Capillary electrophoresis of Cr(VI) reducer Arthrobacter oxydans

Rapid and effective separation of bacteria Arthrobacter oxydans was performed using capillary electrophoresis. For optimal separation of bacteria the influence of buffer concentration, pH and applied voltage were studied. It was found that the most appropriate conditions for electrophoretic mobility measurements are as follows: applied voltage 6-14 kV; buffer concentration 5-10 mM pH 6-8. At the stationary phase of growth there are always two main heterogeneous peaks. They are connected with the morphology of bacteria as well as with cell aggregation. The heterogeneity of samples may be explained by surface modifications of bacterial cells.     

Speciation studies by capillary electrophoresis- Simultaneous determination of chromium(III) and chromium(VI)

A method for the simultaneous determination of chromium(III) and chromium(VI) by capillary electrophoresis (CE) has been developed. The chromium(III) has been chelated with 1,2-cyclohexanediaminetetraacetic acid (CDTA) in order to impart a negative charge and similar mobility to both the chromium(III) and the chromium(VI) species. The effects of the amount of the reagent, pH and heating time required to complete the complexation have been studied. Factors affecting the CE behaviour such as the polarity of electrodes and the pH of electrophoretic buffer have been investigated. The separated species have been monitored by direct UV measurements at 214 nm. The detection limits achieved are 10 g/l for Cr(VI) and 5 g/l for Cr(III) and linear detector response is observed up to 100 mg/l. The procedure has been applied to the determination of both chromium species in industrial electroplating samples and its accuracy was checked by comparing the results (as total chromium) with those of atomic absorption spectrometry. No interference occurred from transition metal impurities under optimized separation conditions. The method is also shown to be feasible for determining Cr(III) as well as other metal ions capable to form complexes with CDTA (like iron(III), copper(II), zinc(II) and manganese(II)) in pharmaceutical preparations of essential trace elements.     

Speciation studies by capillary electrophoresis- Simultaneous determination of chromium(III) and chromium(VI)

A method for the simultaneous determination of chromium(III) and chromium(VI) by capillary electrophoresis (CE) has been developed. The chromium(III) has been chelated with 1,2-cyclohexanediaminetetraacetic acid (CDTA) in order to impart a negative charge and similar mobility to both the chromium(III) and the chromium(VI) species. The effects of the amount of the reagent, pH and heating time required to complete the complexation have been studied. Factors affecting the CE behaviour such as the polarity of electrodes and the pH of electrophoretic buffer have been investigated. The separated species have been monitored by direct UV measurements at 214 nm. The detection limits achieved are 10 microg/l for Cr(VI) and 5 microg/l for Cr(III) and linear detector response is observed up to 100 mg/l. The procedure has been applied to the determination of both chromium species in industrial electroplating samples and its accuracy was checked by comparing the results (as total chromium) with those of atomic absorption spectrometry. No interference occurred from transition metal impurities under optimized separation conditions. The method is also shown to be feasible for determining Cr(III) as well as other metal ions capable to form complexes with CDTA (like iron(III), copper(II), zinc(II) and manganese(II)) in pharmaceutical preparations of essential trace elements.     

Kinetic studies of the oxidation of chromium(III) by N-bromosuccinimide

Summary A kinetic study of the oxidation of chromium(III) by N-bromosuccinimide (NBS) in aqueous solutions and H₂O-MeOH solvent mixtures were performed. The kinetics in aqueous solutions obeyed the rate law: d[Cr^VI]/dt = {k ₄ K _h K ₂[NBS][Cr^III]_T}/[₊]{1 + K _h/[H⁺] + (K ₁ + K _h K ₂/[H⁺][NBS])} where K _h, K ₁ and K ₂ are the hydrolysis constant of [Cr^III(H₂O)₆]³⁺, and pre-equilibrium formation constants for the protonated and deprotonated precursor complexes, respectively. An innersphere mechanism is proposed. An argument based on isokinetic correlations among activation parameters for the oxidation of a series of cobalt(II) and chromium(III) complexes including [Cr(H₂O)₆]³⁺ is presented in support of a common mechanism for these reactions. Abstracted from the Ph.D. Thesis (Ain Shams University) of A. E.-D. M. Abdel-Hady.     

Organic-inorganic hybrid of chitosan/organoclay bionanocomposites for hexavalent chromium uptake

Organic-inorganic hybrid of chitosan and nanoclay (Cloisite 10A) was chosen to develop a nanomaterial with combine properties of hydrophilicity of an organic polycation and adsorption capacity of inorganic polyanion. The chitosan/clay nanocomposite (CCN) was prepared by solvent casting method. The material synthesis was found most efficient in adsorbent behavior was studied in detail taking Cr(VI) as representative ion. The chemical, structural and textural characteristics of the material were determined by FTIR, XRD, TEM, SEM and EDAX analysis. XRD and TEM results indicated that an exfoliated structure was formed with addition of small amounts of MMT-Na+(montmorillonite-Na(+)) to the chitosan matrix. These composite material were used for the removal of chromium(VI) from aqueous solution. The conditions for the adsorption by the composite have been optimized and kinetics and thermodynamic studies were performed. Though the adsorption takes place in wide pH range, pH 3 was found most suitable and at this pH the adsorption data were modeled using the Langmuir and Freundlich isotherms at 15 °C and 35 °C, where the data fitted satisfactorily to Langmuir isotherms, the R(2) values being 0.998 and 0.999 respectively indicating unilayer adsorption. Based on Langmuir model, Q(o) was calculated to be 357.14 mg/g. The adsorption showed pseudo second order kinetics with a rate constant of 8.0763 × 10(-4) g mg(-1) min(-1) at 100 ppm Cr(VI) concentration.     

An ENAA and PGAA comparative study of anoxic Black Sea sediments

A comparative study performed by means of both epithermal neutron activation analysis (ENAA) and prompt gamma neutron activation analysis (PGAA) concerning the distribution of 9 elements (Na, Al, Cl, K, Ca, Ti, Mn Fe and Sm) in unconsolidated sediments collected from the Black Sea anoxic region is presented. Principal component analysis (PCA) was used as a supplementary method in interpreting experimental data. The final results have shown that eight of them (Na, Al, Cl, K, Ti, Mn, Fe and Sm) have concentrations which differ by less than 30% from the corresponding concentrations of Upper Continental Crust (UCC), confirming the continental origin of sediments. At the same time, Ca concentration was about six times higher than that of UCC, this significant enrichment being due to the presence of the neighbouring oxygenated zone of considerable deposits of shell debris, characteristic of the Western Black Sea Continental Platform. Excepting Na and Mn, the concentrations of all other elements involved in this intercomparison were coincident within one standard deviation which attests the accuracy of this test. At the same time, PCA revealed the presence of five clusters, which could be put into concordance with the mineralogical composition of sediments, each of them consisting in equal proportions of elements determined by both methods.     

Calorimetric studies of cementitious materials — chromium interaction

Cementitious systems based on portland cement are used for immobilization of toxic and hazardous wastes. The addition of waste material may impact the hydration reaction in cement matrix and consequently the setting and hardening process. The progress of reaction can be monitored by heat evolution measurements and the calorimetric results can indicate the declination from standard behaviour.In this study the microcalorimetry was used to evaluate the heat output during the hydration of cements in the presence of different chromium containing salts, viz. CrCl₃, Cr₂(SO₄)₃, Na₂CrO₄ and K₂CrO₄.     

Fluorides of ruthenium and chromium: Theoretical studies

The electronic and structural characteristics of CrF₅, CrF₄, RuF₅ and RuF₄ were studied. Ab initio (SCF-CI) calculations were performed with different structures and spin states for each complex. The favored conformation always corresponds to the highest multiplicity: doublet for CrF₅ in D_3h, triplet for CrF₄ in T_d, quadruplet for RuF₅ in C_4v and quintuplet for RuF₄ in D_4h symmetry.     

Comparison of chromium and nickel uptake of plants grown in different soils

The chromium and nickel uptake of ryegrass has been examined in pot experiments in extremely different soils, poor sandy and fertile black chernozem. The effect of calcium carbonate doses and nitrogen supply on heavy metal uptake of the plant has been studied for chromium and nickel loadings (0–100 mg/kg Cr³⁺ or Ni²⁺) applied as inorganic salts. The ability to uptake Cr³⁺ and Ni²⁺ differs significantly and is highly affected by the characteristics of soils, and depends on the metal investigated. The heavy metal uptake of the plant differs significantly in acid, colloid deficient sandy soils; while artificial chromium contamination did not modify the dry-matter production in the pots in either soil, a large quantity of nickel reduced the yields significantly. Nitrogen application did not change significantly the uptake of heavy metals. Lime application reduced the Ni²⁺ uptake of plants considerably, especially in sandy soil. In case of a calcium carbonate addition the dry-matter production of the plant was not affected by nickel. In chernozem soil the effect of lime application – i.e., the reduction of nickel uptake – was of a lesser degree. The significantly lesser Cr³⁺ uptake was further limited by a calcium carbonate application for both soils studied. A graphic presentation of these effects is given.     

Comparison of chromium and nickel uptake of plants grown in different soils

The chromium and nickel uptake of ryegrass has been examined in pot experiments in extremely different soils, poor sandy and fertile black chernozem. The effect of calcium carbonate doses and nitrogen supply on heavy metal uptake of the plant has been studied for chromium and nickel loadings (0–100 mg/kg Cr³⁺ or Ni²⁺) applied as inorganic salts. The ability to uptake Cr³⁺ and Ni²⁺ differs significantly and is highly affected by the characteristics of soils, and depends on the metal investigated. The heavy metal uptake of the plant differs significantly in acid, colloid deficient sandy soils; while artificial chromium contamination did not modify the dry-matter production in the pots in either soil, a large quantity of nickel reduced the yields significantly. Nitrogen application did not change significantly the uptake of heavy metals. Lime application reduced the Ni²⁺ uptake of plants considerably, especially in sandy soil. In case of a calcium carbonate addition the dry-matter production of the plant was not affected by nickel. In chernozem soil the effect of lime application – i.e., the reduction of nickel uptake – was of a lesser degree. The significantly lesser Cr³⁺ uptake was further limited by a calcium carbonate application for both soils studied. A graphic presentation of these effects is given.     

Density functional studies on chromium catalyzed ethylene trimerization

Mechanism of ethylene trimerization using chromium catalyst is investigated using density functional methods. Recent experimental results indicate Cr-based homogeneous catalysts to follow metallacycle pathway in ethylene tri-, teta- and oligomerization reactions. Given the importance of chlorinated Cr-based active catalysts in these reactions, we have used “bare” minimal ligands like Cl^? and considered catalytic cycles with neutral or cationic intermediates starting with [Cr(II)Cl₂(ethylene)₂] and [Cr(II)Cl(ethylene)₂]⁺, respectively. We have compared both ‘Cossee’ and the ‘metallacycle’ mechanisms on these model systems utilizing density functional computations at B3LYP/LANL2DZ(d,p) level. The metallacycle mechanism with cationic Cr(II)–Cr(IV) intermediates is found to be the most favored path, with oxidative coupling of two coordinated ethylene to form the chromacyclopentane being the rate determining step (RDS). We also found that with neutral intermediates the Cossee pathway rather than the metallacycle mechanism is followed. Thus in spite of the simplicity of using just Cl^? as ligand in the model catalytic intermediates, our computational results match remarkably well with many recent and important experimental findings.     

Corrosion studies of a chromium steel in imitated seawater

A series of in-door experiments was performed to get some insight into the corrosion behavior of a commercial alloy Fe-12% Cr (3CR12) exposed to imitated seawater. Applying different analytical methods, the main corrosion process was found to be the formation of flakes on the surface which, peel off after they have reached a certain size. Some Cr is dissolved in the solution, its relative concentration with respect to Fe is higher than in the bulk material. The flakes consist mainly of mixed oxihydroxides of the type FeOOH containing some Cr and Mg. The oxidic layer on the interface is very thin, behaves essentially stationary with a slight growth of about 0.05 nm/day. It consists of Cr oxide with some inclusions of Fe and Mg and is not of a chromite type. Immediately below this oxidic layer, the metallic substrate exhibits a thin layer depleted in Cr and behaving like a-Fe (bcc). As compared with stainless steel, potentiostatic current vs. time records at anodic potentials below the pitting potential indicate a very different stability of the surface films for 3CR12. The kinetics of the passive layer formation on 3CR12 was found to follow a parabolic law initially and to change later (after 10...100 seconds in deaerated solution and even earlier in aerated solution) to a linear law. After some time, pitting corrosion and/or cracks due to internal stresses play the dominant role. Cr does not form a protective oxidic layer. The surface morphology of samples exposed at -200 mV for 20 and 80 minutes has been studied by scanning electron microscopy and scanning Auger microprobe. The results reflect the competing formation of oxidic layers and pitting, the participation of Cr in the dissolution process. They also suggest that Mg, which is a component of the solution was incorporated into the rust and some Mg was also found on the metallic surface.     

Application of tracer packet technique for multielemental uptake studies by ceric vanadate

We have synthesized and characterized an inorganic ion-exchanger, ceric vanadate. The exchanger is stable towards thermal, chemical and radiation doses within the appreciable working range. The molecular formula of the ceric vanadate has been formulated as 4CeO₂·5V₂O₅·12H₂O. Multi elemental uptake studies by the newly synthesized exchanger have been carried out using “tracer packet” technique. It has been found that the exchanger is highly suitable in removing Tl, Pb, Bi and Po. However, the uptake of Cu and As by the exchanger is moderate while that of Hg, Zn, Ga, Ge, and Se has been found negligible.     

Effects of several compounds on the chromium uptake from surrounding medium by HeLa cells (author's transl)

Radiochromium uptake from surrounding by HeLa cells was examined, the results were as follows: 1) The chromium uptake by the cells after a certain period of incubation in Ca-Mg free phosphate buffered solution (PBS) with radiosodiumchromate (Na251CrO4) was higher than that in serum free Eagle's minimum essential medium with the same concentration of the radiochromate. 2) When the various amount of L-ascorbic acid was added to the above rabiochromate containing PBS, the chromium uptake by the cells decreased with dependence on the concentration of the acid in the surrounding medium. However, when sodiumthiosulfate was added to the medium, no remarkable effect was found. 3) When cells were incubated in the radiochromic chloride(51CrCl3)containing medium with 6.5 microgram/ml of sodium oxalate, sodium acetate or sodium nitrate, the chromium uptake by the cells increased in comparison with the control. Above results suggested that the chromium uptake by the HeLa cells from surrounding medium was affected by several chemicals and the uptake or binding capacity of chromium was closely related to the reported cytotoxicity of the chromium compounds.     

Kinetic studies on the oxidation of [N-phenylethylethylenediaminetriacetate]chromium(III) by periodate

Summary Kinetic studies of the oxidation of [Cr^IIIZ(H₂O)](Z=N-phenylethylethylenediaminetriacetate) by periodate ion, to produce chromium(VI), were carried out in aqueous solutions. The reaction is first order with respect to both total chromium(III) and total periodate, and the rate is inversely dependent upon H⁺ in the 5.43–7.02 pH range. The reaction may follow a two-step inner-sphere electron transfer mechanism. The activated parameters are reported. Steric effects of the phenyl ring account for the smaller electron-transfer rate constants for [Cr^IIIZ(H₂O)] compared to [Cr^III(TOH)(H₂O)], (TOH=N-(2-hydroxyethyl)ethylenediaminetriacetate).     

Infrared spectroscopic studies of the acidity of chromium molybdate catalysts

NH₃ adsorption on chromium molybdate at 25 and 350°C has been studied. Ammonia chemisorption has been found on electron-acceptor and Brønsted centers. The acceptor capacity of cations is shown to change as compared to individual oxides and upon high-temperature interaction with ammonia.

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NH₃ 25° 350°. . .

     

X‐ray photoelectron spectroscopy studies of chromium compounds

Photoelectron spectra of a number of chromium oxides and other compounds were studied under high spectral resolution conditions chosen to reduce the possibility of differential charging. Some of the suite of Cr(III) compounds chosen for study produced Cr 2p spectra containing fine structure that could be identified with multiplet splitting. The splitting patterns produced were similar for all trivalent binary and ternary oxides and sulphides whose patterns closely reproduced the splitting predicted for the Cr(III) free ion by Gupta and Sen. The fine structure observed for compounds such as chromium (III) chloride had a distinctly different pattern. A number of other chromium (III) compounds were studied that did not exhibit the fine structure described above; nonetheless, well‐defined line shapes and reproducible peak centroids were obtained by fitting protocols. The use of such information to determine surface chemistry on chromated steels is described, based on the spectral knowledge of chromium (III) oxides and hydroxides and the chromium (VI) oxide systems. Copyright © 2004 John Wiley & Sons, Ltd.     

Determination of chromium by atomic absorption spectrophotometry of chromium acetylacetonate : Determination of chromium in sea water

A sensitive atomic absorption method is described for the determination of chromium at sub-microgram levels. Chromium(III) is converted to its acetylacetone complex and extracted into MIBK. The atomic absorption sensitivity is thus en hanced two-fold compared to chromium (III) in aqueous medium. The detection limit of chromium is 0.015 p.p.m. with an acetylene-air flame; the sensitivity obtained with other chromium, methods under the same instrumental conditions is compared. The method can be applied to the determination of chromium in sea water; at a level of ca. 1.6 μg Cr/1, the precision is ±0.06 μg/1.