The Mechanism Of Ultrasound-enhanced Electrochemistry Degradation Of Organic Pollutants With Boron-doped Diamond (BDD) Electrode

Advanced Oxidation Processes (AOPs) are of great interest because of the increasing variety and amount of non-biodegradable pollutants with complex structures and high toxicity, but it is difficult for a single oxidation process to meet the requirement for wastewater treatment in some cases. Hence, the combination of several oxidation processes has been becoming one of the active research fields in recent years.Ultrasound-enhanced electrochemical oxidation is a promising treatment technology. It has been demonstrated that ultrasound can improve electrochemical oxidation rate effectively, but there still exist two problems. First, the key of electrochemical oxidation is electrode. Many electrodes are not suitable for ultrasound-enhanced electrochemical oxidation, or show the enhanced effect of ultrasound. Boron-doped diamond (BDD) electrode has received wide attention in recent investigations for its excellent properties, such as wide electrochemical potential window, high oxygen evolution potential, low background current and high anodic stability, which is a suitable material for water treatment in the future. But whether BDD electrode can work steadily and efficiently with ultrasound has not been proved. Second, degradation on electrode is a complex process. The influences of US on electrochemical oxidation have not been understood clearly up to now, especially lacking of quantitative analysis.In this paper, ultrasound-enhanced electrochemical oxidation is studied from the two points above, which is of great significance on the applications of this combinative process in environmental engineering. Several study results have been gained:(1) The mechanism of ultrasound-enhanced electrochemical oxidation process for phenol (Ph) and phthalic acid (PA) is investigated from the three steps of electrochemical (EC) oxidation including mass transport process, adsorption and desorption, and electrochemical reaction roundly. Ultrasound (US) has remarkable influences on all steps, which cause the enhancement of degradation for both pollutants. Mass transport process can be greatly accelerated by US. The adsorption amount of Ph decreases with desorption of polymer intermediates promoted by US, which do benefits to direct oxidation. For PA, the adsorption amount decreases by US, with no direct oxidation happens. US can efficiently reduce the average electrochemical oxidation energy consumption, and the enhancement on degradation of Ph is more effective, because mass transport process does not affect degradation efficiency, but the difference in electrochemical properties leads to different oxidation pathway.(2) The enhancement of electrochemical oxidation of Ph on BDD and Pt electrodes assisted by US are compared in detail, and BDD electrode is demonstrated a suitable electrode for ultrasound-enhanced electrochemistry oxidation technology with excellent performance. the difference is explained from mass transport, adsorption and desorption, and electrochemical reaction. The enhanced effect on BDD electrode by US is much greater than that on Pt electrode, and the reason for the difference between BDD and Pt electrodes is explained. Diffusion process is found the controlling step of Ph degradation on both electrodes, and US enlarges the diffusion coefficient on BDD electrode by 375%, much higher than 42% on Pt electrode. Without US, both electrodes are inactivated with large adsorption amount of Ph. With US, the surface of BDD electrode is renewed and activated more effectively. The improvement on electrochemical reaction of BDD electrode by US is much more significant. The variety of intermediates produced on BDD electrode is found less than that on Pt electrode. US can increase production and degradation rates of intermediates, and reduce the time to reach the highest concentration and promote their degradation, without changing the variety of intermediates.(3) The mechanism of degradation of benzene on BDD electrode by electrochemical oxidation and ultrasound-enhanced electrochemical oxidation is compared. US can accelerate desorption of intermediates and promote oxidation on electrode surface. US can improve oxidation efficiency of benzene with the high oxidation potential of 2.8 V largely, and the time needed for total degradation is decreased by 49.1%. In electrochemical process, benzene is oxidated into aromatic compounds such as Ph, hydroquinone, resorcin, pyrocatechin and benzoquinone, then comes to maleic acid and oxalic acid, and is mineralized into CO₂ and H₂O finally. In ultrasound-enhanced electrochemical process, ultrasound can promote producing and degradation of these intermediates.