UNLV Retrospective Theses & Dissertations

1-1-2008

The effects of electromagnetic fields on the detection of mercury and radionuclides

Yogendra M Panta University of Nevada, Las Vegas

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This Dissertation has been accepted for inclusion in UNLV Retrospective Theses & Dissertations by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. THE EFFECTS OF ELECTROMAGNETIC FIELDS ON THE

DETECTION OF MERCURY AND RADIONUCLIDES

by

Yogendra M. Panta

Master of Science in Mechanical Engineering Youngstown State University, Youngstown, Ohio 2004

Bachelor of Engineering in Mechanical Engineering Tribhuvan University, Kathmandu, Nepal 2000

Bachelor of Science in Physical Science Tribhuvan University, Kathmandu, Nepal 1995

A dissertation submitted in partial fulfillment of the requirements for the

Doctor of Philosophy Degree in Mechanical Engineering Department of Mechanical Engineering Howard R. Hughes College of Engineering

Graduate College University of Nevada, Las Vegas August 2008 UMI Number: 3338521

Copyright 2008 by Panta, Yogendra M.

All rights reserved.

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ProQuest LLC 789 E. Elsenhower Parkway PC Box 1346 Ann Arbor, Ml 48106-1346 Copyright by Yogendra M. Panta 2008 Ali Rights Reserved Dissertation Approval IJNTV The Graduate College University of Nevada, Las Vegas

May 1 4 ,2008

The Dissertation prepared by

Yogendra M Panha

Entitled The Effects of Electromagnetic Fields on the Detection of

Mercury and Radionuclides

is approved in partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Mechanical Engineering

Examination Committee Chair

Dean of the Graduate College

Ext ym\Cornmittee Member \ w Examination Committee Member Examination Committee Member

Graduate College Faculty Representative

11 ABSTRACT

The Effects of Electromagnetic Fields on the Detection of Mercury and Radionuclides

by

Yogendra M. Panta

Dr. Shizhi Qian, Examination Committee Chair Assistant Professor of Mechanical Engineering University of Nevada, Las Vegas

Mercury and many radionuclides are extremely toxic, and their monitoring and detection are of importance for human health and environmental protection. This dissertation focuses on the detection of mercury (II) ions and radionuclides including uranium and plutonium (^^^Pu), and americium (^“^'Am) in aqueous solutions under the effects of electromagnetic fields. The stripping electrochemical method has been widely used to detect mercury (II) ions in aqueous solutions. To enhance the detection sensitivity of the conventional electrochemical method, mechanically stirring the solution is usually performed to enhance the rate of the ionic mass transport. Instead of mechanically stirring the solution; the electrochemical cell is positioned in a magnetic field provided by permanent magnets. The interaction between the current density transmitted through the solution and the magnetic field induces Lorentz force, thus magneto-hydrodynamic (MHD) flow, which can be used to stir and mix the sample solution uniformly without the use of any mechanical moving parts or rotating electrodes.

This research develops an ultra sensitive mercury sensor by integrating the MHD with

iii the electrochemical analyzer, and with the surface plasmon resonance (SPR) instrument, and its detection limit is obtained down to 1 ppb. To improve the resolution of the alpha spectrum during the detection of ^^^Pu, and ^""Am, hydrous oxides of the radionuclides were electrochemically deposited onto the surface of a steel planchet in the presence of a magnetic field. The steel planchet was then washed and dried, and the energy spectrum emitted was measured with the alpha spectrometer. A homogeneous and thin film with larger grain size was formed during the deposition step due to the induced

MHD flow leading to a higher resolution in the detection sensitivity and the alpha spectrum.

IV TABLE OF CONTENTS ABSTRACT...... iii

LIST OF ABBREVIATIONS...... viii

LIST OF SYMBOLS ...... ix

LIST OF FIGURES...... xi

ACKNOWLEDGEMENTS...... xvii

CHAPTER 1 INTRODUCTION...... 1 1.1 Motivation ...... 1 1.2 Heavy Metals ...... 2 1.3 Mercury ...... 3 1.3.1 History of Mercury Poisoning ...... 3 1.3.2 Effects of Mercury Poisoning ...... 4 1.3.3 Mercury Cycle ...... 5 1.4 Radiation ...... 7 1.4.1 Nuclear and Radiation Disasters ...... 7 1.4.2 Ionizing Radiation ...... 7 1.4.3 Effects of Radiation ...... 10 1.4.4 Radionuclides ...... 10 1.5 Organization of Dissertation ...... 11

CHAPTER 2 LITERATURE REVIEW...... 12 2.1 Toxicity of Mercury...... 12 2.2 Detection of Mercury under MHD Convection ...... 13 2.2.1 Electrochemical Detection of Mercury ...... 13 2.2.2 Electrochemical SPR Detection of Mercury ...... 16 2.3 Radiation Hazards to Human Health ...... 19 2.4 Detection of Radionuclides under MHD Convection ...... 20 2.4.1 Electrodeposition of Radionuclides ...... 20 2.4.2 Alpha Spectrometric Measurement of Radionuclides Sources ...... 24 CHAPTER 3 ELECTROCHEMICAL DETECTION OF MERCURY UNDER MHD CONVECTION...... 25 Summary ...... 25 3.1 Materials and Methods ...... 26 3.1.1 Materials...... 26 3.1.1.1 Chemicals and Reagents ...... 26 3.1.1.2 Electrodes ...... 27 3.1.1.3 Electrode Polishing ...... 27 3.1.1.4 Magnets for MHD Convection ...... 29 3.1.2 Methods ...... 32 3.2 Results and Discussion ...... 35 3.2.1 Reduction and Oxidation of Mercury ...... 35 3.2.2 Effects of MHD on Electrochemical Detection ...... 36 3.2.3 Optimization of Local Parameters ...... 38 3.2.3.1 Deposition Potential ...... 38 3.2.3.2 Deposition Time ...... 39 3.2.3.3 Deposition and Stripping Steps ...... 41 3.2.3.4 Magnetic Flux Densities ...... 43 3.2.4 Enhancement of Peak Current under MHD Convection...... 46 3.2.5 Effects of Supporting Electrolyte ...... 49 Conclusions...... 50

CHAPTER 4 ELECTROCHEMICAL SPR DETECTION OF MERCURY UNDER MHD CONVECTION...... 52 Summary ...... 52 4 .1 Materials and Methods ...... 53 4.1.1 Materials...... 53 4.1.1.1 SPR Instrument ...... 53 4.1.1.2 SPR Electrochemical Cell...... 55 4.1.1.3 Electrodes and Accessories ...... 56 4.1.2 Methods ...... 58 4.2 Results and Discussion ...... 63 4.2.1 Reduction and Oxidation of Mercury ...... 63 4.2.2 Effects of MHD on SPR Detection ...... 63 4.2.3 Comparison Between Electrochemical and SPR Results ...... 65 4.2.4 Optimization of Local Parameters ...... 67 4.2.4.1 Deposition Time ...... 67 4.2.4.2 Deposition Potential ...... 68 4.2.5 Effects of MHD on SPR Angle Shift ...... 69 4.2.6 Enhancement of SPR Angle Shifts under MHD Convection ...... 70 Conclusions ...... 73

VI CHAPTER 5 ELECTRODEPOSITION OF RADIONUCLIDES UNDER MHD CONVECTION FOR ALPHA SPECTROMETRIC DETECTION 74 Summary ...... 74 5.1 Materials and Methods ...... 75 5.1.1 Materials...... 75 5.1.1.1 Electrodeposition Cell ...... 75 5.1.1.2 Electrodes and Accessories ...... 76 5.1.1.3 Tracers and Reagents ...... 78 5.1.2 Methods ...... 78 5.1.2.1 Electrodeposition Cell under MHD Convection ...... 80 5.1.2.2 Theory of MHD Convection ...... 82 5.1.2.3 Measurement of Radionuclides Yield...... 83 5.2 Results and Discussion ...... 8 6 5.2.1 Electrodeposition of Radionuclides ...... 8 6 5.2.2 Alpha Spectrometric Measurements of Radionuclides Sources ...... 87 5.2.3 Effects of MHD on Energy Peaks of Radionuclides ...... 8 8 5.2.4 Optimization of Local Parameters ...... 91 5.2.4.1 Deposition Time ...... 91 5.2.4.2 SEM Images of Electrodeposited Surfaces ...... 94 5.2.4.3 Magnetic Flux Densities ...... 96 5.2.4.4 Constant Current Electrodeposition ...... 98 5.2.4.5 pH of Solution ...... 100 5.2.5 Effects of MHD on Alpha Spectrometric Determination ...... 102 Conclusions ...... 104

CHAPTER 6 SUMMARY AND CONCLUSIONS...... 106

CHAPTER 7 SUGGESTED FUTURE WORK ...... 109

BIBLIOGRAPHY...... 110

VITA...... 123

vn LIST OF ABBREVIATIONS

MHD Magneto-hydrodynamic WHO The World Health Organization US-EPA The United States Environmental Protection Agency US-FDA The United States Food and Drug Administration amu Atomic Mass Unit SPR Surface Plasmon Resonance E-SPR Electrochemical Surface Plasmon Resonance ASV Anodic Stripping Voltammetry CSV Cathodic Stripping Voltammetry LSSV Linear Sweep Stripping Voltammetry DPSV Differential Pulse Stripping Voltammetry SWSV Square Wave Stripping Voltammetry PCR Polymerase Chain Reaction DNA Deoxyribonucleic Acid AFM Atomic Force Microscopy

vm LIST OF SYMBOLS

CHsHg" Methylmercury Elemental Mercury Mercurous Ions I Mercuric Ions Hg(II) Mercuric Ions ppb Parts Per Billion ppm Parts Per Million M Mole mM Milli-Mole Concentration pM Micro-Mole Concentration nM Nano-Mole Concentration pM Pico-Mole Concentration fM Femto-Mole Concentration 234 u Uranium Isotope- 234 238V Uranium Isotope- 238 239Pu Plutonium- 239 ^""Am Americium- 241 HgCl2 Mercuric Chloride a Alpha J Current Density B Magnetic Flux Density JxB Lorentz Force Pb2+ Lead Ions c d 2+ Cadmium Ions Cd 2+ Cupric Ions TI+ Thallium Ions R The Reflectivity of the Probe Light in SPR 0 Incident Angle of the Probe Light Beam in SPR measurement T Tesla V Voltage A Ampere pA Micro-Ampere E Electric Fields or Potential HNO3 Nitric Acid KNO3 Potassium Nitrate GC Glassy Carbon Pt Platinum Ag/AgCl Silver/Silver Chloride KCl Potassium Chloride

IX I-V Current-Voltage mV/s Milli-Volt Per Second s Second ïp Peak Current EDL Electric Double Layer DL Debye Length M+ Metal Ions M° Elemental Metal ne' Number of Electrons nA Nano-Ampere Hg(N03)2 Mercuric Nitrate Ed Potential Corresponding to Peak Current in Electrochemical Measurement Bd Magnetic Flux Density during Deposition Step Bs Magnetic Flux Density during Stripping Step

IP, B=Bd ,B=Bs Stripping Peak Current for B=B d and B=Bs

IP, B=0, B=0 Stripping Peak Current in the Absence of a Magnetic Field during Both the Deposition and Stripping Steps (i.e., B d=B s=0). ÔL Relative Peak Current Enhancement 2+ -"Hg Concentration of the Hg^^ Ions

IPS, B—Bq ,B—Bg Peak Current for B=B d and B=Bs in the Presence of the Supporting Electrolyte

P, B—Bn ,B—Be Peak Current for B=Bd and B=Bs

51PS Relative Peak Current Enhancements, -I (I PS, B=B„,B=B, P, B=B„, B=B, )/Ip,B=Bd,B=Bj xlOO% nm Nano-Meter mDeg Milli-Degree mL Milli-Liter n Refractive Index ~ 1.515 NaCl Sodium Chloride dpm Disintegrations Per Minute Bq/mL Becquerel Per Milli-Liter dpm/mL Disintegrations Per Minute Per Milli-Liter "He Helium Nuclei MeV Mega Electron Volts VDC DC Potential U(0H)4 Uranium Hydrous Oxide Pu (0 H) 3 Plutonium Hydrous Oxide Am(OH) 4 Americium Hydrous Oxide td Deposition Time pCi Pico-Curie LIST OF FIGURES

Figure 1-1 Periodic table showing mercury, uranium, plutonium, and americium [1] ...2

Figure 1-2 Sources and pathways of mercury in the environment [23, 2 4 ] ...... 6

Figure 1-3 Types of ionizing radiation [36] ...... 9

Figure 3-1 Electrochemical cell with electrodes...... 26

Figure 3-2 Electrodes used for the detection of mercuric ions ...... 27

Figure 3-3 Electrode polishing kit (CHI 120, CH Instruments Inc.) ...... 28

Figure 3-4 Magnets with magnetic flux densities B=0.27 T, 0.53 T, and 0.71 T 29

Figure 3-5 Electrochemical experiment measuring aqueous mercuric ions...... 30

Figure 3-6 Schematic representation of a cylindrical electrochemical cell positioned on a permanent magnet with a magnetic flux density B. Working, counter, and reference electrodes are vertically inserted into the electrochemical cell containing a sample solution to be detected. A current density J transmitted through the solution results when a potential is applied across the working and reference electrodes. The interaction between the current density and the magnetic field induces Lorentz forces JxB which ultimately induce fluid motion. The induced MHD convection can be used to enhance the mass transport of the electroactive species to/from the surface of the working electrode without the use of any mechanical stirrers ...... 31

Figure 3-7 Stripping anodic current vs. potential of 100 nM Hg^^ ionic solution in the absence (a) and presence (b) of the supporting electrolyte, 30 mM HNO3 and 0.1 M KNO3 , when the magnetic flux densities are, respectively, B=0 (solid line) and B=0.71T (dashed line) during the deposition and stripping steps. The scan rate, deposition potential, and deposition time are, respectively, 5 mV/s, td=180 s, and Ea= -0.3 V ...... 37

Figure 3-8 Peak current (Ip) of 100 nM Hg(N0 s) 2 solution as a function of the deposition potential (Ed) in the presence of the supporting electrolytes, 30 mM HNO3 and 0.1 M KNO3 , when the magnetic flux densities are, respectively, B=0 (solid line) and B=0.71 T (dashed line). Linear sweep with a scan rate 5 mV/s was performed after a deposition time td =180 s....40

XI Figure 3-9 Peak current (Ip) of 100 nM Hg(N0 3 ) 2 solution as a function of the deposition time (td) in the presence of the supporting electrolyte, 30 mM HNO3 and 0.1 M KNO 3 , when the magnetic flux densities are, respectively, B=0 (solid line) and B=0.71 T (dashed line). The scan rate and deposition potential are, respectively, 5 mV/s and Ed =-0.3 V ...... 41

Figure 3-10 Peak current enhancement (5Ip) of 100 nM Hg(N 0 3 ) 2 solution in the absence (open bars) and presence (shaded bars) of the supporting electrolyte, 30 mM HNO3 and 0.1 M KNO 3 . The magnetic field B= (Bd, Bs) in the x-axis represents that the magnetic fields in the deposition and stripping steps are, respectively, B=Bd and B=Bs. The scan rate, deposition potential, and deposition time are, respectively, 5 mV/s, td=180 s, and Ed= -0.3 V 43

Figure 3-11 Peak current enhancement (ôlp) of 100 nM Hg(N 0 3 ) 2 solution as a function of the magnetic flux density (B) in the absence (solid line) and presence (dashed line) of the supporting electrolyte, 30 mM HNO3 and 0.1 M KNO3 . The deposition potential and time are, respectively, Ed= -0.3 V and 180 s. Linear sweep with a scan rate 5 mV/s was performed during the stripping step. The lines represent the linear curve fitting of the experimental data ...45

Figure 3-12 Peak current (Ip) as a function of the concentration of the Hg^" ions (Cng^") in the absence of supporting electrolyte when B=0 (solid line with open circles) and B=0.71 T (dashed line with open circles), in the presence of the supporting electrolyte when B=0 (solid line with solid circles) and B=0.71 T (dashed line with solid circles) ...... 46

Figure 3-13 The relative peak current enhancement (81?) as a function of the concentration of the Hg^^ ions (Cng^ ) in the absence (open bars) and presence (shaded bars) of the supporting electrolyte, 30 mM HNO3 and 0.1 M KNO3 . The magnetic flux density B=0.71 T was applied during both the deposition and stripping steps ...... 48

Figure 3-14 The relative peak current enhancement (5Ips) due to the presence of the supporting electrolyte as a function of the concentration of the Hg^^ ions (Chs^^)- The solid line with triangles and the dashed line with circles represent, respectively, the conditions when B=0 and B=0.71 T during the deposition and stripping steps ...... 50

Figure 4-1 Mechanism of SPR measurement for the detection of mercuric ions 54

Figure 4-2 Electrochemical SPR instrument ...... 55

Figure 4-3 Quadrupled shaped cylindrical electrochemical cell ...... 55

Figure 4-4 Electrodes: (a) Gold chip, (b) Pt counter, and (c) Ag/AgCl (3 M NaCl gel) ...... 56

Xll Figure 4-5 (a) Matching fluid with syringe and (b) electrode holder ...... 57

Figure 4-6 Sample vials containing mercuric ionic solutions ...... 57

Figure 4-7 SPR experiment measuring aqueous mercuric ions...... 59

Figure 4-8 E-SPR experiment measuring aqueous mercuric ions...... 60

Figure 4-9 Schematic representation of a cylindrical SPR electrochemical cell positioned in a magnetic field provided by a permanent magnet. Counter and reference electrodes are inserted into the electrochemical cell containing a sample solution to be detected and the gold working electrode is placed on the bottom of the cell. A current density transmitted through the solution occurs when a potential is applied across the working and reference electrodes. The orthogonal interaction between the current density and the magnetic field induces Lorentz forces which ultimately induce fluid motion. The induced MHD convection can be used to enhance the mass transport of the electroactive species to/from the surface of the working electrode without the use of any mechanical stirrers ...... 62

Figure 4-10 (a) Current vs. time (b) potential vs. time, and (c) SPR angle shift vs. time of 1 nM Hg^^ ionic solution in the presence of the supporting electrolyte, 1 mM HNO3 and 10 mM KNO 3 , when the magnetic flux densities are, respectively, B=0 (solid line) and B=0.71T (dashed line) during the deposition and stripping steps. Dotted line in Figure 4-10 (c) represents the SPR angle of the supporting electrolyte without Hg^^ ions. The scan rate, deposition time, and deposition potential are, respectively, 10 mV/s, td=300 s, and Ed= -0.3 V ...... 64

Figure 4-11 (a) Current vs. potential and (b) SPR angle shift vs. potential of 1 nM Hg^^ ionic solution in the presence of the supporting electrolyte, 1 mM HNO3 and 10 mM KNO3 , when the magnetic flux densities are, respectively, B=0 (solid line) and B=0.71T (dashed line) during the deposition and stripping steps. The scan rate, deposition time, potential window, and deposition potential are, respectively, 10 mV/s, td=300 s, -0.3V to 0.5 V negative scan,

and Ed= -0.3 V ...... 6 6

Figure 4-12 SPR angle shift vs. deposition time of 1 nM Hg^^ ionic solution in the presence of the supporting electrolyte, 1 mM HNO 3 and 10 mM KNO 3 , when the magnetic flux densities are, respectively, B=0 (solid line with open circles) and B=0.71T (dashed line with solid circles) during the deposition and stripping steps. The scan rate, potential window, and deposition potential are, respectively, 10 mV/s, -0.3V to 0.5 V, and Ed= -0.3 V 67

Xlll Figure 4-13 SPR angle shift vs. deposition potential of 1 nM Hg^^ ionic solution in the presence of the supporting electrolyte,1 mM HNO3 and 10 mM KNO3 , when the magnetic flux densities are, respectively, B=0 (solid line with open circles) and B=0.71 T (dashed line with solid circles). The scan rate, potential window, and deposition time are, respectively, 10 mV/s, -0.3V to 0.5 V, and ta= 300 s ...... 6 8

Figure 4-14 Enhancement in SPR angle shift vs. magnetic flux densities of 1 nM Hg^" ionic solutions in the presence of the supporting electrolyte, 1 mM HNO3 and 10 mM KNO3 . The scan rate, deposition time, potential window, and deposition potential are, respectively, 10 mV/s, td=300 s, -0.3V to 0.5 V, and Ed=-0.3 V ...... 70

Figure 4-15 SPR angle shift as a function of the concentration of the Hg^" ions (Cng^") from 1 fM to 1 pM in the presence of supporting electrolyte, 1 mM HNO3 and 10 mM KNO3 when B=0 (solid line with open circles) and B=0.71 T (dashed line with solid circles). The scan rate, deposition time, potential window, and deposition potential are, respectively, 10 mV/s, td=300 s, -0.3V to 0.5 V, and Ed=-0.3 V ...... 71

Figure 4-16 Enhancement in SPR angle shift in percentage as a function of the concentration of the Hg^" ions (Cng^ ) from 1 fM to 1 pM in the presence of supporting electrolyte, 1 mM HNO 3 and 10 mM KNO 3 when B=0.71 T. The scan rate, deposition time, potential window, and deposition potential are, respectively, 10 mV/s, td=300 s, -0.3V to 0.5 V, and Ed= -0.3 V ...... 72

Figure 5-1 (a) Components of electrodeposition cell (b) assembled views of the cell with a magnet, and (c) assembled views of the cell with no magnet 75

Figure 5-2 Spiral shaped platinum anode rinsed in dilute HNO3 solution ...... 76

Figure 5-3 Spiral shaped anode (made of platinum) with splash guard ...... 77

Figure 5-4 Disc shaped cathode (made of steel) ...... 77

Figure 5-5 Electrodeposition of radionuclides with no magnetic field ...... 80

Figure 5-6 Electrodeposition of radionuclides with magnetic fields ...... 81

Figure 5-7 Electrodeposited surface of the steel disc under MHD convection...... 81

XIV Figure 5-8 Schematic representation of an integrated electrodeposition cell placed a permanent magnet with a magnetic flux density B. Anode made of a platinum electrode is inserted from the top of the cell and electrodeposition of radionuclides is performed on a steel planchet placed at the bottom of the cell. When a constant current is supplied across the electrodes, a current density J transmits through the solution. When an external magnetic field is applied, Lorentz forces JxB are induced as a result of the orthogonal interaction between the current density J and the magnetic field B. Thus induced fluid convection due to the Lorentz forces can enhance the mass transport of the radionuclides to the cathodic surface of the steel planchet with no other mechanical stirrers ...... 82

Figure 5-9 Alpha detectors used for the measurement of the radionuclides yields 85

Figure 5-10 Energy spectrums during alpha counting of electrodeposited radionuclides ...... 85

Figure 5-11 Alpha counts vs. energy (MeV) plot of radionuclides in the presence of supporting electrolyte solutions as observed in the alpha spectrometric determination when the magnetic flux densities are, respectively, B=0 (dotted line), and B=0.71 T (solid line). The branching peaks of respective radionuclide are denoted by arrows. Compared to the energy spectrum without MHD Electrodeposition, the energy spectrum using MHD electrodeposition resembles the whole abundance for each radionuclide. Applied initial DC potential, pH range, constant current, and deposition time, respectively, were 12 V, 2 to 2.3, 0.5 A, and 60 min. under all other identical conditions ...... 8 8

Figure 5-12 (a) Proportional counts of each radionuclide, expressed as undeposited radionuclides % vs. deposition time (min) under magnetic field B=0.71 T. Solid lines with solid circles, open circles, solid diamonds, and open diamonds denote, respectively, the alpha counts of ^^^Pu, and ^"'Am. (b) proportional counts of mixed radionuclides are expressed as total undeposited radionuclides % vs. deposition time (min) ...... 90

Figure 5-13 Potential drop (V) vs. time (min) during the deposition of the mixed radionuclides when B=0 (solid line with open circles) and B=0.71 T (solid line with solid circles). Applied initial DC potential, pH range, magnetic flux density, and deposition time, respectively, were 12 V, 2 to 2.3, 0.71 T, and 60 min ...... 91

XV Figure 5-14 Alpha counts vs. deposition time (min) plot of (a) each radionuclide 234u^ 239pu^ Am are, respectively, represented by solid lines with solid circles, open circles, solid diamonds, and open diamonds, and (b) mixed radionuclides represented by solid lines with solid triangles. Applied initial DC potential, pH range, constant current, and magnetic flux density, respectively, were 12 V, 2 to 2.3, 0.5 A, and 0.71 T ...... 92

Figure 5-15 Scanning Electron Microscope (SEM) images of electrodeposited surfaces of steel planchet, when the magnetic flux densities and SEM magnification are, respectively, (a) B=0, low magnification (b) B=0, high magnification, (c) B=0.27 T, low magnification (d) B=0.27 T , high magnification. Applied initial DC potential, pH range, constant current, and deposition time, respectively, were 12 V, 2 to 2.3, 0.5 A, and 60 min. under all other identical conditions ...... 95

Figure 5-16 Alpha counts vs. magnetic flux densities (T) plot of (a) each radionuclides 238u, 234y^ and ^"*Am are, respectively, represented by solid lines with solid circles, open circles, solid diamonds, and open diamonds, and (b) mixed radionuclides represented by solid lines with solid triangles. Applied initial DC potential, pH range, constant current, and deposition time, respectively, were 12 V, 2 to 2.3,0.5 A and 60 min ...... 97

Figure 5-17 Alpha counts vs. current (A) plot of (a) each radionuclide ^^^Pu, and ^"’Am, are respectively, represented by solid lines with solid circles, open circles, solid diamonds, and open diamonds, and (b) mixed radionuclides represented by solid lines with solid triangles. Applied initial DC potential, magnetic flux density, and deposition time, respectively, were 12 V, 0.71 T and 60 min ...... 99

Figure 5-18 Alpha counts vs. pH plot of (a) each radionuclide ^^^Pu, and ^"^Am, are respectively, represented by solid lines with solid circles, open circles, solid diamonds, and open diamonds, and (b) mixed radionuclides represented by solid lines with solid triangles. Applied initial DC potential, constant current, magnetic flux density, and deposition time, respectively, were 12 V, 0.5 A, 0.71 T and 60 min ...... 101

Figure 5-19 Alpha counts vs. the concentration of radionuclide activities (dpm/mL) plot of, respectively, (a) when B=0 (dashed line with open circles), and B=0.71 T (solid line with solid circles), (b) when B=0 (dashed line with open circles), and B=0.71 T (solid line with solid circles), (c) ^^^Pu when B=0 (dashed line with open circles), and B=0.71 T (solid line with solid circles), and (d) ^"^Am when B=0 (dashed line with open circles), and B=0.71 T (solid line with solid circles). Applied initial DC potential, constant current, radionuclides pH range, and deposition time, respectively, were 12 V, 0.5 A, 2 to 2.3 and 60 min ...... 103

XVI ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to my academic advisor and examination committee chair, Assistant Professor Dr. Shizhi Qian, for providing invaluable thorough guidance to research and constant support during my doctoral studies. I deeply appreciate

Dr. Qian for introducing me to scientific research, presenting opportunities to advance my research skills, and encouraging me to work in the field of microfluidics and Lab on a

Chip (LOC) technology for applications in various areas such as environmental, biomedical, and health physics. His expertise in these areas along with his vision of lab on a chip technology has been the source of inspiration for me to further research in this field. The present research work would, therefore, have never been accomplished without his proper guidance, regular supervision, and continuous encouragement.

I would like to thank Professors Dr. Robert F. Boehm and Dr. Ajit K. Roy fi-om the

Department of Mechanical Engineering, Assistant Professor Dr. Marcos A. Cheney from the Department of Health Physics and Research Associate Professor Dr. James V.

Cizdziel from the Harry Reid Center for Environmental Studies for serving on my dissertation supervisory committee. I am grateful to them for their careful and critical reading of my dissertation and constructive, invaluable suggestions. Their comments and suggestions not only helped to enhance my research skills but also served to develop deeper insight for future research. I would also like to specially thank Dennis F. Farmer from the Department of Health Physics for his invaluable suggestions, guidance.

xvn encouragement and cooperation in the laboratory activities. I am thankful to two undergraduate students, Mussie Mehederekal and Kristina Regmi, for their help in the laboratory.

I would also like to thank my parents, Gehendra Panta and Bed Kumari Panta, all my brothers specially Nagendra Panta, and my best friend Narayan Poudyal for encouraging me throughout my doctoral studies. I would like to sincerely thank my wife, Manju

Tandukar, and our daughter, Maya Panta, for their encouragement and motivation throughout my study and research. Finally, I would also like to thank department secretary Joan Conway for her unwavering strength and friendship.

This work was partially supported by the National Science Foundation (NSF) through

Nevada EPSCoR sensor program.

X V lll CHAPTER 1

INTRODUCTION

1.1 Motivation

The motivations for the work presented in this dissertation are fundamentally stated in two-fold. First, a novel, sensitive, and economic technique is presently in demand for the solution of an increasing concern of mercury detection for environmental and human health protection agencies to save people and the environment from mercury poisoning.

The goal of this research is to use a novel and state of the art magneto-hydrodynamic

(MHD) technique to ultra-detect low concentration of mercury in aqueous solutions.

More common but sophisticated methods such as electrochemical and surface plasmon resonance were used in combination with MHD fluid convection to enhance the detection of mercury. Second, the need exists for innovative techniques for the detection of radionuclides in the environment. A rapid and ultrasensitive electrodeposition novel method was developed for high resolution alpha spectroscopic determination of mixed radionuclides such as some of the most dangerous radio isotopes: uranium, plutonium, and americium. During the preparation of alpha sources of these radionuclides, MHD technique was employed to improve the uniformity and selectivity of the electrodeposition of radionuclides that lead to their precise detection. Therefore, the novel concept of convective fluid flow with an application of MHD was utilized for electroplating and detection purposes of mercury and radionuclides; the dissertation

1 details the procedures, results, and discussion. Figure 1 - 1 depicts the location of mercury, uranium, plutonium, and americium in the periodic table [ 1 , 2 ].

1.008 4.003

16.00 19.0020.18

IB ZB 30.97 32.06 35.45 36 Kr 838079.90

131.3 mag

Mercury

Lanthaiudes

Actuudes msmTb T " t Î Uranium Plutonium Americium

Figure 1-1 Periodic table showing mercury, uranium, plutonium, and americium [1]

1.2 Heavy Metals

The growing concern for eradication of environmental pollution has triggered research on the toxicology of heavy metals. While the toxic effects of these metals are a tremendous concern in the modem industrial context, most environmental poisoning is anthropogenic, which has been recorded throughout in history. Virtually all metals are considered to produce toxicity when ingested in sufficient amounts, but some heavy metals such as lead, mercury, arsenic etc. can produce toxicity even at very low concentrations. Complexes of heavy metals or ligands combined with organic compounds are their toxic forms. The most common groups such as oxygen, sulfur, nitrogen, and methyl are usually considered for ligands formation. Thus, modified biological molecules

in the living beings due to the formation of ligands become unable to function properly,

and affected cells will become malfunctioning and may cause to death of cells [3,4].

1.3 Mercury

There are many trace metal ions present in world water systems. Among the heavy metals present in world water systems, mercury is one of the most researched and concerned environmental pollutants compared to other pollutants such as tin, lead, and

arsenic. Because of its toxicity in the form of methylmercury (CHsHg^) and the mobility

in the environment through the mercury cycle, mercury research is becoming a great

concern. Mercury can be transported easily through the atmospheric cycle making it a

significant global pollutant [5].

1.3.1 History of Mercury Poisoning

Since the 19*^ century, there have been several well-documented outbreaks of methylmercury poisoning. First, mercury was known as a “Mad Hatter Disease” in the

United Kingdom (19* Century). Second, the large scale of mercury poisoning occurred in

Minamata Bay, Japan (1950-1960) and in Iraq (1970) known as pink disease. The

expression 'Mad as a Hatter' originates from the hat-makers in the United Kingdom who were chronically exposed to mercury compounds used in the production of such headdress [ 6 ]. In Japan, methylmercury, used as a catalyst in the acetaldehyde factory, leaked over many years into the Minamata Bay resulting in aquatic methylmercury and bioaccumulated by fish, over 3000 victims have been affected after consumption [7]. In the case of pink disease in Iraq, large quantities of grain treated with methylmercury fungicide were accidentally distributed to countryside people, and the grains were consumed, which as a consequence, 6500 people became sick and 500 died [ 8 ].

It was discovered that environmental mercury can be transformed into methylmercury, one of the most toxic forms of mercury, through the process of biomethylation, which can enter into the food chain, impacting both the environment and the human body [9]. The latest fatal poisoning of dimethylmercury happened to Karen

Wetterhahn, Professor of Chemistry at Dartmouth College (1996-1997), ten months after an exposure caused by an accidental spillage of mercury through her latex glove-covered hand [ 1 0 , 1 1 ].

1.3.2 Effects of Mercury Poisoning

The effects of chronic exposure of mercury through the inhalation of elemental mercury (Hg°) or ingestion of mercuric (Hg^^) compounds are neurological and psychiatric which can damage the central nervous system and nervous tissues. The reaction of mercury with sulfur atoms present in brain proteins and enzymes assists in perturbation of their functions [12]. In high doses, mercury poisoning causes many symptoms such as abdominal pain, anorexia, lack of coordination, headache, vomiting, bloody diarrhea, excessive salivation and ultimately leads to death. Also, upon contact with the skin, mercury can be absorbed in blood vessels, ultimately leading to a poisonous interaction [13-16]. 1.3.3 Mercury Cycle

Mercury exists in the environment, at low concentrations, in three valence states: Hg^,

Hg^\ and Hg^^ [15-17]. Mercury can form inorganic and organic complexes by combining with inorganic and organic ligands, such as calomel and methylmercury [17-

19]. Phenylmercury and dimethylmercury were the commercially used forms of organic mercury as fungicides in the 1960s but were later baimed due to their discovered toxicities [18]. Due to the potential poisoning to water ecosystems, uses of methylmercury in industrial applications have raised serious concerns. Usually, sulfate reducing bacteria convert inorganic mercuric ions to a toxic form, methylmercury, through the process of biometylation. This is the transformation of mercuric ions to methylmercury and is primarily converted by the bacteria of aquatic systems [20-22]. The reactions between the labile mercuric complex compounds and methylating agents can form the methylmercury in the ambient air which then goes through photo-degradation or the mercury cycle to the water system as shown in Figure 1-2 [23, 24]. Therefore, this study is mainly focused on the determination of Hg (II) ions in aqueous solutions rather than in other forms of mercury. The main pathway of mercury contaminations in the human body exists through the food chain; the ingestion of fish containing bioaccumulated methylmercury. In most cases, mercury contained in fish is found in the form of methylmercury.

The World Health Organization (WHO) indicates that doses should not exceed 1.6

|xg/ kg of mercury in the body per week for minimal risk [25]. The United States

Environmental Protection Agency (US-EPA) has set up a limiting dose of 0.1 pg/ kg of mercury in the body per day to prevent neurobiobehavioral development [26]. DEPOSITIONaod VOVATILIZATK3N (Mgmk CHgHg)pi^>

- ^ . ■«**

Atmospheric An|[iro;:o(jotiic ^{j Hg G lobal deposition

^ C - - lerrestrlal mmrinR Local and A" regional ^ Hccmitlcd Rocnvtioc deposition^ aninrcpogcnic anthropogenic, ,anC natural Hg ard raturai Mg

MeihylatGd Ho

Figure 1-2 Sources and pathways of mercury in the environment [23, 24] Concerning Hg*^, 0.2 gg/m^ in the atmospheric air is regulated by WHO to prevent the

intoxication of elemental mercury [27]. The United States Food and Drug Administration

(US-FDA) sets up the eoneentration level for mercury as one part per million (ppm) in

fish [28].

1.4 Radiation

Radiation serves as a broad meaning that includes visible, infrared, ultraviolet light, and radio waves. However, in radiation studies, ionizing radiation is most often indicated by the term “Radiation” which changes the physical state of atoms as it decays or unites, becoming electrically charged or simply regarded as ionized [29].

1.4.1 Nuclear and Radiation Disasters

If radionuclides are released into the environment through accidental means or poor

disposal they can immediately have the potential to cause harmful effects associated with radioactive contamination. They can also cause damage if they are excessively used

during treatment or in other ways applied to living beings. This exposure is called radiation poisoning. Two major nuclear incidents occurred around the world, Hiroshima

& Nagasaki bombing in Japan in 1945 [30-32] and the Chernobyl accident in Ukraine in

1986 [33]. Several casualties were reported in both the disasters.

1.4.2 Ionizing Radiation

Ionizing radiation is considered a serious health hazard to humans as the presence of

electrically charged ionizing radiation can disrupt regular biological processes and damage and kill the cells and tissues of living beings [34]. Types of ionizing radiation are depicted in Figure 1-3. Ionizing radiation has enough energy to disturb the electrical balance as it passes through the atoms of substances. The following are various types of ionizing radiation; each of them has different characteristics [34-37]:

(i) Alpha radiation: It constitutes heavy positively charged helium emitted by atoms of radionuclides. Alpha radiation is capable of penetrating the surface of the skin, and it can be blocked completely by an ordinary sheet of paper. These alpha emitting radionuclides are considered dangerous if they are exposed to an open wound or ingested or inhaled through one’s gastrointestinal tract or respiration; they can affect internal tissues directly.

(ii) Beta radiation: It consists of electrons emitted by radionuclides. These have more penetrating capabilities than alpha particles; a sheet of a few millimeters of aluminum can block them completely. Tritium is produced in nuclear test fallouts, is a source of beta radiation.

(iii) Gamma radiation: It consists of energetic photons. These rays are electromagnetic radiations, similar to X-rays, light and radio waves. They have the capabilities of more penetration than alpha and beta radiation. They can be completely blocked by one meter of concrete block.

(iv) X-rays: They are similar forms of electromagnetic radiation, having a limited power of penetration. X-rays generally exist in the form of focused beams, and lead blocks their penetration.

(v) Neutrons: They are produced during processes of splitting atoms in the fuel of nuclear power plants, and they have the highest degree of penetrating capabilities. Water can provide an efficient shield against neutrons.

Radioactive materials are harmful when they are emitted into the human body.

Gamma rays are less ionizing than either alpha or beta particles, and protection against gammas requires thicker shielding. The damage they produce is similar to that caused by

X-rays, and includes bums and also cancer, through cell mutations. Some exposure to ionizing radiation cannot be avoided. Exposures can be natural or man-made. Natural sources include cosmic rays and naturally-occurring radionuclides in the environment and are considered background radiation. Anthropogenic sources include medical X-rays and coal-fired power plants. Other sources of radiation include fallout from nuelear explosives testing and radionuclides emitted from nuclear installations in the course of normal operation [34-40].

Penetrating Powers of Alpha Particles, Beta Particles, Gamma and X-Rays

ALPHA Particles stopped byastieet of paper

BETA Particles I by layer of I o r by a few millimeters of a substance such as aluminum

GAMMA Rays and X-Rays stopped by several feet of concrete or a few Incffes of lead

Figure 1-3 Types of ionizing radiation [36]

Many radioisotopes in nature exist in stable form; rarely changing their forms. Other isotopes, both natural and anthropogenic, are radioactive; they are not stable and can

OG c ^ G /I convert into other forms e.g., U decays to Th and release an alpha particle during the decay. The process of spontaneously disintegrating or decaying an atom’s nucleus over a period of time and releasing energy is known as radioactivity [39].

1.4.3 Effects of Radiation

The amount and the type of radiation absorbed can be easily quantified using radiation measurement instruments. The biological impact of absorbing a given amount of radiation varies depending on the type of radiation absorbed. Radioaetive elements or particles can enter the human body by ingestion, such as by eating, drinking and breathing, or through an open wound. When a particle of radiation penetrates the human body and does not interact with bodily tissue, no damage is inflicted. It is when the particles deposit some of their energies in tissues that damage could occur [34-41].

1.4.4 Radionuclides

A radionuclide is an atom of a radioactive element with an unstable nucleus characterized by excess energy which is available to be imparted either to a newly- created radiation particle within the nucleus or to an atomic electron. Radioactive materials consist of atoms that are the sources of radiation and are therefore pronounced as radionuclides. In view of the potential radiological weaponry, Am and ^^^Pu are two radioactive isotopes that are of significant interests. ^"^’Am is a decayed daughter of plutonium and, itself, an alpha emitter. The main threat of ^"“Am is the heavy metal poisoning which can cause whole body irradiation if swallowed or ingested.

Gastrointestinal absorption of Am is less likely to occur, but it can be absorbed quickly from open wounds and rapidly spread through the blood and lymph circulations. ^^®Pu is produced from uranium reactors, and it is the main fissionable material used in nuclear warheads. Plutonium radiation is obtained in the form of alpha particles, usually mixed

10 with americium, and is introduced into the human body through inhalation, open wounds,

• 's n o O'XA or by ingestion. U and U are can cause both acute and chronic syndromes if an elevated dose is introduced. Thus, ionizing radiation interacts with living cell on a molecular level, with the potential to cause cell damage that can not be repaired. Such irreparable damage results in a variety of health problems, including cancer [34-44]. In this study, results and discussions of the electrodeposition and detection of radionuclides,

238u, and Am, are presented.

1.5 Organization of Dissertation

The brief introduction has highlighted the histories of mercury poisoning and radiation disasters that contribute to the importance of the detection of mercury and radionuclides. This dissertation focuses on the use of MHD technique coupled with standard electrodeposition method to detect mercuric ions and radionuclides in aqueous solution. Electrochemical and electrochemical SPR methods were employed separately and in combination with MHD to detect the mercuric ions. Similarly, standard electrodeposition method with MHD technique was employed to prepare radionuclides sources for alpha spectrometric measurement. The dissertation is organized as follows.

Chapter 2 details the literature review of mercury and radionuclides. Chapters 3 and 4 present the details of mercury detection with local parametric studies using, respectively,

MHD based electrochemical and MHD based electrochemical SPR techniques. The rapid, sensitive, and selective identification of radionuclides measured in an alpha spectrometer based on MHD electrodeposition of alpha sources are discussed in chapter 5. Chapters 6 and 7 conclude with summary and suggested future work respectively.

11 CHAPTER 2

LITERATURE REVIEW

2.1 Toxicity of Mercury

Mercury is a highly toxic element that is found in the environment as a result of human activities [17]. The combination of other toxins with the emission of mercury to the environment such as coal combustion processes have increased mercury cycling in aquatic systems. There are well-documented instances of regional population poisoning, high-level occupational exposure and global scale contamination [18, 19]. Mercury exists in nature, at low concentrations, in three valence states: Hg®, Hg'^, and Hg^^, which may then combine with inorganic and organic ligands to form various complexes, such as mercuric chloride (HgCL) and methylmercury [17-19]. The most common commercial uses (e.g. industrial and medical) of organic mercury are in the form of phenylmercury and dimethylmercury [18]. These compounds were utilized as fungicides in the 1960s but were later banned in the 1970s, however, due to their adverse effects on human health

[18]. Methylmercury is still one of the most common forms of mercury used in the industry, and this issue has raised concerns due to its potential toxicity to aquatic systems caused by inadequate disposal. In soils or in sediments, bacteria convert inorganic Hg (II) to the more toxic form, methylmercury, via biometylation. Mercury biomethylation is the

12 transformation of divalent inorganic mercury (Hg^^) to methylmercury (CHsHg^) and is primarily carried out by sulfate-reducing bacteria that live in anoxic regions of aquatic environments [20-22]. Methylmercury may also be formed in the atmosphere by a reaction of labile Hg (II) complexes and methylating agents. Then, it finds its way into aquatic systems where it may undergo photodegradation or a bacterial transformation through a variety of pathways. Due to the conversion of Hg^^ ions into CHsHg^, which is a neurotoxin form of mercury, organic mercury compounds are more readily absorbed via ingestion than inorganic mercury compounds [18-20, 45]. The detection of mercury (II) ions in aquatic systems has been an increasing priority for law enforcement, environmental protection agencies in order to protect human health. Therefore, in this current study, the determination of Hg^^ ionic solutions is the focus.

2.2 Detection of Mercury under MHD Convection

2.2.1 Electrochemical Detection of Mercury

While the detection of mercury (II) ions has been studied extensively, rapid, reliable, sensitive, and cost-effective techniques and devices for detection are in demand.

Previously developed and most common techniques for the detection of mercury (II) ions in natural samples include molecular, atomic, or fluorescence spectrometry [46,47], ehromatography [48], enzyme-based biosensors [49], fiber optic probes and optical sensors [50, 51], pool optode [52], stripping electrochemistry [53-56], and stripping electrochemistry coupled with piezoelectric sensor technology [57]. The spectrometric techniques require long procedures of sample pretreatment, an expensive instrument, and skilled technicians. The chromatography method requires to flowing a sample containing

13 the analytes such as a typical chemical form of mercury unidirectionally in a fixed-length

column equipped with a stationary phase. The optimal column length that is needed for

efficient separation depends, however, on the analytes that one is attempting to separate.

Unfortunately, existing chromatography for the detection of mercury in water consists of

a fixed length eolumn that cannot be adjusted according to the separation task at hand.

The enzyme-based technique requires the immobilization of enzymes such as xanthine

oxidase, peroxidase, and urease using complex designs either in terms of their reaetion

schemes or their immobilization of enzymes and transduction procedures which often

cause a loss in enzyme aetivities and give poor reproducibility [58]. The fiber optic probe

technique provides high sensitivity and selectivity, but its applications are limited due to

the interferences from electroactive species, ions, and the turbidity of the samples [47].

Compared to other existing techniques, the anodie stripping voltammetry (ASV)

technique offers advantages including the ability to determine several analytes

simultaneously, the determination of very low eoncentrations, performance of on-line

monitoring/in situ measurement, and relatively inexpensive instrumentation. Therefore,

the ASV is a widely used technique for the determination of the concentrations of mercury (II) ions in aqueous solutions [53-57]. Stripping voltammetry is one of the

sensitive electroanalytical techniques widely used for the determination of traee amounts

of metals in aqueous solutions. This technique mainly consists of two basic steps even

though it may have an intermediate step known as a quiet step. During the first deposition

step, which is also called the preconcentration step, metal ions are deposited onto the

surface of a working electrode which is held at a suitable deposition potential for a

certain deposition time. The solution is stirred during this step to maximize the amount of

14 metal deposited on the eleetrode’s snrfaee. During the second stripping step, the deposited metals are stripped from the working electrode by a positive or a negative potential sean, creating either an anodic current (anodie stripping voltammetry, ASV) or a cathodic current (eathodic stripping voltammetry, CSV), respectively. The resulting anodic or cathodic current is directly related to the amount of metal in the aqueous solution. In addition to the direction of the potential scan, various waveforms of the potential such as the linear scan, differential pulse, and square wave ean be used in the stripping step. The simplest one is linear sweep stripping voltammetry (LSSV) where the potential is searmed linearly as a function of time. Furthermore, another two techniques such as differential pulse stripping voltammetry (DPSV) and square wave stripping voltammetry (SWSV) have a lower detection limit than LSSV. One of the major advantages of the ASV technique is that the preconcentration step leads to signifieant enhancements in the resulting anodic current compared to a conventional voltammetric analysis of the solution. The magnitude of the resulting anodic peak current is proportional to the amount of the analyte electrodeposited onto the surface of the working electrode during the preconcentration step [59]. Therefore, meehanical stirring of the solution is usually performed to enhance rates of mass transport during the deposition step. However, during the analysis of small sample volumes present in a micro scale electroehemical eell, the mechanical stirring becomes impractical. In this current study, we demonstrated that one can enhance the mass transport by positioning the electrochemical cell under a magnetic field without using a mechanical stirrer.

When a potential difference is applied across the working and referenee eleetrodes during the deposition and stripping steps, a current density J transmits through the

15 electrolyte solution. When the electrochemical cell is positioned under an external magnetic field B whieh can be provided by a permanent magnet (used in our experiments) or an electromagnet, the current density J and the magnetic field B interact to produce a Lorentz force of density JxB which induces the fluid motion that can be used to stir the fluid present in the electrochemical reactor [60-65]. The induced MHD convection exhibits several advantages over many other stirring methods including no moving parts, and its capability to bring more analytes to be deposited onto the surface of the working electrode during the deposition step, enhancing the peak anodie current and the sensitivity of the determination of the analyte concentrations. Such MHD convection has been used to enhanee the detection of various heavy metal ions such as lead (Pb^^), cadmium (Cd^^), cupric (Cu^^), and thallium (TI^) ions with the ASV technique [ 6 6 , 67].

In the previously reported work, mercuric nitrate was added to the analytes solution and mercury was deposited onto the surface of the working electrode to form a thin film of mercury electrode for detecting other heavy metal ions. However, the detection of Hg^^ ions with the linear scan ASV technique under MHD convection has not been reported in the previous works.

2.2.2 Electrochemical SPR Detection of Mercury

Even an extremely small amount of mercury (II) ions in an aquatic environment is potentially disastrous and needs to be detected. A detection technique for mercury (II) ions that is ultra-sensitive and requires a small volume of test samples for an in-situ analysis to avoid contamination of samples and lengthy process time is desirable. As mentioned previously, eonventional methods for detecting mercury (II) ions, such as molecular, atomic, or fluorescence spectrometry, chromatography, enzyme-based

16 biosensors, fiber optic probes and optical sensors, pool optode, stripping

electrochemistry, and stripping electrochemistry coupled with piezoelectric sensor

technology are not highly appropriate due to a large sample volume and the long process

of sample pretreatment required [ 6 8 , 69] or poor reproducibility [58], among other

drawbacks. Thus, it is important to explore alternative techniques that are more accurate

and less time-consuming. Surface plasmon resonance (SPR) [70-75] is a sensitive and

powerful optical technique for measuring the thickness and structure of ultra-thin

adsorbate layers and for quantifying the adsorbed concentration of metal films [76-91]. A

Combination of the SPR technique and the electrochemical method has been suggested

[46-53] and realized for highly sensitive heavy-metal detection, including mercury [92,

93].

Electrochemical SPR consists of the electrodeposition of mercury onto a gold

electrode surface [94-97] with optimum deposition potential and time and a subsequent

stripping process by creating either an anodic current (ASV) or a cathodic current (CSV),

during which the SPR technique is used to quantify the electrochemical reaction. When a

/ 7 -polarized electromagnetic radiation propagates from an optically dense medium, total

internal reflection occurs for incident angles greater than a critical value, and an

evanescent wave is generated in the sparse medium. When this evanescent wave couples

with electrons in the sparse medium, the intensity of the reflected light is reduced. This

phenomenon is called SPR. The SPR technique involves an optical excitation of surface plasmon resonant waves on gold with mercury film and uses the Kretschmann

configuration. The reflectivity R can be measured as a function of the incident angle 0 of

the probe light beam [98-100]. At the same time, the resonant condition is extremely

17 sensitive the refraetive index of the medium adjacent to the gold plate, and thus the mercury can be detected accurately by monitoring the changes. The SPR technique is currently used in a variety of chemical and biological sensors [101-104], in studying the conformational changes of molecules and the detection of heavy metal ions such as Pb^^

[105, 106].

In this study, electrochemical SPR technique is enhanced further by incorporating the

MHD convection. Although the electrochemical SPR accommodates highly accurate deteetion of heavy-metal ions, sensitivity in the order of femto molar concentration, especially for mercury (11) ions in aqueous solutions, has not been reported. Here, we report the MHD electrochemical SPR technique that detects mercury (11) ions down to 1 fM and requires extremely small detection sample.

Stirring the solution may be applied to increase the mass-transport rate during the deposition process to bring more analytes onto the working gold electrode. Mechanical stirring, however, may be inefficient and inappropriate in the analysis of small-volume samples, such as those in a micro-scale electrochemical cell. MHD convection can induce extremely fine mixing [60-65] and it is non-intrusive, obviating the need to insert any moving part into the electrochemical cell. The fluid motion is generated by the Lorenz force, induced by the interaction of an external magnetic field and the eleetric current in the cell. The effectiveness of the MHD convection has been shown in the deteetion of heavy-metal ions Pb^\ Cd^^, Cu^^, and TI^ in conjunction with the ASV teehnique [ 6 6 ,

67]. In previous work [107], the MHD convection was used to enhance the electroehemieal technique to detect the mercury (II) ions down to 1 nM. In this study, the

MHD electroehemical SPR technique is reported for the first time detection of 1 fM

18 mercury (II) ions in an aqueous solution. The technique can be used in a portable micro­

device for monitoring various trace metal ions in drinking water, wastewater, and

groundwater as well as the detection of toxic metal ions or other electrochemically active

speeies in blood, saliva, and urine.

2.3 Radiation Hazards to Human Health

A radionuclide is an atom of a radioactive element with an unstable nucleus

characterized by excess energy which is available to be imparted either to a newly-

created radiation particle within the nucleus or to an atomie electron. The radionuclide, in

this process, undergoes radioactive decay. Energy traveling through waves or particles is

known as radiation. Depending upon the kind and the range of energies produced,

radiation sueh as microwaves, visible light, and X-rays categorizes. Some atoms of

elements can spontaneously produce high-energy radiation either as released particles or

traveling rays, which is known as radioactivity. Radioactive materials consist of atoms

that are the sources of radiation and are therefore pronouneed to be radionuclides. In view

of the potential radiological weaponry, ^^®Pu and ^"^'Am are two radioaetive isotopes that

require specific awareness. Americium-241 is a decayed daughter of plutonium and,

itself, an alpha emitter. The main threat of ^''^Am is the heavy metal poisoning whieh can

cause whole body irradiation if swallowed or ingested. Gastrointestinal absorption of

^"^*Am is rare, but it can be absorbed quickly from open wounds and rapidly spread through the blood and lymph circulations. ^^®Pu is produced from uranium reactors, and

it is the main fissionable material used in nuclear warheads. Plutonium radiation is

obtained in the form of alpha particles, usually mixed with amerieium, and is introduced

19 to the human body through inhalation, open wounds, or by ingestion. and are responsible for both aeute and chronic syndromes if an elevated dose is introduced. Thus, ionizing radiation interacts with living cells on a molecular basis, with the potential to cause cell damage that ean not be repaired. Such irreparable damage results in a variety of health problems, including a broad range of cancers [108-111]. In this study, the detection and measurement of radionuclides ^^®Pu, and ^""Am are performed.

2.4 Deteetion of Radionuclides under MHD Convection

2.4.1 Electrodeposition of Radionuclides

Global terrorism coupled with the difficulty in preparing thin layer sources of mixed radionuelides such as ^^®Pu, and ^"^*Am for alpha spectrometric determination has made alpha-spectrometry an attractive field of investigation. Thus, new and improved methods for the preparation of thin layer of radionuclide sourees on a dise to eliminate the self-absorption of alpha-particles which results in the degradation of alpha-spectra are needed. Previously reported techniques to prepare alpha emitting radionuclide sources for alpha spectrometric measurements inelude vaeuum sublimation [ 1 1 2 ], direet evaporation

[113], electrodeposition [114-122] and micro-coprecipitation [123, 124].

Uranium isotopes, plutonium, amerieium and higher radionuelides are usually electroplated using the solution of ammonium sulfate [116, 118, 1 2 0 - 1 2 2 ], ammonium chloride [114, 119], ammonium ehloride-ammonium oxalate [117] or dimethylsulfoxide

[115]. There are still some disadvantages present in these electrodeposition systems whieh have not been previously addressed. Basic factors of our concern that can improve and make the radionuclides detection more sensitive, reliable, more rapid; and assist with

20 the homogeneity electrodeposited surface morphology are addressed here with the

application of magnetohydrodynamics based fluid convection used during the preparation

of radionuclide sources.

The electrodeposition technique is one of the most commonly used methods for the preparation of radionuclides sources, and the pioneer of all is the one reported by Talvitie

[125] for the source preparation of ^^®Pu, and ^"^'Am. Although the Talvitie

method has good reproducibility for radionuclides, it lacks a good resolution of the

individual energy lines due to uneven deposition of radionuclides. Electrodeposition is

assumed to be the best method to prepare uniform radionuclide sources for high-

resolution alpha spectrometric measurements. These alpha particles can only travel in a

short range, and are positively charged helium nuclei (4 amu). Moreover, alpha particles

are the primary and the identifying spectral characteristic for the low-level energy

analysis of many of the world’s most dangerous radioactive species, including the

plutonium and uranium isotopes used in nuelear weapons: such as ^^®Pu and

respectively.

There are many reports in the literature on the use of magnetic fields to investigate

the electrochemical mass transport [125-127], iron electrodeposition [128], mercury

electrodeposition [129], and silver deposition [130]. In our previous study, the application

of magnetic field was used successfully to enhance ionic mass transport in the

electrochemical technique to detect mercuric ions in aqueous solutions [131]. Together

with this study of radionuclide detection using magnetically enhanced fluid convection,

MHD based convection was employed for the detection of mercury by using the

electrochemical SPR technique [132]. It has been known for a number of years that the

21 application of low-strength (< 1 Tesla), oriented magnetic fields during the microfluidic processes could accomplish what previously seemed undoable and also speed up the limiting process of simple molecular diffusion in solutions that are difficult or impossible to mix through physical means. Magnetic flux density applied orthogonally to the current density can be used to create advective or the chaotic mixing of ions present in aqueous solution at the micro-scale level and may even be better at nano-scale structure. For example: magnetic-enhancement has been used to improve such diverse microfluidic processes as continuous-flow polymerase reactions (PCR) [133] for DNA amplification

[134]; making better semi-conductor masking templates [135-137]; producing more consistent magnetic data storage media [138]; and even hardening critical wear surfaces on small mechanical parts [139,140].

In recent years, there has been an increase in the application of high magnetic fields in material processing [141, 142]. Material processing applied to MHD or micro-MHD effect is newly developed as is magneto-electrolysis [143-145] and magneto-plating

[146]. The MHD and the micro-MHD effects imply the following phenomena. In electrochemistry, the reaction rates are greatly influenced by the fluid motions (referred to as MHD and micro-MHD flows) arising from the Lorentz force, and these motions can greatly accelerate the electrodeposition reaction, helping to form a uniform electrodeposition of existing ions in the solution [147].

What is not reported in the current literature is to what extent the effect imposed magnetic fields might have on the thin layer electrodeposition of heavy radionuclides at the very low concentrations that need to be detectable to track the movement and fate of these dangerous radioactive materials in the environment or during a radiological

22 emergency. To test this, we applied a series of progressively more powerful permanent magnetic fields in conjunction with an electrodeposition process for preparing thin-layer radioactive sources on the upper surface of an approximately 2 0 mm diameter polished stainless steel disk.

The process involves inducing micro-scale convective mixing of the ions in the plating solution by the interaction of current density (J) and magnetie flux density (B) with a resultant decrease in the thickness of the controlling diffusion layer and decreasing solution resistance in a maintained current density where critical electrodeposition reactions are taking place.

Magnetically induced mixing or MHD fluid convection is correlated with improvements in both the deposition reaction kinetics and ion selectivity. In effect, everything else being equal, the development of a more homogeneous deposition layer is favored by the application of an oriented magnetic field to an existing microanalysis process. Although the exact physiochemical processes occurring at the atomic and sub­ atomic level in various systems are not completely understood, it is likely that some parts of the improvements seen are due to an increase in ionic mass transport of radionuclides in the solution.

The best quality electroplating outcomes obtained thus far in our experiments with heavy radioaetive metals occurred with the current density and the magnetic flux density at orthogonal orientation; however, other orientations and related chemical variables were also being tested. A particular interesting early finding of this work was that, unlike more conventional methods, the simultaneously and proportional deposition of multiple radioactive elements (uranium, plutonium, and amerieium) occurs almost immediately

23 during magnetically-enhanced electroplating. This advance that was unexpected in the past and holds promise for developing broad-scope, rapid analytical methods in the future and may have applieations for developing more rapid methods of measuring airborne radioaetivity if related work, on chemically or otherwise isolating the necessary sample fractions from ambient air, can be developed.

2.4.2 Alpha Spectrometric Measurement of Radionuclides Sources

In this study, a simple, novel MHD based electrodeposition teehnique to prepare the radionuclides alpha source for alpha spectrometric measurement is presented. To determine the optimal conditions for plating radionuclides sources, experimental parameters affecting the electrodeposition locally such as electrodeposition time, current, pH, and magnetic flux densities were investigated. The optimal conditions for the electrodeposition of the radionuclides was applied to natural samples of uranium isotopes as well to find the linearity between the radionuclides (uranium isotopes 238 and 234:

and^^"^U, plutonium 239: ^^®Pu, and americium 241- ^''’Am) concentration activities and the alpha spectrometric counting. The result has created a dramatic improvement in the quality of the energy spectra, surface texture, and morphology of the deposited surfaee that can be obtained for measuring the alpha-particles spontaneously emitted during radioactive decay by most radionuclides.

This study widely reveals the use of the magnetie field to enhance the electrochemical mass transport during the preparation of alpha sources.

24 CHAPTER 3

ELECTROCHEMICAL DETECTION OF MERCURY UNDER MHD CONVECTION

Summary

Inorganic mercury (II) ions are ubiquitous contaminants of world water systems, and their determination and removal from the environment are important. The effects of the magnetic field on the stripping analysis of mercury (II) ionic solutions have been experimentally investigated. During the stripping analysis, a potential difference is applied across the working and reference electrodes positioned in the working sample and a current density transmits through the electrolyte solution. When the electrochemical cell is exposed to a magnetic field, provided by a permanent magnet, the interaction between the current density and the magnetic field induces Lorentz forces, which, in turn, induce fluid motion. The induced MHD convection enhances the ionic mass transport during the deposition and stripping steps, which leads to larger anodic currents during the stripping step and obtains higher detection sensitivity during the determination of the mercury (II) ions. The Hg^^ ionic solutions with concentrations ranging from I nM to I pM in the presenee and absenee of the supporting electrolytes, 30 mM nitric acid (HNO3 ) and 0.1 M potassium nitrate (KNO3 ), under various magnetie flux densities (B=0, 0.27 T, 0.53 T, and 0.71 T), were measured with the linear sweep stripping voltammetry (LSSV) technique.

25 The experimental results demonstrated that the stripping signals of the Hg^^ ions are enhanced, respectively, more than 10% and 30% in the absence and presence of the supporting electrolyte under a magnetic flux density B=0.71T as compared to the cases in the absence of the magnetic field with identieal eonditions.

3.1 Materials and Methods

3.1.1 Materials

3.1.1.1 Chemicals and Reagents

All chemicals were reagent grade and were used as received. Aqueous solutions were prepared by dissolving a certain amount of chemicals into high-purity de-ionized (DI) water (MilliQ water system). Mercuric nitrate, nitric acid, and potassium nitrate were purehased from Fisher Scientific. Experiments were performed in the absence and presence of the supporting electrolytes, 30 mM HNO 3 and 0.1 M KNO 3 . In each experiment, the total sample volume in the electrochemical cell was 4 mL. Pictorial views of the electrochemical cell are shown in Figure 3-1.

Figure 3-1 Electrochemical cell with electrodes

26 3.1.1.2 Electrodes

In these experiments, the working, counter, and reference electrodes are, respectively,

3-mm-diameter glassy carbon, 2-mm-diameter platinum wire, and Ag/AgCl (saturated

KCl) constructed with 0.5 mm diameter wire. All the electrodes were purchased from CH

Instruments Inc. Lengths of the working, counter, and reference electrodes dipped into the working sample were 9 mm each throughout the entire set of experiments. Electrodes used during the electrochemical detection of mercury are depicted as shown in Figure 3-

2 .

Working electrode

Reference electrode

Counter electrode

Figure 3-2 Electrodes used for the detection of mercuric ions

3.1.1.3 Electrode Polishing

Before conducting each experiment, a potential of 1.0 V for 30 seconds, 0.5 V for 20

seconds, and 0.2 V for 10 seconds was applied to the working electrode to make sure that

the mercury film deposited on the electrode from the previous experiment was stripped.

Subsequently, the working electrode was polished with the electrode polishing kit (CHI

27 120, CH Instruments Ine.) and was then rinsed sufficiently with Dl water. The electrode polishing kit is depicted in Figure 3-3.

Figure 3-3 Electrode polishing kit (CHI 120, CH Instruments Ine.)

Then, the working electrode was used to perform the stripping analysis of Dl water to make sure that there was no mercury left on the surfaee of the electrode. Similarly, the counter and reference electrodes were thoroughly rinsed and cleaned with Dl water between two conseeutive experiments. When contaminants became apparent, such as if any peaks appear in the voltammograms of Dl water in the absence of mercury (11) ions, the Pt wire counter electrode was held at 1.5 V vs. another Pt wire electrode in 30 mM

28 HNOs and 0.1 M KNO3 electrolyte solution to oxidize any of the mercury film that may have remained on the counter electrode. The electrodes were vertically inserted into a cylindrical electrochemical cell made of Pyrex glass for clear visibility and general chemical inertness.

3.1.1.4 Magnets for MHD Convection

To investigate the effects of the MHD convection on the peak anodic peak current, the electrochemical cell is positioned on one of the permanent magnets with its north pole facing towards the bottom of the electrochemical cell. The permanent magnets with magnetic flux densities B=0.27 T, 0.53 T, and 0.71 T were purchased from K&J

Magnetics Inc. The magnets are pictorially shown in Figure 3-4. The sizes of the permanent magnets are much larger than that of the electrochemical cell, thus their magnetic flux densities are approximately uniform in the direction parallel to the axis of the electrochemical cell. In such a way, the directions of the current densities across the working and counter electrodes are perpendicular to the external magnetic field.

Figure 3-4 Magnets with magnetic flux densities B=0.27 T, 0.53 T, and 0.71 T

29 During the deposition step, a deposition potential Ed= -0.3 V is applied to the working electrode for a deposition time td=180 s. Both the deposition potential and deposition time were under the optimal conditions, which will be discussed later. During the stripping step, the current-voltage (I-V) curves with a potential scan rate of 5 mV/s and were recorded through an electrochemical workstation (Model 832B, CH Instruments

Inc.) controlled by software (CHI Version 6.05, CH Instruments Inc.) [148]. Each experiment was repeated at least twice, and the experimental results were obtained reproducible with relative errors below 5% from its average value. Figure 3-5 depicts the experimental setup of the electrochemical workstation.

2008/04/24 14 63

Figure 3-5 Electrochemical experiment measuring aqueous mercuric ions

30 The background current behavior for the different electrolytes employed here was first examined by cyclic voltammetry with a slow potential scan rate of 5 mV/s. In the potential window from 0 to +1.0 V, HNO 3 and KNO 3 produced similar voltammograms where other activities due to oxidation on the surface of the working electrode were not visible.

Reference Electrode

Working Electrode Counter Electrode

Cell Lid J x B (Lorentz Force)

F lo w

Sample Solution

M agnet

Figure 3-6 Schematic representation of a cylindrical electrochemical cell positioned on a permanent magnet with a magnetic flux density B. Working, counter, and reference electrodes are vertically inserted into the electrochemical cell containing a sample

solution to be detected. A current density J transmitted through the solution results when a potential is applied across the working and reference electrodes. The interaction between the current density and the magnetic field induces Lorentz forces JxB which ultimately induce fluid motion. The induced MHD convection can be used to enhance the mass transport of the electroactive species to/ffom the surface of the working electrode without the use of any mechanical stirrers.

31 In our experiments, the stripping peak current (Ip) corresponding to the Hg^^ ions in the absence of the supporting electrolyte appears between +0.590 V and +0.593 V.

Schematic representation of the electrochemical cell with electrodes [Figure 3-1] is depicted in Figure 3-6.

The stripping peak current of the Hg^^ ions in the presence of the supporting electrolyte occurs between +0.568 V and +0.592 V. Therefore, the potential window

(0-1.0 V) is appropriate for the analysis. There were no visible complexes formed between the mercury (II) ionic solution and the supporting electrolyte during the experiments.

3.1.2 Methods

Superior to other methods, ASV technique has the advantages of relatively higher sensitivity, lower cost and portability. There are two basic steps involved with this technique. The first step of this method involves the accumulation of trace metals on the surface of the working electrode during the period of deposition time which is called preconcentration. The second step involves quantification of the metal contents present in the electrode during which the metals are stripped from the electrode surface back to the bulk solution of the sample. So, ASV is a voltammetric technique in which current measured at a working electrode is plotted as a function of the potential range or window applied. In the first step, known as pre-concentration, the reduction of metal occurs on the working electrode surface with a potential applied to the solution. The applied potential in the pre-concentration step is maintained over a period of time of deposition, also called the deposition step, as the reduced metals deposit onto the electrode surface. To effectively deposit the reduced metals onto the working electrode, a stronger stirring

32 mechanism can help transport the metal ions from the bulk solution to the electrode

surface. In viewing these requirements, the MHD technique was introduced, effectively

stirring the sample solution without using any moving parts. This MHD technique works with an orthogonal interaction of magnetic fields to the electric field present in the

solution. This will be discussed later in more detail. Following reactions involving

reduction and oxidation occur during the electrochemical determination on the surface of the working electrode:

(1) Reduction of metal ions:

M^ + ne' ► M° (Deposition Step)

(2) Oxidation of elemental metal:

M^ ----- ► M^ + ne' (Stripping Step)

For the detection of mercuric ions:

(1) Reduction of mercuric ions:

Hg^^ + 2 e----- ' ►Hg'^ (Deposition Step)

(2) Oxidation of elemental mercury :

Hg'’ ► Hg^^ + 2e' (Stripping Step)

The second step of the ASV is responsible for quantifying the metal ions deposited on

the working electrode in the form of elemental metal. Once the metal is oxidized, it goes back to the bulk solution from the surface of the electrode and remains as the metal ion in

the solution. Thus, the voltammogram is generated by the current measured during the

stripping step versus the potential scanned during the stripping step. Previously, it has been well documented that the metal concentration in the solution is proportional to the

33 oxidation current produced during the stripping step. In the stripping step, when a potential scan is applied to the working electrode, the deposited metal is oxidized.

Deposition potential and the stripping potential scan waveform are applied to the surface of the working electrode where the reaction of metal ions occurs. As depicted in

Figure 3-5, when the potential is across the working and the reference electrode

(Ag/AgCl saturated dipped into KCl), mercuric ions are reduced into their elemental form and electroplated at the working electrode. The counter electrode is usually a platinum wire, which helps complete the current path in the solution. The external potentiostat helps control the voltage and then current flows through the solution. A separate stirring mechanism is replaced by the MHD based technique which is arranged by putting a permanent magnet underneath the electrochemical cell. This is done in such a way that the current and magnetic flux densities are perpendicular to each other and induce chaotic fluid convection in the solution according to the Lorentz force.

In brief, ASV combined with the MHD technique has excellent detection limits for trace metal analysis (especially mercury (II) ions) with an appropriate choice of local parameters to optimize the detection limits and the resolution of the voltammogram. It is well known that the deposition mechanism applied to deposit the metal during the first step of ASV can affect the detection limit and the shape of the voltammogram as well.

The detection limit as well as the resolution of the current vs. potential plot can be greatly improved and excellently achieved by selecting important controllable variables such as deposition time, deposition potential and stripping parameters.

34 3.2 Results and Discussion

The effects of the MHD convection on the stripping analysis of ionic mercury (II) with various concentrations in the absence and presence of the supporting electrolyte were experimentally investigated under various deposition potentials, deposition times, and magnetic flux densities.

3.2.1 Reduction and Oxidation of Mercury

During the deposition step, the mercury (II) ions are deposited onto the surface of the working electrode through the following redox reaction, Hg^^ + 2e- o Hg”. During the stripping step, the deposited mercury film is oxidized back to the bulk solution, inducing a flow of an anodic current. The peak anodic current obtained from the current-potential

(I-V) curve is proportional to the scan rate and the thickness of the mercury film deposited during the deposition step [59, 149-151]. Figure 3-7 depicts the I-V curves of

100 nM mercury (II) ionic solutions in the absence [Figure 3-7 (a)] and presence [Figure

3-7 (b)] of the supporting electrolyte when the magnetic flux density B=0 (solid line) and

B=0.71 T (dashed line) is applied during both the deposition and stripping steps. The I-V curves are obtained after the deposition time td=180 s at the deposition potential Ed= -0.3

V. In the absence of the magnetic field, the peak currents of 100 nM Hg^^ ionic solutions in the absence and presence of the supporting electrolyte are, respectively, 23.0 nA and

63.3 nA. Note that the value of the peak current reported here represents the difference between the current corresponding to the mercury (II) ions and the baseline current both in the absence of the magnet.

35 3.2.2 Effects of MHD on Electrochemical Detection

In the presence of a magnetic field with B=0.71 T applied during both steps, the peak currents of 100 nM Hg^^ ionic solutions in the absence and presence of the supporting electrolyte are, respectively, 24.5 nA and 8 6 nA.

As mentioned earlier in the previous case, the value of the peak current reported here also represents the difference between the current corresponding to the mercury (II) ions and the baseline current both in the presence of the magnet. The resulting anodic current increases with higher magnetic flux density mainly due to the increase in the thickness of the deposited mercury film on the surface of the working electrode during the deposition step, which will be discussed later. In the absence of the supporting electrolyte [Figure 3-

7(a)], the anodic current exponentially increases with the potential until it peaks at about

0.59 V and then it declines as the potential increases further [151]. In the presence of the supporting electrolyte [Figure 3-7(b)], the peak current occurs at about 0.57 V. The presence of the supporting electrolyte, 30 mM HNO3 and 0.1 M KNO3 , significantly enhances the total current level under all other identical conditions. The scan rate, deposition potential, and deposition time are, respectively, 5 mV/s, ta=180 s, and Ed=

-0.3 V.

36 25 (a)

15 < c gc 3 U

0.5 0.6 0.7

Potential (V) vs. Ag/AgCI (saturated KCl)

1.95

S i.85

0.5 0.6 0.7

Potential (V) vs. Ag/AgCI (saturated KCl)

Figure 3-7 Stripping anodic current vs. potential of 100 nM Hg^^ ionic solution in the absence (a) and presence (b) of the supporting electrolyte, 30 mM HNO 3 and 0.1 M

KNO3, when the magnetic flux densities are, respectively, B=0 (solid line) and B=0.71T

(dashed line) during the deposition and stripping steps. The scan rate, deposition potential, and deposition time are, respectively, 5 mV/s, td=180 s, and Ed= -0.3 V.

37 3.2.3 Optimization of Local Parameters

The anodic currents depend on the deposition potentials and deposition times during the deposition step. The effects of the deposition potential on the stripping peak current

of the 100 nM Hg^^ ionic solution with the supporting electrolyte, in the absence (i.e.,

B=0) and presence (i.e., B-0.71 T) of a magnetic field during both the deposition and

stripping steps, are investigated. Figure 3-8 depicts the peak current of the 100 nM mercury (II) ionic solution as a function of the deposition potential when the magnetic

flux densities are, respectively, B=0 (solid line) and B=0.71 T (dashed line), and the deposition time is td=180 s. Similar results were obtained for other concentrations of mercury (II) ionic solutions under different magnetic flux densities.

3.2.3.1 Deposition Potential

When the deposition potential is below a certain threshold value (i.e., Ed<-0.5 V), as the deposition potential decreases, the stripping peak current corresponding to the mercury (II) ions decreases. The peak current is almost independent of the deposition potential in the range -0.5 V

Ed>-0.2 V. Similar results for deposition potential were obtained for other deposition times ranging from 120 s to 210 s. The resulting peak current is directly related to the

amount of the deposited mercury, which is governed by the ionic mass transport

(convection, diffusion, and migration) and the surface electrochemical reaction during the

deposition step. Figure 3-8 implies that the deposition potential in the range -0 .5

V

deposition step. Figure 3-8 also depicts that the peak current in the presence (dashed line)

38 of a magnetic field is much higher than that in the absence (solid line) of a magnetic field under all other identical conditions. The increase in the peak current is due to the enhancement of the ionic mass transport through the induced MHD convection during both the deposition and stripping steps.

3.2.3.2 Deposition Time

Next, the effects of the deposition time on the stripping peak current corresponding to the Hg^^ ions in the presence of the supporting electrolyte was investigated when the deposition potential is maintained at Ed= -0.3 V. Figure 3-9 depicts the stripping peak current of the 100 nM mercury (II) ionic solution as a function of the deposition time when B=0 (solid line) and B=0.71 T (dashed line) during both the deposition and stripping steps. Since the stripping peak current is proportional to the thickness of the mercury film deposited on the surface of the working electrode, as expected, when the deposition time is relatively small, the thickness of the deposited mercury film and thus the stripping peak current would be small.

As the deposition time increases, the thickness of the deposited mercury film increases, leading to the increase in the stripping current. Figure 3-9 depicts that the peak current does not significantly increase after the deposition time 180 s. Therefore, the deposition time td=I80 s was chosen in all other experiments. Note that the optimal deposition time also depends on the deposition potential. When the deposition time is long enough for all the mercury ions to be deposited onto the surface of the working electrode, the thickness of the deposited mercury film and thus the stripping peak current saturate.

39 100

-î — — î “ —

- 0.6 -0.5-0.4 -0.3 - 0.2 - 0.1 Deposition potential (V)

Figure 3-8 Peak current (Ip) of 100 nM Hg(N0 3 ) 2 solution as a function of the deposition potential (Ed) in the presence of the supporting electrolytes, 30 mM HNO3 and 0.1 M

KNO3 , when the magnetic flux densities are, respectively, B=0 (solid line) and B=0.71 T

(dashed line). Linear sweep with a scan rate 5 mV/s was performed after a deposition time td =180 s.

The choice of both the deposition time and the deposition potential is an interesting optimization problem that is not addressed here. The deposition time (td=180 s) and potential (Ed= -0.3 V) used in the experiments may have not been were not under the global optimal condition, and a further increase in detection sensitivity would be possible.

Under identical conditions. Figure 3-9 depicts that the stripping peak current in the presence of a magnetic field is higher than that in the case of B=0, under which there is no convection to enhance the ionic mass transport during both the deposition and

40 stripping steps. Figures 3-7 to 3-9 have obviously demonstrated the effects of the magnetic field on the stripping current of the mercury (II) ions.

100

75 < c c 2 50 3

I 25

0 60 120 180 240 Deposition time (s)

Figure 3-9 Peak current (Ip) of 100 nM Hg(N0 3 ) 2 solution as a function of the deposition time (td) in the presence of the supporting electrolyte, 30 mM HNO 3 and 0.1 M KNO 3 , when the magnetic flux densities are, respectively, B=0 (solid line) and B=0.71 T (dashed line). The scan rate and deposition potential are, respectively, 5 mV/s and Ed — 0.3 V.

3.2.3.3 Deposition and Stripping Steps

To identify during which step the effects of the magnetic field on the stripping peak current of the mercury (II) ions is more significant, following experiments were conducted under the following four cases; (a) B=0 during both the deposition and stripping steps; (b) B=0 during the deposition step, and B=0.71 T during the stripping

41 step; (c) B=0.71 T during the deposition step and B=0 during the stripping step; and (d)

B=0.71 T during both the deposition and stripping steps.

Figure 3-10 depicts the relative peak current enhancement during the above four

cases, where, § Ip ~ ( Ip _b=Bd,b=Bs ~ ^ p, b=o, b=o ) / ^ p , b=o, b=o ^ • ^p,B=Bn, B=Bg represents the obtained stripping peak current when the strengths of the magnetic fields during the deposition and stripping steps are, respectively, B=Bd and B=Bg. Similarly, Ip g^g represents the obtained stripping peak current in the absence of a magnetic field during both the deposition and stripping steps (i.e., B d= B s=0). The open and shaded bars in

Figure 3-11 represent, respectively, the relative peak current enhancement, ôlp, of 100 nM mercury (II) ionic solution in the absence and presence of the supporting electrolyte,

30 mM HNO3 and 0.1 M KNO 3 . In the absence of the supporting electrolyte (open bars), the relative peak current enhancements in the cases (a), (b), (c), and (d) are, respectively,

0 %, 3.48 %, 7.83 %, and 11.74 %. In the presence of the supporting electrolyte, the relative peak current enhancements in the cases (a), (b), (c), and (d) are, respectively, 0

%, 9.32 %, 25.91 %, and 35.86 %. The results demonstrated that the use of a magnet during the deposition step is more effective to enhance the stripping peak current than the use of the magnet during the stripping step.

42 40

0) 30 E s

.c 20 ï

ü 10

l

0.0 0,0.71T 0.71T, 0 Q.71T, 0.71T Magnetic flux density (B d, Bs) during the deposition (B=Bd) and stripping (B=Bg) steps

Figure 3-10 Peak current enhancement (ôlp) of 100 nM Hg(N 0 3 ) 2 solution in the absence

(open bars) and presence (shaded bars) of the supporting electrolyte, 30 mM HNO3 and

0.1 M KNO3 . The magnetic field B= (B d, Bg) in the x-axis represents that the magnetic fields in the deposition and stripping steps are, respectively, B = B d and B-Bg. The scan rate, deposition potential, and deposition time are, respectively, 5 mV/s, td=180 s, and

Ed=-0.3V.

3.2.3.4 Magnetic Flux Densities

The effects of the magnetic field on the stripping peak current primarily arise from the enhancement of the ionic mass transport by the induced MHD convection and the thickness of the mercury film deposited onto the surface of the working electrode during the deposition step since the stripping peak current is proportional to the thickness of the formed mercury film at the end. The enhancement of the ionic mass transport through the induced MHD convection during the stripping step also enhances the stripping current.

43 despite its effect being not as significant as that in the deposition step. Therefore, a higher

stripping current is obtained when the magnetic field is positioned during both the

deposition and stripping steps. Figure 3-10 also depicts the important roles of the

supporting electrolyte.

Figure 3-11 depicts the relative peak current enhancement as a function of the

magnetic flux density for the 100 nM Hg^^ ionic solution in the absence (solid triangles)

and presence (solid circles) of the supporting electrolyte. In the presence of a magnetic

field, the magnet is positioned during both the deposition and stripping steps. The lines in

Figure 3-11 represent the linear curve fitting of the experimental data. The equations of the linear curve fitting are, respectively, ôlp = 14.97 B and ôlp = 50.30 B with

correlation coefficients R=0.98 and 0.99 in the absence and presence of the supporting

electrolyte. The slope of ôlp vs. B curve in the presence of the supporting electrolyte is

3.4 times greater in the absence of the supporting electrolyte. The relative peak current

enhancement increases almost linearly as the magnetic flux density increases since the

induced MHD convection linearly increases with the applied magnetic field [150-158]. In

the presence of the supporting electrolyte, the effects of the magnetic field on the

enhancement of the stripping peak current become more significant since the total current

level in the presence of the supporting electrolyte is higher than that in the absence of the

supporting electrolyte as depicted in Figures 3-7 (a) and 3-7 (b), leading to higher

Lorentz forces and more significant fluid motion to enhance the ionic mass transport.

44 5 E 8 mc £ 0)c c g 3 I

0.25 0.5 0.75 M agnetic flux den sity (T)

Figure 3-11 Peak current enhancement (ôlp) of 100 nM Hg(N 0 3 ) 2 solution as a function

of the magnetic flux density (B) in the absence (solid line) and presence (dashed line) of

the supporting electrolyte, 30 mM HNO3 and 0.1 M KNO3 . The deposition potential and

time are, respectively, Ed= -0.3 V and 180 s. Linear sweep with a scan rate 5 mV/s was performed during the stripping step. The lines represent the linear curve fitting of the

experimental data.

Under all other identical conditions, the presence of the supporting electrolyte

increases the total current transmitted through the electrolyte solution as shown in Figure

3-7, which results in higher Lorentz forces through the interaction between the current

density and the magnetic field, thereby a more significant fluid motion is induced to

enhance the ionic mass transport. Next, the effects of the induced MHD convection on

the determination of the mercury (11) ions with concentrations ranging from 1 nM to 1

pM were investigated. Figure 3-12 depicts the peak current (1?) as a function of the

45 concentration of the ions (Cng^ ) in the absence of the supporting electrolyte when

B=0 (solid line with open circles) and B=0.71 T (dashed line with open circles) and in the presence of the supporting electrolyte when B=0 (solid line with solid circles) and B=0.71

T (dashed line with solid circles).

1000

750 - r < c c 500 - Ë

0 250 500 750 1000 Concentration of mercuric ions (nM)

Figure 3-12 Peak current (Ip) as a function of the concentration of the Hg^^ ions (Cng^ ) in the absence of supporting electrolyte when B=0 (solid line with open circles) and B=0.71

T (dashed line with open circles), in the presence of the supporting electrolyte when B=0

(solid line with solid circles) and B=0.71 T (dashed line with solid circles).

3.2.4 Enhancement of Peak Current under MHD Convection

Under all other identical conditions, the peak currents in the absence and presence of the supporting electrolyte are enhanced, respectively, by about 10-12 % and 34-40 %

46 when B=0.71 T. Thus, the enhancements of the peak currents under the magnetic field

B=0.71 T are much higher in the presence of the supporting electrolyte than its absence.

The linear regression analyses of the experimental data depicted in Figure 3-12 are listed in Table 3-1. The peak currents are linearly proportional to the concentrations of the mercury (II) ions in the sample. In the absence of the supporting electrolyte, the slope when B=0.71 T is a factor of 1.1 higher than that in the absence of the magnetic field. In the presence of the supporting electrolyte, the slope, when B=0.71 T, is 1.4 times higher than that in the absence of the magnetic field.

Table 3-1 Linear regression analysis of the experimental data of Figure 3-12

Presence of the Symbols in Magnetic flux Linear Correlation supporting Figure 3-12 density regression coefficient electrolyte

Solid line with B=0 No Ip=0.23CHg^^ 1 open circles

Dashed line with B=0.71 T No Ip=0.26CHg^+ 1 open circles

Solid line with B=0 Yes Ip=0.65CHg^+ 1 solid circles

Dashed line with B=0.71 T Yes Ip=0.9CHg^+ 1 solid circles

47 In the absence of the supporting electrolyte, the relative peak current enhancements

for 1 nM, 10 nM, 100 nM, and 1 pM Hg^^ ionic solutions are, respectively, 9.9 %, 10.3

%, 11.76 %, and 12.12 %. In the presence of the supporting electrolyte, their relative peak current enhancements are, respectively, 33.6 %, 34.8 %, 35.86 %, and 40.12 % as

depicted in Figure 3-13. For both in the absence and presence of the supporting

electrolyte, the effects of the magnetic field on the stripping signal increase as the

concentrations of the Hg^^ ions increase. This is due to the fact that the current

transmitted through the solution increases with the concentration of the mercury (II) ions

[148] leading to higher Lorentz force and stronger fluid motion to enhance the ionic mass transport.

40

■E 30 0) E 8 1 20 5

l o U 3 Q.

10” 10* 10' 10^ Concentration of mercuric ions (nM)

Figure 3-13 The relative peak current enhancement (ôlp) as a function of the

concentration of the Hg^^ ions (Cng^^) in the absence (open bars) and presence (shaded bars) of the supporting electrolyte, 30 mM HNO3 and 0.1 M KNO3 . The magnetic flux

density B=0.71 T was applied during both the deposition and stripping steps.

48 3.2.5 Effects of Supporting Electrolyte

To demonstrate the importance of the supporting electrolyte, Figure 3-14 depicts the

relative peak current enhancements, ôlps=(lps b=b„ b=Bs “ Ip,b=b„ b=bJ/Ip,b=b„ b=Bs x 100%,

due to the presence of the supporting electrolyte, as a function of the concentration of the

Hg^^ ions. I PS represents the peak current when the magnetic field B=Bd during

the deposition step and B=Bg during the stripping step in the presence of the supporting

electrolyte. Similarly, Ip g_g^ represents the peak current when the magnetic field

B=Bd during the deposition step and B=Bs during the stripping step in the absence of the

supporting electrolyte. The solid line with triangles and the dashed line with circles

represent, respectively, ôlpg when Bo-Bs-O and ôlpg when Bd=Bs=0.7I T. The results

showed that the presence of the supporting electrolyte enhances the peak currents by 73-

180 % and 110-250 % for the Hg^^ ionic solutions with concentrations ranging from 1

nM to IpM when B=0 and 0.71 T, respectively.

In the absence and presence of the magnetic field during the deposition and stripping

steps, when the concentration of the Hg^^ ions is relatively low, the enhancement of the peak current due to the presence of the supporting electrolyte increases as the

concentration of the Hg^^ ions increases. When the concentration of the Hg^^ ions is

above a certain threshold value, the signal enhancement due to the supporting electrolyte becomes independent of the concentration of the Hg^^ ions. More speeifically, there is an

optimal concentration for the supporting electrolyte for a given concentration of the Hg'^

ions.

49 250 — t»

I E 8 c 150 £n 0)c c Ë 3 S Q.

10° Concentration of mercuric ions (nM)

Figure 3-14 The relative peak current enhancement (ôlps) due to the presence of the supporting electrolyte as a function of the concentration of the Hg^^ ions (Cng^^). The solid line with triangles and the dashed line with circles represent, respectively, the conditions when B=0 and B=0.71 T during the deposition and stripping steps.

Conclusions

During the determination of Hg^^ ions in an aqueous solution using the traditional

ASV technique, a current density transmitted through the solution results when a potential is applied during both the preconcentration and the stripping steps. The resulting current depends on the rate of the ionic mass transport of the electroactive species to/ffom the surface of the electrode. In the presence of a magnetic field, the interaction between the resulting current and the magnetic field induces Lorentz forces which induce fluid motion. The induced MHD convection can be used to enhance the mass transport of the

Hg^^ ions towards the surface of the working electrode without the use of any mechanical

50 stirrers or rotating electrodes. The effects of the induced MHD convection on the

determination of Hg^^ ions have been experimentally demonstrated. The experimental

results demonstrated that the sensitivity of the detection of samples containing 1 nM Hg^^

ions without a supporting electrolyte can be increased by more than 1 0 % in the presence

of a magnetic field, B-0.71 T. The detection sensitivity is enhanced more than 30% in the

presence of the supporting electrolyte, 30 mM HNO 3 and 0.1 M KNO 3 . The effects of the

magnetic field on the stripping peak currents mainly arise from the enhancement of the

ionic mass transport by the induced MHD convection during the deposition step. The results also showed that the use of the supporting electrolyte enhances the stripping peak

currents for 1 nM to 1 pM Hg^^ ions by 73-180 % and 110-250 %, respectively, when

B=0 and B=0.71 T.

The obtained results demonstrated that the MHD can be used to enhance the detection

sensitivity of mercuric ions without using any mechanical stirrers or rotating electrodes

which may be useful to enhance the detection sensitivity of many other heavy metal ions

in various applications.

51 CHAPTER 4

ELECTROCHEMICAL SPR DETECTION OF

MERCURY UNDER MHD CONVECTION

Summary

A high-precision technique that can detect mercury (II) ions down to I femto-mole concentration in aqueous solutions is introduced. The technique combines the conventional electrochemical method, surface plasmon resonance (SPR), and magnetohydrodynamic (MHD) mixing. Mercury (II) ions are electroplated onto a gold

SPR sensing surface in the form of elemental mercury and then detected quantitatively by applying a potential scan with stripping voltammetry. Both the SPR angular shift and electrochemical current signal are recorded for identification and quantification of mercuric ion content in the solution. The detection sensitivity is further enhanced by applying the MHD convection in the presence of a magnetic field, which does not require any moving parts intruding in the aqueous solution. This is advantageous for small detection volume. In the presence of supporting electrolytes; I mM nitric acid (HNO3 ) and 10 mM potassium nitrate (KNO3 ), Hg^^ ionic solutions with concentrations ranging from 1 fM to 1 pM are tested under different magnetic flux densities of B=0 T, 0.27 T,

0.53 T, and 0.71 T. The experimental results demonstrate that the stripping signals of the

1 fM to 1 pM Hg^^ ions are enhanced by 10- 60 % with the flux density B=0.71T.

52 4.1 Materials and Methods

4.1.1 Materials

4.1.1.1 SPR Instrument

Surface plasmons (electron gases) travel with a wave vector that depends upon the properties of the substrate (i.e. composition, thickness, optical properties) and the properties of the adsorbed layer of the analytes (i.e. surface coverage, thickness, optical properties). The adsorbed layer’s properties can be observed as the substrate’s properties remain unchanged. When the wave vector of the incident light matches the wave vector of the plasmons, then surface plasmons get excited. The reflected light beam from a thin

~50 nm gold film is observed as a function of the incident angle during a typical SPR experiment. In the Kretschmann-Raether configuration, a high index of the refraction prism and the thin gold film are optically coupled. When it crosses the total internal reflection, an evanescent electromagnetic wave travels into the gold film. Therefore, the shifts in the position of the angle of minimum reflectivity and the thickness of the analytes deposited on the substrate surface are correlated linearly. This is depicted in

Figure 4-1. The key of the technique is to excite and detect collective oscillations of free electrons in the metal film referred to as surface plasmons. The excitation and detection of metal film on the gold chip is achieved through the Kretschmann configuration, which is first focused by source through a glass prism and then detects the reflected light

[Figure 4-1 (a)]. At a particular resonance angle, the plasmons start resonating with the light, producing the absorption of light when a dark line appeared in the reflected beam

[Figure 4-1 (b)]. A molecular binding exists onto the gold chip, or resonance angle shift

(in mDeg) can be observed by a conformational change in the analytes and gold chip

53 binding. Angle shift as a function of time can be monitored during the deposition and stripping step of analytes onto the surface of the gold chip if the SPR is coupled with an electrochemical workstation [Figure 4-1 (c)]. Electrochemical measurements are obtained in an I-V graph for the mercuric ions present in the aqueous solution. The detection sensitivity of SPR can be obtained in pDeg if it is used very precisely.

> - M etal film

Incident light Reflected light Angular sMt

Time

Figure 4-1 Mechanism of SPR measurement for the detection of mercuric ions

Figure 4-2 is the depiction of the SPR instrument used for the detection of mercuric ions from the aqueous solutions. Three electrodes were inserted into an electrochemical cell containing mercuric ionic solutions.

54 2008/ 04/24 16:23

Figure 4-2 Electrochemical SPR instrument

4.1.1.2 SPR Electrochemical Cell

An electrochemical cell was used to test the sample solution containing mercuric ions. The internal volume of the cylindrical electrochemical cell, shown in Figure 4-3, is

1.2mL, with 0.7 mL occupied hy a sample solution to be tested.

Figure 4-3 Quadrupled shaped cylindrical electrochemical cell

55 4.1.1.3 Electrodes and Accessories

The bottom of the electrochemical cell acting as the working electrode is a glass slide made of BK7 PEEK material (refractive index n~ 1.515 and thickness 350 gm-lSO gm), depicted as in Figure 4-4 covered with a 2 nm chromium coat underneath a 46 nm gold coating. It is placed on the refractive surface of a prism using refractive index matching fluid (refractive index n~ 1.515) [Figure 4-5]. A Teflon 0-ring, with internal and external diameters of 15 mm and 19 mm, respectively, is tightly fitted at the bottom to prevent leakage between the bottom and the cell wall. Sample solutions of mercury (II) ions are prepared with supporting electrolytes, 1 mM HNO3 and 10 mM KNO3 , and high- purity de-ionized (Dl) water from MilliQ water system [Figure 4-6]. All chemicals are reagent grade, purchased from Fisher Scientific, and used directly without any further contamination.

Figure 4-4 Electrodes: (a) Gold chip, (b) Ft counter, and (c) Ag/AgCl (3 M NaCl gel)

56 The counter and reference electrodes, as shown in Figure 4-4, are a platinum wire of

0.508 mm in diameter and 8 mm in length along with an Ag/AgCl wire, dipped into a saturated solution 3 M NaCl gel, measured at 0.5 mm in diameter.

Figure 4-5 (a) Matching fluid with syringe and (b) electrode holder

Figure 4-6 Sample vials containing mercuric ionic solutions

57 4.1.2 Methods

Compared to gold electrodes, the detection of Hg^^ is not easier by using carbon electrodes due to the weaker bonding between elemental mercury and the carbon electrode surface and the slow mechanism of nucléation and growth deposition of mercury on the carbon electrodes. Therefore, this requires more time to deposit mercury onto the carbon electrode surface than on the gold surface. People have been using the rotating electrode to enhance the mass transport of Hg^^ ions or synthesizing the electrode with an auxiliary element to increase the Hg'’ deposition. A novel state of the art technique to dramatically enhance the mass transport of the mercury ions was proposed to achieve this goal. As mentioned earlier, MHD, an induced fluid convective force produced with an orthogonal interaction of current and magnetic fields, is utilized to enhance the mass transport of the mercury ions to/from the working electrode in order to achieve the low detection limit of mercury.

The detection of Hg^^ at glassy carbon electrodes requires longer deposition times and a large magnitude of negative deposition potentials as is experimented in the electrochemical detection of mercury with glassy carbon electrode for 180 s of the deposition time and -0.3 V of deposition potential. Gold electrodes generally require shorter deposition times than the use of glassy carbon as it combines with mercury quicker than the binding of glassy carbon and mercury. Figure 4-7 depicts the pictorial views of a typical SPR experiment measuring the mercuric ions.

Electrochemical techniques have been long used for various applications in the detection of metal. SPR combined with the electrochemical technique measures changes in the optical properties of an electrode surface or molecules adsorbed on the electrode.

58 Figure 4-7 SPR experiment measuring aqueous mercuric ions

Combined SPR and electrochemical measurements can provide additional insights into various electrochemical phenomena and proeesses taking place at the solid-liquid interfaces. Examples include: charge-induced molecular adsorption/desorption and structural changes of adsorbents, electrodeposition, and anodic stripping. Biosensing

Instruments Incorporated provides SPR instruments that are compatible with major electrochemical instruments, allowing the seamless integration of SPR with electrochemical measurements. By controlling the potential of the “working electrode”

(SPR metal sensor) with respect to a reference electrode using a potentiostat, electrochemical processes can be studied by monitoring both the current and the SPR signal at the electrode. Figure 4-8 depicts a typical electrochemical SPR (E-SPR) measuring mercuric ion from aqueous solutions.

59 m

Figure 4-8 E-SPR experiment measuring aqueous mercuric ions

Surface plasmon resonance measurements are performed using the BI-SPRIOOO system purchased from Biosensing Instruments Inc., along with the electrodes, the electrochemical cell, and other accessories. The submerged depth of the counter and reference electrodes is maintained at 5 mm for all experiments. At each interval a potential sequence of 1.0 V for 30 seconds, 0.5 V for 20 seconds, and 0.2 V for 10 seconds is applied to the working electrode to ensure that the mercury film deposited in the preceding experiment is completely stripped. Subsequently, it is rinsed with DI water, followed by a series of cleaning with ethanol and thorough wiping with cotton tips. The working electrode is then tested with a stripping analysis of DI water to confirm no mercury is left on its surface. Similarly, the counter and reference electrode are thoroughly rinsed and cleaned with DI water many times at each interval. With any sign of contamination, the Pt-wire counter electrode is held at 1.5 V against another Pt-wire

60 electrode in 1 mM HNO 3 and 10 mM KNO 3 electrolyte solution to oxidize any remaining mercury.

A permanent magnet among those of flux density B=0.27 T, 0.53 T, and 0.71 T, purchased from K&J Magnetics Inc. [Figure 3-4], is placed in such a way that its flux is

orthogonal to the electric current density, as shown in Figure 4-9. The size of all magnets used is much larger than that of the electrochemical cell, so their magnetic flux densities

are approximately uniform and perpendicular to the axis of the electrochemical cell.

During the deposition step, a deposition potential Ed= -0.3 V is applied to the working gold electrode for a deposition time td=300 s. During the stripping step, the time history of angle shifts and voltage are recorded by the SPR reflectometer with the BI-

SPR Control Version 0.7.2. Similarly, current-voltage (I-V) curves with a potential scan rate of 10 mV/s are recorded through an electrochemical workstation (Model 832B as

shown in Figure 3-5, CH Instruments Inc.) controlled by a software (CHI Version 6.05,

CH Instruments Inc.) [159]. Each experiment is performed multiple times until the reproducibility of output data is ensured with a maximum mean deviation of 5%.

The background current behavior for electrolytes is first examined by cyclic voltammetry with a slow potential scan rate of 10 mV/s. In the potential window of -0.3 to +5.0 V, HNO3 and KNO 3 produced similar voltammograms while other activities due

to oxidation on the surface of the working electrode were not visible. The angle shifts,

corresponding to the various concentrations of Hg^^ ions from 1 fM to 1 pM in the presence of the supporting electrolyte, are recorded. Consequently, oxidizing and reducing stripping peak currents (Ip) in the cyclic voltammogram, corresponding to the

Hg^^ ions in the presence of the supporting electrolyte, are, respectively, between - 0 . 0 2

61 and -0.01 V and between +0.20 V and +0.21 V. The potential window -0.3 to 0.5 V is appropriate for the analysis. Schematic representation of the electrochemical SPR cell is depicted in Figure 4-9.

Reference Counter electrode electrode

SPR electrochemical cell

I® ® ® Mercury thin film Working electrode M agnet

Prism Light To so u rc e d e tec to r

Figure 4-9 Schematic representation of a cylindrical SPR electrochemical cell positioned in a magnetic field provided by a permanent magnet. Counter and reference electrodes are inserted into the electrochemical cell containing a sample solution to be detected and the gold working electrode is placed on the bottom of the cell. A current density transmitted through the solution occurs when a potential is applied across the working and reference electrodes. The orthogonal interaction between the current density and the magnetic field induces Lorentz forces which ultimately induce fluid motion. The induced

MHD convection can be used to enhance the mass transport of the electroactive species to/from the surface of the working electrode without the use of any mechanical stirrers.

62 4.2 Results and Discussion

4.2.1 Reduction and Oxidation of Mercury

During the deposition step, the mercury (II) ions are deposited onto the working electrode through the redox reaction, Hg^^ + 2e- o Hg“, which is more rapid than binding kinetics of Hg with other elements. During the oxidation step, the deposited mercury is oxidized to the bulk solution, inducing a flow of an oxidizing current signal in the cyclic voltammogram. Superior to other electrochemical methods, cyclic voltammetry is appropriate to visualize the reduction and oxidation of mercury in a cyclic loop.

4.2.2 Effects of MHD on SPR Detection

The peak oxidizing current, obtained from the oxidization part of the I-V curve, is proportional to the scan rate and the thickness of the mercury film deposited during the deposition step [54, 150, and 151]. Figure 4-10 shows the transient current, potential, and SPR angle shift for 1 nM mercury (II) ionic solutions in the absence (B=0, solid line) and presence (B=0.71 T, dashed line) of the MHD convection. These results were obtained from electrochemical SPR measurement of mercuric ions. SPR instrument measures the angle shift vs. time and electrochemical analyzer measures current vs. potential. A combination of both results show the graphical comparison as depicted in

Figure 4-10.

63 0.1 0.5 (a)

0.3

- 0.2

- 0.1

-0.4 -0.3 300 400 500 Time (s) 300 400 500 Time (s)

S’ 4 Q E

w 0) 2 tm Q: (OCL 0 300 400 500

Time (s)

Figure 4-10 (a) Current vs. time (b) potential vs. time, and (c) SPR angle shift vs. time of

1 nM Hg^^ ionic solution in the presence of the supporting electrolyte, 1 mM HNO 3 and

10 mM KNO3 , when the magnetic flux densities are, respectively, B-0 (solid line) and

B=0.71T (dashed line) during the deposition and stripping steps. Dotted line in Figure 4-

10 (c) represents the SPR angle of the supporting electrolyte without Hg^^ ions. The scan rate, deposition time, and deposition potential are, respectively, 10 mV/s, td=300 s, and

E(j~ —0.3 V.

64 4.2.3 Comparison Between Electrochemical and SPR Results

The data are recorded at time increments of 0.1 s after the initial deposition time td=300 s with the deposition potential Ed= -0.3 V. The peak currents in the absence and presence of the magnetic field are 0.07 |uA and 0.08 pA, respectively, as shown in the

Figure 4-11. Figure 4-11 (a) and Figure 4-11 (b) were obtained from electrochemical and

SPR measurements respectively. The values for the peak current reported here represent the difference between the current corresponding to the mercury (II) ions and the baseline current in the absence of mercury. Thermal effects may be accountable at such low signals of SPR angle shift which is not performed here. So, the results obtained would be better appropriated by using a temperature control mechanism while measuring the SPR angle shift.

The peak current increases with the magnetic flux density mainly due to the increase in the thickness of the mercury deposit. In the case where B=0.71 T, a 15% increase is observed due to the MHD convection. The SPR angle shifts, which is proportional to the amount of the mercury deposit; however, it shows a significant jump fi-om 3.0 mDeg to

4.5 mDeg with B=0.71 T, which indicates 50% increase. The SPR clearly is more sensitive than the electrochemical measurement. Similar trends are observed by reducing the mercury (II) ion concentration to 1 nM, beyond which the peak currents are not distinguishable and only the SPR technique becomes appropriate. Though there are many electrochemical methods such as anodic stripping voltammetry etc.; cyclic voltammetry is chosen to clearly visualize the reduction and oxidation of mercury.

65 0.05

< 0.15

0.35 ■0.3 0.1 0.5

£ (0 ® O) 2 mc K Q. (O

0 ■0.3 0.1 Potential (V)

Figure 4-11 (a) Current vs. potential and (b) SPR angle shift vs. potential of 1 nM Hg^^ ionic solution in the presence of the supporting electrolyte, 1 mM HNO 3 and 10 mM

KNO3 , when the magnetic flux densities are, respectively, B=0 (solid line) and B=0.71T

(dashed line) during the deposition and stripping steps. The scan rate, deposition time, potential window, and deposition potential are, respectively, 10 mV/s, ta=300 s, -0.3V to

0.5 V negative scan, and Ea= -0.3 V.

66 4.2.4 Optimization of Local Parameters

4.2.4.1 Deposition Time

Figure 4-12 shows the SPR angle shift as a function of the deposition time with the

deposition potential maintained at Ea= -0.3 V, for 1 nM mercury (II) solution in the

absence (solid line) and presence (dotted line) of the MHD convection. For either case,

the angle shift increases monotonically with the deposition time until it asymptotes to a

saturation value.

5

O) O E Ë 4 .cM 0) D> C n Q£ W 3

2 0 200 400 600 Deposition time (s)

Figure 4-12 SPR angle shift vs. deposition time of 1 nM Hg^^ ionic solution in the presence of the supporting electrolyte, 1 mM HNO 3 and 10 mM KNO 3 , when the magnetic flux densities are, respectively, B=0 (solid line with open circles) and B=0.71T

(dashed line with solid circles) during the deposition and stripping steps. The scan rate, potential window, and deposition potential are, respectively, 10 mV/s, -0.3V to 0.5 V,

and Ed= -0.3 V.

67 4.2A.2 Deposition Potential

For the deposition potential used, the saturation appears to occur at approximately

180 s. The enhancement due to the MHD convection is clearly seen with a 60% increase in sensitivity. Figure 4-13 depicts the resulting SPR angle shift as a function of the deposition potential when the scan rate is 10 mV/s and the deposition time td=300 s.

When the deposition potential is below a threshold value (i.e., Ed<-0.5 V for B=0 and

Ed<-0.4 V for B=0.71 T), the SPR angle shift decreases.

4 o

w 0) O) c ra Q£ Q. (0

- 0.6 - 0.5 - 0.4 - 0.3 - 0.2 - 0.1 Deposition potentiai (V)

Figure 4-13 SPR angle shift vs. deposition potential of 1 nM Hg^^ ionic solution in the presence of the supporting electrolyte, 1 mM HNO 3 and 10 mM KNO 3 , when the magnetic flux densities are, respectively, B=0 (solid line with open circles) and B=0.71 T

(dashed line with solid circles). The scan rate, potential window, and deposition time are, respectively, 10 mV/s, -0.3V to 0.5 V, and td= 300 s.

68 The SPR angle shift is almost independent of the deposition potential in the range

-0.5 V -0 .2 V for B = 0 and E d> -0.3 V for B =0.71 T. The deposition potential Ed= -0 .3 V , thus is a suitable value for high sensitivity. Again, MHD enhancement is highly conspicuous.

4.2.5 Effects of MHD on SPR Angle Shift

Figure 4-14 depicts the relative SPR angle shift as a function of the magnetic flux density for 1 nM mercury (II) ionic solutions. It is seen that in the presence of the supporting electrolyte, the MHD enhancement of the SPR angle shift appears almost linear for the range of magnetic flux density considered. The linear increase in the

Lorenz force with the magnetic flux density, giving rise to more significant fluid convection and thus ionic mass transport, seems to have produced a linear response in the sensitivity. Further increase in the magnetic flux density, in search of nonlinear saturation in MHD effect, is not undertaken because the magnet with B=0.71 T has provided sufficient MHD enhancement in the detection of mercury (II) ions of femto- mole concentration in order, as discussed below. The linearly increasing trend line of angle shift vs. magnetic flux density indicates that the angle shift may increase with the use of higher magnetic flux density. This is due to the fact of higher ionic mass transport to/from the electrode during the detection. This can enhance the detection sensitivity of mercuric ions in aqueous solution.

69 1.6

1.2 (0 0) O) 2 0.8 a. (0

I O-i

îc ra JZ c m 0 0.25 0.5 0.75 Magnetic flux density (T)

Figure 4-14 Enhancement in SPR angle shift vs. magnetic flux densities of 1 nM Hg^^ ionic solutions in the presence of the supporting electrolyte, 1 mM HNO 3 and 10 mM

KNO3 . The scan rate, deposition time, potential window, and deposition potential, are, respectively, 10 mV/s, td=300 s, -0.3V to 0.5 V, and E

4.2.6 Enhancement of SPR Angle Shifts under MHD Convection

The MHD enhancement in the detection of extremely low concentrated mercury (11) ion is illustrated in Figures 4-15 and 4-16. The concentration is varied from 1 fM to 1 pM, and the corresponding SPR angle shift is recorded with and without the magnet

(B=0.71 T). These data are plotted on a logarithmic scale in Figure 4-15. The SPR angle shift seems logarithmically proportional to the concentrations of the mercury (II) ions in the sample solutions. The MHD convection consistently enhances the sensitivity significantly for the entire concentration range.

70 g Q E 10®

10 •2

10 ® 10 ^ lo '* 10'^ 10 ^ 10 1 10® 10^ 10^ 10^

Concentration of Hg^* (nM)

Figure 4-15 SPR angle shift as a function of the concentration of the Hg^^ ions 2+x from 1 fM to 1 |uM in the presence of supporting electrolyte, 1 mM HNO3 and 10 mM

KNO3 when B=0 (solid line with open circles) and B=0.71 T (dashed line with solid circles). The scan rate, deposition time, potential window, and deposition potential are, respectively, 10 mV/s, td=300 s, -0.3V to 0.5 V, and Ed= -0.3 V.

The percentage enhancement by MHD is shown in Figure 4-16. The relative SPR angle shift enhancements for 1 fM, 10 fM, 100 fM, 1 pM, 10 pM, 100 pM, 1 nM, 10 nM,

100 nM, and 1 pM Hg^^ ionic solutions are, respectively, 10 %, 16.7 %, 22.7 %, 31.1 %,

37.8 %, 45.3 %, 49.9 %, 54.3 %, 56.7 %, and 57.1 %. The MHD boost on the SPR angle shifts increases as the concentration of Hg^^ ions increases and seems to saturate in micro-molar order. The current transmitted through the solution increases with the

71 concentration of the mercury (II) ions [148], which leads to a higher Lorentz foree and stronger fluid motion to enhance the ionic mass transport. Relative enhancements do not have exact linear relationship with the concentrations of mercury.

40

20

0

10'^ 10'^ 1 0 1 0 '^ 10'^ 10 * 10“ 10* 10^ 10^ Concentration of (nM)

Figure 4-16 Enhancement in SPR angle shift in percentage as a funetion of the concentration of the Hg^^ ions (Cng^^) from I fM to 1 pM in the presence of supporting electrolyte, 1 mM HNO 3 and 10 mM KNO 3 when B=0.71 T. The scan rate, deposition time, potential window, and deposition potential are, respectively, 10 mV/s, td=300 s, -

0.3V to 0.5 V, and Ej= -0.3 V.

72 Conclusions

During the detection of Hg^^ ions in an aqueous solution using the SPR and the electrochemical technique, an electrical current is transmitted through the solution due to the potential applied during the deposition and the stripping steps. The entrent depends on the rate of ionic mass transport of the electroactive species between the electrodes.

The presence of a magnetic field applied externally generates the Lorentz forces that induce fluid motion, which, in turn, considerably enhance the mass transport of the Hg^^ ions toward the working electrode without the use of any mechanical stirrers or rotating electrodes.

The test results reported in the present study clearly show consistent enhancement in the detection sensitivity for all values of control parameters involved. The MHD convection does not require any mechanical stirrers or rotating electrodes, and can be applied to micro-devices with a small test cell. Since the electric current is already present in test solutions, the MHD convection is induced without additional power consumption. The MHD electrochemical SPR technique introduced here has a potential for further enhancement, beyond the 10% in femto-molar order, by the use of magnets with a higher flux density and can be a subject of many future studies.

73 CHAPTER 5

ELECTRODEPOSITION OF RADIONUCLIDES UNDER MHD

CONVECTION FOR ALPHA SPECTROMETRIC DETECTION

Summary

In recent years alpha spectrometry has received renewed interests due to global terrorism and in preparing thin layer sources required for high quality alpha spectrum.

The importance of a thin layer source is that it prevents the self-absorption of alpha particles, generates better resolution and produce better yields. There are a myriad of methods in the literature for the electrodeposition of radionuclides; however, such methods are tedious to reproduce and still needs improvement. In this study, a new method was developed to prepare thin layer sources that are highly effective and reproducible. The novel approach coined MHD electrodeposition consists of MHD convection coupled with electrodeposition for the preparation of mixed ^**U, ^*^Pu, and ^"^'Am sources for alpha spectrometric determination. The method was validated by all its application to natural samples of ^^*U and ^*"*U using ^^^U as a tracer. The yields and quality of alpha spectrum after using the new method were higher than those of the conventional methods available in the literature for the last twenty years, and the time required for source preparation is less than 60 min. The effects of magnetic flux density, constant current, pH and deposition time were also investigated.

74 5.1 Materials and Methods

5.1.1 Materials

5.1.1.1 Electrodeposition Cell

An electrodeposition cell was designed and constructed using 20 mL disposable

polyethylene liquid scintillation vials (VWR, Co.) with plastic threaded end caps lined

with aluminum foil. The components of the electrodeposition cell and the assembled

views of the cell are depicted in Figures 5-1 (a), 5-1 (b), and 5-1 (c).

Electrodeposition ceii

M agnet

Figure 5-1 (a) Components of electrodeposition cell (b) assembled views of the cell with a magnet, and (c) assembled views of the cell with no magnet

75 A 42.5 mm hole was bored on the side of the caps for the introduction of a 19.1 mm copper washer connector. The aluminum foil was removed and replaced hy a 22 mm steel planchet (A.F. Murphy Die & Co., Inc.) sandwiched between the copper washer and the mouth of the vial. The washer served as an electrical contact for the planchet cathode.

The bottom of the vial was removed for the introduction of the anode which consisted of a coil platinum-iridium wire electrode (12.7 mm diameter and 102 mm length) and a small plastic cone used as a splash guard for the unit.

5.1.1.2 Electrodes and Accessories

Before each experiment, the platinum anode was rinsed with dilute HNO 3 , Dl water, and then cleaned with ethanol several times. Figure 5-2 depicts platinum anode dipped into dilute HN 0 3 Solution. Similarly, the cathode made of stainless steel was thoroughly rinsed and cleaned with Dl water and ethanol many times prior to the experiments.

Figure 5-2 Spiral shaped platinum anode rinsed in dilute HNO3 solution

76 The pH measurements were made using a pH meter (Coming Inc.) and pH electrode

(Calomel combination Electrode) calibrated with pH 2, 4 and 6 buffers. A power supply

(Hewlett Packard Model E3610A) was used to provide a constant current of 0.5 A with a range from 0-12 V and 0-3 A. The pH indicator strips 0-2.5 pH units (EMD) were used to verify the salmon pink endpoint of the thymol blue indicator. Electrodes made of platinum and steel were used as anode and cathode. The electrodes used in the experiments are depicted in Figures 5-3 and 5-4.

.:.C- :

Splash guard

Platinum anode

Figure 5-3 Spiral shaped anode (made of platinum) with splash guard

Steel Cathodic surface

Figure 5-4 Disc shaped cathode (made of steel)

77 5.1.1.3 Tracers and Reagents

The radionuclides (Dilution of US-EPA 1843-2 ampoule initially made by

NIST dated Oct 1, 1993), ^*^Pu (Dilution of US-EPA RS # 94002 dated February 2,

1995) and ^'*’Am (Dilution of US-EPA RS # 9704 dated April 30, 1997) used for the electroplating experiments were prepared by the dilution of the following primary and secondary standards: ^*^Pu, ^**U-natural and ^^'Am. The final activity of the solution used in the electroplating experiments was ^**U (125 dpm), ^*“*U (118 dpm), Pu (110 dpm) and ^"^’Am (112 dpm) per mL of stock solution. The total activity was 465 dpm/mL.

[Tracer ^'*^Pu was prepared from National Institute of Standards and Technology (NIST) traceable standard reproduced in US-EPA SRM # 4334G, June 7, 1994). For the working standards, a known volume of the tracer was transferred to a volumetric flask followed hy dilution with approximately 2 N HNO 3 . The concentration of the working standards was in the order of 100 dpm/mL or 1.5-2.0 Bq/mL]. All chemicals were reagent grade materials, used as received and diluted with 18 MQ water.

5.1.2 Methods

Samples used in the electroplating experiments were prepared by transferring % mL of the stock solution to a 50 mL beaker; adding 2 mL of concentrated nitric acid followed by the addition of 0.5 mL of concentrated sulfuric acid heating at a high temperature (on a hot plate) until the nitric acid volatilized and the sulfuric acid evolved into dense white fumes. The residual sulfuric acid in the beaker was allowed to cool and then diluted with double washes of approximately 3~4 mL each of purified water followed by the addition

of 4 to 6 drops of a 0.02 % thymol blue indicator (as sodium salt). The pH of the solution was then adjusted to a range of 2.0 to 2.3 by the drop-wise addition of concentrated

78 ammonium hydroxide until the salmon-pink end point of the thymol blue indicator was reached. The endpoint was also verified using a pH meter on selected samples, primarily at the beginning of the experiments. The pH-adjusted solution was then poured into the electrodeposition cell, assembled as shown in Figure 5-1, and the beaker rinsed with 2 -3 mL of purified water, giving a final volume in the electroplating cell of approximately 10 to 12 mL. The solution was electroplated at a constant current of 0.5 A and after an initial voltage drop to between 5.5 and 6.5 VDC, the voltage reading dropped slowly as the resistance of the cell decreased.

Electrodeposition experiments were carried out at 3, 5 10, 20, 30, 45 and 60 min, respectively. The optimum electrodeposition time was identified to be 60 min and this time was used for all experiments. At the end of the deposition step, the reaction was quenched by adding 10 mL of 1.5 M ammonium hydroxide to fill the cell and then the cell was allowed to stand for 3 to 5 min. The electrodeposition experiment was terminated by turning off the power supply, raising the anode and pouring out the solution into a waste container. The cell was then disconnected and the planchet rinsed with methanol and blow dried with hot air at a temperature of -150 °C. The typieal experimental views for the electrodeposition of radionuclides with no magnets are depicted in Figures 5-5.

79 ^ Electrodeposition cell

^ P ow er su p p ly unit

Figure 5-5 Electrodeposition of radionuclides with no magnetic field

5.1.2.1 Electrodeposition Cell under MHD Convection

Before the electrodeposition experiment was initiated, permanent magnet were placed vertically upright on opposite sides (outside) of each electrodeposition cell with their

north and south poles facing perpendicular to the cathode so that their fluxes ran

orthogonal to the current density flowing through the solution during each

electrodeposition experiment. The ahove procedure was repeated using magnets (K&J

Magnetics Inc.) of different intensities; B = 0.27 T, 0.53 T, and 0.71 T, respectively

[Figure 3-4]. The diameters of the permanent magnets are much larger than the diameter

of the electrochemical cell, thus their magnetic flux densities are approximately uniform

in the direetion parallel to the axis of the electrochemical cell. Placed in this way, the

directions of the current densities across the electrodes are perpendicular to the external magnetic field. Similarly, the typical experimental views for the electrodeposition of radionuclides with a magnet are depicted in Figures 5-6.

80 Electrodeposition ceii | - M agnet

■ Power supply unit

Figure 5-6 Electrodeposition of radionuclides with magnetic fields

Figure 5-7 depicts an electrodeposited steel disc under MHD when B=0.71 T. The thin layer surfaces on the top of the disc are the electrodeposited surfaces of the planchet.

The thin layer surface was further analyzed by alpha spectrometer for energy spectrum of the radionuclides. Atomic Force Microscopy (AFM) can measure the thickness of the

electrodeposited thin film, which is not performed here.

Eiectrodeposited surface

Figure 5-7 Eiectrodeposited surface of the steel disc under MHD convection

81 5.1.2.2 Theory of MHD convection

Figure 5-8 shows the schematic representation of an integrated electrodeposition cell and a permanent magnet with a magnetic flux density B. The anode was made of platinum-iridium wire in a coiled shape which was inserted from the top of the cell. The schematic side view of Pt-anode looks like a disc electrode in the figure but it is a coiled shape electrode. The cathodic electrodeposition of the radionuclides was accomplished on a steel planchet that was kept at the bottom of the cell. The surface area of the steel planchet was almost covered by the surface area of the platinum-iridium electrode.

Splash guard

0 Magnet Pt electrode 12 \-DC 0.5 A Constant current RadtonucMes solution 0 Radronuchdes hydrous oxides

Steel planchet

Figure 5-8 Schematic representation of an integrated electrodeposition cell placed a permanent magnet with a magnetic flux density B. Anode made of a platinum electrode is inserted from the top of the cell and electrodeposition of radionuclides is performed on a steel planchet placed at the bottom of the cell. When a constant current is supplied across the electrodes, a current density J transmits through the solution. When an external magnetic field is applied, Lorentz forces JxB are induced as a result of the orthogonal interaction between the current density J and the magnetic field B. Thus induced fluid convection due to the Lorentz forces can enhance the mass transport of the radionuclides to the cathodic surface of the steel planchet with no other mechanical stirrers.

82 5.1.2.3 Measurement of Radionuclides Yield

Alpha spectrometer measures the amount of radionuclides present in a sample source

by detecting the emitted alpha particles from the source. Alpha-emitting radionuclides

spontaneously emit alpha particles (or "*He- Helium nuclei) usually between 3 and 9 MeV

of characteristic energy spectrums. These heavy charged alpha particles can be blocked

simply by a single sheet of paper or by human skin, so any physical medium present

between the alpha source of the radionuclide and detector surface can easily capture a

portion of the alpha particle energy. Precautions should be taken to decrease the tailing of

the energy peaks of the radionuclides by adjusting the appropriate distance between the

radionuclides source and the detector which can avoid self absorption of the alpha particles emitted from the source.

The preparation of alpha sources of the radionuclides is very important to achieve the

resolution of alpha spectrum results from the alpha spectrometer during alpha counting.

Better resolution, proportional deposition of the mixed radionuclides, and reduction of

self absorption would be obtained if radionuclides are uniformly deposited. The major

steps of preparation of the alpha source involve preliminary treatment, electrodeposition

of the radionuclide, and source mounting on the alpha spectrometer for measurement. It

should be understood that the best possible resolution of the radionuclide sources can be

obtained with an alpha spectrometer, if the alpha source is thin, flat, uniformly deposited

on the steel planchet disc. MHD, the state of the art novel technique, was applied to

achieve these objectives.

To determine the concentration of a radionuclide, a tracer of known concentration is

mixed while preparing the sample source of the radionuclides. The tracer radionuclide is

83 a radioisotope of the same element under study i.e., for and ^^Pu for

^^^Pu; however, different radionuclides can be used as a tracer element to find the

concentration of unknown concentration of the radionuclides. The same radionuclide is preferred as the radioisotopes of the same element behave chemically alike, so the lost percent of tracer in the detection processes is equal to the percent of the sample lost, under the assumption that the tracer and the sample radionuclide are mixed homogeneously to create a chemical equilibrium. Testing was carried out using high- purity solutions of several commonly analyzed radioactive elements, including uranium, plutonium, and americium both in combination and individually. The chemical recoveries

(yield) and peak resolution were determined by measuring the alpha emission rate from

the surface of the plates.

The yield obtained from the electrodeposition of each radionuclide was analyzed by

alpha spectrometry using a system consisting of five alpha analysts units (Canberra

Instruments) equipped with 450 mm^ Ultra™ detectors. The samples were placed 10-12 mm below the detector, and the spectra was collected and analyzed using Alpha Vision*^

version 5.3 and MAESTRO version 32-MCA (ORTEC Instruments).

Prior to the analysis, a routine energy calibration of the detectors was initiated using

3-9 MeV/Channel. Given the source, the absolute efficiency was calculated to be 20-25

%. Figures 5-9 and 5-10 depict the views of alpha detectors and a typical alpha counting

experiment of the radionuclides.

84 Figure 5-9 Alpha detectors used for the measurement of the radionuclides yields

Figure 5-10 Energy spectrums during alpha counting of eiectrodeposited radionuclides

85 5.2 Results and Discussion

5.2.1 Electrodeposition of Radionuclides

Electrodeposition of radionuclides was successfully performed using the method of standard electrodeposition cell. MHD technique is incorporated with the basic electrodeposition method at a constant current of 0.5 amperes over a deposition time of approximately 60 min. The original methodology of electrodeposition used to employ constant voltage conditions (12 VDC/ 1.0 ampere) over a deposition time of more than

90 min. This also required an operator to make minor adjustments to the overall plating circuit resistance using a large adjustable potentiometer because resistance varied in the electroplating cell [116]. Previously depicted Figures 5-5 and 5-6 differentiate between a conventional method and a new MHD method of electrodepositing radionuclides for alpha spectrometric measurement. The use of a constant current approach and a fully regulated power supply allow unattended operation of the electrodeposition cell and also ensure the magnetic and electric field interaction relatively constant over the entire plating period. The effects of the MHD convection on the preparation of alpha sources of radionuclides over the period of electrodeposition time on a steel planchet in the presence of 1 M ammonium sulfate were experimentally investigated under various experimental parameters such as deposition currents, deposition times, magnetic flux densities, and pH. All the further experiments were run under the local optimum parameters.

During the electrodeposition step, mixed radionuclides were deposited onto the surface of the steel planchet in the form of hydrous oxides typically; U(0H)4, Pu(0H)3, and Am(0H)4, respectively, which were later analyzed by alpha spectrometer.

86 5.2.2 Alpha Spectrometric Measurements of Radionuclides Sources

The energy resolutions of the ^^^Pu, and ^"^’Am sources were observed to have selectively higher deposition yield under magnetohydrodynamically enhanced electrodeposition. Various experimental parameters were optimized to produce uniform and thin layer deposition. First, electrodeposition of radionuclides was made with and without magnetic fields separately to study the MHD effect in the electrodeposition.

Second, the proportional plating and the radionuclides yields were measured by alpha spectrometer [160]. These experiments postulated that the coupling of MHD with the traditional electrodeposition method achieves the proportional plating and the whole abundance of each radionuclide as shown in Figure 5-11.

The identification and concentrations of the radionuclides are analyzed by the alpha spectrometric detector in terms of counts vs. energy. Peak locations of individual radionuclides in the energy vs. counts are unique in nature. This is due to the fact that the emitted peak energies by alpha particles of each radionuclide are specifically significant and different. This identifies and quantifies each radionuclide from the mixed alpha sources.

Figure 5-11 shows the alpha counts vs. energy plot of the radionuclides measured by alpha spectrometric determination when the magnetic flux densities are, respectively,

B=0 and B=0.71 T during electroplating. Proportional electrodepositions of the uranium, plutonium, and americium were obtained in approximately ten min. and essentially complete chemical recovery (more than 98 % deposition) occurred in about 60 min.

87 241 Ann

1.5 234 238 239 Pu 0 E 1 1 I £n Q. < 0.5

235

0 4 4.2 4.6 4.8 5 5.2 5.4 5.64.4

Energy (MeV)

Figure 5-11 Alpha counts vs. energy (MeV) plot of radionuclides in the presence of supporting electrolyte solutions as observed in the alpha spectrometric determination when the magnetic flux densities are, respectively, B=0 (dotted line), and B=0.71 T (solid line). The branching peaks of respective radionuclide are denoted by arrows. Compared to the energy spectrum without MHD Electrodeposition, the energy spectrum using MHD electrodeposition resembles the whole abundance for each radionuclide. Applied initial

DC potential, pH range, constant current, and deposition time, respectively, were 12 V, 2 to 2.3, 0.5 A, and 60 min. under all other identical conditions.

5.2.3 Effects of MHD on Energy Peaks of Radionuclides

The fine structure or branching ratio of the major uranium peaks (^^*U and is evident in the top portion of Figure 5-11 when B=0.27T, and the analysis of plutonium

88 alone, removing the proportionally large mass-effect of uranium, reveals a level of within-peak resolution well beyond what has been previously attainable with standard deposition protocols first published and subsequently refined by others [116]. Basie differences between the previous methods and the newly developed method are the applications of constant current electrodeposition with MHD instead of previously used constant voltage electrodeposition with no MHD. An orthogonal orientation of a constant current density and a uniform magnetic flux density in the electrodeposition cell can produce a constant magnitude of Lorentz forces. Lorentz forces enhance the ionic mass transport of radionuclides onto the surface of cathode. The seen effects are due to the application of MHD that can add the electrochemical mass transport of radionuclides onto the surface of the cathodic disc. Thus, prepared thin, thin layer, and uniformly deposited alpha sources produce a better deposition yield, clear surface morphology, and better detection sensitivity.

Figure 5-12 (a) shows the comparative proportional plating counts of each radionuclide, expressed as undeposited % vs. deposition time (min). Figure 5-12 (b) demonstrates the undeposited % of mixed radionuclides vs. deposition time (min). The percentage of undeposited mixed radionuclides are determined on the surface of the steel planchet for the optimal conditions of magnetic flux density, initial DC potential, pH range, constant current, and deposition time, respectively, B=0.71 T, 12 V, 2 to 2.3, 0.5

A, and 60 min. A solid line with solid triangles denotes the alpha counts of mixed radionuclides.

89 100

80 ■DS Ï Oc 60 '- V 2 I 40 I = 20

0 15 30 45 60 Deposition time (min)

100

80 5 ô 1 2 I 40 1 ë

0 15 30 45 60 Deposition time (min)

Figure 5-12 (a) Proportional counts of each radionuclide, expressed as undeposited radionuclides % vs. deposition time (min) under magnetic field B=0.71 T. Solid lines with solid circles, open circles, solid diamonds, and open diamonds denote, respectively, the alpha counts of ^^^Pu, and (b) proportional counts of mixed radionuclides are expressed as total undeposited radionuclides % vs. deposition time

(min).

90 5.2.4 Optimization of Local Parameters

5.2.4.1 Deposition Time

In the presence of a magnetic field at a constant current deposition of 0.5 A, resistance of the radionuclides solution increases as the potential drops. This confirms that electroplating of the radionuclides is rapidly occurring during the first 15 to 30 min. as shown in Figure 5-13. This is due to the fact that MHD enhances the ionic mass transport of the radionuclides onto the surface of the steel planchet.

6.5

n c o 0. 5.5

0 30 4515 60 Deposition time (min)

Figure 5-13 Potential drop (V) vs. time (min) during the deposition of the mixed radionuclides when B=0 (solid line with open circles) and B=0.71 T (solid line with solid circles). Applied initial DC potential, pH range, magnetic flux density, and deposition time, respectively, were 12 V, 2 to 2.3, 0.71 T, and 60 min.

91 2.0

O % 1.4

c 3 8 n

15 30 45 60 75 90

Deposition time (min)

7

O 5 IL £ c 3 8 10 £a < 3

1 15 30 45 60 75 90

Deposition time (min)

Figure 5-14 Alpha counts vs. deposition time (min) plot of (a) each radionuclide

234u^ 239pu^ Am are, respectively, represented by solid lines with solid circles, open

circles, solid diamonds, and open diamonds, and (b) mixed radionuclides represented by

solid lines with solid triangles. Applied initial DC potential, pH range, constant current, and magnetic flux density, respectively, were 12 V, 2 to 2.3, 0.5 A, and 0.71 T.

92 Figure 5-13 clearly shows the potential drop vs. time of the mixed radionuclides during the electrodeposition time in the presence of supporting electrolyte solutions. In the first 15 min., the potential drop with MHD largely dominates the potential drop without MHD. This indicates that the deposition of radionuclides is much higher with

MHD than with its absence.

The effects of the deposition time on the alpha counts corresponding to the thin layers of radionuclide hydrous oxide were investigated. Figure 5-14 (a) shows the alpha counts vs. deposition time (min) plot of each radionuclide ^^^Pu, and ^"^'Am, respectively. Similarly, Figure 5-14 (b) shows the alpha counts vs. deposition time (min) of the mixed radionuclides.

Since the alpha counts are proportional to the thickness of the radionuclides hydrous oxide film deposited, as expected, when the deposition time is negligible, the thickness of the deposited film, and thus the alpha counts would be smaller. As the deposition time increases until the planchet surface is saturated with the hydrous oxides of mixed radionuclides, the thickness of the deposited hydrous oxides increases which leads to the increase in the alpha counts. This is because it emits more alpha particles to the detector during alpha spectrometric determination. When the deposition time is long enough for almost all the hydrous oxides to be deposited, the thickness of the radionuclides films saturate and would not be enhanced beyond 60 min.

Compared to the work performed [116], the novel technique improved the yield of same radionuclides in half the deposition time. Figure 5-14 depicts the alpha counts do not significantly increase after the deposition time of 60 min. even with the use of magnetic fields. Therefore, the deposition time 60 min. is obtained to be enough time to

93 complete electrodeposition, the deposition time td- 60 min. is chosen in all other experiments. Note that the optimal deposition time may also depend on the deposition potential. The combination of both the selection of deposition time and the deposition potential may be an interesting optimization problem that is not addressed here. The deposition time (ta=60 min.) and constant current (Ia= 0.5 A) used in the experiments were identified under the local optimal conditions obtained in the laboratory, and a further increase in detection sensitivity would be possible.

S.2.4.2 SEM Images of Eiectrodeposited Surfaces

Figures 5-15 (a) and (b) show the scanning Electron Microscope (SEM) images of eiectrodeposited surfaces when the magnetic flux densities are B=0, low magnification, and B=0, high magnification, respectively. Similarly, Figures 5-15 (c) and (d) show the scanning Electron Microscope (SEM) images of eiectrodeposited surfaces, when the magnetic flux densities are B=0.27 T, low magnification, and B=0.27 T, high magnification, respectively.

Scanning electron microscopy (SEM) images reveal the uniform depositions with more grain sizes formed using magnetically enhanced alpha sources. These images are unlike those obtained from samples made without the benefit of magnetic enhancement, which exhibited biases and oriented structure at the nano-scale level as shown by the accumulation of deposited material along grain imperfections or striations in the stainless steel plating substrate. The physical differences seen in the SEM images are responsible for the corresponding improvements in the experimentally produced alpha spectra. Alpha particles continuously emitted from the surface of the plate at rates proportional to the radioactive half-lives of the various species are less likely to collide with the substrate or

94 other atoms and lose energy (be attenuated or absorbed) and thus peaks with closely related energies are resolved much more cleanly and inter-peak interferences are reduced.

(a) B=0, low magnification (b) B=0, high magnification

(c) B=0.27 T, iow magnification (d) B=0.27 T, high magnification

Figure 5-15 Scanning Electron Microscope (SEM) images of eiectrodeposited surfaces of steel planchet, when the magnetic flux densities and SEM magnification are, respectively,

(a) B=0, low magnification (b) B=0, high magnification, (c) B=0.27 T, low magnification

(d) B=0.27 T , high magnification. Applied initial DC potential, pH range, constant current, and deposition time, respectively, were 12 V, 2 to 2.3, 0.5 A, and 60 min. under all other identical conditions.

95 5.2.4.3 Magnetic Flux Densities

Figure 5-16 (a) depicts the alpha counts as a function of magnetic flux densities (T) plot of each radionuclides ^^^Pu, and Am, respectively. Similarly, Figure 5-

16 (b) shows the alpha counts vs. magnetic flux densities (T) of mixed radionuclides. The enhancement in alpha counts increases almost linearly as the magnetic flux density increases. As mentioned earlier, the induced MHD convection linearly increases with the intensity of the applied magnetic field during electrodeposition. The effects of the magnetic field on the enhancement of the alpha counts become more significant since the presence of the magnetic field leads to higher Lorentz forces and more significant fluid motion to enhance the ionic mass transport during electrodeposition.

Figures 5-11 to 5-16 have demonstrated the effects of the magnetic field on metrology and the surface morphology of the alpha sources. The effects of the magnetic field on the counts primarily occur from the enhancement of the ionic mass transport by the induced MHD convection and the thickness of the hydrous oxides of radionuclides deposited. Then, the corresponding alpha counts are proportional to the thickness of the film formed during deposition. Under all other identical conditions, the presence of the magnetic field increases the alpha counts quantitatively and qualitatively, which results with the interaction between the current density and the magnetic flux density, and thereby, a more significant fluid stirring is induced.

96 o

•2. 1.6 -

■£ 1.4

1.2 i 0 0.5 0.750.25 Magnetic flux density (T)

6.2

o 5.8 % î c 3 8 ra .c 5.4 Q. <

5 0 0.25 0.5 0.75 M agnetic flux d en sity (T)

Figure 5-16 Alpha counts vs. magnetic flux densities (T) plot of (a) each radionuclides

238u^ 234y^ arc, respectively, represented by solid lines with solid circles, open circles, solid diamonds, and open diamonds, and (b) mixed radionuclides represented by solid lines with solid triangles. Applied initial DC potential, pH range, constant current, and deposition time, respectively, were 12 V, 2 to 2.3, 0.5 A and 60 min.

97 5.2.4.4 Constant Current Electrodeposition

The peak energy speetra during alpha measurement depend on the deposition currents and deposition times during the electrodeposition. The effects of different magnitudes of constant current on the electrodeposition of the radionuclides in the absence (i.e., B=0) and presence (i.e., B=0.71 T) of a magnetic field were investigated. Figure 5-17 (a) shows the alpha counts vs. current plot of each radionuclide ^^^Pu, and Am, respectively. Similarly, Figure 5-17 (b) shows the alpha counts vs. current of the mixed radionuclides.

The results of such alpha counts for each radionuclide and the mixed radionuclides with various current values show that 0.5 A is an optimum condition for electrodeposition. Values above or below 0.5 A do not give better yield of the alpha counts of radionuclides because there may be significant gas evolution and electrode corrosion at higher current densities and the lower current densities. This situation can create unfavorable condition for electrodepositing the radionuclides.

Next, the effects of the induced MHD convection on the determination of the natural samples of mixed uranium-238 and uranium-234, and of 4.24 pCi were investigated with tracer radionuclide uranium-232, of known concentration activity.

This will be discussed later. Under all other identical conditions, larger alpha counts and proportional electrodepositing in the presence of the magnetic field B=0.71 T during electrodeposition were obtained compared with no magnetic field. This is because the magnetic fields have electrochemically enhancing effects on the alpha metrology.

98 1.5

o>

0.1 0.3 0.5 0.7 Current (A)

£ 5 2 4

0.1 0.3 0.5 0.7 Current (A)

Figure 5-17 Alpha counts vs. current (A) plot of (a) each radionuclide ^^^Pu, and ^^'Am, are respectively, represented by solid lines with solid circles, open circles, solid diamonds, and open diamonds, and (b) mixed radionuclides represented by solid lines with solid triangles. Applied initial DC potential, magnetic flux density, and deposition time, respectively, were 12 V, 0.71 T and 60 min.

99 5.2.4.5 pH of Solution

Figure 5-18 (a) shows the alpha counts vs. pH of solution for each radionuclide

234|j^ 239pu^ respectively. Similarly, Figure 5-18 (b) shows the alpha counts vs. pH of solution for the mixed radionuclides. The initial hydrolysis of the radionuclide ions depends on the pH of the solution and should occur at the cathodic surface of the steel disc to produce the maximum desired deposition of the radionuclides. Figures 5-18

(a) and 5-18 (b) show the change in deposition yield with variation in the pH of the solution for each individual and the mixed radionuclides.

All the radionuclides exhibit the optimum deposition yield at pH between 2 and 2.3, and then the yield decreases above or below pH 2 to 2.3, as shown in Figure 5-18. The results are in good agreements with the work published [116]. The deposition yield for uranium-238 and uranium-234 has the same trend but plutonium-239 and americium-241 indicates some changes in the nature of the deposition trends. The energy resolution and homogeneity of the eiectrodeposited sources using MHD compared with no MHD under identical conditions. The MHD results show the better resolutions, proportional electrodeposition, more alpha counts, and more homogeneity of the eiectrodeposited hydrous oxides of radionuclides.

100 1.5

o

X

3 8 re

0.5

1.01.5 2.0 2.5 pH of solution

1.0 1.5 2.0 2 5 pH of solution

Figure 5-18 Alpha counts vs. pH plot of (a) each radionuclide ^^^Pu, and

are respectively, represented by solid lines with solid circles, open circles,

solid diamonds, and open diamonds, and (b) mixed radionuclides represented by solid lines with solid triangles. Applied initial DC potential, constant current, magnetic flux density, and deposition time, respectively, were 12 V, 0.5 A, 0.71 T and 60 min.

101 5.2.5 Effects of MHD on Alpha Spectrometric Determination

Figure 5-19 depicts the alpha counts of each radionuclide from a mixed solution as a function of their concentration when B=0.71 T (solid line with solid symbols) and B=0

(dashed line with open symbols). The comparative slopes of the counts vs. concentration with and without magnetic fields show an increase of almost 30 % detection sensitivity of each radionuclide with the application of the magnetic field during the electrodeposition.

The deposition rate of the hydrous oxides of the radionuclides was not measured here.

Subsequently, alpha eounts with and without magnetic fields are increasing with the increase of the concentration of each radionuclide. This is due to the fact that the current density transmitted through the solution increases with the concentration of the radionuclides. Figures 5-19 (a) and 5-19 (b) depict the alpha counts vs. concentration of

^ 'J Q O /I natural samples of U and U. These results were obtained by using tracer radionuclide The detection sensitivities of these two uranium isotopes were enhanced by about 30 %. However, the enhancements of their corresponding alpha counts were not uniform even though they were observed at better resolution. The detection sensitivity of natural samples of radionuclides with a tracer was not so suceessful with MHD due to possible interactions with other radionuclides. These can possible future works.

102 9000 9000

6000 Itf) 6000 1a < 3000 < 3000

20 40 80060 100 0 20 40 6080 100 Concentrations of (dpm/mL) Concentrations of^^^U (dpm/mL)

9000 9000

6000 6000 c t < 3000 ^ 3000

0 20 40 60 80 0 20 40 60 80 100 Concentrations of Pu (dpm/mL) Concentrations of^^^Am (dpm/mL)

Figure 5-19 Alpha counts vs. the concentration of radionuclide activities (dpm/mL) plot

of, respectively, (a) when B=0 (dashed line with open circles), and B=0.71 T (solid

line with solid circles), (b) when B=0 (dashed line with open circles), and B=0.71 T

(solid line with solid circles), (c) ^^^Pu when B=0 (dashed line with open circles), and

B=0.71 T (solid line with solid circles), and (d) ^'^'Am when B=0 (dashed line with open

circles), and B=0.71 T (solid line with solid circles). Applied initial DC potential,

constant current, radionuclides pH range, and deposition time, respectively, were 12 V,

0.5 A, 2 to 2.3 and 60 min.

103 Conclusions

The effects of the magnetic field on the detection sensitivity of radionuclides enhanced by the electrodeposition process were investigated. The deposition rate linearly increased with the magnitude of the magnetic flux density due to the increased ionic mass transport in the diffusion layer near the surface of the steel planchet due to the action of the induced Lorentz forces.

During the electrodeposition of radionuclides onto the cathodic surface under a MHD fluid convection, a constant current density transmitted through the aqueous solution was maintained. Under the application of an external magnetic field, the orthogonal interaction between the resulting current density and the external magnetic flux density induces Lorentz forces which ultimately generate fluid motion and act as a stirring action with no moving parts. The induced MHD convection can be used to enhance the mass transport of the radionuclides towards the surface of the steel planchet. The detection sensitivities of the radionuclides with supporting electrolytes are enhanced and each of them is proportionally eiectrodeposited from the mixed radionuclides solution. The effects of the magnetic field on the alpha counts mainly arise from the enhancement of the ionic mass transport by the induced MHD convection during the electrodeposition.

The deposition conditions for electroplating radionuclides have been locally optimized for experimental parameters such as constant current of 0.5 A, pH range of 2 to 2.3, deposition time of 60 min, and a higher magnetic flux density of 0.71 T. The alpha spectrometric yields of the optimized technique for electrodeposition of radionuclides were higher, more selective, and proportional. This modified MHD electrodeposition method made it possible to prepare ultra thin layer, uniform alpha particle sources. Thus

104 produced radionuclides source can generate better resolution under reproducible conditions. The detection sensitivity was enhanced by about 30 % with the use of MHD convection.

The obtained results demonstrated that the MHD can be used to enhance the detection sensitivity of other radionuclides without using any mechanical stirrers or rotating electrodes.

105 CHAPTER 6

SUMMARY AND CONCLUSIONS

With the application of the electrochemical technique, a current density is transmitted through the solution of ions in an aqueous solution using the traditional ASV technique, when a potential is applied during both the preconcentration and stripping steps. The resulting current depends on the rate of the ionic mass transport of the electroactive species to/from the surface of the electrode. In the presence of a magnetic field, the interaction between the resulting current and the magnetic field induces Lorentz forces which induce fluid motion. The induced MHD convection can be used to enhance the mass transport of the Hg^^ ions towards the surface of the working electrode without the use of any mechanical stirrers or rotating electrodes. The effects of the induced MHD convection on the determination of Hg^^ ions have been experimentally demonstrated.

The experimental results by electrochemical technique demonstrated that the sensitivity of the detection of samples containing 1 nM Hg^^ ions without supporting electrolytes can be increased by more than 10% in the presence of a magnetic field,

B=0.71 T. The detection sensitivity is enhanced more than 30% in the presence of the supporting electrolyte,30 mM HNO3 and 0.1 M KNO3 . The effects of the magnetic field on the stripping peak currents mainly arise from the enhancement of the ionic mass transport by the induced MHD convection during the deposition step. The results also showed that the use of the supporting electrolyte enhances the stripping peak currents for

106 1 nM to 1 pM ions by 74-180 % and 110-250 %, respectively, when B=0 and

B-0.71 T.

A current density is transmitted through the solution of Hg^^ ions in an aqueous sample using SPR coupled with the traditional CV technique when a potential is applied during both the preconcentration and stripping steps. The resulting current depends on the rate of the ionic mass transport of the electroactive species to/from the surface of the electrode. The induced convection due to MHD can be used to enhance the mass transport of the Hg^^ ions towards the surface of the working electrode without the use of any mechanical stirrers or rotating electrodes as mentioned in the previous technique. The effects of the induced MHD convection on the determination of Hg^^ ions in the form of an SPR angle shift have been experimentally measured. The experimental results demonstrated that the sensitivity of the detection of samples containing 1 fM Hg^^ ions with supporting electrolytes can be increased by more than 10% in the presence of a magnetic field, B=0.71 T. The detection sensitivity of 1 pM Hg^^ ions with supporting electrolytes is enhanced nearly by 60% in the presence of the supporting electrolyte, 1 mM HNO3 and 10 mM KNO3 . The effects of the magnetic field on the SPR angle shifts mainly arise from the enhancement of the ionic mass transport by the induced MHD convection during the deposition step. The results also showed that the use of the magnetic field enhances the angle shifts for 1 fM to 1 pM Hg^^ ions by nearly 10-60 %, respectively, when B=0.71 T than in the case of B==0 under all other identical conditions.

The magnetic field effects on depositing hydrous oxides of radionuclides during the electrodeposition process were investigated. The deposition rate linearly increased as the magnetic flux density increased due to the increased ionic current density flown in the

107 diffusion layer near the surface of the steel planchet by the action of the induced Lorentz forces. During the electrodeposition under a MHD fluid convection, a constant current density transmitted through the aqueous solution is produced. Under the application of an external magnetic field, the orthogonal interaction between the resulting current density and the external magnetic flux density induces Lorentz forces which ultimately generate fluid motion that circulate the solution as a stirring action with no moving parts. The detection sensitivities of the radionuclides are enhanced dramatically, and each of them is proportionally electrodeposited from the mixed radionuclides. The effects of the magnetic field on the alpha counts mainly arise from the enhancement of the ionic mass transport by the induced MHD convection during the electrodeposition. The deposition conditions for electroplating radionuclides have been locally optimized for experimental parameters such as current, the pH, and deposition time. The alpha spectrometric yields of the optimized technique for electrodeposition of radionuclides were higher, more selective, and proportional.

These obtained results demonstrated that the MHD can be used to enhance the detection sensitivity of mercury ions and radionuclides such as uranium, plutonium, americium, without using any mechanical stirrers or rotating electrodes which may be useful to enhance the detection sensitivity of many other heavy metal ions and radionuclides. MHD has numerous potential applications that can be employed for the detection of heavy metal ions and radionuclides. MHD can also be employed for mixing of analytes in aqueous solutions.

108 CHAPTER 7

SUGGESTED FUTURE WORK

Future work may include redesigning MHD with a chaotic mixing mechanism for ultra detection purposes of single or multiple heavy metal ions and radionuclides. Chaotic mixing mechanism can be designed with multiple electrodes for an on/off circuit of mixing switch in milliseconds or in microseconds. Chaotic mixing can greatly enhance the process of detection. In addition, to miniaturize the sensors, micro or nano-scale cells and channels can be designed that may further increase detection sensitivity and reduce the sample volumes of analytes.

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122 VITA

Graduate College University of Nevada, Las Vegas

Yogendra M. Panta

Local Address: 1455 E. Rochelle Ave. Apt. # 53 Las Vegas, Nevada 89119

Degrees: Master of Science in Mechanical Engineering, 2004 Youngstown State University Youngstown, Ohio

Bachelor of Engineering in Mechanical Engineering, 2000 Tribhuvan University Kathmandu, Nepal

Bachelor of Science in Physical Science, 1995 Tribhuvan University Kathmandu, Nepal

Special Honors and Awards: • National Science Foundation (NSF), Nevada EPSCoR Graduate Research Fellowship in Sensor Technology, University of Nevada, Las Vegas, 2006- 2008

• Merit Recognition, G.A. Excellence in Teaching, University of Nevada, Las Vegas, 2007

Publications: (1) Y.M. Panta, S. Qian, M.A. Cheney, Determination of mercury (II) ions under redox-based magnetohydrodynamic convection, J. Coll. and Interface Sci., 2008 (Manuscript in preparation).

(2) Y.M. Panta, J. Liu, M.A. Cheney, S.W. Joo, S. Qian, Ultra-sensitive detection of mercury (II) ions using electrochemical surface plasmon resonance (SPR) with magnetohydrodynamic convection. Analyst, 2008 (In review).

(3) Y.M. Panta, S. Qian, M.A. Cheney, Stripping analysis of mercury (II) ionic solutions under magneto-hydrodynamic convection, J. Coll. and Interface Sci., vol. 317, pp. 175, 2008.

123 (4) Y.M. Panta, S. Qian, J.V. Cizdziel, C.L. Cross, Mercury content of whole eigarettes, cigars and chewing tobacco packets using pyrolysis atomic absorption spectrometry with gold amalgamation, J. Analytical and Appl. Pyrolysis, 2008 (In Press).

(5) Y.M. Panta, S. Qian, M.A. Cheney, The use of magneto-hydrodynamic convection for the determination of mercury (II) ions in aqueous solutions, American Physical Society, 60* Annual Meeting of the Division of Fluid Dynamics, Salt Lake city, Utah, 2007.

(6) Y.M. Panta, H.W. Kim, H. Marie, Numerical flow analysis of a hydraulic gear pump, American Physical Society, 60* Annual Meeting of the Division of Fluid Dynamics, Salt Lake city, Utah, 2007.

(7) Y.M. Panta, S. Qian, M.A. Cheney, Stripping analysis of mercury (II) ionic solutions under magneto-hydrodynamic convection, 12U* AO AC Annual Meeting & Exposition, Anaheim, California, 2007.

(8) Y.M. Panta, S. Adhikari, Rural energy technology. Engineering Vision- Department of Mechanical Engineering, Pulchowk Campus, Institute of Engineering, Tribhuvan University, Nepal, 2”‘* ed., pp. 36, 1999.

Dissertation Title: The Effects of Electromagnetic Fields on the Detection of Mercury and Radionuclides

Dissertation Examination Committee: Chairperson, Dr. Shizhi Qian, Ph.D. Committee Member, Dr. Robert F. Boehm, Ph.D. Committee Member, Dr. Ajit K. Roy, Ph.D. Committee Member, Dr. Marcos A. Cheney, Ph.D. Graduate Faculty Representative, Dr. James V. Cizdziel, Ph.D.

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