Coal has been a source of energy since the industrial revolution and within the United States the Appalachian region has supplied coal to the Nation for more then 200 years (Cecil and Tewalt 2005), however many of the benefits from coal have not always out numbered the environmental implications of mining. Surface mining has disturbed about 1.8 million ha in the Appalachian region since 1930 and in West Virginia about 610,000 ha have been mined by underground methods, along with 276,000 ha have been surfaced mined (Demchak et al 2004). One of the major problems arising from coal mining is acid mine drainage, for about 10,000 km of streams have been affected in Pennsylvania, Maryland, Ohio and West Virginia (USEPA 1995).
Acid mine drainage occurs when pyrite is exposed to the atmosphere where it is oxidized which is extremely acidic (Evangelou 1998). This mechanism for reaction is as follows (Singer and Stumm 1970);
FeS2 + 7/2O2 + H2O à Fe2+ + 2SO42- + 2H+
Fe2+ + 1/4O2 + H+ à Fe3+ + 1/2H2O
Fe3+ + 3H2O à Fe(OH)3(s) + 3H+
FeS2 + 7Fe2(SO4)3 + 8H2O à 15 FeSO4 + 8H2SO4
Pyrite is present in coal seams and the rock layers overlying coal seams and the formation of acid mine drainage arises in surface mining, in which the overlying rocks are broken and removed to the extracted the coal. In the above mechanism, it is the second reaction which is known as the rate-limiting step and Thiobacillus ferrooxidans can increase the rate of Fe2+ oxidation by a factor of 106 (Singer and Stumm 1970). T. ferrooxidans is a chemoautotrophic and acidophilic organism that is able to oxidize Fe2+, S, metal sulfides and other reduced inorganic sulfur compounds (Evangelou 1998). Another bacterium found in acid mine wastes is Thiobacillus thiooxidans and it is able to oxidize both elemental sulfur and sulfide to sulfuric acid (Brierley 1982), nevertheless T. thiooxidans cannot oxidize Fe2+ (Harrison 1984). Pyrite oxidation by bacteria is classified into either direct metabolic reactions which requires physical contact between bacteria and pyrite particles or indirect metabolic reactions in which the bacteria oxidizes Fe2+ therefore regenerating the Fe3+ required for the chemical oxidation of pyrite (Singer and Stumm 1970).
There are however several methods of determining the potential of pyretic material to produce acid mine drainage. The first of these methods is the determination of potential acidity in the overburden. In this method a direct determination of the acid-producing potential is a pyrite oxidation technique using 30% H2O2 and the acid produced from the H2O2 is termed the potential acidity (Evangelou 1998).
FeS2 + 7.5 H2O2 à Fe(OH)3(s) + H2SO4 + 4H2O
As seen in the reaction above complete pyrite oxidation frees 2 moles of H2SO4 for each mole of pyrite and thus for each mole of pyrite oxidized, 2 moles of calcium carbonate is needed to neutralize the acid (Evangelou 1998). Another method is acid-base counting, which one of the most widely used methods for characterizing overburden in geochemistry (Evangelou 1998). The idea behind acid-base accounting is to account for the acid producing potential due to pyrite and the neutralizing potential due to alkaline materials, such as carbonates and the differences between the two potentials specify if there is enough base to neutralize the acid formed from the pyrite material (Evangelou 1998). However this method has been “criticized because it dose not consider differences between the rate of pyrite oxidation and the rate of carbonate dissolution” (Evangelou 1998). The last method is simulated weathering, where leaching overburden in laboratory scale experiments. The effluent is then collects from the leaching process in the laboratory and tested for pH, sulfate and iron and the results are used to evaluate acid drainage formation potential (Evangelou 1998).
In 1977, the Surface Mining Control and Reclamation Act was passed which “provided standards for environmental protection during mining operations and placed the responsibility of AMD [acid mine drainage] control and treatment on the operator” (Demchak et al 2004), since then there has been development of different ways to treat acid mine drainage. Even before the passage of the Surface Mining Control and Reclamation Act, the State of Pennsylvania passes strict effluent standards for mining operations and companies started to use chemicals, such as calcium carbonate, sodium hydroxide and sodium bicarbonate, to raise the pH of the effluent and decrease the solubility of dissolved metals (Department of Environmental Protection Bureau of Abandoned Mines 2005). However chemicals are expensive and so the c idea of passive treatment began to be researched in 1978. Passive treatment theoretically is to allow natural occurring chemical and biological reactions to aid in the treatment of acid mine drainage n a controlled environment.
The first passive treatment systems used the natural Sphangum wetlands and lead to research in anaerobic wetlands to treat acid mine drainage. Typically wetlands are designed conservatively and can treat discharges that contain dissolved oxygen, Fe3+, Al3+ and acidity less than 300 mg/L. The wetland acts as reducing wetland where organic substrates promote chemical and microbial process that generate alkalinity and increase the pH. The compost in the system removes the oxygen, which allows for the sulfate to be reduced and keeps the metal from oxidizing and the microbial respiration within the substrate reduces the sulfates to water and hydrogen sulfide (Department of Environmental Protection Bureau of Abandoned Mines 2005). The size of the wetlands has been determined by the United States Bureau of Mines,
Minimum wetland size (m2) = acidity loading (g/day)/ 7
Another method of passive treatment is open limestone channels. These can be constructed in two ways, the first being to construct a drainage ditch of limestone and the acid mine drainage will collect in the ditch or the second method would be to place limestone fragments directly into a contaminated stream. The limestone dissolves and neutralized the pyrite acidity to raise the pH (Department of Environmental Protection Bureau of Abandoned Mines 2005).
Also, diversion wells are another way of using limestone to raise the pH of contaminates waters. The contaminated water is piped into a downstream well where is mixes with crushed limestone aggregate using the hydraulic force of the pipe flow. However this method, the diversion wells require frequent refilling with new limestone. Yet another method using limestone is anoxic limestone drains, which are buried beds of limestone that are constructed to intercept subsurface mine water flows (Department of Environmental Protection Bureau of Abandoned Mines 2005).
However the pyroulsute process is a patented process that uses cultured microbes to remove iron, manganese and aluminum from acid mine drainage. This treatment uses a shallow bed of limestone aggregate introduced with microorganism by inoculation ports throughout the bed. These microorganism grow on the surface of the limestone where they oxidize metal contaminates while chipping way at the limestone, thus increasing the alkalinity (Department of Environmental Protection Bureau of Abandoned Mines 2005).
Abandoned mine lands generate more then 90% of the acid mine drainage in streams and rivers in the Appalachian region and most acidic drainage flows from underground mines (Demchak et al 2004). Nevertheless the unavailability of water resources and its accompany impaired aesthetics and degradation is the largest cost to the public. Consequently, the need for simple and inexpensive treatment, such as passive treatment systems need to be researched along with a better understanding of the natural process within mines that affect water quality over time. (Demchak et al 2004).
Works Cited
Brielery, C.L. 1982. Microbiological Mining. Scientific America 247: 42-51
Cecil, Blaine C and Tewalt, Susan J. 2005. “Coal Extraction—Environmental Prediction” U.S. Geological Survey Fact Sheet
Demchak, J., Skousen, J. and McDonald, L.M. 2004. “Longevity of Acid Discharges from Underground Mines Located above the Regional Water Table” J. Environ Qual. 33:656-668
Department of Environmental Protection Bureau of Abandoned Mines. 2005. “The Science of Acid Mine Drainage and Passive Treatment”
Evangelou, V.P. 1998. Environmental Soil and Water Chemistry: Principles and Applications. Wiley-Interscience Publications, New York, NY.
Harrison, A.P. 1984. “The Acidophilic thiobacilli and other acidophilic bacteria that share their habitat. Annual Review of Microbiology 38: 265-292
USEPA. 1995. “Streams with fisheries impacted by acid mine drainage in MD, OH, PA, VA and WV” USEPA Region III, Wheeling, WV.
written for IS: Environmental Chemistry
12th December 2005
s.e.w
The plot as quoted from the bbc.uk - "High school teacher Mr. Harvey (Timothy Spall, Shooting the Past, Secrets & Lies) is completely misunderstood — and not just in the classroom. He's been carrying a secret, which has shaped his entire identity, but he is about to shed it. The kids, and his colleagues, think he's always been an old, humorless, fuddy-duddy. But before the end of a field trip to a 13th century cathedral, they'll learn about the secret — and students, staff, and Mr. Harvey himself will experience an epiphany."
