Technical Field

The present invention relates to a thin film composite hollow fibre membrane and a method of making the same.

Background

Greenhouse gas emissions and highly fluctuated oil prices are two major issues towards earth sustainability. Intensive research has been made to explore clean and renewable energy productions as alternative energy sources. One possible energy source is from pressure retarded osmosis (PRO). By applying a semi-permeable membrane between two solutions of different salinity, a salinity-gradient energy is extracted by an energy transfer device when the water transports across the membrane from the low-salinity side (i.e., feed solution) to the high-salinity side (i.e., draw solution). Either a hydro turbine or a pressure exchanger could be used in PRO to generate electricity or transfer energy.

Phase inversion and interfacial polymerization methods have been employed to form the selective layer of PRO membranes. The membranes fabricated by phase inversion usually suffer from a low water flux and a low salt rejection because of the thick selective layer and membrane materials available. Interfacial polymerization enables a higher degree of control on membrane morphology and performance by separately tuning the selective layer and the polymeric support. The resultant thin-film composite (TFC) membranes consist of an ultrathin polyamide selective layer on top of a porous and mechanically strong substrate, providing a high water flux and a low reverse salt flux. Thus, most PRO studies are based on TFC membranes.

Most TFC PRO membranes are in either flat sheet or inner-selective hollow fibre configurations. Another form of TFC PRO membranes is an outer-selective TFC hollow fibre membrane. Such membranes are advantageous because there is more membrane area per fibre/module, less mechanical deformation due to the elimination of feed spacers, less mass transfer resistance because the highly pressurized draw solution flows through the module shell side, and it is easier to clean off foulants introduced by a feed solution. However, the challenge lies in forming a good polyamide layer on the outer surface of hollow fibres, as none of the conventional approaches such as roller rolling, air purging and solvent treatment can be applied to effectively remove the excess amine solution before the interfacial polymerization of the outer layer.

Previous attempts at forming outer-selective TFC hollow fibre membranes have produced compromised membranes. For example, Sun and Chung (Environ. Sci. Technol., 2013, 47:13167-13174) have developed a vacuum-assisted interfacial polymerization to synthesize a polyamide layer on top of the outer surface. A vacuum pressure of 800 mbar was employed on the lumen side of a hollow fibre bundle to remove the excess amine solution. The resultant membrane exhibited a peak power density of 7.63 W m ^"2 at 20 bar using 1 M NaCI and deionised (Dl) water as feeds. Nonetheless, the membrane's high water permeability was achieved but with a compromised salt permeability. Since a large salt permeability may cause an instant drop in initial water flux and accelerate flux decline under high hydraulic pressures, it would reduce the optimal operating pressure and the maximum power density.

There is therefore a need for an improved outer-selective TFC PRO hollow fibre membrane with a reasonably high water permeability and a small salt permeability is attractive for real high pressure PRO operations.

Summary of the invention

The present invention seeks to address these problems, and/or to provide an improved thin film composite hollow fibre membrane which is suitable for various applications such as osmotic power generation, low-pressure reverse osmosis of seawater desalination, nanofiltration of pharmaceutical active compounds and removal of heavy metal ions and other emerging contaminants in an aquatic environment.

In general terms, the invention relates to a thin film composite hollow fibre membrane comprising a robust spongy-like hollow fibre support which provides minimal water transport resistance and high pressure tolerance, and a defect-free polyamide selective layer with low salt permeability formed on an outer surface of the hollow fibre support. In particular, the thin film composite hollow fibre membrane has low reverse salt flux and small slope of water flux decline along ΔΡ increases for PRO applications. The present invention also relates to a method of making the thin film composite hollow fibre membrane with simplified fabrication steps, is cost effective and minimises the potential failure of forming a defective polyamide selective layer. According to a first aspect, the present invention provides a thin film composite hollow fibre membrane comprising: a porous hollow fibre support layer formed of polyethersulfone, polysulfone, polyphenylsulfone, polyacrylonitrile, polyimide, polyether imide, polyamide- imde, polyvinylidene fluoride, cellulose triacetate, polyetherketone, polyetheretherketone or a combination thereof, the hollow fibre support layer having a thickness of 100-350 μπι; and

a selective layer on an outer circumferential surface of the hollow fibre support layer, the selective layer formed of a cross-linked polyamide and having a thickness of 100-500 nm,

wherein the thin film composite hollow fibre membrane has a salt permeability of 0.028-

0.042 L m ^"2 h ^"1 and a collapse pressure of at least 20 bar.

According to a particular aspect, the porous hollow fibre support layer may be formed from polyethersulfone (PES).

According to a particular aspect, the thickness of the porous hollow fibre support layer may be 100-350 μιη. In particular, the thickness of the support layer may be 120-330 μ ^η ι, 150-300 μιτι, 170-280 μιη, 200-275 μίτι, 220-270 μιη, 230-260 μιη, 250-255 μηι. Even more in particular, the thickness of the support layer may be about 300 μιη.

The selective layer may have a thickness of 100-500 nm. In particular, the thickness of the selective layer may be 120-480 nm, 150-450 nm, 175-425 nm, 200-400 nm, 220- 380 nm, 250-350 nm, 275-325 nm, 300-310 nm. Even more in particular, the thickness of the selective layer may be about 100-300 nm.

The thin film composite hollow fibre membrane may have a suitable water permeability rate. For example, the pure water permeability rate may be 1.0-2.0 L m ^"2 h ^"1 bar ^"1 . In particular, the water permeability rate may be 1.15-1.75 L m ^'2 h ^"1 bar ^"1 , 1.2-1.5 L m ^"2 h ^* bar ^~ 1.3-1.4 L m ^"2 h ^"1 bar ^"1 . Even more in particular, the water permeability rate may be 1.42 L m ^"2 h ^"1 bar ^"1 .

The thin film composite hollow fibre membrane may have a suitable power density. For example, the power density may be 7.0-10.5 W m ² . The power density may vary depending on the pressure applied during the osmosis process. In particular, the power density may be 7.2-10.0 W m ² , 7.5-9.5 W m ² , 7.8-9.3 W m ² , 8.0-9.0 W m ² , 8.2-8.8 W m ² , 8.5-8.7 W m ² . Even more in particular, the power density may be about 7.81 W m ² at 20 bar pressure and about 7.57 W m ² at 15 bar pressure.

The thin film composite hollow fibre membrane exhibits favourable salt rejection rates. In particular, the salt rejection rate is 80-90%, 82-89%, 84-88%, 85-87%. Even more in particular, the salt rejection rate is about 88.4%.

According to a second aspect, there is provided a method of preparing a thin film composite hollow fibre membrane of the first aspect. The method comprises:

- preparing a module comprising at least one porous hollow fibre support layer potted in the module; and

- forming a selective layer on an outer circumferential surface of the hollow fibre support layer through interfacial polymerization, wherein the forming comprises contacting the surface of the porous hollow fibre support layer with a first solution comprising a polyamine, removing excess of the first solution by applying vacuum and subsequently contacting the surface of the porous hollow fibre support layer with a second solution comprising a polyfunctional acyl halide.

According to a particular aspect, the method may comprise first preparing the at least one porous hollow fibre support layer. The porous hollow fibre support layer may be prepared by:

- providing a dope solution comprising a polymer solution, a solvent/non- solvent mixture and water to an annulus of a spinneret, the polymer solution comprising: polyethersulfone, polysulfone, polyphenylsulfone, polyacrylonitrile, polyimide, polyether imide, polyamide-imde, polyvinylidene fluoride, cellulose triacetate, polyetherketone, polyetheretherketone or a combination thereof;

providing a bore solution to an inner tube of the spinneret; and

- extruding the dope solution and bore solution through the spinneret into a coagulation bath, thereby obtaining a porous hollow fibre support layer. According to a particular aspect, the porous hollow fibre support layer may be formed from polyethersulfone.

The first solution may be any solution comprising a suitable polyamine. According to a particular aspect, the first solution may comprise a polyamine selected from the group consisting of but not limited to: m-phenylenediamine (MPD), p-phenylenediamine, p- xylylenediamine, cyclohexanediamine, piperazine, branched or dendrimeric polyethylenimine, and a combination thereof.

The first solution may further comprise a surfactant. Any suitable surfactant may be added to the first solution. For example, the surfactant may be selected from, but not limited to, sodium dodecyl sulphate (SDS), trimethylamine (TEA), camphorsulfonic acid (CSA), and a combination thereof.

The vacuum may be applied at any suitable trans-membrane pressure. For example, the trans-membrane pressure applied may be 150-1000 mbar. In particular, the transmembrane pressure applied may be 200-900 mbar, 250-850 mbar, 300-800 mbar, 350- 750 mbar, 400-700 mbar, 450-650 mbar, 500-600 mbar. Even more in particular, the trans-membrane pressure applied may be about 800 mbar.

The second solution may be any solution comprising a suitable polyfunctional acyl halide. According to a particular aspect, the second solution may comprise a polyfunctional acyl halide selected from the group consisting of but not limited to: trimesoyl chloride (TMC), isophthaloyl chloride, terephthaloyl chloride, 1 ,3,5- cyclohexane tricarbonyl chloride, 1 ,2,3,4-cyclohexane tetracarbonyl chloride, and a combination thereof.

The second solution may further comprise an organic solvent. Any suitable organic solvent may be added to the second solution. For example, the organic solvent may be selected from, but not limited to, hexane, heptane, and a combination thereof.

Brief Description of the Drawings

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings: Figure 1 shows a schematic process of preparing the TFC hollow fibre membrane according to one embodiment of the invention;

Figure 2 shows the FESEM morphologies of PES hollow fibre support layers as functions of bore fluid composition;

Figure 3 shows the AFM images of the outer surface of the PES hollow fibre support layers where R _a is the mean roughness;

Figure 4 shows the probability density function curves of the PES hollow fibre support layers;

Figure 5 shows the pure water permeability vs. the applied pressure of PES hollow fibre support layers;

Figure 6 shows the FESEM morphologies of the polyamide selective layers of the PES- TFC hollow fibre membranes;

Figure 7 shows the membrane performance in PRO tests using Dl water as the feed solution and 1 M NaCI as draw solution. (A) to (C) show the water flux and reverse salt flux while (D) to (F) show the power density as a function of hydraulic pressure difference;

Figure 8 shows a schematic process of preparing the TFC hollow fibre membrane according to one embodiment of the invention;

Figure 9 shows the FESEM morphology of PES hollow fibre support layer;

Figure 10 shows the probability density function curves of the PES hollow fibre support layer;

Figure 11 shows the pure water permeability vs. the applied pressure of the PES hollow fibre support layer;

Figure 12 shows the FESEM morphology of the polyamide selective layer of the PES- TFC hollow fibre membrane; and Figure 13 shows the membrane performance in PRO tests using Dl water as the feed solution and 1 M NaCI as draw solution. (A) shows the water flux and reverse salt flux while (B) shows the power density as a function of hydraulic pressure difference.

Detailed Description

As explained above, there is a need for improved thin film composite hollow fibre membranes for use in various applications such as in pressure retarded osmosis (PRO) for generating electricity or transferring energy for further use. For this purpose, it is essential for the PRO membranes to have characteristics such as: (i) a thin selective layer allowing a high water flux with a reasonably low reverse salt flux; (ii) a robust support layer to sustain high pressure operations with small transport resistance; and (iii) low affinity to foulants to maintain high power generation in real environments.

The present invention provides an improved thin film composite hollow fibre membrane which may be used in various applications including PRO. In particular, the thin film composite hollow fibre membrane of the present invention comprises a porous hollow fibre support layer and a selective layer formed on an outer circumferential surface of the hollow fibre support layer. The thin film composite hollow fibre membrane is robust and possess reasonable water permeability and an impressively low salt permeability.

According to a first aspect, there is provided a thin film composite hollow fibre membrane comprising: a porous hollow fibre support layer formed of polyethersulfone, polysulfone, polyphenylsulfone, polyacrylonitrile, polyimide, polyether imide, polyamide- imde, polyvinylidene fluoride, cellulose triacetate, polyetherketone, polyetheretherketone or a combination thereof, the hollow fibre support layer having a thickness of 100-350 μιτι; and

a selective layer on an outer circumferential surface of the hollow fibre support layer, the selective layer formed of a cross-linked polyamide and having a thickness of 100-500 nm,

wherein the thin film composite hollow fibre membrane has a salt permeability of 0.028-

0.042 L m ^"2 h ^'1 and a collapse pressure of at least 20 bar. The collapse pressure may be defined as the highest pressure sustained before structural failure. According to a particular aspect, the collapse pressure of the porous hollow fibre support layer may be at least 15 bar, more preferably at least 30 bar. According to another particular aspect, the collapse pressure of the thin film composite hollow fibre membrane may be at least 10 bar, more preferably at least 20 bar.

The thin film composite hollow fibre membrane may have suitable characteristics such as a high water permeability rate, low salt permeability rate and high salt rejection rate. The calculations of the water permeability rate, salt permeability rate and salt rejection rate will be described in detail with specific reference to a particular thin film composite hollow fibre membrane. However, it would be understood by a person skilled in the art that the calculations may apply to other thin film composite hollow fibre membranes prepared according to the method of the present invention and not restricted to the specific support layer and selective layer described below.

According to a particular embodiment, the porous hollow fibre support layer may be formed from polyethersulfone (PES).

According to a second aspect, there is provided a method of preparing a thin film composite hollow fibre membrane of the first aspect. The method adopts a simplified vacuum-assisted interfacial polymerization method to prepare the thin film composite hollow fibre membrane. In particular, the method comprises: preparing a module comprising at least one porous hollow fibre support layer potted in the module; and

- forming a selective layer on an outer circumferential surface of the hollow fibre support layer through interfacial polymerization, wherein the forming comprises contacting the surface of the porous hollow fibre support layer with a first solution comprising a polyamine, removing excess of the first solution by applying vacuum and subsequently contacting the surface of the porous hollow fibre support layer with a second solution comprising a polyfunctional acyl halide.

The module may comprise a housing accommodating the at least one porous hollow fibre support layer. In particular, both ends of the porous hollow fibre support layer may be potted in the module. The module may comprise a shell side and a lumen side. The first solution may be any solution comprising a suitable polyamine. According to a particular aspect, the first solution may comprise a polyamine selected from the group consisting of but not limited to: m-phenylenediamine (MPD), p-phenylenediamine, p- xylylenediamine, cyclohexanediamine, piperazine, branched or dendrimeric polyethylenimine, and a combination thereof.

The first solution may further comprise a surfactant. Any suitable surfactant may be added to the first solution. For example, the surfactant may be selected from, but not limited to, sodium dodecyl sulphate (SDS), trimethylamine (TEA), camphorsulfonic acid (CSA), and a combination thereof.

According to a particular embodiment, the first solution comprises MPD and SDS. For example, the first solution may comprise 1-4 wt% MPD and 1-3 wt% SDS. In particular, the first solution comprises a 2 wt% MPD aqueous solution and 2 wt% SDS solution.

The contacting with the first solution comprises introducing the first solution to the outer surface of the porous hollow fibre support layer for a p re-determined period of time.

The removing excess of the first solution is then carried out by applying vacuum at a suitable trans-membrane pressure. For example, the trans-membrane pressure applied may be 150-1000 mbar. In particular, the trans-membrane pressure applied may be 200-900 mbar, 250-850 mbar, 300-800 mbar, 350-750 mbar, 400-700 mbar, 450-650 mbar, 500-600 mbar.. Even more in particular, the trans-membrane pressure applied may be about 800 mbar. The vacuum may be applied from the module lumen side.

The second solution may be any solution comprising a suitable polyfunctional acyl halide. According to a particular aspect, the second solution may comprise a polyfunctional acyl halide selected from the group consisting of but not limited to: trimesoyl chloride (TMC), isophthaloyl chloride, terephthaloyl chloride, 1 ,3,5- cyclohexane tricarbonyl chloride, 1 ,2,3,4-cyclohexane tetracarbonyl chloride, and a combination thereof.

The second solution may further comprise an organic solvent. Any suitable organic solvent may be added to the second solution. For example, the organic solvent may be selected from, but not limited to, hexane, heptane, and a combination thereof. According to a particular embodiment, the second solution comprises TMC and hexane. In particular, the second solution comprises a 0.05-0.3 wt% TMC in hexane. Even more in particular, the second solution comprises a 0.15 wt% TMC in hexane.

The contacting with the second solution comprises introducing the second solution to the outer surface of the porous hollow fibre support layer saturated with the first solution for a pre-determined period of time to form the selective layer on the porous hollow fibre support layer.

The method may further comprise draining off the second solution, stabilising the selective layer formed and/or rinsing the thin film composite hollow fibre membrane.

The method of the present invention may comprise first preparing the at least one porous hollow fibre support layer. The porous hollow fibre support layer may be prepared by: providing a dope solution comprising a polymer solution, a solvent/non- solvent mixture and water to an annulus of a spinneret, the polymer solution comprising: polyethersulfone, polysulfone, polyphenylsulfone, polyacrylonitrile, polyimide, polyether imide, polyamide-imde, polyvinylidene fluoride, cellulose triacetate, polyetherketone, polyetheretherketone or a combination thereof;

- providing a bore solution to an inner tube of the spinneret; and

- extruding the dope solution and bore solution through the spinneret into a coagulation bath, thereby obtaining a porous hollow fibre support layer.

According to a particular aspect, the porous hollow fibre support layer may be formed from polyethersulfone (PES).

In particular, the dope solution may comprise the polymer in N-methyl-2-pyrrolidone (NMP), polyethylene glycol (PEG) and water. NMP may act as a solvent for the polymer. PEG may be employed as a weak non-solvent additive to improve pore connectivity and to enhance pore formation. Water may be added in a relatively small amount to increase dope viscosity and lead the polymer solution close to a bimodal decomposition, resulting in sponge-like porous structure. Even more in particular, the dope solution comprises PES, NMP, PEG 400 and water. For example, the dope solution may comprise PES, NMP, PEG 400 and water in the following composition: 20-22/36.9-39.0/36.9-39.0/2.0-4.2 wt % based on the total weight of the dope solution. In particular, the dope solution comprises PES, NMP, PEG 400 and water in the following composition: 22/38/38/2 wt% based on the total weight of the dope solution.

The bore solution may be any suitable bore solution for the purposes of the present invention. For example, the bore solution may be, but not limited to, tap water, deionised (Dl) water, NMP, or a combination thereof. In particular, the bore solution is Dl water.

The coagulation bath may comprise any suitable coagulant. For example, the coagulation bath may comprise water, NMP or a combination thereof. In particular, the coagulation bath comprises water.

The porous hollow fibre support layer formed from the method may comprise an asymmetric sandwich structure with double dense skins and a porous substructure in the middle. The dense skins may be formed as a result of water used as a both the bore fluid and the external coagulant. The porous substructure may be formed as a result of the fast phase separation process. The porous substructure minimises water transport resistance and is therefore advantageous in an osmosis driven process.

The method described above enables robust hollow fibre support layers of various dimensions and morphologies to be formed with dense and smooth outer surfaces using a single-layer spinneret. Further, almost defect-free polyamide selective layers were formed on the stop surface of the support layers with the aid of a vacuum pressure, preferably of about 800 mbar, to effectively remove excess MPD residuals during interfacial polymerization.

The properties of the thin film composite hollow fibre membranes of the present invention make them suitable for use in various applications such as PRO. The thin film composite hollow fibre membrane of the present invention, which are outer-selective hollow fibre membranes, are superior to PRO membranes of the prior art. For example, compared to inner-selective hollow fibre membranes, the membrane of the present invention has more advantages. Generally, outer-selective hollow fibre membranes are more challenging to make compared to inner-selective hollow fibre membranes. In particular, the morphology of the selective layer is highly dependent on the pore characteristics of the hollow fibre support layer. Support layers with small or large pores tend to result in a polyamide selective layer with (i) globules and few worm-like or (ii) flake-like morphology, respectively. The water permeability of the TFC hollow fibre membranes also rely on the morphology of the support layer and polyamide selective layer. The thin film composite hollow fibre membranes of the present invention display impressive low salt permeabilities of 0.022 - 0.042 L m ^"2 h ^"1 , thereby confirming that the method of the present invention can produce a nearly defect-free polyamide selective layer.

Further, the membrane of the present invention is able to achieve a high peak power density of about 10 W m ^"2 despite being an outer-selective hollow fibre membrane. This is comparable to the best known inner-selective hollow fibre membranes being used for PRO applications.

Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention.

Having now generally described the invention, the same will be more readily understood through reference to the following examples which are provided by way of illustration, and are not intended to be limiting.

EXAMPLE

Example 1

1.1 Materials

Radel ^® A polyethersulfone (PES, Solvay Advanced Polymer, L.L.C. GA), N-methyl-2- pyrrolidone (NMP, >99.5%, Merck), polyethylene glycol 400 (PEG, Mw = 400 g mol ^"1 , Acros Organics) and deionized water (Dl water, ELGA MicroMEG) were used, as the polymer, solvent, pore former and non-solvent, respectively, to prepare the spinning dope solution. A 50/50 wt% mixture of glycerol (Industrial grade, Aik Moh Paints & Chemicals Pte. Ltd., Singapore) and Dl water was prepared to post-treat the as-spun hollow fibre supports before drying for storage and module fabrication. Polyethylene glycol and polyethylene oxide at various molecular weights (PEG, Mw = 2,000 g mol ^"1 , 4,000 g mol ^"1 , 6,000 g mol ^"1 , 10,000 g mol ^"1 , 12,000 g mol ^"1 , 20,000 g mol ^"1 , 35,000 g mol ^"1 and PEO, Mw = 100,000 g mol ^"1 , 200,000 g mol ^"1 , 300,000 g mol ^"1 respectively, Sigma-Aldrich) were employed to characterize the molecular weight cut-off (MWCO), mean pore size and pore size distribution of the hollow fibre supports. Sodium dodecyl sulphate (SDS, >97%, Fluka), m-phenylenediamine (MPD, >98%, T.C.I.), 1 ,3,5- benzenetricarbonyl trichloride (TMC, 98%, Sigma-Aldrich), and hexane (99.9%, Fisher Chemicals) were applied to perform interfacial polymerization. Sodium chloride (NaCI, 99.5%, Merck) was purchased for membrane transport characterizations and PRO performance tests. All chemicals were used as received.

1.2 Fabrication of PES hollow fibre support layer and post-treatments

The PES polymer was first dried in a vacuum oven at 105°C overnight to remove moisture and then added into a NMP/PEG mixture and stirred at 65°C overnight. After complete dissolution, the dope solution was cooled down to room temperature and a small amount of Dl water was slowly added. Subsequently, the dope solution was continuously stirred for another few hours until a transparent solution was obtained. Afterwards, the prepared spinning dope solution was loaded into a 500 mL ISCO syringe pump and kept overnight for degassing before spinning.

A dry-jet wet spinning technique as described in G Han et al (Environ. Sci. Technol., 2013, 47:8070-8077) was performed to spin the PES hollow fibre support layer. By altering the spinning conditions, such as the bore fluid composition, bore fluid flowrate, air-gap distance and take-up speed, three different hollow fibre supports with various dimensions and morphologies were obtained as listed in Table 1.

Table 1 : Spinning parameters of PES hollow fibre support layer

The addition of hydrophilic non-solvent additive PEG into the spinning dopes aims to enhance pore formation and improve pore connectivity. A small amount of water is added to increase dope viscosity and move the dope closer to the binondal curve, resulting in interconnected open-cell pores. The as-spun hollow fibres were immersed in tap water for two days to remove the residual solvent and PEG 400. Part of the fibres were kept in a water bath in order to perform a series of characterizations as described below. The others were soaked in a glycerol/water mixture (50/50 wt %) for another two days, and then dried in ambient conditions. The aqueous glycerol treatment prevents pore collapse during drying.

To make hollow fibre modules, four to six pieces of support layers were carefully spaced and housed in a 1/2 inch perfluoroalkoxy (PFA) tubing with two Swagelok stainless male run tees connected to each side. Both ends were capped with cotton and potted with a slow curing epoxy resin (EP 231 , Kuo Sen, Taiwan). Prior to module fabrication, the two ends of each fibre were sealed with a fast curing epoxy resin (Araldite® Rapid, Huntsman, Belgium) and subsequently coated with the slow curing epoxy resin to leave a clean fibre length of 3-4 cm for PRO tests. These procedures ensure an accurate calculation of membrane area, and eliminate the formation of a defective polyamide selective layer on the outer surface of fibre ends contacting with cotton. 1.3 Fabrication of PES-TFC hollow fibre membranes

The TFC membranes were synthesized via interfacial polymerization on the outer surface of PES hollow fibre supports as illustrated in Figure 1. The steps were as follows: (1 ) the membrane module was positioned vertically and a 0.5 wt% PEG 400 aqueous solution was recirculated on the module shell side for 30 min at a flowrate of 5.35 ml_ min ^*1 ; (2) A 2 wt% MPD aqueous solution comprising a SDS concentration of 2 wt% was introduced to the outer surface of hollow fibres from bottom to top for 3 min at a flowrate of 5.35 ml_ min ^"1 ; (3) After that, the excess MPD residual was removed by vacuum at a transmembrane pressure of 800 mbar applied from the module lumen for 6 min; (4) A 0.15 wt% TMC in hexane at a flowrate of 6.11 ml_ min ^"1 was brought into contact with the outer surface of MPD saturated membranes for 2 min; (5) Then, a compressed air was purged on the module shell side for a few seconds. The resultant TFC membranes were left under ambient condition for 2.5 hr to stabilize the polyamide selective layer before being rinsed with Dl water several times and stored in Dl water for further tests.

1.4 Characterization - Morphology, topology, porosity, MWCO, pore size and pore size distribution of PES hollow fibre support layer

Membrane morphology was examined using field emission scanning electronic microscopy (FESEM, JEOL JSM-6700). Prior to FESEM characterizations, the PES supports and TFC hollow fibre membranes were freeze dried, fractured in liquid nitrogen and platinum coated with a Jeol JFC-1100E ion sputtering device.

Membrane surface topology was studied by atomic force microscopy (AFM, Bruker). Each membrane sample was scanned at a rate of 1.0 Hz in the tapping mode. Mean roughness (Ra) was determined using NanoScope Analysis (1.40, Bruker) from the average of 10 sample measurements.

For porosity measurements, the wet PES hollow fibre supports were cut into 5 cm shorts (/, cm) and dried by a freeze dryer (Christ Alpha 2-4 LD plus) overnight before being weighted. Every three shorts were weighted together (m, g) and at least 5 measurements were taken for each short bundle. The overall porosity (ε, %) was determined using the following equation: where OD and ID are the outer and inner diameters, respectively and p _p \s the polymer density of 1.37 g cm ^"3 .

The MWCO, pore size and pore size distribution of PES hollow fibre support layers were characterized by the solute transport method based on the procedures described in KY Wang et al (J. Membr. Sci., 2004, 240:67-79). Briefly, 200 ppm PEG or PEO aqueous solutions with various molecular weights were prepared to test the solute rejection under a trans-membrane pressure of 1 bar. The PEG or PEO concentrations of the collected permeates were analyzed by a total organic carbon analyzer (TOC ASI-5000A, Shimadzu, Japan). The effective solute rejection (R) was calculated from the measured feed concentration (C _f ) and permeate concentration (C _p ) as follows:

Since the solute diameter (d _s , nm) as a function of molecular weight M _w obeys the following equations: d _s = 3.35xl0 ^~2 x ° ^'557 (for PEG) (3)

2.09 xl0 ^~2 xM ⁸⁷ (for PEO) (4) a straight line can be employed to regress R vs. d _s on a log-normal probability graph. The MWCO was obtained at R=90% and the mean effective pore size (μ _ρ ) was acquired by assuming it to be the same as the solute geometric mean size (μ ₅ ) at R=50%. The geometric standard deviation (σ _ρ ) is defined as the ratio between d _s at R=84.13% and that at R=50%. The pore size distribution of a membrane may be expressed based on μ _ρ and σ _ρ as following: 5 Determination of pure water permeability and collapse pressure of PES hollow fibre support layer

The pure water permeability (PWP) of the PES hollow fibre support layers under different pressures were tested by using a lab-scale RO setup as described in KY Wang et al (J. Membr. Sci., 2004, 240:67-79). Dl water serving as the feed solution was pumped into the module shell side at a flowrate of 0.6 ml_ min ^~1 with a series of pressures in an ascending order. At each pressure stage, the permeate was subsequently collected from the lumen side for 10 min to calculate PWP using the equation below:

PWP = -Q— (6) where Q is the permeate volumetric flowrate, A _m is the effective filtration membrane area and ΔΡ is the trans-membrane pressure. The collapse pressure of a membrane is defined as the highest pressure which the hollow fibre support could sustain just before its structural failure.

1.6 Determination of water permeability and salt permeability of PES-TFC hollow fibre membranes

The water permeability (A) and salt rejection (R _s ) of the TFC hollow fibre membranes were measured using the same setup for pore size characterizations (X Li and TS Chung, Appl. Energy, 2014, 114:600-610). A was obtained by recirculating Dl water through the module shell side at a trans-membrane of 1 bar, while R _s was acquired by pressurizing a 1000 ppm NaCI feed solution at a trans-membrane pressure of 1 bar. The permeate conductivity was measured by a conductivity meter (Metrohm 856 conductivity module) and the salt rejection was calculated as

R _s = xl00% (7)

Cd _f Where Cd _p and Cd _f refer to the conductivity of the permeate and feed, respectively. The salt permeability {B) was calculated based on the equation derived from the solution- diffusion theory (K L Lee et al, J. Membr. Sci., 1981 , 8:141 -171 ): jl - R^Ap - Απ)

B = A (8) where Δπ is the osmotic pressure difference across the membrane.

1.7 PRO performance tests of PES-TFC hollow fibre membranes

The osmotic power generation tests were conducted by using a setup descried in G Han et al (Environ. Sci. Technol., 2013, 47:8070-8077) under the PRO mode where the selective layer faces the draw solution. A high-pressure pump (Minneapolis, MN) was employed to recirculate the 1 M NaCI draw solution through the module shell side at 0.6 L min ^"1 (0.056 m s ^"1 ) under various hydraulic pressures from zero to their corresponding highest stable pressures in series. Then the pressure gradually reduces to zero during the second run followed by increasing it up again for the third run. While a variable-speed peristaltic pump (Cole-Palmer, Vernon Hills, IL) was used to recirculate the feed Dl water through the lumen side at 0.01 to 0.05 L min ^"1 (0.368 to 0.910 m s ^"1 ) to maintain its local pressure build-up in the lumen less than 1 .00 psi. In other words, the average in pressure gauge readings located at the inlet and outlet of the lumen is less than 1 .00 psi.

For each TFC hollow fibre membrane, at least three modules were tested and the average value was reported. The water flux (J _w ) was calculated based on the weight loss of the feed solution within 15 min and the reverse salt flux (J _s ) was determined by monitoring the conductivity change. The power density (W) can be obtained using the following equation:

W = J _w xAP (9) where ΔΡ is the hydraulic pressure difference across the membrane. The structural parameter (S), describing the resistance of the support layer in osmosis processes, is defined by the tortuosity (τ , thickness (t) and porosity (ε) of the membrane support as:

S = - (10)

ε

It can also be obtained from the measurements of forward osmosis (FO) experiments by setting the selective layer of the membrane facing the feed Dl water (i.e., the FO mode):

where D is the solute diffusion coefficient, J™ is the measured water flux, and is the osmotic pressure of the bulk draw solution.

1.8 Characteristics of PES hollow fibre support layer

The morphologies of PES hollow fibre support layers spun from different spinning conditions (Table 1 ) are shown in Figure 2. Due to the fast demixing induced by the external coagulant, all hollow fibre supports exhibit dense and smooth outer surfaces with small pores suitable for the deposition of a thin and less defective polyamide selective layer. The inner surfaces of fibres II and III possess similar morphologies comparable to their outer surfaces because water is used as both the internal and external coagulant. The inner surface of fibre I has a few large pores of 200 - 300 μιτι, estimated from its FESEM picture, due to the delay demixing caused by the bore fluid of 80/20 NMP/water.

These three fibres have different cross-section morphologies. Fibre I has a single layer of finger-like macrovoids, while fibres II and III have two layers of finger-like structure across the hollow fibre walls. The latter is caused by the water penetration from both inner and outer surfaces. An equal depth distribution of each finger-like layer is observed for fibre II. As the air gap distance increases, it takes time for the outer coagulant to contact the outer surface and then intrude in, whereas the inner surface immediately contacts with the bore fluid once leaving the spinneret. Hence, the depth of the inner finger-like layer grows longer than that of the outer finger-like layer for fibre III. The addition of NMP into the bore fluid retards the phase inversion rate of the inner surface and results in only one finger-like layer for fibre I. In addition, the NMP mixture induces a close-cell structure in the inner edge of the cross-section, rather than the highly porous sponge-like structure of fibres II and III. This is due to the fact that the inner part was re-dissolved after spinning by the NMP mixture trapped inside the lumen side and re-precipitated. The sponge-like cross-section structure is critical to minimize the water transport resistance, being favourable in osmosis drawn process.

Figure 3 displays the AFM images and the mean roughness of the outer surfaces of these hollow fibre support layers. All support layers exhibit similar surface topologies with a mean roughness of 3 - 5 nm over a 5 * 5 prn ² area, which is in the suitable range for interfacial polymerization. A clear ascending trend in roughness is observed as the air gap distance increases from fibres I to III. A higher air gap distance brings a better contact of air moisture on the outer surface, it may develop larger gradients of NMP and polymer mass fraction near the dope/air interface. As a result, the NMP concentration increases while the polymer mass fraction decreases sharply. This localized polymer dilution may perturb the interface and result in a rougher outer surface.

Figure 4 shows the probability density function curves of pore size characteristics for the PES hollow fibre supports, and Table 2 summarizes the mean pore diameter, MWCO as well as geometric standard deviation.

Table 2: Characteristics of PES hollow fibre support layers

The air gap distance plays an important role on membrane nanostructure. Because of rapid phase inversion without much chain relaxation, fibre I spun from the shortest air gap distance of 0.5 cm possesses a small mean pore diameter of 3.93 nm and a sharp pore size distribution with a geometric standard deviation of 1.48. As the air gap distance increases, die swell relaxation occurs. The former induces a larger elongational stress along the nascent fibre, while the latter results in chain relaxation. The combined effects can either align the polymer chains with small interstitial space or relax as well as disentangle chains with large pores depending on which factor is dominant. As a result, fibre II has a larger mean pore diameter of 11.61 nm, a much broader pore size distribution and a geometric standard deviation of 1.86 compared to fibre I. The MWCO increases from 17.8 kDa of fibre I to 183.2 kDa of fibre II. With a further increase in air gap distance to 20 cm, fibre III shows an enhanced MWCO of 224.8 kDa and a geometric standard deviation of 2.03. Because of the elongational stretch by gravity, fibre III has smaller OD and ID, and a larger mean pore diameter than fibre II.

Figure 5 elucidates the PWP of each hollow fibre support as a function of applied hydraulic pressure. Generally, all PWPs decline near-linearly followed by accelerated decays because of membrane compaction under pressure. A further increase in pressure may result in a complete change of membrane physical properties. The collapse pressure of each hollow fibre support is defined as the highest pressure which the hollow fibre support could tolerate without structural failure as indicated in Figure 5. It is noticed that fibres II and III have similar initial PWPs around 130 L m ^"2 h ^"1 bar ^"1 at 3 bar, which is much higher than that of fibre I of around 60 L m ^"2 h ^"1 bar ^"1 . This is due to the effects of the inner close-cell structure as well as smaller and narrower pore sizes of fibre I. When increasing the hydraulic pressure, fibre III shows a rapid PWP decline and has a relatively low collapse pressure of 20 bar, while fibres I and II exhibit much slower PWP declines and have higher collapse pressures of 26 and 30 bar, respectively. Clearly, hollow fibres with a small OD (e.g. fibre I) or thicker wall (e.g. fibre II) are beneficial for higher mechanical tolerance.

1.9 Characteristics and PRO performance of PES-TFC hollow fibre membranes

Figure 6 discloses the polyamide selective layer was successfully formed on the outer surface of PES hollow fibre supports via interfacial polymerization. Three types of morphologies could be clearly distinguished. TFC-I has a smooth surface covered by small globules and few worm-like domains with a typical "ridge-and-valley" polyamide morphology. In contrast, both TFC-II and III have much rougher surfaces. The ridge curls in TFC-II have been significantly enlarged and have a "flake-like" shape. The number of ridge curls in TFC-III is reduced. A relatively smooth dense layer with some protuberances can be found beneath the ridge curls in the outer cross-section of TFC- III. In other words, the morphology of TFC-III can be treated as a combination of those of TFC-l and II.

The polyamide morphology is significantly influenced by the support layer's pore size. Since fibre I has the smallest surface pore of 3.93 nm with the sharpest pore size distribution, it tends to firmly hold the absorbed MPD solution inside the pores. This retards the diffusion of MPD towards the TMC solution for interfacial polymerization. As a consequence, a smooth polyamide layer of around 100 nm thick with small globules and few worm-like domains is formed. On the contrary, large pores with a broader pore size distribution such as fibres II and III promote the rapid migration of MPD molecules vigorously, increasing the flow turbulence and enlarging the reaction contact area. Thus, a rough polyamide layer consisting of many "flake-like" ridge curls with a larger thickness of 300 - 500 nm is formed. Since fibre III has a higher percentage of pore sizes smaller than 6 nm compared to fibre II as illustrated in Figure 4, some regions of fibre III form small globules and worm-like domains, while other regions grow the "flakelike" ridge curls.

Table 3 summarizes the transport properties of the three TFC hollow fibre membranes.

^* : 1000 ppm NaCI as the feed solution in RO tests under an applied pressure of 1 bar.

: Structural parameter was calculated based on FO mode performance of FO experiment using 1 NaCI and Dl water as feeds.

Table 3: Transport properties of PES-TFC hollow fibre membranes

All TFC membranes show good salt rejections of about 80 - 90 %, with impressively low salt permeabilities of 0.022 - 0.042 L m ^"2 h ^"1 . These values are found to be about one order of magnitude lower than those of other reported PRO membranes shown in Table 4. However, their water permeabilities vary from 0.609 L m ^"2 h ^"1 bar ^"1 of TFC-I to 1.420 L m ^"2 h ^"1 bar ^'1 of TFC-II. This might be explained by the larger effective membrane area provided by the "flake-like" ridge curls in TFC-II. The cavities inside the ridge curls also facilitate water transport across the polyamide layer. As the morphology of TFC-III is between those of TFC-I and II, it has a moderate water permeability of 1.122 L m ^"2 h ^"1 bar ^"1 . Comparing to TFC-II and TFC-III, TFC-I possesses the largest structural parameter of 2819 pm, which might be caused by its close-cell structure near the inner surface. As a result, the high S value indicates this membrane is not suitable for osmosis driven processes.

: Estimated from the figure plotted

b: Slope of the water flux decline obtained as the slope of the linear regression line of the data points plotted in a PRO performance figure (water flux vs hydraulic pressure difference). The first available PRO performance data point (normally at 0 bar) was chosen as the starting point and the data point where the maximum power density occurs was chosen as the ending point of the linear regression line. c: Peak J _s /J _w corresponds to the ratio between reverse salt flux and water flux at the point where the maximum power density occurs.

Table 4: Comparisons of properties of PRO membrane in prior art

The references referred to in Table 4 are as follows:

[I] S.P. Sun, T.S. Chung, Environ. Sci. Technol. 47 (2013) 13167-13174; [2] P.G. Ingole et al, Desalination 345 (2014) 136-145;

[3] F.J. Fu et al, J. Membr. Sci. 469 (2014) 488-498;

[4] S. Chou et al, J. Membr. Sci. 448 (2013) 44-54;

[5] S. Zhang et al, Chem. Eng. J. 241 (2014) 457-465;

[6] G. Han et al, Environ. Sci. Technol. 47 (2013) 8070-8077;

[7] X. Li and T.S. Chung, Appl. Energy 1 14 (2014) 600-610;

[8] Y. Cui et al, Chem. Eng. J. 242 (2014) 195-203;

[9] X. Song et al, Energy Environ. Sci. 6 (2013) 1199-1210;

[10] Y. Li, J. Membr. Sci. 488 (2015) 143-153; and

[I I] N.N. Bui and J.R. McCutcheon, Environ. Sci. Technol. 48 (2014) 4129-4136.

The key performance of the TFC hollow fibre membranes in terms of water flux, reverse salt flux and power density as a function of hydraulic pressure difference in PRO tests is displayed in Figure 7. Generally, the water flux declines with increasing AP due to the reduction in effective driving force, while the reverse salt flux rises. TFC-I possesses a low initial water flux of 13.30 LMH because of its small A and big S values. A sudden water flux drop and reverse salt flux upturn are observed when AP increases to 5 bar. The performance would further be deteriorated during the second and third runs as the water flux even gets lower, showing no potential for PRO applications.

For TFC-II and III, the repeated data from the second and third runs after the first run confirm the membrane stability. The water flux of TFC-II at AP = 0 is slightly lower than that of TFC-III due to its larger structural parameter (Table 3). If the slope of water flux decline is measured from AP = 0 to where the maximum power density occurs, both TFC-II and III exhibit remarkably small slopes of the water flux decline at 0.22 LMH bar ^"1 and 0.33 LMH bar ^"1 , respectively, compared to other PRO membranes as listed in Table 4. This is due to the fact that that they have superior low J</J _w ratios even at very high hydraulic pressure differences (i.e., 0.44 g L ^"1 of TFC-II at 20 bar and 0.40 g L ^"1 of TFC-III at 15 bar). As a consequence, the effective osmotic driving force across the membrane is well maintained at elevated PRO pressures, leading TFC-II and III with maximum power densities of 7.81 W m ^"2 at 20 bar and 7.57 W m ^"2 at 15 bar, respectively. The extremely low J/J _w ratio and high performance stability make the TFC-II and III membranes suitable for PRO applications in comparison to other PRO membranes.

Example 2

2.1 Fabrication of PES hollow fibre support layer

A further PES hollow fibre support layer was formed using the method described in 1.2 of Example 1 above. The spinning parameters utilised are as shown in Table 5. In particular, the polymer solution was changed slightly compared to Hollow Fibre ID II of Example 1.

Table 5: Spinning parameters of PES hollow fibre support layer

2.2 Fabrication of PES hollow fibre support layer

To make hollow fibre modules, four to six pieces of the formed fibre support layers were carefully spaced and housed in a 1/2 inch perfluoroalkoxy (PFA) tubing with two Swagelok stainless male run tees connected to each side. Both ends were capped with cotton and potted with a slow curing epoxy resin (EP 231 , Kuo Sen, Taiwan). Prior to module fabrication, the two ends of each fibre were sealed with a fast curing epoxy resin (Araldite® Rapid, Huntsman, Belgium) and subsequently coated with the slow curing epoxy resin to leave a clean fibre length of 3-4 cm for PRO tests. These procedures ensure an accurate calculation of membrane area, and eliminate the formation of a defective polyamide selective layer on the outer surface of fibre ends contacting with cotton.

The TFC membranes were synthesized via interfacial polymerization on the outer surface of the PES hollow fibre supports as illustrated in Figure 8. The steps are similar to that described in relation to Figure 1 at 1.3 of Example 1 , except that a final step of heat treatment at about 65°C for about 10 minutes is applied to the formed TFC membranes. The heat treatment effectively sealed defects, if any, in the polyamide selective layer and enhanced the repeatability and reproducibility of the formation of the TFC membranes.

2.3 Characterization - Morphology, porosity, MWCO, pore size and pore size distribution of PES hollow fibre support layer, as well as determination of pure water permeability (PWP) and collapse pressure of PES hollow fibre support layer

The methods described at 1.4 of Example 1 were applied to characterize the PES hollow fibre support layer formed in 2.1 , while the method of determining the PWP was using the method described in 1.5 of Example 1. Results similar to those obtained for hollow fibre ID II of Example 1 were obtained. For example, Figure 9 shows the morphologies of the different regions of the hollow fibre support layer.

Figure 10 shows the probability density function curves of pore size characteristics for the PES hollow fibre support formed in 2.1 , while Table 6 summarises the mean pore diameter, MWCO as well as geometric standard deviation.

Table 6: Characteristics of PES hollow fibre support layer

Figure 11 shows the PWP of the hollow fibre support as a function of applied hydraulic pressure. 2.4 Determination of water permeability and salt permeability of PES-TFC hollow fibre membranes and PRO performance tests of the membranes

The methods described at 1.6 and 1.7 of Example 1 were applied to determine the various characteristics of the PES-TFC hollow fibre membranes formed in Example 2. Figure 12 discloses the polyamide selective layer was successfully formed on the outer surface of the PES hollow fibre support layer via interfacial polymerization. The results are comparable to those obtained for TFC-II of Example 1. Table 7 summarises the transport properties of the PES-TFC hollow fibre membranes formed.

*: 1000 ppm NaCI as the feed solution in RO tests under an applied pressure of 1 bar.

*: Structural parameter was calculated based on FO mode performance of FO experiment using 1 M NaCI and Dl water as feeds.

Table 7: Transport properties of PES-TFC hollow fibre membrane

Table 8 provides the other properties of the PES-TFC hollow fibre membrane which can be seen to be comparable or superior to the properties of PRO membranes in the prior art as shown in Table 4.

: Slope of the water flux decline obtained as the slope of the linear regression line of the

data points plotted in a PRO performance figure (water flux vs hydraulic pressure

difference). The first available PRO performance data point (normally at 0 bar) was

chosen as the starting point and the data point where the maximum power density occurs

was chosen as the ending point of the linear regression line.

c: Peak J _s /J _w corresponds to the ratio between reverse salt flux and water flux at the point where the maximum power density occurs.

Table 8: Comparisons of properties of PRO membrane in prior art The key performance of the TFC hollow fibre membrane in terms of water flux, reverse salt flux and power density as a function of hydraulic pressure difference in PRO tests is displayed in Figure 13. Compared to TFC-II, the peak power density of the PES-TFC hollow fibre membrane formed in Example 2 has increased from 7.81 W m ^"2 at 20 bar to 10.05 W m ^"2 at 22 bar, which is the highest power density to be recorded for outer- selective TFC PRO hollow fibre membranes.