Environmental Engineering Reference
In-Depth Information
(All rate constants are
in L ∙ mol
-1
∙ s
-1
.)
OH
-
O
3
k
1
= 140
× 10
9
k
2
= 1.6
O
-•
⇔ HO
•
O
2
k
6
≈ 10
10
O
3
OH
O
2
O
3
+ O
2
+ H
2
O
k
5
= 3 × 10
9
O
-•
OH
-
k
7
= 4 × 10
8
CO
2-
OH
•
H
+
×
10
10
O
3
k
3
= 5
×
10
5
k
4
= 1.4
HO
•
HCO
-
OH
-
Inhibition
reactions
O
2
×
10
7
k
8
= 1.5
FIGURE 6.21
Radical chain reaction mechanism for ozone decomposition in pure water.
Inhibition by carbonate alkalinity is also shown. (Reprinted from Staehelin, J. and Hoigne, J.
1985.
Environmental Science and Technology
19, 1206-1213. American Chemical Society.
With permission.)
byStaehelinandHoigne(1985)(Figure6.22).AsoluteMcaninterferewiththeradical
chain reaction involving the ozonide and hydroxyl radical in four different ways: (i)
Solute M can react directly with ozone to produce the ozonide radical and an M
+
ion with a rate constant
k
13
. (ii) Solute M can be oxidized to M
ox
by direct reaction
with O
3
with a rate constant
k
14
. (iii) As discussed in Figure 6.21, the solute M can
react with the hydroxyl radical in two ways. The reaction can lead to the scavenging
of OH
•
as in the case of CO
2
3
or HCO
3
. This step is generally represented by the
formation of the product
Φ
in Figure 6.22 with a rate constant
k
12
. (iv) In some cases,
the solute M reacts with OH
•
to form the peroxo radical ROO
•
, which further adds
O
2
to give M
ox
and re-forming the HO
2
. This is characterized by the rate constants
k
10
and
k
11
in Figure 6.22.
Let us now ascribe to the above reaction scheme (Figure 6.22) a reaction rate law
by using steady-state approximations for O
−
2
and OH
•
species, and consider the loss
of ozone by all possible reaction pathways. The rate of ozone reaction with M directly
is
k
14
[M][O
3
], and the total rate of initiation of chain reactions (i.e., the formation of
O
−•
2
and O
−•
3
)is
k
13
[M][O
3
]+2
k
1
[OH
−
][O
3
]. The rate at which OH
•
is converted to
HO
−
2
is
k
9
[M][OH
•
], and the rate of scavenging of OH
•
byMis
k
12
[M][OH
•
]. The
rate of ozone consumption via chain reactions is given by
k
2
O
•
2
[
d
[
O
3
]
d
t
=
−
O
3
]
.
(6.109)
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