As you probably know, mining for precious metals of various kinds (gold, silver, copper and so forth) is a messy job. when mining metals, the desired metal is not generally found in nice, big, pure chunks. The largest gold nugget ever found was reputed to weigh about 70 Kilograms. But most gold, as with all other precious metals, is generally hard to see with the naked eye, mixed in the ground with dirt, rocks, and other ground debris—the ore from which the gold must be extracted (note that the ore in a good copper mine, for instance, will typically consist of less than 1% copper). The extraction process, when done with chemicals, is called “leaching.” For years, the leaching of copper, for example, was done with acid which is not very good for the environment. In fact, most leaching technologies have resulted in toxic leftovers.
Well, guess what? Today approximately 10 to 20 percent of copper mined in the United States is extracted from low grade ore with the assistance of biofilms. And mining companies are making a considerable investment to extend this process to the extraction of other precious metals.
How is a biofilm engineered to accomplish this job? Again, one must find a bacteria with a particular appetite—one that would eat the ore, say, that encased copper particles, thus releasing the copper to be recovered. This idea has led to the most common biofilm supported leaching process, called “heap leaching.” Low grade ore is placed in a “heap,” and sprayed with a mildly acidified water solution that encourages the growth of a particular bacterium that oxidizes the ore, releasing water soluble cupric ion (copper) that can then be recovered from the water.
Biomining, for example of chalcopyrite for copper recovery, is a more sustainable biotechnological process that exploits the capacity of acidophilic microbes to catalyse solid metal sulphide dissolution to soluble metal sulphates. A key early stage in biomining is cell attachment and biofilm formation on the mineral surface that results in elevated mineral oxidation rates. Industrial biomining of chalcopyrite is typically carried out in large scale heaps that suffer from the downsides of slow and poor metal recoveries.
Further reading on biofilms and metals mining
Heap leaching guide. Science Direct, 2011. https://www.sciencedirect.com/topics/materials-science/heap-leaching.
Buetti-Dinh, A., Herold, M., Christel, S. et al. Systems biology of acidophile biofilms for efficient metal extraction. Sci Data 7,215 (2020). https://doi.org/10.1038/s41597-020-0519-2.
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