Acid mine drainage,acid and metalliferous drainage (AMD), oracid rock drainage (ARD) is the outflow ofacidic water frommetalmines andcoal mines.[citation needed]
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 ofsulfide minerals. Areas where the earth has been disturbed (e.g. construction sites orhighway construction) may create acid rock drainage. In many localities, the liquid that drains fromcoal stocks, coal handling facilities, coal washeries, andcoal wastetips can be highly acidic, and in such cases it is treated as acid rock drainage. These, combined with reduced pH, have a detrimental impact on the streams' aquatic environments.[citation needed]
The same type of chemical reactions and processes may occur through the disturbance ofacid sulfate soils formed under coastal or estuarine conditions after the last majorsea level rise, and constitutes a similarenvironmental hazard.[citation needed]
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.[1] 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".[2] Both AMD and ARD refer to lowpH or acidic waters caused by the oxidation ofsulfide minerals, though ARD is the more generic name.
In cases where drainage from a mine is not acidic and has dissolved metals ormetalloids, 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.
Sub-surface mining often progresses below thewater table, so water must be constantly pumped out of the mine in order to prevent flooding. When a mine is abandoned, the pumping ceases, and water floods the mine. This introduction of water is the initial step in most acid rock drainage situations.Tailings piles or ponds,mine waste rock dumps,[3] andcoal spoils are also an important source of acid mine drainage.
After being exposed to air and water,oxidation of metalsulfides (oftenpyrite, which is iron-sulfide) within the surrounding rock andoverburden generates acidity. Colonies ofbacteria andarchaea greatly accelerate the decomposition of metal ions, although the reactions also occur in anabiotic environment. These microbes, calledextremophiles for their ability to survive in harsh conditions, occur naturally in the rock, but limited water andoxygen supplies usually keep their numbers low. Extremophiles known asacidophiles especially favor the lowpH levels of abandoned mines. In particular,Acidithiobacillus ferrooxidans is a key contributor to pyrite oxidation.[5]
Metal mines may generate highly acidic discharges where theore is a sulfide mineral or is associated with pyrite. In these cases the predominant metalion may not beiron but ratherzinc,copper, ornickel. 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]
The chemistry of oxidation of pyrites, the production offerrous ions and subsequentlyferric 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]
The oxidation of the sulfide to sulfate solubilizes the ferrous iron (iron(II)), which is subsequently oxidized to ferric iron (iron(III)):
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]
The net effect of these reactions is to release H+, which lowers the pH and maintains the solubility of the ferric ion.
Water temperatures as high as 47 °C (117 °F)[9] have been measured underground at theIron 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 thecoal mine discharges inPennsylvania have pH under 5.[12] However, a portion of mine drainage in both thebituminous andanthraciteregions of Pennsylvania is alkaline, becauselimestone in theoverburdenneutralizes acid before the drainage emanates.[citation needed]
When the pH of acid mine drainage is raised past 3, either through contact with fresh water orneutralizing minerals, previously soluble iron(III) ions precipitate asiron(III) hydroxide, a yellow-orange solid colloquially known asyellow boy.[13] Other types of iron precipitates are possible, including iron oxides and oxyhydroxides, and sulfates such asjarosite. All these precipitates can discolor water and smother plant and animal life on the streambed, disruptingstreamecosystems (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.[14]
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 aslead,arsenic,aluminium, andmanganese. 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 southWales valleys in theUK highly acidic nickel-rich discharges from coal stocking sites have proved to be particularly troublesome.[citation needed]
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]
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]
In the United Kingdom, many discharges from abandoned mines are exempt from regulatory control. In such cases theEnvironment Agency andNatural Resources Wales working with partners such as theCoal Authority have provided some innovative solutions, includingconstructed wetland solutions such as on theRiver Pelenna in the valley of theRiver Afan nearPort Talbot and the constructed wetland next to theRiver Neath atYnysarwed.
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 achievepH 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 betweenCA$2 billion andCA$5 billion.[19] Over a period of eight years, MEND claims to have reduced ARD liability by up toCA$400 million, from an investment ofCA$17.5 million.[20]
Often,limestone rocks or appropriatecalcareousstrata that could contribute to neutralize acid effluents are lacking, or insufficiently accessible (too short contact time with acidic waters flowing too fast, too lowspecific surface area, insufficient contact...), at sites affected by acidic rock drainage. In such cases, crushed limestone can be dumped on site as neutralizing agent.
However, although limestone is an unprocessed raw material available in large quantities and the least expensive neutralisation agent, it can suffer from a number of disadvantages possibly limiting its applications. Indeed, smallcalcium carbonate grains of crushed limestone can be prone to the formation of a coating ofgypsum (CaSO4·2H2O) surrounded by a thin impermeable and protective film of less soluble Fe-Al hydroxysulfate. This coating is sometimes referred to in the literature as anarmor (shield, encrustation, rim, rind...).[21] When present, itpassivates the limestone surface, preventingcalcitedissolution and the further release ofbicarbonate in solution.[21]
This might explain why atCwm Rheidol in midWales, the positive impact of limestone application has been much less than anticipated because of the formation of a poorly solublecalcium sulfate layer onto the surface of limestone chips, binding the material and preventing further neutralization.
Depending on the volume and flowrate of acid effluents to be neutralised and the scale of industrial installations, a commonly but more expensive commercial process for treating acid mine drainage is precipitation with lime in a high-density sludge (HDS) process. In this application, a slurry oflime (CaO –Ca(OH)2 after hydration) is dispersed into a tank containing acid mine drainage and recycled sludge to increase water pH to about 9. At this pH, most toxic metals become insoluble and precipitate, aided by the presence of recycled sludge. Optionally, air can be injected in the tank to oxidize iron(II) and manganese(II) and assist in their precipitation. The resulting slurry is directed to a sludge-settling vessel, such as aclarifier. In that vessel, clean water will overflow for release, whereas settled metal precipitates (sludge) are recycled to the acid mine drainage treatment tank, with a sludge-wasting side stream. A number of variations of this process exist, as dictated by the chemistry of ARD, its volume, and other factors.[22] Generally, the products of the HDS process also containgypsum (CaSO4) and unreacted lime, which enhance both its settleability and resistance to re-acidification and metal mobilization.
A general equation for this neutralisation process is:
Less complex variants of this process, such as simple lime neutralization, may involve no more than a lime silo, a mixing tank and a settling pond. These systems are far less costly to build, but are also less efficient (longer reaction times are required, and they produce a discharge with higher trace metal concentrations, if present). They would be suitable for relatively small flows or less complex acid mine drainage.[23]
Acalcium silicate feedstock, made from processed steelslag, can also be used to neutralize active acidity in AMD systems by removing free hydrogen ions from the bulk solution, thereby increasing pH. As the silicate anion captures H+ ions (raising the pH), it forms monosilicic acid (H4SiO4), a neutral solute. Monosilicic acid remains in the bulk solution and play many roles in correcting the adverse effects of acidic conditions. In the bulk solution, the silicate anion is very effective in neutralizing H+ cations in the soil solution.[24] 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[clarification needed] of 90–100% and its relative neutralizing value of 98%[clarification needed].[25]
In the presence of heavy metals, calcium silicate reacts in a different way than limestone. As limestone raises the pH of the bulk solution, when heavy metals are present, precipitation of the poorly soluble metal hydroxides is accelerated and the tendency for an impermeable metal hydroxide coating, termedarmoring, to form on limestone grains surface increases significantly. Limestone grains become coated by a rind of gypsum encapsulated itself in a thin external film of impermeable and protective Fe-Al hydroxysulfate.Armoring slows theCaCO3 dissolution and prevents the limestone grains from releasing additionalalkalinity in solution.[21] In the calcium silicateaggregates, as silicic acid species areadsorbed onto the metal hydroxide surface, the development of silica layers (mono- and bi-layers) lead to the formation ofcolloidal complexes with neutral or negative surface charges. These negatively charged colloids are electrostatically repelled by each other (as well as with the negatively charged calcium silicate granules). The sequestered metal colloids are stabilized and remain in a stabledispersed state – effectively interrupting metal precipitation and reducing vulnerability of the material toarmoring (formation of an impervious crust around material grains preventing their dissolution and decreasing their reactivity).[24]
Cation exchange processes have previously been investigated as a potential treatment for acid mine drainage. The principle is that anion-exchange resin can remove potentiallytoxic metals (cationic resins), or chlorides, sulfates and uranyl sulfate complexes (anionic resins) frommine water.[26] Once the contaminants areadsorbed, the exchange sites on resins must be regenerated, which typically requires acidic and basic reagents and generates abrine that contains the pollutants in a concentrated form. A South African company[clarification needed] that won the 2013IChemE award for water management and supply (treating AMD) has developed a patented ion-exchange process that treats mine effluents (and AMD) economically.[citation needed]
Constructed wetlands systems have been proposed during the 1980s to treat acid mine drainage generated by the abandoned coal mines in EasternAppalachia.[27] Generally, the wetlands receive near-neutral water, after it has been typically neutralized by 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 ofreducing 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 Mineconstructed 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 easilyoxidised and remobilised when exposed to atmospheric oxygen (e.g.,copper sulfide or elementalselenium), and it is very important that the wetland sediments remain largely and permanently submerged to maintain them insoluble and immobile. Prolongeddroughts caused byclimate warming might compromise the proper functioning and the safety of some constructed wetlands if during extremely hot summer period water supply decreases and evaporation accelerates causing them to dry up.
An example of an effective constructed wetland is on theAfon Pelena in theRiver Afan valley abovePort Talbot where highly ferruginous discharges from theWhitworth mine have been successfully treated.[citation needed]
Most base metals in acidic solution precipitate in contact with free sulfide, e.g. from H2S or NaHS. Solid-liquid separation after reaction would produce a base metal-free effluent that can be discharged or further treated to reduce sulfate, and a metal sulfide concentrate with possible economic value.
As an alternative, several researchers have investigated the precipitation of metals using biogenic sulfide. In this process,sulfate-reducing bacteria (SRB)oxidizeorganic matter usingsulfate asterminal electron acceptor, instead ofoxygen. Theirmetabolic products includebicarbonate produced by organic matter oxidation, which can neutralize water acidity, andhydrogen 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.[29]
Many technologies exist for the treatment of AMD.[30]
With the advance oflarge-scale sequencing strategies,genomes ofmicroorganisms 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 ofacidophiles in acid mine drainage remains rudimentary: we know of many more species associated with ARD than we can establish roles and functions.[32]
Scientists have recently begun to explore acid mine drainage andmine 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 theCenter for Pharmaceutical Research and Innovation, suggests these extreme environments to be an untapped source for new discovery.[34][35]
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.
