Sonolytic and sonocatalytic degradation of azo dyes by low and high frequency ultrasound

Abstract

The study describes degradation of two azo dyes at low and high frequency ultrasound (US) to compare their reactivity and to assess the impacts of frequency, OH , chemical structure and soluble/nonsoluble additives. Low frequency US alone was found totally ineffective for bleaching the dyes even after 2-h irradiation, while high frequency provided significant color decay in 30-min contact. The result was attributed to larger number of oscillations at high frequency that allowed a larger fraction of OH ejection to the bulk liquid. The difference in the rates of dye degradation was due to different substituents around the azo bonds that dictated the reactivities of the dyes with OH and other species. The performance of low frequency US was remarkably improved and exceeded that of high frequency in the presence of CCl 4, nano-sized TiO 2 and zero-valent copper. The effect was attributed to the advantage of low frequency for long bubble-life time, high collapse temperatures, turbulent flow conditions and high sonoluminescence intensity. The efficacies of the additives in terms of the reduction in dye concentration per unit mass of additive were: TiO 2 > CCl 4 > Cu, regardless of the dye structure and the operation frequency. Much better performance of TiO 2 than Cu was attributed to its larger surface area with a slight positive charge on it and to the effect of stable sonoluminescence that may have induced photocatalytic properties on semiconductor surface.