Dependence of the FeII/IIIEDTA
complex on pH
Ryan Hutcheson and I. Francis Cheng*
Department of Chemistry, University of Idaho
Moscow, ID 83844
ifcheng@uidaho.edu
March 30, 2004
Ryan Hutcheson
University of Idaho
1
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
2
FeIIIEDTA Speciation Diagram
FeIIIEDTA
FeIII(OH)2EDTA
1.00E-03
FeIII(OH)EDTA
9.00E-04
8.00E-04
Concentration (M)
7.00E-04
6.00E-04
5.00E-04
4.00E-04
3.00E-04
2.00E-04
FeIIIHEDTA
1.00E-04
0.00E+00
2
3
4
5
6
7
8
9
10
11
pH
March 30, 2004
Ryan Hutcheson
University of Idaho
3
FeIIEDTA Speciation Diagram
FeIIEDTA
1.00E-03
FeII(OH)2EDTA
Free Fe+2
FeII(OH)EDTA
9.00E-04
8.00E-04
Concetration (M)
7.00E-04
6.00E-04
5.00E-04
FeIIHEDTA
4.00E-04
3.00E-04
FeIIH2EDTA
2.00E-04
1.00E-04
0.00E+00
2
3
4
5
6
7
8
9
10
11
pH
March 30, 2004
Ryan Hutcheson
University of Idaho
4
Electrocatalytic (EC’) Mechanism
and Cyclic Voltammetry
FeIII-L + e-  FeII-L
FeII-L +H2O2  FeIII-L OH• +OHE: O + ne- = R
C’: 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
5
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
6
Cyclic Voltammagrams of
II/IIIEDTA
Fe
1mM Fe EDTA
III
-0.000005
0.1M buffer
5mV/s scan rate
FeIIIEDTA + e- → FeIIEDTA
-0.000004
Current (A)
-0.000003
-0.000002
-0.000001
pH 5.5
-0.000005
0
FeIIIEDTA + e- ← FeIIEDTA
0.000001
-0.000004
0.3
0.1
-0.1
0.000002
Current (A)
-0.000003
-0.3
-0.5
-0.7
Potential (V)
pH 2
-0.000002
-0.000001
pH 11
0
0.000001
0.3
0.000002
March 30, 2004
0.1
-0.1
-0.3
-0.5
-0.7
Potential (V)
Ryan Hutcheson
University of Idaho
7
E1/2 vs. pH (FeIIIEDTA)
FeIII(OH)2EDTA
1.00E-03
0.05
FeIIIEDTA
FeIII(OH)EDTA
9.00E-04
0
8.00E-04
7.00E-04
-0.1
6.00E-04
5.00E-04
-0.15
4.00E-04
-0.2
3.00E-04
-0.25
2.00E-04
FeIIIHEDTA
-0.3
1.00E-04
0.00E+00
-0.35
2
March 30, 2004
3
4
5
6
7
pH
Ryan Hutcheson
University of Idaho
8
9
10
11
8
Potential (V)
Concentration (M)
-0.05
E1/2
E1/2 vs. pH (FeIIEDTA)
FeII(OH)2EDTA
1.00E-03
FeIIEDTA
Free Fe+2
9.00E-04
0.1
0.05
8.00E-04
0
FeII(OH)EDTA
7.00E-04
-0.05
6.00E-04
-0.1
5.00E-04
-0.15
FeIIHEDTA
4.00E-04
Potential (V)
Concetration (M)
E1/2
-0.2
3.00E-04
FeIIH2EDTA
2.00E-04
-0.25
1.00E-04
-0.3
0.00E+00
-0.35
2
3
4
5
6
7
8
9
10
11
pH
March 30, 2004
Ryan Hutcheson
University of Idaho
9
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
10
Reaction Vessel
Air flow
2.0 mL 50/50 hexane/ethyl acetate
(extraction only)
10.0 mL water
0.44mM EDTA
0.44mM Xenobiotic
pH 5.5 – 6.5, unbuffered.
Stir bar
0.5g Fe; 20 or 40-70 mesh
Noradoun,C., Industrial and Engineering Chemistry Research, (2003), 42(21), 5024-5030.
March 30, 2004
Ryan Hutcheson
University of Idaho
11
Xenobiotic Oxidation Studies
H2O2
+
O2 + 2H+
Fe2+
EDTA
FeIIEDTA
Iron
particles
0.1-1 mm
FeIIIEDTA + HO- + HO.
Aqueous
Xenobiotic
LMW acids
Noradoun,C., Industrial and Engineering Chemistry Research, (2003), 42(21), 5024-5030.
March 30, 2004
Ryan Hutcheson
University of Idaho
12
Proposed O2 Reduction
Mechanism by Van Eldik
FeIIEDTAH(H2O) + O2  FeIIEDTAH(O2) + H2O
FeIIEDTAH(O2)  FeIIIEDTAH(O2-)
FeIIIEDTAH(O2-) + FeIIEDTAH(H2O)  FeIIIEDTAH(O22-)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
Van Eldik, R. Inorg. Chem, 1997, 36, 4115-4120
March 30, 2004
Ryan Hutcheson
University of Idaho
13
Van Eldik’s O2 Reduction
Van Eldik, R. Inorg. Chem, 1997, 36, 4115-4120
March 30, 2004
Ryan Hutcheson
University of Idaho
14
FeIIIEDTA (CN = 7)
FeIIEDTA
FeIIHEDTA
CN = 7
Structures
FeIIIHEDTA (CN = 6)
O
O
O
O
-
-
-
O
-
N
O
O
-
Fe
-
O
HN
-
Octahedral
O
N
N
O
Fe
O
O
O
O
HO
O
O
O
O
-
N
-
O
O
N
Fe
O
O
HN
N
O
O
-
-
O
Monocapped trigonal prismatic (MCP)
Pentagonal-bipyramidal (PB)
Square Pyramidal
O
O
March 30, 2004
Miyoshi, K., Inor. Chem. Acta., 1995, 230, 119-125.
Heinemann, F.W., Inor. Chem. Acta., 2002, 337, 317-327.
Ryan Hutcheson
University of Idaho
15
Structures cont’d
FeIIEDTA
FeIIHEDTA
pH 3 – pH 4
> pH 4
MCP
PB
Active
site
Active
site
Free Fe+2
< pH 3
Miyoshi, K., Inor. Chem. Acta., 1995, 230, 119-125.
March 30, 2004
Ryan Hutcheson
University of Idaho
16
Fenton Reaction
FeIIIL +e-→ FeIIL
E°’=depends on ligand
H2O2 + e- → HO• + OHE°=0.32V SHE @pH 7
FeIIL + H2O2 → FeIIIL + HO• + OHOnly 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
17
Electrocatalytic CV
0.1mM FeIIIEDTA
20mM H202
0.1M buffer
5mV/s scan rate
0.00007
pH 4
pH 3.5
FeIIIEDTA + e- → FeIIEDTA
0.00006
0.00005
0.00004
0.00003
0.3
0.1
-0.1
-0.3
-0.5
-0.7
Current (A)
pH 4
pH 4.5
pH 3
0.00002
0.00001
0.3
0.1
-0.1
-0.3
-0.5
0
-0.7
pH 2.5
pH 2
Potential (V)
March 30, 2004
Ryan Hutcheson
University of Idaho
18
Fenton Reactivity vs. pH
Free Fe+2
1.00E-04
FeIIEDTA
0.00018
9.00E-05
0.00016
8.00E-05
0.00014
7.00E-05
6.00E-05
0.0001
FeIIHEDTA
5.00E-05
0.00008
FeIIH2EDTA
4.00E-05
Current (A)
Concentration (M)
0.00012
0.00006
3.00E-05
0.00004
2.00E-05
0.00002
1.00E-05
0.00E+00
0
2
2.5
3
3.5
4
4.5
5
pH
Each data point was collected 9 times.
March 30, 2004
Ryan Hutcheson
University of Idaho
19
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
20
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
21
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
22
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
23
FeIIIEDTA Model
EDTA-4 + H+ → HEDTA-3
HEDTA-3 + H+ → H2EDTA-2
H2EDTA-2 + H+ → H3EDTAH3EDTA- + H+ → H4EDTA
H4EDTA + H+ → H5EDTA+
H5EDTA+ + H+ → H6EDTA+2
EDTA-4 + Fe+3 → FeIIIEDTAFeIIIEDTA- + H+ → FeIIIHEDTA
FeIIIEDTA- + H20 → FeIII(OH)EDTA-2 + H+
2FeIII(OH)EDTA-2 → FeIII2(OH)2EDTA2-4
FeIII(OH)EDTA-2 + 2H2O → FeIII(OH)2EDTA-3 + 2H+
H+ + OH- → H2O
Fe+3 + OH- → FeIII(OH)+2
Fe+3 + 2OH- → FeIII(OH)2+
Fe+3 + 3OH- → FeIII(OH)3
Fe+3 + 4OH- → FeIII(OH)42Fe+3 + 2OH- → FeIII2(OH)2+4
3Fe+3 + 4OH- → FeIII3(OH)4+8
March 30, 2004
Ryan Hutcheson
University of Idaho
log β = 9.52
log β = 6.13
log β = 2.69
log β = 2.00
log β = 1.5
log β = 0.0
log β = 25.1
log β = 1.3
log β = 17.71
log β = 38.22
log β = 4.26
log β = 13.76
log β = 11.27
log β = 23.0
log β = 29.77
log β = 34.4
log β = 24.5
log β = 49.7
24
FeIIEDTA Model
EDTA-4 + H+ → HEDTA-3
HEDTA-3 + H+ → H2EDTA-2
H2EDTA-2 + H+ → H3EDTAH3EDTA- + H+ → H4EDTA
H4EDTA + H+ → H5EDTA+
H5EDTA+ + H+ → H6EDTA+2
EDTA-4 + Fe+2 → FeIIEDTA-2
HEDTA-3 + Fe+2 → FeIIHEDTAH2EDTA-2 + Fe+2 → FeIIH2EDTA
FeIIEDTA-2 + OH- → FeII(OH)EDTA-3
FeII(OH)EDTA-3 + OH- → FeII(OH)2EDTA-4
Fe+2 + OH- → FeII(OH)Fe+2 + 2OH- → FeII(OH)2
Fe+2 + 3OH- → FeII(OH)3Fe+2 + 4OH- → FeII(OH)4-2
March 30, 2004
Ryan Hutcheson
University of Idaho
log β = 9.52
log β = 6.13
log β = 2.69
log β = 2.00
log β = 1.5
log β = 0.0
log β = 14.3
log β = 6.82
log β = 13.41
log β = 18.93
log β = 13.03
log β = 4.2
log β = 7.5
log β = 13
log β = 10
25