Acidic Mine Drainage (AMD), or Acid Rock Drainage (ARD), describes a set of redox processes in the environment which principally involve the oxidative dissolution of the mineral pyrite, FeS2. The amount of pyrite which dissolves into H2SO4 and Fe2+ is principally controlled by the supply of oxygen. However, when we consider more of the detail in these systems, we see that the pyrite usually oxidizes as a result of interacting with oxidized iron as Fe3+. It is the regeneration of the ferric iron (Fe3+) which limits how fast pyrite is oxidized bacause at low pH, Fe2+ reacts with O2 very slowly. This fact is why microbes can exist in these environments - they take advantage of this energy available from having Fe2+ and O2 together.
There are a total of 15 electrons requires to bring the iron and sulfur in pyrite to their most oxidized forms in water, and almost all of those electrons are because of the reduced sulfur. There are many intermediate forms of sulfur known to occur, and several of these are thought to be important in the reactions which take the iron and sulfur in pyrite to sulfate and ferric iron. Just as their are microbes that utilize the presence of Fe2+ and O2 for energy, there are microbes which can utilize these intermediate sulfur species with O2 or Fe3+ for energy. Below are some of the inorganic intermediate sulfur species of interest in these systems.
If we look at some of the ideas which are thought to govern the transformation of sulfur in pyrite oxidizing at low pH, we see a number of possibilities. At the surface of pyrite the sulfur must oxidize, at some point it would become detached from the surface of the mineral and enter the surrounding fluid. At what point and by what mechanism this happens may be governed by the solution's chemical and physical characteristics and/or the mineral surface structure. A popular idea, put forward quite eleoquently by George Luther in 1987, is that thiosulfate (S2O32-) detached from pyrite as a result of oxidation by ferric iron. This is supported in part by the observation of thiosulfate in solution in experiments that oxidized pyrite. Williamson and Rimstidt then showed that thiosulfate quickly reacted with ferric iron to form tetrathionate. It was assumed for a while that tetrathionate simply oxidized very quickly after that, which is why we never saw much of it in experiments that oxidized pyrite. Druschel then showed that tetrathionate in fact does not oxidize that fast and if thiosulfate is a primary product of pyrite oxidation that is RELEASES to solution, then we should see a builbup of tetrathionate in those experiments. The key here may lie in what happens at the surface of pyrite as it oxidizes at low pH. Borda's recent paper has started to describe some of what may be happening - he found that surface-bound sulfoxy species similar to thiosulfate do indeed form on oxidizing pyrite. Now the question lies in the fate of that surface-bound thiosulfate-like molecule on the surface of pyrite --> what happens to it under different conditions and ca that describe some of what we see with respect to the intermediate sulfoxyanion species formed in the course of pyrite oxidation?
