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|J. Michael Engle|
Acid mine drainage (also known as Abandoned Mine Drainage) is the outflow of acidic water from mines in Pennsylvania, West Virginia, and other mostly East Coast states. These areas have a large abundance of pyrite FeS2 (also known as "fool's gold") found within the coal. Normally the pyrite is underground "locked away" and isolated from the atmosphere and water. But when the pyrite is exposed to water and air sulfuric acid is released into the water resulting in the low pH. The sulfide oxidizes to sulfate which turns to sulfuric acid H2SO4 when exposed to air. The acid can dissolve other harmful metals (such as arsenic, copper, zinc, cadmium, mercury, and lead) and then bring them to the surface. Water in mines is a common problem. In fact the first use of the steam engine was to run a pump to remove water from active mines. This acidic outflow can continue for years and years. There are some ancient Roman mines in Great Britain that are still releasing AMD more than 2000 years after mining has ceased. The released acidic water can destroy organisms growing in watersheds. Another difficulty that occurs with AMD is the release of "yellow boy", an orange-yellow substance composed of dissolved iron precipitates. This is what produces the orange color that lines the beds of AMD impacted streams. This precipitate can form a sludge-like material which inhibits algae, insect, and fish growth.
There are generally three types of treatment:
Here the idea is to prevent the sulfuric acid from forming. The goal is to prevent contact between the pyrite and oxygen, water, or air. This is accomplished by keeping the pyrite under water. While not a solution this process dramatically slows the release of the acid. Eventually the water reaches the surface as AMD.
This concept aims to collect the runoff or seepage. This only works when there is minimal water released as in seepage. The collected water must sill be transported and disposed of. Many more times the water released is far too great to collect so this approach is not used very often.
This process aims to neutralize the acid once the water has emerged. There are two methods used here, active and passive.
Active treatment involves the use of a base to neutralize the acid. Bases that are often used include: hydrated lime, sodium hydroxide (NaOH), sodium carbonate, ammonia, and crushed limestone, Eventually all the base is used up and needs to be replaced. This makes active treatment quite costly but it can is done in a small amount of land area / space. One problem with active treatment is that the metals are not removed. What often happens is as the pH is raised the metals begin to precipitate out. The precipitated metals then coat the base which prevents the acid from contacting the base and being neutralized.
Passive treatment was first introduced by the Department of Interiors Bureau of Mines in 1982. Here artificial wetlands are created and the AMD directed to flow through. The plants take up the metals. One "pond" is created with an outward flow from the bottom which is covered with limestone is first placed on the bottom and covered with compost. AS the water flows through the compost microorganisms begin to neutralize the acid. What is not neutralized by the microorganisms is neutralized by the limestone. The flow through is brought up through an aeration apparatus, which is usually a tall pipe with holes in it. As the water flows through the holes it becomes aerated and the iron precipitates. Thus the "pond" following the aerator turns orange. While passive treatment is less costly it does require much more land area than active treatment. Usually it takes about 15 square meters of wetland for every liter per minute of flow. Flow may also limit passive treatment. Some passive treatment sites can handle large volumes of water (100 gallons/min). But most passive treatment sites handle 5-20 gallons/min.
The last point and our area of interest is what microorganisms live and thrive is this harsh environment. This is the focus of our research. We are using a metagenomics approach. Basically this involves isolating the DNA from all the organisms present and then using next generation DNA sequencing to determine what organisms are present. We are working with a group of undergraduate colleges and universities called GCAT-SEEK which aims to make available next generation sequencing to undergraduate students.