Mining with bacteria
Professor Scott Dunbar and colleagues have created bacteriophages – viruses that infect bacteria – that adhere to, and segregate, the minerals.
Biologists screened a library of one billion heptapeptides (organic compounds consisting of various numbers of amino acids) to find phages with a peptide sequence that binds specifically to the zinc and copper minerals sphalerite and chalcopyrite. This was achieved by exposing the library to mineral particles and washing it with water, which carried away the phages that did not bind. The adhered heptapeptides were then mixed with the E. coli bacteria to replicate them quickly, resulting in higher concentrations of binding sequence peptides. After four rounds of this, a number of phages with the desired binding properties were created.
When applied to a slurry of metals, the phages not only stick to the desired mineral, but also cause it to agglomerate. ‘We are not sure why this happens, but it is likely that the phage particles are binding to each other as they are attached to the mineral particles,’ explains Dunbar.
A method of collecting the agglomerated minerals has yet to be devised, although he believes an upscaled technique could be created. ‘The phages appear to impart an electrical charge to the mineral particles, so an electric field could be used to attract the mineral particles, with the phage attaching to the electrodes.’
Such a system would not require costly chemicals and could be more detailed in its separation than traditional froth flotation, claims Dunbar.
Traditional detergents used in flotation to separate minerals ‘can be made specific to the mineral of interest, but not so in the case of sphalerite and galena [a lead mineral that often occurs with sphalerite],’ says Dunbar. Other methods, in addition to flotation, are usually required to separate galena from sphalerite, but Dunbar says his bacteriophages could draw them apart in one step.
Professor Jan Cilliers, Chair in Mineral Processing at Imperial College London, UK, says mining with bacteria has been carried out before, although this technique appears to be more technically involved. He believes the specifying characteristic of the bacteriophages is interesting, ‘but it is not going to replace anything we currently use for a while,’ as it will take several years for an industrial system to be created.
Dunbar is working on understanding the peptide’s binding mechanism, as this could lead to synthetic chemical binding tools that are easier to work with. He is also exploring how different protein coatings on the peptide may create a stronger bond.
Materials World Magazine, 01 Sep 2009