Electrochemical Reduction Of Cr Research Articles

A honeycomb-like micro-needle sensor with gold nanoparticles embedded for the determination of hexavalent chromium in seawater

As one of the most toxic heavy metals, hexavalent chromium (Cr(VI)) is a typical environmental pollutant that directly affects human health. In this work, a novel micro-needle sensor with honeycomb-like structure and embedded gold nanoparticles was proposed for the stable and sensitive determination of Cr(VI). The honeycomb-like structure provided larger specific surface area to support more gold nanoparticles which exhibited excellent catalytic activity towards the electrochemical reduction of Cr(VI) to Cr(Ⅲ). In addition, gold nanoparticles were embedded into the holes of the honeycomb-like structure, which can effectively prevent them from falling off from the electrode surface due to the protection of the holes. Combining the honeycomb-like structure of micro-needle electrode with the excellent catalytic activity of gold nanoparticles, the fabricated sensor exhibited outstanding stability and sensitivity for the cathodic stripping determination of Cr(VI). A linear range of Cr(VI) from 0.5 to 300 μmol/L was obtained, with the detection limit of 0.15 μmol/L. The recovery of the sensor for the determination of hexavalent chromium in seawater samples was also investigated. Moreover, the sensor might be further adapted for the determination of other heavy metals with different nanoparticles embedded.

Green synthesis of Pd@biochar using the extract and biochar of corn-husk wastes for electrochemical Cr(VI) reduction in plating wastewater

In this study, Corn Husks, as an organic waste, were utilized to synthesize palladium nanoparticles (PdNPs) using a green approach and the related biochar as a support material for PdNPs to produce Pd@biochar nanocatalyst. Pd@biochar nanocatalysts were synthesized using the extracted liquid from corn husks and PdCl2 salt, and biochar was obtained by a hydrothermal process conducted on chapped corn husks. FESEM, UV-Vis, XRD, and FTIR were used to evaluate the physicochemical properties of the nanocatalyst and the formation of PdNPs using the implemented approach. The effect of the Pd@biochar mass ratio was explored, and nanocomposite with a mass of 0.06 palladium to biochar was the most favorable catalyst. The Design Expert software was applied to investigate and optimize the effect of parameters, such as pH, initial Cr(VI)concentration, and contact time, using the Response Surface Box Behnken method. The proposed model of Design Expert software indicated a 99.9% reduction of Cr(VI) at optimum conditions ( pH=1, initial Cr(VI) concentration= 100 mg L-1, contact time= 60 min). The kinetics of the electrochemical reaction showed a high correlation with the pseudo-first-order model at different concentrations of Cr(IV). The applicability of synthesized nanocatalyst in six cycles of the experiment was tested, and results pointed out only a 10% decrease in nanocatalyst efficiency, proving its high stability. The electrochemical reduction of Cr(VI) in real plating wastewater was investigated at optimum conditions, which resulted in a 99.7% reduction and 0.35 mg L-1 residual of Cr(IV), reaching Iran’s standard for industrisal wastewater discharge.

Efficient reduction of Cr(VI) and elimination of total Cr with S-bridged Ni3S2/MoS2 nanowire electrode

Electrochemical reduction of Cr(VI) is eco-friendly and cost-effective for eliminating the environmental risk of Cr-contaminated water. Herein, a novel Ni3S2/MoS2 nanowire electrode with a unique stem-leaf structure was synthesized for this purpose. Self-supported ultra-long Ni3S2 nanowires bearing MoS2 nanoflakes were in-situ grown from Ni foam with a hydrothermal method, in which Ni3S2 and MoS2 grow accompanied with each other by sharing some S atoms. Consequently, highly conductive out-stretched MoS2 nanoflakes wrap the Ni3S2 nanowires, providing aligned pathways for directional electron transfer and huge networks for the efficient reduction of Cr(VI) and high uptake of generated Cr(OH)3. The highest Cr(VI) removal rate attained at − 1.2 V approaches 0.005 mg/cm2 min, and elimination efficiency of total Cr is high up to 90.81 %. Although the adsorbed Cr(OH)3 on Ni3S2/MoS2 decreases the activity after repeated runs, the electrode can be restored by a simple pickling process with dilute acid, which has promising application prospects in the electrochemical reduction of Cr-contained wastewater.

Electrochemical reduction of Cr (VI) using a palladium/graphene modified stainless steel electrode.

In this study, a palladium/graphene modified stainless steel electrode was successfully prepared and applied in an electrochemical reduction device to remove Cr (VI) from the wastewater. Pd was modified onto the electrode mainly via interacting with the carboxyl group of graphene. The Cr (VI) removal efficiency was up to 99.70 ± 0.00% under the optimal condition (Pd content proportion of 3%, electrode potential of -0.9 V, pH = 2 and electrolyte concentration of 6 g/L). It was found that Cr (VI) was removed via the following processes: (1) direct electrochemical reduction by accepting electrons, (2) indirect electrochemical reduction by H2O2 that was generated from H2 in the presence of Pd, (3) adsorption through hydrogen bond, and (4) chemical reduction through alkoxy groups donating electrons. The indirect electrochemical reduction considerably promoted the Cr (VI) removal while a small amount of Cr (VI) was removed via adsorption and chemical reduction. The method could not only be used as a pretreatment technology to solve the problem of excessive Cr (VI) concentration of industrial wastewater, but also could provide reference for the electrochemical reduction of similar metal ions.

Electrochemical Reduction of Cr (VI) Using a Graphene-Modified Stainless Steel Electrode

Electrochemical reduction using a metal-based electrode is a novel method for treatment of Cr (VI)-containing wastewater. However, the removal efficiency with a metal-based electrode is limited by the low conductivity and specific surface area. In this study, a stainless steel (SS) electrode modified with graphene was fabricated and used to reduce Cr (VI) in an electrochemical system. Under the optimal conditions (i.e., the graphene loading of 1.50 mg·cm−2, −0.7 V, pH 1.5 and the electrolyte concentration (Na2SO4) of 10 g·l−1), this composite electrode removed 99.7 ± 0.00% of Cr (VI) with an initial concentration of 10 mg·l−1 in 80 min Cr (VI) was removed by the following pathways: (1) under the additive electric field, Cr (VI) was mainly reduced into Cr (III) directly by accepting electrons, (2) Cr (VI) was reduced to Cr (III) by −OH of graphene, (3) −COOH on the surface of graphene underwent bonding with H+ thereby producing hydrogen bonding to adsorb Cr (VI). Meanwhile, COO− was produced via losing H+ to adsorb Cr (III).

Impact of the choice of buffer on the electrochemical reduction of Cr(vi) in water on carbon electrodes.

Hexavalent chromium is a contaminant of concern in water. Electrochemical methods are being developed to reduce toxic Cr(vi) to benign Cr(iii) at the point of generation or point of use. The effectiveness of glassy carbon electrodes to detect and reduce Cr(vi) in cyclic voltammetry was recently demonstrated. Herein, we report that the nature of the buffer system used, at a fixed pH, has unexpected impacts on the electrochemical reduction of Cr(vi) in water. At low concentrations of Cr(vi), the buffer influences the PCET step gating Cr(vi) reduction on the timescale of cyclic voltammetry experiments. At higher concentrations of Cr(vi), the effect is more complex. Data suggests impacts on both the chemical steps of Cr(vi) reduction and the nature of the products formed, hypothesized to be due to chelation effects. In particular, evidence of adsorption on the electrode surface is seen through cyclic voltammetry studies in certain buffers. Chronoamperometry studies confirm the adsorption of chromium containing species on the electrode surface during Cr(vi) electroreduction. XPS confirms Cr(iii) as the product. The activity of the electrode is regained after an acid wash step, without the need for re-polishing. This work provides a framework to understand the impact of the presence of small organic acids on Cr(vi) reduction for water purification.

Electrochemical reduction of Cr(VI) in water: lessons learned from fundamental studies and applications.

Converting toxic Cr(vi) to benign Cr(iii) would offer a solution to decontaminate drinking water. Electrochemical methods are ideally suited to carry out this reduction without added external reductants. Achieving this transformation at low overpotentials requires mediating the transfer of protons and electrons to Cr(vi). In this review thermodynamic parameters will be discussed to understand Cr(vi) speciation in water and identify reduction pathways. The electrochemical reduction of Cr(vi) at bare electrodes is reviewed and mechanistic considerations are discussed. Works on modified electrodes are compared to identify key parameters influencing the reduction. An overview of current applications to Cr(vi) reduction is briefly discussed to link fundamental studies to applications.

Electroplating of Thick Hard Chromium Coating from a Trivalent Chromium Bath Containing a Ternary Complexing Agent: A Methodological and Mechanistic Study

Trivalent chromium electroplating is considered a greener alternative to hexavalent chromium electroplating. However, the electrochemical reduction of Cr(III) in aqueous solution is extremely unstable and the thickness of the coating can hardly grow beyond 10 μm, which limits its industrial applications. We herein report a trivalent chromium bath containing a ternary complexing agent, which yields bright and uniform chromium coatings with thickness exceeding 30 μm, as well as high hardness and corrosion resistance. The electroplating behaviors and first principal calculations reveal that the Cr(II) intermediate plays a vital role in sustained electroplating in Cr(III) baths. The composition of complexing agents has a profound effect on the geometry and electron accepting ability of the Cr(II) complex ions. The failure to grow thick Cr coating is attributed to the accumulation of inactive hydroxo-bridged complexes. In the bath with ternary complexing agent, the Cr(II) complex ions are dsp3 or dsp2 hybridized with high electron affinity, which can be readily reduced to metallic Cr. The bath also shows a strong competitive ligand-binding effect that renders Cr(II) to preferentially bind with an organic ligand rather than a hydroxy ligand. Furthermore, the planar geometry of dsp2 hybridization does not favor the formation of hydroxo-bridged complexes because of the steric hindrance effect. The above merits of the ternary complexing agent lead to sustained electroplating, and thick Cr coatings are obtained.

Electrochemical reduction of Cr(VI) in the presence of sodium alginate and its application in water purification

Chromium (Cr) is used in many manufacturing processes, and its release into natural waters is a major environmental problem today. Low concentrations of Cr(VI) are toxic to human health and living organisms due to the carcinogenic and mutagenic nature of this mineral. This work examined the conversion of Cr(VI) to Cr(III) via electrochemical reduction using gold electrode in an acidic sodium alginate (SA) solution and subsequent removal of the produced Cr(III)-SA by the polymer-enhanced ultrafiltration (PEUF) technique. A solution of SA in nitric acid was used both as an electrolytic medium during the voltammetric measurements and bulk electrolysis and as an extracting agent during the PEUF technique. The electroanalysis of Cr(VI) was performed by linear sweep voltammetry in the presence of acidic SA solution to study its voltammetric behavior as a function of the Cr(VI) concentration, pH, presence of Cr(III), SA concentration and scan rate. In addition, the quantitative reduction of Cr(VI) to Cr(III) was studied through the bulk electrolysis technique.The results showed efficient reduction with well-defined peaks at approximately 0.3 V vs. Ag/AgCl, using a gold working electrode. As the pH increased, the reduction signal strongly decreased until its disappearance. The optimum SA concentration was 10 mmol/L, and it was observed that the presence of Cr(III) did not interfere in the Cr(VI) electroanalysis. Through the quantitative reduction by bulk electrolysis in the presence of acidic SA solution, it was possible to reduce all Cr(VI) to Cr(III) followed by its removal via PEUF.

Electrocatalytic reduction of Cr(VI) over heterophase MoS2 film electrode

Electrochemical reduction and treatment of Cr(VI)-contaminated wastewater normally suffer from a high overpotential, due to the extravagant consumption of electrons by the adverse reactions of hydrogen evolution and oxygen reduction. In this work, the MoS2 film electrode prepared by electrochemical deposition-annealing method was applied into the electrochemical reduction of Cr(VI), with the aim to suppress the over-potential of the reduction and improve the utilization of electrons. Results demonstrated that the 2H/1T′ heterostructure of MoS2 enhanced the reduction efficiency of Cr(VI) up to 99% at a lower potential of −1.0 V vs. Ag/AgCl. The in-situ electrochemically generated hydrogen peroxide was found to play a critical role in accelerating the reduction of Cr(VI). Selective oxygen reduction reaction (ORR) via a two-electron pathway on the MoS2 cathode was responsible for the formation of hydrogen peroxide, where the MoS2 film electrode annealed at 700 °C can obtain a high productivity of 360 μM hydrogen peroxide within 60 min. The plenty of preserved interfaces between 2H and 1 T′ in the heterophase MoS2 were crucial to the selective electrocatalytic activity. In addition, this electrode also presented an excellent electrocatalytic activity in hydrogen evolution reaction (HER), which could form atomic hydrogen on the surface of cathode to facilitate the reduction of Cr(VI) under the anoxic condition. This study paves a novel pathway to in-situ deposit the MoS2 film electrode for the sustainable electrochemical reduction and treatment of Cr(VI)-containing wastewater.

Electrochemical and Raman Spectroscopic Investigations on the Speciation and Behavior of Chromium Ions in Fluoride Doped Molten LiCl-KCl

The speciation and behavior of chromium ions in LiCl-KCl-CrF3 and LiCl-KCl-LiF-CrF3 melts were investigated by using cyclic voltammetry, square wave voltammetry, chronopotentiometry and Raman spectroscopy. Electrochemical reduction of Cr(III) to Cr(0) in LiCl-KCl-CrF3 is a two-step process mediated by the formation of Cr(II). Most Cr(III) ions exist in the form of CrCl6 3− while CrCl6-xFx 3− (x ≤ 3) ions are minor in the molten salt medium. Based on the cyclic voltammetric results obtained at different potential scanning rates, the diffusion coefficients of Cr(III) and Cr(II) in LiCl-KCl-CrF3 melt were determined between 723 and 873 K, and their dependence on temperature can be described as lnDCr(III) = −4.90-4322.66/T and lnDCr(II) = −4.71-4007.09/T respectively. The diffusion coefficients of chromium ions in LiCl-KCl-CrF3 are higher than those in LiCl-KCl-CrCl3, which can be explained by the hopping mechanism. In addition, a series of apparent standard potentials of the Cr(III)/Cr(II) and Cr(II)/Cr(0) couples were obtained over the same temperature range using the open-circuit chronopotentiometry technique. With the addition of LiF into the LiCl-KCl-CrF3 mixture, the dominant chromium species change from CrCl6 3− to CrCl6-xFx 3− (x ≤ 6), which is supported by the appearance of the new peaks in both square wave voltammograms and Raman spectra.

Electrochemical reduction of hexavalent chromium on titania nanotubes with urea as an anolyte additive

The present study reports an energy efficient electrochemical strategy to remediate hexavalent chromium Cr(VI) to less toxic Cr(III). The novelty in this system is the use of titania nanotubes (TNT) synthesized by anodization as cathode for the reduction of Cr(VI) with alkaline urea as an anolyte additive. The electrochemical reduction of Cr(VI) was evaluated using bare Ti and porous TNT/Ti cathodes, both with and without urea as an anolyte additive. The experimental results showed that the dual combination of alkaline urea oxidation at anode and acidic Cr(VI) reduction on TNT cathode showed the best performance, with enhanced degradation efficiency up to 97% at 5 V within 15 min for an initial concentration of 100 mg L−1 Cr. The optimized urea-TNT combination was evaluated for the effect of catholyte pH, temperature, initial metal ion concentration and the effect of other anions, such as Cl−, NO3−, PO42− and CO32−, which are commonly present in wastewater discharged from tannery or electroplating industries. Furthermore, the performance when evaluated with cow urine as an anolyte additive confirmed that the dual combination could be a promising in-situ treatment technique for simultaneous urine oxidation and Cr(VI) reduction.

Scrap iron anode packed in a Ti mesh for electrochemical reduction of Cr(VI) in electroplating wastewater

Scrap iron anode packed in a Ti mesh for electrochemical reduction of Cr(VI) in electroplating wastewater

The electrochemical reduction of Cr(VI) ions in acid solution at titanium and graphite electrodes

Cyclic voltammetry (CV) and rotating disc electrode (RDE) linear sweep voltammetry were used to study the reduction of Cr(VI) ions on a titanium electrode at 298K. Diffusion controlled reduction of Cr(VI) ions occurred between −0.22V and −0.45V vs. Ag/AgCl and appears to take place in a single, 3-electron step. The diffusion coefficient was 1.2×10−5cm2s−1, which agrees with data reported in the literature. Following a study in a three-electrode cell, the electrolysis of 2×10−3moldm−3 Cr(VI) in 0.1moldm−3H2SO4 solution at −1.0V vs. Ag/AgCl, on titanium and −0.65V vs. Ag/AgCl on carbon electrodes were carried out. The area of the electrodes was 64cm2 and the mean linear flow velocity of electrolyte past the cathode varied between 10 and 80cms−1 (630–5680cm3min−1). The parasitic reactions of oxygen reduction and hydrogen evolution, which decreased the current efficiency, were observed during the reduction of Cr(VI) ions at titanium electrodes.

Sensitive and selective electrochemical detection of chromium(vi) based on gold nanoparticle-decorated titania nanotube arrays

Owing to the severe toxicity and mobility of Cr(VI) in biological and environmental systems, it is of great importance to develop convenient and reliable methods for its detection. Here we report on a facile and effective electrochemical technique for monitoring Cr(VI) concentrations based on the utilization of Au nanoparticle-decorated titania nanotubes (TiO2NTs) grown on a titanium substrate. It was found that the electrochemical reduction of Cr(VI) at the Ti/TiO2NT/Au electrode exhibited an almost 23 fold improvement in activity as compared to a polycrystalline gold electrode, due to its nanoparticle/nanotubular heterojunction infrastructure. As a result, the Ti/TiO2NT/Au electrode demonstrated a wide linear concentration range from 0.10 μM to 105 μM, a low detection limit of 0.03 μM, and a high sensitivity of 6.91 μA μM(-1) Cr(VI) via amperometry, satisfying the detection requirements of the World Health Organization (WHO). Moreover, the Ti/TiO2NT/Au electrode exhibited good resistance against interference from coexisting Cr(III) and other metal ions, and excellent recovery for Cr(VI) detection in both tap and lake water samples. These attributes suggest that this hybrid sensor has strong potential in applications for the selective detection of Cr(VI).

Azo dyes as electron transfer mediators in the electrochemical reduction of Cr(VI) using boron-doped diamond electrodes

Reducing hexavalent chromium in industrial wastewater to its less toxic trivalent state is an important but challenging problem. Among the available techniques, electrochemical reduction has many advantages. Within electrochemical cells, boron-doped diamond (BDD) electrodes are advantageous in that they are inert and have a high overpotential. However, even at optimal pH of 2 with a supporting electrolyte, a BDD electrochemical cell could only achieve a reduction of 36% of aqueous Cr(VI). However, small amounts of azo dyes enhanced the process by serving as an electron transfer mediator which increases the electrochemical effectiveness. The three azo dyes thionine (Th), methyl orange (MO), and methylene blue (MB) all increased the effectiveness of the electrochemical reduction, with MO increasing it to about 90%. The electrochemical process also oxidized 98% of the azo dye. Other than electrolytes and acid, the only chemical reagent added was the MO, which was decomposed in the process, and no sludge was produced, so the process is quite environmentally friendly.

Electroreduction of Cr(III) Ions in Methanesulphonate Solution on Pb Electrode

The electrochemical reduction of Cr(III) methanesulphonate complexes were investigated on the lead electrode. The Cr(III) electroreduction process proved to be irreversible. The standard rate constants as well as the standard electrode potentials of Cr(III) ions in methanesulphonic acid (MSA) solution are less than those corresponding to the perchlorate electrolytes.

Cr(VI) reduction in wastewater using a bimetallic galvanic reactor

The electrochemical reduction of Cr(VI)–Cr(III) in wastewater by iron and copper–iron bimetallic plates was evaluated and optimized. Iron has been used as a reducing agent, but in this work a copper–iron galvanic system in the form of bimetallic plates is applied to reducing hexavalent chromium. The optimal pH (2) and ratio of copper to iron surface areas (3.5:1) were determined in batch studies, achieving a 100% reduction in about 25 min. The Cr(VI) reduction kinetics for the bimetallic system fit a first order mechanism with a correlation of 0.9935. Thermodynamic analysis shows that the Cr(VI) reduction is possible at any pH value. However, at pH values above 3.0 for iron and 5.5 for chromium insoluble species appear, indicating that the reaction will be hindered. Continuous column studies indicate that the bimetallic copper–iron galvanic system has a reduction capacity of 9.5890 mg Cr(VI) cm −2 iron, whereas iron alone only has a capacity of 0.1269 mg Cr(VI) cm −2. The bimetallic copper–iron galvanic system is much more effective in reducing hexavalent chromium than iron alone. The exhausted plates were analyzed by SEM, EDS, and XRD to determine the mechanism and the surface effects, especially surface fouling.

Optimization of Cr(VI) electroreduction from synthetic industrial wastewater using reticulated vitreous carbon electrodes modified with conducting polymers

In order to maintain high values of current efficiency and space–time yield during the electrochemical reduction of Cr(VI) to Cr(III), a new methodology based on the control of the applied current throughout the process was tested. The electrochemical process was carried out using a reticulated vitreous carbon electrode modified with polyaniline. Experiments to determine the reaction rate of Cr(VI) depletion as a function of current and flow velocities were performed in order to establish the relationship between the transition concentration and these operational variables. The transition concentration was the basis of the optimization procedure proposed here and it is defined as being the concentration where the kinetics changes from linear to exponential concentration dependence. The results demonstrate that for a previously optimized flow velocity, control of the current was very effective in maintaining a high space–time yield down to very low concentrations of Cr(VI) and, consequently, the energy consumption was minimized.

Application of a renewable silver based mercury film electrode to the determination of Cr(VI) in soil samples

The design and experimental results of the application of a renewable mercury film silver based electrode to the determination of Cr(VI) in soil samples are presented. The main feature of this procedure is that it can be used in field measurements. The procedure is based on the extraction of total Cr(VI) exploiting the complexation property of diethylenetriaminepentaacetic acid (DTPA) followed by electrochemical reduction of Cr(VI) to Cr(III) with the formation of Cr(III)-H2DTPA complex adsorbed on mercury film electrode. The voltammetric signal is caused by reduction of this complex. The validation of the proposed procedure was made by Cr(VI) determination in the certified reference material “Chromium VI in soil”. The protocol for Cr(VI) determination has also been applied to the analysis of Rendoll soil samples with satisfying precision.