Environmental Engineering Reference
In-Depth Information
Fe
2
+
+
HO
•
→
Fe
3
+
(3.13)
+
HO
−
(3.14)
HO
•
+
H
2
O
2
→
HO
2
•
+
H
2
O
Fe
2
+
+
HO
2
•
→
Fe
3
+
(3.15)
+
HO
2
•−
Fe
3
+
+
O
2
−
→
Fe
2
+
+
O
2
(3.16)
Recently, the reaction rate constants of the Fenton system have been measured
at various pH values, an issue that will be discussed later (Kwan and Voelker
2002
;
Duesterberg et al.
2008
). At low [H
2
O
2
]/[Fe
2
+
] ratios in both reactions (
3.12
) and
(
3.13
), only the reactions are important, and the overall process is second-order with
respect to the reactants. At high [H
2
O
2
]/[Fe
2
+
] ratios, there is also an important contri-
bution from the competitive reactions (
3.15
,
3.16
) (Rush and Bielski
1985
). The ear-
lier studies have examined the Fenton reaction at low pH (<1.0) with the reactants at
mM levels. The production of HO
•
from Fe
2
+
+
H
2
O
2
is also important in AOTs, as
it allows the use of transition metals to catalyze the oxidation of organic compounds.
Recent studies demonstrate that the formation of HO
•
increases linearly with
the H
2
O
2
concentration (Lindsey and Tarr
2000
). Experiments carried out using
ESR spin trapping together with water labeled with
17
O suggest that HO
•
is derived
exclusively from hydrogen peroxide, and that there is no exchange of oxygen atoms
between H
2
O
2
and the water solvent (Lloyd et al.
1997
). It has been demonstrated
that fulvic and humic acid reduce the HO
•
formation in the Fenton reaction under
most conditions, but fulvic acid increases HO
•
formation at certain pH values
(Lindsey and Tarr
2000
; Voelker and Sulzberger
1996
). Fulvic acid can inhibit the
degradation of dissolved aromatic compounds (e.g. phenol, fluorene and phen-
anthrene) by the Fenton reagent in aqueous solution (Lindsey and Tarr
2000
).
Accordingly, natural organic matter could inhibit the remediation of pollutants by the
Fenton process in water and soil environments. However, it has also been shown that
humic acids are able to enhance the degradation of phenol in the second step of the
Fenton process. Indeed, after the reaction between Fe
2
+
and H
2
O
2
is completed, fur-
ther degradation of the organic substrate can be directed by the reduction of Fe(III) to
Fe
2
+
. The latter process is enhanced by humic substances (Vione et al.
2004
).
Indeed, the ferric ion (Fe
3
+
) catalyzes the decomposition of H
2
O
2
into HO
•
and
•
Fe
2
+
(Fe
3
+
+
H
2
O
2
→
Fe
2
+
+
HO
2
+
H
+
) (Barb et al.
1951
; Walling and Weil
1974
; Lee et al.
2003
; Lee and Sedlak
2009
). The rate of the Fenton process is
greatly enhanced when the temperature is raised from 10 to 50 °C, because of the
high activation energy (
≈
126 kJ mol
-1
) of the reaction (Lee et al.
2003
). A high
production of Fe
2
+
causes a correspondingly high production of HO
•
in the reac-
tion system (Eq.
3.12
).
The effective catalytic oxidation of organic compounds by the system Fe
3
+
/
H
2
O
2
is usually limited to the acidic pH region, because of the low solubility
of Fe
3
+
and of the low efficiency of the oxidant generation at neutral pH values
(Lee and Sedlak
2009
). The addition of polyoxometalate ions (POM) greatly
