Kinetic Analysis Applied to Iron in a Natural Water Model Containing Ions,Organic Complexes,Colloids, and Particles

Abstract

Kinetic analysis using complex formation reactions is applied to “mixtures” containing (variously), Fe³⁺, Fe²⁺, iron complexed to a fulvic acid, hydrous oxide colloids, and non-settleable particulate iron. Such mixtures can be directly resolved if the kinetics of complex formation reactions are pseudo first order and differences among rate constants are large enough. At pH = 4, it is found that fulvic acid causes substantial reduction of Fe³⁺ to Fe²⁺ and that it causes complete dissolution and depolymerization of colloidal and particulate iron at 1:1 ratio. Addition of one equivalent of phosphate causes precipitation of ferric phosphate even in the presence of fulvic acid. This system is very useful for modeling natural water because the kinetic technique allows convenient analysis of components of varying particle size. The present results are strongly indicative of the role of fulvic acid in mediating metal ion chemistry in natural water.     

pH-dependent magnetic phase transition of iron oxide nanoparticles synthesized by gamma-radiation reduction method

In this study, the influence of pH variation on structural and magnetic phase transition of gamma radiolytic synthesized iron oxide nanoparticles is investigated. The structure and magnetic properties of irradiated samples are characterized using X-ray diffraction, Fourier transfer infrared spectroscopy, transmission electron microscopy, and vibrating sample magnetometer. It was found that, in acidic irradiated solution, Fe³⁺ ions make various complexes with polyvinyl alcohol and water molecules which exhibit a multiphase magnetic property as a mixture of dia and paramagnetic materials. On the other hand, in basic condition, rate of radiation induced reduction of Fe³⁺ ions increased which leads to the formation of superparamagnetic Fe₃O₄ nanoparticles. By increasing pH value, in strong basic condition, the tendency of paramagnetic iron (III) oxy-hydroxide formation was high compared to other phases. This variation in the magnetic properties was explained based on iron ions reduction mechanism and the variation of the ligands’ properties during formation of nanoparticles under irradiation.     

Investigating the oxidation state of Fe from LiFePO4-based lithium ion battery cathodes via capillary electrophoresis

A capillary electrophoresis (CE) method with ultraviolet/visible (UV–Vis) spectroscopy for iron speciation in lithium ion battery (LIB) electrolytes was developed. The complexation of Fe²⁺ with 1,10-phenantroline (o-phen) and of Fe³⁺ with ethylenediamine tetraacetic acid (EDTA) revealed effective stabilization of both iron species during sample preparation and CE measurements. For the investigation of small electrolyte volumes from LIB cells, a sample buffer with optimal sample pH was developed to inhibit precipitation of Fe³⁺ during complexation of Fe²⁺ with o-phen. However, the presence of the conducting salt lithium hexafluorophosphate (LiPF₆) in the electrolyte led to the precipitation of the complex [Fe(o-phen)₃](PF₆)₂. Addition of acetonitrile (ACN) to the sample successfully re-dissolved this Fe²⁺-complex to retain the quantification of both species. Further optimization of the method successfully prevented the oxidation of dissolved Fe²⁺ with ambient oxygen during sample preparation, by previously stabilizing the sample with HCl or by working under counterflow of argon. Following dissolution experiments with the positive electrode material LiFePO₄ (LFP) in LIB electrolytes under dry room conditions at 20°C and 60°C mainly revealed iron dissolution at elevated temperatures due to the formation of acidic electrolyte decomposition products. Despite the primary oxidation state of iron in LFP of +2, both iron species were detected in the electrolytes that derive from oxidation of dissolved Fe²⁺ by remaining molecular oxygen in the sample vials during the dissolution experiments.     

Thermal study of polypyrrole complexes with vermiculites of different layer charge

We have studied the synthesis of polypyrrole-clay nanocomposites by the in situ oxidative polymerization of pyrrole in the interlayer space of vermiculites with different layer charges from Santa Olalla and Ojén, Spain. Moreover, the influence of different interlayer cations (Na⁺, Mg²⁺, Fe³⁺) on the interaction between pyrrole and the vermiculties was studied. The resulting materials were characterized by means of DTA-TG, XRD, FTIR and Mössbauer spectroscopy. In all samples polymerization of pyrrole was observed, presumably triggered by the structural iron. In most cases it was found to be externally deposited. An uptake of pyrrole in the interlayer space and PPy formation is observed in the case of the Fe³⁺-intercalated Ojén vermiculite, which has a lower layer charge than the Santa Olalla vermiculite.     

Time-space evolution of the cloud of ion-electron pairs formed by Auger electrons around Mossbauer ion <Superscript>57</Superscript>Fe<Superscript><Emphasis Type="Italic">n</Emphasis>+</Superscript>

It is shown that the emission of energetic Auger electrons during formation of the Mossbauer nucleus ⁵⁷Fe leads not only to the formation of multicharged ion ⁵⁷Fe ⁿ⁺, but also to the formation of a cloud of several hundred (200–300) ion-electron pairs (H₂O⁺, e ^?) around the Fe ion. This cloud is called an Auger blob. Its size (radius) is approximately ～100 Å. Fast radiation-chemical reactions in an Auger blob determine the experimentally observable ratio of yields of final chemically stable states (Fe³⁺ and Fe²⁺) of the Mossbauer ion. Knowledge of this ratio is important for an adequate interpretation of the results of Mossbauer emission spectroscopy. Although our assessments relate to iron nuclei in frozen aqueous solutions, they can be easily adjusted for media of other chemical composition.     

Interaction of Phosphate with Iron(III) in Acidic Medium,Equilibrium and Kinetic Studies

Equilibrium reactions of iron(III) with phosphate were studied spectrophotometrically by UV-Vis in the pH range of ～ 1.0-2.20. The STAR-94 Program was used to determine the number of absorbing species as well as the stoichiometries and formation constants of the complex species. Some literature values were further confirmed and new values of different stoichiometries were obtained. The kinetics and mechanism of Fe(III) with phosphate were studied in acidic medium. The reactive phosphate species were found to be only H₃PO₄ and H₂PO^? ₄ and for Fe(III) were only Fe³⁺, FeOH²⁺ and Fe(OH)⁺ ₂. The observed rate constants were pH as well as T_phos (total concentration of phosphate) dependent, i.e. K_obs,i = A _i + B _i T_phos + C _i T² _phos (at a given pH).     

Oxidation of NO mediated by water-soluble iron porphyrin

An anodic voltammetric wave of NO catalyzed by meso-tetra(N-methyl-4-pyridyl) iron(III) pentachloride ([Fe^III(TMPyP)]⁵⁺) was found in a phosphate buffer solution (pH 7.4). The current was 10 times larger than the diffusion-controlled current of NO without the iron porphyrin. The current can be used to detect NO in aerobic physiological environments. Spectroelectrochemical measurements suggested the formation of iron-nitrosyl complex, which is responsible for the catalytic oxidation of NO. The intermediate of the catalytic oxidation was detected by spectroelectrochemistry.     

Structural studies on some iron 8-oxi-quinolinates using Mössbauer spectroscopy

Using Mössbauer spectroscopy the quinolinates of iron/II/ and iron/III/ have been studied. In iron/II/ quinolinate three sublattices were evidenced, two of them being attributed to Fe²⁺ ions and the third to Fe³⁺ impurities. In the iron/III/ quinolinate five structural sublattices were found, two of them containing Fe³⁺ ions, the other two Fe²⁺ ions and the fifth may be attributed to the interstitial Fe³⁺ ions.     

Fe-assisted formation of α-Al2O3, starting from sol-gel precursors

The role of Fe³⁺ ions in the transformations from boehmites and pseudoboehmite xerogels via transition aluminas to corundum was studied here. Especially, the active iron species responsible for the decrease of the temperature of transformation to corundum were looked for. To enable the formation of various Fe³⁺ and Fe²⁺ species, samples were subjected to thermal treatments in different atmospheres as well as mechanically activated. Thermal analysis and ESR spectroscopy served to follow the processes and to characterise the resulting products. It was found that (i) isolated Fe³⁺ ions can indicate local structural changes but have (almost) no influence on the temperature of corundum formation, (ii) the temperature of corundum formation decreases in the result of action of small α-Fe₂O₃ particles and (iii) during thermal treatments Fe³⁺ ions are distributed between different phases or precursors thereof: transition aluminas, corundum, Fe₂O₃, and a Fe³⁺ pool.     

Facile synthesis of tough metallosupramolecular hydrogels by using phosphates as temporary ligands of ferric ions to avoid inhibition of polymerization

Forming carboxyl-Fe³⁺ coordination bonds as physical crosslinks is an effective strategy to develop tough hydrogels. Considering the inhibition of ferric ions on free-radical polymerization, these coordination bonds cannot be formed during the reaction, and a soaking process of preformed hydrogels is usually required for mechanical enhancement, resulting in uncontrollable gradient structure, long preparation time, and unnecessary waste of metallic ions. A facile strategy is reported here to prepare tough metallosupramolecular hydrogels by polymerization and in situ formation of coordination bonds with phosphates as the temporal ligands of Fe³⁺ ions. The phosphate ligands in the precursor solution form coordination complexes with the Fe³⁺ ions, which avoids the inhibition and ensures the polymerization. After swelling the resultant hydrogel in water, the ligands are substituted by carboxyl groups of the gel matrix due to the variation of local pH. The equilibrated hydrogel with carboxyl-Fe³⁺ coordination bonds as the physical crosslinks possesses excellent mechanical properties that can be tuned over a wide range by adjusting the polymer compositions and the concentrations of phosphate ligands and Fe³⁺ ions. This strategy should be applicable to other systems to enable synthesis of functional hydrogels with Fe³⁺ ions as the additive toward specific applications in engineering and biomedical fields.     

A study on vertical distribution of elements and their chemical states in Yatsu tideland sediments

We have clarified vertical distribution of thirty and more elements in the Yatsu tideland sediment by instrumental neutron activation analysis (INAA) and prompt g-ray analysis (PGA). The distributions of elements were categorized into three groups according the tendency of each vertical distribution. Chemical states of iron in the sediments were investigated by Mössbauer spectroscopy. In the distribution of iron species, maximum pyrite distribution was found in the middle layer. Paramagnetic high-spin Fe²⁺ distributed complementarily to pyrite, suggesting that the Fe²⁺ was used for pyrite formation.     

Anion influence on the hydrated oxides formed by Fe3+ and Fe2+ precipitation

The anion influence on the hydrated oxides formed by Fe³⁺ and Fe²⁺ precipitation simulating the aqueous radioactive waste treatment has been investigated by Mössbauer spectroscopy. Fe₃(SO₄)₂, Fe(NO₃)₃, FeCl₃, FeSO₄ and FeCl₂ were used as iron sources and neutralized by NaOH. The obtained products contain several kinds of amorphous and/or poorly crystallized hydrated oxides, depending on the anion type which exists in solution and on the initial iron valence. The pH influence on the final precipitate is taken into account.     

Activation parameters of ferryl ion reactions in aqueous acid solutions

The temperature dependence of the oxidation kinetics of Fe²⁺ by O₃ at pH 0–3 was studied by stopped-flow technique in the temperature range 5–40°C. Activation parameters of the reactions involved in formation and decay of the ferryl ion (iron(IV)), FeO²⁺ are determined. The reaction of Fe²⁺ + FeO²⁺ was found to branch into two channels forming iron(III)-dimer, Fe(OH)₂Fe⁴⁺, and Fe³⁺. The yield of the dimer, Fe(OH)₂Fe⁴⁺, increases with temperature on the expense of the Fe³⁺ yield. On the basis of the overall rate constant and relative yield of Fe(OH)₂Fe⁴⁺ the activation energy is determined for both channels. The activation parameters of the hydrolysis of the ferryl ion and its reaction with H₂O₂ were also determined. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 17–24, 1997.     

镁铁和镁铁铝催化剂氢还原过程的研究

以水滑石为前体 ,制备了镁铁和镁铁铝复合氧化物催化剂 ,运用原位穆斯堡尔谱研究了催化剂在H2 气氛中的还原行为。结果表明 :由于Mg、Al的加入和固溶体的形成 ,相对地稳定了FeO物相 ,阻碍了H2 对铁离子的还原 ,使得Fe2 进一步还原为金属Fe0 的能力减弱 ;在还原过程中催化剂首先生成含Fe2 的固溶体FeO MgO或FeO MgO Al2O3,然后再完全还原成金属Fe0。     

Surface analysis of mixed-conducting ferrite membranes by the conversion-electron Mössbauer spectroscopy

Conversion-electron Mössbauer spectroscopy analysis of iron surface states in the dense ceramic membranes made of ⁵⁷Fe-enriched SrFe_0.7Al_0.3O_3−δ perovskite, shows no traces of reductive decomposition or carbide formation in the interfacial layers after operation under air/CH₄ gradient at 1173 K, within the limits of experimental uncertainty. The predominant trivalent state of iron cations at the membrane permeate-side surface exposed to flowing dry methane provides evidence of the kinetic stabilization mechanism, which is only possible due to slow oxygen-exchange kinetics and enables long-term operation of the ferrite-based ceramic reactors for natural gas conversion. At the membrane feed-side surface exposed to air, the fractions of Fe⁴⁺ and Fe³⁺ are close to those in the powder equilibrated at atmospheric oxygen pressure, suggesting that the exchange limitations to oxygen transport are essentially localized at the partially reduced surface.     

UV-absorption and radiation effects in different glasses doped with iron and tin in the ppm range

Intrinsic and extrinsic ultraviolet absorption and radiation-induced effects were investigated in different glass types, fluorides, phosphates and borosilicates. High-purity glass samples were prepared and their intrinsic absorption was measured in the vacuum ultraviolet region. The influence of doped iron and tin species in the ppm range on the ultraviolet absorption and radiation-induced effects were studied. The maximum of the dominating Fe³⁺ charge transfer band has the lowest energy (4.8 eV) and intensity in the fluoride glass and the highest energy (5.6 eV) and intensity in the borosilicate glass samples. The charge-transfer band for Fe²⁺ has much lower intensity and higher energy (∼5.7 eV) than those for Fe³⁺ in all glasses investigated. Photo-oxidation of Fe²⁺ to (Fe²⁺)⁺ hole centres and glass-matrix-related electron centres by UV irradiation increases the UV absorption drastically in all glasses. The kinetics was measured and simulated depending on the glass matrix. In fluoride and phosphate glasses, Fe³⁺ complexes are very stable against UV irradiation and do not participate in UV-radiation-induced processes. Only in silicate glasses, Fe³⁺ is able to form a (Fe³⁺)^– electron centre defect which decreases the charge transfer absorption of Fe³⁺ near 220 nm, but increase the absorption of hole centre defects, with a maximum at 280 nm. So, the defect generation in the ultraviolet region increases drastically with increasing Fe content in the range 10–200 ppm. Three or four electronic s → p transitions for Sn²⁺ were detected by optical absorption and luminescence spectroscopy shifted to longer wavelength in the range fluoride → phosphate → silicate glass samples. Sn⁴⁺ absorption bands were found at shorter wavelength in the vacuum ultraviolet region in all cases investigated. Sn²⁺ ions are photo-oxidised under UV radiation very fast, which leads to an decrease of absorption near 200 nm and to an increase near 250 nm. Both Sn²⁺ and Sn⁴⁺ are involved in the radiation-induced processes. In contrast to phosphate and silicate glasses, tin-doped fluoride glasses are very resistant against UV lamp but not against UV laser irradiation. The mechanisms are very complicated, with maximums and minimums in the defect formation curves.     

Iron-siRNA Nanohybrids for Enhanced Chemodynamic Therapy via Ferritin Heavy Chain Downregulation

Ferrous iron (Fe²⁺) has more potent hydroxyl radical (⋅OH)-generating ability than other Fenton-type metal ions, making Fe-based nanomaterials attractive for chemodynamic therapy (CDT). However, because Fe²⁺ can be converted by ferritin heavy chain (FHC) to nontoxic ferric form and then sequestered in ferritin, therapeutic outcomes of Fe-mediated CDT agents are still far from satisfactory. Here we report the synthesis of siRNA-embedded Fe⁰ nanoparticles (Fe⁰-siRNA NPs) for self-reinforcing CDT via FHC downregulation. Upon internalization by cancer cells, pH-responsive Fe⁰-siRNA NPs are degraded to release Fe²⁺ and FHC siRNA in acidic endo/lysosomes with the aid of oxygen (O₂). The accompanied O₂ depletion causes an intracellular pH decrease, which further promotes the degradation of Fe⁰-siRNA NPs. In addition to initiating chemodynamic process, Fe²⁺-catalyzed ⋅OH generation facilitates endo/lysosomal escape of siRNA by disrupting the membranes, enabling FHC downregulation-enhanced CDT.     

A Sensitive Fluorescence Quenching Method for the Determination of Iron( Ⅱ） with 1,10-Phenanthroline

A sensitive and selective fluorescence quenching method for the determination of Fe²⁺ with 1,10‐phenanthroline was developed. The fluorescence intensity of 1,10‐phenanthroline at λ_ex of 266 run and λ_em of 365 nm was constant in the range of pH 4.0 to 10.0 and decreased linearly upon addition of Fe²⁺ to its solution. This decrease was mainly due to a static quenching effect caused by the formation of a non‐fluorescent complex of Fe²⁺ with 1, 10‐phenanthroline. The total amount of iron was determined by using hydroxylamine hydrochloride to reduce Fe³⁺ to Fe²⁺. The linear range was from 5.0 × 10–⁷ to 2.0 × 10–⁵ mol/L with a detection limit of 2.4 × 10–⁸ mol/L at 3s?. The quenching constant of Fe²⁺ to 1,10‐phenanthroline was calculated to be (5.70 × 0.05) × 10⁴ L/mol at 25 °C. Effects of foreign ions on the determination of Fe²⁺ were investigated. The results of the new method for the determination of iron in tap water and natural water samples were in good agreement with those obtained by graphite atomic absorption spectrometry.     

Simultaneous spectrophotometric determination of iron species using orthogonal signal correction–generalized partial least squares calibration method after vortex-assisted dispersive liquid–liquid microextraction

A vortex-assisted dispersive liquid–liquid microextraction method in combination with UV–Vis spectrophotometry was developed for the simultaneous extraction and determination of iron species. In this method, Fe²⁺ and Fe³⁺ were complexed with pyridine-2-amidoxime, neutralized through ion pair formation with sodium dodecyl sulfate, and extracted into the fine droplets of chloroform. After centrifugation, the absorbance of the extracted complexes was recorded in the wavelength range of 360–700 nm. The parameters affecting the extraction efficiency such as the pH, the type and volume of the extraction solvent, ligand concentration, and sample volume were optimized. The individual iron species was then determined by means of the orthogonal signal correction–generalized partial least squares method. Under the optimized conditions, the calibration curves were linear over the range of 2.0–100 and 3.0–200 µg L^?1 with detection limits of 0.4 µg L^?1 for Fe²⁺ and 0.8 µg L^?1 for Fe³⁺, respectively. The relative standard deviations for intra- and inter-day assays (n = 5) were 2.3 and 4.0 for Fe²⁺ at 50 µg L^?1 and 2.7 and 4.3 for Fe³⁺ at 30 µg L^?1, respectively. The enhancement factors of 77 and 69 were achieved for Fe²⁺ and Fe³⁺, respectively. The proposed method was successfully applied to the determination of iron species in water samples.     

Regularities of formation of iron(III) nanocrystalline oxides and oxyhydroxides during oxidation of iron(II) compounds in an alkaline medium

Regularities of formation of nanocrystalline iron(III) oxides and oxyhydroxides via oxidation of iron(II) compounds in an alkaline pH range (pH ≥ 12) were studied using pH and E _h measurements, chemical analysis, electron microscopy, and X-ray diffraction. When the molar ratio [OH⁻]/[Fe^II] ∼ 2 (pH ∼ 12–12.5), the oxidation process yields cube-shaped magnetite Fe₃O₄ particles. An excess of an alkaline agent with an overstoichiometric concentration equal to or higher than 0.5 mol/L (pH ≥ 13.5) induces the formation of anisotropic particles of nanocrystalline goethite α-FeOOH over the entire range of the synthesis parameters studied. Reaction products (Fe₃O₄ and/or α-FeOOH) are formed immediately as the initial Fe(OH)₂ starts oxidizing by the dissolution-oxidation-precipitation mechanism near the surface of Fe(OH)₂ precursor particles. Carbonate ions considerably change the structure and shape of newly formed α-FeOOH particles.