Environmental Engineering Reference
In-Depth Information
with Barbara Sherwood Lollar of the University of Toronto; unfortunately, the grant was not funded.
*
The study would have pursued measurement of the degree of isotopic fractionation occurring in
aerobic biodegradation of 1,4-dioxane (and anaerobic, if verii able) following approaches used to
apply CSIA to biodegradation of another ether, MTBE. The remainder of this discussion of applica-
tion of CSIA to 1,4-dioxane is paraphrased from the Oregon State University CSIA study outline,
in which the following approach was proposed:
1. Develop analytical methods for gas or liquid chromatography-isotope ratio mass spec-
trometry (GC-IRMS or LC-IRMS) for 1,4-dioxane and its expected biodegradation
products.
2. Determine fractionation factors (the shifts in isotope ratio per fraction contaminant
degraded) for both aerobic and anaerobic microcosms using pure and mixed cultures.
3. Apply the fractionation factors to i eld data to assess the extent to which the 1,4-dioxane
has biodegraded and under what conditions (e.g., aerobic or anaerobic).
4. Develop protocols to ensure that analytical measurements of both concentrations and iso-
tope values in the laboratory accurately rel ect the status of i eld and microcosm samples.
Isotopic fractionation resulting from biodegradation follows the Rayleigh model, in which the
extent of measured fractionation is proportional to the extent of biodegradation by the following
relationship:
(
δ
13
C
x
+ 1000
)
= 1000 × (α − 1) ln
F
= ε ln
F
,
___________
δ
1000
×
ln
1000
(9.6)
13
C
0
+
where
δ
13
C
x
and
δ
13
C
0
are the
13
C/
12
C isotope ratios determined from microcosm experiments at
time
x
and time 0,
is the carbon fractionation factor, and
F
is the ratio of contaminant concentrations at time
x
and 0 (e.g., the fraction remaining). Carbon
isotopic fractionation for nondegrading physical- or phase-transfer processes such as dissolution,
vaporization, or sorption for
dissolved
TCE and PCE is not signii cant at equilibrium (i.e.,
ε
is the carbon isotope enrichment factor,
α
0.5‰).
Therefore, shifts in contaminant isotopic ratios are useful as indicators of biodegradation (Slater
et al., 2000).
†
The fractionation factors (
<
) for each reaction can be calculated by posting the data for labora-
tory degradation experiments on a plot of ln
F
versus 1000
α
×
ln(
δ
13
C
x
/
δ
13
C
0
). Through the use of
least-squares regression, the fractionation factor,
, is determined as the slope of the plotted data.
Researchers applying CSIA in biodegradation studies use fractionation enrichment factors from
laboratory microcosm experiments and the kinetic isotope effect (KIE) described later in this sec-
tion. Measurements of isotope ratios for contaminants in i eld samples, when combined with the
laboratory-determined enrichment factors, permit the calculation of
F
, the ratio of contaminant
remaining relative to the initial concentration, which is a direct measure of the extent of biodegrada-
tion of the contaminant.
Researchers have found that different microorganisms grown with different electron acceptors
generated fractionation factors for chlorinated ethenes and for aromatic hydrocarbons in a relatively
narrow range. This general agreement of fractionation factors for organisms of different origin or
organisms using different electron acceptors suggests that different microbial populations in differ-
ent environmental conditions degrade the same contaminants via similar mechanisms, which results
in similar isotopic fractionation.
α
*
Personal communication with Jennifer A. Field of the Department of Environmental and Molecular Toxicology, Oregon
State University, December 2, 2005, regarding NSF Grant Proposal for 1,4-Dioxane CSIA Study.
†
The negligible isotopic shift for dissolved chlorinated ethenes is in contrast to the expected enrichment in the vapor phase
for these solvents in the vapor degreaser or dry-cleaner setting, where solvents are not in aqueous solution and a signii -
cant isotopic shift is expected.
Search WWH ::
Custom Search