Acid Mine Drainage

Acid mine drainage (air asam tambang in Bahasa Indonesia) is water that commonly contaminated when pyrite, chalcopyrite or any  iron sulfides, then highly exposed and reacts with air and water. This contaminant form sulfuric acid and dissolved iron. Some of or all of this iron contaminated can precipitate to form the red, orange, or yellow sediments that falls in the bottom of streams containing mine drainage. This kind of solutions is highly acid runoff and dissolves some of heavy metal particle such as lead, mercury, cooper that might be dispersion into ground or surface water. The intends action of certain bacteria (commonly sulfuric and an aerob bacteria) in acid mine drainage process also influenced the rate and degree of acid water.

Reactions for Acid Mine Drainage

Acid mine drainage caused some of environmental problem in mining atmosphere, It disrupts growth and reproduction of aquatic plants and animals, diminishes valued recreational fish species, degrades surface and groundwater drinking supplies, and causes acid corrosion of infrastructure like wastewater pipes. The degree of acid water can reach 3 or 4 pH.

 

Yellowboy as result of Acid Mine Drainage

 

Mining known as an activity that produced acid mine drainage, both of underground and open pit mine. Beside acid mine water that come out from coal mine, ore mine (ex: cooper and gold mine, lead mine) also. More over, acid mine from gold mine more dangerous that coal mine. Drainage from these ore mines can contain high levels of these metals.

Acid mine drainage at Tinto River, Spain

Two condition that highly impact acid mine drainage were chemical and physical condition. Some of chemical parameters that influenced of contaminating acid mine drainage were pH, temperature, oxygen concentration, dissolved water, Fe³⁺ ionic activity, and land area of sulfuric  that exposed. Mean while physical parameter were acid reaction rate, weather, rock permeability, porous ground pressure  and hydrologic condition. 

The danger of acid mine that contaminating ground and surface water

There are two commonly method that applied to eliminate or reduce by acidity and heavy metals components. They are passive treatment that uses alkaline chemicals to neutralize acid-polluted waters and passive treatment method uses a treatment system that employs naturally occurring chemical and biological reactions to minimize acid mine drainage with small maintenance.

Active treatment system:

Some method of active treatment system were using limestone (calcium carbonate) which is to enrich water with calcium then rise up the pH into balance. The advantages of using limestone include low cost, ease of use, and formation of a dense, easily handled sludge. The disadvantages include slow reaction time, loss in efficiency of the system because of coating of the limestone particles with iron precipitates, difficulty in treating acid mine drainage with a high ferrous-ferric ratio, and ineffectiveness in removing manganese.

Others method using a hydrated lime (calcium hydroxide) as neutralizing agent that match to applied in coal mining because it is easy and safe to use, effective, and relatively inexpensive. The disadvantages is the sludge. Soda Ash (sodium carbonate) that seems match to small acid mine water treatment that flows but it is poor of settling the properties of sludge. Caustic soda that use for manganese acid mine water but it is dangerous to environment because of hard to handling the chemical, poor sludge properties and hard freeze in cold water.

 

The passive treatments:

Passive treatment is a treatment that occurred in wetland area and still a new technology. Passive treatments can be combined with several technologies to treats difficult effluents. Some of commonly passive treatment that applied on are aerobic wetland, anaerobic wetland, successive alkalinity producing system, anoxic limestone drains, and limestone pond.

Anoxic Limestone Drains is simply constructed passive treatment method that uses open ditches filled with limestone (anoxic drains are covered). The dissolution of limestone adds alkalinity and raising pH, but a coating of limestone by iron and aluminum precipitates affects the performance of this treatment method.

 

Anoxic Limestone Drains Method

Aerobic wetlands are typically designed to promote precipitation of iron hydroxide. Limestone can be added to the organic substrate for additional treatment via limestone dissolution. The wetlands are usually 1 to 6 acres in size, but depend upon the flow rate and may require periodic dredging. These treatments are limited to cases where the discharge has a pH greater than 4 and are often used as a final polishing treatment.

Anaerobic wetlands used to neutralizing acid rate and reduce metals particle to the sulfide form. Anaerobic wetlands may be lined or unlined shallow ponds filled with organic matter, such as compost, and underlain by limestone gravel. Microorganisms such as anaerobic bacteria, facilitate this reaction by first consuming oxygen, alkalinity and H2S are produced. If the system is improperly sized, if flow dries up, or if extended low temperatures are encountered, the microorganisms will die and the performance will be decreased. Some anaerobic wetlands discharge a sulfide “sewage” effluent.

The advantages of passive treatment for acid mine drainage are:


• do not require electrical power


• do not require any mechanical equipment, hazardous chemicals, or buildings


• do not require daily operation and maintenance


• more natural and aesthetic in their appearance and may support plants and wildlife


• less expensive