites has the potential to release PAHs into the surrounding environment. For example, according to the findings of Li et al. (2020), waste biomass ashes originating from the Lhasa landfill have been identified as a substantial source of PAH emissions in the Tibetan Plateau. According to the authors, this could potentially increase the total emissions of PAHs in the pristine area by more than 25%. Similarly, Lou et al. (2016) and Petrovic et al. (2018) identified that biomass ashes led to significant level of PAHs pollution in landfill cover soils. They also found evidence of biomass ash transporting PAHs from landfills to the surrounding air and soils respectively. Furthermore, the levels of PAHs detected in the surface soil adjacent to a municipal solid waste landfill in Poland exceeded the permissible threshold (Melnyk et al. 2015). The highest level of PAHs in this region was found to be 3.5 mg/kg overall. In majority of the soil samples, the most prevalent compounds were PAHs that consisted of 2–3 rings in each molecule.

Landfill leachate has also consistently shown high levels of PAHs in many different studies (Wowkonowicz & Kijeńska 2017; Fang et al. 2018). Wet deposition by stormwater, treatment sludges, waste foundry sand, coal ash, and anaerobic decomposition of organics might all be sources of PAHs in the leachate. Two categories of PAHs including low molecular weight PAHs (two to four aromatic rings) and high molecular weight PAHs (five or more rings) are used to classify compounds that include two (naphthalene) or more fused benzene rings (Lee 2010). There are two main causes for the increase in PAHs. Firstly, it is possible that some high molecular weight micropollutants in leachate will thermally crack or pyrolyze at high temperatures and low dimensionless oxidant dose values, producing targeted micropollutants. Second, the processes involved in the creation of organic substances may have caused some simple chemical molecules to change into PAHs. Due to an inadequate oxidant dose in the first experimental set, this phenomenon was very noticeable. Under both subcritical and supercritical circumstances, a number of processes, including steam reform, synthesis, hydrolysis, thermal cracking, pyrolysis, etc., may be involved in the creation of PAHs (Calzavara et al. 2005; Kruse & Dinjus 2007). The Diels–Alder reaction mechanism, consists of dehydrogenation and cyclization of alkanes, as well as recombination of cyclopentadiene during pyrolysis are responsible reactions for the identified increases in PAHs (Böhm et al. 1998; Zhang et al. 2007). Furthermore, it has been demonstrated that the growth of high-ring PAHs can occur by the addition of C2H2 and H + separation/addition, or by the combination of cyclopentadienyl, which is identical to the formation of naphthalene (Richter & Howard 2000).

There are 16 high priority PAH pollutants which are main focus in literature. These PAHs can be categorized into two groups including the low molecular weight PAHs (naphthalene, acenaphtylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benz(a)anthracene, chrysene), and high molecular weight PAHs (benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, dibenzo(a,h)anthracene, benzo(ghi)perylene, indeno(123-cd)pyrene). Soil is considered as polluted with PAHs when these 16 priority PAHs have a mean concentration > 0.60 mg/kg according to the classification of Institute of Soil Science of Poland (Melnyk et al. 2015). While, according to Zhengyu et al. (2013), PAHs contamination in soil is slight with a concentration range of 0.200–0.600 mg/kg, 0.600–1.000 mg/kg for medium contamination, and severe contamination when concentrations are above 1.000 mg/kg. Although PAHs are concerning and they are common contaminants identified in and near landfill sites globally, only one study focused on the PAHs contamination in soils near a landfill site in Iran. Mohammadi et al. (2021) identified that surface soils collected near the Arad-Kouh landfill in Tehran, Iran, exhibited a range of ∑16PAHs values from 0.863 to 1.384 mg/kg, with an average value of 1123 mg/kg. The results indicated that the soil samples were contaminated with PAHs.

On the other hand, studies of PAHs contamination in landfill sites have been conducted globally with confirmed presence of contamination. Findings from Tenodi et al. (2020) inferred that the concentrations of PAHs in soil samples surrounding a landfill in Serbia were generally found to be at low to moderate levels. The study suggested that lower values are indicative of a reduced influence of pyrogenic sources on the overall content of PAHs over time. All 16 PAHs were detected in the soil samples taken from a former landfill site in Brighton, UK. Higher molecular mass compounds, particularly 4-ring compounds including fluoranthene and pyrene, which comprised 40.1–48.3% of the overall PAH burden in the soil samples, contributed a substantial proportion of the soil PAH load in this area (Zhou et al. 2014). A significant quantity of chemical compounds, including PAHs, which have been proven to pose risks to both human health and the environment, were detected in the leachate and topsoil of a dumpsite located in Lagos State, Nigeria. In the topsoil, concentrations of PAHs ranged from 0.94 to 2.79 mg/kg, whilst the values ranged from 0.85 to 1.47 mg/L in the leachate samples (Oketola & Akpotu 2015). Similarly, sediments surrounding the Ruseifa landfill area in Jordan contained a total concentration of 16 PAHs ranging from 0.286 to 1.704 mg/kg, with a mean value of 0.751 mg/kg. Furthermore, the value of PAHs in the leachate ranged from 0.10 to 0.40 mg/L, with a mean concentration of 0.29 mg/L (Jiries et al. 2005). The distribution of PAH compounds in soil samples obtained from the closed Kubang Badak landfill in Selangor, Malaysia was generally uniform, with the exception of phenanthrene and anthracene. On the contrary, the distribution of PTEs was found to be non-uniform, particularly in areas with elevated levels of Zn (Kamil & Abdul-Talib 2010).

One of the potential hazards associated with illegal landfills is the occurrence of fires, which can be ignited through deliberate acts of arson or spontaneous combustion caused by heated waste, lightning strikes, landfill equipment, smoking or sparks, failure of landfill gas systems, reactive materials, or chemical reactions of waste (Jeff 2018). Landfill fires are increasingly prevalent and hazardous on a global scale, with their frequency and severity escalating annually (Stańczyk-Mazanek et al. 2019; Vaverková et al. 2020). Wild landfills are commonly found in various locations such as forest margins, ditches, and the outskirts of areas that are inhabited (Vaverková et al. 2019). The proper disposal of items such as batteries, rusted metal, fuel, solvents, and oil in landfills are necessary due to their potential to cause substantial damage to the environment. However, this process can be expensive to perform (Esa et al. 2017; Lu 2019). Elevated temperatures can lead to the degradation of volatile compounds, resulting in the emission of dense black smoke (Rykała et al. 2022). The uncontrolled combustion of waste poses the risk for rapid spread beyond the boundaries of the landfill and can result in the contamination of air. Following this incident, a significant amount of smoke can disperse over several kilometers, posing a potential threat to residents in nearby areas. A potential chemical risk can also occur as a result of a landfill fire. The incineration of waste materials on soil lacking proper protection can result in soil degradation, both physically and chemically, potentially leading to the migration of contaminants to aquifers (Koda et al. 2018; Abiriga et al. 2020). Used car tires are an excellent example of highly hazardous solid waste commonly found in wild landfills. They are mainly composed of steel wire, nylon, extender oils, carbon black, bromated butyl rubber, poly (butadiene), and styrene-butadiene (Wang et al. 2019). Sulfur is a vital constituent in tires due to its presence in vulcanization agents (Wik & Dave 2009). As a result, there is a possibility that polycyclic aromatic sulfur heterocycles (PASHs) are generated during tire combustion following an aromatization reaction of the Diels–Alder type involving PAHs and sulfur (Williams & Bottrill 1995).

Only limited scientific researches have focused on the environmental implications of potential landfill fires. For example, high levels of PAHs were found in soil samples affected by uncontrolled tire fires at the Seseña landfill in Toledo, Spain, which is one of the largest landfills in Europe. The most common and significant groups of PAHs in the examined samples from this area were those with 3–5 rings and their alkyl-derivatives (Escobar-Arnanz et al. 2018). After a tire landfill fire in Lithuania, it was observed that the levels of total PAHs were considerably higher within the fire zone compared to the surrounding soils which exhibited significantly lower concentrations. In addition, high levels of Zn were found in the fire zone, along with increased levels of Cr, Ni, and Cu (Raudonyte-Svirbutaviciene et al. 2022). On the other hand, no PAH compounds were identified in the leachate during or after a spontaneous landfill fire in Western Norway (Øygard et al. 2005). It has been reported that even soil samples collected long time after the initiation of the landfill fire contain substantial levels of hazardous organic compounds, specifically PAHs. For example, four years after a landfill fire in Trzebinia, located in southern Poland, it was reported that the soil samples obtained from the site had a significant concentration of PAHs (Rykała et al. 2022).

Several crucial aspects contribute to the level of risk associated with PAH exposure, including exposure route, exposure duration, quantity of the substance and the potential interactions with other chemicals that the body may be exposed to (Kim et al. 2013). Soil has been identified as the primary sinks and reservoir for PAHs in terrestrial ecosystems owing to their capacity for adsorbing soil organic matter and their rapid binding with suspended particles. Thus, it is not unusual to observe and detect these chemicals in soil samples taken from both remote industrial areas and residential areas (Perra et al. 2011; Tipmanee et al. 2012). Furthermore, the potential harmful health impacts can arise from the accumulation of PAH chemicals in soil as a result of pollution in food chains (Jiang et al. 2011). PAHs have the ability to enter the body through various routes, including the respiratory and digestive systems, as well as through direct contact with substances such as soot and tar on the skin (Kim et al. 2013). Persistent organic compounds, such as PAHs are lack of associated carcinogenic or mutagenic properties. However, their metabolites have been shown to exhibit such effects (White 2002).

Microplastics

Landfills are used as reservoirs for the accumulation of plastic waste originating from households and industrial sources (Zhu et al. 2023). There has long been suspicion that MPs from landfills are discharged into the environment; their formation, accumulation, and release in landfills are long-term procedures (He et al. 2019). The MPs can be classified into two distinct classification, namely primary and secondary, which are determined by their particular formation processes. Primary MPs are synthetically produced MPs utilized in the manufacturing of pharmaceuticals, personal care items, plastic goods, textiles, etc. (Atugoda et al. 2021). Secondary MPs are generated through the disintegration of bigger plastic waste, including bottles, bags, and packaging materials (Chamanee et al. 2023). These MPs eventually, not only infiltrate into the soil by runoff but also float in the air and settle across a larger area of land (Amato-Lourenco et al. 2021). For example, MP films and foams mostly originate from the degradation of plastic bags and packing supplies, which are vital commodities in the everyday routines of humans (Zhou et al. 2020).

Based on an initial assessment, it has been determined that approximately 79% of the municipal solid waste found in landfills can be attributed to its plastic composition (Chamanee et al. 2023). The majority of these plastic wastes are disposed in landfills, where they undergo a process of gradual degradation over an extended period of time. Under certain environmental conditions, plastic materials experience substantial chemical alterations that cause them to lose some of their properties. The degradation process is greatly influenced by the polymeric characteristics. These characteristics include substituents present in the structure, mobility, functional groups, crystallinity, molecular weight, and additives that have been incorporated into the polymers. It is still unclear what happens to these polymers in landfills and how long it will take for them to completely mineralize into CO2. Polymers have the potential to undergo degradation through chemical, photodegradation, and biological processes, resulting in the potential emergence of secondary MPs pollution (Wojnowska-Baryła et al. 2022).

The environmental conditions in landfills can cause plastics to break down into smaller particles known as secondary MPs. The degradation processes that lead to the production of secondary MPs in landfills are influenced by the location of the plastic. The degradation of particles on the surface is primarily caused by the strong adsorption and scattering of UV rays. On the other hand, particles located in deeper layers of the landfill are degraded due to the acidity that is leached out and the chemical activity of compounds existing in those layers (Teuten et al. 2009). In addition to that, primary MPs are also found prominently in landfills due to the waste generated by households. Primary MPs are emitted directly from their sources, such as synthetic fibers originating from clothing and polymers in the form of microbeads and pellets. These MPs are commonly found in various personal care, pharmaceutical, and industrial products (Cole et al. 2011). Large specific surface area, and considerable hydrophobicity are the most notable characteristics of MPs, which facilitate their absorption of pollutants present in the leachate (Chamanee et al. 2023). Therefore, considerable scientific interest has been directed towards the process of leaching of MPs and the pollutants they contain from landfills, primarily driven by concerns regarding environmental contamination, the potential for bioaccumulation, and the detrimental effects on biota (Issac & Kandasubramanian 2021). The chemical compounds emitted through the process of plastic degradation have the potential to disperse into the surrounding environment, resembling a landfill leachate. The extent of this diffusion is dependent upon the pore size of the plastic material and the molecular size of the added substances (Wong et al. 2007). The leachate originating from landfills is identified by a significant presence of several pollutants, including MPs (Su et al. 2019). Thus, landfill leachate becomes a potentially significant contributor of MPs to the surrounding environment, given that leachate is frequently released into the environment without undergoing any form of treatment in the majority of instances (He et al. 2019). Although there are reviews discussing the MPs abundance in landfill leachate, there has only been a limited quantity of scientometric analysis of MPs in landfill leachate (Chamanee et al. 2023).

The few studies that investigated the presence of MPs in landfills and surrounding areas in Iran, identified high amounts of MPs. The mean concentration of MPs in shallow and deep soils in residential areas adjacent to the Tehran landfill was estimated to be 76 ± 34.98 and 24.7 ± 19.79 particles/kg soil, respectively. The predominant MP particles observed in this region were identified as fragment-shaped, having a particle size ranging from 0.1 to 0.5 mm (Shirazi et al. 2022). The degree of pollution by MPs in the soils surrounding two landfills and a municipal solid waste transfer station in Ahvaz City has been identified to be higher than the levels found in many similar landfills and MSW transfer stations globally. This area revealed an elevated quantity of MPs at 325.9 ± 26.8 items/100 g soil (Mahdavi Soltani et al. 2022). It has been reported that landfill site nearby Bushehr port has the potential to discharge significant levels of MPs into the surrounding environment if adequate protection strategies are not implemented. This study identified the presence of MPs smaller than 25 μm in leachate (Mohammadi et al. 2022). The mean concentration of MPs in shallow and deep soil samples from the Tehran landfill was found to be 863 ± 681 and 225 ± 138 particles/kg soil. In this investigation, MPs mainly existed in the form of films, white and black in color, with sizes ranging from 0.1 to 0.5 mm (Shirazi et al. 2023).

From other parts of the world, the mean concentrations of MPs in leachate and refuse taken from landfill sites in Shanghai, China, were found to be 8 ± 3 items/L and 62 ± 23 items/g, respectively (Su et al. 2019). Similarly, the mean concentration of MPs in soil samples and leachates at dump sites situated near the Gulf of Thailand was determined to be 1457.99 ± 489.71 Items/1kg and 20.90 ± 4.96 Items/1kg, respectively (Puthcharoen & Leungprasert 2019). Soil adjacent to Landfills in the Republic of Korea had mean MP concentrations of 73.4 MPs(ea)/kg and 97.8 MPs(ea)/kg. In this study area three primary forms of MPs, namely fragments, fibers, and films were identified. Additionally, a substantial level of secondary plastics was found (Kim et al. 2023). The soil samples collected from the Solid Waste Landfills in the Akmola Region of North Kazakhstan revealed ultimate retention of MP particles. These particles were found to be primarily caused by rainwater runoff and leachate (Salikova et al. 2023). Soil samples taken from Aminbazar Sanitary landfill sites in Bangladesh indicated an abundance of MPs in various forms of high-density polyethylene, and cellulose acetate as well as low density polyethylene (Afrin et al. 2020).

Approximately 25% of the plastic generated in Europe ultimately ending up into landfills. It has been observed that the concentration of MPs in the Lapes regional landfill in Lithuania at a depth of 10–20 m reaches as high as 55 items/g or 52.8 g/kg (Sholokhova et al. 2023).

The accumulation of MPs within soil aggregates can have significant impacts on the biological, chemical, and physical characteristics of the soil. Furthermore, it can impact the accuracy of estimating soil carbon sequestration (Rillig et al. 2017; Moreno & Moreno 2008; Atuanya et al. 2012; Santos et al. 2017). In addition, the research conducted by Atuanya et al. (2012) revealed that the introduction of plastic granules into soil resulted in a notable rise in the overall organic carbon content of the soil. This can be attributed to the fact that existing methods utilized to measure soil organic carbon also considered for the detection of invisible MPs within soil aggregates, such as polystyrene or polyethylene, which are composed of approximately 90% carbon. Accordingly, Rillig (2018) suggested the necessity for a reassessment of the true level of soil carbon storage in soils contaminated with plastic.

As previously pointed out, the interaction between MPs and soil's reactive components, as well as its primary extracellular biological molecules can have an influence on soil functionality. This, in turn, it has the potential to directly impact soil fertility, subsequently influencing crop yield and quality. Nevertheless, the environmental relevance of MPs in soil has only been confirmed by a limited number of studies. Microplastics have the potential to interact with different functional groups existing in the dissolved fraction, which could be attributed to inorganic contaminants (Chen et al. 2017; Sun et al. 2017). As a result of this process, organic complexes with high toxicity that have the ability to migrate within the soil profile will be formed (Chen et al. 2018). Additionally, humic substances have been shown to interact with MPs, as well as PTEs such as Pd and Cd, when they were adsorbed onto a clay mineral referred to bentonite (Zhang et al. 2015). Furthermore, the interactions among MPs and organic compounds are determined by the age of the plastic. However, conflicting variations in reactivity have been found between aged and the relative original plastics (Liu et al. 2019). The interactions between MPs and organic matter can have effects on the availability of nutrients to organisms in soil. This can be identified through a reduction in the concentrations of dissolved organic N and dissolved organic P forms (Liu et al. 2017).

Conclusions and future directions

An improper disposal of municipal solid waste, such as directly depositing in landfills or open dump areas without proper segregation and treatment, is a significant global environmental concern which leads to soil pollution by PTEs, PAHs, and MPs. This review focuses on the scientific studies conducted in Iran that investigated the chemical analysis of soils impacted by dumpsites and non-sanitary landfills. Since the studies evaluated in this review only focused on a limited number of dumpsites and non-sanitary landfills in Iran, it can be presumed that other inadequate disposal areas in the country might also be impacting soils and causing risks to the environment and public health.

The significant rise in waste deposition in landfills leads to the migration of PTEs towards the adjacent regions, resulting in soil contamination and subsequent absorption by plants. The accumulation of PTEs in soil leads to a decline in soil quality, reduced soil fertility, groundwater contamination, and irreversible harm to soil biota. Based on the published studies from Iran, soils in landfill sites were mostly pollutant by Cd and Pb. PAHs can be emitted from landfill sites into soils through various routes, such as leachates and fires involving burning tires and biomass ashes. To date, various investigations have been conducted to evaluate the presence of PAH compounds in diverse environmental settings, including water, dust, sediments, soils, sewage sludge samples, and atmospheric deposition. However, limited research has been undertaken to identify the sources, determination, and assessment of health risks associated with PAHs in surface soil samples taken from nearby regions of municipal solid waste landfills. These compounds are important for evaluation because of their resistance to biodegradation, susceptibility to bioaccumulation and also carcinogenic, teratogenic, genotoxic, and mutagenic potentials. Landfill leachate has become a substantial reservoir of MPs because of the large amount of plastic waste accumulated from municipal and industrial sources in landfills. The discharge of MPs originating from landfill leachate into the surrounding environment can potentially result in negative impacts for humans and biota. Nevertheless, the issue of landfills as a pollution source and hotspot for MPs have received less attention in Iran and worldwide compared to other pollutants.

Variations in pollutant concentrations in different regions influenced by landfill types and their management techniques, highlight the importance of comprehending local contexts when implementing mitigation strategies. While studies and the presence of PTEs appears to be more pronounced in Iran, there is limited research on PAHs and MPs. Thus, further investigations are required to enhance existing understanding of the management of municipal solid waste without jeopardizing soil resources. For future studies related to contamination caused by inadequate municipal solid waste disposal areas, the following recommendations should be considered:

It is recommended for future research endeavors to undertake a comprehensive assessment of the physico-chemical and biological characteristics in order to estimate the level of PTEs in the soil surrounding landfills in Iran.

The presence of soil pollution in the vicinity of landfills indicates the potential for food contamination, which poses a threat to human health. Therefore, it is imperative to carry out consistent monitoring and establish an increased level of awareness for future research endeavors.

In order to enhance our understanding and management of MPs pollution in landfills, both in Iran and globally, it is crucial to fully understand all sources of MPs and their corresponding fate and degradation pathways/ mechanisms.

The issue of PAHs being released from landfills into soil has received limited attention in Iran and many other countries across the world. Therefore, it is extremely important to prioritize further studies on the distribution and transformation of PAHs and their derivatives in landfills.