Environmental Engineering Reference
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inhibits the oxidation reaction, which might cause decomposition of H
2
O
2
to oxygen and water, and the recombination of OH radicals. And in addi-
tion to this, higher concentrations of hydrogen peroxide might act as free-
radical scavenger itself [62].
The reason for its being an optimum value can be concluded as follows:
At moderate concentrations, the OH radicals attack the dye molecules,
whereas at excess H
2
O
2
concentration the scavenging of OH radicals may
occur and hence the degradation may decrease [69,81,94].
In addition to this, besides considering the process efficiency, the selec-
tion of an optimum H
2
O
2
concentration is also recommended due to envi-
ronmental aspects and the cost of H
2
O
2
[94,96].
Temperature
Increasing the temperature has a positive effect on the removal of dyes. Fenton's
reaction can be accelerated by raising the temperature, which improves the
generation rate of OH and therefore enhances the removal of dyes [97].
However, as temperatures increase above a certain value (generally
40-50 C), the efficiency of H
2
O
2
utilization decreases. This is due to the
possible decomposition of H
2
O
2
into oxygen and water. As a practical mat-
ter, temperatures between 20 and 40 C are recommended for most appli-
cations of Fenton's reagent [93].
3.2.1.3
Wet Air Oxidation
The wet air oxidation (WAO) process, which was first patented by
Zimmerman over 50 years ago, removes organic compounds in the liquid
phase by oxidizing them using an oxidant such as oxygen or air at high
temperatures (120-300°C) and pressures (0.5-20 MPa) [103,104].
With the help of WAO processes, the organic contaminants dissolved
in water are in turn partially degraded by means of an oxidizing agent
into biodegradable intermediates or mineralized into innocuous inor-
ganic compounds such as carbon dioxide, water and inorganic salts, which
remain in the aqueous phase [105].
In this method, by setting an appropriate temperature and pressure and
injecting determined values of oxidant (air, oxygen, hydrogen peroxide,
ozone, etc.), the oxidation operation of organic matter is performed [106].
The severe operating conditions and high costs sometimes limit its
application in wastewater treatment. Catalytic wet air oxidation (CWAO)
introduces a catalyst to the system of traditional WAO, thus it enables mild
reaction conditions, increases the oxidation capacity of oxidants, improves
the formation of hydroxyl radicals and shortens reaction time, thereby reduc-
ing the investment in operating costs [105,107-109]. The reaction pathway
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