The application of ultrasound on the photocatalysis provides a prominent influence on the degradation of most organic contaminants selected in this study. Yet the extent of the influence mainly depends on the natures and characteristics of these compounds. In this work, the effects of ultrasonication as well as other factors on photocatalytic destruction of several organic compounds were studied and discussed. These organics include phenol, 4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol, and two simulants of chemical warfare agents (CWA) -dimethyl methylphosphonate (DMMP) and 2-(butylamino) ethanethiol (BAET).
The “synergistic” effect due to the application of ultrasound on the photocatalytic degradation of phenol was demonstrated in the presence of Hombikat TiO2suspensions in a sonophotocatalytic reactor. Noticeable enhancement of the reaction rate was found as ultrasound was utilized in the photocatalysis of phenol. By reducing the reaction volume or increasing the average ultrasonic power density, the enhancement was intensified considerably. The average ultrasonic power density of around 0.7 W/mL seems to be an optimal usage of ultrasonic energy in our system for achieving efficient sonophotocatalytic degradation of phenol. The influence of ionic strength and anions on sonophotocatalysis was evaluated. It was shown that chloride anions (Cl-) inhibit the action of sonophotocatalysis at low pH. Investigation of chlorophenols, namely, 4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol, was involved in this study as well, leading to the suggestions that (1) the generation of chloride radicals (Cl•) may assist the degradation of the organic compounds before substantial chloride anions (Cl-) were formed to inhibit the photocatalysis, and (2) the solubility of the examined organic compounds has a significant influence on the performances of photocatalytic and sonochemical reactions.
The same application with the aid of ultrasound was utilized in the photocatalytic degradation of dimethyl methylphosphonate (DMMP), a non-toxic compound simulating chemical structures of organophosphorous nerve agents sarin, soman, and VX. Ultrasonic irradiation obviously increased the rate of DMMP photocatalytic mineralization over TiO2 aqueous suspension more than twofold. The same intermediates were detected in both cases of photocatalysis and sonophotocatalysis, which testifies to the same reaction mechanism.
2-(butylamino) ethanethiol (BAET), which simulates nerve agent VX, was also evaluated in the procedure. However, the presence of ultrasound barely improved the photocatalytic degradation rate of BAET. The optimal reaction condition for the degradation of this compound in our system was found at pH 9 with catalyst (Degussa P25) concentration of 500 mg/L and BAET concentration of 150 mg/L. Thus, it is suggested that the effect of ultrasound on the photocatalysis depends mainly on the natures of organic compounds and that the detail chemical mechanisms of these compounds under sonophotocatalysis need to be investigated before practical applications.