Environmental Engineering Reference
In-Depth Information
The overall rate of production of HBr is
2
k
2
(k
1
/k
5
)
1
/
2
1
/
2
d
[
HBr
]
[
H
2
][
Br
2
]
=
.
(5.100)
d
t
1
+[
(k
4
/k
3
)
·
(
[
HBr
]
/
[
Br
2
]
)
]
Note that this expression gives the correct dependencies at both initial times and later
times. The constants
k
and
k
in the earlier equation can be identified as related to
the individual rate constants.
Chain reactions such as the above are of significance in environmental engineering
and will be frequently encountered in atmospheric (gas-phase) reaction chem-
istry (Seinfeld, 1986), in catalytic reactions in wastewater treatment, heterogeneous
catalysis of atmospheric solution chemistry (Hoffmann, 1990), and in combustion
engineering. An interesting example of a free radical chain reaction discussed by
Hoffmann (1990) is the oxidation of S(IV) by Fe(III), which is a prevalent metal in
atmospheric particles.
Fe
3
+
+
SO
2
−
3
Fe
2
+
+
SO
•
3
−→
.
(5.101)
E
XAMPLE
5.10 C
HAIN
R
EACTION FOR THE
O
XIDATION OF
O
RGANIC
C
OMPOUNDS IN
N
ATURAL
W
ATERS
An example of a chain reaction is the oxidation of organic compounds by peroxides
in water, sediment, and atmospheric environments (Ernestova et al., 1992). Let RH
represent an organic compound and AB an initiator (e.g., hydrogen peroxide, metal
salts, or organic azo compounds). H
2
O
2
is an excellent oxidant in natural water. There
are at least three possible initiation steps in this case:
AB
h
ν
,
T
A
•
+
B
•
,
−−−→
fast
(5.102)
RH
+
A
•
−−→
R
•
+
AH,
RH
+
O
2
−→
R
•
+
HOO
•
.
The initiation can be induced by sunlight, ionizing radiation (cosmic rays for example),
acoustic waves, and temperature fluctuations. Dissolved gases such as ozone can also
initiate free radicals. In shallow water bodies, the principal factor is sunlight. H
2
O
2
has
been recognized as an important component in the self-purification of contaminated
natural waters. It also provides OH radical in the atmosphere that reacts with most
other organics, which leads to increased oxidation, aqueous solubility, and scavenging
of organics. The hydroxyl radical is appropriately termed the
atmosphere's detergent
.
The last reaction given above is an initiation of oxygen in the absence of any AB.
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