Environmental Engineering Reference
In-Depth Information
Petasne and Zika
1997
) may follow two pathways. First, photolytic decomposi-
tion of H
2
O
2
can occur in seawater (e.g., filtered Vineyard Sound waters) to
yield O
2
. The photodecomposition was approximately 5 % of the correspond-
ing photoproduction (Moffett and Zafiriou
1990
). However, H
2
O
2
decomposition
typically does not occur in oligotrophic seawater after 2 h of irradiation. This sug-
gests that the contaminants associated with H
2
O
2
synthesis in Vineyard Sound
samples might be susceptible to the photolytic decomposition of H
2
O
2
(Moffett
and Zafiriou
1990
). Second, H
2
O
2
and ROOH can photolytically form free radi-
cals (R′OOH
+
h
υ
→
RO
•
•
where R′
=
H or R). For example, ROOH
compounds are lower in surface seawater than in the deeper layers (Sakugawa
et al.
2000
). The ROOH compounds are negatively correlated with solar intensity
(Sakugawa et al.
2000
). This suggests that ROOH may be decomposed by pho-
tolytic processes in surface seawater. This result can be justified by the observa-
tion of a significant correlation between H
2
O
2
and HO
′
+
HO
•
generated photolytically
in experiments conducted on river waters, standard Suwannee River Fulvic Acid
and DAS1 using a solar simulator Mostofa KMG and Sakugawa H (unpublished),
which indicates the photoinduced formation of HO
•
from H
2
O
2
. Therefore, decay
of peroxides by photolytic processes is a typical phenomenon that may signifi-
cantly occur in natural waters.
Formation of H
2
O
2
and ROOH by chemical processes may include several
chain-reactions among various reactant species (Eqs. (
3.2
-
3.5
,
3.10
-
3.12
,
3.27
).
The decomposition of peroxides by chemical processes may involve the Fenton
reaction (H
2
O
2
+
Fe
2
+
→
Fe
3
+
+
HO
•
+
OH
−
) (Fenton
1894
), photo-Fenton
reaction (H
2
O
2
+
Fe
2
+
+
h
ν
→
Fe
3
+
+
HO
•
+
OH
−
) (Zepp et al.
1992
), photo-
ferrioxalate reaction (Fe
II
(C
2
O
4
)
+
H
2
O
2
+
h
ν
→
Fe
III
(C
2
O
4
)
+
HO
•
+
OH
−
)
(Safazadeh-Amiri et al.
1997
) and other chain reactions (Eqs.
3.7
,
3.8
,
3.16
). Free
radical oxidation of H
2
O
2
by transition metal ions is one of the most important
chemical decomposition processes of H
2
O
2
in natural waters (Jeong and Yoon
2005
; Fenton
1894
; Millero and Sotolongo
1989
).
4.7 Physical Mixing Processes
The rates of production and decay of peroxides may be influenced by physical pro-
cesses, such as the mixing by strong waves in the surface mixing zone (Mostofa
KMG and Sakugawa H, unpublished; Scully et al.
1998
). Physical mixing by
strong waves can facilitate the contact of the reactants and increase the reaction
rates. For example, the production rate of H
2
O
2
was increased by mechanical stir-
ring during irradiation of seawater (86 nM h
−
1
) and standard Suwannee River
Fulvic Acid (445 nM h
−
1
) samples, compared to the same samples that were not
stirred (51 and 211 nM h
−
1
, respectively). The photoexperiments on site were car-
ried out with a solar simulator Mostofa KMG and Sakugawa H (unpublished).
Mixing phenomena can contribute to the relatively elevated H
2
O
2
concentration
that is often observed in the mixing zone or in the upper surface layers of lake or
