Environmental Engineering Reference
In-Depth Information
in the sedimentary environment. Ngabe, Bidleman, and Falconer (1993)
studied the hydrolysis rate of the
α
-isomer using buffered distilled water
where all other processes including biotransformations and photo-assisted
processes were suppressed. The data obtained were as follows:
k
(min
−1
)
T
(K)
pH
10
−
7
278
9.04
5.01
×
4.19
×
10
−
6
288
9.01
10
−
5
303
9.01
9.74
×
6.45
×
10
−
4
318
9.01
4.86
×
10
−
5
318
7.87
7.52
×
10
−
6
318
7.01
Given that
α
-HCH is resistant to acid attack, use the above data to obtain
(i) the specific neutral and base-catalyzed hydrolysis rate constants, (ii)
the relative contribution of each mechanism, and (iii) the activation energy
for the base-catalyzed reaction.
If a local lake in Baton Rouge, Louisiana, has an average pH of
7.2 and a temperature of 14EC, what is the time required to reduce the
α
-HCH concentration in the lake to 75% of its original value? Neglect any
sedimentation or biotransformation in the lake.
5.19
3
Consider a body of water containing 10
−
4
M of a pollutant A that is in
contactwithairthatcontainshydrogensulfide.ThedissolvedH
2
Sconverts
compoundA to its reduced form according to the equationA
ox
+
H
2
S
→
A
red
with an
E
H
of 0.4V. Given that the total gas
+
aqueous concentration
of hydrogen sulfide is [H
2
S]
T
10
−
2
M and that the p
K
a
of H
2
Sis7at
298 K, obtain the following: (i) the redox potential at a pH of 8.5 (note:
E
H
H
2
S
(
aq
))
=+
0.14V at 298 K and (ii) the fraction of A
ox
converted
at pH 8.5.
5.20
3
Chlorine dioxide (ClO
2
)
is used as an oxidant in wastewater treatment
and drinking water disinfection processes. It is preferred over other oxi-
dants since it produces no chloroform and very few other chloroorganics.
It also has its limitations since one of its reduced forms (chlorite) is a
blood poison at high concentrations. The reaction of ClO
2
with a pol-
lutant A follows the stoichiometric equation: ClO
2
+ α
A
→
products,
with
α =
(
Δ
A
)/(
Δ
ClO
2
)
, and follows the rate law
−
d
[
ClO
2
]
/
d
t
=
k
tot
[ClO
2
]
n
[A
0
]
n
. In a series of experiments Hoigne and Bader (1994) used
[A
0
]
5
[
ClO
2
]todeterminetherateconstantsfortheabovereactionwith
several pollutants. The reaction with H
2
O
2
is of particular importance.
Since H
2
O
2
dissociates with pH changes,
k
tot
=
k
HA
+ β
(k
A
−
−
k
HA
)
,
with HA denoting the undissociated H
2
O
2
and A
−
its dissociated form
HO
2
.
β
is the fraction in the dissociated form. The p
K
a
of H
2
O
2
is 11.7.
10
−
5
L/mol s at 298 K with [H
2
O
2
]inthe
range 0.3-20 mM and pH between 2 and 11. Determine the half-life of a
10
μ
MH
2
O
2
solution at a pH of 8 if the water is disinfected with 1
μ
M
of ClO
2
.
5.21
3
Barryetal.(1994)studiedtheoxidationkineticsofFe(II)(aprevalentmetal
in most natural waters) in an acidic alpine lake. The total rate of oxidation
wasfoundtobeacombinationofthefollowing:(i)homogeneousoxidation
k
HA
=
0.1 L/mol s,
k
A
=
1.3
×
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