78

Bioremediation for Sustainable Environmental Cleanup

on soil contaminated with polycyclic aromatic hydrocarbons was also studied. The concentration of

phenanthrene was reduced to greater extent with the integrated application of black locust and soil

amendments after 1 yr of plant growth. The uptake of trace elements Cu, As, Cd, Pb and Sb to leaves

was low (Wawra et al. 2018).

5.3.1.3 Phytovolatilization

Phytovolatilization has been observed for numerous contaminants including volatile organic

compounds and inorganic contaminants. Based on the possible mechanisms, the technique has

been divided into two categories, i.e., direct phytovolatilization by stems or leaves and indirect

phytovolatilization from the root zone. In direct phytovolatilization, the pollutants are absorbed by

roots, transported through the xylem and excreted to the atmosphere from the aerial plant parts in the

volatile form. However, in indirect phytovolatilization process, volatile organic contaminants flux

increases from the subsurface due to plant roots activities, e.g., increasing soil permeability, chemical

transport via hydraulic redistribution or water table fluctuations (Limmer and Burken 2016). This

strategy may prove advantageous as the contaminant can be transformed from more a toxic form to

a relatively lesser toxic substance, however, there might be the possibility that the resultant form is

still potentially toxic, and thus resettle into the environment. The phytovolatilization can be applied

for contaminants present in soil, sediment or water, particularly, for organic contaminants. Till date,

this technique has been used for heavy metals (Shrestha et al. 2006, Sakakibara et al. 2010).

5.3.1.4 Phytodegradation

‘Phytodegradation’ is a subset technique of phytoremediation under which organic contaminants are

absorbed by the roots followed by degradation due to catalytic action of enzymes which are involved

in metabolism of the plant (Newman and Reynolds 2004, Sharma and Pandey 2014). The enzymes

involved in the phytodegradation process are dehalogenase, peroxidase, nitroreductase, nitrilase and

phosphatase (Chatterjee et al. 2013). Phytodegradation is primarily used for the remediation sites

by contamination with organic pollutants like and PAHs (Muthusaravanan et al. 2018). To date,

numerous plants have been used for phytodegradation of organic contaminants. Lupinus luteus in

association with endophytic bacteria has shown immense phytodegradation potential in landfill soils

of the Iberian Peninsula contaminated with PAHs (Gutiérrez-Ginés et al. 2014). He and Chi (2019)

investigated the phytodegradation of phenanthrene and pyrene using aquatic plants, Vallisneria

spiralis and Hydrilla verticillata, in PAH-polluted sediments. Among them, sediments planted with

V. spiralis showed the highest dissipation of phenanthrene and pyrene (85.9 and 79.1%) as compared

to sediments planted with H. verticillata and unplanted sediment. Phytodegradation of phenanthrene

and pyrene using the maize plant has been confirmed using GC-MS analysis. The degradation rate

of phenanthrene was found to be faster than that of pyrene and prominently occurred in the roots

(Houshani et al. 2021).

5.3.1.5 Rhizofiltration

‘Rhizofiltration’or ‘phytofiltration’refers to the process that uses both terrestrial and aquatic plants in

adsorbing contaminants in the surrounding root zone (rhizosphere), concentrating and precipitating

them on or within the root (Rahman and Hasegawa 2011). The plants’ roots are harvested after

becoming saturated. The plant species which possess high tolerance towards metal toxicity and

have a high surface area for absorption of metal are preferred (e.g., Salix spp., Populus spp.,

Brassica spp.). Terrestrial plants have been reported as a suitable candidate for rhizofilteration as

they possess a more developed and fibrous root structure, thus provide a higher surface area for

absorption of contaminants (Cule et al. 2016). This strategy can be applied for the remediation of

contaminated sites loaded with heavy metals and radionuclides. The factors that limit the application

of this technique are pH adjustment, hydroponic cultivation in a greenhouse, frequent harvests and

proper disposal of the plants (Kristanti et al. 2020).