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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, Erik Rupp, Marty Willinger, Theresa Foley, Suzanne Bell, Brian Barbaris, Robert Arnold, Eduardo Sáez, Eric BettertonBrian 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 PressPaper in Press: Applied Catalysis B: Environmental, 2007: 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
The University of Arizona
Yellow contours Yellow contours representrepresent
PCE concentration PCE concentration in groundwater in groundwater
from 100 ppb to 1 from 100 ppb to 1 ppb.ppb.
1 ppb
10 ppb
100 ppb
1000 ft
Harmful Health Effects ofPCE & 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.Ground Water
Contaminant Plume
VaporVadose Zone
GAC
Column
Released into atmosphere
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
metal catalyst
metal catalyst
• Catalyst categories:- Supported and unsupported noble metals
• Issues - Rapid deactivation through coking
- High cost of H2
Catalytic Destruction – Reduction
• 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 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
Water Bath
PCE filled U TubeMFC
MFC (Mass Flow Controller )
N2
H2
O2
Waste
Tube Furnace
Reactor
GCwaste
Honeycomb
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 CO2, H2 and O2:
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 O2, 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%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
50 100 150 200 250 300 350 400 450 500
Catalyst Temperature (oC)
PCE Conversion
a
H2/O2 2.15H2/O2 1.18H2/O2 0.67H2/O2 0.26H2/O2 0.0
• 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!
T = 180°C T = 280°C T=380°C T = 450°C H2-only (6%) Conversion
decreased from 15% to zero in 30 min.
Conversion decreased from 98% to 38% in 1 h 50 min.
O2-only (5%) Conversion was ~ 30% for 4 h.
Conversion was ~ 30% for 4 h.
Redox (6% H 2, 3% O 2)
Conversion was steady at 67% for 5 h 30 min.
Conversion was steady at 72% for 2 h.
Conversion was steady at 84% for 30 h.
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!
0
10
20
30
40
50
60
70
80
90
100
75 125 175 225 275 325 375 425 475
Catalyst Surface Temperature (°C)
% PCE Conversion
a
H2/O2 = 2.2
Catalyst Surface T ~ Furnace T
0
50
100
150
200
250
300
350
400
450
500
0 100 200 300 400 500Furnace Temperature (°C)
Catalyst Surface Temperature (°C)
.
H2/O2 = 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
Temperature (°C) vs Fraction of PCE Removed
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
150 250 350 450 550 650 750
Temperature (°C)
Fraction of PCE Removed
Methane Model
Methane Experimental
Ethane Model
Ethane Experimental
Propane Model
Propane Experimental
Butane Model
Butane Experimental
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.
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
80.0%
340.0 360.0 380.0 400.0 420.0 440.0 460.0 480.0
Catalyst Temperature (0C)
PCE Conversion
a
0% C3H80.4% C3H80.6% C3H81.0% C3H8
• 1 Lpm Flow Rate • 5% O2 (vol)
•Varying Propane
•N2 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)100 L/min through each reactor (3.5 cfm)
300 L/min (10 cfm)300 L/min (10 cfm)
• 2 Alumina supported Pt/Rh catalysts– 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% C3H8 (vol)
• ~ 0.2 s Residence Time
• ~ 520 oC Catalyst Temperature
0
10
20
30
40
50
60
70
80
90
100
110
120
0 20 40 60 80 100 120 140 160 180 200 220 240
Elapsed Time (days)
Concentration (ppm)
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
100.0
% Removal of Target Compound
PCE Inlet Conc.TCE Inlet Conc.PCE RemovalTCE Removal
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 decrease with [PCE])
• Granular Activated Carbon 200 ppmv PCE, 50 cfm, 85 F– GAC-only treatment costs:
(Siemens Water Technologies, Sept. 2006) $7/lb PCE absorbed (increaseincrease with [PCE])
PCE Treatment Cost ~ Soil Vapor [PCE] (ppmv)
0.0
5.0
10.0
15.0
20.0
25.0
0 100 200 300 400 500 600 700
SVE PCE conc (ppmv)
PC
E t
rea
tme
nt
co
st
($/l
b)
Redox catalysis
GAC
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*
RClx + * ↔ RClx*
RClx* + H* ↔ RHClx-1* + Cl*
RHClx-1* ↔ RHClx-1 + * (etc.)
H*+ Cl* ↔ HCl + 2 *
• Concerted/parallel mechanism
RClx* + x H*→ RHx + 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?