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8/3/2019 Saki -Electronic Thesispla
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Micellar-Enhanced Ultrafiltration of Palladium and Platinum
Anions
BY
Gwicana Sakumzi
A dissertation submitted in fulfilment of the requirements for the degree of
MASTER OF TECHNOLOGY: CHEMISTRY
in the Faculty of Science at the
NELSON MANDELA METROPOLITAN UNIVERSITY
January 2007
Supervisor: Dr N.M. Vorster
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TABLE OF CONTENTS
Acknowledgements.................................................................................. i
Summary….............................................................................................. ii
Abbreviations ........................................................................................... iv
List of Figures........................................................................................... v
List of Tables............................................................................................ viii
CHAPTER ONE
1. Introduction .......................................................................................... 1
1.1. Background to waste treatment .................................................... 1
1.2. Definition of a hazardous waste.................................................... 2
1.3. Overview of platinum group metals............................................... 2
1.3.1. Occurrence and Origin........................................................... 3
1.3.2. Abundance and Distribution................................................... 3
1.3.3. Chemical properties............................................................... 5
1.3.4. Basic coordination chemistry of platinum and palladium....... 5
1.3.5. Industrial applications of PGMs ............................................. 6
1.3.5.1. Catalysis......................................................................... 6
1.3.5.2. Jewellery ........................................................................ 7
1.3.5.3. Electronics...................................................................... 7
1.4. Surfactants.................................................................................... 8
1.4.1. Overview................................................................................ 8
1.4.2. Classification of surfactants ................................................... 12
1.4.3. Industrial applications of surfactants...................................... 12
1.5. Membranes................................................................................... 13
1.5.1. Overview................................................................................ 13
1.5.2. Typical membrane properties ................................................ 15
1.5.3. Classification of membranes.................................................. 15
1.5.4. Membrane modules............................................................... 18
1.5.5. Pressure-driven membrane filtration processes .................... 19
1.5.6. Ultrafiltration system .............................................................. 20
1.5.6.1. Micellar-enhanced ultrafiltration system ......................... 21
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1.6. Research objectives...................................................................... 23
CHAPTER TWO
2. Experimental and Key Performance Aspects...................................... 24
2.1. Materials ....................................................................................... 24
2.2. Synthetic procedures .................................................................... 25
2.2.1. Preparation of surfactant solutions ........................................ 25
2.2.2. Preparation of surfactant/metal ion solutions......................... 25
2.2.3. Membrane module preparation.............................................. 25
2.3. Apparatus...................................................................................... 27
2.3.1. Ultrafiltration system set-up ................................................... 27
2.3.2. Major components of ultrafiltration system ............................ 29
2.3.2.1. Membrane module ......................................................... 29
2.3.2.2. Peristaltic pump.............................................................. 30
2.3.2.3. Controlled temperature bath........................................... 30
2.3.2.4. Operation of MEUF......................................................... 30
2.4. Analytical techniques.................................................................... 31
2.4.1. Ultraviolet-visible spectrophotometer (UV-VIS) ..................... 31
2.4.1.1. Cetylpyridinium chloride calibration curve ...................... 31
2.4.2. Inductively Coupled Plasma Mass Spectrometry (ICP-MS)... 32
2.4.3. Conductometer ...................................................................... 33
2.5. Key Performance Aspects of the Ultrafiltration system................. 33
2.5.1. Principle and Operation of MEUF system.............................. 33
2.5.2. Assessment and monitoring of the membrane condition....... 34
2.5.3. Regeneration of an ultrafiltration membrane ......................... 35
2.5.4. Fouling in membrane processes............................................ 36
2.5.4.1. Factors affecting fouling ................................................. 36
2.5.4.2. Common fouling problems.............................................. 36
2.5.5. Membrane cleaning ............................................................... 37
CHAPTER THREE
3. Preliminary investigation of metal ion and surfactant retention............ 39
3.1. Definitions ..................................................................................... 39
3.2. Influence of various parameters.................................................... 40
3.2.1. Effect of pressure variation .................................................... 40
3.2.2. Effect of temperature variation............................................... 42
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3.2.3. Effect of cetylpyridinium chloride concentration variation ...... 44
3.3. Micellar-enhanced ultrafiltration of Pt (lV) and Pd (ll) anions ........ 45
3.3.1. Micellar-enhanced ultrafiltration of Pt (lV) and Pd (ll) anions
in acidic medium ................................................................... 45
3.3.2. Micellar-enhanced ultrafiltration of Pt (lV) and Pd (ll) anions
in neutral medium.................................................................. 48
3.3.3. Micellar-enhanced ultrafiltration of a mixture of Pt (lV) and
Pd (ll) anions in neutral medium............................................ 50
3.3.4. Effect of an electrolyte on metal ion and surfactant retention 52
3.4. Membrane interaction ................................................................... 53
3.5. Summary of preliminary investigations ......................................... 55
CHAPTER FOUR
4. Investigation of cetylpyridinium chloride retention................................ 56
4.1. Overview....................................................................................... 56
4.2. Investigation of the effects of an electrolyte.................................. 56
4.2.1. Effects of hydrochloric acid concentration variation............... 57
4.2.2. Effects of nitric acid concentration variation .......................... 60
4.2.3. Effects of sodium chloride concentration variation ................ 62
4.3. Summary of CPC retention investigations .................................... 64
CHAPTER FIVE
5. The concept of micellisation and conductivity investigations ............... 65
5.1. The concept of micellisation.......................................................... 65
5.2. Conductivity study......................................................................... 66
5.2.1. Experimental determination of the critical micelle
concentration of cetylpyridinium chloride ............................... 66
5.2.2. Experimental procedure......................................................... 66
5.2.3. Determination of the degree of ionization .............................. 68
5.3. Influence of an electrolyte on the cmc value and degree of
ionisation ............................................................................... 69
5.3.1. Determination of cmc in the presence of HCl ........................ 69
5.3.2. Determination of cmc in the presence of Pd (ll) anions and
aqua regia .............................................................................. 71
5.3.3. Determination of cmc in the presence of Pt (lV) anions and
aqua regia .............................................................................. 72
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5.5. Summary of the conductivity investigations.................................. 74
CHAPTER SIX
6. Improved conditions for the retention of metal ion and surfactant........ 75
6.1. Micellar-enhanced ultrafiltration of individual metal ions............... 75
6.1.1. Ultrafiltration of platinum (lV) anions...................................... 75
6.1.2. Ultrafiltration of palladium (ll) anions...................................... 77
6.2. Micellar-enhanced ultrafiltration of Pt (lV)-Pd (ll) mixture.............. 79
6.3. Membrane response during Pt/Pd mixture ultrafiltration............... 82
6.3.1. CPC retention ........................................................................ 82
6.3.2. Flux variation ......................................................................... 83
6.4. Summary of the investigation of improved conditions……………..84
CHAPTER SEVEN
7. Conclusive remarks.............................................................................. 85
7.1. Conclusion.................................................................................... 85
7.2. Recommendations........................................................................ 86
References .............................................................................................. 87
APPENDIX A ........................................................................................... 89
APPENDIX B ........................................................................................... 97
APPENDIX C ........................................................................................... 99
APPENDIX D ........................................................................................... 101
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i
ACKNOWLEDGEMENTS
I wish to express my sincere thanks to:
• Dr Nicole Vorster for her wonderful support, guidance and motivation.
• Prof EP Jacobs and Prof P. Loyson for their support.
• Dr E. Hosten for his assistance.
• A special tribute to my friends, Lwandile Mcingana, Akhona, Sivuyile,
Chanda and Mtheza for their assistance and support throughout this study.
• My fellow students and co-workers at NMMU, and the entire PETCRU
personnel for their assistance.
• Dr C. Viviers and Mr A. Joubert and co-workers at HCSA for their
understanding and support.
• CCETSA, NRF and PET for their financial support.
• My mom and the entire family for their encouragement and support tocarry out this work during the trying times.
The “Almighty” for making everything possible.
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ii
EXECUTIVE SUMMARY
The project was concerned with studying the capability of a micellar-enhanced
ultrafiltration system (MEUF) to remove platinum group metal ions namely Pt
(lV) and Pd (ll) chloro anions from aqueous industrial waste effluents. South
Africa has the world’s largest reserves of platinum group metals (PGMs) and
other valuable metals such as manganese, chrome ores, titanium minerals
etc. which are required for new automotive and other technologies, including
fuel cells, catalytic converters and lighter components. The consistent loss
with the industrial waste stream and the toxicological effects of these precious
metals led to the need to develop new and effective methods to recover them
from industrial waste effluents.
With such a wide variety of fields where these PGMs are used and the failure
of the traditional techniques namely sedimentation, fermentation etc. to
effectively reduce or recover these highly toxic and precious metal ions prior
to discharging industrial waste effluents, it is necessary to explore other
techniques such as membrane technology that can be used to recover these
valuable species from industrial waste streams.
The present study involved the use of a cationic surfactant, viz cetylpyridinium
chloride, which was introduced into an aqueous solution containing palladium
and platinum metal anions. The surfactant forms charged micelles above a
certain critical concentration value. The metal anions adsorb onto the
available oppositely charged sites on the micelle surfaces and are then able
to be retained by a suitable membrane. Hollow fibre ultrafiltration membranes
with the MWCO of +/- 10 kD and +/-30nm pore size were used as a filter
component in this study. For this MEUF system to be effective, it was vital thatthe anionic metal ion species adsorbed sufficiently onto the available
oppositely charged sites of the micelles and that the micelles were retained
efficiently by the membrane.
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iii
Results obtained during the investigation made it possible to make certain
predictions about the micellisation process. It was also found that, it was not
only the metal ion: surfactant (M:S) ratio that was critical, but the presence of
other electrolytes in the aqueous stream proved to have a huge impact on the
capability of the MEUF system.
Findings of this research study showed that the MEUF system using
cetylpyridinium chloride (CPC) can be used to recover or retain Pt (lV) and Pd
(ll) anions from industrial waste effluents. It was also found that PtCl62-, due to
its greater adsorption capabilities onto the micelle surface than PdCl42- or
PdCl3(H2O)-, was preferentially retained in neutral medium. This may be
exploited as a possible means of separating the two metal ions.
The developed system offers the following advantages over some traditional
and current methods: simplified unit operation line flow process, smaller
amounts of chemical usage and no solid toxic sludge to be disposed of.
Applications of this work could be of vital importance in catalytic converter
recycling, especially in Port Elizabeth where extensive automobile parts
manufacturing occurs.
KEYWORDS:
Platinum, palladium, micellar-enhanced ultrafiltration, surfactant,
micellisation, cetylpyridinium chloride
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iv
ABBREVIATIONS
MEUF Micellar-Enhanced Ultrafiltration
mM Millimolar
CPC Cetylpyridinium chloride
PGMs Platinum group metals
cmc Critical micelle concentration
NaCl Sodium chloride
HCl Hydrochloric acid
HNO3 Nitric acidICP-MS Inductively Coupled Plasma Mass Spectrophotometry
UV-VIS Ultraviolet visible
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v
LIST OF FIGURES
Figure 1.1: Worldwide distribution of PGMs 4
Figure 1.2: Micelle formation in dilute solution 9
Figure 1.3: Schematic diagram of a micelle 9
Figure 1.4: Schematic diagram showing the occurrence of cmc with 10
changing solution parameters
Figure 1.5: A typical diagram showing the occurrence of a cmc value 11
at the Kraft point with changing solution parameters
Figure 1.6: Cetylpyridinium chloride structure 12
Figure 1.7: Hollow fibre module 18
Figure 1.8: Tubular module 18
Figure 1.9: Pressure-driven membrane separation processes 19
Figure 1.10: A mechanistic scheme showing the adsorption of an 22
anionic metal species such as Pt (lV) onto a cationic micelle
Figure 2.1: SEM image of a cross section of the polysulfone hollow fibre 26
membrane
Figure 2.2: SEM image of a part of the outer surface of the polysulfone 26
hollow fibre membrane
Figure 2.3: Photo of a laboratory scale MEUF system 28
Figure 2.4: Schematic diagram of an ultrafiltration system 29
Figure 2.5: CPC calibration curve 32
Figure 2.6: A typical layout of an ultrafiltration system 34
Figure 2.7: Pure water flux measurements taken after various 35
experiments
Figure 3.1: Effects of pressure variation at 30°C and 10 mM CPC 41
Figure 3.2: Effects of temperature variation 150 kPa and 10 mM CPC 42Figure 3.3: Variation of CPC concentration at 30°C and 150 kPa 44
Figure 3.4: Investigation of Pt and Pd anions retention at 150 kPa and 46
30°C in acidic medium as a function of CPC concentration
Figure 3.5: Investigation of Pt and Pd anions retention at 150 kPa and 48
30°C in neutral medium as a function of CPC concentration
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vi
Figure 3.6: Species distribution curves of Pd (ll)/chloride system 49
Figure 3.7: Investigation of Pt and Pd anions separation in acidic 51
medium at 150 kPa, 30°C as a function of CPC concentration
Figure 3.8: Effects of sodium chloride on Pt and Pd anions retention 52
Acidic medium at 150 kPa and 30°C using 40 mM CPC
Figure 3.9: Effects of various species on flux variation at 150 kPa and 54
30°C in neutral medium
Figure 4.1: Variation of hydrochloric acid concentration at 30°C, 58
150 kPa and 10 mM CPC
Figure 4.2: Variation of nitric acid concentration at 30°C,150 kPa and 59
10 mM CPC
Figure 4.3: Study of variation of sodium chloride concentration at 61
30°C, 150 kPa and 10 mM CPC
Figure 4.4: Surface tension of a 10 mM CPC solution as a function of 62
HNO3 concentration
Figure 4.5: Study of variation of sodium chloride acid concentration 63
at 30°C, 150 kPa and 10 mM CPC
Figure 5.1: Determination of the cmc of CPC and its degree of 67
ionisation
Figure 5.2: Plot of the conductivity vs CPC concentration in the 70presence of HCl
Figure 5.3: Plot of the conductivity vs CPC concentration in the 71
presence of Pd (ll) anions in aqua regia
Figure 5.4: Plot of the conductivity vs CPC concentration in the 73
presence of Pt (lV) anions in aqua regia
Figure 6.1: MEUF of Pt (lV) anions at 40 mM CPC, 30°C and 150 kPa 76
in acidic medium
Figure 6.2: MEUF of Pd (lV) anions at 40 mM CPC, 30°C and 150 kPa 78
in acidic medium
Figure 6.3: MEUF of a mixture of Pd (lV)/Pt (lV) anions at 40mM CPC, 80
30°C and 150 kPa in acidic medium
Figure 6.4: MEUF of a mixture of Pt (lV)/Pd (lV) anions at 40 mM CPC, 81
30°C and 150 kPa in acidic medium
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vii
Figure 6.5: Effects of metal ion ratio on CPC retention 82
Figure 6.6: Flux variation during Pt (lV)/Pd (lV) anions ultrafiltration 84
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viii
LIST OF TABLES
Table 1.1: Examples of common surfactants 12
Table 1.2: Description of different types of membranes 16
Table 1.3: Summary of membrane separation processes 20
Table 2.1: List of Chemicals used for synthesis and analysis 24
Table 2.2: Major components of a laboratory scale MEUF system 28
Table 2.3: Membrane module specifications 29
Table 2.4: Peristaltic pump specifications 30
Table 2.5: ICP-MS settings 33
Table 2.6: Membrane foulants and control measures 37Table 6.1: Summary of varying metal ion concentration ratios 79
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1
CHAPTER ONE
INTRODUCTION
In this chapter, a brief overview on waste treatment, the chemistry and
industrial applications of platinum group metals, surfactants and
membranes will be given. The scope of this research study will also be
outlined.
1.1 Background to waste treatment
In recent years, waste treatment has become a necessary activity of
various industries, most importantly, of chemical industries. The major
purpose of wastewater treatment is to remove as much of the
suspended solids and hazardous materials present in waste streams
prior to their disposal back to the environment. The emphasis on
wastewater treatment has not only been generated by the negative
impact that wastewater constituents impose on the community due to
their toxic and harmful effects, but also by the huge profits that can be
enjoyed by chemical industries from the recovery of waste materials. For
some chemical industries, the recovered components can be
reprocessed, thus leading to higher yields of their products.
Stringent regulations after the introduction of an Environmental
Management System (EMS) standard, namely the ISO 14001 series,
require all industries to comply with the regulatory systems and to
continuously improve wastewater treatment activities.1, 2 Since the costs
and constraints of treatment systems can be unbearable, the necessity
to reduce the content of these highly toxic as well as non-biodegradable
constituents prior to discharging the waste stream into sewage is of vital
importance. 3
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2
It has been noted that the use of traditional techniques, namely lime
precipitation, ion exchange, activated carbon adsorption, electrolytic
processes etc. are ineffective in reducing the levels of carcinogenic
components to the required levels as stipulated in the Environmental
Management System. 2, 4 It has also been noted that the use of
membrane separation processes in wastewater treatment is an
attractive, as well as a more preferred technique, especially in the
chemical industry, than the previously mentioned methods.
1.2 Definition of a hazardous waste
Hazardous waste is defined as an organic or inorganic element or
compound that due to its chemical, physical and toxicological properties
may be deemed harmful and/or brings chronic impacts to human health
and the environment.1 It can be generated from a variety of agricultural,
commercial, domestic and industrial activities and may be in the form of
a liquid, sludge or a solid. Within this hazardous waste there are
inorganic pollutants such as platinum group metals that are considered
to be highly toxic, non-biodegradable and which have definite
carcinogenic effects. 3 These features contribute not only to the degree
of hazard, but are also of great importance in the ultimate choice of a
safe and environmentally reasonable method of disposal or treatment.
1.3 Overview of platinum group metals (PGMs) 5, 6
Platinum group metals form part of the transition metal series. The group
is made up of the following metals, namely, palladium, platinum, iridium,
osmium, rhodium and ruthenium. Theoretically, platinum group metal
ores are assumed to contain 20% of each of palladium, platinum,
osmium and ruthenium and 6% of iridium and rhodium.
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1.3.1 Occurrence and origin
The platinum group metals in the lithosphere have been transferred from
the earth’s interior. Tectonic movements of the earth’s crust followed by
the eruption of magma have led to the presence of PGMs in regions
closer to the surface. Chemical interaction with silicate layers such as
sulphides, arsenides, antimonide etc. at high temperatures has played a
major role in the origination of platinum group metals. However, the
PGMs later separated from ultrabasic magmas, hence the ores are
sulphide free. Platinum and palladium, and sometimes together with
nickel or copper, have undergone some hydrothermal reactions with
chlorides in the earth’s interior which have led to the formation of the
primary deposits of the platinum-bearing rock. Activities in those primary
deposits depend on the following factors, namely, concentration of
platinum metals present, accessibility, size of deposits, their value and
the economic potential of the accompanying materials.
The platinum group metals occur mainly in ore deposits in a large
number of minerals. Workable deposits usually contain mainly sperrylite
(PtAs2), cooperite (PtS), ferroplatinum (Fe-Pt), polyxene (Fe-Pt-other
PGMs) etc. These minerals are often associated with specific carrier
materials such as iron pyrites, nickel iron pyrites, or chrome iron pyrites.
These minerals as well as the platinum group metals are rarely found in
exact stoichiometric ratios. 7, 8, 9 The platinum group metals, as already
stated, often occur in a very complex ore body in which they are bonded
mainly to the so-called soft ligands such as sulphides or polysulphides,
arsenides and selenides.
1.3.2 Abundance and Distribution
The abundance of platinum group metals, which occupy an intermediate
position, based on their atomic number and atomic mass would be
expected to be 10-4 ppm based on the manner of formation of their
atomic nuclei. 8 A global distribution diagram of the platinum group
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metals is shown in Fig. 1.1. South Africa is a leading producer of
platinum group metals because it has the world’s largest reserves of
PGMs. The worldwide distribution of the PGMs shows that there are only
two countries, namely, South Africa and Russia, that produce a
significant amount of these PGMs. 8 The main ore bodies in South Africa
are owned by two companies, namely, Anglo American and Impala
Platinum. The mining, refining and processing of these PGMs in South
Africa does not only serve as a source of income for the country, it also
brings about economic stability. 2
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
South Africa Russia Finland Zimbabwe
USA Canada China Colombia
Figure 1.1: Worldwide distribution of platinum group metals
It is also said 6, 9 that, in South Africa, PGMs together with other by-
products namely gold, silver, nickel, copper and cobalt occur in
economic concentrations in three extensive layered reefs associated
with the mafic rocks of the Rustenburg layered suite of the Bushveld
Complex. They are the Merensky reef, the UG2 Chromitite layer and the
Platreef. Small quantities of the PGMs are also produced from golddeposits of the Witwatersrand Basin and from the copper ores of the
Phalaborwa Complex. South Africa’s mineral wealth and the increasing
demands of the platinum group metal products throughout the world,
makes the process of hydrometallurgy vital. Also, the chemistry related
to the dissolution of the ore of the PGMs, their successive separation
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and ultimate production of the pure individual PGMs are thus very
important for the economy of this country.
1.3.3 Chemical properties
The chemical properties of the platinum group metals resemble those of
the late 4d and 5d transition metals as listed below:
• Platinum group metals have multiple oxidation states viz. Pt (ll), Pt
(lV), Pd (ll), Pd (lV) etc.
• They show high covalent character in bonding.
• They have slow ligand exchange due to kinetic stability of their
complexes.
• In their metallic form, the metals are inert and show great stability.
These above-mentioned properties greatly contribute towards the
problematic nature of the dissolution, isolation and processing of PGMs.
Also, these same characteristics make these metals suitable for a variety
of applications, namely, catalysis, synthesis, etc.7, 8, 10, 11
1.3.4 Basic coordination chemistry of platinum and palladium 7, 8
Palladium is a member of the 4d transition metal series while platinum
falls within the 5d series. Palladium and platinum have the same number
of electrons in their valence shell, viz. ten electrons in excess of the
preceding noble gas (s0d10 for Pd and s1d9 for Pt). These metals have
similar electron configurations since they belong in the sub-group that
constitutes nickel, palladium and platinum. The most significant
characteristic properties of the two metals are the kinetic stability of their
complexes and the high covalent character of their bonding. These
metals are known to prefer the “soft” donor ligands with covalent
properties as opposed to “hard” donor ligands such as ammonia and
aliphatic amines, and thus they show a greater preference for pi
acceptor ligands like chloride, sulphur, arsenic and phosphorous donors.
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Furthermore, the divalent metal complexes of these two metals are
almost, without exception, spin-paired square planar d8 systems, while
their tetravalent compounds are spin-paired octahedral d6 systems.
Based on the previously mentioned identical behaviours of these two
metals, it can be stated that these metals have similar preferences with
respect to ligand selectivity and symmetry of their coordination
compounds; and have similar electronic as well as magnetic properties.
1.3.5 Industrial applications of platinum group metals 8, 12
1.3.5.1 Catalysis
Platinum group metals are renowned for contributing towards a pollution
free environment. They are extensively used in automobile catalytic
converters where they catalyse the detoxification of nitrates and
reduction of hydrocarbons in automobile exhaust gas emissions to
acceptable low levels. They are also used in various complex catalytic
processes such as aromatization, cracking, cyclisation, desulphurisation,
hydrocracking, hydroprocessing and reforming in the petroleum industry
to manufacture a wide variety of products.
Nitric acid that is used in the production of nitrogenous fertilizers (78%),
and explosives (12%), is produced by catalytic oxidation of ammonia
over gauzes woven from very fine platinum (90%)-rhodium/palladium (5-
10%) alloy wire. 9
Similar catalysts are employed in the Andrussen and Degussa
processes for the manufacturing of hydrogen cyanide from methane and
ammonia. These plants produce nitrous oxides that are reduced by a
platinum-impregnated catalyst contained either in a ceramic honeycomb,
alumina-pellet or nickel-chromium alloy ribbon support.
PGMs are also used in processes involved in the manufacture of liquid-
phase chemicals which include:
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• Highly specific hydrogenation catalysts.
• Complex catalysts using polymeric supports to catalyze a whole
spectrum of organic reactions.
• Metal-cluster catalysts in which organo-metallic compounds form
cluster structures that can model biological catalysts.
• Oil from coal processes.
1.3.5.2 Jewellery 8, 12
In jewellery making processes, platinum offers the following advantages:
• It is harder (4.3 on mohs scale) than gold and does not nick or
scratch easily.
• It does not tarnish like silver.
• It holds gemstones better because of its superior strength and
hardness.
However, it has one disadvantage in the form of a high melting point and
thus has to be alloyed with rhodium in order to be suitable for jewellery
setting.
1.3.5.3 Electronics 5, 12
Platinum is employed in electrical contacts that are used under severe
conditions such as heavy-duty relays, switches, thermostats, voltage
regulators and slip-ring assemblies. It is also used in making thin-film
circuits, resistance elements, multilayer capacitors and oxygen sensors.Thermocouples employed in the steel and glass smelting industry use a
fine-wire platinum/rhodium alloy for the positive electrode and a pure
platinum wire for the negative electrode to provide a long life span and
the highest degree of accuracy.
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Palladium is used in capacitors as contact materials applying to any
electrical contacts operating at low currents and voltages under low
contact forces due to the reasons listed below:
• Freedom from oxide films and tarnish.
• High melting temperature which results in a high resistance to arc
erosion and to the welding of contact surfaces such as telephone
switching relays, electrical and electronic apparatus, resistors, gas
turbine engines and atomic reactors, spinnerets to produce rayon
fibre, heating pads, high voltage regulators, relays, thermostats,
sliding contacts, magnetos, vibrators and signs.
1.4. Surfactants
1.4.1 Overview
Surfactants are defined as molecules that are usually surface active in
aqueous solutions. 13 These molecules adsorb strongly at the water-air
interface and subsequently lower the surface energy of water (However,
inorganic electrolytes that are desorbed at the water air interface behave
differently as they tend to slightly increase the surface energy of water).
Surfactant molecules are amphiphilic in nature, that is, they have a
charged head group that is hydrophilic and a hydrophobic hydrocarbon
tail. It is this unique feature that makes surfactants adsorb so effectively
at a surface interface. In dilute solutions, surfactant molecules are in the
form of monomers. The number of monomers adsorbed at the surface
increases with increasing surfactant concentration until the surface
becomes saturated. At this saturation point called the critical micelle
concentration, the monomers in the bulk solution aggregate to form
micelles in order to minimize the exposure of the hydrophobic tail to the
hydrophilic environment while maintaining the maximum interaction of
the charged head group with the water. The resulting micelles are
usually spherical in shape and range from 50 to 100 monomers as
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shown in Fig. 1.2 below. An increase in surfactant concentration tends to
increase the number of micelles per unit volume.
Figure 1.2: Micelle formation in dilute solution
For a particular spherical micelle, the head (hydrophilic) groups are not
closely packed on the surface of the micelle and calculations indicate
that the head groups occupy not more than a third of the micelle surface
(see Fig. 1.3). 13
Figure 1.3: Schematic diagram of a micelle 13
+ + + + + + + +
+ +
+ + +
+ +
+ +
+ + + +
+
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Repulsion between the head groups tends to increase the surface area
hence some of the counter ions remain associated with the micelles so it
is not fully ionized.
Above the critical micelle concentration (cmc) some of the surfactant
solution properties such as conductivity, surface tension and viscosity
suddenly change. This abrupt change in solution properties can be used
as a means of determining the cmc (see Fig. 1.4).
Figure 1.4: Schematic diagram showing the occurrence of the cmc with
changing solution parameters
Micelles are soluble in water but only above a certain temperature called
the Kraft temperature. This is the triple point on the phase diagram of an
ionic surfactant (see Fig. 1.5 below). Below this temperature and above
the cmc the micelles form gels. The solubility of a surfactant generally
depends on how easily it dissociates in water and how well the head
group is solvated. The solubility generally increases with increasing
temperature.
Surface tension
Electricalconductivity
S o l u t i o n p a r a m e t e r
Concentration of surfactant
cmc
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Figure 1.5: A typical phase diagram of an ionic surfactant showing the
cmc value at the Kraft point
There are a number of possible micelles shapes, namely, spherical, rod-
shaped or disk-shaped that can result when micellisation takes place.
The shape of a micelle is determined mostly by the ratio of the area
requirement of the head group of the surfactant under the prevailing
conditions to the volume of the hydrophobic tail group. If this ratio is
large, as in the case of n-alkyl surfactants, spherical surfactants are
formed, whereas small ratios, such as in the case of nonionic alkyl
polyglycol ethers, lead to the formation of rod-shaped micelles. Rod-
shaped micelles can also be formed by the addition of an electrolyte into
ionic surfactant solutions that leads to the screening of the head groups’
electrostatic repulsive forces which result in a reduction of the head-
group area. Spherical micelles can also transform at higher surfactant
concentrations into rod-shaped or disk-shaped micelles and this change
is indicated by a second critical micelle concentration where there is a
rapid change of colligative properties of a surfactant.13
Gel
Micellar solution
cmc
Monomersolution
T
c
Kraftpoint
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1.4.2 Classification of surfactants
Surfactants can be classified into the following groups, namely, anionic
(negatively charged head groups), cationic (positively charged head
groups) and nonionic (no surface charge). Examples of each type are
listed in Table 1.1 below.
Table 1.1: Examples of common surfactants
Type Examples Chemical Formula
Sodium dodecyl sulphate CH3 (CH2)11SO4-Na+
Anionic Sodium dodecyl benzene
sulphonate (SDS)CH3 (CH2)11C6H4SO3
-Na+
Cetyltrimethylammonium
bromide (CTAB)CH3 (CH2)15N(CH3) 3
+Br- Cationic
Cetylpyridinium chloride C21H38NCl
Nonionic Polyethylene oxide CH3 (CH2)7(O.CH2CH2)8OH
The structure of the cationic surfactant, namely, cetylpyridinium chloride
that was used for this research study is shown in Fig 1.6 below.
Figure 1.6: Cetylpyridinium chloride structure
1.4.3 Industrial applications of surfactants 14
Surfactants are considered to be of significant importance in various
industries, namely, catalysis, detergent, emulsification, lubrication, oil
recovery industry and the pharmaceutical industry. Some applications
are listed below:
N + C H 3 Cl -
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• They have been used as solubilizing agents and as probes for the
hydrophobic binding sites in the investigation of molecular properties
of membrane proteins and lipoproteins.
• Surfactants have been used in the investigation of the denaturation
and thermal stability experiments of bacteriorhodopsin.
• They are also employed to promote dissociation of proteins from
nucleic acids on extraction from biological material.
• Surfactants have been used as useful reagents in analytical chemistry
techniques, namely, chromatography and luminescence
spectroscopy.
1.5 Membranes
1.5.1 Overview
Industrial and scientific development during the last 200 years has
brought about a variety of industrial-scale separation techniques such as
distillation, precipitation, crystallization, extraction, adsorption and ion
exchange. However, processes that employ semi-permeable
membranes as separation barriers have recently supplemented these
traditional techniques. Although membranes and membrane processes
were initially introduced as an analytical tool in chemical and biomedical
laboratories, they later developed into industrial products and methods
with greater technical and commercial impact.15,16 A membrane is
defined as an interphase that influences the transport of chemical
species in a particular manner. A membrane structure has lateral
dimensions much greater than its thickness, through which mass
transfer may occur under a variety of driving forces.
17
There are several kinds of membranes that are currently in use, namely:
16
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• Membranes that are made from synthetic polymers (also copolymers
and blends).
• Inorganic membranes that use inorganic porous supports and
inorganic colloids such as zirconium oxide (ZrO2) or alumina with
appropriate binders.
• Melt-spun “thermal inversion” membranes like hollow fibre
membranes and track-etched membranes.
The great majority of analytically important ultrafiltration membranes
belong to the first type, that is, synthetic polymers. These membranes
are usually made of polycarbonate, cellulose (esters), polyamide, and
polysulphone. The greater interest recently in membrane separation
processes originates from the simplicity of the systems, affordability in
terms of cost, as well as their application in various fields namely,
chemical, food, pharmaceutical as well as wastewater treatment
industries.18 There are unique characteristics that influence the
membrane performance namely: selectivity, speed (flow rate with which
the feed solution passes through the membrane) and the stability that
relies mainly on the components that were used for the manufacturing of
the membrane. Membranes are formed by various methods from
numerous materials both of which profoundly influence the morphology
and structure of the resulting product. Some of the industrial applications
of membranes are listed below. 16
• Membranes (e.g. ultrafiltration (UF), nanofiltration (NF), microfiltration
(MF) and reverse osmosis (RO)) are used to treat industrial waste
effluents to recover valuable constituents.
• They are used to concentrate, purify and/or fractionatemacromolecular solutions in various industries e.g. (UF and MF).
• They are used to produce potable water from seawater.
• They are used to remove urea and some other toxins from the
bloodstream in the medical field (RO).
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1.5.2 Typical membrane properties
• A membrane can be heterogeneous or homogeneous and/or
asymmetric or symmetric in structure.
• It may be liquid or solid, and can carry neutral, negative or positive
charges.
• A membrane can have varying functional groups with different
binding or complexing capabilities.
• Membrane thicknesses range from 100 nm to more than 1 cm.
• The electrical resistance of a membrane may vary from thousands of
mega ohms to a fraction of an ohm.
1.5.3 Classification of membranes 15, 16
Membranes can be classified into five groups, namely, asymmetric,
electrically charged barriers, homogeneous, liquid films with selective
carriers and microporous membranes as shown in Table 1.2.
Membrane transport can be achieved by diffusion of individual molecules
or through convection induced by the following variables, namely,
concentration, pressure, temperature, or electrical potential gradient.16, 18
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Table 1.2: Description of different types of membranes
Membrane
type
Structure
description
Separation
process
Preparation
techniques
Microporous Solid matrix
Pore diameter:
(20 µm – 1 nm)
Materials used:
Ceramics, Metals,
and Polymers
Sieving mechanism
determined by pore
diameter and
particle size.
Sintering of
powders,
stretching of films,
irradiation and
etching of films
and phase
inversion
techniques.
Homogeneous Dense film
through which a
mixture of
chemical species
is transported
under the driving
force of
concentration,
pressure, etc.
Mass transport
occurs strictly by
diffusion.
Prepared from
polymers, metals,
or metal alloys by
film-forming
techniques.
Asymmetric Thin skin layer
(0.1 – 1 µm) on a
highly thick
porous
substructure (100
– 200 µm)
Separation
performance
determined by the
nature of the
membrane
material or pore
size
Phase inversion
process that
leads to an
integral structure
deposition of a
thin polymer film
on a microporous
substructure
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Table 1.2 contd: Description of different types of membranes
Membrane typeStructure
description
Separation
process
Preparation
techniques
Ion exchange Highly swollen
gels carrying
fixed positive or
negative
charges.
Different organic
polymer
matrices and
functional
groups
determine the
ion exchange
properties.
Matrix that
consists of a
hydrophobic
polymer such as
polystyrene,
polyethylene, or
polysulphone.
Fixed ionic
moiety can be
one of the
following: SO3, -
COO, AsO3, -
NH3, -NH2, -NH,
-N
Liquid films Comprise a thin
oil film
separating two
phases
consisting of
aqueous
solutions or gas
mixtures.
Use carriers to
selectively
transport
components
such as metal
ions at a
relatively high
rate across the
membrane
interphase.
The pores of a
microporous
membrane are
filled with the
selective liquid
barrier material
or the
membrane is
stabilised as a
thin oil film by a
surfactant.
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1.5.4 Membrane modules 19
Membranes can be formed into different configurations e.g. flat sheet,
capillaries or hollow fibre. The advantage of a hollow fibre configuration
is increased surface area. A number of capillary or hollow fibres can be
bundled together to form a module. Fig. 1.7 and Fig. 1.8 show two types
of membrane modules.
Figure 1.7: Hollow fibre module
Figure 1.8: Tubular module
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1.5.4 Pressure-driven membrane filtration processes
Membrane separation processes can differ significantly with respect to
membrane type, driving forces, areas of application and industrial or
economic relevance. Ultrafiltration, microfiltration and reverse osmosis
are the most important pressure-driven separation processes. The
performance of a membrane in a pressure-driven separation process is
determined by its filtration rate (i.e. transmembrane flux at certain
hydrostatic pressure) and its mass separation properties (i.e. retention
capability).
A summarized schematic diagram of pressure-driven membrane
separation processes and examples of species that can be retained
using those processes is shown in Fig. 1.9 below.
Figure 1.9: Pressure driven membrane separation processes 19
Membrane filtration processes can be applied to the treatment of
industrial waste containing inorganic constituents, such as metal ions, at
low concentrations.
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The different membrane filtration processes are classified according to
the type of chemical species that is to be retained, the corresponding
membrane pore size that is required to retain that species and the
applicable driving forces as shown in Table 1.3 below which lists the
membrane processes in order of the type of species to be retained.
Table 1.3: Summary of pressure-driven membrane separation
processes 16
ProcessMembrane
type
Driving
force
Separation
methodApplication
Microfiltration Symmetric
microporous
membrane,
0.1 to 10
µm pore
radius
Hydrostatic
pressure
difference,
10 to 500
kPa
Sieving
mechanism
due to pore
radius and
adsorption
Sterile
filtration,
clarification
Ultrafiltration Asymmetric,
microporous
membrane,
1 to 10 nm
pore radius
Hydrostatic
pressure
difference,
0.1 to 1
MPa
Sieving
mechanism
Separation
of macro-
molecular
solutions
Reverse
osmosis
Asymmetric
skin-type
membrane
Hydrostatic
pressure
difference,
2 to 10
MPa
Solution
diffusion
mechanism
Separation
of salts and
microsolutes
from
solutions
1.5.6 Ultrafiltration
Ultrafiltration is one of the several pressure-driven membrane
technologies. It can be used to separate small colloids and large
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molecules from water and other aqueous media. The system is similar to
microfiltration and differs only in the size of the separated particles and
the membranes used. The ultrafiltration process falls between reverse
osmosis and microfiltration in terms of the size of the particles removed,
with ultrafiltration removing particles in the 0.002 to 0.1 micron range,
and typically rejecting organics over 1000 molecular weight while the
ions and smaller organics are allowed to pass through. A feed solution
containing a mixture of components of different sizes is brought to the
surface of a semi- permeable membrane. Under the driving force of a
hydrostatic pressure gradient, solvent or small solutes permeate the
membrane as filtrate while the larger particles or molecules and unbound
ions are retained by the membrane and concentrated in the retentate.
The separation is based solely on a sieving effect and particles are
separated solely according to their dimensions. This process is regarded
as an alternative to conventional and other traditional clarification
techniques like flocculation, sedimentation, etc.20
1.5.6.1 Micellar-enhanced ultrafiltration for metal ion retention
Supported liquid membranes, where a complexing agent is added to an
organic solvent contained in a porous hydrophobic membrane support
have been applied to metal ion separation, but the use of these methods
on an industrial scale is limited by the inherent instability of such
membranes. Other techniques such as reverse osmosis and
nanofiltration membrane systems that are able to retain metal ions can
also be applied but have limitations that are mainly due to their high
costs which result from high transmembrane pressures and product
fluxes which tend to be generally low.3
Micellar-enhanced ultrafiltration has been shown to be an effective
method for the removal of low levels of toxic heavy metal ions and
organic compounds from industrial waste effluents.4
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In micellar-enhanced ultrafiltration, a surfactant is added to an aqueous
solution containing the solutes of interest (metal ions) at a concentration
higher than its critical micelle concentration (cmc) in order to capture
ionic solutes. Once in solution, the surfactant forms large amphiphilic
aggregate micelles with charged surfaces, ranging from 50 to 100
surfactant molecules. This phenomenon of micelle formation is called
micellisation. The metal ions of opposite charge adsorb onto the charged
micelle surface. The micelles increase the hydrodynamic size of the
metal ion solutes that brings about their retention by the ultrafiltration
membranes. See Fig 1.10 below.
Figure 1.10: A mechanistic scheme showing the adsorption of an anionic
metal species such as Pt (lV) onto a cationic micelle
Among the advantages of this method are the low-energy requirements
of the ultrafiltration process and its high removal efficiency owing to the
effective trapping of the metal ion solutes by the micelles. MEUF has
been used to separate organic pollutants, heavy metals, chromates,
nitrates and sulphates using both cationic and anionic surfactants as well
as polyelectrolytes as additives.4, 21
Cationic surfactant monomer
Retentate
PtCl62-
Permeate
_
_
_
+
+
+
+++
++
+
+ +++ +_
_
_
_
_
+ Cationic surfactant monomer
Retentate
PtCl62-
Permeate
_
_
_
+
+
+
+++
++
+
+ +++ +_
_
_
_
_
+
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1.6 Research objectives
In this research project, the ability of a membrane ultrafiltration system
that is commonly used for the separation as well as the recovery of
dissolved molecules or colloids in aqueous medium on the basis of
molecular size was investigated for the retention of platinum group
metals, namely, palladium and platinum. Within this membrane
ultrafiltration system, a suitable cationic surfactant, namely,
cetylpyridinium chloride (CPC), was introduced in order to form colloidal
aggregates that would increase the hydrodynamic size of the metal ions
thus improving their retention by the system. The resulting process is
referred to as micellar-enhanced ultrafiltration (MEUF). Variables such
as pressure, temperature, pH, surfactant: metal ratio (S:M) and the effect
of using electrolytes like sodium chloride were investigated in order to
establish the conditions that can achieve the highest retention of
platinum and palladium anions as well as surfactant from industrial
waste streams.
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CHAPTER TWO
EXPERIMENTAL AND KEY PERFORMANCE ASPECTS
In this chapter, the reagents, apparatus or equipment as well as
analytical and synthetic procedures that were used during this research
study will be described. The key performance aspects of the membrane
module will also be explained.
2.1 MaterialsAll materials that were used in the preparation of synthetic solutions with
their sources and respective grades are listed in Table 2.1 and were
used without purification.
Table 2.1: List of chemicals used for synthesis and analysis
CHEMICAL NAME FORMULA SOURCE GRADE
Cetylpyridinium chloride
(98%)
C21H38ClN.H2O Aldrich AR
Dichloromethane CH2Cl2 Saarchem AR
Sodium hydroxide NaOH Aldrich AR
Hydrochloric acid (32%) HCl Saarchem AR
Nitric acid (55%) HNO3 Saarchem AR
Sodium
hexachloroplatinate (lV)
Na2PtCl6 Aldrich CP
Potassium
hexachoropalladate (IV)
K2PdCl6 Aldrich CP
Sodium hypochlorite NaOCl Saarchem AR
Potassium chloride KCl Saarchem AR
Sodium chloride NaCl Saarchem AR
AR: Analytical reagent
CP: Chemically pure
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2.2 Synthetic procedures
2.2.1 Preparation of surfactant solutions
A calculated amount of cetylpyridinium chloride salt was weighed and
dissolved in deionized water to make a 100 mM surfactant stock
solution. Aliquot amounts were taken from the prepared stock solution to
prepare surfactant solutions of different concentrations for ultrafiltration
experiments.
2.2.2 Preparation of surfactant/metal solutions
1.0 mM metal stock solutions were prepared by dissolving calculated
amounts of either Na2PtCl6 or K2PdCl6 salt in 5.0 ml aqua regia solution
(vol. ratio HCl:HNO3 = 3:1) or other acid mixture in a small beaker and
transferring into a 500 ml volumetric flask. These prepared stock
solutions were used to prepare surfactant/metal solutions by taking
aliquots of the metal stock solutions and adding them to various amounts
of cetylpyridinium chloride. Other experimental variables such as pH,
electrolyte concentration and temperature etc. were adjusted as required
by the reaction conditions prior to the ultrafiltration process.
2.2.3 Membrane module preparation
A bundle of double skinned polysulphone hollow fibre membranes
(twelve per unit) of approximately 30 nm pore sizes, 10 kD MWCO
manufactured at the Institute of Polymer Science at Stellenbosch
University was prepared and properly fitted into a stainless steel
membrane tube. A quickset epoxy resin was used to seal the
membranes into the tube in order to ensure that there were no leaks of
metal feed solution during an ultrafiltration experiment. SEM images of
the cross-section and surface of one of the hollow fibres were obtained
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using the electron microscope in the Physics Department of the NMMU.
These images are found in Figs. 2.1 and 2.2 respectively.
Figure 2.1: SEM image of a cross-section of the polysulphone hollow
fibre membrane
Figure 2.2: SEM image of a part of the outer surface of the polysulphone
hollow fibre membrane
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2.3 Apparatus
A laboratory scale membrane ultrafiltration system was used in all
ultrafiltration experiments carried out in this research study.
2.3.1 Ultrafiltration system set-up
The design of a membrane filtration system is very important as it
determines the rates of physical transport phenomena associated with
the chemical process or reaction, which in turn determines the outcome
of the chemical reaction.22, 23 To have an ideal and uniform micellisation
process occurring during the ultrafiltration experiments, the feed
reservoir should have a sufficient volume to contain a certain amount of
matter for a certain period of time. It should also offer the possibility for
sufficient contact between the reactants. The feed metal ion solution was
contained in a 300 cm3 beaker that was housed in a controlled
temperature water bath and mechanically agitated. A four-bladed stirrer
was used for agitation at approximately 300 rpm. A peristaltic pump was
used to pump the feed solution through the membrane module, with the
feed, retentate and permeate samples taken at specified intervals. A
photograph of the laboratory scale ultrafiltration unit is shown in Fig. 2.3
and a diagrammatic representation is depicted in Fig. 2.4. Table 2.2 lists
the major components of the system.
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90
Figure 2.3: Photo of a laboratory scale MEUF system
Table 2.2: Major components of lab scale MEUF system
Component Name
A Feed vessel
B Peristaltic pump
C Pressure gauge
D Membrane module
E Permeate vessel
F Pressure release valve
G Overhead stirrer
H Thermostat bath
B
A
C D
E
F
G
H
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PermeateFeed
solution
Retentate
Pressure
gauge
Membrane (hollow fibre) in S/S casing
Pressure
control
valve
Thermostat bath
Stirrer
Figure 2.4: Schematic diagram of MEUF system
2.3.2 Major components of Ultrafiltration system
2.3.2.1 Membrane module (D)
This unit is a stainless steel pipe that was used to house the
polysulphone membranes used during the ultrafiltration process. It is
about 38 cm long and is held firmly at a slight angle to the horizontal to
allow continuous and uniform flow of the feed solution through the
membrane and also to prevent the membranes from folding inside it and
subsequently being damaged. A list of membrane filter unit
specifications is shown in Table 2.3.
Table 2.3: Membrane module specifications
Variable Value (units)
No of fibres 12
Length 38 cm
Outside diameter 2.0 mm
Membrane Area 0.024 m2
Peristaltic pump
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2.3.2.2 Peristaltic pump (B)
The pump was used to pump the feed solution through the membranes
from the inside to the outside of each hollow fibre during the ultrafiltration
process. It was driven at a constant speed of 45 rpm in all the
ultrafiltration experiments. The pump played an important role as it
helped to maintain the consistent flow of the feed solution in all the
experiments. Peristaltic pump specifications are shown in Table 2.4
below.
Table 2.4: Peristaltic pump specifications
Variable Value (units)
Model Gilson’s Minipulse 3
Speed 0 - 48 rpm
Operating Pressure Range 0 - 600 kPa
2.3.2.3 Controlled temperature bath (H)
A stainless steel controlled temperature water bath was used to control
and maintain a constant temperature in all the ultrafiltration experiments.
2.3.2.4 Operation of MEUF
The feed solution was pumped axially across the membrane surface to
ensure tangential flow that reduces the concentration polarization
phenomenon near the membrane surface. Permeate and retentate
streams were recycled into the feed reservoir to maintain steady state
conditions. Initial feed, retentate and permeate samples were collected
after the completion of each ultrafiltration run and analysed for metal ion
and surfactant content by Inductively Coupled Plasma Mass
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Spectrometry (ICP-MS) and Ultraviolet-visible spectrophotometry
respectively. Every UF run was performed in duplicate, and the results
averaged.
2.4 Analytical techniques
The methods that were employed for the analyses of the surfactant and
platinum group metals together with the parameters and the instrumental
settings are described below.
2.4.1 Ultraviolet visible spectrophotometry (UV-VIS)
UV-visible analysis of feed and permeate samples were performed on a
Beckman DU-650 spectrophotometer to determine the surfactant
content. A 0.1 cm quartz cell was used. 24, 25 The surfactant,
cetylpyridinium chloride absorbs in the UV region. The wavelength of
maximum absorption occurs at 260 nm and this was thus chosen as the
best wavelength for spectral analysis. Feed samples (20 ml) were taken
from the untreated solution containing platinum group metals, prior to an
ultrafiltration run while the permeate samples were taken at the
permeate outlet at the end of each ultrafiltration run.
2.4.1.1 CPC calibration curve
Fig. 2.5 below shows the CPC calibration curve that was used
throughout this study for the conversion of CPC absorbance data to
concentration in units of mM. Calculated amounts of CPC salt were
weighed and dissolved in deionised water to make up standards and
analysed for CPC content by UV-VIS spectrophotometry at 260 nm
against water as blank. The data pertaining to this experiment can be
found in Appendix A, Table A1.
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y = 4.219x
R2 = 0.9992
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.05 0.1 0.15 0.2 0.25
CPC concentration (mM)
a b s
Figure 2.5: CPC calibration curve
2.4.2 Inductively coupled plasma mass spectrometry (ICP-MS)
Due to its sensitivity and low detection limit 8, 9, 26, a Perkin-Elmer Sciex
6100 ICP-MS with a Perkin-Elmer AS 90 autosampler was chosen asthe best instrument to perform analysis of the PGMs investigated in this
research. A “spectrascan” certified multi element standard Au, Ir, Os, Pd,
Pt, Re, Rh and Ru at 100 µg/ml was used to prepare the standards
ranging from 5.0 to 1000 µg/L in 1% (v/v) of hydrochloric acid. Also, an
In 1000 µg/ml standard solution was used to prepare a 100 µg/L In
solution in 1% (v/v) hydrochloric acid. This In standard solution was
mixed online and the In was used as an internal standard to counter the
matrix effect. The instrumental settings of the Perkin Elmer ICP-MS are
listed in Table 2.5 below.
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Table 2.5: ICP-MS settings
Parameter Value (units)
Nebuliser Flow rate 0.71 L/min
RF power 1400 W
Detection limit 10 – 20 µg/L
All the metal ion/surfactant samples were diluted either 100 or 200 times
in order to fall within the detectable range of the ICP-MS for metal
analysis and a second set diluted 40 or 100 times for UV-VIS surfactant
analysis.
2.4.3 Conductometer
In this study, a Metrohm 660 conductometer with conductivity cell (model
no. 6.0908.110) coupled with a platinum thermocouple electrode was
used for conductivity measurements of the surfactant solutions with
various additives.
2.5 Key Performance Aspects of the Ultrafiltration system
2.5.1 Principle and Operation of MEUF system
An untreated solution containing the micelles and the solutes of interest
(PGMs) is forced against a semi-permeable membrane. The feed
solution flows through the hollow fibre ultrafiltration membrane with pore
sizes small enough to block the passage of micelles. The rejection of
micelles brings about the removal of the solutes of interest while the
unbound ions and surfactant monomers are able to pass through the
ultrafiltration membrane to the permeate outlet. The operation of the
MEUF system was performed in batch mode of 20 minute runs. Fig. 2.6
shows a typical batch mode operation.
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Figure 2.6: A typical layout of a batch Ultrafiltration system 32
2.5.2 Assessment and monitoring of the membrane condition
It is vital that the membrane condition is kept the same for all
ultrafiltration experiments, thus the membrane performance is measured
prior and after each experiment by taking pure water flux (PWF)
measurements. The pure water flux measurements before and after
various ultrafiltration runs and after back flushing at 100 kPa are plotted
against time as shown in Fig. 2.7 below. The pure water flux after each
run with either Pt, Pd or CPC alone was significantly reduced indicating
fouling of the membrane surface. However, after back flushing the
membrane for about 15 minutes, the original pure water flux was
restored. This indicates that the fouling is reversible.
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0
20
40
60
80
100
0 20 40 60 80 100 120
Time (minutes)
P e r m e a t e F l u x ( L / M 2 h )
Original PWF After Pd run After Pt run
After backflushing After CPC run
Figure 2.7: Pure water flux measurements at 100 kPa taken after various
ultrafiltration experiments
2.5.3 Regeneration of an ultrafiltration membrane 18, 30
During the ultrafiltration process, a membrane can either be permanently
or temporarily damaged depending on the nature of the species that is
being purified or recovered. This damage of the membrane can be due
to membrane pore clogging which occurs as a result of a cake or gel
layer formation on the membrane surface. However, a temporary gel
layer due to concentration polarization onto the membrane surface is
sometimes desirable as it helps to improve the retention of the targeted
species. Although a temporary gel layer on the membrane surface can
be reversed by back-flushing the membrane, a permanent cake or gel
layer is not desired as it reduces the membrane performance.
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2.5.4 Fouling in membrane processes
Fouling is the result of irreversible deposition or adsorption of solutes
onto the membrane surface or pores. It has been established that an
irreversible fouling may lead to the following:
• Flux decline
• Change in membrane selectivity and / or retention
2.5.4.1 Factors affecting fouling
The following factors can lead to fouling of a membrane:
• Unfavourable hydrodynamics
• Unfavourable membrane-solute interaction
• Membrane surface topography
• Hydrophilicity of the membrane
• Pore morphology and size distribution
• Membrane surface modification
2.5.4.2 Common fouling problems
A number of foulants that have been found to have a major impact in
membrane fouling in various ultrafiltration processes are listed in Table
2.6 below.
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Table 2.6: Membrane foulants and control measures
Process Foulants Control measures
Generic
Improve
hydrodynamics.
Shear back flush. Air
scouring. Controlled
flux operation.
Hydrocarbon surfactants Limit concentration
Proteins
Control ionic
environment, pH, use
porous membranes
Ultrafiltration and
Microfiltration
Biological solids, bacteria,
Flux control
Prefiltration
Use hydrophilic and
more porous
membranes
2.5.5 Membrane cleaning 18
Membrane cleaning is an essential requirement in membrane process
applications. The frequency of cleaning entirely depends on the rate of
fouling and operating protocol used, hence the cleaning may either be
necessary after each ultrafiltration process or after months of continuous
use of the membranes. The condition and performance of an
ultrafiltration membrane are maintained by closely monitoring the pure
water flux measurements. This can be approached in several ways, with
the main aim being to minimize the adsorption of substances onto the
membrane surface, thereby reducing the risk of shortening the lifespan
of an ultrafiltration membrane, the amount of a cleansing reagent
required, as well as the frequency of cleaning. Studies have shown that
there is no specific technique that can prevent certain substances, such
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as proteins, lipids, and phenolic substances from adsorbing onto the
membrane surface.
Pure water flux measurements were taken prior to ultrafiltration
experiments and used as a reference and also after each experimental
run to monitor the condition of the ultrafiltration membrane. Upon
observing that the pure water flux measurements deviated significantly
from what they were prior to ultrafiltration experiments, necessary steps
that involved the following were taken, namely:
• Linear flush that can be used, for any membrane module for loosely
bound species and is a useful step before applying chemical
cleaning processes.
• Backflushing the entire system with pure water with, or without,
applied pressure.
• Using 0.1 mM of sodium hydroxide followed by 1.4 mM of
hypochlorite and flushing with 1 mM of hydrochloric acid.
The procedure used for cleaning was found to be effective as it was able
to restore the pure water flux measurements to their original and normal
values that were used as reference measurements throughout the
research studies.
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CHAPTER THREE
PRELIMINARY INVESTIGATION OF SURFACTANT AND
METAL ION RETENTION
This chapter involves the preliminary investigation of the factors which
influence the retention of cetylpyridinium chloride by a specific
ultrafiltration membrane. The capability of the micellar-enhanced
ultrafiltration (MEUF) system to retain palladium (ll) and platinum (lV)
anionic species with the aid of a cationic surfactant, namely,
cetylpyridinium chloride, from synthetic aqueous solutions was also
investigated as well as the possibility of separating these two metal ions
from a synthetic mixture typical of the industrial waste effluent. The
results are presented here in the form of graphs, with the data tables
pertaining to each graph contained in Appendix A.
3.1 Definitions 18
Some of the key concepts or terms that will be used for discussion in this
research study are defined below:
• A membrane is a thin barrier or film between two phases that allows
preferential transport of some species over others.
• Concentration polarization is the build-up of concentration of the
species to be rejected near the membrane surface (Cm) leading to a
higher concentration of the species near the membrane surface, (Cm)
compared to the species concentration in the bulk of the solution,
(Cb). This can lead to the formation of a temporary cake layer or in
the worst case the formation of a permanent cake layer on the
membrane surface.
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• Flux is the throughput of solution per unit area and is defined as
follows:
Jv = permeability x driving force, and is expressed as
Litres/m2h (LMH)
• Retention (rejection) = (1 – Cp /Cf) x 100%
Cp: permeate concentration
Cf : feed concentration
• Molecular weight cut off (MWCO) is the molecular mass of
solute with 90% retention by the membrane.
3.2 Influence of various parameters
In this section, an investigation of the factors influencing the retention of
the surfactant was done. Among the factors that were investigated were:
pressure variation, temperature variation and the effect of increasing
surfactant concentration.
3.2.1 Effect of pressure variation
This experiment was done in order to determine the pressure at which
the best compromise between retention of the surfactant and permeate
flux could be obtained. A set of cetylpyridinium chloride solutions of
constant concentration (10 mM) was prepared by dissolving a calculated
amount of CPC salt in water in a 250 ml volumetric flask and then
transferring it into a suitable feed container for the ultrafiltration
experiments. Ultrafiltration runs were done at various pressures ranging
from 50 to 250 kPa. The temperature was kept constant at 30°C in order
to prevent any precipitation of the surfactant. Each ultrafiltration
experiment was run for 20 minutes and at the end of each experiment,
20 ml permeate (P) and feed (F) samples were collected for CPC
analysis by UV-VIS spectrophotometry. The results for this investigation
are shown in Fig. 3.1 below with the data in Table A2 in Appendix A.
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0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0
100.0
0 50 100 150 200 250 300 Pressure (kPa)
C P C r e t e n t i o n ( %
)
0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0
P e r m e a t e F l u x ( L / m
2 h )
CPC retention Permeate flux Figure 3.1: Effect of pressure variation at 10 mM CPC and 30°C
From Fig. 3.1 it can be seen that cetylpyridinium chloride retention
decreased slightly initially and then more dramatically beyond 150 kPa
while the permeate flux increased continuously with increasing pressure
up to 200 kPa after which it decreased. The decreasing retention with
increasing pressure is a consequence of greater mass transport at
higher pressures allowing more surfactant to pass through the
membrane. This is also reflected by the increasing flux measurements
with increasing pressure, up to 200 kPa. The sharp decline in flux at
pressures greater than 200 kPa is due to concentration polarization near
the membrane surface. A pressure of 150 kPa was chosen for all further
experiments as this was seen to be the best compromise between
permeate flux and retention.
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3.2.2 Effect of temperature variation
This investigation was done in order to establish a suitable temperature
that would enable complete micellisation of CPC which would in turn
lead to optimum retention of the surfactant. A set of cetylpyridinium
chloride solutions of constant concentration (10 mM) was prepared by
dissolving a suitable amount of CPC salt in deionised water in a 250 ml
volumetric flask and then transferred into a suitable container that was
used for ultrafiltration experiments. Ultrafiltration runs were done at
various temperatures ranging from 10 to 50°C. The pressure was kept
constant at 150 kPa. Each ultrafiltration experiment was run for 20
minutes and at the end of each experiment, 20 ml permeate and
retentate samples were collected for CPC analysis by UV-VIS
spectrophotometry. Results for this investigation are shown in Fig. 3.2
below with the data in Table A3 in Appendix A.
Figure 3.2: Effect of temperature variation at 10 mM CPC and 150 kPa
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
0.0 10.0 20.0 30.0 40.0 50.0 60.0
Temperature (degrees)
C P C
r e t e n t i o n ( % )
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
200.0
P e r m e a t e F l u x ( L / m 2 h )
CPC retention Permeate flux
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Looking at Fig 3.2 above, it can be seen that at temperatures below
20°C, cetylpyridinium chloride retention is very low, despite choosing a
concentration value (10 mM) that is well above the critical micelle
concentration that is required for this surfactant to form micelles. The low
retention in this temperature region (0°C to 20°C) can be attributed to
the fact that ionic surfactants only form micelles when the hydrocarbon
chains are sufficiently fluid at temperature above the chain melting
temperature. This temperature region is thus below the Kraft
temperature, the point below which a surfactant becomes insoluble.
A significant increase in retention occurred at temperatures above 20°C,
with maximum retention, greater than 80%, achieved at around 30°C.
The higher retention observed at this point can be attributed to complete
micellisation assumed to have occurred. However, beyond 30°C a
significant decrease in CPC retention was observed and this might be
due to greater mass transport as a result of higher temperatures that
permits the passage of the monomers and some micelles through the
membrane pores.
However, while the CPC retention showed three different behaviours
with increasing temperature, the flux measurements showed a
continuous decrease with increasing temperature with a gradual
levelling-off at higher temperatures. It can be assumed that the initial flux
decline was due to increased numbers of micelles in solution and
concentration polarization at the membrane surface which correlates
with the increasing retention of surfactant. The latter levelling-off effect
could be a result of partial membrane pore clogging due to the presence
of micelles in the pores.
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3.2.3 Effect of cetylpyridinium chloride concentration variation
A study to determine the surfactant concentration at which sufficient
micellisation would occur was done. A set of cetylpyridinium chloride
solutions of varying concentrations ranging from 0 to 45 mM was
prepared by weighing suitable amounts of CPC salt. The respective salts
were dissolved in deionised water in 250 ml volumetric flasks and then
transferred into a suitable container that was used for ultrafiltration
experiments. Ultrafiltration runs were done at a constant temperature
(30°C) and at a pressure of 150 kPa. A temperature of 30°C was chosen
as this was the temperature at which maximum retention of CPC
occurred. Each ultrafiltration experiment was run for 20 minutes and at
the end of each experiment, 20 ml permeate and retentate samples were
collected for CPC analysis by UV-VIS spectrophotometry. The results for
this investigation are shown in Fig. 3.3 below with the data in Table A4 in
Appendix A.
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0
100.0
0.0 10.0 20.0 30.0 40.0 50.0 CPC concentration (mM)
C P C r e t e n t i o
n ( % )
0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0
P e r m e a t e F l u x ( L / m 2 h )
CPC retention) Permeate flux
Figure 3.3: Effect of CPC concentration variation at 30
C and 150 kPa
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From Fig. 3.3 above, it is quite clear that an increase in cetylpyridinium
chloride concentration led to a continuous gradual increase in
cetylpyridinium chloride retention. This consistent increase started at
above 75% cetylpyridinium chloride retention in the lower concentration
range of cetylpyridinium chloride (0 to 10 mM) with the maximum
retention observed just above 95% at 45 mM cetylpyridinium chloride.
The high cetylpyridinium chloride retention at greater CPC concentration
can be ascribed to two factors, namely, concentration polarization and
sufficient micellisation. As has been stated earlier, concentration
polarization on the membrane surface is sometimes beneficial as it helps
to enhance the retention of species of particular interest. It can also be
said that the higher retention of cetylpyridinium chloride confirms that the
very small number of monomers present in solution is due to fairly
complete micellisation that is achieved with increasing concentration of
cetylpyridinium chloride. While it was possible to achieve the desired
high retention of cetylpyridinium chloride of more than 95% at 45 mM of
cetylpyridinium chloride, it was also observed that, the flux
measurements decreased sharply with increasing surfactant
concentration and levelled at high CPC concentration. This product flux
decline can be attributed initially to the increased number of micelles in
solution as a result of an increased CPC concentration and finally to the
concentration polarization phenomenon close to the membrane surface.
3.3 Micellar-enhanced ultrafiltration of Pt (lV) and Pd (ll) anions
Several experiments were performed in order to establish the effect of
parameters such as the surfactant: metal ion ratio (S : M), the pH of the
solution and the presence of an electrolyte on the retention of both the
metal ions and the surfactant.
3.3.1 MEUF of Pt (lV) and Pd (ll) anions in acidic medium
This experiment was done in order to study the potential of the system
to remove the metal anions of interest, viz, PtCl62- and PdCl4
2- from a
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synthetic aqueous solution. The synthetic solutions were prepared by
dissolving a suitable pre-weighed PGM chloride salt in aqua regia (vol
ratio of HNO3 : HCl = 1: 3) solution and introducing various amounts of
CPC solution as required per ultrafiltration run. For these experiments,
the metal ion concentration was kept constant at 0.1 mM while the CPC
concentration was varied from 0 to 40 mM. The temperature and
pressure were kept constant at 30°C and 150 kPa respectively. Each
ultrafiltration experiment was done in duplicate and the samples were
analysed as stated in section 2.4. The analysis data was averaged and
used to construct the curves as shown in Fig. 3.4 below. The data can
be obtained in Tables A5 and A6 in Appendix A.
0
20
40
60
80
100
120
0 5 10 15 20 25 30 35 40 45
CPC concentration (mM)
M e t a l i o n
r e t e n t i o n
( %
)
Pt retention CPC retention in Pt (lV)
Pd retention CPC retention in Pd (lV)
Figure 3.4: Investigation of Pt and Pd anions retention at 150 kPa and
30°C in acidic medium as a function of CPC concentration
Looking at the curves in Fig. 3.4 above, it is evident that, the MEUF
system is able to retain the platinum group metal ions namely Pt (lV) and
Pd (ll) from a synthetic solution similar to that of an industrial waste
effluent. The retention of the two metal ions was very similar with a slight
decrease in retention with increasing CPC concentration being observed
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in both cases. The retention values for Pt (lV) decreased from 98.9% to
94.4% and for Pd (ll) from 98.8% to 95.5% as CPC values increased
from 1 to 40 mM. However, although the MEUF system was able to
achieve such high retentions for both metal ions, the retention of the
surfactant (CPC) in the presence of each metal ion decreased
significantly with the highest retention obtained at only 60% and the
lowest retention well below 20%. This dramatic decline in cetylpyridinium
chloride retention may be due to the presence of HCl and HNO3 in the
solution. These acids are electrolytes and as stated in section 1.4.1 on p
11 electrolytes may affect the micellisation process and might cause the
micelles to change their shape from a spherical shape to a rod-like
structure with larger surface area but smaller diameter.13 These rods
might be small enough to be able to pass through the membrane pores.
The decline in CPC retention was greater in the Pt (lV) solution than in
the Pd (ll) solution.
A possible explanation for the fact that the metal ion retention was close
to 100% even though the surfactant retention was very low could be that
the larger metal anions are partly responsible for the change in micelle
shape due to their need for a larger surface area. Since the metal ion
concentration is very low compared to the surfactant concentration, and
their tendency to adsorb onto the metal surface is greater than that of
supporting electrolyte, practically all metal ions are adsorbed from the
solution. The hydrodynamic size of a micelle containing PdCl42- or PtCl6
2-
anions is larger than that of a micelle containing only chloride or nitrate
anions, thus the metal-containing micelles are retained by the membrane
while the remainder of the possibly rod-shaped micelles and
unaggregated monomers pass through the pores. Another possible
explanation for the fact that the metal ions are retained so well could be
the interaction of the metal anions with the charged head groups of
surfactant molecules adsorbed at the membrane surface.3,4,21
The significant decline in cetylpyridinium chloride retention in the
presence of the metal ions, led to a further investigation of the influence
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of the different electrolytes on the retention of CPC. This will be dealt
with in Chapter 4.
3.3.2 MEUF of Pt (lV) and Pd (ll) anions in neutral medium
Ultrafiltration of the PGMs in neutral medium was done in order to study
the effect of pH on metal ion retention. The ultrafiltration solutions were
prepared in the same manner as in 3.3.1 above except that the pH of the
solutions was adjusted to a value of 7 +/- 0.02 using a 10% sodium
hydroxide solution prior to starting each ultrafiltration experiment. The
ultrafiltration runs were performed in duplicate and the data obtained
from sample analysis were averaged and used to construct the curves
as shown in Fig. 3.5 below. The data can be found in Tables A7 to A9 in
Appendix A.
88.0
90.0
92.0
94.0
96.0
98.0
100.0
0 20 40 60 80 100 120
CPC concentration (mM)
m
e t a l i o n
r e t e
n t i o n (
%
)
Pt retention Pd retention
Figure 3.5: Investigation of Pt and Pd anions retention at 150 kPa and
30°C in neutral medium as a function of CPC concentration
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0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8
p Cl
% o
f P
d ( I I )
It can be seen in Fig. 3.5 that in neutral medium there is a difference in
the retention behaviour between Pd (ll) and Pt (lV), with Pt (lV) being
retained to a greater extent than Pd (ll). This difference becomes more
significant at higher CPC concentrations. However, both metal ions
experience a decrease in retention with increasing CPC concentration.
This could be related once again to the electrolyte effect, which has a
bigger effect on retention values, in this case due the added Na+ ions.
Since it is well known that Pd (lV) immediately reduces to Pd (ll) in
solution due to the instability of Pd (lV) solutions, species distribution
curves of the Pd (ll) / Cl – system as shown in Fig 3.6 below will be used
to predict and explain the observations that occurred during the
ultrafiltration of a mixture of Pt (lV) and Pd (ll) anions in aqueous
solutions.
Figure 3.6: Species distribution curves of Pd/Chloride system (generated
by IUPAC Stability Constant Database speciation software¡
IUPAC
and Academic Software 2000)
Pd2+
PdCl+
PdCl2
PdCl3-
PdCl42-
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From the species distribution curve, it can be seen that at pCl = 1
(chloride range in which solutions are prepared) Pd (ll) is present as
PdCl42- and PdCl3(H2O)-. In neutral medium, OH activity is greater than
in acidic medium, thus the possibility of forming hydrated mixed
chloro/hydroxo species is greater. Also, ligand exchange of Pd (II) is
105 times faster than that of Pt (IV) since exchange can take place via
an associative mechanism with a 5-coordinate intermediate for square-
planar Pd (II), which is 4-coordinated, whereas Pt (IV) tends to be
octahedral (6-coordinated). Thus it is more likely that hydrated species
are formed for Pd (II) rather than Pt (IV). Hydrated species are more
hydrophilic and interact with water by hydrogen bonding and thus have a
smaller tendency to associate with a micelle. This could account for the
lower retention of Pd (ll) in neutral medium. 8,24,26
3.3.3 MEUF of a mixture of Pt (lV) and Pd (ll) anions in neutral
medium
The experimental solutions were prepared by mixing equal quantities
(0.1 mM each) of the two metal ions and varying the CPC concentration
in the range of 0 to 100 mM. The pH of the ultrafiltration solutions was
adjusted to a pH value of 7± 0.2. Other experimental variables, namely,
the pressure and temperature were kept at 150 kPa and 30°C
respectively. Samples were drawn at the end of each ultrafiltration run
and prepared for the analysis of both metal ions (Pt and Pd) and the
surfactant (CPC) as described in section 2.4. The results are presented
in Fig. 3.7 below. The data can be found in Table A10 in Appendix A.
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0.0
20.0
40.0
60.0
80.0
100.0
120.0
0 20 40 60 80 100 120
[CPC] mM
m e t a l i o n r e t e n t i o
n ( % )
Pd retention Pt retention
Figure 3.7: Investigation of Pt and Pd anions separation in neutral
medium at 150 kPa, 30°C as a function of CPC concentration
From a closer look at Fig. 3.7 above, it is evident that the system can be
used to partially separate the two metal ions in a mixture at high CPC
concentrations. High retention for Pt (lV) that was close to 100%
throughout the studied CPC concentration range was obtained, while Pd(ll) was retained similarly at CPC concentration up to 20 mM with a
significant decrease in its retention after 20 mM CPC concentration to
reach a minimum value of around 80% at CPC concentration of 100 mM.
Also, it can be noted that there was a maximum of about 20% difference
in terms of the retention of the two metal ions, and this difference
indicates a competition for adsorption on the available oppositely
charged sites on the micelle surface with PtCl62- anions being
preferentially adsorbed onto the micelles. This would be due to the
reasons discussed in section 3.3.2.
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3.3.4 Effect of an electrolyte on metal ion and surfactant retention
The experimental solutions were prepared as in section 3.3.1 above
except that the CPC concentration was kept constant at 40 mM and
varying amounts ranging from 0 to 100 mM of an electrolyte namely,
sodium chloride were introduced into the metal ion (0.1 mM) and
surfactant solutions prior to diluting them to the mark. The pH of the
solutions was not adjusted and ranged between 4.6 and 5.3. Duplicate
ultrafiltration runs were carried out as usual. Samples were taken and
prepared for metal ion and CPC analysis. Sample analysis data was
compiled accordingly and the averaged values were used to construct
the curves shown in Fig. 3.8 below. The corresponding data tables are
Table A11 (1) and Table A11 (4) in Appendix A.
0
20
40
60
80
100
120
0.00 20.00 40.00 60.00 80.00 100.00 120.00
[NaCl] (mM)
m e t a l i o n / C P
C
r e t e n t i o n ( % )
Pd retention Pt retention CPC with Pd CPC with Pt
Figure 3.8: Effects of sodium chloride on retention of Pt and Pd anions
in acidic medium at 150 kPa and 30°C using 40 mM CPC
It is well known that the industrial waste streams often contain some
electrolytes that are used during chemical processing. Also, it is well
known that the presence of electrolytes can influence the properties of a
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surfactant13 and may affect the removal of the metal ions. It has been
reported in literature,3,4,21 that the introduction of an electrolyte reduces
the critical micelle concentration at which micelles form thereby
increasing the number of micelles in solution and thus the micelle
retention. Increased micelle retention was not observed here as can be
seen in the curves in Fig. 3.8 above. The retention of the two metal ions
remained fairly constant at above 90% throughout the sodium chloride
concentration range. However, the retention of the surfactant was greatly
reduced in the presence of sodium chloride especially during the
ultrafiltration of platinum as it significantly decreased from just above
40% to just below 10%. This dramatic decrease in cetylpyridinium
chloride retention cannot be attributed to the disruption of micellisation
since the metal ion retention was still high in the presence of an
electrolyte but might be due to a change of shape of the micelles from
spherical to rod-like as it has been noted in section 3.3.1. Although
during the ultrafiltration of palladium, the cetylpyridinium chloride
retention was still higher at lower sodium chloride concentration range (0
to 30 mM) than in the ultrafiltration of platinum, it showed a consistent
decrease after 40 mM sodium chloride concentration from just above
80% to close to 40%. It is clear that the Pt (lV) anion has a much greater
effect on the retention of the CPC micelles than the Pd (ll) anions. This is
possibly due to the double charge of the PtCl62- compared with
PdCl3(H2O)-; the PtCl62- having a greater screening effect than
PdCl3(H2O)- and thus might more readily promote a change of shape of
the micelle.
3.4 Membrane interaction
Membrane interaction with a variety of species was also studied by care-
fully monitoring the variation of the permeate flux measurements during
all the ultrafiltration runs. Flux data was compiled from duplicate
measurements and the averaged values were used to construct the
curves shown in Fig. 3.9 below. The data is found in Table A12.
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0 20 40 60 80
100 120 140
0.0 20.0 40.0 60.0 80.0 100.0 120.0 [CPC] mM
P e r m e a t e F l u x ( L
/ M 2 h r )
CPC alone CPC + 0.1 mM Pd CPC + 0.1 mM Pt CPC + 0.1 mM Pd/Pt mixture
Figure 3.9: Effect of various species on flux at 150 kPa and 30°C
Flux decline followed the usual trend with the decrease in permeate flux
observed with increasing surfactant concentration (from 0 to 100 mM).
The initial steep flux decline can be attributed to an increased number of
micelles in solution with increasing CPC concentration and the
subsequent flatter portion of the curves is due to concentration
polarization near the membrane surface. Based on these findings, it can
be seen that working in a lower CPC concentration range up to 20 mM is
more advantageous in order to achieve higher fluxes. Also, after each
experimental run the membranes can be backwashed and/or cleaned
with an alkali mixture in order to restore the membrane condition.
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3.5 Summary of the preliminary investigations
• CPC retention was linearly dependent on pressure up to 150 kPa
after which the retention became less dependent on pressure
increases.
• It was found that it is important to maintain a temperature of 30 oC in
order to enhance the micellisation process.
• CPC retention increased with increasing initial surfactant
concentration and it was found that retention close to 100 % can be
achieved above 40 mM surfactant concentration.
• The system was able to achieve a high retention of both Pd (ll) and
Pt (lV) metal anions in acid medium.
• Pt (lV) retention in neutral medium was slightly greater than that of
Pd (ll) and this was attributed to its greater adsorption onto the
micelle surface than Pd (ll) due to Pd (ll) species being partially
hydrated.
• Introduction of an electrolyte, namely, sodium chloride failed to
improve the retention of both the metal ion and the surfactant, but in
fact dramatically worsened the retention of CPC.
• The membrane responded differently in various mediums. This wasshown by the flux variations of the various species.
• Although the system showed a great potential to retain both Pd (ll)
and Pt (lV) anions from an aqueous solution, it failed to achieve
optimum retention of the surfactant. This led to the investigation of
conditions which affect the retention of CPC and the establishment of
improved experimental conditions that would achieve higher
retention of both the metal ion and surfactant. This investigation is
described in the following chapter.
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CHAPTER FOUR
INVESTIGATION OF CETYLPYRIDINIUM CHLORIDE
RETENTION
4.1 Overview
This chapter focuses on an investigation of the effects of different
electrolytes on cetylpyridinium chloride retention. This investigation was
necessitated by the realization that cetylpyridinium chloride retention by
the polysulphone membranes used during the ultrafiltration experiments
was negatively affected by the presence of electrolytes and metal ions
during preliminary investigations of both the palladium (ll) and platinum
(lV) synthetic solutions. Though, it had already been observed that
cetylpyridinium chloride was able to adsorb and effectively increase the
hydrodynamic size of the Pt (lV) and Pd (ll) ions in aqueous solution as
required during the micellisation process, it was also critical to keep the
amount of the surfactant going to the waste stream at a minimal level in
order to avoid further pollution of the waste stream by the surfactant. A
series of experiments was conducted where the effect of electrolytes
such as nitric acid, hydrochloric acid and sodium chloride were carefully
studied in order to establish the optimum conditions required in order to
achieve maximum retention of the surfactant.
4.2 Investigation of the effects of an electrolyte
In this section, an investigation of the cause of the deviating retention of
the cetylpyridinium chloride in the presence of electrolytes such as
hydrochloric acid, nitric acid and sodium chloride in synthetic solutions of
the surfactant will be done. The content of cetylpyridinium chloride
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solutions was kept constant at 10 mM in all the ultrafiltration experiments
while increasing amounts of different electrolytes were added to the
surfactant solutions. The choice of the concentration was based on the
findings that were obtained in section 3.2.3. The results will be shown
here in graphical form while the data can be found in tables in Appendix
B.
4.2.1 Effect of hydrochloric acid concentration variation
The influence of hydrochloric acid addition on the CPC retention was
investigated. This investigation was necessitated by the realisation in
preliminary experiments in Chapter Three that CPC retention deviated
significantly in the presence of metal ion solutions containing
electrolytes. A set of cetylpyridinium chloride solutions of constant
concentration (10 mM) was prepared by dissolving a suitable amount of
CPC salt in deionised water in a 250 ml volumetric flask, with varying
amounts of hydrochloric acid (ranging from 0 to 20 mM) added.
Ultrafiltration runs were done at constant temperature (30oC) and
pressure (150 kPa). Each ultrafiltration experiment was run for 20
minutes and at the end of each experiment, 20 ml permeate and
retentate samples were taken for CPC analysis by UV-VIS
spectrophotometry. The results for this investigation are shown in Fig.
4.1 below with the data in Table B1 in Appendix B.
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0.0
20.0
40.0
60.0
80.0
100.0
0.0 5.0 10.0 15.0 20.0 25.0 Hydrochloric acid concentration (mM)
C P C r e t e n t i o n ( % )
0.0
20.0
40.0
60.0
80.0
100.0
P e r m e a t e F l u x ( L / m 2 h )
CPC retention Permeate flux Figure 4.1: Variation of hydrochloric acid concentration at 30°C, 150
kPa and 10 mM CPC
From the curves in Fig. 4.1 above, it can be seen that cetylpyridinium
chloride retention decreased dramatically from just above 80% to less
than 20% with increasing electrolyte (HCl) concentration ranging from 0
mM to 20 mM. The flux measurements showed a slight increase from
approximately 60 L/m2h to just above 80 L/m2h at 0 mM to 5 mM acidic
medium and remained fairly constant with increasing hydrochloric acid
concentration.
In order to determine what changes in solution properties occur upon
addition of hydrochloric acid, surface tension and viscosity
measurements were taken of solutions prepared in the same way as
those used in the ultrafiltration runs and covering the acid concentration
range used. Surface tension measurements were made manually using
a tensiometer with torsion ring while viscosity measurements were made
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with an Ostwald viscometer. This data is plotted as a function of acid
concentration in Fig. 4.2.
0.04
0.041
0.042
0.043
0.044
0.045
0 5 10 15 20 25[HCl] mM
s u r f a c e t e n s i o n ( N / m )
6.9
6.95
7
7.05
7.1
7.15
7.2
v i s c o s i t y ( N / m )
surface tension
viscosity
Figure 4.2: Surface tension and viscosity of a 10 mM CPC solution as a
function of HCl concentration
From the curves in Fig. 4.2 above, it can be seen that both
cetylpyridinium chloride surface tension and viscosity decreased
dramatically with the addition of hydrochloric acid and then reached a
minimum value after 5 mM HCl was added. This can be explained interms of surface effects, in the case of surface tension data, and effects
in the bulk of the solution, in the case of viscosity measurements.
At 10 mM CPC the solution surface is already saturated with adsorbed
monomers since the concentration of CPC exceeds the cmc value.
Addition of chloride ions causes a screening effect of these ions on the
positively charged head groups of the monomers. This allows the head
groups which normally repel each other, to come closer together thus
creating additional space at the surface for the adsorption of more
monomers from the solution thereby decreasing the surface tension
further, until a new saturation point is reached (in this case, around 5
mM HCl). The screening effect of the additional chloride ions on the
micelles in the bulk of the solution is similar, allowing the head groups on
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the micelle surface to come closer together thereby decreasing the ratio
of the head group area to tail volume. If this effect occurs to a very large
extent, that is, the ratio becomes lower than a certain critical value, the
micelle can change its shape from a spherical to a more rod-like
structure. This change is reflected in the abrupt viscosity change at
around 5 mM HCl.
The abrupt changes in both the surface tension and viscosity at around 5
mM HCl could thus be indicating a saturation point and a change in
micelle structure and could be the reason for the decreased retention of
the surfactant solution. This abrupt change also corresponds with the
point at which permeate flux reaches a maximum constant value (see
Fig. 4.1).
4.2.2 Effect of nitric acid concentration variation
The influence of nitric acid addition on CPC retention was investigated. A
set of cetylpyridinium chloride solutions of constant concentration (10
mM) was prepared by dissolving a suitable amount of CPC salt in
deionised water in a 250 ml volumetric flask, with varying amounts of
nitric acid (ranging from 0 to 20 mM) added. Ultrafiltration runs were
done at constant temperature (30oC) and pressure (150 kPa). Each
ultrafiltration experiment was run for 20 minutes and at the end of each
experiment, 20 ml permeate and retentate samples were taken for CPC
analysis by UV-VIS spectrophotometry. The results are shown in Fig. 4.3
with the data in Table B2 in Appendix B.
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0.0
20.0
40.0
60.0
80.0
100.0
120.0
0.0 5.0 10.0 15.0 20.0 25.0
Nitric acid concentration (mM)
C P C
r e t e n t i o n
( %
)
0.0
20.0
40.0
60.0
80.0
100.0
120.0
P e r m
e a t e
F l u x
( L / M
2 h )
CPC retention Permeate flux
Figure 4.3: Variation of nitric acid concentration at 30°C, 150 kPa and 10
mM CPC
A closer look at the curves in Fig. 4.3 show that hydrochloric acid and
nitric acid behaved differently with respect to retention of cetylpyridinium
chloride. With nitric acid variation, it can be seen that the retention of the
surfactant eventually increased after an initial decrease, from nearly 80%
to above 90% with increasing acid concentration, whereas flux
measurements showed a dramatic decline from approximately 100
L/m2h to just above 40 L/m2h. It is evident that nitric acid concentration
did not affect the micellisation process as much as hydrochloric acid,
since the system was still able to achieve a high enough retention of the
surfactant.
Surface tension measurements (see Fig. 4.4) show that compared with
HCl, surface tension decreased continually with increasing HNO3
concentration and there was no abrupt change in the measurements
over the concentration range measured. This could possibly be
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explained as follows: although nitrate and chloride ions are both
monovalent the nitrate ion has more electron lone pairs than the chloride
ion thus providing a larger screening effect of the surfactant head groups
which means that the surface saturation point of the additional surfactant
molecules being adsorbed is reached at higher nitrate concentration.
0.034
0.035
0.036
0.037
0.038
0.039
0.04
0.041
0.042
0.043
0.044
0.045
0 5 10 15 20 25[HNO3] mM
s u r f a c e
t e n s i o n ( N / m )
HNO3
HCl
Figure 4.4: Surface tension of a 10 mM CPC solution as a function of
HNO3 concentration
4.2.3 Effect of sodium chloride concentration variation
The influence of sodium chloride addition on CPC retention was
investigated. A set of cetylpyridinium chloride solutions of constant
concentration (10 mM) was prepared by dissolving a suitable amount of
CPC salt in deionised water in a 250 ml volumetric flask, with varying
amounts of sodium chloride (ranging from 0 to 20 mM) added.
Ultrafiltration runs were done at constant temperature (30oC) and
pressure (150 kPa). Each ultrafiltration experiment was run for 20
minutes and at the end of each experiment, 20 ml permeate and
retentate samples were taken for CPC analysis by UV-VIS
spectrophotometry. The results are shown in Fig. 4.5 below.
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0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
0.0 5.0 10.0 15.0 20.0 25.0
Sodium chloride concentration (mM)
C P C
r e t e n t i o n
( %
)
0.0
20.0
40.0
60.0
80.0
100.0
120.0
P e r m
e a t e
F l u x
( L / M
2 h )
CPC retention Permeate flux
Figure 4.5: Study of variation of sodium chloride acid concentration at
30°C, 150 kPa and 10 mM CPC
It is evident from Fig. 4.5 above that introduction of an electrolyte like
sodium chloride failed to improve the retention of a surfactant, but in fact
decreased it dramatically; a similar effect that was observed with
hydrochloric acid addition. These results do not agree with the work of
other researchers 3 who claim that introducing sodium chloride to
surfactant solutions reduces the critical micelle concentration leading to
increased micellisation and increased retention. This is not the case
here, and the results can be explained in the same way as for
hydrochloric acid addition (section 4.2.1).
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4.3 Summary of CPC retention investigations
• Introduction of chloride ions reduces the surfactant retention
significantly and the effect has been shown by viscosity and surface
tension measurements to be probably due to a change in micelle
structure.
• Introduction of nitrate ions did not have a significant negative impact
on the surfactant retention. This might be explained in terms of a
second change in micelle structure .
• It is concluded that the use of HCl to dissolve the metal salts should
be kept to a minimum, thus the volume ratio of HNO3:HCl should be
increased in the acid mixture used to dissolve Pt and Pd salts.
• In order to shed further light on the reduced surfactant retention in
the presence of electrolytes, the following chapter investigates the
determination of the cmc value of cetylpyridinium chloride under
various experimental conditions.
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CHAPTER FIVE
THE CONCEPT OF MICELLISATION AND CONDUCTIVITY
INVESTIGATIONS
In this chapter the focus will be on studying the extent to which
micellisation occurred during the ultrafiltration experiments in the
previous chapters. Conductivity studies will be used to confirm the
theoretical cmc value of cetylpyridinium chloride and to determine the
degree of ionisation of CPC in different media. The effects of various
electrolytes namely, aqua regia, hydrochloric acid and nitric acid on the
conductivity measurements of CPC solutions will also be investigated.
5.1 Micellisation
It is important that certain conditions namely, critical micelle
concentration (cmc), Kraft temperature point and the solubility of the
surfactant are known in order to maximise the micellisation effect. The
critical micelle concentration of a surfactant can be experimentally
determined by employing a number of methods including, conductivity,
surface tension measurements and capillary electrophoresis (CE). 13, 14,
29 These techniques can help to determine the extent or the degree to
which micellisation has occurred. It has been reported that the addition
of an electrolyte such as sodium chloride to an aqueous solution
containing the surfactant facilitates the reduction of the critical micelle
concentration that in turn increases the micellisation process. 4
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5.2 Conductivity study
In this study, a Metrohm 660 conductometer with conductivity cell (model
no. 6.0908.110) coupled with a platinum thermocouple electrode was
used to confirm and establish the influence of the following factors as
listed below:
• Determination and confirmation of the cmc value of CPC.
• Addition of varying amounts of different electrolyte(s) and
synthetic mixtures containing either Na2PtCl6 or K2PdCl6 to
cetylpyridinium chloride solutions to observe the effect of these
on conductivity measurements.
5.2.1 Experimental determination of the cmc of CPC
This experiment was done in order to confirm the theoretical cmc value
(0.9 mM) of cetylpyridinium chloride obtained from the literature. 3, 13 Of
the various methods that can be employed namely, conductivity, surface
tension and capillary electrophoresis, conductivity was chosen for this
study due to the accessibility and simplicity of the equipment.
5.2.2 Experimental procedure
• The conductometer was calibrated with 0.1 M KCl solution.
• After calibration, the cell constant of the conductivity cell at a
specific temperature was calculated and other parameters and
applicable settings of the conductometer were set prior to taking
the conductivity measurements.
Cetylpyridium chloride solutions of varying concentrations (0.2 mM to 2
mM) were prepared by dissolving a calculated amount of CPC salt in
deionized water and diluting this stock solution to the required
concentration.
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The conductivity measurements of the prepared CPC solutions were
recorded as shown in Table C1, in Appendix C, and a plot showing the
relationship between conductivity and CPC concentration is shown in
Fig. 5.1 below:
R2
= 0.9956
R2
= 0.9966
0
20
40
60
80
100
120
140
160
180
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2
[CPC] mM
c o n d u c t i v i t y ( u S / c m
Slope = 94
Slope = 43.36
Figure 5.1: Determination of the cmc of CPC and its degree of ionisation
The formation of the micelles leads to an abrupt change in conductivity
of solution due to removal of ions from solution since the counter ions
are adsorbed onto the micelle surface; thus the point at which the slope
of the conductivity curve changes can be regarded as the onset of
micellisation. Therefore the point of intersection of the two straight line
portions of the plot in Fig. 5.1 is taken as the cmc value.
Looking at the curve in Fig. 5.1 above, it can be seen that the obtained
cmc value is in agreement with the reported literature value (0.9 mM), 3,
13 thus the chosen technique and the methodology followed were
correct. The slopes of each plot in Fig. 5.1 above were also used to
determine the degree of ionisation of the micelles as explained in
section 5.2.3 below.
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5.2.3 Determination of the degree of ionisation
The determination of the degree of ionisation of CPC micelles was done
in order to investigate the available adsorption sites on the micelle
surface. The degree of ionisation of a micelle was determined by
measuring the change in slope of the solution electrical conductivity (¢
)
vs total concentration (C) as the solution goes through the cmc 13, 28, 29,
assuming the following conditions:
• ¢
is only due to free chloride ions in solution
• ¢
due to CP+ and micelles is negligible, thus¢
£
CCl
• When C < cmc:¢
= ACCl , since CPC monomer is fully ionised,
where A = constant
• When C > cmc, CCl = cmc +¤
(C – cmc)
where¤
is the degree of ionisation of a micelle.
• Thus¢
= A (cmc +¤
(C – cmc))
• Gradients above and below the cmc can be calculated as
follows:
dk /dC ( C > cmc ) =¤
A and dk /dC ( C < cmc ) = A
• Thus the ratio of these slopes gives¤
, which is the degree of
ionisation.
A closer look at Fig. 5.1 above also shows that the ratio of the two
slopes, viz, 43.36 and 94.00, respectively, gives a 46% degree of
ionisation. This means that 54% of the chloride counter ions are
associated with the micelle surface or adsorbed onto the micelle
surface.
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5.3 Influence of electrolytes on the cmc value and degree of
ionisation
Conductivity data is not useful when large amounts of acid are present in
the surfactant solution because then the conductivity due to the acid is
too high and small changes due to CPC are not observed, thus only
small amounts of acid, to give a pH of around 3, were added to the CPC
solutions. The results pertaining to this investigation are presented in
graphical form below.
5.3.1 Determination of cmc in the presence of HCl
Surfactant solutions of various concentrations were prepared by
dissolving calculated amounts of CPC salt in deionised water. HCl was
added in the same volumetric flask (100 ml) prior to filling up to the mark
with deionised water, to give a final concentration of 0.5 mM. The
concentrations of the CPC solutions ranged from 0 mM to 2.2 mM.
Conductivity measurements were made using a pre-calibrated (see
section 5.2.1) Metrohm 660 conductivity meter. The conductivity meter
measurements were plotted against CPC concentration in Fig. 5.2 and
the data is given in Table C2 in Appendix C. The ratio of the two slopes
was used to predict the degree of ionisation and the cmc value was also
determined.
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0.1
0.2
0.3
0.4
0.5
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2
[CPC] mM
c o n d u c t i v i t y ( m S / c m
Slope = 0.2286
Slope = 0.098
Slope = 0.0369
Figure 5.2: Plot of the conductivity vs CPC concentration in the presence
of 0.5 mM HCl
A closer look at Figure 5.2 above shows that when a fixed amount of HCl
is added to each CPC solution, conductivity measurements show three
abrupt changes with the first cmc value occurring at around 0.25 mM
CPC. This is in agreement with the accepted theory that electrolytes
lower the cmc value of surfactants by reducing the head group area due
to electrostatic screening of the head group charges by the additional
counter ions. As a result of this, micellisation occurs sooner than it
normally occurs at lower surfactant concentrations. A second cmc value
occurs at around 1.2 mM CPC. This could indicate the point at which the
spherical micelles transform into rod-shaped micelles with increasing
surface area and corresponding greater degree of counter anion
adsorption.
The degree of ionisation (obtained from the ratio of the two slopes in Fig.
5.1) is around 46% for CPC alone, but is reduced to 43% when HCl is
added. The excess Cl ions in solution are now attracted to the charged
micelles causing them to be removed from solution, thus leading to the
abrupt change in conductivity, and the lower ionisation value. At the
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second cmc value, the degree of ionisation is further reduced to 37.6%
which shows that even more ions are associated with the micelle
structure which now has a larger surface area.
5.3.2 Determination of cmc in the presence of Pd (ll) anions and
aqua regia
CPC solutions were prepared in the same manner as described in 5.3.1
except that aliquots of the 1 mM Pd (ll) stock solution used for the
preparation of the MEUF solutions were added to each solution, to give
0.01 mM concentration of Pd (ll), prior to making up to the mark with
deionised water in the volumetric flasks (100 ml). The plot of solution
conductivity against CPC concentration is shown in Fig. 5.3. The data is
in Table C3 in Appendix C.
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 1 2 3 4 5 6 7 8 9
[CPC] mM
c o n d u c t i v i t y ( m S / c m )
Slope = 0.0164
Slope = 0.5424
Slope = 0.1209
Figure 5.3: Plot of the Conductivity vs CPC concentration in the
presence of 0.01 mM Pd (ll) anions in aqua regia
In the presence of Pd with aqua regia, the conductivity of increasing
amounts of CPC showed interesting results. The first cmc value is not
shown in Fig. 5.3 since it is out of the concentration range studied. (In
acid medium we have seen that it occurs at around 0.2 mM CPC.) It can
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be seen that there is a flattening off of the conductivity between 0.2 and
2 mM CPC indicating the adsorption of counter-ions onto the micelle
surfaces. This is followed by an abrupt change in conductivity indicating
a cmc value of around 2 mM. This could be the second cmc, the point at
which the micelles could be changing shape. Between 2 and 3 mM CPC
there is a sharp increase in conductivity. This could indicate a transition
state where there is a change of structure. At around 3 mM there is
another cmc value. The degree of ionisation i.e. the ratio of the two
slopes around the cmc value of 3 mM is 22%. This value is considerably
less than that of the micelles in the pure surfactant solution (46%)
indicating that the palladium ions are adsorbed onto the micelle surface.
5.3.3 CMC determination in the presence of Pt (lV) anions and
aqua regia
CPC solutions were prepared in the same manner as described in 5.3.1
except that aliquots of the 1 mM Pt (lV) stock solution used for the
preparation of the MEUF solutions were added to each solution, to give
0.01 mM concentration of Pt (lV), prior to making up to the mark with
deionised water in the volumetric flasks (100 ml). The plot of solution
conductivity against CPC concentration is shown in Fig. 5.4. The data is
in Table C4 in Appendix C.
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0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
0 1 2 3 4 5 6 7 8 9
[CPC] mM
c o n d u c t i v i t y ( m S
/ c m )
Slope = 0.0376
Slope = 0.34
Slope = 0.0535
Figure 5.4: Plot of the conductivity vs CPC concentration in the presence
of 0.01 mM Pt (lV) anions in aqua regia.
Looking at Fig 5.4 above, it can be seen that as was the case with Pd,
two cmc values are observed. The first cmc value is close to 2 mM while
the second cmc value is around 3 mM. The ratio of the two slopes
around 3 mM CPC (0.0535/0.34) gives 15.7%. This value is less than
that obtained for Pd (ll) (22%), indicating that there are fewer ions in
solution i.e. more counter ions are associated with the micelle surface as
can be observed from the flatter slope. This can mean that PtCl62- ions
promote the transformation of the micelles to rod-shapes to a greater
extent than PdCl42- ions with perhaps greater overall surface area
leading to more elongated or longer rods.
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5.4 Summary of the conductivity investigations
• Findings on this research show that the presence of electrolytes
lowers the initial cmc value of the surfactant and creates a second
cmc value which possibly indicates a change in micelle shape or
structure.
• Also, it was found that the presence of the metal ions brings about
a further change in conductivity through a third cmc value which
could indicate a further possible change of micelle structure.
• The ratios of the slopes around the cmc values give an indication of
the degree of ionisation of the micelles and thus the amount of ions
associated with the micelle surface.
• The values of the slopes show that the Pt (IV) ions are absorbed to
a greater extent than the Pd (II) ions onto the micelle surface.
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CHAPTER SIX
IMPROVED CONDITIONS FOR THE RETENTION OF
METAL ION AND SURFACTANT
In this chapter, a complete study of the optimum retention of both metal
ions (Pt (lV) and Pd (ll)) and surfactant (CPC) will be investigated. Also
the possibility of separating Pt (lV) from Pd (ll) will be explored. These
investigations will be based on the findings obtained in the previous
chapters that focused on establishing the reasons for the reduced
retention of surfactant. The results for this chapter are illustrated in the
form of graphs with the data contained in tables in Appendix D.
6.1 Micellar-enhanced ultrafiltration of individual metal ions
6.1.1 Ultrafiltration of platinum (lV) anions
Ultrafiltration of platinum metal ions alone as a function of metal ion
concentration was done in order to determine the capability of the MEUF
system to retain different amounts of metal from the synthetic solution.
The synthetic solutions used for these experiments were prepared as
follows:
• A platinum stock solution was prepared by weighing a suitable
amount of the metal ion salt as mentioned previously.
• The weighed salt was dissolved with a mixture of 2.5 mM HCl and 5
mM HNO3 and deionized water in a 500 ml volumetric flask.
• 2.5 mM HCl was the minimum amount of HCl that was necessary to
dissolve PdCl62- salt and is considerably less than the amount in
aqua regia.
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• Varying amounts of the metal solution were taken from the prepared
stock solution to prepare the solutions for the ultrafiltration
experiments and the appropriate amount of CPC was added as
required by each ultrafiltration experiment.
Fig. 6.1 shows the retention of both Pt (lV) and CPC as well as the
permeate flux as a function of metal ion concentration for experiments
performed at 150 kPa, 30°C and using 40 mM CPC. The data is found in
Tables D1 and D2 in Appendix D.
92.0
93.0
94.0
95.0
96.0
97.0
98.0
99.0
100.0
0.00 0.10 0.20 0.30 0.40 0.50
Pt concentration (mM)
R e t e n t i o n
%
0.0
20.0
40.0
60.0
80.0
100.0
P e
r m
e a t e
F l u x
L / m
2 h r
Pt retention CPC retention Flux variation
Figure 6.1: MEUF of Pt (lV) anions at 40 mM CPC, 30°C, and 150 kPa in
acidic medium
From the curves in Fig. 6.1 above, it can be seen that the retention of Pt
(lV) was between 98% and 100% with a slight increase observed as
initial Pt (lV) concentration in the solution increased. The CPC retention
was 99% at low metal concentration and decreased to around 93% at
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higher metal concentration. The higher retention can be attributed to
sufficiently large micelles which form at 40 mM CPC and a low enough
electrolyte concentration to prevent the electrolyte effect that was
discussed in the previous chapter. However, the slight decrease in CPC
retention observed with increasing Pt (lV) concentration can be attributed
once again to a slight electrolyte effect, since the higher metal
concentration solutions also contained proportionately more electrolytes.
The flux measurements decreased slightly from just above 45 L/m2h and
remained fairly constant with increasing metal ion concentration (0 to 0.4
mM) at constant surfactant concentration (40 mM). This slight decline in
flux measurements can be attributed to concentration polarization on
the membrane surface that occurs increasingly with increasing metal ion
concentration.
6.1.2 Ultrafiltration of Palladium (ll) anions
Ultrafiltration of palladium metal ion alone was done in order to
determine the capability of the MEUF system to retain the metal from the
synthetic solution with increasing palladium concentration. Palladium
synthetic solutions used in these ultrafiltration experiments were
prepared exactly in the same manner as described in section 6.1.1
above.
Fig. 6.2 shows the retention of both Pd (ll) and CPC, as well as the
permeate flux as a function of metal ion concentration for experiments
performed at 40 mM CPC, 150 kPa and 30°C. The data is found in
Tables D3 and D4 in Appendix D.
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90.0
92.0
94.0
96.0
98.0
100.0
102.0
0.00 0.10 0.20 0.30 0.40 0.50
Pd concentration (mM)
m
e t a l i o n r
e t e n t i o n
( %
)
0.0
20.0
40.0
60.0
80.0
100.0
P e r m
e a t e
F l u x ( L
/ M
2 h )
Pd retention CPC retention Permeate flux
Figure 6.2: MEUF of Pd (ll) anions at 40 mM CPC, 30 °C, and 150 kPa in
acidic medium
Looking at the curves of the palladium-cetylpyridinium chloride system in
Fig. 6.2, it is evident that Pd (II) was retained at 100% irrespective of the
initial Pd (II) concentration. However, it was also observed that the
retention of cetylpyridinium chloride gradually decreased from 98 to 90%
with increasing metal ion concentration. Although the overall surfactant
retention was high, the slight decrease would be due to increasing
electrolyte concentration as metal ion concentration increases. Also, it
was found that flux measurements decreased slightly initially and later
remained fairly constant with increasing palladium concentration from 0
to 0.4 mM, as was found with Pt (lV) ultrafiltration experiments.
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6.2 Micellar-enhanced ultrafiltration Pt (lV)-Pd (ll) mixture
The possibility of separating the two metal ions in an acidic synthetic
mixture containing varying ratios (1:1 to 1:6) of the two metal ions, was
investigated. The total metal concentration of the synthetic mixture was
kept constant at 1 mM throughout, while the ratios of the metal ions were
varied. Table 6.1 shows the exact metal ion ratios that were used in
these ultrafiltration experiments.
Table 6.1: Summary of varying metal ion concentration ratios
Metal ion
ratio
Exact *MA
concentration
(mM)
Exact *MB
concentration
(mM)
1:1 0.05 0.05
1:2 0.0333 0.0666
1:3 0.025 0.075
1:4 0.02 0.08
1:6 0.015 0.085
*Can be any of the two metal ions, that is Pt or Pd
Fig. 6.3 shows the micellar-enhanced ultrafiltration of Pt (lV)/Pd (ll)
mixture in acidic medium when Pd (ll) concentration ratio is kept
constant. The data is found in Table D7 in Appendix D.
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92.0
93.0
94.0
95.0
96.0
97.0
98.0
99.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
metal ion ratio (1 : x)
m e t a l i o n r e t e n t i o
n ( % )
Pt re te ntion Pd re te ntion
(1 = constant Pd concentration ratio, x = changing Pt concentration ratio)
Figure 6.3: MEUF of a mixture of Pd (ll) - Pt (lV) anions at 40 mM CPC,
30°C, and 150 kPa in acidic medium
Pt (lV) anion retention increases sharply initially until it reaches 98% at
Pd:Pt ratio of 1:2 and later decreases to below 96% at Pd:Pt ratio of 1:4
ratios and increases again to 98% at Pd:Pt ratio of 1:6. Pd (ll) retention
follows the same trend as well but at an overall lower retention. This
difference in metal ion retention shows that there is competition between
the two metal ions for available sites on the surfactant surface with the
platinum ions being preferentially adsorbed onto the micelles. In each of
these solutions except the 1 : 1 ratio, the concentration of platinum ions
is greater than that of palladium ions, hence the separation factor
increases with increased platinum concentration.
Fig. 6.4 shows the micellar-enhanced ultrafiltration of Pt (lV)/Pd (ll)
mixture in acidic medium when Pt (lV) concentration ratio is kept
constant and the amount of palladium relative to platinum is increasing.
The data is found in Table D8 in Appendix D.
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95.0
96.0
97.0
98.0
99.0
100.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
metal ion ratio (1 : x)
M
e t a l i o n
r e t e n t i o n
%
Pt retention Pd retention
(1 = constant Pt concentration ratio, x = changing Pd concentration ratio)
Figure 6.4: MEUF of a mixture of Pt (lV)-Pd (ll) anions at 40 mM CPC,
30°C, and 150 kPa in acidic medium
Looking at Fig. 6.4 above, it is evident that palladium retention increases
steadily from just above 96% to nearly 99% with increasing metal ion
concentration ratio (Pt: Pd), that is, with more Pd (ll) present in the
system compared to Pt (lV) , the Pd (ll) is increasingly retained whilst
platinum retention decreases slightly from 98% to 97% in the same
metal ion concentration ratio region (1:1 to 1:3). Also, it was observed
that the retention of the two metal ions increased slightly again beyond1:4 Pt:Pd concentration ratio with platinum achieving more than 99.5%
while palladium retention remained below 98%. The variation in retention
of the two metal ions can be attributed to the differing adsorbing
capabilities that they have towards the available charged adsorbing sites
on the surfactant surface. However, the differences remain very small.
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6.3 Membrane response during Pt/Pd mixture ultrafiltration
Membrane response during the micellar-enhanced ultrafiltration of the
mixture of Pt (lV) and Pd (ll) was carefully monitored by monitoring the
cetylpyridinium chloride retention and permeate flux variation.
6.3.1 CPC retention
The curves in Fig. 6.5 below were constructed from the sample analysis
data that was obtained during the ultrafiltration experiments of the Pt/Pd
synthetic mixtures. The samples were analysed for CPC content by UV-
VIS spectrophotometry. The data pertaining to this investigation can be
obtained in Tables D5 and D6 in Appendix D.
88.0
89.0
90.0
91.0
92.0
93.0
94.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0metal ion ratio
C P C r e t e n t i o n ( % )
CPC retention when Pt ratio is constant
CPC retention when Pd ratio is constant
Figure 6.5: Effects of metal ion ratio on CPC retention in acidic medium
in the presence of various mol ratios of Pd and Pt
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In the presence of fixed Pd (ll) in the metal ion concentration range of 1:1
to 1:4 Pd : Pt, it can be seen that CPC retention decreased sharply from
just above 92% to just above 88%, while in the same region of metal ion
ratio in the presence of constant Pt (lV), CPC retention slightly increased
initially from just above 92% to close to 94% and later decreased and
followed the same trend as in the case where Pd (ll) concentration was
kept constant. For the two metal ions above 1:4 metal ratios, CPC
retention increased sharply from as low as 88% to nearly 92%. However,
though the CPC retention was affected by the changing metal ion ratio,
the system where Pt (lV) ratio was kept constant was the most affected
as it only achieved a maximum of just more than 92% while in the
system where Pd (ll) ratio was kept constant close to 94% retention of
CPC was achieved. The electrolyte content is constant in these
experiments since the total metal ion concentration was held constant.
The only change is the relative amounts of Pd (ll) and Pt (lV). It is then
evident that the metal ions themselves influence the shapes of the
micelles due their large sizes and need for a larger surface area. The
PtCl62- anion has a larger effect on the micelle structure than the PdCl4
2-
as can be seen from the lower retention of CPC when there are more Pt
(lV) ions relative to Pd (ll) ions.
6.3.2 Flux variation
The curves in Fig. 6.6 are constructed from the sample analysis data
that was obtained during the ultrafiltration runs of Pt/Pd synthetic
mixtures. The data is found in Tables D7 and D8 in Appendix D.
A closer look at the curves in Fig. 6.6 shows that for both systems, thus
when either Pt : Pd = 1 : x or Pd : Pt = 1 : x, the flux measurements
continually decreased throughout. However, although the flux variations
in both systems followed the same trend, flux measurements for the Pd :
Pt =1 : x system were still higher than in the system of Pt : Pd = 1 : x.
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0.0
10.0
20.0
30.0
40.0
50.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
metal ion ratio (mM)
P e r m
e a t e
F l u x
v a r i a t i o n
( L / M
2 h )
Permeate flux when Pt concentration ratio is constant
Permeate flux when Pd concentration ratio is constant
Figure 6.6: Flux variation during Pt(lV)/Pd (lV) ultrafiltration
The differences in flux measurements can be attributed to the manner in
which these two metal ions adsorb on the available oppositely charged
sites of the surfactant surface. The slight decrease in flux measurementscan also be due to different ratios of the metal ion and the way that they
influence a micelle shape which influences flux.
6.4 Summary of the investigation of improved conditions
• The conditions for improved micellisation to occur in this
investigation lead to the use of 40 mM of the CPC surfactant, a
pressure of 150 kPa, and a temperature of 30°C.
• It was also found that controlling or minimizing the electrolyte
content, especially chloride content, led to optimum retention of
both the metal ion(s) and the surfactant.
• Quantitative separation of the two metal ions under these
conditions could, however, not be achieved.
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CHAPTER SEVEN
CONCLUSIONS
7.1 Conclusion
Based on the findings of this study, it can be concluded that the MEUF
system using cetylpyridinium chloride (CPC) can be used to recover or
retain Pt (lV) and Pd (ll) anions from industrial waste effluents. It can also
be said that PtCl62-
due to its greater adsorption capabilities onto themicelle surface than PdCl4
2- or PdCl3 (H2O)- was preferentially retained in
neutral medium. This may be exploited as a possible means of
separating the two metal ions.
However, although the system was able to achieve optimum retention of
both metal ions, the CPC retention was very poor during initial
investigations. This was attributed to the electrolyte effect which
influences the micellisation process, possibly changing the shape or
structure of the micelles. These findings necessitated the investigation of
suitable experimental conditions that would permit the optimum retention
of both the metal ion(s) and the surfactant. The study considered the
influence of different electrolytes on the physical properties of CPC
solutions to establish the conditions under which surfactant retention
was a maximum.
The conditions which were found for optimum micellisation to occur in
this investigation lead to the use of 40 mM of the CPC surfactant, 150
kPa, and 30°C. It was also found that controlling or minimizing the
electrolyte content led to optimum retention of both the metal ion(s) and
the surfactant.
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7.2 Recommendations
Continued investigation of this project could be carried out, viz.:
• Investigation of the capability of a different membrane that would
provide different specifications namely MWCO and membrane pore
size.
• Using a different cationic surfactant with different properties that may
not be as sensitive to electrolyte effects.
• Investigating the retention of other platinum group metals.
• Focusing on the separation of Pt (lV) and Pd (ll) by incorporating into
the surfactant micelles a specific separating agent such as an
alkylpyrazole, for example, which is known to be Pd (ll) specific.
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REFERENCES
1. Thomas M.P. (2000). The Utilization of toxic paint waste generated
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2. Bennen W. (2002).The evaluation of waste minimization/waste
treatment strategies for a commercial production process of 4-
methyl-3-thiosemicarbazide. M Tech Thesis, Port Elizabeth
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3. Gzara L. Dhahbi M., (2000). Desalination, 137: 241 - 250
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267
5. www.webelements.com/Pt
6. Cowey A., “Mining and metallurgy in South Africa”. Mintek, 1994
7. Hartley F.R. “Chemistry of Platinum Group Metals”. Elsevier, 1991.
8. Vogts S.E., (2004). The separation of Palladium and Platinum from
hydrochloric acid medium. PhD. Thesis, University of Port
Elizabeth. (pp 1 - 30)
9. Rao C.R.M., Reddi G.S., (2000). Trends in Analytical Chemistry: 19
(9), 565 - 585
10. Louw T., (2004). The development and application of platinum
selective separating agents. MSc. Thesis, University of Port
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11. Viviers C., (1995). Facets of Rhodium and Iridium separation. PhD
Thesis, University of Port Elizabeth. (pp 1 -36)
12. www.google.com/platinum, Platinum-Wikipedia, The free
encyclopedia.
13. Pashley M.R., Karaman E.M., “Applied Colloid and Surface
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14. www.kcpc.usyd.edu.au/discovery
15. www. petrosepmembrane.com/membranes
16. Ullmans Encyclopedia of Industrial Chemistry, A16, pp 187 - 264
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17. Filtration and Ultrafiltration Techniques:
http//membranes.nist.gov.com
18. Jacobs E. P., Fane T. (2003). Shortcourse on “Introduction to
membrane science and technology” presented at P.E. Technikon.
19. Zydney A., (2005). Workshop on Membranes for Liquid
Separations, Including Fouling Issues (MF,UF,NF,RO)” presented
at International Congress on Membranes and Membrane
Processes. Seoul, Korea.
20. Twitchell J., (2000). What is Ultrafiltration?. www.dost.gov.ph,
20051003.
21. Baek K., Lee H.H., Yang J.W., (2003). Desalination, 158:157 - 166
22. Mtyopo B.M., (2004). Optimisation of a manufacturing process for
Atrazine with a focus on waste minimization. M Tech Thesis, Port
Elizabeth Technikon. (pp 35 – 36)
23. Zeelie B., “Laboratory Process Development”. 2003, pp 60 - 66
24. Holler F. J., Skoog D.A., West D.M. “Fundamentals of Analytical
Chemistry” 7th Edition, Saunders College Publishers, New York,
1991.
25. Konkel J. “Analytical Chemistry for Technicians”, Lewis Publisher
Inc. Lincoln Nebraska, 1991.
26. Beamish F.E. “The Analytical Chemistry of Noble Metals”,
Pergamon,Oxford, 1966.
27. Swart P., Maartens A., Engelbrecht J., Jacobs E.P.(1999). “The
Development and Implementation of Biological Cleaning
Techniques for Ultrafiltration and Reverse Osmosis Membranes
fouled by Organic Substances”. WRC Report No 660/1/99.
28. Attwood D., Florence A.T., “Surfactant systems: Their chemistry,
pharmacy and biology”. Chapman and Hall, London, 1983.
29. Everett D.H., “Basic Principles of Colloid Science “. Royal Society
of Chemistry, 1988.
30. Hunter R.J. “Introduction to modern colloid science”. Oxford
University Press Inc., New York, 1993.
31. Zeelie B. (1996). Iodo Complexes. MSc Thesis, University of Port
Elizabeth. (pp 12-14)
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APPENDIX A
Preliminary investigations of metal ion and surfactant retention
Table A1: CPC calibration curve
CPC
(mM)Absorbance
0.00098 0.0048
0.00196 0.01
0.00392 0.0191
0.0049 0.0232
0.0098 0.0452
0.0196 0.0948
0.0392 0.175
0.098 0.424
0.196 0.8182
Table A2: Effect of pressure variation
Table A2 (1): CPC data
CPC concentration (mM)Experiment
number
[CPC]I
(mM)
Pressure
(kPa)Feed Permeate
#1 10 50 9.755 0.549
10 100 9.958 0.854
10 150 9.921 1.287
10 200 9.949 2.763
10 250 9.919 4.543
#2 10 50 9.968 0.521
10 100 10.086 0.823
10 150 9.769 1.194
10 200 9.846 2.99010 250 9.824 4.733
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Table A2 (2): Averaged data: Effect of pressure variation
Flux (L/m2h) % CPC Retention[CPC]
(mM)
Pressure
(kPa) # 1 # 2 Av # 1 # 2 Av
10 50 69.28 69.03 69.16 94.37 94.78 94.58
10 100 99.03 95.17 97.09 91.42 91.84 91.63
10 150 120.82 119.39 120.10 87.03 87.78 87.41
10 200 141.07 138.78 139.92 72.23 69.63 70.93
10 250 107.04 106.02 106.53 54.20 51.82 53.01
Table A3: Effect of temperature variation
CPC concentration (mM)[CPC]
(mM)
Temperature
°°°°C
Flux
(L/m2
h) Feed Permeate
Retention
%
10 10 184.04 9.96 7.81 21.61
10 20 160.98 10.10 7.40 26.79
10 30 107.05 9.91 1.73 82.60
10 40 93.98 10.02 3.50 65.08
10 50 83.12 9.48 3.98 57.97
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Table A4: CPC concentration variation
CPC concentration
(mM)Experiment
number
[CPC]i
(mM)
Flux
(L/m2h)
Feed Permeate
Retention
%
0 103.58 10.178 1.754 82.77
2.5 146.28 2.448 0.618 74.77
5.0 146.28 4.950 1.029 79.22
7.5 146.28 7.565 0.832 89.00
10 136.08 10.163 1.075 89.42
12.5 80.67 13.875 1.413 89.82
15.0 66.79 14.909 1.562 89.52
25 53.83 23.830 1.238 94.80
#1
45 49.90 45.054 1.554 96.55
0 103.58 10.178 1.754 82.77
2.5 147.91 2.662 0.872 67.24
5.0 98.01 5.400 1.255 76.76
7.5 85.82 7.769 1.311 83.12
10 82.35 10.001 1.628 83.72
12.5 80.67 13.875 1.413 89.82
15.0 66.28 15.734 1.282 91.85
25 53.829 23.830 1.238 94.80
#2
45 49.900 45.054 1.554 96.55
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Table A5: Pd ultrafiltration in varying CPC concentration in acidic
medium
Pd concentration (ppm)
Experiment # 1
Pd concentration (ppm)
Experiment # 2[Metal]
mM
[CPC]
mMF* R* P* F* R* P*
0.1 1 7.307 1.015 0.089 7.307 1.015 0.089
0.1 2 7.587 2.302 0.255 7.587 2.302 0.255
0.1 3 6.236 1.469 0.075 6.236 1.469 0.075
0.1 4 7.726 2.746 0.177 7.726 2.746 0.177
0.1 5 8.109 1.99 0.735 8.109 1.99 0.735
0.1 10 7.806 4.304 3.623 7.813 9.146 0.347
0.1 20 7.826 4.509 1.812 8.898 10.147 0.257
0.1 40 7.378 7.853 1.419 9.569 9.888 0.014
0.1 60 9.641 8.452 0.509 10.026 16.576 1.051
0.1 80 8.111 8.806 1.011 8.834 10.23 0.945
0.1 100 9.774 9.459 0.726 7.307 1.0156 0.089
*F = Feed concentration, R = Retentate concentration, P = Permeate concentration
Table A6: Averaged data of Pt and Pd ultrafiltration with varying
CPC concentration in acidic medium
Platinum-CPC system
% Retention
Palladium-CPC system
% Retention
[Metal]
mM
[CPC]
mMPt CPC Pd CPC
0.1 1 98.85 59.43 98.78 56.28
0.1 2 98.79 41.86 98.46 36.30
0.1 3 98.87 31.94 99.53 33.63
0.1 4 98.56 24.64 97.98 31.10
0.1 5 98.25 21.60 97.10 35.28
0.1 10 98.06 18.26 97.77 30.60
0.1 15 97.59 15.14 96.38 29.61
0.1 20 95.84 18.21 97.45 43.680.1 30 95.66 18.75 96.13 35.21
0.1 40 94.41 7.63 95.52 42.16
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Table A7: Pt ultrafiltration with varying CPC concentration in
neutral medium
Pt concentration (ppm)
Experiment # 1
Pt concentration (ppm)
Experiment # 2[Metal]
mM
[CPC]
mMF R P F R P
0.1 1 14.23 4.34 0.133 14.23 4.343 0.132
0.1 2 14.69 3.62 0.201 14.69 3.623 0.201
0.1 3 16.85 4.17 0.226 16.85 4.171 0.226
0.1 4 17.85 4.74 0.289 17.85 4.74 0.289
0.1 5 17.59 6.54 0.297 17.59 6.54 0.297
0.1 10 23.99 0.27 0.766 22.61 15.72 0.534
0.1 20 24.51 3.06 0.196 21.75 13.77 0.741
0.1 40 19.16 16.19 0.658 21.22 13.84 0.689
0.1 60 18.39 13.34 0.704 19.77 10.87 1.291
0.1 80 19.31 14.98 0.650 20.7 9.497 1.078
0.1 100 18.25 17.11 0.683 14.23 4.343 0.132
Table A8: Pd ultrafiltration with varying CPC concentration in
neutral medium
Pd concentration (ppm)
Experiment # 1
Pd concentration (ppm)
Experiment # 2[Metal]
mM
[CPC]
mM
F R P F R P
0.1 1 7.307 1.016 0.089 7.307 1.016 0.089
0.1 2 7.587 2.302 0.256 7.587 2.302 0.256
0.1 3 6.236 1.469 0.076 6.236 1.469 0.076
0.1 4 7.726 2.746 0.177 7.726 2.746 0.177
0.1 5 8.109 1.990 0.736 8.109 1.990 0.736
0.1 10 7.806 4.304 3.623 7.813 9.146 0.348
0.1 20 7.826 4.509 1.812 8.898 10.147 0.258
0.1 40 7.378 7.853 1.419 9.569 9.888 0.014
0.1 60 9.641 8.452 0.509 10.026 16.576 1.0510.1 80 8.111 8.806 1.011 8.834 10.230 0.945
0.1 100 9.774 9.459 0.726 7.307 1.016 0.089
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Table A9: Averaged data of Pt and Pd ultrafiltration with varying
CPC concentration in neutral medium
Metal ion- CPC sytem
% Retention[Metal]
(mM)
[CPC]
(mM)Pt Pd
0.1 1 99.07 98.88
0.1 2 98.63 98.79
0.1 4 98.38 97.70
0.1 5 98.31 95.55
0.1 10 97.26 90.31
0.1 40 96.66 89.52
0.1 60 96.17 89.30
0.1 80 95.52 98.88
0.1 100 95.05 98.79
Table A10: Separation study of Pt/Pd mixture in acidic medium
Pd concentration
(ppm)
Pt concentration
(ppm)% Retention[Metal]
mM
[CPC]
mMF R P F R P Pt Pd
0.1 10 11.14 2.08 0.44 20.64 4.24 0.44 97.86 96.03
0.1 20 10.10 5.83 0.53 19.91 12.33 0.42 97.88 94.76
0.1 40 10.58 6.49 2.09 20.60 11.40 0.66 96.77 80.280.1 80 10.25 8.10 2.61 18.55 13.42 0.99 94.65 74.55
0.1 100 10.53 8.47 2.35 20.28 14.29 0.58 97.12 77.72
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Table A11: Effect of an electrolyte in metal ion and surfactant
retention
Palladium-cetylpyridinium chloride-sodium chloride system
[CPC]
mM
Palladium concentration
(ppm)
% Retention[Metal]
mM
[NaCl]
mMF P F R P Pd CPC
0.1 0 - - - - - 90.31 93.34
0.1 10.00 41.13 4.61 8.307 6.724 0.357 85.79 88.79
0.1 25.00 36.03 7.18 6.482 5.139 0.437 84.95 80.07
0.1 50.00 39.64 8.89 4.849 4.288 0.752 84.46 77.58
0.1 100.00 38.68 21.83 6.710 3.627 1.044 84.20 43.56
Platinum-cetylpyridinium chloride-sodium chloride system
[CPC]
mM
Platinum concentration
(ppm)% Retention[Metal]
mM
[NaCl]
mMF P F R P Pt CPC
0.1 0 - - - - - 96.66 38.37
0.1 10.00 37.64 27.51 15.21 11.69 0.69 95.05 27.59
0.1 25.00 40.70 35.83 15.27 12.43 0.57 95.86 12.03
0.1 50.00 44.23 41.70 16.15 8.12 0.76 95.24 5.73
0.1 100.00 44.13 41.66 13.40 4.90 0.77 94.15 5.55
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Table A 12: Flux variation with CPC concentration in various
experimental conditions (Membrane response)
Flux (L/m2h)
Experiment CPC (mM)Batch # 1 Batch # 2 Average
10 62.75
20 57.91
40 39.64
80 27.7
CPC alone
100 27.43
10 96.05 89.99 93.02
20 48.11 49.54 48.83
40 37.62 36.48 37.05
80 25.70 29.95 27.83
CPC + Pd
100 25.45 28.72 27.09
10 126.90 122.05 124.48
20 60.41 63.22 61.82
40 54.49 47.29 50.89
80 33.88 34.28 34.08
CPC + Pt
100 27.95 24.59 26.27
10 60.03
20 58.01
40 51.89
80 38.87
Pd + Pt
100 36.33
Table A 13: Flux measurements after ultrafiltration of various
species
Flux (L/m2h)
Time
(minutes) Original After CPCAfter
Pd
After
Pt
After
backflush
20 94.14 74.19 64.19 70.21 94.1940 94.14 74.16 64.20 70.20 94.19
60 94.14 74.15 64.20 70.20 94.19
80 94.14 74.13 64.20 70.20 94.19
100 94.14 74.12 64.20 70.20 94.19
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APPENDIX B
Investigation of cetylpyridinium chloride retention
Table B1: Variation of hydrochloric acid concentration
CPC concentration
(mM)Experiment
number
[HCL]
mM
Flux
(L/m2h)
Feed Permeate
Retention
%
0 66.839 10.178 1.754 82.77
5 86.330 10.123 2.981 70.55
10 86.636 9.999 5.060 49.39#1
15 86.687 10.039 7.078 29.49
20 86.738 10.118 8.210 18.86
0 66.890 10.178 1.754 82.77
5 86.228 10.316 4.183 59.45
10 86.330 10.236 5.693 44.38
15 86.432 10.300 6.525 36.65
#2
20 86.636 10.241 8.241 19.53
Table B2: Variation of nitric acid concentration
CPC concentration
(mM)Experiment
number
[HNO3]
mM
Flux
(L/m2h)Feed Permeate
Retention
%
0 103.576 10.178 1.754 82.77
5 84.850 9.795 4.131 57.83
10 70.819 10.261 3.097 69.81#1
15 70.666 10.254 1.691 83.50
20 49.951 10.297 1.497 85.46
0 106.484 10.178 1.754 82.77
5 88.830 10.427 3.780 63.75
10 76.279 10.591 3.368 68.20
15 69.646 10.304 1.812 82.42
#2
20 49.237 10.147 1.537 84.85
0 106.484 10.178 1.754 82.77
5 84.29 9.518 3.886 59.169
10 71.43 9.461 2.078 78.034
15 67.40 9.654 1.567 83.767#3
20 50.10 9.759 1.236 87.331
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Table B3: Variation of sodium chloride concentration
CPC concentration
(mM)Experiment
number
[NaCl]
mM
Flux
(L/m2h)
Feed Permeate
Retention
%
0 102.20 11.293 1.754 84.47
2.5 55.51 9.849 6.917 29.77
5.0 53.01 9.653 7.058 26.89
7.5 51.84 9.792 7.093 27.57
10.0 51.18 9.806 7.500 23.51
15 50.61 10.503 8.034 23.51
#1
20 65.77 9.666 6.710 30.58
0 107.30 10.519 1.799 82.89
2.5 60.67 10.560 4.418 58.16
5.0 56.84 10.305 4.618 55.18
7.5 52.50 10.528 5.014 52.37
10.0 51.69 10.464 5.467 47.76
15 51.43 10.503 7.845 25.31
#2
20 65.77 9.666 6.710 30.58
0 107.30 10.519 1.799 82.89
2.5 64.75 10.570 5.565 47.35
5.0 63.73 9.849 4.217 57.18
7.5 88.37 9.789 4.969 49.24
10.0 63.32 9.797 7.066 27.87
15 62.71 9.796 4.126 57.88
#3
20 65.77 9.666 6.710 30.58
0 107.30 10.519 1.799 82.89
2.5 98.07 9.844 3.121 68.29
5.0 88.83 9.773 5.551 43.20
7.5 88.37 9.789 4.969 49.24
10.0 84.49 9.525 5.788 39.24
15 82.40 9.637 7.402 23.20
#4
20 80.82 9.581 7.465 22.09
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APPENDIX C
Conductivity studies
Table C1: Determination of the cmc of CPC
cell constant = 0.9365
[CPC] conductivity Temp
mM¥
S/cm °°°°C
0.2 37.3 28.4
0.4 55.7 28.4
0.6 77.2 28.5
0.8 92.8 28.5
1 109.6 28.4
1.2 121 28.4
1.4 129.6 28.2
1.8 145 28.3
2 154.4 28.2
degree of ionisation = 0.461
Table C2: Determination of the cmc of CPC in the presence of 0.5
mM HCl
[CPC]mM
ConductivitymS/cm
0 0.294
0.16 0.33
0.24 0.349
0.32 0.357
0.4 0.3645
0.48 0.3725
0.56 0.3815
0.64 0.391
0.8 0.4025
1 0.42251.2 0.446
1.4 0.462
1.6 0.4565
1.8 0.4705
2 0.491
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Table C3: Determination of cmc of CPC in the presence of Pd and
aqua regia
[CPC]mM
conductivitymS/cm
0.2 1.04
0.4 0.812
0.6 0.82
0.8 0.83
1 0.83
1.2 0.83
1.4 0.84
1.8 0.84
2 0.84
2.4 1.19
2.8 1.34
3 1.39
3.5 1.49
4 1.44
4.5 1.57
5 1.65
6 1.9
7 1.93
8 1.92
Table C4: Determination of cmc of CPC in the presence of Pt and
aqua regia
[CPC]
mM
conductivity
mS/cm
0.2 1.333
0.4 0.72
0.6 0.819
0.8 0.867
1 0.869
1.2 0.87
1.4 0.873
1.8 0.879
2 0.832
2.4 0.975
2.8 1.1043 1.114
3.5 1.125
4 1.129
4.5 1.136
5 1.237
6 1.252
7 1.339
8 1.345
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APPENDIX D
Improved experimental conditions for the metal ion and surfactant
retention
MEUF of Pt (lV) anions in acidic medium
Table D1: CPC data
Averaged absorbanceCPC concentration
(mM)Sample no.
Feed Permeate Feed Permeate
Retention
%
1 0.9709 0.0201 46.025 0.477 98.96
2 0.9805 0.0208 46.480 0.493 98.94
3 0.9939 0.0787 47.115 1.865 96.04
4 1.0895 0.1310 51.647 3.105 93.99
5 1.0261 0.1416 48.642 3.357 93.10
Table D2: Metal ion data
Metal ion concentration
(mM)Sample no.
Feed Permeate
Flux
(L/m2h)
Retention
%
1 0.0332 0.0008 48.60 97.73
2 0.0869 0.0014 43.70 98.38
3 0.1354 0.0021 38.90 98.48
4 0.1902 0.0012 36.24 99.40
5 0.2557 0.0012 35.63 99.55
MEUF of Pd (ll) anions in acidic medium
Table D3: CPC data
Averaged absorbanceCPC concentration
(mM)Sample no.
Feed Permeate Feed Permeate
Retention
%
1 1.0293 0.0394 48.794 0.933 98.092 0.9945 0.0493 47.144 1.168 97.52
3 0.9927 0.0721 47.059 1.708 96.37
4 0.9923 0.0914 47.040 2.165 95.40
5 0.9242 0.1755 43.811 4.160 90.51
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Table D4: Metal ion data
Metal ion concentration
(mM)Sample no.
Feed Permeate
Flux
(L/m2h)
Retention
%
1 0.0237 3.4E-05 45.18 99.86
2 0.0354 4.6E-05 31.90 99.87
3 0.0350 9.8E-06 31.14 99.97
4 0.0636 1.6E-05 30.07 99.97
5 0.1712 2.4E-05 29.56 99.99
Pt/Pd mixture ultrafiltration in acidic medium
Table D5: CPC data obtained when [Pt] : [Pd] = 1 : x
Averaged absorbance CPC concentration
(mM)
Metal ion
ratio
Pt : Pd Feed Permeate Feed Permeate
Retention
%
1 : 1 0.9963 0.1564 47.229 3.707 92.15
1 : 2 0.9863 0.1312 46.755 3.110 93.35
1 : 3 1.0023 0.1353 47.514 3.207 93.25
1 : 4 0.9819 0.1753 46.547 4.155 91.07
1 : 6 0.9919 0.1603 47.021 3.799 91.92
Table D6: CPC data obtained when [Pd] : [Pt] = 1 : x
Averaged absorbanceCPC concentration
(mM)
Metal ion
ratio
Pd : Pt Feed Permeate Feed Permeate
Retention
%
1 : 1 1.0212 0.1582 48.410 3.750 92.25
1 : 2 1.0166 0.1627 48.192 3.856 92.00
1 : 3 1.0062 0.1847 47.699 4.378 90.82
1 : 4 0.9202 0.2031 43.622 4.814 88.96
1 : 6 1.0302 0.1672 48.836 3.963 91.89
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Table D7: Metal ion data when [Pd] : [Pt] = 1 : x
Table D8: Metal ion data when [Pt] : [Pd] = 1 : x
[Platinum]
(mM)
[Palladium]
(mM)
Retention
%RatioFlux
(L/m2h)
F P F P Pt Pd
1 : 1 28.84 0.05 4.8E-04 0.05 9.5E-04 96.6 93.3
1 : 2 27.77 0.0666 3.8E-04 0.0333 3.5E-04 98.2 96.9
1 : 3 25.27 0.075 5.6E-04 0.025 2.9E-04 97.3 95.1
1 : 4 22.15 0.08 9.8E-04 0.02 5.3E-04 95.8 92.4
1 : 6 20.67 0.085 5.7E-04 0.015 5.2E-04 98.3 94.1
[Platinum]
(mM)
[Palladium]
(mM)
Retention
%RatioFlux
(L/m2h)
F P F P Pt Pd
1 : 1 40.17 0.05 4.6E-04 0.05 4.7E-04 98.1 96.3
1 : 2 38.44 0.033 3.5E-04 0.0666 2.5E-04 97.8 98.3
1 : 3 35.48 0.025 3.1E-04 0.075 2.0E-04 97.1 98.8
1 : 4 29.66 0.02 3.6E-04 0.08 5.0E-04 96.2 97.0
1 : 6 25.42 0.015 4.3E-04 0.085 4.3E-04 99.4 97.8