CrVI Species Research Articles

One-Pot Separation and Preconcentration for the Determination of Chromium Species Using an Aqueous Biphasic System

The widespread use of the trace metal chromium in industrial processes and the known toxicity of CrVI species have stimulated interest in chromium speciation analysis. However, some analytical detection systems are not suitable for the quantification of CrIII and CrVI species. In the present study, an aqueous biphasic system (ABS) was employed as an environmentally safe, simple, and economical alternative. The ABS allows the implementation of an efficient separation step before quantifying of the species of interest by flame atomic absorption spectrometry. A robust speciation and preconcentration method was developed, which can be applied to samples with a wide concentration range, without the use of any extractants and interconversion of the species during the procedure. The proposed method was successfully used to determine the chromium species present in tap, treatment plant, river, and electroplating wastewater samples. Recovery percentages of 87.9-107% were achieved, suggesting minor water matrix interference. The proposed method had acceptable limits of detection and quantification of 5.38 and 16.3 μg kg-1, respectively.

Retention and release of hexavalent and trivalent chromium by chitosan, olive stone activated carbon, and their blend.

Shrimp shells and waste of olive stones were used as feedstock for the preparation of chitosan and activated carbon. The adsorption of CrVI and CrIII species in aqueous solution by the materials prepared and their blend were studied by using the well-known kinetic and isotherm models, Fourier transform infrared spectroscopy and scanning electron microscope. It was demonstrated that the rates of adsorption were controlled by diffusion inside particles and throughout the liquid film, and adsorption occurred spontaneously (- 26 < ∆G° < - 15kJ/mol) in the range of 298-333K, except for that involving CrIII species and activated carbon. The maximum amounts of Cr species retained by the composite (146mg of CrVI/g and 33mg of CrIII/g at 298K) were three times greater than those of the basic constituents. Adsorption was markedly affected by temperature and pH, and CrVI species were substantially desorbed in acid mediums, particularly in acetic acid solution. The recovery of CrIII species varied according to the adsorbent and the solution used. The immobilization of CrVI species (HCrO4- and CrO42-) and CrIII species (Cr(OH)2+ and Cr3(OH)45+) by chitosan was accomplished by means of amine moieties and hydroxyls of D-glucosamine units of the biopolymer. The adsorption of CrVI species on activated carbon involved π electrons of aromatic rings as well as oxygenated sites (C-OH, C=O, C-O-C). In such a condition, CrVI was partially reduced into CrIII. For the composite, the amino functional groups of chitosan and hydroxyls of both constituents were implicated in the linkage of the biopolymer and activated carbon, and the C-O-H and C-O-C functional groups of chitosan were involved in the retention of CrVI species. For CrIII species, adsorption occurred preferentially on hydroxyls of the components, and consequently, the chains of the biopolymer recovered some flexibility.

In situ Carbothermal Synthesis of Nanoscale Zero‐Valent Iron Functionalized Porous Carbon from Metal–Organic Frameworks for Efficient Detoxification of Chromium(VI)

Owing to the high toxicity of CrVI species to human health and the ecosystem, much attention has been paid to the development of efficient adsorbents for CrVI. Herein, MIL‐100(Fe) was successfully utilized as a precursor for the carbothermal reductive synthesis of nanoscale zero‐valent iron functionalized porous carbon (nZVI@C) for the effective removal of CrVI. The organic ligands in MIL‐100(Fe) were transformed into a porous carbon matrix, whereas the Fe–O clusters were reduced in situ to nZVI owing to the strong reducibility of pyrolytic carbon. Additionally, the nZVI was distributed uniformly in the carbon support with a high loading and controllable particle size. Highly toxic CrVI species were efficiently degraded into less toxic CrIII species at the nZVI reductive sites, and then the CrIII species precipitated as (CrxFe1–x)(OH)3 in the porous hybrids. The CrVI adsorption capacity of the developed nZVI@C reached 206 mg g–1 under optimal conditions. Furthermore, the excellent magnetic performance of nZVI@C makes it convenient for CrVI remediation through simple magnetic separation. These outstanding characteristics indicate the promising potential of the developed material as an adsorbent for the efficient removal of CrVI from industrial wastewater.

Ligands Make the Difference! Molecular Insights into CrVI/SiO2 Phillips Catalyst during Ethylene Polymerization.

Operando-sensitive spectroscopic techniques were employed for investigating the changes in the molecular structure of the Cr sites in the CrVI/SiO2 Phillips catalyst during ethylene polymerization. Practically, the most arduous barrier to be overcome was the separation of the chromates reduction carried out by ethylene from the subsequent polymerization. By carefully tuning the experimental parameters we succeeded in observing these two events separately. We found that the sites involved in ethylene polymerization are mainly divalent Cr ions in a 6-fold coordination, in interaction with the oxygenated byproduct (mostly methylformate, generated from the disproportionation of two formaldehyde molecules). Unreduced CrVI species are also present during ethylene polymerization as well as reduced Cr species (either CrII or CrIII) acting as spectators. Our results challenge the old vision of "naked" chromium species (i.e., low coordinated) as the active sites and attribute a fundamental role to external (and flexible) oxygenated ligands that resemble the ancillary ligands in homogeneous polymerization catalysis.

Oxidation mechanisms and chemical bioavailability of chromium in agricultural soil – pH as the master variable

Chromium (Cr) is a heavy metal that exists in soils in two stable oxidation states, +III and +VI. The trivalent species is an essential nutrient, whereas the hexavalent species is highly toxic. This study investigated the environmental impact of CrIII potentially released into soil from wastes and various materials by determining the risk of oxidation of initially soluble inorganic CrIII into hazardous CrVI. The principal aim was to describe the pH-dependent mechanisms that regulate 1) the formation of CrVI from the easily soluble CrIII and 2) the potential bioavailability of CrIII and that of CrVI species produced in the oxidation of CrIII in agricultural soil (fine sand, organic carbon 3.2%). The amount of CrVI formed in oxic soil conditions was regulated by two counteracting reactions: 1) oxidation of CrIII into CrVI by manganese oxide (MnIVO2) and 2) the subsequent reduction of CrVI by organic matter back to CrIII. The effect of pH on this net-oxidation of CrIII and on the chemical availability of both CrIII and CrVI species was investigated in soil samples incubated with or without excessive amounts of synthetic MnO2, over the chemically adjusted pH range of 3.9–6.3 (+22 °C, 47 d). In soil subsamples without added MnO2, the net-oxidation of CrIII into CrVI (1 mM CrCl3 in soil suspensions, 1:10 w/V) was negligible. As for the MnO2-treated soils, at maximum only 4.7% of added CrIII was oxidized – regardless of the high oxidation potential of these subsamples. The lowest production of CrVI was observed under acidic soil conditions at pH ∼4. At low pH, the net-oxidation diminished as result of enhanced reduction of CrVI back to CrIII. At higher pHs, the oxidation was limited by enhanced precipitation (or adsorption) of CrIII, which lowered the overall amount of CrIII susceptible for oxidation. Moreover, the oxidation reactions by MnO2 were inhibited by formation of Cr(OH)3 coverage on its surface. The pH-dependent chemical bioavailability of added CrIII differed from that of the CrVI formed. At elevated pHs the chemical availability of CrIII decreased, whereas that of CrVI produced increased. However, the risk of CrVI formation through oxidation of the easily soluble inorganic CrIII was considered to be low in agricultural soils high in organic matter and low in innate MnO2.

Pre-reduction of the Phillips CrVI/SiO2 catalyst by cyclohexene: A model for the induction period of ethylene polymerization

The pre-reduction of the industrially relevant CrVI/SiO2 Phillips catalyst by cyclohexene at room temperature was investigated by means of operando XANES, Diffuse Reflectance UV–Vis and transmission FT-IR spectroscopies. It was found that cyclohexene efficiently reduces the surface chromates sites (CrVI) at room temperature to give mainly divalent chromium sites and aldehyde by-products, which are rapidly converted in ester species. These by-products remain in strong interaction with the reduced chromium sites, defining a complex and tunable ligand sphere, which controls the entrance of incoming molecules, including ethylene. Unlike the CrVI/SiO2 catalyst, the cyclohexene-reduced Cr/SiO2 catalyst polymerizes ethylene already at room temperature with no induction time. The reactivity of cyclohexene with the CrVI species mimics that of ethylene during the induction period on CrVI/SiO2 catalyst, simplified by the absence of the further polymerization step. Hence, the results discussed in this work are potentially useful to understand what happens during the induction period in the presence of ethylene.

Radiation-Induced Formation of Chromium Oxide Nanoparticles: Role of Radical Scavengers on the Redox Kinetics and Particle Size

The formation of Cr2O3 nanoparticles by γ-irradiation of dichromate solutions was investigated in the presence of radical scavengers, tert-butyl alcohol, nitrous oxide, and dissolved oxygen. The concentrations of CrVI and CrIII species and the chemical composition and sizes of particles that were formed were measured as a function of irradiation time. A mechanism is proposed that is consistent with the observed effects of scavengers on the kinetics of reduction of CrVI in solution to solid CrIII and the final size of Cr2O3 nanoparticles that are formed. The chromium reduction occurs in three stages: Stage I involves homogeneous aqueous reduction of CrVI(aq) to CrIII(aq) followed by spontaneous condensation of Cr(OH)3, providing nucleation sites onto which CrVI and CrIII coprecipitate and form mixed CrVI/CrIII oxide/hydroxide particles. In Stage II, the aqueous reduction of CrVI(aq) and CrIII(aq) reaches steady state but the adsorption of CrVI continues, growing the mixed CrVI/CrIII oxide/hydroxide particles. Stage 3 involves the solid-state conversion of CrVI and CrIII, and the particles slowly convert from a mixed CrVI/CrIII oxide/hydroxide to Cr(OH)3 and then to Cr2O3. Adding a radical scavenger in solution affects the kinetics of particle formation and growth at different stages differently. This information can be used to tailor the final sizes of particles.

Preparation and Characterization of Chitosan Thin Films on Mixed-Matrix Membranes for Complete Removal of Chromium.

Herein we present a new approach for the complete removal of CrVI species, through reduction of CrVI to CrIII, followed by adsorption of CrIII. Reduction of chromium from water is an important challenge, as CrIV is one of the most toxic substances emitted from industrial processes. Chitosan (CS) thin films were developed on plain polysulfone (PSf) and PSf/TiO2 membrane substrates by a temperature-induced technique using polyvinyl alcohol as a binder. Structure property elucidation was carried out by X-ray diffraction, microscopy, spectroscopy, contact angle measurement, and water uptake studies. The increase in hydrophilicity followed the order: PSf < PSf/TiO2 < PSf/TiO2/CS membranes. Use of this thin-film composite membrane for chromium removal was investigated with regards to the effects of light and pH. The observations reveal 100 % reduction of CrVI to CrIII through electrons and protons donated from OH and NH2 groups of the CS layer; the reduced CrIII species are adsorbed onto the CS layer via complexation to give chromium-free water.

Low-Cost Synthesis of Flowerlike α-Fe2O3 Nanostructures for Heavy Metal Ion Removal: Adsorption Property and Mechanism

Flowerlike α-Fe(2)O(3) nanostructures were synthesized via a template-free microwave-assisted solvothermal method. All chemicals used were low-cost compounds and environmentally benign. These flowerlike α-Fe(2)O(3) nanostructures had high surface area and abundant hydroxyl on their surface. When tested as an adsorbent for arsenic and chromium removal, the flowerlike α-Fe(2)O(3) nanostructures showed excellent adsorption properties. The adsorption mechanism for As(V) and Cr(VI) onto flowerlike α-Fe(2)O(3) nanostructures was elucidated by X-ray photoelectron spectroscopy and synchrotron-based X-ray absorption near edge structure analysis. The results suggested that ion exchange between surface hydroxyl groups and As(V) or Cr(VI) species was accounted for by the adsorption. With maximum capacities of 51 and 30 mg g(-1) for As(V) and Cr(VI), respectively, these low-cost flowerlike α-Fe(2)O(3) nanostructures are an attractive adsorbent for the removal of As(V) and Cr(VI) from water.

Low-level determination of silicon in steels by anodic stripping voltammetry on a hanging mercury drop electrode

The sensitive differential pulse anodic stripping voltammetry (DPASV) proposed originally by Ishiyama et al. (2001) has been revised and improved to allow the accurate measurement of silicon on a hanging mercury drop electrode (HMDE) instead of a glassy carbon electrode. We assessed the rate of formation of the partially reduced β-silicododecamolybdate and found that metallic mercury promotes the reaction in the presence of a large concentration of Fe 3+. The scope of the method has been broadened by carrying out the measurements in the presence of a constant amount of Fe 3+. The limit of detection (LOD) of the method described in the present paper is 100 μg Si g −1 of steel, with a relative precision ranging from 5% to 12%. It can be further enhanced to 700 ng Si g −1 of steel provided the weight of the sample, the dilution factors, the duration of the electrolysis and the ballast of iron are adequately revised. The tolerance to several interfering species has been examined, especially regarding Al 3+, Cr 3+ and Cr VI species. The method was validated using four low-alloy ferritic steels certified by the National Institute of Standards and Technology (NIST). Its application to nickel base alloys as well as to less complicated matrixes is straightforward. It has also been successfully applied to the determination of free silicon into silicon carbide nano-powder.

New Open‐Framework Mixed‐Valence Chromium(III) Cerium(III)/(IV) Sulfate: CrCeIII7CeIV6(HSO4)6(SO4)21·75H2O

AbstractThe title compound, synthesized from CrVI and CeIV species, crystallizes in the space group P63 with a = 19.3015(2), c = 25.2684(4) Å, and Z = 2. The structural design of the title compound differs essentially from structures of known mixed‐valence cerium compounds which all form three‐dimensional networks. In the title compound the cerium atoms, interlinked by sulfate groups, extend to form layers held together by hydrogen‐bonding contacts. Perpendicular to these layers there are 10 Å wide channels. One third of the channels host solvent water only, while the other two thirds of the channels also host [Cr(H2O)6]3+ and [Ce(SO4)3(H2O)6]3– units, blocking each of these channels once per unit cell, resulting in 14 Å long voids.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2007)

Relationship between tribological properties of hard chromium coatings and performances of chromium plating electrolyte

Hard chromium coatings are commonly used for applications requiring low friction coefficient and wear resistance. The aim of this work is to study the influence of electroplating conditions (electrolyte temperature and cathodic current density) on the efficiencies of both cathodic and anodic reactions and the tribological properties of chromium electrodeposits. First, the mass balance allows the evolution of the electrolyte composition to be followed, especially the decrease in CrVI species, due to their reductions to chromium metal and trivalent chromium. Experimental results highlight the significant effects of the current density and the electrolyte temperature on the cathodic efficiencies. The strong link between plating electrolyte temperature and coating properties (roughness and tribological characteristics) is then shown.

Application of xanthine functionalized resin in speciation of chromium in natural water

Separation and preconcentration of two forms of chromium viz. Cr III and Cr VI in natural water was carried out applying a resin containing xanthine moiety anchored onto polystyrene-divinylbenzene (8%) by azo (-N=N-) function. The maximum exchange capacity of Cr III was found to be 0.6718 mmol g -1 at pH 6.5 and that for Cr VI was 1.1324 mmol g -1 at pH 1.0. The sorption of both Cr III and Cr VI onto resin follows a second order kinetics with rate constant values of 0.047 and 0.410 mmol -1 min -1 for Cr111 and Cr VI respectively. The two species Cr III and Cr VI are eluted completely by 3 M and 4 M HCl respectively. The detection limits of 4.3 and 5.1 ng ml -1 for Cr III and Cr VI were achieved. The optimum flow rate, sample breakthrough volume of column operation for Cr III and Cr VI species were found to be 1.2 ml min -1 and 3000 ml respectively. Finally the developed method using this resin was applied for determination of these two species in synthetic binary mixtures, natural water samples and validated comparing with results using previously developed method.

Adsorption of Cr VI on commercially available ash powder - A kinetic approach

Among heavy metal pollutants, chromium plays a major role in polluting environment. Several techniques are available and among these, adsorption process is found suitable for the removal of Cr VI species from wastewater. In the present work, the investigator has chosen the commercially available ash powder (CAP), which is being used as a sacred material by Hindus called Vibhoothi or Thiruneeru as the adsorbent. The adsorbent was characterized according to the Bureau of Indian Standards. It has also been planned to determine the adsorption potential of the adsorbent to assess the suitability of the material for the removal of Cr VI from waste waters. Batch mode adsorption experiments were carried out to study the effect of initial concentration of chromium solution, effect of the dose of the adsorbent, and pH of the solution. Various models such as Lagergren, Bhattacharya and Venkobachar were tested and the results are interpreted.

Oxidation of d-glucose in the presence of 2,2′-bipyridine by Cr VI in aqueous micellar media: a kinetic study

The kinetics of Cr VI oxidation of d-glucose to the corresponding lactone in the presence and absence of 2,2′-bipyridine (bipy) has been carried out under the conditions, [ d-glucose] T ≫ [Cr VI] T at different temperatures in aqueous micellar media. The monomeric Cr VI species has been found to be kinetically active in the absence of bipy whereas in the bipy-catalysed path, the Cr VI–bipy complex has been found to be the active oxidant. In the bipy-catalysed path, the Cr VI–bipy complex undergoes nucleophilic attack by the substrate to form a ternary complex. The ternary complex spontaneously experiences a redox decomposition (through two-electron transfer) in the rate-determining step leading to the product lactone and Cr IV–bipy complex. The Cr IV–bipy complex then takes part in faster steps in the further oxidation of d-glucose and is ultimately converted into a Cr III–bipy complex. In the uncatalysed path, the Cr VI-substrate ester experiences acid catalysed redox decomposition (two-electron transfer) in the rate-determining step. The uncatalysed path shows a second order dependence on [H +] and a first order dependence on each of the reactants [ d-glucose] T and [Cr VI] T. In contrast, the bipy-catalysed path shows a first order dependence on each of the reactants [H +], [ d-glucose] T and [Cr VI] T. The bipy-catalysed path is first order in [bipy] T. These observations remain unaltered in the presence of externally added surfactants. The effect of the cationic surfactant, N-cetylpyridinium chloride (CPC) and anionic surfactant, sodium dodecyl sulfate (SDS) on both the uncatalysed and bipy-catalysed path has been studied. CPC inhibits both the uncatalysed and bipy-catalysed path, while SDS catalyses these reactions. The observed micellar effects have been explained by considering hydrophobic and electrostatic interactions between the surfactants and reactants.

Nanostructured coatings approach for corrosion protection

Nanostructured surface treatment coatings based on the Self-assembled Nanophase Particle (SNAP) approach were investigated as potential replacement for chromate-based surface treatments on aircraft aluminum alloys. In the traditional sol–gel method, hydrolysis-condensation processes are followed by condensation polymerization upon film application. This process sequence provides a low temperature route to the preparation if thin coatings which are readily applied to most metallic substrates. The recent discovery of a method of forming functionalized silica nanoparticles in situ in an aqueous sol–gel process, and then cross-linking the nanoparticles to form a thin film, is an excellent example of a nanoscience approach to coatings. This Self-assembled Nanophase Particle (SNAP) process can be used to form thin, dense protective organic surface treatment coatings on Al aerospace alloys. The ability to design coating components from the molecular level upward offers tremendous potential for creating multifunctional coatings. The important components of Al alloy corrosion inhibition by chromate are storage and release of Cr VI species, inhibition of cathodic reactions (primarily oxygen reduction), and inhibition of attack at active sites in the alloy. Unlike chromate-based treatments, current SNAP coatings provide barrier-type corrosion resistance but do not have the ability to leach corrosion inhibitors upon coating damage and minimize corrosion of the unprotected area. In this study, organic inhibitors were tested for corrosion protection of aluminum alloys in combination with the (SNAP). Scanning Vibrating Electrode Technique, anodic polarization, electrochemical impedance spectroscopy, and salt spray test were used to study this new approach for chromate replacement.

Specific extraction of chromium as tetrabutylammonium-chromate and spectrophotometric determination by diphenylcarbazide: speciation of chromium in effluent streams.

A very specific, selective, simple, and inexpensive procedure was developed for the speciation of CrVI and CrIII. This method is based on the quantitative extraction of chromate and CrIII (previously oxidized to CrVI) as a tetrabutylammonium-chromate ion-pair in methyl isobutyl ketone (MIBK), and then back extraction and preconcentration with an acidic diphenylcarbazide (DPC) solution. Back extraction was applied to achieve further preconcentration by a final factor of 20. The CrVI-DPC complex was determined in back-extract by a spectrophotometer at 548 nm. Under these extraction conditions, most of the probable concomitant cations and anions remained in the first inorganic phase. The calibration curve was linear up to 0.14 microg L(-1) of CrVI with a detection limit of 2.22 ng L(-1). The developed procedure was found to be suitable for the determination of the CrVI and CrIII species in various natural water samples with a relative standard deviation of better than 1.6%. The method was successfully applied to the speciation of chromium in spiked natural water samples, and also samples of effluent from a leather treatment plant.

In situ Raman spectroscopic investigation of chromium surfaces under hydrothermal conditions

Three chromium coupons were exposed to air-saturated water at pressures of ca. 25 MPa and temperatures up to 545°C in an optically accessible flow cell. In situ Raman spectra were collected at different temperatures as the coupons were heated and then cooled. Coupons were also characterized ex situ with Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry, and X-ray diffraction. Coupons heated to either 374 or 505°C exhibited the same corrosion product, tentatively identified as α-CrOOH that originated as corrosion product released from the optical cell and/or flow system. A coupon heated to 545°C exhibited a variety of Cr III, mixed chromium valence Cr III/Cr VI, and/or Cr VI species during heating. During cooling, the same corrosion product was observed at all temperatures on this coupon and was tentatively identified as Cr 2O 3 with Cr VI incorporated into the Cr 2O 3 Cr III–oxygen network. The difference in observed corrosion processes among the three coupons was attributed to a difference in water purity to which they were exposed.

Raman spectroscopic analysis of the speciation of dilute chromate solutions

The relative concentrations of various Cr VI species are important to several mechanisms for corrosion inhibition by chromate conversion coatings and chromate containing primers. The pH dependent conversion of CrO 4 −2 to Cr 2O 7 −2 is well known, but the existence and importance of bichromate, HCrO 4 −, is more controversial. Past reports have described the pH and concentration dependence of the CrO 4 −2/HCrO 4 −/Cr 2O 7 −2 distribution, while recent reports have concluded that HCrO 4 − does not exist in aqueous media. The current work shows that HCrO 4 − does indeed exist at a pH below 6 and at low total Cr VI concentration. The Raman signature of the Cr–O–Cr bond in dichromate decreases relative to the Cr–O symmetric stretch as [Cr VI] decreases. The results are quantitatively consistent with previous reports on the CrO 4 −2/HCrO 4 −/Cr 2O 7 −2 equilibrium, and establish that HCrO 4 − is the dominant Cr VI species for low [Cr VI]. In the low concentration and slightly acidic conditions likely to be present during corrosion inhibition by Cr VI in the field, HCrO 4 − is the most prevalent form of Cr VI in water.

Certification of the contents of the chromium(III) and chromium(VI) species and total chromium in a lyophilised solution (CRM 544)†

The preparation is described of a batch of 1100 vials containing a lyophilised solution, with CrVI and CrIII contents as in natural or tap water and a total Cr content <50 µg l–1 following the European Community Directive 80/778/EEC, L229/20, D48 on the quality of drinking water. The homogeneity and stability studies and the analytical work performed for the certification of the contents of the CrIII and CrVI species and the total Cr content are also described. The paper contains all the results and gives the methods used to certify the measured concentrations of CrVI (22.79 µg l–1), CrIII (26.77 µg l–1) and total Cr (49.34 µg l–1) after reconstitution in 20 ml. A brief description of the preliminary feasibility study is also given.