highly soluble form, whereas reduced form of actinides is insoluble and immobile
under aqueous condition (Humphries and Macaskie 2002; Istok et al. 2004).
Therefore, these reduced elements are often found in precipitated form. Under
in vitro condition, Desulfovibrio vulgaris have the ability to reduce U(VI) and Cr
(VI) to U(IV) and Cr(III) where H 2 act as electron donor and cytochrome c 3 as Cr
reductase (Lovley and Phillips 1994; Lovley et al. 1993). In 2002, another
experiment was performed to understand the involvement of cytochrome c 3 and
hydrogenase protein in metal reduction. It was found that the c 3 mutant
D. desulfuricans strain G20 was able to reduce uranium along with lactate or
pyruvate as electron donor, but the rate of reduction was decreased as compared
to the wild-type. From this study, it was concluded that cytochrome c 3 is a part of
metal reduction along with hydrogenase, and it can be bypassed by additional
pathways (Payne et al. 2002).
2. Indirect immobilization of radionuclides
Indirect immobilization means immobilization of primary molecules via
bioreduction of a secondary molecule. For example, iron Fe(III) and sulphur S
(VI) can be reduced by microbes into Fe(II) and S(II) form and the oxidation of
that bioreduced Fe & S can reduce a primary molecule and transform them into
mobile to immobile molecule (Prakash et al. 2013). Technetium-99 [Tc(VII)] is
an example of higher risk driving radioactive waste. Indirect mechanism play
important role in immobilization of Tc(VII), where bioreduced Fe(II) directly
donate electron to Tc(VII). After accepting electron from Fe(II) the reduced Tc
(VII) becomes immobile (Kumar et al. 2007). Geobacter metallireducens has the
ability to reduce Fe(III) as ferrihydrite Fe(II) enzymatically, when the cell is
exposed to the highly soluble U(VI), it converts U(VI) to poorly soluble U
(IV) (Lloyd 2003). Another indirect immobilization process involves the
siderophore production and complexation. For example, Microbacterium
flavescens developed under radioactive waste condition secretes various organic
acids, siderophores, extracellular metabolites which mix with and assemble the
radionuclides in the form of dirt (Banerjee et al. 2018; Kumar et al. 2007).
13.2.1.2 Biomineralization
The term biomineralization refers to the process of metal precipitation at the
microbial cell surface with the help of ligands such as sulphides, carbonates,
phosphates and hydroxides generated by microbes (Jiang et al. 2019). Bacteria
like Citrobacter species and Serratia species showed efficient uranium biomineralization (Ding et al. 2019). It was observed that under glycerol phosphate condition,
the cell shows phosphatase activity and releases inorganic phosphates which ultimately form complexes with uranium in the form of hydrogen uranyl phosphate at
the cell surface (Beazley et al. 2007). Similar uranium biomineralization was noticed
earlier when Pseudomonas species was supplied with tributylphosphate (Thomas
and Macaskie 1996). Bacterial cells covered with uranium phosphates were isolated
from uranium contaminated soils, which suggest that biomineralization is a naturally
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U. K. Vandana et al.
under aqueous condition (Humphries and Macaskie 2002; Istok et al. 2004).
Therefore, these reduced elements are often found in precipitated form. Under
in vitro condition, Desulfovibrio vulgaris have the ability to reduce U(VI) and Cr
(VI) to U(IV) and Cr(III) where H 2 act as electron donor and cytochrome c 3 as Cr
reductase (Lovley and Phillips 1994; Lovley et al. 1993). In 2002, another
experiment was performed to understand the involvement of cytochrome c 3 and
hydrogenase protein in metal reduction. It was found that the c 3 mutant
D. desulfuricans strain G20 was able to reduce uranium along with lactate or
pyruvate as electron donor, but the rate of reduction was decreased as compared
to the wild-type. From this study, it was concluded that cytochrome c 3 is a part of
metal reduction along with hydrogenase, and it can be bypassed by additional
pathways (Payne et al. 2002).
2. Indirect immobilization of radionuclides
Indirect immobilization means immobilization of primary molecules via
bioreduction of a secondary molecule. For example, iron Fe(III) and sulphur S
(VI) can be reduced by microbes into Fe(II) and S(II) form and the oxidation of
that bioreduced Fe & S can reduce a primary molecule and transform them into
mobile to immobile molecule (Prakash et al. 2013). Technetium-99 [Tc(VII)] is
an example of higher risk driving radioactive waste. Indirect mechanism play
important role in immobilization of Tc(VII), where bioreduced Fe(II) directly
donate electron to Tc(VII). After accepting electron from Fe(II) the reduced Tc
(VII) becomes immobile (Kumar et al. 2007). Geobacter metallireducens has the
ability to reduce Fe(III) as ferrihydrite Fe(II) enzymatically, when the cell is
exposed to the highly soluble U(VI), it converts U(VI) to poorly soluble U
(IV) (Lloyd 2003). Another indirect immobilization process involves the
siderophore production and complexation. For example, Microbacterium
flavescens developed under radioactive waste condition secretes various organic
acids, siderophores, extracellular metabolites which mix with and assemble the
radionuclides in the form of dirt (Banerjee et al. 2018; Kumar et al. 2007).
13.2.1.2 Biomineralization
The term biomineralization refers to the process of metal precipitation at the
microbial cell surface with the help of ligands such as sulphides, carbonates,
phosphates and hydroxides generated by microbes (Jiang et al. 2019). Bacteria
like Citrobacter species and Serratia species showed efficient uranium biomineralization (Ding et al. 2019). It was observed that under glycerol phosphate condition,
the cell shows phosphatase activity and releases inorganic phosphates which ultimately form complexes with uranium in the form of hydrogen uranyl phosphate at
the cell surface (Beazley et al. 2007). Similar uranium biomineralization was noticed
earlier when Pseudomonas species was supplied with tributylphosphate (Thomas
and Macaskie 1996). Bacterial cells covered with uranium phosphates were isolated
from uranium contaminated soils, which suggest that biomineralization is a naturally
342
U. K. Vandana et al.