Environmental Engineering Reference
In-Depth Information
200
(a)
(b)
y = 0.0009x
2
-
0.2903x + 32
R
2
= 0.996
1800
y = 0.00011x
2
+ 0.0712x-0.7
R
2
= 0.999
1500
150
1200
100
900
600
50
300
0
0
0
200
400
600
800
0
300
600
900
1200
1500
1800
100
100
(c)
(d)
y = -0.0002x
2
+ 0.2011x-5.7
R
2
= 0.989
y = 0.0002x
2
+ 0.088x-2.5
R
2
= 0.995
75
75
50
50
25
25
0
0
0
100
200
300
400
500
0
100
200
300
400
500
H
2
O
2
production (nM)
•
Fig. 3
Relationship between H
2
O
2
and HO
in situ produced from river waters and standard
organic substance during the 10 h of irradiation period in photoexperiments conducted using a
solar simulator. The relationships of the (
a
,
b
,
c
and
d
) are the same samples of Fig.
1
.
Data
source
Mostofa KMG and Sakugawa H (unpublished data)
•
-
is protonated to form HO
•
At pH < 12 in aqueous solution, O
:
k
3.5
k
−
3.5
O
•−
+
H
2
O
HO
•
+
HO
−
(3.5)
•
where
k
3.5
=
1.7
×
10
6
M
−
1
s
−
1
for the HO
formation reaction and
k
−
3.5
=
1.2
×
10
10
M
−
1
s
−
1
for the reverse reaction. The radical HO
•
can significantly recombine
•
and NO
2
—
; such reactions are very fast (diffusion-controlled) in aqueous
media (Mack and Bolton
1999
):
with NO
(3.6)
HO
•
+
NO
•
→
HNO
2
where
k
3.6
=
1.0
×
10
6
M
−
1
s
−
1
.
HO
·
+
NO
2
→
NO
2
·
+
OH
−
(3.7)
where
k
3.7
=
1.0
×
10
10
M
−
1
s
−
1
. These reactions can limit the steady-state con-
centration of HO
•
and, therefore, the ability of the hydroxyl radical to take part in
photooxidation reactions of organic compounds in natural waters. Note, however,
that the main HO
•
scavengers are DOM in freshwater and bromide in seawater
(Takeda et al.
2004
).
