Microcosm
experiments to assess microbial reductive dechlorination
of chlorinated aliphatic hydrocarbons typically experience 5–50%
mass loss due to frequent sampling events and diffusion through septa.
A literature review, however, reveals that models fit to such experiments
for kinetic constant estimation have generally failed to account for
experimental mass loss. To investigate possible resultant bias in
best-fit parameters, a series of numerical experiments was conducted
in which Monod kinetic models with and without mass loss were fit
to more than 1300 synthetic data sets, generated using published microcosm
data. Models that failed to account for mass loss resulted in significant
fitted parameter bias. Bias ranged from 5 to 45% of the parameter
magnitude for Monte Carlo simulations with low (approximately 10%)
mass loss to 20–120% for simulations with high (approximately
40%) mass loss. In addition, for high mass loss simulations, best-fit
values consistently fell along the bounds of the optimization range.
These results suggest that failure to properly account for mass loss
in microcosms may lead to inaccurate estimation of kinetic constants
and may explain some of the literature-reported variability in these
parameters. A model is presented that provides a method for including
sampling and diffusional mass losses to improve kinetic constant estimation
accuracy.
Perfluoroalkyl acids (PFAAs) have been shown to inhibit
biodegradation
(i.e., organohalide respiration) of chlorinated ethenes. The potential
negative impacts of PFAAs on microbial species performing organohalide
respiration, particularly Dehalococcoides mccartyi (Dhc), and the efficacy of in situ bioremediation
are a critical concern for comingled PFAA-chlorinated ethene plumes.
Batch reactor (no soil) and microcosm (with soil) experiments, containing
a PFAA mixture and bioaugmented with KB-1, were completed to assess
the impact of PFAAs on chlorinated ethene organohalide respiration.
In batch reactors, PFAAs delayed complete biodegradation of cis-1,2-dichloroethene (cis-DCE) to ethene.
Maximum substrate utilization rates (a metric for quantifying biodegradation
rates) were fit to batch reactor experiments using a numerical model
that accounted for chlorinated ethene losses to septa. Fitted values
for cis-DCE and vinyl chloride biodegradation were
significantly lower (p < 0.05) in batch reactors
containing ≥50 mg/L PFAAs. Examination of reductive dehalogenase
genes implicated in ethene formation revealed a PFAA-associated change
in the Dhc community from cells harboring the vcrA gene to those harboring the bvcA gene.
Organohalide respiration of chlorinated ethenes was not impaired in
microcosm experiments with PFAA concentrations of 38.7 mg/L and less,
suggesting that a microbial community containing multiple strains
of Dhc is unlikely to be inhibited by PFAAs at lower,
environmentally relevant concentrations.
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