Biogenesis Of Metals And Minerals

Transcription

1 Lecture 2 Biogenesis Of Metals And Minerals Keywords: Biomineralization, Metal-bacteria Cycles, Biogenesis Many types of bacteria (autotrophic, heterotrophic, aerobic-anaerobic, mesophile-thermophile, acidophilic-neutrophilic), fungi, yeasts and algae, inhabit mineral deposits, even under extreme environmental conditions. Ubiquitous presence of a plethora of microorganisms in mining environments is pointer towards biogenesis and biomineralization. In figure 2.1 below, bluish copper sulfate emanating from chalcopyrite containing rocks could be seen. Natural biooxidation and dissolution of chalcopyrite by native Acidithiobacillus bacteria is a pointer towards biogenesis. Fig.2.1: Bluish copper sulfate generated by indigenous Acidithiobacillus in copper-bearing rocks Geomicrobiology of gold has been well studied. Many microorganisms can solubilise and precipitate gold. Bacteria and Archaea are involved in the gold biogeochemical cycle. Gold 1

2 formation, mineralization, reconcentration and precipitation are catalyzed by bacteria. Goldbearing sulfide minerals such as arsenopyrite, pyrite and chalcopyrite may be formed in deep subsurface environments through the activities of sulfate reducing bacteria. Aerobic autotrophs such as At.ferrooxidans and At.thiooxidans can break-up gold-bearing sulfide minerals. Microbial metabolism also generates many types of solvents such as amino-acids, cyanides and thiosulfates which can solubilise gold. Cyanobacteria promote formation of secondary crystals of gold. Bacteriform gold occurs associated with quartz. Similarly, association of gold with organic and inorganic carbon can be biomediated. Usage of microorganisms as indicators of gold occurrences is an interesting outcome. Presence of bacterial spores containing tiny gold particles on top soils may indicate presence of gold, underneath. Microbially-induced gold cycle in nature can be assessed at different mine sites. [4] Microbial activity is an essential constituent in the natural iron cycle. Many iron-involved redox reactions stimulate bacterial growth which promotes iron dissolution and precipitation. Biogenic iron oxides generally occur as nano - crystals possessing varying morphology and mineralogy. Direct bacterial metabolism or biosorption and nucleation processes can result in the formation of iron minerals. Acidophilic and neutrophilic, aerobic bacteria promotes oxidation of ferrous iron to ferric oxide. Anaerobic organisms reduce ferric to ferrous and ferrous oxides and sulfides are formed. Various types of iron oxides such as ferrihydrite, goethite, lepidocrocite, magnetite and hematite are found in sediments. Iron oxide particulates can occur closely associated with cell walls containing exopolymers. Extracellular biogenic oxides of iron can be differentiated from intracellular mineral formations. Iron-oxidizing microbes are isolated from iron rich seepages. Various types of organic and inorganic intracellular polymeric substances are generated by microorganisms. Production of intracellular magnetite has been reported in magnetotactic bacteria, which includes various prokaryotes including anaerobic, sulfate reducing bacteria. Banded iron formations may represent biogenic iron mineralization. [5-6] Various microorganisms indigenous to iron ore deposits are isolated to bring about beneficial iron ore processing to remove impurities such as phosphorus, alkalis, silica and alumina. Fungi 2

3 and organic and inorganic acid producing heterotrophs and autotrophs isolated from iron ore mines were found to be useful in dephosphorization. Microbial formation and degradation of limestone is an important example of biogenesis. Biological fixation of carbon in carbonates is performed by several bacteria, fungi and algae. Achromatium oxaliferum is involved in intracellular deposition of calcium carbonate. Calcium carbonate is a skeletal support structure in many invertebrates, sponges and mollusks. Biogenic calcium-magnesium carbonates formation in the form of chalk and calcite deposits in nature is well established. Biodissolution and degradation of limestone can also be brought about by microorganisms. This will lead to breakdown of cements and concrete structures. Nitrifying and sulfur-oxidizing bacteria could be detected in limestone samples after degradation. Cyano bacteria, fungi and algae can cause localised dissolution of limestone, creating pits and fissures. Such microorganisms bore into limestone and can destroy calcified coatings. Silicon is taken up and concentrated by organisms. Clays containing silicon influences microbial growth and attachment. Clays can modify microbial habitat and may influence activity of bacterial enzymes. Certain bacteria can accumulate silicon, so also some fungi and diatoms. Microbial silicon mobilizations play a role in weathering of rocks. Silicate minerals can be biodissolved. Phosphorous is fundamental to life as a nutritional and structural component of all organisms. Microbial conversion of organic into inorganic phosphorous and the synthesis of phosphate compounds is known. Soil organisms like Bacillus subtilis, B.megaterium, Arthrobacter and fungi such as Aspergillus can participate in natural phosphorous cycle. Microbes can solubilise many phosphate minerals such as apatite. Microbes involved in phosphate solubilisation include B.megaterium, Acidithiobacillus, Nitrifying bacteria, Pseudomonads and fungi such as A.niger and A.flavus. Microorganisms can also cause phosphate immobilization. Phosphorites in nature are biogenic. Pentavalent phosphorous can be reduced to lower valence states by microbes. Although arsenic is toxic to life, several microorganisms can grow and metabolize in the presence of larger concentrations. Many acidophiles inhabit arsenopyrite and orpiment mineral 3

4 forms in nature. Bacteria may develop resistance to arsenic, both through genetic and mutational changes. Arsenite-oxidizing bacterial strains have been isolated from soils. Arsenic- bearing minerals containing iron, copper and sulfur are oxidized by bacteria. For example, arsenopyrite and enargite are oxidized by Acidithiobacillus to generate As (III) and As (V) under acidic conditions. Geomicrobiology of iron is of great interest in metallurgy. Iron is biologically important as a nutritional source. Acidophiles such as At.ferrooxidans and L.ferrooxidans utilize ferrous ions as energy source and oxidize them to ferric state. Thermophiles such as Sulfobacillus and neutrophiles like Gallionella participate in enzymatic iron oxidation. Ferric ion reduction by bacteria is also known. Examples include Paenibacillus polymyxa, B.circulans and B.megaterium. The term, iron bacteria is used to represent the organisms that oxidize iron enzymatically. There are also iron accumulators. Magnetotactic bacteria produce magnetite through reduction of ferric iron. Magnetite can also be formed extracellularly. Many sedimentary iron deposits are biogenic. So also banded iron formations. Bacteria and fungi can mobilize large amounts of iron from minerals. Sulfide forms of iron such as pyrite and pyrrhotite are biogenic.[5-6] Many manganese oxide formations in nature are biogenic. Some manganese-oxidising bacteria include. Hyphomicrobium Pseudomonas Arthrobacter Leptothrix Metallogenium 4

5 Mn (IV) reducing bacteria include Paenibacillus polymyxa Bacillus mesentericus Pseudomonas liquefaciens Some microorganisms can accumulate manganese oxides. Microbial manganese deposition in nature is important in biogenesis in soil and water environments. Manganese-oxidizing bacteria have been detected in marine environments. Bacteria participate in manganese cycle in sea. Ferromanganese nodules in the ocean floors are formed due to biogenic reactions. The nodules provide manganese and iron as energy source to Mn (II) oxidising bacteria. Geomicrobiology of sulfur in nature is important in extractive metallurgy. Bacterial oxidation of sulfur and its compounds and reduction of sulfate is relevant in bioleaching and bioremediation of acid mine drainage. Biogenesis and biodegradation of sulfide minerals in nature lead to formation of many nonferrous metal sulfides and sulfates. Sedimentary metal sulfides are of biogenic origin. Hydrogen sulfide is formed from bacterial reduction of sulfates. Many metal sulfides such as those of iron, zinc, copper and nickel are formed through such biogenic reactions. 5

6 References (Lectures 1 and 2): 1. R.F. Decker, Biotechnology / Materials: The growing interface, Met. Trans, A, Vol. 17A (1986), pp S.Mann, Biomimetic Materials Chemistry, VCH Publishers Inc. N. Y. (1996). 3. S.Mann, Biomineralization and Biomimetic Materials Chemistry, in Biomimetic Materials Chemistry, Chapter 1, VCH (New York) (1996). 4. F. Reith, M. F. Lengke, D. Falconer, D. Craw and G. Southam, The Geomicrobiology of gold, The ISME Journal (2007), 1, D. Fortin and S. Langley, Formation and occurrence of biogenic iron-rich minerals, Earth Science Reviews, 72 (2005), H.L. Ehrlich, Geomicrobiology, Marcel Dekker Inc., N. Y. (1990). 6