Page 118 - Partition & Adsorption of Organic Contaminants in Environmental Systems

BACKGROUND IN SORPTION STUDIES 109

Despite the fact that the relationship observed between soil uptake and
SOM content had greatly simplified assessments on the uptake of nonionic
organic compounds from water by soils, there was no single widely accepted
view on the sorptive mechanism with SOM in the literature before 1979. Prior
to that time, one popular view considered SOM as a high-surface-area adsor-
bent (Bower and Gschwend, 1952; Bailey and White, 1964) capable of adsorb-
ing nonionic organic compounds by hydrophobic interactions (Weed and
Weber, 1974; Browman and Chesters, 1977; Mingelgrin and Gerstl, 1983). Such
a hydrophobic adsorption concept, however, is not supported by common
adsorption criteria and in particular by the observed soil sorption data in
aqueous systems. Moreover, the earlier accepted surface area for SOM (550
2
to 800m /g), reported by Bower and Gschwend (1952) based on the ethylene
glycol (EG) retention method, was later shown to be largely an artifact of the
high solubility of polar EG in relatively polar SOM (Chiou et al., 1990, 1993).
Using some high-organic-content soils (peat and muck) as a model for SOM,
the surface area of SOM as measured by the standard BET method (with N 2
2
vapor as the adsorbate) is actually only about 1m /g (Chiou et al., 1990;
Pennell et al., 1995), which is nearly three orders of magnitude lower than the
value assumed earlier.
In attempts to reconcile the inconsistency in reported SOM surface areas,
Pennell and Rao (1992) consider that the large difference between the values
obtained by the polar-solvent retention method and by the standard BET-N 2
method represents the internal surface of the SOM. Although the term inter-
nal surface has been used concurrently in surface science, it is well understood
there that the internal surface, which extends inward the porous channels of
a solid (e.g., activated carbon), is freely accessible to an inert gas such as N 2,
as noted in Chapter 6. Therefore, if an assumed internal surface is impervious
to an inert gas, it is more a reflection of solvent penetration into the SOM solid
matrix, as pointed out earlier by Brunauer (1945). As we will see later, the
excess uptake of an organic vapor (especially, a polar vapor) over that of N 2
gas by SOM is more properly interpreted in terms of the vapor partition.
Along with these unique features for soil uptake from water, we also recall
that water vapor exhibits a generally much greater adsorption than an organic
(benzene) vapor on various dry minerals, as elucidated in Chapter 6. Based on
this disparity, one would then expect water, as a solvent, to strongly suppress
the adsorption of an organic solute onto a soil mineral, because the adsorption
process is competitive. Thus, the many outstanding features in the sorption of
organic solutes from water solution, such as the virtual isotherm linearity and
the dependence of the uptake on soil organic content, can only be reconciled
readily and logically with the partition-dominated solute uptake by SOM.

7.2.2 Soils as a Dual Sorbent for Organic Compounds
A major advance in the description of the sorption process of organic
compounds with soil (or sediment) started with the proposition by Chiou and