Desulfurication

As a sulfate-reducing bacteria (lat. Sulfur : sulfur), and sulfate respiration or dissimilatory or bacterial sulfate reduction refers to the reduction of sulfate to sulfide and hydrogen sulfide by certain bacteria and archaea ( Desulfurizierer ). Various organic substances or elemental hydrogen (H ₂ ) are used as reducing agents . Sum equations for examples of such redox reactions are:

• ${\displaystyle \mathrm {SO_{4}^{2-}+4\ H_{2}\longrightarrow HS^{-}+3\ H_{2}O+OH^{-}} }$

Sulphate and hydrogen converted to hydrogen sulphide, water and a hydroxide ion . Sulfur is reduced from oxidation level + VI to -II, hydrogen from level O to level + I.

Under standard conditions at pH = 7, energy released per mole of converted sulfate corresponding to the change in free enthalpy :

 ΔG ₀ '= - 112 kJ / mol.

• ${\displaystyle \mathrm {SO_{4}^{2-}+2\ Lactat\longrightarrow HS^{-}+2\ Acetat+CO_{2}+HCO_{3}^{-}+H_{2}O} }$

Sulphate and lactate converted to sulphide, acetate , carbon dioxide , hydrogen carbonate and water

Energy released per mole of sulfate converted under standard conditions corresponding to the change in free energy:

ΔG ₀ '= - 157 kJ / mol.

• ${\displaystyle \mathrm {SO_{4}^{2-}+Acetat\longrightarrow HS^{-}+2\ HCO_{3}^{-}} }$

Sulphate and acetate converted to sulphide and hydrogen carbonate

Energy released per mole of sulfate converted under standard conditions corresponding to the change in free energy:

ΔG ₀ '= -47.6 kJ / mol.

These conversions are exergonic and serve as an energy source for the sulphate-reducing microorganisms . Like aerobic respiration, sulfate reduction takes place over several intermediate stages.

For comparison: the aerobic oxidation of glucose provides a free enthalpy of 1140 kJ / mol.

Desulfurizers and their properties

Desulfovibrio vulgaris : negative contrasting, transmission electron microscope image, graduation mark = 0.5 µm

Desulfurizers are obligatory anaerobic bacteria, occur only in anoxic environments and mainly use substances as reducing agents that are produced as end products in the fermentative breakdown of organic substances by fermentative bacteria, especially alcohols , organic acids and elemental hydrogen. Due to their ability to use sulfate as an oxidizing agent, they can gain energy from the oxidation of substances that cannot be used by fermentative bacteria and are therefore excreted as end products.
Related bacterial genera have the ability to reduce sulfate. This can mean that sulfate reduction was developed at an early stage in the evolution of organisms.

Within the proteobacteria, they are found in the delta group in the orders Desulfobacterales , Desulfovibrionales and Syntrophobacterales . Examples of individual genera of sulfate-reducing bacteria of the Deltaproteobacteria are: Desulfovibrio , Desulfuromonas , Desulfobulbus , Desulfobacter , Desulfococcus , Desulfosarcina , Desulfonema and Desulfotomaculum .

Apart from the deltaproteobacteria, sulfate respiration occurs in the Phylum Thermodesulfobacteria and in the order Clostridiales of the Firmicutes department (genus Desulfotomaculum ).

There are also desulfurizers in the Archaea domain , for example the genus Archaeglobus .

Ecological importance

Bacterial sulfate reduction is an important part of the sulfur cycle in the upper layers of the earth. Along with volcanic degassing, it is one of the main sources of hydrogen sulfide there. As the gross equations show, the pH value is increased by desulfurication (formation of OH ^- ions or consumption of protons). Desulfurization takes place in almost all anoxic areas that contain sulfate and usable organic substances or elemental hydrogen. Hydrogen sulphide is toxic to living things . The desulfurizers are also sensitive to the hydrogen sulfide they produce themselves. In natural habitats , however, the hydrogen sulfide formed is mostly harmless, since it with many metal ions poorly water-soluble metal sulfide forms. In natural habitats, the precipitation of iron and sulfide ions as black iron monosulfide FeS is particularly important:

${\displaystyle \mathrm {Fe^{2+}+H_{2}S\longrightarrow FeS+2\ H^{+}} }$

Removing black mud from a pond

This process is the cause of the blackening of anoxic water sediments , such as sludge sediments of ponds and lakes and some deeper layers of Watt mud . The black sediments of the Black Sea are known , which is why it is said to have received its name. Through further action of hydrogen sulfide on iron monosulfide, iron disulfide FeS ₂ can be formed over several intermediate stages, i.e. the minerals pyrite or marcasite .

Technical importance

Elemental hydrogen is formed on the surface of base metals in contact with water with the formation of metal ions. As an example, choose iron or an iron alloy, e.g. steel :

${\displaystyle \mathrm {Fe+2\ H_{2}O\longrightarrow Fe^{2+}+H_{2}+2\ OH^{-}} }$

The hydrogen layer formed in this way mostly adheres to the surface of the iron alloy and prevents the process from proceeding (“passivation”). Under anoxic conditions in the presence of sulfate, however, corrosion can occur due to sulfate-reducing bacteria, which oxidize the hydrogen with sulfate and thus expose the metal surface, so that further hydrogen formation and further metal dissolution ( corrosion ) occurs:

${\displaystyle \mathrm {4\ Fe+5\ H_{2}O+SO_{4}^{2-}\longrightarrow 4\ Fe^{2+}+HS^{-}+9\ OH^{-}} }$

Steel corrosion caused by sulphate-reducing bacteria is important in oil production , among other things . In addition, in crude oil deposits and in crude oil extraction plants, the formation of hydrogen sulphide through bacterial sulphate reduction can occur, whereby the hydrogen sulphide formed in this way does not only accumulate in the reservoir water, but also in the oil and the accompanying natural gas ("acidification", " sour gas "). This leads to disturbances because of the toxicity of hydrogen sulfide and because of its corrosive effect. In addition, a sulfur content in the oil and natural gas in the combustion of which is the emission of unwanted sulfur dioxide (SO ₂ ) result (see acid rain ), so the crude oil and natural gas desulfurization must be ( crude oil desulfurization , gas sweetening ).

literature

• Larry Barton: Sulfate-reducing bacteria . In: Biotechnology handbooks . Plenum Press, New York et al. O. 1995, ISBN 0-306-44857-2 .
• J. Martin Odom, Rivers Singleton (Ed.): The sulfate-reducing bacteria: Contemporary perspectives . In: Brock / Springer series in contemporary bioscience . Springer-Verlag, New York et al. O. 1993, ISBN 0-387-97865-8 and ISBN 3-540-97865-8 .
• John R. Postgate: The sulphate-reducing bacteria . 2nd Edition. Cambridge University Press, Cambridge GB 1984, ISBN 0-521-25791-3 .

Web links

• Bacteria corrode iron Max Planck Institute for Marine Microbiology
• Sulfate, Hydrogen Sulfide, Sulfate Reducing Bacteria - How to Identify and Manage Water Research Center
• Sulfur-Reducing Bacteria Pubget Medical Subject Headings (MESH) - A list of recent research reports.