Electro-oxidation tests with different electrolytes (Na 2 SO 4 , NaCl, H 2 SO 4 ) and anode types (Pt, Ti lined with Ir and Ta oxides, PbO 2 , activated carbon) were performed on aqueous solutions containing phenol to assess the mechanism and nature of electrode deactivation phenomena. For the Pt electrode, the nature of the electro-deposited organic species was investigated by ATR-FTIR and FESEM-EDS analyses, which showed adsorption of intermediate oxidation products (e.g. benzoquinone, hydroquinone) is likely responsible for the early deactivation stages. Conversely, in the longer term, formation of polymeric films is promoted. Potentiostatic tests showed that anode regeneration can be achieved by anodic polarisation above 1.1 V (vs Hg/Hg 2 SO 4 ). This reactivation was found to be easier in the presence of significant amounts of chloride ions. Conversely, the deactivated state is maintained for the Ti/IrO 2 /Ta 2 O 5 electrode even though anodic polarisation at high positive potentials is applied. Cyclic voltammetric curves on PbO 2 electrodes did not provide satisfactory results as the intensity of the leaddioxide reduction peak was so high that peaks for phenol oxidation were hardly detectable. Finally, the activated carbon based electrode was found to be promising as it enables simultaneous adsorption of the organic pollutant and oxidation of the pollutant itself to constitute a sort of self-regenerating adsorber unit.
The electrooxidation of nitrogen-containing molecules (urea, reactive dye blue 4, acetonitrile, formamide,
guanidine, and pyridazine) over Pt−Ti and Ti/RuO2 anodes is investigated. The product distribution of the
electrochemical mineralization of nitrogen is monitored. Nitrogen mineralization generally leads to inorganic
pollutants (NH3/NH4
+ and/or nitrite and nitrate ions). Amidic and aminic compounds were found to react
according to both homogeneous (acid hydrolysis) and heterogeneous (direct electrooxidation) pathways, with
a rate depending on the original state of oxidation of nitrogen. Heterocyclic and multiple-bond carbon−nitrogen molecules were converted but with negligible mineralization of nitrogen due to the stability of their
first oxidation intermediates. The obtained results can be useful for the design of further treatment routes
after electrooxidation or the definition of simultaneous strategies for inorganic pollutants elimination.
The direct oxidation of urea at high electrode potentials over platinized titanium electrodes in sulphate media generates a high amount of nitrates, representing an environmental problem. Coupling direct and indirect oxidation (using chloride salt), the total amount of generated nitrates can be lowered due to competition between anodic generation of chlorine and nitrates. A qualitative description of the influence of applied current density (ranging from 0.5 to 30 A m -2 ) and the nitrogen-to-chlorine ratio N/Cl (ranging from 0.5 to 3 by weight) on the distribution of urea electroxidation by-products was obtained. In a highly acidic environment, a decrease in nitrate concentration was observed when operating at relatively high current densities, which suggests the occurrence of a complex heterogeneous reaction involving nitrates and yielding mainly molecular nitrogen.
The electro-oxidation, over platinized titanium and ruthenium oxide anodes, of nitrogen containing molecules (urea, reactive Blue 4 dye, acetonitrile, formamide, guanidine and pyridazine) was investigated, monitoring the products distribution. The Nmineralization leads to have inorganic pollutants (NH 3 /NH 4 + and/or NO 2 -/NO 3 -). Amidic and aminic compounds react both in homogeneous (acid hydrolysis) and in heterogeneous phase (direct electroxidation) with a rate depending on the original state of oxidation of nitrogen. Heterocyclic and multiple-bond carbon-nitrogen molecules were effectively converted with negligible mineralization of nitrogen due to the stability of their first oxidation intermediates. The obtained results (high rate of nitrate generation) evidence the need of coupling of the direct electroxidation with other process to limit the nitrate concentration to an accepted level; in accordance, dialysis (of the ammonia cation) and indirect oxidation (chlorine-mediated) were proved to be valid alternatives.
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