Revealing the nuclearity of iron citrate complexes at biologically relevant conditions.

Citric acid plays an ubiquitous role in the complexation of essential metals like iron and thus it has a key function making them biologically available. For this, iron(III) citrate complexes are considered among the most significant coordinated forms of ferric iron that take place in biochemical processes of all living organisms. Although these systems hold great biological relevance, their coordination chemistry has not been fully elucidated yet. The current study aimed to investigate the speciation of iron(III) citrate using Mössbauer and electron paramagnetic resonance spectroscopies. Our aim was to gain insights into the structure and nuclearity of the complexes depending on the pH and iron to citrate ratio. By applying the frozen solution technique, the results obtained directly reflect the iron speciation present in the aqueous solution. At 1:1 iron:citrate molar ratio, polynuclear species prevailed forming most probably a trinuclear structure. In the case of citrate excess, the coexistence of several monoiron species with different coordination environments was confirmed. The stability of the polynuclear complexes was checked in the presence of organic solvents.

Debye Temperature Evaluation for Secondary Battery Cathode of α-SnxFe1-xOOH Nanoparticles Derived from the 57Fe- and 119Sn-Mössbauer Spectra.

Debye temperatures of α-SnxFe1-xOOH nanoparticles (x = 0, 0.05, 0.10, 0.15 and 0.20, abbreviated as Sn100x NPs) prepared by hydrothermal reaction were estimated with 57Fe- and 119Sn-Mössbauer spectra measured by varying the temperature from 20 to 300 K. Electrical properties were studied by solid-state impedance spectroscopy (SS-IS). Together, the charge-discharge capacity of Li- and Na-ion batteries containing Sn100x NPs as a cathode were evaluated. 57Fe-Mössbauer spectra of Sn10, Sn15, and Sn20 measured at 300 K showed only one doublet due to the superparamagnetic doublet, while the doublet decomposed into a sextet due to goethite at the temperature below 50 K for Sn 10, 200 K for Sn15, and 100 K for Sn20. These results suggest that Sn10, Sn15 and Sn20 had smaller particles than Sn0. On the other hand, 20 K 119Sn-Mössbauer spectra of Sn15 were composed of a paramagnetic doublet with an isomer shift (δ) of 0.24 mm s-1 and quadrupole splitting (∆) of 3.52 mm s-1. These values were larger than those of Sn10 (δ: 0.08 mm s-1, ∆: 0.00 mm s-1) and Sn20 (δ: 0.10 mm s-1, ∆: 0.00 mm s-1), suggesting that the SnIV-O chemical bond is shorter and the distortion of octahedral SnO6 is larger in Sn15 than in Sn10 and Sn20 due to the increase in the covalency and polarization of the SnIV-O chemical bond. Debye temperatures determined from 57Fe-Mössbauer spectra measured at the low temperature were 210 K, 228 K, and 250 K for Sn10, Sn15, and Sn20, while that of α-Fe2O3 was 324 K. Similarly, the Debye temperature of 199, 251, and 269 K for Sn10, Sn15, and Sn20 were estimated from the temperature-dependent 119Sn-Mössbauer spectra, which were significantly smaller than that of BaSnO3 (=658 K) and SnO2 (=382 K). These results suggest that Fe and Sn are a weakly bound lattice in goethite NPs with low crystallinity. Modification of NPs and addition of Sn has a positive effect, resulting in an increase in DC conductivity of almost 5 orders of magnitude, from a σDC value of 9.37 × 10-7 (Ω cm)-1 for pure goethite Sn (Sn0) up to DC plateau for samples containing 0.15 and 0.20 Sn (Sn15 and Sn20) with a DC value of ~4 × 10-7 (Ω cm)-1 @423 K. This non-linear conductivity pattern and levelling at a higher Sn content suggests that structural modifications have a notable impact on electron transport, which is primarily governed by the thermally activated via three-dimensional hopping of small polarons (SPH). Measurements of SIB performance, including the Sn100x cathode under a current density of 50 mA g-1, showed initial capacities of 81 and 85 mAh g-1 for Sn0 and Sn15, which were larger than the others. The large initial capacities were measured at a current density of 5 mA g-1 found at 170 and 182 mAh g-1 for Sn15 and Sn20, respectively. It is concluded that tin-goethite NPs are an excellent material for a secondary battery cathode and that Sn15 is the best cathode among the studied Sn100x NPs.

Photocatalytic and Cathode Active Abilities of Ni-Substituted α-FeOOH Nanoparticles.

The present study investigates the relationship between the local structure, photocatalytic ability, and cathode performances in sodium-ion batteries (SIBs) and lithium-ion batteries (LIBs) using Ni-substituted goethite nanoparticles (NixFe1-xOOH NPs) with a range of 'x' values from 0 to 0.5. The structural characterization was performed applying various techniques, including X-ray diffractometry (XRD); thermogravimetry differential thermal analysis (TG-DTA); Fourier transform infrared spectroscopy (FT-IR); X-ray absorption spectroscopy (XANES/EXAFS), both measured at room temperature (RT); 57Fe Mössbauer spectroscopy recorded at RT and low temperatures (LT) from 20 K to 300 K; Brunauer-Emmett-Teller surface area measurement (BET), and diffuse reflectance spectroscopy (DRS). In addition, the electrical properties of NixFe1-xOOH NPs were evaluated by solid-state impedance spectroscopy (SS-IS). XRD showed the presence of goethite as the only crystalline phase in prepared samples with x ≤ 0.20, and goethite and α-Ni(OH)2 in the samples with x > 0.20. The sample with x = 0.10 (Ni10) showed the highest photo-Fenton ability with a first-order rate constant value (k) of 15.8 × 10-3 min-1. The 57Fe Mössbauer spectrum of Ni0, measured at RT, displayed a sextet corresponding to goethite, with an isomer shift (δ) of 0.36 mm s-1 and a hyperfine magnetic distribution (Bhf) of 32.95 T. Moreover, the DC conductivity decreased from 5.52 × 10-10 to 5.30 × 10-12 (Ω cm)-1 with 'x' increasing from 0.10 to 0.50. Ni20 showed the highest initial discharge capacity of 223 mAh g-1, attributed to its largest specific surface area of 174.0 m2 g-1. In conclusion, NixFe1-xOOH NPs can be effectively utilized as visible-light-activated catalysts and active cathode materials in secondary batteries.

New aspects of the photodegradation of iron(III) citrate: spectroscopic studies and plant-related factors.

Iron (Fe) is an essential cofactor for all livings. Although Fe membrane transport mechanisms often utilize FeII, uncoordinated or deliberated ferrous ions can initiate Fenton reactions. FeIII citrate complexes are among the most important complexed forms of FeIII especially in plants that, indeed, can undergo photoreduction. Since leaves as photosynthetic organs of higher plants are generally exposed to illumination in daytime, photoreaction of ferric species may have biological relevance in iron metabolism, the relevance of which is poorly understood. In present work FeIII citrate transformation during the photodegradation in solution and after foliar application on leaves was studied by Mössbauer analysis directly. To obtain irradiation time dependence of the speciation of iron in solutions, four model solutions of different pH values (1.5, 3.3, 5.5, and 7.0) with Fe to citrate molar ratio 1:1.1 were exposed to light. Highly acidic conditions led to a complete reduction of Fe together with the formation of FeII citrate and hexaaqua complexes in equal concentration. At higher pH, the only product of the photodegradation was FeII citrate, which was later reoxidized and polymerized, resulting in the formation of polynuclear stable ferric compound. To test biological relevance, leaves of cabbage were treated with FeIII citrate solution. X-ray fluorescence imaging indicated the accumulation of Fe in the treated leaf parts. Mössbauer analysis revealed the presence of several ferric species incorporated into the biological structure. The Fe speciation observed should be considered in biological systems where FeIII citrate has a ubiquitous role in Fe acquisition and homeostasis.

Thermally Induced Solid-Phase Quasi-Intramolecular Redox Reactions of [Hexakis(urea-O)iron(III)] Permanganate: An Easy Reaction Route to Prepare Potential (Fe,Mn)Ox Catalysts for CO2 Hydrogenation.

Research on new reaction routes and precursors to prepare catalysts for CO2 hydrogenation has enormous importance. Here, we report on the preparation of the permanganate salt of the urea-coordinated iron(III), [hexakis(urea-O)iron(III)]permanganate ([Fe(urea-O)6](MnO4)3) via an affordable synthesis route and preliminarily demonstrate the catalytic activity of its (Fe,Mn)Ox thermal decomposition products in CO2 hydrogenation. [Fe(urea-O)6](MnO4)3 contains O-coordinated urea ligands in octahedral propeller-like arrangement around the Fe3+ cation. There are extended hydrogen bond interactions between the permanganate ions and the hydrogen atoms of the urea ligands. These hydrogen bonds serve as reaction centers and have unique roles in the solid-phase quasi-intramolecular redox reaction of the urea ligand and the permanganate anion below the temperature of ligand loss of the complex cation. The decomposition mechanism of the urea ligand (ammonia elimination with the formation of isocyanuric acid and biuret) has been clarified. In an inert atmosphere, the final thermal decomposition product was manganese-containing wuestite, (Fe,Mn)O, at 800 °C, whereas in ambient air, two types of bixbyite (Fe,Mn)2O3 as well as jacobsite (Fe,Mn)T-4(Fe,Mn)OC-62O4), with overall Fe to Mn stoichiometry of 1:3, were formed. These final products were obtained regardless of the different atmospheres applied during thermal treatments up to 350 °C. Disordered bixbyite formed first with inhomogeneous Fe and Mn distribution and double-size supercell and then transformed gradually into common bixbyite with regular structure (and with 1:3 Fe to Mn ratio) upon increasing the temperature and heating time. The (Fe,Mn)Ox intermediates formed under various conditions showed catalytic effect in the CO2 hydrogenation reaction with <57.6% CO2 conversions and <39.3% hydrocarbon yields. As a mild solid-phase oxidant, hexakis(urea-O)iron(III) permanganate, was found to be selective in the transformation of (un)substituted benzylic alcohols into benzaldehydes and benzonitriles.

57Fe Mössbauer Spectroscopy as a Tool for Study of Spin States and Magnetic Interactions in Inorganic Chemistry.

In this mini-review of our research group's activity, the application of 57Fe Mössbauer spectroscopy in studies of electronic structure, coordination environment, and magnetic interactions in an interesting series of Fe(II/III) compounds selected is discussed. We selected two prominent phenomena that arose during investigations of selected groups of compounds carried out at different periods of time: (1) very high magnetic hyperfine fields observed at low temperatures; (2) changes in the oxidation state of the central iron atom of complexes in the solid state during interactions with gaseous O2/H2O mixtures, resulting in spin crossover (SCO).

Evidence for ferritin as dominant iron-bearing species in the rhizobacterium Azospirillum brasilense Sp7 provided by low-temperature/in-field Mössbauer spectroscopy.

For the ubiquitous diazotrophic rhizobacterium Azospirillum brasilense, which has been attracting the attention of researchers worldwide for the last 35 years owing to its significant agrobiotechnological and phytostimulating potential, the data on iron acquisition and its chemical speciation in cells are scarce. In this work, for the first time for azospirilla, low-temperature (at 80 K, 5 K, as well as at 2 K without and with an external magnetic field of 5 T) transmission Mössbauer spectroscopic studies were performed for lyophilised biomass of A. brasilense (wild-type strain Sp7 grown with (57)Fe(III) nitrilotriacetate complex as the sole source of iron) to enable quantitative chemical speciation analysis of the intracellular iron. In the Mössbauer spectrum at 80 K, a broadened quadrupole doublet of high-spin iron(III) was observed with a few percent of a high-spin iron(II) contribution. In the spectrum measured at 5 K, a dominant magnetically split component appeared with the parameters typical of ferritin species from other bacteria, together with a quadrupole doublet of a superparamagnetic iron(III) component and a similarly small contribution from the high-spin iron(II) component. The Mössbauer spectra recorded at 2 K (with or without a 5 T external field) confirmed the assignment of ferritin species. About 20% of total Fe in the dry cells of A. brasilense strain Sp7 were present in iron(III) forms superparamagnetic at both 5 and 2 K, i.e. either different from ferritin cores or as ferritin components with very small particle sizes.

Hexakis(urea-O)iron Complex Salts as a Versatile Material Family: Overview of Their Properties and Applications.

Due to their Fe- and N-containing reactive urea ligand content, the hexakis(urea-O)iron(II) and hexakis(urea-O)iron(III) complexes were found to be versatile materials in various application fields of industry and environmental protection. In our present work, we have comprehensively reviewed the synthesis, structural and spectroscopic details, and thermal properties of hexakis(urea-O)iron(II) and hexakis(urea-O)iron(III) salts with different anions (NO3-, Cl-, Br- I-, I3-, ClO4-, MnO4-, SO42-, Cr2O72-, and S2O82-). We compared and evaluated the structural, spectroscopic (IR, Raman, UV-vis, Mössbauer, EPR, and X-ray), and thermogravimetric data. Based on the thermal behavior of these complexes, we evaluated the solid-phase quasi-intramolecular redox reactions of anions and urea ligands in these complexes and summarized the available information on the properties of the resulting simple and mixed iron-containing oxides. Furthermore, we give a complete overview of the application of these complexes as catalysts, reagents, absorbers, or agricultural raw materials.

Emission ((57)Co) Mössbauer spectroscopy as a tool for probing speciation and metabolic transformations of cobalt(II) in bacterial cells.

The emission ((57)Co) variant of Mössbauer spectroscopy, rarely used in biology-related studies, was applied to study binding and possible transformations of (57)Co(II) traces in live and dead (hydrothermally treated) cells of the rhizobacterium Azospirillum brasilense (strain Sp7) at T=80 K in frozen aqueous suspensions and as their dried residues. The Mössbauer parameters calculated from the spectra were compared with the similarly obtained data reported earlier for another A. brasilense strain, Sp245 (which differs from strain Sp7 by the ecological niche occupied in the rhizosphere and was found earlier to exhibit different metabolic responses under similar environmental conditions). Similarly to strain Sp245, live cells of strain Sp7, rapidly frozen 2 min and 1 h after their contact with (57)Co(2+) (measured in frozen suspensions), showed marked differences in their Mössbauer parameters, reflecting metabolic transformations of (57)Co(2+) occurring within an hour. However, the parameters for strains Sp7 (this work) and Sp245 (reported earlier), obtained under similar conditions, were found to significantly differ, implying dissimilarity in their metabolic response to Co(2+). This is in line with their different metabolic responses to several heavy metals, including Co(2+), detected earlier using Fourier transform infrared spectroscopy.

Apoplast utilisation of nanohaematite initiates parallel suppression of RIBA1 and FRO1&3 in Cucumis sativus.

Nanoscale Fe containing particles can penetrate the root apoplast. Nevertheless, cell wall size exclusion questions that for Fe mobilisation, a close contact between the membrane integrating FERRIC REDUCTASE OXIDASE (FRO) enzymes and Fe containing particles is required. Haematite nanoparticle suspension, size of 10-20 nm, characterized by 57Fe Mössbauer spectroscopy, TEM, ICP and SAED was subjected to Fe utilisation by the flavin secreting model plant cucumber (Cucumis sativus). Alterations in the structure and distribution of the particles were revealed by 57Fe Mössbauer spectroscopy, HRTEM and EDS element mapping. Biological utilisation of Fe resulted in a suppression of Fe deficiency responses (expression of CsFRO 1, 2 & 3 and RIBOFLAVIN A1; CsRIBA1 genes and root ferric chelate reductase activity). Haematite nanoparticles were stacked in the middle lamella of the apoplast. Fe mobilisation is evidenced by the reduction in the particle size. Fe release from nanoparticles does not require a contact with the plasma membrane. Parallel suppression in the CsFRO 1&3 and CsRIBA1 transcript amounts support that flavin biosynthesis is an inclusive Fe deficiency response involved in the reduction-based Fe utilisation of Cucumis sativus roots. CsFRO2 is suggested to play a role in the intracellular Fe homeostasis.

Magnetic Anisotropy and Microstructure in Electrodeposited Quaternary Sn-Fe-Ni-Co Alloys with Amorphous Character.

Sn-Fe-Ni-Co quaternary alloys, in the composition range of 37-44 at% Sn, 35-39 at% Fe, 6-8 at% Ni and 13-17 at% Co, were prepared by direct current (DC) and pulse plating (PP) electrodeposition. The alloy deposits were characterized by XRD, 57Fe and 119Sn conversion electron Mössbauer spectroscopy, SEM-EDX and magnetization measurements. XRD revealed the amorphous character of the quaternary alloy deposits. The dominant ferromagnetic character of the deposits was shown by magnetization and Mössbauer spectroscopy measurements. Room temperature Mössbauer spectra showed minor paramagnetic phases, where their occurrences (~3-20%) are correlated to the electrodeposition parameters (Jdep from -16 to -23 mA/cm2 for DC, Jpulse from -40 to -75 mA/cm2 for PP), the composition and the saturation magnetization (~52-73 emu/g). A considerable difference was found in the magnetization curves applying parallel or perpendicular orientation of the applied fields, indicating magnetic anisotropy both in DC and pulse plated alloy coatings.

Mössbauer Spectroscopy with a High Velocity Resolution in the Studies of Nanomaterials.

The present review describes our long experience in the application of Mössbauer spectroscopy with a high velocity resolution (a high discretization of the velocity reference signal) in the studies of various nanosized and nanostructured iron-containing materials. The results reviewed discuss investigations of: (I) nanosized iron cores in: (i) extracted ferritin, (ii) ferritin in liver and spleen tissues in normal and pathological cases, (iii) ferritin in bacteria, (iv) pharmaceutical ferritin analogues; (II) nanoparticles developed for magnetic fluids for medical purposes; (III) nanoparticles and nanostructured FINEMET alloys developed for technical purposes. The results obtained demonstrate that the high velocity resolution Mössbauer spectroscopy permits to excavate more information and to extract more spectral components in the complex Mössbauer spectra with overlapped components, in comparison with those obtained by using conventional Mössbauer spectroscopy. This review also shows the advances of Mössbauer spectroscopy with a high velocity resolution in the study of various iron-based nanosized and nanostructured materials since 2005.

Change in Magnetic Anisotropy at the Surface and in the Bulk of FINEMET Induced by Swift Heavy Ion Irradiation.

57Fe transmission and conversion electron Mössbauer spectroscopy as well as XRD were used to study the effect of swift heavy ion irradiation on stress-annealed FINEMET samples with a composition of Fe73.5Si13.5Nb3B9Cu1. The XRD of the samples indicated changes neither in the crystal structure nor in the texture of irradiated ribbons as compared to those of non-irradiated ones. However, changes in the magnetic anisotropy both in the bulk as well as at the surface of the FINEMET alloy ribbons irradiated by 160 MeV 132Xe ions with a fluence of 1013 ion cm-2 were revealed via the decrease in relative areas of the second and fifth lines of the magnetic sextets in the corresponding Mössbauer spectra. The irradiation-induced change in the magnetic anisotropy in the bulk was found to be similar or somewhat higher than that at the surface. The results are discussed in terms of the defects produced by irradiation and corresponding changes in the orientation of spins depending on the direction of the stress generated around these defects.

Detailed spectroscopic, thermodynamic, and kinetic studies on the protolytic equilibria of Fe(III)cydta and the activation of hydrogen peroxide.

The crystal structure of the as-yet-unknown salt K[Fe(III)(cydta)(H(2)O)].3H(2)O, where cydta = (+/-)-trans-1,2-cyclohexanediaminetetraacetate, has been resolved: orthorhombic space group Pbca with R1 = 0.0309, wR2 = 0.0700, and GOF = 0.99. There are two independent [Fe(III)(cydta)(H(2)O)](-) anions in the asymmetric unit, and the ligand is (R,R)-cydta in both cases. The coordination polyhedron is a seven-coordinate capped trigonal prism where the quadrilateral face formed by the four ligand donor oxygen atoms is capped by the coordinated water molecule. The speciation of [Fe(III)(cydta)(H(2)O)](-) in water was studied in detail by a combination of techniques: (i) Measurements of the pH dependence of the Fe(III/II)cydta redox potentials by cyclic voltammetry enabled the estimation of the stability constants (0.1 M KNO(3), 25 degrees C) of [Fe(III)(cydta)(H(2)O)](-) (log beta(III)(110) = 29.05 +/- 0.01) and [Fe(II)(cydta)(H(2)O)](2-) (log beta(II)(110) = 17.96 +/- 0.01) as well as pK(III)(a1OH) = 9.57 and pK(II)(a1H) = 2.69. The formation enthalpy of [Fe(III)(cydta)(H(2)O)](-) (DeltaH degrees = -23 +/- 1 kJ mol(-1)) was measured by direct calorimetry and is compared to the corresponding value for [Fe(III)(edta)(H(2)O)](-) (DeltaH degrees = -31 +/- 1 kJ mol(-1)). (ii) pH-dependent spectrophotometric titrations of Fe(III)cydta lead to pK(III)(a1OH) = 9.54 +/- 0.01 for deprotonation of the coordinated water and a dimerization constant of log K(d) = 1.07. These data are compared with those of Fe(III)pdta (pdta = 1,2-propanediaminetetraacetate; pK(III)(a1OH) = 7.70 +/- 0.01, log K(d) = 2.28) and Fe(III)edta (pK(III)(a1OH) = 7.52 +/- 0.01, log K(d) = 2.64). Temperature- and pressure-dependent (17)O NMR measurements lead to the following kinetic parameters for the water-exchange reaction at [Fe(III)(cydta)(H(2)O)](-) (at 298 K): k(ex) = (1.7 +/- 0.2) x 10(7) s(-1), DeltaH(++) = 40.2 +/- 1.3 kJ mol(-1), DeltaS(++) = +28.4 +/- 4.7 J mol(-1) K(-1), and DeltaV(++) = +2.3 +/- 0.1 cm(3) mol(-1). A detailed kinetic study of the effect of the buffer, temperature, and pressure on the reaction of hydrogen peroxide with [Fe(III)(cydta)(H(2)O)](-) was performed using stopped-flow techniques. The reaction was found to consist of two steps and resulted in the formation of a purple Fe(III) side-on-bound peroxo complex [Fe(III)(cydta)(eta(2)-O(2))](3-). The peroxo complex and its degradation products were characterized using Mossbauer spectroscopy. Formation of the purple peroxo complex is only observable above a pH of 9.5. Both reaction steps are affected by specific and general acid catalysis. Two different buffer systems were used to clarify the role of general acid catalysis in these reactions. Mechanistic descriptions and a comparison between the edta and cydta systems are presented. The first reaction step reveals an element of reversibility, which is evident over the whole studied pH range. The positive volume of activation for the forward reaction and the positive entropy of activation for the backward reaction suggest a dissociative interchange mechanism for the reversible end-on binding of hydrogen peroxide to [Fe(III)(cydta)(H(2)O)](-). Deprotonation of the end-on-bound hydroperoxo complex leads to the formation of a seven-coordinate side-on-bound peroxo complex [Fe(III)(cydta)(eta(2)-O(2))](3-), where one carboxylate arm is detached. [Fe(III)(cydta)(eta(2)-O(2))](3-) can be reached by two different pathways, of which one is catalyzed by a base and the other by deprotonated hydrogen peroxide. For both pathways, a small negative volume and entropy of activation was observed, suggesting an associative interchange mechanism for the ring-closure step to the side-on-bound peroxo complex. For the second reaction step, no element of reversibility was found.

Mössbauer characterization and in situ monitoring of thermal decomposition of potassium ferrate(VI), K2FeO4 in static air conditions.

Solid orthorhombic crystals of potassium ferrate(VI) (K(2)FeO(4)) of a high-chemical purity (>99.0%) were characterized by low-temperature (1.5-5 K), high-temperature (463-863 K), and in-field (1.5 K/3 T) Mössbauer spectroscopy. Potassium ferrate(VI) reveals a Néel magnetic transition temperature (TN) of approximately 3.8 K and a saturation hyperfine magnetic field of 13.8 T at 1.5 K. Spectral line intensities recorded below TN in an external magnetic field of 3 T manifest a perfect antiferromagnetic ordering. For the in situ monitoring of the thermal behavior of K(2)FeO(4), high-temperature Mössbauer data were combined with those obtained from thermogravimetry, differential scanning calorimetry, and variable-temperature X-ray diffraction measurements. Such in situ approach allowed the identification of the reaction products and intermediates and yielded the first experimental evidence for the participation of CO2 in the decomposition process. As the primary conversion products, KFeO(2) and two potassium oxides in equivalent molar ratio, KO2 and K(2)O, were suggested. However, the KO2 phase is detectable with difficulty as it reacts very quickly with CO2 from air resulting in the formation of K(2)CO(3). The presented decomposition model is consistent with thermogravimetric data giving the mass loss of 8.0%, which corresponds to the participation of 1/6 mol of CO2 and liberation of 3/4 mol of O2 per 1 mol of K(2)FeO(4) (K(2)FeO(4) + 1/6CO2 --> KFeO(2) + 1/3K(2)O + 1/6K(2)CO(3) + 3/4O2). An explanation of the multistage reaction mechanism has an important practical impact for the optimization of the solid-state synthesis of potassium ferrate(VI).

Cobalt(II) complexation with small biomolecules as studied by 57Co emission Mössbauer spectroscopy.

In the emission (57Co) variant of Mössbauer spectroscopy (EMS), the 57Co radionuclide (with a half-life of 9months) is used that undergoes a nuclear decay 57Co→57Fe via electron capture followed by the emission of a γ-quantum, the energy of which is modified by the chemical state and the close coordination environment of the parent 57Co atom. While EMS has been used largely in materials science and nuclear chemistry, its high sensitivity can also be of great advantage in revealing fine structural features and for speciation analysis of biological complexes, whenever the 57Co2+ cation can be used directly as the coordinating metal or as a substitute for native cobalt or other metal ions. As such EMS applications are yet rare, in order to reliably interpret emission spectra of sophisticated 57Co2+-doped biosystems, model EMS studies of simple cobalt biocomplexes are necessary. In this work, EMS spectroscopic data are analysed and discussed for 57Co2+ complexes with a range of small biomolecules of different structures, including 4-n-hexylresorcinol, homoserine lactone and a few amino acids (spectra measured in rapidly frozen dilute aqueous solutions or in the dried state at T=80K). The EMS data obtained are discussed with regard to the available literature data related to the coordination modes of the biocomplexes under study.

Characterization and 10Be content of iron carbonate concretions for genetic aspects - Weathering, desert varnish or burning: Rim effects in iron carbonate concretions.

The research investigated three iron carbonate (siderite) sedimentary concretions from Nagykovácsi, Úri and Délegyháza, Hungary. To identify possible source rocks and effects of the glaze-like exposed surface of the concretions, we carried on comparative petrological, mineralogical, geochemical and isotopic studies. The samples were microbially mediated siderite concretions with embedded metamorphous and igneous mineral clasts, and had specific rim belts characterized by semi-concentric outer Fe-oxide layers, fluffy pyrite-rich outer belts and siderite inner parts. We investigated the cross section of the Fe-carbonate concretions by independent methodologies in order to identify their rim effects. Their surficial oxide layers showed evidence of degassing of the exposed surface caused most probably by elevated temperatures. The inner rim pyrite belt in the concretions excluded the possibility of a prolonged wet surface environment. Microtextural and mineralogical features did not support desert varnish formation. 10Be nuclide values of the Nagykovácsi and Uri concretions were far above the level of terrestrial in-situ cosmogenic nuclides, but they were consistent with the lowest levels for meteorites. Though the data were not conclusive to confirm any kind of known origin, they are contradictary, and open possibilities for a scenario of terrestrial meteorite origin.

Speciation and structure of tin(II) in hyper-alkaline aqueous solution.

The identity of the predominating tin(ii)-hydroxide complex formed in hyper-alkaline aqueous solutions (0.2 ≤CNaOH≤ 12 mol dm(-3)) is determined by potentiometric titrations, Raman, Mössbauer and XANES spectroscopy, supplemented by quantum chemical calculations. Thermodynamic studies using a H2/Pt electrode up to free hydroxide concentrations of 1 mol dm(-3) showed the presence of a single monomeric complex with a tin(II) : hydroxide ratio of 1 : 3. This observation together with Raman and Mössbauer spectroscopic measurements supplemented by quantum mechanical calculations proved that the predominating complex is [Sn(OH)3](-), and that the presence of the other possible complex, [SnO(OH)](-), could not be proven with either experiments or simulations. The structure of the trihydroxidostannate(II) complex, [Sn(OH)3](-), was determined by EXAFS and was found to be independent of the applied hydroxide and tin(II) concentrations. The mean Sn-O bond distance is short, 2.078 Å, and in very good agreement with the only structure reported in the solid state. It is also shown that at pH values above 13 the speciation of the predominant trihydroxidostannate(II) complex is not affected by the presence of high concentrations of chloride ions.

The structure of Fe(III) ions in strongly alkaline aqueous solutions from EXAFS and Mössbauer spectroscopy.

To establish the structure of ferric ions in strongly alkaline (pH > 13) environments, aqueous NaOH solutions supersaturated with respect to Fe(III) and the solid ferric-hydroxo complex salts precipitating from them have been characterized with a variety of experimental techniques. From UV measurements, in solutions of pH > 13, only one kind of Fe(III)-hydroxo complex species was found to be present. The micro crystals obtained from such solutions were proven to be a new, so far unidentified solid phase. Mössbauer spectra of the quick-frozen solution and that of the complex salt indicated a highly symmetrical ferric environment in both systems From the EXAFS and XANES spectra, the environment of the ferric ion in these solutions (both native and quick-frozen) and in the complex salt was found to be different. In the complex salt, the bond lengths are consistent with an octahedral coordination around the ferric centres. In solution, the coordination geometry of Fe(III) is most probably tetrahedral. Our results demonstrate that in strongly alkaline aqueous solutions, ferric ions behave very similarly to other structurally related tervalent ions, like Al(III) or Ga(III).