Fast kinetics of fe oxidation in packed-bed reactors

Thiobacillus ferrooxidans was used in fixed-film bioreactors to oxidize ferrous sulfate to ferric sulfate. Glass beads, ion-exchange resin, and activated-carbon particles were tested as support matrix materials. Activated carbon was tested in both a packed-bed bioreactor and a fluidized-bed bioreactor; the other matrix materials were used in packed-bed reactors. Activated carbon displayed the most suitable characteristics for use as a support matrix of T. ferrooxidans fixed-film formation. The reactors were operated within a pH range of 1.35 to 1.5, which effectively reduced the amount of ferric iron precipitation and eliminated diffusion control of mass transfer due to precipitation. The activated-carbon packed-bed reactor displayed the most favorable biomass holdup and kinetic performance related to ferrous sulfate oxidation. The fastest kinetic performance achieved with the activated-carbon packed-bed bioreactor was 78 g of Fe oxidized per liter per h (1,400 mmol of Fe oxidized per liter per h) at a true dilution rate of 40/h, which represents a hydraulic retention time of 1.5 min.     

Characterization of Jarosite Formed upon Bacterial Oxidation of Ferrous Sulfate in a Packed-Bed Reactor

A packed-bed bioreactor with activated-carbon particles as a carrier matrix material inoculated with Thiobacillus ferrooxidans was operated at a pH of 1.35 to 1.5 to convert ferrous sulfate to ferric sulfate. Despite the low operating pH, trace amounts of precipitates were produced in both the reactor and the oxidized effluent. X-ray diffraction and chemical analyses indicated that the precipitates were well-ordered potassium jarosite. The chemical analyses also revealed a relative deficiency of Fe and an excess of S in the reactor sample compared with the theoretical composition of potassium jarosite.     

Ferrous iron oxidation by foam immobilized Acidithiobacillus ferrooxidans: Experiments and modeling

Ferrous iron bio‐oxidation by Acidithiobacillus ferrooxidans immobilized on polyurethane foam was investigated. Cells were immobilized on foams by placing them in a growth environment and fully bacterially activated polyurethane foams (BAPUFs) were prepared by serial subculturing in batches with partially bacterially activated foam (pBAPUFs). The dependence of foam density on cell immobilization process, the effect of pH and BAPUF loading on ferrous oxidation were studied to choose operating parameters for continuous operations. With an objective to have high cell densities both in foam and the liquid phase, pretreated foams of density 50 kg/m³ as cell support and ferrous oxidation at pH 1.5 to moderate the ferric precipitation were preferred. A novel basket‐type bioreactor for continuous ferrous iron oxidation, which features a multiple effect of stirred tank in combination with recirculation, was designed and operated. The results were compared with that of a free cell and a sheet‐type foam immobilized reactors. A fivefold increase in ferric iron productivity at 33.02 g/h/L of free volume in foam was achieved using basket‐type bioreactor when compared to a free cell continuous system. A mathematical model for ferrous iron oxidation by Acidithiobacillus ferrooxidans cells immobilized on polyurethane foam was developed with cell growth in foam accounted by an effectiveness factor. The basic parameters of simulation were estimated using the experimental data on free cell growth as well as from cell attachment to foam under nongrowing conditions. The model predicted the phase of both oxidation of ferrous in shake flasks by pBAPUFs as well as by fully activated BAPUFs for different cell loadings in foam. Model for stirred tank basket bioreactor predicted within 5% both transient and steady state of the experiments closely for the simulated dilution rates. Bio‐oxidation at high Fe²⁺ concentrations were simulated with experiments when substrate and product inhibition coefficients were factored into cell growth kinetics. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Scanning electron microscopic examination of Thiobacillus ferrooxidans on different support matrix materials in packed bed and fluidized bed bioreactors

Summary Thiobacillus ferrooxidans was used in fixed-film bioreactors to oxidize ferrous iron to ferric iron. The test support matrix materials included activated carbon particles, glass beads, and ion-exchange resin particles. The experimental systems included a fluidized bed approach, which was evaluated with activated carbon only, and a packed bed approach which was tested with each of the support matrix materials. The colonization of the matrix surface was examined with scanning electron microscopy. There were contrasting differences in the bacterial colonization and accumulation of Fe(III) precipitates on the matrix surface among the test materials. The packed bed activated carbon bioreactor displayed the fastest kinetics and the highest amount of cell sorption as well as the roughest and most porous matrix surface.     

Biotic factor does not limit operational pH in packed-bed bioreactor for ferrous iron biooxidation

Ferrous ion biooxidation is a process with many promising industrial applications: mainly regeneration of ferric ion as an oxidizing reagent in bioleaching processes and depuration of acid mine drainage. The flooded packed-bed bioreactor (FPB) is the design that leads to the highest biooxidation rate. In this bioreactor, biomass is immobilized in a biofilm that consists of an inorganic matrix, formed by precipitated ferric compounds, in the pores of which cells are attached. This biofilm covers the surface of particles (siliceous stone) that form the bed. The chemical stability of this inorganic matrix defines the widest possible pH range in FPBs. At pH below 1, ferric matrix is dissolved and cells are washed out. At pH higher than 2, ferric ion precipitates massively, greatly hindering mass transfer to cells. Thus, among other parameters, pH is recognised as a key factor for operational control in FPBs. This paper aims to explain the effect of pH on FPB operation, with an emphasis on microbial population behaviour. FPBs seeded with mixed inocula were assayed in the pH range from 2.3 to 0.8 and the microbial population was characterised. The microbial consortium in the bioreactor is modified by pH; at pH above 1.3 Acidithiobacillus ferrooxidans is the dominant microorganism, whereas at pH below 1.3 Leptospirillum ferrooxidans dominates. Inoculum can be adapted to acidity during continuous operation by progressively decreasing the pH of the inlet solution. Thus, in the pH range from 2.3 to 1, the biotic factor does not compromise the bioreactor performance.     

Degradation of massive pyrite: Physical,chemical, and bacterial effects

Massive pyrite was shown to produce soluble iron, hydrogen, and sulfate ions on exposure to air and water. The rate of this process was directly proportional to the surface area of the mineral; it was unaffected by a drop in the pH and the presence of the ferrous and sulfate ions formed. Cupic ion had no effect but ferric ion accelerated pyrite degradation until all the ferric ion was consumed, in accordance with FeS₂ + 2Fe³⁺ —>‐3Fe²⁺ + 2S°. Thiobacillus ferrooxidans increased pyrite degradation considerably; the presence of Thiobacillus thiooxidans had no influence on pyrite degradation.     

Effect of ferric sulfate on activity of moderately thermophilic acidophilic iron-oxidizing microorganisms

The effect of high ferric sulfate concentrations on the organisms predominating in biohydrometallurgical processes (bacteria of genus Sulfobaсillus and archaea of the genus Acidiplasma) was studied. Ability of the studied strains to grow and oxidize ferrous iron in the media with 125 to 500 mM ferric sulfate was determined. High concentrations of ferric sulfate significantly inhibited the oxidative activity and growth of the studied microorganisms. Bacteria of the genus Sulfobaсillus were found to be incapable of active iron oxidation in the presence of ferric iron sulfate at concentrations exceeding 250 mM. Archaea of the genus Acidiplasma oxidized ferrous iron completely in the presence of 500 mM Fe³⁺. Microbial growth was suppressed by relatively low ferric sulfate concentrations. Almost no growth occurred at ferric sulfate concentrations exceeding 199 mM, while lysis of the cells of all studied strains was observed at higher Fe³⁺ concentrations. Archaea (genus Acidiplasma, family Ferroplasmaceae) were shown to be more tolerant to high ferric sulfate concentrations than bacteria of the genus Sulfobaсillus. The results obtained may be used for improvement of biohydrometallurgical technologies and are also important for the understanding of the patterns of formation of microbial communities carrying out the technological processes.     

Significance of Oxygen Supply in Jarosite Biosynthesis Promoted by Acidithiobacillus ferrooxidans

Jarosite [(Na⁺, K⁺, NH₄ ⁺, H₃O⁺)Fe₃(SO₄)₂(OH)₆] is an efficient scavenger for trace metals in Fe- and SO₄ ^2--rich acidic water. During the biosynthesis of jarosite promoted by Acidithiobacillus ferrooxidans, the continuous supply of high oxygen levels is a common practice that results in high costs. To evaluate the function of oxygen in jarosite production by A. ferrooxidans, three groups of batch experiments with different oxygen supply levels (i.e., loading volume percentages of FeSO₄ solution of 20%, 40%, and 70% v/v in the flasks), as well as three groups of sealed flask experiments with different limiting oxygen supply conditions (i.e., the solutions were not sealed at the initial stage of the ferrous oxidation reaction by paraffin but were rather sealed at the end of the ferrous oxidation reaction at 48 h), were tested. The formed Fe-precipitates were characterized via X-ray powder diffraction and scanning electron microscope-energy dispersive spectral analysis. The results showed that the biosynthesis of jarosite by A. ferrooxidans LX5 could be achieved at a wide range of solution loading volume percentages. The rate and efficiency of the jarosite biosynthesis were poorly correlated with the concentration of dissolved oxygen in the reaction solution. Similar jarosite precipitates, expressed as KFe₃ (SO₄) ₂(OH)₆ with Fe/S molar ratios between 1.61 and 1.68, were uniformly formed in unsealed and 48 h sealed flasks. These experimental results suggested that the supply of O₂ was only essential in the period of the oxidation of ferrous iron to ferric but was not required in the period of ferric precipitation.     

Rate Equations and Kinetic Parameters of the Reactions Involved in Pyrite Oxidation by Thiobacillus ferrooxidans

Rate equations and kinetic parameters were obtained for various reactions involved in the bacterial oxidation of pyrite. The rate constants were 3.5 μM Fe²⁺ per min per FeS₂ percent pulp density for the spontaneous pyrite dissolution, 10 μM Fe²⁺ per min per mM Fe³⁺ for the indirect leaching with Fe³⁺, 90 μM O₂ per min per mg of wet cells per ml for the Thiobacillus ferrooxidans oxidation of washed pyrite, and 250 μM O₂ per min per mg of wet cells per ml for the T. ferrooxidans oxidation of unwashed pyrite. The K_m values for pyrite concentration were similar and were 1.9, 2.5, and 2.75% pulp density for indirect leaching, washed pyrite oxidation by T. ferrooxidans, and unwashed pyrite oxidation by T. ferrooxidans, respectively. The last reaction was competitively inhibited by increasing concentrations of cells, with a K_i value of 0.13 mg of wet cells per ml. T. ferrooxidans cells also increased the rate of Fe²⁺ production from Fe³⁺ plus pyrite.     

Ferrous ion oxidation by Thiobacillus ferrooxidans immobilized in calcium alginate

Summary Thiobacillus ferrooxidans was immobilized by entrapment into calcium alginate matrix. The immobilized bacteria were used in packed-bed column reactors for the continuous oxidation of ferrous ion at pH 1.5. The presence of mineral salts resulted in a shorter lag period before a steady-state of about 95% iron oxidation was achieved. Parallel shake flask experiments were used to evaluate pH, mineral salts, and alginate toxicity as factors influencing biological iron oxidation. Manometric experiments indicated that the previous growth history of T. ferrooxidans was important in determining the rate of iron oxidation. Scanning electron microscopy and energy dispersive analysis of X-rays were used to characterize bacteria entrapped in calcium alginate and the enrichment of iron in the matrix.     

Role of a Ferric Ion-Reducing System in Sulfur Oxidation of Thiobacillus ferrooxidans

The properties of a ferric ion-reducing system which catalyzes the reduction of ferric ion with elemental sulfur was investigated with a pure strain of Thiobacillus ferrooxidans. In anaerobic conditions, washed intact cells of the strain reduced 6 mol of Fe³⁺ with 1 mol of elemental sulfur to give 6 mol of Fe²⁺, 1 mol of sulfate, and a small amount of sulfite. In aerobic conditions, the 6 mol of Fe²⁺ produced was immediately reoxidized by the iron oxidase of the cell, with a consumption of 1.5 mol of oxygen. As a result, Fe²⁺ production was never observed under aerobic conditions. However, in the presence of 5 mM cyanide, which completely inhibits the iron oxidase of the cell, an amount of Fe²⁺ production comparable to that formed under anaerobic conditions was observed under aerobic conditions. The ferric ion-reducing system had a pH optimum between 2.0 and 3.8, and the activity was completely destroyed by 10 min of incubation at 60°C. A short treatment of the strain with 0.5% phenol completely destroyed the ferric ion-reducing system of the cell. However, this treatment did not affect the iron oxidase of the cell. Since a concomitant complete loss of the activity of sulfur oxidation by molecular oxygen was observed in 0.5% phenol-treated cells, it was concluded that the ferric ion-reducing system plays an important role in the sulfur oxidation activity of this strain, and a new sulfur-oxidizing route is proposed for T. ferrooxidans.     

Coal Depyritization by the Thermophilic Archaeon Metallosphaera sedula

The kinetics of pyrite oxidation by Metallosphaera sedula were investigated with mineral pyrite and two coals with moderate (Pittsburgh no. 8) and high (New Brunswick, Canada) pyritic sulfur content. M. sedula oxidized mineral pyrite at a greater rate than did another thermophile, Acidianus brierleyi, or a mesophile, Thiobacillus ferrooxidans. Maximum rates of coal depyritization were also greater with M. sedula, although the magnitude of biological stimulation above abiotic rates was notably less than with mineral pyrite. Coal depyritization appears to be limited by the oxidation of pyrite with ferric ions and not by the rate of biotic oxidation of ferrous iron, as evidenced by the maintenance of a high ratio of ferric to ferrous iron in solution by M. sedula. Significant precipitation of hydronium jarosite at elevated temperature occurred only with New Brunswick coal.     

Competitive Inhibition of Ferrous Iron Oxidation by Thiobacillus ferrooxidans by Increasing Concentrations of Cells

The oxidation of ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) with dioxygen (O₂) by various strains of Thiobacillus ferrooxidans was studied by measuring the rate of O₂ consumption at various Fe²⁺ concentrations and cell concentrations. The apparent K_m values for Fe²⁺ remained constant at different cell concentrations of laboratory strains ATCC 13661 and ATCC 19859 but increased with increasing cell concentrations of mine isolates SM-4 and SM-5. The latter results are explained by the competitive inhibition of the Fe²⁺-binding site of a cell by other cells in the reaction mixture. Possible mechanisms involving cell surface properties are discussed.     

Ferric Iron Reduction by Acidophilic Heterotrophic Bacteria

Fifty mesophilic and five moderately thermophilic strains of acidophilic heterotrophic bacteria were tested for the ability to reduce ferric iron in liquid and solid media under aerobic conditions; about 40% of the mesophiles (but none of the moderate thermophiles) displayed at least some capacity to reduce iron. Both rates and extents of ferric iron reduction were highly strain dependent. No acidophilic heterotroph reduced nitrate or sulfate, and (limited) reduction of manganese(IV) was noted in only one strain (Acidiphilium facilis), an acidophile which did not reduce iron. Insoluble forms of ferric iron, both amorphous and crystalline, were reduced, as well as soluble iron. There was evidence that, in at least some acidophilic heterotrophs, iron reduction was enzymically mediated and that ferric iron could act as a terminal electron acceptor. In anaerobically incubated cultures, bacterial biomass increased with increasing concentrations of ferric but not ferrous iron. Mixed cultures of Thiobacillus ferrooxidans or Leptospirillum ferrooxidans and an acidophilic heterotroph (SJH) produced sequences of iron cycling in ferrous iron-glucose media.     

Effect of ferrous iron concentration on the catalytic activity of immobilized cells of Thiobacillus ferrooxidans

 The kinetics of continuous oxidation of ferrous iron by immobilized cells of Thiobacillus ferrooxidans was studied in a packed-bed bioreactor. Polyurethane foam biomass support particles were used as carriers for cell immobilization. Effects of ferrous iron concentration and its volumetric loading on the kinetics of the reaction were investigated. Media containing different concentrations of ferrous iron in the range 5–20 kg m^-3 were tested. For each medium the kinetics of the reaction at different volumetric loadings of ferrous iron, at a constant temperature of 30^°C, were determined. With media containing 5 kg m^-3 and 10 kg m^-3 Fe²⁺, the fastest oxidation rates of 34.25 kg m^-3 h^-1 and 32 kg m^-3 h^-1 were achieved at a dilution rate of around 6 h^-1, which represents a residence time of 10 min. Employing a higher concentration of ferrous iron (20 kg m^-3) in the medium resulted in lower oxidation rates, with a maximum value of 10 kg m^-3 h^-1, indicating an inhibitory effect of ferrous iron on growth and activity of T. ferrooxidans. The reliable performance of the bioreactor during the course of the experiments confirmed the suitability of polyurethane foam biomass support particles as carriers for T. ferrooxidans immobilization. Received: 5 December 1995/Received revision: 21 April 1996/Accepted: 29 April 1996     

Biooxidation of ferrous iron by immobilized Acidithiobacillus ferrooxidans in poly(vinyl alcohol) cryogel carriers

PVA-cryogels entrapping about 10⁹ cells of Acidithiobacillus ferrooxidans per ml of gel were prepared by freezing-thawing procedure, and the biooxidation of Fe²⁺ by immobilized cells was investigated in a 0.365 l packed-bed bioreactor. Fe²⁺ oxidation fits a plug-flow reaction model well. A maximum oxidation rate of 3.1 g Fe²⁺ l^–1 h^–1 was achieved at the dilution rate of 0.4 h^–1 or higher, while no obvious precipitate was determined at this time. In addition, cell-immobilized PVA-cryogels packed in bioreactor maintained their oxidative ability for more than two months under non-sterile conditions. Nomenclature: C _A0 – Concentration of Fe²⁺ in feed stream (g l^–1) C _A – Concentration of Fe^{2 +} in outlet stream (g l^{– 1}) D – Dilution rate of the packed-bed bioreactor (h^–1) F – Volumetric flow rate of iron solution (l h^–1) F _A0 – Mass flow rate of Fe²⁺ in the feed stream (g h^–1) K – Kinetic constant (l l^–1 h^–1) r _A – Oxidation rate of Fe²⁺ (g l^–1 h^–1) V – Volume of packed-bed bioreactor (l) X _A – Conversion ratio of Fe²⁺ (%)     

Oxidative Dissolution of Arsenopyrite by Mesophilic and Moderately Thermophilic Acidophiles

The purpose of this work was to determine solution- and solid-phase changes associated with the oxidative leaching of arsenopyrite (FeAsS) by Thiobacillus ferrooxidans and a moderately thermoacidophilic mixed culture. Jarosite [KFe₃(SO₄)₂(OH)₆], elemental sulfur (S⁰), and amorphous ferric arsenate were detected by X-ray diffraction as solid-phase products. The oxidation was not a strongly acid-producing reaction and was accompanied by a relatively low redox level. The X-ray diffraction lines of jarosite increased considerably when ferrous sulfate was used as an additional substrate for T. ferroxidans. A moderately thermoacidophilic mixed culture oxidized arsenopyrite faster at 45°C than did T. ferroxidans at 22°C, and the oxidation was accompanied by a nearly stoichiometric release of Fe and As. The redox potential was initially low but subsequently increased during arsenopyrite oxidation by the thermoacidophiles. Jarosite, S⁰, and amorphous ferric arsenate were also formed under these conditions.     

A potentiometric and kinetic study on the respiratory chain of ferrous-iron-grown Thiobacillus ferrooxidans

The type and number of respiratory chain components present in membranes of Thiobacillus ferrooxidans have been investigated. These redox components were resolved potentiometrically and kinetically. Using optical techniques two cytochromes a₁, multiple cytochromes c and two cytochromes b were detected. By using electron paramagnetic resonance, two copper-containing centres, high and low spin ferric haems and a ferredoxin centre were detected. Based on the combination of a potentiometric resolution and a kinetic study a model for the sequence of the respiratory chain components in the Fe²⁺ to O₂ branch of the T. ferrooxidans respiratory chain is proposed.     

Iron Oxidation and Precipitation of Ferric Hydroxysulfates by Resting Thiobacillus ferrooxidans Cells

The oxidation of ferrous ions, in acid solution, by resting suspensions of Thiobacillus ferrooxidans produced sediments consisting of crystalline jarosites, amorphous ferric hydroxysulfates, or both. These products differed conspicuously in chemical composition and infrared spectra from precipitates formed by abiotic oxidation under similar conditions. The amorphous sediments, produced by bacterial oxidation, exhibited a distinctive fibroporous microstructure when examined by scanning electron microscopy. Infrared spectra indicated outer-sphere coordination of Fe(III) by sulfate ions, as well as inner-sphere coordination by water molecules and bridging hydroxo groups. In the presence of excess sulfate and appropriate monovalent cations, jarosites, instead of amorphous ferric hydroxysulfates, precipitated from bacterially oxidized iron solutions. It is proposed that the jarositic precipitates result from the conversion of outer-sphere (T_d) sulfate, present in a soluble polymeric Fe(III) complex, to inner-sphere (C_3v) bridging sulfate. The amorphous precipitates result from the further polymerization of hydroxo-linked iron octahedra and charge stabilized aggregation of the resulting iron complexes in solution. This view was supported by observations that bacterially oxidized iron solutions gave rise to either amorphous or jarositic sediments in response to ionic environments imposed after oxidation had been completed and the bacteria had been removed by filtration.     

An ultraviolet spectrophotometric method for the determination of pyrite and ferrous ion oxidation by Thiobacillus ferrooxidans

Summary An ultraviolet spectrophotometric method was used to monitor the formation of soluble ferric iron in acid culture solutions of Thiobacillus ferrooxidans. This methodology was demonstrated to be applicable for determining both pyrite and ferrous ion oxidation. Kinetic parameters of Fe²⁺ oxidation determined with the use of this method were in close agreement with those previously obtained by measurement of oxygen uptake rates.