EPR and kinetic investigation of free cyanide oxidation by photocatalysis and ozonation

Abstract

Oxidation of free cyanide in aqueous suspensions of three commercial TiO2 specimens, with different anatase crystal size, has been carried out in a batch photoreactor by simultaneously applying ozonation and photocatalysis. Dissolved ozone participates both in homogeneous and catalytic reactions with cyanide; the extents of these two processes are comparable to that of the photodegradation with oxygen. The reactivity results are well described by the Langmuir-Hinshelwood kinetic model, providing the values of the kinetic and equilibrium adsorption constants for the catalytic and photocatalytic reactions contributing to cyanide oxidation. The cyanide concentration decreases faster with time for catalysts with increasing anatase crystal size, being more marked under UV irradiation. EPR studies on gaseous ozone adsorption on the three samples in the dark have shown stronger ozone interactions with Ti4+ and O2− ions of the samples with largest anatase crystal size, leading to the formation of significant signals of Ti3+ and s O−−O2 radicals than with the anatase with the lowest crystal size, where ozone was mainly adsorbed on water molecular arrangements covering its surface. The hampering of the ozone and/or cyanide adsorption by the water molecular arrangements covering the surface of the catalyst with the lowest crystal size would justify the low cyanide degradation rate observed for this sample.