Advanced Oxidation Processes for Removal of Dyes from Aqueous Media - Green Chemistry for Dyes Removal from Wastewater - page 90

3.2.1.1.1 Factors Influencing the Ozonation Efficiency
Researchers have demonstrated that the rate of decomposition of ozone
is correlated with initial pH and inlet ozone concentration, reaction time,
temperature, catalyst and H 2 O 2 concentration.
pH
The initial pH reaction is an important parameter influencing the ozona-
tion process [27] since the oxidizing agent generated depends on the pH of
the solution; ozone decomposition in water is strongly pH dependent and
occurs faster with an increase of pH. As mentioned before, degradation of
dyes is either due to direct oxidation of ozone or radical oxidation by OH
radical, and this mainly depends on the pH of the solution. Consequently,
the oxidizing agent is known to be ozone in acidic solution, whereas in
neutral and basic solution it is the hydroxyl radicals [14,29,34,35]. Since the
oxidation potential of OH radicals is higher than that of the ozone mol-
ecule, the decomposition reaction is faster in basic conditions [27,36-39].
Inlet Ozone Concentration
The ozone concentration that is used in the reaction is an important
parameter affecting the extent of oxidation. The effect of the amount of
ozone concentration on decolorization and degradation of different dyes
has been studied extensively by other researchers. They have concluded
that an increase in the inlet ozone concentration considerably increases the
removal efficiency [40,41].
However, it is very important to calculate the optimum ozone concen-
tration before the oxidation process in order to avoid overdosing. Ozone
is a gas that is very unstable in the liquid phase and hence it is difficult to
obtain saturation in the regions where mass transfer is limiting. This results
in a steady state concentration that often is much lower than the saturation
concentration. It is therefore important to carefully determine optimum
ozone concentration. If ozonation is carried out at ambient temperature
and atmospheric pressure, the concentration of ozone in aqueous phase
can be calculated according to Henry's Law [42,43].
Reaction Time
The studies reported in literature have shown that that the reaction time in
the ozonation process has a positive effect on the treatment efficiency. The
researchers observed that removal of both color and COD increased with
an increase in reaction time [44,45].
Temperature
Ozonation reactions are recommended to be carried out at ambient tem-
perature. Even though higher temperatures generally increase the reaction