Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 161

carbon and energy below 150,000 μ g/L and that there was incomplete biodegradation at concentrations

greater than 150,000 g/L, indicating production of an inhibitory toxic by-product of 1,4-dioxane.

In a follow-on paper (Roy et al., 1995), the growth of microorganisms on 1,4-dioxane was modeled
to estimate biodegradation kinetic parameters by using respirometric data. The growth data were
obtained from experiments that used microbes from an acclimatized industrial waste to investigate
the biodegradability of pure 1,4-dioxane. Growth of the microbes was measured by monitoring
oxygen uptake in a respirometer. Experiments were performed on a pure solution of 1,4-dioxane
(300,000 and 670,000

μ

g/L) in electrolytic respirometers as part of a 1989 Master's degree thesis
(Anagnostu, 1989). The modeling assumed no stable by-products, no toxicity to microorganisms
from dioxane, and Monod kinetics. From a best i t to experimental data of 1,4-dioxane depletion and
oxygen uptake, the maximum specii c growth rate,

μ

μ max , was found to be 0.127 day −1 , the saturation

g/L, and the cell yield was 0.218.
Grady et al. (1997) reported that 1,4-dioxane was very difi cult to biodegrade because of the
stability of the two ether bonds in the 1,4-dioxane saturated heterocyclic structure. Their study
coni rmed that 1,4-dioxane could serve as a sole source of carbon and energy through a series of
bioreactor experiments using 1,4-dioxane concentrations up to 150,000

constant, K s , was 182,000 μ

g/L. Growth on dioxane
alone was sustained for 5 weeks within a complex bacterial community with several bacterial genera
present; however, they were unable to isolate a pure culture capable of converting carbon to energy
from 1,4-dioxane alone. Biodegradation of 1,4-dioxane followed Monod kinetics with no indication
t h a t e it h e r s u b s t r a t e o r p r o d u c t i n h i b it io n o c c u r r e d a t 1, 4 - d iox a n e c o n c e n t r a t io n s u p t o 2 ,10 0 , 0 0 0

μ

g/L.
Grady et al. determined that growth is particularly sensitive to temperature, as proi led in Table 3.20.
At low temperatures, the growth rate of microbes on 1,4-dioxane would be too slow to maintain itself
without getting l ushed out of the bioreactor. This kinetics-based failure to biodegrade 1,4-dioxane
occurred especially in winter when bioreactor temperatures decreased to 20°C; it was attributable
neither to an inability of bacteria to degrade 1,4-dioxane nor to any inhibitory toxic characteristics.
Hyman (1999) studied the structural features that inl uence the reactivity of a single, nonspecii c
monooxygenase enzyme toward a variety of ether compounds. Hyman noted that many ether-
bonded compounds are oxidized by microorganisms that cannot further metabolize the products of
those reactions and that relatively few microorganisms appear to be able to utilize ether-containing
compounds as growth substrates. Hyman's study examined the rates of cometabolism of six ethers
and the degradation products obtained by bacteria and fungi. The study also investigated the physi-
ological consequences of ether cometabolism on bacteria. Hyman found that the alkane monooxy-
genase (AMO) enzyme is largely unreactive to cyclic ethers such as tetrahydropyran (THP) and
dioxane isomers, even though this enzyme oxidizes several sulfur analogs of the cyclic ethers. The
main focus of Hyman's study was ether oxidation by propane-oxidizing bacteria. AMO is a bacterial
enzyme; its capacity for oxidizing specii c carbon-hydrogen bonds in some organic compounds
leads to cometabolic degradation of recalcitrant contaminants.
Hyman's study used Mycobacterium vaccae JOB-5 grown on straight-chain (C3-C8) and branched
alkanes to cometabolically degrade the cyclic ethers THF, THP, hexamethylene oxide (HMO), and

μ

TABLE 3.20
Effect of Temperature on 1,4-Dioxane Biodegradation Kinetics in Bioreactor Experiments

Parameter Temperature

40°C 35°C 30°C 25°C

0.022 0.043 0.014 0.010

μ max , h −1

K S , mg/L 0.30 1.04 9.93 13.51

Y , mg biomass COD /mg dioxane COD 0.70 0.64 0.23 0.33

Source: From Grady, C.P.L., Jr., Sock, S.M., and Cowan, R.M., 1997, In: G.S. Sayler, J. Sanseverino, and K.L. Davis (Eds),
Biotechnology in the Sustainable Environment . New York: Plenum Press. With permission.