Reduction Of Toxic Hexavalent Chromium Research Articles

Mechanically pulverized covalent organic framework as a metal-free photocatalyst for fenton-like degradation of organic pollutants and hexavalent chromium reduction

Visible light active, metal-free heterogeneous photocatalysts are required to achieve sustainable and green chemical processes. Development of such a catalyst is reported here. Mechanical pulverization of a covalent organic framework (COF) resulted in enhanced visible light photocatalytic activity for environmental remediation. The pulverized COF have achieved the apparent quantum yield (AQY) up to 41% for photo-Fenton-like degradation of organic dyes. Photocatalytic activity of the pulverized COF is one of the highest amongst the metal-free catalysts reported so far. The photocatalyst can operate under a broad range of pH, unlike metal-based photocatalysts. The catalyst does not lose its activities on recycling over several cycles. The exceptional performance of the pulverized COF is due to the high dispersibility, efficient visible light absorption, and excellent charge mobility. Moreover, the photocatalyst have also been used in continuous flow photo-Fenton-like degradation. The photocatalytic space time yield (PSTY) in continuous flow was boosted by twenty-eight times than the conventional batch photocatalysis. Moreover, the pulverized COF showed an outstanding photocatalytic reduction of toxic hexavalent chromium (Cr6+) to non-toxic trivalent (Cr3+).

Robust, magnetic, and effective Pd-based nanocatalyst for reduction of toxic hexavalent chromium and nitrophenols

A serial of magnetic and robust nanocatalysts of Pd nanoparticles with a mean size of 4 nm evenly supported on N-doped porous carbon with Co wrapped (Co-NC) were fabricated by a facile route at room temperature. Among these, 18 %Pd/Co-NC has regular morphology and exhibits the highest activity towards reduction of Cr (VI) with HCOOH and nitrophenols with NaBH4 in aqueous solution. What is more, 18 %Pd/Co-NC has a higher catalytic activity than that of 20 % commercial Pd/C towards the reduction of Cr(VI). The excellent performance may be due to the Pd nanoparticles of small size, uniform dispersion on Co-NC, large BET surface area, abundant active sites, nitrogen doping, and synergetic effect between Pd and the support of Co-NC. The introduction of Co nanoparticles to the support not only provides magnetism but also may prevent the oxidation of Pd, thus facilitating the separation of the catalyst from the solution and providing more active Pd species to catalyze the reduction reaction. The catalyst can be separated easily by an external magnet and remain the activity without obvious decline after the recycling experiment, indicating the catalyst has a good stability in acidic or basic solution. What is more, K2Cr2O7 can be catalytically reduced by Pd/Co-NC in an industrial environment, confirming the catalyst has a potential application in the remediation of industrial wastewater.

Optical and Paper-based Dual Sensing of Hg 2+ and Colorimetric Reduction of Cr(VI) by Green Synthesized Silver Nanoparticles Prepared from the Bark Extract of Sweetinia mahagoni and Their Promising Antimicrobial Applications

This study was conducted to identify promising applications of green silver nanoparticles (AgNPs) prepared from a bark extract of <i>Sweetinia mahagoni</i> (Sm). The green synthesized Sm-AgNPs were characterized using various spectroscopy methods. AgNPs were first investigated using ultraviolet–visible spectroscopy, and the metal nanoparticles exhibited an intense surfaceplasmon resonance (SPR) peak at different wavelengths. The green synthesized Sm-AgNPs had an SPR peak at 430 nm, which confirms the formation of Sm-AgNPs. In addition, Fourier transform infrared (FTIR) spectroscopy was conducted to determine the bioactive compounds of bark extract that actively participate in the reduction of Sm-AgNPs, and the results revealed O−H stretching of free hydroxyl alcohol and phenols, N−H bonds of primary amines, S＝O stretching of sulfoxide in aromatic groups, C−I stretching due to aliphatic iodo compounds, and C−Br stretching by halo compounds of the bark extract which might reduce and stabilize Sm-AgNPs. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) results revealed that Sm-AgNPs were approximately irregular spheres. EDS results revealed the complete reduction of silver to elemental silver. The particle size analysis of Sm-AgNPs was conducted using dynamic light scattering (DLS), and the results revealed that Sm-AgNPs were polydisperse with an average size range from 35.8 to 47.8 nm, an average mean size of 41.3 nm, and a <i>Z</i> average of 37.7 nm. Sm-AgNPs had a negative zeta potential value of −19.0 mV, indicating that Sm-AgNPs were very stable in colloidal form. Further studies were carried out to demonstrate their usefulness in industrial and biomedical applications. In these studies, Sm-AgNPs exhibited a very good antibacterial activity against both Gram-negative and Gram-positive bacteria. In addition to regular assays, we also investigated important industrial applications such as the reduction of toxic hexavalent chromium to a nontoxic form and sensing of Hg²⁺ ions. The results revealed that Sm-AgNPs had an excellent performance in biosensor applications such as sensing and detecting mercury at parts per million/parts per billion levels. In conclusion, green Sm-AgNPs are promising materials in therapeutic and industrial applications.

Treated activated carbon as a metal-free catalyst for effectively catalytic reduction of toxic hexavalent chromium

In this work, activated carbon treated in N2 atmosphere, as a non-metallic catalyst, exhibits excellent catalytic performance in reduction of Cr (VI) to Cr (III) using HCOOH as the reducing agent at room temperature. A series of characterizations and control experiments were carried out to deduce the possible reaction mechanism. The results showed that the improved catalytic performance can be attributed to the enhanced graphitization degree and basic sites such as pyrone-like, which favor electron transferring and activation of reactant. The reaction rate constant observed herein for the C-800 was 22 and 6 times more than that for C-0 and Pd/C catalyst, respectively. In addition, C-800 showed good recycle performance, and no loss of activity was observed after 5 cycles. This study broadens the application of nonmetallic catalyst and provides an easy-available and cost-effective catalytic material for removing toxic Cr (VI).

A Study on Survival and Chromate Reducing Ability of Bacterial Isolates in Tannery Effluent

Rapid industrialization coupled with rapid growth of human population has brought about hazards of environmental pollution. The tanning industry is a major source of water pollution and a threat to the environment due to indiscriminate use of chromium. The reduction-mediated immobilization of Cr (VI) to Cr (III) is a useful process for remediation of Cr (VI) affected environments. The conventional physico-chemical methods for the treatment of Cr (VI) contaminated soil and groundwater are relatively lengthy and expensive as they are energy intensive and require large quantities of chemical reagents which can sometimes generate secondary wastes that require subsequent disposal and do not completely reduce chromium (VI). In view of these treatment shortcomings microbial reduction of toxic hexavalent chromium can be obtained by utilizing microbes in waste water which is cost-effective process and does not produce secondary wastes. The present study aimed to identify and evaluate chromate reducing strains of bacteria from tannery effluent. Seven bacterial strains were isolated from tannery effluent and were identified as members of genera Staphylococcus, Bacillus, Citrobacter and Pseudomonas. The hexavalent chromium reduction and its simultaneous conversion to chromium (III) by the strains were studied in minimal broth medium (with glucose) containing potassium dichromate at various concentrations ranging from 250 mg/l to 1000 mg/l. All the isolates could grow as well as reduce Cr (VI) even at concentration of 1000 mg/l. Qualitative analysis of Cr (III) was also performed.

An overview of palladium supported on carbon-based materials: Synthesis, characterization, and its catalytic activity for reduction of hexavalent chromium

Palladium plays a pivotal role in most of the industrial heterogeneous catalysts, because of its unique properties such as well-defined structure, great intrinsic carrier, outstanding electronic, mechanical and thermal stability. The combination of palladium and various porous carbons (PCs) can widen the use of heterogeneous catalysts. This review highlights the advantages and limitations of carbon supported palladium-based heterogeneous catalyst in reduction of toxic hexavalent chromium (Cr(VI)). In addition, we address recent progress on synthesis routes for mono and bimetallic palladium nanoparticles supported by various carbon composites including graphene-based materials, carbon nanotubes, mesoporous carbons, and activated carbons. The related reaction mechanisms for the Cr(VI) reduction are also suggested. Finally, the challenge and perspective are proposed.

Synthesis and Catalytic Applicability of Pt–Pd ITO Grown Nano Catalyst: An Excellent Candidate for Reduction of Toxic Hexavalent Chromium

In this study, we report the synthesis of indium tin oxide (ITO) decorated Pt–Pd nanoparticle by a simple and facile liquid phase deposition protocol and their application as a heterogeneous catalyst for the reduction of toxic hexavalent chromium. Different characterization techniques were used to confirm the synthesized nanoparticles are honeycomb-like structure. The average size of nanoparticles was found to be 2–5 nm and uniformly distributed over ITO surface. During this study, the parameter was optimized such as the effect of different reducing agent and their concentration, effect of microwaves radiation and dose of nanoparticles. It was observed that the kinetic mechanism was best describes by the pseudo-first order. The synthesized Pt–Pd nanoparticles were successfully applied as an excellent heterogeneous catalyst for reduction of Cr(VI) to Cr(III) in water. The synthesized nanocatalyst was good in term of fast kinetic, high catalytic efficiency, excellent recyclability, and analytical applicability for fast and efficient reduction of toxic hexavalent chromium as compared to conventional methods of nanocatalysts. Schematic illustration of hexavalent chromium reduction

Biomass Derived Sheet-like Carbon/Palladium Nanocomposite: An Excellent Opportunity for Reduction of Toxic Hexavalent Chromium

Palladium nanoparticles (Pd NPs) immobilized on a garlic skin-derived activated carbons (GACs) is reported. The morphology, structure, surface compositions, and textural properties of the GACs and Pd@GAC catalyst were investigated by a variety of physicochemical characterization techniques, which revealed a dispersion of Pd NPs with average particle size of ca. 21 nm on sheet-like graphitized GACs with surface areas and pore volumes as high as 1836 m2 g–1 and 0.89 cm3 g–1, respectively. As a result, the Pd@GAC with a Pd loading of ca. 1–2 wt% exhibited superior activity for catalytic reduction of toxic Cr(VI) to Cr(III) surpassing most metal-based catalysts reported in the literature. As evidenced by a comprehensive UV–vis spectrophotometric study, the eco-friendly Pd@GAC catalyst reported herein, which can be facilely prepared with biowaste feedstocks, also showed excellent catalytic performances for efficient reduction of Cr(VI) with extraordinary stability and recyclability over at least five repeated ...

High-capacity adsorption of dissolved hexavalent chromium using amine-functionalized magnetic corn stalk composites

Easily separable amine-functionalized magnetic corn stalk composites (AF-MCS) were employed for effective adsorption and reduction of toxic hexavalent chromium [Cr(VI)] to nontoxic Cr(III). The saturated magnetization of AF-MCS reached 6.2emu/g, and as a result, it could be separated from aqueous solution by a magnetic process for its superparamagnetism. The studies of various factors influencing the sorption behavior indicated that the optimum AF-MCS dosage for Cr(VI) adsorption was 1g/L, and the maximum adsorption capacity was observed at pH 3.0. The chromium adsorption perfectly fitted the Langmuir isotherm model and pseudo second order kinetic model. Furthermore, characterization of AF-MCS was investigated by means of XRD, SEM, TEM, FT-IR, BET, VSM and XPS analysis to discuss the uptake mechanism. Basically, these results demonstrated that AF-MCS prepared in this work has shown its merit in effective removal of Cr(VI) and rapid separation from effluents simultaneously.

Inhibition of CO poisoning on Pt catalyst coupled with the reduction of toxic hexavalent chromium in a dual-functional fuel cell.

We propose a method to enhance the fuel cell efficiency with the simultaneous removal of toxic heavy metal ions. Carbon monoxide (CO), an intermediate of methanol oxidation that is primarily responsible for Pt catalyst deactivation, can be used as an in-situ reducing agent for hexavalent chromium (Cr (VI)) with reactivating the CO-poisoned Pt catalyst. Using electro-oxidation measurements, the oxidation of adsorbed CO molecules coupled with the concurrent conversion of Cr (VI) to Cr (III) was confirmed. This concept was also successfully applied to a methanol fuel cell to enhance its performance efficiency and to remove toxic Cr (VI) at the same time.

Hexavalent Chromium Reduction under Fermentative Conditions with Lactate Stimulated Native Microbial Communities

Microbial reduction of toxic hexavalent chromium (Cr(VI)) in-situ is a plausible bioremediation strategy in electron-acceptor limited environments. However, higher [Cr(VI)] may impose stress on syntrophic communities and impact community structure and function. The study objectives were to understand the impacts of Cr(VI) concentrations on community structure and on the Cr(VI)-reduction potential of groundwater communities at Hanford, WA. Steady state continuous flow bioreactors were used to grow native communities enriched with lactate (30 mM) and continuously amended with Cr(VI) at 0.0 (No-Cr), 0.1 (Low-Cr) and 3.0 (High-Cr) mg/L. Microbial growth, metabolites, Cr(VI), 16S rRNA gene sequences and GeoChip based functional gene composition were monitored for 15 weeks. Temporal trends and differences in growth, metabolite profiles, and community composition were observed, largely between Low-Cr and High-Cr bioreactors. In both High-Cr and Low-Cr bioreactors, Cr(VI) levels were below detection from week 1 until week 15. With lactate enrichment, native bacterial diversity substantially decreased as Pelosinus spp., and Sporotalea spp., became the dominant groups, but did not significantly differ between Cr concentrations. The Archaea diversity also substantially decreased after lactate enrichment from Methanosaeta (35%), Methanosarcina (17%) and others, to mostly Methanosarcina spp. (95%). Methane production was lower in High-Cr reactors suggesting some inhibition of methanogens. Several key functional genes were distinct in Low-Cr bioreactors compared to High-Cr. Among the Cr resistant microbes, Burkholderia vietnamiensis, Comamonas testosterone and Ralstonia pickettii proliferated in Cr amended bioreactors. In-situ fermentative conditions facilitated Cr(VI) reduction, and as a result 3.0 mg/L Cr(VI) did not impact the overall bacterial community structure.

Potential of Novel Bacterial Consortium for the Remediation of Chromium Contamination

This study was aimed to examine the efficiency of a novel bacterial consortium on the reduction of toxic hexavalent chromium [Cr(VI)] to non-toxic trivalent Cr [Cr(III)]. Six Cr(VI)-resistant bacteria (IS1-IS6) were isolated from a tannery waste disposal site at Mount Barker, South Australia, of which three viz., IS1, IS2 and IS3 were selected based on Cr(VI) reduction ability in minimal salt medium. The isolates were identified as Bacillus endophyticus (IS1), Microbacterium paraoxydans (IS2) and Bacillus simplex (IS3) by 16S rRNA gene sequencing. All three isolates were able to tolerate chromium (Cr(VI), 300–400 mg L−1), arsenic (As(V), 1,000 mg L−1), copper (Cu(II), 300–400 mg L−1) and lead (Pb(II), 1,000 mg L−1). The isolates were evaluated both as an individual and as a consortia for Cr(VI) reduction in minimal salt medium and storm water, both spiked with 100 mg Cr(VI) L−1. In both cases, the rate of Cr(VI) reduction was found to be significantly higher in the bacterial consortium inoculation (t½ = 8.45 for minimal salt medium; 6.02 h for storm water), compared to inoculation with individual isolates (t½ = 53.3–115.5 h for minimal salt medium; 8.77–9.76 h for storm water). The rate of Cr(VI) reduction in both minimal salt medium and storm water was found to be higher in bacterial consortium inoculation (IS1 + IS2 + IS3) than in individual isolate inoculation. This experiment demonstrated that bacterial consortium prepared by using B. endophyticus, M. paraoxydans and B. simplex was more effective in Cr(VI) detoxification than application of individual bacterium. This experiment also proved that a bacterial consortium was more effective in Cr(VI) detoxification than the application of individual bacterial strain.

Photocatalytic reduction of hexavalent chromium at gold nanoparticles modified titania nanotubes

N-[3-(Trimethoxysilyl)propyl]ethylenediamine (EDAS) silicate supported titanium dioxide nanotubes-gold ((TiO2 NTs-Au)NCM) nanocomposite material (EDAS/(TiO2 NTs-Au)NCM) was prepared by deposition–precipitation method and characterized by diffuse reflectance spectra, X-ray diffraction pattern, Brunauer–Emmett–Teller surface area analysis, transmission electron micrographs, scanning electron micrographs and energy-dispersive X-ray spectra analysis. The photocatalytic activity of the EDAS/(TiO2 NTs-Au)NCM in the film form was investigated towards the reduction of toxic hexavalent chromium (Cr(VI)) into trivalent chromium (Cr(III)) in the presence of oxalic acid as an electron donor. The EDAS/(TiO2 NTs-Au)NCM film exhibited higher photocatalytic activity when compared to the photocatalytic activities of pristine TiO2 nanoparticles and TiO2 nanotubes (TiO2 NTs) which can be attributed to the effective photoinduced interfacial charge transfer from the (TiO2 NTs-Au)NCM to Cr(VI) through Au nanoparticles (Aunps). The Aunps present in the TiO2 NTs act as an electron sink for the photogenerated electrons that minimizes the charge recombination process at the TiO2 NTs. The Aunps on the TiO2 NTs surface facilitates the transfer of photogenerated electrons to the Cr(VI) leading to the formation of Cr(III) ions.

Ultrasound mediated reduction of Cr(VI) using sludge obtained during electrocoagulation

This study presents the ultrasound mediated reduction of toxic hexavalent chromium (Cr(VI)) into non‐toxic trivalent chromium (Cr(III)) using sludge obtained during the electrocoagulation of pharmaceutical wastewater (EC sludge). Experiments were carried out with and without ultrasound to explore its influence. The effect of initial concentration of Cr(VI) (100–200 mg/l), pH (6–8) and EC sludge dosage (5–10 g/l) on Cr(VI) reduction was studied. The morphological and functional group changes of EC sludge due to Cr(VI) reduction were analysed with scanning electron microscopy and Fourier transform infrared, respectively. The reduction rates observed in the presence of ultrasound were found to be higher than the rates observed for without ultrasound. The results concluded that the EC sludge can be successfully used for reduction of Cr(VI) at lower cost.

Reduction of toxic hexavalent chromium by Ochrobactrum intermedium strain SDCr-5 stimulated by heavy metals

A Cr(VI) resistant bacterial strain SDCr-5, identified as Ochrobactrum intermedium on the basis of 16S rRNA gene sequencing, was tolerant to high concentrations of Cr(VI) up to 15 mg ml −1 in acetate minimal medium. O. intermedium SDCr-5 reduced Cr(VI) under a wide range of concentrations from 100 to 1500 μg ml −1 and reduction was optimum at 37 °C and pH 7. It reduced 200 and 721 μg ml −1 Cr(VI) within 72 and 96 h, respectively. The rate of Cr(VI) reduction increased with concentration from 100 to 1500 μg ml −1. The presence of heavy metal cations such as Cu 2+, Co 2+, Mn 2+ and Ni 2+ stimulated Cr(VI) reduction. Strain SDCr-5 might be useful for Cr(VI) detoxification under a wide range of environmental conditions.

Reaction rate and electrochemical stability of conducting polymer films used for the reduction of hexavalent chromium

The reduction of toxic hexavalent chromium to its trivalent state at conducting polymers films was studied. The process is based on the capacity of conducting polymers to spontaneously donate electrons to the Cr(VI) species. Electrodes of polyaniline and polypyrrole, deposited as films on reticulated vitreous carbon (RVC), were prepared. The process of Cr(VI) reduction was studied under open and closed circuit conditions. It was observed that polyaniline was only suitable for use only under closed circuit conditions, whereas polypyrrole did not perform well under either open or closed circuit conditions. The electrocatalytic power of polyaniline for Cr(VI) reduction was demonstrated when the reaction rates for bare RVC and RVC/PANI were compared.

Kinetics and modeling of hexavalent chromium reduction in enterobacter cloacae

Kinetics of bacterial reduction of toxic hexavalent chromium (chromate: CrO(4) (2-)) was investigated using batch and fedbatch cultures of Enterobacter cloacae strain HO1. In fedbatch cultures, the CrO(4) (2-) feed was controlled on the basis of the rate of pH change. This control strategy has proven to be useful for avoiding toxic CrO(4) (2-) overload. A simple mathematical model was developed to describe the bacterial process of CrO(4) (2-) reduction. In this model, two types of bacterial cells were considered: induced, CrO(4) (2-)-resistant cells and uninduced, sensitive ones. Only resistant cells were assumed to be able to reduce CrO(4) (2-). These fundamental ideas were supported by the model predictions which well approximated all experimental data. In a simulation study, the model was also used to optimize fed-batch cultures, instead of lengthy and expensive laboratory experiments.

A survey of effective electron donors for reduction of toxic hexavalent chromium by enterobacter cloacae (strain HO1).

In the present study, we conducted reduction experiments with growing and washed cells in a basal medium containing a variety of organic substances to survey appropriate electron donors for CrO 4 2− reduction in E. cloacae HOl

Bacterial reduction of toxic hexavalent chromium using a fed-batch culture of Enterobacter cloacae strain HOl

Bacterial reduction of hexavalent chromium (chromate: CrO 4 2−) was investigated using fed-batch cultures of Enterobacter cloacae strain HOl. In the fed-batch cultures, toxic CrO 4 2− was continuously added in small doses to minimize the toxic effect. The fed-batch process was proved to be an effective biological method for detoxifying CrO 4 2− in aqueous solutions.