9.8 Microbes and the geological environment
Microbes are increasingly recognised as important catalytic agents in certain geological processes, e.g. mineral formation, mineral degradation, sedimenta-tion, weathering and geochemical cycling.
One of the most detrimental examples of microbal involvement with min-erals occurs in the production of acid mine waters. This occurs from microbial pyrite oxidation when bituminous coal seams are exposed to air and moisture during mining. In many mining communities, the huge volumes of sulphuric acid produced in this way have created pollution on an unprecedented scale.
Other examples of the detrimental effects of microbes include the microbial weathering of building stone such as limestone, leading to defacement or structural changes.
In contrast to these harmful effects, microbes are increasingly used benefi-cially to extract commerbenefi-cially important elements by solubilisation (bioleach-ing). For example, metals like cobalt, copper, zinc, lead or uranium can be more easily separated from low-grade ores using microbial agents – mining with microbes.
The biological reactions in extractive metal leaching are usually concerned with the oxidation of mineral sulphides. Many bacteria, fungi, yeasts, algae and even protozoa are able to carry out these specific reactions. Many minerals exist in close association with other substances, e.g. sulphur, and iron sulphide, which must be oxidised to free the valuable metal. A widely used bacterium Thiobacillus ferrooxidans can oxidise both sulphur and iron, the sulphur in the ore wastes being converted by the bacteria to sulphuric acid. Simultaneously, the oxidation of iron sulphide to iron sulphate is enhanced.
The commercial process involves the repeated washing of crushed ore (normally in large heaps, Fig. 9.8) with a bioleaching solution containing live microorganisms and some essential nutrients (phosphate/ammonia) to encourage their growth. The leach liquor collected from the heaps contains the essential metal, which can easily be separated (downstream processing) by sulphuric acid extraction.
In the USA, almost 10% of total copper production is obtained by this method. Countries such as India, Canada, the USA, Chile and Peru are rou-tinely extracting copper at a worldwide annual rate of 300 000 tonnes using microbes; with low-grade ores, bioleaching costs a half to a third as much as direct smelting.
Large-scale bioleaching of uranium ores is widely practised in Canada, India, the USA and the USSR. By means of bacterial leaching it is possible to recover uranium from low-grade ore (0.01–0.5% U3O8), which would
Leach liquor ponds
Asphalt pad Make-up
liquor
Recycle of leach liquor
Valve
Pump
Low grade ore heap
Liquor collection pond
Leach liquor dikes
Fig. 9.8 The principle of ‘mining with microbes’.
be uneconomic by any other known process. The USA alone extracts 4000 tonnes of uranium per year in this manner. Uranium is primarily used as a fuel in nuclear power generation, and microbial recovery of uranium from otherwise useless low-grade ores can be considered as an important contribu-tion to energy produccontribu-tion (Table 9.4). Bioleaching of uranium ores is seen to have an important contribution to the economics of nuclear power stations by providing also a means of recovery of uranium from low-grade nuclear wastes.
Continuous processes have been developed, and the control of the essential bacterial populations is easily achieved because of the acidity and limited substrate availability. Leaching technology will continue to offer more efficient and cheaper ways of extracting the increasingly scarce metals necessary for modern industry. The principal disadvantage of bioleaching is the relative slowness of the process.
Another important potential application for bacterial bioleaching is the removal of the sulphur-containing pyrite from high-sulphur coal. Little use is now made of high-sulphur coal because of the sulphur dioxide pollution that occurs with burning. However, as more and more reserves of coal are brought into use, high-sulphur coals cannot be overlooked. Thus the bacterial removal of pyrite (which contains most of the sulphur) from high-sulphur coal could well have huge economic and environmental significance.
9.8 Microbes and the geological environment 177
Table 9.4. Chemical reactions associated with microbial bioleaching of low-grade uranium ores
Indirect oxidation of uranium ore with ferric ion catalysed by Thiobacillus ferrooxidans
UO22−tetravalent uranium, insoluble oxide
UO2SO24−hexavalent uranium (uranyl ion, UO22−), soluble sulphate UO2+ 2Fe3++ SO2−4 → UO2SO4+ 2Fe2+
(U4++ 2Fe3+→ U6++ 2Fe2+)
Thiobacilllus ferrooxidans re-oxidises the Fe2+.
Aliphatic hydrocarbon-utilising bacteria are also being used for prospecting for petroleum deposits. Microbes will soon be commercially used to release petroleum products from oil shelf and tar sands. In all these systems there is rarely any formalised containment vessel or bioreactor. Instead, the natural geological site becomes the bioreactor, allowing water and microorganisms to flow over the ore and to be collected after natural seepage and outflow.
Recycling by mechanical pumping can also be used.
Microorganisms can also be used as metal (bio) accumulators from dilute solutions. The microorgansms, bacteria, yeasts and moulds can actively uptake the metals in various ways, and such processes have a potential use in extracting rare metals from dilute solution, but it is still to be seen whether it will become an important technology.
In a similar way, microorganisms are being used to extract toxic metals from industrial effluents and reduce subsequent environmental poisoning.
Some plants have been shown to accumulate heavy metals such as nickel, cobalt, cadmium, nickel and even gold, and studies are now being carried out to assess whether such plants could be used to extract metal from soils or ores that are subeconomic for conventional mining. This area of study is called ‘phytomining’ and will depend on the use of hyperaccumulating plants.
It is envisaged that hyperaccumulating plants would be harvested from soil containing metal, the plant material burnt to give a small volume of plant ash (bio-ore) containing high concentrations of the target metal, and the final bio-ore smelted to yield metal. Such processes are not yet commercially viable.
Phytomining could well appeal to conservation movements as an alternative to opencast mining of low-grade ores.
In all these activities, multidisciplinary approaches are necessary, and new biotechnological techniques, such as designing an organism for a specific function, could yield further benefits. The overall picture of this area of
biotechnology is one of rapid and exciting development. There is a grow-ing awareness of the value of an unpolluted environment.
9.9 Environmental sustainability and clean technology