Kinetics Of Iron Oxidation Research Articles

Ferrous iron oxidation efficiency and kinetics by indigenous iron-oxidizing bacteria in acid mine drainage

Biological Fe(II) oxidation by iron-oxidising bacteria (FeOBs) at low pH is a cost-effective treatment for acid mine drainage (AMD). However, treatments based on this process are limited because of uncertainties regarding the ability and rate of oxidation of Fe(II) from AMD. In the present study, an indigenous FeOBs consortium was enriched in AMD, and its ability to oxidise Fe(II) is described. The bio-oxidation rate of Fe(II) was 39.1 mg/(L·h) under optimal culture conditions [35 ℃, pH 2.0, 500 mg/L Fe(II)]. In addition, the oxidation rate equation of Fe(II) could be fitted to a zero-order kinetic model, indicating that Fe(II) was oxidised at a constant rate. Furthermore, a continuous-flow bioreactor was developed to simulate the Fe(II) oxidation efficiency of indigenous FeOBs for in situ AMD biological treatment. The maximum Fe(II) oxidation rate was 22.8 mg/(L·h) when the influent Fe(II) load was 32.3 mg/(L·h). Acidithiobacillus and Acidiphilium were the dominant species contributing to Fe(II) oxidation in the bioreactor, accounting for 67.7 % and 32.8 %, respectively. The results will help promote the application of FeOBs in AMD treatment.

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.

Kinetics study comparing bacterial growth and iron oxidation kinetics over a range of temperatures 5–45 °C

This comprehensive study summarizes kinetic data from experiments performed over a wide temperature range that overlaps the physiological operating temperature windows for most mesophilic Fe-oxidizers. Three acidophilic autotrophic bacterial strains Acidithiobacillus (At.) ferrivorans, At. ferrooxidans and Leptospirillum (L.) ferriphilum were investigated in this work in order to compare their growth and iron oxidation kinetics over a temperature range of 5–45 °C. On-line gas analysis was applied to determine the O2 and CO2 consumption rates during the culture incubation. Despite the genotypic differences these three strains share many common physiological traits. At t = 30 °C, the specific growth rate (μ) at all three species reached 0.11 h−1 which is equivalent to generation time 6.5 h. Maximum ferrous‑iron oxidation rate per unit of biomass was achieved with L. ferriphilum at t = 40 °C and approached 10 mmol Fe2+ (C-mmol) h−1. The temperature of incubation had a profound effect on the specific rates, while the kinetic parameter Ks/Ki at the Acidithiobacilli strains remained unaffected. Cultures with Leptospirillum incubated at temperatures >35 °C were capable of iron oxidation at redox potentials close to +950 mV and kept constant specific rates until Fe2+ depletion. The parameter Ks/Ki at L. ferriphilum was two order of magnitude lower than for the Acidithiobacilli. However, this feature disappears at temperatures below 30 °C, where Ks/Ki has a value comparable to Acidithiobacilli, which suggests that L. ferriphillum uses different Fe oxidation systems depending on temperature conditions. Strong correlation was observed between ferrous iron oxidation, and carbon dioxide fixation in the exponentially growing cultures with the yield coefficient (YSX) approaching 14 μmol C fixed per mmol Fe2+ oxidized. The kinetic parameters differed significantly between bacterial strains under “extreme” temperatures (t < 10 °C; t > 35 °C). All three bacterial strains were capable of oxidizing iron over the entire range (5–45 °C) of incubation temperatures. Neither At. ferrivorans nor At. ferrooxidans showed exponential growth at the temperature 40 °C. Bacterial iron oxidation not associated with growth was observed during incubation at 5 °C. At. ferrivorans, was the only species that can grow exponentially at a temperature of 5 °C. Moreover, it differed from the other species by greater sensitivity to low pH which seems to be more pronounced at temperatures t < 10 °C, with minimum of pH 1.9 for growth.

BIOKINETIC CHARACTERIZATION OF FERROPLASMA ACIDIPHILUM

Abstract. Recently a mixed culture dominated by the iron-oxidizing microorganisms Leptospirillum and Ferroplasma has been used in a large-scale microbial fuel cell for electrical power generation. There are many factors that affect the kinetics of iron oxidation by the mixotroph Ferroplasma acidiphilum. This study investigated the effects of pH, temperature, and substrate and yeast extract concentrations in order to arrive at kinetically favorable operating conditions with minimal jarosite precipitation. Furthermore, F. acidiphilum was cultured with Leptospirillum sp. in order to determine its viability as a species capable of limiting the organic by-products of chemolithotrophic microbial growth. Bacterial characterization in the culture was accomplished using fluorescent in situ hybridization (FISH). It was found that the ferrous iron oxidation was most favorable at yeast extract concentration of 0.02% (w/v), pH of 1.6, temperature of 35 oC and an initial ferrous iron concentration of 1 g/L yielding a maximum specific growth rate of 0.0351-0.042 h-1. Moreover, F. acidiphilum displayed a symbiotic relationship with its chemolithotrophic counterpart, Leptospirillum sp. in that they were able to utilize the metabolic organic products of the chemolithotroph and limit the organic concentration to ~20 ppm total organic carbon (TOC), well below the threshold concentration of 250 ppm for chemolithotroph activity.

Effect of Phosphate and Ferritin Subunit Composition on the Kinetics, Structure, and Reactivity of the Iron Core in Human Homo- and Heteropolymer Ferritins.

Ferritins are highly conserved supramolecular protein nanostructures that play a key role in iron homeostasis. Thousands of iron atoms can be stored inside their hollow cavity as a hydrated ferric oxyhydroxide mineral. Although phosphate associates with the ferritin iron nanoparticles, the effect of physiological concentrations on the kinetics, structure, and reactivity of ferritin iron cores has not yet been explored. Here, the iron loading and mobilization kinetics were studied in the presence of 1-10 mM phosphate using homopolymer and heteropolymer ferritins having different H to L subunit ratios. In the absence of ferritin, phosphate enhances the rate of ferrous ion oxidation and forms large and soluble polymeric Fe(III)-phosphate species. In the presence of phosphate, Fe(II) oxidation and core formation in ferritin is significantly accelerated with oxidation rates several-fold higher than with phosphate alone. High-angle annular dark-field scanning transmission electron microscopy measurements revealed a strong phosphate effect on both the size and morphology of the iron mineral in H-rich (but not L-rich) ferritins. While iron nanoparticles in L-rich ferritins have spherical shape in the absence and presence of phosphate, iron nanoparticles in H-rich ferritins change from irregular shapes in the absence of phosphate to spherical particles in the presence of phosphate with larger size distribution and smaller particle size. In the presence of phosphate, the kinetics of iron-reductive mobilization from ferritin releases twice as much iron than in its absence. Altogether, our results demonstrate an important role for phosphate, and the ferritin H and L subunit composition toward the kinetics of iron oxidation and removal from ferritin, as well as the structure and reactivity of the iron mineral, and may have an important implication on ferritin iron management in vivo.

Efficiency of using ozone for extraction of metals from mineral raw materials

The paper summarizes the results of studies on the use of ozone for the extraction of non-ferrous, rare and noble metals from ores, beneficiation concentrates and technogenic raw materials identified from world scientific publications and in patent literature since the early 20th century. Ozone is a strong oxidizing agent with an oxidizing potential 1.5 times higher than the potential of chlorine in an acidic environment. Even refractory metals and minerals dissolve with ozone. Metal extraction from mineral raw materials using ozone does not contaminate processed products or generate any hazardous waste. The paper presents a significant number of studies on the use of ozone to dissolve gold and other noble metals in mineral acids showing an increase in the extraction of metals into the solution. The cyanide and thiourea leaching of gold from mineral raw materials with the replacement of oxygen with ozone was investigated. The paper provides the results of the vat and heap leaching of non-ferrous and noble metals using ozone obtained by air or oxygen irradiation with ultraviolet light, in particular with the use of photoelectrochemical treatment. These results were used as a basis for patenting new technologies. The effectiveness of ozone used in the flotation concentration of mineral raw materials, purification and detoxification of solutions and solid products of metallurgical processing, regeneration of other oxidants, metal extraction from process solutions was evaluated. The results of studies on using ozone for the vat leaching of metals from refractory sulfide ores and sulfide beneficiation concentrates in an acid solution, and the study of the ozone-assisted oxidation kinetics of copper, iron, zinc, and molybdenum sulfide minerals are summarized. The paper provides and analyzes the results of using ozone in a combination with other oxidants – hydrogen peroxide and iron (III) ions – for metal extraction from sulfide mineral raw materials in a sulfuric acid. According to the results of most of the studies carried out, it can be concluded that the use of ozone is effective for metal extraction from mineral raw materials as it improves process performance (metal extraction into the solution, selectivity of metal extraction from complex raw materials), and reduces processing time.

A quantitative analysis of the ignition characteristics of fine iron particles

Ignition of iron particles in an oxidizing environment marks the onset of self-sustained combustion. The objective of the current study is to quantitatively examine the ignition characteristics of fine iron particles (i.e., 1µm- to 100µm-sized) governed by the kinetics of solid-phase iron oxidation. The oxidation rates are inversely proportional to the thickness of the oxide layer (i.e., following a parabolic rate law) and calibrated using the experimentally measured growth of iron-oxide layers over time. Steady-state (i.e., Semenov’s analysis) and unsteady analysis have been performed to probe the dependence of the critical gas temperature required to trigger a thermal runaway (namely, the ignition temperature Tign) on particle size, initial thickness of oxide layer, inert gas species, radiative heat loss, and the collective heating effect in a suspension of particles. Both analyses indicate that Tign depends on δ0, i.e., the ratio between the initial oxide layer thickness and particle size, regardless of the absolute size of the particle. The unsteady analysis predicts that, for δ0≲0.003, Tign becomes independent of δ0. Under standard conditions in air, Tign is approximately 1080 K for any particle size greater than 5µm. The ignition temperature decreases as the thermal conductivity of the oxidizing gas decreases. Radiative heat loss has a minor effect on Tign. The collective effect of a suspension of iron particles in reducing Tign is demonstrated. The transition behavior between kinetic-controlled and external-diffusion-controlled combustion regimes of an ignited iron particle is systematically examined. The influences of initial oxide-layer thickness and particle temperature on the ignition delay time, τign, of iron particles are parametrically probed. A d2-law scaling between τign and particle size is identified. Possible sources of inaccuracy are discussed.

The Effect of Metal Ions on the Growth and Ferrous IronOxidation by Leptospirillum ferriphilum CC Isolated from Armenia Mine Sites

The aim of this study is to investigate the potential of newly isolated strain Leptospirillum (L.) ferriphilum CC for bioleaching of pyrite and chalcopyrite in pure or mixed culture with other iron- and/or sulfur-oxidizing bacteria. In this paper, kinetics of ferrous iron (Fe2+) oxidation by newly isolated strain Leptospirillum (L.) ferriphilum CC was studied. The effect of initial Fe2+ in the concentration range of 50–400 mM on bacterial growth and iron oxidation was studied. It was shown that microbial Fe2+ oxidation was competitively inhibited by Fe3+. The influence of copper, zinc, nickel and cobalt ions on the oxidation of Fe2+ by L. ferriphilum CC was also studied. Minimal inhibitory concentrations (MIC) for each metal ion were determined. The toxicity of the ions was found to be as follows: Co &gt; Zn &gt; Ni &gt; Cu. The comparison of iron oxidation kinetic parameters of L. ferriphilum CC with other strains of L. ferriphilum indicates the high potential of strain L. ferriphilum CC for biogenic regeneration of concentrated ferric iron (Fe3+) in bioleaching processes of ores and ore concentrates. Bioleaching tests indicated that the newly isolated L. ferriphilum CC can be a prospective strain for the bioleaching of sulfide minerals in pure culture or in association with other iron- and/or sulfur-oxidizing bacteria.

Variability in iron (II) oxidation kinetics across diverse hydrothermal sites on the northern Mid Atlantic Ridge

One of the recently recognized main sources of iron to the deep ocean inventory is the hydrothermal activity associated with mid-ocean ridges. Little is known about the oxidation kinetics of iron(II) within these environments, especially the dependence on physico-chemical parameters such as temperature (T), pH, particle size-fractionation and the effect of organic matter.Following sample collection during the GA13 section cruise, the iron(II) oxidation at six hydrothermal vent sites (Menez Gwen, Lucky Strike, Rainbow, Lost City, Broken Spur and TAG) along the Mid-Atlantic Ridge were investigated, revealing high variability. The Fe(II) oxidation rate constant analysis from multiple stations at two sites (Rainbow and TAG), revealed that factors, other than T and pH, controlled the oxidation process. Experiments on the effect of particle size-fractionation and organic matter at different pH showed that the presence of organic ligands and colloidal size particles delayed the oxidation process, while not affecting the overall pH dependency.Extending our analysis to the broader relationship between the Fe(II) oxidation rate constants across a range of temperatures (between 2 and 25 °C) and pH (between 7 and 8) on a set of selected hydrothermal samples allowed us to derive a multiparametric equation to model the iron(II) oxidation rate constants in the ocean. This equation covers a larger range of temperatures than previous published equations, improving its applicability for global biogeochemical models.

Resistive Oxidation Kinetics of Iron(II) Thiochelate used as a Nitric Oxide Absorbent in Flue Gas

Iron(II) thiochelates, especially iron(II) 2,3-bis(sulfanyl)propane-1-sulfonate (Fe(II)(DMPS)2), have received particular attention as nitric oxide (NO) absorbents because they maintain high NO removal efficiency even in the presence of O2 in flue gases. Herein, we explored the O2-inducing oxidation kinetics of Fe(II) and thiol groups (R–SH) in Fe(II)(DMPS)2 and analyzed the resultant products to understand the resistive behavior toward O2. Quantitative analysis revealed that 4 mM of Fe(II) in 10 mM Fe(DMPS)2 solution was maintained without further oxidation even after 12 h under 5% O2, but instead thiol group concentration continued to decrease. In comparison to the commonly used Fe(II)(EDTA) absorbent, in which all the 10 mM Fe(II) was fully oxidized within 40 min reaction by 5% O2, this distinctive oxidative behavior of iron(II) thiochelate was attributed to the self-sacrificing thiol-containing ligands. This phenomenon was verified by the rapid and almost complete redox reaction between the oxidized Fe(III) and thiol groups in DMPS. Fourier transform infrared spectra indicated that the oxidation products of thiols were disulfide (S–S) and some sulfonates (−SO3–). Finally, based on all the experimental data, possible oxidation pathways of Fe(II)(DMPS)2 by O2 were proposed. This study provides both fundamental and applied insights of harnessing iron(II) thiochelate as a competitive NO absorbent for mitigating air pollution.

Thermodynamic and Kinetic Studies of the Interaction of Nuclear Receptor Coactivator-4 (NCOA4) with Human Ferritin.

Ferritinophagy is a ferritin autophagic degradation process mediated by the selective nuclear receptor coactivator-4 (NCOA4). NCOA4 binds to ferritin and delivers it to nascent autophagosomes, which then merge with the lysosomes for ferritin degradation and iron release. Earlier studies have demonstrated a specific association of NCOA4 with ferritin H-subunits, but not L-subunits. However, neither the thermodynamics of this interaction nor the effect of NCOA4 on iron oxidation, iron mineral core formation, or iron mobilization in ferritin has been explored. Using isothermal titration calorimetry, light absorption spectroscopy, and a soluble fragment (residues 383-522) of human NCOA4 expressed in Escherichia coli, we show that the NCOA4 fragment specifically binds H-rich ferritins with a binding stoichiometry of ∼8 NCOA4 molecules per ferritin shell, and Kd values of ∼0.4 and ∼2 μM for homopolymer H-chain ferritin and heteropolymer H-rich ferritin, respectively. The binding reaction was both enthalpically and entropically favored. Whereas the iron oxidation kinetics were not affected by the presence of NCOA4, iron mobilization from ferritin by two different reducing agents (FMN/NADH and sodium dithionite) showed a strong inhibitory effect that was dependent on the concentration of NCOA4 present in solution. Our results suggest that the binding of NCOA4 to ferritin may interfere in the electron transfer pathway through the ferritin shell and may have important biological implications on cellular iron homeostasis.

An experimental and modeling study of the chain initiation reaction in heterogeneous Fenton systems with zero valent iron

The main aim of this manuscript was to investigate the kinetics of zero valent iron (ZVI) oxidation to produce Fe2+ in hydrogen peroxide-water solutions. The experiments were performed in a batch reactor by submerging an iron plate in agitated aqueous H2O2 at 25 °C and pH ~2–4. A shrinking core model described properly the kinetics of iron oxidation. The overall rate of this reaction was governed by the rate of transport of protons through an iron (hydr)oxide film formed over the iron plate. To validate the model, additional kinetic experiments were carried out at different surface area of ZVI, initial concentration of H2O2, and pH. The determined concentrations of total iron, Fe2+, Fe3+ and H2O2 were reproduced by a detailed kinetic model for the Fenton-like process with ZVI that comprised 14 species and 22 reactions. Tests of oxidation of chlorpyrifos confirmed the reliability of the model to describe the Fenton-like process with ZVI.

Equilibrium and kinetic simulation of groundwater demanganation and deironing

Analysis of chemical equilibria among iron and manganese aqueous species at various Eh-pH conditions and aqueous CO2 concentration is done. Thermodynamic and equilibrium-kinetic simulation of iron and manganese aqueous species oxidations is developed for groundwater demanganation and deironing.&#x0D; Numerical simulation of chemical interactions in the system groundwater-aqueous oxygen-rock minerals-aqueous carbon dioxide is shown that deironing is effective enough but aqueous manganese(II) concentration is increased. It occurs because (Fe,Mn)CO3 solubility rate is too slow and (Fe,Mn)CO3 dissolution and removal of aqueous iron species results in secondary MnCO3 formation.&#x0D; Using published experimental data on carbonate dissolution kinetics, iron and manganese oxidation kinetics and the critical values of rate constants of iron and manganese homogenous oxidation, iron and manganese carbonates solubility, manganese homogenous catalytical oxidation on iron hydroxide suspension are chosen. The kinetics-thermodynamics model of underground oxidation of iron and manganese by dissolved oxygen have been developed. By numerical simulation of chemical interactions in the system groundwater saturated by oxygen-stratal water-intake rock minerals shows that deironing occurs effective enough but aqueous manganese concentration increased. It happens due to aqueous manganese slow oxidation and dissolution of (Fe,Mn)CO3. Also secondary MnCO3 formation is possible due to removal of aqueous iron specis. So underground demanganation is possible if there is no (Fe,Mn)CO3 among intake rock minerals or inconvenience of water contact with it.

Ferritin exhibits Michaelis-Menten behavior with oxygen but not with iron during iron oxidation and core mineralization.

The excessively high and inconsistent literature values for Km,Fe and Km,O2 prompted us to examine the iron oxidation kinetics in ferritin, the major iron storage protein in mammals, and to determine whether a traditional Michaelis-Menten enzymatic behavior is obeyed. The kinetics of Fe(ii) oxidation and mineralization catalyzed by three different types of ferritins (recombinant human homopolymer 24H, HuHF, human heteropolymer ∼21H:3L, HL, and horse spleen heteropolymer ∼3.3H:20.7L, HosF) were therefore studied under physiologically relevant O2 concentrations, but also in the presence of excess Fe(ii) and O2 concentrations. The observed iron oxidation kinetics exhibited two distinct phases (phase I and phase II), neither of which obeyed Michaelis-Menten kinetics. While phase I was very rapid and corresponded to the oxidation of approximately 2 Fe(ii) ions per H-subunit, phase II was much slower and varied linearly with the concentration of iron(ii) cations in solution, independent of the size of the iron core. Under low oxygen concentration close to physiological, the iron uptake kinetics revealed a Michaelis-Menten behavior with Km,O2 values in the low μM range (i.e. ∼1-2 μM range). Our experimental Km,O2 values are significantly lower than typical cellular oxygen concentration, indicating that iron oxidation and mineralization in ferritin should not be affected by the oxygenation level of cells, and should proceed even under hypoxic events. A kinetic model is proposed in which the inhibition of the protein's activity is caused by bound iron(iii) cations at the ferroxidase center, with the rate limiting step corresponding to an exchange or a displacement reaction between incoming Fe(ii) cations and bound Fe(iii) cations.

High-Temperature Atmospheric Corrosion of Fe-Cr-Si Alloys Studied by Mössbauer Spectroscopy

The high-temperature atmospheric corrosion of iron-based Fe-Cr-Si alloys was studied using 57Fe transmission Mössbauer spectroscopy (TMS) and conversion electron Mössbauer spectroscopy (CEMS). The Mössbauer measurements allow one to determine the chemical composition changes as well as the content of iron oxides in 300 nm subsurface layer and the bulk of studied samples during exposure to air at 870 K. The results indicate that the oxidation of iron atoms slows with increasing Si and Cr content in the specimens and as one could expect, the oxidation process in the bulk is much slower than in the 300 nm subsurface layer of studied materials. The parameters of the measured spectra made it possible to compute the iron oxidation kinetics. The comparison of the obtained kinetics for the Fe0.96Cr0.03Si0.01, Fe0.94Cr0.03Si0.03, and Fe0.90Cr0.05Si0.05 samples with corresponding data obtained previously for the binary Fe-Si and Fe-Cr alloys suggests that an addition of a relatively small amount of Cr to the dilute iron-based Fe-Si alloys has a negative effect on high-temperature corrosion resistance of the systems. At the same time, the present investigations show that the iron oxidation process is practically not observed when the alloy that is richer in Cr, Fe0.85Cr0.10Si0.05, is exposed to air at 870 K. Finally, the additional TMS measurements were performed for Fe0.85Cr0.10Si0.05 sample after exposure to air at 1,070 K and 1,270 K.

Inhibition Kinetics of Iron Oxidation by &lt;i&gt;Leptospirillum ferriphilum&lt;/i&gt; to Residual Thiocyanate Present in Bioremediated Cyanidation Tailings Wastewater

Inhibition of iron oxidation by Leptospirillum ferriphilum in the presence of thiocyanate (SCN-) was studied in small-scale batch experiments. The L. ferriphilum culture was challenged with SCN- over a range of 0 mg/l to 10 mg/l. The data showed that L. ferriphilum was able to utilise ferrous iron at low-level concentrations of SCN- (0.25 mg/l to 1.75 mg/l), however exhibited a reduction in oxidation rate relative to the control (0 mg/l). Moreover, introduction of SCN- at low-level concentrations resulted in a lag in iron oxidation activity, specifically at concentrations of 1 mg/l, 1.25 mg/l and 1.75 mg/l. No iron oxidation was observed at SCN- concentrations above 1.75 mg/l, indicating complete inhibition. As L. ferriphilum is the dominant iron oxidising bacteria within biooxidation tanks, evidence of sustained iron oxidation activity at low-level SCN- concentrations affirms the potential of recycling bioremediated cyanidation wastewater within biooxidation circuits in bioprocesses for gold recovery from sulfidic minerals as residual SCN- concentrations in remediated effluent are reportedly lower than 0.25 mg/l. The inhibition kinetics of this system need to be explored further in order to develop a deeper understanding of the system such that it may be applied to inform process operation.

Variation of iron redox kinetics and its relation with molecular composition of standard humic substances at circumneutral pH.

Oxidation and reduction kinetics of iron (Fe) and proportion of steady-state Fe(II) concentration relative to total dissolved Fe (steady-state Fe(II) fraction) were investigated in the presence of various types of standard humic substances (HS) with particular emphasis on the photochemical and thermal reduction of Fe(III) and oxidation of Fe(II) by dissolved oxygen (O2) and hydrogen peroxide (H2O2) at circumneutral pH (pH 7–8). Rates of Fe(III) reduction were spectrophotometrically determined by a ferrozine method under the simulated sunlight and dark conditions, whereas rates of Fe(II) oxidation were examined in air-saturated solution using luminol chemiluminescence technique. The reduction and oxidation rate constants were determined to substantially vary depending on the type of HS. For example, the first-order rate constants varied by up to 10-fold for photochemical reduction and 7-fold for thermal reduction. The degree of variation in Fe(II) oxidation was larger for the H2O2-mediated reaction compared to the O2-mediated reaction (e.g., 15- and 3-fold changes for the former and latter reactions, respectively, at pH 8). The steady-state Fe(II) fraction under the simulated sunlight indicated that the Fe(II) fraction varies by up to 12-fold. The correlation analysis indicated that variation of Fe(II) oxidation is significantly associated with aliphatic content of HS, suggesting that Fe(II) complexation by aliphatic components accelerates Fe(II) oxidation. The reduction rate constant and steady-state Fe(II) fractions in the presence of sunlight had relatively strong positive relations with free radical content of HS, possibly due to the reductive property of radical semiquinone in HS. Overall, the findings in this study indicated that the Fe reduction and oxidation kinetics and resultant Fe(II) formation are substantially influenced by chemical properties of HS.

Comparison of the Oxidation Process in Pure Iron and Diluted Fe\u2013Si Alloys: A Transmission M\xf6ssbauer Spectroscopy Study

The room temperature study of polycrystalline iron, Fe0.99Si0.01 and Fe0.97Si0.03 alloys, was performed using the 57Fe transmission Mössbauer spectroscopy (TMS). The TMS measurements allow us to determine changes of chemical composition and content of iron oxides in the studied samples during exposure to air at 570, 870 and 1070 K. The results indicate that the chemical composition of samples changes significantly and it is strongly dependent on the temperature at which the studied samples are exposed to air. Moreover, it was shown that the addition of few atomic percent of Si impurities to iron leads to a significant slowdown of the internal oxidation process. The iron and silicon internal oxidation kinetics for Fe0.99Si0.01 and Fe0.97Si0.03 alloys were calculated using the Deal-Grove model. The activation energies EA for the internal oxidation of iron atoms in studied Fe–Si alloys were estimated: EA = 1.46 (3) eV for Fe0.99Si0.01 and EA = 1.62 (6) eV for Fe0.97Si0.03.

Oxidation Kinetics and Oxygen Capacity of Ti-Bearing Blast Furnace Slag under Dynamic Oxidation Conditions

The oxidation kinetics of low valence titanium and iron in Ti-bearing blast furnace slag were investigated, the activation energies were calculated, which are 461.1 and 437.3 kJ/mol, respectively. The results illustrate that the oxidation process of Ti3+ in the slag is controlled by chemical reactions. However, the chemical reaction between oxygen and iron, between slag and gas, is the determining step of the iron oxidation process. The effects of the isothermal oxidation on the content of Fe2O3, FeM, FeO, and FeT in the slag are discussed. The Fe2+ and Ti3+ in the molten slag were oxidized at high temperatures. Oxygen affinity of the slag can be described using oxygen capacity. The oxygen capacity of Ti-bearing blast furnace slag was investigated during the dynamic oxidation process, the results indicates that the oxygen capacity of the slag decreased with increasing oxidation time during the dynamic oxidation process.

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.