1. Catalytic Destruction of Gas-Phase PCE and TCE in Groundwater and Soils - Laboratory Study & Field Investigation Departments of Atmospheric Sciences & Chemical and Environmental Engineering, The University of Arizona, Tucson, AZ 85721 Erik Rupp, Marty Willinger, Theresa Foley, Suzanne Bell, Brian Barbaris, Robert Arnold, Eduardo Sáez, Eric Betterton Song Gao, Erik Rupp, Marty Willinger, Theresa Foley, Suzanne Bell, Brian Barbaris, Robert Arnold, Eduardo Sáez, Eric Betterton Desert Remedial Action Technologies Workshop - Phoenix October 3, 2007

2. Overview of This Talk Demonstrate the validity of a new remediation method to destroy chlorinated solvents: Redox Catalysis. Explore reaction mechanisms and kinetics involved. Describe the successful application of this method in a pilot field study at a State Superfund site in Tucson. Estimate treatment costs and illustrate the potential of this method for low-cost, large-scale remediation. Paper in Press: Applied Catalysis B: Environmental, 2007

3. Chlorinated Solvents are Widespread Contaminants in Soils and Groundwater in the US PCE & TCE are among the top 31 CERCLA (Superfund) Priority List of Hazardous Substances. PCE and TCE are the 1 st and 3 rd most frequently detected solvents in groundwater at concentrations greater than their respective MCLs. Moran et al. 2007

4. Widespread Contamination by Chlorinated Solvents Regional level: Are primary contaminants at 29 out of 33 of Arizona’s WQARF (“State Superfund”) sites & at 13 out of the 14 National Superfund sites. Local Level: The Park-Euclid site in Tucson is contaminated by PCE and TCE that are derived from long-defunct dry cleaning operations and affect the local community.

5. Park-Euclid PCE Plume The University of Arizona Yellow contours represent PCE concentration in groundwater from 100 ppb to 1 ppb. 1 ppb 10 ppb 100 ppb 1000 ft

6. Harmful Health Effects of PCE & TCE Can cause cancers in animals. Are probably human carcinogens (DHHS). Necessity to develop efficient & economic remediation technologies to destroy chlorinated solvents.

7. Previous Methodologies Incineration (oxidation) - air pollution; formation of toxic substances Solidification - not destructive in nature Pump and Treat (for groundwater) - high cost; contaminant rebound Soil Vapor Extraction (SVE) - high cost; not destructive in nature; further treatment

8. SVE followed by Activated Carbon Adsorption The cost of such operations can be heavily influenced by carbon recovery or replacement costs, particularly when spent carbon must be treated off site as a hazardous waste. Ground Water Contaminant Plume Vapor Vadose Zone GAC Column Released into atmosphere

9. Catalytic Destruction - Oxidation C 2 Cl 4 + 2O 2 2CO 2 + 2Cl 2 4HCl + O 2 2H 2 O + 2Cl 2 metal catalyst Catalyst categories: - supported noble metals (e.g. Pt, Pd); base metal oxides (e.g., Cu, Mn); noble metal/metal oxide combinations. Issues - High temperatures (>500 o C) - Deactivation through chlorine poisoning (blocking active sites) - Production of furans and dioxins (incomplete oxidation)

10. C 2 Cl 4 + 5H 2 C 2 H 6 + 4HCl Cl 2 + H 2 2HCl metal catalyst Catalyst categories: - Supported and unsupported noble metals Issues - Rapid deactivation through coking - High cost of H 2 Catalytic Destruction – Reduction

11. Hypothesis - Simultaneous reducing and oxidizing (“redox”) conditions may overcome the issues arising from reduction or oxidation alone? Lab study of redox catalysis - Reaction temperatures low enough? - Efficient destruction of PCE and TCE? - Catalyst deactivation avoided? - Good alternatives for H 2 as the reductant? Field study - Explore feasibility of redox method in field operations - Estimate treatment costs Objectives

12. Lab Process Flow Diagram 1” Diameter 1” Length

13. Catalyst Cut from an automobile catalytic converter (cylindrical: 1” diameter x 1” length) Pt/Rh are supported (3:1) on a monolithic honeycomb Honeycomb is composed of cordierite (90%) and washcoat (10%), containing alumina, cerium, zirconium and other trace constituents Cross section of catalyst’s channels: 2mm x 2mm

14. Reactor System

15. Analytical Measurements HP 5890 Gas Chromatograph Measure chlorinated and de-chlorinated hydrocarbons: A 0.53μm wide-bore capillary column with a flame ionization detector (FID) Measure CO 2, H 2 and O 2 : A Supleco packed column with a thermal conductivity detector (TCD)

16. Experiments Initial furnace temperature was 75°C. Furnace temperature was ramped to the desired final temperature at 2°C/min. T change was slow enough to assume steady state reactions at any given T. Influent and effluent gas streams were periodically sampled and analyzed for composition. At end of each experiment, all gas streams were turned off except for O 2, and the furnace T was held at 450°C for 8 hours in order to clean the catalyst surface. This regeneration process proves to be effective in maintaining catalyst activity for over two years!

17. Multiple Reduction & Oxidation Reactions in a Redox System Major reactions leading to end products C 2 Cl 4 + 5H 2  C 2 H 6 + 4HCl C 2 Cl 4 + 2O 2  2CO 2 + 2Cl 2 C 2 H 6 + 3.5O 2  2CO 2 + 3H 2 O 2 H 2 + O 2  2H 2 O Additional reactions (involving intermediates) 2C 2 Cl 4 + 7H 2  2C + 8HCl + C 2 H 6 C + O 2  CO 2 4HCl + O 2  2Cl 2 + 2H 2 O H 2 + Cl 2  2HCl

18. PCE Conversion under Redox and O 2 -only Conditions 0.5 Lpm Flow Rate 5% O 2 (vol) Varying H 2 N 2 Remainder 0.7 s Residence Time (400 °C) 800 ppmv PCE

19. Effects of H 2 /O 2 ratio and T PCE conversion increases with both H 2 /O 2 ratio and T. Under O 2 -only condition, PCE conversion does not take off until 350°C. Under redox condition, there is substantial conversion (≥ 50%) at relatively low temperatures (≥ 300°C). Optimum condition (PCE conversion ≥ 90%): H 2 /O 2 ≥ 2.2 and T ≥ 400 °C.

20. PCE Conversion ~ Catalyst Deactivation: Role of Reaction Condition! PCE = 800ppmv, Residence Time ~ 1.5 s (25 °C), ~ 0.7 s (400°C)

21. Catalyst Poisoning is Minimized by the Simultaneous Presence of H 2 and O 2 Low-T (< 300 °C) conversions were mainly due to reduction. Declines in conv. (130 ~200 °C) indicated poisoning; Recovery of conv. (> 200 °C): “self-cleaning” due to Redox! Conv. rose steadily (H 2 /O 2 ≥2.2): heat prevents coke deposition and catalyst poisoning entirely! H 2 /O 2 = 2.2

22. Catalyst Surface T ~ Furnace T H 2 /O 2 = 2.2

23. Homologous Alkanes as Alternative Reductants Replacing H 2 Oxidation ReactionC-H Bond Dissociation Energy (kJ/mol) Methane CH 4 + 2O 2  CO 2 + 2H 2 O439.3 +/- 0.4 Ethane C 2 H 6 + 3.5O 2  2CO 2 + 3H 2 O420.5 +/- 1.3 Propane C 3 H 8 + 5O 2  3CO 2 + 4H 2 O410.5 +/- 2.9 n-Butane C 4 H 10 + 6.5O 2  4CO 2 + 5H 2 O400.4 +/- 2.9 * CRC Handbook of Chemistry & Physics, 86 th edition (2005-2006)

24. Experimental and Modeling Results of PCE Conversion under O 2 /alkane Conditions 175-200 ppm PCE 1 L/min total flow Resid. Time ~ 0.5 sec Assume first-order reaction rate with respect to PCE; Assume activation energy is a linear function of alkane’s BDE; Three-parameter fits eventually yield conversions reproducing experimental data reasonably well.

25. 1 Lpm Flow Rate 5% O 2 (vol) Varying Propane N 2 Remainder 0.25 s Resid. Time 800 ppmv PCE Lab (Redox): Use of Propane as Reductant

26. Field (Redox): Use of Propane as Reductant Park-Euclid Site, SBIR Phase I Propane SVE pump Catalytic converters Effluent stream Heater control Catalytic converters Scrubber tower Effluent stream 100 L/min through each reactor (3.5 cfm) 300 L/min (10 cfm)

27. 2 Alumina supported Pt/Rh catalysts –2" long x 4.7" major axis; 3.15" minor axis Temperature Range: 450 – 650 o C SVE Gas –10 – 100 ppmv PCE; 5 – 20 ppmv TCE –15% – 20% Oxygen –Diesel –Water Vapor 100 Lpm total flow rate –0.2 s Residence Time 1.0 – 2.0% Propane by volume Field Conditions

28. Field – Extended Operation 100 Lpm Flow Rate SVE Gas ~ 2% C 3 H 8 (vol) ~ 0.2 s Residence Time ~ 520 o C Catalyst Temperature

29. Treatment Costs Catalytic Converter –  200 ppm PCE –2% v/v propane @ $1.70/gal (DOE, 2005) –Propane-only treatment costs decrease  $10/lb PCE destroyed (decrease with [PCE]) Granular Activated Carbon –  200 ppmv PCE, 50 cfm, 85  F –GAC-only treatment costs: (Siemens Water Technologies, Sept. 2006) increase  $7/lb PCE absorbed (increase with [PCE])

30. PCE Treatment Cost ~ Soil Vapor [PCE] (ppmv)

31. Conclusions Redox catalysis is highly efficient in destroying chlorinated solvents at moderate temperatures. Catalyst activity can be maintained for extended periods using mild, convenient regeneration procedures. Alkanes can replace H 2 as the reductant in the redox system for efficient removal of target compounds. PCE reaction rate appears to be directly related to the C-H bond dissociation energy of the alkane used. We achieved success in applying this method in a pilot field study. Redox catalysis holds potential for low-cost, large-scale field operation as an alternative remediation technology.

32. Future Work Lab –Further determine reaction mechanisms –Examine adsorption behaviors of reactants and products –Quantify reaction rates and model the processes –Optimize operating conditions Field –Carry out a larger-scale field project (Phoenix area) –Improved scrubber design; larger flow rates; other target compounds (Freons)?

33. Acknowledgements National Institute of Environmental Health Sciences, NIH U of A Superfund Basic Research Program

34. Extra

35. Hydrodechlorination (reduction by H 2 ) General Reaction Mechanisms Sequential/serial mechanism H 2 + 2 * ↔ 2 H* RCl x + * ↔ RCl x * RCl x * + H* ↔ RHCl x-1 * + Cl* RHCl x-1 * ↔ RHCl x-1 + * (etc.) H*+ Cl* ↔ HCl + 2 * Concerted/parallel mechanism RCl x * + x H*→ RH x + x Cl* * refers to an active site on the catalyst surface; or an adsorbed species that is activated.

36. After displaying all Redox reactions possible, state that “it would seem to be a mess that we are in – ok, what reaction happen, to what extent, and what converts to what…” Well, all this is under way to being fully understood through doing detailed and systematic experiments, but phenomenologically, we can focus on observing two things to meet our initial purposes, i.e., How efficiently is PCE destructed? How stable is the catalyst’s activity?