those electron donors (Payne et al. 2002). However, being merely detoxified rather
than removed, metals can be reoxidized chemically by oxygen or can serve as
electron (energy) sources for endogenous lithotrophic microorganisms, thus such
detoxification can be viewed as only temporary (Wall and Krumholz 2006).
24.3.3.3 Enzyme-Enhanced (Bio)Remediation
Enzymes can be used for enhancing other bioremediation methods. In many cases,
enzymatic attacks on toxic pollutants lead to both reduction of their toxicity and
increase in their biodegradability. Thus, microbial bioremediation can be fundamentally improved, especially in the case of recalcitrant pollutants with heteroatoms,
e.g., organophosphates, chlorinated and nitro compounds. For example, undefined
mixtures of extracellular enzymes produced by a microbial consortium under induction by diesel as a substrate were applied during microbial degradation of hydrocarbons on three different oil-contaminated sites. A diesel hydrocarbon degradation of
approximately 90% was achieved in 60 h for enzyme-enhanced tests, whereas as
many as 175 h were needed to match this efficiency for the degradation tests
involving only microbial cells (Jiménez-T et al. 2011).
An application of nano-bio decontamination based on sequential reduction–oxidation was reported by Bokare et al. (2010) for triclosan detoxification under anaerobic
conditions (Bokare et al. 2010). A rapid reductive dechlorination by palladized zerovalent iron nanoparticles generated 2-phenoxyphenol, followed by its transformation
into a non-toxic polymer using laccase derived from Trametes versicolor in the
presence of a natural redox mediator, syringaldehyde (Bokare et al. 2010).
24.3.3.4 Enhanced Applications of Enzymes
A number of ways to immobilize/stabilize enzymes on (nano)carriers, e.g., entrapment, encapsulation, adsorption, covalent binding, and self-immobilization as well as
many types of nanocarriers, both natural or artificial, e.g., mesostructured silica
materials, magnetite nanoparticles, caolinite, porous carbon tubes, and alginate have
been described so far (Fernández-Fernández et al. 2013). For example, the laccase of
Trametes versicolor was immobilized with only a slight loss of activity on
functionalized nanoparticles SBA-15 with the average diameter smaller than 10 nm,
which are suitable for applications to soil bioremediation (Fernando Bautista et al.
2010). To decrease the anthracene toxicity by conversion to anthroquinone, the laccase
from Trametes versicolor was immobilized on functionalized (silicated) kaolinite
using glutaraldehyde. In both cases, significant enzyme stabilization was shown.
Among other applications, Hong et al. (2017) developed an enzyme-silicate
conjugate material consisting of a self-assembled molecular-size thin silicate network cage encasing each individual enzyme molecule (α-chymotrypsin and lipase).
Due to the near molecular size of these conjugates, both minimized substrate
diffusion limitations and high enzyme stability were achieved. While the k cat /K m
24 Modern Bioremediation Approaches: Use of Biosurfactants, Emulsifiers. . .
511
than removed, metals can be reoxidized chemically by oxygen or can serve as
electron (energy) sources for endogenous lithotrophic microorganisms, thus such
detoxification can be viewed as only temporary (Wall and Krumholz 2006).
24.3.3.3 Enzyme-Enhanced (Bio)Remediation
Enzymes can be used for enhancing other bioremediation methods. In many cases,
enzymatic attacks on toxic pollutants lead to both reduction of their toxicity and
increase in their biodegradability. Thus, microbial bioremediation can be fundamentally improved, especially in the case of recalcitrant pollutants with heteroatoms,
e.g., organophosphates, chlorinated and nitro compounds. For example, undefined
mixtures of extracellular enzymes produced by a microbial consortium under induction by diesel as a substrate were applied during microbial degradation of hydrocarbons on three different oil-contaminated sites. A diesel hydrocarbon degradation of
approximately 90% was achieved in 60 h for enzyme-enhanced tests, whereas as
many as 175 h were needed to match this efficiency for the degradation tests
involving only microbial cells (Jiménez-T et al. 2011).
An application of nano-bio decontamination based on sequential reduction–oxidation was reported by Bokare et al. (2010) for triclosan detoxification under anaerobic
conditions (Bokare et al. 2010). A rapid reductive dechlorination by palladized zerovalent iron nanoparticles generated 2-phenoxyphenol, followed by its transformation
into a non-toxic polymer using laccase derived from Trametes versicolor in the
presence of a natural redox mediator, syringaldehyde (Bokare et al. 2010).
24.3.3.4 Enhanced Applications of Enzymes
A number of ways to immobilize/stabilize enzymes on (nano)carriers, e.g., entrapment, encapsulation, adsorption, covalent binding, and self-immobilization as well as
many types of nanocarriers, both natural or artificial, e.g., mesostructured silica
materials, magnetite nanoparticles, caolinite, porous carbon tubes, and alginate have
been described so far (Fernández-Fernández et al. 2013). For example, the laccase of
Trametes versicolor was immobilized with only a slight loss of activity on
functionalized nanoparticles SBA-15 with the average diameter smaller than 10 nm,
which are suitable for applications to soil bioremediation (Fernando Bautista et al.
2010). To decrease the anthracene toxicity by conversion to anthroquinone, the laccase
from Trametes versicolor was immobilized on functionalized (silicated) kaolinite
using glutaraldehyde. In both cases, significant enzyme stabilization was shown.
Among other applications, Hong et al. (2017) developed an enzyme-silicate
conjugate material consisting of a self-assembled molecular-size thin silicate network cage encasing each individual enzyme molecule (α-chymotrypsin and lipase).
Due to the near molecular size of these conjugates, both minimized substrate
diffusion limitations and high enzyme stability were achieved. While the k cat /K m
24 Modern Bioremediation Approaches: Use of Biosurfactants, Emulsifiers. . .
511