Novel thin-film reverse osmosis membrane with MXene Ti3C2Tx embedded in polyamide to enhance the water flux, anti-fouling and chlorine resistance for water desalination

Journal of Membrane Science 603 (2020) 118036

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Novel thin-film reverse osmosis membrane with MXene Ti3C2Tx embedded in polyamide to enhance the water flux, anti-fouling and chlorine resistance for water desalination Xiaoying Wang a, Qingqing Li a, Jianfeng Zhang a, c, *, Haimeng Huang a, Shaoyu Wu b, Yan Yang c a b c

College of Mechanics and Materials, Hohai University, Nanjing, 211100, China Nanjing Delnamem Technology Co., Ltd, Nanjing, 210000, China Jiangsu Engineering Research Center on Utilization of Alternative Water Resources, Hohai University, Nanjing, 211100, China

A R T I C L E I N F O

A B S T R A C T

Keywords: MXene Ti3C2Tx Reverse osmosis (RO) membranes Water permeability Salt rejection Anti-fouling Chlorine resistance

The development of high performance reverse osmosis (RO) membranes still remains a big challenge due to not only the trade-off between water flux and salt rejection, but also their easy fouling and chlorine attacking. In this paper, a novel thin-film nanocomposite RO membrane with two-dimensional MXene Ti3C2Tx embedded in the polyamide (PA) layer was fabricated by in-situ interfacial polymerization with m-phenylenediamine aqueous solution and trimesoyl chloride. Due to the outstanding diffusion regulating effect of Ti3C2Tx in accordance with the Fick’s first law, the water permeability was elevated to a maximum value of 2.53 L m 2 h 1 bar 1 at a high NaCl salt rejection of 98.5%. Simultaneously, the enhanced anti-fouling, and chlorine resistance of the asobtained PA-Ti3C2Tx membrane were also obtained, outperforming the pristine PA and other related RO membranes in literature. Especially after a chlorine-resistance test at 10000 ppm h, a high salt rejection of 97.1% for the PA-Ti3C2Tx membrane was still retained. Based on the microstructural observation and mechanism analysis, such an improvement in chlorine resistance was mainly ascribed to the interaction between surface functional groups of Ti3C2Tx nanosheets with active chlorine, which effectively protected the PA matrix from being chlorine attacked. This work illustrates successfully a very new use of Ti3C2Tx to improve the compre­ hensive performance for water desalination membranes for practical applications.

1. Introduction Despite the fact that two thirds of the earth’s surface is covered with water, the global social and economic development has been seriously hindered by the fresh water shortage all over the world [1–4]. Reverse osmosis (RO) membrane has been widely realized promising for water desalination because of its high separation efficiency and operability [5]. But a trade-off exists here since the water permeability and salt rejection of the commercial RO membranes are always opposite with each other. Meanwhile, harmful foulants are easily deposited or adsor­ bed on RO membrane surfaces immersed in wastewater, seawater, tap water, etc. Through such a specific or non-specific interaction, the fou­ lants gradually accumulated on the RO membrane as a cake layer, leading to a resistance to the water pressure and decrease of water flux [6–8]. Therefore, active chlorine is commonly utilized in many disin­ fection and sterilization occasions to eliminate such foulants on RO

membranes. However, the chlorine ions in the solution will destroy the structure of polyamide, the mostly used polymer matrix of the RO membrane, by orton rearrangement [9], and seriously decrease the salt separation ability. In order to elevate comprehensive performance of the RO mem­ branes, several modification methods have been explored, including surface coating [10], grafting [11], in-situ surface modification [12], surface bio-adhesion [13], and incorporation of nanomaterials into membranes. The injection of some hydrophilic nanoparticles like oxides [14,15], carbon nanotubes [16] and graphene [17,18] into the PA membrane has attracted much more attention because of processing simplicity and excellent modification effects. Especially in recent years, the applications of two-dimensional nanomaterials in RO membranes have been studied as a hot point due to their high specific surface area and abundant active sites, providing more nanochannels favorable for water to pass through while maintaining the repulsion of salt. Chae et al.

* Corresponding author. College of Mechanics and Materials, Hohai University, Nanjing, 211100, China. E-mail address: [email protected] (J. Zhang). https://doi.org/10.1016/j.memsci.2020.118036 Received 18 January 2020; Received in revised form 20 February 2020; Accepted 9 March 2020 Available online 12 March 2020 0376-7388/© 2020 Elsevier B.V. All rights reserved.

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reported the application of two-dimensional MXene Ti3C2Tx-based films for removal of heavy metal ions from water. The removal capacity of 84, 890, 1241 and 1172 mg g 1 was obtained for Cr(VI), Pd(II), Au(III) and Ag(I), respectively. Liu et al. [27] prepared a novel composite GO/Ti3C2Tx nanofiltration membrane with a unique heterogeneous structure. The membranes were found to have an excellent removal of methylene blue solution (15 mg L 1) while maintaining a high water flux at 28.94 � 0.74 L m 2⋅h 1, which was attributed to the two-dimensional interlayer channels and hydrophilicity of Ti3C2Tx. In this study, we attempted to explore a facile strategy to enhance comprehensive performance of RO membranes by incorporation of Ti3C2Tx into the PA layer while maintaining the desalination ability. To the best of our knowledge, this is the first report to use Ti3C2Tx as an additive to modify the RO membranes, although Michael Ghidiu et al. [28] have already proposed the possibility of Ti3C2Tx in water purifi­ cation. Herein, the effects of Ti3C2Tx on the water permeability, anti-fouling, and chlorine resistance were investigated. The water permeability, anti-fouling and chlorine resistance mechanisms were also discussed in detail.

Table 1 Summary of concentrations of MPD, SDS, TEA, TMC and Ti3C2Tx in IP for preparation of different membranes. Sample name

MPD (wt/ v %)

SDS (wt/ v %)

TEA (v/v %)

Ti3C2Tx (wt/ v %)

TMC (wt/ v %)

PA PA05 PA10 PA15 PA20

2.0 2.0 2.0 2.0 2.0

0.1 0.1 0.1 0.1 0.1

4.2 4.2 4.2 4.2 4.2

0.000 0.005 0.010 0.015 0.020

1.0 1.0 1.0 1.0 1.0

[19] fabricated a thin-film composite membrane with GO embedded in its PA layer and exhibited a high water permeability with 16.6 L m 2⋅h 1 at 225 psi. The biovolume of cells attached on the membrane decreased by approximately 98% with the incorporation of GO. The change in the salt rejection (~97%) of the PA-GO membrane was also decreased compared to that of the bare membrane (~91%) after chlo­ rination. Li et al. [20] prepared few-layered MoS2 sheets by liquid-phase exfoliation and incorporated it with different content into the PA membrane matrix via interfacial polymerization. In their work, 0.01 wt % MoS2-PA membrane exhibited an optimal water permeability (6.2 L m 1⋅h 1⋅bar 1) and a superior fouling resistance against proteins as foulants (normalized flux reached to 0.91 after BSA 60ppm for 14 h). Since 2011, a new series of two-dimensional transition metal car­ bides and nitrides MXenes have emerged [21]. The first reported MXene, Ti3C2Tx (T ¼ O, F, OH), was derived from etching Ti3AlC2, a layered ternary carbide Mnþ1AXn phase where M is an early transition metal, A and X respectively represent a III or Ⅳ A-group element and carbon and/or nitrogen, n ¼ 1, 2, 3 [22]. Hydrophilic Ti3C2Tx in zwitterionic inorganic nanosheets with functional groups and unique properties [23] is chemically versatile to be used in electromagnetic shielding, super­ capacitance and electrocatalysis [24,25]. Meanwhile some articles about Ti3C2Tx used in water purification also appeared recently due to its excellent penetration performance and stability. Xie et al. [26]

2. Experimental procedure 2.1. Materials and chemicals The chemicals, such as m-phenylenediamine (MPD, 99.5%), tri­ mesoyl chloride (TMC, 98%), camphorsulfonic acid (CSA, 99%), bovine serum albumin (BSA, 96%), sodium hypochlorite (NaClO, 6–14%) and lithium fluoride (LiF, 99%) were purchased from Shanghai Aladdin BioChem Technology Co., Ltd. (China); Sodium dodecylsulphate (SDS, 98%) and hydrochloric acid (HCl) were purchased from Yonghua Chemical Co., Ltd. (China); Triethylamine (TEA) (99%), n-hexane (laboratory reagent, 97%) and sodium chloride (NaCl, > 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All the above chemicals were used without further purification.

Fig. 1. (a) X-ray diffraction patterns of Ti3AlC2 and Ti3C2Tx; (b) TEM image of Ti3C2Tx; Surface morphology characterization of Ti3C2Tx, (c) AFM image and (d) height profile. 2

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Fig. 2. (a) Raman spectra of Ti3C2Tx; (b) XPS spectrum of Ti3C2Tx; (c) Ti 2p, (d) O1s, (e) C 1s and (f) F 1s XPS spectrum of Ti3C2Tx.

Ti3AlC2 (>99%, 400 mesh) was purchased from Lianli New Tech­ nology Co., Ltd. Beijing, China; Polysulfone ultrafiltration membrane (AquaCup Co., Ltd. Jiangsu, China) was used as the support for PA and PA-Ti3C2Tx membranes. Deionized water was used throughout the experiments.

preserved in deionized water for subsequent studies. Table 1 showed the summary of concentrations of MPD, SDS, TEA, TMC and Ti3C2Tx in interface polysulfone for preparation of different PA and PA- Ti3C2Tx membranes. 2.3. Microstructural characterization

2.2. Fabrication of Ti3C2Tx and preparation of membranes

The phase composition and morphology of Ti3C2Tx and RO mem­ branes were observed by X-ray diffractometer (XRD, Bruker D8 Advance), field emission scanning electron microscopy (FESEM, S-4800 II), high resolution transmission electron microscope (HRTEM, Tecnai G2 F30 S-Twin), atomic force microscopy (AFM, Bruker Multimode) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). Raman spectra were measured with a Raman microscope (HORIBA LabRAM HR 800) with 514 nm laser in the region from 4000 cm 1 to 100 cm 1. Fourier transform infrared (FT-IR, PerkinElmer Frontier) spectra were conducted from 650 cm 1 to 4000 cm 1 with 32 scans at a resolution of 2.0 cm 1 to analyze the chemical functional groups of membranes. Zeta potential was determined using a laser particle size and zeta potential analyzer (Malvern Panalytical ZEN3600). The water contact angle was measured by an optical contact angle tester (KRUSS DSA25).

2.2.1. Fabrication of Ti3C2Tx Firstly, 0.5 g Ti3AlC2 was added to a mixed solution of 0.5 g LiF and 12 mL HCl (4.5 mol/L) for 72 h at 60 � C. The suspension was centrifuged under 5000 rpm for 5 min. The precipitates were then washed by deionized water for several times until a pH value of about 7. Finally, the precipitates were vacuum dried at 60 � C for 24 h to get the Ti3C2Tx powders as used. 2.2.2. Preparation of PA and PA-Ti3C2Tx membranes The interface polymerized PA membranes were prepared on the polysulfone membrane [29,30]. The polysulfone membrane was rinsed with deionized water three times, and immersed in deionized water for 24 h to remove NaNO2 contamination. Then after immersed in MPD, SDS and TEA solution for 4 min, for 1 min in 1.0 wt% TMC n-hexane solution, and heat treated at 60 � C for 5 min. CSA was used to adjust pH to 11. Ti3C2Tx was uniformly dispersed into MPD solution after ultra­ sonic 30 min at a content of 0.005–0.02%. The membranes were

2.4. Evaluation of membrane properties The separation performance of membrane was tested by a triple high 3

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Fig. 3. (a) Schematic illustration for Ti3C2Tx embedded polyamide RO membrane preparation; (b) Reaction mechanism of the interfacial polymerization.

Fig. 4. Surface SEM images of (a) PA, (b) PA05, (c) PA10, (d) PA15 and (e) PA20 membranes.

pressure flat membrane test equipment (FlowMem0021-HP), and the membrane pool was in the form of radial cross flow. The sodium chlo­ ride solution with concentration of 2000 ppm was prepared as a feed solution, and the pressure difference across the membrane was 1.6 MPa. After the membrane was pre-operated in the device for 2–3 h, the water permeability and salt rejection were calculated through the following equations [31]: Jw ¼ A¼

V S⋅Δt

3

(2) 2

1

1

Here R is the gas constant (83.1 cm bar mol K ), T is the absolute temperature (K) and ΔCs is the salt concentration difference (ΔCs ¼ Cf Cp ). Rsalt ¼

Δπ

(3)

Δπ ¼ ΔCs � R � T

(1a)

Jw Δp

pressure different across the membrane (bar) and Δπ is the osmotic pressure differential across the membrane (bar), which was calculated from equation (3).

Cf

Cp Cf

� 100%

(4)

where Rsalt is salt rejection (%), Cp and Cf are the salt concentration of permeate and feed solution (mg/L), respectively. Anti-fouling performance of the membrane surface was tested by a method of dynamic pollution dissolved at 60 ppm BSA in feed solution

1

where Jw is water flux (L m h ), V is the volume of permeate solution (L) collected over a period of time Δt (h), S is the effective membrane area (m2), A is the water permeability (L m 2 h 1 bar 1), Δp is the 4

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Fig. 5. (a) FT-IR spectrum of different membranes; (b) XPS fractionated PA and PA15 membranes; (c) Ti 2p and (d) F 1s XPS spectrum of PA15 RO membrane.

Eg) [33]. The Ti 2p1, F 1s, O 1s and C 1s peaks of Ti3C2Tx XPS spectrum results were shown in Fig. 2 (b). As shown in the Ti 2p spectrum (Fig. 2 (c)), apart from the peaks centering at 463.9 eV, 458.1 eV and 454.5 eV that corresponded to Ti–O 2p 1/2, Ti–O 2p 3/2 and Ti–C 2p 3/2, respec­ tively [28,35]. The O 1s XPS spectrum (Fig. 2 (d)) showed the peaks at 530.7 eV and 529.7 eV could be assigned to O–C and O–Ti, respectively [36]. The XPS spectrum of C 1s (Fig. 2(e)) could be deconvoluted into three pesks corresponding to C–OH (285.9 eV), C–C (284.7 eV), C–Ti (282.9 eV) [37]. In the core level spectrum of F 1s region of MXene, the binding energy at 685.4 eV was assigned to F–Ti (Fig. 2(f)). In a word, the XPS spectrum proved that we had successfully synthesized Ti3C2Tx with functional groups of –OH and –F. Owing to these functional groups (-OH/-F), the average zeta potential of Ti3C2Tx was 30.2 mV (Fig. S1). Higher negative charge advantage of Ti3C2Tx makes the membrane su­ perior in anti-fouling performance.

for 6 h and the normalized flux was calculated every half an hour. Chlorine resistance of the membrane was tested by using NaClO as the main solute of active chlorine. The prepared membrane was exposed in NaClO solution (2000 ppm) by adopting a static chlorination method. After each exposure time of 1 h, the sample was repeatedly washed by deionized water for 3–4 times. 3. Results and discussion 3.1. Microstructures of Ti3C2Tx Fig. 1 (a) shows the X-ray diffraction patterns of Ti3AlC2 and Ti3C2Tx, respectively. After etching by LiF and HCl, the characteristic peaks of Ti3AlC2 almost disappeared completely, indicating that the Al layer was corroded and peeled off after being etched by hydrofluoric acid [32]. The angle of Ti3C2Tx diffraction peak decreased and shifted to the left due to the atomic cluster introduced between layers and the interlayer spacing became larger. Fig. 1 (b) shows TEM images of Ti3C2Tx with a two-dimensional thin layer structure by 3–4 layers in thickness. The AFM image shown in Fig. 1 (c) revealed that the lateral size of Ti3C2Tx was between 200 nm and 800 nm with pore defects on the surface. These structure defects were confirmed by the sharp in-depth in the height profile in Fig. 1 (d). The thickness of Ti3C2Tx was measured to be 3.5 nm. In consideration that the thickness of a single layer Ti3C2Tx was reported to be 0.83 nm, the layer number of nanosheets was estimated to be about 4 in consistence with the analysis result of TEM diagram (Fig. 1(b)) and XRD results (Fig. 1(a)). Fig. 2 (a) shows the Raman spectra for prepared laminar Ti3C2Tx with two sharp Raman characteristic bands at 148 cm 1 and 205 cm 1, which could be ascribed to the Eg and A1g modes, respectively [33,34]. There were two broad peaks at ~400 cm 1 (ω7, Eg) and ~600 cm 1 (ω4,

3.2. Microstructures of PA and PA-Ti3C2Tx membranes Fig. 3 shows (a) schematic illustration for Ti3C2Tx embedded poly­ amide RO membrane preparation and (b) reaction mechanism of the interfacial polymerization. Based on the optimized parameters, the PA separation layer, Ti3C2Tx and m-phenylenediamine were combined with van der Waals force and hydrogen bonding to form a separation com­ posite membrane to study desalination performance. It should be kept in mind that the reaction time and pH of aqueous phase has been optimized to ensure outstanding separation performance of PA membrane as shown in Figs. S2–S3. The SEM images of membranes with different Ti3C2Tx contents are shown in Fig. 4. The polysulfone support had a uniform spongy pore structure with a smooth surface (as shown in Fig. S4). In contrast, the surface of PA membrane prepared on the polysulfone showed the shape 5

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Fig. 6. Surface AFM images of different membranes.

of dead tree leaves and was relatively rough (as in Fig. 4 (a)). Mean­ while, Fig. S5 shows SEM images of polyamide layers with non-porous and dense characteristics. Fig. 4 (b)–(c) show membranes embedded with Ti3C2Tx have nodular structures due to the dispersion of Ti3C2Tx on the surface. When gradually increasing the addition amount of Ti3C2Tx

(Fig. 4 (d)), the leaf-like folding of the membrane surface slightly increased. It was because that dispersion of Ti3C2Tx was controlled by changing the diffusion of aqueous phase and organic phase to control the interface polymerization reaction, so that the surface structure of the membrane was changed [38]. When the Ti3C2Tx content reached to 6

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Ti3C2Tx in PA membranes was conducted as shown in Fig. S7, where the fakes are found to clearly cover the surface to reduce the density of the membrane. Surface functional groups present in the membranes were examined by the FT-IR spectra presented in Fig. 5 (a). The peaks at 1660 cm 1 was – O stretching vibrations of amide I, C–O stretching and O–H bending C– of carboxylic acid tension at 1450 cm 1 peaks [39–41]. When the modification of Ti3C2Tx was added, the peak strength at 1610 cm 1 (N–H stretching of amide), 1540 cm 1 (in-plane N–H bending) and C–N stretching vibrations of amide II became weaker, while stretching vi­ brations of the O–H groups at 3300 cm 1 increased slightly, which may be due to nanomaterial’s effection. The hydroxyl functional groups made the –OH more, while the polyamide exposed to the surface became less, and the characteristic peaks of the amide bonds were decreased. Fig. 5 (b) showed the XPS survey spectrum of pure PA and PA15 membranes. There were three major peaks at 284.1 eV, 531 eV and 399.2 eV that could be ascribed to the binding energies of C1 s, O 1 s and N 1 s, respectively [4,42]. New characteristic peaks of F 1s and Ti 2p were appeared in the XPS spectrum of PA15 membrane, meanwhile the peaks of C 1s and O 1s were strengthen owing to the incorporation of Ti3C2Tx. Fig. 5 (c) showed three peaks at 463.2 eV, 457.4 eV and 455.7 eV, which can be attributed to the doublet of Ti–C 2p 1/2, Ti–O 2p 3/2 and Ti–C 2p 3/2. The binding energy for F 1s was 685.4 eV as shown in Fig. 5 (d) [43]. The discovery of Ti and F indicated that Ti3C2Tx was success­ fully combined on the surface of PA membrane. The incorporation of Ti3C2Tx decreased the roughness of the PA membrane until the lowest value of Rq (RMS) ¼ 90 nm (Fig. 6) when the mass percent of Ti3C2Tx was 0.02%. At a certain concentration, Ti3C2Tx affected the diffusion rate and made the membrane surface smooth, but when the content of Ti3C2Tx was high (PA20 in Fig. 6), the uneven dispersion and agglomeration of the nanoplates resulted in the increased roughness again. Since the analysis of contact angle is affected by sur­ face roughness, we correct for the increase in surface area due to roughness and use a modified form of the Young-Dupre equation to evaluate relative hydrophilicity of membranes [44,45].

Table 2 Relative surface area and solid-liquid interfacial free energy values of different membranes. Sample name

PA

PA05

PA10

PA15

PA20

Relative surface area (Δ) Solid-liquid interfacial free energy ( ΔGSL /mJ/m2)

1.79 81.1

1.63 71.3

1.12 75.3

1.42 91.7

1.69 78.1

Table 3 Flux decline value of different membranes. Sample name

PA

PA05

PA10

PA15

PA20

Flux decline value (%)

22.72

14.23

13.64

11.11

16.67

Fig. 7. Water contact angle and roughness (RMS) of different RO membranes.

ΔGSL ¼ γL ð1 þ cosθ = ΔÞ

(1b)

where ΔGSL is the solid–liquid interfacial free energy, γL (72.8 mJ/m2 for pure water at 25 � C) is the liquid surface tension, θ is the average contact angle and Δ is relative surface area (Δ ¼ actual surface area divided by the projected surface area) [46]. Table 2 shows the relative surface area and solid-liquid interfacial free energy values of different membranes. A higher ΔGSL indicates that the surface of the membrane is more hydrophilic. As one of the important indexes to evaluate the anti-fouling of the RO membrane, the ΔGSL was raised from 81.1 mJ/m2 to 91.7 mJ/m2, likely owing to the greater amount of the hydrophilic carboxyl groups on its surface for PA-0.015 wt%Ti3C2Tx, also suggesting the enhanced anti-fouling capability by the incorporation of Ti3C2Tx. 3.3. Separation performance of PA-Ti3C2Tx membranes Preliminary experiments on conditions (reaction time and pH) optimization was tested to guarantee fine separation performance in Fig. S8. The water permeance and salt rejection of the membranes were evaluated as shown in Fig. 8. The PA-Ti3C2Tx membrane shows a good water permeability (2.3–2.5 L m 2⋅h 1⋅bar 1) higher than that of the bare PA membrane (1.7 L m 2⋅h 1⋅bar 1), while the salt rejection (97.9–98.5%) was almost the same as that of the bare PA membrane (98.6%), demonstrating the beneficial features of Ti3C2Tx to inhibit the trade-off effect [47,48]. This can be attributed to the increased cross-linking of the PA-Ti3C2Tx membrane selective layer due to the –OH groups produced by the Ti3C2Tx and the excessive TMC reaction. How­ ever, when the concentration was higher than 0.01 wt%, the water flux decreased due to the agglomeration phenomenon of two-dimensional

Fig. 8. Performance comparison (water permeability and salt rejection) of the RO membranes in this study and literature. (NaCl: 2000 ppm, Water pressure: 16.0 bar, Temperature: 25 � C).

0.02% (Fig. 4 (e)), the membrane surface cluster material was obvious owing to two-dimensional nanoparticle concentration increasing in the reaction. Meanwhile, the thickness of separation layer became thinner after integrating Ti3C2Tx (from 280-455 nm to 205–375 nm as shown in Fig. S6 (a)–(b)), likely owing to the effects of additives on aqueous phase and organic phase diffusion and interface reactions, which was benefi­ cial to the improvement of water flux [19]. A further higher addition of 7

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Fig. 9. (a) Normalized water flux of different membranes; (b) Salt rejection of different membranes. (BSA: 60 ppm; NaCl: 2000 ppm; Water pressure: 16.0 bar; Temperature: 25 � C).

Ti3C2Tx nanomaterials. This similar phenomenon was also observed in PA membranes embedded with other nanomaterials like GO, MoS2 [20, 39]. The performance with higher additions of Ti3C2Tx was discussed instantly in Fig. S9. The salt rejection ability decreased further due to the loose structure caused by excessive addition of Ti3C2Tx (as in Fig. S7). Fig. 8 also shows the comparison of the separation performance re­ sults from this study, commercial RO and other literatures for mem­ branes modified with nanomaterials. Commercial RO [44] had a slightly higher salt rejection (99.2%) but very low water permeability (1.4 L m 2⋅h 1⋅bar 1). PA-GO [39,49] exhibited a characteristic of a trade-off between water permeability and salt rejection (0.9 L m 2⋅h 1⋅bar 1 at 96.4% rejection or 2.8 L m 2⋅h 1⋅bar 1at 93.8% rejection). CNFs [50], TiO2 [51], MWCNs [16], Silica [15], Alumina [52] and Zeolite [53] were also reported to enhance the desalination performance. Due to some reasons, such as excessive particle size, loose microstructure, poor hydrophilicity, and the impact on membrane-forming densities, the desired comprehensive properties were still not achieved up to now. In contrast, the PA-Ti3C2Tx obtained in this study represented by the red star in Fig. 8 (b) displays superior performance, indicating the addition of Ti3C2Tx solved the trade-off effect of RO membrane apparently [47, 48]. As a pore-free membrane, the penetrant transport process through the RO membrane is in accordance with Fick’s first law of diffusion [54]. J¼

DP

dC dx

water quality, and increasing energy consumption [56–58]. Hence, we evaluated the attenuation of water flux decline of PA-Ti3C2Tx membrane and PA membrane after adding organic BSA. Compared with PA mem­ brane, the normalization flux of Ti3C2Tx embedded PA membrane improved obviously after 6 h of test (Fig. 9 (a)), the maximum value of flux decline was reduced to 11.1% (Table 3). The salt rejection decreased slightly, remained at about 97% (Fig. 9 (b)). According to the properties of protein pollutants and their existence on the surface of RO membrane [59], it can be found that the anti-fouling of foulants can be enhanced by changing the hydrophilicity, roughness and potential of the membrane surface [60,61]. The force of water flow on the foulants attached to the membrane surface increased and the surface tended to be smooth, at the same time, the surface charge of the modified RO membrane could effectively reduce the deposition and adhesion of foulants on the membrane (Fig. 10 (a) (b)). The surface morphology of PA and PA15 after 6 h BSA fouling test were investigated by SEM. A large number of protein foulants were accumulated on the surface of PA membrane, but in comparison, there were less foulants on the surface of PA15 membrane (Fig. 10 (c)). When the content was 0.015 wt% (PA15), the anti-fouling was the best, and the roughness and contact angle can also be confirmed (Fig. 7). Then the membrane was exposed in a 2000 ppm NaClO solution for 1 h and five times to determine the chlorine resistance of the different RO membranes. The water flux of PA membrane increased greatly and the salt rejection decreased after exposing in NaClO (Fig. 11). The active chlorine solution caused great damage to the structure of PA, and the pores between the functional layers led to the infiltration of water molecules and ions together. On the contrary the water flux of PATi3C2Tx membrane increased after five chlorination, the salt rejection remained above 97% (better than 94% of PA membrane). Fig. 12 showed the changes of morphology and structure of PA and PA15 membranes after chlorination. At the 10000 ppm h chlorination test, the yellowing degree of the PA membrane was more serious than that of the PA15 as showed in the left corner of SEM images (Fig. 12 (a) (b)). A large number of pores in the separation layer of PA membrane were clearly seen, the amide bond was seriously destroyed and C, N, O elements were lost (Fig. 12 (c) (d)). But no obvious pores were found in the modified PA15 membrane (Fig. 12 (b)). It could be seen from the FT-IR and XPS (Fig. 12 (c) (d)) that the change of peak before and after chlorination was smaller. At the same time, the incorporation of Ti3C2Tx improved the stability of PA structure and obviously reduced the effect of active chlorine on the bond cross-linking degree of PA. In addition, the Ti3C2Tx could protect the underlying PA from chlorine attack in consistence with the literature [19].

(1c)

where J is diffusion flux, DP is diffusivity of penetrant, C is volume concentration of component and dC dx is concentration gradient. For salt desalination, the diffusivity (DP) value should be kept at the superiority of DP,H2O to DP,NaCl, and the bigger the difference, the better. That is why a pressure is usually necessary for the RO desalination process to enhance the water separation and purification. It should also be pointed out here the close dependence of the DP value on the char­ acteristics of the RO membranes [55]. In this study, Ti3C2Tx has been firstly proved to be a very excellent diffusion regulator resolving the trade-off dilemma between the water permeability and salt rejection of the RO membranes, which was ascribed to the hydrophilicity and negative electricity of Ti3C2Tx surface terminated with –OH functional groups [20,39]. 3.4. Anti-fouling and chlorine resistance of membranes Membrane fouling was consistently a bottleneck restricting its pro­ cessing efficiency by reducing water permeation flux, deteriorating 8

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Fig. 10. (a) The factors to enhance the anti-fouling ability of PA-Ti3C2Tx RO membrane; (b) Desalination and anti-fouling process; (c) Surface morphology of PA and PA15 membrane after fouling test. (BSA: 60 ppm; NaCl: 2000 ppm; Water pressure: 16.0 bar; Temperature: 25 � C).

Table 4 also compares the comprehensive performance of the PA-RO membranes in literature and this study. Typically, Carbon nanotubes [16] had the potential to resist fouling and chlorine, but salt rejection was lower than 90% because of the larger nanochannels. The NaCl rejection of PA-CNFs membrane [50] could also be maintained only at 92% after exposure to 6000 ppm h of active chlorine. Although the incorporation of TiO2 [14,51] with photocatalytic effects rendered the membrane significant anti-fouling performance, its use in actual pro­ duction is very difficult as the steps for UV action on the RO cells were complicated. The anti-fouling ability of PA-GO membrane [49]was almost the same with PA-Ti3C2Tx, but the chlorine resistance of the former was weaker than the later. The PA-Ti3C2Tx membrane was found to possess far more superior performance in terms of fouling and chlo­ rine resistance, outperforming other nanomaterials (such as GO, MWCNs, TiO2, Silica, Alumina, Zeolite) modified RO membranes man­ ufactured under the similar conditions in water permeability and salt rejection. Therefore, the PA-RO membranes embedded by Ti3C2Tx is quite promising for real applications in desalination with a superior

comprehensive performance and long-time stability. 4. Conclusions Novel PA-RO membranes containing two-dimensional Ti3C2Tx nanosheets (0–0.02 wt%) were prepared by an in-situ interface poly­ merization process. The layered Ti3C2Tx was observed to play an important role in membrane surface modification, rendering it improved surface hydrophilicity and decreased surface roughness. The water permeability increased from 1.74 L m 2⋅h 1⋅bar 1 to 2.53 L m 2⋅h 1⋅bar 1 while salt rejection kept almost the same. Using 60 ppm BSA as a protein foulant for 6 h, 91% of the normalized water flux was maintained for 0.015 wt% PA-Ti3C2Tx membrane, suggesting an apparently improved antifouling ability. Remarkable stability of the PATi3C2Tx membrane to chlorine was also observed, with only a very slight decrease of salt rejection from 98.5% to 97.1% after being exposed to a 2000 ppm NaClO solution for totally 5 h. The high water-salt selectivity, superior fouling and chlorine resistance of PA-Ti3C2Tx RO membrane 9

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Journal of Membrane Science 603 (2020) 118036

Fig. 11. (a) Normalized water flux effect of chlorine cleaning cycles for different membranes; (b) Salt rejection effect of chlorine cleaning cycles for different membranes. (NaClO: 2000 ppm; NaCl: 2000 ppm; Water pressure: 16.0 bar; Temperature: 25 � C).

Fig. 12. (a) Surface morphology of PA membrane after chlorination test; (b) Surface morphology of PA15 membrane after chlorination test; (c) FTIR and (d) XPS spectrum of PA and PA15 membranes after chlorination test. (NaClO: 2000 ppm; NaCl: 2000 ppm; Water pressure: 16.0 bar; Temperature: 25 � C).

outperforms many of the PA-based membranes in literature. The inter­ action between surface functional groups of Ti3C2Tx nanosheets with active chlorine was proposed to contribute to the improvement in chlorine resistance.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 10

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Journal of Membrane Science 603 (2020) 118036

Table 4 Performance of RO membranes by this experiment and literatures. Membranes

Desalination performance Water permeability coefficient, A (L⋅m 2 h bar 1)

1

Anti-fouling

Chlorine-resistant

Ref.

NaCl rejection (%)

Test conditions

Normalized flux

Flux decline value (%)

Test conditions (ppm⋅h)a

NaCl rejection (%)

0.85 /

~15 / ~12.82

6000 6000 /

92.3 92 /

[49] [50] [14, 51]

0.94

/

3500

90

[16]

/

~10 /

/ /

/ /

[15] [52] [53] This work This work

PA-GO PA-CNFs PA-TiO2

0.90 1.96 0.93

96.4 95.6 96.6

PA-MWCNs

1.75

>90.0

PA-Silica PAAlumina PA-Zeolite PA

1.14 0.50

~95 88.0

BSA 100ppm for 12 h / E. coli cell dilution and nutrient broth for 3 days þ UV for 4 h per day Ca(HCO3)2 31ppm and BSA 3ppm for 2 days 5 h of continuous filtration /

1.37 1.74

93.9 98.6

/ BSA 60ppm for 6 h

/ 0.71

/ 22.72

/ 10000

/ 94.3

PA-Ti3C2Tx

2.53

98.5

BSA 60ppm for 6 h

0.89

11.11

10000

97.1

a

ppm⋅h: equivalent to the product of the NaClO concentration and exposure time.

CRediT authorship contribution statement

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