WATER PURIFICATION

Abstract

This invention relates to methods of purifying water using graphene oxide laminates which are formed from stacks of cross-linked individual graphene oxide flakes. The laminates also comprise graphene and/or at least one cross-linking agent. The invention also relates to the laminate membranes themselves.

Claims

1. A graphene oxide laminate membrane comprising graphene oxide (GO) flakes and graphene flakes.

2. A membrane of claim 1, wherein the graphene flakes are from 0.5 wt % to 10 wt % of the flakes of which the graphene oxide laminate membrane is comprised.

3. A membrane of claim 1, wherein the graphene flakes are pristine graphene.

4. A membrane of claim 1, wherein the graphene oxide laminate is supported on a porous material.

5. A membrane of claim 1 having a thickness greater than about 100 nm and wherein the graphene oxide flakes of which the membrane is comprised have an average oxygen:carbon weight ratio in the range of from 0.2:1.0 to 0.5:1.0 and wherein the membrane.

6. A membrane of claim 1, wherein the graphene oxide flakes of which the laminate is comprised have an average oxygen:carbon weight ratio in the range of from 0.3:1.0 to 0.4:1.0.

7. A membrane of claim 1, wherein the graphene oxide laminate membrane has a thickness between 1 ?m and 15 ?m.

8. A membrane of claim 1, wherein the graphene oxide laminate further comprises at least one cross-linking agent.

9. A method of reducing the amount of one or more solutes in an aqueous mixture to produce a liquid depleted in said solutes, the method comprising: a) contacting a first face of a graphene oxide laminate membrane with the aqueous mixture comprising the one or more solutes; b) recovering the liquid from or downstream from a second face of the graphene oxide laminate membrane; wherein the graphene oxide laminate membrane comprises GO flakes and graphene flakes.

10. A method of claim 9, wherein the graphene flakes are from 0.5 wt % to 10 wt % of the flakes of which the graphene oxide laminate membrane is comprised.

11. A method of claim 9, wherein the graphene flakes are pristine graphene.

12. A method of claim 9, wherein the graphene oxide laminate membrane has a thickness greater than about 100 nm and wherein the graphene oxide flakes of which the membrane is comprised have an average oxygen:carbon weight ratio in the range of from 0.2:1.0 to 0.5:1.0 and wherein the membrane.

13. A method of claim 9, wherein the graphene oxide laminate membrane further comprises at least one cross-linking agent.

14. A method of reducing the amount of one or more solutes in an aqueous mixture to produce a liquid depleted in said solutes, the method comprising: a) contacting a first face of a graphene oxide laminate membrane with the aqueous mixture comprising the one or more solutes; b) recovering the liquid from or downstream from a second face of the graphene oxide laminate membrane; wherein the graphene oxide laminate membrane has a thickness greater than about 100 nm and wherein the graphene oxide flakes of which the membrane is comprised have an average oxygen:carbon weight ratio in the range of from 0.2:1.0 to 0.5:1.0 and wherein the membrane comprises GO flakes and at least one cross-linking agent.

15. A method of claim 14, wherein the cross-linking agent is a polymer.

16. A method of claim 15, wherein the cross-linking agent is a charged polymer.

17. A method of claim 14, wherein the d-spacing of the hydrated graphene oxide laminate membrane is below 12 ?.

18. A method of claim 17, wherein the d-spacing of the hydrated graphene oxide laminate membrane is below 10 ?.

19. A method of claim 9, wherein the method is a process of selectively reducing the amount of a first set of one or more solutes in an aqueous mixture without significantly reducing the amount of a second set of one or more solutes in the aqueous mixture to produce a liquid depleted in said first set of solutes but not depleted in said second set of solutes.

20. A method of claim 9, wherein the method is continuous.

21. A method of claim 9, wherein pressure is applied to push the aqueous mixture through the graphene oxide membrane.

22. A method of claim 9, wherein the graphene oxide membrane is supported on a porous material.

23. A method of claim 9, wherein the graphene oxide flakes of which the laminate is comprised have an average oxygen:carbon weight ratio in the range of from 0.3:1.0 to 0.4:1.0.

24. A method of claim 9, wherein the graphene oxide laminate membrane has a thickness between 1 ?m and 15 ?m.

25. A method of claim 9, wherein the solutes the amounts of which are reduced in the aqueous mixture include NaCl.

26. A method of reducing the amount of one or more predetermined solutes having a hydration radius in the range of from about 3.5 ? to about 4.5 ? in an aqueous mixture to produce a liquid depleted in the predetermined solutes, the method comprising; a) determining the identity of one or more solutes in the aqueous mixture which are to be selected for exclusion by the membrane; b) correlating the required d-spacing in the graphene oxide membrane with the hydration radius of the or each predetermined solute; c) forming a graphene oxide laminate membrane comprising GO flakes and also comprising monolayer or few layer graphene flakes and/or at least one cross linking agent and having a reduced d-spacing relative to a membrane which does not comprise the cross-linking agent; d) contacting a first face of a graphene oxide laminate membrane with the aqueous mixture comprising one or more solutes; and e) recovering the liquid from or downstream from a second face of the membrane.

27. A method of tuning the d-spacing of a cross-linked graphene oxide laminate size exclusion filtration membrane, the method comprising: a) selecting at least one cross-linking agent and/or monolayer or few layer graphene flakes which provides a membrane having a desired capillary size when the membrane is hydrated; and b) forming a graphene oxide laminate membrane comprising GO flakes and also comprising monolayer or few layer graphene flakes and/or the at least one cross linking agent.

28. A method of limiting the d-spacing of a hydrated graphene oxide laminate size exclusion filtration membrane to below 12 ?, the method comprising: forming a graphene oxide laminate membrane comprising GO flakes and also comprising monolayer or few layer graphene flakes and/or at least one cross linking agent.

29. A use of monolayer or few layer graphene flakes and/or at least one cross linking agent to limit the d-spacing of a hydrated graphene oxide laminate size exclusion filtration membrane to below 12 ?.

30. A graphene oxide laminate membrane comprising GO flakes and a charged polymer as a cross-linking agent.

31. A graphene oxide laminate membrane comprising GO flakes and at least one cross-linking agent and having, when hydrated, a reduced pore size relative to a hydrated graphene oxide membrane which does not comprise the cross-linking agent, and wherein the pore size in the hydrated membrane is operative to substantially exclude at least the passage of solutes having a hydration radius in the range of from about 3.5 ? to about 4.5 ? when present in an aqueous mixture.

32. A filtration device comprising a graphene oxide laminate membrane of claim 1.

33. A method of producing a graphene oxide laminate membrane comprising GO flakes and graphene flakes, the method comprising: a) providing a suspension of graphite flakes and graphite oxide flakes in an aqueous medium; b) subjecting the graphite flakes and graphite oxide flakes in the aqueous medium to energy to obtain an aqueous suspension comprising graphene flakes and graphene oxide flakes; c) optionally removing any graphite or undesired few-layered graphene flakes from the suspension; and d) filtering the suspension through a porous material to provide a graphene oxide laminate membrane comprising GO flakes and graphene flakes, the laminate membrane being supported on the porous material.

34. A method of claim 14, wherein the method is a process of selectively reducing the amount of a first set of one or more solutes in an aqueous mixture without significantly reducing the amount of a second set of one or more solutes in the aqueous mixture to produce a liquid depleted in said first set of solutes but not depleted in said second set of solutes.

35. A method of claim 14, wherein the method is continuous.

36. A method of claim 14, wherein pressure is applied to push the aqueous mixture through the graphene oxide membrane.

37. A method of claim 14, wherein the graphene oxide membrane is supported on a porous material.

38. A method of claim 14, wherein the graphene oxide flakes of which the laminate is comprised have an average oxygen:carbon weight ratio in the range of from 0.3:1.0 to 0.4:1.0.

39. A method of claim 14, wherein the graphene oxide laminate membrane has a thickness between 1 ?m and 15 ?m.

40. A method of claim 14, wherein the solutes the amounts of which are reduced in the aqueous mixture include NaCl.

41. A filtration device comprising a graphene oxide laminate membrane of claim 30.

42. A filtration device comprising a graphene oxide laminate membrane of claim 31.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0096] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

[0097] FIG. 1 shows the x-ray diffraction peaks for selected cross-linked and non-cross-linked laminate membranes before hydration and the corresponding observed d-spacings.

[0098] FIG. 2 shows the d-spacing of selected cross-linked and non-cross-linked laminate membranes both before (Bf Hyd) and after (Af Hyd) hydration.

[0099] FIG. 3 shows the water flux of selected cross-linked and non-cross-linked laminate membranes (with an applied pressure of 25 bar).

[0100] FIG. 4 shows the NaCl rejection of selected cross-linked and non-cross-linked laminate membranes as an average across all measurements.

[0101] FIG. 5 shows the NaCl rejection of selected cross-linked and non-cross-linked laminate membranes in terms of the value obtained for each measurement.

[0102] FIG. 6 shows the MgCl.sub.2 rejection of selected cross-linked and non-cross-linked laminate membranes.

[0103] FIG. 7 shows (a) Cross sectional and (b) in-plane scanning electron micrograph of Gr-GO membrane. Flaky features in the in-plane SEM image are from the exfoliated graphene in the membrane (c) represents the schematic of the structure of Gr-GO membrane (longer linesGO and shorter linesgraphene).

[0104] FIG. 8 shows Gr-GO Suspensions. (a) Photograph of GO and Gr-GO aqueous colloidal suspensions (concentration ?100 ?g/ml) with increasing wt % of graphene from left to right. (b) The concentration and estimated wt % of exfoliated graphene in Gr-GO membrane as a function of initial graphite oxide/graphite weight ratio. (c) Shift in two-theta position of the (001) diffraction peak of GO and Gr-GO membranes in dry and wet states. (d) Permeation rate of K.sup.+, Na.sup.+, Li.sup.+ and Mg.sup.+2 ions through pristine GO, Gr-GO membranes made of 1:2 and 1:9 dispersions. The shown permeation rates are measured using 5 ?m thick membranes.

DETAILED DESCRIPTION

[0105] The present invention involves the use of graphene oxide laminate membranes. The graphene oxide laminates and laminate membranes of the invention comprise stacks of individual graphene oxide flakes, in which the flakes are predominantly monolayer graphene oxide. Although the flakes are predominantly monolayer graphene oxide, it is within the scope of this invention that some of the graphene oxide is present as two- or few-layer graphene oxide. Thus, it may be that at least 75% by weight of the graphene oxide is in the form of monolayer graphene oxide flakes, or it may be that at least 85% by weight of the graphene oxide is in the form of monolayer graphene oxide flakes (e.g. at least 95%, for example at least 99% by weight of the graphene oxide is in the form of monolayer graphene oxide flakes) with the remainder made up of two- or few-layer graphene oxide. Without wishing to be bound by theory, it is believed that water and solutes pass through capillary-like pathways formed between the graphene oxide flakes by diffusion and that the specific structure of the graphene oxide laminate membranes leads to the remarkable selectivity observed as well as the remarkable speed at which the ions permeate through the laminate structure.

[0106] Graphene oxide flakes are two dimensional heterogeneous macromolecules containing both hydrophobic graphene regions and hydrophilic regions with large amounts of oxygen functionality (e.g. epoxide, carboxylate groups, carbonyl groups, hydroxyl groups)

[0107] In one illustrative example, the graphene oxide laminate membranes are made of impermeable functionalized graphene sheets that have a typical size L??m and the interlayer separation, d, sufficient to accommodate a mobile layer of water.

[0108] The solutes to be removed from aqueous mixtures in the methods of the present invention may be defined in terms of their hydrated radius. Below are the hydrated radii of some exemplary ions and molecules.

TABLE-US-00001 TABLE 1 Hydrated Hydrated Ion/molecule radius (?) Ion/molecule radius (?) K.sup.+ 3.31 Li.sup.+ 3.82 Cl.sup.? 3.32 Rb.sup.+ 3.29 Na.sup.+ 3.58 Cs.sup.+ 3.29 CH.sub.3COO.sup.? 3.75 NH.sub.4.sup.+ 3.31 SO.sub.4.sup.2? 3.79 Be.sup.2+ 4.59 AsO.sub.4.sup.3? 3.85 Ca.sup.2+ 4.12 CO.sub.3.sup.2? 3.94 Zn.sup.2+ 4.30 Cu.sup.2+ 4.19 Ag.sup.+ 3.41 Mg.sup.2+ 4.28 Cd.sup.2+ 4.26 propanol 4.48 Al.sup.3+ 4.80 glycerol 4.65 Pb.sup.2+ 4.01 [Fe(CN).sub.6].sup.3? 4.75 NO.sub.3.sup.? 3.40 sucrose 5.01 OH.sup.? 3.00 (PTS).sup.4? 5.04 H.sub.3O.sup.+ 2.80 [Ru(bipy).sub.3].sup.2+ 5.90 Br.sup.? 3.30 Tl.sup.+ 3.30 I.sup.? 3.31

[0109] The hydrated radii of many species are available in the literature. However, for some species the hydrated radii may not be available. The radii of many species are described in terms of their Stokes radius and typically this information will be available where the hydrated radius is not. For example, of the above species, there exist no literature values for the hydrated radius of propanol, sucrose, glycerol and PTS.sup.4?. The hydrated radii of these species which are provided in the table above have been estimated using their Stokes/crystal radii. To this end, the hydrated radii for a selection of species in which this value was known can be plotted as a function of the Stokes radii for those species and this yields a simple linear dependence. Hydrated radii for propanol, sucrose, glycerol and PTS.sup.4? were then estimated using the linear dependence and the known Stokes radii of those species.

[0110] There are a number of methods described in the literature for the calculation of hydration radii. Examples are provided in Determination of the effective hydrodynamic radii of small molecules by viscometry; Schultz and Soloman; The Journal of General Physiology; 44; 1189-1199 (1963); and Phenomenological Theory of Ion Solvation; E. R. Nightingale. J. Phys. Chem. 63, 1381 (1959).

[0111] The term aqueous mixture refers to any mixture of substances which comprises at least 10% water by weight. It may comprise at least 50% water by weight and preferably comprises at least 80% water by weight, e.g. at least 90% water by weight. The mixture may be a solution, a suspension, an emulsion or a mixture thereof. Typically the aqueous mixture will be an aqueous solution in which one or more solutes are dissolved in water. This does not exclude the possibility that there might be particulate matter, droplets or micelles suspended in the solution. Of course, it is expected that the particulate matter will not pass through the membranes of the invention even if it is comprised of ions with small radii.

[0112] Particularly preferred solutes for removing from water include hydrocarbons and oils, biological material, dyes, organic compounds (including halogenated organic compounds), complex ions, NaCl, heavy metals, ethanol, chlorates and perchlorates and radioactive elements.

[0113] The graphene oxide or graphite oxide for use in this application can be made by any means known in the art. In a preferred method, graphite oxide can be prepared from graphite flakes (e.g. natural graphite flakes) by treating them with potassium permanganate and sodium nitrate in concentrated sulphuric acid. This method is called Hummers method. Another method is the Brodie method, which involves adding potassium chlorate (KClO.sub.3) to a slurry of graphite in fuming nitric acid. For a review see, Dreyer et al. The chemistry of graphene oxide, Chem. Soc. Rev., 2010, 39, 228-240.

[0114] Individual graphene oxide (GO) sheets can then be exfoliated by dissolving graphite oxide in water or other polar solvents with the help of ultrasound, and bulk residues can then be removed by centrifugation and optionally a dialysis step to remove additional salts.

[0115] In a specific embodiment, the graphene oxide of which the graphene oxide laminate membranes of the invention are comprised is not formed from wormlike graphite. Worm-like graphite is graphite that has been treated with concentrated sulphuric acid and hydrogen peroxide at 1000? C. to convert graphite into an expanded worm-like graphite. When this worm-like graphite undergoes an oxidation reaction it exhibits a higher increase the oxidation rate and efficiency (due to a higher surface area available in expanded graphite as compared to pristine graphite) and the resultant graphene oxide contains more oxygen functional groups than graphene oxide prepared from natural graphite. Laminate membranes formed from such highly functionalized graphene oxide can be shown to have a wrinkled surface topography and lamellar structure (Sun et al; Selective Ion Penetration of Graphene Oxide Membranes; ACS Nano 7, 428 (2013) which differs from the layered structure observed in laminate membranes formed from graphene oxide prepared from natural graphite. Such membranes do not show fast ion permeation of small ions and a selectivity which is substantially unrelated to size (being due rather to interactions between solutes and the graphene oxide functional groups) compared to laminate membranes formed from graphene oxide prepared from natural graphite.

[0116] The preparation of graphene oxide laminate supported on a porous membrane can be achieved using filtration, spray coating, casting, dip coating techniques, road coating, inject printing, or any other thin film coating techniques

[0117] For large scale production of supported graphene based membranes or sheets it is preferred to use spray coating, road coating or inject printing techniques. One benefit of spray coating is that spraying GO solution in water on to the porous support material at an elevated temperature produces a large uniform GO film.

[0118] Graphite oxide consists of micrometer thick stacked graphite oxide flakes (defined by the starting graphite flakes used for oxidation, after oxidation it gets expanded due to the attached functional groups) and can be considered as a polycrystalline material. Exfoliation of graphite oxide in water into individual graphene oxide flakes was achieved by the sonication technique followed by centrifugation at 10000 rpm to remove few layers and thick flakes. Graphene oxide laminates were formed by restacking of these single or few layer graphene oxides by a number of different techniques such as spin coating, spray coating, road coating and vacuum filtration.

[0119] Graphene oxide membranes according to the invention consist of overlapped layers of randomly oriented single layer graphene oxide sheets with smaller dimensions (due to sonication). These membranes can be considered as centimetre size single crystals (grains) formed by parallel graphene oxide sheets. Due to this difference in layered structure, the atomic structure of the capillary structure of graphene oxide membranes and graphite oxide are different. For graphene oxide membranes the edge functional groups are located over the non-functionalised regions of another graphene oxide sheet while in graphite oxide mostly edges are aligned over another graphite oxide edge. These differences unexpectedly may influence the permeability properties of graphene oxide membranes as compared to those of graphite oxide.

[0120] A layer of graphene consists of a sheet of sp.sup.2-hybridized carbon atoms. Each carbon atom is covalently bonded to three neighboring carbon atoms to form a honeycomb network of tessellated hexagons. Carbon nanostructures which have more than 10 graphene layers (i.e. 10 atomic layers; 3.5 nm interlayer distance) generally exhibit properties more similar to graphite than to mono-layer graphene. Thus, throughout this specification, the term graphene is intended to mean a carbon nanostructure with up to 10 graphene layers. A graphene layer can be considered to be a single sheet of graphite.

[0121] In the context of this disclosure the term graphene is intended to encompass both pristine graphene (i.e. un-functionalised or substantially un-functionalised graphene) and reduced graphene oxide. When graphene oxide is reduced a graphene like substance is obtained which retains some of the oxygen functionality of the graphene oxide. It may be however that the term graphene is excludes both graphene oxide and reduced graphene oxide and thus is limited to pristine graphene. All graphene contains some oxygen, dependent on the oxygen content of the graphite from which is it derived. It may be that the term graphene encompasses graphene that comprises up to 10% oxygen by weight, e.g. less than 8% oxygen by weight or less than 5% oxygen by weight.

[0122] Throughout the description and claims of this specification, the words comprise and contain and variations of them mean including but not limited to, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0123] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0124] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Example 1Cross-Linked GO Laminate Membranes

[0125] Graphite oxide was prepared from natural graphite through modified Hummer's method using sulphuric acid and potassium permanganate. The graphite oxide was then dispersed in water by ultrasonication to obtain the stable aqueous graphene oxide (GO) dispersion. The unexfoliated graphite oxide and few layer graphene oxide flakes were removed by centrifugation and the supernatant containing the single layer GO sheets was used for the membrane preparation. Then, a cross linker selected from poly vinyl alcohol (PVA), ethylenediamine (EDA), poly (styrene-4-sulfonate) (PSS), poly Allylamine (PAA) and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (poly AMPS) (20% with respect to the weight of the GO present in solution) was dissolved in GO suspension and left for overnight stirring at room temperature. By adjusting the volume of each solution, GO-PVA, GO-EDA, GO-PSS, GO-PAA and GO-polyAMPS membranes of thicknesses ?500 nm, were prepared on the polyethersulfone (PES) membrane (diameter of 47 mm with pore size ?0.2 ?m) using vacuum filtration. The membranes were dried in a vacuum desiccator in prior to use for the pressure filtration experiments.

[0126] X-ray diffraction (XRD) was used to measure the inter-layer d spacing (capillary width) of the GO membranes. The d-spacing values are calculated from the peak position in XRD pattern using Bragg's law n?=2d sin ?. For XRD experiments, GO-PVA, GO-EDA, GO-PSS, GO-PAA and GO-polyAMPS membranes (of thickness ?5 ?m) are prepared by vacuum filtration of each solution through Anodisc alumina membranes with a pore size of 0.02 ?m. These membranes were dried under vacuum to peel a free standing GO membrane with different linker molecules for the XRD measurements. Bruker D8-Discover X-ray diffractometer was used to estimate the d-spacing of the fabricated free standing membranes in both dry and wet states. XRD pattern (5<2?<25) of the each free standing membrane was obtained at room temperature and room humidity and left these membranes in water for 24 hrs. Further, the XRD measurements were conducted on soaked membranes in the same 2? range to estimate the swelling effect. From the XRD measurements of all the GO membranes with different linker molecules, GO-polyAMPS membrane has shown very small increase in the d-spacing from 8.6 ? (in dry state) to 9.1 ? (in wet state).

[0127] In the present study, we have used Sterlitech HP4750 stirred cell for pressure filtration experiments. Various GO membranes with different linkers prepared on the PES were placed in the pressure filtration cell with a porous metal support and performed the pressure filtration experiments for 2 mg/ml MgCl.sub.2 and NaCl solutions by applying 26 bar pressure using a compressed gas cylinder. The solution permeated through the membranes was collected from the permeate tube fixed to the pressure filtration cell. Among all the GO membranes with different linkers, GO-polyAMPS membrane has shown flux rate of 10 L m.sup.?2 h.sup.?1 with a salt rejection ?50%. The data obtained is shown in FIGS. 1 to 6.

Example 2Graphene Oxide/Graphene Composite Laminate Membranes

[0128] 250 mg of graphite oxide (prepared as in Example 1) and 125 mg of pristine graphite powder were sonicated in 250 ml of DI water for 24 hrs to prepare a Gr-GO dispersion. The Gr-GO suspension was then centrifuged at 2500 rpm to remove the unexfoliated graphite oxide and graphite particles with the supernatant containing the mono and few layers of GO and graphene flakes (this sample was denoted as denoted as Gr-GO-2500). In this method, graphene oxide helps exfoliated graphene to disperse in water to form a stable aqueous suspension.

[0129] Gr-GO membranes were prepared by vacuum filtration of Gr-GO dispersion through an Anodisc membrane filter (47 mm in diameter, 0.2 mm pore size) similarly to the method described in Example 1. Gr-GO membranes with Anodisc support were glued onto copper plates which exposes an effective area of ?1 cm.sup.2 of the membrane. The copper plate was then placed in a permeation setup containing the feed and permeates compartments. In a typical experiment, feed compartment filled with 1 M aqueous solution of various salts and the permeate compartment was filled with DI water and kept undisturbed for 24 hrs. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to find the ion species concentration in the permeate cell. Also these results were cross checked by carefully weighing the left over material after the evaporation of water in permeate compartment. It is found that the permeation rate for Mg.sup.+2 and Na.sup.+ ions for the Gr-GO membrane is ?2?10.sup.?3 and 3?10.sup.?3 mol/h/m.sup.2 which is 1000 times smaller when compared to that of the GO laminate membrane which does not comprise graphene or a cross-linking agent. In another permeation experiment with GO-polyAMPS, the permeation rate of Mg.sup.+2 ions found to be ?1?10.sup.?2 mol/h/m.sup.2.

[0130] The amount of graphene present in the Gr-GO suspension can be controlled by centrifuging the dispersion obtained from sonication of the graphite and graphite oxide mixture at differing speeds. Thus, samples obtained from sonication of the graphite and graphite oxide mixture as described above were centrifuged at 5000, 7500 and 10000 rpm and the resultant suspension was formed into a laminate membrane as described above. From the permeation experiments with Gr-GO membranes made of the Gr-GO dispersion centrifuged at 5000, 7500 and 10000 rpm (denoted as Gr-GO-5000, Gr-GO-7500 and Gr-GO-10000), it was found that permeation rate of the Mg.sup.+2 ions (given in the table below) increased for Gr-GO membranes prepared with the Gr-GO dispersion centrifuged at higher speeds. Permeation rate of Mg.sup.+2 ions in the GO/graphene-10000 is 10 times more than that of in the GO/graphene-2500. It is expected that lower centrifugation rates result in a higher proportion of the membrane containing few layer graphene too.

TABLE-US-00002 Permeation Membrane rate (mol/h/m.sup.2) Gr-GO-2500 2.18 ? 10.sup.?3 Gr-GO-5000 1.88 ? 10.sup.?2 Gr-GO-7500 1.95 ? 10.sup.?2 Gr-GO-10000 2.47 ? 10.sup.?2

Example 3Graphene Oxide/Graphene Composite Laminate Membranes

[0131] Further, four different concentrations of Gr-GO aqueous dispersions were prepared by exfoliating the graphite flakes and graphite oxide in the weight ratio (graphite oxide/graphite) of 1:1, 1:2, 1:5 and 1:9. 0.175 g of graphite oxide was sonicated in 120 ml deionised water along with different weights of graphite flakes varying as 0.175 g, 0.35 g, 0.875 g and 1.575 g for 50 hrs. Supernatant of the resulting dispersion was collected after few hours to avoid the unexfoliated graphite and unstable aggregates which settles down gradually. Subsequently, the supernatant was centrifuged twice for 25 mins at 2500 g to obtain the homogenous Gr-GO aqueous dispersion containing mono and few layers GO and graphene flakes. The Gr-GO membranes were prepared by vacuum filtration of Gr-GO dispersion through an Anodisc membrane filter (47 mm diameter, 0.02 ?m pore size) and dried in a vacuum desiccator.

[0132] For the permeation experiments, Gr-GO membranes with Anodisc support were glued onto copper plates in such a way that an effective area of ?1 cm.sup.2 of the membrane is exposed [15]. A typical permeation experiment was carried out for 24 hrs by fixing the membrane attached copper plate in a permeation setup where feed compartment filled with 1 M aqueous solution of various salts (KCl, NaCl, LiCl and MgCl.sub.2) and the permeate compartment was filled with deionised water. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to find the ion species concentration in the permeate cell and these results were cross checked by carefully weighing the left over material after the evaporation of water in permeate compartment.

[0133] FIG. 8a shows the optical photograph of 100 ?g/ml concentrated GO and Gr-GO aqueous colloidal suspensions with increasing wt % of graphene (from left to right). The pale brown coloured GO suspension gradually turns into dark with increasing initial amount of graphite starting material which suggests the increased amount of exfoliated graphene in Gr-GO dispersions in the case of higher initial graphite content. Weight of graphite oxide is kept constant and varied the initial weight of graphite flakes for preparation of each solution in order to estimate the actual wt % of graphene exfoliated into GO suspension. FIG. 8b shows the concentration and actual wt % of exfoliated graphene in GO and Gr-GO dispersions as a function of initial graphite oxide/graphite weight ratio. Three membranes (GO and Gr-GO) prepared from the known amount of volume of each dispersion were carefully weighed using ?g precision microbalance to determine the concentration of GO and Gr-GO dispersions. Subsequently, actual wt % of exfoliated graphite in the different Gr-GO dispersion is estimated from the concentration values and found that ?5.5 wt %, 4.2 wt %, 2.2 wt % and 1.3 wt % of exfoliated graphene (with respect to the weight of GO) is present in the membranes made from the Gr-GO dispersions of 1:9, 1:5, 1:2 and 1:1 initial graphite oxide/graphite ratio, respectively.

[0134] X-ray diffraction technique has been used to analyse the changes in the interlayer spacing of GO and Gr-GO membranes in both dry and wet states. In the dry state, both pristine GO and Gr-GO membranes show similar (001) diffraction peak at ?10.5?0.5? indicating similar laminar structures for both the membranes. To determine the swelling behaviour, GO and Gr-GO membranes were soaked for a day in deionised water. As expected, the interlayer spacing (?8.4 ? in dry state) of GO membrane increased to 14 ? after soaking. In contrast to the GO membranes, Gr-GO membranes having a higher wt % of exfoliated graphene flakes have shown less swelling. For example, the interlayer spacing of Gr-GO membranes with 5.5 wt % and 2.2 wt % of exfoliated graphene is respectively ?10.3 ? and 11.4 ? in the wet state. This indicates that incorporation of exfoliated graphene in the GO membrane controls the swelling of GO membrane by controlling the amount of water in the interlayer space. Without wishing to be bound by theory, this could be due to the more hydrophobic nature of exfoliated graphene which lowers the amount of water in the interlayer spaces of the membrane.

[0135] Homogeneity of the Gr-GO membranes was further confirmed by the SEM investigations. FIGS. 7a and 7b show the cross-sectional and in-plain SEM image of Gr-GO membrane respectively. Distribution of exfoliated graphite (flaky features in FIG. 7b) in the membrane is found to be very uniform and they are shown to be assembled in a layered structure. FIG. 7c shows the schematic structure of layered structure of Gr-GO membrane.

[0136] FIG. 8d summarizes the permeation rate for different ions (K.sup.+, Na.sup.+, Li.sup.+ and Mg.sup.+2) through the GO and Gr-GO membranes made from 1:2 and 1:9 dispersions. From FIG. 8d, it is apparent that the value of permeation rate observed for Mg.sup.+2 ions through Gr-GO membrane made of 1:9 dispersion is ?1000 times smaller than the permeation rate through pristine GO membrane. This can be explained by the lesser swelling effect in 1:9 Gr-GO membrane with respect to the pristine GO membrane. Interlayer distance of 1:9 Gr-GO membrane increases to 10 ?, whereas it is 14 ? for the pristine GO membrane after soaking in water. Similarly, significant decrease (?100 to 1000 times) in the permeation rate for K.sup.+, Na.sup.+ and Li.sup.+ ions is observed in the case of 1:9 Gr-GO membranes. The water permeation rate through GO and Gr-GO membrane has also been measured by monitoring the osmotic height difference and it was found that addition of graphene to GO membrane did not change the osmotic height difference, indicating similar water permeation rate for both GO and Gr-GO membranes.

[0137] NaCl salt rejection properties of 1:9 Gr-GO membranes were further measured using forward osmosis technique by keeping concentrated sugar solution as a draw solute. Salt rejection was calculated using the equation 1-C.sub.p/C.sub.f where C.sub.p is the concentration of NaCl in transmitted water and C.sub.f is the concentration of NaCl in feed side. This analysis yields 96% salt rejection for the 1:9 Gr-GO membranes. The salt rejection of GO only membranes is around 70%.