Page 195 - Partition & Adsorption of Organic Contaminants in Environmental Systems

186 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

This would have a negative effect on soil remediation and a positive effect on
soil contaminant uptake or immobilization. Anionic surfactants with large
charges usually show relatively weak adsorption on mineral soils (Rouse et
al., 1993). This would seem to make anionic surfactants more desirable for soil
remediation. Currently,however,available data are insufficient to compare the
effects of anionic surfactants and other surfactants on the sorption coefficients
of organic contaminants in soil–water systems.
The influence of emulsified material on contaminant sorption coefficient
has been investigated using a PSO surfactant (Petronate L) as reference mate-
rial (Sun and Boyd, 1993). With naphthalene (S w = 31mg/L), phenanthrene
(S w = 1.3mg/L), and 2,2¢,4,4¢,5,5¢-hexachlorobiphenyl (2,2¢,4,4¢,5,5¢-PCB) (S w =
1.0mg/L) as model solutes and Oshtemo silt loam as the reference soil (f om =
d
0.0017), Sun and Boyd (1993) found that with applied X em = 50mg/L the K*/K d
drops to about 0.02 for 2,2¢,4,4¢,5,5¢-PCB and to about 0.75 for phenanthrene,
but remains at about 1 for more water-soluble naphthalene (Figure 7.34).Thus,
(w) (s)
even at low X em, the data indicate that X emK em is much larger than f emK em /K d
for highly insoluble solutes (high K d), but the difference decreases rapidly
with increasing solute S w or with decreasing K d. For an emulsified material in
(w)
(s)
soil–water systems, it is reasonable to assume that K em K em , since the sorbed
material on solid surfaces would probably be in a separate phase similar to
that in water. Therefore, if the emulsified material exhibits only a low-to-
(s)
moderate sorption on a solid (i.e., if f em/X em is not very large), f emK em/K d may
(w)
then be less than X em K em ,or f em/K dX em < 1,for solutes with large K d. For solutes
of low K d value, the f em/K dX em increases and appears to approach 1 for naph-
thalene with a mineral soil. Overall, the addition of emulsified materials to
water for remediation of subsaturated contaminants in soil would be most
effective for highly insoluble contaminants and presumably for soils with high
SOM content, both leading to high K d.
The influence of normal surfactants on K* is expectedly more complicated.
d
A few studies with Triton X-100 (TX100), a nonionic surfactant, have been
reported. Sun et al. (1995) found that the K*/K d of 1,2,4-trichlorobenzene
d
(TCB) (S w = 18mg/L) on Oshtemo silt loam (f om = 0.0017) increases initially
at X << CMC, reaches a maximum of about 5 at X ª 2 CMCs and then declines
slowly thereafter with X; the K*/K d values for highly insoluble 2,2¢,4,4¢,5,5¢-
d
PCB (S w = 1.0mg/L) and DDT (S w = 5.5mg/L) on the same soil exhibit a
maximum of 2.4 and 3.3, respectively, at X ª CMC and then decline sharply
at higher X, with K*/K d ª 0.5 at X ª 2 CMCs (Figure 7.35) (see Table 7.16 for
d
the CMC value). In a similar study, Deitsch and Smith (1995) found that with
Triton X-100 the K* of relatively soluble trichloroethylene (TCE) (S w =
d
1100mg/L) on an organic-rich soil ( f oc = 0.24) exhibits no significant change
over K d up to moderately high X (300mg/L) and that the K* shows a signifi-
d
cant reduction relative to K d only at X >> CMC. An obvious consequence of
the K* – K d relation is that K* < K d for all contaminants when X exceeds
d
d
greatly the CMC. This is because the sorption of surfactant to a soil reaches
the saturation level at some point while the surfactant concentration in water
(X) increases continuously past the CMC, making (1 + X mn K mn + X mc K mc ) >>