André Savall

Electrochemical degradation of phenol in aqueous solution on bismuth doped lead dioxide: a comparison of the activities of various electrode formulations

This paper describes the development of electrochemical processes for the oxidative degradation of toxic organic chemicals in waste waters. Doped bismuth lead dioxide anodes have been tested by the kinetic study of phenol anodic oxidation in aqueous solution. The main products during oxidative degradation of phenol are 1,4- benzoquinone, maleic acid and carbon dioxide. Several deposits of Bi2O5–PbO2 on Ti/(IrO2–Ta2O5) substrates have been prepared by anodic oxidation of Pb2+ and Bi3+ in aqueous solutions containing perchloric acid to increase the solubility of bismuth. To study the effect of perchlorate ions, the efficiency of the PbO2 deposit prepared from lead nitrate in an aqueous solution (pure PbO2) was compared with that of a deposit prepared from perchloric acid solution (perchlorate doped PbO2). Although the phenol is oxidized at the same rate on the two deposits, the charge corresponding to the total elimination of 1,4-benzoquinone is three times higher for perchlorate doped PbO2 than for pure PbO2. Phenol degradation is more efficiently carried out on a PbO2 anode doped with perchlorate and with bismuth than on the same electrode doped only with perchlorate. Among the electrodes tested in this work, the pure PbO2 anode is the most efficient for phenol degradation. It is assumed that certain active sites on the anode occupied by perchlorate ions do not participate in the transfer of oxygen atoms and that for the PbO2 electrode doped with bismuth, oxygen evolution is favoured to the detriment of oxygen atom transfer.

Electrochemical incineration of cresols: A comparative study between PbO2 and boron-doped diamond anodes

The electrooxidation of aqueous solutions containing 5mM of o-, m- and p-cresol at pH 4.0 has been investigated using a flow filter-press reactor with a boron-doped diamond (BDD) under galvanostatic electrolysis. All cresols are degraded at similar rate up to attaining overall mineralization. Comparable treatment of the m-cresol effluent on PbO(2) leads to partial electrochemical incineration. However, this pollutant is more rapidly removed with PbO(2) than with BDD. The decay kinetics of all cresols follows a pseudo-first-order reaction. Aromatic intermediates such as 2-methylhydroquinone and 2-methyl-p-benzoquinone and carboxylic acids such as maleic, fumaric, pyruvic, malonic, tartronic, glycolic, glyoxylic, acetic, oxalic and formic, have been identified and followed during the m-cresol treatment by chromatographic techniques. From these oxidation by-products, a plausible reaction sequence for m-cresol mineralization on both anodes is proposed. The energy consumption for the corresponding electrochemical process is also calculated.

Preparation of oxygen evolving electrodes with long service life under extreme conditions

Among the numerous base metals tested for DSA® type electrodes (e.g., titanium and its alloys, zirconium, niobium etc.), tantalum is a potentially excellent substrate owing to its good electrical conductivity and corrosion resistance, and the favourable dielectric properties of its oxide. Nevertheless, a DSA® type electrode fabricated on a tantalum substrate would be very expensive due to the high cost of the metal. To prepare an anode combining the excellent properties of tantalum at reasonable price, a new material has been developed in our laboratory. This consists of a common base metal (e.g., Cu) covered with a thin tantalum coating. This tantalum layer was obtained by molten salt electroplating in a LiF–NaF–K2TaF7 melt at 800°C. Thus, an anode of the type Metal/Ta/Ta2O5–IrO2 with a surface load of 22gm-2 IrO2, submitted to the severe test conditions used in this work, exhibits a standardized lifetime tenfold greater than one made with ASTM grade 4 titanium base metal. Thus, this type of electrode might be advantageously employed as an oxygen evolution anode in acidic solutions.

The electrochemical regeneration of Fenton's reagent in the hydroxylation of aromatic substrates: batch and continuous processes

Fentons reagent, regenerated by the electrochemical reduction Fe3++e-→Fe2+ has been used for the hydroxylation of benzene into phenol in both continuous and batch processes. In a batch process, the current yield for this reaction is relatively low because of the oxidation, in the aqueous phase, of phenol and its oxidation products by HO0 radicals. When the phenol is continuously extracted in the form of phenate ions, the current yield is found to increase from 20 to 70% for a nine-fold rise in the rate of benzene circulation. Varying the sulphuric acid concentration in the range 0.05 to 0.8m has little effect on current yield, while varying the Fe3+ concentration gives a maximum yield of about 70% for a concentration of 0.1m. At higher Fe3+ concentrations the rate of production of HO0 radicals becomes too high in comparison with the rate of benzene hydroxylation. The presence of a co-solvent like CCl4, although unreactive with HO0, reduces the yield. A numerical model taking into account the 13 reactions for which rate constants could be found or estimated, was developed for the continuous process. This model can only predict the general trends in yield and is not quantitatively predictive. Attempts at hydroxylating fluorobenzene using Fentons reagent with electrochemical regeneration did not give appreciable yields of fluorophenols.

Tantalum protective thin coating techniques for the Chemical Process Industry : Molten salts electrocoating as a new alternative

Abstract A comparison of corrosion resistance and basic properties of solid tantalum with other high-performance materials used in the Chemical Process Industry (CPI) is given. The corrosive chemicals taken into consideration are strong acidic media. Secondly, it is pointed out that tantalum, which exhibits excellent corrosion resistance, owing to a rapid build-up of passivating protective film in oxidizing conditions, also has good mechanical, thermal and electrical properties which suggest its use when little or no metallic corrosion is tolerated. Thirdly, tantalum thin-layer, coated onto a usual base metal, which offers the same protection as solid metal and avoids its expensive use, is treated. Fourthly, numerous tantalum-coating techniques for clad-vessel and CPI devices are reviewed and compared. Amongst these coating techniques, this paper focuses mainly on two techniques which give a very thin, protective coating against corrosion. Thus, Chemical Vapor Deposition (CVD) and Molten Salt Electro-deposition (MSE) are especially enhanced. Finally, MSE which is still not widely used for manufacturing clad-vessels is examined in greater detail.

Water as Solvent for Nickel‐2,2′‐Bipyridine‐Catalysed Electrosynthesis of Biaryls from Haloaryls

Reductive homocoupling of aryl halides into biaryls was achieved by electrolysis of aqueous emulsions, either in an undivided cell fitted with a sacrificial anode, or in a divided diaphragm cell, and in the presence of nickel-2,2′-bipyridine as catalyst. Reactions were also run in a filter-press cell.

Kinetic Study of the Electrochemical Mineralization of m-Cresol on a Boron-Doped Diamond Anode

The kinetics of the electrooxidation of m-cresol in aqueous solution was investigated in a one-compartment flow electrochemical cell with a boron-doped diamond electrode (BDD). Cyclic voltammograms recorded on BDD revealed that cresol oxidation takes place at a potential very close to the discharge of water. Under potentiostatic conditions, at a working potential lower than water discharge, a passive layer was rapidly formed on the electrode surface due to cresol polymerization. The anode fouling was not observed during electrolysis performed with the flow electrochemical cell operating under galvanostatic conditions. In this case, the decay of mcresol concentration followed a pseudo-first-order kinetics. The abatement of chemical oxygen demand (COD) showed that the kinetics of m-cresol oxidation was limited by mass transfer and that a full mineralization was achieved. A good agreement between predicted and experimental COD and instantaneous current efficiency values was obtained, although some deviations were observed at high current since the experimental data decreased faster than those predicted ones. These deviations can be explained by the occurrence of oxygen evolution which increases the mass transfer coefficient.

Treatment of dilute methylene blue-containing wastewater by coupling sawdust adsorption and electrochemical regeneration.

In the present work, the coupling of adsorption and electrochemical oxidation on a boron-doped diamond (BDD) electrode to treat solutions containing dyes is studied. This coupling may be convenient for the treatment of diluted pollutant that is limited by the low rate of electrooxidation due to mass-transfer limitation. A pre-concentration step by adsorption could minimize the design of the electrochemical reactor. The adsorbent chosen was mixed with softwood sawdust, and methylene blue was chosen as the model dye molecule. Isotherms of adsorption and kinetics were investigated as well as the effects of current density and regeneration time. The BDD electrochemical oxidation of methylene blue adsorbed onto sawdust led simultaneously to its degradation and sawdust regeneration for the next adsorption. It was observed that multiple adsorption and electrochemical regeneration cycles led to an enhancement of adsorption capacity of the sawdust. This study demonstrated that adsorption–electrochemical degradation coupling offers a promising approach for the efficient elimination of organic dyes from wastewater.

Anodic oxidation of p-methoxytoluene in acetic acid on graphite

The direct oxidation of p-methoxytoluene in acetic acid medium was studied on a graphite anode for the preparation of p-anisaldehyde and p-anisyl acetate. The effect of the water contents of the solvent on the reaction selectivity was determined for solvent mixtures containing from 0 to 20 vol % water. In anhydrous medium p-anisyl acetate was the main product. When the water contents increased the yield of p-anisaldehyde increased. It was shown that the higher the proportion of water, the lower the p-methoxytoluene oxidation rate. The formation of products of consecutive oxidation steps decreased as the temperature increased from 60 to 90 °C.

Numerical models for reactions catalysed by homogeneous mediators: the case of Fenton's reagent*

A numerical model has been developed to describe the behaviour of a batch reactor in which Fentons reagent is used for hydroxylating aromatic hydrocarbons under conditions of electrochemical regeneration. The test reaction considered is the conversion of benzene into phenol. Comparison is made with previously published experimental results.