Reduction Of CrVI Research Articles

Visible light photodegradation of organic dyes, reduction of CrVI and catalytic oxidative desulfurization by a class of polyoxometalate-based inorganic-organic hybrid compounds

A new family of polyoxometalates-based inorganic–organic hybrid compounds, namely, [Ag4(H2O)(L)3(SiW12O40)] (1), [Zn(L)(H2O)]2[SiW12O40]·3H2O (2), [Cu(L)(H2O)]2[SiW12O40] (3), and [Cu2(L)2(HPWVI10WV2O40)]·4H2O (4), have been synthesized (L = 1,4–bis(3-(2-pyridyl)pyrazol)butane). This series of inorganic–organic hybrid compounds exhibit variable supramolecular architectures. The photodegradation of organic dyes malachite green and methylene blue with the use of compounds 1–4 as photocatalysts was studied under visible light irradiation. Strikingly, the photocatalytic result demonstrates that compound 1 is an efficient photocatalyst for the removal of malachite green and methylene blue. Remarkably, compound 1 serves as an active and recyclable photocatalyst for the reduction of CrVI with isopropanol as scavenger at ambient temperature. The catalytic oxidative desulfurization of thioanisole was also investigated with the use of compounds 1–4 as catalysts and tert-butyl hydroperoxide as oxidant. The catalytic results indicate that compounds 3 and 4 behave high catalytic performance for the oxidation of the thioanisole.

Theoretical analyses of organic acids assisted surface-catalyzed reduction of CrVI on TiO2 nanowire arrays

Assisted by low molecular weight organic acids (LOAs), photocatalytic reduction of CrVI was studied in the presence of TiO2 nanowire arrays (NWs) under the visible light. Theoretical calculations indicates that the decisive factors for this conversion are the stable adsorption behavior of LOA on TiO2 and orbital overlapping between the LOA HOMO and TiO2 LUMO. The orbital configuration interprets that covalent bonds are formed between the LOA and TiO2. Direct electron transfer occurs from LOA to TiO2 surface which facilitates the reducing of CrVI. Tartaric acid (TA) is proved to be the optimum organic acid for the surface-catalyzed reduction of CrVI. The TA can be easily excited by the light with wavelength higher than 1033nm and be broken into two same molecular segments. The molecular segments accelerate the CrVI reduction which follows a two-stage pseudo-first-order kinetics that is slow at initial stage and fast at the second stage.

Sulfate and chromate increased each other's uptake and translocation in As-hyperaccumulator Pteris vittata

We investigated the effects of chromate (CrVI) and sulfate on their uptake and translocation in As-hyperaccumulator Pteris vittata. Plants were exposed to 1) 0.1 mM CrVI and 0, 0.25, 1.25 or 2.5 mM sulfate or 2) 0.25 mM sulfate and 0, 0.5, 2.5 or 5.0 mM CrVI for 1 d in hydroponics. P. vittata accumulated 26 and 1261 mg kg−1 Cr in the fronds and roots at CrVI0.1, and 2197 and 1589 mg kg−1 S in the fronds and roots at S0.25. Increasing sulfate concentrations increased Cr root concentrations by 16–66% and helped CrVI reduction to CrIII whereas increasing CrVI concentrations increased frond sulfate concentrations by 3–27%. Increasing sulfate concentrations enhanced TBARS concentrations in the biomass, indicating oxidative stress caused lipid peroxidation in plant cell membranes. However, addition of 0.25–2.5 mM sulfate alleviated CrVI's toxic effects and decreased TBARS from 23.5 to 9.46–12.3 μmol g−1 FW. Though CrVI was supplied, 78–96% of CrIII was in the biomass, indicating efficient CrVI reduction to CrIII by P. vittata. The data indicated the amazing ability of P. vittata in Cr uptake at 289 mg kg−1 h−1 with little translocation to the fronds. These results indicated that P. vittata had potential in Cr phytoremediation in contaminated sites but further studies are needed to evaluate this potential. The facts that CrVI and sulfate helped each other in uptake by P. vittata suggest that CrVI was not competing with sulfate uptake in P. vittata. However, the mechanisms of how sulfate and CrVI enhance each other's accumulation in P. vittata need further investigation.

Au-Pd Bimetallic Nanoparticle Electrodes for Direct Electroreduction of Hexavalent Chromium Complexes

This paper reports a simple, low-cost, and effective electrochemical technique for sensing and reducing CrVI based on a Au-Pd bimetallic nanoparticle (BNP)-decorated indium tin oxide (ITO) conducting glass electrode. It was observed that the Au-Pd BNP-decorated ITO electrode could significantly boost the electrochemical reduction of CrVI when compared with either Au nanoparticle- or Pd nanoparticle-decorated ITO electrodes. These BNP-decorated electrodes exhibited a wide linear concentration range of 0.001–100 μM, a very low detection limit (signal-to-noise ratio = 3) of 0.3 nM, and a high sensitivity of 1.701 μA μM–1. From electrochemical impedance spectroscopy, it was revealed that this significant improvement was mainly due to the reduction in the charge-transfer resistance, which leads to faster free exchange of the reaction intermediates. The proposed Au-Pd BNP electrode also demonstrated excellent stability, selectivity, repeatability, and reproducibility.

Reduced Phosphomolybdates as Molecular Catalysts for Hexavalent Chromium Reduction

AbstractHybrid phosphomolybdates consisting of highly reduced {MnII(PV4MoV6O31)2} clusters have been reported as heterogeneous molecular catalysts for the reduction of toxic CrVI to nontoxic CrIII using formic acid (FA) as the reducing agent. The experimental results show that these types of nonprecious hybrids are highly active for the catalytic reduction of CrVI under mild conditions. In comparison with noble metal Pt/Pd nanoparticle (NP) catalysts that are widely used for the reaction, the phosphomolybdate hybrids are inexpensive and easy to prepare. Importantly, their structures can be designed and optimised. This study highlights noble metal‐free phosphomolybdates as a novel type of heterogeneous catalyst for the treatment of CrVI‐containing wastewater.

Modeling the transport and bioreduction of hexavalent chromium in aquifers: Influence of natural organic matter

Hexavalent chromium (CrVI) is a carcinogenic heavy metal that is a product of many industrial processes and an important contaminant in aquifers. The study investigates alternative remediation options for CrVI through the integration of effective contaminant transport and microbial biofilms. Bioactive sand reactor studies were conducted using columns charged with Shewanella oneidensis MR-1 biofilm growth in sand media. A biofilm reactor dynamics model was employed to predict CrVI reduction in bioactive sand columns from the standpoint of aquifer remediation and water treatment. The model utilized biokinetic parameters determined from independent microbial batch reactor studies using the Monod model. The biokinetic and reactor dynamics models further demonstrated that humic acid acted as electron shuttle and electron acceptor with lactate as electron donor, enhancing biological activity and CrVI reduction. The reactor model accurately forecast process dynamics for both scenarios with and without humic acid. Model sensitivity studies provided additional insight into the influence of various parameters on system dynamics and the means for improving process efficiencies.

Radiation-Induced Formation of Chromium Oxide Nanoparticles: Role of Radical Scavengers on the Redox Kinetics and Particle Size

The formation of Cr2O3 nanoparticles by γ-irradiation of dichromate solutions was investigated in the presence of radical scavengers, tert-butyl alcohol, nitrous oxide, and dissolved oxygen. The concentrations of CrVI and CrIII species and the chemical composition and sizes of particles that were formed were measured as a function of irradiation time. A mechanism is proposed that is consistent with the observed effects of scavengers on the kinetics of reduction of CrVI in solution to solid CrIII and the final size of Cr2O3 nanoparticles that are formed. The chromium reduction occurs in three stages: Stage I involves homogeneous aqueous reduction of CrVI(aq) to CrIII(aq) followed by spontaneous condensation of Cr(OH)3, providing nucleation sites onto which CrVI and CrIII coprecipitate and form mixed CrVI/CrIII oxide/hydroxide particles. In Stage II, the aqueous reduction of CrVI(aq) and CrIII(aq) reaches steady state but the adsorption of CrVI continues, growing the mixed CrVI/CrIII oxide/hydroxide particles. Stage 3 involves the solid-state conversion of CrVI and CrIII, and the particles slowly convert from a mixed CrVI/CrIII oxide/hydroxide to Cr(OH)3 and then to Cr2O3. Adding a radical scavenger in solution affects the kinetics of particle formation and growth at different stages differently. This information can be used to tailor the final sizes of particles.

Influence of oxygenation on chromium redox reactions with manganese sulfide (MnS(s)).

Manganese sulfide (MnS(s)) minerals exist in sulfidic environments and can have unique reactive abilities because of sulfide, which is a known reductant, and Mn, the oxyhydroxides of which are known oxidants. This study elucidated the role of MnS(s) in controlling Cr speciation with implications on its fate and toxicity in the natural environment, specifically sulfidic sediments that undergo biogeochemical changes due to sediment resuspension during dredging, bioturbation, and flood events. In continuously mixed batch reaction experiments, aqueous CrVI reduction under anaerobic conditions occurred primarily on the surface of MnS(s) displaying a biphasic behavior- the initial rapid removal of CrVI from solution was followed by a slow decline due to surface passivation by reaction products, mainly sorbed or precipitated CrIII. The reaction progress increased with MnS(s) surface area loading but decreased on increasing CrVI concentration and pH, suggesting that surface site regeneration through product desorption was the rate-controlling mechanism. Below circum-neutral pH, higher solubility of MnS(s) resulted in additional CrVI reduction by reduced sulfur species in solution, whereas increased CrIII solubility lowered surface passivation allowing for more reactive sites to participate in the reaction. Aeration of MnS(s) at pH≥7 caused the formation of a heterogeneous MnIII(hydr)oxide that was composed of hausmanite and manganite. CrVI reoccurrence was observed on aeration of CrVI-spiked MnS(s) from the oxidation of product CrIII. The reoccurrence at pH≥7 was attributed to the oxidation of product CrIII by MnIII(hydr)oxide, whereas the reoccurrence at pH<7 was hypothesized from the oxidation of product CrIII by intermediate aqueous MnIII and/or sulfur species. Just as with Cr, MnS(s) may play an important role in speciation, fate, and transport of other environmental contaminants.

Preparation and Characterization of Chitosan Thin Films on Mixed-Matrix Membranes for Complete Removal of Chromium.

Herein we present a new approach for the complete removal of CrVI species, through reduction of CrVI to CrIII, followed by adsorption of CrIII. Reduction of chromium from water is an important challenge, as CrIV is one of the most toxic substances emitted from industrial processes. Chitosan (CS) thin films were developed on plain polysulfone (PSf) and PSf/TiO2 membrane substrates by a temperature-induced technique using polyvinyl alcohol as a binder. Structure property elucidation was carried out by X-ray diffraction, microscopy, spectroscopy, contact angle measurement, and water uptake studies. The increase in hydrophilicity followed the order: PSf < PSf/TiO2 < PSf/TiO2/CS membranes. Use of this thin-film composite membrane for chromium removal was investigated with regards to the effects of light and pH. The observations reveal 100 % reduction of CrVI to CrIII through electrons and protons donated from OH and NH2 groups of the CS layer; the reduced CrIII species are adsorbed onto the CS layer via complexation to give chromium-free water.

Biotreatment of simulated tannery wastewater containing Reactive Black 5, aniline and CrVI using a biochar packed bioreactor

Azo dyes and CrVI in tannery wastewater can be treated by redox active bacteria. Dye decolorization and CrVI reduction are simultaneous under anaerobic conditions. Biochar is an effective support matrix for packed bed bioreactors used to treat dyes and CrVI.

Use of Biodiesel Press Cake Waste to Prepare Fe/Carbon Reactive Composites for Environmental Applications: Removal of Hazardous CrVIContaminants

In this work, Ricinus communis L. press cake waste from biodiesel production was used to produce a versatile reactive material for environmental applications. The biodiesel cake waste was impregnated with FeIII at different concentrations (19-45 wt.%) and treated at 400, 600 and 800 °C. Mossbauer, X-ray diffraction (XRD), thermogravimetric analysis (TG/DTA), scanning electron microscope (SEM), elemental analysis CHN, Raman, potentiometric titration, saturation magnetization and Brunauer-Emmett-Teller (BET) analyses showed that the materials prepared at 400 °C are composed mainly of a graphitic and amorphous carbon containing dispersed magnetite (Fe3O4). At higher temperatures, metallic iron (Fe0) and iron carbide (Fe3C) are the predominant phases well dispersed in the carbon porous structure. These reduced iron species, Fe3O4 and Fe0, are active for the reduction of different hazardous contaminants. Preliminary experiments showed very high activities for the reduction and removal of aqueous CrVI.

Effect of trace metals and electron shuttle on simultaneous reduction of reactive black-5 azo dye and hexavalent chromium in liquid medium by Pseudomonas sp.

This study demonstrates the role of electron shuttles and trace metals in the biotransformation of azo dye reactive black-5 and hexavalent chromium (CrVI) that are released simultaneously in tannery effluent. Previously isolated bacterial strain Pseudomonas putida KI was used for the simultaneous reduction of the dye (100mgL−1) and CrVI (2mgL−1) in a mineral salts medium (MSM). Among various trace metals, only Cu(II) had a stimulating effect on the bacterial-mediated reduction process. Application of electron shuttles such as hydroquinone and uric acid at a low concentration (1mM) had a positive effect on the reduction process and caused simultaneous reduction of 100% dye and 97% CrVI in 12–18h. Mannitol, EDTA and sodium benzoate at all concentrations (ranging from 1 to 9mM) showed an inhibitory effect on the reduction of reactive black-5 and CrVI. An inverse linear relationship between the velocity of reaction (V) and the concentration [S] of electron shuttles was observed. The results imply that both types and concentration of an electron shuttle and trace metals can affect the simultaneous reduction of reactive black-5 and CrVI.

Multifunctional Floating Titania-Coated Macro/Mesoporous Photocatalyst for Efficient Contaminant Removal.

A multifunctional floating photocatalyst consisting of highly active well-ordered mesoporous TiO2 (OMT) as a catalyst and polyurethane foam (PUF) as a floating substrate is synthesized successfully through a low-temperature ultrasonic and deposition approach. With the synthesized OMT/PUF composites, the multifunctionality of contaminant adsorption, dye degradation, and heavy metal removal can be achieved simultaneously. The prepared photocatalyst is characterized in detail by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, scanning electron microscopy, and N2 adsorption. The results indicate that the OMT/PUF presents a hierarchical macro/mesoporous structure, which is favorable for the adsorption and diffusion of pollutants and for the efficient utilization of sunlight. The degradation of several model dyes including rhodamine B, methyl orange, methylene blue, and simultaneous reduction of hexavalent chromium are examined over the floating photocatalysts with sunlight irradiation under certain conditions. A synergistic effect between the degradation of dyes and the reduction of CrVI is observed for the dye-CrVI coexistence system. This novel floating OMT/PUF photocatalyst is very promising in the field of wastewater treatment.

Chromate and phosphate inhibited each other's uptake and translocation in arsenic hyperaccumulator Pteris vittata L.

We investigated the effects of chromate (CrVI) and phosphate (P) on their uptake and translocation in As-hyperaccumulator Pteris vittata (PV). Plants were exposed to 1) 0.10 mM CrVI and 0, 0.25, 1.25, or 2.50 mM P or 2) 0.25 mM P and 0, 0.50, 2.5 or 5.0 mM CrVI for 24 h in hydroponics. PV accumulated 2919 mg/kg Cr in the roots at CrVI0.10, and 5100 and 3500 mg/kg P in the fronds and roots at P0.25. When co-present, CrVI and P inhibited each other's uptake in PV. Increasing P concentrations reduced Cr root concentrations by 62–82% whereas increasing CrVI concentrations reduced frond P concentrations by 52–59% but increased root P concentrations by 11–15%. Chromate reduced P transport, with more P being accumulated in PV roots. Though CrVI was supplied, 64−78% and 92−93% CrIII were in PV fronds and roots. Based on X-ray diffraction, Cr2O3 was detected in the roots confirming CrVI reduction to CrIII by PV. In short, CrVI and P inhibited each other in uptake and translocation by PV, and CrVI reduction to CrIII in PV roots served as its detoxification mechanism. The finding helps to understand the interactions of P and Cr during their uptake in PV.

Binder-free carbon nanotube electrode for electrochemical removal of chromium.

Electrochemical treatment of chromium-containing wastewater has the advantage of simultaneously reducing hexavalent chromium (CrVI) and reversibly adsorbing the trivalent product (CrIII), thereby minimizing the generation of waste for disposal and providing an opportunity for resource reuse. The application of electrochemical treatment of chromium is often limited by the available electrochemical surface area (ESA) of conventional electrodes with flat surfaces. Here, we report the preparation and evaluation of carbon nanotube (CNT) electrodes consisting of vertically aligned CNT arrays directly grown on stainless steel mesh (SSM). We show that the 3-D organization of CNT arrays increases ESA up to 13 times compared to SSM. The increase of ESA is correlated with the length of CNTs, consistent with a mechanism of roughness-induced ESA enhancement. The increase of ESA directly benefits CrVI reduction by proportionally accelerating reduction without compromising the electrode's ability to adsorb CrIII. Our results suggest that the rational design of electrodes with hierarchical structures represents a feasible approach to improve the performance of electrochemical treatment of contaminated water.

Effects of Plants for Reduction and Removal of Hexavalent Chromium from a Contaminated Soil

The comparative effectiveness for hexavalent chromium reduction and removal from irrigation water, using three selected plant species (Phragmites australis, Salix viminalis, and Ailanthus altissima) planted in soil contaminated with hexavalent chromium, has been studied in the present work. The above plant species were irrigated, in a continuous mode, with water, contaminated with 10 mg/L of hexavalent chromium. Hexavalent chromium and total chromium have been measured in all plant tissues species and in the drainage water. Total chromium removal from water was ranging from 56 % (Phragmites) to 70 % (Salix). After 360 days of growth, the chromium content of the contaminated soil dropped from 70 (initial) to 32, 36, and 41 mg Cr/kg(dry soil), for Salix, Phragmites, and Ailanthus, respectively. Salix and Phragmites accumulated the highest amount of chromium in the roots (2,029 and 1,800 mg Cr/kg(dry tissue), respectively), compared with 358 mg Cr/kg(dry tissue) for Ailanthus roots. Most of chromium was found in trivalent form in all plant tissues. Ailanthus had the lowest affinity for CrVI reduction in the root tissues. Phragmites indicated the highest chromium translocation potential, from roots to stems, while Salix indicated the highest chromium translocation from roots to leaves. Toxicity effects, expressed as root growth rate inhibition, indicated that Salix is the most chromium-tolerant species, with Ailanthus in the antipode.

Biogeochemical Controls on Hexavalent Chromium Formation in Estuarine Sediments

Predicting the aquatic and human health impacts of chromium (Cr) necessitates one to determine its speciation as either relatively nontoxic Cr(III) or toxic Cr(VI) and elucidate the influence of biogeochemical changes on its behavior and fate. In the Baltimore Harbor, Cr predominantly exists as Cr(III) associated with sediments. While reduction of Cr(VI) to Cr(III) is dominant in these anoxic sediments, the potential of Cr(III) oxidation and Cr(VI) reoccurrence during sediment resuspension and oxygenation resulting from dredging, bioturbation, and flood events poses a serious concern. In batch experiments, aqueous Cr(VI) spiked into continuously mixed anoxic suspensions was reduced to product Cr(III) under anaerobic conditions. No Cr(VI) reoccurrence was observed when conditions remained anaerobic. Aeration caused Cr(VI) reoccurrence from the abiotic oxidation of product Cr(III). Rates of aeration-driven Cr(VI) reoccurrence increased with pH, and Cr(VI) reoccurrence positively correlated with dissolved manganese (Mn) decline at pH ≥ 7. Aeration-driven oxidation of Mn(II) to Mn(III,IV)(hydr)oxides was the underlying mechanism causing product Cr(III) oxidation. Cr(VI) reoccurrence decreased with sediment loading and negatively correlated with the acid volatile sulfide (AVS) concentration. Although sediment resuspension and oxygenation may create temporary conditions conducive to Cr(VI) formation, long-term Cr(VI) persistence is unlikely in the presence of sediment reductants. While such natural attenuation in reducing environments mitigates the risk associated with Cr toxicity, this risk may still persist in Mn-rich and reductant-deficient environments.

Structure‐Enhanced Photocatalytic Removal of CrVI by a TiO2 Superstructure with Ultrathin Rutile Nanorods and Abundant {110} Faces

AbstractSize and shape, in particular of the external surfaces, are very important factors in determining the photocatalytic activity of TiO2 nanostructures. Here, we demonstrate that the sea‐urchin‐like rutile superstructure with ultrathin nanorods and exposed {110} faces is an excellent photocatalyst in the removal of CrVI ions from plating wastewater. The superstructure consists of ultrathin rutile nanorods with a specific surface area of 224.4 m2 g–1; the nanorods have a diameter of several nanometers, and the exposed faces are predominantly {110} faces. This special morphology exhibits a structure‐enhanced catalytic activity because it facilitates the transport of photoelectrons to the rutile nanorod {110} faces and prevents them recombining with holes. The rutile superstructure proved excellent at removing chromium from plating wastewater by photocatalytic reduction of CrVI to CrIII in sunlight, and at adsorption of the CrIII ions. The removal effectiveness is nearly 100 % at initial Cr concentrations below 53.7 ppm, and the removal capacity can reach about 1000 mg g–1 under irradiation by sunlight for 3 hours.

A Comparison Between Phragmites australis and Helianthus annuus in Chromium Phytoextraction

A comparison of chromium abatement from irrigation water, by the use of two selected plant species, Phragmites australis and Helianthus annuus, planted in chromium-contaminated soil, was studied in the present work. The above plant species were irrigated, in a continuous mode, with 10 mg CrVI/L contaminated tap water. More than 90 % of hexavalent chromium was reduced to trivalent chromium, from both plant species, as measured in the drainage water. Moreover, total chromium removal ranged from 54 % (Phragmites) to 70 % (Helianthus). After 90 days, the total chromium content of the contaminated soil dropped from 70 to 32 and 34 mg Cr/kgdry soil, for Helianthus and Phragmites, respectively. Helianthus accumulated higher amount of chromium in the roots (2,730 mg Cr/kgdry tissue) as compared to 1,800 mg Cr/kgdry tissue for Phragmites. Most of CrVI was reduced to CrIII in all plant tissues, with Phragmites showing lower affinity for CrVI reduction in the root tissues but higher chromium translocation potential from roots to stems, while Helianthus showed higher chromium translocation from roots to leaves. Toxicity effects, expressed as root growth rate inhibition, indicated that Phragmites were the most tolerant specie to chromium effects. Both plant species showed high potentialities to be used in phytoremediation installations for chromium removal.

Reduction of aqueous Crvi using nanoscale zero-valent iron dispersed by high energy electron beam irradiation

High energy electron beam (HEEB) irradiation was used to disperse nanoscale zero-valent iron (NZVI) for reduction of CrVI to CrIII in aqueous solution. Pore size distribution, scanning electron microscopy and X-ray diffraction characterizations demonstrated that HEEB irradiation could effectively increase the dispersion of NZVI resulting in more active reduction sites of Crvi on NZVI. Batch reduction experiments indicated that the reductive capacity of HEEB irradiation-modified NZVI (IMNZVI) was significantly improved, as the reductive efficiency reached 99.79% under the optimal conditions (electron beam dose of 30 kGy at 10 MeV, pH 2.0 and 313 K) compared with that of raw NZVI (72.14%). Additionally, the NZVI was stable for at least two months after irradiation. The modification mechanism of NZVI by HEEB irradiation was investigated and the results indicated that charge and thermal effects might play key roles in dispersing the NZVI particles.