Page 194 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 185

related to the amount adsorbed and mineral surface properties (Gu et al.,
1992; Rutland and Senden, 1993; Nayyar et al., 1994). The surfactant parti-
tioned into SOM is not subject to aggregation, in which case, K sf 0. Although
surfactants partitioned into SOM may modify the SOM properties, the amount
partitioned to SOM is usually not substantial,and thus the intrinsic SOM prop-
erties are probably not changed significantly. The magnitude of (1 + X mnK mn +
X mcK mc) in Eq. (7.29) is determined by surfactant type and its concentration
in water as well as by contaminant properties.
By the same reasoning as above, the relation between K* and K d for a (sub-
d
saturated) contaminant in a soil–water mixture with a microemulsion may be
described as

* + s ()
K d 1 fK em K d
em
= () (7.30)
1 + w
K d XK em
em
(s)
where f em is the fraction of sorbed emulsified material in soil, K em the solute
(w)
sorption coefficient between sorbed microemulsion and water, and K em the
corresponding coefficient between aqueous-phase microemulsion and water.
It is evident from Eqs. (7.29) and (7.30) that the applied surfactant (or
microemulsion) imposes two opposing effects: The sorbed surfactant (or
microemulsion) increases contaminant sorption, whereas the unsorbed sur-
factant promotes contaminant dissolution in water. The prerequisite condition
for soil remediation (K* < K d) requires that (1 + f sfK sf /K d) be smaller than
d
(s)
(1 + X mnK mn + X mcK mc) if a surfactant is used, or that (1 + f emK em/K d) be smaller
(w)
than (1 + X emK em ) if a microemulsion is involved.
(s)
To characterize K sf with respect to f sf (or K em in respect to f em), detailed sorp-
tion information must be made available for various surfactants on various
soils and solids in order to account for the extent and state of the sorbed
surfactant. This requirement would seem too formidable a task to be accom-
plished realistically. Instead, we choose an empirical approach to develop the
knowledge from a large set of relevant data. Because the molecular proper-
ties of surfactants vary considerably, the sorption of surfactants on different
soils or solids is expectedly more complicated than that of nonionic organic
compounds.
The sorption of cationic surfactants on clays and soils that contain signifi-
cant cation exchange sites is known to take place primarily by a cation
exchange process (Theng et al., 1967; Barrer, 1978; Lee et al., 1989; Xu and
Boyd, 1994). Incorporation of a significant amount of long-chain alkylammo-
nium cations onto charge exchange sites of either clays or soils creates an addi-
tional microscopic nonpolar phase for solutes to partition (i.e., to give large
f sf K sf ), which makes K* >> K d for nonpolar solutes (Boyd et al., 1988; Smith
d
et al., 1990b). Therefore, the addition of cationic surfactants to soil–water
systems would normally increase (rather than decrease) the uptake of organic
pollutants on soil and hence reduce pollutant migration through a soil profile.