Mol Of Cr Research Articles

Mechanisms of chromium(VI) removal from solution by zeolite and vermiculite modified with iron(II).

Mechanisms of Cr(VI) reduction by Fe(II) modified zeolite (clinoptilolite/mordenite) and vermiculite were evaluated. Adsorbents were treated with Fe(SO4)·7H2O to saturate their exchange sites with Fe(II). However, this treatment decreased their CEC and pHPZC, probably due to the dealumination process. Vermiculite (V-Fe) adsorbed more Fe(II) (21.8mgg-1) than zeolite (Z-Fe) (15.1mgg-1). Z-Fe and V-Fe were used to remove Cr(VI) from solution in a batch test to evaluate the effect of contact time and the initial concentration of Cr(VI). The Cr(VI) was 100% reduced to Cr(III) by Z-Fe and V-Fe in solution at 18mg L-1 Cr(VI) after 1min. Considering that 3mol of Fe(II) are required to reduce 1mol of Cr(VI) (3Fe+2 + Cr+6 → 3Fe+3 + Cr+3), the iron content released from Z-Fe and V-Fe was sufficient to reduce 100% of the Cr(VI) in solutions up to 46.8mg L-1 Cr(VI) and about 90% (V-Fe) and 95% (Z-Fe) at 95.3mg L-1 Cr(VI). The Fe(II), Cr(III), Cr(VI), and K+ contents of the adsorbents and solutions after the batch tests indicated that the K+ ions from the [Formula: see text] solution were the main cation adsorbed by Z-Fe, while vermiculite did not absorb any of these cations. The H+ of the acidic solution (pH around 5) may have been adsorbed by V-Fe. The release of Fe(II) from Z-Fe and V-Fe involved cation exchange between K+ and H+ ions from solution, respectively. The reduction of Cr(VI) by Fe(II) resulted in the precipitation of Cr(III) and Fe(III) and a decrease in the pH of the solution to < 5. As acidity limits the precipitation of Cr(III) ions, they remained in solution and were not adsorbed by either adsorbent (since they prefer to adsorb K+ and H+). To avoid oxidation, Cr(III) can be removed by precipitation or the adsorption by untreated minerals.

From polyethylene waxes to HDPE using an α,α'-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridyl-chromium(iii) chloride pre-catalyst in ethylene polymerisation.

Five examples of α,α'-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridyl-chromium(iii) chlorides (aryl = 2,6-Me2Ph Cr1, 2,6-Et2Ph Cr2, 2,6-i-Pr2Ph Cr3, 2,4,6-Me3Ph Cr4, 2,6-Et2-4-MePh Cr5) have been synthesized by the one-pot template reaction of α,α'-dioxo-2,3:5,6-bis(pentamethylene)pyridine, CrCl3·6H2O and the corresponding aniline. The molecular structures of Cr1 and Cr4 reveal distorted octahedral geometries with the N,N,N-ligand adopting a mer-configuration. On activation with an aluminium alkyl co-catalyst, Cr1-Cr5 exhibited high catalytic activities in ethylene polymerization and showed outstanding thermal stability operating effectively at 80 °C with activities up to 1.49 × 107 g of PE (mol of Cr)-1 h-1. Significantly, the nature of the co-catalyst employed had a dramatic effect on the molecular weight of the polymeric material obtained. For example, using diethylaluminium chloride (Et2AlCl) in combination with Cr4 gave high density/high molecular weight polyethylene with broad molecular weight distributions (30.9-39.3). By contrast, using modified methylaluminoxane (MMAO), strictly linear polyethylene waxes of lower molecular weight and narrow molecular weight distribution (1.6-2.0) were obtained with vinyl end-groups.

Role of chemical hardness in the adsorption of hexavalent chromium species onto metal oxide nanoparticles

We employed chemical hardness to explain the adsorption behaviour of Cr(VI) anions on metal oxide sorbents. In the absence of electrostatic attraction at pH values around the point of zero charge, silica and titania nanoparticles adsorbed a very small amount of Cr(VI) anions. This lack of adsorption is because the magnitude of the chemical hardness is too high to pair with the less hard Cr(VI) anions. In contrast, maghemite nanoparticles with relatively weak values of hardness adsorbed 0.021mol of Cr(VI) per one mole of OH groups on the particle surface, 2.7 times that of titania. The magnitude of the chemical hardness decreases in the order of SiO2≫TiO2>γ-Fe2O3, which is determined by the hardness of constituting metal cations. This result indicated that the adsorption onto an oxide surface is governed by metal cations in that their chemical hardness determines the basicity of surface OH groups, which regulates adsorption as well as protonation/deprotonation. The hardness match between a sorbent and a sorbate may be interpreted as their chemical affinity. We demonstrated that the chemical affinity is the principal driving force for adsorption of Cr(VI) anions onto the oxides.

Investigation into chromium dioxide nanopowders obtained under hydrothermal conditions in the presence of molybdenum and antimony modifiers

This paper presents the results of studies of magnetic CrO2 nanopowders obtained under hydrothermal conditions in the presence of Mo and Sb modifying compounds. The dependences of the composition, morphology, and structural and magnetic characteristics on the amount of introduced Sb modifier on 1 mol of Cr are presented.

Preparation, Structure, and Ethylene Polymerization Behavior of Bis(imino)pyridyl Chromium(III) Complexes

The synthesis, characterization, and ethylene polymerization behavior of a family of chromium(III) complexes of formula [2,6-bis(imino)pyridyl]CrCl3 is reported. The X-ray diffraction studies of two of the new compounds show that the geometry around the chromium atom is octahedral, with the three chlorine ligands in a mer disposition. The distance between the metal and one of the trans-disposed chlorine atoms is significantly longer than the other two Cr−Cl distances. Treatment of the complexes [2,6-bis(imino)pyridyl]CrCl3 with methylaluminoxane (MAO) leads to very active ethylene polymerization catalysts that afford highly linear polyethylene. The substituents at the ortho position of the N-aryl groups of the 2,6-bis(imino)pyridyl ligands modulate both the catalytic activity and the molecular weights of the resulting polyethylene. The most active catalysts are those with two substituents at the ortho position of the N-aryl groups (activities up to 4 × 107 g (mol of Cr)-1 bar-1 h-1 are achieved). Regarding the size of the substituents, the activity and the molecular weights follow an opposite trend. Systems with two small substituents lead to very active systems, but the molecular weight of the polyethylene is lower than when bulkier substituents are present.

Disproportionation and Nuclease Activity of Bis[2-ethyl-2-hydroxybutanoato(2−)]oxochromate(V) in Neutral Aqueous Solutions1

Complex 1, [Cr(V)O(ehba)2]- (ehba = 2-ethyl-2-hydroxybutanoate(2-)) is the most studied model compound of relevance to the biological activity of Cr(V) with regard to Cr-induced cancers. The first detailed kinetic study of disproportionation of 1 under neutral pH conditions (pH 6.0-8.0, [NaClO4] = 1.0 M, 37 degrees C) is reported. Kinetic data were collected by stopped-flow and conventional UV-vis spectroscopies and processed by the global analysis method. The disproportionation, which follows the stoichiometry 3Cr(V) --> 2Cr(VI) + Cr(III) (1), leads to release of 5 mol of H+/3 mol of Cr(V). Reaction 1 is accelerated by phosphate, but is not affected by acetate, HEPES, or Tris buffers. Initial rates of Cr(V) decay are directly proportional to [Cr(V)]0 (0.020-1.0 mM); they increase with an increase in the pH values and decrease in the presence of a large excess of ehba ligand. The first direct evidence for the formation of Cr(IV) intermediates in reaction 1 has been obtained; however, their UV-vis spectral properties were different from those of the well-characterized Cr(IV)-ehba complexes. The Cr(III) products of reaction I in phosphate buffers differ from those in the other buffers. A mechanism is proposed for reaction 1 on the basis of kinetic modeling. Influences of the reaction time and conditions on the extent of plasmid DNA cleavage induced by 1 have been studied under conditions corresponding to those of the kinetic studies. A comparison of the kinetic and DNA cleavage results has shown that direct interaction of 1 with the phosphate backbone of DNA is the most likely first step in the mechanism of DNA cleavage in neutral media. Small additions of Mn(II) ((0.01-0.1)[Cr(V)]0) did not affect the rate and stoichiometry of reaction 1, but suppressed the formation of Cr(IV) intermediates (presumably due to the catalysis of Cr(IV) disproportionation). However, much higher concentrations of Mn(II) ((0.1-1.0)[Cr(V)]0) were required to inhibit DNA cleavage induced by 1. Thus, contrary to previous reports (Sugden, K. D.; Wetterhahn, K. E. J. Am. Chem. Soc. 1996, 118, 10811-10818), inhibition by Mn(II) does not indicate a key role of Cr(IV) in Cr(V)-induced DNA cleavage.

NAD(P)H-dependent chromium (VI) reductase of Pseudomonas ambigua G-1: a Cr(V) intermediate is formed during the reduction of Cr(VI) to Cr(III).

An NAD(P)H-dependent Cr(VI) reductase (molecular weight = 65,000) was purified from a Cr(VI)-resistant bacterium, Pseudomonas ambigua G-1. Stoichiometric analysis of the enzymatic reaction showed that the enzyme catalyzed the reduction of 1 mol of Cr(VI) to Cr(III) while consuming 3 mol of NADH as an electron donor. Chromium(VI) was reduced to Cr(V) by one equivalent NADH molecule in the absence of the enzyme. Electron spin resonance analysis showed that Cr(V) species (g = 1.979) was formed during the enzymatic reduction. The amount of Cr(V) species formed was about 10 times larger than that of the nonezymatic reduction. These findings show that the Cr(VI) reductase reduced Cr(VI) to Cr(III) with at least two reaction steps via Cr(V) as an intermediate.

Adsorption and desorption of metals on ferrihydrite: reversibility of the reaction and sorption properties of the regenerated solid

The feasibility of recycling ferrihydrite in a metal adsorption process was investigated. In this process, metal ions are removed from dilute solution by sorption onto ferrihydrite and are then desorbed into a more concentrated solution at lower pH. The ferrihydrite thus becomes available for reuse in subsequent sorption operations. Copper, lead, nickel, zinc, cadmium, and chromium(III) can be quantitatively sorbed onto ferrihydrite at pH 9.5. Lowering the pH to 4.5 substantially desorbs the metals. However, for all metals except cadmium, a measurable fraction of the bound metal is not easily desorbed. This fraction increases with increasing pH and duration of the high-pH stage and increases more or less continuously in sequential cycles. The retention of metals in the solid does not interfere with sorption in subsequent cycles, within the range of concentrations investigated. This included retention of up to 0.7 mol of Cr(III)/mol of Fe in the adsorbent. This process has the potential to provide the advantages of sorption processes, including low residual soluble metal concentrations, moderate pH requirements, and simultaneous removal of several metals, without excessive sludge production.

Heat capacities and thermodynamic properties of Cr 3Te 4 and Ni 3Te 4 from 298 to 950 K. Structural order-disorder transitions

The heat capacities of Cr3Te4 and Ni3Te4 have been determined in the range 298 to about 950 K by adiabatic-shield calorimetry with intermittent electrical energy inputs and temperature equilibration between inputs. Values of heat capacity, enthalpy, and entropy are tabulated for selected temperatures after inclusion of earlier low-temperature results. For Cr3Te4 a λ-type transition occurs with heat capacity maximum at 903 K, presumably connected with disordering of vacancies in the grossly defective structure. The estimated order-disorder entropy change is 2.2 J K−1 mol−1 for Cr0.75Te, which compares reasonably well with the assumed disordering of 1/4 mol of Cr and vacancies (ΔS = 2.88 J K−1 mol−1). For Ni3Te4 no transition occurs, except for a slightly enhanced heat capacity in the region 350 to 500 K, indicating only a slight tendency for vacancy ordering under the conditions in question. The vacancies thus probably remain largely disordered in every other metal layer parallel to 001 down to the lowest temperatures, and the zero-point configurational entropy is accordingly approximately 1.5 J K−1 mol−1 for 1/7 Ni3Te4.