Hydroxyl Radical Generation and Partitioning in Degradation of Methylene Blue and DEET by Dual-Frequency Ultrasonic Irradiation

摘要

Ultrasonic (US) irradiation is effective for the degradation of a variety of problematic pollutants, and simultaneous US low and high frequencies can lead to improved degradation yields. Hydroxyl radical (center dot OH) is critical to the efficiency of ultrasonic oxidation processes. We monitored center dot OH production and carried out calorimetric measurements during dual-frequency ultrasonic (DFU) irradiation using probe- (20 kHz) and transducer (640 kHz)-type sources. The conditions were optimized based on calorimetric measurements to determine real power density dissipation to the solution for our dual-frequency ultrasonic reactor (DFUR) (20 and 640 kHz) with a power density of 10.1 W center dot mL-1 in which the observed synergistic index reached a maximum of 0.991, whereas it was only 4.1 and 8.1 W center dot mL-1, respectively, for 20 and 640 kHz individually. The production of center dot OH was measured using coumarin (COU) as trapping agent under simultaneous and sequential operation of low and high frequencies under different gas saturating conditions (Ar, O2, and N2). Distribution of center dot OH within the cavitational zones was assessed by comparing center dot OH trapping by hydrophobic COU and ionic terephthalic acid (TA) under DFU. Based on the approximately 6 times more effective trapping of center dot OH by COU compared to TA during DFU irradiation, the majority of center dot OH leading to degradation appears to be generated at the gas-liquid interface. Methylene blue (MB) and N, N-diethyl-meta-toluamide (DEET) were selected as hydrophilic and hydrophobic model target compounds to probe individual degradation zones within the cavitation process during DFU irradiation. The addition of peroxides H2O2 (PO), persulfate (PS), and monoperoxysulfate (MPS) to the DFU reactor had minimal or modest effects on the DFU-induced degradation of target compounds. The results revealed that combining low and high frequency US has a positive effect on enhancing the