Acid mine drainage

{{Short description|Outflow of acidic water from metal or coal mines}} {{Use dmy dates|date=July 2015}} [[File:Rio tinto river CarolStoker NASA Ames Research Center.jpg|thumb|Rio Tinto in Spain presents an acid drainage of both natural and artificial origin (mining)]] [[File:Rocks on Shamokin Creek.JPG|thumb|Rocks stained by ferrous precipitate from acid mine drainage on Shamokin Creek in Northumberland County, Pennsylvania]]

'''Acid mine drainage''', '''acid and metalliferous drainage''' ('''AMD'''), or '''acid rock drainage''' ('''ARD''') is the outflow of acidic water from metal mines and coal mines.

Acid rock drainage occurs naturally within some environments as part of the rock weathering process but is exacerbated by large-scale earth disturbances characteristic of mining and other large construction activities, usually within rocks containing an abundance of sulfide minerals. Areas where the earth has been disturbed (e.g. construction sites or highway construction) may create acid rock drainage. In many localities, the liquid that drains from coal stocks, coal handling facilities, coal washeries, and coal waste tips can be highly acidic, and in such cases it is treated as acid rock drainage. These, combined with reduced pH, have a detrimental impact on the streams' aquatic environments.

The same type of chemical reactions and processes may occur through the disturbance of acid sulfate soils formed under coastal or estuarine conditions after the last major sea level rise, and constitutes a similar environmental hazard.

==Nomenclature== Historically, the acidic discharges from active or abandoned mines were called acid mine drainage, or AMD. The term acid rock drainage, or ARD, was introduced in the 1980s and 1990s to indicate that acidic drainage can originate from sources other than mines.<ref>Dowding, B. & Mills, C,: ''[http://technology.infomine.com/enviromine/ard/Introduction/Natural.htm Natural acid rock drainage and its impact upon background metal concentrations] {{Webarchive|url=https://web.archive.org/web/20140915091444/http://technology.infomine.com/enviromine/ard/Introduction/Natural.htm |date=15 September 2014 }}'', [http://www.infomine.com/ InfoMine.com]. Accessed 23 September 2013.</ref> For example, a paper presented in 1991 at a major international conference on this subject was titled: "The Prediction of Acid Rock Drainage – Lessons from the Database".<ref>Ferguson, K.D. and Morin, K.A. The Prediction of Acid Rock Drainage - Lessons from the Database. ''Proceedings: Second International Conference on the Abatement of Acidic Drainage.'' Sept 16 to 18, 1991, Montreal, Quebec.</ref> Both AMD and ARD refer to low pH or acidic waters caused by the oxidation of sulfide minerals, though ARD is the more generic name.

In cases where drainage from a mine is not acidic and has dissolved metals or metalloids, or was originally acidic, but has been neutralized along its flow path, then it is described as "neutral mine drainage",<ref name="GARD">[http://www.gardguide.com/index.php/Main_Page Global Acid Rock Drainage Guide (GARD Guide)] INAP: The International Network for Acid Prevention. Accessed 23 September 2013.</ref> "mining-influenced water"<ref>Gusek, J.J., Wildeman, T.R. and Conroy, K.W. 2006. Conceptual methods for recovering metal resources from passive treatment systems. ''Proceedings of the 7th International Conference on Acid Rock Drainage (ICARD)'', March 26–30, 2006, St. Louis MO.</ref> or otherwise. None of these other names have gained general acceptance.

==Occurrence== thumb|In this case, the pyrite has dissolved away yielding a cube shape and residual gold. This break down is the main driver of acid mine drainage. Sub-surface mining often progresses below the water table, so water must be constantly pumped out of the mine in order to prevent flooding. When a mine is abandoned, the pumping ceases, and water floods the mine. This introduction of water is the initial step in most acid rock drainage situations. Tailings piles or ponds, mine waste rock dumps,<ref name="GARD" /> and coal spoils are also an important source of acid mine drainage.

After being exposed to air and water, oxidation of metal sulfides (often pyrite, which is iron-sulfide) within the surrounding rock and overburden generates acidity. Colonies of bacteria and archaea greatly accelerate the decomposition of metal ions, although the reactions also occur in an abiotic environment. These microbes, called extremophiles for their ability to survive in harsh conditions, occur naturally in the rock, but limited water and oxygen supplies usually keep their numbers low. Extremophiles known as acidophiles especially favor the low pH levels of abandoned mines. In particular, ''Acidithiobacillus ferrooxidans'' is a key contributor to pyrite oxidation.<ref>{{cite journal| author=Mielke, R.E. |author2=Pace, D.L. |author3=Porter, T. |author4=Southam, G.| title=A critical stage in the formation of acid mine drainage: Colonization of pyrite by Acidithiobacillus ferrooxidans under pH-neutral conditions| journal= Geobiology| volume= 1|issue=1|pages= 81–90|year=2003 |doi=10.1046/j.1472-4669.2003.00005.x|bibcode=2003Gbio....1...81M |s2cid=129323041 }}</ref>

Metal mines may generate highly acidic discharges where the ore is a sulfide mineral or is associated with pyrite. In these cases the predominant metal ion may not be iron but rather zinc, copper, or nickel. The most commonly mined ore of copper, chalcopyrite, is itself a copper-iron-sulfide and occurs with a range of other sulfides. Thus, copper mines are often major culprits of acid mine drainage.

At some mines, acidic drainage is detected within 2–5 years after mining begins, whereas at other mines, it is not detected for several decades.{{Citation needed|date=September 2013}} In addition, acidic drainage may be generated for decades or centuries after it is first detected. For this reason, acid mine drainage is considered a serious long-term environmental problem associated with mining.{{Citation needed||date=September 2013}}

==Chemistry== {{further|Acidophiles in acid mine drainage}}

The chemistry of oxidation of pyrites, the production of ferrous ions and subsequently ferric ions, is very complex, and this complexity has considerably inhibited the design of effective treatment options.<ref>{{cite journal|author=Blodau, C.| title=A review of acidity generation and consumption in acidic coal mine lakes and their watersheds|journal= Science of the Total Environment|volume=369|issue=1–3|pages= 307–332|year=2006|pmid=16806405|doi=10.1016/j.scitotenv.2006.05.004| bibcode=2006ScTEn.369..307B}}</ref>

Although a host of chemical processes contribute to acid mine drainage, pyrite oxidation is by far the greatest contributor. A general equation for this process is:<ref>[Acid Mine Drainage https://www.westech-inc.com/solutions/mining-and-minerals/acid-mine-drainage]</ref>

:{{chem2|2 FeS2_{(s)} + 7 O2_{(g)} + 2 H2O_{(l)} -> 2 Fe(2+)_{(aq)} + 4 SO4(2-)_{(aq)} + 4 H+_{(aq)}|}}

The oxidation of the sulfide to sulfate solubilizes the ferrous iron (iron(II)), which is subsequently oxidized to ferric iron (iron(III)):

:{{chem2|4 Fe(2+)_{(aq)} + O2_{(g)} + 4 H+_{(aq)} -> 4 Fe(3+)_{(aq)} + 2 H2O_{(l)}|}}

Either of these reactions can occur spontaneously or can be catalyzed by microorganisms that derive energy from the oxidation reaction. The ferric cations produced can also oxidize additional pyrite and reduce into ferrous ions:<ref>{{Cite journal |last1=Johnson |first1=D. Barrie |last2=Hallberg |first2=Kevin B. |date=2005-02-01 |title=Acid mine drainage remediation options: a review |url=https://www.sciencedirect.com/science/article/pii/S0048969704006199 |journal=Science of the Total Environment |series=Bioremediation of Acid Mine Drainage: The Wheal Jane Mine Wetlands Project |volume=338 |issue=1 |pages=3–14 |doi=10.1016/j.scitotenv.2004.09.002 |pmid=15680622 |bibcode=2005ScTEn.338....3J |s2cid=24245069 |issn=0048-9697|url-access=subscription }}</ref>

:{{chem2|FeS2_{(s)} + 14 Fe(3+)_{(aq)} + 8 H2O_{(l)} -> 15 Fe(2+)_{(aq)} + 2 SO4(2-)_{(aq)} + 16 H+_{(aq)}|}}

The net effect of these reactions is to release H+, which lowers the pH and maintains the solubility of the ferric ion.

==Effects==

===Effects on pH=== thumb|right|Yellow boy in a stream receiving acid drainage from surface coal mining Water temperatures as high as {{cvt|47|°C|0}}<ref>Nordstrom, D.K. & Alpers, C. N.: [http://www.pnas.org/content/96/7/3455.full Negative pH, efflorescent mineralogy, and consequences for environmental restoration at the Iron Mountain Superfund site, California] PNAS, vol. 96 no. 7, pp 3455–3462, 30 March 1999. Retrieved 4 February 2016.</ref> have been measured underground at the Iron Mountain Mine, and the pH can be as low as −3.6.<ref>{{cite journal |author1=D. K. Nordstrom |author2=C. N. Alpers |author3=C. J. Ptacek |author4=D. W. Blowes | title = Negative pH and Extremely Acidic Mine Waters from Iron Mountain, California | journal = Environmental Science & Technology | volume = 34 | issue = 2 | year = 2000 | pages = 254–258 | doi = 10.1021/es990646v |bibcode=2000EnST...34..254N |s2cid=95350834 |url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1488&context=usgsstaffpub |url-access=subscription}}</ref>

Organisms which cause acid mine drainage can thrive in waters with pH very close to zero. Negative pH<ref>{{cite journal | last1 = Lim | first1 = Kieran F. | year = 2006 | title = Negative pH Does Exist | journal = Journal of Chemical Education | volume = 83 | issue = 10 | page = 1465 | doi=10.1021/ed083p1465| bibcode = 2006JChEd..83.1465L | doi-access = | s2cid = 94970470 }}</ref> occurs when water evaporates from already acidic pools thereby increasing the concentration of hydrogen ions.

About half of the coal mine discharges in Pennsylvania have pH under 5.<ref>[https://pa.water.usgs.gov/projects/energy/amd/index.php USGS > Pennsylvania Water Science Center > Coal-Mine-Drainage Projects in Pennsylvania] Accessed 17 April 2012.</ref> However, a portion of mine drainage in both the bituminous and anthracite regions of Pennsylvania is alkaline, because limestone in the overburden neutralizes acid before the drainage emanates. {{Citation needed|date=May 2012}}

===Yellow boy=== When the pH of acid mine drainage is raised past 3, either through contact with fresh water or neutralizing minerals, previously soluble iron(III) ions precipitate as iron(III) hydroxide, a yellow-orange solid colloquially known as ''yellow boy''.<ref>Sam Alcorn (2007): [http://www.bucknell.edu/x38124.xml Professor paints a bright picture with 'yellow boy'] Bucknell University > News, September 2007. Accessed 4 January 2012. {{Webarchive|url=https://web.archive.org/web/20140714204951/http://www.bucknell.edu/x38124.xml |date=14 July 2014 }}</ref> Other types of iron precipitates are possible, including iron oxides and oxyhydroxides, and sulfates such as jarosite. All these precipitates can discolor water and smother plant and animal life on the streambed, disrupting stream ecosystems (a specific offense under the Fisheries Act in Canada). The process also produces additional hydrogen ions, which can further decrease pH. In some cases, the concentrations of iron hydroxides in yellow boy are so high the precipitate can be recovered for commercial use in pigments.<ref>Robert S Hedin, [http://www.hedinenv.com/pdf/Recovery%20of%20Marketable%20Iron%20Oxide%20from%20Mine%20Drainage.pdf RECOVERY OF MARKETABLE IRON OXIDE FROM MINE DRAINAGE, 2002 National Meeting of the American Society of Mining and Reclamation] {{Webarchive|url=https://web.archive.org/web/20081121045427/http://www.hedinenv.com/pdf/Recovery%20of%20Marketable%20Iron%20Oxide%20from%20Mine%20Drainage.pdf |date=21 November 2008 }}, Lexington KY, June 9–13, 2002. Published by ASMR, 3134 Montavesta Rd., Lexington, KY 40502</ref>

===Trace metal and semi-metal contamination=== Many acid rock discharges also contain elevated levels of potentially toxic metals, especially nickel and copper with lower levels of a range of trace and semi-metal ions such as lead, arsenic, aluminium, and manganese. The elevated levels of heavy metals can only be dissolved in waters that have a low pH, as is found in the acidic waters produced by pyrite oxidation. In the coal belt around the south Wales valleys in the UK highly acidic nickel-rich discharges from coal stocking sites have proved to be particularly troublesome.{{citation needed|date=November 2012}}

=== Effects on aquatic wildlife === Acid mine drainage also affects the wildlife living within the affected body of water. Aquatic macroinvertebrates living in streams or parts of streams affected by acid mine drainage show fewer individuals, less diversity, and lower biomass. Many species of fish also cannot tolerate the pollution.<ref>{{Cite journal|last1=Letterman|first1=Raymond|last2=Mitsch|first2=William|date=1978|title=Impact of Mine Drainage on a Mountain Stream in Pennsylvania|journal=Environmental Pollution|volume=17|pages=53–73|doi=10.1016/0013-9327(78)90055-1}}</ref> Among the macroinvertebrates, certain species can be found at only certain levels of pollution, while other species can be found over a wide range.<ref>{{Cite journal|last1=Rasmussen|first1=Keld|last2=Lindegaard|first2=Claus|date=1988|title=Effects of Iron Compounds on Macroinvertebrate Communities in a Danish Lowland River System|journal=Water Research|volume=22|issue=9|pages=1101–1108|doi=10.1016/0043-1354(88)90004-8|bibcode=1988WatRe..22.1101R }}</ref>

== Identification and prediction == In a mining setting it is leading practice to carry out a geochemical assessment of mine materials during the early stages of a project to determine the potential for AMD. The geochemical assessment aims to map the distribution and variability of key geochemical parameters, acid generating and element leaching characteristics.<ref name="ret">[http://www.ret.gov.au/resources/documents/lpsdp/lpsdp-acidhandbook.pdf] {{Webarchive|url=https://web.archive.org/web/20130515084247/http://www.ret.gov.au/resources/Documents/LPSDP/LPSDP-AcidHandbook.pdf|date=15 May 2013}} Department of Industry, Tourism and Resources - Managing Acid and Metalliferous Drainage: Leading Practice Sustainable Development Program for the Mining Industry (PDF) Australian Government handbook, 2007: pg 28 - 40</ref>

The assessment may include:<ref name=ret/> # Sampling; # Static geochemical testwork (e.g. acid-base accounting, sulfur speciation); # Kinetic geochemical testwork - Conducting oxygen consumption tests, such as the OxCon, to quantify acidity generation rates<ref>P.J. Schmieder, J.R. Taylor and N. Bourgeot (2012), Oxygen Consumption Techniques to Quantify Acidity Generation Rates, 1st International Acid and Metalliferous Drainage Workshop in China – Beijing 2012, http://earthsystems.com.au/wp-content/uploads/2013/05/Schmieder-et-al-2012_OxCon.pdf</ref> # Modelling of oxidation, pollutant generation and release; and # Modelling of material composition.

==Treatment==

===Oversight=== In the United Kingdom, many discharges from abandoned mines are exempt from regulatory control. In such cases the Environment Agency and Natural Resources Wales working with partners such as the Coal Authority have provided some innovative solutions, including constructed wetland solutions such as on the River Pelenna in the valley of the River Afan near Port Talbot and the constructed wetland next to the River Neath at Ynysarwed.

Although abandoned underground mines produce most of the acid mine drainage, some recently mined and reclaimed surface mines have produced ARD and have degraded local groundwater and surface-water resources. Acidic water produced at active mines must be neutralized to achieve {{val|p=pH |6|–|9}} before discharge from a mine site to a stream is permitted.

In Canada, work to reduce the effects of acid mine drainage is concentrated under the Mine Environment Neutral Drainage (MEND) program. Total liability from acid rock drainage is estimated to be between {{CA$|2 billion}} and {{CA$|5 billion}}.<ref>[http://www.nrcan.gc.ca/mms/canmet-mtb/mmsl-lmsm/enviro/effluents/effspecserv-e.htm] {{webarchive|url=https://web.archive.org/web/20080423224403/http://www.nrcan.gc.ca/mms/canmet-mtb/mmsl-lmsm/enviro/effluents/effspecserv-e.htm|date=23 April 2008}}</ref> Over a period of eight years, MEND claims to have reduced ARD liability by up to {{CA$|400 million}}, from an investment of {{CA$|17.5 million}}.<ref>[http://www.nrcan.gc.ca/mms/canmet-mtb/mmsl-lmsm/mend/default_e.htm] {{webarchive|url=https://web.archive.org/web/20080604053644/http://www.nrcan.gc.ca/mms/canmet-mtb/mmsl-lmsm/mend/default_e.htm|date=4 June 2008}}</ref>

===Methods===

====Neutralization with calcium carbonate==== Often, limestone rocks or appropriate calcareous strata that could contribute to neutralize acid effluents are lacking, or insufficiently accessible (too short contact time with acidic waters flowing too fast, too low specific surface area, insufficient contact...), at sites affected by acidic rock drainage. In such cases, crushed limestone can be dumped on site as a neutralizing agent.

However, although limestone is an unprocessed raw material available in large quantities and the least expensive neutralization agent, it can suffer from a number of disadvantages possibly limiting its applications. Indeed, small calcium carbonate grains of crushed limestone can be prone to the formation of a coating of gypsum ({{Chem2|CaSO4*2H2O}}) surrounded by a thin impermeable and protective film of less soluble Fe-Al hydroxysulfate. This coating is sometimes referred to in the literature as an ''armor'' (shield, encrustation, rim, rind...).<ref name="Hammarstrom2003" /> When present, it passivates the limestone surface, preventing calcite dissolution and the further release of bicarbonate in solution.<ref name="Hammarstrom2003" />

This might explain why at Cwm Rheidol in mid Wales, the positive impact of limestone application has been much less than anticipated because of the formation of a poorly soluble calcium sulfate layer onto the surface of limestone chips, binding the material and preventing further neutralization.

====Neutralization with lime==== Depending on the volume and flowrate of acid effluents to be neutralised and the scale of industrial installations, a common but more expensive commercial process for treating acid mine drainage is precipitation with lime in a high-density sludge (HDS) process. In this application, a slurry of lime (CaO – {{Chem2|Ca(OH)2}} after hydration) is dispersed into a tank containing acid mine drainage and recycled sludge to increase water pH to about 9. At this pH, most toxic metals become insoluble and precipitate, aided by the presence of recycled sludge. Optionally, air can be injected in the tank to oxidize iron(II) and manganese(II) and assist in their precipitation. The resulting slurry is directed to a sludge-settling vessel, such as a clarifier. In that vessel, clean water will overflow for release, whereas settled metal precipitates (sludge) are recycled to the acid mine drainage treatment tank, with a sludge-wasting side stream. A number of variations of this process exist, as dictated by the chemistry of ARD, its volume, and other factors.<ref>Zinck, J.M. and Griffith, W.F. (2000). An assessment of HDS-type lime treatment processes – efficiency and environmental impact. In: ICARD 2000. Proceedings from the Fifth International Conference on Acid Rock Drainage. Society for Mining, Metallurgy, and Exploration, Inc. Vol II, 1027–1034</ref> Generally, the products of the HDS process also contain gypsum (CaSO4) and unreacted lime, which enhance both its settleability and resistance to re-acidification and metal mobilization.

A general equation for this neutralization process is:

: H2SO4 + {{Chem2|Ca(OH)2}} → CaSO4 + 2 H2O

Less complex variants of this process, such as simple lime neutralization, may involve no more than a lime silo, a mixing tank and a settling pond. These systems are far less costly to build, but are also less efficient (longer reaction times are required, and they produce a discharge with higher trace metal concentrations, if present). They would be suitable for relatively small flows or less complex acid mine drainage.<ref>{{cite web|url=http://www.wvu.edu/~agexten/landrec/chemtrt.htm|title=Overview of Acid Mine Drainage Treatment with Chemicals|access-date=13 July 2009|archive-url=https://web.archive.org/web/20110524070234/http://www.wvu.edu/~agexten/landrec/chemtrt.htm|archive-date=24 May 2011|df=dmy-all}}</ref>

====Neutralization with calcium silicate==== A calcium silicate feedstock, made from processed steel slag, can also be used to neutralize active acidity in AMD systems by removing free hydrogen ions from the bulk solution, thereby increasing pH. As the silicate anion captures H+ ions (raising the pH), it forms monosilicic acid (H4SiO4), a neutral solute. Monosilicic acid remains in the bulk solution and play many roles in correcting the adverse effects of acidic conditions. In the bulk solution, the silicate anion is very effective in neutralizing H+ cations in the soil solution.<ref name="steel slag">{{cite web|last=Ziemkiewicz |first=Paul |title=The Use of Steel Slag in Acid Mine Drainage Treatment and Control |url=http://wvmdtaskforce.com/proceedings/98/98zie/98zie.htm |access-date=25 April 2011 |archive-url=https://web.archive.org/web/20110720064631/http://wvmdtaskforce.com/proceedings/98/98zie/98zie.htm |archive-date=20 July 2011 }}</ref> While its mode-of-action is quite different from limestone, the ability of calcium silicate to neutralize acid solutions is equivalent to limestone as evidenced by its CCE value{{Clarify|date=July 2024}} of 90–100% and its relative neutralizing value of 98%{{Clarify|date=July 2024}}.<ref>''Calcium Silicon-Based Mineral CSA.'' Harsco Minerals.</ref>

In the presence of heavy metals, calcium silicate reacts in a different way than limestone. As limestone raises the pH of the bulk solution, when heavy metals are present, precipitation of the poorly soluble metal hydroxides is accelerated and the tendency for an impermeable metal hydroxide coating, termed ''armoring'', to form on limestone grains surface increases significantly. Limestone grains become coated by a rind of gypsum encapsulated itself in a thin external film of impermeable and protective Fe-Al hydroxysulfate. ''Armoring'' slows the {{Chem2|CaCO3}} dissolution and prevents the limestone grains from releasing additional alkalinity in solution.<ref name="Hammarstrom2003">{{cite journal|last=Hammarstrom|first=Jane M.|author2=Philip L. Sibrell|author3=Harvey E. Belkin|date=2003|title=Characterization of limestone reacted with acid-mine drainage|journal=Applied Geochemistry|issue=18|pages=1710–1714|url=https://mine-drainage.usgs.gov/pubs/AG_18-1705-1721.pdf|access-date=30 March 2011|archive-date=5 June 2013|archive-url=https://web.archive.org/web/20130605231303/http://mine-drainage.usgs.gov/pubs/AG_18-1705-1721.pdf}}</ref> In the calcium silicate aggregates, as silicic acid species are adsorbed onto the metal hydroxide surface, the development of silica layers (mono- and bi-layers) lead to the formation of colloidal complexes with neutral or negative surface charges. These negatively charged colloids are electrostatically repelled by each other (as well as with the negatively charged calcium silicate granules). The sequestered metal colloids are stabilized and remain in a stable dispersed state – effectively interrupting metal precipitation and reducing vulnerability of the material to ''armoring'' (formation of an impervious crust around material grains preventing their dissolution and decreasing their reactivity).<ref name="steel slag"/>

====Removal of toxic metals by ion exchange==== Cation exchange processes have previously been investigated as a potential treatment for acid mine drainage. The principle is that an ion-exchange resin can remove potentially toxic metals (cationic resins), or chlorides, sulfates and uranyl sulfate complexes (anionic resins) from mine water.<ref>M. Botha, L. Bester, E. Hardwick "Removal of Uranium from Mine Water Using Ion Exchange at Driefontein Mine"</ref> Once the contaminants are adsorbed, the exchange sites on resins must be regenerated, which typically requires acidic and basic reagents and generates a brine that contains the pollutants in a concentrated form. A South African company{{Clarify|reason=What is the name of this company? Please also add a reference to support it.|date=July 2024}} that won the 2013 IChemE award for water management and supply (treating AMD) has developed a patented ion-exchange process that treats mine effluents (and AMD) economically.{{Citation needed|date=July 2024}}

====Constructed wetlands==== Constructed wetlands systems have been proposed during the 1980s to treat acid mine drainage generated by the abandoned coal mines in Eastern Appalachia.<ref>{{cite web |url=http://technology.infomine.com/enviromine/wetlands/coal.htm |title=Constructed wetlands for treatment of mine drainage - Coal-generated AMD |author=André Sobolewski |publisher=Wetlands for the Treatment of Mine Drainage |access-date=2010-12-12 |archive-date=23 April 2015 |archive-url=https://web.archive.org/web/20150423061659/http://technology.infomine.com/enviromine/wetlands/coal.htm }}</ref> Generally, the wetlands receive near-neutral water, after it has been typically neutralized by a limestone-based treatment process.<ref>{{cite web|url=http://www.wvu.edu/~agexten/landrec/passtrt/passtrt.htm|title=Overview of Passive Systems for Treating Acid Mine Drainage|access-date=13 July 2009|archive-url=https://web.archive.org/web/20090906074838/http://www.wvu.edu/~agexten/landrec/passtrt/passtrt.htm|archive-date=6 September 2009|df=dmy-all}}</ref> Metal precipitation occurs from their oxidation at near-neutral pH, complexation with organic matter, precipitation as carbonates or sulfides. The latter results from sediment-borne anaerobic bacteria capable of reducing sulfate ions into sulfide ions. These sulfide ions can then bind with heavy metal ions, precipitating heavy metals out of solution and effectively reversing the entire process.{{Citation needed|date=August 2012}}

The attractiveness of a constructed wetlands solution lies in its relative low cost. They are limited by the metal loads they can deal with (either from high flows or metal concentrations), though current practitioners have succeeded in developing constructed wetlands that treat high volumes (see description of Campbell Mine constructed wetland) and/or highly acidic water (with adequate pre-treatment). Typically, the effluent from constructed wetland receiving near-neutral water will be well-buffered at 6.5–7.0 and can readily be discharged. Some of the metal precipitates retained in sediments are easily oxidised and remobilised when exposed to atmospheric oxygen (e.g., copper sulfide or elemental selenium), and it is very important that the wetland sediments remain largely and permanently submerged to keep them insoluble and immobile. Prolonged droughts caused by climate warming might compromise the proper functioning and the safety of some constructed wetlands if during an extremely hot summer water supply decreases and evaporation accelerates causing them to dry up.

An example of an effective constructed wetland is on the Afon Pelena in the River Afan valley above Port Talbot where highly ferruginous discharges from the Whitworth mine have been successfully treated.{{Citation needed|date=July 2024}}

====Precipitation of metal sulfides==== Most base metals in acidic solution precipitate in contact with free sulfide, e.g. from H2S or NaHS. Solid-liquid separation after reaction would produce a base metal-free effluent that can be discharged or further treated to reduce sulfate, and a metal sulfide concentrate with possible economic value.

As an alternative, several researchers have investigated the precipitation of metals using biogenic sulfide. In this process, sulfate-reducing bacteria (SRB) oxidize organic matter using sulfate as terminal electron acceptor, instead of oxygen. Their metabolic products include bicarbonate produced by organic matter oxidation, which can neutralize water acidity, and hydrogen sulfide, which forms highly insoluble precipitates with many toxic metals. Although promising, this process has been slow in being adopted for a variety of technical reasons.<ref>{{cite journal|title=Operational Lessons Learned During Bioreactor Demonstrations for Acid Rock Drainage Treatment|first1=Diana|last1=Bless|first2=Brian|last2=Park|first3=Suzzann|last3=Nordwick|first4=Marek|last4=Zaluski|first5=Helen|last5=Joyce|first6=Randy|last6=Hiebert|first7=Charles|last7=Clavelot|date=1 December 2008|journal=Mine Water and the Environment|volume=27|issue=4|pages=241–250|doi=10.1007/s10230-008-0052-6|bibcode=2008MWE....27..241B |s2cid=108962729 }}</ref>

=== Technologies === Many technologies exist for the treatment of AMD.<ref>{{cite journal |doi=10.1016/j.jclepro.2017.03.082 |title=Acid mine drainage: Prevention, treatment options, and resource recovery: A review |year=2017 |last1=Kefeni |first1=Kebede K. |last2=Msagati |first2=Titus A.M. |last3=Mamba |first3=Bhekie B. |journal=Journal of Cleaner Production |volume=151 |pages=475–493 |bibcode=2017JCPro.151..475K }}</ref>

== Metagenomic study ==

With the advance of large-scale sequencing strategies, genomes of microorganisms in the acid mine drainage community are directly sequenced from the environment. The nearly full genomic constructs allows new understanding of the community and able to reconstruct their metabolic pathways.<ref>{{cite journal |journal=Nature |date=2004-03-04 |volume=428 |issue=6978 |pages=37–43 |title=Community structure and metabolism through reconstruction of microbial genomes from the environment |author=Tyson GW |pmid=14961025 |doi=10.1038/nature02340 |name-list-style=vanc |display-authors=1 |last2=Chapman |first2=Jarrod |last3=Hugenholtz |first3=Philip |last4=Allen |first4=Eric E. |last5=Ram |first5=Rachna J. |last6=Richardson |first6=Paul M. |last7=Solovyev |first7=Victor V. |last8=Rubin |first8=Edward M. |last9=Rokhsar |first9=Daniel S. |bibcode=2004Natur.428...37T|s2cid=4420754 }}</ref> Our knowledge of acidophiles in acid mine drainage remains rudimentary: we know of many more species associated with ARD than we can establish roles and functions.<ref>{{cite journal |journal=Journal of Environmental Management |date=2019-12-01 |volume=251 |article-number=109581 |title=A composite taxonomical and functional framework of microbiomes under acid mine drainage bioremediation systems |author=Villegas-Plazas M |pmid=31563048 |doi=10.1016/j.jenvman.2019.109581 |name-list-style=vanc |display-authors=1 |last2=Sanabria |first2=Janeth |last3=Junca |first3=Howard|bibcode=2019JEnvM.25109581V |s2cid=203592485 }}</ref>

== Microbes and drug discovery == Scientists have recently begun to explore acid mine drainage and mine reclamation sites for unique soil bacteria capable of producing new pharmaceutical leads. Soil microbes have long been a source for effective drugs<ref>{{cite journal | last1 = Dias | first1 = D.A. | last2 = Urban | first2 = S. | last3 = Roessner | first3 = U. | year = 2012 | title = A Historical Overview of Natural Products in Drug Discovery | journal = Metabolites | volume = 2 | issue = 4| pages = 303–336 | doi=10.3390/metabo2020303| pmid = 24957513 | pmc = 3901206 | doi-access = free }}</ref> and new research, such as that conducted at the Center for Pharmaceutical Research and Innovation, suggests these extreme environments to be an untapped source for new discovery.<ref>{{cite journal | last1 = Wang | first1 = X. | last2 = Elshahawi | first2 = S.I. | last3 = Shaaban | first3 = K.A. | last4 = Fang | first4 = L. | last5 = Ponomareva | first5 = L.V. | last6 = Zhang | first6 = Y. | last7 = Copley | first7 = G.C. | last8 = Hower | first8 = J.C. | last9 = Zhan | first9 = C.-G. | last10 = Kharel | first10 = M.K. | last11 = Thorson | first11 = J.S. | year = 2014 | title = Ruthmycin, a new tetracyclic polyketide from ''Streptomyces'' sp. RM-4-15 | journal = Org. Lett. | volume = 16 | issue = 2| pages = 456–459 | doi=10.1021/ol4033418| pmid = 24341358 | pmc = 3964319 }}</ref><ref>{{cite journal | last1 = Wang | first1 = X. | last2 = Shaaban | first2 = K.A. | last3 = Elshahawi | first3 = S.I. | last4 = Ponomareva | first4 = L.V. | last5 = Sunkara | first5 = M. | last6 = Copley | first6 = G.C. | last7 = Hower | first7 = J.C. | last8 = Morris | first8 = A.J. | last9 = Kharel | first9 = M.K. | last10 = Thorson | first10 = J.S. | year = 2014 | title = Mullinamides A and B, new cyclopeptides produced by the Ruth Mullins coal mine fire isolate ''Streptomyces'' sp. RM-27-46 | journal = J. Antibiot. | volume = 67 | issue = 8| pages = 571–575 | doi=10.1038/ja.2014.37| pmid = 24713874 | pmc = 4146655 }}</ref>

==List of selected acid mine drainage sites worldwide== This list includes both mines producing acid mine drainage and river systems significantly affected by such drainage. It is by no means complete, as worldwide, several thousands of such sites exist.

===Africa=== * West Rand Goldfield,<ref>{{cite web|url=http://www.fse.org.za/downloads/Decant%20of%20acid%20mine%20drainage%20in%20the%20West.ppt|title=Overview of acid mine drainage impacts in the West Rand Goldfield|date=2 February 2009|work=Presentation to DG of DWAF|access-date=2 July 2014|archive-url=https://web.archive.org/web/20120313033906/http://www.fse.org.za/downloads/Decant%20of%20acid%20mine%20drainage%20in%20the%20West.ppt|archive-date=2012-03-13}}</ref> Witwatersrand, South Africa

===Europe=== *Avoca, County Wicklow, Ireland *Aznalcollar mine on the Guadiamar, Spain *Wheal Jane, Cornwall, England *Tinto River, Spain *Odiel River, Spain *Libiola's mine,<ref>IMWA Symposium 2007: Water in Mining Environments, R. Cidu & F. Frau (Eds), 27–31 May 2007, Cagliari, Italy</ref> Italy *Spree River, Germany * The Lusatian Lake District and the Central German Lake District, both the product of open pit lignite mining, have to deal with acid mine drainage

===North America=== *Argo Tunnel, Idaho Springs, Colorado, US *Berkeley Pit superfund site, covering the Clark Fork River and 50,000 acres (200 km2) in and around Butte, Montana, US *The Summitville Mine in Rio Grande County, Colorado. The area has both natural and mining-exacerbated acid drainage flowing into the Wrightman Fork, then into the Alamosa River, which flows into the San Luis Valley *Britannia Beach, British Columbia, Canada *Clinch-Powell River system, Virginia and Tennessee, US *Iron Mountain Mine, Shasta County, California, United States *Monday Creek, Ohio, US *[http://amrclearinghouse.org/Sub/SCARLIFTReports/IrwinSyncline/Section%201.pdf The Irwin Syncline in Southwestern Pennsylvania] *[https://web.archive.org/web/20050903181524/http://www.ccrs.nrcan.gc.ca/ccrs/rd/apps/geo/mine/uranium_e.html Pronto mine tailings site], Elliot Lake area, Ontario, Canada *[http://www.kyheadwaters.org/ North Fork of Kentucky River], Kentucky, US *Old Forge borehole, Lackawanna River, Pennsylvania. Discharges 40–100 million gallons of acid mine drainage per day.<ref name=Pollution>{{cite web|title=Old Forge borehole drains mines for 50 years|url=http://thetimes-tribune.com/news/old-forge-borehole-drains-mines-for-50-years-1.1421199|work=The Scranton Times Tribune|access-date=18 March 2013|author=David Falchek|date=26 December 2012}}</ref> *[http://gallery.cheat.org/AMD-Pictures Cheat River Watershed] {{Webarchive|url=https://web.archive.org/web/20200218135348/http://gallery.cheat.org/AMD-Pictures |date=18 February 2020 }}, West Virginia, US *Copperas Brook Watershed, from the Elizabeth Mine in S. Strafford, Vermont, impacting the Ompompanoosuc River *[http://www.umass.edu/biocomplexity/ Davis Pyrite Mine in NW Massachusetts] *Hughes bore hole, Pennsylvania * Gold King Mine, Colorado, US *[https://maphub.net/db67/OntarioAMD AMD in Ontario], Canada

===Oceania=== *Brukunga, South Australia<ref>[http://outernode.pir.sa.gov.au/minerals/mines__and__developing_projects/former_mines/brukunga_mine_site DMITRE Minerals >...> Former Mines > Brukunga mine site] {{Webarchive|url=https://web.archive.org/web/20110402150225/http://outernode.pir.sa.gov.au/minerals/mines__and__developing_projects/former_mines/brukunga_mine_site |date=2 April 2011 }} Accessed 6 December 2011.</ref> *Grasberg mine, Papua province, Indonesia<ref>[https://www.nytimes.com/2005/12/27/international/asia/27gold.html?pagewanted=8 Jane Perlez and Raymond Bonner (2005): Below a Mountain of Wealth, a River of Waste. ''The New York Times'', 27 December 2005] Accessed 6 December 2011.</ref> *McArthur River zinc mine, Northern Territory, Australia<ref>[http://www.abc.net.au/news/2017-12-21/mcarthur-river-mine-toxic-waste-rock-ongoing-problem-new-report/9278922 McArthur River Mine: Toxic waste rock ongoing problem, security bond inadequate, report finds], ''ABC News'', 21 December 2017. Retrieved 20 April 2018.</ref> *Mount Morgan Mine, Queensland, Australia<ref>[http://www.abc.net.au/news/2018-03-24/central-queensland-farmers-disgusted-mining-proposal-canned/9581976 Farmers 'disgusted' as proposal at abandoned central Queensland gold mine canned] ''ABC News'', 16 March 2018. Retrieved 24 March 2018.</ref> *Ok Tedi environmental disaster caused by Ok Tedi Mine, Ok Tedi River, Papua New Guinea<ref name="Marychurch">{{cite web|last=Marychurch |first=Judith |author2=Natalie Stoianoff |title=Blurring the Lines of Environmental Responsibility: How Corporate and Public Governance was Circumvented in the Ok Tedi Mining Limited Disaster |work=Australasian Law Teachers Association – Refereed Conference Papers |location=Victoria University, Melbourne, Victoria, Australia |date=4–7 July 2006 |url=http://www.alta.edu.au/pdf/conference/published_papers/marychurch_j_stoianoff_n_2006_alta_conference_paper_blurring_lines_of_environmental_responsibility.pdf |access-date=6 December 2011 |archive-url=https://web.archive.org/web/20111007034627/http://www.alta.edu.au/pdf/conference/published_papers/marychurch_j_stoianoff_n_2006_alta_conference_paper_blurring_lines_of_environmental_responsibility.pdf |archive-date=7 October 2011 }}</ref> * Tui mine, an abandoned mine on the western slopes of Mount Te Aroha in the Kaimai Range of New Zealand, considered to be the most contaminated site in the country *West Coast mineral fields, Tasmania, Australia<ref>[http://www.mrt.tas.gov.au/pls/portal/docs/PAGE/MRT_INTERNET_PAGE_GROUP/MRT_GEOLOGICAL_INFORMATION/MRT_GROUNDWATER/MRT_ACID_DRAINAGE/UR2001_05_REPORT.PDF] {{Webarchive|url=https://web.archive.org/web/20070927163655/http://www.mrt.tas.gov.au/pls/portal/docs/PAGE/MRT_INTERNET_PAGE_GROUP/MRT_GEOLOGICAL_INFORMATION/MRT_GROUNDWATER/MRT_ACID_DRAINAGE/UR2001_05_REPORT.PDF|date=27 September 2007}} Accessed 6 December 2011.</ref>

==References== {{Reflist|2}}

==External links== {{Commons category}} *[http://trove.nla.gov.au/work/27355420?q&versionId=42597054 Managing Acid and Metalliferous Drainage: Leading Practice Sustainable Development Program for the Mining Industry] Trove: Australian Government Dept. of Industry, Tourism and Resources handbook, 2007. {{ISBN|0-642-72512-8}} Accessed 21 May 2016. *[https://web.archive.org/web/20041010190448/http://amrclearinghouse.org/Sub/AMDbasics/ZZoverview.htm AMRClearinghouse.org] *[https://web.archive.org/web/20110727152624/http://www.orangewaternetwork.org/index.php?name=Content&pa=showpage&pid=50 OrangeWaterNetwork.org (EPCAMR's Website)] *[https://web.archive.org/web/20090419045324/http://www.parliament.uk/commons/lib/research/rp99/rp99-010.pdf Assessment of treatment methods (PDF)] *[http://www.imwa.info/ IMWA – International Mine Water Association] *[http://www.inap.com.au/ INAP – International Network of Acid Prevention] *[http://www.gardguide.com/index.php/Main_Page INAP – Global Acid Rock Drainage Guide] *[https://web.archive.org/web/20070622204200/http://www.mines.edu/fs_home/jhoran/ch126/amd.htm Overview of chemical processes involved] *[http://www.padre.imwa.info/ PADRE – Partnership for Acid Drainage Remediation in Europe] *[https://www.dep.pa.gov/OurCommonWealth/pages/Article.aspx?post=92 The Science of Acid Mine Drainage and Passive Treatment] *[https://web.archive.org/web/20070908203355/http://geology.er.usgs.gov/eastern/environment/drainage.html USGS Mine Drainage] *[https://ca.water.usgs.gov/projects/iron_mountain/environment.html World's Most Acidic Waters are Found Near Redding, California (pH -3.6)] *[http://www.miwer.org/ MiWER - Mine Water and Environment Research Centre (based in Australia)] *[https://web.archive.org/web/20120313033906/http://www.fse.org.za/downloads/Decant%20of%20acid%20mine%20drainage%20in%20the%20West.ppt Overview of acid mine drainage impacts in the West Rand Goldfield]

{{DEFAULTSORT:Acid Mine Drainage}} Category:Environmental impact of mining Category:Mining waste Category:Water management in mining

Acid mine drainage

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