Role of microbial iron reduction in the dissolution of iron hydroxysulfate minerals

Iron-hydroxysulfate minerals can be important hosts for metals such as lead, mercury, copper, zinc, silver, chromium, arsenic, and selenium and for radionuclides such as 226Ra. These mineral-bound contaminants are considered immobilized under oxic conditions. However, when anoxic conditions develop, the activities of sulfate- or iron-reducing bacteria could result in mineral dissolution, releasing these bound contaminants. Reduction of structural sulfate in the iron-hydroxysulfate mineral jarosite by sulfate-reducing bacteria has previously been demonstrated. The primary objective of this work was to evaluate the potential for anaerobic dissolution of the iron-hydroxysulfate minerals jarosite and schwertmannite at neutral PH by iron-reducing bacteria. Mineral dissolution was tested using a long-term cultivar, Geobacter metallireducens strain GS-15, and a fresh isolate Geobacter sp. strain ENN1, previously undescribed. ENN1 was isolated from the discharge site of Shadle Mine, in the southern anthracite coalfield of Pennsylvania, where schwertmannite was the predominant iron-hydroxysulfate mineral. When jarosite from Elizabeth Mine (Vermont) was provided as the sole terminal electron acceptor, resting cells of both G. metallireducens and ENN1 were able to reduce structural Fe(III), releasing Fe+2, SO4-2, and K+ ions. A lithified jarosite sample from Utah was more resistant to microbial attack, but slow release of Fe+2 was observed. Neither bacterium released Fe+2 from poorly crystalline synthetic schwertmannite. Our results indicate that exposure of jarosite to iron-reducing conditions at neutral pH is likely to promote the mobility of hazardous constituents and should therefore be considered in evaluating waste disposal and/or reclamation options involving jarosite-bearing materials.