Showing papers on "Chromium published in 2008"

Stoichiometry of Cr(VI) Immobilization Using Nanoscale Zerovalent Iron (nZVI): A Study with High-Resolution X-Ray Photoelectron Spectroscopy (HR-XPS)

Abstract: Remediation of chromium-contaminated sites presents both technological and economic challenges as conventional methods are often too expensive in removing chromium in the soil matrix such as chromium ore process residue (COPR). In this work, reduction and precipitation of hexavalent chromium [Cr(VI)] by nanoscale zerovalent iron (nZVI) are evaluated. Cr(VI) is rapidly reduced and immobilized on the iron nanoparticle surface. In the pH range of 4 to 8, the nZVI has a chromium removal capacity ranging from 180 to 50 mg Cr/g nZVI. Under similar conditions, microscale iron particles (100 mesh) typically have a capacity of less than 4 mg Cr/g Fe. Characterizations with high-resolution X-ray photoelectron spectroscopy (HR-XPS) indicate that Cr(VI) is reduced to Cr(III), which is subsequently incorporated into the iron oxyhydroxide shell of nZVI and form alloy-like Cr−Fe hydroxides with a representative formula approximating (Cr0.67Fe0.33)(OH)3 or Cr0.67Fe0.33OOH. The Cr−Fe hydroxide shell is relatively stable a...

Mechanisms of bacterial resistance to chromium compounds.

Abstract: Chromium is a non-essential and well-known toxic metal for microorganisms and plants. The widespread industrial use of this heavy metal has caused it to be considered as a serious environmental pollutant. Chromium exists in nature as two main species, the trivalent form, Cr(III), which is relatively innocuous, and the hexavalent form, Cr(VI), considered a more toxic species. At the intracellular level, however, Cr(III) seems to be responsible for most toxic effects of chromium. Cr(VI) is usually present as the oxyanion chromate. Inhibition of sulfate membrane transport and oxidative damage to biomolecules are associated with the toxic effects of chromate in bacteria. Several bacterial mechanisms of resistance to chromate have been reported. The best characterized mechanisms comprise efflux of chromate ions from the cell cytoplasm and reduction of Cr(VI) to Cr(III). Chromate efflux by the ChrA transporter has been established in Pseudomonas aeruginosa and Cupriavidusmetallidurans (formerly Alcaligenes eutrophus) and consists of an energy-dependent process driven by the membrane potential. The CHR protein family, which includes putative ChrA orthologs, currently contains about 135 sequences from all three domains of life. Chromate reduction is carried out by chromate reductases from diverse bacterial species generating Cr(III) that may be detoxified by other mechanisms. Most characterized enzymes belong to the widespread NAD(P)H-dependent flavoprotein family of reductases. Several examples of bacterial systems protecting from the oxidative stress caused by chromate have been described. Other mechanisms of bacterial resistance to chromate involve the expression of components of the machinery for repair of DNA damage, and systems related to the homeostasis of iron and sulfur.

Efficient Removal of Cr(VI) from Aqueous Solution with Fe@Fe2O3 Core−Shell Nanowires

Abstract: The batch removal of Cr(VI) from simulated wastewater with Fe@Fe2O3 core-shell nanowires (FCSNs) was investigated in this study. Itwas found that each gram of the FCSNs could remove 7.78 mg of Cr(VI) from simulated wastewater containing 8.0 mg L(-1) of Cr(VI) with an initial pH of 6.5 at room temperature. The Freundlich adsorption isotherm was applicable to describe the removal processes. Kinetics of the Cr(VI) removal was found to follow pseudo-second-order rate equation. Furthermore, the as-prepared and Cr(VI)-adsorbed FCSNs were carefully examined by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopic analysis (XPS). The characterization results suggested that the adsorbed Cr(VI) was partially reduced to Cr(lll) in Cr2O3/Cr(OH)3 on the FCSNs. The possible mechanism of removal of Cr(VI) on FCSNs was proposed, which involved the dominant Cr(VI) adsorption, followed by the partial reduction of Cr(VI) to Cr(III) (chromium(III) oxyhydroides) on the surface of FCSNs. These Fe@Fe2O3 core-shell nanowires with high specific surface area and strong magnetic property are very attractive for the removal of Cr(VI) from wastewater.

XAS and XPS studies on chromium-binding groups of biomaterial during Cr(VI) biosorption.

Abstract: The reduction of toxic Cr(VI) to the less or nontoxic Cr(III) may be an useful detoxification technique for the treatment of Cr(VI)-contaminated waters. Recently, the protonated biomass of brown seaweed, Ecklonia, was shown to completely reduce Cr(VI) to Cr(III) in the pH range 1-5. The reduction of Cr(VI) to Cr(III) appeared to occur at the surface of the biomass. In this study, abiotic Cr(VI) reduction by the biomass was performed with various contact times, pHs and initial Cr(VI) concentrations, and surface and bulk characteristics of the Cr-laden biomass was then investigated using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The XPS spectra indicated that the Cr(VI) bound to the biomass was completely reduced to Cr(III) at tested various conditions. XANES and EXAFS spectra of the Cr-laden biomass were very similar to those of Cr(III)-acetate, which means that the Cr bound to the biomass during Cr(VI) reduction had an octahedral geometrical arrangement. The bonding distance of the chromium oxygen atoms was approximately 1.97-1.99 A. In conclusion, it was obvious that oxygen containing groups, such as carboxyl and phenolic groups, play a major role in the binding of the Cr(III) resulting from the abiotic reduction of Cr(VI) by the biomass.

Adsorption of hexavalent chromium from aqueous solutions by wheat bran

Abstract: In this research, adsorption of chromium (VI) ions on wheat bran has been studied through using batch adsorption techniques. The main objectives of this study are to 1) investigate the chromium adsorption from aqueous solution by wheat bran, 2) study the influence of contact time, pH, adsorbent dose and initial chromium concentration on adsorption process performance and 3) determine appropriate adsorption isotherm and kinetics parameters of chromium (VI) adsorption on wheat bran. The results of this study showed that adsorption of chromium by wheat bran reached to equilibrium after 60 min and after that a little change of chromium removal efficiency was observed. Higher chromium adsorption was observed at lower pHs, and maximum chromium removal (87.8 %) obtained at pH of 2. The adsorption of chromium by wheat bran decreased at the higher initial chromium concentration and lower adsorbent doses. The obtained results showed that the adsorption of chromium (VI) by wheat bran follows Langmuir isotherm equation with a correlation coefficient equal to 0.997. In addition, the kinetics of the adsorption process follows the pseudo second-order kinetics model with a rate constant value of 0.131 g/mg.min The results indicate that wheat bran can be employed as a low cost alternative to commercial adsorbents in the removal of chromium (VI) from water and wastewater.

Removal of chromium (VI) by acid-washed zero-valent iron under various groundwater geochemistry conditions.

Abstract: The hexavalent chromium (Cr(VI)) removal capacity of acid-washed zerovalent iron (AW-Fe0) was evaluated under different groundwater geochemistry conditions through column experiments. It was found that each gram of the AW-Fe0 could remove 0.65-1.76 mg of Cr(VI) from synthetic groundwater in the absence of bicarbonate (HCO3-), magnesium and/or calcium ions. Groundwater geochemistry was found to exert various degrees of impact on Cr(VI) removal by the AW-Fe0, in which HCO3- alone gave the mildest impact whereas the copresence of calcium and HCO3- exerted the greatest impact In comparison with the unwashed Fe0, the AW-Fe0 showed a poorer Cr(VI) removal capacity and was also more susceptible to the influence of the dissolved groundwater constituents on Cr(VI) removal,thereby indicating the unsuitability of using AW-Fe0 in permeable reactive barriers for remediation of Cr(VI)-contaminated groundwater. On the AW-Fe0 surface, where the indigenous iron precipitates were almost erased, trivalent chromium including chromium (III) oxides, hydroxides, and oxyhydroxides in irregular strip, chick footmark-liked or boulder-liked forms as well as Cr(III)-Cr(VI) mixed oxides were detected.

Visible light induced photocatalytic activity of rare earth titania nanocomposites

Abstract: Rare earth metal oxides doped TiO2 were prepared by an incipient wetness impregnation method by varying different mol% of La, Nd and Pr and characterized by various advanced techniques such as powder X-ray diffraction (PXRD), BET-surface area, N2 adsorption–desorption measurements, UV–vis DRS, FTIR and scanning electron microscope (SEM). Analytical results demonstrated that the TiO2 nanoparticles are mesoporous in nature and doping of lanthanides could inhibit the phase transformation, increases the surface area and decreases the crystallite size of mesoporous structures of TiO2. We investigated the visible light induced photocatalytic activities of these materials towards reduction of hexavalent chromium and degradation of methylene blue for the first time. La3+-TiO2 containing 0.4 mol% lanthanum, activated at 673 K showed highest surface area (124.8 m2/g), lowest crystallite size (8 nm) and exhibits highest photocatalytic activity. The reduction of hexavalent chromium and methylene blue degradation rate followed first-order kinetics.

Chromium(VI) reduction kinetics by zero-valent iron in moderately hard water with humic acid: iron dissolution and humic acid adsorption.

Abstract: In zerovalent iron treatment systems, the presence of multiple solution components may impose combined effects that differ from corresponding individual effects. The copresence of humic acid and hardness (Ca2+/Mg2+) was found to influence Cr(VI) reduction by Fe0 and iron dissolution in a way different from their respective presence in batch kinetics experiments with synthetic groundwater at initial pH 6 and 9.5. Cr(VI) reduction rate constants (kobs) were slightly inhibited by humic acid adsorption on iron filings (decreases of 7–9% and 10–12% in the presence of humic acid alone and together with hardness, respectively). The total amount of dissolved Fe steadily increased to 25 mg L−1 in the presence of humic acid alone because the formation of soluble Fe−humate complexes appeared to suppress iron precipitation. Substantial amounts of soluble and colloidal Fe−humate complexes in groundwater may arouse aesthetic and safety concerns in groundwater use. In contrast, the coexistence of humic acid and Ca2+/Mg2...

Metal-metal distances at the limit: a coordination compound with an ultrashort chromium-chromium bond.

Abstract: The nature of the chemical bond is of fundamental importance, and has always fascinated scientists. Metal–metal bonds are of particular interest, as bond orders greater than four are known and are of considerable current interest. The quest for the shortest metal–metal bond is strongly linked with the element chromium and has very recently been reinitiated after the first observation of a bond order greater than four for this metal in a stable compound. Soon afterwards, the shortest metal–metal bond with a chromium–chromium distance of 1.80 ( was observed in a dimeric chromium complex with such a high bond order. Detailed studies on ArCrCrAr complexes (Ar= aryl) performed at the same time showed that such small values can be obtained for this class of compounds as well. Some years ago, we started working with aminopyridinato complexes of chromium and herein report the synthesis and the (electronic) structure of a bimetallic Cr2 complex with a drastically shortened metal– metal distance. The very short metal–metal bond of only 1.75 ( results from a combination of Power3s concept for the stabilization of bond orders higher than four, Hein– Cotton3s principles on the realization of extremely short metal–metal bonds with bridging anionic ligands of type XYZ, and a minimization of additional metal–ligand interactions by optimal steric shielding (Scheme 1). The deprotonation of 1 with potassiumhydride leads to potassium [6-(2,4,6-triisopropylphenyl)pyridin-2-yl](2,4,6-trimethylphenyl)amide, which readily reacts with [CrCl3(thf)3] affording complex 2 (Scheme 2). Compound 2 can be isolated as a green crystalline material in good yield. In the H NMR spectrum, only broad signals can be observed, and magnetic susceptibility experiments show a magnetic moment meff(300 K)= 3.2 mB. When 1 is deprotonated with BuLi and allowed to react with CrCl2 in THF, the Cr II 2 complex 3 is obtained in good yield as a green crystalline material after removal of the solvent and subsequent extraction with

Electrofinishing of metals using eutectic based ionic liquids

Abstract: Ionic liquids are molten salts that have low melting points. This review concentrates upon eutectic mixtures of quaternary ammonium salts and metal salts (usually halides). These so-called eutectic based ionic liquids can be used for the electrodeposition and dissolution of a wide range of metals and alloys. The review highlights the technological challenges involved in the application of ionic liquids to metal finishing.

Rare earth-first-row transition metal perovskites as catalysts for the autothermal reforming of hydrocarbon fuels to generate hydrogen

Abstract: Perovskite oxides (ABO 3 ) containing rare earth elements on the A-site and first-row transition metal elements on the B-site were studied as catalysts for autothermal reforming of liquid hydrocarbon fuels to produce hydrogen for fuel cell systems. Experiments were conducted in a fixed bed microreactor at temperatures of 600–800 °C and gas-hourly space velocities (GHSV) ranging from 4600 to 28,000 h −1 using 2,2,4-trimethylpentane (isooctane) as a surrogate fuel. We have found that the two binary oxides, LaNiO 3 and LaCoO 3 , produced high yields of H 2 , but were not structurally stable. These perovskites decomposed to La 2 O 3 and Ni/NiO or Co/CoO under the reducing conditions present in the reformer. Three other binary oxides, LaCrO 3 , LaFeO 3 , and LaMnO 3 , were structurally stable but significantly less active than LaNiO 3 and LaCoO 3 . The partial substitution of chromium, iron, aluminum, gallium, or manganese on the B-site of LaNiO 3 to yield LaB x Ni 1− x O 3 was shown to improve the structural stability without a significant decrease in the H 2 yield. The effects of substituting rare earth elements for La and the substitution of alkaline earth elements on the A-site (La 1− y A y B x Ni 1− x O 3 ) on catalyst performance and stability were also investigated. Finally, La 0.8 Sr 0.2 M 0.9 Ni 0.1 O 3 catalysts (where M = Cr, Mn, or Fe) were tested with a “benchmark fuel” mixture containing from 0 to 50 ppmw sulfur. These tests showed that using chromium as a stabilizing element in LaNiO 3 imparts the most sulfur tolerance.