Jijo ABRAHAM, et al.
Graphene Oxide Desalination
Nature Nanotechnology volume 12, pages 546–550 (2017)
Tunable sieving of ions using graphene oxide membranes
Jijo Abraham, Kalangi S. Vasu, Christopher D. Williams, Kalon Gopinadhan, Yang Su, Christie T. Cherian, James Dix, Eric Prestat, Sarah J. Haigh, Irina V. Grigorieva, Paola Carbone, Andre K. Geim & Rahul R. Nair
Graphene oxide membranes show exceptional molecular permeation properties, with promise for many applications1,2,3,4,5. However, their use in ion sieving and desalination technologies is limited by a permeation cutoff of ~9 Å (ref. 4), which is larger than the diameters of hydrated ions of common salts4,6. The cutoff is determined by the interlayer spacing (d) of ~13.5 Å, typical for graphene oxide laminates that swell in water2,4. Achieving smaller d for the laminates immersed in water has proved to be a challenge. Here, we describe how to control d by physical confinement and achieve accurate and tunable ion sieving. Membranes with d from ~9.8 Å to 6.4 Å are demonstrated, providing a sieve size smaller than the diameters of hydrated ions. In this regime, ion permeation is found to be thermally activated with energy barriers of ~10–100 kJ mol–1 depending on d. Importantly, permeation rates decrease exponentially with decreasing sieve size but water transport is weakly affected (by a factor of <2). The latter is attributed to a low barrier for the entry of water molecules and large slip lengths inside graphene capillaries. Building on these findings, we demonstrate a simple scalable method to obtain graphene-based membranes with limited swelling, which exhibit 97% rejection for NaCl.

Water Filtration
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[0001] The invention relates to graphene oxide laminate membranes that are physically constrained. This invention also relates to methods of purifying water using said membranes and methods of making said membranes.

[0002] The removal of solutes from water finds application in many fields.
[0003] This may take the form of the purification of water for drinking or for watering crops or it may take the form of the purification of waste waters from industry to prevent environmental damage. Examples of applications for water purification include: the removal of salt from sea water for drinking water or for use in industry; the purification of brackish water; the removal of radioactive ions from water which has been involved in nuclear enrichment, nuclear power generation or nuclear clean-up (e.g. that involved in the decommissioning of former nuclear power stations or following nuclear incidents); the removal of environmentally hazardous substances (e.g. halogenated organic compounds, heavy metals, chlorates and perchlorates) from industrial waste waters before they enter the water system; and the removal of biological pathogens (e.g. viruses, bacteria, parasites, etc) from contaminated or suspect drinking water.

[0004] In many industrial contexts (e.g. the nuclear industry) it is often desirable to separate dangerous or otherwise undesired solutes from valuable (e.g. rare metals) solutes in industrial waste waters in order that the valuable solutes can be recovered and reused or sold.

[0005] Graphene is believed to be impermeable to all gases and liquids. Membranes made from graphene oxide are impermeable to most liquids, vapours and gases, including helium. However, an academic study has shown that, surprisingly, graphene oxide membranes having a thickness around 1 µ?? composed of oxygen rich functionalities are permeable to water even though they are impermeable to helium. These graphene oxide sheets allow unimpeded permeation of water (10<10>times faster than He) (Nair ef al.

Science, 2012, 335, 442-444). Such GO laminates are particularly attractive as potential filtration or separation media because they are easy to fabricate, mechanically robust and offer no principal obstacles towards industrial scale production.

[0006] Sun et al (Selective Ion Penetration of Graphene Oxide Membranes; ACS Nano 7, 428 (2013)) describes the selective ion penetration of graphene oxide membranes in which the graphene oxide is formed by oxidation of wormlike graphite. The membranes are freestanding in the sense that they are not associated with a support material. The resultant graphene oxide contains more oxygen functional groups than graphene oxide prepared from natural graphite and laminates formed from this material have a wrinkled surface topography. Such membranes differ from those of the present invention because they do not show fast ion permeation of small ions and also demonstrate a selectivity which is substantially related to chemical and electrostatic interactions rather than size of ions.

[0007] This study found that sodium salts permeated quickly through GO membranes, whereas heavy metal salts permeated much more slowly. Copper sulphate and organic contaminants, such as rhodamine B are blocked entirely because of their strong interactions with GO membranes. According to this study, ionic or molecular permeation through GO is mainly controlled by the interaction between ions or molecules with the functional groups present in the GO sheets. The authors comment that the selectivity of the GO membranes cannot be explained solely by ionic-radius based theories. They measured the electrical conductivities of different permeate solutions and used this value to compare the permeation rates of different salts. The potential applied to measure the conductivities can affect ion permeation through membranes.

[0008] Other publications (Y. Han, Z. Xu, C. Gao. Adv. Fund. Mater. 23, 3693 (2013); M. Hu, B. Mi. Environ. Sci. Technol. 47, 3715 (2013); H. Huang et al. Chem. Comm. 49, 5963 (2013)) have reported filtration properties of GO laminates and, although results varied widely due to different fabrication and measurement procedures, they reported appealing characteristics including large water fluxes and notable rejection rates for certain salts. Unfortunately, large organic molecules were also found to pass through such GO filters. The latter observation is disappointing and would considerably limit interest in GO laminates as molecular sieves. In this respect, we note that the emphasis of these studies was on high water rates that could be comparable to or exceed the rates used for industrial desalination. Accordingly, a high water pressure was applied and the GO membranes were intentionally prepared as thin as possible, 10-50 nm thick. It may be that such thin stacks contained holes and cracks (some may appear after applying pressure), through which even large organic molecules could penetrate.

[0009] Recently, Joshi et al have described the use of graphene oxide laminate membranes as size exclusion membranes (R. K. Joshi et al., 2014, Science, 343, 752-754; see also WO2015/075451). These membranes selectively excluded solutes having a hydration radius greater than about 4.5 A. allowing solutes with a smaller radius to pass through. Molecular permeation through GO membranes is believed to occur along graphene channels that develop between GO sheets, and their sieving properties are defined by the interlayer spacing, d, which depends on the humidity of the surrounding. Immersing GO membranes in liquid water leads to intercalation of 2-3 layers of water molecules between individual GO sheets, which results in swelling and d ~ 13.5 A. The effective pore-size of 9 A in these swollen membranes (excluding the space occupied by carbon atoms) is larger than a typical size of hydrated ions and restricts possible uses of GO for size-exclusion based ion sieving. Unfortunately, many solutes which might be desirable to be able to filter out, including for example NaCI, have hydration radii which are below 4.5 A and are not effectively excluded from passing through the membrane.

[0010] WO2016/ 189320 (PCT/GB2016/051539) describes how graphene oxide laminate membranes could be modified, either by including graphene flakes or by including cross-linking agents, to improve the level of exclusion of solutes that have hydration radii which are below 4.5 A, e.g. NaCI.

[0011] In a first aspect of the invention is provided a water filtration membrane, said membrane comprising a graphene oxide (GO) laminate comprising a plurality of graphene oxide flakes the planes of which are orientated parallel to one another, said GO laminate having a first pair of oppositely disposed faces which are oriented parallel to the planes of the plurality of graphene oxide flakes, said GO laminate also having a second pair of oppositely disposed faces which are oriented perpendicular to the planes of the plurality of graphene oxide flakes and a third pair of oppositely disposed faces which are oriented perpendicular to the planes of the plurality of graphene oxide flakes; wherein the GO laminate membrane is enclosed by a first encapsulating material that covers each of the first pair of faces of the GO laminate and each of the second pair of oppositely disposed faces of the GO laminate and wherein the third pair of oppositely disposed faces are either not enclosed or are enclosed by a second encapsulating material, said second encapsulating material being porous...
Science 343, 752–754 (2014).
Science  14 Feb 2014:
Vol. 343, Issue 6172, pp. 752-754
DOI: 10.1126/science.1245711  
Precise and ultrafast molecular sieving through graphene oxide membranes.
Joshi, R. K. et al.
Graphene-based materials can have well-defined nanometer pores and can exhibit low frictional water flow inside them, making their properties of interest for filtration and separation. We investigate permeation through micrometer-thick laminates prepared by means of vacuum filtration of graphene oxide suspensions. The laminates are vacuum-tight in the dry state but, if immersed in water, act as molecular sieves, blocking all solutes with hydrated radii larger than 4.5 angstroms. Smaller ions permeate through the membranes at rates thousands of times faster than what is expected for simple diffusion. We believe that this behavior is caused by a network of nanocapillaries that open up in the hydrated state and accept only species that fit in. The anomalously fast permeation is attributed to a capillary-like high pressure acting on ions inside graphene capillaries.

Membranes based on graphene can simultaneously block the passage of very small molecules while allowing the rapid permeation of water. Joshi et al. (p. 752; see the Perspective by Mi) investigated the permeation of ions and neutral molecules through a graphene oxide (GO) membrane in an aqueous solution. Small ions, with hydrated radii smaller than 0.45 nanometers, permeated through the GO membrane several orders of magnitude faster than predicted, based on diffusion theory. Molecular dynamics simulations revealed that the GO membrane can attract a high concentration of small ions into the membrane, which may explain the fast ion transport.
Environ. Sci. Technol., 2016, 50 (7), pp 3875–3881
DOI: 10.1021/acs.est.5b06032
Selective removal of technetium from water using graphene oxide membranes
Williams, C. D. & Carbone, P.

The effective removal of radioactive technetium (99Tc) from contaminated water is of enormous importance from an environmental and public health perspective, yet many current methodologies are highly ineffective. In this work, however, we demonstrate that graphene oxide membranes may remove 99Tc, present in the form of pertechnetate (TcO4–), from water with a high degree of selectivity, suggesting they provide a cost-effective and efficient means of achieving 99Tc decontamination. The results were obtained by quantifying and comparing the free energy changes associated with the entry of the ions into the membrane capillaries (?Fperm), using molecular dynamics simulations. Initially, three capillary widths were investigated (0.35, 0.68, and 1.02 nm). In each case, the entry of TcO4– from aqueous solution into the capillary is associated with a decrease in free energy, unlike the other anions (SO42–, I–, and Cl–) investigated. For example, in the model with a capillary width of 0.68 nm, ?Fperm(TcO4–) = -6.3 kJ mol–1, compared to ?Fperm(SO42–) = +22.4 kJ mol–1. We suggest an optimum capillary width (0.48 nm) and show that a capillary with this width results in a difference between ?Fperm(TcO4–) and ?Fperm(SO42–) of 89 kJ mol–1. The observed preference for TcO4– is due to its weakly hydrating nature, reflected in its low experimental hydration free energy.!divAbstract
Chem. Soc. Rev. 39, 228–240 (2010).
The chemistry of graphene oxide.
Dreyer, D. R., Park, S., Bielawski, C. W. & Ruoff, R. S.
The chemistry of graphene oxide is discussed in this critical review. Particular emphasis is directed toward the synthesis of graphene oxide, as well as its structure. Graphene oxide as a substrate for a variety of chemical transformations, including its reduction to graphene-like materials, is also discussed. This review will be of value to synthetic chemists interested in this emerging field of materials science, as well as those investigating applications of graphene who would find a more thorough treatment of the chemistry of graphene oxide useful in understanding the scope and limitations of current approaches which utilize this material (91 references).
Adv. Mater. 27, 249–254 (2015).
Facile fabrication of freestanding ultrathin reduced graphene oxide membranes for water purification.
Huiyuan Liu
Freestanding ultrathin rGO membranes, with thicknesses down to 17 nm, are fabricated via a facile approach using hydroiodic acid vapor and water-assisted delamination. These unique membranes provide the potential for addressing the key challenge that limits the performance of current forward osmosis membranes.