Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 166

1,4-Dioxane degradation did not occur in the absence of molecular oxygen. Stoichiometric uptake
of dissolved oxygen accompanied 1,4-dioxane degradation by CB1190 within the i rst minute.
Mahendra and Alvarez-Cohen reported that Monod growth kinetics was observed for CB1190;
the maximum dioxane degradation rate ( k ) and half-saturation concentration ( K s ) were computed as
1.1

g/L, respec-
tively, as calculated by a nonlinear regression i t of the model to the data. Similarly, k and K s values
for Strain B5 were calculated as 0.1

± 0.008 mg of dioxane per hour per milligram of protein and 160,000 ± 44,000 μ

± 0.006 mg of dioxane per hour per milligram of protein and

330,000 ± 82,000 μ g/L, respectively. The cell yields for Strains CB1190 and B5 were 0.09 ± 0.002 mg

of protein per milligram of dioxane and 0.03 0.002 mg of protein per milligram of dioxane,

respectively. The electron equivalents ( e − eq) of these yields were 0.07 e − eq protein per e − eq
dioxane and 0.008 e − eq protein per e − eq dioxane, respectively.
Measured growth rates were relatively low because cell yields of all strains growing on
1,4-dioxane were low. Mahendra and Alvarez-Cohen (2006) noted that the yields were consis-
tently low for all cultures reported in the literature, even on an e − eq/ e − eq basis. The theoretical
dioxane transformation capacity of a microbial consortium was reported to be 1.0

±

0.36 mg of
dioxane per milligram of protein (Zenker et al., 2000). Mahendra and Alvarez-Cohen's study was
the i rst to dei nitively show the role of monooxygenases in dioxane degradation by using several
independent lines of evidence. The complete biodegradation pathway of 1,4-dioxane by
monooxygenase-expressing bacteria observed in the study by Mahendra et al. (2007) is presented
in Figure 3.9 .
Cai et al. (2008) isolated Strain D4 from a polyester production sewage aeration pond in China.
They determined that Strain D4 could use 1,4-dioxane as the sole carbon source. Observations of
morphology and physiology as well as biochemical tests and 16S rDNA genetic analysis were used
to identify Strain D4 as Bacillus pumilus. Ideal conditions for 1,4-dioxane degradation were a pH
of 7.0, a temperature of 30°C, and an inoculating quantity of 10%. Degradation rates of 83.7% and
85.6% were determined after 24 and 48 h from an initial concentration of 1000 mg/L.
Skinner (2007) completed an extensive study of biodegradation of ether compounds by the
eukaryotic alkanotroph Graphium sp. (ATCC 58400). Graphium sp. does not grow directly on
1,4-dioxane; however, it is able to cometabolize 1,4-dioxane after growth on either THF or alkanes
such as propane when mycelia are grown at 27

±

3°C. Both n -alkane- and THF-grown mycelia
oxidize 1,4-dioxane. Graphium sp. utilizes THF as a sole source of carbon and energy under aerobic
conditions via the same THF metabolic pathway used by Rhodococcus ruber and two Pseudonocardia
strains. The cometabolic growth rate of Graphium sp. on THF was approximately 10 times greater
than on 1,4-dioxane, which itself supports growth about twice the rate of 1,3-dioxane. Growth rates
of Graphium sp. from Skinner's study are presented in Table 3.21 .
Skinner's dissertation explained that physiological and inhibitor studies suggest that THF degra-
dation is initiated by the same cytochrome P450 responsible for oxidizing n -alkanes, diethyl ether,
and MTBE. There may be an even greater overlap between the enzyme activities involved in
n -alkane and THF oxidation. Growth of Graphium sp. on THF was fully inhibited by acetylene,
ethylene, propylene, or propyne (0.5% vol/vol gas phase), whereas growth on either

±

γ -butyrolactone

or succinate was unaffected by the presence of these gases at these concentrations.
Although Graphium sp. was grown on cyclic ethers containing a single oxygen atom (e.g., THF),
no growth was observed on cyclic ethers containing two oxygen atoms (e.g., 1,3-dioxane and 1,4-
dioxane), and no growth was observed for any of the nitrogen heterocycles tested as potential growth
substrates (morpholine, pyrrolidine, piperazine, and piperadine).
Evidence that the alkane and ether oxidation pathways are superimposed was determined by
measuring rates of THF, MTBE, and 1,4-dioxane degradation by Graphium mycelia incubated
under a spectrum of inductive conditions. Propane-induced mycelia were able to degrade ether
compounds (and vice versa) without either a lag phase or the buildup of metabolic intermediates.
The oxidation of alkanes and ethers by Graphium sp. is linked through a common catalyst,
a cytochrome P450 AMO enzyme that mediates the i rst step of these pathways. The study