Shock-induced cavitation as a way of accelerating phenol oxidation in aqueous media

Abstract

Graphical Abstract2 Highlights  Cavitation induced by the impact of a solid piston on liquid surface  Shock-waves of several bars amplitude exciting a wide frequency range  Phenol degradation initiated at ambient temperature and absence of oxidants  High phenol mineralization extents measured upon H2O2 addition  Accelerated active radical formation in the presence of shock-induced cavitation AbstractShock-induced cavitation phenomena, resulting from the propagation of the trail of shockwaves generated upon the impact of a solid piston on a liquid surface, was used as an innovative way of intensifying the oxidation of phenol in aqueous media. The amount of energy communicated by the impact was found to be proportional to the impact height, and was directly related to the maximal pressure attained, i.e. in the order of several tens of bars. This peak was followed by a rarefaction period that results in the origination of several cavitation phenomena, which continued upon piston rebound and further shock-wave generation and propagation. The multi-frequency nature of shock-induced cavitation, capable of exciting a wide range of frequencies between 1 kHz and 100 kHz, was confirmed by means of wavelet analysis. Under shock-induced cavitation conditions, phenol degradation was found to be possible even in the absence of any oxidizing agent. High extents of mineralization, i.e. 45.3 and 56.2% TOC elimination, 50 mg/L solution, neutral pH, were obtained in the presence of H2O2 (aprox. 10% vol.), pointing to an acceleration of the oxidation reaction based in a faster and more efficient creation of radical species.

Physical and chemical characterization of shock-induced cavitation

“…S2) further evidences a linear relationship between these two parameters. This is in complete agreement with the predictions based on the water hammer model, as observed previously for other similar shock-induced cavitation setups [23,24]. The water hammer phenomenon is indeed the hydraulic shock that occurs when a fluid in motion is suddenly forced either to stop or to modify its direction.…”

“…Hypothesis that shock-induced wave has multiple frequencies was already verified with a wavelet transformation of pressure signals in previous studies [23,24]. In this work, a diagram of frequency distribution was obtained and reveals frequency from 0.1 to 100 kHz limited by used pressure sensor.…”

“…Several modifications were introduced to the studied shock-induced reactor compared to the previously studied setups [23,24]. Therefore, the influence of the different operating parameters and water hammer model was first checked in order to validate that both apparatus have a similar response to the experimental conditions and to anticipate the impact effects.…”

“…To the best of our knowledge only few studies of shock-induced cavitation have been reported so far [21,22]. Bubble formation and collapse upon the impact of a piston on a liquid surface were recently evidenced, as well as its contribution to the acceleration of oxidation reactions [23,24]. The impact of the piston generates a trail of shockwaves that propagates within the bulk of the liquid, inducing pressure variations that result in the formation of cavitation bubbles.…”

“…It was reported recently that shock-induced cavitation can be accurately described by the water hammer model [29,30] (i.e. the Joukowsky equation [31]) and, contrary to ultrasonic cavitation, the shock wave propagated in liquid phase has multiple frequencies [23]. On the other hand, chemical effects of shock-induced cavitation have never been studied up to date.…”