Fabrication of hybrid ultrafiltration membranes with improved water separation properties by incorporating environmentally friendly taurine modified hydroxyapatite nanotubes

Journal of Membrane Science 577 (2019) 274–284

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Fabrication of hybrid ultrafiltration membranes with improved water separation properties by incorporating environmentally friendly taurine modified hydroxyapatite nanotubes

T

Yongfeng Mua, Kai Zhua, Jiashuang Luana, Shuling Zhanga, Chongyang Zhanga, Ruiqi Naa, ⁎ ⁎ Yanchao Yanga, Xi Zhangb,c, , Guibin Wanga, a b c

College of Chemistry, Key Laboratory of High Performance Plastics, Ministry of Education, Jilin University, Changchun 130012, PR China Department of Hand Surgery, The Second Hospital of Jilin University, Changchun 130041, PR China Department of Burn Surgery, The First Hospital of Jilin University, Changchun 130021, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Hydroxyapatite nanotubes Ultrafiltration membrane Antifouling

In this work, hydroxyapatite nanotubes (HANTs) were explored as a novel environment-friendly additive for the fabrication of ultrafiltration (UF) membranes. Hydrophilic modification was conducted to the prepared HANTs, which includes the polydopamine (PDA) coating and the following grafting of taurine (TA), a kind of amino acid with -SO3H group. Subsequently, the modified HANTs (HANTs-DA-TA) were mixed with polyethersulfone (PES) to fabricate nanocomposite UF membranes. The prepared membranes were fully characterized, including the porosity, hydrophilicity, membrane morphology, and practical UF capability. The fabricated PES/HANTs-DA-TA membranes exhibited remarkably elevated water flux (up to 439 L m−2 h−1), which was ~2.6 times that of PES membrane (169 L m−2 h−1), whereas the rejection of PES/HANTs-DA-TA membranes was still kept at a high state. The improved flux mainly benefited from the more porous membrane structure, declined skin layer thickness, and the enhanced hydrophilicity. Meanwhile, the PES/HANTs-DA-TA membranes exhibited increased antifouling capability and the highest flux recovery ratio (FRR) reached 77.9% due to the improved surface hydrophilicity. Furthermore, the short rod-like nano hydroxyapatite (HA) was prepared and functionalized the same way as the HANTs-DA-TA. It was found that compared to rod-like HA, the tubular nano HA was more efficient in facilitating the membrane permeation due to the superiority of nanotubular structure. This work reveals that as a novel green additive, the HANTs have great application potential in fabricating water treatment membranes and can be employed for further studies.

1. Introduction Nowadays, the world's water security, which is considered as an indispensable part for the survival and development of human beings, is being threatened by such reasons as human activities and anthropogenic climate change. At present, nearly 80% of wastewater all over the world is directly discharged without treatment, extremely endangering the natural water systems and further causing approximate 750 million people worldwide absence to enough safe water [1,2]. The aggravating water contamination has become a problem of great urgency to be solved, urging for advances in water purification technologies. Although a number of water purification technologies have been proposed and developed, which include flocculation, adsorption, distillation etc., certain requirements containing sophisticated equipment, high energy consumption and operation cost are basically needed for

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most of these techniques, probably with lots of chemicals used and produced, largely hindering their practical applications in some cases [3]. Owing to its high selectivity, relatively low energy consumption, and little or no use of chemicals, membrane separation technique has drawn people's attention in recent years, which involves a wide range, containing microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), and so on [4,5]. Among numerous membrane separation technologies, UF has been considered as a resultful option for the removal of colloids and macromolecules. However, as the key component in this technique, UF membranes mostly made by polymers are still subjected to intractable problems during practical use like low filtration flux and membrane fouling [6,7]. In the pursuit of membranes with superior performance, various means have been adopted so far, of which the fundamental strategies can generally be divided into three types. First is the pre

Corresponding authors.

https://doi.org/10.1016/j.memsci.2019.01.043 Received 21 July 2018; Received in revised form 14 January 2019; Accepted 27 January 2019 Available online 29 January 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.

Journal of Membrane Science 577 (2019) 274–284

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and biodegradability of PDA, which provides a probability to the construction of green nano additives for water treatment membranes [42,43]. In this study, we explored the possibility of HANTs as a novel additive in the preparation of UF membranes. We tried to construct a kind of green additive, so the environmentally friendly PDA and taurine, an amino acid with -SO3H group, were utilized to modify the nanotubes. The HANTs were decorated by the coating of PDA first, which is proved to be a facile method in the functionalization of nanomaterials [44–46]. Then, according to Michael addition or Schiff base reaction, taurine (TA) was grafted. A series of PES/HANTs-DA-TA nanocomposite membranes were fabricated by blending with different contents of the modified HANTs. The chemical composition and morphology of the nanotubes were completely characterized in terms of X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Transmission electron microscopy (TEM). The effects of the additive on the membrane structure and performance were studied in detail. Moreover, the permeability of the membranes prepared with the short rod-like HA, which was functionalized nearly the same way as the HANTs-DA-TA, was investigated and the results revealed tubular nano HA displayed much better effects.

functionalization of the polymers by introducing hydrophilic segments to their chemical structures, such as sulfonic acid groups [8], zwitterionic groups [9] and poly(ethylene oxide) (PEO) chains [10,11]. Second is direct modification of membrane surface via approaches like chemical grafting [12,13], UV induced grafting [14,15], plasma treatment [16,17] and mussel-inspired chemistry [18–20]. Third strategy is the blending of other materials into the membrane matrix to construct composite UF membranes. Thereinto, blending of other materials to fabricate composite UF membranes, as an effective and facile method, has been extensively applied and investigated [4,21–23]. Varieties of materials have been used as additives, including small molecules [24,25], hydrophilic or amphiphilic polymers [23,26,27] and nanomaterials. In particular, nanomaterials have received widespread attention so far owing to their remarkable effect on membrane performance [22,28]. From previous reports, it can be seen that both the permeation and antifouling property can be prominently enhanced after blending these nanomaterials into polymer membranes. Wu et al. fabricated GO-SiO2 nanohybrid and added the nanohybrid into polysulfone (PSF) to fabricate novel nanocomposite membranes. When the content of GO-SiO2 reached 0.3 wt%, the water flux achieved nearly twice that of the pristine membrane. Meanwhile, the hybrid membranes exhibited enhanced fouling resistance ability [29]. Sun et al. constructed poly(sulfobetaine methacrylate) (PSBMA)-functionalized metal-organic framework (MOF) and employed its potential value in the fabrication of UF membranes. The results revealed that incorporation of the well dispersed and superhydrophilic MOF could greatly enhance both the flux and antifouling ability of the UF membranes [30]. Admittedly, the application of nanomaterials is conductive to the enhancement of membrane performance. However, as the membranes containing nanomaterials are used for a long time, some of the nanomaterials in the membrane matrix may be rinsed off. The effects of these nanofillers on the environment are not well considered and some of them may have negative influence [31–33]. Accordingly, constructing eco-friendly additives for the water treatment membranes is of great significance. Hydroxyapatite (Ca10(PO4)6(OH)2, HA), as the essential inorganic mineral constituent of bones, is considered to be a kind of natural, low cost and non-toxic material [34]. The synthesis of nanosized hydroxyapatite has been widely explored, of which its multiform morphologies include nanorods, nanowires, nanosheets, nanotubes, etc [35–37]. Recently, pure HA single crystalline nanotubes (HANTs) were synthesized [37], which are speculated to be a potential green nanofiller for the fabrication of water treatment membranes. According to previous work, nanotubes such as carbon nanotubes (CNTs) and halloysite nanotubes are highly effective on enhancing membrane performance [21,28,38]. Compared with nanorods, nanomaterials with tubular structure possess higher specific surface area, larger aspect ratio, and additional water channels, which are supposed to attain better effects as nano additives in the fabrication of UF membranes. However, on one hand, the intrinsic hydrophilicity of HANTs is not enough, mainly offered by the hydroxyl groups, which is of great importance in improving the antifouling ability of UF membranes. On the other hand, due to the poor compatibility between organic polymers and inorganic nanomaterials, directly blending pure HANTs in the casting solution is not advisable to the mixing effect, which leads to the aggregation of HANTs and the blending failure. To be a qualified nanofiller for the fabrication of water treatment membranes, both the hydrophilicity and the compatibility problem need to be solved. Inspired by mussel-adhesion performance in nature, it has been found that under alkaline conditions dopamine can proceed a selfpolymerization process to give polydopamine (PDA) [39], forming tight adhesion on various substrates. Attributed to its convenient modification process and the supplement of a universal way for secondary reactions, the self-polymerization of dopamine, as a surface modifier, has been broadly exploited for use [20,40,41]. Moreover, this method is proved to be environment-friendly due to the excellent biocompatibility

2. Experimental 2.1. Materials In the preparation of modified hydroxyapatite nanomaterials, calcium dihydrogen phosphate (Ca(H2PO4)2, > 95.0%) was supplied by Adamas Reagent, Ltd. Calcium chloride (CaCl2, > 96.0%), diammonium phosphate (NH4H2PO4), ammonium hydroxide, dopamine hydrochloride, tri(hydroxymethyl) aminomethane (Tris), taurine, hydrochloric acid and anhydrous ethanol (99.8%) were all purchased from Aladdin Reagent Co. Ltd. Shanghai, China and used without further purification. Dodecylamine (> 97.0%) and hexadecylamine (> 95.0%) were obtained from TCI Shanghai Development Co. Ltd. Methylamine chloride was purchased from Sigma Aldrich. In the fabrication of membranes, PES (Mw=62,000–64,000 g/mol) provided by Solvay Chemical was used as the basic membrane forming material. Polyvinylpyrrolidone (PVP, K30, Mw=30000 g/mol) obtained from Sigma Aldrich and N-methylpyrrolidinone (NMP) supplied by Aladdin were employed as additive and solvent, respectively. Bovine serum albumin (BSA, pI=4.8, Mw=67,000 g/mol) was from Shanghai LanJi technology development Co. Ltd. Na2HPO4·12H2O and NaH2PO4·2H2O obtained from Aladdin were used to prepare the phosphate buffer solution (pH=7.4). 2.2. Preparation of modified hydroxyapatite nanotubes 2.2.1. Synthesis of hydroxyapatite nanotubes Hydroxyapatite nanotubes were synthesized according to a biomimetic process described by Guo et al. [37]. Typically, 1.01 g of calcium dihydrogen phosphate and 0.629 g of calcium chloride were completely dissolved into 80 ml of deionized (DI) water, and subsequently 24 ml pre-prepared ethanol solution containing dodecylamine and hexadecylamine was dropwise added into the above aqueous solution for the next 16 h under constant stirring. Afterwards, the obtained suspension was transferred into a Telflon-lined stainless steel autoclave and was hydrothermally treated at 120 °C for 48 h. Finally, the mixture was centrifuged and the white solid products were washed repeatedly. The products were dried at 40 °C. The removal of organic amine was conducted using ions exchange method. The above dried products were well distributed in an aqueous solution of methylamine chloride by sonication for 30 min and stirred for 2 h. Subsequently, the precipitates were collected by centrifugation and the above washing process was done for six times. After being calcined at 400 °C for 12 h, the hydroxyapatite nanotubes were finally prepared. 275

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Fig. 1. Schematic representation of PDA-based modification procedure of HANTs.

photograph of the nanotubes before and after modification was displayed in Fig. S1A.

Table 1 Compositions of the casting solution used to prepare the UF membranes. Membrane

M0 M1 M2 M3 M4 M1′ M2′ M3′ M4′ M5′

Casting solution compositions (wt%) PES

PVP

NMP

HANTs-DA-TA

HA-DA-TA

16 16 16 16 16 16 16 16 16 16

4 4 4 4 4 4 4 4 4 4

80 80 80 80 80 80 80 80 80 80

– 0.1 0.2 0.3 0.4 – – – – –

– – – – – 0.2 0.4 1 2 3

2.3. Preparation of modified short rod-like HA nanoparticles Briefly, 50 ml of 0.05 M CaCl2 aqueous solution and 30 ml of 0.05 M NH4H2PO4 aqueous solution were mixed and the mixed solution was stirred for 10 min. Afterwards, a given amount of ammonium hydroxide was added under continuous stirring and the hydroxyapatite nanoparticles were gradually emerged in the solution. Finally, the solution turned into a white suspension and the nanoparticles were purified by centrifugation and washed. The white powders were produced after calcining treatment at 400 °C for 8 h. Same methods to modify HANTs were carried out to the nano HA and the final taurine modified HA was marked with HA-DA-TA.

It should be noted that the values corresponding to HANTs-DA-TA and HA-DATA are the mass ratios of the nanofillers to PES. The symbol – means the absence of substance.

2.4. Membrane preparation

2.2.2. Preparation of PDA-coated HANTs PDA modified HANTs were prepared by the typical self-polymerization of dopamine [44,47]. HANTs (0.1 g) were first dispersed in Tris-HCl buffer (pH=8.5, 100 ml) by sonication for 30 min. Then, dopamine hydrochloride (0.2 g) was added into the suspension and the reaction was conducted at 25 °C for 24 h. After repeatedly washed with deionized water and ethanol, the resulting black precipitate was dried at 40 °C and PDA modified HANTs were obtained. The obtained PDAcoated HANTs were designated as HANTs-DA.

PES membrane and PES/HANTs-DA-TA nanocomposite membranes were fabricated according to the method described in our previous work [48,49]. The detailed formation of the casting solutions was summarized in Table 1. Firstly, desired amount of HANTs-DA-TA was added in NMP and sonicated for 30 min. Next, PES and PVP were dissolved in the suspension at room temperature under continuous stirring. Afterwards, the obtained casting solution was sonicated for another 30 min. After fully degasification, the above solutions (Fig. S1C) were cast onto a clean glass plate by a cast knife with a gap of 150 µm. Then, the glass plate was straightly immersed in the coagulation bath for precipitation and stayed for 30 min. To eliminate the residual PVP and NMP, the membranes were fully washed with deionized water repeatedly. The membranes prepared with HANTs-TA were marked with M0, M1, M2, M3, M4, respectively. Digital photographies photographs of PES membrane and the PES/HANTs-DA-TA nanocomposite membranes are depicted in Fig. S1B. As control group, the hybrid membranes prepared with HA-DA-TA were respectively designated as M1′, M2′, M3′, M4′, M5′ and the formulations of casting solutions were listed in Table 1 as well.

2.2.3. Preparation of taurine modified HANTs Based on the secondary reaction, taurine was grafted on the HANTsDA [46]. First, 0.05 g HANTs-DA were dispersed in Tris-HCl buffer (45 ml) by sonication and the temperture temperature of the suspension was kept at 60 °C. After the addition of the pre-prepared taurine aqueous solution (50 mg/ml, 5 ml), the mixture was stirred at 60 °C for 6 h. Subsequently, the precipitate was collected by centrifugation and washed for several times with deionized water and ethanol. Lastly, the product was dried at 40 °C before use. The obtained taurine modified HANTs were designated as HANTs-DA-TA. Schematic diagram of the whole modification process was presented in Fig. 1 and digital 276

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2.5. Characterization

J=

V A×T

(3)

2.5.1. Characterization of nanotubes Powder XRD patterns were obtained by using an X-ray diffractometer (Empyrean, PANalytical B.V.) to characterize the crystallographic structures of the samples. FTIR spectra were measured to analyse the chemical modification of prepared HANTs using a Nicolet Impact 410 Fourier transform infrared spectrophotometer within the range of 4000–400 cm−1. TEM images recorded on a JEM 2100 F Electron Microscope were used to study the microstructure of HANTs. XPS (ESCALAB 250) was applied to characterize the surface composition of the samples. Thermogravimetric analysis (TGA) was conducted on a Perkin Elmer Pyris 1 analyzer. The analysis was carried out with a rate of 10 °C/min up to 800 °C under air atmosphere. The specific surface area (SBET) of HANTs was determined based on the N2 adsorption test at 77 K using an adsorption apparatus (ASAP 2010 Micrometrics instrument, USA). The aspect ratio was calculated using SEM images (FESEM, FEI Nova NanoSEM 450 microscope). The length and diameter of each sample were averaged from 3 times of measurement.

h ), V denotes the volume of Here, note that J is the flux (L m the permeated solution (L), A represents the effective membrane area (m2), and t is the time of permeation (h). Subsequently, the feed solution was replaced by a pre-prepared 1 mg/ml of BSA solution (pH=7.4) and the following filtration process continued for 2 h. The flux of BSA was recorded as Jp. The BSA rejection (R) of the membranes was calculated by Eq. (4):

2.5.2. Water contact angle measurement Water contact angle measurement was conducted using a water contact angle meter (Drop Shape Analysis System DSA-30) at room temperature. Each membrane was measured five times and the average value was recorded.

Jw,2 ⎞ FRR (%) = ⎜⎛ ⎟ × 100 ⎝ Jw,1 ⎠

(5)

Jw,1 − Jp ⎞ Rt (%) = ⎜⎛ ⎟ × 100 ⎝ Jw,1 ⎠

(6)

Jw,2 − Jp ⎞ Rr (%) = ⎜⎛ ⎟ × 100 ⎝ Jw,1 ⎠

(7)

Rir = Rt − Rr

(8)

−2

Cp R (%) = ⎜⎛1− ⎟⎞×100 ⎝ Cf ⎠

where Jw,2 and Jw,1 are the water flux of the fouled membrane and the initial membrane. Jp is the flux of BSA solution. 3. Results and discussion

2.5.4. Overall porosity and mean pore radius Briefly, wet membranes (2 cm2) were weighted first and then dried at 50 °C until the membranes were fully dried. The overall porosity (ε) of each membrane was calculated by Eq. (1):

Ww − Wd × 100% A×l×ρ

3.1. Characterization of HANTs and modified HANTs 3.1.1. XRD analysis XRD spectra were recorded to investigate the crystalline property of the modified HANTs. As shown in Fig. 2A, Bragg diffraction peaks of raw HANTs were almost consistent with those of the pure HA (PDF # 09–0432) marked with the green line. The diffraction peaks at 25.8°, 31.7°, 32.2°, 32.9°, 39.8°, 46.7° and 49.5 °correspond to the (0 0 2), (2 1 1), (1 1 2), (3 0 0), (3 1 0), (2 2 2) and (2 1 3) planes, respectively. Compared with pristine HANTs, the patterns of HANTs-DA and HANTsDA-TA remained unchanged in general, indicating the crystal structure of HANTs was not affected by the following modification process.

(1)

where Ww and Wd are the weights of the wet and dry membranes, A is the area of the membrane (cm2), l denotes the thickness of the membrane (cm) and ρ represents the density of pure water (g cm−3). For each membrane, the final result was averaged from 3 samples so as to minimize the experimental error. Mean pore radius (rm) was calculated on the basis of Guerout-ElfordFerry equation (Eq. (2)) [21]:

rm =

(2.9 − 1.75ε ) × 8ηlQ ε × A×∆P

(4)

Here, Cf and Cp in the above formula denote the concentrations of BSA in the feed and permeate, respectively. After the filtration of BSA solution, the fouled membranes were taken out and immediately washed with pure water. Finally, water flux was recorded again (Jw,2). The flux recovery ratio (FRR), total fouling ratio (Rt), irreversible fouling ratio (Rir), and reversible fouling ratio (Rr) were calculated based on Eqs. (5)–(8):

2.5.3. Morphology of membranes The morphologies of membranes were inspected using field emission scanning electron microscopy (FESEM, FEI Nova NanoSEM 450 microscope). Samples of membranes were fractured in liquid nitrogen for observation of the cross-sectional morphologies. To measure the surface roughness of the membranes, the atomic force microscopy (AFM) images were obtained on a NSK SPA300HV instrument using silicon wafers as substrate.

ε=

−1

3.1.2. FT-IR analysis Fig. 2B shows the FTIR spectra of HANTs and the modified HANTs. It can be seen that FTIR spectra of HANTs and modified HANTs presented the similar characteristic peaks of the hydroxyl group (3570 and 633 cm−1), PO43− (1095, 1030, 962, 604 and 563 cm−1) and adsorbed water (3430 cm−1) [36]. Three new peaks appeared at about 1612 cm−1, 1509 cm−1 and 1290 cm−1 in the spectra of HANTs-DA, which correspond to the C˭C stretching vibration, N–H deformation vibration and C–N stretching vibration in the structure of PDA, respectively [45]. After the grafting of taurine, the peak at 1645 cm−1 verified the Schiff Base reaction between PDA and taurine, which is associated with the stretching vibration of C˭N [50]. The peak observed at 734 cm−1 was attributed to the stretching vibration of S−O band associated with the −SO3H group, while the vibration peaks belonging to the O˭S=O of -SO3H were not obviously viewed because the overlapping effect by the broad and strong peaks of HANTs at 1030 cm−1 [51,52].

(2) −4

Pa s), Q is the volume of whereηis the water viscosity (8.9 ×10 permeate pure water per unit time (m3 s−1) andΔP is the operation pressure (0.1 MPa). 2.5.5. Ultrafiltration performance study In this work, filtration experiments were proceeded to assess the UF performance of membranes using a flat-sheet cross-flow filtration equipment with an effective filtration area of 7.1 cm2. Each sample was pressurized with deionized water at 0.15 MPa for 30 min prior to commencing the test. Then the operational pressure was adjusted to 0.1 MPa. Values of flux were recorded every 5 min and the measurement of initial water flux was conducted for 1 h. The initial permeation flux (Jw,1) was calculated according to the following Eq. (3): 277

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Fig. 2. (A) XRD patterns, (B) FTIR spectra, (C) XPS spectra of HANTs, HANTs-DA, HANTs-DA-TA.

length was several micrometers and the inner and outer diameters were 8–50 nm and 20–80 nm, respectively, which agreed well with the work before [37]. With respect to Fig. 3(B), it was obvious that the structure of HANTs was slightly damaged under electron. As shown in Fig. 3(C) and (D), it can be seen that an obvious PDA shell was formed for the modified HANTs and the coating layer on the surface of HANTs was nearly defect-free.

3.1.3. XPS analysis XPS was applied to research the surface composition of the nanotubes (Fig. 2C). The XPS data of HANTs, HANTs-DA and HANTs-DA-TA are listed in Table 2. Clearly, for pristine HANTs, the peaks of O 1 s (532 eV), Ca 2p (347 eV), and P 2p (134 eV) were included in the spectra, which are the main elements of hydroxyapatite, further verifying the successful synthesis of HANTs [36]. The peak of N 1 s (400 eV) indexed to the characteristic signal of PDA was observed for the HANTs-DA and the content of nitrogen (N1s) was about 6.19% [45,46]. Moreover, it was found that the peaks of Ca 2p and P 2p almost disappeared, indicating that the coating layer on the surface of HANTs was nearly defect-free, which can further be confirmed in the TEM analysis (Fig. 3) [41]. After the HANTs-DA were functionalized by taurine, spectra of HANTs-DA-TA gave rise to the S 2p peak at 168 eV, confirming the presence of -SO3H groups [53].

3.1.5. Thermal stability of the nanotubes TGA was carried out to investigate the actual yield of PDA and taurine on the surface of HANTs. From TGA data (Fig. 4), pure HANTs presented high thermal stability and a final weight loss of 5.5% was determined. After modification, there was a weight loss of 21.2% for the HANTs-DA from approximately 250° to 700°C due to the decomposition of PDA layer. After the grafting of taurine, more weight loss can be seen and the content of grafted taurine was about 2.9% [45].

3.1.4. TEM analysis The typical tubular morphology of HANTs and HANTs-DA-TA was characterized by TEM (Fig. 3). Image Fig. 3(A) and (B) indicated that the synthesized HANTs displayed a tubular structure, of which the

3.1.6. Aspect ratio and specific surface area of HANTs In this work, two kinds of nano HA were prepared and the HANTs displayed better effects on facilitating the permeability of UF membranes than the short rod-like HA nanoparticles, which were supposed to benefit from larger specific surface area and aspect ratio belonging to the nanotubular structure [21,28,38]. The aspect ratios of two kinds of nano HA was were calculated using the SEM images (Fig. 5). As shown in Table 3, nano HA with tubular structure owns higher aspect ratio than the short rod-like HA nanoparticles. Results of N2 adsorptiondesorption isothermal analysis and pore size distribution are shown in Fig. S2. The specific surface area of HANTs calculated on the basis of the Brunauer–Emmett–Teller (BET) model was 133.19 m2 g−1, which was 1.9 times that of the short rod-like HA nanoparticles (68.98 m2 g−1). By comparison, HANTs own higher aspect ratio and larger

Table 2 Elemental analysis on surface of pristine HANTs and functionalized HANTs from XPS. Samples

HANTs HANTs-DA HANTs-DA-TA

atomic percent (atom %) C 1s

N 1s

O 1s

S 2p

40.09 77.32 75.67

– 6.19 6.47

59.91 16.49 16.72

– – 1.14

The symbol – means absence of the element. 278

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Fig. 3. (A) (B) TEM images of HANTs and (C) (D) HANTs-DA-TA. Table 3 Aspect ratio of HA nanotubes and short rod-like HA nanoparticles. Samples

Diameter (nm)

Length (nm)

Aspect ratio

A1 A2 A3 A4 A5 a1 a2 a3 a4 a5

64.0 72.1 73.5 64.5 76.5 62.1 69.4 63.9 52.7 70.7

311.2 414.6 502.9 726.4 1224.0 128.3 245.5 192.7 205.0 204.3

4.9 5.7 6.8 11.3 16.0 2.1 3.5 3.0 3.9 2.9

appropriate amount of nanomaterials such as halloysite nanotubes, CNTs, metal−organic framework nanoparticles (MOF), attained larger porosity compared with the pristine membranes, resulting in a better permeation performance [30,44]. The overall porosity and mean pore radius of the PES/HANTs-DA-TA nanocomposite membranes are listed in Table 4. It can be seen that with the increase of HANTs-DA-TA content, the porosity and mean pore size of the membranes presented an increasing tendency first and then decreased, achieving the peak value at 0.3% nanofiller dosage. The introduction of hydrophilic HANTs-DA-TA decreased the thermodynamic stability of the casting solution and a rapid phase separation process occurred. However, the

Fig. 4. TGA curves of HANTs, HANTs-DA, HANTs-DA-TA.

specific surface area. 3.2. Membrane overall porosity and mean pore radius It is generally accepted that the membrane permeation property has a close relationship with the membrane porosity. From previous reports, it is found that hybrid membranes fabricated by incorporating

Fig. 5. (A1)–(A5) SEM images of HANTs and (a1)-(a5) short rod-like HA nanoparticles. 279

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as the nanotube content increases, the finger-like pores became longer and wider, of which the tendency lasted till the nanotube content reaches 0.3%, and then slightly shrank. The morphology transformation of the membranes was consistent with the variation of porosity, which got to the peak value 73.6% and then decreased (Table 4). In the high magnification SEM images, some of the nanotubes distributed on the pore walls can be found, which indicated good compatibility between HANTs-DA-TA and the PES matrix. In addition, the pores of the hybrid membranes became larger and a more porous structure was achieved compared with the PES membrane. Moreover, the thicknesses of the skin layer of PES membrane and hybrid membranes were marked in Fig. S3, and it can be seen that when 0.3% of the modified nanotubes were added, the thickness decreased to 196.8 nm in average. It is well known that a dense skin layer for UF membranes offers a poor permeability, thus the thickness diminution of skin layer is one main reason of the better permeability for the nanocomposite membranes [30,49]. The above variation in the morphology of skin layer and cross section was attributed to the accelerated phase inversion process caused by the introduction of hydrophilic nanotubes. Specifically, the addition of hydrophilic nanotubes accelerated the exchange rate between solvent and nonsolvent, thus resulting in wider finger-like pore structure and thinner skin layer [30,51]. Meanwhile, the viscosity of casting solution also grew as the nanotube dosage increased and to certain extent a delayed phase inversion took place, which inhibited the extension of the pore structure.

Table 4 Porosity, mean pore radius and roughness of PES/HANTs-DA-TA nanocomposite membranes. Membrane

M0 M1 M2 M3 M4

Porosity (%)

55.4 64.3 69.7 73.6 67.8

± ± ± ± ±

2.6 2.4 2.6 1.9 2.9

Mean pore radius (nm)

34.1 40.1 44.6 43.2 41.6

± ± ± ± ±

1.8 1.5 1.3 2.0 1.2

Roughness Ra (nm)

Rq (nm)

Rz (nm)

9.95 12.83 15.93 19.37 21.17

12.16 15.56 19.20 23.91 26.10

62.65 73.85 92.44 104.1 140.8

addition of HANTs-DA-TA also led to the growth of viscosity and when the viscosity effect reached a certain degree, a slower phase separation process took place, resulting in lower porosity and mean pore radius. 3.3. Hydrophilicity of the membranes Hydrophilicity of UF membranes plays a crucial role due to its affinity to permeability and antifouling properties. It is generally accepted that good hydrophilicity is favorable to promote the antifouling properties for UF membranes, which finally lengthens the working life of the membranes [44,49,51]. As can be seen in Fig. 6, the hybrid membranes had smaller water contact angles due to the existence of hydrophilic nanofillers. Meanwhile, water contact angles descended as the nanofiller dosage increased, which suggests a continuous incremental hydrophilicity. In the course of phase inversion, part of the hydrophilic HANTs-DA-TA migrated to the membrane surface (Fig. 7). Moreover, the top surface photographs of the fabricated membranes are presented in Fig. S1B, it can be observed that the hybrid membrane surfaces became darker, indicating the presence of the nanotubes on membrane surface.

3.4.2. AFM observation The roughness of the PES/HANTs-DA-TA membranes was characterized and the results are displayed in Table 4 and Fig. 8. Roughness parameters were determined in a scanning area of 5 µm × 5 µm. The results show that adding the nanotubes to the PES matrix gave rise to a growth in the membrane roughness and similar variation trend was observed in the titanate nanotubes blended membranes [54], which was attributed to the accelerated exchange rate between solvent and nonsolvent and the increased amount of nanotubes on membrane surface [30]. It is generally approved that the permeation and the antifouling capabilities of UF membranes can be affected by the surface roughness of membranes, which are discussed more in Section 3.5.

3.4. Membrane morphology 3.4.1. SEM morphology Fig. 7 shows the SEM images of the surface and cross section of the pure PES and PES/HANTs-DA-TA nanocomposite membranes. It can be observed that the surface of prepared membranes exhibited a compact morphology with no noticeable defects and some of the nanotubes distributed on the membrane surface can be found (Fig. 7), enhancing the membrane surface hydrophilicity. Meanwhile, the amount of nanotubes distributed on membrane surface gradually increased with the increment of blending proportion. Typical cross-section morphology of UF membranes consisting of a dense top layer and the layer of fingerlike pores can be observed for the prepared membranes. It is found that

3.5. Ultrafiltration performance As seen in Fig. 9A, with the addition of HANTs-DA-TA loading, the pure water flux was remarkably improved for the hybrid membranes and attained a maximum (439 L m−2 h−1) at HANTs-DA-TA content of 0.3 wt%, which is ∼2.6 times that of PES membrane (169 L m−2 h−1). Meanwhile, the rejection of BSA for the membranes still maintained a high level. In general, there are three main aspects promoting the membrane permeability. First, the overall porosity and mean pore radius of the membranes were improved because of faster phase inversion process, which led to an accelerated exchange rate between solvent and water [44,46]. Second reason is the variation in membrane surface. On one hand, the thicknesses of skin layer for the hybrid membranes were reduced, which decreased the filtration resistance of the water [30]. On the other hand, incremental surface roughness was also a conducive factor, which promotes the efficient filtration areas of membranes, improving the membrane permeability (Fig. 8) [49]. Moreover, the increment of water flux also benefited from the improved hydrophilicity of the membranes. As observed in Fig. 6, the gradually decreased water contact angles indicate that the membranes became more hydrophilic, which is conductive for water molecules to pass through [44,49]. However, as the nanotube content continued to ascend, the viscosity of the casting solution grew and aggregation of nanotubes began to happen [44]. When the loading in the membrane matrix reached 0.4 wt%, the exchange between solvent and nonsolvent was suppressed, which ultimately resulted in a decrease in water flux.

Fig. 6. Water contact angle of PES/HANTs-DA-TA nanocomposite membranes with different contents of HANTs-DA-TA. 280

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Fig. 7. The top surface and cross-section SEM images of hybrid membranes with different amount of HANTs-DA-TA.

antifouling study. The flux of the prepared membranes is displayed in Fig. 9B. First, the water flux of the membranes gradually decreased, which was attributed to membrane compaction under constant compression [44]. After 60 min of compaction, pure water flux of the

During practical applications, the UF membranes exposed in the environment of effluent are mostly susceptible to fouling, which highly affects the reusability of the membranes. Protein is one of the most common contaminants and here, BSA was utilized to conduct the

Fig. 8. AFM three dimensional surface image of M0, M1, M2, M3 and M4. 281

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Fig. 10. A) FRR of the PES/HANTs-DA-TA nanocomposite membranes, B) Summary of Rt, Rr, and Rir of membranes with BSA as pollutant.

Fig. 9. A) Water flux and rejection tested at 0.1 MPa operation pressure. B) Time-dependent water permeation fluxes of membranes with different contents of nanofillers after three cycles of UF test.

flux of the membranes prepared with different content of the rod-like HA-DA-TA is shown in Fig. S5, which reached the peak value (288 L m−2 h−1) at 2% nanofiller content. As a contrast, when HA with nanotube structure was employed, only 0.2% addition dosage can promote the flux to 439 L m−2 h−1. Obviously, nano HA with a tubular structure attained better effect on enhancing the membrane permeation. Compared with the short rod-like structure, the nano HA of tubular structure owns much larger specific surface area and aspect ratio. Even when small amount of modified HANTs was added, the thermodynamic instability of the casting solution was greatly increased, which led to an accelerated phase inversion process and a much more porous membrane structure was formed, resulting in a highly enhanced permeability.

membranes reached a steady state. Then, there was a quick reduction during the BSA filtration, which was ascribed to the accumulation and adsorption of protein on the membrane surface and pores. After the rinsing process, the water flux of the membranes can only be partly recovered due to the fact that some of the adhering BSA molecules can not be removed, leading to the blockage of membrane pores. To evaluate the stability of the hybrid membranes, consecutive UF tests were conducted and it can be seen that after 3 cycles of UF test, the hybrd hybrid membranes presented slight flux attenuation, which indicated the hybrid membranes had superior reusability. It is generally accepted that the antifouling capability is highly relevant to membrane surface, including the surface hydrophilicity and surface roughness [30,49]. Usually, membranes which possess stronger hydrophilicity and lower surface roughness represent an improved antifouling property. On one hand, hydration layers can be formed on hydrophilic surfaces, minimizing the chance for direct adsorption of BSA. On the other hand, the foulants are more easily trapped in valleys of the membranes with coarser surfaces. The FRR of the hybrid membranes was all superior to that of PES membrane (Fig. 10) due to the better surface hydrophilicity (Fig. 6). The highest value of FRR (77.9%) and Rr (46.7%) was reached for the hybrid membrane with 0.1% nanotubes, exhibiting the best antifouling ability. However, when the nanotube content is beyond 0.1%, the FRR appeared to be a decreased tendency, which is ascribed to the growth of the surface roughness (Fig. 8). Furthermore, permeability of the membranes prepared with the short rod-like HA, which was functionalized nearly the same way as the HANTs-DA-TA, was studied and the results are displayed in Fig. S4 and Fig. S5. From the TEM image, the HA nanoparticles exhibited a cylindrical structure and an obvious PDA layer can be observed. Water

4. Conclusions In this study, hydrophilic functionalization of HANTs was conducted via the PDA modification and the secondary modification of taurine (HANTs-DA-TA). The modification of HANTs made the nanotubes more hydrophilic and also contributed to the compatibility between PES and HANTs. Then, the modified nanotubes were introduced into PES for the preparation of hybrid UF membranes by immersion precipitation method. The results indicate that hybrid membranes exhibited preferable hydrophilicity and enhanced porosity. The permeation experiments show that water fluxes of the hybrid membrane were significantly improved, reaching the peak value (439 L m−2 h−1) at HANTs-DA-TA content of 0.3 wt%, which displayed ∼2.6 times that of PES membrane (169 L m−2 h−1). The modified membranes also exhibited enhanced antifouling ability with a FRR of 77.9%. Overall, the prepared HANTs-DA-TA are proved to be a novel efficient additive to prepare UF membranes, which endow the membrane enhanced permeability and antifouling property without the attenuation of the BSA 282

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rejection. This study offers a valuable reference for the design of HANTs nanocomposites for water treatment membranes.

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Acknowledgments

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The authors would like to thank the National Natural Science Foundation of China (No. 21774052) and the Foundation of Scholars of Changbai Mountain, Jilin Province.

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Appendix A. Supporting information [26]

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.memsci.2019.01.043.

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