of the Problem
Dense Non-Aqueous Phase Liquids (DNAPLs) are chemicals
that do not mix well with water. When they spill,
they can sink deep within aquifers and form highly
concentrated "source zones" that are extremely
difficult to clean up. One possible clean-up approach
is in situ bioremediation -- a method where native
or introduced microbes feed on the chemicals and
convert them into harmless end products. In situ
bioremediation is an attractive solution because
it potentially has lower costs than other cleanup
technologies, particularly ones that require pumping
contaminants to the surface.
WRHSRC researchers Seungho Yu and Lewis Semprini
are investigating one biochemical process that could
be exploited for in situ bioremediation of contaminant
source zones. The process, called anaerobic reductive
dechlorination, degrades one common type of DNAPL,
the chlorinated ethylenes. The toxic solvents
Tetrachloroethylene (PCE) and trichloroethylene (TCE)
are both members of this class of chemicals.
Anaerobic Reductive Dechlorination
contaminated with high concentrations of chlorinated solvents
such as TCE and PCE is extremely
difficult to clean up. One process that shows promise for in
is anaerobic reductive dechlorination. In this process, anaerobic
microbes trigger a sequence of reactions that transform PCE
and TCE to the harmless substance ethene.
on highlighted words and definitions and illustrations
will pop up.
A first step toward developing reductive dechlorination for
bioremediation is to define the kinetics and inhibition of the
transformation reactions. WRHSRC researchers Lewis Semprini and
Seungho Yu are working on this problem. They have defined kinetic
parameters for each step in the reaction sequence and developed
kinetic models that successfully predict dechlorination rates
wide range of PCE and TCE concentrations. Their work is described
in a forthcoming paper in Biotechnology and Bioengineering.
Defining Kinetic Parameters
Anaerobic reductive dechlorination of TCE and PCE occurs as
a sequence of reactions (Figure
1). Microbes replace chloride molecules
in the chemical's structure with hydrogen molecules. Replacement
of one chloride molecule transforms
PCE to TCE. Replacement of another transforms TCE to cis-dichloroethylene
(cis-DCE), then cis-DCE to vinyl chloride, and finally vinyl
chloride to the harmless substances ethene and chloride.
Yu and Semprini carried out a series of stepwise experiments
to determine kinetic parameters for each reaction in the sequence
(Yu, 2003). The experiments used batch kinetic reactors -- small
anaerobic conditions and contained enriched
For each experiment, they introduced a known concentration of
a compound, for example c-DCE, and measured its disappearance
and the production of its daughter compound. They then purged
the reactor and added a higher concentration of the parent compound
and repeated the measurements (Figure
2). They repeated this procedure in
steps and determined kmax
Ks values from regression of the transformation
rate vs. concentration results.
Modeling Transformation and Inhibition
The team used the kinetic parameters to develop models of
the complete dechlorination reaction sequence. The models mathematically
predicted the transformation rate for each intermediate product
and the production rate of the end product ethene. They compared
the models predictions with results from experiments with a wide
range of starting concentrations of PCE and TCE. They
were particularly interested in whether they could successfully
model dechlorination for high concentrations, up to the solubility
limit of PCE and half the solubility limit of TCE.
The team’s initial model used competitive Michaelis-Menten
kinetics to predict dechlorination rates. This model took into
account the ability of some products in the reaction sequence
to inhibit other reaction steps. For example, in earlier experiments
they found that the more chlorinated compounds inhibited dechlorination
of the less chlorinated compounds. When they compared their model
predictions with laboratory experiments, they found good agreement
for dechlorination of low concentrations of PCE and TCE. However,
the model did not predict the transformation rates they observed
for high contaminant concentrations -- model transformation rates
were much higher than those observed in the experiments. This
suggested that an additional inhibition mechanism affects reaction
rates when PCE and TCE concentrations
are very high. Yu and Semprini then developed a second model
which included both competitive and Haldane inhibition
itself inhibits its own transformation. This model was successful
-- it predicted the transformation rates they observed for experiments
with very high concentrations of PCE and TCE (Figure
The researchers are now experimenting with the cultures in continuous-flow
columns (Figure 4). These
studies will help evaluate the potential of anaerobic reductive
dechlorination under flow conditions
representative of in-situ remediation. They will also use molecular
techniques to identify and monitor the microbial cultures. Each
research step brings the scientists closer to a practical cleanup
solution for groundwater contaminated with high concentrations
of chlorinated solvents.
For More Information
Contact Dr. Lewis Semprini
or read more in the following references:
Yu, S., 2003, Kinetic and modeling investigations of the
anaerobic reductive dechlorination of chlorinated ethylenes
using single and binary mixed cultures and silicon-based
organic compounds as slow-release substrates, Ph.D. Dissertation,
Department of Civil, Construction, and Environmental Engineering,
Oregon State University.
Yu, S. and Semprini, L., 2004, Kinetics and modeling of
reductive dechlorination at high PCE and TCE concentrations,
Biotechnology and Bioengineering, in press.
Yu, S. and Semprini, L., 2002a, Comparison of trichloroethylene
reductive dehalogenation by microbial communities stimulated
on silicon-based organic compounds as slow-release anaerobic
substrates, Water Research, vol. 36, p.4985-4996.
Yu, S. and Semprini, L., 2002b, Dechlorination of PCE
DNAPL with TBOS Using a Binary Mixed Culture. In Remediation
of Chlorinated and Recalcitrant Compounds, Battelle Press,
Columbus, OH, Paper 2B-49.
ATSDR, 1997, Public Health Statement for Tetrachloroethylene,
CAS# 127-18-4, http://www.atsdr.cdc.gov/toxprofiles/phs18.html .
Squillace, P.J., Scott, J.C., Moran, M.J., Nolan, B.T.,
and Kolpin, D.W., 2002, VOCs, pesticides, nitrate, and their
mixtures in groundwater used for drinking water in the United
States: Environmental Science & Technology, v. 36, no.
9, p. 1923-1930.(report
--pdf version). Supplemental
material (MS Excel format).