Biosorption involves the sequestration of positively charges metal ions to the
negatively charged cell membranes and polysaccharides secreted on the outer
surfaces of bacteria through capsule and slime formation (Praksh et al. 2013).
Several microorganisms such as Citrobacter freundii and Firmicutes have been
reported radionuclide biosorbents (Haferburg et al. 2007; Xie et al. 2008).
Biosorption alone may not be sufficient to remove radionuclides unless the ground
biomass content is enhanced. Biostimulation using specific communities of microorganism can also enhance the bioremediation of radionuclides. Nitrate serves as an
energetically favourable electron acceptor for metal-reducing bacterial in nitric acid
co-contaminated sediments (DiChristina 1992). Finneran et al. (2002) reported that
the lack of microbial reduction in U(VI) due to presence of nitrate as a
co-contaminant in sediment. Wu et al. (2006) reported that this issue can be resolved
by the ex situ treatment and removal of nitrate and heavy metals before in situ
biostimulation to reduce the U(VI). A number of microorganisms such as
Desulfovibrio sp., Geobacter sp. and Shewanella sp. have been shown to carry out
reductive precipitation of radionuclides. Some microorganisms such as Citrobacter
sp. can interact with metals ions and immobilize for transformation or generate
biofilms to bind metallic ions, hence serving as a platform for the precipitation of
insoluble minerals (Keasling et al. 2000). Fredrickson et al. (2000) have shown that
the microorganism Deinococcus radiodurans can detoxify Cr(VI), Tc(VII) and U
(VI) from soil. Brim et al. (2003) reported that the microorganisms such as
Deinococcus geothermalis, Deinococcus murrayl have high resistance against
chronic irradiation (50 Gy h
À1 ) and are able to grow at higher temperature
(55
C). Lloyd et al. (2003) has shown that microbial family Geobacteraceae has
potential for radioactive metal reduction.
Thus, the study of the molecular mechanisms behind the extremophilic microbial
transformation of radionuclides and exploiting them in bioremediation would help in
tracking the responsible microbial metabolic products towards cell-free bioremediation and further assist in efficient removal of radionuclides from the contaminant
environments.
12.5.5 Bioremediation of Wastewater Treatment
The main substances found in wastewater are organic and inorganic compounds,
dyes and salts. The primary objective of a wastewater treatment plants is to reduce
the concentrations of pollutants to the level at which the discharge of the effluent will
not adversely affect the environment or pose a health threat. The leftover sludge at
wastewater treatment plants is treated through anaerobic digestion which is one of
the most promising and favourable technology. Breakdown of sewage effluent are
normally carried out by microorganisms which are able to live in the sludge of
treatment plants. They obtain nutrients by degrading the solids in wastewater to
various compounds. Biological treatments of wastewater involve not only carbon
removal, but also elimination of other nutrients such as nitrogen and phosphorus.
316
S. Kaushik et al.
negatively charged cell membranes and polysaccharides secreted on the outer
surfaces of bacteria through capsule and slime formation (Praksh et al. 2013).
Several microorganisms such as Citrobacter freundii and Firmicutes have been
reported radionuclide biosorbents (Haferburg et al. 2007; Xie et al. 2008).
Biosorption alone may not be sufficient to remove radionuclides unless the ground
biomass content is enhanced. Biostimulation using specific communities of microorganism can also enhance the bioremediation of radionuclides. Nitrate serves as an
energetically favourable electron acceptor for metal-reducing bacterial in nitric acid
co-contaminated sediments (DiChristina 1992). Finneran et al. (2002) reported that
the lack of microbial reduction in U(VI) due to presence of nitrate as a
co-contaminant in sediment. Wu et al. (2006) reported that this issue can be resolved
by the ex situ treatment and removal of nitrate and heavy metals before in situ
biostimulation to reduce the U(VI). A number of microorganisms such as
Desulfovibrio sp., Geobacter sp. and Shewanella sp. have been shown to carry out
reductive precipitation of radionuclides. Some microorganisms such as Citrobacter
sp. can interact with metals ions and immobilize for transformation or generate
biofilms to bind metallic ions, hence serving as a platform for the precipitation of
insoluble minerals (Keasling et al. 2000). Fredrickson et al. (2000) have shown that
the microorganism Deinococcus radiodurans can detoxify Cr(VI), Tc(VII) and U
(VI) from soil. Brim et al. (2003) reported that the microorganisms such as
Deinococcus geothermalis, Deinococcus murrayl have high resistance against
chronic irradiation (50 Gy h
À1 ) and are able to grow at higher temperature
(55
C). Lloyd et al. (2003) has shown that microbial family Geobacteraceae has
potential for radioactive metal reduction.
Thus, the study of the molecular mechanisms behind the extremophilic microbial
transformation of radionuclides and exploiting them in bioremediation would help in
tracking the responsible microbial metabolic products towards cell-free bioremediation and further assist in efficient removal of radionuclides from the contaminant
environments.
12.5.5 Bioremediation of Wastewater Treatment
The main substances found in wastewater are organic and inorganic compounds,
dyes and salts. The primary objective of a wastewater treatment plants is to reduce
the concentrations of pollutants to the level at which the discharge of the effluent will
not adversely affect the environment or pose a health threat. The leftover sludge at
wastewater treatment plants is treated through anaerobic digestion which is one of
the most promising and favourable technology. Breakdown of sewage effluent are
normally carried out by microorganisms which are able to live in the sludge of
treatment plants. They obtain nutrients by degrading the solids in wastewater to
various compounds. Biological treatments of wastewater involve not only carbon
removal, but also elimination of other nutrients such as nitrogen and phosphorus.
316
S. Kaushik et al.