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Advanced Oxidation Technologies for Wastewater Treatment: An Overview 157

This exposes pollutant molecules to the cavitating conditions for
a longer time. The divergent angle should be optimized for the desired
cavitational intensity because a venturi with a larger divergent angle
will behave as an orifice plate and will not enhance cavity growth.
An optimum divergent angle should be between 10 and 15
(Bashir et al., 2011).

3.2.2.2.3 Effect of Physicochemical Properties of Liquid
and Operating pH
Liquid phase properties is one of the very important aspect that affect cavita-
tional processes, although the magnitude of the effect of all the liquid prop-
erties may not be the same. Most of the liquid properties affect cavitation in
more than one way. For example, while an increase in the surface tension of
the liquid increases the threshold pressure for cavitation, making the gener-
ation of cavities more difficult, the collapse of cavities is more violent. The
opposing effects of liquid properties give ample scope for optimization. It
should also be noted that the physicochemical properties of the liquid also
decide the initial size of the nuclei, and the effect of the initial radius must
also be considered when choosing a particular liquid medium and the pro-
cess conditions. The state of the molecules, i.e., hydrophobic or hydrophilic,
also plays an important role in the degradation of pollutant using HC. It has
been stated that hydrophobic and more volatile compounds are more easily
degraded through HC when compared to hydrophilic compounds. This is
due to the fact that hydrophobic and volatile compounds can easily enter the
gas-water interface region of cavities because of their hydrophobic nature.
•
Thus, these compounds are more readily subjected to the OH radical attack
and also to thermal decomposition. Thus, the overall decomposition of pol-
lutant molecules is attributed to the pyrolysis and free radical attack occur-
ring at both the cavity-water interface and in the bulk liquid medium.
The hydrophilic compounds remain in the bulk liquid and thus can only
•
degrade through the OH radicals reaching the bulk solution, and only about
10% of total generated OH radicals diffuse into the bulk liquid medium and
•
rest are recombined to form H 2 O 2 . Thus, the concentration of OH radicals
remains low in the bulk liquid medium, thereby giving a lower degradation
rate for pollutant molecules. The solution pH and the presence of ionic spe-
cies can alter the state of the molecules and their effect depends on the type of
pollutant present in the solution. Therefore, experimental studies need to be
conducted at the laboratory level to establish the effect of solution pH and
ionic species on the degradation of pollutants to be treated using HC.