Bioleaching is the extraction of specific metals from their ores through the use of bacteria. Bioleaching is one of several applications within biohydrometallurgy and several methods are used to recover copper, zinc, lead, arsenic, antimony, nickel, molybdenum, gold, cobalt.
The extraction of gold from its ore can involve numerous ferrous and sulfur oxidizing bacteria, including Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans (formerly known as Thiobacillus). For example, bacteria catalyse the breakdown of the mineral arsenopyrite (FeAsS) by oxidising the sulfur and metal (in this case arsenic ions) to higher oxidation states whilst reducing dioxygen by H2 and Fe3+. This allows the soluble products to dissolve.
This process actually occurs at the cell membrane of the bacteria. The electrons pass into the cells and are used in biochemical processes to produce energy for the bacteria to reduce oxygen molecules to water.
In stage 2, bacteria oxidise Fe2+ to Fe3+ (whilst reducing O2).
They then oxidise the metal to a higher positive oxidation state. With the electrons gained, they reduce Fe3+ to Fe2+ to continue the cycle.
The gold is now separated from the ore and in solution.
Copper (Cu2+) ions are removed from the solution by ligand exchange solvent extraction which leaves other ions in the solution. The copper is removed by bonding to a ligand, which is a large molecule consisting of a number of smaller groups each possessing a lone pair. The ligand is dissolved in an organic solvent such as kerosene and shaken with the solution producing this reaction:
The ligand donates electrons to the copper, producing a complex - a central metal atom (copper) bonded to 2 molecules of the ligand. Because this complex has no charge, it is no longer attracted to polar water molecules and dissolves in the kerosene, which is then easily separated from the solution. Because the initial reaction is reversible, it is determined by pH. Adding concentrated acid reverses the equation, and the copper ions go back into an aqueous solution.
Then the copper is passed through an electro-winning process to increase its purity: an electric current is passed through the resulting solution of copper ions. Because copper ions have a 2+ charge, they are attracted to the negative cathodes and collect there.
The copper can also be concentrated and separated by displacing the copper with Fe from scrap iron:
The electrons lost by the iron are taken up by the copper. Copper is the oxidising agent (it accepts electrons), and iron is the reducing agent (it loses electrons).
Traces of precious metals such as gold may be left in the original solution. Treating the mixture with sodium cyanide in the presence of free oxygen dissolves the gold. The gold is removed from the solution by adsorbing (taking it up on the surface) to charcoal.
Some advantages associated with bioleaching are:
Some disadvantages associated with bioleaching are:
Currently it is more economical to smelt copper ore rather than to use bioleaching, since the concentration of copper in its ore is generally quite high. The profit obtained from the speed and yield of smelting justifies its cost. However, the concentration of gold in its ore is generally very low. The lower cost of bacterial leaching in this case outweighs the time it takes to extract the metal.
The physiology and energetic manipulation of metal leaching organisms are extremely significant, especially for gold, copper,uranium leaching and recovery of metals such as arsenic, silver, and mercury.Organisms that thrive in extreme environments such as those described above, are of interest in the production of highly stable enzymes and in the development of certain innovative bioprocesses. One area of environmental/biotechnological research would be the realization of the biocatalytic potential of these extremophilic microbes, which thrive at very high (boiling) temperatures, high pressures, highly saline or acidic environments. An area of interest would be the development of environmentally relevant (bio)technology based on microbial degradation of the recalcitrant pollutants. This calls for characterization of single microbes and mixed cultures that can survive amidst high concentrations of the pollutants.Essential evaluation of microbial physiology in conjunction with industrially relevant molecular bioprocess design is required for development of these system which can aid as great learning/research tools in arenas of enzyme or cell processing applications along with mining, waste-water treatment, bioremediation. Understanding of these "hyperthermophilic anaerobes" that encompass a widely metabolic variety can be utilized for novel applications such as high-temperature 'anaerobic digestors',aiding the conversion of the waste to useful products, molecular engineering of enzymes etc.