March 30, 2004 Ryan Hutcheson University of Idaho
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Dependence of the FeII/IIIEDTA complex on pH
Ryan Hutcheson and I. Francis Cheng*Department of Chemistry, University of Idaho
Moscow, ID [email protected]
March 30, 2004 Ryan Hutcheson University of Idaho
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Importance
• First study of the pH dependence of FeII/IIIEDTA
• Green chemistry – optimization of O2 activation and pH dependence of the Fenton Reaction
• Antioxidants : FeII/IIIEDTA is a good model for low molecular weight biological ligands
March 30, 2004 Ryan Hutcheson University of Idaho
3
0.00E+00
1.00E-04
2.00E-04
3.00E-04
4.00E-04
5.00E-04
6.00E-04
7.00E-04
8.00E-04
9.00E-04
1.00E-03
2 3 4 5 6 7 8 9 10 11
pH
Co
nc
en
tra
tio
n (
M)
FeIIIEDTA Speciation DiagramFeIIIEDTA
FeIIIHEDTA
FeIII(OH)EDTA
FeIII(OH)2EDTA
March 30, 2004 Ryan Hutcheson University of Idaho
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0.00E+00
1.00E-04
2.00E-04
3.00E-04
4.00E-04
5.00E-04
6.00E-04
7.00E-04
8.00E-04
9.00E-04
1.00E-03
2 3 4 5 6 7 8 9 10 11
pH
Co
nc
etr
ati
on
(M
)
FeIIEDTA Speciation DiagramFeIIEDTA
Free Fe+2
FeIIHEDTA
FeIIH2EDTA
FeII(OH)2EDTA
FeII(OH)EDTA
March 30, 2004 Ryan Hutcheson University of Idaho
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Electrocatalytic (EC’) Mechanismand Cyclic Voltammetry
FeIII-L + e- FeII-L
FeII-L +H2O2 FeIII-L OH• +OH-
E: O + ne- = RC’: R + Z = O + Y
Regeneration of the FeIIIEDTA within the vicinity of the electrode causes amplification of the CV wave
March 30, 2004 Ryan Hutcheson University of Idaho
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Conditions
• All scans– 10mL aqueous sol’n purged w/ N2 for 10-15min– 0.1M Buffer - HOAcCl, HOAc, HEPES– 5mV/s sweep rate– BAS carbon disk electrode – BAS Ag/AgCl reference electrode– Spectroscopic graphite rod counter electrode– BAS CV-50w potentiostat
• Cyclic Voltammetric scans of FeIIIEDTA– 1mM FeIIIEDTA
• Catalytic scans (Fenton Reaction)– 0.1mM FeIIIEDTA catalytic scans– 20mM H2O2
March 30, 2004 Ryan Hutcheson University of Idaho
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Cyclic Voltammagrams of FeII/IIIEDTA
-0.000005
-0.000004
-0.000003
-0.000002
-0.000001
0
0.000001
0.000002
-0.7-0.5-0.3-0.10.10.3
Potential (V)
Cu
rren
t (A
)
-0.000005
-0.000004
-0.000003
-0.000002
-0.000001
0
0.000001
0.000002
-0.7-0.5-0.3-0.10.10.3
Potential (V)
Cu
rren
t (A
)
pH 2
pH 11
pH 5.5
FeIIIEDTA + e- → FeIIEDTA
FeIIIEDTA + e- ← FeIIEDTA
1mM FeIIIEDTA0.1M buffer5mV/s scan rate
March 30, 2004 Ryan Hutcheson University of Idaho
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0.00E+00
1.00E-04
2.00E-04
3.00E-04
4.00E-04
5.00E-04
6.00E-04
7.00E-04
8.00E-04
9.00E-04
1.00E-03
2 3 4 5 6 7 8 9 10 11
pH
Co
nc
en
tra
tio
n (
M)
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
Po
ten
tial (V
)
E1/2 vs. pH (FeIIIEDTA)FeIIIEDTA
FeIIIHEDTA
FeIII(OH)EDTA
FeIII(OH)2EDTA
E1/2
March 30, 2004 Ryan Hutcheson University of Idaho
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0.00E+00
1.00E-04
2.00E-04
3.00E-04
4.00E-04
5.00E-04
6.00E-04
7.00E-04
8.00E-04
9.00E-04
1.00E-03
2 3 4 5 6 7 8 9 10 11
pH
Conc
etra
tion
(M)
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
Pote
ntia
l (V)
E1/2 vs. pH (FeIIEDTA)FeIIEDTA
Free Fe+2
FeIIHEDTA
FeIIH2EDTA
FeII(OH)2EDTA
FeII(OH)EDTA
E1/2
March 30, 2004 Ryan Hutcheson University of Idaho
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O2 Activation
• First example of abiotic RTP oxygen activation able to destructively oxidize organics.
• Oxygen activation is pH dependent.
Noradoun,C., Industrial and Engineering Chemistry Research, (2003), 42(21), 5024-5030.
March 30, 2004 Ryan Hutcheson University of Idaho
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Reaction Vessel
0.5g Fe; 20 or 40-70 mesh
0.44mM Xenobiotic
10.0 mL water
Air flow
2.0 mL 50/50 hexane/ethyl acetate(extraction only)
Stir bar
0.44mM EDTApH 5.5 – 6.5, unbuffered.
Noradoun,C., Industrial and Engineering Chemistry Research, (2003), 42(21), 5024-5030.
March 30, 2004 Ryan Hutcheson University of Idaho
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Xenobiotic Oxidation Studies
Ironparticles0.1-1 mm
Fe2+
O2 + 2H+ H2O2
EDTAFeIIEDTA
+
FeIIIEDTA + HO- + HO.
Aqueous Xenobiotic
LMW acidsNoradoun,C., Industrial and Engineering Chemistry Research, (2003), 42(21), 5024-5030.
March 30, 2004 Ryan Hutcheson University of Idaho
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Proposed O2 Reduction Mechanism by Van Eldik
Van Eldik, R. Inorg. Chem, 1997, 36, 4115-4120
FeIIEDTAH(H2O) + O2 FeIIEDTAH(O2) + H2O
FeIIEDTAH(O2) FeIIIEDTAH(O2-)
FeIIIEDTAH(O2-) + FeIIEDTAH(H2O) FeIIIEDTAH(O2
2-)FeIIIEDTAH + H2O
FeIIIEDTAH(O22-)FeIIIEDTAH + H2O + 2H+ 2FeIIIEDTAH(H2O) + H2O2
2FeIIEDTAH(H2O) + H2O2 2FeIIIEDTAH(H2O) + H2O
*Proposes H2O2 as intermediate*Saw no evidence of H2O2
March 30, 2004 Ryan Hutcheson University of Idaho
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Van Eldik’s O2 Reduction
Van Eldik, R. Inorg. Chem, 1997, 36, 4115-4120
March 30, 2004 Ryan Hutcheson University of Idaho
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Structures
O-
O-
O-
N N
O-
Fe
O
O
O
O
NH
O-
O-
O-
NH
FeOH
O
O
O
O
FeIIIEDTA (CN = 7)FeIIIHEDTA (CN = 6)
O-
O-
O-
O- N
N Fe
O
O
O
O
FeIIEDTAFeIIHEDTACN = 7
OctahedralSquare Pyramidal
Monocapped trigonal prismatic (MCP) Pentagonal-bipyramidal (PB)
NO
N
OO
O
Miyoshi, K., Inor. Chem. Acta., 1995, 230, 119-125.Heinemann, F.W., Inor. Chem. Acta., 2002, 337, 317-327.
March 30, 2004 Ryan Hutcheson University of Idaho
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Structures cont’d
< pH 3
pH 3 – pH 4 > pH 4
PB MCP
Free Fe+2
FeIIEDTAFeIIHEDTA
Miyoshi, K., Inor. Chem. Acta., 1995, 230, 119-125.
Active site
Active site
March 30, 2004 Ryan Hutcheson University of Idaho
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Fenton Reaction
FeIIIL +e-→ FeIIL E°’=depends on ligand
H2O2 + e- → HO• + OH- E°=0.32V SHE @pH 7
FeIIL + H2O2 → FeIIIL + HO• + OH-
Only iron complexes with E0’ negative of 0.32 V are thermodynamically capable of hydrogen peroxide reduction. However, Fenton inactivity may result from kinetic factors as well.
March 30, 2004 Ryan Hutcheson University of Idaho
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Electrocatalytic CV
0
0.00001
0.00002
0.00003
0.00004
0.00005
0.00006
0.00007
-0.7-0.5-0.3-0.10.10.3
Potential (V)
Cu
rre
nt
(A)
-0.7-0.5-0.3-0.10.10.3
pH 4pH 3.5
pH 2
pH 2.5
pH 3
pH 4
pH 4.5
FeIIIEDTA + e- → FeIIEDTA
0.1mM FeIIIEDTA20mM H202
0.1M buffer5mV/s scan rate
March 30, 2004 Ryan Hutcheson University of Idaho
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Fenton Reactivity vs. pH
FeIIEDTAFree Fe+2
FeIIHEDTAFeIIH2EDTA
Each data point was collected 9 times.
0.00E+00
1.00E-05
2.00E-05
3.00E-05
4.00E-05
5.00E-05
6.00E-05
7.00E-05
8.00E-05
9.00E-05
1.00E-04
2 2.5 3 3.5 4 4.5 5
pH
Co
nc
en
tra
tio
n (
M)
0
0.00002
0.00004
0.00006
0.00008
0.0001
0.00012
0.00014
0.00016
0.00018
Cu
rren
t (A
)
March 30, 2004 Ryan Hutcheson University of Idaho
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Conclusion
• E1/2 of the FeII/IIIEDTA complex depends on pH, corresponding to the pH distribution diagram.
• Fenton reactivity increases around pH 3.5 due to geometric rearrangement of the FeIIEDTA complex (MCP to PB).
March 30, 2004 Ryan Hutcheson University of Idaho
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Future
• pH dependence of Fenton reactivity at higher pH values
• Expand van Eldik’s O2 activation to higher pH values
March 30, 2004 Ryan Hutcheson University of Idaho
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Acknowledgments
•National Institute of Health
•National Science Foundation
•University of Idaho
•Malcom and Carol Renfrew
•Dr. Cheng Group
•Dr. Mark Engelmann
March 30, 2004 Ryan Hutcheson University of Idaho
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Nernst Equations E1/2
• pH 2 to pH 3.5– E1/2(mV) = 83mV – 69.5mV*(pH )
• pH 3.5 to 7– E1/2(mV) = -89.5mV ± 5.6mV
• pH 7 to 9– E1/2(mV) = 202.8mV – 41.8mV*(pH)
• pH 9 to 11– E1/2(mV) = 409.1mV – 64.6mV*(pH)
March 30, 2004 Ryan Hutcheson University of Idaho
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FeIIIEDTA ModelEDTA-4 + H+ → HEDTA-3 log β = 9.52HEDTA-3 + H+ → H2EDTA-2 log β = 6.13 H2EDTA-2 + H+ → H3EDTA- log β = 2.69H3EDTA- + H+ → H4EDTA log β = 2.00H4EDTA + H+ → H5EDTA+ log β = 1.5H5EDTA+ + H+ → H6EDTA+2 log β = 0.0EDTA-4 + Fe+3 → FeIIIEDTA- log β = 25.1FeIIIEDTA- + H+ → FeIIIHEDTA log β = 1.3FeIIIEDTA- + H20 → FeIII(OH)EDTA-2 + H+ log β = 17.712FeIII(OH)EDTA-2 → FeIII
2(OH)2EDTA2-4 log β = 38.22
FeIII(OH)EDTA-2 + 2H2O → FeIII(OH)2EDTA-3 + 2H+ log β = 4.26H+ + OH- → H2O log β = 13.76Fe+3 + OH- → FeIII(OH)+2 log β = 11.27Fe+3 + 2OH- → FeIII(OH)2
+ log β = 23.0Fe+3 + 3OH- → FeIII(OH)3 log β = 29.77Fe+3 + 4OH- → FeIII(OH)4
- log β = 34.42Fe+3 + 2OH- → FeIII
2(OH)2+4 log β = 24.5
3Fe+3 + 4OH- → FeIII3(OH)4
+8 log β = 49.7
March 30, 2004 Ryan Hutcheson University of Idaho
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FeIIEDTA ModelEDTA-4 + H+ → HEDTA-3 log β = 9.52HEDTA-3 + H+ → H2EDTA-2 log β = 6.13H2EDTA-2 + H+ → H3EDTA- log β = 2.69H3EDTA- + H+ → H4EDTA log β = 2.00H4EDTA + H+ → H5EDTA+ log β = 1.5H5EDTA+ + H+ → H6EDTA+2 log β = 0.0EDTA-4 + Fe+2 → FeIIEDTA-2 log β = 14.3HEDTA-3 + Fe+2 → FeIIHEDTA- log β = 6.82H2EDTA-2 + Fe+2 → FeIIH2EDTA log β = 13.41FeIIEDTA-2 + OH- → FeII(OH)EDTA-3 log β = 18.93FeII(OH)EDTA-3 + OH- → FeII(OH)2EDTA-4 log β = 13.03Fe+2 + OH- → FeII(OH)- log β = 4.2Fe+2 + 2OH- → FeII(OH)2 log β = 7.5Fe+2 + 3OH- → FeII(OH)3
- log β = 13Fe+2 + 4OH- → FeII(OH)4
-2 log β = 10
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