Environmental Engineering Reference
In-Depth Information
for shorter wavelength than for longer one [(4.7-6.1)
×
10
-11
Ms
-1
at 355 nm] in a
variety of lake waters (Table
2
) (Grannas et al.
2006
). Thus, the production of HO
•
greatly depends on the light wavelength.
In seawater, the HO
•
production rate is higher [(4.3-42.0)
×
10
-12
M s
-1
]
in coastal waters than in the open ocean [(2.8-3.1)
×
10
-12
M s
-1
], and it is
very low [(1.04-10.3)
×
10
-13
M s
-1
] in Antarctic waters (Table
2
). The
production rates of HO
•
are lower in surface seawater and gradually increase
with increasing depth (Takeda et al.
2004
; Zafiriou and Dister
1991
). For exam-
ple, the production rate of HO
•
is (5.7-15.0)
×
10
-12
M s
-1
at 0-20 m depth,
4.9
×
10
-11
M s
-1
at 30 m, and 7.2
×
10
-11
M s
-1
at 60 m in Seto Inland Sea
and the Yellow Sea (Takeda et al.
2004
), obviously under the same irradiation
conditions that is not the case of the real environment. The lower production
rates at the surface compared to the deeper layers may be caused by the fact
that the natural solar radiation is active in surface waters where it produces
HO
•
and other reactive transients. These species can lead to the photo-degra-
dation of DOM (e.g. photo-bleaching that reduces the ability of DOM to absorb
sunlight), a process to which the direct photolysis could also contrib-
ute. Therefore, the sources of HO
•
in surface waters can be reduced, e.g. by
decreasing the concentration of NO
2
-
and the capability of DOM to photo-gen-
erate HO
•
. The intensity of solar radiation that reaches the deeper layers is quite
limited. Therefore, the deep water has the double feature of generating the high-
est potentiality to produce HO
•
, but also of being involved to a very limited
•
extent into HO
photo-production in the real natural environment. An impor-
tant exception could be represented by the sites where the deep oceanic water
emerges to the surface.
Studies observe that the sea-salt particulate matter (SS PM) extracted from
coastal seawaters can demonstrate substantially high HO
•
production (rate:
~2778-27778 M s
-1
), approximately 3-4 orders of magnitude greater than HO
•
photoformation rates in surface seawater (Anastasio and Newberg
2007
). The
results show that photolysis of nitrate is a dominant source of HO
•
(on average
59
±
25 %) in the SS PM whilst other source is presumably considered the organic
compounds. The fact behind the other phenomenon is that irradiated organic
compounds or DOM can induce photoinduced production of H
2
O
2
that is a HO
•
source via photolysis or the Fenton reaction, and the photoinduced generation of
H
2
O
2
is enhanced by salinity. Salinity or NaCl solutions are capable of generat-
ing high production of aqueous electrons (e
aq
−
) photolytically in aqueous media
(Gopinathan et al.
1972
; Assel et al.
1998
) that may enhance the H
2
O
2
produc-
tion from DOM components in waters (Mostofa and Sakugawa
2009
; Moore et
al.
1993
) (see also chapter
“
Photoinduced and Microbial Generation of Hydrogen
Peroxide and Organic Peroxides in Natural Waters
”
). In fact, photogeneration of
H
2
O
2
from river DOM was substantially increased with salinity, from 15 to 368
nM h
-1
at circumneutral pH that may enhance the H
2
O
2
production from DOM
components in waters (Osburn et al.
2009
). Salinity effect on irradiated CDOM
might be another most important source of high photoproduction of HO
•
in sea-
salt particulate matter in seawaters.
