Pauliina Nurmi

Oxidation of elemental sulfur, tetrathionate and ferrous iron by the psychrotolerant Acidithiobacillus strain SS3.

Mesophilic iron and sulfur-oxidizing acidophiles are readily found in acid mine drainage sites and bioleaching operations, but relatively little is known about their activities at suboptimal temperatures and in cold environments. The purpose of this work was to characterize the oxidation of elemental sulfur (S(0)), tetrathionate (S4O6(2-)) and ferrous iron (Fe2+) by the psychrotolerant Acidithiobacillus strain SS3. The rates of elemental sulfur and tetrathionate oxidation had temperature optima of 20 degrees and 25 degrees C, respectively, determined using a temperature gradient incubator that involved narrow (1.1 degrees C) incremental increases from 5 degrees to 30 degrees C. Activation energies calculated from the Arrhenius plots were 61 and 89 kJ mol(-1) for tetrathionate and 110 kJ mol(-1) for S(0) oxidation. The oxidation of elemental sulfur produced sulfuric acid at 5 degrees C and decreased the pH to approximately 1. The low pH inhibited further oxidation of the substrate. In media with both S(0) and Fe2+, oxidation of elemental sulfur did not commence until all available ferrous iron was oxidized. These data on sequential oxidation of the two substrates are in keeping with upregulation and downregulation of several proteins previously noted in the literature. Ferric iron was reduced to Fe2+ in parallel with elemental sulfur oxidation, indicating the presence of a sulfur:ferric iron reductase system in this bacterium.

Temperature Effects on the Iron Oxidation Kinetics of a Leptospirillum ferriphilum Dominated Culture at pH Below One

In this study, ferrous iron oxidation rates of a Leptospirillum ferriphilum dominated culture were determined over the temperature range of 2-50oC at pH below one. The results show that at pH 0.9 the culture oxidizes iron within the temperature range of 10°C to 45°C. Using the Arrhenius equation, an Ea value of 89.9 ± 6.75 kJ/mol was calculated. From the data fitted to Ratkowsky Equation, the optimum, minimum and maximum temperatures were 35 ± 1.5, 9.96 ± 1.72 and 42.93 ± 0.64 °C for this culture, respectively. The redox potential of the solution becomes more positive, which was the maximum (650-700 mV) at temperatures between 19-40 oC due to completing biological oxidation and increasing in ferric iron concentration.

Thermodynamic Modelling of Iron Solubility in Sulphide Mineral Leaching

In this study the effects of pH and temperature on iron solubility were predicted using the geochemical modelling code PHREEQC and the thermodynamic database WATEQ4F. The modelling results demonstrated that the solubility of secondary solid phases formed under acid bioleaching conditions decreases with increasing temperature and also with increasing pH. Modelling calculations showed that bioleaching solutions are typically supersaturated with respect to K-, Na-, NH4-, and H3O-jarosites and the precipitates are typically solid solutions containing their mixtures. Jarosite solubility modelling results were also compared with a data set from jarosite synthesis experiments. Model-derived temperature dependence of hydronium-jarosite correlated very well with the actual experimental data.

High-Rate Fluidized-Bed Ferric Sulfate Generation for Hydrometallurgical Applications

An overview is presented of a multi-year research effort on developing high-rate fluidized-bed bioprocesses for ferric sulfate production to be used as a unit process in various hydrometallurgical applications including indirect tank leaching of ore concentrates, regeneration of heap leach liquors and control of iron containing acidic mine wastewater. Iron oxidation rates of over 26 kg m-3 h-1 were achieved at hydraulic retention times of less than 1 h at 37 °C. Oxygen supply became the rate-limiting factor even with 99.5% dioxygen aeration. Fe2+ oxidation proceeded at pH below 1 even in the presence of 60 g Fe3+ L-1 allowing the regeneration of concentrated ferric sulphate solutions required in indirect tank leaching of sulfidic ore concentrate applications. Of several tested FBR carrier materials activated carbon was the most suitable based on its availability, long-term durability and the achieved high iron oxidation rates. Jarosite precipitates accumulating to the top of the inert carrier materials played an important role in the FBR biomass retainment. For regeneration of synthetic and actual sulfidic ore heap leaching liquors, a gravity settler was installed in the recycle line of the FBR. The system produced iron precipitates with good settling characteristics and settling tank effluent with low turbidity and suspended solids concentrations. These results revealed the potential of FBR process in both heap leach liquor regeneration and controlling the iron containing waste streams. The PCR-DGGE-partial seguencing of the 16S rRNA gene protocol revealed that the FBR culture at 25-37 °C remained dominated by Leptospirillum ferriphilum over a range of operational conditions studied over the years. A modeling approach for managing Fe3+ production by FBR in combination with heap leaching was based on an artificial neural network-back propagation algorithm (ANN-HEAP) and resulted in excellent match between the measured and the predicted concentrations. High-rate fluidized-bed iron oxidation is amenable to regeneration of tank and heap leaching solutions as well as controlling iron containing waste streams.

Thermodynamic Modelling of Phosphate and Chloride Effects on Solid and Solution Phase Ferric Iron Speciation

The purpose of this study was to model, based on thermodynamic equilibrium constants, the effects of chloride and phosphate ion on the speciation of ferric iron in solution and on Fe(III)-precipitates. The thermodynamic modelling was based on the geochemical modelling code PHREEQC and the thermodynamic database WATEQ4F. Increasing phosphate levels (g per L range) increase the complexation of ferric ion with phosphate (FeH2PO42+) with a parallel decrease in ferric sulphate complex (FeSO4+) and release of sulphate as SO42- in solution. Chloride ion at comparable levels and under otherwise similar conditions had negligible effects on the speciation of soluble iron species. In the solid phase analysis, jarosite and goethite species declined with increasing phosphate levels, whereas chloride did not affect the relative proportions of secondary Fe(III) minerals in the solid phase. Saturation index values for jarosites and goethite were dependent on the temperature with the range of phosphate levels (0–20 g/L) examine in this study.

Thermodynamic Simulation of Sulphide Mineral Leaching

Geochemical modeling program PHREEQC was used to simulate generic bioleaching processes. Carbonate minerals (e.g., calcite) dissolve in acid solution, increasing the solution pH and Ca concentration while the concentration of CO2 may be controlled by the equilibrium with the atmospheric CO2. Non-oxidative dissolution of Fe-monosulphides was demonstrated to release H2S and increase the pH. In the absence of ferric iron precipitation (goethite), the oxidation of pyrite decreased the solution pH from 2 to ~1.4, while the oxidation of Fe-monosulphide and chalcopyrite increased the solution pH to ~3.2-3.4. Assuming equilibrium precipitation of goethite, oxidative leaching decreased the solution pH for all three minerals from pH ~2 to ~0.9-1.2. Adjustment of the solution pH to 1.8 or 2.0 with KOH with concurrent equilibrium precipitation of K-jarosite resulted in low dissolved iron concentrations.