of the Problem
Some of the most pervasive groundwater pollutants
in the United States are volatile organic compounds
(VOCs). VOCs are hydrocarbons that are used in the
production of paints, plastics, adhesives, gasoline,
and as degreasing agents. In a national study of 1500
drinking water wells, the US Geological Survey found
that 44% contained at least one VOC (Squillace, 2002).
One of the most common VOCs, the halogenated
compound tetrachloroethylene (PCE), has been found
at more than half of the sites identified for cleanup
by the federal Superfund Program. Potential health
effects of exposure to PCE include liver and kidney
damage, birth defects, and cancer (ATSDR, 1997).
Despite the pervasiveness of VOCs, few cost-effective
cleanup technologies are available. This research
brief focuses on palladium catalysis -- a promising
new technique that may expand the treatment options
for halogenated VOCs such as PCE.
Researchers at the Western Region Hazardous Substance
Research Center (WRHSRC) are pioneering a new technique to cleanup
groundwater contaminated with halogenated
volatile organic compounds (VOCs). The technology uses palladium,
a metal, as a catalyst to trigger the chemical transformation
of the contaminants into benign end products.
Click on highlighted words and definitions and illustrations
will pop up.
Halogenated VOCs such as trichloroethylene (TCE) and tetrachloroethylene
(PCE) are among the most common groundwater contaminants (see
box). The chemicals are used widely for applications such as dry
cleaning and degreasing. If they leak from storage or disposal
sites, they can leach into aquifers and become long-lasting groundwater
In the mid-1990’s, WRHSRC researchers Dr. Martin Reinhard
and Dr. Paul Roberts began to test the potential application of
palladium catalysts for treating groundwater contaminated with
halogenated VOCs. Palladium was a known catalyst in organic chemistry
applications and European and American researchers were just beginning
to identify potential environmental applications.
The WRHSRC team first tested the reaction in tap water mixed
with chlorinated compounds such as PCE and vinyl chloride (Schreier
and Reinhard, 1995). When they added hydrogen gas and the palladium
catalysts, the contaminants were destroyed in minutes. The reaction
replaces chlorine atoms in the contaminant molecules with hydrogen
atoms. In the case of TCE (Figure
1), the double bond between carbon atoms also is broken and
ethane and hydrochloric acid result. The reaction usually produces
concentrations of acid that are low enough to be buffered by the
The technology offers many advantages over existing treatment
methods. First, palladium catalysts chemically transform the target
contaminants. Many existing technologies transfer the contaminants
to another medium (such as granular activated carbon) rather than
destroy them outright. Second, palladium catalysts trigger fast
reactions. This reaction rate makes it possible to treat the water
with in-well reactors and avoid the need to pump water to the
ground's surface for treatment. Third, the technology is applicable
to deep aquifers with high contaminant concentrations, situations
where other treatment technologies may not be feasible.
During the late 1990’s, Dr. Reinhard, a professor of environmental
engineering at Stanford University, and his team continued to
study palladium catalysts. They investigated which contaminants
are amenable to the process, reaction rates under different conditions,
and ways to maintain the catalyst activity over time (see below
for references). They also collaborated on a field study with
Walter McNab and Roberto Ruiz from Lawrence Livermore National
Laboratories (LLNL) (McNab et. al, 2000). LLNL has a Superfund
Site with a variety of contaminants including TCE and PCE.
The 1990's experiments and field tests all indicated a high potential
for palladium catalysts. However, they also prompted new research
questions – topics the Dr. Reinhard's team continues to
work on with funding from the WRHSRC’s new grant cycle and
with a technology demonstration grant from the Department of Defense
Current Field Studies
The DOD grant funds a field study that will determine cost and
performance data and develop ways to maximize treatment efficiency
of the catalysts. Dr. Jeffrey Cunningham, an Engineering Research
Associate at Stanford, will manage the project which will take
place at Edwards Air Force Base, California (Figure
The field study incorporates two novel technologies – the
palladium catalysts and horizontal flow treatment wells (HFTWs),
a tool developed by another WRHSRC researcher and Stanford professor,
Dr. Perry McCarty (McCarty et al., 1998). The HFTWs consist of
two wells each with two screened sections (Figure
3). The wells pump in opposite directions pulling water through
one screened section, past an in-well palladium reactor, and out
the other screened section. The opposite flow directions in the
two wells set up a circular flow pattern (Figure
4). In an aquifer with a lateral gradient, a portion of the
“cleaned” flow will move down gradient, while the
remainder will cycle back through the wells for further cleaning.
Multiple passes through the reactors increases contaminant removal
Catalyst Deactivation and Current Lab Studies
One of the factors which most affect the cost of the treatment
system is the activity of the catalyst. In the field test at LLNL,
the reactors maintained long-term performance for well over one
year. However, the research team could only operate the system
for 10 hours a day before the contaminant removal efficiency began
to decline. After this period, the palladium had to be regenerated
by exposure to air for at least 14 hours.
The cause of deactivation is the focus of Dr. Reinhard’s
current laboratory research. He and Dr. John Westall, professor
of chemistry at Oregon State University, are leading a WRHSRC
project studying the chemical and physical mechanisms responsible
for changes in catalyst activity. Their goal is to develop convenient
and economical methods to regenerate catalysts in situ.
As part of this project, graduate student Naoko Munakata is building
a pilot reactor system, in which she can compare water sources
with different chemistries to understand how water quality influences
catalyst activity. Using catalyst samples from the pilot system,
she will analyze the catalyst surface with X-ray photoelectron
spectroscopy to learn more about the surface chemistry of the
catalysts as a function of their activity.
Both in the lab and at the Edwards Air Force Base field site,
the research team will explore ways to regenerate the catalysts
in situ. In earlier studies, they found that they could regenerate
the catalysts by exposing them to sodium hypochlorite. They will
compare the effectiveness of sodium hypochlorite treatment with
the use of hydrogen peroxide, and air-saturated water.
For More Information
Contact Professor Martin Reinhard at firstname.lastname@example.org
or read more in the following references:
Lowry, GV and Reinhard, M , 2001, Pd-catalyzed TCE dechlorination
in water: effect of H sub(2)](aq) and H sub(2)-Utilizing
Competitive Solutes on the TCE Dechlorination Rate and Product
Distribution, Environmental Science & Technology, v.
35, no. 4, pp. 696-702.
Lowry, G.V. and Reinhard, M., 2000, Pd-catalyzed TCE dechlorination
in groundwater: solute effects, biological control, and
oxidative catalyst regeneration, environmental science and
technology, v. 34, no. 15, p. 3217-3223.
McNab, W., Ruiz, R., and Reinhard, M., 2000, In-situ destruction
of chlorinated hydrocarbons in groundwater using catalytic
reductive dehalogenation in a reactive well: testing and
operational experiences, environmental science and technology,
v. 34, no. 1, p. 149-153.
Lowry, G.V. and Reinhard, M., 1999, Hydrodehalogenation
of 1- to 3-carbon halogenated organic compounds in water
using a palladium catalyst and hydrogen gas, Environmental
Science & Technology, v. 33, no. 11 (June 1 1999) p.
Siantar, D., Schreier, C., Reinhard, M., 1996, Treatment
of 1,2-dibromo-3-chloropropane and nitrate-contaminated
water with zero-valent iron or hydrogen/palladium catalysts,
Water Research, v. 30, p. 2315-2322.
Schreier, C. and Reinhard, M., 1995, catalytic hydrodehalogenation
of chlorinated ethylenes using palladium and hydrogen for
the treatment of contaminated water, Chemosphere, v. 31,
ATSDR, 1997, Public Health Statement for Tetrachloroethylene,
CAS# 127-18-4, http://www.atsdr.cdc.gov/toxprofiles/phs18.html
McCarty, P. et al., 1998
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).