The results of natural microbial leaching have been known since ancient times.

Pliny the

Elder (23-73 AD), who had a passion for observing the wonders of nature, discusses the

" vitreolus quasi vitrum" - a glass-like substance - found on rocks in his treatise on natural

history. One of the earliest records of utilizing the effects of bioleaching is from the island of

Cyprus. Galen, a Greek physician from Pergamum, in 162 A.D., is reported to have collected

cuperiferous solutions from mine water from the mines of Skouriotissa and concentrated

them by evaporation to form crystals of copper sulfate. Recent findings have revealed

evidence that predates this account. Indeed, observations have been made on the natural

leaching of copper and the formation of "gall springs" during the East Han Dynasty (206 BC-

220 AD) in China. The "Gall-Copper Process" was recorded as being used during the Song

Dynasty (960-1271AD). Copper was precipitated from solution by dipping iron into the blue

vitriol solution - a process identified as early as 150 BC in China. Therefore, presumably, the

recognition of a natural copper leaching process can be identified as early as that date.

Iron-rich acidic waters draining from abandoned coal and metal mines as well as from

unmined mineralized areas are another evidence of microbial leaching. In fact, history

records that mine water problems began at the same time that mining activities began. For

example, in the area of the Iberian Pyritic Belt, exploited since prehistoric times, the

production of acid mine drainage gave names to rivers like Tinto, Tintillo, Aguas Teñidas,

referring to the river's characteristic color. In fact, at Rio Tinto, in Spain, 17th-century

records describe the occurrence of copper-bearing waters. Formal microbial leaching

continued at the Rio Tinto mine until the late 1970s. The UK-based mining company, Rio

Tinto, which was formed in 1873, owes its name to these copper-bearing waters.

It was not until 1947 that these phenomena were attributed to bacteria. Once identified,

however, rapid steps were taken to commercialize the process. Commercial application of
 bacterial leaching began in the late 1950s at the Kennecott Utah Copper Company's Bingham

Canyon Mine near Salt Lake City, Utah where it was observed that blue copper-containing

solutions were running out of waste piles that contained copper sulfide minerals - something

that should not have happened in the absence of powerful oxidizing agents and acid.

Investigation revealed that naturally occurring bacteria were oxidizing iron sulfides in the

piles,

and the resulting ferric sulfate and sulfuric acid was acting as an oxidizer and leachant for

copper sulphides. These bacteria were given the name ferrooxidans for their ability to oxidize

iron sulfides. A second set of bacteria was also identified and given the name thiooxidans for

their ability to oxidize sulfur to yield sulfuric acid. The bacteria, which were native to the

soil, in effect created a completely natural metallurgical processing plant.

Until recently, conventional acid-leaching of copper was utilized as a means of recovering

copper from low-grade materials such as waste rock and flotation tailings and was conducted

in dumps that were already in place. At least five factors have impeded the acceptance of

leaching as a major commercial process:

1. The lack of an efficient way of recovering metallic copper from solutions.

2. The very slow rate of leaching.

3. The inability to process sulfide mineral concentrates.

4. The inability to process chalcopyrite - the most abundant and, thus, the most economically

important copper mineral.

5. The inability to recover precious metals

Solutions are now in sight to rectify most of these impediments.

Metals extracted from bioleaching include:

 Gold
 Copper
 Silver
 Cobalt
 Uranium
 Zinc
 Nickel
Bioleaching can involve numerous ferrous iron and sulfur oxidizing bacteria,

including Acidithiobacillus ferrooxidans (formerly known as Thiobacillus ferrooxidans)

and Acidithiobacillus thiooxidans (formerly known as Thiobacillus thiooxidans). As a

general principle, Fe3+ ions are used to oxidize the ore. This step is entirely independent of

microbes. The role of the bacteria is the further oxidation of the ore, but also the regeneration

of the chemical oxidant Fe3+ from Fe2+. For example, bacteria catalyse the breakdown of the

mineral pyrite (FeS2) by oxidising the sulfur and metal (in this case ferrous iron, (Fe 2+))

using oxygen. This yields soluble products that can be further purified and refined to yield the

desired metal.

Pyrite leaching (FeS2): In the first step, disulfide is spontaneously oxidized to thiosulfate by

ferric ion (Fe3+), which in turn is reduced to give ferrous ion (Fe2+):

FeS2 + 6Fe3+ +3H2O = 7Fe2+ + S2O32- + 6H+ (spontaneous)

The ferrous ion is then oxidized by bacteria using oxygen:

4Fe2+ + O2 + 4H+ = 4Fe3+ + 2H2O (iron oxidizers)

Thiosulfate is also oxidized by bacteria to give sulfate:

S2O32- + 2O2 + H2O = 2SO42- + 2H+ (sulfur oxidizers)

The ferric ion produced in reaction (2) oxidized more sulfide as in reaction (1), closing the

cycle and given the net reaction:

2FeS2 + 7O2 + 2H2O = 2Fe2+ + 4SO42- +4H+

The net products of the reaction are soluble ferrous sulfate and sulfuric acid.

The microbial oxidation process 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 while reducing oxygen to water. The critical reaction is the oxidation of sulfide

by ferric iron. The main role of the bacterial step is the regeneration of this reactant.

The process for copper is very similar, but the efficiency and kinetics depend on the copper

mineralogy. The most efficient minerals are supergene minerals such as chalcocite, Cu2S

and covellite, CuS. The main copper mineral chalcopyrite (CuFeS2) is not leached very

efficiently, which is why the dominant copper-producing technology remains flotation,

followed by smelting and refining. The leaching of CuFeS 2 follows the two stages of being

dissolved and then further oxidised, with Cu2+ ions being left in solution.

Chalcopyrite leaching:

CuFeS2 + 4Fe3+ = Cu2+ + 5Fe2+ + 2S0 spontaneous

4Fe2+ + O2 + 4H+ = 4Fe3+ + 2H2O (iron oxidizers)

2S0 + 3O2 + 2H2O = 2SO42- + 4H+ (sulfur oxidizers)

net reaction:

CuFeS2 + 4O2 = Cu2+ + Fe2+ + 2SO42-

In general, sulfides are first oxidized to elemental sulfur, whereas disulfides are oxidized to

give thiosulfate, and the processes above can be applied to other sulfidic ores. Bioleaching of

non-sulfidic ores such as pitchblende also uses ferric iron as an oxidant (e.g., UO 2 + 2

Fe3+ ==> UO22+ + 2 Fe2+). In this case, the sole purpose of the bacterial step is the

regeneration of Fe3+. Sulfidic iron ores can be added to speed up the process and provide a
source of iron. Bioleaching of non-sulfidic ores by layering of waste sulfides and elemental

sulfur, colonized by Acidithiobacillus spp., has been accomplished, which provides a strategy

for accelerated leaching of materials that do not contain sulfide minerals.

Further processing

The dissolved 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 electron pair. The ligand-copper complex is extracted from the solution using

an organic solvent such as kerosene:

Cu2+(aq) + 2LH(organic) → CuL2(organic) + 2H+(aq)

The ligand donates electrons to the copper, producing a complex - a central

metal atom (copper) bonded to 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:

Cu2+(aq) + Fe(s) → Cu(s) + Fe2+(aq)

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.

Feasibility

Economic feasibility

Today, 15% of the world’s copper production is done by bioleaching process. Bioleaching is

in general simpler and, therefore, cheaper to operate and maintain than traditional processes,

since fewer specialists are needed to operate complex chemical plants. And low

concentrations are not a problem for bacteria because they simply ignore the waste that

surrounds the metals, attaining extraction yields of over 90% in some cases.

These microorganisms actually gain energy by breaking down minerals into their constituent

elements. The company simply collects the ions out of the solution after the bacteria have

finished. There is a limited amount of ores.

Bioleaching can be used to extract metals from low concentration ores as gold that are too

poor for other technologies. It can be used to partially replace the extensive crushing and

grinding that translates to prohibitive cost and energy consumption in a conventional process.

Because the lower cost of bacterial leaching outweighs the time it takes to extract the metal.

High concentration ores like copper is more economical to smelt rather than to use

bioleaching because the profit obtained from the speed and yield of smelting justifies its cost

due bacterial leaching process being very slow compared to smelting. This brings in less

profit as well as introducing a significant delay in cash flow for new plants. Nonetheless, at
the largest copper mine of the world, Escondida in Chile the process seems to be favorable.

Economically it is also very expensive and many companies once started can not keep up

with the demand and end up in debt.

In space

In 2020 scientists showed, with an experiment with different gravity environments on

the ISS, that microorganisms could be employed to mine useful elements from basaltic rocks

via bioleaching in space.