Catalytic Destruction of Gas-Phase PCE and TCE in Groundwater and Soils - Laboratory Study & Field Investigation Departments of Atmospheric Sciences &

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

gao

at bench scale; in a fixed-bed reactor

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

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Also, PCE and TCE are the most frequently occurring mixture of solvents in goundwater. They are often found as mixtures due to their co-use in industrial processes or their degradation from one to another.

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Samples of groundwater taken from more than 5000 wells thourhgout the US, by US Geological Survey.

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.

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Of the 50 remediation technologies identified by NRC (majority are physical containment or biological in nature), only 5 chemical reaction technologieds were listed. This shortage of alternative remediation technologies is having the undesirable effects of limiting remediation efforts to only those sites that post an immediate health risk. There is also mounting pressure to relax standards at other sites because there is no cost-effective alternatives.

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Chlorinated solvents have been implicated in skin, liver and kidney cancers, nervous system dysfuction, and fetal heart defects.

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

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When semi-volatile contaminants are present in an extensive vadose zone, SVE is frequently selected to recover pollutants.SVE applications yield contaminated gas streams that are frequently cleaned up via carbon adsorption before release to the atmosphere - unadsorbed hydrocarbons may still post a threat. 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 hazardous waste.

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Pump and treat is more expensive than SVE, since pressure driven gas recovery (SVE) is more economical than water recovery from porous media (p&t).

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

gao

This is an example of how intuition can lead to surprising, and surprisingly wonderful, discoveries.

gao

Do we have an edge over other technologies, not only in terms of effectiveness of treatment, but economics of treatment?

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

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The honeycomb is ~ 20cm from the upstream of furnace (38cm in length). This gives the gas enough residence time to warm up to the furnace T before it enters the honeycomb. This is verified by an independent measurment.

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First describe reactor; then describe PCE generation, naturally leadning to next slide - gas measurements.

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)

gao

He used as carrier gas. Injector and detector T were typically 200C and 250C, respectively.

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!

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mild and convenient!

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.

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In Figure 3's experiments, as long as H2/O2 > 1.2, PCE conversion is above 50% at just above 300C.

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We did experiments with H2/O2 ratios larger tahn 2.2. Results show that eventually at the operating T (400 C), the conversion remains the same as in H2/O2 = 2.2 case.

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

gao

H2O/O2 reactions were not the primary cause, according to our experiments. The conversions at low T, from PCE/O2/H2O, were much lower than those in this figure.

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)

gao

Obviously there are other reactions in addition to alkane oxidation. But following H2/O2, alkane/O2 is probably the main heat provider for the reaction system. And as we have seen before, the stoichiometric ratio of alkane/O2 is a good indicator of how much alkane we should use for optimum PCE conversion.

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.

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Plug flow reactor.

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assume E(a) = aE(D) + b, assume there exist a and b applying to all alkanes, then k = k(0)exp(-(aE(D) + B)/RT).At each T, calculate k with a three-parameter fit (k(0), a, b) and compare with k from experim. When k(exp) and k(mod) have the least deviations (minimum square approach), the optimum parameters are found.Turns out this simple assumption can model the PCE conversion very well, meaning the BDE of alkane used in the reaction plays a major, even decisive. role in PCE reaction rate.

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Alkanes were at half the stoichiometric ratio to O2 (21%).

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)

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Hydro Geo Chem Inc. in collaboration

• 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])

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Propane consumption is independent of PCE concentration (always at 2% of total SVE gas volume). 2% propane is always in 1/2 stoichiometric ratio to O2 (1:10), optimum for conversion.Therefore, the higher PCE concentration is to be treated, the lower the cost/lb PCE is!

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)?

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So far from lab work, the best we have gleaned about the reaction mechanisms is that reduction steps probably precede oxidation ones and the importance of oxidation lies in its elimination of intermediates that would otherwise lead to rapid catalyst poisoning.

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?

Catalytic Destruction of Gas-Phase PCE and TCE in Groundwater and Soils - Laboratory Study & Field Investigation Departments of Atmospheric Sciences &

Documents

treatment slide

atmosphere slide

ft slide

us pce tce

pce concentration

phase pce

parkeuclid pce

hcl o