Redox Equilibria Of Iron Research Articles

Effect of Iron Redox Equilibrium on the Foaming Behavior of MgO-Saturated Slags

In this study, the foaming index of CaO-SiO2-FetO and CaO-SiO2-FetO-Al2O3 slags saturated with MgO was measured to understand the relationship between their foaming behavior and physical properties. The foaming index of MgO-saturated slags increases with the FetO content due to the redox equilibrium of FetO. Experimental results indicated that MgO-saturated slag has relatively high ferric ion concentration, and the foaming index increases due to the effect of ferric ion. Therefore, the foaming behavior of MgO-saturated slag is more reasonably explained by considering the effect of ferric ion on the estimation of slag properties such as viscosity, surface tension, and density. Specifically, the estimation of slag viscosity was additionally verified by NBO/T, and this is experimentally obtained through Raman spectroscopy.

Geochemistry and speciation of Fe(II) and Fe(III) in natural geothermal water, Iceland

The geochemistry of Fe(II) and Fe(III) was studied in natural geothermal waters in Iceland. Samples of surface and spring water and sub-boiling geothermal well water were collected and analyzed for Fe(II), Fe(III) and Fetotal concentrations. The samples had discharge temperatures in the range 27–99 °C, pH between 2.46 and 9.77 and total dissolved solids 155–1090 mg/L. The concentrations of Fe(II) and Fe(III) were determined in the <0.2 μm filtered and acidified fraction using a field-deployed ion chromatography spectrophotometry (IC-Vis) method within minutes to a few hours of sampling in order to prevent post-sampling changes. The concentrations of Fe(II) and Fe(III) were <0.1–130 μmoL/L and <0.2–42 μmoL/L, respectively. In-situ dialysis coupled with Fe(II) and Fe(III) determinations suggest that in some cases a significant fraction of Fe passing the standard <0.2 μm filtration method may be present in colloidal/particulate form. Therefore, such filter size may not truly represent the dissolved fraction of Fe but also nano-sized particles. The Fe(II) and Fe(III) speciation and Fetotal concentrations are largely influenced by the water pH, which in turn reflects the water type formed through various processes. In water having pH of ∼7–9, the total Fe concentrations were <2 μmoL/L with Fe(III) predominating. With decreasing pH, the total Fe concentrations increased with Fe(II) becoming increasingly important and predominating at pH < 3. In particular in waters having pH ∼6 and above, iron redox equilibrium may be approached with Fe(II) and Fe(III) possibly being controlled by equilibrium with respect to Fe minerals. In many acid waters, the Fe(II) and Fe(III) distribution may not have reached equilibrium and be controlled by the source(s), reaction kinetics or microbial reactions.

Estimation for Iron Redox Equilibria in Multicomponent Slags

AbstractThe knowledge of redox equilibria of iron in multicomponent molten slags is of significant importance to understand the viscosity, electrical conductivity and structure of iron-containing slags. However, the available data of molar ratio of ferric ion to ferrous ion are limited due to the difficulty of experiment and heavy workload. In this study, a model was established to estimate the $${{{X_{{\rm{FeO}}_2^ -}}} \mathord{\left/{\vphantom {{{X_{{\rm{FeO}}_2^ -}}} {{X_{{\rm{F}}{{\rm{e}}^{2 +}}}}}}} \right. \kern-\nulldelimiterspace} {{X_{{\rm{F}}{{\rm{e}}^{2 +}}}}}}$$ (normally, most of ferric ions exist in the form of complex anions such as $${\rm{FeO}}_2^ - $$) ratio in CaO–MgO–Al2O3–SiO2–“FeOt” slags, which can give good estimation results compared to the experimental measured values. From the model, by increasing oxygen partial pressure or decreasing temperature, the $${{{X_{{\rm{FeO}}_2^ -}}} \mathord{\left/{\vphantom {{{X_{{\rm{FeO}}_2^ -}}} {{X_{{\rm{F}}{{\rm{e}}^{2 +}}}}}}} \right. \kern-\nulldelimiterspace} {{X_{{\rm{F}}{{\rm{e}}^{2 +}}}}}}$$ ratio will increase. Different components have different influences on $${{{X_{{\rm{FeO}}_2^ -}}} \mathord{\left/{\vphantom {{{X_{{\rm{FeO}}_2^ -}}} {{X_{{\rm{F}}{{\rm{e}}^{2 +}}}}}}} \right. \kern-\nulldelimiterspace} {{X_{{\rm{F}}{{\rm{e}}^{2 +}}}}}}$$ ratio: CaO and MgO are beneficial for the increase of this ratio, but Al2O3 and SiO2 have reverse effects.

Physicochemical Properties of Vitrified Forms for LILW Generated From Korean Nuclear Power Plant

ABSTRACTSince 1994, the KHNP has developed a vitrification technology to treat the LILW generated from Korean nuclear power plant. To vitrify the LILW including combustible Dry Active Waste (DAW) and Ion Exchange Resin (IER) containing Zeolite, two borosilicate glasses are formulated. One of the formulated glass, DG2, is for the DAW vitrification solely and the other one, AG8W1, is for the blended wastes (DAW &amp; IER) vitrification in a commercial vitrification facility in HanUl (former Ulchin) nuclear power plant. The physicochemical properties of the two glasses have been evaluated. To evaluate the processability of the glasses, the viscosities and electrical conductivities of the glass melts were measured in the laboratory within a temperature range between 950 and 1,350 degrees C, respectively. The liquidus temperatures of the glasses were evaluated using a gradient furnace for DG2 and data from heat treatment for AG8W1. The Mössbauer spectroscopy for AG8W1 was employed to evaluate the relations between the redox equilibria of iron. In addition, to verify the waste acceptance criteria for the final disposal of the vitrified forms, the compressive strengths of the vitrified forms were tested after an immersion test, a thermal cycling test, and an irradiation test. To verify the chemical durability of the glasses, several tests such as PCT, ISO, VHT, Soxhlet, MCC-1, and ANS16.1 were carried out. The PCT showed leach rates of B, Na, Li and Si were much less than those of the benchmark glass. The ISO test was performed at 90 degrees C for 1,022 days and Cumulative Fraction Leached of all elements in the glasses were analyzed. According to the VHT, the glasses had an outstanding chemical resistance under humid environment at 200 degrees C for 7 days. The Soxhlet leaching was performed on rectangular glass samples at 98 degrees C for 30 days. To analyze the forward dissolution rates of major glass elements, the MCC-1 was conducted at temperatures of 40, 70, and 90 degrees C for three weeks in pH buffer solutions ranging from pH 4 to 11. The processability of the glasses was in the desired ranges. And the product quality of the glasses met all regulatory guidelines. Using two glasses, the CCIM commissioning tests in the UVF were successfully performed and they showed good workability.

Iron redox equilibrium, structure and properties of zinc iron phosphate glasses

Iron redox equilibrium, structure and properties were investigated for the 10ZnO–30Fe 2O 3–60P 2O 5 (mol%) glasses melted at different temperatures. The structure and valence states of the iron ions in these glasses were investigated using Mössbauer spectroscopy, Raman spectroscopy and differential thermal analysis. Mössbauer spectroscopy indicated that the concentration of Fe 2+ ions increased in the 10ZnO–30Fe 2O 3–60P 2O 5 (mol%) glass with increasing melting temperature. The Fe 2+/(Fe 2+ + Fe 3+) ratio increased from 0.18 to 0.38 as the melting temperature increased from 1100 to 1300 °C. The measured isomer shifts showed that both Fe 2+ and Fe 3+ ions are in octahedral coordination. It was shown that the dc conductivity strongly depended on Fe 2+/(Fe 2+ + Fe 3+) ratio in glasses. The dc conductivity increases with the increasing Fe 2+ ion content in these glasses. The conductivity arises from the polaron hopping between Fe 2+ and Fe 3+ ions which suggests that the conduction is electronic in nature in zinc iron phosphate glasses.

Ferric-ferrous equilibria in K 2O-FeO-Fe 2O 3-SiO 2 melts

Seventy-five K 2O-FeO-Fe 2O 3-SiO 2 liquids were equilibrated in air between 1123°C and 1596°C. The bulk compositions of the quenched glasses range from 35 to 85 wt.% SiO 2, 3 to 41 wt.% Fe 2O 3, 0 to 5 wt.% FeO, and 11 to 34 wt.% K 2O. Despite the volatilization of significant amounts of potassium in many of the high-temperature runs, no gradients in K 2O, SiO 2, or total iron were observed from electron microprobe transects of the 75 quenched glass beads. This result strongly suggests that the rate of K diffusion in the melt is faster than the rate of K volatilization at the gas/melt interface. The individual quenched glass beads were homogenous in their ferric-ferrous ratios as well, which further suggests that iron-redox equilibrium was achieved in these experiments. The ferric-ferrous data were used to derive a symmetric, regular solution model, with a standard error of 0.18 wt.% in the prediction of FeO in peralkaline K 2O-FeO-Fe 2O 3-SiO 2 liquids. Compositional trends for redox equilibrium in the potassic silicate liquids contrast with those found in sodic and calcic silicate liquids, as well as magmatic liquids. The effect of increasing K 2O is to decrease the ferric-ferrous ratio in peralkaline, iron-rich (>3 wt.%), K 2O-FeO-Fe 2O 3-SiO 2 liquids. These results are discussed in terms of three compositional effects that influence iron-redox equilibria in silicate melts: 1) charge-balancing of Fe 3+; 2) coordination change of Fe 3+; and 3) the activity of O 2− in the melt.

Iron redox equilibrium, structure and properties of iron phosphate glasses

Iron redox equilibrium, structure and properties were investigated for 40Fe 2O 3–60P 2O 5 (mol%) glasses melted at different temperatures. The Fe 2+/(Fe 2++Fe 3+) ratio increased from 17% to 50% as the melting temperature changed from 1150°C to 1400°C. The equilibrium constant, K, for the reaction of Fe 3+ being reduced to Fe 2+ varied with temperature as ln K=9.40–1.58×10 4/ T. The Raman and infrared spectra indicated that the basic iron pyrophosphate structure of the 40Fe 2O 3–60P 2O 5 (mol%) glasses did not change as the Fe 2+/(Fe 2++Fe 3+) ratio changed. All of the properties did not change to any major degree with increasing the melting temperature. The molar volume decreased while the density increased with increasing Fe 2+/(Fe 2++Fe 3+) ratio. It was found by DTA and XRD that two phases, Fe 3(P 2O 7) 2 and Fe 4(P 2O 7) 3, crystallized from the glass when the glass was heated in nitrogen. The crystallization behavior suggested that the amount of the crystal, Fe 3(P 2O 7) 2, may increase with increasing Fe 2+/(Fe 2++Fe 3+) ratio, which supported the opinion that there are some structural similarities between the iron phosphate glass and the crystalline Fe 3(P 2O 7) 2 in terms of the iron coordination number and bonding of the phosphate groups. The decrease in dc resistivity and increase in dielectric constant and dielectric loss tangent, which occurred with increasing the Fe 2+/(Fe 2++Fe 3+) ratio, were attributed to the increase of the electronic hopping from Fe 2+ ions to Fe 3+ ions.

Complexation of iron(III) and iron(II) by citrate. Implications for iron speciation in blood plasma

Estimates of the concentrations and identity of the predominant complexes of iron with the low-molecular-mass ligands in vivo are important to improve current understanding of the metabolism of this trace element. These estimates require a knowledge of the stability of the iron–citrate complexes. Previous studies on the equilibrium properties of the Fe(III)–citrate and Fe(II)–citrate are in disagreement. Accordingly, in this work, glass electrode potentiometric titrations have been used to re-determine the formation constants of both the Fe(III)– and Fe(II)–citrate systems at 25 °C in 1.00 M (Na)Cl and the reliability of these constants has been evaluated by comparing the measured and predicted redox potentials of the ternary Fe(III)–Fe(II)–citrate system. The formation constants obtained in this way were used in computer simulation models of the low-molecular-mass iron fraction in blood plasma. Redox equilibria of iron are thus included in large models of blood plasma for the first time. The results of these calculations show the predominance of Fe(II)–carbonate complexes and a significant amount of aquated Fe(II) in human blood plasma.

Iron redox equilibria in CaO-Al2O3-SiO2 and MgO-CaO-Al2O3-SiO2 slags

Measurements have been made of the ratio of ferric to ferrous iron in CaO-Al2O3-SiO2 and MgO-CaO-Al2O3-SiO2 slags at oxygen activities ranging from equilibrium with pCO2/pCO≈0.01 to as high as air at temperatures of 1573 to 1773 K. At 1773 K, values are given by $$\begin{gathered} \log {\text{ }}\left( {\frac{{Fe^{3 + } }}{{Fe^{2 + } }}} \right) = 0.3( \pm {\text{ }}0.02){\text{ }}Y + {\text{ }}0.45( \pm {\text{ }}0.01){\text{ }}\log \hfill \\ \left( {\frac{{pCO_2 }}{{pCO}}} \right) - 1.24( \pm {\text{ }}0.01) \hfill \\ \end{gathered} $$ where Y=(CaO+MgO)/SiO2, for melts with the molar ratio of CaO/SiO2=0.45 to 1.52, 10 to 15 mol pct Al2O3, up to 12 mol pct MgO (at CaO/SiO2≈1.5), and with 3 to 10 wt pct total Fe. Available evidence suggests that, to a good approximation, these redox equilibria are independent of temperature when expressed with respect to pCO2/pCO, probably from about 1573 to 1873 K. Limited studies have also been carried out on melts containing about 40 mol pct Al2O3, up to 12 mol pct MgO (at CaO/SiO2≈1.5), and 3.6 to 4.7 wt pct Fe. These show a strongly nonideal behavior for the iron redox equilibrium, with $$\frac{{Fe^{3 + } }}{{Fe^{2 + } }} \propto \left( {\frac{{pCO_2 }}{{pCO}}} \right)^{0.37} $$ The nonideal behavior and the effects of basicity and Al2O3 concentration on the redox equilibria are discussed in terms of the charge balance model of alumino-silicates and the published structural information from Mossbauer and NMR (Nuclear Magnetic Resonance) spectroscopy of quenched melts.

The role of phosphorus in rhyolitic liquids as determined from the homogeneous iron redox equilibrium

The redox ratio of iron is used as an indicator of solution properties of silicate liquids in the system (SiO−Al2O3−K2O−FeO−Fe2O3−P2O5). Glasses containing 80–85 mol% SiO2 with 1 mol% Fe2O3 and compositions covering a range of K2O/Al2O3 were synthesized at 1400°C in air (fixed fO2). Variations in the ratio FeO/FeO1.5 resulting from the addition of P2O5 are used to determine the solution behavior of phosphorus and its interactions with other cations in the silicate melt. In 80 mol% SiO2 peralkaline melts the redox ratio, expressed as FeO/FeO1.5, is unchanged relative to the reference curve with the addition of 3 mol% P2O5. Yet, the iron redox ratio in the 85 mol% SiO2 potassium aluminosilicate melts is decreased relative to phosphorus-free liquids even for small amounts of P2O5 (0.5 mol%). The redox ratio in peraluminous melts is decreased relative to phosphorus- free liquids at P2O5 concentrations of 3 mol%. In peraluminous liquids, complexing of both Fe+3−O−P+5 and Al+3−O−P+5 occur. The activity coefficient of Fe+3 is decreased because more ferric iron can be accommodated than in phosphorus-free liquids. In peralkaline melts, there is no evidence that P+5 is removing K+ from either Al+3 or Fe+3 species. In chargebalanced melts with 3 mol% Fe2O3 and very high P2O5 concentrations, phosphorus removes K+ from K−O−Fe+3 complexes resulting in a redox increase. P2O5 should be accommodated easily in peraluminous rhyolitic liquids and phosphate saturation may be suppressed relative to metaluminous rhyolites. In peralkaline melts, phosphate solubility may increase as a result of phosphorus complexing with alkalis. The complexing stoichiometry may be variable, however, and the relative influence of peralkalinity versus temperature on phosphate solubility in rhyolitic melts deserves greater attention.

Redox equilibria of iron in Ti-bearing calcium silicate quenched glasses

Mössbauer spectroscopy has been employed to evaluate relations between redox equilibria of iron, Ti content and degree of polymerization of quenched glasses in the system CaOTiO 2SiO 2FeO. The data show that Fe 3+/ εFe is positively correlated with Ti/(Ti+Si), with iron content and with NBO/T (non-bridging oxygen per tetrahedrally coordinated cation) of the quenched glasses. In general, isomer shifts of ferric and ferrous iron are consistent with tetrahedrally coordinated Fe 3+ (isomer shift near 0.3 mm/s relative to metallic Fe at 298 K) and octahedrally coordinated Fe 2+ (isomer shifts between 1 and 1.1 mm/s). Neither isomer shift nor quadrupole splitting is sensitive to the bulk compositional variables (Ti and Fe content) with the exception of the quadrupole splitting of Fe 2+. The latter parameter shows positive correlation with the Ti/(Ti+Si) of the quenched glasses, suggesting increasing distortion of the Fe 2+-O polyhedra. The ln(Fe 3+/Fe 2+) is linearly correlated with 1/ T ( T is absolute temperature) and with ln( f O2) (oxygen fugacity) resulting in an enthalpy of reduction of ferric to ferrous iron in the range of 100–200 kJ/mol. This enthalpy increases with increasing Ti and with increasing total iron content. From the log(Fe 3+/Fe 2+) vs. oxygen fugacity data, the activity coefficient ratio of Fe 3+/Fe 2+ is negatively correlated with NBO/T and with Ti content. As the oxygen fugacity decreases below that of air (and, therefore, the Fe 3+/ εFe also decreases), the isomer shift of ferric iron begins to increase (from ∼ 0.3 mm/s to > 0.4 mm/s) so that, when Fe 3+/ εFe < 0.3, the isomer shift values are consistent with ferric iron being in octahedral coordination, whereas with Fe 3+/ εFe > 0.5, Fe 3+ is in tetrahedral coordination. In the Fe 3+/ εFe range 0.3–0.5, Fe 3+ probably exists both in tetrahedral and octahedral coordination in the quenched glasses. The data are interpreted to suggest that solution of Ti 4+ in iron-bearing silicate quenched glasses enhances the stability of Fe 3+ relative to Fe 2+. This enhancement is the result of complex solution mechanisms involving both ferrous iron titanate complexing and formation of highly polymerized Ti-complexes in the melts.

Iron and phosphorus in calcium silicate quenched melts

Mössbauer and Raman spectroscopy has been used to describe the relationships between redox equilibria of iron and phosphorus content and anionic structure of calcium silicate quenched melts. Phosphorus was added as Ca 3(PO 4) 2 [P-series of glasses where Ca ( Si + P) is constant] and as P 2O 5 [PW-series of glasses where Ca ( P + Si) decreases with increasing P ( P + Si) ]. Raman spectroscopic data show that in either case, phosphorus is dissolved in Fe-free melts as an orthophosphate (PO 3− 4) complex with a narrow ∼20–25-cm −1 full width at half height) P-0 stretch band near 950 cm −1. Its frequency as well as those of the Raman bands assigned to SiO stretching modes, are not affected by variations in P ( P + Si) in the range studied (0.00–0.16). There is a positive correlation between ln( Fe 3+ Fe 3+ ) and P ( P + Si) in the P-series of Fe-bearing quenched melts, whereas in the PW-series, the correlation is negative at the same temperature and oxygen fugacity. This different behavior results from two competing effects on Fe 3+ Fe 2+ . Phosphate complexing with Fe causes an increase in the redox ratio as observed in the P-series of glasses. In the PW-series, the degree of polymerization increases with P ( P + Si) , and this increase results in a decrease in the redox ratio. For quenched Fe-bearing melts equilibrated in air (under these conditions Fe 3+ Σ Fe = 0.58–0.75 depending on temperature and composition), the hyperfine parameters (from Mössbauer spectroscopy) of both ferric and ferrous iron are insensitive to composition (for 298-K spectra, isomer shift, IS Fe 3+ = 0.314 ± 0.007 mm s −1, quadrupole splitting, QS Fe 3+ = 1.165 ± 0.021 mm, IS Fe 2+ = 1.012 ± 0.012 mm s −1 and QS Fe 2+ = 1.952 ± 0.011 mm s −1), consistent with tetrahedrally coordinated Fe 3+ and probably octahedrally coordinated Fe 2+. When Fe 3+ Σ Fe is reduced below 0.4, the IS Fe 3+ increases to above 0.5 mm s −1, consistent with a transformation of Fe 3+ from tetrahedral to octahedral coordination.

The compressibility of silicate liquids containing Fe2O3 and the effect of composition, temperature, oxygen fugacity and pressure on their redox states

Ultrasonic longitudinal acoustic velocities in oxidized silicate liquids indicate that the pressure derivative of the partial-molar volume of Fe2O3 is the same in iron-rich alkali-, alkaline earth- and natural silicate melt compositions at 1 bar. The dV/dP for multicomponent silicate liquids can be expressed as a linear combination of partial-molar constants plus a positive excess term for Na2O−Al2O3 mixing. Partial-molar properties for FeO and Fe2O3 components allow extension of the empirical expression of Sack et al. (1980) to permit the calculation of Fe-redox equilibrium in a natural silicate liquid as a function of composition, temperature, fo2 and pressure; a more formal thermodynamic expression is presented in the Appendix. The predicted equilibrium fo2 of natural silicate melts, of fixed oxygen content, closely parallels that defined by the metastable assemblage fayalite+magnetite+β-quartz (FMQ), in pressure-temperature space. A silicate melt initially equilibrated at 3 GPa and FMQ, will remain within approximately 0.5 log10 units of FMQ during its closed-system ascent. Thus, for magmas closed to oxygen, iron-redox equilibrium in crystal-poor pristine glassy lavas represents an excellent probe of the relative oxidation state of their source regions.

A catalyst function for MPTP in superoxide formation

We demonstrate that 1-methyl-4-phenyl-1,2-dihydropyridine (MPDP) can be generated, in an alternate pathway, from the catalyst action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) upon the iron redox equilibrium reaction. Superoxide and ferric iron are instantaneously produced after addition of MPTP to a solution of ferrous iron. This reaction is oxygen and pH dependent. Superoxide, through a iron dependent Haber-Weiss reaction with peroxide, can generate the cytotoxic hydroxyl radical. A small portion of the superoxide reacts with MPTP to produce the reactive species X • which, in the presence of Fe +3 can also generate MPDP.