Ion selectivity in carrier-mediated dialysis and electrodialysis

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Ion Selectivity in Carrier-Mediated Dialysis and

Electrodialysis

by

Steven Paul Hansen

BSc, University of Victoria, 1987

A Dissertation Submitted in Partial Fulfillment of the

Requirements for the Degree of

DOCTOR OF PHILOSOPHY

in the Department of Chemistry

© Steven Paul Hansen, 2012

University of Victoria

All rights reserved. This dissertation may not be reproduced in whole or in

part, by photocopy or other means, without the permission of the author.

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Supervisory Committee

Ion Selectivity in Carrier-Mediated Dialysis and Electrodialysis

by

Steven Paul Hansen BSc, University of Victoria, 1987

Supervisory Committee

Dr. Alexandre Brolo, Department of Chemistry

Supervisor

Dr. David Harrington, Department of Chemistry

Departmental Member

Dr. Dave Berg, Department of Chemistry

Dr. Stephen Evans, Department of Biochemistry

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Abstract

Supervisory Committee

Dr. Alexandre Brolo, Department of Chemistry

Supervisor

Dr. David Harrington, Department of Chemistry

Dr. Dave Berg, Department of Chemistry

Dr. Stephen Evans, Department of Biochemistry

Outside Member

Membrane transport processes underlie many purification technologies. The efficiency of a membrane separation process depends upon material throughput (flux), and the degree to which the membrane discriminates amongst species in the feed stock (selectivity). In a supported liquid membrane, flux may be enhanced by carrier molecules, which act as catalysts of translocation. Carrier molecules also confer selectivity, via differential molecular recognition of the substances in the feed stock. The effect of electrical potential on the flux and selectivity of carrier-containing supported liquid membranes is not well documented. We elected to study the effect of electrical potential on supported liquid membranes containing valinomycin, a potassium ionophore, and a calixarene ester, a sodium ionophore. In these systems, the open circuit membrane potential could be made positive or negative by the choice of anion. With both of these carriers, we observed that selectivity for potassium or sodium salts was dependent on the open circuit membrane potential. To confirm that electrical potential was responsible for the observed selectivity variance, we applied a potential across the membrane using a potentiostat. The applied potential created conditions for carrier-mediated electrodialysis, where oxidation and reduction reactions on either side of the membrane act as the driving force for transmembrane flux of charged species. In chronoamperometry experiments, we found that selectivity for potassium or sodium ion was dependent on the applied electrical potential. Subject to some constraints, selectivity and flux could be controlled by the application of positive or negative electrical potentials. Linear sweep voltammetry experiments allowed for the rapid

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prediction of the potential that must be applied to achieve optimal selectivity. We also found that membrane potential measurements, as well as the magnitude of current that flows in chronoamperometry experiments, could be interpreted to predict Eisenman and Hofmeister sequences. These results are novel, and await a convincing theoretical justification. The results also suggest that a separation technology could be developed around the idea of modulating selectivity with electrical potential. In this regard, carrier-mediated electrodialysis may be suitable for the sequestration of toxic or radioactive heavy metals, and a large number of carrier molecules for metal ions are currently known. The technique may also be suitable for separating organic molecules, such as high-value chiral pharmaceuticals. Supported liquid membranes are a useful research tool, but industrial applications may require a more stable membrane architecture.

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List of Tables

Table 1.1 Membrane Processes... 3

Table 2.1 Summary of Celgard® 2400 film ... 25

Table 2.2 NPOE properties. ... 28

Table 2.3 Summary of membrane properties. ... 29

Table 3.1 Case 1 titration experiments. ... 47

Table 3.2 Case 2 titrations. ... 50

Table 3.3 Calix[4]ester single-salt titrations. ... 58

Table 3.4 Valinomycin single-salt titrations. ... 58

Table 3.5 Calix[4]ester two-salt titrations. ... 59

Table 3.6 Valinomycin two-salt titrations. ... 59

Table 3.7 Calix[4]ester two-salt titration. ... 60

Table 3.8 Dehydration energy for group I cations. ... 66

Table 3.9 Anion dehydration energy. ... 67

Table 3.10 Eisenman sequence experiment. ... 69

Table 3.11 Eisenman sequence experiment. ... 69

Table 3.12 Hofmeister sequence experiments. ... 70

Table 3.13 Hofmeister sequence experiments. ... 71

Table 3.14 Membrane potential reproducibility. ... 74

Table 4.1 Kex and Dm determined by fitting experimental data to equation 4.4. ... 84

Table 4.2 Association equilibrium constants of Na+ and K+ with valinomycin. ... 86

Table 4.3 Kex and Dm determined by fitting experimental data to equation 4.4. ... 88

Table 4.4 Kex and Dm using three different film supports. ... 90

Table 4.5 Relationship of membrane potential, K+/Na+ selectivity, and K+ and Na+ flux in competition experiments with 3 different anions. Valinomycin membrane. ... 93

Table 4.6 Relationship of membrane potential, Na+/K+ selectivity, and K+ and Na+ flux in competition experiments with 3 different anions. Calix[4]ester membrane. ... 94

Table 4.7 Measured and calculated cation selectivity’s (equation 4.6) using perchlorate salts. ... 96

Table 4.8 Determination of Kex and Dm by fitting equation 4.4 to experimental data. .... 97

Table 4.9 Additional statistics for Kex and Dm determination... 98

Table 4.10 Determination of Kex and Dm by fitting experimental points to equation 4.4. ... 98

Table 4.11 Additional statistics for Kex and Dm determination ... 99

Table 5.1 Relationship of flux to current flow in two electrode mode. Flux determined by conductivity measurements. ... 113

Table 5.2 Measured ion flux in ion accounting experiments. AE used for cation determinations, and anion chromatography for anion determinations. ... 120

Table 5.3 Comparison of the charge registered by the potentiostat ... 120

Table 5.4 Proportion of charge carried by the cation and anion in the ion accounting experiments. Calculated transference numbers. ... 122

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Table 5.5 Eisenman sequence experiment. Charge passed at + 0.5 and – 0.5 V.

Valinomycin membrane. ... 126

Table 5.6 Experimentally determined Kex. ... 126

Table 5.7 Eisenman sequence experiments. Charge passed at + 0.5 and – 0.5 V. ... 127

Table 5.8 Extraction equilibrium constants. Group I metal picrate’s ... 128

Table 5.9 Selectivity sequences of chloride salts... 130

Table 5.10 Hofmeister sequence experiments. Charge passed at + 0.5 and – 0.5 V. Valinomycin membrane. ... 132

Table 5.11 Hofmeister sequence experiments. Charge passed at + 0.5 and – 0.5 V. Calix[4]ester membrane. ... 133

Table 5.12 Anion mobility in NPOE. ... 134

Table 5.13 Hofmeister sequences. Increasing order of ... 136

Table 5.14 Carrier concentration in valinomycin and calix[4]ester membranes. ... 136

Table 5.15 Comparison of calix[4]ester and valinomycin anion sequences. ... 141

Table 5.16 Eisenman sequence experiments. Charge passed at + 0.5 V. ... 142

Table 5.17 Cation competition experiments. Valinomycin membrane. Chloride salts of K+ and Na+. ... 146

Table 5.18 Cation competition experiments. Valinomycin membrane. Nitrate salts of K+ and Na+. ... 147

Table 5.19 Cation competition experiments. Valinomycin membrane. Perchlorate salts of K+ and Na+. ... 149

Table 5.20 Comparison of potassium flux and selectivity of chloride, nitrate and perchlorate salts. Valinomycin membrane. ... 149

Table 5.21 Cation competition experiments. Calix[4]ester membrane. Chloride salts of K+ and Na+. 10 μM LiCl receiving phase. ... 150

Table 5.22 Cation competition experiments. Calix[4]ester membrane. ... 151

Table 5.23 Cation competition experiments. Calix[4]ester membrane. Chloride salts of K+ and Na+. 1 mM LiCl receiving phase. ... 151

Table 5.24 Cation competition experiments. Calix[4]ester membrane. Nitrate salts of K+ and Na+. ... 152

Table 5.25 Cation competition experiments. Calix[4]ester membrane. Perchlorate salts of K+ and Na+. ... 153

Table 5.26 Comparison of sodium flux and selectivity of chloride, nitrate and perchlorate salts. Calix[4]ester membrane. ... 154

Table 5.27 K+/Na+ selectivity at various potentials. Valinomycin membrane. ... 155

Table 5.28 Na+/K+ selectivity at various potentials. Calix[4]ester membrane. ... 156

Table 6.1 Parameters used in the calculation of activity coefficient. ... 181

Table 6.2 Calculated values of B and C. ... 182

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List of Figures

Figure 1.1 Cation facilitated gradient pumping. Symport case. ... 6

Figure 1.2 Cation facilitated gradient pumping. Antiport case. ... 7

Figure 1.3 BLM in a U-tube configuration. CHCl3 organic phase. All phases stirred. ... 8

Figure 1.4 Dibenzo-18-crown-6... 8

Figure 1.5 2.2.2 – Cryptand. ... 9

Figure 1.6 Example synthetic carrier molecules used by the Reinhoudt group. ... 10

Figure 1.7 Electrolysis of aqueous NaCl. ... 11

Figure 1.8 Purification of water via electrodialysis using ion exchange membranes. Aqueous Na2SO4 feed stock. ... 13

Figure 1.9 Carrier-mediated electrodialysis. Ion movement is linked to electrode redox reactions, and electron movement in an external wire. ... 14

Figure 1.10 Nonactin. ... 16

Figure 1.11 A porphyrin thiocyanate ionophore. ... 17

Figure 1.12 Five compartment electrodialysis cell used by Sadyrbaeva. ... 18

Figure 1.13 Common carriers in metal extraction electrodialysis research. ... 19

Figure 2.1 Delrin® housing, glass cells and gaskets. This apparatus was used for all experiments. ... 24

Figure 2.2 Structures of valinomycin, 1, and calix[4]ester, 2. ... 26

Figure 2.3 Double junction reference electrode. ... 32

Figure 3.1 Development of two boundary potentials. Diffusion potential, ED, is zero. 38 Figure 3.2 Development of a positive diffusion potential. Boundary potential, EB, is zero. ... 38

Figure 3.3 Single-salt and two-salt titration experiments. Concentrated salt solutions added to the left compartment, and double junction reference electrodes measure the resultant potential. ... 43

Figure 3.4 Case 1 single-salt titrations, showing a single dominant cation (3.4a), or anion (3.4b). ... 47

Figure 3.5 Two-salt titrations. Case 2 Nicolsky-Eisenman behaviour. ... 50

Figure 3.6 Case 3 single-salt titrations with sodium salts. ... 52

Figure 3.7 Case 3 single-salt titrations with potassium salts. ... 53

Figure 3.8 Case 3 single-salt titrations with potassium salts. ... 54

Figure 3.9 Case 3 single-salt titrations with sodium salts. ... 55

Figure 3.10 Case 3 two-salt titrations with potassium salts. Figures show the activity at which the anion reaches its detection limit. ... 56

Figure 3.11 Case 3 two-salt titrations with sodium salts. Figures show the activity at which the anion reaches it’s detection limit. ... 57

Figure 3.12 Case 3 two-salt titration. Cation dominance switching to anion dominance. ... 58

Figure 3.13 Case 4 two-salt titration? ... 60

Figure 3.14 Calix[4]ester. Superimposed one and two-salt K+ salt titrations. ... 61

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Figure 3.16 Valinomycin. Superimposed one and two-salt Na+ salt titrations. ... 63 Figure 3.17 Valinomycin. Superimposed one and two-salt K+ salt titrations. ... 64 Figure 3.18 Apparatus and conditions for determining Eisenman and Hofmeister

sequences in membrane potential experiments. A pH meter reading in mV (not shown), was attached to the double junction reference electrodes. ... 68 Figure 3.19 0.1 M KCl on left. Membrane potential at five different valinomycin

concentrations. ... 72 Figure 3.20 Membrane potential at various valinomycin and calix[4]ester concentrations. ... 73 Figure 4.1 Diffusion and extraction in carrier-mediated dialysis. Figure shows the dimensions of the Nernst layer and interface, the diffusion of ions through the various interfaces, and the extraction equilibria in the membrane interface. ... 78 Figure 4.2 Apparatus and conditions for determining the rate of salt transport by

conductivity... 82 Figure 4.3 Flux vs activity for various salts. Curves are fits to equation 4.4.

Valinomycin membrane. ... 83 Figure 4.4 Comparison of KClO4 and NaClO4 flux in a valinomycin membrane. Curves

are fits to equation 4.4. ... 84 Figure 4.5 Relationship of the Log Kex and anion dehydration energy. ... 85

Figure 4.6 Flux vs activity for various salts. Curves are fits to equation 4.4.

Calix[4]ester membrane. ... 87 Figure 4.7 Comparison of NaClO4 and KClO4 flux in a calix[4]ester membrane. Curves

are fits to equation 4.4. ... 88 Figure 4.8 Relationship of anion dehydration energy to Log Kex. Calix[4]ester. ... 89

Figure 4.9 Relationship of membrane potential to K+/Na+ selectivity using various anions. Valinomycin membrane. ... 93 Figure 4.10 Relationship of cation flux with various anions to K+/Na+ selectivity.

Valinomycin membrane. ... 94 Figure 4.11 Relationship of membrane potential to Na+/K+ selectivity using various anions. Calix[4]ester membrane. ... 95 Figure 4.12 Relationship of cation flux with various anions to Na+/K+ selectivity. ... 95 Figure 4.13 Relative flux of KClO4 through a valinomycim membrane determined by

conductivity measurements. Fluxes are relative to the swatch at the lower left.

Orientation of pores shown. ... 100 Figure 5.1 Example electrodialysis cell with typical conditions for electrolyte and

membrane carrier concentration. ... 105 Figure 5.2 Two-electrode mode of potentiostat operation. ... 107 Figure 5.3 Linear sweep voltammetry at 5 mV/s. Mode 2 │ - 2 V to + 2 V │ KCl 0.1 M │ valinomycin │ LiCl 1 mM. ... 107 Figure 5.4 Four-electrode mode of potentiostat operation with salt bridges. ... 110 Figure 5.5 Linear sweep voltammetry. Mode 4 │ - 3 V to + 3 V │ CsCl 0.1 M │

calix[4]ester │ 1 mM LiCl. Pt electrodes immersed directly in the cells. ... 111 Figure 5.6 Linear Sweep Voltammetry. Mode 2 │ - 2 V to + 2 V │ KNO3 0.05 M │

valinomycin│ LiCl 10 μM. ... 113 Figure 5.7 Chronoamperometry. Mode 4 │ + 0.2 V, - 0.2 V │ NaCl 0.5 M │no carrier │ Li2SO4 0.1 M. ... 114

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Figure 5.8 Chronoamperometry. Mode 4 │+ 0.1 V, - 0.1 V │NaSCN 0.5 M │

valinomycin │ NaSCN 0.5 M. ... 115 Figure 5.9 Chronoamperometry. Mode 4 │ + 0.1 V, - 0.1 V │ KCl 2 M │ valinomycin │ NaSCN 2 M. ... 116 Figure 5.10 Chronoamperometry. Mode 4 │ Various V │NaSCN 0.5 M │

calix[4]ester│ Li2SO4 0.01 M. ... 116

Figure 5.11 Ion Accounting Experiment. Mode 4 │ + 1.5 V │ KCl 0.1 M │

valinomycin │ NaNO3 0.1 M. ... 117

Figure 5.12 Movement of charge across a reference plane. ... 119 Figure 5.13 Reconciling charge measured by the potentiostat with that calculated from ion flux analysis. ... 121 Figure 5.14 Single-salt electrodialysis apparatus for determination of Eisenman and Hofmeister sequences. ... 124 Figure 5.15 Eisenman sequences. Single-salt chronoamperometry with group I cation chloride salts. Mode 4 │ + 0.5 V, - 0.5 V │ MCl 0.1 M │ valinomycin │ LiCl 1 mM. ... 125 Figure 5.16 Eisenman sequences. Single-salt chronoamperometry with group I cation chloride salts. Mode 4 │ + 0.5 V, - 0.5 V │ MCl 0.1 M │ calix[4]ester │ LiCl 1 mM. ... 127 Figure 5.17 Linear sweep voltammetry with group I chloride salts. Mode 4 │ - 3 V to + 3 V │ Alkali metal chloride salts 0.1 M │ calix[4]ester or valinomycin│ LiCl 1 mM. 128 Figure 5.18 Hofmeister sequence experiments. Single-salt variable anion

chronoamperometry with potassium salts. Mode 4 │+ 0.5 V, - 0.5 V │ KX 0.1 M │ valinomycin │ LiCl 1 mM. ... 132 Figure 5.19 Hofmeister sequence experiments. Single-salt variable anion

chronoamperometry with sodium salts. Mode 4 │ + 0.5 V, - 0.5 V │aX 0.1 M │

calix[4]ester │ LiCl 1 mM. ... 133 Figure 5.20 Linear sweep voltammetry with anions as the variable. Mode 4 │ - 3 V to + 3 V │ Na+

or K+ with various anions 0.1 M │ calix[4]ester or valinomycin│ LiCl 1 mM. ... 135 Figure 5.21 Charge carried through a valinomycin membrane at various carrier

concentrations. Mode 2 │ + 1.5 V │ KCl 0.1 M │ various [valinomycin] │ NaCl or KClO4, 0.1 M. ... 137

Figure 5.22 Charge carried through a valinomycin membrane at various carrier concentrations. Mode 2 │ - 1.5 V / KCl 0.1 M │ various [valinomycin] │ NaCl or KClO4, 0.1 M. ... 137 Figure 5.23 Charge carried through a calix[4]ester membrane at various carrier

concentrations. Mode 2 │ + 1.5 V │ NaCl 0.1 M │ various [calix[4]ester] │ KCl 0.1 M. ... 138 Figure 5.24 Charge carried through a calix[4]ester membrane at various carrier

concentrations. Mode 2 │ - 1.5 V │ NaCl 0.1 M │ various [calix[4]ester] │ KCl 0.1 M. ... 139 Figure 5.25 Experimental apparatus and conditions for cation competition experiments. Four-electrode 24 hour CA experiment. ... 143

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Figure 5.26 Competition experiments. Chloride salts of K+ and Na+.

Chronoamperometry. Mode 4 │ Vary V │ NaCl 0.05 M, KCl 0.05 M │ Valinomycin │

LiCl 10 μM. ... 145

Figure 5.27 Competition experiments measuring K+/Na+Selectivity. Nitrate salts of K+ and Na+. Chronoamperometry. Mode 4 │ Vary V │ NaNO3 and KNO3 vary M│ Valinomycin │ LiCl 10 μM. ... 147

Figure 5.28 Competition experiments measuring K+/Na+ selectivity. Perchlorate salts of K+ and Na+. Chronoamperometry. Mode 4 │ Vary V │ NaClO4 0.05 M, KClO4 0.05 M │ Valinomycin │ LiCl 10 μM. ... 148

Figure 5.29 Competition experiments measuring Na+/K+ selectivity. Chloride salts of sodium and potassium. Chronoamperometry. Mode 4 │ Vary V │ NaCl 0.05 M, KCl 0.05 M │ Calix[4]ester │ LiCl 10 μM, LiNO3 1 mM. ... 150

Figure 5.30 Competition experiments measuring Na+/K+ selectivity. Chloride salts of sodium and potassium. Chronoamperometry. Mode 4 │ Vary V │ NaCl 0.05 M, KCl 0.05 M │ Calix[4]ester│ LiCl 1 mM. ... 151

Figure 5.31 Competition experiments measuring Na+/K+ selectivity. Nitrate salts of sodium and potassium. Chronoamperometry. Mode 4 │ Vary V │ NaNO3 0.05 M, KNO3 0.05 M │ Calix[4]ester│ LiCl 1 mM. ... 152

Figure 5.32 Single-salt LSV for KCl and NaCl. Mode 4 │ -3 V to + 3 V │ KCl or NaCl, 0.1 M │ valinomycin│ LiCl 1 mM. ... 154

Figure 5.33 Single-salt LSV for KCl and NaCl. Mode 4 │ - 3 V to + 3 V │ KCl or NaCl, 0.1 M │ calix[4]ester │ LiCl 1 mM. ... 155

Figure 6.1 Development of a liquid junction potential with double junction reference electrodes. ... 184

Figure 6.2 Two of four possible AE calibration lines. ... 187

Figure 6.3 Four-electrode chronoamperometry set up. ... 189

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List of Symbols and Abbreviations

If applicable, units are given in brackets. ϴ Porosity (%)

Potentiometric selectivity coefficient

‘ Left compartment

“ Right compartment

a Activity (context dependant)

AE Atomic emission

aq Aqueous

as Activity in the source phase

BLM Bulk liquid membrane

C Constant or concentration (context dependant) (mV or M) Č Slope modifier, pronounced ceewedge

Ď Anion term slope modifier, pronounced deewedge Db Diffusion coefficient in bulk solution (m2 s-1)

DEHPA di(2-ethylhexyl)phosphoric acid

Di,x Diffusion coefficient of ion, i or x (m2 s-1)

Dm Diffusion coefficient in the membrane (m2 s-1)

e- Electron

E0 Standard potential (V)

EB Boundary (Donnan) potential (V)

ED Diffusion (Henderson) potential (V)

Em Membrane potential (V)

I Ionic strength (context dependent) (M) I Current (context dependent) (A)

I+ , i Cation I+, or i when used as a subscript. i may also refer to a generic ion IC Ion Chromatography

ISE Ion selective electrode

J+, j Cation J+, or j when used as a subscript Ji,j Flux of species i or j (mol m-2 s-1)

Ka Association constant of cation and carrier in the membrane (M-1)

Kex Extraction equilibrium constant (M-1)

Kp Salt partition equilibrium constant

Ks Constant correlating with titration curve shape

m membrane, when used as a subscript NBS National Bureau of Standards NPOE 2-nitrophenyl octyl ether ocp Open circuit potential (V) PAR Princeton Applied Research PTFE Polytetrafluoroethylene PVC Polyvinylidenechloride

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Q Charge (C) rD Debye length (M)

RO Reverse osmosis S Carrier molecule

t- Anion transference number

t+ Cation transference number

TOA Trioctyl amine

ui Mobility of ion I (m2 s-1 V-1)

uj Mobility of ion J (m2 s-1 V-1)

X-, x Anion X-, or x when used as a subscript Y- Anion Y-

z Charge number

ΔGdehyd. Gibbs free energy of dehydration (kJ mol-1)

ε0 Permittivity of a vacuum (F m-1)

εr Dielectric constant

τ Tortuosity

Electrical potential (V)

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Acknowledgments

My supervisor, noted Brazilian polymath Dr. Alexandre Brolo, was a great pleasure to work with and a constant inspiration. This work also owes its genesis to the kind assistance and advice of Drs. David Harrington, Stephen Evans, Pat MacKenzie and Robin Hicks. Much thanks and gratitude to Alex and David for editing the prose contained herein.

Many thanks are due Dr. Jakub Drnec, who discussed many ideas with me early in this project, and introduced me to the potentiostat.

The staff at UVic are first rate and very welcoming. Graduate secretary Carol Jenkins efficiently steered me through the complexities of grad school, and I very much appreciated the general awesomeness of Patricia Ormond. Analytical chemist Nichole Taylor was very generous with her time, and helped me measurably with the cation and anion determinations. John Paul Gogniat in the machine shop, Sean Adams in glassblowing and Mario Ivanov, Shuba Hosalli, Andrew MacDonald, Terry Wiley and Bob Dean in the instrument shop, were all a great help to me. Thanks also to Dr. Peter Marrs, with whom I worked for many years as a teaching assistant. Teaching can be very gratifying and humbling, although marking hundreds of kilos of lab reports may not be an activity I will miss. Time will tell.

Perry Brewer - yachtsman, connoisseur, raconteur, pilot and fisherman - has been a good friend to me. I looked forward to lunch at the University Club, where Perry could be counted on to make my day a bit brighter. The world needs more people like him.

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Dedication

This is for Helge L. Hansen, Lorie Jean Miller-Hansen and Rebecca Pritchett. Thank you all for your kind and encouraging words. Also, for my late friend, Harm de Haan. He thought entering a PhD program at the age of 43 was a bit ambitious/crazy, but I know he would have been very happy to see me succeed in this highly challenging adventure.

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Chapter 1 General Introduction

1.1 Introduction

The objective of this dissertation is to describe how the addition of an ionophore (carrier molecule) to a liquid membrane can effect movement of material through the membrane. Material flux will be examined at open circuit potentials, and under the influence of an applied transmembrane electrical potential. The goal of doing so is to augment the available separation technologies with one that provides additional ways of controlling material flux and selectivity. The process uses electrical potential as the primary driving force, and is referred to as carrier-mediated electrodialysis.

Selective transport of solvent or solute through synthetic membranes is the basis of several important and familiar separation processes. The most common of these processes are reverse osmosis, membrane distillation, dialysis and electrodialysis. Each of these processes makes use of a different driving force and requires unique membrane types. A brief word about the materials, forces and fluxes at play in these separation processes will serve to place carrier-mediated electrodialysis in context.

Reverse osmosis (RO), is probably most familiar as a technology used to make fresh water from brackish water or seawater. Hydrostatic pressure is used to overcome osmotic pressure and to force water molecules through a thin-film composite membrane. Ions and small molecules are retained on the pressurized side of the membrane. Membranes are not perfectly efficient, and most operations manage to achieve a salt rejection of 99 – 99.5 %. The membrane generally consists of a porous polyamide active layer bonded to a polysulphone support layer, and is wound up in a compact cartridge. Pressures range from 30 to 1,200 psi depending on the degree of salinity, with saltier water having a greater osmotic pressure and thus requiring more hydrostatic pressure to reverse the normal flow. As a general rule, 1 psi of hydrostatic pressure is required to overcome the osmotic pressure of 100 ppm of dissolved solids. Flux of water through the membrane

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varies with salinity, pressure and the amount of solute rejected, but is generally on the order of 400 liters per square meter per day in commercial operations[1, 2].

Membrane distillation is also typically used to produce fresh water from salty water, although there are also applications in separating volatiles, such as ethanol, from water. A temperature gradient is the driving force. The liquid to be purified is heated such that a vapour is created. This vapour passes through the pores of a membrane and is condensed on the other side. In the more common case of water purification/desalination, the membranes are made from hydrophobic and thermally resistant polymers such as polytetrafluoroethylene (PTFE), polypropylene or poly(vinylidene fluoride.) Membranes are wound in a spiral architecture similar to that of a reverse osmosis membrane cartridge. Temperatures are kept below the boiling point, and can be as low as 30 0C, although temperatures up to 90 0C are frequently used. Flux of fresh water depends on temperature, membrane porosity and pore size, and is typically only a quarter that of RO[3].

Dialysis is most familiar as a technology used to remove impurities from blood (hemodialysis.) Low molecular weight salts and small molecules, such as the metabolic waste product urea, are removed from the blood by diffusion through a semi-permeable dialysis membrane. The driving force is concentration gradient. Larger proteins and cellular structures are left behind, and returned to the patient. Commercial dialysis membranes are hydrophilic and generally cellulose based, but there are many different polymers and polymer blends in use. Dialysis units may accommodate flat sheet or tubular membranes, but almost all commercial dialyzers now use a hollow fibre configuration. Flux depends on the thickness of the membrane, the concentration difference between the blood and the on one side of the membrane and the dialysate fluid on the other side, as well as the size of its pores[4]. A negative pressure is often applied to the dialysate side. This has the effect of drawing more small molecules and fluids through the small membrane pores. This latter process is called ultrafiltration.

Electrodialysis is a process usually used for the production of fresh water from brackish water, but it can also be adapted to make other products such as chlorine gas, acids, bases and concentrated salt solutions. The driving force for electrodialysis is an electrical potential difference. Most commercial applications of electrodialysis involve

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the use of ion exchange membranes. The membranes are made from crosslinked copolymers based on divinylbenzene and polystyrene or polyvinyl pyridine. Most processes make use of both cation and anion exchange membranes. Cation exchange membranes contain fixed negatively charges sites such as sulphite groups, and are highly selective for the passage of cations and the exclusion of anions. Anion exchange membranes contain fixed positively charges sites such as quaternary ammonium groups, and are highly selective for the passage of anions and the exclusion of cations. Both types of membranes work on the Donnan exclusion principle, which is the reduction in concentration of mobile ions within an ion exchange membrane owing to the presence of fixed sites having the same charge. In a typical commercial electrodialysis unit engineered for the production of fresh water, cation and anion exchange membranes alternate in a stack that may contain dozens or hundreds of membranes[1, 4].

A summary of the four major membrane processes[1, 3, 4], is below in Table 1.1.

Table ‎1.1 Membrane Processes.

Membrane Process Membrane Type Driving Force Example Application

Reverse osmosis Composite microporous

Hydrostatic Pressure Removal of salt and small molecules from water

Membrane distillation Microporous Temperature Separation of water from solutes

Dialysis Microporous Concentration

gradient

Hemodialysis.

Electrodialysis Ion exchange Electrical potential Removal of ions from water

Each of the processes in Table 1.1 may be modified to perform other applications

than those listed. For example, RO can be configured to purify ethanol from mixtures of higher alcohols and alkanes[5]. Membrane distillation may be used to concentrate non-volatile acids such as sulfuric and phosphoric acid, to isolate ethanol from aqueous solution, or to recover hydrochloric acid from spent pickling liquors[6]. These modifications may require significant alterations to the processing equipment and membrane.

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1.1.1 Flux and selectivity

Flux is the throughput of material in a given time per unit of membrane area. High fluxes are important in making separation processes economically viable. Engineering a membrane consists of making practical compromises between resistance to mechanical, thermal and chemical stresses, and the need for rapid material processing. In any process, the easiest and most obvious way to increase flux is to decrease the membrane thickness, and so membranes are usually made as thin as is practically possible. Flux may also be enhanced by increasing the driving force on the membrane. However, there is a limit to how much any driving force can be increased before either destroying the membrane, or seeing diminishing returns that make the application of the additional force uneconomical.

Selectivity is the ability to distinguish and separate the compounds in the feedstock. Selectivity relates to the relative membrane permeation rates of the species to be separated. These permeation rates are a function of the concentration of substrate in the feed stock, the magnitude of the driving force and the physical characteristics of the membrane. While thinner membranes may enhance flux through the membrane, the impact on selectivity may be largely negative. High fluxes and good separations do not usually go hand in hand. Increases in flux generally result in a decrease in separation efficiency, and better separations generally require lower fluxes[1].

Reverse osmosis, membrane distillation and hemodialysis use porous membranes. Precisely controlled pore sizes are critical to the production of a pure product. Pore dimension also has a great deal to do with the rate at which material can pass through the membrane. An important feature of all three of these processes is that they do not require that any part of the feedstock interact chemically or electrostatically with any component of the membrane. These separation technologies rely on the physical exclusion of one or more feedstock components from the membrane pores.

Conventional electrodialysis uses non-porous ion exchange membranes. Flux through the cation and anion exchange membranes of an electrodialysis cell is largely dependent on the electrical potential applied across the membranes. Up to a point, greater potentials translate to more ion flux. Selectivity in electrodialysis is on the basis

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of ionic charge. Selectivity depends upon the degree to which ions are repelled from membranes having the same charge. In electrodialysis, any ion is as likely as any other of the same charge to pass through either the cation or the anion exchange membrane. There is little discrimination on the basis of the size or shape of the substrate molecule.

1.1.2 Liquid membranes

Liquid or semi-liquid membranes can provide a third way in which selectivity may be conferred[7]. A liquid membrane is generally an organic liquid that is immiscible in the surrounding aqueous environment. Flux of material through the membrane may be facilitated by a freely moving carrier molecule that is dissolved in the organic membrane phase. At the interface between the membrane and aqueous phases, the carrier molecule binds to specific components in the feedstock. These carriers enhance the flux of material through the membrane by acting as catalysts of translocation. The carriers confer selectivity through the phenomenon of molecular recognition. Molecular recognition occurs when carrier molecules bind preferentially to specific chemical species. Molecular recognition is the result of a complementary interaction between host (carrier) and guest (substrate) molecule. This complementary interaction is frequently the result of the substrate having the correct size and shape, and/or electrical charge for optimum stabilization by the carrier. Selectivity in carrier-containing liquid membranes is thus due to both the size, shape and charge characteristics of the substrate.

Both dialysis and electrodialysis have been adapted to work with carrier-containing liquid membranes.

1.1.3 Carrier-mediated dialysis background

As with regular dialysis using a porous plastic membrane, the driving force in carrier-mediated dialysis is a concentration gradient. The simple picture is of a carrier binding with the species of interest at one interface, then diffusing down a concentration gradient to the other side of the membrane. Once the carrier and bound species (host/guest pair)

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have reached the other side, the carrier molecule releases the guest species to the environment of lower concentration that prevails on that side. The carrier molecule may then diffuse back across the membrane driven by the concentration gradient in unbound carrier, renewing the cycle.

Without the carrier molecule being present, little or no transport takes place. The carrier molecule acts as a catalyst of translocation, and the process continues at varying rates so long as a concentration gradient in transportable species is present. Specificity is conferred on the process by the degree to which the carrier molecule prefers binding with one species over another. Figure 1.1 shows the transport of a salt, I+X-

,

facilitated by a neutral cation carrier, S.

Left Membrane SI+ + X -I+ + X -I+ + X -Right S

Figure ‎1.1 Cation facilitated gradient pumping. Symport case.

In Figure 1.1, salt I+X- is being transported from the left aqueous phase where the concentration of salt is high, to the right aqueous phase where the concentration is low. Because transport of species I+ and X- is in the same direction, this is referred to as being a symport cycle. The antiport case is shown in Figure 1.2.

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Left Membrane SJ+ Right J+ J+ I+ SI+ I+

Figure ‎1.2 Cation facilitated gradient pumping. Antiport case.

In antiport facilitated by a cation carrier, cations move through the membrane in opposite directions. In Figure 1.2, the transport of cation I+ is coupled to the counter-transport of cation J+.

In actuality, the guest species are not thought to be associated with the same carrier throughout the entire journey across the membrane. Because the cation complex lifetime (- approximately 1 ms for relatively strong complexes like valinomycin/potassium[8]), is much shorter than the time required to move across the membrane, diffusion consists of many association/dissociation steps[9]. Carrier-mediated migration of cations is, therefore, thought to occur via a hopping or relay mechanism, where the cation may be associated with many different carriers[10].

There are several different types of liquid membranes, including bulk liquid membranes (BLM’s), and supported liquid membranes (SLM’s). The first important studies of carrier-mediated transport were published by Cussler and Rausch[11] in 1973, and used a BLM. A BLM consists of an organic solvent membrane with aqueous phases on both sides. Figure 1.3 shows a BLM in a U-tube configuration, one of several different ways to make BLM’s that were originally described by J. Schulman[12].

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Organic

Aq. Aq.

Figure ‎1.3 BLM in a U-tube configuration. CHCl3 organic phase. All phases stirred.

The authors used the newly synthesized carrier molecule dibenzo-18-crown-6, a first generation macrocylic ionophore, and one of a family synthesized by Charles Pedersen[13]. Dibenzo-18-crown-6 is the archetypal cyclic ionophore. Cations are bound and stabilized in the central cavity by interaction with atoms bearing lone pairs of electrons. The size of the cavity is fixed by the closed structure of the molecule. This feature results in an ionophore with an affinity for ions that best fit within the cavity. The structure of dibenzo-18-crown-6 is shown in Figure 1.4.

O O O O O O Figure ‎1.4 Dibenzo-18-crown-6.

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Cussler and Rausch used alkali metal cations as the transported species. Their work is considered to be seminal in the field, as it introduced the theoretical concepts that have given rise to all subsequent models of carrier-mediated dialysis[14]. Lamb, Christensen and Izatt extended this work in the late 1970’s, using a large number of crown ether derivatives, as well as the natural products valinomycin and monensin[15-17] as carrier molecules. They extended transport to divalent metals, and began to study the effect of the anion on transport rate. Contemporaneously with Lamb et al, Behr, Kirch and the Nobel Prize winner Jean-Marie Lehn worked with BLM’s using crown ethers and cryptands to further refine the theoretical constructs underpinning dialysis[18-20]. A typical cryptand molecule is shown below in Figure 1.5.

N O O N O O O O Figure ‎1.5 2.2.2 – Cryptand.

The Lehn group identified the relationship of carrier complex formation and flux for both symport and antiport cases. They extended the range of carriers used to those capable of forming complexes with larger organic anions. In the mid 1990’s the Reinhoudt group examined the transport of catecholamine’s with crown boronic acid derivatives[21], also using BLM’s.

SLM’s were the next type of membrane to be used extensively by carrier-mediated dialysis researchers. These consist of a porous plastic support, 25 to 200 μM in

thickness, into which a carrier-containing organic liquid has been imbibed. The

membrane is sandwiched in a holder, with aqueous compartments on either side. From roughly the mid 1980’s to the mid 1990’s, the Reinhoudt group did extensive studies involving the transport of cation complexes with a variety of novel synthetic carrier

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molecules, focussing on producing kinetic and selectivity data[22-25]. Several of the carriers used by the Reinhoudt group are shown in Figure 1.6.

O O O O R R O O O O O O O O O O

A calix[4] crown ether. R = CH3, CH2CH3 A calix[4] spherand.

Figure ‎1.6 Example synthetic carrier molecules used by the Reinhoudt group.

In collaboration with a group led by Rocco Ungaro, Reinhoudt investigated the transport characteristics of a carrier molecule able to simultaneously bind with both cations and anions[26]. They also did significant work in developing equations and techniques to determine transport parameters, such as the diffusion coefficient and extraction coefficient, and derived equations that made the calculation of these parameters a simpler task than it had been previously[14, 27].

SLM’s degrade through a process of carrier molecule or membrane solvent leaching into the aqueous phase, or the aqueous phase dissolving into the membrane. Although significant commercial applications have so far been limited by membrane instability

[28-30]

, studies on the separation of toxic or radioactive metals[31-33] suggest that carrier-mediated separations have the potential to be commercially viable, providing the

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instability issue can be overcome. At the present time there are some commercial applications involving extraction of cadmium, copper and chromium(IV) from waste streams[31, 34], and radioactive elements from nuclear waste water[32, 33] in the early stages of development.

1.1.4 Carrier-mediated electrodialysis background

A few weeks after Alessandro Volta invented the Voltaic pile in 1800, William Nicholson and Anthony Carlisle used it to discover that water could be split into hydrogen and oxygen gas. Water electrolysis sparked research into the electrolysis of many chemicals, and by 1830 Michael Faraday had established the science of electrochemistry.

In an electrolysis cell, charged species migrate through aqueous or molten solutions towards oppositely charged electrodes. Cations migrate toward the cathode where they are either reduced or neutralized, and anions migrate toward the anode where they are either oxidized or neutralized. The diagram below shows electrolysis of aqueous NaCl.

Anode Cl- → ½Cl2 + e -+

_

Na+ Na+ Na+ Cl -Cl -Cl -DC source

Cathode H2O + e → -OH + ½H2

Figure ‎1.7 Electrolysis of aqueous NaCl.

In the scheme shown in Figure 1.7, an applied DC potential drives the electrode reactions and ion migration through the electrolyte. Chloride migrates to the anode and is

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oxidized to chlorine gas, while sodium ions migrate to the cathode and are neutralized by hydroxide produced by the reduction of water. Electrons flow in an external wire between electrodes. Electron flow is coupled to ion migration through the electrolyte, and redox reactions at the electrodes.

The process of electrodialysis was invented approximately 60 years ago by people working for a company called Ionics. In conventional electrodialysis, stacks of alternating cation and anion exclusion membranes are interposed between the electrodes of an electrolysis cell. An electrodialysis unit is depicted in Figure 1.8. The picture has been simplified; the normal situation is to have several hundred membranes in a plate and frame stack.

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+

_

+ + _ _ DC Source Na₂SO_{4 (aq)}Feed Conc. Na₂SO_{4 (aq)} Fresh Water Cathode Anode Na+ Na + Na+ e- e -SO₄ 2-SO₄ 2-1/2 SO₄ 2-Na+ Na+ Anode Cathode ½H2O → H + + e- + ¼O2 H2O + e → -OH + ½H2

Figure ‎1.8 Purification of water via electrodialysis using ion exchange membranes. Aqueous Na2SO4 feed stock.

Flux through a electrodialysis unit is controlled by the amount of voltage drop across

the membrane stacks. The electrodialysis example above uses a sodium sulphate solution as the feed source. In response to an applied electrical potential, cations move through cation exchange membranes toward the cathode on the right and anions move through anion exchange membranes toward the anode on the left. Redox reactions at the

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electrodes in the right and left-most compartments neutralize the incoming ions. The anode compartment produces acid and the cathode compartment produces base. Alternating cation and anion exchange membranes confine salts to every alternate cell by impeding the further migration of ions by Donnan exclusion. Fresh water is produced in the adjacent cells.

Carrier-mediated electrodialysis may be thought of as being conventional electrolysis, such as depicted in Figure 1.7, but with a liquid membrane containing a carrier inserted between the cathode and anode. Equivalently, carrier-mediated electrodialysis may be thought of as being conventional electrodialysis, as illustrated in Figure 1.8, but with all the cation and anion exchange membranes replaced by a single liquid membrane containing a carrier molecule.

Carrier-mediated electrodialysis is shown below in Figure 1.9.

1/2 H2O H++ 1/4O2 _H 20 OH- + 1/2H2 I+ Membrane + _ e-Electrolyte I+X- Electrolyte J+Y -e- _e -SJ+Y -SI+X -J+ X -Y -1. 2.

Figure ‎1.9 Carrier-mediated electrodialysis. Ion movement is linked to electrode redox reactions, and electron movement in an external wire.

The carrier-mediated electrodialysis cycle begins with partitioning of electrolytes into the membrane. Two processes are shown in Figure 1.9. In the first process, I+X- from the left compartment partitions into the membrane, forming a complex with the carrier. In response to an applied electric field, the cation moves into the right compartment, and the anion re-enters the left compartment. In the second process, J+Y- from the right compartment partitions into the membrane and forms a complex with the carrier, with the cation and anion also moving in opposite directions in response to an applied electric

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field. Cation and anion movement is linked to electrode reactions. On the left, an oxidation reaction produces a proton and one electron. The proton neutralizes the negative charge carried by the anion that has moved into that compartment. On the right, reduction produces hydroxide, which neutralizes the positive charge carried by the cation. Redox reactions, electron flow and ion movement through the membrane are linked, and are thought of as occurring contemporaneously.

Processes 1 and 2 illustrated in Figure 1.9 occur independently of each other, and are in no way linked. By this we mean that the transport of I+ to the right compartment is not linked to the transport of Y- or J+ to the left compartment. Ion movement in electrodialysis is thus not a case of antiport transport.

The distinguishing feature of carrier-mediated electrodialysis is that completing the electrical circuit requires that ions move through the liquid membrane via a mechanism that involves the carrier molecule. Molecular recognition thus plays a part in the degree to which one substrate is transported over another. As in the dialysis case, carrier/substrate complex lifetimes are short relative to the time it takes to transit the membrane[35]. The cation proceeds across the membrane in a series of hops between carrier molecules[36].

The first reports of carrier-mediated electrodialysis using liquid membranes were published in 1969 by Wilhelm Simon et al of the Swiss Federal Institute of Technology in Zurich, Switzerland[37, 38]. At that time, there was great interest in determining the ways in which ions traverse biological cell membranes. The carrier mechanism for which some antibiotics were thought to gain their activity was being actively debated but was not yet known[39]. However, there was evidence that certain antibiotics functioned by somehow facilitating the transport of ions through the cell membrane. It was known at the time that neutral antibiotics, such as valinomycin, nonactin, monactin, gramicidin and the enniatins, displayed a high degree of cation specificity. Since complexes with ions are charged, George Eisenman et al speculated in 1967 that the antibiotic-mediated transport of cations across a cell membrane would be expected to occur in the presence of a transmembrane potential[40]. Further, he speculated that the specificity of the antibiotics for specific ions would be expected to be retained under conditions of an imposed transmembrane potential.

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These first studies used bulk liquid membranes, which consisted of a stack of three thick (0.75 mm) porous (85 %) PVC supports imbued with 2-octanol saturated with dilute HCl. The carrier molecules were a mixture of the cyclic antibiotic ionophores monactin and nonactin. The structure of nonactin (an ammonium ionophore), is shown below.

Figure ‎1.10 Nonactin.

The experiments ran at a potential difference of 30 V. Selective transport of potassium over sodium was observed, and the mechanism of transport was shown to be a relay or hopping mechanism. This mechanism was confirmed by a Japanese group in 1985[41]. They also speculated that the selectivity of ion transport was partially a function of the dielectric properties of the membrane solvent. These were among the first biomimetic chemistry experiments, where synthetic membranes and carriers were successfully used to study and draw conclusions about the mechanism of ion transport in natural systems.

In the mid-seventies, Simon and his colleague W. E. Morf began working with liquid polymer membranes, otherwise known as a solvent polymeric membranes or polymer inclusion membranes. This type of membrane is made by dissolving PVC or cellulose triacetate in a suitable solvent, along with a plasticizer and a carrier molecule. The solution is then poured onto glass where it is contained by a ring of metal, inert plastic or glass. The solvent is allowed to evaporate, leaving a membrane of 30 to 200 μm in thickness. The membrane is then inserted into a suitable holder, where it acts as a semi-permeable barrier between two aqueous phases. Using transmembrane potentials of up to 30 V, Morf and Simon developed increasingly sophisticated models of ion flux[42]. Simon also collaborated with colleagues in the biochemistry department at the Swiss

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Federal Institute of Technology. They used planar lipid bilayers subjected to 30 mV potentials to examine the selectivity characteristics of a Ca2+ ionophore. They found that the lower dielectric of the bilayer reduced selectivity of Ca2+ over Na+, as compared with the higher dielectric of a BLM[43, 44]. In the mid 1980’s, Simon and colleagues manufactured asymmetric BLM’s, where one side of the membrane was manufactured with a solvent of low dielectric constant and the other side with a high dielectric constant. They showed that selectivity changed between Ca2+ and Na+, depending on which side of the membrane was exposed to the electrolyte[45, 46]. These results confirmed their earlier ideas that selectivity could be manipulated by altering the membrane dielectric[37, 38]. In the early 1990’s until his untimely death in 1992[47], Simon published research on the use of anion ionophores as the carrier molecule in liquid polymer membranes[48, 49]. A neutral porphyrin ruthenium anion ionophore used by the Simon group is pictured in Figure 1.11. N N N N Ru L CO

L = Furan. Anions, such as SCN

-, displace furan in this complex

Figure ‎1.11 A porphyrin thiocyanate ionophore.

The authors found that transport of anions through the membrane did not follow Hofmeister anion lipophilicity sequences. This means that the most lipophilic anions (the ones that are easiest to dehydrate), were not necessarily transported in the greatest

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amount. They concluded that the reason for this was because the carriers favoured certain anions over others. This result is analogous to cation carriers preferring certain cations. Simon also used non-cyclic di- and tri-alkyl tin anion ionophores in these studies.

Because there are far fewer anion carriers with suitable selectivity characteristics available than cation carriers, and fewer anions of analytical interest, carrier-mediated electrodialysis research in this area has not been pursued to any great degree. A Russian chemist at Riga Technical University is the notable exception. In 2006, T. Zh. Sadyrbaeva used a bulk liquid membrane to separate PtCl62- from PdCl42-. The

membrane consisted of 2 mL of 1,2-dichloroethane sandwiched between porous cellophane. The solution in the left compartment (feed solution), contained the metal salts. The solution in the right compartment (stripping solution), contained 1 M HCl. Cation exchange membranes separated the electrode compartments from the feed and stripping solutions. This set-up is shown below in Figure 1.12.

+

_

DC source

Membrane

Feed Strip

Figure ‎1.12 Five compartment electrodialysis cell used by Sadyrbaeva.

The carriers used were the non-cyclic basic ionophores diphenylthiourea and di-o-tolylthiourea. The feed solution contained the platinum and palladium chloride complexes in a 1 M HCl solution. The acid served to protonate the carriers at the interface with the membrane. The protonated carriers were then able to act as anion ionophores. None of the compartments were stirred. In this study, the author demonstrated that the selectivity of the separation is a function of applied potential[50]. Interestingly, the ratio of Pt/Pd increased as potential increased up to a maximum, at

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which point the selectivity dropped with further increases in potential. This is the first and only unambiguous published account of potential dependent selectivity available in the literature.

Current activity in carrier-mediated electrodialysis research is focussed primarily on developing commercially viable metal extraction technologies. This activity is concentrated almost entirely in Russia and the Baltic states, with the exception of one Mexican group that has studied the extraction of copper from a copper, nickel and iron sulfate solution[51]. This group had a setup similar to that in Figure 1.12, although the membrane phase was stirred and the Pt electrodes were placed directly in the feed and stripping compartments. In order to minimize the cost of the membrane and carrier, this group, in common with virtually all the others, uses di (2-ethyl hexyl) phosphoric acid (DEHPA), as the carrier. This carrier is frequently combined with formamide or tri-n-octylamine (TOA), in order to increase the conductivity in the membrane phase. TOA is also frequently used as the sole carrier.

HO P O O O N

Di(2-ethylhexyl)phosphoric acid DEHPA Tri-n-octylamine TOA

Figure ‎1.13 Common carriers in metal extraction electrodialysis research.

When DEHPA and TOA are combined in the membrane, proton transfer from the acid can give a negatively charged organophosphorus ligand capable of stabilizing metal ions. When TOA is used alone, the lone pairs on the nitrogen atom fulfill this same function. The membrane solvent is generally either kerosene or 1,2-dichloroethane. Both of these are relatively inexpensive, and much more economical than 2-nitrophenyl octyl ether (NPOE), or related compounds that have been used in dialysis research using SLM’s.

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Since the mid 1990’s, Sadyrbaeva, alone or in collaboration with B. Purin (or Purins. The literature has both spellings), has been by far the most active researcher of carrier-mediated electrodialysis. Judging from SciFinder abstracts, the topics she has written about in Russian[52-60] appear to be represented in English language journals[50, 61-73]. The research has been concentrated primarily on the extraction of transition metals such as Mn(II), Co(II), Ni(II), Cu(II), Pd(II), Ag(I), Fe(III) and Pt(IV), from acidic solutions. Approximately half of these papers focus only on maximizing the rate of throughput from a feed source containing only a single transition metal salt. As such, selectivity between metals is not addressed. Several other authors, including B. Purin(s), have published in non-English journals (primarily Russian), [74-81]. With the exception of a paper published in 2007[75], these contributions began in the mid 1970’s and ceased in the early 1990’s. Based on the English abstracts found on SciFinder, the authors of these papers were engaged in studies very similar to those of Sadyrbaeva.

Sadyrbaeva notes in a 2011 paper that electrodialysis research using liquid membranes is rare, relative to research on other membrane extraction techniques[72]. Supporting this view, the most highly cited recent review on membrane separation technologies (57 SciFinder citings as of September 2011), mentions carrier-mediated dialysis but has no mention of carrier-mediated electrodialysis at all[82]. This review explicitly did not discriminate between those technologies which were commercially viable and those which were just developing. Since this 1997 review, the few papers that have been published on the subject of carrier-mediated electrodialysis describe the application of the well-established methods of Sadyrbaeva and co-workers to yet more separations of transition metals.

1.2 Objective of the dissertation

There is no indication in the literature that electrodialysis has been investigated using carrier-containing SLM’s. Material flux through SLM’s is expected to be high, relative to the thick unstirred BLM’s of Sadyrbaeva. Cyclic ionophores should offer greater control of the selectivity characteristics of the membrane, relative to open chain molecules having greater conformational mobility. Preorganization of the carrier

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molecule may be the key to designing carriers capable of extremely high degrees of selectivity, such as is required for enantiomeric separations.

This study was undertaken to determine the answers to some basic questions about carrier-mediated electrodialysis as a separation technology. What are the conditions under which carrier-mediated electrodialysis actually functions as a separation technology? Is selectivity a function of electrical potential? How does carrier-mediated electrodialysis compare to carrier-mediated dialysis in terms of material flux and selectivity? Is there an optimum potential at which both flux and selectivity are maximized?

The present study used alkali metal cations as the substrates to be transported, but, in principle, any charged species is a potential candidate for electrodialytic transport. The substrate itself need not be charged, only the carrier/substrate complex. While we used only neutral carriers for this study, charged carriers have the potential to act as transporters for neutral molecules.

Carrier-mediated electrodialysis, should it prove effective, has the potential to be of great value in two areas. First, the process has already proven to be capable of selectively transporting various transition metals in BLM’s and SLM’s. Because several of these metals, such as Cr(IV) and Pb(II), are toxic, potential uses include remediating waste streams or decontaminating drinking water[83, 84]. Second, the process may also be suitable for the purification of pharmaceuticals, including enantiomers. In many cases only one of the enantiomers of a racemate is bioactive or safe for use (e.g. epinephrine, propranolol, thalidomide[85].) In 1992 the FDA began to actively discourage the selling of racemic mixtures, unless it could be shown that both enantiomers were both active and non-harmful[86]. Pharmaceutical companies devote enormous resources to synthesizing only the active enantiomer, or to separating it from its racemic partner. By the year 2000, worldwide sales of chiral drugs topped US$100 billion, with sales of these products accounting for approximately 30% of the sales of all pharmaceutical drugs[86]. The value of enantiomerically pure drugs has surely increased since that time, and there is thus a large economic incentive to maximize either the selectivity of the synthesis, or to develop efficient enantiomeric separation technologies.

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This dissertation consists of 6 chapters and an appendix. In the first chapter, the four main driving forces used in membrane separation technologies are discussed. Carrier-mediated electrodialysis is presented as a hybrid of carrier-Carrier-mediated dialysis and electrodialysis. Its effectiveness as a method to separate alkali and transition metals in various BLM configurations is described. It is noted that supported liquid membranes containing macrocyclic carriers may offer superior flux and selectivity characteristics to those of BLM’s. Possible commercial benefits are presented as the rationale for why the study of carrier-mediated electrodialysis using SLM’s impregnated with macrocyclic carriers is worth doing.

The second chapter is a reference to materials and procedures that are common to two or more chapters in this dissertation. Details of how the electrodialysis cell was constructed are given, including the preparation of SLM’s and salt solutions. Analytical methods for the determination of cations and anions are discussed. There is a brief explanation of how double junction reference electrodes function. The properties of the carriers, membrane film and membrane solvent are given. Much of this dissertation deals with the sign of the membrane potential, an applied potential, an electrical current or an accumulated charge. Keeping these signs straight can be difficult, so there is an explanation of the sign convention that can be referred to if and when confusion arises. The third chapter is the first of three chapters of experimental work. This chapter deals with the measurement and causes of membrane potential – the electrical potential that develops when asymmetric electrolytes are separated by a semi-permeable membrane. Several empirical mathematical models are introduced which model the change in potential as the concentration of ions is altered. The magnitude and sign of the membrane potential is discussed in relation to the nature of the electrolyte and the carrier molecule. Membrane potential is discussed as a method to quickly probe Eisenman sequences (relative preference of the carrier molecule for various cations), and Hofmeister sequences (anion lipophilicity).

The fourth chapter is on carrier-mediated dialysis. The transmembrane flux of a variety of different salts with two different carriers is measured at various electrolyte activities. From this flux/activity relationship, the extraction equilibrium constant and diffusion constant are determined. The magnitude of these constants is related to the