Principles Governing Microbe-Mineral Interfacial Phenomena

Transcription

1 Lecture 27 Principles Governing Microbe-Mineral Interfacial Phenomena Keywords: Biomineral Beneficiation, Bioreagents, Cell Surface Hydrophobicity Utility of various microorganisms in bioleaching was discussed so far. Yet another application of mining microorganisms is mineral beneficiation. Principles governing microbe-mineral interfacial phenomena and applicability of microbially-induced mineral beneficiation are discussed in lectures Use of autotrophic and heterotrophic microorganisms including bacteria, fungi and yeasts is illustrated with respect to beneficiation of sulfide and oxide minerals [ ]. Depletion of richer ore deposits coupled with higher demand and consumption for valuable metals have necessitated the development of alternative efficient beneficiation processes. Commercial applications of Microbially-mediated mineral extraction processes have been demonstrated in the extraction of copper, uranium, cobalt and gold. Several microorganisms have been identified to be involved in natural mineral formation and conversion processes. The divergent role of microbes in the field of mineral processing starting from mining, beneficiation, metal extraction to efficient waste disposal has been well recognized. Increased environmental awareness and stringent regulations have promoted the applicability of biotechnology in mineral beneficiation. Utility of microorganisms and bioreagents in many metallurgical applications has become possible. For example, microbially induced flocculation or flotation of minerals, remediation of toxic chemicals discharged from mineral processing operations, degradation of cyanide, etc. As different from bioleaching, biobeneficiation, by definition, refers to selective removal of undesirable mineral constituents from an ore through interaction with microorganisms or bioreagents thus enriching it with respect to the desired valuable minerals. Unlike conventional techniques, microbially induced flotation and flocculation would be more energy efficient, economically viable and an environmentally benign process for the effective separation 1

2 of various ore minerals. Conventional beneficiation techniques involve use of various toxic reagents, which could be safely replaced by environment friendly bioreagents. Some consequences of microbe-mineral interactions relevant to microbially-induced mineral beneficiation are Adhesion to mineral substrates resulting in biofilm formation, Bio-catalised oxidation and reduction reactions, and Interaction of metabolic products. Such biogenic reactions result in surface modification, dissolution of mineral constituents as well as bioaccumulation and remediation. Microbially-induced beneficiation is thus brought about by microorganisms and bioreagents, resulting in surface chemical and physico-chemical changes, such as Changes in surface chemistry of minerals and surface hydrophobicity. Generation of surface active chemicals and bioreagents. Selective dissolution of mineral phases in an ore matrix and Sorption, accumulation and precipitation of metal ions and products. Several types of autotrophic and heterotrophic bacteria, fungi, yeasts and algae can be used in mineral beneficiation processes. Selective dissolution, flotation and flocculation are some of the processes involved in biobeneficiation. Microbiological, surface-chemical as well as physico-chemical factors governing microbiallyinduced beneficiation are discussed. [179] Microbially induced mineral beneficiation processes are influenced by surface properties of bacterial cells, their adhesion behaviour towards minerals and the surface chemical changes of the minerals consequent to microbe-mineral interaction. Bacterial cell wall architecture Structure of the bacterial cell wall plays a significant role in adhesion to mineral surfaces and resulting consequences. 2

3 Bacteria are classified into two major categories with respect to cell wall structure, namely, Gram-positive and Gram-negative. The chemical composition and structural format of cell walls are different in these two types of bacteria. Gram-positive bacteria usually possess a well defined, rigid outer cell wall nm thick, and an inner, closely applied, cell-limiting plasma membrane. The wall constitutes 15 to 30% dry weight of the cell and the rigidity is due to major polymeric component, peptidoglycan. Cell wall may also contain one or more accessory / secondary polymers such as teichoic acids and teichuronic acids. The cell walls of Gram-negative bacteria contain a more general structural format. They have an outer membrane placed above a thin peptidoglycan layer. These lipid-protein bilayer membranes contain, proteins, phospholipids, and lipopolysaccharides and separates the outer environment from the periplasm. In between the outer and the plasma membranes, a gel-like matrix (the periplasm) is observed in the periplasmic space. Different from Gram-positive bacteria cells where cell membrane is in close association with the undersurface of the cell wall, Gramnegative bacteria possess a gap between cell and outer cell membrane. The plasma membrane and the cell wall (outer membrane, peptidoglycan layer, and periplasm) together form the Gramnegative envelope. Biologically produced reagents Both organic and inorganic reagents are generated through bacterial metabolism which are useful in mineral beneficiation. As shown in Table 27.1, different types of mineral acids, fatty acids, polymers and chelating agents are generated by bacteria, fungi and yeasts. A large variety of fatty acids produced by microorganisms have been tested in the flotation of various ores. The microbial products can function as flotation collectors, depressants or dispersants. Many other inorganic and organic surfactants of utility in mineral processing are also secreted by microorganisms as illustrated in Tables 27.2 and Bacterial polysaccharides and enzymes would be useful in flotation, flocculation and mineral leaching processes. 3

4 Table 27.1: Metabolic products of different microorganisms Microbe Functions Useful products Acidophilic sulphur and Oxidation of mineral Ferric ions, sulphuric acid iron bacteria sulphides Nitrifying bacteria Acidulation Nitric acid Fermenting bacteria Reduction, Dissolution Formic acid and chelation Fungi Complexation and dissolution Oxalic, citric, formic and gluconic acids Clostridia Acidulation, Dissolution Acetic acid Heterotrophic bacteria Polymerization, dispersion; agglomeration and metal solubilization exopolysaccharides, bioproteins and organic acids. Table 27.2: Microbial Exoenzymes Bacteria Type of enzyme Bacillus subtilis Paenibacillus polymyxa Clostridium thermocellus Bacillus megaterium Amylase (starch) Pectinase (pectin) Cellulose Proteinase (peptides) 4

5 Table 27. 3: Some Biological Exopolymers Bacteria Polymer Bacillus megaterium Acetobacter xylinum Agrobacterium tumefaciens Paenibacillus polymyxa Leuconostoc spp. Streptococcus spp. Polyglutamic acid Cellulose Glucan Polysaccharides Dextrans Bacterial surface charge, electrical double layer and cell surface hydrophobicity Most bacterial cells possess an overall negative charge on the surface of their cell wall at netural ph, due to the presence of peptidoglycan, which is rich in carboxyl and amino groups. Teichoic acids containing phosphate rich component also contribute to the negative charge. From a physico-chemical point, microorganisms can be considered living colloidal particles. The cells acquire charge through the ionization of amino, carboxylate and phosphate groups, which is ph dependent. Competition among electrical and chemical forces control surface charge neutralization. Counter ions can bind to such charged groups. An electrical double layer will be thus established at the interface. The surface chemical properties of microorganisms can be characterized by zeta potential and the isoelectric point (IEP). In mineral-bacteria-solution systems as in beneficiation processes, mineral-solution as well as mineral-bacterial cell interfaces are formed. Bacterial cells can act as living colloidal particles at the mineral-solution interface. Increasing ionic strength favours bacterial adhesion to a mineral surface. Bacterial dispersion and flocculation are governed by electrical forces as in the case of mineral particles. Different amounts of lipids present on the cell wall are responsible for surface hydrophobicity. Hydrophobicity is attained by hydrophobic molecules present on cell surfaces. Amino groups 5

6 confer hydrophobicity. Surface hydrophobicity is important in interfacial interactions among bacterial cells and mineral particles. Hydrophobic bacteria can readily adhere to surfaces due to repulsion from the polar water molecule. The effect of electrostatic repulsive force decreases with increasing cell hydrophobicity. The role of hydrophobicity in bacterial adhesion to mineral surfaces has been studied. Since surface charge enhances the chances of polar interactions with water molecules, the more charged the cell surfaces, the less hydrophobic they would be. Any process that decreases the cell negative charge can enhance its hydrophobic character. Cells having higher hydrophobicity and lower electrophoretic mobility are more adherent. Cell surfaces are also covered by a large number of positively and negatively charged moieties, which lower the hydrophobicity. Cell appendages such as fimbriae and pili are charged. Cell surface hydrophobicity depends on the chemical composition and architecture of the cell wall outer layers. Surface proteins and polysaccharides are involved in bacterial adhesion to minerals. Relatively hydrophobic portions of protein or hydroxyl-deficient polysaccharide molecules can be differently oriented. Molecules having both hydrophobic and hydrophilic portions may confer dual properties. Many organisms possess both hydrophilic and hydrophobic surface regions. Cell surface hydrophobicity is not a permanent property of the cell envelope and can be modified depending on environmental conditions. Interfaces offer a better environment for nutrition and growth of organisms. Bacterial adhesion is dependent on the bio - and surface chemical properties of the microorganism and also on the interfacial properties of the mineral in an aqueous system. When grown in a liquid medium, the cells acquire appropriate surface properties to survive in a liquid medium. If grown on a solid substrate, they will alter their cell wall and membrane characteristics accordingly. 6

7 Why bacterial adhesion? Bacterial adhesion to substrates may be a survival strategy allowing microorganisms in a nutritionally favourable environment such as the mineral-solution interfaces. Many specialized structures and complex ligand interactions have evolved in prokaryotes specifically designed for recognition of surfaces and biofilm formation. Adhesion of microorganisms to mineral substrates significantly influences microbially-induced mineral beneficiation processes. 7