The metals can be recovered by the microorganisms by two processes: (1) Bioleaching and (2) Bio-Sorption!
Soil microorganisms are very closely involved as catalytic agents in many geological processes. These include mineral formation, mineral degradation, sedimentation and geochemical cycling. In recent years, a new discipline of mineral science namely bio-hydrometallurgy or microbial mining (mining with microbes) is rapidly growing. Broadly speaking, bio-hydrometallurgy deals with the application of biotechnology in mining industry. In fact, microorganisms can be successfully used for the extraction of metals (e.g., copper, zinc, cobalt, lead, uranium) from low grade ores. Mining with microbes is both economical and environmental friendly.
The term metal is used to any substance that is hard, possessing silvery lusture, and is a good conductor of heat and electricity. Some of the metals, however, are relatively soft, malleable and ductile e.g. sulfur. An ore is a naturally occurring solid mineral aggregate from which one or more minerals can be recovered by processing. Majority of microorganisms can interact with metals.
The metals can be recovered by the microorganisms by two processes.
1. Bioleaching or microbial leaching:
This broadly involves the extraction or solubilization of minerals from the ores by the microorganisms.
2. Bio-sorption:
It deals with the microbial cell surface adsorption of metals from the mine wastes or dilute mixtures.
Method # 1. Bioleaching:
In microbial leaching (bioleaching), metals can be extracted from large quantities of low grade ores. Although recovery of metals (e.g. copper) from the drainage water of mines has been known for centuries, the involvement of microbes in this process was recognized about 40 years ago.
The bacteria which are naturally associated with the rocks can lead to bioleaching by one of the following ways.
1. Direct action of bacteria on the ore to extract metal.
2. Bacteria produce certain substances such as sulfuric acid and ferric iron which extract the metal (indirect action).
In practice, both the methods may work together for efficient recovery of metals.
Organisms for bioleaching:
The most commonly used microorganisms for bioleaching are Thiobacillus ferrooxidans and Thiobacillus thiooxidans. Thiobacillus ferrooxidans is a rod-shaped, motile, non-spore forming, Gram-negative bacterium. It derives energy for growth from the oxidation of iron or sulfur. This bacterium is capable of oxidising ferrous iron (Fe2+) to ferric form (Fe3+), and converting sulfur (soluble or insoluble sulfides, thiosulfate, elemental sulfur) to sulfate (SO2-4). Thiobacillus thiooxidans is comparable with T. ferrooxidams, and grows mostly on sulfur compounds.
Several studies indicate that the two bacteria T. ferrooxidans and T. thiooxidans, when put together, work synergistically and improve the extraction of metals from the ores. Besides the above two bacteria, there are other microorganisms involved in the process of bioleaching. A selected few of them are briefly described below.
Sulfolobus acidocaldarius and S. brierlevi are thermophilic and acidophilic bacteria which can grow in acidic hot springs (>60°C). These bacteria can be used to extract copper and molybdenum respectively from chalcopyrite (CuFeS2) and molybdenite (MoS2).
A combination of two bacteria Leptospirillum ferrooxidans and Thiobacillus organoparpus can effectively degrade pyrite (FeS2) and chalcopyrite (CuFeS2). The individual organisms alone are of no use in extracting metals.
Pseudomonas aeruginosa can be employed in mining low grade uranium (0.02%) ore. This organism has been shown to accumulate about 100 mg uranium per one liter solution in less than ten seconds. Another organism, Rhizopus arrhizus is also effective for extracting uranium from waste water.
Certain fungi have also found use in bioleaching. Thus, Aspergillus niger can extract copper and nickel while Aspergillus oryzae is used for extracting gold. Among the various microorganisms, T. ferrooxidans and T. thiooxidans are the most widely used in bioleaching. The utilization of many of the other organisms is still at the experimental stage.
Mechanism of bioleaching:
The mechanism of bioleaching is rather complex and not well understood. The chemical transformation of metals by microorganisms may occur by direct or indirect bioleaching.
Direct bioleaching:
In this process, there is a direct enzymatic attack on the minerals (which are susceptible to oxidation) by the microorganisms. For instance, certain bacteria (e.g., T. ferrooxidans) can transfer electrons (coupled with ATP production) from iron or sulfur to oxygen. That is these organisms can obtain energy from the oxidation of Fe2+ to Fe3+ or from the oxidation of sulfur and reduced sulfur compounds to sulfate as illustrated below.
4FeSO4 + 2H2SO4 + O2 → 2Fe2(SO4)3 + 2H2O
2S° + 3O2 + 2H2O → 2H2SO4
2FeS2 + 7O2 + 2H2O
As is evident from the third reaction given above, iron is extracted in the soluble form the iron ore pyrite (FeS2).
Indirect bioleaching:
In this indirect method, the bacteria produce strong oxidizing agents such as ferric iron and sulfuric acid on oxidation of soluble iron or soluble sulfur respectively. Ferric iron or sulfuric acid, being powerful oxidizing agents react with metals and extract them. For indirect bioleaching, acidic environment is absolutely essential in order to keep ferric iron and other metals in solution. It is possible to continuously maintain acidic environment by the oxidation of iron, sulfur, metal sulfides or by dissolution of carbonate ions.
Commercial Process of Bioleaching:
The naturally occurring mineral leaching is very slow. The microbial bioleaching process can be optimized by creating ideal conditions— temperature, pH, and nutrient, O2 and CO2 supply etc. A diagrammatic representation of general bioleaching process is depicted in Fig. 32.1.
The desired microorganisms with nutrients, acid etc., are pumped into the ore bed. The microorganisms grow and produce more acid. The extracted leach liquor is processed for the metal recovery. The leach liquor can be recycled again and again for further metal extraction.
In commercial bioleaching, three methods are commonly used-slope leaching, heap leaching and in situ leaching (Fig. 32.2).
Slope leaching:
The ore is finally ground and dumped in large piles down a mountainside (Fig. 32.2A). This ore is then subjected to continuous sprinkling of water containing the desired microorganism (T. ferrooxidans). The water collected at the bottom is used for metal extraction. The water can be recycled for regeneration of bacteria.
Heap leaching:
In this case, the ore is arranged in large heaps (Fig. 32.2B) and subjected to treatments as in slope leaching.
In situ leaching:
The ore, in its original natural place is subjected to leaching (Fig. 32.2C). Water containing the microorganisms is pumped through drilled passages. In most cases, the permeability of rock is increased by subsurface blasting of the rock. As the acidic water seeps through the rock, it collects at the bottom which is used for metal extraction. This water can be recycled and reused.
Selected examples of microbial bioleaching are briefly described below:
Bioleaching of Copper:
Copper ores (chalcopyrite, covellite and chalcocite) are mostly composed of other metals, besides copper. For instance, chalcopyrite mainly contains 26% copper, 26% iron, 33% sulfur and 2.5% zinc.
Bioleaching of copper ore (chalcopyrite) is widely used in many countries. This is carried out by the microorganism Thiobacillus ferrooxidans which oxidizes insoluble chalcopyrite (CuFeS2) and converts it into soluble copper sulfate (CuSO4). Sulfuric acid, a byproduct formed in this reaction, maintains acidic environment (low pH) required for growth of the microorganisms.
Copper leaching is usually carried out by heap and in situ process (details given above). As the copper-containing solution (i.e., copper in the dissolved state) comes out, copper can be precipitated and the water is recycled, after adjusting the pH to around 2.
Extraction of copper by bioleaching is very common since the technique is efficient, besides being economical. It is estimated that about 5% of the world’s copper production is obtained via microbial leaching. In the USA alone, at least 10% of the copper is produced by bioleaching process.
Bioleaching of Uranium:
Bioleaching is the method of choice for the large-scale production uranium from its ores. Uranium bioleaching is widely used in India, USA, Canada and several other countries. It is possible to recover uranium from low grade ores (0.01 to 0.5% uranium) and low grade nuclear wastes.
In situ bioleaching technique is commonly used for extracting uranium. In the technique employed, the insoluble tetravalent uranium is oxidized (in the presence of hot H2SO4/Fe3+ solution) to soluble hexavalent uranium sulfate.
UO2 + Fe2(SO4)3 → UO2SO4 + 2FeSO4
Bioleaching of uranium is an indirect process since the microbial action is on the iron oxidant, and not directly on the uranium. The organism Thiobacillus ferrooxidans is capable of producing sulfuric acid and ferric sulfate from the pyrite (FeS2) within the uranium ore.
For optimal extraction of uranium by bioleaching, the ideal conditions are temperature 45-50°C, pH 1.5-3.5, and CO2 around 0.2% of the incoming air.
The soluble form of uranium from the leach liquor can be extracted into organic solvents (e.g., tributyl phosphate) which can be precipitated and then recovered.
Heap leaching process is sometimes preferred instead of the in situ technique. This is because the recovery of uranium in much higher with heap leaching.
Bioleaching of Other Metals:
Besides copper and uranium, bioleaching technique is also used for extraction of other metals such as nickel, gold, silver, cobalt, molybdenum and antimony. It may be noted that removal of iron is desirable prior to the actual process of leaching for other metals. This can be done by using the organism Thiobacillus ferrooxidans which can precipitate iron under aerobic conditions.
Bioleaching is also useful for the removal of certain impurities from the metal rich ores. For instance, the microorganisms such as Rhizobium sp and Brady rhizobium sp can remove silica from bauxite (aluminium ore).
Bioleaching in desulfurization of coal:
The process of removal of sulfur containing pyrite (FeS2) from high sulfur coal by microorganisms is referred to as bio desulfurization. High sulfur coal, when used in thermal power stations, emits sulfur dioxide (SO2) that causes environmental pollution.
By using the microorganisms Thiobacillus ferrooxidans and T. thiooxidans, the pyrite which contains most of the sulfur (80-90%) can be removed. Thus, by employing bioleaching, high sulfur coal can be fruitfully utilized in an environment friendly manner. In addition, this approach is quite economical also.
Advantages of Bioleaching:
When compared to conventional mining techniques, bioleaching offers several advantages. Some of them are listed below.
1. Bioleaching can recover metals from low grade ores in a cost-effective manner.
2. It can be successfully employed for concentrating metals from wastes or dilute mixtures.
3. Bioleaching is environmental friendly, since it does not cause any pollution (which is the case with conventional mining techniques).
4. It can be used to produce refined and expensive metals which otherwise may not be possible.
5. Bioleaching is a simple process with low cost technology.
6. It is ideally suited for the developing countries.
The major limitation or disadvantage of bioleaching is the slowness of the biological process. This problem can, however, be solved by undertaking an in depth research to make the process faster, besides increasing the efficiency.
Method # 2. Bio Sorption:
Bio sorption primarily deals with the microbial cell surface adsorption of metals from the mine wastes or dilute mixtures. The microorganisms can be used as bio sorbents or bio accumulators of metals. The process of bio sorption performs two important functions.
1. Removal of toxic metals from the industrial effluents.
2. Recovery of valuable but toxic metals.
Both the above processes are concerned with a reduction in environmental poisoning/pollution.
A wide range of microorganisms (bacteria, algae, yeasts, moulds) are employed in bio sorption. In fact, some workers have developed bio sorbent-based granules for waste water/industrial effluent treatment, and metal recovery.
In general, the microbial cell membranes are negatively charged due to the presence of carboxyl (COO–), hydroxyl (OH–) phosphoryl (PO3-4) and sulfhydryl (HS–) groups. This enables the positively charged metal ions (from solutions) to be adsorbed on to the microbial surfaces. The different groups of microorganisms used in bio sorption processes are briefly described below.
Bacteria:
Several bacteria and actinomycetes adsorb and accumulate metals such as mercury, cadmium, lead, zinc, nickel, cobalt and uranium. For example, Rhodospirullum sp can accumulate Cd, Pb and Hg. Bacillus circulans can adsorb metals such as Cu, Cd, Co, and Zn. By use of electron microscopy, deposition of metals on the bacterial cell walls was recorded. It appears that the cell wall composition plays a key role in the metal adsorption.
Fungi:
There is a large scale production of fungal biomass in many fermentation industries. This biomass can be utilized for metal bio sorption from industrial effluents. Immobilized fungal biomass is more effective in bio sorption due to increased density, mechanical strength and resistance to chemical environment. Further, immobilized biomass can be reused after suitable processing.
The fungus Rhizopus arrhizus can adsorb several metallic cations e.g. uranium, thorium. Pencillium lapidorum, P. spimuiosum are useful for the bio sorption of metals such as Hg, Zn, Pb, Cu. Several fungi were tried with some degree of success to selectively adsorb uranium e.g. Aspergillus niger, A. oryzae, Mucor haemalis, Penicillium chrysogenum.
Edible mushrooms were also found to adsorb certain metals. For instance, fruit bodies of Agaricus bisporus can take up mercury while Pleurotus sajor- caju can adsorb lead and cadmium. Many yeasts, commonly used in fermentation industries, are capable of adsorbing and accumulating metals. For instance, Saccharomyces cerevisae and Sporobolomyces salmonicolour can respectively adsorb mercury and zinc.
Algae:
Several species of algae (fresh water or marine) can serve as bio accumulators of metals. For instance, Chlorella vulgaris and C. regularis can accumulate certain metals like Pb, Hg, Cu, Mo and U. The green algae Hydrodictyon reticulatum adsorbs and accumulates high quantities of Pb, Fe and Mn. Some workers are in fact trying to use marine algae (e.g., Luminaria, Ulva, Codium sp) as bioaccumulators to reduce the metal pollution in rivers.
Higher plants in control of metal pollution:
Besides the microorganisms described above, there are some higher aquatic plants (i.e., aquatic macrophytes) that can accumulate potential toxic wastes including many metals. Water hyacinth (Eichornia crassipes), duck weeds (Spirodel sp), water lettuce (Pistia stratiotes) and certain ferns (Salvinia sp) are important in the control of metal pollution.
Microbial Recovery of Petroleum:
By the conventional technique used in the oil fields, approximately one-third of the oil can be recovered. However, the oil recovery can be enhanced by using solvents, and surfactants. Certain polymers produced by microorganisms (e.g., xanthan gum), when added to oil wells are capable of increasing oil recovery. Xanthan gum can pass through small pore spaces and promote the release of more trapped oil.
In recent years, oil technologists are trying to directly use microorganisms in situ for increasing the oil recovery. In this process, there is no formalized use of a bioreactor. The natural geological site itself is the bioreactor. This allows the water and microorganisms to flow over the ore which are collected after seepage and outflow.
The microorganisms, through surfactant production, gas formation or by other microbial activities reduce the viscosity of the oil so as to enhance its recovery. However, the success in this direction is not very commendable. Continued further research may one day help to use microbes for commercial release of oil from oil wells or tar sands.