Geoscience Reference
In-Depth Information
Empirical mathematical relationships among
K
ps
,
f
oc
, and
K
ow
have been derived
for various combinations of compounds and natural sorbents:
K
ps
=
f
oc
⋅
K
oc
=
f
oc.
b
⋅
(
K
ow
)
b
(4.72)
Values reported for a and b include a
=
1.00, b
=
0.48
×
10
−6
for polycyclic
aromatic hydrocarbons; a
=
0.52, b
=
4.4
×
10
−6
for a variety of pesticides; and a
=
0.72, b
10
−6
for alkylated and chlorinated benzenes.
120,146,168
From currently
available data, it can be concluded that the values of a and b are primarily determined
by the type of compounds (i.e., the compound class(es)) on which the relationship
was established and only to a much smaller extent by the type of natural sorbents
used. Thus, the reported relationships are deemed reliable for predicting the equi-
librium partition coefficients of a great number of neutral hydrophobic organic
compounds between water and natural sorbents of very different origins.
=
3.2
×
4.2.4.4
Hydrolysis
Reactions catalyzed by oxonium and/or hydroxyl ions are called hydrolysis. The
rate of reaction
R
is given by the equation
R
=
~
dc
/
dt
⋅
K
H
- [
A
]
=
K
A
⋅
[H
+
]
⋅
[
A
]
+
K
B
⋅
[OH
-
]
⋅
[
A
]
+
K
N
⋅
[H
2
O]
⋅
[
A
]
(4.73)
where
K
H
pseudo-first-order rate constant at a given pH
K
A
and
K
B
=
=
second-order rate constants
K
N
=
second-order rate constant for neutral reaction of a chemical compound
with water, which may be expressed as a pseudo-first-order rate constant
This equation indicates that the rate of hydrolysis is strongly dependent on pH,
unless
K
A
and
K
B
are equal to zero.
The kinetic expression for hydrolysis is
dC
/
dt
=
-
k
a
[H
+
]
C
-
k
n
C
-
k
b
[OH
−
]
C
(4.74)
where
C
=
concentration of the toxicant being hydrolyzed [mole l
−1
]
k
a
=
acid-catalyzed hydrolysis reaction rates constant [l mole
−1
day
−1
]
k
b
=
base-catalyzed hydrolysis reaction rates constant [l mole
−1
day
−1
]
k
n
=
neutral-catalyzed hydrolysis reaction rates constant [day
−1
]
[H
+
]
=
hydrogen ion concentration [mole l
−1
]
[OH
−
]
=
hydroxyl ion concentration [mole l
−1
]
In order to evaluate
k
a
and
k
b
, several nonneutral pH hydrolysis experiments must
be conducted as depicted in
Figure 4.8.
In general, hydrolysis reaction rates are
highly dependent on pH.
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