Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 146

(in m/year), and L is the length (in m) of the contaminated soil area that is parallel to groundwater
l ow (HDOH, 2007).

3.3.2.9 Sorption of Vapor-Phase 1,4-Dioxane
Because 1,4-dioxane is hydrophilic and has a very low Henry's law constant, it is unlikely to migrate
signii cantly in the vapor phase once it has dissolved into water. Vapor transport may be of interest
in contamination source areas with thick unsaturated zones where vapor intrusion into occupied
buildings is a concern (Little et al., 1992). The extensive literature on sorption includes several
studies in which sorption of 1,4-dioxane vapor was measured. A 1994 study by Chiou and Kile
measured the vapor adsorption of several solvent-stabilizer compounds dissolving into low-mineral-
content samples of peat soil from the Florida Everglades, including nitroethane, 1,4-dioxane, and
acetonitrile. The isotherm of acetonitrile was found to be about 4.2 times greater than the isotherm
for 1,4-dioxane, and the isotherm for nitroethane was approximately 3.5 times higher greater than
the isotherm for 1,4-dioxane (Chiou and Kile, 1994). The comparatively low sorption to peat, which
is composed of 86.4% organic carbon, * suggests a lower potential for 1,4-dioxane vapors to be
sorbed to the much smaller fraction of soil organic matter found in most vadose zone soils.
Scientists at the Swiss Institute of Biogeochemistry and Pollutant Dynamics rigorously measured
the humic acid/air partition coefi cients for 188 polar and nonpolar organic compounds at different
temperatures and relative humidities by using a dynamic l ow-through technique. Humic acid/air
partition experiment data rel ect the concentration sorbed relative to the concentration in air. The
measurements were made by using inverse gas chromatography with the humic acid sorbent serving
as the chromatographic column stationary phase in a gas chromatograph and the organic compound
of interest injected onto the column under isothermal conditions. † The measured retention rel ects
the tendency of the compound to sorb to this particular variety of humic acid at different tempera-
tures and humidities. ‡ The Swiss experiments provide the most comprehensive and reliable data set
for sorption into humic material available (Niederer et al., 2006a). The humic acid/air partition
coefi cients for a few compounds that may be present in solvent wastes as solvent stabilizers are
provided in Table 3.12 , and Table 3.13 lists humic acid/soil-vapor distribution coefi cients at 98%
relative humidity. For example, the following equation describes the 1,4-dioxane partitioning
between air and humic acid:

È ( n / m ) ˘

V

1,4-dioxane humic acid

K = 4680 = 4680

air

, (3.39)

Í ˙

air/humic acid 1,4-dioxane

( n / V ) m

-

Î ˚

1,4-dioxane air humic acid

where n is the number of moles, m is the mass (in kg), and V is the volume (in L).
The 1,4-dioxane aqueous-phase/soil organic carbon partition coefi cient is comparatively low, but
in the vapor-phase 1,4-dioxane partitions easily into humic acid, the major component of soil organic
matter. The variation in humic acid/soil-vapor distribution coefi cients with humidity may rel ect
changes in the polarity of the humic substance with water saturation (Chiou and Kile, 1994).

3.3.2.10 High-Strength 1,4-Dioxane Solutions May Cause Clay Swelling
At high concentrations, some organic compounds, including 1,4-dioxane, can induce swelling and
l occulation of smectite (i.e., montmorillonite) clays (Bradley 1945; MacEwan, 1948; Brindley and
Tsunashima, 1972). Chemical alteration of clay-mineral arrangement in natural soils and engineered
clay liners can affect permeability and diffusive and advective transport (Wu et al., 1994).

* Measured on a dry-weight basis. The Florida Everglades peat is a reference sample of the International Humic Substances
Society.
† See Chapter 4 for a brief explanation of chromatographic column stationary phases and gas chromatography.
‡ The Swiss study used the “leonardite humic acid” with an organic carbon content of 63.8%.