Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 133

3.3.1 A QUEOUS S OLUBILITY OF S TABILIZER C OMPOUNDS AND
S TABILIZER -S OLVENT -W ASTE M IXTURES
Solubility, the maximum mass of a compound that will dissolve in pure water at a specii c tempera-
ture before phase separation occurs, is for many compounds the most important physical property
governing subsurface fate. Highly soluble compounds are less prone to partition into the vapor
phase and are generally less likely to be adsorbed to organic and mineral surfaces. Consequently,
hydrophilic and highly soluble compounds are not signii cantly retarded in groundwater l ow and
will migrate rapidly in groundwater. Relative retardation of contaminant migration is discussed
further in Section 3.4 .
In general, liquids obey the “like dissolves like” rule; polar molecules (e.g., alcohols, ketones,
and some ethers) are soluble in polar solvents (e.g., water), and nonpolar molecules (e.g., benzene)
are soluble in nonpolar solvents (e.g., oil).
Natural water contains mineral salts that may cause a minor change in the capacity of water to
dissolve organic compounds. In most groundwater, ionic strength contributed by dissolved minerals
will have a negligible effect on organic compound solubility. For groundwater having higher ionic
strength, a correction factor derived from experimental data for aromatic compounds can be applied
(Hashimoto et al., 1984):

log 10 S sw = [(0.0298 I + 1) log 10 S w ] − 0.004 I , (3.24)

where S sw is the solubility in seawater (in mol/L), S w is the freshwater solubility (in mol/L), and I is
the ionic strength, dei ned by

1

()

Â

2

I = C Z ,

2

i i

(3.25)

where C i is the concentration and Z i is the charge of the i th ionic species, summed over all ionic
species in solution (Lewis and Randall, 1921).
Chemical properties of organic compounds and the geochemical characteristics of groundwater
are the primary determinants of pollutant solubility, and molecular structure is the main determi-
nant of most chemical properties. Molecular structures that include branching have increased water
solubility for many compounds. Ring formation increases water solubility. Inclusion of a double
bond in the molecule, ring, or chain also increases water solubility. The presence of a second or
third double bond in a hydrocarbon proportionately increases water solubility, and a triple bond in
a chain molecule imparts greater solubility than two double bonds (Verschueren, 1996).
The technical literature lists a wide range of solubilities for the same compounds. Older literature
typically lists higher solubility values because older products were not as pure as today's chemicals
and because analytical methods have improved (Verschueren, 1996). In addition to laboratory
experiments, a number of estimation methods have been developed to calculate the expected com-
pound solubility based on molecular structures (QSARs) and a variety of other approaches.
The WATERNT program uses the atom/fragment contribution (AFC) estimation methodology
developed by Syracuse Research Corporation. The AFC estimation methodology is based on a
“fragment constant” method wherein coefi cients for individual fragments and groups were derived
by multiple regression of 1000 reliably measured water solubility values. WATERNT retrieves
experimental water solubility values from a database containing more than 6200 organic com-
pounds with measured values. When a compound structure matches a database structure via an
exact atom-to-atom connection match, the experimental water solubility value is retrieved (USEPA,
2007a). Table 3.9 lists literature values for measured and calculated solubilities for solvent-stabilizer
compounds and chlorinated solvents. The large differences between commonly cited solubility