Polycyclic aromatic hydrocarbon contamination in soils and sediments: Sustainable approaches for extraction and remediation
Graphical abstract
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.
Section snippets
PAH extraction methods used for quantification
Quantification of the PAHs in contaminated environmental matrices is essential to understanding the extent of PAH contamination. Soxhlet extraction, ultrasonic extraction, liquid-liquid and solid-phase extraction are some established techniques regularly used to extract PAHs from soils and sediments. However, consumption of large solvent volumes, longer extraction times, limited extraction efficiency, requirement of purification steps and analyte loss complicate their application (Khan et al.,
Bioremediation of PAH contaminated soils and sediments
PAHs in the environment can be removed through bioremediation, photocatalytic oxidation, chemical oxidation, electrokinetic and thermal technologies, soil washing and by adsorption using carbonaceous sorbents (Bianco Prevot, Gulmini, Zelano and Pramauro, 2001; Gan et al., 2009). In certain situations, the implementation of widely used remediation technologies is offset by their non-environmentally friendly existence, energy intensiveness and treatment expense.
Adsorption of PAHs onto
Conclusion
Of the PAH extraction technologies discussed here, MAE, SFE, plant oil-assisted extraction and several microextraction techniques seem to be the most effective extraction techniques. SFE however, is more suitable than MAE due to high selectivity, less usage of solvents and reduced matrix interferences. On the other hand, plant oil assisted extraction is a low-cost and straight forward extraction method. Better extraction times could be achieved by coupling SFE and oil-assisted extraction and
Credit author statement
Thiloka Kariyawasam: Conceptualization, Writing – original draft Gregory S. Doran: Conceptualization, Supervision, Writing – review & editing, Julia A. Howitt: Conceptualization, Supervision, Writing – review & editing Paul D. Prenzler: Conceptualization, Supervision, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Thiloka Kariyawasam acknowledges Charles Sturt University for the provision of Australian Government Research Training Program Scholarship (AGRTP) for her PhD study. Julia Howitt, who contributed greatly for this paper unfortunately passed away before this paper was submitted. She will be deeply missed by her loved ones.
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