Modeling Pd-Catalyzed Destruction of Chlorinated Ethenes
in Groundwater
Chris M. Stoppel
1
and Mark N. Goltz
2
Abstract: Groundwater contamination by chlorinated ethenes is a widespread environmental problem. Shortcomings in conventional
remediation methods have motivated research into novel treatment technologies. A palladium/alumina catalyst in the presence of molecu-
lar hydrogen gas referred to hereafter as the Pd/H
2
system has been demonstrated to destroy chlorinated ethenes in contaminated
groundwater. This study presents a model for aqueous-phase destruction of chlorinated ethenes in contaminated groundwater using the
Pd/H
2
system that includes catalyst deactivation and regeneration. The model is validated using published data from laboratory column
experiments from Stanford University. The model is then coupled with an analytical groundwater flow model to simulate application of
in-well Pd/H
2
reactors for in situ treatment of chlorinated ethene contaminated groundwater in a recirculating horizontal flow treatment
Well HFTW system. Applying the model under realistic conditions results in approximately 130 days of HFTW system operation
without significant catalyst deactivation. This suggests catalyst deactivation will not significantly affect system performance in a real
remediation scenario. The model presented in this study, which simulates deactivation kinetics and regeneration of an in-well catalyst that
is a component of a recirculating well system designed for in situ treatment of contaminated groundwater, represents an important step in
transitioning the Pd/H
2
technology to the field.
DOI: 10.1061/ASCE0733-93722003129:2147
CE Database keywords: Ground-water pollution; Models; Water treatment.
Introduction
Chlorinated ethenes, such as tetrachloroethene PCE and trichlo-
roethene TCE, are prevalent groundwater contaminants. Current
remediation technologies, such as pump-and-treat and permeable
reactive barriers, have shortcomings that have motivated research
into the development of innovative treatment technologies. Re-
cent studies conducted at Stanford University have shown rapid
destruction of chlorinated ethenes using a palladium Pd catalyst
in the presence of molecular hydrogen gas hereafter referred to
as the Pd/H
2
systemLowry and Reinhard 1999, 2000. In this
system, contaminant destruction is accomplished at the catalyst
surface with hydrogen gas acting as an electron donor. The chlo-
rinated ethene contaminant molecule is destroyed through the
process of hydrodehalogenation, whereby the halide atoms on the
molecule are replaced by hydrogen. Due to the very fast kinetics
of the Pd-catalyzed hydrodehalogenation reaction, a Pd reactor
can potentially be deployed in well to effect in situ contaminant
destruction that is, contaminant destruction without the need to
pump contaminated groundwater to the surface. As the contami-
nant is treated in situ, the in-well system reduces pumping costs,
risk of contaminant exposure, and the overall size of the above-
ground treatment facility. In addition, the treatment well pumps
maintain positive control of contaminant migration. Successful
application of the Pd/H
2
system has been demonstrated at
Lawrence Livermore National Laboratory McNab and Ruiz
2000.
One potential problem with in-well deployment of the Pd/H
2
system is catalyst deactivation, resulting in decreased contami-
nant destruction efficiency. Catalyst deactivation due to the pres-
ence of sulfur containing species (HS
-
and SO
3
2 -
) and formation
of HCl at the catalyst surface has been observed Lowry and
Reinhard 2000. Lowry and Reinhard 2000 showed that flushing
the catalyst column for 90 min with a dilute sodium hypochlorite
solution resulted in complete recovery of catalyst activity. One
proposed strategy for in-well application of the Pd/H
2
system is to
install the catalytic reactors as components of a horizontal flow
treatment well HFTW system. In an HFTW system, multiple
dual-screened wells, alternately operating in upflow and down-
flow directions, are used to create recirculating flow patterns or
interflow between wells. As seen in Fig. 1, this configuration
allows a portion of contaminated groundwater to recirculate, re-
sulting in recycling of contaminated water through the in-well
treatment reactors, thereby increasing overall contaminant re-
moval efficiency comparing contaminant concentrations upgradi-
ent and downgradient of the treatment system. An HFTW system
has been successfully demonstrated at Edwards Air Force Base
AFB, where TCE-contaminated groundwater was recirculated
through stimulated bioactive treatment zones located between a
single well pair McCarty et al. 1998. The HFTW demonstration
at Edwards AFB involved injection of toluene into the subsurface
to stimulate indigenous bacteria that biologically degraded the
TCE. An advantage of the proposed treatment strategy, using
1
Air Force Center for Environmental Excellence, 3207 Sidney
Brooks, Building 532, Brooks Air Force Base, TX 78235-5344. E-mail:
chris.stoppel@brooks.af.mil
2
Air Force Institute of Technology, 2950 P Street, Building 640,
Wright-Patterson Air Force Base, OH 45433-7765. E-mail:
mark.goltz@afit.edu
Note. Associate Editor: Wendell P. Ela. Discussion open until July 1,
2003. Separate discussions must be submitted for individual papers. To
extend the closing date by one month, a written request must be filed with
the ASCE Managing Editor. The manuscript for this paper was submitted
for review and possible publication on August 28, 2001; approved on
March 7, 2002. This paper is part of the Journal of Environmental
Engineering, Vol. 129, No. 2, February 1, 2003. ©ASCE, ISSN 0733-
9372/2003/2-147–154/$18.00.
JOURNAL OF ENVIRONMENTAL ENGINEERING / FEBRUARY 2003 / 147