Page 200 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 191

With the rationale above, the observation that the K*/K d values of BTEX
d
are all greater than 1 at the applied X range should be a result of their small
(1 + X mnK mn + X mcK mc) values, due to their high S w; the increase in K*/K d with
d
decreasing f om is attributed to an increased mass of adsorbed and aggregated
TX100 on mineral matter. In this case, the small increase in K*/K d for BTEX
d
with peat, to which a significant mass of TX100 is sorbed, may be realized on
the account that most sorbed TX100 partitions to SOM and the extent of
surface aggregation is small. The increase in K*/K d from p-xylene to benzene
d
on all solids is consistent with an increase in S w. In light that (1 + f sfK sf /K d) >
(1 + X mnK mn + X mcK mc) for BTEX and TCE on low-f om solids with TX100, the
finding that the K d */K d values are greater for BTEX than for TCE may reflect
an improved partition of more-aromatic BTEX to aggregated, partially aro-
matic TX100 (Lee et al., 2000). The finding that the K* value for TCE with a
d
high-f om soil is about the same as K d over a large range of X (Deitsch and
Smith, 1995) is understood on the basis that the term (1 + X mnK mn + X mcK mc)
should be small (i.e., close to 1) over the applied range of X because TCE is
relatively water soluble and that the adsorbed and aggregated TX100 on the
solid surface should be small because of the high solid f om, making the term
(1 + f sfK sf/K d) close to 1.
For the less soluble chlorinated solutes, the variation in K*/K d follows basi-
d
cally the same pattern as noted for BTEX, in which the K*/K d decreases with
d
increasing solid f om (or increasing K d) and become eventually less than 1 for
the solutes on high-f om solids (CSMS and peat). The transition in K*/K d,as
d
found either for different solutes on a solid or for a given solute on different
solids, escapes recognition in other studies (e.g., Deitsch and Smith, 1995;
Sun et al., 1995) because these systems consisted mainly of either low-f om solids
or relatively water-soluble solutes. The noted difference in K*/K d between
d
lindane on TCS (f om = 0.024) (Table 7.20) and TCB on Oshtemo silt loam (f om
= 0.0017) (Figure 7.35), where the two solutes have similar logK ow values, man-
ifests the intimate effect of solid f om on solute K* when TX100 is applied. The
d
impact of solid f om on the relative order of (1 + f sfK sf /K d) and (1 + X mnK mn +
X mcK mc) for different solutes is detected more readily if the solid has a signif-
icant f om. Thus, whereas the K*/K d are much greater than 1 for DDT and
d
2,2¢,4,4¢,5,5¢-PCB with Oshtemo silt loam at low X (Sun et al., 1995), the
reverse effect would most likely occur if the solid has a much higher SOM
content. A more comprehensive analysis of all pertinent system parameters
has been given by Lee et al. (2000).
Although the K*/K d characteristics observed for contaminants with one sur-
d
factant do not suffice for generalization of the potential effects by all surfac-
d
tants, the recognized major features as to the direction to which the K*/K d
ratio varies with soil/solid and contaminant properties should be applicable to
most surfactants. As illustrated with TX100 surfactant and PSO microemul-
sion, the K*/K d ratio increases with increasing solute water solubility (S w ) and
d
decreases largely with increasing solid SOM content (f om ). This behavioral
pattern should aid in our assessment of the contaminant distribution in natural