released to the environment as solutes or particles that can reach high concentrations,
especially near the discharge site (Krishna and Govil 2007). In general, heavy metals
accumulate in aquatic environments, mainly in sediments, debris and organic matter,
where they may be consumed by fish, which tend to accumulate metals in the gills
and intestines magnifying their concentration to a toxic level. This generates serious
problems in health and the trophic chain of organisms affecting all kinds of
ecosystems.
In actuality exist the needed to implement technological actions in order to
diminish the pollution problems. Among these technological actions, the biotechnology processes have the great advantage of high specificity for the removal of
chemical compounds as heavy metals with low energy consumption. The
microorganisms are the biological agents most used on the treatments of polluting
compounds, due to their degradation capacities of organic and inorganic pollutants,
but the metals are not degraded but only can be modified in their redox state,
converting them to less toxic forms (Wood and Wang 1983). Precipitation and
biosorption are the most employed process for metal ion removal from water.
Microalgae, fungi and bacteria have been used successfully in the removal of metals,
but in the last decade, sulfate-reducing bacteria (SRB) have taken great relevance in
the processes of metal removal from wastewater (López-Pérez et al. 2016). The
removal of heavy metals is promoted when hydrogen sulfide is produced; it reacts
with metal ions and forms metal sulfides, which are insoluble and tend to precipitate;
however, it is not the only mechanism by which metal ions are removed, but
biosorption mechanisms can also be carried out (with biomass and production of
exo-polysaccharides), immobilization and enzymatic reduction of the metal ion to
less toxic and insoluble forms, but this will depend on the type of bacteria and their
tolerance to metals (Li et al. 2018). Specifically, the species of the genus
Desulfovibrio have been the most studied for metal removal with a high efficiency,
where the main mechanism is the precipitation of metal sulfides between others. In
particular, D. alaskensis 6SR exhibits a high metallic resistance with respect to other
sulfate-reducing bacteria, including other microorganisms. Therefore, this strain is
considered as a model for the removing of metals under anaerobic conditions.
13.2 Heavy Metal Removal
Heavy metals are chemical elements; in principle, these are the simpler substances of
the matter. The chemical elements are found ordered and classified in the periodic
table. Currently, this contains 118 chemical elements; the most were discovered and
some synthesized. Metals are the most abundant chemical elements, and these are
classified in the periodic table as alkali metals, alkaline earth metals, transition
metals, lanthanides and actinides.
However, the term “heavy metals” has been widely used to refer to any metallic
or semimetallic chemical element. In the scientific literature, an authoritative definition is not found. But the authors have considered the density to refer to the term
“heavy”; some of them have proposed densities among 3.5–7.5 g/cm. Therefore, a
13 Heavy Metal Removal Processes by Sulfate-Reducing Bacteria
369
especially near the discharge site (Krishna and Govil 2007). In general, heavy metals
accumulate in aquatic environments, mainly in sediments, debris and organic matter,
where they may be consumed by fish, which tend to accumulate metals in the gills
and intestines magnifying their concentration to a toxic level. This generates serious
problems in health and the trophic chain of organisms affecting all kinds of
ecosystems.
In actuality exist the needed to implement technological actions in order to
diminish the pollution problems. Among these technological actions, the biotechnology processes have the great advantage of high specificity for the removal of
chemical compounds as heavy metals with low energy consumption. The
microorganisms are the biological agents most used on the treatments of polluting
compounds, due to their degradation capacities of organic and inorganic pollutants,
but the metals are not degraded but only can be modified in their redox state,
converting them to less toxic forms (Wood and Wang 1983). Precipitation and
biosorption are the most employed process for metal ion removal from water.
Microalgae, fungi and bacteria have been used successfully in the removal of metals,
but in the last decade, sulfate-reducing bacteria (SRB) have taken great relevance in
the processes of metal removal from wastewater (López-Pérez et al. 2016). The
removal of heavy metals is promoted when hydrogen sulfide is produced; it reacts
with metal ions and forms metal sulfides, which are insoluble and tend to precipitate;
however, it is not the only mechanism by which metal ions are removed, but
biosorption mechanisms can also be carried out (with biomass and production of
exo-polysaccharides), immobilization and enzymatic reduction of the metal ion to
less toxic and insoluble forms, but this will depend on the type of bacteria and their
tolerance to metals (Li et al. 2018). Specifically, the species of the genus
Desulfovibrio have been the most studied for metal removal with a high efficiency,
where the main mechanism is the precipitation of metal sulfides between others. In
particular, D. alaskensis 6SR exhibits a high metallic resistance with respect to other
sulfate-reducing bacteria, including other microorganisms. Therefore, this strain is
considered as a model for the removing of metals under anaerobic conditions.
13.2 Heavy Metal Removal
Heavy metals are chemical elements; in principle, these are the simpler substances of
the matter. The chemical elements are found ordered and classified in the periodic
table. Currently, this contains 118 chemical elements; the most were discovered and
some synthesized. Metals are the most abundant chemical elements, and these are
classified in the periodic table as alkali metals, alkaline earth metals, transition
metals, lanthanides and actinides.
However, the term “heavy metals” has been widely used to refer to any metallic
or semimetallic chemical element. In the scientific literature, an authoritative definition is not found. But the authors have considered the density to refer to the term
“heavy”; some of them have proposed densities among 3.5–7.5 g/cm. Therefore, a
13 Heavy Metal Removal Processes by Sulfate-Reducing Bacteria
369