Bacteria responsible for iron corrosion"
Naturally iron is present as an elemental form in the earth's crust. About 5% iron as an element in Earth's crust. Organic matter contains organic form of iron like tannins and lignins. Groundwater contains iron in dissolved state (Fe+2) while as surface and shallow water contains partially or fully combined form of iron. Corrosion is actually spontaneous electrochemical process that happens with any metal. Rusting or corrosion of iron is the most common example. The process consists of conversion of Fe to Fe+2 (2e-) to Fe+3 (1e-). Electrons produced are used to reduce oxygen. Oxygen reacts with metal via water leading to formation of iron oxides (red) or hydroxides (green) which we term as rust. Only the iron oxides which are formed by iron corrosion are called as rust; oxides of other metals are not referred as rust. In nature, iron corrosion is activity of special groups of microbes, bacteria in particular. Let's know the correlation of these bacterial groups to iron corrosion.
Corrosion takes place in both aerobic and anaerobic conditions. Iron bacteria or siderophilic bacteria are aerobic, filamentous bacteria which use dissolved iron for their growth. Their enzymes reduce insoluble ferric oxide to soluble ferrous hydroxide. It means that energy is obtained by oxidizing dissolved ferrous iron. Resulting ferric oxide (we refer it as rust) is insoluble brown colored and stains the material coming in contact with it. Colonies of iron bacteria are often heavily encrusted with ferric oxide. Extra iron oxide is stored in sheaths or stalk of cells. Because of this morphological character, iron/siderophilic bacteria have been placed in Group III 'The Sheathed Bacteria' of Kingdom Prokaryota of Bergey's Manual of Systematic Bacteriology. They proliferate in water containing iron concentration as low as 0.1mg/lit and at least 0.3 ppm dissolved oxygen which catalyses oxidation of ferrous iron. Thus the niches where the concentration of organic material exceeds concentration of dissolved oxygen are inhabited by iron bacteria. Thiobacillus ferroxidans, Leptospirillum ferroxidans, Gallionella, Sphaerotilus, Leptothrix and Crenothrix are examples of siderophilic bacteria. Gallionella ferruginea and Leptothrix ochracea are functional at neutral pH and requires microaerophilic environment. The genus Thiobacillus was originally called as Ferrobacillus (Ferro: iron). But it is also known to oxidize inorganic sulfur compounds hence it has been reclassified as Thiobacillus (Thio: sulfur). It is also known to be extremophilic genus found in aquatic systems with pH 3. It produces corrosive mineral acid, sulfuric acid (H2SO4) and thus acidifies rivers draining the mine area. Mineral sulphides such as iron and copper pyrites are then oxidized by acid mine waters. Sulphides are normally protected from bacteria by anaerobic environment but by this way they are exposed to attack. Sulfate reducing bacteria like Desulfovibrio and Desulfotomaculum corrode iron under anaerobic conditions. These bacteria are highly corrosive; they not only produce H2SO4 but also H2S and sulfates. During anaerobic respiration of these bacteria, Fe+3 acts as electron acceptor Anaerobic photosynthetic Rhodopseudomonas is able to convert Fe+2 to NADH during CO2 fixation. Sulfur oxidising bacteria Acidothiobacillus thiooxidans and Ferrobacillus ferroxidans produces H2SO4 and oxidizes iron to iron oxides and hydroxide rust. Fungi, slime molds and algae also produce corrosive organic acids leading to galvanic/iron corrosion.
Effects and significance of bacterial iron corrosion: Iron bacteria corrode material made up of stainless steel. The wells, screens, pipes, plumbing fixtures, steel piping components of water supply and components of automated water systems are always prone to corrosion by iron bacteria. Rust colored deposits that develop on these structures are the results of brown slimy bacterial growth. They often build up in well systems and reduce well yields by clogging the pipes. Localized corrosion is usually initiated with pitting that start at some points on steel surface; it also induces formation of patchy surface colonies and accelerates the rate of corrosion. Corrosion especially in water systems is often followed by unpleasant taste, off-odors and colors of water. Biofouling or odors like swampy, oily or petroleum, cucumber, fishy, sewage, rotten vegetation and musty indicates active growth of iron bacteria. Colors like yellow, orange, red and brown or rust, oil like sheen, rainbow coloration on water with sticky slime are also the characteristic features of iron corrosion bacteria. In standing water body filamentous or feathery growth indicates progressive iron oxidation. Iron corrosion also favors multiplication of other bacteria like sulfur bacteria. Different types of physical, chemical and even ecofriendly biological anticorrosion treatments have been investigated and employed to control growth of corrosion causing iron bacteria. But anticorrosion methods used by our ancestors were very effective and these iron structures are still found in uncorroded state. Some historical monuments in India such as 1600 years old Iron pillar in New Delhi, iron beams from Surya (Sun) temple (built in 13th century) and Mookambika temple (1200 years old) in South India have not corroded since ages. But it has been yet not possible to discover their anticorrosive formulation.
I request the readers of this article to explain anticorrosive formulation of such structures if it is known to them. I would also like to know if there are more such ancient structures present elsewhere in the world.
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