The solubility of magnetite and the hydrolysis and oxidation of Fe2+ in water to 300�C

Kinetics of the reaction of ferrous ions with hydroxyl radicals in the temperature range 25-300 °C

The kinetics and mechanism of the reaction between OH radicals and ferrous ions in the temperature range 25-300 °C were studied using pulse radiolysis. At temperatures <150 °C the rate of reaction is essentially independent of temperature, while at temperatures >150 °C the activation energy is 45.8 ± 3.0 kJ mol-1. The change in activation energy is attributed to a change in the dominant mechanism from hydrogen atom transfer (HAT) to dissociative ligand interchange. The kinetic isotope effect (KIE) was measured by repeating experiments in heavy water. A value of 2.9 was measured at room temperature where HAT is the dominant mechanism. The KIE decreases to zero at temperatures > 150 °C as ligand interchange becomes dominant and the O-H bond is no longer involved in the reaction.

Microstructural Characterization of the Corrosion Product Deposit in the Flow-Accelerated Region in High-Temperature Water

The clogging behavior of the micro-orifice under a flow accelerated condition was investigated after 500 h of immersion in high-temperature water. The results indicated the residual area of the micro-orifice was reduced to one-third of its original size after 500 h of immersion due to the deposition of corrosion products. In this process, the clogging behavior of micro-orifice can be divided into three stages: the stable deposition stage, the quick recovery stage, and the dynamic equilibrium stage. The corrosion products were porous and consisted of many deposited particles. The process of particle deposition and removal was carried out simultaneously.

Thermodynamic Analysis and Crystallographic Properties of MFe2O4, MCr2O4 and MAl2O4 (M = Fe, Ni, Zn) Formed on Structural Materials in Pressurized Water Reactor Primary Circuit under Zinc and Zinc-aluminum Water Chemistry

Zinc injection technology (zinc water chemistry, ZWC) was widely applied in pressurized water reactor (PWR) primary circuits to reduce radiation buildup and improve corrosion resistance of structural materials. The simultaneous injection of zinc-aluminium (ZAWC) is a novel implement created to replace part of Zn2+ by Al3+. It was reported ZAWC can improve further corrosion resistance of carbon steels and stainless steels. However, ZAWC sometimes showed even negative effects on Nickel-alloys. In this study, mechanism of formation of oxide film on metals was investigated. The reactions of Fe2+ Ni2+ in oxide films replaced by Zn2+, or Fe3+ replaced by Al3+ in ZAWC were analysed. The thermodynamic data and solubility of mixed oxides (ZnFe2O4, ZnCr2O4, and ZnAl2O4), the products of replace reactions, were calculated. According to the Gibbs free energy difference between products and reactants, values of the formation reaction of ZnFe2O4, ZnCr2O4, and ZnAl2O4 are extremely negative. Solubility of ZnAl2O4 is the lowest among mixed oxide products, which implies the oxide film composites of ZnAl2O4 may show a lower corrosion rate. In addition, the preferential formation of NiAl2O4 on Ni-based-alloy, under ZAWC, was discussed based on crystallographic properties of spinel, which was considered as the cause of negative effects of ZAWC on corrosion resistance of Nickel-alloys. This research provides an analytical basis for the study of thermodynamic stability of oxide films under different chemical chemistry and a theoretical basis for improving corrosion resistance of different metals and optimizing the chemical conditions of PWR primary circuit.

Thermodynamically Stable Colloidal Solids: Interfacial Thermodynamics from the Particle Size Distribution

True thermodynamic stability of a solid colloidal dispersion is generally unexpected, so much that thorough experimental validation of proposed stable systems remains incomplete. Such dispersions are under investigated and would be of interest due to their long-term stability and insensitivity to preparation pathway. We apply classical nucleation theory (CNT) to such colloidal systems, providing a relationship which links the size-dependent interfacial free energy density of the particles to their size distribution, and use this expression in the fitting of previously reported size distributions for putatively thermodynamically stable nanoparticles. Experimental data from a gold-thiol system exhibiting inverse coarsening or "digestive ripening" can be well-described in terms of a power-law dependence of the interfacial free energy γ on radius based on capacitive charging of the nanoparticles, going asr - 3 , as suggested by prior authors. Data from magnetite nanoparticles in highly basic solutions also can be well-fit using the CNT relation, but with γ going asr - 2 . Slightly better fits are possible if the power of the radius is non-integral, but we stress that more complex models of γ will require richer data sets to avoid the problem of overfitting. Some parameters of the fits are still robustly at odds with earlier models that implicitly assumed absolute thermodynamic stability: first, the extrapolated free energy density of the flat surface in these systems is small and positive, rather than strongly negative; second, the shape of the distributions indicates the solution phase to be supersaturated in monomer relative to the bulk, and thus that these two systems may only be metastable. For future work, we derive expressions for the important statistical thermodynamic and chemical parameters of the interface energy in terms of 1) the surfactant concentration, 2) the temperature dependence, and 3) the concentrations of particles in the tail of the distribution.

Molar Ratio Effect of Sodium to Chloride Ions on the Electrochemical Corrosion of Alloy 600 and SA508 in HCl + NaOH Mixtures

This study aims to investigate the molar ratio effect of sodium to chloride ions on the corrosion of an Alloy 600 steam generator tube and an SA508 tubesheet material. The corrosion behavior was evaluated in solutions with three different molar ratios of sodium to chloride ions using a potentiodynamic polarization method. The corrosion potentials and corrosion rates of both the two materials were significantly decreased as the molar ratio increased from 0.1 to 10. Therefore, it is recommended that the molar ratio control to a value of 1 is beneficial only when the crevice chemistry has a low molar ratio with an acidic pH. The corrosion potentials and corrosion rates were little affected by the total sodium and chloride ion concentrations. SA508 acted as an anode and its corrosion rate was accelerated by galvanic coupling with Alloy 600.

Optimizing the removal of strontium and cesium ions from binary solutions on magnetic nano-zeolite using response surface methodology (RSM) and artificial neural network (ANN)

The feasibility of using magnetic nano-zeolite (MNZ) to remove cesium and strontium from their binary corrosive solutions was investigated by considering the multi-variant/multi-objective nature of the process. RSM (Response Surface Methodology) and ANN (Artificial Neural Network) were used to model and optimize the removal system and assess sensitive parameters that can affect the process reliability. MNZ is characterized by its high surface area and cation exchange capacity and possesses good regeneration behavior for both elements using citric acid. Its stability is comparable to other sorbents in acidic media and the stability increases in alkaline media, where dissolution rate follow first order reaction on heterogeneous sites. MNZ removes both contaminants simultaneously with small tendency toward Cs, where MNZ is suggested for application in pre-treatment of highly contaminated alkaline solutions. The percentage removal, decontamination factors, and separation factors have different dependency on the effluent/process conditions; this dependency is the same for both contaminants. Sorption kinetics is initially controlled by external mass transfer through the boundaries then intra-particle diffusion dominates the reactions. The process sensitivity to pH changes is attributed to changes in structural elements -species distribution at the solid/aqueous interface. Cs⁺ and Sr⁺² are exchanged with Na⁺ and H⁺, regardless the effluent pH value, and with Al and Fe cations at specific pH. Isosteric heat of sorption calculations indicated that the total heat needed to complete the reaction was considerably reduced by operating the process at optimized temperature.

Innovative combination of Fe2+-BAF and ozonation for enhancing phosphorus and organic micropollutants removal treating petrochemical secondary effluent

Two sets of BAF - ozonation systems, with (System-A) and without (System-B) Fe²⁺ dosing into BAF, were used for organic micropollutants (OMPs) and phosphorus removal in the advanced treatment of petrochemical secondary effluent. Pilot scale study showed that when the influent COD, TP and TSS were 73.0, 1.40 and 24.6mgL^-1, the effluent COD, TP and TSS were 48.9, 0.32, 7.78mgL^-1 for System-A with the ozone and Fe²⁺ dosage of 25mgL^-1 and 0.08mmolL^-1, and 60.2, 1.16, 6.44mgL^-1 for System-B with the ozone dosage of 25mgL^-1. The dosage of Fe²⁺ into the BAF significantly enhanced the TP and OMPs removal ability. Exactly, the residual Fe²⁺ in the BAF-A effluent stimulated the catalytic ozonation in the following ozone unit. Therefore, the OMPs removal in System-A was obviously better than that in System-B. The dosage of Fe²⁺ from 0.02 to 0.10mmolL^-1 had slight adverse effect on the biofilm activity in BAF. Based on the TP and OMPs removal ability, the System-B is selected as a proposed process for the advanced treatment of petrochemical secondary effluent.

On the role of Fe3+ ions in FexOy/C catalysts for hydrogen production from the photodehydrogenation of ethanol

FexOy/C photocatalysts at different iron content were prepared by the incipient wet impregnation method and calcined at 773 K. The photocatalysts were characterized by means of nitrogen adsorption-desorption isotherms, surface fractal dimension, non-local density functional theory, X-ray diffraction, Rietveld refinement and UV-vis spectroscopy. The photocatalytic activity was evaluated using the photodehydrogenation of ethanol as a model reaction for the production of hydrogen. The specific surface areas of FexOy/C substrates, with 15, 20 and 30 wt% iron content, diminished from 638 to 490 m(2)/g, as the iron content increased. X-ray diffraction analysis showed that iron oxides coexist as wustite and magnetite in samples with Fe contents of 15 and 20 wt%; for sample with 30 wt% Fe, wustite, magnetite and hematite phases were observed. The photophysical, textural and structural properties were modified by the hematite phase formed by thermal treatment. The Rietveld refinements denoted changes in occupancy of Fe(3+) and Fe(2+) in FexOy crystallites. A relationship between the Fe(3+) ions content and the reactivity for the hydrogen production from the photodehydrogenation of ethanol (from 1360 to 2125 μmol h(-1)), was evidenced.

Surface complexation and oxidation of Sn(II) by nanomagnetite

The long-lived fission product 126Sn is of substantial interest in the context of nuclear waste disposal in deep underground repositories. However, the prevalent redox state, the aqueous speciation as well as the reactions at the mineral-water interface under the expected anoxic and reducing conditions are a matter of debate. We therefore investigated the reaction of Sn(II) with a relevant redox-reactive mineral, magnetite (Fe(II)Fe(III)2O4) at <2 ppmv O2, and monitored Sn uptake as a function of pH and time. Tin redox state and local structure were investigated by Sn–K X-ray absorption spectroscopy (XAS). We observed a rapid uptake (<30 min) and oxidation of Sn(II) to Sn(IV) by magnetite. The local structure determined by XAS showed two Sn–Fe distances of about 3.15 and 3.60 Å in line with edge and corner sharing arrangements between octahedrally coordinated Sn(IV) and the magnetite surface, indicative of formation of tetradentate inner-sphere complexes between pH 3 and 9. Based on the EXAFS-derived surface structure, we could successfully model the sorption data with two different complexes, (Magn_sO)4Sn(IV)(OH)2–2 (logK(2,0)(–2) −14.97 ± 0.35) prevailing from pH 2 to 9, and (Magn_sO)4Sn(IV)(OH)2Fe (logK(2,1)(0) −17.72 ± 0.50), which forms at pH > 9 by coadsorption of Fe(II), thereby increasing sorption at this high pH.

Iron (III) hydrolysis and solubility at 25 degrees C

UV-vis spectrophotometric measurements, potentiometric titrations, and solubility measurements were performed to evaluate the hydrolysis constants for aqueous Fe(III) and the solubility of 2-line ferrihydrite over a wide concentration range (0-3 M NaClO4 and p[H+] 1.54-11.23). From these measurements, Fe3+ was found to hydrolyze to form FeOH2+, Fe2(OH)24+, Fe(OH)2+, Fe(OH)3(0), and Fe(OH)4-. The hydrolysis and solubility constants of these species were determined together with their dependence on ionic strength. The iron (III) hydrolysis constants at infinity dilution were (logbeta(1,1) to logbeta(1,4) and logbeta(2,2))-2.19 +/- 0.02, -5.76 +/- 0.06, -14.30 +/- 0.32, -21.71 +/- 0.24, and -2.92 +/- 0.02, respectively. The solubility product for 2-line ferrihydrite was (logK(s,0)) +3.50 +/- 0.20. The results have been compared with literature values.

A spectrophotometric study of Fe(II)-chloride complexes in aqueous solutions from 10 to 100°C

The absorption spectra of Fe(II)-chloride solutions were measured in both the UV (ultraviolet) and near-IR (near infrared) regions at temperatures ranging from 10 to 100°C with chloride concentrations from 0.1 to 16 mol kg-1. The stability constants of all Fe(II)-chloride complexes were derived from the spectra using a non-negative nonlinear least-squares computer program (SQUAD). Earlier work on this system reported in the literature was rigorously reassessed. The activity coefficients of the ionic species were calculated using both the Pitzer model and the Helgeson model. The results obtained with UV and near-IR spectra and with different activity coefficient calculation models are in general agreement. Other useful thermodynamic data, including the Gibbs energies, enthalpies, and entropies for complex formation, were also obtained. It was found that the Fe(II)-chloride complexes gradually undergo a configuration transformation from octahedral to tetrahedral coordination as the temperature and (or) chloride concentration increases. This coordination change is of significant importance to the nuclear reactors, as the presence of the tetrahedral complex can increase the solubility of iron in steam generator crevices.Key words: Fe(II)-chloride complexes, stability constants, solution thermodynamics, spectrophotometry, reactor chemistry.

Inorganic species in geologic fluids: correlations among standard molal thermodynamic properties of aqueous ions and hydroxide complexes

Correlations among experimentally determined standard partial molal thermodynamic properties of inorganic aqueous species at 25 degrees C and 1 bar allow estimates of these properties for numerous monatomic cations and anions, polyatomic anions, oxyanions, acid oxyanions, neutral oxy-acid species, dissolved gases, and hydroxide complexes of metal cations. Combined with correlations among parameters in the revised Helgeson-Kirkham-Flowers (HKF) equation of state (Shock et al., 1992), these estimates permit predictions of standard partial molal volumes, heat capacities, and entropies, as well as apparent standard partial molal enthalpies and Gibbs free energies of formation to 1000 degrees C and 5 kb for hundreds of inorganic aqueous species of interest in geochemistry. Data and parameters for more than 300 inorganic aqueous species are presented. Close agreement between calculated and experimentally determined equilibrium constants for acid dissociation reactions and cation hydrolysis reactions supports the generality and validity of these predictive methods. These data facilitate the calculation of the speciation of major, minor, and trace elements in hydrothermal and metamorphic fluids throughout most of the crust of the Earth.