Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 54

TCE was replaced with methyl chloroform when it became apparent that TCE was contribut-
ing to smog in the Los Angeles basin and possibly to worker-exposure-related illness. In the
mid-1980s, methyl chloroform fell out of favor when it was recognized as an ozone-depleting
chemical. Many facilities switched from methyl chloroform to dichloromethane, perchloro-
ethylene, or TCE in conjunction with adopting upgraded degreasers and ultrasonic cleaning
equipment. A facility may have an old or recent release of TCE, followed or preceded by a
1,4-dioxane-containing methyl chloroform release. The abiotic half-life of methyl chloroform,
approximately two years, is considerably shorter than either the abiotic or biodegradation
half-life of TCE. Consequently, most of the methyl chloroform from an old spill will have
converted to 1,1-dichloroethylene or 1,2-dichloroethane, while TCE will persist. This situ-
ation can result in the appearance that 1,4-dioxane was present in TCE. At some sites, the
1,4-dioxane could have been used separately for another purpose.
Of greatest signii cance is the fact that TCE is inherently more stable toward reactions with
aluminum and other alkali metals than methyl chloroform. Smaller quantities of stabilizers
were required for TCE, and TCE was more easily stabilized against metal reactions by using
other chemicals. Tests of TCE stabilization using 0.1% 1,4-dioxane did not measurably
improve TCE stability against reactions with iron, zinc, copper, and aluminum (Willis and
Christian, 1957). The smaller boiling-point difference between 1,4-dioxane and methyl chlo-
roform would probably not have produced partitioning in as pronounced a manner as occurs
for 1,4-dioxane in the boiling sump of a methyl chloroform degreaser.
Ofi cials at DOW Chemical assertively emphasize that 1,4-dioxane was not a constituent of
DOW's TCE grades (J.A. Mertens of Midland, Michigan, Dow Chemical Corporation, per-
sonal communication, 2000). Finally, none of the extensive material collected for this topic
and used to coni rm the presence of specii c stabilizer compounds in chlorinated solvents
assertively states that 1,4-dioxane was used to stabilize TCE.
1.2.7 F ATE OF S TABILIZERS IN I NDUSTRIAL A PPLICATIONS AND S OLVENT R ECYCLING
The premise of “contaminant archeology” (as detailed in Chapter 9) is that understanding the nature
of the industrial process in which the contaminant was used will inform the environmental scientist
of the likely composition and properties of the waste industrial efl uent that was released to soil and
groundwater. Understanding the demands on solvents for various uses will provide clues regarding
the types of stabilizers that were needed for a particular process. Contrasting the physicochemical
properties of stabilizers with those of their host solvents reveals which stabilizers would more likely
end up in the vapor degreaser still bottoms and stored in waste tanks that ultimately leaked. Contaminant
archeology focuses on the clues that suggest the whole composition of the waste and how that com-
position may drive the subsurface behavior of the waste mixture, as well as the constituents of con-
cern that should be considered for investigating a release of the waste solvent mixture. The properties
that have the greatest bearing on the separation, concentration, or elimination of stabilizers in chlori-
nated solvents include boiling point, solubility in solvent and water, vapor pressure, organic carbon
partition coefi cient, thermal stability, and consumption in stabilizing reactions. These stabilizer and
solvent properties and their consequences are discussed in more detail in this section.
1.2.7.1 Boiling-PointDifferences
As described in Section 1.2.3.1 , a stabilizer should not partition differentially: the stabilizer should
have the same concentration in the vapor phase as it has in the liquid phase. Two or more organic
solvent or stabilizer compounds, when brought to a boil, will distribute themselves differently
between the two phases. The compound with a higher vapor pressure, or lower boiling point, will
concentrate in the vapor phase. The compound with the lowest vapor pressure, or highest boiling