Forensic Applications for 1,4-Dioxane and Solvent Stabilizers - Environmental Investigation and Remediation - page 442

Reaction rates for different isotopes vary because of mass-dependent differences in activation
energies for the respective reactions. This variation is called the KIE, which can be used to predict
the degree of both carbon and hydrogen isotopic fractionation during biodegradation. KIE describes
the ratio of the reaction rates for the heavy, H k , and light, L k , isotopes

L k

__

KIE =

H k .

(9.7)

Published KIE values from known reactions can be used to estimate the degree of fractionation
for biodegradation of a compound whose KIE has not yet been measured (Jennifer A. Field, Oregon
State University, personal communication, 2005).
KIE can be classii ed as primary (a large ratio, resulting from the atom being directly involved
in the reaction) or secondary (a small ratio, resulting from the atom being bonded to the reactive
center, but not immediately involved in the reaction). On the basis of KIE values for the hydroxyla-
tion reaction in biodegradation of other contaminants, only a moderate secondary carbon isotope
effect (KIE

1.015) is expected for 1,4-dioxane as the carbon atom is not immediately involved in
the reaction. A substantial primary hydrogen isotope effect (KIE

=

3 to 8) is expected as the hydro-
gen atom is thought to be directly involved in the reaction. Hydroxylation is proposed as the initial
degradation step in one possible pathway for 1,4-dioxane degradation, in which hydrolysis to dieth-
ylene glycol occurs (see Figure 3.7) (Steffan et al., 2007). Other degradation pathways may be
present and produce greater or lesser degrees of fractionation.
Given the established relationships between reaction pathway and KIE, it is possible to hypoth-
esize reaction mechanisms from the degree of isotopic fractionation observed for a given atom.
1,4-Dioxane biodegradation may undergo an initial step of proton abstraction. If proton abstraction
is involved, a substantial primary hydrogen isotope effect is expected with a theoretical KIE ranging
from 3 to 8 (Elsner et al., 2005). The large difference in KIE between carbon and hydrogen results
from the hydrogen atom undergoing a primary isotope effect whereas the carbon atom fractionation
is a secondary isotope effect.
At sites contaminated with 1,4-dioxane from methyl chloroform releases, it is likely that 1,4-di-
oxane is present in the highly anoxic or anaerobic portions of the release near the source zone, at the
oxygenated edges of the solvent plume, and at more distant locations where 1,4-dioxane has migrated
beyond the solvent plume. It is therefore relevant to use CSIA to determine the biodegradability of
1,4-dioxane under both aerobic and anaerobic conditions (Field, 2005). Both pure and mixed cul-
tures that possess the ability to degrade 1,4-dioxane aerobically and one study of anaerobic biodeg-
radation are described in Chapter 7.
The rate of anaerobic biodegradation of 1,4-dioxane is expected to be slow because the anaerobic
biodegradation of structurally similar compounds, such as MTBE, is slow (Kuder et al., 2005).
Under anaerobic systems, the addition of proportionately larger concentrations of another biode-
gradable ether may be a viable strategy for promoting anaerobic cometabolism of 1,4-dioxane
(Field, 2005). For example, water-soluble polyethylene glycol (PEG) is degraded by various bacteria
in the absence of molecular oxygen (Yeh and Pavlostathis, 2005). The addition of PEG may stimu-
late “etherase” activity in the subsurface capable of cleaving the 1,4-dioxane ring (Field, 2005).
Nakamiya et al. (2005) (see Chapter 7) reported that a i lamentous fungus degrades 1,4-dioxane via
etherase-type reactions to sequentially produce ethylene glycol, glycol-aldehyde, glycolic acid, and
oxalic acid.
By combining isotope ratio measurements and biodegradation studies under aerobic and anaero-
bic conditions, CSIA methods could be used to assess in situ 1,4-dioxane biodegradation at enhanced
bioremediation sites and MNA sites. This information would be highly useful for risk-based
remediation decision making by facility owners, regulators, and the environmental engineering
community. Numerous published studies have shown that CSIA is an excellent tool for providing
the evidence of in situ biodegradation and to quantify the extent of in situ biodegradation of chlori-
nated solvents at i eld sites; it remains to be seen whether this will also be the case for 1,4-dioxane.

=