inorganic pollutants to safer or less toxic intermediates (Dash and Das 2012).
Ligninolytic enzymes (e.g., manganese peroxidase, lignin peroxidase and laccase),
monooxygenase, chrome reductase, azo-reductase and dioxygenase secreted by
rhizospheric microbes have been found to be effectively used in bioremediation of
various pollutants (Dash and Das 2012). Microbial bioremediation of organic
compounds implicates either partial or complete mineralization by complex and
genetically regulated physiological reactions (Joutey et al. 2013).
Metal(loid)s cannot be destroyed like organic contaminates, rather they are either
removed or biotransformed to a stable form (Tangahu et al. 2011). Remediation of
metal(loid)s includes biosorption, bioleaching, biomineralization, intracellular accumulation and enzyme-catalyzed transformation through redox reactions (Lloyd and
Lovely 2001). Besides, microorganisms develop certain mechanisms such as uptake,
adsorption, oxidation, reduction and methylation to protect them from metal toxicity
(Igiri et al. 2018). Several metal-resistant bacteria (Bacillus sp., Pseudomonas sp.,
Saccharomyces sp., etc.) have been reported to thrive under metal-stressed condition
by their accumulation and complexation into less toxic form (Díaz-Ramírez et al.
2008).
b. Phytoremediation
The term “phytoremediation” was initiated by the Environmental Protection
Agency (EPA) in 1991 and was first addressed in open technical literature in 1993
by Cunningham and Berti. Phytoremediation, also referred to as botano-, green-,
vegetative- or agro-remediation, is collectively used for green plant-based
technologies for in situ and ex situ remediation of contaminated soil (Mahjoub
2013). It is publicly appealing remediation technology that utilizes plants, associated
microbiota, various soil amendments and agronomic techniques to extract, contain,
degrade, detoxify or immobilize organic (hydrocarbons, pesticides and chlorinated
solvents) and inorganic contaminants in the soil (Helmisaari et al. 2007;
Vyslouzilova et al. 2003). For developing countries, phytoremediation is an
eco-friendly, aesthetically pleasing and cost-effective approach, but despite its
potentiality, it is yet to become a commercially viable technology (Ghosh and
Singh 2005; Sharma et al. 2021). Phytoremediation approach is further classified
into various applications such as phytostabilization, rhizodegradation or
phytodegradation, rhizofiltration or phytofiltration, phytoextraction and
phytovolatilization (Fig. 16.6).
Phytostabilization The approach utilizes plant roots to confine the mobility and
availability of contaminants in soil (Jadia and Fulekar 2009). Pollutants from soil are
absorbed through roots and restricted in rhizospheric zone, rendering them harmless
by preventing their leaching (Ekta and Modi 2018). Phytostabilization was found to
be a most suitable technique to remediate Cu, Zn, Cd, Cr and As (Moosavi and
Seghatoleslami 2013). Bacchetta et al. (2018) showed that Helichrysum
microphyllum subsp. tyrrhenicum grown on mine wastes dumps of Sardinia in
Europe can tolerate high concentration of Zn, Pb and Cd, thus appearing as a species
476
M. Gautam et al.
Ligninolytic enzymes (e.g., manganese peroxidase, lignin peroxidase and laccase),
monooxygenase, chrome reductase, azo-reductase and dioxygenase secreted by
rhizospheric microbes have been found to be effectively used in bioremediation of
various pollutants (Dash and Das 2012). Microbial bioremediation of organic
compounds implicates either partial or complete mineralization by complex and
genetically regulated physiological reactions (Joutey et al. 2013).
Metal(loid)s cannot be destroyed like organic contaminates, rather they are either
removed or biotransformed to a stable form (Tangahu et al. 2011). Remediation of
metal(loid)s includes biosorption, bioleaching, biomineralization, intracellular accumulation and enzyme-catalyzed transformation through redox reactions (Lloyd and
Lovely 2001). Besides, microorganisms develop certain mechanisms such as uptake,
adsorption, oxidation, reduction and methylation to protect them from metal toxicity
(Igiri et al. 2018). Several metal-resistant bacteria (Bacillus sp., Pseudomonas sp.,
Saccharomyces sp., etc.) have been reported to thrive under metal-stressed condition
by their accumulation and complexation into less toxic form (Díaz-Ramírez et al.
2008).
b. Phytoremediation
The term “phytoremediation” was initiated by the Environmental Protection
Agency (EPA) in 1991 and was first addressed in open technical literature in 1993
by Cunningham and Berti. Phytoremediation, also referred to as botano-, green-,
vegetative- or agro-remediation, is collectively used for green plant-based
technologies for in situ and ex situ remediation of contaminated soil (Mahjoub
2013). It is publicly appealing remediation technology that utilizes plants, associated
microbiota, various soil amendments and agronomic techniques to extract, contain,
degrade, detoxify or immobilize organic (hydrocarbons, pesticides and chlorinated
solvents) and inorganic contaminants in the soil (Helmisaari et al. 2007;
Vyslouzilova et al. 2003). For developing countries, phytoremediation is an
eco-friendly, aesthetically pleasing and cost-effective approach, but despite its
potentiality, it is yet to become a commercially viable technology (Ghosh and
Singh 2005; Sharma et al. 2021). Phytoremediation approach is further classified
into various applications such as phytostabilization, rhizodegradation or
phytodegradation, rhizofiltration or phytofiltration, phytoextraction and
phytovolatilization (Fig. 16.6).
Phytostabilization The approach utilizes plant roots to confine the mobility and
availability of contaminants in soil (Jadia and Fulekar 2009). Pollutants from soil are
absorbed through roots and restricted in rhizospheric zone, rendering them harmless
by preventing their leaching (Ekta and Modi 2018). Phytostabilization was found to
be a most suitable technique to remediate Cu, Zn, Cd, Cr and As (Moosavi and
Seghatoleslami 2013). Bacchetta et al. (2018) showed that Helichrysum
microphyllum subsp. tyrrhenicum grown on mine wastes dumps of Sardinia in
Europe can tolerate high concentration of Zn, Pb and Cd, thus appearing as a species
476
M. Gautam et al.