Mixture Of Cr Research Articles

Synthesis of MIL-102, a Chromium Carboxylate Metal−Organic Framework, with Gas Sorption Analysis

A new three-dimensional chromium(III) naphthalene tetracarboxylate, CrIII3O(H2O)2F{C10H4(CO2)4}1.5.6H2O (MIL-102), has been synthesized under hydrothermal conditions from an aqueous mixture of Cr(NO3)3.9H2O, naphthalene-1,4,5,8-tetracarboxylic acid, and HF. Its structure, solved ab initio from X-ray powder diffraction data, is built up from the connection of trimers of trivalent chromium octahedra and tetracarboxylate moieties. This creates a three-dimensional structure with an array of small one-dimensional channels filled with free water molecules, which interact through hydrogen bonds with terminal water molecules and oxygen atoms from the carboxylates. Thermogravimetric analysis and X-ray thermodiffractometry indicate that MIL-102 is stable up to approximately 300 degrees C and shows zeolitic behavior. Due to topological frustration effects, MIL-102 remains paramagnetic down to 5 K. Finally, MIL-102 exhibits a hydrogen storage capacity of approximately 1.0 wt % at 77 K when loaded at 3.5 MPa (35 bar). The hydrogen uptake is discussed in relation with the structural characteristics and the molecular simulation results. The adsorption behavior of MIL-102 at 304 K resembles that of small-pore zeolites, such as silicalite. Indeed, the isotherms of CO2, CH4, and N2 show a maximum uptake at 0.5 MPa, with no further significant adsorption up to 3 MPa. Crystal data for MIL-102: hexagonal space group P(-)6 (No. 169), a = 12.632(1) A, c = 9.622(1) A.

Heavy metal accumulation and metallothionein concentration in the frog Rana ridibunda after exposure to chromium or a mixture of chromium and cadmium

The accumulation of two heavy metals (chromium (Cr) and cadmium (Cd)) in the liver, kidney and gut of Rana ridibunda exposed to Cr or to a mixture of Cr and Cd was investigated. The concentration of metallothioneins (MTs) in the same tissues was also studied. Both metals accumulated mainly in the kidney. Cr accumulation in the liver and gut was not affected by the presence of Cd. Furthermore, Cr concentration in the kidney was doubled when Cd was present. MT concentration did not increase after Cr treatment but it increased two- to six-fold over control values in mixture-exposed frogs, the highest value being observed in the gut. MTs in the gut could act as a barrier preventing ingested heavy metals from entering the blood stream. MT concentration correlated positively with Cd concentrations in both the liver and the gut of mixture-exposed animals.

Light‐induced formation of curved needle texture by circularly polarized light irradiation on a discotic liquid crystal containing a racemic chromium complex

We have found that the discotic nematic liquid crystal, hexakis(4‐nonylphenylethynyl)benzene (HNEB), doped with the racemic chromium complex Cr(Ocacac)3, shows a novel straight‐needle texture with hexagonal columnar alignments, changing to a curved‐needle texture under irradiation of circularly polarized light (CPL). This novel phenomenon is specific to the mixture of HNEB and Cr(Ocacac)3. The formation of curved needles means that chiroselective photoinversion of racemic Cr(Ocacac)3 by CPL irradiation induces a needle direction change in a discotic liquid crystal. The change in chirality of Cr(Ocacac)3 in HNEB induced by CPL irradiation, and the resulting nano‐segregation of its enantiomers during cooling from the isotropic to mesophase of HNEB, are considered to influence changes in the alignment of columns and/or small domains of column aggregates in the discotic liquid crystal.

Siderophore production by using free and immobilized cells of two pseudomonads cultivated in a medium enriched with Fe and/or toxic metals (Cr, Hg, Pb)

Pseudomonads are serious candidates for siderophore production applied to toxic metal (TM) solubilization. The bioaugmentation of contaminated soils by these TM-solubilizing bacteria combined with phytoextraction is an emerging clean-up technology. Unfortunately, siderophore synthesis may be drastically reduced by soluble iron in soils and bacteria can suffer from TM toxicity. In this study, we compared siderophore production by Pseudomonas aeruginosa and Pseudomonas fluorescens by using free and immobilized cells in Ca-alginate beads incubated in a medium containing Fe and/or TM (mixture of Cr, Hg, and Pb in concentrations which represented the soluble fraction of a contaminated agricultural soil). Free cell growth was stimulated by Fe, whatever the microorganism, the inoculum size and the presence or not of TM might have been. P. aeruginosa was less sensitive to TM than P. fluorescens. By comparison with free cells, immobilization with the high inoculum size showed less sensitivity to TM most probably because of lower metal diffusion in beads. Indeed, a maximum of 99.1% of Cr, 57.4% of Hg, and 99.6% of Pb were adsorbed onto beads. The addition of iron in the culture medium reduced significantly siderophore production of free cells while it led only to a low decrease with their immobilized counterparts, in particular with P. aeruginosa. In culture medium enriched with Fe and/or TM, siderophore-specific production of immobilized cells was higher than for free cells.

X‐Ray Diffraction Investigation of Phase Stability in the Co—Cr—O and the Fe—Co—Cr—O Systems in Air at 1323 K.

Abstract Mixtures of Co 3 O 4 , Cr 2 O 3 , and Fe 2 O 3 were annealed in air for times up to 147 h at 1323 K. Phase analysis was performed using X-ray diffraction. The mixtures of Cr 2 O 3 and Co 3 O 4 powders reacted forming mixed oxides with the spinel structure. In addition to an oxide with the spinel structure an oxide with the rock salt structure was identified in the cobalt-rich mixtures, suggesting that these compositions belong to a two-phase field in the Cr–Co–O phase diagram. In the Cr-rich mixtures, both non-reacted Cr 2 O 3 and CoCr 2 O 4 were identified. The decomposition temperatures of Co 3 O 4 , Co 2 CrO 4 , and Co 1.5 Cr 1.5 O 4 were estimated with thermogravimetry. On the basis of the results obtained a part of the Cr–Co–O phase diagram was constructed. Incorporation of Fe cations in the Co–Cr spinel was observed to expand the spinel lattice. Qualitative phase analyses of the Fe–Cr–Co oxide mixtures indicate that connecting the phase fields in the Fe–Co–O and Cr–Co–O system gives a fair prediction of the phase fields in the Fe–Cr–Co–O system at 1323 K (in air).

Combined Removal of Chlorinated Ethenes and Heavy Metals by Zerovalent Iron in Batch and Continuous Flow Column Systems

The combined removal of chlorinated ethenes and heavy metals from a simulated groundwater matrix by zerovalent iron (ZVI) was investigated. In batch, Ni (5-100 mg L(-1)) enhanced trichloroethene (TCE, 10 mg L(-1)) reduction by ZVI (100 g L(-1)) due to catalytic hydrodechlorination by bimetallic Fe0/Ni0. Cr(VI) or Zn (5-100 mg L(-1)) lowered TCE degradation rates by a factor of 2 to 13. Cr(VI) (100 mg L(-1)) in combination with Zn or Ni (50-100 mg L(-1)) inhibited TCE degradation. Addition of 20% H2(g) in the headspace, or of Zn (50-100 mg L(-1)), enhanced TCE removal in the presence of Ni and Cr(VI). Sorption of Zn to ZVI alleviated the Cr(VI) induced inhibition of bimetallic Fe0/Ni0 apparently due to release of protons necessary for TCE hydrodechlorination. In continuous ZVI columns treating tetrachloroethene (PCE, 1-2 mg L(-1)) and TCE (10 mg L(-1)), and a mixture of the metals Cr(VI), Zn(II), and Ni(II) (5 mg (L-1)), the PCE removal efficiency decreased from 100% to 90% in columns operated without heavy metals. The PCE degradation efficiency remained above 99% in columns receiving heavy metals as long as Ni was present. The findings of this study indicate the feasibility and limitations of the combined treatment of mixtures of organic and inorganic pollutants by ZVI.

Simultaneous synthesis and consolidation of chromium carbides (Cr 3C 2, Cr 7C 3 and Cr 23C 6) by pulsed electric-current pressure sintering

Chromium carbides (Cr 3C 2, Cr 7C 3 and Cr 23C 6) have been synthesized and consolidated simultaneously from mixtures of Cr and amorphous carbon powders by pulsed electric-current pressure sintering (PECPS). Dense ceramics thus obtained were composed of chromium carbides with a small amount of Cr 2O 3, which originates from a trace amount of oxygen adsorbed on the as-received starting Cr powder. Synthesis and consolidation processes, which were observed from their shrinkage curves during PECPS, have been examined using X-ray diffraction (XRD) and scanning electron microscopy for the powder compacts. Cr 3C 2 ceramics sintered at 1300 °C for 10 min under 30 MPa have a 98.9% of theoretical density and fine structures with a 3.6 μm grain size. They exhibit excellent mechanical properties: a bending strength σ b of 690 MPa, a Vickers hardness H v of 18.9 GPa and a fracture toughness K IC of 7.1 MPa m 1/2.

Effects of Exposure to a Mixture of Cadmium and Chromium on Detoxification Enzyme (GST, P450-MO) Activities in the Frog Rana ridibunda

The effect of two heavy metals, namely chromium and cadmium, on GST and P450-MO activities in the liver, kidney and gut of the frog Rana ridibunda, and the use of these detoxification enzyme systems as possible markers of heavy metal contamination of rivers in Northern Greece was investigated. In control animals, the highest P450-MO activity was observed in the liver and the lowest activity in the gut. As regards GST, the liver and kidneys showed the highest activity. Exposure of Rana to either Cr or a mixture of Cr and Cd caused a decrease in liver GST and P450-MO and renal GST activities. An important finding was that both GST and P450-MO activities were negatively correlated with concentrations of both metals in the liver of mixture-exposed animals. The results suggest that the determination of GST and P450-MO activities, in combination, could serve to indicate heavy metal contamination in rivers of Northern Greece.

The interaction of cysteine with chromium(VI) ions under UV irradiation

In 0.1 M phosphate buffer (pH 7.2), the interaction of chromium(VI) with cysteine in the presence and absence of UV irradiation was studied by cyclic voltammetry and electronic spectroscopy techniques. The reduction of Cr(VI) by cysteine takes place through the formation of Cr(VI)-thioester intermediate. On the cyclic voltammograms of cysteine and Cr(VI) mixture, the peaks at −0.315 and −0.800 V were observed, and these peaks are corresponding to the reduction of Cr(VI)-thioester and thiyl radical, respectively. In the cysteine solution exposed to UV irradiation, the formation of free cystine was observed at −0.792 V. In the cysteine and Cr(VI) mixture exposed to UV irradiation, the peak current of thiyl radical increases while the peak current of Cr(VI)-thioester reaches a maximum at 15 min and then decreases by increasing UV irradiation time. The formation of the thioester in the reaction between Cr(VI) and cysteine in aqueous media has been studied by monitoring the decrease of Cr(VI) at 370 nm. It was observed that the reaction is catalyzed by the UV irradiation of the Cr(VI) and cysteine mixture.

Reversed-phase HPLC with UV detection for the determination of N-acetylaspartate and creatine

Reversed-phase HPLC with UV detection for the determination of N-acetylaspartate and creatine

One-through selective separation of copper, chromium and zinc ions by hollow fiber supported liquid membrane

The separation of a mixture of Cu(II), Cr(VI) and Zn(II) simultaneously from a sulfate media using two consecutive hollow fiber microporous liquid membrane extraction systems has been studied. The experiments were made in the one-through mode. LIX84-I and Aliquat336 were used as carrier extractants for copper and chromium in the first and second hollow fiber modules, respectively. Pure copper and pure chromium ions are extracted and stripped from first and second hollow fiber modules, respectively, but zinc ions cannot be extracted by both extractants due to pH conditions used in this study and remain in the raffinate. Results indicate that the percentage of extraction is highly dependent on pH of the feed solution and the pH value is 2.5 for the maximum extraction. The percentage of extraction of copper and chromium ions is enhanced when the concentration of LIX84-I and Aliquat336 is increased of which both maximum value is 0.5 M, and these results also occur in stripping phase. The influence of sulfuric acid and sodium hydroxide concentration in strip solution of each column was examined, and it was found that the percentage of extraction and stripping is further increased due to the difference of driving force. The maximum percentage of extraction for copper is 33% and 92% for chromium. The experimental results indicated the feasibility of separation and recovery of these metals from the dilute solution by using membrane technology

Synthesis and Characterization of a Chromium(V) cis-Dioxo Bis(1,10-phenanthroline) Complex and Crystal and Molecular Structures of Its Chromium(III) Precursor

The first structurally characterized Cr(V) dioxo complex, cis-[CrV(O)2(phen)2](BF4) (2, phen=1,10-phenanthroline) has been synthesized by the oxidation of a related Cr(III) complex, cis-[Cr(III)(phen)2(OH2)2](NO3)3.2.5H2O (1, characterized by X-ray crystallography), with NaOCl in aqueous solutions in the presence of excess NaBF4, and its purity has been confirmed by electrospray mass spectrometry (ESMS), EPR spectroscopy, and analytical techniques. Previously reported methods for the generation of Cr(V)-phen complexes, such as the oxidation of 1 with PbO2 or PhIO, have been shown by ESMS to lead to mixtures of Cr(III), Cr(V), Cr(VI), and in some cases Cr(IV) species, 3. Species 3 was assigned as [CrIV(O)(OH)(phen)2]+, based on ESMS and X-ray absorption spectroscopy measurements. A distorted octahedral structure for 2 (CrO, 1.63 A; Cr-N, 2.04 and 2.16 A) was established by multiple-scattering (MS) modeling of XAFS spectra (solid, 10 K). The validity of the model was verified by a good agreement between the results of MS XAFS fitting and X-ray crystallography for 1 (distorted octahedron; Cr-O, 1.95 A; Cr-N, 2.06 A). Unlike for the well-studied Cr(V) 2-hydroxycarboxylato complexes, 2 was equally or more stable in aqueous media (hours at pH=1-13 and 25 degrees C) compared with polar aprotic solvents. A stable Cr(III)-Cr(VI) dimer, [Cr(III)(Cr(VI)O4)(phen)2]+ (detected by ESMS), is formed during the decomposition of 2 in nonaqueous media. Comparative studies of the oxidation of 1 by NaOCl or PbO2 have shown that [Cr(V)(O)2(phen)2]+ was the active species responsible for the previously reported oxidative DNA damage, bacterial mutagenicity, and increased incidence of micronuclei in mammalian cells, caused by the oxidation products of 1 with PbO2. Efficient oxidation of 1 to a genotoxic species, [Cr(V)(O)2(phen)2]+, in neutral aqueous media by a biological oxidant, hypochlorite, supports the hypothesis on a significant role of reoxidation of Cr(III) complexes, formed during the intracellular reduction of Cr(VI), in Cr(VI)-induced carcinogenicity. Similar oxidation reactions may contribute to the reported adverse effects of a popular nutritional supplement, Cr(III) picolinate.

Potential Remediation of Waters Contaminated with Cr(III), Cu, and Zn by Sorption on the Organic Polymeric Fraction of Olive Mill Wastewater (Polymerin) and Its Derivatives

A study on the individual sorption of Cr(III), Cu, and Zn on polymerin, the humic-acid-like fraction of olive mill wastewater, and its derivatives, K-polymerin and an Fe(OH)x-polymerin complex, showed that these heavy metals were strongly sorbed on polymerin and K-polymerin in the order Cr(III) > Cu > Zn. The sorption on Fe(OH)x-polymerin was to a lower extent compared with that of the other two sorbents, but to a higher extent compared with ferrihydride [Fe(OH)x]. Combined atomic absorption spectrometry and diffuse reflectance infrared Fourier transform spectroscopy analyses showed that the selected heavy metals were individually sorbed on polymerin by means of a cation exchange mechanism, which was consistent with the replacement of Ca, Mg, K, and H bound to the carboxylate groups of the biosorbent and the concomitant chelation of the heavy metals by the OH groups of polymerin polysaccharide component. In binary combination and equimolar ratio, Cu was sorbed by polymerin more selectively than Zn. In ternary combination and equimolar ratio, Cr(III), Cu, and Zn were sorbed by polymerin in the order Cr(III) > Cu > Zn. The sorbing capacity of Zn and Cu was strongly influenced by Cr(III), whereas the sorbing capacity of Cr(III) was not affected bythe presence of the other two metals. The overall sorbing capacity of the binary and ternary mixtures of the three metals on polymerin proved to be considerable and much greater than that on Fe(OH)x-polymerin. Simulated wastewaters contaminated with Cu and Zn were purified after three sorption cycles by polymerin renewed at each cycle, whereas those containing a mixture of Cr(III), Cu, and Zn showed residues of Zn after five cycles. We briefly discuss environmental and industrial advantages for a possible exploitation of polymerin.

Issues of low activation brazing of SiC f/SiC composites by using alloys without free silicon

The paper presents a novel low activation brazing technique for SiC f/SiC composites. The brazing alloy does not contain free silicon and is based on the use of a Si–44Cr at.% eutectic and the intermetallic CrSi 2 (melting temperatures 1390 and 1490 °C, respectively). These are advantageous because the melting point is low enough to avoid degradation of the advanced fibres and of the interphases in the composite, and the Si–Cr intermetallics are chemically compatible with silicon carbide. Both the eutectic and the intermetallic were prepared before brazing operations by melting a Si–Cr mixture. The joining was performed under vacuum (about 10 −4 Pa). Systematic investigations of the microstructure and of the nanochemistry (TEM, EELS, ELNES) of the Si–Cr joints reveal that direct chemical Si–Si, Cr–C and Si–Cr bonds across the interface are responsible for the adhesion: the interfaces were proved to be nearly atomically sharp and adhesive. Altogether, this brazing procedure enables joints with sufficient strength and with a microstructure comparable with that of the starting powders to be obtained.

Electrodeposition, composition and structure of Zn–Cr alloys

The effect of polyethylene glycol (PEG 1500) as additive and of deposition conditions on Zn—Cr alloy electrodeposition from an acidic sulfate electrolyte at room temperature, without agitation was investigated. PEG polarizes the overall cathodic reaction and inhibits Zn deposition. Cr codeposition with Zn starts at a cathodic potential of about −1,95 V vs Hg/Hg2SO4, which is reached at current density of about 20 A dm−2 in galvanostatic conditions. Zn—Cr alloy coatings containing up to 28 at % Cr were obtained depending on the plating conditions. SEM observations showed an island-like structure, formed by the local growth of crystals, which covered the surface during further deposition. In the first stages of electrodeposition the powder diffraction spectra contain lines of b.c.c. Γ-(Zn,Cr) phase (a Γ ≈ 3.02 A). After 30 s deposition time weak lines of Zn-based η phase (a η ≈ 2.67 A, c η ≈ 4.90 A) appear, and become clearly visible in coatings deposited for 90 s. The average Cr content in the alloy coatings decreases with advancing deposition. The as-plated surface contains C in organic compounds and Zn(OH)2. After 50 min sputtering, Zn and a mixture of Cr, Cr2O3 and Cr7C3 were found. The presence of organic C and O, probably from inclusions of PEG, were also detected.

Cr−Mo solid solutions forced by high-energy ball milling

Mixtures of Cr and Mo elemental powders, with the nominal compositions Cr25Mo75, Cr50Mo50, and Cr75Mo25, are processed by high-energy ball milling at ambient temperature. Milling is observed to force the mixing of the immiscible bcc elements Cr and Mo into solid solutions. The lattice parameter of these solid solutions, measured by X-ray diffraction (XRD), displays the expected positive deviation from Vegard’s law. These deviations are compared to the ones predicted by Eshelby’s inclusion model for dilute alloys. The conventional Williamson-Hall approach is shown to fail to determine the grain size in as-milled samples, probably due to the high density of dislocations. Annealing at 700 °C for 10 hours under argon leads to a large reduction in structural defect density, without inducing any significant decomposition. The mixing measured in Cr-Mo is discussed in the broader context of the mechanical mixing forced by ball milling in moderately immiscible systems.

Atom transfer as a preparative tool in coordination chemistry. Synthesis and characterization of Cr(V) nitrido complexes of bidentate ligands.

The transfer of a terminal nitrido ligand from Mn(V)(N)(salen) to Cr(III) complexes is explored as a new preparative route to Cr(V) nitrido complexes. Reaction of Mn(V)(N)(salen) with labile CrCl(3)(THF)(3) in acetonitrile solution precipitates [Mn(Cl)(salen)].(CH(3)CN) and yields a solution containing a mixture of Cr(V) nitrido species with only labile auxiliary ligands. From this solution Cr(V) nitrido complexes with bidentate monoanionic ligands can be obtained in high yields. Five coordinate complexes of 8-hydroxoquinolinate (quin), 1,3-diphenylpropane-1,3-dionate (dbm), and pyrrolidinedithiocarbamate (pyr-dtc) have been structurally characterized: Cr(N)(quin)(2) (1) crystallizes as compact orange prisms in the triclinic space group P with cell parameters a = 7.2450(6) A, b = 8.1710(4) A, c = 13.1610(12) A, alpha = 80.519(6) degrees, beta = 75.721(7) degrees, gamma = 75.131(5) degrees, V = 725.47(10) A(3), Z = 2. Cr(N)(dbm)(2) (2) crystallizes as green rhombs in the orthorhombic space group Pbca with cell parameters a = 14.6940(6) A, b = 16.4570(18) A, c = 19.890(3) A, V = 4809.8(8) A(3), Z = 8. Cr(N)(pyr-dtc)(2) (3) crystallizes as orange prisms in the monoclinic space group P21/c with cell parameters a = 14.8592(14) A, b = 8.5575(5) A, c = 11.8267(12) A, beta = 106.528(7) degrees, V = 1441.7(2) A(3), Z = 4. Complexes 2 and 3 represent new coordination environments for first row transition metal nitrido complexes. The d-orbital energy splitting in these systems with relatively weak equatorial donors differs significantly from the pattern in vanadyl and the previously known first row transition metal nitrido complexes. The d(x)2(-)(y)2 orbital in 2 and 3 is lower in energy and well resolved from the M-N pi orbitals [d(zx),d(yz)].

Effect of oxygen content and stearate additives on sintering characteristics of Cu-25Cr composite

The sintering behaviour of Cu-25 wt-%Cr contact materials produced by powder metallurgy is investigated with respect to the processing variables, such as oxygen content, sintering temperature, time, stearate additives and microstructural features. The sintering temperature of the mixture is determined from the temperature of the liquid phase formation by differential thermal analysis (DTA). Properties of sintered products prepared with and without the reduction treatment of chromium powders are compared in terms of sintered density and electrical conductivity. The initial oxygen content of the chromium powders is decreased from 4500 ppm to 200 ppm by a new method of reduction treatment developed previously and it is observed that the sintered density is increased from 95% theoretical density to 98% theoretical density while the corresponding electrical conductivity increased from 28% IACS to 39% IACS (International Annealed Copper Standard). The sintered density further increased to 99% theoretical density with an electrical conductivity level of 45% IACS by the addition of 0·5 wt-% lithium stearate to the Cu-25 wt-%Cr (200 ppm O2) mixture. By cold working and age hardening, the electrical conductivity is increased to 55% IACS with accomplishing a hardness value of 154 HV5 in the 50% cold worked samples.

Biosorption of binary mixtures of Cr(III) and Cu(II) ions by Sargassum sp

The adsorption of two metal ions, Cr(III) and Cu(II), in single-component and binary systems by Sargassum sp., a brown alga, was studied. Equilibrium batch sorption studies were carried out at 30ºC and pH 3.5. Kinetic tests were done for a binary mixture (chromium + copper) for a contact time of 72 hours to guarantee that equilibrium was reached. The monocomponent equilibrium data obtained were analyzed using the Langmuir and Freundlich isotherms. The binary equilibrium data obtained were described using four Langmuir-type and Freundlich isotherms. The F-test showed a statistically significant fit for all binary isotherm models. The parameters for isotherms of the Langmuir-type were used to determine the affinity of one metal for the biosorbent in the presence of another metal. The chromium ion showed a greater affinity for Sargassum sp. than the copper ion.

Synthesis and characterisation of heterometallic deca- and dodeca-nuclear metal wheels

The synthesis and crystallographic characterisation of two new cyclic molecules are reported. The molecules have the general formula [M(OMe) 2(O 2CMe)] n where n=10 or 12, and M is a mixture of Mn and Cr. For the decanuclear complex elemental analysis indicates, on average, presence of equal quantities of Mn and Cr. Chromatography combined with electrospray mass spectrometry suggests complexes containing differing ratios of the metal ions can be separated.