Environmental Engineering Reference
In-Depth Information
in addition to this,
OH radical formation is inhibited due to the small
amount of soluble iron (Fe
3+
) at pH below 3 [1,87].
A decrease in efficiency for pH values in alkaline conditions attributed
to the precipitation of iron hydroxide (Fe (OH)
3
) occurs, decreasing the
concentration of dissolved Fe
3+
and consequently of Fe
2+
species [88,89].
In addition, in such conditions hydrogen peroxide is less stable and hence
decomposes into water and oxygen [88,89]; therefore fewer hydroxyl radi-
cals are formed, reducing the process efficiency.
Iron (Fe
2+
) Concentration
It is well known that dye degradation efficiency by Fenton process is influ-
enced by the concentration of Fe
2+
ions, which catalyze hydrogen peroxide
decomposition resulting in OH radical production and hence the degra-
dation of organic molecule. According to the literature, it is recommended
to determine an optimum concentration for iron [62].
In the absence of iron, there is no evidence of hydroxyl radical forma-
tion and hence there is no removal or the removal efficiency is consider-
ably lower.
But as the concentration of iron is increased, dye removal accelerates until
a point is reached where further addition of iron becomes inefficient [62].
When Fe
2+
concentration is increased, the catalytic effect also increases
accordingly, and when its concentration is higher, a great amount of Fe
3+
is
produced. Fe
3+
undergoes a reaction with hydroxyl ions to form Fe (OH)
2+
,
which has a strong absorption band causing higher absorbance and results
in a decrease in OH generation and hence removal efficiency decreases
[91-93].
H
2
O
2
Concentration
In the Fenton process, hydrogen peroxide plays an important role as a source
of hydroxyl radical generation. Generally, it can be concluded that increasing
H
2
O
2
concentration improves the degradation and decolorization of dyes.
Previous studies on Fenton oxidation have highlighted the existence of
an optimum in the H
2
O
2
concentration with respect to achievable rates of
removal.
In many studies performed by Fenton oxidation, the researchers con-
cluded that when a low amount of H
2
O
2
is added, the intermediate com-
pounds of degradation are still present in the reaction mixture even after
enough reaction time.
When a higher amount of H
2
O
2
is used, it will generate more hydroxyl
radicals, which are available for oxidation.
However, when H
2
O
2
is overdosed, this does not improve the degra-
dation efficiency. The excess H
2
O
2
reacts with the hydroxyl radicals and
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