Faculty of Bioscience Engineering

Academic year 2013 – 2014

Are membranes implemented with nanoparticles able to provide a breakthrough in water purification?

Ruben Nackaerts

Promotor: Prof. dr. ir. Arne Verliefde Tutor: Msc. Mahlangu Themba Oranso

Master dissertation presented to obtain the degree of Master in Bioscience Engineering: Environmental Technology

The author and the promoters give permission to use this thesis for consultation and to copy parts of it for personal use. Every other use is subject to the copyright laws, more specifically the source must be extensively specified when using results from this thesis. De auteur en promotoren geven de toelating deze scriptie voor consultatie beschikbaar te stellen en delen ervan te kopiëren voor persoonlijk gebruik. Elk ander gebruik valt onder de beperkingen van het auteursrecht, in het bijzonder met betrekking tot de verplichting uitdrukkelijk de bron te vermelden bij het aanhalen van de resultaten uit deze scriptie. Ghent, 6th

of June 2014 The author, The promoter, Ruben Nackaerts Prof. dr. ir. Arne Verliefde

III

Acknowledgements

"It always seems impossible until it is done." - Nelson Mandela The past year was surreal for me. If I look back, I'd never thought that I would be writing know the last words of my thesis. In August I left for Jo'burg/Jozi, South-Africa (after being there you'll never use Johannesburg ever again) to start my thesis research. It was the best experience of my life. There were some ups and downs. Being alone in a big city in a strange country is never easy and also research was not always going like planned. Instruments dropped down like flies as if they didn't want me to get results! But I survived. I have met the most kind people who helped me out during my stay. Prof. Sabelo Mhlanga and dr. Edward Nxumalo, thank you for all the effort you guys have put in me during my stay: guiding me around, consulting on achieved results, providing me with comfortable accommodation and giving me all the opportunities to let me grow as a young scientist. The whole department of Applied Chemistry at the University of Johannesburg was a very welcoming environment. Someone was always there in case I needed help in the lab. A special group of people for me will be the best South-African Ultimate Frisbee team ever: Ultitude. Their acceptance of me in their team made my stay in South-Africa really feel like home. Especially due to "my personal drivers" Ché and Durban Dave, never shy of helping me out to get to practice. Leaving South-Africa and returning to Belgium didn't seem easy. But my loving parents and my family made me feel like I had never left home. Also my friends let me blend in perfectly again in the life I was used to. During the weeks after, more stress came as results needed to be delivered and the thesis had to be written. Thank you, Arne and Oranso for being patient with me and letting me do my thesis at my own pace. Due to the guidance by both of you, I have delivered something of which I am now proud. Of course I need to thank everyone at PaInT with helping me out with setting up my experiments, provision of chemicals and nice conversations in between. So yeah, looking back over the past months: it seemed indeed always impossible until it is done, which has just happened now. Finally I want to thank VLIR-UOS and the Belgian Development Cooperation for providing me with a scholarship for my stay in South-Africa. It is good to see that support goes to projects for young students who want to cross borders and blend in with different cultures, while being unified by scientific research.

IV

Abstract

The quality and quantity of available fresh water is declining all across the world. Membranes provide an ideal separation technology for easily scalable (small and large installations possible) water treatment installations. They have low energy requirements and are able to produce water of high purity. Membranes that are used for water purification purposes are made of hydrophobic polymers such as polyethersulfone. These polymers make the membranes quite water repellent and prone to fouling. Fouled membranes have been reported to have poor performance in terms of water flux and rejection of solutes. Due to fouling the applied pressure is normally raised to maintain water flux. This means more energy usage which makes the technology cost-ineffective. This therefore results in the need to modify the polymeric membranes for enhanced performance. To counteract the effect of fouling on membrane application, polyethersulfone (PES) membranes were successfully implemented with graphene oxide – zinc oxide (GO ZnO) nanoparticles to produce nanocomposite membranes with increased performance and anti-fouling properties. The successful synthesis of the nanoparticles was confirmed through FTIR, Raman, EDS and TEM analysis. Membrane production was eventually achieved by adding different loadings of the nanoparticles to the casting solution. The membranes were then synthesised following phase inversion techniques. Another series of membranes were produced as well through adding also a pore forming agent, polyvinylpyrrolidone (PVP), to the casting solution. All membranes were produced with 22 wt% PES and varied loading of PVP (0 wt% or 4 wt%) and nanoparticles (0 wt%; 0,2 wt%; 0,05 wt%; 0,0125 wt%). After synthesis the membranes were characterised for various properties such as pure water fluxes, trans-membrane resistance, salt rejection and anti-fouling properties. They were then applied for treatment of feed water spiked with trace organic contaminants. The best performing membrane was the PES PVP 0,02% GO ZnO membrane, although it had not the lowest contact angle with its 68±5°. It showed higher permeability (76 ± 15 L.m-2.h-1.bar-1) compared to the bare PES membrane (43,6 ± 0,9 L.m-2.h-1.bar-1), but its permeability was 40% less of that of the bare PES PVP membrane (122 ± 40 L.m-2.h-1.bar-1). The reduced water permeability was probably due to partial blocking of the pores by the nanoparticles. The PES PVP 0,02% GO ZnO membrane was resistant to fouling by organics and colloids compared to the rest of the synthesised membranes. It had a relative flux 20% higher than that of the PES PVP membrane after fouling. The membrane also rejected a handful of the 23 different trace organic compounds, that were dosed in a cocktail in the feed, with an total average rejection of 27,4% for all the compounds. The solutes were believed to be removed through a combination of steric hindrance by same charge species, hydrophilic/hydrophobic interactions and size exclusion mechanisms based on spatial arrangement instead of molecular weight. The PES PVP 0,02% GO ZnO membrane showed that adding PVP together with the nanoparticles was beneficial because it was performing better than the membrane with the same loading nanoparticles but without PVP.

V

Samenvatting

De kwaliteit en de kwantiteit van het beschikbare zoet water is in de wereld aan het verminderen. Membranen zijn een ideale scheidingsmethode voor eenvoudig schaalbare (zowel kleine als grote installaties mogelijk) waterzuiveringinstallaties. Ze hebben een lage energiebehoefte en zijn in staat water met een hoge zuiverheid te produceren. Membranen die in de waterzuivering gebruikt worden, zijn gemaakt van hydrofobe polymeren zoals polyethersulfon. Deze polymeren maken het membraan redelijk water afstotend en gevoelig voor fouling. Het is gekend dat gefoulde membranen slecht water doorlaten en opgeloste stoffen tegenhouden. Door fouling moet de druk over het membraan opgevoerd worden om een constante stroming van water te kunnen behouden. Dit heeft als gevolg dat er meer energie gebruikt moet worden, wat de technologie kosteninefficiënt maakt. Om de effecten van fouling in membraan toepassingen tegen te gaan werden polyethersulfon (PES) membranen succesvol geïmplementeerd met grafeen oxide - zink oxide (GO ZnO) nanodeeltjes om zogenaamde nanocomposite membranen te produceren met versterkte prestatie en anti-fouling eigenschappen. Het succesvol maken van deze deeltjes werd bevestigd door FTIR, Raman, EDS en TEM analyse. Membraanproductie werd uiteindelijk bereikt door verschillende hoeveelheden van de nanodeeltjes toe te voegen aan de gietoplossing. Het membraan werd dan gesynthetiseerd door de techniek van fase omwisseling toe te passen. Een tweede reeks membranen werd gemaakt door het toevoegen van een porievormende agent, polyvinylpyrrolidon (PVP), toe te voegen aan de gietoplossing. Alle membranen werden dus gemaakt met 22 wt% PES, een variërende hoeveelheid PVP (0 wt% of 4 wt%) en nanodeeltjes (0 wt%; 0,2 wt%; 0,05 wt%; 0,0125 wt%). Na de synthese werden de membranen gekarakteriseerd op basis van verschillende eigenschappen als de flux van zuiver water, de membraanweerstand, het tegenhouden van zouten en anti-fouling eigenschappen. Ze werden tenslotte toegepast op het zuiveren van een voedingsstroom gespiked met organische spoorcontaminaties. Het best presterende membraan was het PES PVP 0,02% GO ZnO membraan, ondanks het feit dat dit membraan niet de laagste contact hoek had met zijn 68±5°. Het toonde wel een hogere permeabiliteit (76 ± 15 L.m-2.h-1.bar-1) in vergelijking met het PES membraan zonder nanodeeltjes (43,6 ± 0,9 L.m-2.h-1.bar-1), maar zijn permeabiliteit lag wel 40% lager dan dat van het PES PVP membraan zonder nanodeeltjes (122 ± 40 L.m-2.h-1.bar-1). De verminderde permeabiliteit voor water was waarschijnlijk het gevolg van het blokkeren van de poriën door de nanodeeltjes. Het PES PVP 0,02% GO ZnO membraan was ook resistent voor organische en colloïde fouling in vergelijking met de andere gemaakte membranen. Na fouling lag zijn flux 20% hoger dan het PES PVP membraan. Het membraan weerhield ook enkele van de 23 verschillende organische spoorcomponenten, die gedoseerd werden als een cocktail in de voedingsoplossing, met een gemiddelde weerhouding van 27,4% voor alle componenten. Deze opgeloste organische stoffen werden waarschijnlijk tegengehouden door een combinatie van sterische hindering door gelijke ladingen, hydrofiele/hydrofobe interacties en uitsluiting op basis van grootte (niet op basis van het moleculair gewicht, maar eerder de ruimtelijke structuur). Dit membraan toonde ook aan dat het toevoegen van PVP als extra additief bevorderend was voor de prestatie van het membraan, omdat het betere resultaten vertoonde dan het membraan met dezelfde hoeveelheid nanodeeltjes maar zonder PVP.

VI

Table of Contents

ACKNOWLEDGEMENTS................................................................................................................ III

ABSTRACT ................................................................................................................................... IV

SAMENVATTING........................................................................................................................... V

TABLE OF CONTENTS ................................................................................................................... VI

LIST OF FIGURES ........................................................................................................................ VIII

LIST OF TABLES ............................................................................................................................ IX

LIST OF ABBREVIATIONS ............................................................................................................... X

1. INTRODUCTION ..................................................................................................................... 1

2. LITERATURE REVIEW .............................................................................................................. 3

2.1 WATER SUPPLY PROBLEM .............................................................................................................. 3 2.1.1 Water Scarcity ..................................................................................................................... 3 2.1.2 Water Pollution .................................................................................................................... 3

2.2 CONVENTIONAL WATER TREATMENT METHODS ............................................................................... 4 2.3 MEMBRANE PROCESSES FOR REMOVAL OF TRACE ORGANICS ................................................................ 5

2.3.1 Membrane separation ......................................................................................................... 5 2.3.2 Removal of trace organics by high-pressure membranes (NF/RO) ..................................... 7 2.3.3 Membrane manufacturing .................................................................................................. 8

2.4 CHALLENGES AND POTENTIAL SOLUTIONS FOR MEMBRANES IN WATER TREATMENT – REMOVAL OF TRACE

ORGANICS ............................................................................................................................................... 9 2.4.1 Challenges ............................................................................................................................ 9 2.4.2 Membrane Modifications to limit organic fouling ............................................................. 10 2.4.3 Nanocomposite Membranes ............................................................................................. 11 2.4.4 Materials of interest .......................................................................................................... 12

2.4.4.1 Polyethersulfone ....................................................................................................... 12 2.4.4.2 Polyvinylpyrrolidone .................................................................................................. 13

2.4.5 Nanoparticles ..................................................................................................................... 14 2.4.5.1 Graphene Oxide ......................................................................................................... 14 2.4.5.2 Zinc Oxide .................................................................................................................. 16 2.4.5.3 Graphene Oxide/Zinc Oxide nanohybrid ................................................................... 17

2.5 GAPS IN KNOWLEDGE – GOAL OF THE THESIS................................................................................... 18

3. MATERIALS AND METHOD ................................................................................................... 20

3.1 MATERIALS ............................................................................................................................... 20 3.2 SYNTHESIS AND CHARACTERISATION OF GRAPHENE OXIDE - ZINC OXIDE NANOPARTICLES ........................ 20

3.2.1 Synthesis of Graphene Oxide (GO) nanoparticles .............................................................. 20 3.2.2 Synthesis of Graphene Oxide – Zinc Oxide Nanoparticle hybrid ........................................ 21 3.2.3 Characterisation of GO and GO ZnO nanoparticles ........................................................... 21

3.3 MEMBRANE SYNTHESIS AND CHARACTERISATION ............................................................................. 22 3.3.1 Membrane synthesis .......................................................................................................... 22 3.3.2 Membrane characterisation .............................................................................................. 23

VII

3.3.3 Surface and Cross-sectional morphology .......................................................................... 24 3.4 MEMBRANE FILTRATION TESTS ..................................................................................................... 24

3.4.1 Filtration setups ................................................................................................................. 24 3.4.1.1 Dead-end Setup ......................................................................................................... 24 3.4.1.2 Cross-Flow Setup ....................................................................................................... 25

3.4.2 Filtration Protocol .............................................................................................................. 25 3.4.2.1 Dead-end Experiments .............................................................................................. 25 3.4.2.2 Cross-flow Experiments ............................................................................................. 26

3.4.3 Trace contaminants and analysis ...................................................................................... 27 3.4.3.1 Solid Phase Extraction ............................................................................................... 27 3.4.3.2 Sample Analysis ......................................................................................................... 28

3.4.4 Fouling experiments .......................................................................................................... 28

4. RESULTS & DISCUSSION ....................................................................................................... 29

4.1 CHARACTERISATION OF NANOPARTICLES ........................................................................................ 29 4.1.1 FTIR .................................................................................................................................... 29 4.1.2 TEM ................................................................................................................................... 31 4.1.3 EDS ..................................................................................................................................... 32 4.1.4 Raman Spectroscopy ......................................................................................................... 33

4.2 MEMBRANE PRODUCTION AND CHARACTERISATION ......................................................................... 34 4.2.1 Contact Angle and Interfacial Free Energies ..................................................................... 34 4.2.2 Pure Water Flux and Membrane Permeability .................................................................. 37 4.2.3 Salt Rejection ..................................................................................................................... 40 4.2.4 SEM images ....................................................................................................................... 43

4.3 FOULING BEHAVIOUR ................................................................................................................. 45 4.3.1 Organic Fouling ................................................................................................................. 45 4.3.2 Colloidal Fouling ................................................................................................................ 48

4.4 APPLICATION IN REJECTION OF TRACE ORGANIC POLLUTANTS ............................................................. 49

5. CONCLUSIONS AND RECOMMENDATIONS ........................................................................... 53

5.1 CONCLUSIONS ........................................................................................................................... 53 5.2 RECOMMENDATIONS FOR FUTURE RESEARCH ................................................................................. 54

REFERENCES ............................................................................................................................... 55

VIII

List of Figures

Figure 2.1: Molecular structure of PES (Ahmad, et al., 2013) ............................................................... 12

Figure 2.2: Structure of polyvinylpyrrolidone (Haaf, Sanner, & Straub, 1985) ..................................... 13

Figure 2.3: Molecular structure for GO (Hong et al., 2011) .................................................................. 15

Figure 2.4: Example of the production route for GO ZnO nanoparticles (Li et al., 2012) .................... 18

Figure 3.1: Scheme from the reaction of potassium permanganate in sulfuric acid to diamanganese

heptoxide (Dreyer et al., 2010) ............................................................................................................. 20

Figure 3.2: Membrane filtration dead-end setup .................................................................................. 25

Figure 3.3: Membrane filtration cross-flow setup ................................................................................ 25

Figure 4.1: FTIR spectrum of graphite, graphene oxide nanoparticles and the nanohybrid graphene

oxide - zinc oxide ................................................................................................................................... 29

Figure 4.2: Zoomed in FTIR spectrum of graphene oxide and graphene oxide - zinc oxide ................. 30

Figure 4.3: TEM images of graphene oxide - zinc oxide nanohybrid31

Figure 4.4: ED - spectra (spectrum 2 and spectrum 5) for graphene oxide - zinc oxide ....................... 32

Figure 4.5: Raman spectrum for graphene oxide and graphene oxide - zinc oxide .............................. 33

Figure 4.6: Water contact angles with the different membranes ......................................................... 36

Figure 4.7: Pure water flux under different pressures for PES membranes in a dead-end setup ........ 37

Figure 4.8: Pure water flux under different pressures for PES PVP membranes in a dead-end setup . 38

Figure 4.9: Pure water flux under different pressures for PES membranes in a cross-flow setup ....... 39

Figure 4.10: Pure water flux under different pressures for PES membranes in a cross-flow setup ..... 39

Figure 4.11: Salt rejection for PES membranes in a cross-flow setup ................................................... 41

Figure 4.12: Salt rejection for PES PVP membranes in a cross-flow setup............................................ 41

Figure 4.13: SEM images of cross sections of different membranes .................................................... 44

Figure 4.14: Influence of organic fouling (alginate) on water flux for PES membranes........................ 46

Figure 4.15: Influence of organic fouling (alginate) on water flux for PES PVP membranes ................ 46

Figure 4.16: Influence of organic fouling on the rejection of NaCl for PES membranes ....................... 47

Figure 4.17: Influence of organic fouling on the rejection of NaCl for PES PVP membranes ............... 48

Figure 4.18: Influence of colloidal fouling on the relative flux for PES membranes ............................. 48

Figure 4.19: Influence of colloidal fouling on the relative flux for PES PVP membranes ...................... 49

Figure 4.20: Rejection of pharmaceuticals (ranked by decreasing MW) for PES membranes .............. 50

Figure 4.21: Rejection of pharmaceuticals (ranked by decreasing MW) for PES PVP membranes ...... 50

Figure 4.22: Rejection of pharmaceuticals (ranked by charge) for PES membranes ............................ 51

Figure 4.23: Rejection of pharmaceuticals (ranked by charge) for PES PVP membranes ..................... 51

Figure 4.24: Rejection of pharmaceuticals (ranked by log D value) for PES membranes ..................... 52

Figure 4.25: Rejection of pharmaceuticals (ranked by log D value) for PES PVP membranes .............. 52

IX

List of Tables

Table 2.1: List of pressure driven membrane processes (Shirazi et al., 2010) ........................................ 6

Table 3.1: Content in wt% for the eight different casting solutions ..................................................... 22

Table 3.2: Surface tension components of water, diiodomethane and glycerol (Brant & Childress,

2002) ..................................................................................................................................................... 24

Table 3.3: Trace organics and their selected physiochemical properties ............................................. 27

Table 4.1: Water, glycerol and diiodomethane contact angles for the different membranes ............. 34

Table 4.2: Components of the interfacial free energy for the different membranes ........................... 37

Table 4.3: Pure water flux, salt rejection, membrane resistance and membrane permeability for the

different membranes in a dead-end setup ........................................................................................... 40

Table 4.4: Pure water flux, salt rejection, membrane resistance and membrane permeability for the

different membranes in a cross-flow setup .......................................................................................... 40

Table 4.5: Calculated concentratians at membrane surfaces for concentration polarisation effect ... 42

Table 4.6: Energy of adhesion between the organic foulant (alginate) and the membranes (Motsa et

al., 2014) ................................................................................................................................................ 45

Table 4.7: Average rejection of TrOCs for the different membranes ................................................... 49

X

List of Abbreviations

DBP Disinfection Byproducts ED Energy Dispersive EDS Energy Dispersive X-Ray Spectroscopy FESEM Field Emission Scanning Electron Microscope FTIR Fourier Transform Infrared

GO GO ZnO H-ESI HRMS IR LC - MS MF MW NF NMP NOM

PES POP PSf

PVP RO SEM SPE TEM TrOC UF UV WWTP ZnO

Graphene Oxide Graphene Oxide Zinc Oxide Heated Electro Spray Ionisation Interface High Resolution Mass Spectrometer Infra Red Liquid Chramatography - Mass Spectrometry Microfiltration Molecular Weight Nanofiltration N-methyl-2- pyrrolidone Natural Organic Matter Polyethersulfone Persistant Organic Pollutants Polysulfone Polyvinylpyrrolidone Reverse Osmosis Scanning Electrone Microscope Solid Phase Extraction Transmission Electron Microscope Trace Organic Compounds/Chemicals Ultrafiltration Ultraviolet Waste Water Treatment Plant Zinc Oxide

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1. Introduction

The stress on fresh water is rising yearly. Societies often dispose some of their waste streams in a improper way in rivers, seas and lakes. This results in major decrease in water quality. Increasing population, urbanisation and changing weather patterns are only some examples of all the parameters that are making the quantity of available fresh water of decent quality disappear very quickly. There is clearly a big challenge to sustain everyone with access to safe drinking water in the near future. In this dissertation a possible solution to this huge water problem is provided by combining two emerging and promising technologies in the field of water treatment. The idea is based on combining membrane technology and nanotechnology to provide water of high purity. Nanotechnology is based on nanoscale particles with better properties in adsorption, stability,

photocatalytic activity in comparison with their larger counterparts. The tons of different

nanoparticles that already have been produced promise a bright future for many applications as drug

delivery, biosensing, durable energy production and water treatment. Membrane technology has

already proven to be able to provide water treatment in an energy-efficient way on both small and

large scale. The application of membranes is challenged by several drawbacks that are withholding

them from worldwide implementation. Some of the drawbacks include high fouling behaviour and

inability to completely remove trace organics from polluted water. This master dissertation will look

into the two different technologies of nanoparticles and membranes and investigate if their

combination would be able to give a definite answer to the drawbacks of membrane technology in

the first place and the water problem in the second place.

First a review will provide information about past research works and the gaps that are still there to

be filled in. Answers for the next questions based on those gaps will be sought:

Is it possible to reproduce the production of the graphene oxide - zinc oxide nanoparticles

based on literature?

Will the nanoparticles blend and disperse homogenously in the PES casting solution?

Can a nanocomposite membrane be made from a PES casting solution with nanoparticles?

What will be the effect of the nanoparticles with and without PVP on the hydrophilicity of

the membrane?

What will be the effect of the nanoparticles with and without PVP on the performance of the

membrane regarding permeability and rejection?

Will the additives make the membrane more fouling resistant?

These questions will be solved by striving to achieve following goals and objectives for the thesis:

Successfully synthesise GO ZnO nanoparticles

Characterise these GO ZnO nanoparticles

Succesfully produce GO ZnO nanocomposite PES membranes

Increase hydrophilicity of the PES membrane surface

Enhance permeability of the PES membrane through modification while at least keeping salt

rejection at the same level as the unmodified membrane

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Enhance membrane permeability even more with the addition of PVP while keeping rejection

performance at the same level

Reduce the decline of water flux due to organic and colloidal fouling of the membrane

Improve the removal of a mixture of several trace organic compounds

In the second part a detailed description will be given about the methodology used for achieving

these goals. Eventually the results achieved will be discussed and conclusions will be drawn to see if

the goals were met and the questions answered.

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2. Literature Review

2.1 Water supply problem Water supply is threatened across the globe due to water scarcity (physical lack of water) and water pollution. With water being essential for all life on this planet (human, animal and vegetal), it is important to come up with solutions to ensure continuous provision of potable water (Dubreuil et al., 2013). The water balance in the human body needs to be maintained around a certain level to ensure proper functioning of the metabolism on a physiological level. Water is also essential in the daily life for cooking, washing and hygienic purposes (Plappally & Lienhard, 2012). It is also used in industrial processes and in the production of food.

2.1.1 Water Scarcity The physical lack of water is influenced by various parameters such as population growth, increasing urbanisation, political instability, policy choices and climate change, amongst other factors (Rio Carrillo & Frei, 2009). These factors affect the availability of fresh and clean water for people. It is obvious that more people (increasing global population) will use more water and put more stress on the water availability this way. The effect of climate change on the availability of potable water remains relatively unclear. Scientists are expecting an influence on the hydrological cycle of the planet leading to more extreme weather patterns (extreme droughts and flooding), and thus an irregular supply of water, even in countries with abundant water sources. Already some small changes in local climate have caused the hydrological cycle to change and the water level to drop. For example in the Colorado river in USA, which is now on average 100 feet lower than recorded historic levels (Siddiqi & Anadon, 2011). Politics and policy are also influencing the water availability due to the decision-makers who have the influence on the allocation of budgets, water resources and infrastructure (Appelgren, 1997). In Australia there is a project running that identifies and investigate the links between climate, energy and water policies, because they recognize that climate change and fresh water availability is linked to each other. Policy makers can decide on which form of energy that a country should pursue to address the climate change problem (biofuel, solar energy, hydro energy etc.). But each form of energy also consumes water in its production. This is a way how policy is able to influence water availability (AUSCEW, 2012).

2.1.2 Water Pollution Not only the quantity of fresh water is declining, also the quality (Appelgren, 1997). This is caused by many pollutants that are present in aqueous systems due to natural die-off, erosion and/or improper disposal by humans. Pollutants are components present that can have harmful effects for the environment. These include heavy metals such as arsenic and lead, inorganic molecules such as nitrates and phosphates, but also organic molecules like natural organic matter, pesticides and pharmaceuticals to name but a few. Regarding the organic pollutants, the most disturbing ones are categorised as persistent organic pollutants (POPs). Some of these POPs have been known to be stable for years or even decades, and hence they are a global threat to human health and the environment (Jones & de Voogt, 1999). With prolonged stability, the chances of these substances ending up having influence on human lives are significant. All the POPs are highly persistent, can travel long distances through the air and water and can bio-accumulate easily in fat (Tang, 2013). Even at low concentrations, some of them are highly toxic when ingested (Jones & de Voogt, 1999).

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Another class of organic pollutants that is emerging in water sources in developed countries are the pharmaceuticals. These are entering into the environment through leaching of landfills, aquaculture (antibiotics for fishes) and improper disposal of hospital, industrial and domestic waste (Li, 2014). Pharmaceuticals are often incompletely metabolised in the human body and become released through excretion. This is the main route through which pharmaceuticals get into the environment( Li, 2014). Pharmaceuticals are present in trace concentrations (ng.L-1 to µg.L-1) hence they are often classified together with other organic substances occurring in trace concentrations as trace organic pollutants (TrOCs) or organic micropollutants. They possess a potential risk for ecosystems, humans and animals due to their quite high mobility in the soil and their persistence, at least for some of them (Sires & Brillas, 2012). The potential health effects of pharmaceuticals in trace concentrations in potable water still remain uncertain but they are believed to potentially impair animal health at every level of the food chain when disposed in these small concentrations in surface water. Different types of pharmaceuticals exist. These include, but are not limited to central nervous system stimulants, anti-epileptic drugs, beta-blockers and antibiotics. Depending on the source of pollution, it is possible to find a cocktail of pharmaceuticals in the same water source. These kinds of pollutants need to be completely removed from the water before it reaches the consumers, due to the so-called “precautionary principle”: since health risks are not yet known, it’s better to be safe than sorry. In the next section, some conventional water treatment techniques will be reviewed, taking a closer look at their (in) abilities to properly remove pharmaceuticals from polluted water.

2.2 Conventional Water Treatment Methods The most conventional way of municipal waste water treatment is in centralised municipal waste water treatment plants. This method occupies a lot of space and consumes a lot of energy. It is a multi stage process requiring much time to complete to complete all the reactions which are necessary to obtain a good end product. Solutions need to be found to make this much more efficient by reducing area, operation period and amount of steps (Draw et al., 2012). For the production of drinking water, a disinfection step to remove pathogens like bacteria and viruses is applied (Plappally & Lienhard, 2012). This disinfection step uses mostly ozone or chlorine as additives in the water to destroy the pathogens. Ozone has a high oxidation potential and easily oxidises organics to smaller compounds which can be toxic (Yang et al., 2012). Ozone and other possible oxidising agents help in removing odour, colour, bad taste and algae bloom during the waste water treatment process (Yang et al., 2013). Ozone has to be formed on site and added to the waste water immediately after the production because of its very high reactivity. Ozone does not stay present for a long time once it is released (Yang et al., 2012). The use of chlorine is another option for disinfection and keeping the level of bacteria at tolerable concentrations. Although it is a very common disinfection procedure, chlorination with aqueous free chlorine can interact with natural organic matter (NOM) present in the waste water to form halogenated disinfect by-products (DBP's). Disinfect by-products are forming a mixture of hundreds of different chemical molecules. These compounds are known for having low volatility and high stability, which gives them a high persistence in the environment. People become highly exposed to DBP's because they stay present in the water coming out of the tap (Hrudey, 2009). Disinfect by-products are a health concern to animals. They can cause cancer in different organs like the liver and kidneys. They also have effect on the reproductive system resulting in pregnancy loss, decreased sperm motility and fetotoxicity. Studies have shown that there is an association between human health effects (such as adverse birth outcomes) and exposure to DBP's (Hamidin et al., 2008). Because of the uncertainty on the exposure of humans and the toxic levels of these DBP's it is best to look into better methods to remove pathogens and NOM from water so that the formation of DBP's is minimised or possibly stopped.

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How do the conventional methods hold up in the removal of trace organic compounds (TrOCs). In conventional wastewater treatment plants, the removal of TrOCs are fluctuating from 12,5 to 100% (Luo et al., 2014). Removal of these TrOCs is happening via hydrolysis, biotransformation or sorption on flocs, suspended solids or activated sludge (Monsalvo et al., 2014). After which they are removed from the water through sedimentation. Parameters to be considered in this process are temperature and sludge and hydraulic retention time (more time in the waste water treatment plant = more time for the TrOCS to get adsorb or undergo reactions) (Luo et al., 2014). Kosma et al. compared different WWTP with a typical activated sludge secondary treatment setup with nitrogen and phosphate removal. Removal efficiencies deemed not sufficient with concentration levels ranging from 9,3 to 96648,3 ng.L-1 in the influents and from 6,6 to 1076,0 ng.L-1 in the effluents. This indicates a high acute and chronic risk for freshwater communities in which the effluent is released (Kosma et al., 2014). But this is not always the case, removal of TrOCs in WWTP has been reported high enough so that (possibly) no risk for the environment was posed (Van De Steene et al., 2010). Ternes et al. found even maximum concentrations of only 70 ng.L-1 of estrogens in different WWTP, indication a very high removal efficiency. Because the results are not consistent and TrOCs posses a high risk for human health, other removal processes (advanced and tertiary processes like nanofiltration, reverse osmosis, ozonation, UV oxidation and activated carbon adsorption) are used to try to remove these organics. These advanced treatment methods are able to remove 60 -100% of the trace organics, providing a better certainty in removal when comparing with WWTP (Luo et al., 2014). Ozone and a combination of ozone and peroxide were able to reduce the amount of TrOCs with 50 to 100% depending on the dosage of the oxidation agent (Pisarenko et al., 2012). Activated carbon is showing similar removal efficiencies (Altmann et al., 2014). In next section membrane filtration will be discussed. This process has got advantages over the conventional methods (Bai et al., 2012). Membrane filtration is easier to operate. Filtration is carried out in a single step and no chemical addition is required. Will this method provide the solution for the removal of trace organic compounds?

2.3 Membrane processes for removal of trace organics

2.3.1 Membrane separation Membrane filtration is a separation technology where the membrane is used as selective barrier for the passage of fluids (gasses or liquids) and/or solutes. While selectively trying to pass one component, the passage of certain other components needs to be restrained. To achieve passage through the membrane, a dynamic driving force must be applied across the membrane. This can be a pressure gradient, a chemical potential or an electric potential. Membranes are applied in gas separations, food industry (separation of emulsions), energy production (batteries, fuel cells, biofuel production, biorefineries etc.), biomedical industry (blood purification using dialysis,) and of course water treatment. They are widely used because they offer the possibility to concentrate, fractionate and purify products in one step. In drinking water production and water treatment processes in general, membranes are used to selectively pass water and restrain pollutants (organic and inorganic) of interest. Membranes offer some advantages in comparison to conventional methods. Membrane separation allows achievement of higher water quality standards, reduced environmental impact of effluents, reduced land and energy requirements and the possibility to develop mobile treatment units (Shirazi et al., 2010). Depending on the type of membrane used, typical components can be removed. The most typically used pressure-driven membranes and their applications are shown in Table 2.1 below.

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Most membranes are made from polymeric materials like polysulfone, polyethersulfone, polyetherimide, polyamide, polyvinylidene fluoride, polyvinyl alcohol and many more (Ahmad et al., 2013). Therefore, their properties (such as charge, roughness and hydrophobicity) are controlled by the material used. The structure of a membrane is controlled by the membrane production process. There are some characteristics which a membrane needs to possess to be used on full industrial scale (Pinnau & Freeman, 1999). A lot of research has been done and is still on-going to achieve these characteristics. A membrane should ideally have:

High flux

High selectivity (solute rejection)

Mechanical stability

Low manufacturing cost

Tolerance to temperature variations (so that there is no physical or chemical change of the

membrane is observed with change in temperature)

Tolerance to components present in the feed stream (chemically inert)

Fouling resistance

Manufacturing reproducibility

ability to be used in high surface area modules

Low operational costs (long term operation)

Newly developed types of membranes are always characterised to gain knowledge of these characteristics. A couple of these characteristics are linked to each other. For example a high flux means that more feed can be treated with less membrane area, resulting in lower capital costs. A high selectivity will also give a more purified product which is also more desirable. It has to be noted that a membrane can be fine-tuned to highly remove organics while allowing the passage of salts. Pressure driven membranes (such as nanofiltration (NF) membranes and reverse osmosis (RO) membranes) are mostly applied in water treatment (Van der Bruggen & Vandecasteele, 2003). Application of non-pressure driven membrane processes such as forward osmosis (FO) is still limited, due to some inherent limitations in membranes used for FO. Pressure Driven Membrane Processes In water purification by membrane filtration, pressure driven membranes are mostly used as stated before. Pressure driven membranes can be divided into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) (see Table 2.1). Their classification is based on selectivity, pore size and operating pressure. However these properties often overlap. Table 2.1: List of pressure driven membrane processes (Shirazi et al., 2010)

Pressure driven membranes can be porous or non-porous. Microfiltration, UF and loose NF membranes fall under the category of porous membranes. Tight end NF and RO membranes are considered non-porous. Porous membranes remove solutes from the feed (often called rejection) through size exclusion or sieving mechanisms. Non-porous membranes utilise a separation mechanism where the solubility or diffusivity between the solvent and the solute in the membrane

Membrane Type

Pore Size (nm) Operating pressure (KPa) Applications

MF 50 - 2000 10 - 50 Separation of particles and bacteria from other smaller solutes

UF 2 - 50 50 - 200 Separation of colloids from solutes such as sugarsand salts

NF <2 - non-porous 200 - 1000 Separation of multivalent salts, pesticides and herbicides from water

RO non-porous 1000 - 10 000 Separation of monovalent salts, small molecules and solvents from water

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leads to separation (Shirazi et al., 2010). Both porous and non-porous pressure driven membranes can have different shapes: flat-sheet or hollow-fibre membranes (Zhao et al., 2013). In industrial treatment of water, hollow-fibre shaped membranes are preferred. This is because they provide a larger surface area/volume ratio, leaving less dead space. Therefore their application is more cost-effective than flat-sheet membranes. Although flat-sheet membranes are less applied in industry, they are excessively utilised in most research works on new types of membranes (Zhao et al., 2013). This is because of the easy and flexible production process for flat sheet membranes. Several techniques are used in the synthesis of membranes. In the next section we will discuss some the methods practised in membrane production.

2.3.2 Removal of trace organics by high-pressure membranes

(NF/RO) Trace organic chemicals (TrOCs) are becoming a concern due to their potential impact on human health and environment. Conventional waste water treatment processes remove TrOCs poorly so new ways to remove these chemicals have been looked for to be able to protect public health (Simon et al., 2013). That is why research has turned towards high-pressure membranes (nanofiltration and reverse osmosis) for finding an adequate, reliable treatment method. NF and RO membranes were tested in a membrane bioreactor for their removal of TrOCs. The bioreactor was able to remove the hydrophobic and biodegradable trace organics. But what was still present (the most hydrophilic compounds) needed to be removed with the assistance of NF/RO membranes. The removal caused the concentrations to drop beneath the detection limit of the trace organics (Ali Aturki et al., 2010). To get a better understanding of the process of filtration of TrOCs the influence of temperature of the feed was changed in the process. At 20°C the NF-membrane was able to reject the TrOCs between 35 and 100%. At 40°C this range decreased to 25 and 98% (Dang et al., 2014). Simon et al. also tested NF membranes on their rejection. Rejection of a cocktail of TrOCs had a range between 20 and 100% with a difference between neutral and negatively charged membrane, and hydrophilic or hydrophobic TrOCs. Also a time dependent factor was observed, the rejection after 24h was lower due to increased convection and diffusion (Simon et al., 2013). All the done research combined together gives a good image on the mechanism behind the rejection of trace organic compounds. Because it is clear that not every trace organic compound is removed efficiently, there is a big difference in rejection percentage between compounds. The rejection mechanism is not only based on size exclusion (or steric hindrance), electrostatic interactions (same charge repulsion) is also involved (Dang et al., 2014). Hajibani et al. observed the next rejection mechanisms for nonionic solutes: adsorption onto macromolecules (present as foulants on the membrane) result in increased rejection and the presence of cake enhanced concentration polarisation decreased rejection. The decrease in rejection was the dominant observed effect. Surface charge densities of the (fouled) membranes played an important role in rejection. Electrostatic repulsion played an important role in this research together with adsorption of the TrOCs (Hajibabania et al., 2011). The transport of TrOCs (hydrophobic/non-polar, hydrophilic or charged) is mainly happening through convection for the most compounds, certainly in the more looser membranes. Diffusion is an important factor too, but only for the more hydrophobic non-polar compounds. The higher the initial concentration the more dominant the convection transport will be due to increase solute fluxes. This gives the confirmation that rejection and compound transport are inversely related (Kim et al., 2007).

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2.3.3 Membrane manufacturing There are a lot of techniques available for the production of membranes. These include phase inversion, interfacial polymerisation, stretching, track-etching and electro-spinning (Lalia et al., 2013). Because the focus is in this thesis is on the use of flat sheet membranes, phase inversion techniques and interfacial polymerisation will be highlighted. Most polymeric flat-sheet membranes are made utilising these two methods (Zhao et al., 2013). First a short description of the interfacial polymerisation (IP) method will be given. This is a main technique to prepare nanofiltration membranes. The interfacial polymerization occurs at the interface between two immiscible phases (one phase is water, the other organic). This will give an ultrathin active layer attached to a porous substrate (Zhao et al., 2014). The porous substrate is very often a membrane that has been made using the phase inversion method. A lot of different variables play a role in IP and the formation and morphology of the interfacial film like the concentration of the reactants, their partition coefficients, the solubility of the nascent polymer in the solvent phase, the diffusion rates of the reactants, the presence of byproducts and their reactions, cross linking reactions and the post reactions or post treatment of the resulting thin layer (Petersen, 1993). The materials that are used for the active layer and substrate are different and there is usually no strong linkage between them. The original interfacial polymerisation process consisted out of the next steps. First the support layer (the porous substrate) was soaked in an aqueous solution of another polymer. The porous substrate will get impregnated with this polymer during the soaking. In the next step this membrane was immersed in organic phase solution. Cross linking of the layers was than achieved through heath treatment at 110 °C (Pinneau et al., 1999). IP is known to produce membranes with high water permeation flux and high salt rejection for RO/NF purposes (Abu Seman et al., 2011). The thin layer is determining the properties of the membrane like rejection and flux, which is a major advantage because the thin layer can be optimized for a membrane process through playing with the different parameters of the IP process. A main concern in IP is the structural stability of this film on the porous layer. Increasing this stability requires multiple steps, which increases the total complexity of the IP technique (Zhao et al., 2014). The polymer density on the surface is also difficult to control, causing highly non-uniform patterns of surface characteristics of the membrane (Abu Seman et al., 2010). Next to interfacial polymerisation, phase inversion is the major technique to produce flat sheet membranes. Phase inversion as a technique to make membranes can be subdivided into different types, depending on how the inversion of phases occurs. There is immersion precipitation, thermally induced and vapour - and evaporation induced phase separation (van de Witte et al., 1996). In the process of phase inversion through immersion, which is the most commonly applied for pressure-driven processes, a casting solution consisting of a polymer dissolved in a solvent is spread out on a support plate. This is then immersed into coagulation bath containing the non-solvent. This non-solvent can be any liquid in which the polymer used for the membrane is not soluble in. Water is the most common non-solvent used. Phase inversion will occur due to the diffusion that will interchange the solvent and non-solvent. This will make the polymer contact with its non-solvent. Therefore, a phase inversion of the polymer will occur which leads to precipitation in the non-solvent and thus the formation of the membrane (Lalia et al., 2013). The morphology and the structure of the resultant membrane depend on a lot of process and material parameters (Pinnau & Freeman, 1999) which include:

choice of polymer (molecular weight) and solvent. Especially the solvent – non-solvent

exchange rate.

composition and temperature of the casting solution

thickness of the film that is casted before immersion

speed of casting

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choice and temperature of the non-solvent

if present: the choice of membrane support material or additive

drying conditions of the membrane

conditions of the environment (temperature and humidity amongst others)

Many parameters are involved so to be able to selectively investigate the influence of one single parameter all the other ones have to be kept as constant as possible. This means that for applying interfacial polymerisation perfect control has to be obtained of the phase inversion process to produce the porous substrate and the IP process that will put a thin layer on this porous substrate. To reduce the complexity for the membrane production process and to have to control only one process, for the scope of this thesis it was chosen to use the phase inversion process by immersion as membrane production method. With phase inversion it is possible to make both MF/UF membranes as NF membranes. The most easy way to control the type of membrane made, is through varying the polymer concentration in the casting solution. Low polymer concentration will lead to low porosity and pore size. These are UF membranes and the polymer concentration to obtain these membranes is 12 -20%. Increasing the polymer concentration to above 20% is giving NF type membranes (Lalia et al., 2013). The thickness of the layer that is spread out is also a parameter that can shift MF/UF membranes to NF membranes. The thicker the layer that is spread out with the casting knife, the membranes are getting les porous (Stropnik et al., 2002). Although membrane processes have gained use in recent years, their application is challenged by several parameters which make the use of membranes in water treatment less suitable. In the next section some of the challenges will be looked at.

2.4 Challenges and potential solutions for membranes in water treatment – removal of trace organics

2.4.1 Challenges Membrane technology provides great opportunities and nice prospects for the future regarding waste water treatment and production of drinking water. Nonetheless, despite the relatively advanced state of the membrane industry, there are still issues that need to be tackled since they are holding back the use of membranes on an even larger scale in the industry. The majority of the problems can be brought back to one main cause. The most frequently used polymers for the synthesis for membranes for water treatment are quite hydrophobic in nature. Making them attractive surfaces for hydrophobic compounds in the water, which will cause fouling of the membrane (see next paragraph). The hydrophobic nature also causes the permeability of the membrane to be lower than desired. One of the most important aspects is fouling. Membrane filtration is prone to fouling, i.e., unwanted deposition of foulants on/in the membrane. Fouling is often attributed to the adsorption/deposition of rejected organic compounds on the membrane surface, referred to as organic fouling. However, also fouling by microorganisms and their associated biofilms (biofouling), fouling by deposition of inorganic salts that surpass their solubility limits ( scaling) and particulate fouling are still issues (Van der Bruggen et al., 2008). Fouling causes the performance of the membrane to decline. The water permeation through the membrane will decrease and also the rejection of solutes will be influenced in an uncertain way (depending on the type of fouling), but fouling typically leads to a decrease in product quality and quantity. To ensure stable operation of the installation, the membrane has to be cleaned to remediate fouling, or even replaced when fouling becomes too severe. This indicates the

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reduced membrane life-span caused by fouling (Katsoufidou et al., 2007). As a result, increasing operating costs are an apparent result of membrane fouling, since energy consumption, system down time, required membrane area, labour, operation time and material costs for the cleaning/backwashing/replacing of the membrane all increase (Listiarini et al., 2009). The cause of organic fouling typically lies within the relatively hydrophobic characteristics of the typical polymers used in the synthesis of membranes. This is because most foulants (such as sodium alginate and humic acid) are also relatively hydrophobic (Van der Bruggen et al., 2008). Due to foulant – membrane hydrophobic (typically Van der Waals) interactions, organic fouling is promoted. (Organic) fouling also typically depends on other membrane characteristics like surface morphology, pore size and surface charge (Katsoufidou et al., 2007). How the foulant will interact with the surface characteristics of the membrane will influence how severe the fouling will be. Inorganic fouling is caused by salts present in the feed water. Because of concentration polarisation (the effect that near the surface of the membrane the concentration of solutes tend to be much higher than in the bulk of the solution) local concentrations can exceed the solubility of the inorganic solutes. This will give particle precipitation on the membrane surface and blockage of the pores. This can cause irreversible fouling which requires chemical cleaning or membrane replacement (Shirazi et al., 2010). Biofouling is a last major form of fouling and results from biofilm formation, accumulation of assimilable organics and growth of microorganisms on the membrane surface (Zhang et al., 2012). Parameters influencing biofouling are physicochemical like hydrodynamics, solution chemistry and interfacial forces. Once microorganisms attach to the surface, they can reproduce and for a matrix of extra-cellular polymers which will protect them from even biocides (Subramani et al., 2008). By making the membrane surface more hydrophilic or negatively charged biofouling can be impeded (Lee et al., 2013). It is needless to say that fouling is a major threat for the use of membranes on large scale and that fouling needs to be controlled or prevented. Fouling can be prevented or reduced by production of membranes with surfaces that will interact less with foulants in the feed. These membranes are generally obtained by modification of the top surfaces of the membranes. One may think, why not tackle the problem by changing the material of which the membrane is made instead of modifying the membrane? This is because the polymers used posses properties like chemical inertness, high mechanical and thermal stability which makes them excellent (see also section 2.4.3). In the next section we will discuss some of the modification techniques that have already been applied for the production of anti-fouling membranes.

2.4.2 Membrane Modifications to limit organic fouling To tackle the problems mentioned in section 2.4.1 (fouling and hydrophobic nature of polymeric membranes) modification of the membrane can be done. There are three global types of membrane modification techniques. The choice of the technique to use depends on the stage of the membrane production process where interference will take place. Different modification techniques currently in use are polymer modification, blending the polymer with a modifying agent (additive) and surface modification of a prepared membrane (Zhao et al., 2013). The possibilities of modification methods are endless of which blending the membrane casting solution with an additive or surface adaptation by interfacial polymerisation are the most common. Less common methods include grafting with gamma ray radiation, plasma, electron beams (Zhao et al., 2013). The goal of these methods is always to improve hydrophilicity of the membrane to achieve flux enhancement and fouling reduction, while still containing their rejection abilities. The interfacial polymerisation does not only change the physical morphology (i.e. roughness, pore size, porosity etc.) of the membrane surface, it also significantly affects the chemistry properties (i.e.

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hydrophilicity, surface charge, etc.) of the membrane. Both physical and chemical properties indirectly affect the membrane performance (permeability and rejection) and certain degree of fouling (Abu Seman et al., 2011). Out of practical consideration, the blending technique is used a lot because of its simplicity. There are no additional steps required during the membrane production process and it doesn't involve extra instruments to generate plasma's or radiation like in the more complex modification. An extra component is mixed with the polymer in the casting solution. This mixed casting solution is used then to produce the membrane with the desired method. The possibilities are endless using blending due to the many different components that can be added to the membrane this way: inorganic nanoparticles, inorganic compounds, polymeric nanoparticles, other polymers than the ones used for the membrane (Ahmad et al., 2013). An example of a polymer additive is PVP. Polyvinylpyrrolidone is normally added to the casting solution to prevent or reduce the formation of macrovoids (Susanto & Ulbricht, 2009). The polymer delays demixing and results in formation of membranes with higher water fluxes, water adsorption and lower contact angle than the bare membrane without PVP. Another polymer additive used was β-cyclodextrin polyurethane (β-CDPU) in polysulfone (PSf) membrane. This modified membrane showed 30% better rejection in of natural organic matter and a ten times better resistance against fouling by natural organic matter, while the flux increased from 12 to 137 L.m-2 (Adams et al., 2014). A last example of successful blending is provided by Kalaiselvi et al. (2013). Poly 6-methyl 2-vinyl pyridinium sulphate (PMVPS) was incorporated in a polysulfone membrane with the goal of increasing rejection of heavy metal ions. The pure water flux was 9 times higher after modification and the rejection of metal ions like Cu2+, Ni2+, Pb2+ and Cd2+ increased from an average of 80% to 95%. The blending technique has provided good results for different purposes as increased water flux, increased rejection and reduced fouling. A new generation of membranes that is getting more attention is incorporating nanoparticles. These membranes are often referred to as nanocomposite membranes and will be highlighted in the next section.

2.4.3 Nanocomposite Membranes From all the different membrane modification possibilities, nanocomposite membranes have received considerable attention recently. Nanoparticles are added to this type of membrane through blending with the casting solution before the phase inversion or as surface modification afterwards (Vatanpour et al., 2012). The surface modification is achieved by dip coating the membrane in a suspension of nanoparticles or by adding nanoparticles in the reaction bath during interfacial polymerisation (nanoparticles will end up in the thin film). The possibility to make nanocomposite membranes in easy, convenient conditions (both dip coating as phase inversion do not involve difficult/complex processes) make them attractive to investigate for enhanced membrane performance. Nanoparticles are interesting because they have unique physico-chemical properties that are setting them apart from the bigger, bulk materials. These properties include high specific surface area, high reactivity and strong sorption (Qu et al., 2013). Incorporation of bulk materials like zeolites and SiO2

has been practiced for a while. The resultant mixed matrix membranes have shown to have improved membrane permeabilities and reduced fouling behaviors. A zeolite filler was able to increase the water flux with 164% of a polysulfone membrane and it showed 60% less flux decline after fouling compared to the control membrane (Leo et al., 2013). A similar approach is adopted to form nanocomposite membranes. Instead of using bulk molecules, nano-scale materials are utilised. These smaller materials can enhance the interaction between polymer and particle because the nanoparticles can be better distributed throughout the membrane. This makes the membrane more fouling resistant and strongly enhance water flux (Li et al., 2009).

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The nanoparticles also offer the possibility to add certain functionalities to the membrane. Materials with photocatalytic properties have been incorporated in membranes (Bergamasco et al., 2011). Membranes with TiO2 nanoparticles, which posses photocatalytic activity, have been used for the purpose of reducing fouling with the assistance of UV light. After radiation with UV light the TiO2 incorporated membrane had groups of active oxidant reagents on their surface with the decomposition and removal of foulants as a consequense (Madaeni et al., 2007). UV light and TiO2 were also used to clean a membrane after biofouling. The membrane with the photocatalyst was able to completely clean the membrane while assisted by UV light. When only UV light was used, 35% of the bacteria (E. Coli) were still on the membrane (Kim et al., 2003). Antimicrobial nanoparticles have also been used. Membranes with these materials have shown to be resistant to biofouling (Zhang et al., 2012). This was done by testing the attachment of cultures of Escherichia coli and Pseudomonas aeruginosa. The membranes implemented with silver nanoparticles were able to resist biofilming forming on the membrane surface during a 9 week test, while the control membrane was heavily fouled. Other materials that can be used at nano-scale include but are not limited to silica, carbon nanotubes, alumina and gold (Qu et al., 2013). Caution has to be kept when doing research on nanocomposite membranes. There is currently a lack of knowledge about the leaching behavior of nanoparticles from these membranes and the (eco)toxicity of the nanoparticles when released to the environment (Kim & Van der Bruggen, 2010). Their bioavailability might be enhanced because of their small size. There is evidence of the effect of engineered nanoparticles on different organisms. E. Coioides, a very important aquaculture species in China and east Asia, had observed tissue damage due to copper nanoparticles (Wang et al., 2014). Silver nanoparticles are also toxic, they can induce mitochondrial membrane damage in C. elegans (Ahn et al., 2014). In addition, the effect of nanoparticles properties on their toxicological effect and methods to measure their concentrations remain unknown. This means that while conducting research on nanocomposite membranes, the release and potential release of nanoparticles to the environment should be minimised. Several materials have been used in literature for synthesis of membranes. In the next section we will briefly look at some of the materials reported in literature and discuss some of the advantages of using them.

2.4.4 Materials of interest In this section a couple of materials will be highlighted. This includes the polymer that will be used to produce the flat sheet membrane that is going to be modified with additives.

2.4.4.1 Polyethersulfone

Polyethersulfone [PES or poly(oxy-4-phenylenesulfony-4-phenylene)] is a polymer with a molecular structure shown in the Figure 2.1.

Figure 2.1: Molecular structure of PES (Ahmad, et al., 2013)

Polyethersulfone is available in many different forms like a resin or powder. It has a glass transition temperature of 220°C (Lecouvet et al., 2013). It is well known for its heat resistance (so it stays stable under elevated temperatures), chemical resistance and strength in comparison with other polymers. It retains its properties throughout a broad temperature range (Aurilia et al., 2010). Polyethersulfone is a good electrical insulator, very good at retarding fire and is resistant to hydrolysis (Lecouvet et al.,

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2013).The polymer is resistant against water and most aqueous acids, bases and other inorganic solutions. It is only affected by concentrated oxidising acids like nitric and sulphuric acid (Saunders). Materials out of PES can be cleaned using most of the typical halogenated cleaning solvents, making them interesting for use in membrane applications. Contact of PES with aromatic solvents should be avoided. Some will only influence the PES after long exposure (benzene, xylene and toluene). Polar aromatic solvents are the main solvents for PES, so contact needs to be avoided if dissolution of PES is not desired. If sterilisation of the PES is required, typical sterilisation solutions can be used. Also steam is an option, because PES is resistant against hot water(BASF, 2007). A last option is the use of radiation such as beta, gamma and X-rays. As a polar polymer PES can be dissolved in solvents like N,N-Dimethylformamide, Dimethyl Sulphoxide and N-Methylpyrrolidone (Saunders). Polyethersulfone is widely used in many different applications because of the good properties it has. It is found in relay bases, switch bases, fuse cases, valve joints in hot water environments and gearbox bearing retainers. It is also used in the aircraft industry, automobile industry, medical equipment and electrical/electronic parts (Li et al., 2003). The most interesting regarding the subject of this literature review is the use of PES for the formation of separation membranes. In the biomedical field these membranes are found in devices that are used for blood purification like hemodialysis and hemodiafiltration (Zhao et al., 2013). Polyethersulfone keeps its interesting properties as a membrane. Membranes formed from PES have outstanding oxidative and mechanical stability. The membranes are inert and stable in water (SumitomoChemical, 2010). Therefore the use of PES for membrane formation (to be used in water treatment) is a good choice (BASF, 2007). However, PES membranes are a classic example of easily fouling, hydrophobic membranes with low water fluxes in wastewater treatment. This is the major disadvantage of using PES for membrane synthesis. This problem can be solved if the PES membranes are modified to enhance their performance in water treatment (Zhao et al., 2013). There are other polymers that can also be used for membrane fabrication. In the next section we will discuss polyvinylpyrrolidone and look at its application in membrane synthesis for water treatment.

2.4.4.2 Polyvinylpyrrolidone

Polyvinylpyrrolidone (PVP) is a linear nonionic polymer made from acetylene and formaldehyde as starting products. After a chain of reactions vinylpyrrolidone is formed which can be polymerised to N-vinylpyrrolidone-2(1-(2-oxo-pyrrolidinyl)-ethylene) (Folttmann & Quadir, 2008). Figure 2.2 shows the structure of PVP. Because of the polymerisation process used in the production, PVP is available in different mean molecular weights (Haaf et al., 1985). It has a high polarity and is amphiphilic. Polyvinylpyrrolidone has some interesting properties: it is soluble in all the conventional solvents (water and other polar solvents), it is adhesive and has good binding powers and it possesses excellent film formation ability and affinity to hydrophilic and hydrophobic surfaces (ISP). The polymer is also good in forming

Figure 2.2: Structure of polyvinylpyrrolidone (Haaf, Sanner, & Straub, 1985)

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complexes. In the food industry it is used to stabilise beverages and in the pharmaceutical industry PVP is also a binder, found in coating, solubiliser for suspensions. Polyvinylpyrrolidone is able to enhance the bioavailability of the products which it is complexed with or coated on (Folttmann & Quadir, 2008). This polymer is a versatile molecule with a wide range of excellent properties and a large number of different applications. Over the years it has found its way to the membrane sector where it is now mainly used as an additive to casting solutions. The polymer typically serves as a pore forming agent. In the area of membranes, PVP is used in many membrane types for different kinds of applications like desalination, hemodialysis and gas separation (ISP). Due to its good compatibility and cross-linking properties, its ability to complex with a broad variety of compounds and due to its strong polar character and hydrophilicity, PVP is used to improve the hydrophilic characteristics of hydrophobic membranes. When added to casting solutions used for the synthesis of PES membranes, PVP is reported to increase pore size and porosity while suppressing the formation of macrovoids (Susanto & Ulbricht, 2009) (hence the classification of PVP as pore forming agent). However, despite the pore forming abilities, the opposite has also been observed in literature: depending on the polymer used for membrane synthesis, the type of solvent used, and the concentration and molecular weight of the pore forming agent, together with the conditions under which the phase inversion process was conducted, enlargement of the macrovoids instead of suppression of macrovoid formation was observed (Susanto & Ulbricht, 2009). Addition of PVP has resulted also in more remarkable results. With the desired effect of increased water permeability, the membranes became less performing in rejection of solutes due to increased molecular weight cut off (MWCO). Without the addition of PVP, the MWCO of typical PES membranes is lower. With increasing additive content, the membrane tensile strength decreases. This is due to the significant influence of PVP on the morphology and mechanical properties of the membrane. This is typically a consequence of using higher PVP concentrations that suppress macrovoid formation which is problematic for the strength of the membrane and its operation under high pressures (Al Malek et al., 2012). In other literature reports, a trade-off was observed between decreasing rejection (of proteins) and increasing flux when using PVP (Vatsha et al., 2013). However, due to the addition of PVP, the degree of fouling by proteins decreased slightly. When using PVP, it is clear that there is a threshold on the amount of PVP that can be added to the casting solution. When adding higher PVP concentrations, the positive effects of PVP are lost and a more dense membrane, and less hydrophilic (i.e., more hydrophobic) membrane is formed which can lead to declining water flux (Vatsha et al., 2013). When PVP is added as a pore forming agent, caution has to be taken to not exaggerate the amount so that advantageous effects are observed and not the adverse ones.

2.4.5 Nanoparticles As previous mentioned nanoparticles implemented in a membrane are able to enhance membrane performance. There are a couple of nanoparticles who are very interesting in this regard, but didn't receive much attention in research for their application in nanocomposite membranes. These nanoparticles will be used for the nanocomposite membrane in this thesis. Why they are so interesting, will be discussed in the next sections.

2.4.5.1 Graphene Oxide

Graphene oxide (Figure 2.3) is a two-dimensional nanoparticle. It possesses on its surface a variety of functional groups like epoxy and hydroxyl groups on the basal plane and carboxylic acid groups on the edges of the sheet. It is a highly oxidised, single atom thick layer of graphene (Goenka et al.,

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2014). Although there are a lot functional groups on the surface of the sheet, graphene oxide keeps a significant amount of carbon that is still sp²-hybridized as backbone (Rattana et al., 2012). This gives the graphene oxide its planarity and a high surface:volume ratio.

Apart from the functional groups, the basal plane of graphene oxide also has some free surface π electrons. These electrons are originating from the parts where the graphene structure keeps unmodified and makes graphene oxide a very good conductor (Goenka et al., 2014). Graphene oxide nanoparticles find a lot of applications in the biomedical sector because of their good biocompatibility. They are used for drug delivery and biosensing purposes (Hong et al., 2011). Recently they have been used as additives in membranes for water purification purposes. The use of graphene oxide in water treatment has already been proven efficient as an adsorbent by second-order mechanism giving monolayer adsorption of dyes (Zhang et al., 2013). Graphene oxide is quite hydrophilic because of the abundant oxygen containing functional groups. This makes it ideal for the purpose of membrane modification. There are already some research reports on the implementation of graphene oxide in polyethersulfone and polysulfone (PSf) membranes for water treatment purposes (Ganesh et al., 2013; Lee et al., 2013; Zinadini et al., 2014). Incorporation of these nanoparticles caused an overall performance enhancement of the membranes. As expected, the pure water flux was significantly higher for the nanocomposite membrane. Lee et al. reported a change in contact angle of their polysulfone membrane from 92° to 82°, while the flux increased with a factor two. Even the rejection salts improved up to 30% with the addition of GO to the membrane while the contact angle decreased from 72° to 55° (Ganesh et al., 2013). Zinadine et al. (2014) observed an improved dye rejection for a PES GO of 10% so that the dye was almost completely rejected compared to the membrane without GO. The improved performance by incorporation of GO is not only due to the higher hydrophilicity, but also to the electronic repulsion caused by the presence of the functional groups on the GO. The addition of GO to the membranes has been reported to give significant improvement in resistance to biofouling. Because of the increased surface hydrophilicity the biofilm thickness decreased from 107 µm to 55µm ( Lee et al., 2013). Not only was the membrane resistant to fouling, also the cleaning of a fouled membrane (by a milk powder solution) gave a higher flux recovery after fouling from 20 to 80% as observed by Zinadini et al. (2014). The addition of GO also had a significant impact on the change of the pore morphology and surface roughness of the membrane. The reason was the enhanced demixing rate during the phase inversion process due to thermodynamic instability caused by the presence of GO. Larger, flux enhancing pores were formed because of this. Adding too much GO increased viscosity of the casting solution which had the opposite effect. GO addition also improved the strength of the membrane shown by a increase of Young's modulus from 150 MPa tot 185 MPa (Lee et al., 2013). The conclusion of all this is that GO is a very interesting molecule that can be added into a membrane casting solution to improve membrane performance.

Figure 2.3: Molecular structure for GO (Hong et al., 2011)

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Lee et al. (2013) added GO in a PSf-NMP casting solution and dispersed it well before casting the membrane using the phase inversion technique. Ganesh et al. (2013) chose to first disperse the GO in a little bit of the solvent before adding it to the casting solution. After good mixing of the nanoparticles in the casting solution, the membrane was made. The authors also used the phase inversion process. Zinadini et al. (2014) followed a similar method. Before phase inversion, the GO must be well dispersed in the casting solution. No difficulties regarding this membrane modification by blending was reported. These three methods all gave improved membrane performance as discussed further below. There are other nanoparticles that can also be used as membrane additives. In the next section we will discuss the use of zinc oxide as a membrane additive for improved performance.

2.4.5.2 Zinc Oxide

Zinc oxide (ZnO) is a molecule that has been used in industry for decades because of its interesting properties. In the rubber industry, ZnO finds its major application as a vulcanizing activator which reduces the time and the amount of sulfur used in the vulcanization process (Moezzi et al., 2012). Adding ZnO to rubber also increases its lifetime. In the ceramics and concrete industry ZnO is used as an additive to improve the elasticity of glazes which prevents crazing and shivering. Many other industries with major ZnO applications are pigments, cosmetics, medical, catalysts, oil and gas well drilling fluids. As an n-type semiconductor with a wide band-gap of 3,37 eV (Bai et al., 2012), and with excellent physical and chemical properties (such as high electrical and thermal conductivity, optical absorption in the UV region, high photosensitivity, excellent electric properties, high thermal stability, stability at neutral pH and antimicrobial action) ZnO is an interesting photocatalyst. It is also readily available because it is already used in many industries, which also reduces its price (Moezzi et al., 2012). Important to notice regarding the issues reported on nanoparticles and their toxicity is that ZnO is non-toxic (Li et al., 2012). The size distribution and surface area of ZnO have been shown not to be related to a change in toxicity. The toxicity of zinc in general has been attributed to its dissolved form and ZnO nanoparticles tend to aggregate in water forming flocs with increased size and decreased bio-availability (Balta et al., 2012). This makes ZnO quite safe to use. In environmental applications ZnO has proven to be useful as a photocatalyst to degrade organic pollutants in water and air (Li et al., 2012). The compound can offer the necessary driving force for reduction and oxidation. Recently, a new application for ZnO was looked into. This new outlook was based on the use of ZnO nanoparticles for the enhancement of membrane properties. The previously mentioned attractive features make ZnO an interesting choice for fill in the objectives that need to be met by nanocomposite membranes. Its lower cost and better/equal photocatalytic performance compared to similar nanoparticles like TiO2 makes ZnO makes it a good candidate for nanocomposite membranes (Balta et al., 2012). The addition of ZnO in (very) low concentrations (starting from 0,035%) gave an improvement of the permeability, fouling resistance (20% higher relative flux) and 20% higher rejection of dyes with a MW between 200 and 450 Da for phase-inverted PES membranes (Balta et al., 2012). Briefly, the membrane was casted after addition and dispersion of the ZnO nanoparticles in the casting solution, similar like the method used for preparing graphene oxide containing membranes. The rejection of salts stayed the same while the permeability increased. This was due to the increased hydrophilicity of the ZnO-PES membrane, as shown by the decrease in contact angle from 80° to 55° formed between the membrane and a droplet of water. The increased fouling resistance was indicated by a lower flux decline of only 7,8% when the membrane was fouled by BSA (Shen et al., 2012). Also an increase in porosity from 73% to more than 80% was observed. Addition of ZnO improved the

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thermal stability of the membrane, as shown by a higher thermal decomposition temperature (Balta et al., 2012; Shen et al., 2012). It can be concluded that ZnO nanoparticles are perfect competitors with other metaloxide nanoparticles in a lot of applications. They can be used in membrane applications where fouling is a major problem. Their cost-effectiveness and anti-fouling properties make them good candidates for application in synthesis of antifouling membranes.

2.4.5.3 Graphene Oxide/Zinc Oxide nanohybrid

Due to the advantages of one kind of nanoparticles over the other, trying to combine two different nanoparticles has been rising to improve the properties of the nanoparticles. This section will discuss the combination of GO and ZnO in a new type of nanohydride particle with ZnO implemented on the surface of GO sheets. ZnO has limitations in its catalytic efficiency which also limits its performance in photo-catalyst applications. These limitations are caused by the fact that the recombination of the charge carrier occurs in a very short time frame, as short as nanoseconds (Fu et al., 2012; Li et al., 2012). This quick recombination is faster than the redox reactions that occur at the surface of the photocatalyst. If the recombination could be retarded, the efficiency and the applications would be enhanced strongly. This would continue the production of reactive oxygen species that can degrade organic pollutants. A second disadvantage of ZnO as photocatalytic NP, is that visible light is not useable because of the threshold in band edge absorption. For this reason, most research has been looking in to the use of heterogeneous photocatalysts when using ZnO. Modifications with noble metals, polymers, metal ions and semiconductors or electron accepting materials already gave improvements. Graphene oxide provides good semiconductive properties. Because of the p-conjugated and oxygenated sp2-domains, GO provides good mobility of charge carries and transport of electrons while having a large surface area as mentioned earlier (Chen et al., 2013). With its reactive functional groups on the surface, it is easy to use GO for chemical functionalisation and production of hybrid materials (Fu et al., 2012). These GO hybrids are reported to increase adsorption of pollutants, extend the light absorption range and have efficient charge transport and separation. Zinc oxide has been combined with GO to increase photocatalytic efficiency. This property is increased by reducing the recombination of the charge carriers, increasing the surface area and extending the light absorption range. Effective electron transmission from ZnO in an excited state to GO was confirmed (Wang et al., 2011). The GO ZnO was tested on its ability to degrade dyes, proving its application as a photocatalyst. GO ZnO was able to completely reduce methylene blue under UV radiation in 40 minutes while ZnO reduced only 50% after 100 minutes and GO 20% after 100 minutes (Li et al., 2012). It was found that GO ZnO showed enhanced photocatalytic activity compared to ZnO. The intimate contact between the GO and the ZnO was believed to enhance the adsorption property, the electron transfer from excited ZnO to the GO sheets and activation of the degrading dyes due to interaction between the dye and the GO. The GO is involved in the facilitation of the charge collection and transport in the GO ZnO composite (Wang et al., 2011). The nanohybrid works as a photocatalyst, even when under visible light.

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Figure 2.4: Example of the production route for GO ZnO nanoparticles (Li et al., 2012)

Figure 2.4 is showing a possible synthesis route for GO. Generalised the following steps are followed. Graphite is first oxidised and exfoliated to form GO. With the addition of a Zn-salt and reacting this on a high temperature, bonds between Zn2+ and the reactive functional groups are formed. The annealing of this product makes the ZnO crystallise on the GO giving the desired end product. Different researchers have followed this global approach with only slight differences like choice of Zn-salt (Chen et al., 2013; Li et al., 2012; Liu et al., 2011). As shown above, GO ZnO nanohybrids have improved properties compared to the GO or ZnO on its own. Therefore, it may be hypothesised that incorporation of this hydrid into PES membranes would yield membranes with improved properties. This is based from the fact that the doping of a PES membranes with GO or ZnO have already shown good improvements compared to PES membranes without the addition of additives. It is possible that a PES-GO ZnO membrane may be hydrophilic and foul less. However, so far, no GO ZnO nanocomposites have been used in membrane manufacturing yet. Other GO-nanohybrids have already found its way towards nanocomposite membranes. GO TiO2 nanohybrids have already been used in membranes. The GO TiO2membranes proved to be multifunctional with high tensile strength, flexibility and anti-fouling properties. They also had high water fluxes and degraded (photo degradation) dyes and humic acid (Gao et al., 2013). Besides TiO2, gold was used to form a GO hybrid (GO-Au) which was implemented after the nanoparticle synthesis in a polysulfone membrane. This membrane had major improvements in permeability with a factor 70 and rejection with a factor 2. Gradual reducing tensile strength of the membrane was observed with increased loading, making the membrane weaker (Crock et al., 2013). As such, it could be hypothesized that the incorporation of GO ZnO nanocomposites in PES/PSf membranes could improve their performance, also at lower cost than other nanocomposites.

2.5 Gaps in knowledge – goal of the thesis Although both NPs (ZnO and GO) separately have beneficial properties, a hybrid between these two has not been looked into as a potential solution for the future. In the previous section it was already discussed that ZnO and GO are enhancing their properties when combined in a hybrid nanoparticle. If the GO ZnO nanohybrid will be applied to develop a nanocomposite membrane, this would be a novel membrane because it has never been done before. Previous section showed that other hybrid

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nanoparticles based on graphene oxide already have successfully been used in membrane technology. It is clear that the performance of membranes needs to be improved to minimise drawbacks such as high fouling tendency, low rejection and low fluxes. Polyethersulfone membranes are easy to manufacture, cheap and robust, but they are facing these drawbacks (in terms of fouling and low flux) because of their hydrophobic nature. The good properties of PES membranes are currently outweighed by the bad ones resulting in the need for some modifications. Addition of nanoparticles could be a very interesting option here because selected NPs are known to enhance membrane permeability without decreasing the rejection of solutes, while also being capable of showing some inherent functionalities (like photocatalytic activity). Combining the need for modification of the PES membranes, the rising interest in nanocomposite membranes and the novelty of the GO ZnO nanohybrid, makes this an interesting subject for research. As last perspective the influence of a combination of different additives is also lacking. That is why it is interesting to add also a pore forming agent like PVP to the membrane together with the nanoparticles with the objective to try to enhance performance even more.

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3. Materials and method

In this section, a brief description of the materials and methods used in this thesis will be highlighted.

3.1 Materials Table 3.1 shows a list of materials used in the synthesis of the graphene oxide and graphene oxide - zinc oxide nanoparticles (see section 3.2) and the membranes/nanocomposite membranes (see section 3.3). The different suppliers of the materials are noted too.

Table 3.1: List of materials and chemicals used

Material Supplier

Graphite (<20µm, Synthetic grade) Sigma Aldrich, Switzerland H2SO4 (95% solution) VWR Chemicals BDH Prolabo, Australia NaNO3 VWR Chemicals BDH Prolabo, Australia KMnO4 Union Chemique Belgium H2O2 (30% solution) Chem-Lab NV, Belgium HCl (32% solution) VWR Chemicals BDH Prolabo, Australia Zn(NO3)2.6H2O (≥ 99%) Sigma Aldrich, Croatia NH3.H2O ( 25% solution) VWR Chemicals BDH Prolabo, Australia Polyethersulfone (PES) Veradel 3000P Solvay, Belgium Polyvinylpyrrolidone (PVP) Sigma Aldrich, China N-methyl-2- pyrrolidone (NMP, 99% solution) Sigma Aldrich, Netherlands

3.2 Synthesis and characterisation of Graphene oxide - Zinc oxide nanoparticles

3.2.1 Synthesis of Graphene Oxide (GO) nanoparticles Graphite was used as a precursor material for the synthesis of the graphene oxide nanoparticles. By oxidation of graphite, graphene oxide is formed. For this oxidation a modified Hummers method was used (Hummers et al., 1958). 8 g of graphite grains was put in a 1 L beaker together with 6 g of sodium nitrate (NaNO3). The beaker was put in an ice bath and 270 mL of a 95% sulphuric acid (H2SO4) was added. The ice bath is necessary to control the general temperature of the H2SO4- graphite dispersion at low temperatures. It will take the heat produced by the chemical reactions away from the dispersion. The dispersion was stirred vigorously and 36 g of potassium permanganate (KMnO4) was added gradually over 1h. It is important to do this gradually and in the ice bad to keep the temperature low. This is because the exothermic reaction in Figure 3.1 generates a lot of heat.

Figure 3.1: Scheme from the reaction of potassium permanganate in sulfuric acid to diamanganese heptoxide (Dreyer et al., 2010)

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The suspension was continuously stirred in the ice bath for an additional 2 h after which it was taken out of the ice bath and stirred at room temperature for 5 days to complete the oxidation reaction as far as possible. An aqueous solution of 5% H2SO4 (400 mL) was added over a period of 1 h after the 5 days of stirring. After the addition of the 5% H2SO4 the solution was heated gradually over a period of an hour to a temperature of 98 °C. Upon reaching this temperature, the suspension was stirred for 2 h. Finally the suspension was cooled down to room temperature and 80 mL of 30 wt% hydrogen peroxide (H2O2) solution was added, followed by stirring for 2 more hours. The H2O2will react with the excess of KMnO4 (will be reduced) that is still present for easier purification purposes. After this the graphene oxide was separated by centrifugation (Sigma 3-16P) at 4200 rpm for 20min. The obtained graphene oxide was washed several times to remove impurities like the ions that were still present from all the reagents used in the oxidisation reaction. To wash the graphene oxide the supernatant was decanted and replaced with an aqueous solution of 5% hydrochloric acid (HCl). The graphene oxide was suspended again and shaken vigorously, after which it was centrifuged again. This cycle was repeated 3 times. The washing cycle was then repeated 3 more times with deionised water instead of the HCl solution to remove leftover acid. Once the graphene oxide was washed, it was kept in an oven at 60°C until it was dry (18 hours).

3.2.2 Synthesis of Graphene Oxide – Zinc Oxide Nanoparticle hybrid The synthesis of the nanohybrid material started with dispersion of 0,5 g of the previously produced and dried graphene oxide in 200 mL of deionised water. Next, the graphene oxide was exfoliated using supersonication for 30 minutes. After this, 0,2 M zinc nitrate (Zn(NO3)2) was added dropwise to the graphene oxide suspension under heavy stirring to make sure that there was good contact between the graphene oxide and the zinc salt. This suspension was ultrasonicated one more time for 30 minutes to optimise the contact between the zinc and the graphene oxide sheets. The suspension was then stirred for 2h. While stirring, the pH of this suspension was checked and adjusted to pH 7 with a 25% ammonia solution (NH4OH) to avoid acidic catalysed reduction on elevated temperatures. After the stirring the suspension was kept static at 90° C in the oven for 9 h to improve the reaction. After the reaction was complete, the suspension was centrifuged at 4200 rpm for 30 minutes and the supernatant was decanted. The graphene oxide - zinc oxide (GO ZnO) nanohybrid was dried overnight at 80 °C in the oven. When the nanocomposite was dry, it was calcinated in a furnace for 2,5 h at 500°C to crystallise the ZnO on the GO and give the final product. The products were then characterised as described in section 3.2.3.

3.2.3 Characterisation of GO and GO ZnO nanoparticles The nanoparticles were characterised using TEM, EDS, Raman and FTIR. TEM was performed with a FEI Tecnai T12 TEM (USA), with EDS option. The samples were prepared for analyses by coating them with copper. Raman spectroscopy was performed using the Raman Micro200 (Perkin Elmer, USA) with a 750nm laser. A small amount of the product was put with the tip of a spatula on a glass slide. This glass slide was put under a microscope (Olympus BX51, Japan), with which the product could be viewed at the desired magnification. Than the laser was applied through the microscope on the sample of the product, during which the sample is analysed for its Raman scattering spectrum. The Fourier transform infrared spectroscopy (Perkin Elmer, FT-IR 100, PerkinElmer Inc, USA) was utilised to record infrared (IR) spectra of the nano materials in the range of wavenumber 800 – 4000 cm-1 at 25 ˚C. This was done for graphite, graphene oxide and graphene oxide - zinc oxide. A small amount of the product (tip of a spatula) was put on the sample holder and then pressure was applied to the sample with a pressure arm.

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3.3 Membrane synthesis and characterisation After characterisation of the synthesised nanoparticles, the novel membranes (both with and without nanoparticles) were synthesised using the phase inversion method. The membranes were then characterised and tested for some parameters. The next section describes the steps taken in synthesis of the nanocomposite membranes followed by a few characterisation techniques and their application in filtration.

3.3.1 Membrane synthesis Eight different casting solutions were made to give 8 different membrane types. The content of the casting solutions is found in Table 3.2. The amount presented in the table is the weight percentage of each component in the total weight of the casting solution. Each casting solution contains 22 wt% polyethersulfone (PES). Polyethersulfone was chosen as base material because of its excellent properties that still apply when PES is transformed to a membrane (see Literature Review). The load of GO ZnO nanoparticles in the casting solution varied from 0 to 0,2%. This concentration range is chosen because concentrations of nanoparticles in a casting solution don't need to be higher to observe a significant effect in the performance of the membrane. This is shown for zinc-oxide (Balta et al., 2012; Shen et al., 2012) and graphene oxide (Zinadini et al., 2014; Lee et al., 2013). Within this range it is expected to see an improvement as well for the membrane with the GO ZnO nanoparticles. To investigate the influence of polyvinylpirrolidone (PVP) as pore forming agent on the membranes, all membrane types were casted with and without PVP. When PVP was added to the casting solution, its concentration was 4 wt%. This concentration was chosen to achieve the positive effect of PVP, without having any risk of getting adverse effects due to a too high PVP concentration in the casting solution (see Literature Review). N-Methyl-2-pyrrolidone (NMP) was added with an amount to achieve a total of 100% based on an initial set weight for the casting solution. Using the density of NMP (1,028 g.mL-1), the weight could be converted to a volumetric amount so that NMP could be added easily with a pipette.

Table 3.2: Content in wt% for the eight different casting solutions

The preparation of the casting solutions was performed in two separate steps. First, a solution is made with PES (and if needed PVP) in NMP. This solution is stirred until the polymers are completely dissolved in the solvent. In a second step, the dispersion of the nanoparticles in a little bit of NMP solvent (5 to 10 mL) is prepared. This dispersion is ultrasonicated for 30 minutes and continuously stirred vigorously and then added to the polymer solution. This will eventually become the casting solution. The casting solution is stirred until the nanoparticles are well dispersed. This casting solution is then put in a sealed recipient and left to rest overnight to let the air bubbles settle out of the casting solution.

Membrane number PES PVP NMP GO ZnO

I 22 4 73,8 0,2 II 22 0 77,8 0,2 III 22 4 73,95 0,05 IV 22 0 77,95 0,05 V 22 4 73,9875 0,0125 VI 22 0 77,9875 0,0125 VII 22 4 74 0 VIII 22 0 78 0

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Once the casting solution is prepared, the membrane can be casted using a glass plate, a casting knife (Elcometer 3530, Elcometer, Belgium) and a automatic film applicator (Elcometer 4340 Automatic film applicator, Elcometer, Belgium). The casting process starts with cleaning and drying the casting knife and the glass plate thoroughly so that no artefacts will end up in the membrane. A coagulation bath with deionised water at room temperature is prepared so that the glass plate with casted polymer solution can immediately be immersed for the phase inversion to occur. The casting solution is spread out on the glass plate and the film applicator and casting knife are used to spread the solution out evenly with a thickness of 200 µm. This glass plate is put immediately in the coagulation bath so that the phase inversion process can start. The membrane will form and will be kept for 15 minutes in this coagulation bath. To make sure to the phase inversion occurs to completion, after casting the membranes are kept in airtight bags filled with water that gets refreshed frequently. The membranes are ready to use after storage in the bags for at least 12 hours. Some important remarks about the above described method are listed here:

Air bubbles need to be completely removed from the casting solution because they can

influence the phase inversion process in a significant and unpredictable way.

Everything from the recipient to the glass plate and casting knife with which the casting

solution comes into contact needs to be completely dry. If not, phase inversion will occur on

the wet spots and the membrane will be flawed.

The membranes that are made can be classified according to their composition. This will reflect in terminology used in the next sections of this thesis. The membranes without nanoparticles will be addressed as bare PES (membrane VIII) or bare PES PVP (membrane VII) membranes. Nanocomposite PES membrane (membrane II, IV and VI) or nanocomposite PES PVP membranes (membrane I, III and V) will be used when talking about the membranes with GO ZnO in different loadings. PES membranes (membrane II, IV, VI and VIII) and PES PVP membranes (membrane I, III, V and VII) are the membranes with and without nanoparticles. These last two are used to make a separation between the addition or absence of PVP in the membrane.

3.3.2 Membrane characterisation After synthesis, the membranes were characterised for hydrophilicity/hydrophobicity and surface tension components by measuring contact angles using a goniometer (DSA 10-MK2, Kruss, Germany). Contact angles were measured using the sessile drop method. Three types of liquids, Milli Q water, glycerol and diiodomethane, were used. For each membrane, 10 drops per liquid were placed on the surface of each membrane sample using a microlitre syringe and the contact angle between the drop and the membrane was measured by means of video imaging. From the measured contact angles (± standard deviation), surface tension components were calculated using the Derjaguin-Landau-Verwey-Overbeek theory (Brant & Childress, 2002). The membranes were also characterised as hydrophilic (with contact angles < 90°) or hydrophobic (with contact angles > 90°) based on the contact angle measured with water. Generally, the total surface tension component of any material is the sum of the Liftshitz-van der

Waals component ( ) and Lewis acid-base components

(

with

being the electron acceptor and electron donor, respectively:

(3.1)

Measurement of contact angles (θ) with three liquids of known surface tension components makes calculation of

possible through:

(

(3.2)

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where: subscripts s and l are the solid surface and test liquid, respectively. For calculation of the interfacial free energies, it is necessary that contact angle of the surface of interest is measured with three liquids of well characterised surface tension components. Table 3.3 shows surface tension components of water, diiodomethane and glycerol. These liquids were used in this thesis for measurement of contact angles and calculation of surface tension components as well as interaction energies.

Table 3.3: Surface tension components of water, diiodomethane and glycerol (Brant & Childress, 2002)

Term Water (mJ.m-2) Diiodomethane (mJ.m-2) Glycerol (mJ.m-2)

72,80 50,80 64,00

21,90 50,80 34,00

51,00 0,00 30,00

25,50 0,00 3,92

25,50 0,00 57,40

3.3.3 Surface and Cross-sectional morphology Surface and cross-sectional images of the membranes were obtained using a scanning electron microscope (JSM 7600 FESEM, JEOL USA). An irradiation beam of15 KV was applied. The membrane was first dried in a desiccator for at least 24 h until completely dry and gold coated before analysis to make the conductivity of the membrane for the electrons optimal. Samples were sputter-coated with gold using an SCD 005 Cool Sputter Coater (Bal-Tec, Germany) at a current of 25 µA for 50 s.

3.4 Membrane filtration tests

3.4.1 Filtration setups Two types of membrane filtration setups were used for membrane characterisation: a dead-end and a cross-flow setup.

3.4.1.1 Dead-end Setup

The scheme for the dead-end setup can be found in Figure 3.2. The dead-end cell was only used for quick pure water permeation and salt rejection tests of the synthesised membranes. The feed for this setup is put in the cell. If stirring is required, a magnetic stirrer can be put in the dead-end cell so that the feed stays homogenous. The membrane is in a special compartment at the bottom of the cell. It has an contact area of 8,04 cm² with the feed solution. Nitrogen gas is used to apply the necessary operating pressure on the liquid column in the dead-end cell. Different types of feed solutions are used, depending on the type of test. More details on the experiments done with this setup will be provided below.

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Figure 3.2: Membrane filtration dead-end setup: 1 - Nitrogen gas cylinder; 2 - dead-end cell; 3 - stirrer; 4 - membrane sheet; 5 - magnetic stirrer; 6 - permeate collection vessel;

3.4.1.2 Cross-Flow Setup

A schematic representation of the cross-flow setup can be found in Figure 3.3. The membrane cell was custom made and had a total surface area of 21 cm². This is also the area of the membrane that is in contact with the feed solution. The feed solution is fed from a 10 L tank by means of high pressure pump (Hydracell; Wanner Engineering, Minneapolis, Minnesota). The feed pressure and feed flow are controlled by a needle valve on the concentrate and the feed pump. The feed flow was kept constant for all the experiments. Feed pressure is measured using a pressure gauge (ERIKS, Belgium).The content of the feed solution depends on the test that was running (see below for more details). The permeate was always recovered to the feed solution except when a sample or measurement was taken.

Figure 3.3: Membrane filtration cross-flow setup: 1 - Feed; 2 - Pump; 3 - Inlet Valve; 4 - Membrane Cell; 5 - Concentrate; 6

- Pressure Gauge; 7 - Permeate; 8 - Outlet Valve

3.4.2 Filtration Protocol

3.4.2.1 Dead-end Experiments

All eight types of membranes (see above) were characterised in the dead-end setup for pure water permeability and salt rejection. Before starting the experiments the membranes were compacted at a pressure of 6 bars with deionised water until the flux was stable. After the compaction, pure water fluxes for the compacted membrane were measured using deionised water.

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Different pressures (1, 2, 4 and 6 bars) were used to measure pure water permeability. To calculate water fluxes a specific volume of permeate was collected and the duration of this collection measured. Permeate flux is then calculated by dividing the permeate flow (volume per time)by the membrane area.

(3.3)

Where Jw is the pure water flux, is applied pressure, is the viscosity , is the membrane resistance and Lp is the pure water permeability:

(3.4)

After measurement of pure water fluxes, the feed was replaced with a solution of 2 g.L-1 MgSO4 to measure salt rejection. A pressure of 4 bar was applied for the rejection experiment. The conductivities of feed ( ) and permeate ( ) were measured and salt rejection ( ) was calculated

from Equation 3.5.

(3.5)

3.4.2.2 Cross-flow Experiments

In the cross-flow setup, pure water permeability, salt rejection and rejection of trace pollutants were determined. These experiments were performed with all eight types of membranes. After compacting the membranes at 5 bar until the flux was stable, the pure water flux was measured in a similar way as in the dead-end setup. A lower pressure for compaction was chosen than in the dead-end setup because problems with rupturing of membranes in the cross-flow setup occurred at higher pressures. Flux measurements were carried out at pressures of 1, 3, 4 and 5 bar. After pure water permeability, salt rejection was measured using a 2 g.L-1 MgSO4 solution as feed solution. Salt rejection was tested at 1, 3, 4 and 5 bar. For salt rejection and pure water permeability, calculations were described in the previous section. Rejection of 23 trace organic pollutants (see Table 3.4) was tested at a fixed pressure of 3 bar. The organics were dosed at concentrations of 5 µg.L-1 in Milli-Q water. Samples of feed and permeate were collected after 1 hour of running the trace organic feed solution through the setup. Let the setup run one hour with the trace organics in the feed before collecting is important to condition the membrane for the feed solution. If the collection happens immediately, adsorption of the trace organics on the membrane would influence the measurements significantly. By letting the organics adsorb first on the membrane this variable is cancelled out. The samples taken need preparation for analysis.

27

Table 3.4: Trace organics and their selected physiochemical properties

Compound MW(g.mol-1) Formula Charge (pH=7)

Log Kow Log D

Atrazine 215,68 C8H14ClN5 neutral 2,198 2,2198 Bezafibrate 361,819 C19H20ClNO4 - 4,25 -1,09 Carbamazepine 236,27 C15H12N2O neutral 2,766 2,766 Chloridazon 221,643 C10H8ClN3O neutral 1,106 1,106

Clofibric acid 214,645 C10H11ClO3 - 2,899 -0,630 Diclofenac 296,148 C14H11Cl2NO2 - 4,259 0,730

Diglyme 134,17 C6H14O3 neutral 0,030 0,030 Diuron 233,09 C9H10Cl2N2O neutral 2,533 2,533 Hydrochlorothiazide 297,74 C7H8ClN3O4S2 neutral -0,576 -0,579

Ketoprofen 258,120 C16H14O3 + 3,613 0,084

Lincomycin 406,538 C18H34N2O6S + -0,317 -1,330

Metoprolol 267,364 C15H25NO3 + 1,759 -0,791

Pentoxifylline 278,31 C13H18N4O3 neutral 0,29 0,48

Phenazone 188,2258 C11H12N2O neutral / 0,44

Pirimicarb 238,29 C11H18N4O2 neutral 1,797 1,793 Primidone 218,252 C12H14N2O2 neutral / 0,83

Propranolol 259,34 C16H21NO2 + / 0,79

Simazine 201,66 C7H12ClN5 neutral 1,781 1,781 Sulfamethoxazole 253,279 C10H11N3O3S neutral 0,791 0,151

Terbutaline 225,284 C12H19NO3 + 1,348 -1,031

Theophylline 180,164 C7H8N4O2 - -0,769 -0,809

Triclopyr 256,46 C7H4Cl3NO3 - 2,703 -0,827 Trimethoprim 290,32 C14H18N4O3 +/neutral / 0,47

3.4.3 Trace contaminants and analysis 23 organic pollutants were used in the rejection experiments. These pollutants were selected with a wide range of solute physico-chemical properties to assess their effect on rejection. Trace organic solutes and their properties are shown in Table 3.4. For some trace organics the log Kow-value could not be found. An organic compound is classified as hydrophobic when the log D is higher than 3 and hydrophilic when it is lower than 3 (Dang et al., 2014; Alturki et al., 2010).

3.4.3.1 Solid Phase Extraction

Solid phase extraction was carried out to concentrate the trace organics before analysis. Calibration standards were prepared. The standards were prepared in a way that their concentrations covered the concentration range of organics in permeate and feed samples (0,05 - 0,1 - 0,3 - 1,0 - 2,5 - 6,0 µg.L-1). Internal standards were added to all standards and samples at equal concentrations. The SPE cartridges were first conditioned with 3 mL LC-MSn grade methanol (Sigma Aldrich, Belgium). Standards and permeate and feed samples (all 25 mL) were then loaded onto the cartridges in a drop wise manner. After loading the samples on the cartridges, the cartridges were rinsed twice with Milli-Q water (2,5 mL each time) and vacuum dried for 10 min. Elution was then performed twice under gravity with 4 mL MeOH (LC-MSn grade). After elution all standards and samples were evaporated to 0,7 mL with nitrogen. Evaporated eluates were transferred into 1,5 mL HPLC vials for chromatographic analysis.

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3.4.3.2 Sample Analysis

The concentration of the trace organics was measured using a U-HPLC-HRMS (Benchtop Exactive Orbitrap Massaspectrometer) from Thermo-Scientific (San José, CA, USA).This system exists of an Accelaautosampler (maintained at 15°C) and an Accela 1250 pump which pumps the solvents and injected sample over the column. The injected mixture (injection volume = 10 µl) is separated on a Nucleodur C18 Pyramid (100 mm x 2.1 mm, 1.8 µm particle size) column of Machery-nagel (PA, USA) at 25°C. The solvents used as mobile phase are: 0,08% HCOOH in ultrapure water (solvent A) and MeOH (solvent B), and the following gradient was maintained:

1 minute 98% A, 2% B

3.5 minutes raising the share of solvent B to a composition of 10% A and 90% B

2 minutes raise of the share of solvent B to a composition of 0% A en 100% B

1.5 minute 100% B

1,5 minute back to initial conditions 98% A and 2% B, to prepare the column for the next

injection.

All of this at a flow of 300 µL.min-1. After separation of the mixture on the column, the components were ionized using a H-ESI II (Heated Electro Spray Ionisation) interface, which alternated (every 500 ms) in positive and negative scan modes. The H-ESI II interface was set to a Spray voltage of 4000 V, a Capillary temperature of 250°C and Capillary voltage of 82,50 V. The Sheath gas flow rate was set to 30 arbitrary units, no Auxiliary gas or Sweep gas was used. The Tube lens voltage and Skimmer voltage were set to 120 V and 20 V respectively. The Vaporizer heater temperature was set to 350 °C. For the detection, an Orbitrap HRMS of Thermo-Scientific was used, which operated in a scan range of 100,0-700,0 m/z. Analysis of analytes was done using the Thermo Xcalibur 2.1.0.1140 software package of Thermo-Scientific. The analytes were identified based on the accurate mass of their precursor ions: [M-H+] or [M-H-] adducts, or for diatrizoic acid the [M-NH4

+] adduct. The maximum mass tolerance was set to 5,0 ppm.

3.4.4 Fouling experiments After the filtration experiments, the membranes were tested for their fouling resistance in the cross-flow setup. Two kinds of foulants were selected to investigate their fouling behaviour. They were sodium alginate (Sigma Aldrich, United Kingdom) and Snowtex ST-ZL silica oxide (Nissan Chemicals, USA). Sodium alginate (SA) represented organic foulants while silica represented colloidal foulants. The fouling experiments were carried out as follows: first, the membranes were compacted at 5 bar until the flux was stable using deionised water. Then, the pump was stopped and NaCl and the foulant were added to the feed solution. The amount that needed to be added was calculated in such way that the total salt concentration in the feed was 10 mM and the concentration of the foulants 50 mg.L-1. Depending on the experiment conducted the foulant added was sodium alginate or silica oxide. So in total 16 separate experiments were conducted. All the eight type of membranes were tested on fouling of both foulants (one experiment with alginate and one with silica). New samples of membranes were used for each experiment so that previous conducted experiments wouldn't have any influence on the performance. The fouling experiments were carried out at 3 bar for 6 hours. To investigate the effects of the presence of foulants in the feed solution, the flux were taken after 1h, 2h, 4h and 6h of fouling. The effect of the fouling on the rejection of NaCl was tested by measuring the conductivity of the feed and permeate and calculating the resulting rejection using Equation 3.5.

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4. Results & Discussion

In the following sections the results of the synthesis of the nanoparticles, the membrane synthesis and the experiments using the synthesised membranes will be presented and discussed.

4.1 Characterisation of Nanoparticles Before implementing the nanoparticles in the membranes, confirmation is needed about the nature of the synthesised nanoparticles to confirm if the desired graphene oxide - zinc oxide nanohybrid has been formed successfully.

4.1.1 FTIR The first method used for characterisation was Fourier Transform Infrared Spectroscopy (FTIR). The IR-spectrum was determined for the starting material (graphite), for the graphene oxide nanoparticles and for the graphene oxide - zinc oxide nanoparticles. This indeed gives a nice overview of the synthesis process and the evolution of the functionalisation of the nanoparticles. Figure 4.1 shows these spectra.

Figure 4.1: FTIR spectrum of graphite, graphene oxide nanoparticles and the nanohybrid graphene oxide - zinc oxide

The sprectrum of graphite is showing adsorbance along the whole range, but without showing any real significant peak. Oxidising graphite to graphene oxide clearly made a big impact on the spectrum as expected. After oxidation, a lot of different peaks are showing up on the IR spectrum, which point to the functional groups that are being formed on the carbon backbone structure. Figure 4.2 is showing the peaks more into detail. The most dominant peak is the one around 3450 cm-1, which can be assigned to the -O-H group and its stretching vibration. The carbon - oxygen single bond is clearly pictured by the peak at 1000 cm-1. This C-O stretching vibration is also causing the peak at 1350 cm-1. The carbon-oxygen double bon (C=O) is giving rise to the two peaks at 1750 and 1600 cm-1. This points to carboxylic or carbonyl containing functional groups. That the peaks of C-O and C=O are quite broad is caused by skeletalvibrations in the unoxidised parts of the graphene oxide. There is a

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small peak at 2800 cm-1 that is caused by C-H stretching of in the graphene oxide, but it is not showing up very sharp. This indicates that the amount of C-H groups is reduced due to the oxidation of graphite. Conclusion of this spectrum is that graphite was oxidised succesfully. By exfolliating graphite oxide, graphene oxide is obtained due to the exfolliation of graphite oxide. Figure 4.1 show the spectrum of the end product, GO - ZnO. It is immedeatly clear that the spectrum is of very low intensity and that there is not a lot of absorbance by functional groups. Indicating that the initially formed GO lost its functional groups or that the GO is shield off from the light beam. The more detailed spectrum in Figure 4.2 does show some indications.The peak at 2390 cm-1 is an indication of the presence of CO2 and is an artifact caused by manual interference while handlingthe instrument during analysis (i.e., breathing out).The broad peak at 3450 cm-1 could also be attributed to the adsorption of water during the handling of the nanoparticles which is causing the peaks for hydroxil groups to appear (Hong et al., 2009; Salavati-Niasar et al., 2011). It seems as if the GO ZnO NPs are influencing the FTIR in some way, possibly by reflecting the lightbeam so that no absorption of the lightbeam can take place. It is also possible that ZnO is formed completely over the surface of the GO so that FTIR of the nanohybrid is not able to show the spectrum of GO anymore or in reduced intensity. Although the absorbance is low for the GO ZnO nanoparticles, the peaks that do appear resemble the peaks that are found for the graphene oxide intermediate, however, no sound conclusions can be drawn.The end-product formed is also expected to have ZnO on the surface of the nanoparticles. Zinc oxide has a very typical, strong peak starting around 550 cm-1 with the top of the peak around 460 cm-1, which should thus show up in the FTIR of the GO ZnO nanoparticle (Chen et al., 2013). It was not possible to record the spectrum clearly in this area because the noise was disturbing the spectrum heavily past 600 cm-1. The spectrum is showing the formation of a peak from 625 cm-1 that could not be completed because of the noise. This could be the ZnO peak that was searched for. It must be stated that the FTIR results alone are inconclusive to state whether ZnO-GO nanoparticles had actually been produced.

Figure 4.4.12: Zoomed in FTIR spectrum of graphene oxide and graphene oxide - zinc oxide

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4.1.2 TEM

Figure 4.3: TEM images of graphene oxide - zinc oxide nanohybrid; a-d different images for the nanoparticles

Images of the graphene oxide - zinc oxide nanoparticles were taken with a transmission electron microscope (see Figure 4.3 a till d). These images shed some light on the size and shape of the nanoparticles produced. Figure 4.3d with the highest maginifcation is clearly showing different sheet-like structures as one would expect from nanoparticles derived from graphene oxide. Some separate shapes can be distinguished to give an estimate of the size of the size of the sheets. These vary roughly from 25 - 50nm in length and 10 - 25nm in width. The other images at lower magnification are also showing flat sheetlike structures but not as nicely seperated as in Figure 4.3d. Figure 4.3c shows asurface that is quite rough in comparison with the flat smooth structures seen at higher magnification in 4.3d. The roughness can be the result of unorganized cristalization of ZnO on the GO surface. It is not possible to detect any clear shape in it. Figure 4.3a and b show the presence of flat structures. In 4.3b these structures are layered on top of each other, while in 4.3a also more seperate round and rod-like structures can be seen (see red circle on image for example). These structures can be the result of the copper coating applied for the TEM analysis. From the TEM-data, there is nothing conclusive to say about the shape or size, nor clearly about the presence of ZnO on the GO sheets.

a) a)

d) c)

b)

32

4.1.3 EDS Simultaneous with taking the TEM images of the nanoparticles, energy-dispersive X-ray spectroscopy (EDS) was performed on the nanoparticles (see Figure 4.4). This spectrum is giving the elemental analysis of the nanoparticles. The elements of interest are Zn, C and O. These are the components that are part of the GO ZnO nanoparticles. The spectra are confirming their presence. Both in spectrum 2 and spectrum 5 the abundance of Zn is high with a typical peak at 8,630 keV and one at 1,01 keV. With a characteristic value of 0,28 keV C is shown clearly without any interference. Its intensity is higher in spectrum 2 than in spectrum 5 showing that the relative presence of C is higher for this spectrum, indicating a changing amount of Zn present on the surface of the nanoparticles. Oxygen is expected in high abundance due to the functialisation of graphite with ZnO and different carbon - oxygen functional groups. Oxygen is indeed showing with good intensity (without interference in spectrum 2, with interference in spectrum 5).

Figure 4.4: ED - spectra (spectrum 2 and spectrum 5) for graphene oxide - zinc oxide

33

4.1.4 Raman Spectroscopy The last method used for characterisation of the nanoparticles (GO and GO ZnO) was Raman spectroscopy. The Raman spectra were produced with a laser of 750 nm (Figure 4.5). Three major bands are present in the spectrum: a broad one around 300 cm-1 and two sharper ones at 1350 cm-1

and 1600 cm-1. The band at 1600 cm-1 is the G-band and is the result of the graphitic carbon in the structure. The band at 1350 cm-1 is the D-band and is associated with defects or disordered domains in the graphitic domain (Chen et al., 2013). The D-band is very strong which confirms the structural defects of the basal planes by presence of functional groups. The G and D band are caused by first-order scattering from the E2g phonon of sp2 carbon (Rattana et al., 2012). The band at 300 cm-1 - 400 cm-1 can be assigned to the presence of ZnO crystals (Ameen et al., 2013). Different peaks have been reported for ZnO. Around 440 cm-1 a peak corresponding to E2 (high) mode is found frequently for ZnO (Pant et al., 2013). This peak is an indication for the hexagonal wurtzite phase of ZnO. This peak is not showing in Figure 4.5. But this report was made for pure ZnO and during the modification to GO ZnO shifts of the peaks can occur. A peak at 330 cm-1 is also possible and is known to originate from vibration modes due to multiple-phono scattering processes (Li et al., 2007). A peak that can show up at 383 cm-1 is caused by the A1 (TO) mode (Li et al., 2007). In the spectrum of Figure 4.5 one big band is showing from 250 - 450 cm-1 without any really distinctive peaks. This position of the broad peak is typically in the part of the spectrum were ZnO is expected in literature, though it is not clear which Raman shift is dominant. Conclusion is that ZnO is present looking at the Raman spectrum. The lack of a dominating peak in this area of the spectrum that can tell something about the morphology of the ZnO may point towards no clear morphology of the ZnO crystallised on the GO (Pant et al., 2013), indicating random crystallisation. A quick look at the spectrum of plain graphene oxide that was taken, shows that the D and G gap are not changing much. There is a change in intensity what can be attributed to the presence of ZnO on the surface of the GO-sheets and a small red shift in the place of the peaks by 6 cm-1. A change in the ratio of the intensities of the D and G gap (IG/ID) between GO and GO ZnO can indicate a change in the amount of graphetisation or the presence of sp2- domains (Wang et al., 2011). No big change is detected here, indicating that ZnO is not having a big influence on the sp2-domains of GO.

Figure 4.5: Raman spectrum for graphene oxide and graphene oxide - zinc oxide

All the previous methods for characterisation and their results considered, the conclusion can be drawn that ZnO - GO was successfully synthesized. There is confirmation that graphite was oxidised to graphene oxide and that Zn is also present on the nanoparticles as ZnO. Only the shape and size, as well as the crystal structure of the ZnO on the nanoparticle with ZnO implemented on the GO stays relatively unclear. More characterisation of the nanoparticles is possible, but this is out of the scope of this thesis and is not required for the continuation of the research.

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4.2 Membrane production and characterisation Having confirmed the nature of the nanoparticles, these particles can now be implemented in the membrane. The membranes can then be characterised and tested for application in water purification. The results are shown below. The discussion on these results will always have the problems stated at the end of the literature review in mind that PES membranes are hydrophilic which is not beneficial for water flux and makes the PES membrane also prone to fouling. The membranes are produced with the purpose of addressing these problems.

4.2.1 Contact Angle and Interfacial Free Energies Contact angles were measured for the eight different membranes that were made. The contact angels were determined for three different probe liquids: water, glycerol and diiodomethane. The results of the contact angle measurements and the standard deviation of these measurements are presented in Table 4.1. The contact angles for water are visualised and presented separately in Figure 4.6, because the contact angle for water is a first important assessment for the hydrophobicity or hydrophilicity of a membrane. The water contact angle is not only able to classify the membrane in a hydrophobic or hydrophilic category, it is a first important indicator of membrane performance too. In general (with exceptions), the lower the contact angle with water, the more hydrophilic the membrane, the better its permeability for water and its resistance to fouling.

Table 4.1: Water, glycerol and diiodomethane contact angles for the different membranes

Water (°) Glycerol (°) Diiodomethane (°)

PES

69 ± 7 53 ± 7 30 ± 5

PES - 0,2% GO ZnO

58 ± 7 56 ± 8 34 ± 6

PES - 0,05% GO ZnO

61 ± 6 48 ± 9 23 ± 4

PES - 0,0125% GO ZnO

64 ± 6 55 ± 10 27 ± 6

PES PVP

69 ± 5 65 ± 6 35 ± 5

PES PVP - 0,2% GO ZnO

68 ± 5 60 ± 5 40 ± 4

PES PVP - 0,05% GO ZnO

70 ± 5 66 ± 5 35 ± 7

PES PVP - 0,0125% GO ZnO

70 ± 5 62 ± 5 27 ± 5

The values of the contact angles for water are ranging from 58 ° for PES - 0,2% GO ZnO to 69° - 70° for most of the other membranes. From Figure 4.6 it is clear that there is no significant difference between the average contact angles of the different membranes, considering the standard deviation. Although there is no statistical significant difference, some kind of trend is showing up in the measurements. For the membranes purely consisting of PES (the first four bars from the left of Figure 4.6) the contact angle with water is decreasing with increasing load of GO ZnO nanoparticles. Because of the error on the measurement, this is not significant, but it gives a good first indication that the membrane is becoming more hydrophilic with the addition of GO - ZnO. A more hydrophilic character with increasing GO ZnO loading is expected due to the many polar functional groups that are present on the surface of the graphene oxide sheets and the polar character of ZnO. These nanoparticles should thus indeed make the membrane more hydrophilic. The other membranes, consisting out of PES and PVP (the last four bars of Figure 4.6), are not showing this kind of trend. Even if the error on the measurements is ignored, the contact angles differ only by 2°. The PES - PVP membranes are not showing any decrease in contact angle whatsoever. Compared with the bare PES membrane, the contact angle for the bare PES PVP is the same. It was expected that PVP would make the PES membrane more hydrophilic, resulting in a decrease in contact angle (Al Malek et al., 2012; Susanto et al., 2009).

35

A similar trend is seen for the other probe liquids: the differences between the different membranes is not statistically significant and does not really allow clear conclusions. This leaves to wonder what can be the cause of these results. A contact angle measurement is only measuring the surface of the membrane, where the contact is taking place with the water droplet. An explanation could be that for the PES PVP membranes the surface consists only out of PES and that the additives (PVP, GO ZnO NPs) are not present here to cause a difference in the contact angle. This can be caused due to only minor physical changes taking place on the surface and no chemical ones. Flux tests should show if the addition of PVP has positive effects on water flux. Improved membrane flux has been observed for PVP without affecting the contact angle significantly (Malek et al., 2012). It is only by having the nanoparticles on the surface of the membrane that the functional groups are available to attract the water molecules to the surface. The PES nanocomposite membranes have lower contact angles, so here the nanoparticles are likely more on the surface of the membrane. Why this difference between the PES and the PES PVP nanocomposite membranes is observed, can be caused by the morphology changes in PES PVP membranes induced by PVP. It is possible that they are similar for all the PES PVP membranes due to the same PVP loading, causing similar contact angles. Another possibility would be more leaching/loss of the nanoparticles during the phase inversion when PVP is added, but it is reported that PVP is able to keep nanoparticles better in the membrane (Basri et al., 2011). The absence of nanoparticles being consequently present at the surface can be caused due to imperfect blending of the nanoparticles in the casting solution. This causes the nanoparticles to not be present equally across the 200 µm thick casting solution that is going to be immersed. If they are not on the surface in that stage of the casting process, they will not be in the membrane. Another possibility is that the instant phase inversion reaction between the top layer of the casting solution and the water pushes out the nanoparticles, causing them to be absent at the surface. It would have been interesting to see what happened if the loading was increased more. Would it give a more significant lower contact angle? Another observation that needs to be addressed is the error on the contact angle measurements that is causing the difference in contact angles between the membranes to be insignificant. The error on a measurement varies from 7% minimum to 12% maximum for at least 10 measurements each taken on two different membrane samples. When the nanoparticles are not evenly spread across the surface of the membrane, there will be parts of the membrane where a low contact angle will show and parts where there will be no nanoparticles present at the surface, causing a higher contact angle. This was clearly observed during the measurements and gives a lot of deviation in the contact angle values. Local differences in surface roughness can also play a part here. The measurements are showing that the membranes where the nanoparticles are causing a lower contact angle also have a larger percentage of error in the measurements. This might possibly be due to the fact that the nanoparticles are not consistent present at the surface of the membrane. As stated above, something that also has to be taken into account is that contact angle is not only influenced by surface chemistry but also by the physical properties (like roughness) of the membrane surface (Qiang et al., 2013). How this is influencing the contact angle measurements of the membranes here is not certain, since no AFM measurements could be carried out to assess surface roughness.

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Figure 4.6: Water contact angles with the different membranes

With the contact angles measured for water, diiodomethane and glycerol, the interfacial free energy components of the membranes can be calculated as described in the materials and methods part. The different components that make up the total interfacial free energy are the Lifshitz-van der Waals (γLW), Lewis acid parameter (γ+) and Lewis base parameter (γ-) (the latter two can be combined in the polar part of the surface tension (γAB). For each membrane, the Lifshitz-van der Waals component is dominating the total surface tension of the membranes. This component is responsible for 89% of the total surface tension for PES 0,05% GO - ZnO membrane, up to 99% for the PES PVP 0,05% GO - ZnO. This means that dispersive, non-polar forces are the most important ones for adhesion of molecules on the membrane surface of the membranes synthesized here, but that there can also be some polar interactions. The PVP membranes have a lower total surface tension than the PES membranes. The component responsible for this is that the PES membranes are showing especially higher values for the Lifshitz-van der Waals component than the PVP membranes. Within the PES membranes, the Lewis base (electron donor) parameter is higher when nanoparticles are added and the highest for the highest loading of nanoparticles. This means that with the nanoparticles the PES membranes are showing more polar characteristics and the electron pairs that can be donated are increasing. This is what is expected when nanoparticles are added whit a lot of oxygen containing functional groups. This can be coupled to what was observed for the water contact angle of the PES membranes. Expectations are that this would reduce the amount of fouling caused by foulants due to the more hydrophilic surphace of the membrane on which hydrophobic foulants are less attracted to.

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Table 4.2: Components of the interfacial free energy for the different membranes

γLW (J.m-²) γ+ (J.m-²)

γ- (J.m-²) γtot (J.m-²)

PES 44 ± 7 0,9 ± 0,2

7 ± 2 49 ± 8

PES 0,2% GO - ZnO 42 ± 7 0,1 ± 0,2

22 ± 6 45 ± 9

PES 0,05% GO - ZnO 47 ± 8 0,8 ± 0,4

12 ± 2 52 ± 8

PES 0,0125% GO - ZnO 45 ± 8 0,2 ± 0,3

13 ± 3 49 ± 8

PES PVP 42 ± 7 0,00

14 ± 3 42 ± 7

PES PVP 0,2% GO - ZnO 40 ± 6 0,3 ± 0,3

12 ± 2 44 ± 8

PES PVP 0,05% GO - ZnO 42 ± 7 0,00

13 ± 3 42 ± 9

PES PVP 0,0125% GO - ZnO 45 ± 8 0,1 ± 0,2

11 ± 2 46 ± 8

4.2.2 Pure Water Flux and Membrane Permeability From the characteristics measured in section 4.2.1, it is interesting to see how these membranes will behave when used in filtration. Figure 4.7 and Figure 4.8 show the pure water flux through the membranes in a dead-end setup under different pressures. For all the membranes the flux increases with increasing pressure. This increase is linear and is observed in a cross-flow setup as well (Figure 4.9 and Figure 4.10).

Figure 4.7: Pure water flux under different pressures for PES membranes in a dead-end setup

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Figure 4.8: Pure water flux under different pressures for PES PVP membranes in a dead-end setup

The most important thing to observe is how the nanoparticles and PVP are influencing the pure water flux, and to check whether this can be linked to the values obtained for the water contact angle. From previous research it is known that the membranes with the lowest contact angles do not necessarily need to have the highest flux: the value of the contact angle with water and the pure water flux is not proportional (Balta et al., 2012; Vatanpour et al., 2012). This is also the case in this study: the membrane with the lowest contact angle (PES 0,2% GO - ZnO) has a permeability of 6,7 L.m².h-1.bar-1 in the cross-flow and 16,5 L.m².h-1.bar-1 in the dead-end (see Table 4.3 and Table 4.4 for the permeabilities of the membranes). The bare PES membrane and all the PES PVP membranes showed the highest contact angles, but are having a higher flux than the membranes with a lower contact angle. This means that the contact angle is not predicting the permeability of a membrane very well. And that higher permeability is not linked to a more hydrophilic surface in this thesis. Interestingly, despite the only very slight differences in contact angles, the membranes with nanoparticles have a drastically different behaviour from the bare membranes. None of the membranes with nanoparticles are showing any permeability improvement compared with the bare membranes. For every setup the bare PES and the bare PES PVP membranes are clearly having superior permeability. Surprisingly, presence of NPs decreases the flux, and this decrease seems to become worse with decreased loading of the nanoparticles. The nanocomposite membranes with the highest loading (0,2%) of GO - ZnO in their casting solution have the highest permeability amongst the nanocomposites. It seems a contradiction that the permeability of membranes with nanoparticles is lower than membranes without, but then when nanoparticles are added the membranes with the highest loading of GO - ZnO have the highest permeability. The nanoparticles are causing a flux decline. A possibility is that they are blocking the pores that are formed during phase inversion, so that the passage for water through the membrane is blocked by the nanoparticles. It is plausible that during phase inversion when the pores are formed, the nanoparticles which are well dispersed in the casting solution are also forced out of the polymer due to differences in solubility and end up in the pores of the membrane when these are formed. Another hypothesis is that the nanoparticles are influencing the formation of pores in such a way that smaller pores are formed due to more hydrophilic character of the NPs which makes the phase inversion faster. These smaller pores are not beneficial for water flux. Why the membranes with a lower loading of GO ZnO also have a lower flux is not clear. It is expected that when pore blocking

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would occur, the membranes with less GO -ZnO would have less blocked pores and thus a higher flux. An explanation could be that membranes with high and low loading of GO ZnO both are experiencing blocked pores leading to a similar flux decline but that for high loading this is compensated a little bit due to more GO ZnO at the surface of the membrane, making the membranes more hydrophilic. This would lead than to better adsorption of the water molecules onto the membrane before letting the pass through the membrane. This can help explain also the observations of the values of the contact angles. A suggestion to improve this, is to change the membrane synthesis and use interfacial polymerisation to be sure that the nanoparticles are present in the active top layer of the membrane.

Figure 4.9: Pure water flux under different pressures for PES membranes in a cross-flow setup

Figure 4.10: Pure water flux under different pressures for PES membranes in a cross-flow setup

The other additive used apart from the nanoparticles is polyvinylpyrrolidone (PVP). This additive is reported as a pore forming agent that can improve the flux significantly when added to PES membranes (Vatsha et al., 2013). The influence of PVP on the membranes in this thesis is flux- enhancing, as expected. Only one membrane without PVP is showing a significant higher flux than its counterpart with PVP. This is the PES - 0,0125% GO ZnO membrane with a flux of 32 L.m².h-1 compared with 24 L.m².h-1 for the PES PVP membrane with the same nanoparticle loading (in the dead-end setup). In general PVP has a positive effect on the water flux, despite the negligible

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differences in contact angle measurements on the surface. PVP enhances the flux with at least a factor two. The positive effect of PVP can be subscribed to the fact that PVP is counteracting with the pore blocking by the nanoparticles. This is also confirmed by the more gradual flux decline in PES PVP membranes when nanoparticles are added. With the addition of nanoparticles in PES membranes the permeability drops down hard with at least 75% and stays around the same value for the other PES membrane with nanoparticles. PES PVP membranes are seeing a more gradual flux decline with decreasing nanoparticle loading.

4.2.3 Salt Rejection Not only flux experiments were carried out, but also salt rejection experiments with Mg SO4. From the flux experiments, the pure water permeability of the membrane of the membrane can be calculated. This was done for all the membranes in both setups (Table 4.3 for dead-end and Table 4.4 for cross-flow). Table 4.3: Pure water flux, salt rejection, membrane resistance and membrane permeability for the different membranes

in a dead-end setup

Jw at 4 bar(L.m-2.h-1) Salt Rejection (%) Rm (1012. m-1) Lp (L.m-2.h-1.bar-1)

PES 266 5,2 ± 1,0 6,0 ± 1,2 66 ± 13

PES 0,2% GO ZnO 66 28 ± 2 24,0± 1,5 16,5 ± 1,0

PES 0,05% GO ZnO 49 23 ± 3 28 ± 10 14 ± 5

PES 0,0125% GO ZnO 32 22 ± 2 49 ± 19 8 ± 3

PES PVP 540 7,7 ± 1,1 2,97 ± 0,18 133 ± 8

PES PVP 0,2% GO ZnO 258 6,8 ± 0,6 6,2 ± 1,4 64 ± 14

PES PVP 0,05% GO ZnO 103 21,5 ± 1,7 14,7 ± 1,6 27 ± 3

PES PVP 0,0125% GO ZnO 24 28 ± 3 60 ± 8 6,6 ± 0,9

Table 4.4: Pure water flux, salt rejection, membrane resistance and membrane permeability for the different membranes

in a cross-flow setup

Jw at 4 bar (L.m-2.h-1) Salt Rejection (%) Rm (1012. m-1) Lp (L.m-2.h-1.bar-1)

PES 174 1,7 ± 0,3 9,07 ± 0,19 43,6 ± 0,9

PES 0,2% GO ZnO 28 8 ± 5 59 ± 7 6,7 ± 0,8

PES 0,05% GO ZnO 18 3,0 ± 1,0 104 ± 33 3,8 ± 1,2

PES 0,0125% GO ZnO 9 28 ± 3 197 ± 79 2,0 ± 0,8

PES PVP 567 2,6 ± 0,2 3,2 ± 1,0 122 ± 40

PES PVP 0,2% GO ZnO 327 5 ± 3 5,2 ± 1,0 76 ± 15

PES PVP 0,05% GO ZnO 10 24 ± 5 158 ± 51 2,5 ± 0,8

PES PVP 0,0125% GO ZnO 67 5 ± 3 24 ± 2 16,4 ± 1,4

Despite the clear differences in pure water permeability, salt rejection and salt permeability are not showing any trend. The membranes with the highest flux, permeability and lowest resistance (like the bare PES and bare PES PVP membrane) have lower salt rejection than the membranes with lower flux. However, it is not a general trend that a membrane with lower permeability will have a better salt rejection. In the cross-flow setup there is significant membrane permeability difference between the PES PVP 0,2% GO ZnO, PES PVP 0,0125% GO ZnO and PES 0,2% GO ZnO, but their salt rejection is not significantly different.

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The dead-end setup is showing generally higher salt rejections than the cross-flow setup. Other research has shown that typically dead-end set-ups have lower salt rejection than cross-flow set-ups because in cross-flow the flow regime is causing more shear stress at the membrane surface, which decreases salt build-up (lower concentration polarisation) and increases rejection (Tansel et al., 2012). This is of course in an ideal situation where the membranes used are having exactly the same characteristics. In this thesis the samples of the membranes used in the dead-end (small scale) are not showing the same characteristics (membrane resistance and permeability) as the samples used in cross-flow (larger scale). This is an indication that the membranes that have been produced are not continuous in their properties across their membrane surface. It is indeed difficult to up-scale phase inversion membranes, even with an automatic casting knife. Therefore, performance of the membranes will depend on which sample of the membrane is cut out, certainly when working with rather small membrane area's like in this thesis. This is something that has to be checked in future experiments. However, it needs to be mentioned that in general, the relations between the different membranes between cross-flow and dead-end results are rather similar.

Figure 4.11: Salt rejection for PES membranes in a cross-flow setup

Figure 4.12: Salt rejection for PES PVP membranes in a cross-flow setup

For the cross-flow setup, salt rejection was measured at multiple pressures to see if the pressure influences salt rejection. Typically, rejection increases with increasing pressure, due to a higher

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dilution of the salts by the higher water permeation at higher fluxes. Figure 4.11 and 4.12 are showing the same trend for all the membranes. The rejection is the highest at the lowest pressure and then drops for the higher pressures to a more constant level, thus contradictory to the theory. For two PES membranes the salt rejection keeps constant throughout the pressure gradient. As stated, typically salt rejection increases with increasing pressure due to the higher dilution at higher fluxes. However, at a certain point, when the flux is very high, concentration polarization will occur in a large increase in solute concentration at the membrane surface at higher fluxes, which will have a negative effect on rejection. As a result, rejection stays constant (since there is a competing effect of more dilution and increasing concentration polarization).In this thesis, however, the rate of rejection decline is declining rapidly with increasing pressure. The reason for this observation can be high concentration polarisation. To see if this can be the case some calculations were done (see Table

4.5). For calculations the next formula was used (van den Berg et al., 1989; Wijmans et

al., 1985). Cb is the salt concentration in the feed solution (2 g.L-1), k is a mass transfer coefficient (10-

5 m.s-1 was chosen), Jv is the measured flux of the membranes at operating pressure of 3 bars. For the high flux membranes the concentration polarisation effect is high. Which can indeed explain the observation the observation of decreased rejection with increased pressure. Because a lot of salt is present at the membrane surface.

Table 4.5: Calculated concentratians at membrane surfaces for concentration polarisation effect

Cm (g.L-1)

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4.2.4 SEM images

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Figure 4.13: SEM images of cross sections of different membranes: a - PES 0,05% GO ZnO; b - PES-PVP 0,0125% GO ZnO; c - PES-PVP 0,2% GO ZnO; d - PES 0,2% GO ZnO; e - PES-PVP 0,0125% GO ZnO; f - bare PES membrane; g – bare PVP

Figure 4.13 shows SEM images of some of the membranes. The two bare membranes (image g and f) can be used as base comparing for morphology changes. Taken into account that image g has a twice as high magnification compared to image f, there can be seen that the PES PVP membrane has nice clear big finger like pores as internal structures. The PES membrane got twisted on the sample holder but the structure looks like more macrovoid pores. For the PES membrane the dense top layer is also visible. All the other images are of membranes implemented with nanoparticles. a and d are both PES membranes with nanoparticles. They look quite similar in structure. The images have more or less the same magnification and that is also what is seen in the dimension of the pore structures which have the same width. The PES 0,2% GO ZnO is having shorter porous structures than the one with 0,05% GO ZnO. Comparing this with the bare PES membrane, it looks as if the addition of nanoparticles has shortened the internal pore structures with the consequence of hindered flux of water. The magnification is not large enough to clearly see the nanoparticles in the structure. The membrane that stands out the most is the PES PVP 0,2% GO ZnO membrane. The finger-like pores from the other images has changed into more spongy membrane. A significant change in morphology has taken place here. This hasn't influenced the permeability of the membrane. The pores are numerous present and are having an open structure. Sponge like structures points towards fast demixing of the solvent and the non-solvent during phase inversion. Maybe this can be caused due to the combination of the loading of nanoparticles and presence of PVP. More research regarding the influence of multiple additives on membrane morphology is certainly needed. The PES PVP 0,0125% membrane resembles a lot the bare membrane in structure. No significant changes are noticed here. Comparing this last membrane with the PES membranes shows that the PES-PVP membranes (except from the one in image c) have bigger pore structures than the PES membranes, which would explain the better flux. However, nothing in the morphology can explain why the nanocomposite membranes are having lower permeabilities than the bare membranes. Image e shows a close up in which little pores within the finger like pores showing. Even when using this high magnification no sign was found of presence of the nanoparticles (the gold that was used for coating the sample was showing though).

g)

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4.3 Fouling Behaviour After flux and salt rejection testing, it was interesting to see how the membranes would behave in fouling experiments with different kinds of foulants. Two types of foulants were used: an organic foulant (alginate) and a colloidal foulant (silica). Data is shown in terms of relative fluxes. With relative flux being the ration of the measured flux J at time t over the measured initial flux at time t = 0.

4.3.1 Organic Fouling With the values of the different components of the interfacial free energy for the membranes obtained from Table 4.2, predictions about the organic fouling can be made. From Motsa et al. (2014) the values of the interfacial free energy components for alginate in a 0,1 M NaCl solution could be obtained. In this solution alginate is in a stable state (cohesion energy value 9,66 mJ.m-²), meaning that alginate will not form aggregates easily (Motsa et al., 2014) . To do the calculations the next formulas were used (Kim & Hoek, 2007):

(4.1)

(4.2)

(4.3)

The subscript 1 and 2 are for solid materials immersed in liquid medium 3. describes than the

attraction or repulsion between two solid materials interacting through a liquid medium.

Table 4.6: Energy of adhesion between the organic foulant (alginate) and the membranes (Motsa et al., 2014)

PES -22,48

PES 0,2% GO - ZnO -2,41

PES 0,05% GO - ZnO -15,07

PES 0,0125% GO - ZnO -12,82

PES PVP -10,03

PES PVP 0,2% GO - ZnO -12,46

PES PVP 0,05% GO - ZnO -11,26

PES PVP 0,0125% GO - ZnO -15,93

The more negative the value for the interfacial free energy of adhesion the more stable the adhesion of the foulant on the membrane is. Comparing this value, high fouling is expected for PES and the lowest fouling for the PES 0,2% GO ZnO membrane (which also had the lowest contact angle for water). Now the results of the practical experiment will be discussed after which they can be compared with what was expected from the calculated values of Table 4.6.

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Figure 4.14: Influence of organic fouling (alginate) on water flux for PES membranes

Figure 4.15: Influence of organic fouling (alginate) on water flux for PES PVP membranes

The influence of the organic foulant (sodium alginate) on the flux is shown in Figure 4.14 and Figure 4.15. The same membrane samples used in the previous section for the cross flow system were used. The absolute starting flux for the operating pressure of 3 bar can thus be found in Figure 4.9 and Figure 4.10. For all the membranes flux is declining in more or less the same way. In the first hour, a lot of flux decline is taking place after which for the next hours the flux stays constant. This is what is observed in other research too (Listiarini et al., 2011). PES and PES - 0,2% GO ZnO are showing a more gradual decline in flux over the whole period rather than a sudden drop in flux. The most important observation is here how the nanocomposite membranes are performing in comparison with the bare membranes. For the PES as well as the PES PVP membranes the membrane with the highest loading of nanoparticles is having the lowest flux decline. After 6 hours the PES 0,2% GO ZnO membrane has still 63% of its initial flux and the PES PVP 0,2% GO ZnO membrane 53% of its initial flux. This means that the foulant is less likely to attach to the membrane with the nanoparticles. This confirms the observations from the contact angles and surface tension components that indicated that these membranes more nanoparticles are present at the surface of

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the membrane. It is known that sodium alginate fouls the membrane mainly by adsorption on its surface. This is dominated by surface characteristics of the membrane (like hydrophobicity) (Katsoufidou et al., 2007). The functional groups of the nanoparticles made the surface of the membranes more hydrophilic, causing the fouling to decrease. No big difference in fouling is detected between the membranes with and the membranes without PVP. There is also no difference in fouling between the other nanocomposite membranes with a load of 0,05% or 0,0125% GO ZnO. This again seems to indicate that in these membranes the nanoparticles are less placed at the surface of the membrane. Looking back to the calculated adhesive interfacial free energy, it can be concluded that the calculation could predict the fouling to some extent. This is because the PES PVP nanocomposite membranes had quite high contact angles, so it was not expected that they would show hydrophilic properties at the surface with the observed, reduced fouling as a consequence. The membrane-foulant combination showing the least negative adhesive interfacial free energy is also in the experiments showing good resistance against fouling. The salt (NaCl) rejection during the fouling runs was recorded. Figure 4.16 and 4.17 are showing the effect of fouling on the rejection of NaCl over the period of the experiment. In figure 4.16 a slight but significant increase of rejection is observed during the experiment. Fouling is increasing the resistance of total layers that solutes have to pass (Listiarini et al., 2009), so it is expected that solutes would have more trouble passing the membrane (i.e., an “extra” membrane is created). This is not observed (or to a lesser extent) in Figure 4.17. A last correlation between the fouling of the membranes and the rejection is that the more heavily fouled membranes are having higher rejections of salt in comparison with the less fouling membranes. The nanocomposite membranes with 0,2% GO ZnO are showing the lowest rejections with 6,2% for the PES PVP type and 2,4% for the PES type.

Figure 4.16: Influence of organic fouling on the rejection of NaCl for PES membranes

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Figure 4.17: Influence of organic fouling on the rejection of NaCl for PES PVP membranes

4.3.2 Colloidal Fouling Silica is used to investigate the influence of colloids on the flux of membranes (Figure 4.18 and 4 .19). For the PES PVP membranes in Figure 4.18, there is no clear effect of fouling observed for the PES PVP - 0,05% GO ZnO membrane (100% flux after 6 hours), only a slight effect of fouling on the PES PVP - 0,2 and 0,0125% GO ZnO (around 80% of the initial flux after 6 hours) and a significant effect for the bare membrane (58% of the initial flux after 6 hours). This means that there is a positive effect on silica fouling too caused by the nanoparticles. It has been reported that nanoparticles can reduce the fouling of silica to a low level (Razali et al., 2013) so that the relative flux stays above 80% due to increased hydrophilicity of the membrane surface. However, fouling is occurring in a much less significant way than the organic fouling, although it was expected that SiO2 would have a much bigger influence on the flux due to the formation of a dense layer of silica particles on the surface (Lin et al., 2014).

Figure 4.18: Influence of colloidal fouling on the relative flux for PES membranes

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Figure 4.19: Influence of colloidal fouling on the relative flux for PES PVP membranes

4.4 Application in rejection of trace organic pollutants The last experiment that was run, was the rejection of trace organic compounds (TrOCs). In Figure 4.20 and Figure 4.21 the rejection of the TrOCs can be found. The TrOCs are ranked from left to right in the order of decreasing molecular weight to see if this influences their rejection. A quick look learns that there is no clear pattern showing in which TrOCs are rejected more or less based on their molecular weight. The nanocomposite membranes are clearly showing higher TrOC rejection than the bare membrane, similar to what was observed for the salt rejection. There are a couple of TrOCs like ketoprofen and primicarb which are only being rejected by nanocomposite membranes and not by the bare membranes. Looking at the nanocomposite membranes there are no clear differences between the membranes. From the overall average rejections in Table 4.7 it is clear that there are a couple good rejection membranes like PES - 0,05% GO ZnO, PES- 0,2% GO ZnO and its PVP counterpart. These higher rejections are due to a combination of effects discussed further in this section. Since all membranes reject certain components well, and others less good, conclusion stay that there is no clear trend to find, except that the nanocomposite membranes are rejecting the TrOCs better. This is probably due to the nanoparticles that are blocking the pores of the membranes causing a sieve effect. The fact that there are differences between the membranes in the type of compounds that are being rejected, shows that other effects (besides solute size) are affecting rejection.

Table 4.7: Average rejection of TrOCs for the different membranes

Average Rejection (% )

Average Rejection (% )

PES 14,7 PES PVP 9,9

PES - 0,2% GO ZnO 30,7 PES PVP - 0,2% GO ZnO 27,4

PES - 0,05% GO ZnO 27,0 PES PVP - 0,05% GO ZnO 19,9 PES - 0,0125% GO ZnO

8,2 PES PVP - 0,0125% GO ZnO

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Figure 4.20: Rejection of pharmaceuticals (ranked by decreasing MW) for PES membranes

Figure 4.21: Rejection of pharmaceuticals (ranked by decreasing MW) for PES PVP membranes

The TrOCs can therefore also be classified by their charge (Figure 4.22 and Figure 4.23). It can be quickly concluded that the neutral TrOCs are better rejected than the charged ones. This is an indication that steric hindrance or other charge related forces are not responsible for rejecting more or less TrOCs. This can be an indication that the membranes used in this thesis are not highly charged (which is logic since PES was used). But that a sieve mechanism based on another parameter is causing the rejection. Since molecular weight and charge are not the main factors that determine TrOC rejection, the reason for differences in rejection is probably related to different solute-membrane affinity interactions. However, no clear differences could be found between the membranes from the contact angle measurements, mainly due to the large deviations on the measured contact angles (see above). In addition, it is not the molecular weight that is important for rejection, but rather the molecular shape of the compound. A long, stretched but heavy molecule could still pass through the pores in the length-direction, while a bulky light one can get rejected.

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The last properties of the TrOC that can be compared are their log D values. Figure 4.24 and Figure 4.25 are showing the rejections based on these values. An organic molecule is regarded hydrophobic when the log D is higher than 3 and hydrophilic when it is lower than 3 (Dang et al., 2014; Alturki et al., 2010). All the TrOC that were used are hydrophilic. Similar as with fouling, hydrophobic organic molecules will be absorbed by the surface of a more hydrophobic membrane, leading to better rejection of these organic molecules. But if the membrane gets saturated this effect stops and more organics will diffuse again through the membrane, leading to less rejection (Simon et al., 2013). In this thesis the experiment was run for 1 hour before taking the sample. This would let the trace organics adsorb first on the membranes and decrease the measured rejection. If the two graphs are compared it shows that with decreasing hydrophilicity the solutes are rejected better. Hydrophilic solutes can also be rejected more because they tend to have a big water mantle, which increases their size. This will then make size exclusion a more appropriate sieve mechanism. It can be concluded that the rejection of the trace organic compounds is complex interaction between the different size, charge and hydrophilic properties of the molecules.

Figure 4.22: Rejection of pharmaceuticals (ranked by charge) for PES membranes

Figure 4.23: Rejection of pharmaceuticals (ranked by charge) for PES PVP membranes

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Figure 4.24: Rejection of pharmaceuticals (ranked by log D value) for PES membranes

Figure 4.25: Rejection of pharmaceuticals (ranked by log D value) for PES PVP membranes

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5. Conclusions and Recommendations

5.1 Conclusions From the research conducted in this thesis it can be concluded that graphene oxide - zinc oxide nanoparticles were synthesised successfully. This was confirmed by a combination of characterisation techniques such as TEM, FTIR, EDS and Raman spectroscopy. Implementing this nanohybrid in a polyethersulfone membrane made the surface of the membrane more hydrophilic with increasing nanoparticle loading. The improvement in hydrophilicity was confirmed by contact angle measurements except for the PES PVP membranes. All the nanocomposite membranes were however found to have lower fluxes than the bare membrane. The nanocomposite membrane with the highest flux (PES PVP - 0,2% GO ZnO) had 40% less permeability than the bare PES PVP membrane. It was believed that the nanoparticles blocked the pores resulting in decrease in water flux. Based on fouling experiments and trace organic rejection experiments, it was confirmed that the addition of nanoparticles increased surface hydrophilicity of other membranes than shown by the contact angle measurements. The PES PVP - 0,2% GO ZnO membrane showed 20% less flux decline (as a result of fouling) than the bare PES PVP membrane. Generally organic foulants adsorb better on hydrophobic surfaces and foul the membranes. The results of reduced fouling indicate that the hydrophobicity of the surface has decreased i.e. the membrane became more hydrophilic. Colloidal fouling resulted in lower flux decline than organic fouling (around 20% instead of 45%). This indicates too the increased surface hydrophilicity. When carrying out rejection experiments of trace organic compounds (TrOCs), it was observed that the molecular weight of the TrOCs had no effect on their rejection behaviour. Charge on the other hand could have played a role, but also here the effect was significant. Neutral TrOCs were rejected better than charged ones. Negatively charged TrOCs were rejected more than the positive TrOCs due to the possible attraction between the negatively charged membranes and the positively charged solutes and the charge repulsion with the negatively charged solutes. Hydrophobic and polar non-electrostatic interactions between the membranes and solutes were also important to consider. Every TOC was classified either as hydrophilic or hydrophobic. Hydrophilic TrOCs were rejected less than hydrophobic compounds by the membranes. The most hydrophilic membrane (PES PVP - 0,2% GO ZnO) rejected better hydrophilic TrOCs due to polar interactions. Hydrophilic compounds also tend to have a larger water mantle and increase in size. Hence they were then rejected more based on size exclusion effect. Based on the performance of all the different nanocomposite membranes, it can be concluded that PES PVP - 0,2% GO ZnO was the best performing nanocomposite membrane. This membrane had the highest flux of all the nanocomposite membranes (but lower than the bare membranes), a similar to slightly better salt rejection compared to the bare membranes, a good resistance to fouling and one of the best rejections of the trace organics. However this was not indicated in the beginning of the research by the contact angle and interfacial free energy calculations. This also implies that PVP was used successful to enhance the performance of a membrane with nanoparticles.

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5.2 Recommendations for Future Research Based on flux results presented here, it would be suggested that other membrane synthesis techniques such as dip coating or interfacial polymerisation could be used for synthesis of similar membrane to see if the nanoparticles are enhancing flux if a different production approach is used, instead of blocking out the pores as is probably happening during the phase inversion used here. Another concern regarding the membrane production is the reproducibility of the membranes. The novel membranes made should have stable performance over and over again so that the step can be made to big scale applications. Graphene oxide - zinc oxide nanoparticles are photocatalysts, i.e., they have photocatalytic properties therefore the photocatalytic ability of the membrane should also be investigated. This can be done by cleaning a biofouled membrane under a UV light to see if the initial flux before fouling could be recovered or by testing the membrane in a UV-membrane reactor to see if the membrane is capable of reducing organic pollutants. To address concerns regarding toxicity of the nanoparticles used, leaching of the nanoparticles from the membranes during an experiment should be investigated as well. In addition the stability of the nanoparticles in the membranes also needs to be tested.

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References

Adams, F. V., Nxumalo, E. N., Krause, R. W. M., Hoek, E. M. V., & Mamba, B. B. (2014). Application of polysulfone/cyclodextrin mixed-matrix membranes in the removal of natural organic matter from water. Physics and Chemistry of the Earth, Parts A/B/C, 67–69(0), 71-78.

Ahmad, A. L., Abdulkarim, A. A., Ooi, B. S., & Ismail, S. (2013). Recent development in additives modifications of polyethersulfone membrane for flux enhancement. Chemical Engineering Journal, 223, 246-267.

Ahn, J.-M., Eom, H.-J., Yang, X., Meyer, J. N., & Choi, J. (2014). Comparative toxicity of silver nanoparticles on oxidative stress and DNA damage in the nematode, Caenorhabditis elegans. Chemosphere, 108(0), 343-352.

Al Malek, S. A., Abu Seman, M. N., Johnson, D., & Hilal, N. (2012). Formation and characterization of polyethersulfone membranes using different concentrations of polyvinylpyrrolidone. Desalination, 288, 31-39.

Altmann, J., Ruhl, A. S., Zietzschmann, F., & Jekel, M. (2014). Direct comparison of ozonation and adsorption onto powdered activated carbon for micropollutant removal in advanced wastewater treatment. Water Research, 55(0), 185-193.

Alturki, A. A., Tadkaew, N., McDonald, J. A., Khan, S. J., Price, W. E., & Nghiem, L. D. (2010b). Combining MBR and NF/RO membrane filtration for the removal of trace organics in indirect potable water reuse applications. Journal of Membrane Science, 365(1–2), 206-215.

Ameen, S., Shaheer Akhtar, M., Seo, H.-K., & Shik Shin, H. (2013). Advanced ZnO–graphene oxide nanohybrid and its photocatalytic Applications. Materials Letters, 100, 261-265.

Appelgren, B. G. (1997). Keynote paper - management of water scarcity: national water policy reform in relation to regional development cooperation. Paper presented at the Second expert consulationon national water policy reform in the near east, Caïro.

Aurilia, M., Sorrentino, L., Sanguigno, L., & Iannace, S. (2010). Nanofilled polyethersulfone as matrix for continuous glass fibers composites: Mechanical properties and solvent resistance. Advances in Polymer Technology, 29(3), 146-160.

AUSCEW. (22-10-2012). Australia - United States Climate, Energy and Water Nexus Project (AUSCEW). Retrieved 08-04, 2014, from http://www.water.anu.edu.au/project/auscew/

Bai, H., liu, Z., & Sun, D. D. (2012). Hierarchical ZnO nanostructured membrane for multifunctional environmental applications. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 410, 11-17.

Balta, S., Sotto, A., Luis, P., Benea, L., Van der Bruggen, B., & Kim, J. (2012). A new outlook on membrane enhancement with nanoparticles: The alternative of ZnO. Journal of Membrane Science, 389, 155-161.

56

BASF. (2007). Ultrason E, Ultrason S: the material of choice for demanding high temperature and filtration needs.

Basri, H., Ismail, A. F., & Aziz, M. (2011). Polyethersulfone (PES)–silver composite UF membrane: Effect of silver loading and PVP molecular weight on membrane morphology and antibacterial activity. Desalination, 273(1), 72-80.

Bergamasco, R., da Silva, F. V., Arakawa, F. S., Yamaguchi, N. U., Reis, M. H. M., Tavares, C. J., de Amorim, M.T.P.S., & Tavares, C.R. (2011). Drinking water treatment in a gravimetric flow system with TiO2 coated membranes. Chemical Engineering Journal, 174(1), 102-109.

Brant, J. A., & Childress, A. E. (2002). Assessing short-range membrane-colloid interactions using surface energetics. Journal of Membrane Science, 203, 257-273.

Chen, J., Yao, B., Li, C., & Shi, G. (2013). An improved Hummers method for eco-friendly synthesis of graphene oxide. Carbon, 64, 225-229.

Chen, Y.-L., Zhang, C.-E., Deng, C., Fei, P., Zhong, M., & Su, B.-T. (2013). Preparation of ZnO/GO composite material with highly photocatalytic performance via an improved two-step method. Chinese Chemical Letters, 24(6), 518-520.

Crock, C. A., Rogensues, A. R., Shan, W., & Tarabara, V. V. (2013). Polymer nanocomposites with graphene-based hierarchical fillers as materials for multifunctional water treatment membranes. Water Research, 47(12), 3984-3996.

Dang, H. Q., Price, W. E., & Nghiem, L. D. (2014a). The effects of feed solution temperature on pore size and trace organic contaminant rejection by the nanofiltration membrane NF270. Separation and Purification Technology, 125(0), 43-51.

Dang, H. Q., Price, W. E., & Nghiem, L. D. (2014b). The effects of feed solution temperature on pore size and trace organic contaminant rejection by the nanofiltration membrane NF270. Separation and Purification Technology, 125(0), 43-51.

Draw, J., Hallett, K., DeWolfe, J., & Venner, I. (2012). Energy Efficiency Strategies for Municipal Waste Water Treatment Facilities: National Renewable Energy Laboratory.

Dreyer, D. R., Park, S., Bielawski, C. W., & Ruoff, R. S. (2010). The chemistry of graphene oxide. [10.1039/B917103G]. Chemical Society Reviews, 39(1), 228-240.

Dubreuil, A., Assoumou, E., Bouckaert, S., Selosse, S., & Maïzi, N. (2013). Water modeling in an energy optimization framework – The water-scarce middle east context. Applied Energy, 101, 268-279.

Folttmann, H., & Quadir, A. (2008). Polyvinylpyrrolidone (PVP) - One of the Most Widely Used Excipients in Pharmaceuticals: an Overview. Drug Delivery Technology, 8(6), 22-27.

Fu, D., Han, G., Chang, Y., & Dong, J. (2012). The synthesis and properties of ZnO–graphene nano hybrid for photodegradation of organic pollutant in water. Materials Chemistry and Physics, 132(2-3), 673-681.

57

Ganesh, B. M., Isloor, A. M., & Ismail, A. F. (2013). Enhanced hydrophilicity and salt rejection study of graphene oxide-polysulfone mixed matrix membrane. Desalination, 313, 199-207.

Gao, P., Liu, Z., Tai, M., Sun, D. D., & Ng, W. (2013). Multifunctional graphene oxide–TiO2 microsphere hierarchical membrane for clean water production. Applied Catalysis B: Environmental, 138-139, 17-25.

Goenka, S., Sant, V., & Sant, S. (2014). Graphene-based nanomaterials for drug delivery and tissue engineering. Journal of Controlled Release, 173, 75-88.

Haaf, F., Sanner, A., & Straub, F. (1985). Polymers of N-Vinylpyrrolidone: Synthesis, Characterization and Uses. Polymer Journal, 17(1), 143-152.

Hajibabania, S., Verliefde, A., McDonald, J. A., Khan, S. J., & Le-Clech, P. (2011). Fate of trace organic compounds during treatment by nanofiltration. Journal of Membrane Science, 373(1–2), 130-139.

Hamidin, N., Yu, Q. J., & Connell, D. W. (2008). Human health risk assessment of chlorinated disinfection by-products in drinking water using a probabilistic approach. Water Research, 42(13), 3263-3274.

Hong, B. J., Compton, O. C., An, Z., Eryazici, I., & Nguyen, S. T. (2011). Successful Stabilization of Graphene Oxide in Electrolyte Solutions: Enhancement of Biofunctionalization and Cellular Uptake. ACS Nano, 11, 63-73.

Hong, R. Y., Li, J. H., Chen, L. L., Liu, D. Q., Li, H. Z., Zheng, Y., & Ding, J. (2009). Synthesis, surface modification and photocatalytic property of ZnO nanoparticles. Powder Technology, 189(3), 426-432.

Hrudey, S. E. (2009). Chlorination disinfection by-products, public health risk tradeoffs and me. Water Research, 43(8), 2057-2092.

Hummers, W. S., & Offeman, R. E. (1958). Preparation of Graphitic Oxide. Journal of the American Chemical Society, 80(6), 1339-1339.

ISP. PVP - Polyvinylpyrrolidone Polymers.

Jin, L. M., Yu, S. L., Shi, W. X., Yi, X. S., Sun, N., Ge, Y. L., & Ma, C. (2012). Synthesis of a novel composite nanofiltration membrane incorporated SiO2 nanoparticles for oily wastewater desalination. Polymer, 53(23), 5295-5303.

Jones, K. C., & de Voogt, P. (1999). Persistent organic pollutants (POPs): state of the science. Environmental Pollution, 100, 209-221.

Kalaiselvi, G., Maheswari, P., Balasubramanian, S., & Mohan, D. (2013). Synthesis, characterization of polyelectrolyte and performance evaluation of polyelectrolyte incorporated polysulfone ultrafiltration membrane for metal ion removal. Desalination, 325, 65-75.

Katsoufidou, K., Yiantsios, S. G., & Karabelas, A. J. (2007). Experimental study of ultrafiltration membrane fouling by sodium alginate and flux recovery by backwashing. Journal of Membrane Science, 300(1-2), 137-146.

58

Katsoufidou, K. S., Sioutopoulos, D. C., Yiantsios, S. G., & Karabelas, A. J. (2010). UF membrane fouling by mixtures of humic acids and sodium alginate: Fouling mechanisms and reversibility. Desalination, 264(3), 220-227.

Kim, E.-S., Hwang, G., Gamal El-Din, M., & Liu, Y. (2012). Development of nanosilver and multi-walled carbon nanotubes thin-film nanocomposite membrane for enhanced water treatment. Journal of Membrane Science, 394-395, 37-48.

Kim, J., & Van der Bruggen, B. (2010). The use of nanoparticles in polymeric and ceramic membrane structures: review of manufacturing procedures and performance improvement for water treatment. Environmental Pollution, 158(7), 2335-2349.

Kim, S., & Hoek, E.M.V. (2007). Interactions controlling biopolymer fouling of reverse osmosis membranes. Desalination, 202, 333 – 342.

Kim, S. H., Kwak, S.-Y., Sohn, B.-H., & Park, T. H. (2003). Design of TiO2 nanoparticle self-assembled aromatic polyamide thin-film-composite (TFC) membrane as an approach to solve biofouling problem. Journal of Membrane Science, 211(1), 157-165.

Kim, T.-U., Drewes, J. E., Scott Summers, R., & Amy, G. L. (2007). Solute transport model for trace organic neutral and charged compounds through nanofiltration and reverse osmosis membranes. Water Research, 41(17), 3977-3988.

Kosma, C. I., Lambropoulou, D. A., & Albanis, T. A. (2014). Investigation of PPCPs in wastewater treatment plants in Greece: Occurrence, removal and environmental risk assessment. Science of The Total Environment, 466–467(0), 421-438.

Kudin, K. N., Ozbas, B., Schniepp, H. C., Prud'homme, R. K., Aksay, I. A., & Car, R. (2007). Raman Spectra of Graphite Oxide and Functionalized Graphene Sheets. Nano Letters, 8(1), 36-41.

Lalia, B. S., Kochkodan, V., Hashaikeh, R., & Hilal, N. (2013). A review on membrane fabrication: Structure, properties and performance relationship. Desalination, 326, 77-95.

Lecouvet, B., Sclavons, M., Bourbigot, S., & Bailly, C. (2013). Thermal and flammability properties of polyethersulfone/halloysite nanocomposites prepared by melt compounding. Polymer Degradation and Stability, 98(10), 1993-2004.

Lee, J., Chae, H.-R., Won, Y. J., Lee, K., Lee, C.-H., Lee, H. H., Kim, I.-C., & Lee, J.-M. (2013). Graphene oxide nanoplatelets composite membrane with hydrophilic and antifouling properties for wastewater treatment. Journal of Membrane Science, 448, 223-230.

Leo, C. P., Ahmad Kamil, N. H., Junaidi, M. U. M., Kamal, S. N. M., & Ahmad, A. L. (2013). The potential of SAPO-44 zeolite filler in fouling mitigation of polysulfone ultrafiltration membrane. Separation and Purification Technology, 103(0), 84-91.

Li, B., Liu, T., Wang, Y., & Wang, Z. (2012). ZnO/graphene-oxide nanocomposite with remarkably enhanced visible-light-driven photocatalytic performance. Journal of Colloid and Interface Science, 377(1), 114-121.

59

Li, C., Lv, Y., Guo, L., Xu, H., Ai, X., & Zhang, J. (2007). Raman and excitonic photoluminescence characterizations of ZnO star-shaped nanocrystals. Journal of Luminescence, 122–123(0), 415-417.

Li, J.-F., Xu, Z.-L., Yang, H., Yu, L.-Y., & Liu, M. (2009). Effect of TiO2 nanoparticles on the surface morphology and performance of microporous PES membrane. Applied Surface Science, 255(9), 4725-4732.

Li, M.-j., Liu, C.-m., Xie, Y.-b., Cao, H.-b., Zhao, H., & Zhang, Y. (2014). The evolution of surface charge on graphene oxide during the reduction and its application in electroanalysis. Carbon, 66(0), 302-311.

Li, Q., Pan, X., Qu, Z., Zhao, X., Jin, Y., Dai, H., Yang, B., & Wang, X. (2013). Understanding the dependence of contact angles of commercially RO membranes on external conditions and surface features. Desalination, 309(0), 38-45.

Li, W. C. (2014). Occurrence, sources, and fate of pharmaceuticals in aquatic environment and soil. Environmental Pollution, 187, 193-201.

Li, X.-G., Shao, H.-T., Bai, H., Huang, M.-R., & Zhang, W. (2003). High-Resolution Thermogravimetry of Polyethersulfone Chips in Four Atmospheres. Applied Polymer Science, 90, 3631-3637.

Lin, Y. L., Chiou, J. H., & Lee, C. H. (2014). Effect of silica fouling on the removal of pharmaceuticals and personal care products by nanofiltration and reverse osmosis membranes. J Hazard Mater.

Listiarini, K., Chun, W., Sun, D. D., & Leckie, J. O. (2009). Fouling mechanism and resistance analyses of systems containing sodium alginate, calcium, alum and their combination in dead-end fouling of nanofiltration membranes. Journal of Membrane Science, 344(1-2), 244-251.

Listiarini, K., Tan, L., Sun, D. D., & Leckie, J. O. (2011). Systematic study on calcium–alginate interaction in a hybrid coagulation-nanofiltration system. Journal of Membrane Science, 370(1-2), 109-115.

Liu, Y., Han, G., Li, Y., & Jin, M. (2011). Flower-like zinc oxide deposited on the film of graphene oxide and its photoluminescence. Materials Letters, 65(12), 1885-1888.

Luo, M., Wen, Q., Liu, J., Liu, H., & Jia, Z. (2011). Fabrication of SPES/Nano-TiO2 Composite Ultrafiltration Membrane and Its Anti-fouling Mechanism. Chinese Journal of Chemical Engineering, 19(1), 45-51.

Luo, Y., Guo, W., Ngo, H. H., Nghiem, L. D., Hai, F. I., Zhang, J., Liang, S., & Wang, X. C. (2014). A review on the occurrence of micropollutants in the aquatic environment and their fate and removal during wastewater treatment. Science of The Total Environment, 473–474(0), 619-641.

Macedonio, F., & Drioli, E. (2008). Pressure-driven membrane operations and membrane distillation technology integration for water purification. Desalination, 223(1-3), 396-409.

Madaeni, S. S., & Ghaemi, N. (2007). Characterization of self-cleaning RO membranes coated with TiO2 particles under UV irradiation. Journal of Membrane Science, 303(1–2), 221-233.

60

Moezzi, A., McDonagh, A. M., & Cortie, M. B. (2012). Zinc oxide particles: Synthesis, properties and applications. Chemical Engineering Journal, 185-186, 1-22.

Monsalvo, V. M., McDonald, J. A., Khan, S. J., & Le-Clech, P. (2014). Removal of trace organics by anaerobic membrane bioreactors. Water Research, 49(0), 103-112.

Motsa, M. M., Mamba, B. B., D’Haese, A., Hoek, E. M. V., & Verliefde, A. R. D. (2014). Organic fouling in forward osmosis membranes: The role of feed solution chemistry and membrane structural properties. Journal of Membrane Science, 460(0), 99-109.

Pant, H. R., Park, C. H., Pokharel, P., Tijing, L. D., Lee, D. S., & Kim, C. S. (2013). ZnO micro-flowers assembled on reduced graphene sheets with high photocatalytic activity for removal of pollutants. Powder Technology, 235(0), 853-858.

Pinnau, I., & Freeman, B. D. (1999). Formation and Modification of Polymeric Membranes: Overview. ACS Symposium Series, 744, 1-22.

Pisarenko, A. N., Stanford, B. D., Yan, D., Gerrity, D., & Snyder, S. A. (2012). Effects of ozone and ozone/peroxide on trace organic contaminants and NDMA in drinking water and water reuse applications. Water Research, 46(2), 316-326.

Plappally, A. K., & Lienhard V, J. H. (2012). Energy requirements for water production, treatment, end use, reclamation, and disposal. Renewable and Sustainable Energy Reviews, 16(7), 4818-4848.

Qu, X., Alvarez, P. J., & Li, Q. (2013). Applications of nanotechnology in water and wastewater treatment. Water Research, 47(12), 3931-3946.

Rattana, T., Chaiyakun, S., Witit-anun, N., Nuntawong, N., Chindaudom, P., Oaew, S., Kedkeaw, C., & Limsuwan, P. (2012). Preparation and characterization of graphene oxide nanosheets. Procedia Engineering, 32, 759-764.

Razali, N. F., Mohammad, A. W., & Hilal, N. (2013). Effects of polyaniline nanoparticles in polyethersulfone ultrafiltration membranes: Fouling behaviours by different types of foulant. Journal of Industrial and Engineering Chemistry(0).

Rio Carrillo, A. M., & Frei, C. (2009). Water: A key resource in energy production. Energy Policy, 37(11), 4303-4312.

Salavati-Niasari, M., Davar, F., & Khansari, A. (2011). Nanosphericals and nanobundles of ZnO: Synthesis and characterization. Journal of Alloys and Compounds, 509(1), 61-65.

Saunders. PES Chemical Resistance.

Shen, L., Bian, X., Lu, X., Shi, L., Liu, Z., Chen, L., Hou, Z., & Fan, K. (2012). Preparation and characterization of ZnO/polyethersulfone (PES) hybrid membranes. Desalination, 293, 21-29.

Shirazi, S., Lin, C.-J., & Chen, D. (2010). Inorganic fouling of pressure-driven membrane processes — A critical review. Desalination, 250(1), 236-248.

61

Siddiqi, A., & Anadon, L. D. (2011). The water–energy nexus in Middle East and North Africa. Energy Policy, 39(8), 4529-4540.

Simon, A., McDonald, J. A., Khan, S. J., Price, W. E., & Nghiem, L. D. (2013a). Effects of caustic cleaning on pore size of nanofiltration membranes and their rejection of trace organic chemicals. Journal of Membrane Science, 447(0), 153-162.

Simon, A., McDonald, J. A., Khan, S. J., Price, W. E., & Nghiem, L. D. (2013b). Effects of caustic cleaning on pore size of nanofiltration membranes and their rejection of trace organic chemicals. Journal of Membrane Science, 447(0), 153-162.

Sires, I., & Brillas, E. (2012). Remediation of water pollution caused by pharmaceutical residues based on electrochemical separation and degradation technologies: a review. Environment International, 40, 212-229.

Stropnik, C., & Kaiser, V. (2002). Polymeric membrane preparation in wet phase separation: mechanism and elementary processes. Desalination, 145, 1-12.

Subramani, A., & Hoek, E. M. V. (2008). Direct observation of initial microbial deposition onto reverse osmosis and nanofiltration membranes. Journal of Membrane Science, 319(1–2), 111-125.

SumitomoChemical. (2010). Sumikaexel Polyethersulfone.

Susanto, H., & Ulbricht, M. (2009). Characteristics, performance and stability of polyethersulfone ultrafiltration membranes prepared by phase separation method using different macromolecular additives. Journal of Membrane Science, 327(1-2), 125-135.

Tang, H. P.-o. (2013). Recent development in analysis of persistent organic pollutants under the Stockholm Convention. Trends in Analytical Chemistry, 45, 48-66.

Tansel, B., Sager, J., Rector, T., Garland, J., Strayer, R. F., Levine, L., Roberts, M., Hummerick, M., & Bauer, J. (2006). Significance of hydrated radius and hydration shells on ionic permeability during nanofiltration in dead end and cross flow modes. Separation and Purification Technology, 51(1), 40-47.

Ternes, T. A., Stumpf, M., Mueller, J., Haberer, K., Wilken, R. D., & Servos, M. (1999). Behavior and occurrence of estrogens in municipal sewage treatment plants — I. Investigations in Germany, Canada and Brazil. Science of The Total Environment, 225(1–2), 81-90.

Van De Steene, J. C., Stove, C. P., & Lambert, W. E. (2010). A field study on 8 pharmaceuticals and 1 pesticide in Belgium: Removal rates in waste water treatment plants and occurrence in surface water. Science of The Total Environment, 408(16), 3448-3453.

van de Witte, P., Dijkstra, P. J., van den Berg, J. W. A., & Feijen, J. (1996). Phase separation processes in polymer solutions in relation to membrane formation. Journal of Membrane Science, 117, 1-31.

van den Berg, G. B., Rácz, I. G., & Smolders, C. A. (1989). Mass transfer coefficients in cross-flow ultrafiltration. Journal of Membrane Science, 47(1–2), 25-51.

62

Van der Bruggen, B., Mänttäri, M., & Nyström, M. (2008). Drawbacks of applying nanofiltration and how to avoid them: A review. Separation and Purification Technology, 63(2), 251-263.

Van der Bruggen, B., & Vandecasteele, C. (2003). Removal of pollutants from surface water and groundwater by nanofiltration: overview of possible applications in the drinking water industry. Environmental Pollution, 122, 435-445.

Vatanpour, V., Madaeni, S. S., Khataee, A. R., Salehi, E., Zinadini, S., & Monfared, H. A. (2012). TiO2 embedded mixed matrix PES nanocomposite membranes: Influence of different sizes and types of nanoparticles on antifouling and performance. Desalination, 292, 19-29.

Vatsha, B., Ngila, J. C., & Moutloali, R. M. (2013). Preparation of antifouling polyvinylpyrrolidone (PVP 40K) modified polyethersulfone (PES) ultrafiltration (UF) membrane for water purification. Physics and Chemistry of the Earth.

Wang, H., Wang, L., Qu, C., Su, Y., Yu, S., Zheng, W., & Liu, Y. (2011). Photovoltaic properties of graphene oxide sheets beaded with ZnO nanoparticles. Journal of Solid State Chemistry, 184(4), 881-887.

Wang, T., Long, X., Cheng, Y., Liu, Z., & Yan, S. (2014). The potential toxicity of copper nanoparticles and copper sulphate on juvenile Epinephelus coioides. Aquatic Toxicology, 152(0), 96-104.

Wijmans, J. G., Nakao, S., Van Den Berg, J. W. A., Troelstra, F. R., & Smolders, C. A. (1985). Hydrodynamic resistance of concentration polarization boundary layers in ultrafiltration. Journal of Membrane Science, 22(1), 117-135.

Yang, X., Guo, W., & Lee, W. (2013). Formation of disinfection byproducts upon chlorine dioxide preoxidation followed by chlorination or chloramination of natural organic matter. Chemosphere, 91(11), 1477-1485.

Yang, X., Guo, W., Zhang, X., Chen, F., Ye, T., & Liu, W. (2013). Formation of disinfection by-products after pre-oxidation with chlorine dioxide or ferrate. Water Research, 47(15), 5856-5864.

Yang, X., Peng, J., Chen, B., Guo, W., Liang, Y., Liu, W., & Liu, L. (2012). Effects of ozone and ozone/peroxide pretreatments on disinfection byproduct formation during subsequent chlorination and chloramination. Journal of Hazardous Materials, 239-240, 348-354.

Yang, Y., & Liu, T. (2011). Fabrication and characterization of graphene oxide/zinc oxide nanorods hybrid. Applied Surface Science, 257(21), 8950-8954.

Zhang, M., Zhang, K., De Gusseme, B., & Verstraete, W. (2012). Biogenic silver nanoparticles (bio-Ag 0) decrease biofouling of bio-Ag 0/PES nanocomposite membranes. Water Research, 46(7), 2077-2087.

Zhang, X., Cheng, C., Zhao, J., Ma, L., Sun, S., & Zhao, C. (2013). Polyethersulfone enwrapped graphene oxide porous particles for water treatment. Chemical Engineering Journal, 215-216, 72-81.

Zhao, C., Xue, J., Ran, F., & Sun, S. (2013). Modification of polyethersulfone membranes – A review of methods. Progress in Materials Science, 58(1), 76-150.

63

Zinadini, S., Zinatizadeh, A. A., Rahimi, M., Vatanpour, V., & Zangeneh, H. (2014). Preparation of a novel antifouling mixed matrix PES membrane by embedding graphene oxide nanoplates. Journal of Membrane Science, 453, 292-301.