The electrochemical oxidation of phenol in basic media using a diamond thin-film electrode has been studied. Within the parameter ranges used (temperature: 15-60 °C, initial total carbon concentration: 360-1450 mg of C dm -3 ; current density: 15-60 mA cm -2 ), almost complete mineralization of the organic waste is obtained. The mineralization rate increases with current density and temperature. Current efficiency depends mainly on mass transfer limitations: in the absence of mass transfer limitations, instantaneous current efficiencies of 1 are obtained. The main intermediates formed are maleic, fumaric, and oxalic acids. A simple model based on mass transfer and kinetic considerations, which involves four species (phenol, maleic/fumaric acid, oxalic acid, and carbon dioxide), can be used to explain the experimental behavior of the system, regardless of the conditions applied.
The electrochemical oxidation of acidic aqueous phenol wastes has been studied using boron-doped diamond thin-film ͑BDD͒ and AISI 304 stainless steel ͑SS͒ electrodes. A voltammetric study shows marked differences in the electrochemical behavior of these two electrodes. The surface of the SS electrodes undergoes significant changes when this material is used as the anode in the treatment of aqueous wastes, even in the potential region of electrolyte stability. These changes have important effects on the waste treatment process. Conversely, the BDD electrode does not undergo any appreciable change during the electrochemical oxidation of the wastes. An electrolysis study highlighted significant differences between the behavior of the two electrodes. First, the oxidation performed using a BDD electrode leads to the rapid sequential formation of aromatic compounds ͑hydroquinone, benzoquinone͒, carboxylic acids ͑maleic, fumaric, and oxalic͒, and carbon dioxide. The oxidation performed using SS electrodes, on the other hand, involves a slower sequential formation of the same compounds, indicating a lower energetic efficiency of these electrodes in the destruction of the organic matter, and to the formation of some insoluble compounds resulting from the electrocoagulation of organic matter with iron dissolved from the electrode.
The electrochemical oxidation of diluted cyanide aqueous wastes has been studied in a single compartment electrochemical flow cell. It has been determined that the anode material influences greatly the process's performance. Boron doped diamond and PbO 2 anodes can oxidize these wastes in the presence of both sulfate or chloride anions. On the contrary, dimensional stable anodes cannot oxidize cyanide in sulfate-containing wastewaters, and require the presence of chloride ions. The oxidation of cyanides leads to the formation of cyanate in a first step, and later to the formation of carbon dioxide and nitrogen. There is a net consumption of hydroxyl ions during the process. Energy consumptions in the range 20-70 kWh m −3 are required to decrease the initial pollutant load by 70-80%. Global current efficiencies in the range 3-8% are obtained. These low current efficiencies are justified by the low cyanide concentrations that the wastes used in this work contain.
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