Sulfate-reducing bacteria
Contents
- 1 SRB for treatment of acid mine drainage
- 2 Hydrogen sulphide
- 3 Classification of SRB
- 4 Extremophilic SRB
- 5 Acidophilic SRB
- 6 Thermophilic SRB
- 7 Cold-adapted SRB
- 8 Sources of energy for SRB
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Sulfate-reducing bacteria (SRB) form one group of sulfate reducing prokaryotes. Main genus is Desulfovibrio. Desulfovibrio desulfuricans is often used to immobilize dissolved heavy metals as metallic sulfides.
Beijerinck
[1] showed in 1895 that living matter could reduce sulphate to sulphide
in sediments under anaerobic conditions. Although many bacteria can
produce sulphide, only a few do so at a sufficient rate for application in high-rate processes. These rapid sulphide-generating bacteria are able to conserve energy by the reduction of sulfur oxyanions
[2], and they are generally termed sulphate-reducing bacteria (SRB). A typical overall conversion equation is (neglecting the small amount of organic material required to produce biomass):
- SO42- + CH3COOH + 2 H+ → HS- + 2 HCO3- + 3 H+ (1)
Eight electrons are transferred from the energy source acetic acid
to the electron acceptor sulphate in order to produce sulphide. The
reaction equation shows that in the same process also alkalinity is
produced. This leads to an increase in the
pH of the water, often to a near neutral value.
Typically, a certain amount of metals is present together with
the sulfate. These metals will react with the dissolved sulfide to form
highly insoluble metals sulfides.
- HS- + Me2+ → MeS + H+ (2)
Me
2+ can for example be
copper,
zinc etc.
Combining the action of SRBs and sulfide oxidizing microbes.
Sulfate reduction by SRBs and sulfide oxidation by oxidising microbes.
To the left - one SRB bacterium with elemental sulfur particles on the
cell membrane
SRB for treatment of acid mine drainage
Sulphate-rich wastewaters are generated by many industrial processes and cause an unbalance in the natural sulphur cycle.
AMD in the Bolivian Andean region.
The effluents produced in sulphide ore mines, defined as acid mine drainage (AMD), also contain large amounts of heavy metals.
Mining and industrial drainage containing sulphate and heavy metal
negatively affects terrestrial and aquatic ecosystems in several
countries around the world.
Sulphate-reducing bacteria (SRB) can be used to biologically treat
sulphate-rich wastewater. SRB comprise several groups of bacteria that
reduce sulphate to sulphide and produce carbonate which increase the pH.
In AMD treatment processes this chemically stabilizes the toxic metal ions as solid metal sulphides
[3].
Hydrogen sulphide
The reduction product of reaction I, hydrogen sulphide, is a volatile gas. The form in which sulphide occurs depends on the pH:
- H2S → HS- + H+ → S2- + 2H- (3)
HS
- and S
2-which occur at neutral and high pH respectively are both water soluble. H
2S is the predominant form at low pH (<6)
[4][5].
Sulphide is distributed over the gas phase (g) and the liquid phase (l) according to:
- [H2S]l =α*[H2S]g (mol/m3) (4)
α is a dimensionless distribution coefficient. The unionised H
2S concentration also depends on the
temperature. Sulphide is highly reactive, corrosive and toxic to microorganisms
[6].
The toxicity increases at low pH while only the un-ionised hydrogen
sulphide form is able to permeate through the cell membrane. H2S affects
the intracellular pH of the microorganism and impedes its metabolism
[7][8].
Classification of SRB
SRB are obligate anaerobes and members of a heterogeneous group of eubacteria and archaebacteria which are able to carry out dissimilatory sulphate reduction
(Colleran et al.,1995; Hansen, 1994). The SRB can be subdivided into
two groups depending on their oxidative capability: the genera that
completely oxidise the organic substrate to CO2, and the bacteria that oxidise the organic compound incompletely usually with acetate
as an end product. The species able to completely oxidise organic
carbon sources mainly prefers fatty acids, lactate and succinate as energy sources. Incomplete oxidation is due to the absence of a mechanism for acetyl-Co-A oxidation. Such bacteria generally prefer simple substrates such as
hydrogen, lactate and primary alcohols (Alvarez, 2005; Kolmert, 1999; Vallero, 2003).
SRB can survive in a wide range of pH conditions but commonly have a pH optimum for growth between pH 5-9 (Jong et al., 2006). SRB populations have been obtained at temperatures ranging from the [psychrophilic]] to the hyperthermophilic range (Kolmert, 1999).
Extremophilic SRB
Regarding the applications for biological treatment processes, the
significance of some extremophilic bacteria should be emphasized. Among
the diversity of sulphate-reducing prokaryotes the acidophilic, thermophilic and psychrotolerant bacteria are extremophiles that could improve the performance of existing treatment systems.
Acidophilic SRB
During mining activities oxygen is introduced into deep geological environments and cause
chemical and biological oxidation processes. Sulphate and hydrogen ions are produced which
lower pH significantly (Kolmert, 1999; Madigan et al., 2000). The pH is generally between 2
and 4 and commonly less than 3. Current biological acid mine drainage treatment systems
mainly use neutrophilic SRB, highly sensitive to acidic water; resulting in few successful
applications (Jong et al., 2006). To run the system “off line” is a method to circumvent this
problem. The SRB grow in an isolated neutral tank where hydrogen sulphide is produced and
the effluent is transferred to a second reactor. This tank contains the contaminated water
which results in precipitation of metal sulphide. Acidophilic or acido-tolerant bacteria are able
to grow in direct contact with the acidic liquid in a single reactor tank. This could simplify the
system and be a less expensive solution to the two tanks treatment system existing today
(Kimura et al., 2006; Kolmert et al., 2001).
Thermophilic SRB
Wastewater containing sulphur compounds is generated by several industrial processes.
Examples of industries contributing to imbalances in the natural sulphur cycle are those using
sulphuric acid or sulphate-rich feed stocks, such as the food and fermentation industry. Some
industrial wastewaters are discharged at high temperatures of 50 to 70ºC and even above 90ºC.
The use of thermophilic SRB to treat such wastewater holds some advantages and may be an attractive
alternative to treat the discharge mesophilically. It eliminates the cooling of the process water
and allows direct use of the treated water without additional re-heating. Besides, the termophilic
systems produce less sludge and are capable of treating higher organic loading rates with feasible
removal efficiency (Vallero, 2003; Pender et al., 2004).
Cold-adapted SRB
Treatment of AMD and industrial wastewater functional in low temperatures are of potential
interest in countries with cold environments. It could be realized by the use of psychrotolerant
sulphate-reducing prokaryotes. Psychrophilic SRB generally have a growth optimum
temperature of 18ºC while the optimum for sulphate reduction is 28ºC. However, bacteria
reducing sulphate below 4°C have been identified. SRB are sometimes less active in low
temperatures and the lower reaction rates of the process could be compensated by an increased
number of bacteria (Knoblauch et al., 1999; Sahm et al., 1999).
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