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Bioremediation for Sustainable Environmental Cleanup
lethal than any other form (Ullrich et al. 2001). Selenium removal from contaminated environments
by biomethylation has also been reported (Adriano et al. 2004).
9.3.3 Oxidation/Reduction Mechanism
Microbes use oxidation/reduction reactions to decrease Cr, Se, Hg and As. Dissimilatory and
assimilatory reactions are the two main classes into which the oxidation/reduction reactions are
separated. In dissimilatory reactions, the metal(loid)s do not play any specific function in the
growth of microbes (Bolan et al. 2014). They are found by accidental reductions linked to microbial
oxidations to produce H2, alcohols, simple organic acids and aromatic compounds (Holden and
Adams 2003). In assimilatory reactions, the growth of microbes is promoted by metal(loid) to assist
as the terminal e- acceptor (Holden and Adams, 2003).
Bacteria reduce the toxicity of Cr(VI) and Hg(II) to less toxic forms by enzymatic reduction
(Choppala et al. 2015). Astudy has shown that Se(VI) reduces to Se(0) from wastewater by anaerobic
bacteria (Nejad et al. 2018). This remediation strategy has been proven as a successful approach
for wastewater treatment. The oxidation mechanism’s capability to transmute As(III) to As(V) has
been found in archaebacterium Sulfolobus acidocaldarius (Lindström and Sehlin 1989). It has been
reported that in an aqueous medium, in the presence of Fe(III), the oxidation rate of As(III) to As(V)
is enhanced. As(V) are less toxic than As(III) and firmly bound with the inorganic soil components
that result in immobilization and bioremediation through microbial oxidation (Lindström and
Sehlin 1989). A similar reduction mechanism is followed by Cr(VI) to Cr(III) in the soil component.
A study reported that Bacillus sp. isolated from the Cr-contaminated landfill reduces the potent
Cr(VI) to a lesser toxic form Cr(III) (Bolan et al. 2003). In situations with a convenient source of
e- (Fe(II)), chromate (Cr(VI)) can be reduced to Cr(III), and microbial Cr(VI) reduction takes place
in the presence of organic matter as an e- donor (Bolan et al. 2014).
9.3.4 Precipitation
In polluted soils with basic concentrations and several anions present including phosphate, carbonate,
sulfate and hydroxide, the precipitation mechanism of metal(loid)s removal has been discovered
(Ok et al. 2010). Metal(loid) precipitation like Pb and Cu with carbonate and phosphate is an
immobilization mechanism of bioremediation for elimination from soil or wastewater (McGowen
et al. 2001). Studies show that phosphate reduced the discharge of Zn, Pb and Cd (McGowen et al.
2001). Similarly, additional research presented the precipitation of Cr(III) by the addition of lime
by enhancing the soil pH (Bolan et al. 2003). The existence of iron oxyhydroxides causes changes
in the surface chemicals present on the substrate and often leads to co-precipitation of metal(loid)s
(Bolan et al. 2014, McGowen et al. 2001). The Pb(II) precipitates at pH 4 by the effect of ferric
oxyhydroxides with hydroxide chloride [Pb(OH)Cl], chloride (PbCl2) and carbonate (PbCO3),
while reacting with Mg/Al in an aqueous solution with hydroxides (Violante et al. 2007). Usually,
phosphate compounds are added to the soil to prevent heavy metal(loid) leaching (Bolan et al.
2014). Stability of metallic phosphates is found in the following order Pb > Cu > Zn (Bolan et al.
2003).
9.3.5 Biological Transformation
Metal(loids)s solubility can be enhanced by microbial processes (Krebs et al. 1997). Microbes raise
the bioavailability that results in immobilization (Park et al. 2011). Solubilization of the metalloids
by microbes is grouped into two categories: Autotrophic (chemolithotrophic) and heterotrophic
(chemoorganotrophic) (Krebs et al. 1997). Immobilization via microbes of metals could possibly
be brought into the framework by the means of reduction, precipitation, biosorption, accumulation,
sequestration and localization (Gadd 2010). Metal is removed through adsorption when metal(loid)s