Advanced Materials Research Vols. 20-21

Abstract: Applicability of Ohshima’s soft-particle model to evaluate the surface potential of Acidithiobacillus ferrooxidans cells is discussed here. The electrokinetic properties were examined by electrophoretic mobility measurements and analyzed using the soft particle electrophoresis theory. As the ionic concentration increased, the mobility of the bacterial cells converged to nonzero values suggesting that the particles exhibited typical soft-particle characteristics. Also, cell surface potentials based on soft-particle theory were lower than those estimated using the conventional Smoluchowski theory. Effect of removal of LPS from the cell surface on surface softness and charge density are investigated and discussed.

Abstract: Biofilm development plays a pivotal role in the bioleaching process. The attachment of the acidophilic chemolithotrophic Acidithiobacillus ferrooxidans to mineral surfaces is mediated by extracellular polymeric substances (EPS) involved in biofilm development. Previous work suggests that EPS composition of A. ferrooxidans is adapted to the energy source and, accordingly, the bacterium must be able to sense the surface to which attachment occurs with the consequent triggering of the expression of different EPS-genes. Quorum sensing (QS) is recognized as one of the main regulators of biofilm formation. A. ferrooxidans possesses a functional QS type AI-1 system and the analysis of culture supernatants revealed us that this bacterium is able to synthesize nine different homoserine lactones (AHLs) whose acyl-chain lengths oscillate between 8 and 16 carbons and include an alcohol or a ketone function at the C3 position. The transcription levels of the afeI gene encoding for the AHL synthase are higher in cells grown in sulfur and thiosulfate media than in iron-grown cells, suggesting that biofilm formation in A. ferrooxidans would be regulated by the QS type AI-1 system. In the present study, the effect of several synthetic AHLs and analogues on the attachment of A. ferrooxidans to pyrite was analyzed. Preliminary results suggest that some of these molecules are changing the bacterial attachment to pyrite.

Abstract: To enhance our understanding of effects of microbially mediated pyrite dissolution and the influence parameters such as varied metabolism and crystallographic orientation of pyrite surfaces some dissolution experiments were performed. Microbial etching experiments on pyrite surfaces of different orientation, including {111} and {210} were devised. The experiments were performed using two strains of thermophilic Archaea (Sulfolobus metallicus, Metallosphaera sedula). Epifluorescence microscopy observations showed that the strains attach to the mineral surface. Studies with Scanning Electron Microscopy (SEM) showed cell attachment and etching effects after one week of incubation. Surface alteration produced structures following crystallographic orientation up to several 10 μm in size. For all incubated pyrite samples it became apparent that surface alteration was more pronounced with M. sedula than with S. metallicus.

Abstract: The catalytic influence of Sulfolobus metallicus in the bioleaching of pure chalcopyrite at 70° C and pH 1.5 was studied in shake flask experiments. In order to evaluate separately the influence of planktonic and attached cells on the catalytic process, in some experiments the microorganisms were prevented from reaching chalcopyrite surface by keeping them in a chamber separated by a 0.1 Millipore membrane. Leaching, which was conducted with a -80 # + 120# chalcopyrite sample, was characterized from monitoring Eh, pH, copper, ferrous and ferric ion concentration in solution. In addition, the concentration of different sulphur species dissolved in solution either in aerated-abiotic and aerobic - biotic conditions, was determined with HPLC. Maximum copper dissolution was reached in experiments where all the microorganisms could reach chalcopyrite. In experiments where microorganisms were maintained separated from the sulphide, the population of microorganisms still increased. However, in this case copper dissolution was only 50 % of the maximum reached when contact of microorganisms with chalcopyrite was allowed, similar to that obtained in aerobic-abiotic conditions. On the other hand, in aerobic-abiotic conditions there was formation of bisulfite (HSO3)-, bisulfate (HSO4)- and sulfate (SO4)2-, while in leaching experiments where microorganisms could reach chalcopyrite the formation of thiosulfate (S2O3)2- and sulfite (SO3)2- was additionally observed. It can be concluded that bioleaching of chalcopyrite in the presence of Sulfolobus metallicus is the result of the cooperative action of attached cells which catalize copper dissolution through formation of thiosulfate, sulfite and bisulfite, and planktonic cells which further oxidize these intermediate compound to bi-sulfate and sulfate.

Abstract: A sample of black shale fraction of copper ore from the Lubin deposit, Poland, was subjected to pretreatment by different microorganisms to improve the copper recovery during the subsequent flotation. Chemolithotrophic bacteria grown on So and S2O3 2- were used to depress pyrite before flotation by means of ethyl xantate as collector. Sulphate-reducing bacteria were used to perform a prior sulphidization of the black shales before flotation by means of the same collector. Two-stage flotation experiments were carried out in which initially the gangue minerals of the ore were depressed by means of “silicate” bacteria and then, during the flotation of the non-floated product from the first stage, the pyrite was depressed by means of pretreatment with So- grown chemolithotrophs. In all these cases the microbial pretreatment had positive effect on the copper recovery.

Abstract: Cell surface hydropobicity plays a significant role in microbe-mineral interactions with special relevance of bioleaching. The present investigation envisages a study on the hydrophobic character of Acidiphilium symbioticum KM2, an acidophilic strain of bioleaching environment, when grown in presence of heavy metals - copper, zinc, cadmium and nickel. The metals, at its sub inhibitory concentrations (MIC50), exhibited profound negative effect on the growth of the bacterium. Inhibition on the culture growth rate was highest due to cadmium followed by zinc, nickel and copper. However, upon successive adaptation in different concentrations of each metal in ascending order, the cells could grow rapidly in the presence of higher concentrations of the metals indicating good metal resistance by the bacterium. Compared to normally grown cells, A. symbioticum KM2, when grown in presence of the metals, became more hydrophobic, which was dependent on the metal and its concentration in the media. Among the four metals studied, the effect of copper was found to be the highest, where up to 74 % increase in the bacterial cell hydrophobicity was observed.

Abstract: Acidithiobacillus ferrooxidans cells grown in ferrous ions were used to study the surface modification of pyrite and chalcopyrite, with focus on coagulation of very fine particles (-5 m). The zeta-potential studies of the minerals, before and after bacterial treatment, showed that the cells have a distinct influence on the surface charge of pyrite and chalcopyrite. The maximum coagulation of particles determined by Turbiscan as a function of pH correlated well with the zetapotential results. Using diffuse reflectance FT-IR spectroscopic studies, the adhesion of cells showed a varied influence on these minerals. The results demonstrate that Acidithiobacillus ferrooxidans interact with pyrite and chalcopyrite differently, allowing selective coagulation of one mineral from the other under different pH conditions.

Abstract: Leaching bacteria attach to their substrates, i.e. mineral sulfides, and form monolayered biofilms. In this study the biofilm formation of Acidithiobacillus ferrooxidans A2 on pyrite was examined using atomic force and epifluorescence microscopy (AFM and EFM, respectively). A novel system by JPK instruments, the BioMaterial WorkstationTM, allows the investigation of the same location on an opaque sample with AFM and EFM. Until recently this was only possible for translucent samples. Sessile bacteria on pyrite coupons were stained with 4’,6-diamidino-2- phenylindol (DAPI) and visualized by EFM as well as AFM. The best imaging conditions for AFM were assessed. Scans of bacteria attached to pyrite were performed in contact mode in air as well as in tapping mode in fluid. Imaging in fluid was more challenging than imaging in air as bacteria tend to detach from their substratum. To avoid the dislocation of microorganisms by the AFM probe the sample was dried in air for 1 h prior to scanning in fluid. Scanning in air was performed with the whole range of cantilever spring constants tested (k = 0.03 N/m to k = 0.65 N/m) while, for scanning in fluid, best results were achieved using stiffer cantilevers (k = 0.65 N/m).

Abstract: Microbially influenced corrosion (MIC) of steel has gained increasing attention in recent years because the damage caused by this process is a significant cost factor for industry. Consequently, inhibition of corrosion and especially the development of corrosion protective films is an important present-day research topic. In this connection, application of microbially produced EPS for mitigating steel corrosion is an innovative idea. However, observations of ”protective” biofilms on metallic surfaces have been previously reported. Their inhibiting effect is generally thought to be caused by oxygen depletion or the formation of passivating layers. In contrast to many conventional corrosion protective methods, EPS or EPS-derived agents would be a cheap and environmentally friendly solution. Extensive research activities are still required, before biofilms or cell-free EPS can be used for corrosion protection on larger scale. In this study, we are developing a novel EPS-based corrosion protection method for unalloyed and corrosion resistant steel in aqueous media, which is based upon the application of microbial metabolic products. EPS of various sulfatereducing bacteria and other microorganisms are investigated for their inhibiting effect. The extent of such inhibition is evaluated in a model test system, in which different steels are subjected to corrosive conditions under sulfate-reducing conditions. To elucidate the protective mechanisms, comparative analyses of the chemical composition of the applied EPS are performed.