Mechanistic Investigations on Sonophotocatalytic Degradation of Textile Dyes with Surface Active Solutes

Abstract

In recent years, two advanced oxidation processes, namely, photocatalysis and sonolysis have been extensively investigated for the degradation of recalcitrant organic pollutants. Simultaneous application of these two techniques, known as sonophotocatalysis has been found to give synergistic enhancement in degradation under specific experimental conditions. The present study attempts to establish the physical mechanism of sonophotocatalytic process by finding the synergy between two techniques that gives enhancement in degradation. Transient collapse of cavitation bubbles gives rise to light emission (known as sonoluminescence), which could provide activation of the photocatalyst. To test this hypothesis, we have conducted experiments on the basis of known effects of surface active solutes on sonoluminescence. Three different textile dyes have been chosen as model pollutants. Experiments have been conducted in the presence of three different surface active solutes, namely, SDS, 2-propanol, and 1-butanol. The rate of degradation reduces drastically with the addition of surface active solutes. The reduction in the degradation process ranges from 5-fold (for Acid Red B) to ∼20% (for Direct Blue 6) for SDS, while for alcohols much higher (∼ 10-fold) reduction is seen for all three dyes. It is revealed that the interaction between photocatalyst and sonolysis is merely of physical nature. The sonoluminescence light from cavitation bubbles is not able to activate the photocatalyst. The role of TiO2 is revealed to be only that of an adsorbent for the dyes. The degradation is caused mostly due to the radicals generated by the cavitation bubble, with negligible role of the radical generation from photocatalyst.