The sulfur found in coal is generally divided into three forms: pyritic, sulfate, and organic sulfur. Originating fromirt-situoxidation of metal sulfides, sulfates axe present in coal at low concentrations. Being soluble in water, they are relatively easy to be leached from the coal. Similarly, microbial depyritization promotes the oxidative conversion of inorganic sulfur compounds to water-soluble products. The pyrite removal results from the combined effects of direct bacterial attack and indirect chemical solubilization. In the former, pyrite (FeS2) is oxidized by bacteria into Fe2(SO4)3; in the latter, ferric iron is the actual oxidizing agent and microorganisms serve to regenerate the ferric iron from ferrous iron.