Acid mine drainage
Introduction
Acid mine drainage (AMD) occurs when metal sulfides, most commonly pyrite, are exposed to and react with air and water. When water flows over or through sulfur-bearing mine tailings, a chemical reaction occurs between the water and rocks resulting in metal-rich water. AMD typically has a bright orange, yellow, or bownish-red color to it. The mine drainage is acidic, as its name states, and found around ore and coal mines. Abandoned and currently operating mining activities contribute to AMD. Metal rich drainage can also occur in areas that have not been mined. Areas like construction sites and other places that have been highly disturbed can exacerbate natural rock weathering processes leading to acidic drainage.
An area heavily mined for coal, therefore greatly impacted by AMD, is the mid-Atlantic region in the middle of the Appalachian Mountains. Here, and everywhere AMD occurs, has a highly altered ecosystem. The greatest consequence of AMD is water pollution. This in turn results in contaminated drinking water, damage to aquatic flora and fauna, and corrosion of man-made infrastructure.
Microorganisms like bacteria and archaea significantly affect AMD. When metal sulfides, usually pyrite, that are contained in rock are exposed to water and air, an oxidation reaction takes place. Microbes speed up the decomposition of these metal ions. Microorganisms also play a huge part in the bioremediation of AMD. Techniques that are being researched include using metal-immobilizing bacteria, biocontrol with bacteria and archaea, and bioleaching.
Physical environment
Physical & Chemical Characteristics
AMD is effected by characteristics such as pore size, particle size, and mineral composition of the materials being oxidized. Water and oxygen availability are the most important factors though.
Oxidation of Pyrite
A lot of mines use the technique of sub-surface mining, where extraction occurs beneath the earth’s surface. With this technique mines are commonly below the water table, requiring water to be pumped out of the mine to stop flooding. When a mine is deserted, water once again fills in its natural position beneath the surface. This water is one of the necessary ingredients for the oxidation of pyrite leading to AMD formation.
Exposing pyrite to oxygen and water leads to an oxidation reaction, where hydrogen and sulfate ions and soluble metal cations are created:
- 2FeS2(s) + 7O2(g) + 2H2O(l) → 2Fe2+(aq) + 4SO42-(aq) + 4H+(aq)
Pyrite oxidation occurs naturally at a slow rate in undisturbed rock. However, the acidity created is buffered by water. Because mining exposes more surface area of these sulfur-bearing rocks, additional acid is produced that is beyond the water’s usual buffering capabilities.
When enough oxygen is available either from dissolved oxygen in the water or the atmosphere, further oxidation of ferrous iron (Fe+2) to ferric iron (Fe+3) occurs:
- 4Fe2+(aq) + O2(g) + 4H+(aq) → 4Fe3+(aq) + 2H2O(l)
Ferric iron (Fe+3) can either precipitate as ochre (Fe(OH)3 ), the reddish-orange precipitate observed in acid mine drainage waters:
- 2Fe3+(aq) + 6H2O(l) <→ 2Fe(OH)3¬(s) + 6H+(aq)
or it can react directly with pyrite to make additional ferrous iron and hydrogen ions:
- FeS2(s) + 14Fe3+(aq) + 8H2O(l) → 15Fe2+(aq) + 2SO42-(aq) + 16H+(aq)
Overall, these reactions release hydrogen ions, which decreases pH leading to an acidic environment. [2]
Metal Contamination
AMD is also known to be contaminated by toxic metals such as copper and nickel and lesser trace metals like lead, aluminum, arsenic, and manganese.
Role of Microbes
Catalyze Oxidation
The general agreement among scientists of microbial-induced pyrite oxidation is that these microorganisms raising the amount of available ferric iron which increases the rate of pyrite oxidation. Acidithiobacillus ferrooxidans, for example, use reduced ferrous iron in AMD areas as an electron donor for energy creation at low pH.
Treatment Methods
- Bioleaching: the process of using microorganisms to extract certain metals from low-grade ores.
- Reactive Permeable Barriers: barriers made of organic matter that are placed in the path of groundwater flow that react with specific substances of concern while allowing water to flow through easily. The barriers can be composed of materials like leaf compost, municipal compost, woodchips, or sawdust. The organic matter encourages the growth of sulfate-reducing bacteria which reduce sulfate to sulfide, resulting in the subsequent formation of insoluble metal sulfides. In the process, pH is increased therefore making the environment less acidic [3].
Subsection 2
Key Microorganisms
Named extremophiles because of their ability to live in severe conditions, microbes living in AMD have to deal with low pH levels (extreme acidophiles) and sometimes high temperatures (extreme thermophiles).
Adaptations
Organisms living in such harsh conditions must have adapted special systems for tolerating their environment. Scientists are particularly interested in the ability of microorganisms to to maintain internal pH homeostasis in such acidic conditions. The most studied acidophilic chemolithotroph is Acidithiobacillus ferrooxidans. A. ferrooxidans grows best in the range of pH from 1.5 to 3.5. But within this range, it is able to retain a cytoplasmic pH near neutral. Some research suggests a reversed membrane potential is responsible for internal pH maintenance, though the specific mechanisms for extreme acidophiles are unknown [4].
Bacteria
- Acicithiobacillus ferrooxidans
- Acidithiobacillus thiooxidans
- Leptospirillum ferrooxidans
- Sulfobacillus thermosulfidooxidans
Archaea
Ferroplasma acidiphilum
Current Research
Iron Mountain
At Iron Mountain near Redding, CA research is currently focusing on a “molecular-level understanding of the metabolism of organisms involved in AMD formation”. The project is using several methods to identify the molecular community and the roles of specific organisms present. DNA sequence analysis is used to learn what organisms are in the environment and then fluorescence in-situ hybridizations (FISH) determines cell type distribution and geochemical conditions. Samples are taking from different locations such as sediments, pore fluids, free-flowing waters, and subaerial biofilms. These field samples then serve as innoculum in various media that are incubated under aerobic, microaerophillic, and anaerobic conditions. Information about growth rates, metabolic capability, and optimal growth conditions is taken from isolates which are then identified through DNA sequence analyses. Currently, analysis has identified Leptospirillum group II, Leptospirillum group III, and Ferroplasma acidarmanus [5].
Treatment Methods
The majority of research in AMD deals with discovering new ways to treat AMD. AMD is one of the biggest environmental problems facing the mining industry. Economic and environmental concern have fueled studies into efficient and sustainable alternatives.
References
Edited by student of Angela Kent at the University of Illinois at Urbana-Champaign.
