The usage of sulfate-reducing bacteria in deposition of metals, conditions of their functioning

Lectures 16-17

Sulfate-reducing bacteria are those bacteria and archaea that can obtain energy by oxidizing organic compounds or molecular hydrogen (H2) while reducing sulfate (SO2−4) to hydrogen sulfide (H2S). In a sense, these organisms "breathe" sulfate rather than oxygen, in a form of anaerobic respiration.

Sulfate-reducing bacteria can be traced back to 3.5 billion years ago and are considered to be among the oldest forms of microorganisms, having contributed to the sulfur cycle soon after life emerged on Earth.

Many bacteria reduce small amounts of sulfates in order to synthesize sulfur-containing cell components; this is known as assimilatory sulfate reduction. By contrast, the sulfate-reducing bacteria considered here reduce sulfate in large amounts to obtain energy and expel the resulting sulfide as waste; this is known as dissimilatory sulfate reduction. They use sulfate as the terminal electron acceptor of their electron transport chain. Most of them are anaerobes.

Most sulfate-reducing bacteria can also reduce other oxidized inorganic sulfur compounds, such as sulfite, thiosulfate, or elemental sulfur (which is reduced to sulfide as hydrogen sulfide).

In addition, there are sulfate-reducing bacteria that can reduce fumarate, nitrate and nitrite, iron (Fe(III)) and some other metals, dimethyl sulfoxide and even oxygen.

Sulfate occurs widely in seawater, sediment, or water rich in decaying organic material. Sulfate-reducing bacteria are common in anaerobic environments where they aid in the degradation of organic materials. In these anaerobic environments, fermenting bacteria extract energy from large organic molecules; the resulting smaller compounds such as organic acids and alcohols are further oxidized by acetogens and methanogens and the competing sulfate-reducing bacteria.

Sludge from a pond; the black color is due to metal sulfides that result from the action of sulfate-reducing bacteria. The toxic hydrogen sulfide is a waste product of sulfate-reducing bacteria; its rotten egg odor is often a marker for the presence of sulfate-reducing bacteria in nature. Sulfate-reducing bacteria are responsible for the sulfurous odors of salt marshes and mud flats. Much of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides. These metal sulfides, such as ferrous sulfide (FeS), are insoluble and often black or brown, leading to the dark color of sludge.

During the Permian–Triassic extinction event (250 million years ago) a severe anoxic event seems to have occurred where these forms of bacteria became the dominant force in oceanic ecosystems, producing copious amounts of hydrogen sulfide.

In engineering, sulfate-reducing bacteria can create problems when metal structures are exposed to sulfate-containing water: Interaction of water and metal creates a layer of molecular hydrogen on the metal surface; sulfate-reducing bacteria then oxidize the hydrogen while creating hydrogen sulfide, which contributes to corrosion.

Hydrogen sulfide from sulfate-reducing bacteria also plays a role in the corrosion of concrete. It also occurs in sour crude oil.

Some sulfate-reducing bacteria play a role in the anaerobic oxidation of methane:

CH4 + SO42– → HCO3– + HS– + H2O

An important fraction of the methane formed by methanogens below the seabed is oxidized by sulfate-reducing bacteria in the transition zone separating the methanogenesis from the sulfate reduction activity in the sediments. This process is also considered a major sink for sulfate in marine sediments.

In hydrofracturing fluids used to frack shale formations to recover methane (shale gas), biocide compounds are often added to water to inhibit the microbial activity of sulfate-reducing bacteria in order to avoid anaerobic methane oxidation and to minimize potential production loss.

Since the biofilm tends to create non-uniform surface conditions, localized attack might start at some points on the surface leading to localized corrosion, usually in the form of pitting. Industrial systems are likely to contain various structures where microbiologically induced corrosion (MIC) and biofouling can cause problems: open or closed cooling systems, water injection lines, storage tanks, residual water treatment systems, filtration systems, different types of pipes, reverse osmosis membranes, and potable water distribution systems.

Current conceptual models concur that there are three stages in the pitting corrosion: initiation, metastable pitting, and active pitting. When microorganisms are involved in the corrosion of metals, the situation is more complicated than it is in an abiotic environment, because microorganisms not only modify the near-surface environmental chemistry via microbial metabolism but also may interfere with the electrochemical processes occurring at the metal-environment interface.

The anaerobic corrosion of iron was noted in the 19th century and many theories were proposed about its mechanism. Decades of scientific research projects and investigations on the complex influence of microbes on increasing or decreasing corrosion rates have provided a much deeper insight in the role microorganisms play on the life of systems exposed to waters and grounds where they proliferate. The mechanisms potentially involved in MIC are summarized as:

Cathodic depolarization, whereby the cathodic rate limiting step is accelerated by micro-biological action;

Formation of occluded surface cells, whereby microorganisms form "patchy" surface colonies. Sticky polymers attract and aggregate biological and non-biological species to produce crevices and concentration cells, the basis for accelerated attack;

Fixing of anodic reaction sites, whereby microbiological surface colonies lead to the formation of corrosion pits, driven by microbial activity and associated with the location of these colonies;

Underdeposit acid attack, whereby corrosive attack is accelerated by acidic final products of the MIC "community metabolism", principally short-chain fatty acids.

Certain microorganisms thrive under aerobic conditions, whereas others thrive in anaerobic conditions. The pH conditions and availability of nutrients also play a role in determining what type of microorganisms can thrive in a particular soil environment. Microorganisms associated with corrosion damage are classified as:

Anaerobic bacteria that produce highly corrosive species as part of their metabolism;

Aerobic bacteria that produce corrosive mineral acids;

Fungi that may produce corrosive by products in their metabolism, such as organic acids. Apart from metals and alloys these can also degrade organic coatings and wood;

Slime formers that may produce concentration corrosion cells on surfaces.

Control questions:

1. What is the valency of sulfur in sulfur oxide?
2. Is a sulfur candle pure sulfur?
3. What is the percent sulfur in sulfur dioxide?
4. How do you recover sulfur from sulfur sludge?

Дата добавления: 2014-10-15