Alkali-Stable Nanofiltration Composite Membrane and Method of Manufacture Thereof

Abstract

Embodiments of the present invention relate to a nanofiltration composite membrane for use to purify water, the methods for preparing said nanofiltration composite membranes and to the nanofiltration composite membranes prepared accordingly.

Claims

1. A process for preparation of a nanofiltration composite membrane, comprising the steps of a.) solving at least one polymer selected from the group consisting of polysulfone polymer, polyether polymer and mixtures thereof and at least one polyvinyl pyrrolidone polymer in at least one aprotic solvent to prepare a casting solution and b.) casting the casting solution of step a.) on a non-woven fabric to prepare a casted non-woven fabric and c.) bringing the casted non-woven fabric of step b.) into contact with a polar non-solvent to prepare a casted non-woven fabric membrane and d.) bringing the casted non-woven fabric membrane of step c.) into contact with at least one radical polymerization initiator and e.) washing the casted non-woven fabric membrane of step d.) with a non-solvent and f.) cross-linking the casted non-woven fabric membrane of step e.) is to prepare said nanofiltration composite membrane.

2. The process according to claim 1, wherein the nanofiltration composite membrane has an average molecular weight cut-off between 200 g/mol to 1000 g/mol.

3. The process according to claim 1, wherein the polysulfone polymer is is a polysulfone containing the following unit (I): ##STR00003## and n is 45 to 230.

4. The process according to claim 1, wherein the polyether polymer is selected from the group consisting of polyethylene glycol, polypropylene glycol and mixtures of such polymers.

5. The process according to claim 1, wherein the total amount of polymer used in step a.) is 10% weight to 22% weight based on the amount of the casting solution.

6. The process according to claim 1, wherein the polyvinyl pyrrolidone polymer contains the following chemical unit (II) ##STR00004## and m is 110 to 1100.

7. The process according to claim 1, wherein the amount of polyvinyl pyrrolidone polymer used in the casting solution is 1% weight to 10% weight based upon the weight of the casting solution.

8. The process according to claim 1, wherein the non-woven fabric in step b.) is selected from the group consisting of polyethylene, polypropylene, polymethylpentene and mixtures thereof.

9. The process according to claim 1, wherein the weight per unit area of the non-woven fabric is 70 to 100 g/m.sup.2.

10. The process according to claim 1, wherein the aprotic solvent in step c.) is selected from the group consisting of N-methyl pyrrolidone, dimethyl formamide, dimethyl sulfoxide and dimethyl acetamide or mixtures thereof.

11. The process according to claim 1, wherein step c.) will be conducted at a temperature of 3° C. to 6° C.

12. The process according claim 1, wherein after step d.) has been performed step f.) is performed at a temperature of 80° C. to 110° C.

13. The process according to claim 12, wherein step f.) will be performed in 1 to 20 minutes.

14. A nanofiltration composite membranes prepared according to claim 1.

15. A process for purifying of water comprising: filtering water through the nanofiltration composite membranes according to claim 14.

16. The process according to claim 1, wherein the nanofiltration composite membrane has an average molecular weight cut-off between 500 g/mol to 1000 g/mol.

17. The process according to claim 1, wherein the total amount of polymer used in step a.) is 17% weight to 21% weight based on the amount of the casting solution.

18. A process for purifying of surface water, groundwater, borehole water, brackish water or seawater comprising: filtering surface water, groundwater, borehole water, brackish water or seawater through the nanofiltration composite membrane according to claim 14.

19. A process for reclamation of municipal or industrial wastewater comprising: filtering municipal or industrial wastewater through the nanofiltration composite membrane according to claim 14.

Description

EXAMPLES

Example 1

[0078] A nanofiltration composite membrane was prepared and casted from a homogeneous solution of polysulfone and polyvinyl pyrrolidone in dimethyl formamide as follows:

[0079] A homogeneous casting solution was prepared from 18 weight % of a polysulfone of the formula (I) having an average molecular weight of 60.000 g/mol (Ultrason® S 6010, BASF GmbH), 2 weight % of a polyvinyl pyrrolidone of the formula (II) having an average molecular weight of 40.000 g/mol (Povidon K-30 (Sigma Aldrich GmbH)) and 80 weight % dimethyl formamide. The casting solution was maintained at room temperature and membranes were casted on a polypropylene-based non-woven fabric with a weight per unit area of 85 g/m.sup.2. The casted membranes were coagulated immediately in a 5° C. water bath to form microporous membranes. After coagulation, the membranes were soaked in 1 weight % ammonium persulfate solution for 1 minute. Following this, membrane 1B was washed in a water bath at 60° C. for 5 minutes, while Membrane 1A did not receive any washing treatment. Membranes 1A and 1B were then crosslinked in an oven at 90° C. for 10 minutes.

[0080] The resulting membranes were tested for their salt rejection and permeation flux in a crossflow filtration system, containing a feed solution of 2000 mg/L MgSO.sub.4 at an operating pressure of 10.sup.3 kPa and a volumetric crossflow of 4 L/h (see ‘Method for the determination of salt rejection and permeate flux’ for details, measured according to ASTM D4194-03). The feed water pH and temperature was kept at 7 and 25° C., respectively. Table 1 shows the result of the permeate flux and salt rejection measurement after 4 hours of filtration, in order to ensure that equilibrium has been reached.

TABLE-US-00001 TABLE 1 Membrane 1A Membrane 1B Membrane Without washing Washing at 60° C. No. Flux (LMH) Rejection (%) Flux (LMH) Rejection (%) 1 110 23.2 11 93.8

[0081] Membrane 1B has an average molecular weight cut-off of 620 g/mol determined according to modified ASTM E1343-90(2001) as described under “Method for the determination of average molecular weight cut-off”. The membrane 1A does not have nanofiltration properties and exhibit an average molecular weight cut-off higher than 1000 g/mol.

[0082] It has been surprisingly found that after additional polymerization by using a polymerization initiator a washing step with a non-solvent is required before crosslinking has been performed. According to this procedure, nanofiltration composite membranes with high rejection performance and high alkaline stability can be prepared.

Example 2

[0083] A nanofiltration composite membrane was prepared and casted from a homogeneous solution of polysulfone and polyvinyl pyrrolidone in dimethyl formamide as the solvent as follows:

[0084] A homogeneous casting solution was prepared from 17.5 weight % of a polysulfone of the formula (I) having an average molecular weight of 60.000 g/mol (Ultrason® S 6010, BASF GmbH), 2.5 weight % of a polyvinyl pyrrolidone of the formula (II) having an average molecular weight of 40.000 g/mol (Povidon K-30 (Sigma Aldrich GmbH)) and 80 weight % dimethyl formamide. The casting solution was maintained at room temperature and membranes were casted on a polypropylene-based non-woven fabric with a weight per unit area of 85 g/m.sup.2. The casted membranes were coagulated immediately in a 5° C. water bath to form a microporous membrane. After coagulation, the membranes were soaked in 1% ammonium persulfate solution for 1 minutes. Following this, the membranes were washed in a water bath at 25° C. for 15 minutes. The membranes were then crosslinked in an oven at 80° C. for 60 minutes. As shown in Table 2, these membranes have rejection of MgSO.sub.4 of more than 89% and an average molecular weight cut-off of 600 g/mol determined according to modified ASTM E1343-90(2001).

[0085] Alkali treatment in NaOH solution was performed in order to test the membrane stability in high pH solution. The membranes were soaked in 20% NaOH solution at room temperature. pH value for this solution is 14.7 Membrane 2 was treated for 60 days and Membrane 3 was treated for 90 days. The resulting membranes were tested for their salt rejection and permeation flux in a crossflow filtration system, containing a feed solution of 2000 mg/L MgSO.sub.4 at an operating pressure of 10.sup.3 kPa and a volumetric crossflow of 4 L/h, before and after the NaOH treatment (see ‘Method for the determination of salt rejection and permeate flux’ for details measured according to ASTM D4194-03). The feed water pH and temperature was kept at 7 and 25° C., respectively. Table 2 shows the result of the measurement after 4 hours of filtration, in order to ensure that equilibrium has been reached.

TABLE-US-00002 TABLE 2 Membrane Before NaOH treatment After NaOH treatment No. Flux (LMH) Rejection (%) Flux (LMH) Rejection (%) 2 14 89.2 17 83.8 3 12 89.2 19 81.6

[0086] Despite extreme treatment by high concentrated alkaline solutions, rejection and flux of the inventive membrane remains high and nearly stable.

Example 3

[0087] A nanofiltration composite membrane was prepared and casted from a homogeneous solution of polysulfone and polyvinyl pyrrolidone in dimethyl formamide as follows:

[0088] A homogeneous casting solution was prepared from 18 weight % polysulfone (Ultrason® S 6010, BASF GmbH (average molecular weight is 60.000 g/mol), 2.5 weight % polyvinyl pyrrolidone (Povidon K-30 (Sigma Aldrich GmbH (average molecular weight is 40.000 g/mol)) and 79.5 weight % dimethyl formamide. The casting solution was maintained at room temperature and membranes were casted on a polypropylene-based non-woven fabric with a weight per unit area of 85 g/m.sup.2. The casted membranes were coagulated immediately in a 5° C. water bath to form microporous membranes. After coagulation, Membrane 4A was soaked in 1 weight % ammonium persulfate solution for 1 minute, then washed in a water bath at 60° C. for 5 minutes, whereas Membrane 4B was first washed in a water bath at 60° C. for 5 minutes, and then soaked in 1 weight % ammonium persulfate solution for 1 minute. Membranes 4A and 4B were then crosslinked in an oven at 100° C. for 5 minutes.

[0089] The resulting membranes were tested for their salt rejection and permeation flux in a crossflow filtration system, containing a feed solution of 2000 mg/L MgSO.sub.4 at an operating pressure of 10.sup.3 kPa and a volumetric crossflow of 4 L/h (see ‘Method for the determination of salt rejection and permeate flux’ for details, measured according to ASTM D4194-03). The feed water pH and temperature was kept at 7 and 25° C., respectively. Table 3 shows the result of the permeate flux and salt rejection measurement after 4 hours of filtration, in order to ensure that equilibrium has been reached.

TABLE-US-00003 TABLE 3 Membrane 4A Membrane 4B Membrane Catalyst followed by washing Washing followed by catalyst No. Flux (LMH) Rejection (%) Flux (LMH) Rejection (%) 4 14 88.5 94 26.6

[0090] This example shows that the order of catalyst and washing step is important to achieve nanofiltration performance of the membrane.