Palladium-catalyzed TCE Dechlorination in Groundwater: Dechlorination Kinetics, Dissolved Solute Effects, and Catalyst Deactivation
Funding Agency
U.S. EPA
Publications:
Lowry, G.V., Reinhard, M. (2001). "Pd Catalyzed TCE Dechlorination in Groundwater: Effect of [H2](aq) and H2-Utilizing Competitive Solutes on the TCE Dechlorination Rate and Product Distribution", Environ. Sci. Technol. 35 (4), 696.
Lowry, G.V., Reinhard, M. (2000). "Pd Catalyzed TCE Dechlorination in Groundwater: Solute Effects, Biological Control, and Oxidative Catalyst Regeneration", Environ. Sci. Technol. 34 (15), 3217.
Lowry, G.V., Reinhard, M. (1999). "Hydrodehalogenation of 1- to 3-Carbon Halogenated Organic Compounds in Water Using Palladium Catalyst and Hydrogen Gas", Environ. Sci. Technol. 33 (11), 1905.
Abstract: Groundwater contamination by chlorinated solvents and pesticides is a familiar environmental problem. These compounds are known or suspected carcinogens and pose a significant public health threat. The inefficiency and high cost of traditional groundwater remediation technologies such as air sparging and granular activated carbon (GAC) contacting has spurred an effort to create novel, cost effective treatment technologies which transform these contaminants to innocuous compounds in situ.
Pd-based catalytic treatment reactors are an effective alternative to traditional treatment technologies for the aqueous phase destruction of halogenated organic contaminants in groundwater. Some key advantages of catalytic reactors are that they; i) effectively treat a wide range of contaminants, ii) produce few or no toxic reaction by-products, iii) provide rapid in situ contaminant destruction yielding no secondary waste streams, and iv) can be effective in regions with high contaminant concentrations such as DNAPL source zones. Because Pd catalysts rapidly dehalogenate many compounds, small in situ fixed-bed treatment reactors can be placed within the treatment well bore hole, thereby reducing pumping costs, lowering operation and maintenance costs, and eliminating large above ground treatment facilities.
In batch experiments, transformation rates are compared for 12 halogenated compounds. Half-lives of 4-6 min were observed for 1 to 10 mg/L aqueous concentrations of most contaminants using only 0.22 g/L of catalyst. Transformation pathways and mechanisms are postulated for TCE and carbon tetrachloride. Catalyst performance under groundwater remediation conditions was evaluated using laboratory scale column reactors. The effect of individual groundwater solutes (e.g. HCO3-, SO42-) on catalyst activity and lifetime was also determined. Catalyst deactivation was observed when using groundwater from a contaminated site. Most groundwater solutes did not reduce catalyst activity, but SO32- (44 mg/L) and HS- (<1 mg/L) caused rapid catalyst deactivation. Lost catalyst activity was recoverable in all cases using a 90-min pulse of a dilute sodium hypochlorite solution.
The economic viability of using catalytic reactors for groundwater remediation relies on the ability to maintain catalyst activity for many years. The macroscopic data collected in this study can be used to postulate catalyst deactivation and regeneration mechanisms, but detailed studies of the complex contaminant-catalyst interactions occurring on the catalyst surface are necessary to validate these mechanisms.
Greg Lowry Home | Dept. Civil & Env. Engr.
Last Modified: 1 January 2003