Carbon tetrachloride (CCl4) sonochemistry: A comprehensive mechanistic and kinetics analysis elucidating how CCl4 pyrolysis improves the sonolytic degradation of nonvolatile organic contaminants

In spite of the great number of experimental works conducted on CCl4 sono-degradation, several controversies are observed, these including the main species generated upon CCl4 decomposition and the mechanism by which CCl4 accelerated the sonolytic removal of nonvolatile contaminants. In this paper, a new mechanism of carbon tetrachloride pyrolysis inside an argon bubble is proposed. In this model, the effects of heat exchange, mass transport and the chemical reactions heat on the bubble dynamics are taken into account. This model was used to explain the formation of the different species created by the pyrolytic decomposition of CCl4 inside the bubble at 355 kHz for various acoustic intensities (In = 0.7, 1 and 1.5 W/cm(2)), where the whole range of CCl4 concentration in the bulk solution is covered (from 0 to 5.2 x 10(-3) mol/L, at 20 degrees C). It has been found that the increase of CCl4 mole fraction inside the bubble decreases its maximal temperature. However, the chemical bubble yield increases proportionally with the increase of CCl4 mole fraction. It has been demonstrated that CCl4 enhances the sonolytic degradation of nonvolatile contaminants through the formation of reactive chlorine species (RCS: (CCl3)-C-center dot, :CCl2 and Cl-center dot and HOCl) coupled with an increase of (OH)-O-center dot radicals yield when sonolysis is performed in the presence of CCl4. Besides, the contributing effect of (OH)-O-center dot radicals and HOCl is lower than that of the other RCS. For the acoustic intensity of 1.5 W/cm(2) and CCl4 mole fractions within the bubble lower than similar to 0.007 (CCl4 concentration <= 10(-4) mol/L in the bulk solution), the amounts of RCS and (OH)-O-center dot radicals are approximately in the same range. However, for higher CCl4 mole fractions (>0.007), the production of RCS becomes dominant. On the other hand, for the acoustic intensity of 1 W/cm(2), the yield of RCS is dominant whatever the initial mole fraction of CCl4 inside the bubble. It was found that the decrease of the solution pH is due to HCl and HOCl formation inside the bubble, where hydrochloric acid is produced in important quantity compared to hypochlorous acid for both cases 1 and 1.5 W/cm(2).

Automated summary

This paper proposes a new mechanism of carbon tetrachloride pyrolysis inside an argon bubble, taking into account factors such as heat exchange, mass transport, and chemical reactions heat. It has been found that CCl4 enhances the sonolytic degradation of nonvolatile contaminants through the formation of reactive chlorine species and an increase of (OH)-O-center dot radicals yield. The study also shows that the increase of CCl4 mole fraction inside the bubble decreases its maximal temperature but increases the chemical bubble yield.