Cavitation Chemistry of Polychlorinated Biphenyls:  Decomposition Mechanisms and Rates

Abstract

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 μM), 4-PCB (5.4 μM), and 2,4,5-PCB (7.6 × 10-2 μM) were 2.1 × 10-3 ± 2.8 × 10 -5 s-1, 1.6 × 10-3 ± 3.4 × 10-5 s-1, and 2.6 × 10-3 ± 9.4 × 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.

Sonochemical Treatment of Water Polluted by Chlorinated Organocompounds. A Review

“…In addition, specific studies, such as the evolution of the pH [39] to acidic values, or the correlation of the molecule properties (vapor pressure) with the kinetics of the process and degradation mechanism [13,40,41,42,43] have been reported. Not only aliphatic compounds but also chlorinated aromatic compounds such as chlorotoluenes [ 44 ], chlorobenzenes [ 45 , 46 , 47 , 48 , 49 ] mixtures of chlorobenzene with trichloroethylene [ 50 , 51 , 52 ], p-chlorobenzoic acid [ 53 ], PCBs [ 54 ], chlorophenols [3,35,48,55,56,57,58,59,60,61] and chlorinated pesticides [58,62,63] have also been the subjects of study, providing a wide range of examples from volatile, nonvolatile, hydrophobic and hydrophilic compounds and their respective behaviors. In the following, the general trends of the operational variables reported in literature and those specifically observed for chlorinated compounds are presented.…”

“…Both the intensity and the emission spectra of sonoluminescence are altered when cavitation is induced at different ultrasonic frequencies. The mechanism by which ultrasonic frequency enhances reaction rates has not been definitively established [54] and normally the explanations found in literature are partial and specifically related to the rate constants of OH · production and the formation of H 2 O 2 by coupling of OH · radicals [64,71]. A critical qualitative analysis on the influence of the frequency has been proposed by Hua and Hoffmann [72].…”

“…However, no controversy has been found in the case of nonvolatile compounds and the increase of the degradation rate with frequency has been routinely reported for PCBs [54], 4-chlorophenol [55], pentachlorophenol [58], trichlorophenol [61] or alachlor [62] and, if the range of studied frequencies reaches values higher than 1 MHz, a maximum around 350 kHz has also been detected. Even so, different tendencies have been reported for degradation efficiency: on one hand, slight independence for PCBs, i.e., no tendency [54] and, on the other hand, a maximum around 360 kHz in the case of the trichlorophenol degradation [61].…”

“…Similar behavior was reported for nonvolatile compounds. For example, Zhang and Hua [54] also reported that the rate of degradation of PCBs at 20 kHz was accelerated at higher sound intensities. Gondrexon et al [59] reported, for a continuous flow ultrasonic reactor, an increase of the rate conversion of pentachlorophenol with increasing power input, reaching values of rate conversion close to 80%.…”

“…As a conclusion, the authors reported that vapor pressure was found to be the most important parameter affecting the sonodegradation kinetics and efficiency, while the bond dissociation energy and the hydrophilic/hydrophobic characteristics of the trihalomethanes compounds were found to be of secondary importance, providing a specific difference between volatile and hydrophobic features. Other chlorinated organocompounds, now non-volatile such as p-chlorobenzoic acid [53], or even highly hydrophobic and non-volatile such as PCBs, have also been studied [54]. Chlorophenols have also been extensively studied [56,57].…”