# Presentation: 261767

## 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*
_**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

## 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*

## 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 1st and 3rd _most frequently detected_ solvents in groundwater at concentrations greater than their respective MCLs.

- Moran et al. 2007

## 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.

## Park-Euclid PCE Plume

**Yellow contours represent**
**PCE concentration **
**in groundwater **
**from 100 ppb to 1 ppb.**

## 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.

## 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

## 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.

## Catalytic Destruction - Oxidation

- C2Cl4 + 2O2       	   2CO2+ 2Cl2
- 4HCl +   O2         	   2H2O + 2Cl2

**metal catalyst**

**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 (>500oC)
- - Deactivation through chlorine poisoning (blocking active sites)
- - Production of furans and dioxins (incomplete oxidation)

## C2Cl4 + 5H2              	C2H6 + 4HCl 
Cl2 + H2		          2HCl

- Cl2 + H2		          2HCl

**metal catalyst**

**metal catalyst**

**Catalyst categories:**
- 	- Supported and unsupported noble metals
**Issues **
- - Rapid deactivation through coking
- - High cost of H2

**Catalytic Destruction – Reduction **

## Objectives

*- 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 H2 as the reductant?
**Field study**
- - Explore feasibility of redox method in field operations
- - Estimate treatment costs

**Objectives**

## Lab Process Flow Diagram

** ****1” Diameter**
** ****1” Length**
** **

## 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

## Reactor System

## 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)

## 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!***

## Multiple Reduction & Oxidation Reactions in a Redox System

**Major reactions leading to end products**
- 	C2Cl4 + 5H2  C2H6 + 4HCl
- 	C2Cl4 + 2O2  2CO2 + 2Cl2
- 	C2H6 + 3.5O2  2CO2 + 3H2O
- 	2 H2 + O2  2H2O
**Additional reactions (involving intermediates)**
- 	2C2Cl4 + 7H2  2C + 8HCl + C2H6
- 	C + O2  CO2
- 	4HCl + O2  2Cl2 + 2H2O
- 	 H2 + Cl2  2HCl

## PCE Conversion under Redox and O2-only Conditions

** **0.5 Lpm Flow 		Rate
-  5% O2 (vol)
- Varying H2
- N2 Remainder
- 0.7 s Residence
-      Time (400 °C)
- 800 ppmv PCE
** **

## Effects of H2/O2 ratio and T

- PCE conversion increases with both H2/O2 ratio and T.
- Under O2-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%):
- 		H2/O2 ≥ 2.2 and T ≥ 400 °C.

## PCE Conversion ~ Catalyst Deactivation:Role of Reaction Condition!

- PCE = 800ppmv, Residence Time ~ 1.5 s (25 °C), ~ 0.7 s (400°C)

## Catalyst Poisoning is Minimized by the Simultaneous Presence of H2 and O2 

- _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 (H2/O2≥2.2): heat prevents coke deposition and catalyst poisoning entirely!

**H****2****/O****2**** = 2.2**

## Catalyst Surface T ~ Furnace T

**H****2****/O****2**** = 2.2**

## Homologous Alkanes as Alternative Reductants Replacing H2

|  | Oxidation Reaction | C-H Bond Dissociation Energy (kJ/mol)  |
| --- | --- | --- |
| Methane | CH4  +  2O2  CO2  +  2H2O
 | 439.3 +/- 0.4 |
| Ethane | C2H6  +  3.5O2  2CO2 +  3H2O | 420.5 +/- 1.3
 |
| Propane | C3H8  +  5O2  3CO2  +  4H2O | 410.5 +/- 2.9
 |
| n-Butane | C4H10  +  6.5O2  4CO2 + 5H2O | 400.4 +/- 2.9
 |

- * CRC Handbook of Chemistry & Physics, 86th edition (2005-2006)

## Experimental and Modeling Results of PCE Conversion under O2/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.

## Lab (Redox): Use of Propane as Reductant

** ****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**

## 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)**

## Field Conditions

- 2" long x 4.7" major axis; 3.15" minor axis
- Temperature Range: 450 – 650oC
- 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**

## 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**
** **

## Treatment Costs

**Catalytic Converter**
-  200 ppm PCE
- 2% v/v propane @ $1.70/gal (DOE, 2005)
- Propane-only treatment costs
-  _$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)
-  _$7/lb PCE absorbed (__**increase**__ with [PCE])_

## PCE Treatment Cost ~ Soil Vapor [PCE] (ppmv)

## 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 H2 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.**

## 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)?

## Acknowledgements

**National Institute of Environmental Health Sciences, NIH **
**U of A Superfund Basic Research Program**

## Extra

## Hydrodechlorination (reduction by H2) General Reaction Mechanisms

**Sequential/serial mechanism**
- 	H2 + 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.

## 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?