Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 121

from aqueous solutions to air for i xed concentrations. Comparison of relative mass transfer rates
inferred from ratios of the Henry's law constants for chlorinated solvents and their stabilizers sug-
gests that water-to-air partitioning after a waste is discharged to a surface-water body may favor the
ether, epoxide, nitroalkane, and alcohol stabilizers remaining in water, whereas the solvents and
cyclohexane volatilize to the atmosphere. The Henry's law constant is also a key parameter for
designing groundwater and wastewater treatment systems where air stripping is viable. Chapter 7
discusses the suitability of air stripping for remediation of 1,4-dioxane.
A more tangible means of relating the Henry's law constant to the environmental fate of stabiliz-
ers discharged to surface water is to estimate their rates of volatilization from rivers and lakes.
Models are available to predict the volatilization half-life of a compound in water. Modeled half-
lives for stabilizer and chlorinated solvent compounds are proi led in Table 3.4 . The model used was
Estimation Programs Interface Suite, EPI Suite™ (a trademark of ImageWare Systems ® , Inc.), which
predicts physical-chemical properties of compounds entered by their Chemical Abstracts Service
Registry Number or by their SMILES notation. * The model within EPI Suite for estimation of vola-
tilization from water, WVOLNT, or Water Volatilization Program is based on the method outlined
in Handbook of Chemical Property Estimation Methods (Thomas, 1990).
The accuracy of the properties estimated by EPI Suite and the associated estimates of chemical
phase partitioning depends on the chemical's class, the quality of the available chemical data used
by the model as a training set, and whether the chemical's properties fall within the range of the 353
chemicals in the training data set. In general, EPI Suite is able to predict the measured property
value within an order of magnitude of measured values, provided the chemical is appropriate for the
types of regression and other estimation techniques used (USEPA, 2007a).
3.1.4 A TMOSPHERIC F ATE OF S TABILIZER C OMPOUNDS
Chemical reactions in the atmosphere may occur in the vapor phase as gas-phase collisions between
molecules, on the surfaces of airborne solid particulate matter, and in aqueous solution in water
droplets. Reactions in water droplets are predominately of acid-base type. Reactions on particle
surfaces are of minor importance because of their short residence time in the atmosphere. Gas-phase
reactions are the dominant chemical reaction that transforms chemicals released to the atmosphere
(Seinfeld, 1986).
Physical reactions—including condensation and precipitation, dissolution into water droplets
followed by rain, and reactions with sunlight—can also remove, transform, or eliminate stabilizer
compounds emitted to the atmosphere. The longevity of a chemical in the atmosphere is referred to
as its atmospheric half-life, that is, the length of time before half the mass of the emitted compound
is removed, transformed, or eliminated by chemical or physical reactions. Most studies of atmo-
spheric half-lives for chemicals involve laboratory bench-top testing of compounds reacting with
simulated sunlight or atmospheric oxidants such as hydroxyl radicals, nitrate, ozone, and chlorine.
Modeling studies are also used to predict the atmospheric fate of compounds subjected to chemical
and physical reactions.
The most important atmospheric reactions affecting stabilizer compounds are photolysis, in
which ultraviolet (UV) light reacts directly with the compound to break it down, and photo-oxidation,
a reaction with hydroxyl radicals, chlorine, and other oxidants in the atmosphere. These reactions
are described in more detail in the following sections.
3.1.4.1 Photolysis
Photolysis occurs when a chemical absorbs light energy and undergoes chemical transformation.
Light is said to excite electrons and destabilize bonds in some compounds. Absorption of light does
* SMILES is “Simplii ed Molecular Input Line Entry System,” a notation system for describing molecular structure. See
Reinhard and Drefahl (1999) for a brief introduction.