Microbiological Mining
The central role of bacteria in the leaching of copper from low grade Ore long went unrecognized.
The minerals Industry now stand to gain from the application of novel methods of microbiological technology by- Corale L. Brierley
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 autetrophic, 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 (112 degrees Fahrenheit), and a few strains have been found at temperatures the boiling point of water.
For many years the only microorganisms thought to be important in the leaching of metals from ores was the rod shaped bacteriam Thiobacillus ferrooxidans :a microorganism was discovered in acidic water draining coal .
The microbiological 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, carboxyl, sulfhydryl (HS-) and hydroxvl (OH-) groups and are responsible for the adsorption of positively charged metal ions from solution. The adsorption is typically rapid, reversible and independent of temperature and energy metabolism. The common beer Yeast Saccharomyces cerevisiae and rhe fungus Rhizopus arrhizus have recently been shown to adsorb uranium from waste Water. For S. cereoisiae the acidity level at which the surface binding of uranium is optimal suggests that the positively charged uranium ions are attracted to the negatively charged ligands on the cell. Uranium concentrations representing between 10 and 15 percent of the dry weight of the cell have been recorded for this yeast. R,arrhizus adsorbs uranium to the extent that the metal amounts to 18.5 percent of the dry weight of the cell, This is more than twice the uptake of a commercially available ion-exchange resin.
The deposition of insoluble metals has been observed at the surface some microorganisms. The sheathed, filamentous bacteria of the Sphaerorilus Leptothrix group and the polymorphic Hyphomicrobium can become encrusted with oxides of manganese. Sphaerorilus Letrothrix and Gallionella are called iron bacteria because they deposit a sheath Iron their twisted stalks.
The most bizarre phenomenon of this kind is the reported intracelluar accumulation of very high concentrations of toxic metals. T he common soil and water bacterium Pseudomonas aeruginosa accumulates nearly 100 milligram of uranium per liter of solution in less than 10 seconds.
Other microbiological mechanisms with a potential for wastewater treatment eliminate metals by precipitating them in chelates, or cage like 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.
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. 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 algae Synechococcus a comparatively small cadmium-binding metallothionein can bind an average of 1.28 atoms of cadmium per molecule of protein.
EXTRA CELLULAR ACCUMULATION of uranium ions by cells of the common beer yeast Saccharomyces cereviciae is recorded-in an electron micrograph made by Gerald W. Strandberg of the Oak Ridge National Laboratory. The positively charged uranium ions in the test solution are attracted to negatively charged groups of atoms associated with the membrane and the wall of the y east cells, forming a lacer of needlelike uranium crystals around each cell. The electron micrograph magnifies the yeast cells by approximately 30.000 diameters.
INTRACELLULAR ACCUMULATI0N of uranium ions by the common soil and water bacterium Pseudonomas aeruginosa is another potentially attractive approach to the microbiological restoration of metal-contaminated waste water, According to Workers at Oak Ridge, the bacteria are capable of accumulating as much as 100 milligrams of uranium per liter of solution it, less than 10 seconds. lo the active cells the concentration of uranium can reach 56 % of the dry weight of the cell. The magnification is about 2O,000 diameters.
This is a partial extract from a magazine article I found years back written by Corale L Breirley