Advancements in Composting Technologies for Efficient Soil Remediation of Polycyclic Aromatic Hydrocarbons (PAHs): A Mini Review

Abstract

The release of polycyclic aromatic hydrocarbons (PAHs) into the environment has become a serious concern with rapidly increasing human activities. PAHs are one of the hazardous pollutants generated primarily from the incomplete combustion of fossil fuels, industrial emissions, and the expenditure of vehicles. These toxic compounds are very dangerous to ecosystems and human health due to being persistent, bioaccumulative, and carcinogenic. Composting is considered a form of bioremediation for eliminating PAHs in contaminated soils. The method utilizes microbial communities to break down organic pollutants and is low-cost and environmentally friendly. The efficiency factor depends on many aspects, including soil pH, oxygen, temperature provision, and the diversity of microbes, among others. Thermophilic conditions help in the decomposition of both low- and high-molecular-weight PAHs. This paper focuses on the effectiveness of composting as a bioremediation technology for remediating PAH-contaminated soils and its impact on the environment and human health. Due to its safety and high efficiency, composting should be improved and prioritized for its widespread application as a principal remediation technology for PAH pollution at the earliest opportunity.

1. Introduction

Over the past few decades, anthropogenic activities have caused major problems for the environment, leading to pollution and a general degradation of environmental quality. Soil, one of the most essential components of terrestrial ecosystems, plays a critical role in supporting agriculture, regulating water flow, and sustaining biodiversity. However, it is particularly vulnerable to contamination due to excessive agricultural inputs, industrial discharges, improper waste disposal, and urban development [1]. Among the most concerning soil pollutants are heavy metals, petroleum hydrocarbons, and polycyclic aromatic hydrocarbons (PAHs)—toxic organic compounds that are widely present in contaminated soils.

Table 1. Classification of PAHs.

PAH Name	Molecular Formula	Number of Rings	Common Sources	Toxicity and Concerns
Naphthalene	C10H8	2	Mothballs, cigarette smoke, fossil fuels	Possible human carcinogen, respiratory irritant
Acenaphthylene	C12H8	3	Incomplete combustion, vehicle exhaust	Toxic to aquatic organisms
Acenaphthene	C12H10	3	Coal tar, wood preservatives	Possible skin and eye irritants
Fluorene	C13H10	3	Coal tar, vehicle emissions	Can bioaccumulate in organisms
Phenanthrene	C14H10	3	Crude oil, coal combustion	Potentially harmful to aquatic life
Anthracene	C14H10	3	Wood combustion, coal tar	Low acute toxicity but can cause skin irritation
Fluoranthene	C16H10	4	Vehicle exhaust, industrial emissions	Possible carcinogen, environmental pollutant
Pyrene	C16H10	4	Cigarette smoke, fossil fuels	Can cause DNA damage
Benzo [a]anthracene	C18H12	4	Charred foods, industrial processes	Known carcinogen
Chrysene	C18H12	4	Diesel exhaust, crude oil	Possible human carcinogen
Benzo [a]pyrene	C20H12	5	Grilled meat, tobacco smoke	Strong carcinogen, mutagenic
Dibenz [a,h]anthracene	C22H14	5	Fossil fuel combustion, tobacco smoke	Highly carcinogenic
Indeno [1,2,3-cd]pyrene	C22H12	6	Vehicle emissions, coal burning	Probable human carcinogen
Benzo [g,h,i]perylene	C22H12	6	Industrial combustion, waste burning	Persistent environmental pollutants

shows common types of PAHs that persist in the environment.

PAHs are considered priority pollutants due to their carcinogenic, mutagenic, and teratogenic effects on humans and animals. They can enter the food chain through plant uptake or leaching into water bodies and are associated with numerous health conditions, including cancer, immune suppression, and reproductive toxicity [2]. Their high molecular weight and hydrophobicity make them particularly persistent in soils, where they bind strongly to organic matter and resist natural degradation. As a result, PAHs must be remediated promptly and effectively to minimize their environmental and health risks.

Several methods are currently employed to treat PAH-contaminated soils, including physical (excavation and soil washing), chemical (oxidation and surfactant flushing), thermal (incineration), and biological approaches. Biological treatment, especially composting, has gained prominence due to its low cost, environmental safety, and potential to enhance soil health post-treatment [3]. Composting utilizes microbial activity under controlled aerobic conditions to decompose organic matter and contaminants, converting them into stable humus-like substances. Compared to other techniques, composting is more economical—typically costing USD 50 to USD 140 per ton, whereas slurry-phase bioremediation costs around USD 170 per ton [4,5].

Despite its advantages, the efficiency of composting in PAH remediation is highly variable, depending on several factors such as microbial community structure, temperature, moisture, pH, oxygen availability, and nutrient balance. While previous studies have confirmed the potential of composting for degrading PAHs, many have focused narrowly on the general composting process without providing comprehensive evaluations of its applicability for different types and concentrations of PAHs, especially in diverse soil environments [6,7].

A comprehensive review of composting for PAH-contaminated soil remediation is essential due to the growing prevalence of PAH pollution and the limitations of existing treatments. While composting offers a low-cost, eco-friendly solution, its effectiveness varies with soil type, PAH composition, and operational conditions. The current literature lacks a focused synthesis on optimizing composting parameters for diverse PAH scenarios. This mini review addresses that gap by evaluating recent advancements and highlighting critical factors that influence composting efficiency, guiding future applications and research. Another underexplored area is the integration of innovative reactor technologies, such as in-vessel composting systems or semi-continuous reactors, which could offer improved control over key environmental variables and thereby increase the efficiency and scalability of PAH remediation [8]. Moreover, few reviews have examined the economic viability, energy input–output balance, or long-term environmental impacts of large-scale composting applications in contaminated land restoration.

Other real-world issues include scalability, regulatory constraints, and public uptake, which are often assumed in most current studies. As a matter of fact, these real-world concerns are crucial for scaling up laboratory-scale findings to come up with applicable remediation strategies [9]. This paper fills that gap by providing evidence of the facilitators and barriers for composting to be considered a reliable and sustainable method for remediating soil contaminated with PAH. This mini review aims to bridge these gaps by exploring recent advancements in composting technologies specifically tailored for the remediation of PAHs in soil. It highlights innovative strategies, examines microbial and environmental factors influencing the composting process, and provides insights into future prospects for upscaling composting applications while maintaining environmental and economic sustainability.

2. PAHs’ Characteristics and Source

PAHs are a ubiquitous group of hundreds of chemically related compounds that are highly persistent in the environment with various structures and toxicities, as well as poor water solubility [7]. The classification of PAHs is shown in Table 1. The structure of PAHs is composed of at least two aromatic rings with a pair of carbon atoms shared between the rings. Small PAHs consist of two to six aromatic rings, while any PAHs with more than six rings are considered large PAHs [4]. Two of the simplest forms of PAHs are phenanthrene and anthracene, which contain only three fused aromatic rings [10]. PAHs are commonly produced during combustion processes, whether it be anthropogenic activities or natural occurrences. Anthropogenic activities include open burning of household wastes, the incineration of scheduled wastes, petrol combustion, diesel combustion, and many more [8]. Natural occurrences include volcano eruptions and forest fires. These combustion processes lead to the production of PAHs in the environment. Furthermore, one of the main reasons that PAHs are hard to remove is their persistence in water [9]. This can cause serious environmental contamination issues because PAHs become insoluble in water [11]. This characteristic ultimately causes contaminants to sink into the soil. In addition, PAHs can be harmful to human health, and many of them consist of toxic, carcinogenic, and mutagenic properties, posing large threats to human health. Due to the high lipid solubility properties of PAHs, they can be easily absorbed by mammals via the gastrointestinal tract and rapidly distributed throughout the tissue cells in the human body [12].

There are a lot of activities that contribute to PAHs in the environment. Some of the most common PAH sources in the environment are pyrogenic, petrogenic, and biological processes [13]. Pyrogenic sources can be easily found during and after the pyrolysis process. PAHs are formed due to organic matter being excessively exposed to high temperatures with little to zero oxygen. Examples of activities of pyrolysis are the distillation of coal into coal tar and the formation of lighter hydrocarbons via the thermal cracking of petroleum residuals [14]. These activities are considered intentional processes. On the other hand, unintentional processes contribute to pyrogenic PAHs, such as the incomplete combustion of vehicle fuel, incomplete combustion of forest fires, and incomplete combustion of heat-generating systems. Pyrogenic processes typically occur when temperatures exceed 350 °C [15]. As mentioned previously, when organic matter is exposed to temperatures ranging from 350 °C to 1200 °C, it harms the environment. In some cases, PAHs can be formed at lower temperatures. For instance, they are formed in crude oil at temperatures ranging from 100 °C to 150 °C. These PAH sources are categorized as petrogenic [16]. In addition, petrogenic PAHs are often found in oil spills in the ocean or in freshwater, underground storage tanks, and accumulations of gasoline and motor oil due to incomplete combustion from vehicles. PAHs are reportedly found in petroleum-related products as well. Biological sources are slightly different from pyrogenic and petrogenic sources. For instance, biological PAHs can be produced by vegetative decay when certain plants biodegrade. This type of PAH formation can occur either naturally or due to human activities. PAH sources can be categorized into natural and anthropogenic sources. It is important to identify the source, whether it is natural or anthropogenic. Identifying the sources can help to avoid and mitigate the formation of PAHs [17]. Some common examples of naturally formed PAHs are forest fires, volcano eruptions, the seepage of petroleum into soil, and the decomposition of vegetation. On the other hand, anthropogenic activities contributing to PAHs can be categorized into large-point sources and small-point sources. An example of large-point sources is incomplete combustion processes in industries/incinerators, while an example of small-point sources is the combustion of fuel, which leads to the emission of PAHs and includes vehicle emissions, smoking cigarettes, and open burning of municipal solid wastes [18].

3. Identification and Fate of PAHs in the Environment

PAHs can be difficult to identify without using the correct method. One of the easier methods is to identify real environmental samples based on the PAH content via chemical fingerprinting [19]. Chemical fingerprinting adapts several different techniques to help identify the PAH sources of different groups, such as coal-based and oil-based sources. In addition, chemical fingerprinting can speed up the identification of non-point sources of PAHs. The examination of hydrocarbon rings is a method that can be used to distinguish petrogenic and pyrogenic PAHs. The examination process studies the five-member hydrocarbon rings in the PAHs. The five-member rings can be easily found in petrogenic PAHs because they are more abundant in petrogenic PAHs. The process of the marshaling of the five-member rings in petrogenic PAHs requires a longer period of time [19]. However, pyrogenic PAHs are generally more stable because the source material is converted into a six-member ring instead of a five-member ring. Thus, this can be used to distinguish the type of PAHs in the environment. Temperature is one of the key factors that can easily lead to the identification of PAHs. As mentioned previously, pyrogenic PAHs and petrogenic PAHs can be formed when the temperature is high [18]. Thus, it is easier to identify PAH sources, especially those in industrial stack effluent. This is because stack effluents are formed rapidly due to high temperatures. The pattern of the effluent can change depending on the PAH component. As PAHs harm the environment and human health, it is important to understand how PAHs are dispersed and transported. There are various ways that PAHs can be transported. For example, PAHs can be easily transported into the human body when a person inhales air containing PAHs. Figure 1 shows an overall diagram of how PAHs are formed and how they can be transported.

3.1. PAHs in Soil

PAHs are normally emitted from the incomplete combustion of organic matter. An example of a common source is industrial stack effluent, which emits PAHs into the atmosphere in the vapor phase and in the solid phase. These PAHs ultimately end up being deposited in the soil when rain occurs. PAHs from automotive exhaust can also be carried far to different regions through the air. When PAHs are deposited in soil, they can gain mobility because most PAHs can attach to soil particles. These contaminated soil particles can be easily transferred from one place to another without prior notice that they are contaminated [20,21]. PAHs’ mobility can be affected according to the properties of the subsurface particles in soil. The sorbent particle size and the size of the pores of the soil are key factors affecting mobility. Smaller-pore-size soil particles can restrict PAHs that are sorbed to move through the soil particles, thus minimizing the movement of PAHs [20,22].

3.2. PAHs in Food

PAHs can be found not only in soil and the atmosphere but also in foods. Although the concentration is not high, the cumulative ingestion of foods with PAHs can harm the human body. PAHs can be very toxic. In fact, their toxicity increases when exposed to ultraviolet light [23,24]. PAHs can have a high level of acute toxicity toward marine species and birds. However, PAHs are less harmful to mammals when compared to marine species. PAHs can exist in the environment for a long time as they are highly persistent, and they can be bioaccumulated. Concentrations of PAHs are higher in marine species such as fish and shellfish. Hence, it is important to treat PAHs so that acute disease can be prevented [23,25].

3.3. PAHs in Sediment

PAHs in sediment are usually PAHs that are dispersed and transported through industrial effluents or runoff from rainstorms. In rural areas, these PAHs attach to other atmospheric particles, which then settle onto the surface of oceans, lakes, and rivers. PAHs are then transported to other areas and become integrated with sediments at the bottom of oceans, lakes, and rivers. In contrast, PAHs in urban areas mostly come from industrial sewage effluents [26,27]. A previous study has shown that PAHs are sorbed to particles that settle and become sediment. Unlike soil surfaces, PAHs can lose mobility when incorporated into sediments due to non-polar structures inhibiting them from dissolving in water. However, PAHs are partially soluble, especially those with a lower molecular weight. Therefore, a minimal number of PAHs can still dissolve and mix with water, which can be harmful when ingested [27,28]. The sources, transport, deposition, and environmental impact of PAHs in soil, food, and sediment is shown in

Table 2. Sources, transport, deposition, and environmental impact of PAHs in soil, food, and sediment.

Category	Sources	Transport and Deposition	Environmental Impact	Refs.
PAHs in Soil	Incomplete combustion of organic matter; industrial stack emissions; automotive exhaust	Atmospheric transport (vapor and solid phases), deposition via rainfall, sorption to soil particles, transport through soil depending on pore size and particle characteristics	Persistent contamination of soil; mobility varies with soil texture; poses a risk of spreading contamination	[20,21,22]
PAHs in Food	Deposition from air and soil into agricultural products; bioaccumulation in marine organisms	Ingestion through contaminated foods (fish, shellfish); UV exposure increases toxicity	Toxic to humans and wildlife; bioaccumulative and persistent; especially harmful to marine species and birds	[23,24,25]
PAHs in Sediment	Industrial effluents; runoff; atmospheric deposition in rural areas; sewage in urban areas	Settling onto water surfaces; sorption to particles; incorporation into bottom sediments	Reduced mobility due to non-polar structure; still partly soluble—low-molecular-weight PAHs may dissolve and pose health risks	[26,27,28]

.

4. Composting

Composting is a type of biological treatment that hastens the degradation process by providing extra nutrients to microorganisms. Some microorganisms involved in PAH biodegradation during composting are summarized in

Table 3. Microorganisms involved in PAH biodegradation during composting.

Microorganism Group	Example Species	Role in PAH Biodegradation	Citation
Bacteria	Pseudomonas putida, Mycobacterium sp., Sphingomonas sp., Rhodococcus sp., Bacillus sp., Acinetobacter sp., Arthrobacter sp.	Capable of degrading both low- and high-molecular-weight PAHs through enzymatic oxidation. Pseudomonas and Sphingomonas produce dioxygenases that initiate PAH breakdown. Mycobacterium can degrade persistent PAHs such as benzo [a]pyrene.	[31,32]
Fungi	Phanerochaete chrysosporium, Aspergillus sp., Trametes versicolor, Fusarium sp., Penicillium sp., Pleurotus ostreatus	Extracellular ligninolytic enzymes such as laccase, manganese peroxidase, and lignin peroxidase are produced, which break down PAHs. White-rot fungi (Phanerochaete chrysosporium and Trametes versicolor) degrade high-molecular-weight PAHs effectively.	[31,33]
Actinomycetes	Streptomyces sp., Nocardia sp., Micromonospora sp., Thermomonospora sp., Rhodococcus sp.	Break down complex PAH structures under aerobic conditions by producing hydrolytic enzymes and biosurfactants. Rhodococcus species can metabolize PAHs in extreme conditions.	[34,35]
Yeast	Candida sp., Rhodotorula sp., Yarrowia lipolytica, Saccharomyces cerevisiae, Trichosporon sp.	Produce biosurfactants that increase PAH solubility and enhance microbial access. Yarrowia lipolytica can degrade phenanthrene and pyrene effectively.	[33,36]

. The treatment process can be defined as aerobic as it requires oxygen to complete it. Other factors affecting the composting efficiency are moisture content, porosity, and temperature [25]. Composting has been widely used for many types of treatment, such as the biodegradation of solid wastes, agricultural wastes, sewage sludge, and many more. In recent years, composting has been studied and shown to be effective in removing PAHs in contaminated soil. An example of composting is mushroom compost. Mushroom compost uses chicken manure, wheat straw, and gypsum in a manufactured gas plant (MGP) soil with a thermal insulation composting chamber installed. To obtain the best results from composting, the temperature and oxygen content should be optimal to obtain a good removal rate of PAHs. The degradation process of PAHs can take up to 54 days to reduce the PAH concentration, ranging from 20% to 60%, while the removal rate can range from 37% to 80% for a 100-day compost process. Compost was also found to be effective in degrading PAHs with three to four rings of hydrocarbon and even more effective for PAHs with five and six rings of hydrocarbon [29,30].

4.1. Bioremediation of PAHs by Composting

Composting technology is known to be one kind of ex situ bioremediation and among those that have great potential for the treatment of contaminated soils, especially when they are contaminated with persistent organic compounds, such as PAHs. In the last few decades, composting has received more and more attention because it is relatively cheap to operate in terms of money, it is environmentally sustainable, and it is proven to degrade organic pollutants. Through this method, not only are wastes broken down, but it also creates healthy valuable compost from wastes, which is a step towards circular waste management. Composting was proven in numerous studies to be able to accelerate the microbial degradation of PAHs, by means of the enrichment of the microbial population and an improvement in physicochemical properties. This is supposed to be a less energy-intensive chemically based remediation approach. In the future, through integrated applications, the treatment of organic waste together with composting will pave a new way for remediating PAH-contaminated soils that is cleaner and greener. This synergy would work from the aspect of waste management by diverting organic waste from landfills and by increasing biodegradation capacity via the stimulation provided by the composting matrix. The success of composting in the remediation of PAH-contaminated soils has been attributed to the optimization of key environmental factors that govern microbial activity. These factors are soil pH, moisture content, temperature, oxygen availability, and nutrient levels. If managed properly, the systems within composting would generate a way to create the optimum conditions for these microorganisms to proliferate—to degrade PAHs, thereby increasing the hydrophobic compounds’ bioavailability and improving overall degradation kinetics. Composting has demonstrated an exceptional potential for the rectification of certain drawbacks that are intrinsic to other bioremediation techniques, such as their shorter treatment times and poor pollutant bioavailability. This can increase microbial diversity and enzymatic activity by providing organic amendments to accelerate the transformation of PAHs into less toxic or harmless compounds. It is increasingly being considered as a viable, scalable, and eco-efficient method for the remediation of contaminated soils, especially in scenarios prioritizing sustainability and resource recovery [32,33].

4.2. Factors Affecting Composting

Some factors affecting the composting process of PAHs are temperature, oxygen supply, moisture content, nutrient availability, PAH properties, microbial community, pH level, and bioaugmentation (

Table 4. Factors affecting the composting process.

Factor	Description	Elaboration	Reference
Temperature	Influences microbial activity, enzymatic function, and PAH degradation. Optimum temperatures enhance biodegradation.	- Composting phases: Mesophilic (moderate temperature), thermophilic (high temperature), and curing phase. - Higher temperatures increase enzymatic activity, accelerating PAH breakdown. - Prolonged thermophilic conditions improve total petroleum hydrocarbon (TPH) removal.	[37,38]
Oxygen Supply	Essential for aerobic microbial activity and PAH decomposition. Oxygen availability affects biodegradation rates.	- Aerobic conditions enhance PAH degradation, while oxygen limitation can slow the process. - Oxygen supply needs to be balanced to prevent excess CO2 buildup and anaerobic zones.	[39]
Moisture Content	Affects microbial metabolism and enzymatic activity in composting.	- Optimal moisture range (50–60%) is required for microbial activity. - Excess moisture leads to anaerobic conditions, slowing degradation. - Insufficient moisture reduces microbial activity, affecting PAH breakdown.	[21]
Nutrient Availability	Nutrients (C:N:P ratio) influence microbial growth and degradation efficiency.	- Carbon, nitrogen, and phosphorus ratios impact microbial metabolism. - Imbalance leads to slow microbial growth and incomplete PAH degradation. - Proper compost amendments (e.g., manure and green waste) enhance PAH removal.	[32]
pH Levels	pH influences microbial activity and enzyme function in composting.	- Optimal pH (6.5–8.0) promotes microbial activity. - Acidic or highly alkaline conditions inhibit enzymatic processes and microbial degradation of PAHs.	[37]
Physical and Chemical Properties of PAHs	PAHs’ molecular weight and solubility affect their degradation efficiency.	- Low-molecular-weight PAHs degrade faster than high-molecular-weight PAHs due to better solubility. - Hydrophobic PAHs resist microbial degradation, limiting bioavailability. - Requires surfactants or co-composting agents to improve solubility.	[21,40]
Microbial Community	Diversity and adaptability of microbes determine PAH biodegradation success.	- Native vs. introduced microbes affect degradation rates. - Bioaugmentation with specialized PAH-degrading microbes can enhance efficiency. - Competition between indigenous and exogenous microbes can impact success.	[31,32]
Bioaugmentation	Adding specific microorganisms to accelerate PAH biodegradation.	- Enhances PAH removal efficiency in cases in which indigenous microbes are ineffective. - Example: Candida Catenulate CM1 increased hydrocarbon removal from 48% to 84%. - Potential barriers include competition with native microbes and environmental adaptation issues.	[31,32]

). A detailed understanding of each factor is essential to optimize the composting process.

4.2.1. Temperature

Temperature is one of the most crucial factors affecting composting efficiency. Composting with an optimum temperature can enhance the biodegradation process. This is because temperature can affect the compost’s metabolic activity, solubility, diffusion rate, and bioavailability [33]. As solubility increases along with temperature, this decreases oxygen solubility, which causes the metabolic activity to decrease. In addition, the optimum temperature can help to determine the type of dominant microorganisms and the enzymatic activity during the degradation of PAHs [33]. In composting, there are three important phases: the mesophilic phase, the thermophilic phase, and the curing phase. These phases are sensitive to the surrounding temperatures and react differently based on the temperature. For instance, the mesophilic and thermophilic phases have been found to change the enzymatic activity of the microorganism due to temperature changes. As the temperature increases, the enzymatic activity of microorganisms also increases. This promotes the degradation rate of hydrocarbons from PAHs [34,41]. Furthermore, the thermophilic phase has been studied to enhance PAHs’ biodegradation. A previous study stated that the composting of hydrocarbon-contaminated sediments was used with different organic co-substrates, and a higher temperature was achieved to maintain the thermophilic temperature longer, which resulted in a greater total petroleum hydrocarbon (TPH) removal rate. Regarding the observations above, it can be seen that temperature is impacting the removal rate of PAHs in contaminated soil due to the physiochemical properties of PAHs [41,42].

4.2.2. Oxygen Supply

Adequate oxygen is essential for sustaining aerobic microbial activity, which drives the degradation of PAHs during composting. Oxygen availability directly affects the rate of biodegradation—under aerobic conditions, PAHs are broken down more effectively, whereas oxygen-deficient conditions can slow the process or even shift it toward inefficient anaerobic pathways [32]. Continuous monitoring and controlled aeration are necessary to avoid the formation of anaerobic zones and the buildup of excess CO₂, which could disrupt the composting environment. A balanced oxygen stream confirms the degradation process is effective while maintaining microbial viability and activity when composting [43].

4.2.3. Moisture Content

Moisture plays an important role in microbial metabolism and enzymatic reactions. An optimal moisture range of 50–60% is considered ideal for composting, supporting microbial activity without creating excess water that might reduce aeration. If moisture levels exceed this range, anaerobic conditions can develop, leading to unpleasant odors and reduced degradation rates. Conversely, insufficient moisture limits microbial growth and slows enzymatic functions, impeding PAH breakdown [23]. Maintaining moisture within the optimal range requires regular monitoring and may involve adjusting the mix of composting materials or adding water or dry bulking agents as needed [42,43].

4.2.4. Nutrient Availability

The availability of essential nutrients—particularly carbon, nitrogen, and phosphorus—significantly influences microbial growth and the effectiveness of composting. A balanced C:N:P ratio supports efficient microbial metabolism, which is key to PAH degradation. Imbalances, such as excess carbon or nitrogen deficiency, can lead to slow microbial proliferation and the incomplete decomposition of contaminants. Amendments like manure, green waste, or other organic additives can help restore the balance, enhance microbial diversity, and promote more a complete degradation of PAHs [34,40]. Nutrient supplementation should be tailored to the specific characteristics of the composting materials and the contamination profile of the soil [42,43].

4.2.5. pH Level

While not elaborated in the above data, pH remains an important factor and should be mentioned here for completeness. The optimal pH range for microbial activity in composting is typically between 6.5 and 8. A pH outside this range may inhibit microbial growth or shift community structure away from those species most effective at degrading PAHs. Monitoring and adjusting pH—through lime, sulfur, or buffer amendments—can help maintain an ideal environment for biodegradation [34,42].

4.2.6. Physicochemical Properties

The physical and chemical properties of PAHs have a large effect on the biodegradation rate. For example, the microbial assimilation and biodegradation of PAHs are dependent on their solubility [35]. However, most organic compounds, including PAHs, have poor solubility, especially when the molecular weight increases, increasing the hydrophobicity and affecting the composting efficiency. Many previous studies have shown that the biodegradation of PAHs in soil is much more effective when the molecular weight is negligible. At the same time, the removal rate of contaminants decreases as the molecular weight increases. In addition, high-molecular-weight PAHs were observed to have a removal rate of less than 50% [35,42].

4.2.7. Microbial Community

The diversity and adaptability of microbial communities are crucial for the successful biodegradation of PAHs in composting systems. Indigenous (native) microbial populations often play a key role in PAH degradation; however, their efficiency may be limited in some contaminated soils. In such cases, bioaugmentation—the introduction of specialized exogenous microbes with known PAH-degrading capabilities—can enhance the remediation process [36,44]. While this approach has shown promise, its effectiveness can be hampered by competition between native and introduced microbes. Indigenous microbes may outcompete the added strains for nutrients or ecological niches, thereby reducing the success of bioaugmentation. A balanced microbial ecology that supports both diversity and adaptability is essential to maximize composting efficiency and the long-term stability of PAH degradation [36,45].

4.2.8. Bioaugmentation

Bioaugmentation is used when native microbial populations in composting are insufficient to effectively degrade pollutants such as PAHs. This technique involves introducing specialized external microbial strains known for their strong metabolic capabilities to break down complex organic contaminants [36,44]. The added microbes are expected to enhance the degradation process by supplementing or replacing the limited indigenous activity. However, various studies have indicated that the actual impact of bioaugmentation on improving PAH degradation in composting systems can be minimal. Factors such as competition between native and introduced microbes, environmental constraints, and the adaptability of microbial strains can limit its effectiveness. While improvements in pollutant removal are sometimes observed, they are not always significant, raising questions about the reliability and consistency of this approach in practical applications [36,45].

Composting does not work as an individual factor in the degradation of PAHs, but rather as multiple interactions between several environmental and biological parameters. For example, temperature affects microbial growth and enzyme activity; more precisely, it also depends on the availability of oxygen since aerobic conditions are required for high-temperature microbial metabolism to prevail. In the same way, moisture content should be maximized for nutrients to diffuse and microbes to move, but at excessive levels, it reduces aeration, thereby creating anaerobic zones which then inhibit PAH biodegradation. The nutrients available should readily provide a good balance between carbon and nitrogen to facilitate microbial synthesis without causing toxicity. pH is also important since it affects the solubility of PAHs as well as enzyme stability. The molecular weight and hydrophobicity of PAH compounds influence their bioavailability and degradation pathways. Further factors are the microbial community composition and strategies such as bioaugmentation because they further interact with these factors. The optimization of conditions for specific microbial consortia results in enhanced degradation [31,32,37,38,40].

5. Challenges Faced by Composting

As mentioned and discussed in the previous section, composting is a biological remediation technique, and it is a more environmentally friendly practice to remediate contaminated soils than other physical and chemical techniques. Composting is also known as an ex situ treatment technology. In addition to its function as an alternative for waste management to reduce the weight and volume of organic waste, it has also been widely used to address heavy metal contamination in agricultural soils [25]. Even though composting has been primarily recognized globally for its capability to remediate contaminated soils, composting practices have a few drawbacks, resulting in non-fully treated contaminated soils (

Table 5. Challenges faced by composting in soil remediation.

Challenge Category	Description	Reference
Limited Degradability	Some inorganic substances, such as heavy metals (e.g., lead, cadmium, and mercury) and radionuclides, cannot be biologically degraded. While certain metals may be immobilized or adsorbed onto organic matter, they remain in the environment in different chemical forms, posing a risk of leaching into groundwater or becoming bioavailable under changing soil conditions.	[25]
Microbial Degradation Factors	The efficiency of PAH degradation depends on microbial activity, which is influenced by various environmental conditions. Key factors include temperature (optimal microbial activity occurs between mesophilic and thermophilic conditions), pH levels (affecting enzyme function), moisture content (required for microbial metabolism), oxygen levels (aerobic vs. anaerobic degradation pathways), nutrient availability (carbon, nitrogen, and phosphorus balance), and contaminant toxicity (some PAHs and other chemicals inhibit microbial growth).	[31]
Different Degradation Rates	The rate of contaminant breakdown varies depending on chemical structure and environmental conditions. For example, low-molecular-weight PAHs (e.g., naphthalene) degrade faster in aerobic conditions, while high-molecular-weight PAHs (e.g., benzo [a]pyrene) require specialized microbial consortia. Highly chlorinated compounds, such as polychlorinated biphenyls (PCBs), degrade more efficiently under anaerobic conditions before undergoing aerobic oxidation.	[38]
Cost and Resource Demand	Composting is low in cost as a remedial method, though the actual cost will depend on the ratio of compost to soil used, level of contamination, and cost of the amendments. The aeration and monitoring add to the cost, but it is still cheaper compared to incineration or chemical treatment. In the form of optimization and scaling, composting can result in quite significant savings and sustainability, particularly in low-resource settings when the specific conditions are good.	[46,47]
Operational and Scaling Issues	The logistics and environmental challenges of scaling composting as a means of PAH remediation are land requirements, complex waste handling, and monitoring costs. Along with the large volumes of soil and amendments which would further complicate operations, their byproducts are toxic, and heavy metal concentration by organic carbon loss does not enhance mobilization effectiveness. These issues have to be addressed for large-scale applications to be successful.	[46]
Environmental trade-offs	Composting is environmentally friendly for soil restoration but comes with the following side effects: greenhouse gassing, toxic intermediates from incomplete degradation of PAHs, and heavy metal accumulation, which create long-term ecological risks. Thus, such practice should be monitored to avoid secondary pollution to make the practice safe and sustainable.	[48,49]

).

Firstly, there are several organic substances and inorganic substances that cannot be broken down or remediated. Examples of inorganic chemicals that cannot be remediated by composting are metals and radionuclides. But, some metals can be adsorbed into less biodegradable forms rather than not being degraded at all. To determine the rate at which microorganisms are capable of degrading soil contaminants, a few factors must be investigated, such as temperature, pH, moisture, nutrient supply, oxygen supply, the concentration of contaminants, and the presence of substances that are toxic to the microorganisms [31].

Notable drawbacks of composting as a remedial method are seen in the variability displayed by its degradation endpoints in different studies. Basically, this stems from the broad chemical nature of the contaminants and the site-specific operational conditions within which composting systems work. Different authors have reported highly different degradation efficiencies even for the same class of contaminants. This reflects how complex the interaction is between microbial activity, environmental conditions, and the properties of contaminants. For example, the PAHs of quite pronounced hydrophobic organic pollutants display relatively facile degradation under the conditions of composting, provided that thermophilic temperatures (generally 50–70 °C) are attained and sustained. Greater-than-normal temperatures further facilitate microbial metabolism and enzymatic activities, more specifically for thermophilic and mesophilic microorganisms in complex hydrocarbon assimilation. However, even in studies on PAHs, there has been noted variation in the rate of degradation due to the initial concentrations of PAHs, substrates used during composting, moisture content, and available microbial consortia. Some cite greater than 90% degradation within a few weeks; others indicate prolonged periods with a lower efficiency, which is normally ascribed to inadequate aeration or improper carbon-to-nitrogen ratios. In contrast, highly chlorinated organic compounds, such as polychlorinated biphenyls or chlorinated pesticides, do not undergo fast degradation and need different treatment conditions for their effective treatment. Studies have shown that such compounds might be best treated with sequential anaerobic–aerobic treatments, in which the anaerobic phase involves reductive dechlorination and the subsequent aerobic composting phases involve complete mineralization. The variation in treatment performance across studies indicates how these compounds are extremely sensitive to moisture levels, redox potential, and microbial specificity. Some studies have reported the partial transformation of chloro-organic compounds, in which complete mineralization was not achieved. Some other studies have reported significant degradation when structured co-substrates or special inocula were applied [24,26,38].

A major gap in the literature lies in the limited understanding of microbial dynamics during the composting of PAH-contaminated soils. While it is known that microbial consortia play a vital role in PAH degradation, few studies have detailed the succession patterns of microbial populations or identified specific functional genes and metabolic pathways involved in the biodegradation process [7]. Additionally, little is known regarding the use of engineered microbial inoculants or compost additives to enhance degradation efficiency and adaptation to site-specific conditions [10].

Another challenge is that the costs are varied for remediation via composting, depending on the mix ratio of soil to compost, the number of soils that need to be treated, the types of contaminants and their process design, and the availability and cost of amendments. Other challenges include developing laboratory tests to pilot studies and full-scale operations; the possible increase in the total volume of materials that need to be handled, which would require having a very large space available to compost several thousands of cubic yards of contaminated soils; the potential presence of even higher levels of toxicity in the intermediate decomposition products during the biodegradation process; the probability for hazardous residues in the soils that do not tend to degrade and become non-extractable, which eventually return to the soil matrix; and the potential increase in the metal concentrations due to the carbon loss when composting [46].

6. Conclusions

Understanding the fate of PAHs in the environment is important for assessing their impact on human health and other organisms. Chemical fingerprinting, coupled with hydrocarbon ring examinations, proves invaluable in discerning PAH sources and types. Temperature variations are the main aspect in distinguishing pyrogenic and petrogenic PAHs, especially in industrial stack effluents. PAHs, found in various environmental compartments like soil, food, and sediment, necessitate effective remediation strategies. Composting, a biological treatment method, shows promise in degrading PAHs, with mushroom compost proving effective for higher-ring hydrocarbons. However, challenges persist, including the limitations posed by inorganic substances like metals and radionuclides, varying degradation rates, and associated costs. Addressing these challenges is crucial for optimizing composting as a sustainable and effective method for PAH-contaminated soil remediation. Future research should concentrate on the integration methods of advanced microbial consortia and co-composting strategies to improve not only the efficiency of the degradation of PAHs but also overcome current limitations in large-scale applications.