From BioMineWiki
At low pH, the oxidation of acid-insoluble metals such as pyrite will produce thiosulfate and is hence called the thiosulfate mechanism. The acid-soluble metals will produce polysulfide and this is called the polysulfide mechanism.
Comparison of the polysulfide and thiosulfate mechanisms
 1. Some of the stages of RISC oxidation can be catalysed by microbes. |
 2. The leaching of sulfides would stop after a while if the leaching-chemical ferric iron was only consumed. Without ferric iron the thiosulfate or polysulfide mechanisms can't proceed. |
 3. To recycle ferric iron from ferrous iron, a very powerful oxidising agent will be needed, even more powerful than ferric iron itself. Oxygen is the only electron acceptor able to spontaneously oxidise ferrous iron. This spontaneous reaction is very slow at low pH. But at a pH below 4 prokaryotic microbes can use the oxygen to speed up the regeneration of ferric iron out of ferrous iron. |
 4. Then, since ferric iron is regenerated, the leaching of metal sulfide can continue. This reaction is considered to be rate-limiting for oxidative dissolution of pyrite by ferric iron.(Singer & Stumm 1970, Moses & Herman 1991). |
