Phytoremediation of pyrene-contaminated soils: A critical review of the key factors affecting the fate of pyrene
Graphical abstract
Introduction
Polycyclic aromatic hydrocarbons (PAHs) are widespread as the result of human-related and natural activities, having both pyrogenic (combustion) or petrogenic (originating from petroleum) origins (Hamid et al., 2018). Gas expelled from volcanoes and wildfires represent the main natural sources of PAHs, while fossil fuel combustion in vehicles, coking plants and industrial productive activities are the anthropogenic sources that mostly increase the quantity of PAHs in the environmental matrices. PAHs can be additionally formed during the slow transformation of complex organic molecules in soil (Cachada et al., 2016), or during industrial thermal processes and, as a consequence, petroleum, natural gas, and bio-gas are rich in these compounds (Hu et al., 2020).
PAHs belong to the group of persistent organics pollutants (POPs) and consist of two or more fused aromatic rings organized in a linear, angular or a cluster structure. In general, low-molecular-weight PAHs with less than 4 rings are easier to degrade than high-molecular-weight PAHs, which are particularly recalcitrant to chemical and biological degradation (Bianco et al., 2020; Gupta et al., 2020b; Sun et al., 2010). Humans can be exposed to PAHs contamination through different ways such as ingestion, inhalation, or skin adsorption, with the ingestion of contaminated food being the predominant path of exposure (Polachova et al., 2020). Although more than a hundred PAHs exists, the United States Environmental Protection Agency (USEPA) listed sixteen compounds of priority concern, based upon their toxicity, easier exposure, and persistency in the contaminated sites (USEPA, 1993). In the last decades, this list of the 16 PAHs has played a significant role mainly for environmental science and policy issues, but questions are recently opened on the update of the environmental regulatory system based on recent toxicology studies, possibly extending the classification to additional PAHs in the future (Andersson and Achten, 2015; Keith, 2015).
Among PAHs, pyrene (4 fused rings) has gained attention as a result of its high toxicity, mutability, ubiquitous presence in all environmental matrices, high persistence and recalcitrance to biodegradation (IARC Working Group on the Evaluation of Carcinogenic Risks to Humans, 2010). Particularly related to the subject of this article, recent research has shown a negative influence of pyrene on plant life cycle in function of the global increased temperature, posing the problem of a possible future aggravation of the risk caused by pyrene in the environment due to climate change (Zhang et al., 2018). Pyrene is repeatedly found in lands intended for agriculture as the predominant PAH due to the industrial practices described above, urgently demanding for the implementation of remediation technologies (Lu et al., 2019; Yakovleva et al., 2020). The success of these technologies is intimately correlated to some factors (e.g. pH, site temperature, bioavailability), thus knowing the environmental behavior of pyrene is crucial to assess the efficiency and select the most suited remediation technique. Apart from the pyrene concentration in soil and the additional variables cited above, the environmental and ecotoxicological properties of pyrene strongly depend on the content of natural organic matter in soil (Cachada et al., 2016). Indeed, hydrophobicity and bioaccumulating characteristics make pyrene highly prone to be adsorbed onto suspended particulates and biota, and accumulated in soil and sediments (Dong et al., 2010; Umeh et al., 2020).
Despite the existence of different thermal and chemical methods to remediate pyrene-polluted soils, such as incineration, solvent extraction and oxidation by chemical agents, green biological technologies offer several advantages (Sakshi et al., 2019). For instance, phytoremediation has been successfully applied due to its cost-effectiveness, the possibility to remediate pollutants from different environmental compartments (Truu et al., 2015) and the eco-friendly interaction with the surrounding environment (Lama et al., 2020). Although phytoremediation is a consolidate technology, current knowledge needs a review that includes a critical approach about the emerging phytoremediation results for the removal of pyrene from contaminated sites (Bao et al., 2018; Kim et al., 2019) and the main mechanisms occurring between the contaminant and the plant. Also, the use of soil amendments requires an in-depth analysis as a strategy to enhance the phytoremediation of pyrene-contaminated sites in terms of both removal percentage and rate. Hence, this study aims to provide a guidance for future research on pyrene removal from soils through phytoremediation and to boost the development of new methods capable to improve the efficiency of the process.
Section snippets
Pyrene: properties, distribution, and removal
Pyrene, C16H10, is composed of 4 fused rings assembled in a flat aromatic structure (IUPAC, 1998). Pyrene belongs to group 4 in the classification for carcinogenicity provided by International Agency Research on Cancer (IARC), meaning that the agent (mixture or circumstances) does not show evidence for carcinogenic induced effects to humans. In contrast, studies report that pyrene can meet transformation processes and be transformed into more hazardous compounds such as benzo(a)pyrene, which
Phytoremediation of pyrene contaminated soils
Phytoremediation offers different advantages against the conventional physico-chemical methods, as the latter may cause serious chemical modification of the site and interference with ongoing activities (Vasavi et al., 2010). Overall, the most considerable advantages of phytoremediation deal with the possibility to concomitantly remove different contaminants, the non-interference with the ecosystem and the site, and the low operating costs. Indeed, the estimated costs for phytoremediation are
Pyrene bioavailability
During phytoremediation of pyrene-contaminated soils, bioavailability of pyrene is an essential key factor to consider as it represents the relevant exposure dose available for degradation by living organisms (Fernández-López et al., 2020), but also a possible toxicity exposure for plants and associated microorganisms in soil (Ortega-Calvo et al., 2015). The accessible quantity of pyrene for biodegradation is the portion available to cross the cellular membrane of organisms (Wei et al., 2017)
Additives used to strengthen phytoremediation of pyrene-contaminated soils
The performance of phytoremediation of pyrene-contaminated soils can be improved by adding compounds (e.g. nutrients, chelating agents, bacterial growth stimulators) or seeding rhizospheric and endophytic bacteria to soil (Salehi et al., 2020; Zhang et al., 2017). As illustrated in Fig. 3, the aim is to improve the quality of the soil treated by increasing the bioaccessibility of the contaminant, the biomass growth and the biological degradation, thus the success of the treatment (Kumar et al.,
Conclusions and potential future investigations
Phytoremediation is an effective, sustainable, and reliable technique for the remediation of pyrene-contaminated soils, involving only simple equipment, resulting in low operating costs, and having no-interference with the ecosystem. The present review pointed out that pyrene is mostly removed through rhizodegradation (i.e. the action of microorganisms in the rhizosphere region), significantly promoted by the specific plant species involved. The phytoremediation efficiency strongly depends on
Credit author statement
Ilaria Gabriele: Conceptualization, Data curation, Investigation, Writing – original draft, Writing – review & editing. Marco Race: Conceptualization, Supervision, Writing – review & editing. Stefano Papirio: Conceptualization, Supervision, Writing – review & editing. Giovanni Esposito: Supervision, Writing – review & editing, Administration, Funding acquisition.
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.
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