Browsing by Author "van Hille, Robert P"

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    Attachment of Acidithiobacillus ferrooxidans and Leptospirillum ferriphilum cultured under varying conditions to pyrite, chalcopyrite, low-grade ore and quartz in a packed column reactor
    (Springer Verlag, 2012) Africa, Cindy-Jade; van Hille, Robert P; Harrison, Susan T L
    The attachment of Acidithiobacillus ferrooxidans and Leptospirillum ferriphilum spp. grown on ferrous medium or adapted to a pyrite mineral concentrate to four mineral substrata, namely, chalcopyrite and pyrite concentrates, a low-grade chalcopyrite ore (0.5 wt%) and quartzite, was investigated. The quartzite represented a typical gangue mineral and served as a control. The attachment studies were carried out in a novel particle-coated column reactor. The saturated reactor containing glass beads, which were coated with fine mineral concentrates, provided a quantifiable surface area of mineral concentrate and maintained good fluid flow. A. ferrooxidans and Leptospirillum spp. had similar attachment characteristics. Enhanced attachment efficiency occurred with bacteria grown on sulphide minerals relative to those grown on ferrous sulphate in an ore-free environment. Selective attachment to sulphide minerals relative to gangue materials occurred, with mineral adapted cultures attaching to the minerals more efficiently than ferrous grown cultures. Mineral-adapted cultures showed highest levels of attachment to pyrite (74% and 79% attachment for A. ferrooxidans and L. ferriphilum, respectively). This was followed by attachment of mineral-adapted cultures to chalcopyrite (63% and 58% for A. ferrooxidans and L. ferriphilum, respectively). A. ferrooxidans and L. ferriphilum exhibited lower levels of attachment to low-grade ore and quartz relative to the sulphide minerals.
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    Development of a method to assay the microbial population in heap bioleaching operations
    (Elsevier, 2005) Coram-Uliana, Nicolette J; van Hille, Robert P; Kohr, William J; Harrison, Susan T L
    Heap bioleaching is an economically viable approach to the mining of low-grade ores. Oxidation is microbially assisted, involving a consortium of microorganisms that together span the mesophilic to extreme thermophilic range of temperatures (25–80 °C). Temperatures inside the heap are not externally regulated, making the microbial interactions difficult to predict. In order to gain insight into these interactions, a qualitative and quantitative assay of the microorganisms that colonise the ore surface or are present in the liquid phase between the ore clusters at different levels within a heap has been developed. This method was developed using crude ore and liquid samples obtained from the GeoBiotics temperature controlled mesophilic heap operation at the Agnes Gold Mine in Barberton, South Africa, and the high temperature test columns at SGS Lakefield Research, Johannesburg, South Africa. This method of sample analysis may be applied to bioheap leach operations with and without temperature control. Ease of application, reproducibility and turn around time influenced technique design in order to provide a useful assay for commercial bioleaching operations. Following microbial removal from the solid phase using successive washes with detergent and acidified water, the cells were enumerated and genetic DNA was isolated. Microbial identification was achieved via restriction endonuclease analysis of the 16S rRNA genes, as well as 16S rRNA gene sequencing where necessary. Quantification was achieved using species-and genus-specific probes through fluorescent in situ hybridisation (FISH).
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    Effect of culture conditions on the competition between lactate oxidisers and fermenters in a biological sulfate reduction system
    (Elsevier, 2012) Oyekola, Oluwaseun O; Harrison, Susan T L; van Hille, Robert P
    Kinetic constants (μmax and Ks) describing the predominance of lactate oxidation and fermentation were determined in chemostat cultures. The kinetics of sulfate reduction and lactate utilization were determined from 0.5 to 5 d residence times at feed sulfate concentrations of 1.0–10.0 g l−1. The kinetics of lactate fermentation in the absence of sulfate were investigated at residence times of 0.5–5 d. The lactate oxidizers (LO) were characterized by a μmax of 0.2 h−1 and Ks value of 0.6 g l−1 compared with a μmax of 0.3 h−1 and Ks of 3.3 g l−1 for the lactate fermenters (LF). Using mathematical models, it was shown that LO competed more effectively for lactate at low lactate concentrations (⩽5 g l−1) and high sulfide concentrations (0.5 g l−1). Lactate fermenters outcompeted the oxidizers under conditions of excess lactate (>5 g l−1) and low sulfide (0.014–0.088 g l−1).
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    The effect of nitrogen limitation on lipid productivity and cell composition in Chlorella vulgaris
    (Springer Verlag, 2014) Griffiths, Melinda J; van Hille, Robert P; Harrison, Susan T L
    Chlorella vulgaris accumulates lipid under nitrogen limitation, but at the expense of biomass productivity. Due to this tradeoff, improved lipid productivity may be compromised, despite higher lipid content. To determine the optimal degree of nitrogen limitation for lipid productivity, batch cultures of C. vulgaris were grown at different nitrate concentrations. The growth rate, lipid content, lipid productivity and biochemical and elemental composition of the cultures were monitored for 20 days. A starting nitrate concentration of 170 mg L−1 provided the optimal tradeoff between biomass and lipid production under the experimental conditions. Volumetric lipid yield (in milligram lipid per liter algal culture) was more than double that under nitrogen-replete conditions. Interpolation of the data indicated that the highest volumetric lipid concentration and lipid productivity would occur at nitrate concentrations of 305 and 241 mg L−1 , respectively. There was a strong correlation between the nitrogen content of the cells and the pigment, protein and lipid content, as well as biomass and lipid productivity. Knowledge of the relationships between cell nitrogen content, growth, and cell composition assists in the prediction of the nitrogen regime required for optimal productivity in batch or continuous culture. In addition to enhancing lipid productivity, nitrogen limitation improves the lipid profile for biodiesel production and reduces the requirement for nitrogen fertilizers, resulting in cost and energy savings and a reduction in the environmental burden of the process.
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    The effect of nitrogen limitation on lipid productivity and cell composition in Chlorella vulgaris
    (Springer Verlag, 2014) Griffiths, Melinda J; van Hille, Robert P; Harrison, Susan T L
    Chlorella vulgaris accumulates lipid under nitrogen limitation, but at the expense of biomass productivity. Due to this tradeoff, improved lipid productivity may be compromised, despite higher lipid content. To determine the optimal degree of nitrogen limitation for lipid productivity, batch cultures of C. vulgaris were grown at different nitrate concentrations. The growth rate, lipid content, lipid productivity and biochemical and elemental composition of the cultures were monitored for 20 days. A starting nitrate concentration of 170 mg L−1 provided the optimal tradeoff between biomass and lipid production under the experimental conditions. Volumetric lipid yield (in milligram lipid per liter algal culture) was more than double that under nitrogen-replete conditions. Interpolation of the data indicated that the highest volumetric lipid concentration and lipid productivity would occur at nitrate concentrations of 305 and 241 mg L−1 , respectively. There was a strong correlation between the nitrogen content of the cells and the pigment, protein and lipid content, as well as biomass and lipid productivity. Knowledge of the relationships between cell nitrogen content, growth, and cell composition assists in the prediction of the nitrogen regime required for optimal productivity in batch or continuous culture. In addition to enhancing lipid productivity, nitrogen limitation improves the lipid profile for biodiesel production and reduces the requirement for nitrogen fertilizers, resulting in cost and energy savings and a reduction in the environmental burden of the process.
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    The effect of nutrient supplementation on growth and leaching performance of bioleaching bacteria
    (Trans Tech Publications, 2009) van Hille, Robert P; Bromfield, L V; Botha, S S; Jones, G; van Zyl, A W; Harrison, Susan T L
    Heap bioleaching operations are often faced with extended and unpredictable lag periods after inoculation, prior to the establishment of a stable oxidising environment, during which the heap is fully colonised or the inoculum overcomes the sub-optimal conditions resulting from acid agglomeration. Supplementation of laboratory scale (4kg ore) leach columns with soluble nitrogen, particularly as yeast extract, significantly reduced the lag time and promoted bacterial growth, resulting in a 50-95% increase in copper recovery post-inoculation. The effect of yeast extract addition to Acidithiobacillus ferrooxidans in controlled oxidation tests was investigated. Initial exposure of a stock culture to yeast extract resulted in a transient, dose dependent inhibition at concentrations of 0.5 g.l-1 and below. At 1.25 g.l-1 inhibition was complete over the time scale of the experiment. The inhibition phase was characterised by observable changes in cell morphology and ultrastructure. Despite the initial inhibition, the biomass yield at the end of the experiments was equivalent, or higher, in the presence of yeast extract. Cultures were adapted to growth on yeast extract as the sole nitrogen source and adapted cultures showed the highest rates of iron oxidation and cell growth, in the presence of 0.5 and 1 g.l-1 of yeast extract.
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    Open Access
    Evaluation of the ASTERTM process in the presence of suspended solids
    (Elsevier, 2014) van Zyl, Andries W; Huddy, Robert; Harrison, Susan T L; van Hille, Robert P
    The ability to recycle and reuse process water is a major contributing factor toward increased sustainability in the mining industry. However, the presence of toxic compounds has prevented this in most bioleaching operations. The ASTERTM process has been used for the bioremediation of cyanide (CN) and thiocyanate (SCN−) containing effluents at demonstration and commercial scale, increasing the potential for recycling of the treated effluent. The process relies on a complex consortium of microorganisms and laboratory tests have shown that the biomass retention, in suspended flocs or attached biofilm, significantly improved SCN− degradation rates. The current research evaluated the process performance in the presence of suspended solids (up to 5.5% m/v) ahead of implementation at a site where complete tailings removal is not possible. Experiments were performed in four 1 l CSTRs (with three primary reactors in parallel at an 8 h residence time, feeding one secondary reactor at a 2.7 h residence time). Stable operation at the design specifications (5.5% solids, 100 mg/l SCN− feed, effluent SCN− <1 mg/l) was achieved within 50 days, including a period of adaptation. The pH had the most significant effect on performance, with significant inhibition below pH 6. The presence of gypsum and anhydrite phases in the fresh tailings was most likely responsible for the observed decrease in pH. A maximum SCN− degradation rate of >57 mg/l/h was achieved, despite no obvious floc formation. Microbial ecology studies (16S rRNA clone library) revealed reduced diversity relative to reactors operated without suspended solids.
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    The generation of toxic reactive oxygen species (ROS) from mechanically activated sulphide concentrates and its effect on thermophilic bioleaching.
    (Elsevier, 2011) Jones, Gavin C; Corin, Kirsten C; van Hille, Robert P; Harrison, Susan T L
    Two types of laboratory mills, planetary and vibratory, were used to activate sulphide mineral concentrates mechanically before thermophilic (bio)leaching. These samples were analysed in terms of particle size, surface area, density, SEM, XRD line profile analysis and reactivity. The product particle size distributions indicated different particle breakage mechanisms of the two mills. The surface area for pyrite milled with the planetary mill was three fold that milled in the vibratory mill for the same length of time. Planetary milled samples showed lower densities, up to 4% less for pyrite samples, compared to vibratory milled samples. Particle surface oxidation, observed by SEM, occurred post milling. Surface oxidation products were more prevalent with planetary milled sulphide samples. XRD line profile analysis showed more line broadening effects with the planetary mill. This indicated that more bulk particle-related structural defects were present in the planetary milled samples. The reactivity in acidic solution was measured in terms of the generation of toxic reactive oxygen species (ROS): hydrogen peroxide and hydroxyl radicals. The ROS generation from milled sulphides, normalised to constant surface area loading, increased with increased mechanical activation. The planetary milled samples generated greater ROS per sample surface area than vibratory milled samples, more than 4-fold for pyrite after 60 min of milling. Increased ROS generation was postulated to result from increased surface area defects, solubilisation of iron oxidation products and bulk particle-related defects. The effect of mechanical activation on performance on thermophilic leaching and bioleaching tests was investigated using milled samples at 2% (w/v) pulp density. Short mill times improved leach rates from both mills, up to 7-fold cf. unactivated feed leach rates. Poor bioleaching performance resulted following long periods of mechanical activation (20–60 min). Pyrite and chalcopyrite bioleaching performance decreased dramatically above surface area loadings of 25 and 125 m2 /L respectively. Planetary milled samples were less amenable to bioleaching. For pyrite milled for 20 and 60 min and chalcopyrite milled for 40 min, no viable cells were observed following inoculation via fluorescence microscopy, suggesting culture death supported by compromised ferrous iron oxidation. The generation of ROS was postulated to cause poor bioleaching performance under these conditions.
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    In situ investigation and visualisation of microbial attachment and colonisation in a heap bioleach environment: the novel biofilm reactor
    (Elsevier, 2010) Africa, Cindy-Jade; Harrison, Susan T L; Becker, Megan; van Hille, Robert P
    In this paper, the development of a novel means of investigating the attachment and subsequent biofilm formation of mineral bioleaching micro-organisms to mineral surfaces in situ is described. The protocol was developed to investigate the interactions of micro-organisms with sulfide minerals and low-grade chalcopyrite ore under conditions resemblant of a bioheap environment. The method makes use of a biofilm reactor in which thin sections of mineral ore are mounted. The reactor is operated as a continuous flow-through system. Attachment of pure and mixed cultures of Acidithiobacillus ferrooxidans and Leptospirillum ferriphilum is assessed. The technique allows for the investigation of microbial ecology with special regard to microbe–mineral attachment, site and mineral specific associations of micro-organisms and spatial organisation of microbial communities present through the use of fluorescent microscopy techniques. Preliminary fluorescent in situ hybridisation (FISH) analysis of the attachment of L. ferriphilum and A. ferrooxidans to massive chalcopyrite sections, as well as to low-grade chalcopyrite containing ore sections is presented. In the case of both low-grade and massive sulfide mineral samples, attachment of mixed micro-colonies was observed in regions where surface defects were prevalent. In low-grade samples, preferential attachment was observed in regions where sulfide minerals were present. The density of the attached micro-colonies increased with an increase in contacting time (from 20, 72 and 96 h) and was indicative of an actively growing mono-layered biofilm.
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    Interference by pigment in the estimation of microalgal biomass concentration by optical density
    (Elsevier, 2011) Griffiths, Melinda J; Garcin, Clive; van Hille, Robert P; Harrison, Susan T L
    Optical density is used as a convenient indirect measurement of biomass concentration in microbial cell suspensions. Absorbance of light by a suspension can be related directly to cell density using a suitable standard curve. However, inaccuracies can be introduced when the pigment content of the cells changes. Under the culture conditions used, pigment content of the microalga Chlorella vulgaris varied between 0.5 and 5.5% of dry weight with age and culture conditions. This led to significant errors in biomass quantification over the course of a growth cycle, due to the change in absorbance. Using a standard curve generated at a single time point in the growth cycle to calculate dry weight (dw) from optical density led to average relative errors across the growth cycle, relative to actual dw, of between 9 and 18% at 680 nm and 5 and 13% at 750 nm. When a standard curve generated under low pigment conditions was used to estimate biomass under normal pigment conditions, average relative errors in biomass estimation relative to actual dw across the growth cycle were 52% at 680 nm and 25% at 750 nm. Similar results were found with Scenedesmus, Spirulina and Nannochloropsis. Suggested strategies to minimise error include selection of a wavelength that minimises absorbance by the pigment, e.g. 750 nm where chlorophyll is the dominant pigment, and generation of a standard curve towards the middle, or across the entire, growth cycle.
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    Investigating heap leaching – the effect of feed iron concentration on bioleaching performance
    (Elsevier, 2010) van Hille, Robert P; van Zyl, Andries W; Spurr, Nicholas R L; Harrison, Susan T L
    This paper describes an investigation into the effect of iron concentration in the leach solution on the bioleaching of a low grade copper ore, where chalcopyrite was the dominant copper sulphide. The concentration of dissolved iron is primarily controlled by pH and the relative proportion of ferric to ferrous iron, with significant jarosite precipitation occurring above pH ≈ 1.8 in a highly oxidised system. The solution pH may be increased by the dissolution of acid soluble gangue and when iron oxidation is significantly higher than sulphur oxidation. The study was approached using two experimental systems. In the former, the leach solution was recycled through an ore bed of low aspect (reactor height divided by diameter) ratio for a portion of the experiment. During the recycle phase, no acid was added to the system and acid consumption by gangue material led to a pH increase (1.6–2.2). The resulting jarosite precipitation reduced soluble iron from 2.5 g/l to less than 250 mg/l. Copper recovery decreased, but not in proportion to the decrease in iron. This was partly attributed to adsorption on, or entrainment within, the jarosites. To study the effect of reduced iron concentration on leach performance under more controlled conditions, bioleaching was performed in packed bed column reactors with feed iron concentrations ranging from 5 g/l to 200 mg/l. Observations indicated an initial decreased rate of copper liberation with reduced iron concentration in the feed. The relationship between available Fe3+ concentration and copper liberation was not proportional. However, with time, the liberation of copper became independent of iron concentration in the percolation liquor. Further, the specific rate of copper liberation was consistently below the theoretical value on a basis of ferric iron concentration. The highest values of copper liberation were reported at the lowest iron concentrations. In summary, while increased iron concentration in solution may enhance the initial rate of leaching, mineral availability appears to dominate CuFeS2 leach kinetics through the majority of the leach. Furthermore, high iron concentrations in solution aggravate jarosite formation with concomitant retention of copper in the ore bed.
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    Investigation and in situ visualisation of interfacial interactions of thermophilic microorganisms with metal-sulphides in a simulated heap environment
    (Elsevier, 2013) Africa, Cindy-Jade; van Hille, Robert P; Sand, Wolfgang; Harrison, Susan T L
    This study sought to provide a better understanding of the dynamics of microbial-metal sulphide interfacial processes relevant to heap bioleaching. Attachment and subsequent biofilm formation by Metallosphaera hakonensis (M. hakonensis) on the surface of massive chalcopyrite and pyrite samples, as well as a low-grade chalcopyritic whole ore were investigated. The method made use of a biofilm reactor in which thin sections of mineral ore were mounted. Operating conditions in the reactor simulated those of a bioheap in terms of fluid-flow and mineralogy, where the low-grade chalcopyrite ore sections were used. Pure cultures of M. hakonensis were used to inoculate the reactors and the attachment and subsequent biofilm development visualised in situ after 2, 4 and 8 days using a combination of atomic force and epifluorescent microscopy (AFM–EFM) as well as confocal scanning laser microscopy (CSLM). This revealed insights into biofilm structure and architecture. The effect of varying temperature on the extent of attachment and biofilm development was also assessed after 4 days using three temperature regimes: room temperature (20 ± 1 °C), 45 °C and 65 °C. The density of the attached micro–colonies increased with an increase in time, indicative of an actively growing biofilm. The extent of surface coverage and proliferation of the biofilm was dependent on the temperature, with surface coverage being more extensive at 65 °C, near the optimal temperature for growth. Preferential attachment and biofilm formation to sulphide minerals was observed, with increased surface coverage of pyrite mineral surfaces relative to chalcopyrite and low-grade ore. The AFM–EFM technique enhanced the level of detail at which site specific associations of microorganisms with mineral surfaces could be assessed. Spatial orientation and density of attached micro-colonies were noted.
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    Kinetic analysis of biological sulphate reduction using lactate as carbon source and electron donor: Effect of sulphate concentration.
    (Elsevier, 2010) Oyekola, Oluwaseun O; van Hille, Robert P; Harrison, Susan T L
    This study investigated the effect of feed sulphate concentration on the kinetics of anaerobic sulphate reduction by a mixed SRB culture, using lactate as the sole carbon source and electron donor. Chemostat cultures were operated across a range of residence times (0.5–5 d) and feed sulphate concentrations (1.0–10.0 g l1 ). Similar phenomena were observed at feed sulphate concentrations of 1.0 and 10.0 g l1 with the volumetric sulphate reduction rate increasing linearly with increasing volumetric sulphate loading rate. These reactors were characterised by higher specific volumetric sulphate reduction rates with maximum values of 0.24 and 0.20 g h1 g1 . Contrastingly, the reactors fed with sulphate concentrations of 2.5 and 5.0 g l1 showed distinctly different trends in which the volumetric sulphate reduction rate passed through a maximum at the dilution rates of 0.014 and 0.021 h1 , respectively, followed by a decline with further increase in sulphate loading rate. The maximum specific volumetric sulphate reduction rates observed were 2–6-fold lower than those observed at 1.0 and 10.0 g l1 feed sulphate concentrations. Profiles of specific volumetric sulphate reduction rate and biomass concentration suggested a shift in lactate utilisation from oxidation to fermentation at high dilution rates, implying a change in the dominant components of the microbial consortium. The data suggest that population structure was influenced by lactate affinity and dissolved sulphide concentration. The trends observed were attributed to the greater ability of lactate oxidisers to scavenge lactate under limiting concentrations of the substrate and their greater resilience to dissolved sulphide species in comparison to lactate fermenters.
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    Lipid productivity, settling potential and fatty acid profile of 11 microalgal species grown under nitrogen replete and limited conditions
    (Springer, 2012) Griffiths, Melinda J; van Hille, Robert P; Harrison, Susan T L
    Microalgae are a promising alternative source of oil for biodiesel production. Identification of a species with desirable characteristics is a key component towards achieving economic feasibility for the process. This has been compromised by a lack of data allowing effective interspecies comparison. Eleven species of microalgae, selected on the basis of available literature data, were tested for lipid productivity, gravity sedimentation and the suitability of their fatty acid profiles for biodiesel production. The response to nitrogen limitation was species-specific. Lipid yields and productivity were higher at 150 mg L−1 nitrate than at 1,500 mg L−1 for all species tested except Spirulina platensis. The Chlorophyta, particularly Chlorella vulgaris and Scenedesmus, had the highest growth rates and showed the greatest increase in lipid content in response to nitrogen limitation. Cylindrotheca fusiformis, S. platensis, Scenedesmus and Tetraselmis suecica had the fastest settling rates and highest biomass recoveries after 24 h of gravity sedimentation. For most species, the fuel would need to be blended or culture conditions to be optimised to achieve the correct lipid profile in order for microalgal fuel to meet the European standards for biodiesel production (EN 14214). The most promising species overall were the freshwater algae Scenedesmus and C. vulgaris and the marine algae C. fusiformis and Nannochloropsis.
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    Open Access
    The microbial ecology of moderately thermophilic mineral leaching reactors: the effect of solids loading and organic carbon supplementation on reactor performance
    (Trans Tech Publications, 2015-11) Dean, Jonathan; van Hille, Robert P; Harrison, Susan T L
    Tank bioleaching of refractory sulphidic gold ores is well established, with potential to expand application to base metal concentrates. With increasing commercial tank bioleaching operations, understanding their microbial consortia is essential for process robustness. Recently, it has been shown that the consortia implicated in tank mineral bioleaching of gold-containing pyritic ores are dynamic, responding to the leaching environment. Factors driving the microbial dynamics of these consortia are under investigation, with emphasis also placed on determining the metabolic role of the key players in these consortia. Here, the combined influence of solids loading and organic carbon availability on microbial community dynamics and performance has been studied in agitated, aerated slurry bioreactors at 45C. The control (autotrophic) and experimental (organic carbon added) reactors performed comparably at low solids loadings (4%, 7% and 10%). At 20% solids loadings, higher ferric iron concentrations (31 g/L vs 25g/L) and sulphide oxidation (66% vs 45%) were observed in the experimental reactor over the control. Under operating conditions used, a shift from bacterially-dominated cultures with L.ferriphilum as major species towards increasing archaeal abundance was observed. Archaeal abundance was higher in the organic carbon supplemented reactor at all solids loadings. The increased microbial diversity with organic supplementation appears to contribute to increased community robustness and associated leaching with increasing stress.
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    The microbial ecology of moderately thermophilic mineral leaching reactors: the effect of solids loading and organic carbon supplementation on reactor performance
    (Trans Tech Publications, 2015) Dean, Jonathan; van Hille, Robert P; Harrison, Susan T L
    Tank bioleaching of refractory sulphidic gold ores is well established, with potential to expand application to base metal concentrates. With increasing commercial tank bioleaching operations, understanding their microbial consortia is essential for process robustness. Recently, it has been shown that the consortia implicated in tank mineral bioleaching of gold-containing pyritic ores are dynamic, responding to the leaching environment. Factors driving the microbial dynamics of these consortia are under investigation, with emphasis also placed on determining the metabolic role of the key players in these consortia. Here, the combined influence of solids loading and organic carbon availability on microbial community dynamics and performance has been studied in agitated, aerated slurry bioreactors at 45°C. The control (autotrophic) and experimental (organic carbon added) reactors performed comparably at low solids loadings (4%, 7% and 10%). At 20% solids loadings, higher ferric iron concentrations (31 g/L vs 25g/L) and sulphide oxidation (66% vs 45%) were observed in the experimental reactor over the control. Under operating conditions used, a shift from bacterially-dominated cultures with L.ferriphilum as major species towards increasing archaeal abundance was observed. Archaeal abundance was higher in the organic carbon supplemented reactor at all solids loadings. The increased microbial diversity with organic supplementation appears to contribute to increased community robustness and associated leaching with increasing stress.
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    Study of anaerobic lactate metabolism under biosulphidogenic conditions
    (IWA Publishing, 2009) Oyekola, Oluwaseun A; van Hille, Robert P; Harrison, Susan T L
    Biological sulfate reduction (BSR) has been reported to have potential for the treatment of acid mine drainage (AMD). The provision of a suitable carbon source and electron donor for this process remains a challenge. Lactate offers potential advantages as carbon source and electron donor in the biological sulfate reduction process. As this substrate is utilized by both fermentative bacteria and oxidative sulfate-reducing bacteria (SRB), the effect of feed sulfate concentration on the lactate pathways utilized under biosulfidogenic conditions was investigated. Studies were carried out in chemostat bioreactors across a range of residence times, using an enriched culture of SRB. The stoichiometry of biological sulfate reduction was affected by feed sulfate concentration and dilution rate. Incomplete oxidation of lactate was dominant at low feed sulfate concentration (1.0 g/L), while the yield of propionate from lactate metabolism increased at feed sulfate concentrations of 2.5–10.0 g/L, indicating the occurrence of lactate fermentation. Furthermore, at each sulfate feed concentration, in the range 2.5–10.0 g/L, the ratio in which lactate was metabolized by the oxidative and fermentative pathways varied with varying dilution rates. Lactate oxidation was higher at a feed sulfate concentration of 10.0 g/L relative to 2.5 and 5.0 g/L. The volumetric lactate utilization rate was enhanced by increasing the feed sulfate concentration. However, the proportion of total lactate consumed that was channelled into providing electrons for other activities apart from sulfate reduction also increased over the range of increasing sulfate concentrations studied and appeared to be a function of residual lactate and sulfide concentrations.
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    Sulfide mineral induced oxidative stress as a limiting factor in tank bioleaching performance
    (Trans Tech Publications, 2009) Jones, Gavin; van Hille, Robert P; Harrison, Susan T L
    In tank bioleaching, decreasing particle size of milled concentrates has been shown to improve leaching performance owing to increased mineral surface area and mechanical activation effects of fine mineral particles. However, evidence suggests a critical lower limit of particle size distribution exists below which the performance of the thermophilic iron and sulfur oxidizer Sulfolobus metallicus is compromised and complete culture death may result. This paper proposes an explanation for these observations at fine fractions through identifying a relationship between mineral composition of six sulfide concentrates, their extent of milling and the generation of the reactive oxygen species (ROS) hydrogen peroxide (H2O2) and hydroxyl radicals (•OH) in slurries. The effect of oxidative stress induced in the absence of minerals on growth and bioleaching performance of S. metallicus is studied.
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    The effect of sulfide concentrate mineralogy and texture on Reactive Oxygen Species (ROS) generation
    (Elsevier, ROS) Jones, Gavin C; Becker, Megan; van Hille, Robert P; Harrison, Susan T L
    The generation of Reactive Oxygen Species (ROS), H2O2 and radical dotOH, has been observed from sulfide mineral containing particles in acidic solutions. The implications of this phenomenon, as a potential microbial stress-causing effect, have been studied previously with respect to thermophilic bioleaching performance in the presence of finely milled pyrite and chalcopyrite concentrates. In this study, the effect of sulfide mineralogy on ROS generation in the absence of microbes under physicochemical conditions typical for the bioleach environment was investigated. The mineralogical and elemental composition of eleven different samples containing sulfide mineral was obtained. These Au, Cu and other base metal-containing sulfide mineral concentrates as well as a milled whole ore of low Cu grade were tested for ROS generation. The whole ore sample and two refractory Au concentrates containing approximately 50% pyrite, generated significantly less ROS compared to the base metal-containing concentrates when compared on a constant surface area loading basis. Sulfide mineral-related variables were correlated with ROS generation. A significant difference was observed between FeS2 and CuFeS2 grades separately, whereas a combined measure of both minerals present in samples showed a consistently strong correlation to ROS generation. The Cu grade, total Cu-containing sulfides and the chalcopyrite content of Cu-containing samples correlated well with ROS generation. However, a common deterministic variable with a strong association to increased ROS generation was not found. A sub-set of samples were subjected to QEMSCAN® for textural analysis. Results suggested that a decrease in sulfide mineral liberation, caused by gangue silicate mineral occlusion to solution, resulted in decreased reactivity as shown in one of the Au-containing samples. Well-liberated chalcopyrite and pyrite phases corresponded to increased reactivity of samples. Pyrite, which was present in all of the reactive samples, was shown to be associated with other sulfide minerals, implicating its importance in galvanic interactions. Micro-analysis of chalcopyrite and pyrite phases from highly reactive samples showed an abundance of particles with extensive cracking and the possible presence of secondary transformation phases (szomolnokite). These results suggest that sulfide mineralogy, liberation and extent of physical processing affect sulfide mineral concentrate reactivity in acidic solutions.