Ferric hydroxide and ferric hydroxysulfate precipitation by bacteria in an acid mine drainage lagoon

Abstract

The spontaneous precipitation of amorphous iron hydroxide and ferrichydroxysulfate has generally been considered to be an inorganic processinvolving the oxidation of ferrous iron with or without the presence ofsulfate. However, our study of bacterial communities growing in an acidmine drainage lagoon sediment has confirmed that microorganisms werealso capable of facilitating this mineral precipitation. Transmissionelectron microscopy revealed that bacteria growing at the surface hadiron-rich capsules, along with detectable amounts of Zn, Ti, Mn and Kincorporated into the mineralised matrix. In the subsurface, more cellswere associated with granular, fine-grained mineral precipitates,composed almost exclusively of iron and sulfur. Pore water profilesindicated that no discernible sulfate reduction had taken place,suggesting that these authigenic minerals were `ferric hydroxysulfate',and not iron sulfide. Energy dispersive X-ray spectroscopy furtherindicated that the subsurface minerals had variable composition, withthe Fe:S ratio decreasing with depth from 3.5:1 at 15 cm to 1.9:1 at 30cm. This indicates the high reactivity of ferric hydroxide for dissolvedsulfate. Because iron reduction was limited to sediment depths between3-10 cm, it is conceivable that these minerals are not amenable tobacterial reduction, and hence, the ability of bacteria to bind and formsuch precipitates may provide a natural solution to cleansing acidifiedwaters with a high dissolved metal content.