Stabilization of chromium(VI) by hydroxysulfate green rust in chromium(VI)-contaminated soils

Chromium (Cr)-contaminated soils pose a great environmental risk, with high solubility and persistent leaching of Cr(VI). In this study, hydroxysulfate green rust (GR_SO4), with the general formula Fe(II)₄Fe(III)₂(OH)₁₂SO₄·8H₂O, was evaluated for its efficiency in Cr(VI) stabilization via Cr(VI) reduction to Cr(III) in four representative Cr(VI)-spiked soils. The initial concentrations of phosphate buffer-extractable Cr(VI) (Cr(VI)_b) in soils 1, 2, 3, and 4 were 382.4, 575.9, 551.3, and 483.7 mg kg^-1, respectively. Reduction of Cr(VI) to Cr(III) by structural Fe(II) (Fe(II)_s) in GR_SO4 in all studied soils was fast, wherein the application of GR_SO4 markedly decreased the amount of Cr(VI)_b at the Cr(VI)_b/Fe(II)_s stoichiometric mole ratio of 0.33. The kinetics of Cr(VI) reduction by GR_SO4 could not be determined as this reaction coincided with the release of Cr(VI) from soil during the experiment. The concentration of Cr(VI)_b decreased, as the Cr(VI)_b/Fe(II)_s ratio decreased from 0.46 to 0.20, generally to below 10 mg kg^-1. Back-transformation of the generated Cr(III) was examined in the presence of manganese oxide birnessite at the birnessite/initial Cr(III) mole ratio of 4.5. The results of batch tests showed that only 5.2% of the initial Cr(III) was converted to Cr(VI) after two months, while under field capacity moisture conditions, less than 0.05% of the initial Cr(III) was oxidized to Cr(VI) after six months. The results illustrated that remediation of Cr(VI)-contaminated soils would be fast, successful, and irreversible with an appropriate quantity of fresh GR_SO4.     

Critical Influence of Culture Medium and Cr(III) Quantification Protocols on the Interpretation of Cr(VI) Bioremediation by Environmental Fungal Isolates

The influence of culture medium composition on chromium(VI) quantification according to diphenylcarbazide (DPC) colorimetric determination was evaluated. Considering the eventual biospeciation of Cr(VI) as a mechanism of microbial bioremediation, the possibility to quantify Cr(III) in culture medium was also explored. Yeast nitrogen base (YNB) was identified as the least interferent culture medium for Cr(VI) quantification by DPC and it was applied to compare different strategies for Cr(III) oxidation. The most appropriate oxidation protocol consisted in the reaction with 80 mM KIO₄ at room temperature for 30 min prior to DPC. Parameters like basal culture medium (vitamins + salts + oligoelements), C and N source were systematically evaluated, either independently or in combination. Results demonstrated that C source was the most interferent culture medium component, being the use of sucrose preferable to glucose. A medium arbitrarily named as YNB′ (YNB without amino acids and ammonium sulfate plus 50 g L^?1 sucrose and 0.6 g L^?1 (NH₄)₂SO₄) was defined for Cr(VI)-amended fungal cultures. Kinetics of growth, Cr(VI) removal, and nutrient consumption for isolates A. pullulans VR-8, filamentous fungus PMF-1, and Lecythophora sp. NGV-1 were obtained. The order of Cr(VI) removal efficiency was as follows: A. pullulans VR-8 > Lecythophora sp. NGV-1 > filamentous fungus PMF-1, and a similar trend was observed for biomass yield and nutrients consumption. Studies on biospeciation by means of the selected Cr(III) oxidation protocol were unsuccessful, leading to Cr(VI) values much lower than expected. It revealed that this kind of protocols should be cautiously evaluated when studying microbial Cr(VI) bioremediation.     

A simple and inexpensive method for chromium speciation in soil extracts

Abstract

Chromium (Cr) appears in two stable forms in nature as Cr(III) and Cr(VI). Hexavalent chromium (CrO₄ ^2‐; Cr₂O₇ ^2‐) is very toxic and carcinogenic, while inorganic Cr(III), however, is essential for mammals. Only two methods, atomic absorption and inductively coupled plasma atomic emission (ICP) spectrometry, provide information on the total amount of Cr in a test solution. This is the reason that several efforts have been made with regard to Cr speciation. Either an acidic or a basic activated aluminum oxide and a reversed phase C‐18 column or an ion exchanger column are used for the separation of chromium(III) from chromium(VI) in FIA and HPLC analyses. In our experiments, acidic‐activated aluminum oxide was used for separation. This alumina was placed into a silicon tube and connected to an ICP spectrometer between the nebulizer and peristaltic pump. The average grain size of the alumina was large enough that the solution could be pumped through the micro column. Acidic‐activated aluminum oxide in the 2.0 to 8.0 pH range adsorbs the chromate anion but not the Cr(III) cation. During this stage, the Cr(III) content of a sample is measurable. The adsorbed chromate can be eluted with a strong acid. The height or area of the elution peak can be used for the calibration of Cr(VI). Detection limits of 4 μg/kg and 0.5 μg/kg were obtained for Cr(III) and Cr(VI), respectively. The effects of sulphate and phosphate anions on the surface of the alumina on chromate adsorption were also evaluated. This method was used for the measurement of Cr(VI) concentration by 0.01M CaCl₂ extraction of soil.     

Kinetic and Equilibrium Modeling for Cr(III) and Cr(VI) Removal from Aqueous Solutions by Citrus reticulata Waste Biomass

The pulp left after the extraction of juice from Citrus reticulate (kinnow), is a waste material, which was used as a potential sorbent for Cr(III) and Cr(VI) in the present study. The effect of experimental parameters such as pH, biosorbent dosage, biosorbent particle size, initial metal concentrations, temperature, shaking speed and sorption time on the Cr removal is apparent from the obtained results. The Freundlich isotherm and pseudo second order kinetic models fitted well to the data of Cr(III) and Cr(VI) biosorption by Citrus reticulata waste biomass. Effect of several pretreatments such as gases, natural coagulant and many other chemicals on Cr(III) and Cr(VI) sorption capacity of Citrus reticulata waste biomass was first time analyzed in the present study. The metal sorption capacity of Citrus reticulata waste biomass after a specific pretreatment was not only related to the nature of chemical but also strongly dependent on the oxidation state of the metal.     

Potential transformations of chromium in natural waters

A study has been conducted on the transformation of Cr(III) and Cr(VI) in simulated natural water conditions. It has been found that these forms are readily interconvertible under natural water conditions. The results of this study indicate that Cr(VI) is reduced by Fe(II), dissolved sulfides, and certain organic compounds with sulfhydryl groups, while Cr(III) is oxidized by a large excess of MnO₂ and at a slow rate by Oz under conditions approximating those in natural waters. Based on the results of these studies, water quality standards for Cr should be based on total Cr rather than on Cr(VI), as has been frequently done in the past.     

Molecular characterization of chromium (VI) reducing potential in Gram positive bacteria isolated from contaminated sites

Hexavalent chromium [Cr(VI)] is highly toxic, teratogenic and carcinogenic to man and other animals. Some bacterial species have the ability to reduce Cr(VI) to a stable speciation state of trivalent chromium [Cr(III)], which is insoluble and comparatively less toxic. Therefore, the reduction of Cr(VI) thus provides potential as a means for environmental bioremediation of Cr(VI) pollution. In the present study bacteria isolated from chromium and diesel contaminated sites were found to have the ability to rapidly reduce highly toxic concentrations of Cr(VI) to Cr(III) when grown in minimal medium supplemented with glucose as the sole carbon source. Partial chromate reductase gene sequences were retrieved after PCR amplification of genomic DNA extracted from three Gram positive isolates which were highly similar (>99% sequence similarity) to chromate reductase genes found in Gram negative bacteria, more specifically those identified from Escherichia coli and Shigella spp. whole-genome studies. The isolated bacteria were putatively identified by 16S rRNA gene sequencing as Arthrobacter aurescens strain MM10, Bacillus atrophaeus strain MM20, and Rhodococcus erythropolis strain MM30.     

Toxicity of selected chromium compounds in microbial bioassay system

The short term acute toxicity of potassium chromate, potassium dichromate and chromium sulphate has been compared in a simple microbial bioassay. The test parameters were, decrease in viability, genotoxicity and metal uptake. The LC₅₀ values of Cr(III), dichromate Cr(VI) and chromate Cr(VI) for Escherichia coli were 16, 10 and 1.2 μg mL^?1, respectively. Among the test substances potassium chromate was most toxic and showed no bioaccumulation while potassium dichromate was less toxic but resulted in significant bioaccumulation. Chromium sulphate was least toxic. As evident from loss of plasmid, genotoxicity was exhibited only by Cr (VI).     

Removal of Pb(II), Cr(III) and Cr(VI) from Aqueous Solutions Using Alum-Derived Water Treatment Sludge

The sorption of Pb(II), Cr(III) and Cr(VI) from aqueous solution using alum-derived water treatment sludge was investigated using the batch adsorption technique. Samples of sludge from two separate water treatment plants were used (one where alum was used alone and one where it was used in combination with activated C). The sorption characteristics of the two samples were generally very similar. Sorption isotherm data for all three ions fitted equally well to both Freundlich and Langmuir equations. Maximum sorption capacity and indices of sorption intensity both followed the order: Cr(III)?>?Pb(II)?>?Cr(VI). Kinetic data correlated well with a pseudo-second-order kinetic model suggesting the process involved was chemisorption. Sorption was pH-dependant with percentage sorption of Cr(III) and Pb(II) increasing from <30% to 100% between pH?3 and 6 whilst that of Cr(VI) declined greatly between pH?5 and 8. HNO₃ at a concentration of 0.1?M was effective at removing sorbed Cr(III) and Pb(II) from the sludge surfaces and regeneration was successful for eight sorption/removal cycles. It was concluded that water treatment sludge is a suitable material from which to develop a low-cost adsorbent for removal of Cr and Pb from wastewater streams.     

万寿菊：铬植物修复的潜在价值

Environmental pollution with chromium(Cr) is harmful to humans, animals and plants, while in plants it causes diminished growth,anatomical alterations and death. In the present study, the potential value of marigold(Tagetes erecta) in the phytoremediation of Cr has been investigated. The randomized experimental design involved the exposure of plants to nutrient solutions containing 0.00,0.04, 0.08, 0.12, 0.16 or 0.24 mmol L~(-1)Cr(Ⅲ). Chromium toxicity was observed at Cr(Ⅲ) concentrations ≥ 0.12 mmol L~(-1) as demonstrated by diminished growth of the aerial parts and reduced density of the root system. Increasing Cr(Ⅲ) concentrations in the nutrient solution resulted in a higher bioaccumulation of total Cr in the tissues, although translocation from roots to aerial parts was not efficient(maximum value of 25% at 0.12 mmol L~(-1)Cr(Ⅲ)). The Cr bioaccumulation was up to 11-fold greater in roots than in the aerial parts. Tagetes erecta exhibited leaf plasticity when exposed to Cr, indicating the existence of a tolerance mechanism to Cr in this species. Chromium caused a reduction in xilem vases, resulting in a plastic effect in T. erecta leaves that increased the metal tolerance in culture solution. Tagetes spp. are potential Cr hyperaccumulators; at Cr(Ⅲ) concentrations up to 0.12 mmol L~(-1), the plants accumulated levels above that proposed for hyperaccumulators and still maintained a considerable growth and even flourished. However, this study was conducted in nutrient solution, and studies on species confirmation as Cr hyperaccumulator should be conducted in soils for further clarification.     

Bioreduction of chromium (VI) by Bacillus sp. isolated from soils of iron mineral area

Extensive use of chromium in industry has caused environmental contamination. Chromium-resistant bacteria are capable of reducing toxic Cr (VI) to less toxic Cr (III). Eight isolates, which can grow on LB agar containing 500 mg/L of Cr (VI), were isolated from soil samples of iron mineral area. The bacterial isolates were identified as Bacillus sp. by the 16S rRNA gene sequences. Phylogenetic tree analysis indicates the isolates can be divided into two groups. The bacterial isolates can be resistant to other heavy metals and reduce Cr (VI) at different levels. One bacterial isolate (MDS05), which can tolerate 2500 mg/L Cr (VI) and was able to reduce almost 100% of Cr (VI) at the concentration of 10 mg/L in 24 h, was selected to study the effects of some environmental factors such as pH, temperature, and time on Cr (VI) reduction and growth. The cell growth of MDS05 was affected by the presence of Cr (VI), especially at the concentration of 100 mg/L. It reduced more amount of Cr (VI) under a wide range of concentrations from 5 to 50 mg/L, and reduction was optimum at 37 °C and pH 8. MDS05 showed great promise for use in Cr (VI) detoxification under a wide range of environmental conditions.     

Sawdust: Cost Effective Scavenger for the Removal of Chromium(III) Ions from Aqueous Solutions

Cr(III) ions sorption onto sawdust of spruce (Picea smithiana) has been studied thoroughly using radiotracer technique. Maximum sorption (94%) of Cr(III) ions (8.98×10^?5 M) onto sorbent surface is achieved from deionized water in 20 min agitation time using 200 mg of sawdust. The sorption data followed the Freundlich, Dubinin-Radushkevich (D-R) and Langmuir isotherms. Freundlich constants l/n = 0.86 ± 0.07 and C _e = 85.0 ± 25.8 mmole g^?1 have been estimated. Sorption capacity, X _m = 0.82± 0.3 mmole g^?1, β = ?0.00356± 0.00017 kJ² mole^?2 and energy, E = 11.9± 0.3 kJ mole^?1 have been evaluated using D-R isotherm. The Langmuir constants Q = 5.8± 0.2 μmole g^?1 and b = (7.4± 0.5)×10⁴ dm³ mole^?1 have been calculated. The variation of sorption with temperature yields thermodynamic parameters Δ H = ?11.6± 0.3 kJ mole^?1, Δ S = ?16.2± 0.9 J mole^?1 K^?1 and Δ G = ?6.8± 0.3 kJ mole^?1 at 298 K. The negative value of enthalpy and free energy reflect the exothermic and spontaneous nature of sorption respectively. Among the anions studied oxalate, citrate, carbonate and borate have reduced the sorption. The cations Y(III), Ce(II) and Ca(II) suppressed sorption. The sawdust column can be used to separate Cr(III) ion from Cs(I), I(I),Tc (VII) and Se (IV).     

Uptake and apoplastic retention of EDTA‐ and phytosiderophore‐chelated chromium(III) in maize

Increasing the mobilization and root uptake of chromium (Cr) by synthetic and plant‐borne chelators might be relevant for the design of phytoremediation strategies on Cr‐contaminated sites. Short‐term uptake studies in maize roots supplied with ⁵¹CrCl₃ or ⁵¹Cr(III)‐EDTA led to higher apoplastic Cr contents in plant roots supplied with ⁵¹CrCl₃ and in Fe‐sufficient plants relative to Fe‐deficient plants, indicating that Fe stimulated co‐precipitation of Cr. Concentration‐dependent retention of Cr in a methanol:chloroform‐treated cell‐wall fraction was still saturable and in agreement with the predicted tendency of Cr(III) to precipitate as Cr(OH)₃. To investigate a possible stimulation of Cr(III) uptake by phytosiderophores, Fe‐deficient maize roots were exposed for 6 d to Cr(III)‐EDTA or Cr(III)‐DMA (2'‐deoxymugineic acid). Relative to plants without Cr supply, the supply of both chelated Cr species in a subtoxic concentration of 1 µM resulted in alleviation of Fe deficiency–induced chlorosis and higher Cr accumulation. Long‐term Cr accumulation from Cr(III)‐DMA was similar to that from Cr(III)‐EDTA, and Cr uptake from both chelates was not altered in the maize mutant ys1, which is defective in metal‐phytosiderophore uptake. We therefore conclude that phytosiderophores increase Cr solubility similar to synthetic chelators like EDTA, but do not additionally contribute to Cr(III) uptake from Cr‐contaminated sites.     

Intracellular chromium accumulation by <Emphasis Type="Italic">Streptomyces</Emphasis> sp. MC1

Streptomyces sp. MC1, previously isolated from sugar cane, has shown ability to reduce Cr(VI) in liquid minimal medium and soil samples. The objective of this work was to demonstrate the intracellular chromium accumulation by Streptomyces sp. MC1 under different culture conditions. This strain was able to accumulate up to 3.54 mg of Cr(III) per gram of wet biomass, reducing the 98% of Cr(VI) and removing 13.9% of chromium from the culture medium supernatants. Streptomyces sp. MC1 chromium bioaccumulation ability was corroborated by using Timm’s reagent technique, a low-cost method, which has been used by first time to detect chromium deposits in bacteria. The results of atomic absorption spectrometry, scanning electron microscopy, and energy dispersive X-ray analysis suggest that the mechanism of Cr(VI) resistance observed in Streptomyces sp. MC1 includes adsorption coupled with reduction to Cr(III), and finally, Cr(III) bioaccumulation. This mechanism have special relevance to remediation of Cr(VI) contaminated environments by Streptomyces sp. MC1.     

The Monitoring of Cr(III) and Cr(VI) in Natural Water and Synthetic Solutions: An Assessment of the Performance of the Dgt and Dpc Methods

The technique of diffusive gradients in thin films or diffusive gradient in thin films (DGT) has been used in this work for the in situ measurement of labile Cr(III) and Cr(VI) species. Direct measurement of Cr(VI) was also carried out in parallel with a field-based colourimetric technique based on the EPA 7196 diphenyl-carbohydrazide (DPC) method. The efficiency of the DGT and DPC methods were tested (a) in the laboratory, using synthetic solutions in the presence of realistic concentrations of Cr, humic substances (HS), and ethylenediaminetetraacetic acid (EDTA), and (b) in the field, in river water affected by effluents discharged by the tannery industry. The main advantage of the DGT method is that it allows the in situ separation of labile species of Cr(III) and Cr(VI), though there are still uncertainties about its performance in field conditions. The DPC method proved to be a fast, accurate, and relatively economical option for the field-based determination of Cr(VI). Sample acidification and ageing of unacidified samples from contaminated aquatic environments, produced significant errors in the determination of ‘dissolved’ Cr. The concentration of Cr(VI) determined by either the DGT or the DPC method exceeds recommended international guidelines.     

The solubility and plant uptake of chromium from heated soils

Abstract

The effects of heating on the solubility and plant uptake of Cr from three soils variously influenced by ultramafic parent materials were evaluated. Chromium extracted by 2 M HNO₃ and by M KCl increased with degree of serpentine influence and with temperature of ignition in air. In contrast, solubility of the element was only slightly influenced by ignition in a N₂ atmosphere. Heating enhanced the solubility of soil Cr by at least two oxidative reactions: (1) the destruction of a relatively heat‐stable, probably organic, complex with the release of Cr(III), and (2) the oxidation of free Cr(III) to Cr(VI).

Corn (Zea mays L.) grew poorly in the ignited soils, but normally in ignited and leached soils. Growth depression from ignition was related to shoot Cr levels (r² = 0.494) and is attributed to the readily absorbed and translocated chromate formed at elevated temperatures. Under more usual soil conditions, as in the untreated and ignited then leached soils, the less readily soluble forms of the element are the principal contributors to available Cr.     

As(III) removal from aqueous solution by adsorption

The removal of As(III) from aqueous solutions at different concentrations, pH and temperatures by haematite has been carried out successfully. The maximum removal was found to be 96 % at concentration 13.34 μmol L^?1, temperature 20 °C and pH 7.0. The process of uptake follows first-order adsorption rate expression and obeys the Langmuir's model of adsorption. The removal of As(III) by haematite is also partially diffusion controlled and mass transfer coefficients, diffusion coefficients and thermodynamic parameters have been determined to explain the results.     

Biosorption of Chromium (III) and Chromium (VI) by Untreated and Pretreated Cassia fistula Biomass from Aqueous Solutions

The present study explained the effect of pretreatments on the biosorption of Cr (III) and Cr (VI) by Cassia fistula biomass from aqueous solutions. For this purpose Cassia fistula biomass was pretreated physically by heating, autoclaving, boiling and chemically with sodium hydroxide, formaldehyde, gluteraldehyde, acetic acid, hydrogen peroxide, commercial laundry detergent, orthophosphoric, sulphuric acid, nitric acid, and hydrochloric acid. The adsorption capacity of biomass for Cr (III) and Cr (VI) was found to be significantly improved by the treatments of gluteraldehyde (95.41 and 96.21 mg/g) and benzene (85.71 and 90.81 mg/g) respectively. The adsorption capacity was found to depend on pH, initial metal concentration, dose, size, kinetics, and temperature. Maximum adsorption of both the Cr (III) and Cr (VI) was observed at pH 5 and 2. When Freundlich and Langmuir isotherms were tested, the latter had a better fit with the experimental data. The kinetic studies showed that the sorption rates could be described better by a second order expression than by a more commonly applied Lagergren equation.     

茶树枝叶制备生物炭负载纳米零价铁净化水中Cr(VI)

茶树废弃物引起的环境破坏和病虫害爆发问题日益突出,对其进行无害化和资源化利用具有重要意义。该研究以修剪的茶树枝叶提取液作为还原剂和封端剂,以提取后的残渣作为炭源,成功制备了一种可高效去除水中六价铬(Cr(Ⅵ))的生物炭负载纳米零价铁复合材料(nanoscale zero-valent iron embedded tea leaves,TLBC-nZVI)。分析了材料用量、溶液初始pH值和温度等对Cr(Ⅵ)去除效果的影响;利用扫描电子显微镜结合能量色散X射线光谱仪(SEMEDS)、傅立叶变换红外光谱仪(FTIR)、X射线粉晶衍射仪(XRD)和X射线光电子能谱仪(XPS)等对材料进行表征,结合吸附动力学、吸附等温线和吸附热力试验探讨了去除机制。结果表明酸性条件、高温、增加材料用量有利于TLBC-nZVI对Cr(Ⅵ)的去除。TLBC-nZVI吸附过程符合准二级动力学模型、颗粒内扩散模型和Freundlich吸附等温模型,该吸附是自发的化学吸热过程。TLBC-nZVI与Cr(Ⅵ)的反应机制为吸附在材料上的Cr(Ⅵ)被零价铁(Fe⁰)和还原性官能团还原为三价铬(Cr(Ⅲ))...     

Cr(III) oxidation coupled with Mn(II) bacterial oxidation in the environment

Purpose  

Cr(III) oxidation to Cr(VI) significantly increases Cr mobility and toxicity and thus its environmental risks. Manganese (Mn) oxides may serve as the potential oxidants of Cr(III) in environment. Natural Mn oxides in the environment are believed to be derived from bacterial oxidation. The objective of this study was to examine the Cr(III) oxidation capacity of biogenic Mn oxide and the role of Mn-oxidizing bacteria in Cr(III) oxidation.     

Catalysis of Dissolved and Adsorbed Iron in Soil Suspension for Chromium（Ⅵ） Reduction by Sulfide

The kinetics of Cr（Ⅵ） reduction by sulfide in soil suspensions with various pHs, soil compositions, and Fe（Ⅱ） concentrations was examined using batch anaeroblc experimental systems at constant temperature. The results showed that the reaction rate of Cr（Ⅵ） reduction was in the order of red soil 〈 yellow-brown soil 〈 chernozem and was proportional to the concentration of HCl-extractable iron in the soils. Dissolved and adsorbed iron in soil suspensions played an important role in accelerating Cr（Ⅵ） reduction. The reaction involved in the Cr（Ⅵ） reduction by Fe（Ⅱ） to produce Fe（ⅡI）, which was reduced to Fe（Ⅱ） again by sulfide, could represent the catalytic pathway until about 70% of the initially present Cr（Ⅵ） was reduced. The catalysis occurred because the one-step reduction of Cr（Ⅵ） by sulfide was slower than the two-step process consisting of rapid Cr（Ⅵ） reduction by Fe（Ⅱ） followed by Fe（Ⅲ） reduction by sulfide. In essence, Fe（Ⅱ）/Fe（Ⅲ） species shuttle electrons from sulfide to Cr（Ⅵ）, facilitating the reaction. The effect of iron, however, could be completely blocked by adding a strong Fe（Ⅱ）-complexing ligand, 1,10-phenanthroline, to the soil suspensions. In all the experiments, initial sulfide concentration was much higher than initial Cr（Ⅵ） concentration. The plots of In e[Cr（Ⅵ）] versus reaction time were linear up to approximately 70% of Cr（Ⅵ） reduction, suggesting a first-order reaction kinetics with respect to Cr（Ⅵ）. Elemental sulfur, the product of sulfide oxidation, was found to accelerate Cr（Ⅵ） reduction at a later stage of the reaction, resulting in deviation from linearity for the In c[Cr（Ⅵ）] versus time plots.