Biooxidation of iron in elevated pressures and production of iron oxidizing biomass for a pilot-scale bioreactor
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Securing the future´s metal demand through traditional metal recovery methods is often economically not viable because of the low metal content of the readily available ores. Although biological metal recovery from low-grade ores can be potential alternative, the recently used approaches such as heap and tank bioleaching still require the extraction and crushing of the ores. Therefore, an environmentally friendly approach that would work with low-grade ores at the natural occurrence of metals known as deep in situ bioleaching is under investigation. Studying the pressure tolerance of a mixed acidophilic iron oxidizing microbial community (Leptospirillum ferriphilum and Sulfobacillus sp.) that could be used in deep in situ application was the main objective of this thesis. Furthermore, production of activated carbon-bound iron oxidizing biomass for pilot-sale demonstration of in situ bioleaching was also conducted. Experiments with a pressure reactor (1 L) showed pressure tolerance of the acidophilic culture at 40 bar (with initial 0.3 bar oxygen partial pressure (pO2), while the pressure was induced with N2 gas) above atmospheric pressure. The 10 bar/min pressure increase/decrease rate was not inhibitory to the iron oxidation activity of the microorganisms. When the elevated pressure was induced with technical air, the highest tolerated pressure where biotic iron oxidation still occurred was +3 bar (pO2=0.63 bar). From the elevated pressures tested, the highest biotic iron oxidation rate (0.78 g/L/d) was obtained at +3 bar, which was approximately half of the rate obtained at atmospheric pressure (1.7 g/L/d) in shake flask cultures. The abiotic iron oxidation rate linearly increased with the increase of oxygen partial pressure. During the biomass production for the pilot reactor, it was shown that the iron oxidation rate decreased as the reactor volume got larger. In order to reach iron oxidation efficiency of 90% took approximately 0.3, 3 and 4 days in the fluidized bed reactor (900 mL), shake flasks (100 mL) and semi-pilot reactor (~600 L), respectively. This work demonstrated that in situ iron oxidation by acidophilic microbial community of this study in culture suspension is possible up to +3 bar (pO2= 0.63 bar). Abiotic iron oxidation in deep subsurface is an option if oxygen can be provided there. To achieve the highest possible iron oxidation rate and maintain the microbial community structure, fully controlled environment (pH, temperature, pressure, mixing, aeration) and continuous operation are required.