Page 127 - Partition & Adsorption of Organic Contaminants in Environmental Systems

118 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

which can be expressed as

d ln S w d ln C e
> (7.4)
dT dT

Since one finds that

d ln S w D H w
= (7.5)
dT RT 2

and that

d ln C e D H d D H
= = - (7.6)
dT RT 2 RT 2

H
with DH d denoting the molar heat of desorption (i.e., DH d =-D ), one there-
fore finds that
DH > -DH w (7.7)

which means that the heat liberated when 1 mole of solute is sorbed to the
soil is less exothermic than the solute’s reverse heat of solution in water. Equa-
tion (7.7) explains the small exothermic heats of sorption of DDT (Pierce et
al., 1974), lindane and b-BHC (Mills and Biggar, 1969), parathion (Yaron and
Saltzman, 1972), and 1,1,1-trichloroethane (Chiou et al., 1979) on soil.
Equation (7.7) is actually a consequent form of Eq. (3.23) for the molar
heat of partition of a solute between an organic solvent and water. Since the
heats of solution of organic compounds in an organic phase (DH o ) are gener-
ally positive but small due to their improved compatibilities, DH will be small
for compounds with low positive DH w values and may even become positive
(endothermic) for compounds with abnormal (negative) DH w values. For
example, Chiou et al. (1979) showed that the DH for 1,2-dichlorobenzene
sorption by soil from water is nearly zero because of its low DH w and that the
DH for 1,1,1-trichloroethane is positive because of its negative DH w in the
temperature range 3.5 to 20°C. One may conclude from these data that in
systems where the DH values are negative, such exothermic heats originate
primarily from condensation of the solutes from water (-DH w) and that with
DH o being normally positive, interactions between SOM and solute (DH o) are
normally endothermic, as usually is the case for the heat of solution. One finds
H
small and nearly constant exothermic heats (D ) (and hence small tempera-
ture coefficients) in solute partition equilibria as a result of the partial
cancellation in heat between DH o and DH w according to Eq. (3.23).
The sorption data of p,p¢-DDT are especially worth noting because DDT
is a solid with a large heat of fusion (DH fus), about 25kJ/mol (Plato and