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The microbially mediated recovery of valuable metals contained in mining waste presents an economical alternative to conventional hydrometallurgical processes. In order to investigate the effect of bacterial adaptation and biological oxidation on bioleaching, the microbially mediated bioleaching of a pyrrhotite sample from mine waste, with indigenous bacteria existing in acid mine drainage, was studied. The indigenous bacteria were sub-cultured repeatedly for iron adaptation, and Acidithiobacillus ferrooxidans was identified as the dominant member of the microbial consortium. The point of zero charge (PZC) of pyrrhotite sampled from mine waste was determined as 3.0. The performance of bioleaching by contact and non-contact biological oxidation was compared by conducting bioleaching under different initial pH (pHini) conditions (2.8 and 3.2). Negatively charged bacteria could be attached onto the pyrrhotite, which has a positive surface charge at lower pHini (2.8) than the PZC (3.0). Bacteria attachment and corrosion pits on the surface of the pyrrhotite residues were observed at pHini of 2.8. Under bacteria-adapted conditions, the leaching concentration of Fe (44.2 mg/L) at pHini of 2.8 was 2.1 times greater than that (21.3 mg/L) at pHini of 3.2. Under non-adapted bacteria conditions, the extent of Fe leaching was not significantly different between the pHini of 2.8 and 3.2. This could be attributed to the fact that the adapted bacteria could more easily attach onto the pyrrhotite surfaces at pHini 2.8, allowing contact biological oxidation during the bioleaching experiments. We demonstrate here that the bioleaching of pyrrhotite could increase Fe recovery through bacterial adaptation and contact biological oxidation.

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Numerous low-level and intermediate-level radioactive wastes are generated from the decommissioning processes of nuclear power plants; these wastes are immobilized to prevent the release of radionuclides under disposal conditions. In this study, we investigated the leaching behavior of Cs, Sr, Co, and Eu, which are common in immobilized decommissioning wastes. Ordinary Portland cement (OPC) was used as an immobilization agent. During the test, leaching of the nuclides occurred in the order of Cs, Sr, Co, and Eu and decreased over time. The results showed that 41.4% of the total Cs leached over 90 days, although the other elements leached in quantities less than 1.5%. CaCO3 was precipitated by the release of cementitious materials, indicating carbonation of the leachate. The leachability indexes in all cases exceeded the acceptable criteria (>6). The results of the present study suggest that OPC can be effectively used as a binding material to immobilize nuclides (Cs, Sr, Co, and Eu) contained in decommissioning wastes.