Showing papers on "Chromium published in 2003"

Removal of hexavalent chromium from wastewater using a new composite chitosan biosorbent.

Abstract: A new composite chitosan biosorbent was prepared by coating chitosan, a glucosamine biopolymer, onto ceramic alumina. The composite bioadsorbent was characterized by high-temperature pyrolysis, porosimetry, scanning electron microscopy, and X-ray photoelectron spectroscopy. Batch isothermal equilibrium and continuous column adsorption experiments were conducted at 25 degrees C to evaluate the biosorbent for the removal of hexavalent chromium from synthetic as well as field samples obtained from chrome plating facilities. The effect of pH, sulfate, and chloride ion on adsorption was also investigated. The biosorbent loaded with Cr(VI) was regenerated using 0.1 M sodium hydroxide solution. A comparison of the results of the present investigation with those reported in the literature showed that chitosan coated on alumina exhibits greater adsorption capacity for chromium(VI). Further, experimental equilibrium data were fitted to Langmuir and Freundlich adsorption isotherms, and values of the parameters of the isotherms are reported. The ultimate capacity obtained from the Langmuir model is 153.85 mg/g chitosan.

Corrosion Inhibition of Aluminum and Aluminum Alloys by Soluble Chromates, Chromate Coatings, and Chromate-Free Coatings

Abstract: This review covers the aqueous chemistry of chromium and the origins of chromium toxicity. Evidence from older and more recent studies is presented showing that inhibition of Al corrosion is derived from both inhibition of oxygen reduction and inhibition of metal dissolution reactions mainly due to a delay in the onset of pitting. Inhibition of corrosion by chromates appears to be closely linked to their ability to irreversibly adsorb on to metal and oxide surfaces. With respect to chromate conversion coatings (CCCs), the evolution of modern coating formulations is presented with a focus on key advances that led to process simplification and improved coating performance. The current understanding of CCC formation, protection, and aging is presented. In the last section of the paper, processing and properties of selected chromate-free conversion coating chemistries is discussed. This discussion focuses on coatings that offer particularly attractive attributes such as self-healing and excellent adh...

Removal of hexavalent chromium with a lignocellulosic substrate extracted from wheat bran.

Abstract: In this paper, a new recovery system of the toxic hexavalent chromium Cr(VI) is proposed that uses a lignocellulosic substrate derived from the industrial treatment process of wheat bran. We studied the adsorption mechanism of Cr(VI) onto the lignocellulosic substrate and showed that the adsorption reaction consumes a large amount of protons goes along the reduction of Cr(VI) into Cr(III). The oxidation of lignin moieties takes place concurrently to the chromium reduction and leads to the formation of hydroxyl and carboxyl functions. The latter contribute to an increase in the number of ion-exchange sites for the reduced chromium. The maximum adsorption capacity for hexavalent chromium was found at about 35 mg g-1 in an acidic medium. This is comparable to other natural substrates and ordinary adsorbents.

Fabrication of nickel and chromium nanoparticles using the protein cage of apoferritin

Abstract: The iron storage protein, apoferritin, has a cavity in which iron is oxidized and stored as a hydrated oxide core. The size of the core is about 7 nm in diameter and is regulated by the cavity size. The cavity can be utilized as a nanoreactor to grow inorganic crystals. We incubated apoferritin in nickel or chromium salt solutions to fabricate hydroxide nanoparticles in the cavity. By using a solution containing dissolved carbon dioxide and by precisely controlling the pH, we succeeded in fabricating nickel and chromium cores. During the hydroxylation process of nickel ions a large portion of the apoferritin precipitated through bulk precipitation of nickel hydroxide. Bulk precipitation was suppressed by adding ammonium ions. However, even in the presence of ammonium ions the core did not form using a degassed solution. We concluded that carbonate ions were indispensable for core formation and that the ammonium ions prevented precipitation in the bulk solution. The optimized condition for nickel core formation was 0.3 mg/mL horse spleen apoferritin and 5 mM ammonium nickel sulfate in water containing dissolved carbon dioxide. The pH was maintained at 8.65 using two buffer solutions: 150 mM HEPES (pH 7.5) and 195 mM CAPSO (pH 9.5) with 20 mM ammonium at 23 degrees C. The pH had not changed after 48 h. After 24 h of incubation, all apoferritins remained in the supernatant and all of them had cores. Recombinant L-ferritin showed less precipitation even above a pH of 8.65. A chromium core was formed under the following conditions: 0.1 mg/mL apoferritin, 1 mM ammonium chromium sulfate, 100 mM HEPES (pH 7.5) with a solution containing dissolved carbon dioxide. About 80% of the supernatant apoferritin (0.07 mg/mL) formed a core. In nickel and chromium core formation, carbonate ions would play an important role in accelerating the hydroxylation in the apoferritin cavity compared to the bulk solution outside.

DMAP/Cr(III) Catalyst Ratio: The Decisive Factor for Poly(propylene carbonate) Formation in the Coupling of CO2 and Propylene Oxide

Abstract: Highly efficient formation of poly(propylene carbonate) can be achieved in the coupling of CO 2 and propylene oxide assisted by 4-(N,N-dimethylamino)pyridine (DMAP) and catalyzed with salen chromium(III) chloride by using DMAP/Cr ratios of less than 2. Under these conditions a possible backbiting mechanism is suppressed, leading to only minor amounts of cyclic carbonate as a side product.

Chromium remediation or release? Effect of iron(II) sulfate addition on chromium(VI) leaching from columns of chromite ore processing residue.

Abstract: Chromite ore processing residue (COPR), derived from the so-called high lime processing of chromite ore, contains high levels of Cr(III) and Cr(VI) and has a pH between 11 and 12. Ferrous sulfate, which is used for remediation of Cr(VI) contamination in wastewater and soils via reduction to Cr(III) and subsequent precipitation of iron(III)/chromium(III) hydroxide, has also been proposed for remediation of Cr(VI) in COPR. Instead, however, addition of FeSO4 to the infiltrating solution in column experiments with COPR greatly increased leaching of Cr(VI). Leached Cr(VI) increased from 3.8 to 12.3 mmol kg(-1) COPR in 25 pore volumes with 20 mM FeSO4, reaching solution concentrations as high as 1.6 mM. Fe(II) was ineffective in reducing Cr(VI) to Cr(III) because it precipitated when it entered the column due to the high pH of COPR, while Cr(VI) in solution was transported away with the infiltrating solution. The large increase in leaching of Cr(VI) upon infiltration of sulfate, either as FeSO4 or Na2SO4, was caused by anion exchange of sulfate for chromate in the layered double hydroxide mineral hydrocalumite, a process for which scanning electron microscopy with energy-dispersive X-ray microanalysis provided direct evidence.

Role of Water Vapor in Chromia-Scale Growth at Low Oxygen Partial Pressure

Abstract: The oxidation behavior of pure chromium and ODS-Cr alloys in Ar-H2-H2O and Ar-O2-H2O was studied at 1000°C. At high oxygen potentials, the addition of H2O to the gas had negligible effect on the scaling behavior. However, at low oxygen potentials, when the pH2O/pH2 ratio was held constant, the oxidation rate increased with water partial pressure. Increasing values of pH2O/pH2 led to more rapid rates. At fixed pH2O values, the rate increased with increasing pH2. Compact scales were formed under all conditions. In addition Cr2O3 blades grew on the scale surface when pure chromium was reacted with H2O/H2 mixtures, but not in reaction with O2/H2O. These blades did not form when Y2O3 dispersion-strengthened material was reacted. A model, in which oxide growth was sustained by diffusion of chromium vacancies and adsorption of H2O on oxide exposed to low oxygen-activity gas led to the formation of hydroxyl species, explained most of the complex effects of gas composition on scale growth and blade formation. However, it failed to account for the observed increase in scaling rate with pH2 at fixed pH2O. The latter effect is ascribed to alteration of an additional contribution to diffusion from chromium interstitials.

Boron solubility in Fe–Cr–B cast irons

Abstract: Boron solubility in the as-cast and solution treated martensite of Fe-Cr-B cast irons, containing approximately 1.35 wt.% of boron, 12 wt.% of chromium, as well as other alloying elements, has been investigated using conventional microanalysis. The significant microstructural variations after tempering at 750 degreesC for 0.5-4 h, compared with the original as-cast and solution treated microstructures, indicated that the matrix consisted of boron and carbon supersaturated solid solutions. The boron solubility detected by electron microprobe was between 0.185-0.515 wt.% for the as-cast martensite and 0.015-0.0589 wt.% for the solution treated martensite, much higher than the accepted value of 0.005 wt.% in pure iron. These remarkable increases are thought to be associated with some metallic alloying element addition, such as chromium, vanadium and molybdenum, which have atomic diameters larger than iron, and expand the iron lattice to sufficiently allow boron atoms to occupy the interstitial sites in iron lattice. (C) 2002 Elsevier Science B.V. All rights reserved.

Chromium speciation by hyphenation of high-performance liquid chromatography to inductively coupled plasma-mass spectrometry—study of the influence of interfering ions

Abstract: This study presents the development of a speciation method for chromium redox species by HPLC-ICP-MS. Chromatographic separation is performed with CG5A-CS5A columns set (Dionex) containing both anionic and cationic groups to retain Cr(VI) and Cr(III), respectively, associated with a nitric acid elution. After optimisation, detection limits lower than 0.5 µg L−1 were obtained for both Cr species. This method is free from interferences possibly linked to chloride and organic or inorganic carbon on the two most abundant chromium isotopes (52Cr+ and 53Cr+). A modification of the chemistry of Cr(III) was, however, observed in the presence of hydrogencarbonate ions. In this medium, for trivalent chromium, three peaks instead of one are present on the chromatograms. This modification was attributed to the hydrolysis of Cr(III) that occurs at these pH values. The accuracy of the analytical procedure was verified by analysing a certified reference material (BCR-CRM 544) and spiked natural waters. The concentrations found for Cr(VI) were in good agreement with the certified and spiked values. Because of the high reactivity of Cr(III) in alkaline medium, recoveries were not completely quantitative.

The influence of complexing agent and proteins on the corrosion of stainless steels and their metal components.

Abstract: The present work is devoted to the problem of biodegradation of orthopaedic implants manufactured from stainless steel. In vitro simulations of the biocompatibility of two types of stainless steel, AISI 304 and AISI 316L, and their individual metal components, i.e. iron, chromium, nickel and molybdenum, were carried out in simulated physiological solution (Hank's) containing complexing agents. Knowledge of the effects of the chemical and biological complexing agents, EDTA and proteins, respectively, on the corrosion resistance of a metal should provide a better understanding of the processes occurring in vivo on its surface. The behavior of stainless steels and metal components was studied under open circuit and under potentiostatic conditions. The concentration of dissolved corrosion products in the form of released ions was determined by differential pulse polarography (DPP) and atomic emission spectrometry using inductively coupled plasma (ICP-AES). The composition of solid corrosion products formed on the surface was analyzed by energy dispersive X-ray spectroscopy (EDS) and their morphology was viewed by scanning electron microscopy (SEM). The addition of EDTA and proteins to physiological solution increased the dissolution of pure metals and stainless steels. The effect of particular protein differs on different metals and alloys.

Aerobic oxidations catalyzed by chromium corroles.

Abstract: Oxochromium(V) complexes of 5,10,15-tris(pentafluorophenyl)corrole and brominated derivatives oxygenate substrates (triphenylphosphine and norbornene) with concomitant production of chromium(III). Regeneration of CrVO by reaction of dioxygen with CrIII completes an aerobic catalytic cycle, with very large solvent effects; in acetonitrile, rapid initial turnovers observed initially are shut down by formation of CrIVO, while in toluene, THF, and methanol, relatively slow reactions are further inhibited by product formation.

Characterization of chromium species in catalysts for dehydrogenation and polymerization

Abstract: Chromium catalysts, prepared with different chromium contents, supports and precursor compounds, were characterized by O2 and CO chemisorption, in order to quantify the Cr3+ and Cr2+ sites, active species to dehydrogenation and ethylene polymerization reactions. Both probe molecules, O2 and CO, showed to be selective to the chromium active species, Cr3+ and Cr2+, respectively, and a valid method to quantify these sites. Distinct chromium species, Cr6+, amorphous Cr3+, crystalline Cr3+ and Cr2+, were characterized by XRD, DRS, TPR, FT-IR and DRIFTS in the calcined and reduced catalysts. The distribution of chromium species, which depends on the content, precursor compound and support, was related to the catalytic activity in the dehydrogenation and polymerization reactions. The catalytic activity to cyclohexane dehydrogenation depends on the dispersion of amorphous Cr3+ species. The activity in the ethylene polymerization is related to the amount of CrA2+ and CrB2+ species. Both sites showed similar activities.

The influence of complexing agent and proteins on the corrosion of stainless steels and their metal components

Abstract: The present work is devoted to the problem of biodegradation of orthopaedic implants manufactured from stainless steel. In vitro simulations of the biocompatibility of two types of stainless steel, AISI 304 and AISI 316L, and their individual metal components, i.e. iron, chromium, nickel and molybdenum, were carried out in simulated physiological solution (Hank's) containing complexing agents. Knowledge of the effects of the chemical and biological complexing agents, EDTA and proteins, respectively, on the corrosion resistance of a metal should provide a better understanding of the processes occurring in vivo on its surface. The behavior of stainless steels and metal components was studied under open circuit and under potentiostatic conditions. The concentration of dissolved corrosion products in the form of released ions was determined by differential pulse polarography (DPP) and atomic emission spectrometry using inductively coupled plasma (ICP-AES). The composition of solid corrosion products formed on the surface was analyzed by energy dispersive X-ray spectroscopy (EDS) and their morphology was viewed by scanning electron microscopy (SEM). The addition of EDTA and proteins to physiological solution increased the dissolution of pure metals and stainless steels. The effect of particular protein differs on different metals and alloys.