Advanced Oxidation Processes (AOP)—Effective Innovative …
189
by combined AOPs like O 3 /hydrogen peroxide, UV/H 2 O 2 , UV/H 2 O 2 /ozone, etc.
The combination of UV/H 2 O 2 has been discussed earlier in this chapter. To reduce
the cost of H 2 O 2 processes, research is going on to produce the H 2 O 2 prior to the
advanced oxidation process by cathodic generation of H 2 O 2 .
3.5 Fenton Reagent
Fenton, in the year 1894, discovered the Fenton reaction that the activation of
hydrogen peroxide is possible by Fe
2+ salts to oxidize tartaric acid. In late 1960s,
studies were carried out to oxidize the organic pollutants. Fenton process is an efficient advanced oxidation process, involving generation of hydroxyl radical due to
the reaction between hydrogen per oxide and aqueous ferrous ions. The principle of
removal or degradation of the organics is by radical oxidation and flocculation. The
hydroxyl radical generated during the reaction of ferrous ion catalysing the hydrogen
peroxide effectively oxidizes the organics in the effluent, and effective results can be
obtained like generation of CO 2 , water and other inorganic salts, while ferrous ions
generated in the wastewater by Fenton act as a flocculant.
There are two mechanisms suggested for degradation of organic pollutants by
Fenton process [50].
One mechanism considers active hydroxyl species (*OH) produced in Fenton
reaction degrades the organic pollutants.
Second mechanism considers generation of strong oxidizing iron substance like
FeO
2+ and FeO
3+ in Fenton reaction instead of hydroxyl radical generation.
Hydroxyl radical merely involves in initiating the reaction [51].
The Fenton process is normally operated at the pH 3, and the oxidation activity
of hydroxyl radical is dependent on the pH of the solution. In general, the oxidation
potential and the oxidation capacity of the hydroxyl radical increase with decrease in
the pH. The activity of Fenton reagent is less due to increase in the pH because of lack
of Fe
2+ species which results in the formation of ferric hydroxides and iron oxohydroxides which are inactive. Moreover, auto decomposition of hydrogen peroxide
happens at higher pH levels
Fenton is highly considered in wastewater treatment due to its high performance
and simple operating conditions like room temperature and atmospheric pressure. It
is also non-toxic, and the hydrogen peroxide in the wastewater can be broken down
to simple compounds like water and oxygen. Though it has several advantages, the
process involves maintenance of pH, higher consumption of hydrogen peroxide and
formation of ferric sludge that hinders the oxidation during the process. To overcome
these problems, both homogeneous and heterogenous catalysts are used to replace
the Fe
2+ are under current research.
189
by combined AOPs like O 3 /hydrogen peroxide, UV/H 2 O 2 , UV/H 2 O 2 /ozone, etc.
The combination of UV/H 2 O 2 has been discussed earlier in this chapter. To reduce
the cost of H 2 O 2 processes, research is going on to produce the H 2 O 2 prior to the
advanced oxidation process by cathodic generation of H 2 O 2 .
3.5 Fenton Reagent
Fenton, in the year 1894, discovered the Fenton reaction that the activation of
hydrogen peroxide is possible by Fe
2+ salts to oxidize tartaric acid. In late 1960s,
studies were carried out to oxidize the organic pollutants. Fenton process is an efficient advanced oxidation process, involving generation of hydroxyl radical due to
the reaction between hydrogen per oxide and aqueous ferrous ions. The principle of
removal or degradation of the organics is by radical oxidation and flocculation. The
hydroxyl radical generated during the reaction of ferrous ion catalysing the hydrogen
peroxide effectively oxidizes the organics in the effluent, and effective results can be
obtained like generation of CO 2 , water and other inorganic salts, while ferrous ions
generated in the wastewater by Fenton act as a flocculant.
There are two mechanisms suggested for degradation of organic pollutants by
Fenton process [50].
One mechanism considers active hydroxyl species (*OH) produced in Fenton
reaction degrades the organic pollutants.
Second mechanism considers generation of strong oxidizing iron substance like
FeO
2+ and FeO
3+ in Fenton reaction instead of hydroxyl radical generation.
Hydroxyl radical merely involves in initiating the reaction [51].
The Fenton process is normally operated at the pH 3, and the oxidation activity
of hydroxyl radical is dependent on the pH of the solution. In general, the oxidation
potential and the oxidation capacity of the hydroxyl radical increase with decrease in
the pH. The activity of Fenton reagent is less due to increase in the pH because of lack
of Fe
2+ species which results in the formation of ferric hydroxides and iron oxohydroxides which are inactive. Moreover, auto decomposition of hydrogen peroxide
happens at higher pH levels
Fenton is highly considered in wastewater treatment due to its high performance
and simple operating conditions like room temperature and atmospheric pressure. It
is also non-toxic, and the hydrogen peroxide in the wastewater can be broken down
to simple compounds like water and oxygen. Though it has several advantages, the
process involves maintenance of pH, higher consumption of hydrogen peroxide and
formation of ferric sludge that hinders the oxidation during the process. To overcome
these problems, both homogeneous and heterogenous catalysts are used to replace
the Fe
2+ are under current research.