AWRA MeetingPhiladelphia, PAMarch 21, 2013

Adrienne DonaghueBrian P. ChaplinVillanova University

Department of Civil & Environmental Engineering

Electrochemical Oxidation for Water Treatment and the

Limitation of Hazardous Byproducts

Introduction

Electrochemical oxidation has become promising for treatment of recalcitrant and biorefractory waste streams

Advantages:• Easy installation and operation

• Cost effective

• Environmentally friendly

2

Oxineo ®

Environmental Applications

Electrochemical Oxidation Pilot plant for landfill leachate in Cantabria, Spain.

3

Electrochemical Reactions

Power Supply+ _

Anode Cathode

e-

OHH 0.5eOH 22H2O OH + H+ + e-

OH

OH

OH

OH

OH

OH

OH

4

Electrochemical OxidationDestruction of pollutants occurs through 2 mechanisms:

1. Direct electron transfer (DET)2. Indirect oxidation via hydroxyl radicals (OH●)

* Electrochemical material plays important role in the effectiveness of oxidation!

5

Indirect electrochemical oxidation

Anode Adsorbed •OH

current

•OH

Free •OH

R

ROR or RO

CO₂ + H₂O

Oxygen Evolution

Direct electrochemical oxidation

e-

Zhu et. al, 2008.

Boron Doped Diamond Electrode

• Boron-doped diamond (BDD) film grown on p-silicon substrate using CVD (Advanced Diamond Technologies).

• Boron doping @ ppm levels provides electrical conductivity.• Inert surface and low adsorption properties• Remarkable corrosion satiability• Produces large amount of OH●

(weakly adsorbed)

• Emerging AOP technology.• Can oxidize perfluorinated

compounds

Note! These compounds can not be degraded by

other AOP technologies 6

Farrell et al. (2008)

Perfluorooctane Sulfunate (PFOS)

(C₈F₁₇SO₃⁻)

7

By-product/Perchlorate (ClO4-) Formation

• Is a multi-step process• Hazardous to human health• EPA set an advisory limit of 15 ppb for drinking

water sources• CA and MA drinking water limits of 2 and 6 ppb

Cl- OCl- ClO₂- ClO₃- ClO₄-

8

Rate-limiting step

9

By-product/ClO4- Formation Cont.

Azizi et. al, 2011

2 step process:

Cl- OCl- ClO₂- ClO₃- ClO₄-

Rate-limiting step

Reaction Zone

Anod

e

ClO₃⁻

OH●

e-

ClO3●

ClO4-

1.

2.

Research Objectives

• Understand how the reactivity of certain organics effect perchlorate formation at the anode surface

• Use “model” p-substituted phenols to determine the importance of each step in the two step process of perchlorate formation.

• Model organic behavior with in the diffuse and reaction zones to understand mechanisms of inhibition of ClO4

- at the anode surface

10

1)

2)

Experimental Setup

11

Batch ReactorRotating Disk Electrode (RDE)

Organic compounds

p-nitrophenol (p-NP)

p-methoxyphenol (p-MP)

p-benzoquinone (p-BQ)

Oxalic acid (OA)

Solutions were tested at: Kinetically Control: 1.0 mA/cm² Mass-transfer Control: 2.4 mA/cm²,

10.0 mA/cm²

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Results: ClO₄⁻ Formation

OH• Rate Constant* Log Kow

(L mol⁻¹ s⁻¹)

p-nitrophenol (p-NP) 3.8x10⁹ 1.91

p-benzoquinone (p-BQ) 6.6x10⁹ 0.2

p-methoxyphenol (p-MP) 2.6x10¹⁰ 1.34

oxalic acid (OA) 1.4x10⁶ -0.81

p-NP p-BQ p-MP OA0

20

40

60

80

100

12099.6 96.1 93.3

5.3

93.6 92.1

53.5

0.0

29.27

85.04

12.96

0.00

1.0 mA cm⁻² 2.4 mA cm⁻² 10 mA cm⁻²

Inhi

bitio

n of

ClO

₄⁻ F

orm

ation

(%)

* Buxton et al. 1988

Initial Organic Concentration = 250 μM

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Results: Organic Reactivity

C

x/L

Anode Diffuse Layer

COMSOL ®

Anod

e Su

rface

OH●

ClO₃●

RB

Diffusion ZoneReaction Zone

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2.0E-03

5.2E-18

2.0E-03

4.0E-03

6.0E-03

8.0E-03

1.0E-02 High Current Density

p-NP p-BQp-pmeth

x/μm

Conc

entr

ation

(mol

/m³)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

0.05

0.1

0.15

0.2

0.25Low Current Density

p-NP p-BQ p-pmeth

x/μm

Conc

entr

ation

(mol

/m³)

2 μm 5 μm

Results: ClO₄⁻ Formation

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p-NP p-BQ p-MP OA0

20

40

60

80

100

12099.6 96.1 93.3

5.3

93.6 92.1

53.5

0.0

29.3

85.0

13.0

0.0

Experimental1.0 mA cm⁻²2.4 mA cm⁻²10 mA cm⁻²

Inhi

bitio

n of

ClO

₄⁻ F

or-

mati

on (%

)

p-NP p-BQ p-MP OA0

20

40

60

80

100

120

95 97 99

1

85 8696

16 5 5 1

Model1.0 mA cm⁻²

2.4 mA cm⁻²

10 mA cm⁻²

Inhi

bitio

n of

ClO

₄⁻

Form

ation

(%)

Conclusions:

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Limiting ClO₄- formation• Rate limiting step is a

two step process• Reactions occur right at

surface• Organic reactivity is

importantFor Low Current Densities:

Scavenging occurs on surface

For High Current Density:Location becomes important

Anod

e

ClO₃⁻

OH●

e-

ClO3●

ClO4-

Step 1

Step 2

Conclusion Cont.

• Operating under MT conditions is the most effective means to limit ClO4

- formation.• In addition, operating at these conditions is

cost effective.• EC is viable technology for refractory organic

pollutants but in order for it to be integrated into environmental applications, ClO4

- must be inhibited below advisory levels.

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Acknowledgements

This research was funded by Advanced Diamond Technologies (ADT) in Romeoville, IL via NSF SBIR Phase II grant.

Special thanks to my advisor Dr. Brian P. Chaplin

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Questions?

Results: LSV of p-substituted phenols

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0.50 1.00 1.50 2.00 2.50 3.00 3.500.00

0.20

0.40

0.60

0.80

1.00

1.20p-methoxyphenol

Potential (V/ SHE)

Curr

ent D

ensit

y (m

A/cm

^2)

0.50 1.00 1.50 2.00 2.50 3.000.00

0.20

0.40

0.60

0.80

1.00

1.20

p-nitrophenol

Potential (V/SHE)

Curr

ent D

ensit

y (m

A/cm

²)

0.50 1.00 1.50 2.00 2.50 3.00 3.50-0.30

0.20

0.70

1.20

1.70 p-benzoquinone

Potential (V/SHE)

Curr

ent D

ensit

y (m

A/cm

²)

Blank

Blank

Blank

0.50 1.00 1.50 2.00 2.50 3.00 3.500.00

0.20

0.40

0.60

0.80

1.00

1.20

Oxalic Acid

Potential (V/SHE)

Curr

ent D

ensit

y (m

A/cm

²)

Blank

1 mM

5 mM

10 mM

0.75 mM

1.0 mM

0.25 mM

0.50 mM

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Measured Rates vs. Mass Transfer

0 200 400 600 800 10000.0E+005.0E+021.0E+031.5E+032.0E+032.5E+033.0E+033.5E+034.0E+03

p-NP

Conc (µM)

Rate

(µm

ole/

m³/

min

)

0 200 400 600 800 10000.0E+00

5.0E+02

1.0E+03

1.5E+03

2.0E+03

2.5E+03

3.0E+03

3.5E+03

4.0E+03 p-BQ

Conc. (µM)

Rate

(µm

ole/

m³/

min

)0 200 400 600 800 1000

0.0E+005.0E+021.0E+031.5E+032.0E+032.5E+033.0E+033.5E+034.0E+03

p-MP

Conc (µM)

Rate

(µm

ole/

m³/

min

)

OH●

OH●

OH●

OH●

R

R

RClO3

●

ClO3●

ClO3●

ClO₄⁻

ClO₄⁻

Anod

e

Reaction Zone

Anod

e

ClO₃⁻

OH●

e-

ClO3●

ClO4-