Bioextraction (reductive dissolution) of iron from low-grade iron ores. Fundamental and applied studies.

Results of the present study indicate that S. putrefaciens 200 may be a suitable Fe(3+)-reducing microorganism for commercial application in a microbially catalyzed iron ore bioextraction (reductive dissolution) process. The proposed scheme of the bioextraction process (Fig. 1) entails the addition of a suitable iron ore to anaerobic, batch cultures of aerobically grown S. putrefaciens 200, with subsequent recovery of Fe2+ in the product stream. Although batch growth under low oxygen tension is known to induce expression of the high-rate Fe3+ reduction system in S. putrefaciens, such growth conditions do not appreciably enhance the rate at which S. putrefaciens catalyzes the reductive dissolution of iron from low-grade iron ore. As a result, strict monitoring of dissolved oxygen levels during batch growth is not required. Highly aerobic growth conditions are most desirable because such conditions maximize microbial growth rates. Commercial application of the proposed process is made more attractive by the ability to grow S. putrefaciens aerobically on a relatively inexpensive organic substrate (filter-sterilized, primary effluent wastewater) as sole carbon and energy source. Physical and chemical factors that accelerate overall reductive dissolution rates include (i) pulverization of the iron ores before their addition to the anaerobic, batch cultures, and (ii) subsequent addition of an Fe(III)-chelating agent to the anaerobic iron ore-microorganism slurry. Recycle of residual ore remaining in the initial reactor vessel after a one-hour incubation is recommended, since overall reductive dissolution rates decrease dramatically after that time. Significant enhancement of the overall reductive dissolution rates may reside in the ability to genetically engineer a more robust Fe(3+)-reducing microorganism. Preliminary genetic studies presented here indicate that S. putrefaciens is a suitable model microorganism for studying the molecular basis of microbial Fe3+ reduction. Mutagenesis experiments demonstrated that the Fe3+ reduction system of S. putrefaciens is physiologically uncoupled from other electron-accepting processes carried out by this bacterium, and that a distinct ferrireductase enzyme is expressed after growth under either highly aerobic or microaerobic conditions. An array of S. putrefaciens mutants (Class I), deficient only in their ability to grow anaerobically on Fe3+ as sole terminal electron acceptor, were isolated and a single mutant selected for subsequent gene cloning (complementation) experiments. Restriction enzyme analysis of putative, complemented clones (i.e., transconjugates in which the ability to grow anaerobically on Fe3+ had been restored) revealed the presence of a common cloned DNA insert.(ABSTRACT TRUNCATED AT 400 WORDS)