1999 Progress Report: Palladium Catalyzed Reductions of Water Contaminants under Environmental Conditions

EPA Grant Number: R825421
Title: Palladium Catalyzed Reductions of Water Contaminants under Environmental Conditions
Investigators: Reinhard, Martin , Lowry, Greg , Sriwatanapongse, Watanee
Current Investigators: Reinhard, Martin
Institution: Stanford University
EPA Project Officer: Lasat, Mitch
Project Period: November 1, 1996 through October 31, 1999 (Extended to October 31, 2001)
Project Period Covered by this Report: November 1, 1998 through October 31, 1999
Project Amount: $366,667
RFA: Exploratory Research - Water Engineering (1996) RFA Text |  Recipients Lists
Research Category: Water , Land and Waste Management , Engineering and Environmental Chemistry


The project aims to evaluate metal-catalyzed dehalogenation and hydrogenation for the reductive treatment of fluidized waste streams. A broad base of batch kinetic data will be obtained to determine the treatability of various specific compounds and compound classes of interest in groundwater remediation, particularly halogenated methanes, ethanes, ethenes, and aromatics. Kinetic studies will include the identification of intermediates and products. Emphasis also will be placed on determining competing or inhibitory compounds present as natural groundwater solutes. Methods of circumventing these interfering ions then will be evaluated.

Progress Summary:

Treatability Study. The efficacy of palladium catalysts and hydrogen gas for destroying organic environmental pollutants has been demonstrated. Rapid transformation rates were observed for many priority pollutants, including chlorinated methanes, ethenes, chlorofluorcarbons, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), chlorobenzenes, and chlorocyclohexanes (Lindane). The normalized first order rate constant and half life for each compound tested are summarized in Table 1. Transformation rates for tetrachloroethylene (PCE), trichloroethylene (TCE), 1,1-dichloroethylene (1,1-DCE), 1,2-dichloroethylene (1,2-DCE), 1,1-dichloroethane (1,1-DCA), 1,2-dichloroethane (1,2-DCA), carbon tetrachloride, chloroform, and Freon-113 were determined in batch tests using 0.22 g/L of a 1 percent by weight palladium on alumina powder (200-400 mesh size) in Milli-Q water at ambient temperature and pressure. The dechlorination reaction rates for PCE, TCE, 1,1-DCE, 1,2-DCE, carbon tetrachloride, and Freon-113 are very rapid with half lives on the order of 5-6 minutes. The chlorinated ethanes (1,1-DCA and 1,2-DCA) showed very little reactivity even at elevated catalyst concentrations and methylene chloride showed no reactivity.

No production of lesser chlorinated intermediate compounds was observed with the following exceptions. Chloroform appears as a reactive intermediate in the transformation of carbon tetrachloride (25 percent maximum) and continues to transform, but at a slow rate relative to the other compounds tested. All three DCE isomers and vinyl chloride appear as reactive intermediates in the dechlorination of TCE using pure palladium powder as a catalyst, but do not appear when using palladium supported on alumina. These reactive intermediates are present in trace amounts (just at detectable limits) and ultimately degrade to ethane. Carbon mass balances of 95-100 percent have been achieved verifying that ethane is the only major product.

Table 1. Normalized (0.22 g/L catalyst) First Order Transformation Rate Constants and
Half Lives for Several Classes of Chemicals With Major Environmental Impact

F.O. Trans. Rate Constant in water
(L min-1 gPd-1)
Half Life
Chlorinated Ethenes
Chlorinated methanes
Carbon tetrachloride
Methylene chloride
no rxn
Chlorinated ethane/propane
too slow to measure
too slow to measure
y-hexachlorocyclohexane (Lindane)

Batch kinetic experiments testing the reactivity of naphthalene, phenanthrene, 4-chlorobiphenyl, 1,2-dichlorobenzene, and symbol y-hexachlorocyclohexane (Lindane) used 1 g/L of a 1 percent palladium on alumina catalyst, had an initial contaminant concentration of around 2 mg/L (unless noted), and were run at ambient temperature and pressure. Transformation reactions, either dehalogenation or hydrogenation or a combination of both, were observed for all compounds. A 95 percent transformation of naphthalene to tetrahydronaphthalene was observed within 35 minutes. Phenanthrene (starting concentration of 0.65 mg/L) transformed within 2 hours to the two octahydrophenanthrene isomers with 8,9-dihydrophenanthrene and 1,2,3,4-tetrahydrophenanthrene being transient intermediates

Dechlorination was, in general, much faster than the hydrogenation reaction. 1,2-Dichlorobenzene reacted via chlorobenzene to benzene within minutes. The first dechlorination step for 4-chlorobiphenyl occurred within 8 minutes to form the biphenyl (95% conversion), but the complete reaction to dicyclohexyl (with cyclohexylbenzene as a second intermediate) took about a week. y-Hexachlorocyclohexane reacted to benzene within approximately12 minutes and no intermediates were detected. Preliminary results using microporous hydrophobic support materials (zeolite Y with high silicon to aluminum ratios) showed that with increasing hydrophobicity of the support, the stability of the catalyst against deactivation by ionic species like sulfite could be greatly enhanced. These findings merit further investigation which is in progress.

Catalyst Deactivation. Inhibitory and deactivating effects of naturally occurring groundwater solutes was investigated using groundwater samples from two sites?Lawrence Livermore National Laboratory (LLNL), CA, and Dover Air Force Base (AFB), DE. Column studies were used to determine the catalyst deactivation rate and lifetime under treatment conditions in groundwater and compared to a solute-free system (DI). Stainless steel columns (9cm x 1cm) were filled with glass (14 g) and catalyst (0.5 g) beads and used as upflow column reactors. The groundwater was saturated with hydrogen gas and TCE was added to give an influent TCE concentration of 4 mg/L. TCE conversion through the column was used as a surrogate for catalyst activity. The 0.5 g of catalyst provided an initial TCE conversion of 50 percent through the column at a flow rate of 1 mL/minute (4.3 minutes retention time).

The clean (DI) system performed well, with the initial TCE conversion of 52 percent reduced to 46 percent after 8,862 pore volumes. TCE conversion in Dover groundwater dropped from 51 percent to 22 percent after 7,387 pore volumes had passed through the column. LLNL groundwater had the fastest deactivation rate with TCE conversion dropping from 55 percent to 33 percent in only 2,829 pore volumes. The reason for the drop in catalyst activity is still unclear, but it may correlate with dissolved solids as the total dissolved solids (TDS) of the three waters increases in the order DI<Dover<LLNL.

The effects of common groundwater solutes were tested individually to determine which solutes were responsible for the deactivation. The following solutes were used: bicarbonate, sulfate, dichromate, chloride, lead, humic substances, phosphate, and nitrate, as well as acidic (HCl) and basic (NaOH) conditions. Catalyst beads (0.4 g) were placed in solutions containing each of these solutes (25 mL) for several months. The catalyst beads were then put into the column reactors to determine catalyst activity using TCE conversion as a surrogate. None of the solutes showed significant differences in catalyst activity from controls with the exception of phosphate and nitrate, which showed an increase in catalyst activity. The reason for the increase in catalyst activity is under investigation.

Future Activities:

Future work will focus on statistical analysis of the current data to verify the apparent innocuous behavior of most of the solutes tested. Also, the apparent increase in catalyst activity caused by nitrate and phosphate is under investigation. The loss of catalyst activity due to palladium dissolution from the catalyst support also is being considered.

Journal Articles on this Report : 1 Displayed | Download in RIS Format

Other project views: All 20 publications 5 publications in selected types All 5 journal articles
Type Citation Project Document Sources
Journal Article Lowry GV, Reinhard M. Hydrodehalogenation of 1-to 3-carbon halogenated organic compounds in water using a palladium catalyst and hydrogen gas. Environmental Science & Technology 1999;33(11):1905-1910. R825421 (1999)
R825421 (Final)
not available

Supplemental Keywords:

waste streams, reductive treatment, metal-catalyzed hydrogenation, palladium, treatability, contact time, reactor configuration, inhibitory compounds, circumventing interfering processes, TCE, PCE, DCE, DCA, Dover Air Force Base, DE, Lawrence Livermore National Laboratory, CA, dehalogenation, chlorinated solvents., Scientific Discipline, Water, Ecology, Wastewater, Environmental Chemistry, Chemistry, Engineering, Engineering, Chemistry, & Physics, waste treatment, metal catalysts, inhibitory compounds, hydrocarbon, hydrogenated hydrocarbon, chemical composition, chemical transport modeling, chemical kinetics, water treatment, water contaminants, reductive treatement

Progress and Final Reports:

Original Abstract
  • 1997
  • 1998
  • 2000 Progress Report
  • Final Report