Biomimetic membrane for desalination and water reuse 1

Driven by the extraordinary permeability and selectivity of AQP towards water molecules, novel biomimetic membranes consisting of AquaporinZ AqpZ for desalination and water reuses are ta

BIOMIMETIC MEMBRANES FOR DESALINATION

AND WATER REUSE

SUN GUOFEI

NATIONAL UNIVERSITY OF SINGAPORE

2013

BIOMIMETIC MEMBRANES FOR DESALINATION

AND WATER REUSE

SUN GUOFEI

(B.Eng (Hons.), NUS)

A THESIS SUBMITTED FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL AND BIOMOLECULAR

ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE

2013

: I have duly acknowledged all the sources of :,ill' *-r " -e-

a-ACKNOWLEDGEMENTS

This dissertation would not have been possible without the help of many people First and foremost, I would like to express my deepest appreciation to my supervisor Prof Chung Tai-Shung, in the Department of Chemical and Biomolecular Engineering at National University of Singapore (NUS), for his invaluable support and enthusiastic encouragement throughout my PhD study I would like to thank him for offering me the opportunity to carry out my research in this special topic, and enlightening me

in exploring the academic area

I would like to thank my mentor Dr Y Li who guided me through the first year of

my PhD He was always approachable whenever I have any doubts in research, and was always patient to help

A special thank you to Dr A Armugam (Department of Biochemistry, NUS) for her efforts in producing high quality aquaporins and also her valuable suggestions in protein handling

I would also like to thank all the collaborators in this biomimetic membrane project, Prof W Meier (University of Basel), Prof S Hua (The State University of New York-Buffalo), Prof K Jeyaseelan (Department of Biochemistry, NUS), Prof Q Lin (Department of Biological Sciences, NUS), and Prof Y W Tong (Department of Chemical and Biomolecular engineering, NUS), for their generous supports in this project This project was financially supported by the Environment and Water Industry Programme Office (EWI) (NUS grant number: R-279-000-293-272) under the Singapore National Research Foundation (NRF)

I wish to take this opportunity to thank Dr H Wang, Dr Z Zhou, Dr S Zhang, Dr W

Xie, Dr J.C Su, Dr H Zhou, Ms P.S Zhong, Mr J Yong and all my group members

who have helped me in one way or another through the years of my PhD study

Finally, I am most grateful to my parents and husband for their unconditional love

and support

TABLE OF CONTENT

ACKNOWLEDGEMENTS i

TABLE OF CONTENT iii

SUMMARY vii

LIST OF TABLES x

LIST OF FIGURES xi

Chapter 1 Introduction 1

1.1 Membranes for desalination and water treatment 1

1.1.1 RO, NF and FO 2

1.1.2 Traditional membranes 6

1.1.3 New generation membranes 8

1.2 Aquaporin 9

1.2.1 The aquaporin family 9

1.2.2 The aquaporin structure 10

1.2.3 Production of AqpZ 14

1.2.4 Functional characterization 15

1.3 Biomimetic membrane 16

1.3.1 Langmuir-Blodgett technique 18

1.3.2 Vesicle adsorption technique 22

1.3.3 Block copolymer membrane 24

1.4 Biomimetic membrane for water treatment 25

1.4.1 Designs and strategies 25

1.5 Motivation, challenges and objectives 28

Chapter 2 AquaporinZ Incorporation via Binary-Lipid Langmuir Monolayers 30

2.1 Introduction 30

2.2 Materials and methods 31

2.2.1 Materials 31

2.2.2 Surface pressure measurement 31

2.2.3 Surface tension measurement 32

2.2.4 LB film deposition and AFM scanning 33

2.3 Results and discussion 34

2.3.1 DDM effects on DPPC monolayer 34

2.3.2 AqpZ incorporation via DPPC-DOGSNTA monolayers 38

2.4 Conclusion 43

Chapter 3 Stabilization and Immobilization of Aquaporin Reconstituted Lipid Vesicles for Water Purification 45

3.1 Introduction 45

3.2 Materials and methods 47

3.2.1 Materials 47

3.2.2 Preparation of Vesicles 48

3.2.3 Characterization of polymerized vesicles 48

3.2.4 Vesicle permeability measurements 49

3.2.5 Preparation of vesicle immobilized membranes 49

3.2.6 Field-emission scanning electron microscopy (FESEM) 50

3.2.7 Nanofiltration studies 50

3.3 Results and discussion 51

3.3.1 Characterization of polymerized vesicles 51

3.3.2 Vesicle immobilization on PDA coated silicon surface 53

3.3.3 Water permeability of polymerized vesicles 54

3.3.4 Vesicle immobilized nanofiltration membranes 57

3.4 Conclusion 60

Chapter 4 Aquaporin-embedded Mixed Matrix Membrane: A Layer-by-Layer Self-assembly Approach 62

4.1 Introduction 62

4.2 Materials and methods 65

4.2.1 Materials 65

4.2.2 Vesicle preparation and characterization 65

4.2.3 Vesicle permeability measurement using stopped-flow 66

4.2.4 Liposome-embedded LbL on a mica surface 67

4.2.5 Liposome-embedded LbL membranes for nanofiltration 67

4.2.6 Nanofiltration studies 68

4.3 Results and discussion 69

4.3.1 PLL adsorption on liposomes 69

4.3.2 Water permeability measurement by stopped-flow 71

4.3.3 Embedding PLL-covered liposome in LbL films 72

4.3.4 Liposome embedded membrane for nanofiltration 75

4.4 Conclusion 80

Chapter 5 Highly Permeable Aquaporin-embedded Biomimetic Membrane Featured with a Magnetic-aided Approach 81

5.1 Introduction 81

5.2 Experimental 83

5.2.1 Materials 83

5.2.2 Magnetic nanoparticle synthesis 84

5.2.3 Magnetic liposome preparation 84

5.2.4 Field emission transmission electron microscopy (FETEM) 85

5.2.5 Liposome-embedded LbL membranes formation 86

5.2.6 Vesicle adsorption study by confocal Laser scanning microscope (CLSM) 87

5.2.7 Forward osmosis measurement 87

5.3 Results and discussion 89

5.3.1 Characterization of magnetic liposomes 89

5.3.2 Characterization of the LbL membrane 94

5.3.3 Study the forward osmosis performance 98

5.4 Conclusion 100

Chapter 6 Conclusions and recommendations 102

APPENDICES 105

REFERENCES 106

SUMMARY

Water is transported rapidly through most biological cell membranes Peter Agre and his co-workers revealed the origin of this high water permeability in 1992 with the discovery of the first aquaporin (AQP) protein Inspired from biological membranes where AQPs provide extraordinary water permeability and selectivity, the use of AQPs to fabricate membranes for water purification has recently drawn worldwide attention The discovery of AQPs that facilitate water transport through biological membranes gives us strong support to mimic biological membranes when designing novel membranes for desalination with the aid of these proteins

The conventionally used reverse osmosis process for producing high quality drinkable water is an energy-intensive process Developing innovative high-performance membranes for desalination and water reuse that consume less energy has become an urgent issue in recent years Driven by the extraordinary permeability and selectivity of AQP towards water molecules, novel biomimetic membranes consisting of AquaporinZ (AqpZ) for desalination and water reuses are targeted in this study

In the first part of the work, a new approach of incorporating the transmembrane protein AqpZ into the lipid bilayer has been developed with the aid of the Langmuir-Blodgett (LB) technique Protein incorporation in this study was achieved by combining a pure binary-lipid monolayer with an AqpZ-associated binary-lipid monolayer and a subsequent refolding of AqpZ in the bilayer The binary-lipid monolayer is composed of (1) gel-phase lipids that prevent detergent dissolution and

(2) nickel-chelating lipids that attach the AqpZ from the subphase To gain a better

film integrity and enhance the protein adsorption onto the monolayer, the detergent in

the subphase was removed by BioBeads It is shown that the removal rate can be

determined by the quantity of the BioBeads that are used and the circulation of the

subphase Furthermore, the bilayer that is formed with reconstituted AqpZ is imaged

by AFM and the incorporation mechanism is explained in a three-step process

However, this AqpZ embedded membrane has a large number of defects and cannot

be deposited onto porous substrates for water purification applications Therefore,

better solutions have to be found

The membrane formation strategy was changed in the next step A nanofiltration

membrane was fabricated by the immobilization of AqpZ-reconstituted liposomes

onto a polydopamine (PDA) coated polyacrylonitrile substrat Amine-functionalized

proteoliposomes were first deposited via gentle vacuum suction and subsequently

conjugated on the PDA layer via the formation of amine-catechol adducts The

membrane could sustain hydraulic pressures of up to 5 bar as well as strong surface

agitation that was used in nanofiltration tests because of the existence of a polymer

network within the lipid bilayers, indicating a stable membrane structure Compared

to the membrane without AqpZ incorporation, the membrane with an AqpZ-to-lipid

weight ratio of 1:100 experienced a 65% increase in the water flux from 2.3 to 3.8 L

m-2 h-1 bar-1 with enhanced NaCl and MgCl2 rejections of 66.2% and 88.1%,

respectively

When the vesicles are directly immobilized on the microporous membrane surface,

the aquaporin molecules that are used for water transport are limited to those that are

above the pores In the third part of this study, we developed a mixed matrix biomimetic membrane using the multilayer polyelectrolyte adsorption method to further improve the water flux and the salt rejections of the membrane By encapsulating AqpZ-incorporated vesicles with positively charged poly-L-lysine molecules, the vesicles could be adsorbed onto polyanion films AFM and FESEM studies showed that the amount of adsorbed vesicles was proportional to the percentage of charged lipids that was present in the liposomes The AqpZ embedded layer-by-layer (LbL) membrane demonstrated a water permeability of 6 L m-2 h-1 bar-1and a MgCl2 rejection of more than 95% In comparison with the control LbL membrane that had no embedded liposomes, the newly designed mixed matrix membrane improved the water permeability by 60% because of the presence of AqpZ The LbL approach provides the AqpZ embedded biomimetic membrane with a satisfactory stability and a satisfactory separation performance, which makes the large-scale fabrication of AqpZ-incorporated membranes feasible and practical

In the last part of my PhD work, we adopted the use of magnetic nanoparticles to enhance the amount of proteoliposomes embedded in the LbL membrane and to maximize the potential of AQP for filtration applications Magnetic nanoparticles were encapsulated inside the proteoliposomes and a magnet was used to accelerate the precipitation and the adsorption of these magnetic liposomes onto the membrane matrix The liposome coverage on the membrane surface was greatly improved using this approach and this newly developed biomimetic membrane possessed an extraordinary performance in forward osmosis

LIST OF TABLES

Table 1.1 The characteristics and applications of different membrane separation

techniques 2!Table 1.2 State-of-the-art designs of biomimetic membranes for water treatment 27!Table 3.1 DLS results of control liposomes with and without UV cross-linking 56!Table 3.2 Pure water permeability (PWP) of PAN membrane before and after

modification The standard error of the data is within 8% 59!Table 4.1 The zeta potential and the intensity-weighted mean hydrodynamic

diameter of the intact liposomes and the PLL-covered liposomes The

thickness of the adsorbed PLL layer was calculated from the diameters

obtained Three different liposome samples were used to obtain the

mean and the standard deviation 70!Table 4.2 The intrinsic water permeability and the MgCl2 permeability of the

blank substrate and the substrate with LbL films 77!Table 5.1 Pure water permeability (PWP), molecular weight cut-off (MWCO)

and surface roughness of the hydrolyzed PAN membrane substrate 96!Table 5.2 FO performance of the AqpZ-embedded membrane by using 0.3M

sucrose as the draw solution and MgCl2 as the feed solution The

active layer faces the draw solution 100!

LIST OF FIGURES

Figure 1.1 Schematic comparison of FO and RO processes In the RO process,

an external hydraulic pressure has to be applied to overcome the osmotic pressure 3!Figure 1.2 Novel desalination process by forward osmosis 6!Figure 1.3 Schematic representation of symmetric, integrally skinned

asymmetric and composite membranes 7!Figure 1.4 Schematic representation of the flat-sheet membrane casting process

via phase inversion 8!Figure 1.5 The hourglass model of AQP, the extended 2D structure (left) and

folded 3D structure (right) [40] 11!Figure 1.6 The three mechanisms for solute rejection in AQP [41] 12!Figure 1.7 Illustration of (A) the Grotthuss Mechanism and (B) the water-

dipole re-orientation mechanism in AQP for proton exclusion 13!Figure 1.8 Three membrane models that are commonly adopted to mimic

biological membranes: liposomes (lipid vesicle), supported lipid membranes and black lipid membranes [46] 17!Figure 1.9 The schematic diagram of an LB trough and the vertical monolayer

transfer technique onto a hydrophilic substrate or a hydrophobic substrate [53] 20!Figure 1.10 A pressure-area (π-A) isotherm plot of a phospholipid,

dipalmitoylphosphatidylcholine (DPPC) with an indication of the two-dimensional phases of the monolayer at different regions 21!Figure 1.11 Liposome adsorption techniques: (A) Liposome spreading directly

onto a solid surface to form an SLB; (B) liposome spreading on to

a self-assembled momolayer to form an SLB; (C) liposome spreading on a polymer cushions to form an SLB [46] 23!

Figure 1.12 Examples of synthetic lipo-polymers that are used to build-up

polymer-cushined/tethered SLBs [70] 24!

Figure 2.1 Pressure-area isotherm cycles of DPPC at 23°C after 6 hours

detergent removal with (a) 50mg, (b) 150mg, and (c) 300 mg

BioBeads in the trough (dotted line) and subphase volume of

pure DPPC isotherm (solid line) and isotherm cycle without using

BioBeads (dashed line) 35!

Figure 2.2 AFM topograph of (a) pure DPPC bilayer, (b) disrupted DPPC

bilayer by 2mg L-1 DDM in the subphase, and (c) DPPC bilayer

with removal of subphase DDM All the images were scanned in

ultrapure water environment The dark area corresponds to the

mica surface The typical DPPC thickness should be around 4-5 nm

as shown in (a) and (c) However with DDM disruption, the DPPC

thickness is reduced below 4nm and the “fluid-like” defective

structure is formed in the bilayer The cross-section profiles at the

dashed lines are shown at the bottom of respective images The

scale bars (in solid line) are 1µm in all the three images 37!

Figure 2.3 Pressure-area isotherms of DPPC, DOGSNTA and their mixtures of

different molar ratios (5:1 and 10:1) The temperature of

experiments is 23±1°C 37!

Figure 2.4 Surface tension versus DDM concentration in the AqpZ-free

solution and AqpZ solution The base solution is PBS buffer with

pH=7.4 For pure DDM solutions ( ), the transition point of the

trend indicates the CMC value of DDM For DDM-AqpZ solutions

( ), the first transition point (from left to right) is the CAC value

of the DDM-AqpZ complex system, and the second transition point

is the same with the CMC of DDM At an AqpZ concentration of

0.5mg/L (0.02µM), the CAC of the mixed system is at about

2mg/L DDM, which is well below the CMC value of pure DDM

solution, 85mg/L 40!

Figure 2.5 AFM topograph of AqpZ associated lipid bilayer after 6 hours of

detergent removal (recorded in PBS buffer solution, pH=7.4) The dark area corresponds to the mica surface while the lighter area corresponds to the bilayer surface The thickness of the bilayer is about 4-5 nm (a) Lower magnification topograph of the bilayer shows hints of incorporated AqpZ (arrows) Scale bar is 1µm (b) Higher magnification topograph shows the inserted proteins are distributed in the lipid bilayer Scale bar is 200nm (c) and (d) are the cross-sectional images of protrusions pointed by arrow 1 and arrow 2, respectively 41!Figure 2.6 Schematic presentation of the proposed AqpZ incorporation

mechanism: (a) protein attachment on the DPPC-DOGSNTA monolayer, (b) partial insertion of AqpZ into the DPPC-DOGSNTA monolayer as DDM is slowly removed, (c) protein reconstitution into the lipid bilayer after LS deposition onto the second DPPC-DOGSNTA monolayer 42!Figure 3.1 Schematic presentation of immobilization of the cross-linked

proteoliposome on a PDA coated membrane (not to scale) 47!Figure 3.2 Confocal microscopy images of Rh-PE doped liposomes (a) without

and (b) with UV cross-linking of the bilayer CF can diffuse through the control vesicles and be removed in dialysis process in (a), but with formation of a polymer network within the lipid layer, fluorescein was trapped inside the liposomes as shown in (b) 52!

(b) with UV cross-linking of the bilayer The thickness of the vesicle wall is about 15nm from the images 53!Figure 3.4 AFM images of immobilized liposomes on PDA deposited silicon

surface in amplitude graph (a), 3D topography (b), and a sectional profile (c) of a representative liposome pointed by an arrow

cross-in (b) 54!Figure 3.5 Water permeability of liposomes and proteoliposomes with different

AqpZ-to-lipid weight ratios 55!

Figure 3.6 FESEM images of the PAN membrane (a) before and (b) (c) after

PDA coating, and (d) (e) with vesicle immobilization on the PDA

coating A PDA layer with a thickness of about 100nm can be

observed from the cross-section image of the PDA-PAN membrane

in (c) Two immobilized vesicles were imaged at a higher

magnification in (e) 57!

Figure 3.7 Salt rejection of vesicle immobilized PAN membranes with different

AqpZ-to-lipid weight ratios (Testing pressure: 5 bar Feed solution:

200ppm NaCl or 200ppm MgCl2.) 59!

Figure 4.1 The schematic presentation of the formation procedures for the

liposome-embedded LbL membrane 64!

Figure 4.2 FETEM images of (a) intact liposomes, and (b) PLL-covered

liposomes The liposomes content 15% POPG 70!

Figure 4.3 The liposome permeability measured by stopped-flow light scattering

(a) The normalized light scattering signal of vesicles with different

AqpZ-to-lipid ratios, in the first 80 ms (b) The normalized light

scattering of liposomes without any incorporation of AqpZ (control)

(c) The comparison of the calculated permeability P f of intact

liposomes and PLL-covered liposomes at different AqpZ-to-lipid

weight ratios 72!

Figure 4.4 Intensity-weighted mean diameters of intact liposomes,

PLL-liposomes and PLL-liposome mixed with different polyanions 73!

Figure 4.5 The AFM images of (a) the PLL-lipo15 and (b) the PLL-lipo30

embedded LbL membranes on the mica substrate The dimensions of

both images are 6 ×6 µm 74!

Figure 4.6 The FESEM images of (a) blank hydrolyzed PAN membrane, (b) a

control LbL film without embedding any liposomes, (c) (d) a

embedded LbL film before filtration tests, (e) a

liposome-embedded LbL film after filtration tests The slanted view in (d)

demonstrates that the liposomes are partially embedded in the

polyelectrolyte matrix 75!

Figure 4.7 The water permeability (marker) and the MgCl2 rejection (column) of

the LbL film embedded with reconstituted vesicles or free vesicles The FESEM images demonstrate an increase in liposome adsorption as POPG content is increased 77!Figure 4.8 The rejection of glutathione at various solution pH values (red solid

AqpZ-line) The membrane used was an LbL film that was embedded with PLL-lipo30 (AqpZ-to-lipid ratio was 1:100) Glutathione can be ionized into five different states bychanging the solution pH The fraction of each state is indicated in the figure as dotted curves

pKa1=2.12, pKa2=3.59, pKa3=8.75, pKa4=9.65 79!Figure 5.1 The schematic presentation of fabrication procedures for the

magnetic-aided LbL membrane 83!Figure 5.2 The FETEM images of (a) free MNPs, (b) MNP-encapsulated

liposomes and (c) intact liposomes Scale bar: 100nm The insert in (a) is the MNP size distribution measured by dynamic light scattering 89!Figure 5.3 Stopped-flow light scattering result (a) The normalized light

scattering signal of magnetic liposomes with different AqpZ-to-lipid

weight ratios, (b) the comparison of the calculated permeability P f

of intact liposomes and MNP-encapsulated liposomes at different AqpZ-to-lipid weight ratios 91!Figure 5.4 The Arrhenius plots of water flow across liposomes The kinetics of

water transport were studied in intact liposomes (blue) without AqpZ

(regression equation: ln k = 25.76-6.99/T) and intact proteoliposomes (AqpZ-to-lipid weight ratio: 1:100, regression equation: ln k = 10.63- 1.94/T), and in magnetic liposomes (red) without AqpZ (regression equation: ln k = 25.96-7.06/T) and with AqpZ incorporation (AqpZ- to-lipid weight ratio: 1:100, regression equation: ln k = 10.91- 1.76/T) 92!

Figure 5.5 The FESEM images of ruptured liposomes on membrane surface The

hydrolyzed PAN membrane was firstly covered with a positively charged PAH layer, and then incubated with negatively charged

POPC/POPG/Chol liposomes for two hours The liposomes were

adsorbed onto the oppositely charged surface, but the electrostatic

force induced the rupture of vesicles 93!

Figure 5.6 Study the liposome adsorption process by CLSM Representative

CLSM images of liposome adsorbed LbL film at different liposome

deposition time, (a) in presence, or (b) in absence of the magnetic

driven force (c) The amount of adsorbed vesicles estimated by

counting the number of bright dots in the CLSM images is plotted

against the deposition time 94!

Figure 5.7 The elution profile of magnetic liposome suspension separated by the

cross-linked dextran gel Sephadex G100 and detected by a UV

spectrometer 95!

Figure 5.8 The FESEM images of (a) the top surface and (b) the bottom surface

of a blank 18% PAN membrane, and (c) the top surface and (d) the

cross-section of the PAN membrane with a deposition of a

PAH-PSS/PAA bilayer 96!

Figure 5.9 The FESEM image of the membrane surface deposited with magnetic

liposomes 97!

Figure 5.10 FO performance of the liposome-embedded membrane (a) Water

flux and (b) salt reverse flux of the liposome embedded membrane at

different draw solution concentrations 98

!

Chapter 1 Introduction

1.1 Membranes for desalination and water treatment

A membrane is defined as a separation boundary that selectively transports substances between two phases Membrane technology plays an increasingly important role in a broad spectrum of industrial applications that are related to water, energy, pharmaceutical and life sciences, which can be attributed to three distinct advantages of the membrane separation process Firstly, it is a clean separation process that works without the addition of any other chemicals Secondly, it is energy-efficient and cost-effective compared to other separation processes such as evaporation [1] Lastly, it can be easily scaled up industrially without the requirement for a large footprint [2]

Today, water scarcity has become an issue that has received much attention throughout the world [3] Two thirds of the earth’s surface is covered by ocean, hence seawater desalination has become one of the more common areas that is being investigated to solve the water shortage crisis Membrane filtration has emerged as a viable means of desalination since the 1960s The asymmetric cellulose acetate membranes that was invented by Loeb and Sourirajan in 1963 was a breakthrough in industrial membrane applications [4] Over the years, membrane technology was rapidly developed and became one of the most efficient methods for desalination and wastewater treatment

In a typical membrane process for water purification, the preferential transport of substances across the membrane can be achieved by applying an effective driving

force upon the feed water This driving force can be a concentration difference, a

pressure difference, a temperature difference or an electrical potential difference

across the membrane Based on different membrane structures and driving forces, the

membrane process for water separation can be classified into microfiltration (MF),

ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), forward osmosis (FO),

membrane distillation (MD), and electrodialysis (ED), as shown in Table 1.1

Table 1.1 The characteristics and applications of different membrane separation

techniques

1.1.1 RO, NF and FO

Osmosis was first discovered by Nollet in 1748 [5] and can be frequently observed

in nature As two solutions of different concentration are separated by a

semi-permeable membrane, solvent flows across the membrane from the solution of low

Membrane

process

Pore size (Å)

Separation

Microfiltration 500-50,000 Size exclusion

Hydrostatic pressure

Removal of small suspended solids

Ultrafiltration 10-1,000 Size exclusion

Pre- and treatment of oils, bacteria, and colloids

post-Nanofiltration 5-20

Size exclusion/Donnan exclusion/solution diffusion

Removal of small molecules and divalent salts

Reverse

osmosis < 5 Solution diffusion

Water production from brackish and seawater Forward

osmosis < 5 Solution diffusion Osmotic pressure

Water production from wastewater and seawater Membrane

distillation 100-10,000 Vapor pressure

Temperature difference

Water production from non-volatile solutes Electrodialysis <10 Dielectric

exclusion

Electrical potential difference

Brackish water desalination

concentration to the solution of high concentration until the solute concentration are equalized on the two sides Ideally, the semi-permeable membrane rejects all the solutes and only allows the solvent to pass through The chemical potential is different across the membrane due to the different concentrations of the two solutions The osmotic pressure π arises from the tendency of solvent movement through the membrane due to the difference in chemical potentials The osmotic

pressure Π of a solution can be calculated by the van’t Hoff equation [6]

where i is the van’t Hoff factor, c is the concentration of all solute species in the

solution, R is the gas constant and T is the temperature Seawater that has a salinity

of approximately 3.5% has an osmotic pressure of approximately 27 bar

Figure 1.1 Schematic comparison of FO and RO processes In the RO process,

an external hydraulic pressure has to be applied to overcome the osmotic

pressure

The osmotic pressure difference Δπ is equal to the pressure required to stop osmosis

from occurring In an RO process, a hydraulic pressure that is higher than the

osmotic pressure (Δp > ΔΠ) is applied to reverse the natural water flow direction

that occurs as a result of osmosis (Figure 1.1) Pure water is squeezed out of the

feed solution and collected as the permeate The general equation for water

transport in RO and FO is as follows:

where J w is the water flux, A is the membrane water permeability, and σ is the

reflection coefficient For ideal semipermeable membranes, σ = 1 The direction of

the water flow across the membrane depends on the relationship between Δp

and ΔΠ, which has been characterized in early 1980s by Lee et al [7]

The predominant separation mechanism in RO is solution-diffusion and is different

from other filtration processes such as UF or NF Solution-diffusion mechanism

determines that the membrane efficiency depends on the feed concentration, the

external pressure, the flow rate and the temperature Today, the RO process is the

most successful membrane desalination technology for producing fresh water from

brackish water, seawater [8], wastewater [9] and contaminated ground water [10]

Seawater RO membranes are able to achieve a salt rejection of more than 99% [11]

To overcome the osmotic pressure in the feed solution, a higher external pressure

has to be applied The operating pressure for RO can vary from 6 to 30 bar for

brackish water and 55 to 80 bar for seawater, mainly because brackish water has a

lower osmotic pressure than seawater [11] The major cost in the RO process is the

energy consumption that is required for operating the high-pressure pump As oil

prices have increased rapidly in recent years, the cost for RO desalination may also

increase tremendously in the future

The NF process was developed in the 1980s mainly removing hardness and organic compounds [12] NF membranes are nanoporous membranes and also known as

“loose” RO membranes NF membranes can reject small dissolved organic molecules as well as divalent ions and other multivalent ions such as Ca2+

that contribute to water hardness Although research has shown that the NF process alone cannot produce drinkable water from seawater, the NF process has been successfully applied to treat mildly brackish water and wastewater [13] The separation mechanism of NF membrane includes size exclusion, Donnan exclusion and solution-diffusion as well It offers a higher retention than UF and requires a lower operating pressure than RO (4 to 20 bar) Therefore, in recent years, NF has received a lot of attention in wastewater treatment, the food industry, the chemical and pharmaceutical industry and many other industries [14]

Forward osmosis is a new emerging membrane-based technology for water treatment [15] Table 1.1 shows that FO membranes are characteristically similar to RO membranes, such as possessing small pore sizes and a high rejection towards contaminants However, the FO process is different from the RO and the NF process

as it operates at very low or even zero hydraulic pressure, utilizing the natural osmotic pressure difference between two solutions to facilitate the transport of water across the membrane FO has been widely applied in the food and the pharmaceutical industries for dehydration or concentration purposes [16] The use of FO for seawater desalination has been conceptually demonstrated since the 1970s [17], but was only revived in recent years when commercial FO membranes were developed A complete FO desalination process consists of two steps, as shown in Figure 1.2, where (1) osmotic water is transported from the feed to the draw solution, and (2)