Reduction Of Cr Research Articles

One-step synthesis of highly active Ti-containing Cr-modified MCM-48 mesoporous material and the photocatalytic performance for decomposition of H2S under visible light

A highly photoactive Ti-containing Cr-modified MCM-48 photocatalyst (Si/Ti=3.4, Si/Cr=50) was prepared by a facile one-step method at room temperature. A combination of various physicochemical techniques such as X-ray diffraction (XRD), N2 physisorption, diffuse reflectance UV–vis spectra (DRS) and X-ray photoelectron spectra (XPS) were used to characterize the properties of the synthetic catalysts. The characterization and experimental results indicated that tetrahedral Ti oxide moieties as dominant Ti oxide were loaded into the mesoporous structure and there was a synergistic interaction between the Ti species anchored on the walls and the Cr ions presented in the MCM-48 framework, which was considered to be directly correlated to the photoactivity. The Ti-Cr-MCM-48 sample can remove H2S with the efficiency of 92% under visible light, being the Cr6+ species primarily responsible for this photoactivity. A deactivation was observed as a consequence of sulfate accumulation on the surface of the catalyst and reduction of Cr6+.

Removal Efficiency of Cr6+by IndigenousPichiasp. Isolated from Textile Factory Effluent

Resistance of the indigenous strains P. jadinii M9 and P. anomala M10, to high Cr6+ concentrations and their ability to reduce chromium in culture medium was studied. The isolates were able to tolerate chromium concentrations up to 104 μg mL−1. Growth and reduction of Cr6+ were dependent on incubation temperature, agitation, Cr6+ concentration, and pH. Thus, in both studied strains the chromium removal was increased at 30°C with agitation. The optimum pH was different, with values of pH 3.0 and pH 7.0 in the case of P. anomala M10 and pH 7.0 using P. jadinii M9. Chromate reduction occurred both in intact cells (grown in culture medium) as well as in cell-free extracts. Chromate reductase activity could be related to cytosolic or membrane-associated proteins. The presence of a chromate reductase activity points out a possible role of an enzyme in Cr6+ reduction.

Study on the Heterojunction Photocatalyst Nb<sub>2</sub>WO<sub>8</sub>/ZnO Prepared by Ball Milling Method

Orthorhombic Nb2WO8 was synthesized by solid state reaction in Nb2O5-WO3 system. Heterojunction photocatalyst Nb2WO8/ZnO was prepared by ball milling. The structural and optical properties of the photocatalyst were characterized by X-ray powder diffraction, transmission electron microscopy, UV–vis diffuse reflectance spectroscopy, and fluorescence emission spectroscopy.The photocatalytic activity was evaluated by photocatalytic oxidation of rhodamine B and reduction of Cr6+. The results showed that the photocatalytic activity of the Nb2WO8/ZnO was higher than that of ZnO. When the amount of doped Nb2WO8 was 10 wt.% and the sample was ball milled for 9 h, the Nb2WO8/ZnO showed the optimal photocatalytic activity. Effect of ball milling time on the photocatalytic activity was also investigated. The mechanisms of the increase in the photocatalytic activity were discussed by the valence band principle.

H2S photodegradation by TiO2/M-MCM-41 (M = Cr or Ce): Deactivation and by-product generation under UV-A and visible light

M-MCM-41 molecular sieves (M=Ce or Cr) were prepared by a hydrothermal method and impregnated with TiO2. The materials were characterized by XRD, N2 adsorption–desorption, DRS and XPS. Their potential application to photooxidize H2S in a wet gas stream was tested in a continuous flow reactor operating at a flow rate of 110mLmin−1 at 50% relative humidity and using 30ppmv of the pollutant. The photocatalytic efficiency using UV-A and visible light was compared to the activity of TiO2/MCM-41 without heteroatoms incorporated into the MCM-41 structure. It was found that the incorporation of Ce did not improve the performance of TiO2/MCM-41, but Cr-containing samples presented higher initial efficiency and were able to photooxidize H2S without formation of SO2 as a by-product, in contrast to the other prepared samples and to Degussa P-25 TiO2. Moreover, no other gaseous by-product was detected. The isomorphic incorporation of Cr into the structure of MCM-41 followed by TiO2 incorporation produced photocatalysts that presented good adsorption capacity and were much more active under visible light than under UV-light. This performance represents an important advantage for solar applications. Their photoactivity depended on the concentration of chromium; the highest efficiency was attained with samples with a Si/Cr ratio of 50. Finally, deactivation was observed as a consequence of sulfate accumulation on the surface of the catalyst and reduction of Cr6+.

Structural features of the electrode/solution interface at the reduction of Cr3+(aq) cations on liquid mercury and solid indium electrodes in acidic media

Characteristics of electroreduction of Cr3+ and Cr2+ ions on the dropping mercury (literature data) and indium cathodes are analyzed as a function of pH of the medium. A hypothesis that polymeric hydroxocompounds of chromium, which are present in different aggregate states, represent the main or partial reactants in the \( Cr\left( {III} \right)\xrightarrow{{ + e}}Cr\left( {II} \right)\xrightarrow{{ + 2e}}Cr \) stages is additionally substantiated.

Corrosion Mitigation in Supercritical Water with Chromium Ion

Abstract It was reported that the chromium ion originated from the autoclave material (Ni-44Cr-1Mo) deposited on the specimen surface as chromium oxyhydroxide (CrOOH), and the protective chromium oxide layer mitigated corrosion of the alloys tested in supercritical water containing sulfuric acid (H2SO4). Based on the report, three kinds of tests were conducted at 400°C and 30 MPa in 0.01 mol/kg H2SO4 aqueous solution containing Cr3+ or Cr6+. Corrosion resistance of the specimens became 1.5 ∼ 7.5 times better than specimens without chromium ions, because chromium ions from the aqueous solution was incorporated into the protective chromium oxide on the specimen. It was revealed that reduction of Cr6+ to Cr2O3 (Cr3+) occurred on the specimen surface and in the autoclave. The chemical equations of chromium oxide or chromium oxyhydroxide scale formed on the specimen surface were evaluated.

Modeling of simultaneous unidirectional leaching and reduction of Cr6+ in unfixed CCA treated wood

The unidirectional leaching of unfixed hexavalent chromium (Cr6+) from wood freshly treated with chromated copper arsenate (CCA) is modeled as simultaneous diffusion and first-order chemical fixation reaction. Small wood specimens were coated to restrict preservative movement to one direction, then treated with CCA and immersed in water to evaluate unidirectional leaching characteristics. The directional diffusion coefficients and reaction rate constants of Cr6+ in unfixed CCA-treated red pine (Pinus resinosa Ait.) and southern pine (Pinus sp.) were estimated by least squares fitting of the model to leaching results in each direction independently. The reaction rate constants calculated from the diffusion and leaching model were generally consistent with results from pure fixation experiments, although the computed rate constants increased under the most severe longitudinal leaching conditions. The applicability of using diffusion coefficients and rate constants estimated from leaching results was tested using a finite difference implementation of the diffusion and reaction model to predict unidirectional leaching from small samples immersed intermittently in water. The predicted leachate quantities were consistent with experimental measurements at the end of each of five leaching events of various durations at temperatures alternating between 13 and 23°C over a 9-day period.

Hybrid Properties of Alginate‐PEI Adsorbent for Chromium (VI) Removal from Aqueous Solutions

An adsorbent consisting of polyethyleneimine (PEI) immobilized in calcium alginate gel beads was synthesized and evaluated for Cr6+ removal. An evaluation of the synthesis process showed the importance of the PEI molecular weight on the immobilization efficiency. Polyethyleneimine of 70,000 Da molecular weight displayed the highest immobilization percentage at 52%. Batch kinetics and equilibrium tests showed that alginate‐PEI (APEI) resin displayed considerable affinity for negatively charged Cr6+ complexes at low pH conditions ranging from pH 1.5‐pH 3. The results also indicated the reduction of Cr6+ to less toxic Cr3+ species by the APEI adsorbent. The column adsorption experiments showed the ability of APEI resin to treat a 10 mg/L Cr6+ solution with pH influent adjustment from pH 1.5 to pH 3 to concentrations that satisfy effluent standards for Cr6+ (<0.1 mg/L) and total Cr (<0.5 mg/L). Finally, comparisons with a highly aminated commercial resin Chitopearl CS‐03 highlighted the unique ability of the hybrid APEI beads with its amine and carboxylic groups for the adsorption of Cr6+ as well as the retention of generated Cr3+ ions.

Cultural characteristics of chromium resistant unicellular cyanobacteria isolated from local environment in Pakistan

Many unicellular cyanobacteria were isolated from different places: fields, ponds, polluted water, and soils from Muredkey and Kasur tannery areas, near Lahore, Pakistan. Different media like BG 11 medium, Bold Basal medium, Chu's #10 medium and Gorham's medium, in standard forms and with slight variations of ingredients, and different pH, temperature and light regimes were checked for the optimum growth of the isolates. The isolation procedure was repeated with different concentrations of chromium to select the resistant strains. These selected strains grew on chromium of the range 100–200 μg/ml in BG 11 medium. Cyanobacteria were maintained in solid and liquid media with/without shaking. Cyanobacterial strains were collected from natural habitats that were accompanied by a diversified group of organisms including bacteria, protozoan, and rotifers etc. In order to eliminate these agents termed as contaminants, we used several methods including phenol treatment, use of antibiotic and careful manual picking of unicellular cyanobacteria. Resistance of these strains against different heavy metals (ZnSO4, MnSO4, NiSO4, CoCl2, Pb(NO3)3, CuSO4, HgCl2, AgNO3 and CdCl2) and antibiotics (erythromycin, streptomycin, kanamyci chloramphenicol, neomycin) was evaluated. Optimum temperature was 30°C with variable pH for the reduction of Cr6+ in to Cr3+ in majority of strains.

Inhibition of mild steel by strontium chromate in artificial acid rain solution

The inhibition of mild steel in an artificial acid rain solution (pH 4·5) in low concentrations of strontium chromate pigment has been evaluated using a range of methods: potentiodynamic polarisation, electrochemical impedance, X-ray photoelectron spectroscopy and solution analysis. The individual effects of strontium and chromate have been evaluated and have been used to estimate inhibitor efficiency and film composition, and to determine the possible inhibition mechanism. The results show that, at low concentration, strontium chromate affects the cathodic reaction, with reduction of Cr6+ to Cr3+, and the surface film was composed of magnetite and hydrated chromium hydroxide. The presence of strontium was not found significantly to affect the inhibitory performance.

Formation, Absorption and Emission Spectra of Cr4+ Ions in Li2O-SiO2 System Transparent Glass-Ceramics

The formation mechanism, optical absorption and emission spectra of Cr4+ ions-containing Li2O-SiO2 system transparent glass-ceramics were investigated. In this material, the main crystalline phase was Li2O•2SiO2, and the percent crystallinity and crystal size were 67-72 and 20-33 nm, respectively. The remarkable change in color and absorption spectra was observed upon crystallization. The characteristic emission of tetrahedrally coordinated Cr4+ ions was identified in the near infrared region, 1000-1600 nm. It was found that tetrahedrally coordinated Cr4+ ions exist in this transparent glass-ceramics from absorption and emission measurement. The Cr4+ ions exist in residual high SiO2 glassy phase, and their ligand field parameters are estimated to be: 10Dq=10,610 cm-1, B=690 cm-1 and Dq/B=1.54. This Dq/B value is just below the crossing point of 3T2 and 1E levels (Dq/B=1.6). The Cr4+ ions in Li2O-SiO2 system transparent glass-ceramics occupy the little stronger ligand field sites than those in aluminate glass reported previously (Dq/B=1.2). It is considered that the Cr4+ ions can be formed by the reduction of Cr6+ ions (chromate ion [CrO4]2-) associated with decreasing the basicity of residual glassy phase during crystallization. In this process the behavior of Li+ ions plays a significant role. The [CrO4] formed is equivalent to [SiO4] and substitutes [SiO4] sites in the residual high SiO2 glassy phase.

Retention and plant availability of chromium in soils as affected by lime and organic matter amendments

Chromium is used as Cr(III) in the tannery industry and as Cr(VI) in the timber treatment industry. In this experiment, the effect of 2 liming materials [fluidised bed boiler ash (FBA) and lime] and an organic amendment (Pinus radiata bark) on the retention of Cr by 2 soils (Egmont and Tokomaru) was examined using both ‘batch’ and ‘column’ experiments. The effect of these amendments on the uptake of Cr from the Egmont soil, treated with various levels of Cr (0–3200 mg Cr/kg soil), was examined using sunflower (Helianthus annuus) plants. The transformation of Cr was examined by fractionation of Cr in soils. Addition of FBA and lime increased the retention of Cr(III), but had the opposite effect on the retention of Cr(VI). Addition of bark did not affect the retention of Cr(III), but it increased the retention of Cr(VI). Increasing additions of Cr increased Cr concentration in plants, resulting in decreased plant growth. The liming materials were found to be effective in reducing the phytotoxicity of Cr(III) and the Pinus radiata bark was effective for Cr(VI). In both Cr(III)- and Cr(VI)-contaminated soils, the concentrations of Cr were higher in the organic-bound, oxide-bound, and residual fractions than in the soluble and the exchangeable fractions. The concentrations of Cr in the soluble and the exchangeable fractions were higher in the Cr(VI)-contaminated than the Cr(III)-contaminated soil. Addition of the liming materials decreased the concentration of the soluble Cr(III) and bark decreased soluble Cr(VI) in soil. There was evidence for the reduction of Cr(VI) to Cr(III) in the bark-treated soils.

Removal of Chromium from Abrasive Blast Media by Leaching and Electrochemical Precipitation

ABSTRACT Water is effective in leaching out Cr6+ from a mixture of paint powders and abrasive blast media. However, acids such as HNO3, HCl, and H2SO4 significantly enhance the leaching procedure. Cr ions in the leaching solutions are successfully removed by electrochemical precipitation. The consumable Fe electrodes generate ferrous ions to cause the reduction of Cr6+ to Cr3+. Cr3+ ions along with Fe2+ and Fe3+ are then removed mainly by precipitation as Cr(OH)3, Fe(OH)2, and Fe(OH)3 near the cathode where OH- ions are generated by water electrolysis. The electrochemical process is capable of discharging low levels of Cr6+, less than 1 mg/L, without pH adjustment.

Oxidative dehydrogenation of ethane by carbon dioxide over sulfate‐modified Cr2O3/SiO2 catalysts

The oxidative dehydrogenation of ethane into ethylene by carbon dioxide over unsupported Cr2O3, Cr2O3/SiO2 and a series of Cr2O3/SiO2 catalysts modified by sulfate was investigated. The results show that Cr2O3/SiO2 is an effective catalyst for dehydrogenation of ethane and CO2 in the feed promotes the catalytic activity. Sulfation of silica will influence the catalytic behavior of Cr2O3/SiO2 in dehydrogenation of ethane with carbon dioxide depending on the amount of sulfate. Cr2O3/6 wt% SO42-–SiO2 catalysts exhibit an excellent performance for this reaction, giving an ethylene yield of 55% at 67% ethane conversion at 650°C. Characterizations indicate that addition of sulfate changes the bulk and surface properties of Cr2O3/SiO2, promoting the reduction of Cr6+ to Cr3+ and favoring the catalytic conversion.

Activation of a procarcinogen by reduction: Cr6+→Cr5+→Cr4+→Cr3+ a case study by electron spin resonance (ESR/PMR)*

Chromate is a human carcinogen. Since it does not form a covalent DNA adduct in vitro under physiological conditions, and is not mutagenic in vitro in the presence of cytochrome P450 preparations from liver, reduction of Cr6+ by cellular reductants to lower oxidation states such as Cr5+, Cr4+ is considered to be a critical step in the mechanism of carcinogenesis. Long‐lived paramagnetic chromium species, Cr5+, Cr4+, Cr3+ are formed in the presence of coenzymes such as NAD(P)H, GSH, and cytochromes. These anionic complexes of reduced chromium are considered potential “penultimate” carcinogens. Various in vitro and in vivo studies from our group have demonstrated the formation of these ionic species using a modified paramagnetic spectroscopy approach. In this review, information is provided on the half‐lives of formation and decay, free energy changes, atomic structures and reaction mechanisms of these compounds in situ, in vivo and in vitro, at the molecular, cellular and organismic levels. Hydroxyl radical (·OH) can be generated from the reaction of these Cr5+, Cr4+ compounds with H2O2 by a Fenton‐like reaction, as can be demonstrated by molecular spin traps. In addition to ·OH radical, a number of other free radicals may be generated from reaction of chromium with cellular reductants and peroxides. These radicals, particularly the hydroxyl radical, are considered the ultimate agents in chromium carcinogenesis. They may break phosophodiester bonds of the DNA double strands, and modify 2'‐deoxyguanosine to form promutagenic 8‐hydroxy‐2'‐deoxyguanosine (8‐OHdG) in the DNA structure. Genetic expressions are changed at the transcription level. Changes in genetic information may also be passed onto future generations through cell replications to the daughter cells. Thus, ·OH from the interaction of Cr5+, Cr4+ with H2O2 affects not only differentiation but also cell division, and leads to the development of cancer as a result.

XPS study of coating delamination from non-rinse chromate treated steel

The mechanisms of delamination of an epoxy coated steel system with various substrate non-rinse chromate treatments has been investigated by XPS and TOF-SIMS. First, the chromate surface before epoxy coating was investigated to establish the layer structure produced by the chromate treatments. The pre-treatment layer was found to be a complex material consisting of trivalent and hexavalent chromium, and a cross-linked structure of these ions within chromium oxide. A non-rinse chromate with colloidal SiO2 added has a similar structure of trivalent and hexavalent chromium oxide at the outer surface, but within the inner layer Cr oxide and SiO2 are bonded to each other. This leads to a more complicated structure compared to the chromate treatment. Adsorption isotherms of epoxy resin and curing agent on chromate layers were established by using XPS, in order to evaluate the adsorption characteristics of these molecules, to gain an insight into the initial stages of adhesion. The chromate surface was found to have more bonding sites than the chromate with SiO2 surface.Steel substrates treated in this manner were coated with an epoxy coating and exposed to a saline environment at both the free corrosion potential and a cathodic potential The delamination kinetics of the epoxy coating were recorded. Interfacial failure surfaces from the delamination studies were examined by XPS and TOF-SIMS. By obtaining Cr 2p spectra at high resolution and peak-fitting to resolve the chemical states present, it has proved possible to determine the layer structure produced by the chromate treatment and elucidate their chemical behaviour during the electrochemical tests. In the case of the chromate treatment, failure following cathodic polarisation is within the conversion coating itself and associated with the inner layer, whilst an interfacial failure was observed for the untreated bare steel. All failure surfaces of the chromate treated steel sample were rich in Cr3+ , compared with the initial value obtained before the delamination experiments, and rich in sodium. It is shown that cathodic conditions exist under epoxy coating which have brought about the reduction of Cr6+ to Cr3+ within the chromate layer. This cathodic reduction, in the case of the chromate treated sample, leads to failure by the destruction of the cross-linked structure incorporating Cr6+ and Cr3+ .

In situ UV–VIS diffuse reflectance spectroscopy–on-line activity measurements Significance of Cr n+ species ( n=2, 3 and 6) in n-butane dehydrogenation catalyzed by supported chromium oxide catalysts

The dehydrogenation of n-butane over supported chromium oxide catalysts has been investigated at 500 °C by insitu UV–VIS diffuse reflectance spectroscopy–on-line activity measurements as a function of the reaction time (0–60 min), the support composition (SiO2:Al2O3 ratio) and the Cr loading (0–8 wt.%). The catalytic reaction is characterized by an initial induction period, during which the supported chromium oxides are reduced to Cr3+/2+, and CO/CO2 and cracking products (C1, C2 and C3) are formed. After this initial induction period, but-1-ene and but-2-ene are produced with a selectivity up to 90%. The catalytic activity increases with increasing Cr content and Al2O3 content of the support, and is maximum after 5–15 min on stream. A relation between the dehydrogenation activity and the amount of Cr3+ formed upon the reduction of Cr6+ has been established.

The Effect of Sulphur Dioxide Exposure under Controlled Environmental Conditions on the Challenge Performance of Copper and Chromate Impregnated Active Carbon against Hydrogen Cyanide

An ASC/T (Cu2+, Cr6+, Ag and triethylenediamine impregnated) Whetlerite activated carbon sample was exposed to a flow rate of 1 l/min, 0.746 mg/l SO2 in 80% RH air at 22°C for up to 510 min. Samples were subsequently challenged with 2 mg/l HCN in an identical diluent gas stream. Increasing SO2 exposure resulted in accelerated HCN and (CN)2 bed penetration. The basic shapes of the breakthrough profiles were however essentially unchanged. This observation is in accordance with numerical analysis of these results using Hinshelwood's adsorption model, which suggested that the adsorption rate constant was not significantly affected by SO2 but rather the pollutant exposure resulted in the number of active centres on the carbon surface being reduced and the effective bed depth of the sample being shortened. This loss in active centres was thought most likely to result from the reduction of Cr6+ to Cr3+.

In Situ Diffuse Reflectance Spectroscopy of Supported Chromium Oxide Catalysts: Kinetics of the Reduction Process with Carbon Monoxide

In situ diffuse reflectance spectra of supported chromium oxide catalysts are investigated for the first time at elevated temperatures under controlled reaction conditions using a specially designed diffuse reflection attachment. The obtained results are compared and discussed with those obtained by the classical diffuse reflectance spectroscopy technique. A novel method for studying the reduction kinetics of supported transition metal oxides is proposed. In the case of Cr(VI), the reduction is faster on silica than on alumina. A kinetic model is developed to explain the kinetics. It consists of the activation of CO by adsorption followed by the reduction of Cr6+ with formation of surface carboxylates.

On the reduction of Cr3+ during the thermal decomposition of fluorochromates(III) with nitrogen containing cations

The thermal decomposition of [CO(NH2)2H]CrF6·H2O, (C3N6H8)CrF5·H2O and the solid state reaction of CrF3 and melamine are investigated under non-reciprocal quasi-static conditions and compared with the thermal behaviour of other fluorochromates(III) ([Cr(NH3)6]CrF6, (NH4)3CrF and [C(NH2)3]3CrF6). The comparison of the results shows that the amount of chromium(II) in the final product is determined by the thermal stability and consequently by the decomposition temperature of the intermediates. Neither bonding properties in the starting materials nor the absolute amount of generated NH3 influence the composition of the final product.