Reduction Of Chromium Ions Research Articles

Electrochemical Behavior of Chromium Difluoride in Molten Lithium and Beryllium Fluoride Mixtures

The interaction between CrF2 and a molten binary LiF-BeF2 mixture has been studied using the electrochemical methods. The polarization of the chromium electrode relative to the dynamic beryllium electrode was measured. The obtained voltammetry data elucidates that under the experimental conditions, chromium is observed exclusively in one valence form. According to the cathode polarization dependences, it is established that the reduction of chromium ions in the metal occurs by a two-electron electrode reaction. The equilibrium concentrations of chromium were experimentally measured at the temperatures of 923, 973, and 1023 K at different chromium fluoride concentrations in the melt. The oxidation degree of chromium, dissolved in the melt, was calculated according to the concentration dependences of the equilibrium potentials. A thermodynamic analysis of the chromium difluoride dissolution in a molten mixture of lithium and beryllium fluorides was performed. The formation of dilute solutions of chromium difluoride is accompanied by slight deviations of the salt system from Raoult’s laws.

Comparative study of the adsorption and photo-reduction of hexavalent chromium onto AlPO4-11 and SAPO-31 substituted Fe2O3 from aqueous solutions: Synthesis, characterization, kinetic and thermodynamic studies

ABSTRACT AlPO4-11/Fe2O3 and SAPO-31/Fe2O3 composites were used in the adsorption and reduction of chromium ions in order to compare their catalytic performances. They are characterized by XRD, SEM and EDS. The nature of coordination units of nuclei 27Al, 31P and 29Si was determined by MAS-NMR. The best performances on photo-reduction (99%) were obtained on the SAPO-31/Fe2O3 due to its acidic surface Fe2O3 oxides properties. The estimate of the thermodynamic parameters ΔG°, ΔH° and ΔS° revealed that photo-reduction process was spontaneous, exothermic for AlPO4-11/Fe2O3 and endothermic for SAPO-31/Fe2O3. The process follows the first pseudo order kinetic rationalized by the Langmuir-Hinshelwood (HL) model.

Sorption and reduction of chromium ions by the chelating ion exchanger Diaion CR20

Sorption and reduction of chromium ions by the chelating ion exchanger Diaion CR20

Formation of chromium composite electrochemical coatings from sulfate oxalate solutions based on Cr(III)

The mechanism responsible for the inclusion of Al2O3 and SiC nanoparticles, the mixture of Nb2N and Ta2N (1: 1), MoS2, Cr2O3, and SiO2 with diverse electric conductivity, hydrophilicity, and resistance to solution components in chromium deposits from the sulfate–oxalate suspension solutions based on Cr(III) was studied. The main factors that determine the formation of chromium composite electrochemical coatings, their composition, and surface morphology were determined. The film on the surface of the growing deposit of the intermediates of the reduction of chromium ions plays the key role in the formation of composite coatings from Cr(III) and Cr(VI) sulfate–oxalate suspension solutions. The film can play the role of a structural mechanical barrier that hinders the incorporation of particles in the deposit, or it can fix the particles on the electrode surface by creating hydroxo bridges with chemisorbed hydroxide compounds on the particle surface.

Nature of the cathode polarization during reduction of chromate ions in alkaline solutions

The effect of potential scanning rate and temperature on the kinetics of the reduction of chromate ions in alkaline solutions was studied, and the nature of polarization in separate stages of the cathode process was determined.

Electrode processes and their kinetic parameters in the reduction and oxidation of Cr(III) and Cr(II) in molten CsCl

The reduction of chromium ions in the CsCl-CrCl3 melt in wide concentration and temperature ranges is reported. It is assumed that the melt contains polynuclear chromium complexes. The reduction of Cr(III) to Cr(II) is a reversible one-electron process. The electroreduction of divalent chromium to chromium metal is an irreversible process involving dinuclear and mononuclear chromium complex ions. The electron transfer rate constant has been calculated for various temperatures and chromium chloride concentrations in the salt melt. The activation energy of electron transfer and its dependence on the CrCl3 concentration in the electrolyte have been determined for the Cr(III) ↔ Cr(II) process.

Biosorption of hexavalent chromium by Termitomyces clypeatus biomass: Kinetics and transmission electron microscopic study

Biosorption of Cr +6 by Termitomyces clypeatus has been investigated involving kinetics, transmission electron microscopy (TEM) and Fourier transform infrared spectroscopic (FTIR) studies. Kinetics experiments reveal that the uptake of chromium by live cell involves initial rapid surface binding followed by relatively slow intracellular accumulation. Of the different chromate analogues tested, only sulfate ion reduces the uptake of chromium to the extent of ∼30% indicating chromate ions accumulation into the cytoplasm using sulfate transport system. Metabolic inhibitors, e.g. N,N′-dicyclohexylcarbodiimide, 2,4-ditrophenol and sodium azide inhibit chromate accumulation by ∼30% in live cell. This indicates that accumulation of chromium into the cytoplasm occurs through the active transport system. TEM-EDXA analysis reveals that the chromium localizes in the cell wall and also in the cytoplasm. Reduction of chromate ions takes place by chromate reductase activity of cell-free extracts of T. clypeatus. FTIR study indicates that chromate ions accumulate into the cytoplasm and then reduced to less toxic Cr +3 compounds.

Formation of Superoxochromium(III) by a Novel Mechanism

The reduction of chromate ion by l-cysteine in near-neutral aqueous solutions was studied by u.v.–vis spectroscopy. The rate constants for the formation, decomposition and redox degradation of a CrVI thioester intermediate (both in the absence and in the presence of dissolved O2) were determined by the use of a nonlinear least-squares regression to fit the experimental data to a double-exponential integrated rate law. Superoxochromium(III), CrOO2+, could be observed as a long-lived intermediate by the use of u.v. difference spectroscopy (peaks at 238 and 292 nm). The effects of several reaction variables on the yield of the intermediate CrOO2+ were studied. A striking result was that the yield decreased as the concentration of dissolved O2 increased. In the presence of phosphate buffer, the yield showed a maximum at pH 6.8. Formation of CrOO2+ was suppressed by the additives Mn2+, H2O2, d-ribose, 2-deoxy-d-ribose and a high concentration of Ce3+, whereas a low concentration of the latter enhanced its appearance. A novel mechanism for the formation of CrOO2+ from CrVI and l-cysteine (involving the intramolecular formation of an O–O bond) is proposed, and the possible routes for the decay of this intermediate are discussed. This novel mechanism might offer new insights into the redox chemistry of chromate ions.

Characterization of cathodic film formed during reduction of chromic acid: role of sulphuric acid concentration

AbstractThe surface composition of chromium, electrodeposited from a chromic acid solution with different amounts of sulphuric acid, has been investigated by means of Auger Electron Spectroscopy (AES) and X‐ray Photoelectron Spectroscopy (XPS). The quantity of sulphuric acid is a critical parameter in order to form metallic chromium instead of a non‐reducible chromium (III) oxide layer. The intermediate cathodic film formed on the electrode before the metallic chromium deposition has been investigated and XPS measurements have shown that a chromium oxide film is formed whatever the sulphuric acid concentration. However, in the presence of sufficient amounts of sulphate, this oxide is dissolved or the layer is broken down, giving rise to a free steel surface where the reduction of chromate ions into metallic chromium can take place. Copyright © 2004 John Wiley & Sons, Ltd.

The reduction of chromate ions by Fe(II) layered hydroxides

Abstract. The reduction of chromate ions by Fe(OH)2 and the iron (II)-iron (III) hydroxysulphate green rust, GR(SO42-), was studied to evaluate whether such synthetic layered hydroxides and the corresponding natural green rust mineral could be involved in the natural attenuation of contaminated environments. The resulting Cr (III) bearing phases, which would govern the subsequent behaviour of chromium, were clearly characterised. Both compounds proved to be very reactive and oxidised instantaneously while chromate ions were reduced to Cr (III) as evidenced by X-ray photoelectron spectroscopy. Mass balance (ICP-AES) demonstrated that the Fe/Cr ratio inside the solid end product was equal to the initial Fe/Cr ratio. The solid phases, analysed by X-ray diffraction, Raman and Mossbauer spectroscopies were identified as Cr-substituted poorly crystallised iron (III) oxyhydroxides in both cases, more precisely δ-FeOOH when starting with Fe(OH)2 and ferrihydrite when starting with GR(SO42-).

Trapping of Cr by Formation of Ferrihydrite during the Reduction of Chromate Ions by Fe(II)−Fe(III) Hydroxysalt Green Rusts

Hexavalent chromium, a byproduct of many industrial processes, is toxic and produces mobile aqueous oxyanions, whereas Cr(III) is relatively immobile in the environment and, moreover, essential in human glucidic metabolism. For this reason, Fe(II)-Fe(III) layered double hydroxysalt green rusts, recently identified as a mineral in hydromorphic soils, were evaluated as potential Fe(II)-bearing phases for hexavalent chromium reduction. Both considered synthetic varieties, the hydroxysulfate GR(SO{sub 4}{sup 2{minus}}) and the hydroxychloride GR(Cl{sup {minus}}), proved to be very reactive; their interaction with potassium chromate solutions leads to the rapid and complete reduction of Cr(VI) into Cr(III). The Cr(III)-bearing solid phase, studied by X-ray diffraction, Moessbauer, X-ray photoelectron, and Raman spectroscopies, was determined to be a poorly ordered Cr(III)-Fe(III) oxyhydroxide, similar to the 2 the line ferrihydrite. The comparison between the experimental redox potential and pH values for a theoretical equilibrium diagram bearing Cr and Fe phases indicated that the solubility of this solid solution, which may govern the behavior of chromium in the environment, is of the same order as that of pure Cr(OH){sub 3}.

Reduction of chromate ions by glutathione tripeptide in the presence of sugar ligands

Polarographic and ESR data obtained for the reduction of chromate in the presence of the tripeptide glutathione (GSH) and sugars are presented. The results indicate that in the binary GSH-Cr(VI) system, glutathione binds chromate forming a thioester species which can be reduced efficiently by free tripeptide molecules. In the systems containing chromate, sugars and glutathione, chromium(VI) interacts with the sugar ligand (or with both sugar and GSH) yielding esters which are easily reduced by GSH. The formed Cr(V) ions are then stabilized by the coordination to the sugar ligands. The sugars having pairs of cis hydroxyl groups available for binding to metal are the most effective in the formation, first, of Cr(VI) esters and then, of Cr(V) complexes.

Effect of adsorption of o-cresoxide ion on the kinetics of polarographic reduction of chromate ion

Effect of adsorption of o-cresoxide ion on the kinetics of polarographic reduction of chromate ion

Effect of adsorption of o-cresoxide ion on the kinetics of polarographic reduction of chromate ion

Summary The polarographic reduction of chromate ion is studied in 1 M NaOH with additions of o-cresol. It is observed that the reduction rate is depressed due to the adsorption of o-cresoxide ions formed in solution. The results are interpreted using the theory of irreversible polarographic waves with corrections applied for double layer effects. It is shown that the mechanism of inhibition of the reaction by adsorbed o-cresoxide ion is entirely one of electrostatic interaction in the adsorbed layer.

Fluorinated alkoxides of chromium(V) and vanadium(IV)

The reduction of chromate ion in the presence of perfluoropinacol (PFP) leads to the formation of stable, chelated, alkoxides containing chromium(V) in the ion [GrO(PFP)2]–, the cations being K+, Cs+ or Et4N+; the isoelectronic vanadium(IV) complex, [VO(PFP)2]22–, may be prepared by reaction of PFP with a vanadyl salt.