Remediation Technologies - Environmental Investigation and Remediation - page 339

ultimately yielded. This greater required bacterial effort is consistent with the high energy demand
required for scission of the ether bond, in contrast to the small yield of assimilable carbon. The loss
of biodegrading activity could also be due to a rapid turnover of the active enzyme once the THF
is depleted.

7.6.1.4 Kinetics of Cometabolism
In Zenker et al.'s (2002) third noteworthy contribution to the body of 1,4-dioxane biodegradation
knowledge, the kinetics of cometabolism of cyclic ethers by a mixed culture was modeled by using
data from their 2000 study to estimate the maximum specii c utilization rate of 1,4-dioxane. Note
that in biodegradation studies, total suspended solids (TSS) is often used as a surrogate for biomass.
The maximum specii c utilization rates were determined to be 1.09 mg of THF per milligram of
TSS per day and 0.45 mg of 1,4-dioxane per milligram of TSS per day. The half-saturation
coefi cients ( K S ) were measured as 10.8 and 12.6 mg/L for THF and 1,4-dioxane, respectively. No
evidence of toxic by-products was observed, and Zenker et al. noted that THF biodegradation was
not inhibited by the presence of 1,4-dioxane. However, 1,4-dioxane biodegradation may be competi-
tively inhibited by the presence of THF. The model was capable of predicting the biodegradation of
1,4-dioxane and THF at 1,4-dioxane:THF molar ratios of 0.9:3.3.
The presence of the nongrowth substrate, 1,4-dioxane, did not signii cantly affect the biodegra-
dation of the growth substrate, THF. If the same enzyme is responsible for the biotransformation
of both ethers, then THF apparently binds much more aggressively to the enzyme than does 1,4-
dioxane. Zenker et al. also noted that in systems treating dilute industrial efl uents, 1,4-dioxane
concentrations may be present below the threshold necessary to maintain growth.

7.6.2 B ACTERIA G ENUS : R HODOCOCCUS
Additional work by Cowan et al. (1994) identii ed a bacterium from the genus Rhodococcus that
could degrade 1,4-dioxane as the sole carbon and energy source. They found that the bacterial
growth rate and the associated biodegradation rate were optimal at 35°C and decreased dramati-
cally above and below that temperature. Cowan et al. evaluated the performance of a l uidized bed
reactor inoculated with the Rhodococcus , which effectively reduced an initial 1,4-dioxane concen-
tration of 100,000 to less than 1000

g/L. However, the growth rates were very slow; adequate
removal was only achievable with long residence times.

μ

7.6.3 B ACTERIA G ENUS : P SEUDONOCARDIA

7.6.3.1 Degradation of Multiple Ether Pollutants
Vainberg et al. (2006) published a study of the biodegradation of THF, 1,4-dioxane, 1,3-dioxolane,
bis-2-chloroethylether (BCEE), and methyl tert -butyl ether (MTBE) by Pseudonocardia sp. strain
ENV478. Strain ENV478 degraded 1,4-dioxane after growth on sucrose, sodium lactate, yeast
extract, 2-propanol, and propane, which showed that there was some level of constitutive degrada-
tion activity. The highest rates of 1,4-dioxane degradation occurred after growth on THF. Degradation
of 1,4-dioxane caused the accumulation of 2-hydroxyethoxyacetic acid (2HEAA). Although ENV478
does not grow directly on 1,4-dioxane, ENV478 degraded 1,4-dioxane for more than 80 days in
aquifer microcosms in the presence of THF. The inability of strain ENV478 to grow solely on 1,4-
dioxane is related to its inability to efi ciently metabolize the 1,4-dioxane degradation product
2HEAA. Vainberg et al. proposed that strain ENV478 may nonetheless be useful as a biocatalyst for
remediating 1,4-dioxane-contaminated aquifers.
The Vainberg et al. paper discusses the identii cation of urinary metabolites of 1,4-dioxane
from toxicology studies using rats as indicators of possible bacterial 1,4-dioxane biodegradation
pathways. Two urinary metabolites of 1,4-dioxane are 2HEAA and 1,4-dioxane-2-one (PDX).
δ -Hydroxy acids are generally unstable in aqueous solutions except as salts; they more commonly