Effect of Fouling on Removal of Trace Organic Compounds by

Effect of Fouling on Removal of Trace Organic Compounds by Nanofiltration

By Shima Hajibabania

A thesis is submitted in fulfilment of the requirements for the Degree of Master of Engineering by Research

School of Chemical Engineering University of New South Wales Sydney, Australia March 2010

ORIGINALITY STATEMENT ‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.’

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ABSTRACT In order to predict the rejection efficiency of trace organic compounds by nanofiltration (NF), a better understanding of the rejection mechanisms of these contaminants is required. Previous studies have reported that membrane fouling alters membrane surface properties and thus, considerably influences the rejection of trace contaminants. However, the interactions between trace organics, polymeric membrane, fouling layer and other solutes present in the solution and their individual impact on rejection still need to be fully assessed and characterised. In this study, both model and real organic matters (OM) were used to foul NF membrane. Seventeen trace chemicals representing a wide range of physicochemical properties were spiked at environmental concentration (2µg/L) and their rejection behaviour was assessed by liquid chromatography-mass spectrometry (LC-MS/MS). The new and fouled membranes were systematically characterised for surface charge, hydrophobicity and roughness. It was observed that the development of the fouling layer on the membrane surface generally reduced the membrane surface charge and altered the hydrophobicity and surface roughness. Although constant delivered total organic carbon (TOC) was used for all experiments, the fouling layers featured different densities. For example, humic acid and Suwannee river humic acid increased the normalised transmembrane pressure (TMP) by 18 and 5% respectively. This suggests that the fouling deposit was denser for humic acid. In this study, the rejection of charged trace organics improved due to the increased electrostatic interactions by fouled membranes and the trace organics adsorption onto OM. On the other hand, the removal of nonionic compounds decreased when fouling occurred: removal of caffeine, for example, was observed to decrease from 97 to 46% due to presence of cake enhanced concentration polarisation (CECP). To better assess the impact of the different mechanisms affecting trace organics rejection, a rigorous experimental plan was designed. A mass balance study was conducted to characterise the fate of trace organics during NF treatment. The adsorption levels of trace organics onto the membrane surface and the macromolecules were strongly dependant on the physicochemical properties of the trace organics as well as the nature of the OM. The results also indicated that two counteractive mechanisms were generally involved in the rejection of nonionic solutes: (1) the adsorption onto OM resulting in increased rejection and (2) the presence of CECP resulting in lower retention. ii

ACKNOWLEDGMENTS

I believe that without the help of many people, this work could not have been completed. First, I would like to express my gratitude to Dr. Pierre Le-Clech for his motivating and patient supervision throughout the course of my postgraduate study and his confidence in my ability to finish this thesis in time. I am also grateful to Dr Arne Verliefde and Dr. Stuart Khan for their wise words and time in revising my thesis and to Dr. James McDonald for being kind enough to assist me with analysis of my research samples. Many thanks go to Steven Muliawan and Imam Resmon for their support when I mostly needed it and to Deyan Guang for facilitating my work in the lab and all the lab partners for their assistance and encouragement. Lastly, and most importantly, my overwhelming thanks go to my supportive family for their patience, encouragement and understanding during the busy years of my study.

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LIST OF PUBLICATIONS Journal Articles in Preparation Hajibabania S., Verliefde A.R.D., McDonald J., Stuart Khan Le-Clech P., 2009,“ Effect of Membrane Fouling on Removal of Trace Organic Compounds by NF Hajibabania S., Verliefde A.R.D., McDonald J., Stuart Khan Le-Clech P., 2009,“ Fate of Trace Organic Compounds During Nanofiltration”,

Conference Proceedings Hajibabania S., Verliefde A.R.D., McDonald J., Stuart Khan Le-Clech P., 2009,“ Effect of Membrane Fouling on Removal of Trace Organic Compounds by NF/RO in Water and Wastewater Treatment and Reuse”, The Fifth Conference of Asian Membrane Society,Kobe, Japan.

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TABLE OF CONTENTS ABSTRACT .................................................................................................................................... ii ACKNOWLEDGMENTS ............................................................................................................. iii LIST OF PUBLICATIONS ........................................................................................................... iv TABLE OF CONTENTS................................................................................................................ v LIST OF FIGURES ........................................................................................................................ x LIST OF TABLES ....................................................................................................................... xiv LIST OF ABBREVIATIONS ..................................................................................................... xvii 1

Introduction .......................................................................................................................... 2

1.1

Water shortage and water recycling by membrane processes .......................................... 2

1.2

Justification of this research ............................................................................................. 3

1.3

Aims of this project .......................................................................................................... 4

1.4

Thesis outline ................................................................................................................... 5

2

Literature review .................................................................................................................. 8

2.1

Membrane separation processes ....................................................................................... 8

2.2

NF material and fabrication............................................................................................ 11

2.3

NF membrane configurations ......................................................................................... 12

2.4

NF applications in drinking water treatment .................................................................. 12

2.4.1

Softening ................................................................................................................. 12

2.4.2

Organics removal .................................................................................................... 13

2.4.3

Micropollutant removal .......................................................................................... 13

2.5 2.5.1 2.6

Membrane fouling in NF processes ............................................................................... 13 Fouling mechanisms ............................................................................................... 14 Foulants in natural waters .............................................................................................. 15

2.6.1

Particulate materials ................................................................................................ 16

2.6.2

Dissolved inorganic substances .............................................................................. 16

2.6.3

Dissolved organic substances ................................................................................. 16 v

2.6.4

Biological foulants .................................................................................................. 18

2.6.5

Tools used for NF characterization ......................................................................... 19

2.6.5.1

Zeta potential analysis ..................................................................................... 20

2.6.5.2

Contact angle ................................................................................................... 21

2.6.5.3

Scanning electron microscopy (SEM) ............................................................. 22

2.6.5.4

Atomic force microscopy (AFM) .................................................................... 23

2.7 2.7.1 2.8

Trace organic compounds .............................................................................................. 24 Occurrence of trace organic compounds................................................................. 24 Removal of trace organics by NF ................................................................................... 26

2.8.1

Size exclusion ......................................................................................................... 27

2.8.2

Charge repulsion ..................................................................................................... 28

2.8.3

Solute-membrane affinity and adsorption ............................................................... 30

2.9

Effect of fouling on removal of trace organics by NF ................................................... 33

2.9.1

Rejection of hydrophilic compounds ...................................................................... 35

2.9.2

Rejection of hydrophobic compounds .................................................................... 37

2.10

Conclusions ................................................................................................................ 39

3 3.1

Materials and methods ....................................................................................................... 43 Membranes ..................................................................................................................... 43

3.1.1

NF membrane.......................................................................................................... 43

3.1.2

UF membrane.......................................................................................................... 44

3.2

Feed water ...................................................................................................................... 44

3.3

Trace organic compounds .............................................................................................. 46

3.3.1

Other chemicals ...................................................................................................... 47

3.4

Bench-scale crossflow nanofiltration set up................................................................... 48

3.5

UF experimental set up .................................................................................................. 50

3.6

Dead-end nanofiltration Process .................................................................................... 51

3.7

Analytical methods ......................................................................................................... 52 vi

3.7.1

Membrane characterization ..................................................................................... 52

3.8

Trace organic analysis .................................................................................................... 54

3.8.1

Solid phase extraction ............................................................................................. 54

3.8.2

Liquid Chromatography .......................................................................................... 55

3.8.3

Mass Spectrometry.................................................................................................. 55

3.8.4

Calibration and limits of Detection ......................................................................... 55

4

Effect of fouling on removal of trace organic compounds by NF ..................................... 57

4.1

Materials and methods ................................................................................................... 57

4.2

Effect of compaction of membrane on membrane surface properties ........................... 58

4.3

Rejection of trace organic compounds by virgin membrane.......................................... 59

4.3.1

Impact of trace organic compounds on membrane performance in Milli-Q .......... 60

4.3.2

Assessment of initial adsorption of hydrophobic compounds on NF membrane ... 60

4.3.3

Rejection by new membrane................................................................................... 61

4.3.3.1

Hydrophilic nonionic compounds ................................................................... 61

4.3.3.2

Hydrophobic nonionic compounds.................................................................. 62

4.3.3.3

Hydrophilic nonionic compounds ................................................................... 66

4.3.4 4.4

Interim conclusions ................................................................................................. 67 Fouling of NF membrane ............................................................................................... 68

4.4.1

Feed water characterization .................................................................................... 68

4.4.2

Effect of fouling on membrane performance .......................................................... 70

4.4.3

Effect of fouling on membrane properties .............................................................. 73

4.4.3.1

Humic acid ....................................................................................................... 76

4.4.3.2

Alginic acid...................................................................................................... 77

4.4.3.3

L-lysine ............................................................................................................ 77

4.4.3.4

SRHA............................................................................................................... 78

4.4.3.5

Surface water ................................................................................................... 78

4.4.3.6

MBR permeate................................................................................................. 79 vii

4.5

Rejection of trace organic compounds by fouled membranes ....................................... 79

4.5.1

Ionic compounds ..................................................................................................... 80

4.5.2

Hydrophobic nonionic compounds ......................................................................... 85

4.5.3

Hydrophilic compounds .......................................................................................... 89

4.6 5

Conclusions .................................................................................................................... 94 Fate of trace organics during NF treatment ....................................................................... 98

5.1

Introduction .................................................................................................................... 98

5.2

Materials and methods ................................................................................................... 99

5.3

Selection of UF membrane........................................................................................... 104

5.4

Adsorption behaviour of trace organics ....................................................................... 105

5.4.1

Natural decomposition of trace organics in feed solutions ................................... 105

5.4.2

Adsorption on UF membrane................................................................................ 106

5.4.3

Adsorption onto NF membrane ............................................................................ 108

5.5

Adsorption onto macromolecules................................................................................. 110

5.5.1.1

Hydrophilic nonionic compounds ................................................................. 111

5.5.1.2

Hydrophobic nonionic compounds................................................................ 113

5.5.1.3

Hydrophobic ionic compounds...................................................................... 115

5.5.1.4

Interim conclusions........................................................................................ 117

5.6

Effect of pre-adsorption of trace organics onto OM on their removal by NF .............. 117

5.6.1

Hydrophobic nonionic compounds ....................................................................... 121

5.6.2

Hydrophobic ionic compounds ............................................................................. 123

5.7

Effect of fouling layer on removal of trace organics by pre-fouled membrane ........... 124

5.7.1

Hydrophilic nonionic compounds ......................................................................... 126

5.7.2


5.7.3


5.8 5.8.1

Combined effect of fouling and adsorption on rejection of trace organics .................. 131 Hydrophilic nonionic compounds ......................................................................... 132 viii

5.8.2


5.8.3


5.9 6

Conclusions .................................................................................................................. 139 Conclusions and recommendations.................................................................................. 142

6.1

General rejection trends obtained with nonionic compounds ...................................... 144

6.2

General rejection trends obtained with the ionic compounds ...................................... 145

REFERENCES ........................................................................................................................... 147

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LIST OF FIGURES Figure 2.1- Membrane fouling mechanisms, (a) complete blocking, (b) intermediate blocking, (c) standard blocking and (d) cake filtration. .................................................................. 15 Figure 3.1– Shimatzu800 TOC analyser. ........................................................................ 45 Figure 3.2– Schematic illustration of the experimental set up........................................ 48 Figure 3.3– Filtration protocol for compaction, fouling and rejection test..................... 49 Figure 3.4– Schematic illustration of UF experimental set up in dead-end mode. ......... 50 Figure 3.5– Schematic diagram of the experimental set-up to measure the streaming potential: (a) pump with three-way-valve; (b) Pt-electrodes; (c) glass plate; (d) membrane ......... 53 Figure 3.6– Rame-hart model 200-F1 goniometer. ........................................................ 54 Figure 4.1–Temporal changes of fluoxetine and bisphenol A rejection by new NF270. 61 Figure 4.2– Rejection of hydrophobic nonionic compounds according to their molecular weights. ........................................................................................................................... 64 Figure 4.3– Rejection of hydrophobic nonionic compounds vs. their LogP values. ...... 65 Figure 4.4– Rejection of hydrophilic nonionic compounds according to their molecular weight.............................................................................................................................. 67 Figure 4.5– LC-OCD diagrams for the different feed waters used for the fouling experiments. ......................................................................................................................................... 69 Figure 4.6– NF270 Surface morphology imaged by AFM; (a) virgin ,(b) fouled with humic acid, (c) l-lysine, (d) alginate, (e) SRHA, (f) surface water and (g) MBR permeate. The images were 2×2µm in scan size. ................................................................................... 76 Figure 4.7– Rejection of diclofenac by HA, alginate, lysine, SRHA and surface water fouled membranes as a function of time (NF270 at pH6.5 and 25°C, over 48 hours of filtration). 80 Figure 4.8– Rejection of naproxen by HA, alginate, lysine, SRHA and surface water fouled (NF270 at pH6.5 and 25°C, over 48 hours of filtration)................................................. 80 x

Figure 4.9– Rejection of ibuprofen by HA, alginate, lysine, SRHA and surface water fouled (NF270 at pH6.5 and 25°C, over 48 hours of filtration)................................................. 81 Figure 4.10– Rejection of hydrophobic ionic compounds by membranes fouled by various organic matter. ................................................................................................................ 82 Figure 4.11– Rejection of diclofenac by HA, alginate, lysine, SRHA and surface water fouled NF270 vs. surface charge. ............................................................................................... 83 Figure 4.12– Rejection of diclofenac by HA, alginate, lysine, SRHA and surface water fouled NF270 vs. surface roughness. ......................................................................................... 83 Figure 4.13– Rejection of diclofenac by HA, alginate, lysine, SRHA and surface water fouled NF270 vs. fouled membranes contact angle. .................................................................. 84 Figure 4.14– Rejection of risperidone by HA, alginate, lysine, SRHA and surface water fouled (NF270 at pH6.5 and 25°C, over 48 hours of filtration). .................................... 86 Figure 4.15– The rejection of hydrophobic nonionic compounds by different fouled membranes. ..................................................................................................................... 87 Figure 4.16– Rejection of risperidone by HA, alginate, lysine, SRHA and surface water fouled NF270 vs. fouled membranes contact angle. ....................................................... 88 Figure 4.17– Rejection of trimethoprim by HA, alginate, lysine, SRHA, surface water and MBR permeate fouled (NF270 at pH6.5 and 25°C, over 48 hours of filtration). ........... 90 Figure 4.18– Rejection of paracetamol by HA, alginate, lysine, SRHA, surface water and MBR permeate fouled (NF270 at pH6.5 and 25°C, over 48 hours of filtration). ........... 91 Figure 4.19– The rejection of hydrophilic nonionic compounds by new and fouled membranes. ..................................................................................................................... 91 Figure 4.20– Rejection of trimethoprim by HA, alginate, lysine, SRHA, surface water and MBR fouled NF270 vs surface roughness. ..................................................................... 93 Figure 4.21– Rejection of trimethoprim by HA, alginate, lysine, SRHA, surface water and MBR fouled NF270 vs. contact angle............................................................................. 93

xi

Figure 5.1– Filtration of macromolecules and the adsorbed trace organics by UF membrane. ....................................................................................................................................... 111 Figure 5.2– Fate of caffeine in alginate, BSA and humic acid solutions when filtered by UF ....................................................................................................................................... 112 Figure 5.3– Adsorption of hydrophilic nonionic compounds onto alginate against their Log P values. ........................................................................................................................... 113 Figure 5.4– Adsorption of hydrophobic nonionic compounds onto alginic acid, humic acid and BSA solutions......................................................................................................... 115 Figure 5.5– Adsorption of hydrophobic ionic compounds onto humic acid, alginate and BSA in the bulk solution........................................................................................................ 116 Figure 5.6– Fate of naproxen in (a) alginic acid, (b) humic acid and (c) BSA solutions when filtered by UF (* Value below calculated STD). .......................................................... 116 Figure 5.7– Normalised permeate flux during nanofiltration of UF pre-treated trace organics + macromolecules mixtures. ......................................................................................... 118 Figure 5.8– Nanofiltration of UF pre-treated feed solutions. ....................................... 119 Figure 5.9– Rejection of hydrophilic nonionic compounds in Milli-Q and UF treated feed solutions (with level of NF membrane adsorption (ANF) from Section 5.4.3).............. 121 Figure 5.10– Rejection efficiencies of hydrophobic nonionic compounds in Milli-Q and UF pre-treated feed solutions (with level of NF membrane adsorption (ANF) from Section 5.4.4). ....................................................................................................................................... 122 Figure 5.11– Rejection efficiencies of hydrophobic ionic compounds in Milli-Q and UF pretreated feed solutions (with level of NF membrane adsorption (ANF) from Section 5.4.5). 124 Figure 5.12– Normalised permeate flux during filtration of BSA, humic acid and alginate ([TOC] = 20 mg/L). ...................................................................................................... 125 Figure 5.13– Schematic of filtration of trace organics by OM pre-fouled NF membrane.126

xii

Figure 5.14– Rejection of hydrophilic nonionic compounds in Milli-Q with new and prefouled membranes. ........................................................................................................ 127 Figure 5.15– Rejection of hydrophobic nonionic compounds in Milli-Q and pre-fouled membranes. ................................................................................................................... 128 Figure 5.16– Rejection of hydrophobic ionic compounds by Milli-Q and pre-fouled membranes. ................................................................................................................... 129 Figure 5.17– Schematic of filtration of complex feed solutions by NF........................ 132 Figure 5.18– Rejection of hydrophilic nonionic compounds in Milli-Q and macromolecule solutions (the results for BSA are not shown due to the very analytical error). ........... 132 Figure 5.19– Relative behaviour of hydrophilic nonionic solutes with and without alginate. ....................................................................................................................................... 133 Figure 5.20– Relative behaviours of hydrophilic nonionic solutes with and without humic acid. ............................................................................................................................... 134 Figure 5.21– Rejection of hydrophobic nonionic compounds in Milli-Q and macromolecule solutions. ....................................................................................................................... 135 Figure 5.22– Rejection of hydrophobic ionic compounds in Milli-Q and macromolecule solutions. ....................................................................................................................... 136 Figure 5.23– Relative behaviours of hydrophilic nonionic solutes with and without alginate. ....................................................................................................................................... 138 Figure 5.24– Relative behaviours of hydrophilic nonionic solutes with and without BSA. 138

xiii

LIST OF TABLES Table 2.1– General characteristics of membrane processes (Jun et al., 2001). .............. 10 Table 2.2– Characterisation methods for NF membranes (Schäfer et al., 2003)............ 19 Table 2.3– The zeta potential of various NF membranes. .............................................. 20 Table 2.4– Contact angle behaviours (Wilson et al., 1990)............................................ 21 Table 2.5– Contact angle of clean and fouled membrane (Cho et al., 1998). ................ 22 Table 2.6– Measured maximum concentration of some selected pharmaceuticals in STP outflow and surface water in Australia, Europe and USA (CRC, 2007). ....................... 25 Table 2.7– Rejection of various trace organics by virgin NF and fouled with several NOM ......................................................................................................................................... 34 Table 2.8– Changes in rejection trends of trace organic compounds due to fouling. ..... 39 Table 3.1– Selected membranes properties .................................................................... 44 Table 3.2– Properties of UF-5kDa and the rejection efficiencies of various macromolecules. ......................................................................................................................................... 44 Table 3.3– The ratio of [TOC] content of organic matter............................................... 45 Table 3.4– Properties of selected trace organic compounds ........................................... 47 Table 4.1– Average removal efficiency of trace organic compounds by l-lysine fouled membrane and their calculated confidence level (n = 3). ............................................... 58 Table 4.2– Characteristics of virgin and compacted NF270 (n ≥ 4). .............................. 59 Table 4.3– Rejection of hydrophobic ionic compounds by NF270 at pH=6.5, T=25°C and under constant flux mode. ............................................................................................... 61 Table 4.4– Rejection of hydrophobic nonionic compounds by new NF270 at pH=6.5, T=25°C and under constant flux mode. ........................................................................................ 63

xiv

Table 4.5– Rejection of hydrophilic nonionic compounds by virgin NF270 at pH=6.5, T=25°C and under constant flux operation. .................................................................... 66 Table 4.6– Characteristics of the feed water. .................................................................. 69 Table 4.7– TMP and salt rejection for the virgin and fouled NF270 membranes. ......... 71 Table 4.8– Characteristics for compacted and fouled NF270 in terms of roughness, charge and hydrophobicity. ........................................................................................................ 73 Table 4.9– R-square values obtained for linear regression fitted rejection and various surface characteristics of the fouled membranes. ........................................................................ 84 Table 4.10– R-square values obtained for linear regression fitted rejection of hydrophobic nonionic compounds and fouled membranes roughness and contact angle. .................. 89 Table 4.11– R-square values obtained for linear regression fitted rejection of hydrophilic nonionic compounds as a function of fouled membranes roughness and contact angle 94 Table 5.1– Summary of the experiments, procedure and equations used for mass balances in Chapter 5. ...................................................................................................................... 102 Table 5.2– Characteristics of the organic carbon content of alginic acid, BSA and humic acid. ....................................................................................................................................... 103 Table 5.3– Organic matter rejection efficiencies by UF 5kDa membrane. .................. 104 Table 5.4– Level of decomposition & adsorption of trace organics onto the UF membrane and confidence levels of analyses. ................................................................................ 107 Table 5.5– Relative adsorption of trace organics onto NF membrane reported the confidence levels of analysis. .......................................................................................................... 109 Table 5.6– Relative adsorption (% of initial feed concentration) of hydrophilic nonionic compounds on humic acid, alginic acid and BSA. ....................................................... 112 Table 5.7– Adsorption of hydrophobic nonionic on humic acid, alginic acid and BSA.114 Table 5.8– Adsorption of hydrophobic ionic on humic acid, alginic acid and BSA. ... 115

xv

Table 5.9– Level for adsorption of trace organic compounds to various OM. ............. 117 Table 5.10– Concentration of trace organic compounds in the UF pre-treated feed solutions. ....................................................................................................................................... 119 Table 5.11– Characteristics for virgin and fouled NF270 in terms of roughness, charge and hydrophobicity. ............................................................................................................. 126

xvi

LIST OF ABBREVIATIONS

AFM

Atomic Force Microscopy

BSA

Bovine Serum Albumin

CA

Cellulose Acetate

CECP

Cake-Enhanced Concentration Polarisation

CP

Concentration Polarization

DBP

Disinfection by-products

DIA

Dialysis

DOC

Dissolved Organic Carbon

ED

Electro Dialysis

EDCs

Endocrine Disrupting Compounds

GAC

Granular Activated Carbon

HPLC

High Performance Liquid Chromatography

LC-MS/MS

Liquid Chromatography/Tandem Mass Spectrometry

LC-OCD

Liquid Chromatography-Organic Carbon Detector

LMW

Low Molecular Weight

LOQ

Limit of Quantitation

MBR

Membrane Bio-Reactor

MF

Microfiltration

MTBE

Methyl Tert-butyl Ether

MW

Molecular Weight

xvii

MWCO

Molecular Weight Cut-Off

NF

Nanofiltration

NOM

Natural Organic Matter

OC

Organic Carbon

OM

Organic Matter

PA

Polyamide

PAC

Powdered Activated Carbon

PCPs

Personal Care Products

PES

Polyether Sulfone

PhACs

Pharmaceutically Active Compounds

PI

Polyimide

PS

Polysulfonated Polysulfone

PV

Pervaporation

RO

Reverse Osmosis

SEM

Scanning Electron Microscope

SPE

Solid Phase Extraction

SRHA

Suwannee River Humic Acid

STD

Standard Deviation

SUVA

Specific UV Adsorption

TMP

Trans-Membrane Pressure

TOC

Total Organic Carbon

UF

Ultrafiltration

xviii

Introduction Chapter 1

1

1

Introduction

1.1 Water shortage and water recycling by membrane processes Demand for water increases annually at a rate of at least 3%, and the rapid reduction of subterranean aquifers combined with the increasing salinity of non renewable sources will continue to exacerbate the water shortage problems worldwide (Shaalan, 2003). Particularly, water will be scarce in areas with low rainfall and of relatively high population density. In addition to water shortage, contemporary environmental legislation maintains that discharge is not a favorable option for sewage effluent, unless all other possibilities have been rejected. To overcome these issues, different approaches are proposed including water conservation and recycling. Water conservation can be limited and does not present a response to growing water shortage; however, it is important in the institutional adaptation to increasing scarcity. On the other hand, recycling water is an important aspect of water resource and environment management policies, ensuring reliable alternative water resources. During the last decade, Australia has entered a dryer climate phase and is facing serious problems in the provision of water resources for urban areas. Consequently, most state and territory capital cities are required to implement domestic water restrictions. Water recycling schemes in Australia started to emerge during the late 1980s and the 1990s. Since, there have been significant policy developments in water recycling. Most water recycling schemes are aimed to either off-set current potable water use or expand potable water supplies (Radcliffe, 2006). In indirect potable recycling, treated water is discharged from a plant into a stream and then re-purified in a drinking water treatment facility. There are numerous national examples of indirect potable recycling plants, including Canberra’s Lower Molonglo sewerage treatment plant (STP) discharging into the Molonglo and Murrumbidgee Rivers to Burrinjuck Dam, and Adelaide’s Hahndorf STP discharging into the Onkaparinga River to flow to Mount Bold Reservoir. However, pathogens and chemicals were identified as a primary concern with recycled water (Khan et al., 2007). There are considerable current concerns regarding the potential negative effects of organic chemical contaminants in recycled water on human health, as many pesticides, pharmaceutically active compounds (PhACs) and endocrine 2

disrupting chemicals (EDCs), especially steroid hormones, have been detected in wastewater effluent and in natural water bodies in global scale (Khan, 2002; CRC, 2007). These contaminants are suspected to be carcinogenic, even in very low concentrations (Kolpin et al., 2002). A variety of treatment processes are used in upgrading municipal wastewater treatment plants for water recycling applications. Within these membrane processes are now frequently used in water and waste water treatment applications due to the high quality of the produced water and their small plant size. Where trace organics are to be removed, NF and reverse osmosis (RO) are the appropriate membrane processes. Particularly, NF process application in drinking water industry has extended tremendously (Ventresque et al., 1997).

1.2 Justification of this research Recent studies have widely investigated the removal of trace organic compounds by NF/RO membranes; and it was observed that although high pressure membrane systems efficiently remove most organic micropollutants, some of these compounds are not retained completely (Van der Bruggen et al., 2008). Fouling is an inevitable phenomenon in membrane filtration systems. Studies have reported that a fouling layer alters membrane surface properties and thus, influences the rejection of trace contaminants. Even though the prediction of the removal of any given compound by NF/RO is expected to be based on its physicochemical properties, fundamental understanding of the complex removal mechanisms of trace organics is still required. Finally, the various interactions between trace organics, polymeric membrane, fouling layer and other solutes in the solution still need to be fully assessed and characterised. In order to obtain a better understanding of the interactions between the fouling layer formed on the membrane surface and the trace organic compounds, advanced characterisation techniques will be used to characterise the fouled membranes using a rigorous methodology. Most of previous studies were based on the removal of trace organics present at high concentrations (mg/L); whereas, in full scale plants, the concentrations of trace organics and foulant in water are at environmental levels (µgng/L). Moreover, full scale plants operate at constant flux mode, while, research studies which applying constant flux mode are relatively scarce. In this research, a more 3

realistic approach was chosen to study the phenomena occurring in NF operation, and included: •

Environmental concentrations of trace organics in the feed,

•

Low concentrations of foulants,

•

Relatively long-term filtration runs,

•

Constant flux operation,

•

Full characterisation of the fouled membranes and,

•

Constant feed pH and temperature.

While, literature data reporting the observed low rejection values of hydrophobic solutes by adsorption of these compounds to the membrane is available, this effect has not been quantified yet. The relative contributions of fouling and hydrophobic interactions to the rejection have not been properly distinguished. This thesis will study the relative effects of the different mechanisms involved in the rejection of trace organics through the design of a series of experiments mimicking individual behaviour of these compounds during NF operation.

1.3 Aims of this project This research aims to study the influence of fouling on rejection of trace organic compounds in conjunction with membrane and solute physicochemical properties. The specific objectives of this work are: 1. To critically review published studies relevant to this field of research, 2. To assess the effect of membrane fouling on the retention and accumulation of trace

organics

in

conjunction

with

solute-membrane

physicochemical

characteristics, 3. To investigate the temporal changes of the fouling layer at low concentrations of different organic macromolecules, 4. To characterise the effect of macromolecule natures and properties on the fouled membrane surface properties, 5. To quantify the adsorption of trace organics during NF and to contrast with solute physicochemical properties, and

4

6. To assess the relative effect of the different mechanisms defining fate of trace organics during NF filtration.

1.4 Thesis outline This thesis will comprehensively review previous research conducted on the removal of trace organic compounds by NF and will compare this with new insights gained in this study. The final goal is to gain a better understanding on the effect of membrane fouling on rejection of trace organics based on their physico-chemical properties. Chapter 2 reports the literature review of background which provides information about the membrane separation processes, fouling in NF processes, NF applications in drinking water industry, foulants in natural waters. This chapter further discusses occurrence of trace organic compounds, removal of trace organics by NF and, the current understanding of the effect of fouling on removal of trace organics by NF. Chapter 3 discusses the materials and describes the methods used in this research. In Chapter 4, the influence of membrane fouling on NF rejection of trace organic compounds is investigated. By using different types of organic matter (OM), different types of membrane fouling is achieved, and the influence of these fouling layers on the rejection of organic micropollutants is investigated. The characteristics of the fouled membranes are used to assess and explain NF rejection mechanisms. Chapter 5 discusses the fate of trace organics in different water matrices during NF by quantification of trace organics adsorptive behaviour. Moreover, the effect of adsorption of trace organics onto macromolecules in the feed water matrix and the fouling layer will be discussed. Finally, these results will be compared to the ones obtained from the NF filtration of complex water solutions to evaluate the contribution of adsorption and fouling in the rejection efficiencies of NF. Chapter 6 reports the conclusions of results carried out from this research in chapter 4 and 5, as well as the recommendations for future studies.

5

6

Literature review Chapter 2

7

2

Literature review

In this chapter, the current understanding on membrane processes and applications is discussed. Removal efficiencies of trace organic compounds by NF, as well as the mechanisms and factors affecting the rejection of these solutes will be reviewed. Finally, the effect of fouling and the role of physico-chemical properties of membrane and solutes on removal of these compounds will be particularly evaluated.

2.1 Membrane separation processes The term “membrane filtration” describes a family of separation processes which rely on semi-permeable material to separate fluids, gases, particles and/or dissolved solutes. The variety of membrane systems and the range of materials that can be separated or removed from aqueous/gaseous solutions are broad. Membrane processes can be classified into a number of groups depending on the type of material from which the membrane is made, the nature of the driving force, the separation mechanism, and/or the nominal size of the particles or solutes that can be separated. The driving force used to promote (force) separation, can be gradients in pressure, electrical voltage, temperature, or concentration. In the case of pressure-driven processes, the TMP is responsible for water transport through the membrane, and permeable solutes transport is driven by both their concentration gradient and the transmembrane pressure (a combination of diffusion and convection, respectively). In addition, there are two different filtration modes used in membrane processes: dead-end or crossflow. The velocity tangential to the surface of the membrane is applied to control build-up of fouling material in crossflow mode. One of the main factors determining solute permeation through the membrane is the membrane pore size. Membrane pore sizes range from more than one micrometer to less than one nanometer. The membrane basic filtration principles can be determined using Equations (2.1-4). Transmembrane pressure [bar] Water transport (flux) [L/m2h]

Jw=A . 8

Equation 2.1

Equation 2.2

Solute transport [g/m2h]

Js= .

Rejection %

R% =

Where

is the feed pressure (bar),

Equation 2.3 /

100

the concentrate pressure (bar),

pressure (bar), A the water permeability (L/m2h bar),

Equation 2.4 the permeate

the osmotic pressure (bar), B the

solute permeability (L/m2h bar), K a distribution coefficient, Cm and Cp the concentrations at the membrane surface and permeate (g/L), respectively. In the drinking water industry, membrane processes are used for solutes (dissolved organics and colour) and microbial removal, softening as well as desalination. Common membrane separation processes include microfiltration (MF), ultrafiltration (UF), NF, RO, pervaporation (PV), dialysis (DIA) and electrodialysis (ED). Table 2.1 summarizes the general characteristics of membrane processes used in water treatment including approximate operating ranges, separation mechanisms, driving forces, and constituents removed.

9

Table 2.1– General characteristics of membrane processes (Jun et al., 2001). Membrane process

Driving force

Separation mechanism

Microfiltration

Pressure difference

Sieve

Ultrafiltration

Pressure difference

Sieve

Nanofiltration

Pressure difference

Reverse osmosis

Pressure difference

Electrodialysis

Electromotive

Dialysis

Concentration difference

Sieve, solution/diffusion and electrostatic repulsion Solution/diffusion and electrostatic repulsion Ion exchange with selective membranes Diffusion

Permeation

Typical constituents removed

Water and solutes

Turbidity, protozoa and cyst, bacteria and viruses

Water and small solutes Water, very small solutes and some ions Water and very small solutes

Macromolecules, colloids, proteins, bacteria and viruses Small solutes, color, hardness, viruses Very small solutes, color, hardness, sulfates, nitrates, sodium, other ions

Water and Ions

Ions

Water and small particles

Macromolecules, colloids, most bacteria, viruses

Table 2.1 shows the common pressure driven membrane processes used in drinking water treatment and the various materials found in raw waters. According to Table 2.1, pressure driven membranes can be used to remove a wide range of components, ranging from suspended solids (eg. MF) to small organic compounds and ions (eg. NF/RO). MF membranes have the largest pore size of the membrane processes, ranging from 0.1 to several µm. As a consequence of their large pore size, MF membranes can be operated at low pressure ( 5000 Da), organic colloids and low molecular weight acids (MW < 1000 Da) (Shon et al., 2006). In aquatic environments humic substances are considered to be a major fraction of NOM, and consist of anionic macromolecules of low to moderate MW. These substances contain both carboxylic (60 to 90% of all functional groups) and phenolic groups. As a result, humics are generally negatively charged in the pH range of natural waters. Large MW humics have been reported to be responsible for cake formation on membrane surfaces (Listiarini et al., 2009). Although humic substances are generally negatively charged, studies have shown that a more negatively charged membrane surface did not necessarily limit fouling (Kennedy et al., 2008). This indicates that electrostatic repulsion was not effective for NOM fouling control. Generally, the negative membrane charge originates from the presence of carboxylic functional groups. Consequently, due to increased hydrophobic affinity with the carboxylic groups and hydrogen bonding, humic substances might be drawn to more negatively charged membranes. In addition, multivalent cations enhance fouling by NOM dramatically (Hasson et al., 2001). The influence of inorganic scalants and NOM on NF fouling was reported to be dependent on ion valency (monovalent or divalent), types of inorganic scalants (chloride, carbonate, sulphate and phosphate) and different divalent inorganic scalants (calcium and magnesium). In fact, divalent calcium cations and phosphates in particular, were reported to cause the most severe NOM fouling by pore blockage and cake formation due to bridging (Jarusutthirak et al., 2007). The pH of the feed solution has an important effect on the fouling behaviours of humics. At low pH (less than pH 4), humic acid reduces in size (in terms of molecular configuration) and due to the declined inter-chain electrostatic repulsion and the increased hydrophobicity, these macromolecules pass more easily through the membrane pores which results in their decreased rejection (Al-Amoudi et al., 2007).

17

Furthermore, NOM can be fractionated into hydrophilic and hydrophobic fractions. Hydrophobic fractions were found to be the major factor causing permeate flux decline, while the hydrophilic fraction has relatively small effect (Nilson et al., 1996). This can be due to higher adsorbance tendency of the hydrophobic compounds, compared to hydrophilic species, onto the membrane surface. In addition, it was reported that the hydrophobicity of NOM increases with increasing MW (Hong et al., 1997). The rate and extent of fouling are mainly influenced by operating conditions, such as the applied pressure and the cross-flow velocity. Applied pressure governs the initial permeate flux and the resulting convective transport of foulants towards the membrane surface. A higher permeate flux results in more severe fouling due to higher permeation drag and therefore, a more compressed fouling layer (Hong et al., 1997). Organic fouling could cause either reversible or irreversible flux decline. Depending on the extent and nature of the fouling the reduced flux can be restored partially or fully by standard or more rigorous use of chemical cleaning agents (Al-Amoudi et al., 2007).

2.6.4

Biological foulants

Fouling by biological material (bio-fouling) is problematic in (certain) membrane filtration systems. In general, membrane biofouling is due to the dynamic accumulation and growth of bacteria and to a lesser degree, fungi and other micro-organisms (LappinScott et al., 1989). During filtration, micro-organisms are transported to the membrane surface, where they can attach with sufficient force. Once the microbial cell is attached to the surface of the membrane, it starts consuming nutrients from the feed water to grow and reproduce thus forming a biofilm layer. Biofouling is favoured by several factors including reduced feed water cross-flow velocities, elevated operating pressures and temperatures, high feed water concentrations of organics, smaller feed channels, and membrane materials with enhanced bacterial affinities (Horsch et al., 2005). Usually the membrane system responds to biofouling problems by an increase in TMP and/or decrease in permeate flux. Moreover, the pressure drop from feed to concentrate can also increase. However, this response can be caused by other fouling mechanisms as 18

well. Controlling biofouling can be achieved by removal of easily degradable components from the feed water, by disinfection, or by performing regular active cleanings (Horsch et al., 2005).

2.6.5

Tools used for NF characterisation

NF membranes vary in materials, morphologies and applications. Characterisation of the membrane plays an important role in membrane selection for the intended application. Generally, NF membranes are characterised by three parameters: performance, morphology and charge. Based on the mentioned criteria, the appropriate membrane is selected. Several characterisation methods are summarized in Table 2.2. Table 2.2– Characterisation methods for NF membranes (Schäfer et al., 2003)

Methods

Characteristic Hydrodynamics

Permeability measurement

N/A

Membrane resistance Morphology

Retention measurements with N/A

Pore size/MWCO

N/A

Pore size

uncharged molecules Gas adsorption/desorption

Field emission microscopy (FEM)

Characterisation of foulant depositions Microscopy

Scanning electron microscopy (SEM)

Characterisation of foulant depositions Surface roughness

Atomic force microscopy (AFM) Surface chemistry

Spectroscopy

ATR-FTIR ESR/NMR Raman spectroscopy XPS(ESCA)

Chemical composition (of clean and fouled membranes)

EDAX Contact angle

Captive bubble method Sessile drop method

Hydrophobicity

Electro-kinetic measurements

N/A

Zeta potential

Titration

N/A

Impedance spectroscopy

N/A

Ion exchange capacity Ionic conductivity

19

2.6.5.1 Zeta potential analysis The electrical characteristics of a membrane surface (surface charge) are often expressed in terms of membrane zeta potential, which can be estimated using streaming potential measurements. It is possible to determine the membrane surface charge both quantitatively and qualitatively using this method. Streaming potential is the potential induced when an electrolyte solution flows across a stationary, charged surface. In streaming potential measurements, a pressure difference is applied across the membrane, which is located in an electrolyte solution. As a consequence, the sheer of fluid displaces the electrical double layer. Thus, an electrical potential is generated between the ends of the measurement channel. The potential difference is proportional to the charge on the surface and to the pressure applied (Kim et al., 1996). Streaming potential measurements can also be determined over a range of solution pH reflecting the membrane surface isoelectric point. Polymeric membranes are generally negatively charged due to the presence of carboxylic functional groups in the membrane active layer or material (Schäfer et al., 2003) and thus the membrane charge occurs due to dissociation of the functional groups (as well as adsorption of ions and charged macromolecules to the membrane surface) and is dependent on pH. Table 2.3 shows various commercial NF membranes composed from different materials and their zeta potential values. Table 2.3– The zeta potential of various NF membranes. Membrane

Zeta potential (mV)

Material

pH 4

pH 6

pH 8

Reference

NF90

PA

-

-

-31

(Kim et al., 1996)

NF270

PA

-

-

-25

(Nghiem et al., 2008)

CE

CA

-3

-11

-15

(Childress et al., 1996)

ESNA

PA

-

-

-11

(Yoon et al., 2005)

TFC-SR2

PA

-5

-9

-10


As shown in Table 2.3, pH affects the membrane charge. Zeta potentials for most membranes has been observed in many studies to become increasingly more negative as pH is increased. For instance, TFC-SR2 membrane zeta potential is reduced from -4.7 to -10.4 mV at pH 4 and 8 respectively (Nghiem et al., 2008). This is due to the 20

dissociation of the functional groups at higher pH values. Electrostatic interactions between the membrane and charged compounds strongly influence the filtration properties. Therefore, electrical characteristics of membranes should be evaluated. 2.6.5.2 Contact angle Contact angle (θ) is the angle formed at the interface of three phases (solid-liquidvapour) tangential to the drop deposited on the membrane surface and a common technique to assess the hydrophobicity of the membrane by measuring the wettability of the surface. Lower contact angle values indicate high affinity between the liquid and the solid surface (Adham et al., 2006). The most common contact angle methods in membrane applications are the sessile drop, Wihelmy plate, captive air bubble and capillary rise methods. Sessile drop method is the most common technique used to measure contact angle on flat surfaces. Usually, this technique is accompanied by the drop profile technique, i.e. the measurement of advancing and receding contact angle. When a drop of liquid is placed on a solid surface, the drop will spread out until equilibrium between liquid cohesion and its adhesion to the solid is reached. In advancing contact angle, the droplets are added successively until a plateau is obtained while in receding contact angle, the droplets are retracted successively (Chau, 2009). Table 2.4 shows the variations in contact angles and their behaviours. Table 2.4– Contact angle behaviours (Wilson et al., 1990). Figures (AWWA et al., 1996)

Contact angle (θ)

Liquid/solid interaction

Spreading behaviour or wettability

1800

No affinity

No Spreading

>900

Weak affinity

Poor wetting

10.5, resulting in a higher removal efficiency (Schäfer et al., 2001).

2.8.3

Solute-membrane affinity and adsorption

Hydrophobicity is defined as a physical property that is repelled by water. Hydrophobic molecules are not electrically polarized. Thus, they are repelled by water, which is electrically polarized (Gross et al., 2003). The hydrophobicity of solutes can be correlated and quantified with the logarithm of the octanol-water partition coefficient (Log Kow or LogP). The LogP values are determined as the log of the ratio of the solute concentration in an octanol phase over the concentration in a water phase at adjusted pH. Molecules with LogP > 2 are usually referred to as hydrophobic. 30

As mentioned earlier, relative membrane surface hydrophobicity can be determined by contact angle measurements with water. Previous studies determined that hydrophobic membranes with larger contact angles could reject and adsorb more mass per unit area of hydrophobic organic compounds than hydrophilic membranes with smaller contact angles (Chang et al., 2002). Hydrophobic interactions between membrane and solute are an important interaction mechanism between solute and membrane in NF (Van der Bruggen et al., 2006). Adsorption can occur due to the high affinity between hydrophobic solutes and hydrophobic membranes, when Van-der Waals attraction is present (Verliefde et al., 2009). However, adsorption is a time-dependent phenomenon that only occurs until the adsorption capacity of the membrane is exceeded. Afterwards, solute-membrane interactions (and thus rejection) are not only dependent on Van der Waals attraction, but also on hydrogen bonding and Lewis acid-base interactions. Hydrogen bonding and hydrophobic interactions can act independently or concurrently and it is difficult to separate the two independent effects (Schäfer et al., 2003). The interplay of a polar (Van der Waals) and polar interactions can lead to attraction of solutes towards the membrane surface and the membrane pores, as well as repulsion away from the membrane surface (and membrane pores). In a recent study, the free energy of interaction between a solute and the membrane (∆Gi) in water phase was considered to determine the attractive or repulsive solutemembrane affinity interaction (Verliefde, 2008). The ∆Gi ∆ values were calculated using surface tensions of the solutes, membrane and liquid derived from contact angle measurements on the membrane with the compounds in liquid pure form using the Young-Dupré equation. Three other liquids with known surface tensions (Milli-Q water, diodemethane and glycerol) were used to find the membrane surface tension. A negative value of ∆Gi ∆ indicated high affinity between solute and membrane, resulting in attraction between solute and membrane, and thus a facilitated solute transport through the membrane and a lower rejection. For positive ∆Gi, the solute-membrane interaction is repulsive, and the solute does not “partition” into the membrane matrix as easily. A lower partitioning in the membrane matrix and the membrane pores results in a higher rejection. However, the calculation of solute-membrane affinity can be relatively difficult for non-liquid trace organic solutes. 31

Very hydrophobic compounds were reported to be largely removed by hydrophobic membranes. However, as stated before, this high removal was mainly due to adsorption and thus this high removal was only present as long as the adsorptive capacity of the membrane was not exhausted. Therefore, for these compounds, rejection was observed to decrease considerably after the membrane was saturated after several hours of filtration, which was referred to as initial adsorption (Bellona et al., 2004). This indicates that the initial adsorption of the hydrophobic molecules on the membrane surface can cause high rejections at first, which then decreases when the solute concentration on the membrane reaches saturation. After the initial adsorption, solutemembrane affinity and size exclusion govern solute rejection. Feed water chemistry that affects the hydrophobicity of the molecules, such as pH, since membrane functional groups are dependent on pH and can also influence solutemembrane interactions (Verliefde et al., 2009). Furthermore, the presence of NOM in the feed water influences the rejection mechanism and thus, can alter the removal rate of trace organics. It was reported that the removal of pharmaceuticals increased, due to their association with MBR effluent macromolecules (Kimura et al., 2009). The effect of tertiary effluent on rejection of trace organics was reported to be marginal in this study. However, the level of adsorption of trace organics on macromolecules was not investigated. In another study, a significant increase in EDCs rejection by tight NF membranes treating membrane bioreactor (MBR) effluent and natural waters was observed (Jin et al., 2009). This improvement was attributed to estrone molecules binding to hydrophobic, negatively charged fractions of effluent organic matter through hydrogen bonds, resulting in a reduction of the passage of estrone molecules by enhanced sieving and charge repulsion. Similarly, rejection of pesticides was observed to improve as a result of adsorbance of these solutes onto the humic acid in feed water resulting in increased size as well as electrostatic interactions with the membrane (Majewska-Nowak et al., 2002). This hypothesises of adsorption of trace organics onto NOM was supported by showing the decreased concentration of estrone in the feed solution when humic acid was present. However, the adsorption level of estrone was not further investigated in this study. Furthermore, increases in ionic strength of the feed can cause the molecular structure of NOM to change and consequently alter the presentation of sites for association of trace organic compounds, leading to lower adsorption of trace organics onto NOM and lower 32

rejection. Association of trace organic compounds with NOM is suspected to be due to hydrogen bonding and hydrophobic interactions (Comerton et al., 2009). Carboxylic acids were also reported to influence the removal of hydrophobic compounds through formation of hydrogen bonds with the target compound resulting in an increase their rejections (Tödtheide et al., 1997).

2.9 Effect of fouling on removal of trace organics by NF Besides affecting the permeate flux, membrane fouling can alter membrane surface properties. For instance, the deposition of hydrophobic or hydrophilic foulants on a membrane surface can alter the overall hydrophobicity of the membrane. Adsorption of more hydrophilic molecules was reported to decrease the hydrophobicity of the membrane and vice versa (Palacio et al., 1999). Thus, there is a direct relationship between the hydrophobicity of solute particles and fouled membranes. Depending on the foulant material, membrane surface properties such as charge, hydrophobiciy and roughness can vary during fouling and cake layer formation. It was reported that membrane surface charge decreased and contact angle increased, after fouling with surface water (Verliefde et al., 2009). Moreover, a significant increase of membrane surface roughness (by 5 to 6 times) was observed as a result of protein adsorption on the membrane (Bowen et al., 2003). As mentioned previously, membrane surface properties as well as feed water matrix, can affect the rejection of organic contaminants. Examples of rejection values of various trace organics by virgin and fouled membranes are summarised in Table 2.7.

33

Table 2.7– Rejection of various trace organics by virgin NF and fouled with several NOM Feed water

∆Rejection*

composition

(%)

Activated sludge

+29

Raw sewage

-10

Humic acid

+11

NF90

Alginate

+5

Caffeine

NF200

Alginate

+12

Bisphenol A

NF270

Naproxen

NF200

Sulfamethoxazole

Compounds Estrone

Carbamazepine

Paracetamol

Membrane NF270

Suwannee river

Reference (Schäfer et al., 2001)

(Yangali-Quintanilla et al., 2009)

+5


Alginate

+4


NF270

Humic acid

+54


NF90

Humic acid

+2


NF200

Alginate

0


NF270

Humic acid

-7


TS80

Lake Ontario

+56

(Comerton et al., 2009)

MBR effluent

+45

Alginate

+10

NF90

humic acid


* The differences between the rejection of new and fouled membranes. According to Table 2.7, the NF rejection of trace organics increased up to 29% in the case of estrone in feed containing activated sludge. Generally, an increase in rejection of trace organics after membrane fouling was observed, which is often explained by enhanced electrostatic interactions and size exclusion. For example, it was observed that when the membrane was fouled with untreated surface water, the rejection of negatively charged solutes increased noticeably whereas positively charged compounds removal declined although membrane surface charge was reported to decrease (Verliefde et al., 2009). Alteration of the membrane surface hydrophobicity can also change the hydrophobic interactions between hydrophobic compounds and the membrane, which might lead to overestimation in removal of these compounds (i.e. increased hydrophobicity of the membrane can result in higher adsorption and therefore, partitioning of the hydrophobic compounds through the membrane into the permeate). 34

In the following sections, the effect of fouling on removal of trace organics is reviewed based on the physicochemical properties of these compounds.

2.9.1

Rejection of hydrophilic compounds

Effects of fouling on removal of trace organics have been studied and correlated to the alteration of membrane characteristics after fouling (Ng et al., 2004). The influence of membrane fouling on the rejection of organic contaminants was, amongst others, explored by using different feed water types such as secondary wastewater treated effluent, organic matter or landfill leachate organic matter. Synthetic water types have also been used, containing varying concentrations of model organic foulants such as humic acid, alginate or model colloidal particles. As discussed in Section 2.8, nonionic hydrophilic compounds are rejected mainly by size exclusion. Nghiem et al. (2009) reported that membrane pore size plays an important role in governing the effects of fouling on rejection of hydrophilic compounds. Therefore, it was hypothesized that rejection of the hydrophilic compounds would increase after organic matter fouling, due to pore blocking and possibly deposition of organic matter on the pore walls (Nghiem et al., 2009). Severely fouled membranes (more than 50% flux decline) result in enhanced sieving effect of the membrane, resulting in higher rejection of these solutes (Xu et al., 2006). Similar results were reported for various membranes with different pore sizes, however, this increase was observed to be less pronounced for membranes with smaller pore sizes. The increase in rejection of hydrophilic compounds after membrane fouling was also reported for two NF membranes (with different pore sizes) fouled by varying concentrations of alginate. This increase was explained to be due to enhanced size exclusion as a result of a fouling (i.e. pore restriction) which was more pronounced for the membranes with larger pore size, confirming the observed trends obtained by Xu et al. (Zazouli et al., 2009). In both studies, the selected hydrophilic compounds were both charged and non-charged; however, the electrostatic repulsion of ionic solutes by the membrane and the effect of fouling on surface charge were not considered. Drewes et al. (2006) also studied the effect of membrane fouling on rejection of nonionic contaminants. In that study, it was stated that the rejection of hydrophilic non35

ionic contaminants (before fouling) by different NF membranes depended (only/mainly) on the MWCO of the membranes. However, when membranes were fouled, the rejection of solutes was decreased by 8 to 41 % as most of the membranes became more negatively charged (Drewes et al., 2006). It was first expected that the sieving effect increased due to clogging, however, the experimental results suggested that the MWCO of the fouled membrane was larger than the virgin membrane due to swelling. It was further explained that this decrease in the rejection of hydrophilic non-ionic compounds after fouling could be due to the increased negative charge of the fouled membrane (i.e. -5 to -15 mV), leading to membrane swelling. According to another study (Bellona et al., 2004), when the zeta potential of the surface becomes more negative at higher pH, the pore radii of the membrane are expected to increase due to increasing electrostatic repulsion within the membrane pores, resulting in membrane swelling. However, the deposition of a fouling layer on the membrane surface is less likely to cause membrane swelling since the increase in surface charge is due to physicochemical characteristics of the foulant. Hence, it was proposed that membrane fouling is likely to result in two opposite effects on transport of ionic compounds across the membrane: one the one hand, membrane fouling is expected to increase the rejection of ionic compounds by increased electrostatic repulsion due to the more negative surface charge. On the other hand, an increase in the negative surface charge can possibly lead to a decrease in rejection due to membrane expansion or swelling, as mentioned previously. Also other studies investigated the rejection of several trace organic compounds by clean and pre-fouled NF membranes (Yangali-Quintanilla et al., 2009). Rejection of neutral hydrophilic compounds was reported to decrease for fouled membrane, in comparison to clean membranes, due to CECP after alginate fouling was established on the membrane surface. In the CECP, or cake-enhanced concentration polarisation model, it is hypothesized that the foulant layer hinders back diffusion of solutes from the membrane surface to the bulk solution, leading to an increased concentration polarisation. The accumulated solutes on the membrane surface increase the concentration gradient across the membrane leading to an increase in permeate concentration and thereby a decrease in rejection (Elimelech et al., 2004).

36

The decrease in rejection of hydrophilic compounds by an alginate-fouled NF, was reported to be due to cake-enhanced concentration polarization in another study as well (Steinle-Darling et al., 2008). Furthermore, the CECP effect appeared to be more significant for tighter membranes possibly due to a thicker fouling layer or a higher concentration of retained solutes at the membrane surface. On the other hand, hydrophobic interactions of the membrane were hindered due to the shielding effect of fouling layer, resulting in the lower repulsive interactions between hydrophilic solutes and the membrane.

2.9.2

Rejection of hydrophobic compounds

Size exclusion plays an important role in the transport of hydrophobic nonionic compounds as well. It was observed that the rejection of hydrophobic nonionic compounds was enhanced after the membrane was fouled with humic acid due to restriction of membrane pores (Nghiem et al., 2008). Additionally, the effect of pore blockage seemed to be more obvious when higher concentrations of foulant were used or when calcium was added to the feed water, resulting in a more compact fouling layer, and thus a higher rejection of compounds. Since the hydrophobic compounds were not charged, feed ionic strength did not affect the solutes rejection whereas fouling became more severe (28% flux decline) due to bridging by calcium ions. Moreover, it was noted that the magnitude of this rejection enhancement caused by severe fouling was more pronounced for membranes with large pore sizes. However, as mentioned previously, rejection of small hydrophobic nonionic solutes, does not only depend on molecular weight or size, but is also related to adsorptive interactions (hydrogen bonding and affinity). In a previous study, an increase in retention of hydrophobic compounds after NF membrane fouling was reported (Nghiem et al., 2009), and it was hypothesised that this was due to the formation of a fouling layer which could isolate and thus hinder hydrophobic interactions between membrane and solute. This hindrance of the hydrophobic interactions was hypothesized to reduce the partitioning of highly hydrophobic solutes into the membrane polymeric phase, hence limiting diffusion across the membrane. Xu et al, (2006), also observed a significant increase in rejection of hydrophobic nonionic solutes after the membrane was fouled with waste water secondary effluent. From a mass balance between the feed, concentrate and permeate flow, the adsorbed mass of trace organics was calculated for 37

both virgin and fouled membranes. It was reported that although the hydrophobicity of the fouled membrane decreased as compared to virgin membrane, the mass adsorption of the solutes was doubled. Therefore, it was concluded that the enhanced rejection of hydrophobic solutes was due to isolation of the membrane from hydrophobic interactions by the fouling layer, resulting in less solute partitioning and diffusing across the membrane. It is noteworthy that a high initial rejection of hydrophobic compounds was not observed the proposed study, in contrast to clean membranes. In a recent study, it was shown that rejection of hydrophobic ionic compounds decreased after alginate fouling, even though the membrane surface charge became more negative (Yangali-Quintanilla et al., 2009). It was stated that the alginate fouling layer was expected to be relatively sparse at pH 7, ultimately leading to elevated concentrations of trace organics near the membrane surface by convective transport which ultimately favours solute transport across the membrane. Thus, it was concluded that occurrence of cake enhanced concentration polarization was responsible for the decrease in rejection of hydrophobic ionic compounds. However, the more negative surface charge of the alginate fouled membrane could have been due to an error during the zeta potential measurement. In summary, fouling can both improve and decrease the rejection of both hydrophilic and hydrophobic nonionic compounds. Improved rejections are hypothesized to be due to pore blockage and enhanced sieving effects and enhanced adsorption as well as a shielding of the hydrophobic interactions leading to decreased partitioning. Decreasing rejections after fouling were mainly explained by the CECP phenomenon and an increased pore size of the membrane due to swelling. For hydrophilic and hydrophobic ionic compounds no clear conclusion regarding the effect of fouling on rejection of these solutes could be reached: as well as for uncharged solutes, both increasing as well as decreasing rejections were observed. Table 2.8 shows a summary of reported changes in rejection for different types of trace organics and foulants, as well as their main (hypothesized) rejection mechanism and the parameters that were used to explain these observations. According to Table 2.8, the nature of foulant can significantly influence the parameters which were reported to influence the rejection of trace organic compounds. 38

Table 2.8– Changes in rejection trends of trace organic compounds due to fouling. Type of trace organics

Foulant

Effect on rejection

Mechanism

Hydrophilic nonionic

Waste water secondary effluent

Increased

Size exclusion

Hydrophilic ionic

Waste water secondary effluent

Decreased

Electrostatic repulsion

Hydrophobic ionic nonionic Hydrophobic ionic

MBR effluent Tertiary effluent Alginate

non-ionic Hydrophobic ionic

Hydrophobic non-ionic

Hydrophobic non-ionic Hydrophobic nonionic

2.10

Increased

Size exclusion Adsorption

Humic acid

Humic acid+ Ca

Enhanced sieving due to decreased membrane pore size Increased MWCO as a result of membrane swelling as a result of increased surface electronegativity Enhanced sieving due to binding of compounds to NOM

Electrostatic repulsion, size exclusion Size exclusion

Enhanced sieving

Decreased

Electrostatic repulsion

CECP

Decreased Increased

Size exclusion Adsorption

CECP Disturbed sorption diffusion

Increased

Size exclusion, adsorption

Increased Decreased

Alginate Humic acid BSA Colloid Alginate Humic acid BSA Colloid

Parameters effecting

Decreased Size exclusion Increased

CECP

Absorption to humic acid Pore blacking CECP Enhanced sieving due to pore blocking

Conclusions

With the increasing scarcity of clean water resources, water reuse and recycling have been proposed as major solutions. However, recently, the presence of trace organic contaminants in water bodies and their possible negative effects on human health has caused growing concern and are a limiting factor in reuse applications. To ensure a high quality of produced water from recycling, advanced water and wastewater treatment technologies are required to remove trace organics. During the last decade, the application of membrane technology in water treatment has increased as membranes have become more cost-effective and reliable, and can produce high quality water and provide sufficient removal of trace organic solutes.

39

However, the rejection mechanisms of trace organics by NF/RO membranes are still not well understood due to the complexity of interactions between the solutes and the membrane matrix. Furthermore, it is evident that fouling (which is an inevitable phenomenon in membrane filtration processes) and presence of macromolecules can also considerably influence the rejection of trace organic compounds. Many studies have focused on the influence of fouling on rejection of trace organic compounds by membranes and inconsistent results have been reported. One of the main mechanisms proposed to explain the influence of NOM fouling on rejection was the alteration of the membrane properties by the fouling layer, leading to pore restriction and CECP. Several studies have investigated effects of organic and colloidal fouling and other feed water matrix properties on membrane performance, with emphasis on permeate flux decline and salt rejection, but also on removal of trace organics. However, research on fouling in conjunction with membrane properties is limited. A detailed characterisation of fouled membranes is an absolute prerequisite to better understand effects of fouling on membrane surface properties and confirm hypotheses used to explain the changing rejection behaviour of trace organics after fouling. Previous studies on the effects of fouling on rejection of trace organics have only focused on the modified membrane charge at various feed water pH, and on the alteration of membrane pore size. These authors attribute the difference in rejection values between virgin and fouled membranes to changing membrane characteristics but the membrane was never extensively characterised for surface properties in these studies. Therefore, systematic research on the effect of various types of fouling on rejection of trace organic compounds is needed. Moreover, the influence of the feed solution matrix in terms of adsorptive interactions between trace organic solutes and macromolecules in the feed water, and the resulting effect on rejection, is poorly understood. Particularly the relative influences of the effects of both fouling and adsorbance by macromolecules on removal of trace organics have not yet been studied. In this thesis, the effects of fouling on the rejection of trace organic compounds by NF will be investigated by focusing on physicochemical interactions between trace organic 40

compounds and the fouled membranes. The effect of the nature of the organic foulants and of different membrane surface characteristics on both fouling and rejection will be assessed in conjunction with the influence of filtration time on the trace organics rejection efficiency. Furthermore, the fate of trace organic compounds in presence of NOM and adsorptive behaviour of these interacting compounds according to their physicochemical properties will be systematically investigated. The outcome of this study will clarify the individual contributions of adsorptive interactions between trace organic solutes and NOM, as well as of membrane fouling on the rejection of trace organics.

41

Materials and methods Chapter 3

42

3

Materials and methods

This Chapter describes the equipment (membrane module membrane characteristics), the feed water matrices, the filtration and fouling protocols as well as the sampling and analytical methods used in Chapters 4 and 5. The influence of organic fouling on removal of selected trace organic compounds by NF membranes was investigated in Chapter 4. The fate of trace organics in the feed solutions and effect of adsorption and fouling were studied in Chapter 5. The experiments in Chapter 4 involved filtration of feed solutions under constant flux. Subsequently, a full characterisation of the membrane was carried out to assess the variations due to fouling in the membrane surface properties and the effect of these alterations on the rejection of trace organic compounds. The area of research covered in Chapter 5 involved two major sections. First, adsorption of trace organics onto macromolecules was studied by removing the macromolecules using UF membranes. Then, changes in rejection of trace organics by NF membrane due to presence of different types of macromolecules in the feed solution were assessed. The adsorption of trace organics on the UF and NF membranes were also investigated.

3.1 Membranes 3.1.1

NF membrane

A commercial NF membrane, NF270 (Dow/FilmTec), which is a polyamide thin film composite on a polysulfone microporous support was used in this study. NF270 was used in this study because of high water permeability and high relative flux as well as high NOM removal and a medium to high salt passage (De la Rubia et al., 2008). The membranes were received as flat sheets and stored at 4°C before use. The membrane specimen was rinsed with Milli-Q water prior to use, in order to remove the preservatives. Then, the membranes were characterised in terms of pure water permeability. Table 3.1 illustrates the main membrane properties.

43

Table 3.1– Selected membranes properties Membrane

NF270

Manufacturer

Dow/Filmtec

MWCO

Pure water

NaCl

Contact

Zeta potential

(Da)

permeability

rejection

angle

(mV)

(L/m2hbar)

(%)

(°)

300

13.6

40–60

35

24

The MWCO value was provided by the membrane manufacturer. The membrane zeta potential was determined at neutral pH (6.5) in 1mM KCl, 0.01 KOH and 0.25 HCl solutions using streaming potential equipment (Surpass, Anton Paar).

3.1.2

UF membrane

For the adsorption experiments, the UF membrane was supplied and manufactured of polyether sulfone (PES) with a nominal molecular weight cut-off of 5 kDa by Millipore Corporation (USA). The membranes were received as flat sheet samples. Properties of UF-5kDa membrane and its rejection efficiencies for humic acid (HA), alginate and BSA are shown in Table 3.2. Table 3.2– Properties of UF-5kDa and the rejection efficiencies of various macromolecules. UF membrane

UF-5 kDa

Pure water

Rejection of

Rejection of

Rejection of

permeability

HA

Alginate (%)

BSA

(L/m2.h.bar)

(%)

41.5

98

(%) 99

99

3.2 Feed water As mentioned in Section 2.6.3, the common organic fractions present in natural waters are humic substances polysaccharides and proteins, organic colloids and low molecular weight acids. Hence, in this research foulants were selected to represent those common organic fractions. The feed solution for the experiments reported in Chapter 4 contained different NOM components including humic acid (Aldrich), l-lysine (Aldrich), alginic acid sodium salt (Ajax) and Suwannee river humic acid (International Humic Substances Society) in addition to buffer solution (KH2PO4 and NaOH 0.1M) and sodium azide. The buffer solution was added to maintain the solution pH at 6.5 during filtration, as feed water pH variation was reported to influence the rejection of trace 44

organics dramatically (Bellona et al., 2004). Sodium azide was added at a concentration of 0.02 weight% to prevent biological growth in the feed water solution.

Figure 3.1– Shimatzu800 TOC analyser. Fouling solution was prepared by dissolving individual foulant with Mill-Q water. Concentration of total organic carbon (TOC) for each NOM substances was 5 mg/L which was quantified using a Shimatzu800 TOC analyser (Japan) with auto-sampler (Figure 3.1). Table 3.3 shows a summary of TOC concentration in 1 mg/L of organic matter used in this study. Table 3.3– The ratio of [TOC] content of organic matter Organic matter

[TOC] per mg/L of compound (mg/L)

HA

0.32

SRHA

0.39

L-lysine

0.46

Alginic acid

0.28

BSA

0.45

Since the constant flux mode was used in experiments reported in Chapter 4 and also [TOC] = 5 mg/L of foulant in the feed solution, the same TOC was delivered to the membrane surface in experiments conducted in Chapter 4, for a given volume of permeate. In these experiments, 15 L of solutions filtered during 96 hr, resulting in TOC 45

delivered of 0.01 mg/cm2. Constant delivered TOC was employed for all experiments (calculated by Eq 3.1). Delivered TOC mg/cm

Equation 3.1

Where V (L) is the permeate volume collected during filtration, C (mg/L) is the feed TOC concentration and A (cm2) is the membrane surface area. The filtration experiments were also conducted with real water samples including untreated surface water (Waroona dam, NSW) and MBR permeate (pilot plant, UNSW Water Research Centre) as feed water. Prior to characterisation, water samples were filtered through a 0.45µm filter to separate large particles. The detailed characterisation of these solutions was conducted by liquid chromatography-organic carbon detector (LC-OCD) detector (DOC-Labor, Germany). The LC-OCD technique involves using size-exclusion chromatography to separate classes of dissolved organic materials and also, measuring UV-A absorbance at 254 nm. SUVA (specific UV-A) of the total dissolved organic matter (DOC), which provides insight into the aromaticity of the DOC, was calculated from the DOC concentration and the measured UV-A adsorbance also by LC-OCD analyser. Experiments in Chapter 5 were carried out using humic acid, alginic acid the sodium salt and bovine serum albumin (BSA purchased from Sigma Aldrich) at [TOC] = 20 mg/L in the feed solution, also calibrated using TOC analyser. Since these experiments were short-term (2-3 hr), biological growth was not an issue.

3.3 Trace organic compounds Target organic solutes were selected to represent a wide range of physicochemical properties such as molecular weight, pKa and logP. The physicochemical properties of the selected compounds are presented in Table 3.4. All the chemicals were purchased from Supelco (St Louis, USA). A mixture of the target trace organic compounds as concentrated stock solution was prepared in methanol, and was stored at 90%). Compounds with lower MWs such as: bisphenol A and carbamazepine (226 and 236 Da) were removed by 38 and 73% respectively. Table 4.4– Rejection of hydrophobic nonionic compounds by new NF270 at pH=6.5, T=25°C and under constant flux mode. Compound

MW

Log P

Rejection by new membrane (%)

Bisphenol A

228

3.43

37.8

Carbamazepine

236

2.67

73.5

Triclosan

289

5.17

93.4

Fluoxetine

345

4.09

98.0

Risperidone

410

2.89

97.3

Simvastetine

418

4.42

97.2

The relationship between the MW and rejection of the solute is shown in Figure 4.2. Compounds with larger MW were removed more efficiently than the smaller solutes. As the solute MW approached the MWCO of the membrane, compounds rejection trend reached a maximum and all solutes with MW larger than the MWCO were retained similarly. From the observations of the rejection of hydrophobic nonionic compounds obtained in this study, it can be concluded that size exclusion was the main rejection mechanism. These observations can be supported by the investigations of Nghiem et al. (2009), who also reported similar rejection efficiencies and mechanisms for hydrophobic nonionic compounds using NF270 in various pH values.

63

100

Triclosan

Rejection (%)

80

Fluoxetine Simvastetin Risperidone

Carbamazepine

60

Reported MWCO of NF270 40

Bisphenol A 20

0 150

200

250

300

350

400

450

Molecular weight (Da)

Figure 4.2– Rejection of hydrophobic nonionic compounds according to their molecular weights. It is noteworthy that although bisphenol A and carbamazepine featured similar MWs, the difference between rejections of these compounds was significant (38 and 73% respectively). To explain the gap between rejection of bisphenol A and carbamazepine, their reported molecular widths (0.83 and 0.92 nm respectively) and lengths (1.25 and 1.20 nm respectively) were also considered (Yangali-Quintanilla et al., 2009). Although carbamazepine had a slightly larger molecular width, this difference was not considered to be significant. Thus, it is not likely that a difference in size was responsible for 35% higher rejection of carbamazepine, compared to Bisphenol-A. On the other hand, bisphenol A had a larger Log P value. The Log P value of the compounds is plotted against their rejection in Figure 4.3 to assess the relationship between these parameters.

64

Fluoxetine

100 80 Rejection %

Triclosan

Risperidone Carbamazepine

Simvastetine

60 40

Bisphenol A

20 0 0

1

2

3

4

5

6

LogP

Figure 4.3– Rejection of hydrophobic nonionic compounds vs. their LogP values. There was no significant correlation between compound with MW > MWCO rejection and their LogP (Figure 4.3) which was due to the overwhelming effect of the steric hindrance. For the smaller solutes (carbamazepine & bisphenol-A), there is a significant correlation between log P and rejection. Poor correlation between rejection of hydrophobic trace organics and LogP was also reported in previous studies (Kiso et al., 2001; Yangali-Quintanilla et al., 2009). However, it is noteworthy that Kiso et al. (2001) observed a good correlation between adsorption and LogP. As discussed in Section 4.3.2 Bisphenol A featured initial adsorptive behaviour in the experiment. The long-term rejection value of bisphenol A, however, followed the general trend observed for hydrophobic nonionic compounds. Therefore, the adsorptive behaviour of bisphenol A might be able to explain its lower rejection compared to carbamazepine (since Log P for bisphenol A is much larger). It can thus be concluded that although size exclusion was clearly the main rejection mechanism for rejection of hydrophobic nonionic compounds for solutes with similar size, adsorption (i.e. solute-membrane affinity) can offset this main rejection mechanisms for the most hydrophobic solutes.

65

4.3.3.3 Hydrophilic nonionic compounds Rejection of hydrophilic nonionic compounds by the virgin NF270, as well as their MW and Log P values are shown in Table 4.5. Rejection of these compounds (paracetamol, caffeine, sulfamethoxazole and trimethoprim) ranged from 4 to 99%. Table 4.5– Rejection of hydrophilic nonionic compounds by virgin NF270 at pH=6.5, T=25°C and under constant flux operation. Compound

MW

Log P

Rejection by new membrane (%)

Paracetamol

151

0.49

3.9

Caffeine

194

-0.07

63.4

Sulfamethoxazole

253

0.89

88.7

Trimethoprim

290

0.79

98.7

Rejection trends for hydrophilic and hydrophobic nonionic solutes were observed to be similar in that removal of these solutes significantly improved with the size of the compounds. Figure 4.3 shows the rejection values of hydrophilic nonionic compounds, plotted against their MWs. Trimethoprim with the largest MW (290 Da) in this group, was removed almost completely (99%). Paracetamol on the other hand was not retained by NF270 due to the small MW of this compound (151 Da). From these results, it can be concluded that the main rejection mechanism for hydrophilic nonionic solutes was size exclusion. This is similar to the findings of Yangali-Quintanilla et al. (2009), who attributed rejection of nonionic compounds to the sieving phenomenon.

66

100 Trimethoprim

Rejecction (%)

80

Sulfamethoxazole

60 Caffeine

Reported MWCO of

40

20 Paracetamol 0 150

170

190

210

230

250

270

290

310

Molecular weight (Da)

Figure 4.4– Rejection of hydrophilic nonionic compounds according to their molecular weight.

4.3.4

Interim conclusions

Among trace organic compounds with similar MW, the highest rejections were observed for the negatively charged compounds. For example, naproxen (Table 4.3) and bisphenol A (Table 4.4) - both having MW around 230 Da- were rejected by 92 and 37% respectively. This remarkable difference in rejection efficiencies of these two compounds is that, naproxen is negatively charged (pKa = 4.48) and bisphenol A is neutral (pKa = 9.73) in pH 6.5. Therefore, naproxen and bisphenol A were respectively rejected via electrostatic repulsion and size exclusion. Bisphenol A was only partially removed as it was smaller than the MWCO of the membrane and also tended to adsorb onto the membrane. The main rejection mechanism observed for the ionic compounds on the virgin membranes was electrostatic repulsion, although solute-membrane affinity resulted in lower rejection efficiencies for strongly hydrophobic compounds. For both hydrophobic and hydrophilic nonionic solutes, the main rejection mechanism was size exclusion. Affinity between hydrophobic compounds and the membrane was only clear for bisphenol A amongst the neutral trace organics. This affinity resulted in the low rejection of bisphenol-A. Although the temporal changes of rejection did not show any

67

sign of membrane saturation due to adsorption of trace organics during the filtration time, it did not provide any information on the level of adsorption of the compounds. Rejection of larger hydrophobic nonionic solutes (MW 228-263 Da) did not increase with increasing molecular size to the same extend as the hydrophilic nonionic compounds with lower MW (MW 151-194 Da). This might be due to higher affinity of the hydrophobic compounds for the membrane which leads to increased partitioning of the solute in the membrane matrix, and thus increased transport through the membrane matrix.

4.4 Fouling of NF membrane 4.4.1

Feed water characterisation

The fouling experiments were carried out with different synthetic foulants (i.e., humics, proteins, hydrocarbons), as well as with different natural water types (i.e., MBR permeate and untreated surface water). The model foulants differed considerably in their physicochemical properties and hence, allowed for systematic examination of the role of different organic foulants in influencing the rejection of trace organics. As mentioned in Chapter 3, both the synthetic and real water solutions were investigated using LC-OCD to determine the size distribution and exact organic composition of the foulants (Figure 4.5 and Table 4.6). Smaller molecules have longer retention times on the detector column of the LC-OCD. As such, by comparing the retention times of the different foulant peaks, the size distribution of the macromolecules in a feed sample can be compared. The peak for the foulant alginic acid peak already appeared after 28 min, which means alginic acid features the largest MW amongst the macromolecules used in this study. On the other hand, surface water and MBR demonstrated the largest fractions of small organics.

68

Signal Response

LMM Acids

Building Blocks

25

Humics

Biopolymers

30

LMW Neutrals

Alginic acid SHRA

20

Surface water Humic acid

15

MBR permeate

10 5 0 0

20

40

60

80

100

Retention Time (min)

Figure 4.5– LC-OCD diagrams for the different feed waters used for the fouling experiments. Macromolecules such as alginic acid, with a lower number of different OC fractions and thus a limited size variety, are expected to form more uniform fouling layers. Size distribution and organic composition of the macromolecules in the feed solutions are thus the main factors influencing the characteristics of the membrane fouling layer (e.g., the density, hydrophobicity and roughness). Table 4.6– Characteristics of the feed water. Organic carbon fraction % Bio-polymers*

Humic acid n.q.

Alginic acid 100

SRHA n.q.

Surface water 3.1

MBR permeate 11.1

Humic Subst. (HS)

67.0

n.q.

80.2

33.1

32.4

Building Blocks**

14.8

n.q.

8

17.3

16.6

LMW *** Neutrals

18.2

n.q.

11.8

12.5

39.9

LMW*** Acids

n.q.

n.q.

n.q.

34.0

0.0

*Biopolymers = polysaccharides, proteins, aminosugars **Building Blocks = breakdown products of humics *** LMW = low-molecular weight **** n.q. = not quantifiable (< 1ppb calculated) 69

The chromatogramable DOC in each sample is divided into 5 different organic fractions by LC-OCD. There is a hydrophobic fraction, consisting of humic substances (HS), building block smaller than 0.1 µm and low molecular weight (LMW) neutrals; and a hydrophilic fraction consisting of bio-polymers and LMW acids. For example, according to Table 4.6, the model foulant alginic acid was fractionated into only one fraction (the biopolymers) and did not show any traces of humic substances. This also indicates that alginic acid has the largest MW among the foulants used in this study. The model foulant humic acid consisted of 67% humic substances and also contained low molecular weight fractions (Baker et al., 2003). The model foulant Suwannee River Humic acid (SRHA), on the other hand, contained HS fraction (69%), but also 80%) lower quantities of building blocks and LMW neutrals. Surface water was characterised by a wide range of organic compounds with MWs from less than 350 to more than 20,000 Da. Hydrophilic species and LMW neutrals were present in relative quantities of about 56% and 30% respectively. MBR permeate had both hydrophilic (11%) and hydrophobic fractions (89%). The chromatogram for L-lysine could not be integrated by the LC-OCD software and thus, the characterisation of this compound could not be reported.

4.4.2

Effect of fouling on membrane performance

As mentioned in Chapter 3, fouling experiments with different organic foulants were performed under constant flux. The deposition of foulants on the membrane surface during the fouling runs was monitored by measuring the TMP and salt rejection for new and fouled membranes. An increase in TMP confirms the formation of fouling on membrane surface. The TMP increase during the fouling runs was calculated as in Eq 4.1: TMP

Normalised TMP increase (%) =

TMP

TMP

100

TMP

TMP

TMP

100

Equation 4.1

Where TMPt and TMP0 (bar) are the recorded TMP at times t and the start of the filtration respectively.

70

The salt rejection was also measured for each experiment to investigate the occurrence of CP and fouling, by comparing the removal values obtained with virgin and fouled membranes. Salt rejection was calculated as in Eq4.2: Salt rejection (%) =

100

100

Equation 4.2

Where condf and condp (µS) were conductivity of feed and permeate respectively. The TMP increase and salt rejection of virgin and fouled membranes with humic acid, llysine, alginate, SRHA and surface water are shown in Table 4.7. Table 4.7– TMP and salt rejection for the virgin and fouled NF270 membranes. Experiments

Normalised TMP increase (%)

Salt rejection (%)

Compacted membrane

–

59.2

HA

18

56.3

L-lysine

10

67.6

Alginate

13

60.5

SRHA

5

53.7

Surface water

6

30.9

MBR permeate

7

24.6

According to Table 4.7, the normalised TMP generally increased by 5 to 18% after 96 hr of filtration for the different foulants. This indicates that the extent of fouling was significant but not severe. The changes in membrane performance after fouling varied noticeably for the different macromolecules despite the constant amount of TOC delivered to membrane in all experiments (on a mass basis). SRHA and humic acid resulted in the lowest and highest TMP increase (18 and 5%) respectively. Hence, the effect of fouling on membrane performance was always influenced by the foulant nature (such as macromolecules properties and MW range). Fouling propensity for humic acid was observed to be higher than those of the other foulants applied in this study, whereas the humic acid fouling only resulted in slight decrease (3%) in salt rejection. The higher fouling propensity was due to the higher

71

affinity of the humic acid for the membrane polymer and the resulting formation of a dense fouling layer on the membrane. For SRHA, the normalised TMP increased by 5% after 48 hr of filtration. Salt rejection of the membrane fouled by SHRA decreased slightly from 59 to 53%. These results suggest that the fouling formed a porous cake layer which did not influence the TMP in the case of SRHA. The existence of this porous cake layer might have attributed to the decrease in salt rejection by causing cake-enhanced CP (CECP): the CP within the fouling increased due to entrapment of small solutes within the cake layer which hinders the back diffusion of the solute into the bulk solution. The observation for SHRA above was consistent with previous studies where it was reported that membrane fouling by NOM resulted in low hydraulic resistance and thus a porous cake layer (Li et al., 2004; Nghiem et al., 2009). However, the observation of completely different fouling phenomena for SRHA and humic acid was not in agreement with the LC-OCD results reported in Section 4.4.1, which indicated that the organic matter in humic acid and SRHA were not significantly different. It is noteworthy that SRHA consists of humic substances derived directly from river water whereas the humic acid used in this study was from soil origin. Although both humic acids consist of a large fraction of humic substances, they apparently do feature different physicochemical properties. LC-OCD fractionation is mainly based on the size of the compound and does not provide enough details regarding other physicochemical properties of these compounds (such as charge and polar/apolar characteristics). The membranes fouled by surface water and MBR permeate resulted in small TMP increases (6 and 7%, respectively), whereas their salt rejection decreased considerably (from 59 to 30 and 25% respectively). In comparison to SRHA, the extent of salt rejection decrease was much more pronounced for surface water and MBR permeate. However, the influence of fouling on TMP was almost identical for these three feed waters. LC-OCD diagrams showed that the organic matter in surface water and MBR permeate were quite broad in composition, consisting of all five organic carbon fractions (with different sizes) in different concentrations. Therefore, the fouling layer formed by surface water and MBR permeate will not be as uniform as for feed waters with a uniform type of foulant, and as such, the fouling layers might contain cavities due to the irregular stacking of macromolecules with varying sizes in the feed water. 72

These cavities ultimately led to the significant decrease in salt rejection due to severe CECP. After the NF270 membrane was fouled with L-Lysine, salt rejection improved by 9%. Alginic acid fouling also improved the salt rejection slightly (around 1%). For L-lysine and alginate, TMP increased by 10 and 13% respectively (Table 4.7). These observations lead to the hypothesis that the fouling by L-Lysine and alginic acid resulted in pore blocking, in addition to compaction of the cake layer, which resulted in higher rejection of salts. It can be concluded that if fouling noticeably increased the TMP and improved the salt rejection, this probably indicated that a compacted layer acting as an additional barrier was formed on the membrane surface. On the other hand, when fouling resulted in a small TMP increase and a considerable decrease in salt rejection, a rather porous cake layer was formed, which enhances the occurrence of CECP and thus result in a lower rejection (Nghiem et al., 2010).

4.4.3

Effect of fouling on membrane properties

Membrane fouling experiments were conducted using different organic foulants, under constant flux operation and constant TOC concentrations. The goal was to obtain different membrane surface properties by forming fouling layers of various natures. A full membrane characterisation was carried out on the membranes, before and after fouling. The new and fouled membrane characteristics are presented in Table 4.8. Table 4.8– Characteristics for compacted and fouled NF270 in terms of roughness, charge and hydrophobicity. Experiments Compacted membrane HA L-Lysine Alginate SRHA Surface water MBR permeate

Roughness (nm)

Contact angle with water(°)

Zeta potential at pH 6.5 (mV)

20 ± 4

58 ± 3

-21

25 ± 1 61 ± 1 13 ± 6 27 ± 3 31 ± 4 10± 3

57 ± 5 60 ± 5 36± 4 35± 3 78± 4 66± 2

-17 -22 -48 -18 -13 -22

73

The characterisation of the fouled membranes confirmed that their surface properties significantly changed compared with the virgin (compacted) membrane. The new NF270 membrane was significantly negatively charged (-21 mV) at pH 6.5. However, fouled membranes generally demonstrated significant variations from that value (-13 to -48 mV). This indicates that the surface charge was mainly determined by the foulant deposits. However, the measured zeta potential of the membranes does not necessarily reflect the zeta-potential of the fouling layer: it is possible that a combination of both the fouling layer and the membrane surface is measured, particularly when low concentrations of foulant are used and the fouling does not completely cover the surface. Table 4.8 also indicates that membrane hydrophobicity was altered after fouling occurred, and the change in hydrophobicity was dependent on the properties of the fouling material. For example, the contact angle of humic acid and l-lysine fouled specimens (57 and 60°) was similar to that of the compacted membrane (58°), indicating that these fouling layers featured intermediate hydrophobicity. However, the membrane became more hydrophobic due to fouling by surface water as its contact angle with water increased to 78°. Also, according to the characterisation data, both alginate and SRHA (36° and 35°, respectively) fouling layers resulted in a decrease in contact angle, and thus a more hydrophilic membrane surface. The roughness data were obtained from images derived from AFM measurements. The images are shown in Figure 4.6. From these images, it can be observed that the smooth NF270 membrane (20 nm) became noticeably rougher after fouling (25 to 61 nm), except when being fouled with alginate and MBR permeate (13 and 10 nm, respectively). Alginic acid and MBR permeate both contain “gel-like” material (i.e. EPS). Apparently, this results in a smooth fouling layer. For smooth surfaces like NF270, the foulants are expected to build up on the membrane resulting in higher roughness values. In the following sections, the nature of fouling material is discussed in order to explain the changes in membrane surface properties after fouling.

74

(a)

(b)

(c)

(d)

(e)

(f)

75

(g) Figure 4.6– NF270 Surface morphology imaged by AFM; (a) virgin ,(b) fouled with humic acid, (c) l-lysine, (d) alginate, (e) SRHA, (f) surface water and (g) MBR permeate. The images were 2×2µm in scan size. 4.4.3.1 Humic acid Humic materials are ubiquitous in the aquatic environment. They originate from decay of plant and animal residues, often found in soil and natural waters. This group of macromolecules is a very complex and heterogeneous association of many molecules. Possessing several functional groups including aliphatic, aromatic, carboxylic and phenolic groups, the physicochemical properties of humic acid are expected to vary greatly depending upon the solution chemistry (e.g., the feed water pH). Ionization of carboxylic groups of humic acid results in electrostatic charges which can induce intramolecular interactions in the humic acid complex. While carboxylic and phenolic functional groups render humic acid quite hydrophilic, it is also possible for humic substances to exhibit hydrophobic properties due to the presence of aromatic rings and non-polar aliphatic carbon chains (Nghiem et al., 2008). A dark brown layer of humic foulant attached to the membrane surface was observed subsequent to all fouling experiments. It was suggested that the electric charge of humic acid is relatively low due to the high relative occurrence of aromatic and aliphatic components (Nghiem et al., 2008). Consequently, the humic acid fouling layer would be expected to reduce the membrane surface charge and increase hydrophobicy.

76

4.4.3.2 Alginic acid Alginic acid is a hydrophilic anionic polysaccharide that is produced by algae and bacteria and features a large MW (20,000-2,000,000 Da) containing predominantly carboxylic functional groups (Tansel, 2008). This macromolecule swells in water and it is capable of adsorbing up to 200 times its own weight of water. At pH 6.0, the carboxylic groups of alginate molecules in bulk solution and on the membrane surface were reported to almost completely deprotonate. Deprotonation leads to an increase in the electrostatic repulsion between alginate molecules, thus lessening intermolecular adhesion amongst these alginate molecules. The increase in intermolecular adhesion initiates the formation of alginate fouling layer on the membrane surface (Lee et al., 2006). Alginate solutions create a clear/transparent hydrogel layer on the membrane surface (Ye et al., 2005). The alginate fouling layer significantly increased the negative surface charge of NF270 from -21 to -48 mV. According to AFM images (Figure 4.6, a and d), the roughness value of the compacted NF270 was 20 nm, and surface roughness reduced to 13 nm after fouling with alginic acid. A layer of negatively charged alginate might mask the surface of NF270 and consequently, dissociated acidic groups of foulant increased the membrane negative charge. Moreover, the macromolecules were trapped in the valleys of the compacted NF270 which led to reduced surface roughness. The reduced roughness of alginate fouled membrane can also be a result of inadequate AFM measurements on the dried sample. From visual observation of the fouled membrane, it could be seen that the alginate deposit was a hydrogel-like layer which became smooth and glass-like after being dried in a dessicator prior to AFM imaging. Therefore, the measured roughness for this specimen might not represent the real surface morphology of alginate fouling in wet conditions. 4.4.3.3 L-lysine L-lysine is a type of protein which is derived from horse plasma and has its isoelectric point at pH 9.7 (Hong et al., 2006). Proteins tend to deposit more extensively and less reversibly on hydrophobic surfaces compared to hydrophilic surfaces. This can lead to a compact deposit on the membrane surface. Moreover, it was reported that the 77

topography of the adsorbed protein layer is usually dependent on the membrane characteristics (roughness, surface chemistry, hydrophilicity and surface charge distribution) (Bowen et al., 2003). In this study, l-lysine was chosen for its high isoelectric point and its potential to create a positively charged fouling layer. However, l-lysine fouled membrane surface charge was similar to the new membrane. However, the L-lysine fouling layer significantly increased the membrane surface roughness to 61 nm. According to AFM image (Figure 4.6, a and c), the protein deposition formed deep valleys which resulted in a rougher membrane surface. This image also confirmed that l-lysine formed a compact fouling layer on the membrane surface. Membrane hydrophobicity did not change noticeably when fouled by this protein. 4.4.3.4 SRHA SRHA is the isolated humic substances from Suwannee river, with larger MW (ranging from 1000 to 5000 Da) than the typical aquatic humic acid (Tang et al., 2007). SRHA contains carboxylic and phenolic functional groups which deprotonate at pH 7, and it has a high carboxylic acidity (Hong et al., 1997). The SRHA fouling increased the membrane surface roughness slightly to 27 nm, due to accumulation of the foulant molecules on the smooth membrane surface. Moreover, the membrane became notably more hydrophilic after fouling with SHRA whereas the membrane surface charge decreased to -18 mV. 4.4.3.5 Surface water Surface water fouling decreased the membrane surface charge more than the synthetic solutions. The results from LC-OCD (as mentioned in Figure 4.5) showed that the surface water sample contained a large amount of LMW acids as well as humic substances, which is consistent with the determined zeta potential data. The significant increase in membrane surface roughness after being fouled by surface water was possibly due to the humic substances. According to Table 4.6, the OM in surface water was mainly hydrophobic (67%), and thus the fouling layer showed hydrophobic properties.

78

4.4.3.6 MBR permeate Fouling by MBR permeate did not change the membrane surface charge compared to the compacted membrane. The results from LC-OCD (as mentioned in Figure 4.5) showed that the MBR permeate contained a large amount of neutral compounds as well as humic substances. The sample contained all the OC fractions except for LMW acids. The largest fraction of these compounds in MBR permeate were the LMW neutrals, explaining why the membrane zeta potential did not change (-22 mV). The significant decrease in membrane surface roughness after being fouled by MBR permeate, was possibly due to the large concentration of small organics, which could “fill” the “gaps” on the rough virgin membrane surface. In summary, the characterisation of the fouled membranes confirmed that their surface properties changed, due to formation of a fouling layer. The fouled membranes generally featured a lower surface charge (i.e., less negative) and higher roughness and hydrophobicity (characterised as contact angle with pure water), except for the membrane fouled with alginate. The pure water contact angle of the membrane fouled with alginic acid was lower than the compacted membrane, while the charge became noticeably more negative.

4.5 Rejection of trace organic compounds by fouled membranes Membrane fouling was reported to result in alteration of membrane properties and thereby, affect the rejection of pharmaceuticals (Xu et al., 2006). However, the exact mechanisms by which fouling influences rejection of trace organics is not clear and thus, further work is required. In this study, low concentrations of macromolecules (5 mg/l) were used to foul the membrane and afterwards the fouled membranes were spiked with a mix of trace organic compounds with a wide range of physicochemical properties (Table 3.4). Temporal changes of the fouling layer and the rejection level of the trace organic compounds were studied and compared for all different types of foulants to gain a better understanding of the interactions between the trace organics and the different fouled membranes with various surface properties. For the discussion, the trace organic compounds were divided into three groups: the ionic solutes, the hydrophobic non-ionic and the hydrophilic non-ionic solutes. 79

4.5.1

Ionic compounds

The results obtained for the removal of the ionic trace organics with Milli-Q (i.e., in the baseline experiment) were discussed in Section 4.3. In general, adsorption phenomenon due to hydrophobic interactions can be identified by studying changes in rejection during extended filtration periods. If no temporal changes in rejection are observed, then no adsorption is expected to occur. The temporal changes in rejection of diclofenac, naproxen and ibuprofen by the different (fouled) membranes during 48 hr filtration runs are plotted in Figure 4.7, 4.8 and 4.9).

Rejection%

100 90 80 70 60 0

10 Milli Q

HA

20 Time (hr) Alginate Lysine

30

40

SRHA

SW

50

MBR

Figure 4.7– Rejection of diclofenac by HA, alginate, lysine, SRHA and surface water fouled membranes as a function of time (NF270 at pH6.5 and 25°C, over 48 hours of filtration).

Rejection %

100 90 80 70 60 0

10

20

30

40

50

Time (hr) Milli Q

HA

Alginate

Lysine

SRHA

SW

MBR

Figure 4.8– Rejection of naproxen by HA, alginate, lysine, SRHA and surface water fouled (NF270 at pH6.5 and 25°C, over 48 hours of filtration). 80

The rejection values of all three ionic compounds were observed to be relatively stable and constant during filtration and thus, the ionic solutes did not seem to adsorb onto the fouling layers. This is probably due to the difficulty for negatively charged solutes to approach the negatively charged membrane surface (due to electrostatic repulsion). Solutes that can not approach the membrane surface can not experience hydrophobic interactions with this surface (and thus can not adsorb onto it) (Verliefde et al., 2009). 100

Rejection%

90

80

70

60 0

10

20

30

40

50

Time (hr) Milli Q

HA

Alginate

Lysine

SRHA

SW

MBR

Figure 4.9– Rejection of ibuprofen by HA, alginate, lysine, SRHA and surface water fouled (NF270 at pH6.5 and 25°C, over 48 hours of filtration). The rejection values for negatively charged hydrophobic compounds by fouled membranes are plotted as a function of MWs in Figure 4.10. Diclofenac and ibuprofen were rejected more than 85%, whereas as much as 92% of naproxen was rejected by humic-acid-, alginate-, l-lysine- and MBR-permeate-fouled membranes. However, for the SHRA and surface water fouled NF270, the removal of diclofenac and ibuprofen decreased to 78 and 66% respectively. The results for these two fouling layers were anomalies in the rejection trends observed for ionic compounds in this study. The reduced charge of the membrane surface due to surface water and SRHA fouling cannot explain the significant decrease in rejection of naproxen, as this decrease in rejection did not appear for the other two charged compounds in this group.

81

100

Rejection %

80

60 40 20 0 Ibuprofen MW = 206 Da Milli Q

HA

Naproxen MW = 230 Da Alginate

Lysine

SRHA

Diclofenac MW = 296 Da SW

MBR

Figure 4.10– Rejection of hydrophobic ionic compounds by membranes fouled by various organic matter. There was no significant correlation between the rejection and the MW of ionic solutes. The main rejection mechanism for these solutes was electrostatic repulsion, as discussed in Section 4.3.3. The average rejection of ibuprofen by the fouled membranes was 94% which was almost similar to rejection obtained with new membrane (93%). For diclofenac, however, fouling improved the average rejection from 85 to 95%. Comparing the effect of different types of membrane fouling, the greatest increase in rejection was observed for diclofenac with the protein- and surface water-fouled membranes. However, as mentioned in Section 4.4.3, generally, fouling reduced the membrane surface charge (except for alginate) and this was not consistent with the increase in rejection (Figure 4.11), where electrostatic repulsion is expected to be the main removal mechanism.

82

SRHA

100

L-lysine

SW 90

Alginate MBR

Rejec tion%

HA Milli-Q

80

70 y = 0.6711x + 106.06 R² = 0.267

60 0

-10

-20

-30

-40

-50

Zeta potential (mV)

Figure 4.11– Rejection of diclofenac by HA, alginate, lysine, SRHA and surface water fouled NF270 vs. surface charge. As shown in Figure 4.12, the rejection values of diclofenac by the different fouled membranes correlated quite well with the membrane surface roughness of the different membranes. This can be explained by the increased overall charge density of the fouled membrane with increasing roughness (i.e. membrane surface area). For rough membranes, negatively charged solutes that are forced into the “crevices” of the rough fouling layer on the membrane surface by convective flow, are surrounded by the negative charges of the membrane in these crevices, resulting in an increase in electrostatic repulsion. As discussed previously, protein and surface water fouled membranes were the roughest, which enhanced the removal of diclofenac for these membranes more significantly. 100

SW Lysine Alginate

Rejection%

90

HA

SRHA

Milli_Q

80 70

y = 0.094x + 92.484 R² = 0.5868

60 0

10

20

30

40 50 Roughness (nm)

60

70

80

Figure 4.12– Rejection of diclofenac by HA, alginate, lysine, SRHA and surface water fouled NF270 vs. surface roughness. 83

As discussed in section 4.4.3, the hydrophobicity of the membrane surface changed after fouling. As adsorption of diclofenac to the virgin membrane was observed, it was expected that more hydrophobic membrane surfaces would influence the rejection of this compound. It was already observed in literature that membranes with larger contact angles could reject and adsorb more mass per unit area of a hydrophobic compound than membranes characterised by smaller contact angles (Bellona et al., 2004). The fouled membrane contact angle values are plotted against the rejection of the hydrophobic ionic compound diclofenac in Figure 4.13. The trend observed for the other hydrophobic ionic compounds was similar. No correlation between fouled membranes contact angle and rejection was observed (Table 4.9). 100

SRHA HA

Lysine

SW

Rejection%

90 Alginate

Milli-Q

80

70

60 0

10

20

30

40

50

60

70

80

Contact angle (°)

Figure 4.13– Rejection of diclofenac by HA, alginate, lysine, SRHA and surface water fouled NF270 vs. fouled membranes contact angle. Naproxen rejection decreased by 13 and 18% for membranes fouled with SRHA and surface water, respectively. These trends seem to be an anomaly in the generally observed rejection trends for ionic solutes with fouled membranes. As shown in Table 4.9, naproxen rejection correlated well with surface charge of the different membranes, but no relation was found with membrane roughness. Ibuprofen rejection did not show any correlations with any of the membrane parameters surface charge, roughness and hydrophobicity. Table 4.9– R-square values obtained for linear regression fitted rejection and various surface characteristics of the fouled membranes. 84

Compound

Zeta potential (mV)

Roughness (nm)

Contact angle (°)

Diclofenac

R² < 0.5

R² = 0.6

R² < 0.5

Naproxen

R² = 0.6

R² < 0.5

R² < 0.5

Ibuprofen

R² < 0.5

R² < 0.5

R² < 0.5

It can thus be concluded that electrostatic repulsion was the main, rejection mechanism for charged solutes. Although fouling seemed to reduce the membrane surface charge for most foulants, the removal of negatively charged solutes increased due to rougher surface (i.e. relative larger charge density) of the fouled membranes. The rejection of hydrophobic ionic compounds did not show a significant correlation with fouled membrane surface charge or roughness. However, it was the combination of these two parameters that explained the slight differences in rejection.

4.5.2

Hydrophobic nonionic compounds

Similar to the observations for the virgin membrane, temporal changes in rejection of hydrophobic nonionic compounds by the fouled membranes were also monitored to give an indication on the adsorptive behaviour of the hydrophobic non-ionic compounds onto the fouled membranes. Figure 4.14 shows the changes in rejection of risperidone by HA, alginate, lysine, SRHA and surface water fouled membranes (during a 48 hr filtration run). The trends for the other non-ionic hydrophobic compounds were similar to the results for risperidone. It was shown before that the main rejection mechanism for risperidone by the clean membrane was size exclusion. The effect of fouling on this size exclusion was also investigated. Generally, in the first 4 hr of the rejection test, the rejection of risperidone by both new and fouled membranes increased slightly. As the duration of this initial stage was relatively short, the increase in rejection can not be due to enhanced size exclusion by fouling. One explanation for this increase in rejection might be the (adsorptive)

interaction

of

the

hydrophobic

nonionic

macromolecules in the feed water and the fouling layer.

85

compounds

with

the

After the 4 initial hours of the experiment, rejection of risperidone slightly decreased during the remaining filtration time. This rejection trend agrees with the observations made by Kimura et al. (2003), who reported that the rejection of hydrophobic compounds decrease significantly after the initial “adsorption phase”, due to membrane saturation. The slight decrease in rejection for risperidone occurred for most membranes, except for the membranes fouled with humic acid and MBR permeate which was possibly due to lower adsorption of risperidone onto these fouling layers. 100

Rejection %

90

80

70

60 0

10 Milli Q

20 Time (hr) HA

Alginate

Lysine

30 SRHA

40 SW

50

MBR

Figure 4.14– Rejection of risperidone by HA, alginate, lysine, SRHA and surface water fouled (NF270 at pH6.5 and 25°C, over 48 hours of filtration). The rejection values of the hydrophobic non-ionic solutes by new and fouled membranes after 48h of filtration time are plotted against their MWs in Figure 4.14. Risperidone, fluoxetine, carbamazepine and bisphenol A were rejected in increasing order, according to their MW. This was already observed in the experiment with MilliQ (Section 4.3). Fluoxetine was not detected in feed and permeate samples for the last four fouling experiments, possibly due to its decomposition in the concentrated stock solution. As can be seen in Figure 4.15, fouling generally reduced the removal of the non-ionic hydrophobic trace organics. Humic acid fouling only resulted in slight decreases in rejection; SRHA fouling on the other hand, resulted in 40% decrease in rejection. Surface water-fouling on the membrane surface resulted in a decrease of risperidone, rejection from 81 to 68%. Fouling with MBR permeate did not have any significant

86

effect on the rejection of bisphenol A, but reduced rejection of carbamazepine and risperidone by 27 and 24% respectively. 100

Rejection %

80 60 40 20 0 Bisphenol A MW = 228 Milli Q

Carbamazepine MW = 236 HA

Alginate

Fluxetine MW = 345 L-lysine

SRHA

Risperidone MW= 410 SW

MBR

Figure 4.15– The rejection of hydrophobic nonionic compounds by different fouled membranes. Two possible explanations are proposed to explain the decrease in rejection of the nonionic hydrophobic solutes as the result of fouling. Firstly, the decrease in rejection can be due to the enhanced adsorption of the hydrophobic compounds onto the fouling layer. It was reported that adsorption of compounds resulted in a high initial rejection which decreased dramatically after the membrane became saturated due to increased partitioning and diffusion of solutes through the membrane (Bellona et al., 2004). Secondly, the decrease in rejection can be due to the increased concentration gradient of these solutes inside the cake layer as a result of their convective transport towards the membrane. Once the solutes enter the cake layer, they can not diffuse back into the bulk solutions due to the hindered back diffusion in the cake. As a result, the concentration of the solutes in the fouling layer builds up, leading to increased concentration polarisation (CECP). As mentioned in Section 4.4.1, the cake layer resulting from filtration of surface water comprised organic matter with a vast range of MW, preventing efficient stacking of the cake layers, and thus resulting in a relatively loose cake layer. The gradual increase of the concentration of hydrophobic solutes (CECP) in the loose cake layer decreased their removal.

87

As shown in Figure 4.15, the extent of the decrease in rejection of hydrophobic nonionic compounds was different for different foulants. This can be explained by the interaction between the solutes and the different foulants: to demonstrate this, the rejection of risperidone is plotted against the fouled membrane contact angle values in the figure below. Other compounds rejections were also plotted against their contact angle and the values obtained from linear regression fitted for these parameters are reported in Table 4.10. MIlli-Q HA

Rejection%

100

80

L-lysine

Alginate

60

MBR

SRHA

SW

y = 0.4819x + 56.26 R² = 0.1926

40 0

20

40

60

80

Contact angle (°)

Figure 4.16– Rejection of risperidone by HA, alginate, lysine, SRHA and surface water fouled NF270 vs. fouled membranes contact angle. According to Figure 4.16, fouled membranes with higher contact angles resulted in a higher rejection of risperidone except for the surface water-fouled membrane. The reason for this anomaly with surface water might be due to more severe CECP for this membrane. Amongst the hydrophobic nonionic compounds, only carbamazepine did not show correlation with the membrane contact angle (Table 4.10). The low Log P value (2.67) of this compound could explain its lower affinity with the hydrophobicity of the membrane surface. Amongst the hydrophobic nonionic compounds, only carbamazepine did not show a significant correlation with the membrane contact angle (Table 4.10). The lower Log P value (2.67) of carbamazepine compared to the other solutes could explain its lower affinity for the membrane surfaces.

88

The relationship between the rejection of the hydrophobic nonionic compounds and membrane roughness for the different fouled membranes was also assessed. No significant relationship between surface roughness and rejection of these compounds was observed. Table 4.10 shows a summary of the regression coefficients visualising the relations between the rejection values of the hydrophobic nonionic compounds and the membranes roughness and contact angle values of the fouled membranes. Table 4.10– R-square values obtained for linear regression fitted rejection of hydrophobic nonionic compounds and fouled membranes roughness and contact angle. Compound

Contact angle (°)

Roughness (nm)

Bisphenol A

R² = 0.67

R² = 0.09

Risperidone

R² = 0.75

R² = 0.09

Fluxetine

R² = 0.85

R² = 0.44

Carbamazepine

R² = 0.26

R² = 0.43

4.5.3

Hydrophilic compounds

The temporal changes in the rejection of the hydrophilic nonionic compounds were also monitored to investigate the possible effect of adsorption on rejection for these compounds. In the baseline experiment with Milli-Q, it was observed that adsorption was limited for the hydrophilic solutes, and that size exclusion was the main rejection mechanism. The effect of fouling formation on size exclusion was therefore also investigated. Figure 4.17 shows the rejection of trimethoprim during 48 hr of filtration. The rejection trends obtained for other hydrophilic compounds were similar. The rejection of all hydrophilic nonionic compounds was observed to be stable during the filtration time, indicating that the hydrophilic solutes do not adsorb onto the fouling layer.

89

100

Rejection %

80 60 40 20 0 0

10

20

30

40

50

Time (hr) Milli Q

HA

Alginate

Lysine

SRHA

SW

MBR

Figure 4.17– Rejection of trimethoprim by HA, alginate, lysine, SRHA, surface water and MBR permeate fouled (NF270 at pH6.5 and 25°C, over 48 hours of filtration). The effect of fouling on rejection efficiencies of these compounds was more pronounced compared to the hydrophobic solutes (Figure 4.17). For most solutes, a decrease in rejection was observed after fouling, which could be due to the occurrence of CECP. Among the hydrophilic compounds, paracetamol showed different rejection behaviours compared to the other solutes, which was due to its small size. The temporal changes in the rejection of paracetamol by different fouled membranes during 48 hr of filtration are shown in Figure 4.18. Rejection of paracetamol did not seem to change significantly during the course of the experiments, except for the alginate-fouled membrane. This might be due to the nature of alginate hydro-gel which led to entrapment of paracetamol molecules in the fouling layer.

90

100

Rejection %

80

60

40

20

0 0

10

20

30

40

50

Time (hr) Milli Q

HA

Alginate

Lysine

SRHA

SW

MBR

Figure 4.18– Rejection of paracetamol by HA, alginate, lysine, SRHA, surface water and MBR permeate fouled (NF270 at pH6.5 and 25°C, over 48 hours of filtration). The rejection efficiencies of the hydrophilic nonionic solutes by new and fouled membranes are shown as a function of their MW in Figure 4.19. Trimethoprim, sulfamethoxazole, caffeine and paracetamol and caffeine were rejected according to their MW by the virgin membrane, as discussed previously. As can be seen in Figure 4.19, fouling generally reduced the removal of trace organics, except for paracetamol and caffeine (with certain organic foulants) which showed improved rejections. 100

Rejection%

80 60 40 20 0 Paracetamol MW=151 Da Milli Q

Caffeine MW= 194 Da HA

Alginate

Sulfamethoxazole MW=253 Da Lysine

SRHA

SW

Trimethoprim MW=290 Da MBR

Figure 4.19– The rejection of hydrophilic nonionic compounds by new and fouled membranes. 91

Trimethoprim, which was the largest solute in this group, experienced a significant decrease in rejection from 98 to 46% after the NF membrane was fouled. Paracetamol (the smallest compound of this group) removal, however, was increased by fouling. For paracetamol alone, fouling enhanced size exclusion, with the extent of the increase in rejection varied depending on the foulant. The largest increases in rejection for paracetamol were observed for the surface water and MBR permeate-fouled membranes (the foulant types with the largest fraction of low molecular weight organic carbon content). Rejection values of caffeine (with a molecular weight between paracetamol and sulfamethoxazole) showed an increase after fouling for humic acid and alginate fouled membranes. For the other foulants, however, caffeine rejection decreased after fouling. It is noteworthy that the observed increase in rejection for humic acid and alginate was in the range of the standard deviation calculated for this solute. As discussed previously, the main rejection mechanism for the hydrophilic nonionic compounds was size exclusion. In general, the formation of fouling layer did not seem to enhance the sieving process for the larger solutes. The reason for reduced rejection of large solutes in this group was most likely the occurrence of CECP (due to hindered back diffusion of these compounds within the fouling layer). As it was discussed previously, different OM resulted in fouling layers with varying porosities (Section 4.4.2). Porous structures of fouling layers enhance the occurrence of CECP and consequently lead to increased diffusion of solutes through the membrane. For instance, humic acid and lysine fouling decreased the rejection of trace organics less than the other OM used in this study, and both fouling layers resulted in a higher TMP increase and salt rejection, indicating that humic acid and lysine form closely-stacked fouling layers, which form extra barriers instead of increasing CECP. Alginate formed a gel-like fouling which impeded with the back diffusion of the solutes and resulted in more severe CECP. The porous structure of the SRHA and surface water fouling layers caused a dramatic decrease in removal of hydrophilic nonionic compounds due to severe CECP and increased diffusion of solutes through the membrane. CECP resulted in a decrease in rejection which was more pronounced for larger solutes (since for these solutes, the hindered back-diffusion is more pronounced). 92

As stated above, alginate and humic acid formed a dense fouling layer on the membrane that appears to enhance the sieving effect (e.g., of caffeine which has an intermediate MW). Other fouling layers with lower densities resulted in CECP which led to decrease in rejection efficiency of e.g., caffeine. Milli-Q

100 80 Rejection %

HA L-lysine 60

SW

Alginate MBR

40

SRHA 20 y = 0.3445x + 53.842 R² = 0.0846

0 0

10

20

30

40

50

60

70

Roughness (nm)

Figure 4.20– Rejection of trimethoprim by HA, alginate, lysine, SRHA, surface water and MBR fouled NF270 vs surface roughness.

Milli-Q

100

Rejection %

80

L-lysine

HA

60

SW

Alginate

40

MBR

SRHA

20 y = 0.2759x + 47.674 R² = 0.0459 0 0

10

20

30

40

50

60

70

80

Contact angle (°)

Figure 4.21– Rejection of trimethoprim by HA, alginate, lysine, SRHA, surface water and MBR fouled NF270 vs. contact angle. To investigate the effect of fouled membrane characteristics on the rejection of hydrophilic nonionic compounds, the rejection of these solutes was plotted against the roughness and contact angles of the fouled membranes. Figure 4.20 and 4.21 plot the 93

rejection of trimethoprim as a function of this roughness and contact angle (similar trends were observed for the other compounds as well; a summary of R-square values obtained for linear regression fitted rejection of the other solutes as a function of roughness and contact angle is given in Table 4.11). From the Figure 4.20 and the Table, 4.11 it is clear that there is no correlation between the rejection of hydrophilic nonionic compounds and membrane surface roughness and hydrophobicity. Therefore, size exclusion still seems to be the most important mechanism for rejection of hydrophilic solutes and no further mechanisms regarding rejection of hydrophilic nonionic compounds were identified. Table 4.11– R-square values obtained for linear regression fitted rejection of hydrophilic nonionic compounds as a function of fouled membranes roughness and contact angle Compound

Roughness (nm)

Contact angle (°)

Paracetamol

R² = 0.06

R² = 0.42

Caffeine

R² = 0.05

R² = 0.10

Sulfamethoxazole

R² = 0.03

R² = 0.08

Trimethoprim

R² = 0.08

R² = 0.05

4.6 Conclusions The influence of fouling on rejection of trace organic compounds was investigated in this chapter, and relationships between rejection and membrane and solute physicochemical properties, but also foulant properties were also investigated. The feed waters covered a wide range of synthetic and real solutions containing environmental concentrations of trace organic compounds. Target compounds represent different physicochemical properties and were categorized in three families which were namely: hydrophilic nonionic, hydrophobic nonionic and hydrophobic ionic. Membrane surface characteristics (i.e. hydrophobicity, charge and roughness) of the fouled membranes varied depending on the type of organic foulant used. Fouling layer structure (density also differed for each foulant, and denser fouling layers resulted in a higher TMP increase and lower salt rejection. Low density fouling layers resulted in lower salt rejections due to the occurrence of CECP.

94

Fouled membrane generally had a lower surface charge and higher roughness and hydrophobicity compared to virgin membranes, leading to alteration in rejection of trace organic compounds. Changes in solute adsorptive behaviour, and significant adsorption of trace organic solutes onto fouled membranes were not observed in this study, since no temporal changes in rejection during the filtration run were observed (except for ibuprofen). However, the level of adsorption of trace organics onto the membrane and/or the macromolecules could not be exactly quantified from these results; and specially designed experiments are still required to assess the relative adsorptive behaviour of the trace organics Fouling did not significantly change the rejection of hydrophobic ionic solutes, even though slightly improved for the more hydrophobic diclofenac. Although fouling seemed to reduce the membrane surface charge, improved removal of diclofenac was due to higher charge density of the rougher fouled membrane. No relationships could be found, however, between the rejection and membrane surface charge or roughness individually. The influences of roughness and surface charge on rejection of ionic trace organics occurred simultaneously. For hydrophobic nonionic compounds, CECP resulted in a decrease in rejection which was more pronounced for larger solutes, which is logical, since the back transport of large molecules is more difficult. Coupling the rejection trends with the fouled membrane surface characteristics showed that generally fouled membranes with higher contact angles had a positive effect on the rejection of solutes. The trends observed for rejection of hydrophilic nonionic compounds changed with the size of the solute. Increased rejections of very small solutes were observed, whereas changes in rejection of medium-sized compounds (still below the MWCO of the membrane) compared to the virgin membrane, depended highly on the type of organic foulant. This was again due to density of fouling layer formed by different organic foulants. Lager hydrophilic compounds were rejected less by the fouled membranes due to CECP. No correlation could however be found between the fouled membrane surface characteristics and the rejection of hydrophilic compounds. 95

From this study, it can be concluded that the effect of fouling on removal of trace organics was more pronounced for nonionic compounds, as these compounds can really interact with the fouling layer (they can really approach and enter the fouling layer, in contrast to ionic solutes, which are repelled by the negatively charged fouling). Less dense fouling layers resulted in severe CECP and lower rejections of nonionic solutes. Similar trends for hydrophobic and hydrophilic compounds confirmed the occurrence of CECP. Adsorption of non-ionic solutes onto the fouling layers could not be assessed in these experiments (although there were indications that adsorption seemed to improve the rejection of some compounds, i.e. diclofenac). No trends between rejection by fouled membranes and a single parameter used to describe surface properties, was observed (i.e., no correlation with only surface roughness of the fouled membranes was found, since other parameters had an effect as well). A more in-depth study on adsorption of trace organics will be undertaken in the next chapter where the fate of trace organic compounds will be rigorously investigated using different organic foulants in a dead-end NF filtration process.

96

Fate of trace organic compounds during Nanofiltration Chapter 5

97

5

Fate of trace organics during NF treatment

5.1 Introduction Rejection mechanisms of trace organics by NF membrane in real water matrices (i.e. in the presence of organic macromolecules and salts) are very complex. Amongst the proposed rejection mechanisms, the influence of adsorption of the trace organics onto macromolecules in the feed solution and/or onto the membrane polymer has not been fully characterised yet. Additionally, knowledge is limited about the interactions occurring within the complex feed water matrixes. It is therefore especially important to study the relative influence of trace organics adsorption onto macromolecules on the overall removal performance of trace organics by NF. Therefore, there is a need to further expand the understanding of adsorptive behaviour of different classes of trace organics, both onto membrane and onto macromolecules, as well as the influence of this adsorption on NF rejection mechanisms. The adsorption of compounds onto NF membranes is generally assessed through the monitoring of temporal changes. As seen in the previous section, compounds which show a high initial rejection, followed by a sharp decline are expected to be significantly adsorbed on the membrane. The discussion conducted in Chapter 4 was based on the quantitative assessment of fouling and rejection obtained during the cross flow filtration of complex solutions (OM + trace organics). In order to validate the trends obtained with the first series of experiments, a new series of tests were designed to better assess the relative impact of trace organics adsorption onto membrane materials and onto OM. The level of adsorption of trace organics onto membrane has already been quantified by Xu and co-workers (Xu et al., 2006). However, the experimental methodology proposed here will also allow the characterisation of the relative impact of trace organics adsorption on OM. Once trace organics are present in complex water matrices, containing macromolecules, they might also adsorb onto these macromolecules. The potential natural degradation of these compounds from the feed solution can also affect the results in those experiments.

98

This Chapter focuses on the adsorption of trace organics onto organic macromolecules as well as onto the membrane surface. The influence of the adsorptive behaviour of trace organics on their rejection efficiency by NF membranes will also be investigated. Furthermore, the results from this study will allow distinguishing between the individual effects of fouling and adsorption on rejection of trace organics during NF treatment. This study is based on a series of experiments in which a rigorous methodology is proposed to characterise the fate of trace organics in NF applications. To assess the impact of different mechanisms, a rigorous experimental plan was designed to allow a total mass balance study assessing the relative quantity of trace organics removed in each step of the NF tests. Possible interactions of trace organics in this system includes: •

natural degradation in the feed solution,

•

adsorption onto macromolecules in the feed solution,

•

adsorption onto the membrane surface

•

adsorption onto the fouling layer on the membrane surface

5.2 Materials and methods In order to study the fate of trace organics during NF, the role of adsorption as well as the effect of fouling was investigated. The detailed descriptions of the materials and the experimental protocols used in this study are given in Chapter 3.

Unlike the

experiments in Chapter 4, all the experiments in this Chapter were carried out in deadend configuration (using UF 5-kDa and NF270 membranes). Therefore, variations between the observed rejection efficiencies of trace organics by NF membrane in Chapter 4 and 5 were expected. A baseline rejection (R1) test was performed using trace organics in Milli-Q at concentration to compare the rejections obtained, with the rejections when macromolecules were present in the feed solution. The concentrations of trace organics in Milli-Q (Cf1) and the NF permeate (Cp1) was used to determine rejection. R1 in this experiment includes the main rejection mechanism plus the adsorption level of trace organics onto the NF membrane (ANF).

99

Also, preliminary tests were carried out to assess the natural degradation (D) in the feed solution and static adsorption of trace organics onto UF and NF membranes (AUF and ANF, respectively). In the degradation test, concentrations of the trace organics from the feed solutions at the beginning of the test (Ct0) was compared with the level of trace organics still present in the solution after 48 hr storage at 3 °C (Ct48). In the static adsorption test, a membrane (same area as used in the dead-end cell) was immersed in the trace organics in Milli-Q solution and the mixture was stored in a fridge for 48 hr. Samples were taken from the initial feed water (trace organics in MilliQ without the membrane, Cb) and the mixture at the end of the test (Cm). This test was carried out for both UF and NF membranes. In this chapter, three types of feed solutions were filtered by the NF membrane. Firstly, concentrations of different UF permeates after ultrafiltration of macromolecule solutions (UF pre-treated feed solution, Cp/UF) were used to assess the effect of OM presence on rejection of trace organics without fouling (R2) on the NF membrane (Section 5.6). The UF membrane was used to separate the OM (and the adsorbed trace organics) from the non-adsorbed trace organics (Section 5.5). This allows quantification of the level of adsorption of trace organic compounds onto organic macromolecules (AOM) in the feed solution. A mass balance between the concentrations of trace organics in the UF feed (Cf/UF) and UF permeate (Cp/UF) was used to quantify the mass of trace organics adsorbed onto the removed macromolecules. Before the filtration tests, all feed solutions were stored in a fridge (3°C) for 48 hr in order to allow trace organics adsorption on macromolecules. Secondly, the NF membrane was pre-fouled using macromolecule solutions ([TOC] = 20 mg/L). Afterwards, this filtration process was followed by filtration of a solution of 2µg/L of trace organics in Milli-Q water. This allows the exclusive observation of the effect of fouling on rejection of trace organics solutes (Section 5.7). The rejection of trace organics by the pre-fouled membrane (R3) was calculated using the concentration of trace organics in Milli-Q (Cf1) and permeate (Cp3) samples. Cf1 was used instead of the concentrations of trace organics in the OM solutions to avoid the possible errors that can occur in the analysis of complex feed solutions due to interference of OM. To assess the relative impact of the fouling layer on rejection of trace organics (I), a mass balance

100

between R3 and R1 was carried out. Since in the fouling experiments, the trace organics did not interact with the NF membrane surface, the ANF was subtracted from R1. Thirdly, the feed solutions containing both macromolecules and trace organics were filtered by NF membrane to assess the simultaneous effect of the presence of macromolecules (and thus adsorption of TO onto macromolecules) and fouling on the rejection of the trace organics by NF (Section 5.8). The rejection (R4) of trace organics in this experiment was calculated using the concentration of trace organics in the complex feed (Cf4) and permeate (Cp4) samples. Three different macromolecule solutions were used in these experiments, as specified in Section 3.7. The organic carbon fractions of the used foulants are summarised in Table 5.2. The effect of macromolecule nature on trace organics adsorption will also be assessed. The experiments and procedures, as well as the mass balance equations are summarised in Table 5.1.

101

Table 5.1– Summary of the experiments, procedure and equations used for mass balances in Chapter 5. Experiment

Procedure

Mass balance

Natural degradation (D) in

Trace organics solution in Milli-Q D= 1

the feed (Section 5.4.1)

stored for 48 hr (3°C)

Trace organics adsorption

UF membrane submerged in trace AUFi= 1

onto UF membrane (AUF)

organics solution for 48 hr (3°C)


NF membrane submerged in trace ANFi= 1

onto NF membrane (ANF)

organics solution for 48 hr (3°C)

100

C

100

C /UF

C

100

C /NF

ANF = ANFi - D

(Section 5.4.3)

onto macromolecules

AUF = AUFi - D

(Section 5.4.2)


C C

C /UF

UF filtration of trace organics + AOMi = 1 OM

C /UF

100

AOM= AOMi – (D + Am)

(AOM) in the bulk solution (Section 5.5) Baseline

NF filtration of trace organics in R1= 1

C C

100

Milli-Q water Rejection (R2) of trace

NF filtration of UF pre-treated R2= 1 organics in UF pre-filtered feed solutions

C /UF C

100

feed solution (Section

5.6) Impact (I) of NF fouling

NF filtration of trace organics in R3= 1

layer on rejection (Section

Milli-Q solution by pre-fouled NF

5.7)

membrane

Rejection (R3) of trace

NF filtration of trace organics + R4= 1

organics in complex feed

OM

C C

100

I = R3 – (R1- Am/NF) C C

100

solution (Section 5.8)

These experiments were carried out with synthetic water containing model OM compounds consisting of humics, proteins, and hydrocarbons. The feed solutions were characterised according to their organic carbon composition and size distribution using LC-OCD, as described in detail in Section 4.5.1. According to the results from the LCOCD chromatograms, the average MWs for alginic acid, BSA and humic acid were 8416, 5385 and 1034 Da respectively. Alginic acid featured the largest average MW 102

amongst the macromolecules used in this study. The organic carbon fractions of the macromolecules are shown in Table 5.2. Table 5.2– Characteristics of the organic carbon content of alginic acid, BSA and humic acid. Sample’s organic carbon

Humic acid

Alginic acid (%)

BSA (%)

fractions

(%)

Bio-polymers*

0.1

100

77.3

Humic Subst. (HS)

67.0

n.q.

n.q.

Building Blocks**

14.8

n.q.

10.0

LMW *** Neutrals

18.0

n.q.

13.7

LMW*** Acids

n.q.

n.q.

n.q.

*Biopolymers = polysaccharides, proteins, aminosugars **Building Blocks = breakdown products of humics *** LMW = low-molecular weight n.q. = not quantifiable (< 1ppb calculated) As mentioned in Section 4.5.1, the chromatogramable DOC (the DOC of sample after going through a 0.1 filter) of the sample falls into 5 different organic fractions by LCOCD. According to Table 5.2, alginic acid sample only consisted into biopolymers. The humic acid consisted of 67% humic substances and also contained low molecular weight (LMW) fractions. BSA contained mainly biopolymer fractions (77%) and lower quantities of building blocks and LMW neutrals. The occurrence of building blocks in the BSA is probably due to presence of a certain amount of LMW molecules which are classified as building blocks and neutrals according to LC-OCD standard retention time. In the LC-OCD data analysis, biopolymers are considered as hydrophilic compounds and the rest of the fractions feature hydrophobic properties. Alginic acid consisted of only hydrophilic substances, whereas BSA contained both hydrophilic and hydrophobic species. Humic acid fractions were all in the hydrophobic zone. Therefore, it is expected that BSA will show a different behaviour in these experiments as it contains a wide range of properties (size and hydrophobicity).

103

5.3 Selection of UF membrane The efficient removal of macromolecules was necessary to assess the absorbance of trace organics onto OM. Therefore, the MWCO of the UF membrane had to be chosen so that the trace organics would permeate while the majority (> 95%) of macromolecules would be retained during filtration. With trace organics generally smaller than 500 Da and large macromolecules, MWCO of 5kDa was expected to retain most of macromolecules without rejecting trace organics (except the fraction adsorbing onto the UF membrane). Furthermore, it was assumed that the adsorption of trace organics onto the UF membrane was the same with and without presence of OM which made it possible to consider the effect of trace organics adsorption onto the membrane in all experiments. It might, however, be possible that this assumption was not correct and the interactions between the trace organics and the membrane were expected to slightly change due to the presence of OM in the feed solutions. Since efficient removal of macromolecules from the feed was required for this experiment, the rejections of chosen macromolecules by the UF membrane were studied. Rejection of all macromolecules was determined by filtering a macromolecule solution (1 L) through the UF membrane at 1.5 bar. The TOC analysis of the macromolecules and their rejection efficiencies are shown in Table 5.3. Table 5.3– Organic matter rejection efficiencies by UF 5kDa membrane. Organic matter

Initial TOC (mg/L)

Final TOC (mg/L)

% Rejection

Alginic acid

20

0.23

98.8

Humic acid

20

0.46

97.7

BSA

20

0.15

99.2

According to Table 5.3, TOC-analysis of the UF permeate confirmed that the UF 5-kDa was suitable for this specific research application, as it removed the macromolecules by more than 98%. The detection limit of the TOC analyser was 0.2 mg/L, therefore BSA concentration in the UF permeate was below detection limit. Humic acid was rejected slightly lower than alginic acid and BSA. This was due the smaller size of humic acid, as reflected in characterisation results by LC-OCD. The humic acid contained higher concentrations of break-down products (building blocks and LMW neutrals) that could permeate through the UF membrane pores resulting in a lower rejection. 104

5.4 Adsorption behaviour of trace organics The decreased concentration of trace organics in the permeate of UF membranes compared to the feed can be due to various parameters such as their natural decomposition, possible adsorption onto the membrane and adsorption onto macromolecules (followed by removal of these macromolecules by the membrane). The tendency of some solutes to degrade over time was already observed in Chapter 4. The natural decomposition of the trace organic compounds was assessed in a preliminary test, to check whether the removal of trace organics in this study could be due to biological degradation or evaporation. A first set of experiments was performed in order to assess the adsorption of trace organics onto UF and NF membranes (solutes only in contact with membrane). A second set was carried out to assess the adsorption of trace organics onto OM, by checking the differences in removal of trace organics by UF membrane before and after addition of macromolecules.

5.4.1

Natural decomposition of trace organics in feed solutions

The feed solutions containing trace organics and macromolecules were prepared 48 hours prior to the experiment, allowing sufficient time for adsorption to occur. To investigate the effects of storage conditions (48 hr at 3 °C) on degradation of trace organic solutes, samples from the blank feed solution (containing only trace organics, no membrane nor macromolecules) were collected at the beginning and end of the storage. The calculated natural degradation of the trace organics used in this study is shown in Table 5.4. The confidence level of the analysis, as confirmed by the STD values, ranged between 0 to 10%. The STD% was based on the deviation in triplicate samples from the feed solution and the average concentration of each compound. The natural decomposition of the trace organics varied for different compounds and ranged from below the confidence limit (c.l.) up to 23%. All groups of trace organic compounds showed similar degradation levels. However, clozapine and risperidone 105

degradation levels (20 and 23 % respectively) were significantly higher than those of the other compounds.

5.4.2

Adsorption on UF membrane

In this Section, the adsorption of trace organics solutes onto the UF membrane polymeric material was assessed. The macromolecules concentration was 20mg/L in terms of TOC. The confidence level of the analyses, as characterised by the standard deviation (STD) values, ranged between 0 and 10%. The STD % was based on the deviation obtained from triplicate samples from the feed solution and the average concentration of each compound. For example, measured ketoprofen concentrations in the feed solution samples were 1730, 1697 and 1650 ng/L, resulting in an average concentration of 1692 ng/L and an STD of 3.3%. The level of trace organics adsorption on the UF membrane was assessed by submerging it in the solution containing only trace organics and no macromolecules for 48 hr. The amount adsorbed was calculated from a mass balance, taking into account the mass naturally degraded for each compound during the 48 hr test. The level of decomposition and the adsorption onto the UF membrane in these experiments are shown in Table 5.4 for the different solutes, together with the standard deviations on this decomposition and adsorption. Amongst all compounds, clozapine and resperidone featured the highest degradation level which was probably due to low solubility of these compounds in water, leading to precipitation or adsorption to the experimental set up. All compounds were observed to significantly adsorb on the UF membrane, although the adsorption behaviour was different depending on the compound physicochemical properties. Adsorption of hydrophilic nonionic compounds onto the UF membrane ranged from 3 to 10%. Hydrophobic nonionic compounds showed higher adsorption (11-28%). This was due to the higher affinity between these compounds and the hydrophobic UF membrane polymeric material (contact angle = 68.8 °). However, hydrophobic ionic solute adsorption levels (< 10%) onto the UF membrane were comparable to hydrophilic compounds. This was probably due to the electrostatic

106

repulsion by the charged UF membrane (-14 mV) prevented the ionic solutes from to approaching the membrane and therefore adsorbing onto it. Table 5.4– Level of decomposition & adsorption of trace organics onto the UF membrane and confidence levels of analyses. Compounds

MW (Da)

Log P

Natural degradation (%)

TO adsorption to UF (%)

STD (%)

Hydrophobic ionic Diclofenac Ibuprofen Ketoprofen Naproxen

296 206 254 230

4.51 8.1 3.97 0.5 3.12 1.7 3.18 4.5 Hydrophilic nonionic

4.9 9.2 10.1 7.7

3.3 0.8 3.3 0.2

Caffeine Paracetamol primidone Sulfamethoxazol e Trimethoprim

194 151 218

-0.13 0.49 0.91

3.2 9.2 8.2

3.2 5.8 6.5

0.1 3.5 0.9

253

0.89

< c.l.*

10.3

3.2

290

0.91 6.1 Hydrophobic nonionic

9.5

3.9

Amtriptyline Atrazine Bisphenol A Carbamazepine Clozapine Linuron Risperidone * Confidence level

277 215 228 236 326 248 410

4.92 2.61 3.32 2.45 3.23 3.20 3.49

16.5 18.5 27.7 11.1 14.3 17.8 24.9

0.5 1.0 0.5 0.1 3.2 0.1 4.5

6.4 3.1 1.9 11.4 20.6 6.4 23.2

These results confirmed the observations of Yoon et al. (2006), who also reported hydrophobic compounds to adsorb more onto UF membranes and adsorption of hydrophobic ionic trace organics to be lower due to their deprotonation. However, the effect of charge repulsion was not considered in that study, which was most likely the reason for lower adsorption of ionic compounds.

107

5.4.3

Adsorption onto NF membrane

Assessment of trace organics adsorption onto the NF membrane polymeric material can contribute to a better understanding of their rejection mechanisms. The level of trace organics adsorption onto the NF membrane was assessed by submerging it in the solution containing only trace organics and no macromolecules for 48 hr. To quantify the adsorption of trace organics onto the NF membrane, a mass balance was calculated based on the difference in concentration of each solute in solution before and after the test, minus the amount decomposed naturally (data obtained in Section 5.4.1). The confidence level of the analysis was also applied to the membrane adsorption experiment since this set of experiments was based on the detected concentrations in the blank and the feed samples. The decomposition, adsorption to NF membrane and the confidence limits in these experiments are shown in Table 5.5. Most of the tested compounds (except for paracetamol and primidone) were observed to adsorb on the NF membrane, and the adsorption behaviour varied depending on the physicochemical properties of the compounds. The adsorption of hydrophilic compounds onto NF was generally below 5%. Since these values were mostly in the range of the STD of the analysis, their adsorption on the NF membrane was considered to be negligible. It is noteworthy that trimethoprim (the solute with the largest MW in this group) was observed to adsorb the most amongst the hydrophilic compounds (17%). It was not expected that the large MW of trimethoprim could be the only reason for this high adsorption. Further tests are recommended to confirm the trends observed here. Trimethoprim has about the same size as the membrane pores, and therefore, it possibly adsorbs more easily into the pores.

108

Table 5.5– Relative adsorption of trace organics onto NF membrane reported the confidence levels of analysis. Compounds

Trace organics adsorption to NF (%)

STD (%)

Hydrophobic ionic

Diclofenac Ibuprofen Ketoprofen Naproxen

6.4 9.5 4.8 5.6

3.3 0.8 3.3 0.2

1.6 < c.l.* < c.l.

0.1 3.5 0.9

4.9 17.4

3.2 3.9

27.2 6.8 14.3 14.0 15.3 7.9

0.5 1 0.5 3.2 0.1 4.5

Hydrophilic nonionic

Caffeine Paracetamol Primidone Sulfamethoxaz ole Trimethoprim Hydrophobic nonionic

Amtriptyline Atrazine Bisphenol A Clozapine Linuron Risperidone * Confidence level

On the other hand, hydrophobic nonionic compounds were observed to adsorb on the membrane more significantly (from 8 to 27%). Amtriptyline featured the highest adsorption level in this group, possibly due to its very high Log P value (4.92), and thus its very hydrophobic character. Hydrophobic solutes are expected to adsorb to a high level onto the hydrophobic NF membrane due to high affinity between these compounds and the membrane polymer. However, no linear relationship between Log P value and the adsorption of these compounds was confirmed within the data obtained in these experiments. Hydrophobic ionic compounds showed lower affinity with the membrane; however, their adsorption levels (5 to 9%) onto the NF membrane were still observed to be higher than those obtained for the hydrophilic compounds. The higher adsorption levels of hydrophobic compounds in general, compared to hydrophilic solutes was due to their higher affinity with the hydrophobic membrane. The lower adsorption level of hydrophobic ionic compounds on the membrane was due to the electrostatic repulsion 109

between these compounds and the membrane which hindered their adsorption. Amongst ionic compounds ibuprofen showed the highest adsorption onto the membrane (9%). The adsorption of atrazine onto the NF270 membrane was previously studied by stirring the membrane in atrazine solution for 24 hr (Plakas et al., 2006). The adsorption level of atrazine (4%) reported in that study agreed with the obtained results in this experiment. The adsorption of hydrophobic compounds onto NF membranes was also observed by Xu et al. (2006). Although the UF and NF membranes were manufactured from different polymers, the observed trends for the adsorption of trace organic compounds to these membranes were similar. This was due to the hydrophobic nature of both membranes resulting in higher adsorption of hydrophobic solutes. However, the NF membrane tended to adsorb less mass (per unit area) of trace organics than the UF. This can be due to the higher surface area of the UF membrane as a result of the larger pore size than NF. Yoon et al. (2006) also quantified the adsorption of trace organics by UF and NF membranes. Similar trends were observed compared with this study regarding the adsorption of trace organics, and the UF membrane also showed higher adsorption compared to the NF.

5.5 Adsorption onto macromolecules To assess the adsorption of trace organics onto the macromolecules, solutions containing a mixture of trace organics and OM (humic acid, alginic acid and BSA) were filtered by the UF membrane at 1.5 bar. Filtration by UF resulted in removal of OM (Table 5.3) and therefore, the trace organics adsorbed onto these macromolecules (Figure 5.1). The permeate contains < 2% of the initial OM concentration, and it also contains the non-adsorbed trace organics. Adsorption of the different groups of trace organics on to humic acid, alginic acid and BSA is discussed in the following Sections.

110

Figure 5.1– Filtration of macromolecules and the adsorbed trace organics by UF membrane. The adsorption values for the TO onto the macromolecules were calculated using feed and permeate concentrations, as well as the data for degradation and adsorption onto the UF membrane. The confidence limit of these experiments is taken as the value of STD. The filtration of humic acid was carried out in triplicate in order to calculate the STD which was applied to all of the UF experiments. The STD% was based on the calculated deviation and the average concentration of each compound. The results for adsorption of trace organic compounds as well as their STD are reported according to the solute physicochemical properties in the following sections. 5.5.1.1 Hydrophilic nonionic compounds The adsorption levels of hydrophilic nonionic compounds on humic acid, alginic acid and BSA are shown in Table 5.6. Only the adsorption levels of hydrophilic compounds onto alginic acid were significant and exceeded the confidence levels. This was consistent with the LC-OCD characterisation that showed alginic acid was only consisting of biopolymers which feature hydrophilic properties. Moreover, alginate swells in water and it is capable of adsorbing up to 200 times its own weight of water (Le-Clech et al., 2007) and therefore hydrophilic compounds.

111

Table 5.6– Relative adsorption (% of initial feed concentration) of hydrophilic nonionic compounds on humic acid, alginic acid and BSA. Hydrophilic nonionic Compounds Caffeine Paracetamol Primidone Sulfamethoxazole Trimethoprim

MW (Da)

Log P

194 151 218 253 290

-0.13 0.49 0.91 0.89 0.91

Adsorption (%) Alginate 58.3 24.9 16.1 17.5 33.4

Humic acid

Effect of Fouling on Removal of Trace Organic Compounds by

Effect of Fouling on Removal of Trace Organic Compounds by

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