PAHs in Terrestrial Environment and their Phytoremediation 75

into less toxic forms under both aerobic as well as anaerobic conditions. In aerobic degradation, CO2

and water are formed as by-products, while in anaerobic conditions, methane is formed. Some of

the bacterial species involved in PAHs remediation are Pseudomonas, Rhodococcus, Bacillus and

Mycobacterium (Imam et al. 2022). Gamma-proteobacteria and actinobacteria are the two major

bacterial lineages that degrade PAHs in PAH-contaminated soils (Chaudhary et al. 2015). Apart

from bacteria, fungi are also employed for remediation known as mycoremediation due to their co­

metabolic activity (Srivastava and Kumar 2019).

Phytoremediation is a broader term, it includes many techniques where plants are used to

remove, detoxify or immobilize contaminants like PAHs in soil. It is even more efficient, eco-friendly

and cost-effective than bioremediation. Phytoextraction, rhizoextraction, phytovolatilization,

phyto stabilization, phyto stimulation, phyto transformation, phyto assimilation, phyto reduction,

etc., are various green technologies of phytoremediation. Plants involved in the phytoremediation

process are called “hyperaccumulators” or “phytoremediators”. The process in which plants

remove contaminants from soil by storing them in tissues is called phytoextraction/rhizoextraction/

phytofilteration. In the phytovolatilization process, plants remove the contaminants through

volatilization. In phyto stabilization, plants reduce the mobility of contaminants. Phyto stabilization

followed by the addition of microbes degrade the immobile contaminants. The physical, chemical

or biological remediation methods can be employed both in-situ and ex-situ. In in-situ, remediation

is done at the original contaminated soil whereas in ex-situ, contaminated soil is transferred to

another location for remediation. Bionano-remediation is also an emerging technique for PAHs

degradation. The use of nanoparticles like Nano zero-valent iron and metal oxides, carbon-based

and polymer-based materials with biotechnological tools could be a promising approach for PAHs

remediation. It is observed that the addition of nanoparticles in soil positively affects the mobility

of PAHs, decreases the phytotoxicity and microbial flora remain unaffected (Mazarji et al. 2021).

Combinations of physical-biological, chemical-biological or all the three (physical-chemical­

biological) known as integrated methods are also used as remediation techniques for PAHs (Patel

et al. 2020). Although various remediation technologies are available for PAH contaminated soils,

but out of them, the most suitable and sustainable technology is the phytoremediation (Dolatabadi

et al. 2021).

5.3 Phytoremediation of PAHs from Soil

Phytoremediation is a process that involves the use of plant/parts of plants to remove, degrade

or stabilize the pollutants from soil/water. By the process of phytoremediation (phytodegradation,

rhizodegradation, phytoextraction, rhizofilteration, etc.) different PAHs can be removed (Bose et al.

2022). The term “Phytoremediation” refers to the technique that uses plants to clean up soil and water

contaminated with harmful substance (Ashraf et al. 2019, Dhaliwal et al. 2020). Phytoremediation

has gained the attention of the scientific community for environmental remediation as it provides

an alternative to the conventional methods that are energy intensive, use expensive chemicals and

instruments and alter the soil properties (Ali et al. 2013, Dal Corso et al. 2019). Thus, to achieve

site-specific remedial goals, phytoremediation is considered as a cost-effective and sustainable

approach that has a profound restoration effect without disturbing the native microflora (Wan

et al. 2016, Nedjimi 2021). The technology uses green plants (trees, shrubs, grasses and aquatic

plants) because of their transport capacity and accumulation of contaminants which enables them to

remove, degrade or isolate toxic environmental contaminants (Yan et al. 2020). These contaminants

include heavy metals, organic compounds such as polyaromatic hydrocarbons (PAHs) and

radioactive substances in soil or water. Till date, various plants have been used for remediation of

various contaminants (Table 5.5).