Cavitation chemistry of polychlorinated biphenyls: Decomposition mechanisms and rates

Polychlorinated biphenyls (PCBs) are pollutants of environmental concern due to their widespread presence and persistence. The sonolytic destruction of aqueous polychlorinated biphenyls (PCBs) is reported in this article. The kinetics and transformation pathways were investigated at multiple frequencies and with various analytical tools. The pseudo-first-order rate constants for 2-PCB (4.6 mu M), 4-PCB (5.4 mu M), and 2,4,5-PCB (7.6 x 10(-2) mu M) were 2.1 x 10(-3) +/- 2.8 x 10(-5) s(-1), 1.6 x 10(-3) +/- 3.4 x 10(-5) s(-1), and 2.6 x 10(-3) +/- 9.4 x 10(-5) s(-1), respectively, when Ar-saturated solutions were sonicated at 20 kHz and an acoustic intensity of 30.8 W cm(-2). Chlorine recovery as chloride ion was 77%, 79%, and 70% for 2-PCB, 4-PCB, and 2,4,5-PCB under the same conditions. Electron spin resonance (ESR) experiments demonstrated that phenyl radical was formed during sonolysis of 2-PCB and 4-PCB. The formation of biphenyl, which was detected by mass spectroscopy, also indicates PCB dechlorination during sonication. Ethyl benzene, diethylbiphenyl, dibutylbiphenyl, phenol, propylphenol, and di-tert-butyl phenol were also detected by mass spectroscopy as products of PCB sonication. The kinetics of 2-PCB composition as well as the role of aqueous hydroxyl radical were examined at 20, 205, 358, 618, and 1071 kHz. The ultrasonic frequency significantly impacted the reaction rate and chloride recovery.