Electrochemical Oxidation for Water Treatment and the Limitation of Hazardous Byproducts
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Transcript of Electrochemical Oxidation for Water Treatment and the Limitation of Hazardous Byproducts
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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
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Introduction
Electrochemical oxidation has become promising for treatment of recalcitrant and biorefractory waste streams
Advantages:• Easy installation and operation
• Cost effective
• Environmentally friendly
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Oxineo ®
Environmental Applications
Electrochemical Oxidation Pilot plant for landfill leachate in Cantabria, Spain.
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Electrochemical Reactions
Power Supply+ _
Anode Cathode
e-
OHH 0.5eOH 22H2O OH + H+ + e-
OH
OH
OH
OH
OH
OH
OH
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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!
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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.
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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
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Farrell et al. (2008)
Perfluorooctane Sulfunate (PFOS)
(C₈F₁₇SO₃⁻)
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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-
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Rate-limiting step
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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.
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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
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1)
2)
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Experimental Setup
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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
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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
(%)
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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
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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?
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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
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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
)
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OH●
OH●
OH●
OH●
R
R
RClO3
●
ClO3●
ClO3●
ClO₄⁻
ClO₄⁻
Anod
e
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Reaction Zone
Anod
e
ClO₃⁻
OH●
e-
ClO3●
ClO4-