salt separation

Desalination 479 (2020) 114343

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Tannic acid assisted interfacial polymerization based loose thin-film composite NF membrane for dye/salt separation

T

Qin Li, Zhipeng Liao, Xiaofeng Fang, Jia Xie, Linhan Ni, Dapeng Wang, Junwen Qi, Xiuyun Sun, ⁎ Lianjun Wang, Jiansheng Li Key Laboratory of New Membrane Materials, Ministry of Industry and Information Technology, Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Loose nanofiltration membrane Interfacial polymerization Tannic acid Dye/salt separation

In this work, a novel tannic acid assisted interfacial polymerization (TAIP) method was developed for preparing loose nanofiltration membranes (LNMs). Piperazine (PIP) and tannic acid (TA), two kinds of common monomer for interfacial polymerization (IP), were employed to deposit on substrate for the TAIP process. The TAIP process resulted in enlarged pores in the skin layer due to the longer monomers, thus achieved considerable permselectivity in separating salts from dye/salt mixture solution. The optimal TAIP membrane (M4) exhibited high permeability (32.57 LMH·bar−1 to Congo Red solution), reasonable rejection to dyes (99.40% and 99.19% for Congo Red and Rose Bengal, respectively) and satisfactory transport to salts (90.59% to Na2SO4 and 97.75% to NaCl). Meanwhile, this membrane retained high performance in dye/salt separation (7.16 LMH·bar−1 of permeability, 97.47% of CR rejection and 2.55% of NaCl retention) under even as high as 6 wt% of salt concentration. Moreover, TAIP LNM showed high stability throughout a 30-hour-long running test to separate salts from dye/salt mixture solution. We anticipated the TAIP methodology, together with the consequential LNM, to provide a potential candidate for dye/salt separation application.

⁎ Corresponding author at: School of Environmental and Biological Engineering, Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, Nanjing University of Science and Technology, Nanjing 210094, PR China. E-mail address: [email protected] (J. Li).

https://doi.org/10.1016/j.desal.2020.114343 Received 17 November 2019; Received in revised form 28 December 2019; Accepted 18 January 2020 0011-9164/ © 2020 Published by Elsevier B.V.

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1. Introduction

will be obtained using such longer monomers via IP process. Herein, inspired by the properties of TA, we report a tannic acid assisted interfacial polymerization (TAIP) methodology for LNMs fabrication for the first time. By simply mixing TA and PIP, two common water soluble monomers, on substrate, longer monomers were formed via reactions between TA and PIP. After reacting with TMC/n-hexane solution, skin layer with enlarged pores can be developed via TAIP process with the participation of the longer monomers. The resultant membrane exhibited reasonable rejection to dyes, enhanced water permeability and salt transport compared with traditional IP membranes (merely using PIP or TA as monomer). The influence of salt concentration to dye/salt separation performance and the TAIP membrane long-term stability were also investigated. Moreover, the loose nanofiltration mechanism of TAIP membrane was explored. TAIP was expected as a potential approach to fabricate LNMs for practical application.

Water has been intensively used in textile industry. A mass of textile wastewater, which includes carcinogenic organic dyes and inorganic salts, is thus produced [1,2]. Although traditional adsorption or coagulation approaches are able to remove dyes efficiently, the resultant solid waste can cause secondary pollution predictably [3,4]. In addition, the existence of salt hinders textile wastewater from biodegrading [5]. Therefore, this effluent is extremely harmful to environment and humanity if not treated correctly. Nanofiltration (NF), known as one of the typical membrane technologies, played an important role in wastewater treatment, pharmaceutical reclamation, water softening and heavy metal removal, etc. [6–10] NF membranes commonly possessed approximate pore size of 0.5–2.0 nm, which allowed the membranes to remove multivalent salts or organic matters with molecular weight of 200–1000 Da effectively [11]. Polymeric NF membranes usually appeared as thin-film composite (TFC) membrane. A highly cross-linked thin film of polymer, which had considerable selectivity, was developed upon the porous polymeric substrate. Among lots of methods to develop such TFC NF membrane, interfacial polymerization (IP) was believed as one of the most important technologies, and piperazine-trimesoylchloride (PIP-TMC) chemistry was ranked as the basis of TFC NF membranes [12,13]. The dense polyamide (PA) layer established via IP process generally manifested high retention to both multivalent salts and organic matters like dyes. However, such TFC NF membranes suffered when treating typical textile wastewater, which included organic dyes and inorganic salts (usually NaCl or Na2SO4). Traditional NF properties, namely high rejection to both dyes and salts, unfortunately, became stranglers to textile wastewater treatment. The high rejections to dyes and salts raised the osmotic pressure up to a higher level, which compromised the membrane permselectivity [14,15]. Worse still, valuable inorganic salts were wasted since they could not be separated from the textile wastewater [16]. Loose nanofiltration membranes (LNMs) have emerged as a potential candidate to recover salts from saline textile wastewater since 2004 [17]. Due to the looser skin layer compared with traditional TFC NF membrane, LNMs exhibited efficient permeation to salts and higher permeability, while maintained high rejection to organic dyes [1,18]. Efforts were made to obtain aforementioned skin layer via IP process [19–23]. These works focused on designing longer monomers or introducing suitable nanofillers to loosen the skin layer. Although excellent loose NF performances were achieved, inevitable drawbacks, such as complicated pre-synthesis process of particular monomers, agglomeration of nanoparticles, and the lack of interactions between fillers and matrix, existed [24]. Therefore, it remains challenging to develop facile and stable methodologies for LNMs preparing. Polyphenol has been soundly proved useful alternatives for polymeric membrane synthesis and modification [14,25–29]. Catechol or pyrogallol structures of polyphenols rendered them as valid reagents for surface engineering and functionalization. Tannic acid (TA), as a kind of easily available and low-cost polyphenol, showed its versatility in membrane fabricating. TA was able to serve as additive in casting solution of non-solvent induce phase separation (NIPS) [30–32], modifier of polymeric membrane [25,33,34], as well as monomer of IP [28,35]. Moreover, TA can be oxidized to TA-quinone under alkaline condition followed by reacting with amino groups via Michael addition/Schiff base reaction [36,37]. In aqueous phase, such reactions were widely applied in functional coating [26,38–40], surface immobilization [41] and selective layer developing [42–44]. These properties endow TA with ability to react with amino monomers (e.g. with PIP, see Fig. 1) and such products are undoubtedly longer than sole PIP or TA. Due to the large polymer chain and slow diffusion rate, longer monomers will hinder the active layer from being packed densely [45,46], hence result in looser active layer and larger pores after polymerizing with TMC. Therefore, it can be expected that looser skin layer with larger pores

2. Materials and methods 2.1. Chemicals and materials Piperazine (PIP), Congo Red (CR, purity: > 99%), Rose Bengal (RB, purity: > 99%), Acid Yellow 17 (AY17, purity: > 99%), magnesium sulfate (MgSO4), sodium sulfate (Na2SO4), sodium chloride (NaCl) and magnesium chloride (MgCl2) were purchased from Sinopharm Chemical Reagent Co., Ltd. Tannic acid (TA) was provided by Shanghai Macklin Biochemical Co., Ltd. Trimesoylchloride (TMC) was purchased from Aladdin Co., Ltd. (China). n-Hexane was provided by Merck Chemical Reagent Co., Ltd. (Germany). Polyethersulfone (PES) UF substrates (UE020) were obtained from Rising Sun Membrane Technology Co., Ltd. (China). 2.2. Membrane fabrication The TFC membranes were prepared via TAIP method as follows. First, designed mass of PIP and TA was dissolved in phosphate buffer aqueous solution (pH ≈ 8.0), respectively. Meanwhile, the PES membrane was loaded in a holder which allowed the skin layer of the membrane to contact with air. Then the fresh solutions were mixed uniformly (total content of PIP + TA in mixed solution was maintained at 2 g/L) and poured on the membrane top for 5 min. During this period, longer monomers were formed and deposited onto the substrate. Afterwards, the solution was drained off and the residual matter was eliminated by rolling a soft rubber roller upon the membrane top. Then the membrane surface was gently contacted with 1 g/L of TMC/nhexane solution for 1 min, followed by washing with DI water thoroughly, and stored in DI water. For comparison, TFC membrane merely based on PIP-TMC and TA-TMC IP processes were also fabricated via similar approaches. The membrane IDs and their ingredients were presented in Table 1. 2.3. Characterizations Membrane surface morphologies were studied via field emission scanning electron microscope (FE-SEM, Quanta 250, FEI). The surface roughness of membranes were investigated via atomic force microscopy (AFM) device (Bruker MultiMode8, Germany) with the scan area of 5 μm × 5 μm and the scanning of each sample was under smart mode. Fourier transform-infrared (FT-IR) spectra of membranes were collected after scanning 32 times per sample via a Perkin Elmer 100 FT-IR spectrometer. The scanning range of wavenumbers is from 4000 to 650 cm−1. Water contact angle (WCA) of membranes was detected using a drop shape analysis equipment (Krüss DSA30, German). An electrokinetic analyzer (SurPASSIII, AntonPaar) was employed to identify the membrane surface charges. 2

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O

OH OH R

OH

buffer pH 8.0

Michael NH

HN R

Tannic Acid (TA)

OH

TA-quinone

N

Piperazine (PIP)

Organic phase

TA content (g/L)

PIP content (g/L)

TA/PIP Ratio

TMC content (g/L)

0 0.40 0.67 1.00 1.33 1.60 2.00

2.00 1.60 1.33 1.00 0.67 0.40 0

0:1 1:4 1:2 1:1 2:1 4:1 1:0

1.00 1.00 1.00 1.00 1.00 1.00 1.00

N

OH

R

OH

addition

Longer monomer

The membrane was pre-compacted with feed at 6 bar for 30 min prior to each filtration test and maintained at 4 bar for NF test. Pure water, 0.2 g/L dye aqueous solution (RB, CR or AY17) or 2 g/L salt aqueous solution (MgSO4, Na2SO4, MgCl2 or NaCl) was employed as feed to investigate the filtration performances of membranes. Each test was replicated for 3 times under the same condition, the average of the data were calculated to obtain more reliable results. Moreover, the standard deviation appeared as error bars. Antifouling properties of membranes were studied using 1 g/L BSA solution as fouling agent. The membrane was first pre-compacted at 6 bar for 30 min and then used to filtrating pure water at 4 bar for 2 h, the initial permeability was recorded as J0. Afterwards, fouling period was engaged using 1 g/L BSA solution and last for 5.5 h, the last recorded permeability in this period was marked as J1. Then, pure water replaced the BSA solution for another 2.5 h to detect the flux recovery and the final permeability was noted as J2. Calculation methodologies of filtration performance and antifouling related indexes (flux recovery ratio (FRR), total fouling ratio (Rt), irreversible fouling ratio (Rir) and reversible fouling ratio (Rr)) were exhibited in Supplementary Information.

Table 1 Formula of IP/TAIP processes. Aqueous phase

Fig. 1. Schematic illustration of reaction between TA and PIP.

OH

O

Membrane ID

M1 M2 M3 M4 M5 M6 M7

2.4. Molecular weight cutoff (MWCO) testing Considering the current lack of approaches detecting pore size of NF membrane directly, filtrating solutions containing neutral solute and learn their rejections was regarded as the most common method [47]. To help understand the separation mechanism, solutions contain different molecular weight of polyethylene glycol (PEG) (PEG-200, 400, 600, 1000, 1500, 1 g/L) were prepared to figure out the membrane MWCO curves. The operation processes were the same as mentioned above, while the detecting of PEG content was finished by a total organic carbon (TOC) analyzer (Vario TOC, Elementar, Germany).

2.6. Dye/salt separation and long-term stability Filtration tests of dye/salt mixture solution were measured via the same processes and operation parameters as Section 2.5. Each solution contained 0.2 g/L dye together with designed salt concentration and was employed as feed to reveal how does salt concentration influence dye/salt separation efficiency. Mixture solutions (contained 0.2 g/L dye together with 2 g/L salt) were prepared for testing long-term stability of TAIP membrane in dye/salt separation during a 30-hour-long period.

2.5. Filtration performance Membrane filtration performances were tested using a cross-flow device of lab-scale with an effective filtration area of 1.256 × 10−3 m2.

Fig. 2. SEM and AFM images of IP and TAIP membranes surface. a, d) M1; b, e) M4; c, f) M7. 3

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3. Results and discussion

of ester and amide groups.

3.1. Membrane characterizations

3.2. Membrane permeability and rejections

The surface morphologies of IP (M1 and M7) and TAIP (M4) membranes were given in Fig. 2 in the form of SEM and AFM images. Typically, M1 exhibited a nodular morphology (Fig. 2a), which has been also reported in literatures [48,49]. A much smoother surface was observed on M7 (Fig. 2c). Compared with M1 and M7, M4 showed striped Turing-like structures (Fig. 2b), which were rougher and benefit for achieving high permeability [50]. AFM measurements further revealed the membrane surface morphologies. Similar with the SEM results, M1 (Fig. 2d) and M7 (Fig. 2f) were both smoother than M4 (Fig. 2e), indicating the coherence between the observations of AFM and SEM images. The surface roughness values of M1, M4 and M7 were listed in table S2. It was obvious that M4 possessed the highest roughness, and this further confirmed what we observed from Fig. 2. Therefore, we can conclude that TAIP method was likely to create rougher surface. Furthermore, cross section of the three membranes was exhibited (Fig. S1). A layer of thin film can be observed on each PES support. The active layer thickness of M4 (Fig. S1b, 105 nm) was between that of M1 (Fig. S1a, 120 nm) and M7 (Fig. S1c, 70 nm), indicating that both PIP and TA attend TAIP process. FTIR spectra of PIP–TMC membrane (M1), TA–TMC membrane (M7) and TAIP membrane (M4) were exhibited in Fig. 3. A broad band around 3420 cm−1 (Fig. 3a) and a dash-line denoted band at 1610 cm−1 (Fig. 3b) appeared in all of three membrane spectra indicated –OH stretching [22] and aromatic C]C resonance vibration [51], respectively. Because of the abundance of –OH groups and aromatic rings in TA, each of the two bands appeared as the most intensive one in M7 than that in M1 and M4. Corresponding bands in M1 were revealed as the weakest ones, because the aromatic rings were only brought by TMC, while the –OH groups entirely derived from hydrolysis of TMC. Intensities of above-mentioned bands were found to be moderate in M4. It demonstrated that both PIP and TA attended the TAIP process. A band which can only be observed in M1 and M4 spectra at 1567 cm−1 indicated the stretching of CeN groups [52–54], and this also confirmed the presence of PIP in M1 and M4. In M7, however, this band cannot be observed, because there was no nitrogen in TA or TMC. Hydrophilicity of M1, M4 and M7 were shown in Fig. 4. The WCA followed the order of M1 (38.45o) < M4 (53.45o) < M7 (63.26o), hence their hydrophilicity followed the reverse order. M7 possessed the lowest hydrophilicity because of ester groups, which derived from hydroxyl groups in TA and acyl chloride groups in TMC [55,56]. M4 held a moderate hydrophilicity between M1 and M7 due to the co-existence

Solution permeability and dye rejection of membranes were studied. As shown in Fig. 5, M1 or M7 both exhibited high rejection to dyes (99.71%, 96.90% to CR, AY17 for M1 and 99.86%, 96.98% for M7) while relatively low permeability (13.81 LMH·bar−1 for M1 and 8.38 LMH·bar−1 for M7). From M2 to M6, interestingly, none of their filtration performance were between those of M1 and M7, though their synthesis formulas were the convex combination of M1 and M7. The solution permeability increased with the increasing of TA/PIP ratio and then deceased at over high ratio. M4, which had the TA/PIP mass ratio of 1:1, showed maximum permeability (32.57 LMH/bar to CR solution and 34.62 LMH/bar to AY17 solution). With the increasing of TA/PIP ratio from 0 to 1:1, more long monomers were formed and the selective layer became looser, thus resulted in promoted permeability. With further rise of TA/PIP ratio, excessive TA controlled the IP process, the resultant membrane performed more similar with M7, hence the membrane permeability decreased. As for dye rejection, opposite trends were observed. It should be noticed that the lowest rejections to CR and AY17 were still at acceptable levels (99.28% to CR and 86.97% to AY17, M5). The TAIP method resulted in the increasing of solution permeability and the compromising of dye rejection of M2–M6. Firstly, longer monomers of TA-PIP complex enlarged the pores of the skin layer. Thus, it was easier for water and dye molecules to pass through the membrane. Moreover, the Turing-like membrane surface structure derived excess surface area brought TAIP membrane more permeate sites, thus also contributed to higher permeability of TAIP membrane. The two reasons resulted in much enhanced permeability as well as slightly decreased rejection to dyes. Dyes (0.2 g/L) or salts (2 g/L) solutions were employed to investigate NF performance of single component solution. As shown in Fig. 6a, evidently higher permeability was observed from M4 than that from both M1 and M7. However, as revealed in Fig. S1, the active layer of M4 was not the thinnest of M1, M4 and M7, indicating that the high permeability of M4 wasn't mainly due to the thickness of the skin layer. For the same membrane, the permeability of dye solutions was commonly lower than that of salt solutions due to larger size of solute. Fig. 6b exhibited the rejections to different solutes of these membranes. As expected, M1, M4 and M7 all had high rejection to both CR (99.71%, 99.40%, 99.85% for M1, M4 and M7, respectively) and RB (99.31%, 99.19%, 98.82% for M1, M4 and M7, respectively) while only M4 achieved high pass to salts. Salt rejection rates of M4 follows the order of R(MgSO4, 12.29%) > R(Na2SO4, 9.41%) > R(MgCl2, 6.79%) > R

a)

b)

TA-PIP-TMC (M4)

TA-PIP-TMC (M4) TA-TMC (M7)

TA-TMC (M7) PIP-TMC (M1)

PIP-TMC (M1) C-N

aromatic C=C

4000

3000

2000

1800

1000

1600

1400

1200

Wavenumber (cm-1)

-1

Wavenumber (cm )

Fig. 3. FTIR patterns of PIP-TMC membrane (M1), TA–TMC membrane (M7) and TAIP membrane (M4). a) Wider wavenumber range; b) narrower wavenumber range. 4

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Fig. 4. Photographs of membranes WCA. a) M1; b) M4; c) M7.

80 60

Table 2 Salt/dye selectivity of M1, M4 and M7 (based on Fig. 6).

100

90 Rejection Permeability

CR CR

AY17 AY17

80

40

Rejection(%)

Permeability(LMH bar-1)

100

0

SNaCl/CR

SNaCl/RB

SNa2SO4/CR

SNa2SO4/RB

M1 M4 M7

2.69 12.74 4.19

2.68 12.71 4.15

1.22 10.57 1.91

1.22 10.54 1.89

Although the antifouling performance of M4 was inferior to that of M1, it surpassed that of M7. Moreover, the flux of M4 was significantly superior to that of M1 and M7 during the whole test period, signifying its higher efficiency on water treatment.

70

20

Membrane ID

60 M1

M2

M3

M4

M5

M6

M7

3.3. Influence of salt concentration

Membrane name

Loose nanofiltration performances under different salt content were also revealed using M4. NaCl or Na2SO4 were dissolved together with dye (CR or RB) to prepare 4 kinds of different salinity feed solutions, respectively. As shown in Fig. 8a, with the increasing of NaCl content from 0 g/L to 60 g/L, the permeability of CR/NaCl mixture solution dropped from 31.52 LMH·bar−1 to 7.16 LMH·bar−1, while solute rejections also decreased (for NaCl, 5.41% to 2.55% and for CR, 99.40% to 97.47%, Fig. 8b). Similar trends were also observed for permeability and solute rejections among the other three kinds of feed solutions (detail data in Table S3). It was believed that dye molecules were likely to develop clusters in aqueous solutions and adding salts would lessen this agglomeration so that dyes were dispersed uniformly [57]. Uniformly dispersed dye particles exhibited smaller size and thus easier to go through the membrane pores as well as absorb on pores wall of membrane. This led to elevated dye transport and narrower effective pores size, finally resulted in decreased permeability and dye rejection. Besides, adding more salts caused severer concentration polarization (CP), and higher concentration gradient would give rise to lower salt rejection [58]. Moreover, the permeability of CR + Na2SO4 dropped faster than that of CR + NaCl (Fig. 8a), and CR rejection reduced less with Na2SO4 than that with NaCl when increasing salt content (Fig. 8b). According to the ionic strength formula (see Supplementary Information), Na2SO4 solution possesses higher ionic strength than NaCl solution with the same content (g/L). With increasing ionic strength, dye

Fig. 5. Permeability and dye rejection performance of M1–M7. (Dye content: 0.2 g/L).

(NaCl, 2.25%). In other words, M4 achieved high transport of 88.71%, 90.59%, 93.21% and 97.75% to the 4 kinds of inorganic salts, respectively. Based on Fig. 6 and Eq. (7) (see Supplementary Information), salt/dye selectivity of M1, M4 and M7 were calculated and listed in Table 2. M1 and M7 exhibited limited salt/dye selectivity, which were mainly owing to high rejections to salts. Surprisingly, M4 presented much higher selectivity due to elevated salt permeation. Enlargement of skin layer pores derived from TAIP process accounted for the high pass of salt. Given such high rejections to dyes and low retention to salt of M4, its efficient desalination performance of textile wastewater can be expected. Antifouling properties of M1, M4 and M7 were displayed in Fig. 7. It can be observed that BSA fouling dropped the permeability of all the three membranes down (Fig. 7a). Antifouling indexes were calculated (see calculation in Supplementary Information) and exhibited in Fig. 7b. FRR, Rt and Rir of M4 were between that of M1 and M7. Generally, M4 manifested a moderate antifouling property in the middle of M1 and M7. This was consistent with the order of membrane hydrophilicity, which was revealed by WCA measurements (Fig. 4).

RB CR AY17 MgSO4 Na2SO4 NaCl MgCl2 H 2O

50 40 30 20 10

b)

RB CR AY17 MgSO4 Na2SO4 NaCl MgCl2

100

Rejection(%)

Permeability(LMH bar-1)

a)

80 60 40 20

0 M1

M4

M7

0 M1

Membrane name

M4

M7

Membrane name

Fig. 6. NF performance of dyes or salts solutions and pure water. a) permeability; b) rejection of single solute (dye or salt). 5

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b) 80

40

M1 M4 M7

30 20 10 0

FRR

Rt

Rir

Rr

60

Index (%)

Permeability (LMH bar-1)

a)

40

20 water

water

BSA solution

0 0

2

4

6

8

10

M1

M4

M7

Time (h) Fig. 7. Antifouling properties (a) and relate indexes (b) of M1, M4 and M7.

3.4. Long-term stability of TAIP membrane M4 was employed for testing the long-term stability of TAIP membrane in dye/salt separation. Solutions that consisted of 0.2 g/L dyes (CR or RB) and 2 g/L salts (NaCl or Na2SO4) were used as feed. As shown in Fig. 9, for all the four kinds of mixtures, the solution

CR + NaCl CR + Na2SO4

20

10

30

98

25 20

96

CR with NaCl

CR with Na2SO4

NaCl rejection

Na2SO4 rejection

10 92

5 0

90

0 0

10

20

30

40

50

0

60

10

30

40

50

60

d) 100 RB + NaCl RB + Na2SO4

20

10

RB rejection (%)

Permeability (LMH bar-1)

20

Salt content (g/L)

Salt content (g/L) c) 40

30

30 25

98

20 96

RB with Na2SO4

RB with NaCl

Na2SO4 rejection

NaCl rejection

94

5

90

0 10

20

30

40

50

0 0

60

10

20

30

40

50

Salt content (g/L)

Salt content (g/L)

Fig. 8. Influence of salt concentration on dye/salt separation. a, c) Permeability; b, d) rejections. 6

15 10

92

0

15

94

60

Salt rejection (%)

30

b) 100

CR rejection (%)

Permeability (LMH bar-1)

a) 40

Salt rejection (%)

permeability dropped obviously within first 5 h due to the membrane fouling and then kept stable. As for solute rejections, CR and RB were both efficiently rejected (> 99.10% for CR (Fig. 9a and b), > 98.60% for RB (Fig. 9c and d)) during the 30 h tests. As expected, high passages of salts were observed (~93% for NaCl and ~90% for Na2SO4). The results indicated good stability of M4 in dye/salt mixture separation, and further verified the potential of M4 in practical application (Fig. S2). In comparison to other works which aimed at loose NF, the TAIP membrane manifested superior performance (Table S4). Given the results in Sections 3.3 and 3.4, we have reasonable ground to believe that the TAIP membrane could potentially be further used to recycle salt from other dyes contained textile wastewater.

molecules tend to form smaller aggregates, which make the cake layer denser and denser cake layer helps achieve lower flux and higher rejection [59]. It was worth noting that even under as high as 6 wt% of salt con, which was close to the salinity of typical textile wastewater [60], our TAIP membrane still exhibited higher than 97.00% of dye rejection together with no lower than 95.00% of salt pass and showed potential in practical application.

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b)

80 60

0.2g/L CR + 2g/L Na2SO4

40 20

20 0

0 0

5

10

15

20

25

Permeability CR rejection NaCl rejection

60

60 0.2g/L CR + 2g/L NaCl

40

40 20

20 0

0

30

0

5

10

Time(h)

15

20

25

30

Time(h) d)

c) Permeability RB rejection Na2SO4 rejection

60

80 60

0.2g/L RB + 2g/L Na2SO4

40

40 20

Permeability(LMH bar-1)

100

80

Rejection(%)

Permeability(LMH bar-1)

80

20

0

5

10

15

20

25

Permeability RB rejection NaCl rejection

60

80 60

0.2g/L RB + 2g/L NaCl

40

40 20

20 0

0

0

0

100

80

Rejection(%)

40

100

80

Rejection(%)

Permeability CR rejection Na2SO4 rejection

60

Permeability(LMH bar-1)

100

80

Rejection(%)

Permeability(LMH bar-1)

a)

0

30

5

10

15

20

25

30

Time(h)

Time(h)

Fig. 9. Long-term stability of TAIP membrane (M4). a) CR + Na2SO4; b) CR + NaCl; c) RB + Na2SO4; d) RB + NaCl.

To investigate the mechanism of loose nanofiltration brought by TAIP, MWCO of M1, M4 and M7 were measured. As shown in Fig. 11, M1 possessed a MWCO of 540 Da while 840 Da for M7. Interestingly, for M4, which was prepared via TAIP, its MWCO was 1370 Da. This demonstrated relatively loose skin layer was originated from the TAIP process. Although the MWCO of M4 was higher than dye MW, the dye rejection of M4 was still sufficient. As we mentioned before, dye molecules were likely to develop clusters in aqueous solutions, and a dye cluster was likely to possess a MW of > 1370. Furthermore, zeta potential of M4 was between that of M1 and M7 and they were all negatively charged at around neutral pH (Fig. S3). Moreover, the results manifested no big differences between the zeta potential of three membranes, thus they exhibit similar exclusion to dyes and salts. However, M4 was able to transfer much more salts than M1 and M7. Therefore, we can conclude that Donnan exclusion help reject dyes and

3.5. The mechanism of loose nanofiltration Generally, typical IP process using solely PIP or TA can form pores that meet the requirement of traditional NF application, namely removing multivalent salts from water. We hypothesized that longer monomers were able to form enlarged pores with TMC (Fig. 10a). Such pores were prerequisite for loose nanofiltration. Fig. 10b further manifested the reaction between TA and PIP. TA and PIP solutions were both colorless (although TA solution with higher concentration appeared as light yellow), while their mixture solution turned green rapidly and exhibited an evident UV–Vis absorption peak at the wavelength around 600 nm. It is noteworthy that such reaction happens without other harsh conditions (no need for UV, catalyst or high temperature), indicating the facility of the longer monomer formation strategy.

b)

Abs.

0.15

PIP TA PIP+TA

0.10

0.05

0.00 500

550

600

650

700

750

800

Wavelength (nm) Fig. 10. a) Schematic of pores forming in IP and TAIP processes; b) the UV–vis absorption spectra of PIP, TA and PIP + TA solutions. 7

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4. Conclusions

100

In summary, we developed a TAIP strategy for high performance LNM preparation. The TAIP methodology employed easily available monomers (PIP and TA) to form longer monomers and then polymerized with TMC. The TAIP membrane showed Turing-like structure together with rougher surface compared with traditional IP membrane. PIP and TA were both revealed to attend the TAIP process. In dye/salt separation tests, the optimal TAIP membrane (M4) achieved promoted permeability (32.57 LMH·bar−1 to CR solution), high rejection to dyes (99.40% and 99.19% for CR and RB, respectively), reasonable transport to inorganic salts (90.59% to Na2SO4 and 97.75% to NaCl), as well as good salt tolerance and stability. Moreover, the investigation of membrane MWCO and the filtration tests of IP membranes under low monomer content were measured to study the loose nanofiltration mechanism. Results manifested that the pore enlarging effect played a leading role in dye/salt separation. The TAIP technology, accompanied by the as-prepared LNM, can serve as potential candidates to recycle salts from typical textile wastewater.

PEG rejection (%)

90 80 M1 M4 M7

70 60 540 Da

50

840 Da 1370 Da

40 30 0

400

800

1200

1600

PEG molecular weight (Da) Fig. 11. Molecular weight cutoff (MWCO) curves of M1, M4 and M7.

CRediT authorship contribution statement

salts but isn't the main mechanism of loose NF performance of M4. However, here remained unclear on the causes of the formation of loose skin layer, because reaction between TA and PIP consumed the active sites for IP of monomers, and less active sites may also contribute to loosening the skin layer. Further, to clarify the forming mechanism of the skin layer, we prepared TFC membranes via traditional IP process under lower concentrations of solely used monomer and studied their NF performances. In the cases of lower monomer content, namely lower active sites content, inefficient separation of dye/salt mixture was observed whether solely using PIP (Fig. 12a) or TA (Fig. 12b) as watersoluble monomer. The solute (dye or salt) rejections synchronously increased with the increasing of monomer content and didn't evince enough differences with each other. Under no circumstances can sufficient dye rejection and high salts transport be achieved simultaneously. Therefore, simply reducing the amount of the active sites for IP cannot achieve loose packing of the skin layer, thus the resultant membrane cannot fractionate dye and salt effectively. Only with longer PIP-TA monomers, the polymer chains were prolonged and the diffusion rate was decelerated, then the skeletons of the longer monomers hindered the skin layer from packing tightly [45,46], the final skin layer was loose packed and possessed larger pores. Thus, we were able to conclude that the loose skin layer and larger pores played a leading role in loose nanofiltration.

Qin Li: Methodology, Investigation, Writing - original draft. Zhipeng Liao: Methodology, Investigation. Xiaofeng Fang: Investigation. Jia Xie: Methodology. Linhan Ni: Formal analysis. Dapeng Wang: Validation. Junwen Qi: Investigation, Validation. Xiuyun Sun: Formal analysis. Lianjun Wang: Writing - review & editing. Jiansheng Li: Supervision, Conceptualization, Project administration.

Declaration of competing interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant no. 51678307).

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.desal.2020.114343.

100

b)

RB Na2SO4 NaCl

100

80

Rejection(%)

Rejection(%)

a)

60 40 20

RB Na2SO4 NaCl

80 60 40 20

0

0

0.4

0.8

1.2

1.6

0.4

solely PIP content (g/L)

0.8

1.2

1.6

solely TA content (g/L)

Fig. 12. Filtration performance of IP membrane based on a) solely low PIP content; b) solely low TA content. 8

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