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

Abstract

• A new mechanism of carbon tetrachloride pyrolysis inside an argon bubble is proposed. • The chemical bubble yield increases proportionally with the increase of CCl 4 mole fraction. • CCl 4 pyrolysis enhanced the formation reactive chlorine species (RCS) and OH radicals. • The contribution of RCS and OH depends on acoustic intensity. • For 1.5 W/cm 2 and %CCl 4 < 0.007, the amounts of RCS and OH radicals are similar. • For %CCl 4 > 0.007, the production of RCS becomes dominant. In spite of the great number of experimental works conducted on CCl 4 sono-degradation, several controversies are observed, these including the main species generated upon CCl 4 decomposition and the mechanism by which CCl 4 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 CCl 4 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 CCl 4 concentration in the bulk solution is covered (from 0 to 5.2 × 10 −3 mol/L, at 20 °C). It has been found that the increase of CCl 4 mole fraction inside the bubble decreases its maximal temperature. However, the chemical bubble yield increases proportionally with the increase of CCl 4 mole fraction. It has been demonstrated that CCl 4 enhances the sonolytic degradation of nonvolatile contaminants through the formation of reactive chlorine species (RCS: CCl 3 , :CCl 2 and Cl and HOCl) coupled with an increase of OH radicals yield when sonolysis is performed in the presence of CCl 4 . Besides, the contributing effect of OH radicals and HOCl is lower than that of the other RCS. For the acoustic intensity of 1.5 W/cm 2 and CCl 4 mole fractions within the bubble lower than ~0.007 (CCl 4 concentration ≤10 −4 mol/L in the bulk solution), the amounts of RCS and OH radicals are approximately in the same range. However, for higher CCl 4 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 CCl 4 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 .