Mobility Of Cr Research Articles

Silicon and chromium stable isotopic systematics during basalt weathering and lateritisation: A comparison of variably weathered basalt profiles in the Deccan Traps, India

Global biomass production is fundamentally affected by the hydrological cycling of elements at the Earth's surface. Continental weathering processes are the major source for most bio-essential elements in marine environments and therefore affect primary productivity. In addition, critical zone biomass depends on energy and chemical exchange reactions in weathering profiles. The latter reservoirs are in turn influenced by different climatic conditions that control weathering and pore water parameters like pH and Eh, which regulate mineral break down rates and dictate the mobility and mass flux of elements.Two Deccan Traps basalt weathering profiles of contrasting age and alteration intensity provide a natural laboratory for investigating the effects of rock alteration on Si and Cr and their isotopic systematics. The Quaternary Chhindwara profile has progressed to a moderate degree of alteration (saprolite), while the Paleogene Bidar example displays an extremely altered laterite. The Chhindwara saprolite profile shows a near uniform Cr and Si concentration and isotopic composition, whereas the Bidar laterite profile is characterised by an intense loss of Si, a large enrichment of Cr within the most altered uppermost levels, and a wide range of Cr stable isotope ratios (−0.85 to 0.36‰ δ53/52Cr). A co-variation between Si and Cr isotopes, as well as their co-variation with iron content, provides empirical evidence that iron redistribution within the profile has a large effect on Cr mobility and Si isotopic fractionation. Therefore, it is concluded that iron oxides exert a primary control over the isotopic composition of both Cr and Si in pore waters of laterites.Since laterite formation is promoted by tropical climates, the results of this study provide new evidence to suggest that the hydrological Cr and Si fluxes originating from continental weathering have changed in accordance with large-scale, deep time climate variation and continental plate configuration. An increased flux of Si and greater magnitude of Cr mobility and isotopic fractionation are possibly amplified under CO2-rich, greenhouse episodes and/or when large landmasses were tectonically arranged at near equatorial latitudes.

Changes of chromium speciation and organic matter during low-temperature pyrolysis of tannery sludge.

The application or disposal of char derived from tannery sludge is directly influenced by the mobility and bioavailability of Cr during pyrolysis process. This study focused on the changes of Cr speciation and organic matter in tannery sludge during low-temperature pyrolysis (100-400°C) to evaluate the toxicity of char in terms of the leaching possibility of Cr. The results showed that (1) lower char yield and more porous structure were observed after pyrolysis. (2) Higher pyrolysis temperature increased Cr content in the char; however, Cr in this case was converted into the residual fraction which minimized its bioavailability therefore lowers its potential risk to the environment. (3) Organic matters in the acid and alkali leachates were mainly humic acid-like substance, and condensed organic matter might appear at 200°C and then destruct. (4) Despite the comparatively high content of Cr in the char, the leaching toxicity of char was within the security range according to the national standard of China. The Cr content in the acid and alkali leachates decreased to the range of 16.5-35.3 and 0.2-6.8mg/L, respectively. It was suggested that the potential toxicity of tannery sludge from Cr could be reduced before utilization or disposal by pyrolysis, especially under 400°C.

Kinetic stable Cr isotopic fractionation between aqueous Cr(III)-Cl-H2O complexes at 25 °C: Implications for Cr(III) mobility and isotopic variations in modern and ancient natural systems

The stable Cr isotope fractionation preserved in natural substances has been attributed predominantly to Cr(III)-Cr(VI) redox transformations. However, non-redox reaction pathways (e.g., ligand-promoted dissolution, ligand exchange, adsorption of Cr(III)) are liable to contribute to isotopic fractionation in natural systems given that soluble Cr(III)-ligands have been directly documented or modeled in several marine, continental, and hydrothermal environments.This study isolates the stable Cr isotope fractionation accompanying Cl-H2O ligand exchange during the transformation of three aqueous species in the Cr(III)-Cl-H2O system, [CrCl2(H2O)4]+aq (abr. CrCl2+ or S1), [CrCl(H2O)5]2+aq (abr. CrCl2+ or S2), and [Cr(H2O)6]3+aq (abr. Cr3+ or S3), at low pH (≤2). In dilute HCl (0.01 to 1 M), Cr3+ is the kinetically favoured species and transformation of CrCl2+ to CrCl2+ to Cr3+ via 2 steps of dechlorination/hydrolyzation begins immediately upon dissolution of a Cr(III)-Cl solid. Individual species are separated with cation exchange chromatography at different stages of transformation and inter- and intra-species (across an elution peak of one species) isotopic fractionation of up to 1 and 2‰ (δ53/52Cr), respectively, is documented. Comparison of peak elution characteristics with Cr-Cl-H-O isotopologue mass abundances suggests mass-dependent sorting of isotopologues alone cannot explain intra-species fractionation, supporting a previously published proposal that preferential adsorption of light Cr isotopes on the resin is driven by vibrational energy effects. The transformation of CrCl2+ to CrCl2+ is faster than CrCl2+ to Cr3+ and the rates of both transformations increase with solution pH. Preferential reaction of light Cr(III) isotopes into product species occurs during each transformation, consistent with closed-system, kinetic fractionation during Cl-H2O ligand exchange. Inter-species fractionation is assessed using time-series experiments beginning from the dissolution of two Cr(III)-Cl solids (dried NIST SRM979 standard and commercial CrCl3·6H2O salt) in 0.01 M HCl (pH ≈ 2). The ε53/52Cr(CrCl2+/CrCl2+) for the CrCl2+ to CrCl2+ reaction is −0.19‰ (SRM979) and −0.38‰ (salt) and the ε53/52Cr(Cr3+/CrCl2+) for the CrCl2+ to Cr3+ reaction is consistent for both experiments at −0.49‰ (SRM979) and −0.51‰ (salt). Experiments where SRM979 is dissolved in 0.1 and 1 M HCl for a longer aging period provide preliminary evidence that the Cr3+/CrCl2+ Cr(III) isotopic fractionation scales with HCl concentration (transformation rate). Chromium(III) dissolved in 6 M HCl and aged 5 months still yields an inter-species Cr isotope distribution that is apparently inherited from kinetic effects (light Cr isotopes in Cr3+), attesting to the slow development of inter-species isotopic equilibrium, which instead predicts progressively heavier Cr isotopes from CrCl2+ to CrCl2+ to Cr3+.The kinetic Cr(III) isotopic fractionation documented herein is proposed to be relevant to understanding systems where aqueous Cr(III)-Cl species may be temporarily stable (e.g., metamorphic and hydrothermal systems or lateritic weathering). Further, the complexation of Cr(III) with other ligands (e.g., CO32−, organics), combined with additional kinetic effects of Cr(III) potentially occurring in soils or sediment, must be explored prior to establishing the significance of empirical stable Cr isotope signatures in marine and continental environments. Further understanding of non-redox effects may lead to stable Cr isotopes developing as a proxy for system pH or ligand chemistry.

Fluids are bound to be involved in the formation of ophiolitic chromite deposits

Chromite orebodies within the harzburgitic upper mantle have been investigated in several ophiolitic complexes, with a focus here on New Caledonia. The relationship between harzburgite and a dunite envelope around pods of chromitite points to a passive metasomatic transformation. The replacement of orthopyroxene by olivine leads to the necessary mobilization of Cr 3+ , not accepted by the neoformed olivine. A symplectitic intergrowth of enstatite + chromite at the interface between harzburgite and dunite may result from a silica-enriched fluid phase. Chromite in the orebodies contains micro-inclusions of olivine, clinopyroxene, orthopyroxene (rare), pargasite, aspidolite and nepheline. The olivine is hypermagnesian (up to 97 mol% Fo) and Ni-rich (0.25–1.05 wt%) and crystallized from a reducing fluid phase, not a magma. The pyroxenes and pargasite also are highly magnesian. The presence of Na in the environment accounts for pargasite, aspidolite and anomalously sodic nepheline. A Ni–Cu alloy is present. Chromite in the orebodies contains fluid inclusions, whereas chromite disseminated in harzburgite does not. The mixed fluid contains H 2 O (5 wt% equivalent of NaCl), with CO 2 and CH 4 in a 10 to 1 molar ratio. Values of δ 13 C VPDB (−22.1‰ for CO 2 and −28.1‰ for CH 4 , respectively) are typical of shallow marine hydrothermal systems. Experiments reveal that the precipitation of chromite from a fluid phase requires a reducing fluid and a temperature of ≤1050 °C. The appearance of olivine in the envelope results from the incongruent dissolution of enstatite in the fluid. The reaction is proven experimentally to be very efficient at a modest temperature (750 °C) and a low confining pressure (1.5 kbar). It leads to a shrinkage of the solid fraction, making the reaction self-propagating. Large volumes of harzburgite can be efficiently processed in an open, fluid-dominant reducing system. Relevant experiments show that the dissolution will increase in efficiency at higher temperatures and pressures. We do not negate the widely accepted explanation of chromitite formation by localized melt–harzburgite or melt–lherzolite interaction, but do point out that on the grounds of efficiency, such a process should be of secondary importance.

Chelant-enhanced washing of CCA-contaminated soil: Coupled with selective dissolution or soil stabilization

Remediation of CCA-contaminated soil (Cr, Cu, and As) by biodegradable chelant-enhanced washing (EDDS, S,S-ethylene-diamine-disuccinic-acid) needs further enhancement. This study investigated the effectiveness of coupling with pre-treatment by selective dissolution and post-treatment by soil amendments, respectively. Three groups of reagents (reductants, alkaline solvents, and organic ligands) were adopted in the pre-treatment to dissolve the oxide minerals before EDDS extraction. In the post-treatment, soil amendments (coal fly ash (CFA), acid mine drainage sludge (AMDS), green waste compost (GWC)), and their mixtures) were used for a 2-month stabilization after 2-h EDDS washing. Multi-endpoint evaluation was performed by assessing the chemical state, leachability, mobility, bioaccessibility, and plant-availability of residual metal(loid)s as well as the cytotoxicity, enzyme activities, and available nutrients of the treated soils. Pre-treatment by dithionite-citrate-bicarbonate significantly enhanced extraction efficiency, but also increased the leachability of As and Cr and bioaccessibility of Cr in the treated soils. While sodium hydroxide removed the majority of As without increasing its leachability and bioaccessibility, it increased the cytotoxicity and inhibited the acid phosphatase activity. Post-treatment with AMDS and CFA effectively controlled the mobility and leachability of residual As and Cr after EDDS washing. However, destabilized Cu was only marginally immobilized by GWC due to strong Cu-EDDS complexation. The bioaccessibility and phytoavailability of Cu was primarily reduced by EDDS washing, while those of As and Cr could be attenuated by AMDS and CFA. This study indicates that coupling chemical extraction with subsequent soil amendment plays complementary roles in mitigating effects of residual metal(loid)s and improving environmental quality.

Isoproterenol exacerbates hyperglycemia and modulates chromium distribution in mice fed with a high fat diet

Background and purposeIsoproterenol (ISO), a nonselective β-adrenoceptor agonist for treating bradycardia and asthma, has been proposed to raise blood glucose level. Little is known regarding the relationship between ISO treatment, the induced chromium (Cr) redistribution, and changes in glucose metabolism. We aimed to characterize the effects of a single dose of ISO on glucose homeostasis and Cr level changes in an obesity mouse model. MethodsMice (C57BL6/j strain) were first fed for a continuous period of 12 weeks with either a high fat diet (HFD), to develop an obesity animal model, or a standard diet (SD), to develop a lean animal model as controls. These groups were each separated into two subgroups to receive either a single dose of ISO or saline (control). We measured in vivo their metabolic parameters, fasting glucose level, area under the curve (AUC) for glucose level time profile, insulin level time profile, insulin sensitivity index, and chromium distribution. ResultsAfter a single dose of ISO, the SD-fed mice had slightly higher blood glucose levels compared with the SD controls, when the level was measured 30 and 60min after injection. By contrast, the ISO-treated HFD-fed mice had significantly higher blood glucose levels and AUC during the entire 120min following one administration compared with the HFD control group. Additionally, they had a substantially lower HOMA-IR index, whereas insulin levels remained unchanged. The Cr level in their bones and liver was decreased, and loss of Cr through urinary excretion was elevated. ConclusionThe results demonstrated that ISO exacerbated hyperglycemic syndrome in the obesity animal model. ISO induced a net negative Cr balance as a result of increased urinary excretion, leading to Cr mobilization that was not desirable to overcome the hyperglycemia.

Chemistry of magnetite-apatite from albitite and carbonate-hosted Bhukia Gold Deposit, Rajasthan, western India – An IOCG-IOA analogue from Paleoproterozoic Aravalli Supergroup: Evidence from petrographic, LA-ICP-MS and EPMA studies

The Bhukia gold (+copper) deposit hosted by albitite and carbonates that occur within the Paleoproterozoic Aravalli-Delhi Fold Belt (ADFB) in western India consists of magnetite, graphite, apatite and tourmaline along with sulfide mineralization. Ubiquitous presence of magnetite and apatite in gold-sulfide association, alteration patterns and shear controlled mineralization suggest it to be IOCG (Iron-oxide copper gold) type deposits. The detailed mineral chemistry of magnetite and apatite are generated and interpreted in terms of their genetic significance, hydrothermal and magmatic origin vis-à-vis their affiliation with IOCG deposition. The data suggest that the magnetite has hydrothermal affiliation. The Ni/Cr ratio is greater than 1, which is explained by differences in solubility and mobility of Ni and Cr in hydrothermal fluids and is corroborated with other key evidences including that of wide ranging Mg concentration further supports a strong hydrothermal input that is envisaged for the deposition of magnetite. Concentration of vanadium in magnetite is generally <1000ppm in case of barren hydrothermal occurrences while in the study area, it is relatively higher as it is attributed to the gold-sulfide-Cu mineralization. Ti vs Ni/Cr, Ni/(Cr+Mn) vs Ti+V, Ca+Al+Mn vs Ti+V and Al+Mn vs Ti+V variations are interpreted in terms of magnetite genesis. EPMA data suggests that apatite present in Bhukia is of fluorapatite variety with F content >1wt% and F/Cl >1. Higher concentration of F and moderate Mn along with lower concentration of Cl attests their magmatic hydrothermal character and its derivation from meta-volcano sedimentary source. REE patterns obtained from LA-ICP-MS analysis suggest enrichment of LREE relative to MREE and HREE with negative Eu anomaly. Y/∑REE, La/Sm, Ce/Th and Eu/Eu∗ vs Ce/Ce∗ values of apatite is indicative of their origin in a highly oxidized environment. Presence of magnetite along with apatite is a common feature in IOCG-IOA (Iron-Oxide Apatite) association. Bhukia Gold Deposit has many similarities with Kiruna type Iron-Oxide Apatite (IOA) deposits particularly with respect to their similar tectonic setting, alteration patterns, mineral assemblages such asabundance of magnetite, apatite and presence of late stage sulfides based on EPMA and Laser ablation ICP-MS (LA-ICP-MS) studies. Lithological, petro-mineralogical and geochemical signatures of magnetite and apatite infer that the Bhukia is a possible IOCG-IOA type gold deposit typically associated with sulfides and graphite which may be used as petrogenetic indicators and pathfinders for exploration.

Preliminary Evaluation of Animal Bone Char as Potential Metal Stabilization Agent in Metal Contaminated Soil.

The aim of this study was to evaluate the potential effect of animal bone char (ABC) addition on metal mobility in mine tailings. The mobility of metals after addition of ABC to tailings at four different application rates (0.6 g, 1.2 g, 1.8 g and 3.6 g ABC per 100 g of tailings) was evaluated by Toxicity Characteristic Leaching Procedure (TCLP) one step extraction. The obtained results indicated that the mobility of Pb, Cr and Cd gradually decreased with increasing quantity of added ABC. According to the TCLP, mobile concentrations of Pb in tailings exceeded threshold values for almost eight times. After ABC addition, Pb TCLP-extractable concentrations decreased from 39 mg L-1 in tailings to lower than the TCLP limit values of 5 mg L-1 at all ABC application rates, except in mixtures with the lowest addition of ABC. We concluded that ABC could be a successful metal stabilization agent for multi-metal contaminated soil, although attention should be paid at highly As contaminated soil.

Role of Dissolved Organic Matter in the Release of Chromium from Schwertmannite: Kinetics, Repartition, and Mechanisms.

Dissolved organic matter (DOM) is an important factor influencing mineral biogeochemistry, although the role of labile DOM in the release of chromium (Cr) from schwertmannite, a mineral with high surface area, is unclear. In this study, the interaction of DOM with synthetic CrO-schwertmannite was investigated to better understand the potential fate of Cr in high-DOM environments. Minerals and their products were analyzed using Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. Experiments were conducted at pH 3.2 or 6.5 for different lengths of time, with a shrinking core model developed to describe kinetic processes. The concentration of total Cr in solution reached a maximum when the pH was 6.5 and the concentration of L-tryptophan was 5 mM. The newly formed minerals were confirmed to be mixtures involving residual schwertmannite, goethite, ferrihydrite, and jarosite. A possible mechanism is proposed to be a ligand-controlled binary system, accompanied by possible reduction at acidic pH conditions (3.2), including mass transfer and charge transfer processes. This study gives a new perspective for understanding the reactivity and stability of schwertmannite in the environment; it also provides some predictions on the mobility and fate of Cr. These findings will help design remediation strategies for Cr contamination.

Effects of Biochar-Derived Sewage Sludge on Heavy Metal Adsorption and Immobilization in Soils.

The object of this study was to evaluate the effect of sewage sludge biochar on adsorption and mobility of Cr, Mn, Cu, and Zn. Biochar (BC400) was produced via pyrolysis of municipal sewage sludge at 400 °C. Maximum adsorption capacities (qm) for Zn, Cr, Mn, and Cu were 5.905, 5.724, 5.681, and 5.342 mg·g−1, respectively, in the mono-metal solution and 2.475, 8.204, 1.01, and 5.415 mg·g−1, respectively, in the multi-metal solution. The adsorption capacities for Mn, Cu, and Zn decreased in the multi-metal solution due to competitive adsorption, whereas the capacity for Cr increased. Surface precipitation is an important mechanism in the sorption of these metals on BC400. The 360-day incubation experiment showed that BC400 application reduced metal mobility in contaminated soils, which was attributed to the substantial decreases in the acid-soluble fractions of Cr, Mn, Cu, and Zn (72.20%, 70.38%, 50.43%, and 29.78%, respectively). Furthermore, the leaching experiment using simulated acid rain indicated that the addition of BC400 enhanced the acid buffer capacity of contaminated soil, and the concentration of Cr, Mn, Cu, and Zn in the leachate was lower than in untreated soil. Overall, this study indicates that sewage sludge biochar application reduces the mobility of heavy metal in co-contaminated soil, and this adsorption experiment is suitable for the evaluation of biochar properties for remediation.

Phytoavailability of Cr in Silene vulgaris: The role of soil, plant genotype and bacterial rhizobiome

Understanding the metal behavior at the soil-root interface is of utmost significance for a successful implementation of phytoremediation. In this study, we investigated the differences in chromium (Cr) uptake, chemical changes in soil solution and the shifts in rhizosphere bacterial communities of two genotypes of Silene vulgaris (SV21, SV38) with different tolerance to Cr. A greenhouse experiment was performed in two soils that differed on pH and organic matter (OM) content. An industrial sludge with high content in Cr was used as pollution source. The soil solution in the rhizosphere was sample by Rhizon Soil Moisture Samplers. The total concentration of Cr reached the highest values in soil solution samplers from calcareous soils with poor contents in OM. Plants grown in this soil also increased the Cr uptake in roots of both genotypes, but the concentration was higher in genotype SV-38 than in SV21. The clustering analysis of denaturing gradient gel electrophoresis (DGGE) of 16S rRNA fragments revealed major differences in bacterial community structure related to Cr pollution, followed by soil type and finally, plant genotype. Diversity indices based on DGGE profiles were the highest in alkaline soil, and between genotypes, values were significantly greater in SV38. Canonical correspondence analysis (CCA) showed that changes in bacterial community structure of rhizosphere were highly correlated with total Cr concentration and soil solution pH. The isolation and identification of S. vulgaris bacterial rhizosphere revealed a different composition according to soil type and plant genotype. Results suggested the potential role of Pseudomonas fluorescens on Cr mobilization and therefore, on enhanced metal bioavailability and may provide a starting point for further studies aimed at the combined use of tolerant plants and selected metal mobilizing rhizobacteria, in the microbial-assisted phytoremediation of Cr-polluted soils.

Mobility of As, Cr and Cu in a contaminated grassland soil in response to diverse organic amendments; a sequential column leaching experiment

Sequential column leaching coupled with batch sequential extraction assays were used to assess the fate of arsenic, chromium and copper in a wood ash contaminated grassland soil amended with biochar and its non-pyrolysed origin source material. Application of both amendments resulted in a general reduction of copper and arsenic mobility. Chromium, confirmed to be highly mobile under the elevated pH conditions of the experimental soil, was also stabilised with both amendments, though its mobility was more significantly reduced in soils amended with the source material. This was attributable in part to lower pH in the source material amended soils when compared to the biochar amended soils which facilitated the reduction of the more mobile chromium (VI) to less mobile chromium (III), as confirmed by ion exchange chromium speciation. In this study, the use of biochar vs source material was beneficial only for select metals, thus highlighting the importance of considering the specific physico-chemical conditions and metal (loid) properties in contaminated soils during the evaluation of the suitability of individual materials for onward remediation actions.

Various soil amendments and environmental wastes affect the (im)mobilization and phytoavailability of potentially toxic elements in a sewage effluent irrigated sandy soil

Contamination of long-term sewage effluent irrigated soils by potentially toxic elements (PTEs) is a serious concern due to its high environmental and health risk. Our scientific hypothesis is that soil amendments can cause contradictory effects on the element mobilization and phytoavailability depending on the type of element and amendment. Therefore, we aimed to assess the impact of the application (1%) of several low cost amendments and environmental wastes on the (im)mobilization, availability, and uptake of Al, Cd, Cr, Cu, Fe, Mn, Ni, and Zn by sorghum (Sorghum bicolor) in a long term sewage effluent irrigated sandy soils collected from Egypt. The used materials include activated charcoal (AC), potassium humate (KH), phosphate rock (PR), phosphogypsum (PG), triple superphosphate (TSP), phosphoric acid (PA), sulfur (S), sugar beet factory lime (SBFL), cement bypass kiln dust (CBD), egg shell (ES), bone mill (BM), brick factory residual (BFR), ceramic powder (CP), and drinking water treatment residual (WTR). The mobilization and availability of the elements in the soil were extracted using NH4NO3 and ammonium bicarbonate- diethylene triamine penta acetic acid (AB-DTPA), respectively. The above-ground biomass samples were analyzed for the elements studied.The results confirmed our hypothesis and concluded that although some amendments like S, PA, and TSP can be used for reducing the plant uptake of Al, Cr, and Fe, they might be used with KH for enhancing the phytoextraction of Cd, Cu, Mn, and Ni. Moreover, several wastes such as BFR and WTR might be used for enhancing the phytoextraction of Al, Cd, Cr, Cu, Fe, and Ni and reducing the uptake of Mn from the studied soil. Although SBFL decreased the plant uptake of Al, Fe, Mn, and Zn, it's increased the plant uptake of Cd, Cu, and Ni. Therefore, the amendments which reduce the plant uptake of an element might be suitable candidates for its immobilization, while the amendments which increase the plant uptake of an element might be used for enhancing its phytoextraction when using bioenergy crops like sorghum in similar contaminated sandy soils. The studied materials offered the potential for effective and low cost media for the treatment of PTEs contaminated sewage effluent irrigated sandy soils. These results should be verified in a field study.

Arsenic, chromium, molybdenum, and selenium: Geochemical fractions and potential mobilization in riverine soil profiles originating from Germany and Egypt

The fractionation and potential mobilization of As, Cr, Mo, and Se in four floodplain soil profiles collected along the Nile (Egypt) and Wupper (Germany) Rivers were assessed using the BCR sequential extraction procedure. The concentrations of total and the geochemical fractions (acid soluble (F1), reducible (F2), oxidizable (F3), and residual (F4) fraction) of the elements were determined. The Wupper soils had the highest total concentrations (mg kg−1) of As (378) and Cr (2,797) while the Nile soils contained the highest total Mo (12) and Se (42). The residual fraction of As, Cr, Mo, and Se was dominant in the Nile soils suggesting the geogenic source of the elements in these soils. The residual fraction of As and Mo and the oxidizable fraction of Cr and Se were dominant in the Wupper soils. Among the non-residual fractions (potential mobile fractions; PMF = ∑F1−F3), the oxidizable fraction was dominant for Cr, Mo, and Se in the Nile soils and for Mo in the Wupper soils, while the reducible fraction was dominant for As in both soils. The PMF of As, Cr, and Se was higher in the Wupper than in the Nile soils which might reflect the anthropogenic sources of these elements in the Wupper soils, while the opposite was the case for the PMF of Mo. The high PMF of Se (87%), Cr (87%), and As (21%) in the Wupper soils suggested that a release of these toxic elements may happen which increase the potential environmental risks in the anthropogenically polluted soils.

Steel pickling rinse water sludge: Concealed formation of Cr(VI) driven by the enhanced oxidation of nitrite

Steel processing hydroxide sludge (SPHS) is a waste from the treatment of rinse water generated during stainless steel processing. Cr(VI) concentrations of up to 20mg/L were observed in the leachate of a landfill mono-section receiving SPHS. This was surprising since the rinse water is free of Cr(VI) and compliance tests of the fresh SPHS gave no indication of Cr(VI) mobilization. The aim of this study was to identify the processes conducive to the formation of Cr(VI) with emphasis on the effect of H2O2 used in the elimination of NO2−. Rinse water, SPHS, and the landfill leachate were characterized in samples collected over a period of several months. The water treatment process was downscaled to laboratory conditions and the generated SPHS was investigated for Cr(VI) desorption. Results showed that overdosing H2O2 with respect to NO2− caused the formation of Cr(VI) which adsorbed to the SPHS and was only desorbed in presence of a competing anion. This hampers the recognition of the true leaching potential in compliance testing. Therefore, Cr(VI) leaching from the landfill is attributed to patchy zones of deposited SPHS that was generated from process water treatment at overdosed H2O2. In conclusion, Cr(VI) formation may circumvented by an automated process control based on the monitoring of either NO2− or H2O2.

Critical metals distribution in Tethyan karst bauxite: The cretaceous Italian ores

Bauxite is a residual rock, consisting mainly of a mixture of aluminium hydroxides, whose industrial significance is primarily due to its profitably exploitable alumina contents. In the last decades, bauxite, mainly karst bauxite, has been also considered as possible resource of a great number of economically interesting elements including some critical metals such as rare earth elements, Sc, Co, Ni, and Nb. In this study, we present results of univariate and multivariate (R-mode factor analysis) statistics performed on a large data set including chemical composition of the principal Cretaceous karst bauxites from central and southern Italy with the twofold aim to evaluate the inter-elemental relationship among major oxides and critical metals, and describe factors affecting their distribution. Univariate statistics reveal that some critical metals Co, Ni, and LREE, have a significant number of outliers. The Co-Ni relationship associated to the outliers dataset suggests that Co is likely hosted in Ni-rich phases whereas Ce accumulation in authigenic minerals, carbonate-fluoride and phosphate, is at the origin of LREE outliers. Multivariate R-mode statistics, applied to the outliers-devoid database, instead demonstrate that in absence of specific mineralization events, the distribution of most critical metals is controlled by Al-, Fe-, and Ti-oxi-hydroxides and, to a lesser extent, by detrital phases. Among the critical metals, Cr and Y are the exceptions. Their geochemical behaviour seems to be influenced primarily by their own chemical features that are responsible for the mobility of Cr during bauxitization and for the decoupling of Y from the REEs.

Chromium isotope systematics in the Connecticut River

Limited constraints on Cr isotope fractionation during weathering and river transport is currently a gap in our understanding of the chromium (Cr) isotope system, which is an emerging proxy in environmental and paleoenvironmental studies. Here, we investigate Cr mobility and isotope fractionation from the temperate Connecticut River, USA, including Cr concentrations and isotopic compositions of river water, suspended particles, riverbed sediments, and weathering profiles. The δ53Cr values of the Connecticut River water range from −0.17‰ to +0.92‰, which are similar to or higher than the weathered rocks in the catchment (−0.08‰ to −0.29‰). We find seasonal variations in dissolved δ53Cr values in some but not all tributaries, suggesting that dissolved δ53Cr is not a simple function of seasonality but may also be influenced by sub-catchment heterogeneity in lithology. In contrast to dissolved Cr, we found consistent seasonal difference in suspended Cr concentration and δ53Cr. Suspended δ53Cr is 0.1‰ higher than the unfractionated BSE in the fall (0.01–0.13‰), but indistinguishable from the BSE in the spring (−0.11‰ to 0.00‰). The suspended Cr concentration is also lower in the spring, and with higher Al-Mn-Fe concentrations. The lower suspended Cr concentration and δ53Cr in spring may be linked to increased silicate and oxide load with depleted Cr due to stronger hydrological flux. Building from our dataset, there is not a consistent correlation with climate zones in a compilation of δ53Cr and Cr concentration data from river water and weathering profiles, suggesting that climate is not a dominating factor controlling Cr isotopic behavior during weathering, suggesting that other factors (e.g., local catchment conditions and dissolved organic matter) may also be responsible for the observed river water δ53Cr variability.

Nano-sized Fe2O3/Fe3O4 facilitate anaerobic transformation of hexavalent chromium in soil–water systems

The purpose of this study is to investigate the effects of nano-sized or submicro Fe2O3/Fe3O4 on the bioreduction of hexavalent chromium (Cr(VI)) and to evaluate the effects of nano-sized Fe2O3/Fe3O4 on the microbial communities from the anaerobic flooding soil. The results indicated that the net decreases upon Cr(VI) concentration from biotic soil samples amended with nano-sized Fe2O3 (317.1±2.1mg/L) and Fe3O4 (324.0±22.2mg/L) within 21days, which were approximately 2-fold of Cr(VI) concentration released from blank control assays (117.1±5.6mg/L). Furthermore, the results of denaturing gradient gel electrophoresis (DGGE) and high-throughput sequencing indicated a greater variety of microbes within the microbial community in amendments with nano-sized Fe2O3/Fe3O4 than the control assays. Especially, Proteobacteria occupied a predominant status on the phylum level within the indigenous microbial communities from chromium-contaminated soils. Besides, some partial decrease of soluble Cr(VI) in abiotic nano-sized Fe2O3/Fe3O4 amendments was responsible for the adsorption of nano-sized Fe2O3/Fe3O4 to soluble Cr(VI). Hence, the presence of nano-sized Fe2O3/Fe3O4 could largely facilitate the mobilization and biotransformation of Cr(VI) from flooding soils by adsorption and bio-mediated processes.

Sorption Characteristics of Hexavalent Chromium in the Soil Based on Batch Experiment and Their Implications to the Environment

The objective of this experimental study was to determine the kinetics and equilibrium sorption of Cr(VI) in soils collected from Hengshui City of Hebei Province, China, based on batch experiments. The main concentration for this paper is on the effect of soil pH, solute concentration and ionic strength as the variable factors in the sorption of Cr(VI) in soils and the assessment of their implications to the environment. Atomic Absorption Spectrophotometer (AAS) was used for Cr(III) analysis; UV-VIS Spectrophotometer for quantification of Cr(VI) in soil samples and determination of electrical conductivity and temperature of the soil samples; and Automatic Laser Particle Size Analyzer LS230 for the determination of soil physical characteristics. Results from this study show that adsorption and reduction are major reactions accounting for removal of Cr(VI) from soil solution. It is concluded that chemical reactions such as reduction, strongly influence Cr(VI) mobility in soil. Adsorption kinetics experiments indicated that Cr(VI) removal from soil solution increases with increasing solute concentration, with decreasing pH and with decreasing ionic strength. Adsorption reactions reached equilibrium within 12 hours in batch reactors. Increasing background electrolyte concentration (KCI) decreases Cr(VI) adsorption on soil. The Cr(VI) adsorption isotherm for this soils conforms well to the Langmuir isotherm at constant pH. Two Cr(VI) adsorption parameters: the maximum sorption capacity (Qo) and Langmuir adsorption constant (KL), were determined as 1.0135 × 10-4 mol/g and 0.0622 mg/L, respectively. The reduction of Cr(VI) into less toxic Cr(III), means reduction of significant environmental problems. Finally, this study advises relevant environmental governing authorities to observeperiodic monitoring of the status of Cr(VI) in soils.

Heavy metals in sludge during anaerobic sanitary landfill: Speciation transformation and phytotoxicity

Sanitary landfill persists as a predominant sludge disposal method in particular in China. In this study, successive subsurface sludge samples (0.3 m deep) were collected from a bioreactor landfill unit where an anaerobic process took place during a 500 d period. The sludge samples were analyzed for total concentrations of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) and their species distributions, together with selected sludge chemical properties. In addition, the sludge samples were subjected to phytotoxicity testing. The results showed that the mobilities of Cd and Ni were decreased with landfill time, while the mobilities of Cr, Cu, Pb and Zn remained consistently low over the landfill period. The residual amounts of all these heavy metals were increased with landfill time, suggesting that the anaerobic sanitary landfill is an effective sludge stabilization method to reduce the ecological risk of these heavy metals. The increase in the sludge pH value with landfill time tended to promote the precipitation of heavy metals. Moreover, the sludge stabilization was found to be indicated by the formation of humic substance (HS) and volatilization of volatile matter (VM). The germination index (GI) values of barley (Hordeum vulgare L.) and Chinese cabbage (Brassica rapa chinensis) seeds grown in the 500 d anaerobically stabilized sludge were approximately 5.2- and 4.1-times higher than the values of those grown in the fresh sludge. The final sludge extract did not cause any significant inhibitory effect on the germination of the two types of seeds.