Page 201 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Oxidation Technologies for Wastewater Treatment: An Overview 175

efficiency of photocatalytic oxidation. On the other hand, HC has the
potential for scale-up and gives a higher cavitational yield if larger volumes
are to be treated as compared to acoustic cavitation. Thus, overall it appears,
that combining hydrodynamic cavitation with photocatalysis would be more
effective as compared to the combination of ultrasound and photocatalysis.

3.5.4 Photo-Fenton (Fenton Process in the Presence
of UV Light)
A combination of H 2 O 2 and UV radiation with Fe(II) or Fe(III) oxalate
ion, the so-called photo-Fenton process, produces more hydroxyl radicals
in comparison to the conventional Fenton method (Fe(II) with H 2 O 2 )or
photolysis, thus promoting the rates of degradation of organic pollutants
(Kusic et al., 2006; Ma et al., 2005; Papic et al., 2009; Song et al.,
2006). It is well known that under acidic conditions some ferrous ions
2+
would exist in the form of Fe(OH) , which can easily get converted into
• 2+
OH radicals and Fe under UV irradiation as indicated in Equa-
tion (3.42). This can establish a cycle of reactions generating additional
hydroxyl radicals and regenerating catalyst because Fe 2+ ions are restored.
•
In addition to this, H 2 O 2 can alsodissociateintomorereactive OH
radicals in the presence of UV light. Hence, in the combined process of
•
Fenton and UV radiation, OH radicals are generated in three ways and
•
hence the concentration of OH radicals in the solution enhances signifi-
•
cantly. The higher concentration of OH radicals means higher chances of
•
pollutant molecules reacting with OH radicals and hence a higher rate of
mineralization of the pollutant molecules. The following are the major
reactions taking place during the photo-Fenton process.
2+ • 3+
Fe +H 2 O 2 ! OH + OH +Fe (3.40)
Fe 3+ +H 2 O ! Fe OHÞ 2+ +H + (3.41)
ð
2+ 2+ • +
ð
Fe OHÞ +UVÞ ! Fe + OH + H (3.42)
ð
•
H 2 O 2 +UVð Þ ! 2 OH (3.43)
Figure 3.11 shows the reaction pathways for the degradation starting
with the primary photo-reduction of the dissolved Fe(III) complexes to
Fe(II) ions followed by the Fenton reaction and the subsequent oxidation
of organic compounds. Additional hydroxyl radicals generated in the first
step also take part in the oxidation reaction. For better efficiency of this pro-
cess, the recombination of the free radicals and scavenging by H 2 O 2 , if pre-
sent in excess, should be minimized.