Environmental Engineering Reference
In-Depth Information
a catalyst and an acid as an optimum pH adjuster. This particular reagent
mixture is called Fenton's Reagent.
The main advantages of Fenton's reagent oxidation compared to other
AOPs are its simplicity, the lack of toxicity of the reagents and the cost-
effective source of hydroxyl radicals it offers, since the chemicals are always
available at moderate cost and there is no need for special equipment [61].
The general mechanism using Fenton's reagents, via which the hydroxyl
radicals are produced, is a number of cyclic reactions, which utilize the Fe
2+
or Fe
3+
ions as a catalyst to decompose the H
2
O
2
[62].
There are numerous parallel and consecutive reactions, but are generally
described by the following two dominant reactions [63]:
Fe
2+
+ H
2
O
2
Fe
3+
+ OH + OH
-
(3.5)
Fe
3+
+ H
2
O
2
Fe
2+
+ OOH + H
+
(3.6)
The principal active component of Fenton's reagent is the hydroxyl free
radical which is produced by catalytic chemical reaction (Reaction 3.5)
between hydrogen peroxide and ferrous iron under an optimum pH con-
dition [64,65]. In this reaction, the ferrous ion initiates and catalyses the
decomposition of H
2
O
2
and hydroxyl radicals are generated [63]. In addi-
tion to the oxidation, the iron (III) ions generated during the oxidation
stage promote the removal of other pollutants by coagulation and sedi-
mentation. Reaction 3.6 is known to be several orders of magnitude slower
than Reaction 3.5 [67]. The slow regeneration of Fe
3+
to Fe
2+
is the rate-
determining step of the overall reaction. Thus, in AOP, rate of dye deg-
radation is fast in the beginning due to high initial concentration of Fe
2+
.
However, subsequently the rate is drastically reduced due to the drop in the
concentration of Fe
2+
and poor rate of its regeneration [68].
The Fenton oxidation process can be operated both homogeneously or
heterogeneously under various combinations.
The classical homogeneous Fenton oxidation may be the most exten-
sively studied method. However, there are two major drawbacks of the
process. The process is limited by the acidic pH required (pH = 2-4) and
it needs high amounts of iron ions in the solution, and the iron ions need
to be separated from the system at the end of the reaction, which means an
additional removal process [69-75].
To overcome these disadvantages, increasing attention has been paid
to research on the heterogeneous Fenton systems. The use of heteroge-
neous Fenton and Fenton-like catalysts has recently received much atten-
tion, such as with zeolites,clays and activated carbon (from organic waste)
loaded with the iron is recommended [69,76-80].
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