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2D nanochannels and huge specific surface area offer unique ways for water remediation and adsorption: assessing the strengths of hexagonal boron nitride in separation technology


This review highlights the advantages of incorporating hexagonal Boron Nitride (BN) into the current membrane-based architectures for water remediation over other well-explored 2D nanomaterials such as graphene, graphene oxide, molybdenum sulphide, MXenes. BN has an interlayer spacing of 3.3A0 which is similar to that of graphene, but smaller than that of the other 2D nanomaterials. BN is bioinert, and stable under harsh chemical and thermal conditions. When combined with thin film composite and mixed matrix membrane architectures, BN can help achieve high permeance, dye rejection, and desalination. Laminar membranes assembled by BN nanosheets do not swell uncontrollably in aqueous environments unlike graphene oxide. BN nanomaterials have a large specific surface area which implies more adsorption sites, and are inherently hydrophobic in nature, which means the adsorbent in its powder form can be easily separated from contaminated water. BN adsorbents can be regenerated by treating with chemicals or heating to high temperatures to remove the adsorbate, without damaging the BN, due to its thermal and chemical inertness. BN nanomaterials have the potential to circumvent the current shortcomings of membranes and adsorbents, while greatly enhancing the performance of membranes and adsorbents for water remediation.

Graphical Abstract


The availability of clean water is one of the most critical issues globally, now more so than ever. With the ever-expanding population of the world and the diminishing sources of fresh groundwater, it is proving increasingly challenging to meet the clean water needs of the modern world [1]. There is an urgent need for innovation and technology to face the global water challenge. Amongst the most prevalent ways of water remediation, membrane-based filtration methods hold the reins compared to their counterparts due to their high separation efficiency, low thermal energy input, and high-quality output. Polymer-based membranes are the most prevalent in all the filtration regimes – microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), as these are easy to process on an industrial scale, and their properties can be tuned to suit the end requirement. In the RO and NF regimes, polyamide-based thin-film composite (PA-TFC) membranes currently rule the market as these are easy to process, offer control over the thickness of the active polyamide layer, and most importantly, have high rejection rates of salts and other micropollutants. However, the current state-of-the-art polyamide desalination membranes are only twice as porous as they were 20 years ago, with the same selectivity.

These membranes are still chlorine intolerant, which makes disinfection cumbersome and increases membrane fouling and thus reduces its filtration capacity [1]. Water transport occurs through the solution-diffusion mechanism- a relatively slow process. NF and RO membranes suffer from low flux, severe fouling, and high energy input. In addition, RO systems require post filtration steps like remineralization. These are some of the reasons that led to the Government of India to completely ban the domestic usage of RO, unless required.

Although the research on PA-TFC membranes continues to move ahead at full throttle, these shortcomings have prompted researchers to search for alternative materials. Considering that there is a need for a breakthrough membrane technology with high water-to-solute selectivity if the needs of future generations are to be met, 2D nanomaterials seem to be the ideal candidates for this purpose. The isolation of graphene in 2004 at the University of Manchester sparked a revolution in the study and application of 2D nanomaterials in various disparate fields. Membrane technology is no exception. The last decade has seen an explosion of research on water filtration membranes based on these nanomaterials. 2D nanomaterials possess nanometer level thickness, leading to higher flux, large surface area, which results in numerous transport pathways and greater area for solute-surface interaction, vast array of tunable properties that can be tailored to meet the filtration needs, and nanometer-thick transport channels and pores. These nanomaterials can be incorporated into the polymer matrix to obtain mixed matrix membranes, they can be free-standing porous 2D membranes, or can be deposited as an active layer atop a support membrane [2, 3]. The numerous ways in which nanomaterials can be used to enhance the performance of existing membranes and to fabricate completely new high-performance membrane systems is what propels the thriving research. These materials have been the focus of extensive research in the last few years. This review aims to bring the reader up to date by highlighting the work done on BN-based water purification membranes specifically and Graphene, MXenes and MoS2 based membranes generally. Unless mentioned otherwise, BN here implies hexagonal boron nitride.

BN is made up of boron and nitrogen arranged alternatingly forming a hexagonal pattern in each monolayer. The monolayers follow an ABAB stacking pattern, due to the van der Waals forces between the monolayers as well the partial positive and negative charges on boron and nitrogen respectively. In the following sections, a few studies on graphene and graphene oxide, dichalcogenides, MXenes are discussed followed by a thorough survey of membrane applications and the role as an adsorbent of hexagonal boron nitride, which is the material of focus of this article. This review for the most part includes only the experimental studies involving these nanomaterials. As this review focuses on BN, the reader is referred to other general works on GO, MXenes, Dichalcogenides in their respective sections.

2D nanomaterials for water purification (membranes)

While the transport of solutes, water and other solvents occurs through solution-diffusion in RO membranes, and through convection in the UF and MF membranes and a combination of convection and diffusion in NF membranes, the transport in 2D NMs occurs through molecular sieving through nanochannels and nanopores, and electrostatic interactions. 2D nanomaterials possess a host of desirable properties. A few have even been found to be anti-bacterial. The sections on each of the nanomaterials will illustrate, through studies, the properties that make these materials superior in water filtration applications.

Graphene and graphene oxide

Studies on membranes based on graphene and its derivatives have burgeoned in the last few years because of their potential to enhance, if not replace, the performance of current water purification membranes. This can be attributed to the thinness, large surface area, vast array of tunable properties, pliability in terms of functionalization, mechanical strength, antimicrobial activity, and low density of graphene and its derivatives [3, 4]. In pristine graphene, carbon atoms that are sp2 hybridized are arranged in a hexagonal pattern in a single layer, about 0.34 nm thick [5]. The atomic-level thickness of the layers results in very thin laminar membranes with high flux, even thinner than the typical 100 nm polyamide active layer [6]. Additionally, graphene shows better resistance to chlorine compared to polyamide. In its pristine form, graphene is impermeable to water. This can be circumvented by introducing nanopores into the sheets, as demonstrated in a modelling study by Suk and Aluru. Water flux observed was greater than that of CNT when the pore diameter was larger than 0.8 nm [7]. Although other studies have corroborated the high water permeability values along with salt rejection via sieving through the nanopores [8], fabrication of large, leak-free single layer graphene with controlled pore size and pore size distribution on an industrial scale remains an issue. On the other hand, graphene oxide (GO) is a derivative of graphene made by chemical oxidation and ultrasonic exfoliation of graphite and is easy to fabricate on a large scale. Depending on the degree of oxidation, GO consists of graphene sheets with hydroxyl, epoxide, carboxyl, and carbonyl groups in varying fractions, which increases the interlayer spacing to over 0.6 nm. These sheets can be easily stacked to form laminar structures held together by hydrogen bonding [5, 9]. Multilayer GO membranes are highly permeable to water because of the porous microstructure which includes the space between the edges of adjacent sheets, wide channels at wrinkles, and nanopores in the sheets. The latter contributes negligibly to multilayer structures. The selectivity of these membranes depends on the interlayer spacing, electrostatic interactions between the negatively charged sheets and ions, and ion adsorption effects like cation-π and metal coordination to the sheets [10,11,12]. The major shortcoming of GO membranes, due to the presence of hydrophilic groups, is their uncontrollable swelling and instability on hydration, which largely increases the interlayer spacing and affects sieving ability. This can be controlled to an extent by reducing or chemically cross-linking the GO sheets, or stitching them to a polyamide layer [13, 14]. Other shortcomings of GO multilayered membranes include low mechanical stability and low desalination capacity [15, 16]. Extensive work has been done on membranes based on graphene and its derivatives in the past decade. For detailed reviews of the same, the reader is directed to references [4, 5, 10, 11, 15, 16].

Transition metal dichalcogenides (TMD)

TMDs are a class of materials with the formula MX2 where M is a transition metal element from groups IV, V or VI and X is S, Se or Te, i.e. chalcogens. These materials form laminar structures of the form X—M—X, with the chalcogen atoms in two hexagonal planes separated by a plane of metal atoms. Adjacent layers are held together by weak van der Waals forces [17]. One TMD that has been extensively studied for applications in electronics, and more recently in membranes is MoS2. With the S atoms above and below the Mo plane, these layers are smooth due to the absence of functional groups unlike in GO, which can result in unimpeded water flow due to low hydraulic resistance. The absence of extruding hydrophilic functional groups also means that van der Waals forces dominate instead of weak electrostatic forces and lead to stability in aqueous media. As each sheet is made up of 3 atomic layers, MoS2 is robust under pressure unlike GO where the nanochannels tend to undergo elastic deformation under pressure. Due to an abundance of S atoms, MoS2 has highly negative zeta potential which imparts good dispersibility. There are numerous ways to obtain the 2D nanosheets of these materials, which have been discussed in detailed in refs [17, 18].

Laminar structured MoS2 membranes can be fabricated for molecular separation. Sun et al. were the first to report such a membrane through vacuum filtration of chemically exfoliated MoS2 sheets on to a polycarbonate porous support. The resulting membrane showed 3–5 times higher permeance than GO, with similar rejection performance for 3 nm sized molecules for the same membrane thickness. The thin sheets partially overlap leading to many empty spaces which serve as the nanochannels. These nanochannels were found to be unperturbed even under high pressure, unlike in GO where the nanochannels suffer elastic deformation [19, 20].

Another study which corroborated these observations was conducted by Wang et al. An MoS2 membrane was fabricated by vacuum filtration onto a PES porous support post organolithium intercalation and forced hydration. The interlayer spacing was 1.2 nm (free spacing 0.9 nm) post hydration, which remained unchanged even after soaking in water for 3 days. In comparison, the interlayer spacing of GO of similar thickness increased from 0.8 nm to 5.2 nm following excessive swelling. On drying, the interlayer spacing was 0.62 nm (free spacing 0.3 nm), same as that of bulk MoS2 due to the restacking of the sheets, which makes it almost impermeable to water [19]. Ries et al. reported stable MoS2 free-standing membranes through covalent functionalization with acetamide, methyl, and ethyl-2-ol groups using organohalides. The membranes exhibited superior desalination and organic molecule rejection compared to GO based laminate membranes. Through molecular dynamics simulation of water flow in the nanochannels, the team observed that the water velocity is enhanced in methyl functionalized membrane compared to the pristine one. This was attributed to the pinning of water molecules right underneath the S atoms due to the interactions between highly polarized O–H and Mo-S bonds, which slowed the local velocity in the pristine MoS2 membrane.

On the other hand, the methyl groups substituted some of the pinning points and enhanced the overall velocity [21]. MoS2 sheets with nanopores also hold the potential to work as desalination membranes. In a simulation study by Heiranian et al., a 0.2–0.6 nm2 area pore in an MoS2 monolayer could reject 88% ions and predict a pure water flux 2–3 times that of other nanoporous 2D materials [18].

Many other studies have been carried out with MoS2 nanosheets- one group recently designed an MoS2 nanosheets-silk nanofibril hybrid membrane inspired by nacre shell structure [22], another group developed a MoS2/GO hybrid membrane for dye and salt removal [23]. Numerous studies have also reported the ability of MoS2 nanosheets to adsorb heavy metals from water [24,25,26].


MXenes (pronounced Maxenes) are a class of 2D transition metal carbides, nitrides, and carbonitrides that were discovered in 2011 by researchers at Drexel University [27]. MXenes have a general formula of Mn+1XnTx (n = 1–3), where M, as an early transition metal (such as W, Mo, Cr, Ta, V, Nb, Hf, Zr, Ti, Y, or Sc), X is carbon and/or nitrogen, and T denotes the surface termination groups such as fluorine (–F), oxygen (–O), chlorine (–Cl) and hydroxyl (–OH), and x represents the number of surface functionalities. The surface functionalities are a result of the etching and exfoliation of the MAX phase generally in aqueous media [27, 28]. In the precursor MAX phase, the A (Al or Si) layer is sandwiched within octahedral Mn+1Xn, where the strong M-X bond has a mixed covalent/metallic/ionic character whereas the weak M-A bond is metallic in nature. Exploiting the weak nature of the M-A bond, the A layers can be selectively etched out keeping the structure intact going from the 3D MAX phases to a 2D morphology. What makes MXenes stand out is the numerous possible combinations of transition metals. The goal is to find the stable ones, currently an area of great interest for researchers. Titanium based MXenes have been explored the most, such as Ti3C2Tx, Ti2C1Tx with nitrides, carbides and carbonitrides. Ti3C2Tx can be obtained, for example, by etching the MAX phase Ti3AlC2 with hydrogen fluoride. The unique structure of MXenes imparts various characteristics such as high surface area, hydrophilicity, biocompatibility, high electrical conductivity, ease of functionalization. These properties make them ideal for use in energy storage applications, electronics, sensors, catalysis, environmental remediation, water purification [28, 29]. Like other 2D nanomaterials, MXenes can be used as a filler in polymeric composites, or directly as a membrane on top of a support layer. MXenes have also been shown to be antibacterial, good adsorbents, and have ultrafast water transport and ionic sieving abilities.

Ren et al. created a Ti3C2Tx membrane though vacuum assisted filtration of its colloidal solution containing 1 nm thick sheets, on to a PVDF (polyvinylidene fluoride) support. The membrane showed greater selectivity for metal cations than GO. The ion selectivity was based on charge as well as the hydrated size of the ions, indicating the potential use of MXenes for water softening. MXenes are negatively charged. MXenes reject cations with hydration shell bigger than the interlayer spacing. MXenes can selectively aid ion permeation for cations which have a hydrated diameter smaller than the interlayer spacing such as Na+ and Mg2+. The Na+ ions intercalate into the MXene gallery easily and get attached to the sheets on either sides, eventually leading the sheets covered with Na+ to repel each other and slightly expands the gallery, resulting in higher permeation. On the other hand, for divalent and trivalent cations, the mechanism is slightly different. When these multivalent ions penetrate into the MXene gallery, there is a slight expansion in the gallery first, but as more multivalent ion enter the galley the electrostatic attraction between the cations and the negatively charged MXene sheets overtakes, tightening the gallery and slowing down the permeation of multivalent cations. Higher permeance was observed compared to GO membrane of the same thickness, which could be because of the narrow, uniform 2D nanochannels of 6.4 A0, due to the high aspect ratio of the sheets [30].

Biofouling is a major problem in membrane filtration, which is why membrane materials that exhibit antibacterial properties are desired. In a study by Rasool et al., antibacterial action of Ti3C2Tx colloidal solution was observed against Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis). The MXenes damaged bacterial cell membranes, releasing its cytoplasmic material. They were also found to have higher antibacterial activity than GO, for the same colloidal concentration [31]. Although it has been experimentally demonstrated that MXenes show antibacterial activity, the exact mechanisms for the same are still under active research. Rasool et al. observed that the bactericidal activity of MXenes is concentration dependent, and damages the bacterial cell membrane through direct penetration. This study does not however conclude on the contribution of reactive oxygen species, if any, on the bacteriocidal activity of MXenes [31]. Further, another work by Rsool et al. extended the above mentioned work to study the effect of MXene membrane aging on its bactericidal properties by keeping the membrane in ambient atmosphere for 1 month. Surprisingly, they observed enhanced bactericidal activity after aging the membrane, compared to the freshly made membranes. This was attributed to the formation of anatase TiO2 and defective 2D carbon on aging of the MXene membrane, which along with the MXenes aid in physical penetration of the bacterial cell membrane. Additionally, the group mentions that reactive oxygen species could have been generated from TiO2 which could have aided in the enhancement as well, but the effect of the latter has not been quantitatively demonstrated [32]. A simulation study by One-Sun Lee et al. investigated the possible mechanism for antibacterial activity of MXenes using molecular dynamics. The study observed that the MXenes get adsorbed on the bacterial cell membrane and cause a phase transition leading to a phase boundary defect, which further causes the intracellular material to leak, damaging the bacteria [33]. For a comprehensive study on the proposed mechanisms for the antibacterial activity of MXenes, the reader is directed to the recent review article on the same by Salimiyan Rizi [34].

MXenes have also been fabricated into membranes for oil–water separation, dye removal and desalination [35]. Another interesting application was illustrated by Zha et al., where a flexible, solar-driven water purification membrane was fabricated using MXenes and cellulose fibres [36]. Insanullah has discussed more applications of MXenes in water remediation in reference [27, 28]. MXenes are a relatively new entrant into the 2D nanomaterials market and there exist significant issues that need be addressed to translate the lab research to an industrial scale. Some of these include the use of toxic chemicals for etching out the Al from Ti3AlC2, limited knowledge of the potential toxicity and biocompatibility of MXenes and based membranes, and most importantly, their short shelf life as they degrade quickly in the open [28, 37]. Clearly, this promising nanomaterial’s potential is yet to be fully explored once its shortcomings are addressed.

Structure, properties, and synthesis of hexagonal boron nitride

Boron nitride has a structure similar to graphene, in its 2D form, so it is also called ‘white graphene’. The boron and nitrogen atoms are covalently bonded with a partial positive and partial negative charge on boron and nitrogen respectively and arranged alternatingly, in a hexagonal manner, in each (mono)layer. The nanosheets consist of a few hexagonal boron nitride layers with an interlayer spacing of 0.33 nm, held together by weak van der Waals forces. Due to the electronegativity difference, the electron pairs of the B-N σ bond are more confined towards nitrogen than boron. In the case of graphene, there is complete delocalization and equal sharing of the π electrons between the C atoms, whereas the lone pair of the pz orbital of nitrogen is only partially delocalized with that of boron, resulting in partial positive and negative charges on boron and nitrogen atoms respectively. This results in a stacking pattern where the B of each layer is between the N of the layers above and below it [38]. Typically, exfoliated BN is in the form of flakes consisting of 2–5 monolayers, but can vary according to the synthesis, exfoliation and further functionalization routes. The lateral dimensions of the flakes are 100–300 nm, while the lateral size of bulk BN can be 2-10 µm. BN has a density of about 2.2 g/cc, a melting point of around 2800 ℃ [39,40,41]. BN has high mechanical strength, making it ideal as a reinforcing agent in certain polymers [42]. Pristine BN is inherently hydrophobic in nature.

BN possesses high chemical inertness and oxidative resistance due to its structure. 2D laminar membranes made by stacking BN flakes have numerous nanochannels with large surface area because of the flakes’ high length to thickness (aspect) ratio. The higher aspect ratio makes the nanochannels tortuous for contaminants. It also simultaneously increases the likelihood that the contaminants come in contact with the surface of the hBN flakes, to prevent or enhance their permeation through specific interactions, depending on the end requirement. The surface of BN is overall negatively charged, which has been attributed to the presence of adsorbed hydroxyl groups, according to Pendse et al. At a higher pH, an abundance of hydroxyl groups results in higher negative charge [43]. These unique set of properties renders BN ideal for a host of applications ranging from water remediation and osmotic energy conversion to electronics and drug delivery.

BN is a very good adsorbent for heavy metals such as Cr(III), Cd(II), dyes like rhodamine B (RB) and organic molecules like tetracycline and a wide range of oils. The excellent adsorption capacity has been attributed to the polarity of the B-N bond (which makes it particularly suitable for metal ion chemisorption), high specific surface area, and surface defects. A major advantage of BN as an adsorbent for organics is its high thermal stability, making the regeneration relatively easy by simply heating to high temperatures to burn the adsorbed organics, making it reusable. Additionally due to the inertness of hBN to harsh chemicals, substances adsorbed onto hBN can be removed by treated them with these harsh chemicals, while BN remains intact [44,45,46,47]. Studies have shown that BN is biocompatible and more than carbon nanomaterials and can be used as implant materials. One recent study demonstrated that BN, as a therapeutic agent, accelerated wound healing. The potential for BN in medicinal applications is attributed to its excellent chemical stability and mechanical strength, absorption in the UV region, and low toxicity [38, 48].

The structure and properties of 2D nanomaterials are determined by techniques used to synthesize them, and the processing steps thereafter. For membrane applications, a high yield of BN with large lateral dimensions is desired to stack the sheets to make galleries which create tortuous pathways for contaminants, as mentioned earlier. There are many methods for fabricating BN, including chemical vapor deposition, epitaxy, mechanical and chemical exfoliation. Each technique has its advantages and disadvantages. Chemical vapor deposition and epitaxy involve high temperatures and gaseous atmospheres which produce BN with inherent defects over which there is little control, but the number of layers and the dimensions over a large scale can be controlled in these methods. On the other hand, mechanical and chemical exfoliation, though relatively cost-effective and good for large scale production, have low yield and produce flakes with a broad size distribution and offer no control over the number of layers. Other techniques include pulsed laser deposition, physical vapor deposition and pyrolysis of boron and nitrogen containing compounds [39]. A detailed review of these techniques for synthesis of BN can be found in reference [39]. To tailor the properties of BN to the application, it can be functionalized with different functional groups and polymers. The numerous ways demonstrated for functionalizing BN nanostructures have been detailed in the review references [38, 39]. Going forward, the sections on the applications of BN in water purification membranes and adsorbents will give a better insight into the effect of structure, properties, and the synthesis routes of BN on the performance of membranes and adsorbents in the various works mentioned.

Applications of BN in water purification

BN based membranes

As mentioned earlier, the thin sheet-like structure, high surface area, small interlayer spacing, excellent thermal and chemical stability, and oxidative resistance, negative surface charge, amongst other qualities make BN an ideal candidate for applications in water purification membranes. Modelling and simulations studies on water and ion transport through nanopores in the BN monolayers and through the interlayer spacing, have demonstrated enhanced transport performance. Simulation studies by Gorbanfekr et al. show that the water transport is faster between the layers of BN due to low friction (Fig. 1) [49]. Another simulation study by Tocci et al. discusses the transport of water at nanoscale, in contact with BN sheets [50]. Yet another simulation study by Liu et al. proposes that the nanoslit patterns in BN monolayers can affect the water transport and the ion rejection performance of BN nanosheet membranes [51]. Jafarzadeh et al. in their simulation study of the ion rejection performance of nanoporous BN nanosheets, demonstrate that functionalizing the nanopore with fluorine and hydroxyl groups can improve its filtration performance [52]. Since this review focuses on the experimental studies on filtration performance of BN based membrane architectures, only a few simulation studies are mentioned on the same. From the above studies, it can be concluded that the absence of functional groups lead to a friction free transport of water through the nanochannels made by stacking hBN sheets into a gallery, assuming non porous sheets. If the hBN sheets have pores, it can further aid in the transport of water by reducing the path traveled by the water molecules through the hBN gallery, while rejecting ions with hydration shell greater than the size of the nanopores. Further modification of the nanopores with functional groups such as hydroxyl or fluorine has been proposed to aid in the ion rejection through the nanopores through specific interactions between these and the ions.

Fig. 1
figure 1

System of nanoconfined water molecules (red and white balls) between BN membranes (blue and pink balls) with nanometric interlayer distance h for typical a Density Functional Theory and b Molecular Dynamics supercells. Reprinted with permission from reference [49]. Copyright 2020 American Chemical Society

With a combination of various synthesis, exfoliation and functionalization strategies, this decade has seen numerous works on the use of BN as a membrane, a foam, a reinforcing agent, an adsorbent, a surface modifier, and more. Following are experimental studies conducted on applications of BN in water filtration membranes, followed by a section on its applications as an adsorbent. Each of these works will help in understanding the properties of BN that are specifically applicable for water purification, through various architectures.

Lei et al. synthesized porous BN nanosheets of high porosity and specific surface area to demonstrate its sorption properties for oils, organic solvents, and dyes (Fig. 2). The porous nanosheets showed fast absorption kinetics and high mass uptake (up to 33 times their own weight for ethylene glycol) compared to other common adsorbents of oils and organic solvents. This has been attributed to adsorption, capillary effects, and intercalation because of the hydrophobic nature and polarity of the B-N bond. The porous nanosheets outperformed other absorbents for dye sorption as well. The absorbent was easily collected as the nanosheets floated on water and were regenerated by heating in air or solvent extraction, which were possible because of the excellent thermal and chemical stability of BN. This regeneration capacity over multiple absorption cycles makes BN stand out [53]. Lei et al. also developed aerogels and flexible, free-standing membranes from amine functionalized BN. The latter was prepared through vacuum filtration. This is notable because the group used a high yield and the relatively simple and scalable exfoliation and functionalization process of ball-milling BN with urea. This is a mechano-chemical exfoliation in which shear forces exfoliate the BN sheets and simultaneously decompose urea, prompting the NH2 groups to attach to the edges and defects of the BN layers [40].

Fig. 2
figure 2

Exfoliated few-layer BN. a TEM image of few-layer BN, with the selected-area electron diffraction pattern (inset) indicating a layered BN structure. Scale bar, 50 nm. b, c HRTEM images of the edge folding of two few-layer BN sheets with three and six BN layers, respectively. Scale bars, 2 nm (b) and 5 nm (c). d HRTEM and the fast Fourier transform images of a few-layer BN sheet. Scale bar, 2 nm. e, f Atomic force microscopy image and corresponding line-scan profile of few-layer BN. g XRD patterns of few-layer BN and pristine h-BN. (Reprinted from ref. [40] under Creative Commons License)

Like other 2D nanomaterials, BN can be infused into a polymer matrix to enhance its properties. Liu et al. developed a composite porous BN/PVDF membrane through non-solvent induced phase inversion, by adding BN to the PVDF dope solution. The permeability of both water and organic solvents obtained was thrice that of neat PVDF due to the highly cross-linked networks between porous nanosheets and PVDF. A 99.99% separation was obtained for oil/water emulsions, pharmaceuticals were filtered up to 14L/g and organic dyes up to 9.3L/g. The filtration performance was attributed to the polarity of the B-N bond, high porosity, and specific surface area of the nanosheets. The membranes could be recycled as well [54].

Marichy et al. developed a well-crystallized, self-standing BN membrane using a combined atomic layer deposition (ALD) and polymer derived ceramic (PDC) route with control of through porosity in terms of size and shape. The cylindrical pores obtained ranged from a few tens to hundreds of nanometers. These membranes, even though brittle, could be inserted into a suitable porous support and indicate potential use in osmotic energy conversion and water filtration [55].

A thin film nanocomposite nanofiltration membrane was fabricated by Abdikheibari et al., where 0.004wt% amine functionalized BN (modified through ball-milling with urea) was added to the aqueous solution of piperazine and interfacial polymerization was carried out with trimesoyl chloride on a porous polyethersulfone support. The nanosheets, embedded in the polyamide layer, enhanced the hydrophilicity and the free volume available for water flow, thus increasing the flux as well as the fouling resistance. A 13.4% improvement in the flux and a 5.2% increase in natural organic matter (NOM) rejection was observed. 0.97 of the initial flux was maintained when a humic acid solution was filtered [42]. Another architecture developed by the same group involved deposition of amine functionalized BN(0.003wt%) on top of the polyamide layer of piperazine and trimesoyl chloride on polyethersulfone porous support. This resulted in a smooth, hydrophilic surface with a negative charge, which led to a 59% increase in flux and 50% increase in resistance to fouling by NOM. Sodium alginate and bovine serum albumin rejection was above 92% [56].

The earlier works incorporated BN into the matrix of the polymer membranes by adding them to the dope solutions. Gonzalez-Ortiz et al., on the other hand, developed a BN/polyvinylalcohol (PVA) microfiltration membrane through pickering emulsion templating. Here, BN was used as an emulsion stabilizer and to enhance the mechanical and antifouling properties due to its high hardness, corrosion, and oxidation resistance. PVA was added to an aqueous dispersion of exfoliated BN, followed by ethyl benzoate, which resulted in a homogeneous emulsion post sonication. The BN sheets would adsorb at the liquid–liquid interface, form a rigid shell around the droplets, and prevent coalescence. This was cast onto a glass substrate, cured, and immersed in a bath containing ethanol, HCl and glutaraldehyde for cross-linking, as PVA is water soluble otherwise. The curing time was found to greatly affect the pore size and porosity of the resulting membrane (Fig. 3). For a certain curing time and corresponding pore size, the membrane was found to reject 76% of polystyrene latex nanoparticles of size 0.1 μm and 99.7% of the particles of size 1.2 μm. The membrane showed higher permeance and rejection capabilities than the PVA based membranes fabricated through other conventional methods such as thermally induced phase separation or Electrospinning. More importantly, this method was green—free from harmful reactants and waste generation [57].

Fig. 3
figure 3

SEM images of the h-BNNS/PVA porous membranes depending on the curing time before crosslinking: a cross-section of single PVA, b surface of single PVA, c cross-section of BNP-24 h, d surface of BNP-24 h, e cross-section of BNP-3 h, f surface of BNP-3 h, g cross-section of BNP-1 h and h surface of BNP-1 h. Reproduced from Ref. [57] with permission from the Royal Society of Chemistry

Low et al. fabricated a polyethersulfone/BN mixed matrix nanofiltration membrane through non-solvent induced phase separation. An aqueous dispersion of BN was sonicated with sodium dodecyl sulfate and added to the dope solution. The resulting membrane was more hydrophobic, more negatively charged, due to the inherent hydrophobicity of BN and its negative charge. While the neat PES membrane only recovered 80% water flux on fouling, the PES/BN membrane recovered the flux completely. The permeance increased but the rejection, as tested with the dye Rose Bengal, decreased by a small amount owing to BN acting as a plasticizer [58].

The addition of BN to the membrane matrix generally makes the surface denser, rougher, more hydrophobic and increases the permeance. The rugged surface is a result of the nodular structures on the membrane surface due to solid–liquid de-mixing, in which BN acts as nuclei for crystallization and partly because of aggregation of BN nanosheets. To demonstrate this, Zahirifar et al. incorporated BN into PVDF to develop a membrane for air-gap membrane distillation via non-solvent induced phase separation and BN was added to the casting solution. It also enhanced the thermal stability of the membrane along with an 18% increase in the tensile strength. These improvements could be credited to the excellent thermal and mechanical strength of BN itself [59].

Although relatively easy to fabricate, mixed matrix membranes may not always fully exploit the properties of 2D nanomaterials such as their high aspect ratio and specific surface area, nanoscale interlayer distance, etc. On the other hand, laminar membranes formed by stacking sheets of 2D nanomaterials offer desirable molecular separation based on electrostatic interactions and sieving, through nanopores and nanochannels. Following are studies on the same, giving insight into the mechanisms of transport in such membranes.

Chen et al. developed a multipurpose thin functionalized BN (FBN) membrane through vacuum filtration onto a nylon support to separate mixtures. BN powder was functionalized by ball-milling with urea. The flakes were 1.2 nm high and 100–300 nm in lateral dimensions. Each flake consisted of 4–5 monolayers. The thickness of the FBN layer could be varied by changing the volume of their aqueous dispersion. Permeation tests of toluene, p-xylene, pyrene, and rhodamine B were conducted in ethanol and water. RhB was fully blocked, while permeation of acetone in water and toluene in water was 5.8 and 2.2 times that through GO, respectively. Permeation was on par with that of MXene laminar membranes. The short and abundant transport pathways in the FBN membrane rendered such high flux possible, along with the excellent thermal and chemical stability of BN itself, which kept the structure stable. Membranes were stable even after solvation for 24 h in acetone, ethanol, hexane, toluene, and water as well as in extreme pH environments [60].

Another work by Chen et al. involved pressure driven filtration studies of the FBN membrane on a nylon support in organic solvents. Dye rejection tests were performed with Methylene Blue, Acid Fuschin, Evans Blue and Congo Red in Methanol. A 0.4 µm thick FBN layer showed 99% rejection for gold nanoparticles (5 nm) and > 99% rejection for congo red. The water flux was 1.5 and 16 times greater than of MXenes (0.1–0.4 µm thick) and reduced graphene oxide (2 µm thick) membranes respectively, reported in literature. The performance was ascribed to the hydrophilicity, the stability of the flakes and the spaces between them, and the structure of the nanochannels, which depended on the lateral size of the flakes, the height of the nanochannels, and the gap size between the edges of the flakes [61].

Ultrafast permeation of ions was observed through an FBN membrane on a cellulose ester support by Chen et al. The 200 nm thick FBN membrane essentially acted as a mesh with a pore size of 8.6 A0, as it completely rejected ions with hydrated radii greater than 4.3 A0. The membrane exhibited ultrafast permeation with a permeation rate of nearly 9 mol m-2 h-1 for sodium ions for smaller hydrated ions, nearly 25 times faster than the theoretical diffusion rate. Small ions such as Na+, K+, Mg2+, Ca2+ and Zn2+ permeated quickly while larger ions and molecules such as Mn2+, sucrose and glycerol were much slower to permeate or did not permeate at all (Fig. 4). Their observations indicated that permeation rates were influenced by the mass, size, charge, and hydration radii of the ions and the concentration of feed solution and surface charge of the membrane [41].

Fig. 4
figure 4

a Photograph of FBN water dispersion. b Photograph of the FBN membrane. The FBN membrane is highlighted with a red dash line to distinguish it from the support membrane. c TEM image of the edge of an FBN flake. d Cross-sectional SEM image of the FBN membrane with a thickness of 200 nm supported on a filter membrane. e Schematic diagram of the permeation of different ions through FBN membranes. f Tested species, the left box shows the ions that can pass through the FBN membrane, whereas the right box illustrates the hydrated ions and molecules that are blocked by the FBN membrane. Reprinted (adapted) with permission from reference [41]. Copyright 2019 American Chemical Society

The above experiments involved functionalizing BN with amine groups, which reduces the negative surface charge of BN, as Pendse et al. argue. They created a membrane where exfoliated BN, via bath and probe sonication, was vacuum filtered onto an AAO (Anodized Aluminium Oxide) porous support to get a 10 µm thick layer. By not functionalizing the BN, the group chose to exploit the highly negative surface charge of BN while reducing the processing steps. To demonstrate that the filtration property depends on the size of the sieving channels as well as the surface charge, filtration tests were performed with dyes and salts across a wide range of charges and molecular sizes. The estimated size of the nanochannels was 1.2–5 nm and zeta potential of BN was calculated to be −60 to −70 mV between a pH of 8 and 10 [43]. The negative surface charge was estimated to be caused by the adsorption of hydroxyl ions on the surface of the BN, which would only increase at a higher pH. The membrane outperformed other membranes based on functionalized BN, MXenes, dichalcogenides, graphene and derivatives in terms of small anion rejection and water permeability [43].

Hybrid membranes fabricated using two different nanomaterials can offer a synergistic effect and circumvent the limitations that might show up when using each material separately. For instance, Lin et al. developed an amine functionalized BN and GO hybrid membrane on a polydopamine coated porous PES support. The major problem of instability and excessive swelling of GO in aqueous media was solved by alternately depositing GO and BN resulting a tightly packed laminar stacking. The covalent bonding between the edges of the two nanomaterials stabilized and prevented the GO from swelling. This also created sufficient nanochannels for water permeance and dye rejection. The resulting membrane was found to be stable across solutions of different pH. A rejection rate > 99.5% was observed for methylene blue across hybrid membranes of varying GO and BN concentration. The water flux varied from 1–4.5 LMH at 300psi, across different compositions [62].

Osmotic energy generation from seawater is a promising alternative source of energy with low environmental impact. Also called salinity gradient energy or blue energy, it make use of the salinity gradient between seawater and river water which causes an osmotic pressure difference. The membranes used to generate power from salinity gradients must have high mechanical strength, thermal and chemical stability- to withstand the lifecycle of osmotic energy generation plants- combined with high salt rejection values for pressure retarded osmosis. Stable nanochannels formed by the interlayer gallery of membranes made from 2D nanomaterials present an opportunity to exploit this blue energy through reverse electrodialysis and pressure retarded osmosis. Chen et al. created a cartilage inspired BN-aramide nanofiber composite membrane via layer-by-layer assembly to meet these requirements, making a hybrid of 2D nanomaterials and 1D nanofibres. BN was chosen because of its superior thermal and chemical stability, and stability in a variety of solvents and extreme pH environments in addition to its ultrafast water and ion transport properties, which are the major requirements for membranes used for osmotic energy generation. BN was ball-milled with glucose for hydroxyl groups. Aramid nanofibres were added to impart mechanical robustness to prevent microcracks under pressure and to create additional transport pathways. A glass substrate treated with poly(diallyldimethylammonium) chloride polycation (PDDA) was immersed in aramid nanofiber and BN suspensions alternately for a certain number of steps (n) to create a multilayered nanocomposite membrane which they termed as ABNn. ABN30 was flexible and only 1 µm thick. The layers are held together strongly by dense hydrogen bonds between the nanosheets and the fibers. This resulted in a tensile strength of 370 MPa, which was much greater than that of membranes made only from BN or aramide nanofibers. They demonstrated that ABN30 membranes in tandem could power a digital timer [63].

Yazda et al. fabricated a BN/SiN hybrid membrane with nanopores in the sub 10 nm range through a technique called tip-controlled local breakdown, using an atomic force microscope, for osmotic power generation. With a pore-to-pore spacing of 500 nm, they concluded that the mechanical stability provided by SiN and the high surface charge density of BN resulted in a power density of 15 Wm−2 when using KCl as the electrolyte. Their technique allowed for precise control over the size and the distribution of pores in the BN layer [64].

Keshebo et al. fabricated a BN lamellar membrane by vacuum filtration of its aqueous dispersion containing tannic acid onto a Nylon support membrane. Tannic acid acted as a green exfoliating agent by specifically interacting with the BN layers and thereby reducing the BN interlayer attraction. It also helps the BN nanosheets adhere to the Nylon support membrane. The resulting membrane showed > 99% rejection of congo red, and a high antifouling tendency against humic acid even after 7 cyles [65].

It can be concluded from the above works that exploiting the properties of BN relevant to the applications- such as its hydrophobic nature, tunable sheet size as well as surface area according to the synthesis routes, its capacity to withstand high temperatures, harsh chemical environments, and its mechanical strength- the overall properties of the membranes could be significantly enhanced. Although the research on applications of BN in membrane-based technology is relatively in early stages, it suggests optimistic results in effective nanofiltration and desalination membranes for the future.

BN based adsorbents

The adsorption performance of any adsorbent is evaluated in terms of the following factors. Ideally, the adsorbents should demonstrate high separation selectivity and high adsorption capacity, quick uptake, easy regeneration, and multiple cycling ability. Unless mentioned otherwise, 1 cycle includes an adsorption as well as a desorption cycle. The absorbent should also exhibit high gravimetric capacity, should be easy to deploy and recover after the adsorption process. Since boron and nitrogen are light elements, boron nitride absorbents would have large gravimetric capacity. BN has high chemical and thermal stability. This means that BN adsorbents can be regenerated by simply heating at high temperatures to remove the adsorbed organics, without destroying the structural integrity of the adsorbent, or can be washed with various solvents and chemicals to remove the adsorbed contaminants. The adsorbed substances can also be removed using solvent extraction or broken down using harsh chemicals, without damaging BN since it exhibits very high chemical inertness. Since nanomaterials have a large specific surface area, adsorbents made from BN nanomaterials would be ideal as they combine the properties of nanoscale materials with properties of BN [47, 53].

Das et al. fabricated a non-functionalized BN membrane through vacuum filtration of a suspension of boron nitride nanosheets in NMP and IPA onto a PVDF support membrane. After performing filtration tests with BPA (a neutral solute), MO and DR-80(anionic dyes), for a certain optimum thickness of the lamellar membrane, they concluded that the retention was driven by the adsorption of these pollutants to the BN sheets. They found the effects of size exclusion or Donnan exclusion to be negligible in this case. The membrane showed high stability even in an acidic environment, and recovered 90% of its dye absorption capacity after 4 cycles of rejection indicating the reusability of the membrane [66].

Lei et al. synthesized boron nitride nanosheets using boron trioxide and guanidine hydrochloride as the boron and nitrogen precursors respectively. Since a high temperature of 1100 ℃ was used for the synthesis, nitrogen, hydrogen chloride and ammonia gases are released from the decomposition of guanidine hydrochloride, which created pores of a few tens of nanometers in the boron nitride nanosheets over a micron in their lateral dimensions. Here, the porosity is due to the pores in the nanosheets as well as the spaces between the sheets themselves. After the oils and organic solvents were adsorbed from water, the group studied the XRD patterns of the BN adsorbent before and after the adsorption process. According to the XRD patterns, the interlayer spacing in the porous boron nitride nanosheets increases slightly as the oils and organic solvent molecules intercalate into interlayer spacing due to its porosity and inherent hydrophobicity. The percentage change in the interlayer spacing would depend on the adsorbate and the adsorbent-adsorbate affinity as well. Comparatively, the interlayer spacing of commercial bulk BN remains unchanged even after saturation with oil and organic solvent. This ability of porous BN to slightly swell and accommodate adsorbent molecules while still maintaining its structural stability has been correlated to its greater adsorption capacity compared to the commercial bulk BN [53]. Liang, G. et al. came up with a creative route to synthesize a nanonet from BN nanofibers, with NH4BF4 and NaN3 as precursors while using CS2 as a catalyst through a solvothermal process at 260 °C (Fig. 5). The nanonet, which was made up of 8 nm thin BN nanofibers, was able to adsorb methylene blue from water, and sieve Au nanoparticles with respect to their size when used as a membrane [67].

Fig. 5
figure 5

Schematic Illustration of the Formation Process of BN Ultrathin Fibrous Nanonets. BN ultrathin fibrous nanonets prepared by one-step solvothermal process with CS2 as the catalysts. a, b Typical SEM images and c, d the corresponding TEM images of BN nanonet with different magnification ratios; e, f HRTEM images of a typical fibrous knot (circled) and a BN nanofiber with diameter of 8 nm. Reprinted with permission from reference [67]. Copyright 2013 American Chemical Society

Lian Gang et al. synthesized BN nanospheres with outer diameters of 50–300 nm using a template free route, for hydrogen storage as well as organic pollutant removal from water. The nanospheres showed preferential adsorption for basic yellow 1, an aromatic dye molecule even in the presence of CuSO4, a heavy metal ion. This has been attributed to the specific non-covalent interaction between the aromatic rings and hBN, similar to the π-π interactions observed in graphene and its derivatives [68]. Liu et al. also synthesized boron nitride spheres high absorption capacity for a range of heavy metals and organic dyes [69]. Theoretical studies have reported that BN shows only weak interactions with hydrogen, but some experimental works have shown otherwise, so the use of BN in hydrogen storage needs to be delved into further [70, 71]. Along with H2 adsorption and storage, hBN has also been touted as a prospective material for CO2 capture [72,73,74].

Li, J., Xiao, X., Xu, X. et al. synthesized “activated BN” by adding a surfactant to the precursors during the synthesis of BN. The activated BN had a ribbon like structure, which were porous and rough. The BN obtained here had a d-spacing of 0.35 nm, which is slightly greater than that of hBN (0.33 nm). The activated BN performed better than porous BN and activated carbon as well, in the case of adsorption of Co2+, Ni2+, Ce3+ and Pb2+, tetracycline, methyl orange and congo red. According to the group, the surfactant added during synthesis helped increase the surface area and pore volume and the adsorption ability, but the exact mechanism is yet to be elucidated [44]. Apart from the presence of functional groups, the number of defect sites and the degree of crystallinity also influences the performance of BN. Li, Jie, et al. synthesized BN fibres which was more crystalline in nature which they claim imparts stability to the adsorbent during the absorption–desorption process, compared to the activated BN using a surfactant in their previous work (discussed in the previous paragraph). They further activated these fibres with a mixture of sulphuric acid and nitric acid treatment, which created defects and functional groups at the edges of BN while maintaining its crystallinity mostly intact (as corroborated by their XRD plots and TEM micrographs) and increasing the absorption capacity [75]. Rod like porous hBN has been synthesized by Song et al. for antibiotic adsorption, specifically tetracycline-based compounds here. The microporosity and mesoporous nature is concluded from the adsorption isotherms and the adsorption–desorption hysteresis curves [76]. hBN has been synthesized in various forms for adsorption of different antibiotics, and bisphenol A as well, which is a toxic substance prominent in the food packaging industries [77, 78].

In the above-mentioned work, the different forms of BN were synthesized as a powder, and used as a powder for the adsorption experiments, which tends to agglomerate during adsorption and desorption, which can limit its performance. Hence, Jie Li et al. synthesized activated BN embedded with NaOH groups, in a 3D form to increase the surface area of interaction of the pollutants with the adsorbent, while maintaining structural stability. NaOH-3D BN was prepared to adsorb HCOH and simultaneously act as a support for its catalytic degradation. Its porous structure has been attributed to the interfacial spaces between the nanoflakes. HCHO removal on NaOH-3D BN were much higher than those of the NaOH-embedded commercially activated carbon and the NaOH-embedded activated BN. NaON-3D BN was regenerated just by using an electric hair dryer and retained its HCHO adsorption capacity of 90.5% for 15 cycles. The embedded NaOH converts the toxic HCHO to relatively less toxic methoxy and formate salts [79].

Yanming Xue et al., fabricated a boron nitride porous monolith (BNPM) for dye and heavy metal removal as well as oil–water separation. The group first prepared a boric acid-dissolved formaldehyde-dicyandiamide resin as the precursor in the form of a rod-like monolith, which was porous because of the gases formed during the resin solidification reaction, which bubbled through to escape and resulted in a porous precursor monolith. This precursor solid was then pyrolyzed above 1000 ℃ in ammonia atmosphere under ammonia flow to obtain a BN porous monolith. The BNPM showed excellent oil adsorption capacity and could be easily regenerated by burning the adsorbed organic substances in air. BNPM also showed high adsorption for rhodamine B dye and Copper (II) ions from water due the hydroxy and amine groups residing on the walls of micropores and holes of BNPM. The dye was removed from BNPM using ethanol. The group explained how the hierarchical structure enhances the adsorption of oil, dye, or organic molecules onto boron nitride. Although the boron nitride is in its turbostatic form, the concept of a hierarchical structure for adsorption can be applied for applied across different materials, and boron nitride in its hexagonal form as well. The macropores allow unrestricted pathways for pollutants to diffuse though. The large number of micropores then provide abundant surfaces for the pollutants to adsorb onto. Since boron nitride is inherently hydrophobic, the micropores allow oils to flow into and fill up the pores through capillary action. Next, the surface interaction plays the major role in adsorption of dye molecules and heavy metal ions. The pore walls, pore network, nodes and defects present in the structure of boron nitride, as well the functional groups (if any) influence the dye and heavy metal adsorption [45]. The role of hierarchical structure of porous architectures of adsorbents is discussed in the work by Jun Yin et al. on the boron nitride foam synthesized using CVD, discussed in the following paragraph.

Jun Yin et al. fabricated BN foams with a network of hollow tubes via CVD using borazane as the precursor and a Nickel foam as the template. The foam could recover completely even after compression up to 70%. As shown in the SEM and TEM micrographs of the foam in Fig. 6, it is made ofhierarchical structures- foam-like macrostructure, hundreds of micrometers porous network, hollow-tube-like branches and nodes at tens of micrometers, branch wall consisting of a few BN layers, and atomic lattice of hexagonal BN (Fig. 6) [80].

Fig. 6
figure 6

Hierarchical structures of the free-standing BN foam. a Photograph of a free-standing BN foam. b SEM image of a BN foam network. c Magnified SEM images of two branches and a node. d High-resolution TEM images of BN sheets near their folded edges, where the number of dark lines for the edges indicates the thickness of 12 and 4 layers, respectively. The interlayer spacing obtained from the edges is 0.34 nm. e High-magnification TEM image and its fast Fourier transform (inset), indicating the typical hexagonal lattice of the BN sheet. Reprinted with permission from reference [80]. Copyright 2013 American Chemical Society

2D nanomaterials have large specific surface area but using the adsorbent in a powder form has certain drawbacks as well. Due to the aggregation tendency of these powders during absorption process, their uptake could be hindered. To circumvent this, three-dimensional (3D) architectures can be made with 2D nanomaterials. These 3D architectures are lightweight, have very low densities, and can create hierarchical porous structures while retaining the inherent features of 2D nanomaterials. These 3D forms are also relatively easier to handle compared to the powder form of BN, that is, to deploy and recover because of their structural stability [81, 82]. Zhao, H. et al. synthesized a 3D BN foam which they called “3D WG”, where WG stands for white graphene, as hexagonal boron nitride is isostructural to graphene but is white in color. Using thiourea and aminothiourea as vesicants which were added to ammoniaborane as the precursor. The CS2, NH3 and NCNS gases released aided in the foaming. they obtained a low-density 3D WG foam with pore sizes ranging from a few nanometers to microns. The foam could absorb a range of organic solvents up to 70–190 times its weight and had higher organic dye adsorption in comparison to other forms of BN such as hollow spheres [68] fibrous nanonets [67], nanocarpets [83] and porous nanosheets [53]. Liu, Z., Fang, Y., Jia, H. et al. prepared a 3D “cheese-like” C-BN with pore size ranging 2-100 nm, which could adsorb methylene blue and congo red, as well as Cr3+, Cd2+, Ni2+ from water. The group also demonstrated successful oil–water separation performance of C-BN with salad oil, gasoline, and pump oil. The adsorbent could be regenerated by heating at certain temperatures for a certain amount of time, and reused, as shown by the multiple regeneration-adsorption cycles (Fig. 7). The group combined the high adsorption capacity of boron nitride with that of carbon, with the C-BN hybrid material. Interesting point to note- the C-BN adsorbent was synthesized as a foam. But for dye and heavy metal removal tests it was ground into a powder, while for the oil–water separation tests the foam was used as is [84].

Fig. 7
figure 7

ac The process of adsorbing gasoline by 3D C-BN; d Original appearance of the 3D C-BN; e Burning the sample after absorbing gasoline; f The appearance of 3D C-BN after six times recycling. Reproduced from reference [84] under the Creative Commons license.

Composites of BN with polymers also offer excellent adsorption properties by exploiting the combined properties of BN as well as those of the polymer. Like in the case of mixed matrix membranes discussed in the previous section, dispersing various forms of BN in a polymer matrix to fabricate a foam, or directly depositing BN nanomaterials on already synthesized foams imparts structural stability to the entire architecture while enhancing the adsorption properties of the composite foams. For instance, Yu et al. fabricated a porous composite of BN nanosheets and PVDF for oil–water separation and demonstrated its application as a membrane as well as an adsorbent, exploiting the inherent hydrophobicity of BN and PVDF [85]. Krishna Kumar et al. fabricated polyethyleneimine modified hBN nanosheets, on which magnetite (Fe3O4) nanoparticles were grown, and were blended with polyvinyl alcohol and freeze dried to create an aerogel for heavy metal removal and organic dye removal. The magnetite nanoparticles enhanced the capacity for heavy metal adsorption [86]. Liu et al. coated BN nanosheets on cotton fabric and melamine sponge using a layer by layer assembly for enhanced oil–water separation and dye removal [87]. Similarly, Zhou et al. made BN deposited melamine foams for excellent oil–water separation and dye adsorption [88]. These studies demonstrate the application of BN as an adsorbent as is or by incorporating it into existing architectures, such as in the 3D porous structures, to exploit its properties that make it ideal for use as an adsorbent- low density, lightweight, high surface area to volume ratio, inherent hydrophobicity, ability to withstand high temperatures and harsh chemical environments without damaging its structural integrity, and high mechanical strength.

Table 1 reports the architectures and as well as the application of BN in water remediation in the references in this review, along with their corresponding performance.

Table 1 Recent works involving applications of BN in separation applications


The future of BN nanomaterials in water purification membranes looks promising. However, numerous challenges must be overcome to realize their full potential. The synthesis routes for fabricating most nanomaterials are complex, long-winded, and expensive, and often involve hazardous chemicals. For example, hydrogen fluoride is used to etch out Aluminium from the MAX phase of Ti3C2Tx [28]. In the case of BN, its chemical inertness is a double-edged sword. On the one hand, it makes BN adsorbents easy to regenerate, prevents swelling and retains the sieving performance of laminar membranes in various solvents, but on the other hand renders the functionalization of BN quite difficult [40]. More effective, high-yielding methods of functionalization of BN are needed to alleviate these issues. When incorporated into a polymer matrix, aggregation of the nanomaterials presents a problem, which can be controlled by modifying the latter to aid matrix-filler interaction and dispersion. Laminar membranes formed by stacking these nanomaterials swell or disperse on hydration, which can affect the separation efficiency and durability. Unstable structures can also be leached out during filtration. Attaching additional functional groups, covalently cross-linking the sheets, and using the right porous support would solve these issues. Most filtration studies are conducted with a feed solution of one or few model micropollutants, which is far from the real feed water composition. Experiments, along with modelling and simulation studies must be carried out with complex feed matrices containing different types and levels of pollutants representative of the real compositions. This helps understand the selectivity of the 2D BN nanomaterials-based membrane for one pollutant in the presence of others.

Although laminar membranes of 2D nanomaterials show high water permeance, their desalination capacity has not yet reached that of polyamide membranes. Their selectivity depends on the electrostatic interaction and size-based exclusion. Engineering the surface charge and the nanochannels according to the desalination needs without compromising on the water permeance, would lead to the ideal desalination membranes. Transport mechanisms in 2D nanomaterials is an attractive field of study, as there is lot more yet to be understood, through modelling and simulation studies aided by carefully designed experiments [89].

Large scale, cost-effective manufacturing of defect-free BN nanomaterials with controlled dimensions, pore size and pore size distribution remains a challenge as quality of the material varies considerably across manufacturers [90]. The limited data on the environmental and health risks of engineered nanoparticles, although much less than perceived, can hinder plant operators and regulators from adopting nanomaterials-based products in treatment plants. Therefore, more studies must be conducted on the environmental impacts of each of these engineered nanomaterials, especially in the presence of other pollutants [91]. From the review of the ongoing research efforts on BN nanomaterials, it can be concluded that BN nanomaterials possess remarkable potential in leading the new generation of water purification membranes and adsorbents, and it is worth investing our time and effort to overcome barriers to herald a new generation of architectures for efficient water purification.

Availability of data and materials

Third party data has been used with permission, which will be provided upon request.


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The authors would like to thank the Science and Engineering Research Board (SERB), Government of India for the financial support.

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SA wrote the main manuscript text and prepared the figures SB: Gave critical inputs and insights to various section of this review. All the authors reviewed the manuscript. The author(s) read and approved the final manuscript.

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Correspondence to Suryasarathi Bose.

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Avasarala, S., Bose, S. 2D nanochannels and huge specific surface area offer unique ways for water remediation and adsorption: assessing the strengths of hexagonal boron nitride in separation technology. Functional Composite Mater 4, 5 (2023).

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  • Hexagonal boron nitride
  • 2D nanomaterials
  • Membrane
  • Adsorbent
  • Dye and antibiotics removal
  • Desalination
  • Antifouling
  • Oil–water separation