Importance of microbiology in the development of sustainable technologies for mineral processing and wastewater treatment

Mining companies have become increasingly aware of the potential of microbiological approaches for recovering base and precious metals from low-grade ores, and for remediating acidic, metal-rich wastewaters that drain from both operating and abandoned mine sites. Biological systems offer a number of environmental and (sometimes) economical advantages over conventional approaches, such as pyrometallurgy, though their application is not appropriate in every situation. There are two major areas in which biological systems are currently utilised in full-scale operations by the mining industries: metal extraction (“biomining”) and treatment of acid mine drainage (bioremediation). A third, much-researched area, biosorption of metals, has yet to become established as a proven biotechnology. Mineral processing using microorganisms has been exploited for extracting gold, copper, uranium and cobalt, and current developments are targeting other base metals. Engineering systems ranging from crude heap leaching systems to temperature-controlled bioreactors have been used, depending on the nature of the ore and the value of the metal product. Recently, there have been major advances on the microbiology these (and related) ecosystems, which will allow greater control (e.g. of redox potentials) of bioleaching operations, and thereby greater efficiencies and faster rates of metal extraction. Biological treatment of acidic mine effluents mostly involves the use of constructed wetlands. More recently, compost bioreactors have been shown to be effective in both generating alkalinity and removing heavy metals (e.g. as sulfides). Although these “passive” systems generally require little maintenance once constructed, their performance may be variable and they do not allow recovery and reuse of metals from the waste streams. In contrast, “active” bioremediation systems, based on microbial sulfidogenesis, offer both system control and separation (and recovery) of metals. This paper presents a concise review of current applications and developments in the field of biohydrometallurgy, and of how projected developments could allow future expansion of microbiological applications in the mining industries.