Superfund Research Program

Model for Catalytic Reductive Dehalogenation

Project Leader: Robert G. Arnold
Grant Number: P42ES004940
Funding Period: 1995 - 2000

Project-Specific Links

Final Progress Reports

Year:   1999 

During the past year, project researchers concentrated on the simultaneous development of three alternative technologies for the destruction of halogenated solvents.

Modification of fuel cell technology
The primary difficulty in finding fundamental kinetic relationships for transformation of halogenated solvents in the modified fuel cell is inability to fix or measure the cathode potential. Such information is necessary to support detailed calculations for full-scale reactor design. Consequently, reactor modifications were undertaken in which a mixture of ethanol and water was substituted for dihydrogen gas as the primary reactant in the anode compartment. This enabled us to insert a Luggin probe in the anode compartment and to measure the potential on the anode surface. Because cell potential (difference between the potentials of the anode and cathode) is normally fixed, it is now possible to know the cathode potential. The modified reactor was thoroughly tested using molecular oxygen in the cathode compartment. Both the anode reaction and charge transfer through the membrane separating the two reactor compartments are fast and will not impede the dehalogenation of target contaminants when they are delivered to the cathode in place of molecular oxygen. Engineering calculations suggest that the modified fuel cell will provide an inexpensive method for the destruction of gas-phase halogenated solvents. With some additional engineering, the system can be adapted for treatment of aqueous-phase organics and will serve as substitute for relatively expensive separation/destruction activities such as carbon adsorption and thermal destruction. Half times for the destruction of gas-phase contaminants in the reactor cathode are on the order of seconds.

Catalysis of thermochemical reduction of gas-phase VOCs
The reaction of dihydrogen with gas-phase chlorinated VOCs such as trichloroethylene and tetrachloroethylene can be catalyzed on the surface of a number of metals including palladium and platinum. In order to take advantage of the simplicity and rapid kinetics of such a system, we designed a reactor in which traces of platinum are added to the surface of glass beads or quartz sand particles. These were then packed in column reactors for the treatment of contaminated gas streams, such as those originated from soil vapor extraction or air sparging operations. Hydrogen gas was added to the gas stream just ahead of the column. Although experiments conducted to date are incomplete, the reductive dehalogenation of chloroethylenes to predominantly ethane is very rapid, with half times in the column reactor of just a few seconds. Kinetic results are very sensitive to temperature and somewhat sensitive to the amount of catalyst added. The accumulation of hydrochloric acid in the reactor can impair reactor performance. By running the reactor above ambient temperature, it may be possible to volatilize the acid as rapidly as it is formed, thus avoiding the problem. Reaction kinetics in the presence of molecular oxygen have not yet been studied.

Photo-initiated reductive dehalogenation of chlorinated ethenes
The photo-initiated destruction of halogenated ethylenes was studied in mixtures of alcohols and ketones. It was previously shown that reduction of carbon tetrachloride to chloroform depends on the presence of each solvent and does not proceed in the dark. It was hypothesized that the ketone is necessary to absorb light in the ultraviolet or near-ultraviolet range. The photo-promoted ketone can then participate in a hydrogen abstraction involving the alcohol—usually isopropanol in these experiments. The radical so produced participates in dechlorination reactions. Products of reactions involving trichloroethylene and tetrachloroethylene are largely unidentified. It is known, however, that approximately two of the four chlorines in tetrachloroethene are liberated in the overall transformation process, that less halogenated homologues are not produced, and that the primary products include several relatively large chlorinated compounds. The results of laboratory tests were sufficiently encouraging to motivate a field scale trial of the technology. To this end, researchers withdrew gases from a soil vapor extraction system that was recently installed at the Harrison Landfill in Tucson. The primary contaminant in the gas stream to be treated is tetrachloroethylene. The system that was designed and built to treat a sidestream that was withdrawn from the primary facility consisted of a gas scrubber, which transferred tetrachloroethylene and other VOCs to a liquid-phase mixture of acetone and isopropanol, and a sunlight-driven treatment unit. Results to date indicate that both the scrubber and treatment units perform as anticipated when run independently.