A FILTER AND A METHOD OF MAKING A FILTER

Abstract

The present invention relates to a filter and a method of making a filter. The filter includes a porous substrate and a graphene oxide membrane and can be used to filter fluids. The graphene oxide membrane includes a crosslinking additive that reduces degradation of the graphene oxide membrane on exposure to chlorine.

Claims

1-50. (canceled)

51. A filter including a porous substrate and a graphene oxide membrane on the porous substrate, wherein the graphene oxide membrane includes a crosslinking additive between graphene oxide sheets which reduces degradation of the graphene oxide membrane by chlorine.

52. The filter according to claim 51, wherein the crosslinking additive is selected from a group comprising: i) a polymer having at least one epoxide group, ii) a molecule having at least one epoxide group, iii) a cationic polymer having at least one quaternary ammonium group, iv) a polymer having at least two amine groups, or v) a molecule having at least two amine groups.

53. The filter according to claim 51, wherein the crosslinking additive includes an epoxide group for reacting with the graphene oxide.

54. The filter according to claim 51, wherein the crosslinking additive includes multiple epoxide groups, including diepoxide; preferably the multiple epoxide groups include: poly(ethylene glycol) diglycidyl ether, 1,4-butanediol diglycidyl ether, and poly(dimethylsiloxane) that is diglycidyl ether terminated

55. The filter according to claim 51, wherein the crosslinking additive has an epoxide group and an alkoxysilane group for reacting with the graphene oxide; preferably the crosslinking additive having an epoxide group and a hydrolysable silanol group is 3-glycidyloxypropl trimethoxy silane, which is also known as glymo.

56. The filter according to claim 51, wherein the crosslinking additive includes a cationic polymer, and preferably wherein the cationic polymer has cationic functionality provided by a quaternized nitrogen atom and preferably wherein the quaternized nitrogen atom is a quaternary ammonium group.

57. The filter according to claim 56, wherein the cationic polymer includes at least one of: cationic polyvinyl alcohol, cationic polyacrylamide, cationic poly-urea-ammonium-ether, cationic hydroxyethyl cellulose, cationic guar, cationic polyDADMAC.

58. The filter according to claim 56, wherein the cationic polymer is a modified polyvinyl alcohol incorporating one or more quaternary ammonium groups, such as GOHSENX K.

59. The filter according to claim 56, wherein the cationic polymer includes a polymer compound with at least one quaternized ammonium structure within the principal chain, in which the quaternized ammonium structure includes: pyridinium, piperidinium, piperazinium and aliphatic ammonium, and preferably wherein the cationic polymer includes a cationic polyacrylamide which includes one or more cationic monomers such as acryloyloxyethyltrimethyl ammonium halide (preferably chloride; DAC), methyacryloyloxyethyltrimethyl ammonium halide (preferably chloride; DMC), or diallyl dimethyl ammonium halide; preferably chloride (DADMAC).

60. The filter according to claim 51, wherein the filter includes an adhesive additive for adhering the graphene oxide membrane to the porous substrate, and wherein the adhesive additive is resistant to chlorine degradation, and preferably wherein the adhesive additive includes cationic functionality provided by a quaternized nitrogen atom, and preferably wherein the quaternized nitrogen atom is a quaternary ammonium group.

61. The filter according to claim 60, wherein the cationic polymer is selected from a group including cationic polydiallyldimethylammonium chloride (polyDADMAC) and cationic polyvinylalchohol (such as GOHSENX K).

62. The filter according to claim 51, wherein the crosslinking additive is a cationic polymer selected from cationic polyacrylamide, cationic poly urea-ammonium-ether, cationic hydroxyethyl cellulose, Cationic guar and cationic polyDADMAC, and when exposed to 10,000 ppm.Math.h of chlorine, the filter has rejection values ranging from 90% to 98.5% under alkaline conditions when measured using the probe molecule Rose Bengal in the cross-flow apparatus, and preferably wherein the filter has permeance values ranging from 13.7 to 29.8 (L/m.sup.2/h/bar) when measured using the probe molecule Rose Bengal in the cross-flow apparatus, and preferably the filter has rejection value ranging from 86.9% to 99.2% under acid conditions when measured using the probe molecule Rose Bengal in the cross-flow apparatus.

63. The filter according to claim 51, wherein the filter includes an adhesive additive for adhering the graphene oxide membrane to the porous substrate, and wherein the adhesive additive is resistant to chlorine degradation, and the adhesive additive is selected from a group including cationic polydiallyldimethylammonium chloride (polyDADMAC) and cationic polyvinylalchohol (such as GOHSENX K).

64. The filter according to claim 51, wherein the filter has a crosslinking additive including a cationic polymer and an adhesive additive including GOHSENX K, and when exposed to 10,000 ppm.Math.h of chlorine, the filter has a permeance value ranging from 13.7 to 29.8 L/m2/h/bar and a rejection value ranging from 90.7% to 98.5% under alkaline conditions when measured using the probe molecule Rose Bengal in the cross-flow apparatus, and preferably the filter has a permeance value ranging from 11.0 to 23.8 L/m2/h/bar and a rejection value ranging from 86.6% to 99.5% under acidic conditions when measured using the probe molecule Rose Bengal in the cross-flow apparatus.

65. The filter according to claim 51, wherein the filter has a crosslinking additive including a diamine polymer with at least two reactive amine groups, and an adhesive additive for adhering the membrane to the porous substrate.

66. The filter according to claim 51, wherein the porous substrate is selected from the group comprising a metallic substrate, a ceramic substrate, or a polymeric substrate such as polyvinylidene difluoride.

67. A method of making a filter having a porous substrate and a graphene oxide membrane, wherein the method includes: applying a composition containing graphene oxide sheets to the porous substrate to form the graphene oxide membrane, and incorporating a crosslinking additive in the graphene oxide filter membrane to form crosslinks between graphene oxide sheets to reduce degradation on exposure to halides such as chlorine.

68. The method according to claim 67, wherein the method includes selecting the crosslinking additive and the crosslinking additive is selected from a group comprising: i) a polymer having at least one epoxide group, ii) a molecule having at least one epoxide group iii) a cationic polymer having at least one quaternary ammonium group, iv) a polymer having at least two amine groups, or v) a molecule having at least two amine groups.

69. The method according to claim 67, wherein the crosslinking additive includes a cationic polymer having a quaternary ammonium group, and preferably wherein the cationic polymer includes at least one of: cationic polyvinyl alcohol, cationic polyacrylamide, cationic poly-urea-ammonium-ether, cationic hydroxyethyl cellulose, cationic guar, cationic polyDADMAC.

70. The method according to claim 67, wherein the step of incorporating the crosslinking additive in the graphene oxide filter membrane includes a post treatment step of applying the crosslinking additive to the graphene oxide membrane once dried after the membrane has been applied to the porous substrate, and preferably wherein the post treatment step includes activating the crosslinking additive to complete crosslinking between the graphene oxide sheets, and preferably wherein activating the crosslinking additive includes heating the substrate and the membrane on the substrate above 50 C. for at least 1 hour, and to a temperature of 75 C. for at least 2 hours, and preferably wherein activating the crosslinking additive includes applying a catalyst, such as aluminium acetylacetonate to the graphene oxide membrane, and preferably wherein the step of incorporating the crosslinking additive in the graphene oxide filter membrane may include adding the crosslinking additive to a suspension of graphene oxide sheets prior to the suspension being applied to the porous substrate, and preferably wherein the method includes applying an adhesive additive to a porous substrate to facilitate adhesion of the graphene oxide membrane to the substrate, and preferably wherein the adhesive additive is cationic polymer having a quaternary ammonium group, wherein the adhesive additive is a cationic polyvinylalchohol which is commercially available under the trade name GOHSENX K Series from Mitsubishi Chemical, and preferably wherein the method includes the steps of i) adding a modifying agent to a composition containing the graphene oxide feed suspension and ii) modifying the graphene oxide by mixing the composition under elevated temperature conditions so that the modifying agent reacts with the graphene oxide sheets to create imperfections in the graphene oxide sheets, wherein progress of the reaction is stopped by reducing the temperature of the composition to stabilise the composition so that the composition can be applied to a substrate to form a graphene oxide filtration membrane, and preferably wherein the step of adding the modifying agent includes adding the modifying agent to the graphene oxide mass in the feed suspension is in a range of less than or equal to 15 to 1, 12 to 1, 10 to 1, 9 to 1, 8 to 1, 7 to 1, 6 to 1, 5 to 1, 4 to 1, 3 to 1, or 2 to 1, and preferably wherein the step of modifying the graphene oxide includes heating the composition to a temperature in the range of from 50 C. to about 200 C., or preferably about 80 C. to about 150 C., or preferably in the range of 50 to 98 C., and ideally in the range of 80 to 90 C. whilst mixing wherein the step of mixing the composition occurs for a period of 0.5 hr to 7 hrs, and suitably for a period from 1 hr to 6 hrs, and preferably wherein the step of modifying the graphene oxide is carried out for at least 1 to 5 hours after a maximum in viscosity of the composition has occurred, and preferably from 3 to 5 hours after a maximum in viscosity of the composition, and preferably wherein the modifying agent is selected from a group including hydrogen peroxide, peracetic acid, benzoyl peroxide, sodium perborate, ammonium hydroxide, or alkali hydroxides such as sodium hydroxide and potassium hydroxide.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0128] Embodiments of the present invention will now be described with reference to the accompanying Figures which may be summarised as follows.

[0129] FIG. 1 is a block diagram of the steps of preparing a graphene oxide composition and a filter including a substrate and the composition for filtering fluids according to a preferred embodiment.

[0130] FIG. 2 is a block diagram of the steps of preparing a graphene oxide composition and a filter including a substrate and the composition for filtering fluids according to another embodiment.

[0131] FIG. 3 is a block diagram of the steps of selecting a suitable graphene oxide suspension and making a filter including a substrate and the suspension for filtering fluids according to another embodiment.

[0132] FIG. 4 is a block diagram of the steps of preparing a graphene oxide suspension and the making a filter including a substrate and a composition include the suspension according to another embodiment.

[0133] FIG. 5 is a schematic cross-sectional view of the filter according to an embodiment that can be made in accordance with the steps outlined in FIGS. 1 to 4.

DESCRIPTION OF THE DRAWINGS

[0134] FIG. 1 is a block diagram of an embodiment for making a filter 200 (shown in FIG. 5) including the graphene oxide filtration membrane 90. FIG. 1 has steps that include modifying the substrate 110 in step 70 by applying an adhesive agent 100 to a substrate 110 to assist in adhering/bonding a graphene oxide filtration membrane 90 to the substrate 110, and applying a crosslinking additive 120 to the dried graphene oxide filtration membrane 90 in a post treatment step 50. The crosslinking additive 120 provides a level of chlorine resistance to the graphene oxide filtration membrane 90. The suitability of a graphene oxide feed suspension that can be included in a composition that can be printed or applied to form a graphene oxide filtration membrane 90 on the substrate 110 is a function of many variables. One possible variable is the level or concentration of impurities in the feed suspension. Step 10 of FIG. 1 can including selecting a graphene oxide suspension, and depending on the known properties of the suspension, step 10 can include the optional steps of assessing the level of impurities in the feed suspension and one method for doing so is measuring the electrical conductively of the feed suspension. Step 10 of FIG. 1 may also include an optional step of combining dry graphite oxide with water and mixing, or combining dry graphene oxide with water and mixing. For example, a feed suspension may be prepared by combining a graphene oxide cake containing 435 wt % graphene oxide with water and mixing. Water may be added to the feed suspension to adjust the graphene oxide concentration into a range from 0.1 to 15 wt %.

[0135] Step 20 of FIG. 1 is optional can include treating or modifying the feed suspension to create imperfections in the graphene oxide. Specifically, Step 20 includes adding an active modifying agent to the feed suspension to form a composition, and mixing the composition under elevated temperature and then reducing the temperature of the composition to stop progress of the reaction between the modifying agent and the graphene oxide so that the composition becomes stable. In this situation all of, or nearly all of, the modifying agent may have reacted with the graphene oxide sheets. Step 20 can include determining the amount of modifying agent(s) to be added to the feed suspension, including determining a desired mass ratio of modifying agent to the graphene oxide sheets in the feed suspension. In addition, Step 20 can be carried out under heated conditions, which may or may not be accompanied by pressurised conditions, to increase the reaction rate between the modifying agent(s) and the graphene oxide. The period of mixing the composition may be reduced by increasing the temperature and pressure conditions of Step 20. Conversely the period of mixing the composition may be increased by lowering the temperature to ambient temperature.

[0136] In a situation in which the modifying agent comprises hydrogen peroxide in isolation with no other modifying agent, we have found that a 30 wt % hydrogen peroxide solution can be added to the feed suspension at a ratio of equal to or less than 5 parts hydrogen peroxide to 1 part feed suspension. In this embodiment the suspension comprises approximately 1 mg/ml of graphene oxide. When used in this range, the hydrogen peroxide is reacted with the graphene oxide for a sufficient reaction period. These conditions provide a graphene oxide with the required permeance, avoids subsequent separation of excess modifying agent from the feed suspension, and can be readily scaled up into commercial operating batches. Step 20 may be carried out in a batch operation.

[0137] Step 20 of FIG. 1 may include determining when the reaction between the modifying agent and the graphene oxide has progressed far enough by measuring, for example, any one or a combination of the viscosity of the composition, colour and colour changes of the composition, and infrared spectrum analysis of the composition. The reaction between the modifying agent and the graphene oxide may also produce a gas, such as carbon dioxide, and determining whether the reaction has progressed far enough may be determined by measuring the amount of the gas produced during the mixing step.

[0138] The composition may then be treated in Step 30 of FIG. 1 to facilitate the composition being printed by a microgravure printing machine or other application methods. Properties of the composition, such as viscosity and surface tension may then be measured and adjusted in Step 30 to make the composition suitable for printing. For example, the surface tension may be controlled using surfactants such as Triton available from DOW Inc. In another example, the viscosity of the composition may also be controlled by adding polyphenol and/or ethanol. Adjusting the viscosity and/or the surface tension enables the composition to be applied as a filtration membrane using high speed printing machinery, such as gravure printing or microgravure printing. In other words, not only is the composition suitable for making filtration membranes, but in addition, the composition can be applied at a manufacturing speed at a large scale.

[0139] One of the benefits of the method shown in FIG. 1 is that no crosslinking materials need to be added to the composition prior to applying the composition to the substrate.

[0140] Step 60 of FIG. 1 can include selecting the porous substrate 110 such as a porous film including polymeric, metallic or ceramic films. Selecting an appropriate substrate 110, is typically based on the desired permeance and flexibility.

[0141] Step 70 of FIG. 1 can include modifying the substrate 110 to improve the adhesion of the graphene oxide membrane 90 to the porous substrate. For example, the adhesive additive 100, such as either one or a combination of, adhesives such as polyDADMAC and GOHSENXK can be applied to the substrate in Step 70, and dried. The rate at which the adhesive additive 100 dries may be increased by heating once the adhesive additive 100 has been applied to the substrate 110. The adhesive additive 100 may be applied using dip coating, printing, spraying, or other suitable techniques.

[0142] Step 40 of FIG. 1 can include printing or applying the composition prepared in Step 30 to the substrate 110 to form the filtration membrane 90. This may be done by a gravure printing machine such as a micro-gravure printing machine, or other techniques such as dip coating, rod coating, knife coating, blade coating, vacuum filtration or spraying to form a membrane of the composition on the substrate. Step 40 may include drying the composition which may be done in ambient conditions. However ideally, the rate at which the composition dries may be increased by exposure to radiant heat or by convection, for example, by means of a stream of heated air. The adhesion of the composition to the substrate is less prone to degradation as a result of the adhesive additive applied in Step 70.

[0143] Step 50 of FIG. 1 can include a post treatment of the filtration membrane by applying a crosslinking additive 120 to the dried graphene oxide membrane 90. The crosslinking additive 120 may include a cationic polymer, suitably having quaternary ammonium functionality such as polyDADMAC, or epoxide based crosslinking additive, such as GLYMO. The crosslinking additive 120 provides a level of chlorine resistance and pH resistance to the graphene oxide membrane. The term pH resistance refers to the rejection of the filtration membrane being less variable over a broader pH range than if no crosslinking additive 120 was added to the composition. The term chlorine resistance refers to the resistance of the membrane to degradation caused by sodium hypochlorite, or other hypochlorite salt.

[0144] Step 50 of FIG. 1 can also include a permeance-enhancing or permeance setting step by exposing the membrane 90 to a solution of sodium hypochlorite for a period of time. For example, the membrane 90 can be submerged in a 10 g/L solution of sodium hypochlorite for a period of 30 minutes (5,000 ppm.Math.h) to increase the permeance.

[0145] FIG. 2 is a block diagram of an embodiment for making a filter 200 including the graphene oxide filtration membrane 90. The embodiment includes modifying a substrate 110 in step 70 by applying an adhesive additive 100 to a substrate 110 to assist in adhering a graphene oxide filtration membrane 90 to the substrate 110, and adding a crosslinker additive 120 to a composition containing the graphene oxide prior to the composition being applied to the substrate 110, and optionally activating of the crosslinker 120 in a post-treatment step after the composition has been applied to the substrate to provide chlorine resistance, and some pH resistance. Step 10 of FIG. 2 can include the same procedures described above in relation to Step 10 of FIG. 1. Ideally, the feed suspension has or is adjusted to have a graphene oxide concentration into a range of 0.1 to 15 wt %.

[0146] Step 20 of FIG. 2 can include the same procedures described above in relation to Step 20 of FIG. 1. That is Step 20 includes treating the feed suspension to create imperfections in the graphene oxide and form a composition. Step 20 can include adding an active modifying agent to the feed suspension to form the composition, and mixing the composition under elevated temperature and then reducing the temperature of the composition to stop progress of the reaction between the modifying agent and the graphene oxide to stabilise the composition. In this situation all of, or nearly all of, the modifying agent may have reacted with the graphene oxide. In addition, Step 20 of FIG. 2 can include determining whether the reaction can be stopped to form a stable composition.

[0147] For instance, Step 20 of FIG. 2 may include determining when the reaction between the modifying agent and the graphene oxide has progressed far enough by measuring, for example, any one or a combination of the viscosity of the composition, colour and colour changes of the composition, and infrared spectrum analysis of the composition. The reaction between the modifying agent and the graphene oxide may also produce a gas, such as carbon dioxide, and determining whether the reaction has progressed far enough may be determined by measuring the amount of the gas produced during the mixing step.

[0148] The composition may then be treated in Step 30 of FIG. 2 to facilitate the composition being printed using gravure printing machines such as microgravure printing machines, or other application methods. For instance, viscosity and surface tension may then be measured and adjusted in Step 30 by adding surfactants such as Triton available from DOW Inc. The viscosity of the composition may also be controlled by adding polyphenol and/or ethanol. Adjusting the viscosity and/or the surface tension enables the composition to be applied as a filtration membrane using high speed printing machinery, such as gravure printing including microgravure printing.

[0149] In addition, Step 30 of FIG. 2 can include step 80 of adding a crosslinking additive 120 to the composition such as either one or a combination of epoxide containing crosslinkers, such as GLYMO or GTAC (glycidyl trimethylammonium chloride). These crosslinking additive 120 provide stabilization to the rejection properties of the filtration membrane over a broader range of pH than if no crosslinking additive was included. At a structural level the crosslinkers manage the spatial separation between the graphene oxide sheets.

[0150] Step 60 of FIG. 2 can include the same procedures described above in relation to Step 60 of FIG. 1. Specifically, Step 60 can include selecting a porous substrate 110 such as a porous film including polymeric, metallic or ceramic films.

[0151] Step 70 of FIG. 2 can include modifying the substrate 110 to improve the adhesion of the graphene oxide membrane 90 to the porous substrate 110. For example, an adhesive additive 100, such as either one or a combination of, polymers such as polyDADMAC and GOHSENXK can be applied to the substrate in Step E, and dried. The rate at which the adhesive additive 100 dries may be increased by heating once the adhesive additive 100 has been applied to the substrate 110. The adhesive additive 100 may be applied using dip coating, printing, spraying, or other suitable techniques.

[0152] Step 40 of FIG. 2 can include the same steps as Step 40 described in relation to FIG. 1. For instance, Step 40 of FIG. 2 can include printing or applying the composition prepared in Step 30 to the substrate 110 to form a graphene oxide membrane 90 provided by the composition. This may be done by a gravure printing machine such as a microgravure printing machine, or other techniques such as dip coating or spraying to form a membrane of the composition on the substrate. Step F may include drying the composition which may be done in ambient conditions.

[0153] Step 50 of FIG. 2 can include a post treatment step in which a crosslinking additive including a cationic polymer, suitably having quaternary ammonium functionality, or epoxide based crosslinking additive, such as GLYMO. Step 30 may include activating the crosslinking additive 120 to complete the crosslinking between the graphene oxide sheets. In one example, activation can be carried out by heating the substrate and composition on the substrate above 50 C. for at least 1 hour, and suitably to a temperature of 75 C. for at least 2 hours. In another example, a catalyst such as aluminium acetylacetonate (1 g/L in 2-propanol) can be applied to the graphene oxide membrane by dip-coating, gravure printing, microgravure printing, or rod coating. For dip-coating, a coupon of the graphene oxide membrane 120 was submerged in a bath of the catalyst solution for a period of 5 minutes, after which the coupon was removed from the bath and dried at ambient temperature without washing. Step 50 of FIG. 2 can also include a permeance-enhancing step of exposing the membrane 90 to a solution of sodium hypochlorite for a period of time. For example, the membrane 90 can be submerged in a 10 g/L solution of sodium hypochlorite for a period of 30 minutes (5,000 ppm.Math.h) to increase the permeance.

[0154] FIG. 3 is a block diagram of an embodiment for making a filter 200 including the graphene oxide filtration membrane 90. The embodiment includes selecting a composition that can be applied to, including printed on, a substrate 110 to form the graphene oxide filtration membrane 90. FIG. 3 has particular steps that include modifying the substrate in step 70 by applying an adhesive additive 100 to a substrate 110 to assist in adhering the graphene oxide filtration membrane 90 to the substrate 110, and applying a crosslinking additive 120 to the dried graphene oxide filtration membrane 90 in a post treatment step 50. The crosslinking additive 120 provides a level of chlorine resistance to the graphene oxide filtration membrane. Similarly, the adhesive additive 100 also has chlorine resistance.

[0155] Step 10 of FIG. 3 includes selecting a graphene oxide suspension for use as composition to form a graphene oxide filter membrane 90. Depending on the known properties of the graphene oxide suspension, Step 10 may optionally also include adjusting the concentration of the graphene oxide to a range from 0.1 to 15 wt %, and optionally adjusting the viscosity and surface tension of the suspension. Step 10 of FIG. 3 does not include treatment of the graphene oxide sheet with the oxidising agent which occurs in Step 20 of FIGS. 1 and 2.

[0156] Step 60 of FIG. 3 can include the same procedures described above in relation to Step 60 of FIG. 1. Specifically, Step 60 can include selecting a porous substrate 110 such as a porous film including polymeric, metallic or ceramic films.

[0157] Step 70 of FIG. 3 includes modifying the substrate 110 to improve the adhesion of the graphene oxide membrane 90 to the substrate 110. For example, an adhesive additive 100, such as either one or a combination of, polyDADMAC and GOHSENXK can be applied to the substrate 110 in Step 70, and dried. The rate at which the adhesive additive 100 dries may be increased by heating once the adhesive additive 100 has been applied to the substrate 110. The adhesive additive 100 may be applied using dip coating, printing, spraying, or other suitable techniques.

[0158] Step 40 of FIG. 3 includes applying, including printing, the composition prepared in Step 30 to the substrate 110 to form a graphene oxide membrane 90. This may be done by a gravure printing machine such as a microgravure printing machine, or other techniques such as dip coating or spraying so as to form a membrane of the composition on the substrate. Step F may include drying the composition which may be done in ambient conditions or by using heating or ventilation means.

[0159] Step 50 of FIG. 3 includes an optional step of treating the graphene oxide membrane 90, suitably after the graphene oxide membrane 90 has been dried, and hence it may be referred to a post-treatment step. The post treatment step may include applying a crosslinking additive 120 including a cationic polymer, suitably having quaternary ammonium functionality, or epoxide based crosslinking additive, such as GLYMO. Step 30 may include activating the crosslinking additive 120 to complete crosslinking between the graphene oxide sheets. Optionally, Step 50 may also include element of Step 50 in FIGS. 1 and 2.

[0160] FIG. 4 is a block diagram of the steps of preparing a graphene oxide suspension and the making a filter 200 including a substrate 110 and a composition include the suspension. Step 10 of FIG. 1 can including selecting a graphene oxide suspension, and depending on the known properties of the suspension, step 10 can include the optional steps preparing a suitable feed suspension by assessing the level of impurities in the feed suspension by measuring the electrical conductively of the feed suspension. Step 10 of FIG. 1 may also include an optional step of preparing a suitable feed suspension by combining dry graphite oxide with water and mixing. Water may be added to the feed suspension to adjust the graphene oxide concentration into a range from 0.1 to 15 wt %. FIG. 4 includes Step 80 of adding crosslinking additives to the suspension such as either one or a combination of epoxide containing crosslinkers, such as GLYMO or GTAC (glycidyl trimethylammonium chloride) to form a composition. These crosslinking additives provide stabilization to the rejection properties of the filtration membrane over a broader range of pH than if no crosslinking additive was included.

[0161] Step 60 of FIG. 4 can include the same procedures described above in relation to Step 60 of FIG. 1. Specifically, Step 60 can include selecting a porous substrate 110 such as a porous film including polymeric, metallic or ceramic films. Although not shown, FIG. 4 may include the optional step of modifying the substrate 110 to improve the adhesion of the graphene oxide to the porous substrate. For example, an adhesive additive 100, such as either one or a combination of, adhesives such as polyDADMAC and GOHSENXK can be applied to the substrate in Step 70, and dried.

[0162] Step 40 of FIG. 4 includes printing or applying the composition to the substrate 110 to provide a graphene oxide membrane 90. This may be done by a gravure printing machine such as a microgravure printing machine, or other techniques such as dip coating or spraying to form a membrane of the composition on the substrate. Step 50 may include drying the composition which may be done in ambient conditions or by using heating or ventilation means.

[0163] Step 50 of FIG. 4 includes an optional step of treating the graphene oxide membrane 90, suitably after the graphene oxide membrane 90 has been dried, and hence it may be referred to a post-treatment step. The post treatment step may include applying a crosslinking additive 120 including a cationic polymer, suitably having quaternary ammonium functionality, or epoxide based crosslinking additive, such as GLYMO. Step 30 may include activating the crosslinking additive 120 to complete crosslinking between the graphene oxide sheets. Optionally, Step 50 may also include element of Step 50 in FIGS. 1 and 2

[0164] FIG. 5 is a schematic cross-section view of the filter including a graphene oxide membrane 90 applied to a substrate 110. The substrate 110 can be selected in accordance with Step 60 and optionally modified in accordance with Steps 70 by applying an adhesive additive 100. The graphene oxide membrane 90 that is applied to the substrate 110 includes the suspension or composition of Step 10, and the composition may optionally be further characterised by the Steps 20 and 30. The composition applied to the substrate may include a crosslinking additive 120 which is suitably applied to a dried graphene oxide membrane 90 formed on the substrate 110 in accordance Step 50. Optionally, crosslinking additive 120 may also be added to the composition containing the graphene oxide prior to the composition being applied to the substrate 110, such as in Steps 30 and 80.

Examples

Example 1

[0165] In this example, a composition and a filtration membrane were made in accordance with the Steps of FIG. 1. The permeance and the rejection of this filtration membrane were measured, in accordance with the membrane performance test described below, to be 14 L/m.sup.2/h/bar and 96% respectively. These measurements are included in Table 3A.

[0166] A composition containing modified graphene oxide (8.6 g/L), i.e., the GO composition was prepared by reacting graphene oxide (10 g/L) with hydrogen peroxide (5:1 mass ratio H.sub.2O.sub.2:GO) for 6.0 h at 90 C. in an autoclave.

[0167] A porous substrate membrane, which was commercially available under the trade name Solecta PVDF 400, was prepared by printing a thin film of GOHSENXK onto the substrate using a microgravure printer to act as an adhesive additive. GOHSENXK was in the form of a 50% aqueous ethanol (4.3 g/L) solution. After drying, a thin film of the modified GO composition was applied to the treated porous membrane support using a microgravure printer at a density of approximately 0.1 g/m.sup.2. The modified GO composition contained 4.3 g/L of modified graphene oxide. After drying, a thin film of crosslinking additive comprising polyDADMAC (average M.sub.w<100,000, Sigma-Aldrich product code 522376) was applied at a density of approximately 0.01 g/m.sup.2 to the graphene oxide membrane using a microgravure printer, from a 50% aqueous ethanol solution containing 5 g/L polyDADMAC.

Example 2

[0168] In this example, a composition and a filtration membrane were made in accordance with the Steps of FIG. 2. The permeance and the rejection of this filtration membrane were measured, in accordance with the membrane performance test described below, to be 27 L/m.sup.2/h/bar and 95% respectively. These measurements are included in Table 3A.

[0169] A composition containing modified graphene oxide (8.6 g/L) was prepared by reacting graphene oxide (10 g/L) with hydrogen peroxide (5:1 mass ratio H.sub.2O.sub.2:GO) for 6.0 h at 90 C. in an autoclave. 100 ml of the modified graphene oxide suspension was diluted to 5 g/L by adding 70 mL of 2-propanol. 0.43 g of glymo, (3-glycidyloxypropyl)trimethoxysilane, was added to the diluted suspension (dropwise), while the suspension was being vigorously stirred.

[0170] A porous membrane substrate was prepared by printing a thin film of an adhesive additive comprising GOHSENXK onto a substrate at a density of approximately 0.1 g/m.sup.2. The substrate comprised Solecta PVDF 400 and an aqueous solution of GOHSENXK (5 g/L) was applied using a microgravure printer. After drying, a thin film of the modified graphene oxide composition was applied to the treated porous membrane substrate using a microgravure printer at a density of approximately 0.09 g/m.sup.2. After drying, the membrane was immersed in a solution of aluminium acetylacetonate (1 g/L in 2-propanol) for 5 minutes which acts as a catalyst for the glymo to form crosslinks between the graphene sheets. The membrane was then removed from the catalyst solution and dried at ambient temperature without washing.

Example 3

[0171] In this example, a composition and a filtration membrane were made in accordance with the Steps of FIG. 1.

[0172] A graphene oxide membrane can also be formed on a hollow fibre porous support, such as an ultrafiltration or microfiltration hollow fibre made of polyvinylidene fluoride, polypropylene, polyacrylonitrile, polysulphone, or ceramic.

[0173] The fibre is dipped in an adhesive additive comprising GOHSENXK (4 g/L in 50% aqueous ethanol) for 1 minute, then dried for 1 hour in a dehydrator at 70 C. (application of adhesive).

[0174] A composition containing modified graphene oxide (8.6 g/L) was prepared by reacting graphene oxide (10 g/L) with hydrogen peroxide (5:1 mass ratio H.sub.2O.sub.2:GO) for 5.5 h at 90 C. in an autoclave, then diluting to 1 g/L graphene oxide with water. The adhesive-coated substrate was then submerged in the composition and connected to a vacuum pump. A vacuum is applied to the adhesive-coated membrane until the permeate is colourless, indicating that a graphene oxide film had formed on the fibre substrate, preventing further passage of graphene oxide. The graphene oxide-coated fibre was then dried overnight at 70 C. in a dehydrator.

[0175] A crosslinking additive was then applied to the film of graphene oxide by being dipped in a solution of polyDADMAC (5 g/L in 50% aqueous ethanol) for 1 minute, then dried for 2 hours in a dehydrator at 70 C.

[0176] The treated hollow fibre was then submerged in a solution of Rose Bengal (200 mg/L) and a vacuum applied. The area of the fibre was unknown so the permeance could not be calculated, but the concentration of the feed and permeate showed that the rejection was 99%.

Filtration Membrane Performance Testing

[0177] We have tested the permeance and rejection of the filtration membranes mentioned in Examples 1 and 2 above, and several other samples of filtration membranes that have been made in accordance with the embodiments shown in FIGS. 1 and 2. Results of the tests are summarised in Tables 3A, 3B, 3C and 3D.

[0178] The testing procedure included flat samples of the filtration membranes being held in a cross-flow mode at 2 bar transmembrane pressure using Rose Bengal (4,5,6,7-tetrachloro-2,4,5,7-tetraiodofluorescein disodium salt) as the probe molecule (200-300 mg/L) at pH 9 (unless otherwise specified) using a Sterlitech CF042 cell. The membrane area was 42.1 cm.sup.2. Rejection was calculated from the concentration of Rose Bengal in the feed and the permeate according to the following equation:

[00001]$\begin{array}{c}\mathrm{Rejection}=\left[1-\left({\mathrm{RB}}_{permeate}/{\mathrm{RB}}_{\mathrm{feed}}\right)\right]100\%\\ \mathrm{Where}:\\ {\mathrm{RB}}_{\mathrm{feed}}=\mathrm{concentration}\mathrm{of}\mathrm{Rose}\mathrm{Bengal}\mathrm{in}\mathrm{the}\mathrm{feed}\\ {\mathrm{RB}}_{permeate}=\mathrm{concentration}\mathrm{of}\mathrm{Rose}\mathrm{Bengal}\mathrm{in}\mathrm{the}\mathrm{permeate}\end{array}$

[0179] The concentration of Rose Bengal was measured by UV-visible spectrophotometry, using the Beer-Lambert Law to convert the absorbance at 549 nm to concentration in mg/L.

[0180] The flow rates of the feed and the permeate were measured gravimetrically for calculation of the permeance using the following equation:

[00002]$\begin{array}{c}\mathrm{Permeance}\left(L/m^2/h/\mathrm{bar}\right)=V_{permeate}/\left(AtP\right)\\ \mathrm{Where}:\\ V=\mathrm{volume}\mathrm{of}\mathrm{permeate}\left(L\right)\\ A=\mathrm{area}\mathrm{of}\mathrm{membrane}\left(m^2\right)\\ t=\mathrm{time}\left(h\right)\mathrm{during}\mathrm{which}\mathrm{permeate}\mathrm{was}\mathrm{collected}\\ P=\mathrm{transmembrane}\mathrm{pressure}\left(\mathrm{bar}\right)\end{array}$

[0181] NaOCl post-treatment was carried out by exposing membrane coupons to a 10000 mg/L solution of NaOCl for 30 minutes.

[0182] Chlorine resistance was evaluated by exposing membrane coupons to a solution of NaOCl at the specified pH for a period of time. For example, 10,000 ppmh=1 hour of exposure to a 10000 mg/L NaOCl solution. Following chlorine exposure, the membrane coupon was rinsed with deionised water and then tested using Rose Bengal as above.

TABLE-US-00003 TABLE 3A Rejection of Rose Bengal with various combinations of adhesive and crosslinker Made in Adhesive additive Crosslinking Permeance Rejection accordance Sample Substrate added to substrate additive L/m.sup.2/h/bar % with: 1a Solecta GOHSENX-K polyDADMAC 14 96% FIG. 1 PVDF400 2a Solecta GOHSENX-K glymo 27 95% FIG. 2 PVDF400 3a Solecta polyDADMAC glymo 23 99% FIG. 2 PVDF400 4a Solecta polyDADMAC GOHSENX-K 34 98% FIG. 1 PVDF400 5a Synder Bx GOHSENX-K polyDADMAC 12 99% FIG. 1

[0183] The measurements in Table 3A shows that the filtration membranes made in accordance with FIGS. 1 and 2 with combinations of adhesive and crosslinking additives have viable rejections of equal to, or greater than 90%, and in the range from 96% to 99%. In addition, the permeance is measured to range from 12 to 34 L/m.sup.2/h/bar.

TABLE-US-00004 TABLE 3B Improved permeance after NaOCl post-treatment and maintaining high rejection Made in Adhesive additive Crosslinking Post- Permeance Rejection accordance Sample Substrate added to substrate additive treatment L/m2/h/bar % with 7a Solecta GOHSENX-K glymo None 27 95% FIG. 2 PVDF400 7b Solecta GOHSENX-K glymo NaOCl 37 99% FIG. 2 PVDF400 8a Solecta GOHSENX-K polyDADMAC None 14 96% FIG. 1 PVDF400 8b Solecta GOHSENX-K polyDADMAC NaOCl 31 94% FIG. 1 PVDF400 9a Solecta polyDADMAC glymo None 14 98% FIG. 2 PVDF400 9b Solecta polyDADMAC glymo NaOCl 20 94% FIG. 2 PVDF400 10a Synder Bx GOHSENX-K polyDADMAC None 12 99% FIG. 1 10b Synder Bx GOHSENX-K polyDADMAC NaOCl 35 93% FIG. 1

[0184] Table 3B includes measurements that compare the filtration membranes without a post treatment and filtration membranes that include a bleach post treatment. The combinations of substrate, adhesive additive and crosslinking additive show that the filtration membranes were resistant to the chlorine in the bleach post treatment, whilst maintaining viable rejection rates of greater than 90%. In addition, the bleach post treatment was shown to increase the permeance of the filtration membranes.

TABLE-US-00005 TABLE 3C Stable rejection and permeance after exposure to bleaching solution Exposure Made in Porous Adhesive Crosslinking to NaOCl Permeance Rejection accordance Sample Substrate additive additive pH ppm .Math. h L/m2/h/bar % with 13a Solecta GOHSENX-K polyDADMAC 5.5 0 12 97% FIG. 1 13b PVDF400 10000 14 96% FIG. 1 13c 20000 17 95% FIG. 1 13d 50000 14 93% FIG. 1 13e 100000 26 91% FIG. 1 14a 12 0 12 97% FIG. 1 14b 10000 20 96% FIG. 1 14c 20000 40 93% FIG. 1 14d 50000 26 95% FIG. 1 14e 120000 52 88% FIG. 1 15a polyDADMAC glymo 5.5 0 23 96% FIG. 2 15b 25000 26 96% FIG. 2 15c 50000 28 94% FIG. 2 15d 75000 31 94% FIG. 2 15e 100000 32 94% FIG. 2 16a 12 0 24 97% FIG. 2 16b 25000 29 94% FIG. 2 16c 50000 33 90% FIG. 2 16d 75000 36 83% FIG. 2 16e 100000 42 78% FIG. 2

[0185] Filter samples 13a to 16e were tested for chlorine resistance, by being submerged in a bleach solution at a pH of either 5.5 or 12, which represent typical acidic and alkaline conditions in which the filter may be washed. Samples 13a, 14a, 15a and 16a were used as control samples that were not exposed to bleach. The performance of the samples after being in contact with bleach was tested to determine permeance and rejection using Rose Bengal as the probe molecule as outlined above for test performance procedure. It will be appreciated that the test procedure may be conducted using other probe molecules and similar results can be achieved. For instance, probe molecules having similar molecular weights in the range of 100 or 200 or 300 or 400 to 1,000, or 2,000 or 3,000 or 4000 can be used.

[0186] The samples were submerged in the bleach solution comprising 10,000 mg/L of NaOCl for various periods. For exposures of 10,000 ppm.Math.h, the samples were submerged for 1 hour, and similarly, for exposures of the 20,000 ppm.Math.h, 50,000 ppm.Math.h, 75,000 ppm.Math.h and 100,000 ppm.Math.h the samples were submerged for 2 hours, 5 hours, 7.5 hours and 10 hours respectively.

[0187] The results show for exposures of 100,000 ppm.Math.h of NaOCl the samples retained acceptable filtering properties for Rose Bengal, or molecules of equivalent size. Generally speaking, for exposures of up to 100,000 ppm.Math.h, rejections of equal to or greater than 78%, and suitably equal to or greater than 88%, or even more preferably equal to greater than 90% were measured. This represents an acceptable rejection of the probe molecules on the membrane.

[0188] In some examples, an acceptable rejection of at least 78% can be achieved on exposure to at least 100,000 ppm.Math.h of chlorine, and in addition an increase in permanence, or preferably a doubling in permeance can also be achieved. For instance, with reference to the samples comprising polyDADMAC adhesive additive and glymo crosslinking additive, sample 15e had a permeance of 32 L/m2/h/bar and a rejection of 94% at a pH of 5.5. Sample 16c had a permeance of 33 L/m2/h/bar and a rejection of 90% at a pH of 12.

[0189] With reference to the samples 13a to 13e comprising GOHSENX-K adhesive additive and polyDADMAC crosslinking additive, sample 13e had a permeance of 26 L/m2/h/bar and a rejection of 91% at a pH of 5.5. Sample 14d was exposed to 50,000 ppm.Math.h of NaOCl had a permeance of 26 L/m2/h/bar and a rejection of 95%.

[0190] The results show that the permeance of the filter may increase by equal to or less than 14 L/m2/h/bar when exposed to 100,000 ppm.Math.h of NaOCl under acid conditions. For instance, from 12 to 26 L/m2/h/bar as in case of samples 13a to 13e.

[0191] The results show that the permeance of the filter may increase by equal to or less than 40 L/m2/h/bar when exposed to 100,000 ppm.Math.h of NaOCl under alkaline conditions. For instance, from 12 to 52 L/m2/h/bar as in the case of samples 14a to 14e.

[0192] Table 4 includes samples 17 to 23 of membranes that were prepared in accordance with the procedure outlined in Example 1 and FIG. 1 and test for pH resistance. The membranes were prepared by coating a porous substrate comprising PVDF from TOMAC Corporation (Japan) with an adhesive additive comprising a GOHSENXK aqueous solution. A thin film of the adhesive additive, at a density of approximately 0.1 g/m.sup.2, was applied to the substrate using a microgravure printer and dried. A thin film of the graphene oxide composition was then applied at a density of approximately 0.09 g/m.sup.2 using a microgravure printer. The membranes were then dip coated with aqueous solutions of various crosslinking additives as shown in Table 4 at a density of approximately 0.001 g/m.sup.2, and the permeance and rejection of the membranes were tested under particular pH conditions. No crosslinking additive was added to sample 24 which provided a Control.

[0193] The membranes were tested for pH resistance in the cross-flow apparatus Sterlitech CF042, using a solution of Rose Bengal, at pH 4, pH 7 and pH 10. The permeance and the rejection were measured. The data set out in Table 4 shows that all of the cationic polymers tested as crosslinking additives produced membranes with higher rejection than the Control that had no crosslinking additive at each of the pH values. That is to say, the crosslinking additives had a pronounced effect on the ability of the membranes to continue to perform under various pH conditions, specifically from pHs values ranging from 4 to 10. The difference was most pronounced at pH 10, which is indicative of layers of the graphene oxide sheets swelling, which decreases rejection. The permeance increased as the rejection decreased.

TABLE-US-00006 TABLE 4 Stable performance of membranes including cationic polymers as crosslinkers Crosslinking additive Conc Permeance (L/m.sup.2/h/bar) Rejection (%) Sample Trade name Class/Description (g/L) pH 4 pH 7 pH 10 pH 4 pH 7 pH 10 17 SINOFLOC 680* Cationic 1 6.7 8.1 10.9 96.3% 95.0% 91.9% polyacrylamide 18 Polyquaternium-2* Cationic poly urea- 5 6.9 8.6 10.0 99.3% 99.1% 98.4% ammonium-ether 19 Hydroflux HB-2705* Cationic 1 9.0 10.4 12.9 98.6% 97.9% 95.8% polyacrylamide 20 Polyquaternium-10* Cationic hydroxyethyl 1 11.0 13.7 16.3 95.8% 92.9% 76.7% cellulose 21 Jaguar Optima* Cationic guar 1 11.6 16.1 19.8 91.9% 86.4% 65.5% 22 Polyquaternium-6* Cationic polyDADMAC 5 13.7 17.5 25.1 99.5% 99.1% 98.8% 23 PEI Polyamine 5 9.8 12.8 17.6 98.7% 97.6% 95.7% 24 N/A Control n/a- 15.8 18.8 34.3 90.0% 62.1% 36.1% *indicates quaternary functionality

[0194] The membranes of the samples 17 to 23 were prepared in accordance with Example 1 and FIG. 1, and a Control, sample 24, was also tested for chlorine resistance. A First set of the samples 17 to 24 were treated with a solution of sodium hypochlorite (5 g/L) at pH 4 or at pH of 10 for 2 hours, that is 10,000 ppm.Math.h of NaOCl. The membranes were then rinsed with deionised water to remove residual NaOCl, and then tested in the cross-flow apparatus using a solution of Rose Bengal, at pH 4 and pH 10. For comparison, another set of the membranes, namely the Second Set of samples 17 to 23 were made in accordance with Example 1 and FIG. 1, and Control 24 was prepared and tested without being exposed to chlorine, that is 0 ppm.Math.h of NaOCl. The permeance and rejection of the Second Set of samples 17 to 24 were tested in the cross-flow apparatus using a solution of Rose Bengal at pH 4 and pH 10.

[0195] Table 5 comprises performance data of the First and Second Sets of samples 13 to 24 at a pH of 10. The data of the samples having a cationic polymer as crosslinking additive produced membranes with higher rejection than the Control which had no crosslinking additive after exposure to chlorine, such as sodium hypochlorite. The data also showed that the membranes having cationic polymers with quaternary ammonium groups as crosslinking additives produced membranes with improved resistance to degradation by chlorine. Specifically, the first set of samples 1 to 13 had permeance values after exposure to 10,000 ppm.Math.h of NaOCl ranging from 13.7 to 29.8 (L/m.sup.2/h/bar) when measured using the probe molecule Rose Bengal in the cross-flow apparatus at a pH of approximately 10, ie under alkaline conditions. Under alkaline conditions, ie a pH of approximately 10, the rejection was greater than 90% with a cross linking additive selected from cationic polyacrylamide, Cationic poly urea-ammonium-ether, Cationic polyacrylamide, polyDADMAC. By comparison, the permeance of the second set of samples 13 to 30 which were not exposed to chlorine had a permeance values ranging from 10.9 to 25.1 (L/m.sup.2/h/bar). That is to say, the permeance, of the membranes, after exposure to 10,000 ppm.Math.h of NaOCl increased in range from 2.8 to 5.3 (L/m.sup.2/h/bar) according to the Rose Bengal cross flow under alkaline conditions, whilst rejection ranged from 45.1% to 98.5%, and suitably 61.8% to 98.5%, and even more suitably from 90% to 98.5%.

TABLE-US-00007 TABLE 5 Chlorine resistance of membranes with quaternary ammonium crosslinkers at pH 10. Second Set of samples First Set of samples Filtration at pH 10 after Filtration at pH 10 after Crosslinking additive 0 ppm .Math. h 10,000 ppm .Math. h Sample Conc Permeance Rejection Permeance Rejection numbers Trade name Class/Description (g/L) (L/m.sup.2/h/bar) % (L/m.sup.2/h/bar) % 17 SINOFLOC 680* Cationic polyacrylamide 1 10.9 91.9% 13.7 90.7% 18 Polyquaternium-2* Cationic poly urea- 5 10.0 98.4% 14.1 93.2% ammonium-ether 19 Hydroflux HB-2705* Cationic polyacrylamide 1 12.9 98.4% 16.3 91.6% 20 Polyquaternium-10* Cationic hydroxyethyl 1 16.3 76.7% 19.7 61.8% cellulose 21 Jaguar Optima* Cationic guar 1 19.8 65.5% 25.1 45.1% 22 Polyquaternium-6* Cationic polyDADMAC 5 25.1 98.8% 29.8 98.5% 23 PEI Polyamine 5 17.6 95.7% 49.1 19.3% 24 None Control 34.3 36.1% 51.3 20.5% *indicates quaternary functionality

[0196] In summary, when the crosslinking additive is a cationic polymer selected from cationic polyacrylamide, cationic poly urea-ammonium-ether, cationic hydroxyethyl cellulose, cationic guar and cationic polyDADMAC, and when exposed to 10,000 ppm.Math.h of chlorine, the filter has rejection values ranging from 90% to 98.5% under alkaline conditions. In this instance, the filter has permeance values ranging from 13.7 to 29.8 (L/m.sup.2/h/bar) when measured using the probe molecule Rose Bengal in the cross-flow apparatus.

[0197] When the filter has a crosslinking additive including a cationic polymer and an adhesive additive including GOHSENXK, and when exposed to 10,000 ppm.Math.h of chlorine, the filter has a permeance value ranging from 13.7 to 29.8 L/m2/h/bar and a rejection value ranging from 90.7% to 98.5% under alkaline conditions.

[0198] Table 6 comprises performance data at pH of 4. The data of the samples having a cationic polymer as crosslinking additive produced membranes with higher rejection than the Control which had no crosslinking additive after exposure to chlorine, such as sodium hypochlorite. The data also showed that the membranes having cationic polymers with quaternary ammonium groups as crosslinking additives produced membranes with improved resistance to degradation by chlorine. Specifically the two far right columns shows permeance values after exposure to 10,000 ppm.Math.h of NaOCl range from 11.0 to 23.8 (L/m2/h/bar) when measured using The probe molecule Rose Bengal in the cross-flow apparatus at a pH of approximately 4, ie under acidic conditions. By comparison, the permeance of the second set of samples 13 to 30, which was not exposed to chlorine had permeance values ranging from 10.9 to 25.1 (L/m.sup.2/h/bar). The permeance value of the membranes after exposure to 10,000 ppm.Math.h of NaOCl increased by an amount of equal to or less than 10.1 (L/m.sup.2/h/bar), and suitably less than 5.0, and suitably by an amount in the range of 1.1 to 4.8 (L/m.sup.2/h/bar) according to the Rose Bengal cross flow under acid conditions and rejection values range from 86.9% to 99.2%, and suitably 90% to 99%, and even more suitably from 90% to 97%, and even more suitably still form 90% to 95%.

TABLE-US-00008 TABLE 6 Chlorine resistance of membranes with quaternary ammonium cationic crosslinkers at pH 4 Second Set of samples First Set of samples Filtration at pH 4 after Filtration at pH 4 after Crosslinking additive 0 ppm .Math. h 10,000 ppm .Math. h Sample Conc Permeance Rejection Permeance Rejection number Trade name Class/Description (g/L) (L/m.sup.2/h/bar) % (L/m.sup.2/h/bar) % 17 SINOFLOC 680* Cationic polyacrylamide 1 6.7 96.3% 11.3 93.3% 18 Polyquaternium-2* Cationic poly urea- 5 6.9 99.3% 11.0 95.0% ammonium-ether 19 Hydroflux HB-2705* Cationic polyacrylamide 1 9.0 98.6% 13.8 93.9% 20 Polyquaternium-10* Cationic hydroxyethyl 1 11.0 95.8% 12.1 90.9% cellulose 21 Jaguar Optima* Cationic guar 1 11.6 91.9% 13.7 86.9% 22 Polyquaternium-6* Cationic polyDADMAC 5 13.7 99.5% 23.8 99.2% 23 PEI Polyamine 5 9.8 98.7% 14.5 87.8% 24 None Control 15.8 90.0% 20.2 88.2% *indicates quaternary functionality

[0199] In summary, the crosslinking additive is a cationic polymer selected from cationic polyacrylamide, cationic poly urea-ammonium-ether, Cationic hydroxyethyl cellulose, Cationic guar, and cationic polyDADMAC, and when exposed to 10,000 ppm.Math.h of chlorine, the filter has rejection value ranging from 86.9% to 99.2% under acid conditions.

[0200] When the cross linking additive is selected from cationic polyacrylamide, cationic poly urea-ammonium-ether, Cationic hydroxyethyl cellulose, and cationic polyDADMAC, and when exposed to 10,000 ppm.Math.h of chlorine the filter has rejection values greater than 90% to 99.2, under acid conditions. The filter has permeance values ranging from 11.0 to 23.8 (L/m.sup.2/h/bar) when measured using the probe molecule Rose Bengal in the cross-flow apparatus.

[0201] When the filter has a crosslinking additive includes a cationic polymer and an adhesive additive including GOHSENXK, and when exposed to 10,000 ppm.Math.h of chlorine, the filter has a permeance value ranging from 11.0 to 23.8 L/m2/h/bar and a rejection value ranging from 86.6% to 99.5% under acidic conditions

[0202] When the filter has a crosslinking additive is a diamine polymer with at least two reactive amine groups, and an adhesive additive for adhering the membrane to the porous substrate. Similarly, when the adhesive additive includes GOHSENXK, and when exposed to 10,000 ppm.Math.h of chlorine, the filter has a rejection value at least 85%, and suitably a rejection in the range of 85% to 88% and a permeance value ranging from 13 to 15 L/m2/h/bar, and suitably a permeance value ranging approximately 14.5 L/m2/h/bar under acid conditions.

[0203] In addition, membranes prepared in accordance with Example 1 and FIG. 1 were also prepared to determine the point at which the reaction between a modifying and the graphene oxide sheets had progressed sufficiently far enough so the temperature of the composition can be reduced to form a stable composition. That is to say, all of, or nearly all of, the modifying agent had reacted with the graphene oxide sheets.

[0204] The procedure included adding i) 2.5 L of graphene oxide at a concentration of 10 g/L and ii) 420 mL of H.sub.2O.sub.2 at a concentration of 30 wt % to an autoclave which was stirred at 500 rpm and heated up to 90 C. at 0.8 C./min, and then held at 90 C. for 7 hours. Once the temperature reached 90 C., samples of the composition was withdrawn at time equals zero hrs. At this point, there had been no appreciable reaction between the graphene oxide and the modifying agent and graphene oxide is regarded as unmodified. Samples of the composition were withdrawn at one hour intervals until the reaction was stopped after 7 hours by turning off the heater and turning on the chiller.

[0205] The viscosity of the samples was measured at 20 C. using a Brookfield LVT viscometer. Due to the shear thinning nature of the graphene oxide composition, the reported viscosity is the apparent viscosity at a shear rate of 30 rpm.

[0206] Filter membranes were then prepared using the samples of the composition in accordance with Example 1 and FIG. 1 and are labelled as Samples 25 to 32 in Table 7 below. Filter membranes were prepared using TOMAC PVDF for the porous substrate, GOHSENX-K for the adhesive additive and polyDADMAC for the crosslinking additive. The Samples 25 to 32 comprised coupons of 42.1 cm.sup.2 in area and the permeance and rejection of each were tested using a 300 ppm solution of Rose Bengal in a cross-flow apparatus operating at 2 bar.

TABLE-US-00009 TABLE 7 Apparent viscosity of unmodified and modified graphene oxide composition and performance of membranes prepared therefrom Reaction Apparent time viscosity Rejection Permeance Sample hr mPa .Math. s % L/m.sup.2/h/bar 25 0 104 96.4% 6.5 26 1 830 99.5% 9.6 27 2 1800 N/A N/A 28 3 1160 99.8% 8.6 29 4 470 97.1% 17.8 30 5 170 95.5% 17.8 31 6 30 96.2% 17.8 32 7 <30 95.9% 16.9

[0207] Table 7 shows that the viscosity increases as the reaction proceeds, up to a maximum after 2 hours of reaction time and thereafter the viscosity decreases. The data shows that a functioning membrane can be produced from any of samples 25 to 32, each having a rejection of Rose Bengal greater than 90%. That is to say, a stable composition was obtained for each. In addition, for the filter to have an increased permeance, the viscosity of the composition has ideally reached and passed a maximum viscosity. A maximum in the viscosity occurred somewhere in between 1 and 3 hours. For instance, a total reaction time of 4, 5, 6 and 7 hrs is preferrable. In other words, the modifying step was carried out for at least 1 to 5 hours after a maximum in the viscosity of the composition has occurred. Suitably, the modifying step was carried out for at least 2 to 4 hours after a maximum in the viscosity of the composition has occurred. In the case of sample 27, the viscosity of the composition prevented the composition from being printed onto the substrate.

[0208] In addition to steps in the examples 1 to 3, the method carried out to make may include options steps such as the following.

Optional Addition of Reducing Agent

[0209] This step involves adding and mixing a reducing solution with the hGO suspension. Specifically, a reducing solution containing an amount of red grape skin polyphenol equal to 25% of the GO mass is measured and added to the hGO dispersion whilst the dispersion is being sheared. Typically, 5.36 g of polyphenol is added to the 2.917 L of hGO suspension while stirring the composition vigorously at a rate of 6000 rpm. The polyphenol is added slowly in stages and the composition is sheared for 15 minutes or until the composition is homogenised and a desired viscosity is achieved.

[0210] This composition was then heated to 75 C. and stirred at 100 rpm for 1 hour. The suspension obtained is then notionally referenced as an intermediate suspension.

Optional Addition of Physical Property Modifiers

[0211] To provide a composition that is conducive to printing, an organic solvent can be added. For example, 100 ml of a graphene oxide suspension containing 8.57 g/L of graphene oxide was mixed with 100 ml of ethanol to form a 50% aqueous ethanol suspension. This resulted in changes to viscosity and surface tension that improved the application of the graphene oxide to the porous substrate.

Optional Post-Treatment

[0212] Finally, fine control of the permeance of the substrate was carried out with a NaOCl treatment. Specifically, setting the permeance may include treating the filtration membrane by submerging the substrate in a solution of NaOCl for a period. For example, submerging the substrate in a solution of 10000 mg/L NaOCl for period of 30 minutes, increased the permeance of a PVDF-GO membrane e.g. from 20 to 30 L/m.sup.2/hr/bar.

[0213] Those skilled in the art of the present invention will appreciate that many variations and modification can be made to the example described herein without departing from the spirit and scope of the present invention.

[0214] In the claim which follows, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word comprise and variations such as comprises or comprising are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the apparatus and method as disclosed herein.