Page 107 - Partition & Adsorption of Organic Contaminants in Environmental Systems

98 ADSORPTION OF VAPORS ON MINERALS AND OTHER SOLIDS

Rutherford (1997), it has been shown, however, that after the initial cation sol-
vation by EGME or water, the expanded clay interlayer spacings create more
voids, on which polar molecules may adsorb at high P/P°. Evidently, the very
large initial uptake of EGME (or other polar solvents) by a strongly solvat-
ing clay is driven by a powerful cation solvation force.
In a similar study by Mooney et al. (1952) on the water vapor uptake by
montmorillonites in various cationic forms, the calculated BET surface areas
with water data are likewise orders of magnitude larger than the values based
on N 2 data, as would be anticipated. Since the uptake of EGME (or other
polar vapors) by an expanding clay depends sensitively on the specific clay
cation (Chiou and Rutherford, 1997), the Q m(EGME) ap value or the resulting
2+
surface area would vary sensitively with the cation in the clay (e.g., Ca versus
+
K ), even when the surface areas by N 2 data of the clay in different cationic
forms are fairly comparable. Thus, although K-SAz-1 montmorillonite has a
somewhat higher BET-N 2 surface area than Ca-SAz-1 montmorillonite, as
shown earlier, the latter clay exhibits a much greater water-vapor uptake than
2+
does the former, to be illustrated later, due to the fact that Ca is a far more
+
powerful hydrating cation than K (Cotton and Wilkinson, 1966).
The virtually linear EGME isotherm on peat, together with the peat’s very
2
small BET-N 2 surface area (1.26m /g), is good evidence for the presumption
that the EGME uptake by peat at room temperature occurs primarily by
partition (i.e., penetration) into the peat’s organic matter matrix (Chiou et al.,
1993). Further elaboration of the organic compound partition into soil organic
matter is presented in Chapter 7. Since the EGME isotherm is practically
linear (rather than type II), the BET model does not apply, and there is no
theoretical Q m (EGME) ap. The partition effect, as reflected by the relatively
linear uptake of EGME on peat, is analogous to the solubilization of organic
substances into amorphous polymers (Flory, 1941; Huggins, 1942). The extrap-
olated limiting capacity of EGME with peat at P/P° = 1, normalized to the
organic content of the peat, is 250mg/g, which is comparable in magnitude with
the finding of Bower and Gschwend (1952) that 170 to 250mg of EG is
retained by 1g of soil organic matter under some evacuation condition. Bower
2
and Gschwend thus obtained a value of 560 to 800m /g as the apparent surface
area of soil organic matter by calculations in which the observed uptake of
EG on soil organic matter was ascribed to surface adsorption rather than to
bulk solubility.
The data for illite and natural hydrous iron oxide, which show moderate
ratios of Q m (EGME) ap to Q m (EGME) eq , reflect a moderate amount of EGME
penetration, probably due to some combined effect of dissolution into organic
matter and cation solvation with the clay component in the sample. On illite,
the small number of exchangeable cations and a small amount of organic
impurity (ca. 1.5% organic matter) could lead to the discrepancy observed.
For the natural hydrous iron oxide, which is a relatively impure material, the
nonsurface uptake by small amounts of expanding clay and organic matter
could easily give a Q m (EGME) ap to Q m (EGME) eq ratio of 1.86. For Woodburn