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The Rio Tinto mine, in the province of Huelva in the southwestern corner of Spain, has been a source of copper for some 2000 years. In 1752, engineers investigating the possibility of reopening the Roman workings noticed streams of blue-green liquid running from the mountains of excavated rock that surround the site. When this fluid ran across old iron implements or other metal it left a brown film. Scraping it off, they realised that it was pure copper.

At first, people thought that the copper and other metals were leached from the tailings dumps – the waste left over from ore-crushing – as a result of an inorganic chemical reaction, like those used to extract metals from ores. But in 1947, US microbiologists discovered that the transformation was in fact the work of a microorganism known as Thiobacillus ferrooxidans. These bacteria live by oxidising the sulphur that binds copper, zinc, lead and uranium into their respective sulphide minerals. This releases the valuable metal.

Humans can achieve the same result only by smelting ores at high temperatures, a far more polluting and energy-intensive approach. But as we learn more about these biological processes, it is becoming clear that they can be used to process ores – a technology that in the future could transform the metals industries and bring enormous environmental benefits.

Research laboratories around the world are witnessing a marriage between biotechnology and metallurgy, creating a new discipline known as biohydrometallurgy. This is not just a case of researchers trying to hitch a ride on the environmental bandwagon. The method was first used commercially in the late 1970s to extract the uranium left in old mines in Canada. At roughly the same time in South Africa it was applied in two gold mines.

By the mid-1980s, the copper industry in the US was on its last legs. High-grade ore bodies were becoming exhausted, so the industry was having to move, crush and smelt more and more lower-grade rock to end up with the same amount of copper. Coupled with this, the international price of copper had dropped from $1.45 per pound in 1980 to $0.58 in 1985. And by the mid-1980s, regulatory agencies were beginning to restrict emissions of sulphur dioxide, a major cause of acid rain, adding to the cost of metal production. To stay in business, firms began to look to technologies that were more cost-effective and less polluting; these proved to be biological and now 30 per cent of the copper produced in the US is extracted in this way.

Smelting copper by traditional methods had cost between $60 and $90 per pound. The introduction of biohydrometallurgy cut the cost to less than $30 per pound. Smelting one tonne of copper typically results in two tonnes of sulphur dioxide being pumped into the atmosphere. Biological extraction avoids this.

Biohydrometallurgy is straightforward when applied to copper production. First, the low-grade ore – and tailings left from any earlier conventional mining – is piled up in an area where the ground has been made impermeable. It is then sprayed with a leaching solution that contains iron, in the form of the Fe ³+ ion, sulphate ions (SO4 ²-) and T. ferrooxidans. These bacteria are ‘acidophilic autotrophs’, meaning that they are able to live solely on sulphides and thrive in an acidic environment. The sulphate ions in the leaching solution form sulphuric acid, giving a suitably acidic solution.

Copper is released when T. ferrooxidans catalyses chemical reactions that yield copper (as Cu ²+), iron (as Fe ²+) and sulphate. Because the piles sit on an impermeable base layer, it is easy to drain off the solution carrying the copper; the metal is then removed from it with another solvent. The remaining leaching solution flows into an open pond, where T. ferrooxidans catalyses a reaction that oxidises Fe ²+ ions to Fe ³+ ions and reduces the sulphide to sulphate. This recharges the leaching solution, which is pumped back to the top of the pile for the cycle to begin again.

The copper, meanwhile, is extracted as sheets through an ‘electrowinning’ process, in which electricity is passed through the copper solution. The metal collects on the negative terminals. This part of the process is still costly in energy, but research is under way to develop ‘bioabsorption filters’ such as algae, which could be used to make the process entirely biological.

Biohydrometallurgy may provide a method of underground mining, without the environmental damage associated with conventional techniques. There is now a mine in San Manuel in Arizona, consisting of five holes drilled into an ore deposit, which was fractured by detonating an explosive charge underground. Instead of standard mining practice, a mixture of acidic water and T. ferrooxidans is pumped down the central hole where the bacteria do their work. The resulting solution, rich in valuable copper, is pumped from the other four holes, processed and recycled.

Despite the potential of these methods, the mining industry is reluctant to use them. So far, they have been applied only as a last resort to recover low-grade metals from sites where traditional methods are not profitable. The problem lies in the slowness of the biological processes: T. ferrooxidans have not yet come to appreciate the importance of rate of return on capital. According to Keith Debus from the Centre for Interfacial Microbial Engineering at Montana State University, ‘conventional processes can recover most metal from an ore body in a matter of months or years, depending on the size of the deposit and the level of resources applied to production, but biological metal recovery may take decades,’ he says. ‘Where both techniques have been evaluated, biological approaches have often been found to be cheaper, but delay in cash flow from slower production has hindered their adoption.’

John Madgwick and his team at the School of Biotechnology at the University of New South Wales is trying to speed things up. He has been working on the extraction of manganese by so-called heterotrophic leaching, a process carried out under air-starved conditions similar to the anaerobic fermentation stage of brewing. By adding carbohydrate in the form of sugar to a solution containing an ore, Madgwick speeded up significantly the extraction of manganese. Heterotrophic organisms, unlike autotrophs, feed on carbohydrates produced by other plants and animals. In this process, according to Madgwick, ‘the mineral itself becomes a substitute for oxygen, acting as an electron acceptor at the end of an oxidation of sugar’. It is possible that genetically engineered micro-organisms could make such reactions run even faster.

A second feature of Madgwick’s research brings a bonus for biohydrometallurgy in general. As well as finding more efficient methods of extracting manganese from its ore, he and his team are also finding ways to remove manganese from the separated solution. This process, bioabsorption, uses bacteria and algae to oxidise dissolved manganese. As well as separating metals for commercial purposes, this method might also be used to filter low concentrations of polluting heavy metals from waterways.

Microorganisms are also involved in naturally occurring electrostatic processes that alter the concentration of minerals, with potentially profitable results for humanity. Many microorganisms have a cell envelope or sheath that can absorb a wide range of metals and other toxic materials, even when they are present at very low concentrations. These bacteria, which are common constituents of soil, may play an important role in the formation of some gold and silver deposits in sediments laid down by rivers – alluvial deposits. In the past year, Gunter Bischoff, Robert Coenraads and John Lusk of Macquarie University, near Sydney, have suggested that alluvial gold in Venezuela could have originated from bacterial action. They think that a negatively charged polymer on the outside of the bacteria attracts the positively charged gold particles in the soil so that they clump together as grains. In 1979, W. C. Ghiorse and P. Hirsch of the Idaho National Engineering Laboratory in Boise demonstrated that metals continue to accumulate on the negatively charged polymers even after the bac-teria have died; the process could continue indefinitely. There is even speculation that large gold nuggets could have formed in this way.

If biological processes are important in producing the millions of tonnes of alluvial gold, silver and other metals that generations of miners have panned from rivers and streams, it would make sense if mining engineers had some biological education. This is rarely the case and, according to Debus, the mining industry has a woeful record in research of any sort. ‘They spend less that 2 per cent of sales on research and development – compared with 3.5 per cent for US industry overall – and almost nothing on biological processes,’ he says.

Political pressure on the metal industries could force the pace of change. Widespread application of these new techniques may come from research carried out at the Idaho Laboratory, using microorganisms like T. ferrooxidans to remove the sulphur from coal. Extracting sulphur in this way could be a relatively cheap way of reducing the emis-sions of sulphur dioxide from coal-fired power stations. Applied in countries such as India and China, where industry is developing rapidly, this technique could bring enormous environmental benefits.

Microorganisms are already proving to be cost-effective as biological filters to remove heavy metals and other toxic material from polluted water. When the citizens of Arcata, on Humboldt Bay in northern California, were confronted with the need to treat the industrial waste water and sewage that has been pouring into the bay for decades, they decided against the obvious route – building a $30 million chemical treatment plant. Instead they chose to spend $5 million on creating 38 hectares of wetlands and marshes to do the job. Rushes such as cat’s-tails (Typha latifolia) collect microorganisms that metabolise heavy metals in the sediment around their roots. By selecting plant species which would absorb low-level pollutants, Arcata cleaned up its water enough to render fish from Humboldt Bay edible for the first time in decades. As a bonus, the wetland has turned into a major bird sanctuary and tourist attraction. And if the polluting heavy metals can be extracted and used again, dealing with waste water in this way could make a profit.

There are signs that such extraction could succeed. Madgwick, in conjunction with the giant mining company BHP, is considering the use of single-celled algae to target specific metals in waste water. Harvesting algae rich in zinc, mercury or manganese, for example, could make the process of cleaning up water cost-effective or even profitable. And if miners can extract metals from ores of lower grade than is possible with conventional techniques, they can transform old tailing sites from polluting nuisances into valuable sources of raw materials. But research on biological techniques of metal extraction remains rare. Debus sees stricter environmental regulation as the key to encouraging research. ‘If we were to have left environmental regulations in the US at what they were 50 years ago we would see little use for these biological techniques,’ he says. ‘In essence it is an economic issue: the industry has been spilling their waste on the rest of society which imposes a cost on us, and what’s happening is the cost is now being put back on them.’

As might be expected, the mining industry rejects this suggestion. It argues that it has always been pushing the research barriers aside in the search for new techniques; industry has funded much of the research in biohydrometallurgy, which is now beginning to pay back the investment. If society wants to make mining protect the environment, industry’s demand for profit will mean that we may have to accept higher prices. But increasing environmental concern, coupled with the cost-effective techniques of biohydrometallurgy, could change the face of the mining industry for good.

John Merson is a senior lecturer in the School of Science and Technology Studies at the University of New South Wales and a member of the Australian Broadcasting Corporation Science Unit.