Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 156

plant are unchanged in efl uent (Skadsen et al., 2004). The general conclusion of the growing body of
research into biodegradation of 1,4-dioxane is that it does not biodegrade appreciably under ambient
conditions; however, in laboratory-controlled environments, 1,4-dioxane can be degraded completely
with enhancements. The “holy grail” in the pursuit of 1,4-dioxane biodegradation is the means to
induce signii cant and sustainable in situ biodegradation under ambient temperatures. Table 3.18
summarizes key research on the capacity of microbes to degrade 1,4-dioxane.
Most of the research into 1,4-dioxane biodegradability has focused on aerobic biodegradation.
Anaerobic biodegradation of cyclic ethers is not widely reported. Microcosm studies on soils from
dioxane-contaminated sites in Maryland and New York showed no degradation under anaerobic
conditions (Steffan, 2006). USEPA's guidance document (1999) for implementation of monitored
natural attenuation remedies describes protocols for conducting microcosm tests for anaerobic
biodegradation. To dissolve hydrophobic or nonaqueous-phase liquids, the guidance recommends
using 1,4-dioxane instead of alcohol, because alcohols will biodegrade anaerobically whereas
1,4-dioxane will not (USEPA, 1999).
Aliphatic ethers such as methyl tert -butyl ether (MTBE) can be degraded anaerobically (Pruden
et al., 2005). MTBE will mineralize in anoxic conditions at slow but measurable rates, beginning
with cleavage of the ether linkage (Zenker, 2006). Cyclic ethers like 1,4-dioxane may also be cleav-
able under strongly anaerobic conditions. Tests of anaerobic biodegradation of the cyclic ether THF
with methanotrophic bacteria showed less than 30% of the theoretical gas production (Battersby
and Wilson, 1989). Humic acids have many ether linkages in their structure and may be a candidate
for stimulating cometabolic breakdown of 1,4-dioxane in both oxic and anoxic conditions (Zenker,
2006). Anaerobic biodegradation of 1,4-dioxane under humic and iron-reducing conditions was
recently reported (Pan and Chen, 2006; Shen et al., 2008).
The ether linkage, C-O-C, is commonly encountered in nature (lignins) and in anthropogenic
compounds (agrochemicals, detergents) and is generally resistant to biological mineralization (White
et al., 1996). The intrinsic refractivity of the ether linkage to biological destruction was recognized
early in the history of biodegradation research (Alexander, 1965). Bacterial scission of ether bonds is
not thermodynamically favored, as noted in a seminal paper surveying the topic (White et al., 1996):
“Microbial cleavage of the ether bond is a remarkable phenomenon, since the C-O bond energy * is
360 kJ/mol and necessarily demands an appreciable investment of energy to effect its i ssion in rela-
tion to the sometimes relatively small yield of assimilable carbon.” The enzymes responsible for
bacterial scission of ether bonds were called etherases in early work and were believed to catalyze a
hydrolysis reaction that cleaves the bond; however, catalysis of the ether bond division is now attrib-
uted to a heterogeneous group of enzymes that exhibit a variety of mechanisms to break the ether
bond. Hydrolysis does not play an important role in cleaving the ether bond (White et al., 1996).
Table 3.18 summarizes the research into metabolism and cometabolism of 1,4-dioxane by pure
and mixed cultures of bacteria and fungi. Where biodegradation has been documented, the rate and
sustainability must also be determined. Biodegradation that occurs too slowly will not have a sig-
nii cant impact on 1,4-dioxane concentrations in the subsurface environment if migration rates
substantially exceed degradation rates. Intermediate products that are toxic to and inhibit the
growth of microl ora will also limit the degree to which biodegradation lowers concentrations of
1,4-dioxane in groundwater.
Rates of biodegradation of 1,4-dioxane can be described by the commonly used general equation
(Chappelle, 1993; Evans et al., 2006):

m XS

max

r = , (3.40)

YK ( + S )

s

* kJ = kilojoules; 1 kJ ≈ 0.239 kilocalories (kcal); 360 kJ/mol = 86 kcal/mol. Alexander (1994) lists the ether linkage bond
energy as 85.5 kcal/mol.