The Sonication of Water

The choice of the solvent has a profound influence over the observed sonochemistry as well. The effect of vapor pressure has already been mentioned. Other liquid properties, such as surface tension and viscosity, will alter the threshold of cavitation (8), but this is generally a minor concern. The chemical reactivity of the solvent is often much more important. As discussed below, aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone. No solvent is inert under the high temperature conditions of cavitation even linear alkanes will undergo pyrolytic-like cracking during high intensity sonication (89). One may minimize this... 

Most investigations of sonochemistry for the purposes of water pollution control are performed with reagent grade water, which does not closely mimic environmental matrices. Natural waters and most industrial process streams contain solid impurities. Therefore, it is necessary to assess the viability of sonication in heterogeneous systems. 

A synthetically important N-acylation reaction is the formation of Boc protected amines [129]. A rapid and simple method has been reported using sonication which allows this reaction to be effected in the absence of water using solid sodium hydrogen carbonate as the base in a methanol suspension (Eq. 3.29). 

Cavitation induced in any liquid system will result in the formation of radicals (see Section4.2.2). In the case of water sonication one chemical product is hydrogen peroxide which, together with the radical species provides a powerful bactericide and oxidant. 

Studies of the combined process involving ultrasound and ozone have shown faster degradation rates for a range of chemical contaminants than either method applied alone. Sonolytic ozonation has also been found effective for the disinfection of water but in these cases sonication also has a number of direct effects on the bacteria and viruses (see above). 

Besides small gas bubbles, other nucleation sites (e.g., at minute dust particles) may give rise to the cavitation phenomenon. Normally, the surface tension of water is too high to allow the formation of water vapor bubbles at the relatively small negative pressures created by the sonic field. However, at the surface of the dust particles the surface tension of water may be sufficiently low to create a water vapor bubble in the sonic field and thus start the cavitation process. 

The high temperatures and pressures produced lead to the formation of free radicals and other compounds thus, the sonication of pure water causes its thermal dissociation into H atoms and OH radicals, the latter forming hydrogen peroxide by recombination [9,11,12]. Table 3.1 shows the main reactions occurring in water irradiated with ultrasounds. If the water contains some salt such as potassium iodide, iodine free radicals are also released in addition to the previous species [4]. 

McLean and Mortimer [187] have studied the variations in HO free radical production during the sonication of aqueous solutions at different powers at 970 kHz. A typical curve is given in Figure 36. From this it is clear that a threshold exists for radical production, after which there is a linear correlation with acoustic power up to a limiting value which probably corresponds with surface cavitation . Acoustic power was calibrated with a radiation balance and a PVDF hydrophone. Repeatability on experiments performed on the same day was less than 15%, but day-to-day variations could be as much as 50%, probably mainly due to small uncontrolled changes in the alignment of the reaction chamber (a test tube dipped in a water tank) with the ultrasonic source which was an acoustic horn. 

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