Bioleaching is simply defined as the solubilization of metals from their ores using microorganisms. It is the dissolution of metals from their natural mineral sources or ores by certain group of naturally-occurring microbes known as bioleaching microbes; and bioleaching is good for the efficient extraction of metals from low-grade metal ores and other mineral concentrates. Bioleaching is the use of microbes to transform elements so that these elements can be extracted from their ores when water is filtered through it. In bioleaching, solid metals in their natural ores are converted to water-soluble forms through the metabolic activities of bioleaching microbes. Bioleaching is the extraction of metals from their ores through the use of microorganisms including mineralytic bacteria and fungi. It is a preparatory step in the recovery of metals from their ores; and in the subsequent stages of metal recovery, the actual metals are finally recovered from the leachate resulting from the microbial activities carried out on the ores. A leachate is the liquid substance that has percolated through a solid material (e.g. metal ore) and leached or percolated out some of its constituents.
Leaching is the process of extracting substances such as metals from a solid like metal ore by dissolving them in a liquid medium, either in nature or through an industrial or chemical process. It is the loss or extraction of certain materials such as metals from a carrier into a liquid (usually, but not always a solvent). Leaching is a process where metal ore is soluble and impurities are insoluble. Leaching is widely used in extractive metallurgy techniques which convert metals into soluble salts in aqueous media. The extraction of metals from their ores naturally is much cleaner and preferred than the traditional heap leaching using chemicals such as cyanide because the former (i.e. bioleaching) leaves little or no effect on the environment than the later (chemical leaching) which affects the environment. The advantages of bioleaching are enormous. Bioleaching is a relatively low cost procedure, compared to other leaching options such as roasting or smelting; and it does not also contaminate the environment and so it is an eco-friendly process. It is much better and cleaner than the heap leaching using cyanide.
One way to mitigate the environmental consequences of mining activities and quarrying in particular is through the use of bioleaching microbes; and such rock-eating microbes like Acidithiobacillus and Leptospirillum bacterium can clean up abandoned mine sites and quarrying sites. Bioleaching is used to recover several elements and/or metals such as copper, zinc, lead, arsenic, antimony, nickel, molybdenum, gold, silver and cobalt; and it is an efficient and an ecological friendly mining process commonly used in mining industries as an alternative method to roasting or smelting, especially when there are lower concentrations of metal in the ore. In bioleaching activities, natural materials that are native to the environment such as water, air and microorganisms are used for the leaching activities instead of chemically synthesized chemicals. Bioleaching is the process of using bacteria to dissolve metals instead of chemical solutions. It is the extraction of a metal from sulfide ores or concentrates using natural materials such as water and microbes. Bioleaching is leaching where the extraction of metal from solid minerals into a solution is facilitated by the metabolism of certain microbes generally known as bioleaching microbes (rock-eating microbes).
Bioleaching microbes are special class of microorganisms that “eat rocks”; and the ability of these microbes to eat rocks or solubilize metals from their ores are applied in metallurgy/mining activities (metal extraction processes) for the sustainable extraction of metals from their ores naturally. Typical examples of bioleaching microbes include microorganisms in the group prokaryotes, archaea and some eukaryotic organisms. These microbes are known as bioleaching bacteria, bioleaching fungi and bioleaching archaea. Thiobacillus species, Leptospirillum ferrooxidans, Acidithiobacilus thiooxidans, Sulfobacillus acidophilus, thermophilic bacteria and heterotrophic bacteria and fungi (which require organic supplements for growth and energy supply) are some notable examples of bioleaching microbes. The microbes found in bioleaching environments perform various activities through the metabolic activities of the organisms. While some rock-eating microbes produce leaching chemicals that aid in leaching activities, the others do not produce leaching chemicals but instead they support the activities of the microbes that produce leaching chemicals/reagents.
Microbes that produce leaching reagents or chemicals such as iron III (Fe3+) are known as true bioleaching microbes while those microbes that support the true bioleaching microbes are known as bioleaching supporting microbes or bioleaching supporters. Both true bioleaching microbes and bioleaching supporting microbes are found in bioleaching environments. Sub-optimal growth conditions found in bioleaching environments such as incorrect pH conditions; unavailability of appropriate macronutrients and micronutrients for microbial growth; limitations of carbon and oxygen; and the presence of organic compounds with inhibitory effects on rock-eating microbes are some of the factors that impede on the optimal growth of true bioleaching microbes from efficiently leaching metals from their ores (Table 1).
To achieve a higher yield of metal extraction from the ore, the leaching conditions of the leaching environment must be supportive to the optimal growth of the bioleaching microbes – since the leaching efficiency depends largely on the chemical and mineral compositions of the ore to be leached as well as on the effectiveness of the bioleaching microorganisms. And to be effective in carrying out the leaching, the leaching environment must provide all necessary factors that support the optimal growth of the bioleaching microbes at optimal amounts (Table 1). The biologically produced leaching reagents (Fe3+) attack the minerals and leach the metals in the ore via oxidation-reduction reactions. Fe3+ is one important leaching reagent which is regenerated by the oxidation of Fe2+ (Figure 1). Bioleaching supporting microbes assist true bioleaching microbes (which produce leaching reagents/chemicals) by feeding on organic wastes products in the bioleaching environments, and thus remove the organic waste products that usually build up to affect the activities of the true bioleaching microbes.
The organic wastes products present in bioleaching environments are detrimental to the survival of true bioleaching microbes. Some of the best known bioleaching reactions include when metal is released from sulfide minerals by iron III (Fe3+) oxidation of the metal sulfide bond and is catalyzed by acidophilic iron oxidizing microorganisms such as Thiobacillus ferrooxidans that convert the resulting iron II (Fe2+) back to Fe3+ (Figure 1). The process of bioleaching usually involves the microbes (e.g. bacteria) feeding on the nutrients in the metal ore thereby separating the metal from its ore. After the microbe have finished feeding on the nutrients found in the ore, the metal can then be collected from the bottom of the solution; and this process in which bacteria eat rocks is possible because of the unique ability of some microbes to react and breakdown the mineral deposits in the ore.
These bacteria including Thiobacillus ferrooxidans, Leptospirillum ferrooxidans and Thermophilic speciesof Sulfobacillus, Acidianus and Sulfolobus leach (percolate) metals of value such as Copper, Zinc, Uranium, Nickel and Cobalt from sulphide mineral ores after feeding on them. These microbes are able to carry out this degradation activities because they are able to tolerate acids and metabolize sulphur while feeding on metal ores. The bacteria act as a catalyst to accelerate the natural processes inside the metal ore; and they usually do this through an oxidation-reduction reaction carried out by oxidizing bacteria as aforementioned. Bioleaching microbes or rock-eating microbes carryout mineralytic effects on metal ores; and the culmination of these mineralytic effects leads to the leaching of the metals from their natural ores.
The mineralytic effects of rock-eating microbes include:
- The formation of organic or inorganic acids (protons).
- Oxidation and reduction reactions which releases metals from sulphide minerals. In oxidation-reduction reaction, autotrophic bacteria such as iron-oxidizing bacteria gains energy that is utilized for leaching metals from their ores.
- Excretion of complexing agents that aid in the leaching process.
The mineralytic effects of microbes are usually based on acidolysis, complexolysis and redoxolysis – all of which are responsible for rock-eating microbes to mobilize metals during bioleaching. And these factors are also responsible for the formation of organic or inorganic acids; oxidation-reduction reactions and the excretion of complexing agents by bioleaching microorganisms as aforementioned. The recovery of metals from sulphide minerals for example, is usually based on the metabolic activities of chemolithotrophic bacteria such as Thiobacillus species including T. thiooxidans and T. ferrooxidans which are notable in converting insoluble metal sulphides to soluble metal sulphates.
Other non-sulphide metal ores and mineral concentrates can be treated with heterotrophic bacteria and fungi – in order to extract their metals. Metals are released from their ores or sulphide minerals and other mineral concentrates by either direct bacterial leaching or indirect bacterial leaching. In direct bacterial leaching, the bioleaching bacteria are in direct physical contact with the surface of the mineral sulphide; and the process is enzymatically catalyzed by bioleaching microbes via oxidation-reduction reaction. Indirect bacterial leaching involves the production of chemical substances which chemically oxidizes the sulphide mineral; and the bioleaching bacteria do not need to be in direct physical contact with the mineral surface (as is applicable in direct bacterial leaching).
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