Advanced Materials Research Vols. 71-73

Abstract: The interrelation between cells and extracellular polymeric substances (EPS) from the acidophilic bacterium Acidiphilium 3.2Sup(5) was investigated on two different carbon surfaces (carbon fibre cloth and graphite rods). This bacterium was chosen due to its ability to directly transfer electrons to carbon surfaces in aerobic conditions, which makes its use especially attractive in microbial fuel cells (MFC). The characterization of the bacterial adhesion and interrelation with the EPS was carried out using a combination of scanning (SEM) and transmission (TEM) electron microscopy. The extraction of the EPS was performed using EDTA and their characterization accomplished by chemical analyses and FTIR spectroscopy. The cellular lysis provoked by the extraction of EPS was determined by the protein/carbohydrate ratio. Chemical analyses showed that the main components of the EPS were proteins and carbohydrates, whereas FTIR spectroscopy showed the presence of a great majority of carboxyl, hydroxyl and amino groups. The tendency of cells was to adhere to superficial carbon imperfections, which after certain time were covered by a matrix of EPS.

Abstract: Ferrous iron oxidation studies in the presence of activated carbon were conducted at 30 °C in basal medium at pH 1.6 with a pure strain of Acidithiobacillus ferrooxidans. Two-chamber modified shake flasks were used in these experiments, which prevented direct contact between the microorganisms and the carbon contained in the flasks. This design permitted an accurate determination of bacterial population during the experiment and enabled the involvement of ferric iron reduction with carbon to be evidenced. Notably, iron was initially added as ferric iron in a concentration of 3 g/L. It could be observed that bacteria could grow in this condition evidencing that bacteria was in fact oxidizing ferrous ion produced from reduction of ferric by carbon. From complementary experiments in which activated carbon was contacted with abiotic solutions containing ferric ion in the concentration range 0.1 – 1.2 g/l, the chemical reductive action of carbon of ferric iron was confirmed and a kinetic expression for this reaction was determined. A mathematical model was developed which incorporated expressions for the kinetic of bacterial oxidation of ferrous ion and the chemical reduction of ferric ion. This model enabled the prediction of the rate of bacterial growth and ferrous ion oxidation in a bioreactor as a function of the initial concentrations of iron, activated carbon and bacterial population. Results in this work imply that the observed variations in activity observed by other authors during bacterial oxidation of ferrous iron with A. ferrooxidans adsorbed on carbon can be in fact related to bacterial utilization of supplementary ferrous iron produced by the chemical action of carbon, phenomenon which is not explicitly accounted for.

Abstract: The understanding of mechanisms, by which microorganisms are effecting the dissolution of metal sulfides, what in general is called bioleaching, has progressed largely during the last 15 years. Whereas in the beginning of the nineties of the last century the direct and the indirect mechanisms were state of the knowledge, nowadays it is generally accepted that contact and non-contact mechanisms besides ferric iron, perhaps with some organic sulfhydryl groups, are the relevant players [1]. Consequently, since the introduction of these ideas/hypotheses, research in this field has been revived considerably, as is obvious from the literature[2]. Also molecular biology has started to contribute to the understanding of these processes due to the clarification of the basic reactions [3, 4, 5]. Summarizing the main elements of the contact and non-contact mechanisms, we know now that in case of the contact mechanism bacteria need to attach to metal sulfide surfaces by means of their extracellular, polymeric substances (EPS) due to a primary electrostatic interaction between iron(II)ions complexed by uronic acid residues. This first interaction is followed by hydrophobic interactions (more than half of the EPS of A. ferrooxidans are lipids) and perhaps covalent bonding (speculative). These forces are active in the Angstrom range (Å). Consequently, the space between bacterial outer membrane and the surface of a metal sulfide, which is filled by EPS, ranges between 10 and 50 nm only. In this space the dissolution reactions occur.

Abstract: This paper gives a brief introduction to the modelling of bioleach processes developed from a careful analysis of the fundamental process steps at the gas-liquid, biological and mineral interfaces and how these interact in a given reactor environment (tanks and heaps). The insights gained from modelling work can guide both engineers in the optimisation of processes and scientist in directing their research at areas not yet well understood. Some perspectives of future directions of the bioleaching field are offered.

Abstract: The importance of comprehensive laboratory evaluation for development of an ore body to commercial processing using biohydrometallurgy cannot be understated. Laboratory evaluation for a biohydrometallurgical process must include the microbiological component and definition of operating parameters for the engineers to design the commercial plant. Failure to meet commercial production at a mine site can be a consequence of incomplete understanding of biohydrometallurgical technologies for processing a specific ore. One example is the inability of a copper bioleach process to meet the design criteria in part because of lack of sufficient testing to demonstrate the ramifications of fluoride toxicity to the microbial component of the bioleach process. Laboratory research has demonstrated toxicity of low levels of fluoride to Acidithiobacillus species. However, laboratory determined toxicity values are not always relevant to field conditions at commercial bioleach operations. This is the case with fluoride toxicity where complexing reactions increase the amount of fluoride required for toxicity. Consequently, the toxic fluoride concentrations at field sites can be significantly higher than toxic levels reported in the laboratory, but still achieve concentration inhibitory for the microorganisms.

Abstract: Conventionally, physico-chemical methods are used in mineral processing for recovering value minerals from ores. The ageing of ore processing tailings and waste rocks, and mining tailings contamination by chemical reagents constitute a major threat to the environment. It is imperative to develop novel economically more efficient and environmentally benign methods of flotation and waste processing, exploiting the intriguing and exciting ability of bacteria to selectively modify the surface properties of solids. Microorganisms have not only facilitate hydrometallurgical leaching operations but have also show a great promise in mineral beneficiation processes such as flotation and flocculation. Several laboratory investigations revealed that microorganisms could function similar to traditional reagents. Microorganisms have a tremendous influence on their environment through the transfer of energy, charge, and materials across a complex biotic mineral-solution interface. The bio-modification of mineral surfaces involves the complex action of microorganism on the mineral surface. The manner, in which bacteria affect the surface reactivity and the mechanism of bacteria adsorption, is still unknown and accumulation of the primary data in this area is only starting. The bio-flotation and bio-flocculation processes concern the mineral response to the bacterium presence, which is essentially interplay between microorganism and the physicochemical properties of the mineral surface, such as the atomic and electronic structure, the net charge/potential, acid-base properties, and wettability of the surface. There is an urgent need for developing basic knowledge that would underpin biotechnological innovations in the natural resource (re)processing technologies that deliver competitive solutions.

Abstract: Bioleaching is the biological conversion of an insoluble metal compound into a water soluble form. In this process metal sulfides are oxidized to metal ions and sulfate by acidophilic microorganisms capable of oxidizing Fe2+ and/or sulfur-compounds. The metal solubilization from sulfide minerals is a chemical process which requires Fe3+ reduction. It is an environmentally friendly technique and an economical method for recovering metals that requires low investment and operation costs. In this work we studied the bioleaching of two kinds of acid-soluble copper sulfides, one easily leached by mesophilic bacteria (covellite), and the other one refractory to their activity (chalcopyrite), in acidic media with or without Fe2+ ions. We studied attached and planktonic populations of autotrophic bacteria, such as Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans in pure or mixed cultures. The influence of a heterotrophic microorganism, Acidiphilium cryptum, was also studied. Attachment was evaluated with fluorescence staining and FISH using four specific probes. L. ferrooxidans showed highest initial attachment in all cases. The presence of Ap. cryptum increased the cell attachment compared with the autotrophic pure cultures. It was possible to correlate experimental data with a mechanism of bacterial-metal sulfide oxidation, the polysulfide pathway for acid- soluble metal sulfides.

Abstract: The aim of this study was to investigate interspecies interaction by quantifying pyrite dissolution and to visualize the colonization of metal sulfides in pure and mixed cultures of leaching bacteria. Strains, such as chemolithoautotrophic Acidithiobacillus ferrooxidans ATCC 23270 (type strain) and chemoorganoheterotrophic Acidiphilium cryptum JF-5, were used. Sessile, pyrite-detached and planktonic cells were visualized by epifluorescence microscopy using DAPI staining and FISH. Additionally, atomic force microscopy was used for investigations on cell morphology, bacterial distribution on pyrite and mineral surface topography. In pure At. ferrooxidans cultures and in mixed cultures with Ap. cryptum JF-5, it could be shown that the bacterial morphology of sessile cells differed significantly from those of planktonic cells by decreased cell sizes and enhanced production of extracellular polymeric substances in case of sessile cells. Interspecies interaction in mixed cultures resulted in increased pyrite leaching and production of extracellular polymeric substances and consequently, enhanced biofilm formation.

Abstract: The aim of the study was to quantify and to visualize colonization of metal sulfides by pure and mixed cultures. Strains of the genera Acidithiobacillus and Leptospirillum were tested. Sessile and planktonic cells were visualized by fluorescence microscopy using 4',6-diamidino-2-phenylindole (DAPI) and FISH. Additionally, atomic force microscopy was used for the investigations on cell morphology, spatial arrangement of cells on metal sulfides and mineral surface topography. It was shown that the morphology of sessile cells was totally different as compared with planktonic ones. Interactions of different species resulted in increased production of extracellular polymeric substances (EPS) or caused negligible-attaching bacteria to be incorporated into a biofilm by the good attaching ones. Consequently, biofilm formation was furthered.

Abstract: Ex-situ and in-situ Tapping Mode AFM were used to investigate responses of attached bacteria to stressful conditions. For ex-situ measurements, the AFM was equipped with a customised re-positioning stage and sample mount to permit re-examination of the same surface area. For in-situ measurements, the inoculated pyrite coupon was immersed in solution in a flow through cell. Initial experiments using Sulfobacillus thermosulfidooxidans indicated that increased acidity promoted EPS production but increased salinity resulted in cell detachment.