›The Effects of Oxidant

(Nitrate) on Trichloroethylene

Degradation by Granular Iron

›WaterTech 2017June Lu, SNC-Lavalin

Sung-Wook Jeen, Chonbuk National University

Lai Gui, Health Canada

Robert W. Gillham, University of Waterloo

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Background

Background

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The effects of co-existing oxidants (nitrate, O2, Cr6+, etc.)

on long-term performance of PRBs (permeable reactive

barriers) include:

› Interference with TCE degradation

› Competition for electrons

› Changes in corrosion potential (OCP) and pH

› Formation of iron oxide film

Nitrate is often found in groundwater as a co-contaminant of TCE

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Background

Iron surface film

Fe2O3 + Fe0 + 6H+ → 3Fe2+ + 3H2O reductive dissolution

3 Fe2O3 + 2 H+ + 2 e- → 2 Fe3O4 + H2O reductive conversion

› Outer layer can be removed by auto-reduction

› Double layer of oxide

› inner layer Fe3O4

› outer layer Fe2O3

Iron

In the presence of nitrate

› A decrease in TCE degradation rates was observed

› Two mechanisms were proposed by previous studies

› Formation of passive oxide film as a consequence of an increase in

corrosion potential

› Competition between TCE and nitrate

Background

Ritter et al. 2003, Farrell et al. 2000, Schlicker et al. 2000

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Reference

electrodes

Solution

Reservoir

Effluent

A

B

C

D

E

F

G

Electrical connector

L

H

Peristaltic pump

Influent

Iron

Column setup

Results and Discussion Reaction Kinetics1.TCE reduction

Effective half-life

)ln(

)2ln(

2

1

Ce

Cot

Increased from

2.9 hr to 116 hr

after 160 PV1. Profile shifted up

2. No longer 1st order

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TCE reduction intermediates, products, and mass balance

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Nitrate reduction

› Similar trends as TCE reduction

› Effective half-life increased from

› 8.0 hr @28 PV to 28.9 hr @ 160 PV

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Nitrate reduction products, and mass balance

Results and DiscussionCorrosion potential and changes in surface film

Corrosion potential changes

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NO3- conc.

(mg/L)

Corrosion potential* (mV)

Before NO3-

addition

After NO3-

additionChanged

4.7 -530 -450 +80

47 -500 -400 +100

100 -540 -390 +150

Corrosion potential changes (cont.)

* Measurements were taken from port L

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Green rusts

Magnetite

CarbonGraphite

Hematite

Iron surface film changes

– Dry material and after running Millipore water

Raman Shift / cm-1

Inte

nsity /

arb

. unit

Dry

Iron

Millipore

Water

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Iron surface film changes

– after receiving 150 PV of TCE and nitrate

Green rust disappeared

Fe(OH)3

formed

Hematite Magnetite

Raman Shift / cm-1

Inte

nsity /

arb

. unit

15

Influent NO.Fe2+,

Fe2+/ Fe3+Fe3+

Effective

Half-life (hr)

OCP

(mV) at

port L

Millipore water 10 19 1 -------- -580

10 mg/L TCE +

Millipore

10 10 ----- 2.9-3.2 -510

10 mg/L TCE +

Millipore

+ 100 mg/L NO3-

15 15 7 220

(80 PV)

-400

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Experimental Results

Results and Discussion– Recovery of iron reactivity

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Recovery of Corrosion Potential

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Iron surface film changes

– after receiving ca. 223 PV TCE after nitrate removal

Effluent

Influent

Magnetite

Green rusts

Raman Shift / cm-1

Inte

nsity /

arb

. unit

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Influent NO.Fe2+,

Fe2+/ Fe3+Fe3+

Effective

Half-life

(hr)

OCP

(mV) at

port L

Millipore water 10 19 1 -------- -580

10 mg/L TCE +

Millipore

10 10 ----- 2.9-3.2 -510

10 mg/L TCE +

Millipore

+ 100 mg/L NO3-

15 15 7 220

(80 PV)

-390

10 mg/L TCE +

Millipore

After NO3 - removal

10 12 1 3.4

(90 PV)

-525

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Experimental Results

Conclusions

› The presence of nitrate causes

› Decrease in TCE reduction rate (same as Nitrate)

› Formation of passive iron oxide film

(confirmed by Raman measurements)

› Higher nitrate concentration → greater potential change

› Changes in iron surface condition and loss of reactivity

due to nitrate is a reversible process

› Concurrent adverse effect of both TCE and nitrate

degradation suggests that competition between nitrate

and TCE for available electrons on the iron surface may

not be the main reason for the decline in iron reactivity

towards TCE.

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Implications

The findings of this study suggest that the use of an iron

PRB for remediation of groundwater containing high levels

of nitrate, is not suitable due to its adverse effects

on iron corrosion potentials and surface film formation.

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Plume

Iron Permeable Reactive barrier

Modified environmetal technologies inc. www.eti.ca

Clean GroundwaterNitrate

At low nitrate levels or for seasonal nitrate plumes, the effect of

nitrate should be taken into account for PRB design. A thicker wall

may be required to ensure satisfactory TCE removal.

Implications

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Questions

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