Remediation of soils and sediments polluted with polycyclic aromatic hydrocarbons: To immobilize, mobilize, or degrade?
Graphical abstract
Introduction
PAHs are a category of organic chemicals with two, or more than two, bonded benzene rings, and 16 of them have been cited as priority contaminants by the Environmental Protection Agency of the United States (US EPA) owing to their oncogenic, teratogenic, and mutagenic characteristics (Zhang et al., 2021b, Lukić et al., 2016). PAHs in the natural environment originate from incomplete combustion of organic substances, including fossil fuel and biomass (Patel et al., 2020, Balmer et al., 2019), agricultural, mining, and industrial activities (Yuan et al., 2021, Bao et al., 2020, Tarafdar and Sinha, 2019), or from natural geogenic inputs (McGrath et al., 2019). Soils and sediments are contaminated with PAHs with concentrations greater than those that are a risk to humans and ecosystems (Xing et al., 2020, Yavar Ashayeri et al., 2018). PAHs pollution can cause adverse health impacts on humans and other living organisms (Yuan et al., 2021, Hussain et al., 2018) (Table 1). Low molecular weight (LMW) PAH compounds, consisting of 2–3 rings, have been reported to cause acute toxicity but are not carcinogenic (Olayinka et al., 2019). In contrast, high molecular weight (HMW) PAHs, consisting of 4–7 rings are relatively lower in toxicity but have carcinogenic, mutagenic, or teratogenic properties (Bauer et al., 2018, Ghosal et al., 2016).
PAHs are ubiquitous contaminants, and, due to their lipophilicity, they can easily sorb onto soils and sediments and persist over there for a long duration (Wu et al., 2019, McGrath et al., 2019). Thus, soils and sediments are considered to be the ultimate sinks of PAHs in terrestrial and aquatic ecosystems (Barhoumi et al., 2019, Alegbeleye et al., 2017). The level of PAHs in soils depends on the distance from the emission source, with higher quantities in industrial and urban areas than in suburban and rural areas (Yuan et al., 2015). The spatial dispersal of PAH compounds in soils is also affected by dissipation processes, such as sorption-desorption, abiotic degradation, volatilization, biodegradation, leaching, and bioaccumulation (Li et al., 2010, Su and Zhu, 2008). These processes are influenced by physicochemical properties of PAHs (e.g., molecular weight, the octanol-water partition coefficient (Kow), and the organic carbon-water partition coefficient (Koc)), along with soil characteristics, such as pH, soil carbon concentrations, texture, and moisture content (Omores et al., 2017, Zhang et al., 2013). Environmental conditions (e.g., temperature and precipitation) are also factors that disturb PAHs distribution in the soil (Hong et al., 2020, Zhao et al., 2015). Similar to soils, sediments are considered as sinks for PAHs in aquatic ecosystems. When PAHs enter the riverine system, they are adsorbed onto particulate matter and immobilized on the surface sediments (Gong et al., 2018). Sediment-adsorbed PAHs in overlying waters are a threat for aquatic organisms (Idowu et al., 2020a, Soukarieh et al., 2018). PAHs enter in the flora and fauna and can be transferred to humans via the food chain and display biomagnification (Honda and Suzuki, 2020).
Increased concentration of PAHs in the environmental, along with their eco-toxicity and health impact on humans, have resulted in several investigations concerning removing them from the environment using various physical, chemical, biological, and integrated or hybrid technologies (Zhang et al., 2021a, Cui et al., 2020, Idowu et al., 2020b, Kuppusamy et al., 2017). Due to the heterogeneity of soils and other factors, a huge percentage of the overall existing PAH compounds are non-bioavailable to living beings. Bioavailability is reduced as PAHs age within a complex soil and sediment matrix (Maletić et al., 2019, Yang et al., 2016). For instance, the remediation rate of PAHs can be either rapid or slow reliant on the soil and sediment properties, bioavailability, and the time it takes to generate derivatives (oxygenated derivatives) that are more noxious than the initial PAH compounds (Zang et al., 2021, Idowu et al., 2020a, Kuppusamy et al., 2017). Specific characteristics of PAHs, such as their high hydrophobicity and low water solubility, reduce their bioavailability and subsequent degradation (Ranjbar Jafarabadi et al., 2020; Idowu et al., 2020b; Zang et al., 2020). Because of these constraints, the need for evolving methods to remediate PAHs is evident. Avenues have developed to design novel remediation technologies that can override the prevailing technological limitations. Assessment built on bioavailability is considered as an important tool in risk-based tactics for remediation of PAHs polluted sites (Duan et al., 2015).
There have been a number of publications on PAH contamination and its remediation technologies (Dai et al., 2020, Lu et al., 2019, Ma et al., 2018; Kuppusamy, 2017; Lawal, 2017) (Fig. 1). However, only a limited number of reviews have reported contamination of soils and sediments by PAHs and remediation approaches based on their bioavailability. This review discusses methods to remediate soils and sediments contaminated with PAHs using conventional and advanced immobilization, mobilization, and degradation techniques. In addition, case studies are presented giving the source of PAHs and how the PAHs interact with soils and sediments. Also, this review covers the limitations of current technologies. Finally, future research is discussed, which is needed to evolve innovative approaches to improve the remediation of soils and sediments polluted with PAHs.
Section snippets
Sources, source identification techniques, and interaction of PAHs in soils and sediments
PAHs sources can be categorized as pyrogenic, petrogenic, and biological or natural (Abdel-Shafy and Mansour, 2016). When biomass is processed at high temperatures in low or no oxygen conditions, pyrogenic PAHs are emitted. (Balmer et al., 2019). The transformation of coal into coke/coal tar, or the pyrolytic refining of petroleum residues into lower hydrocarbons, leads to emission of pyrogenic PAHs (Guarino et al., 2019, Abdel-Shafy and Mansour, 2016). Petrogenic sources of PAHs include
Bioavailability of PAHs in soil and sediments
The bioavailability of PAHs in soils represents the bioavailable fraction of PAHs, when they are in a bound state (Beckles et al., 2007). Compared with the total amount of pollutants, the bioavailable fraction of pollutants is considered as the fraction that is accumulated in the food chain. Therefore, bioavailability is a reasonable assessment in determining environmental risks of PAHs (Yang et al., 2016). Bioavailability of PAHs varies in different environments (Peng et al., 2013; Lanno et
Remediation of PAH in soils and sediments
Information on techniques used to remediate soils and sediments contaminated with PAHs is critical to limit environmental and living organism health risks (Guarino et al., 2019, Gitipour et al., 2018). Various methodologies have been adopted to remediate PAHs in soils and sediments (Fig. 3). They can be immobilized, mobilized, and degraded from a solid medium (e.g., soils, sediments, or wastes) via selective chemicals or materials, or their concentration can be decreased by plant uptake and by
Case studies of field application of remediation technologies
Several studies using laboratory-based, batch-scale, and pilot-scale remediation technologies have demonstrated their ability to remediate soils and sediments polluted with PAHs (Table 3). Nevertheless, few field experiments have been carried out to show the importance of the above-discussed remediation technologies. Below, field trials using these remediation technologies are discussed.
Knowledge gaps and prospective
These days several technologies have been emerged out to remediate PAHs contaminated soils and sediments and transmute PAHs compounds in to less toxic, and/or nontoxic forms as discussed in this review comprehensively. These technologies are successfully adopted to eliminate or diminish PAHs from the contaminated media at minimum environmental cost. These technologies, also minimizes the carcinogenic, mutagenic and teratogenic properties of PAHs prevailing in soils and sediments. Still, the
Conclusions
Worldwide, soils and sediments continue to be polluted with PAHs. This review has discussed the advantages and challenges associated with immobilization, mobilization and degradation-based remediation approaches of soils and sediments polluted with PAHs. Many technologies and methods are available for remediation of polluted sites. The adoption of a technology depends upon the type of contamination, level of contamination and their bioavailability, environmental conditions, duration of the
CRediT authorship contribution statement
Manish Kumar, PhD: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Validation, Visualization. Nanthi S. Bolan, Professor: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Validation, Visualization, Supervision. Son A. Hoang, PhD student: Writing - original draft, Writing - review & editing. Ankush D. Sawarkar, PhD student: Writing - original draft, Writing - review & editing. Tahereh Jasemizad, PhD student: Writing -
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.
Acknowledgement
The authors are thankful to Director, CSIR-National Environmental Engineering Research Institute for providing necessary facilities for this work.
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