The central role of bacteria
in the leaching of copper from low-grade ore long went
by Corale L. Brierley (Published in: Scientific American, 247, 42 - 50, 1982, here without accompanying figures)
|The recovery of copper from the
drainage water of mines was probably a widespread practice in the Mediterranean
basin as early as 1000 B.C. Although such mining operations are difficult
to document, it is known that the leaching of copper on a large scale was
well established at the Rio Tinto mines in Spain by the 18th century. What
none of the miners engaged in this traditional method of mineral extraction
realized until about 25 years ago is that bacteria take an active part
in the leaching process. They help to convert the copper into a water-soluble
form that can be carried off by the leach water Today bacteria are being
deliberately exploited to recover millions of pounds of copper from billions
of tons of low-grade ore. Copper obtained in this way accounts for more
than 10 percent of the total U.S. production. In recent years bacterial
leaching has also been applied to the recovery of another nonferrous metal:
Recent progress in the genetic manipulation of microorganisms for industrial purposes promises to revitalize not only the bacterial leaching of metal-bearing ores but also the microbiological treatment of metal-contaminated waste water. The enthusiasm of the microbiologists working on the development of the new biomining" techniques is matched by a need in the minerals industry to find alternatives to conventional methods of mining, ore processing and waste-water treatment. The need arises from recent trends in the industry: the continued depletion of high-grade mineral resources, the resulting tendency for mining to be extended deeper underground, the growing awareness of environmental problems associated with the smelting of sulfide minerals and the burning of sulfur-rich fossil fuels and the rising cost of the prodigious amounts of energy required in the conventional recovery methods. The current methods will surely prevail for many years to come, but biological processes are generally less energy-intensive and less polluting than most nonbiological ones, and so the role of biological technology in mining, ore processing and waste-water treatment is likely to become increasingly important.
To understand the applications of microbiology to the minerals industry several questions must be answered. What are the microorganisms that extract metals from rocks and from metal-bearing solutions, and where are they found? What biochemical functions do the microorganisms perform, and what do they need in the way of nutrients and environmental conditions to maintain their activity? What are the constraints on the commercial exploitation of such biological techniques? What impact will the new tools of genetic engineering have on the future of biomining? I shall address each of these questions in turn.
The bacteria involved in the leaching of metals from ores are among the most remarkable life forms known. The microorganisms are said to be chemolithotrophic ("rock-eating"); they obtain energy from the oxidation of inorganic substances. Many of them are also autotrophic, that is, they capture carbon for the synthesis of cellular components not from organic nutrients but from carbon dioxide in the atmosphere. The leaching bacteria live in environments that would be quite inhospitable to other organisms; for example, the concentration of sulfuric acid and of soluble metals is often very high. Some thermophilic, or heat-loving, species require temperatures above 50 degrees Celsius (122 degrees Fahrenheit), and a few strains have been found at temperatures near the boiling point of water.
For many years the only microorganism thought to be important in the leaching of metals from ores was the rod-shaped bacterium Thiobacillus ferrooxidans. This microorganism was discovered in the acidic water draining coal mines; it was not until 1957 that a correlation was recognized between the presence of the bacterium and the dissolution of metals in copper-leaching operations. Since then a great deal of information has been amassed on T. ferrooxidans and on its vital role in the leaching of metals.
T. ferrooxidans is acidophilic, or acid-loving; it tends to live in environments such as hot springs, volcanic fissures and sulfide ore deposits that have a high concentration of sulfuric acid. It is also moderately thermophilic, thriving in the temperature range between 20 and 35 degrees C. The bacterium gets energy for growth from the oxidation of either iron or sulfur. The iron must be in the ferrous, or bivalent, form (Fe++), and it is converted by the action of the bacterium into the ferric, or trivalent, form (Fe++). Several forms of sulfur can be attacked. They include both soluble and insoluble sulfides (compounds containing the bivalent sulfur ion S--), elemental sulfur and soluble compounds that incorporate either the thiosulfate ion (S203-- ) or the tetrathionate ion (S406--). In each case the product of the transformation is a substance in which the sulfur atom has fewer valence electrons, culminating in the formation of the sulfate ion (S04-- ). T. ferrooxidans obtains carbon autotrophically from atmospheric carbon dioxide.
Although T. ferrooxidans is essential to the bacterial leaching of metals, it is by no means the only microorganism with an important role in the process. Among the other microorganisms taking part is T. thioxidans, a rod-shaped bacterium not unlike T. ferrooxidans that grows on elemental sulfur and some soluble sulfur compounds. Studies by Donovan P. Kelly and his associates at the University of Warwick have confirmed the importance of mixed cultures of bacteria in the extraction of metals from ores. T. ferrooxidans and T. thiooxidans combined, for example, are more effective in leaching certain ores than either organism is alone. Similarly, the combination of Leptospirillium ferrooxidans and T. organoparus can degrade pyrite (FeS2) and chalcopyrite (CuFeS2), a feat neither species can accomplish alone.
In acidic environments supporting leaching bacteria one can often isolate a number of heterotrophic microorganisms: bacteria and fungi that scavenge the small amounts of organic matter present in these environments or that survive on the organic by-products of other organisms' autotrophic metabolism. The role of the heterotrophic microorganisms in the leaching process is largely undetermined. Thiobacilli that attack some sulfide minerals and certain soluble sulfur compounds under neutral conditions (that is, neither acidic nor alkaline) are often found in sulfide ore deposits and in other habitats where sulfur is available. Thiobacilli of this type may be responsible for the initial increase in acidity that establishes an environment conducive to the growth of the more acidophilic leaching bacteria.
At temperatures between 60 and 75 degrees C. and under neutral conditions the filamentous bacterium Thermothrix thiopara oxidizes sulfhydryl ions (HS-), sulfite ions (SO3-), thiosulfate ions and elemental sulfur to form sulfate ions. There is increasing evidence of the widespread existence of Thermothrix species and similar filamentous, sulfur-oxidizing bacteria in thermal springs and near volcanic fissures. Few leaching sites have been tested for the presence of these bacteria. Their existence in sulfur-bearing springs, however, suggests they could colonize sulfidic ores and thereby prepare such environments for the more acidophilic species.
Among the most interesting of the leaching microorganisms are the moderately thermophilic and acidophilic bacteria designated TH (for thermophilic and Thiobacillus-like). Studies by James A. Brierley of the New Mexico Institute of Mining and Technology and
Norman W. Le Roux of the Warren Spring Laboratory in Britain have established that strains of TH bacteria grow on ferrous iron or on minerals such as pyrite, chalcopyrite, covellite (CuS) and pentlandite [(Fe,Ni)9S8]. An organic supplement to the mineral diet is apparently required by these organisms for growth. Although the TH strains are similar in form to the rod-shaped thiobacilli, their distinctive temperature domain near 50 degrees C., their very different biochemical activities and their inability to metabolize carbon dioxide rule out a close relation with T. ferrooxidans. TH strains have been isolated from acidic hot springs and from leaching environments where copper sulfide is present. The role of the organisms in extractive processes has not been fully elucidated; from an industrial point of view, however, they must be considered potentially important to the development of any high-temperature biomining process.
The most robust of the leaching microorganisms are the extremely thermophilic and acidophilic species of the genus Sulfolobus. These bacteria flourish 'in acidic hot springs and volcanic fissures at temperatures that can exceed 60 degrees C. Some strains of Sulfolobus have been observed in springs at temperatures near the boiling point of water. The cell wall of the Sulfolobus bacteria has a different structure from that of most bacteria. Microorganisms of this type are thought to belong to the Archaebacteria, a group of unusual bacteria proposed as a separate kingdom of life forms [see "Archaebacteria," by Carl R. Woese; SCIENTIFIC AMERICAN, June, 1981].
Sulfolobus acidocaldarius and S. brierleyi oxidize sulfur and iron for energy, relying on either carbon dioxide or simple organic compounds for carbon. Ordinarily oxygen is required by Sulfolobus, - as in other aerobic organisms, the oxygen serves as the ultimate acceptor of the electrons removed in the process of chemical oxidation. Sulfolobus bacteria can also grow anaerobically, however. It has been demonstrated that molybdenum (Mo6+) and ferric iron can serve as electron acceptors in the absence of air. Minerals that resist most microorganisms, such as chalcopyrite and molybdenite (MoS2), are readily attacked by Sulfolobus, and the resulting soluble metals are not toxic to the organism. Molybdenum, which is extremely toxic even to the metal-tolerant thiobacilli, is readily endured by S. brierleyi in concentrations as high as 750 milligrams per liter. Sulfolobus has not been isolated from commercial leaching operations, but laboratory studies confirm the ability of the organism to proliferate in such environments. The potential of Sulfolobus species to leach metals from ores is only now being recognized: because of the extraordinary ability of these organisms to attack resistant mineral structures, however, they are certain to be among the leaching bacteria that will be successfully exploited in the future.
How exactly does the growth of the leaching bacteria result in the extraction of metals from rocks? By convention bacterial leaching has been divided into two approaches: direct and indirect. Direct bacterial leaching entails an enzymatic attack by the bacteria on components of the mineral that are susceptible to oxidation. In the process of obtaining energy from the inorganic material the bacteria cause electrons to be transferred from iron or sulfur to oxygen. In many cases the more oxidized product is more soluble. It should be noted that the inorganic ions never enter the bacterial cell; the electrons released by the oxidation reaction are transported through a protein system in the cell membrane and thence (in aerobic organisms) to oxygen atoms, forming water. The transferred electrons give up energy, which is coupled to the formation of adenosine triphosphate (ATP), the energy currency of the cell.
Indirect leaching, in contrast, does not proceed through a frontal attack, by the bacteria on the atomic structure of the mineral. Instead the bacteria generate ferric iron by oxidizing soluble, ferrous iron; ferric iron in turn is a powerful oxidizing agent that reacts with other metals, transforming them into the soluble, oxidized form in a sulfuric acid solution. In this reaction ferrous iron is again produced and is rapidly reoxidized by the bacteria. Indirect leaching is usually referred to as bacterially assisted leaching. In an acidic solution without the bacteria ferrous iron is stable, and leaching mediated by ferric iron would be slow. T ferrooxidans can accelerate such an oxidation reaction by a factor of more than a million.
Direct and indirect leaching by bacteria are difficult to differentiate quantitatively because most minerals include some iron. Even if leaching were to begin with the direct process exclusively, the iron would be released from the mineral and would establish an indirect leaching cycle. Direct leaching by thiobacilli has been demonstrated in the laboratory with iron-free synthetic metal sulfides.
In practice the leaching of metals is far more complex than the above analysis might suggest; there are numerous processes in addition to direct enzymatic oxidation and bacterial generation of ferric iron. Some chemical reactions between ferric iron and metal-sulfide minerals result in the formation of secondary minerals and elemental sulfur., which can "blind," or inactivate. the reactive surfaces. When sulfur is formed, T. thiooxidans plays an indispensable role in oxidizing the sulfur to sulfuric acid, thus exposing the metal for further leaching.
The control of acidity is of utmost importance in leaching, because an acidic environment must be maintained in order to keep ferric iron and other metals in solution. Acidity is controlled by the oxidation of iron, sulfur and metal sulfides, by the dissolution of carbonate ions and by the decomposition of ferric iron through reaction with water. The last reaction promotes leaching by generating hydrogen ions (which make the solution more acidic), but it may also be detrimental because precipitates of basic ferric sulfates may inactivate the surfaces of metal-sulfide minerals and in some cases may even prevent the flow of the leaching solution. The chemical and biological processes are part of a complex system whose functioning depends on elements of hydrology, geology, physics and engineering.
In commercial dump-leaching operations millions of tons of sulfidic over-burden and waste rock-, containing small but valuable quantities of copper and other metals, are transported by truck or
train from open-pit copper mines to the dump site. The dump is often built in a valley to take advantage of natural slopes for maintaining the stability of the pile and for facilitating the recovery of the applied solutions. Such a dump can be immense, with slopes up to 1,200 feet high holding four billion tons of material. To these impressive formations thousands of gallons of acidified water are applied by flooding or sprinkling the top surface. Sprinkling introduces air, a vital component of both the chemical and the biological oxidation reactions, into the solution.
The dumps are not inoculated with the leaching bacteria. The organisms are ubiquitous, and when conditions in the rock pile become suitable for their growth, they proliferate. Rock samples collected near the top of a leach pile typically harbor more than a million bacteria of the species T. ferrooxidans per gram; T thiooxidans bacteria are present in somewhat smaller numbers. The leach solution percolates through the leach dump, and the "pregnant," or metal-laden, solution is collected in catch basins or reservoirs at the foot of the dump. The copper is removed from solution either by a cementation reation, in which ferrous iron replaces the copper in solution, or by solvent extraction, in which the copper is concentrated by transferring it from the aqueous leaching solution to an organic solution. The "barren," or copper-free solution, is then recycled to the top of the dump.
In a large dump-leaching operation the ore is processed for many years to recover as much of the copper as possible. Because of the construction methods employed and the volume of the solid material treated, dump leaching is a crude operation. The placement of the dump in a natural valley can impede the flow of air to the interior of the pile. The large size of some of the rocks limits contact among the metal-sulfide minerals, the oxidizing solution and the bacteria. During the dumps construction large ore haulers compact the surface, creating impermeable zones in the pile. Gypsum (CaS04), ferric hydroxide [Fe(OH)3] and basic ferric sulfate precipitates also decrease permeability, further reducing the contact of the solution with the sulfide rocks.
Although studies of bacteria in the dump environment have barely begun. some factors that may adversely affect the populations are known. They include high metal concentrations, particularly of ions such as silver and mercury that are known to be toxic to the organisms, lack of air, and temperatures higher than those tolerated by the organisms. Because the limitations of dump leaching are more clearly defined today than they were in the past, considerable forethought now goes into the construction of the leaching piles. Special "finger" dumps allow greater air circulation, and haulage is controlled to minimize compaction. From a biological viewpoint. however, dump leaching remains an essentially uncontrolled process.
Extractive methods other than dump leaching offer somewhat more regulation of biological, chemical and engineering factors. Heap leaching. for example, is used to extract metals from sulfide and oxide minerals in ores of a somewhat higher grade than those subjected to dump leaching. In heap leaching the rocks are often crushed to avoid the solution-contact problems encountered when leaching large boulders, and the heaps are built up on impermeable pads to prevent loss of the solution into the underlying soil. Aeration systems have been installed to increase the flow of air in the piles.
In-place leaching is a promising technique for the recovery of metals from low-grade ores in inaccessible sites. This technology, which has minimal impact on the environment, is currently employed to extract residual minerals from abandoned mine workings and to recover uranium from low-grade deposits. To
leach metals from depleted mine workings the leaching solution is applied directly to the walls and the roof of an intact stope (an underground excavation from which ore has been removed) or to the rubble of fractured workings. In place leaching techniques have been successful in the recovery of both copper and uranium.
In the extraction of uranium the bacteria do not directly attack the uranium mineral; instead they generate ferric iron from pyrite and soluble ferrous iron. Ferric iron readily attacks minerals incorporating quadrivalent uranium (U4+), converting this ion into hexavalent uranium (U6+), which is soluble in dilute sulfuric acid.
The bacterially assisted leaching of uranium could in principle be applied not only to the recovery of residual metals but also to the in-place leaching of low-grade uranium ore bodies. The e are a number of low-grade deposits in the western and southwestern U.S., but they are low in pyritic material and include rocks that tend to neutralize acids. Such mineralogical conditions are not conducive to bacterial leaching, and an exclusively chemical method of leaching has been adopted instead. Wells are drilled into the isolated ore bodies at depths ranging from tens to hundreds of feet. A carbonate solution containing an oxidant is then injected into the mineralized zone, where the uranium is solubilized. The uranium-bearing solution is withdrawn from the formation through a precisely engineered pattern of recovery wells.
The same practices might be applied in formations suitable for in-place bacterial leaching. Further study is needed, however, to assess such factors as the effect of hydrostatic pressure on the leaching bacteria and the loss of permeability in the formation resulting from bacterial growth.
Vat leaching is employed mainly for the chemical extraction of copper from oxide ores. The technology is based on the controlled agitation of concentrated ore particles with precisely determined amounts of acid. Vat leaching is seldom used for sulfide ores owing to the need for an oxidant and to the long leaching times; these requirements, however, do not preclude the vat leaching of sulfide ores under certain circumstances. Studies by A. Bruynesteyn of British Columbia Research and by Arpad Torma of the New Mexico Institute of Mining and Technology indicate that the bacterial leaching of sulfidic concentrates can be competitive with existing methods of extraction.
Although bacterial leaching is currently exploited only for the recovery of copper and uranium, the appetite of the leaching bacteria is fairly nonspecific. The organisms readily degrade other sulfide minerals, yielding zinc from sphalerite (ZnS) and lead from galena (PbS). The leaching bacteria can therefore be considered for the extraction of many other metals. The bacteria readily catalyze the dissolution of inorganic sulfur from coal, and recent advances indicate that organic sulfur may also be vulnerable to microbiological attack.
Progress in this area suggests that the precombustion desulfurization of sulfur-laden coal is on the horizon.
Given the seemingly limitless capability of bacterial leaching for the recovery of metals, why has such a potentially powerful tool not been more widely exploited? Probably the main reason is that the technology was not much needed until now, when energy is costly and the supply of accessible, high-grade metal ores is limited. Now that a genuine need exists for biomining, the technology lacks refinement. The study of bacterial leaching has largely been in the hands of a few investigators who have been systematically examining the organisms and the reactions they carry out. There has been no coordinated engineering effort to harness the organisms and their beneficial reactions in controlled fermentation tanks. To do so will require a carefully directed effort and large sums of money. The effort will probably be financed by private industry. The difficulties in development are likely to include those that can befall any engineering project as well as a number of other problems heretofore unknown. Unlike the carefully nurtured microorganisms that yield chemical and health-care products, the leaching bacteria will be subject to adversities such as severe weather, erratic fluctuations in acidity, a continual onslaught of competitive "wild type" bacteria and variations in the concentration and type of the mineral feedstock.
It seems probable that at some time in the future genetic manipulation may produce leaching microorganisms with highly desirable characteristics. Two of the qualities sought are an enhanced tolerance of toxic metals and a greater ability to generate the oxidant, ferric iron. The genetic modification of the organisms may prove difficult, however, because little is known about the genetics of the leaching bacteria or the exact mechanisms they rely on for the degradation of minerals. The latter problem is particularly troublesome, since it is uncertain which bacterial characteristics should be genetically altered to enhance the organisms' leaching capabilities.
Increasing the leaching rate of the bacteria is certainly one of the most sought-after improvements. The major drawback of bacterial leaching has been the slowness of the biological process compared with some chemical methods of extraction. The disadvantage is most striking when the microbiological leaching
rate is compared with some high-temperature, high-pressure extraction processes in which strong oxidants a on finely ground particles. Valid comparisons between biological and chemical processes, however, must be based on economic factors.
Bacterial leaching is likely to find greatest application in the controlled treatment of vast quantities of solid materials, such as discarded waste rock, overburden and tailings, in which small quantities of valuable metals are widely dispersed. For example, the U.S. has vast sulfide deposits, including more than seven billion tons of material with an average nickel content of .2 percent. At current prices the nickel is worth approximately $60 billion. The nickel is not mined because of inefficiencies in current mining and extractive technology and because of possible detrimental effects on the environment. Advances in biomining technology may make it possible to recover not only some of the nickel but also some of the 400 million pounds of cobalt (worth $10 billion at current prices) found in the same sulfidic rnaterial. In this situation the emphasis is not so much on rapid reaction rates as it is on lower capital investment, greater recovery of metal and reduced environmental damage.
In spite of the many uncertainties associated with the genetic engineering of leaching bacieria and the scale-up of microbiological leaching processes, there is little doubt that such technology will be developed. Interest in industrial microbiology, a need to develop more cost-effective methods for the recovery of metals from low-grade mineral deposits and the desire to utilize process that are environmentally benign all favor this development.
The controlled use of microorganisms for the extraction of metals from ores and from solid waste materials paralleled by the application of biological technology to the restoration of particulate-laden and metal-contaminate industrial waste water. Such technology not only would aid in clarifying the waste water but also would allow the recovery of valuable metals. Now under investigation are bacteria, algae an fungi that readily accumulate inorganic ions present in dilute concentration in waste streams. The microbiologic processes for the removal of metals from solution can be divided into three categories: the adsorption of metal ions onto the surface of a microorganism the intracellular uptake of metals and the chemical transformation of metals by biological agents.
Most microorganisms have a negative
electric charge owing to the presence of negatively charged groups of atoms
on the cell membrane and the cell wall. The charged groups, or ligands,
include phosphoryl(PO4---),carboxyl(COO ) sulfhydryl
(HS ) and hydroxyl (OH-) groups and are responsible for the
adsorption of positively charged metal ions from solution. The adsorption
The deposition of insoluble metals has been observed at the surface of some microorganisms. The sheathed, filamentous bacteria of the Sphaerotilus- Leptothrix group and the polymorphic Hyphomicrobium can become encrusted with oxides of manganese. Sphaerotilus-Leptothrix and Gallionella are called iron bacteria because they deposit a sheath of iron on their twisted stalks.
Microorganisms ordinarily take up some ions that are necessary for cellular activity. The transport systems for the ions are dependent on both temperature and energy. Examples of trace substances that are readily transported into the cells of microorganisms are magnesium (Mg++), calcium (Ca++), potassium (K+), sodium (Na+) and sulfate (SO4--). Although the mechanisms by which the cells assimilate the ions are highly selective, substitutions are possible. For example, the negatively charged chromate (Cr04--), selenate (Se04--), vanadate (VO4--), tungstate (WO4-) and molybdate (Mo04 --) ions can be transported into the cells of some microorganisms by the system that ordinarily carries sulfate ions.
The most bizarre phenomenon of this Kind is the reported intracellular accumulation of very high concentrations of toxic metals. The common soil and water bacterium Pseudomonas aeruginosa accumulates nearly 100 milligrams of uranium per liter of solution in less than 10 seconds. With the aid of an electron microscope Starling E. Shumate II and Gerald W. Strandberg of the Oak Ridge National Laboratory found that only 44 percent of the cells accumulated the uranium. This observation implies that in the active cells uranium accounts for as much as 56 percent of the dry weight of the cell. The mechanism that drives the cells to virtually commit suicide by the accumulation of toxic metals is not known; nevertheless, the phenomenon may someday be exploited for the restoration of metal-contaminated waste water.
Other microbiological mechanisms with a potential for waste-water treatment eliminate metals by precipitating them in chelates, or cagelike compounds, and by incorporating them into volatile compounds that can later be evaporated. Many microorganisms synthesize specific chelation compounds that immobilize heavy metals. More commonly in natural systems, however, metals adsorbed or absorbed by plants and microorganisms become sequestered in sediments following the death of the organisms.
Precipitation results when hydrogen sulfide (H2S), which is generated by sulfate-reducing bacteria, combines with a metal to form an insoluble metal sulfide. The exploitation of microorganisms such as Desulfovibro for the generation of hydrogen sulfide has considerable potential for application in the treatment of metal-contaminated effluents. The generation of ferric iron by T. ferrooxidans serves to remove soluble iron from acidic mine wastes. The organisms, attached to plastic disks, are immersed in acidic waters containing ferrous iron. Through oxidation the bacteria convert the iron into the less soluble ferric form. This pretreatment is expected to reduce the cost of neutralizing the acids in the drainage water.
The methylation of metals (that is, the substitution of a metal atom for the hydrogen atom of the hydroxyl group of a methyl alcohol molecule) can result in the metal's becoming volatile. Metals and metalloids known to be subject to biomethylation include mercury, selenium, tellurium, arsenic, tin, lead and cadmium. It has been predicted that platinum. palladium, gold and thallium can also be transformed in this way. The commercial applicability of biomethylation is questionable, however, owing to the difficulty of trapping the volatile compounds and the extreme toxicity of some of them.
The microbiological processes mentioned above for the recovery of metals and metalloids from solution have been observed in the laboratory and in natural environments where conditions are suitable for specific types of biological activity. An example is Schist Lake in Manitoba, which receives metal-contaminated tailings from a mining and smelting operation and sewage effluent from a small town. Nutrients in the sewage promote algal blooms, and the algae in turn accumulate metals. The decay of the metal-laden algae is mediated by microorganisms that generate hydrogen sulfide, which precipitates the metals as sulfides.
Observations of such natural cleansing systems have encouraged workers in the mining industry to try to imitate them. An artificial meandering stream draining a tailing pond in the New Lead Belt of Missouri is inhabited by the algae Spirogyra, Rhizoclonium, Hydrodictyon and Cladophora. The microorganisms are credited with the removal of nutrients and soluble heavy metals and the entrapment of suspended mineral particulates, A sedimentation pond and a baffled outlet prevent the algae from escaping into the receiving stream. At a uranium mine in the Grants Uranium District of New Mexico the mine water is treated by means of an interconnected system of impoundments in which the algae Spirogyra, Oscillatoria, Rhizoclonium and Chara accumulate soluble ions of molybdenum, selenium, uranium and radium. The sediments of the impoundments are enriched with the metals and metalloids, suggesting an ion-removal scheme analogous to the natural process observed at Schist Lake. Large populations of sulfate-reducing bacteria in the sediments indicate that hydrogen sulfide may have a role in the sedimentation of the ions. Uranyl carbonates [U02(CO3)-- and U02(CO3)34-] probably interact with Chara, which is encrusted with crystalline hydrous carbonates, to form the mineral liebigite [Ca2U02(CO3)3 × 10H20].
The microbiological processes currently exploited by the minerals industry are fairly simple in engineering design, and their effectiveness is sensitive to seasonal changes and sudden alterations in the chemistry of the system. Recent developments in the study of the uptake of metals by S. cerevisiae, R. arrhizus and P. aeruginosa make it probable that these microorganisms can be utilized in precisely engineered processes for the recovery of metals from waste-water streams. The new tools of genetic engineering may well lead to the creation of modified organisms with, greater effectiveness in metal removal. It was noted above that only 44 percent of the P. aeruginosa cells take part in the uranium-uptake process, Shumate and Strandberg speculate that if the factor or factors controlling the uptake can be identified, the bacteria may be genetically altered to increase the population that accumulates the metal.
The accumulation of metals by microorganisms, whether the process is intracellular uptake or surface accumulation, is fairly nonspecific: negatively charged groups of atoms on the surface of microorganisms attract any positively charged ions in the solution. Many organisms have cellular components that are highly metal-specific. One of the best-understood metal-binding agents is the protein metallothionein. Structural studies of metallothionein indicate there is a high concentration of sulfur-containing amino acid units, which, when they are brought into juxtaposition by the folding of the protein chain, form a sulfhydryl (HS-) chelation site. In the marine blue-green alga Synechococcus a comparatively small cadmium-binding metallothionein can bind an average of 1.28 atoms of cadmium per molecule of protein. The identification of the gene or the genes that specify the structure of the metallothionein of this organism or any other may enable geneticists to isolate and clone the genes in selected microorganisms. Cells carrying the cloned genes could be directed to synthesize massive quantities of metallothionein with a specific metal-binding capacity. The small protein could be immobilized on an inert carrier and waste water contaminated with metals could be passed over the fixed protein. Further studies of metallothioneins with the ability to bind specific metals may provide clues for the laboratory synthesis of simple compounds with an increased metal-binding capacity. Based on nature's own workings, on controlled laboratory studies done by many workers and on current applications in the field, it is clear that microorganisms and their versatile activities will help man to lay claim to mineral wealth buried deep in the ground or available in amounts not economically feasible to recover at present. These small servants of man promise to help in cleaning the air and water while retrieving valuable metal resources.