Investigating the reactivity of TiO_x and BDD anodes for electro-oxidation of organic pollutants by experimental and modeling approaches

The reactivity of two non-active anodes (BDD and TiOx) for the removal of organic pollutants from water was investigated and compared. A mathematical model for electro-oxidation of organic compounds was developed by considering (i) an analytical expression of the spatial distribution of (OH)-O-center dot concentration at non-active anodes, (ii) mass transport through the diffusion boundary layer and (iii) both direct electron transfer and (OH)-O-center dot-mediated oxidation for degradation of organic pollutants. The model was calibrated from experimental data using, for example, terephthalic acid as (OH)-O-center dot probe molecule or ethanol as (OH)-O-center dot quencher during paracetamol degradation. Calculation of the fraction of current ascribed to the different possible reactions as well as a sensitivity analysis allowed for highlighting the key mechanisms and limiting phenomena. Degradation of organics occurs almost exclusively through (OH)-O-center dot-mediated oxidation under mass transport limitation and for sufficiently high reaction rate constants. Improvement of degradation kinetics strongly depends on mass transport enhancement of organic compounds to the reaction zone at the electrode surface. Competition between the mother molecule and degradation by-products for reaction with (OH)-O-center dot did not significantly affect degradation kinetics under mass transport limitation. Direct electron transfer can potentially become important in presence of large amounts of (OH)-O-center dot scavengers or for organic compounds that react slowly with (OH)-O-center dot (k < 10(5) m(3) mol(-1) s(-1)). Results also showed that the implementation of a suitable (OH)-O-center dot quenching experiment required the use of a concentration of ethanol of at least 100 mM for experiments performed at 5 mA cm(-2) and with a concentration of target pollutant of 0.1 mM.