Page 203 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Oxidation Technologies for Wastewater Treatment: An Overview 177

•
Fe 2+ +H 2 O 2 ! Fe 3+ +OH + OH (3.44)

The resulting Fe 3+ ions can react with H 2 O 2 (Equation 3.45) to produce
2+
the intermediate complex (Fe-OOH ), which can easily get converted into
•
Fe 2+ and HO under cavitating conditions (Equation 3.46)(Merouani et al.,
2
2010; Pang et al., 2011).
Fe 3+ +H 2 O 2 ! Fe OOH 2+ +H + (3.45)

cavitation
Fe OOH 2+ !Fe 2+ +HO • (3.46)
2
2+
Generated Fe ions can again react with H 2 O 2 to generate even more
•
OH radicals. In addition to this, some part of H 2 O 2 directly decomposes to
hydroxyl radicals in the presence of cavitation (as explained earlier) as shown
in the following equation.
cavitation • •
H 2 O 2 ! OH + OH (3.47)
Thus, the combination of cavitation and Fenton process accelerates the
rate of generation of hydroxyl radicals and the degradation of organic pol-
lutants. Various reports suggest that the efficiency of the Fenton process
can be enhanced significantly with the use of cavitation. Wang et al.
(2008) have shown that the degradation rate of reactive brilliant red
K-BP is substantially enhanced by the addition of Fenton reagent to the
ultrasound system. The degradation rate through the Fenton reaction was
increased by almost sixfold when it was used in combination with ultra-
sound, and the synergistic coefficient of 2.27 suggest that combination of
Fenton and cavitation gives higher efficiency than when used individually.
Similar trends have been reported in the literature for the degradation of
some other organic pollutants by applying a combination of HC and Fenton.
Joshi and Gogate (2012) have investigated the degradation of dichlorvos
using a combination of HC and the Fenton process and have reported that
the rate of degradation of dichlorvos increases significantly with increasing
2+
concentrations of Fe , giving a maximum degradation of 91.5% at 3:1
loading of FeSO 4 :H 2 O 2 .
The efficiency of this combined process is very much dependent on the
•
solution pH, rate of consumption of generated OH radicals by the pollutant
molecules, presence of other ionic species, loading of Fe ions and H 2 O 2 , and
cavitational intensity. The effect of all these parameters on the degradation
efficiency has to be studied on a laboratory scale for the case of specific
effluents.