Lanthanide-Polyaminopolycarboxylate Complexation Kinetics in High Lactate Media

LANTHANIDE-POLYAMINOPOLYCARBOXYLATE COMPLEXATION KINETICS IN

HIGH LACTATE MEDIA: INVESTIGATING THE AQUEOUS PHASE OF TALSPEAK

DEREK MACKENZIE BRIGHAM

A dissertation submitted in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

WASHINGTON STATE UNIVERSITY Department of Chemistry

May 2013

To the Faculty of Washington State University:

The members of the Committee appointed to examine the dissertation of DEREK MACKENZIE BRIGHAM find it satisfactory and recommend that it be accepted.

______Kenneth L. Nash, Ph.D., Chair

______Scot E. Wherland, Ph.D.

______Sue B. Clark, Ph.D.

______Jeremy J. Lessmann, Ph.D.

ACKNOWLEDGMENTS

“Faith in your partner, your fellow men, your friends, is very important, because without it there's no mutual component to your relationship, and relationships are important. So faith plays an important role, but faith in people you don't know, faith in religious or political leaders or even people on stages, people who are popular in the public eye, you shouldn't have faith in those people. You should listen to what they have to say and use it. It might give you some ideas on how to view the world, but ultimately you have to base your views on evidence. Evidence comes from your own eyes and ears.” Dr. Greg Graffin

When I first arrived at WSU to begin my undergraduate studies I had very different ambitions and could not have predicted where I would end up. I certainly would not have guessed that I would still be in Pullman nearly ten years later. While this journey has been long and at times arduous, I am glad for all the experiences it has afforded me.

I want to thank my friends and family back home in Oregon for always being there for me. I certainly would not be where I am today without the ever present love and support of my parents, Larry and Marie Brigham. I am especially grateful of their patience with me throughout my life. I am deeply appreciative for all they have done for me, and hope to always make them proud. When I would come home to visit my friends made it feel like I had never left. This always gave me something to look forward to, and left me refreshed and ready to return. My life would not be the same without them.

My time at WSU has left me with some of the closest friends I could have ever asked for.

The ones I have trained with, studied with, laughed, cried, suffered, and been triumphant with, I will never forget them. It is rare to make this kind of connection, and it is something I will always cherish.

iii

I wish to express my deepest gratitude to my committee members. I would first like to thank Dr. Sue Clark, not only for being on my committee but for helping to set me on the path that has brought me to this point. If not for her involvement in the undergraduate chemistry program I would not have been introduced to radiochemistry. Dr. Jeremy Lessmann has been a welcome presence throughout both my undergraduate and graduate experience. It has been a pleasure to work for and to work with him. I am especially grateful of Dr. Scot Wherland. Not only has he been an excellent teacher, who taught me most of what I know about kinetics, but he was always welcoming and insightful when I would come to his door unannounced to discuss my results. I would not have come this far without his assistance. And finally I wish to thank Dr.

Ken Nash, who has been an excellent mentor, teacher, and friend. I truly believe I would not have made it through the graduate program without his belief in, and patience with, me.

I wish to make special mention of the late James “Sully” Sullivan. He was the one to first foster my interest in chemical kinetics, and he showed me what a true passion for science was.

As the last student Sully took on, I hope to honor his memory and pass on his passion for research and discovery.

LANTHANIDE-POLYAMINOPOLYCARBOXYLATE COMPLEXATION KINETICS IN

HIGH LACTATE MEDIA: INVESTIGATING THE AQUEOUS PHASE OF TALSPEAK

Abstract

by Derek Mackenzie Brigham, Ph.D. Washington State University May 2013

Chair: Kenneth L. Nash

In advanced nuclear fuel reprocessing schemes, the TALSPEAK (Trivalent Actinide

Lanthanide Separation with Phosphorus-reagent Extraction from Aqueous Komplexes) process has been proposed as a means to separate Am and Cm from the lanthanides. One significant limitation of the TALSPEAK process is slow phase transfer kinetics of the lanthanides to the organic phase. Increasing the lactic acid buffer concentration is found to improve the solvent extraction kinetics. However, concentrations of greater than 1 M are necessary to achieve rates of mass transfer fast enough for TALSPEAK to be applicable on an industrial scale. The

TALSPEAK process employs diethylenetriaminepentaacetic acid (DTPA) as an aqueous phase complexant to selectively bind to the actinides and prevent their extraction, however, DTPA also binds with the lanthanides. Understanding the mechanism of the interaction between DTPA and the lanthanides in high total lactate will help to explain the accelerative effect of increased total lactate on TALSPEAK mass transfer rates.

This dissertation describes the homogeneous aqueous complexation kinetics of the lanthanides Pr3+, Nd3+, Sm3+-Lu3+ and the polyaminopolycarboxylate ligands DTPA,

v ethylenediaminetetraacetic acid (EDTA), and (hydroxyethyl)ethylenediaminetriacetic acid

(HEDTA) in 1 M total lactate aqueous media similar to that found in the aqueous phase of a

TALSPEAK separation system. Temperature studies on the interactions of select lanthanides with DTPA, EDTA, and HEDTA were performed to obtain activation parameters associated with the complex formation reaction. Additional studies on the interaction of Eu3+ with DTPA were performed under varying total lactate conditions at several different constant concentrations of lactate ion and pH values to determine the mechanistic role of the species in the lactate/lactic acid buffer system. Kinetic data were obtained using the method of equilibrium perturbation by ligand displacement via stopped-flow spectrophotometry employing the colorimetric dye arsenazo III.

This work provides an increased understanding of lanthanide interactions with polyaminopolycarboxylate ligands in TALSPEAK-like aqueous media. From the insights gained in these studies, a possible explanation for the accelerative effect of lactate on TALSPEAK phase transfer rates is proposed. The overall conclusion of this work is that under high concentrations of total lactate the lactate ion governs the aqueous phase complexation kinetics.

TABLE OF CONTENTS

ACKNOWLEDGMENTS ...... iii

ABSTRACT ...... v

LIST OF TABLES ...... ix

LIST OF FIGURES ...... xi

CHAPTER

1. INTRODUCTION ...... 1

Electricity in the U.S ...... 1

Nuclear Power ...... 1

Nuclear Fuel Reprocessing ...... 2

TALSPEAK ...... 4

Kinetics of Extraction in TALSPEAK ...... 6

Research Aims ...... 9

2. EXPERIMENTAL ...... 13

Reagents ...... 19

Procedure ...... 20

Trans-lanthanide Study ...... 21

Temperature Studies ...... 22

Total Lactate Experiments ...... 22

AAIII Independence...... 23

3. EXPERIMENTAL RESULTS...... 28

DTPA Lanthanide Series Study ...... 29

Temperature Study ...... 33

vii

EDTA Lanthanide Series Study ...... 35

Temperature Study ...... 39

HEDTA Lanthanide Series Study ...... 41

Temperature Study ...... 45

High Ligand Concentration Study ...... 47

4. TRANS-LANTHANIDE STUDY DISCUSSION ...... 53

Trans-lanthanide Trends ...... 53

Isokinetic Relationship...... 59

Calculated Stability Constants ...... 63

Lactate Complexation ...... 66

High Ligand Concentration...... 75

5. RESOLVING THE MECHANISTIC ROLE OF THE LACTATE BUFFER ...... 78

Constant pH ...... 80

Constant Lactate Ion ...... 84

Role of Lactic Acid ...... 88

High [DTPA] Under Constant Lactate Ion Conditions ...... 90

Data Fitting ...... 94

Conclusion ...... 100

CONCLUSION ...... 102

APPENDIX A ...... 112

viii

LIST OF TABLES Chapter 3 3.1 Lanthanide-DTPA second order rate constant of complex formation. 1.0 M NaLac variable HClO4 2.0 M NaClO4 ionic strength 25 °C ...... 30

3.2 Lanthanide-DTPA first order rate constant of complex dissociation. 1.0 M NaLac variable HClO4 2.0 M NaClO4 ionic strength 25 °C ...... 30

3.3 Activation parameters for the Ln-DTPA complexation reaction in 1.0 M NaLac 0.9 M HClO4 and 2 M ionic strength. Additionally the second order rate constant of complex formation at 25 °C is listed ...... 35

3.4 Lanthanide-EDTA second order rate constant of complex formation. 1.0 M NaLac variable HClO4 2.0 M NaClO4 ionic strength 25 °C ...... 37

3.5 Lanthanide-EDTA first order rate constant of complex dissociation. 1.0 M NaLac variable HClO4 2.0 M NaClO4 ionic strength 25 °C ...... 37

3.6 Activation parameters for the Ln-EDTA complexation reaction in 1.0 M NaLac 0.9 M HClO4 and 2 M ionic strength. Additionally the second order rate constant of complex formation at 25 °C is listed ...... 41

3.7 Lanthanide-HEDTA second order rate constant of complex formation. 1.0 M NaLac variable HClO4 2.0 M NaClO4 ionic strength 25 °C ...... 43

3.8 Lanthanide-HEDTA first order rate constant of complex dissociation. 1.0 M NaLac variable HClO4 2.0 M NaClO4 ionic strength 25 °C ...... 43

3.9 Activation parameters for the Ln-HEDTA complexation reaction in 1.0 M NaLac 0.9 M HClO4 and 2 M ionic strength. Additionally the second order rate constant of complex formation at 25 °C is listed ...... 47

Chapter 4 4.1 Activation enthalpies, entropies, and rate constants at 25 °C for the complex formation reaction between the Ln’s and PAPC ligands in 1.0 M NaLac, 0.9 M HClO4 and 2 M ionic strength ...... 60

4.2 Calculated kinetic equilibrium constants from the rate constants obtained in1.0 M NaLac 0.9 M HClO4 2 M ionic strength. Thermodynamic calculated equilibrium constants calculated from literature data at 0.1 M ionic strength and 25 °C ...... 68

4.3 Average number of lactate ions coordinated to Ln3+ in 1.0 M NaLac at a given pH ...... 69

Chapter 5

5.1 Rate constants of complex formation (kf) and dissociation (kd) under constant pH with different total lactate conditions ...... 83

5.2 Rate constants of complex formation (kf) and dissociation (kd) under constant lactate ion concentrations with different total lactate conditions. Respective pH - - values at 1.0, 0.9, and 0.8 M Lactot; 0.1 M Lac : 2.72, 2.77,and 2.83; 0.2 M Lac : 3.07, 3.13, and 3.19; 0.3 M Lac-: 3.30, 3.37, and 3.45 ...... 87

Appendix A.1 Upper and lower sensitivity limits of calculated fitting parameters resulting in a 20% increase in the residual sum of squares. *Zero value for this parameter results in a 13% increase in RSS...... 114

LIST OF FIGURES Chapter 1 1.1 Structure of di(2-ethylhexyl) phosphoric acid (HDEHP) ...... 5

1.2 Structure of lactic acid ...... 5

1.3 Structure of diethylenetriaminepentaacetic acid (DTPA) ...... 6

1.4 Lanthanide extraction at 30 minutes by 0.3 M HDEHP in n-dodecane from 0.05 M DTPA and () 0, (●) 0.1, and (■) 1.0 M total lactate at pH 3.5. Reproduced from [8] ...... 7

1.5 Rate of complexation for the reaction of La3+ with DTPA in 0.1 (■), 0.2 (●), and 0.3 () M total lactate at pH 3.5. Reproduced from [9] ...... 8

1.6 Structure of ethylenediaminetetraacetic acid (EDTA) ...... 9

1.7 Structure of (hydroxyethyl)ethylenediaminetriacetic acid (HEDTA) ...... 10

Chapter 2 2.1 Structure of the Arsenazo III...... 15

2.2 Change in the visible spectrum upon mixing of EuAAIII with DTPA. 0.05 mM Eu 0.0125 mM AAIII and 8 mM DTPA in 1.0 M NaLac, pH 2.6, 2 M ionic strength 25 °C ...... 16

2.3 Complexation of 0.05 mM Gd and EDTA 1.0 M NaLac 0.9M HClO4 2 M ionic strength 25 °C with [AAIII] (■) 0.0125 mM and (●) 0.025 mM at pH 2.7 and 2.8 respectively ...... 24

Chapter 3 3.1 Observed rate constants of the reaction between Eu3+ and DTPA in 1.0 M NaLac 0.9 (■), 0.85 (●), and 0.8 () M HClO4 2 M ionic strength at 25 °C ...... 29

3.2 Ln-DTPA kf values across the Ln series (Pr-Lu) in 1 M NaLac, 0.9 (■), 0.85 (●), and 0.8 () M HClO4, 2 M ionic strength at 25 °C ...... 31

3.3 Ln-DTPA kd values across the Ln series (Pr-Lu) in 1 M NaLac, 0.9 (■), 0.85 (●), and 0.8 () M HClO4, 2 M ionic strength at 25 °C ...... 32

3.4 Temperature dependence of the reaction between Eu3+ and DTPA 1 M NaLac 0.9 M HClO4 2 M ionic strength at 25 °C (■), 20 °C (●), 15 °C () and 10 °C ()...... 34

3.5 Observed rate constants of the reaction between Eu3+ and EDTA in 1.0 M NaLac and 0.9 (■), 0.85 (●), and 0.8 () M HClO4 2 M ionic strength at 25 °C ...... 36

3.6 Ln-EDTA kf values across the Ln series (Pr-Lu) in 1 M NaLac, 0.9 (■), 0.85 (●), and 0.8 () M HClO4, 2 M ionic strength at 25 °C ...... 38

3.7 Ln-EDTA kd values across the Ln series (Pr-Lu) in 1 M NaLac, 0.9 (■), 0.85 (●), and 0.8 () M HClO4, 2 M ionic strength at 25 °C ...... 39

3.8 Temperature dependence of the reaction between Eu3+ and EDTA 1 M NaLac 0.9 M HClO4 2 M ionic strength at 30 °C (■), 25 °C (●), 20 °C () and 15 °C ()...... 40

3.9 Observed rate constants of the reaction between Eu3+ and HEDTA in 1.0 M NaLac and 0.9 (■), 0.85 (●), and 0.8 () M HClO4 2 M ionic strength at 25 °C ...... 42

3.10 Ln-HEDTA kf values across the Ln series (Pr-Lu) in 1 M NaLac, 0.9 (■), 0.85 (●), and 0.8 () M HClO4, 2 M ionic strength at 25 °C ...... 44

3.11 Ln-HEDTA kd values across the Ln series (Pr-Lu) in 1 M NaLac, 0.9 (■), 0.85 (●), and 0.8 () M HClO4, 2 M ionic strength at 25 °C ...... 45

3.12 Temperature dependence of the reaction between Eu3+ and HEDTA 1 M NaLac 0.9 M HClO4 2 M ionic strength at 33 °C (■), 33 °C (●), 27 °C () and 25 °C ()...... 46

3+ 3.13 Complexation of Eu with (■) DTPA, (●) EDTA, and () HEDTA in 1.0 M NaLac, 0.9 M HClO4, and 2 M ionic strength at 25 °C. Lines added as a visual guide ...... 48

3.14 Observed rate constants of the reaction between Eu3+ and DTPA in 1.0 M NaLac 0.9 (■), 0.85 (●), and 0.8 () M HClO4 2 M ionic strength at 25 °C. Lines added as a visual guide...... 49

Chapter 4 4.1 Rate of complex formation across the lanthanides with (■) DTPA, (●) EDTA, and () HEDTA in 1.0 M NaLac 0.9 M HClO4 2 M ionic strength at 25 °C ...... 54

4.2 Rate of complex formation of the lanthanides with (■) DTPA, and (●) EDTA in 0.3 M Lactot pH 3.6 (reproduced from [1]); (□) DTPA and (○) EDTA in 1.0 M Lactot pH 3.0 at 25 °C ...... 56

4.3 Rate of complex dissociation across the lanthanides with (■) DTPA, (●) EDTA, and () HEDTA in 1.0 M NaLac 0.9 M HClO4 2 M ionic strength at 25 °C ...... 57

4.4 Rate of complex formation of the lanthanides with (■) DTPA, and (●) EDTA in 0.3 M Lactot pH 3.6; (□) DTPA and (○) EDTA in 1.0 M Lactot pH 3.0 at 25 °C ...... 58

4.5 Isokinetic relationship for the complexation reaction between the lanthanides Nd3+, 3+ 3+ 3+ 3+ 3+ Eu , Tb , Ho , Tm , and Lu with (■) DTPA, (●) EDTA, and () HEDTA ...... 61

xii

4.6 Rate constant of complex formation of Eu3+ with (■) DTPA and (●) EDTA as a function of the average number of lactate ions coordinated to Eu3+ prior to mixing ...... 69

4.7 Rate constant of complex formation between the lanthanides with DTPA as a function of the average number of coordinated lactate ions. 1.0 M NaLac, pH 2.6, at 25 °C ...... 70

4.8 Rate constant of complex formation between the lanthanides with EDTA as a function of the average number of coordinated lactate ions. 1.0 M NaLac, pH 2.6, at 25 °C ...... 71

4.9 Rate constant of complex formation between the lanthanides with HEDTA as a function of the average number of coordinated lactate ions. 1.0 M NaLac, pH 2.6, at 25 °C ...... 72

4.10 Rate constant of complex formation with ligand vs. average number of bound lactates in (■) 0.3 M total lactate pH 3.6 with the lanthanides La3+-Lu3+ [1] and (●) 1.0 M total lactate pH 3.0 with the lanthanides Pr3+-Lu3+ ...... 73

4.11 Rate constant of complex dissociation between the lanthanides with EDTA as a function of the average number of coordinated lactate ions. 1.0 M NaLac, pH 2.6, at 25 °C ...... 74

Chapter 5 5.1 Speciation of DTPA in 2 M ionic strength ...... 79

5.2 Observed rate constants for the reaction between Eu3+ with DTPA under a constant pH of 2.72 in 1.0 (■), 0.9 (●), and 0.8 () M total lactate ...... 81

5.3 Observed rate constants for the reaction between Eu3+ with DTPA under a constant pH of 3.07 in 1.0 (■), 0.9 (●), and 0.8 () M total lactate ...... 82

5.4 Observed rate constants for the reaction between Eu3+ with DTPA under a constant pH of 3.30 in 1.0 (■), 0.9 (●), and 0.8 () M total lactate ...... 83

5.5 Observed rate constants for the reaction between Eu3+ with DTPA under a - constant [Lac ] of 0.1 M in 1.0 (■), 0.9 (●), and 0.8 () M total lactate with pH values of 2.72, 2.77, and 2.83 respectively ...... 85

5.6 Observed rate constants for the reaction between Eu3+ with DTPA under a - constant [Lac ] of 0.2 M in 1.0 (■), 0.9 (●), and 0.8 () M total lactate with pH values of 3.07, 3.13, and 3.19 respectively ...... 86

5.7 Observed rate constants for the reaction between Eu3+ with DTPA under a - constant [Lac ] of 0.3 M in 1.0 (■), 0.9 (●), and 0.8 () M total lactate with pH values of 3.30, 3.37, and 3.45 respectively ...... 87

xiii

5.8 Observed rate constants for the reaction between Eu3+ with DTPA under the conditions of constant pH of 3.07 (left) and a constant [Lac-] of 0.2 M (right) in 1.0 (■), 0.9 (●), and 0.8 () M total lactate ...... 89

5.9 Observed rate constants for the reaction between Eu3+ with DTPA under a - constant [Lac ] of 0.1 M in 1.0 (■), 0.9 (●), and 0.8 () M total lactate with pH values of 2.72, 2.77, and 2.83 respectively ...... 91

5.10 Observed rate constants for the reaction between Eu3+ with DTPA under a - constant [Lac ] of 0.2 M in 1.0 (■), 0.9 (●), and 0.8 () M total lactate with pH values of 3.07, 3.13, and 3.19 respectively ...... 92

5.11 Observed rate constants for the reaction between Eu3+ with DTPA under a - constant [Lac ] of 0.3 M in 1.0 (■), 0.9 (●), and 0.8 () M total lactate with pH values of 3.30, 3.37, and 3.45 respectively ...... 93

5.12 Experimental data and calculated fits for the constant [Lac-] condition of 0.1 M 2 in 1.0 (■), 0.9 (●), and 0.8 () M total lactate with respective R values for the fits of 0.976, 0.999, and 0.978. Fitting parameters – a: 3813; b: 2.29•10-3; c: 1.22 ...... 96

5.13 Experimental data and calculated fits for the constant [Lac-] condition of 0.2 M 2 in 1.0 (■), 0.9 (●), and 0.8 () M total lactate with respective R values for the fits of 0.964, 0.994, and 0.938. Fitting parameters – a: 4173; b: 4.57•10-3; c: 2.19 ...... 97

5.14 Experimental data and calculated fits for the constant [Lac-] condition of 0.3 M 2 in 1.0 (■), 0.9 (●), and 0.8 () M total lactate with respective R values for the fits of 0.968, 0.998, and 0.967. Fitting parameters – a: 4961; b: 4.32•10-3; c: 4.62 ...... 98

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DEDICATION

In loving memory of Martha Ann Brigham

Chapter 1

Introduction

Electricity in the U.S.

In 2010 the United States accounted for 21% of global electricity consumption. In 2011

68% of the electricity generated in the U.S. was through combustion of fossil fuels.1 With the rising concern over the adverse effects of carbon dioxide emission to the atmosphere on the global climate, there is increased pressure for implementing alternative methods of power production. With federal government incentives, wind power generation has seen increased use in the past several years, accounting for 3% of total power generation in 2011, up from 0.8% in

2007.1 While renewable energy sources such as wind and solar will play an important role in the reduction of reliance on fossil fuels, the intermittent nature of these renewable energy sources means their use on a large energy grid will require complementary alternative forms of power generation, which often is supplied by a conventional thermal power plant run on coal, oil, or natural gas.

Nuclear Power

Of presently available large capacity options, the best means available to reduce CO2 emissions from electricity generation is by increased implementation of nuclear power. In 2011 nuclear power accounted for 19% of the electricity produced in the United States.1 It is the only

CO2 emission-free method of production that is both well established and capable of replacing a significant percentage of the power generated by fossil fuel combustion. Conventional nuclear power reactors moderated and cooled by light water generate electricity by utilizing uranium fuel

1 that has been enriched to between three and five percent 235U. The neutrons generated in the fissioning of 235U (and later by 239Pu that is bread from 238U) maintain the chain reaction. This exothermic process hears water to produce steam that runs turbines that generate electricity.

Besides reducing the amount of carbon dioxide released to the atmosphere as a result of power production, nuclear power generation could reduce the amount of radioactive material released to the environment from electricity production. In 2011 42% of the electricity generated in the U.S. was done by burning coal.1 Coal contains small quantities of uranium, thorium, and their radioactive daughter products. In a study by McBride et. al. it was found that populations received a greater dose of radiation from coal-fired power plants than from nuclear power plants.2

Nuclear Fuel Reprocessing

The main drawback to nuclear power generation is the highly radioactive waste that remains after the uranium fuel elements have gone through one (typically four year) cycle in the reactor. The vast majority of this material (>95%) can be recycled for additional power generation, however the remaining material still presents a substantial challenge. Without treatment the long term radiotoxicity hazard requires sequestration of the used fuel on a time scale reaching hundreds of thousands of years.3 However, this time span can be greatly reduced by reprocessing the spent nuclear fuel.

Although no reprocessing of used fuel is performed in the U.S., the U.K., Russia, Japan, and France recycle their spent nuclear fuel, mainly by some form of the PUREX process. The

PUREX (Plutonium Uranium Recovery by EXtraction) process was developed by the U.S. during the Manhattan Project, primarily as a means for the production of plutonium for use in

2 nuclear weapons. The PUREX liquid-liquid separation scheme employs the extractant tributyl phosphate (TBP) to separate uranium and plutonium from the fission products and other actinides.3 Solutions of 30% TBP in kerosene are contacted with 3-4 M nitric acid aqueous solutions of the dissolved spent nuclear fuel. In the first cycle more than 99.8% of uranium and plutonium are extracted (in hexavalent and tetravalent oxidation states respectively) leaving more than 99% of the fission products in the aqueous raffinate. In later stages, plutonium is reduced to the trivalent state by contact with a solution containing a suitable reductant, such as

U(IV). The Pu(III) then strips to the aqueous phase, separating from the uranium. At this point, the PUREX solvent extraction method produces a stream of clean weapons grade plutonium and as such raises concerns over nuclear proliferation.

UREX+, an advanced nuclear reprocessing scheme developed at Argonne National

Laboratory, employs a suite of solvent extraction methods to reduce the risk of proliferation as well as the length of time the waste is hazardous to the environment.4 The first step is the UREX

(Uranium Extraction) process, a modification of the PUREX process. UREX avoids producing pure plutonium by adding the reductant acetohydroxamic acid to the aqueous feed. The Pu(IV) is reduced to Pu(III) which is poorly extracted and remains in the aqueous phase while the uranium is extracted to the organic phase by TBP. The UREX aqueous raffinate is then introduced to the the CCD-PEG segment. A mixture of chlorinated cobalt dicarbollide (CCD) and polyethylene glycol (PEG) extract cesium and strontium respectively, which are the dominant fission products.

The CCD-PEG aqueous raffinate then moves on to the NPEX stage. The NPEX (Neptunium

Plutonium Extraction) process thermally destroys the acerohydroxamic acid reductant/complexant introduced in the UREX stage and increases the concentration of nitric acid to convert plutonium and neptunium to the extractable tetravalent oxidation state. Pu(IV)

3 and Np(IV) are then extracted by TBP in a process similar to that of UREX and PUREX. The aqueous raffinate of NPEX then moves on to the TRUEX (TRansUranium EXtraction) segment.

TRUEX employs the carbamoyl organophosphorus extractant octyl(phenyl)-N,N- diisobutylcarboylmethyl-phosphine oxide (CMPO) and TPB to extract americium, curium, and the fission product lanthanides from the remaining fission products.

The product stream of TRUEX contains the most important isotopes contributing to the long term radiotoxicity of the spent fuel, the actinides americium and curium. Removing these elements and returning them to reactors for transmutation to shorter lived isotopes significantly reduces the amount of time the fuel elements need to be isolated from the biosphere. One obstacle to this method, however, is the presence of the trivalent lanthanides, which act as neutron poisons and reduce the efficiency of the actinide transmutation. Unfortunately the trivalent lanthanides have chemical behavior very similar to the trivalent actinides and are quite difficult to separate from one another. The separation of these elements has been a major factor in nuclear fuel cycle research.

TALSPEAK

One method for separating the trivalent actinides from the trivalent lanthanides is the solvent extraction system known as the TALSPEAK (Trivalent Actinide Lanthanide Separation with Phosphorus-reagent Extraction from Aqueous Komplexes) process. Developed at Oak

Ridge National Laboratory in the 1960’s by Weaver and Kappelman5, the TALSPEAK process employs the liquid cation exchange extractant di(2-ethylhexyl) phosphoric acid (HDEHP) (figure

1.1) to selectively extract the trivalent lanthanides into the organic phase from aqueous solutions.

The pH of the aqueous medium in conventional TALSPEAK is maintained between pH 3.0 and

3.5 using a lactic acid buffer (figure 1.2). The trivalent actinides are held back in the aqueous phase by the complexing agent diethylenetriaminepentaacetic acid (DTPA) (figure 1.3). While the trivalent actinides have very similar chemistry to that of the trivalent lanthanides, the f- orbitals of the actinides extend slightly out of the d-orbital subshell while the lanthanide f- orbitals are completely shielded. This gives the actinides some soft Lewis-acid character which leads to greater interactions with soft Lewis-base electron donors, in this case the amine donor atoms of DTPA. The TALSPEAK process takes advantage of this characteristic of the actinides to achieve separation between the actinides and lanthanides by using DTPA as an aqueous hold- back reagent. The softer nitrogen-donors of DTPA interact with the trivalent actinides more strongly than the lanthanides. As metal-DTPA complexes are non-extractable, and DTPA will preferentially bind to the actinides, the lanthanides are able to be selectively extracted by

HDEHP to the organic phase.

Figure 1.1: Structure of di(2-ethylhexyl) phosphoric acid (HDEHP).

Figure 1.2: Structure of lactic acid.

Figure 1.3: Structure of diethylenetriaminepentaacetic acid (DTPA).

Kinetics of Extraction in TALSPEAK

The TALSPEAK process has been subject to extensive study since its introduction.6

Nevertheless, the chemistry underpinning the process is not yet completely understood.

Although TALSPEAK seems to be quite promising for application to nuclear fuel reprocessing, the process is not without limitations. One such complication is that the phase transfer kinetics of

TALSPEAK are very slow at low concentrations of the lactic acid buffer. It has been observed that increasing the concentration of total lactate in the system increases the rate of lanthanide extraction.7 The data in figure 1.4 show the effect of increasing lactate on the extraction kinetics in TALSPEAK across the lanthanide series.8 For the phase transfer kinetics to be fast enough for

TALSPEAK to be applicable as an industrial process, concentrations of lactate greater than 1.0

M are necessary. The need for such high concentrations of carboxylic acid buffer have been established as one contributing factor to the complexity of the TALSPEAK process.

105

104

103

10 D

101

100

10-1 0.84 0.86 0.88 0.90 0.92 0.94 0.96 0.98 1.00 1.02 1.04 Inverse ionic radius (Å-1) CN=8

Figure 1.4: Lanthanide extraction at 30 minutes by 0.3 M HDEHP in n-dodecane from 0.05 M DTPA and () 0, (●) 0.1, and (■) 1.0 M total lactate at pH 3.5. Reproduced from [8].

Increasing the concentration of lactic acid is seen to increase the rate of lanthanide extraction in the TALSPEAK process, however the reason for this accelerative effect is unclear.

An investigation of the complexation kinetics in TALSPEAK-like aqueous media, performed by

Nash et. al., could provide insight to the lactate effect on the extraction kinetics.9 However, the results of this study seem to present further complications. In a pseudo first-order study of the complexation kinetics between La3+ and DTPA in various concentrations of total lactate at pH

-1 -1 3.5, it was found that the rate constant of complex formation, kf (M s ), is constant at different concentrations of total lactate. Figure 1.5 reproduces these results, where under pseudo first- order conditions the slope gives the kf value and the intercept gives the kd value (derivation of this relationship provided in chapter 2). It was noted that the rate constant of complex

-1 dissociation, kd (s ), decreases with increasing total lactate.

0.06

0.05

0.04

) -1

0.03

(ms

obs k 0.02

0.01

0.00 0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 DTPA (M)

Figure 1.5: Rate of complexation for the reaction of La3+ with DTPA in 0.1 (■), 0.2 (●), and 0.3 () M total lactate at pH 3.5. Reproduced from [9].

The observation that the Ln-DTPA rate constant of complex dissociation decreases with an increase in total lactate concentration indicates a greater kinetic stability of the complex.

Greater kinetic stability of the DTPA complexes means the lanthanides will spend more time bound to the aqueous complexant, which is a non-extractable species and therefore should slow the rate of phase transfer in a TALSPEAK-like process. This observed effect of increased concentration of total lactate on the aqueous complexation kinetics appears to be in conflict with the observation that higher concentrations of total lactate increase the rate of lanthanide extraction in TALSPEAK. Clearly, more complex interactions must be at work.

Research Aims

The apparent conflict between the experimental results of increased total lactate concentration on the kinetics of lanthanide extraction and the aqueous complexation kinetics of the lanthanides motivated behind the work that is to follow. In examining the experiments on the extraction kinetics to the aqueous complexation kinetics, it is noteworthy that the aqueous lanthanide-DTPA complexation kinetics experiments occur in 0.1-0.3 M total lactate. This range is considerably lower than the 1.0 M concentration where the most significant effect on the extraction kinetics is observed. In order to be more representative of the TALSPEAK aqueous phase, the complexation kinetics experiments need to be performed in solutions of higher total lactate concentration. Therefore, the work that is to follow investigates the complexation kinetics of the lanthanides with polyaminopolycarboxylate (PAPC) ligands in 1.0 M total lactate by stopped-flow spectrophotometry. Three PAPC ligands selected for their decreasing denticity and bonding strength will be used in the following complexation kinetics studies, DTPA (figure 1.3), ethylenediaminetetraacetic acid (EDTA) (figure 1.6), and

(hydroxyethyl)ethylenediaminetriacetic acid (HEDTA) (figure 1.7). The systematic changes in the structures and binding pockets of these ligands should provide some insight to the effect of the different ligands on the complexation kinetics.

Figure 1.6: Structure of ethylenediaminetetraacetic acid (EDTA).

Figure 1.7: Structure of (hydroxyethyl)ethylenediaminetriacetic acid (HEDTA).

Temperature studies of the complexation kinetics of these PAPC ligands with the lanthanides will provide a great deal of information on how the mechanism of the complexation reaction changes between the different ligands. Completing these studies across the lanthanide series will allow for correlations to be developed between rates of complexation, ligand denticity, thermodynamic stability and the cation size in transiting the lanthanide series, as the radii shrink by about 20% across the. As the total lactate in these systems are comprised of the lactic acid and lactate ion species and their concentrations are dependent on the pH of the system, these complexation kinetics were investigated under three different acidity conditions to gain a cursory understanding of the effect of these different species of lactate on the kinetics.

However, to successfully interpret the kinetic role of the lactic acid, lactate ion, and hydrogen ion species in these systems, more complex experiments with varying concentrations of total lactate will be required. Understanding the roles of these different species allows for mechanistic interpretation of the aqueous complexation reactions that take place in TALSPEAK.

The goal of the experiments to follow is to gain a better understanding of the aqueous complexation kinetics in high lactate media so that the conflict between the results of previous complexation kinetics and extraction kinetics experiments may be resolved. Additionally, a greater mechanistic understanding of how the lanthanides interact with the aqueous hold-back

10 reagent DTPA in high lactate potentially provides insight as to how the complicated TALSPEAK extraction process operates.

References

1. U.S. Energy Information Administration. http://www.eia.gov/, February 2013.

2. McBride, J. P.; Moore, R. E.; Witherspoon, J. P.; Blanco, R. E. Radiological Impact of Airborne Effluents of Coal and Nuclear Plants. Science, 1978, 202, 1045.

3. Choppin, G.R.; Liljenzin, J-O.; Rydberg J. Chapter 21: Nuclear fuel cycle. Radiochemistry and Nuclear Chemistry, 3rd edition; Butterworth-Heinemann: Woburn, MA, 2002; 583-641.

4. Vandegrift, G. F.; Regalbuto, M. C.; Aase, S. C.; Bakel, A. J.; Battisti, T. J.; Bowers, D. L.; Byrnes, J. P.; Clark, M. A.; Emery, J. W.; Falkenberg, J. R.; Gelis, A. V.; Pereira, C.; Hafenrichter, L. D.; Tsai, Y.; Quigley, K. J.; Vander Pol, M. H. Designing and demonstration of the UREX+ process using spent nuclear fuel. Proceedings of Atatlante 2004: Advances of Future Nuclear Fuel Cycles, Nimes, France, June 21-24, 2004; 012- 01.

5. Weaver, B.; Kappelmann, F. A. Talspeak, A new method of separating americium and curium from the lanthanides by extraction from an aqueous solution of an aminopolyacetic acid complex with a monoacetic organophosphate or phosphonate. August 1964, ORNL-3559.

6. Nilsson, M.; Nash, K. L. Review Article: A Review of the Development and Operational Characteristics of the TALSPEAK Process. Solv. Extr. Ion Exch., 2007, 25, 665.

7. Kolarik, Z.; Koch, G.; Kuhn, W. Acidic Organophosphorus Extractants-XVIII The Rate of Lanthanide(III) Extraction by Di(2-ethylhexyl) Phosphoric Acid From Complexing Media. J. Inorg. Nucl. Chem. 1974, 36, 905.

8. Nilsson, M.; Nash, K. L. Manuscript in preparation.

9. Nash, K. L.; Brigham, D.; Shehee, T. C.; Martin, A. The kinetics of lanthanide complexation by EDTA and DTPA in lactate media. Dalton Trans. 2012, 41, 14547.

Chapter 2

Experimental

The rates of complex formation and dissociation of f-elements and polyaminopolycarboxylate (PAPC) ligands have been investigated previously primarily using either metal ion or ligand exchange reactions. A variety of techniques have been employed including spectophotometry,1-3 NMR spectroscopy,4-7 electrochemistry,8 and radiometric methods.9-12 The exchange reactions are mainly used for determining dissociation rates as the complex formation reactions for f-elements and PAPC ligands are generally very rapid. Direct measure of complex formation reactions have been reported (in dilute acetate buffer media) for only trivalent La2 and trivalent Am3 complexation by trans-1,2-diaminocyclohexane tetraacetic acid (DCTA). In these cases the reaction was slow enough to monitor directly due to the structural rigidity of the cyclohexyl backbone of DCTA, which hinders the conformational change of ligand that must take place in order to bind at all of its chelation sites to the metal. It is interesting that the prearrangement of ligand donor atoms in what should be a favorable conformation (complex stability is increased about 100-fold for DCTA relative to EDTA) results in slower kinetics of complexation.

Spectrophotometric experiments for measuring the kinetics of rapid reactions often employ stopped-flow techniques. In a stopped-flow experiment, two syringes, each containing one component of the reaction, are set up parallel with a single driving piston on the plungers.

When signaled to fire, the driving piston acts on the syringes causing equal volumes of the reagent solutions to flow into a mixing chamber then onto an observation cell. The cessation of solution flow triggers the electronics to acquire an optical signal that is correlated with the

13 chemical reaction of interest. This method allows for the measurement of reactions with half- lives on the scale of milliseconds. In the series of experiments to follow the direct measurement of the complexation reaction between the lanthanides and PAPC ligands is achieved by employing stopped-flow spectrophotometry using a technique reported previously that makes these reactions kinetically accessible. As these reactions are otherwise too fast to be measured with such techniques, this result is primarily ascribed to the unusual aqueous media used in these experiments of high total concentrations of lactate, the effects of which will be discussed in detail in subsequent chapters.

The experimental method of equilibrium perturbation by ligand displacement that is applied in these series of experiments has been demonstrated in previous studies of examining different classes of actinide-complexing ligands.13-17 The lanthanides do not typically form strongly colored complexes or demonstrate significant changes in their absorption spectra from complexation with the PAPC ligands of interest in this study. Therefore, a strongly colored auxiliary reagent is required for spectrophotometric techniques such as stopped-flow to monitor the growth of the Ln-PAPC complexes. The colorimetric indicator dye 2,2’-(1,8-dihydroxy-3,6- disulfo-2,7-napthalene-bis(azo)dibenzenarsonic acid, known as Arsenazo III (hereafter AAIII)

(Figure 2.1) is used in these experiments for monitoring the rate of complexation of the lanthanides and PAPC ligands. It will be established that AAIII plays no significant role in the

Ln-PAPC complexation kinetics.

Figure 2.1: Structure of the Arsenazo III.

Free AAIII is magenta in color with a broad, intense absorption centering around 550 nm.

It has eight ionizable protons, for which acid dissociation constants have been reported (pK1 =

−2.5, pK2 = 0, pK3 = pK4 = 2.5, pK5 and pK6 = 5.3, pK7 = 7.5 and pK8 = 12.4); the most basic hydrogen ions are likely associated with the phenolic groups, the weakest with the sulfonate groups. AAIII complexes with the lanthanides are strongly colored blue with absorption bands at

610 and 658 nm. Their features have been investigated in some detail.18-22 The extinction

4 -1 -1 coefficients of the bands at the λmax= 658 nm are greater than 10 M cm , allowing the spectrophotometric monitoring of lanthanide concentration at 10-4 M or less, a critical design element for this investigation. It is found that AAIII forms 1:1 complexes with the lanthanides under most conditions and is believed to coordinate the lanthanide cation in a tetradentate manner with one oxygen atom from each arsenate and the phenol groups. AAIII is especially useful in analytical applications as it will complex with the lanthanides even under acidic conditions.

It has been demonstrated in previous studies that AAIII and Chlorophosphonazo III

(CLIII) interact with tetra,penta- and hexavalent actinide ions at a stopped-flow accessible

15 rate.23−27 However, studies looking at the interaction of AAIII with lanthanide ions found the complexation to mainly occur at or near the diffusion-controlled limit, too fast for monitoring with stopped-flow under normal conditions.28 This rapid interaction is advantageous in that the

Ln-AAIII complex rapidly exchanges between the complexed and free species, which allows for the PAPC ligand to complex with the free lanthanide. The subsequent decrease in the Ln-AAIII absorption spectrum can then be correlated to the growth of the Ln-PAPC complex, allowing for direct measurement of the forward complexation reactions between the lanthanides and PAPC ligands. Figure 2.2 shows the spectral change of the Eu-AAIII complex when mixed with DTPA.

0.6

0.4

Abs

0.2

0.0 560 580 600 620 640 660 680 700 Wavelength (nm)

Figure 2.2: Change in the visible spectrum upon mixing of EuAAIII with DTPA. 0.05 mM Eu 0.0125 mM AAIII and 8 mM DTPA in 1.0 M NaLac, pH 2.6, 2 M ionic strength 25 °C.

It should be noted that in the high lactate media employed in this series of experiments the Ln-

AAIII absorption band at 610 nm overlaps with the free AAIII spectra, however the 610 peak is found to be present after deconvolution of the spectra.

In the experiments that are to follow, an otherwise significantly complex system is simplified by operating under pseudo-first order conditions to monitor the approach to equilibrium between the lanthanides and the PAPC ligands. Under pseudo-first order conditions the concentration of the ligand is many times greater than that of the lanthanide ion, such that the concentration of the ligand is essentially constant over the course of the reaction. This allows for simple first-order analysis of the kinetic data of an otherwise second order system, yielding a first-order observed rate constant (kobs). The value of kobs will be dependent on the concentration of the ligand. This pseudo-first order approach to equilibrium method of kinetic analysis is a very powerful tool for investigating higher order reactions. It is instructive to first consider the fundamental mathematics of the kinetics of a first order approach to equilibrium reaction, as follows.

In a first order reversible reaction where reactant A goes to product P, the approach to equilibrium can be described in the rate of disappearance of the reactant. The system can be described as follows:

k 1 (1) k-1

(2)

(3)

(4)

Combining equations 2, 3, and 4 gives eq.5. Manipulation of the variables gives eq.6, and simple integration leads to eq.7.

(5)

(6)

(7)

It is seen here that a plot of ln([A]t - [A]e) with time will be linear, the negative of the slope of which is the observed rate constant of the reaction and is the sum of the forward and reverse rate constants.

This same treatment can be applied to a second order reversible approach to equilibrium reaction where reactants A and B go to product P, given the pseudo-first order condition of

[B]o >> [A]o. The system can be described as follows:

k 1 (8) k-1

(9)

(10)

(11)

(12)

Combination of equations 9, 10, and 11 and application of the pseudo-first order condition described in eq. 12 give eq. 13, manipulation of the variables and integration gives eq. 14.

(13)

(14)

Again it is seen here that a plot of ln([A]t - [A]e) with time will be linear, the negative slope of which is the observed rate constant of the pseudo-first order reaction:

(15)

This result shows that a plot of kobs against [B]o will give a straight line, the slope of which is the second-order rate constant of the forward reaction and the intercept is the first order rate constant of the reverse reaction. In the experiments that are to follow k1 will be the second-order rate

-1 -1 constant of complex formation kf (M s ) and k-1 will be the first-order rate constant of complex

-1 dissociation kd (s ). Equation 16 reflects the analysis of all the kinetic experiments to be presented.

(16)

Reagents

Lanthanide perchlorate stock solutions have been prepared, using 99.999% Ln2O3 from

Arris International Corp., through dissolution of the solid in HClO4. Stock solutions were standardized by ICP-OES analysis and sample acidity determined by potentiometric titration.

These lab stocks were used to prepare each lanthanide sample in all experiments. AAIII stock solutions were prepared by weight using material obtained from Alfa Aesar. Stock solutions of

EDTA were prepared by weight using 99.9% disodium EDTA salt from J.T. Baker. Stock solutions of DTPA and HEDTA were prepared by weight from the solid acid form and were

19 dissolved in two mole equivalents of sodium hydroxide to help with dissolution. The DTPA and

HEDTA solids were obtained from Alfa Aesar and Fluka Chemika respectively. Both were assayed at greater than 98%. Sodium lactate stock solutions were prepared by mass using 60% by mass sodium DL-lactate from Alfa Aesar. Solutions were acidified using stock solutions of perchloric acid prepared with 70% HClO4 from Fisher Scientific, and were standardized with

NaOH by indicator titration. NaOH solutions were prepared by mass using 50% NaOH from

Sigma Aldrich and standardized by KHP titration. The ionic strength of all solutions was adjusted to 2 M using a re-crystallized NaClO4 solution prepared in house.

Procedure

The kinetics of the reaction of the lanthanides with PAPC chelating agents in lactate media were investigated by stopped-flow spectrophotometry. Samples were thermally equilibrated for at least five minutes in the thermostated water bath of the stopped-flow unit prior to mixing. All kinetic experiments were performed using an OLIS U.S.A. stopped-flow system with a 20 mm optical cell paired with an OLIS RSM-1000 scanning spectrophotometer, except for experiments where the final DTPA concentration after mixing was greater than 6 mM. In this case, the stopped-flow system was paired with an OLIS Cary-14 spectrophotometer. The RSM-

1000 monitors a 220 nm wide window of wavelengths at speeds up to 1000 scans per second, and in all experiments this window was centered at 660 nm. The Cary-14 monitors a single wavelength and therefore can obtain a large number of data points for that wavelength in a short period of time. As such the Cary-14 was employed in experiments where the reaction reached completion in under 0.1 seconds. The Cary-14 was set to monitor at 660 nm in all experiments.

Kinetic data from the RSM-1000 was analyzed using the OLIS RSM Robust Global Fitting software package and observed rate constants were fit using first-order parameters. Kinetic data

20 from the Cary-14 was exported to Excel and observed rate constants were then fit in Origin 8 using a first-order decay model.

Trans-lanthanide Study

The complexation reaction of the ligands DTPA, EDTA, and HEDTA in high concentration lactate media was investigated with the lanthanides Pr3+, Nd3+, Sm3+-Lu3+ at

25.0 °C. In all cases the samples were made in solutions of 1.0 M NaLac, with the total ionic strength of the solution adjusted to 2.0 M with NaClO4. The lanthanide series was investigated under these conditions with each of the ligands at three different total added acid contents of 0.9,

0.85, and 0.8 M HClO4. At equilibrium these solutions have pH values of approximately 2.6, 2.8, and 3.0 respectively. The lanthanide solutions were made at 0.1 mM in all cases. The lanthanides were found to have a decreasing interaction with AAIII going across the series, thus it was necessary to increase the concentration of AAIII with heavier lanthanides. The AAIII concentration was 0.025 mM for Pr3+-Gd3+, 0.05 mM for Tb3+-Ho3+, 0.1 mM for Er3+-Yb3+ and

0.2 mM for Lu3+. The concentrations of DTPA and EDTA were made up to 4, 6, 8, 10 and 12 mM for reaction with the lanthanides. As the HEDTA reactions were found to be considerably slower, solutions were made at concentrations of 10, 20, 30, 40, and 50 mM in order to extrapolate a trend out of the change in the observed rate constants. For comparison, experiments with Eu3+ and higher concentrations of DTPA and EDTA were performed at 20, 30, 40, 50, and

60 mM at pH values of 2.6, 2.8, 3.0 with DTPA and pH 2.6 with EDTA. In all experiments equal volumes of the reacting solutions were mixed separately for pH measurement. The pH values were determined via pH probe calibrated with 0.01 and 0.001 M HClO4 standards made up in

2.0 M NaClO4.

Temperature Studies

In order to obtain activation parameters associated with the complex formation reaction, temperature studies were performed with the lanthanides Nd3+, Eu3+, Tb3+, Ho3+, Tm3+, and Lu3+ in 1.0 M NaLac, pH 2.6, and 2 M ionic strength with each of the ligands investigated at the same concentrations used in the trans-lanthanide study. As the rates were found to be significantly different between the different ligands and across the lanthanide series different temperature ranges were required for several of the different systems. In the DTPA temperature studies of

Nd3+ were run at 17.0, 20.0, 23.0 and 25.0 °C; Eu3+ at 10.0, 15.0, 20.0 and 25.0 °C; Tb3+ at 17.0,

20.0, 23.0, and 25.0 °C; Ho3+ at 20.0, 23.0, 25.0, 30.0, and 33.0 °C; Tm at 25.0, 27.0, 30.0, and

33.0 °C; Lu3+ at 25.0, 30.0, 35.0 and 40.0 °C. In the EDTA system, temperature studies of Nd3+ were run at 20.0, 25.0, 27.0, and 30.0 °C; Eu3+ and Tb3+ were run at 15.0, 20.0, 25.0, and

30.0 °C; Ho3+, Tm3+, and Lu3+ were run at 25.0, 27.0, 30.0, and 33.0 °C. In the HEDTA temperature studies the same range was used for all the lanthanides investigated at 25.0, 27.0,

30.0, and 33.0 °C. Reported temperatures have uncertainty of ±0.1 °C. In all cases the second- order rate constants were used in linear least squares fit Arrhenius plots and treated using

Activated Complex Theory (ACT) to obtain the activation parameters of the complex formation reaction.

Total Lactate Experiments

Often in kinetic experiments the role of one species in the reaction will be determined by maintaining constant concentrations of all the other reagents and varying the concentration of the species of interest. However in the high lactate system this is not possible as the concentration of lactic acid, lactate ion and hydrogen ion are all coupled. Since the concentration of one of these

22 species cannot be changed without changing the concentration of the others, a series of experiments were designed to determine the role of each of these species. Experiments examining the complexation kinetics between Eu3+ and DTPA were done under three different total lactate conditions of 1.0, 0.9, and 0.8 M. Each of these [Lac]tot systems were adjusted to have either the same lactate ion concentration or the same pH. This was done by calibrating a pH electrode under 2.0 M ionic strength conditions via a Gran titration. The results of the electrode calibration where then used to calculate the requisite mV reading for the desired pH in each system. Solutions were prepared and then adjusted with HClO4 or NaOH to within 1 mV of the calculated value. Solutions of 0.1 mM Eu3+ 0.025mM AAIII were mixed with DTPA concentrations of 4, 6, 8, 10, 12, 20, 30, 40, 50, and 60 mM under each [Lac]tot system adjusted to [Lac-] of 0.1, 0.2, and 0.3 M and the same pH of 2.72, 3.07, and 3.30.

AAIII Independence

In this series of experiments the kinetics of complexation between the lanthanides and

PAPC ligands is being measured by the perturbation of the equilibrium of the Ln-AAIII complex. The complexation reaction between hydrated lanthanides and AAIII has been reported as too fast to monitor by stopped-flow techniques at 25 °C.28 Therefore these experiments were operated with the assumption that the reaction mechanism was independent of the concentration of AAIII. However it is still prudent to test the system for a dependence on AAIII. Figure 2.3 shows the complexation of 0.05 mM Gd with EDTA in 1 M NaLac at 0.9 M HClO4 at two concentrations of AAIII.

It is seen in figure 2.3 that doubling the AAIII concentration has no effect on the kf value

-1 -1 (~2800 M s ), and the difference in the kd value can be attributed to the slight difference in the

23 pH as an acid dependant dissociation pathway for the lanthanides and PAPC ligands has been observed previously.29 These observations suggest that AAIII is not involved in the complexation reaction in these systems and therefore allows for the perturbation of the Ln-AAIII equilibrium as a method for monitoring the forward reaction of the lanthanides with the PAPC ligands.

) 12

-1

(s 10 obs k 8

0 0.000 0.001 0.002 0.003 0.004 0.005 0.006 EDTA (M)

Figure 2.3: Complexation of 0.05 mM Gd and EDTA 1.0 M NaLac 0.9M HClO4 2 M ionic strength 25 °C with [AAIII] (■) 0.0125 mM and (●) 0.025 mM at pH 2.7 and 2.8 respectively.

References

1. Nash, K. L.; Choppin, G. R., Sullivan, J. C. A Kinetic Study of Americium(III) and trans- 1,2 Diaminocyclohexane Tetraacetate. Inorg. Chem.1978, 17, 3374.

2. Nyssen, G. A.; Margerum, D. W. Multidentate Ligand Kinetics. XIV. Formation andDissociation Kinetics of Rare Earth-Cyclohexylenediaminetetraacetate Complexes. Inorg.Chem. 1970, 9, 1814.

3. Ryhl, T. Kinetics studies of lanthanoid carboxylate complexes. I. The dissociation rates of Ytterbium, Lanthanum and Copper EDTA complexes. Acta Chem. Scand. 1972, 26, 3955.

4. Siddall, T. H.; Stewart, W. E. Proton magnetic resonance studies of ethylenediaminetetraacetate complexes of lanthanides. Inorg. Nucl. Chem. Letters 1969, 5, 421.

5. Ryhl, T. Kinetics studies of lanthanoid carboxylate complexes. II. PMR investigation of the lanthanum and lutetium EDTA complexes. Acta Chem. Scand. 1972, 26, 4001.

6. Alsaadi, B. M.; Rossotti, F. J. C.; Williams, R. J. P. Hydration of complexon complexes of lanthanide cations. J. Chem Soc. Dalton Trans. 1980, 2151.

7. Gennaro, M. C.; Mirti, P., Casalino, C. NMR study of intramolecular processes in EDTA metalcomplexes. Polyhedron 1983, 2, 13.

8. Ryhl, T. Kinetics studies of lanthanoid carboxylate complexes. III. The dissociation rates of Praseodymium, Neodymium, Europium, and Erbium EDTA complexes. Acta Chem. Scand. 1973 27, 303.

9. Choppin, G. R.; Williams, K. R. Kinetics of Exchange Between Americium (III) and Europium Ethylenediaminetetraacetate. J. Inorg. Nucl. Chem. 1973, 35(12), 4255.

10. Williams K. R.; Choppin, G. R. J. Kinetics of exchange of trivalent actinide ions with europium ethylenediaminetetraacetate. Inorg. Nucl. Chem. 1974, 36, 1849.

11. Muscatello, A. C.; Choppin, G. R.; D'Olieslager, W. Kinetics of dissociation of trivalent actinide chelates of trimethylenediamine-N,N,N',N'-tetraacetic acid [TMDTA]. Inorg. Chem. 1989, 28(6), 993.

12. Glentworth, P.; Wiseall, B.; Wright, C. L.; Mahmood, A. J. A Kinetic Study of Isotopic Exchange Reactions Between Lanthanide Ions and Lanthanide Polyaminopolycarboxylic Acid Complex Ions. I. Isotopic Exchange Reactions of Ce(III) with Ce(HEDTA), Ce(EDTA)-, Ce(DCTA)-, and Ce(DTPA)2-. J. Inorg. Nucl. Chem. 1968, 30(4), 967.

13. Hines, M. A.; Sullivan, J. C.; Nash, K. L. Study of the Complexation Kinetics of Dioxouranium(VI) with Selected Diphosphonic Acids in Acidic Solutions. Inorg. Chem. 1993, 32, 1820.

14. Fugate, G. A.; Nash, K. L.; Sullivan, J. C. Kinetic Study of the Reactions of Np(V) and U(VI) with Diphosphonic Acids in Acetate Buffer Solutions. Radiochim. Acta 1997 79, 161.

15. Friese, J. I.; Nash, K. L.; Jensen, M. P.; Sullivan, J. C. Kinetic Study of the Reactions of Np(V) and U(VI) with Oxydiacetic Acid. Radiochim. Acta 1998, 83, 175.

16. Friese, J. I.; Nash, K. L.; Jensen, M. P.; Sullivan, J. C. Interaction of Np(V) and U(VI) with Dipicolinic Acid. Radiochim. Acta 2001, 89, 35

17. Hall, H.; Sullivan, J.C.; Rickert, P.G.; Nash, K.L. Kinetics of the Reaction of U(VI ) With Benzene-1,2-Diphosphonic Acid. Dalton Trans. 2005, 2011.

18. Rowatt, E.; Williams, R. The Interaction of Cations with the Dye Arsenazo III. Biochem.J. 1989, 259, 295.

19. Rohwer, H.; Collier, N; Hosten, E. Spectrophotometric study of arsenazo III and its interactions with lanthanides. Anal. Chim. Acta 1995, 314, 219.

20. Rohwer, H.; Hosten, E. pH dependence of the reactions of arsenazo III with the lanthanides. Anal. Chim. Acta 1997, 339, 271.

21. Hosten, E.; Rohwer, H. Interaction of anions with arsenazo III-lanthanide (III) complexes. 1997, 345, 227.

22. Lu, Y. W.; Laurent, G.; Pereira, H. A novel methodology for evaluation of formation constants of complexes: example of lanthanide-Arsenazo III complexes. Talanta 2004, 62, 959.

23. Pippin, C. G.; Sullivan, J. C.; Wester, D. W. A kinetic study of the reactions between tetravalent actinide ions and Arsenazo(III). Radiochim. Acta 1984, 37, 99.

24. Pippin, C. G.; Sullivan, J. C. A kinetic study of the complexation reaction between the dioxouranium(VI) ion and Arsenazo III. Radiochim. Acta 1989, 48(1-2), 37.

25. Feil Jenkins, J. F.; Sullivan, J. C.; Nash, K. L. Kinetics of the Complexation of Dioxouranium(VI) with Chlorophosphonazo III. Radiochim. Acta 1995, 68, 209.

26. Feil Jenkins, J.; Nash, K. L.; Sullivan, J. C. Kinetics of the Complexation Reactions Between Th(IV), Zr(IV) and Chlorophosphonazo III. Radiochim. Acta 1995, 68, 215.

27. Fugate, G.; Feil-Jenkins, J. F.; Sullivan, J. C.; Nash, K. L. Actinide Complexation Kinetics: Rate and Mechanism of Dioxoneptunium(V) Reaction with Chlorophosphonazo III. Radiochim. Acta 1996, 73, 67.

28. Shi, Y; Eyring, E; van Eldik, R. Kinetic analysis of the complexation of aqueous lanthanides(III) ions by arsenazo III. Dalton Trans. 1998, 6, 967.

29. Choppin, G. Complexation kinetics of f-element and polydentate ligands. Journal of Alloys and Compounds 1995, 225, 242.

Chapter 3

Experimental Results

As mentioned in the previous chapter, the rates of complex formation and dissociation of f-elements and PAPC ligands have been examined using a variety of techniques.1-12 The studies investigating Ln-PAPC reactions looked at complex dissociation rates as the formation reactions were too fast to monitor. Efforts to calculate formation rate constants between lanthanides and

EDTA from dissociation rate data by Laurenczy and Brücher13 found rate constants on the order of 106 M-1s-1 and 108 M-1s-1 for diprotonated and monoprotonated EDTA respectively. They determined a formation rate constant for monoprotaonated EDTA with Gd3+ of 3.0 x 108 M-1s-1 which is comparable to, but slightly slower than, the reported value for Gd3+ water exchange at

10.6 x 108 s-1.14 Additionally, values for the rate constants of complexation between the lanthanides and DCTA, calculated by Nyssen and Margerum, are found to be rapid despite the conformational hindrance of the cyclohexyl-backbone, with values on the order of 107 M-1s-1.2

These consistently rapid rates of reaction suggest that the complexaiton of lanthanides with

PAPC ligands takes place at or near the diffusion limit under normal aqueous conditions. It is notable then that in the experimental results to follow, the complexation of the lanthanides with the ligands DTPA, EDTA and HEDTA are directly measured. Being able to directly monitor these reactions speaks to the considerable effect that the high concentration of lactate has on these systems. Reactions that are normally on a time scale accessible only by NMR are slowed to the point of being able to be monitored by stopped-flow spectrophotometry. The experiments presented here aim to elucidate the significant effect the lactate has on these systems. Errors associated with all reported values correspond to a 95% confidence interval.

DTPA

Lanthanide Series Study

The Eu-DTPA complexation results shown in figure 3.1 are representative for the

lanthanide series. Table 3.1 shows the second order rate constant of complex formation and table

3.2 shows the first order rate constant of complex dissociation between the lanthanides and

DTPA in 1.0 M NaLac for the three acidities studied at 2.0 M ionic strength and 25.0 °C. It is

seen that across the lanthanides studied there is a trend of decreasing rate constants for both

formation and dissociation with decreasing acidity. This pattern is strictly adhered to for the kd

values, while the kf values mainly follow the trend. For lanthanides where the trend in kf values is

50 )

-1 40

(s obs

k 30

0 0.000 0.001 0.002 0.003 0.004 0.005 0.006 DTPA (M)

Figure 3.1: Observed rate constants of the reaction between Eu3+ and DTPA in 1.0 M NaLac 0.9 (■), 0.85 (●), and 0.8 () M HClO4 2 M ionic strength at 25 °C.

-1 -1 kf (M s )

Ln 0.9 M HClO4 0.85 M HClO4 0.8 M HClO4 Pr 6900 (114) 7170 (370) 6880 (150) Nd 7020 (145) 7200 (10) 6730 (200) Sm 10410 (190) 10070 (270) 9080 (110) Eu 11310 (90) 10430 (80) 9790 (90) Gd 9620 (20) 9540 (110) 8520 (220) Tb 7370 (50) 6670 (190) 5810 (60) Dy 5300 (80) 5410 (40) 3910 (50) Ho 2670 (20) 2780 (20) 2590 (20) Er 1820 (60) 1530 (20) 1080 (20) Tm 1420 (40) 980 (30) 670 (100) Yb 950 (30) 670 (30) 530 (30) Lu 470 (10) 500 (10) 560 (20) Table 3.1: Lanthanide-DTPA second order rate constant of complex

formation. 1.0 M NaLac variable HClO4 2.0 M NaClO4 ionic strength 25 °C.

-1 kd (s )

Ln 0.9 M HClO4 0.85 M HClO4 0.8 M HClO4 Pr 21.0 (0.6) 16.7 (1.1) 15.1 (0.7) Nd 11.0 (0.3) 9.4 (0.1) 8.6 (0.6) Sm 7.4 (0.8) 5.3 (0.8) 4.5 (0.2) Eu 5.4 (0.2) 3.4 (0.4) 2.1 (0.4) Gd 5.0 (0.1) 1.4 (0.2) 0.8 (0.5) Tb 4.6 (0.1) 2.2 (0.5) 1.3 (0.2) Dy 3.1 (0.3) 1.6 (0.1) 0.6 (0.3) Ho 2.7 (0.1) 1.2 (0.1) -0.1 (0.1) Er 2.8 (0.1) 1.4 (0.1) 1.2 (0.1) Tm 2.7 (0.2) 1.4 (0.1) 1.0 (0.4) Yb 2.8 (0.1) 1.6 (0.1) 0.8 (0.1) Lu 3.3 (0.1) 1.5 (0.1) 0.1 (0.1) Table 3.2: Lanthanide-DTPA first order rate constant of complex dissociation.

1.0 M NaLac variable HClO4 2.0 M NaClO4 ionic strength 25 °C. not followed, the kf values at the different acidities are close to one another i.e. Dy at 0.9 M

-1 -1 -1 -1 HClO4 and 0.85 M HClO4 with kf values of 5300 ± 80 M s and 5410 ± 80 M s respectively.

Ln-DTPA complex formation and dissociation rate constants are plotted against the inverse ionic radii of the lanthanides in figures 3.2 and 3.3, respectively. It is seen in figure 3.2 that the second order rate constant of complex formation is essentially constant for Pr3+ and Nd3+

3+ 3+ at all acidities, followed by a large increase in kf at Sm which peaks at Eu . This sudden increase in the rate of complex formation may be attributed at least in part to the change in the

12000

10000

8000

)

-1

s -1

6000

f k 4000

2000

0 0.84 0.86 0.88 0.90 0.92 0.94 0.96 1/r (CN=9, Angstoms-1)

Figure 3.2: Ln-DTPA kf values across the Ln series (Pr-Lu) in 1 M NaLac, 0.9 (■), 0.85 (●), and 0.8 () M

HClO4, 2 M ionic strength at 25 °C. coordination environment of the lanthanides that occurs midway through the series due to the decreasing ionic radii.15 Sm3+ and Eu3+ are in equilibrium between species of coordination number (CN) of 9 and 8, and those ions with a CN of 9 will more easily lose a coordinated water allowing DTPA to more readily bind with the metal. The steady and significant decrease in kf that occurs after Eu3+ would not be expected considering the decrease in the ionic radii that

)

-1

(s d

k 10

0 0.84 0.86 0.88 0.90 0.92 0.94 0.96 1/r (CN=9, Angstroms-1)

Figure 3.3: Ln-DTPA kd values across the Ln series (Pr-Lu) in 1 M NaLac, 0.9 (■), 0.85 (●), and 0.8 () M

HClO4, 2 M ionic strength at 25 °C. occurs across the lanthanide series, which leads to an increase in charge density. With an increase in charge density it is reasonable to expect an increase in the interaction between the trivalent lanthanide ion and the negatively charged carboxylate groups on the DTPA leading to faster coordination. This behavior could be expected from the interaction between DTPA and hydrated lanthanide ions, however given the high concentration of lactate in the system, starting from the hydrated lanthanide ions may not be appropriate. Taking the lanthanide-lactate species into consideration for their role in the kinetics will be shown to have a significant effect. This argument will be developed in great detail in the discussion chapter.

In contrast to the behavior of the kf values, the trend of decreasing kd values that is observed going across the lanthanide series is behavior that would be expected with the decrease in ionic radii. The resulting increase in charge density leads to stronger interaction with the binding groups of DTPA thus the lanthanide is less likely to dissociate. Additionally, the observed trend of decreasing kd with decreasing acidity would be expected. An acid catalyzed pathway has been observed regularly in the dissociation of the lanthanides and many PAPC ligands, thus in systems with lower hydrogen ion concentrations slower rates of dissociation are should be expected.16

Temperature Study

To calculate activation parameters for the complexation reaction between DTPA and the lanthanides Nd3+, Eu3+, Tb3+, Ho3+, Tm3+, and Lu3+ temperature studies were completed. As the rates of complexation for these lanthanides cover a wide margin, different temperature ranges were required as described in the experimental chapter. Figure 3.4 shows the reaction of Eu3+ with DTPA at the temperatures of 25.0, 20.0, 15.0, and 10.0 °C and is representative for the lanthanides studied. Table 3.3 lists the activation parameters for the Ln-DTPA complex formation reaction in 1.0 M NaLac and 0.9 M HClO4 at 2 M ionic strength. It is seen for all the lanthanides studied that the activation entropy (ΔS‡) is highly positive, which is indicative of a dissociative reaction mechanism. Surprisingly, the activation enthalpy (ΔH‡) for these reactions are all very similar and do not follow the expected trend of lower activation energies for faster rates of reaction. This surprising result is well illustrated by comparing the activation enthalpies and the rate constants of complex formation at 25 °C of Eu3+ and Lu3+ which are ΔH‡ = 96 ± 3

-1 -1 ‡ -1 -1 kJ/mol, kf = 11300 ± 90 M s and ΔH = 107 ± 4 kJ/mol, kf = 470 ± 10 M s respectively. The rate of complexation for Lu3+ is 24 times slower than that of Eu3+ however the activation

33 parameters are comparatively close to one another, as is the case for all the lanthanides studied.

The similar activation parameters across the lanthanide series may be suggestive of a very similar mechanism for complex formation between the different metals, despite the significant difference in the rates of reaction across the series.

60 )

-1 40

obs k

0 0.000 0.001 0.002 0.003 0.004 0.005 0.006 DTPA (M)

3+ Figure 3.4: Temperature dependence of the reaction between Eu and DTPA 1 M NaLac 0.9 M HClO4 2 M ionic strength at 25 °C (■), 20 °C (●), 15 °C () and 10 °C ().

E ΔG‡ ΔH‡ ΔS‡ k a ln(A) f Ln kJ/mol kJ/mol kJ/mol J/mol*K 25 °C M-1s-1 Nd 97 (4) 48 (2) 52 (5) 95 (4) 146 (13) 7020 (145)

Eu 99 (3) 49 (1) 51 (4) 96 (3) 156 (10) 11300 (90)

Tb 100 (2) 49 (1) 51 (3) 97 (2) 155 (8) 7370 (50) Ho 90 (2) 44 (1) 53 (7) 88 (2) 117 (5) 2870 (20) Tm 83 (6) 42 (2) 55 (9) 80 (6) 85 (21) 1420 (40) Lu 109 (4) 50 (2) 56 (6) 107 (4) 164 (15) 470 (10) Table 3.3: Activation parameters for the Ln-DTPA complexation reaction

in 1.0 M NaLac 0.9 M HClO4 and 2 M ionic strength. Additionally the second order rate constant of complex formation at 25 °C is listed.

EDTA

Lanthanide Series Study

The Eu-EDTA complexation results shown in figure 3.5 are representative for the lanthanide series. Table 3.4 shows the second order rate constant of complex formation and table

3.5 shows the first order rate constant of complex dissociation between the lanthanides and

EDTA in 1.0 M NaLac for the three acidities studied at 2.0 M ionic strength and 25.0 °C. As with DTPA, it is seen that across the lanthanides studied there is a trend of decreasing rate constants for both formation and dissociation with decreasing acidity. This pattern is strictly

3+ adhered to for the kf values, except for at Ho where at 0.85 and 0.8 M HClO4 the kf values are within error of each other. The trend with acidity is followed in the kd values for the early and mid lanthanides, which indicates an acid catalyzed dissociation pathway as has been observed

) 12

-1

(s 10 obs k 8

0 0.000 0.001 0.002 0.003 0.004 0.005 0.006 EDTA (M)

Figure 3.5: Observed rate constants of the reaction between Eu3+ and EDTA in 1.0 M NaLac and 0.9 (■), 0.85 (●), and 0.8 () M HClO4 2 M ionic strength at 25 °C.

previously in PAPC ligands.16 At the heavy lanthanides the rate of dissociation becomes very

slow and the values at all acidities are within error of each other. While this may indicate the

acid dependency falls off at the end of the series care must be taken in interpreting these results

as some of the values are not statistically different from zero. The dissociation rates with the

heaviest lanthanides may be too slow to reliably measure with this extrapolative process.

Ln-EDTA complex formation and dissociation rate constants are plotted against the

inverse ionic radii of the lanthanides in figures 3.6 and 3.7, respectively. EDTA and DTPA

demonstrate similar behavior as it is seen in figure 3.6 that the second-order rate constant of

Ln 0.9 M HClO4 0.85 M HClO4 0.8 M HClO4 Pr 1350 (20) 1250 (20) 1080 (20) Nd 1310 (30) 1180 (50) 1040 (10) Sm 2340 (50) 1910 (20) 1720 (20) Eu 2650 (10) 2310 (30) 2080 (30) Gd 2760 (50) 2410 (120) 2060 (40) Tb 2830 (50) 2220 (70) 2090 (30) Dy 2390 (60) 2060 (30) 1620 (50) Ho 1810 (20) 1010 (10) 1040 (20) Er 1160 (20) 880 (20) 670 (10) Tm 840 (50) 610 (10) 410 (10) Yb 500 (10) 320 (10) 290 (10) Lu 280 (10) 200 (10) 160 (10) Table 3.4: Lanthanide-EDTA second order rate constant of complex

formation. 1.0 M NaLac variable HClO4 2.0 M NaClO4 ionic strength 25 °C.

Ln 0.9 M HClO4 0.85 M HClO4 0.8 M HClO4 Pr 7.7 (0.1) 5.6 (0.1) 4.3 (0.1) Nd 5.6 (0.2) 4.3 (0.1) 3.4 (0.1) Sm 4.2 (0.1) 3.4 (0.1) 2.9 (0.1) Eu 4.6 (0.1) 3.3 (0.1) 2.8 (0.1) Gd 4.3 (0.3) 3.1 (0.4) 2.3 (0.1) Tb 3.3 (0.1) 3.2 (0.2) 1.9 (0.1) Dy 2.3 (0.2) 1.4 (0.1) 1.6 (0.2) Ho 1.0 (0.1) 0.8 (0.1) 1.0 (0.1) Er 0.5 (0.1) 0.5 (0.1) 0.4 (0.1) Tm 0.4 (0.2) 0.2 (0.1) 0.3 (0.1) Yb 0.2 (0.1) 0.3 (0.1) 0.1 (0.1) Lu 0.2 (0.1) 0.2 (0.1) 0.1 (0.1) Table 3.5: Lanthanide-EDTA first order rate constant of complex dissociation.

1.0 M NaLac variable HClO4 2.0 M NaClO4 ionic strength 25 °C.

3+ 3+ complex formation is constant for Pr and Nd between the different acidities. An increase in kf is observed at all acidities beginning at Sm3+ which levels out at Eu3+ and the values do not change significantly until Dy3+ followed by a steady decrease for the rest of the series. The

3000

2500

2000

)

-1

s -1

1500

f k 1000

500

0 0.84 0.86 0.88 0.90 0.92 0.94 0.96 1/r (CN=9, Angstroms-1)

Figure 3.6: Ln-EDTA kf values across the Ln series (Pr-Lu) in 1 M NaLac, 0.9 (■), 0.85 (●), and 0.8 () M

HClO4, 2 M ionic strength at 25 °C.

3+ 3+ increase in kf that occurs between Nd and Eu can partially be attributed to the change in the coordination environment that occurs as the lanthanide ionic radius decreases across the series as

15 was seen for DTPA. Also like DTPA, the decrease in kf that occurs for the heavy lanthanides would not be expected considering the increase in charge density that occurs across the lanthanide series, which should increase the strength of the interaction between the metal ion and the carboxylate groups of EDTA. This is again indicative of more complex behavior in the lactate system than would be expected from the interaction between the ligand and hydrated lanthanide ions.

5 )

-1 4

(s d k 3

0.84 0.86 0.88 0.90 0.92 0.94 0.96 1/r (CN=9, Angstroms-1)

Figure 3.7: Ln-EDTA kd values across the Ln series (Pr-Lu) in 1 M NaLac, 0.9 (■), 0.85 (●), and 0.8 () M

HClO4, 2 M ionic strength at 25 °C.

Temperature Study

Temperature studies were completed on the reaction between EDTA and the lanthanides

Nd3+, Eu3+, Tb3+, Ho3+, Tm3+, and Lu3+. Figure 3.7 shows the reaction of Eu3+ with EDTA at the temperatures of 30.0, 25.0, 20.0, and 15.0 °C and is representative for the lanthanides studied.

Table 3.6 lists the activation parameters for the Ln-EDTA complex formation reaction in 1.0 M

NaLac and 0.9 M HClO4 at 2.0 M ionic strength. As with DTPA, a dissociative reaction mechanism is indicated in all the lanthanides studied based on the highly positive ΔS‡ values.

Additionally both the ΔS‡ and ΔH‡ values for the different lanthanides are identical within the limits of uncertainty. As the kf values across the lanthanide series change considerably the similarity of the resulting activation parameters does not readily provide an explanation for the disparate values of the complex formation rate constants. There is an order of magnitude difference between the complex formation second order rate constant of Tb3+ and Lu3+, however the activation enthalpy values are 95 ± 3 kJ/mol and 96 ± 4 kJ/mol and the activation entropy values are 138 ± 10 J/(mol*K) and 134 ± 12 J/(mol*K) respectively. While these results do not explain the change in the kf values across the series, the similar activation parameters may suggest that the lanthanides complex to EDTA via a similar mechanism.

)

-1

(s 20

obs k 15

0 0.000 0.001 0.002 0.003 0.004 0.005 0.006 EDTA (M)

3+ Figure 3.8: Temperature dependence of the reaction between Eu and EDTA 1 M NaLac 0.9 M HClO4 2 M ionic strength at 30 °C (■), 25 °C (●), 20 °C () and 15 °C ().

E ΔG‡ ΔH‡ ΔS‡ k a ln(A) f Ln kJ/mol kJ/mol kJ/mol J/mol*K 25 °C M-1s-1 Nd 102 (11) 48 (4) 55 (15) 99 (11) 148 (35) 1310 (33) Eu 94 (2) 46 (1) 54 (3) 92 (2) 128 (7) 2650 (10) Tb 97 (3) 47 (1) 54 (4) 95 (3) 138 (10) 2830 (50) Ho 99 (2) 48 (1) 54 (3) 97 (2) 142 (7) 1810 (20) Tm 99 (4) 47 (1) 56 (5) 96 (4) 134 (12) 840 (50) Lu 98 (4) 46 (2) 56 (5) 96 (4) 126 (12) 280 (10) Table 3.6: Activation parameters for the Ln-EDTA complexation reaction in

1.0 M NaLac 0.9 M HClO4 and 2 M ionic strength. Additionally the second order rate constant of complex formation at 25 °C is listed.

HEDTA

Lanthanide Series Study

The Eu-HEDTA complexation results shown in figure 3.9 are representative for the lanthanide series. It is notable that the HEDTA concentrations used in these studies are considerably higher than those for the DTPA and EDTA systems. Under the experimental conditions the lanthanide complexation reaction with HEDTA is considerably slower and a larger range in the ligand concentration was required in order to see a substantial enough change in kobs to obtain adequate precision in the values of kf and kd.

5.0

4.5

4.0

3.5

3.0

) -1

2.5

(s obs

k 2.0

1.5

1.0

0.5

0.0 0.000 0.005 0.010 0.015 0.020 0.025 HEDTA (M)

Figure 3.9: Observed rate constants of the reaction between Eu3+ and HEDTA in 1.0 M NaLac and 0.9 (■), 0.85 (●), and 0.8 () M HClO4 2 M ionic strength at 25 °C.

Table 3.7 shows the second order rate constant for complex formation and table 3.8

shows the first order rate constant of complex dissociation between the lanthanides and HEDTA

in 1.0 M NaLac for the three acidities studied at 2.0 M ionic strength and 25.0 °C. In contrast to

the other two ligands studied, the kf values do not exhibit a trend with acidity. While the errors

on these values are fairly small in some cases, the scattering of similar values between the

different acidities for all the lanthanides suggests the complex formation reaction is independent

of acidity in the range investigated. The kd values display similar behavior except at the

3+ 3+ lanthanides Eu through Ho where the kd value for 0.9 M HClO4 are higher than for the two

lower acidities, the values of which fall within error of each other. This suggests that an acid

catalyzed dissociation pathway becomes important for the mid lanthanides at higher acidities (pH

< 2.6), that becomes less significant at lower acidity and at the heaviest of the lanthanides. The

Ln 0.9 M HClO4 0.85 M HClO4 0.8 M HClO4 Pr 130 (8) 140 (6) 140 (8) Nd 120 (5) 110 (10) 120 (10) Sm 140 (3) 135 (1) 140 (1) Eu 130 (1) 135 (2) 140 (1) Gd 110 (2) 115 (1) 120 (2) Tb 110 (1) 105 (3) 105 (1) Dy 95 (1) 90 (1) 85 (2) Ho 70 (1) 80 (4) 65 (2) Er 65 (8) 60 (7) 65 (3) Tm 55 (11) 50 (3) 60 (3) Yb 55 (1) 45 (5) 40 (1) Lu 35 (7) 45 (5) 45 (8) Table 3.7: Lanthanide-HEDTA second order rate constant of complex

formation. 1.0 M NaLac variable HClO4 2.0 M NaClO4 ionic strength 25 °C.

Ln 0.9 M HClO4 0.85 M HClO4 0.8 M HClO4 Pr 5.4 (0.2) 5.3 (0.1) 5.0 (0.1) Nd 3.4 (0.1) 3.5 (0.2) 3.4 (0.2) Sm 1.7 (0.1) 1.7 (0.1) 1.7 (0.1) Eu 1.8 (0.1) 1.4 (0.1) 1.4 (0.1) Gd 1.8 (0.1) 1.4 (0.1) 1.3 (0.1) Tb 2.1 (0.1) 1.7 (0.1) 1.6 (0.1) Dy 2.4 (0.1) 2.0 (0.1) 1.8 (0.1) Ho 3.0 (0.1) 2.1 (0.1) 2.2 (0.1) Er 2.5 (0.1) 2.4 (0.1) 2.5 (0.1) Tm 2.5 (0.1) 2.5 (0.1) 2.4 (0.1) Yb 1.4 (0.1) 1.5 (0.1) 1.5 (0.1) Lu 1.1 (0.1) 0.9 (0.1) 0.9 (0.2) Table 3.8: Lanthanide-HEDTA first order rate constant of complex

dissociation. 1.0 M NaLac variable HClO4 2.0 M NaClO4 ionic strength 25 °C.

lack of an apparent acid catalyzed dissociation pathway in this system is of interest, as it runs

counter to the observations made in the DTPA and EDTA systems, as well as what has been

reported in the literature.16

Ln-HEDTA complex formation and dissociation rate constants are plotted against the

inverse ionic radii of the lanthanides in figures 3.10 and 3.11, respectively. It is seen in figure

3.10 that the second order rate constant of complex formation stays essentially constant from

Pr3+ through Eu3+ and then steadily decreases through the rest of the series. The changing

coordination environment at the mid lanthanides does not appear to effect kf in the manner that

was seen for DTPA and EDTA. However the decrease in the rate of complex formation occurs at

the mid to late lanthanides is consistent with what was seen with the other ligands.

160

140

120

100

)

-1

s -1

f k 60

0 0.84 0.86 0.88 0.90 0.92 0.94 0.96 1/r (CN=9, Angstroms-1)

Figure 3.10: Ln-HEDTA kf values across the Ln series (Pr-Lu) in 1 M NaLac, 0.9 (■), 0.85 (●), and 0.8 () M HClO4, 2 M ionic strength at 25 °C.

4 )

-1 3

d k 2

0 0.84 0.86 0.88 0.90 0.92 0.94 0.96 1/r (CN=9,Angstroms-1)

Figure 3.11: Ln-HEDTA kd values across the Ln series (Pr-Lu) in 1 M NaLac, 0.9 (■), 0.85 (●), and 0.8 () M HClO4, 2 M ionic strength at 25 °C.

Temperature Study

Temperature studies were completed to calculate activation parameters for the

complexation reaction between HEDTA and the lanthanides Nd3+, Eu3+, Tb3+, Ho3+, Tm3+, and

Lu3+. Figure 3.12 shows the reaction of Eu3+ with HEDTA at the temperatures of 33.0, 30.0,

27.0, and 25.0 °C and is representative for the lanthanides studied. Table 3.9 lists the activation

parameters for the Ln-HEDTA complex formation reaction in 1.0 M NaLac and 0.9 M HClO4 at

2.0 M ionic strength. As seen with the other ligands studied, the highly positive ΔS‡ values are

indicative of a dissociative reaction mechanism. Although the rates of complex formation

between the lanthanides and HEDTA are considerably slower in the lactate system than DTPA or

)

-1

(s 6

obs k

0 0.000 0.005 0.010 0.015 0.020 0.025 HEDTA (M)

3+ Figure 3.12: Temperature dependence of the reaction between Eu and HEDTA 1 M NaLac 0.9 M HClO4 2 M ionic strength at 33 °C (■), 33 °C (●), 27 °C () and 25 °C (). EDTA the same condition of similar activation parameters for different rates is present. Of the lanthanides investigated in the temperature study Eu3+ has the fastest rate constant of

-1 -1 3+ -1 -1 complexation at 130 ± 1 M s and Lu has the slowest kf value at 35 ± 7 M s and yet their

ΔH‡ and ΔS‡ are identical within the limits of uncertainty at 99 ± 2 kJ/mol and 126 ± 5 J/(mol*K) for Eu3+ and 105 ± 4 kJ/mol and 135 ± 14 J/(mol*K) for Lu3+. The similar activation parameters again suggest a similar mechanism for complex formation between the lanthanides and HEDTA in the lactate system. However as with the other ligands studied, the similar activation parameters do not provide an explanation for the decreasing kf values going across the lanthanide series. As this condition is seen in all the ligands studied, possible explanations for this behavior will be developed further in the discussion chapter.

‡ ‡ ‡ k Ea ΔG ΔH ΔS f ln(A) -1 -1 Ln kJ/mol kJ/mol kJ/mol J/mol*K 25 °C M s Nd 89 (3) 41 (1) 61 (5) 87 (3) 86 (10) 120 (5) Eu 101 (2) 46 (1) 61 (2) 99 (2) 126 (5) 130 (1) Tb 95 (3) 43 (1) 61 (4) 93 (3) 105 (9) 110 (1) Ho 93 (9) 42 (4) 53 (7) 91 (9) 96 (30) 70 (1) Tm 90 (5) 41 (2) 62 (7) 88 (5) 85 (17) 55 (11) Lu 107 (4) 47 (2) 64 (6) 105 (4) 135 (14) 35 (7) Table 3.9: Activation parameters for the Ln-HEDTA complexation reaction

in 1.0 M NaLac 0.9 M HClO4 and 2 M ionic strength. Additionally the second order rate constant of complex formation at 25 °C is listed.

High Ligand Concentration Study

It was seen in the HEDTA system that high concentrations of ligand were required to obtain values for kf and kd. A large increase in the concentration of any component of a reaction can potentially lead to a significant change in the reaction mechanism. To compare the HEDTA results with those of DTPA and EDTA, it is prudent to examine the DTPA and EDTA systems under the condition of high concentrations of the PAPC ligand. Figure 3.13 compares the results

3+ of the Eu complexation with DTPA, EDTA, and HEDTA in 1.0 M NaLac 0.9 M HClO4 and 2

M ionic strength at 25 °C.

In figure 3.13 it is seen that EDTA behaves similarly at high and low concentrations, however the behavior of DTPA changes going from low to high concentration realms. The increase in kobs with increasing [DTPA] begins to plateau at [DTPA]t > 15 mM which suggests a saturation effect at high concentrations of DTPA, in effect the plateau indicates that the reaction is becoming independent of [DTPA]t in the high concentration limit.

160

140

120

100 )

-1 80

(s obs k 60

0 0.000 0.005 0.010 0.015 0.020 0.025 0.030 [PAPC]

Figure 3.13: Complexation of Eu3+ with (■) DTPA, (●) EDTA, and () HEDTA in 1.0 M NaLac, 0.9 M HClO4, and 2 M ionic strength at 25 °C. Lines added as a visual guide.

Figure 3.14 shows the complexation of Eu3+ with the full range of DTPA concentrations at all acidities studied. It can be seen in figure 3.14 that kobs levels off at high [DTPA] at all the acidites studied, beginning at 15 mM DTPA. It is also observed that decreasing the acidity of the system decreases the value at which kobs plateaus. In a simple saturation system, it could be expected to see kobs level off at the same value for all acidities if the effect was dependant on the concentration of DTPA alone, however the behavior observed here suggests a more complex system.

160

140

120

100 )

-1 80

(s obs k 60

0 0.000 0.005 0.010 0.015 0.020 0.025 0.030 DTPA (M)

Figure 3.14: Observed rate constants of the reaction between Eu3+ and DTPA in 1.0 M NaLac 0.9

(■), 0.85 (●), and 0.8 () M HClO4 2 M ionic strength at 25 °C. Lines added as a visual guide.

It is worth reiterating that the preceding results probed reactions that have previously been characterized as too fast to monitor with stopped-flow spectrophotometry. The addition of lactate to the system allows for the complexation reactions to be directly monitored, however it also complicates the system considerably. This higher degree of complexity has yielded unexpected results in the trans-lanthanide studies as well as in the temperature studies. In the following chapter these results will be analyzed in greater detail to in an attempt to better understand the roles each of the different species has in the complexation kinetics of these complex systems.