Environmental Engineering Reference
In-Depth Information
TABLE 6.4
Photochemical Transient Species in Natural Waters
Typical Steady-state
Transient Species
Source of Transient
Concentration (mol/L)
Typical Half-life (h)
1
O
2
10
−
14
-10
−
12
DOM
≈
3.5
e
aq
10
−
11
DOM
≈
≈
4.3
OH
•
NO
3
,NO
2
,H
2
O
2
10
−
17
-10
−
15
≈
170
O
−•
2
10
−
8
-10
−
7
DOM
≈
2
Source:
From Zepp, R.G. 1992. In: J.L. Schnoor (ed.),
Fate of Pesticides and Chemicals in the
Environment
, pp. 127-140. NewYork: Wiley.
0.01-0.03 in UV and blue spectra. Superoxide ions and hydrogen peroxide are found
in both lakes and atmospheric moisture (fog and rain). They are longer lived than
other transients and are powerful oxidants for most organics. Algae and other biota
are known to quench the action of superoxide ions.
In general, the kinetics and concentrations of photochemical transients in natural
waters can be understood through the application of the steady-state theory. The
formation of transient (Tr) is via the reaction
S
O
2
−
hv
Tr
(6.121)
with the rate
r
f,Tr
= φ
I
. The transient species can either decay by itself or by reaction
with a solute molecule A:
Tr
k
Tr
−→
P,
(6.122)
k
r
−→
Tr
+
A
P.
The total rate of disappearance of the transient is
r
d,Tr
=
k
r
[
A
][
Tr
]+
k
Tr
[
Tr
]
.
(6.123)
At steady state,
r
d,Tr
=
r
f,Tr
, and hence
φ
I
ss
[
Tr
]
=
k
Tr
.
(6.124)
k
r
[
]+
A
The overall rate of the reaction of A is
k
r
d
[
]
d
t
=
A
φ
I
r
A
=−
[
A
]
.
(6.125)
k
r
[
A
]+
k
Tr
If the concentration of A is sufficiently low, as is the case in most natural systems,
then
k
r
[
]
A
k
Tr
, and hence we have
k
r
k
Tr
φ
k
r
[
r
A
=
[
]=
]
I
A
A
,
(6.126)
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