Electrochemical Regeneration Research Articles

Electrochemical Acid-Catalyzed Desorption and Regeneration of MDEA CO2-Rich Liquid by Hydroquinone Derivatives (Tiron)

The acid-catalyzed desorption of the MDEA CO2-rich liquid under the electrochemical reaction can lower the desorption temperature, thereby reducing the energy consumption for regeneration. In this work, disodium 4,5-dihydroxy-1,3-benzenedisulfonate (tiron) was used as an acidic medium for the electrocatalytic regeneration of the MDEA CO2-rich solution. The results show that the CO2 desorption efficiency reaches more than 90% at 25 °C for the 0.3 M MDEA (0.1 M tiron) system. At 50 °C, compared with 3 M MDEA (without tiron), the maximum CO2 desorption rate and desorption efficiency of the 3 M MDEA (with 0.1 M tiron) CO2-rich solution increased by 27.4 and 18.3%, respectively. Elevating the temperature can increase the quantity of CO2 desorption and the degree of MDEAH+ deprotonation as well as the reactivity of tiron on the electrode, so the desorption rate of CO2 can be further improved by the oxidation of tiron releasing H+ in coordination with thermal desorption.

Kinetics-Based Approach to Developing Electrocatalytic Variants of Slow Oxidations: Application to Hydride Abstraction-Initiated Cyclization Reactions.

Electrochemical oxidant regeneration is challenging in reactions that have a slow redox step because the steady-state concentration of the reduced oxidant is low, causing difficulties in maintaining sufficient current or preventing potential spikes. This work shows that applying an understanding of the relationship between intermediate cation stability, oxidant strength, overpotential, and concentration on reaction kinetics delivers a method for electrochemical oxoammonium ion regeneration in hydride abstraction-initiated cyclization reactions, resulting in the development of an electrocatalytic variant of a process that has a high oxidation transition state free energy. This approach should be applicable to expanding the scope of electrocatalysis to include additional slow redox processes.

VFB Maintenance Methods: Technical and Economic Issues

The VFB system has been extensively studied for almost 30 years. Several plants are installed around the world, with power and energy exceeding some MW and some MWh respectively and new companies are entering the growing market. However, a real widespread application of this technology is hindered by its high capital cost. One method to make these batteries more competitive on the market is to increase their cyclability by means of appropriate maintenance procedures. Some procedures are focused on physical treatments such as the remixing of the positive electrolyte with the negative one, which causes heat generation. Other methods are focused on chemical and electrochemical regeneration procedures which make use of chemical reducing agents, catalysts or electrochemical processes. The latter requires the use of an electrolysis system in order to restore the vanadium oxidation state to the correct ratio in the positive and negative electrolyte. In the first part of this work, an extensive description of VFB technology is presented while in the second part a description of the most important and realizable maintenance procedures with their impact on the system cost is shown, considering both operative and economical points of view.

Biomimetic Metal-Free Hydride Donor Catalysts for CO2 Reduction

The catalytic reduction of carbon dioxide to fuels and value-added chemicals is of significance for the development of carbon recycling technologies. One of the main challenges associated with catalytic CO2 reduction is product selectivity: the formation of carbon monoxide, molecular hydrogen, formate, methanol, and other products occurs with similar thermodynamic driving forces, making it difficult to selectively reduce CO2 to the target product. Significant scientific effort has been aimed at the development of catalysts that can suppress the undesired hydrogen evolution reaction and direct the reaction toward the selective formation of the desired products, which are easy to handle and store. Inspired by natural photosynthesis, where the CO2 reduction is achieved using NADPH cofactors in the Calvin cycle, we explore biomimetic metal-free hydride donors as catalysts for the selective reduction of CO2 to formate. Here, we outline our recent findings on the thermodynamic and kinetic parameters that control the hydride transfer from metal-free hydrides to CO2. By experimentally measuring and theoretically calculating the thermodynamic hydricities of a range of metal-free hydride donors, we derive structural and electronic factors that affect their hydride-donating abilities. Two dominant factors that contribute to the stronger hydride donors are identified to be (i) the stabilization of the positive charge formed upon HT via aromatization or by the presence of electron-donating groups and (ii) the destabilization of hydride donors through the anomeric effect or in the presence of significant structural constrains in the hydride molecule. Hydride donors with appropriate thermodynamic hydricities were reacted with CO2, and the formation of the formate ion (the first reduction step in CO2 reduction to methanol) was confirmed experimentally, providing an important proof of principle that organocatalytic CO2 reduction is feasible. The kinetics of hydride transfer to CO2 were found to be slow, and the sluggish kinetics were assigned in part to the large self-exchange reorganization energy associated with the organic hydrides in the DMSO solvent. Finally, we outline our approaches to the closure of the catalytic cycle via the electrochemical and photochemical regeneration of the hydride (R-H) from the conjugate hydride acceptors (R+). We illustrate how proton-coupled electron transfer can be efficiently utilized not only to lower the electrochemical potential at which the hydride regeneration takes place but also to suppress the unwanted dimerization that neutral radical intermediates tend to undergo. Overall, this account provides a summary of important milestones achieved in organocatalytic CO2 reduction and provides insights into the future research directions needed for the discovery of inexpensive catalysts for carbon recycling.

Regeneration of activated carbon adsorbent by anodic and cathodic electrochemical process

The efficiency of saturated Activated Carbon (AC) electrochemical regeneration was evaluated. Reactor configurations in cathodic/anodic process, applied current effect on the properties of AC and Methylene Blue MB-AC interactions and adsorption/desorption mechanisms were studied. The efficiency was measured by adsorption capacity adsorption/regeneration cycles and changes in the adsorbent characteristic were analyzed through FTIR, BET, Optical Microscopy (OM) and SEM before and after the regeneration. The energy consumption was analyzed to assess the economic feasibility. It was found that the electrochemical treatment was efficient in returning the AC adsorption capacity, maximum efficiencies of approximately 79% and 84% were reached when the Carbon Fiber Cloth-AC electrode was subjected to anodic and cathodic currents, respectively, at a current of 0.1 A. In addition, the material demonstrated good stability through adsorption-regeneration cycles by cathodic process, with the material's adsorption capacity being almost totally recovered for 8 consecutive cycles. The removal percentage around 80% was kept after the cycles, with optimal conditions when the electrode was subjected to cathodic currents, NaCl as electrolyte at a current of 0.1 A for 2 h. The characterization techniques provided results that allow to explain the higher and prolonged efficiency of the cathodic current. Furthermore, these reactor configurations achieved an electrical energy consumption of 2.3 and 2.1 kWh kg −1 of material for cathodic and anodic, respectively. These results indicate that the electrochemical is an economical and environmental suitable technique capable of restore carbon adsorbents to be reusable for several water treatment cycles.

Building efficient biocathodes with Acidithiobacillus ferrooxidans for the high current generation

The development of biocathodes is highly fascinating in microbial electrochemical technologies research. In this study, iron-oxidizing bacterium Acidithiobacillus ferrooxidans -based biocathodes were developed under the constant polarization of the electrochemical reactors at −0.2 V vs. Ag/AgCl with a pH of 2. On the 15 th day of the 21-day batch experiment, A. ferrooxidans -based biocathode produced a maximum current density of −38.61 ± 13.16 A m −2 when the reactors were supplemented with 125 mM Fe 2+ ions as an electron donor and 9 mM citrate as an iron chelator to buffer the iron-rich medium. Oxidation of Fe 2+ to Fe 3+ by A. ferrooxidans and its electrochemical regeneration at the cathode were mainly responsible for the high current generation. Furthermore, in the presence of iron, A. ferrooxidans develop a multi-layer biofilm on the cathode surface, which could potentially perform an indirect electron transfer mechanism. • Highly improved Acidithiobacillus ferrooxidans -based biocathodes were developed. • Biocathodes had shown the highest current density of −38.61 ± 13.16 A m −2 . • Fe 2+ /Fe 3+ redox transformation is mainly responsible for the highest current densities. • A. ferrooxidans develop a multi-layer biofilm in the presence of iron. • A. ferrooxidans could perform both direct and indirect electron transfer mechanisms.

Fabrication of a biocathode for formic acid production upon the immobilization of formate dehydrogenase from Candida boidinii on a nanoporous carbon

The immobilization of the non-metallic enzyme formate dehydrogenase from Candida boidinii (CbFDH) into a nanoporous carbon with appropriate pore structure was explored for the bioelectrochemical conversion of CO2 to formic acid (FA). Higher FA production rates were obtained upon immobilization of CbFDH compared to the performance of the enzyme in solution, despite the lower nominal CbFDH to NADH (β-nicotinamide adenine dinucleotide reduced) cofactor ratio and the lower amount of enzyme immobilized. The co-immobilization of the enzyme and a rhodium complex as mediator in the nanoporous carbon allowed the electrochemical regeneration of the cofactor. Preparative electrosynthesis of FA carried out on biocathodes of relatively large dimensions (ca. 3 cm × 2 cm) confirmed the higher production rate of FA for the immobilized enzyme. Furthermore, the incorporation of a Nafion binder in the biocathodes did not modify the immobilization extent of the CbFDH in the carbon support. Coulombic efficiencies close to 46% were obtained for the electrosynthesis carried out at −0.8 V for the biocathodes prepared using the lowest Nafion binder content and the co-immobilized enzyme and rhodium redox mediator. Although these values may yet be improved, they confirm the feasibility of these biocathodes in larger scales (6 cm2) beyond most common electrode dimensions reported in the literature (ca. a few mm2).

Bulk Electrocatalytic NADH Cofactor Regeneration with Bipolar Electrochemistry.

Electrochemical regeneration of reduced nicotinamide adenine dinucleotide (NADH) is an extremely important challenge for the electroenzymatic synthesis of many valuable chemicals. Although some important progress has been made with modified electrodes concerning the reduction of NAD+ , the scale-up is difficult due to mass transport limitations inherent to large-size electrodes. Here, we propose instead to employ a dispersion of electrocatalytically active modified microparticles in the bulk of a bipolar electrochemical cell. In this way, redox reactions occur simultaneously on all of these individual microelectrodes without the need of a direct electrical connection. The concept is validated by using [Rh(Cp*)(bpy)Cl]+ functionalized surfaces, either of carbon felt as a reference material, or carbon microbeads acting as bipolar objects. In the latter case, enzymatically active 1,4-NADH is electroregenerated at the negatively polarized face of the particles. The efficiency of the system can be fine-tuned by controlling the electric field in the reaction compartment and the number of dispersed microelectrodes. This wireless bioelectrocatalytic approach opens up very interesting perspectives for electroenzymatic synthesis in the bulk phase.

Wastewater-powered high-value chemical synthesis in a hybrid bioelectrochemical system

SummaryA microbial electrochemical system could potentially be applied as a biosynthesis platform by extracting wastewater energy while converting it to value-added chemicals. However, the unfavorable thermodynamics and sluggish kinetics of in vivo whole-cell cathodic catalysis largely limit product diversity and value. Herein, we convert the in vivo cathodic reaction to in vitro enzymatic catalysis and develop a microbe-enzyme hybrid bioelectrochemical system (BES), where microbes release the electricity from wastewater (anode) to power enzymatic catalysis (cathode). Three representative examples for the synthesis of pharmaceutically relevant compounds, including halofunctionalized oleic acid based on a cascade reaction, (4-chlorophenyl)-(pyridin-2-yl)-methanol based on electrochemical cofactor regeneration, and l-3,4-dihydroxyphenylalanine based on electrochemical reduction, were demonstrated. According to the techno-economic analysis, this system could deliver high system profit, opening an avenue to a potentially viable wastewater-to-profit process while shedding scientific light on hybrid BES mechanisms toward a sustainable reuse of wastewater.

Selective Electrochemical Regeneration of Aqueous Amine Solutions to Capture CO2 and to Convert H2S into Hydrogen and Solid Sulfur

Removing CO2 from natural gas or biogas in the presence of H2S is technically challenging and expensive as it often requires separation of both acid gases from the gas, typically using an aqueous amine solution, followed by separation of CO2 from H2S and conversion of H2S into solid S. In this work, the proof of concept of electrochemical, instead of thermal, regeneration of an aqueous amine solution is developed. This invention might be a very promising technology and has several advantages. It has H2S versus CO2 selectivity of 100%, can directly convert H2S into S and H2, and is economically competitive with CO2 desorption energy around 100 kJmol−1 and H2S conversion around 200 kJmol−1. If renewable energy is used for electrochemical regeneration, CO2 emissions due to the CO2 capture process can be significantly reduced.

Electrochemically driven efficient enzymatic conversion of CO2 to formic acid with artificial cofactors

Enzymatic reduction of CO2 to formic acid with the enzyme formate dehydrogenase (FDH) and a cofactor is a promising method for CO2 conversion and utilization. However, the natural cofactor nicotinamide adenine dinucleotide (NADH) shows some drawbacks such as a low reduction efficiency and forms isomers or dimers (1,6 - NADH or NAD dimer) in the regeneration reaction. To overcome them and to improve the production of formic acid, in this work, the artificial cofactors, i.e., the bipyridinium-based salts of methyl viologen (MV2+), 1,1’-dicarboxymethyl-4,4’-bipyridinium bromine (DC2+), and 1,1’-diaminoethyl-4,4’-bipyridinium bromine (DA2+), were used to replace NADH, and the effect of different functional groups on the electrochemical regeneration and catalytic performance in the enzymatic reaction was studied systematically. Also, studies using the natural cofactor NADH were carried out for comparison. It was found that the cofactor with amino groups showed the highest catalytic efficiency (kcat/Km) of 0.161 mM-1min-1, which is 536 times higher than that of the natural cofactor NADH. Molecular Dynamics simulations were conducted to give further molecular insight into the behavior of the cofactors. Analyzing the free energy profiles of the complexes between CO2 in the FDH active site with different artificial cofactors indicated that the artificial cofactor with the amino groups had the highest affinity for CO2, being consistent with the experimental observations.

Electrochemically-driven regeneration of iron (II) enhances Fenton abatement of pesticide cartap

Cartap is a carbamate insecticide intended to protect crops such as rice, tea, and sugarcane. Cartap in the environment presents a serious threat to non-target organisms through direct exposure or via biomagnification. Electro-assisted Fenton technology taps the potential of Fenton reagents to degrade cartap. Electrochemical reduction of iron accelerates catalyst regeneration. Cartap degradation was first investigated by varying reaction pH, as well as the initial H2O2 and Fe2+ dosage, followed by optimization studies using central composite design. Parametric results indicate the highest cartap removal of 98.10% was achieved at 1.6 pH, 3.0 mM Fe2+, and 40 mM H2O2 at I = 1.0 A and t = 30 min. These results notoriously surpass conventional Fenton that only achieved 53.8% cartap removal under similar conditions. The hybridization of Fenton process through electrochemical regeneration enhances removal and increases degradation kinetic up to a pseudo-first-order rate constant value of 21.30 × 10–4 s−1. Effects of coexisting inorganic salts PO43–, NO3–, and Cl– at 1 mM and 10 mM concentrations were investigated. These results demonstrate that Fenton electrification as process intensification alternative can enhance the performance and competitiveness of conventional Fenton by ensuring higher availability of iron catalyst while minimizing sludge production.

Selective Adsorption of Potassium in Seawater by CoHCF Thin Film Electrode and Its Electrochemical Desorption/Regeneration.

Cobalt Hexacyanoferrate (CoHCF) was tested for the selective uptake of K from seawater and the electrochemical method was adopted for the desorption and regeneration of the material. Powder form CoHCF could adsorb about 6.5 mmol/g of K from the seawater. For the ease of the electrochemical desorption and regeneration, CoHCF thin film was coated onto the Indium Tin Oxide (ITO) glass to obtain a CoHCF electrode. K adsorption kinetics on CoHCF thin film was found to be well fitted with the intraparticle diffusion model, which was a two-step process. Five consecutive adsorption-desorption-regeneration cycles were carried out to know the gradual decrease in the adsorption capacity owing to changes in the redox states of two metals, Co and Fe, in the material. Fourier Transform Infrared Spectroscopy (FT-IR) and Ultraviolet-Visible (UV-Vis) measurement results corresponded to the color change of CoHCF thin film, indicating the valence change of transition metals and the exchange of alkali metal cations happened on the CoHCF at different operation stages. In order to elucidate the reaction mechanism, composition of the material was analysis in the following steps: adsorption, desorption, and regeneration. It was proved that the system based on CoHCF thin film modified electrode had the potential of recovering potassium from seawater.

Cr(VI) reduction coupled with Cr(III) adsorption/ precipitation for Cr(VI) removal at near neutral pHs by polyaniline nanowires-coated polypropylene filters

BackgroundLimited work was done about remediation of Cr(VI))-contaminated water at near neutral pHs (6-8), such as drinking water resources. MethodsPolyaniline nanowires-coated polypropylene filter (PANI-PP filter) was fabricated for easy recollection and reuse of spent adsorbents. The physicochemical properties of PANI nanowires were characterized by SEM, FTIR, BET, and XPS. The effect of solution pH, contact time, and Cr(VI) concentration on Cr(VI) reduction and total Cr removal are investigated with batch and column adsorption experiments. FindingsThe PP filter provided micro-sized fiber for PANI deposition, and the deposited PANI nanowires exhibited higher surface area and Cr(VI) removal ability than the PANI powders formed in synthetic solution. At near neutral pHs, Cr(VI) removal by PANI-PP filter was attributed to the Cr(VI) adsorption by the amine groups, Cr(VI) reduction to Cr(III), and subsequent Cr(III) adsorption/precipitation on the PANI nanowires, and the maximum removal capacity of Cr(VI) and total Cr were in the range of 55.7 to 113.8 mg/g PANI. The Cr(VI) reduction was accompanied with the oxidation of amine groups to imine groups of PANI. The electro-chemical regeneration promoted the reduction of imine groups back to amine groups, and after 10 regeneration cycles, around 105% Cr(VI) removal efficiency was still achieved.

Bipolar Membranes with Systematically Varied Interfacial Areas in the Junction Region

Bipolar membranes (BPMs) have been historically deployed in electrodialysis for pH adjustment of process streams and the production of acids and bases. More recently, it has seen increasing popularity for fuel cell and electrolysis technologies. Under reverse bias, BPMs allow for disparate pH control in electrochemical cells and regeneration of electrolyte. The ability to maintain disparate pH in the cell expands the palette of electrocatalysts for a given cell configuration. Despite the enticing potential BPM electrolyte separators offer, they often cannot facilitate large current density values normally attained with single-ion conducting membranes (i.e., CEM or AEMs) in fuel cells and electrolyzers. This challenge has motivated three general strategies to improve the performance of BPMs: i.) judicious selection of the water-dissociation catalyst in the junction region; ii.) facilitating improved water transport to the junction region; and iii.) fabricating high surface area interfaces between the AEM and CEM in the junction region.This talk presents soft-lithography as a method to generate micropatterned membrane surfaces with a defined interfacial area that is retained during BPM fabrication. The approach is conducive for a variety of commercially available CEM and AEM chemistries (e.g., Nafion, perfluorinated AEM, Orion poly(arylene) AEM, and SPEEK). These chemistries are stable at elevated temperatures, extreme pHs, and in the presence of strong oxidizers. Our work demonstrates a 250 mV in the on-set potential for water-splitting while also increasing the current density by 15% when increasing the interfacial area by 2.28x. The improved water-splitting observations are primarily explained by calculating the electric field in the junction region using the Poisson equation. The marginal increase in current density was attributed to the BPM being under mixed reaction-diffusion control. Finally, we demonstrate a block copolymer lithography approach for generating nanopatterned surfaces on ion-exchange membranes. Taking advantage of the tunability of block copolymer lithography, the feature sizes and geometry of the nanostructure at the interface can be systematically controlled.This work was supported from NSF Award # 1703307. Figure 1

A hybrid electrochemical flow reactor to couple H2 oxidation to NADH regeneration for biochemical reactions

AbstractBeta‐nicotinamide adenine dinucleotide (NAD+/NADH) is an important enzymatic co‐factor that can be efficiently regenerated using a rhodium‐based catalyst as electron transfer mediator (ie, [Cp*Rh(bpy)Cl]+, where Cp* = pentamethylcyclopentadienyl and bpy = 2,2‐bipyridine). Here, the above mediated regeneration of NADH is implemented in a redox flow bioreactor hybridized with a gas diffusion electrode for hydrogen oxidation. The reactor was initially optimized with respect to rhodium complex and NAD+ concentrations, humidification of the hydrogen gas, flow rates of both H2 gas and electrolytic solution, and solution pH. The integration of an enzymatic reaction consuming the generated NADH was then investigated in a flow process, combining in series the electrochemical reactor to a biochemical cell with immobilized l‐lactic dehydrogenase for the conversion of pyruvate to lactate. A high activity was achieved with a turnover number up to 370 h−1 for NADH regeneration. Coupled electrochemical regeneration to enzymatic reaction led to total turnover number values of 2000 and 6.3 × 106 for NADH electrochemical regeneration and bioconversion, respectively.

Green quaternary ammonium nitrogen functionalized mesoporous biochar for sustainable electro-adsorption of perchlorate

In this work, functional mesoporous bean dregs-based biochar electrodes were prepared by electrophoretic deposition for perchlorate (ClO4-) electro-sorption. We characterized the textural and electrochemical properties of the biochar and determined the kinetics of electro-adsorption. The results show that 15% concentration of ZnO nanoparticle modification is recommended to increase the mesoporous volume, specific surface area, hydrophilicity and conductivity of biochar materials. After betaine functionalization, the obtained biochar (BK-Z15%-N) possesses an excellent electrochemical performance with high current density, charging capacity, and low ohmic impedance. As expected, it presents an electro-adsorption capacity of 28.31 mg g-1 toward 20 mg L-1 ClO4- at 1 V, which is four times higher than that without voltage and much better than other electrodes in this study and most reported data. The electro-adsorption of ClO4- on BK-Z15%-N electrode fits the fractal model, which is a complex process controlled by the electrical force and independent of solution pH. BrO3- and Cl- are the most competitive anion to ClO4- electro-sorption. Notably, it maintains a sustainable electrochemical regeneration ability due to the reversible redox activity of quaternary amine groups. Therefore, this study offers a remarkable and reliable method for improving the electro-sorption performance of perchlorate.

Chemical-Electrochemical Process Concept for Lead Recovery from Waste Cathode Ray Tube Glass.

This paper presents a novel approach for the recovery of lead from waste cathode-ray tube (CRT) glass by applying a combined chemical-electrochemical process which allows the simultaneous recovery of Pb from waste CRT glass and electrochemical regeneration of the leaching agent. The optimal operating conditions were identified based on the influence of leaching agent concentration, recirculation flow rate and current density on the main technical performance indicators. The experimental results demonstrate that the process is the most efficient at 0.6 M acetic acid concentration, flow rate of 45 mL/min and current density of 4 mA/cm2. The mass balance data corresponding to the recycling of 10 kg/h waste CRT glass in the identified optimal operating conditions was used for the environmental assessment of the process. The General Effect Indices (GEIs), obtained through the Biwer Heinzle method for the input and output streams of the process, indicate that the developed recovery process not only achieve a complete recovery of lead but it is eco-friendly as well.

Hazardous wastewater treatment by low‐cost sorbent with in situ regeneration using hybrid solar energy‐electrochemical system

Hazardous industrial wastes negatively impact the environment by creating issues for aquatic as well as human's life. This study investigates the treatment of hazardous industrial wastewater using cost-effective graphite adsorbent along with electrochemical regeneration integrated with renewable solar energy. The synthetic industrial effluent containing crystal violet dye was treated using an adsorbent (Nyex™ 1000) having a surface area of 1.0m2 g-1 . The efficiency of removing solute was found to be more than 90%. The adsorbent regeneration efficiency was achieved at 99.5% by passing a charge of 100Cg-1 at current density of 10mAcm-2 for 1h. Solar energy was integrated with electrochemical reactor for the regeneration of adsorbent to make the system cost-effective and self-sustainable. PRACTITIONER POINTS: Industrial hazardous wastewater treatment with a cost-effective graphite integrated adsorbent. Development of renewable solar energy-integrated with electrochemical system for regeneration. Regeneration efficiency of adsorbent Nyex™ 1000 was achieved around 99.5% with integrated system. Sustainable system was introduced to incorporate with renewable energy for waste water treatment.

Electrochemical regeneration of hexavalent chromium from aqueous solutions in a gas sparged parallel plate reactor

ABSTRACT In this study anodic oxidation of Cr2(SO4)3 was carried out in an air-sparged divided parallel plate cell. Variables studied were current density, Cr2(SO4)3 concentration, and superficial air velocity. The rate constant of Cr2(SO4)3 oxidation was found to increase with increasing current density and Cr2(SO4)3 concentration. The effect of air sparging was found to depend on Cr2(SO4)3 concentrations, at high Cr2(SO4)3 concentration (> 0.1 M) air sparging does not affect the rate constant of the reaction denoting that the reaction is charge transfer controlled. As Cr2(SO4)3 concentration decreases below 0.1 M the reaction becomes under mixed diffusion and chemical control and the rate constant increases with increasing air superficial velocity, the lower Cr2(SO4)3 concentration the higher the contribution of diffusion to the reaction rate. The current efficiency of the process ranged from 20 to 85% depending on current density and Cr2(SO4)3 concentration. Electrical energy consumption which ranged from 1.8 to 14.4 kW h/kg of Cr6+ was found to increase with increasing current density and decreases with increasing Cr2(SO4)3 concentration. Air sparging was found to decrease electrical energy consumption in the case of dilute solutions << 0.1 M Cr2(SO4)3.