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The present work deals with the use of iron/sulfur-oxidizing bacterium Acidithiobacillus ferrooxidans for the desulfurization of a coal sample from Assam, India. The coal sample contains 3.87% w/w of organic sulfur, 1.4% w/w pyritic sulfur, and 1.29% w/w of sulfate sulfur which altogether are contributing to its total sulfur content of 6.56% w/w. The microbial process followed the oxidative-desulfurization mechanism for the removal of sulfur from the coal sample. The bacterium performed cleavage of the sulfide bonds in pyrite to release the inorganic sulfur. Mass spectroscopic analysis revealed that the bacterial strain A. ferrooxidans performed oxidative desulfurization of a model sulfur-containing organic compound dibenzothiophene to 2-hydroxy biphenyl through the formation of dioxy-dibenzothiophene. A. ferrooxidans cells played the catalytic role to release the sulfur present in organic and inorganic form in the coal sample. The microbial desulfurization process conducted in a shake flask scale (500 mL) removed about 79% of total sulfur from the coal sample in 14 days. [ABSTRACT FROM AUTHOR]