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Abstract
Sediments recovered from the flooded mine workings of the Penn Mine, a Cu-Zn mine abandoned since the early 1960s, were cultured for anaerobic bacteria over a range of pH (4.0 to 7.5). The molecular biology of sediments and cultures was studied to determine whether sulfate-reducing bacteria (SRB) were active in moderately acidic conditions present in the underground mine workings. Here we document multiple, independent analyses and show evidence that sulfate reduction and associated metal attenuation are occurring in the pH-4 mine environment. Water-chemistry analyses of the mine water reveal: (1) preferential complexation and precipitation by H2S of Cu and Cd, relative to Zn; (2) stable isotope ratios of 34S/32S and 18O/16O in dissolved SO4 that are 2–3 ‰ heavier in the mine water, relative to those in surface waters; (3) reduction/oxidation conditions and dissolved gas concentrations consistent with conditions to support anaerobic processes such as sulfate reduction. Scanning electron microscope (SEM) analyses of sediment show 1.5-micrometer, spherical ZnS precipitates. Phospholipid fatty acid (PLFA) and denaturing gradient gel electrophoresis (DGGE) analyses of Penn Mine sediment show a high biomass level with a moderately diverse community structure composed primarily of iron- and sulfate-reducing bacteria. Cultures of sediment from the mine produced dissolved sulfide at pH values near 7 and near 4, forming precipitates of either iron sulfide or elemental sulfur. DGGE coupled with sequence and phylogenetic analysis of 16S rDNA gene segments showed populations of Desulfosporosinus and Desulfitobacterium in Penn Mine sediment and laboratory cultures.
Highlights
7% of the total sulfur was extracted in a dilute bicarbonate-carbonate buffer, a sulfur fraction associated with soluble sulfates
An hypothesis for the formation of similar ZnS spheroids in an underground mine environment with near-neutral pH was presented by Labrenz et al [50] and Labrenz and Banfield [51], who considered nanometer-sized ZnS grains in bacterial slime of sulfate-reducing bacteria (SRB) to be produced by aerotolerant microbes as a mechanism for H2S regulation
Geochemical models for formation of low-temperature Cu-Pb-Zn deposits have been proposed, based on metal fixation of H2S produced by SRB [53]
Summary
Acid mine drainage (AMD) is caused primarily by the oxidation of sulfide minerals and is characterized by high aqueous concentrations of metals and low pH values in the absence of neutralizing agents such as carbonates. To determine whether microbially mediated sulfate reduction could occur at pH 4, we cultured microbial samples from sediments collected from the mine workings over a range of pH values, and investigated the molecular biology of the cultures to evaluate whether or not known SRB were present. During pumping from each discrete depth from which a water sample was collected, a flow-through chamber with probe inserts was used to monitor pH, specific conductance, temperature, oxidation-reduction potential [ORP, converted to Eh; see Additional file 2], and dissolved oxygen of the water. Stable isotope ratios of sulfur (34S/32S) and oxygen (18O/ 16O) in aqueous sulfate were determined at the USGS laboratory in Denver, CO, using continuous-flow isotoperatio mass spectrometry techniques [40,41]. ZhFaingvceursbueelef1indeasbpahcetreurlieumprecipitated on the surface of what may Zinc sulfide spherule precipitated on the surface of what may have been a bacterium
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