Page 128 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 119

Glasgow, 1969), and is highly incompatible with water, which should make
DH w much greater than 25kJ/mol. The sorption coefficient of DDT with soil
or sediment normalized to the SOM content (K om) is approximately 1.5 ¥ 10 5
H
(Pierce et al., 1974; Shin et al., 1970), while the heat of sorption (D ) at equi-
librium is about -8.4 to -16.8kJ/mol (or -12.6 ± 4.2kJ/mol) (Pierce et al.,
1974). Thus the observed DH is far less exothermic than -DH w, as would be
expected for a solid solute with large DH fus in partition equilibrium [see Eq.
(3.23) and the discussion thereafter]. Based on these values, one can calculate
the standard entropy change for the transfer of DDT from water to the SOM
as

G (7.8)
D ° =-RT lnK om

and

DS∞= ( DH∞- DG∞) T (7.9)

G
where D ° is the (molar) standard free energy change for the transfer of 1
mole (or a unit mass) of the solute from water at unit concentration to the
H
S
SOM phase at unit concentration, and D ° and D ° are the corresponding
enthalpic and entropic changes at the said standard state. Since the DH for a
solute in a partition process is largely independent of the solute concentra-
tion, the D ° value at the standard state is essentially equal to the DHH value
5
at the point of equilibrium. Now, if one takes K om 1.5 ¥ 10 and D ° DHH
=-12.6kJ/mol for DDT, one gets D ° 58J/mol·K at T = 298K. Although
S
the calculated D ° value for DDT is subject to some uncertainty because of
S
the inaccuracy of the DH value, it is nonetheless indicative of a relatively small
change in molar entropy for the transfer of DDT from water into SOM at the
standard state, as would be expected for a partition process. Such a small
entropy change is in sharp contrast to a usually very large entropy decrease
when a trace component adsorbs strongly from a solvent (water) onto an
adsorbent. In analyzing the sorption process with entropy, it is important that
the D ° at the standard state, rather than the DSS at equilibrium, be employed.
This is because the DS values for solutes at equilibrium between any two
phases (where DG = 0) will always be negative whenever the process (adsorp-
tion or partition) proceeds exothermically. We shall consider later the heat
effect associated with the soil sorption of organic compounds in nonaqueous
systems.
Accountability of the solute partitioning into the SOM phase is further sub-
stantiated by the estimated magnitude of the solute solubility in SOM. Since
the isotherm is practically linear, the solubility of a solute in SOM may be
determined by

(7.10)
S om = S w ·K om