Tao Zheng ,Xiuyang Zou ,Meisheng Li,*,Shouyong Zhou ,Yijiang Zhao,*,Zhaoxiang Zhong

1 Jiangsu Engineering Laboratory for Environment Functional Materials,Jiangsu Key Lab for Chemistry of Low-Dimensional Materials,School of Chemistry and Chemical Engineering,Huaiyin Normal University,Huaian 223300,China

2 State Key Laboratory of Materials-Oriented Chemical Engineering,National Engineering Research Center for Special Separation Membrane,Nanjing Tech University,Nanjing 210009,China

Keywords:Two-dimensional (ZD)g-C3N4 Membranes Separation

ABSTRACT Recent years,membrane separation technology has attracted significant research attention because of the efficient and environmentally friendly operation.The selection of suitable materials to improve the membrane selectivity,permeability and other properties has become a topic of vital research relevance.Two-dimensional (2D) materials,a novel family of multifunctional materials,are widely used in membrane separation due to their unique structure and properties.In this respect,as a novel 2D material,graphitic carbon nitride(g-C3N4)have found specific attention in membrane separation.This study reviews the application of carbon nitride in gas separation membranes,pervaporation membranes,nanofiltration membranes,reverse osmosis membranes,ion exchange membranes and catalytic membranes,along with describing the separation mechanisms.

1.Introduction

Membrane separation technology emerged at the beginning of the 20th century,and it is a vital technology for separation,concentration and purification of materials [1].This technology has the characteristics of high efficiency,energy saving,environmental friendliness,molecular level filtration and easy to control filtration process [2].Therefore,the membrane separation technology has been widely employed in diverse fields,such as food,medicine,biology,environmental protection,chemical industry,metallurgy,energy,petroleum,water treatment,electronics,bionicsetc.[3-5].Overall,it has become one of the most important means of separation,along with leading to significant economic and social benefits [1,2].Traditional three-dimensional (3D) membranes prepared from 3D materials have stable structure and outstanding performance.However,Compared to 2D membrane,3D membrane is difficult to prepare and its structure is complex and difficult to control.At the some time,the membrane-forming performance of 2D materials is better than that of 3D materials [6,7].In fact,an increasing number of new materials have been used in the preparation of membranes to achieve additional functionalities(photoelectric conversion,catalysis,etc.)[8,9].For instance,Tsapatsiset al.(2011) prepared a new separation membrane (ZSM-5)with zeolite nanosheets [10].Since then,the development of high-performance separation membranes from 2D materials has become a topic of significant research interest.In contrast with 3D membranes,2D membranes can be one-atom to a few atoms thick,therefore,these exhibit less resistance and high flux.Particularly,a high extent of separation is possible by adjusting the pore size of 2D nanosheets and interlayer channels of adjacent nanosheets [10-12].Thus these advantages have made 2D nanosheets membranes the superb candidates for highperformance separation.Graphene and its derivatives are one of the most widely studied 2D materials.Since 2012,graphenebased separation membranes have received widespread attention[13,14].Apart from graphene and its derivatives,other emerging 2D nanosheet materials including 2D zeolites,2D metal-organic frameworks (MOFs),2D covalent-organic frameworks (COFs),MXenes and graphitic carbon nitrides(g-C3N4)have also exhibited an unprecedented potential in microfiltration,nanofiltration,ultrafiltration,reverse osmosis,pervaporation and gas separation [15-17].g-C3N4,a novel non-metallic photocatalytic material,is mainly used in the field of catalysis because of its superior catalytic properties as compared to the existing materials [18,19].g-C3N4has unique photoelectric properties and is easily stimulated by light to produce electron-hole pairs.g-C3N4is a planar 2D lamellar structure similar to graphene and the flexibility of g-C3N4is better than 3D materials.g-C3N4nanosheets with single atomic layers can be prepared.In recent years,the potential of g-C3N4in the field of membrane separation has been continuously explored due to its unique structure and excellent performance[20,21].Form Fig.1,it can be seen that the g-C3N4has been applied in gas separation membranes,pervaporation membranes,nanofiltration membranes,reverse osmosis membranes,ion exchange membranes,catalytic membranes,and so on.

This review is structured as follows:Section 2 reviews the structure and methods to prepare g-C3N4,whereas;Section 3 discusses the industrial applications of g-C3N4in gas separation membranes,pervaporation membranes,nanofiltration membranes,reverse osmosis membranes,ion exchange membranes and catalytic membranes.The role of g-C3N4in these membranes has been correlated with the structure and properties of the membranes.Finally,the conclusions and some guidelines for future work are presented in Section 4.

2.Structure and Preparation of g-C3N4

Carbon nitride(C3N4)is known to be one of the oldest synthetic compounds.In 1996,Teteret al.employed the first-principles calculations and proposed that the C3N4has 5 structures including g-C3N4[28].g-C3N4exhibits a 2D planar lamellar structure similar to graphene,with triazine (C3N3) and 3-s-triazine (C6N7) rings as the basic structural units,extending indefinitely to form a network structure [29,30].Krokeet al.reported that the structure of 3-striazine ring is more stable than the triazine ring and the 2D nanosheet layers are held together by van der Waals forces with using density functional theory (DFT) [31].Due to the presence of 3-s-triazine ring structure,the g-C3N4layer is observed to have many pores [29-31].Fig.2 shows that the distance between the two adjacent atomic layers of g-C3N4is observed to be 0.326 nm,whereas the molecular kinetic diameter of the pore is 0.311 nm[28-30].

There are four main preparation methods for g-C3N4,including solid phase reaction,solvent-thermal,electrochemical deposition and thermal polymerization [32-36].Synthesis of g-C3N4by solid-phase reaction generally selects compounds containing triazine structure as reaction precursors,such as melamine and melamine,and uses LiN3and NaN3as nitrogen sources.g-C3N4should be prepared by solid phase reaction at a certain temperature.The method can flexibly adjust the C/N molar ratio and the nanostructure and morphology of the controllable material.Generally,g-C3N4is prepared by solvent thermal method with melamine,melamine and chlorine as raw materials,and NH2NH2and Et3N as solvent under a certain temperature.The method has the advantages of mild reaction conditions,easy control and good uniformity of the system.In recent years,electrochemical deposition has been applied to the preparation of g-C3N4thin films.This method is simple,easy to control and can reduce the temperature of the reaction system.Thermal polymerization is a direct and simple material preparation method,which forms g-C3N4through the condensation polymerization of precursors induced by heat.The thermal polymerization method has evolved as the preferred way to prepare g-C3N4on a large scale due to its several advantages,including ability to employ different precursors,low cost,moderate equipment and control requirement,simple operation and easy amplification.Initially,g-C3N4was applied in the field of catalysis[37,38].Compared to conventional metal oxide catalysts(Fe2O3,TiO2,ZnO,etc.) [39-42],g-C3N4has a wider spectrum of absorption [43,44].Especially,g-C3N4can perform photocatalysis in normal visible light without the need for ultraviolet light.As a classical 2D material,g-C3N4has unique structural characteristics different from traditional 3D materials.g-C3N4has good thermal stability,chemical stability,acid and alkaline resistance,and it is non-toxic and environmentally friendly.In recent years,g-C3N4has gained widespread research attention due to its unique structure and excellent performance.Therefore,the potential of g-C3N4in the field of energy is constantly being explored [45,46].

Fig.1.(a) The schematic diagram of mixed gas permeation setup [22];(b) The proposed mechanism for proton transport in vanadium ion permeation process in hybrid membrane [23];(c) The schematic of water-salt separation process by nanoporous g-C3N4 membranes [24];(d) The schematic illustration of the preparation of graphene oxide and polyelectrolyte complex nanohybrid membranes[25];(e)The schematic of water-salt separation process and antifouling process of PA/CNs membrane[26];(f)The schematic illustration of oil/water separation and photocatalytic degradation process in the composite membrane [27].

Fig.2.Molecular structure diagram of g-C3N4 [28-30]:(a) C3N3 and (b) C6N7;The schematic diagram of g-C3N4 molecular model:(c) Layers and (d) Pores.(1?=0.1 nm)

3.Application of g-C3N4 in Membranes

3.1.Gas separation membranes

A gas separation membrane is mainly used to screen gas molecules of different sizes through pores or holes [47-49].The same function can be achieved through the holes developed in materials[48,49].Therefore,g-C3N4has been used to separate gases using the pores on the surface.The gas separation mechanism for the g-C3N4membrane is proposed in Fig.3.Also,g-C3N4can be used to modify the original pores of the membranes to prepare mixed matrix membranes (MMMs) to improve the separation performance [50,51].Alternatively,the pores can also absorb gas molecules,and the adsorption effect is different for different molecules,thus,leading to their separation due to this characteristic [52,53].At present,most of the research focuses on H2separation and purification.Due to the structure,only molecules (H2,H2O,etc.) smaller than the pore size of g-C3N4can pass freely through the g-C3N4sheets.So,the separation and purification of H2by g-C3N4is feasible and easy to be realized.

Fig.3.Schematic diagram of gas separation membranes prepared by g-C3N4.

In 2015,Liet al.[22]demonstrated using first-principles calculations that as-synthesized g-C3N4exhibited high efficiency of helium separation from the gas molecules (H2,N2,CO and CH4)in natural gas and the noble gas molecules (Ne and Ar).For the transition state calculations,the authors performed the minimum energy path profiling using the climbing image nudged elastic band method (CNEB) implemented in the VASP transition state tools.The most energetically favorable configuration of a gas molecule on the g-C3N4membrane was determined through structural optimization.The authors observed that the gas molecules prefer to reside right above the pores at different heights depending on the specific gas.Through the energy barrier analysis,the diffusion rates and selectivities of different molecules for g-C3N4at different temperature were studied.In addition,the authors analyzed the steered molecular dynamics (MD) simulation for separating He from H2and3He from4He.The energy barrier for the He molecule passing through the g-C3N4membrane was calculated to be 0.354 eV,however,the selectivity over H2at room temperature was predicted to be as high as 107.More interestingly,g-C3N4served as an efficient quantum sieving membrane for3He/4He separation with a predicted transmission ratio of 18 at 49 K.The observed results offer a promising membrane system for separating3He from natural gas,which is quite crucial for cryogenic industries.It was for the first time that g-C3N4could be applied to gas separation membranes by using the theoretical calculations.In the following years,further research efforts proved the feasibility of such process.By using the first-principles calculations and molecular dynamics simulations,Jiet al.concluded that the porous g-C3N4monolayer works as an efficient and highly selective hydrogen purification membrane [54].The results indicated that the g-C3N4nanosheets exhibit a significant potential for separating H2from undesirable gases.Using a combination of density functional theory(DFT)and MD simulations,de Silvaet al.demonstrated that g-C3N4can effectively purify H2from CO2and CH4[55].The theoretical analysis employed by the authors indicated that the flux can be significantly improved by widening the pore area by applying biaxial strains as low as 2.5% and 5%.It was also observed that the strain tuning only improves the H2permeability of the membrane,while its excellent H2/CO2and H2/CH4selectivity is not compromised.In 2019,Guoet al.studied the separation of H2from the impurity gases (H2,N2,H2O,CO,Cl2,and CH4) by using the bilayer porous g-C3N4membrane with DFT[56].The studies highlighted a new approach towards achieving high permeance and high selectivity molecular-sieving membranes for simple structural engineering.

Fig.4.The transmission rate (a) and selectivity (b) of gas molecules passing through the pores on the g-C3N4 membrane as a function of temperature [22];The plots of (c)selectivity and (d) permeance versus temperature for H2,O2,N2,Cl2,CO,CH4 and H2O molecules passing through the bilayer g-C3N4 [56].

From Fig.4,it can be noted that the separation and purification of hydrogen from common components can be effectively achieved with g-C3N4.Thus,it is theoretically proved that g-C3N4can be used to prepare gas separation membranes.In addition to the theoretical calculations,the researchers have carried out experimental efforts to explore the use of g-C3N4in gas separation in recent years.In 2016,Tianet al.prepared novel mixed matrix membranes(MMMs)by incorporating the g-C3N4nanosheets into the polymer matrices exhibiting intrinsic microporosity(PIM-1,Fig.5)[49].The pure gas permeation tests of the MMMs were conducted for CO2,CH4,N2and H2.

The results showed that g-C3N4displayed a pronounced effect,on the gas transport within PIM-1/g-C3N4MMMs.Firstly,the 2D structured g-C3N4effectively influenced the packing of the polymer chains owing to the high aspect ratio.Correspondingly,the gas permeability of MMMs was significantly enhanced at a relatively low g-C3N4mass loading of 1%,as compared with the pure PIM-1 membrane.Moreover,the periodic ultramicropores of g-C3N4with size-sieving effect preferentially facilitated the transport of smaller molecules (especially,H2),and the selectivities for H2/CH4and H2/N2were observed to increase without compromising the gas permeability,as compared with pure PIM-1.The thermal and mechanical properties of the MMMs were also noted to improve with the incorporation of g-C3N4.Furthermore,the PIM-1/g-C3N4MMMs demonstrated superior long-term performance even at low g-C3N4loading (see Tables 1 and 2).

Fig.5.Chemical structure of PIM-1 [49].

Table 1Mechanical properties of pure PIM-1 and PIM-1/g-C3N4 MMMs [49].

Table 2Selectivity of as-cast pure PIM-1 and PIM-1/g-C3N4 MMMs for various gas pairs [49]

Houet al.[57]proposed rapid in-situ fabrication of zeolitic imidazolate framework-8(ZIF-8)hybrid membrane by using the 2D g-C3N4nanosheets at room temperature.The chemical structure of g-C3N4and the preparation process are shown in Fig.6.The negatively charged g-C3N4nanosheets with abundant nitrogen coordinating sites are capable of capturing and anchoring Zn2+,thus,providing a large number of heterogeneous nucleation sites for the formation of initial ZIF-8 crystals.During the cyclical spin coating of the Zn2+/g-C3N4nanosheets and ligand(2-methylimidazole),the in-situ formed ZIF-8 crystal nuclei on the g-C3N4nanosheets promote the further growth of the continuous defect-free membranes.The ZIF-8/g-C3N4membranes with a thickness of 240 nm can be obtained using this methodology within 30 min at room temperature,which represents an attractive attribute for fast and facile preparation of the molecular sieving membranes.Moreover,a promising H2/CO2separation performance with the selectivity up to 42 are observed,which is superior to the other ZIF-8 membranes.

Fig.6.(a)The chemical structure of g-C3N4;(b)The schematic illustration of the preparation of the molecular sieving membrane with assistance of the g-C3N4 nanosheets via cycled spin coating [57].

Fig.7.The schematic of the synthesis of:(a) protonated g-C3N4;(b) oxygen plasm treatment used to introduce hydroxylamine structure (N-OH) on the surface of g-C3N4 (hydroxylaminated g-C3N4);and (c) hydrazinated g-C3N4 [58].

For the separation of CO2/CH4system,Mariaet al.[58] protonized,hydroxylated and hydrated g-C3N4.As shown in Fig.7,The authors subsequently doped Matrimid with protonated g-C3N4to yield Matrimid@/g-C3N4MMMs for improving the separation performance.Due to the presence of abundant-C-N-bonds in the g-C3N4framework,it could be easily protonated by HCl,thus,resulting in surface charge modification from negative to positive.g-C3N4is a laminar material with non-oxidized aromatic and aliphatic phases containing phenolic,carboxyl and oxygen epoxide groups,which make it hydrophilic in aqueous media.This behavior can be modified by treatment with oxygen plasma or hydrazine monohydrate.Use of plasma with oxygen results in dissociation to generate oxygen-containing radicals,which incorporate -OH as hydroxylamine groups,on the surface of g-C3N4,thus,improving its dispersibility.

In the reported work,the novel MMMS was observed to improve the gas separation by enhancing the selectivity for CO2/CH4by up to 36.9%at 0.5%(mass)filler doping.With a view to further enhance the performance of the composite membrane due to the incorporation of g-C3N4,oxygen plasma and hydrazine monohydrate treatments were also assayed as alternatives to protonation.Hydroxylamination by oxygen plasma treatment enhanced the selectivity for CO2/CH4by up to 52.2% (at 2% (mass) doping)and O2/N2by up to 26.3% (at 0.5% (mass) doping).In comparison,the hydrazination process can achieve enhancements in the CO2/CH4separation by up to 11.4%,but the result is lower.Chitosan(CS) modified g-C3N4/ZIF-8/polyethersulfone (PES) membranes were synthesized for enhanced CO2/CH4separation by Abolfazlet al.in 2019[59].The layer by layer method of membrane fabrication was employed for the fabrication of ZIF-8 on PES support.The results indicated that the chitosan modified g-C3N4incorporated ZIF-8 membranes exhibit superior CO2/CH4ideal selectivity(24.2) as compared with the pristine ZIF-8 membrane.Due to the formation of the C-N bond between g-C3N4and imidazole group of ZIF-8,the flexibility of the modified ZIF-8 membranes was observed to increase remarkably,thus,leading to a superior performance as compared to other membranes.Both theoretical and experimental results indicate that g-C3N4can be used in the gas separation membranes with a superior separation effect.But,there are challenges as well.Due to the pores of g-C3N4,there are fewer gas molecules that can pass directly through the g-C3N4layers.More separation of gas systems can be achieved by forming channels with other materials.Selectivity is the most important.Only prepare the appropriate structure,high selectivity can be required.Though the research in this field is still in infancy,the observed findings in the recent years are envisaged to result in a significant research effort in this area.In the near future,g-C3N4is expected to be used in gas separation membranes at a large scale.

3.2.Pervaporation membranes for solvent dehydration

Membrane technology,with molecular level separation of substances under mild and low-cost conditions,has led to overcome serious challenges in the fields of energy,water resources and environment [60,61].Pervaporation organic dehydration and recovery is considered as the most important application of the pervaporation technology.Ideal membrane materials generally use hydrophilic polymeric materials with rigid chains and ability to form ion-dipole interactions or hydrogen bonds with water[62].Obviously,during application of pervaporation,there is a mutually constrained relationship between the permeability and selectivity of the membrane [25,63].As the selectivity increases,the permeability of the components decreases,correspondingly,i.e.,the ‘‘trade-off” effect.One approach to overcome the ‘‘tradeoff” effect of the pervaporation membranes is to prepare the organic/inorganic nano-hybrid membranes.

By adding the commonly used inorganic particles including carbon quantum dots [64],graphene and its derivatives [65,66],organometallic frameworks [67],etc.,the mechanical properties,thermal stability and solvent resistance of the pervaporation membrane can be enhanced.As shown in Fig.8,compared with the pervaporation membranes prepared by using graphene oxide,the g-C3N4pervaporation membranes demonstrate the following advantages:(1) the water nanochannels in the g-C3N4membrane are stable and rigid under increasing pH conditions and pressures;(2) the N atoms in g-C3N4have the possibility to form hydrogen bonds with the water molecules or other hydroxyl groupcontaining solvent molecules in the inter-layer,thus,leading to a unique phenomenon.In our previous study[68],it was noted that the hydroxyl groups of ethanol are closer to the surface of g-C3N4than the methyl groups.This phenomenon is completely opposite to the report by Gao [69],where the methyl groups in ethanol are observed to be closer to graphene than the hydroxyl groups.The observed phenomenon is due to the lack of the hydrogen bond between ethanol and graphene surface.Therefore,the interaction between g-C3N4and ethanol is stronger,which is conducive for the separation of ethanol and water.Pervaporation hybrid membranes includes polymer phase (Polyvinyl Alcohol),inorganic phase (g-C3N4) and polymer-inorganic interface phase.Polyvinyl alcohol (PVA) was formed on the surface of nitride by interfacial polymerization.The inorganic particles and polymer matrix exhibit four main interfacial morphologies,such as micro-phase separation,strong binding,medium binding and weak binding morphologies(See Fig.9).The current research efforts focus on obtaining the high-performance pervaporation membranes by adjusting the interaction of the organic-inorganic interface.

Fig.8.Schematic diagram of solvent dehydration with g-C3N4.

By creating pores,the properties of g-C3N4can be tuned to selectively allow the water molecules to pass through g-C3N4,while rejecting salts and dyes.For instance,Wang and coworkers developed a g-C3N4membrane with artificial nanopores and self-supporting spacers by acidifying g-C3N4with concentrated hydrochloric acid [69].The pore size of the g-C3N4nanosheets must be accurately tuned to allow the water molecules to pass,while rejecting the other molecules and ions.In this respect,the g-C3N4membrane characterized by artificial nanopores (sizes between 1.5-3 nm) is observed to have an optimal separation performance.For the membrane thickness of 160 nm,the water flux of the g-C3N4free-standing membrane was observed to be 29 L·m-2·h-1·bar-1(1bar=0.1MPa),with a rejection rate of 87% (molecules larger than 3 nm).In 2015,Caoet al.[70] added g-C3N4to sodium alginate (SA) for the first time to develop a pervaporation hybrid membrane with excellent performance.The weak binding morphology (intramolecular hydrogen bond) was observed between -NH2and -NH in g-C3N4and -OH and COOin the SA chain.At 76 °C,the hybrid membrane with g-C3N4content of 3%(mass)was used for the dehydration analysis employing 90% (mass)ethanol solution.The water flux of the membrane was observed to be 2469 g·m-2·h-1,whereas the separation factor was noted to reach up to 1653.In addition,the tensile strength of the membrane reached a maximum of 120 MPa,along with a superior thermal stability (～215 °C).Similarly,Wanget al.[71] fabricated the pervaporation hybrid membrane by incorporating the g-C3N4nanosheets into a succinic acid-crosslinked PVA matrix.The addition of succinic acid tuned the organic-inorganic interfacial strength of the pervaporation membrane,and the hydrophilicity and heat-resistance properties of the membrane were correspondingly improved.The water flux of the g-C3N4membrane was noted to be 6332 g·m-2·h-1,with the separation factor reaching 30.7 for 90% (mass) ethanol solution at 75 °C,as compared with 2337 g·m-2·h-1and 11.2 for the cross-linked pure PVA membrane,thus,breaking the ‘‘trade-off effect” of the pervaporation membrane.Although the addition of carbon nitride enhances both membrane flux and separation factor,the dispersion and compatibility of g-C3N4in the hydrophilic polymer matrices require further investigation.By performing the surface chemical modification of g-C3N4to regulate the organic-inorganic interfacial microstructure in the pervaporation separation layer,a highperformance pervaporation membrane can be obtained [72].In our earlier work,different types of g-C3N4and its derivatives including g-C3N4[63],hydrogen peroxide activated g-C3N4(O-g-C3N4) and polydopamine modified g-C3N4(PDA@O-g-C3N4) [72]were added to the cross-linked PVA for preparing the pervaporation membranes,as shown in Fig.10.For the cross-linked PVA/Og-C3N4(CPVA-O-g-C3N4) hybrid membranes,the interfacial interaction between the PVA matrix and O-g-C3N4nanosheets exhibits not only weak hydrogen bonding,but also rigid chemical bonding interactions.The -COOH group in O-g-C3N4reacts with the OH group in the PVA chain to form an ester group,thus,confirming the role of O-g-C3N4as a crosslinking agent.The permeation flux and separation factor of the CPVA-O-g-C3N4membrane were noted to be 3392 g·m-2·h-1and 28.9,respectively.The interaction at the polymer-inorganic interfaces in the CPVA-PDA@O-g-C3N4hybrid membranes result in flexible chemical bonds,thereby leading to further grafting of PDA.The formation of the flexible chemical bonds reduces the number of non-selective interface pores in the hybrid membrane,thus,significantly improving the membrane’s permeability selectivity towards aqueous ethanol solution.The permeation flux and separation factor of the CPVA-PDA@O-g-C3N4membrane were noted to be 2328 g·m-2·h-1and 57.9,respectively.Fig.11 compares the separation performances of different PVA-based pervaporation membranes for ethanol dehydration.

Fig.9.Interface morphology of polymer and inorganic particles [68].

Fig.10.The reaction mechanisms of hybrid membranes prepared with PVA,Sa and g-C3N4 [72].

Fig.11.Separation performance of PVA-based membranes for ethanol dehydration as reported in literature and in this work.Membranes reported in previous studies:CS-PVA (% (mass));Glutaraldehyde crosslinked PVA,Sulphated crosslinked PVA,PVA/sericin blend,PVA/7% (mass) sulphosuccinic acid and PVA/5% (mass) Li+sulphosuccinate;PVA(3)-PES(15);PVA-zeolite 4A;PVA-APTES/TEOS;PVA/PTEE and PVA-A4-80 [72].

The g-C3N4composite membrane breaks the ‘‘trade-off” effect between the permeation flux and separation factor as compared to the traditional polymer membranes.Moreover,the hybrid composite pervaporation membranes exhibit high swelling resistance and mechanical stability.For a certain mixture,the permeation rate depends mainly on the properties of the membrane.The membrane with a high permeation rate to one component and a low permeation rate to the other component can be obtained by using appropriate membrane materials and manufacturing methods.The prepared 2D g-C3N4and modified g-C3N4need to be further refined and modified.High performance pervaporation membranes can be obtained by modifying g-C3N4.

3.3.Nanofiltration and reverse osmosis membranes for water treatment

In addition to solvent dehydration by pervaporation,g-C3N4can be used for the removal of dyes and metal ions by virtue of its excellent adsorption properties and reusability.The dyes or metal ions entrapment mechanism by g-C3N4is given in Fig.12.To ensure high dye and metal ion selectivity and water permeability,the membranes must have a narrow pore size distribution,porous structure and low thickness[73-75].It is well known that various factors,such as pore size,ion size,interaction between ions and membranes,hydration of nanopores and polymer membrane surface affect the dye and metal ion selectivity and water permeability of the membranes [76-78].The nanopores can be generated on g-C3N4to allow the channels of water,while rejecting salts and dyes[79].The stacking of the g-C3N4nanosheets as well as their hydrophobic nature result in ultra-low friction water channels for the rapid flow of water across the membrane [80].Therefore,either g-C3N4naonosheets with nanopores or stacked g-C3N4can be employed for water purification application.The following sections discuss the application of g-C3N4for the purpose of water purification,especially for salt rejection,water permeation capability and anti-pollution performance of membrane.

3.3.1.The improvement of salt rejection through nanoporous g-C3N4

Compared with the conjugated double bonds in the tri-striazine units in g-C3N4,the single bonds (N-(C)3) connecting the N atoms and tri-s-triazine units are weaker,thus,enabling their easy elimination to adjust the pore size [81].Fig.13 presents the typical schematic of the water-salt separation process by nanoporous g-C3N4membranes.

Using classical molecular dynamics,Liu and coworkers reported the permeation of water and ions through the g-C3N4nanoporous membranes with different pore sizes[24].By accurately preparing the nanopores on the g-C3N4nanosheet,the g-C3N4membrane was observed to sieve out the monovalent cations (at least 70%) and reject the bivalent cations(Ca2+)entirely,while allowing the water molecules to penetrate.The cross-section area of the nanopores in the g-C3N4nanosheet was observed to be 1.74 nm2,3.93 nm2and 6.99 nm2,respectively,and the corresponding converted radius was 0.74 nm,1.12 nm and 1.49 nm,respectively.In another study,Xiaoet al.[82] prepared the 2D ultra-thin free-standing polymerized carbon nitride membranes with a unique layered structure to control the ion transport and salinity gradient energy conversion.The g-C3N4free-standing membrane were negatively charged at both pH 5.8 and 10 due to unreacted amino and imine residues,thereby improving the ion selectivity at low concentrations.Although porous g-C3N4demonstrates a high extent of dye retention,it is still a technical challenge to generate uniformly sized nanopores on a large area of the nanosheets.In addition,the current research studies have reported that porous g-C3N4separates water,ions or dyes solely by pore size.In order to further improve the salt retention and selectivity,functionalized porous g-C3N4should be considered.

Fig.12.Schematic diagram of particle entrapment with g-C3N4.

Fig.13.The schematic of the water-salt separation process by nanoporous g-C3N4 membranes [24].

3.3.2.Water permeability improvement through interlayer nanochannels in g-C3N4

The g-C3N4membranes demonstrated ultra-high water diffusion coefficients due to the ultrafast transportation of the water molecules through confinement between the g-C3N4nanosheets[83].Fig.14 shows that the schematic of the water-salt separation process and antifouling process of the PA/CNs membrane[26].The experimental analysis and MD simulations showed that the velocity of the water molecules through the two-layer g-C3N4nanosheets was faster thann-hexane molecules (though the viscosity of water is larger thann-hexane).Meanwhile,the flux of the water molecules through the two-layer g-C3N4nanosheets was noted to be similar to that of the four-layer g-C3N4nanosheets.This can be attributed to the ultralow friction of the water molecules between the channels.

Wang and coworkers tuned the interlayer distance in the g-C3N4membranes by inserting the anions between the g-C3N4nanosheets for the selective separation of the organic species in the aqueous solution [84].The intercalation of the sulfate anion enhanced the interlayer distance of the membrane to～1.08 nm,thus,resulting in a water flux of 111 L·m-2·h-1·bar-1(1 bar=0.1 MPa),along with a highly enantioselective permeation towards limonene racemate with an enantiomeric excess value of 89%.In another study,Gaoet al.[21] incorporated acidified g-C3N4in the polyamide (PA) layerviainterface polymerization.The membrane water flux was enhanced due to the formation of the nano-water channels in the PA selective layer and enhancement of the hydrophilicity at the membrane surface.The membrane incorporated with acidified g-C3N4exhibited a permeate flux of 45.0 L·m-2·h-1(1.8 MPa) with a NaCl rejection of 98.6%.Similarly,Abdul Aziz and coworkers fabricated the composite membranes by incorporating g-C3N4or protonated g-C3N4(pCN)to achieve salt separation in the aqueous solution [85].After protonation,the g-C3N4nanosheets exfoliated to pCN,thus,resulting in small size,high water dispersibility and an enhanced extent of defects on the edges of the pCN network.The high water dispersibility of pCN improved the water permeability by preventing the closure of the nanochannels.In addition,the defect-rich topology of pCN provided a shorter path for rapid water diffusion.Therefore,the acidified or protonated g-C3N4enhanced the overall membrane flux.

Yeet al.[86] prepared the g-C3N4nanofiltration membrane by doping the g-C3N4nanosheets in the homogeneous dispersion of polyethylenimine (PEI) and dopamine (PDA).The g-C3N4nanofiltration membrane exhibited high permeability ((28.4 ± 1.2)L·m-2·h-1·bar-1) due to the formation of the nano-channels in the PDA/PEI layer.Wanget al.[87] fabricated the g-C3N4-PAA hybrid membranes by incorporating polyacrylic acid (PAA) in the g-C3N4nanosheets.The thickness of the membrane increased from 405 to 680 nm on enhancing the PAA content from 0.1 to 0.5(mass ratio of PAA to the g-C3N4nanosheets).Though the thickness of the membrane was enhanced,the membrane flux did not decrease(from 117 to 143.1 L·m-2·h-1) due to the larger nanochannels formed on incorporating PAA.By adding the nanoparticles of different dimensions,the surface roughness and microstructure of the membranes could be modified,thus,enhancing the pure water flux of the membranes.In another study,Liuet al.[88]transferred g-C3N4,halloysite nanotubes and piperazine monomers in the PSF substrate by vacuum filtration,followed by polymerization with trimesoyl chloride by interfacial polymerization to prepare the thin film nanocomposite (TFN) membranes.The nanomaterial addition enhanced the surface roughness and hydrophilicity of the TFN membranes.As a result,the water flux of the TFN membranes was observed to improve further (205 L·m-2·h-1·MPa-1).Table 3 presents the desalination parameters of various g-C3N4based nanofiltration membranes.

Table 3Comparison of divalent and monovalent rejection as well as water permeation abilities of various g-C3N4 incorporated membranes (1 bar=0.1 MPa)

In summary,by tuning the membrane channels and pores on g-C3N4,the flux and selectivity of the membranes could be effectively controlled.Nanofiltration and reverse osmosis membranes have high ion retention rate.Compared with the graphene membrane,the charge distribution of the g-C3N4membrane exhibited a significant impact on the penetration of ions through the nanopores and nanochannels.So,It has a lot of limitations in ion intercept for g-C3N4.The charge distribution of the material should be changed to achieve a high ion retention rate.

Fig.14.The schematic of the water-salt separation process and antifouling process of the PA/CNs membrane [26].

3.4.Ion exchange membranes

The ion exchange membranes,as a kind of polymer films,have selective transmissibility towards ions.The major performance indicators include ion exchange capacity (IEC),conductivity and selective transmittance [93-95].As shown in Fig.15,g-C3N4is resistant to strong acids as well as alkalinity and produces electron-hole pairs swiftly [94,96].Therefore,g-C3N4is added to the ion exchange membranes to improve their performance [97].There are a large number of amino groups on the surface of g-C3N4,so g-C3N4can be combined with other materials to form ion transport channels.Simultaneously,due to the characteristics of g-C3N4,the structural properties of the ion exchange membranes are also improved [93].

Fig.15.Schematic diagram of ion transport mechanism with g-C3N4.

Hybrid membranes (SPEEK/g-C3N4) composed of sulfonated poly (ether ether ketone) (SPEEK) and g-C3N4were fabricated by Niuet al.in 2016 by employing a solution-casting method for vanadium redox flow battery(VRB)[23].The authors observed that the physicochemical properties,such as swelling ratio (SR),ion exchange capacity,proton conductivity and vanadium ion permeability,changed with the content of the g-C3N4nanosheets.

The structure-property performance of the SPEEK/g-C3N4hybrid membranes was analyzed,whereby the physicochemical properties such as swelling ratio,water uptake (WU),electrolyte uptake(EU),proton conductivity(δ)and vanadium ion permeability were studied as a function of the content of the g-C3N4nanosheets.The desired ratio of g-C3N4in the SPEEK/g-C3N4hybrid membranes varied from 0.5 to 2.5% (0.5%,1%,1.5%,2% and 2.5%(mass)) (see Table 4).

The results indicated that the interfacial interaction was regulated by the incorporated g-C3N4nanosheets,which effectively controlled the ion selectivity,vanadium ion permeation and structure stability of the hybrid membranes.The mechanisms for the proton transport and vanadium ion permeation in the hybrid membrane were proposed in Fig.16.The acid-base pairs and unique nanoporous structure of the g-C3N4nanosheets promoted the proton transport and restricted the vanadium ion permeation.In another similar study,Wanget al.[96] prepared a composite membrane in 2017 by adding oxidized g-C3N4(OCN).The bulk g-C3N4was acid-treated by using a mixture of concentrated sulfuric acid and nitric acid to produce OCN at room temperature.A large number of amino groups on the surface of the g-C3N4were oxidized by the strong acids and correspondingly replaced by the hydroxyl groups.The permeability and selectivity of vanadium were tested to ascertain the properties of the membrane (see Table 5).

The uniformly dispersed OCN powder in the SPEEK matrix not only decreased the diffusion of the vanadium ions significantly,but also enhanced the ion selectivity of the composite membrane.Meanwhile,a superior cycling stability of the hybrid membrane was obtained.Ion exchange membranes are most widely used inbatteries.In this respect,the use of g-C3N4to modify the ion exchange membrane in batteries to improve their performance has attracted a significant research attention.A series of proton exchange membranes have been fabricated by Liuet al.in 2019 with the g-C3N4nanosheets as nanofiller and sulfonated polysulfone (SPSF) as membrane matrix by employing the simple solution-casting method [97] (see Table 6).

Table 4WU,SR,EU,IEC and σ of Nafion 117,SPEEK and SPEEK/g-C3N4 hybrid membranes [23].

Table 5The physicochemical properties of Nafion 117,pure SPEEK and composite membranes [96].

Table 6WU,SR,IEC,and σ of Nafion 117,SPSF and SPSF/g-C3N4 composite membranes [97].

Fig.16.Proposed mechanism for the proton transport and vanadium ion permeation in the hybrid membrane [23].

The ion selectivity,permeability,proton conductivity and mechanical properties were observed to be significantly enhanced on incorporation of g-C3N4in the SPSF matrix.SPSF/g-C3N4-1 exhibited the highest ion selectivity and lowest permeability as compared to Nafion 117 and pristine SPSF.An optimal balance between selectivity and permeability of the high-performance acid-base composite membranes could,thus,be promoted for vanadium redox flow batteries.In summary,by virtue of its physical and chemical properties,g-C3N4improves the selectivity of the ion exchange membranes,while maintaining an optimal permeability.Further,the conductivity and IEC can be improved,along with the cycle time and service life of the membranes.The amino group on the surface of g-C3N4is the key to the application of ion exchange membranes.Ion transport channels can be obtained when the g-C3N4combined with other materials.

3.5.Catalytic separation membranes

During the actual operation,especially with the organic components,the surface of the membrane absorbs a large number of organic substances after a period of use,thus,resulting in the membrane pollution and decreased flux,thus,weakening the separation effect [98,99].To overcome this issue,the substances are usually added to the membrane to reduce the adsorption of the organic matter on the surface in order to improve the antipollution ability of the membrane [98-101].

Fig.17.Schematic illustration of the photo excited electron -hole pairs in g-C3N4 with possible decay pathways.A and D denote electron acceptor and electron donor,respectively [102].

Fig.18.Schematic diagram of catalytic separation membrane prepared with g-C3N4.

As a novel non-metallic photocatalytic material,g-C3N4exhibits a wide range of absorption spectrum and can play a photocatalytic role in the ordinary visible light without the need of the ultraviolet light [102-104].As shown in Fig.17,g-C3N4effectively activates the molecular oxygen and produces the superoxide radicals,which can be used for the photocatalytic transformation of the organic functional groups and photocatalytic degradation of the organic pollutants.Due to its photocatalytic properties,g-C3N4is added to catalyze the decomposition of the organic components on the membrane surface in Fig.18 [105].In 2016,Zhaoet al.[106]assembled the graphitic carbon nitride nanosheet/reduced graphene oxide (g-C3N4NS/RGO) composite based photocatalyst on a commercial cellulose acetate (CA) membrane by vacuum filtration and high-pressure processing for water treatment.The photocatalytic efficiency of the g-C3N4NS/RGO composite with different RGO contents was evaluated through the degradation of Rhodamine B (RhB) in a quartz reactor.The g-C3N4NS/RGO/CA composite photocatalytic membrane exhibited high efficiency for the removal of the organic contaminants in the integrated process of filtration and visible light photocatalysis.The membrane was observed to demonstrate antibacterial properties as well.In a similar study,a RGO/PDA/g-C3N4-CA composite membrane was prepared by Liet al.in 2017 for the catalytic decomposition and oilin-water emulsion separation [27].The synthesis of the RGO/PDA/g-C3N4composites is shown in Fig.19.The RGO/PDA/g-C3N4membrane was fabricated by carrying out the dopamine modification,followed by the assembly of the RGO/PDA/g-C3N4composite on the surface of commercial CA membrane.

The photocatalytic efficiency of the free-standing RGO/PDA/g-C3N4composite membranes with different g-C3N4content were evaluated through the degradation of methylene blue (MB).The oil/water emulsions were prepared by mixing the diesel oil and water.The oil/water emulsion and MB aqueous solution containing H2O2were filtered through the RGO/PDA/g-C3N4/CA composite membrane using a vacuum filtration setup and visible-light irradiation.The oil/water separation and photocatalytic degradation process in the composite membrane are shown in Fig.20.The novel RGO/PDA/g-C3N4composite membranes with flow-through catalytic degradation of the soluble organic dye molecules and oil/water emulsion separation were successfully prepared by using the vacuum filtration method.The authors attributed the excellent performance to the compact 2D-2D structure of RGO and g-C3N4with dopamine modification.RGO with superior electron transport properties efficiency separated the photoelectrons by g-C3N4production under light illumination.As shown in Fig.21,a highly efficient oilfield produced water treatment,the photocatalytic nanofiber-coated alumina hollow fiber membranes were prepared by Aliaset al.[107] in 2018.The membranes were fabricated by coating the polyacrylonitrile (PAN) nanofibers incorporated with photocatalytic g-C3N4on Al2O3hollow fiber membrane.The highly porous coating consisting of smooth hydrophilic nanofibers facilitated the water permeation,with the coating effectively capturing the oil droplets in its opening,thus,resulting in an effective rejection efficiency against oil contaminants.In another study,GCNembedded PAN nanofibers were electrospuned on top of the Al2O3hollow fiber membrane surface using direct electrospinning technique.The permeability and pollution resistance of the membrane were investigated for the treatment of oilfield produced water (OPW).

Fig.19.Schematic of the synthesis of the RGO/PDA/g-C3N4 composites [27].

Fig.20.The schematic illustration of oil/water separation and photocatalytic degradation process in the composite membrane [27].

Fig.21.Crossflow filtration of OPW using NF-nsGCN/Al2O3 membrane [107].

Fig.22.(a)Removal of RhB by different membranes under diverse conditions[106];(b)E.coli removal under different conditions.Photocatalytic degradation of MB by free standing membrane without H2O2 (c) and with H2O2 (d) under visible-light irradiation [27].

The crossflow filtration using the fabricated membranes demonstrated a significant improvement in the pure water flux,OPW permeate flux and oil rejection percentage as compared to the bare Al2O3membrane.Sparse mesh structure,high water affinity and smooth nanofiber morphology were observed to be the key parameters for the nanofiber coatings leading to significantly improved membrane performance.The photodegradation ability of the NF-nsGCN nanofibers enabled the coating to degrade the captured oil contaminants under UV irradiation,which helped to maintain the high permeate flux and rejection in the repeated filtration system.

Overall,the literature studies confirm that the excellent photocatalytic performance of g-C3N4has a significant impact on the anti-pollution(Fig.22(a),(b))and catalytic decomposition abilities(Fig.22(c),(d)) of the membrane.When g-C3N4is applied to catalytic separation membranes,catalytic performance of g-C3N4will be affected.The key to this research is how to balance these two properties.

4.Conclusions

The motivation for this review originates from the abundant research studies and extensive developments in the membrane technology during the last decade.Specifically,the 2D membranes from g-C3N4are focused in this review due to their strong potential of commercial application,though only a few reports are currently available on the application of g-C3N4in membranes.For this purpose,a few prominent C3N4systems were chosen and analyzed in detail.g-C3N4exhibits the characteristics of visible light catalysis and unique 2D microstructure.So,by incorporating g-C3N4to membranes,the performance of the membranes is expected to be significantly improved.However,the addition of carbon nitride was not particularly effective for all membranes.Comparing recent studies,we think that g-C3N4is more suitable for nanofiltration membranes or reverse osmosis membranes.Because g-C3N4has a higher retention rate for ions and macromolecules than other particles.At the same time,the catalytic performance of the bulk g-C3N4is not good and can be improved when the g-C3N4was treated,such as g-C3N4nanosheets.Nevertheless,the catalytic performance of g-C3N4is lower than that of traditional metal catalysts.In terms of materials,the performance of bulk g-C3N4is not particularly outstanding and must be modified to give full play to its superior performance.Green and environmental protection is now the trend of research.As a stable,non-toxic and pollution-free material,g-C3N4will be applied in more fields and scenarios in the future not only membranes.The preparation of high performance g-C3N4is a research hotspot in the future.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are grateful for the financial support of the National Natural Science Foundation of China (No.21878118),Natural Science Foundation of the Jiangsu Higher Education Institutions of China(21KJA530002,19KJA150009),Natural Science Foundation of Jiangsu Province (BK20211368) and Jiangsu Province Qing Lan Project for the Young Academic Leaders (Meisheng Li,2021).