Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 130

such as ethers, alcohols, alkanes, alkenes, aromatics, and heterocyclic compounds. Compounds sus-
ceptible to hydrolysis include amines, amides, epoxides, nitriles, esters, and several other compounds
not found among the stabilizer or chlorinated solvent compounds (Harris, 1990b). There are no
hydrolysable functional groups on the 1,4-dioxane molecule; therefore, 1,4-dioxane is not expected
to hydrolyze signii cantly (NICNAS, 1998).
Hydrolysis is a i rst-order reaction whose rate varies directly with the concentration of the organic
species, RX, as shown:

d[RX]

______

d t

= − k [RX], (3.19)

where k is the i rst-order hydrolysis rate constant in units of inverse time (in s −1 ), giving a disappear-
ance rate of moles per liter per second. The i rst-order rate constant describes the loss of RX at any
concentration of RX and at constant pH, pressure, and temperature. At any time t , the concentration
of RX, [RX] t , is given by

[RX] t = [RX] 0 e − kt , (3.20)

where [RX] 0 is the starting concentration of RX, t is time in seconds, k is the i rst-order rate con-
stant, and e is the natural logarithm (USEPA, 1998a; Bennett, 2004a). The time needed to decrease
the concentration by one-half, that is, the half-life ( t 1/2 ), is

ln 2 0.693

___ _____

t 1/2 =

k

=

k

. (3.21)

The hydrolysis reaction described above is the unimolecular form. A bimolecular form of the
hydrolysis reaction involves kinetics that depend on both the concentration of the organic molecule
RX and the nucleophile, H 2 O, H + , and OH − . Hydrolysis reactions are therefore sensitive to the pH of
the water, so that a difference of 2 pH units would cause the reaction rate to change 100-fold.
Depending on whether hydroxide (OH − ) or hydrogen (H + ) is the dominant species at a given pH,
bimolecular hydrolysis reactions are characterized as base-catalyzed hydrolysis or acid-catalyzed
hydrolysis (Hemond and Fechner, 1994). Catalysis by microbial or other agents may further compli-
cate the kinetics of hydrolysis; therefore, citations of hydrolysis rate constants from the literature or
from predictive modeling must be used with caution, as natural systems are likely to be a great deal
more complex than the system measured or modeled.
Equations and software to estimate hydrolysis rate constants for those compounds susceptible to
acid-catalyzed or base-catalyzed hydrolysis are freely available (USEPA, 2000a). The HYDROWIN
module in the EPIWIN Suite only estimates aqueous hydrolysis rate constants for esters, carbamates,
epoxides, halomethanes, and selected alkyl halides (USEPA, 2000a). HYDROWIN does not esti-
mate neutral hydrolysis rate constants, which may be the dominant hydrolysis rate for some epoxide
compounds, leading to underestimation of the rate constant. Molecular structure features are used to
estimate the acid- and base-catalyzed rate constants, which are then used to calculate hydrolysis half-
lives (Mill et al., 1987; USEPA, 2000a). Table 3.7 summarizes estimates for those solvent-stabilizer
compounds susceptible to hydrolysis. Literature values for two chlorinated solvents are included to
indicate their immunity to hydrolysis. Ethers are generally resistant to hydrolysis; HYDROWIN does
not estimate hydrolysis rate constants for ether compounds, including 1,4-dioxane.

3.2.2 A CID D ISSOCIATION P OTENTIAL
Organic compounds that are prone to form ions by dissociating when dissolved in water will behave
differently when in pure form. Ionization of an organic compound will affect its solubility and