Microbially mediated leaching of low-sulfur coal in experimental coal columns

The leaching of a low-sulfur bituminous coal was investigated with experimental coal columns subjected to simulated rainfall events. Leachates from the columns became dominated by iron-oxidizing bacteria as evidenced by specific enrichment cultures and measurements of CO(2) assimilation. Heterotrophic microorganisms were also present in the coal leachates, but their numbers and activity decreased with decreasing pH. This pattern could be reversed by increasing the pH of the coal with lime. Organosulfur-utilizing bacteria made up a substantial portion of the heterotrophic community. Measurements of microbial activity in coal cores indicated that although much of the microbial community remained associated with coal particles, the relative abundance of heterotrophs and autotrophs in leachate seemed to reflect that in coal cores. When bacterial growth was delayed by autoclaving coal samples, acid production and leaching of iron and sulfur were also delayed. Rapid leaching of materials from coal thus appears to be strongly dependent on the presence of the natural bacterial microflora.