Microbiology and Chemistry of Ard

Microbiology and Chemistry of Ard

	Microbiology and Chemistry of Ard Acid Rock Drainage is the leachate resulting from the oxidation of sulphide minerals exposed to water, air, and bacteria and the resultant products from the interaction of acid, metal-bearing solutions reacting with alkaline rocks and water. ARD is the result of both bacterial and chemical activity. When sulphide minerals (pyrites) are exposed to air and water in rocks during mining operations, the pyrite chemically oxidises, creating a slightly acidic environment conducive for the development of Thiobacillus ferrooxidans which colonises the exposed mineral surfaces. It derives energy from the oxidation of inorganic sulphur and iron-containing compounds.
Bacterial oxidation of pyrite produces ferric iron, a strong oxidant that chemically oxidises mineral sulphides including pyrite. The ferrous iron resulting from this reaction is regenerated to ferric iron by T. ferrooxidans. The ferric iron is then available to oxidise more pyrite, and the cycle continues. Apart from T. ferroxidans are the chemoautotrophic acidophilic Thiobacillus thiooxidans, which oxidises reduced sulphur and Leptospirillum ferroxidans, which oxidises reduced iron compounds are also present in the same environment.
The oxidation of pyrite is an exothermic reaction. In waste rock piles with active pyrite oxidation, sufficient heat is produced and retained within the pile for heat to build up. It is not unusual for temperatures to exceed 60°C. Because Thiobacillus and Leptospirillum species are mesophilic, oxidising inorganic substrates in the temperature range of 10-40°C, the high temperatures that occur in some sulphidic waste piles eventually limit these organisms. At lower temperatures (10-40°C) Thiobacillus and Leptospirillum species will predominate. At about 40°C, mesophilic chemoautotrophs will begin to die and moderately thermophilic, acidophilic chemoautotrophic bacteria will appear.
At 30-40°C, mesophilic and moderately thermophilic bacteria will coexist. Oxidising iron and sulphur compounds at a temperature range of about 40-60°C is a diverse group of organisms that are not yet well characterised. At a temperature of approximately 55°C, the moderate thermophiles are succeeded by the extremely thermophilic, acidophilic Sulfolobus, Metallosphaera, Sulfobacillus and Sulphurococcus archaea. These organisms oxidise iron and sulphur compounds under acid conditions at temperatures ranging from about 55-85°C. When the temperatures exceeds the upper limits of the bacteria colonising the moist and acidic areas of the sulphidic rock pile, all bacterial activity ceases and the temperature of the pile will decrease.
	Because of the bacterial catalysis in ARD formation, efforts have been made to inhibit the growth of Thiobacillus and Leptospirillum species and the thermophilic bacteria by adding surfactants and slow release biocides. But frequent applications are required. Once ARD is initiated, it is virtually unstoppable. To prevent the initiation of ARD, sulphide minerals must be isolated from air and water. This stops chemical oxidation and also inhibits the growth and activity of the bacteria that catalyse the reactions accelerating formation of ARD.

Applications of Microbial Interactions Part 2
Causes of Corrosion
Oxygen Influencing Corrosion
Anaerobic Microbial Corrosion
Pitting Corrosion
Protection from Corrosion
Microbially Induced Concrete Corrosion
Bio-Mining
Microbiology and Chemistry of Ard
Bioleaching and Bio-Oxidation
Bio-Accumulation
Xenobiotics
Bio-Geochemical Cycles
Carbon Cycle
Sources of Carbon Cycle
Scources of Organic Carbon
Mineralisation of Carbon
Decomposition and Carbon Dioxide Evolution
Carbon Assimilation
Nitrogen Cycle
Steps in Nitrogen Cycle

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