The cooperative electrochemical oxidation of chlorophenols in anode-cathode compartments

By using a self-made carbon/polytetrafluoroethylene (C/PTFE) O2-fed as the cathode and Ti/IrO2/RuO2 as the anode, the degradation of three organic compounds (phenol, 4-chlorophenol, and 2,4-dichlorophenol) was investigated in the diaphragm (with terylene as diaphragm material) electrolysis device by electrochemical oxidation process. The result indicated that the concentration of hydrogen peroxide (H2O2) was 8.3 mg/L, and hydroxyl radical (HO) was determined in the cathodic compartment by electron spin resonance spectrum (ESR). The removal efficiency for organic compounds reached about 90% after 120 min, conforming to the sequence of phenol, 4-chlorophenol, and 2,4-dichlorophenol. And the dechlorination degree of 4-chlorophenol exceeded 90% after 80 min. For H2O2, HO existed in the catholyte and reduction dechlorination at the cathode, the mineralization of organics in the cathodic compartment was better than that in the anodic compartment. The degradation of organics was supposed to be cooperative oxidation by direct or indirect electrochemical oxidation at the anode and H2O2, HO produced by oxygen reduction at the cathode. High-performance liquid chromatography (HPLC) allowed identifying phenol as the dechlorination product of 4-chlorophenol in the cathodic compartment, and hydroquinone, 4-chlorocatechol, benzoquinone, maleic, fumaric, oxalic, and formic acids as the main oxidation intermediates in the cathodic and anodic compartments. A reaction scheme involving all these intermediates was proposed.     

Removal of color from real dyeing wastewater by Electro-Fenton technology using a three-dimensional graphite cathode

This work investigates the removal of color from wastewater that contains low dyestuff concentrations by the Electro-Fenton process. The color was removed by in situ electrogenerated hydrogen peroxides at a three-dimensional graphite cathode with added ferrous sulfates. Experimental runs were conducted to evaluate the effect of the operating parameters, such as the oxygen contact mode, the oxygen sparging rate, the applied current density, the concentration of ferrous ions, the solution temperature and the pH among others, on the removal of color. The removal efficiency of the color in the cathodic chamber reached 70.6% under specified operation conditions in 150 min. The removal efficiency was controlled by the mass transfer when the oxygen-sparging rate was less than 0.3 dm(3)/min for the reactor configuration herein. The optimal applied current density was 68 A/m(2) when the energy consumption was considered. The highest removal efficiency was obtained by adding 20 mM Fe(II) to the solution. The optimal solution pH was 3 in this work. The temperature negatively affected color removal.     

Development of a novel electro-dialysis based technique for lead removal from silty clay polluted soil

In this study, a novel electro-dialysis based technique has been developed and used to treat a silty clay soil polluted by lead. The effect of chemical reagents, i.e. tap water at pH 4 (reagent 1) and sodium acetate at pH 5 (reagent 2), on enhancing electro-dialysis extraction of lead from the tested soil was examined. Specimens were prepared by mixing soil with 1000 ppm of lead and were compacted in the dialysate at predetermined dry density and moisture content. Then, specimens were subjected to a predetermined level of current. In the dialysate compartment, anions and cations were removed by charge transport in opposite directions to the anodic and cathodic cells. Meanwhile, in the anodic and cathodic cells, ion concentrations were increased. Thus, concentrated electrolyte streams were produced in alternating cells and cleaned soils were obtained in the dialysate. Both soil pH and lead concentrations were uniformly distributed within the compacted soil specimen during testing. Total lead removal of 80 and 92% was obtained for reagents 1 and 2, respectively. The high removal efficiency was attributed to the separation of electrode reactions from the soil and inclusion of ion selective membranes (ISM), which restrict the movement of counter charged species.     

Electrochemical reactions oh iron silicide surfaces in sulphuric acid

The electrochemical behaviour of pure FeSi₂ and FeSi electrodes has been examined in sulphuric acid solutions over a wide range of cathodic and anodic potentials. The potentiodynamic profiles exhibit a structureless hysteresis and thence shed no light on the possible pseudo-faradaic processes that might be involved.

Potentiostatic steady-state curves for the hydrogen evolution reaction (h.e.r.) show that the h.e.r. can be carried out at these electrodes at very high rates without affecting their electrochemical stability; for the sake of comparisons, the electroactivity of these electrodes is much less than that of smooth platinum but much higher than that of the commercially-available Ebonex electrodes made of ceramic titanium dioxide.

On the anodic side, oxide growth, rather than the evolution of oxygen, is the favoured process. Also, little activity is detected, at low electrode potentials (<3 V), for the anodic oxidation of HCOC⁻, CH₃COO⁻, CH₃OH, NH₂−NH₂ and Cl⁻. When the electrochemical reactions are forced at high rates through a galvanostatic arrangement, oxide growth to very high electrode potentials and dielectric breakdown are observed in all solutions except those containing chloride ions; in the latter case anodic evolution of chlorine is observed.     

Multi-walled carbon nanotube-based glucose/O2 biofuel cell with glucose oxidase and laccase as biocatalysts

This study describes a multi-walled carbon nanotube-based glucose/O2 biofuel cell with glucose oxidase and laccase as the anodic and cathodic biocatalysts, respectively. Upon being functionalized with L-a-phosphatidylcholine, one kind of lipid, multi-walled carbon nanotubes can serve as a support for glucose oxidase to form a three-dimensional, conducting and uniform bioanode that possesses a good bioelectrocatalytic activity toward the oxidation of glucose biofuel with solution-phased ferrocene monocarboxylic acid as the mediator to shuttle the electron transfer between glucose oxidase and multi-walled carbon nanotubes. In a similar way, the lipid-functionalized multiwalled carbon nanotubes can also be used to support the cathodic biocatalyst, i.e., laccase, and, more remarkably, to facilitate the direct electron transfer of laccase. As a result, the prepared biocathode is very active toward the reduction of oxygen without using any electron-transfer mediators. The biofuel cell has a 0.45 V open circuit potential and 34 microA/cm2 short circuit current density in phosphate buffer (pH 6.0) separated with Nafion-117 membrane with anodic compartment containing 15 mM glucose and 2 mM ferrocene monocarboxylic acid and with cathodic compartment being saturated with O2 at room temperature. A maximum power density of 3.2 microW/cm2 is obtained with ca. 0.2 V potential output.     

The effects of cathodic and anodic voltages on the characteristics of porous nanocrystalline titania coatings fabricated by microarc oxidation

Porous nanocrystalline titania films were prepared by microarc oxidation (MAO) of a titanium alloy keeping anodic voltage at 230–410 V and cathodic voltage at 20–45 V in a Na₂CO₃ and Na₂SiO₃ electrolytic solutions using an asymmetric pulse alternating current power supply. XRD, EDS and SEM were employed to characterize the phase, composition and microstructure of the films. It is found that the films consist of dominant rutile and little anatase phases. The phase, pore size and thickness of the coatings strongly depend on the applied voltage, consistent with the previous reports, and the cathodic voltage has an intense effect on the films' pitting corrosion performance in sodium chloride solution. The films prepared by keeping the anodic and cathodic voltages at 320 and 45 V, respectively, for 30 min were porous, with 200 nm to 4 μm pores and the pore walls composed of 100–300 nm rutile crystallites.     

A new method to control electrolytes pH by circulation system in electrokinetic soil remediation

To simultaneously avoid a decrease of electro-osmotic flow by hydrogen ions and to increase heavy metal precipitation due to hydroxide ions, simulated electrokinetic remediation was conducted in saturated kaolinite specimens loaded with lead(II) using an electrolyte circulation method to control electrolyte pH. At an electrolyte circulation rate of 1.1 ml/min, it was possible to increase the anolyte pH from 2 to 4 and decrease the catholyte pH from 12 to 8. Using electrolyte circulation, it was observed that the rate of decrease of clay pH due to the change of electrolyte pH was reduced. As a result, the operable period was extended and the removal efficiency for lead(II) was also increased. It was observed that most of the effluent lead(II) from the cathode compartment was electroplated onto the cathode and that residual effluent lead(II) did not precipitate onto, or adsorb to, the clay at the anode compartment during circulation. Therefore, there was no need to treat the electrolyte because there was virtually no effluent from the cathode compartment in the circulation system. It was also found that the electrolyte volume required to sustain the electrolytic reaction was sufficient for the whole electrokinetic remediation process.     

Anodic destruction of 4-chlorophenol solution

The electrochemical oxidation of 4-chlorophenol solutions was studied using a dimensional stable anode (DSA), made of pure titanium sheet mesh coated with Ti/TiO(2) and RuO(2) film. An electrochemical cell with one working electrode and two counter-electrodes was designed. A gas collecting system to collect the electrolysis gaseous products was also designed. The influence of current density (6.51-21.58 mA/cm(2)), pH (2.0-12.6) and initial 4-chlorophenol concentration (25-100 mg/l) on the destruction was investigated. Complete elimination was successfully achieved within 2 h for most investigated conditions. Highest rates of elimination were achieved at a pH of 12.6.A new approach to calculate the current efficiency (CE) of the cell was proposed. The volumes of the gases produced at the anode and at the cathode were the basis for the new CE calculations. It was observed that the worst CE was approximately 20% and the best CE was approximately 89%. The most efficient pH was at 12.6 and the most efficient current density was at 11.39 mA/cm(2).     

Oxalic acid mineralization by electrochemical oxidation processes

In this study, two electrochemical oxidation processes were utilized to mineralize oxalic acid which was a major intermediate compound in the oxidation of phenols and other aromatic compounds. The anode rod and cathode net were made of a titanium coated with RuO(2)/IrO(2) (Ti-DSA) and stainless steel (S.S. net, SUS304), respectively. First, the Fered-Fenton process, which used H(2)O(2) and Fe(2+) as additive reagents, achieved 85% of TOC removal. It proceeded with ligand-to-metal charge-transfer (LMCT), which was evidenced by the accumulation of metallic foil on the selected cathode. However, in the absence of H(2)O(2)/Fe(2+), it showed a higher TOC removal efficiency while using Cl(-) only as an additive reagent due to the formation of hypochlorite on the anode. It was also found that the mineralization of oxalic acid by electrolysis generated hypochlorite better than the dosage of commercial hypochlorite without electricity. Also, pH value was a major factor that affected the mineralization efficiency of the oxalic acid due to the chlorine chemistry. 99% TOC removal could be obtained by Cl(-) electrolysis in an acidic environment.     

pH effect on the potentiodynamic behaviour of titanium electrodes

Potentiodynamic measurements on mechanically polished titanium electrodes were performed in different solutions in the pH range 0.55–12. Two anodic peaks and one small cathodic peak were observed. Comparison with thermodynamic data revealed the possibility of formation of Ti₂O₃ or TiO₂ at potentials corresponding to the first anodic peak probably directly from titanium hydride. The second anodic peak is interpreted as the oxidation of incorporated H to form H⁺ ions which partly remain inside the metal. The small cathodic peak represents the back reduction of the incorporated H⁺ to the metal hydride.     

pH- and concentration-programmable electrodialytic buffer generator

We have presented in a companion paper a suppressor-based electrodialytic buffer generator (EBG) that can produce programmable pH gradients. Here we demonstrate a three-electrode EBG. In this three-compartment flow-through device, the central compartment is separated from the outer compartments with a cation-exchange membrane (CEM) and an anion-exchange membrane (AEM), respectively. One platinum electrode is disposed in each compartment. The flows through each compartment are independent. With appropriate solutions in each compartment, independent potentials are applied to the CEM and AEM electrodes with respect to the grounded central electrode. The CEM current and the AEM current can be independently manipulated to generate buffers with variable concentration and pH in the central compartment. Both the CEM and AEM currents can be positive or negative. For the CEM, a positive current (i(cat)(in)) indicates that cations are coming in from the CEM channel to the center. A negative current (i(cat)(out)) takes cations out of the center to the CEM channel. Similarly for the AEM, currents governing anion transport into the center channel from the AEM channel (AEM electrode negative) or the reverse (AEM electrode positive) are respectively denoted by i(an)(in) or i(an)(out). Most examples herein involve inward ion transport, referred to as the additive mode. Depending on whether i(cat)(in) i(an)(in), H(+)/O(2) and OH(-)/H(2) are respectively produced at the central electrode to maintain electroneutrality. Any gas formed is subsequently removed by a gas removal device. The pH of the central channel effluent is related to the ratio of the currents through the two membranes, while the generated concentration is controlled by the absolute value of the currents. The buffer concentration and pH can be varied in a controlled predictable manner. A pH span of 3-12 was attained and a phosphate buffer concentration up to 140 mM was generated. We demonstrate a variety of combined pH/concentration gradients from a mixture of ethylenediamine, citrate, and phosphate by manipulating i(cat)(in), which controls introduction of the ethylenediammonium ion, and i(an)(in), which controls the introduction of citrate and phosphate ions. We also demonstrate an additive-subtractive mode of operation where both inward and outward currents are used to add one type of ion while removing another type of ion to reproducibly generate pH/concentration gradients.     

An approach to the effect of hydrogen loading on the anodic behaviour of Ti in NaOH solutions

The voltammetric behaviour of titanium electrode in 0.5 M NaOH solution was investigated. The I-E curves revealed the presence of a well-defined O₂ evolution peak. The peak current density decreases to a great extent by pre-loading the metal with hydrogen by cathodic polarization, indicating that pre-loading the metal with hydrogen suppresses the O₂ evolution. The oxidation of hydrogen loaded inside the metal was suggested to be responsible for reducing the O₂ evolution peak current density during Ti anodization. The charge consumed during the anodic polarization of Ti, below O₂ evolution potential, was used to estimate the anodization coefficient and capacitance measurement was conducted for measuring the dielectric constant of the oxide film. Pre-loading the metal with hydrogen was found to decrease the oxide film formation rate as well as the dielectric constant.     

Complete degradation of Orange G by electrolysis in sub-critical water

Complete degradation of azo dye Orange G was studied using a 500 mL continuous flow reactor made of SUS 316 stainless steel. In this system, a titanium reactor wall acted as a cathode and a titanium plate-type electrode was used as an anode in a subcritical reaction medium. This hydrothermal electrolysis process provides an environmentally friendly route that does not use any organic solvents or catalysts to remove organic pollutants from wastewater. Reactions were carried out from 30 to 90 min residence times at a pressure of 7 MPa, and at different temperatures of 180-250°C by applying various direct currents ranging from 0.5 to 1A. Removal of dye from the product solution and conversion of TOC increased with increasing current value. Moreover, the effect of salt addition on degradation of Orange G and TOC conversion was investigated, because in real textile wastewater, many salts are also included together with dye. Addition of Na(2)CO(3) resulted in a massive degradation of the dye itself and complete mineralization of TOC, while NaCl and Na(2)SO(4) obstructed the removal of Orange G. Greater than 99% of Orange G was successfully removed from the product solution with a 98% TOC conversion.     

Studies on process parameters for chlorine dioxide production using IrO2 anode in an un-divided electrochemical cell

Chlorine dioxide is potentially a powerful oxidant with environmentally compatible application in several strategic areas relating to pollution control typically for water disinfection, and its sustained production is a key factor for its successful application. Although increased attention has been paid for on-line chlorine dioxide generation by several chemical and electrochemical methods, the details are mostly confined as patents. We studied in this work the electrochemical generation of chlorine dioxide from an un-buffered solution of sodium chlorite and sodium chloride mixture in an un-divided electrochemical cell under constant current mode, with a view to optimize various process parameters, which have a direct bearing on the chlorine dioxide formation efficiency under laboratory conditions. The effect of feed flow rate (10-150 ml min(-1)), feed solution pH (2.3-5.0), concentration of sodium chloride (0-169.4mM), concentration of sodium chlorite (0-7.7 mM), and the applied current (100-1200 mA) on the formation of dissolved ClO(2) gas in solution and the pH of the product-containing solution was investigated by performing single pass experiments, with no circulation, in a cell set-up with Ti/IrO(2) anode and Ti/Pt cathode. The current efficiency and the power consumption were calculated for the optimized conditions.     

Nanostructured layer formed on CP-Ti by plasma electrolysis (effect of voltage and duty cycle of cathodic/anodic direction)

The present work reports the usage of nanocrystalline plasma electrolytic saturation (PES) by applying pulsed current in an organic electrolyte based on Glycerol. Response surface methodology was applied to optimize the operating conditions for small nanocrystallite sizes of coatings. The levels studied were peak of applied cathodic voltage range between 500 and 700 V, peak of applied anodic voltage between 200 and 400 V and the ratio of duty cycle of cathodic direction to duty cycle of anodic direction at 0.25–0.35. The usage of high applied cathodic voltages and low anodic voltages and also low ratio of duty cycle of cathodic direction to duty cycle of anodic direction is more suitable for achieving lower sizes of complex nanocrystallites. The samples with high height to width ratio of distribution curves of nanocrystallites have simultaneously, smaller average sizes and lower length to diameter ratio of nanocrystallites.     

Microfluidic protein preconcentrator using a microchannel-integrated nafion strip: experiment and modeling

We propose a simple microfluidic device for protein preconcentration based on the electrokinetic trapping principle. It comprises a narrow Nafion strip that is simply cut from a commercial membrane and is integrated into a molded poly(dimethylsiloxane) (PDMS) microfluidic structure using a guiding channel. Mechanically clamping the PDMS/Nafion assembly with a glass substrate results in a rapid prototypable, leak-tight, and easily disposable device. Our device preconcentrates negatively charged fluorescent proteins located at the anodic microfluidic compartment side of the Nafion strip within a few minutes and up to a concentration factor of 10(4). Moreover, we present a numerical study of the preconcentration effect by solving the coupled Poisson, Nernst-Planck, and Navier-Stokes equations for our type of device, which provides microscopic insight into the mechanism of preconcentration. The electrical field across the ion-permselective Nafion generates concentration polarization, i.e., ion depletion at the anodic side and ion enrichment at the cathodic side for both types of ions, with a local excess of mobile positive ions in the depleted concentration polarization zone, inducing a nonequilibrium electrical double layer in close proximity to the Nafion membrane. A voltage difference applied over the anodic compartment is used to generate the electrophoretic flow velocity of the negatively charged tracer biomolecules. This, in combination with the electroosmotic flow in the opposite direction, which originates from the fixed charges on the channel walls and the induced space charge near the membrane, provides the basis for the local preconcentration of the negative tracer biomolecules.     

Investigation of the effect of different electrodes and their connections on the removal efficiency of 4-nitrophenol from aqueous solution by electrocoagulation

This study investigates the influence of variables on the removal efficiency of solution containing 4-NP (4-nitrophenol) by D. C. electrocoagulation (EC). The efficiency of different electrode connections and materials (steel 310, Fe, Al, graphite and steel 304) for 4-NP removal is compared. Current density, time of electrolysis, interelectrode distance, supporting electrolyte concentration and stirring rate of the solution were the variables that mostly influenced the 4-NP removal. Initially, a simple electrochemical cell was prepared with an anode and a cathode. Then the effect of each variable was studied separately using aqueous 4-NP in a batch mode. For a solution of 20 mg/L 4-NP+300 mg/L NaCl with chemical oxygen demand (COD) of approximately 40 mg O2/L, almost up to 99% 4-NP and 65% COD were removed, when the pH was about 9, time of electrolysis was approximately 10 min, current density was 100 A m(-2), interelctrode distance was 15 mm and stirring rate was 400 rpm. In the second series of experiments, the efficiency of EC cells with monopolar electrodes in series and parallel connections and an EC cell with bipolar electrodes was compared with that of a simple electrochemical cell. The best results obtained when steel 310 and Fe are used as anodes and employing Al and graphite as anodes would not be satisfactory. Also findings show that the types of sacrificial electrodes are not very significant in the removal of 4-NP. In the real wastewater obtained from Tabriz petrochemical plant 52% removal could be achieved after 10 min with using steel 310 as anode and steel 304 as cathode.     

An electrodeposition method of calcium phosphate coatings on titanium alloy

Calcium phosphates coatings were deposited onto titanium alloy discs via en electrodeposition method. Titanium alloy discs were blasted with calcium phosphate particles, then etched in a mixture of nitric and fluoric acids and rinsed in demineralized water. The titanium alloy disc (cathode) and platinum mesh (anode) were immersed in a supersaturated calcium phosphate electrolyte buffered at pH 7.4 and connected to a current generator. The microstructure, chemical composition and crystallinity of the electrodeposited coatings were studied as function of time 10–120 min, temperature 25–80°C, current density 8–120 mA/cm², magnesium and hydrogen carbonate amounts (0.1–1 mM). Uniform calcium phosphate coatings were obtained in 30 min but coating thickness increased with deposition time. Raising the temperature of electrolyte resulted in more uniform coatings as ionic mobility increased. Low current density was preferable due to hydrogen gas evolving at the cathode, which disturbed the deposition of calcium phosphate crystals on titanium. The amounts of magnesium and hydrogen carbonate ions affected both the homogeneity and morphology of the coatings. This study showed that the electrodeposition method is efficient for coating titanium with osteoconductive calcium phosphate layers.     

Decolorization of dye solution containing Acid Red 14 by electrocoagulation with a comparative investigation of different electrode connections

This study was performed to investigate the variables that influence the efficiency of decolorization of a solution containing an azo dye (Acid Red 14) by electrocoagulation (EC) in order to compare the efficiency of different electrode connections for color removal. Current density, time of electrolysis, interelectrode distance, and pH of the solution were the variables that most influenced color removal. Initially, a simple electrochemical cell was prepared with an anode and a cathode. Then the effect of each variable was studied separately using synthetic wastewater in a batch mode. The efficiency of the method tested was determined by measurement of color removal and reduction of Chemical Oxygen Demand (COD). For dye solutions with COD of approximately 30 ppm and dye concentrations less than 150 ppm, high color removal (93%) was obtained when the pH ranged from 6 to 9, time of electrolysis was approximately 4 min, current density was approximately 80 A/m2, the temperature was approximately 300 K, and interelectrode distance was 1 cm. During the EC process under these conditions, the COD decreased by more than 85%. In the second series of experiment, the efficiency of EC cells with monopolar electrodes in series and parallel connections and an EC cell with bipolar electrodes were compared with results using a simple electrochemical cell. The experimental results showed that an EC cell with several electrodes was more effective than a simple electrochemical cell in color removal. The results also showed that an EC cell with monopolar electrodes had a higher color removal efficiency than an EC cell with bipolar electrodes. Furthermore, within an EC cell, the series connection of the monopolar electrodes was more effective for the treatment process than the parallel connection in color removal.     

Nafion-coating of the electrodes improves the flow-stability of the Ag/SiO2/Ag2O electroosmotic pump

When a current or a voltage is applied across the ceramic membrane of the nongassing Ag/Ag(2)O-SiO(2)-Ag/Ag(2)O pump, protons produced in the anodic reaction 2Ag(s) + H(2)O → Ag(2)O(s) + 2H(+) + 2e(-) are driven to the cathode, where they are consumed by the reaction Ag(2)O(s) + H(2)O + 2e(-) → 2Ag(s) + 2 OH(-). The flow of water is induced by momentum transfer from the electric field-driven proton-sheet at the surface of the ceramic membrane. About 10(4) water molecules flowed per reacted electron. Because dissolved ions decrease the field at the membrane surface, the flow decreases upon increasing the ionic strength. For this reason Ag(+) ions introduced through the anodic reaction and by dissolution of Ag(2)O decrease the flow. Their accumulation is reduced by applying Nafion-films to the electrodes. The 20 μL min(-1) flow rate of 6 mm i.d. pumps with Nafion coated electrodes operate daily for 5 min at 1 V for 1 month, for 70 h when the pump is pulsed for 30 s every 30 min, and for 2 h when operating continuously.