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

Although most papers in the field of sonochemical degradation of volatile o rganics in aqueous media describe experiments at the millimolar concentrati on 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 concentrat ions. For example, the pseudo-first-order reaction rate constant of CB incr eases by a factor of 14.3, if the initial concentration drops from 3440 to 1 muM. Previous work in the millimolar range has shown that the degradation of these volatiles is mainly due to pyrolytic reactions. The enhancement o f the reaction kinetics at lower concentrations, in this work, could no lon ger 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 degr adation, next to pyrolysis. The model, fitting the experimental results, il lustrated that at micromolar concentrations the OH radical induced degradat ion becomes significant. Simulations showed that at initial concentrations of CB > 1000 muM degradation is due to pyrolysis for over 99.97%, but it wa s also demonstrated that at concentrations between 1 and 5 muM, the OH radi cal mechanism contributed 48.5% of the total degradation. (C) 2001 Elsevier Science B.V. All rights reserved.