Oxygen-induced concurrent ultrasonic degradation of volatile and non-volatile aromatic compounds

Abstract

Acoustic cavitation, induced by ultrasound, can be used to eliminate organic pollutants from water. This type of ultrasonic treatment of polluted water can be grouped with those generally referred to as advanced oxidative processes since it involves hydroxyl radicals. In this case these highly active species are generated from the dissociation of water and oxygen dissociation caused by cavitation bubble collapse. The cavitation induced degradation rates of organic compounds in water are mainly linked to their vapor pressure and solubility and here we will further explore these links by examining the degradation of a mixture of two materials with different physical properties, chlorobenzene and 4-chlorophenol. The results obtained when a dilute solution of a mixture of these compounds saturated with argon is subjected to sonication at 300 kHz, parallels previous observations achieved in an aerated aqueous medium at 500 kHz. The two compounds exhibit sequential degradation with the more volatile chlorobenzene entering the cavitation bubble and being destroyed first. The 4-chlorophenol degradation occurs subsequently only when the chlorobenzene has been completely destroyed. The two compounds exhibit different behavior when sonicated in water saturated with oxygen. Under these conditions the two compounds are degraded simultaneously, a remarkable result for which two explanations can be proposed, both of which are based on the formation of additional OH radical species: • There is a shell of supercritical water that surrounds the residual bubble (hot spot) on the point of collapse and the presence of oxygen could increase the production of OH radicals in this shell. • Additional OH radicals could also be derived from the combustion of chlorobenzene within the cavitation bubble itself. By this mechanism the more volatile component (chlorobenzene) could induce the generation of more OH radicals which then degrade the organic with lower volatility. The ability to produce conditions for the simultaneous elimination of two organic compounds by the use of oxygen is of great importance in the developing field of ultrasonic water treatment.