Polycyclic aromatic hydrocarbon contamination in soils and sediments: Sustainable approaches for extraction and remediation

Highlights

• The fate of PAHs in contaminated soils and sediments was summarized.

• Greener methods of PAH extraction for quantitative analysis were reviewed.

• Sustainable approaches to remediate PAH contaminated soils/sediments were discussed.

• Biosurfactant-mediated remediation efficiency and toxicity needs further research.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic environmental pollutants that are extremely hydrophobic in nature and resistant to biological degradation. Extraction of PAHs from environmental matrices is the first and most crucial step in PAH quantification. Extraction followed by quantification is essential to understand the extent of contamination prior to the application of remediation approaches. Due to their non-polar structures, PAHs can be adsorbed tightly to the organic matter in soils and sediments, making them more difficult to be extracted. Extraction of PAHs can be achieved by a variety of methods. Techniques such as supercritical and subcritical fluid extraction, microwave-assisted solvent extraction, plant oil-assisted extraction and some microextraction techniques provide faster PAH extraction using less organic solvents, while providing a more environmentally friendly and safer process with minimum matrix interferences. More recently, more environmentally friendly methods for soil and sediment remediation have been explored. This often involves using natural chemicals, such as biosurfactants, to solubilize PAHs in contaminated soils and sediments to allow subsequent microbial degradation. Vermiremediation and microbial enzyme-mediated remediation are emerging approaches, which require further development. The following summarises the existing literature on traditional PAH extraction and bioremediation methods and contrasts them to newer, more environmentally friendly ways.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants generated through incomplete combustion and pyrolysis of organic matter. Natural and anthropogenic sources, including volcanic eruptions, forest fires, incomplete combustion of fossil fuels in vehicles, wood, and fuel oils in heating systems, contribute heavily to environmental PAH contamination (Abdel-Shafy and Mansour, 2016). PAHs consist of two or more fused benzene rings and possess low water solubility, which decreases almost linearly with increasing molecular weight (Kuppusamy et al., 2017; Mohan et al., 2006). Hence, PAHs generally associate primarily with the particulate phase, specifically with organic matter in soils and sediments (Wilson and Jones, 1993).

Vapour and particulate matter such as soot, pollens, dust, pyrogenic metal oxides and fly-ash, can act as airborne PAH carriers. Atmospheric PAHs settle on the soil and enter water bodies via dry deposition, precipitation and dissolution associated with wet deposition. Over 90% of the environmental PAH burden is in soils and sediments (Wild and Jones, 1995). In soil, PAHs remain sorbed to soil particulate matter, making them less mobile and less bioavailable for microbial degradation. Loss or removal of PAHs from the soil can occur via processes such as leaching to groundwater, irreversible sorption to soil organic matter, volatilisation, photooxidation, abiotic losses and uptake by plants or microbial degradation (Okere and Semple, 2012). The deposition of PAHs in sediments is quite similar to that in the soil. PAHs in the particulate phase in the atmosphere settle on water reservoirs and eventually accumulate in the sediment phase (Boitsov et al., 2009). Sediment bound PAHs enter pore water through the pores in sediments and attach to colloids. Colloid bound PAHs contribute to the most significant proportion of the bioavailable fraction in the environment.

PAHs cause teratogenic, mutagenic and carcinogenic effects to humans and other organisms (Kuppusamy et al., 2016). While many chemicals are classified as PAHs, only 28 of them are considered hazardous by the US Environmental Protection Agency (EPA) in 2008, and among them, 16 are considered EPA priority pollutants in terms of their toxicity (Gan et al., 2009). According to the classifications of the International Agency for Research on Cancer, benzo[a]pyrene (Group 1), naphthalene, chrysene, benzo[a]anthracene, benzo[k]fluoranthene and benzo[b]fluoranthene (Group 2B) are probably carcinogenic to humans (Cancer & Organization). Terrestrial invertebrates show a higher accumulation of PAHs, mainly from contaminated soils. Aquatic organisms are more prone to PAH contamination due to the direct contamination with PAHs in water and sediments and absorption from plants (Meador et al., 1995; Tao et al., 2004). PAHs can accumulate in mammals through inhalation, ingestion, or dermal contact, affecting processes such as reproduction, development, and immunity (Abdel-Shafy and Mansour, 2016).

Hence, developing appropriate remediation methods is required to mitigate the possible risk of PAHs on environmental and human health. However, remediation of PAHs in soils using industrial chemicals and high temperatures can be problematic due to their cost, energy intensiveness and environmentally unsustainable nature. Therefore, bioremediation is gaining importance as a safer, more economical, and more environmentally sustainable technology to remediate PAH contaminated soils. Biosurfactant mediated-remediation of PAH contaminated soils and sediments is less explored and will be extensively discussed in the current review.

Understanding the extent of PAH contamination through an efficient PAH extraction method is necessary, before applying a remediation approach. Typical PAH extraction techniques used for quantification utilise a large amount of hazardous organic solvents, which are highly volatile and difficult to contain. Hence, organic solvents themselves can be pollutants and show health hazards. From a green chemistry perspective, the use of hazardous solvents for PAH extraction and determination should be replaced with solvents that are less hazardous. However, cleaner techniques for the extraction of PAHs from soils and sediments have not been systematically reviewed.

This paper aims to review the eco-sustainable techniques used for PAH extraction and remediation of contaminated soils and sediments. The review covers PAH extraction techniques that utilise sustainable solvents or low concentrations of hazardous organic solvents during quantification, and remediation methods that are less harmful to the environment.