Aligned Small Diameter Single-Walled Carbon
Nanotube Membrane for Reverse Osmosis Desalination
of Water
Blake Wilson
Department of Chemistry
The University of Texas at Dallas
December 11, 2012
i
Contents
1
Specific Aims
2
2
Background and Significance
3
2.1
Water Desalination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
2.1.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
2.1.2
Current Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
2.1.3
Reverse Osmosis Membranes . . . . . . . . . . . . . . . . . . . . . . . .
4
Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2.2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2.2.2
Mass Transport Through Carbon Nanotubes . . . . . . . . . . . . . . . .
8
2.2
3
Research Design and Methods
3.1
11
Fabricate the Membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Main Approach 1: Sub-nm SWCNTs . . . . . . . . . . . . . . . . . . . . 11
Main Approach 2: Ultra Dense SWCNT Forest . . . . . . . . . . . . . . . 13
4
3.2
Characterize the Membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.3
Methods Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Merit
17
1
1
Specific Aims
As freshwater supplies slowly dwindle and many regions of the world continue to face water shortages, the need for an efficient and cost effective means of desalination of ocean and brackish water
is ever growing. Current commercial desalination techniques generally lack in efficiency and are
often costly to implement. As a solution to creating more efficient desalination filters, water transport through the interior of single-walled carbon nanotubes (SWNTs) has been studied theoretically, computationally, and experimentally [1, 2, 3, 4, 5, 6]. It has been shown computationally that
water has a high osmotic flux across the interior of SWNTs [6]. It has also been shown that with
some specific SWCNT diameters 100% ion rejection can be achieved while maintaining high flow
rates of water [3, 6]. These studies suggest that SWCNTs aligned as the pores in a membrane may
provide an efficient means of reverse osmotic desalination of water. However, an aligned SWCNT
membrane with highly selective pores and high pore density has yet to be constructed. The long
term goal of this proposal is to determine the performance of an aligned SWCNT membrane for
reverse osmosis desalination of water. The central hypothesis is that such a membrane will have
much greater efficiency than current reverse osmosis (RO) desalination membranes. This hypothesis was formulated based on studies of water transport through SWCNTs [1, 2, 3, 4, 5, 6]. This
proposal will be executed with these specific aims:
Aim 1: Fabricate the membrane
Aim 2: Characterize the membrane
2
2
Background and Significance
2.1
Water Desalination
2.1.1
Introduction
Desalination is a water purification technique that removes salts from sea or brackish water to
create fresh water [7, 8, 9]. Approximately 97% of the world’s water supply is composed of
sea and brackish water, while the remaining 3% of the world’s water supply is considered fresh
[7, 10, 9]. Of this small amount of fresh water, only about 0.25% is contained in readily accessible
forms such as rivers and lakes, while the remaining fresh water is either trapped in frozen forms,
such as glaciers, or contained in aquifers [9]. The readily accessible fresh water sources pale in
comparison to the availability of salt water. Water desalination provides a way to tap into the vast
supply of salt water and convert it into potable water. Techniques for the desalination of water have
existed for hundreds of years, with one of the first examples of desalination accounted in CE 200
[9]. From such early uses of desalination, the technique has become much more refined and the
commercial uses continue to expand.
2.1.2
Current Technology
Today many desalination plants exist around the world, especially in arid regions where fresh water
is not readily available. There are several techniques used to desalinate water, which can be broken
into two main categories: phase-change processes and membrane processes [8, 9]. Some of the
most important techniques associated with these two categories are listed below in Table 1.
Multi-stage flash (MSF), multiple-effect boiling (MEB), and vapour compression (VC) are
all distillation techniques. These techniques use thermal energy to distill salt water. Of these
three thermal techniques, MSF is commercially the most dominant, comprising approximately
3
Phase-change Processes
Membrane Processes
1. Multi-stage flash (MSF)
1. Reverse Osmosis (RO)
2. Multiple effect boiling (MEB) 2. Electrodialysis (ED)
3. Vapour compression (VC)
Table 1: Most common desalination techniques [8, 9].
93% of thermal process production and approximately 44% of total worldwide production. Reverse
osmosis (RO) and electrodialysis (ED) take advantage of selective membranes to desalinate water.
RO accounts for 42% of the worldwide production and approximately 88% of membrane process
production [9].
2.1.3
Reverse Osmosis Membranes
Osmosis can be defined as the passage of solvent molecules, but not those of the solute, through
a semipermeable membrane from a less concentrated solution to a more concentrated solution
[11, 7, 2]. In reverse osmosis (RO) a pressure is applied such that solvent flows in reverse of the
natural osmotic flow. This causes the solvent molecules to travel from the more concentrated solution across the semipermeable membrane into the less concentrated solution. Osmosis and reverse
osmosis are schematically shown in Figure 1. These processes have been known for many years
and studies date back as far as 1748 [11]. However, using the process of RO for water desalination
is a more recent advance, taking place in the last few decades. Reid showed in the 1950’s that a
cellulose acetate RO membrane was capable of removing salt from water; and the first asymmetric
cellulose acetate RO membranes with feasible flux rates were developed in the early 1960’s by
Loeb and Sourirajan [11]. Since those achievements, many advances have been made in RO membranes. Currently used commercial RO membranes can be separated into two basic categories:
cellulose acetate (CA) and thin film (TF) membranes. CA membranes have relatively high chlorine and fouling resistance but their compressibility under high pressures typically limits their use
4
Figure 1: Schematic representation of osmotic and reverse osmotic flow systems with direction of
water flow shown by the horizontal arrows across the semi-permeable membranes [12].
to brackish water [11]. TF membranes can be used for both brackish and seawater desalination. TF
membranes have been used desalinate seawater with high flux values, high salt rejection, and up to
60% water recovery [11]. With their remarkable mass transport properties, CNT RO membranes
may be able to achieve greater efficiency than current TF membranes. In general, RO membranes
have a few specific advantages over other desalination techniques. They avoid the use of energyintensive phase-changes and costly solvents or adsorbents. ED membrane processes only work for
desalination of brackish water whereas RO membranes can be used for both brackish and sea water. Compared with other common techniques RO is considered a rather simple process to design
and run; and typically can be easily integrated into hybrid desalination systems [11].
5
2.2
Carbon Nanotubes
2.2.1
Introduction
Carbon Nanotubes (CNTs) have been a topic of intensive research over the past couple of decades.
They exhibit many interesting structural, mechanical, optical, and electronic properties. The first
publication describing CNTs was published in 1991 by Iijima, who observed multi-walled carbon
nanotubes (MWCNTs) [13, 14]. The first single walled carbon nanotubes were reported two years
later in 1993 [13]. CNTs are composed of sp2 hybridized carbon networks and are in essence
rolled up sheets of graphene, as Figure 2 depicts.
Figure 2: Schematic representation of the folding of graphene into a CNT [15].
MWCNTs are composed of two or more concentric SWCNTs, while SWCNTs consist of just
a single folded graphene layer as shown in Figure 3.
6
Figure 3: Left: Single walled carbon nanotube(SWNT). Right: Multi-walled carbon nanotube
(MWCNT)
SWCNTs are described by the chiral vectors which are denoted by the integers n and m [16,
17]. These integers n and m are referred to as the chiral indices and are given as (n,m). The chiral
vectors are represented in Figure 4; and can be thought as representing the ”folding vectors” to roll
up graphene sheets [18].
Figure 4: Graphene sheet with respective chiral vectors for folding into CNT [18].
There are three basic constructions of SWNTs, which are determined by the chiral indices; the
basic constructions are shown in Figure 5. The first two: the zigzag and the armchair configurations
are not chiral. The zigzag structure is given by chiral indices (n,0) and the armchair structure is
7
given by indices n = m or (n,n). The third type of configuration is the chiral SWCNT, which is
given by chiral indices n > m > 0.
Figure 5: The three basic constructions of CNTs [19].
A large range of SWCNT diameters are now achievable and range from about 0.5 nm to greater
than 10 nm. The length of CNTs also has a wide range going from less than 10 nanometers to
greater than 100 µm.
2.2.2
Mass Transport Through Carbon Nanotubes
A very interesting phenomena observed is fast mass transport of several gases and of water through
the interior of CNTs. For example, many gases hve been shown to pass through CNTs with flux
values orders of magnitude greater than equivalent zeolite pores [20, 4, 21, 22]. Holt et al reported
the fast mass transport through sub-2nm CNTs with water transport rates reaching more than 3
orders of magnitude greater than as predicted by the bulk continuum Hagen-Poisselle equation[4].
Indeed the flow of water through CNTs is particularily interesting. By nature the phenomena seems
quite peculiar as water molecules are highly polar and CNTs are rather hydrophobic. The fast
flow rates through CNTs has been attributed to the atomic smoothness of CNTs [20, 21, 3, 1, 4].
Molecular ordering phenomena also occurs, and contributes to the fast molecular transport of water
8
[4, 1]. In fact water molecules have been shown to travel through CNTs with almost no friction,
and the main energy barriers being the entering and exiting of the tube [6]. This effect of near
frictionless travel through the interior of CNTs has been shown to be nearly independent of the
the tube length [6]. Kalra et al performed molecular simulation of SWCNT membranes under
large osmotic gradients, shown in Figure 6, and showed that water moves very rapidly through the
nanotubes; water transport rates approached 5.8 water molecules per ns per nanotube [6].
Figure 6: Simulation snapshot of SWCNT membrane. SWCNTs are hexagonally packed and the
blue beads are N a+ , yellow beads are Cl− , red beads are oxygen and white beads are hydrogen
[6].
Corry conducted molecular simulation of four SWCNT diameters to determine the desalination
efficiency of each tube diameter. The SWCNTs used in this study are listed below in Table 2.
All the SWCNTs that were tested by Corry, pictured in Figure 7, showed some salt rejection. The
(5,5) and (6,6) SWCNTs showed 100% salt rejection. The water conductances reported in Table
2 are under extreme hydrostatic pressure difference but Corry also simulated these SWCNT under
a 5.5 MPa hydrostatic pressure and osmotic pressure of 2.4 MPa, which mimics the conditions
9
Chirality (n,m)
(5,5)
(6,6)
(7,7)
(8,8)
Diameter (nm)
0.66
0.81
0.93
1.09
Ion Rejection (percent)
100
100
95
58
Water Conductance (pt pns)
10.4
23.3
43.7
81.5
Table 2: SWCNT chiral indices and their respective diameters, salt rejection, and water conductance per tube per nanosecond(under 208 MPa pressure and 250 mM NaCl) as reported by Corry
[3].
often seen in the desalination of seawater. Assuming a pore density of 2.5 x 1011 nanotube pores
per cm2 , it was shown that under these typical desalination conditions that flow rate improvements
could be expected from 2.42 (smallest tube) to 9.76 (largest tube), over one specific commercial
RO membrane, with corresponding salt rejections of 100% to 58%[3].
Figure 7: Water and ions in the nanotubes.(A) Snapshots from molecular dynamics simulations
showing the configuration of water in each of the (5,5), (6,6), (7,7), and (8,8) CNTs are shown as
viewed from the plane of the CNT membrane.(B) Top views of the nanotubes show the differing
sizes of the tubes as well as the structure of water in the pores Density values range form low
(orange) to high (blue).(C) Location and hydration structure of N a+ ions that are pulled into the
center of the pores [3].
10
These fast mass transport phenomena of CNTs provides a very promising premise for the
design of highly efficient CNT membranes.
3
Research Design and Methods
The development of highly efficient aligned SWCNTs RO membranes could revolutionize the RO
industry and provide a better opportunity for retrieval of potable water through desalination. This
project will set out to accomplish these specific aims:
3.1
Fabricate the Membrane
Two different main approaches will be taken to achieve this specific aim.
Main Approach 1: Sub-nm SWCNTs This approach will utilize sub-nanometer (sub-nm) SWCNTs. The sub-nm SWCNTs will be synthesized using the method of Loebick et al, which well
characterized a scalable method to produce sub-nm SWCNTs; This method uses CVD with CoMn
bimetallic catalysts supported on MCM-41 silica templates [23]. The sub-nm SWCNTs generated
will then be used in two different different sub-approaches.
Sub-approach 1
Sub-nm SWCNTs will be treated with a mixture of H2 SO4 /HN O3 , which
will break down the SWCNTs into shorter lengths and transform most into MWCNTs [20]. This
acid treatment will maintain the same sub-nm pore size, while increasing the outer diameter of
the CNTs; This will allow the CNTs to be aligned by vacuum filtration. After acid treatment the
altered SWCNTs will be dispersed in THF and subsequently vacuum filtered over a PTFE filter
which will align most nanotubes with a pore density on the order of 1010 pores per cm2 [20, 24].
It is also possible to perform filtering while the SWCNT suspension is under high magnetic field
11
to further induce CNT alignment [25]. CNTs remaining on the filter will be coated with a thin
layer of polysulfone (PSF), creating the composite membrane. The filter alignment and coating
is schematically shown in Figure 8. PSF has been shown to have high wettability with CNTs
Figure 8: Schematic representation of CNT alignment by vacuum filtration over a PTFE membrane
and coating with polymer matrix [20].
and also shows good mechanical strength [20]. The resulting CNT/PSF composite film will be
removed from the PTFE filter. The composite film will then be sandwiched between two thicker
highly porous PSF layers to offer additional support, completing the basic membrane structure.
Sub-approach 2
Sub-nm SWCNTs will be treated in a manner similar to that of Wu et al,
but the procedure will be modified to increase the CNT loading to 5% [26]. The SWCNTs will
be mixed with Epon 862 epoxy resin, hardener methylhexahydrophthalic anhydride, catalyst 1-
12
cyanoethyl-2ethul-4methylimidazole and 0.1 g of surfactant Triton-X 100. The components will
be mixed using a centrifugal shear mixer. Mixing under shear force has been shown to induce CNT
alignment [27, 28]. The subsequent CNT/epoxy composites will be cured at 85 o C under high
magnetic field to increase alignment of SWNTs in the composite [27, 28]. The cured composite
will then be microtomed to approximately 5 microns and each side of the composite will be treated
with water plasma oxidation to remove residual debris and ensure opening of the SWCNTs [26].
Main Approach 2: Ultra Dense SWCNT Forest
This approach will utilize recent advances in
CNT forest synthesis, in which ultra dense SWCNT forests will be synthesized and formed into
a membrane. The membrane structure is schematically shown in Figure 9. The method of Zhong
Figure 9: Schematic representation of aligned CNT/Si3 N4 composite membrane with carboxylated
CNT tips [5].
et al will be followed to generate ultra-high density SWCNT forests, shown in Figure 10, with
SWCNT density on the order of 1013 SWCNTs per cm2 and average diameter of 1.2 nm [29]. This
method uses chemical vapor deposition (CVD) at 700 o C with nanolaminate Fe-Al2 O3 catalyst
design which consists of Al2 O3 , Fe, and Al2 O3 . The lower layer of Al2 O3 is densified by oxygen plasma treatment allowing a thinner catalyst layer to be used [29]. The synthesized SWCNT
forests will then be coated with low-stress low pressure chemical vapor deposited silcon nitride to
13
Figure 10: Cross section of ultra dense SWCNT forest obtained by scanning electron microscope
[29].
fill all the gaps between the SWCNTs and generate the membrane [4]. Excess silicon nitride will
then be removed with argon ion beam etching, and the nanotube pores will subsequently be opened
by using water plasma oxidation. This will also ensure that the tips of the SWCNTs will be functionalized with carboxyl groups. It has been predicted that neutral SWCNTs with diameters of 1
nm or larger will have little ion rejection [3]. However, significant ion rejection has been shown by
membranes composed of CNTs with average diameter of 1.6 nm and charged functional groups on
the tube tips [26, 5]. Addition of carboxyl groups to the tube tips in the membrane should generate
charged species in pH’s greater than 5.5, which coupled with the smaller average tube diameter
should show significant ion rejection.
14
3.2
Characterize the Membrane
Fabricated membranes will be imaged using electron microscopy (EM) to assess initial characteristics and quality. The membranes will be checked for cracking by EM. EM along with Raman
spectroscopy will be used to characterize the average diameters of SWCNTs in the membrane
along with the pore density. Then membranes will then be tested under RO conditions using NaCl
at various osmotic gradients and operating pressures. This will test the integrity of the membrane
for use in RO processes. During the RO testing the flux values for water and the salt rejection will
be thoroughly measured; the efficiency of the membrane will be gauged.
3.3
Methods Analysis
Multiple methods have been proposed to achieve the first specific aim of this project. These differing methods each have potential advantages and problems. The use of sub-nm SWCNTs could
provide a means of achieving 100% salt rejection, thus being highly selective. The limiting factor
for high water flux is the pore density, which is dependent the number of aligned SWCNTs in the
membrane. The first sub approach of main approach one has been shown to generate pore densities
of approximately 7 x 1010 pores per cm2 [4]. Comparing this pore density to calculations done by
Corry, who assumed a pore density of 2.5 x 1011 pores per cm2 , a membrane constructed using
sub-approach 1 would be expected to achieve 100% salt rejection, but there may be little improvement in flux values of water across the membrane [3]. Sub-approach 2 provides a different method
to use sub-nm SWCNTs as the pores in the membrane, which may lead to higher pore densities. In
the past, achieving a high degree of alignment of CNTs by shear flow or high magnetic field alone,
was often limited to low loading densities [27, 28]. By subjecting the composite mixture to two
forms of alignment, this method may be able to achieve a much higher degree of CNT alignment at
15
loading densities necessary to achieve pore densities on the order of 1011 pores per cm2 or greater.
Main approach 2 uses ultra-dense SWCNT forests. This method could generate membranes with
SWCNT pore densities of approximately 1.25 x 1013 pores per cm2 , which may provide for extremely high flux values of water across the membrane [29, 3]. The concern in this method is the
average diameter of SWCNTs produced (1.2 nm) by this method, may lead to a lack in selectivity.
It has been predicted that the neutral tubes with diameters greater than 1.08 nm may have little to
no salt rejection [3]. However, Fornasiero et al. developed a membrane with average CNT pore
size of 1.6 nm and showed that addition of charged groups, such as deprotonated carboxyl groups,
greatly enhanced salt rejection up to around 60% [5]. Therefore the addition of carboxyl groups to
provide charged groups on the SWCNT tips, along with smaller average diameter of the SWCNTs,
should provide significant ion rejection. The implementation of all proposed methods will thus
provide for comparisons of salt rejection versus water flux for the produced membranes.
These methods proposed here are designed to provide proof of concept and are meant for testing on a small scale. The membranes produced from these methods may only be on the order of
(mm−cm)2 in size, although larger membranes may be produced by putting the small membranes
into a gridded membrane structure. Further work will be needed to optimize the production and
reduce the costs of membranes produced from these methods. The costs of many of the materials and instruments needed for fabrication and characterization of SWCNT membranes by these
methods are great. CNTs in high purity are typically 100+ USD per gram. High magnetic field
generators are often highly specialized multi-million USD instruments; and microtoming machines
range from several thousand to 20,000+ USD. Also, both electron microscopes and CVD equipment are typically 100,000+ USD. Although membranes fabricated by the proposed methods may
have greater efficiency than many current RO membranes, they would likely not be a cost effective
16
alternative to currently employed desalination technology at this time. However, this project could
provide a launch pad for further development of ever more efficient and cost effective desalination
technology.
4
Intellectual Merit
The intellectual merit of this project is its multi-pronged approach, which allows for the comparison of SWCNT diameter and membrane pore density on water flux values and salt rejection.
A potential far reaching effect of this project is the development of new RO membranes with
extremely high water flux and excellent salt rejection. This project will also provide a basis of
comparison to the previously computationally exclusive data. A better understanding of size and
tip functionalization effects on water and ion transport through the interior of SWCNTs will be
gained, which will provide greater insight into the fundamental description of these processes.
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List of Figures
1
Schematic representation of osmotic and reverse osmotic flow systems with direction of water flow shown by the horizontal arrows across the semi-permeable
membranes [12]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
2
Schematic representation of the folding of graphene into a CNT [15]. . . . . . . . .
6
3
Left: Single walled carbon nanotube(SWNT). Right: Multi-walled carbon nanotube (MWCNT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
4
Graphene sheet with respective chiral vectors for folding into CNT [18]. . . . . . .
7
5
The three basic constructions of CNTs [19]. . . . . . . . . . . . . . . . . . . . . .
8
19
6
Simulation snapshot of SWCNT membrane. SWCNTs are hexagonally packed
and the blue beads are N a+ , yellow beads are Cl− , red beads are oxygen and
white beads are hydrogen [6]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
9
Water and ions in the nanotubes.(A) Snapshots from molecular dynamics simulations showing the configuration of water in each of the (5,5), (6,6), (7,7), and (8,8)
CNTs are shown as viewed from the plane of the CNT membrane.(B) Top views of
the nanotubes show the differing sizes of the tubes as well as the structure of water
in the pores Density values range form low (orange) to high (blue).(C) Location
and hydration structure of N a+ ions that are pulled into the center of the pores [3].
8
10
Schematic representation of CNT alignment by vacuum filtration over a PTFE
membrane and coating with polymer matrix [20]. . . . . . . . . . . . . . . . . . . 12
9
Schematic representation of aligned CNT/Si3 N4 composite membrane with carboxylated CNT tips [5]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10
Cross section of ultra dense SWCNT forest obtained by scanning electron microscope [29]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
List of Tables
1
Most common desalination techniques [8, 9]. . . . . . . . . . . . . . . . . . . . .
2
SWCNT chiral indices and their respective diameters, salt rejection, and water
4
conductance per tube per nanosecond(under 208 MPa pressure and 250 mM NaCl)
as reported by Corry [3]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
20