Showing papers on "Chromium published in 1991"

Processes affecting the remediation of chromium-contaminated sites.

Abstract: The remediation of chromium-contaminated sites requires knowledge of the processes that control the migration and transformation of chromium. Advection, dispersion, and diffusion are physical processes affecting the rate at which contaminants can migrate in the subsurface. Heterogeneity is an important factor that affects the contribution of each of these mechanisms to the migration of chromium-laden waters. Redox reactions, chemical speciation, adsorption/desorption phenomena, and precipitation/dissolution reactions control the transformation and mobility of chromium. The reduction of CrVI to CrIII can occur in the presence of ferrous iron in solution or in mineral phases, reduced sulfur compounds, or soil organic matter. At neutral to alkaline pH, the CrIII precipitates as amorphous hydroxides or forms complexes with organic matter. CrIII is oxidized by manganese dioxide, a common mineral found in many soils. Solid-phase precipitates of hexavalent chromium such as barium chromate can serve either as sources or sinks for CrVI. Adsorption of CrVI in soils increases with decreasing chromium concentration, making it more difficult to remove the chromium as the concentration decreases during pump-and-treat remediation. Knowledge of these chemical and physical processes is important in developing and selecting effective, cost-efficient remediation designs for chromium-contaminated sites.

Passivity breakdown and pitting corrosion of binary alloys

Abstract: PITTING corrosion—the localized dissolution of a passivated (oxide-covered) metal in the presence of a solution of certain anionic species—is a major cause of failure of metal structures. The breakdown of extremely thin (∼1 nm thick), highly stable passivating layers typically occurs in a sporadic, localized and stochastic fashion1,2, rather than as a catastrophic, global process. Using a model of a random binary iron–chromium alloy, we have shown previously3–6 that experimental observations of passivity of stainless steels can be explained by assuming that it is controlled by the selective dissolution of iron7. Thus if the chromium content is above a certain threshold (the percolation limit8), clusters of iron are finite and dissolution will proceed for a while and then stop. Oxidation of surface chromium atoms to form Cr—O—Cr linkages then creates a passive state in which the entire surface is covered with such a layer3,5. Here we show that, by adding to this model the further assumption that there is a small but finite dissolution rate for surface chromium atoms, one obtains a mechanism for the triggering of pitting corrosion of stainless steels that is consistent with experimental studies.

Reaction of chromium(VI) with glutathione or with hydrogen peroxide: identification of reactive intermediates and their role in chromium(VI)-induced DNA damage.

Abstract: The types of reactive intermediates generated upon reduction of chromium(VI) by glutathione or hydrogen peroxide and the resulting DNA damage have been determined. In vitro, reaction of chromium(VI...

The oxidation of iron-chromium-manganese alloys at 900C

Abstract: The oxidation of nine ternary iron-chromium-manganese alloys was studied at 900°C in an oxygen partial pressure of 26.7 kPa. The manganese concentration was set at 2, 6, and 10 wt. %, and chromium at 5, 12, and 20 wt. %. The scales formed on the low-chromium alloys consisted of (Mn,Fe)2O3, α-Fe2O3, and Fe3O4. These alloys all exhibited internal oxidation and scale detachment upon cooling. The scales formed on the higher-chromium alloys were complicated by nodule formation. Initially, these scales had an outer layer of MnCr2O4 with Cr2O3 underneath, adjacent to the alloy. With the passage of time, however, nodules formed, and the overall reaction rate increased. This tendency was more marked at higher manganese contents. Although these alloys contained a high chromium content, the product chromia scale usually contained manganese. It was concluded that the presence of manganese in iron-chromium alloys had an adverse effect on the oxidation resistance over a wide range of chromium levels.

The chemistry of chromium and some resulting analytical problems.

Abstract: Chromium, named for its many-colored compounds, exists in the oxidation states of -2 to +6 inclusively. The compounds exhibit a wide range of geometries including square planar, tetrahedral, octahedral, and various distorted geometries. Chromium is found in nature principally as the chromite ore FeCr2O4 in which chromium is in the +3 state. The existence of a particular oxidation state is dependent on many factors including pH, redox potentials, and kinetics. Thermodynamically, +3 and +2 are the most stable states, while the +3 and +6 oxidation states are the most common ones found in aqueous solution. Kinetically, chromium +3 is substitutionally inert: for water exchange k(sec-1) = 2.5 x 10(-6), due to the presence of the half-filled d(t2g)3.4A2g state. On the other hand, protonation/deprotonation is quite rapid. Polymerization is very slow but is promoted at higher pHs; acid cleavage of the protonated oligomers is also quite slow. Chromium +6 as the chromate ion is strongly oxidizing at low pHs and less so in basic solution. The chromate ion does form some polyacids and polyanions. These factors must be considered in analyzing samples for total chromium and for the amounts of each oxidation state.

Complexing of actin and other nuclear proteins to DNA by cis-diamminedichloroplatinum(II) and chromium compounds

Abstract: Actin was found to be the major protein crosslinked to the DNA of intact Chinese hamster ovary cells that were treated with either potassium chromate (hexavalent) or cis-diamminedichloroplatinum(II) (cis-platinum). This protein was identified as actin by its mol. wt (45 kd), its isoelectric point (pI = 5.4), positive reactivity with an actin antibody, and by protease mapping. Additionally, a purified actin standard migrated to the same location in a two-dimensional gel system as p45. Actin comprised approximately 20% of the protein component in chromate-induced DNA-protein crosslinks. In addition, to actin, several other major proteins (e.g. 53 kd, pI = 5.2, 50 kd, pI = 9) were crosslinked to DNA following exposure to cis-platinum or chromate. These proteins were abundant in the nuclear matrix fraction. Hexavalent chromate is the toxicologically active form because it is readily taken up into cells by an anion transport system. In contrast, trivalent chromium is considerably less toxic because it cannot enter the cell; however, most of the hexavalent form is eventually reduced to the trivalent form inside the cell. Previous studies have suggested that the trivalent form of chromium participates in complexing DNA with proteins. DNA-protein crosslinks were formed in isolated nuclei or in mixtures of purified DNA and protein incubated with trivalent chromium. However, the formation of these complexes required at least 16 h of incubation to exchange the parent compound ligands. Hexavalent chromate did not form these complexes in vitro under similar conditions. Incubation of trivalent chromium with purified actin and DNA resulted in DNA-actin crosslinks as detected by an electrophoretic mobility different from that of either free actin or DNA when the complex was transferred from a gel to nitrocellulose and stained for protein. These studies describe a new technique for detecting DNA-protein complexes and demonstrate that actin-DNA structures in intact cells create sites that selectively react with metal DNA-protein crosslinking agents.

Adsorption op cadmium and chromium from wastewater by flyash

Abstract: The removal of cadmium and chromium from wastewater by adsorption on fly ash was investigated to determine the effects of contact time, pH, and temperature. Removals of cadmium and chromium attained an equilibrium in three hours. The alkaline (pH 7 to 8) aqueous medium favoured the removal of cadmium by fly ash, while chromium removal was maximum in the pH range of 2.0 to 3.0. Batch adsorption experiments conducted at 5°C, 10°C, and 21°C showed that adsorption capacity of fly ash with respect to cadmium and chromium decreased with an increase in temperature. The data for the adsorption of cadmium by fly ash fitted well the Langmuir isotherm while the removal of chromium by fly ash followed generally the Freundlich isotherm. The maximum removals of cadmium and chromium by fly ash were 93% and 44% respectively.

Chromium(III) bound to DNA templates promotes increased polymerase processivity and decreased fidelity during replication in vitro.

Abstract: Carcinogenic chromium [Cr(VI)] compounds are reduced intracellularly to DNA- and protein-reactive chromium(III) species. However, the role of Cr(III) ions in chromium-induced genotoxicity remains unclear. We have investigated the effects of chromium(III) binding on DNA replication and polymerase processivity in vitro. Chromium ions bind slowly and in a dose-dependent manner to DNA. Micromolar concentrations of free chromium inhibit DNA replication, but if the unbound chromium is removed by gel filtration, the rate of DNA replication by polymerase I (Klenow fragment) on the chromium-bound template is increased greater than 6-fold relative to the control. This increase is paralleled by as much as a 4-fold increase in processivity and a 2-fold decrease in replication fidelity. These effects are optimum when very low concentrations of chromium ions are bound to the DNA [3-4 Cr(III) ions per 1000 nucleotide phosphates]. Increased concentrations of chromium lead to the production of DNA-DNA cross-links and inhibition of polymerase activity. These results suggest that low levels of DNA-bound chromium(III) ions may contribute to chromium mutagenesis and carcinogenesis by altering the kinetics and fidelity of DNA replication.

Activation of chromium(VI) by thiols results in chromium(V) formation, chromium binding to DNA and altered DNA conformation.

Abstract: The ability of the thiols glutathione, cysteine, beta-mercaptoethanol and dithiothreitol to effect chromium(VI)-induced DNA damage in vitro has been investigated. Reaction of pBR322 DNA with chromium(VI) in the presence of the thiols led to formation of chromium(V) and chromium--DNA adducts. The extent of chromium binding to DNA differed by several orders of magnitude among the thiols tested, in the order dithiothreitol greater than beta-mercaptoethanol much greater than cysteine greater than or equal to glutathione. The maximal level of chromium(V) formed also differed among the thiols tested, in the order beta-mercaptoethanol greater than dithiothreitol much greater than glutathione greater than or equal to cysteine. Electronic spectral studies of these reactions indicated that the rate of reduction of chromium(VI) is dependent on the thiol tested, in the order cysteine greater than dithiothreitol greater than glutathione greater than beta-mercaptoethanol. Electron paramagnetic resonance studies of these reactions indicate that a significant level of chromium(III) is detected only with cysteine. Chromium--DNA adducts formed by reaction of chromium(VI) in the presence of glutathione or cysteine did not lead to DNA conformational changes detectable upon agarose gel electrophoretic analysis. Changes in DNA conformation were detected as altered electrophoretic mobility of pBR322 DNA on agarose gels after reaction with chromium(VI) in the presence of dithiothreitol or beta-mercaptoethanol. Effects on DNA electrophoretic mobility, which depended on whether the initial conformation of the plasmid was linear or supercoiled, included altered and heterogeneous mobility, as well as complete inhibition of migration of the plasmid. Transmission electron microscopy of chromium--DNA complexes revealed aggregates of several plasmids, as well as condensation of individual plasmids into compact kinked forms. These effects may be due to cross-linking of DNA induced by chromium metabolites. These studies indicate that the levels of chromium bound to DNA are related to the levels and stabilities of the chromium(V) species formed upon reaction of chromium(VI) with the various thiols. Chromium--thiol interactions may play an important role in chromium(VI) genotoxicity.

Effects of vitamins on chromium(VI)-induced damage.

Abstract: The effects of vitamin E and vitamin B2 on DNA damage and cellular reduction of chromium(VI) were investigated using Chinese hamster V-79 cells. Pretreatment with alpha-tocopherol succinate (vitamin E) resulted in a decrease of DNA single-strand breaks produced by Na2CrO4, while similar treatment with riboflavin (vitamin B2) enhanced levels of DNA breaks. In contrast, levels of DNA-protein crosslinks induced by Na2CrO4 were unaffected by these vitamins. Electron spin resonance (ESR) studies showed that incubation of cells with Na2CrO4 resulted in the formation of both chromium(V) and chromium(III) complexes, and cellular pretreatment with vitamin E reduced the level of the chromium(V) complex, whereas pretreatment with vitamin B2 enhanced the level of this intermediate. However, the levels of chromium(III) were unchanged by these vitamins. The uptake of chromate was not affected by vitamin E or vitamin B2, nor were the levels of glutathione or glutathione reductase activity, which are both capable of reducing chromate. ESR studies demonstrated that a chromium(V) species was formed by the reaction of Na2CrO4 with vitamin B2 and that vitamin B2 enhanced the formation of hydroxyl radicals during the reaction of Na2CrO4 and hydrogen peroxide. Treatment cells with Na2CrO4 resulted in a decrease of glutathione reductase activity, and pretreatment with vitamin E restored the enzyme activity suppressed by this metal. However, pretreatment with vitamin B2 enhanced the inhibition of this enzyme by Na2CrO4. Using a colony-forming assay, pretreatment with vitamin E dramatically decreased the cytotoxicity of Na2CrO4, while pretreatment with vitamin B2 was found to result in only a decrease of cell lethality of this metal.(ABSTRACT TRUNCATED AT 250 WORDS)

Infrared spectroscopic evidence for second-sphere hydration in aqueous solutions of aluminum(3+), chromium(3+) and rhodium(3+)

Abstract: The hydration of the chromium(III), rhodium(III), and aluminum(III) ions in aqueous perchlorate solution has been studied by vibrational spectroscopy in both the OD (2,900-1,800 cm{sup {minus}1}) and OH (4,000-2,800 cm{sup {minus}1}) stretching regions, using an infrared absorption double-difference method. By isotopic substitution of hydrogen atoms it has been possible to extract spectra of ion-perturbed HDO molecules. The OD and OH stretching spectra obtain bands that can be assigned to the first and second hydration spheres of the cations and are consistent with recent results from solution X-ray and diffusion studies.

Stereoselective synthesis of homochiral alpha substituted o-methoxybenzyl alcohols via nucleophilic additions to kinetically resolved homochiral tricarbonyl (η6-o-anisaldehyde)chromium(0).

Abstract: Tricarbonyl(η6-o-anisaldehyde)chromium(0) has been kinetically resolved via the selective hydrolysis of one of theL-valinol derived imine presence of deactivated alumina. An X-ray crystal structure analysis on the adduct formed between L-valinol and homochiral (+)-tricarbonyl(η6-o-anisaldehyde)chromium(0) unambiguously established the presence of the corresponding imine and not oxazolidine and confirmed the absolute configuration of the aldehyde complex. Addition of Grignard reagents to homochiral tricarbonyl(η6-o-anisaldehyde)chromium(0) occurs completely stereoselectively to give, following decomplexation, homochiral alpha substituted o-methoxybenzyl alcohols.

Sulfur segregation during the high-temperature oxidation of chromium

Abstract: A TEM and STEM examination has been carried out of cross-sectioned specimens of convoluted chromia scale formed by oxidizing chromium at 950° C. Sulfur was detected at the oxide/metal interface and the oxide grain boundaries (apart from low-angle grain boundaries), but not within the oxide grains. These results are consistent with the “sulfur effect” theory.

Formation of metastable L12 phases in Al3Zr and Al-12.5%X-25%Zr (X ≡ Li, CR, Fe, Ni, Cu)

Abstract: The intermetallic Al3Zr crystallizes in the tetragonal DO23 structure but can be produced as a metastable cubic L12 structure by rapid solidification We have studied the effects of mechanical alloying, a low-temperature isothermal processing method, and ternary additions of lithium, chromium, iron, nickel and copper on the phase stability of the L12 phase Mechanically alloyed Al3Zr is single-phase L12 which is stable up to 550 °C The additions of lithium or chromium increase the stability of the L12 phase to 750 and 740 °C, respectively The addition of iron or nickel leads to the formation of amorphous, rather than L12, phases Mechanically alloyed Al-125%Cu-25%Zr is single-phase L12 and this phase is stable up to at least 1300 °C