radionuclides by addition or removal of electrons, leading to increase the mobility of
the contaminants and thus allowing it to be easily flushed from the environments
(Amachi et al. 2010; Green et al. 2012). This microbial-mediated biotransformation
presents opportunities for bioremediation of radionuclides in the environments,
either to immobilize them in place or to accelerate their removal.
Bioremediation of environmental niches (soil, sediments and water contaminated
with radionuclides) can be achieved by changing in the oxidation state through
biologically encoded biomolecules. Similarly, alternation in solubility, transport
properties and toxicity of radionuclides can take place by changing in speciation,
e.g. detoxification of mercury by methylation (Wang et al. 2012). Enzymatic reduction through oxidation-reduction, changes in pH, biodegradation of radionuclides,
biosorption by mass or biomass can bring about changes in solubility of radionuclides (Holker et al. 2002; Law et al. 2010; Hegazy and Emam 2011). Microbial
activity is mostly influenced by acceptors and electron donors, nutrients and other
environmental factors during the biotransformation of radionuclides.
As the reduced species are greatly insoluble and occur as precipitate, the oxidized
forms of radionuclides being soluble in aqueous medium are mobile in ground water.
Enzymatic reduction of soluble U(VI) by a c-type cytochrome protein in the
periplasm to insoluble species on the surface of the microorganism Shewanella
putrefaciens is reported by Wildung et al. (2000). A homologous cytochrome
(PpcA), a trihaem periplasmic cytochrome c7 of the Fe(III)-reducing bacterium
Geobacter sulfurreducens that may also play a role in U(VI) reduction in vitro
was reported by Lloyd et al. (2003).
99
Tc is long-lived radionuclide with half-life
2.13 Â 10
5 years and occurs in nuclear wastes. Tc(VII) is very difficult to remove
from solution using conventional chemical methods due to poor ligand-complexing
capabilities. The studies on the microorganisms which can reduce Tc(VII) and
precipitate the radionuclide into low-valency oxide Tc(IV) was demonstrated by
Pignolet et al. (1989). Lloyd and Macaskie (1996) observed the direct microbial
enzymatic reduction of Tc(VII) using Shewanella putrefaciens and Geobacter
metallireducens. The use of immobilized cells of sulphate-reducing bacteria such
as Desulfovibrio fructosovorans, which are capable of treating low concentration of
nitrate ions commonly occurring in nuclear waste, was demonstrated on the development of a process to decontaminate water with Tc(VII) species (Lloyd et al. 1999).
Tc and U are normally the highest-priority radionuclide contaminants in most
radioactive wastes, but other actinides including Th, Np, Pu and Am are also present
at the polluted sited (Lloyd and Macaskie 2000; Tamponnet and Declerck 2008).
These pollutants can be enzymatically reduced by iron-reducing bacteria such as
Rhodoferax ferrireducens and Geobacter sp. (Kim et al. 2012). The enzymatic
reduction of radionuclides can be triggered through indirect reduction of soluble
pollutants in soil or sedimentary environments by sulphate or iron-reducing microorganisms. For instance, Fe(III) can be bioreduced into Fe(II) and sulphur S(IV) into
S(II) in the form of hydrogen sulphide. Microbacterium flavescens grown in the
presence of nuclides such as U, Th, Am and Pu produced compounds such as
siderophores, organic acids and extracellular metabolites which are capable of
dissolving and mobilizing radionuclides with the cells (John et al. 2001).
12 Potential of Extremophiles for Bioremediation
315
the contaminants and thus allowing it to be easily flushed from the environments
(Amachi et al. 2010; Green et al. 2012). This microbial-mediated biotransformation
presents opportunities for bioremediation of radionuclides in the environments,
either to immobilize them in place or to accelerate their removal.
Bioremediation of environmental niches (soil, sediments and water contaminated
with radionuclides) can be achieved by changing in the oxidation state through
biologically encoded biomolecules. Similarly, alternation in solubility, transport
properties and toxicity of radionuclides can take place by changing in speciation,
e.g. detoxification of mercury by methylation (Wang et al. 2012). Enzymatic reduction through oxidation-reduction, changes in pH, biodegradation of radionuclides,
biosorption by mass or biomass can bring about changes in solubility of radionuclides (Holker et al. 2002; Law et al. 2010; Hegazy and Emam 2011). Microbial
activity is mostly influenced by acceptors and electron donors, nutrients and other
environmental factors during the biotransformation of radionuclides.
As the reduced species are greatly insoluble and occur as precipitate, the oxidized
forms of radionuclides being soluble in aqueous medium are mobile in ground water.
Enzymatic reduction of soluble U(VI) by a c-type cytochrome protein in the
periplasm to insoluble species on the surface of the microorganism Shewanella
putrefaciens is reported by Wildung et al. (2000). A homologous cytochrome
(PpcA), a trihaem periplasmic cytochrome c7 of the Fe(III)-reducing bacterium
Geobacter sulfurreducens that may also play a role in U(VI) reduction in vitro
was reported by Lloyd et al. (2003).
99
Tc is long-lived radionuclide with half-life
2.13 Â 10
5 years and occurs in nuclear wastes. Tc(VII) is very difficult to remove
from solution using conventional chemical methods due to poor ligand-complexing
capabilities. The studies on the microorganisms which can reduce Tc(VII) and
precipitate the radionuclide into low-valency oxide Tc(IV) was demonstrated by
Pignolet et al. (1989). Lloyd and Macaskie (1996) observed the direct microbial
enzymatic reduction of Tc(VII) using Shewanella putrefaciens and Geobacter
metallireducens. The use of immobilized cells of sulphate-reducing bacteria such
as Desulfovibrio fructosovorans, which are capable of treating low concentration of
nitrate ions commonly occurring in nuclear waste, was demonstrated on the development of a process to decontaminate water with Tc(VII) species (Lloyd et al. 1999).
Tc and U are normally the highest-priority radionuclide contaminants in most
radioactive wastes, but other actinides including Th, Np, Pu and Am are also present
at the polluted sited (Lloyd and Macaskie 2000; Tamponnet and Declerck 2008).
These pollutants can be enzymatically reduced by iron-reducing bacteria such as
Rhodoferax ferrireducens and Geobacter sp. (Kim et al. 2012). The enzymatic
reduction of radionuclides can be triggered through indirect reduction of soluble
pollutants in soil or sedimentary environments by sulphate or iron-reducing microorganisms. For instance, Fe(III) can be bioreduced into Fe(II) and sulphur S(IV) into
S(II) in the form of hydrogen sulphide. Microbacterium flavescens grown in the
presence of nuclides such as U, Th, Am and Pu produced compounds such as
siderophores, organic acids and extracellular metabolites which are capable of
dissolving and mobilizing radionuclides with the cells (John et al. 2001).
12 Potential of Extremophiles for Bioremediation
315