Environmental Engineering Reference
In-Depth Information
Hence, a meaningful comparison should be based on the
energychangeperunitarea
.
On this basis, the replacement of cyclohexane on the surface by water involves an
enthalpy decrease of about 21-23 kJ/mol.The entropy change was relatively small for
both molecules. Thus the enthalpy change per unit area is the major driving force for
the displacement of cyclohexane from the metal surface. Such large enthalpy changes
are possible for water molecules because of their special tetrahedral coordination and
H-bonding tendency, which are lacking in cyclohexane molecules.
The adsorption of several organic molecules on solids from their aqueous solutions
is reported in the literature.The data are invariably reported as mass of solute (sorbate)
adsorbed per unit mass of solid sorbent,
W
OC
i
W
min
i
+
W
i
(
mg sorbate/kg sorbent
)
C
i
w
(
mg sorbate/L solution
)
=
K
sw
(
L
/
kg
)
=
.
(4.72)
C
i
w
The overall partition constant,
K
sw
, has two contributions - one from mineral matter
(min) and the other from organic matter (oc). Thus,
K
oc
K
min
K
sw
=
+
sw
.
(4.73)
sw
If
f
oc
is the fraction of organic carbon on the solid (g/g) and the mineral surface area is
given by
S
a
(m
2
/kg),
K
oc
(L/kg), and
K
min
(mg/m
2
)
represent the partition constants
for a compound normalized to oc and min, respectively.
K
oc
sw
f
oc
K
min
sw
S
a
K
oc
=
,
K
min
=
(4.74)
Schwarzenbach Gschwend, and Imboden (1993) suggest that for hydrophobic com-
pounds, which are driven to the mineral surface via exclusion from the water structure,
an LFER should exist between
K
min
and
γ
i
. The form of the LFER is
γ
i
log
K
min
=
a
log
+
b
.
(4.75)
Based on a limited set of data, this relationship appears to hold for a particular sorbent
adsorbing different nonpolar compounds under identical conditions of pH and ionic
strength (Mader, Goss, and Eisenreich, 1997).
The amount adsorbed on mineral surfaces is indeed very small. However, if the
mineral surface is coated with even a small percent of organic macromolecules, the
adsorptioncapacityisenhancedbyseveralordersofmagnitude.Mineralmatterdevel-
ops these coatings rapidly and hence natural soil and sediment can be considered to
provide dual sorbent sites. The bare mineral surfaces are characterized by low
K
min
.
The organic matter coating the mineral provides a highly compatible medium with
which a neutral hydrophobic compound can associate and thereby limit its interaction
with water.
The organic matter in sediments and soils is composed of approximately one half
of the total carbon that is directly measured as organic carbon. Hence, sorption is
often keyed to organic carbon rather than organic matter through the approximate
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