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Kinetics of EDTA degradation induced by dioxygen activation in the Fe(0)/air/water (ZEA) systemFrank Cheng* Tina Noradoun
University of IdahoChemistry DepartmentMoscow, ID [email protected]
3/15/05 Cheng & Noradoun, University of Idaho
2
The search for a “green” oxidant Problems with chlorine based bleaching
methods.
Oxygen is the ultimate green oxidant.
Oxygen is kinetically stable
3/15/05 Cheng & Noradoun, University of Idaho
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Molecular Oxygen O2 is kinetically stable
Oxygen’s two unpaired electrons make it difficult to accept a bonding pair
Partially reduced oxygen
3/15/05 Cheng & Noradoun, University of Idaho
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Reactive Oxygen Forms
HO-OH b.o. = 1 50 kcal/mol
G
•O=O• b.o. = 2 120 kcal/mol
+4e-
+4H+
2H2O
•O-O• b.o. = 1.5 80 kcal/mol
+e-
+2e-
+2H+
3/15/05 Cheng & Noradoun, University of Idaho
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The Fenton Reaction
H2O2 + e- HO• + HO- Fe(II) Fe(III) + e-
Fe(II) + H2O2 Fe(III) + HO• + HO-
H.J.H. Fenton. J. Chem. Soc. 1894, 65, 889.F. Haber and J.J. Weiss. Proc. Roy. Soc. London, Ser. A. 1934, 147, 332.
3/15/05 Cheng & Noradoun, University of Idaho
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Oxygen Activation Biological
cytochrome P450 enzymes, monooxygenase
3/15/05 Cheng & Noradoun, University of Idaho
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Molecular Oxygen as an Oxidant
Diagram showing reaction oxygen intermediates between O2 and H2O. H+ left out for simplicity
Most attractive oxidant for green oxidations is O2 from air.
OHH2O2
O2 OH2
OHH2O2
O2
•-
O2
•-
hydroxyl radical
hydrogen peroxide
superoxide radical
+ e-+ e- + e-
+ e-
- e-
- e-- e-
- e-
3/15/05 Cheng & Noradoun, University of Idaho
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Fe°, EDTA, Air (ZEA) system
IF Cheng, et al, Ind. & Eng. Chem. Res. 2003, 42(21), 5024-5030.
Fe0
FeIIEDTA
FeIIIEDTA
e-
+ H2O2 + OH- +FeIIEDTA FeIIIEDTA
O2
O2
.-+ +2H
+O2
.-
Fe2++ EDTA
HO·Fe(0), EDTA, & air
The only nonbiological system know to date that can activate O2 under RTP and produce a facile oxidizing species capable of extensively degrading xenobiotics
Organophosphorous agentsHalocarbonsOrganics
3/15/05 Cheng & Noradoun, University of Idaho
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Outline Introduction
Environmental Impact of EDTA The RTP Dioxygen Activation
General Reaction Scheme Zero-valent iron/EDTA/air (ZEA) system
Degradation Kinetics and Reaction Products EDTA
Chlorinated phenols Organophosphorus and UXO compounds
Mechanisms Rate-limiting Step
Conclusions
3/15/05 Cheng & Noradoun, University of Idaho
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EDTA
•Finds widespread use in food, cosmetic and pharmaceutical preparations
•Newer uses – manufacture of textiles and in paper-pulp bleaching
Swedish pulping industry where the use of EDTA has increased from 2,000 metric tons to about 8,000 metric tons per year from 1990 to 2000.
Ekland, B; Bruno, E; Lithner, G.; Hans, B.; “Use of Etheylenediaminetetraacetic acid in Pulp Mills and Effects on Metal Mobility and Primary Products”; Environ. Toxicol. Chem.; 2002; 21(5), 1040-1051.
NN
CH2COOH
CH2COOH
HOOCH2C
HOOCH2C
3/15/05 Cheng & Noradoun, University of Idaho
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EDTA Control of metal ion activities, Fe, Mn, Zn, Cu,
Mg, Ca
-Corrosion
-Catalysis
-“Green” bleaching - H2O2
Sillanpaa, M; Pirkanniemi, K.; Sorokin, A.; “Degradative hydrogen peroxide oxidation of chelates catalysed by metallophthalocyanines”; The Science of the Total Environment; 2003; 307, 11-18.
3/15/05 Cheng & Noradoun, University of Idaho
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Concerns about EDTA Many industrial chelating agents are not degradable by methods
currently found in wastewater treatment facilities
Not readily biodegradable
Considerable quantities of EDTA pass through wastewater treatment facilities in the form of FeIIIEDTA, as high as 18µM.
Sillanpaa, Mika; Orma, Marjatt; Ramo, Jaakko; Oikair; “The importance of ligand speciation in environmental research: a case study”; The Science of the Total Environment; 2001; 267, 23-31.
Sillanpaa, M; Pirkanniemi, K.; “Recent Developments in Chelate Degradation”; Environmental Technology, 2001, 22, 791.
Kari, F. G.; Giger W; Modeling the Photochemical Degradation of Ethylenediaminetetraacetate in the River Glatt; Environmental Science and Technology; 1995, 29, 2814.
Nirel, P. et. al.; Method for EDTA Speciation Deteremination: Application to Sewage Treatment Plant Effluents; Wat. Res; 1998; 32, 3615.
Kari, F. G. and Giger W.; Speciation and fate of EDTA in municipal wasterwater treatment. Wat. Res., 1996, 30, 122-134.
3/15/05 Cheng & Noradoun, University of Idaho
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Concerns about EDTA
Questions regarding the ability to mobilize metals in the environment. Currently not being monitored or treated at waste
water treatment facilities Concern for heavy metal mobility and longer
bioavailability of metals to aquatic plants and animals Stable in aquatic environment
EDTA is anthropogenic and long-lived.
3/15/05 Cheng & Noradoun, University of Idaho
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Goals•The destruction or neutralization of EDTA (xenobiotics)
•Search for in situ conditions that will aid in the reduction in the release of EDTA in emerging green chemistries.
Inexpensive & Safe Processes.
•Room Temperature and Pressure Conditions (RTP)•Common Reagents – Long Term Storage•No Specialized Catalysts•System that may be incorporated into existing water treatment systems
3/15/05 Cheng & Noradoun, University of Idaho
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Overall Goals of Our Green Oxidation Program The destruction or neutralization of
xenobiotics, including nerve agents and chlorinated pesticides using green oxidation chemistry.
Focus on non-biological oxygen activation to eliminate the need for tricky enzyme based systems
3/15/05 Cheng & Noradoun, University of Idaho
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Oxygen activation system
The ZEA system uses only zero-valent iron, EDTA and air
The only nonbiological system know to date that can activate O2 under RTP and produce a facile oxidizing species capable of extensively degrading xenobiotics
Industrial and Engineering Chemistry Research 2003, 42, 5024-5030
3/15/05 Cheng & Noradoun, University of Idaho
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Experimental Setup
Bioanalytical Systems – RPM controlled stir plate
stir bar
2.5 g Fe°
125 ml round bottom flask
1 mM EDTA (Total Vol. 50mL)
2.5g Fe° 30-40 mesh Aldrich
Open to the Atmosphere
Aliquots were taken directly from reaction vessel, diluted, filtered and injected into HPLC
3/15/05 Cheng & Noradoun, University of Idaho
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Evidence for Production of Reactive Oxygen Species
ROS O2
-•, H2O2, HO., FeIV=O, etc.
Two Analyses were performed Thiobarbituric acid-reactive
substances (TBARS) assay Addition of known radical
scavenger, 1-butanol
O2
F e 0O2
.- O2.-+ + 2H+
e-
+ H2O2 + OH-+ OHFeIIEDTA FeIIIEDTA
Fe2++ EDTA
II: Homogeneous O2 Activation
F e 0
O2
O2.- O2
.-+ + 2H+
+ H2O2 + OH-+ OHFeIIEDTA FeIIIEDTA
FeIIEDTA
FeIIIEDTA
e-
Fe2++ EDTA
I: Heterogeneous O2 Activation
3/15/05 Cheng & Noradoun, University of Idaho
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Thiobarbituric acid reactive substances assay (TBARS)
Nonselective detection of reactive oxygen species oxidizing species.
HO·, FeIV=O
Malonaldehyde bis(dimethyl acetal)
TBA
Deoxyribose
534 nm
Junqueira VB; Mol Aspects Med. 2004 Feb-Apr;25(1-2):5-16. Hader D; Photochem Photobiol Sci. 2002 Oct;1(10):729-36.
3/15/05 Cheng & Noradoun, University of Idaho
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TBARS results
30 minutes of reaction time with 0.10 g 40-70 mesh Fe(0), under aerobic conditions.
Absorbance Units at 534 nm
Control 1 – 0 mM deoxyribose, 2.39 mM EDTA
0.0
Control 2 – 3.18 mM deoxyribose, 0 mM EDTA, - also N2 flow, -No Fe(0)
0.149
3.18 mM deoxyribose, 2.39 mM EDTA 0.846
Ind. & Eng. Chem. Res. 2003, 42(21), 5024-5030.
3/15/05 Cheng & Noradoun, University of Idaho
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Suppression of EDTA degradation with
the addition of Radical Scavenger
(■) kobs = -1.11 M-
1hr-1
(▲)kobs = -0.08 M-
1hr-1 with 5mM 1-butanol
(2.5 g ZVI g, 1.00mM EDTA, open to air)
Mantzavinos D; Water Res. 2004 Jul;38(13):3110-8. J Hazard Mater. 2004 Apr 30;108(1-2):95-102.
-10
-9.5
-9
-8.5
-8
-7.5
-7
-6.5
0 1 2 3 4 5 6
Time (hrs)
ln [
Fe
III E
DT
A]
Control (no Fe)
EDTA, Air
5 mM 1-butanol
Linear ( EDTA,Air)Linear (5 mM 1-butanol)
1-butanol is a •OH radical scavenger
3/15/05 Cheng & Noradoun, University of Idaho
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Summary of ZEA system and O2 Both TBARS and butanol tests indicate that
ZEA system is able to produce facile oxidant from air at RTP
Form of abiotic O2 activation at RTP Identity of oxidant isn’t clear
HO· FeIV=O
3/15/05 Cheng & Noradoun, University of Idaho
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Degradation of EDTA by ZEA reaction
1 mM EDTA (50mL, aqueous)2.5g Fe° + air
NN
COOH
COOH
HOOC
HOOC
NHCOOH
COOH
COOH
COOHHOOC
CO2/HCO3-
3/15/05 Cheng & Noradoun, University of Idaho
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Products of ZEA System
None of the products of EDTA are significant metal chelation agents.
All are more easily biodegraded.
The ZEA system has proven successful at the degradation of other organic xenobiotics.
Halocarbons Organophosphorus Organics
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Carbon Balance - Total Organic Carbon*, ESI-
MS**, HPLC# Table 1 : Carbon Balance 1mM EDTA, 2.5g ZVI, air, reaction volume 50mL, 6hrs
% C
CO2 35% (± 5)*
Iminodiacetic Acid 28% (± 3)**
Oxalic acid 17% (± 2)**
Propionic Acid 14 % (± 2)**
EDTA 2% (± 2)#
Total 96%
Trapping of the volatile gases using Tenax® showed no volatile organic carbon of molecular weight C4 and above released from the
system during the course of the reaction
3/15/05 Cheng & Noradoun, University of Idaho
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Degradation products for -EDTA-Malathion-4-chlorophenol-pentachlorophenol-phenol
iminodiacetic acid (degrades after
12 hrs)
succinic acid bicarbonate
propionic acid Oxalic acid
Kinetically stable organic products from ZEA degradation.
3/15/05 Cheng & Noradoun, University of Idaho
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Summary of EDTA degradation EDTA is degraded by the ZEA system to
LWM acids and inorganic carbons All products are more biodegradable than
EDTA RTP O2 activation
Next: Kinetic studies
3/15/05 Cheng & Noradoun, University of Idaho
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Overall Scheme (simplified) Metal dissolution: Fe(0) Fe2+ + 2e- (1)
Complex formation Fe2+ + EDTA FeIIEDTA (2)
Homogeneous O2 activation: 2FeIIEDTA + O2 + 2H+ 2FeIIIEDTA + H2O2 (3)
Fenton Reaction FeIIEDTA + H2O2 FeIIIEDTA + OH• + OH- (4)
EDTA degradation: OH• + FeEDTA Fe2+/3+ + EDTA* (5)
EDTA* = damaged EDTA
Redox Cycling: FeIIIEDTA + e- FeIIEDTA (6)
3/15/05 Cheng & Noradoun, University of Idaho
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Kinetic Parameters ExaminedEffects of
EDTA concentration Possible Scale-up
Fe° mass (surface area) Omitted
Rate of mixing Omitted
Temperature Rate-limiting Step
3/15/05 Cheng & Noradoun, University of Idaho
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Kinetics of EDTA degradation [EDTA]initial = 1 mM
R2 = 0.9998
-10
-9.5
-9
-8.5
-8
-7.5
-7
-6.5
0 0.5 1 1.5 2 2.5 3Time (hrs)
ln [
FeII
I ED
TA
]
kobs = 1.22 /M hr
1mM EDTA, 2.5 g Fe° and air (▲), control in the absence of iron (■)
Pseudo-first order plot showing linearity for EDTA degradation from 10min-2.5hrs.
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0 1 2 3 4 5 6 7
Time (hr)
[Fe
III E
DT
A]
Air
control (No Fe)
3/15/05 Cheng & Noradoun, University of Idaho
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EDTA degradation rate is effected by initial EDTA concentration
[EDTA] (M)
0 2x10-3 4x10-3 6x10-3 8x10-3 10x10-3
k ob
s(M
-1h
-1)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
2.5 g Fe°, open to atmosphere, 450 rpm, total rxn volume 50mL
•Narrow optimal range for EDTA concentration.
•Important point: adding more EDTA will not speed up reaction.
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How does [EDTA]i effect the ZEA reaction rate?
F e 0F e 0
Fe-oxide
+ H2O2 + OH- + OHFeIIEDTA FeIIIEDTA
EDTA2-
Interaction of EDTA and Fe-Oxide layer
FeIIEDTA
FeIIIEDTA
Fe2++ EDTA
Corrosion Rate
Rate of FeIIIEDTA reduction by Fe(0).
Rate of Fenton Reaction
O2 activation
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Corrosion studies
Electrochemical Studies - Tafel Analysis
i0 is the exchange current which is the rate of the corrosion.
RT
F
RT
Fiitotal
)1(expexp0
3/15/05 Cheng & Noradoun, University of Idaho
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Tafel Corrosion AnalysisCorrosion normally occurs at a
rate determined by an equilibrium between opposing electrochemical reactions.
Anodic reaction: metal oxidized, releasing electrons into the metal.
Fe° Fe2+ + 2e-
Cathodic reaction: solution species (often O2 or H+) reduced, removing electrons from the metal.
2H+ + 2e- H2
3/15/05 Cheng & Noradoun, University of Idaho
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Corrosion Cell•Working Electrode: Fe° (99%), 3/8" diameter by 1/2" length (surface area 5.22 x 10-4 m2)
•Counter Electrode: high density graphite rod
•Reference: Standard Calomel Electrode (SCE), glass luggin capillary
•1 liter glass cell
•Polished working electrode with 600 grit sandpaper between sample runs
•Used 50mM KNO3 as electrolyte in all samples
3/15/05 Cheng & Noradoun, University of Idaho
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Corrosion Rate
[EDTA] (mM)
0 2 4 6 8 10 12 14 16
Cor
rosi
on R
ate
(mm
/yr)
0
2
4
6
8
10
12
14
N2 purge
Air purge
• Addition of EDTA does enhance dissolution rates to ~5mM
• Overall corrosion rates for in the presence of N2 are higher than air
• Passivation layer forming on the Fe° surface in the presence of O2 in air
• Important point is the dissolution is not hindered by excess EDTA
3/15/05 Cheng & Noradoun, University of Idaho
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Comparison of Corrosion to ZEA rates
[EDTA] (mM)
0 2 4 6 8 10 12 14 16
Co
rro
sio
n R
ate
(m
m/y
r)
0
1
2
3
4
5
k obs
(M-1
h-1)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
• Red: ZEA reaction
• Blue: Corrosion studies
• EDTA probably stabilizes Fe2+
• Inversely correlated!
• Slower rxn kinetics with higher [EDTA] is not attributable to effects on corrosion.
3/15/05 Cheng & Noradoun, University of Idaho
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Other Possibilities for effects of [EDTA] on ZEA reaction rates Kinetic barriers
Electrochemical FeIIIEDTA + e- = FeIIEDTA
Fenton Rxn FeIIEDTA + H2O2 FeIIIEDTA +HO- + HO·
Oxygen Reduction/Activation FeIIEDTA + O2 FeIIIEDTA + O2
.-
Other routes
3/15/05 Cheng & Noradoun, University of Idaho
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Excess EDTA does not affect the rate of FeIIIEDTA reduction
Cyclic voltammograms of 0.1 mM FeIIIEDTA
0.1 M HEPES pH 7.4 10 mV/s carbon disk electrode
A) 0.1 mM B) 10 mM EDTA
10 V/s
3/15/05 Cheng & Noradoun, University of Idaho
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Other Possibilities
Kinetic barriers ElectrochemicalElectrochemical
FeFeIIIIIIEDTA + eEDTA + e-- = Fe = FeIIIIEDTAEDTA
Fenton Rxn FeIIEDTA + H2O2 FeIIIEDTA +HO- + HO·
Oxygen Reduction FeIIEDTA + O2 FeIIIEDTA + O2
.-
Other routes
3/15/05 Cheng & Noradoun, University of Idaho
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Excess EDTA inhibits the Fenton Rxn. Cyclic
voltammograms of 0.1 mM FeII with 10 mM H2O2. 10 mV/s
[FeII]:[EDTA] (A) 1:1 (B) 1:10
3/15/05 Cheng & Noradoun, University of Idaho
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[FeIII]:[EDTA] Fenton Rxn trends
EDTA may act as an anti- or pro-oxidant.
Highly dependent on [EDTA]i
3/15/05 Cheng & Noradoun, University of Idaho
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Other Possibilities
Kinetic barriers ElectrochemicalElectrochemical
FeFeIIIIIIEDTA + eEDTA + e-- = Fe = FeIIIIEDTAEDTA
Fenton RxnFenton Rxn
FeFeIIIIEDTA + HEDTA + H22OO22 Fe FeIIIIIIEDTA +HOEDTA +HO-- + + HOHO·
Oxygen Reduction (future work) FeIIEDTA + O2 FeIIIEDTA + O2
.-
Other routes
3/15/05 Cheng & Noradoun, University of Idaho
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Temperature Experiments
An Arrhenius plot - activation energy
May allow us to determine rate-limiting step Metal Dissolution Complex Formation Homogeneous O2 Activation Fenton Rxn EDTA oxidation Heterogeneous Redox Cycling
3/15/05 Cheng & Noradoun, University of Idaho
45
Arrhenius plot - dependence of observed rate constants on temperature
1/T (1/K)
0.0031 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037
k ob
s (
1/h
)
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Activation Energy25.5 kJ/mole
2.5g Fe°, 1mM EDTA, 50ml total volume, reactions conducted using a temperature bath and a water-jacketed cell
k = A exp(-Ea/RT)
3/15/05 Cheng & Noradoun, University of Idaho
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Summary of Arrhenius Studies• This study: Ea = 25.5 kJ/mol
• Under vigorous stirring; Mass transport limited region
• This activation energy includes all steps.
• How may this determine which of the 6 processed is rate-limiting?
• Comparison with literature regarding:• O2 reduction by FeIIEDTA
3/15/05 Cheng & Noradoun, University of Idaho
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Overall Scheme (simplified) Metal dissolution: Fe(0) Fe2+ + 2e- (1)
Complex formation Fe2+ + EDTA FeIIEDTA (2)
Homogeneous O2 activation: 2FeIIEDTA + O2 + 2H+ 2FeIIIEDTA + H2O2 (3)
Fenton Reaction FeIIEDTA + H2O2 FeIIIEDTA + OH• + OH- (4)
EDTA degradation: OH• + FeEDTA Fe2+/3+ + EDTA* (5)
EDTA* = damaged EDTA
Redox Cycling: FeIIIEDTA + e- FeIIEDTA (6)
3/15/05 Cheng & Noradoun, University of Idaho
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Comparison to kinetics of homogeneous O2 reduction by FeIIEDTA in literatureR. Van Eldik; Inorg. Chem.; 1990, 29, 1705-1711.
[FeIIEDTA] = 0.02M pH=5
FeIIEDTA + O2 FeIIEDTAO2 k1 = 107/MsFeIIEDTAO2 FeIIIEDTA + O2
- k2 = 102/MsFeIIEDTAO2 + H+ FeIIIEDTA + HO2 k3 = 1010/Ms
Rate limiting step is the activation of oxygen at the iron coordination site
Activation energy, 33.9 kJ/mol*
3/15/05 Cheng & Noradoun, University of Idaho
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Summary of Van Eldik Study
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 or Fenton Rxn
3/15/05 Cheng & Noradoun, University of Idaho
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Comparison to kinetics of homogeneous O2 reduction by FeIIEDTA in literatureBeenackers, A.; Ing. Eng. Chem. Res. 1992, 32, 2580
[FeIIEDTA]=0.10 M pH=7.5
2FeIIEDTA + O2 + H+ 2FeIIIEDTA + H2O2 (overall) Activation energy, 27.2 kJ/mol
3/15/05 Cheng & Noradoun, University of Idaho
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Overall Scheme (simplified) Metal dissolution: Fe(0) Fe2+ + 2e- (1)
Complex formation Fe2+ + EDTA FeIIEDTA (2)
Homogeneous O2 activation: 2FeIIEDTA + O2 + 2H+ 2FeIIIEDTA + H2O2 (3)
Fenton Reaction FeIIEDTA + H2O2 FeIIIEDTA + OH• + OH- (4)
EDTA degradation:
OH• + FeEDTA Fe2+/3+ + EDTA* (5) EDTA* = damaged EDTA
Redox Cycling: FeIIIEDTA + e- FeIIEDTA (6)
This study 25.5 kJ/mol
Beenackers 27.2 kJ/mol
Van Eldik 33.9 kJ/mol
•Step 3 is Rate-Limiting
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Conclusions This system is a viable option for the destruction of a variety of pollutants and has
a strong possibility for scale up. The only system known to date that can obtain non-biological Oxygen Activation
at room temperature and pressure to produce reactive oxygen species that are capable of fully degrading pollutants to LMW carboxylates and inorganic forms
Due to the duality of EDTA acting as both a pro-oxidant and antioxidant, controlling the [EDTA] is imperative to the success of the process.
Rate-limiting steps is (are) oxygen activation
FeIIEDTA may provide insights into biological oxygen activation
This study may provide insights as to how EDTA release can be controlled Future studies – how many oxidative hits & redox cycles required to
degrade EDTA to inorganic forms
Search for more suitable reducing agents
3/15/05 Cheng & Noradoun, University of Idaho
53
Acknowledgments
Thank You!
Christina Noradoun
Funding NSF BES-0328827 UI Foundation Seed Grant
NIH EPRI
Dr. Malcolm and Mrs. Renfrew Renfrew Scholarships
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54
The ZEA system is durable
Time (hrs)0 2 4 6 8 10 12 14
[Fe
IIIE
DT
A]
(M)
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
1mM EDTA Aliquot Added
ZVI maintains EDTA degradation without significant loss in the observed rate over a time period of several hours
All systems mixed at 450 rpm, open to atmosphere, unbuffered using 2.5g ZVI.
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55
Comparison Studies
Auto-oxidation of FeII to FeIII by O2 in aqueous solutions Significantly enhanced by EDTA FeII:EDTA ratios were important
1:1 ratios were reported as optimal 1:20 ratios showed a significant decrease in the
autoxidation process
R. Van Eldik; Inorg. Chem.; 1990, 29, 1705-1711. (* 0.02M [Fe(EDTA)])
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56
ZVI mass (g/L)
0 10 20 30 40 50
k ob
s(M
-1h
-1)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Role of Fe° mass/surface area in observed rate constant
0.10g - 2.5 g Fe°, 1.00mM EDTA, open to atmosphere, 450 rpm, total rxn volume 50mL
BET surface area analysis 0.1106 m2/g : Porous Material Inc., Ithaca, NY
surface area, kobs
(0.29 m2, -1.11 /Mh)
Increased levels of Fe°, enhance the rate of degradation by maintaining a balance between the Fe2+ and [EDTA]
(0.028 m2, -0.014 /Mh)
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Maintaining proper Fe°to EDTA ratios Interactions between EDTA and Fe2+ are important factor
controlling the degradation rates Due to the duality of EDTA acting as both a pro-oxidant
and antioxidant, controlling the [EDTA] is imperative to the success of the process.
Rate-limiting step 1. Surface chemistry : Reduction of FeII/III at the iron
surface inhibited by excess EDTA 2. Solution chemistry: High FeII/III:EDTA ratios inhibiting
Fenton reactivity.
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58
General Model Mass Transport-limited Kinetics
1) mass transport of FeIIIEDTA to the Fe° surface 2) FeIIIEDTA + e- FeIIEDTA 3) mass transport of FeIIEDTA to the bulk soln.
“A common criterion for detecting mass transport-limited kinetics is variation in reaction rate with intensity of mixing. Rates that are controlled by chemical reaction step should not be affected, where as aggressive mixing usually accelerates diffusion-controlled rates by reducing the thickness of the diffusion layer.”
Leah Matheson and Paul Tratnyek; ES&T. 1994, 28 2045-2053.
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59
Effect of mixing rate on observed degradation rate constant for EDTA
2.5 g Fe° g, 1.00mM EDTA, open to air, total rxn volume 50mL
-11
-10.5
-10
-9.5
-9
-8.5
-8
-7.5
-7
-6.5
0 1 2 3 4 5 6
Time (hrs)
ln [
Fe
IIIE
DT
A]
50 rpm200 rpm350 rpm450 rpmLinear (50 rpm)Linear (200 rpm)Linear (350 rpm)Linear (450 rpm)
0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400 500 600
rpmk o
bs
(M-1
h-1
)
Good indication that rate-limiting step of EDTA degradation involves mass transport and not chemical reactions occurring in the bulk solution
3/15/05 Cheng & Noradoun, University of Idaho
60
If reaction is mass transport controlled rate limiting step likely: FeII/IIIEDTA reduction at iron
surface Can not rule out the
heterogeneous O2 activation Mass transport of oxygen
from the bulk solution to the reacting iron surface is enhanced by the fluid flow.
Typical bulk oxygen concentrations at room temperature in aqueous solutions are 0.25mM (8ppm).
II: Homogeneous O2 Activation
F e 0
O2.- O2
.-+ + 2H+
+ H2O2 + OH-+ OHFeIIEDTA FeIIIEDTA
FeIIEDTA
FeIIIEDTA
e-
Fe2++ EDTA
O2
F e 0O2
.- O2.-+ + 2H+
e-
+ H2O2 + OH-+ OHFeIIEDTA FeIIIEDTA
Fe2++ EDTA
I: Heterogeneous O2 Activation
O2
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HPLC conditions for FeIIIEDTA detection
EDTA non-extractable using organic solvent must use direct aqueous injection
EDTA alone not absorb, however FeIIIEDTA complex does at 258nm
Mobile phase: 0.02M formate buffer, pH 3.3 Containing: TBA-Br (0.001M) and acetonitrile (8%) Flow rate: 1ml/min Temp: ambient temp UV = 258 nm Sample volume 20µL Column RP-C18
Nowack et. al.; Anal. Chem. 1996, 68, 561
TBA-Br
+
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TNT surrogate, nitrobenzene (985 ppm) was decomposed in 24 hours.
VX surrogate, malathion (49 ppm) was consumed in 4 hours, to give diethyl succinate. Malathion was the only pollutant to give a by-product detectable by GC-FID.
N+
O-O
nitrobenzene
S
OCH3
O
O
CH3
O
S
P
O
OCH3
CH3
malathion
SO
P N
CH3CH3
CH3
CH3
O
CH3
CH3
VX
CH3
N+
O
O-
N+
O
-O
N+
O-O
TNT
Organophosphorous Nerve Agents and Nitrated Explosive Surrogates
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Malathion DegradationO
CH3 O
O CH3
O
PO43-
+ SO42-
S
OCH3
O
O
CH3
O
S
P
O
OCH3
CH3
malathionDES
malaoxon
O
CH3
O
O
CH3
O
S
P
O
O
CH3
O
CH3
max: 4-6 hrs
Max: 7 hrs
SO42- :0.0593mM (14% yield) (24hrs)
PO42- : 0.0825 mM (19 % yield) (24hrs)
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Kinetics of Malathion Degradation
GC/FID chromatograph: each data point indicates an individual reaction vial extracted using 50/50 hexane/ethyl acetate, error bars indicate the standard deviation between three measurements of each sample vial.
MalathionDiethyl Succinate (DES)
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Reaction Conditions
0.44mM Malathion
0.44mM EDTA
0.5g FeO Air
25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400m/z0
100
%
140.9
124.9
60.9
60.0
96.978.9
156.9
291.1
163.9
203.0187.0273.1
315.1
292.1329.1 335.1
25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400m/z0
100
%
x2
61.0
59.0
140.9
73.9
88.0 131.9
88.9
125.9 154.0157.0
Time: 0 hrs
Time: 12 hrs
EDTA
Malathion (m/z 329)
Iminodiacetic Acid
HCO3-
oxalate
propionic acid
Malaoxon (m/z 315)
ESI-MS
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ESI-MS0,C:\MASSLYNX\noradoun.PRO\Data\,FEEDTA1,RAW,1,1,1,0
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440m/z0
100
%
0
100
%
0
100
%
x6
62.043626496
61.026985472 96.9
10873856
x6 96.9618070016
61.060764160
60.019196928
62.015208448
344.1140525568
98.934430976
300.119373056132.0
7510272273.0
6948864
255.04822528
342.18591360
345.120764672
x661.0
98426880
60.013406208
73.065916928
132.043970560
88.041005056
96.920006912
154.011402240 344.1
6284032273.0
3862016
background
No Fe°, N2
1mM FeSO4
1mM EDTA
4hrs
Fe°, Air1mM EDTA4hrs
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0.5g Fe; 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 EDTA
General reaction conditions for Xenobiotic degradation
Beaker Rxn
@ 25°C, pH 5.5-6.5
Ind. & Eng. Chem. Res. 2003, 42(21), 5024-5030
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0.5g Fe; 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 EDTA
General reaction conditions for Xenobiotic degradation
One reaction vessel was generated for each data point.
Degradation curves represent 8-15 individual reaction vials each extracted and analyzed using GC-FID or HPLC.
@ 25°C, pH 5.5-6.5
Ind. & Eng. Chem. Res. 2003, 42(21), 5024-5030
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69Stumm, W; “Chemistry of the Solid-Water Interface”; John Wiley & Sons, Inc. NY, © 1992, p204
EDTA and other dicarboxylic acids enhance dissolution by shifting electron density towards the metal ion and simultaneously enhancing surface protonation therefore weakening the Fe-oxygen lattice bonds.
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Time (hrs)
0 1 2 3 4 5 6 7
ln [
Fe
III E
DT
A]
-11
-10
-9
-8
-7
-6
-5
-4
Addition of 10mM Ca2+
Calcium Addition The addition of 10mM Ca2+ did not effect degradation rate.
10mM EDTA, 2.5g Fe °
kobs = 0.042 M-1h-1 (with Ca2+)
2.5 g Fe°,open to air, total rxn volume 50mL
kobs = -0.044 M-1 h-1kobs = -0.042 M-1 h-1
1mM EDTA, 2.5g Fe °
kobs = -1.11 M-1h-1
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Calcium Addition cont.
Ca2+ addition had no overall effect on the rate of degradation The added Ca2+ also did not help sequester excess EDTA in
solution
Therefore there was no improvement of Fenton Reactivity with the Ca2+ addition
Alternative way of examining the problem was to hold EDTA concentration constant and vary amount of Fe° present