Mössbauer kinetics of formation of goethite in the basic medium: Effect of the particle size of ferrous hydroxide

The formation of goethite in the basic medium is simulated by stirring, in the presence of air, a mixture of FeSO₄.7H₂O and NaOH in the 1∶5 ratio at different initial concentration C of ferrous ions. The initial oxidation product is ferrous hydroxide and its further oxidation leads mainly to goethite through active α′-FeOOH. The kinetics of the process is studied from Mössbauer spectra.     

Preparation and Eh--pH diagrams of Fe(II)--Fe(III) green rust compounds; hyperfine interaction characteristics and stoichiometry of hydroxy-chloride, -sulphate and -carbonate

Fe(II)--Fe(III) hydroxy-chloride, -sulphate and -carbonate were prepared by oxidation of a ferrous hydroxide precipitate in anion-containing aqueous solutions. The compounds are characterized by monitoring the redox potential E_h and the pH of stochiometric suspension vs time with the appropriate concentration ratios. X-ray diffraction allows us to characterize the crystal structure by distinguishing “green rust one” (GR1) from “green rust two” (GR2). Since green rusts (GRs) are of a pyroaurite-sjögrenite-like structure, i.e., consisting of intercalated foreign anions and water molecules in the interlayers between the brucite-like layers of Fe(OH)₂, their chemical formulae can be determined from the Mössbauer spectra. Three quadrupole doublets are observed: D₁ and D₂ correspond to a ferrous state with isomershift IS of about 1.27 mm s^-1 and quadrupole splittings QS of about 2.85 and 2.60 mm s^-1, respectively, whereas D₃ corresponds to a ferric state with IS and QS of about 0.4 mm s^-1. The hyperfine parameters of these doublets are similar from one green rust to another but their intensity ratios vary considerably. Finally, E_h and pH equilibrium diagrams of the Fe species in the presence of chloride, sulphate and carbonate anions contained within the water solution are drawn and the thermodynamic conditions of existence and degrees of oxidation of green rusts are discussed.     

Development of the collinear laser beam line at TRIUMF

Several commercially available medicaments containing ferrous fumarate (FeC₄H₂O₄) and ferrous sulfate (FeSO₄), as a source of ferrous iron, were studied using a high velocity resolution Mössbauer spectroscopy. A comparison of the ⁵⁷Fe hyperfine parameters revealed small variations for the main components in both medicaments indicating some differences in the ferrous fumarates and ferrous sulfates. It was also found that all spectra contained additional minor components probably related to ferrous and ferric impurities or to partially modified main components.     

The substitution of Fe2+ ions by Ni2+ ions in the green rust 2 compound studied by Mössbauer effect

Green rust 2 is usually obtained by oxidizing an initial mixture of FeSO₄ and NaOH solutions and a complete oxidation leads mainly towards γ oxyhydroxide known as lepidorocite. By adding some NiSO₄ one can stop at the first stage and Mössbauer spectra reveral only ferric doublets. This implies that the initial formula 4Fe(OH)₂, 2FeOOH, FeSO₄ of green rust 2 must be replaced byxNi(OH)₂, (6?x)FeOOH, NiSO₄, wherex scans from 2 to 4. It also means that all initial ferrous ions become oxidized into the ferric state leaving the Ni₂₊ ions unchanged. Therefore the end product of oxidation is the nickel containing green rust 2 at the place of the usual lepidocrocite.     

Influence of temperature and initial ratio of the reactants on the formation of rusts in sulphated aqueous media

The investigation of the rusts obtained in sulphated aqueous media is done using the Mössbauer spectroscopy by simulating the corrosion process through the oxidation of ferrous hydroxide obtained by mixing FeSO₄ and NaOH in different initial ratios R=[FeSO₄]/[NaOH]. The influence of the temperature of oxidation is studied from 10°C to 60°C for various values of R scanning from 0.2 to 3. One notes strickingly that goethite is always present whereas magnetite and lepidocrocite are never observed together.     

The simultaneous presence of green rust 2 and sulfate reducing bacteria in the corrosion of steel sheet piles in a harbour area

Mössbauer spectroscopy and X-ray diffraction analysis allow to detect the presence of green rust 2, the ferrous-ferric sulfated compound of composition, 4Fe(OH)₂,2FeOOH,FeSO₄,nH₂O, mixed with magnetite at the surface of steel sheets corroded in a harbour area where the presence of sulfate reducing bacteria are also detected.     

Mechanism of formation of magnetite from ferrous hydroxide in aqueous corrosion processes

The stoichiometric conditions for the formation of ferrous hydroxide Fe(OH)₂, by mixing Fe²⁺ ions with caustic soda NaOH, leads by oxidation to magnetite, irrelevant of the foreign anions, e.g. Cl^? or SO₄ ^2?, as demonstrated from Mössbauer spectroscopy. The electrochemical potential E_h and pH value of the initial conditions correspond to the drastic change from basic to acidic medium, observed when varying the initial Fe²⁺/OH^? ratio. Mössbauer analysis of the end products of oxidation at various temperatures shows that magnetite is only obtained at stoichiometry at very low temperature, but extends off stoichiometry at higher temperatures. The mechanism of formation of magnetite through an intermediate compound is discussed.     

Study of the kinetics of bacterial synthesis of iron minerals by Mössbauer spectroscopy

The kinetics of formation of iron-containing minerals by thermophilic dissimilatory iron-reducing bacteria (strain Z-0001) under controlled laboratory conditions has been investigated by Mössbauer spectroscopy. The content of Fe(III) hydroxide that is used by bacteria as an electron acceptor was 90 mmol l^?1 and the percentages of CO₂ and N₂ in the gas phase were, respectively, 20 and 80%. The Mössbauer measurements were performed at room and low (77 and 4.2 K) temperatures and in a magnetic field of 6 T. It is established that the Fe(III) hydroxide reduction by bacteria occurs in several stages with the formation of a mixture of nonstoichiometric magnetite and maghemite.     

Transformation of Fe(II)-Fe(III) Hydroxysulphite into Hydroxysulphate Green Rusts

Fe(II)-Fe(III) hydroxysulphite Green Rust 1, GR1(SO₃ ^2?), can be obtained by oxidation of Fe(OH)₂ precipitates in aqueous solution and characterised by X-ray diffraction and Mössbauer spectroscopy. In contrast to other Green Rusts (GRs) which then oxidise directly into ferric oxyhydroxides and magnetite, GR1(SO₃ ^2?) first oxidises into a Green Rust two compound, as identified by X-ray diffraction. Mössbauer analysis reveals that this GR2 is the Fe(II)-Fe(III) hydroxysulphate, GR2(SO₄ ^2?). Such an oxidation process, GR1(SO₃ ^2?) → GR2(SO₄ ^2?) confirms that the average oxidation number of Fe increases according to the chemical formulae previously proposed, [Fe^II ₆Fe^III ₂(OH)₁₆]²⁺[SO₃ · mH₂O]^2?, and [Fe^II ₄Fe^III ₂(OH)₁₂]²⁺[SO₄ · nH₂O]^2? for GR1(SO₃ ^2?) and GR2(SO₄ ^2?) with oxidation numbers of 2.25 and 2.33, respectively. The process implies likely the presence of sulphate ions inherent to the oxidation of sulphite in the solution.     

Characterization of Iron Oxides Commonly Formed as Corrosion Products on Steel

For fundamental studies of the atmospheric corrosion of steel, it is useful to identify the iron oxide phases present in rust layers. The nine iron oxide phases, iron hydroxide (Fe(OH)₂), iron trihydroxide (Fe(OH)₃), goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), feroxyhite (δ-FeOOH), hematite (α-Fe₂O₃), maghemite (γ-Fe₂O₃) and magnetite (Fe₃O₄) are among those which have been reported to be present in the corrosion coatings on steel. Each iron oxide phase is uniquely characterized by different hyperfine parameters from M?ssbauer analysis, at temperatures of 300K, 77K and 4K. Many of these oxide phases can also be identified by use of Raman spectroscopy. The relative fraction of each iron oxide can be accurately determined from the M?ssbauer subspectral area and recoil-free fraction of each phase. The different M?ssbauer geometries also provide some depth dependent phase identification for corrosion layers present on the steel substrate. Micro-Raman spectroscopy can be used to uniquely identify each iron oxide phase to a high spatial resolution of about 1 μm. This revised version was published online in August 2006 with corrections to the Cover Date.     

57Fe Mössbauer spectra and magnetic data from the kagomé antiferromagnet H3O-jarosite

⁵⁷Fe Mössbauer spectra are presented from (H₃O)Fe₃(SO₄)₂(OH)₆ (or H₃O-jarosite), which is a model kagomé antiferromagnet which features geometrical frustration and spin-glass-like behaviour. Dynamic scaling of the freezing temperature as a function of frequency is observed over a large frequency range, which indicates the presence of a spin-glass transition. A fast relaxation model between “up” and “down” states, separated by an energy gap, is presented to account for the shape of the Mössbauer spectra below the freezing temperature. From a calculation of the Electric Field Gradient tensor, it is suggested that H₃O-jarosite is an XY-Heisenberg antiferromagnet, where the Fe³⁺ moments lie in the kagomé planes.     

Identification of Green Rust Compounds in the Aqueous Corrosion Processes of Steels; the Case of Microbially Induced Corrosion and Use of 78 K CEMS

Fe(II)-Fe(III) hydroxy-sulphate Green Rust 2, GR2(SO₄ ^–), is obtained by microbially induced corrosion of steel. Transmission Mössbauer spectroscopy (TMS) was used to characterise the corrosion products of steel sheet piles under the biofilm at low sea-water level in a harbour. To understand the process, iron coupons maintained in aqueous solutions of 4 M NaCl and 0.1 M NaHCO₃ of pH 7.4 were studied by X ray diffraction and conversion electron Mössbauer spectroscopy (CEMS) at 78 K. The Fe(II)-Fe(III) hydroxy-carbonate, GR1(CO₃ ^–), covers the surface, as predicted by the E_h-pH diagram.     

Mössbauer study on the gamma radiolysis of anhydrous cesium tris (oxalato) ferrate(III)

The final product of the gamma radiolysis of anhydrous cesium tris(oxalato) ferrate(III) has been identified by Mössbauer spectroscopy as Cs₂Fe(ox)₂. The radiolytic decomposition proceeds as a first-order process due to the original compound depletion and to the radiolytic stability of the ferrous compound. Lamb-Mössbauer factors measurements indicate that the recoilless fractions of the iron species are practically unaffected by the radiolysis.     

Mössbauer spectroscopical investigation of amorphous Fe–Y alloy ribbons prepared by melt spinning

Fe–Y amorphous alloy ribbons were prepared by the melt spinning method and characterized by X-ray diffraction, Mössbauer spectroscopy and inelastic neutron scattering. X-ray diffraction demonstrates that the Fe_0.7Y_0.3 ribbons are completely amorphous, whereas the Fe_0.3Y_0.7 ribbons contain a small fraction of crystalline Y precipitates in the amorphous Fe–Y matrix. Mössbauer spectroscopy between 4.2 to 300 K reveals the amorphous nature of the Fe–Y matrix and the Fe_0.7Y_0.3 ribbons. The preliminary neutron scattering results S(Q, ω) show excess low energy vibrational modes which gives rise to the so called “boson peak” in this amorphous material.     

Mössbauer spectroscopy in studying magnetite formed by iron- and sulfate-reducing bacteria

The kinetics of iron mineral formation by thermophilic dissimilatory iron reducing bacteria using Fe(III) amorphous hydroxide and acetate CH₃COO^? as an electron acceptor and donor, respectively, was investigated by Mössbauer spectroscopy. The effects of various physical and chemical conditions and the presence of an inert organic substance on the formation of biogenic minerals were considered. The production of magnetite due to microbial sulfate reduction by hyperthermophilic dissimilatory sulfate-reducing microorganisms was investigated by Mössbauer spectroscopy methods.     

An57Fe Mössbauer effect study of the highT c superconductor GdBa2Cu3O7−y

Split source⁵⁷Fe Mössbauer effect spectroscopy has been performed between 4 K and 295 K on the superconducting perovskite GdBa₂Cu₃O_7?y. No evidence is seen for magnetic splitting at low temperatures as reported in some split absorder⁵⁷Fe Mössbauer experiments on this material. There is evidence for phonon mode softening, as observed for¹¹⁹Sn Mössbauer spectra of some other highT _c superconductors.     

The oxidation of Fe(OH)2 in the presence of carbonate ions: Structure of carbonate green rust one

Sodium carbonate Na₂CO₃ is added to a solution containing an Fe(OH)₂ precipitate in order to study the influence of CO ₃ ^2– ions on the oxidation of ferrous hydroxide. The first stage of the reaction leads to a ferrous-ferric compound, the carbonate green rust one (GR1), identified by its X-ray diffraction pattern. The Mössbauer spectrum at 78 K of this GR1 displays two ferrous doublets and one ferric doublet in the 312 abundance ratio. The quadrupole splittingsQS are 2.91, 2.58 and 0.42 mm/s, respectively, and the isomer shifts 75 are 1.25, 1.25 and 0.47 mm/s respectively. These values are very close to those of the three doublets of the chloride GR1, 3Fe(OH)₂Cl·Fe(OH)₂Cl·nH₂O. This fact confirms that the crystallographic structures of these two GR1s are similar, formed by the stacking of hydroxide layers and interlayers containing the considered anions (Cl^– or CO ₃ ^2– ) and water molecules. The chemical formula of carbonate GR1 is Fe ₄ ^(II) Fe ₂ ^(III) (OH)₁₂CO₃·nH₂O, and its standard chemical potential -853 900 cal/mol ifn=0. The second stage of the reaction is the oxidation of GR1, which leads to -FeOOH goethite.     

Mechanism of solid-state oxidation of FeSO4·H2O: model of simultaneous reactions

The mechanism of the thermal transformation of FeSO₄·H₂O in air has been studied under isothermal conditions at temperatures (150–460)°C using mainly⁵⁷Fe Mössbauer spectroscopy and X-ray powder diffraction (XRD). Two trends are typical for the thermal behaviour of FeSO₄·H₂O in air, a tendency toward oxidation and dehydration. We suggested a new transformation model consisting of two ways of oxidation, direct one and indirect one. Fe(OH)SO₄ was identified as a product of the direct way, Fe₂(SO₄)₃ and superparamagnetic nanoparticles ofγ-Fe₂O₃ as products of the indirect way. The suggested model of simultaneous reactions explains the unusual non-monotonous dependence of the oxidation level of the thermally treated samples on temperature.     

Mössbauer effect studies on the formation of iron oxide phases synthesized via microwave–hydrothermal route

Microwave–hydrothermal (MH) route was employed to synthesize various iron oxide phases in ultra-fine crystalline powders by using ferrous sulphate and sodium hydroxide as starting chemicals. All chemical reactions were carried out under identical MH conditions, namely, at 190°C, 154 psi, 30 min, by varying the molar ratio (MR) of FeSO₄/NaOH in the aqueous solutions. The variation of MR has a dramatic effect on the crystallization behavior of various phases of iron oxides under MH processing conditions. For example, spherical agglomerates of Fe₃O₄ powder were obtained if MR equal to 0.133 (pH?>?10 sample A). On the other hand non-stoichiometric Fe₃O₄ powders (Sample B) were obtained for all higher MR of FeSO₄/NaOH between 0.133 and 4.00 (6.6?2O₃ powders (sample C) were produced. Fe⁵⁷ Mössbauer spectra were recorded for all the three sets of samples at room temperature. In the case of sample B, temperature dependent Mössbauer spectra were recorded in the range of 77–300 K to understand the non-stoichiometric nature of Fe₃O₄ powders. All these results are discussed in the present paper.