Reduction Process Of Cr Research Articles

Synchronous or selective removal of nitrate and Cr(VI) by montmorillonite supported sulfidized nanoscale zerovalent iron: Role of Fe(II) and S0

The reduction process of Cr(VI) and NO3−-N by sulfidized nanoscale zerovalent iron (S-nZVI) under coexistence conditions has yet to be investigated. This study innovatively confirmed that NO3−-N and Cr(VI) could be reduced synchronously or selectively by synthesizing diverse montmorillonite-supported S-nZVI (S-nZVI@MMT) to meet different remediation requirements. Concretely speaking, S-nZVI@MMT at low S/Fe could synchronously reduce NO3−-N and Cr(VI) to NH4+-N and Cr(III). S-nZVI@MMT at high S/Fe selectively removed Cr(VI). The vital roles of Fe(II) and S0 in S-nZVI@MMT were clearly revealed. Fe(II) mainly involved in the generation of Fe3O4 during the reaction at low S/Fe, while participating in the transformation of amorphous phases of Fe3O4, FeS, and FeS2 at high S/Fe. The Fe0 and converted Fe3O4 were the primary reactive sites for the synchronous removal of NO3−-N and Cr(VI) while the surface-bound Fe(II) and converted amorphous phases of Fe3O4 and FeSx determined the selective removal of Cr(VI). Meanwhile, the high contents of S0 in S-nZVI@MMT at high S/Fe mediated the generation of FeSx during the reaction and blocked the adsorption and reduction of NO3−-N which was critical for the selective removal of Cr(VI). The variations in the reactive species of S-nZVI@MMT at different S/Fe led to discrepant Cr(VI) reaction pathways. The findings provided new insights and strategies for the practical applicability of S-nZVI. SynopsisStudy provides a new insight into the synchronous/selective removal of Cr(VI) and NO3−-N by S-nZVI to meet the remediation requirements for different application scenarios.

Green synthesis of nZVI-modified biochar significantly enhanced the removal of Cr(VI) from aqueous solution.

Water contamination by hexavalent chromium (Cr(VI)) seriously jeopardizes human health, which is a pressing environmental concern. Biochar-loaded green-synthesized nZVI, as a green and environmentally friendly material, can efficiently reduce Cr(VI) to Cr(III) while removing Cr(VI) from water. Therefore, in this study, an efficient green-modified biochar material (TP-nZVI/BC) was successfully prepared using tea polyphenol (TP) and sludge biochar (BC) using a low-cost and environmentally friendly green synthesis method. The preparation conditions of TP-nZVI/BC were optimized using response surface methodology (RSM), revealing that the dosage of tea polyphenols plays a crucial role in the removal performance (R2 = 1271.09), followed by reaction time and temperature. The quadratic regression model proved accurate. The optimal preparation conditions are as follows: tea polyphenols (TP) dosage at 48 g/L, reaction temperature at 75 ℃, and a reaction time of 3 h. TP-nZVI/BC removed Cr(VI) from water at a rate 7.6 times greater than BC. The pseudo-second-order kinetic model (R2 = 0.987) accurately describes the adsorption process, suggesting that chemical adsorption predominantly controls the removal process. The adsorption of Cr(VI) by TP-nZVI/BC can be well described by the Langmuir model, and the maximum adsorption capacity reached 105.65 mg/g. FTIR and XPS analyses before and after adsorption demonstrate that nZVI plays a crucial role in the reduction process of Cr(VI), and the synergistic effects of surface adsorption, reduction, and co-precipitation enhance Cr(VI) removal. In summary, using green-modified biochar for Cr(VI) removal is a feasible and promising method with significant potential.

Modeling and visualizing the transport and retention of cationic and oxyanionic metals (Cd and Cr) in saturated soil under various hydrochemical and hydrodynamic conditions

Cationic and oxyanionic metals are widely existed in the aquatic and soil environment with the process of industrialization and they may behave different transport properties in aquifer systems due to the opposite charges. In this study, the comparative transport behaviors of Cd2+ and CrO42− in water-saturated soil columns were investigated under a variety of hydrochemical and hydraulic conditions such as pH, ionic strength (IS), and flow rate. The transport mechanisms of Cd(II) and Cr(VI) were explored by fitting the breakthrough curves with a two-site non-equilibrium transport model. Results indicated that high solution pH inhibited the transport of Cd(II) due to the enhanced electrostatic interaction. In contrast, the migration of Cr(VI) was promoted with the least amount of Cr(VI) (1.23 mg) being retained in soil at high pH, ascribing to the stronger electrostatic repulsion between anions and soil surface. Meanwhile, high pH conditions were not favorable for the participation of reduced iron in the reduction process of Cr(VI), resulting in the least amount of Cr(III) detected (22%). The increase in ionic strength decreased the negativity of the potential at the adsorption plane, which enhanced the transport of cationic Cd(II) and the retardation of anionic Cr(VI). In addition, the increase in flow rate facilitated the transport of Cd(II) and Cr(VI), mainly due to the decreasing contacting with porous media and enhanced dispersion effect. These findings demonstrated that the fate and environmental behavior of metal cations and anions differed with the change of hydrochemical and hydrodynamic properties, which should be considered for the risk assessment and remediation of metal contaminated sites.

Rapid and efficient reduction of chromate by novel Pd/Fe@biomass derived from Enterococcus faecalis

Efficient reduction of chromate is highly desirable for its detoxification and remediation of the contaminated environment. This study described a fusion of the concepts of precious metal biorecovery and fabrication of Pd/Fe@biomass derived from simulated wastewater. The effectiveness of Pd/Fe@biomass during reduction process of Cr(VI) was evaluated by comparing with pure nZVI, E. faecalis and Pd@biomass. Results showed that Pd(II) could be recovered by E. faecalis with Fe(II) as the electron donor, and precipitation could yield nZVI anchored onto Pd-loaded E. faecalis. The nano particles (NPs) on Pd/Fe@biomass were well-dispersed, which provided 2.70 folds specific surface area comparing with nZVI. Efficient Cr(VI) reduction could be achieved at a higher catalyst dosage, the most appropriated Pd/Fe molar ratio of 2% and a wide pH range. Typically, 0.5 mM Cr(VI) could be completely reduced in 5 min driven by Pd/Fe@biomass under the conditions of dosage of 1.0 g/L and pH 3. Moreover, the mechanisms of Cr(VI) reduction by Pd/Fe@biomass were proposed, which intimately related to nZVI electron donating capacities, Pd catalysis for hydrogenation and galvanic cell effects between Fe and Pd. Therefore, Pd/Fe@biomass could be an alternative for rapid and complete reduction of Cr(VI).

Remediation of soil contaminated with high levels of hexavalent chromium by combined chemical-microbial reduction and stabilization

In order to solve the problem of re-oxidation after chemical remediation of soil contaminated with high levels of hexavalent chromium (Cr(VI)), we investigated the use of chemical reduction combined with microbial stabilization to remediate soils contaminated with high Cr(VI) concentration. The leaching toxicity and microbial diversity of Cr(VI)-contaminated soil and the leaching toxicity of remediated soil oxidized by potassium permanganate (KMnO4) were measured. The results indicate that the conversion rate of Cr(VI) reached 97 %, and the concentration of Cr(VI) in toxic solutions leaching can be reduced by 95 % after 40 days of microbial stabilization. Sterilization experiments showed that the reduction of Cr(VI) by microorganisms is stable. The results of microbial diversity analysis indicate that bacterial community changed more than fungal community during the reduction process of Cr(VI), and the species abundance and species evenness of bacteria decreased. Bacillus spp. and Halomonas spp. were the dominant species in this study.

Chromium(VI) reduction and accumulation on the kaolinite surface in the presence of cationic soil flocculant

PurposeHeavy metal soluble forms pose a threat to plants, soil microflora, and microfauna. To limit their toxicity and mobility, various immobilizing additives are being developed. The main aim of the study was to determine the influence of soil flocculant (cationic polyacrylamide (CtPAM)) on the hexavalent chromium ion reduction and accumulation on the kaolinite surface. In this way, the efficiency of the selected polymer conditioner in the soil remediation was determined.Materials and methodsThe adsorbed amounts of Cr(VI) and CtPAM on the kaolinite surface was determined spectrophotometrically (spectrophotometer UV/Vis) using specific indicators. The structure of electrical double layer formed on the kaolinite surface in the mixed system of cationic polyacrylamide and Cr(VI) ions was described based on the adsorptive measurements, zeta potential measurements, as well as potentiometric titration. The kaolinite aggregation in the presence of CtPAM and/or Cr(VI) ions was determined spectrophotometrically and proved using SEM microscopy. The reduction process of Cr(VI) to Cr(III) occurring in the adsorption layer was determined by diffuse reflectance spectroscopy (DRS).Results and discussionThe obtained results indicated that cationic polyacrylamide contributes to Cr(VI) reduction to Cr(III) form in the kaolinite suspension. This heavy metal ions are strongly adsorbed by the applied clay mineral, especially at pH 5 (then the Cr(VI)-adsorbed amount equals 5.42 mg/g). The CtPAM addition favors the Cr(VI) ion immobilization on the kaolinite surface. In the conditioner presence, the heavy metal-adsorbed amount is even at 7.34 mg/g. The adsorption of CtPAM and chromium(VI) ions induces changes in the kaolinite surface charge density and electrokinetic potential. What is more, both substances favors the kaolinite aggregation.ConclusionsCationic polyacrylamide may be considered a substance reducing the toxicity of chromium(VI) ions in the natural environment and decreasing their availability for soil organisms. It can improve the soil structure (by stimulating aggregation) and reduce environment pollution with heavy metals simultaneously.Graphical abstract

Enhanced reduction of sulfate and chromium under sulfate-reducing condition by synergism between extracellular polymeric substances and graphene oxide

Microbial reduction of sulfate and metal were simultaneously enhanced in the presence of graphene oxide (GO)-like nanomaterials, however, the mechanism remained unclear. In this study, bio-reduction of Cr was compared between free-living bacterium BY7 and immobilized BY7 (BY-rGO) on reduced GO particles. The role of extracellular polymeric substances (EPS) and rGO material on reduction of sulfate and Cr was investigated. Cr(VI) was reduced to Cr(III) and elemental Cr by BY-rGO particles up to 51% and 28%, respectively. EPS produced by the bacterium BY7 mainly consisted of proteins, polysaccharides, nucleic acids and humic substances. Concentration of EPS was sharply increased (about 54%) with the addition of graphene oxide, while the composition of EPS components was strongly affected by the exposure to Cr. By removing surface EPS without breaking the cells, reduction activities of sulfate and chromium by both BY-rGO particles and free-living BY7 cells were decreased. In contrast, reduction of sulfate and Cr by the free-living BY7 cells was enhanced with external addition of extracted EPS. Based on electrochemical analysis, the reduction peak indicating enhanced electron transfer was lost after removing EPS. Moreover, the contribution of each EPS fractions on sulfate and Cr reduction followed an order of polysaccharides > proteins > humic substances. Therefore, microbial sulfate and Cr reduction processes in the presence of BY-rGO particles were enhanced by the increasing amounts of EPS, which likely mediated electron transfer during sulfate and Cr reduction, and relieved bacteria from metal toxicity. Nevertheless, the presence of rGO was crucially important for elemental Cr production under sulfate-reducing condition, which might contribute to lowering electric potential or reducing activation energy for Cr(III) reduction. This work provided direct evidences for enhancing sulfate and Cr reduction activities by supplement of EPS as an additive to increase treatment efficiency in environmental bioremediation.

Cr(VI) reduction in chromium-contaminated soil by indigenous microorganisms under aerobic condition

Abstract Bioremediation plays an increasingly important role in the remediation of chromium-contaminated soil because it is an environmentally friendly technology. To investigate the Cr(VI) reduction process by indigenous microorganisms in soil, a batch of incubation experiments were carried out in a bioreactor under aerobic conditions. The results showed that in the presence of indigenous microorganisms, the Cr(VI) concentration in the chromium-contaminated soil decreased from 1521.9 to 199.2 mg/kg within 66 h with culture medium addition, while a slight decrease in the Cr(VI) concentration was found in the sterilized soil, implying that the indigenous microorganisms contributed to the Cr(VI) reduction. In the microbial remediation process, Cr(VI) microbial reduction occurred after the reduction of, Mn 4+ and Fe 3+ and, before reduction. The reduction process of Cr(VI) can be divided into two phases, characterized by the exponential equation model of microbial reduction and the linear equation model of the combined effect of the major ions. It can be concluded that indigenous Cr(VI)-reducing bacteria have a potential application for in-situ remediation of Cr(VI)-contaminated soil.

Effect factors, kinetics and thermodynamics of remediation in the chromium contaminated soils by nanoscale zero valent Fe/Cu bimetallic particles

Nanoscale zero valent iron (nZVI) was used to remediation contaminated soil. But nZVI has poor stability and tends to agglomerate easily. In this work, nanoscale valent iron–copper bimetal (nZVI/Cu) was synthesized and characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). The nZVI/Cu was applied to reduce hexavalent chromium at a concentration of 120mg/kg in contaminated soil. A number of factors, including nZVI/Cu dose, initial pH, time and temperature were assessed to determine their impact on the reduction of Cr(VI) in soils. Results showed that Cr(VI) reduction exceeded 99% at a pH of 5, temperature of 303K, and nZVI/Cu dose of 0.06g. The Cr(VI) reduction kinetics was consistent with the mechanism of Langmuir–Hinshelwood first order kinetic model. The thermodynamic parameters (ΔG0<0, ΔH0>0, ΔS0>0) showed that the reduction process of Cr(VI) was spontaneous and endothermic. Therefore, this work could provide a reference for remediation of Cr(VI) contaminated soil.

Remediation of Cr(VI) via combined self-reduction and adsorption bychemically modified carbon sorbents

Hexavalent chromium, Cr(VI), is a highly toxic species, while trivalent chromium, Cr(III), is an essential trace element for humans and other organisms. In the present work, a simple, rapid, and efficient procedure for water treatment of Cr(VI) is described. Active carbon (AC) was chemically treated with HCl and NaOH for the formation of surface modified Acidic-AC and Basic-AC sorbents, respectively. In addition, AC sorbent was treated with HNO$_{3}$ as an oxidizing agent and sodium sulfite as a reducing agent for the formation of Ox-AC and Rd-AC sorbents, respectively. The modified AC sorbents were tested to identify their potential applications in removal of Cr(VI) under various controlling factors using the batch technique. The results indicated that these sorbents can be used in water treatment due to their excellent and efficient reduction of Cr(VI) into Cr(III) with percentage transformation values in the range of 90%-100%. Acidic-AC was found to be the superior sorbent in the reduction process of Cr(VI). The produced Cr(III) was removed from aqueous solutions via adsorption on the surface of these sorbents. The reduction and adsorption processes of Cr(VI)-Cr(III) were evaluated and optimized under different experimental parameters. The potential applications of these sorbents in real water treatment were also explored.

Determination of the accumulation, spatial distribution and reduction of Cr in unsaturated Pseudochrobactrum saccharolyticum LY10 biofilms by X-ray fluorescence and absorption methods

A Cr(VI)-reducing bacteria (Pseudochrobactrum saccharolyticum LY10) was selected to simulate the biofilm grown in unsaturated habitats containing Cr(VI) and the X-ray fluorescence and absorption methods were used to characterize the temporal and spatial distributions of metal elements in the unsaturated biofilms in the present study. The results indicated that Cr and other elements (Fe, Mn, Ca, K, and Zn) were selectively accumulated in the unsaturated biofilms. Synchrotron radiation micro X-ray fluorescence microscopy (SR-μXRF) analysis revealed that Cr mainly existed in the 10–45μm layer near the bottom biofilm–membrane interface after 24h of Cr exposure. The analysis of Cr speciation by X-ray absorption near-edge structure (XANES) spectroscopy analysis further confirmed the bioreduction of Cr(VI) to Cr(III), and the precipitate formed in the biofilms after reduction were most similar to CrPO4. This systematic understanding of the accumulation, distribution and reduction process of Cr in the unsaturated biofilms will elaborate to enhance the bioremediation efficiencies for Cr pollution in soil and other environment.

Influence of operational parameters and kinetics analysis on the photocatalytic reduction of Cr(VI) by immobilized ZnO

In the present work the performance of immobilized ZnO on a glass plate with the heat attachment method has been described for photoreduction of Cr(VI) to the less harmful Cr(III) at different operational parameters. The photoreduction of Cr(VI) on the surface of the immobilized ZnO catalyst was studied as a function of the pH of solution, initial Cr(VI) concentration and ultraviolet (UV) light intensity. Results indicated that the reduction rate decreases with increasing initial concentration of Cr(VI) and initial pH of solution, whereas it increases with increasing UV light intensity. The photoreduction rate of Cr(VI) on the surface of the immobilized ZnO in the presence of O2 as mobile gas is more than Ar and N2. The reduction process of Cr(VI) by immobilized ZnO also could be done under visible light irradiation. Pseudo first-order kinetics were observed for the photoreduction of Cr(VI) at different operational conditions. With non-linear regression analysis a mathematical kinetics model was developed for the pseudo first-order constant (k ap) as a function of operational parameters.

Identification and characterization of the chromium(VI) responding protein from a newly isolated Ochrobactrum anthropi CTS-325

A Gram-negative, chromium(VI) tolerant and reductive strain CTS-325, isolated from a Chinese chromate plant, was identified as Ochrobactrum anthropi based on its biochemical properties and 16S rDNA sequence analysis. It was able to tolerate up to 10 mmol/L Cr(VI) and completely reduce 1 mmol/L Cr(VI) to Cr(III) within 48 h. When the strain CTS-325 was induced with Cr(VI), a protein increased significantly in the whole cell proteins. Liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis revealed that this protein was a superoxide dismutase (SOD) homology. The measured superoxide dismutase activity was 2694 U/mg after three steps of purification. The SOD catalyzes the dismutation of the superoxide anion (O2*-) into hydrogen peroxide and molecular oxygen. This protein is considered to be one of the most important anti-oxidative enzymes for O. anthropi as it allows the bacterium to survive high oxygen stress environments, such as the environment produced during the reduction process of Cr(VI).

The involvement of ATP sulfurylase in Se(VI) and Cr(VI) reduction processes in the fission yeast Schizosaccharomyces pombe.

The response of Schizosaccharomyces pombe towards the oxyanions selenate [Se(VI)] and dichromate [Cr(VI)] was investigated in order to establish the involvement of the yeast ATP sulfurylase in their reduction. An ATP sulfurylase-defective/selenate-resistant mutant of S. pombe (B-579 Se(R) -2) and an ATP sulfurylase-active/selenate-sensitive strain of S. pombe (B-579 Se(S)) were included in this study. The inhibitory effect of Se(VI) and Cr(VI) oxyanions on growth and bioaccumulation was measured. The sensitive strain showed natural sensitivity to selenate while the resistant mutant tolerated a 100-fold higher concentration of selenate. These results indicate that selenate toxicity to microorganisms is connected with the reduction of selenate to selenite. Both strains showed similar sensitivity to Cr(VI) and in this study there was no evidence that ATP sulfurylase participates in the reduction process of Cr(VI).

Mass Transfer Model of Chromium Reduction in a Fluidized Bed Electrochemical Reactor

A batch recycling fluidized bed electrochemical reactor system was used to recover Cr(VI) from a very dilute solution containing 2.5 g Cr/L at pH 2. It was found that the electrowinning rate is controlled by mass transfer and that the reduction process of Cr(VI) is interrupted by the formation of Cr(H2O)3 3+. The Cr2O7 2− and Cr(H2O)6 3 concentration-time relationship can be predicted by a plug flow model. A finite difference method and a complex optimization technique were used to estimate the mass transfer coefficient. At Rep between 10 and 30, the Cr2O7 2− and Cr(H2O)6 3+ mass transfer coefficients lie between 1.52-3.20 × 10−6 and 1.17-2.50 × 10−6 cm/s, respectively, and increase with the Reynolds number based on the particulate cathode.