Kinetics Of Ferrous Iron Oxidation Research Articles

Development of Technology for the Bioleaching of Uranium in a Solution of Bacterial Immobilization

This study presents findings regarding the kinetics of ferrous iron oxidation in solution mediated by Acidithiobacillus ferrooxidans bacteria within a continuous-flow bioreactor employing diverse types of immobilizers. The objective is to augment the rate of ferrous iron oxidation in solutions utilizing an immobilizer for Acidithiobacillus ferrooxidans strains. Immobilization represents a promising avenue for enhancing the efficiency of Fe2⁺ oxidation via acidophilic ferrooxidizing bacteria, leading to a several-fold increase in oxidation rate. A comparative analysis was conducted to evaluate the efficacy of different types of immobilizer in facilitating iron oxidation within a continuous-flow bioreactor, including the application of wood chips coated with Fe(OH)3. The results indicate that wood chips coated with iron hydroxide serve as effective type of immobilizer, facilitating the robust attachment of Acidithiobacillus ferrooxidans via electrostatic interactions between negatively charged bacteria and positively charged surfaces. Experimental investigations were conducted using novel immobilization matrices in pilot-scale tests simulating the underground borehole leaching (UBL) of uranium. The bioactivation of leaching solutions enhances the efficiency and environmental compatibility of UBL compared to conventional chemical oxidation methods. The relationships between redox potential and ferric iron content in bioactivated solutions during the UBL of uranium were delineated. The significance of this study lies in its elucidating the pivotal role of Fe2⁺ oxidation in uranium extraction processes, particularly in the context of UBL. By employing bioactivation mediated by Acidithiobacillus ferrooxidans, the study demonstrates not only enhanced uranium extraction efficiency, but also markedly improved environmental sustainability compared to traditional chemical oxidation methods. The findings reveal crucial correlations between redox potential and ferric iron concentration in bioactivated solutions.

A geochemical characterization of cold-water natural acid rock drainage at the Zn–Pb XY deposit, Yukon, Canada

Acid rock drainage (ARD) is considered to be temperature-limited due to the diminished activity of Fe(II)-oxidizing microbes at low temperatures. Nonetheless, ARD streams are present in cold climates. This study presents a geochemical characterization of a cold climate ARD creek at the Zn–Pb XY deposit in Yukon, Canada, which showed highly elevated concentrations of dissolved zinc (up to 475mg/L). Acid rock drainage at the XY deposit is likely generated via subsurface abiotic and biotic oxidation of sulfide minerals, and then exits as seeps at the headwaters of the creek. The uppermost reaches of the creek have the lowest pH levels (pH 3.3) and highest metal concentrations, with prolific precipitation of iron-hydroxysulfate and -oxyhydroxide mineral precipitates (schwertmannite, jarosite, and goethite), present as terraced iron formations (TIFs) at one sampling location. The lower reaches of the creek show a progressive pH increase (up to pH level 4.9) which occurs due to Fe(III)- and Al-hydrolysis, the neutralizing influence of carbonate-rich strata and/or ground waters, and dilution by surface waters entering the creek. Progressive pH neutralization causes a change in precipitate mineralogy to X-ray amorphous Al-hydroxysulfates, with a composition similar to aluminite and hydrobasaluminite, and amorphous Al(OH)3. Natural attenuation of Cd, Zn, and Pb occurred downstream from the headwater seeps, which was likely influenced by adsorption reactions involving both metal-sulfate anions and metal-sulfate ternary complexes. Generally, the concentrations of Cd, Zn, and Pb in the mineral precipitates followed trends in pH, aqueous concentration, predicted speciation, and mineralogy of the precipitates. However, anomalous trace metal concentrations were observed in the precipitates at 120m downstream and potentially reflect a local ground water input. Overall the geochemistry of this cold climate ARD creek was similar to ARD in more temperate climates, although low temperatures appear to retard the kinetics of ferrous iron oxidation and seasonal hydrodynamics may influence the formation of TIFs in these highly saturated waters.

A novel microwell-based analytical technique for studying ferrous iron biooxidation activity

The broad range of complexities in bioleaching includes the use of mixed microbial communities with diversity of species and strains with different windows of operating conditions. Empirical approaches to characterise these currently use cumbersome experimental systems; hence the need to develop a high throughput research tool, analogous to the techniques used in high throughput pharmaceutical research. In this study, a microwell research tool was evaluated as a growth and measurement tool for mixed autotrophic bioleaching cultures. The tool was assessed by comparing its performance to conventional shake flask apparatus. Mixed mesophilic cultures of predominantly Leptospirillum ferriphilum and Acidithiobacillus ferrooxidans were used. Growth and ferrous iron oxidation kinetics were quantified and assessed. Microwell plates performed similarly to conventional shake flasks with respect to growth and iron oxidation kinetics. The microwell plate apparatus was also used as a measurement system in combination with a microwell plate reader (measuring absorbance change at 428nm over time). Progressive colour change of growth experiments correlated to ferrous iron oxidation within a defined operating window. We conclude that, using this measurement as a proxy for trends iron oxidation, the microwell research tool is well-suited for high throughput scoping studies to map operating windows for different cultures, in both an unadapted and adapted context. This was confirmed through an activity test utilising fluoride as an inhibitor. Where absorbance measurements at 428nm are used to track oxidation progress, the research tool has limitations with respect to pH (<2.0) and total iron concentration (<8.0gl−1).

Bench-scale evaluation of ferrous iron oxidation kinetics in drinking water: Effect of corrosion control and dissolved organic matter

Corrosion control strategies are important for many utilities in maintaining water quality from the water treatment plant to the customers’ tap. In drinking water with low alkalinity, water quality can become significantly degraded in iron-based pipes if water utilities are not diligent in maintaining proper corrosion control. This article reports on experiments conducted in bicarbonate buffered (5 mg-C/L) synthetic water to determine the effects of corrosion control (pH and phosphate) and dissolved organic matter (DOM) on the rate constants of the Fe(II) oxidation process. A factorial design approach elucidated that pH (P = 0.007, contribution: 42.5%) and phosphate (P = 0.025, contribution: 22.7%) were the statistically significant factors in the Fe(II) oxidation process at a 95% confidence level. The comprehensive study revealed a significant dependency relationship between the Fe(II) oxidation rate constants (k) and phosphate-to- Fe(II) mole ratio. At pH 6.5, the optimum mole ratio was found to be 0.3 to reduce the k values. Conversely, the k values were observed to increase for the phosphate-to- Fe(II) mole ratio >1. The factorial design approach revealed that chlorine and DOM for the designated dosages did not cause a statistically significant (α=0.05, P>0.05) change in rate constants. However, an increment of the chlorine to ferrous iron mole ratio by a factor of ∼2.5 resulted in an increase k values by a factor of ∼10. This study conclusively demonstrated that the lowest Fe(II) oxidation rate constant was obtained under low pH conditions (pH ≤ 6.5), with chlorine doses less than 2.2 mg/L and with a phosphate-to-Fe(II) mole ratio≈0.3 in the iron water systems.

A Novel Apparatus to Determine the Bio-Oxidation Kinetics of Sessile &lt;i&gt;Leptospirillum ferriphilum&lt;/i&gt;

A novel apparatus was developed to test the ferrous iron oxidation kinetics of Leptospirillum ferriphilum in predominantly sessile culture, by combining a CSTR under wash-out conditions with a packed bed of inert ceramic saddles. Results indicate that a dense culture of sessile bacteria is established rapidly, which achieves high oxidation rates in all experiments with a yield in terms of CO2 uptake rates comparable or higher to what has been measured in planktonic culture. However, the yield in terms of cell counts changes dramatically, indicating a substantial shift in carbon utilisation in the sessile culture. The apparatus is feasible as a method to study iron oxidation kinetics of sessile cultures, providing it is operated at sufficiently high recycle ratios.

Kinetics of ferrous iron oxidation by batch and continuous cultures of thermoacidophilic Archaea at extremely low pH of 1.1–1.3

The extreme acid conditions required for scorodite (FeAsO4·2H2O) biomineralization (pH below 1.3) are suboptimal for growth of most thermoacidophilic Archaea. With the objective to develop a continuous process suitable for biomineral production, this research focuses on growth kinetics of thermoacidophilic Archaea at low pH conditions. Ferrous iron oxidation rates were determined in batch-cultures at pH 1.3 and a temperature of 75°C for Acidianus sulfidivorans, Metallosphaera prunea and a mixed Sulfolobus culture. Ferrous iron and CO2 in air were added as sole energy and carbon source. The highest growth rate (0.066 h−1) was found with the mixed Sulfolobus culture. Therefore, this culture was selected for further experiments. Growth was not stimulated by increase of the CO2 concentration or by addition of sulphur as an additional energy source. In a CSTR operated at the suboptimal pH of 1.1, the maximum specific growth rate of the mixed culture was 0.022 h−1, with ferrous iron oxidation rates of 1.5 g L−1 d−1. Compared to pH 1.3, growth rates were strongly reduced but the ferrous iron oxidation rate remained unaffected. Influent ferrous iron concentrations above 6 g L−1 caused instability of Fe2+ oxidation, probably due to product (Fe3+) inhibition. Ferric-containing, nano-sized precipitates of K-jarosite were found on the cell surface. Continuous cultivation stimulated the formation of an exopolysaccharide-like substance. This indicates that biofilm formation may provide a means of biomass retention. Our findings showed that stable continuous cultivation of a mixed iron-oxidizing culture is feasible at the extreme conditions required for continuous biomineral formation.Electronic supplementary materialThe online version of this article (doi:10.1007/s00253-011-3460-7) contains supplementary material, which is available to authorized users.

Kinetics of ferrous iron oxidation by Sulfobacillus thermosulfidooxidans

The biological oxidation of ferrous iron is an important sub-process in the bioleaching of metal sulfides and other bioprocesses such as the removal of H 2S from gases, the desulfurization of coal and the treatment of acid mine drainage (AMD). As a consequence, many Fe(II) oxidation kinetics studies have mostly been carried out with mesophilic microorganisms, but only a few with moderately thermophilic microorganisms. In this work, the ferrous iron oxidation kinetics in the presence of Sulfobacillus thermosulfidooxidans (DSMZ 9293) was studied. The experiments were carried out in batch mode (2L STR) and the effect of the initial ferrous iron concentration (2–20 g L −1) on both the substrate consumption and bacterial growth rate was assessed. The Monod equation was applied to describe the growth kinetics of this microorganism and values of μ max and K s of 0.242 h −1 and 0.396 g L −1, respectively, were achieved. Due to the higher temperature oxidation, potential benefits on leaching kinetics are forecasted.

Batch kinetics of ferrous iron oxidation by Leptospirillum ferriphilum at moderate to high total iron concentration

A study of the kinetics of ferrous iron oxidation by a free suspended culture of the bacterium Leptospirillum ferriphilum in batch regime at moderate to high iron concentrations was conducted. A circulating bed airlift bioreactor was used in order to obtain reliable biokinetic data, unaffected by biofilm growth. The two major factors in consideration were the effects of the pH and the total iron concentration in the range of 5–40 g/L. The optimal pH was found between 1.05 and 1.80. In this range a strictly growth associated biooxidation with constant yield coefficient was proven, while at suboptimal pH values non-growth associated iron biooxidation was shown at pH as low as 0.4. This effect was taken into consideration for the derivation of a Monod-type kinetic model, derived on first principles from the electrochemical-enzymatic model for ferrous iron biooxidation. Our model shows a linear dependence between the apparent half-saturation constant ( K app) and the total iron concentration in studied range of iron concentration.

The kinetics of ferrous-iron oxidation by Leptospirillum ferriphilum in continuous culture: The effect of temperature

A typical bioleach heap is characterized by wide variation of temperature across the heap bed, leading to oxidation of target minerals occurring at different rates. Previous studies on the effect of temperature on the microbial oxidation of ferrous-iron were limited to a narrow range of temperatures (30–40 °C) near optimum conditions and mostly to Acidithiobacillus ferrooxidans. The kinetics of ferrous-iron oxidation by Leptospirillum ferriphilum were studied in continuous culture. In this paper we focus on the effect of temperature (18–45 °C) on these kinetics. The study was based on the assumption that the effect of temperature can be studied independently of other, equally important factors such as pH, dissolved salts, etc. and independent of the reactor context. The experimental data were correlated using both, a simplified ferric-iron inhibitory model and the Pirt Equation. The results showed that the maximum specific ferrous-iron oxidation rate, q F e 2 + max increased with increasing temperature to a maximum at 42 °C. This trend can be described adequately by the Arrhenius Equation with an activation energy, E a of 34.46 kJ mol −1 and frequency factor, K 0 of 1.05 × 10 7 mmol Fe 2+(mmolC) −1 h −1. An increase in temperature slightly reduces the steady state carbon biomass in the reactor, while the apparent affinity constant, K ′ F e 2 + increases. The investigation further suggests that at low temperature (18 °C) and beyond the maximum temperature (42 °C), the culture cannot be sustained in a continuous mode. The maximum biomass yield followed a linear decline with increasing temperature, while cell maintenance on ferrous-iron followed a quadratic trend, although the small values indicates that it is not significant, as would be expected in continuous culture. The results indicate that L. ferriphilum is likely to perform optimally, at warm temperatures (25–42 °C) in heap bioleach operations before being taken over by thermophiles at higher temperatures.

Kinetics of Ferrous Iron Oxidation by Moderately Thermophilic Microorganisms

Kinetics of Ferrous Iron Oxidation by Moderately Thermophilic Microorganisms

The Effect of Total Iron Concentration and Iron Speciation on the Rate of Ferrous Iron Oxidation Kinetics of Leptospirillum ferriphilum in Continuous Tank Systems

In this study the results from a systematic study of the oxidation kinetics of Leptospirillum ferriphilum in continuous culture at total iron concentrations ranging from 2 to12 g/L are reported. In all experiments the steady-state concentrations of ferrous iron were small and comparable, and at least 97% of was as ferric. Surprisingly, the specific ferrous iron utilisation rate decreased with increasing total iron concentration, while yield coefficients increased. It was noted that the biomass concentration in the reactor (as measured by both CO2 uptake rate and cell counts) dramatically increased with increasing total iron concentrations, whereas it stayed more or less the same over a wide range of dilution rates at a given total iron concentration. The experimental data was re-analysed in terms of ferrous iron kinetics using Monod kinetics with a ferric inhibition term. The results confirm that the maximum specific iron utilisation rate is itself a function of ferric iron concentration, declining with increasing concentration. It thus appears that high concentrations of ferric iron stimulate microbial growth while at the same time inhibiting the rate of ferrous iron oxidation. It is postulated that these phenomena are related, i.e. that more growth occurs to reduce the load on the individual cell, possibly by sharing some metabolic functions.

Kinetics of iron oxidation by Leptospirillum ferriphilum dominated culture at pH below one

The kinetics of ferrous iron oxidation by Leptospirillum ferriphilum (L. ferriphilum) dominated culture was studied in the concentration range of 0.1-20 g Fe(2+)/L and the effect of ferric iron (0-60 g Fe(3+)/L) on Fe(2+) oxidation was investigated at pH below one. Denaturing gradient gel electrophoresis of PCR amplified 16S rRNA genes followed by partial sequencing confirmed that the bacterial community was dominated by L. ferriphilum. In batch assays, Fe(2+) oxidation started without lag phase and the oxidation was completed within 1 to 60 h depending on the initial Fe(2+) concentration. The specific Fe(2+) oxidation rates increased up to around 4 g/L and started to decrease at above 4 g/L. This implies substrate inhibition of Fe(2+) oxidation at higher concentrations. Haldane equation fitted the experimental data reasonably well (R(2) = 0.90). The maximum specific oxidation rate (q(m)) was 2.4 mg/mg VS . h, and the values of the half saturation (K(s)) and self inhibition constants (K(i)) were 413 and 8,650 mg/L, respectively. Fe(2+) oxidation was competitively inhibited by Fe(3+) and the competitive inhibition constant (K(ii)) was 830 mg/L. The time required to reach threshold Fe(2+) concentration was around 1 day and 2.3 days with initial Fe(3+) concentration of 5 and 60 g/L, respectively. The threshold Fe(2+) concentration, below which no further Fe(2+) oxidation occurred, linearly increased with increasing initial Fe(2+) and Fe(3+) concentrations. Fe(2+) oxidation proceeds by L. ferriphilum dominated culture at pH below 1 even in the presence of 60 g Fe(3+)/L. This indicates potential of using and biologically regenerating concentrated Fe(3+) sulfate solutions required, for example, in indirect tank leaching of ore concentrates.

Modeling the kinetics of ferrous iron oxidation by monochloramine.

The maintenance of disinfectants in distribution systems is necessary to ensure drinking water safety. Reactions with oxidizable species can however lead to undesirable disinfectant losses. Previous work has shown that the presence of Fe(II) can cause monochloramine loss in distribution system waters. This paper further examines these reactions and presents a reaction mechanism and kinetic model. The mechanism includes both aqueous-phase reactions and surface-catalyzed reactions involving the iron oxide product. In addition, it considers competitive reactions involving the amidogen radical that lead to a nonelementary stoichiometry. Using the method of initial rates, the aqueous-phase reactions were found to have first-order dependencies on Fe(II), NH2Cl, and OH- and a rate coefficient (kNH2Cl,soln) of 3.10 (+/-0.560) x 10(9) M(-2) min(-1). The surface-mediated reactions were modeled by assuming the formation of two surface species: >FeOFe+ and >FeOFeOH. Using numerical techniques, combined rate coefficients for the surface-mediated processes were determined to be 0.56 M(-3) min(-1) and 3.5 x 10(-18) M(-4) min(-1), respectively. The model was then used to examine monochloramine and Fe(II) stability under conditions similar to those observed in distribution systems. Our findings suggest the potential utility of monochloramine as an oxidant for Fe(III) removal in drinking water treatment.

The consequences of hydroxyl radical formation on the stoichiometry and kinetics of ferrous iron oxidation by human apoferritin

Despite previous detection of hydroxyl radical formation during iron deposition into ferritin, no reports exist in the literature concerning how it might affect ferritin function. In the present study, hydroxyl radical formation during Fe(II) oxidation by apoferritin was found to be contingent on the “ferroxidase” activity (i.e., H subunit composition) exhibited by apoferritin. Hydroxyl radical formation was found to affect both the stoichiometry and kinetics of Fe(II) oxidation by apoferritin. The stoichiometry of Fe(II) oxidation by apoferritin in an unbuffered solution of 50 mM NaCl, pH 7.0, was approximately 3.1 Fe(II)/O 2 at all iron-to-protein ratios tested. The addition of HEPES as an alternate reactant for the hydroxyl radical resulted in a stoichiometry of about 2 Fe(II)/O 2 at all iron-to-protein ratios. HEPES functioned to protect apoferritin from oxidative modification, for its omission from reaction mixtures containing Fe(II) and apoferritin resulted in alterations to the ferritin consistent with oxidative damage. The kinetic parameters for the reaction of recombinant human H apoferritin with Fe(II) in HEPES buffer (100 mM) were: K m = 60 μM, k cat = 10 s −1, and k cat/ K m = 1.7 × 10 5 M −1 · s −1. Collectively, these results contradict the “crystal growth model” for iron deposition into ferritin and, while our data would seem to imply that the ferroxidase activity of ferritin is adequate in facilitating Fe(II) oxidation at all stages of iron deposition into ferritin, it is important to note that these data were obtained in vitro using nonphysiologic conditions. The possibility that these findings may have physiological significance is discussed.

Bacterial ferrous iron oxidation of acid mine drainage as pre-treatment for subsequent metal recovery

Mining activities at Falu copper mine in Sweden started around the year 1080 AD and continued until 1993. During all these centuries, the acid mine drainage has caused low pH values and high metal levels in the nearby Falu river and it's surroundings. This work is part of a plan to achieve a long-term solution to this environmental problem, where the aim is to treat the mine water overflow in such a way that a neutral pH and an essentially metal free aqueous effluent is obtained. The idea is to first oxidise the ferrous iron with bacteria, in order to be able to make a selective ferric iron precipitation at a pH of around 3. One possible process option is to use the precipitated ferric hydroxide for the production of red pigment. Thereafter, zinc will be recovered in such a way that it will be suitable for recycling. In this paper, only the initial stage of bacterial ferrous iron oxidation is discussed. The ferrous iron oxidation was initially studied in laboratory scale using batch cultures of mesophilic (35°C), moderate thermophilic (45°C) and extreme thermophilic (65°C) microorganisms. The ferrous iron oxidation kinetics was determined with two different concentrations of ferrous iron. The moderate thermophilic culture did not grow well on ferrous iron only. Since the mesophilic and extreme thermophilic microorganisms showed approximately the same oxidation kinetics, the mesophilic bacteria was selected for further studies in pilot scale, due to their lower operating temperature which reduces the heating cost. A pilot plant was erected at the mine site with three 500 l reactors in series with a treatment capacity of up to approximately 500 l h −1. The reactors were filled with plastic bodies in order to support the formation of a permanent biofilm to avoid bacterial washout. The pilot plant has been operating continuously for several months and the influence of flow rate, ferrous concentration, pH and temperature was investigated. The feasibility for a biological ferrous iron oxidation step was successfully demonstrated. With a ferrous concentration of 3.5 g l −1, 35°C, pH 1.8 and a flow rate of 330 l h −1 a ferrous iron oxidation rate of 750 mg l −1 h −1 was achieved. The results obtained will be used to calculate the capital and operating cost for a full scale plant, capable of treating 25 m 3 of acid mine drainage per hour. Before the final decision, regarding the ferrous iron oxidation step is made, alternative processes such as hydrogen peroxide and ozone oxidation will be evaluated.

The effect of temperature on the continuous ferrous-iron oxidation kinetics of a predominantly Leptospirillum ferrooxidans culture.

The ferrous-iron oxidation kinetics of a bacterial culture consisting predominantly of Leptospirillum ferrooxidans were studied in continuous-flow bioreactors. The bacterial culture was fed with a salts solution containing 12 g/L ferrous-iron, at dilution rates ranging from 0.01 to 0.06 l/h, and temperatures ranging from 30 to 40 degrees C, at a pH of 1.75. The growth rate, and the oxygen and ferrous-iron utilization rates of the bacteria, were monitored by means of off-gas analysis and redox-potential measurement. The degree-of-reduction balance was used to compare the theoretical and experimental values of r(CO(2)), -r(O(2)) and -r(Fe(+2)), and the correlation found to be good. The maximum bacterial yield on ferrous-iron and the maintenance coefficient on ferrous-iron, were determined using the Pirt equation. An increase in the temperature from 30 to 40 degrees C did not appear to have an effect on either the maximum yield or maintenance coefficient on ferrous-iron. The average maximum bacterial yield and maintenance coefficient on ferrous-iron were found to be 0.0059 mmol C/mmol Fe(2+) and 0.7970 mmol Fe(2+)/mmol C)/h, respectively. The maximum specific growth rate was found to be 0.077 l/h. The maximum specific ferrous-iron utilization rate increased from 8.65 to 13.58 mmol Fe(2+)/mmol C/h across the range from 30 to 40 degrees C, and could be described using the Arrhenius equation. The kinetic constant in bacterial ferrous-iron oxidation increased linearly with increasing temperature. The ferrous-iron kinetics could be accurately described in terms of the ferric/ferrous-iron ratio by means of a Michaelis-Menten-based model modified to account for the effect of temperature. A threshold ferrous-iron level, below which no further ferrous-iron utilization occurred, was found at a ferric/ferrous-iron ratio of about 2500. At an overall iron concentration of 12 g/L, this value corresponds to a threshold ferrous-iron concentration of 78.5 x10(-3) mM.

Comparison of the oxidation kinetics of different pyrites in the presence of Thiobacillus ferrooxidans or Leptospirillum ferrooxidans

The question whether the overall bacterial oxidation rate of a pyrite is determined by both the chemical reactivity of the pyrite as well as the affinity of a bacterial strain to the ferrous to ferric iron ratio, was examined. Bioleaching studies were carried out with a pure culture of Thiobacillus ferrooxidans and an enrichment culture of Leptospirillum bacteria on a framboidal pyrite from Germany and a euhedral pyrite from Prieska, South Africa. It was found that the Leptospirillum bacteria were able to oxidise euhedral pyrite and framboidal pyrite, whereas T. ferrooxidans was able to oxidise framboidal pyrite but not euhedral pyrite. The German, framboidal pyrite has a granular and irregular surface structure, which is probably more chemically reactive than the highly crystalline surface structure of euhedral, Prieska pyrite. In previous work it has been shown that the oxidation of pyrite by Leptospirillum bacteria is determined by a mechanism which consists of two sub-processes: (i) the chemical oxidation of pyrite with ferric iron to ferrous iron and sulphate, and (ii) the bacterial oxidation of ferrous iron. By using roughly estimated kinetic constants from a batch culture of T. ferrooxidans on framboidal pyrite, and the independently determined ferrous iron oxidation kinetics of T. ferrooxidans, it was possible to explain why this strain is not able to oxidise the refractory euhedral pyrite. The results are explained in terms of the relative affinities of T. ferrooxidans and the Leptospirillum bacteria for the ferrous/ferric-iron ratio, and susceptibility of the two types of pyrite to ferric leaching.

The ferrous iron oxidation kinetics of Thiobacillus ferrooxidans in continuous cultures

The oxidation and growth kinetics of ferrous iron with Thiobacillus ferrooxidans in continuous cultures was examined at several total iron concentrations. On-line off-gas analyses of O2 and CO2 were used to measure the oxygen and carbon dioxide consumption rates in the culture. Off-line respiration measurements in a biological oxygen monitor (BOM) were used to measure directly the maximum specific oxygen consumption rate, qO2,max, of cells grown in continuous culture. It was shown that these reproducibly measured values of qO2,max vary with the dilution rate. The biomass-specific oxygen consumption rate, qO2, is dependent on the ratio of the ferric and ferrous iron concentrations in the culture. The oxidation kinetics was accurately described with a rate equation for competitive ferric iron inhibition, using the value of qO2,max measured in the BOM. Accordingly, only the kinetic constant Ks/K i needed to be fitted from the measurements. A new method was introduced to determine the steady-state kinetics of a cell suspension in a batch culture that only takes a few hours. The batch culture was set up by terminating the feeding of a continuous culture at its steady state. The kinetic constant K s/K i determined in this batch culture agreed with the value determined in continuous cultures at various steady states.

The ferrous iron oxidation kinetics of Thiobacillus ferrooxidans in batch cultures

The ferrous iron oxidation kinetics of Thiobacillus ferrooxidans in batch cultures was examined, using on-line off-gas analyses to measure the oxygen and carbon dioxide consumption rates continuously. A cell suspension from continuous cultures at steady state was used as the inoculum. It was observed that a dynamic phase occurred in the initial phase of the experiment. In this phase the bacterial ferrous iron oxidation and growth were uncoupled. After about 16 h the bacteria were adapted and achieved a pseudo-steady state, in which the specific growth rate and oxygen consumption rate were coupled and their relationship was described by the Pirt equation. In pseudo-steady state, the growth and oxidation kinetics were accurately described by the rate equation for competitive product inhibition. Bacterial substrate consumption is regarded as the primary process, which is described by the equation for competitive product inhibition. Subsequently the kinetic equation for the specific growth rate, μ, is derived by applying the Pirt equation for bacterial substrate consumption and growth. The maximum specific growth rate, μmax, measured in the batch culture agrees with the dilution rate at which washout occurs in continuous cultures. The maximum oxygen consumption rate, qO2,max, of the cell suspension in the batch culture was determined by respiration measurements in a biological oxygen monitor at excess ferrous iron, and showed changes of up to 20% during the course of the experiment. The kinetic constants determined in the batch culture slightly differ from those in continuous cultures, such that, at equal ferric to ferrous iron concentration ratios, biomass-specific rates are up to 1.3 times higher in continuous cultures.

Effect of pH on ferrous-iron oxidation kinetics of Leptospirillum ferrooxidans in continuous culture

The ferrous-iron oxidation kinetics of Leptospirillum ferrooxidans were studied at dilution rates ranging from 0.01 to 0.10 h −1, and pH values ranging from pH 1.10 to 1.70. The growth rate, and the oxygen and ferrous-iron utilisation rates of the bacteria, were monitored by means of off-gas analysis and redox potential measurement. The degree-of-reduction balance was used to compare the theoretical and experimental values of − r 2+ Fe, − r O 2 and r CO 2 , and the correlation found to be good. The bacterial culture maintained at pH 1.30 achieved the greatest measured maximum growth rate. An increase in the pH from pH 1.10 to pH 1.70 did not affect the maximum yield and maintenance coefficients, or the maximum specific ferrous-iron and oxygen utilisation rates. However, the kinetic constants in bacterial ferrous-iron oxidation, viz. K Fe 2+ and K O 2 , increased linearly with increasing pH, across the range from pH 1.10 to 1.70. The kinetics could be described in terms of the ferric/ferrous-iron ratio using a Michaelis–Menten based model modified to account for the effect of pH on K Fe 2+ and K O 2 . Furthermore, the threshold concentration of ferrous-iron increased with a decrease in the pH of the solution.