Structure and energetics of solvated ferrous and ferric ions: Car-Parrinello molecular dynamics in the DFT+U formalism

We implemented a rotationally-invariant Hubbard U extension to density-functional theory in the Car-Parrinello molecular dynamics framework, with the goal of bringing the accuracy of the DFT+U approach to finite-temperature simulations, especially for liquids or solids containing transition-metal ions. First, we studied the effects on the Hubbard U on the static equilibrium structure of the hexa-aqua ferrous and ferric ions, and the inner-sphere reorganization energy for the electron-transfer reaction between aqueous ferrous and ferric ions. It is found that the reorganization energy is increased, mostly as a result of the Fe-O distance elongation in the hexa-aqua ferrous ion. Second, we performed a first-principles molecular dynamics study of the solvation structure of the two aqueous ferrous and ferric ions. The Hubbard term is found to change the Fe-O radial distribution function for the ferrous ion, while having a negligible effect on the aqueous ferric ion. Moreover, the frequencies of vibrations between Fe and oxygen atoms in the first-solvation shell are shown to be unaffected by the Hubbard corrections for both ferrous and ferric ions.Comment: 13 pages, 2 figures, 1 table. Submitted to Journal of Electroanalytical Chemistr

Calcium Pyrophosphate Crystal Deposition: The Effect of Soluble Iron in a Kinetic Study Using a Gelatin Matrix Model

The kinetics of calcium pyrophosphate dihydrate (CPPD) crystal growth was studied by allowing calcium and pyrophosphate (PPi-4) ions to diffuse through a denatured collagen matrix (biological grade gelatin) in the presence of either ferric or ferrous ions. Ferric and, to some extent, ferrous ions blocked the migration of the PPi-4 diffusion gradient. This retardation in the [PPi-4] gradient led to numerous changes in the patterns of CPPD crystal formation. At the initial stages of crystal growth, the iron ions induced more crystal growth compared to control. At later incubation times, ferrous and ferric ions enhanced crystal growth at the expense of crystal nucleation. The presence of both ferrous and ferric ions resulted in the more rapid formation of the two crystals observed in vivo, triclinic CPPD and monoclinic CPPD. Further, both ferrous and ferric ions also reduced the solubility of the crystalline material in the broad diffuse band which formed when the Ca+2 and PPi-4 gradients first met. In this system, the presence of either ferrous or ferric ions increased the amount of hydroxyproline included in the crystalline precipitates. Iron was also incorporated into the crystals, particularly into the triclinic CPPD and monoclinic CPPD crystals

Proton Spin-Lattice Relaxation of Water Molecules in Ferrous-Ferric/Agarose Gel System

Proton spin-lattice relaxation time (T1) of water in aqueous solutions of ferrous and ferric ions and in the corresponding agarose gel systems have been studied in the light of NMR relaxation theory. The theoretical analysis of 1/T1\u27s has revealed that, at the microscopic level, changes in the solvation states of paramagnetic ions in aqueous or gel environment are greater than difference in the paramagnetism between ferric and ferrous ions. The former change is the primary factor for the exhibition of radiation effect. At the phenomenological level, we have confirmed and demonstrated that: (1) Radiation effect is almost exclusively exhibited through changes in 1/T1 caused by the interactions between water proton and ferrous or ferric ions; and (2) fraction of conversion of ferrous to ferric ions induced by radiation is the “true” representation of the spatial distribution of radiation dose

Sampling the oxidative weathering products and the potentially acidic permafrost on Mars

Large areas of Mars' surface are covered by oxidative weathering products containing ferric and sulfate ions having analogies to terrestrial gossans derived from sulfide mineralization associated with iron-rich basalts. Chemical weathering of such massive and disseminated pyrrhotite-pentlandite assemblages and host basaltic rocks in the Martian environment could have produced metastable gossaniferous phases (limonite containing poorly crystalline hydrated ferric sulfates and oxyhydroxides, clay silicates and opal). Underlying groundwater, now permafrost on Mars, may still be acidic due to incomplete buffering reactions by wall-rock alteration of unfractured host rock. Such acidic solutions stabilize temperature-sensitive complex ions and sols which flocculate to colloidal precipitates at elevated temperatures. Sampling procedures of Martian regolith will need to be designed bearing in mind that the frozen permafrost may be corrosive and be stabilizing unique complex ions and sols of Fe, Al, Mg, Ni and other minor elements

Screen-printed platinum electrodes for the detection of cupric and ferric ions in high chloride backgrounds

Screen-printed platinum electrodes developed for use in corrosion monitoring applications have been used to detect cupric and ferric ions both individually and as mixtures in a background of 3.5% by weight sodium chloride and in the presence of dissolved oxygen. In single species detection linear responses for the Fe3+/Fe2+ couple were observed over the concentration range 0.3 to 100mM. By contrast, the small size of the working electrode caused a current limiting response for cupric ions over the same concentration range. In mixtures of these ions, the sensors show good differentiation and are able to separate the individual metal ion responses

Mossbauer and optical spectroscopic study of temperature and redox effects on iron local environments in a Fe-doped (0.5 mol% Fe2O3)18Na2O–72SiO2 glass

Local environments of ferric and ferrous irons were systematically studied with Mössbauer (at liquid helium temperature)and ultraviolet–visible–near infrared spectroscopic methods for various 18Na2O–72SiO2 glasses doped with 0.5 mol% Fe2O3. These were prepared at temperatures of 1300–1600 °C in ambient air or at 1500 °C under reducing conditions with oxygen partial pressures from 12.3 to 0.27 x 10-7 atmospheres. The Mössbauer spectroscopic method identified three types of local environments, which were represented by the Fe3+ sextet, the Fe3+ doublet, and the Fe2+ doublet. The Fe3+ sextet ions were assigned to “isolated” octahedral ions. Under reducing conditions, the octahedral Fe3+ ions were readily converted into octahedral ferrous ions. The Fe3+ doublet exists both in octahedral and tetrahedral environment, mainly as tetrahedral sites in the reduced samples. The tetrahedral ions were found stable against reduction to ferrous ions. The Fe2+ doublet sites existed in octahedral coordination. Combining results from both spectroscopic studies, the 1120- and 2020-nm optical bands were assigned to octahedral ferrous ions with a different degree of distortion rather than different coordinations. Further, we assigned the 375-nm band to the transition of octahedral ferric ions that are sensitive to the change of oxygen partial pressure in glass melting and 415-, 435-, and 485-nm bands to the transitions of the tetrahedral ferric ions that are insensitive to oxidation states of the melt. The effect of ferric and ferrous ions with different coordination environments on the glass immiscibility was elucidated

Investigating the ferric ion binding site of magnetite biomineralisation protein Mms6

The biomineralization protein Mms6 has been shown to be a major player in the formation of magnetic nanoparticles both within the magnetosomes of magnetotactic bacteria and as an additive in synthetic magnetite precipitation assays. Previous studies have highlighted the ferric iron binding capability of the protein and this activity is thought to be crucial to its mineralizing properties. To understand how this protein binds ferric ions we have prepared a series of single amino acid substitutions within the C-terminal binding region of Mms6 and have used a ferric binding assay to probe the binding site at the level of individual residues which has pinpointed the key residues of E44, E50 and R55 involved in Mms6 ferric binding. No aspartic residues bound ferric ions. A nanoplasmonic sensing experiment was used to investigate the unstable EER44, 50,55AAA triple mutant in comparison to native Mms6. This suggests a difference in interaction with iron ions between the two and potential changes to the surface precipitation of iron oxide when the pH is increased. All-atom simulations suggest that disruptive mutations do not fundamentally alter the conformational preferences of the ferric binding region. Instead, disruption of these residues appears to impede a sequence-specific motif in the C-terminus critical to ferric ion binding

Kinetic studies of the dissolution of copper in ferric chloride solutions

M.S.Walter H. Burrow

Tunable Effect of Metal Ions on Polyelectrolyte Mechanics

Polyelectrolyte based hydrogel fibers can mimic extracellular matrix and have applications such as drug delivery and tissue scaffolding. Metal ions play a critical role in hydrogel fiber stability via electrostatic interactions, but knowledge of how they modulate mechanical properties of individual polyelectrolyte polymers is lacking. In this study, electrospun polyacrylic acid with chitosan is used as a model system to evaluate ferric ion effect on nanofiber mechanics. Using dark field microscopy imaging and persistence length analysis, we demonstrate that ferric ions modulate the bending stiffness of nanofibers. Young\u27s modulus of individual nanofibers is estimated at values of a few kilopascals, suggesting that electrospun nanofibers possibly exist in a hydrated state. Furthermore, Fourier Transform Infrared (FTIR) spectra indicate the effect of ferric ions on polyacrylic acid molecular bonds. Our results suggest that metal ions can regulate single nanofiber stiffness, thereby providing designs to fabricate hydrogels in a tunable fashion

The changes in various iron compounds in culture media for algae. [Translation from: Archiv fur Hydrobiologie Supplemente Band 38(1/2) 151-169, 1970.]

The purpose of this work is a contribution to the quantitative record of the use of iron by planktonic algae. Preliminary experiments with Chlorella to determine the rate of iron intake in the presence of inorganic sources of iron did not produce the desired result. The crucial point of this work is the investigation of the influence of various external factors on the stability of FeEDTA (FeEDTA = Ferric(III)-compound of ethylene-diamine tetra-acetic acid), since this compound appears to be particularly well-suited as a source of iron for planktonic algae (e.g. TAMIYA et al. 1953). Cultures of Chlorella fusca in a light thermostat were used in experimental research. Methods and results are discussed