1 kinetics and mechanism of xenobiotic degradation induced by dioxygen activation christina noradoun...

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Kinetics and Mechanism of Xenobiotic Degradation Induced by Dioxygen Activation

Christina NoradounUniversity of IdahoChemistry DepartmentMoscow, ID 83843-2343

2

Outline Introduction

Environmental Problem The Importance of Dioxygen Activation

Biological vs. Chemical Activation General Reaction Scheme Degradation Kinetics and Reaction Products

Xenobiotic: Environmental Pollutants Chlorinated phenols Organophosphorus and nitrated compounds EDTA

Mechanism of Degradation Conclusions

3

Research Problem The disposal of organic pollutants and common chemical

warfare agents is a matter of increasing concern.

The 1997 Chemical Weapons Convention Treaty mandated the eradication of all chemical weapons by the year 2007, later extended to 2012.

As of November 2003 only 11% of the 70,000 metric tons of chemical weapons stored worldwide had been destroyed.

US and Russia are holding 95% of all stockpiles and are unlikely to meet 2012 deadline.

Lapses in arms disclosures and delays in chemical weapons destruction prompt proliferation fears

Pifer, A.; et.al. J. Am. Chem. Soc. 1999, 121, 7485-7492. C&EN News; May 5, 2004

4

Chlorinated Pollutants PCBs (polycholorinated biphenyls)

290 million pounds are located in landfills and storage facilities in the USA

PCP (Pentachlorophenol, wood preservation) Pesticides

Aldrin/Dieldrin (termiticide, banned 1974) Chlordane (EPA banned sales 1988) DDT (EPA banned all public uses 1972)l Heptachlor (banned 1983) Hexptachlorobenzene (banned 1984)

Agency for Toxic Substance and Disease Registry; www.atsdr.cdc.govhttp://www.epa.gov/history/topics/pcbs/01.htm

5

Incineration: Up in Smoke

The only approved EPA method for stored nerve agents and most common method for chlorinated waste removal

Economically prohibitive (3000 ºC)

Source of polycylic aromatic hydrocarbons (PAHs), such as chlorinated or brominated dioxins

Disposal concerns of the tons of toxic bottom and fly ash.

TRANSPORTATION

U.S. Army operated incineration plant in

Anniston, AL

Ember, Louis; C&EN News; 2004, 82, 25. Wang, Lin-Chi; et.al; ES&T, 2002, 36, 3420-3425. Soderstrom, Gunilla; et. al; ES&T. 2002, 36, 1959-1964.

6

Dioxins The term “dioxin” signifies the family of

polychlorinated dibenzo-p-dioxins and furans The most toxic subgroup is chlorinated in the 2,3,7,8

positions (ex. 2,3,7,8-tetrachloro-benzo-p-dioxin) These compounds can form in combustion of

chlorine-containing organic materials

Cl

Cl

O

O Cl

Cl

TCDD

C&EN; Oct 8, 2004, 82, 40.

7

Alternative Available Methods

Biological Oxidation Often incomplete short catalytic lifetime Not applicable to high

pollutant concentrations thermal sensitivity

Ember, Lois; C&EN News, 2004, 82, 8. Wang, Lin-Chi; et.al; ES&T, 2002, 36, 3420-3425. Soderstrom, Gunilla; et. al; ES&T. 2002, 36, 1959-1964, Fighting Nerve Agent Chemical Weapons with Enzyme Technology; Annals of the New York Academy of Sciences 864:153-170 (1998).

This 1-ton tank contains aging mustard gas that will be destroyed at Tooele Army Facility in Utah, beginning of summer 2005.

8

Chemical OxidationThese chemical oxidation techniques are successful at oxidizing

nerve agents although there are drawbacks Peroxygen Oxidizers

perborate, peracetic acid, m-chloroperoxybenzoic acid In complete oxidation

Caustic Bleaching Agents In 1917 the Germans used bleaching powder to neutralize

mustard agent. Requires solubilization therefore large quantities of solvent that

must be dealt with Supercritical H2O

Material and energy costs are large when considering any large-scale demilitarization processes

Annals of the New York Academy of Sciences 864:153-170 (1998), Formulations for the Decontamination of CB Warfare Agents; Annual Report MOD2001-1008-M February 2001; Sandia National Lab.

9

Overall GoalThe destruction or neutralization of xenobiotics,

including nerve agents and chlorinated pesticides using green oxidation chemistry.

Produce a low cost alternative to incineration by working at Room Temperature and Pressure Conditions (RTP)

Common Reagents with Long Term Storage

Focus on non-biological oxygen activation to eliminate the need for specialized catalysts

10

Molecular Oxygen How does one tap into the

seemingly stable energy source? Oxygen’s two unpaired electrons

make it difficult to accept a bonding pair, hence the reluctance to react by forming new chemical bonds

Two ways of overcome this Oxygen can absorb energy from

other molecules that have been excited by heat or light

Add electrons to oxygen one at a time Iron which also has an unpaired

electron is efficient at donating electrons to oxygen

11

Molecular Oxygen as an Oxidant

Diagram showing reaction oxygen intermediates between O2 and H2O. Hydrogen 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-

12

Hydroxyl Radical and The Fenton Reaction

H2O2 + e- HO• + HO- Fe(II) Fe(III) + e-

Fe(II) + H2O2 Fe(III) + HO• + HO-

The impact of Ferrous salts on H2O2 reduction was discovered over 100 years ago by Henry Fenton.5

The Fenton reaction in form above, including the hydroxyl radical, was suggested over 75 years ago.6

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.

13

Oxygen Activation

Biological cytochrome P450 enzymes, monooxygenase

Chemical GIF reaction TAML ligand - hydrogen peroxide activator

14

Cytochrome P450 enzymes: Nature’s Oxidative Workhorse

•Large family of enzymes

•Catalyzes redox-processes

•The major system for drug and xenobiotic metabolism.

•Highest concentration in the liver.

•The CO complex absorbs at 450 nm.

•Active center: Protoporphyrin + Fe(III) + Cys.

15

Catalytic cycle for cytochrome P450 monooxygenations

Chem. Soc. Rev., 2000, 29, 375–384

From an inorganic chemist's perspective, P450 enzymes are fascinating due to their ability to activate molecular oxygen to react with organic substrates with a selectivity and efficiency unparalleled in synthetic systems.

16

Non-biological oxygen activation The pioneering research

focused on iron porphyrins as biological mimics.

Barton and co-workers developed a non-porphyrin iron catalyst system that has come to be known as the “Gif system”.

Main thrust of this research was centered around highly selective oxygenation for industrial synthesis.

Protoporphyrin-IX

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•Gif reactions are capable of catalyzing monooxgenation of carbon-hydrogen bonds, to produce ketones.

RH + O2 (2 H+, 2e-, Mn+) ROH + H2O

• General requirements: reducing agent (electron source), protons, a catalytically active metal ion (Fe2+, Cu2+), oxygen and a solvent.

• The major disadvantage of Gif-type reactions for environmental remediation is the expense and toxicity of the necessary solvent pyridine.

Barton, D.H.R; Doller, D. Acc. Chem. Res. 1992, 25,504-512; Stravropoulos, P.; Celenligil-Cetin, R.; Tapper, A.; Acc. Chem. Res. 2001,34,745-752.

General Gif Reaction

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Hydrogen Peroxide ActivatorsDr. Terrence Collins at the Institute of Green Oxidation Chemistry has pursued the design and synthesis of hydrogen peroxide activating catalyst for the past 20 years.

Collins, T.; Acc. Chem. Res. 2002,35,782-790. Collins, T.; Acc. Chem. Res. 1994, 27,279-285.

TAML ligands can activate H2O2 to strong oxidizing agents capable of breaking down pollutants in aqueous and non-aqueous solutions.

TAML ligands are uniquely designed to be inert to internal ligand oxidation.

CH3

CH3N

N

NFe

N

O

O

CH3

CH3

CH3

CH3

O

Cl

Cl

O

TAML ligand

2+

19

9 minutes, 99% degradation of TCP (2,4,6-trichlorophenol)

TAML/TCP ratios--1:2000

H2O2/TCP ratios—100:1

pH 10 and 25C

Gupta, S.; Stadler, M.; Noser, C.; Ghosh, A.; Steinhoff, B.; Dieter, L.; Horwitz, C.; Schramm, K.; Collins,T.; Science, 2002, 296, 326-328.

%C %Cl CO +CO2 35 (+/-5) Cl- 83 (+/-2) Oxalic acid 11 Formic acid 5 Chloromaleic acid 16 8 Malonic acid 7.5 Hydroxymalonic acid 6.5 Chloromalonic acid 4.5 3 Chlorinated aromatics 2 1 Total 87.5(+/-5)% 95(+/-2)%

Table 1: Mass balance after TCP treated with Fe-TAML activated H2O2.

38

TAML Degradation

20

Review of current oxygen activation systems Biological oxygen activation, P450 enzymes Gif and TAML the major drawbacks are the

requirement of expensive reagents and incomplete degradation

The proposed system uses only zero valent iron, EDTA and air

Is the only system know to date that can take O2 and convert it to potent oxidizing species capable of extensively degrading xenobitics

21

Reaction Scheme: Fe°, EDTA, Air

II: Homogeneous O2 Activation

Fe 0

e-

+ H2O2 + OH- + OHFeIIEDTA FeIIIEDTA

Fe2++ EDTA

F e 0

FeIIEDTA

FeIIIEDTA

e-

I: Heterogeneous O2 Activation

O2

O2

.-+ + 2H

+O2

.-


O2

O2

.-+ + 2H

+O2

.-

Fe2++ EDTA

Fe°

• releases Fe2+

• site for reduction FeIIIEDTA

EDTA

• promotes Fenton reaction

• promotes FeII solubility

• enhance dissolution of Fe2+

22

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

Noradoun, C; et.al. Ind. & Eng. Chem. Res. 2003, 42(21), 5024-5030.

23

Decay Curve (phenol)

y = 1E-05e-0.938x

R2 = 0.9885

0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

0 0.5 1 1.5 2 2.5 3

time (hrs)

co

nc (

M)

Phenol Degradation

First order Kinetics (phenol)

y = -0.938x - 11.413R2 = 0.9885

-14.5

-14

-13.5

-13

-12.5

-12

-11.5

-11

0 1 2 3

Time (hrs)

LN

Co

nc

(M)

Using: Iron metal, EDTA, and air

Results have shown >90% degradation of 1.26 x 10-3 M phenol in under 3 hours.

Pseudo-First Order Rate constant: -0.94 /M hr

Direct aqueous injection using HPLC

HPLC Mobile Phase: 60/40 water/ methanol (1% Hac) Flow rate (1ml/min)UV: 270nm , C18 column

24

4-chlorophenol (4CP) Degradation

y = 1.23E-03e-1.16E+00x

R2 = 9.66E-01

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0 0.5 1 1.5 2 2.5 3 3.5 4

Time (hours)

Co

nce

ntr

atio

n (

Mo

lari

ty)

Reaction Curve

Expon. (Reaction Curve)

y = -1.1081x - 6.8133R2 = 0.9878

-1.10E+01

-1.00E+01

-9.00E+00

-8.00E+00

-7.00E+00

-6.00E+00

0 1 2 3 4

Time (hrs)

ln(c

on

c)

First OrderKinetics

Linear (First OrderKinetics)

Results have shown >95% degradation of 1mM 4-chlorophenol (4CP) in under 4 hours. (hexane/ethyl acetate extraction GC-FID)

Pseudo-first order rate constant: -1.11 /M hr @ 25°C, pH 5.5


25

ESI-MS of 4CP after 4 hours of degradation

0,C:\Masslynxold\andrzej.PRO\Data\,SAMPLE1,RAW,1,1,1,0

50 100 150 200 250 300 350m/z0

100

%

73

61

60

132

89

11791

102

344

286

143157

261244 287 342

345

360

FeIIIEDTAIminodiacetic Acid

HCO3-

propionic acid

oxalatesuccinic acid


None of the degradation products were detected in the organic extractions (GC-FID) or direct aqueous injections (HPLC), therefore ESI-MS was conducted.

M-1 ion peaks. Results show the complete degradation of 4CP (m/z 127), as well as the absence of any chlorinated products.

26

•0.5 g of 40-70 mesh Fe°•10 mL of solution•1mM EDTA•1mM 4-chlorophenol•4 hour reaction time•GC-FID and ESI-MS•RTP

4-chlorophenol (4CP) Degradation cont.

OH

Cl

COOHHOOC COOHCO2 + HCO3- + C2O4

2- + +succinic acid propionic acid

OH

O

OH

O

N

N

OH

O

OH

O

EDTA

NH

OH

O

OH

O

Iminodiacetic Acid (IDA)

27

Summary of ESI-MS analysis of 4CP reaction Complete destruction of 4CP after 4 hours

No chlorinated products produced during any time of the reaction (1hr-4hrs).

Ring opening produces low molecular weight acids, succinic, oxalic etc.

No evidence of Cl- was found in any of the ESI-MS, even when NaCl was spiked into the sample. The chloride is thought to be adsorbed to the iron surface.

28

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

29

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)

30

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)

31

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

32

EDTA an Environmental Concern Control studies show EDTA degradation as

well as xenobiotic degradation. Why is EDTA an environmental concern?

Used as a metal chelation agent in a wide variety of applications including: Paper-pulp bleaching Photochemical processing Lumber industry Cosmetics, Detergents

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 Anthropogenic

OH

O

OH

O

N

N

OH

O

OH

O

EDTA

33

Experimental Setup

BAS stir plate

stir bar

2.5 g Fe°

125 ml round bottom flask

1 mM EDTA (Total Vol. 50mL)

2.5g Fe°

Open to the Atmosphere

Aliquots were taken directly from reaction vessel, diluted, filtered and injected into HPLC

34

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

+

35

EDTA degradation

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)

36

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

37

Degradation products for -EDTA-Malathion-4-chlorophenol-pentachlorophenol-phenol

iminodiacetic acid succinic acid bicarbonate

propionic acid oxalate

Kinetically stable organic species in the presence of aqueous Fe(0)/EDTA/O2

38

A More in Depth Investigation…

Longevity Understanding Reaction Mechanism

Reactive Oxygen Intermediate species Reaction Kinetics

Optimization of Experimental Parameters

39

Prolonged Degradation of EDTA

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.

40

Reactive Oxygen Species

ROS O2

-•, OH-, FeV=O, etc.

Two Analyses were performed Thiobarbituric acid-

reactive substances (TBARS) assay

Addition of known radical scanvenger, 1-butanol

O2

F e 0O2

.- O2.-+ + 2H+

e-

+ H2O2 + OH-+ OHFeIIEDTA FeIIIEDTA

Fe2++ EDTA


F e 0

O2

O2.- O2

.-+ + 2H+


FeIIEDTA

FeIIIEDTA

e-

Fe2++ EDTA


41

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.

42

TBARS cont.

Results of HO· radical trapping by deoxyribose/thiobarbituric acid system forming a chromgen (534 nm). The conditions were 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


43

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)

Alcohols such as 1-butanol are known to be •OH radical scavengers

44

TBARS assay indicates reactive oxygen species are present

While 1-butanol studies indicate that •OH radicals are an important part of the reaction mechanism.

Further studies using the newly acquired departmental ESR, would give insight as to the specific type of radical species present.

45

Kinetic Parameters ExaminedFuture industrial scale up would require the knowledge

of how do these parameters effect the observed reaction rate?

EDTA concentration Fe° mass (surface area) Rate of mixing Temperature

A better understanding of the rate-limiting step in the reaction sequence could allow one to possibly speed up the reaction.

46

Rate limiting step

Homogeneous O2 Activation

F e 0

O2

O2.- O2

.-+ + 2H+


FeIIEDTA

FeIIIEDTA

e-

Fe2++ EDTA

1) Fe° → Fe2+ (dissolution)

2) Fe2+ → FeIIEDTA (Fe-EDTA formation)

Homogenous chemical steps

3) FeIIEDTA + O2 ↔ FeIIEDTA-O2

4) FeIIEDTA-O2 → FeIIIEDTA + O2•-

5) O2•- + O2•- + H+ → H2O2

6) FeIIEDTA + H2O2→ FeIIIEDTA + HO• +HO- (Fenton rxn)

7) Xenobiotic degradation

Heterogeneous reduction steps

8) FeIIIEDTA FeIIEDTA

47

How does Fe-oxide layer and adsorbed EDTA effect rate?

F e 0F e 0

Fe-oxide


EDTA2-

Interaction of EDTA and Fe-Oxide layer

FeIIEDTA

FeIIIEDTA

Fe2++ EDTA

Experimental setup:

Hold Fe° mass constant and vary concentration of EDTA and measure the observed rate constant

48Stumm, 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.

49

EDTA degradation rate effected by 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

Theory would suggest [Fe2+] released should be proportional to [EDTA], therefore more [EDTA] should enhance degradation rates. If degradation rates are based upon Fe dissolution rates.

The opposite was found experimentally.

Important point: Simply adding more EDTA will not speed up reaction.

50

Possible mechanisms for suppression of the reaction by excess EDTA

Surface Controlled EDTA hindering dissolution at

high concentrations

Reduction of FeII/III at the iron surface inhibited by excess EDTA

Non-Surface controlled Fenton Chemistry: High

FeII/III:EDTA ratios in solution has been shown to inhibit Fenton reactivity*.

*Engelmann, M; et.al. Biometals, 2003, 16, 519.

F e 0F e 0

Fe-oxide


EDTA2-

Interaction of EDTA and Fe-Oxide layer

FeIIEDTA

FeIIIEDTA

Fe2++ EDTA

51

EDTA hindering dissolution at high concentrations

Measurement of the dissolution rate was done using an electrochemical cell designed specially to measure corrosion rates at metal surfaces

Experimental design Varying EDTA concentration, while maintaining

constant Fe° mass (surface area).

52

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

53

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

54

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 a certain point (~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

• Rate-limiting step is not Fe° dissolution

55

[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

1.2If EDTA does not hinder the dissolution, what causes the reaction rate to decrease?

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.

56

High FeII/III:EDTA ratios inhibit Fenton reactivity

Previous work by earlier groups members has shown cases of ratios of FeII/III:EDTA more than 1:3 in which Fenton reactivity is hindered

Which explains the duality of EDTA acting as both a pro-oxidant and an antioxidant.

It was shown that Ca2+ metal could be added to sequester the excess EDTA.

Fenton reactivity was then shown to return due to the return of the optimal values of FeII/III:EDTA (1:1).

The exact coordination chemistry of FeII/III:EDTA in aqueous solutions remains uncertain

*Engelmann, M; et.al. Biometals, 2003, 16, 519.

Fe

N

O

N

OO

O

O

O

O

O

57

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

58

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

59

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)

60

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.

61

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.

62

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 [

FeIII

ED

TA

]

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

63

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).


F e 0

O2.- O2

.-+ + 2H+


FeIIEDTA

FeIIIEDTA

e-

Fe2++ EDTA

O2

F e 0O2

.- O2.-+ + 2H+

e-


Fe2++ EDTA


O2

64

Temperature Experiments

The last kinetic parameter investigated was the effect of temperature on the reaction mixture

Temperature was varied using a temperature bath and a jacketed water cell

An Arrhenius plot was constructed to obtain the activation energy

65

Arrhenius plot shows 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)

66

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)])

67

FeIIEDTA + O2 FeIIEDTAO2 k1 = 107/Ms

FeIIEDTAO2 FeIIIEDTA + O2- k2 = 102/Ms

FeIIEDTAO2 + H+ FeIIIEDTA + HO2 k3 = 1010/Ms

Rate limiting step is the activation of oxygen at the iron coordination site

Activation energy of 33.9 kJ/mol*

Similiar studies show the rate limiting step as FeIIEDTA + O2 + H+ FeIIIEDTA + H2O2

Activation energy of 27.2 kJ/mol**

R. Van Eldik; Inorg. Chem.; 1990, 29, 1705-1711. (* 0.02M [Fe(EDTA)],pH=5), Beenackers, A.; Ing. Eng. Chem. Res. 1992, 32, 2580.(**[EDTA]=100mol/m3 pH=7.5)

68

Rate limiting step1) Fe° → Fe2+ (dissolution)

2) Fe2+ → FeIIEDTA (Fe-EDTA formation)

Homogenous chemical steps

3) FeIIEDTA + O2 ↔ FeIIEDTAO2

4) FeIIEDTAO2 → FeIIIEDTA + O2•-

5) O2•- + O2•- + H+ → H2O2

6) FeIIEDTA + H2O2→ FeIIIEDTA + HO• +HO- (Fenton rxn)

7) Xenobiotic degradation

Heterogeneous reduction steps

8) FeIIIEDTA + e- FeIIEDTA

Potential rate limiting step with an activation energy of 25 kJ/mol

Can’t rule out heterogeneous rate limiting step with mass transport limited kinetics

69

Conclusions Take home message: This system is a viable option for

environmentally remediation 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

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 are controlled by oxygen activation and transport characteristics

70

Acknowledgments

Dr. Frank Cheng Cheng Group Dr. Malcolm and Mrs.

Renfrew Synder and Renfrew

Scholarships National Science

Foundation

1 kinetics and mechanism of xenobiotic degradation induced by dioxygen activation christina noradoun...

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