210
A. Naz et al.
their environmental expenditure with minimum financial footprint. The most effective technique explored at the tropical countries like India is the microbe mediated
removal/reduction of Cr
6+ as the optimum temperature in these regions is suitable
for natural microbial growth, and external energy is not required for maintaining
the growth rate of the organisms. The uses of pure and mixed culture for the Cr
6+
remediation are a topic of debate between researchers working in this topic across
the globe (Sugiyama et al. 2012; Doganli and Dogan 2013).
Recent developments of microbial techniques like aerobic and anaerobic suspended sludge bioreactor system, activated sludge system, upflow sludge anaerobic
bioreactor (USAB), microbial fuel cell are widely used for bioremediation of trace
metal contaminated effluents (Venkata Nancharaiah et al. 2012; Naz and Gupta 2013;
Lytras et al. 2017; Kumari et al. 2017). Several microorganisms including aerobic
and anaerobic have been identified which can be utilized for the chemical transformation of toxic chromium (Cr
6+ ) into non-toxic form (Cr
3+ ). Thus, the treatment
of wastewater can be done by the semi-conventional microbial treatment systems,
and the microbial fuel cell can be utilized to remediate the Cr
6+ -contaminated vadose
zone and aquifer water table (Molokwane 2010; Hsu 2011; Kumar et al. 2009). Thus,
we can achieve a full-fledged Cr
6+ remediation process to decontaminate effluent,
surface water, and groundwater. Though microbial remediation technique is efficient
for the chromium contaminated wastewater, the commercial use of this technology
is still large-scale application trials. The main reason behind the limited application
of microbial remediation is lack of knowledge, less popularity, and low demand.
Thus, this knowledge gap in identifying microbial strains with Cr− remediation
potential and their possible utilization in wastewater treatment is the focus of this
chapter. Thus, the chapter deals with the widely used microbial strain for the removal
of Cr
6+ from the contaminated water, their removal mechanism, microbial tolerance
for Cr
6+ , and the application of mixed microbial culture for the removal of Cr
6+ from
contaminated wastewater and utility of hybrid microbial bioremediation systems like
microbial fuel cell to make this green process more applicable for commercial and
industrial use.
9.2 Hexavalent Chromium Reducing Microbes
There is a wide variety of microbial species isolated from the different sources,
which are not only adaptable in the metal-rich condition, but can also transform
the toxic Cr− species to its less or non-toxic forms (Kasan and Baccker 1989).
Microorganisms such as Pseudomonas fluorescens have genetic characteristics to
tolerate metal stress because of their inherent characteristics in their nuclear and
extranuclear plasmids. The microbial species found in heavy metal contaminated
sites have plasmid or chromosomal-borne resistance to metal toxicity, and they are
generally mutants among the natural population. Some of the microorganisms do
not have any plasmid-borne characteristics to reduce metals, yet they can address
this problem through certain inherent or spontaneous chromosomal mutations, e.g.
A. Naz et al.
their environmental expenditure with minimum financial footprint. The most effective technique explored at the tropical countries like India is the microbe mediated
removal/reduction of Cr
6+ as the optimum temperature in these regions is suitable
for natural microbial growth, and external energy is not required for maintaining
the growth rate of the organisms. The uses of pure and mixed culture for the Cr
6+
remediation are a topic of debate between researchers working in this topic across
the globe (Sugiyama et al. 2012; Doganli and Dogan 2013).
Recent developments of microbial techniques like aerobic and anaerobic suspended sludge bioreactor system, activated sludge system, upflow sludge anaerobic
bioreactor (USAB), microbial fuel cell are widely used for bioremediation of trace
metal contaminated effluents (Venkata Nancharaiah et al. 2012; Naz and Gupta 2013;
Lytras et al. 2017; Kumari et al. 2017). Several microorganisms including aerobic
and anaerobic have been identified which can be utilized for the chemical transformation of toxic chromium (Cr
6+ ) into non-toxic form (Cr
3+ ). Thus, the treatment
of wastewater can be done by the semi-conventional microbial treatment systems,
and the microbial fuel cell can be utilized to remediate the Cr
6+ -contaminated vadose
zone and aquifer water table (Molokwane 2010; Hsu 2011; Kumar et al. 2009). Thus,
we can achieve a full-fledged Cr
6+ remediation process to decontaminate effluent,
surface water, and groundwater. Though microbial remediation technique is efficient
for the chromium contaminated wastewater, the commercial use of this technology
is still large-scale application trials. The main reason behind the limited application
of microbial remediation is lack of knowledge, less popularity, and low demand.
Thus, this knowledge gap in identifying microbial strains with Cr− remediation
potential and their possible utilization in wastewater treatment is the focus of this
chapter. Thus, the chapter deals with the widely used microbial strain for the removal
of Cr
6+ from the contaminated water, their removal mechanism, microbial tolerance
for Cr
6+ , and the application of mixed microbial culture for the removal of Cr
6+ from
contaminated wastewater and utility of hybrid microbial bioremediation systems like
microbial fuel cell to make this green process more applicable for commercial and
industrial use.
9.2 Hexavalent Chromium Reducing Microbes
There is a wide variety of microbial species isolated from the different sources,
which are not only adaptable in the metal-rich condition, but can also transform
the toxic Cr− species to its less or non-toxic forms (Kasan and Baccker 1989).
Microorganisms such as Pseudomonas fluorescens have genetic characteristics to
tolerate metal stress because of their inherent characteristics in their nuclear and
extranuclear plasmids. The microbial species found in heavy metal contaminated
sites have plasmid or chromosomal-borne resistance to metal toxicity, and they are
generally mutants among the natural population. Some of the microorganisms do
not have any plasmid-borne characteristics to reduce metals, yet they can address
this problem through certain inherent or spontaneous chromosomal mutations, e.g.