DK201900343A1 - Forward osmosis membrane obtained by using sulfonated polysulfone (sPSf) polymer and production method thereof - Google Patents

Description

DESCRIPTION

FORWARD OSMOSIS MEMBRANE OBTAINED BY USING

SULFONATED POLYSULFONE (sPSf) POLYMER AND PRODUCTION METHOD THEREOF

Field of the Invention

The present invention relates to producing a forward osmosis membrane by forming a nanofiber support membrane layer on a polyester nonwoven material using sulfonated polysulfone (sPSf) polymer, and by coating this sulfonated polysulfone nanofiber support membrane layer with a thin film composite of polyamide.

Background of the Invention

Osmosis can be explained as movement/passage of solvents from a lower solute concentration to a higher solution concentration through a semi-permeable membrane without consuming energy. Although osmosis is a frequently encountered process in the nature, the most important part thereof is that it does not need any external driving power for passage of the solvent. Osmosis frequently takes place with water, which is the most common solvent in nature, and it can be expressed that, particularly in cells, when the cell is present in a solution with a higher concentration (e.g. sea water) than its own internal concentration, it will get more concentrated as the water therein will move out through the cell membrane. In other words, it can be said that it is diffusion of water from a less salty medium to a more salty medium without consuming energy.

This concept in nature can be used for controlled water filtration and this filtration process is called “forward osmosis process”. In this process, a forward osmosis (FO) membrane is used as the semi-permeable membrane, a draw solution containing a high salt concentration is used as the higher concentration medium and feed water is used as the lower concentration medium. The feed water may be water such as waste water, greywater, sea water or well water. Concentrated solutions of various salts such as chloride and sulfate salts can be used as the draw solution. The forward osmosis membrane serves here as a semi-permeable membrane and allows passage of the water while rejecting passage of the minerals and particles.

There are two basic characterization parameters for FO membranes. These are; the water flux (Jw) denoting the amount of water passing through a unit area and the reverse salt flux (Js/Jw) denoting the amount of salt in a unit weight which diffuses back from the draw solution to the feed solution per the amount of the water in a unit volume passing through the membrane. Here, Js denotes the amount of salt diffusing from the draw solution to the feed water. Reverse salt flux unit is given as g/L. In other words, it is the amount of salt in grams lost from the draw solution per a liter of the obtained product water. The higher the water flux and the lower the reverse salt flux of the FO membranes are, the better their performance will be.

It is observed that the state-of-the-art membranes are either produced in a polymer-dense structure by using polymers such as cellulose triacetate or by applying a thin layer of dense polymer coating on a microporous support membrane. Phase transformation method is widely used to form this polymer dense structure or microporous support layer in the membranes.

It is known that in the recent years, as an alternative to these microporous support membranes, forward osmosis membranes are produced by forming a thin film composite by interfacial polymerization of aromatic acid halide with aromatic amine on a nanofiber support membrane. (Wang et al. (2005), US20130105395 A1)

There are two basic characterization parameters for FO membranes. These are; the water flux (Jw) denoting the amount of water passing through a unit area and the reverse salt flux (Js/Jw) denoting the amount of salt in a unit weight which diffuses back from the draw solution to the feed solution per the amount of the water in a unit volume passing through the membrane. Here, Js denotes the amount of salt diffusing from the draw solution to the feed water. Reverse salt flux unit is given as g/L. In other words, it is the amount of salt in grams lost from the draw solution per a liter of the obtained product water. The higher the water flux and the lower the reverse salt flux of the FO membranes are, the better their performance will be. In the prior art, an excess amount of reverse salt flux was observed in forward osmosis membranes and therefore water flux could not be increased to high levels.

Korean patent document no. KR20130078827, an application known in the art, discloses a hollow fiber type forward osmosis membrane and a method of manufacturing the same. This method of manufacturing the forward osmosis membrane comprises the steps of forming hollow fiber by simultaneously spinning a doping solution containing 0.5-5% by weight of sulfonated polysulfone-based polymer and a hollow fiber forming core solution to polymer, exposing the resultant product to the air, impregnating the product into a coagulating bath, and drying the product; and forming a polyamide layer on the hollow fiber by impregnating the hollow fiber in an aqueous solution containing multifunctional or alkylated aliphatic amine, washing the excessive amount of the aqueous solution, processing the hollow fiber with an organic solution containing multifunctional acid in order to generate interfacial polymerization between the compounds. The prepared hollow fiber is immersed in an aqueous solution containing phenylenediamine (MPD), and compression is applied to the surface layers in order to remove the excess amount of water. The hollow fiber is immersed in an organic solvent containing trimesoyl chloride (TMC) in order to form a polyamide layer. In order to form hollow membranes, spinning is performed on the polymer dope solution and a hollow-forming core solution for containing the sulfonated polysulfone-based polymer 0.5 to 5% by weight. The core solution includes a mixture of organic polar solvent (solvent A) and water (solvent B) with a mixing ratio of 4:6 to 9:1. One of the said organic polar solvents is dimethylacetamide (DMAc). Cross-sectional thicknesses of the hollow fibers are 30 to 250 micrometers.

Korean patent document no KR20030088090, an application known in the art, discloses a method for producing sulfonated polysulfone for cation exchange membranes. This method comprises the steps of (a) causing reaction between chlorosulfonic acid and trimethylchlorosilane, (b) dissolving polysulfone in a solvent, (c) adding triethylamine to the polysulfone solution, and (d) mixing the complex sulfonation agent of the step (a) and the polysulfone of the step (c) to cause reaction therebetween.

United States patent document no. US2013026091, an application known in the art, discloses a method for improving the performance of forward osmosis membrane. The document describes thin film composite (TFC) membranes for use in forward osmosis (FO) and pressure reduced osmosis (PRO) processes. This membrane is comprised of two layers: a composite layer combining a backing layer and a porous, polymer-based support into a single layer, and a rejection layer disposed on top of the composite layer. The rejection layer is formed from a thin coating of a hydrophilic polymer. The composite layer's surface may be coated with a pre-formed polymer or a polymer may be formed via in situ polymerization. One of the polymers which may be used is sulfonated polysulfone. Alternatively, a polymer such as polyamide may be polymerized in situ on the composite layer to form the rejection layer. For the in situ polymerization of polyamide, the composite layer is first soaked in an aqueous solution of m-phenylenediamine (m-PDA). Excess m-PDA is removed with a roller or an air knife. Subsequently, a solution of trimesoyl chloride (TMC) in an organic fluid, such as hexane or Isopar G, is applied to the top surface of the processed composite layer. Thus, interfacial polymerization occurs to yield a thin polyamide rejection layer on the composite layer. Thickness of this coating layer can be 1 micron or less (e.g., 0.2 micron).

Chinese patent document no. CN103977718, an application known in the art, discloses a high-water-flux forward-osmosis composite membrane and a production method thereof. This forward-osmosis composite membrane is a polysulfone-sulfonated polysulfone-inorganic filler blended/polyamide composite membrane. The method of producing this composite membrane comprises the steps of: blending a polymer with a modifier to form film casting liquid; performing non-solvent coagulating bath with water; preparing a polysulfone ultrafiltration membrane, airing the polysulfone membrane to dry the surface thereof, and growing a polyamide active layer by performing interfacial polymerization. One of the monomers used for forming polyamide layer is m-phenylenediamine (MPD). Additionally, the reaction oil phase contains trimesoyl chloride dissolved in hexane.

Chinese patent document no. CN102665882, an application known in the art, discloses a forward osmosis membrane of high flux for desalinating seawater and a method for manufacturing the same. This membrane is comprised of a nonwoven fabric layer, a hydrophilic polymer support layer, and a polyamide layer. The said hydrophilic polymer support layer includes 0.1 to 10% by weight of sulfonated polysulfone-based polymer. The polyamide layer is formed on the surface of the hydrophilic polymer support layer. During formation of the polyamide layer, polymerization reaction takes place between its components. The aqueous solution includes 2% by weight of phenylenediamine (MPD), 0.1% by weight of trimesoyl chloride (TMC) in organic solution (dissolved in ISOPAR agent).

Summary of the Invention

The objective of the present invention is to realize a forward osmosis membrane production method by using sulfonated polysulfone (sPSf) polymer, which enables performance improvements in parameters such as water permeability and reverse salt passage in production of thin film composite forward osmosis membranes.

Another objective of the present invention is to realize a forward osmosis membrane production method by using sulfonated polysulfone (sPSf) polymer, which enables to obtain forward osmosis membranes developed to be used in water and waste water treatment (e.g. desalination of sea water) and mining.

Another objective of the present invention is to realize a forward osmosis membrane production method by using sulfonated polysulfone (sPSf) polymer, which enables to obtain forward osmosis membranes developed to be used in energy sectors for energy production by means of pressure-retarded osmosis technology.

Detailed Description of the Invention

The method of producing forward osmosis membrane by using sulfonated polysulfone (sPSf) polymer of the present invention, which enables to prevent problems of reverse salt flux and low water permeability observed in forward osmosis membranes, comprises the steps of - sulfonating the polysulfone polymer; o introducing the polysulfone (PSf) into the reaction vessel, o adding a solvent onto the polysulfone, o placing the mixture in a water bath and stirring, o obtaining the polysulfone solution, o passing inert gas over the solution after it becomes homogeneous, o diluting trimethylsilyl chlorosulfonate reagent solution with dichloromethane at a ratio of 1:1 by volume, o adding the resulting trimethylsilyl chlorosulfonate reagent solution into the previously obtained polysulfone solution, o proceeding until the reaction is completed, o adding the mixture obtained as a result of the reaction dropwise into alcohol thereby precipitating the polysulfone samples, o filtering the mixture, o washing the separated solid samples with alcohol and then drying, - producing nanofiber support membrane layer with sulfonated polysulfone; o allowing the sulfonated polysulfone to dry in a vacuum oven, o mixing the dehumidified polymer solution with dimethylacetamide (DMAc) solvent, o forming the nanofibers by means of electrospinning method; placing the polymer solution onto the collection layer via a nozzle, applying voltage on the nozzle tip, o applying heat treatment to improve mechanical strength of the produced nanofibers, o nanofibers accumulating on the collection layer in bulk forming the nanofiber support membrane layer structure, - allowing the nanofibers to rest in distilled water in order to increase wettability thereof, - coating the produced nanofiber support membrane layer surface with a thin polyamide film, o passing nitrogen gas through the distilled water in order to remove the oxygen dissolved therein, o adding MPD (m-phenylenediamine), pH adjusting agent (camphor sulfonic acid (CSA)), acid removal agent (triethylamine (TEA)) and surfactant (sodium dodecyl sulfate (SDS)) into distilled water and stirring to form the primary solution, o adding TMC (trimesoyl chloride) into hexane to form the secondary solution, o immersing the nanofiber support membrane layer first into the primary solution and then into the secondary solution, o forming the coating by means of the interfacial polymerization performed on the membrane surface, o subjecting the coating to heat treatment, - obtaining the forward osmosis membrane which is the final product.

Within the scope of the invention, first of all sulfonation of the commercial polysulfone polymer is performed. Trimethylsilyl chlorosulfonate is used in sulfonation of commercial polysulfone samples. Sulfonation reactions performed by using trimethylsilyl chlorosulfonate take place in two steps: 1. In the first step, the isopropyl group (CH3-C-CH3) increases activation of the aromatic ring and as a result of this, substitution takes place in the aromatic ring in the dihydroxyl compound. In this first stage the ligated group is not sulfonic acid group but trimethylsilyl ester of sulfonic acid. Since the ester group does not cause an important change in the polarity of the polymer, it does not significantly change dissolution properties of the polymer. Therefore, no precipitation takes place in the reaction medium. Thus, undesired side reactions are prevented; equal ratio of sulfonation is enabled to occur in polymer chains; and furthermore, trimethylsilyl chlorosulfonate does not cause chain scissions due to its low reactivity compared to the other sulfonation agents. 2. In the second step, these ester groups are hydrolyzed and transformed into sulfonic acid groups.

In order to reach the desired degree of sulfonation, assays are conducted by taking necessary starting amounts from trimethylsilyl chlorosulfonate taking into consideration the number of the repeating units of the commercially available polysulfone homopolymer. In order to perform the reactions in a controlled medium, temperature of the reaction medium is aimed to be kept fixed at (4065°C) 15-40°C above the room temperature (25°C). For this purpose, solvents having relatively low boiling point within the range of 40-65 °C are preferred in the reactions. Accordingly, a solvent selected from the group comprising dichloromethane (DCM), chloroform, polysulfone or mixtures thereof is used. Thus, commercially available polysulfone samples are enabled to have a desired degree of sulfonation such that they will not yield undesirable side reactions.

Since sulfonated polymers having 40% sulfonation degree are used within the scope of the present invention, the commercial polysulfones are sulfonated such that their sulfonation degree is 40%. As an example to this sulfonation process, 40% sulfonation of 100 grams of the polysulfone sample can be described as follows. As the ratio of sulfonated polymer increases, the sizes of the vessels and materials that are used will also vary. - A solvent is added onto a polysulfone (PSf) sample (680 mL (900 gr) dichloromethane (DCM) is added onto 100 g polysulfone (PSf) sample) which is introduced into a 3 neck reaction vessel of 2L that is preferably made of glass such that polysulfone:solvent ratio will be 1:9 by mass (w/w), - then, the reaction vessel is placed into a water bath of 35°C and is kept there for a period of 12 to 24 hours and stirred in order for the polysulfone to be completely dissolved, - after the solution becomes completely homogenous, argon gas at fixed temperature (35°C) is passed over it, - in order to achieve a sulfonation degree of 40%, trimethylsilyl chlorosulfonate (TMSCS) is diluted with dichloromethane (DCM) such that trimethylsilyl chlorosulfonate (TMSCS):dichloromethane (DCM) ratio will be 2:1 by volume (v/v) (18.2 g trimethylsilyl chlorosulfonate is diluted with 10 mL DCM), - the resulting trimethylsilyl chlorosulfonate solution is added dropwise to the homogenous polysulfone solution which is obtained previously by the help of a dropping funnel, - the reaction is continued for 72-96 hours in order to obtain high yield, - sulfonation degree can be defined as the number of the repeating units in a polymer chain containing sulfone group. Dissolution characteristics of polymers may vary significantly depending on the increase of the sulfonation degree; for example, a polymer, which in the beginning can easily dissolve in a solvent, may hardly or do not at all dissolve in this solvent due to the sulfone groups included in its structure. Similarly, as the number of sulfone groups in a polymer chain increases, a solvent, which is non-solvent for it in the beginning, may start to dissolve this polymer. Therefore, this situation should be taken into consideration during purification of the sulfonated polymers from the solution via precipitation, and different non-solvent solvent types should be found and applied. The solution obtained at the end of this process is added dropwise under high stirring into an alcohol (selected from a group consisting of methanol, ethanol, isopropyl alcohol or mixtures thereof) suitable to the sulfonation degree and having a volume of 10-15 times more than the volume of the solution, and the sulfonated polysulfone samples are precipitated, - the solid polymer samples which are filtered through a filter paper or a Gooch crucible under vacuum are washed by a suitable alcohol and then dried in an oven at 55°C for 48 hours.

The stage of sulfonation of the polysulfone polymer is followed by the stage of production of the nanofiber support membrane layer by using this polymer. Sulfonated polymer having 40% sulfonation degree is allowed to rest in a vacuum oven overnight at 60-80°C for the last time before proceeding with the other membrane production steps. The solution containing 30% by weight of polymer after the polymer is dehumidified as above is mixed with DMAc solvent at 3040°C for 24-48 hours to be prepared. Electrospinning method is used in the scope of the invention in order to produce a nanofiber. The produced nanofibers are collected on a PET nonwoven support layer which is used as a membrane base. In the electrospinning process, the process conditions are arranged as follows: polymer feed rate: 4 ml/min, applied voltage: 27 kV, distance between the nozzle tip and the collection layer: 19 cm, and ambient temperature 25°C. However, electrospinning conditions may vary according to the machinery that is used and the process medium. In order to improve mechanical strength of the produced nanofibers, heat treatment is applied by allowing them to rest at a temperature of 180-190 °C for 3 hours. Heat post treatment is based on hardening and mechanically improving the polymers by subjecting them to a temperature near or above the glass transition temperature. These nanofibers are allowed to rest for 3 hours at a temperature near glass transition temperature of polysulfone polymer and heat post treatment is applied.

The characterization information related to this nanofiber formed by sulfonated polysulfone is provided in Table 1. Average diameter of the fibers is about 247 nm. Average pore diameter of the resulting nanofiber structure is measured as 1.562 μm. Bubble point flow rate is determined as 0.091 l/min. 67% porosity value of the obtained nanofiber support membrane is found to be high and favorable. Water permeability depending on high porosity is measured as 8820 l/m2.h.bar. Normally, sulfonation process favorably decreases contact angle values of the polysulfone polymer. However, this improvement is not exactly observed in the nanofiber structures due to the air bag effect. Hydrophilicity of the membranes is determined by the method of contact angle measurement. In this measurement method, one distilled water drop is dropped on the membranes, and the angle remaining between this water drop and the membrane surface is measured. The smaller this angle is, the more hydrophilic the membrane is. It is observed in the experimental studies conducted that while the contact angle of nanofibers produced with unsulfonated polymers is found to be around 122°, contact angles of the nanofibers produced with sulfonated polymers is found to be around 117°. An improvement of 5° is observed in the contact angles. Zeta potential value of the nanofiber support membrane layer is found to be lower than the value (pH 7.58.4) -50 mV. This result is favorable in terms of both hydrophilicity and fouling resistance of the membrane.

Table 1 shows the data regarding the characterization of the nanofiber support membrane layer produced with sulfonated polysulfone.

There are a total of three layers in the forward osmosis membranes of the present invention; namely PET nonwoven support layer, nanofiber support membrane layer produced on the former, and a rejection layer, i.e. active layer, produced on the membrane layer. After the nanofiber support membrane layer is produced with sulfonated polysulfone, it is proceeded with the process of thin film composite coating. Prior to this process, the sPSf nanofiber support membrane layer is kept in distilled water for 12-36 hours, preferably 24 hours, for a better wettability. Before preparation of the MPD solution (m-phenylenediamine), nitrogen gas is passed through the distilled water and the dissolved oxygen in the distilled water is removed from the medium. Following this process, MPD and the other auxiliary chemicals are added into the distilled water thereby producing the

primary solution. TMC (trimesoyl chloride) is added into hexane and thereby the secondary solution is formed (Table 2). Firstly, the sPSf nanofiber support membrane layer is immersed in MPD solution and it is allowed to rest for 1 to 10 minutes, preferably 5 minutes, to enable a thorough wetting, and then excess MPD solution is removed by using a silicone or rubber roller. Subsequent to this process, sPSf membranes saturated with MPD are immersed in TMC solution for 0.5 - 3 minutes, preferably 2 minutes. Here, only the surface part of the membrane is reacted with TMC solution. Thus, a polyamide film layer is produced by interfacial polymerization on the surface of the sulfonated polysulfone (sPSf) nanofiber support membrane layer which is immersed in TMC solution after MPD solution. Following this interfacial polymerization process, the membrane is dried and subjected to heat treatment in an oven at 60-80 °C, preferably 70°C, for 5 to 10 minutes, preferably 7.5 minutes. Subsequent to all of the processes, the membrane is stored in distilled water until the characterization tests are conducted. As a result of the characterization tests, thickness of the produced active layer (polyamide film) is found to be 900-930 nanometers.

Table 2 shows the data regarding the characterization of the solutions used for producing active layer.

* 0.1 gram TMC is dissolved in 1 L hexane.

Water permeability and reverse salt flux performances of the forward osmosis membranes produced within the scope of the invention obtained by using 1 M

draw solution are given in Table 3 in comparison to some of the best performance values given in the literature. As seen in Table 3, the membrane in this study has (65.7/313) L/m2 h water flux values in (FO/PRO) operation modes. In addition to these high water flux values, the produced membranes have shown very low reverse salt flux of 2.5/5.3 g/m2 h particularly in PRO mode. As it can also be seen in Table 3, performance values of the forward osmosis membranes of the present invention have been found to be better than those of all of the other membranes given in the comparison. It is seen that the membrane of the invention, due to its high salt retention, has the lowest reverse salt flux value among the other membranes. By using the advantage of these high performance properties, operating costs may be substantially reduced in forward osmosis processes. Additionally, this membrane, by means of its high water flux, has a great potential and advantages both in desalination of sea water and energy production via osmosis.

Table 3 is a representation of the performance comparisons regarding various membranes in the experimental studies conducted within the scope of the invention.

After detection of forward osmosis performances of the membranes by widespread methods in the experimental studies conducted within the scope of the invention, performances thereof with real sea water were desired to be detected. To this end, sea water (15.77 mS/cm) was collected from the Bosphorus on February 2015, and it was used as feed solution. 2 M NaCl solution was used as the draw solution. The sea water, before being used, was subjected to microfiltration process in order to get rid of the suspended substances which may interfere and have negative impacts. Water flux was measured for both FO mode and PRO mode such that flow directions of the draw and feed solutions were opposite to each other. The system was operated for an average of 3 hours and average water flux value was derived.

As can be seen in Table 4, the membrane of the present invention showed high water flux values for FO and PRO modes which are 15.11 and 49.44 LMH respectively. When sea water was used instead of distilled water, water flux

values significantly decreased, because the osmotic pressure difference between the draw solution and the feed solution on both sides of the membrane decreased.

Table 4 is a representation of PRO and FO performances of TFC-FO membranes for sea water having reverse flow direction.

The forward osmosis membrane production method of the present invention is realized for solving the problem of reverse salt flux and low water permeability typically observed in forward osmosis membranes. These two basic parameters, which constitute the biggest obstacle for widespread use of the forward osmosis membranes, are optimized thanks to the present invention.

The present invention enables performance improvement in parameters such as water permeability and reverse salt passage in production of thin film composite forward osmosis membranes. The present invention is based on producing a forward osmosis membrane by forming a nanofiber support membrane layer on a polyester nonwoven material specifically using sulfonated polysulfone (sPSf) polymer, and by coating this sulfonated polysulfone nanofiber support membrane layer with a thin film composite of polyamide; and the performance improvements of this membrane structure. Sulfonation process is carried out by using trimethyl sulfonate. Electrospinning method is used for obtaining nanofibers from sulfonated polymers. In order to produce thin composite polyamide film coating, the nanofiber support membrane layer is first immersed in an aqueous

solution of MPD (m-phenylenediamine) solution, and then is processed with TMC (trimesoyl chloride) dissolved in hexane. Thus, polymerization process is performed on the membrane surface. sPSf nanofiber support membrane layers have a wide variety of areas of use such as water and waste water treatment, mining, and energy sectors including petroleum and natural gas. On the other hand, the forward osmosis membrane structure obtained in the scope of the present invention is designed particularly for desalination of sea water. Additionally, it is also suitable for energy production via pressure-retarded osmosis technology.

The forward osmosis membrane of the present invention can be easily applied and can be used for providing water and treating waste water for both residential areas and the industry. Thus waste water is not only treated but it can also be recovered and used as a product.