The Effect of Ultrasound Alone

After an irradiation time of 300 min, (500 kHz, 30 W, 20 °C) the hydroxylated intermediates disappear and the chlorine atoms are completely mineralised to chloride ions. The concentrations of the final products (CO and CO2) rise slowly and after 400 min they represent 21 % when starting with cMorophenol and 18 % when starting with phenol. These results are in agreement with previous studies in which it was demonstrated that the sonochemical reactivity of an organic compound is related to its vapor pressure and hydrophobicity. [26,27]. These studies have been extended to the destruction of a number of chloroaromatics [25] (Tab. 4.3). 

In general chloroaromatic hydrocarbons decompose faster than the more hydrophilic compounds such as phenols. The destruction of these two kinds of pollutants occurs at different sites in or around the cavitation bubble and follows different pathways. The difference becomes even more pronounced when more volatile contaminants are used. 

Volume treated 250cm, ultrasonic power 30 W, electric power SOW, frequency 500kHz. 

The occurrence of an optimum frequency at 200 kHz was explained through a two step reaction pathway. In the first step water sonolysis produces radicals within the bubble. In step two the radicals must migrate to the bubble interface or into the bulk aqueous medium to form peroxide or react with the phenolic substrate. The authors suggest that the lower frequencies are the most efficient for the decomposition of molecules inside the bubble but a proportion of the radicals recombine inside the bubble at high temperature to form water thereby reducing the overall yield of H2O2 (Eqs.4.1 and 4.2). 

As the frequency increases the pulsation and collapse of the bubble occurs more rapidly and more radicals escape from the bubble. However as the frequency increases the cavitation intensity decreases and this reduces the yield of radicals and consequently the number which reach the interface and bulk solution. 

---