Showing papers on "Chromium published in 1998"

Determination of the concentration of surface hydroxyl groups on metal oxide films by a quantitative XPS method

Abstract: Quantitative XPS has been used to determine the surface concentration of hydroxyl groups in native air-formed oxide films on metals having low surface areas. A mathematical expression has been derived to give the concentration of surface hydroxyl groups as a function of the intensity ratio of the OH to O2− contributions to the O 1s photopeak. This expression is based on modeling the oxide film on a metal to be a multilayer system consisting of an outermost layer of organic contamination, a layer of chemisorbed water, a surface hydroxylated region of the oxide film and the inner portion of the oxide film. The average values of the experimentally determined concentrations of surface hydroxyl groups are 15, 13, 11, 6 and 8 OH nm−2 for oxide-covered aluminum, chromium, titanium, tantalum and silicon, respectively. X-ray photoelectron spectroscopy depth profiles using argon ion sputtering and variable-angle XPS have been utilized in this work. Surface treatments have employed either ultrasonic cleaning with organic solvents or argon plasma treatment. © 1998 John Wiley & Sons, Ltd.

Semiconducting Properties of Passive Films Formed on Stainless Steels Influence of the Alloying Elements

Abstract: Passive films formed on stainless steels in a borate buffer solution (pH 9.2) have been investigated by capacitance measurements and photoelectrochemistry. The study was carried out on films formed on AISI type 304 and 316 stainless steels and high purity alloys with differing chromium, nickel, and molybdenum contents. Complementary research by Auger analysis shows that the passive films are composed essentially of an inner chromium region in contact with the metallic substrate and an outer iron oxide region developed at the film/electrolyte interface. The semiconducting properties of the passive films are determined by those of the constituent chromium and iron oxides which are of p-type and n-type, respectively. Thus the influence of the alloying elements on the semiconducting properties of the passive films is explained by changes in the electronic structure of each of these two oxide regions.

X‐Ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy Study of Passive Films Formed on (100) Fe‐18Cr‐13Ni Single‐Crystal Surfaces

Abstract: X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were combined to investigate the thickness, chemical composition, and structure of passive films formed in 0.5 M H 2 SO 4 on (100)Fe-18Cr-13Ni. The XPS measurements show that aging under polarization at +500 mV/SHE causes a dehydration reaction of the outer chromium hydroxide layer of the passive film. This reaction results in a thickening of the mixed Cr(III) and Fe(III) inner oxide layer and increases the Cr 2 O 3 enrichment. This reaction consumes, in addition to chromium hydroxide of the outer layer, chromium from the metallic phase underneath the passive film. Only traces of nickel (hydroxide) are detected in the passive film, whereas Ni enrichment is observed in the alloy underneath the passive film. High-resolution STM images reveal that aging under polarization causes a crystallization of the inner Cr 2 O 3 oxide layer in epitaxy with the substrate. The epitaxial relationship is (0001) α-Cr 2 O 3 ||(100) Fe-18Cr-13Ni with [2130]α-Cr 2 O 3 ||[011] Fe-18Cr-13Ni. The crystallization proceeds with a faster kinetics than on (110) Fe-22Cr in the same conditions. The crystallization rate is modified by the presence of Ni in the alloy, which is enriched in the metallic phase underneath the film and slows down the formation of Cr 2 O 3 in the inner part of the film. This favors a more complete process of crystallization. Aging under polarization is beneficial to the further stability of the passive film in air.

Sunflower Stalks as Adsorbents for the Removal of Metal Ions from Wastewater

Abstract: Sunflower stalks as adsorbents for the removal of metal ions such as copper, cadmium, zinc, and chromium ions in aqueous solutions were studied with equilibrium isotherms and kinetic adsorptions. The maximum adsorptions of four heavy metals are 29.3 mg/g (Cu2+), 30.73 mg/g (Zn2+), 42.18 mg/g (Cd2+), and 25.07 mg/g (Cr3+), respectively. Particle sizes of sunflower stalks affected the adsorption of metal ions; the finer size of particles showed better adsorption to the ions. Temperature also plays an interesting role in the adsorption of different metal ions. Copper, zinc, and cadmium exhibited lower adsorption on sunflower stalks at higher temperature, while chromium showed the opposite phenomenon. The adsorption rates of copper, cadmium, and chromium are quite rapid. Within 60 min of operation about 60−80% of these ions were removed from the solutions.

Microbial Chromium (VI) Reduction

Abstract: This review was conducted to describe the microbial reduction of hexavalent chromium [Cr(VI)] in general and a potential for biological treatment of Cr(VI)-containing wastes in particular. Cr(VI) is the highest oxidation state of the metal chromium, which is widely used in various industries. Chromate (CrO4 −2) is the prevalent species of Cr(VI) in natural aqueous environments and is the major pollutant from Cr-related industries. Cr(VI) exhibits inhibitory effects on biological wastewater treatment processes. Several microorganisms, however, have been found to be able to resist and/or reduce Cr(VI) within a wide range of Cr(VI) concentrations. The microbial Cr(VI) reduction activities are either plas-mid-or membrane-associated phenomena. Factors affecting microbial Cr(VI) reduction, including biomass concentration, initial Cr(VI) level, carbon source, pH and temperature, oxidation-reduction potential, oxyanions and metal cations, are discussed. The kinetic models for Cr(VI) reduction are described. Biolo...

Influence of Organic Ligands on Chromium(VI) Reduction by Iron(II)

Abstract: Iron(II) is one of the most important reductants that transforms toxic chromium(VI) to essentially nontoxic chromium(III), but the effect of iron speciation on this redox reaction is not well-understood. We determined rate constants for Cr(VI) reduction by a series of Fe(II)−organic complexes, using UV−vis spectroscopy and kinetic fitting. The experiments with 1−20 μM Cr(VI), 1−60 μM Fe(II), and 5−1000 μM organic ligand at pH 4.0−5.5 can be described with the following rate law: −d[Cr(VI)]/dt = ∑LkL[Fe(II)L][Cr(VI)], where kL is pH-dependent. Fe(III)-stabilizing ligands such as bi- and multidentate carboxylates and phenolates generally accelerate the reaction, whereas Fe(II)-stabilizing ligands such as phenanthroline essentially stop the reaction. The rate coefficients increase with decreasing electron reduction potential of the Fe(III)L−Fe(II)L redox couples. The relationship of log kL versus EH°(Fe(III)L) is quite linear over 10 orders of magnitude. Dissolved organic matter extracted from the organic h...

Homogeneous Chromium Catalysts for Olefin Polymerization

Abstract: Chromium-based heterogeneous catalysts are used extensively for the polymerization of olefins. There are few homogeneous model systems that allow investigation of the reaction mechanisms and unique characteristics of chromium catalysts. Coordinatively unsaturated paramagnetic chromium alkyls containing a Cp*CrIIIR fragment (Cp* = η5-C5Me5, pentamethylcyclopentadienyl) catalyze the polymerization of ethylene and α-olefins. Correlations between molecular structure and catalytic activity are reviewed.

Aerobic chromate reduction by Bacillus subtilis.

Abstract: We have studied the reduction of hexavalent chromium (chromate) to the less toxic trivalent form by using cell suspensions and cell-free extracts from the common soil bacterium, Bacillus subtilis. B. subtilis was able to grow and reduce chromate at concentrations ranging from 0.1 to 1 mM K2CrO4. Chromate reduction was not affected by a 20-fold excess of nitrate-compound that serves as alternate electron acceptor and antagonizes chromate reduction by anaerobic bacteria. Metabolic poisons including sodium azide and sodium cyanide inhibited chromate reduction. Reduction was effected by a constitutive system associated with the soluble protein fraction and not with the membrane fraction. The reducing activity was heat labile and showed a Km of 188 microns CrO4(2)-. The reductase can mediate the transfer of electrons from NAD(P)H to chromate. The results suggest that chromate is reduced via a detoxification system rather than dissimilatory electron transport.

Chemistry of a Chromate Conversion Coating on Aluminum Alloy AA2024‐T3 Probed by Vibrational Spectroscopy

Abstract: Infrared and Raman spectroscopy were used to characterize a chromate conversion coating (CCC) on 2024-T3 aluminum aircraft alloy with a long range objective of determining the anticorrosion mechanism of the CCC. Spectra were compared to those from synthetic mixed oxides of aluminum, Cr(III), and Cr(VI) made by treating pure reagents with NaOH. The Fourier transform infrared (FTIR) and Raman spectra of the CCC showed similar behavior to the chromium III/VI mixed oxide for both the initial materials and after heat-treatment. Analysis of the CCC and chromium mixed oxide by UV-vis spectroscopy indicated that both have a 3:1 ratio of Cr(III) to Cr(VI). When the chromium mixed oxide was immersed in pH 4 HNO 3 , the ratio of released H + to released Cr(VI) ranged from 0.98 to 1.07. In addition, a compound with a Raman spectrum very similar to that of a CCC was formed by a reaction of Cr(III) hydroxide with Cr 2 O 7 -2 , CrO 4 -2 , or the Alodine chromating bath. The results indicate a strong structural similarity between the Cr-mixed oxide and a major component of the CCC. A likely structure for this common material involves covalent bonding between polymeric Cr(III) hydroxide and Cr 2 O 7 -2 or CrO 4 2 , This mixed oxide structure may hydrolyze to release H + and soluble chromate.

Critical Evaluation and Selection of Standard State Thermodynamic Properties for Chromium Metal and Its Aqueous Ions, Hydrolysis Species, Oxides, and Hydroxides

Abstract: This review critically evaluates the reported thermodynamic data on chromium metal, oxides, hydroxides, free aqueous ions, and hydrolysis species. Several discrepancies and inconsistencies have been uncovered and resolved to improve equilibrium calculations for chemical modeling and related engineering purposes. A revised set of data is derived from evaluation of electrochemical measurements, silver chromate solubility measurements, and auxiliary post-1980 data, reevaluation of earlier data, and reconsideration of the path for the thermodynamic network. The recommended thermodynamic values for Cr(cr), C , C , Cr , Cr2 , Cr2O3(cr), CrO3(cr), FeCr2O4(cr), CrCl2(cr), CrCl3(cr), and KFe3(CrO4)2(OH)6(cr) at 25 °C, 1 bar, and infinite dilution are given.

Comparison of the Formation of 8-Hydroxy-2‘-deoxyguanosine and Single- and Double-Strand Breaks in DNA Mediated by Fenton Reactions

Abstract: The formation of 8-hydroxydeoxyguanosine (8-OHdG) and both single- and double-strand breaks in DNA by Fenton-type reactions has been investigated. Salmon sperm DNA was exposed to hydrogen peroxide (50 mM) and one of nine different transition-metal ions (25 microM-1 mM). Modified DNA was isolated and subjected to analysis by liquid chromatography coupled to an electrochemical detection system (LC-ECD), to evaluate the formation of 8-OHdG. The highest yield of 8-OHdG was obtained following treatment of DNA with the chromium(III) Fenton reaction (a maximum of 19 400/10(6) nucleotides), followed by iron(II) (13 600), vanadium(III) (5800), and copper(II) (5200). The chromium(VI) Fenton reaction generated a moderate yield of 8-OHdG (3600/10(6) nucleotides), while the yield obtained in DNA treated with cobalt(II), nickel(II), cadmium(II), and zinc Fenton reactions was not significantly higher than in control incubations of DNA with hydrogen peroxide alone. Similar treatment of the double-stranded plasmid pBluescript K+ with hydrogen peroxide (1 mM) and each transition-metal ion (1-100 microM) followed by quantitative agarose gel electrophoresis demonstrated that open-circle DNA, resulting from single-strand breaks, was generated in Fenton reactions involving all nine metal ions. In contrast, linear DNA was only formed in Fenton reactions involving chromium(III), copper(II), iron(II), and vanadium(III) ions. Formation of linear DNA, under conditions that generated relatively few single-strand breaks, suggests that these four transition-metal ions partake in Fenton reactions to generate true double-strand breaks. Furthermore, the generation of 8-OHdG exhibits a good correlation with the formation of double-strand breaks, suggesting that they arise by a similar mechanism.

Chromium(III) complexes bearing N,N-chelate ligands as ethene polymerization catalysts

Abstract: Novel chromium(III) ethene polymerization catalysts bearing bulky monoanionic N,N-chelate ligands are described.

(Cr:Al)N coatings deposited by the cathodic vacuum are evaporation

Abstract: (Cr:Al)N coatings were deposited using two cathodic vacuum arc evaporators fitted with chromium and aluminium cathodes. Both monolayer and multilayer coatings were deposited with different aluminium content. Selected mechanical and tribological properties of the coating were investigated as well as the oxidation behaviour. It is shown that the oxidation rate decreases with increasing aluminium content. The hardness of the (Cr:Al)N coating is higher than that of CrN coatings.

The Structure of the Hydrated Gallium(III), Indium(III), and Chromium(III) Ions in Aqueous Solution. A Large Angle X-ray Scattering and EXAFS Study.

Abstract: The structure of the hydrated gallium(III), indium(III), and chromium(III) ions has been determined in aqueous perchlorate and nitrate solutions by means of the large-angle X-ray scattering (LAXS) and extended X-ray absorption fine structure (EXAFS) techniques. The EXAFS studies have been performed over a wide concentration range, 0.005-1.0 mol.dm(-)(3) (2.6 mol.dm(-)(3) for chromium(III)), while the LAXS studies are restricted to concentrated solutions, ca. 1.5 mol.dm(-)(3). All three metal ions were found to coordinate six water molecules, each of which are hydrogen bonded to two water molecules in a second hydration sphere. The metal-oxygen bond distance in the first hydration sphere of the gallium(III), indium(III), and chromium(III) ions was determined by LAXS and EXAFS methods to be 1.959(6), 2.131(7), and 1.966(8) A. The LAXS data gave mean second sphere M.O distances of 4.05(1), 4.13(1), and 4.08(2) A for the gallium(III), indium(III), and chromium(III) ions, respectively. The perchlorate ion was found to be hydrogen bonded to 4.5(7) water molecules with the O.O distance 3.05(2) A and Cl.O 3.68(3) A. Analyses of the Ga, In, and Cr K-edge EXAFS data of the aqueous perchlorate and nitrate solutions showed no influence on the first shell M-O distance by a change of concentration or anion. The minor contribution from the second sphere M.O distance is obscured by multiple scattering within the tightly bonded first shell. EXAFS data for the alum salts CsM(SO(4))(2).12H(2)O, M = Ga or In, showed the M-O bond length of the hexahydrated gallium(III) and indium(III) ions to be 1.957(2) and 2.122(2) A, respectively.

Chromia/zirconia catalysts with Cr content exceeding the monolayer. A comparison with chromia/alumina and chromia/silica for isobutane dehydrogenation

Abstract: CrO x /ZrO 2 , CrO x /γ-Al 2 O 3 and CrO x /SiO 2 catalysts were prepared by wet impregnation in the loading range 0.05–14 atoms Cr nm −2 , that is below and above the monolayer capacity of support. The samples were characterized by chemical analysis, including carbon monoxide reduction and Cr VI extraction with sodium hydroxide or water, and by XRPD, UV-Vis DRS and ESR. Several Cr species were identified, namely Cr VI , Cr V (ESR γ-signal), Cr III (ESR δ-signal), small Cr III oxide clusters (ESR β-signal) and α-chromia. The relative abundance of these Cr species was found to be mainly controlled by the specific support employed and the Cr concentration. The catalytic activity in the isobutane dehydrogenation was studied at 723 and 773 K in a flow microreactor. It was found that the activity per total Cr atom was markedly higher for the zirconia-based catalysts compared with the other two systems in all the explored Cr concentration range. That is, the CrO x /ZrO 2 interaction maintains its positive effect well beyond the monolayer capacity of this support. Moreover, the catalytic activity correlated with the amount of high-oxidation state chromium (Cr VI and Cr V ), evaluated by the quantity of extractable Cr or by the consumption of carbon monoxide in the catalyst reduction. The larger concentration of mononuclear Cr V species on zirconia, confirmed here, is thought to be responsible for the higher activity of the zirconia-based catalysts, through reduction to active mononuclear Cr III species under reaction conditions.

Steam reforming of naphthalene on Ni–Cr/Al2O3 catalysts doped with MgO, TiO2, and La2O3

Abstract: Magnesium, lanthanum, and titanium oxide-doped nickel–chromium/alumina catalysts were prepared and their performances assessed in the steam reforming of naphthalene (T: 1,023 K; water-to-naphthalene molar ratio: 16, atmospheric pressure, residence time of 0.55 s; and GHSV = 10,080 h−1). Impregnation of nickel and chromium in MgO- and La2O3-doped alumina gave the most active, stable catalysts. Nickel was active in aromatic ring opening, while chromium inhibited the encapsulation of the nickel clystallites by inactive carbon filaments. The use of MgO had a signifkant effect on the robustness of catalyst due to the formation of MgAl2O4 spinelphase. La2O3 was necessaly to drive the reaction toward total gasification. The addition of TiO2 gave poor activity and a soft catalyst. Some of the catalyst preparations were characterized before and after reaction by XRD, SEM, DTA, and surface measurements. The physical and catalytic properties are discussed in light of these characterizations. The catalyst optimized during this study showed high clystallite dispersion, excellent crushing strength, and good thermal stability. When comparing the data obtained using Ni/Al2O3 with Ni–Cr/Al203·MgO·La2O3, it is clear that the improved catalyst reached higher conversion and gas yield performances.

Partitioning of chromium and molybdenum in super duplex stainless steels with respect to nitrogen and nickel content

Abstract: Experimental data of elemental partitioning of Cr, Mo, and Ni in a 5% Mn containing SAF 2507 super duplex stainless steel (SDSS) with varying nitrogen and nickel mass fractions are presented. Experimental results on phase equilibria and alloying element partitioning are compared to values calculated using Thermo-Calc software revealing good agreement. A trilinear model is proposed to describe the partitioning ratio of Cr, Mo and Ni with nitrogen and nickel mass percent in the austenite and the annealing temperature as variables. While nitrogen is found to reduce the partitioning of chromium and molybdenum, nickel enhances the partitioning ratio of these to alloying elements. Based on this semi-empirical model, a guideline for the future development of improved SDSS is formulated. With respect to corrosion resistance, a higher nitrogen level is not beneficial by itself but needs to be paralleled by increasing molybdenum and decreasing chromium contents.

Removal of metal ions using an industrial biomass with reference to environmental control

Abstract: The utility of a waste, dead fungal biomass in the removal of various metal ions such as calcium, iron, nickel and chromium when present individually and in possible combinations has been demonstrated. Although the tests were carried out using synthetic solutions of varying pH and metal ion concentrations, the results are representative of typical waste effluents emanating from leather, paper, paint and mineral processing industries as well as electroplating circuits. The maximum metal uptake was found to be dependent on solution pH (4-5 for Fe, 4-7 for Ca, 6-7 for Ni and 6 for Cr) and increased with biomass loading up to 10 g/l. The adsorption densities for various metal ions could be arranged as Ca > Cr(III) > Ni > Fe > Cr(VI). The presence of co-ions in binary, ternary and quarternary combinations decreased the metal uptake; the Ni uptake being most significantly affected while that of Cr(VI) the least. Possible mechanisms of metal ions removal are outlined along with a flow sheet for potential industrial application.

Mixed lithium manganese oxide and lithium nickel cobalt oxide positive electrodes

Abstract: Positive electrodes including a lithium nickel cobalt metal oxide are disclosed. The lithium nickel cobalt metal oxides have the general formula Li x Ni y CO z M n O 2 , where M is selected from the group consisting of aluminum, titanium, tungsten, chromium, molybdenum, magnesium, tantalum, silicon, and combinations thereof, x is between about 0 and about 1 and can be varied within this range by electrochemical insertion and extraction, the sum of y+z+n is about 1, n ranges between above 0 to about 0.25, y and z are both greater than 0, and the ratio z/y ranges from above 0 to about 1/3. Also disclosed are composite positive electrodes including the above-described lithium nickel cobalt metal oxides together with a lithium manganese metal oxide of the formula Li x Mn 2−r M1 r O 4 , where r is a value between 0 and 1 and M1 is chromium, titanium, tungsten, nickel, cobalt, iron, tin, zinc, zirconium, silicon, or a combination thereof.

Activation of Molecular Oxygen during the Reactions of Chromium(VI/V/IV) with Biological Reductants: Implications for Chromium-Induced Genotoxicities1

Abstract: The first systematic study on the roles of O2 in the reactions of Cr(VI/V/IV) with major intracellular reductants, cysteine (1), glutathione (2), and ascorbic acid (3) as well as with vitamin E analogue Trolox (4), has been performed The reactions of 1−3 with Cr(VI) (aqueous buffer solutions, pH = 45−75, 25 °C) led to a slow O2 consumption (measured by a Clark oxygen electrode) The reactions of 1−3 with the relatively stable Cr(V) and Cr(IV) 2-ethyl-2-hydroxybutanoato complexes under the same conditions were accompanied by fast O2 consumption The O2 consumption during the reactions of Cr(VI/V/IV) with 1−3 did not lead to a significant accumulation of H2O2 (determined with catalase) No significant O2 consumption was detected for the reactions of Cr(VI/V/IV) with 4 To reveal the mechanisms of O2 activation, the kinetics of the Cr(V/IV) reactions with 1−4 at pH 45 and 75 were studied by stopped-flow UV−visible spectrophotometry; and the kinetic data were processed by the global analysis method The

Electrothermal atomic absorption spectrometric determination of aluminium, cadmium, chromium, copper, iron, manganese, nickel and lead in olive oil

Abstract: A simple and fast procedure for the direct ETAAS determination of Al, Cd, Cr, Cu, Fe, Mn, Ni and Pb in olive oil is proposed.N,N-Hexamethylenedithiocarbamic acid, hexamethyleneammonium salt (HMDC-HMA), was used as a universal modifier with two functions: isoformation of the chemical species of the investigated elements, present in olive oil and their thermal stabilisation during the pre-treatment step. Various organic solvents were studied and 1,4-dioxane was found to be the most suitable solvent for oil dilution prior to ETAAS analysis. 1,4-Dioxane improves the decomposition of triglycerides during the pre-treatment step and permits aqueous standard solutions to be used for calibration. Uncoated graphite tubes with platforms are proposed as atomisers for the ETAAS determination of Cd, Cu, Fe, Mn and Fe, and pyrolytic graphite coated graphite tubes with grooves are the most suitable atomisers for Al, Cr and Ni. Instrumental parameters optimised according to pre-treatment and atomisation curves, obtained in the presence of the olive oil matrix and with HMDC-HMA as modifier, are presented. The proposed method permits the determination of 0.1 µg g–1 Fe, 0.05 µg g–1 Ni, 0.02 µg g–1 Al, Cu, Cr and Pb, and 0.01 µg g–1 Cd and Mn with relative standard deviations of about 8–10% for all analytes in this concentration range.

Infrared laser photodetachment of transition metal negative ions: studies on , , and

Abstract: Photodetachment threshold spectroscopy on the negative ions of chromium, molybdenum, copper and silver has yielded values for the electron affinities of 5451.0(10), 6027(2), 9967.2(3) and , respectively. The results agree well with previous measurements, with an improvement in the accuracy of up to a factor of 300.