ABSTRACT
The uranium deposit Curilo, located in Western Bulgaria, for a long period of time was a site of intensive mining activities including both open-pit and underground techniques as well as in situ leaching of uranium. The hexavalent uranium was readily solubilized by the sulfuric acid present in the irrigating solutions or generated in the soil as a result of the oxidation of sulfides, mainly of pyrite. The tetravalent uranium was solubilized as a result of its prior bacterial oxidation to the hexavalent state. The bacterial oxidation of sulfides and tetravalent uranium was carried out by well-known direct and indirect mechanisms. There was a clear tendency for the initially adsorbed uranium and non-ferrous metals to be subjected to reduction in the course of time. Radium also was solubilized as complexes of the same types as these of the uranium. Most of the uranium was precipitated as uraninite also by these bacteria.
