Environmental Engineering Reference
In-Depth Information
Table 7
The reaction rate constants for generation of HO• in Fenton reaction at various pH
ranges in the absence of light
Reaction type
pH 3
pH 4
pH 5
Equations
−
1
s
−
1
)
Reaction rate constant (
k
) (M
•
Fe
(II)
+
H
2
O
2
→
Fe
(III)
+
HO
+
OH
−
1.2
×
10
2
5.7
×
10
2
63
4.18
Fe
(III)
+
H
2
O
2
→
Fe
(II)
+
HO
2
•
−
+
−
3
−
3
−
3
/O
2
+
H
2
×
10
2.5
×
10
2.6
×
10
4.19
•
•
−
3.7
×
10
7
3.3
×
10
7
3.3
×
10
7
H
2
O
2
+
HO
→
HO
2
/O
2
+
H
2
O
4.20
Fe
(III)
+
HO
2
•
−
→
Fe
(II)
+
O
2
+
H
+
7.8
×
10
5
6.8
×
10
6
3.1
×
10
7
/O
2
4.21
Fe
(II)
+
HO
•
→
Fe
(III)
+
OH
−
3.2
×
10
8
3.2
×
10
8
3.2
×
10
8
4.22
Fe
(II)
+
HO
2
•
−
→
Fe
(III)
+
HO
2
•
1.3
×
10
6
2.4
×
10
6
6.6
×
10
6
/O
2
4.23
•
5.2
×
10
9
5.2
×
10
9
5.2
×
10
9
HO•
+
HO
→
H
2
O
2
4.24
•
−
•
−
2.3
×
10
6
1.2
×
10
7
2.3
×
10
7
HO
2
/O
2
+
HO
2
/O
2
→
H
2
O
2
4.25
•
•
−
7.1
×
10
9
7.5
×
10
9
8.9
×
10
9
HO
+
HO
2
/O
2
→
H
2
O
+
O
2
4.26
Data source
(Kwan and Voelker
2002
)
•
•
HO
photoinduced generation from H
2
O
2
is double compared to that of H
2
O
2
photolysis (Hunt and Taube
1952
; Baxendale
and Wilson
1956
; Volman and Chen
1959
).
. Therefore, the quantum yield of HO
4.6 Fenton Reaction: Effect of pH, Temperature and Salinity
The Fenton reaction depends on the presence of Fe
2
+
(or Fe
3
+
) and H
2
O
2
in natu-
ral waters. The oxidation of Fe(II) with H
2
O
2
in seawater depends on pH (2-8.5),
temperature (5-45 °C) and salinity (0-35 g L
-1
) (Wells and Salam
1968
; Moffett
and Zika
1987a
,
b
; Gallard et al.
1998
; Bossmann et al.
1998
; Duesterberg et al.
2008
; Millero and Sotolongo
1989
; de Laat and Gallard
1999
; Duesterberg
and Waite
2006
; Duesterberg et al.
2005
; Farias et al.
2007
; Jung et al.
2009
).
The rate constants of the chain Fenton reactions and the relevant dependence on
pH are presented in Table
7
(Kwan and Voelker
2002
). The reaction rate constant
between Fe
2
+
and H
2
O
2
significantly increases with pH in the range from 3 to 5
(Eq. 4.18; Table
7
), and a similar effect is observed with some of the chain reac-
tions (Eqs. 4.19-4.26; Table
7
).
A recent study that has been carried out in the pH range 2.5-4.0, both in the
presence and absence of a target organic substance (formic acid), also high-
lights the importance of the Fenton system in the catalytic redox cycling of iron
(Duesterberg et al.
2008
). Supply of oxygen can enhance the efficiency of the
Fenton oxidation, which is understandably attenuated by competition with the
organic intermediates in the reaction media (Sychev and Isak
1995
; Duesterberg
et al.
2005
).
It is shown that the addition of phosphotungstate (PW
12
O
40
3
−
), a polyoxo-
metalate, extends the working pH range of the Fenton system (Fe
3
+
/H
2
O
2
) up
