![]() A general equation for this process is: 2 FeS 2 (s) + 7 O 2 (g) + 2 H 2O (l) → 2 Fe 2+ (aq) + 4 SO 2− 4 (aq) + 4 H + (aq) Īlthough a host of chemical processes contribute to acid mine drainage, pyrite oxidation is by far the greatest contributor. The chemistry of oxidation of pyrites, the production of ferrous ions and subsequently ferric ions, is very complex, and this complexity has considerably inhibited the design of effective treatment options. Chemistry įurther information: Acidophiles in acid mine drainage For this reason, acid mine drainage is considered a serious long-term environmental problem associated with mining. In addition, acidic drainage may be generated for decades or centuries after it is first detected. Thus, copper mines are often major culprits of acid mine drainage.Īt some mines, acidic drainage is detected within 2–5 years after mining begins, whereas at other mines, it is not detected for several decades. The most commonly mined ore of copper, chalcopyrite, is itself a copper-iron-sulfide and occurs with a range of other sulfides. In these cases the predominant metal ion may not be iron but rather zinc, copper, or nickel. Metal mines may generate highly acidic discharges where the ore is a sulfide mineral or is associated with pyrite. In particular, Acidithiobacillus ferrooxidans is a key contributor to pyrite oxidation. Extremophiles known as acidophiles especially favor the low pH levels of abandoned mines. These microbes, called extremophiles for their ability to survive in harsh conditions, occur naturally in the rock, but limited water and oxygen supplies usually keep their numbers low. Colonies of bacteria and archaea greatly accelerate the decomposition of metal ions, although the reactions also occur in an abiotic environment. Tailings piles or ponds, mine waste rock dumps, and coal spoils are also an important source of acid mine drainage.Īfter being exposed to air and water, oxidation of metal sulfides (often pyrite, which is iron-sulfide) within the surrounding rock and overburden generates acidity. This introduction of water is the initial step in most acid rock drainage situations. When a mine is abandoned, the pumping ceases, and water floods the mine. Sub-surface mining often progresses below the water table, so water must be constantly pumped out of the mine in order to prevent flooding. This break down is the main driver of acid mine drainage. Occurrence In this case, the pyrite has dissolved away yielding a cube shape and residual gold. None of these other names have gained general acceptance. 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", "mining-influenced water" or otherwise. ![]() 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. 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". ![]() 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. Historically, the acidic discharges from active or abandoned mines were called acid mine drainage, or AMD. The same type of chemical reactions and processes may occur through the disturbance of acid sulfate soils formed under coastal or estuarine conditions after the last major sea level rise, and constitutes a similar environmental hazard. These, combined with reduced pH, have a detrimental impact on the streams' aquatic environments. This liquid often contains highly toxic metals, such as copper or iron. In many localities, the liquid that drains from coal stocks, coal handling facilities, coal washeries, and coal waste tips can be highly acidic, and in such cases it is treated as acid rock drainage. ![]() construction sites, subdivisions, and transportation corridors) may create acid rock drainage. Areas where the earth has been disturbed (e.g. Rio Tinto in Spain presents an acid drainage of both natural and artificial origin (mining) Rocks stained by acid mine drainage on Shamokin Creek, a tributary of the Susquehanna River in Northumberland County, Pennsylvania.Īcid mine drainage, acid and metalliferous drainage ( AMD), or acid rock drainage ( ARD) is the outflow of acidic water from metal mines or coal mines.Īcid 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. Outflow of acidic water from metal or coal mines ![]()
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