Petroleum-contaminated soil: environmental occurrence and remediation strategies

Introduction

Due to the dynamic increase in industrialization, urbanization, and the increasing demand for energy, pollution with persistent organic pollutants (POPs), including polycyclic aromatic hydrocarbons (PAHs), poses a serious threat to all forms of aquatic and terrestrial life (Mojiri et al. 2019).

Petroleum hydrocarbons (PHs) contain hazardous chemicals such as benzene, toluene, ethylbenzene, xylene, and naphthalene, which can be harmful to all parts of the ecosystem, especially the land (Sarkar et al. 2005; Liu et al. 2017; Kuppusamy et al. 2017).

PAHs exhibit persistence in soils; their solubility and biodegradability decrease as their molecular weight and the number of benzene structures increase, making them more toxic (Meador 2008). The US Environmental Protection Agency (USEPA) has classified PAHs as priority pollutants due to their persistence, high toxicity, mutagenicity/carcinogenicity, and teratogenicity for human beings (Rengarajan et al. 2015; Polidoro et al. 2017).

Oil spillage is a serious threat to all parts of the ecosystem (Sarkar et al. 2005). During extraction, transportation, storage, and distribution operations, crude oil and its refined products are frequently spilt, causing soil pollution (Macaulay and Rees 2014).

Soils contaminated with POPs associated with petroleum, such as PAHs, present high potential health risks because of their ability to enter the food chain and their affinity for accumulation in living organisms (Bastami et al. 2013; Honda and Suzuki 2020).

Generally, PAHs can be found in high concentrations in contaminated soil, indicating a potential environmental hazard. PHs alter soil biological properties, affecting microbial diversity and enzymatic activities as well as its physicochemical characteristics (Czarny et al. 2020; Dos-Santos and Maranho 2018).

Therefore, soil contamination emphasizes the need for effective environmental remediation and restoration strategies to preserve the ecosystem. Generally, soil petroleum contamination can be remediated with physical, chemical, thermal, and biological approaches (Kuppusamy et al. 2017). The first two methods have limitations such as high costs, inefficacy, and altering the natural ecosystem (Verma and Haritash 2019). As an alternative, bioremediation offers an eco-friendly process for the removal or reduction of petroleum pollutants in environments using selective microbial flora (Patel et al. 2020).

In this context, mycoremediation, which is fungal-based remediation, is a promising technique for the clean-up of contaminated soil (Kumar and Gopal 2015; Li et al. 2020).

Ligninolytic fungi and their enzymatic oxidative system (especially laccases and peroxidase) have proven their potential in the remediation of several complex aromatic pollutants such as dyes, PAHs and aromatic compounds. For instance, white-rot fungi (WRF) are among the most studied species for their potential to degrade a wide range of xenobiotic compounds, such as PAHs (Daâssi et al. 2021). Moreover, fungi isolated from PAH-contaminated soil can reduce petroleum pollution (Das and Chandran 2010).

Thus, the main purpose of this review is to demonstrate the physicochemical and biological remediation strategies for treating PAH-contaminated soil. The degradation pathways and contributing factors of the microbial PAHs-degradation will also be elucidated.

The impact of PAHs contamination on the soil properties

Petroleum-contaminated soil contains several types of hydrocarbon, including aliphatic (straight-chain) and aromatic (cyclic) structures that may change soil properties, such as texture, moisture, conductivity, total organic carbon, etc. Moreover, halogenated hydrocarbons with nonorganic elements like fluoride, bromide, iodide, or chloride (e.g., carbon tetrachloride), are frequently reported as environmentally persistent, toxic, and hazardous soil pollutants (Klamerus-Iwan et al. 2015; Kuppusamy et al. 2017).

Soil matrix properties and functions are closely related to the different activities occurring in the soil and to xenobiotic structures, like PAHs, associated with petroleum. PAHs’ chemical stability, hydrophobicity, and resistance to microbial degradation mean that spilt oil may damage the biological and physicochemical properties of the soil it pollutes.

Physicochemical properties

Petroleum is considered the major source of PAHs that may be absorbed by soil particles due to their high hydrophobicity and thus replace water molecules, reducing the oxygen and water infiltration in the petroleum-polluted soil (Sakshi and Haritash 2019). According to Terytze et al. (1995), PAHs are characterized by a strong sorption affinity to soil organic matter.

Further, soil geotechnical characteristics may be affected by hydrocarbon contamination, such as permeability, hydraulic conductivity, and compaction, as well as the biological properties (biomass and enzymes) of the soil matrix (Zahermand et al. 2020).

Petroleum polluted areas are characterized by a lower self-purification capacity that reduces the indigenous microbes involved in soil purification processes (Hreniuc et al. 2015).

Biological properties

The presence of hydrocarbons in the soil affects its biological properties and impoverishes microbial diversity (Labud et al. 2007). The persistence and the toxicity of some PAH compounds may inhibit soil microbial communities. Alrumman et al. (2015) reported that oil contamination in the soil matrix influences soil enzymatic activities and microbial biomass carbon, and so biological functions.

Also, certain essential soil functions may be lost due to the high toxicity of such persistent aromatic hydrocarbon structures (Khomarbaghi et al. 2019). Furthermore, spilt oil may instigate anaerobic conditions and asphyxia in soil pores, with their consequent impacts on microbial activities (Sutton et al. 2013).

Petroleum-contaminated areas suffer tremendously from the drastic impact of the toxicity as well as the concentration induced by PAHs’ high molecular weight. Klamerus-Iwan et al. (2015) demonstrated a significant decline of microbial biomass and enzymatic activities (urease and dehydrogenase) in soil polluted by chainsaw oil.

Remediation strategies of PAH-contaminated soils

Remediation of PAH-contaminated soils is a global issue that poses risks to public health. The reclamation or remediation of petroleum-polluted soils is important to remove pollutants from the environment and it can be done with several methods involving the removal, isolation, or alteration of the contaminant. Today, there are several techniques for soil reclamation, including physical, chemical, thermal, and biological remediation methods (ex-situ and in situ).

In-situ remediation occurs at the contaminated site and offers several advantages. It involves lower risk, lower cost, and limited human involvement, and the environmental surroundings can help in the remediation process to transform the contaminants.

Alternatively, ex-situ remediation processes occur off-site and the contaminant is transferred to another location for treatment. It is costly but occurs under a control system, with more human involvement and direct exposure to contaminants.

There are regulatory constraints for ex-situ remediation. Some of these remediation techniques are solvent extraction, UV oxidation, photochemical or photocatalytic degradation, bioremediation, and phytoremediation. The selection of suitable remediation techniques for petroleum-polluted soils depends on many factors such as the type and structure of contaminant, the future use of contaminated soil, the soil type and properties, the budget, etc.

Chemical treatments

Chemical oxidation reactions have been widely used to degrade oil or PAH-contaminated soils by the addition of oxidants to the soil to oxidize contaminants (Tsai and Kao 2009; Rivas 2006).

The chemical method of treating polluted soil involves oxidizing agents such as ozone (Shin et al. 2005; Yu et al. 2007); Fenton’s reagent uses hydrogen peroxide as an oxidant (Flotron et al. 2005). Alternative oxidants like persulfate/Fe(II), peroxymonosulfate (PMS), persulfate, H₂O₂, and permanganate can also be used for the chemical oxidation treatment of diesel and fuel oil (Do et al. 2009; Yen et al. 2011). The chemical treatment converts hazardous contaminants into less toxic or non-hazardous compounds (Verma and Haritash 2019). Furthermore, the combined microbial and chemical treatment of PAHs can be a cost-effective strategy for further application to contaminated sites.

Liao et al. (2018) evaluated chemical oxidation combined with microbial remediation for treating PAH-contaminated soil. Their results demonstrated a complementary improvement in the impact of microbial PAH degradation yields after chemical oxidant peroxidation.

Biological treatments (bioremediation)

Biological treatment is based on the use of living organisms and their derivatives. The effectiveness of bioremediation is often a function of the microbial population or consortium and how it can be enriched and maintained in an environment (Babu et al. 2019).

Generally, the biological method employs the natural potential of microbes, including bacteria and yeasts (bioremediation), algae (phyco-remediation), plants (phytoremediation), or fungi (mycoremediation) to biodegrade PH pollutants.