Ozone enhanced activity of aqueous titanium dioxide suspensions for photocatalytic oxidation of free cyanide ions

Abstract

The ozonization and the heterogeneous photocatalytic method are contemporarily applied for the oxidation of free cyanide ions. A batch photoreactor with immersed lamp was used for treating aqueous suspensions containing polycrystalline TiO2 powders irradiated in the near-UV region. Air or ozone–air mixtures were used as oxidation reactants. At the used experimental conditions, the photoreaction proceeds at a measurable rate until low values of cyanide concentration. In the presence of ozone (O3) several parallel reaction routes are contributing to the overall cyanide degradation: (i) homogeneous oxidation by ozone; (ii) heterogeneous catalytic reaction by ozone; (iii) photocatalytic degradation with oxygen; and (iv) photocatalytic degradation with ozone. The rate of cyanide oxidation by heterogeneous photocatalytic mechanism is enhanced by ozone with respect to that observed in the presence of neat oxygen. A kinetic model based on Langmuir–Hinshelwood equations describes adequately the photoreactivity results and provides the values of the kinetic constants and equilibrium adsorption constants for the catalytic and photocatalytic reactions contributing to cyanide oxidation. In order to gain further information on the interaction of ozone with TiO2 surfaces, either in the dark or under UV-irradiation, an ESR study has been also carried out. The results indicate that ozone strongly interacts with TiO2, leading mainly to the formation of ozonide radicals.