Dominique Morin

Characterization of arsenopyrite oxidizing Thiobacillus. Tolerance to arsenite, arsenate, ferrous and ferric iron

Two strains of Thiobacillus, T. ferrooxidans and T. thiooxidans, have been isolated from a bacterial inoculum cultivated during a one-year period in a 1001 continuous laboratory pilot for treatment of an arsenopyrite/pyrite concentrate. The optimum pH for the growth of both strains has been found to be between 1.7 and 2.5. Because of the high metal toxicity in bioleach pulps, the tolerance of T. ferrooxidans and T. thiooxidans with respect to iron and arsenic has been studied. The growth of both strains is inhibited with 10 g/l of ferric ion, 5 g/l of arsenite and 40 g/l of arsenate. 20 g/l of ferrous iron is toxic to T. ferrooxidans but 30 g/l is necessary to impede the growth of T. thiooxidans.

Precipitation of arsenic sulphide from acidic water in a fixed-film bioreactor

Arsenic (As) is a toxic element frequently present in acid mine waters and effluents. Precipitation of trivalent arsenic sulphide in sulphate-reducing conditions at low pH has been studied with the aim of removing this hazardous element in a waste product with high As content. To achieve this, a 400m L fixed-film column bioreactor was fed continuously with a synthetic solution containing 100mg L(-1) As(V), glycerol and/or hydrogen, at pH values between 2.7 and 5. The highest global As removal rate obtained during these experiments was close to 2.5mg L(-1)h(-1). A switch from glycerol to hydrogen when the biofilm was mature induced an abrupt increase in the sulphate-reducing activity, resulting in a dramatic mobilisation of arsenic due to the formation of soluble thioarsenic complexes. A new analytical method, based on ionic chromatography, was used to evaluate the proportion of As present as thioarsenic complexes in the bioreactor. Profiles of pH, total As and sulphate concentrations suggest that As removal efficiency was linked to solubility of orpiment (As(2)S(3)) depending on pH conditions. Molecular fingerprints revealed fairly homogeneous bacterial colonisation throughout the reactor. The bacterial community was diverse and included fermenting bacteria and Desulfosporosinus-like sulphate-reducing bacteria. arrA genes, involved in dissimilatory reduction of As(V), were found and the retrieved sequences suggested that As(V) was reduced by a Desulfosporosinus-like organism. This study was the first to show that As can be removed by bioprecipitation of orpiment from acidic solution containing up to 100mg L(-1) As(V) in a bioreactor.

Isolation and study of two strains of Leptospirillum-like bacteria from a natural mixed population cultured on a cobaltiferous pyrite substrate

Two strains ofLeptospirillum-like bacteria, L6 and L8, have been isolated from a mixed inoculum, also containingThiobacillus ferrooxidans andT. thiooxidans, cultured for one year with a colbaltiferous pyrite as energy substrate in a 100 I continuous bioleaching laboratory unit. Several physiological properties of the strains are described. The vibrio-shaped microorganisms grew at pH values lower than 1.3. Their growth rate was maximum between 2.5 and 8.0 g l1 ferrous iron. The optimal growth temperature was 37.5° C. Ferric iron had a stimulative effect on bacterial development up to 8 g l−1, and growth was as rapid at 14 g l−1 ferric iron as at 8 g l−1. The negative influence of cobalt on the final cell concentration was observed at 0.5 g l−1, but the growth rate was not affected up to 2 g l−1. The G + C content of strains L8 is 55.6 mol%.

Continuous Bioleaching of a Cobaltiferous Pyrite in Stirred Reactors: Population Dynamics and EPS Production vs. Bioleaching Performances

In the frame of a European project (BioMinE - FP6), a continuous bioleaching operation was carried out in a laboratory-scale unit using a cobaltiferous pyrite. The objective of the work was to use this system to investigate mechanisms of microbial activity and mineral oxidation in continuous stirred bioreactors (1x50L - 3x20L). A combination of scientific and technical approaches (molecular ecology, biochemistry and microscopy) was used and various key operating parameters were tested (temperature, nitrogen source, CO2 availability, designed consortia). An increase of temperature of 10°C (35°C to 45°C) had no major influence on the bioleaching efficiency. When the ammonium source was limiting, there was a negative influence on both bacterial growth and bioleaching efficiency. This result was related to a combination of factors such as less bacterial attachment to the pyrite surface and less precipitate formation. CO2 limitation had a very significant negative effect on the bacterial productivity and consequently on the bioleaching efficiency. Nevertheless, the population composition remained unchanged. An important decrease of EPS (sugar) production was also observed. The bacterial strains, that dominate the culture, originated from the deposit in Uganda. It seems that their bioleaching ability was improved over the time when cultured in continuous mode. The culture composition was very stable. The iron-oxidizer L. ferriphilum was the dominant organism in standard (not limiting) conditions, and was always very well represented during the first 3-4 days of residence time. Sulfobacillus sp. BRGM2 also played an important role in the process. This study gives new insights for the application of this technology, and more specifically on the influence of key operating parameters on bioleaching performances, population dynamics and attachment of bacteria to the solid surfaces.

Adaptation and Evolution of Microbial Consortia in a Stirred Tank Reactor Bioleaching System: Indigenous Population versus a Defined Consortium

To participate in the investigation concerning these key questions, the bioleaching of a cobaltiferous pyrite by two different microbial consortia was studied. The first was an indigenous population taken from industrial bioreactors used to treat a cobaltiferous pyrite at the Kasese Cobalt Company site in Uganda [2]. The second was a defined consortium comprising four organisms which had been found to dominate the indigenous population. These organisms had been maintained in synthetic media as pure cultures in order to observe whether they would loose any adaptational advantages accrued during the period spent in the bioreactor system.

Preservation of ancestral Cretaceous microflora recovered from a hypersaline oil reservoir

Microbiology of a hypersaline oil reservoir located in Central Africa was investigated with molecular and culture methods applied to preserved core samples. Here we show that the community structure was partially acquired during sedimentation, as many prokaryotic 16S rRNA gene sequences retrieved from the extracted DNA are phylogenetically related to actual Archaea inhabiting surface evaporitic environments, similar to the Cretaceous sediment paleoenvironment. Results are discussed in term of microorganisms and/or DNA preservation in such hypersaline and Mg-rich solutions. High salt concentrations together with anaerobic conditions could have preserved microbial/molecular diversity originating from the ancient sediment basin wherein organic matter was deposited.

Bioprecipitation of Arsenic Sulphide at Low pH

A process for the precipitation of trivalent arsenic sulphide in sulphate-reducing condition at low pH would be very attractive due to the high arsenic content (60%) in the final precipitate. A bacterial consortium able to reduce sulphate at pH 4.5 served to inoculate column bioreactors that were continuously fed with As(V) or As(III), glycerol and/or hydrogen, at pH values between 2 and 5. The diversity, functionality and evolution of the consortium colonizing the bioreactors were characterized by means of biomolecular tools, in relation with operating parameters (pH, As, sulphide, acetate). The highest As removal rate obtained during these experiments was close to 3 mg.l-1.h-1 using As(V) as the initial arsenic form, while precipitation rates were improved using As(III). When glycerol was replaced by hydrogen in a bioreactor containing a mature biofilm, sulphate-reducing activity increased roughly. Organisms related to Desulfosporosinus were the only sulphate-reducing bacterium (SRB) detected in the bioreactor. arrA genes, involved in As(V) dissimilatory reduction, were also detected and suggested that As(V) was reduced by a Desulfosporosinus-like organism. Molecular fingerprints evidenced an evolution of the bacterial population structure according to changes in operating conditions.