2D laminar maleic acid-crosslinked MXene membrane with tunable nanochannels for efficient and stable pervaporation desalination

Journal Pre-proof 2D laminar maleic acid-crosslinked MXene membrane with tunable nanochannels for efficient and stable pervaporation desalination Mingmei Ding, Hang Xu, Wei Chen, Guang Yang, Qing Kong, Derrick Ng, Tao Lin, Zongli Xie PII:

S0376-7388(19)33496-9

DOI:

https://doi.org/10.1016/j.memsci.2020.117871

Reference:

MEMSCI 117871

To appear in:

Journal of Membrane Science

Received Date: 15 November 2019 Revised Date:

18 January 2020

Accepted Date: 21 January 2020

Please cite this article as: M. Ding, H. Xu, W. Chen, G. Yang, Q. Kong, D. Ng, T. Lin, Z. Xie, 2D laminar maleic acid-crosslinked MXene membrane with tunable nanochannels for efficient and stable pervaporation desalination, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/ j.memsci.2020.117871. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Mingmei Ding： Data curation, Writing- Original draft preparation, Visualization Hang Xu：Supervision, Funding acquisition, Wei Chen：Conceptualization, Validation Guang Yang：Conceptualization Qing Kong：Visualization Derrick Ng：Investigation Tao Lin：Visualization Zongli Xie：Supervision , Writing- Reviewing and Editing

2D laminar maleic acid-crosslinked MXene membrane with tunable nanochannels for efficient and stable pervaporation desalination Mingmei Ding,a,b,c Hang Xu,a, c, * Wei Chen,a, c Guang Yang,b Qing Kong,a,c Derrick Ng,b Tao Lin,c Zongli Xie b, * a

Ministry of Education Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Hohai

University, Nanjing 210098, PR China. b c

CSIRO Manufacturing, Private Bag 10, Clayton South, Vic. 3169, Australia.

College of Environment, Hohai University, Nanjing 210098, PR China.

Abstract Two-dimensional (2D) laminar materials have shown great promise in membrane applications. Herein, a maleic acid covalent-bridged MXene (MXMA) membrane supported on microporous nylon substrate is fabricated via a facile vacuum-assisted filtration synthesis and studied for pervaporation desalination. The adjacent MXene nanosheets are strongly fixed by the covalent bonding formed by the esterification reaction between the carboxyl groups in maleic acid and hydroxyl groups in MXene surface. In this way, the crosslinked MXMA membrane features enlarged and structurally well-packed nanochannels as well as large slip length, leading to outstanding desalination performance with improved water permeance and nearly complete salt rejection. Moreover, the swelling resistance of MXMA membrane is dramatically enhanced due to the strong chemical bond between MXene layers, which contributes to the excellent stability. Keywords: Pervaporation; MXene; Desalination; Crosslinking; Stability

1

1. Introduction Water scarcity caused by the population explosion, rapid development of urbanization and water contamination has become one of the formidable global challenge of contemporary society [1-3]. Among various possible methods for alleviating water crisis, desalination technology is now universally accepted as an ideal strategy [4, 5]. Membrane separation is considered as one of the most attractive alternative for seawater desalination because of its high efficiency and low energy consumption as compared with the conventional thermal technologies [6, 7]. Compared with the traditional membrane separation methods (such as reverse osmosis (RO), membrane distillation (MD)), pervaporation (PV) has been proved to be a promising and effective processing method for seawater desalination owing to its high selectivity, ability to tolerate high salinity water and superior anti-fouling performance [8]. Currently, the polymeric composite membranes are widely used for seawater desalination attributed to their merits of flexibility, simple fabrication process, and economical efficiency. However, the poor solvent resistance, inferior antifouling ability and short lifetime of polymeric membrane would usually lead to the low rejection and stability [9]. Inversely, the inorganic ceramic membranes have advantages of outstanding chemical stability, high strength, and long lifetime, but face the drawbacks of complex synthesis process, high expense and fragileness [10, 11]. Hence, it is highly imperative to design a versatile membrane that simultaneously integrate fine characters of polymeric and ceramic membranes for efficient desalination. To this end, two dimensional materials (such as graphene [12], graphene oxide (GO) [13, 14] and molybdenum disulfide [15]), possessing both the ceramic-like stability and polymer-like flexibility, have emerged as promising materials to fabricate separation membranes[16-18]. Among various strategies to generate high density of pores across 2D nanomaterial structures such as stacking sheets[19], perforation[20], or pore former addition[21], stacking the 2D materials into the peculiar laminate membrane, in which the water and ion transport are confined in the nanochannels between planar 2D nanosheets, has received significant interest due to the advantages of easy-operation, low-cost and flexible regulation. So far, 2

graphene-based materials are most studied for seawater desalination. However, the application of graphene-based membranes is still limited by the low permeation-flux [4, 22, 23]. Recently, Transition metal carbides, carbonitrides, and nitrides (MXenes) as a new family of 2D materials, with attractive electronic conductivity, mechanical properties, and versatile surface chemistry, have received great attention in a wide range of field [24-26]. MXene is prospective as a novel potential candidate for developing selective membranes for hydrogen separation [27], water desalination [28], and selective molecular separation [29] due to its abundant surface functionalities, hydrophilicity, facile scale-up synthesis and environmental friendliness.[26, 30-32] However, there are limited reports on MXene for pervaporation desalination. Jin and co-workers first reported Ti3C2Tx-based membranes for desalination through pervaporation process and a high water permeance was demonstrated [28]. However, the presence of oxygen-containing groups endow MXene with outstanding hydrophilicity and thus makes it highly possible to absorb water and enlarge the d-spacing among MXene nanosheets in aqueous conditions, which would render the membrane with poor mechanical strength and low selectivity for salt ions [33-35]. Therefore, it is highly desirable to strengthen the MXene interaction and fixed the 2D nanochannels between MXene nanosheets to enhance the separation performance and long term stability. Hitherto, covalent crosslinking has been proven to be an efficient strategy to restrict the swelling effect of hydrophilic laminate membranes [36-38]. In this way, well-packed interchannels fixed by strong covalent bonding could be obtained, exhibiting significant preponderances over nonlinked membranes. However, it is still difficult to simultaneously achieve a high water permeation without a sacrifice in separation efficiency [39]. Besides decreasing the membrane thickness, an ideal 2D laminate membrane for desalination is expected to have an appropriate interlayer spacing that not only can repel the larger hydrated ions, but also maximize the transport of water molecules [40]. According to the slip flow theory, large slip lengths inside membrane channel derived from the hydrophobic region would give rise to a fast water diffusion [41, 42]. Consequently, to achieve the MXene-based membrane with fixed and appropriate 2D capillaries as well as 3

large slip lengths, it is of great importance to choose a crosslinker with larger dimension and abundant reactive terminal groups. Maleic acid, with appropriate size of ~0.6nm (four-carbon atom molecular skeleton) and two carboxyl groups [43] which could act as reactive binding sites to construct more hydrophobic nonoxidized region, is a highly promising candidate to interconnected MXene flakes. In this work, a novel ultrathin maleic acid covalent-bridged MXene membrane (MXMA) with fixed 2D nanochannels for desalination was designed and fabricated through a facile vacuum-assisted filtration method. The maleic acid crosslinker brought about an enlarged interlayer spacing (0.49nm) and large slip lengths via the deoxidization reaction with the MXene nanosheets, which led to the improvement in the membrane permeability without a sacrifice in selectivity. Moreover, the swelling of membrane was significantly restricted and MXMA membrane exhibited excellent long-term stability in real sea water desalination. 2. Experimental 2.1. Materials Ti3AlC2 (MAX) powder (≥ 98%, 300 mesh) was purchased from Yuehuan Company (Shanghai, China). Lithium fluoride (LiF, >99.0%), maleic acid (MA, >99.0%) were purchased from Sigma-Aldrich. Sodium chloride (NaCl, >99.0%), sodium sulfate (Na2SO4, >99.0%), magnesium chloride (MgCl2, >99.0%), magnesium sulfate (MgSO4, >99.0%), hydrochloric acid (HCl, 32%), were obtained from Merck KGaA, Darmstadt, Germany. Potassium chloride (KCl, >99.0%) was sourced from Thermo Fisher Scientific. Nylon (polyamide) microfiltration membranes with a nominal pore size of 0.22 µm provided by Sterlitech were used as the substrates. All reagents are of analytical grade and were used directly without further purification. Milli-Q deionised water (18.1 MΩ﹒cm at 25 °C) was used for the preparation of all solutions used in the experiments. 2.2. Synthesis of Ti3C2Tx nanosheets Ti3C2Tx nanosheets were synthesized through a mild in situ HF method described in details elsewhere[44]. 4

Typically, 1 g of LiF was added into 15 mL of 6M HCl and the mixture was under continuous stirring for 5 min. Then, 1 g of Ti3AlC2 powder was gradually added to the above etchant, and the reaction was held at 35 °C for 24 h. After that, the acid mixture was washed to neutral pH by centrifugation with deionized water for 5 cycles. The black Ti3C2Tx slurry was collected and followed by redispersing into 100mL of deionized water. Afterward, the mixture was kept under ultrasonication in flowing Nitrogen for 1h for delamination, and then the suspension was centrifuged with 3500 rpm for 30 min to obtain the stable MXene colloidal solutions with concentration of 3.5 mg mL−1 2.3 Fabrication of MXene membrane and maleic acid crosslinked MXene membrane The Ti3C2Tx membrane was prepared on the commercial Nylon membrane with pore size of o.22µm by vacuum filtration. A specific volume of MXene solution was added into 80mL deionized water and followed by sonication for 10min to obtain homogeneous Ti3C2Tx flakes suspension. Before filtration, the MXene solution was kept in the filtration flask for 2 min to realize a good contact between the substrate surface and the Ti3C2Tx nanosheets. The vacuum filtration was then proceeded until the no liquid flow to the vacuum side was observed. Afterwards, a certain amount of MA solution (mass ratio of MA and MXene was 1:1) was filtrated through the whole membrane. 1 mL of 3M HCl was then pass through the membrane with the assistance of vacuum pressure, and the crosslinking reaction was held at 70℃ for 1h. Finally, the obtained the MXMA membrane was dried at room temperature overnight for further use. For comparison, the pure MXene (MX) membrane was prepared without maleic acid and HCl treatment. 2.4 Characterization The morphology and thickness of the samples were observed by field emission Scanning Electron Microscope (FESEM, Zeiss Merlin Gemini 2). Energy Dispersive Spectrometers (EDS, Oxford Instrument with JED-2300 Analysis Station) equipped on FESEM was used to detect the elemental distribution of samples. Before FESEM and EDS characterization, the surfaces of membranes were sputtering coated with iridium. Transmission electron microscopy (TEM) was carried out on a TECNAI 12 electron microscope at 5

accelerating voltage 200 kV. The membrane samples were obtained by preparing a thick freestanding membrane peeled form AAO membrane substrate. The interlayer spacing and crystallization properties of membranes was examined using X-ray diffraction (XRD, Bruker D8) within the 2θ range of 5 to 80°at a scan rate of 0.2 s step−1. The surface chemical functional groups of samples were detected by attenuated total reflectance-Fourier Transform Infrared (ATR-FTIR, Perkin-Elmer Spectrum 2000) with frequency range from 4000 to 100 cm-1. Raman spectroscopy (LabRAM HR, HORIBA, France) was used with a confocal microscopic Raman spectrometer (inVia-Reflex, Renishaw) equipped with a 532 nm laser light irradiation in the range of 100 –800 cm−1. Atomic force microscope (AFM, Bruker FastScan atomic force microscope) was applied to analyze the roughness of prepared membranes. The contact angle measurement system (CAM200) was used to detect the static water contact angles of membrane samples for hydrophilicity evaluation. The surface elemental component and chemical state of membranes were examined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo, Germany) equipped with a monochromatic 100-eV Al-Kα X-ray source. The surface charge properties of membranes were characterized by a zeta-potential analyzer (SurPASS, Anton Paar GmbH, Austria) at the pH range of 3-11. 2.5 Membrane performance testing The desalination performances of as-prepared membranes were performed in a home-made pervaporation unit (Fig.S1) reported in our previous study [45]. The membrane was put in a stainless steel membrane module with an effective membrane area of 3.14 cm2. The feed solutions containing NaCl, KCl, Na2SO4, MgCl2, and MgSO4 (concentrations of 3.5wt% each) were preheated to the pre-set temperature and circulated on the fed side of the membrane module by a Masterflex® peristaltic pump. After stabilization for 1 hour, the vapor on the permeate side of the membrane module was collected at certain time intervals by a dry-ice cold trap. The operating feed temperature was monitored by a K-type thermocouple installed in the feed chamber. The permeate pressure was maintained at 1 Torr by a vacuum pump. The membrane desalination performance was evaluated by water flux and salt rejection. The water flux (J) can be calculated 6

by the following equation (1): J=

×∆

.(1)

where M is permeation mass (kg), A is the effective membrane area (m2), and ∆ is the collecting time (h). The salt rejection (R) was determined by ion concentration of the permeation and the feed, and calculated by equation (2): =

. (2)

where Cf and Cp represent the ion conductivity of the permeation and the feed, respectively. The ions concentrations were measured by a pre-calibrated Oakton® Con 110 conductivity meter. 3. Results and Discussion 3.1 Characterization of MXene Ti3C2Tx The SEM image of the precursor Ti3AlC2 powders (Fig. S2a) showed a brick-like structure with tightly stacked layers and hardly to be delaminated by sonication. The MXene synthesis process involves the extraction of Al from Ti3AlC2 particles through wet-chemical etching with LiF-HCl solution, in which Li ions were intercalated into the bulk layer to form the stacked MXene (Ti3C2Tx). After sonication-assisted exfoliation, the MXene bulk was easily delaminated to single layer with a typical Tyndall effect (Fig.1a). The morphology of MXene nanosheets was observed by Field emission SEM (FESEM), AFM and TEM. As shown in the SEM image (Fig.1b), the sheet-like morphology of MXene on the top of anodic aluminum oxide (AAO) membrane was found. The thickness of the MXene nanosheets was in the range of 1.2~1.4 nm, suggesting a single layer structure (Fig.S2b) [25]. The high-quality MXene monolayer nanosheets with flat and nearly transparent surface was directly displayed by TEM image (Fig.1c). Moreover, the fast Fourier transform pattern (FFT, inset in Fig. 1c) shows the hexagonal symmetry of the planes, suggesting that Mxene nanosheets remain the same single crystalline phase as its bulk counterpart, consistent with the previous results[46]. The element constitutes of exfoliated nanosheets were detected by EDS elemental, It can be seen from Fig.1d that Ti, C and O elements were uniformly distributed and Al element was 7

successfully etched from the MAX precursor. The ATR-FTIR spectra of Mxene nanosheet, displayed in Fig. 1e, exhibited two characteristic bands at 3457 cm−1 and 1600 cm−1, corresponding to -OH and -C=O, respectively [47]. The abundant surface-terminating -OH groups endow MXene nanosheets with outstanding hydrophilicity and the potential to react with maleic acid through esterification reaction. The size distribution was displayed in Fig.1f, indicating that lateral size of the MXene nanosheets was in the range of around 0.2 to 1.4 µm.

Fig.1 (a) Digital photograph of MXene nanosheets solution, showing the typical Tyndall effect. (b) SEM image of MXene sheets. (c) TEM image of MXene monolayer. Insert is the corresponding FFT pattern. (d) EDS elemental mapping of an Mxene nanosheets. (e) FTIR spectra of MXene nanosheets. (f) Frequency histograms of MXene lateral size

3.2 Characterization of crosslinked-MXene membranes As shown in Fig.2a, MXene membrane was first fabricated through a facial vacuum-assisted filtration followed by maleic acid and HCl aqueous solution treatment with the aim to adjust the inter-sheet d-spacing and immobilize the adjacent MXene nanosheets. Due to the abundant hydroxyl group on the MXene surface, it is expected that covalent interlinking would occur between MXene sheets and maleic acid molecule through deoxidization reaction. Finally, as shown in Fig.2b, the crosslinked MXene membrane (MXMA) 8

with excellent flexibility and stiffness was obtained. SEM images, shown in Fig. 2c-e, exhibit the topographic and cross-sectional morphologies of the MXMA membranes, respectively. The pristine nylon substrate shows a highly porous structure (Fig. S3). In topographic view, the surface of the MXMA membrane is rough and no obvious defect was observed except for some ripples formed by the edges of MXene nanosheets (Fig.2c). Moreover, distinct boundaries of stacked MXene was demonstrated in Fig. 2d. Compared to MXene membrane (MX) with same amount of MXene loading (Fig. S4a-b), there was more wrinkles on the MXMA membranes surface, which is probably derived from the reaction between MXene nanosheets and MA. AFM analysis (Fig.S5) further revealed the surface roughness of MXMA membrane increased after cross-linking with maleic acid, which is in line with the result of SEM. This result indicates a looser stacking structure for MXMA membrane, which would further influence the permeability [28]. Fig.2d shows the SEM image of cross-section of MXMA membrane, in which the MXene sheets horizontally assembled onto the microporous nylon substrate forming a well-ordered laminar film. The interlamination spaces of this layered structure are supposed to provide 2D channels for molecular transport. Notably, there was no significant change in the thickness of MXMA membrane after MA-cross-linking reaction (Fig.S4c). The typical laminar structure with small wrinkles on the top surface of MXMA membrane was further confirmed by the cross-sectional TEM image (Fig.2f). Moreover, according to the result of FFT pattern (insert of Fig. 2f), the interlayer spacing was calculated to be ~0.5 nm, which is larger than that of pure MXene membrane (Fig.S4d). Since the thickness is one of the crucial factors for membrane performance, several membranes with thicknesses of 20, 30, 70, 190 and 370nm were fabricated by depositing different amount of MXene nanosheets and maleic acid solution on the top of porous substrate, respectively, (Fig.S6 and S7). It is obvious that no detectable cracks, pinholes, or other defects were found in all of the membranes except for the membrane with lowest MXene deposition (thickness of ~20 nm). Interestingly, the corrugations on the membrane surface decrease with the increase of membrane thickness. 9

Fig.2 (a) Scheme of the preparation of MXMA membrane on supported nylon substrate through vacuum-assisted filtration. (b) Digital photo of MXMA membrane with excellent flexibility. (c) and (d) Top-sectional SEM image of the MXMA membrane with different magnifications. Red arrows represent the boundaries of stacked MXene. (e) Cross-sectional SEM image of the MXMA membrane, supported on the nylon substrate. (f) TEM image of the MXMA membrane. Insert is the corresponding FFT pattern.

In order to further investigate the cross linking reaction between MXene nanosheet and maleic acid, the element components of MXMA and pure MX membrane were evaluated by XPS analysis. Obviously, the overall O and C atomic percentages (O/C) for MXMA membrane increased after cross linking treatment (Fig.S8a and Table S1). This is mainly ascribed to the relative high O/C ratio (O/C=1) of MA molecule as well as the consumption of hydroxyl groups from carboxyl group in maleic acid molecule during the 10

esterification reaction between MXene sheets and maleic acid. Correspondingly, the high resolution O1s and C1s XPS spectra of membranes were performed to characterize the chemical bonds. It was observed from Fig. 3a that O1s spectra can be deconvoluted into four characteristic peaks at binding energies of 528.8 eV (TiO2), 529.8 eV (C-Ti-Ox), 531.1 eV (C-Ti-(OH)x) and 532.4 eV (H2Oabs), respectively. Compared to the pristine MX membrane, the relative content of surface hydroxyls (C-Ti-(OH)x) in MXMA surface decreased to 19.3%, accompanied by the increasement of adsorbed molecular water (H2Oabs), which indicates the successful occurrence of dehydration condensation reactions between MXene and maleic acid. In the high-resolution XPS spectra of C1s (Fig.8b), the four fitted peaks located at the binding energies of 281.7, 284.8, 286.2, and 288.4 eV could be assigned to C-Ti, C-C, C-O-C and O-C=O, respectively. Based on the relative areas of the fitted peaks (Table S1), the relative fraction of the O-C=O in MXMA membrane increased from 7.1% to 10.7% in comparison with the pristine MX membrane, further confirming the successful crosslinking between MXene nanosheets and maleic acid molecules through deoxygenated process. Figure 3b shows Raman spectra of MXMA and pristine MX membranes. The peak at 109 cm−1 (Eg) is ascribed to out-of-plane vibrations of C atoms in Ti3C2O2, and 709 cm−1 (A1g) is assigned to out-of-plane vibrations of C atoms in Ti3C2O(OH)[48, 49]. Therefore, crosslinking reaction can be evaluated by the change in the ratio, IA1g /IEg, to some extent. The corresponding IA1g /IEg value for the pristine MXene membrane was 1.17. However, IA1g /IEg ratio decreased to 1.13 for MXMA membrane, which reveals that partial hydroxyl group was eliminated due to chemical interlinks of the maleic acid with the basal plane of the MXene, in accord with the XPS analysis. The surface hydrophilicity of membrane was evaluated by the static water contact angle shown in Fig. 3c. The water contact angle for MXMA membrane, 43.6°, is higher than that of the pristine MXene membrane (26.5°), demonstrating that the MXMA membrane was still hydrophilic due to the residual oxygen-containing functional group on MXene surface. Fig. S9 shows the results of the zeta potential 11

analysis of membranes in the range of pH 3–11, in which both MXMA and MX membrane were negatively charged and exhibited a growing tendency in electronegativity with pH value. This result is predominantly ascribed to the deprotonation of the carboxyl and hydroxyl groups [40]. Notably, the MXMA membrane is less negatively charged, which is probably derived from the consuming of hydroxyl groups and the formation of less polar ester groups in the reactions with maleic acid. The crystal structure and d-spacing of MXMA and MX membranes were determined by XRD as shown in Fig.S10 and Fig.3d. The characteristic (002) peak centered at 2θ of 6.28, corresponding to a d-spacing of 1.37 nm, was observed for pristine MX. Considering the thickness of MXene monolayer of ~1 nm, the interlayer spacing in the MX membrane was calculated to be ~0.37nm, which is in good agreement with the previous literature [50]. For the maleic acid cross-linked MXMA membrane, the d-spacing was slightly changed to 1.49 nm (5.81°) with the corresponding 2D nanochannel size of 0.49 nm. The enlarged interlayer spacing of MXMA is primarily derived from the intercalation of maleic acid molecules, resulting in an increased steric hindrance between MXene nanosheets. Additionally, the types of cross-linking between the layers would be highly important to determine the size of nanochannel in MXMA. It has been reported that crosslinkers inside the nanochannel are prone to be tilted to the membrane nanosheet[51]. As the maleic acid size along the carbon chain direction (~0.6 nm) is larger than the interlayer spacing of MXMA in dry state, it is reasonable to suppose that MA monomer links with MXene layers in tilted orientation. Furthermore, as MXene nanosheet is prone to swelling in water, the sensitivity of membrane toward water was investigated by presoaking the membranes in deionized water for 24 h. The XRD analysis shows that d-spacing of MX expanded by 31.4% to 1.80 nm with a corresponding interlayer size of 0.80 nm, which reveals that water molecule is readily intercalated into the interlayer space of pristine MX membrane owing to its small kinetic diameter (~0.29 nm) and the adsorption from surface hydrophilic oxygen-containing groups, and thus impairing the selectivity performance. On the other hand, the variation of the d-spacing is nearly negligible within 2.7% expansion for MXMA, suggesting that the MXene sheets in MXMA were regularly fixated by 12

covalently bridging with maleic acid, which significantly suppressed the swelling of the interlayer and improved the mechanical strength of the MXMA.

Fig.3. (a) High-resolution XPS spectra of O1s, (b) Raman spectra and (c) Water contact angle for MX and MXMA membranes. (d) XRD diffractogram of membranes before and after immersing into water for 24h.

On the basis of above analysis, the possible chemical reaction between the MXene nanosheets and maleic acid involved in the MXMA membrane fabrication was depicted in Fig.4. As is shown, the hydroxyl groups on MXene planes reacted with the carboxyl groups in maleic acid to form ester bonds, which is expected to provide a stable interlamination spacing of the MXene nanosheets, and thus restricting the swelling of MXMA membrane in aqueous condition, while maintaining the nanochannels exist in the internal of MXene layer.

13

Fig.4 Possible reaction scheme between MXene and maleic acid through esterification reaction.

3.3 Pervaporation performance of membranes for desalination Based on the Hagen–Poiseuille equation, the membrane permeability could be significantly influenced by the thickness of its selective layer [52]. It is well accepted the thinner membrane will achieve better performance. Therefore, the desalination performance of MXMA membranes with different thicknesses was conducted at feed temperature of 30C using 3.5 wt% NaCl solution as the feed, and the results are illustrated in Fig. 5a. It was found that the water permeation shows a negative correlation to the thickness of membrane. As the membrane thickness of MXMA was decreased from 370 to 30 nm, the water flux gradually increased from 5.1 to 22.8 kg m-2 h-1 due to the decreased mass transfer resistance, while the outstanding ions rejection (> 99.7%) was maintained. However, when continuously decreased the membrane thickness by depositing less MXene and MA solution on the top of nylon substrate, the MXMA membrane exhibited a poor separation performance with only ~30% of NaCl rejection, which is primarily attributed to the non-selective defects on the resulting membrane surface. It can be concluded that a defect- free MXMA membrane could be fabricated with a minimum thickness of ~30 nm. The pervaporation desalination performance of MXMA and MX membranes was investigated by using 3.5 wt% NaCl solution as the feed at 30 °C. It can be seen from Fig.5b that MXMA exhibited higher water flux of 22.8 kg m-2 h-1 than that of MX membrane (17.3 kg m-2 h-1) without the compromise of selectivity 14

performance. Primarily, based on the solution-diffusion model, the water affinity of membrane surface and permeates diffusivity in transport channels are two crucial factors affect the pervaporation performance. Despite some hydroxyl groups on MXene surface were consumed due to the cross-linking reaction, the MXMA still maintained high hydrophilic property, which facilitates the adsorption of water molecules into the 2D nanochannels. The improved surface roughness of MXMA membrane is regarded as an enlargement of the effective area, which could facilitate the interaction between water molecules and membrane [53]. In addition, the enlarged interlayer spacing in MXMA endows the intercalation of more than one layer of water molecules between individual MXene flakes, which boosts the improvement of water flux, while maintain the high rejection of ~99.9%. Moreover, analogous to graphene oxide membrane, MXene membrane also can to divided into two types of regions: hydrophilic and hydrophobic. The hydrophilic regions would be in favor of the intercalation of water molecules into interlayer, but it also results in the blockage of water molecules ascribed to the strong interaction between them. Inversely, the nearly frictionless hydrophobic region would accelerate the water transport. Therefore, according to the large slip length theory, the consumption of hydrophilic hydroxyl groups and the generation of hydrophobic ester groups in MXMA membrane provided more frictionless region, which could also be responsible for the fast water flow in transport channels. Furthermore, the separation performances of MXMA membrane in various salt solution systems were measured as well (Fig.S11). Expectedly, the MXMA membrane shows excellent desalination performance with high salt rejections of 99.90% for KCl, 99.97% for MgCl2, 99.98% for Na2SO4, and 99.99% for MgSO4, as well as relatively high water flux. The separation mechanism for MXMA membranes is mainly on the basis of molecular sieving effect, in which the 2D nanochannels could exclude the larger ions (Cl− for 0.66 nm, Na+ for 0.72 nm, SO42− for 0.76 nm, and Mg2+ for 0.86 nm) while make the smaller molecules pass through[54, 55]. Generally, the feed temperature plays an important role in the pervaporation process. As shown in Fig.5c, the water flux for MXMA membrane with the thickness of ~30 nm shows an exponential increase in 15

the feed temperature range of 30 to 65 ℃, and a high water permeance of 79.3 kg m-2 h-1 was achieved at 65℃, which could be ascribed to the increased driving force of pervaporation and the acceleration of the diffusion rate of water molecules[56]. Moreover, all the salt rejections maintain almost unchanged. According to Arrhenius law[57], the activation energy (Ea) of water permeation (3.5 wt% NaCl solution as feed) in the MXMA and MX membrane were calculated to be 27.05 and 31.01 kJ mol-1, respectively (Fig. S12). The lower activation energy suggests that it is easier for water molecules to diffuse through the interlayer channel between MXene sheets in MXMA. Additionally, in comparison with other pervaporation membranes reported in previous literature[28, 45, 56, 58-63], MXMA membrane fabricated here showed good desalination performance, with a higher water flux and separation ability (Fig. 5d and Table S2). Accordingly, it is reasonable to believe that MXMA is a promising candidate for pervaporation desalination.

Fig.5. (a) Water permeation and ion rejection of MXMA membrane as a function of thickness at 30 °C for desalination of 3.5 wt% NaCl solution through pervaporation. (b) Water permeation and ion rejection of MXMA and MX membrane with similar thickness of ~30 nm at 30 °C for desalination of 3.5 wt% NaCl solution through pervaporation. (c) Water 16

permeation and ion rejection of MXMA membrane with thickness of ~30nm as a function of temperature for desalination of 3.5 wt% NaCl solution through pervaporation. (d) Comparison between MXMA membrane and other membranes reported in literature on pervaporation desalination performance.

In order to further evaluate the stability including swelling performance and mechanical strength of membranes, a long-term desalination test was performed by using 3.5% NaCl as feed solution at 30 ℃. As shown in Fig.6a, both the permeation flux and the ion rejection of MXMA membrane remained stable during 30 h successive operation, demonstrating a well structural integrity and separation performance. However, the pristine MX membrane exhibits unstable water flux and continuously declined salt rejection, which is probably due to the combined effect of the membrane swelling boosting the water flux as well as the formation of crystal salt hindering the water diffusion (Fig. S13). Accordingly, XRD analysis was applied to detect the variation in interlayer spacing of membranes before and after immersion in 3.5% NaCl solution (Fig.6b). The interlayer size of MXMA shows a slight expansion of 4.7% to 0.56 nm, which is still smaller than salt size. In comparison, the interlayer spacing in MX enlarged to 0.92 nm with a 40.1% expansion, which would negatively limit the sieving performance. This phenomenon proves again that the swelling of MXMA membrane was effectively inhibited by covalently linking the MXene sheets using MA. Moreover, it is obvious that the planar channel size of membranes in NaCl solution was larger than that in deionized water, which is probably due to the formation of electric double layer caused by penetration of Na+, thus leading to the repulsion of MXene flakes [29]. In addition, the MXMA layer were intact on the surface Nylon substrate with outstanding integrity and mechanical strength (Fig.S14), suggesting a strong adhesion of the active layer with the substrate. EDS technology was used to investigate the formation of salt crystal on both the feed and permeate sides of the membranes after the long-term desalination test. Fig.S15 shows that both MXMA and MX membrane remained high integrity and no salt crystals were detected on the permeate sides of MXMA membrane. Reversely, both Na and Cl elements were observed on the feed and permeate sides of MX membranes, which visibly demonstrates the salt ions were prone to pass through the 17

MX membrane and form the crystals on the downstream side. For practical application, the pervaporation for desalination performance of MXMA membrane was examined by using the real sea water from Black Rock, located in Melbourne, Australia. After 50 h long-term operation, both water flux and salt rejection remained stable and were comparable with the 3.5 wt% NaCl system (Fig. 6d). Remarkably, although the MXMA membrane exhibits outstanding anti-swelling performance and mechanical strength, the sensitivity of MXene to oxidizing agents is still a challenge that would hinder its wide application. Strategies to improve the long term operational stability should be the considered in the future work.

Fig.6. (a) Long-term stability test of MXMA membrane at 30 °C for desalination of 3.5 wt% NaCl solution through pervaporation for 30h. (b) XRD diffractogram of membranes before and after immersing into 3.5 wt% NaCl solution for 24h. (c) Long-term stability test of MXMA membrane at 30 °C for desalination of real sea water for 50h.

4. Conclusions In summary, we developed and fabricated a 2D lamellar maleic acid covalent-bridged MXene membrane (MXMA) supported on microporous nylon substrate by vacuum-assisted assembly strategy. The 18

adjacent MXene nanosheets were covalently linked by MA molecules through the esterification reaction between the carboxyl groups in MA and hydroxyl groups in MXene surface, resulting in structurally well-defined nanochannels with size of ~0.49 nm, which is larger than that of the pristine MX membrane (0.37nm). The resultant MXMA membrane shows enhanced desalination performance with high water flux and salt rejection in the process of pervaporation. In particular, the strong covalent bonds between MXene nanosheets endow the crosslinked MXene membrane with superior swelling resistance and mechanical strength.

Acknowledgements This work was supported by The National Natural Science Foundation of China (NO. 51678213 and NO. 51578209), the Fundamental Research Funds for the Central Universities (2018B53314), the National major water projects (No.2017ZX07201002), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and CSIRO Manufacturing. Mingmei Ding shows gratitude towards the scholarship from China Scholarship Council (CSC). We would also like to acknowledge Mr. Ray Huang, Dr. Julian Ratcliffe, Aaron Seeber and Mark Greaves from CSIRO for assisting with water contact angle test, TEM characterisations, XRD analysis and SEM training

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1. A novel maleic acid-crosslinked MXene membrane was fabricated for pervaporation desalination. 2. Enlarged nanochannels were obtained through crosslinking reaction. 3. MXMA membrane showed improved water permeation and high salt rejection. 4. MXMA membrane exhibited excellent antiswelling performance due to the fixed nanochannels.

Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: