PAHs in Terrestrial Environment and their Phytoremediation 79

Research studies demonstrated that sunflowers have shown greater potential for phytoremediation

of soil contaminated with radionuclides (caesium and strontium) in Chernobyl, Ukraine. The

results indicated that Cs was accumulated in roots, whereas Sr uptake was recorded in the shoots

(Prasad 2007). Another study reported that rhizofiltration using and Phaseolus vulgaris has shown

remediation of uranium contaminated groundwater (Lee and Yang 2010). The aromatic medicinal

plant, Plectranthus amboinicus, has been evaluated as a candidate for rhizofilteration of lead-

containing wastewater. The results indicated that P. amboinicus showed potential tolerance towards

Pb toxicity and accumulates Pb maximum in the roots, whereas translocation in the stem and leaf was

limited (Ignatius et al. 2014). In hydroponic experiments, Typha angustifolia and Acorus calamus

(aquatic) and Pandanus amaryllifolius (terrestrial) plants were evaluated for rhizofiltration of Cd

and Zn. Among these, T. angustifolia showed maximum tolerance towards HM toxicity without loss

in dry biomass production. With maximum HM accumulation in roots, T. angustifolia was found to

be the most suitable candidate plant for phytoremediation in constructed wetlands and aquatic plant

systems (Woraharn et al. 2021).

5.3.1.6 Rhizodegradation

Rhizodegradation is an emerging technique for remediation of contaminated soil that involves

plant roots, plant-supplied nutrients and soil microorganisms. This process occurs through the

plant-supplied substrates such as bacteria, fungi and yeasts which favors the growth of microbial

communities in the rhizosphere to break down organic pollutants (Allamin et al. 2020, Rajkumari

et al. 2021). The factors affecting the rhizodegradation efficiency of processes are the type

of contaminant, the ability of microflora to degrade the contaminants and the bioavailability of

pollutants (Noroozi et al. 2017, Patel and Patra 2017). Plant roots provide air to the soil and release

exoenzymes and nutrients through root exudates. The advantages of rhizodegradation are complete

mineralization of the contaminant, lesser translocation of the contaminant to other plant parts or in

the atmosphere, low installation and maintenance costs (Dos Santos and Maranho 2018). However,

there exist some disadvantages of the process as it is a slow process and effective only on the surface

of contamination (20–25 cm of depth); the limited depth of the roots and requirement of fertilizers

by plants.

Various studies have reported the effective remediation of soils via rhizodegradation

contaminated including PAHs, pesticides and solvents containing benzene ring. Plants such as

Avicennia marina (Jia et al. 2016) or Lolium multiflorum (Hussain et al. 2022) have shown immense

potential towards the degradation of PAH following the rhizodegradation mechanism. The efficiency

of Rhizophora mangle to extract PAHs in mangrove sediment contaminated with crude oil has also

been studied. The soil planted with Rhizophora mangle showed higher removal of 16 PAHs as

compared to natural attenuation (Verâne et al. 2020).

5.3.2 Parameters that affect the Phytoremediation Efficiency

To achieve maximum efficiency of the phytoremediation process, different aspects of certain

parameters should be taken care of. These include the type of contaminant, selection of plant species,

type of soil and other biotic and abiotic conditions (Mudgal et al. 2010).

5.3.2.1 Contaminant Characteristics and Concentration

To date, phytoremediation technology has been used for the removal of a wide range of soil

contaminants including inorganic and organic contaminants (Henry 2000). For organic pollutants, the

hydrophobicity affects the efficiency of its uptake by the plants to a greater extent. The remediation

of soil contaminated with moderately hydrophobic pollutants has been successfully reported due to

efficient uptake and translocation by the plant (Cunningham and Ow 1996). Inorganic pollutants

generally include heavy metals and the efficiency of phytoremediation process for removal of one

metal may vary as compared to a removal of mixture of metals (Dushenkov et al. 1995). Apart