Low Cr Concentrations Research Articles

Effect of Chromium on Microstructure and Mechanical Properties of Nb–Si Hypoeutectic and Eutectic Alloys

The niobium silicide based alloys have been found to be the best alternative over Ni-based superalloys as they provide the balance between both ductility and fracture toughness at low temperature and strength and creep resistance at elevated temperature. The concept of making Nb-silicide based alloys is to form hard silicide phase which is thermodynamically stable and imparts adequate strength to the material at high temperature, within the primary ductile solid solution phase `of niobium. However, the main concern of using these alloys at high temperature is their low oxidation resistance and strength at high temperature. To overcome these limitations, alloying elements can be added, which will improve mechanical properties as well as high temperature oxidation resistance of the alloys. Our present investigation is to study the effect of chromium on the microstructure of Nb–Si hypoeutectic and eutectic alloys. Cylindrical rods of pre-set composition are synthesized by vacuum-arc-melting-cum-suction casting using argon atmosphere. The binary hypoeutectic alloys show the presence of primary (Nb)ss dendrites and lamellar eutectic between (Nb)ss and Nb3Si whereas primary Nb3Si along with eutectic (Nb)ss/Nb3Si are observed in the eutectic binary alloy. In the hypoeutectic Nb–Si–Cr ternary alloys, primary dendritic (Nb)ss, silicide (Nb3Si and/β-Nb5Si3), Laves (Cr2Nb)(depends on chromium concentration) and their eutectic phases are observed whereas Laves phase is found in the microstructure of eutectic Nb–Si–Cr ternary alloys even at very low concentration of Cr (2 at.%). At room temperature, compressive properties of these rods have been reported.

Photosynthesis performance, antioxidant enzymes, and ultrastructural analyses of rice seedlings under chromium stress

The present study was conducted to examine the effects of increasing concentrations of chromium (Cr(6+)) (0, 25, 50, 100, and 200μmol) on rice (Oryza sativa L.) morphological traits, photosynthesis performance, and the activities of antioxidative enzymes. In addition, the ultrastructure of chloroplasts in the leaves of hydroponically cultivated rice (O. sativa L.) seedlings was analyzed. Plant fresh and dry weights, height, root length, and photosynthetic pigments were decreased by Cr-induced toxicity (200μM), and the growth of rice seedlings was starkly inhibited compared with that of the control. In addition, the decreased maximum quantum yield of primary photochemistry (Fv/Fm) might be ascribed to the decreased the number of active photosystem II reaction centers. These results were confirmed by inhibited photophosphorylation, reduced ATP content and its coupling factor Ca(2+)-ATPase, and decreased Mg(2+)-ATPase activities. Furthermore, overtly increased activities of antioxidative enzymes were observed under Cr(6+) toxicity. Malondialdehyde and the generation rates of superoxide (O2̄) also increased with Cr(6+) concentration, while hydrogen peroxide content first increased at a low Cr(6+) concentration of 25μM and then decreased. Moreover, transmission electron microscopy showed that Cr(6+) exposure resulted in significant chloroplast damage. Taken together, these findings indicate that high Cr(6+)concentrations stimulate the production of toxic reactive oxygen species and promote lipid peroxidation in plants, causing severe damage to cell membranes, degradation of photosynthetic pigments, and inhibition of photosynthesis.

Collapse of the low temperature insulating state in Cr-doped V2O3 thin films

We have grown epitaxial Cr-doped V2O3 thin films with Cr concentrations between 0% and 20% on (0001)-Al2O3 by oxygen-assisted molecular beam epitaxy. For the highly doped samples (>3%), a regular and monotonous increase of the resistance with decreasing temperature is measured. Strikingly, in the low doping samples (between 1% and 3%), a collapse of the insulating state is observed with a reduction of the low temperature resistivity by up to 5 orders of magnitude. A vacuum annealing at high temperature of the films recovers the low temperature insulating state for doping levels below 3% and increases the room temperature resistivity towards the values of Cr-doped V2O3 single crystals. It is well-know that oxygen excess stabilizes a metallic state in V2O3 single crystals. Hence, we propose that Cr doping promotes oxygen excess in our films during deposition, leading to the collapse of the low temperature insulating state at low Cr concentrations. These results suggest that slightly Cr-doped V2O3 films can be interesting candidates for field effect devices.

함산소불화 활성탄소섬유를 이용한 저농도 크롬이온의 흡착 특성

In this work, activated carbon fibers (ACFs) were oxyfluorinated and their adsorption ability for the low concentration of hexavalent chromium in an aqueous solution was investigated. The pore structure and surface properties of ACFs were exam- ined by BET and X-ray Photoelectron Spectroscopy (XPS), respectively. Due to the oxyfluorination treatment, the content of (C-O) bond on ACFs surface which influences the adsorption capacity for heavy metal ions increased largely, resulting that Cr 6+ adsorption equilibrium reached quickly within 10 min. In addition, the maximum removal efficiency at the initial Cr 6+ concentration of 20 ppm was observed, which is a 100% improvement compared to that of non-treated ACFs. These results suggest that the oxyfluorination of ACFs can be applied as a good surface treatment for the effective adsorption of the low concentration of Cr 6+ .

Chromium (VI) detoxification by oxidation and flocculation of exopolysaccharides from Arthrobacter sp. B4

The exopolysaccharides from Arthrobacter sp. B4 (B4-EPS) exhibited an excellent chromium (VI) (Cr(VI)) removal capability without any pH adjustment, whereby 50mgL−1 of Cr(VI) could be completely removed by 4gL−1 of B4-EPS. The kinetics tests revealed that the first-order rate constant was 8.3×10−5s−1 and the optimal reaction time was 720min. However, a low initial concentration of Cr(VI) (5–30mgL−1) would accelerate the reaction rate of Cr(VI) removal and shorten reaction time to less than 360min. Meanwhile, the X-ray photoelectron spectroscopy (XPS) and ultraviolet–visible (UV–vis) spectra indicated that Cr(VI) was reduced to Cr(III) by B4-EPS in accordance with the emergence of the green reaction products. Furthermore, the Fourier transform-infrared spectra (FT-IR) and nuclear magnetic resonance (NMR) showed that carboxyl and hydroxyl groups of B4-EPS contributed to Cr(VI) reduction. Additionally, a feasible scheme for Cr(VI) detoxification by oxidation and flocculation of B4-EPS is presented.

Adsorption characteristics of bio-adsorbent on chromium(III) in industrial wastewater.

The removal of chromium(III) (Cr(III)) from industrial wastewater by various low-cost methods has been widely investigated. In this paper, a type of bio-adsorbent was prepared using rice straw modified by fermentation and simple chemical treatment. The aim is to detect the adsorption mechanism and characteristics on Cr(III) ions. The analysis shows that the bio-adsorbent possesses four modified characteristics for Cr(III) adsorption. The first one is the acquired physical adsorption involving concave and convex structures. The second one is the effects of the hydrogen bonding surface hydroxyl groups and the metal chromium ion with complexation. The third one is mainly caused by hydrophilic active groups that possess carboxyl and hydroxyl groups during microbial degradation to combine with ions. The final one is the bio-adsorbent had high adsorption for low concentration of Cr(III) ions. The highest removal of around 97.45% was obtained at pH 5.0, bio-adsorption dosage of 0.5 g L(-1), and initial Cr(III) concentration of 20 mg L(-1). The adsorption process followed the pseudo second-order model (R(2) > 0.99), while the isotherms were fitted to the Freundlich equation (68.1926 mg g(-1)), mainly by chemical adsorption. This study demonstrates the potential of using this biosorbent to remove Cr(III) from both synthetic and industrial wastewater.

Fractionation behavior of chromium isotopes during coprecipitation with calcium carbonate: Implications for their use as paleoclimatic proxy

Interest in chromium (Cr) isotope incorporation into carbonates arises from the observation that Cr isotopic composition of carbonates could be used as a paleoclimate proxy to elucidate past fluctuations of oxygen contents in atmosphere and hydrosphere. The use of Cr isotopes to track paleoenvironmental changes, for example related to the rise of oxygen during the Archaean and Protoerozoic, needs careful assessment of the signal robustness and necessitates a thorough understanding of the Cr cycle in Earth system processes. We conducted experiments testing the incorporation of chromate into the calcite lattice to investigate isotopic changes facilitated by the coprecipitation process. Our experiments indicate enrichment in Cr concentration in the precipitates compared to the solutions, consistent with previous reports of Cr enrichment in chemical sediments compared to ambient seawater. The fractionation of Cr isotopes during calcium carbonate coprecipitation was assumed to be small, based on previously published data of modern seawater and modern non-skeletal marine carbonates. However, results from this study for rapidly precipitated calcium carbonate in the presence of chromate show a tendency for preferential incorporation of heavy Cr isotopes in the precipitates resulting in increasing relative isotope difference between precipitate and initial solution (Δ53Cr[p-is]) from +0.06‰ to +0.18‰, with increasing initial Cr concentration of the solution. Sample precipitation in the presence of chromate also showed the presence of vaterite. Calcium carbonate crystals were also precipitated in a double diffusion silica hydrogel over a longer period of time resulting in samples consisting of micrometric–millimetric calcite crystals, which were again significantly enriched in heavy Cr isotopes compared to the initial solutions. They average, irrespective of the initial Cr concentration, a relative isotope difference (Δ53Cr[p-is]) of +0.29±0.08‰ (2σ), whereas silica hydrogel samples show a preferential retention of light Cr isotopes. These results imply that in previous studies the δ53Cr seawater signals inferred from carbonates may be too positive or, at lower Cr concentrations typical for seawater, marginal to no Cr isotope fractionation can be expected; this might have implications for the use of Cr isotopic signals recorded in ancient marine carbonates in relation to ambient seawater and pave the way for future work to enable a reliable application of the Cr isotope proxy.

Global variability of chromium isotopes in seawater demonstrated by Pacific, Atlantic, and Arctic Ocean samples

Seawater chromium (Cr) isotope and concentration data are presented from multiple sites in the Arctic Ocean, and three locations in the Atlantic and Pacific Oceans. A 2400-m profile illustrates the heterogeneity of δ53Cr in the Arctic Ocean with depth and water-mass source (Pacific vs. Atlantic). The highest δ53Cr values occur in Pacific-sourced waters, which also have the lowest Cr concentration. Chromium concentration and δ53Cr data from these locations, in conjunction with published data for the South Atlantic Ocean, yield a simple logarithmic function, as predicted by Rayleigh fractionation in a closed system. The observed Cr isotope signature is hypothesized to arise from fractionation during the reduction of Cr(VI) in surface waters and oxygen minimum zones, scavenging of isotopically light Cr(III) to deeper water and sediment, and subsequent release of this seawater-derived Cr(III) back into seawater, either as organic complexes with Cr(III) or after oxidation to Cr(VI). The isotopic fractionation factor (ε) associated with Cr cycling in seawater is estimated to be −0.80±0.03‰ (2σ). Samples from the sea-ice affected Surface Mixed Layer of the Arctic Ocean (∼10 m depth) deviate from the general trend, and samples proximal to rivers illustrate geographic variation in δ53Cr values for continental runoff, but prompt loss of this signature away from the source.

Cyclic oxidation behaviour of plasma sprayed metallic coatings

High temperature Co- and Ni-based metallic coatings of various compositions were applied on SS-310 samples using plasma spray technique. The coated samples were exposed to air under cyclic conditions at 900°C for upto 100 h. Adhesion of coatings was measured using standard ASTM C 633-79 method. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy was used to determine the surface morphology and microchemical constitution of coatings. X-ray diffraction was used to identify different phases formed during oxidation. Experimental results indicated that high temperature oxidation of SS-310 alloy was greatly reduced by all coatings. The plasma sprayed metallic coatings on SS-310 alloy performed better than that of bare SS-310 under the selected test conditions. Moreover, when compared among the selected metallic coatings used in the study, the Co-based coatings with higher Cr concentration exhibited superior performance compared to Ni-based coating with lower Cr concentration during the reported cyclic exposure.

Unraveling the mechanism responsible for the contrasting tolerance of Synechocystis and Synechococcus to Cr(VI): Enzymatic and non-enzymatic antioxidants

Two unicellular cyanobacteria, Synechocystis and Synechococcus, showed contrasting tolerance to Cr(VI); with Synechococcus being 12-fold more tolerant than Synechocystis to potassium dichromate. The mechanism responsible for this differential sensitivity to Cr(VI) was explored in this study. Total content of photosynthetic pigments as well as photosynthetic activity decreased at lower concentration of Cr(VI) in Synechocystis as compared to Synechococcus. Experiments with 51Cr showed Cr to accumulate intracellularly in both the cyanobacteria. At lower concentrations, Cr(VI) caused excessive ROS generation in Synechocystis as compared to that observed in Synechococcus. Intrinsic levels of enzymatic antioxidants, i.e., superoxide dismutase, catalase and 2-Cys-peroxiredoxin were considerably higher in Synechococcus than Synechocystis. Content of total thiols (both protein as well as non-protein) and reduced glutathione (GSH) was also higher in Synechococcus as compared to Synechocystis. This correlated well with higher content of carbonylated proteins observed in Synechocystis than Synechococcus. Additionally, in contrast to Synechocystis, Synechococcus exhibited better tolerance to other oxidative stresses like high intensity light and H2O2. The data indicate that the disparity in the ability to detoxify ROS could be the primary mechanism responsible for the differential tolerance of these cyanobacteria to Cr(VI).

High strength and good electrical conductivity in Cu–Cr alloys processed by severe plastic deformation

Ultrafine-grained (UFG) microstructures in Cu–Cr alloys were processed by high pressure torsion (HPT). The improved hardness was accompanied by a reduced electrical conductivity due to the large amount of grain boundaries. The effect of heat-treatment after HPT-processing on the hardness and the electrical conductivity was studied for different chromium contents (0.75, 9.85 and 27wt%). For low Cr concentration (0.75%) the electrical conductivity increased considerably above 250°C, however the hardness decreased concomitantly. At the same time, for high Cr content (9.85% and 27%) the hardness was only slightly reduced even at 500°C, while the electrical conductivity increased to a similar level as before HPT due to grain boundary relaxation and decomposition of Cu–Cr solid solution. Our study demonstrates the capability of SPD-processing and subsequent heat-treatment to achieve a combination of high strength and good electrical conductivity.

Characteristics of dissolved organic matters in submerged membrane bioreactor with low-concentration Cr(VI)

Hexavalent chromium (Cr(VI)) is a common heavy metal, but is hazardous to the environment and human body. Low concentration of Cr(VI) (0.4 mg/L) is ubiquitous in municipal wastewaters of China. However, little is known about its effects on dissolved organic matter (DOM) in submerged membrane bioreactors (MBRs). The effects of low concentration of Cr(VI) on the characteristics of DOM in submerged MBRs were investigated in this study. Compared with control MBR conditions, results showed that low-concentration Cr(VI) caused an increase in concentrations of dissolved organic carbon, polysaccharides, and proteins. Low-concentration Cr(VI) also increased the concentrations of humic substances and protein-like substances, and inhibited the formation of macromolecules (over 500 kDa) in DOM, further enhancing membrane fouling. The low molecular weight (<500 kDa) DOM led to irreversible membrane pore blocking and serious membrane fouling. Additionally, the shifting of DOM functional groups (shift from 3,200 to 3,700 cm−1, shift from 1,640 to 1,626 cm−1, minor change at 1,383.4 and 671–601.4 cm−1) indicated that amino, carboxyl, and sulfonate functionalities were involved in the connection between Cr(VI) and DOM.

Optimization of hexavalent chromium removal from aqueous solution using acid-modified granular activated carbon as adsorbent through response surface methodology

Response surface methodology (RSM) was applied to evaluate the effect of the main operational variables, including initial pH, initial chromium ion concentration, bulk density of GAC and time on the removal of hexavalent chromium Cr(VI) from contaminated groundwater by permeable reactive barriers (PRB) with acid-modified granular activated carbon (GAC) as an adsorbent material. The removal rates of Cr(VI) under different values of these parameters were investigated and results indicated high adsorption capacity at low pH and low initial metal ion concentration of Cr(VI), but the bulk density of GAC slightly influenced the process efficiency. According to the ANOVA (analysis of variance) results, the model presents high R2 values of 94.35% for Cr(VI) removal efficiency, which indicates that the accuracy of the polynomial models was good. Also, quadratic regression models with estimated coefficients were developed to describe the pollutant removals.

Mn–Cr dating of Fe- and Ca-rich olivine from ‘quenched’ and ‘plutonic’ angrite meteorites using Secondary Ion Mass Spectrometry

Angrite meteorites are suitable for Mn–Cr relative dating (53Mn decays to 53Cr with a half life of 3.7Myr) using Secondary Ion Mass Spectrometry (SIMS) because they contain olivine and kirschsteinite with very high 55Mn/52Cr ratios arising from very low Cr concentrations. Discrepant Mn–Cr and U–Pb time intervals between the extrusive or ‘quenched’ angrite D’Orbigny and some slowly cooled or ‘plutonic’ angrites suggests that some have been affected by secondary disturbances, but this seems to have occurred in quenched rather than in slow-cooled plutonic angrites, where such disturbance or delay of isotopic closure might be expected. Using SIMS, we investigate the Mn–Cr systematics of quenched angrites to higher precision than previously achieved by this method and extend our investigation to non-quenched (plutonic or sub-volcanic) angrites. High values of 3.54 (±0.18)×10−6 and 3.40 (±0.19)×10−6 (2-sigma) are found for the initial 53Mn/55Mn of the quenched angrites D’Orbigny and Sahara 99555, which are preserved by Cr-poor olivine and kirschsteinite. The previously reported initial 53Mn/55Mn value of D’Orbigny obtained from bulk-rock and mineral separates is slightly lower and was probably controlled by Cr-rich olivine. Results can be interpreted in terms of the diffusivity of Cr in this mineral. Very low Cr concentrations in Ca-rich olivine and kirschsteinite are probably charge balanced by Al; this substitutes for Si and likely diffuses at a very slow rate because Si is the slowest-diffusing cation in olivine. Diffusion in Cr-rich Mg–Fe olivine is probably controlled by cation vacancies because of deficiency in charge-balancing Al and is therefore more prone to disturbance. The higher initial 53Mn/55Mn found by SIMS for extrusive angrites is more likely to reflect closure of Cr in kirschsteinite at the time of crystallisation, simultaneous with closure of U–Pb and Hf–W isotope systematics for these meteorites obtained from pyroxenes. For the younger angrites Northwest Africa (NWA) 4590 and 4801 we have found initial 53Mn/55Mn values which are consistent with more precise work, at 0.90 (±0.4)×10−6 and 0.13 (±1.1)×10−6 respectively. Our work shows that SIMS can usefully constrain and distinguish the ages of angrites of different petrologic groups. In reviewing the petrology of angrites, we suggest that NWA 2999, 4590, and 4801 underwent a secondary partial melting and Cr (+/−Pb) disturbance event that the sub-volcanic Lewis Cliff 86010, and perhaps the plutonic Angra dos Reis, did not. With our higher initial 53Mn/55Mn for D’Orbigny and Sahara 99555 as well as previous data, a combined quenched angrite initial 53Mn/55Mn of 3.47 (±0.12)×10−6 (2-sigma, MSWD 1.00) yields consistent Mn–Cr and U–Pb intervals between these angrites and Lewis Cliff 86010. Discrepant Mn–Cr timescales for other plutonic and sub-volcanic angrites represents resetting during the secondary partial melting event at ∼4557.2Ma and indicates a relative order of disturbance of isotope systems: Mn–Cr in olivine before U–Pb in pyroxene, with Hf–W in pyroxene being the most resistant.

Resistance of Nanostructured Fe-Cr Alloys to Oxidative Degradation: Role of Zr and Cr Contents

This article investigates the effect of grain size and Cr concentration on the oxidation resistance of nanocrystalline Fe-Cr alloys having varying Cr contents between 2 and 10 wt pct. The results have been compared with microcrystalline Fe-Cr alloys with 10 and 20 wt pct Cr. Pellets of nanocrystalline and microcrystalline Fe-Cr alloys were prepared by mechanical alloying followed by hot compaction and sintering, and then oxidized at 823 K (550 °C) for 150-hours. Oxidation kinetics was determined by measuring the weight gain during oxidation, and the post-oxidation characterization of the oxide scales was performed using secondary ion mass spectroscopy. The chromium content at the inner oxide scale of nanocrystalline Fe-Cr alloys (with >7 pct Cr) was found to be comparable with that of microcrystalline 20 pctCr alloy, which suggests that nanocrystalline grain size distribution can be exploited to develop highly oxidation resistant alloys with much lower amounts of expensive alloying element (Cr). A mechanistic understanding of the high temperature oxidation in nanostructured alloys has been presented and the critical amount of bulk Cr content required to form a protective chromia layer has been calculated. The paper also investigates the role of a reactive element Zr on the high temperature oxidation resistance of nanocrystalline Fe-Cr. The beneficial effect is more pronounced at low bulk Cr (2 to 4 pct) concentrations (compared to higher Cr (>7 pct) concentration), however, at low bulk Cr, insufficient Cr enrichment occurs to establish a protective chromium oxide layer.

Partial melting of thickened continental crust in central Tibet: Evidence from geochemistry and geochronology of Eocene adakitic rhyolites in the northern Qiangtang Terrane

The composition of the deep crust is a key to understanding the formation of the low-velocity zone in the middle to lower crust of the Tibetan Plateau. The Suyingdi rhyolites exposed in the northern Qiangtang Terrane have high Sr (296–384 ppm) and low Y (5.81–7.93 ppm), with therefore high Sr/Y ratios (42–56), showing geochemical features of adakitic rocks. Zircon U–Pb dating yields an eruption age of 38.2±0.8 Ma (MSWD = 0.78). These adakitic rhyolites are high-K calc-alkaline in composition, displaying a weakly peraluminous character. They have low MgO content (0.20–0.70 wt.%) and Mg# values (24–39), as well as low Sc (2.25–2.76 ppm), Cr (8–14 ppm), Co (1.6–3.5 ppm) and Ni (2–3 ppm) concentrations. The rocks are LREE-enriched ((La/Yb)N = 50–62) and display weakly negative Eu anomalies (Eu/Eu⁎ = 0.82–0.95) and pronounced negative Nb and Ta anomalies. They have low initial (87Sr/86Sr)i ratios (0.707860 to 0.708342) and enriched Nd isotopic compositions with εNd(t) values ranging from −8.4 to −5.0, which are indistinguishable from those of Cenozoic potassic and ultra-potassic lavas exposed in northern Tibet. Their much higher SiO2 and lower Fe2O3 contents, yet similar MgO, Cr, Co, Ni, and Mg# values to the potassic and ultra-potassic lavas, however, indicate that the rhyolites are unlikely to have formed by fractional crystallization of these lavas. Because of their low Nb/Ta ratios and similar Sr–Nd isotopic compositions to granulite xenoliths within the Cenozoic potassic rocks, we infer that the Suyingdi adakitic rhyolites were most likely produced by partial melting of a thickened lower crust in the garnet stability field. The magma source is most likely dominated by granulite facies metabasalts and clay-poor metamorphosed sedimentary rocks which indicate that the lower crust of northern Tibet is heterogeneous. In combination with data from previously-reported peraluminous and metaluminous adakitic rocks in the same region, the age and petrogenesis of the Suyingdi adakitic rhyolites suggest that the low-velocity zone in the deep crust of central and northern Tibet was most likely the result of partial melting of thickened crust.

Equilibrium isotopic fractionation and isotopic exchange kinetics between Cr(III) and Cr(VI)

We determined the equilibrium isotope fractionation between Cr(III) and Cr(VI), defined as Δ53CrVI-III=δ53Cr(VI)−δ53Cr(III), and the rates of isotopic exchange between the two redox species under different conditions. In high Cr concentration, low-pH experiments we determined the Δ53CrV-III between CrO42− and Cr(H2O)63+ to be 5.2±0.3‰ and 5.5±0.3‰ at 60°C and 40°C, respectively. At 25°C, the system only progressed 25% toward isotopic equilibrium after 684days. By extrapolating from the 60°C and 40°C experiments we estimated the Δ53CrVI-III between CrO42− and Cr(H2O)63+ to be 5.8±0.5‰ at 25°C. Isotope exchange rates between dissolved Cr(III) and dissolved Cr(VI) at 25°C, 40°C, and 60°C were determined to be 3.13×10−5Mday−1, 6.83×10−4Mday−1, and 8.37×10−3Mday−1, respectively.In low concentration, neutral-pH experiments we determined the isotopic exchange rates between dissolved Cr(VI) and solid Cr(III) oxyhydroxide at 25°C. In these experiments, significant isotopic exchange was found on time scales of months, though the magnitude of isotopic shifts was limited by the small mass of Cr(III) available for exchange on the surfaces of Cr(III) oxyhydroxide particles. Exchange rates were relatively fast, compared to rates obtained from high concentration, low-pH experiments. This faster isotopic exchange is attributed to adsorption of Cr(VI) to Cr(III) particle surfaces, which keeps Cr(III) and Cr(VI), and potentially intermediate species Cr(V), in close proximity long enough to allow multiple electron transfers. The isotopic exchange rate at neutral-pH was found to conform to the rate law R=k·[Cr(VI)]adsorbed, in which R is the isotopic exchange rate (Mday−1); k is the rate constant, determined to be 0.00047day−1; [CrO42−]adsorbed is the concentration of Cr(VI) adsorbed to Cr(III) oxyhydroxide (M). The impact of isotopic exchange on the 53Cr/52Cr ratio of the dissolved Cr(VI) depends on the relative masses and 53Cr/52Cr ratios of the starting Cr(III) and Cr(VI), as well as the fraction of Cr(III) atoms exposed to solution. In many natural settings, isotopic exchange will have little impact, because only very small amounts of Cr(VI) are adsorbed onto Cr(III) oxyhydroxide surfaces. However in certain settings with low Cr(VI) concentrations and abundant, exposed Cr(III)-bearing solids, significant isotopic exchange could happen within about 20years.

Biogeochemistry and geobotany of the serpentine soils of the Rezvanshahr area in northwestern Iran: a preliminary investigation

Ultramafic rocks occur in many areas in Iran. These rocks consist predominantly of partly serpentinised peridotite. The Rezvanshahr area is one of the ultramafic areas in north-western Iran. It has an elevation of 1100–1700 m above sea level and an average rainfall of ~400 mm. In this study, soils and plants from this area were collected, identified and analysed for serpentine-associated metals. Analysis of the total acid-extractable elemental concentrations in the soil showed Ni, Cr, Co, Mn, Fe, Mg and Ca concentrations up to 1823, 727, 73, 2497, 47 778, 98 452 and 6440 μg g–1 soil DW respectively. In this study, 22 plant species belonging to 13 families were collected. The highest concentration of Ni (239 μg g–1 leaf DW) was found in Trifolium tricocephalum Pursh (Fabaceae). The highest concentrations of Cr (266 μg g–1) and Mn (332 μg g–1) were found in Trifolium clusii Godr. &amp; Gren. (Fabaceae). The most significant positive correlations were found between Ni and Mn, and Ni and Co in soil samples. Polygonum persicaria Gray contained low concentrations of Ni, Cr and Co, and may possess exclusion mechanisms to restrict excessive metal uptake.

Microwave-assisted preparation of sepiolite-supported magnetite nanoparticles and their ability to remove low concentrations of Cr(vi)

A sepiolite–nanomagnetite composite obtained by a facile microwave-assisted route can not only remove low concentrations of Cr(vi), but also effectively immobilize the secondary Fe3+in the final solution.

Deposition of chromium in aquatic ecosystem from effluents of handloom textile industries in Ranaghat–Fulia region of West Bengal, India

Accumulation of chromium (Cr) was determined in water, sediment, aquatic plants, invertebrates and fish in aquatic ecosystems receiving effluents from handloom textile industries in Ranaghat–Fulia region of West Bengal in India. Cr was determined in the samples by atomic absorption spectrophotometer and data were analyzed functionally by Genetic Algorithm to determine trend of depositions of Cr in the sediment and water. Area plot curve was used to represent accumulation of Cr in biota. The results indicate that the aquatic ecosystems receiving the effluents from handloom textile factories are heavily contaminated by Cr. The contamination is hardly reflected in the concentration of Cr in water, but sediment exhibits seasonal fluctuation in deposition of Cr, concentration reaching to as high as 451.0μgg−1 during the peak production period. There is a clear trend of gradual increase in the deposition of Cr in the sediment. Aquatic weed, insect and mollusk specimens collected from both closed water bodies (S1 & S2) and riverine resources (S3 & S4) showed high rate of accumulation of Cr. Maximum concentration of Cr was detected in roots of aquatic weeds (877.5μgg−1). Fish specimens collected from the polluted sites (S3 & S4) of river Churni showed moderate to high concentration of Cr in different tissues. Maximum concentration was detected in the liver of Glossogobius giuris (679.7μgg−1) during monsoon followed by gill of Mystus bleekeri (190.0μgg−1) and gut of G. giuris (123.7μgg−1) during summer. Eutropiichthys vacha showed moderately high concentration of Cr in different tissues (65–99μgg−1) while Puntius sarana showed relatively low concentration of Cr (below detection limit to 18.0μgg−1) in different tissues except in gill (64.4μgg−1).