Bioaugmentation

Figure 5.2. Overview of remediation methods for PAHs in soil.

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Bioremediation for Sustainable Environmental Cleanup

5.2.3 PAHs in Terrestrial Ecosystems as Reported in the Literature

Several studies have been conducted to determine the levels of PAHs in different types of soils of

India and other countries that are presented in the Table 5.3. The natural concentration of PAHs

present in the soil is in the range of 0.001–0.010 µg g^–1 or by Dutch standards 0.02–0.05 µg g^–1 DW

(Wang et al. 2010). The concentration of PAHs increases due to anthropogenic inputs in soils around

industrial belts, urban and traffic areas. The level of PAHs can be as high as 200000 µg kg^–1 (DW) in

the soil near oil refineries and in traffic-affected soils it is less than 2000 µg kg^–1 (DW) (Rengarajan

et al. 2015). In some studies, the health risks of PAHs were also assessed. For such assessment, the

USEPA (2005) health risk model is used. PAHs are very toxic, mutagenic and carcinogenic even at

very low concentrations. The carcinogenic potencies of PAHs are measured by determining TEFs

(Toxic Equivalency Factors).

5.2.4 Overview of Remediation measures for PAHs in Soil

PAHs have many environmental implications, as they are resistant to environmental degradation. The

reasons behind their resistance are their hydrophobic nature, low water solubility, low sorption, low

volatility, high oxidative resistance and low bioavailability. They are ubiquitous pollutants. PAHs

can enter the human body by three major ways: inhalation, consumption and epidermal. They are

toxic, mutagenic, carcinogenic, teratogenic and potent immunosuppressants and hence have adverse

impacts on human health and other living organisms. PAHs contaminated-soil negatively affects

the soil microbial community. Microbial activity and biomass get reduced due to the presence of

PAHs in soil pores. It is investigated that the population of microorganisms of phyla Actinobacteria

Alphaproteobacteria Chlorfexi, Crenarchaeota and Deltaproteobacteria gets reduced after the

addition of pyrene in soil (Ren et al. 2015). Therefore, the accumulation of PAHs in the soil is of

great concern. Keeping this in mind, many methods have been developed for the removal of PAHs

from the soil. An overview of remediation methods for PAHs is depicted in Figure 5.2 and Table 5.4.

Physical and chemical methods like solvent extraction, chemical oxidation, air sparging, adsorption,

thermal desorption, photo-oxidation and electrokinetic remediation are available for remediation.

But these methods are expensive, labor-intensive, inefficient and can form toxic intermediates (Gan

et al. 2009). Thus bioremediation and phytoremediation are the two emerging potential approaches

to mitigate the effects of PAHs. In bioremediation, living organisms (mainly microorganisms) and

their products (mostly enzymes), are used to remove persistent pollutants like PAHs. The catabolic

actions of enzymes convert complex PAHs (both high molecular weight and low molecular weight)