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Acid mine drainage, acid and metalliferous drainage (AMD), or acid rock drainage (ARD) is the outflow of acidic water from
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
mines or
coal mines Coal mining is the process of extracting coal from the ground. Coal is valued for its energy content and since the 1880s has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron fro ...
. 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, subdivisions, and transportation corridors) may create acid rock drainage. In many localities, the liquid that drains from
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...
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. This liquid often contains highly toxic metals, such as copper or iron. 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 Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cry ...
, and constitutes a similar
environmental hazard An environmental hazard is a substance, state or event which has the potential to threaten the surrounding natural environment or adversely affect people's health, including pollution and natural disasters such as storms and earthquakes. It can i ...
.


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. 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". 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",Global Acid Rock Drainage Guide (GARD Guide)
INAP: The International Network for Acid Prevention. Accessed 23 September 2013.
"mining-influenced water" or otherwise. None of these other names have gained general acceptance.


Occurrence

Sub-surface mining Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the economic via ...
often progresses below the
water table The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated. T ...
, 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 In mining, tailings are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are different to overburden, which is the waste rock or other material that overli ...
piles or ponds, mine waste rock dumps, and coal spoils are also an important source of acid mine drainage. After being exposed to air and water,
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
of metal sulfides (often
pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...
, which is iron-sulfide) within the surrounding rock and
overburden In mining, overburden (also called waste or spoil) is the material that lies above an area that lends itself to economical exploitation, such as the rock, soil, and ecosystem that lies above a coal seam or ore body. Overburden is distinct from tai ...
generates acidity. Colonies of
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
and archaea greatly accelerate the decomposition of metal ions, although the reactions also occur in an
abiotic In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them under ...
environment. These microbes, called
extremophile An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme temper ...
s for their ability to survive in harsh conditions, occur naturally in the rock, but limited water and
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
supplies usually keep their numbers low. Extremophiles known as
acidophile Acidophiles or acidophilic organisms are those that thrive under highly acidic conditions (usually at pH 5.0 or below). These organisms can be found in different branches of the tree of life, including Archaea, Bacteria,Becker, A.Types of Bacteria ...
s especially favor the low pH levels of abandoned mines. In particular, ''
Acidithiobacillus ''Acidithiobacillus'' is a genus of the ''Acidithiobacillia'' in the " Pseudomonadota". The genus includes acidophilic organisms capable of iron and/or sulfur oxidation. Like all ''"Pseudomonadota"'', ''Acidithiobacillus'' spp. are Gram-negative ...
ferrooxidans'' is a key contributor to pyrite oxidation. Metal mines may generate highly acidic discharges where the
ore Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 Apr ...
is a sulfide mineral or is associated with pyrite. In these cases the predominant metal
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
may not be
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
but rather
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
,
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
, or
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
. The most commonly mined ore of copper,
chalcopyrite Chalcopyrite ( ) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mo ...
, 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. 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.


Chemistry

The chemistry of oxidation of pyrites, the production of
ferrous In chemistry, the adjective Ferrous indicates a compound that contains iron(II), meaning iron in its +2 oxidation state, possibly as the divalent cation Fe2+. It is opposed to " ferric" or iron(III), meaning iron in its +3 oxidation state, suc ...
ions and subsequently
ferric In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+. The adjective ferric or the prefix ferri- is often used to sp ...
ions, is very complex, and this complexity has considerably inhibited the design of effective treatment options. 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: :2FeS2 + 7O2 + 2H2O -> 2Fe^2+ + 4SO4^ + 4H+. The oxidation of the sulfide to sulfate solubilizes the ferrous iron (
iron(II) In chemistry, iron(II) refers to the element iron in its +2 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe2+. The adjective ferrous or the prefix ferro- is often used to s ...
), which is subsequently oxidized to ferric iron (
iron(III) In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+. The adjective ferric or the prefix ferri- is often used to sp ...
): :4Fe^2+ + O2 + 4H+ -> 4Fe^3+ + 2H2O. 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: :FeS2 + 14Fe^3+ + 8H2O -> 15Fe^2+ + 2SO4^ + 16H+. 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 have been measured underground at the Iron Mountain Mine, and the pH can be as low as −3.6. Organisms which cause acid mine drainage can thrive in waters with pH very close to zero. Negative pH occurs when water evaporates from already acidic pools thereby increasing the concentration of hydrogen ions. About half of the
coal mine Coal mining is the process of extracting coal from the ground. Coal is valued for its energy content and since the 1880s has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron from ...
discharges in
Pennsylvania Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...
have pH under 5. However, a portion of mine drainage in both the
bituminous Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term ...
and anthracite
regions of Pennsylvania Geographic regions of the Commonwealth of Pennsylvania. Lehigh Valley The Lehigh Valley is named for the Lehigh River, which flows through it. The region includes Allentown, the third most populated city in Pennsylvania, the neighboring easte ...
is alkaline, because
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
in the
overburden In mining, overburden (also called waste or spoil) is the material that lies above an area that lends itself to economical exploitation, such as the rock, soil, and ecosystem that lies above a coal seam or ore body. Overburden is distinct from tai ...
neutralizes acid before the drainage emanates. Acid rock drainage was a hindrance to the completion of the construction of
Interstate 99 Interstate 99 (I-99) is an Interstate Highway in the United States with two segments: one located in central Pennsylvania, and the other in southern New York. The southern terminus of the route is near exit 146 of the Pennsylvania Turn ...
near
State College, Pennsylvania State College is a home rule municipality in Centre County in the Commonwealth of Pennsylvania. It is a college town, dominated economically, culturally and demographically by the presence of the University Park campus of the Pennsylvania Sta ...
. However, this acid rock drainage didn't come from a mine; rather, it was produced by oxidation of pyrite-rich rock which was unearthed during a road cut and then used as filler material in the I-99 construction. Similar issues with pyritic slate occurred at the Halifax Stanfield International Airport in Canada. When the phenomena is the result of earth-moving operations other than mining it is sometimes called "acid ''rock'' drainage"''.''


Yellow boy

When the pH of acid mine drainage is raised past 3, either through contact with fresh water or
neutralizing A neutralizing antibody (NAb) is an antibody that defends a cell from a pathogen or infectious particle by neutralizing any effect it has biologically. Neutralization renders the particle no longer infectious or pathogenic. Neutralizing antibod ...
minerals, previously soluble iron(III) ions precipitate as
iron(III) hydroxide Iron(III) oxide-hydroxide or ferric oxyhydroxideA. L. Mackay (1960): "β-Ferric Oxyhydroxide". ''Mineralogical Magazine'' (''Journal of the Mineralogical Society''), volume 32, issue 250, pages 545-557. is the chemical compound of iron, oxygen, ...
, a yellow-orange solid colloquially known as ''yellow boy''. Other types of iron precipitates are possible, including iron oxides and oxyhydroxides, and sulfates such as
jarosite Jarosite is a basic hydrous sulfate of potassium and ferric iron (Fe-III) with a chemical formula of KFe3(SO4)2(OH)6. This sulfate mineral is formed in ore deposits by the oxidation of iron sulfides. Jarosite is often produced as a byproduct du ...
. All these precipitates can discolor water and smother plant and animal life on the streambed, disrupting stream
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
s (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.


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 Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
,
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...
,
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
, and
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
. 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 Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...
valleys in the UK highly acidic nickel-rich discharges from coal stocking sites have proved to be particularly troublesome.


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. Among the macroinvertebrates, certain species can be found at only certain levels of pollution, while other species can be found over a wide range.


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.
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
The assessment may include: # 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 # 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 working with partners such as the
Coal Authority The Coal Authority is a non-departmental public body of the United Kingdom government sponsored by the Department for Business, Energy & Industrial Strategy (BEIS). On behalf of the country, it owns the vast majority of unworked coal in Great Brit ...
have provided some innovative solutions, including
constructed wetland A constructed wetland is an artificial wetland to treat sewage, greywater, stormwater runoff or industrial wastewater. It may also be designed for land reclamation after mining, or as a mitigation step for natural areas lost to land development ...
solutions such as on the River Pelenna in the valley of the River Afan near
Port Talbot Port Talbot (, ) is a town and community in the county borough of Neath Port Talbot, Wales, situated on the east side of Swansea Bay, approximately from Swansea. The Port Talbot Steelworks covers a large area of land which dominates the south ...
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. 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.


Methods


Lime neutralization

By far, the most commonly used commercial process for treating acid mine drainage is
lime Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a color between yellow and green Lime may also refer to: Botany ...
( 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 Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and/or thi ...
. 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. Generally, the products of the HDS process also contain
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywal ...
( CaSO4) and unreacted lime, which enhance both its settleability and resistance to re-acidification and metal mobilization. A general equation for this process is: :H2SO4 + CaOCaSO4 + H2O or more precisely in aqueous solution: : + 2 H+ + Ca2+O2−(aq) → Ca2+ + (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.


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. 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%. 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. 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.


Carbonate neutralization

Generally,
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
or other
calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an ad ...
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 Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...
, the positive impact has been much less than anticipated because of the creation of an insoluble
calcium sulfate Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris ...
layer on the limestone chips, binding the material and preventing further neutralization.


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 Pit water, mine water or mining water is water that collects in a mine and which has to be brought to the surface by water management methods in order to enable the mine to continue working. Origin Although all water that enters pit workings or ...
. 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 that won the 2013 IChemE (ww.icheme.org) award for water management and supply (treating AMD) have developed a patented ion-exchange process that treats mine effluents (and AMD) economically.


Constructed wetlands

Constructed wetland A constructed wetland is an artificial wetland to treat sewage, greywater, stormwater runoff or industrial wastewater. It may also be designed for land reclamation after mining, or as a mitigation step for natural areas lost to land development ...
s systems have been proposed during the 1980s to treat acid mine drainage generated by the abandoned coal mines in Eastern Appalachia. Generally, the wetlands receive near-neutral water, after it has been neutralized by (typically) a limestone-based treatment process. 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. 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 A constructed wetland is an artificial wetland to treat sewage, greywater, stormwater runoff or industrial wastewater. It may also be designed for land reclamation after mining, or as a mitigation step for natural areas lost to land development ...
) 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 Port Talbot (, ) is a town and community in the county borough of Neath Port Talbot, Wales, situated on the east side of Swansea Bay, approximately from Swansea. The Port Talbot Steelworks covers a large area of land which dominates the south ...
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,
Sulfate-reducing bacteria Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termina ...
oxidize organic matter using sulfate, instead of oxygen. Their metabolic products include
bicarbonate In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial biochem ...
, 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.


Technologies

Many technologies exist for the treatment of AMD from traditional high cost water treatment plants to simple in situ water treatment reagent dosing methods.


Metagenomic study

With the advance of large-scale sequencing strategies,
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
s 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. Our knowledge of
acidophiles in acid mine drainage The outflow of acidic liquids and other pollutants from mines is often catalysed by acid-loving microorganisms; these are the acidophiles in acid mine drainage. Acidophiles are not just present in exotic environments such as Yellowstone National ...
remains rudimentary: we know of many more species associated with ARD than we can establish roles and functions.


Microbes and drug discovery

Scientists have recently begun to explore acid mine drainage and
mine reclamation Mine reclamation is the process of modifying land that has been mined to ecologically functional or economically usable state. Although the process of mine reclamation occurs once mining is completed, the planning of mine reclamation activiti ...
sites for unique soil bacteria capable of producing new pharmaceutical leads. Soil microbes have long been a source for effective drugs 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.


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,
Witwatersrand The Witwatersrand () (locally the Rand or, less commonly, the Reef) is a , north-facing scarp in South Africa. It consists of a hard, erosion-resistant quartzite metamorphic rock, over which several north-flowing rivers form waterfalls, which ...
,
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...


Europe

* Avoca, County Wicklow, Ireland * Aznalcollar mine on the
Guadiamar The Guadiamar is a river of Andalusia, Spain, and a tributary of the Guadalquivir. Its course runs entirely within the Province of Seville, flowing from Sierra Morena through the eastern border of Doñana National Park. Mining disaster In 1998, t ...
, Spain *
Wheal Jane Wheal Jane is a disused tin mine near Baldhu and Chacewater in West Cornwall, England, United Kingdom. The area itself consisted of a large number of mines. History Wheal Jane was probably seriously worked for tin from the mid-18th century. ...
,
Cornwall Cornwall (; kw, Kernow ) is a historic county and ceremonial county in South West England. It is recognised as one of the Celtic nations, and is the homeland of the Cornish people. Cornwall is bordered to the north and west by the Atlantic ...
,
England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...
* Tinto River, Spain * Odiel River, Spain * Libiola's mine, Italy * Spree River, Germany * The
Lusatian Lake District The Lusatian Lake District (german: Lausitzer Seenland, dsb, Łužyska jazorina, hsb, Łužiska jězorina) is a chain of artificial lakes under construction in Germany across the north-eastern part of Saxony and the southern part of Brandenburg ...
and the
Central German Lake District The Central German Lake District (''German:'' Mitteldeutsches Seenland) is a group of artificial lakes that are the result of extensive open pit lignite mining. The area around Leipzig is sometimes separately marketed as the "Leipzig Lake Distri ...
, both the product of open pit lignite mining, have to deal with acid mine drainage


North America

* Argo Tunnel,
Idaho Springs, Colorado The City of Idaho Springs is the Statutory City that is the most populous municipality in Clear Creek County, Colorado, United States. Idaho Springs is a part of the Denver–Aurora–Lakewood, CO Metropolitan Statistical Area. As of the 2010 ...
, 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 Clinch may refer to: * Nail (fastener) or device to hold in this way * Clinching, in metalworking * Clinch fighting or the clinch, a grappling position in boxing or wrestling, a stand-up embrace * Clinch County, Georgia, USA * Clinch River, near T ...
- Powell River system,
Virginia Virginia, officially the Commonwealth of Virginia, is a state in the Mid-Atlantic and Southeastern regions of the United States, between the Atlantic Coast and the Appalachian Mountains. The geography and climate of the Commonwealth ar ...
and
Tennessee Tennessee ( , ), officially the State of Tennessee, is a landlocked state in the Southeastern region of the United States. Tennessee is the 36th-largest by area and the 15th-most populous of the 50 states. It is bordered by Kentucky to th ...
, US * Iron Mountain Mine,
Shasta County, California Shasta County (), officially the County of Shasta, is a county in the northern portion of the U.S. state of California. Its population is 182,155 as of the 2020 census, up from 177,223 from the 2010 census. The county seat is Redding. Shasta ...
, United States * Monday Creek, Ohio, US
The Irwin Syncline in Southwestern Pennsylvania
Elliot Lake area,
Ontario Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central C ...
, Canada
North Fork of Kentucky River
Kentucky, US * Old Forge borehole,
Lackawanna River The Lackawanna River is a U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map, accessed August 8, 2011 tributary of the Susquehanna River in Northeastern Pennsylvania. It flows through a region of th ...
, Pennsylvania. Discharges 40–100 million gallons of acid mine drainage per day.
Cheat River Watershed
,
West Virginia West Virginia is a state in the Appalachian, Mid-Atlantic and Southeastern regions of the United States.The Census Bureau and the Association of American Geographers classify West Virginia as part of the Southern United States while the B ...
, US *Copperas Brook Watershed, from the Elizabeth Mine in S.
Strafford, Vermont Strafford is a town in Orange County, Vermont, United States. The population was 1,094 at the 2020 census. The town of Strafford was created on August 12, 1761 by way of a royal charter which King George III of Great Britain issued to Governor ...
, impacting the Ompompanoosuc River
Davis Pyrite Mine in NW Massachusetts
* Hughes bore hole,
Pennsylvania Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...
* Gold King Mine,
Colorado Colorado (, other variants) is a state in the Mountain states, Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the wes ...
, US


Oceania

* Brukunga, South Australia *
Grasberg mine The Grasberg mine has one of the largest gold mining, reserves of gold and copper extraction, copper in the world. It is located in Mimika Regency, Central Papua, Indonesia near Puncak Jaya. It is operated by PT Freeport Indonesia (PTFI, ''s ...
, Papua province,
Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...
*
McArthur River zinc mine The McArthur River mine is a zinc-lead mine, situated about 70 kilometres southwest of Borroloola, near the Gulf of Carpentaria in the northeastern Northern Territory, Australia. It is operated by McArthur River Mining (MRM), a subsidiary of t ...
,
Northern Territory The Northern Territory (commonly abbreviated as NT; formally the Northern Territory of Australia) is an Australian territory in the central and central northern regions of Australia. The Northern Territory shares its borders with Western Aust ...
, Australia *
Mount Morgan Mine Mount Morgan Mine was a copper, gold and silver mine in Queensland, Australia. Mining began at Mount Morgan in 1882 and continued until 1981. Over its lifespan, the mine yielded approximately of gold, of silver and of copper. The mine was ...
,
Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , establishe ...
, Australia * Ok Tedi environmental disaster caused by
Ok Tedi Mine The Ok Tedi Mine is an open-pit copper and gold mine in Papua New Guinea located near the headwaters of the Ok Tedi River, in the Star Mountains Rural LLG of the North Fly District of the Western Province of Papua New Guinea. The mine is ...
,
Ok Tedi River The Ok Tedi is a river in New Guinea. The Ok Tedi Mine is located near the headwaters of the river, which is sourced in the Star Mountains. It is the second largest tributary of the Fly River. Nearly the entirety of the river runs through the N ...
,
Papua New Guinea Papua New Guinea (abbreviated PNG; , ; tpi, Papua Niugini; ho, Papua Niu Gini), officially the Independent State of Papua New Guinea ( tpi, Independen Stet bilong Papua Niugini; ho, Independen Stet bilong Papua Niu Gini), is a country i ...
*
Tui mine The Tui mine is an abandoned mine on the western slopes of Mount Te Aroha in the Kaimai Range of New Zealand. It was considered to be the most contaminated site in the country, following the cleanup of the former Fruitgrowers Chemical Company ...
, an abandoned mine on the western slopes of Mount Te Aroha in the
Kaimai Range The Kaimai Range (sometimes referred to as the ''Kaimai Ranges'') is a mountain range in the North Island of New Zealand. It is part of a series of ranges, with the Coromandel Range to the north and the Mamaku Ranges to the south. The Kaimai R ...
of New Zealand, considered to be the most contaminated site in the country * West Coast mineral fields,
Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...
, Australia
Accessed 6 December 2011.


See also

*
Bioleaching Bioleaching is the extraction of metals from their ores through the use of living organisms. This is much cleaner than the traditional heap leaching using cyanide. Bioleaching is one of several applications within biohydrometallurgy and several ...
*
Environmental issues with mining Environmental effects of mining can occur at local, regional, and global scales through direct and indirect mining practices. The effects can result in erosion, sinkholes, Biodiversity loss, loss of biodiversity, or the Soil contamination, cont ...
* International Mine Water Association * Passive treatment system * Uranium acid mine drainage


References


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. Accessed 21 May 2016.
OrangeWaterNetwork.org (EPCAMR's Website)Assessment of treatment methods (PDF)IMWA – International Mine Water AssociationINAP – International Network of Acid PreventionINAP – Global Acid Rock Drainage GuidePADRE – Partnership for Acid Drainage Remediation in EuropeThe Science of Acid Mine Drainage and Passive Treatment
*[http://www.miwer.org/ MiWER - Mine Water and Environment Research Centre (based in Australia)]
Overview of acid mine drainage impacts in the West Rand Goldfield
{{DEFAULTSORT:Acid Mine Drainage Environmental impact of mining Water management in mining