Final Report: Palladium Catalyzed Reductions of Water Contaminants under Environmental ConditionsEPA Grant Number: R825421
Title: Palladium Catalyzed Reductions of Water Contaminants under Environmental Conditions
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 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 overall goal of this research project was to gain better insight into the hydrodehalogenation chemistry of palladium (Pd)-catalyzed systems under conditions that are similar to groundwater treatment conditions. The evaluation of Pd-catalyzed hydrodehalogenation using hydrogen gas as a method for the destructive removal of halogenated hydrocarbon compounds from contaminated water was proposed. Halogenated substrates, such as chlorinated and brominated ethanes, ethylenes, benzenes, and phenols have been known to rapidly undergo hydrodehalogenation in aqueous solutions in the presence of hydrogen gas (a reductant) and Pd (a catalyst) under mild conditions, i.e., ambient temperature and neutral pH. Some inorganic oxyanions, such as nitrate, also can be transformed into less toxic forms if the proper catalyst is employed.
To achieve the project goal, the following specific objectives were pursued: (1) determination of the hydrodehalogenation rate as a function of compound structure; (2) identification of intermediates and products; (3) development of kinetic and mechanistic models; (4) identification of the factors that contribute to catalyst fouling; and (5) quantification of catalyst efficacy as a function of catalyst structure and water quality.
Column tests were used to determine contact time requirements and to study catalyst performance in natural waters. Emphasis was placed on chlorinated ethylenes, especially trichloroethylene (TCE), and its less chlorinated analogs, determining the effect of hydrogen sulfide and biological catalyst fouling, evaluating methods of circumventing interfering processes, and gaining operational experience with Pd-reactors at the bench scale.
The first three specific objectives were addressed in a series of studies that focused on the removal of toxic chlorinated contaminants, such as chlorinated ethanes, ethylenes, some pesticides, and chlorinated and nonchlorinated aromatics from drinking water. Pd catalyzed the rapid hydrodehalogenation of nine 1- to 3-carbon halogenated organic compounds, resulting in little or no production of halogenated intermediates. Initial transformation rates were compared for 12 halogenated organic compounds using 1 w/w percent Pd-on-Al2O3 and metallic Pd catalysts in clean, aqueous batch systems at ambient temperature and pressure conditions. Half-lives of 4-6 minutes were observed for 5-100 µM (1-10 mg/L) aqueous concentrations of tetrachloroethylene (PCE), trichloroethylene (TCE), cis-dichloroethylene (DCE), trans-DCE, 1,1-DCE, carbon tetrachloride, and 1,2-dibromo-3-chloropropane at ambient temperature and pressure with 0.22 g/L of catalyst. Using Pd/Al2O3, TCE transformed quantitatively (97 percent) to ethane without formation of any detectable chlorinated intermediate compounds. This implies a direct conversion of TCE to ethane at the Pd surface. Carbon tetrachloride transformed primarily to methane and ethane and minor amounts of ethylene, propane, and propylene. Chloroform is a reactive intermediate (20 percent). Formation of C2 and C3 products implies a free radical mechanism. Methylene chloride, 1,1-dichloroethane, and 1,2-dichloroethane were nonreactive. Reaction mechanisms and kinetic models were postulated for TCE, carbon tetrachloride, and chloroform transformation.
In a related study, the catalytic transformations of 1,2-dichlorobenzene, chlorobenzene, 4-chlorobiphenyl, gamma-hexachlorocyclohexane (lindane), naphthalene, and phenanthrene over Pd on alumina in hydrogen-saturated water (Pd/Al2O3/H2), were studied under ambient temperature and pressure conditions. The chlorinated benzenes were hydrodechlorinated rapidly and lindane was dehydrochlorinated to benzene. Partial or complete hydrogenation was observed for biphenyl and the polycyclic aromatic hydrocarbons. The phenanthrene ring was cleaved at the 9,10-position. In general, dechlorination reactions were faster than hydrogenation reactions.
The performance under simulated treatment conditions were studied using bench-scale column reactors. The performance of a Pd/Al2O3 catalyst for the dechlorination of TCE was evaluated in synthetic and real groundwater. Low initial TCE conversions were used to provide maximum sensitivity to changes in catalyst activity. TCE conversions of greater than 24 percent were maintained in deionized water for 60 days using 0.5 g of 1 percent (w/w) Pd/Al2O3 and a retention time of 4.3 minutes. Slow catalyst deactivation was reversed by treatment with a dilute sodium hypochlorite solution. The presence of high concentrations of H2CO3, HCO3-, CO32-, SO42-, and Cl- did not adversely affect catalyst activity. TCE conversion increased 30 percent upon increasing the solution pH from 4.3 to 11. The presence of 87 mg/L SO32- or 0.4 mg/L HS- caused rapid catalyst deactivation, presumably due to chemisorption to active sites. Dilute sodium hypochlorite solutions regenerated catalysts deactivated by HS- and SO32-. Sulfide produced by sulfate-reducing bacteria that developed in natural groundwater amended with aqueous-phase hydrogen concentration ([H2](aq)), deactivated the catalyst, but activity was restored after flushing with a dilute sodium hypochlorite solution. TCE conversion remained stable between 20 and 28 percent during a 63-day experiment using natural groundwater and periodic (4- to 7-day intervals) pulses of a dilute sodium hypochlorite solution.
The [H2](aq) and the presence of H2-utilizing competitive solutes affect TCE dechlorination efficiency in Pd-based in-well treatment reactors. The effect of [H2](aq) and H2-utilizing competing solutes (cis-DCE, trans-DCE, 1,1-DCE, dissolved oxygen [DO], nitrite, nitrate) on the TCE transformation rate and product distribution were evaluated using 100 mg/L of a powdered Pd-on-Al2O3 catalyst in batch reactors or 1.0 g of a 1.6-mm Pd-on-gamma-Al2O3 catalyst in column reactors. The TCE dechlorination rate constant decreased by 55 percent from 0.034 +/- 0.006 to 0.015 +/- 0.001 minutes-1, when the [H2](aq) decreased from 1000 to 100 µM and decreased sharply to 0.0007 +/- 0.0003 minutes-1 when the [H2](aq) decreased from 100 to 10 µM. Production of reactive chlorinated intermediates and C4-C6 radical coupling products increased with decreasing [H2](aq). At an [H2](aq) of 10 µM (P/P-o = 0.01), DCE isomers and vinyl chloride accounted for as much as 9.8 percent of the TCE transformed at their maximum but disappeared thereafter, and C4-C6 radical coupling products accounted for as much as 18 percent of TCE transformed. The TCE transformation rate was unaffected by the presence of cis-DCE, trans-DCE, and 1,1-DCE (91 ump), indicating that these compounds do not compete with TCE for catalyst active sites. DO is twice as reactive as TCE but had no effect on TCE conversion in the column below a concentration of 370 µM (11.8 mg/L), indicating that DO and TCE will not compete for active catalyst sites at typical groundwater DO concentrations. TCE conversion in the column was reduced by as much as a factor of 10 at influent DO levels greater than 450 µM (14.3 mg/L) because the [H2](aq) fell below 100 µM due to H2 utilized in DO conversion. Nitrite reacts 2-5 times slower than TCE and reduced TCE conversion by less than 4 percent at a concentration of 6,630 µM (305 mg/L). Nitrate was not reactive and did not affect TCE conversion at a concentration of 1,290 µM (80 mg/L).
To avoid catalyst fouling in the presence of sulfide, a series of Pd-on-zeolite catalysts were studied that exhibited a range optimized for treating contaminated groundwater. Aqueous sulfite was used as the model poison, and the dechlorination of 1,2-dichlorobenzene (1,2-DCB) was used as the model reaction to study the relationship between zeolite hydrophobicity, pore size, and resistance against deactivation. The optimal Pd support is a hydrophobic zeolite Y, with a pore size of 0.74 nm tailored to exclude reactive ions from internal sites, to minimize deactivation and inhibition, and to maximize the transformation rate of halogenated hydrocarbon compounds. Common halogenated contaminants, including chlorinated ethylenes and aromatics, are removed within minutes. Catalyst efficiency under groundwater treatment conditions was demonstrated in a bench-scale column experiment.
In conclusion, results obtained from this research indicated that with Pd/Al2O3 as the catalyst and hydrogen as the reactant, rapid and sustainable dehalogenation rates can be achieved for a wide range of chlorinated and brominated contaminants. For example, TCE is transformed to ethane in the presence of excess hydrogen. Kinetic data indicate that the TCE hydrodechlorination is a direct conversion to ethane without the formation of lesser halogenated byproducts. Carbon tetrachloride transforms primarily into methane (60 percent), with a significant fraction going to chloroform, a transient but less reactive intermediate. Lack of hydrogen can lead to the accumulation of intermediates. Nitrite also can be transformed but the transformation is slower than that of TCE, and products were not identified. Taken together, results demonstrate the feasibility of catalyzed dechlorination using hydrogen over Pd as a treatment technology. The data of this study can serve as a basis for the design of Pd-based groundwater treatment systems.
Journal Articles on this Report : 4 Displayed | Download in RIS Format
|Other project views:||All 20 publications||5 publications in selected types||All 5 journal articles|
||Lowry GV, Reinhard M. Pd-catalyzed TCE dechlorination in groundwater: solute effects, biological control, and oxidative catalyst regeneration. Environmental Science & Technology 2000;34(15):3217-3223.||
||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.||
||McNab WW, Ruiz R, Reinhard M. In-situ destruction of chlorinated hydrocarbons in groundwater using catalytic reductive dehalogenation in a reactive well: Testing and operational experiences. Environmental Science & Technology 2000;34(1):149-153.||
||Schuth C, Reinhard M. Hydrodechlorination and hydrogenation of aromatic compounds over palladium on alumina in hydrogen-saturated water. Applied Catalysis B-Environmental 1998;18(3-4):215-221.||