Mössbauer Spectroscopy of Fe Xanthates

Abstract

Mössbauer spectroscopic studies indicate that, depending on pH, two different precipitates are formed when appropriate solutions of iron ions are mixed with ethyl xanthate solutions. Below pH 3.5, ferric ethyl xanthate is obtained, regardless of whether ferric or ferrous reactants are used; ferrous xanthate can be obtained only with highly concentrated solutions and in strictly neutral or even reducing conditions. Above pH 3.5 a ferric hydroxy-xanthate, with one or more OH groups replacing the xanthate groups, is formed. No analogy with the Cu²⁺ → Cu⁺ reduction and the accompanying stoichiometric oxidation of xanthate to dixanthogen could be observed in the Fe³⁺ → Fe²⁺ systems, in which dixanthogen is detected only as a product of ferric xanthate decomposition in alkaline pH's.     

Influence of process variables on biooxidation of ferrous sulfate by an indigenous Acidithiobacillus ferrooxidans. Part II: Bioreactor experiments

The oxidation of ferrous iron in solution using Acidithiobacillus ferrooxidans has industrial applications in the regeneration of ferric iron as an oxidant agent for the removal of hydrogen sulfide from waste gases, desulphurization of coal, leaching of non-ferrous metallic sulfides and treatment of acid mine drainage. The aim of this attempt was to increase the biooxidation rate of ferrous sulfate by using immobilized cells. Rate of ferrous iron oxidation was determined in a packed-bed reactor configuration with low density polyethylene (LDPE) particles as support material in order to find the most practical system for scale-up. The present work studies the influence of basic parameters on the ferrous iron biooxidation process using an indigenous iron-oxidizing microorganism, namely A. ferrooxidans, in a 2 L packed-bed bioreactor. Effects of several process variables such as initial pH, temperature, dilution rate, initial concentrations of ferrous and ferric ions on oxidation of ferrous sulfate were investigated. Experimental results indicate that in the temperature range of 31–34 °C the biooxidation of ferrous ions to ferric ions could be resulted efficiently. A pH range of 2–2.2 was optimum for the growth of the culture and effective bacterial activity for oxidation of ferrous ions to ferric ions. The highest oxidation rate of 2.9 g Fe²⁺ L⁻¹ h⁻¹ was obtained using a culture initially containing 25 g L⁻¹ Fe⁺² at the dilution rate of 0.4 h⁻¹. This rate is very high compared to that achieved in other bioreactors found in the literature. In addition the biooxidation of Fe²⁺ to Fe³⁺ conversion could be achieved effectively in the presence of the Fe³⁺ in the concentration range of 0.1–0.7 g/L.     

Phase Transformation of Iron Oxide Nanoparticles by Varying the Molar Ratio of Fe2+:Fe3+

Co‐precipitation from a solution of ferrous/ferric mixed salt with the ratio of Fe²⁺:Fe³⁺ = 1:2 in air atmosphere is not a reliable method to synthesize magnetite (Fe₃O₄) nanoparticles because of the fact that Fe²⁺ oxidizes to Fe³⁺ and the molar ratio of Fe²⁺:Fe³⁺ changes. Therefore, the phase composition changes from magnetite to maghemite (γ‐Fe₂O₃). The influence of the initial molar ratio of Fe²⁺:Fe³⁺ on the phase composition of nanoparticles, their crystallinity and magnetic properties was studied. Experimental data from XRD, FTIR, SEM, and VSM reveal that the appropriate method to synthesize magnetite nanoparticles is reverse precipitation from only ferrous salt. It is found that by decreasing the synthesis temperature and by increasing the concentration of alkaline solution and the ratio of Fe²⁺:Fe³⁺ the crystallinity and the specific saturation magnetization (σ_s) are increased.     

Catalytic wet oxidation of phenol with homogeneous iron salts

The catalytic wet oxidation of phenol has been investigated in a 1 L semi‐batch reactor in the presence of both ferrous and ferric salts. Oxidation reactions follow first‐order kinetics with respect to phenol and half‐order kinetics with respect to dissolved oxygen. The activation energy for the reaction was 44.5 and 48.3 kJ mol^?1 for runs employing Fe³⁺ and Fe²⁺, respectively. Rate constants and induction periods were also similar for both catalysts. This result could be explained by analysing the evolution of iron during the oxidation process. For pH > 2, Fe²⁺ was rapidly oxidized under reaction conditions to Fe³⁺, resulting in a unique catalytic redox system Fe²⁺/Fe³⁺. It was also shown that if pH < 2 the dissolved oxygen was unable to oxidize ferrous ion, resulting in a much slower oxidation rate of phenol. The absence of a redox pair resulted in a complete lack of catalytic activity of the dissolved iron salt. Copyright © 2005 Society of Chemical Industry     

Structural Insights into Iron Ions Accumulation in Dps Nanocage

Dps (DNA-binding protein from starved cells) is well known for the structural protection of bacterial DNA by the formation of highly ordered intracellular assemblies under stress conditions. Moreover, this ferritin-like protein can perform fast oxidation of ferrous ions and subsequently accumulate clusters of ferric ions in its nanocages, thus providing the bacterium with physical and chemical protection. Here, cryo-electron microscopy was used to study the accumulation of iron ions in the nanocage of a Dps protein from Escherichia coli. We demonstrate that Fe²⁺ concentration in the solution and incubation time have an insignificant effect on the volume and the morphology of iron minerals formed in Dps nanocages. However, an increase in the Fe²⁺ level leads to an increase in the proportion of larger clusters and the clusters themselves are composed of discrete ~1–1.5 nm subunits.     

Treatment of iron and manganese in simulated groundwater via ozone technology

This paper deals with experimental investigations related to removal of iron and manganese from simulated contaminated groundwater via ozone technology. Ozone as a powerful oxidizing agent, which was used in this study to oxidize iron and manganese converting ferrous ions (Fe²⁺) iron to ferric state (Fe³⁺) and (Mn²⁺) to (Mn⁴⁺) state, the oxidized salts will precipitate as ferric hydroxide and manganese oxide, that to reach the concentrations of these pollutants under their limit values in drinking water. The initial concentrations of (Fe²⁺) and (Mn²⁺) in synthetic water sample under study were 2.6 mg/l and 1 mg/l respectively. The effects of ozone dose concentration, operating temperature, and pH on the percentage removal of (Fe²⁺) and (Mn²⁺) have been discussed. For optimum removal of iron and manganese species the ozone dose has been noted as 3 mg/l at optimum temperature of 20 °C which improved removal of (Fe²⁺) and (Mn²⁺) to more than 96% and 83% respectively. The removal percentage of both metals was also affected by changing pH with the range of 5-12; where the maximum removal of iron and manganese was observed in pH (9-10). Experiments also studied the effects of coagulant type and bicarbonate concentration in raw water, as a result it was found that the optimum concentrations of coagulant was a mixture of 30 mg/l of aluminum sulfate with 10 mg/l of lime.     

Fenton reactions in lipid phases

Metal catalysis of membrane lipid oxidation has been thought to occur only at cell surfaces. However, conflicting observations of the pro-oxidant activity of ferric (Fe³⁺) vs ferrous (Fe²⁺) forms of various chelates have raised questions regarding this dogma. This paper suggests that the solubilities of iron complexes in lipid phases and the corresponding abilities to initiate lipid oxidation there, either directly or via Fenton-like production of reactive hydroxyl radicals, are critical determinants of initial catalytic effectiveness. Partitioning of Fe³⁺ and Fe²⁺ complexes and chelates into bulk phases of purified lipids was quantified by atomic absorption spectroscopy. mM solutions of iron salts partitioned into oleic acid at levels of about micromolar. Ethylenediamine tetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) chelates were somewhat less soluble, while adenosine diphosphate (ADP) chelates, and ferrioxamine were soluble as chelates at greater than 10⁻⁵ M. Solubilities of all iron compounds in methyl linoleate were 10- to 100-fold lower. To determine whether Fenton-like reactions occur in lipid phases, H₂O₂ and either Fe²⁺ or Fe³⁺ and a reducing agent were partitioned into the lipid along with the spintrap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), and free radical adducts were recorded by electron paramagnetic resonance (EPR). Hydroxyl radicals (OH^.) adducts were observed in oleic acid, but in lipid esters secondary peroxyl radicals predominated, and the presence of OH^. adducts was uncertain. Presented at the symposium “Free Radicals, Antioxidants, Skin Cancer and Related Diseases” at the 78th AOCS Annual Meeting in New Orleans, LA, May 1987     

Bio‐oxidation of ferrous ions by Acidithioobacillus ferrooxidans in a monolithic bioreactor

BACKGROUND: The bio‐oxidation of ferrous iron is a potential industrial process in the regeneration of ferric iron and the removal of H₂S in combustible gases. Bio‐oxidation of ferrous iron may be an alternative method of producing ferric sulfate, which is a reagent used for removal of H₂S from biogas, tail gas and in the pulp and paper industry. For practical use of this process, this study evaluated the optimal pH and initial ferric concentration. pH control looks like a key factor as it acts both on growth rate and on solubility of materials in the system. RESULTS: Process variables such as pH and amount of initial ferrous ions on oxidation by A. ferrooxidans and the effects of process variables dilution rate, initial concentrations of ferrous on oxidation of ferrous sulfate in the packed bed bioreactor were investigated. The optimum range of pH for the maximum growth of cells and effective bio‐oxidation of ferrous sulfate varied from 1.4 to 1.8. The maximum bio‐oxidation rate achieved was 0.3 g L^?1 h^?1 in a culture initially containing 19.5 g L^?1 Fe²⁺ in the batch system. A maximum Fe²⁺ oxidation rate of 6.7 g L^?1 h^?1 was achieved at the dilution rate of 2 h^?1, while no obvious precipitate was detected in the bioreactor. All experiments were carried out in shake flasks at 30 °C. CONCLUSION: The monolithic particles investigated in this study were found to be very suitable material for A. ferrooxidans immobilization for ferrous oxidation mainly because of its advantages over other commonly used substrates. In the monolithic bioreactor, the bio‐oxidation rate was 6.7 g L^?1 h^?1 and 7 g L^?1 h^?1 for 3.5 g L^?1 and 6 g L^?1 of initial ferrous concentration, respectively. For higher initial concentrations 16 g L^?1 and 21.3 g L^?1, bio‐oxidation rate were 0.9 g L^?1 h^?1 and 0.55 g L^?1 h^?1, respectively. Copyright © 2008 Society of Chemical Industry     

The Magnetosome Protein,Mms6 from Magnetospirillum magneticum Strain AMB-1, Is a Lipid-Activated Ferric Reductase

Magnetosomes of magnetotactic bacteria consist of magnetic nanocrystals with defined morphologies enclosed in vesicles originated from cytoplasmic membrane invaginations. Although many proteins are involved in creating magnetosomes, a single magnetosome protein, Mms6 from Magnetospirillum magneticum strain AMB-1, can direct the crystallization of magnetite nanoparticles in vitro. The in vivo role of Mms6 in magnetosome formation is debated, and the observation that Mms6 binds Fe³⁺ more tightly than Fe²⁺ raises the question of how, in a magnetosome environment dominated by Fe³⁺, Mms6 promotes the crystallization of magnetite, which contains both Fe³⁺ and Fe²⁺. Here we show that Mms6 is a ferric reductase that reduces Fe³⁺ to Fe²⁺ using NADH and FAD as electron donor and cofactor, respectively. Reductase activity is elevated when Mms6 is integrated into either liposomes or bicelles. Analysis of Mms6 mutants suggests that the C-terminal domain binds iron and the N-terminal domain contains the catalytic site. Although Mms6 forms multimers that involve C-terminal and N-terminal domain interactions, a fusion protein with ubiquitin remains a monomer and displays reductase activity, which suggests that the catalytic site is fully in the monomer. However, the quaternary structure of Mms6 appears to alter the iron binding characteristics of the C-terminal domain. These results are consistent with a hypothesis that Mms6, a membrane protein, promotes the formation of magnetite in vivo by a mechanism that involves reducing iron.     

Cathodic dissolution of the passive film on iron—I. Kinetics and mechanism

The cathodic reduction of passive film on iron has been investigated by the potentiodynamic and galvanostatic techniques in phosphate—borate buffer solutions of various pH values. The reduction was completed through two processes occurring at different potentials. One of them was the simultaneous reduction of ferric oxide to ferrous ion and metallic iron. This process consisted of a number of reaction steps, and the rate-determining step was the charge transfer reaction from FeOH²⁺ to (FeOH)_ads.As another process, it was unreasonable to consider the reduction of magnetite to iron which had been proposed previously; however, the reaction was possibly the reduction of Fe²⁺ to Fe.     

CuI and H2O2 Inactivate and FeII Inhibits [Fe]‐Hydrogenase at Very Low Concentrations

[Fe]‐Hydrogenase (Hmd) catalyzes reversible hydride transfer from H₂. It harbors an iron‐guanylylpyridinol as a cofactor with an Fe^II that is ligated to one thiolate, two COs, one acyl‐C, one pyridinol‐N, and solvent. Here, we report that Cu^I and H₂O₂ inactivate Hmd (half‐maximal rates at 1 μM Cu^I and 20 μM H₂O₂) and that Fe^II inhibits the enzyme with very high affinity (K_i=40 nM ). Infrared and EPR studies together with competitive inhibition studies with isocyanide indicated that Cu^I exerts its inhibitory effect most probably by binding to the active site iron‐thiolate ligand. Using the same methods, it was found that H₂O₂ binds to the active‐site iron at the solvent‐binding site and oxidizes Fe^II to Fe^III. Also it was shown that Fe^II reversibly binds away from the active site iron, with binding being competitive to the organic hydride acceptor; this inhibition is specific for Fe^II and is reminiscent of that for the [FeFe]‐hydrogenase second iron, which specifically interacts with H₂.     

Effect of precursors on surface and catalytic properties of Fe/TiO2 catalysts

Titania‐supported iron (5 wt%) catalysts were prepared by a sol–gel method using different gelation pH and metal precursors (Fe(II) and Fe(III)). Characterization data of calcined catalysts revealed that, irrespective of the nature of the metal precursor, iron is present in all cases as ferric oxide. However, the crystalline phase exhibited by titania does depend on the metal precursor used. The catalytic activity of the catalysts, tested in the combustion of methane at atmospheric pressure, is not related to the dispersion of iron oxide. Thus, Fe³⁺ ions may be obtained in two extreme situations; one highly dispersed in which Fe³⁺ ions are placed in the titania network and another in which large Fe₂O₃ crystals are located on the surface of the catalyst. The former exhibits the best performance in the combustion of methane. © 2002 Society of Chemical Industry     

FENTON AND FENTON-LIKE AOPs FOR ALUM SLUDGE CONDITIONING: EFFECTIVENESS COMPARISON WITH DIFFERENT Fe2+ AND Fe3+ SALTS

Currently, organic polymers are adopted in alum sludge (aluminum-coagulated drinking water treatment sludge) conditioning. However, there are important concerns regarding the use of these polymers because of the unknown and long-term effects of the potential release of excess polymer to the surrounding environment when the sludge is landfilled. Therefore, as an initial action, this study aimed at investigating alternative chemical conditioning methods and focused mainly on exploiting Fenton (Fe²⁺/H₂O₂) and Fenton-like (Fe³⁺/H₂O₂) reagents as the conditioner. Experiments have been conducted to test the effectiveness of Fenton's reagent (containing the ferrous salts of chloride, sulfate, or oxalate), Fenton-like reagent (containing ferric salts of chloride and sulfate), and the coagulation method using FeCl₃ for alum sludge conditioning at constant hydrogen peroxide and iron salt concentrations of 125 and 20 mg/g DS (dry solids), respectively. The effectiveness on dewaterability of the alum sludge demonstrated that the maximum reduction (%) of SRF (specific resistance to filtration) and CST (capillary suction time) of 74% and 47%, respectively, can be obtained when Fenton's reagent was adopted for sludge conditioning. Such reduction of 64% for SRF and 38% for CST can be achieved when Fenton-like reagents were applied.     

Synthèse à haute température d'un glycérolate de fer au départ d'oxalate ferreux et ferrique

Attempts at synthesizing high temperature crystalline organic complexes starting from glycerol and various iron salts have been achieved, particularly with ferrous and ferric oxalate. For these latter, the powdered reaction products at various stages of the synthesis were studied by X-ray diffraction, i.r. spectroscopy, electron microscopy and chemical analysis. At least two steps are involved in the formation of the glycerolate: (1) a rearrangement of the structure of the starting salt with formation of a sheet structure suitable for the glycerolate, and (2) a reduction or oxidation of metallic ions. Relative rates of these two steps depend on the structural stability of starting materials. Rapid decomposition of ferric oxalate produces the formation of a ferric-rich intermediate product: in this case, the limiting step of the reaction is the progressive reduction of the Fe³⁺ ion. On the other hand, ferrous oxalate breaks up at a rather slow rate and the relative oxidation rate of Fe²⁺ may become fast enough to favour the direct formation of the stable organic complex.     

Process intensification in wastewater treatment: ferrous iron removal by a sustainable membrane bioreactor system

Biooxidation of ferrous iron (Fe²⁺) from strongly acidic industrial wastewater with a high Fe²⁺ content by Thiobacillus ferrooxidans in a packed bed reactor and subsequent removal of ferric iron (Fe³⁺) by a crossflow microfiltration (membrane) process have been investigated as functions of wastewater flowrate (54–672 cm³ h^?1), Fe²⁺ concentration (1.01–8.06 g dm^?3), and pH (1.5–5.0). A natural (vegetable) sponge, Luffa cylindrica, was used as support matrix material. The fastest kinetic performance achieved was about 40 g Fe²⁺ dm^?3 h^?1 at a true dilution rate of 19 h^?1 corresponding to a hydraulic retention time of 3.16 min. Steady state conversion was observed to be about 10% higher at pH 2.3 than that at pH 1.5. Increasing the flowrate of the inlet wastewater caused a reduction in conversion rate. The oxidation rate reduced along the reactor height as the wastewater moved towards the exit at the top but conversion showed the opposite trend. Increasing Fe²⁺ concentration up to a critical point resulted in an increased oxidation rate but beyond the critical point caused the oxidation rate to decrease. Luffa cylindrica displayed suitable characteristics for use as a support matrix for formation of a Thiobacillus ferrooxidans biofilm and showed promising potential as an ecological and sustainable alternative to existing synthetic support materials. Membrane separation was shown to be a very effective means of Fe³⁺ removal from the wastewater with removal changing from 92% at pH 2.3 to complete removal at pH 5.0. Copyright © 2003 Society of Chemical Industry     

Characterization and leaching behaviour of lizardite in Yuanjiang laterite ore

Lizardites in Yuanjiang laterite ore were characterized using X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR). Their leaching behaviour in sulphuric acid was investigated. XRD patterns show that there are two different lizardite polytypes in Yuanjiang laterite ore: lizardite-1T and lizardite-1M. The crystal order of the lizardites between soil and rock samples was different, which was demonstrated by XRD and FTIR spectrum analyses. The obvious change of the crystal cell volume was observed in the lizardite-1T of soil sample, with higher iron content, whereas the crystal cell varied little for the lizardite-1T of rock sample, with higher aluminum content. FTIR spectrum, EDS analyses and the leaching experiments show substitution of octahedral nickel, iron and aluminum, as well as tetrahedral aluminum. Iron occurred as ferrous and ferric iron in the lizardite-1T of rock sample, mostly as ferrous iron in the lizardite-1M and as ferric iron in the lizardite-1T of soil and rock fragment samples. The results from leaching experiments also show that the leachability of lizardite-1T in sulphuric acid decreases in the following sequence: lizardite-1T of laterite soil > lizardite-1T of rock fragments > lizardite-1T of rocks. Lizardite-1M leached faster than lizardite-1T of rock fragments. The leaching behaviour differences of lizardites in Yuanjiang laterite ore may be explained by the various crystal structures resulting from the weathering process of laterite ores and the substitution of metal cations such as Ni²⁺, Fe²⁺, Fe³⁺ and Al³⁺. It is concluded that rapid dissolution occurs preferentially for the lizardite with low crystal order, low aluminum content and high iron content while acid leaching laterite ores.     

The SPASIBA Force Field for Studying Iron-Tannins Interactions: Application to Fe3+/Fe2+ Catechol Complexes

The SPASIBA force field parameters have been obtained for Fe³⁺/Fe²⁺-Oxygen interactions occuring between non-heminic iron and hydroxyl groups of polyphenols found in tannins. These parameters were derived from normal modes analyses based on quantum chemical calculations using the Density Functional Theory (DFT). Four models involving complexation of iron with water ([Fe(H₂O)₆]³⁺, [Fe(H₂O)₆]²⁺) and with cathechol molecules ([Fe(cat)₂(H₂O)₂]⁻¹, [Fe(cat)₂(H₂O)₂]⁻²) were studied using the Density Functional Theory and the B3LYP hybrid functional under high spin states of iron.     

Role of ethylamines on the electrochemical behaviour of Fe–Ni alloy films

Cyclic voltammetric experiments were carried out on platinum in acidic solution (pH 3) containing ferrous sulfate, nickel sulfate and ethylamines (EtNH₂, Et₂NH, Et₃N). Spectral ultraviolet absorption studies indicate the complexation of both Fe²⁺ and Ni²⁺ ions with ethylamines. The results under transient polarisation conditions indicate the reduction of Fe²⁺ ions through the intermediate species FeOH⁺, with second electron transfer as a slow step. The higher charge transfer rate of FeOH⁺ over NiOH⁺ reduction causes the anomalous codeposition of Fe–Ni alloy film. Among the ethylamines, Et₃N considerably assists the alloy deposition process. A gradual variation in free energy of alloy formation with Fe²⁺:Ni²⁺ (mol:mol) in the bath suggests the formation of an alloy intermediate phase rich in iron. Stripping voltammetric curves indicate the preferential dissolution of iron from iron rich alloy intermediate phase. X-ray diffraction studies further confirm the phase to be b.c.c. Fe–Ni alloy. The extent of corrosion of the Fe–Ni alloy film in the presence of ethylamines is in the following order: Et₃N > Et₂NH > EtNH₂.     

Nitrate and nitrite reduction of a sulphide-rich environment

A sulphide-rich anaerobic sludge acclimated with a molasses wastewater was used to carry out studies on nitrate and nitrite reductions in continuously stirred batch reactors. It was shown that a COD/N-NO_x ratio as high as 65·6 mg mg⁻¹ did not promote dissimilatory reduction of nitrogen oxides to ammonia. Denitrification was characterized by a probable accumulation of gaseous intermediates, nitric oxide (NO) and nitrous oxide (N₂O), by sulphide consumption with concomitant elemental sulphur production and by an increase of the redox potential. In addition, sulphate reducers were completely inhibited by nitrogenous oxides. Cultures performed without any carbon source proved that denitrifiers were able to use sulphides as electron donors. Furthermore, while a lag phase preceded nitrate denitrification, nitrite was consumed immediately. Chemical reduction of nitrite by ferrous iron (Fe²⁺) was considered to be responsible for this difference. Evidence of such a chemodenitrification has been presented by using a sterilized sludge which kept its ability to reduce nitrite while it lost its capacity to use nitrate. Moreover, this chemical activity was favoured by Fe²⁺ addition. Finally, it has been suggested that during the cultures performed with non-sterilized sludge, a biological reduction of the ferric ions (Fe³⁺) would be coupled to nitrite chemodenitrification and would allow a regeneration of Fe²⁺. © 1998 SCI     

In situ high-temperature ESR characterization of FeZSM-5 and FeSAPO-34 catalysts in flowing mixtures of NO, C3H6, and O2

In situ ESR of FeHZSM-5 shows that samples made by sublimation of FeCl₃ contain mainly ferric ions in tetrahedral and distorted tetrahedral sites. Above 200°C these ions do not chemisorb water, which confirms the resistance to activity deterioration by water vapor claimed for FeHZSM-5, when used in SCR of NO_x. The number of ESR-active Fe³⁺ ions decreases when moving from strongly oxidizing conditions towards stoichiometry at 500°C. In more reducing atmospheres, i.e., when excess reductant is present, the exposure produces an agglomerated ferromagnetic species, presumably magnetite. This process is irreversible. As the trivalent dispersed iron ions are the catalytically active sites, this transformation makes Fe-containing catalysts vulnerable to accidental but irreversible transformation induced by even mildly reducing conditions. The dispersion of ferric ions in FeSAPO-34 is not as good as in FeHZSM-5 and the reactive Fe³⁺ in distorted tetrahedral sites is absent. Catalytic oxidation of ethane allows correlation of the EPR results with activity and stability. This revised version was published online in July 2006 with corrections to the Cover Date.