Palladium-catalyzed reductive dehalogenation of dissolved chlorinated aliphatics using electrolytically-generated hydrogen

Chemosphere, Vol. 37, No. 5, pp. 925-936, 1998 0 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 004%6535/98 $19.00+0.00

Pergamon

PII: SO0456535(98)00095-Z PALLADIUM-CATALYZED CHLORINATED ALIPI-IATICS

REDUCTIVE DEHALOGENATION OF DISSOLVED USING ELECTROLYTICALLY-GENERATED HYDROGEN

Walt W. McNab, Jr. and Roberto Ruiz

Environmental

Restoration Division, Lawrence Livermore Livermore, CA 9455 1, USA

National Laboratory

(Received in Germany 22 December 1997; accepted 27 February 1998)

Abstract Recent studies have shown that reductive dehalogenation of chlorinated hydrocarbons by hydrogen occurs rapidly in the presence of a palladium catalyst. The speed and completeness of these reactions may offer advantages for groundwater remediation. A practical design challenge arises with the need to expeditiously saturate the aqueous phase with hydrogen. To address this, a two-stage treatment column (hydrogen generator and catalytic reactor) has been developed. The first stage consists of an undivided electrolyzer cell which generates hydrogen by electrolyzing the influent water. The second stage contains a catalyst bed of palladium metal supported on alumina beads. This reactor has been tested with groundwater containing various chlorinated aliphatics. Using a flow rate of 300 ml/min and a current of 4 amps under a potential of 8 volts, removal efficiencies greater than 95% were achieved for PCE, TCE, 1,1-DCE, and carbon tetrachloride with residence times on the order of 2 minutes. Chloroform and 1,2-DCA appear less susceptible to dehalogenation by this process. These results imply that dissolved oxygen present in solution does not completely inhibit reduction of the chlorinated hydrocarbons on the catalyst. 01998

Elsev~~

Science Ltd. All rights reserved

Introduction Contamination

of groundwater

is a familiar environmental processes

(e.g., hydrolysis)

with respect

to carbon

problem.

conditions

dehalogenation

dioxide.

Most of these compounds

[ 11.

However,

because

counterparts, Reductive

has been well documented

iron can transform

hydrocarbons

(e.g., PCE, TCE, chloroform)

are resistant to non-redox

halogenated

a potential

dehalogenation

hydrocarbons

by indigenous

[2-61. More recently, the discovery

a variety of halogenated

compounds

chlorinated

using batch reactors

instability

are in an oxidized

reaction pathway exists for degradation soil microbes that zero-valent

[9] have demonstrated

state

through

under anaerobic metals such as

[7, 81 has led to the design of engineered

treatment systems, such as passive treatment walls, for purposes of groundwater Experiments

transformation

and are often difficult to oxidize despite their inherent thermodynamic

relative to their non-halogenated reductive

resources by halogenated

in

situ

plume remediation.

that hydrogen

can effectively

reduce

ethenes in the presence of a palladium catalyst, yielding ethane and HCI,

TCE + 4Hz --> ethane + 3H+ + 3Cl. 925

(Eq. 1)

926 This Hz/Pd process results in rapid, essentially intermediate

transformation

reduction

processes.

hydrocarbons

products

Similarly,

complete

such as vinyl chloride palladium-coated

iron

dissolving

sufficient

quantities

contact time presents engineering water has been evaluated stage process,

been

shown

or metal-based

to transform

chlorinated

treatment methods based on the

particularly

in treatment columns where a

implying a short residence

time.

However.

as an alternative

In this study, in-line generation

of Hz by electrolysis

of suitable electrodes

immersed

in the influent stream under

applied voltage and current to generate H’. Effluent from the first stage, containing

second

bed containing

stage, a packed

measuring

the concentration

efficiencies

chlorinated

hydrocarbons

remediation

tetrachloroethene

a palladium

hydrocarbons.

metal catalyst

of the chlorinated

were conducted

supported

hydrocarbons

Hz

is in turn passed to the on alumina

beads.

By

across the catalyst bed, removal

(PCE),

chloroform,

35

trichloroethene

indicate

carbon tetrachloride

aliphatics

east

of

San

Francisco.

I-2 dichloroethene

(TCE),

( 1,2-DCA).

time on removal efficiency

that the electrolytic-HJPd

process

Contaminants

( I, 1-DCE),

The objectives

to estimate design parameters

slower rate. whereas

with

carbon

and quantifying

impractical.

Chloroform

included

reaction rates for

operational

treatment

unit.

l,l-DCE,

and

treat PCE, TCE,

was also removed

of

tetrachloride

of the experiments

for a scaled-up

will effectively

consisted

at low flow rates where electric current demands and residence time constraints

too high so as to render the process significantly

contaminated

National Laboratory (LLNL), a Superfund site undergoing

miles

and l&dichloroethane

each of the chlorinated The results

with the test reactor using groundwater

at Lawrence Livermore

approximately

measuring the effects of current and residence

efficiency

chlorinated

dissolved

may be quantified.

A series of experiments

(CTET),

and dissolved

differences

of

to the direct injection of Hz gas. This approach involves a two-

with the first stage consisting

of electrolysis)

in such

of Hz in the intluent stream with limited space and with limited

and safety issues.

(and 02 as a byproduct

active

ethenes, whereas

are often noted in microbial has

applications,

high flow rate relative to column volume is desired.

sufficient

of the chlorinated

much more readily than iron metal alone [IO]. Groundwater

HJPd process offer promise for certain remediation

situations,

transformation

are not

by this process but at a

1,2-DCA appeared to be largely resistant altogether.

In addition, process

was observed to generally decline with time, apparently in response to surface reactions involving

the catalyst and inorganic constituents by temporarily

in the groundwater

solution.

This decline was found to be reversed

shutting off the hydrogen source.

Materials, Equipment,

and Methods

ColurnrzDesign The two stages of the treatment column are depicted in Figure 1. The electrolyzer cylindrical

graphite sheet electrodes,

PVC vessel. manner.

The electrodes,

separated

approximately

each 56 cm-long, housed within a 60 cm-long by 7.6 cm diameter clear

each supported

structurally

by an annular distance of 0.5 cm.

1300 cm?. while

unit contains two

that of the inner

by clear PVC tubing, are configured

The outer electrode electrode

in a coaxial

features a total surface area of

is approximately

I100 cm’.

Electrical

927

connections

to an external power supply unit are established

the graphite sheets. resulting

DC current is supplied by a regulated-current

in average current densities

polarity is switched with a frequency calcium carbonate) in a 40 cm-long

through 0.6 cm stainless steel pins attached to

on the electrodes

power supply.

At a current of 4 amps, the

are on the order of 3.3 milliamps/cm’.

of five minutes to prevent the accumulation

Electrode

of mineral deposits

(e.g.,

on the cathode resulting from local increases in pH. The packed catalyst bed is housed by 7.6 cm diameter

clear PVC tube.

The catalyst material consists

of alumina

spheres of a 0.32 cm nominal diameter coated with 1% palladium metal by weight (Prototech, to a porosity of approximately

(Alz03)

Inc.), packed

40%.

CATALYST

BED

‘CphW

WllC r w -

-c

FIGURE

1. Schematic

diagram of test column (not to scale).

Sampling ports are located upstream of the electrolyzer bed, and downstream

of the catalyst bed.

Stainless

cell, between the electrolyzer

and the catalyst

steel valves and tubing were used for connections

and

sampling ports.

Operation

The column was operated at a test stand within an existing groundwater Water

from an extraction

well (W-351)

used to supply contaminated

partially diverted to the test column for the experiments, geochemical

composition

and contaminant

concentrations

treatment facility at LLNL.

groundwater

to the facility

with the well water serving as the electrolyte. characterizing

was The

W-35 1 water are given on Table 1.

928 TABLE 1. Groundwater chemistry measured in W-35 1(chlorinated hydrocarbons, majo rc :ations and anions). Species Cont. (mg/l)* 0.005-0.007 PCE 0.5-0.6 TCE 0.0 150.025 l,l-DCE 0.008-0.015 Carbon tetrachloride 0.004-0.01 Chloroform 0.004-0.01 1,2-DCA 200-220 Bicarbonate alkalinity 50-95 Ca*+ 70-120 cl15-20 Mg?+ 25-30 NOs1-2 K+ 100-I 10 Naf 55-13s SOJ’~ 7.2-8.2 PH 4-5 Dissolved 02 ’ Ranges denote observed

variability

Water was pumped through a particulate ml/min using a diaphragm electrolyzer

with a mechanical

uncertainties

filter and into the electrolyzer

cell at approximately

300

No attempt was made to separate anodic and cathodic waters within the

or within the catalyst bed. A back-pressure

entire column Residence

pump.

over time, not analytical

regulator

times within the electrolyzer

minutes and 2.3 minutes, respectively;

of approximately

to suppress

the formation

2.5 atm was maintained

across the

of gas bubbles during electrolysis.

cell and catalyst bed for most experiments

was approximately

these could be adjusted by changing the flow velocity.

6

Effluent from

the catalyst bed was fed back into the main treatment facility line. A programmable

logic controller coupled with Labview control software (National Instruments,

was used to regulate experiment and electrode

current/voltage

settings, including process water flow control, electrode polarity switching,

settings.

The system was monitored

sensors placed upstream and downstream electrolyzer

Sampling

pH, flow, and pressure

For abnormal operating conditions

over voltage, loss of flow to electrolyzer,

(pump failure,

or high electrolyte

and Anu1~~se.s

were collected

reactor

to monitor

analyzed by gas chromatography 0.4

by temperature,

the system was designed to shut down using interlock controls.

Samples catalytic

of the reactors.

power supply failure, electrolyzer

temperature),

Inc.)

Kg/l or approximately

concentrations

upstream

process

of the electrolyzer

effectiveness.

in accordance

Chlorinated

hydrocarbons

and downstream in water samples

of the were

with EPA Method 601 [ 111. Detection limits were generally

0.4 parts-per-billion

of TCE, where dilution

and both upstream

requirements

(ppb)

except

in influent

raised the detection

samples

containing

limit to 4.0 ppb.

high

Likely end-

929 products

of the reductive

quantified

dehalogenation

sequence

- ethane,

by Method 601. Because of high background

not feasible

to attempt mass balance calculations

methane,

concentrations

on the reductive

and ethene

- are not effectively

of chloride in W-351 water, it was

dehalogenation

reactions

by including

chloride analysis in the experiments.

Results Changes

in contaminant

concentrations

across the column using W-351 water under 4 amps at 8

volts are shown in Figure 2(a) for TCE and Figure 2(b) for l,I-DCE flow rate of 300 ml/min.

Across the catalyst bed, chlorinated

than 95% after approximately residence

time within

and carbon tetrachloride

hydrocarbon

2 minutes of residence time. In comparison,

the electrolyzer,

losses

of only lo-15%

concentrations

(CTET) for a

declined by more

during the approximate

were observed.

probably attributable

to adsorption

of the organics onto the graphite electrodes

as well volatilization

into minute O2 and H2 gas bubbles present in the column.

6-minute

These minor losses

are

and other column materials

FIGURE 2. Removal of chlorinated aliphatics across the two-stage reactor at 8 V and 4 amps under a flow rate of 300 ml/mm. a) TCE; b) 1, I-DCE and carbon tetrachloride. Electrical Requirements The electrolysis

Thermodynamic

reactions occurring at the cathode and anode are, respectively,

(Eq. 2)

2H20 --> O2 + 4H+ + 4e-

(Eq. 3)

arguments suggest that the generation

V, based on the standard voltage potentials

2H20 + 2e- --> H2 + 20H.

potentials

of H2 should commence

for the half-reactions

at a minimum voltage of 1.2

given by Eqs. (2) and (3).

are usually required before Hz is generated because of overvoltage

In practice,

larger

effects on the electrode

930 surfaces [ 121. The minimum current required to saturate the influent stream with HZ within the electrolyzer vessel may be calculated

by the reaction stoichiometry.

100% current efficiency,

is given by:

The theoretical

rate of production

of HZ, assuming

(Eq. 4)

where RHOis the rate in moleskec, 96,500 coulombs/mole.

i the current in amperes (coulombskec),

The factor 2 arises from the stoichiometry

and F the value of the Faraday,

of Eq. (2), which indicates that one mole

of HZ is produced for each Faraday of electricity. The concentration electrolyzer

of hydrogen produced

in the column, CHZ, is a function of the molar flux in the

and the flow rate, f, in liters/set:

The solubility of HZ in water at 2.5 atm and 20°C is approximately

2.0 x 10e3 mole/l.

Thus, for a flow rate

of 300 ml/min, it may be shown that a 1.9 amp current would be required to achieve Hz saturation. Removal efficiency,

RE, is defined based on contaminant

concentration

differences

in the influent

and effluent streams,

C

RE=l-$!hK

Removal efficiency

as a function of current, measured with reference to residence within the catalyst bed, is

shown in Figure 3 for two different

residence

times.

Catalyst residence

times were calculated

the catalyst bed pore volume by the flow rate.

These results suggest a clear relationship

efficiency

efficiency

volts.

and applied current, with maximum

Although

the cell voltage is manipulated

which determines

not achieved

in the experiments,

the quantity of Hz that is generated.

by dividing

between removal

below approximately

4 amps at 8

it is primarily the electrolytic

The observed 4 amp current at maximum

current

efficiency

is higher than the ideal value of 1.9 amps required to saturate the solution with Hz. This suggests that an excess of hydrogen is required to reduce the chlorinated recombination effect.

hydrocarbons.

of Hz and 02 to form water, or the reduction

The lower-than-expected

side reaction rates.

removal efficiency

Possible side reactions involving the

of NOJ- by Hz, may be responsible

observed at 8 amps may be a reflection

for this

of increases

in

931

.

70%

E 5 I

60%

f x B E

50%

6V

16V 7 HP supersaturation

40%

B

0

1

2

4

3 current

FIGURE 3. Removal efficiency

5

7

6

6

(amps)

of TCE versus applied electric current.

Reaction Rates The relationship TCE with current

between

removal efficiency

equal to or greater than 4 amps (applied

Previous workers [9] had used a first-order process

assuming

a constant HZ concentration

order degradation

consistently

Estimated

observed

effective

Nevertheless,

that of chloroform

of a first-

in the test column yielded a mean effective

0.02 sec.‘, corresponding reaction

rates

to a reaction

for the other

chlorinated

first-

half-life

of

aliphatics

(PCE, TCE, and I,l-DCE)

and carbon tetrachloride

is less than these by a factor of at least two. The transformation

observed

across the two-stage

reductive dehalogenation

In all of the experiments

reactor due to mechanisms

conducted,

as well as carbon tetrachloride

water, the production

The

are very similar, while of 1,2-DCA appears to

Given the small losses (lo-15%) other than reductive

dehalogenation,

of 1,2-DCA is suspect.

methyl chloride) were not observed

TCE, l,l-DCE

and other factors.

the relative rates of reaction of the different compounds.

be very slow, with a rate less than l/20 of that of the chloroethenes.

chloride,

Application

02, packing of the catalyst, mixing processes,

they are useful for comparing

rates for the three chloroethenes

significant

by the H$Pd

present in W-351 water are shown on Table 2. Such rates are specific to these experiments

since they reflect the influence of dissolved

typically

greater than or equal to 8 volts).

and a constant catalyst surface area.

rate for TCE of approximately

30 seconds.

potential

time is shown in Figure 4 for

kinetic model to describe reductive dehalogenation

order kinetic model to the removal efficiencies

approximately

and catalyst residence

unequivocal

chlorinated

above applicable detection and chloroform

of some transient chlorinated

intermediates

degradation limits.

products

However,

given that PCE,

are all present as co-contaminants cannot be completely

(e.g., vinyl

ruled out.

in W-351

932

y=eQmr R2= 0.73

FIGURE 4. Removal efficiency amps or higher).

versus catalyst residence time (at 8 V and 4

TABLE 2. Estimated first-order reaction coefficients for chlorinated aliphatics in catalyst bed (4 amps current at 8 volts, l/S-inch (0.32 cm) diameter A1203 spheres coated with 1% Pd by weight). Species Rate constant (set”)’ PCE 20.02 TCE 0.02 1 + 0.005 l,l-DCE 0.019 + 0.002 Carbon tetrachloride 20.02 Chloroform SO.009 1,2-DCA <0.0009

’High analytical detection limits relative to concentrations of PCE, carbon tetrachloride, chloroform, and I ,2-DCA precluded more precise estimates. When given, errors refer to standard deviations. Effect of Dissolved Oxygen In addition to reductive dehalogenation, on the catalyst is the recombination

a competing

hydrogen-consuming

reaction which may occur

of Hz and 02, either in the aqueous or gaseous phase,

O2 + 2H, --> 2H20

(Eq. 7)

This reaction is known to occur vigorously in the presence of platinum [ 131. The solubility of 02 in water at 20°C and 0.5 atm (2.5 atm pressure x 20% mole fraction in air) is approximately concentration

is almost

certainly

background

concentrations

comparison,

the mean concentration

reached

of dissolved

as the influent

passes

through

6.9 x 1O-4mole/l.

the electrolyzer,

given

02 in W-351 water are already greater than 3 x IO-’ mole/l.

of TCE, the most abundant chlorinated

hydrocarbon

This that In

in W-35 1 water, is

933 approximately equivalent

3.8 x 1O-6mole/l.

to the reductive

greatly suppressed

reaction

this issue, an experiment

porous polypropylene

membrane

and cathodic electrolyzer

observed

dehalogenation

reaction given by Eq. (7) were to proceed given by Eq. (1) reductive

at a rate

dehalogenation

would be

or not occur at all.

To address

the anodic

If the recombination

stream,

column

dehalogenation,

catalyst surface to some degree.

design

mixing of the anodic

This action appeared

suggesting

Nevertheless,

to approximately

that 02 does compete

practical engineering

considerations

preclude modifying

Separation of the two electrolytic

a divided-cell

would add complexity

create locally extreme

(i.e. separate anolyte and catholyte)

double

the

with TCE for HZ on the

design of a full-scale reactor to take advantage of this finding. arrangement

in which a

The cathodic stream, enriched in Hz, was fed into the catalyst bed, while

in 02, was discarded.

rates of reductive

with a modified

was placed within the cell to suppress mechanical

streams.

enriched

was conducted

the

streams in

to the design, would

pH values, and would fail to address the issue of remediating

the Hz-poor anodic

water.

Choice of Electrode Material The choice of suitable requirement

to periodically

graphite performed

electrode

switch

adequately

scale unit.

Unfortunately,

the platinum address

Consequently,

identifying

The precious

inhibits the conduction

to prevent

operational

CaCOs precipitation

unit is complicated on the cathode.

Although

showed considerable

replacement

iridium, ruthenium)

metal coating prevents

wear over

would be required in a full-

Titanium metal coated with a member of

is often used in electrolytic the formation

applications

To assess suitability

than using

of one such material in the

treatment column, the graphite electrodes

were replaced with two titanium cylinders of equivalent

each coated on both sides with ruthenium

oxide (obtained from The Electrosynthesis

conducted

twice

concentrations

declining

phenomenon

as the graphite)

indicated

by only about 40% across the catalyst

is that the ruthenium

HzO, depleting dehalogenation.

as conductive

titanium rod which was employed

removal

bed.

to the Pd catalyst

A possible

was tested by replacing

bed, thus reducing

the innermost

as a cathode under a fixed polarity.

proved to be

efficiencies,

with TCE

explanation

oxide surface acts as a catalyst for the recombination

the HZ in the influent This hypothesis

poor

geometry,

Co., Inc.). Initial tests,

with a flow rate of 300 ml/min under 4.1 volts and 4 amps (the Ti-RuOz electrodes

approximately

to

of a TiOx oxide layer which

Using titanium as a base material is less expensive

metal components.

by the

for graphite is difficult because most metals

when used as an anode.

of an electric current.

made entirely of precious

frequent electrode

a suitable replacement

or passivation

metal group (e.g., platinum,

this problem.

electrodes

polarity

in a full-scale

in the pilot tests in this study, the electrodes

the course of the experiments.

tend to undergo corrosion

materials

for this

of 02 and Hz into

the rate of reductive

Ti-RuOz electrode

A porous polypropylene

with a pure membrane

was placed between the anode and the cathode to separate the two waters, with both streams fed into the Pd

934 catalyst bed. The resulting marked improvement

in the removal efficiency

of TCE, up to 8 l%, suggests that

the reaction of 02 and Hz on the ruthenium oxide surface is a likely cause of reduced performance. No convincing the ruthenium-oxide palladium

evidence of reductive dehalogenation coated

and ruthenium

processes.

electrodes.

within the electrolyzer

Thus, it appears

that the two precious

oxide, differ markedly in their catalytic properties

Hz reacts more readily with the chlorinated

itself was observed with

hydrocarbons

metal-based

in application

materials,

in this treatment

than with oxygen

on palladium,

whereas the opposite appears to hold true for ruthenium oxide.

Process De-Activation Influent and effluent TCE concentrations

associated

with the two-stage

reactor over an extended test

period with W-351 water are shown on Figure 5. Although the initial removal efficiency concentrations involving

rise over a period

direct injection

phenomenon, recovers

steadily

of many hours.

of Hz, rather than electrolytic

Similar

generation,

experiments

is high, effluent

conducted

have exhibited

elsewhere

the same deactivation

which apparently involves the Pd/A1203 catalyst itself [ 141. Tests have shown that the catalyst

most or all of its capacity if soaked in deionized

water over a period of hours to days. and may

recover a portion of its capacity if the hydrogen source is simply taken off line for a period of time. The rate of deactivation exacerbate

also appears to depend on water composition;

soaking of the catalyst in deionized

catalyst is not allowed to deactivate extensively addressed,

of bicarbonate

appear to

the problem.

The precise cause of the deactivation periodic

high concentrations

application

phenomenon

is currently

under investigation.

water appears to restore effectiveness, (i.e. below 50%).

particularly

Until the cause is identified

of this may require the design of a regeneration

Nevertheless. when the

and suitably

system based upon periodic deionized

water flushes.

FIGURE 5. Extended performance test of treatment column in W-35 1 water, showing both deactivation and regeneration of catalytic activity.

935 Removal

efficiency

results and rate calculations

time data in each experiment

Discussion

presented

using only fresh or regenerated

in this study were all taken from early

catalyst.

and Summary

Rapid reductive dehalogenation flow-through

treatment reactor consisting

and a packed bed of Pd/AlzOs catalyst. transformation somewhat

of dissolved

with pseudo-first-order

more slowly,

tetrachloride.

contaminated

of an electrolyzer

has been demonstrated

rate approximately

which hold implications

for the full-scale

all appear to undergo

Carbon tetrachloride

half that of the chloroethenes

Reaction products

reacts

and carbon

were not observed.

application

with a

of hydrogen from water

and carbon tetrachloride

reaction rates on the order of 0.02 sec.‘.

with a reaction

groundwater

hydrocarbons

cell for the generation

PCE, TCE, l,l-DCE,

1,2-DCA reacts very slowly, if at all.

other observations

chlorinated

A number of

of this approach

for treating

include:

(1) Reaction rates are lowered by the presence of oxygen, but the effect on process efficiency

is not severe.

(2) The Pd catalyst is subject to a deactivation

but appears to

be related to water composition. deionized

mechanism

This de-activation

which is not yet well-understood

may be reversed by periodically

water.

(3) Electrode

materials

which include

certain platinum

group metals (ruthenium)

choices for use in this process as they tend to catalyze the recombination thus removing

the reducing agent from the reductive dehalogenation

Acknowledgments. Research

soaking the catalyst in

(LDRD) grant from Lawrence

Ken Carroll, Dwayne Coufal, Scott Kawaguchi, were instrumental and the choice

of oxygen and hydrogen into water,

reaction.

This work has been funded through a U.S. Department

and Development

materials.

Livermore

of Energy Laboratory-Directed

National Laboratory.

The efforts of

Steve Orloff, Leon Newton, Stan Rudolf, and Ken Williams

in carrying out the experiments. of electrode

may be inappropriate

Jack Harrar provided

advice on electrochemical

We also wish to thank Jack Harrar, Ken Jackson,

issues

and Martin

Reinhard for their helpful reviews of the manuscript.

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