Treatment of organic pollutants in water using TiO2 powders: photocatalysis versus sonocatalysis

The scope of this work was to explore the application of different TiO2 powders (P25, PC105, PC500 and their calcined forms) for the photocatalytic and sonocatalytic treatment of model wastewaters containing single pollutants and their mixtures; dyes: C.I. Reactive Violet 2 (RV2), C.I. Mordant Yellow 10 (MY10) and oxalic acid. The influence of the applied catalyst type on the photocatalytic and sonocatalytic efficiency was explored with the emphasis on crystalline form and granulometric properties of powders. Generally, TiO2 powders, which were applied as obtained, demonstrated higher photocatalytic activity (~20 % of organic content oxidized in the mixture of dyes and oxalic acid in 30 min) while their calcined forms were shown to be more prominent as sonocatalysts (up to 13 % of organic contents oxidized in 30 min). The XRD analysis of calcined TiO2 powders confirmed the predominant crystal form of rutile. The degradation and mineralization kinetics of dyes RV2, MY10, oxalic acid and their mixtures was studied. In general, experimental results fitted well (R2 > 0.94) to the half and first order reaction rate model, pointing at the two ongoing mechanisms, i.e. reaction with ·OH radicals and direct electron transfer between adsorbed molecules and catalyst surface. A certain deviation is observed for the model solution containing dye MY10. MY10 serve as a filter for the UV-A irradiation (λmax = 365 nm). A detailed kinetic study confirmed the two simultaneous kinetic pathways and the comparable mechanisms for oxalic acid and dyes photocatalysis. The performed study confirmed the similarity of occurring mechanisms in photocatalysis and sonocatalysis due to sonoluminescence, with the extent of acoustic cavitation in ·OH radical generation.