Reductive dissolution of minerals and selective recovery of metals using acidophilic iron- and sulfate-reducing acidophiles

Abstract

Most microbiological applications in biohydrometallurgy use the abilities of some acidophilic bacteria and archaea to catalyze oxidative transformations of metals (e.g. iron) and non-metals (e.g. sulfur), and thereby either to facilitate metal extraction and recovery (bioleaching and bio-oxidation), or to immobilize metals and metalloids (iron, arsenic etc.) in bioremediation of mine wastes. Many acidophiles, including species more well known as iron- and sulfur-oxidizers, can also catalyze reductive transformations of these elements in anoxic or micro-aerobic environments, though the biotechnological potential of the latter has, for the most part, been ignored. Three potential applications of iron- and sulfate-reduction mediated by acidophilic bacteria are described in this review. The first of these uses the chemolithotroph Acidithiobacillus ferrooxidans to accelerate the dissolution of ferric iron oxy-hydroxides by coupling the oxidation of elemental sulfur to iron reduction when, grown under anaerobic conditions. This is the key reaction in the “Ferredox” process for extracting nickel from lateritic ores. Secondly, a continuous flow ferrous iron-generating bioreactor is described in which the heterotrophic acidophile Acidiphilium SJH, immobilized on porous beads, is used to couple the oxidation of glycerol to the reduction of soluble ferric iron in feed liquors. Iron-reducing bioreactors have potential both in mineral bio-processing (e.g. indirect leaching of oxidized ores) and mine water remediation. A laboratory-scale (2L) reactor was demonstrated to reduce between 90 and 99.9% of ferric iron at dilution rates of up to 0.87h−1, with ~1g of ferric iron being reduced h−1 at the highest flow rates. The heterotrophic iron-reducing system also has the advantage of not requiring anoxic conditions to operate efficiently. Thirdly, selective precipitation of copper and zinc from synthetic mine waters containing a variety of soluble metals has been demonstrated in anaerobic bioreactors, operated at pH 2.2–3.8, using novel consortia of acidophilic sulfate-reducing bacteria. Environmentally-benign technologies for remediating acid mine and process waters, using controlled biosulfidogenesis in acidic liquors to ameliorate pH and to recover dissolved metals, have many advantages over conventional remediation strategies.