Environmental Engineering Reference
In-Depth Information
various chemical reactions cause the production of H
2
O
2
and ROOH in the gase-
ous and aqueous phases in the atmosphere.
In the gas phase, H
2
O
2
and ROOH compounds are formed through several
chain reactions (Eqs.
3.1
-
3.5
) as a combined effect of solar radiation on organic
substances, nitrogen oxides (NO
x
), and oxygen (O
2
) (Sakugawa et al.
1990
,
1993
;
Zuo and Hoigné
1992
,
1993
). The relevant processes are shown below:
RCHO + h
ν
O
→
2RO
•
(3.1)
+ CO
(
R = H, methyl, alkyl, etc)
(3.2)
NO
3
−
+
HCHO
→
HNO
3
+
HO
2
•
+
CO
HO
2
•
+
RO
2
•
→
H
2
O
2
+
O
2
(3.3)
HO
2
•
+
HO
2
•
→
H
2
O
2
+
O
2
(3.4)
(3.5)
HO
2
•
+
RO
2
•
→
ROOH
+
O
2
In atmospheric waters the formation and decomposition of H
2
O
2
is mechanis-
tically different compared to the gas-phase reactions (Eqs.
3.6
-
3.9
). A general
scheme can be expressed as follows below (Sakugawa et al.
1990
):
HO
2
•
(
aq
) +
O
2
H
2
O
−→
H
2
O
2
(
aq
) +
O
2
+
OH
−
(3.6)
HSO
3
−
+
H
2
O
2
(
aq
) +
H
+
→
SO
4
2
−
+
2H
+
+
H
2
O
(3.7)
H
2
O
2
(
aq
) +
HO
•
(
aq
) →
H
2
O
+
HO
2
•
(
aq
)
(3.8)
H
2
O
2
(
aq
) +
h
ν →
2HO
•
(
aq
)
(3.9)
H
2
O
2
is also formed by photodecomposition of Fe(III) complexes with oxalic,
glyoxalic and pyruvic acids, under the typical acidic conditions that can be found in
atmospheric waters (Zuo and Hoigné
1992
,
1993
; Faust et al.
1993
. A general mech-
anism for the formation of H
2
O
2
via this route is reported below (Eqs.
3.10
-
3.12
)
(Sakugawa et al.
1990
; Kim et al.
2003
):
FE(III)-L
+
H
ν →
FE(II)
+
L
•
(3.10)
(3.11)
L
•
+
O
2
→
O
2
•−
+
OXIDIZED L
O
2
•−
+
2H
+
→
H
2
O
2
+
O
2
(3.12)
In (Eqs.
3.10
,
3.11
), Fe(III)-L is a complex of Fe(III) with an organic ligand,
h
υ
is the energy of a photon, and L
•
is the organic radical of L. Superoxide ion
•
−
(O
2
) is a major intermediate in many O
2
-mediated oxidations, such as the well-
known Haber-Weiss mechanism of iron oxidation (Haber and Weiss
1934
).
In natural waters, the main sources of H
2
O
2
are fulvic acid (FA), humic acid,
tryptophan amino acid, fluorescent whitening agents (DSBP and DAS1) and their
photoproducts, as well as various unknown organic substances belonging to DOM
(Mostofa and Sakugawa
2009
). There is evidence that H
2
O
2
may be a photoprod-
uct of reaction chains involving dissolved organic matter (DOM) components in
