Cerium(IV)-mediated electrochemical oxidation process for destruction of organic pollutants in a batch and a continuous flow reactor

The mediated electrochemical oxidation (MEO) process with Ce(IV) and nitric acid as the oxidizing medium was employed for the destruction of various model organic pollutants in batch and continuous organic feeding modes. A near complete destruction was observed for all the model organic pollutants studied. The effects of organic concentration, temperature, concentration of Ce(IV), concentration of nitric acid and feeding time on the organic destruction efficiency were investigated. Under the experimental conditions of 80 °C and 0.95 M Ce(IV) in 3 M nitric acid, nearly 90% destruction was achieved based on CO₂ production and 95% based on TOC and COD nearly for all the organic compounds studied in batch organic addition. In the case of continuous organic addition with in situ electroregeneration of Ce(IV) by the electrochemical cell a good destruction efficiency was obtained. For long term organic feeding (120 min) the destruction efficiency was found to be 85% based on CO₂ evolution and 98–99% based on TOC and COD analyses. A model was proposed for calculating the CO₂ formation constant during the continuous process of organic addition. The model predicted a steady state CO₂ evolution pattern for the destruction process during continuous organic feeding. The experimental results obtained confirmed the predicted trends for the destruction process. The changes in enthalpy, entropy, activation energy and free energy for EDTA degradation were found to be 26.7 kJ/mol, −230 J/(mol·K), 29.7 kJ/mol, and 118 kJ/mol respectively.     

Silver ion catalyzed cerium(IV) mediated electrochemical oxidation of phenol in nitric acid medium

Mediated electrochemical oxidation (MEO) is one of the sustainable processes for organic pollutant destruction and has been employed for organic mineralization reactions by many researchers. In the MEO a metal ion capable of exhibiting redox behavior is oxidized from lower oxidation state to higher oxidation state by an electrochemical cell and subsequently used as an oxidant for mineralizing the toxic organics into CO₂ and water. The net result is the consumption of electrical energy for organic mineralization. Therefore, the current efficiency is an important factor and maximizing the current efficiency is one of the ways of reducing the running cost of the MEO process. It has been reported in the literature that the current efficiency could be increased using a metal ion catalyst having a good redox potential. In this study Ce(IV) mediated electrochemical oxidation of phenol was carried out with silver ion catalyst. The current efficiency for the electro-oxidation of cerium(III) in nitric acid was found to be increased by the addition of silver ions. This mixed mediator system was tested for the oxidation of phenol in order to optimize the parameters for organic pollutant destruction. The mineralization efficiency calculated based on the CO₂ evolution was found to be higher for silver catalyzed Ce(IV) mediated oxidation compared to the non-silver catalyzed system.     

Studies on cerium oxidation in catalytic ozonation process: A novel approach for organic mineralization

A novel regenerative catalytic system has been developed using cerium and ozone in nitric acid medium. It was found that cerium(III) was oxidized to cerium(IV) by ozone in nitric acid medium with good conversion yields. The conversion rate of Ce(III) was measured under various parameters viz. ozone–air flow rate, initial concentration of Ce(III), and concentration of nitric acid at 25 °C. It was found that the conversion of Ce(III) increased with increasing ozone flow rate and concentration of nitric acid while decreased with increasing Ce(III) concentration. The pseudo first order kinetic constants were evaluated for Ce(III) oxidation. The efficiency of this hybrid system comprising of ozone and cerium redox pair towards organic mineralization was evaluated taking phenol as the model organic pollutant and compared with Ce(III) catalyzed and uncatalyzed ozonation processes. The presence of Ce(III) catalyst increased the destruction efficiency of phenol compared to uncatalyzed ozonation whereas a synergetic effect was observed between the cerium redox pair (Ce(III) and Ce(IV)) and ozone towards phenol mineralization and a maximum TOC removal was obtained in the latter case. Kinetic interpretations have been made with some simplifying assumptions owing to the much complex nature of ozone and metal ion interactions. This hybrid catalytic ozonation process may find its suitability for continuous organic destruction at room temperature.     

Mediated electrochemical oxidation of phenol in continuous feeding mode using Ag (II) and Ce (IV) mediator ions in nitric acid: A comparative study

Mediated electrochemical oxidation (MEO) process is one among the latest treatment technologies for the destruction of toxic organic pollutants under ambient temperatures and at atmospheric pressure. The process is a further extension of the conventional electrochemical treatment for the removal of toxic organics with powerful mediator oxidants in acidic medium. In this report the experimental results of using silver and cerium as mediator ions were compared with respect to their electro-oxidation behavior within the limitations of each mediator metal ion and their destruction efficiencies were compared for destructing phenol in continuous feeding mode. The following conclusions were drawn: (i) the optimum nitric acid concentration was found to be 8 and 3 M and the optimum temperature was found to be 60 and 80 °C for silver and cerium electro-oxidations, respectively; (ii) in the case of Ag (II)-MEO of phenol the maximum destruction efficiency achieved was 98% at 70 °C based on CO₂ evolved; (iii) for Ce (IV)-MEO of phenol the maximum destruction efficiency achieved was 93% at 90 °C based on CO₂. The results may provide baseline information on the use of suitable mediator metal ion in treating the target organic wastes by MEO process.     

Electrochemical cell current requirements for toxic organic waste destruction in Ce(IV)-mediated electrochemical oxidation process

The electrochemical cell for cerium oxidation and reactor for organic destruction are the most important operation units for the successful working mediated electrochemical oxidation (MEO) process. In this study, electrochemical cells with DSA electrodes of two types, single stack and double stack connected in series, were used. The performances towards the electrochemical generation of Ce(IV) in nitric acid media at 80 °C were studied. The current-voltage curves and cerium electrolysis kinetics showed the dependence on number of cell stacks needed to be connected in series for the destruction of a given quantity of organic pollutant. The presence of an optimum region for Ce(III) oxidation with a contribution of oxygen evolution, especially at low Ce(III) concentration (high conversion ratios), was found. The cells were applied for the Ce(IV) regeneration during the organic destruction. The cell and reactor processes were fitted in a simple model proposed and used to calculate the current needed in terms of Ce(III) oxidation rate and the number of cell stacks required for maintaining Ce(IV)/Ce(III) ratio at the same level during the organic destruction. This consideration was based on the kinetic model previously developed by us for the organic destruction in the MEO process.     

Electrolytic oxidation of Ce(III) in nitric acid and sulfuric acid media using a flow type cell

The electrochemical oxidation behavior of Ce(III) in nitric acid and sulfuric acid media with various concentrations and compositions were investigated by cyclic voltammetry (CV) and potentiostatic electrolysis. In nitric acid media, the peak potential separation for the redox reactions of the Ce(III)/Ce(IV) couple shifted to base side with the increasing concentration of nitric acid from 1 to 6 M. The kinetics of the Ce(III)/Ce(IV) couple is rapid in high concentration nitric acid. The formal potential is independent of both proton and nitrate concentrations while the standard rate constant increases with added protons but is independent of nitrate concentration. Constant-potential electrolysis also shows that a high proton concentration is electrochemically favorable for the electron transfer of the Ce(III)/Ce(IV) couple and for a high Ce(IV) yield in nitric acid media. The current efficiency was ca. 75% for the oxidation process of Ce(III) at 298 K. A Ce(IV) yield of ca. 80% was achieved for the electrolysis of 100 mM Ce(III) in 6 M HNO₃ solution. In sulfuric acid media, the peak potential separation for the redox reactions of the Ce(III)/Ce(IV) couple shifted to noble side with rising concentration of sulfuric acid from 0.1 to 2 M and then shifted to base side with further increase in the concentration. A Ce(IV) yield of ca. 95% was achieved for the potentiostatic electrolysis of 100 mM Ce(III) in 3 M H₂SO₄ solution.     

Organic Waste Destruction by Indirect Electrooxidation

Abstract

The destruction of organic model substances by indirect electrooxidation was investigated. The oxidation agent Co(II1) was used because of the high redox potential of the Co(III)/Co(II) redox couple (E₀ = 1.808 V).

Experiments were performed in a batch and in a continuous electrolytic cell by using various model substances (especially phenol and different chlorophenols). Intermediate and final products of the oxidation were identified and quantified. Organic carbon is ultimately transformed to CO₂ and to small amounts of CO. The residual carbon in the process solution was determined by TOC measurement to be about 20 ppm. Organic chlorine is oxidized via chlorate to perchlorate. The remaining amount of adsorbahle organic halogens (AOX) was less than 3 ppm. Based on these results, a pilot plant was constructed and is presently in operation.     

Electrocatalytic oxidation of methanol on the nickel–cobalt modified glassy carbon electrode in alkaline medium

In this work, Ni–Co alloy coating on the surface of glassy carbon (GC) electrode was performed by cyclic voltammetry. The results showed that the deposition of Ni–Co is an anomalous process. The deposition bath was prepared according to the metal ion Ni/Co ratio of 4:1 using NiSO₄·7H₂O and CoSO₄·8H₂O, and the total concentration of all solutions was 40.0 mM. The pH of the bath solution was adjusted at 2.0 using boric acid at room temperature. The modified electrode was conditioned by potential recycling in a potential range of 100–700 mV (vs. Ag/AgCl) by cyclic voltammetric method in an alkaline solution. The Ni–Co modified electrode showed a higher activity towards methanol oxidation in the Ni (III) and Co (IV) oxidation states. Cyclic voltammetry was used for the electrochemical characterization of the Ni–Co modified electrode and the mechanism of methanol oxidation is proposed. The result of double steps chronoamperometry shows that the methanol electrooxidation is an irreversible reaction. Moreover, the effects of various parameters such as mole ratio of Ni–Co in the alloy in modification step, potential scan rate, methanol concentration and solution temperature on the electro-oxidation of methanol have also been investigated.     

Experimental and theoretical studies of anti-ulcer drugs with benzimidazole rings as corrosion inhibitor for copper in 1 M nitric acid medium

The adsorption effect of esomeprazole (ESP) and lansoprazole (LP) on corrosion behavior of copper in 1 M HNO₃ solution was investigated using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and weight loss measurements. The experimental results indicate that both ESP and LP inhibited the corrosion of copper in nitric acid solution and the inhibition efficiency increased as the concentration of the compounds increased. EIS measurements confirmed that the charge transfer resistance increases on increasing the inhibitor concentration. Polarization measurements showed that the inhibitors are of mixed type. From the weight loss measurements, the inhibition efficiency of the inhibitors was found to vary with concentration, immersion time, and temperature. The adsorption of inhibitors on the copper surface follows Langmuir isotherm. The surface morphology was examined by scanning electron microscope and atomic force microscope. Further, the computational calculations are performed to find a relation between their electronic and structural properties.     

Ozonation of Phenol-Containing Wastewater Using O3/Ca(OH)2 System in a Micro Bubble Gas-Liquid Reactor

The degradation of phenol in aqueous solution was investigated in an integrated process consisting of O₃/Ca(OH)₂ system and a newly developed micro bubble gas-liquid reactor. The effects of operating parameters such as Ca(OH)₂ dosage, reactor pressure, liquid phase temperature, initial phenol concentration and inlet ozone concentration on degradation and mineralization (TOC removal) were studied in order to know the ozonation performance of this new integrated process. It is demonstrated that the degradation and TOC removal efficiency increased with increasing inlet ozone concentration and increasing Ca(OH)₂ dosage before 2 g/L, as well as decreasing initial phenol concentration. The optimum Ca(OH)₂ dosage should exceed Ca(OH)₂ solubility in liquid phase. The reactor pressure and liquid phase temperature have little effects on the removal and TOC removal efficiency. When Ca(OH)₂ dosage exceeded 3 g/L, the degradation and TOC removal of phenol almost reached 100% at 30 and 55 min, respectively. The intensification mechanism of Ca(OH)₂ assisted ozonation was explored through analysis of the precipitated substances. The mechanism for Ca(OH)₂ intensified mineralization of phenol solution is the simultaneous removal of CO₃^2- ions, as hydroxyl radical scavengers, due to the presence of Ca²⁺ ions. Results indicated that the proposed new integrated process is a highly efficient ozonation process for persistent organic wastewater treatment.     

Anodic oxidation of phenol for waste water treatment

The electrochemical oxidation of phenol for waste water treatment applications was investigated on lead dioxide packedbed anodes. Cells were operated in both batch and continuous modes with feed streams up to 1100 mg/l phenol dissolved in aqueous solutions of Na₂SO₄ and H₂SO₄ or NaOH. All the phenol in solution could be readily oxidized but complete total organic carbon (T.O.C.) removal was more difficult. The percent phenol oxidized increased with increasing current density, and decreased as initial phenol concentration, electrolyte flow rate, pH and anode particle size were increased. Results are compared to simple mathematical models.     

Electrochemical oxidation of aniline at boron-doped diamond electrodes

The electrochemical oxidation of aniline at boron-doped diamond (BDD) electrodes was investigated by cyclic voltammetry, steady-state polarization measurements and bulk electrolysis under potentiostatic control. It was found that acidic media is suitable for efficient electrochemical oxidation of aniline, because at low pH, the potential required for avoiding electrode fouling is lower than in neutral and alkaline media. The results of the longtime polarization measurements suggested that more anodic potentials ensure slightly higher efficiency for the conversion of aniline to CO₂, while the direct oxidation process does not play a prominent part in the overall electrochemical incineration of aniline. The current efficiencies (44%) and the efficiency of aniline conversion to CO₂ (80%) favourably compare with those reported for other electrochemical methods for aniline destruction. The results demonstrate the possibility of using BDD as an electrode material for electrochemical wastewater treatment, mainly when very high anodic potentials are required.     

Anodic oxidation of phenol for waste water treatment

The electrochemical oxidation of phenol for waste water treatment applications was investigated on lead dioxide packed-bed anodes. Cells were operated in both batch and continuous modes with feed streams up to 1100 nig/1 phenol dissolved in aqueous solutions of Na₂SO₄ and H₂SO₄ or NaOH. All the phenol in solution could be readily oxidized but complete total organic carbon (T.O.C.) removal was more difficult. The percent phenol oxidized increased with increasing current density, and decreased as initial phenol concentration, electrolyte flow rate, pH and anode particle size were increased. Results are compared to simple mathematical models.     

The oxidation of o-nitrotoluene to o-nitrobenzaldehyde with electrogenerated cobaltic sulphate

Indirect electrochemical oxidation ofo-nitrotoluene too-nitrobenzaldehyde by Co (III) has been studied. The highest aldehyde yield ( 80%) is obtained in dispersing a 70-fold excess ofo-nitrotoluene in a solution of CO₂(SO₄)3 in 60% H₂SO₄ at 12° C. The influence of acid concentration, excess of nitrotoluene and presence of catalyst are discussed and found to be compatible with a mechanism proposed by Bawn.     

Electrooxidation of acetaldehyde on carbon-supported Pt, PtRu and Pt3Sn and unsupported PtRu0.2 catalysts: A quantitative DEMS study

The oxidation of acetaldehyde on carbon supported Pt/Vulcan, PtRu/Vulcan and Pt₃Sn/Vulcan nanoparticle catalysts and, for comparison, on polycrystalline Pt and on an unsupported PtRu_0.2 catalyst, was investigated under continuous reaction and continuous electrolyte flow conditions, employing electrochemical and quantitative differential electrochemical mass spectroscopy (DEMS) measurements. Product distribution and the effects of reaction potential and reactant concentration were investigated by potentiodynamic and potentiostatic measurements. Reaction transients, following both the Faradaic current as well as the CO₂ related mass spectrometric intensity, revealed a very small current efficiency for CO₂ formation of a few percent for 0.1 m acetaldehyde bulk oxidation under steady-state conditions on all three catalysts, the dominant oxidation product being acetic acid. Pt alloy catalysts showed a higher activity than Pt/Vulcan at lower potential (0.51 V), but do not lead to a better selectivity for complete oxidation to CO₂. C–C bond breaking is rate limiting for complete oxidation at potentials with significant oxidation rates for all three catalysts. The data agree with a parallel pathway reaction mechanism, with formation and subsequent oxidation of CO_ad and CH _{x, ad} species in the one pathway and partial oxidation to acetic acid in the other pathway, with the latter pathway being, by far, dominant under present reaction conditions.     

Phenol degradation by Fenton's process using catalytic in situ generated hydrogen peroxide

The recent reported pathway using oxygen and formic acid at ambient conditions has been utilized to generate hydrogen peroxide in situ for the degradation of phenol. An alumina supported palladium catalyst prepared via impregnation was used for this purpose. Almost full destruction of phenol was carried out within 6 h corresponding to the termination of 100 mM formic acid at the same time. In addition, a significant mineralization (60%) was attained. A simulated conventional Fenton process (CFP) using continuous addition of 300 ppm H₂O₂ displayed maximum 48% mineralization. Study of different doses of formic acid showed that decreasing the initial concentration of formic acid caused faster destruction of phenol and its toxic intermediates. The catalytic in situ generation of hydrogen peroxide system demonstrated interesting ability to oxidize phenol without the addition of Fenton's catalyst (ferrous ion). Lower Pd content catalysts (Pd1/Al and Pd0.5/Al) despite of producing higher hydrogen peroxide amount for bulk purposes, did not reach the same efficiency as the Pd5/Al catalyst in phenol degradation. The later catalyst showed a remarkable repeatability so that more than 90% phenol degradation along with 57% mineralization was attained by the used catalyst after twice recovery. Higher temperature (45 °C) gave rise to faster degradation of phenol resulting to almost the same mineralization degree as obtained at ambient temperature. Meanwhile, Pd leaching studied by atomic adsorption proved excellent stability of the catalysts.     

One-step growth of 3-5 nm diameter palladium electrocatalyst in a carbon nanoparticle-chitosan host and characterization for formic acid oxidation

The electrochemical formation of a palladium nanoparticle catalyst composite material has been investigated. A carbon nanoparticle-chitosan host film deposited onto a carbon substrate electrode has been employed to immobilize PdCl₂ as catalyst precursor. A one-step electrochemical reduction process gave Pd nanoparticles within the chitosan matrix with different levels of loading, on different carbon substrates, and with a reproducible catalyst particle diameter of ca. 3-5 nm. High activity for formic acid oxidation has been observed in aqueous phosphate buffer medium. The oxidation of formic acid has been investigated as a function of pH and maximum catalyst activity was observed at pH 6. When varying the formic acid concentration, limiting behaviour consistent with a “resistance effect” has been observed. A flow cell system based on a screen-printed carbon electrode has been employed to establish the effect of hydrodynamic conditions on the formic acid oxidation. Both increasing the convective-diffusion mass transport rate and increasing the concentration of formic acid caused the oxidation peak current to converge towards the same “resistance limit”. A mechanistic model to explain the resistance effect based on CO₂ flux and localized CO₂ gas bubble formation at the Pd nanoparticle modified carbon nanoparticle-chitosan host film has been proposed.     

Highly stable Fe/γ‐Al2O3 catalyst for catalytic wet peroxide oxidation

BACKGROUND: A highly stable Fe/γ‐Al₂O₃ catalyst for catalytic wet peroxide oxidation has been studied using phenol as target pollutant. The catalyst was prepared by incipient wetness impregnation of γ‐Al₂O₃ with an aqueous solution of Fe(NO₃)₃· 9H₂O. The influence of pH, temperature, catalyst and H₂O₂ doses, as well as the initial phenol concentration has been analyzed. RESULTS: The reaction temperature and initial pH significantly affect both phenol conversion and total organic carbon removal. Working at 50 °C, an initial pH of 3, 100 mg L^?1 of phenol, a dose of H₂O₂ corresponding to the stoichiometric amount and 1250 mg L^?1 of catalyst, complete phenol conversion and a total organic carbon removal efficiency close to 80% were achieved. When the initial phenol concentration was increased to 1500 mg L^?1, a decreased efficiency in total organic carbon removal was observed with increased leaching of iron that can be related to a higher concentration of oxalic acid, as by‐product from catalytic wet peroxide oxidation of phenol. CONCLUSION: A laboratory synthesized γ‐Al₂O₃ supported Fe has shown potential application in catalytic wet peroxide oxidation of phenolic wastewaters. The catalyst showed remarkable stability in long‐term continuous experiments with limited Fe leaching, < 3% of the initial loading. Copyright © 2010 Society of Chemical Industry     

Destruction of aniline by mediated electrochemical oxidation with Ce(IV) and Co(III) as mediators

Mediated electrochemical oxidation has been employed to test the feasibility of treating soluble organic wastes. We report Ce(IV)- and Co(III)-mediated electrochemical oxidation of aniline at various electrodes in acidic media as an example of organic waste. Aniline was oxidized by an electrogenerated electron transfer mediator, Ce⁴⁺ or Co³⁺, in the anolyte and carbon dioxide was produced as a final oxidation product. Carbon dioxide was collected by bubbling through a barium hydroxide solution. When a powerful oxidizing agent, Ce(IV) or Co(III), was used as an electron shuttling mediator, parameters affecting the coulombic efficiency for aniline oxidation were the standard oxidation potentials of the mediators, their concentrations and the reaction temperature. Intermediate species produced during the oxidation of aniline were identified by cyclic voltammetric and absorption spectroscopic measurements.     

Electrochemical regeneration of chrome etching solution

A metal surface is chromatized with a chromic acid solution to obtain a good adherence of polymer coatings. In this process Cr(VI) is reduced to Cr(III). The oxidation strength of the solution decreases during use. The chrome solution needs to be regenerated and purified. A new anode material, namely boron-doped diamond, was used to investigate the oxidation of Cr(III) to Cr(VI). It was found that the current efficiency for Cr(III) oxidation decreases with increasing total current density. The current density of Cr(III) oxidation increases linearly with increasing Cr(III) concentration and is practically independent of the Cr(VI) concentration. It was concluded that the diffusion of Cr(III) is the rate-determining step for the Cr(III) oxidation at Cr(III) concentrations form 40 to 160 mol m^–3. The surface of the boron-doped diamond shows no signs of chemical corrosion or mechanical destruction. A filter-press type cell divided into two compartments by a cation exchange membrane was proposed. A cost calculation was carried out for the oxidation of 1.28 mmol s^–1 Cr(III) in a 40 mol m^–3 chrome(III) solution. Factors affecting the feasibility of this process include the costs of chemical waste disposal, the costs of chromic acid, government legislation and to a great extent the costs of the new anode material.