Flux Of Chromium Research Articles

Effects of chironomid larvae and Limnodrilus hoffmeisteri bioturbation on the distribution and flux of chromium at the sediment-water interface

The impacts of chironomid larvae and the tubificid worm Limnodrilus hoffmeisteri on the distribution and flux of the heavy metal chromium (Cr) across the sediment-water interface were investigated with a 21-day laboratory microcosm experiment. The two studied species feature different bioturbation modes involving bioirrigation and upward bioconveyance. The Cr concentrations in the overlying water and pore water were measured and compared using treatments with bioturbation by a single species and by combinations of both species and a treatment with no organisms. The results indicated that both bioturbation modes significantly increased the Cr concentrations in the overlying water and pore water. The overlying water had lower Cr concentrations than the pore water. Little variation in the Cr concentrations was observed in the treatment without organisms. Both species enhanced the Cr flux from the pore water to the overlying water. The worm treatments had a great impact on the Cr concentration in the overlying water through intensive upward conveyance activity, while the chironomid larvae treatments exerted significant effects on the Cr variation in the pore water and Cr flux across the interface via bioirrigation activity. These findings reveal the importance of bioturbation in biogeochemical processes in freshwater ecosystems.

Effect of Atmosphere and Sample Thickness on Kinetics, Microstructure, and Compressive Stresses of Chromia Scale Grown on Ni–25Cr

Most high-temperature alloys rely on the formation of a protective chromia scale during service. Generally, the oxidation resistance is mainly influenced or limited by corrosive environments around the alloy. In the present study, the evaluation of kinetic data is connected to microstructural analyses and stress measurements of the Cr2O3-layer formed on the Ni–25Cr model alloy after 1000 °C of isothermal exposure to atmospheres with and without water vapor and a varying (1 and 0.01%) oxygen content. Significant differences in oxide morphology, scale thickness, and oxide grain appearance could be observed. The results, with respect to oxidation kinetics and microstructures, showed a high tendency of variation when water vapor is present in the test gas. It was found that the addition of water vapor to the dry gas leads to decreased oxidation rates and reduced chromia scale thicknesses. The influence of the gas composition on the defect structure, the chromium flux, and diffusion of gas species is discussed on the basis of classical oxidation theory.

Chromium fluxes and speciation in ultramafic catchments and global rivers

The role of ultramafic rock weathering in global chromium (Cr) budgets and how these may be linked to changes in Earth's climate over geological time was investigated by compiling dissolved Cr speciation and fluxes in (i) 73 of the worlds large rivers, representing ~35% of global river discharge to oceans, and in (ii) an ultramafic catchment (Putah Creek watershed) in the California Coast Range Mountains, USA. Weathering of ultramafic rocks creates ideal conditions for Cr release and redox cycling. Alkaline river water draining ultramafic catchments is naturally enriched in Cr (up to 582nmol/L). Chromium concentrations increase with the extent of rock–water interaction (as indicated by correlations with Mg and HCO3− concentrations and pH), whereas Cr cycling depends on the availability of electron donors and acceptors (e.g. Fe(II), organics, Mn(III/IV)-oxides). Thus, dissolved Cr is exported from ultramafic watersheds as both the soluble hexavalent Cr(VI) species (MgCrO4, CrO42−), and also as trivalent Cr(III) species (CrOH(CO3)22−), Cr(OH)3). The latter Cr(III) species were previously thought to have low solubility. Ultramafic catchments have higher area normalized Cr and major ion fluxes for a given runoff when compared to global rivers and may thus have a disproportional impact on global Cr-budgets, long-term carbon cycling and moderation of the Earth's climate over million-year timescales. Riverine export fluxes of Cr are linearly correlated with fluxes of Ca+Mg and alkalinity in both ultramafic catchments and global rivers. This suggests that silicate weathering is a key control on Cr fluxes and that major element weathering fluxes may be a useful proxy for estimating Cr and other trace element fluxes. Globally, present-day riverine dissolved Cr fluxes to oceans are spatially variable and estimated to be 1.7×109mol/yr, three times higher than previously reported. However, when geochemical reactions in estuaries are considered riverine Cr fluxes may be lower. Throughout Earth history, Cr weathering fluxes also may have varied with changes in the global distribution of ultramafic rocks, Cr concentrations in continental rocks, or climactic conditions (e.g. atmospheric CO2 levels or runoff). Such variation should be considered when interpreting both modern and past Cr seawater isotopic compositions and residence times.

Characterization of bulk hexagonal boron nitride single crystals grown by the metal flux technique

The optical and physical properties of hexagonal boron nitride single crystals grown from a molten metal solution are reported. The hBN crystals were grown by precipitation from a nickel–chromium flux with a boron nitride source, by slowly cooling from 1500°C at 2–4°C/h under a nitrogen flow at atmospheric pressure. The hBN crystals formed on the surface of the flux with an apparent crystal size up to 1–2mm in diameter. Individual grains were as large as 100–200µm across. Typically, the flakes removed from the metal were 6–20µm thick. Optical absorption measurements suggest a bandgap of 5.8eV by neglecting the binding energy of excitons in hBN. The highest energy photoluminescence peak was at 5.75eV at room temperature. The hBN crystals typically had a pit density of 5×106cm−2 after etching in a molten eutectic mixture of potassium hydroxide and sodium hydroxide. The quality of these crystals suggests they are suitable as substrates for two dimensional materials such as graphene and gallium nitride based devices.

Removal of chromium(III) from acidic aqueous solution by polymer inclusion membranes with D2EHPA and Aliquat 336

A facilitated transport of chromium(III) through polymer inclusion membrane (PIM) containing 2-ethylhexylphosphonic acid (D2EHPA) as an ion carrier and tricaprylylmethylammonium chloride (Aliquat 336) as a plasticizer was investigated. In order to optimize conditions of the membrane transport, the initial flux of chromium(III) ions was considered as a function of acidity and temperature of source phase, membrane thickness and amount of plasticizer in the membrane. The highest values of initial flux of chromium(III) ions were obtained for source phase pH equal to 4.0 and for 1.5 cm 3 Aliquat 336 per 1.0 g cellulose triacetate (CTA) in the membrane phase. The activation energy value found from Arrhenius plot to be equal 110 kJ/mol suggests that the transport process is controllable by chemical reaction. The competitive transport of Cr(III) and Cr(VI) across PIMs consisting of 29% CTA, 19% D2EHPA and 52% Aliquat 336 from aqueous source phase with two different pH values 1.0 and 4.0 was also carried out. It was found that at pH 1.0 the removal of Cr(VI) ions is more efficient in comparison with Cr(III), while at pH 4.0 the reverse selectivity was observed.

The oxidation behaviour of the 9 % Cr steel P92in CO2- and H2O-rich gases relevant to oxyfuel environments

Abstract In oxyfuel plants metallic heat exchanging components will be subjected to service environments containing high amounts of CO2 and water vapour. In the present paper, the oxidation behaviour of the ferritic/martensitic 9 % Cr steel P92 was studied in a model gas mixture containing 70 % CO2-30 % H2O in the temperature range 550 – 650 °C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. In the CO2- and/or H2O-rich gases, the steel formed iron-rich oxide scales which possess substantially higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich oxide scales are formed as a result of a decreased flux of chromium in the bulk alloy toward the surface. This is the result of enhanced internal oxidation of chromium in the H2O-containing gases and carburisation in the CO2 gases. The oxide scales allow molecular transport of CO2 towards the metallic surface, resulting in carburisation of the alloy. The presence of water vapour induced buckling in the outer haematite layer, apparently as a result of compressive oxide growth stresses. Buckling did not occur in the H2O-free gas. This has been discussed in terms of the potential for H2O to increase growth stresses and accelerate crack propagation. The oxidation rates in CO2–H2O do not seem to be higher than those observed in flue gases of conventional fossil fuel fired power plants.

Scale formation mechanisms of martensitic steels in high CO2/H2O-containing gases simulating oxyfuel environments

In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO2 and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9–12%Cr steels in a model gas mixture containing 70% CO2–30% H2O was studied in the temperature range 550–700°C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. It was found that in the CO2- and/or H2O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H2O-containing gases and carbide formation in the CO2-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The Fe-base oxide scales formed in CO2(–H2O)-rich gases appeared to be permeable to CO2 molecules resulting in substantial carburization of the steel.

Scale formation mechanisms of martensitic steels in high CO2/H2O-containing gases simulating oxyfuel environments

In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO2 and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9–12%Cr steels in a model gas mixture containing 70% CO2–30% H2O was studied in the temperature range 550–700°C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. It was found that in the CO2- and/or H2O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H2O-containing gases and carbide formation in the CO2-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The Fe-base oxide scales formed in CO2(–H2O)-rich gases appeared to be permeable to CO2 molecules resulting in substantial carburization of the steel.

Dry atmospheric deposition rates of metals along a coastal transect in southern California

While recent studies indicate atmospheric deposition is a significant source of metals to the Santa Monica Bay and coastal river systems of the Los Angeles area, the spatial extent of the atmospheric source along the entire southern California coast has not been measured in 30 years. This study provides measurements of dry atmospheric deposition of chromium, copper, lead, nickel and zinc at eight sites located along the coast between Santa Barbara and San Diego, and compares these data to historic measurements from the 1970s. Median dry deposition fluxes across sites ranged between 0.23 and 3.6 (chromium), 0.21 and 5.4 (nickel), 0.52 and 14 (lead), 0.89 and 29 (copper), and 4.8 and 160 (zinc) μg m −2 day −1. Differences in metal dry deposition rates observed between sites were dominated by proximity to urban areas and/or other nearby sources, with the highest metal fluxes observed near the Los Angeles Harbor (LAH) and San Diego Bay (SDB) sites. Compared with data from the 1970s, lead fluxes were typically one to two orders of magnitude lower in the present study (2006), indicating atmospheric sources of this metal have decreased over the past three decades in southern California. Chromium fluxes were also lower in 2006 compared with the 1970s, although to a lesser extent than for lead. In contrast, copper and zinc fluxes were typically within the same order of magnitude between the two time periods, with some higher measurements observed in 2006 compared with the 1970s at the LAH and SDB sites. This result indicates atmospheric sources of copper and zinc have not decreased over the past three decades in southern California, and have increased near our harbor/urban sites. Differences in sampling conditions (e.g., Santa Ana winds) and measurement techniques may also explain, in part, the differences observed in metal flux rates for these time periods. However, these limitations were most important for those metals with the smallest difference in flux rates measured in the 1970s vs. 2006 (e.g., chromium).

Magnitude and Directional Measures of Water and Cr(VI) Fluxes by Passive Flux Meter

A new configuration of the passive fluxmeter (PFM) is presented that provides for simultaneous measurements of both the magnitude and the direction of ambient groundwater specific discharge qo and Cr(VI) mass flux J(Cr). The PFM is configured as a cylindrical unit with an interior divided into a center section and three outer sectors, each packed with a granular anion exchange resin having high sorption capacity for the Cr(VI) oxyanions CrO4(2-) and HCrO4-. The sorbent in the center section is preloaded with benzoate as the "resident" tracer. Laboratory experiments were conducted in which PFMs were placed in porous packed bed columns, through which was passed a measured volume of synthetic groundwater containing Cr(VI). During the deployment period, some of the resident tracer is depleted while the Cr(VI) is sorbed. The resin was then removed from the four sectors separately and extracted to determine the "captured" mass of Cr(VI) and the residual mass of the resident tracer in each. Cumulative specific discharge, q0t, values were assessed using the residual mass of benzoate retained in the center section. The direction of this discharge theta was ascertained from the mass distribution of benzoate intercepted and retained in the outer three sections of the PFM. Cumulative chromium fluxes, J(Cr)t, were quantified using the total Cr(VI) mass intercepted and retained on the PFM. Experiments produced an average measurement error for direction theta of 3 degrees +/- 14 degrees, while the average measurement errors for q0 and J(Cr) were, respectively, -8% +/- 15% and -12% +/- 23%. Results demonstrate the potential utility of the new PFM configuration for characterizing groundwater and contaminant fluxes.

Changes of chromium behavior in soil during phenanthrene removal by Penicillium frequentans

Soil contamination due to polycyclic aromatic hydrocarbons is often associated with the presence of high levels of potentially toxic metals. Bioremediation is an important option for the clean up of this type of contamination. Changes of chromium fluxes and concentrations during the phenanthrene removal by Penicillium frequentans in soil were investigated. During the bioremediation process, changes in chromium behavior were monitored by Diffusive Gradients in Thin-films (DGT) and by filtration in both sterilized and non-sterilized soils. DGT provided absolute data on fluxes from the solid phase and relative trends of concentrations of the most labile metal species. Filtration provided data on the concentrations of Cr in the solution phase. Together the data provided information about the physical and chemical metal behavior. Results showed that the highest phenanthrene removal was observed in non-sterilized soil (which included the autochthonous microorganisms and P. frequentans inoculum), with a phenanthrene removal of 73 +/- 3.2%. However, in all cases microbial activity increased chromium fluxes and chromium soil solution concentration. The bioremediation of soil by P. frequentans increased the lability and mobility of chromium in soil, with potential consequences for plant uptake and for increased movement of metals into the human food chain.

Oxidation of Stainless Steel in H<sub>2</sub>O/O<sub>2</sub> Environments – Role of Chromium Evaporation

Oxidation of Stainless Steel in H<sub>2</sub>O/O<sub>2</sub> Environments – Role of Chromium Evaporation

Chromium speciation in rainwater: temporal variability and atmospheric deposition.

The concentrations of the various chromium species were determined in 89 rainwater samples collected in Wilmington, NC, from October 1, 1999, to December 31, 2001. Volume-weighted annual average concentrations of Cr(total), particulate Cr, Cr(III)(aq), and Cr(VI)(aq) were 4.6, 2.2, 0.8, and 1.2 nM, respectively. There was distinct seasonal and diurnal variability in the concentrations of the various chromium species. Chromium emissions to the global atmosphere by both natural and anthropogenic sources are estimated to be 2.2 x 10(9) mol/yr. Using our rainwater concentration data along with other published rainwater Cr concentrations and an estimate for total global annual rain, the total global flux of chromium removed from the atmosphere via wet deposition is 2.1 x 10(9) mol/yr. This represents complete removal of Cr and indicates that essentially all chromium released into the global atmosphere is removed via rain. About half this chromium is dissolved with roughly equal concentrations of toxic Cr(VI) and relatively harmless Cr(III) species.

New membrane technology for removal and recovery of chromium from waste waters

AbstractWe have developed new supported liquid membrane (SLM) technology for removal and recovery of chromium from waste waters. The technology not only removes the Cr (VI) to less than 0.05 ppm in the treated effluent allowable for discharge or recycle, but also recovers the product at a high concentration of about 20% (62.3% Na2CrO4) suitable for resale or reuse. In other words, we have achieved the goals of zero discharge and no sludge. The chromium flux in its entire range from low Cr (VI) concentrations in the feed solution to high feed concentrations, at which the carrier saturation of the facilitated transport mechanism occurs, has been in good agreement with a film model. The stability of the SLM has been ensured by strip dispersion, where the aqueous strip solution is dispersed in the organic membrane solution in a mixer. The strip dispersion formed is circulated from the mixer to the membrane module providing a constant supply of the organic solution to the membrane pores.

Factors affecting chromium carbide precipitate dissolution during alloy oxidation

Ferrous alloys containing significant volumefractions of chromium carbides were formulated so as tocontain an overall chromium level of 15% (by weight) buta nominal metal matrix chromium concentration of only 11%. Their oxidation at 850°C inpure oxygen led to either protectiveCr2O3 scale formation accompaniedby subsurface carbide dissolution or rapid growth ofiron-rich oxide scales associated with rapid alloy surface recession, which engulfedthe carbides before they could dissolve. Carbide sizewas important in austenitic alloys: an as-castFe-15Cr-0.5C alloy contained relatively coarse carbides and failed to form aCr2O3 scale, whereas the samealloy when hot-forged to produce very fine carbidesoxidized protectively. In ferritic alloys, however, evencoarse carbides dissolved sufficiently rapidly to provide the chromium flux necessary to formand maintain the growth of a Cr2O3scale, a result attributed to the high diffusivity ofthe ferrite phase. Small additions of silicon to theas-cast Fe-15Cr-0.5C alloy rendered it ferritic and led toprotective Cr2O3 growth. However,when the silicon-containing alloy was made austenitic(by the addition of nickel), it still formed aprotective Cr2O3 scale, showing that the principal function of silicon was inmodifying the scale-alloy interface.

Alloy-grain-size dependence of the effectiveness of silica coatings as oxidation barriers on stainless steel

In order to determine the effect of alloy grain size on the oxidation properties of silica-coated austenitic Fe-18Cr-20Ni stainless steel, both coarse-grain (100-μm grain size) and fine-grain (5-μm grain size) forms of the alloy were produced. A 1-μm-thick vitreous silica coating was deposited by chemicalvapor deposition on the alloys, which were subjected to isothermal and cyclic oxidation in air at 900°C. The coarse-grain alloys underwent widespread oxidation below the silica coating, leading to extensive coating spallation. This was attributed to the inability of the alloy to supply a sufficient outward flux of chromium to prevent oxygen penetration through microcracks in the silica coating. Due to an abundance of chromium available at the surface of the finegrain alloy, chromia formed in the microcracks within the silica layer. As a result, the silica-coated, fine-grain alloy demonstrated superior oxidation resistance and excellent adhesion of the coating.

Chromium fluxes through Mex Bay inshore waters

Chromium is discharged into Mex Bay from an agricultural drain (456 kg Cr d −1), a chlor-alkali plant (7 kg d −1) and the flowing water from Alexandria harbour (41 kg Cr d −1). Elevated Cr concentrations in the Bay waters have been measured. Superficial sediments and edible organisms from different trophic levels were collected from the Bay and analysed by GFAAS for their Cr content. Cr concentrations in the bay sediments ranged from 42 μg g −1 dry wt and 752 μg g −1 dry wt with an average of 243±213 μg g −1 dry wt. No significant correlation was found between chromium concentration and organic carbon (r = + 0.25) in sediments. Chromium concentrations in marine organisms increased in the following order: Boops boops > mixed plankton > Mullus barbatus > Sardina pilchardus > Rhinobatus halavia > Ulva rigida > Enteromorpha intestinalis. Cr did not undergo biomagnification in the marine food webs of the Bay and may actually experience biodepletion from algae and crabs to fish. From the daily total mass emission of Cr to the Bay (504 kg d −1), flushing of the bay to the open sea is responsible for the removal of about 60% and sedimentation within the bay 26%. The remaining 14% assumed to be accumulated in the Bay organisms.