Acidophiles in Acid Mine Drainage - Bacterial Influences On Acid Mine Drainage

Bacterial Influences On Acid Mine Drainage

The oxidation of metal sulfide (by oxygen) is slow without colonization by acidophiles, particularly Acidithiobacillus ferrooxidans (synonym Thiobacillus ferrooxidans). These bacteria can accelerate pyritic oxidation by 10^6 times. In that study, a proposal for the rate at which A.ferrooxidans can oxidise pyrite is the ability to use ferrous iron to generate a ferric iron catalyst :

Fe2+ + 1/4O₂ + H+ → Fe3+ + 1/2H₂O

Under the above acidic conditions, ferric iron (Fe3+) is a more potent oxidant than oxygen, resulting in faster pyrite oxidation rates.

A.ferrooxidans is a chemolithoautotrophic bacteria, due to the oligotrophic nature (low dissolved organic carbon concentration) of acidic environments, and their lack of illumination for phototrophy. Even when in vadose conditions, A.ferrooxidans can survive, if the rock retains moisture and the mine is aerated. In fact in this situation, with pioneer microorganisms, the limiting factor is likely to be the environmental circumneutral pH, which inhibits many acidophile’s growth. However, favourable geochemical conditions quickly develop with an acidic interface between the bacteria and the mineral surface, and pH is lowered to a level closer to acidophilic optimum.

The process proceeds through A.ferrooxidans exhibiting a quorum level for the trigger of acid mine drainage (AMD). At first colonisation of metal sulfides there is no AMD, and as the bacteria grow into microcolonies, AMD remains absent, then at a certain colony size, the population begins to produce a measurable change in water chemistry, and AMD escalates. This means pH is not a clear measure of a mine’s liability to AMD; culturing A.ferrooxidans (or others) gives a definite indication of a future AMD issue.

Other bacteria also implicated in AMD include Leptospirillum ferrooxidans, Acidithiobacillus thiooxidans and Sulfobacillus thermosulfidooxidans.