A different yet similarly hampering limitation of enzymatic remediation is characteristic for metal and metalloid contaminated sites. Enzymes as well as microorganisms can convert toxic metals and metalloids into their less toxic forms, e.g.,
reduced or insoluble, but they cannot physically remove them from a contaminated
site; furthermore, such conversion is reversible (Wall and Krumholz 2006).
One more limitation is that the enzymes used for pollution bioremediation, e.g.,
chitinase, carboxymethyl cellulase, β-glucosidase, protease, acid phosphatase, polyphenol oxidase, laccase, and guaiacol oxidase also exhibit an undesired activity
against organic soil colloids (Wang et al. 2014), thus decreasing the efficiency of the
attack on the target pollutant because of competition.
Natural biodegradability of enzymes can shorten the lifetime of the applied
enzymes because of their decomposition in soil by naturally occurring peptidases
and microorganisms. Also the risk of mutual decomposition, e.g., proteolysis, should
be taken into account when mixtures of enzymes are applied; therefore, repetitive
enzyme application can be essential (Gianfreda et al. 2016). In contrast, specific
features of live cells lacking in enzyme application, namely self-replication, active
movement (chemotaxis), adaptation (changes in cell transport and metabolisms),
evolution (mutation, horizontal gene transfer), and production of supporting substances (biosurfactants for pollutants’ release) may favor their use.
24.3.3 Application Potential of Enzymes in Bioremediation
Although the list of potential disadvantages is relatively long, a number of promising
lab-scale applications of enzymes, either free or immobilized, have been described,
as summarized below. The current focus addressing most of the above-listed limitations is on using nanoscale carriers for bioremediation of toxic and recalcitrant
pollutants.
24.3.3.1 Treatment of Organic Pollutants
As stated above, enzymatic attacks cause detoxification of complex pollutants rather
than their mineralization. For example, bisphenol A was completely removed within
3 h by the action of Coriolopsis gallica laccase; however, GC-MS analyses revealed
its conversion into carboxylic acid derivatives such as tartaric, β-hydroxybutyric, and
pyroglutamic acids (Daâssi et al. 2016). Similarly, several main metabolites, e.g.,
10,11-dihydro-10,11-dihydroxy-CBZ,
10,11-dihydro-10,11-epoxy-CBZ,
and
acridone were identified resulting from the laccase action on carbamazepine. The
concomitant decrease of biotoxicity was confirmed (Ji et al. 2016). In another study,
both free and immobilized laccases decreased the concentration of sulfathiazole and
sulfamethoxazole as well as their biotoxicity in the presence of
1-hydroxybenzotriazole as a laccase mediator. Intermediates were not determined;
however, detoxification rather than mineralization was assumed (Rahmani et al. 2015).
24 Modern Bioremediation Approaches: Use of Biosurfactants, Emulsifiers. . .
509
reduced or insoluble, but they cannot physically remove them from a contaminated
site; furthermore, such conversion is reversible (Wall and Krumholz 2006).
One more limitation is that the enzymes used for pollution bioremediation, e.g.,
chitinase, carboxymethyl cellulase, β-glucosidase, protease, acid phosphatase, polyphenol oxidase, laccase, and guaiacol oxidase also exhibit an undesired activity
against organic soil colloids (Wang et al. 2014), thus decreasing the efficiency of the
attack on the target pollutant because of competition.
Natural biodegradability of enzymes can shorten the lifetime of the applied
enzymes because of their decomposition in soil by naturally occurring peptidases
and microorganisms. Also the risk of mutual decomposition, e.g., proteolysis, should
be taken into account when mixtures of enzymes are applied; therefore, repetitive
enzyme application can be essential (Gianfreda et al. 2016). In contrast, specific
features of live cells lacking in enzyme application, namely self-replication, active
movement (chemotaxis), adaptation (changes in cell transport and metabolisms),
evolution (mutation, horizontal gene transfer), and production of supporting substances (biosurfactants for pollutants’ release) may favor their use.
24.3.3 Application Potential of Enzymes in Bioremediation
Although the list of potential disadvantages is relatively long, a number of promising
lab-scale applications of enzymes, either free or immobilized, have been described,
as summarized below. The current focus addressing most of the above-listed limitations is on using nanoscale carriers for bioremediation of toxic and recalcitrant
pollutants.
24.3.3.1 Treatment of Organic Pollutants
As stated above, enzymatic attacks cause detoxification of complex pollutants rather
than their mineralization. For example, bisphenol A was completely removed within
3 h by the action of Coriolopsis gallica laccase; however, GC-MS analyses revealed
its conversion into carboxylic acid derivatives such as tartaric, β-hydroxybutyric, and
pyroglutamic acids (Daâssi et al. 2016). Similarly, several main metabolites, e.g.,
10,11-dihydro-10,11-dihydroxy-CBZ,
10,11-dihydro-10,11-epoxy-CBZ,
and
acridone were identified resulting from the laccase action on carbamazepine. The
concomitant decrease of biotoxicity was confirmed (Ji et al. 2016). In another study,
both free and immobilized laccases decreased the concentration of sulfathiazole and
sulfamethoxazole as well as their biotoxicity in the presence of
1-hydroxybenzotriazole as a laccase mediator. Intermediates were not determined;
however, detoxification rather than mineralization was assumed (Rahmani et al. 2015).
24 Modern Bioremediation Approaches: Use of Biosurfactants, Emulsifiers. . .
509