Palladium Catalysts

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.

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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 contaminants.

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 aquifer.

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 (DOD).

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 2).

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 efficiency.

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.