Page 209 - Partition & Adsorption of Organic Contaminants in Environmental Systems

200 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

7.5 SORPTION FROM VAPOR PHASE

7.5.1 General Aspects of Vapor Sorption

Following our deliberations on the sorptive effects of SOM and minerals, we
expect the sorption of organic vapors on relatively dry soils to consist of
adsorption on soil minerals (and on HSACM, if present) and concurrent par-
tition into SOM. For ordinary dry soils that are abundant in mineral content,
the adsorptive contribution would undoubtedly predominate the overall soil
uptake. Shipinov (1940) found that the vapor sorption of hydrogen cyanide on
dry soils was nonlinear (BET type II shape). Stark (1948) showed similar non-
linear isotherms for chloropicrin vapor uptake on dry soils and found a close
correlation between sorption capacities and soil clay contents; the sorption
capacity decreased as the soil-moisture content increased. Hanson and Nex
(1953) observed that at a soil moisture content substantially below the wilting
point, ethylene dibromide (EDB) was strongly sorbed by the soil, but that the
sorption decreased sharply to a minimum near the wilting point. Wade (1954)
studied the EDB vapor sorption by three soils having very different clay versus
SOM contents. A much greater EDB sorption was found on dry soils; mois-
ture sharply suppressed the EDB sorption only before the soil-water content
reached saturation, after which the sorption was unaffected by soil-water
content and the EDB isotherms were essentially linear. On wet soils, the EDB
sorption capacities were closely proportional to respective SOM contents, with
no correlation to clay contents.
Jurinak (1957a,b) and Jurinak and Volman (1957) studied the vapor uptake
of EDB and 1,2-dibromo-3-chloropropane on dehydrated clays (which still
retained small amounts of water) in relation to clay types and exchanged
cations. The extent of sorption was related to the surface areas of specific clays
and the sorption data fit the BET adsorption model. The BET monolayer
capacity of EDB increased from 17.5g/kg for nonexpanding Ca-kaolinite
(1.0% water) to 72.1g/kg for expanding Ca-montmorillonite (2.3% water).
Call (1957) studied the dependence of EDB vapor uptake on several soils and
a Ca-montmorillonite on relative humidity (RH). On relatively dry Ca-
montmorillonite and soils (RH < 20%), the EDB isotherms were type II shape,
in which the sorption capacity increased with increasing clay content. An
increase of RH from 0 to 50% progressively suppressed the EDB sorption on
the soils, with a concomitant change of the isotherm shape toward linearity.
On Ca-montmorillonite, however, the EDB sorption increased sharply when
RH increased from 0 to 10%, which was attributed to the clay-layer expan-
sion creating additional surfaces; however, with RH > 10%, the EDB sorption
decreased with increasing RH, and at RH = 90% the sorption became very
small relative to that with the dry clay. By these observations, Call suggested
that sorption of EDB on clays and soils at low RH resulted from adsorption
on mineral surfaces, whereas the sorption on wet soils and soils at high RH
occurred by dissolution in soil water or by adsorption, as a Gibbs surface