Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 148

a window to allow x-ray diffraction measurements of spacing between clay layers and a drain to
retrieve samples of drained solution. The 1,4-dioxane solution was expressed through the i lter by
applying pressure with helium; the expressed solution was drained at atmospheric pressure.
Successive x-ray diffraction measurements at different helium pressures and for different concentra-
tions of 1,4-dioxane showed that 1,4-dioxane converts fully expanded clay layers with spacing of
30 Å to partially expanded layers with a spacing of 15 Å (Wu et al., 1994). *
The researchers in this study postulate that 1,4-dioxane separates from solution and i lls the
space between the partially expanded layers in the sodium montmorillonite clay, that is, a pure-
phase 1,4-dioxane accumulates between the clay layers. 1,4-Dioxane has also been observed to
accumulate between layers of calcium montmorillonite (Zhang et al., 1990). 1,4-Dioxane apparently
rearranges water molecules between the expanded clay layers to produce an open, coordinated
structure that may force layers apart because of the larger volume of the inferred structure (Wu
et al., 1994).
Zhang et al. (1990) studied 1,4-dioxane adsorption to sodium and calcium smectite suspensions
in the

m size fraction by using 14 C-labeled 1,4-dioxane. The 1,4-dioxane sorption isotherm for
a calcium smectite followed the Langmuir pattern, wherein the rate of sorption is initially i rst-order
and subsequently zero-order, producing a steep initial slope in the curve of quantity adsorbed versus
1,4-dioxane concentration, followed by an asymptotic tapering off to a zero-slope plateau. † This
result was markedly different from the sorption isotherm for sodium smectite, for which the
millimoles of 1,4-dioxane adsorbed per gram of clay increased monotonically with increasing
concentration. The Zhang study found that the 1,4-dioxane adsorption reaction with calcium smec-
tite was exothermic, but bonding was not indicated. Adsorption of most other organic compounds is
usually endothermic. Zhang proposed that 1,4-dioxane adsorption onto calcium smectite clay being
exothermic is attributable to its unique structure-breaking effect on the tightly held monolayer of
water on the clay surface (Zhang et al., 1990). The behavior of 1,4-dioxane as a water-structure
breaker has been coni rmed; two 1,4-dioxane rings can form a dimer stabilized by two intermolecu-
lar hydrogen bonds, leaving two oxygen atoms available for interaction with the water molecules
and creating a dipole moment (Mazurkiewicz and Tomasik, 2006).
In solutions with 1,4-dioxane, the interlayer distance in calcium smectite expands from 0.55 to
1.48 nm. In a 1.0 M solution of 1,4-dioxane, the interlayer concentrations in the calcium smectite
were found to be 6.0 M (Zhang et al., 1990). Zhang concluded that 1,4-dioxane is not adsorbed
because of any specii c interaction with the surfaces of the clay layer; instead, 1,4-dioxane is
distributed between the bulk phase and the interfacial phase, but prefers the interfacial phase, which
facilitates its migration through clays. A different conclusion was reached in an earlier study by
Brindley et al. (1969): calcium smectite clays in 1,4-dioxane solutions ranging from 1 to 100 mol%
had constant basal spacings, leading to the conclusion that 1,4-dioxane is strongly and preferentially
adsorbed by the clay. That study has been superseded by newer research under more controlled
conditions, for example, Wu et al. (1994).

< 2 μ

3.3.3 S UBSURFACE V APOR -P HASE T RANSPORT
The migration of contaminants in the vapor phase constitutes an important pathway by which
releases of volatile chemicals to soil may manifest as groundwater contamination. Soil vapors are a
concern when they migrate to and accumulate in enclosed spaces, where they may lead to either
toxic exposure in occupied buildings or create an explosion hazard. Man-made migration pathways
for soil vapors—such as permeable sand or gravel i ll in trenches for buried electrical lines or rapid

* Å = angstrom; 1 Å = 10 −8 m = 0.1 nm.
† In i rst-order processes, the rate is proportional to the concentration of a single reactant or substrate; in a zero-order
process, the rate is independent of the substrate concentration.