Biological rust removal

Under biological rust removal is the removal of rust or scale from metal surfaces using biologically formed rust converter . The active substances are produced by natural cells that get into solution and z. B. convert iron into complex bonds. These substances are acids but also so-called siderophores (iron chelators).

Acids from microbiological production

EMP path as a preliminary stage for biological phosphoric acid production

The chemical formalism for biological rust removal is the same as for conventionally produced acids. A distinction is made between rust converters and pickling agents. The first group includes, for example, phosphoric acid; the second group includes sulfuric acid.

Microbial production of phosphoric acid

Phosphoric acid can be produced indirectly via the path described by Embden , Meyerhof and Parnas ( EMP path ). An important intermediate product is fructose-1,6-bisphosphate , which is why it is also called the fructose diphosphate route. This pathway is part of breathing, in which organic material is broken down into H ₂ O and CO ₂ while generating energy. Here the glucose is absorbed and processed into pyruvate . The resulting intermediates are phosphorylated compounds. One of them, dihydroxyacetone phosphate , can be converted into phosphoric acid and glycerol together with the phosphatase in a second step . This path was investigated with Escherichia coli and Clostridium . Since these also occur in the humus , tests have already been carried out to de-rust surfaces with the help of compost.

Microbial production of sulfuric acid

Microorganisms that draw their energy from inorganic substances, so-called chemo-litho-autotrophic microorganisms, have a metabolic pathway to oxidize iron-containing sulfides to sulfuric acid . The sulfuric acid is able to dissolve metals from the rock. This process is called leaching . The species Acidithiobacillus thiooxidans (formerly Thiobacillus thiooxidans ) and Acidithiobacillus ferrooxidans (formerly: Thiobacillus ferrooxidans ) can be used for this. Both species act catalytically in the reactions . Acidithiobacillus ferrooxidans forms the more easily soluble iron sulfate and sulfuric acid during direct leaching from iron sulfide. In indirect leaching, various metal sulfides are converted into elemental sulfur . In a second reaction, Acidithiobacillus thiooxidans produces sulfuric acid. In contrast to A. ferroxidans, A. thiooxidans works at very low pH values (2 to 3). High yields of sulfuric acid can be achieved by forming a mixed culture from both Acidithiobacillus species.

Direct leach

${\displaystyle FeS_{2}+H_{2}O+3{,}5\ O_{2}\quad {\xrightarrow {Thio.ferrooxidans}}\quad FeSO_{4}+H_{2}SO_{4}}$

Indirect leaching

${\displaystyle 2\ FeSO_{4}+H_{2}SO_{4}+0{,}5\ O_{2}\quad {\xrightarrow {Thio.ferrooxidans}}\quad Fe_{2}(SO_{4})_{3}+H_{2}O}$

${\displaystyle Fe_{2}(SO_{4})_{3}+MeS\quad {\xrightarrow {Thio.ferrooxidans}}\quad 2\ FeSO_{4}+MeSO_{4}+S}$

${\displaystyle S+H_{2}O+1{,}5\ O_{2}\quad {\xrightarrow {Thio.thiooxidans}}\quad H_{2}SO_{4}}$

There are other acids such as citric acid , tartaric acid , malic acid and acetic acid that have complexing properties, but the complexing properties are only weak. To increase the effectiveness of the acids, the acids are mixed together.

Siderophores

Rod model of carboxymycobactin T with iron ion, according to PDB 1X8U

Many cells need iron to build their cells. Since iron is a very common element on earth, but in physiological solution (i.e. aerobic and pH-neutral) it occurs almost exclusively in undissolved form, aerobically growing microorganisms must have a strategy in order to get to bound iron. Bacteria, fungi and plants have so-called siderophores . Siderophores are selective molecules that mainly bind trivalent iron. They belong to the group of " small molecules ". Their small size allows them to be excreted into the solution through the cell wall, to complex trivalent iron from iron compounds (e.g. Fe ₂ O ₃ ) and to transport it back into the cell. The figure shows an iron ion (green) bound to a siderophore.

After reduction of Fe ³⁺ to Fe ²⁺ iron ion can be released from the siderophore. Siderophores are also suitable for removing aluminum oxide layers (Al ₂ O ₃ ) for the purpose of surface treatment. These siderophores have been used in pharmacy since around the middle of the 20th century, specifically for dialysis and iron poisoning. The idea of ​​using siderophores to remove rust already existed in the early 1990s. The advantages of using it are many. The biggest advantage, however, is its complete biodegradability. In addition, siderophores selectively bind the iron ions of the rust layer and do not attack the iron below. They are also recyclable and completely non-toxic. The complexing agents are produced in an environment that is severely depleted of iron, so that the microorganisms are stimulated to produce strong siderophores. In contrast to pharmacy, production for industrial use can be carried out more cost-effectively, since subsequent complex cleaning steps are not necessary. Only those siderophores come into consideration that go into solution in an aqueous environment. There is a patent for this from 1994 and describes the production and use of siderophores as rust removal agents. There are now some products based on siderophores for derusting components. These are offered as an additive for derusting and are used in smaller rinsing baths or as a paste.

Individual evidence

1. ↑ ^{a b c d e f g h i} P. M. Kunz: Umweltbioverfahrenstechnik. Vieweg, Braunschweig / Wiesbaden 1992, ISBN 3-528-06451-X .
2. ↑ W. Fritsche: Microbiology. 2nd Edition. Spectrum Academic Publishing House, Heidelberg / Berlin 1999.
3. ↑ ^{a b} P. M. Kunz: Natural rust removal. In: Bio World. 1, 2001, p. 8ff.
4. ↑ PM Kunz, A. Hämmerl, I. Sommer: Complexation of aluminum oxide by natural chelators for the substitution of pickling acid in luster pickling. Feasibility study by the Karl Völker Foundation at the Mannheim University of Applied Sciences, February 2010.
5. ↑ PM Kunz: (1994) Patent DE 4433 376 C1, Abstract: Natural storage and transport compounds for removing rust from surfaces that occur in microorganisms and the manufacturing process for these compounds.
6. ↑ ASA Spezialenzyme GmbH , accessed November 30, 2011.