Remediation Technologies - Environmental Investigation and Remediation - page 338

of the aerobic and anaerobic bacterial populations and fungi were conducted for growth on the site
contaminants acetone, isopropanol, methanol, toluene, dichloroethane, dichloromethane, cyclo-
hexane, and 1,4-dioxane as the sole source of carbon and energy. All the major site contaminants,
except for 1,4-dioxane, were utilized by the aerobic and anaerobic bacteria and fungi.
Zenker et al. (1999) sought to determine whether cyclic ethers and alkyl ether oxygenates such
as MTBE biodegrade under ambient conditions in soil microcosms. The microcosms used 1,4-
dioxane concentrations of 1000 and 200,000 μ g/L as well as mixtures of 1,4-dioxane and THF at

g/L (Zenker et al., 1999). The researchers noted the complete biodegradation of 1,4-
dioxane and THF at 35°C and high concentrations (200,000

200,000 μ

g/L) with the addition of nitrogen,
phosphorous, and trace minerals. Biodegradation of 1,4-dioxane was dependent upon the presence
of THF. Microcosms using activated sludge and incubated under ambient conditions exhibited no
biodegradation of 1,4-dioxane or THF, which suggests that breakdown of 1,4-dioxane is unlikely
to succeed by intrinsic bioremediation. The latter involves a demonstration that indigenous sub-
surface microl ora do not have the capability to biodegrade the target contaminants without artii -
cial augmentation.

μ

7.6.1.3 Degradation of Tetrahydrofuran and 1,4-Dioxane Together
In the second important paper on the biodegradation of 1,4-dioxane from North Carolina State
University, “Mineralization of 1,4-Dioxane in the Presence of a Structural Analog,” a mixed bacte-
rial culture (a “consortium”) capable of aerobically biodegrading 1,4-dioxane in the obligate pres-
ence of THF was enriched from a 1,4-dioxane-contaminated aquifer sediment. Zenker et al. (2000)
used a continuous-l ow rotating biological contactor (RBC) to inoculate samples from each soil
microcosm bottle at 35°C. The RBC was supplied with 6 L/day of a 25% BSM containing 20 mg/L
of THF and 30 mg/L uniformly 14 C-labeled 1,4-dioxane. Radio-labeled 1,4-dioxane was utilized to
monitor destruction and assimilation (labeling molecules allows the tracing and quantitation of
by-products).
THF and 1,4-dioxane disappeared after three months in one microcosm and after 10 months in
the remaining two. Microcosms incubated without THF did not exhibit 1,4-dioxane biodegradation,
even when respiked with additional 1,4-dioxane. When more THF was added, both compounds
were degraded within 75 days. Zenker explained that these data show that the enrichment culture
can completely oxidize 1,4-dioxane. A small amount (16.1%) of 14 C-labeled 1,4-dioxane was
observed in the particulate fraction, which the researchers considered to be cell mass. The majority
of the 1,4-dioxane (78.1%) was converted to CO 2 , whereas 5.8% remained in the liquid.
Zenker et al. noted that the biodegradation of 1,4-dioxane was dependent on the presence of
THF. Repeated addition of 1,4-dioxane in the absence of THF failed to stimulate biodegradation.
During the 400 days in which the RBC was operated, samples of the bioreactor culture were peri-
odically placed in BSM containing only 1,4-dioxane. No disappearance of 1,4-dioxane or microbial
growth was observed during the course of these experiments.
The toxicity to by-products is due to the formation of HEAA that can denature enzymes or other
cellular components. Therefore, the continued biodegradation of the nongrowth substrate is limited
by the effects of product toxicity. In contrast, 1,4-dioxane biodegradation did not decrease the
observed THF biodegradation rate.
The study showed that 1,4-dioxane does not begin to disappear until THF reaches relatively low
levels. This result suggests competitive inhibition, a commonly observed phenomenon in cometa-
bolic processes. Even though Zenker et al.'s study shows that 1,4-dioxane is completely destroyed,
produces no toxic by-products, and is partially assimilated into biomass, the ability of the bacterial
culture to biodegrade 1,4-dioxane is lost in the absence of THF.
The North Carolina State University research suggests that the loss of biodegradation activity may
be due to the depletion of enzymatic cofactors, or coenzymes. The biodegradation of 1,4-dioxane
may exert an abnormally high demand on the resources of the consortium. For example, the con-
sortium may invest more energy and/or coenzymes in the biodegradation of 1,4-dioxane than is