Abstract
Dyestuffs are often present in industrial wastewaters and can consist of hazardous substances which have a serious impact on the environment and health. The present work shows a system developed to degrades these substances using sonochemical reactions. Ultrasonic frequencies of 118, 224, 404 and 651 kHz and power input values of 20, 40 and 60 W were tested on Rhodamine B and Orange II dyestuff solutions in order to find the best degradation conditions. Ultrasonic irradiation of air saturated solutions produces free radicals that combine and generate hydrogen peroxide; compared to the production of hydrogen peroxide when irradiating water, a decrease was found during irradiation of the dyestuffs solutions, indicating that some of the free radicals were used in the degradation process. Effects of pH, nitric and nitrous acid formation as well as the total organic carbon value (TOC) were also investigated. For ultrasonic frequencies of 224, 404 and 651 kHz, the degradation rates were very similar, however, the 118 kHz system presented a degradation rate for both dyestuffs about one third of the higher frequencies. Rhodamine B solutions decolorized within 2 h of ultrasonic irradiation for all systems with the exception of 118 kHz. In the case of Orange II, except for the 118 kHz system, all solutions were decolorized within 4 h of ultrasonic irradiation. All reactions were carried out at 25°C and the total ultrasonic irradiation time was 10 h.
