Preparation of a novel zwitterionic striped surface thin-film composite nanofiltration membrane with excellent salt separation performance and antifouling property

Abstract

Thin-film composite (TFC) nanofiltration (NF) membranes were fabricated via the co-deposition of taurine, tannic acid (TA), and polyethyleneimine (PEI), followed by subsequent interfacial polymerization with trimesoyl chloride (TMC) on the surface of the polysulfone ultrafiltration substrates. The surface properties, including the roughness, hydrophilicity, surface potential, and NF performances were facilely tuned by varying the taurine content for the prepared TFC membranes. In addition, the as-prepared TFC NF membranes had an excellent antifouling property and flux recovery ratio (FRR) in humic acid (HA), bovine serum albumin (BSA) and sodium alginate (SA) filtration tests. These results also revealed that the taurine content controlled the formation of the striped surface. Thus, this work provided a viable strategy for fabricating TFC NF membranes with high selectivity and outstanding antifouling ability.

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1 Introduction

Nanofiltration (NF) is a pressure-driven membrane separation technology with a molecular weight cut-off (MWCO) ranging from 200 to 1000 Da and a pore size of around 0.5–2.0 nm, which was confirmed as an effective technique for desalination and could also be used in liquid purification, sewage, and industrial wastewater treatments because of its low-pressure requirement.^1–4 However, membrane fouling is now the bottleneck problem in membrane applications since the practical application conditions and separation systems of the membranes are far more complicated.⁵ For wastewater treatment, the membrane foulants mainly arise from macromolecular organic matter, humic substances, hydrocarbons, bacteria, suspended sludge and inorganic colloidal or mineral scaling, which inevitably lead to the unfavorable change of membrane structures and significant deterioration of the separation performance.^6,7 Therefore, the construction of antifouling nanofiltration has a big significance in membrane applications.

In an antifouling membrane, advanced materials (such as silica spheres,⁸ nanoparticles,^9–12 hydrophilic polymers,^13–15 metallics^16,17 as well as some functional carbon materials^18,19) play an important role. Recently, polyphenol tannic acid (TA) has been widely used in membrane fabrication because of its abundance in plant tissue²⁰ and versatility for membrane synthesis. Tannic acid has anti-bacterial, anti-enzymatic and astringent properties.²¹ With these properties, it is an ideal molecule for improving the antifouling performance. Kim et al.²² coated TA–Fe^III coordination complexes onto PES UF membrane surfaces via a facile one-pot assembly process to obtain desirable features, such as antifouling properties against proteins, oils, and microorganisms, as well as antimicrobial and heavy metal ion removal properties. Zhang et al.²³ developed a facile TA-assisted in situ assembly approach during non-solvent induced phase separation (NIPS) to prepare antifouling NF membranes. Xu et al.²⁴ reported on the addition of TA to a PVDF membrane for oil/water emulsion filtration to avoid membrane over-fouling. Aside from tannic acid, zwitterions have attracted much attention in the study of antifouling membranes owing to their high intrinsic hydrophilicity, which forms a hydration layer through the electrostatic interaction between the zwitterions and water molecules.²⁵ This hydration layer provides a repulsive steric barrier to prevent the adsorption of organic molecules and bacteria on the substrate surface.²⁵ Wang et al.²⁶ introduced a zwitterionic copolymer carrying a positive charge into a negatively charged membrane via simple electrostatic adsorption in a one-step dip-coating process. During filtration with lysozyme for 1 h, the flux of this modified membrane was 93% of the initial flux, which was much higher than that of the original membrane. Davari et al.²⁷ tethered the zwitterionic polyelectrolyte poly[1-vinyl-3(2-carboxyethyl)imidazolium betaine] (PVCIB) onto a commercial thin film composite polyamide (TFC PA) membrane, which exhibited remarkable antifouling ability to resist non-specific protein adsorption at neutral and alkaline pH values. However, both PA-PVI and PA-PVCIB membranes exhibited high resistance to the positively charged lysozyme adhesion under acidic pH conditions. In addition, the surface zwitterionization can be constructed on the membrane surface rather than adding a zwitterionic copolymer. Researchers used 1,3-propanesultone to create a zwitterionic surface by ring-opening directly on the nanofiltration membrane prepared from the interfacial polymerization of 3,3′-diamino-N-methyldipropylamine and 1,3,5-benzenetricarboxylic chloride (TMC).²⁸ The zwitterionic N,N-bis(3-aminopropyl)methylamine was synthesized by Mi et al. to prepare an antifouling NF membrane with a 94.9% flux recovery ratio in BSA and SA solutions.²⁹ The zwitterionic structures improved the hydrophilicity of the nanofiltration membrane, thus increasing the water permeability and antifouling performance to proteins and polysaccharides. Zwitterions and tannic acid have been widely applied in the preparation of antifouling membranes, while the effect of their combination still remains unknown.

Striped structures in the nanofiltration membrane were first illustrated by Tan et al. in 2018.³⁰ The nanoscale spotted and striped Turing structures were generated by controlling the concentration of the PVA addition, which changes the diffusion rate of piperazine from the aqueous phase to the organic phase by hydrogen bond.^30,31

In our study, taurine (3-amino-1-propanesulfonic acid) and polyethyleneimine were poured on the polysulfone (PSF) substrate. Taurine was introduced into the membrane to improve the antifouling performance, and tannic acid acted as a cross-linking agent to lock the zwitterion onto the membrane surface with catechol groups.³² Then, this chemical-stable anti-biofouling compound and polyethyleneimine (PEI) were used to build a hydrophilic zwitterionic striped structure membrane by interfacial polymerization with TMC. We first used the strategy in which we attached the hydrophilic material by chemical bond to build the striped structure, which obviously improved the membrane hydrophilicity. The influence of the molecule weight of PEI, the concentration of taurine, as well as the interfacial polymerization conditions on the membrane morphology, separation performance and anti-fouling property was explored. The as-prepared membranes were characterized by FTIR, SEM, EDX, XPS, AFM, contact angle goniometer and zeta potential measurement.

2 Experimental

2.1 Materials and reagents

The commercial PSF UF membrane (MWCO = 20 000 Da) used as the support membrane was provided by Hangzhou Water Treatment Technology Development Center (China). Taurine (3-amino-1-propanesulfonic, 98%), tannic acid (TA, 98%), polyethyleneimine (PEI, MW = 600, 1000, 1800, 2500, 10 000, 99%), polyethyleneimine (PEI, MW = 70 000, 50%), bovine serum albumin (BSA) and sodium alginate (SA) were purchased from Shanghai Macklin Biochemical Co., Ltd. The inorganic salts (i.e., sodium phosphate tribasic dodecahydrate (Na₃PO₄·12H₂O), sodium chloride (NaCl), magnesium chloride hexahydrate (MgCl₂·6H₂O), anhydrous sodium sulfate (Na₂SO₄) and anhydrous magnesium sulfate (MgSO₄)) were provided from Hangzhou Lanbo Industrial Co., Ltd. Sodium dodecyl sulfate (SDS) and humic acid (HA) were purchased from Aladdin Industrial Corporation. Trimesoyl chloride (TMC, ≥98%) was purchased from Qingdao Benzo Chemical Co., China. Isopar L as an organic solvent was purchased from Exxon Mobil Corporation. Deionized (DI) water obtained by the electrodialysis of reverse osmosis water was used for all experiments. All chemical reagents were of analytical grade and implemented without further purification.

2.2 Fabrication of TFC NF membranes

TFC NF membranes were fabricated through a rapid co-deposition of PEI/TA/taurine onto the top surface of a PSF ultrafiltration (UF) substrate, followed by interfacial polymerization (IP) with TMC (Fig. S1†). The detailed fabrication process is illustrated in the ESI (S1).† The synthesized membranes are denoted as “M_a”, where “a” is the weight concentration of taurine (example: 0.1% wt is written as 1). M_b was the membrane without crosslinking by TMC (Table 1).

Table 1 IP parameters corresponding to the labeled TFC NF membrane^a

TFC membrane	TA content (wt%)	PEI content (wt%)	TMC content (w/v)	Taurine content (wt%)
a TA, PEI, taurine contents (wt%) are relative to the total IP solution volume (100 mL).
M0	0.20	0.5	0.1	0
M1	0.20	0.5	0.1	0.1
M2	0.20	0.5	0.1	0.2
M3	0.20	0.5	0.1	0.3
M4	0.20	0.5	0.1	0.4
Mb	0.20	0.5	0	0.2

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2.3 Characterization of the TFC NF membranes

All samples were dried in a vacuum oven at 40 °C for 12 h before characterization.

The functional structure of the TFC membrane surface was examined by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR, Nicolet 6700). For ATR-FTIR analysis of the membrane samples, the resulting NF membranes were characterized by the total reflection of a zinc selenide crystal. The chemical compositions of the NF membranes were measured by X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD). The morphologies of the NF membranes surface were observed by scanning electron microscopy (SEM, Hitachi S-4700). Then, the surface of the membranes was scanned after coating with gold. The NF membrane surface roughness was obtained by atomic force microscopy (AFM, XE-100) with 5 μm × 5 μm scanning range. RMS means the root mean square roughness, and R_a means the arithmetic mean roughness. The water contact angle was measured on a dried membrane surface by a contact angle analyzer (OCA-20, Germany) at ambient temperature. After each drop, the image was captured and the reading was recorded. For each sample, at least 12 readings were obtained, and the average contact angle values were calculated from these readings. Zeta potentials of the membrane samples were measured by a SurPASS analyzer (Anton Paar, Austria). The test reagent used for our study was 0.001 mol L⁻¹ KCl, and the pH value was varied to investigate its effect on the zeta potential.³³

2.4 Membrane performance evaluation

The membrane performance was tested using a laboratory-scale cross-flow apparatus with an effective area of 12.6 cm² at 0.5 MPa and 25 ± 1 °C. Before the test, each membrane was pressurized with DI water at 0.75 MPa and 25 ± 1 °C for 30 min to reach a steady state. Various salts (Na₂SO₄, NaCl, MgSO_4, and MgCl₂) in aqueous solutions with a concentration of 1 g L⁻¹ were used as the feed solution to evaluate the separation performance of the membrane. The flux (J, L m⁻² h⁻¹) and rejection (R, %) were calculated according to eqn (1) and (2), respectively:where ΔV (L), A (m²) and ΔT (h) correspondingly represent the volume of the permeate, the effective membrane area and the time for permeate collection.where C_p (mol L⁻¹) and C_f (mol L⁻¹) refer to the concentration of permeate and feed solution, respectively.

2.5 Antifouling property of the membrane

The antifouling property of the membrane was investigated by using the organic foulant (BSA, SA, and HA) solution (500 mg L⁻¹) in the same cross-flow apparatus at 25 ± 1 °C.³⁴ The membranes were compacted with DI water at 0.5 MPa and 25 ± 1 °C for 30 min to achieve a stable state, and the pure water flux of the original membrane (J₀) was measured for 3 h at 0.5 MPa. Then, the fouling experiment was carried out with 100 mg L⁻¹ BSA, SA or HA aqueous solution as the feed solution for another 10 h, and the steady value of the flux was noted as J₁. The normalized flux J₁/J₀ refers to the flux declining ratio during the fouling process. After that, the fouled membrane was thoroughly flushed by DI water for 30 min at 0.05 MPa. Finally, the pure water flux (J₂) of the cleaned membrane was measured for 3 h at 0.5 MPa. The above procedure was repeated 2 times to evaluate the flux recovery ratio (FRR), which was determined by eqn (3).

The total fouling ratio (R_t), reversible fouling ratio (R_r) and irreversible fouling ratio (R_ir) were calculated by eqn (4)–(6), respectively.

3 Results and discussion

3.1 Characterization of TFC NF membranes

3.1.1 Physical and chemical structures of the TFC membranes. As shown in Fig. 2, the chemical components of the membrane surfaces were characterized by ATR-FTIR. The specific absorption peak at 1039 cm⁻¹ was denoted as the SO₃⁻ groups that were observed in M₂ because of the introduction of taurine.³⁵ The specific absorption peaks at 1658 cm⁻¹ and 1530 cm⁻¹ were attributed to the existence of the formation of –C=O (amide I and amide II).^36,37 With the increase in the concentration of taurine, the intensity of this peak was decreased. The band appearing in the M_b and M₂ membrane spectra at 1645 cm⁻¹ was assigned to the formation of the –C=N bond.^38,39 The region from 2800 cm⁻¹ to 2900 cm⁻¹ in the blue circle was assigned to the specific absorption peak of the primary and secondary amines.⁴⁰ The absorption bands of –OH and N–H were assigned at around 3429 cm⁻¹ in the green circle.⁴¹ Due to the crosslinking by TMC, the broad peak became weaker in M₀ and M₂. M₀ and M₂ showed a newly emerged broad peak at around 1725 cm⁻¹ owing to the stretching vibrations of the ester groups (–COO) provided by the crosslinking of TA and TMC.⁴² Furthermore, the XPS spectrum is shown in Fig. 3. When taurine was introduced, the intensity of the S 2s and S 2p peaks increased, which confirmed that the taurine was locked into the membrane surface. In addition, the XPS C 1s and N 1s spectra in Fig. 4 were further used to prove that the –C=N bond was formed during the membrane immersion in the aqueous phase,³⁹ and the C–S bond was confirmed to be introduced by comparing the species of M₀ and M₂. Table 2 displays the elemental information of various TFC membranes, which was calculated from the corresponding X-ray photoelectron spectroscopy (XPS) spectra in Fig. 3. The ratio of C/O and C/N increased from 3.50 to 3.74 as the amounts of taurine varied from 0 to 0.3%, which was caused by the introduction of the C and N atoms from taurine. The C/O and C/N declined when the taurine content reached 0.4%, which was caused by the reaction of free taurine and TMC. The remaining taurine that did not attach to TA reacted with TMC directly to decrease the surface degree of crosslinking. The change of the C/O and C/N ratio in M₄ gave evidence for the reaction of taurine and TMC. Therefore, the results from ATR-FTIR and XPS demonstrated that the prepared membrane had a zwitterionic surface and the reaction process in IP.

Fig. 1 The mechanism of interface polymerization and zwitterionic striped structure formation.

Fig. 2 The ATR-FTIR spectra of the PSF support membrane, M₀, M₂, and M_b.

Fig. 3 Wide-scan XPS spectrum of the TFC membranes.

Fig. 4 Deconvoluted C 1s and N 1s XPS spectra for membranes including M₀, M₂, and M_b.

Table 2 Chemical compositions of various TFC membranes

Membrane	C (%)	N (%)	O (%)	S (%)	Cl (%)	C/O	C/N
M0	63.84	17.07	18.26	0.81	0.02	3.50	3.74
M1	64.09	15.77	18.17	1.96	0.01	3.53	4.06
M2	65.05	14.85	17.98	2.11	0.01	3.62	4.38
M3	66.60	13.22	17.80	2.36	0.02	3.74	5.03
M4	63.49	15.77	18.23	2.50	0.01	3.48	4.03
Mb	61.01	18.78	18.43	1.78	0	3.31	3.25

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3.1.2 Surface and cross-section morphologies of the membranes. The top surface and cross-section morphologies of each membrane were observed under SEM. From Fig. 5, the nanostrand hybrid morphology appeared in panels (e) and (g). The surface morphology was changed by the contents of the additional taurine. When 0.2% or 0.3% taurine was added, the roughness and surface area of the striped structure increased (as shown in Table 3).³⁰ The membrane surface property and hydrophilicity, in particular, were improved with increasing taurine content. Also, the reaction of PEI, TA, and taurine helped the hydrophilic groups attach to PEI, which hindered the further diffusion of the PEI monomer and efficiently reduced the polyamide thickness.⁴³ These hydrophilic groups also absorbed water to form a water layer to change the surface tension between the aqueous phase and organic phase in the IP reaction to create the striped structure⁴⁴ (Fig. 1). Fig. 1 illustrates that the sulfonic acid group was much more hydrophilic, which caused the PEI to remain at the aqueous phase for as long as possible, and increased the surface tension. So, the striped structure appeared. It is important to highlight that the striped structure disappeared when a larger amount of the taurine was added (0.4%). The IP reaction takes place in the organic phase,⁴⁵ and the micromolecules were easier to diffuse to the organic phase from the aqueous phase than the macromolecules.³⁰ Thus, the redundant taurine molecules (not combined with TA) were easily diffused to the organic phase to immediately react with TMC, and the reaction between PEI and TMC was further decreased. Moreover, the cross-section structure showed that the degree of crosslinking was decreased by the increased taurine content.⁴⁶ Thus, a loose surface morphology could enhance the permeability of the water molecule.⁴⁷ The roughness of the membranes is depicted by AFM in Table 3. The R_a and RMS of M₂ were 4.61 nm and 5.97 nm, respectively. After the taurine was introduced, much rougher surfaces with more nodular structures were observed for membranes M₁ through M₄ in comparison with M₀. The R_a values of membranes M₁ through M₄ were 3.54 nm, 4.61 nm, 5.64 nm, and 7.17 nm, respectively. These results proved the conclusions from the previously discussed SEM images that the formation of the striped structures increased the surface roughness. Therefore, the surface morphology and roughness, which are related to the permeability and anti-fouling property of the membrane, were dominated by the introduction of taurine.

Fig. 5 SEM images of the TFC membranes: M₀ ((a) surface, (b) cross-section), M₁ ((c) surface, (d) cross-section), M₂ ((e) surface, (f) cross-section), M₃ ((g) surface, (h) cross-section), M₄ ((i) surface, (j) cross-section).

Table 3 AFM roughness parameters of the TFC membrane surfaces. Surface roughness values of the 5 × 5 μm² sample scans are represented as R_a (average roughness), RMS (root mean square roughness), and R_m (maximum height)

TFC membrane	Ra (nm)	RMS (nm)	Rm (nm)
M0	3.39	4.29	43.7
M1	3.54	4.54	48.8
M2	4.61	5.97	63.5
M3	5.64	7.53	80.6
M4	7.17	9.59	108

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3.1.3 Water contact angle and surface charge of the membranes. The water contact angles of the TFC membranes are shown in Fig. 6. It is intuitive that the water contact angle of the membrane was decreased by the increase of the taurine content. Amine groups and sulfonic acid groups have a strong affinity to water, which increased the surface hydrophilicity of the membrane.^48,49 Thus, the change of the water contact angle demonstrated that the introduction of taurine improved the surface hydrophilicity, corresponding to the result in Table 3. In addition, the surface charge of the membranes was evaluated by the zeta potential test. The result is displayed in Fig. 7. All membrane surfaces were found to be positively charged when the pH value was at 6.5 (test value) because the polymerization between PEI and TMC led to a polyamide thin-film with a large number of unreacted positive primary and secondary amine groups.⁵⁰ When the concentration of taurine increased from 0 to 0.3%, the isoelectric point was shifted to a higher pH value. One reason is that the increased taurine replaced some amino groups of PEI to react with TA and TMC to release more amino groups of PEI. Meanwhile, taurine itself brought in amino groups. Nevertheless, when the taurine content came to 0.4%, the isoelectric point was shifted to the left. The explanation for this result was that the high concentration of the taurine strongly impeded the reaction between TA and PEI or TMC and PEI (Fig. 1). These unreacted PEIs were washed by the water, causing the amino groups in the membrane surface to decrease. Although the surface had become more positively charged with the introduction of taurine, the sulfonic acid groups still provided negative charges in the membrane to form a zwitterionic surface that could not be evaluated by the surface zeta potential test.⁵¹

Fig. 6 The water contact angle of the M₀, M₁, M₂, M₃, and M₄ membranes.

Fig. 7 Membrane surface charges as a function of pH.

3.2 Membrane performance

The fabrication conditions were evaluated during the preparation process, and the influence of these conditions on membrane performance are described below.

3.2.1 Concentration of taurine. The effect of the taurine content on the resulting membrane separation performance are presented in Fig. S2(a).† The reaction time in the aqueous phase and TMC organic phase was 3 min and 60 s, respectively, and the PEI molecular weight was 70 000, as listed in Table 1. When the taurine content varied from 0 to 0.4%, the rejection of Na₂SO₄ and NaCl increased in the first stage, and subsequently decreased. Furthermore, when the taurine content was at 0.2%, the separation performance of Na₂SO₄ and NaCl was superior and kept a relatively high MgCl₂ rejection. The membrane containing 0.2% taurine with a striped surface also had a larger water contact area,³⁰ which had a considerable water flux. The NF membrane is considered as a charged porous film, and there are two main factors that affect the rejection of the NF membrane that coincides with the Donnan exclusion theory and steric hindrance effect: the surface charge plays a crucial role.⁵² With the addition of taurine, the surface property of the membrane was changed. The high Na₂SO₄ rejection and the reduction of MgCl₂ rejection can be rationalized by the Donnan effect and size exclusion effect, as there was a charge shielding effect in the charged membrane surface and the hydration radii of Na⁺ and Cl⁻ are too small to be rejected.⁵³ The sulfonic acid groups were taken into the membrane with the unreacted amino groups of PEI to construct a zwitterionic surface, which altered the surface potential. The taurine content increased, enhancing the negative potential, so the rejection of Na₂SO₄ first increased. On the other hand, the taurine content dominated the surface menology. When the taurine was introduced, the taurine that did not combine with TA easily reacted with TMC to decrease the degree of surface crosslinking to form a thinner PA layer. This is why the rejection of Na₂SO₄, MgCl₂, and NaCl was reduced when the taurine content increased to more than 0.2%, and the flux of the membrane rose as a result of the increased taurine content. Thus, the membrane with 0.2% taurine exhibited relatively optimistic separation performance and water permeability.

3.2.2 PEI molecular weight. The influence of the PEI molecular weight is presented in Fig. S2(b).† The reaction times in the aqueous phase and TMC organic phase were 3 min and 60 s, respectively, and the taurine content was 0.2%. When the molecular weight increased from 600 to 2500, the rejection of Na₂SO₄, MgCl₂ and NaCl increased. When the molecular weight was further enlarged, the rejection of NaCl was decreased. The water flux also declined with the increased molecular weight of PEI. The reason may be that the selective layer became more dense with the increased molecular weight of PEI.³⁹ In addition, a PEI molecule with a larger molecular weight has more amino groups to provide more positive charges for a high divalent cation rejection.⁵² In addition, the larger molecular weight (10 000 and 70 000) PEI was harder to introduce to the organic phase to react with TMC, but provided more amino groups, which explained the stable Na₂SO₄ rejection and rapid decline of the NaCl rejection.

3.2.3 Co-deposition time. The role of the co-deposition time in membrane performance is displayed in Fig. S2(c).† The reaction time in the TMC organic phase was 60 s. The taurine content was 0.2% and the PEI molecular weight was 70 000. When the time varied from 1 min to 3 min, the water permeability was improved and the rejection of Na₂SO₄, MgCl₂, and NaCl was increased. That may be ascribed to the deposition of more hydrophilic monomers, especially the amino groups of PEI, in the aqueous phase. After extending the deposition time from 3 min to 9 min, the water permeability of the membrane dropped from 57.58 L m⁻² h⁻¹ to 43.82 L m⁻² h⁻¹. The explanation for this phenomenon was that much more hydroxyl groups of TA reacted with the amino groups to form a crosslinking structure, which weakened the surface permeability of the membrane. The rejection of Na₂SO₄ and NaCl showed no great changes, while the rejection of MgCl₂ was decreased since the rejection of salt in the TFC membranes was mainly dominated by the surface charge rather than the surface structure,³⁹ and the amino groups do not have much impact on the divalent rejection.

3.2.4 IP reaction time. The effect of the IP reaction time is illustrated in Fig. S2(d).† The reaction time in the aqueous phase was 60 s. The taurine content was 0.2% and the PEI molecular weight was 70 000. When the reaction time was 3 min, the Na₂SO₄ rejection was >97%, as the reaction rate of the interface polymerization was very fast.⁵⁴ When the reaction time rose from 30 s to 60 s, the rejection of Na₂SO₄, MgCl₂ and NaCl increased rapidly, while the water flux decreased rapidly. When the reaction time increased from 60 s to 150 s, the Na₂SO₄ rejection became steady while that of MgCl₂ decreased, and the NaCl rejection still increased rapidly and the declination tendency of the water flux slowed down. That can be ascribed to the increase of the degree of crosslinking with the expanding reaction time. Therefore, the best condition for the membrane preparation was when the reaction times in the aqueous phase and TMC organic phase were 3 min and 60 s, respectively, the taurine content was 0.2% and the PEI molecular weight was 70 000, which were the preparation conditions of M₂.

3.2.5 The membrane performance of M₀ and M₂ in four different salt solutions. Fig. 8 displays the membrane performance of M₀ and M₂ in four different salt solutions at 1000 ppm. For M₂, the rejections of Na₂SO₄, MgSO₄, MgCl₂ and NaCl were 97.55%, 94.72%, 90.40%, and 36.36%, respectively. For M₀, the rejections of Na₂SO₄, MgSO₄, MgCl_2, and NaCl were 94.29%, 92.00%, 93.32%, and 43.70%, respectively. The rejection mechanism of the nanofiltration membranes was mainly determined by the molecular size (sieve effect) and the charge effect.⁵⁵ The M₂ membrane with the zwitterionic surface had a special charge distribution for divalent ion rejection. The sulfonic acid group helped to increase the rejection of SO₄²⁻ and maintain the relatively high rejection of Mg²⁺. In addition, the water flux of M₂ in these four salt solutions was 49.53 L m⁻² h⁻¹, 52.97 L m⁻² h⁻¹, 55.3 L m⁻² h⁻¹, and 58.16 L m⁻² h⁻¹, respectively. The striped surface increased the surface water contact area to result in a relatively higher water flux. This result showed that M₂ had a better salt separation performance and water permeability than M₀.

Fig. 8 The membrane performance of M₀ and M₂ in four different salt solutions.

3.3 Antifouling performance of the TFC membrane

Generally, nanofiltration membranes easily suffer from membrane fouling in applications, which restricts the lifetime of the membrane. Factors like the electrostatic interaction, hydrogen bonding effect, hydrophobic impact, and van der Waals forces normally have a great impact on membrane fouling.⁵⁶

A comparison of the antifouling performances of M₂ and M₀ is shown in Fig. 9. Fig. 9(a)–(c) display the antifouling performances of M₂ and M₀ at pH = 7 in HA, SA and BSA solutions of 500 ppm, respectively. When the feed solution was replaced by a model foulant solution, the water flux of both membranes declined due to the extra osmotic resistance caused by the deposition of foulants on the membrane surface. Fig. 9(a) shows that the flux of M₀ decreased rapidly and reached a quasi-steady stage of 85% to the initial water flux, while M₂ showed a lower flux reduction ratio (9%) in the HA solution. After being cleaned with DI water, the relative water FRRs of M₀ and M₂ were 91.5% and 97.8%, respectively. These flux variations during the antifouling test indicate that the M₂ membrane had an excellent humic acid antifouling performance. In addition, Fig. 9(b) shows the antifouling performances of M₀ and M₂ in a 500 ppm SA solution. The flux of M₀ decreased rapidly and reached a quasi-steady stage of 82% to the initial water flux and the flux reduction ratio was 10% after being cleaned by DI water, while M₂ showed a relatively lower flux reduction ratio (7%).

Fig. 9 Time-dependent flux for the M₂ and M₀ membranes during the filtration of foulants at 500 ppm: (a) HA, (b) SA, (c) BSA, and the (d) antifouling indices of M₂ in these foulants.

Moreover, the antifouling performances of M₂ and M₀ in the BSA solution are shown in Fig. 9(c). The permeation flux of M₂ and M₀ experienced a marked decrease in 78% and 67%, respectively, demonstrating severe BSA membrane fouling. After being cleaned with DI water, the water permeabilities of these membranes recovered to 90% and 82%, which were much lower than the original pure water flux. The lower recoveries indicated that these membranes had a relatively low flux recovery and reversible fouling. These results were primarily ascribed to the introduction of the sulfonic acid groups on the membrane surface. As we all known, the BSA, HA and SA foulants used in this experiment were all negatively charged under the neutral filtration condition (pH = 7).¹² The introduction of the sulfonic acid groups provided more surface negative charges, which hindered the foulant molecules from contacting the membrane surfaces via electrostatic interactions. Meanwhile, the sulfonic acid groups increased the surface hydrophilicity and helped absorb water molecules. These changes enabled the formation of a hydration layer that prevented the adsorption of organic foulants on the membrane surface due to the higher maximum Gibbs free energy than the unmodified surface.^57,58 Although the roughness of M₂ was greater than M₀ owing to it theoretically being embedded with more contaminants, the surface hydration layer was dense enough to cover the surface gap and help eliminate the influence of the membrane surface roughness. In addition, the zwitterionic surface provides some electrostatic resistance to these charged foulants. Thus, the M₂ membrane showed better antifouling property than M₀. Four parameters (FRR, R_t, R_r, and R_ir) were utilized to evaluate the antifouling performance of M₂ as summarized in Fig. 9(d). The FRR in the HA, BSA, and SA filtration test was 97.8%, 89.9%, and 93.1%, respectively. These results demonstrated that M₂ showed remarkable antifouling properties in the HA solution, while having relatively lower FRR in the BSA and SA solutions. M₂ also had a relatively better antifouling property in the HA solution. The explanation for this was that the BSA and SA solutions had relatively higher molecular weights and stronger viscidity values than HA. Therefore, the prepared membrane M₂ demonstrated excellent performance in the membrane fouling evaluation.

4 Conclusion

In summary, novel TFC NF membranes with zwitterionic striped surfaces were fabricated via the co-deposition of taurine, TA, and PEI onto the PSF ultrafiltration membrane, followed by IP with TMC. The skin layer of the membranes was dense and defect-free; this endowed the TFC membranes with a high rejection (over 90%) for multivalent ions. The pure water flux of the membrane was also more than 61.28 L m⁻² h⁻¹ because of the striped structure. We first used hydrophilic micromolecules to change the diffusion rate of the macromolecules by chemical bonding rather hydrogen bond. In addition, the membrane showed a favorable antifouling property in HA, SA and BSA solutions (FRR > 89%). In conclusion, we propose a novel and effective strategy for promoting both the structural and chemical properties of TFC NF membranes to meet the antifouling demands of practical filtration processes, giving insight into the construction of a zwitterionic striped surface membrane by the introduction of micromolecules to macromolecules in the aqueous phase via chemical bonds.

Conflicts of interest

The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgements

Financial support is acknowledged to the National Key Research and Development Program of China (Grant No. 2017YFC0403702).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra00480d

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