Editing Bioleaching

{{short description|Method of metal extraction}}
'''Bioleaching''' is the extraction or liberation of [[metal]]s from their [[ore]]s through the use of [[living organism]]s. Bioleaching is one of several applications within [[biohydrometallurgy]] and several methods are used to treat ores or concentrates containing [[copper]], [[zinc]], [[lead]],  [[arsenic]], [[antimony]], [[nickel]], [[molybdenum]], [[gold]], [[silver]], and [[cobalt]].

Bioleaching falls into two broad categories.  The first, is the use of [[microorganism]]s to oxidize [[refractory]] minerals to release valuable metals such and gold and silver.  Most commonly the [[mineral]]s that are the target of oxidization are [[pyrite]] and [[arsenopyrite]].

The second category is leaching of [[sulphide]] [[mineral]]s to release the associated metal, for example, leaching of [[pentlandite]] to release [[nickel]], or the leaching of [[chalcocite]], [[covellite]] or [[chalcopyrite]] to release [[copper]].

== Process ==
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, in one proposed method of bacterial leaching known as [[Indirect Leaching]], Fe<sup>3+</sup> ions are used to oxidize the ore. This step is entirely independent of microbes. The role of the bacteria is further oxidation of the ore, but also the regeneration of the chemical oxidant Fe<sup>3+</sup> from Fe<sup>2+</sup>. For example, bacteria [[catalyst|catalyse]] the breakdown of the mineral [[pyrite]] (FeS<sub>2</sub>) by oxidising the [[sulfur]] and metal (in this case ferrous iron, (Fe<sup>2+</sup>)) using [[oxygen]]. This yields [[Solubility|soluble]] [[product (chemistry)|products]] that can be further purified and refined to yield the desired metal.{{Citation needed|date=January 2021}}

'''[[Pyrite]] leaching''' (FeS<sub>2</sub>):
In the first step, disulfide is spontaneously oxidized to [[thiosulfate]] by ferric ion (Fe<sup>3+</sup>), which in turn is reduced to give ferrous ion (Fe<sup>2+</sup>):
:(1)&nbsp;&nbsp; <math>\mathrm{FeS_2 + 6 \ Fe^{\,3+} + 3 \ H_2O \longrightarrow 7 \ Fe^{\,2+} + S_2O_3^{\,2-} + 6 \ H^+}</math> &nbsp;&nbsp;&nbsp;spontaneous

The ferrous ion is then oxidized by bacteria using oxygen:
:(2)&nbsp;&nbsp; <math>\mathrm{4 \ Fe^{\,2+} + \ O_2 + 4 \ H^+ \longrightarrow 4 \ Fe^{\,3+} + 2 \ H_2O}</math> &nbsp;&nbsp;&nbsp;(iron oxidizers)

Thiosulfate is also oxidized by bacteria to give sulfate:
:(3)&nbsp;&nbsp; <math>\mathrm{S_2O_3^{\,2-} + 2 \ O_2 + H_2O \longrightarrow 2 \ SO_4^{\,2-} + 2 \ H^+}</math> &nbsp;&nbsp;&nbsp;(sulfur oxidizers)

The ferric ion produced in reaction (2) oxidized more sulfide as in reaction (1), closing the cycle and given the net reaction:
:(4)&nbsp;&nbsp;<math>\mathrm{2 \ FeS_2 + 7 \ O_2 + 2 \ H_2O \longrightarrow 2 \ Fe^{\,2+} + 4 \ SO_4^{\,2-} + 4 \ H^+}</math>

The net products of the reaction are soluble [[ferrous sulfate]] and [[sulfuric acid]].{{Citation needed|date=January 2021}}

The microbial oxidation process occurs at the [[cell membrane]] of the bacteria. The [[electron]]s pass into the [[cell (biology)|cell]]s 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.{{Citation needed|date=January 2021}}

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]], Cu<sub>2</sub>S and [[covellite]], CuS. The main copper mineral [[chalcopyrite]] (CuFeS<sub>2</sub>) is not leached very efficiently, which is why the dominant copper-producing technology remains flotation, followed by smelting and refining. The leaching of CuFeS<sub>2</sub> follows the two stages of being dissolved and then further oxidised, with Cu<sup>2+</sup> ions being left in solution.{{Citation needed|date=January 2021}}

'''[[Chalcopyrite]] leaching''':
:(1)&nbsp;&nbsp; <math>\mathrm{CuFeS_2 + 4 \ Fe^{\,3+} \longrightarrow Cu^{\,2+} + 5 \ Fe^{\,2+} + 2 \ S_0}</math> &nbsp;&nbsp;&nbsp;spontaneous
:(2)&nbsp;&nbsp; <math>\mathrm{4 \ Fe^{\,2+} + O_2 + 4 \ H^+ \longrightarrow 4 \ Fe^{\,3+} + 2 \ H_2O}</math> &nbsp;&nbsp;&nbsp;(iron oxidizers)
:(3)&nbsp;&nbsp; <math>\mathrm{2 \ S^0 + 3 \ O_2 + 2 \ H_2O \longrightarrow 2 \ SO_4^{\,2-} + 4 \ H^+}</math> &nbsp;&nbsp;&nbsp;(sulfur oxidizers)
net reaction:
:(4)&nbsp;&nbsp;<math>\mathrm{CuFeS_2 + 4 \ O_2 \longrightarrow Cu^{\,2+} + Fe^{\,2+} + 2 \ SO_4^{\,2-}}</math>

In general, [[sulfide]]s are first oxidized to elemental sulfur, whereas [[disulfide]]s 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<sub>2</sub> + 2 Fe<sup>3+</sup> ==> UO<sub>2</sub><sup>2+</sup> + 2 Fe<sup>2+</sup>). In this case, the sole purpose of the bacterial step is the regeneration of Fe<sup>3+</sup>. Sulfidic [[iron ore]]s 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.<ref>{{Cite journal|doi = 10.1021/es900986n|title = Bioleaching of Ultramafic Tailings by ''Acidithiobacillusspp''. For CO2Sequestration|year = 2010|last1 = Power|first1 = Ian M.|last2 = Dipple|first2 = Gregory M.|last3 = Southam|first3 = Gordon|journal = Environmental Science & Technology|volume = 44|issue = 1|pages = 456–462|pmid = 19950896|bibcode = 2010EnST...44..456P}}</ref>

== Further processing ==
The dissolved copper (Cu<sup>2+</sup>) 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 [[functional group|groups]], each possessing a [[lone electron pair]]. The ligand-copper complex is extracted from the solution using an [[organic compound|organic]] solvent such as [[kerosene]]:

:Cu<sup>2+</sup><sub>(aq)</sub> + 2LH(organic) &rarr; CuL<sub>2</sub>(organic) + 2H<sup>+</sup><sub>(aq)</sub>

The ligand donates electrons to the copper, producing a [[complex (chemistry)|complex]] - a central metal [[atom]] (copper) bonded to the ligand. Because this complex has no [[electric charge|charge]], it is no longer attracted to [[polar molecule|polar]] water molecules and dissolves in the kerosene, which is then easily separated from the solution. Because the initial [[chemical reaction|reaction]] is [[reversible reaction|reversible]], it is determined by pH. Adding concentrated acid reverses the equation, and the copper ions go back into an [[aqueous solution]].{{Citation needed|date=January 2021}}

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 [[cathode]]s and collect there.{{Citation needed|date=January 2021}}

The copper can also be concentrated and separated by [[single displacement reaction|displacing]] the copper with Fe from scrap iron:

:Cu<sup>2+</sup><sub>(aq)</sub> + Fe<sub>(s)</sub> &rarr; Cu<sub>(s)</sub> + Fe<sup>2+</sup><sub>(aq)</sub>

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).{{Citation needed|date=January 2021}}

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.<ref>{{cite book |doi=10.1016/B978-0-12-804022-5.00014-1 |chapter=Experimental and Research Methods in Metals Biotechnology |title=Biotechnology of Metals |year=2018 |last1=Natarajan |first1=K.A. |pages=433–468 |isbn=978-0-12-804022-5 }}</ref> The gold is removed from the solution by [[adsorption|adsorbing]] (taking it up on the surface) to [[charcoal]].<ref>{{Cite web|title=Use in Mining {{!}} International Cyanide Management Code (ICMI) For The Manufacture, Transport and Use of Cyanide In The Production of Gold(ICMI)|url=https://www.cyanidecode.org/cyanide-facts/use-mining|access-date=2021-02-03|website=www.cyanidecode.org|archive-date=2012-02-29|archive-url=https://web.archive.org/web/20120229195438/http://www.cyanidecode.org/cyanide_use.php|url-status=dead}}</ref>

==With fungi==
Several species of [[Fungus|fungi]] can be used for bioleaching. Fungi can be grown on many different substrates, such as [[e-waste|electronic scrap]], [[catalytic converter]]s, and [[fly ash]] from municipal waste [[incineration]]. Experiments have shown that two fungal [[strain (biology)|strains]] (''Aspergillus niger, Penicillium simplicissimum'') were able to mobilize Cu and Sn by 65%, and Al, Ni, Pb, and Zn by more than 95%. ''Aspergillus niger'' can produce some organic acids such as [[citric acid]]. This form of leaching does not rely on microbial oxidation of metal but rather uses microbial metabolism as source of acids that directly dissolve the metal.<ref>{{cite journal|last1=Dusengemungu|first1=Leonce|last2=Kasali|first2=George|last3=Gwanama|first3=Cousins|last4=Mubemba|first4=Benjamin|title=Overview of fungal bioleaching of metals|journal=Environmental Advances|volume=5|issue=2021|pages=100083
|publisher=Elsevier Ltd.|date=27 June 2021|language=EN|issn=2666-7657|doi=10.1016/j.envadv.2021.100083|doi-access=free|bibcode=2021EnvAd...500083D }}</ref>

== Feasibility ==
===Economic feasibility===
Bioleaching is in general simpler and, therefore, cheaper to operate and maintain than traditional processes, since fewer specialists are needed to operate complex [[chemical]] [[factory|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 [[microorganism]]s actually gain [[energy]] by breaking down minerals into their constituent elements.<ref>{{Cite web|title=Enterprise Europe Network|url=https://een.ec.europa.eu/partners/bioleaching-technology-and-bioreactors-metal-extraction|access-date=2020-08-28|website=een.ec.europa.eu|language=en}}</ref> The company simply collects the [[ion]]s out of the solution after the bacteria have finished.

Bioleaching can be used to extract metals from low concentration ores such 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.{{Citation needed|date=January 2021}}

High concentration ores, such as copper, are more economical to smelt rather bioleach due to the slow speed of the bacterial leaching process compared to smelting. The slow speed of bioleaching introduces a significant delay in [[cash flow]] for new mines. Nonetheless, at the largest copper mine of the world, [[Escondida]] in [[Chile]] the process seems to be favorable.<ref>{{Cite web |title=Bioleaching: The worldwide copper mining is slowly turning green {{!}} CAR ENGINE AND SPORT |url=https://topgear-autoguide.com/category/tech-future/bioleaching-the-global-copper-mining-is-slowly-turning-green1607835314 |archive-url=https://web.archive.org/web/20230726142318/https://topgear-autoguide.com/category/tech-future/bioleaching-the-global-copper-mining-is-slowly-turning-green1607835314 |url-status=usurped |archive-date=July 26, 2023 |access-date=2022-05-06 |website=topgear-autoguide.com |language=en}}</ref>

Economically it is also very expensive and many companies once started can not keep up with the demand and end up in debt.{{Citation needed|date=January 2021}}

===In space===
{{multiple image
 | footer = 
 | image1 = The BioRock Experimental Unit of the space station biomining experiment that demonstrated rare earth element extraction in microgravity and Mars gravity.webp
 | width1 = 200
 | alt1 = BioRock Experimental Unit of the space station biomining experiment
 | caption1 = The experimental unit of the experiment
 | image2 = Effects of microorganisms on rare earth element leaching.webp
 | width2 = 128
 | alt2 = Effects of microorganisms on rare earth element leaching
 | caption2 = ''[[Sphingomonas desiccabilis|S. desiccabilis]]'' is a microorganisms that showed high efficacy
}}
In 2020 scientists showed, with an experiment with different gravity environments on the [[ISS]], that [[biomining|microorganisms could be employed to mine]] useful elements from [[basalt]]ic rocks via bioleaching in space.<ref>{{cite news |last1=Crane |first1=Leah |title=Asteroid-munching microbes could mine materials from space rocks |url=https://www.newscientist.com/article/2259373-asteroid-munching-microbes-could-mine-materials-from-space-rocks/ |access-date=9 December 2020 |work=New Scientist}}</ref><ref>{{cite journal |last1=Cockell |first1=Charles S. |last2=Santomartino |first2=Rosa |last3=Finster |first3=Kai |last4=Waajen |first4=Annemiek C. |last5=Eades |first5=Lorna J. |last6=Moeller |first6=Ralf |last7=Rettberg |first7=Petra |last8=Fuchs |first8=Felix M. |last9=Van Houdt |first9=Rob |last10=Leys |first10=Natalie |last11=Coninx |first11=Ilse |last12=Hatton |first12=Jason |last13=Parmitano |first13=Luca |last14=Krause |first14=Jutta |last15=Koehler |first15=Andrea |last16=Caplin |first16=Nicol |last17=Zuijderduijn |first17=Lobke |last18=Mariani |first18=Alessandro |last19=Pellari |first19=Stefano S. |last20=Carubia |first20=Fabrizio |last21=Luciani |first21=Giacomo |last22=Balsamo |first22=Michele |last23=Zolesi |first23=Valfredo |last24=Nicholson |first24=Natasha |last25=Loudon |first25=Claire-Marie |last26=Doswald-Winkler |first26=Jeannine |last27=Herová |first27=Magdalena |last28=Rattenbacher |first28=Bernd |last29=Wadsworth |first29=Jennifer |last30=Craig Everroad |first30=R. |last31=Demets |first31=René |title=Space station biomining experiment demonstrates rare earth element extraction in microgravity and Mars gravity |journal=Nature Communications |date=10 November 2020 |volume=11 |issue=1 |pages=5523 |doi=10.1038/s41467-020-19276-w |pmid=33173035 |pmc=7656455 |bibcode=2020NatCo..11.5523C |url=|language=en |issn=2041-1723}} [[File:CC-BY icon.svg|50px]] Available under [https://creativecommons.org/licenses/by/4.0/ CC BY 4.0].</ref>

== Environmental impact ==
The process is more environmentally friendly than traditional extraction methods.<ref>{{Cite journal |last1=Putra |first1=Nicky Rahmana |last2=Yustisia |first2=Yustisia |last3=Heryanto |first3=R. Bambang |last4=Asmaliyah |first4=Asmaliyah |last5=Miswarti |first5=Miswarti |last6=Rizkiyah |first6=Dwila Nur |last7=Yunus |first7=Mohd Azizi Che |last8=Irianto |first8=Irianto |last9=Qomariyah |first9=Lailatul |last10=Rohman |first10=Gus Ali Nur |date=2023-10-01 |title=Advancements and challenges in green extraction techniques for Indonesian natural products: A review |journal=South African Journal of Chemical Engineering |volume=46 |pages=88–98 |doi=10.1016/j.sajce.2023.08.002 |issn=1026-9185|doi-access=free }}</ref>  For the company this can translate into profit, since the necessary limiting of [[sulfur dioxide]] [[air pollution|emissions]] during smelting is expensive. Less landscape damage occurs, since the bacteria involved grow naturally, and the mine and surrounding area can be left relatively untouched. As the bacteria [[biological reproduction|breed]] in the conditions of the mine, they are easily cultivated and [[recycling|recycled]].<ref>{{Cite web |title=Mission 2015: Bioleaching |url=https://web.mit.edu/12.000/www/m2015/2015/bioleaching.html |access-date=2024-01-21 |website=web.mit.edu}}</ref>

[[Toxicity|Toxic]] chemicals are sometimes produced in the process. [[Sulfuric acid]] and H<sup>+</sup> ions that have been formed can leak into the [[groundwater|ground]] and surface water turning it acidic, causing environmental damage. [[Heavy ion]]s such as [[iron]], zinc, and arsenic leak during [[acid mine drainage]]. When the [[pH]] of this solution rises, as a result of [[concentration|dilution]] by fresh water, these ions [[precipitation (chemistry)|precipitate]], forming [[Acid mine drainage#Yellow boy|"Yellow Boy"]] pollution.<ref>{{Cite book|last=Dr. R.C. Dubey|title=A textbook of biotechnology : for university and college students in India and abroad|year=1993|isbn=978-81-219-2608-9|location=New Delhi|pages=442|oclc=974386114}}</ref> For these reasons, a setup of bioleaching must be carefully planned, since the process can lead to a [[biosafety]] failure. Unlike other methods, once started, bioheap leaching cannot be quickly stopped, because leaching would still continue with rainwater and natural bacteria. Projects like Finnish [[Talvivaara]] proved to be environmentally and economically disastrous.<ref>{{Cite news|title=Four charged in Talvivaara toxic leak case|url=https://yle.fi/uutiset/osasto/news/four_charged_in_talvivaara_toxic_leak_case/7485070|date=22 September 2014|publisher=[[Yle]]}}</ref><ref>{{cite journal |last1=Sairinen |first1=Rauno |last2=Tiainen |first2=Heidi |last3=Mononen |first3=Tuija |title=Talvivaara mine and water pollution: An analysis of mining conflict in Finland |journal=The Extractive Industries and Society |date=July 2017 |volume=4 |issue=3 |pages=640–651 |doi=10.1016/j.exis.2017.05.001 |bibcode=2017ExIS....4..640S |s2cid=134427827 |url=https://doi.org/10.1016/j.exis.2017.05.001 |access-date=4 August 2022|url-access=subscription }}</ref>

== See also ==
{{Portal|Biology|Technology}}
* [[Phytomining]]

== References ==
{{reflist}}

== Further reading ==
* ''T. A. Fowler and F. K. Crundwell'' – "Leaching of zinc sulfide with Thiobacillus ferrooxidans"
* ''Brandl H.'' (2001) "Microbial leaching of metals". In: Rehm H. J. (ed.) ''Biotechnology'', Vol. 10. Wiley-VCH, Weinheim, pp.&nbsp;191–224
*{{cite journal | doi = 10.1016/j.hydromet.2006.05.001 | title = The bioleaching of sulphide minerals with emphasis on copper sulphides — A review | year = 2006 | last1 = Watling | first1 = H. R. | journal = Hydrometallurgy | volume = 84 | issue = 1–2 | page = 81| bibcode = 2006HydMe..84...81W }}
*{{cite journal | doi = 10.1007/s00253-003-1404-6 | title = Bioleaching review part B | year = 2003 | last1 = Olson | first1 = G. J. | last2 = Brierley | first2 = J. A. | last3 = Brierley | first3 = C. L. | journal = Applied Microbiology and Biotechnology | volume = 63 | issue = 3 | pages = 249–57 | pmid = 14566430| s2cid = 24078490 }}
*{{cite journal | doi = 10.1007/s00253-003-1448-7 | title = Bioleaching review part A | year = 2003 | last1 = Rohwerder | first1 = T. | last2 = Gehrke | first2 = T. | last3 = Kinzler | first3 = K. | last4 = Sand | first4 = W. | journal = Applied Microbiology and Biotechnology | volume = 63 | issue = 3 | pages = 239–248 | pmid = 14566432| s2cid = 25547087 }}

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