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.

Chloroplasts preferentially take up ferric-citrate over iron-nicotianamine complexes in Brassica napus.

MAIN CONCLUSION: Fe uptake machinery of chloroplasts prefers to utilise Fe(III)-citrate over Fe-nicotianamine complexes. Iron uptake in chloroplasts is a process of prime importance. Although a few members of their iron transport machinery were identified, the substrate preference of the system is still unknown. Intact chloroplasts of oilseed rape (Brassica napus) were purified and subjected to iron uptake studies using natural and artificial iron complexes. Fe-nicotianamine (NA) complexes were characterised by 5 K, 5 T Mössbauer spectrometry. Expression of components of the chloroplast Fe uptake machinery was also studied. Fe(III)-NA contained a minor paramagnetic Fe(II) component (ca. 9%), a paramagnetic Fe(III) component exhibiting dimeric or oligomeric structure (ca. 20%), and a Fe(III) complex, likely being a monomeric structure, which undergoes slow electronic relaxation at 5 K (ca. 61%). Fe(II)-NA contained more than one similar chemical Fe(II) environment with no sign of Fe(III) components. Chloroplasts preferred Fe(III)-citrate compared to Fe(III)-NA and Fe(II)-NA, but also to Fe(III)-EDTA and Fe(III)-o,o'EDDHA, and the Km value was lower for Fe(III)-citrate than for the Fe-NA complexes. Only the uptake of Fe(III)-citrate was light-dependent. Regarding the components of the chloroplast Fe uptake system, only genes of the reduction-based Fe uptake system showed high expression. Chloroplasts more effectively utilize Fe(III)-citrate, but hardly Fe-NA complexes in Fe uptake.

Transformations of ferrates(iv,v,vi) in liquids: Mössbauer spectroscopy of frozen solutions.

The kinetics and mechanism of ferrate(iv), (v) and (vi) transformations in water and in polar organic solvents (namely ethanol and tetrahydrofuran) have been investigated by the method of 57Fe Mössbauer spectroscopy of frozen solutions. Ethanol with a very limited amount of water under an inert atmosphere, significantly slows down the transformation reactions of ferrates(iv and v) and provides direct proof of the existence of intermediate states. Simultaneously, ethanol is oxidized to caboxylates in the close vicinity of the surface of ferrate crystallites as proven by X-ray photoelectron spectroscopy. On the contrary, any transformation of ferrate(vi) in pure ethanol (with a very limited amount of water) was not observed. Mössbauer spectroscopy of frozen solutions enabled us to experimentally identify and quantify intermediates of ferrate(iv) and ferrate(v) transformations for the first time. Sodium ferrate(iv) in its tetrahedral form, Na4FeO4, undergoes a two-step charge disproportionation to Fe(iii) and Fe(vi) via a Fe(v) intermediate without any evolution of oxygen in polar protic and aprotic solvents, specifically 2Fe(iv) → Fe(iii) + Fe(v), and Fe(iv) + Fe(v) → Fe(iii) + Fe(vi), i.e. in sum 3Fe(iv) → 2Fe(iii) + Fe(vi). Ferrate(v) (K3FeO4) transforms to Fe(iii) and Fe(vi) without any indication of the Fe(iv) intermediate within the detection limit of the method. In addition to a charge disproportionation reaction proceeding in polar liquids, 3Fe(v) → Fe(iii) + 2Fe(vi), a competitive reduction of Fe(v) directly to Fe(iii) accompanied by oxygen evolution takes place in water. Oxygen evolution was also measured for ferrate(iv and vi) transformations in water, but to a higher and a smaller extent compared to ferrate(v), respectively. The thermodynamics of the suggested ferrate(iv) and ferrate(v) transformation pathways was examined by DFT calculations.

Revisiting the iron pools in cucumber roots: identification and localization.

MAIN CONCLUSION: Fe deficiency responses in Strategy I causes a shift from the formation of partially removable hydrous ferric oxide on the root surface to the accumulation of Fe-citrate in the xylem. Iron may accumulate in various chemical forms during its uptake and assimilation in roots. The permanent and transient Fe microenvironments formed during these processes in cucumber which takes up Fe in a reduction based process (Strategy I) have been investigated. The identification of Fe microenvironments was carried out with (57)Fe Mössbauer spectroscopy and immunoblotting, whereas reductive washing and high-resolution microscopy was applied for the localization. In plants supplied with (57)Fe(III)-citrate, a transient presence of Fe-carboxylates in removable forms and the accumulation of partly removable, amorphous hydrous ferric oxide/hydroxyde have been identified in the apoplast and on the root surface, respectively. The latter may at least partly be the consequence of bacterial activity at the root surface. Ferritin accumulation did not occur at optimal Fe supply. Under Fe deficiency, highly soluble ferrous hexaaqua complex is transiently formed along with the accumulation of Fe-carboxylates, likely Fe-citrate. As (57)Fe-citrate is non-removable from the root samples of Fe deficient plants, the major site of accumulation is suggested to be the root xylem. Reductive washing results in another ferrous microenvironment remaining in the root apoplast, the Fe(II)-bipyridyl complex, which accounts for ~30 % of the total Fe content of the root samples treated for 10 min and rinsed with CaSO4 solution. When (57)Fe(III)-EDTA or (57)Fe(III)-EDDHA was applied as Fe-source higher soluble ferrous Fe accumulation was accompanied by a lower total Fe content, confirming that chelates are more efficient in maintaining soluble Fe in the medium while less stable natural complexes as Fe-citrate may perform better in Fe accumulation.

Fe-Impregnated Mineral Colloids for Peroxide Activation: Effects of Mineral Substrate and Fe Precursor.

Heterogeneous iron species at the mineral/water interface are important catalysts for the generation of reactive oxygen species at circumneutral pH. One significant pathway leading to the formation of such species arises from deposition of dissolved iron onto mineral colloids due to changes in redox conditions. This study investigates the catalytic properties of Fe impregnated on silica, alumina, and titania nanoparticles (as prototypical mineral colloids). Fe impregnation was carried out by immersing the mineral nanoparticles in dilute Fe(II) or Fe(III) solutions at pH 6 and 3, respectively, in an aerobic environment. The uptake of iron per unit surface area follows the order of nTiO2 > nAl2O3 > nSiO2 for both types of Fe precursors. Impregnation of mineral particles in Fe(II) solutions results in predominantly Fe(III) species due to efficient surface-mediated oxidation. The catalytic activity of the impregnated solids to produce hydroxyl radical (·OH) from H2O2 decomposition was evaluated using benzoic acid as a probe compound under dark conditions. Invariably, the rates of benzoic acid oxidation with different Fe-laden particles increase with the surface density of Fe until a critical density above which the catalytic activity approaches a plateau, suggesting active Fe species are formed predominantly at low surface loadings. The critical surface density of Fe varies with the mineral substrate as well as the aqueous Fe precursor. Fe impregnated on TiO2 exhibits markedly higher activity than its Al2O3 and SiO2 counterparts. The speciation of interfacial Fe is analyzed with diffuse reflectance UV-vis analysis and interpretation of the data in the context of benzoic oxidation rates suggests that the surface activity of the solids for ·OH generation correlates strongly with the isolated (i.e., mononuclear) Fe species. Therefore, iron dispersed on mineral colloids is a significant form of reactive iron surfaces in the aquatic environment.

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.

Impact of inorganic buffering ions on the stability of Fe(vi) in aqueous solution: role of the carbonate ion.

An iron compound of +6 oxidation state (Fe(VI)O4(2-), Fe(vi)) is a green molecule for various applications (water oxidation catalyst, organic transformation for synthesis, and water remediation agent). However, its use is hindered because of its inherent decay in aqueous solution. This study presents a systematic kinetics investigation of the decay of ferrate(vi) in the presence of inorganic buffering ions (borate, phosphate, and carbonate) at a pH range from 6.0 to 9.0. When the heterogeneous decay of Fe(vi) on ferric products was inhibited by phosphate, detailed kinetic analysis revealed that the carbonate anion enhanced the Fe(vi) decay rate, compared to phosphate and borate ions. The order of the Fe(vi) decay rate under neutral solution conditions was carbonate > phosphate ≥ borate. In alkaline solution, the decay rates of Fe(vi) were similar for the studied buffering ions. The decay of Fe(vi) in the presence of the carbonate ion was described by mixed first- and second-order kinetics and the first-order rate constant (k1') had a linear relationship with the concentration of the carbonate ion at a neutral pH (k1' = 0.023 + 3.54 × [carbonate] L mol(-1) s(-1)). The analysis of the Fe(vi) decay intermediates/products (˙O2(-), H2O2, and O2) suggests similar decay pathways in the presence of different buffering anions. The impact of carbonate ions on the size of the nanoparticles of the Fe(iii) precipitate, the final reduced form of Fe(vi), was studied using transmission electron microscopy, (57)Fe Mössbauer spectroscopy, and magnetization measurements. The results indicated that carbonate ions induce the formation of ultrasmall iron(iii) oxyhydroxide nanoparticles (<5 nm), which apparently contribute to increased decay of Fe(vi) due to their larger specific surface area. The described homogeneous reaction of carbonate with Fe(vi) has important implications in the efficiency of environmental Fe(vi) applications. On the other hand, the observed low reactivity of borate with Fe(vi) demonstrates that borate is the least reactive buffer in studies of Fe(vi) reactivity in neutral solutions.

High-valent iron (Fe(VI), Fe(V), and Fe(IV)) species in water: characterization and oxidative transformation of estrogenic hormones.

This paper presents solid state synthesis and characterization of tetra-oxy iron(iv) and iron(v) species in their salt forms (Na4FeO4-Fe(IV) and K3FeO4-Fe(V)). Stability of the synthesized salts, commonly called ferrates, in water was determined by applying the (57)Fe Mössbauer spectroscopy technique. Within 2 s in water, Fe(IV) converted into Fe(III) while Fe(V) transformed into Fe(VI) and Fe(III) at pH = 8.2. Comparatively, Fe(VI) (bought as K2FeO4) remained stable in aqueous solution during the short time period. The oxidative removal efficiency of the high-valent iron species was then tested against five environmentally important estrogenic hormones (estron (E1), 17-ß-estradiol (E2), estriol (E3), 17-α-ethinylestradiol (EE2), and diethylstibestrol (DES)) in effluent water of a wastewater treatment plant. Three dosages of iron species (1, 10, and 100 mg L(-1)) were applied to the effluent water. An increase in the concentration of dosages enhanced the removal of estrogens. Both Fe(V) and Fe(VI) were effective in degrading estrogens, but Fe(IV) showed limited oxidation capacity to transform estrogens. The oxidized products of the estrogens were analyzed using Raman spectroscopy and high-performance liquid chromatography-mass spectrometry (HPLC-MS) techniques. Results demonstrated the transformation of estrogens into low molecular weight oxygenated compounds such as quinone-like and opened-aromatic ring species. A detailed study on E1 by using excess Fe(VI) showed the mineralization of the parent compound. The results demonstrate great potential of high-valent iron species in the degradation of endocrine disruptor chemicals like estrogens with several superior aspects including fast reactions, complete degradation and/or formation of benign organic species, and environmentally-acceptable iron oxide by-products.

Direct evidence of Fe(V) and Fe(IV) intermediates during reduction of Fe(VI) to Fe(III): a nuclear forward scattering of synchrotron radiation approach.

Identification of unstable high-valent iron species in electron transfer reactions of ferrate(VI) (Fe(VI)O4(2-), Fe(VI)) has been an important challenge in advancing the understanding of the oxidative mechanisms of ferrates. This paper presents the first example of distinguishing various phases differing in the valence state of iron in the solid state reduction of Fe(VI) to Fe(III) oxides at 235 °C using hyperfine parameters, isomer shift and hyperfine magnetic field, obtained from nuclear forward scattering of synchrotron radiation (NFS). The NFS technique enables a fast data accumulation resulting in high time resolution of in situ experiments. The results suggest a reaction mechanism, involving Fe(V) and Fe(IV) species, in the thermal decomposition of K2FeO4 to KFeO2. The present study opens up an approach to exploring the unambiguous identification of Fe(VI), Fe(V), Fe(IV), and Fe(III) in electron-transfer reaction mechanisms of ferrates in solid and aqueous phase systems.

Zinc Ferrite Nanoparticle Coatings on Austenitic Alloy Steel.

The phase transition of austenitic stainless steel of commercial label CL20ES and zinc ferrite nanoparticles was studied in an oxidative atmosphere of dry air to develop a low-cost, effective technique for covering-layer fabrication. CL20ES powder and zinc ferrite powder were mechanically mixed. This mixture was studied in an atmosphere of dry air at different annealing temperatures from room temperature to 900 °C. The employed characterization techniques are X-ray powder diffraction, Mössbauer spectroscopy in the transmission geometry, and scanning electron microscopy with elemental mapping. The fabricated layers were also characterized by surface-specific techniques such as conversion electron Mössbauer spectroscopy and grazing incidence X-ray powder diffraction. The analyzed powder mixture shows resistance against oxidation in dry air and high temperatures. These results were employed to produce zinc ferrite covering layers on 3D-printed cylinders of CL20ES. The results show a predisposition of zinc ferrite to be recrystallized at temperatures above 350 °C without the production of corrosive substances on steel. The zinc ferrite layers were analyzed by an ultrasonic hardness tester as well, which proved the hardness enhancement.

Development of tungsten-modified iron oxides to decompose an over-the-counter painkiller, Acetaminophen by activating peroxymonosulfate.

Acetaminophen (APAP) is a well-known type of over-the-counter painkillers and is frequently found in surface waterbodies, causing hepatotoxicity and skin irritation. Due to its persistence and chronic effects on the environment, innovative solutions must be provided to decompose APAP, effectively. Innovative catalysts of tungsten-modified iron oxides (TF) were successfully developed via a combustion method and thoroughly characterized using SEM, TEM, XRD, XPS, a porosimetry analysis, Mössbauer spectroscopy, VSM magnetometry, and EPR. With the synthesis method, tungsten was successfully incorporated into iron oxides to form ferrites and other magnetic iron oxides with a high porosity of 19.7 % and a large surface area of 29.5 m2/g. Also, their catalytic activities for APAP degradation by activating peroxymonosulfate (PMS) were evaluated under various conditions. Under optimal conditions, TF 2.0 showed the highest APAP degradation of 95 % removal with a catalyst loading of 2.0 g/L, initial APAP concentration of 5 mg/L, PMS of 6.5 mM, and pH 2.15 at room temperature. No inhibition by solution pHs, alkalinity, and humic acid was observed for APAP degradation in this study. The catalysts also showed chemical and mechanical stability, achieving 100 % degradation of 1 mg/L APAP during reusability tests with three consecutive experiments. These results show that TFs can effectively degrade persistent contaminants of emerging concern in water, offering an impactful contribution to wastewater treatment to protect human health and the ecosystem.

Mixtures of L-amino acids as reaction medium for formation of iron nanoparticles: the order of addition into a ferrous salt solution matters.

Owing to Mössbauer spectroscopy, an advanced characterization technique for iron-containing materials, the present study reveals previously unknown possibilities using l-amino acids for the generation of magnetic particles. Based on our results, a simple choice of the order of l-amino acids addition into a reaction mixture containing ferrous ions leads to either superparamagnetic ferric oxide/oxyhydroxide particles, or magnetically strong Fe0-Fe2O3/FeOOH core-shell particles after chemical reduction. Conversely, when ferric salts are employed with the addition of selected l-amino acids, only Fe0-Fe2O3/FeOOH core-shell particles are observed, regardless of the addition order. We explain this phenomenon by a specific transient/intermediate complex formation between Fe2+ and l-glutamic acid. This type of complexation prevents ferrous ions from spontaneous oxidation in solutions with full air access. Moreover, due to surface-enhanced Raman scattering spectroscopy we show that the functional groups of l-amino acids are not destroyed during the borohydride-induced reduction. These functionalities can be further exploited for (i) attachment of l-amino acids to the as-prepared magnetic particles, and (ii) for targeted bio- and/or environmental applications where the surface chemistry needs to be tailored and directed toward biocompatible species.

Microwave-Enhanced Crystalline Properties of Zinc Ferrite Nanoparticles.

Two series of ZnFe2O4 mixed cubic spinel nanoparticles were prepared by a coprecipitation method, where a solution of Fe3+ and Zn2+ was alkalised by a solution of NaOH. While the first series was prepared by a careful mixing of the two solutions, the microwave radiation was used to enhance the reaction in the other series of samples. The effect of the microwave heating on the properties of the prepared particles is investigated. X-ray powder diffraction (XRD), 57Fe Mössbauer spectroscopy and magnetometry were employed to prove the cubic structure and superparamagnetic behavior of the samples. The particle size in the range of nanometers was investigated by a transmission electron microscopy (TEM), and the N2 adsorption measurements were used to determine the BET area of the samples. The stoichiometry and the chemical purity were proven by energy dispersive spectroscopy (EDS). Additionally, the inversion factor was determined using the low temperature Mössbauer spectra in the external magnetic field. The microwave heating had a significant effect on the mean coherent length. On the other hand, it had a lesser influence on the size and BET surface area of the prepared nanoparticles.

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.

Microwave Synthesis of Magnetite Nanoparticles and Mg-Doped Magnetite Nanoparticles by Precipitation of Fe2+ Ions.

This study is focused on a simple and fast synthesis of nonstoichiometric magnetite nanoparticles with the chemical formula Fe3-XO4 and magnesium ferrite nanoparticles (Mg1-XFe2+XO4). The nanoparticles were prepared with Fe2+ ions (FeSO4 · H2O) alkalised by KOH under oxidative conditions and in a microwave field. X-ray powder diffraction (XRD) and 57Fe transmission Mössbauer spectroscopy were used to determine the phase composition and crystal structure in detail. The presence of synthetic magnetite, maghemite, goethite, and magnesium ferrite was observed. Room temperature Mössbauer spectroscopy revealed the existence of ferromagnetic sublattices and superparamagnetic fraction. The superparamagnetic component corresponds to magnesium ferrite nanoparticles. Low temperature Mössbauer spectroscopy was used to locate the blocking temperature of superparamagnetic nanoparticles and to separate the sublattices. The presumed spherical morphology of nanoparticles and their size under 100 nm have been confirmed by transmission electron microscopy (TEM). The obtained results were used to provide possible reaction scheme, which serves to tailor the synthesis to a desired application.

Environmentally friendly synthesized and magnetically recoverable designed ferrite photo-catalysts for wastewater treatment applications.

Fenton processes are promising wastewater treatment alternatives for bio-recalcitrant compounds. Three different methods (i.e., reverse microemulsion, sol-gel, and combustion) were designed to synthesize environmentally friendly ferrites as magnetically recoverable catalysts to be applied for the decomposition of two pharmaceuticals (ciprofloxacin and carbamazepine) that are frequently detected in water bodies. The catalysts were used in a heterogeneous solar photo-Fenton treatment to save the cost of applying high-energy UV radiation sources, and was performed under a slightly basic pH to avoid metal leaching and adding salts for pH adjustment. All the developed catalysts resulted in the effective treatment of ciprofloxacin and carbamazepine in both synthetic and real domestic wastewater. In particular, the sol-gel synthesized ferrite was more magnetic and more suitable for reuse. The degradation pathways of both compounds were elucidated for this treatment. The degradation of ciprofloxacin involved attacks to the quinolone and piperazine rings. The degradation pathway of carbamazepine involved the formation of hydroxyl carbamazepine and dihydroxy carbamazepine before yielding acridine by hydrogen abstraction, decarboxylation, and amine cleavage, which would be further oxidized.

Silver nanoparticles by atomic vapour deposition on an alcohol micro-jet.

We achieved sputter deposition of silver atoms onto liquid alcohols by injection of solvents into vacuum via a liquid microjet. Mixing silver atoms into ethanol by this method produced metallic silver nanoparticles. These had a broad, log-normal size distribution, with median size between 3.3 ± 1.4 nm and 2.0 ± 0.7 nm, depending on experiment geometry; and a broad plasmon absorption band centred around 450 nm. We also deposited silver atoms into a solution of colloidal silica nanoparticles, generating silver-decorated silica particles with consistent decoration of almost one silver particle to each silica sphere. The silver-silica mixture showed increased colloidal stability and yield of silver, along with a narrowed size distribution and a narrower plasmon band blue-shifted to 410 nm. Significant methanol loss of 1.65 × 10-7 mol MeOH per g per s from the mature silver-silica solutions suggests we have reproduced known silica supported silver catalysts. The excellent distribution of silver on each silica sphere shows this technique has potential to improve the distribution of catalytically active particles in supported catalysts.

Amorphous iron(III) oxide--a review.

The syntheses of amorphous Fe(2)O(3) nanoparticles of varying size and morphology, their magnetic properties, crystallization mechanism, and applications are reviewed herein. The synthetic routes are classified according to the nature of the sample (powders, nanocomposites, films, coated particles). The contributions of various experimental techniques to the characterization of an amorphous Fe(2)O(3) phase are considered in this review, including some key experimental markers, allowing its distinction from nanocrystalline "X-ray amorphous" polymorphs (maghemite, hematite). We discuss the thermally induced crystallization mechanisms depending on transformation temperature, atmosphere, and the size of the amorphous particles that predetermine the structure of the primarily formed crystalline polymorph. The controversial description of the magnetic behavior, including an interpretation of the low-temperature and in-field Mössbauer spectra, is analyzed.

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).

Degradation of the cyanotoxin microcystin-LR using iron-based photocatalysts under visible light illumination.

In this study, a simple and low-cost method to synthesize iron(III) oxide nanopowders in large quantity was successfully developed for the photocatalytic degradation of microcystin-LR (MC-LR). Two visible light-active iron(III) oxide samples (MG-9 calcined at 200 °C for 5 h and MG-11 calcined at 180 °C for 16 h) with a particle size of 5-20 nm were prepared via thermal decomposition of ferrous oxalate dihydrate in air without any other modifications such as doping. The synthesized samples were characterized by X-ray powder diffraction, 57Fe Mössbauer spectroscopy, transmission electron microscopy, Brunauer-Emmett-Teller (BET) specific surface area analysis, and UV-visible diffuse reflectance spectroscopy. The samples exhibited similar phase composition (a mixture of α-Fe2O3 and γ-Fe2O3), particle size distribution (5-20 nm), particle morphology, and degree of agglomeration, but different specific surface areas (234 m2 g-1 for MG-9 and 207 m2 g-1 for MG-11). The results confirmed higher photocatalytic activity of the catalyst with higher specific surface area. The highest photocatalytic activity of the sample to decompose MC-LR was observed at solution pH of 3.0 and catalyst loading of 0.5 g L-1 due to large amount of MC-LR adsorption, but a little iron dissolution of 0.0065 wt% was observed. However, no iron leaching was observed at pH 5.8 even though the overall MC-LR removal was slightly lower than at pH 3.0. Thus, the pH 5.8 could be an appropriate operating condition for the catalyst to avoid problems of iron contamination by the catalyst. Moreover, magnetic behavior of γ-Fe2O3 gives a possibility for an easy separation of the catalyst particles after their use.