Mechanical agitation accelerated ultrasonication for wastewater treatment: Sustainable production of hydroxyl radicals

Low efficiency in energy conversion has long been the bottleneck in sonochemistry-based water treatment technologies. In this work, we reported a simple and efficient strategy by introducing mechanical agitation into a low powered ultrasonic system to facilitate the production of cavitation bubbles. The coupled system remarkably intensifies the evolution of reactive oxygen species (ROS) for degradation of refractory organic pollutants. We in-situ monitored the generation of hydroxyl radicals (•OH) by selective scavenging tests and chemical trapping experiments. The operational factors such as rotation speed, gas atmosphere, solution temperature and pH were carefully evaluated for their impacts on the degradation of a plastic microcontaminant, diethyl phthalate (DEP). It was found that the degradation efficiency is closely related to the population of cavitation bubbles in the solution, which was collaboratively governed by the aforementioned factors. A high mechanical agitation speed (600 rpm), great solubility of inert gas atmosphere (Argon), and low reaction temperature (15 ºC) are beneficial to the generation of cavitation bubbles and the associated production of ROS. This work shows a facile strategy to intensify the mechanical energy-to-chemical conversion and provides new mechanistic insights into the ultrasound-based advanced oxidation without external chemical inputs.