Acid mine drainage - AbsoluteAstronomy.com

Acid mine drainage or acid rock drainage (ARD), refers to the outflow of acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic water from (usually abandoned) metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

mines

Mining

Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...

or coal mines. However, other areas where the earth has been disturbed (e.g. construction sites, subdivisions, transportation corridors, etc.) may also contribute acid rock drainage to the environment. In many localities the liquid that drains from coal

Coal

Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

stocks, coal handling facilities, coal washeries, and even coal waste tips can be highly acidic, and in such cases it is treated as acid rock drainage. 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 mineral

Sulfide mineral

The sulfide minerals are a class of minerals containing sulfide as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, the sulfarsenides and the sulfosalts...

s.

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 constitute a similar environmental hazard.

Occurrence

Sub-surface mining

Underground mining refers to a group of underground mining techniques used to extract coal, oil shale and other minerals or geological materials from sedimentary rocks...

often progresses below the water table

Water table

The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...

, 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

Tailings

Tailings, also called mine dumps, slimes, tails, leach residue, or slickens, are the materials left over after the process of separating the valuable fraction from the uneconomic fraction of an ore...

piles or ponds may also be a source of acid rock drainage.

After being exposed to air and water, oxidation of metal sulfides (often pyrite

Pyrite

The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...

, which is iron-sulfide) within the surrounding rock and overburden

Overburden

Overburden is the material that lies above an area of economic or scientific interest in mining and archaeology; most commonly the rock, soil, and ecosystem that lies above a coal seam or ore body. It is also known as 'waste' or 'spoil'...

generates acidity. Colonies of bacteria and archaea

Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

greatly accelerate the decomposition of metal ions, although the reactions also occur in an abiotic environment. These microbes, called extremophile

Extremophile

An extremophile is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles...

s for their ability to survive in harsh conditions, occur naturally in the rock, but limited water and oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

supplies usually keep their numbers low. Special extremophiles known as acidophile

Acidophile (organisms)

Acidophilic organisms are those that thrive under highly acidic conditions . These organisms can be found in different branches of the tree of life, including Archaea, Bacteria, and Eukaryotes...

s especially favor the low pH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

levels of abandoned mines. In particular, Acidithiobacillus
Acidithiobacillus
Acidithiobacillus is a genus of Proteobacteria. Like all Proteobacteria, Acidithiobacillus is Gram-negative. The members of this genus used to belong to Thiobacillus, before they were reclassified in the year 2000....

ferrooxidans is a key contributor to pyrite oxidation.

Metal mines may generate highly acidic discharges where the ore

Ore

An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....

is a sulfide mineral or is associated with pyrite. In these cases the predominant metal ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

may not be iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

but rather zinc

Zinc

Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

, copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, or nickel

Nickel

Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

. The most commonly mined ore of copper, chalcopyrite

Chalcopyrite

Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical composition CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.On exposure to air, chalcopyrite...

, is itself a copper-iron-sulfide and occurs with a range of other sulfides. Thus, copper mines

Copper extraction

Copper extraction techniques refers to the methods for obtaining copper from its ores. This conversion consists of a series of chemical, physical, and electrochemical processes. Methods have evolved and vary with country depending on the ore source, local environmental regulations, and other...

are often major culprits of acid mine drainage.

Chemistry

For further information, see Acidophiles in acid mine drainage
Acidophiles in acid mine drainage
Acidophiles are not just present in exotic environments such as Yellowstone National Park or deep-sea hydrothermal vents. Genera such as Acidithiobacillus and Leptospirillum bacteria, and Thermoplasmales archaea, are present in syntrophic relationships in the more mundane environments of concrete...

The chemistry of oxidation of pyrites, the production of ferrous

Ferrous

Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....

ions and subsequently ferric

Ferric

Ferric refers to iron-containing materials or compounds. In chemistry the term is reserved for iron with an oxidation number of +3, also denoted iron or Fe3+. On the other hand, ferrous refers to iron with oxidation number of +2, denoted iron or Fe2+...

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(s) + 7O₂(g) + 2H₂O(l) = 2Fe²⁺(aq) + 4SO₄^2-(aq) + 4H⁺(aq)

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

4Fe²⁺(aq) + O₂(g) + 4H⁺(aq) = 4Fe³⁺(aq) + 2H₂O(l)

Either of these reactions can occur spontaneously or can be catalyzed by microorganisms that derive energy from the oxidation reaction. The ferric irons produced can also oxidize additional pyrite and oxidize into ferrous ions:

FeS₂(s) + 14Fe³⁺(aq) + 8H₂O(l) = 15Fe²⁺(aq) + 2SO₄^2-(aq) + 16H⁺(aq)

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

Traditionally, the character of acid mine drainage is determined by its acidity (mg/L), which is measured by titrating AMD with sodium hydroxide solution from the AMD initial pH till pH 8.3. Then calculate the moles of NaOH that consumed by one liter of AMD, and transfer the mole number into the weight of CaCO₃. It is the value of acidity (mg/L) of AMD. Hence, the direct meaning of acidity is: weight of CaCO₃ needed to neutralize the pH of 1 liter AMD.

However, acidity can not best represent AMD’s characters. Some AMDs have same acidity values, even same pH value, but of different properties. Because the AMD acidity includes two components: hydrogen ions and dissolved metal ions. This can be seen clearly from the AMD acidity titration curves.

The AMD acidity titration curve is shaped like a staircase. Vertical part shows the process OH^- ions neutralizing H⁺ ions, which increases the pH of water. Horizontal part indicates OH^- ions precipitate metal ions into metal hydroxides, which will act as a buffer

Buffer solution

A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a...

, using hydroxide from the titrant, keeping the pH constant for a brief time until a specific metal has completely precipitated.

Most metal hydroxides (except Na and K) are insoluble in water and have specific solubility products. When pH reaches certain level the metal ions will precipitate and be eliminated from the water. This forms the stair steps of the titration curve.

In different AMD the metal concentration may range from 500mg/L to 0.1mg/L. If assuming the highest concentration allowed for each metal is 0.1mmole/L, use metals’ precipitation products, we can calculate the criteria pH for each metal in water. Higher than the pH criteria the metal concentration is lower than 0.1mmole/L.

The following table shows the experimental and theoretical pH criteria of metals in AMD

Metal	Fe⁺³	Al⁺³	Cu⁺²/Mn	Zn⁺²/Ni⁺²	Fe⁺²
pH¹	<3.2	3-4.5	5-6.5	6.5-8	5.5-6.5
pH²	2.93	4.43	6.60/9.33	7.83/8.22	8.95

pH¹ values are experiment data
pH² values are calculated from metal hydroxide solubility products, assuming metal concentration is 0.1mmole/L

In real situation the following factors can affect the metal precipitation process, therefore in the experiment pH of precipitation spans around 1-1.5:

There are microbial and organic in the natural AMDs, which affect metal ion activities greatly.
During the titration process Fe⁺² ions are quickly oxidized into Fe⁺³ in the air and precipitate as more insoluble Fe(OH)₃. This lowers Fe⁺² ions precipitation pH. (usually AMD titrations are not conducted under N2 flow).
For Cu⁺², Ni⁺² and Zn⁺², there are competitions between precipitates of hydroxide and carbonate.
In low pH AMDs Mn⁺² ions are stable, as pH increases during the titration, Mn⁺² is oxidized and precipitated as MnO₂, This also lowers Mn precipitation pH.
When iron and manganese occur together (which is relatively common), manganese will not be removed from solution until all the ferrous iron is removed. This is because ferrous iron reduces - and redissolves - insoluble manganese oxides. Once iron is removed, manganese can be precipitated, either by raising water pH above 9.5 (rapid reaction) or by promoting microbial oxidation at neutral-alkaline pH (slower reaction). As indicated above, manganese oxide precipitates tend to promote manganese precipitation at lower pH (ca. 7.5-8.5), but this reaction is somewhat slower than direct precipitation at elevated pH.

The major advantage of titration curve method is: It is simple, convenient and low cost. It can show water problem quickly and clearly. It helps to explain the AMD forming procedure and helps to find the treat method.

Effects on pH

In some acid mine drainage systems temperatures reach 117 degrees Fahrenheit (47 °C), and the pH can be as low as
-3.6.

Acid mine drainage causing organisms 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 discharges in Pennsylvania

Pennsylvania

The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...

have pH under 5 standard units (source needed). However, a significant portion of mine drainage in both the bituminous

Bituminous coal

Bituminous coal or black coal is a relatively soft coal containing a tarlike substance called bitumen. It is of higher quality than lignite coal but of poorer quality than Anthracite...

and anthracite

Anthracite coal

Anthracite is a hard, compact variety of mineral coal that has a high luster...

regions of Pennsylvania is alkaline, because limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

in the overburden

Overburden

neutralizes acid

Neutralization

In chemistry, neutralization, or neutralisation is a chemical reaction in which an acid and a base react to form a salt. Water is frequently, but not necessarily, produced as well. Neutralizations with Arrhenius acids and bases always produce water:Y and X represent a monovalent cation and anion...

before the drainage emanates.

Acid mine drainage has recently been a hindrance to the completion of the construction of Interstate 99

Interstate 99

Interstate 99 is an intrastate Interstate Highway located in central Pennsylvania in the United States. The southern terminus of the route is at exit 146 of the Pennsylvania Turnpike north of Bedford, where the road continues south as U.S. Route 220 . The northern terminus is at...

near State College, Pennsylvania

State College, Pennsylvania

State College is the largest borough in Centre County in the U.S. state of Pennsylvania. It is the principal city of the State College, Pennsylvania Metropolitan Statistical Area which encompasses all of Centre County. As of the 2010 census, the borough population was 42,034, and roughly double...

, but this acid rock drainage didn't come from a mine: pyritic rock was unearthed during a road cut and then used as filler material in the I-99 construction. A similar situation developed at the Halifax airport in Canada. It is from these and similar experiences that the term acid rock drainage has emerged as being preferable to acid mine drainage, thereby emphasizing the general nature of the problem.

Yellow boy

When the pH of acid mine drainage is raised past 3, either through contact with fresh water or neutralizing

Neutralization

minerals, previously soluble Iron(III) ions precipitate as Iron(III) hydroxide, a yellow-orange solid colloquially known as yellow boy. Other types of iron precipitates are possible, including iron oxides and oxyhydroxides. All these precipitates can discolor water and smother plant and animal life on the streambed, disrupting stream

Stream

A stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...

ecosystem

Ecosystem

An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

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

Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

, arsenic

Arsenic

Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

, aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

, and manganese

Manganese

Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

. In the coal belt around the south Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

valleys in the UK

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

highly acidic nickel-rich discharges from coal stocking sites have proved to be particularly troublesome.

Oversight

In the United Kingdom

United Kingdom

, many discharges from abandoned mines are exempt from regulatory control. In such cases the Environment Agency

Environment Agency

The Environment Agency is a British non-departmental public body of the Department for Environment, Food and Rural Affairs and an Assembly Government Sponsored Body of the Welsh Assembly Government that serves England and Wales.-Purpose:...

working with partners such as the Coal Authority

Coal Authority

The Coal Authority is a non-departmental public body of the United Kingdom government.-History:It was established under the Coal Industry Act 1994 to manage certain functions previously undertaken by the British Coal Corporation , including ownership of unworked coal.It is situated in the south of...

have provided some innovative solutions, including constructed wetland

Constructed wetland

A constructed wetland or wetpark is an artificial wetland, marsh or swamp created as a new or restored habitat for native and migratory wildlife, for anthropogenic discharge such as wastewater, stormwater runoff, or sewage treatment, for land reclamation after mining, refineries, or other...

solutions such as on the River Pelenna in the valley of the River Afan

River Afan

The River Afan is a river in southwest Wales whose river valley formed the territory of the medieval Lords of Afan. The town of Aberavon grew up on the banks of the river, and was later subsumed by the larger centre of population known as Port Talbot...

near Port Talbot

Port Talbot

Port Talbot is a town in Neath Port Talbot, Wales. It had a population of 35,633 in 2001.-History:Port Talbot grew out of the original small port and market town of Aberafan , which belonged to the medieval Lords of Afan. The area of the parish of Margam lying on the west bank of the lower Afan...

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 $5 billion CAD. Over a period of eight years, MEND claims to have reduced ARD liability by up to $400 million CAD, from an investment of $17.5 million CAD.

Lime neutralization

By far, the most commonly used commercial process for treating acid mine drainage is lime 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 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. Generally, the products of the HDS process also contain gypsum and unreacted lime, which enhance both its settleability and resistance to re-acidification and metal mobilization.

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 (i.e., 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 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

or other calcareous

Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

strata

Stratum

In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...

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

, the positive impact has been much less than anticipated because of the creation of an insoluble calcium sulfate

Calcium sulfate

Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...

layer on the limestone chips, binding the material and preventing further neutralization.

Ion exchange

Cation exchange

Ion exchange

Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion...

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 (anionic resins), or chlorides and sulfates (cationic resins) from mine water. Once the contaminants are adsorbed, the exchange sites on resins must be regenerated, which typically requires expensive reagents and generates a brine that is difficult to dispose. A South African company claims to have developed a patented ion-exchange process that treats mine effluents (and AMD) economically, but such claims remain unsubstantiated at present.

Constructed wetlands

Constructed wetland

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

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 between 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

River Afan

valley above Port Talbot

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, Sulfate-reducing bacteria

Sulfate-reducing bacteria

Sulfate-reducing bacteria are those bacteria and archaea that can obtain energy by oxidizing organic compounds or molecular hydrogen while reducing sulfate to hydrogen sulfide...

oxidize organic matter using sulfate, instead of oxygen. Their metabolic products include bicarbonate

Bicarbonate

In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

, which can neutralize water acidity, and hydrogen sulfide

Hydrogen sulfide

Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

, 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.

Metagenomic study of acid mine drainage

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. Our knowledge of Acidophiles in acid mine drainage

Acidophiles in acid mine drainage

Acidophiles are not just present in exotic environments such as Yellowstone National Park or deep-sea hydrothermal vents. Genera such as Acidithiobacillus and Leptospirillum bacteria, and Thermoplasmales archaea, are present in syntrophic relationships in the more mundane environments of concrete...

remains rudimentary: we know of many more species associated with ARD than we can establish roles and functions.

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.

North America

Argo Tunnel
Argo Tunnel
The Argo Tunnel, originally called the Newhouse Tunnel , is a mine drainage and access tunnel with its portal at Idaho Springs, Colorado, USA...

, Idaho Springs, Colorado
Idaho Springs, Colorado
The City of Idaho Springs is a municipality in the U.S. state of Colorado that is the most populous settlement in Clear Creek County, Colorado. Idaho Springs is located in Clear Creek Canyon, in the mountains upstream from Golden, some thirty miles west of Denver...

, USA
Berkeley Pit
Berkeley Pit
The Berkeley Pit is a former open pit copper mine located in Butte, Montana, United States. It is one mile long by half a mile wide with an approximate depth of . It is filled to a depth of about with water that is heavily acidic , about the acidity of cola or lemon juice...

superfund
Superfund
Superfund is the common name for the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 , a United States federal law designed to clean up sites contaminated with hazardous substances...

site, covering the Clark Fork River and 50,000 acres (200 km²) in and around Butte, Montana
Butte, Montana
Butte is a city in Montana and the county seat of Silver Bow County, United States. In 1977, the city and county governments consolidated to form the sole entity of Butte-Silver Bow. As of the 2010 census, Butte's population was 34,200...

, USA
Britannia Beach, British Columbia
Britannia Beach, British Columbia
Britannia Beach is a small unincorporated community in the Squamish-Lillooet Regional District located approximately 30 kilometers north of Vancouver, British Columbia on the Sea-to-Sky Highway on Howe Sound. It has a population of about 300....

, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
Clinch
Clinch River
The Clinch River rises in Southwest Virginia near Tazewell, Virginia and flows southwest through the Great Appalachian Valley, gathering various tributaries including the Powell River before joining the Tennessee River in East Tennessee.-Course:...

-Powell
Powell River (Virginia)
The Powell River in the United States rises in southwest Virginia and flows into East Tennessee.The river rises in rural Wise County, Virginia near the Laurel Grove community northwest of Norton and flows for several miles before the confluence with Roaring Fork in the Kent Junction community...

River system, Virginia
Virginia
The Commonwealth of Virginia , is a U.S. state on the Atlantic Coast of the Southern United States. Virginia is nicknamed the "Old Dominion" and sometimes the "Mother of Presidents" after the eight U.S. presidents born there...

and Tennessee
Tennessee
Tennessee is a U.S. state located in the Southeastern United States. It has a population of 6,346,105, making it the nation's 17th-largest state by population, and covers , making it the 36th-largest by total land area...

, USA
Iron Mountain Mine
Iron Mountain Mine
Iron Mountain Mine, also known as the Richmond Mine at Iron Mountain, is a mine near Redding in Northern California. Geologically classified as a "massive sulfide ore deposit", the site was mined for iron, silver, gold, copper, zinc, and pyrite intermittently from the 1860s until 1963...

, Shasta County, California
Shasta County, California
Shasta County is a county located in the northern portion of the U.S. state of California. The county occupies the northern reaches of the Sacramento Valley, with portions extending into the southern reaches of the Cascade Range. As of the 2010 census, the population was 177,223, up from 163,256...

, USA
Monday Creek
Monday Creek
Monday Creek is a tributary of the Hocking River, 27 miles long, in southeastern Ohio in the United States. Via the Hocking and Ohio Rivers, it is part of the watershed of the Mississippi River, draining an area of 116 square miles on the unglaciated portion of the Allegheny Plateau...

, Ohio, USA
The Irwin Syncline in Southwestern Pennsylvania
Pronto mine tailings site, Elliot Lake area, Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....

, Canada
North Fork of Kentucky River, Kentucky, USA
Cheat River Watershed, West Virginia
West Virginia
West Virginia is a state in the Appalachian and Southeastern regions of the United States, bordered by Virginia to the southeast, Kentucky to the southwest, Ohio to the northwest, Pennsylvania to the northeast and Maryland to the east...

, USA
Copperas Brook Watershed, from the Elizabeth Mine
Elizabeth Mine
The Elizabeth Mine was a copper mine located in Orange County, Vermont, some miles south of the town of Strafford.The ore deposite was discovered in 1793, but mining did not start until 1809. Open pit mining and from 1886 underground mining was conducted. The mine produced up to 8,500,000 lbs...

in S. Strafford, Vermont
Strafford, Vermont
Strafford is a town in Orange County, Vermont, United States. The population was 1,045 at the 2000 census. The town of Strafford was created on August 12, 1761 by way of a royal charter which King George III of England issued to Governor Benning Wentworth of New Hampshire...

, impacting the Ompompanoosuc River
Ompompanoosuc River
The Ompompanoosuc River is a river, about 25 mi long, in eastern Vermont in the United States. It is a tributary of the Connecticut River, which flows to Long Island Sound...
Davis Pyrite Mine in NW Massachusetts
Hughes bore hole
Hughes bore hole
The Hughes borehole is an acid mine drainage site located near the southwest central borough of Portage, Pennsylvania in Cambria County. In the 1920s, a hole was drilled in order to remove water from the myriad coal mines in the area. In the 1950s, the bore hole was capped, but in the 1970s,...

, Pennsylvania
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...

Europe

Avoca
Avoca, County Wicklow
Avoca is a small town near Arklow, in County Wicklow, Ireland. It is situated on the River Avoca.The Avoca area has been associated with its famous copper mines for many years and the valley has been immortalised by Thomas Moore in the famous song The Meeting of the Waters...

, County Wicklow
County Wicklow
County Wicklow is a county in Ireland. It is part of the Mid-East Region and is also located in the province of Leinster. It is named after the town of Wicklow, which derives from the Old Norse name Víkingalág or Wykynlo. Wicklow County Council is the local authority for the county...

, Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
Aznalcollar mine
Aznalcollar mine
The Doñana Disaster, also known as the Aznalcollar Disaster or Guadiamar Disaster , was an industrial accident in Andalusia, southern Spain. On 25 April 1998, a holding dam burst at the Los Frailes mine, near Aznalcóllar, Seville Province, releasing 4–5 million cubic metres of mine tailings...

on the Agrio River
Agrio River
-References:* Rand McNally, The New International Atlas, 1993.*...

, Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
Wheal Jane
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
Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...

, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
Tinto River
Tinto River
The Río Tinto is a river in southwestern Spain that originates in the Sierra Morena mountains of Andalusia. It flows generally south-southwest, reaching the Gulf of Cádiz at Huelva....

, Spain

Africa

West Rand, Witwatersrand
Witwatersrand
The Witwatersrand is a low, sedimentary range of hills, at an elevation of 1700–1800 metres above sea-level, which runs in an east-west direction through Gauteng in South Africa. The word in Afrikaans means "the ridge of white waters". Geologically it is complex, but the principal formations...

, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...

Oceania

Queen River, Tasmania, Australia
King River, Tasmania, Australia

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Occurrence

Chemistry

Effects on pH

Yellow boy

Trace metal and semi-metal contamination

Oversight

Lime neutralization

Calcium silicate neutralization

Carbonate neutralization

Ion exchange

Constructed wetlands

Precipitation of metal sulfides

Metagenomic study of acid mine drainage

List of selected acid mine drainage sites worldwide

North America

Europe

Africa

Oceania

See also

External links