Geomicrobiological and geochemical investigation of pyrite – containing tailings from Kristineberg, Northern Sweden

Pyrite - containing tailings in Impoundment 1 in Kristineberg, northern Sweden were covered in 1996 with a soil cover consisting of 0.3 m compacted till and 1.5 m unspecified till. From the 1940s until 1996, the impoundment was unremediated and sulfide oxidation occurred to depths of 1 m in the tailings. This study focuses on the importance of microorganisms for pyrite oxidation and the release of oxidation products in the remediated tailings. Three cores containing material from the sealing layer and the zones of oxidized and unoxidized tailings were taken in September 2003 and geomicrobiologically and geochemically analyzed. Most-probable-number (MPN) numbers of pyrite-oxidizing, acidophilic Fe(II)-oxidizing microorganisms of the type Acidithiobacillus ferrooxidans were highest with up to 106 cells g-1 dw at the interface between the oxidized and unoxidized tailings, which correlates with maximum metal and sulfate pore water concentrations, a shift of pH and pe, as well as with high potential biological pyrite oxidation rates measured by microcalorimetry at atmospheric oxygen content. The mean proportion of biological pyrite oxidation was 80 % for the oxidized tailings. The potential pyrite oxidation rate for an assumed 0.3 m thick oxidation zone within the oxidized tailings was 4.7 x10-6 mol m-2 s-1 tailings surface (18 kg FeS2 m-2 year-1). The potential chemical pyrite oxidation rate was highest for the unoxidized tailings where Fe(II)-oxidizing microorganisms could not be detected and where the pyrite content was much higher than in the oxidized tailings. The results show that pyrite is biologically oxidized in particular zones of the mine tailings.