Technical Field

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

Background

Membrane -based separation such as nanofiltration (NF) and reverse osmosis (RO) has been widely recognised as an effective water treatment approach to produce clean and/or potable water from various water sources. In particular, clean water may be produced by filtering the contaminated water through a semi-permeable membrane under pressure. RO is preferred for brackish water and seawater desalination in view of its simplicity, good performance and low energy consumption.

The most common RO membrane used are thin film composite (TFC) membranes made from interfacial polymerization. TFC membranes are known to have superior permeation properties, monovalent salt rejection and excellent stability in alkaline and acidic environments. However, there still exist limitations and challenges in the performance of TFC membranes to meet the tightening requirements of water quality and energy efficiency since there is a trade-off relationship between water permeability and solute selectivity.

There is therefore still a need for an improved TFC hollow fibre membrane with higher pure water permeability (PWP) without compromising salt rejection, and an improved method of forming the TFC hollow fibre membrane which is low-cost, environmentally friendly and easily scalable.

Summary of the invention

The present invention seeks to address these problems, and/or to provide an improved thin film composite (TFC) membrane. In particular, the membrane is an improved TFC hollow fibre membrane.

According to a first aspect, the present invention provides a thin film composite (TFC) hollow fibre membrane comprising: a porous hollow fibre support layer; and a selective layer on a surface of the support layer, the selective layer formed of a cross-linked polyamide comprising amino-functionalised carbon quantum dots (N-CQDs).

According to a particular aspect, the TFC hollow fibre membrane may be in inner- selective TFC hollow fibre membrane and therefore, the selective layer may be on an inner circumferential surface of the hollow fibre support layer.

According to a particular aspect, the support layer may be formed of a polymer or a ceramic material. For example, the polymer may comprise, but is not limited to, polyethersulfone, polysulfone, polyphenylsulfone, polyacrylonitrile, polyimide, polyether imide, polyamide-imide, polyvinylidene fluoride, cellulose acetate, cellulose triacetate, polyetherketone, polyetheretherketone, or a combination thereof. In particular, the support layer may be formed of polyethersulfone (PES).

The TFC hollow fibre membrane may have suitable properties. For example, the TFC hollow fibre membrane may have a pure water permeability (PWP) rate of 2-10 L rm ² IT ¹ bar ¹ at 15 bar. The TFC hollow fibre membrane may have a salt rejection rate of > 97%.

The selective layer of the TFC hollow fibre membrane may further comprise chlorine bonded to the cross-linked polyamide. According to a particular aspect, the selective layer may comprise < 15 wt % chlorine. In particular, the selective layer may further comprise chlorine bonded to the cross-linked polyamide after the selective layer may be subjected to hypochlorite treatment.

According to a particular aspect, the hypochlorite treatment may comprise treating the selective layer with hypochlorite solution of a pre-determined concentration and for a pre-determined period of time. The pre-determined concentration may be < 15,000 ppm. The pre-determined period of time may be < 24 hours.

The membrane comprising chlorine bonded to the cross-linked polyamide may have a suitably higher pure water permeability (PWP) rate as compared to the membrane without the chlorine bonded to the cross-linked polyamide. According to a particular aspect, the TFC hollow fibre membrane comprising chlorine bonded to the cross-linked polyamide may have a PWP rate of 4-20 L nr ² IT ¹ bar ¹ at 15 bar. The TFC hollow fibre membrane comprising chlorine bonded to the cross-linked polyamide may have a salt rejection rate of > 97%.

The TFC hollow fibre membrane may be used for any suitable application. In particular, the TFC hollow fibre membrane may be for use in, but not limited to, pressure retarded osmosis (PRO), osmotic power generation, nanofiltration, or reverse osmosis desalination. According to a particular aspect, the TFC hollow fibre membrane comprising chlorine bonded to the cross-linked polyamide, may be for use in reverse osmosis desalination.

According to a second aspect, the present invention provides a method of forming a TFC hollow fibre membranes, the method comprising: providing a porous hollow fibre support layer; and forming a selective layer on a surface of the support layer through interfacial polymerization, wherein the forming comprises contacting the surface of the support layer with a first solution comprising a polyamine and aminofunctionalised carbon quantum dots (N-CQDs), removing excess of the first solution by applying air and subsequently contacting the surface of the support layer with a second solution comprising a polyfunctional acyl halide.

The membrane may be any suitable TFC hollow fibre membrane. For example, the TFC hollow fibre membrane may be as described in the first aspect.

According to a particular aspect, the porous hollow fibre support layer may be prepared by: providing a dope solution comprising a polymer solution or a ceramic solution, a solvent/non-solvent mixture and water to an annulus of a spinneret; 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.

The dope solution may comprise a suitable polymer solution. For example, the polymer comprised in the polymer solution may comprise, but is not limited to, polyethersulfone, polysulfone, polyphenylsulfone, polyacrylonitrile, polyimide, polyether imide, polyamideimide, polyvinylidene fluoride, cellulose acetate, cellulose triacetate, polyetherketone, polyetheretherketone or a combination thereof. In particular, the polymer solution may comprise polyethersulfone (PES).

According to a particular aspect, when the selective layer may be on an inner circumferential surface of the hollow fibre support layer, the method may further comprise: preparing a module comprising at least one porous hollow fibre support layer potted in the module; and forming the selective layer on an inner circumferential surface of the hollow fibre support layer through interfacial polymerization.

The first solution may comprise any suitable polyamine. For example, the polyamine may be, but is not limited to, m-phenylenediamine (MPD), p-phenylenediamine, p- xylylenediamine, cyclohexanediamine, piperazine, branched or dendrimeric polyethylenimine, or a combination thereof.

According to a particular aspect, the first solution may further comprise a surfactant. The surfactant may be any suitable surfactant. For example, the surfactant may be, but not limited to, sodium dodecyl sulphate (SDS), trimethylamine (TEA), camphorsulfonic acid (CSA), or a combination thereof.

The first solution may comprise a suitable amount of N-CQDs. According to a particular aspect, the first solution may comprise 0.1 -5.0 weight % N-CQDs.

The second solution may comprise any suitable polyfunctional acyl halide. For example, the polyfunctional acyl halide may be, but not limited to, trimesoyl chloride (TMC), isophthaloyl chloride, terephthaloyl chloride, 1 ,3,5-cyclohexane tricarbonyl chloride, 1 ,2,3,4-cyclohexane tetracarbonyl chloride, or a combination thereof.

According to a particular aspect, the second solution may further comprise an organic solvent. The organic solvent may be any suitable organic solvent. For example, the organic solvent may be, but not limited to, hexane, heptane, cyclohexane, isoparaffinic hydrocarbon, or a combination thereof. According to a particular aspect, the present invention may further comprise treating the TFC hollow fibre membrane with a hypochlorite solution of a pre-determined concentration and for a pre-determined period of time. The hypochlorite solution may be any suitable solution comprising hypochlorite. In particular, the hypochlorite solution may be sodium hypochlorite solution. For example, the pre-determined concentration may be < 15,000 ppm. For example, the pre-determined period of time may be < 24 hours.

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 FTIR spectra of the amino functionalized CQDs in comparison with the control;

Figure 2(a1) shows FESEM images of cross-section of unmodified TFC membrane; Figure 2(b1) shows FESEM images of surface of the polyamide layers of unmodified TFC membrane; Figure 2(a2) shows FESEM images of cross-section of TFC-(CQD- EA/CA-0)-1; Figure 2(b2) shows FESEM images of surface of the polyamide layers of TFC-(CQD-EA/CA-0)-1; Figure 2(a3) shows FESEM images of cross-section of TFC- (CQD-EA/CA-0.1)-1; Figure 2(b3) shows FESEM images of surface of the polyamide layers of TFC-(CQD-EA/CA-0.1)-1 ; Figure 2(a4) shows FESEM images of crosssection of TFC-(CQD-EA/CA-0.2)-1; and Figure 2(b4) shows FESEM images of surface of the polyamide layers of TFC-(CQD-EA/CA-0.2)-1 ;

Figure 3(a) shows pure water permeability (feed: DI water); and Figure 3(b) shows salt rejection (feed: 2000 ppm NaCI aqueous solution) of the unmodified TFC membrane, TFC-(CQD-EA/CA-0)-1, TFC-(CQD-EA/CA-0.1)-1 and TFC-(CQD-EA/CA-0.2)-1 (measured at 15 bar after the membranes had been stabilized at 20 bar for 1 hour);

Figure 4(a1) shows FESEM images of cross-section of TFC-(CQD-EA/CA-0.1)-1 without any treatment; Figure 4(b1) shows FESEM images of surface of the polyamide layer of TFC-(CQD-EA/CA-0.1)-1 without any treatment; Figure 4(a2) shows FESEM images of cross-section of TFC-(CQD-EA/CA-0.1)-1 treated with a sodium hypochlorite aqueous solution at 2000 ppm for 3 hours; Figure 4(b2) shows FESEM images of surface of the polyamide layer of TFC-(CQD-EA/CA-0.1)-1 treated with a sodium hypochlorite aqueous solution at 2000 ppm for 3 hours; Figure 4(a3) shows FESEM images of cross-section of TFC-(CQD-EA/CA-0.1)-1 treated with a sodium hypochlorite aqueous solution at 4000 ppm for 3 hours; Figure 4(b3) shows FESEM images of surface of the polyamide layer of TFC-(CQD-EA/CA-0.1)-1 treated with a sodium hypochlorite aqueous solution at 4000 ppm for 3 hours; Figure 4(a4) shows FESEM images of cross-section of TFC-(CQD-EA/CA-0.1)-1 treated with a sodium hypochlorite aqueous solution at 8000 ppm for 3 hours; Figure 4(b4) shows FESEM images of surface of the polyamide layer of TFC-(CQD-EA/CA-0.1)-1 treated with a sodium hypochlorite aqueous solution at 8000 ppm for 3 hours;

Figure 5(a) shows pure water permeability (feed: DI water); and Figure 5(b) shows salt rejection (feed: 2000 ppm NaCI aqueous solution) of TFC-(CQD-EA/CA-0.1)-1 without any treatment or treated with a sodium hypochlorite aqueous solution at 2000 ppm, 4000 ppm or 8000 ppm for 3 hours (measured at 15 bar after the membranes had been stabilized at 20 bar for 1 hour);

Figure 6 shows 8-hour separation performance test of TFC-(CQD-EA/CA-0.1)-1 after being post-treated with a 4000 ppm sodium hypochlorite aqueous solution for 3 hours (measured at 15 bar after the membranes had been stabilized at 20 bar for 1 hour); and

Figure 7(a) water permeability, and Figure 7(b) shows salt rejection in 3-month separation performance test of TFC-(CQD-EA/CA-0.1)-1 after being post-treated with a 4000 ppm sodium hypochlorite aqueous solution for 3 hours using real MBR permeate (measured at 15 bar after the membranes had been stabilized at 20 bar for 1 hour).

Detailed Description

As explained above, there is a need for an improved TFC hollow fibre membrane which has improved physiochemical properties and separation performance.

In general terms, the present invention provides a thin film composite (TFC) hollow fibre membrane incorporating carbon quantum dots, particularly amino-functionalised carbon quantum dots, which can significantly improve PWP rate while maintaining a high salt rejection rate at 15 bar as compared to conventional TFC hollow fibre membranes without amino-functionalised carbon quantum dots, thereby making the membrane useful without compromising on the water permeability and salt rejection. The membrane of the present invention may also be used in several applications including, but not limited to, pressure retarded osmosis, reverse osmosis, brackish water reverse osmosis desalination, nanofiltration, and forward osmosis.

The present invention also provides an improved method of forming the TFC hollow fibre membrane, which results in a TFC hollow fibre membrane having improved transport properties so that it would produce higher PWP rate. In particular, the aminofunctionalised carbon quantum dots (N-CQDs) are incorporated into the polyamide selective layer via the conventional interfacial polymerization reaction.

According to a first aspect, the present invention provides a thin film composite (TFC) hollow fibre membrane comprising: a porous hollow fibre support layer; and a selective layer on a surface of the support layer, the selective layer formed of a cross-linked polyamide comprising amino-functionalised carbon quantum dots (N-CQDs).

The TFC membrane may be any suitable hollow fibre membrane. For example, the TFC hollow fibre membrane may be, but not limited to, an inner selective TFC hollow fibre membrane or an outer selective TFC hollow fibre membrane.

According to a particular aspect, the TFC hollow fibre membrane may be an inner- selective TFC hollow fibre membrane. When the TFC hollow fibre membrane is an inner-selective TFC hollow fibre membrane, the selective layer may be on an inner circumferential surface of the hollow fibre support layer.

The support layer may be formed of any suitable material. For example, the support layer may be formed of a polymeric material or a ceramic material. According to a particular aspect, the support layer may be formed of a polymeric material. The polymeric material may comprise any suitable polymer. For example, the polymer may be, but not limited to, polyethersulfone, polysulfone, polyphenylsulfone, polyacrylonitrile, polyimide, polyether imide, polyamide-imide, polyvinylidene fluoride, cellulose acetate, cellulose triacetate, polyetherketone, polyetheretherketone, or a combination thereof. In particular, the support layer may be formed of polyethersulfone (PES).

The support layer may be of a suitable thickness. For example, the thickness of the support layer may be 100-600 .m. In particular, the thickness of the support layer may be 100-500 .m, 150-450 .m, 175-425 .m, 200-400 .m, 250-375 .m, 275-350 .m, 300-325 .m. In particular, the thickness may be 200-300 .m, preferably 150-300 .m.

The selective layer may be of a suitable thickness. The selective layer of the TFC hollow fibre membrane may be thinner as compared to a conventional TFC membrane without N-CQDs. For example, the thickness of the selective layer may be 10-800 nm, 50-500 nm, 100-480 nm, 120-450 nm, 150-400 nm, 175-375 nm, 200-350 nm, 250-325 nm, 275-300 nm. In particular, the thickness may be about 200-350 nm. Even more particular, the thickness may be about 200-500 nm, preferably 300-350 nm.

The TFC hollow fibre membrane according to the first aspect also has a suitably high pure 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 are described in detail with specific reference to a particular TFC hollow fibre membrane in the Example section. However, it would be understood by a person skilled in the art that the calculations may apply to other TFC hollow fibre membranes within the scope of the present invention, and not necessarily restricted to the specific support layer and selective layer described in the Example.

According to a particular aspect, the TFC hollow fibre membrane may have a high pure water permeability rate of 2.0-10.0 L rm ² IT ¹ bar ¹ at 15 bar. In particular, the pure water permeability rate may be 3.5-7.5 L rm ² h' ¹ bar ¹ , 3.7-7.3 L rm ² h' ¹ bar ¹ , 4.0-7.1 L rm ² IT ¹ bar ¹ , 4.2-7.0 L rm ² IT ¹ bar ¹ , 4.5-6.8 L rm ² IT ¹ bar ¹ , 4.7-6.5 L rm ² hr ¹ bar ¹ , 5.0-6.2 L rm ² IT ¹ bar ¹ , 5.2-6.0 L rm ² h' ¹ bar ¹ , 5.5-5.8 L rm ² IT ¹ bar ¹ at 15 bar. Even more in particular, the pure water permeability rate may be about 5.5 L rm ² h' ¹ bar ¹ at 15 bar.

The TFC hollow fibre membrane may have a favourably high salt rejection rate of > 97%. For example, the salt rejection rate of the TFC hollow fibre membrane may be 97- 99%. Even more in particular, the salt rejection rate may be > 98%, particularly 98- 99%. The selective layer of the TFC hollow fibre membrane may further comprise chlorine bonded to the cross-linked polyamide. According to a particular aspect, the selective layer may comprise < 15 wt % chlorine based on the total weight of the selective layer. For example, the selective layer may comprise < 10 wt % chlorine based on the total weight of the selective layer. In particular, the selective layer may comprise 0.1-15 wt %, 0.5-10 wt %, 1.0-9.5 wt %, 1.5-9.0 wt %, 2.0-8.5 wt %, 3.0-8.0 wt %, 3.5-7.5 wt %, 4.0-7.0 wt %, 4.5-6.5 wt %, 5.0-6.0 wt %, 5.5-5.8 wt % chlorine. The selective layer may further comprise chlorine bonded to the cross-linked polyamide after the selective layer may be subjected to hypochlorite treatment.

According to a particular aspect, the hypochlorite treatment may comprise treating the selective layer with hypochlorite solution of a pre-determined concentration and for a pre-determined period of time. The pre-determined concentration may be any suitable concentration. For example, the pre-determined concentration of hypochlorite solution may be < 15,000 ppm, preferably < 10,000 ppm. In particular, the pre-determined concentration may be 500-15,000 ppm, 1 ,000-10,000 ppm, 1 ,500-9,500 ppm, 2,000- 9,000 ppm, 2,500-8,500 ppm, 3,000-8,000 ppm, 3,500-7,500 ppm, 4,000-7,000 ppm, 4,500-6,500 ppm, 5,000-6,000 ppm, 5,500-5,700 ppm. Even more in particular, the predetermined concentration may be 2,000-8,000 ppm.

The pre-determined period of time may be any suitable pre-determined period of time. For example, the pre-determined period of time may be < 24 hours. In particular, the pre-determined period of time may be 0.5-24 hours, 1-20 hours, 2-18 hours, 3-17 ours, 4-16 hours, 5-15 hours, 7-12 hours, 8-10 hours. Even more in particular, the predetermined period of time may be about 3 hours.

The membrane comprising chlorine bonded to the cross-linked polyamide may have a suitably higher pure water permeability (PWP) rate as compared to the membrane without the chlorine bonded to the cross-linked polyamide. According to a particular aspect, the TFC hollow fibre membrane comprising chlorine bonded to the cross-linked polyamide may have a PWP rate of 4.0-20.0 L rm ² IT ¹ bar ¹ at 15 bar. For example, the cross-linked polyamide may have a PWP rate of 7.0-18.0 L rm ² IT ¹ bar ¹ . In particular, the PWP rate may be 7.5-17.5 L rm ² hr ¹ bar ¹ , 8.0-17.0 L rm ² IT ¹ bar ¹ , 8.5-16.5 L rm ² IT ¹ bar ¹ , 9.0-16.0 L rm ² hr ¹ bar ¹ , 9.5-15.5 L rm ² hr ¹ bar ¹ , 10.0-15.0 L rm ² hr ¹ bar ¹ , 10.5-14.5 L rm ² h- ¹ bar ¹ , 11-14 L rm ² hr ¹ bar ¹ , 11.5-13.5 L rm ² IT ¹ bar ¹ , 12.0-13.0 L rm ² hr ¹ bar ¹ at 15 bar. Even more in particular, the PWP rate may be 10.0-10.5 L rm ² h' ¹ bar ¹ , particularly about 10.13 L rm ² IT ¹ bar ¹ .

According to a particular aspect, the TFC hollow fibre membrane comprising chlorine bonded to the cross-linked polyamide may have a salt rejection rate of > 97%. In particular, the salt rejection rate may be > 98%. Even more in particular, the salt rejection rate may be 98-99%.

The TFC hollow fibre membrane may be used for any suitable application. In particular, the TFC hollow fibre membrane may be for use in, but not limited to, pressure retarded osmosis (PRO), osmotic power generation, nanofiltration, or reverse osmosis desalination. Even more in particular, the membrane may be used in PRO applications, particularly for osmotic power generation and desalination. According to a particular aspect, the TFC hollow fibre membrane comprising chlorine bonded to the cross-linked polyamide, may be for use in reverse osmosis desalination.

According to a second aspect, the present invention provides a method of forming a TFC hollow fibre membrane, the method comprising: providing a porous hollow fibre support layer; and forming a selective layer on a surface of the support layer through interfacial polymerization, wherein the forming comprises contacting the surface of the support layer with a first solution comprising a polyamine and aminofunctionalised carbon quantum dots (N-CQDs), removing excess of the first solution by applying air and subsequently contacting the surface of the support layer with a second solution comprising a polyfunctional acyl halide.

The TFC hollow fibre membrane may be any suitable membrane. For example, the TFC hollow fibre membrane may be an inner selective TFC hollow fibre membrane or an outer selective TFC hollow fibre membrane. In particular, the TFC hollow fibre membrane may be an inner selective TFC hollow fibre membrane and the selective layer may be on an inner circumferential surface of the hollow fibre support layer.

The support layer may be as described above. In particular, the support layer may be a polymeric support layer. The porous hollow fibre support layer may be prepared by any suitable method. According to a particular aspect, the method may further comprise first preparing the porous hollow fibre support layer. For example, the porous hollow fibre support layer may be prepared by: providing a dope solution comprising a polymer solution or a ceramic solution, a solvent/non-solvent mixture and water to an annulus of a spinneret; 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.

The polymer solution comprised in the dope solution may comprise any suitable polymer. For example, the polymer comprised in the polymer solution may be, but not limited to: polyethersulfone (PES), polysulfone, polyphenylsulfone, polyacrylonitrile, polyimide, polyether imide, polyamide-imide, polyvinylidene fluoride, cellulose acetate, cellulose triacetate, polyetherketone, polyetheretherketone or a combination thereof. In particular, the polymer comprised in the polymer solution may be PES.

According to a particular aspect, the porous hollow fibre support layer may be formed of 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 relatively small amounts 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: 20-25/30-40/30-40/1-5 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 (DI) water, NMP, or a combination thereof. In particular, the bore solution may be DI 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 may be water.

According to a particular aspect, the TFC hollow fibre membrane may be a TFC inner selective hollow fibre membrane. Therefore, when the selective layer is on an inner circumferential surface of the hollow fibre support layer, the method of forming the TFC hollow fibre membrane may further comprise: preparing a module comprising at least one porous hollow fibre support layer potted in the module; and forming the selective layer on an inner circumferential surface of the hollow fibre support layer through interfacial polymerization.

The forming may comprise contacting the surface of the porous hollow fibre support layer with a first solution comprising a polyamine and N-CQDs, removing excess of the first solution by applying air and subsequently contacting the surface of the porous hollow fibre support layer with a second solution comprising a polyfunctional acyl halide.

The first solution may comprise any suitable polyamine. For example, the polyamine may be, but not limited to, m-phenylenediamine (MPD), p-phenylenediamine, p- xylylenediamine, cyclohexanediamine, piperazine, branched or dendrimeric polyethylenimine, or a combination thereof. In particular, the polyamine comprised in the first solution may be MPD.

The amino-functionalised carbon quantum dots (N-CQDs) comprised in the first solution may be of a suitable size. For example, the size of the N-CQDs may be < 200 nm. In particular, the N-CQDs may have a size of 1-200 nm, 5-150 nm, 10-100 nm, 20- 80 nm, 30-75 nm, 50-60 nm. Even more in particular, the size may be < 10 nm, preferably about 10 nm. The first solution may comprise a suitable amount of N-CQDs. According to a particular aspect, the first solution may comprise 0.1-5.0 weight % N-CQDs based on the total weight of the first solution. In particular, the first solution may comprise 0.5-2.0 weight %, 1.0-1.5 weight % N-CQDs. Even more in particular, the first solution may comprise 1.0 weight % N-CQDs.

The N-CQDs may be formed by any suitable method. For example, the N-CQDs may be formed from quantum carbon dots (QCD). The QCD may be functionalised with at least one amino group. For example, the QCDs may be functionalised by, one or more of, but not limited to, ethanolamine (EA), ethylenediamine, urea, or a mixture thereof. In particular, the QCDs may be functionalised by EA.

According to a particular aspect, the first solution may further comprise a surfactant. The surfactant may be any suitable surfactant. For example, the surfactant may be, but not limited to, sodium dodecyl sulphate (SDS), trimethylamine (TEA), camphorsulfonic acid (CSA), or a mixture thereof. In particular, the surfactant may be SDS.

The contacting with the first solution may comprise introducing the first solution to a surface of the porous hollow fibre support layer for a pre-determined period of time. The removing excess of the first solution may then be carried out by applying air, such as purging air.

The second solution may comprise any suitable polyfunctional acyl halide. For example, the polyfunctional acyl halide comprised in the second solution may be, but not limited to, trimesoyl chloride (TMC), isophthaloyl chloride, terephthaloyl chloride, 1 ,3,5-cyclohexane tricarbonyl chloride, 1,2,3,4-cyclohexane tetracarbonyl chloride, or a mixture thereof. In particular, the polyfunctional acyl halide comprised in the second solution may be TMC.

According to a particular aspect, the second solution may further comprise an organic solvent. The organic solvent may be any suitable organic solvent. For example, the organic solvent may be, but not limited to, hexane, heptane, cyclohexane, isoparaffinic hydrocarbon, or a combination thereof. In particular, the organic solvent may be hexane. According to a particular embodiment, the second solution may comprise TMC and hexane. In particular, the second solution may comprise 0.05-0.3 wt% TMC in hexane. Even more in particular, the second solution may comprise 0.15 wt% TMC in hexane.

The contacting with the second solution may comprise introducing the second solution to the surface of the support layer saturated with the first solution for a pre-determined period of time to form the selective layer on the support layer.

The method may further comprise draining off the second solution, stabilising the selective layer formed and/or rinsing the thin film composite membrane formed. The draining off of the second solution may be by any suitable means. For example, the draining off of the second solution may be by purging air.

According to a particular aspect, the present invention may further comprise treating the TFC hollow fibre membrane with a hypochlorite solution of a pre-determined concentration and for a pre-determined period of time.

The hypochlorite solution may be any suitable solution comprising hypochlorite. In particular, the hypochlorite solution may be sodium hypochlorite solution.

According to a particular aspect, the pre-determined concentration may be any suitable concentration. For example, the pre-determined concentration of hypochlorite solution may be < 15,000 ppm, preferably < 10,000 ppm. In particular, the pre-determined concentration may be 500-15,000 ppm, 1 ,000-10,000 ppm, 1 ,500-9,500 ppm, 2,000- 9,000 ppm, 2,500-8,500 ppm, 3,000-8,000 ppm, 3,500-7,500 ppm, 4,000-7,000 ppm, 4,500-6,500 ppm, 5,000-6,000 ppm, 5,500-5,700 ppm. Even more in particular, the predetermined concentration may be 2,000-8,000 ppm.

The pre-determined period of time may be any suitable pre-determined period of time. For example, the pre-determined period of time may be < 24 hours. In particular, the pre-determined period of time may be 0.5-24 hours, 1-20 hours, 2-18 hours, 3-17 ours, 4-16 hours, 5-15 hours, 7-12 hours, 8-10 hours. Even more in particular, the predetermined period of time may be about 3 hours.

The TFC hollow fibre membrane of the present invention and formed from the method of the present invention may exhibit enhanced water permeability as compared to conventional TFC membranes without losing salt rejection. Carbon quantum dots are a new class of carbon nanomaterials with advantages of excellent hydrophilicity, low toxicity, environmental friendliness, easy synthesis and low cost.

The addition of the N-CQDs into the selective layer of the TFC hollow fibre membrane causes covalent bonds to form between the N-CQDs and the polyamide chains. Since the amino groups of the polyamine in the first solution react with the polyfunctional acyl halide of the second solution during the interfacial polymerization to form cross-linked polyamide, the N-CQDs with amino functionality may participate with the cross-linking reaction during the polymerization, resulting in a polyamide layer with improved physiochemical properties and separation performance. In particular, the improved separation performance may arise from the morphological and composition changes of the polyamide layer, such as a higher surface roughness, larger effective surface area and the existence of more hydrophilic oxygen-containing groups.

With regards to the TFC hollow fibre membrane treated with a hypochlorite solution, the chlorine may be bonded to the polyamide layers, thereby enhancing the repulsion against charged solutes and therefore, making the polyamide network less crosslinked. As a result, the PWP and salt rejection of the TFC hollow fibre membrane may be further improved.

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

Examples

Materials and methods

Citric acid (CA, 99.5%, Sigma-Aldrich), ethanolamine (EA, 99.5%, Sigma-Aldrich) and sodium hydroxide (98%, Sigma-Aldrich) were used to synthesize amino functionalized CQDs. The hollow fibre substrates of the TFC membranes were fabricated from a spinning dope made of Radel® A polyethersulfone (PES, Solvay Advanced Polymer), N-methyl-2-pyrrolidone (NMP, 99.5%, Merck), polyethylene glycol 400 (PEG, Mw = 400g/mol, Acros Organics) and deionized (DI) water (Millipore, USA). The as-spun hollow fibre substrates were post-treated with a mixture of DI water and glycerol (Industrial grade, Aik Moh Paints & Chemicals Pte. Ltd.), before interfacial polymerization.

The polyamide layers of the TFC membranes were prepared via interfacial polymerization reaction between 1, 3, 5-benzenetricarbonyl trichloride (TMC, 98%, Sigma-Aldrich) dissolved in hexane (99.9%, Fisher Chemicals) and m- Phenylenediamine (MPD, 98%, Tokyo Chemical Industry) dissolved in DI water with the sodium dodecyl sulfate (SDS, 99%, Sigma-Aldrich) as the surfactant. A sodium hypochlorite solution (NaCIO, 4.00-4.99%, Sigma-Aldrich) was used to treat the polyamide layers. Sodium chloride (NaCI, 99.5%, Merck) and DI water were used for the tests of membrane separation performance.

Synthesis and characterization of amino functionalized carbon quantum dots (CQDs)

To synthesize the amino functionalized CQDs, mixtures of ethanolamine (EA) and grinded citric acid (CA) at different ratios (0:10, 1 :10 and 1 :5) were heated to 180°C in air for 3 hours in a glass beaker covered with a glass slide. Specifically, 0 g, 3.5 g and 7 g ethanolamine were respectively added into 35 g ground citric acid to achieve the different ratios. Subsequently, the reacted mixtures were dissolved in DI water forming aqueous solutions after being cooled down to room temperature. The solutions were then dialyzed using Slide-A-Lyzer G2 Dialysis Cassettes (2K MWCO) against DI water to remove the remaining molecular precursors. After the dialysis, the residues were neutralized with a sodium hydroxide aqueous solution (5.0 M) to a pH value of 9 and then freeze dried to produce the functionalized CQDs with different amounts of amino groups.

The amino functionalized CQDs were referred to as CQD-EA/CA-0 (control), CQD- EA/CA-0.1 and CQD- EA/CA-0.2, where the numbers 0, 0.1 and 0.2 represent the ratios of ethanolamine to citric acid during the hydrothermal reaction, respectively. The synthesized CQDs were characterized with Fourier transform infrared spectroscopy (FTIR, Vertex 70, Bruker, USA) with a wavenumber range of 4000-500 cm ^-1 and an X- ray photoelectron spectrometer (XPS, Kratos AXIS Ultra ^DLD spectrometer, Kratos Analytical Ltd) with a mono Al Ka X-ray source to investigate the changes of chemical composition.

Fabrication of hollow fibre substrates of thin film composite (TFC) membranes The substrates of the TFC membranes were fabricated by a co-extrusion technique via a dry-jet wet spinning process with a dual layer spinneret. First, the vacuum-dried PES polymer was dissolved in a mixture of NMP and PEG (1:1) at 60 °C. Then, a small portion of DI water was added into the fully dissolved PES polymer solution dropwise. Subsequently, the DI water, homogenous PES polymer solution and NMP were extruded from a dual layer spinneret via the inner, middle and outer channels, respectively, to produce the hollow fibre substrates with a dense layer at the lumen side.

The as-spun membranes were post-treated with tap water for 2 days and a mixture of DI water and glycerol (1:1) for another 2 days before being air dried in room temperature for 2 days. Three pieces of the PES hollow fibre substrates were made into a small lab-scale module with a length of about 14 cm.

Fabrication, modification and post-treatment of polyamide layers of TFC membranes

The polyamide layers of the TFC membranes were fabricated on the inner surface of the hollow fibre substrates via conventional interfacial polymerization reaction. A 2 wt. % MPD aqueous solution with 0.1 wt. % SDS was pumped through the lumen side of the hollow fibre substrates for 3 minutes at a flow rate of 4.25 mL/min. Then, compressed air was applied through the lumen side of the hollow fibre substrates for 5 minutes to get rid of the excessive MPD solution. Subsequently, a TMC solution, which was dissolved in hexane at 0.15 wt. %, was pumped through the lumen side of the hollow fibre substrates for 5 minutes at a flow rate of 2.50 mL/min to react with the MPD aqueous solution remaining on the inner surface. Compressed air was then applied through the lumen side of the hollow fibre substrates for 1 minute to remove the excess TMC solution.

To modify the polyamide layers, the synthesized CQD-EA/CA-0, CQD-EA/CA-0.1 or CQD-EA/CA-0.2 were each added into the MPD aqueous solution at 1 wt. %. The resultant TFC membranes were referred to as TFC-(CQD-EA/CA-0)-1, TFC-(CQD- EA/CA-0.1)-1 and TFC-(CQD-EA/CA-0.2)-1 , respectively, where 1 represents the concentration of the synthesized CQDs in the MPD aqueous solutions. The small labscale modules after interfacial polymerization were left in air overnight and then immersed in DI water for one day before being tested or post-treated. The modified TFC membrane which had the optimal separation performance was treated with a sodium hypochlorite aqueous solution at 2000 ppm, 4000 ppm and 8000 ppm for 3 hours, respectively, to investigate the effects of hypochlorite treatment on separation performance and physicochemical properties of the newly developed TFC membranes incorporated with amino functionalized CQDs.

Characterization of the polyamide layers of TFC membranes

A variety of characterization techniques were employed to investigate the physicochemical changes of the polyamide layers after the modifications (i.e., incorporation of amino functionalized CQDs, and hypochlorite treatment). For sample preparation, the TFC hollow fibre membranes were rinsed with DI water completely to eliminate potential contaminants, before being freeze dried.

Field emission scanning electronic microscopy (FESEM, JEOL JSM-6700) was utilized to examine the morphology of the unmodified and modified polyamide layers, for which the TFC membranes were fractured in liquid nitrogen and coated with platinum using a JOEL JFC-1100E ion sputtering device.

Atomic force microscopy (AFM, Nanoscope Illa, Digital Instrument, USA) was used to determine surface topography (i.e., mean roughness (F?a), root mean square roughness (F?q), and effective surface area) of the unmodified and modified polyamide layers with a scan size of 5 pm x 5 pm, under a tapping mode, at room temperature in air.

An X-ray photoelectron spectrometer (XPS, Kratos AXIS Ultra ^DLD spectrometer, Kratos Analytical Ltd) with a mono Al K _a X-ray source was utilized to study the chemical composition changes of the modified polyamide layers.

Separation performance of the TFC membranes

The separation performance of the unmodified and modified TFC membranes were examined in terms of pure water permeability (PWP or A, L rm ² IT ¹ /bar, LMH/bar) and solute rejection (R, %) using small lab-scale modules with an effective membrane area of about 7.4 - 7.8 cm ² . Prior to each test, all the TFC membranes were stabilized at 20 bar for 1 hour. The stabilization not only increased the membrane surface area, but also stretched the polyamide selective layer (i.e., decrease water transport length and resistance), as well as lowered the substrate tortuosity. To evaluate the PWP of the TFC membranes, DI water with a volume of 4.5 L was circulated through the lumen side of the small lab-scale modules at 15 bar with a flow rate of 200 mL/min. The PWP was calculated with the equation (1) below: where AV (L) is the volume of the water permeate collected from the shell side of the small lab-scale modules during a time interval At (h) under a trans-membrane pressure of AP (bar), and A _m (m ² ) is the effective membrane area of the small lab-scale modules.

To evaluate the salt rejection of the TFC membranes, a 2000 ppm NaCI aqueous solution with a volume of 4.5 L was circulated through the lumen side of the small labscale modules at 15 bar with a flow rate of 200 mL/min. The conductivity of the feed solution and permeate collected from the shell side of the small lab-scale modules was measured to calculate the salt rejection of the TFC membranes using equation (2) below: where C _p and Cf are the salt concentrations (which were determined by a conductivity meter) of the permeate and feed solutions, respectively.

Results and discussion

Characterization of the synthesized CQDs

Figure 1 shows a comparison of the FTIR spectra of surface chemistry on CQDs with and without amino functionalization. For all the synthesized CQDs, a broad absorption of O-H stretching around 3300 cm ^-1 and an absorption of C=O stretching at 1567 cm ^-1 were observed, indicating the existence of carboxyl groups in these CQDs. In contrast with the control (i.e., CQD-EA/CA-0), two characteristic peaks at 1196 cm ^-1 and 1723 cm ^-1 were found in the spectra of the amino functionalized CQDs (i.e., CQD-EA/CA-0.1 and CQD-EA/CA-0.2), which could be attributed to the C-N stretching and C=O stretching of the aliphatic ester, respectively. These confirm that CQD-EA/CA-0.1 and CQD-EA/CA-0.2 were successfully functionalized with amino groups.

The surface chemistry of the synthesized CQDs was further quantitatively investigated by XPS, and the results are summarized in Table 1.

Table 1: XPS results (mass ratio, %) of the amino functionalized CQDs in comparison with the control

All synthesized CQDs had a high oxygen content of more than 30%, which indicates many hydrophilic oxygen-containing groups existing on these CQDs. Since all the synthesized CQDs were neutralized with sodium hydroxide, around 10-15% Na content was present in these CQDs. The existence of oxygen-containing groups and sodium imparted the synthesized CQDs with excellent hydrophilicity. Further, a negligible N content was detected in the control CQDs (i.e. , CQD-EA/CA-0), while the N content in CQD-EA/CA-0.1 and CQD-EA/CA-0.2 was 2.55% and 3.19%, respectively, due to the increased ratio of EA to CA during the hydrothermal reaction. Thus, it can be seen that CQD-EA/CA-0.1 and CQD-EA/CA-0.2 were successfully synthesized with amino functionality, which could react with TMC and play a more active role in the subsequent interfacial polymerization.

Characterization of the polyamide layers incorporated with CQDs

FESEM was employed to characterize the cross-section and surface morphology of the polyamide layers incorporated with CQD-EA/CA-0, CQD-EA/CA-0.1 and CQD-EA/CA- 0.2 in comparison with the unmodified polyamide layer, as shown in Figure 2.

All the TFC membranes had a visible and independent polyamide layer on top of the inner surface of the PES hollow fibre substrates. Figure 2 (a1 and b1) show that the unmodified TFC membrane has a relatively smooth polyamide layer with a nodular-like surface morphology. In contrast, the TFC-(CQD-EA/CA-0)-1, TFC-(CQD-EA/CA-0.1)-1 and TFC-(CQD-EA/CA-0.2)-1 membranes had rougher polyamide layers showing the typical ridge-and-valley surface morphology with more and broader leaves than the unmodified one (Figure 2 (a2-a4, b2-b4)). The morphological change resulted from the presence of CQDs during the interfacial polymerization which interfered with the crosslinking reaction between the two monomers and created more diffusion channels in the nascent polyamide network. As a result, more MPD molecules in the aqueous phase could diffuse across the nascent polyamide layer and react with TMC molecules in the organic phase until the polyamide layer becomes dense and thick enough to stop the MPD diffusion and terminate the crosslinking reaction.

The surface topology of the polyamide layers incorporated with CQD-EA/CA-0, CQD- EA/CA-0.1 and CQD-EA/CA-0.2 was probed by AFM. Table 2 shows the measured Rq, Ra, and effective surface areas.

Table 2: Characteristics of the polyamide layers of the unmodified TFC membrane, TFC-(CQD-EA/CA-0)-1, TFC-(CQD-EA/CA-0.1)-1 and TFC-(CQD- EA/CA-0.2)-1

All the modified TFC membranes had a polyamide layer with higher roughness than the unmodified one. Consistent with the order of N content in the synthesized CQDs (Table 1), the Rq and Ra values follow the order of: TFC-(CQD-EA/CA-0.2)-1 > TFC-(CQD- EA/CA-0.1)-1 > TFC-(CQD-EA/CA-0)-1. Clearly, the increasing amino content in the functionalized CQDs facilitated their participation with the interfacial polymerization reaction. The effective surface areas of TFC-(CQD-EA/CA-0)-1, TFC-(CQD-EA/CA- 0.1)-1 and TFC-(CQD-EA/CA-0.2)-1 were larger than that of the unmodified one (i.e., 35-36 vs 31 pm ² ). It is widely believed that TFC membranes with higher roughness would also have a higher water permeability because the higher roughness results in the larger effective surface area for water transport across the membranes.

Table 3 summarizes the XPS results and reveals the effects of incorporating these CQDs on the chemical compositions of polyamide layers.

Tab e 3: XPS results (mass ratio, %) of the polyamide layers of the unmodified TFC membrane, TFC-(CQD-EA/CA-0)-1, TFC-(CQD-EA/CA-0.1)-1 and TFC-(CQD- EA/CA-0.2)-1

Although all the three kinds of CQDs had Na content of 10-15 %, only 1 wt. % of the CQDs were added into the MPD aqueous solutions. Therefore, there were only negligible Na content in the modified polyamide layers. All the modified polyamide layers had a slightly higher O content than the unmodified polyamide layer because the incorporated CQDs had high oxygen content, as shown in Table 1.

The higher O content in the modified polyamide layers may potentially improve their water permeability. Among the modified TFC membranes, the polyamide layer of TFC- (CQD-EA/CA-0.1)-1 showed the highest oxygen content of 16.22%. In contrast, the polyamide layer of TFC-(CQD-EA/CA-0.2)-1 had a lower oxygen content of 16.09%, because this kind of CQDs had a higher N content of 3.19% and a lower oxygen content of 32.83% (Table 1). Consistent with the order of N content on CQDs (Table 1), the N content in the polyamide layers followed the same trend: the unmodified TFC membrane < TFC-(CQD-EA/CA-0)-1 < TFC-(CQD-EA/CA-0.1)-1 < TFC-(CQD-EA/CA- 0.2)-1. Clearly, when the amino functionalized CQDs reacted with TMC, it would bring more functionalized CQDs into the interfacial polymerization and result in a higher oxygen and nitrogen content in the polyamide layer.

Separation performance of the TFC membranes incorporated with CQDs

Figure 3 shows a comparison of the PWP (Figure 3(a)) and salt rejection (Figure 3(b)) of the TFC membranes with and without the synthesized CQDs. The unmodified TFC membrane has a PWP of 3.87 LMH/bar and a NaCI rejection of 98.6% against a 2000 ppm NaCI aqueous solution at 15 bar. Comparing with the unmodified TFC membrane, TFC-(CQD-EA/CA-0)-1 , TFC-(CQD-EA/CA-0.1)-1 and TFC-(CQD-EA/CA-0.2)-1 had 17.8%, 42.1% and 8.8% higher PWP values, respectively, but had slightly lower NaCI rejections. Generally, the enhanced PWPs of the modified TFC membranes could be attributed to the physicochemical changes of the polyamide layers induced by the incorporated CQDs, such as higher roughness and effective surface area, and the existence of more hydrophilic oxygen-containing groups in the polyamide network. However, since the synthesized CQDs had slightly different chemical compositions from one another, the resultant polyamide layers also showed slightly different performance. Among them, TFC-(CQD-EA/CA-0.1)-1 had the highest PWP of 5.50 LMH/bar at 15 bar. TFC-(CQD- EA/CA-0.1)-1 had a higher PWP than TFC-(CQD-EA/CA-0)-1 because the former had amino functionalized CQDs that could react with TMC and bring more CQDs into the polyamide layer, thus resulting in more performance enhancement than the latter ones. The lower PWP of TFC-(CQD-EA/CA-0.2)-1 than TFC-(CQD-EA/CA-0.1)-1 is due to the fact that the former had less hydrophilic oxygen-containing groups on the polyamide layer than the latter, as indicated by its lower oxygen content (i.e. , 16.09 vs 16.22%) as shown in Table 3. Therefore, the content of N and O elements in the synthesized CQDs must be balanced in order to maximize the effects of the CQDs.

It should be noted that the NaCI rejections of all the modified TFC membranes are still > 98%. The slightly lower NaCI rejections of the modified TFC membranes may be caused by the interference of CQDs on the crosslinking reaction between MPD and TMC monomers. However, the variation in NaCI rejection among all membranes is very small. This implies that the incorporation of 1 wt. % synthesized CQDs into the interfacial polymerization does not induce significant defects to the polyamide layer.

Characterization of polyamide layer of TFC-(CQD-EA/CA-0.1)-1 after hypochlorite treatment

Since the TFC-(CQD-EA/CA-0.1)-1 membrane had the highest PWP with a comparable salt rejection, it was selected for the study of hypochlorite treatment. The polyamide layer was treated with a sodium hypochlorite aqueous solution at 2000 ppm, 4000 ppm and 8000 ppm for 3 hours, respectively.

Figure 4 (a2-a4) and (b2-b4) shows the cross-section and surface morphology of the polyamide layers after hypochlorite treatment. Compared with the one without any post-treatment (Figure 4 (a1) and (b1)), the sodium hypochlorite treated polyamide layers still had the similar typical ridge-and-valley surface morphology. Therefore, there were no obvious surface morphology changes observed for the polyamide layer of TFC-(CQD-EA/CA-0.1)-1 after hypochlorite treatment.

The AFM results tabulated in Table 4 confirms the observation by FESEM. All the polyamide layers with or without hypochlorite treatment have the similar Rq, Ra, and effective surface area.

Table 4: Characteristics of the polyamide layer of TFC-(CQD-EA/CA-0.1)-1 without any treatment or treated with a sodium hypochlorite aqueous solution at 2000 ppm, 4000 ppm or 8000 ppm for 3 hours

Table 5 shows the XPS results reflecting the effects of hypochlorite treatment on the chemical compositions of the polyamide layer of TFC-(CQD-EA/CA-0.1)-1.

Table 5: XPS results (mass ratio, %) of the po yamide layer of TFC-(CQD-EA/CA- 0.1 )-1 without any treatment or treated with a sodium hypochlorite aqueous solution at 2000 ppm, 4000 ppm or 8000 ppm for 3 hours

Generally, the measured Cl content showed an increasing trend with an increase in sodium hypochlorite concentration. The Cl content in the polyamide layer of TFC- (CQD-EA/CA-0.1)-1 increased to 2.68%, 4.53% and 7.34% after being treated with sodium hypochlorite aqueous solutions at 2000 ppm, 4000 ppm and 8000 ppm for 3 hours, respectively. The chlorine is likely bonded to the polyamide layer by replacing the hydrogen on N-H groups, weakening the hydrogen bonds among the polyamide network. This replacement would provide more rotational freedom and flexibility for the polymer chains. Alternatively, the chlorine might be bonded to the ring structure undermining the amide bonds and breaking the polyamide chains, which would result in a less cross-linked polyamide layer. Both chlorination mechanisms would lead to a higher water permeability across the TFC membranes.

Separation performance of TFC-(CQD-EA/CA-0. 1)-1 after hypochlorite treatment

Figure 5 shows the effects of hypochlorite treatment on PWP and salt rejection of the TFC-(CQD-EA/CA-0.1)-1 membrane. After hypochlorite treatment, the PWP value was increased significantly. Compared with the membrane without treatment, PWP increased about 54.2%, 84.2% and 218.7% after the membrane was treated with a sodium hypochlorite aqueous solution at 2000 ppm, 4000 ppm and 8000 ppm for 3 hours, respectively. The increasing PWP trend was consistent with the increasing Cl content observed in Table 5 for the hypochlorite treated polyamide layers. As discussed above, the chlorine bonded to the polyamide layer would either weaken the hydrogen bonding among the polyamide network or undermine the amide bonds of the polymer chains, resulting in a less cross-linked polyamide layer.

In terms of salt rejection, the TFC-(CQD-EA/CA-0.1)-1 membrane without any treatment had a NaCI rejection of 98.0% against a 2000 ppm NaCI feed solution at 15 bar. After being treated with 2000 ppm and 4000 ppm sodium hypochlorite for 3 hours, the NaCI rejection of the membranes increased to 98.8% and 98.9%, respectively. This surprising phenomenon may have resulted from the enhanced charge repulsion effect against charge solutes as a result of the greater Cl content on the polyamide layer despite the reduced degree of cross-linking. Nevertheless, when the sodium hypochlorite concentration was further increased to 8000 ppm, the NaCI rejection of the treated membrane decreased to 97.2%. This could be explained by the highest Cl content of 7.34% observed on the treated polyamide layer, as shown in Table 5. Since the chlorine bonded to the polyamide layer would result in a less cross-linked polyamide layer, the highest Cl content implies the least cross-linked polyamide layer. The aforementioned two mechanisms are competing with each other, having different effects on salt rejection. When the negative effects from the breaking of the polyamide chains with a least cross-linked polyamide layer on NaCI rejection cannot be compensated by the enhanced charge repulsion effects, the NaCI rejection of the TFC membrane starts to decline.

Since a moderate hypochlorite treatment could significantly enhance the PWP and improve the NaCI rejection of the TFC-(CQD-EA/CA-0.1)-1 membrane with an impressive PWP at 10.13 LMH/bar and a NaCI rejection of 98.9%, it could be employed as an effective post-treatment method for the newly developed TFC membrane. Table 6 shows a benchmarking between the newly developed membrane and BWRO membranes consisting of nanomaterials reported in the literature. The newly developed TFC membrane after hypochlorite treatment has superior separation performance to the state-of-the-art BWRO membranes, and therefore has great potential for brackish water desalination.

Separation performance of TFC-(CQD-EA/CA-0.1)-1 after hypochlorite treatment (8- hour test)

Figure 6 shows the water permeability and salt rejection of the TFC-(CQD-EA/CA-0.1)- 1 membrane after being post-treated with a 4000 ppm sodium hypochlorite aqueous solution for 3 hours, for an 8-hour test. In general, the PWP showed a slightly decreasing trend with time during the 8-hour test, but the average value of PWP was consistent at 11.89 LMH/bar. The salt rejection during the 8-hour test was well maintained around 98.6%.

Separation performance of TFC-(CQD-EA/CA-0.1)-1 after hypochlorite treatment (3- month test)

Figure 7(a) shows the water permeability and Figure 7(b) shows the salt rejection of the TFC-(CQD-EA/CA-0.1)-1 after being post-treated with a 4000 ppm sodium hypochlorite aqueous solution for 3 hours during a 3-month test using real membrane bioreactor (MBR) permeate. The initial water flux and salt rejection were 10.77 LMH/bar and 97.8%. The initial water flux was comparable with the PWP when using the DI water as feed, while the initial salt rejection was lower than the salt rejection of 98.4% when using a 2000 ppm NaCI aqueous solution as feed. After 63 days, the water permeability and salt rejection decreased to 5.90 LMH/bar and 95.2% due to severe membrane fouling. There were significant amount of contaminants, such as silica, phosphate, bacteria etc., which existed in the real MBR permeate which would cause membrane fouling. At day 64, the membrane was cleaned with 1 wt. % citric acid aqueous solution. After membrane cleaning, the water permeability increased from 5.90 LMH/bar to 6.08 LMH/bar, and the salt rejection increased from 95.2% to 96.9%. At the end of the 3-month test, the water permeability and salt rejection decreased to 5.43 LMH/bar and 95.6%.

Table 6: Comparison of newly developed TFC membrane with selected BWRO membranes incorporated with nanomaterials reported in literature

Citric acid, HCI, and NaOH solutions were used to clean the fouled membrane after the 3-month test to determine if the deteriorated membrane performance could be recovered to its initial level. It was found that only the NaOH solution could clean the membrane and recover its performance effectively. The results are summarized in Table 7.

Tab e 7: Performance of TFC-(CQD-EA/CA-0.1)-1 (after being post-treated with a 4000 ppm sodium hypochlorite aqueous solution for 3 hours) before the 3-month test and after the 3-month test and NaOH cleaning (Measured at 15 bar using 2000 ppm NaCI aqueous solution as feed)

Before the 3-month test, the water permeability and NaCI rejection of the membrane were 11.86 LMH/bar and 98.3%, respectively. After being cleaned with 0.1 wt. % NaOH for 4 hours, the water permeability of the membrane could be recovered to 10.71 LMH/bar, which is 90.3% of its pristine performance, while the rejection of the membrane is 97.2%, which is slightly lower than its pristine performance.

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.