Title: Western Region Hazardous Substance Research Center Project 1-SU-02
Chemical, Physical and Biological Processes at the Surface of Palladium Catalysts under Groundwater Treatment Conditions

Investigators: Martin Reinhard and John Westall

Institution: Stanford University and Oregon State University

Research Category: Groundwater, treatment, chlorinated solvents

Project Period: January 2002- August 2007

 

Objectives: This project aimed to (1) evaluate the impacts of groundwater on catalyst activity; (2) elucidate the chemical and physical mechanisms responsible for changes in catalyst activity; (3) investigate potential biofouling issues that may result from biological activity expected in long-term treatment applications; (4) develop convenient and economical methods to regenerate catalysts in situ.

Rationale: Batch studies with supported palladium catalysts have demonstrated the potential of the palladium/hydrogen process for treating groundwaters or effluent streams that are contaminated by halogenated compounds. These studies yielded near-complete reductive dehalogenation of chlorinated ethylenes to ethane at room temperature within minutes, with reaction rates that are orders of magnitude higher than zero-valent iron. Other batch studies have shown the ability of palladium to catalyze the reaction of a range of compounds: all six species of chlorinated ethylenes, carbon tetrachloride, chloroform, 1,2-dibromo-3-chloropropane, Freon 113, chlorobenzene, naphthalene and lindane. Laboratory column studies and field tests have indicated that catalyst activity may decline over time because of chemical and biological fouling. This project investigated (1) causes of activity loss, (2) optimization of process operation, and (3) catalyst regeneration under simulated field conditions.

Summary of Findings: A bench-scale column system was developed to that allowed us to observe the rate of TCE transformation in the presence of hydrogen sulfide as a function of time. The column system consisted of pump that supplied water from a tank to a hydrogen contactor. After the contactor, the flow was divided and pumped with three separate pumps to three reactors packed with Pd catalyst. The reactor inflows were connected to auxiliary feed systems that allowed us to augment the reactor feed with TCE, foulant (hydrogen sulfide), or regenerant (bleach, hydrogen peroxide, deionized water) as needed. Catalyst activity was evaluated by measuring comparing the concentrations of TCE at the inlet and outlet of the reactor. The mechanism of catalyst fouling was studied by analyzed using X-ray photoelectron spectroscopy (XPS) for the accumulation of deactivating species at the palladium surface. Observations made at ongoing field project that was executed at Edwards Air Force Base (EAFB), Lancaster, California.

The laboratory portion of the project developed a quantitative model for deactivation kinetics with aqueous sulfide, investigated the effects of pH on a catalyzed dehalogenation reaction and sulfide deactivation, and characterized regeneration with acids, bases, and oxidizing agents. Results obtained with trichloroethylene showed no inherent catalyst deactivation in deionized water. Deactivation increased with sulfide concentration and exposure time. Deactivation was slowly reversible by flushing the catalyst with deionized water at pH 10.4. Treatment with 20 mM sodium hypochlorite quickly and completely regenerated the catalyst, and was significantly more effective than hydroxide, hydrochloric acid, hydrogen peroxide, and air-saturated water. The time required for regeneration increased with increasing sulfide concentrations and exposure times. These results were useful for interpreting reactor behavior and optimizing operating conditions and regeneration procedures at the EAFB field site.

During several prolonged failures of the reactor control system, the catalyst was exposed to high sulfide concentrations for days to weeks. This exposure eliminated catalyst activity nearly 100%. Nearly complete regeneration oft the catalyst was possible by soaking the catalyst in bleach for up to a week. Sustaining catalyst activity was possible by daily bleaching of the reactor for approximately 30 minutes.

X-ray photoelectron spectroscopy (XPS) was performed on supported palladium catalyst and a model catalyst after exposure to EAFB groundwater and sodium hypochlorite. The model catalyst was prepared by depositing palladium onto a polished a-alumina surface. Results from these analyses indicated that organics accumulate on the catalyst surface upon exposure to water, but the accumulation of organic matter did not correlate strongly with catalyst deactivation. The spectroscopic data suggested that sulfide may bind to the Pd surface, and may be oxidized to sulfate with hypochlorite treatment.

Adsorption of trichloroethylene (TCE) on alumina-supported palladium catalysts (Pd/Al2O3) was studied in the presence and absence of hydrogen using 13C-solid state NMR. Carbon-13 NMR spectra indicate that at low coverage strongly adsorbed species are formed while at high coverage additional physisorbed species are present. Carbon-13 spin-echo amplitude data measured as a function of pulse separation, t, was used to determine the 13C-13C intramolecular dipolar coupling and the carbon-carbon bond length of adsorbed species. Results indicate that a substantial fraction of the chemisorbed carbon species had undergone carbon-carbon bond scission forming single-carbon fragments, suggesting that the activation energy for carbon-carbon bond scission is comparable to the heat of adsorption. For the remaining surface species, the double bond is elongated to 1.46 ± 0.03 Å and is suspected to be chemically bonded ethynyl. At room temperature, adding an excess of hydrogen to catalyst that is covered to saturation with TCE precursors produces only in a small amount of ethane, indicating the fraction of surfacespecies that are hydrodehalogenation precursors is small.

The field demonstration results can be demonstrated as follows: Catalytic destruction of TCE in groundwater was demonstrated at Edwards AFB. The site was contaminated with 800 to 1,200 mg L-1 TCE, which was the sole contaminant. A treatment methodology was developed to maintain catalyst activity and keep treated water TCE concentrations at or below the maximum contaminant level (MCL) of 5 mg L-1 without by product formation. The treatment protocol entailed treating 2 gpm in a single catalyst column for 21 h (contact time approximately 1 min) followed by a 3 h bleach cycle to restore and maintain catalyst activity. The maintenance cycle consisted of bleaching of the catalyst for 1 h and flushing with hydrogen-containing groundwater for 2 h. After each maintenance cycle, TCE in the product water was at or below 1 mg L-1 corresponding to 99.9% removal. During a 21 h treatment cycle, effluent TCE concentrations increased slowly to approximately 10-15 mg L-1, corresponding to approximately 99% removal.

Daily bleaching maintained catalyst activity by preventing biological fouling with sulfidogenic bacteria (bacteria oxidizing hydrogen and reducing sulfate to hydrogen sulfide). Operational problems led to episodes of biological sulfide formation and severe catalyst poisoning marked by complete activity loss. Laboratory experiments and field observations demonstrated that the activity of the catalyst is nearly completely recoverable by treating the catalyst with bleach.

Based on data obtained in this demonstration, it is estimated that a capital investment of $572,000 and annual O&M costs of $72,000 (including monitoring & analysis) are sufficient to install and operate a treatment system that creates a barrier approximately 20 m wide in a plume of contaminated groundwater. This estimate applies to sites contaminated with chlorinated ethylenes (PCE, TCE, DCE isomers and vinyl chloride) with a relatively permeable aquifer, shallow water table and low gradient, similar to the Edwards AFB field site. This cost estimate is for a two-well system having a total flow of 2 gpm per treatment well or 4 gpm total. The system operates 87.5% of the time in a daily 21h:3 h treatment:regeneration cycle and remediates a TCE concentration of 1000 mg L-1. The estimate is directly applicable to a full scale system and scalable to multiple sets of two wells. Sites with lower quality water would require more frequent bleaching whereas sites with cleaner (more aerobic) water are expected to require less frequent bleaching. A modification is proposed for continuous (100%) treatment by using two catalytic columns per well whereby one reactor is bleached and reactivated while the other treats the contaminated groundwater. 

In the final phase of the project (spring 2007), we focused on improving the design of the field reactor that was used to treat TCE contaminated groundwater installed at the EAFB. We have shown that by recirculating the dilute bleach solution for catalyst regeneration, the regeneration and reactivation cycle could be shortened, the production of TCE containing rinse water was nearly avoided, and the total daily through put could be minimized.

 

Publications:

Journal Articles

Munakata, N., and M. Reinhard (2007). Palladium-catalyzed aqueous hydrodehalogenation in column reactors: Modeling of deactivation kinetics with sulfide and comparison of regenerants. Applied Catalysis B: Environmental 75 1–10.

Sriwatanapongse, W., M. Reinhard, and C. A. Klug (2006). Reductive hydrodehalogenation of trichloroethylene by palladium-on-alumina catalyst: 13C-NMR study of adsorption. Langmuir, 22, 4158-4164.

 

Book Chapter

Munakata, N., M. Reinhard (2001). Palladium Catalysis for the Treatment of Waters Contaminated with Halogenated Hydrocarbons, Nonhalogenated Aromatics, Oxidized Carbon and Oxidized Nitrogen Species. Physicochemical Groundwater Remediation. J. A. Smith and S. E. Burns, editors, Academic/Plenum Publisher, Dordrecht, The Netherlands: 45-71. 

 

Conference Proceedings and Presentations

Goltz, M. N., R.K. Gandhi, S.M. Gorelick, G.D. Hopkins, C. LeBron, P.L. McCarty, and M. Reinhard (2001). Application of Circulating Wells for In Situ Treatment of Contaminated Groundwater. International Symposium on Soil and Groundwater Contamination Control Strategy, Kyung Hee University, Seoul, Korea.

Munakata, N., J. Cunningham, R. Ruiz, C. Lebron, and M. Reinhard (2002). Pd-Catalyzed Hydrodechlorination of Chlorinated Organic Pollutants in Groundwater. Third International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA; USA (May 20-23).

Munakata, N., J.A. Cunningham, R. Ruiz, C. Lebron, and M. Reinhard (2002). Palladium Catalysis in Horizontal-Flow Treatment Wells: Field-Scale Design and Laboratory Study. Remediation of Chlorinated and Recalcitrant Compounds: Advances in ex situ Treatment of Groundwater, eds. A. Gavaskar and A. Chen, Batelle Press. Third International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA; USA (May 20-23).

Sriwatanapongse, W., M. Reinhard, C.A. Klug (2005). Reductive hydrodechlorination of TCE by palladium on alumina catalyst: A solid state NMR study of the surface reaction.

Abstracts of Papers, American Chemical Society; The 230th ACS National Meeting, in Washington, DC, v.222, pt.1, p.372-372 (Aug 28-Sept 1).

Sriwatanapongse, W., M. Reinhard, C.A. Klug (2005). A solid state NMR study of the surface reaction of the supported palladium catalyzed hydrodechlorination of trichloroethylene. AIChE Annual Meeting, Cincinnati, OH, United States Conference Proceedings p.9962 (Oct 30-Nov 4).

 

Theses

Munakata, Naoko (2005). Palladium-Catalyzed Hydrodehalogenation for Groundwater Remediation: Sulfide Deactivation Model, Regenerant Comparison, and Chemical Surface Changes. Ph.D., Stanford University.

Sriwatanapongse, Wataneee (2005). Reductive Dechlorination of Trichloroethylene by Palladium On Alumina Catalyst: a Solid State NMR Study of the Surface Reaction Mechanism. Ph.D., Stanford University.

 

Supplemental Keywords: groundwater; NAPL; VOCs; chlorinated solvent; remediation technologies; in-situ; technology transfer; Environmental Chemistry