PRODUCING THEREOF

Related Application

[001] The present invention claims priority to Singapore patent application no. 10202251582W filed on 1 November 2022, the disclosure of which is incorporated in its entirety.

Field of Invention

[002] The present invention relates to a thin-film composite (TFC) membrane and a method for producing thereof. In particular, the invention relates to a thin-film composite (TFC) membrane modified with in situ grown layered double hydroxide and a method for producing thereof.

Background

[003] The accessibility of clean water for uses in human civilization is dwindling with the increase in population and expansion in urbanization and industrialization. As estimated by the United Nations, nearly one-fifth of the world populations live in areas of physical water scarcity. Though the two-third of the Earth is made up of water, only around 2.5% thereof is fresh water. Hence to meet the increasing demand for fresh water, sea and brackish water desalination and wastewater reclamation using membranes have been recognized as the promising technology due to the technological cleanliness involved in the process and less energy intrusive nature. Reverse osmosis (RO), as one of membrane technologies, has received unprecedented attentions owing to its simplicity and efficiency, while the thin-film composite (TFC) configuration is dominated in RO membranes and desalination market since its emergence in 1981.

[004] Membrane performance is the key to the successful operation of reverse osmosis plant, high flux and high salt rejection membranes are desirable for cost effective operation of the system. TFC membranes can be facilely fabricated by interfacial polymerizing amine monomers with acyl monomers to form a polyamide matrix on a porous substrate. Although TFC membranes are well featured at high water permeability, salt rejection, and sustainability towards acid/alkaline and pressure, continuous efforts have been given to target better separation performance at a lower energy cost. Fully aromatic polyamide thin film composite reverse osmosis membranes that are capable of delivering high flux and salt rejection are available in the market from different manufacturers but, they are only few in numbers.

[005] Technologies related to TFC membrane preparation and performance have been reported and published in several journals and patents. The development of nanomaterials paves new ways to design novel thin-film nanocomposite (TFN) membranes by means of incorporating nanomaterials into the polyamide matrix. Such nanomaterials can not only regulate the physiochemical properties of the TFN membranes but also endow the membranes with additional water paths through interior or exterior pore architecture. Therefore, the water permeability is enhanced without compromising the selectivity. So far, many nanomaterials have shown their potentials as nanofillers to improve the performance of TFN membranes, including graphene oxide nanosheets/nanodots, nanoclays, silica nanoparticles, and metal-organic frameworks (MOFs).

[006] To introduce the nanomaterials into TFN membranes, different techniques have been implemented. Among them, pre-mixing the nanoparticles with monomer solutions prior to interfacial polymerization. For example, Lin and co-workers dispersed HKUST-1 in the m- phenylenediamine (MPD) aqueous solution to enhance the water permeability of the TFN membranes [Lin et. al.; Thin film nanocomposite hollow fiber membranes incorporated with surface functionalized HKUST-1 for highly-efficient reverses osmosis desalination process; July 2019; Journal of Membrane Science; 589:117249]. Separately, Duan and coworkers pre-mixed ZIF-8 nanoparticles into the trimesoyl chloride (TMC) hexane solution to prepare the TFN membranes [Duan et. al. ; High-performance polyamide thin-film- nanocomposite reverse osmosis membranes containing hydrophobic zeolitic imidazolate framework-8; Journal of Membrane Science; 15 February 2015; 476:303-310]. Further, spray-coating was invented as an alternative approach, where nanoparticles were sprayed onto the substrate to form an interlayer of nanomaterials between the substrate and polyamide layer. Such technique is exemplified in Zhao’s work whereby TFN membranes were fabricated by spray-coating with UiO-66-NH2, which could remarkably improve the water permeability by 50% without sacrificing the selectivity [Zhao et. al. ; Thin-Film Nanocomposite Membranes Incorporated with UiO-66-NH2 Nanoparticles for Brackish Water and Seawater Desalination; Journal of Membrane Science; March 2020;

604(6191):! 18039].

[007] United States of America Patent No. US8029857B2 discloses a process for producing micro- and nano-composite support structure (porous membrane) for reverse osmosis membrane. Nanoparticles are added to the support membrane during casting. The resultant reverse osmosis membrane obtained after coating the micro-nano composite support membrane produced a membrane with more resistant to compaction when compared to the one having no such particle. Further, United States of America Patent No. US7815987B2 also discloses a composite membrane comprising a microporous support, a thin film polyamide layer and a coating located upon a surface portion of the thin film polyamide layer, wherein the coating comprises a reaction product of a polyalkylene oxide compound and a polyacrylamide compound.

[008] The resultant RO membranes showed exceptional separation performance towards brackish water desalination. Although the aforementioned methods were capable of producing high-performance membranes, the utilized nanoparticles had to be synthesized prior to the fabrication of TFN membranes. Usually, the synthesis and preparation of nanoparticles or their solutions were tedious and involved in centrifugation, purification, drying and re-dispersion. All these steps were highly material- and time-consuming.

[009] In recent works, there have been attempts to coat nanomaterials onto the substrate membranes via in-situ growth prior to the initiation of interfacial polymerization. In particular, layered double hydroxides (LDHs) are selected as the primary nanomaterial of interest because of their facile synthesis, good stability, and attractively low cost, which have recently been investigated as nanofillers in TFN membranes. Due to the unique interlayer spacing, LDHs have shown potentials to improve the separation performance by providing both molecular sieving ability and additional water channels. It can thus be seen that there exists a need for a modified thin-film composite (TFC) membrane configured with in situ grown layered double hydroxide and a simple and robust method for producing thereof that can overcome the disadvantages of the existing prior art. Summary

[0010] The following presents a simplified summary to provide a basic understanding of the present invention. This summary is not an extensive overview of the present invention, and is not intended to identify key features of the invention. Rather, it is to present some of the inventive concepts of this invention in a generalised form as a prelude to the detailed description that is to follow.

[0011] The present invention seeks to provide a method for producing a modified thin-film composite (TFC) membrane which utilizes alcohol and alkaline treatments to initiate an in situ growth of layered double hydroxide thereon.

[0012] The present invention also seeks to provide a modified thin-film composite (TFC) membrane comprising a porous substrate and a functional layer configured with an in situ grown layered double hydroxide.

[0013] The present invention also seeks to provide the use of the aforementioned modified thin-film composite (TFC) membrane in brackish water purification. Advantageously, the in situ grown layered double hydroxide improves the hydrophilicity of the TFC membrane surface and provides additional pathways or channels for water molecules transport therethrough, thus enhances the overall water permeability of the TFC membrane without imperilling the salt rejection.

[0014] In one embodiment, the present invention provides a method for producing a modified thin-film composite (TFC) membrane, the method comprising the steps of providing a porous substrate derived from polysulfone, forming a functional layer on a surface of the porous substrate by an interfacial polymerization reaction, swelling the functional layer by treating thereof with a divalent metal salt and a trivalent metal salt in an alcohol solution and treating the swollen function layer with an alkaline solution whereby metal ions derived from the divalent and trivalent metal salts react with hydroxide and carbonate ions of the alkaline solution thereby growing layered double hydroxide in situ.

[0015] In a preferred embodiment of the present invention, it is disclosed that the step of swelling the functional layer is performed by immersing the porous substrate having the functional layer in an alcohol solution comprising a divalent metal salt and a trivalent metal salt.

[0016] Preferably, the divalent metal salt in the alcohol solution is magnesium nitrate, calcium nitrate, cobalt nitrate, zinc nitrate or a combination of two or more thereof.

[0017] Preferably, the trivalent metal salt in the alcohol solution is aluminium nitrate, iron nitrate or a combination thereof.

[0018] Preferably, the divalent metal salt and the trivalent metal salt are present in a molar concentration ranging from substantially 1 mM to 30 mM.

[0019] Preferably, the divalent metal salt and the trivalent metal are present in a molar ratio ranging from substantially 1 : 1 to 3 : 1.

[0020] In the embodiment, it is disclosed that the step of treating the swollen functional layer is performed by immersing the swollen function layer in an alkaline solution comprising sodium hydroxide and sodium carbonate in a molar concentration ranging from substantially 50 mM to 200 mM.

[0021] Preferably, the sodium hydroxide and sodium carbonate in the alkaline solution are present in a molar ratio ranging from substantially 1 : 1 to 5 : 1.

[0022] In the embodiment, it is preferred that the method of the present invention further comprises a step of repeating the steps of swelling the functional layer and treating the swollen functional layer to form a ripened layered double hydroxide on the functional layer.

[0023] In the embodiment, it is preferred that the functional layer on the surface of the porous substrate is formed by reacting a polyfunctional amine with a polyfunctional acid chloride in hexane to form a polyamide layer.

[0024] Preferably, the polyfunctional amine is m-phenylene diamine present in an amount ranging from substantially 1 wt% - 3 wt%.

[0025] Preferably, the polyfunctional acid chloride is trimesoyl chloride present in an amount ranging from substantially 0. 1 wt% - 0.2 wt%. [0026] In another embodiment, the present invention provides a modified thin-film composite (TFC) membrane obtained from the method of the present invention. In a preferred embodiment of the present invention, the modified thin-film composite (TFC) membrane comprising a porous substrate and a functional layer configured with an in situ grown layered double hydroxide (LDH), wherein the LDH is formed by reaction of divalent and trivalent metal ions with hydroxide and carbonate ions in an aqueous condition.

[0027] In the embodiment, it is disclosed that the functional layer has a surface roughness ranging from substantially 100 nm to 200 nm.

[0028] In the embodiment, it is disclosed that the functional layer has a thickness ranging from substantially 390 nm to 500 nm.

[0029] Preferably, the divalent metal ions are magnesium ion (Mg ²⁺ ), calcium ion (Ca ²⁺ ), cobalt ion (Co ²⁺ ), zinc ion (Zn ²⁺ ) or a combination of two or more thereof.

[0030] Preferably, the trivalent metal ions are aluminium ion (Al ³⁺ ), ferric ion (Fe ³⁺ ) or a combination thereof.

[0031] In the embodiment, it is disclosed that the reaction is represented by Formula I: M ²⁺ i _x M ³⁺ _x (OH)2A ⁿ “ x/n .//7H2O; wherein M ²⁺ is a divalent cation comprising magnesium ion (Mg ²⁺ ), calcium ion (Ca ²⁺ ), cobalt ion (Co ²⁺ ), zinc ion (Zn ²⁺ ) or a combination of two or more thereof, M ³⁺ is a trivalent cation comprising aluminium ion (Al ³⁺ ), ferric ion (Fe ³⁺ ) or a combination thereof, A ⁿ " is an anion comprising hydroxide ion (OH ), carbonate (CO3 ² ) or a combination thereof, n is an integer of 1 or greater, x is 0.2 to 0.4, and m is 1 or greater.

[0032] Preferably, the porous substrate is derived from polysulfone.

[0033] Preferably, the functional layer is derived from polyamide.

[0034] In the embodiment, it is disclosed that the membrane of the present invention exhibits a salt rejection in the range of substantially 98% - 99%. [0035] In the embodiment, it is disclosed that the membrane of the present invention exhibits a water permeance in the range of substantially 3.0 liter nr ² h ¹ bar ¹ to 3.5 liter nr ² h ¹ bar ¹ .

[0036] In another embodiment, the present invention provides the use of the modified thin- film composite (TFC) membrane in brackish water purification.

Brief Description of the Drawings

[0037] This invention will be described by way of non-limiting embodiments of the present invention, with reference to the accompanying drawings, in which:

[0038] FIG. 1 illustrates a schematic illustration of the thin-film composite (TFC) membrane, according to an embodiment the present invention;

[0039] FIG. 2 illustrates a flowchart of the fabrication of the TFC membrane via in situ growth of layered double hydroxide, according to an embodiment of the present invention;

[0040] FIG. 3a illustrates SEM images of a surface of the control TFC membrane without any post-treatment (M-C), a TFC membrane after the alcohol treatment (M-E-l and M-E- 2), and after both alcohol and alkaline treatments (M-0-1 and M-0-2), according to an embodiment of the present invention;

[0041] FIG. 3b illustrates SEM images of a cross-section of the control TFC membrane without any post-treatment (M-C), a TFC membrane after the alcohol treatment (M-E-l and M-E-2), and after both alcohol and alkaline treatments (M-0-1 and M-0-2), according to an embodiment of the present invention;

[0042] FIG. 3c illustrates AFM images of the control TFC membrane without any posttreatment (M-C), a TFC membrane after the alcohol treatment (M-E-l and M-E-2), and after both alcohol and alkaline treatments (M-0-1 and M-0-2), according to an embodiment of the present invention;

[0043] FIG. 4 illustrates the high-resolution O Is spectra of the control TFC membrane, according to an embodiment of the present invention; [0044] FIG. 5 illustrates the water contact angles of the control TFC membrane, according to an embodiment of the present invention;

[0045] FIG. 6a illustrates an EDX mapping of the top surface of the TFC membrane synthesized with 10 mM of divalent metal salt for 1 cycle (M-10-1), according to an embodiment of the present invention;

[0046] FIG. 6b illustrates an EDX mapping of the top surface of the TFC membrane synthesized with 20 mM of divalent metal salt for 2 cycles (M-20-2), according to an embodiment of the present invention;

[0047] FIG. 6c illustrates an XRD spectra of the in situ grown layered double hydroxide (LDH), the control TFC membrane, the TFC membrane synthesized with 10 mM of divalent metal salt for 1 cycle (M-10-1) and the TFC membrane synthesized with 20 mM of divalent metal salt for 2 cycles (M-20-2), according to an embodiment of the present invention;

[0048] FIG. 7a illustrates SEM images of a surface of the modified TFC membrane, according to an embodiment of the present invention;

[0049] FIG. 7b illustrates SEM images of a cross-section of the modified TFC membrane, according to an embodiment of the present invention;

[0050] FIG. 7c illustrates AFM images of the modified TFC membrane, according to an embodiment of the present invention;

[0051] FIG. 8 illustrates the high-resolution O Is spectra of the modified TFC membrane, according to an embodiment of the present invention;

[0052] FIG. 9 illustrates the water contact angles of the modified TFC membrane, according to an embodiment of the present invention;

[0053] FIG. 10a illustrates the water permeance and salt rejection of the control TFC membrane, according to an embodiment of the present invention;

[0054] FIG. 10b illustrates the water permeance and salt rejection of the modified TFC membrane, according to an embodiment of the present invention; [0055] FIG. 10c illustrates the 8-hours brackish water reverse osmosis (BWRO) removal test results of the control TFC membrane (M-C) and the modified TFC membrane synthesized with 10 mM of divalent metal salt for 2 cycles (M-10-2), according to an embodiment of the present invention;

[0056] FIG. lOd illustrates the 8-hours boron removal test results of the control TFC membrane (M-C) and the modified TFC membrane synthesized with 10 mM of divalent metal salt for 2 cycles (M-10-2), according to an embodiment of the present invention.

Detailed Description

[0057] One or more specific and alternative embodiments of the present invention will now be described with reference to the attached drawings. It shall be apparent to one skilled in the art, however, that this invention may be practised without such specific details. Some of the details may not be described at length so as not to obscure the invention.

[0058] The present invention relates to a method for producing a modified thin-film composite (TFC) membrane whereby layered double hydroxides are grown in situ on a surface of a functional layer of a pristine TFC membrane. The method of the present invention features a technique of fine tuning the concentrations of layered double hydroxide precursors and optimizing the preparation process conditions for the in situ growth of layered double hydroxide. Advantageously, the in situ grown layered double hydroxide improves the hydrophilicity of the TFC membrane surface by providing additional pathways or channels for water molecules transport therethrough, thus enhances the overall water permeability of the TFC membrane without imperilling the salt rejection. In situ growth of layered double hydroxide in accordance to method of the present invention does not require pre-synthesis of nanoparticle precursors, thereby avoiding material- and time-consuming preparation steps such as centrifugation, purification, drying and re-dispersion.

[0059] In a preferred embodiment of the present invention, the modified TFC membrane can be produced by providing a porous substrate, forming a functional layer on a surface of the porous substrate, swelling the functional layer by treating thereof with a divalent metal salt and a trivalent metal salt in an alcohol solution and treating the swollen functional layer with an alkaline solution whereby metal ions derived from the divalent and trivalent metal salts react with hydroxide and carbonate ions of the alkaline solution thereby growing layered double hydroxide in situ.

[0060] The porous substrate is one as described. Conventionally, the porous substrate may be composed of ceramics or metals. In the context of the present invention, the porous substrate is preferably derived from polymers, more preferably polysulfone. Porous substrate derived from polysulfone provides substantial mechanical support and structural integrity to the TFC membrane. TFC membranes are typically composed of a functional layer that performs the separation function and a porous support layer (in the context of the present invention being the polysulfone substrate) that provides stability and prevents the TFC membrane from collapsing or deforming under pressure. Ideally, polysulfone is hydrophilic which is essential for improved TFC membrane wettability and flux in water purification applications. A hydrophilic porous substrate therefore allows water to pass through more easily, enhancing the overall performance of the TFC membrane.

[0061] Next, the method of the present invention is proceeded with a step of forming a functional layer on a surface of the porous substrate. The functional layer is the most critical part of the TFC membrane which performs the actual separation process. The functional layer is responsible for selectively allowing certain molecules or ions to pass through whilst rejecting others. In the context of the present invention, the functional layer acts as a semipermeable barrier that allows water molecules to pass through whilst rejecting large solutes. In this preferred embodiment of the present invention, the functional layer is formed by an interfacial polymerization reaction between two monomers, namely a polyfunctional amine and a polyfunctional acid chloride in hexane which are mutually immiscible, and the polymerization reaction takes place at the interface to form a polyamide layer. Preferably, the polyfunctional amine is m-phenylene diamine present in an amount ranging from substantially 1 wt% - 3 wt% whereas the polyfunctional acid chloride is trimesoyl chloride present in an amount ranging from substantially 0. 1 wt% - 0.2 wt%.

[0062] Subsequently, the formed functional layer on the porous substrate is treated by immersing thereof in an alcohol solution comprising a divalent metal salt and a trivalent metal salt. The divalent and trivalent metal salts are provided herein as the precursors for in situ growth of layered double hydroxide on the surface of the functional layer. Essentially, the divalent and trivalent metal salts in the alcohol solution generate metal ions which are crucial for nucleation of layered double hydroxide. The divalent metal salt used in the alcohol solution may be magnesium nitrate, calcium nitrate, cobalt nitrate, zinc nitrate or a combination of two or more thereof. The trivalent metal salt used in the alcohol solution used is aluminium nitrate, iron nitrate or a combination thereof. The alcohol solution contains the aforementioned dissolved divalent and trivalent metal salts, and the alcohol solution thereby contains divalent and trivalent metal ions.

[0063] The alcohol solution contains divalent metal salt and trivalent metal salt in a molar concentration preferably ranging from substantially 1 mM to 30 mM and in a molar ratio preferably ranging from substantially 1 : 1 to 3 : 1. These molar concentration and ratio ranges facilitate the nucleation and the in situ growth of layered double hydroxide in a balanced manner and can form a highly-oriented high-density layered double hydroxide- containing functional layer. At a low molar concentration of divalent metal salt and trivalent metal salt, the in situ growth of layered double hydroxide dominates over the nucleation, resulting in a decrease in the number of the layered double hydroxide particles and an increase in the size of the layered double hydroxide particles. At a high molar concentration, the nucleation dominates over the in situ growth of layered double hydroxide, resulting in an increase in the number of the layered double hydroxide particles and a decrease in the size of the layered double hydroxide particles.

[0064] In the present invention, the alcohol solution helps to swell the functional layer of polyamide and assists in anchoring the generated metal ions on the polyamide networks. Furthermore, the alcohol solution etches away the surficial unreacted monomers of the polyfunctional amine and polyfunctional acid chloride and polyamide oligomers from the interfacial polymerization reaction. For example, the alcohol solution may be an ethanol solution.

[0065] In the next stage, the swollen functional layer of polyamide on the porous substrate is further treated with an alkaline solution whereby the divalent and trivalent metal ions derived from the divalent and trivalent metal salts react with hydroxide and carbonate ions of the alkaline solution to thereby initiate the in situ growth of layered double hydroxide on the surface of the functional layer. Preferably, the alkaline treatment is performed by immersing the swollen functional layer in an alkaline solution comprising sodium hydroxide and sodium carbonate in a molar concentration ranging from substantially 50 mM to 200 mM. Without being bound by theory, the layered double hydroxide is formed through a process known as precipitation or ion exchange in which the metal ions generated from the divalent and trivalent metal salts react with the hydroxide ions through a precipitation process under alkaline conditions. The resultant metal hydroxide species precipitate out of the alkaline solution as solid particles that form layers of metal hydroxides, with each layer containing metal ions and hydroxide ions. During the precipitation process, anions such as nitrate from the divalent and trivalent metal salts and carbonate from sodium carbonate intercalate between the metal hydroxide layers to provide layered double hydroxide its characteristic layered structure. It is to be noted that the presence of sodium carbonate in the alkaline solution does not directly react with the divalent and trivalent metal ions to form the layered double hydroxide. As such, the sodium hydroxide and sodium carbonate used in the alkaline solution is present in a molar ratio ranging from substantilly 1 : 1 to 5 : 1.

[0066] The inventors found that the alcohol and alkaline treatments of the functional layer for in situ growth of layered double hydroxide thereon play a significant role in determining the surface morphologies of the functional layer, thereby imperatively affects the surface activity thereof, for example water permeance and salt rejection properties of the resultant TFC membrane. Taking this into consideration, it is therefore desirable to repeat the steps of alcohol and alkaline treatments on the thus functional layer with in situ grown layered double hydroxide to form a substantially ripened layered double hydroxide on the functional layer that reflects improved surface morphologies which substantially improves hydrophilicity of the TFC membrane, which will be further described in the Example section.

[0067] The present invention also discloses a thin-film composite (TFC) membrane produced from the method abovementioned. According to a preferred embodiment of the present invention, the TFC membrane (1) comprises a porous substrate (2) and a functional layer (3) configured with an in situ grown layered double hydroxide (4), wherein the layered double hydroxide (4) is formed by reaction of divalent and trivalent metal ions with hydroxide and carbonate ions in an aqueous condition. A schematic illustration of the TFC membrane (1) of the present invention is depicted in FIG. 1. As described previously, the functional layer (3) deposited on the porous substrate (2) is immersed in an alcohol solution comprising divalent and trivalent metal salts that generate divalent and trivalent metal ions therein and subsequently immersed in an alkaline solution comprising sodium hydroxide and sodium carbonate that generate hydroxide ions and carbonate ions to essentially initiate the in situ growth of layered double hydroxide (4) on the surface of the functional layer (3).

[0068] The layered double hydroxide (4) is represented by the general formula M ²⁺ i- _x M ³⁺ _x (OH)2A ⁿ “ x/n .//7H2O, wherein M ²⁺ represents a divalent metal ion, M ³⁺ represents a trivalent metal ion, A ⁿ " represents an n-valent anion, n represents an integer of 1 or greater, x represents a value of 0.2 to 0.4, and m represents a value of 1 or greater. In the general formula, M ²⁺ may represent any divalent metal ion, preferably magnesium ion (Mg ²⁺ ), calcium ion (Ca ²⁺ ), cobalt ion (Co ²⁺ ), zinc ion (Zn ²⁺ ) or a combination of two or more thereof. M ³⁺ may represent any trivalent metal ion, preferably aluminium ion (Al ³⁺ ), ferric ion (Fe ³⁺ ) or a combination thereof. A ⁿ " may represent an anion, preferably hydroxide ion (OH ), carbonate (CO3 ² ) or a combination thereof.

[0069] The porous substrate (2) in the TFC membrane (1) of the present invention is provided as a mechanical support for deposition of the functional layer (3) in which the layered double hydroxide (4) is grown in situ. The porous substrate (2) may be composed of any material and may have any porous structure. Preferably, the porous substrate (2) has a porous structure that permits water permeability to the porous substrate (2) because, by virtue of such a porous structure, water molecules can be transported therethrough whilst rejecting larger solutes. As described above, the porous substrate (2) may be composed of ceramics or metals. In the context of the present invention, the porous substrate (2) is preferably derived from polymers, more preferably polysulfone.

[0070] In the present invention, the functional layer (3) is preferably derived from polyamide. Polyamide has some level of inherent porosity and therefore is selectively water permeable whilst having low permeability to most solutes, including ions and larger molecules. As described above, the functional layer (3) is subjected to alcohol and alkaline treatments whereby the in situ growth of layered double hydroxide (4) is initiated on the functional layer (3) whilst the surface morphologies of the functional layer (3) are further characterized whereby the water permeability and salt rejection property are substantially improved. In a preferred embodiment of the present invention, the functional layer (3) has a surface roughness in the range of substantially 100 nm to 200 nm and a thickness in the range of substantially 390 nm to 500 nm. It is to be noted that the surface roughness and thickness determine effectiveness of water transport and rejection of unwanted particles of the TFC membrane (1). As such, collectively, the porous substrate (2) and functional layer (3) constituting the TFC membrane (1) of the present invention is suitable for various separation and purification processes. Particularly, the TFC membrane (1) of the present invention is suitable for use in brackish water purification. In an exemplary embodiment of the present invention, the TFC membrane (1) exhibits a salt rejection in the range of substantially 98% - 99% and a water permeance in the range of substantially 3.0 liter nr ² h ¹ bar ¹ to 3.5 liter m ^-2 h ¹ bar ¹ .

Examples

[0071] The following examples are given for illustrative purpose only and therefore these should not be construed to limit the scope of the present invention.

[0072] Example 1: Preparation of Pristine Thin-Film Composite Membranes and Layered Double Hydroxides-Modified Thin-Film Composite Membranes

[0073] The pristine thin-film composite membranes were firstly prepared following a conventional protocol. Typically, the polysulfone substrates were immersed into an aqueous solution containing about 1 wt% to about 3 wt%, preferably about 2 wt% m- phenylenediamine and about 0.15 wt% sodium dodecyl sulfate for about 2 minutes. Then, the substrate surfaces were dried by wiping out the excess solution using a rubber roller, following by introducing to a hexane solution with about 0.1 wt% to about 0.2 wt%, preferably about 0.15 wt% trimesoyl chloride for about 1 minute. The temperature of the hexane solution was kept at around 10 °C in an ice bath. The resultant thin-film composite membranes were further cured in an oven at about 80 °C for about 5 minutes, and then stored in ultrapure water overnight for further procedures.

[0074] Prior to the in- situ growth of layered double hydroxide onto the polyamide layer of the thin-film composite membranes, two stock solutions were prepared: 1) an ethanol solution containing salts with divalent and trivalent metal ions e.g. magnesium nitrate (Mg(NOa)2) and aluminium nitrate ('AI('NOi)i) with designed concentrations (for example, the molar ratio of Mg to Al was kept at about 1 to about 3, preferably about 2; and 2) an aqueous alkaline solution containing about 50 to 200 mM sodium hydroxide (NaOH) and sodium carbonate (Na2COa), wherein the ratio of NaOH to NaiCOs is kept at about 5:1. The pristine thin-film composite membranes were firstly soaked in an alcohol solution, for example, ethanol solution for at least about 5 min and then rinsed with deionized water to remove excess metal ions and loosely attached metal ions. The alcohol solution will also etch away the surficial unreacted monomers and polyamide oligomers from the interfacial polymerization and swell the polyamide and help metal ions anchor on polyamide networks. Other types of divalent metal ions that can be used are calcium, cobalt and zinc. An example of another type of trivalent metal ion that can be used is iron.

[0075] The thin-film composite membranes were subsequently immersed in the alkaline solution for at least about 5 min to initiate the in-situ growth of layered double hydroxides and then rinsed with water. The procedure described above was designated as 1 cycle. The resultant membranes were then stored in ultrapure water and denoted as M-x-y, where x refers to the molar concentration (mM) of Mg(NOa)2 while y means the times of cycle for in-situ growth of layered double hydroxides. For example, M-10-2 is the membrane fabricated with about 10 mM Mg(NOa)2 for 2 cycles. FIG. 2 illustrates the preparation process of the modified thin-film composite membranes via in-situ growth of layered double hydroxides. For comparison, four sets of control modified thin-film composite membranes were prepared following the similar procedures: the membranes treated with only pure ethanol (for example, 0 mM Mg(NOa)2) once and twice are designated as M-E-l and M-E- 2, respectively; the membranes treated with pure ethanol and then the alkaline solution in sequence for once and twice are designated as M-0-1 and M-0-2, respectively. The pristine thin-film composite membranes are referred to as M-C.

[0076] Example 2: Characterizations of Membranes

[0077] Scanning electron microscopy (SEM, JEOL JSM-7610FPlus) was utilized to analyze the morphologies of the top surfaces and cross-sections of membranes. Energy dispersive X-ray (EDX) was employed to reveal the uniformity of nanoparticles distributed on the surface of the membranes. The crystalline of the layered double hydroxide-modified thin- film composite membranes were examined by X-ray diffraction (XRD, Brucker D8) in the 20 region from 5 to 30°. A Bruker Dimension ICON atomic force microscope (AFM, tapping mode in air) was used to measure the surface roughness with a scanning area of 10 x 10 pm ² . The root mean square (RMS) was recorded at 5 different places for each sample. The elemental analyses of the selective layer were conducted by X-ray photoelectron spectroscopy (XPS). The water contact angle of each sample was measured using a goniometer (DataPhysics Instruments, OCA25) with the standard sessile drop mode.

[0078] Example 3: Evaluation of Membrane Separation Performance

[0079] The reverse osmosis (RO) tests were carried out on a cross-flow setup at a feed flow rate of 2 L min ¹ , and the effective filtration area was substantially 15 cm ² . A substantially 2000 ppm sodium chloride (NaCl) aqueous solution was the feed solution for the evaluation of water permeance (P _w , L nr ² h ¹ bar ¹ , LMH bar ¹ ) and salt rejection (R _s , %) in brackish water reverse osmosis (BWRO) tests. The boron rejection (R _b , %) was evaluated with a feed solution containing substantially 2000 ppm NaCl and 15 ppm boron (boron from boric acid, H3BO3) at pH ~8. All the tests were operated at substantially 20 + 1 bar, and the temperature of the feed was maintained at 25 ± 1 °C by a circulating chilling system.

[0080] P _w was calculated by Eq. (1):

[0081] P _w = AP/(A X At X AP) (1)

[0082] where AP (L) is the volume of the permeate water collected through a membrane with an effective filtration area of A (m ² ) for a filtration duration of At (h) at an operation pressure of AP (bar).

[0083] R _s was determined using Eq. (2):

[0084] R _s = (1 - Cp/Cf) X 100% (2)

[0085] where C _f and C _p (mol L ¹ ) are the salt concentrations of the feed and permeate solutions, respectively.

[0086] R _b was obtained by Eq. (3):

[0087] R _b = (1 - Cpb/Cfb) x 100% (3) [0088] where C _rb and C _pb (ppm) are the boron concentrations of the feed and permeate solutions, respectively, which were determined using the reported ISO method (ISO 9390:1990).

[0089] Example 4: Characterizations of the Control Thin-Film Composite Membranes

[0090] To fabricate the layered double hydroxide-modified thin-film composite membranes, the pristine thin-film composite membranes (M-C) were soaked into a metal salt ethanol solution and an alkaline aqueous solution in sequence. Prior to the investigation of the layered double hydroxide-modified thin-film composite membranes, it is necessary to study the influences of ethanol and alkaline solutions on morphologies and physiochemical properties of the resultant membranes. Therefore, four control thin-film composite membranes were prepared and studied; namely, M-E-l and M-E-2 treated with pure ethanol, and M-0-1 and M-0-2 treated with pure ethanol and then alkaline solutions. FIGS. 3a and 3b show their surface and cross-sectional morphologies. From FIG. 3a, all the control thin-film composite membranes possess a typical “ridge-and-valley” polyamide structure with no notable difference in the surface morphologies of polyamide after being treated with ethanol or alkaline. However, a reduction in thickness is observed from FIG. 3b. When being treated with ethanol once, the polyamide thickness is decreased from 385 nm to 343 nm for M-E-l. It is deduced that ethanol could induce mild swelling of polyamide and dissolve the amine- rich polyamide oligomers at the top layer, which is also evidenced by the increase in C-0 content (for example, C-0 is contributed from the carboxyl groups, C=O is contributed from the amide and carboxyl groups) as shown in Table 1 and the high-resolution O ls spectra by XPS in FIG. 4. For M-E-2, the C-0 content decreases, which could be owing to the etching of the carboxyl-rich polyamide at the top layer. Interestingly, the polyamide thickness is subtly increased possibly due to a further swelling of polyamide upon the second time of impregnation in ethanol. When being treated with ethanol and alkaline solutions in sequence, the decrease in polyamide thickness is also observed as expected. In addition, the decrease in C-0 content shown in Table 1 implies that alkali can also contribute to the dissolution of the carboxyl-rich polyamide at the top layer.

[0091] Table 1: Elemental compositions of the control TFC membranes by XPS analysis

* The C-0 content is roughly estimated by the ratio of the area under the C-0 fitting curve to the area under the overall fitting curve from O ls XPS spectra.

[0092] FIG. 3c shows the surface roughness of the control thin-film composite membranes. Comparing with the pristine M-C, the other four control thin-film composite membranes all exhibit slightly rougher surfaces, indicating the etching effect of ethanol and alkaline treatments on the surface layer. However, M-0-1 and M-0-2 tend to have a smoother surface than M-E-l and M-E-2. This is because alkali could etch away the asperities caused by the ethanol treatment and thus flatten the surface. FIG. 5 depicts the water contact angles of the control thin-film composite membranes. Clearly, after post-treatment in ethanol or ethanol/alkaline, the contact angles of the resultant thin-film composite membranes are all reduced, revealing the improved surface hydrophilicity. Moreover, although ethanol and alkaline have an obvious impact on the thickness and surface roughness of the polyamide layer, there is no significant distinction in surface hydrophilicity among the four thin-film composite membranes.

[0093] Example 5: Characterizations of the Layered Double Hydroxide-Modified Thin-Film Composite Membranes

[0094] To verify the successful in-situ growth of layered double hydroxide onto the polyamide layer of membranes, EDX and XRD were utilized. In FIGS. 6a and 6b, it can be visually witnessed from the EDX mapping that magnesium and aluminium ions are evenly distributed on the surfaces of M-10-1 and M-10-2, proving that the membranes are able to adsorb the metal ions through complexation with the carboxyl groups of polyamide networks. In addition, XRD analyses were carried out to examine the crystalline structures of the in-situ grown particles. As shown in FIG. 6c, M-10-1 has an additional peak at around 12° compared with the pristine M-C membrane due to the unique peak of (003) from layered double hydroxide. The peak gradually becomes prominent for M-10-2 due to the ripening of layered double hydroxide crystallinity. These results evidence the successful in-situ growth of layered double hydroxide on the thin-film composite membranes.

[0095] The surface and cross-sectional morphologies of the layered double hydroxide- modified thin-film composite membranes as functions of magnesium concentration and cycle time are displayed in FIGS. 7a and 7b. Although the polyamide layer is modified with layered double hydroxide via in-situ growth, the top surfaces have no much variation from the control membranes, in which the characteristic “ridge-and-valley” structure is well retained. However, for M-10-2, the leaf-like polyamide exhibits an enlarged size. It was postulated that metal ions could easily penetrate into the polyamide leaves and adsorb on them during their swelling in ethanol, thus the layered double hydroxide nanocrystals were ripened during the second cycle and acted as spacers to expand the polyamide. For M-20-2 being prepared with a higher concentration of metal ions, the polyamide expansion was not as severe as that of M-10-2. It was hypothesized that the saturated crystal nuclei formed during the first cycle might hinder further precursor ions from impregnating the polyamide and impede larger crystal formation.

[0096] The thickness of the selective layer for each membrane is revealed in FIG. 7b. Comparing with M-C, the membranes modified with layered double hydroxide possess a thicker selective layer, and a greater thickness is observed for the membranes doped with a higher concentration of metal ions. Especially for M-10-2, due to the severer expansion of the polyamide, the selective layer becomes thicker than the rest. However, as aforementioned, the treatments by pure ethanol and alkaline could partially affect the polyamide microstructure and yield a thinner polyamide layer. Therefore, it is essential to figure out why the thickness increases with the modification of layered double hydroxide. The elemental compositions of the as-prepared layered double hydroxide-modified thin-film composite membranes are investigated and summarized in Table 2. By increasing the dosage of metal salts, both the magnesium concentration and C-0 content (roughly estimated from FIG. 8) in the resultant membranes increase. It is speculated that the complexation formed between the metal ions and carboxyl groups of polyamide networks could prevent the carboxyl-rich polyamide from dissolution in ethanol and alkaline. Therefore, the increasing thickness may result from the synergetic effect of the polyamide protection from the metal ions and the polyamide expansion caused by the formation of layered double hydroxide. [0097] Table 2: Elemental compositions of the layered double hydroxide -modified thin-film composite membranes

[0098] FIG. 7c shows the topography of the layered double hydroxide-modified thin-film composite membranes. All the layered double hydroxide-modified thin-film composite membranes gain a rougher surface than the control thin-film composite membranes, which could be mainly attributed to the polyamide expansion as discussed above. To assess the surface hydrophilicity of the double hydroxide-modified thin-film composite membranes, the water contact angles were measured and depicted in FIG. 9. All the layered double hydroxide-modified thin-film composite membranes show higher hydrophilicity than the pristine M-C. Generally, there is only a slight variation in surface hydrophilicity for the membranes with one cycle of in-situ layered double hydroxide growth. For two cycles upon the ripening of layered double hydroxide crystals, the surface hydrophilicity is notably improved. Among them, M-10-2 is the most hydrophilic possibly due to the highest C-0 content and roughness.

[0099] Example 6: Separation Performance

[00100] The separation performance of the control thin-film composite membranes is firstly investigated with the results summarized in FIG. 10a. The pristine M-C exhibits a water permeance of 1.83 EMH bar ¹ with a salt rejection of about 98.84% when using a 2000 ppm NaCl solution as the feed. When being treated with pure ethanol, the water permeance is significantly improved, where both M-E-l and M-E-2 obtain a water permeance of about 2.40 LMH bar ¹ . In addition, a salt rejection of about 98.8% is still retained, indicating no noticeable trade-off between water permeance and salt rejection. This improvement might benefit from the reduced polyamide layer thickness and the enhanced surface hydrophilicity and roughness, owing to the swelling and etching of polyamide. Moreover, although polyamide is slightly etched by ethanol, no severe defects are induced in the polyamide networks. However, the water permeance is reduced when being treated with pure ethanol and alkaline in sequence. The alkaline treatment may partially erase the benefits from the ethanol treatment because of the flattened surface and reduced effective surface area for water transport, thus leading to the performance deterioration.

[00101] The impact of layered double hydroxide modification on the membrane separation performance is then studied and the results are summarized in FIG. 10b. Compared to all the control thin-film composite membranes, the layered double hydroxide-modified thin-film composite membranes fabricated under all conditions exhibit a higher water permeance. When allowing only one cycle of in-situ layered double hydroxide growth, the resultant layered double hydroxide-modified thin-film composite membranes show a similar water permeance of about 2.75 LMH bar ¹ regardless of the Mg concentration. When proceeding with two cycles of in-situ growth, the water permeance is further increased, and a remarkably enhanced water permeance of about 3.36 LMH bar ¹ is observed for M-10-2. This could be explained as follows: (1) with one cycle of layered double hydroxide deposition, the surface roughness of the membranes is significantly increased, thus providing more surface areas for water passage and leading to a higher water permeance; (2) with two cycles, not only the surface roughness increases, but also the surface hydrophilicity is greatly improved; (3) upon the ripening of the layered double hydroxide crystals, more “shortcuts” are created for water transport due to the unique interlayered structure of layered double hydroxide as shown in FIG. 11 ; (4) M-10-2 has the highest water permeance because of its larger surface roughness and higher hydrophilicity than the others. On the other hand, compared to the pristine M-C, the salt rejection of the layered double hydroxide-modified thin-film composite membranes is slightly lower but still remains above about 98.0%. Yet an obvious decline in salt rejection is noticed for M-20-2, which could result from the defects created during the polyamide dissolution and layered double hydroxide crystallization.

[00102] Since M-10-2 outperforms the rest layered double hydroxide-modified thin-film composite membranes, it is then selected for the about 8-h brackish water reverse osmosis (BWRO) tests to assess its stability along with time. According to FIG. 10c, the pristine M- C shows a stable water permeance of about 1.77 LMH bar ¹ along the 8-h process. For M- 10-2, the water permeance slightly drops but still remains above about 3.05 LMH bar ¹ at the end of the test. Meanwhile, its salt rejection increases from about 98.66% to about 98.85%, complying with the trade-off relationship between water permeance and salt rejection. This phenomenon results from the continuous compression of the selective layer under a high pressure. Clearly, the layered double hydroxide-modified thin-film composite membranes show their capability to operate for long-term use and are comparable to the state-of-the-art thin film composite membranes in desalination.

[00103] The boron removal tests were conducted with M-C and M-10-2 using a feed solution containing 2000 ppm NaCl and 15 ppm boron (pH~8). The boron rejections along with time are illustrated in FIG. lOd. M-10-2 is able to achieve a boron rejection of about 70% after about 8 h, while M-C shows a higher boron rejection of around 75%. This phenomenon may arise from the fact that the molecular size of boric acid is close to water and M-10-2 has a much higher water permeance than M-C (for example, 3.36 vs. 1.83 LMH bar ¹ ). Thus, the negligible size exclusion effect and high water permeance allow boron species to easily cross the membranes, leading to a reduced rejection.

[00104] While specific embodiments have been described and illustrated, it is understood that many changes, modifications, variations and combinations of variations disclosed in the text description and drawings thereof could be made to the present invention without departing from the scope of the present invention.