Acid mine drainage
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
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.
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",[3] "mining-influenced water"[4] or otherwise. None of these other names have gained general acceptance.
Occurrence
After being exposed to air and water,
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] 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]
Chemistry
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.[6]
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:[7]
- 2 FeS2(s) + 7 O2(g) + 2 H2O(l) → 2 Fe2+(aq) + 4 SO2−4(aq) + 4 H+(aq)
The oxidation of the sulfide to sulfate solubilizes the ferrous iron (
- 4 Fe2+(aq) + O2(g) + 4 H+(aq) → 4 Fe3+(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:[8]
- FeS2(s) + 14 Fe3+(aq) + 8 H2O(l) → 15 Fe2+(aq) + 2 SO2−4(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
Water temperatures as high as 47 °C (117 °F)[9] have been measured underground at the Iron Mountain Mine, and the pH can be as low as −3.6.[10]
Organisms which cause acid mine drainage can thrive in waters with pH very close to zero. Negative pH[11] occurs when water evaporates from already acidic pools thereby increasing the concentration of hydrogen ions.
About half of the
Yellow boy
When the pH of acid mine drainage is raised past 3, either through contact with fresh water or
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]
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.[15] Among the macroinvertebrates, certain species can be found at only certain levels of pollution, while other species can be found over a wide range.[16]
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.[17]
The assessment may include:[17]
- 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[18]
- 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 ground-water and surface-water resources. Acidic water produced at active mines must be neutralized to achieve 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 $2 billion and C$5 billion.[19] Over a period of eight years, MEND claims to have reduced ARD liability by up to C$400 million, from an investment of C$17.5 million.[20]
Methods
Lime neutralization
By far, the most commonly used commercial process for treating acid mine drainage is lime (CaO) precipitation in a high-density sludge (HDS) process. In this application, a slurry of lime 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 may be introduced in this tank to oxidize iron and manganese 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) will be 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.[21] 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 process is:
or more precisely in aqueous solution:
- SO2−
4 + 2 H+ + Ca2+O2−(aq) → Ca2+ + SO2−
4(aq) + 2 H+ + O2−(aq)
Less complex variants of this process, such as simple lime neutralization, may involve no more than a lime silo, mixing tank and 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.[22]
Calcium silicate neutralization
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 to play many roles in correcting the adverse effects of acidic conditions. In the bulk solution, the silicate anion is very active in neutralizing H+ cations in the soil solution.[23] 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 of 90–100% and its relative neutralizing value of 98%.[24]
In the presence of heavy metals, calcium silicate reacts in a different manner than limestone. As limestone raises the pH of the bulk solution, and if heavy metals are present, precipitation of the metal hydroxides (with extremely low solubilities) is normally accelerated and the potential of armoring of limestone particles increases significantly.[25] In the calcium silicate aggregate, as silicic acid species are absorbed onto the metal 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 create an electrostatic repulsion with each other (as well as with the negatively charged calcium silicate granules) and the sequestered metal colloids are stabilized and remain in a dispersed state – effectively interrupting metal precipitation and reducing vulnerability of the material to armoring.[23]
Carbonate neutralization
Generally, limestone or other calcareous strata that could neutralize acid are lacking or deficient at sites that produce acidic rock drainage. Limestone chips may be introduced into sites to create a neutralizing effect. Where limestone has been used, such as at Cwm Rheidol in mid Wales, the positive impact has been much less than anticipated because of the creation of an insoluble calcium sulfate layer on the limestone chips, binding the material and preventing further neutralization.
Ion exchange
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.[27] Generally, the wetlands receive near-neutral water, after it has been neutralized by (typically) a limestone-based treatment process.[28] 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 reverting 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]
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 metal precipitates retained in sediments are unstable when exposed to oxygen (e.g., copper sulfide or elemental selenium), and it is very important that the wetland sediments remain largely or permanently submerged.
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.
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,
Technologies
Many technologies exist for the treatment of AMD.[30]
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.[31] 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.[32]
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[33] 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.[34][35]
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
Europe
- Avoca, County Wicklow, Ireland
- Aznalcollar mine on the Guadiamar, Spain
- Wheal Jane, Cornwall, England
- Tinto River, Spain
- Odiel River, Spain
- Libiola's mine,[37] 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
- , US
- Iron Mountain Mine, Shasta County, California, United States
- Monday Creek, Ohio, US
- The Irwin Syncline in Southwestern Pennsylvania
- Pronto mine tailings site, Elliot Lake area, Ontario, Canada
- North Fork of Kentucky River, Kentucky, US
- Old Forge borehole, Lackawanna River, Pennsylvania. Discharges 40–100 million gallons of acid mine drainage per day.[38]
- Cheat River Watershed Archived 18 February 2020 at the Wayback Machine, West Virginia, US
- Copperas Brook Watershed, from the Elizabeth Mine in S. Strafford, Vermont, impacting the Ompompanoosuc River
- Davis Pyrite Mine in NW Massachusetts
- Hughes bore hole, Pennsylvania
- Gold King Mine, Colorado, US
Oceania
- Brukunga, South Australia[39]
- Grasberg mine, Papua province, Indonesia[40]
- Northern Territory, Australia[41]
- Queensland, Australia[42]
- Ok Tedi environmental disaster caused by Ok Tedi Mine, Ok Tedi River, Papua New Guinea[43]
- 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
See also
- Bioleaching
- Environmental issues with mining
- International Mine Water Association
- Passive treatment system
- Uranium acid mine drainage
- Environmental impact of iron ore mining
References
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- ^ 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.
- ^ a b Global Acid Rock Drainage Guide (GARD Guide) INAP: The International Network for Acid Prevention. Accessed 23 September 2013.
- ^ 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.
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- ^ Nordstrom, D.K. & Alpers, C. N.: 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.
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- ^ USGS > Pennsylvania Water Science Center > Coal-Mine-Drainage Projects in Pennsylvania Accessed 17 April 2012.
- ^ Sam Alcorn (2007): Professor paints a bright picture with 'yellow boy' Bucknell University > News, September 2007. Accessed 4 January 2012. Archived 14 July 2014 at the Wayback Machine
- ASMR, 3134 Montavesta Rd., Lexington, KY 40502
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- ^ a b [1] Archived 15 May 2013 at the Wayback Machine 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
- ^ 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
- ^ [2] Archived 23 April 2008 at the Wayback Machine
- ^ [3] Archived 4 June 2008 at the Wayback Machine
- ^ 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
- ^ "Overview of Acid Mine Drainage Treatment with Chemicals". Archived from the original on 24 May 2011. Retrieved 13 July 2009.
- ^ a b Ziemkiewicz, Paul. "The Use of Steel Slag in Acid Mine Drainage Treatment and Control". Archived from the original on 20 July 2011. Retrieved 25 April 2011.
- ^ Calcium Silicon-Based Mineral CSA. Harsco Minerals.
- ^ Hammarstrom, Jane M.; Philip L. Sibrell; Harvey E. Belkin. "Characterization of limestone reacted with acid-mine drainage" (PDF). Applied Geochemistry (18): 1710–1714. Archived from the original (PDF) on 5 June 2013. Retrieved 30 March 2011.
- ^ M. Botha, L. Bester, E. Hardwick "Removal of Uranium from Mine Water Using Ion Exchange at Driefontein Mine"
- ^ André Sobolewski. "Constructed wetlands for treatment of mine drainage - Coal-generated AMD". Wetlands for the Treatment of Mine Drainage. Archived from the original on 23 April 2015. Retrieved 12 December 2010.
- ^ "Overview of Passive Systems for Treating Acid Mine Drainage". Archived from the original on 6 September 2009. Retrieved 13 July 2009.
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- ^ IMWA Symposium 2007: Water in Mining Environments, R. Cidu & F. Frau (Eds), 27–31 May 2007, Cagliari, Italy
- The Scranton Times Tribune. Retrieved 18 March 2013.
- ^ DMITRE Minerals >...> Former Mines > Brukunga mine site Archived 2 April 2011 at the Wayback Machine Accessed 6 December 2011.
- ^ 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.
- ^ McArthur River Mine: Toxic waste rock ongoing problem, security bond inadequate, report finds, ABC News, 21 December 2017. Retrieved 20 April 2018.
- ^ Farmers 'disgusted' as proposal at abandoned central Queensland gold mine canned ABC News, 16 March 2018. Retrieved 24 March 2018.
- Victoria University, Melbourne, Victoria, Australia. Archived from the original(PDF) on 7 October 2011. Retrieved 6 December 2011.
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External links
- 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 0642725128Accessed 21 May 2016.
- AMRClearinghouse.org
- OrangeWaterNetwork.org (EPCAMR's Website)
- Assessment of treatment methods (PDF)
- IMWA – International Mine Water Association
- INAP – International Network of Acid Prevention
- INAP – Global Acid Rock Drainage Guide
- Overview of chemical processes involved
- PADRE – Partnership for Acid Drainage Remediation in Europe
- The Science of Acid Mine Drainage and Passive Treatment
- USGS Mine Drainage
- World's Most Acidic Waters are Found Near Redding, California (pH -3.6)
- MiWER - Mine Water and Environment Research Centre (based in Australia)
- Overview of acid mine drainage impacts in the West Rand Goldfield