The pure cultures of bacteria are used to solubilise the metal ions from their ores. Leaching can be direct or indirect. In case of direct leaching, microorganisms secrete some enzymes, these enzymes oxidise the metals ions and release metal sulphides from the ore. Metal ions are then separated by water which wets the ore. Thiobacillus ferrooxidans oxidises copper sulphide (Chalcolite)
Cu2S +O2 +2H –> 2Cu++ +S +H2O
In case of indirect leaching, microbes help to oxidise various ferrous ions into ferric sulphate and generate energy in the process. Ferric sulphate in turn acts as a strong oxidser that dissolves many metal sulphides as in Chalcopyrite (CuFeS2).
Thiobacillus ferrooxidans has also been used to leach other metals such as cobalt, nickel, zinc. Here the leaching will be indirect and ulphuric acid is generatedduring the oxidation process. Recently, metal extraction of gold and silver is also being tried. Thiobacillus can oxidise elemental sulphur into H2So4 which can in turn solubilise Zwurite, Chrysocolla etc.
Metal recovery can also be done by accumulating microbes such as Pseudomonas maltophilia and Staphylococcus aureus. Heavy metals can be recovered by Desulfo-vibrio, Desulfo tomacilium and Desulfomonas.
The most practical application is of course in copper leaching and has been used for many years. Uranium leaching is brought about by Saccharomyces cerviseae, Rhizopus arrhizus also has been shown to be successful in absorption of uranium.
Some facts about Thiobacillus ferrooxidans-
Gram negative, acidophilic, chemoautotroph
Short, rod shaped cells
Lives in hot springs, volcanoes and sulphide ores deposits
Gets its energy from oxidation of ore of iron or sulphur
Iron must be in ferrous and is converted into ferric
Other important species are-
1. Leptospirillum ferrooxidans
First isolated in 1972
Gram negative, acidophilic, chemolithotrophic
Can bring about ferrous ion oxidation
Thermophilic and acidophilic
Can oxidise sulphur, ferrous ions and metal sulphides