Environmental Engineering Reference
In-Depth Information
Table 1
Oxidation potentials of major oxidants
Free radicals
Oxidation potentials (E°)
(V)
Fluorine
3.03
Hydroxyl radical
2.80
Atomic oxygen
2.42
Ozone
2.07
Hydrogen peroxide
1.78
Perhydroxyl radical
1.7
Permanganate
1.68
Chlorine dioxide
1.57
Hypochlorous acid
1.49
Chlorine
1.36
Data source
Sun et al. (
1997
)
(1.36 V) whilst one (Table
1
) (Sun et al.
1997
). The oxidizing capacity of the
hydroxyl radical can be described in terms of its reduction potential (E), which
allows the comparison with other powerful oxidants (Buettner and Jurkiewicz
1996
;
Buettner
1993
; Ross et al.
1994
). One-electron reduction potentials at pH 7.0 for
selected radical couples are 2.31 V for HO
•
, H
+
/H
2
O; 1.60 V for RO
•
, H
+
/ROH
(aliphatic alkoxyl radical); 1.00 V for ROO
•
, H
+
/ROOH (alkyl peroxyl radical);
0.92 V for GS
•
/GS
-
(glutathione); 0.60 V for PUFA
•
, H
+
/PUFA-H) (
bis
-allylic-H
)
;
0.59 V for HU
•-
, H
+
/UH
2-
(urate); 0.48 V for TO
•
, H
+
/TOH (tocopherol); 0.32 V
for H
2
O
2
, H
+
/H
2
O, HO
•
; 0.28 V for ascorbate
•-
, H
+
/ascorbate monoanion; 0.12 V
for Fe(III)EDTA/Fe(II)EDTA; and
−
3.30 V for O
2
/O
2
•
(Buettner and Jurkiewicz
1996
; Buettner
1993
). The HO
•
reacts with organic compounds at close to diffusion-
limited rate constants, which are the fastest after equilibrium reactions and the rate
constants (
k
obs
) for the reaction of the equilibrium mixture of ascorbic acid spe-
cies (AscH
2
/AscH
-
/Asc
2-
at pH 7.4) are 1.1
×
10
10
M
-1
s
-1
for HO
•
; 1.6
×
10
9
M
-1
s
-1
for tert-Butyl alkoxyl radical (RO
•
); 1-2
×
10
6
M
-1
s
-1
for Alkyl peroxyl
radical, e.g. CH
3
OO
•
(ROO
•
); 1.8
×
10
8
M
-1
s
-1
for ClCOO
•
; 6
×
10
8
M
-1
s
-1
for
glutathiol radical (GS
•
); 1
×
10
6
M
-1
s
-1
for urate radical (HU
•-
); 2
×
10
5
M
-1
s
-1
for tocopheroxyl radical (TO
•
); 2
×
10
5
M
-1
s
-1
for dismutation (Asc
•-
); 1.4
×
10
9
M
-1
s
-1
for chlorpromazine radical action (CPZ
+
•
);
≈
10
2
M
-1
s
-1
for Fe
III
-EDTA/
Fe
II
-EDTA; and 1
×
10
5
M
-1
s
-1
for O
2
•-
/HO
2
•
(Buettner and Jurkiewicz
1996
;
Buettner
1988
; Ross et al.
1994
). The HO
•
radical is formed by a variety of sources
such as NO
2
-
and NO
3
-
under UV irradiation, the Fenton and the photo-Fenton reac-
tion, the photo-ferrioxalate/H
2
O
2
system and so on (Legrini et al.
1993
). It is directly
responsible for a number of important biogeochemical functions in natural waters.
Among other radical species present in natural waters, organic peroxy radicals
(ROO
•
) are intermediates formed photolytically and thermally from organic perox-
ides, or directly from the degradation of dissolved organic matter. These radicals
are short-lived and highly reactive transients. An important process that involves
ROO
•
is the formation of new organic compounds upon rapid combination of
