ANTI-FOULING AND SEMI-PERMEABLE MEMBRANE

Abstract

The present invention relates to an anti-fouling, semi-permeable membrane comprising a porous support layer, a thin film composite (TFC) layer formed on a surface of the support layer, and a cross-linked polyvinyl alcohol (PVA) layer formed on top of the TFC layer, wherein the cross-linked PVA layer is the reaction product of PVA and a cross-linking agent, said cross-linking agent being a polybasic acid comprising three or more acid groups or precursors thereof. The obtained membrane shows a high water flux and a low roughness suitable for an effective membrane notable for feed solution having a tendency of fouling the membrane.

Claims

1. An anti-fouling, semi-permeable membrane comprising a porous support layer, a thin film composite (TFC) layer formed on a surface of the support layer, and a cross-linked polyvinyl alcohol (PVA) layer formed on top of the TFC layer, wherein the cross-linked PVA layer is the reaction product of PVA and a cross-linking agent, said cross-linking agent being a polybasic acid comprising three or more acid groups or precursors thereof.

2. The anti-fouling membrane according to claim 1, wherein the TFC layer comprises aquaporin water channels.

3. The anti-fouling membrane according to claim 1, wherein the aquaporin water channels are present in vesicles.

4. The anti-fouling membrane according to claim 1, wherein the vesicles are polymersomes prepared by one or more polymers.

5. The anti-fouling membrane according to claim 1, wherein the PVA is derived from hydrolysis of polyvinyl acetate, the degree of hydrolysis of the polyvinyl acetate being from 90% to 99.9%.

6. The anti-fouling membrane according to claim 1, wherein the polybasic acid is selected from the group consisting of citric acid, 1,2,3,4-butanetetracarboxylic acid, 2,3,5-hexanetricarboxylic acid, and 1,2,3-butanetricarbaoxylic acid.

7. The anti-fouling membrane according to claim 6, wherein the polybasic acid is citric acid.

8. A method of preparing an anti-fouling, semi-permeable membrane comprising the steps of: providing a semi-permeable membrane comprising a porous support layer having a thin film composite (TFC) layer formed on a surface thereof, applying a layer of an aqueous PVA mixture of polyvinyl alcohol and a cross-linking agent to the TFC layer surface of the membrane, and allowing the mixture to react, wherein the cross-linking agent is a polybasic acid having three or more acid groups or precursors thereof.

9. The method according to claim 8, wherein the semi-permeable membrane comprising a support layer having a thin film composite layer formed on a surface thereof is treated with an aqueous glycerol solution prior to the application of the aqueous PVA mixture.

10. The method according to claim 9, wherein a glycerol solution is applied to the support layer surface of the TFC membrane prior to the application of aqueous PVA mixture.

11. The method according to claim 9, wherein the concentration of the glycerol in the glycerol solution is 13% by weight to 80% by weight.

12. The method according to claim 8, wherein the polybasic acid is selected among the group consisting of citric acid, 1,2,3,4-butanetetracarboxylic acid, 2,3,5-hexanetricarboxylic acid, and 1,2,3-butanetricarbaoxylic acid.

13. The method according to claim 12, wherein the polybasic acid is citric acid.

14. The method according to claim 12, wherein the concentration by weight of polybasic acid is equal to or higher than the concentration by weight of the PVA in the aqueous PVA mixture.

15. The method according to claim 14, wherein the concentration by weight of citric acid is at least twice the concentration of the PVA in the aqueous mixture.

Description

EXAMPLES

Example 1.1: Production of the Support Membrane

[0039] A dope was prepared of 17% polysulfone (Solvay P3500 MB7 LCD) dissolved in 83% DMF (N,N-Dimethylformamide) obtained from TACT Chemie. The dope was mixed at a mixing speed of 90 rpm in a closed container at 45° C. for 8 hours for obtaining a uniform viscosity.

[0040] The dope was casted on a non-woven polyester sheet (model PMB-SKC) obtained from Mitsubishi in a knife over roll casting mode using a casting gap of 230 μm. After an exposure time 1.9 s a phase inversion was performed by quenching in water at 13° C. for 16 s. Subsequently the support membrane was washed in water at 60° C. for 120 s. A thickness of about 130 μm was obtained.

Example 1.2: Production of Aquaporin Water Channel

[0041] Expression of Histidine Tagged Aquaporin from Oryza sativa Japonica (Japanese Rice) in Escherichia coli and its Purification Using Immobilized Metal Affinity Chromatography (IMAC)

[0042] The gene encoding aquaporin from Oryza sativa Japonica (UNIPROT: A3C132) was codon optimized using Geneart's (Subsidiary of Thermo Fischer Scientific) service for improving expression in E. coli. The resulting gene was synthesized with the addition of ten histidine encoding codons C-terminally, along with flanking NdeI/XhoI restriction sites N-terminally and C-terminally, respectively (Gene ID: aquaporin_Oryza_sativa_Japonica). The synthetic gene fragment was digested with NdeI/XhoI restriction enzymes and ligated to NdeI/XhoI—digested and purified vector pUP1909 fragment. The resulting ligation mixture was transformed into Escherichia coli DH10B and kanamycin resistant transformants were selected on LB agar plates with kanamycin. Transformants were confirmed by sequencing of the genetic construct. Isolated vector DNA was subsequently transferred to the production host, Escherichia coli BL21.

[0043] In order to heterologously express aquaporin in E. coli, the production host was grown in minimal medium consisting of 30 g/L Glycerol, 6 g/L (NH4)2HPO4, 3 g/L KH2PO4, 5 g/L NaCl, 0.25 g/L MgSO4.7H2O, 0.4 g/L Fe(III)citrate and 1 mL/L sterile filtered trace metal solution. The trace metal solution consisted of 1 g/L EDTA, 0.8 g/L CoC12.6H2O, 1.5 MnC12.4H2O, 0.4 g/L CuC12.2H2O, 0.4 g/L H3BO3, 0.8 g/L Na2Mo04.2H2O, 1.3 g/L Zn(CH3COO)2.2H2O. After inoculation and overnight growth, additional 0.25 g/L MgSO4.7H2O was added.

[0044] E. coli was cultivated in 3 L Applikon Bioreactors with ez-Control in a batch fermentation process. Protein production was induced by addition of IPTG to a final concentration of 0.5 mM at an optical density (OD 600 nm) of approximately 30. The culture was induced for approximately 24 hours and the bacterial cells were harvested with centrifugation at 5300 g for 20 min.

[0045] The pellets comprising the E. coli cells were resuspended in buffer (aqueous solution of the protease inhibitor PMSF and EDTA) and homogenized at 1000 bar in a Stansted nm-GEN 7575 homogenizer. The temperature was maintained around 10-15° C. The mixture was centrifuged at a maximum speed of 5300 g for 30 minutes. The pellet contains the membrane protein and the supernatant is discarded.

[0046] The pellet was resuspended in a 0.9% sodium chloride solution to obtain a total protein concentration of approximately 50 mg/ml. Solubilization of the membrane protein was performed by adding 28 L TRIS binding buffer and 4.5 liter 5% n-lauryl dimethylamine N-oxide (LDAO) to 5 L of the resuspended pellet material. At room temperature and gentle stirring the mixture was allowed to incubate for 2 to 24 hours.

[0047] After the solubilization process the mixture was centrifuged in 2 L containers at 5300 g for 90 minutes. The supernatant was recovered and the LDAO concentration was adjusted to 0.2% by addition of dilution buffer.

[0048] After solubilization and clarification, the protein was captured using IMAC and eluted in Elution buffer containing 1000 mM imidazole and 0.2% w/v LDAO. The elution fractions were analyzed by SDS Page and only revealed a single major band which migrated at 27 kDa which corresponds to the size of aquaporin from Japanese rice. Furthermore, the result was confirmed by comparison to a negative control purification from E. coli transformed with an empty vector. The negative control resulted in no purified protein. Western blot analysis with antibodies (TaKaRa Bio) specific for the histidine-tag resulted as expected in a clear signal from the purified protein and no signal from the negative control confirming the origin of the purified protein as the histidine tagged membrane protein.

[0049] A stock solution was prepared by adjusting the protein concentration to 5 mg/ml by adding ice cold imidazole-free buffer containing 2% LDAO. Finally, the aquaporin stock solution was sterilized by filtration through 0.45 μM sterilized cup and stored at 4° C. in refrigerator for use within a month or else stored at −80° C. in a freezer.

Example 1.3: Production of Aquaporin Formulation

[0050] 1. Prepare a 0.5% by weight Kolliphor® HS 15 (polyethylene glycol (15)-hydroxystearate) (KHS) solution by dissolving 5 g KHS in 11 PBS (prepared by dissolving 8 g NaCl, 0.2 g KCl, 1.44 g Na2HPO4 and 0.24 g of KH2PO4 in 800 mL MiliQ purified H2O, adjusting the pH to 7.2 with HCl and completing the volume to 1 L). [0051] 2. Prepare a 0.05% by weight LDAO solution in PBS by dissolving 0.05 g LDAO in 100 mL PBS. [0052] 3. In the preparation vessel, weigh 0.5 g poly(2-methyloxazoline)-block-poly(dimethylsiloxane) diblock copolymer (PDMS.sub.65PMOXA.sub.24) per L of prepared formulation. [0053] 4. In the same preparation vessel weigh poly(2-methyloxazoline)-block-poly(dimethylsiloxane) diblock copolymer (PDMS.sub.65PMOXA.sub.32 to reach a concentration on 0.5 g/L of prepared formulation (1:1 weight ratio PDMS.sub.65PMOXA.sub.24 and PDMS.sub.65PMOXA.sub.32) [0054] 5. In the same preparation vessel, add poly(2-methyloxazoline)-block-poly(dimethylsiloxane)-block-poly-(2-methyloxazoline) triblock copolymer PMOXA.sub.12PDMS.sub.65PMOXA.sub.12 to reach a concentration of 0.12 g/L of prepared formulation. [0055] 6. Add LDAO 0.05% prepared in step 2 in the proportion 100 mL/L of prepared formulation. [0056] 7. Add the bis(3-aminopropyl) terminated poly(dimethylsiloxane) having a molecular weight of 2500 Da to reach a final concentration of 0.1%. [0057] 8. Add aquaporin stock solution to reach a concentration of 5 mg/L of prepared formulation and a 1/400 protein:polymer ratio. [0058] 9. Add 3% by weight KHS solution prepared in step 1 to reach the desired volume of prepared formulation subtracting the volumes of LDAO, bis(3-aminopropyl) terminated poly(dimethylsiloxane) and aquaporin added in step 6 and 8. [0059] 10. Stir the mixture from step 9 overnight at 170 rotations per minute (not more than 20 hours) at room temperature to achieve the formulation. [0060] 11. Next morning take the prepared formulation obtained in the sequence of steps 1 to 10 and filter it through 200 nm pore size filters to sterilize it, put it in a closed sealed bottle and keep it at room temperature for not more than 12 months.

Example 1.4: Production of TFC Layer on the Support Membrane

[0061] a. Prepare an aqueous solution by mixing in DI water [0062] i. 3.0% MPD [0063] ii. 0.75% ε-caprolactam (CAP) [0064] iii. 3% aquaporin formulation

[0065] b. Prepare an organic solution with 0.13% TMC in Isopar E.

[0066] c. TFC formation [0067] i. Support membrane was dipped in the aqueous solution for 50 seconds, [0068] ii. Membrane was dried with air gun at 1 bar, [0069] iii. Organic solution was added for 20-22 seconds [0070] iv. Membrane was dried with air gun at about 0.1 bar

[0071] d. The membrane with TFC layer was placed in 70° C. 10% citric acid for 4 min and then in 60° C. DI water for 2 min.

[0072] e. The membrane was dipped in 2000 ppm sodium hypochlorite solution for 2 min, then dipped in room temperature DI water for 1 min, and finally dipping in 1% sodium bisulfite (SBS)

Example 2: Glycerol Treatment Prior to PVA Layer

[0073] Aqueous 10% and 20% glycerol solutions were prepared. In a first run the membranes obtained in example 1.4 were dipped in the glycerol solutions for 1 min, treated with an air knife to remove excess glycerol solution, and then treated by pouring with an aqueous 0.35% PVA solution (Kuraray Poval, type 60-98) for 30 s, treatment with air-knife at 1 bar to remove excess PVA solution, and dried in oven at 60° C. for 4 min. The results are shown in table 1 below:

TABLE-US-00001 TABLE 1 Glycerol dip A [LMH/bar] Rejection [%] 10%, 1 min 5.0 ± 0.5 99.0 ± 0.0 20%, 1 min 5.3 ± 0.4 98.8 ± 0.2

[0074] By changing the application method from dipping the entire membrane in to the glycerol solution to just applying the glycerol solution to the backside of the membrane obtained in example 1.4, another test run was performed. The results are shown in table 2 below:

TABLE-US-00002 TABLE 2 Backside glycerol A [LMH/bar] Rejection [%] 10%, 1 min 4.7 ± 0.5 98.9 ± 0.0 20%, 1 min 5.1 ± 0.4 98.9 ± 0.1

[0075] The results shown in table 1 and 2 indicate that the flux improves when using 20% glycerol compared to 10% glycerol. A slight drop in the rejection for glycerol dip was observed. However, with back-side glycerol, the change in glycerol concentration does not seem to affect the rejection. The deviations in flux are also lesser as compared to glycerol dip.

[0076] The test runs were repeated and including a longer reaction time of 3 min for the glycerol solution. The results are shown in table 3.

TABLE-US-00003 TABLE 3 Backside glycerol A [LMH/bar] Rejection [%] 10%, 1 min 4.7 ± 0.0 98.9 ± 0.0 10%, 3 min 4.9 ± 0.1 98.9 ± 0.1 20%, 1 min 5.1 ± 0.3 98.9 ± 0.1 20%, 3 min 5.6 ± 0.1 98.9 ± 0.0

[0077] The results in Table 3 show that treatment with 20% glycerol solution and a reaction time of 3 min for the PVA solution provides the highest flux and the same rejection.

Example 3: Reaction Time and Type of PVA Solution

[0078] A membrane treated with 20% glycerol on the back side for 3 minutes were poured with a 0.35% PVA 60-98 solution for 10 s or 30 s, treated with air-knife at 1 bar, and then dried in oven at 60° C. for 4 minutes. The results are shown in table 4:

TABLE-US-00004 TABLE 4 Backside glycerol A [LMH/bar] Rejection [%] Control (no PVA) 5.7 ± 0.0 98.9 ± 0.1 PVA coating, 10 s 4.5 ± 0.1 99.3 ± 0.2 PVA coating, 30 s 4.5 ± 0.1 99.2 ± 0.3

[0079] The results show that the flux and the rejection remain essentially unchanged. Therefore, for further investigation, 10 s PVA reaction time was used.

[0080] Various types of PVA obtainable from Kuraray Poval® was screened using the experience from the test runs above, i.e. 20% glycerol back-side treatment for 3 min, 0.35% PVA solution applied the pour method and allowed to react for 10 s, treatment with air-knife at 1 bar to remove excess PVA solution and then oven-dried at 60° C. for 4 min. The results for various PVA types and qualities are shown below in table 5.

TABLE-US-00005 TABLE 5 Types of PVA [N = 5 ] A [LMH/bar] Rejection [%] Control (no PVA) 5.4 ± 0.3 98.9 ± 0.3 PVA 4-98 4.8 ± 0.5 98.5 ± 0.6 PVA 18-88 4.7 ± 0.5 98.5 ± 0.5 PVA 30-75 3.9 ± 0.5 99.3 ± 0.6 PVA 28-99 4.0 ± 0.1 98.9 ± 0.4 PVA 30-99 4.3 ± 0.5 99.3 ± 0.2 PVA 30-98 4.4 ± 0.2 99.2 ± 0.1 PVA 60-98 4.4 ± 0.1 99.3 ± 0.2

[0081] The first number in the nomenclature denotes the viscosity of the 4% aqueous solution at 20° C. as a relative measure for the molar mass of the PVA. The second number denotes the degree of hydrolysis of the polyvinyl acetate from which the PVA is derived.

[0082] The results shown in table 5 show that PVA 30-98 and PVA 60-98 are good candidates as they show high flux and rejection, and at the same time low deviation. These candidates were compared to the commercially available Hypershell DOW membrane (benchmark). The results are shown in table 6 below.

TABLE-US-00006 TABLE 6 N = 4 A [LMH/bar] Rejection [%] Control (no PVA) 5.8 ± 0.1 99.0 ± 0.1 PVA 30-98 4.2 ± 0.1 99.2 ± 0.1 PVA 60-98 4.4 ± 0.2 99.4 ± 0.0 DOW Hypershell 3.5 ± 0.1 99.5 ± 0.1

[0083] Based on the results shown in table 6, PVA 60-98 is selected for further development due to higher flux and rejection compared to PVA 30-98. The rejection is essential at the same level as the benchmark, however, the flux is about 25% higher.

Example 4: PVA Application Method

[0084] It was observed that the PVA layer of the coated membranes was rather thick by a PVA dye test. The relatively thick layer could be the reason for a relatively high drop in flux compared to the control membrane. Therefore, a spraying method for applying the PVA solution was used and compared with the pouring method. The results for the flux is shown in table 7 below.

TABLE-US-00007 TABLE 7 PVA 60-98 DOW Control Spray [LMH/bar] [LMH/bar] [LMH/bar] PWP*.sup.) 6.4 (spray) 4.0 7.4 5.3 (pour) BWRO**.sup.) 4.5 (spray) 2.9 5.2 3.8 (pour) Test conditions: *.sup.)10 bar, 25 C., RO water **.sup.)15.5 bar, 25 C., 2000 ppm NaCl feed, 1.8 LPM flow rate.

[0085] The results of table 7 show that the membrane having the PVA solution applied by spraying improves the PWP flux by about 20% relative to the pouring method and 60% relative to the benchmark from DOW.

Example 5: Citric Acid (CA) Cross-Linker

[0086] In an attempt to improve the flux further, citric acid was added to the PVA solution as a cross-linking or pore forming agent.

[0087] A PVA solution prepared as indicated above was supplemented with 0.42% by weight citric acid, corresponding to 50% crosslinking degree when reacted with the available OH groups of the PVA. The PVA was applied by the spray method and allowed to react for 10 s before excess PVA solution was removed with air knife. The results are indicated in table 8 below.

TABLE-US-00008 TABLE 8 Citric acid A [LMH/bar] Rejection [%] PVA 4.5 99.1 PVA + CA (50%) 4.9 99.1

[0088] Test Conditions: 2000 ppm NaCl Feed, 15.5 Bar, 1.8 LPM Flow Rate

[0089] The results show that an improvement of the flux of about 9% can be obtained by adding citric acid to the PVA solution.

[0090] The surface roughness, Ra (nm), the contact angle and the zeta potential (mV) were investigated for the PVA membrane as well as the PVA+CA(50%) and compared to control membrane not treated with PVA. The results are shown in table 9 below.

TABLE-US-00009 TABLE 9 Average Contact Zeta poten- roughness (nm) angle tial (mV) PVA 35.6 70.4 −9.00 PVA + CA (50%) 22.4 61.2 −6.33 Control 53.9 55.3 −22.05 

[0091] The results in table 9 show that the average roughness decreases when the membrane is coated with a PVA layer. A further decrease in roughness is obtained by citric acid to the PVA solution. Furthermore, the contact angle as well as the zeta potential are significantly reduced when citric acid is added to the PVA solution. A low surface roughness and zeta potential are generally regarded to improve the anti-fouling properties of the membrane. The fact that the surface charge tends towards neutrality when citric acid is added to the PVA-solution is also regarded to improve the anti-fouling characteristics.

[0092] A further experiment was designed to compare 50% and 100% cross-linking degree. The PVA solution for 100% cross-linking degree contained 0.84% by weight of citric acid. The results are indicated table 10 below.

TABLE-US-00010 TABLE 10 Citric acid A [LMH/bar] Rejection [%] PVA + CA (50%) 4.6 99.3 ± 0.2 PVA + CA (100%) 5.0 99.2 ± 0.2

[0093] Test Conditions: 2000 ppm NaCl Feed, 15.5 Bar, 1.8 LPM Flow Rate

[0094] The results indicated in table 10 show that an improved flux of about 9% may be obtained when the amount of citric acid in the PVA solution is increased from 50% cross-linking degree to 100% cross-linking degree.

[0095] The various aspects and implementations have been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed subject-matter, from a study of the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality.