Showing papers on "Chromium published in 2009"

Layered Double Hydroxides as Highly Efficient Photocatalysts for Visible Light Oxygen Generation from Water

Abstract: Oxygen generation through photocatalytic water splitting under visible light irradiation is a challenging process. In this work we have synthesized a series of Zn/Ti, Zn/Ce, and Zn/Cr layered double hydroxides (LDH) at different Zn/metal atomic ratio (from 4:2 to 4:0.25) and tested them for the visible light photocatalytic oxygen generation. The most active material was found to be (Zn/Cr)LDH with an atomic ratio of 4:2 that exhibits two absorption bands in the visible region at λmax of 410 and 570 nm. It was found that the efficiency of these chromium layered double oxides for oxygen generation increases asymptotically with the Cr content. Using iron oxalate as chemical actinometer we have determined that the apparent quantum yields for oxygen generation (Φapparent = 4 × mol oxygen/mol incident photons) are of 60.9% and 12.2% at 410 and 570 nm, respectively. These quantum yields are among the highest values ever determined with visible light for solid materials in the absence of light harvesting dye. The...

Biological Chromium(VI) Reduction in the Cathode of a Microbial Fuel Cell

Abstract: The biocathode of a microbial fuel cell (MFC) offers a promising potential for the reductive treatment of oxidized pollutants. In this study, we demonstrated biological Cr(VI) reduction in the cathode of a MFC and identified putative Cr(VI) reducing microorganisms. The MFC was continuously monitored for Cr(VI) reduction and power generation. Acetate was provided to the anode compartment as substrate and bicarbonate was added to the cathode compartment as the sole external carbon source. The contribution of biomass decay and abiotic processes on Cr(VI) reduction was minimal, confirming that most of the Cr(VI) reduction was assisted by microbial activity in the cathode, which utilizes electrons and protons generated from the oxidation of acetate in the anode compartment. Relatively fast Cr(VI) reduction was observed at initial Cr(VI) concentrations below 80 mg/L. However, at 80 mg Cr(VI)/L, Cr(VI) reduction was extremely slow. A maximum Cr(VI) reduction rate of 0.46 mg Cr(VI)/g VSS.h was achieved, which resulted in a current and power density of 123.4 mA/m(2) and 55.5 mW/m(2), respectively. The reduced chromium was nondetectable in the supernatant of the catholyte which indicated complete removal of chromium as Cr(OH)(3) precipitate. Analysis of the 16S rRNA gene based clone library revealed that the cathode biomass was largely dominated by phylotypes closely related to Trichococcus pasteurii and Pseudomonas aeruginosa, the putative Cr(VI) reducers.

Isolation of Cr(VI) reducing bacteria from industrial effluents and their potential use in bioremediation of chromium containing wastewater.

Abstract: The present study was aimed to assess the ability of Bacillus sp. JDM-2-1 and Staphylococcus capitis to reduce hexavalent chromium into its trivalent form. Bacillus sp. JDM-2-1 could tolerate Cr(VI) (4800 microg/mL) and S. capitis could tolerate Cr(VI) (2800 microg/mL). Both organisms were able to resist Cd2+ (50 microg/mL), Cu2+ (200 microg/mL), Pb2+ (800 microg/mL), Hg2+ (50 microg/mL) and Ni2+ (4000 microg/mL). S. capitis resisted Zn2+ at 700 microg/mL while Bacillus sp. JDM-2-1 only showed resistance up to 50 microg/mL. Bacillus sp. JDM-2-1 and S. capitis showed optimum growth at pH 6 and 7, respectively, while both bacteria showed optimum growth at 37 degrees C. Bacillus sp. JDM-2-1 and S. capitis could reduce 85% and 81% of hexavalent chromium from the medium after 96 h and were also capable of reducing hexavalent chromium 86% and 89%, respectively, from the industrial effluents after 144 h. Cell free extracts of Bacillus sp. JDM-2-1 and S. capitis showed reduction of 83% and 70% at concentration of 10 microg Cr(VI)/mL, respectively. The presence of an induced protein having molecular weight around 25 kDa in the presence of chromium points out a possible role of this protein in chromium reduction. The bacterial isolates can be exploited for the bioremediation of hexavalent chromium containing wastes, since they seem to have a potential to reduce the toxic hexavalent form to its nontoxic trivalent form.

A novel adsorbent for heavy metal remediation in aqueous environments.

Abstract: The objective of this study was to investigate the possibility of using maize tassel as an alternative adsorbent for the removal of chromium (VI) and cadmium (II) ions from aqueous solutions. The effect of pH, solution temperature, contact time, initial metal ion concentration and adsorbent dose on the adsorption of chromium (VI) and cadmium (II) by tassel was investigated using batch methods. Adsorption for both chromium (VI) and cadmium (II) was found to be highly pH dependent compared to the other parameters investigated. Obtained results gave an adsorption capacity of 79.1 % for chromium (VI) at pH 2, exposure time of 1h at 25 °C. Maximum capacity of cadmium of 88 % was obtained in the pH range of 5-6 at 25 °C after exposure time of 1 h. The adsorption capacities of tassel for both chromium (VI) and cadmium (II) were found to be comparable to those of other commercial adsorbents currently in use for the removal of heavy metals from aqueous wastes. These results have demonstrated the immense potential of maize tassel as an alternative adsorbent for toxic metal ions remediation in polluted water and wastewater.

Kinetics and thermodynamics studies of chromium(VI) ions adsorption onto activated carbon from aqueous solutions

Abstract: The removal of chromium(VI) from aqueous solutions by activated carbon has been investigated as a function of solution pH, initial chromium concentration C, solid/liquid ratio R and temperature T. The Freundlich and the Langmuir models have been applied and the equilibrium adsorption was found to best fit the Langmuir adsorption isotherm, where good correlation between theoretical and experimental equilibrium concentration of chromium(VI) ion was observed in most cases. The uptake distribution coefficient, KD indicated that the chromium(VI) removal was the highest at minimum solid–liquid ratio. An empirical modelling was performed by using a 24 full factorial design, and regression equation for adsorption chromium(VI) was determined from the data. The pH and the temperature are the most significant parameter affecting chromium(VI) adsorption, followed by the solid/liquid ratio whereas the initial chromium(VI) concentration has the most negative effect on the process. However, the interaction pH—solid–liquid ratio—temperature has a positive effect. The optimal parameters obtained, pH 3.62; C = 77.35 mg/L; R = 10 and T = 45 °C, have been applied to wastewater from the tannery plant of Rouiba (East Algiers, Algeria) in order to remove the contained chromium. The adsorption rate has been found to be 65.70%. A comparison of kinetic models applied to the adsorption of chromium(VI) ions on the activated carbon was evaluated for the pseudo first-order, the pseudo second-order, and intraparticle diffusion kinetic models, respectively. Results show that the pseudo first-order kinetic model was found to correlate the experimental data well. In other hand, the sorption rates at different temperatures were found to increase with increasing temperature and an activation energy of approximately 9.16 × 10−3 kJ/mol was determined. The findings of this investigation suggested that the physical sorption was the mode controlling the sorption rate. Thermodynamic parameters were calculated. The positive value of standard enthalpy of adsorption, Δ H ads ° = 14.51 × 10 − 3 kJ / mol revealed the endothermic nature of the adsorption process, the positive entropy of adsorption, Δ S ads ° = 104.12 J / mol K reflected the affinity of the adsorbent material toward chromium(VI) and the negative values of Gibbs free energy, Δ G ads ° were indicative of the spontaneity of the adsorption process.

Dose-dependent cytotoxicity of clinically relevant cobalt nanoparticles and ions on macrophages in vitro

Abstract: Despite the satisfactory short-term implant survivorship of metal-on-metal hip resurfacing arthroplasty, periprosthetic soft-tissue masses such as pseudotumours are being increasingly reported Cytotoxic effects of cobalt or chromium have been suggested to play a role in its aetiology The aim of this study was to investigate the effects of clinically relevant metal nanoparticles and ions on the viability of macrophages in vitro A RAW 2647 murine macrophage cell line was cultured in the presence of either: (1) cobalt, chromium and titanium nanoparticles sized 30-35 nm; or (2) cobalt sulphate and chromium chloride Two methods were used to quantify cell viability: Alamar Blue assay and Live/Dead assay The cytotoxicity was observed only with cobalt Cobalt nanoparticles and ions demonstrated dose-dependent cytotoxic effects on macrophages in vitro: the cytotoxic concentrations of nanoparticles and ions were 1 x 10(12) particles ml(-1) and 1000 microM, respectively The high concentration of cobalt nanoparticles required for cytotoxicity of macrophages in vitro suggests that increased production of cobalt nanoparticles in vivo, due to excessive MoM implant wear, may lead to local adverse biological effects Therefore, cytotoxicity of high concentrations of metal nanoparticles phagocytosed by macrophages located in the periprosthetic tissues may be an important factor in pathogenesis of pseudotumours

Hydrogen generation by hydrolysis of alkaline NaBH4 solution with Cr-promoted Co–B amorphous catalyst

Abstract: Cr-modified Co–B (Co–Cr–B) catalyst alloy powders have been synthesized by chemical reduction of cobalt and chromium salt at room temperature to study the hydrogen production by catalytic hydrolysis of NaBH4. The Cr/Co molar ratio was varied in the catalyst in order to study the effect of Cr doping on surface modification and catalytic efficiency of Co–B catalyst. The resulting catalyst powders were characterized by scanning electron microscopy, X-ray diffraction, X-photoelectron spectroscopy, and BET surface area measurement. When the molar ratio χCr = Cr/(Cr + Co) exceeds 9% the BET surface area of the Co–Cr–B catalyst increases by one order of magnitude as compared to that of Co–B catalyst. The catalytic activity of the Co–Cr–B for hydrogen production depends on Cr concentration: specifically, the activity increases by increasing χCr up to about 4% and then it gradually decreases by further increasing χCr. We established that the increased catalytic activity is related to the formation of chromium oxide on the catalyst surface, with the oxide favoring the dispersion of Co–B particles resulting in high catalyst surface area. However as χCr exceeds 4%, Cr starts to cover the Co active sites and the corresponding catalytic activity decreases. The highest catalytic activity was obtained at the optimum Cr-content, χCr = 4%, in Co–Cr–B catalyst, showing nearly 4 times higher H2 generation rate than that of pure Co–B catalyst. Kinetic studies on the hydrolysis reaction of NaBH4 with Co–Cr–B catalyst reveal that the concentrations of both NaBH4 and NaOH have essentially no effects on hydrogen generation rate. The promoting effect of Cr in Co–Cr–B catalyst results in lower activation energy for hydrogen production, which is 37 kJ mol−1 as compared to 45 kJ mol−1 obtained with pure Co–B powder. Finally, the possible role of Cr3+ species in the electron exchange mechanisms involved in NaBH4 hydrolysis with the Co–Cr–B catalyst has been discussed.

Synthesis of Highly Selective Magnetic Mesoporous Adsorbent

Abstract: Magnetic MCM-41 of large surface area (ca. 800 m2 g−1) and high magnetization (ca. 8.3 emu g−1) was prepared at a reasonable iron oxide nanoparticles loading of 10 wt % by a two-step synthesis process. The 8 nm iron oxide nanoparticles (i.e., 30 m2 g−1) were embedded in the MCM-41 with no observable effects on the particle morphology and pore symmetry, but a slight change (i.e., <20%) in the textural properties and surface chemistry was detected. Aminopropyls (2.7 mmol g−1) were grafted on the mesopore channels followed by the adsorption of 1.2 mmol g−1 Fe3+ to prepare selective adsorbents for arsenic(V) and chromium(VI) oxyanions. Detailed analyses indicated that the surface modifications of the magnetic and nonmagnetic MCM-41 by aminopropyls and Fe3+ yield similar site chemistry and accessibility. The Fe3+-magMCM-41 and Fe3+-MCM-41 displayed comparable adsorption capacity and selectivity. Both adsorbed only As(V) and Cr(VI) oxyanions (i.e., 1.0 and 2.0 mmol g−1, respectively), and none of the Cu(II). Th...

Adsorption of Chromium(VI) from Aqueous Solutions Using Cross-Linked Magnetic Chitosan Beads

Abstract: The performance of a cross-linked magnetic chitosan (CMC), which has been coated with magnetic fluids and cross-linked with epichlorohydrin, has been investigated for the adsorption of chromium(VI) from aqueous solutions. Infrared spectra of chitosan before and after modification show that it is a successful coating and that the cross-linking process has been effective. The influences of the pH of solution and the contact time on the adsorption amounts have been discussed, and the appropriate process conditions for the adsorption of Cr(VI) have been obtained. Scanning electron micrographs of the CMC before and after adsorption indicate that the reduction in the number of pores on the surface was due to the adsorption of Cr(VI). The adsorption experimental equilibrium data was found to fit the Langmuir model well, and the uptake of Cr(VI) was 69.4 mg/g. The experimental data for the kinetics of adsorption correlated well with the first-order Langergren rate equation, and Langergren rate constants have been determined. The adsorbed Cr(VI) ions can be removed from the exhausted CMC effectively by being regenerated with a 0.1 mol/L hydrochloric acid solution.