BIOLEACHING OF HEAVY METALS FROM PRISTINE AND NATURALLY LEACHED URANIUM ORE FROM THE CLOSED MINE AT RANSTAD, SOUTHERN SWEDEN

Higher than expected levels of heavy metals have been found in the surroundings of the closed uranium mine in Ranstad, southern Sweden. As these findings cannot be explained entirely by chemistry it was suggested that microbiological activities could be responsible for this. Many microorganisms produce short-chain organic acids and element specific ligands that are able to change pH and enhance chelation resulting in increased mobilization of many trace elements. Other non-essential elements as Tl, lanthanides and actinides may also be mobilized as a result of this. Three common ligand producing bacterial species (Pseudomonas fluorescens, Shewanella putrefaciens and Pseudomonas stutzeri) were incubated in a chemically defined medium supplemented U-ore that had been exposed to natural weathering conditions for 30 years having a content of 0.0013 % U by weight. For comparison, non-leached uranium ore (0.61 % U by weight) from the same area were incubated with P. fluorescens and S. putrefaciens. P. fluorescens is the only species that thrives and manages to mobilize measurable amounts of uranium from the two ores. Despite the extensive increase in pH from 4.7 to 9.3 P. fluorescens supplemented with ore manages to mobilize 0.001-0.005 % of the total amount of U from both ores. The release of U was interpreted to be attributed to the production of pyoverdine chelators, which is a typical ligand produced by fluorescent pseudomonads, as U could not be detected in either sterile controls or in the experiments with the other two bacteria. P. fluorescens also doubles the Cr concentration in solution as compared to the sterile controls whereas P. stutzeri and S. putrefaciens result in a 6-fold increase in Cr concentration. Thallium, Co, Zn, Mn and V concentrations initially follow that of the sterile controls, whereafter a decrease from day 2 of the experiment is observed. However, in cultures with P. stutzeri and S. putrefaciens and naturally leached ore, Fe concentrations are 5-6 times higher than in sterile controls and 4-5 times higher than in cultures with P. fluorescens. The difference in leaching behavior between the bacteria used in this study is likely to be explained by the production of different chelators.