A Novel Mineral Flotation Process Using Thiobacillus ferrooxidans

Oxidative leaching of metals by Thiobacillus ferrooxidans has proven useful in mineral processing. Here, we report on a new use for T. ferrooxidans, in which bacterial adhesion is used to remove pyrite from mixtures of sulfide minerals during flotation. Under control conditions, the floatabilities of five sulfide minerals tested (pyrite, chalcocite, molybdenite, millerite, and galena) ranged from 90 to 99%. Upon addition of T. ferrooxidans, the floatability of pyrite was significantly suppressed to less than 20%. In contrast, addition of the bacterium had little effect on the floatabilities of the other minerals, even when they were present in relatively large quantities: their floatabilities remained in the range of 81 to 98%. T. ferrooxidans thus appears to selectively suppress pyrite floatability. As a consequence, 77 to 95% of pyrite was removed from mineral mixtures while 72 to 100% of nonpyrite sulfide minerals was recovered. The suppression of pyrite floatability was caused by bacterial adhesion to pyrite surfaces. When normalized to the mineral surface area, the number of cells adhering to pyrite was significantly larger than the number adhering to other minerals. These results suggest that flotation with T. ferrooxidans may provide a novel approach to mineral processing in which the biological functions involved in cell adhesion play a key role in the separation of minerals.     

Mechanism of microbial flotation using Thiobacillus ferrooxidans for pyrite suppression

Microbial desulfurization might be developed as a new process for the removal of pyrite sulfur from coal sluries such as coal-water mixture (CWM). An application of iron-oxidizing bacterium Thiobacillus ferrooxidans to flotation would shorten the periods of the microbial removal of pyrite from some weeks by leaching methods to a few minutes. The floatability of pyrite in flotation was mainly reduced by T. ferrooxidans itself rather than by other microbial substances in bacterial culture as additive of flotation liquor. Floatability was suppressed within a few seconds by bacterial contact. The suppression was proportional to increasing the number of cells observed between bacterial adhesion and the suppression of floatability. If 25% of the total pyrite surface area covered with the bacteria, pyrite floatability would be completely depressed. Bacteria that lost their iron-oxidizing activities by sodium cyanide treatment were also able to adhere to pyrite and reduced pyrite floatability as much as normal bacteria did. Thiobacillus ferrooxidans ATCC 23270, T-1, 9, and 11, which had different iron-oxidizing abilities, suppressed floatability to similar-levels. The oxidizing ability of bacteria did not influence the suppressing effect. These results showed the mechanism of the suppression of pyrite floatability by bacteria. Quick bacterial adhesion to pyrite induced floatability suppression by changing the surface property from hydrophobic. The quick adhesion of the bacterium was the novel function which worked to change the surface property of pyrite to remove it from coal. (c) 1993 John Wiley & Sons, Inc.     

Suppression of pyritic sulphur during flotation tests using the bacterium Thiobacillus ferrooxidans

Environmental concern about sulphur dioxide emissions has led to the examination of the possibility of removing pyritic sulphur from coal prior to combustion during froth flotation, a routine method for coal cleaning at the pit-head. The bacterium Thiobacillus ferrooxidans was effective in leaching 80% and 63% -53 mum pyrite at 2% and 6% pulp density in shake flasks in 240 and 340 h, respectively.The natural floatability of pyrite was significantly reduced in the Hallimond tube following 2.5 min of conditioning in membrane-filtered bacterial liquor prior to flotation. The suppression effect was greatly enhanced in the presence of Thiobacillus ferrooxidans. A bacterial suspension in pH 2.0 distilled water showed 85% suppression, whereas in spent growth liquor this value was 95%. The optimum bacterial density was 3.25 x 10(10) cells/g pyrite in 230-ml distilled water (2% pulp density) in the Hallimond tube. The degree of suppression by the cells was related to particle size but not to pH or temperature. The sulphur content of a synthetic coal/pyrite mixture was reduced from 10.9 to 2.1% by flotation after bacterial preconditioning. It is postulated that pyrite removal in coals which are cleaned by froth flotation could be significantly reduced using a bacterial preconditioning stage with a short residence time of 2.5 min.     

Enzyme-Linked Immunofiltration Assay To Estimate Attachment of Thiobacilli to Pyrite

An enzyme-linked immunofiltration assay (ELIFA) has been developed in order to estimate directly and specifically Thiobacillus ferrooxidans attachment on sulfide minerals. This method derives from the enzyme-linked immunosorbent assay but is performed on filtration membranes which allow the retention of mineral particles for a subsequent immunoenzymatic reaction in microtiter plates. The polyclonal antiserum used in this study was raised against T. ferrooxidans DSM 583 and recognized cell surface antigens present on bacteria belonging to the genus Thiobacillus. This antiserum and the ELIFA allowed the direct quantification of attached bacteria with high sensitivity (10⁴ bacteria were detected per well of the microtiter plate). The mean value of bacterial attachment has been estimated to be about 10⁵ bacteria mg⁻¹ of pyrite at a particle size of 56 to 65 μm. The geometric coverage ratio of pyrite by T. ferrooxidans ranged from 0.25 to 2.25%. This suggests an attachment of T. ferrooxidans on the pyrite surface to well-defined limited sites with specific electrochemical or surface properties. ELIFA was shown to be compatible with the measurement of variable levels of adhesion. Therefore, this method may be used to establish adhesion isotherms of T. ferrooxidans on various sulfide minerals exhibiting different physicochemical properties in order to understand the mechanisms of bacterial interaction with mineral surfaces.     

矿物作用下A.ferrooxidans中磁小体相关基因的差异表达

目的:研究氧化亚铁硫杆菌(Acidithiobacillus ferrooxidans,A.f)中与磁小体形成相关的mpsA、magA、thy和mamB基因分别在黄铁矿、黄铜矿、磁黄铁矿和闪锌矿的作用下的表达差异,寻找有利于磁小体形成的最佳培养矿物能源。方法:测量以不同硫化矿为能源时的菌体生长特性,用实时定量PCR方法研究与磁小体形成相关基因的表达差异。结果:在以磁黄铁矿为能源时,菌的生长量及多数基因的表达量优于其它三种矿,四个基因相对表达量分别为1.15、2.35、1.32、2.68。结论:磁黄铁矿是A.f中磁小体形成的最佳矿物能源。     

Bacterial leaching of a sulfide ore by Thiobacillus ferrooxidans and Thiobacillus thiooxidans: I. Shake flask studies

Bacterial leaching of a sulfide ore containing pyrite, chalcopyrite, and sphalerite was studied in shake flask experiments using Thiobacillus ferrooxidans and Thiobacillus thiooxidans strains isolated from mine sites. The Fe(2+)grown T. ferrooxidans isolates solubilized sphalerite preferentially over chalcopyrite leaching 7-10% Cu, 68-76% Zn, and 10-22% Fe from the ore in 18 days. The sulfur grown T. thiooxidans isolates leached Zn much more slowly and very little Fe, with a Cu-Zn extraction ratio twice the value obtained with T. ferrooxidans. The ore adapted T. ferrooxidans started solubilizing Cu and Zn without a lag period. The ore-adapted T. thiooxidans extracted Cu as well as T. ferrooxidans, but the extraction of Zn or Fe was still much slower in the low-phosphate medium, while in the high-phosphate medium it approached the value obtained with T. ferrooxidans. A high Cu-Zn extraction ratio of 0.34 was obtained with T. thiooxidans in the low phosphate medium. In the mixed-culture experiments with T. ferrooxidans and T. thiooxidans, the culture behaved as T. thiooxidans in the low-phosphate medium with a higher Cu-Zn extraction ratio and as T. ferrooxidans in the high-phosphate medium with a lower Cu-Zn extraction ratio. It is concluded that T. ferrooxidans and T. thiooxidans solubilize sulfide minerals by different mechanisms.     

Selective Adhesion of Thiobacillus ferrooxidans to Pyrite

Bacterial adhesion to mineral surfaces plays an important role not only in bacterial survival in natural ecosystems, but also in mining industry applications. Selective adhesion was investigated with Thiobacillus ferrooxidans by using four minerals, pyrite, quartz, chalcopyrite, and galena. Escherichia coli was used as a control bacterium. Contact angles were used as indicators of hydrophobicity, which was an important factor in the interaction between minerals and bacteria. The contact angle of E. coli in a 0.5% sodium chloride solution was 31°, and the contact angle of T. ferrooxidans in a pH 2.0 sulfuric acid solution was 23°. E. coli tended to adhere to more hydrophobic minerals by hydrophobic interaction, while T. ferrooxidans selectively adhered to iron-containing minerals, such as pyrite and chalcopyrite. Ferrous ion inhibited the selective adhesion of T. ferrooxidans to pyrite competitively, while ferric ion scarcely inhibited such adhesion. When selective adhesion was quenched by ferrous ion completely, adhesion of T. ferrooxidans was controlled by hydrophilic interactions. Adhesion of E. coli to pyrite exhibited a liner relationship on langmuir isotherm plots, but adhesion of T. ferrooxidans did not. T. ferrooxidans recognized the reduced iron in minerals and selectively adhered to pyrite and chalcopyrite by a strong interaction other than the physical interaction.     

Mineral and iron oxidation at low temperatures by pure and mixed cultures of acidophilic microorganisms

An enrichment culture from a boreal sulfide mine environment containing a low-grade polymetallic ore was tested in column bioreactors for simulation of low temperature heap leaching. PCR-denaturing gradient gel electrophoresis and 16S rRNA gene sequencing revealed the enrichment culture contained an Acidithiobacillus ferrooxidans strain with high 16S rRNA gene similarity to the psychrotolerant strain SS3 and a mesophilic Leptospirillum ferrooxidans strain. As the mixed culture contained a strain that was within a clade with SS3, we used the SS3 pure culture to compare leaching rates with the At. ferrooxidans type strain in stirred tank reactors for mineral sulfide dissolution at various temperatures. The psychrotolerant strain SS3 catalyzed pyrite, pyrite/arsenopyrite, and chalcopyrite concentrate leaching. The rates were lower at 5 degrees C than at 30 degrees C, despite that all the available iron was in the oxidized form in the presence of At. ferrooxidans SS3. This suggests that although efficient At. ferrooxidans SS3 mediated biological oxidation of ferrous iron occurred, chemical oxidation of the sulfide minerals by ferric iron was rate limiting. In the column reactors, the leaching rates were much less affected by low temperatures than in the stirred tank reactors. A factor for the relatively high rates of mineral oxidation at 7 degrees C is that ferric iron remained in the soluble phase whereas, at 21 degrees C the ferric iron precipitated. Temperature gradient analysis of ferrous iron oxidation by this enrichment culture demonstrated two temperature optima for ferrous iron oxidation and that the mixed culture was capable of ferrous iron oxidation at 5 degrees C.     

Biomining with bacteriophage: selectivity of displayed peptides for naturally occurring sphalerite and chalcopyrite

During mineral processing, concentrates of sulfide minerals of economic interest are formed by froth flotation of fine ore particles. The method works well but recovery and selectivity can be poor for ores with complex mineralogy. There is considerable interest in methods that improve the selectivity of this process while avoiding the high costs of using flotation chemicals. Here we show the first application of phage biotechnology to the processing of economically important minerals in ore slurries. A random heptapeptide library was screened for peptide sequences that bind selectively to the minerals sphalerite (ZnS) and chalcopyrite (CuFeS2). After several rounds of enrichment, cloned phage containing the surface peptide loops KPLLMGS and QPKGPKQ bound specifically to sphalerite. Phage containing the peptide loop TPTTYKV bound to both sphalerite and chalcopyrite. By using an enzyme-linked immunosorbant assay (ELISA), the phage was characterized as strong binders compared to wild-type phage. Specificity of binding was confirmed by immunochemical visualization of phage bound to mineral particles but not to silica (a waste mineral) or pyrite. The current study focused primarily on the isolation of ZnS-specific phage that could be utilized in the separation of sphalerite from silica. At mining sites where sphalerite and chalcopyrite are not found together in natural ores, the separation of sphalerite from silica would be an appropriate enrichment step. At mining sites where sphalerite and chalcopyrite do occur together, more specific phage would be required. This bacteriophage has the potential to be used in a more selective method of mineral separation and to be the basis for advanced methods of mineral processing.     

A new look at microbial leaching patterns on sulfide minerals

Leaching patterns on sulfide minerals were investigated by high-resolution scanning electron microscopy (SEM). Our goal was to evaluate the relative contributions of inorganic surface reactions and reactions localized by attached cells to surface morphology evolution. Experiments utilized pyrite (FeS(2)), marcasite (FeS(2)) and arsenopyrite (FeAsS), and two iron-oxidizing prokaryotes in order to determine the importance of cell type, crystal structure, and mineral dissolution rate in microbially induced pit formation. Pyrite surfaces were reacted with the iron-oxidizing bacterium Acidithiobacillus ferrooxidans (at 25 degrees C), the iron-oxidizing archaeon 'Ferroplasma acidarmanus' (at 37 degrees C), and abiotically in the presence of Fe(3+) ions. In all three experiments, discrete bacillus-sized (1-2 μm) and -shaped (elliptical) pits developed on pyrite surfaces within 1 week of reaction. Results show that attaching cells are not necessary for pit formation on pyrite. Marcasite and arsenopyrite surfaces were reacted with A. ferrooxidans (at 25 degrees C) and 'F. acidarmanus' (at 37 degrees C). Cell-sized and cell-shaped dissolution pits were not observed on marcasite or arsenopyrite at any point during reaction with A. ferrooxidans, or on marcasite surfaces reacted with 'F. acidarmanus'. However, individual 'F. acidarmanus' cells were found within individual shallow (<0.5 μm deep) pits. The size and shape (round rather than elliptical) of the pits conformed closely to the shape of F. acidarmanus (cells) pits on arsenopyrite. We infer these pits to be cell-induced. We attribute the formation of pits readily detectable (by SEM) to the higher reactivity of arsenopyrite compared to pyrite and marcasite under the conditions the experiment was conducted. These pits contributed little to the overall surface topographical evolution, and most likely did not significantly increase surface area during reaction. Our results suggest that overall sulfide mineral dissolution may be dominated by surface reactions with Fe(3+) rather than by reactions at the cell-mineral interface.     

Chemolithotrophic Bacteria in Copper Ores Leached at High Sulfuric Acid Concentration

Extensive bacterial growth was observed when copper sulfide ores were leached with 0.6 N sulfuric acid. The bacterial population developed in this condition was examined by characterization of the spacer regions between the 16S and 23S rRNA genetic loci obtained after PCR amplification of the DNA extracted from the leached ore. The spacers observed had the sizes found in strains of "Leptospirillum ferrooxidans" and Thiobacillus thiooxidans, except for a larger one, approximately 560 bp long, that was not observed in any of the strains examined, including those of Thiobacillus ferrooxidans. The bacteria with this last spacer were selected after culturing in mineral and elemental sulfur media containing 0.7 N sulfuric acid. The spacer and the 16S ribosomal DNA of this isolate were sequenced and compared with those in species commonly found in bioleaching processes. Though the nucleotide sequence of the spacer showed an extensive heterologous region with T. thiooxidans, the sequence of its 16S rDNA gene indicated a close relationship (99.85%) with this species. These results indicate that a population comprised of bacterial strains closely related to T. thiooxidans and of another strain, possibly related to "L. ferrooxidans," can develop during leaching at high sulfuric acid concentration. Iron oxidation in this condition is attributable to "L. ferrooxidans" and not T. ferrooxidans, based on the presence of spacers with the "L. ferrooxidans" size range and the absence of spacers characteristic of T. ferrooxidans.     

Enumeration of Thiobacilli within pH-Neutral and Acidic Mine Tailings and Their Role in the Development of Secondary Mineral Soil

The Lemoine tailings of Chibougamau, Quebec, Canada, were deposited as a pH-neutral mineral conglomerate consisting of aluminum-silicates, iron-aluminum-silicates, pyrite, chalcopyrite, and sphalerite. These tailings are colonized by an active population of Thiobacillus ferrooxidans which is localized to an acid zone occupying 40% of the tailings' surface. This population peaked at 7 × 10⁸ most probable number per gram of tailings during July and August 1990 and extended to a depth of 40 cm from the surface. Examination of samples over this depth profile by transmission electron microscopy and electron dispersive spectroscopy revealed a microbially mediated mineral transition from sulfides (below 40 cm) to chlorides and phosphates (at the surface). Silicate minerals were unaltered by microbial action. Transmission electron microscopy showed a tight association between Thiobacillus species and the sulfide minerals, which helps account for their prominence in tailings environments. Accurate enumeration of T. ferrooxidans from tailings required the disruption of their bonding to the mineral interface. Vortexing of a 10% aqueous suspension of the tailings material prior to most-probable-number analysis best facilitated this release. Even though heavy metals were highly mobile under acidic conditions at the Lemoine tailings, it was evident by transmission electron microscopy and electron dispersive spectroscopy that they were being immobilized as bona fide fine-grain minerals containing iron, copper, chlorine, phosphorus, and oxygen on bacterial surfaces and exopolymers. This biomineralization increased with increasing bacterial numbers and was most evident in the upper 3 cm of the acidic zone.     

Desulfurization of coal by microbial column flotation

Twenty-three strains capable of oxidizing iron were isolated from coal and ore storage sites as well as coal and ore mines, volcanic areas, and hot spring. Four strains were found to have high iron-oxidizing activity. One strain (T-4) was selected for this experiment since the strain showed the fastest leaching rate of iron and sulfate from pyrite among the four strains. The T-4 strain was assigned for Thiobacillus ferrooxidans from its cultural and morphological characteristics.Bacterial treatment was applied to column flotation. An increase of cell density in the microbial column flotation resulted in the increase of pyrite removal from a coal-pyrite mixture (high sulfur imitated coal) with corresponding decrease of coal recovery. The addition of kerosene into the microbial column flotation increased the recovery of the imitated coal from 55% (without kerosene) to 81% (with 50 muL/L kerosene) with the reduction of pyrite sulfur content from 11% (feed coal) to 3.9% (product coal). The kerosene addition could reduce the pyritic sulfur content by collecting the coal in the recovery. However, the addition could not enhance separation of pyrite from the coal-pyrite mixture, since pyrite rejection was not affected by the increase of the kerosene addition. An excellent separation was obtained by the microbial flotation using a long column which had a length-diameter (L/D) ratio of 12.7. The long column flotation reduced the pyritic sulfur content from 11% (feed coal) to 1.8% (product coal) when 80% of the feed coal was recovered without the kerosene addition. The long column flotation not only attained an excellent separation but also reduced the amount of cells for desulfurization to as little as one-tenth of the reported amount. (c) 1994 John Wiley & Sons, Inc.     

Effect of applied potentials on the activity and growth of Thiobacillus ferrooxidans

The effect of applied DC potentials both in the positive and negative range, on the activity and growth of Thiobacillus ferrooxidans, is discussed. In general, application of positive potentials up to +1000 mV in an acid bioleaching medium was found to be detrimental to bacterial activity, while the impression of negative potentials enhanced both their activity and growth through electrochemical regeneration of ferrous ions and an increase in the biomass. Ferrous-ferric ratios in a bioleaching medium could be monitored through Eh measurements.Among the base sulfide minerals such as pyrite, chalcopyrite, and sphalerite, sphalerite could be selectively bioleached if an impressed potential of -500 mV (SCE) could be maintained in the leaching medium. Electrochemical bioleaching tests carried out under an applied potential of -500 mV with sphalerite in the presence and absence of noble minerals such as pyrite and chalcopyrite indicated enhanced zinc dissolution with negligible copper and iron in solution. Probable mechanisms and advantages of the electrochemical bioleaching process developed in the laboratory are outlined.     

Influence of lipopolysaccharides on the attachment of Thiobacillus ferrooxidans to minerals

The loss of part of the lipopolysaccharides (LPS) of the outer membrane of T. ferrooxidans negatively influenced the attachment of the bacteria to minerals and the bioleaching process. LPS previously extracted from T. ferrooxidans and which had come into contact with pyrite inhibited the attachment of cells to minerals and also negatively affected the bioleaching. These results suggest that LPS play an important role in the attachment of the microorganisms and therefore, its presence or absence could affect the bioleaching process.     

Bacterial leaching of manganese ores

Leaching of various types of ores, containing 12-30% manganese, by the thiobacterium Acidithiobacillus ferrooxidans was studied. Leaching of reduced ores (manganocalcite and manganiferous limestone) was mediated mainly by degradation of manganiferous minerals (by sulfuric acid produced in the course of bacterial oxidation of pyrite or sulfur). Bacterial treatment of the ores for 144 and 192 h allowed solubilization of 96-98% of manganese. Inoculation of bacteria into pulp with pyrite increased the rate of leaching of oxide ore (psilomelane) by 37%, and the degree of its extraction within 180 h increased from 80 to 97%.     

Quantification of the intragranular porosity formed in bioleaching of pyrite by Thiobacillus ferroxidans

During the bacterial oxidation of a pure pyrite by Thiobacillus ferrooxidans, a great number of corrosion tunnels appear that are easily revealed by scanning electron microscopy observations. This involves an increase in the surface area without significant granulometric reduction of mineral grains. Thus, the evaluation of intragranular porosity, determined by elution front analysis, allows one to estimate accurately the fraction of oxidized sulphide, because of the development of deep holes (propagating pore mechanism). After 60 days of bioleaching, the intragranular porosity represents about 34% of the initial sulphide volume, which corresponds to 25 km of tunnels (2 mum i.d.) per gram of pyrite. On other hand, the granulometric reduction ( approximately 7%) is responsible for a 23% decrease of the initial sulphide volume. The elution front analysis appears as a nondestructive method for measuring the intragranular porosity of the bioleached pyrite.     

Analysis of Genes and Proteins in Acidithiobacillus ferrooxidans During Growth and Attachment on Pyrite Under Different Conditions

Microbes are able to enhance the sulfide mineral decomposition, which lead to the formation of AMD. Attachment of bacterial cells to the mineral surface is an important process for pyrite oxidation by Acidithiobacillus ferrooxidans. The selective advantage of bacterial adhesion is considered to favor the surface localization of bacterial populations as nutritionally favorable. Environmental factors determine cell accumulation or dissociation of attachment. In our study, the amount of sessile cells increased rapidly during the initial stage of attachment on pyrite. Planktonic cells showed high activity leading to the accumulation of large colonies on the pyrite surface. We found three proteins to be up-regulated significantly. Additionally, by matching the sequences of the three proteins to the Pfam database, we found that they are related to adhesion, pili biosynthesis and movement. When we replaced pyrite with glass to provide an inert surface that abolished electrostatic forces, we found that cell attachment was maintained under nutrient-rich conditions but drastically reduced under conditions of limited nutrients or the presence of the inhibitor homoserinelactone. Our results are consistent with the idea that starvation may lead to inhibition of attachment by an unknown mechanism that allows bacteria to search for nutrient-rich habitats.     

Occurrences at mineral-bacteria interface during oxidation of arsenopyrite by Thiobacillus ferrooxidans

The combination of an improved bacterial desorption method, scanning electron microscopy (SEM), diffuse reflectance and transmission infrared Fourier transform spectroscopy, and a desorption-leaching device like high-pressure liquid chromatography (HPLC) was used to analyze bacterial populations (adhering and free bacteria) and surface-oxidized phases (ferric arsenates and elemental sulfur) during the arsenopyrite biooxidation by Thiobacillus ferrooxidans. The bacterial distribution, the physicochemical composition of the leachate, the evolution of corrosion patterns, and the nature and amount of the surface-oxidized chemical species characterized different behavior for each step of arsenopyrite bioleaching. The first step is characterized by a slow but strong adhesion of bacteria to mineral surfaces, the appearance of a surface phase of elemental sulfur, the weak solubilization of Fe(II), As(III), and As(V), and the presence of the first corrosion patterns, which follow the fragility zones and the crystallographic orientation of mineral grains. After this short step, growth of the unattached bacteria begins, while ferrous ions in solution are oxidized by them. Ferric ions produced by the bacteria can oxidize the sulfide directly and are regenerated by Fe(II) bacterial oxidation. At this time, a bioleaching cycle takes place and a coarse surface phase of ferric arsenate (FeAsO(4) . xH(2)O where x approximately 2) and deep ovoid pores appear. At the end of the bioleaching cycle, the high concentration of Fe(III) and As(V) in solution promotes the precipitation of a second phase of amorphous ferric arsenate (FeAsO(4) . xH(2)O where x approximately 4) in the leachate. Then the biooxidation process ceases: The bacteria adhering to the mineral sufaces are coated by the ferric arsenates and the concentration of Fe(III) on the leachate is found to have decreased greatly. Both oxidation mechanisms (direct and indirect oxidation) have been stopped. (c) 1995 John Wiley & Sons, Inc.     

Toxicity of arsenic during high temperature bioleaching of gold-bearing arsenical pyrite

 A moderately thermophilic mixed culture, MT, and the thermophilic Sulfolobus acidocaldarius strain BC were studied for their response to arsenic in a defined medium and also in media containing a pyrite and an arsenical pyrite flotation concentrate. In defined medium, the individual constituents of the MT culture exhibited a high tolerance to arsenite and arsenate compared to S. acidocaldarius strain BC. When grown on increasing concentrations of the pyrite flotation concentrate, both cultures had similar specific leaching rates over the various concentrations of the mineral substrate. In contrast, S. acidocaldarius strain BC exhibited a decreasing specific leaching rate when grown on the arsenical pyrite while the MT culture was not affected. In addition, arsenic added to cultures of S. acidocaldarius strain BC growing with pyrite as a growth substrate inhibited further growth, while added arsenic had no effect on the MT culture growing on the pyrite. These data indicate that the moderately thermophilic, arsenic-resistant MT culture was able to leach arsenical pyrite more efficiently than was the S. acidocaldarius strain BC culture at high concentrations of the mineral. This emphasizes the fact that proper culture selection is an important parameter when developing commercial processes involving arsenic-containing minerals. Received: 21 June 1995/Received revision: 25 August 1995/Accepted: 7 September 1995