Geomicrobiological investigation of two different mine waste tailings generating acid mine drainage

The impact of microbiological metal sulfide oxidation on acid mine drainage generation was studied for two different mine tailings. Microorganisms were quantified using different methods: (1) SYBR Green II direct counting, (2) TaqMan quantitative, real-time PCR (Q-PCR), (3) catalyzed reporter deposition–fluorescence in situ hybridization (CARD-FISH) and (4) most probable number (MPN) cultivation of acidophilic Fe(II) oxidizers. Potential pyrite or pyrrhotite oxidation rates were measured by microcalorimetry.In the uncovered, pyrrhotite-containing tailings near Selebi-Phikwe, Botswana, acidophilic Fe(II)-oxidizing microorganisms were present in high numbers (MPN) of up to 107 cells g− 1 dw (mean value 3 × 106 cells g− 1 dw) throughout the entire water unsaturated, oxidized zone of about 25 m (at the tailings dam periphery) with a paste pH in the range of 3–4. Mean numbers of living Bacteria (CARD-FISH) and total microorganisms (SYBR Green II) were 1 × 107 cells g− 1 dw and 8 × 107 cells g− 1 dw, respectively. Cell numbers obtained by Q-PCR analysis were in the same range. The average potential pyrrhotite oxidation rate measured by microcalorimetry was 3.4 × 10−4 mol pyrrhotite m− 3 tailings s− 1 at 25 °C. About half of the pyrrhotite oxidation activity was biologically catalyzed.By contrast, in the covered pyrite-containing tailings in Impoundment 1 in Kristineberg, northern Sweden, acidophilic Fe(II)-oxidizing microorganisms (mean value 5 × 105 cells g− 1 dw) were only detected in a distinct zone of oxidized tailings between the cover and the unoxidized tailings where low pH values down to 3 prevailed. Bacterial numbers obtained by Q-PCR analysis were much higher (mean value 3 × 108 cells g− 1 dw). The proportion of biological pyrite oxidation was up to 100% for the oxidized zone. The average potential pyrite oxidation rate was 1.6 × 10−5 mol pyrite m− 3 tailings s− 1 at 10 °C, an order of magnitude lower than that for the pyrrhotite-containing tailings.