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Interactions of bacterial exopolymers with metal ions
286
Biodeterioration Society Abstracts
A Systematic Study on Equilibrium and Kinetics of Biosorptive Accumulation of Heavy Metals. The Case of Ag and Ni M. Tsezos, E. Remoundaki and V. Angeilatou
Metallurgy and Metals Engineering, National Technical University of Athens, Athens, Greece The use of microorganisms for the removal of toxic heavy metals and radionuclides from industrial wastes may represent a potential alternative to existing technologies. Metabolism-independent metal uptake takes place using dead microbial cells, is termed biosorption, and has been shown to be a complex phenomenon. Metal sequestration by different parts of the dead cell can occur via a number of mechanisms. Each one of these mechanisms depends on microorganisms' characteristics, physico-chemical characteristics of the targeted elements, and microenvironmental characteristics of the solutions. The ~levelopment of biosorption techniques for the sequestering and recovery of targeted elements can be possible only after the identification, study and definition of the factors controlling the phenomenon. These objectives can be attained through a systematic, multidisciplinary and concerted research effort. The definition of biosorptive metal uptake equilibrium isotherms, the control of the solution parameters during the experiments, the examination of the reversibility of biosorption, and the study of the biosorption kinetics are the main targets of the systematic approach undertaken by the authors. The present paper reports experimental results on biosorption, desorption and apparent (overall) kinetics for silver and nickel and five selected microbial biomass strains. The biosorptive metal uptake capacity for each metal and strain has been quantitatively evaluated. The strains tested are successful biosorbents for silver; strain BP 7/26 has been proven to be the most efficient. Nickel biosorption uptake capacities by the strains tested exhibit lower values than those corresponding to silver. An interpretation of this behaviour is attempted based on the different physico-chemical characteristics of these elements. Further research is in progress. Biosorption of silver and nickel by the strains tested is an irreversible process. Ninety to 100% of maximum silver biosorption capacity is attained within the first hour, while nickel biosorption uptake capacities attained equilibrium after contact times much shorter than 24 h. Interactions of Bacterial Exopolymers with Metal Ions lwona B. Beech
School of Chemistry, Physics and Radiography, University of Portsmouth, U.K. This communication addresses the phenomenon of complexation of metal ions by bacterial extracellular molecules and its importance for biotechnological processes designed to remove or imobilise metal species from a variety of environments. The emphasis will be placed on discussing the involvement of exopolymers
Biodeterioration SocieO' Abstracts
287
produced by anaerobic sulphate-reducing bacteria (SRB) in binding of metal ion species. In natural environments bacterial cells are present as both planktonic and sessile (attached) populations. The formation of microbial communities on submerged surfaces in aquatic environments creates an assemblage termed the biofilm. In the complex process of biofilm development the initial attachment of microorganisms to a surface is followed by the irreversible adhesion of cells, facilitated by production of extracellular polymeric substances (EPS), often referred to as glycocalyx or slime. The EPS form a three-dimensional heterogeneous matrix consisting of polysaccharides, glycoproteins, proteins and nucleic acids. In planktonic bacteria EPS are often present as capsule. Exopolymers do not have to be associated with cells but can also be released into the bulk liquid. One of the important properties of EPS is their ability to complex metal ions. Many exopolymers of aquatic microorganisms act as polyanions under natural conditions carrying a charge which promotes ionic and electrostatic binding of counterions including metals. Several classes of polymeric molecules participate in EPS/metal interactions by the formation of salt bridges with carboxyl groups on acidic polymers and by forming weak electrostatic bands with hydroxyl groups on neutral polymers. A large number of metals have been reported to cross-link polysaccharides. Although microbial EPS have been shown to exhibit selectivity in complexing metal ions, in many cases, the type of macromolecules playing a key role in metal binding, has not been determined. One group of microorganisms that are of considerable interest to biotechnology are sulphate-reducing bacteria. Indeed, certain aspects of the metabolic activity of SRB, namely the production of hydrogen sulphide, are already explored in anaerobic bioreactors used for the treatment of effluents containing heavy metals. The ability of SRB to produce exopolymers is well documented. It has also been shown that the composition of these exopolymers vary depending on the type of SRB and their growth conditions. The carbohydrate part of EPS synthesised by SRB is rich in mannose. Protein content of EPS vary from 30 to 60% (w/w). Exopolymers released into the bulk phase differ from those secreted within biofilms. For some SRB isolates, EPS produced by the biofilm population have a higher content of uronic acids compared with EPS secreted into the bulk liquid. It has been observed that under appropriate experimental conditions, iron sulphide species generated in SRB batch cultures are enriched with exomolecules. In addition, the results of investigation, aimed to determine the ability of SRB exopolymers released into a bulk phase to bind metal ions such as Fe, Cr, Mo and Ni, demonstrated that EPS participate in the uptake of these ions. There is little doubt that EPS of sulphate reducers play a role in the process of complexation of metal ion species. However, whether this phenomenon can be of any practical value for biotechnological applications remains to be evaluated.
Biodeterioration Society Abstracts
A Systematic Study on Equilibrium and Kinetics of Biosorptive Accumulation of Heavy Metals. The Case of Ag and Ni M. Tsezos, E. Remoundaki and V. Angeilatou
Metallurgy and Metals Engineering, National Technical University of Athens, Athens, Greece The use of microorganisms for the removal of toxic heavy metals and radionuclides from industrial wastes may represent a potential alternative to existing technologies. Metabolism-independent metal uptake takes place using dead microbial cells, is termed biosorption, and has been shown to be a complex phenomenon. Metal sequestration by different parts of the dead cell can occur via a number of mechanisms. Each one of these mechanisms depends on microorganisms' characteristics, physico-chemical characteristics of the targeted elements, and microenvironmental characteristics of the solutions. The ~levelopment of biosorption techniques for the sequestering and recovery of targeted elements can be possible only after the identification, study and definition of the factors controlling the phenomenon. These objectives can be attained through a systematic, multidisciplinary and concerted research effort. The definition of biosorptive metal uptake equilibrium isotherms, the control of the solution parameters during the experiments, the examination of the reversibility of biosorption, and the study of the biosorption kinetics are the main targets of the systematic approach undertaken by the authors. The present paper reports experimental results on biosorption, desorption and apparent (overall) kinetics for silver and nickel and five selected microbial biomass strains. The biosorptive metal uptake capacity for each metal and strain has been quantitatively evaluated. The strains tested are successful biosorbents for silver; strain BP 7/26 has been proven to be the most efficient. Nickel biosorption uptake capacities by the strains tested exhibit lower values than those corresponding to silver. An interpretation of this behaviour is attempted based on the different physico-chemical characteristics of these elements. Further research is in progress. Biosorption of silver and nickel by the strains tested is an irreversible process. Ninety to 100% of maximum silver biosorption capacity is attained within the first hour, while nickel biosorption uptake capacities attained equilibrium after contact times much shorter than 24 h. Interactions of Bacterial Exopolymers with Metal Ions lwona B. Beech
School of Chemistry, Physics and Radiography, University of Portsmouth, U.K. This communication addresses the phenomenon of complexation of metal ions by bacterial extracellular molecules and its importance for biotechnological processes designed to remove or imobilise metal species from a variety of environments. The emphasis will be placed on discussing the involvement of exopolymers
Biodeterioration SocieO' Abstracts
287
produced by anaerobic sulphate-reducing bacteria (SRB) in binding of metal ion species. In natural environments bacterial cells are present as both planktonic and sessile (attached) populations. The formation of microbial communities on submerged surfaces in aquatic environments creates an assemblage termed the biofilm. In the complex process of biofilm development the initial attachment of microorganisms to a surface is followed by the irreversible adhesion of cells, facilitated by production of extracellular polymeric substances (EPS), often referred to as glycocalyx or slime. The EPS form a three-dimensional heterogeneous matrix consisting of polysaccharides, glycoproteins, proteins and nucleic acids. In planktonic bacteria EPS are often present as capsule. Exopolymers do not have to be associated with cells but can also be released into the bulk liquid. One of the important properties of EPS is their ability to complex metal ions. Many exopolymers of aquatic microorganisms act as polyanions under natural conditions carrying a charge which promotes ionic and electrostatic binding of counterions including metals. Several classes of polymeric molecules participate in EPS/metal interactions by the formation of salt bridges with carboxyl groups on acidic polymers and by forming weak electrostatic bands with hydroxyl groups on neutral polymers. A large number of metals have been reported to cross-link polysaccharides. Although microbial EPS have been shown to exhibit selectivity in complexing metal ions, in many cases, the type of macromolecules playing a key role in metal binding, has not been determined. One group of microorganisms that are of considerable interest to biotechnology are sulphate-reducing bacteria. Indeed, certain aspects of the metabolic activity of SRB, namely the production of hydrogen sulphide, are already explored in anaerobic bioreactors used for the treatment of effluents containing heavy metals. The ability of SRB to produce exopolymers is well documented. It has also been shown that the composition of these exopolymers vary depending on the type of SRB and their growth conditions. The carbohydrate part of EPS synthesised by SRB is rich in mannose. Protein content of EPS vary from 30 to 60% (w/w). Exopolymers released into the bulk phase differ from those secreted within biofilms. For some SRB isolates, EPS produced by the biofilm population have a higher content of uronic acids compared with EPS secreted into the bulk liquid. It has been observed that under appropriate experimental conditions, iron sulphide species generated in SRB batch cultures are enriched with exomolecules. In addition, the results of investigation, aimed to determine the ability of SRB exopolymers released into a bulk phase to bind metal ions such as Fe, Cr, Mo and Ni, demonstrated that EPS participate in the uptake of these ions. There is little doubt that EPS of sulphate reducers play a role in the process of complexation of metal ion species. However, whether this phenomenon can be of any practical value for biotechnological applications remains to be evaluated.