High Concentrations Of Ferric Iron Research Articles

The effect of iron status on gadolinium deposition in the rat brain: mechanistic implications.

Introduction: Sites associated with gadolinium (Gd) deposition in the brain (e.g., the globus pallidus) are known to contain high concentrations of ferric iron. There is considerable debate over the mechanism of Gd deposition in the brain. The role of iron transport mechanisms in Gd deposition has not been determined. Thus, we seek to identify if Gd deposition can be controlled by modifying iron exposure. Methods: Female Sprague-Dawley rats were given diets with controlled iron levels at 2-6ppm, 6ppt (20g/kg Fe carbonyl) or 48ppm for 3weeks to induce iron deficiency, overload or normalcy. They were kept on those diets while receiving a cumulative 10mmol/kg dose of gadodiamide intravenously over 2weeks, then left to washout gadodiamide for 3days or 3weeks before tissues were harvested. Gd concentrations in tissues were analyzed by ICP-MS. Results: There were no significant effect of dietary iron and total Gd concentrations in the organs, but there was a significant effect of iron status on Gd distribution in the brain. For the 3-week washout cohort, there was a non-significant trend of increasing total brain deposition and decreasing dietary iron, and about 4-fold more Gd in the olfactory bulbs of the low iron group compared to the other groups. Significant brain accumulation was observed in the low iron group total brain Gd in the 3-week washout group relative to the 3-day washout group and no accumulation was observed in other tissues. There was a strong negative correlation between femur Gd concentrations and concentrations in other organs when stratifying by dietary iron. Discussion: Gd brain deposition from linear Gd-based contrast agents (GBCAs) are dependent upon iron status, likely through variable transferrin saturation. This iron dependence appears to be associated with redistribution of peripheral deposited Gd (e.g., in the bone) into the brain.

Sticky bacteria: Combined effect of galactose and high ferric iron concentration on extracellular polymeric substances production and the attachment of Acidithiobacillus ferrooxidans on a polymetallic sulfide ore surface.

Adaptation and microbial attachment mechanisms for the degradation of sulfide ores are mediated by the production of extracellular polymeric substances (EPS) and their role in biofilm formation. EPS production responds to induction mechanisms associated with environmental conditions. In this study, the double induction of EPS with galactose and high ferric iron concentrations in planktonic cells of Acidithiobacillus ferrooxidans, and their attachment on the surface of a polymetallic sulfide ore from Bella Rica-Azuay in Ecuador were evaluated. A. ferrooxidans cells were previously adapted to different concentrations of galactose [0, 0.15, and 0.25% (w/v)], using two ferrous iron concentrations as an energy source (9 and 18 g L–1) in a 9K culture medium. EPS production and its effect on mineral attachment were determined at the time point of maximal growth. The results obtained show a maximum cell attachment of 94.1% within 2 h at 0.15% of galactose and 18 g⋅L–1 of ferric iron concentration, compared to 71.4% without galactose and 9 g⋅L–1 of ferric iron. The maximum concentration of EPS was obtained with a 0.25% galactose concentration; however, it did not result in greater attachment compared to 0.15% galactose concentration. Through the combined induction of low galactose concentration and high ferric iron concentration, the percentage of bacterial attachment can be increased and, therefore, a possible increase in the rate of biooxidation and bioleaching could be obtained.

Decreased phosphorus incorporation explains the negative effect of high iron concentrations in the green microalga Chlamydomonas acidophila

The green microalga Chlamydomonas acidophila is an important primary producer in very acidic lakes (pH 2.0–3.5), characterized by high concentrations of ferric iron (up to 1 g total Fe L−1) and low rates of primary production. It was previously suggested that these high iron concentrations result in high iron accumulation and inhibit photosynthesis in C. acidophila. To test this, the alga was grown in sterilized lake water and in medium with varying total iron concentrations under limiting and sufficient inorganic phosphorus (Pi) supply, because Pi is an important growth limiting nutrient in acidic waters. Photosynthesis and growth of C. acidophila as measured over 5 days were largely unaffected by high total iron concentrations and only decreased if free ionic Fe3+ concentrations exceeded 100 mg Fe3+ L−1. Although C. acidophila was relatively rich in iron (up to 5 mmol Fe: mol C), we found no evidence of iron toxicity. In contrast, a concentration of 260 mg total Fe L−1 (i.e. 15 mg free ionic Fe3+ L−1), which is common in many acidic lakes, reduced Pi-incorporation by 50% and will result in Pi-limited photosynthesis. The resulting Pi-limitation present at high iron and Pi concentrations was illustrated by elevated maximum Pi-uptake rates. No direct toxic effects of high iron were found, but unfavourable chemical Pi-speciation reduced growth of the acidophile alga.

The Histidine Residue of QseC Is Required for Canonical Signaling between QseB and PmrB in Uropathogenic Escherichia coli.

Two-component systems are prototypically comprised of a histidine kinase (sensor) and a response regulator (responder). The sensor kinases autophosphorylate at a conserved histidine residue, acting as a phosphodonor for subsequent phosphotransfer to and activation of a cognate response regulator. In rare cases, the histidine residue is also essential for response regulator dephosphorylation via a reverse-phosphotransfer reaction. In this work, we present an example of a kinase that relies on reverse phosphotransfer to catalyze the dephosphorylation of its cognate partner. The QseC sensor kinase is conserved across several Gram-negative pathogens; its interaction with its cognate partner QseB is critical for maintaining pathogenic potential. Here, we demonstrate that QseC-mediated dephosphorylation of QseB occurs via reverse phosphotransfer. In previous studies, we demonstrated that, in uropathogenic Escherichia coli, exposure to high concentrations of ferric iron (Fe3+) stimulates the PmrB sensor kinase. This stimulation, in turn, activates the cognate partner, PmrA, and noncognate QseB to enhance tolerance to polymyxin B. We demonstrate that in the absence of signal, kinase-inactive QseC variants, in which the H246 residue was changed to alanine (A) aspartate (D) or leucine (L), rescued a ΔqseC deletion mutant, suggesting that QseC can control QseB activation via a mechanism that is independent of reverse phosphotransfer. However, in the presence of Fe3+, the same QseC variants were unable to mediate a wild-type stimulus response, indicating that QseC-mediated dephosphorylation is required for maintaining proper QseB-PmrB-PmrA interactions.IMPORTANCE Two-component signaling networks constitute one of the predominant methods by which bacteria sense and respond to their changing environments. Two-component systems allow bacteria to thrive and survive in a number of different environments, including within a human host. Uropathogenic Escherichia coli, the causative agent of urinary tract infections, rely on two interacting two-component systems, QseBC and PmrAB, to induce intrinsic resistance to the colistin antibiotic polymyxin B, which is a last line of defense drug. The presence of one sensor kinase, QseC, is required to regulate the interaction between the other sensor kinase, PmrB and the response regulators from both systems, QseB and PmrA, effectively creating a "four-component" system required for virulence. Understanding the important role of the sensor kinase QseC will provide insight into additional ways to therapeutically target uropathogens that harbor these signaling systems.

Resistance of Moderately Thermophilic Acidophilic Microorganisms to Ferric Iron Ions

Resistance of microorganisms predominating in biohydrometallurgical processes including bacteria of the genus Sulfobaсillus and archaea of the genus Acidiplasma to ferric iron ions was studied. Capabilities of the strains for growth and ferrous iron oxidation in the media containing high concentrations of ferric iron ions (of 250 to 1000 mM) were evaluated. Ferric iron ions significantly inhibited oxidative activity and growth of the studied microorganisms. It was revealed that bacteria of the genus Sulfobacillus were not able to oxidize ferrous iron actively when ferric iron concentration exceeded 500 mM, whereas archaea of the genus Acidiplasma completely oxidized ferrous iron in the medium containing 1000 mM of Fe3+. Growth of the microorganisms was inhibited by relatively low concentrations of ferric iron. Microorganisms did not grow in the medium containing more than 750 mM of Fe3+ and cells of all studied strains lysed in presence of high concentrations of ferric iron. It was shown, that archaea of the genus Acidiplasma of the family Ferroplasmaceae were more resistant to high concentrations of ferric iron than bacteria of the genus Sulfobacillus. The results obtained are important for understanding of the regularities of the formation of microbial communities performing technological processes.

Electrolytic Treatment of Synthetic Acid Mine Drainage Containing High Concentrations of Ferric Iron

Electrolytic Treatment of Synthetic Acid Mine Drainage Containing High Concentrations of Ferric Iron

Galactose as Inducer of the Production of Extracellular Polymeric Substances by <i>Acidithiobacillus ferrooxidans</i>

The presence of extracellular polymeric substances (EPS) is important in the building of biofilms on mineral surfaces, increasing the bioleaching activity, as well as protecting the cells from adverse environmental conditions. The objective of this work was to study the effect of galactose in EPS production byAcidithiobacillus ferrooxidans. The experiences were performed in shake flask of 250 mL at 30 °C, 200 rpm and at an initial pH of 1.8. In order to establish the natural tolerance of the strain, its growth behaviour was evaluated at high ferric iron concentrations by adding consecutively the equivalent of 9 g/L of ferrous iron each time it was depleted in the broth. Cell growth stopped once ferric iron concentration increased up to 38 g/L. In order to determine the optimal conditions for EPS production, experiments were run in a chemostat of 0.5 L, operated at a constant dilution rate of 0.03 h-1. Different steady states were obtained varying feeding concentrations of galactose (0.15%; 0.25% and 0.35%) and carbon dioxide (180 ppm and 360 ppm). , Cells grown in the chemostat at optimum operation conditions were used as inoculum to determine oxidative capacity of the microorganisms overproducing EPS. The EPS was quantified using confocal laser scanning microscopy (CLSM), labelling the cells with propidium iodide and EPS carbohydrates with wheat germ agglutinin (WGA). The higher volume production of EPS was observed in cells grown using 360 ppm of CO2and 0.35% of galactose. Also it was observed a size increment of cells, compared to cells grown in culture medium having 9 g/L of ferrous iron where presence of EPS was no detected. The results revealed that EPS overproducing A. ferrooxidansshowed a tolerance to ferric iron concentration almost 9.5 g/L higher than the natural tolerance of cells grown in absence of galactose. Presence of galactose in culture medium stimulated the EPS production.

EPS Production during Adaptation of <i>Acidithiobacillus ferrooxidans</i> to High Ferric Ion Concentration

The ability ofAcidithiobacillus ferrooxidansto get its energy from the oxidation of ferrous iron and the inhibitory effect of high ferric iron concentrations on its growth behaviour has been extensively studied. Furthermore it is known thatA. ferrooxidansexudes organic substances called extracellular polymeric substances (EPS), which could play a role in its protection against adverse environmental conditions. In this context, the aim of this work was to study the production of EPS during adaptation ofA. ferrooxidansto high ferric ion concentrations. The experiments were performed in shake flasks of 250 mL at 30 °C, 200 rpm and at an initial pH of 1.8. In order to establish the natural tolerance of the strain, its growth behaviour was evaluated at high ferric iron concentrations by adding consecutively the equivalent of 9 g/L of ferrous iron each time it was depleted in the broth. Cell growth stopped once ferric iron concentration increased up to 38 g/L. The adaptation consisted in eight sub-cultures run in parallel at initial concentrations of ferrous iron of 18, 27 and 36 g/L. The EPS was quantified as micro volumes using confocal laser scanning microscopy (CLSM), labelling the cells with propidium iodide and EPS carbohydrates with wheat germ agglutinin (WGA). During the adaptation procedure it was observed an increase in the ferric ion volumetric productivity of subcultures run with 27 and 36 g/L, as a result of cell adaptation. The amount of EPS exuded by cells was higher along with those experimental conditions having higher ferric iron concentrations. It was not detected EPS on cells grown on 9 g/L of ferrous iron. This study found that the adapted strain showed higher production of EPS at high ferric ion concentrations and higher ferric ion tolerance than non-adapted ones.

Specific jarosite biomineralization by Purpureocillium lilacinum, an acidophilic fungi isolated from Río Tinto

Río Tinto (Huelva, southwestern Spain) is an extreme environment with a remarkably constant acidic pH and a high concentration of heavy metals, conditions generated by the metabolic activity of chemolithotrophic microorganisms thriving in the rich complex sulfides of the Iberian Pyrite Belt (IPB). Fungal strains isolated from the Tinto basin were characterized morphologically and phylogenetically. The strain identified as Purpureocillium lilacinum specifically induced the formation of a yellow-ocher precipitate, identified as hydronium-jarosite, an iron sulfate mineral which appears in abundance on the banks of Río Tinto. The biomineral was characterized by X-ray diffraction (XRD) and its formation was observed with high-resolution transmission electron microscopy (TEM) and scanning electron microscopy (SEM) coupled to energy-dispersive X-ray spectroscopy (EDX) microanalysis. Jarosite began to nucleate on the fungal cell wall, associated to the EPS, due to a local increase in the Fe(3+) /Fe(2+) ratio which generated supersaturation. Its formation has been also observed in non-viable cells, although with much less efficiency. The occurrence of P. lilacinum in an ecosystem with high concentrations of ferric iron and sulfates such as Río Tinto suggests that it could participate in the process of jarosite precipitation, helping to shape and control the geochemical properties of this environment.

Stibnite (Sb2S3) oxidative dissolution kinetics from pH 1 to 11

The kinetics of oxidative dissolution of natural stibnite (Sb2S3) under environmentally relevant conditions is investigated. The effects of hydrogen ion activity (pH 1.08–10.63), dissolved oxygen saturation (p(O2) from 0 to 80% of atmospheric pressure), and temperature (25–48°C) on the reaction rate were studied using a mixed flow reactor. Trivalent cations (Fe3+, Al3+, Ce3+) in acidic solution, and divalent cations (Mg2+, Ca2+) in basic solution were found to accelerate the rates, with the effect being related to ionic radius. In both acid and basic domains, the stibnite dissolution rate was found to be proportional to fractional powers of hydrogen ion and dissolved oxygen activities. The rate dependence on dissolved oxygen could also be described in terms of Langmuir adsorption, with saturation predicted to occur at oxygen partial pressures in excess of 3.5atm in acid solution and 15.5atm in basic solution. The empirical rate equations are given by:r=k′a(H+)αθ=k′a(H+)αKa(O2)1+Ka(O2)where k′=1.25×10−10±0.02×10−10; α=0.133±0.005; K=2.22×104±0.16×104mol−1l in acid solution (pH 3.0) and k′=8.88×10−12±0.62×10−12; α=−0.133±0.008; K=5.01×103±1.24×103mol−1l in basic solution (pH 9.8), both at 25°C. Activation energies are 7.4±0.5kJmol−1 (pH 3.0, between 25°C and 40°C) and 56.1±5.0kJmol−1 (pH 9.9, between 25°C and 48°C), suggesting diffusion control in acidic solution and surface control in basic solution. Ferric iron in acidic solution strongly catalyzes the dissolution, with an acceleration of the rate by approximately an order of magnitude for a(Fe3+)=4.03×10−4M at pH 2.0. Aluminum ion, which has an ionic radius (54pm) almost identical to that of ferric iron (55pm), produces a similar rate enhancement. The effect of cationic radius on the dissolution rate of stibnite in the acid domain was evaluated using Ce3+ (114pm), which increased the rate even more. In basic solution, Mg2+ and Ca2+ increased the dissolution rate; the effect was stronger with the larger of the two cations (Ca2+). Stibnite dissolution was incongruent to varying degrees, with elemental sulfur forming a residual surface layer, in both acid and basic solution. In the presence of high concentrations of ferric iron, the dissolution became highly incongruent, with a substantial accumulation (7–13 atomic layers) of elemental sulfur. In contrast to Fe3+, neither Ce3+ nor Al3+ had an effect on the formation of elemental sulfur, clearly demonstrating the important role played by Fe3+ in the oxidation of sulfide to elemental sulfur. In the case of iron catalyzed dissolution, the elemental sulfur formed at the reacting surface accounts for a very significant proportion of the stoichiometric total reactive sulfur, so that reaction rates may not be assessed by measuring the concentration of dissolved sulfur species.

Aqueous geochemistry and oxygen isotope compositions of acid mine drainage from the Río Tinto, SW Spain, highlight inconsistencies in current models

The Río Tinto river in SW Spain is a classic example of acid mine drainage and the focus of an increasing amount of research including environmental geochemistry, extremophile microbiology and Mars-analogue studies. Its 5000-year mining legacy has resulted in a wide range of point inputs including spoil heaps and tunnels draining underground workings. The variety of inputs and importance of the river as a research site make it an ideal location for investigating sulphide oxidation mechanisms at the field scale. Mass balance calculations showed that pyrite oxidation accounts for over 93% of the dissolved sulphate derived from sulphide oxidation in the Río Tinto point inputs. Oxygen isotopes in water and sulphate were analysed from a variety of drainage sources and displayed δ 18O (SO4–H2O) values from 3.9 to 13.6‰, indicating that different oxidation pathways occurred at different sites within the catchment. The most commonly used approach to interpreting field oxygen isotope data applies water and oxygen fractionation factors derived from laboratory experiments. We demonstrate that this approach cannot explain high δ 18O (SO4–H2O) values in a manner that is consistent with recent models of pyrite and sulphoxyanion oxidation. In the Río Tinto, high δ 18O (SO4–H2O) values (11.2–13.6‰) occur in concentrated (Fe = 172–829 mM), low pH (0.88–1.4), ferrous iron (68–91% of total Fe) waters and are most simply explained by a mechanism involving a dissolved sulphite intermediate, sulphite–water oxygen equilibrium exchange and finally sulphite oxidation to sulphate with O 2. In contrast, drainage from large waste blocks of acid volcanic tuff with pyritiferous veins also had low pH (1.7), but had a low δ 18O (SO4–H2O) value of 4.0‰ and high concentrations of ferric iron (Fe(III) = 185 mM, total Fe = 186 mM), suggesting a pathway where ferric iron is the primary oxidant, water is the primary source of oxygen in the sulphate and where sulphate is released directly from the pyrite surface. However, problems remain with the sulphite–water oxygen exchange model and recommendations are therefore made for future experiments to refine our understanding of oxygen isotopes in pyrite oxidation.

Bioleaching of molybdenite

Molybdenum was bioleached from molybdenite-containing mine waste samples and from high purity molybdenite in solutions amended with ferrous sulfate. Bioleaching of Mo occurred when solution Eh exceeded 750 mV to 800 mV (s.h.e.). The rate of bioleaching of high purity molybdenite increased with decreasing particle size, averaging 3.22 mg Mo/m 2/d at 24 °C. This surface area-based rate is less than 1% of the bioleaching rate of pyrite. Molybdenite bioleaching increased with temperature by a factor of approximately 2.5 for each 10 °C increase in leaching temperature up to 40 °C. Apparent activation energies were 61.2 to 73.4 kJ/mol for bioleaching of high purity molybdenite and for molybdenite from mineral processing waste streams, respectively. Thermophilic bioleaching at 65 °C did not improve the rate or extent of Mo extraction. Molybdenum extraction was about 50% from mineral process waste stream samples after the first month of bioleaching, and slowed thereafter. Ultimate Mo extraction was approximately 85% after 6 months of bioleaching. High concentrations of ferric iron in bioleach solutions allowed iron-oxidizing microorganisms to grow, oxidize iron and bioleach molybdenite at dissolved Mo concentrations as high as 4.4 g/L. There was no evidence for selection of microbial populations with increased Mo resistance.

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.

Silver-catalyzed bioleaching of low-grade copper ores.: Part I: Shake flasks tests

A study of the effect of different variables (inoculation, pulp density, [Ag], nutrient medium, pH and [Fe 3+]) on the silver-catalyzed bioleaching of a low-grade copper sulfide ore has been carried out in shake flasks. Chalcopyrite was the dominant copper mineral in the ore. Preliminary tests showed that addition of other ions (Sb, Bi, Co, Mn, Ni and Sn) did not enhance the copper dissolution rate. Conversely, an inoculation with mesophilic microorganisms and the addition of silver had a markedly catalytic effect on the extraction of copper. The kinetics of the silver-catalyzed chalcopyritic ore bioleaching was greatly affected by pulp density and silver concentration. Small amounts of silver (14.7 g Ag/kg Cu) dramatically accelerated the copper dissolution process while large amounts (294.12 g Ag/kg Cu) had an inhibitory effect. The copper dissolution rate was slightly affected in the range of pH between 1.2 and 2.5 but was significantly slower at pH 3.0. The effect of [Fe 3+] in the presence of silver was studied both in abiotic and biotic conditions. High ferric iron concentrations in abiotic tests recovered similar copper amounts (∼ 95%) to those obtained without or with low [Fe 3+] in the presence of bacteria. The leaching of copper from the low-grade copper ore can be very effectively enhanced with silver and mesophilic microorganisms. For that system, the onset of oxidizing conditions starts at an Eh value slightly higher than 650 mV. Above that critical value of potential the copper dissolution rate slows down. This also corresponds with the completion of the leaching process. As the potential rises past 650 mV, the copper extraction reaches a plateau.

Geological record of an acidic environment driven by iron hydrochemistry: The Tinto River system

The existence of possible hematitic strata on the surface of Mars demands a search for terrestrial analogues formed in unusual environments. This will help us to recognize and interpret environmental and, perhaps, biological signatures preserved in Mars' hematites. Such an analogue would allow us to establish valid reference systems based on geomicrobial and biogeochemical signatures. Two different aspects place the Tinto River inside the boundaries of a natural extreme system: its high level of biological diversity and the presence of fluvial rocks formed in the same acidic conditions as in the modern system, which could predate the Tertiary. Study of both the modern system and the ancient system is necessary to understand the formation of biosignatures. A chemolithotrophic community that biooxidizes the Iberian Pyritic Belt, acidifying water (pH between 0.9 and 3.0) and favoring high concentrations of ferric iron in solution (up to 20 g·L−1), maintains this iron‐driven system. In spite of these extreme conditions, high microbial diversity was found. Its acidic bacteria, archaea, and eukarya constitute a complex community supported by algal biomass in highly stable hydrochemical conditions, which are achieved through iron buffering. The pH is maintained at constant low levels even at very high water dilution. In these conditions, iron minerals as oxyhydroxides, hydroxides, and sulfates are formed. The modern and recent parageneses contrast with the ancient Tinto River terrace mineral associations, which show dehydrated and desulfated iron oxides. If this dehydration process is considered, these Tinto River ironstones may be a key for knowing some aquatic habitats, which may have hosted a part of the early Mars biosphere.

Crystal structure and Mossbauer spectroscopy of the synthetic amphibole potassic-ferri-ferrorichterite at 298 K and low temperatures (80-110 K)

The crystal structure of a synthetic potassic-ferri-ferrorichterite, ideally K(Ca 0.5 Na 1.5 )Fe 5 Si 8 O 22 (OH) 2 was refined from single-crystal X-ray imaging plate diffraction data at 298 K and 110 K. The title compound is monoclinic, S.G. C 2/ m , Z = 2, with a = 10.145(1) A, b = 18.184(1) A, c = 5.296(1) A, β = 104.42(1)° at 298 K (sample Ri1). The structure is consistent with the general amphibole structure type. There is no crystallographic phase transition down to 110 K. The two nonequivalent T sites and the octahedral M1 and M3 sites behave very rigidly upon cooling. Bond lengths for the distorted M sites slightly decrease as temperature is lowered. The tetrahedral chain kinking increases by 0.9° between 298 K and 110 K. Similar behavior was found for a second sample investigated by single crystal X-ray diffraction. Mossbauer spectra, collected at 298 K, yield rather high ferric iron concentrations up to 2.0 a.p.f.u for the three samples investigated. Fe 3+ prefers the M2 site. Three components were detected within the ferrous quadrupole splitting distribution (QSD) of the C-type sites and where assigned to the M1, M3 and M2 sites respectively. This assignment is consistent with polyhedral distortion and gives correct site occupancies. The magnitude of the quadrupole splitting of Fe 2+ on M1 and M3 sites shows a dependency on the ferric iron content (mean cation radius of the M2 site) of the sample.

An experimental study on iron removal with ferric sludge recycling

An iron removal process, which makes use of the catalytic effect of ferric iron, is proposed. For this purpose, the reaction kinetics derived from the data of the batch experiments was applied to the continuous flow system. Based upon this reaction kinetics, it has been theoretically demonstrated that the volumes of aeration tanks can be significantly reduced by keeping a high concentration of ferric iron in the reactor. However, in natural waters, Fe(II) is found commonly to be in the range of 0.01–10 mg/l. These ferrous iron concentrations are not high enough to maintain the high concentrations of ferric iron in the aeration tank. Therefore, similar to the activated sludge processes used in wastewater treatment, it is suggested that the required Fe(III) concentrations can be maintained by recycling Fe(OH)3 sludge back to the aeration tank. It is known that the oxygenation of ferrous iron is catalyzed by the reaction product, ferric hydroxide. Catalytic action of the ferric iron sludges on the oxidation of ferrous iron by aeration has been identified and the kinetics of this catalytic reaction has been formulated by the authors. The oxidation of Fe(II) was studied in batch reactors in which the concentration of Fe(III) was in the range of 0–600 mg/l. The oxygenation rate increased linearly with the increasing Fe(III) concentrations up to 50 mg/l and a second-order polynomial relationship was found between the reaction rate and the Fe(III) concentrations in the range of 50–600 mg/l. The required volume (V) of the aeration tank and the effluent Fe(II) concentrations were determined as a function of the Fe(III) concentration. The volume of the aeration tank required for the same Fe(II) conversion was reduced by a factor of 15 when the Fe(III) concentration was raised from 0 to 600 mg/l at pH=6.7. No incremental benefit of the increase of Fe(III) concentration was observed at Fe(III) levels beyond the 600 mg/l. This study has experimentally demonstrated that significant savings can be achieved in iron removal systems by recirculating the Fe(III) sludges back to the aeration tank.

The ferric leaching kinetics of arsenopyrite

In this investigation batch, ferric leaching experiments were carried out in a 100 mℓ jacketed vessel maintained at 25°C. The parameters varied during the course of the experimental program included the initial redox potential, the total iron concentration, the solids concentration and the pH of the leaching solution. The initial redox potential used ranged from 625 to 470 mV, the overall iron concentration ranged from 8 to 32 g.ℓ −1, the mineral concentration ranged from 5 to 20 g.ℓ −1 and the initial pH used ranged from 1.10 to 1.45. The redox potential of the leach solution was monitored continuously using a redox probe connected to a computer. The leach rates were calculated from the measured change in the redox potential of the leaching solution. The variation in the ferric leaching rate of the arsenopyrite as a function of the solution redox potential displayed similar trends, irrespective of the conditions employed. The ferric leaching rate of the arsenopyrite decreased with decreasing redox potential of the leaching solution and could be accurately described using a modified Butler–Volmer equation; − r Fe 2+ = r 0(e αβ( E− E′) −e (1− α( β( E− E′) ). High concentrations of ferric iron and protons, and a reduction in the solids concentration were found to impede the leach rate. The `rest potential' ( i.e., the redox potential at which the dissolution of arsenopyrite stops) of the arsenopyrite was found to be higher under these conditions. However, no occluding sulphur layer could be detected on the surface of mineral particles, hence the results suggest that the reactivity of the mineral decreases with an increase in the effective concentration of the ferric iron species. Therefore, although the results suggest the likelihood of an electrochemical mechanism being operative, it is necessary to modify the Butler–Volmer-based model to account for the above observations in order to obtain a model capable of predicting the ferric leaching rate of arsenopyrite across a broad range of operating conditions.

Bacterial oxidation conditions for gold extraction from Olympias refractory arsenical pyrite concentrate

The arsenical pyrite concentrate produced at the Olympias mine, Macedonia, Greece, assays approximately Fe: 40%, S: 40%, As: 12% and Au: 26g/t. Mineralogically, it consists mainly of pyrite (68–70%) and arsenopyrite (23–26%), the former includes both arsenic-free and arsenian pyrite. Gold is mainly associated with the arsenopyrite and arsenian pyrite. The concentrate is highly refractory in nature, as direct cyanidation yields gold recovery lower than 10%. An oxidative pretreatment step is, therefore, necessary before cyanidation, in order to break up the sulphide lattice and liberate gold. The present paper aims at elucidating the effect of the leaching conditions on the bacterial oxidation of the Olympias concentrate. This research subject is of particular interest because selective oxidation of the arsenopyrite and arsenical pyrite fractions may result in high gold recoveries without the need for total sulphur oxidation. Based on the above, this study is focused on the factors that enhance preferential arsenopyrite oxidation. It has shown that preferential oxidation of arsenopyrite is observed especially at short retention times and pulp densities in excess of 10% solids. Arsenopyrite oxidation is complete at EMF values of 480–550 mV, while the oxidation of pyrite is observed to commence at higher EMF values, where the oxidation of arsenopyrite is almost complete. High ferric iron concentrations in solution enhance arsenopyrite but have an adverse affect on pyrite oxidation rates. When operating at constant pH values in the range 1.0–1.2, selective oxidation of arsenopyrite is observed, while pyrite oxidation proceeds at higher pH values, around 1.5. The indirect mechanism is deduced to play a significant role in the bio-oxidation of arsenopyrite, while pyrite oxidation is mainly attributed to direct bacterial attack.

Contact aeration for iron removal—A theoretical assessment

An iron removal process which makes use of the catalytic effect of ferric iron is proposed. Based upon the oxygenation rate equation, it has been theoretically demonstrated that the volumes of aeration tanks can be significantly reduced by keeping high concentration of ferric iron in the reactor. Ferric iron is more effective in reducing the reactor volumes at lower pH values. Recycling of ferric sludge is proposed to maintain the high ferric iron concentrations in the reactor.