Catalytic Reductive Dehalogenation
Catalytic Reductive Dehalogenation (CRD) uses dissolved hydrogen as a reducing agent, in the presence of a palladium-on-alumina catalyst, to chemically transform compounds such as trichloroethylene (TCE) and tetrachloroethylene (PCE) into environmentally benign ethene without the accumulation of intermediate transformation products such as vinyl chloride. The catalyst, called a reactor, can be placed in an extraction well or above ground. As water is pumped through the reactor, it is broken down, very similar to the passive Permeable Reactive Barrier technology. Because of its rapid reaction rates (within several minutes removal efficiencies for most of the chlorinated hydrocarbons are greater than 99%), a treatment unit system can be placed in a dual-screened well, allowing contaminated groundwater to be drawn from one water-bearing zone, treated within the well, and discharged to an adjacent zone.
The extra cost of the palladium catalyst is justified when groundwater has co-contaminants that are safer to leave in the ground, such as radionuclides. The system was developed at Lawrence Livermore National Laboratory (LLNL) so that tritiated groundwater would not be brought to the surface while TCE was destroyed. The groundwater contains high concentrations of TCE (up to 30 parts per million) as well as tritium (up to 20,000 picocuries/L). The presence of tritium in the groundwater complicates conventional pump-and-treat approaches to remediating the chlorinated hydrocarbon contamination because of the potential for accidental releases to the environment as well as the need to dispose of mixed wastes (tritiated spent activated carbon) generated from air stripping.
Limitations and Concerns
With in-well treatment systems, the geology of the site must be thoroughly understood. At LLNL, one of the well screens was in an area that would not accept the volume of water being pumped through the reactor. Therefore, LLNL built a second reactor above ground for ex-situ treatment with re-injection of the contaminated (tritiated) water.
Technology Development Status
LLNL and Stanford University developed this system for rapidly destroying chlorinated hydrocarbon contaminants dissolved in groundwater. There is a full- scale system in operation at LLNL.
Other Resources and Demonstrations
See description of Permeable Reactive Barrier Walls.
See related report, Field Evaluation of a Horizontal Well Recirculation System for Groundwater Treatment: Field Demonstration at X-701B Portsmouth Gaseous Diffusion Plant, Piketon, Ohio, 1998, ORNL/TM—13529, NTIS: DE98058132, 173 pp. This report describes the field-scale demonstration of in-situ treatment of groundwater using horizontal recirculation coupled with treatment modules. Groundwater contamination consists of trichloroethylene and technetium-99.
See http://www-erd.llnl.gov/library/AR-136189.pdf for project description at LLNL.