Sulfate-reducing bacteria

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):

SO₄^2- + 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)

Me²⁺ can for example be copper, zinc etc. 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⁺ → S^2- + 2H^-     (3)

HS^- and S^2-which occur at neutral and high pH respectively are both water soluble. H₂S 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 H₂S 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).