The use of pulsed ultrasound technology to improve environmental remediation: a comparative study

Relatively little is known about the effects of pulsed ultrasound on the facilitation of chemical reactivity and remediation. Previous studies have indicated that sonochemistry using pulses may be either more or less effective than continuous wave (CW) ultrasonic irradiation. However, because of the time-modulated nature of the pulses used in these studies, less acoustic energy in general was transmitted to the solutions compared to CW sonication. The effectiveness of ultrasound when the pulse is adjusted so that the same amount of acoustic energy is input compared to continuous irradiation over a given time has not been previously explored. In this study we have embarked on a comparison of the efficacy of power-modulated pulsed (PMP) sonochemistry with more traditional time-modulated pulsing. As a prototypical reaction, we have explored the effects of pulse type on the degradation of acid orange, a common industrial colorant. An increase in the degradation rate by a factor of three was observed using PMP ultrasound compared with continuous irradiation under conditions of equivalent acoustic input power, while the use of time-modulated pulsed ultrasound from a commercially available direct-immersion (DI) horn-type sonicator exhibited a rate decrease compared to CW sonication. Possible mechanisms for the enhancement are discussed.