BACKGROUND OF THE INVENTION

Nanofiltration (NF) is an emerging technology in water purification, wastewater treatment, and seawater desalination. See Hoek et al., Nanofiltration separations, Dekker Encyclopedia of Nanoscience and Nanotechnology (2nd edition), Taylor and Francis Group, 2009, pp. 2749-2762. Comparing with traditional technologies, nanofiltration offers many benefits including low operation cost, reduced energy consumption, and avenues for integrated and environmentally friendly processing. Progress has been made to perfect this technology in industrial applications. See, e.g., Freger et al., J. Membr. Sci. 209(1 ) (2002) 283. However, many problems still exist, for example, membrane fouling and low membrane stabilities and durability.

There is still a need to develop nanofiltration membranes having improved features.

SUMMARY OF THE INVENTION

One aspect of this invention relates to a nanofiltration membrane, which includes (i) a supporting fabric layer, (ii) an engineered nanofibrous structural layer deposited on the top of the supporting fabric layer, and (iii) a polyamide thin film deposited on the top of the engineered nanofibrous structural layer. The supporting fabric layer is 100- 200 μιη thick, the engineered nanofibrous structural layer is 30-75 μπι thick and contains pores of the size of 0.4-1.5 μπι, and the polyamide thin film is 100-300 ran thick. The pore size used herein is referred to the diameter at the maximum of the pore size distribution. See Nimmo J.R., Encyclopedia of Soils in the Environment, Ed. by Hillel, D., London, Elsevier, 2004, v. 3, p. 295-303. Preferably, the nanofiltration membrane has one or more of the following features: the supporting layer has a thickness of 100-150 μπι and/or an air permeability of 0.0015-0.01 1 cubic foot per minute, the engineered nanofibrous structural layer has a thickness of 30-60 μπι and/or contains pores of the size of 0.7-1 μπι, and the polyamide thin film has a thickness of 100-200 nm and/or contains pores of the size of 0.001-0.1 μπι (or 0.001-0.01 μηι). In one embodiment, the engineered nanofibrous structural layer has a thickness of 50-75 μηι and contains pores of the size of 0.7-1 μηι. In another embodiment, the supporting fabric layer has a thickness of about 100 μπι, the engineered nanofibrous structural layer has a.thickness of about

50 μηι, and the polyamide thin film has a thickness of about 100 nm.

The supporting fabric layer can be made of polyester or polypropylene. The engineered nanofibrous structural layer can be made of polyacrylonitrile, polysulfone, polyethersulfone, Nylon 4,6, Nylon 6, Nylon 6,6, Nylon 12, polyamides,

polyetheretherketone, or a mixture thereof. The polyamide thin film can be made by interfacial polymerization between aromatic or cyclic aliphatic diamine (e.g., ortho-, meta-, or para-phenylenediamine, piperazine, or bipiperidine) and acid halide (e.g., trimesoyl chloride, isophthaloyl chloride, or terephthoyl chloride).

Another aspect of this invention relates to a process of preparing the above-described nanofiltration membrane. The method includes (i) applying nanofibers onto a supporting fabric layer to form a nanofibrous structural layer, (ii) pressing the nanofibrous structural layer and the supporting fabric layer at a predetermined temperature and under a pre-determined pressure, and (iii) coating the pressed nanofibrous structural layer with a polyamide thin layer via interfacial polymerization between diamine and acid halide. The applying step (i) can be performed by electrostatic spinning; melt electrospinning; modified melt spinning;

electroblowing, and splitting of jet of melt/solution. The pressing step (ii) can be performed at a temperature in a range between 10°C below and 10°C above the glass transition temperature of the nanofibers of the nanofibrous structural layer and under a pressure of 0.55-1.1 1 lb/in.

Also within the scope of this invention is a process of treating water by passing it through the nanofiltration membrane of this invention to remove salts or impurities. This process can be performed under atmospheric or elevated pressure or under vacuum. The nanofiltration membrane can be of any shape, e.g., tubular or sheet.

The details of one or more embodiments of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the detailed description of several embodiments and also from the appending claims. DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 is a cross-section illustration of a nanofiltration membrane of this invention. Fig. 2 shows schematic filtration testing set-up.

Figs. 3 A and 3B show membrane stability of a NF membrane prepared without hot-pressing and a NF membrane prepared with hot-pressing, respectively.

Figs. 4A and 4B show filtration results for a NF membrane prepared without hot- pressing and a NF membrane prepared with hot-pressing, respectively.

Fig. 5 shows mechanical testing results of NF membranes DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a nanofiltration membrane as shown in Fig. 1 , which has a supporting fabric layer (1), an engineered nanofibrous structural layer (2), and a polyamide thin film (3). This membrane exhibits superior features, including high energy efficiency, structural durability and pressure tolerance, and operational stability for water treatment applications.

To fabricate the nanofiltration membrane of this invention, one can use

commercially available nonwoven fabric as the supporting fabric layer (1 ). Nonwoven fabric is a fabric-like material which has a random fibrous web structure bonded together by entangling fibers or filaments mechanically, thermally, or chemically. The nonwoven fabric has interconnecting open area throughout cross-section to allow liquid or gaseous fluids, but not particulate, to follow through it. A commonly-used example is

spunbonded calendered nonwoven fabric made of fibers of an average diameter of 30- 40 microns. It has a 20-30 microns mean flow pore ratings, a basis weight ranging from 60 to 70 g/m ² , an air flow permeability of 0.0015-0.01 1 cubic foot per minute, a thickness of 100-200 microns, and a high tear strength and tensile strength across machine direction (e.g., 4028 N/m MD) and cross direction (e.g., 2451.8 N/m CD). The Frazier

permeability of the nonwoven spunbonded calendered fabric can be around 640 L/m ² /sec at 196 Pa differential pressures.

Onto the supporting fabric layer (1), one deposits nanofibers via a known technique such as electrostatic spinning (electrospinning), melt electrospinning, modified melt spinning, electroblowing, splitting of jet of melt/solution and multicomponent fiber spinning or an 'islands-in-the-sea' method to form a nanofibrous structural layer:

Preferably, nanofibers of a diameter of 50-500 nm having very high homogeneity are used. They can be made from polymer such as polyacrylonitrile, polysulfone, polyethersulfone, Nylon 4,6, Nylon 6, Nylon 6,6, Nylon 12, polyamides,

polyetheretherketone, or a mixture thereof. Production and deposition of nanofibers can carried out in a synchronized fashion.

The thus-obtained nanofibrous structural layer is then pressed at a predetermined temperature and under a pre-determined pressure via a conventional method to form the engineered nanofibrous structural layer (2). This step is preferably conducted within ± 20°C (or ± 10°C) of the glass transition temperature of the nanofibers of the

nanofibrous structural layer and under a pressure of 0.55-1.1 1 lb/in. This treatment improves not only the adhesion between the nanofibrous structural layer and the supporting fabric layer, but also the pore interconnectivity in nanofibrous structural layer. After the hot-pressing step, the pore size is reduced.

The engineered nanofibrous structural layer has a thickness of 30-75 μπι (or 30- 60 μπι) and contains pores of the size of 0.4-1.5 μπι (or 0.7-1.0 μπι). It has a water permeability ranging from 30,000 l/(m ² h) to 50,000 l/(m ² h) at transmembrane pressure of 70 psi, mostly near to 36,0001/(m ² h).

Next, the engineered nanofibrous structural layer is coated with a polyamide thin layer (3) via the technique of interfacial polymerization. Preferably, the polyamide thin film is 100-300 nm thick. Interfacial polymerization has been described in, e.g., in Yoon et al., Journal of Membrane Science, 2009. 326(2): p. 484-492. In this reaction, polymerization occurs at or near the interfacial boundary of two solutions, one containing a diamine monomer and the other containing an acid halide monomer.

The diamine monomer can be aromatic diamines (such as ortho, meta and para- phenylenediamine) or cyclic aliphatic diamines (such as piperazine and bipiperidine). The acid halide monomer can be acid chloride (such as trimesoyl chloride, isophthaloyl chloride, or terephthaloyl chloride). Preferably, the diamine monomer and acid halide monomer have different dissociation constants with trace additives so as to create different sizes of directed channels in the polyamide layer, which lead to varying rejection of monovalent, bivalent, and multivalent ions. Examples are

piperazine/bipiperidine dihydrochloride and trimesoyl chloride.

Solvents used in the interfacial polymerization can be selected from water and organic solvents. Preferably, the solvent used to dissolve the diamine and the solvent used to dissolve the acid halide are immiscible. For example, the diamine is dissolved in water and the acid chloride is dissolved in methylene chloride, chloroform, toluene, ester, ether, alkane (e.g., pentane or hexane), a mixture thereof.

After interfacial polymerization, the resulting membrane is dried with convective air circulation flow at a temperature ranging from 65 ^P C to 90°C and then thoroughly washed with de-ionized water.

The membrane of this invention unexpectedly offers several benefits such as structural durability, high pressure tolerance and operational stability, considerable energy saving efficiency due to high permeabilities/fluxes even at relatively lower driving pressure differentials.

One skilled in the art can translate the process parameters described herein to those suitable for commercial manufacture of the nanofiltration membrane of this invention. For example, to continuously produce the membrane, one can synchronize the nanofiber production machineries with thermally and mechanically driven flat presses or rollers, thin film coating machines, washing/rinsing lines, and convective air drying chambers.

The nanofiltration membrane of this invention can be further processed to make various types of articles useful for performing water treatments such as surface, ground, brackish, or sea water treatments, wastewater treatments, partial and hybrid desalination operation, industrial desalting operations, and industrial scale low molecular weight organics separation and purification. For example, a tubular membrane can be used to purify water by separating salts or impurities from water.

Without further elaboration, it is believed that one skilled in the art can, based on the disclosure herein, utilize the present invention to its fullest extent. The following specific embodiments are, therefore, to be construed as merely descriptive, and not limitative of the remainder of the disclosure in any way whatsoever. All publications cited herein are incorporated by reference. Examples

Chemicals/solvents: Nonwoven poly( ethylene terephthalate (PET) (Hollytex 3242 supplied by Ahlstrom Mount Holly Springs, USA), polyacrylonitrile (PAN) average Mw 150,000 (Aldrich Product Number 181315), N,N-Dimethylformamide GR ACS (DMF) (Merck Ltd., Product code 1.03053), piperazine (Sigma-Aldrich Product Number P- 45907), bipiperidine dihydrochloride (Sigma-Aldrich Product Number 180742), triethylamine (Sigma-Aldrich Product Number T0886), NaOH (Sigma-Aldrich Product Number S8045), Water (Milli Q), 1,3,5-benzenetricarbonyl trichloride (trimesoyl chloride or TMC) (Sigma-Aldrich Product Number 147532), hexane anhydrous (Sigma aldrich Product Number 296090), and washing water for membrane (Milli Q), MgS0 ₄ 7 H ₂ 0 (Sino Chemicals Co. Pte Ltd ).

NF 90 and NF270 were supplied by the FilmTec Corporation (Edina, M , USA). NF 90 is an aromatic polyamide, which contains carboxylic acid and primary amines (-NH2), whereas NF 270 is a mixed aromatic, aliphatic polyamide (polypiperazine amide) with secondary amine (-NH) and carboxylic acid. See ArtuAY, et al., Sep. Sci. Technol. 42(13) (2007) 2947. ENSA 1 -LF and ENSA 1 -LF2 membranes were supplied by Hydranautics a Nitto Denko Company (Oceanside, CA, USA). ENSAl-LF and ENSA1- LF2 membranes are composite polyamide type similar to existing reverse osmosis membranes but chemically treated to adjust the hardness rejection and impart fouling resistance. See Bartels et al., Desalination 22 (1-3) (2008) 158.

Preparation of the nanofibrous structural layer over the supporting fabric

Spunbonded polyester sheet structured from the continuous polyester fibers is commonly used for supporting a microporous layer. Calendering imparts high tear strength and tensile strength to the spunbonded polyester fabric across machine direction (MD) and cross direction (CD). Polyester materials of spunbonded sheet show a good thermal and chemical resistance particularly to acids, oxidizing agents and solvents. Balanced structural properties across the machine direction and cross direction and controlled thickness are an important perquisite feature of calendered nonwoven fabric. The properties of such a layer used to support the nanofibrous layer are given in Table 1 below.

Table 1. Properties of the spunbonded polyester sheet

DMF was treated with molecular sieves to remove moisture. PAN was dissolved in dried DMF to form a PAN solution (8 % w/w). This solvent was subsequently subjected to electrospinning to deposit a nanofibrous structural layer onto the spunbonded polyester sheet. The electrospinning conditions were similar to those described in Yoon et al., Journal of Membrane Science, 2009. 326(2): p. 484-492, which are shown in Table 2 below. A Nanon-01 A electrospinning machine (MECC CO, Ltd, Japan) was used for electrospinning. Table 2: Electrospinning conditions

* selected close to 135 rpm as machine resolution was 50 rpm.

After electrospinning, the nanofibrous structural layer and the supporting fabric layer were exposed to mild convective air flow in fumehood for at least 3 hours to remove the residual solvent. The nanofibrous structural layer and the supporting fabric layer were then hot-pressed in thermal transfer press (Hotronix, Carmichaels, PA 15320, USA) at light pressure (setting value of 3) at 85°C (near to glass transition temperature of PAN) for 16.65 mins. The pore size of the nanofibrous structural layer was measured by a capillary flow porometer (CFP-1200-A, PMI- Porous Materials Inc., Ithaca, NY, USA) using Galwick™ of surface tension 0.0159 N/m as wetting liquid.

Deposition of polvamide thin layer

The nanofibrous structural layer was coated with a polyamide film by interfacial polymerization reaction using bipiperidine dihydrochloride and piperazine (as diamine monomers) and TMC (as aromatic acid monomer) according to Yoon et al., Journal of Membrane Science, 2009. 326(2): p. 484-492. Pre-cut nanofibrous structural layer (15 cm x 15 cm) was placed on the flat metallic surface in a tray. Aqueous diamine solution containing bipiperidine dihydrochloride (0.3 %w/v), piperazine (0.7 % w/v), NaOH (1.5 equivalent to bipiperidine dihydrochloride concentration), and triethylamine (1% w/v) was prepared. The nanofibrous structural layer in the tray was wetted by aqueous diamine solution for 1 min. The excess amine solution was drained by keeping the tray in vertical position for 3 minutes. The wetted membrane was taken in separate dried tray and edges of the support membrane were sealed by an adhesive tape (3M, USA). The wetted nanofibrous structural membrane was covered completely by an organic solution containing TMC (0.1 % w/v) for 1 min. After draining TMC solution, the membrane was air dried in fumehood for 15 minutes followed by drying in a hot air oven with convective airflow at temperature of 65 °C for 20 minutes. Finally, the membrane was exposed to convective airflow at 25°C in the fume hood for 3.5 hours. In some cases, membrane was just air dried in fumehood for 1 hr. Dried TFC NF membrane was washed with water three times for 1/2 to 1 day. The dried membrane was used for filtration testing. Each membrane was fabricated three times for filtration testing to account the variations. Preparation of membranes without hot-pressing

The membranes were prepared according to the methods described above except that hot-pressing step was omitted. See also Yoon et al., Journal of Membrane Science, 2009. 326(2): p. 484-492.

Microscopy of membranes

Morphological observations of the membranes were performed by a versatile high performance and low vacuum scanning electron microscope (SEM) (quanta 200F, FEI). The samples were prepared by cutting the membranes into small pieces (approximately 5 mm x 5 mm in size) with a sharp surgical blade. The samples for thickness measurement were prepared by cutting very thin pieces (rectangular approximately 4 mm x 15 mm in size). The samples were fixed on a flat sample holder (stubs). The samples for thickness measurement were hold vertically in a metallic clip. To prevent excessive charging of the samples during analysis, a conductive adhesive carbon tap was applied on the sample holder (except in the thickness measurement) before mounting the membrane samples over it.

The samples for SEM were coated by platinum in the auto fine coater (JFC-1600, JEOL, Japan) for 60-120 sec at the current of 10 mA and vacuum of 8 to 10 Pa. If the samples were not coated enough, they were recoated again. The distance between electron gun and sample, accelerating voltage, spot size and vacuum used for capturing good SEM images were 10 mm, 10 kV, 3 and 0.9 mbar, respectively. The magnification, sharpness and contrast of images were adjusted before capturing the images. The planer images were analyzed for the fiber diameter, orientations of fibers and surface analysis, while the vertical or cross sectional images were analyzed for the thickness of samples. The Java image processing software (Image J 1.29 x (222 commands)) was used for measuring the diameter of fibers and thickness of mat from stored SEM images. At least four pictures were used for calculating the mean values of diameter of fibers and thickness of web. Analysis using Atomic Force Microscope

Membrane surface imaging and roughness measurement was performed by the Atomic Force microscope (AFM) Dimensional 0.0 AFM,.Veeco.Instruments, Edina, M ). Imaging was performed in the tapping mode with silicon probe (Nanosensors™ PPP-NCH, non-contact/tapping mode type probe, Nanosensors, Switzerland). The cantilever had nominal thickness of 4 μπι, nominal mean width of 30 μπι, a nominal spring constant of 42 N/m, nominal resonant frequency of 330 Hz, nominal tip radius of <10 nm and nominal cantilever length of 125 μπι. The air-dried membrane of 10 μπι x 10 μπι area was scanned at scan rate of 1 Hz, tip velocity of 10 μηι/sec, sample/lines 128 and lines 64. The best images were captured by adjusting the amplitude set point, contrast, data scale, and resolution to highest value 512 samples/lines and 512 lines. The captured images were processed by nanoscope ® III, version 6.13 Rl software. The roughness measurement was performed over the entire image. The following

measurements were done to estimate surface topology of the membranes.

(i) Rq: The Root Mean Square (RMS) Roughness is the standard deviation of the Z values within a given area.

^N /

RSM = i=l

N

Z _ave is the average of Z value within the given area, Zi is the current Z value and N is the number of points within a given area.

(ii) Ra: The mean Roughness (Ra) represents the arithmetic average of the deviations from the centre plan.

Z _cp is the Z value of the center plan and Zi is the current Z value and N is the number of points within a given area.

(iii) Rmax: The difference in height between the highest and lowest points on the surface relative to the mean plane. (iv) Surface area (SA): This value is the sum of the area of all triangles formed by three adjacent data points.

(v) Surface Area Difference (SAD): The percentage increase of the three dimensional surface over the two dimensional surface area.

Mechanical strength determination

The mechanical testing of membranes was done using the Instron (UK) make 3345 Single Column Testing Systems. The specimen of size 8 cm x 2 cm was used for tensile strength measurement. The thickness of membrane was measured prior to analysis. The appropriate load cell of 100 N or 1000 N capacity was used for the measurement, depending upon the membrane strength. The test method was established using particular load cell at 10 mm/min stretching rate. The area of 2 cm x 2 cm at both ends of the membrane was used to hold the specimen in the Instron machine. The measurement was done using established test method. The data acquisition/processing was done by using the Bluehill® 2 is a fully-integrated modular software package. The plot of tensile stress (MPa) vs. tensile strain (%) was obtained to compare mechanical strengths of membranes.

Filtration testing

NF membranes of this invention and commercial membranes were cut into circular pieces (0.065 m in diameter) with a membrane cutter. They were fixed in the cross flow cell. The exposed area of the membranes was 0.001963 m . The schematic filtration testing set-up is shown in the Fig. 2.

Permeate controlling valve (V ₃ ) was kept fully open and Trans Membrane Pressure (TMP- the driving force for filtration) was varied by adjusting the feed controlling valve (V i) and retentate controlling valve (V ₂ ). TMP was calculated by using the following equation.

TMP = [(Pi-P ₂ )/2]-P ₃ The aqueous solution of MgS0 ₄ .7H ₂ 0 (2000 ppm) at ambient condition (25 °C) - was used as feed solution. Before starting the filtration experiment, membrane was compacted at TMP pressure of 70 psi using the 2000 ppm MgS0 ₄ feed solution. Each filtration experiment was run for 1 hr at various TMP pressures (70, 100, 130, 160 and 190 psi). Conductivity and temperature of feed before starting the filtration experiment and after the filtration run was measured by the digital conductivity meter (Thermo Scientific Orion 3-star Plus). Conductivity and temperature of permeate was also measured. Volume of retentate (1) and permeate collected (1) was measured. Permeate flux at various TMP was calculated using the values of permeate collected (1), exposed area of membrane (0.001963 m ² ) and time of filtration (1 hr). Rejection of MgS0 ₄ (R %) was calculated using the following expression.

R (%) = [l- (Cp/C _f )]*100 Where, C _p is the conductivity of permeate (μ8/αη) and C _f is the conductivity of feed (μ8/α ) after the filtration.

Results

The capillary flow porosimetry results and pure water flux are summarized in the Table 3 below. The hot pressing treatment to nanofibrous structural layer narrowed the pore size distribution of membrane.

Table 3. Capillary Flow Porosimetry results of nanofibrous structural layer

Measurement Nanofibrous structural Nanofibrous

layer before hot structural layer

pressing treatment after hot pressing

treatment

Mean flow pore 1.7587 μπι 0.7998 μπι

diameter

Standard 1.3861 0.346

deviation of

average pore

diameter

The membranes prepared without hot pressing could not withstand the impact of high pressure, particularly when the TMP was more than 130 psi. See Fig. 3 A and Fig. 4A. MgS0 ₄ rejection was dropped from 88.78 % to 53.60 % and permeate flux was increased from 218.2 to 451.7 1/m h, when the TMP was increased from 130 psi to 160 psi. See Fig. 4A. However, the nanofibrous structural layer (of 1.759 μπι mean flow pore diameter) was capable of supporting the polyamide thin film, as shown from the high values of MgS0 ₄ rejection. See Fig. 4A. When the thickness of thin film

polyamide layer became smaller than the diameter of fibers of supporting film, the polyamide film showed imprints of the nanofiber morphology.

By contrast, the membrane prepared with hot pressing had improved pressure tolerance and mechanical and structural integrity. See Figs. 3B and 4B. It was robust to withstand the higher pressure more than the 130 psi. See Fig. 4B. The membrane prepared with hot pressing had higher flux than commercial membranes (i.e. 10-12 % higher than the best commercial membrane).

Fig. 5 shows tensile stress vs. tensile strain relationships of various commercial membranes and the membrane of this invention. Nanofibrous layer is too weak to impart suitable mechanical strength to the polyamide NF membrane (black curve). However, when it is supported by commercially available engineered nonwoven backing material, it imparts adequate mechanical strength to polyamide NF membrane (violet curve). The membrane of this invention has comparable mechanical property with commercially available NF membranes. OTHER EMBODIMENTS

All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the claims.