Bacteria may be the workhorses of the future, says S.Ananthanarayanan.
Micro-organisms drive the process of fixing nitrogen from the air, the earth’s largest chemical process, which enriches the soil and sustains all vegetation. And again, within the bodies of living things, and processes on the land and sea, and in laboratories and factories, micro-organisms perform myriad tasks. It is estimated that the work-force of bacteria on the earth numbers five million trillion trillion (or 5 x 1030). Could this resource be used to simplify or replace current, polluting, industrial processes?
A paper in the journal, Science Advances, by Louise Hase Gracioso, Janire Peña-Bahamonde, Bruno Karolski, Bruna Bacaro Borrego, Elen Aquino Perpetuo, Claudio Augusto Oller do Nascimento, Hiroki Hashiguchi, Maria Aparecida Juliano, Francisco C. Robles Hernandez, Debora Frigi Rodrigues, from the University of São Paulo, the Federal University of São Paulo, the University of Houston and JEOL Ltd, an enterprise in Tokyo, suggests that it can be done. They propose an application where bacteria are harnessed to undertake extraction of metallic copper from its compounds, an industrial process that consumes energy and uses toxic reagents.
Copper is a pivotal material of the modern world. As it is relatively easy to extract from the ore, copper has been used since prehistoric times. With the discovery of electricity, however, its consumption has skyrocketed, in power lines, generators, motors and transformers. And the future of copper mining? Although abundant in the crust of the earth, only a fraction is within reach, and copper that is mined would suffice only for 50-60 years. For this reason, a method that could tap the distributed sources of copper may be what we need, quite apart from current methods of extraction being energy intensive and polluting.
While it is true that copper exists in nature as a metal, most of it is in ores, in combination with other element, or mixed with impurities. The extraction from ore is a two-step process. The first is to grind, sift and roast the ore in hot air, to get rid of sulphur. It is then melted and treated with fluxes, to get it to a workable state. The final process is purification with the use of electricity. While all processes consume energy, the last, to attain high purity, is truly energy intensive.
The ingots of partly refined copper are immersed in an electrolyte, a conducting solution of copper sulphate, and a current is passed from the copper ingot to a rod of pure copper. The copper atoms in the ingot pass into the medium and collect at the opposite end. Other metals that are present do not participate and collect as a sludge, and the copper is purified.
Another method of extraction is to draw out the copper from the ore by bathing the ore in sulphuric acid. The copper dissolves into a copper sulphate solution, from which copper is recovered by as before. This method can be used with low quality ores, but the high cost of extraction by electricity remains.
Yet another method, one which does not consume energy, is to extract copper by biological means. As copper combines only feebly with other elements, milder heat, and action of microbes can liberate metallic copper. Microbes that seek oxygen in copper salts have hence evolved to be copper-tolerant, and much of natural, free copper may have come through this route. There are already places where bacteria are employed, along with chemicals and electrolysis, to improve the content of low-grade ore which is otherwise discarded as waste.
Bacterium in the mine
The work that the group writing in Science Advances has done is to identify a copper-resistant bacterium which is able to get at metallic copper from the ores, “naturally under aerobic conditions eliminating toxic solvents.” The group has used a sophisticated electron microscope technique and shown “that microbes in acid mine drainages can naturally extract metal ions, such as copper, and transform them into a valuable commodity”
The paper points out that copper finds applications in light sensitive, electro-chemical cells, sensors, solar cells, inks, varnishes, and, as copper is toxic to many microbes, in antimicrobial coatings. Copper, in the form of single atoms just released from copper compounds, even more than in the form of nanoparticles, is also effective for special applications, as in catalysts, where it forms short-lived combination with reagents, or to ‘dope’ materials as a trace impurity. Creating copper atoms in this form, however involves tedious processing and needs toxic chemicals. There are studies where it has been done with the help of bacteria or fungi, the paper says, but they need the support of additives.
In the current study, the authors examine a bacterium, which is found in copper mines, which is able to generate atomic copper, and without the use of toxic solvents or energy. Atomic copper that was isolated in copper mines in a region in Brazil suggested that micro-organisms could extract metal atoms from the environment, the paper says. And this implied also that microbes could clear sites that are contaminated by copper.
That the bacterium concerned could do this, the paper says, was first shown visually, by the change in the green colour of the bacteria growth medium, which contained copper sulphate. In 48 hours, the mixture changed from green to orange – indicating the presence of metallic copper. Sophisticated electron microscopy then showed that the orange mixture contained copper, and in the form of separate atoms.
In electron microscopy, what is shone through the sample is not light, but a beam of electrons. Just as light, in the case of interaction with a solar cell, for instance, behaves like a particle, or a ‘photon’, particles like electrons can exhibit wave behavior, and these waves are of extremely short wavelength. The result is that the electron microscope can make out details at extremely small dimensions – good enough to show that the particles in the sample have the dimensions expected of copper atoms.
The mechanics of how the bacteria went about this process was studied with the help of proteins that were expressed by the bacteria, when in the presence of copper sulphate and otherwise. While 562 proteins were there when there was no copper sulphate, 458 proteins were expressed when copper sulphate was there. Of these 458 proteins, 313 were also there among the 562, but the remaining 145 proteins were expressed only in the presence of copper sulphate. Analysis of these unique proteins then led to understanding of what brought about the extraction and stabilizing of the copper atoms.
That the bacteria from the copper mine are able to isolate copper atoms opens “up exciting strategies to produce in large-scale atomic copper via sustainable manufacturing and at low cost for existing and future applications,” the paper says.” And the beginning of a field of research into using microbes to extract other metals for applications in science, technology, engineering and medicine, the paper says.
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