Journal of Membrane Science 623 (2021) 119076

Available online 15 January 20210376-7388/© 2021 Elsevier B.V. All rights reserved.

Fabrication of surface-charged MXene membrane and its application for water desalination

Baochun Meng 1, Guozhen Liu 1, Yangyang Mao , Feng Liang , Gongping Liu **, Wanqin Jin *

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, 30 Puzhu Road, Nanjing, 211816, PR China

A R T I C L E I N F O

Keywords: MXene membrane Surface charged Nanofiltration Forward osmosis Water desalination

A B S T R A C T

Two-dimensional (2D) materials have been demonstrated as promising building blocks of designing high- performance membranes for molecular separation. Nevertheless, it is a big challenge to apply 2D-material membranes for water desalination. Herein, we proposed a new type of surface-charged MXene (SC-MXene) membrane for water desalination, which was facilely fabricated by laminar stacking of MXene nanosheets and subsequent surface-coating with polyelectrolyte layer. The morphology, physicochemical structure and surface property of the MXene materials and resulted membranes were observed by XPS, SEM, water contact angle test, AFM, IR and XPS. These results demonstrated that coating polyelectrolyte (PEI) successfully tuned the surface charge and enhanced the hydrophilicity without scarifying the laminar structure of MXene membrane. By uti-lizing the effects of electrostatic interaction and size-sieving, the resulting surface-charged MXene (SC-MXene) membrane exhibited high salt rejection and water permeance during either nanofiltration or forward osmosis process, showing great potential for water desalination.

1. Introduction

With the increasing population and emerging economies, water shortage has become a challenge for human [1,2]. To solve this crisis, the utilization of brackish water or seawater is considered as an effective strategy. In the past decades, owing to low energy consumption and high separation efficiency, membrane technology has been proved as an alternative to water purification [3,4]. Conventional membrane mate-rials are dominated by polymers, which are limited by the trade-off ef-fect between permeability and selectivity [5]. Various advanced materials have been developed for membrane separation to further improve the separation efficiency. Recently, two-dimensional (2D) materials such as graphene oxide [6,7], MoS2 [8,9], metal-organic framework [10], etc., have been demonstrated as building block for membranes used for molecular separation. The 2D materials are able to minimize transport resistance and achieve high permeation flux due to atomic-thickness and micrometer lateral size, showing great potential in water purification [11–13]. However, it remains great challenge for 2D-material membranes realizing highly-efficient water desalination, generally because that the excessively large interlayer space and will be

further enlarged when they were swollen in water [14–16]. Many at-tempts had been tried to inhibit the swelling phenomenon via physical and chemical approaches with the aim of precise controlling the inter-layer space at sub-nanometer scale to enhance the ion rejection [17–20]. An alternative way is to develop new 2D-material membranes with enhanced separation performance and stability in water.

As a new addition to the 2D family, MXene have attracted tremen-dous research interests owing to unique physical-chemical properties [21–27]. MXene are transition metal carbides, carbonitrides, and ni-trides with formula Mn+1XnTx (n = 1, 2, 3), where M is the early tran-sition metal, X represents carbon and/or nitrogen and T stands for the functional groups (such as hydroxyl, oxygen or fluorine). Normally, it was generated from its precursor MAX via chemical etching method, where A is the IIIA and IVA groups [28,29]. Meanwhile, the wet-etching method endows abundant oxygen-containing groups on the surface. Until now, owing to its high hydrophilicity and anti-fouling property, MXene nanosheets have been widely considered as membrane material for molecular separation, especially for water treatment [12,30–32]. Until now, most of them focused on the dye rejection and solvent dehydration, while, only a few reports studied MXene-based membranes

* Corresponding author. ** Corresponding author.

E-mail addresses: gpliu@njtech.edu.cn (G. Liu), wqjin@njtech.edu.cn (W. Jin). 1 These authors contribute equally to this work.

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Journal of Membrane Science

journal homepage: http://www.elsevier.com/locate/memsci

https://doi.org/10.1016/j.memsci.2021.119076 Received 14 October 2020; Received in revised form 8 January 2021; Accepted 8 January 2021

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for water desalination [12,13,31–34]. Wang and co-works developed Al3+ and self-crosslinked MXene membranes for forward osmosis (FO) desalination, respectively [31,32]. Excellent salt rejection and water flux were achieved because of enhanced anti-swelling property and regular laminar structure, while the external-pressure driven desalina-tion process remains to be demonstrated. Sun et al. explored high tem-perature (~400 ◦C) treated MXene membrane with adjustable interlayer spacing for ion rejection [35]. However, the rejection rate for most salts such as MgCl2 and NaCl is rather poor (<60%). Very recently, Wang and co-works demonstrated a strategy for stabilizing the MXene laminar architecture by alginate hydrogel pillars [36]. The resulting MXe-ne/alginate hybrid structure with stable interlayer spacing exhibits excellent ion sieving properties. Besides of the interlayer spacing, the MXene membrane could be also functioned via electrostatic effect by utilizing the deprotonation groups of MXene nanosheets [33,37]. Apparently, the electrostatic functionalization of MXene membranes would provide additional opportunities for tuning the membrane sur-face properties (e.g., hydrophilicity and charge), which however was not fully explored for water desalination yet.

Hence, in this study, we proposed a new type of surface-charged MXene membrane (SC-MXene) via a facile polyelectrolyte coating for water desalination (Fig. 1). Wet-etching method was applied to fabricate ultra-thin Ti3C2Tx MXene nanosheets from Ti3AlC2 powders and the laminar membrane structure was constructed via filtration. The thick-ness of membrane could be finely controlled by regulating MXene nanosheets loading. More importantly, surface charge of the MXene membrane was manipulated by coating polyethyleneimine (PEI) layer with the aid of electrostatic interactions between negative charged MXene and positively charged PEI. The desalination property for SC- MXene membrane was investigated via two membrane processes: nanofiltration (external pressure-driven process) and forward osmosis (non-external pressure-driven process).

2. Experimental

2.1. Materials

The precursor Ti3AlC2 powders (200 mesh) were bought from 11 technology Co., Ltd,China. Polyethyleneimine (PEI, Mw = 10000) and lithium fluoride (99.9%) were provided by Aladdin Industrial Corpo-ration, Shanghai, China. Hydrochloric acid (36%–38%) and ethanol (>99.7%) were provided by Yasheng Chemical Co. Ltd. Polyacrylonitrile (PAN) substrate was purchased from Shandong Megavision Membrane Technology & Engineering Co., Ltd., China.

2.2. Fabrication of Ti3C2Tx MXene nanosheets

The synthesis process of mono-layer MXene nanosheets can be found

in our previous papers [34,38]. In detail, specific amount of Ti3AlC2 were immersed into the mixture of HCl (6 M) and LiF, which was stirring at 35 ◦C for 24 h. Then water and ethanol were used to wash the resulting solution via centrifugation until the pH was close to 7. Finally, we could obtain the stacked MXene powders after drying at 40 ◦C for 12 h. The delaminated MXene nanosheets were prepared using sonication method: MXene powders and water were added into the three-necked glass flask and sonicated for 1 h under the Nitrogen atmosphere. Meanwhile, the ice bag was applied to decrease the temperature of so-lution. The un-exfoliated MXene was removed via centrifugation at 3500rpm. The resulting MXene solution was stored at low temperature before using.

2.3. Fabrication of MXene membranes

The laminar MXene membranes were fabricated by vacuum- filtration, where MXene nanosheets will be uniformly deposited on the porous PAN substrate. The SC-MXene membranes were fabricated via coating the PEI solution on the membrane surface using vacuum- filtration device. The PEI solution with concentration varying from 0.025 wt%, 0.05 wt%, 0.075 wt% to 0.1 wt% was poured into the device and kept for 30min without pressure. Finally, the resulted membranes were rinsed with DI water for several minutes to remove extra PEI molecules. All the MXene membranes were treated under vacuum at room temperature before testing.

2.4. Measurement of water desalination performance

The salt rejection and permeation rates of water and ions were car-ried out using home-made nanofiltration and forward osmosis U-shape devices. The effective membrane areas for nanofiltration and forward osmosis are 2.54 and 4.91 cm2, respectively. During the nanofiltration process, the concentration of salt (MgCl2, NaCl, Na2SO4 and MgSO4) in water is 50 ppm and the operating pressure is 0.2 MPa. Each data was collected after 60min when the membrane performance reaches the steady state.

The water flux (J) and rejection (R) are calculated as follows:

J =V

A × p × t(1)

R=

(

1 −Cp

Cf

)

× 100% (2)

where V (L) stands for the water volume of the permeate side. A (m2) refers to the effective membrane area. t (h) is the operation time and p (bar) responds to the operation pressure, respectively. Besides, Cp and Cf are the concentrations of salts in permeate and feed sides, respectively.

For the forward osmosis process, the same volume of 0.2 M of salts solution (NaCl, MgCl2, Na2SO4 and MgSO4) and DI water were used as the draw solution and feed solution, respectively. The membrane surface faced the draw solution. Meanwhile, magnetic stirring was applied to avoid possible concentration gradients. The salts concentration was measured via electrical conductivity. The water permeance Jw (mol⋅m− 2⋅h− 1) and ions permeation rate Js (mol⋅m− 2⋅h− 1) are calculated by:

Jw =Δm

A × Mw × t(3)

Js =(Ct⋅Vt − C0⋅V0)

1000 × A × t × Ms(4)

where Δm (g) represents the mass change in the feed side, A (m2) is the effective membrane area, Mw (g/mol) represents the molecular weight of water and t (h) is the operating time. C0 (mg/L) and V0 (L) stand for the initial salt concentration and solution volume of the feed side,

Fig. 1. Schematic diagram: the fabrication of SC-MXene membrane via vacuum filtration and their application for water desalination.

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respectively. Ct (mg/L) and Vt (L) are the salt concentration and solution volume after operating for a given time t. Ms (g/mol) represents the molecular weight of salts.

The ratio of Jw/Js is defined as the water/salt selectivity [39–41].

2.5. Characterizations

X-ray diffraction (XRD) (Rigaku, Miniflex 600, Japan) was applied to study the crystal phases of powders and membranes. The morphology of powders and membranes were studied by the field emission scanning electron microscopy (FESEM, JSM-7600F, Japan). The MXene nano-sheets thickness, membranes phase diagrams and surface roughness were characterized via Atomic Force Microscope (AFM, XE-100, Park SYSTEMS, Korea), where we measured at least three times to make sure the obtained results. Besides, the chemical property of resulting mem-branes was analysed via X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, USA) and Fourier Transform Infrared Spectrometer

(FTIR, AVATAR-FT-IR-360, Thermo Nicolet, USA). Meanwhile, contact angle (DSA100, Kruss) drop-meter was applied to investigate mem-branes surface property. To confirm the results, the contact angles were obtained via measuring at least three times. The zeta potential of membrane surface was studied by streaming potential measurements with an electrokinetic analyser (SurPASS3, Anton Paar, Austria). The salt concentration was determined via the electrical conductivity (FE38- Standard, METTLER TOLEDO, Switzerland).

3. Results and discussion

3.1. MXene properties

Owing to micro-meter lateral size and hydrophilicity, the most studied MXene material, Ti3C2Tx, has been widely studied for fabri-cating separation membranes [12,13,30–32]. Generally, it was gener-ated from its precursor, Ti3AlC2 powders, which present tightly stacked

Fig. 2. (a) and (b) SEM images and (c) XRD patterns of Ti3AlC2 MAX powders and Ti3C2Tx MXene powders; (d) AFM image of MXene nanosheets; (e) Zeta potential of MXene and PEI solution; (f) The corresponding height profiles of MXene nanosheets in (d).

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layered structure, as shown in Fig. 2a. However, the metallic bond Ti–Al is too strong to be destroyed via conventional mechanical method [22]. Therefore, wet-etching method was applied to selective remove Al element. We selected the mixture of HCl and LiF as the etching agent to form HF in-situ. After selectively etching of Al atoms, the resulting Ti3C2Tx MXene powders showed loosely layered structure, like an ac-cordion, shown in Fig. 2b. The exfoliated MXene nanosheets could be obtained via external force like hand-shaking and sonication. It should be noted that during the etching process, after the etching of Al atom, owing to the reaction between the resulted Ti3C2 and HF or water, many oxygen-containing groups (-OH, –O) generated [28], which could enhance its hydrophilicity and be well dispersed in water, showing po-tential application in water treatment. Meanwhile, characteristic peak located at ~7 o belonging to Ti3C2Tx appeared (Fig. 2c), indicating that the successful transformation from Ti3AlC2 to Ti3C2Tx [12]. Besides, we found that there are several interference peaks of MAX powders between 10-50◦, which were depend on the etching agents. However, these MAX powders can be removed through the following sonication and centri-fugation, which has no effect on fabricating high-quality nanosheets.

After sonication of Ti3C2Tx powders in water, ulta-thin MXene nanosheets could be easily obtained. As shown in Fig. 2d and f, the typical laminar structure with the lateral size of several micro-meters and thickness of ~1 nm could be clearly observed, which are ideal building blocks to assembled laminate membrane for molecular sepa-ration. Moreover, owing to deprotonating of oxygen-containing groups [41], MXene nanosheets are negatively charged (Fig. 2e), which pro-vided the possibility for surface charge control of MXene membrane via introducing positively charged polyelectrolytes such as PEI with zeta potential of ~50 mV.

3.2. Membrane morphology

Owing to excellent mechanical stability and hydrophilicity, PAN substrate was selected to stack MXene nanosheets, which had many uniform pores with a size of several tens of nanometers on its surfaces (Fig. 3a), water and salt molecules could pass through without any resistance. MXene nanosheets were stacked on the substrate to generate laminar membrane structure using vacuum-assisted filtration device. When the MXene deposition amount was 39.81 mg/m2, it can be seen from Fig. 3b that almost all the pores were covered by MXene, indicating a typical wrinkle-like structure of 2D membranes [14,30]. Nevertheless, we still found some visible defects, which would lead the passage of salt molecules and thus sacrifice the membrane selectivity (i.e. salt rejection rate). With further increasing the MXene loading to 55.73 mg/m2, the

PAN substrate pores were uniformly covered by MXene flakes without visible defects (Fig. 3c) and the surface presents apparent surficial rip-ples and fluctuations. Meanwhile, the MXene membrane with thickness of ~50 nm showed laminar structure (Fig. 3d), which possesses abun-dant channels to facilitate the fast transport of water molecules. With PEI coating, N element distributed on the SC-MXene membrane surface uniformly (Figs. S1 and S2). Besides, the SC-MXene membrane surface seemed more flatter (Fig. 3e), which may be attributed to the filling of PEI in the low-lying of the MXene wrinkles, while the membrane thickness appeared negligible change (Fig. 3f).

The morphology of MXene membranes was further observed by AFM. The surface of pristine MXene membrane showed crumpled and wrinkled structure shown in Fig. 4a, which is in line with the SEM image shown in Fig. 3c. Interestingly, we found MXene flake depositing on the surface, which is consistent with the morphologies of nanosheets in AFM images (Fig. 2d). In addition, owing to the surface coating of positive- charged PEI with ultra-thin thickness, the surface roughness of SC- MXene membrane decreased slightly (Fig. 4b). Meanwhile, as shown in Fig. 4c and d, the AFM phase images showed distinct morphologies, which further verified the uniform attachment of PEI on MXene mem-brane surface.

3.3. Membrane physicochemical properties

We further studied the zeta potentials of membrane surface. As shown in Fig. 5a, the pristine MXene membrane is able to maintain negatively charged at pH values ranging from ~4.5 to 9.0. For SC- MXene membrane, due to additional amine groups are introduced on the surface, the membrane is positively charged, indicating that the charge state of MXene membrane can be easily tuned via a simple coating method. We also used XRD to study the physical structure of MXene membranes. The pristine MXene membrane and SC-MXene membrane demonstrated almost the same 2θ value of ~6.0o (Fig. 5a), indicating that the regular laminar structure of MXene membrane was not affected by the charge control via PEI [41]. Besides, the inter-layer space can be calculated according to the Bragg equation:

2dsinθ= nλ (5)

Where the λ is the wavelength of ray (0.154 nm); n is the reflection series (n = 1). d stands for the d-spacing. Here, the d-spacing of membranes was calculated as 1.47 nm. Due to mono-layer MXene nanosheets thickness is ~1 nm [42], the inter-layer height of the MXene membranes was ~0.47 nm, which would reject molecules or hydrated ions with larger kinetic diameter. In addition, the water affinity of MXene

Fig. 3. SEM images: surface of (a) PAN substrate; (b) MXene membrane with the deposition loading of 39.81 mg/m2; (c) MXene membrane with the deposition loading of 55.73 mg/m2 and (e) SC-MXene membrane; Cross-section of (d) MXene membrane corresponding to (c) and (f) SC-MXene membrane corresponding to (e), respectively.

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membranes was verified by water contact angle measurement. The hy-drophilicity of SC-MXene membrane was enhanced compared with the pristine MXene membrane shown in Fig. 5b, which will facilitate the adsorption and fast transport of water molecules preferentially.

We applied IR to analyse the chemical property of resulting mem-branes. As shown in Fig. 6a, there are two peaks of –C––O (~1624 cm− 1) and –OH (~3486 cm− 1) in IR spectra of the pristine MXene membrane, indicating the typical chemical structure of MXene [12,30]. These deprotonated groups endowed the negatively charged surface of pristine MXene membrane (Fig. 2e). Besides, in the IR spectra of SC-MXene membrane, a new peak belonging to -C-N located at ~2223 cm− 1

appeared, attributing to the PEI coating on the MXene membrane sur-face [41]. Furthermore, XPS characterization was used to study the chemical composition of MXene membranes surface. The C1s XPS spectra of the pristine MXene membrane could be fitted with four con-tributions located at ~280.8 eV, ~283.5 eV, ~285.3 eV and ~287.7 eV, belonging to -C-Ti, -C-C, -C-O and –C––O, respectively (Fig. 6b), which are in line with the literatures [12,25]. However, the characteristic peak corresponding to -C-N located at ~285.7 eV generated in the C1s spectra

of SC-MXene membrane (Fig. 6d), indicating the existence of amino groups on the membrane surface. Furthermore, the N1s spectra of SC-MXene membrane showed characteristic peaks corresponding to charged and uncharged primary amine moieties of PEI (Fig. 6c) [41,43]. Besides, we also investigated the vertical distribution of N element through the SC-MXene membrane. As shown in Fig. 6e, with increasing the etching depth, the N atomic percentage decreased from ~15.7% on the membrane surface to ~4.8% at a depth of 15 nm and then remained stable. Although there were no N element in MXene membrane, the signal intensity of N element was still detected (Fig. S3). Therefore, we speculate that the ~4.8% of N atomic percentage at a depth of 15 nm mainly generated from the background correction. Meanwhile, the N1s peak intensity on the membrane surface was the strongest (Fig. 6f), indicating that the N element mainly existed on the membrane surface. Combining with IR and XPS analysis, it can be confirmed that the PEI molecules with positively charged groups were successfully coated on the MXene membrane surface.

Fig. 4. AFM images of (a) MXene membrane with the deposition loading of 55.73 mg/m2 and (b) SC-MXene membrane; The corresponding AFM phase images of (c) MXene membrane and (d) SC-MXene membrane in (a) and (b), respectively.

Fig. 5. (a) Zeta potential of MXene and SC-MXene membranes; (b) XRD pattern of resulting membranes; (c) Contact angle of membrane surface.

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3.4. Membrane separation performance

3.4.1. Effect of fabrication conditions Before the surface charge functionalization, the fabrication of defect-

free pristine MXene laminar structure was optimized. We applied the pristine MXene membranes for nanofiltration process to study the in-fluence of membrane thickness on the separation performance for MgCl2 water solution. The membrane thickness was facilely varied via con-trolling the deposition amount of MXene nanosheet. As shown in Fig. 7, high water permeance (~64 L/(m2 h bar)) and ultra-low MgCl2 rejection (~18.8%) was achieved when MXene membrane thickness was ~25 nm. It indicated that such low MXene loading could not form continuous MXene layer, as confirmed by SEM characterization (Fig. 3b). Mean-while, owing to enhanced transport resistance with increasing

membrane thickness, the water permeance decreased, while the MgCl2 rejection improved. When the membrane thickness reached 50 nm, the ion rejection tends to decrease slightly, which may due to that the pumping force is uneven during the fabrication process of membranes, leading to the generation of few minor defects. Nevertheless, the pristine MXene membrane with negative-charged surface exhibited appreciable rejection, this may due to the hydration radius of Mg2+ (0.856 nm) is larger than the height of inter-layer channels (~0.47 nm). Namely, in this case, the size-sieving effect is the dominant effect rather than electrostatic interaction [14,31]. Although the MXene laminates thick-ness of 50 nm had higher salt rejection with reasonable permeance, given that PEI coating may increase the transport resistance, the resulted SC-MXene membrane showed uncompetitive permeance (~4.3 L/(m2 h bar)). Therefore, ~35 nm was considered as the optimal thickness of MXene laminar for fabricating the SC-MXene membranes.

The surface charge of the above optimized MXene laminar mem-brane was tuned via coating positively charged PEI, transforming from negative to positive. Here, the influence of PEI concentration for the separation performance of the SC-MXene membranes was observed. It can be seen from Fig. 8 that remarkable enhancement for MgCl2 rejec-tion was achieved by introducing PEI molecules onto the surface of MXene membrane compared with that of the pristine MXene membrane. The positively charged SC-MXene membrane with concentration of 0.025 wt% demonstrated MgCl2 rejection of ~72%, which is about two times higher than that of the pristine MXene membrane (~35%). In addition, MgCl2 rejection was increased to ~82% and then hold steady with further increasing the PEI concentration, which could be ascribed to the electrostatic repulsion between positive charges on MXene membrane surface and MgCl2 salts [20]. Meanwhile, the water per-meance was decreased due to the increase of transport resistance with the PEI coating. Thus, the 0.05 wt% was chosen as the optimal PEI concentration for further discussion.

3.4.2. Desalination performance of nanofiltration and forward osmosis We further studied the nanofiltration performance of SC-MXene

Fig. 6. (a) IR spectra of resulting membranes; (b) XPS C1s spectra of the pristine MXene membrane; (c) and (d) XPS N1s and C1s spectra of SC-MXene membrane, respectively; (e) Depth-dependent N element content during XPS depth profiling of the argon ion laser-treated samples and (f) XPS N 1s spectra at different etching depths.

Fig. 7. Effect of membrane thickness on nanofiltration performance of pristine MXene membrane for MgCl2 solution.

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membrane against several typical salts (Na2SO4, NaCl and MgSO4) in water. As shown in Fig. 9, the rejection rate of the positive-charged SC- MXene membrane followed the order of R (MgCl2) > R (NaCl) > R (MgSO4) > R (Na2SO4), which is in accord with Donnan exclusion theory [18,20]. Charged membrane surface repels co-ions, meanwhile, in order to maintain electrical neutrality, the counter-ions are rejected in the feed solution. Besides, the rejection is relevant to the ratio of charge number for the co-ions and counter-ions. It should be noted that there is no significant increase in rejection rate for equal-valent salts such as NaCl and MgSO4, which may be due to that there is a balanced electrostatic interaction with both co-ions and counter-ions, leading to unchanged salts transport behaviors. However, it is suitable for charged membranes to reject salts with high-valent co-ions, such as AB2- (Na2SO4 …) or A2B- (MgCl2 …) type salts, where the electrostatic repulsion of the charged membrane against high-valent co-ions is stronger than the electrostatic attraction between membrane surface charges and a low-valent coun-ter-ion [41]. Hence, the SC-MXene membrane with positive-charged exhibited the highest rejection for MgCl2 of ~82% with a similar water permeance of ~9 L m− 2 h− 1⋅bar− 1.

Besides of nanofiltration, forward osmosis is another useful mem-brane process for water desalination. In this work, we further investi-gated the water desalination performance of the SC-MXene membrane by using forward osmosis process. The permeation rates of water, various ions were measured through SC-MXene membrane by using 0.2 M NaCl, KCl, CaCl2 or MgCl2 as the feed solution. The permeation rate of

Na+, K+, Ca2+ and Mg2+ for the pristine MXene membrane were 0.419, 0.739, 0.167 and 0.157 mol m− 2 h− 1, respectively (Fig. 10a), which is much higher than the reported MXene membrane because of the ultra-thin membrane thickness [31,32]. Notably, the permeation rates of these ions through the MXene membrane were reduced by two orders of magnitude by introducing PEI on the membrane surface. This is mainly because the positively charged SC-MXene membrane surface strongly repels the co-ions. Besides, water/salt selectivity could be also used to investigate the ion transport behavior. As shown in Fig. 10b, owing to ultralow salt permeation, the water/salts selectivity of SC-MXene membrane is much higher than that of MXene membrane. Meanwhile, the permeation rates of these ions showed the tendency: P (K+) > P (Na+) > P (Ca2+) > P (Mg2+), which is a reverse order of their hydration diameters: Mg2+ (8.56 Å) > Ca2+ (8.24 Å) > Na+ (7.16 Å) > K+ (6.62 Å) [13]. Hence, the water/salts selectivity of membranes shows the con-trary tendency due to the similar water permeance, suggesting that the size sieving effect of the interlayer channels of MXene laminate also contributed to the water desalination performance.

We compared the desalination performance of our SC-MXene mem-brane with state-of-the-arts laminar 2D-material membranes reported. In the forward osmosis desalination process (Fig. 11a), one order of magnitude higher water/salt selectivity was achieved in the SC-MXene membrane compared with membranes derived from 2D-materials such as graphene oxide and MoS2 (Table S2) [6,15,32,44]. Meanwhile, the SC-MXene membrane operated with nanofiltration process (Fig. 11b) also showed much higher salt rejection with comparable water per-meance as comparing with 2D-material (Table S1) [14,18,20,35,45–50]. Such outstanding water/salt selectivity and salt rejection can be attrib-uted to the effects of interlayer size sieving and surface electrostatic repulsion in the designed SC-MXene membrane. In addition to the dominant charge effect, the presence of PEI coating could suppress the swelling of MXene membrane in water to maintain the interlayer size sieving property, and also enhance the hydrophilicity to allow more water molecules passing through the membrane. Nevertheless, due to the additional transport resistance of the PEI coating, the per-meance/selectivity trade-off still occurs to the SC-MXene membrane. Future work will focus on optimizing the molecular structure and coating method of the polyelectrolyte to further reduce the negative effect of surface coating on the transport performance of MXene membrane.

4. Conclusions

In this work, a novel surface charged MXene (SC-MXene) membrane for water desalination was designed and fabricated. The PEI surface coating rationally tuned the surface charge property and also enhanced the membrane hydrophilicity without sacrificial the laminar structure of MXene membrane. Owing to the synergic effect of electrostatic repul-sion and size sieving, the resulting SC-MXene membrane exhibited excellent water permeance and high salt rejection either in nano-filtration or forward osmosis process, showing great potential for water desalination and other applications related to selective transport of water and ions.

CRediT authorship contribution statement

Baochun Meng: Investigation, Methodology, Formal analysis, Writing - original draft. Guozhen Liu: Investigation, Formal analysis, Writing - review & editing. Yangyang Mao: Investigation. Feng Liang: Investigation. Gongping Liu: Conceptualization, Writing - review & editing, Supervision. Wanqin Jin: Conceptualization, Writing - review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial

Fig. 8. Effect of PEI concentration for water permeance and MgCl2 rejection of SC-MXene membrane.

Fig. 9. Nanofiltration performance of SC-MXene membrane for different salts separation.

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interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 21922805, 22038006, 21921006), the Innovative Research Team Program by the Ministry of Education of China (No. IRT_17R54), the Postgraduate Research & Practice Innovation Program of Jiangsu Province and the Topnotch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2021.119076.

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Fig. 10. (a) Ion permeation rate and (b) water/salt selectivity of MXene and SC-MXene membranes, draw solutions: 0.2 M NaCl, KCl, CaCl2 or MgCl2, feed solu-tion: water.

Fig. 11. (a) Water desalination performance comparison with state-of-the-arts 2D-material membrane reported for (a) forward osmosis process; (b) nano-filtration process.

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