Ultrasonic degradation of trichloroethylene and chlorobenzene at micromolar concentrations: kinetics and modelling

Abstract

Although most papers in the field of sonochemical degradation of volatile organics in aqueous media describe experiments at the millimolar concentration range, this study focuses on the degradation kinetics of chlorobenzene (CB) and trichloroethylene (TCE) in the micromolar range. It was found that the reaction kinetics increase with decreasing initial substrate concentrations. For example, the pseudo-first-order reaction rate constant of CB increases by a factor of 14.3, if the initial concentration drops from 3440 to 1 microM. Previous work in the millimolar range has shown that the degradation of these volatiles is mainly due to pyrolytic reactions. The enhancement of the reaction kinetics at lower concentrations, in this work, could no longer be explained by this mechanism, even by taking into account the effect of the concentration of the solutes on the reaction temperature. Therefore, a new model was developed, incorporating gas phase OH radical induced degradation, next to pyrolysis. The model, fitting the experimental results, illustrated that at micromolar concentrations the OH radical induced degradation becomes significant. Simulations showed that at initial concentrations of CB > 1000 microM degradation is due to pyrolysis for over 99.97%, but it was also demonstrated that at concentrations between 1 and 5 microM, the OH radical mechanism contributed 48.5% of the total degradation.

Sonochemical Formation of Hydrogen Peroxide

“…The extreme temperature conditions generated by a collapsing bubble can also lead to the formation of radical chemical species. The radicals formed in this reaction are highly reactive and interact rapidly with other radical or chemical species in solution [6]. …”

Sonochemical Treatment of Water Polluted by Chlorinated Organocompounds. A Review

“…From the results, it was stated that the influence on the degradation rate cannot be explained considering only the effect of the average specific heat ratio , but the interaction between the different intermediates was proposed as an additional factor in the mechanism degradation. Further implications were found in the M range, where the effect of initial concentrations on the specific heat ratio was not sufficient to interpret the experimental results, which showed that the degradation rate was faster than predicted from pyrolysis kinetics [51], therefore suggesting additional removal mechanisms. The latter may originate from an OH · radical induced degradation pathway.…”

“…Moreover, the same authors, working with solution of trichloroethylene and chlorobenzene, have distinguished between ranges of concentrations. At mM scale, no influence of the concentration was detected, but at M range they reported an influence as a consequence of a relevant role of the radical induced pathway in addition to the pyrolysis mechanism [50,51]. Another possible explanation for this behavior was reported by Hoffmann et al [31], establishing differences between the reaction sites.…”

“…Hydrophobic and volatile chlorinated organocompounds such as CCl 4 [13,30,31], chloroform [13], trihalomethanes [43] chlorobenzene [45,46,47,49,50,51] and trichloroethylene [38,50,51] have been subject to kinetic analysis and their degradation has been used to develop simulation models of kinetics for compounds which are degraded in the interior of cavitation bubbles by thermolysis (pyrolysis) and, in presence of oxygen, by oxidation reactions. Pyrolysis reactions are expected to follow simple first-order kinetics [44].…”

“…Indeed, Drijvers et al analyzed the mechanism of a mixture of chlorinated compounds in the mM concentration range [50] and Dewulf et al in the M concentration range [51]. In the mM range, a change from the first-order reaction rate to zero-order was found, when a second chlorinated compound was added to a single compound solution.…”