Non-organic solvent prepared nanofiltration composite membrane from natural product tannic acid (TA) and cyclohexane-1,4-diamine (CHD)

Accepted Manuscript Non-organic solvent prepared nanofiltration composite membrane from natural product tannic acid (TA) and cyclohexane-1,4-diamine (CHD) Meng He, Honghong Sun, Haixiang Sun, Xiujie Yang, Peng Li, Q. Jason Niu PII: DOI: Reference:

S1383-5866(18)34563-5 https://doi.org/10.1016/j.seppur.2019.04.064 SEPPUR 15534

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

23 December 2018 17 April 2019 19 April 2019

Please cite this article as: M. He, H. Sun, H. Sun, X. Yang, P. Li, Q. Jason Niu, Non-organic solvent prepared nanofiltration composite membrane from natural product tannic acid (TA) and cyclohexane-1,4-diamine (CHD), Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.04.064

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Non-organic solvent prepared nanofiltration composite membrane from natural product tannic acid (TA) and cyclohexane-1,4-diamine (CHD) Meng He, Honghong Sun, Haixiang Sun, Xiujie Yang, Peng Li*, Q. Jason Niu*, State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao, 266580, P. R. China. *

Corresponding author

Tel: +86 0532 86981850 Address: No. 66 Changjiang Road, Huangdao District, Qingdao, China Author e-mail address: 1) [email protected] (Meng He) ORCID: 0000-0001-7373-9779 2) [email protected] (Honghong Sun) 3) [email protected] (Haixiang Sun) 4) [email protected] (Xiujie Yang) 5) [email protected] (Peng Li) ORCID: 0000-0002-5186-2992 6) [email protected] (Q. Jason. Niu) ORCID: 0000-0002-3450-3453

Abstract Conventional nanofiltration membrane fabrication generally include the usage of toxic organic solvent that pose threats to the environment and public health. Herein, we explored preparing nanofiltration membrane through a non-organic solvent method utilizing the highly reactive natural product, tannic acid (TA), along with cyclohexane-1,4-diamine (CHD). Fabrication process was carried out through sequential immersion of a polyethersulfone (PES) support in two aqueous monomer phases. The absence of a solvent interface facilitated the diffusion and mixing of the two monomers, and the highly reactive quinone derivative of TA enabled the rapid film formation via Michael addition and Schiff base formation. Membrane performance was optimized through investigation of fabrication parameters. The TA-CHD membrane exhibited a negatively charged surface, with a molecular weight cut-off (MWCO) of 600 Da. The membrane prepared at optimal conditions showed approximate 97% and 50.7% rejections towards Na2SO4 and NaCl, with a water flux at approximate 35 L m-2 h-1 (10 bar). Fouling behaviour of the TA-CHD membrane was studied and a chemical cleaning test under two extreme pH was performed to demonstrate the chemical stability. The proposed non-organic solvent method here provides an eco-friendly way to utilize the potential of polyphenol natural products in water purification membrane preparation. KEYWORDS: nanofiltration composite membrane; polyphenol natural product; non-organic solvent method; polymer

1. Introduction Nanofiltration (NF), a pressure-driven membrane separation process, is capable of purifying water solution at high efficiency as well as low energy footprint[1]. In recent years, nanofiltration has been widely used in various application to address the increasingly urgent water issues in different regions and industries[2]. Different from reverse osmosis membrane, which is mainly used in desalination without exhibiting a selectivity towards different solutes, nanofiltration membrane generally has the ability to separate and recycle species depending on various factors, e.g. the molecular size, charge[3, 4] or dielectric exclusion[5, 6]. So far, nanofiltration has exhibited its advantages in selective ion-sieving, dye removal[7-9], and high value solute reclamation, e.g. rare earth element[10], drug molecules[11].

As a prevailing method, the state-of-the-art interfacial polymerization (IP) is generally used in fabricating nanofiltration composite membrane, which is carried out through an interfacial reaction between a multifunctional acid chloride (e.g. trimesoyl chloride, TMC) and multifunctional amine (e.g. piperazine, PIP). The monomers are usually dissolved in two immiscible phases, most typically hexane/water solutions, which generate an interface where the diffusion-reaction film formation process takes place[12]. However, rather paradoxically as membrane technology aims to cope with the water safety issues, the organic solvent in its fabrication process may cause threat to both the environment[13] and human body[14], reducing its benefits. Thus, it is preferable to replace the organic solution with a non-toxic and green counterpart, which most likely could be water owing to its good solubility towards many chemicals. However, conventional organic monomers (e.g. acid chloride, isocyanate[15]) used for separating membrane fabrication suffer from low solubility or hydrolysis in aqueous solution, making it impractical to replace the organic solvent used in current preparation process. Hence, it is estimated that water-based membrane fabrication could only be achieved with monomers of both high reactivity and solubility in aqueous phase. Polyphenol from natural products, like catechol[16, 17], dopamine, tannic acid, contains phenol groups in ortho positions that endow versatile reactivity and good solubility in water, which could be used in membrane fabrication[18, 19]. Especially as for polydopamine (PDA), numerous study has been reported in terms of its application in coating[20] [21], film formation[22] and surface modification of nanomaterials[23, 24]. Tannic acid (TA) is a cheap natural product containing pyrogallol-type phenols in its backbone, and it exhibits a higher reactivity due to the greater amount of phenol groups, which enables a faster chemical modification process[25]. In light of its reactivity, some researchers have investigated the potential of TA as a monomer for separating membrane preparation[26-29]. Respectively, Zhang et al.[28] and Pérez-Manríquez et al.[29] prepared membranes for dye removal and organic solvent nanofiltration via conventional IP method between TA and acid chlorides. Others carried out membrane preparation by means of co-deposition of polyphenols and polyamines in pH buffer solution[17, 30]. For example, Xu et al.[17] co-deposited catechol and polyethyleneimine (PEI) on a polyacrylonitrile (PAN) ultrafiltration support, and the nanofiltration composite membrane exhibited a

positively charged surface with MgCl2 rejection for 85.2% and Na2SO4 rejection for 55.3%. Compared with the relatively time-consuming co-deposition method, the state-of-the-art IP method enabled a practical implementation of these natural products with established membrane preparation process, yet the consumed solvent would remain a hurdle to fulfill the intention of environment protection. Thus, there is still a need to develop novel methods to further utilize the reactivity of natural polyphenol products for nanofiltration membrane fabrication in aqueous phase. Under weak alkaline condition, catechol- and pyrogallol-type phenols have a high tendency to be oxidized and form quinone derivatives[18]. As unstable species with high reactivity, these derivatives could react with primary amines through Schiff base reaction and Michael addition[26]. Based on this, in this study, we attempted preparing NF membrane between tannic acid and cyclohexane-1,4-diamine (CHD). The CHD molecule has two primary amine moieties linked to a cyclohexane ring, and we hypothesize the distorted conformation of the molecule can lead to higher free volume of the result membrane, which improved the selectivity of monovalent ions over multivalent ones. Taking advantage of the reactivity and solubility of TA in aqueous solution, the fabrication process was carried out rapidly in a non-organic solvent method, and the selective layer was assembled on a polyethersulfone (PES) ultrafiltration support through sequential immersions in two aqueous monomer solutions. The reaction mechanism was investigated using spectroscopic methods, and it was confirmed that the membrane was mainly linked via Michael addition and partly by Schiff base reaction. Preparation parameters were optimized and the membrane showed nanofiltration performance towards different electrolytes. Hydrophilicity and morphology of the membrane were characterized, and fouling test was performed to study the fouling behavior using a model foulant. Furthermore, to evaluate the application potential, the chemical stability of membrane was investigated by treating membrane with acidic and alkaline pH solutions similar to common cleaning process of RO or NF operation. The preparation method reported here may allow for a new way to utilize the abundant polyphenols existing in the nature for nanofiltration membrane preparation.

2. Experimental 2.1. Materials and reagents

Polyethersulfone (PES) ultrafiltration membrane was obtained from Vontron Membrane technology Co.,Ltd. The flat sheet support was cut into rectangular pieces and stored in deionized water to remove any storing additives. Tannic acid (TA, AR) and trans-cyclohexane-1,4-diamine (CHD, >98%) were purchased from Aladdin Industrial Corporation (Shanghai). Hydrochloride acid (HCl), magnesium sulphate (MgSO4), sodium chloride (NaCl), sodium sulphate (Na 2SO4) were all of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. Polyethylene glycol (PEG) with molecular weight at 200, 400, 600, 800, 1000 Da were obtained from Shanghai Macklin Biochemical Co., Ltd. Bovine serum albumin (BSA, 98% BR) was purchased from Shanghai Yuanye Biological Technology Co.,Ltd. All chemicals involved were used as received without any further treatment. Deionized water herein was made by a laboratory scale RO water purification system, and the conductivity ranged from 0.76-1.26 μs cm-1 depending on the condition of the tap water. 2.2. Preparation of TA-CHD composite NF membrane The polyethersulfone support used for membrane fabrication was cut into 12*17 cm2 pieces in advance and stored in DI water for at least 24 h to remove the storing agents. To prepare the TA-CHD composite NF membrane, a piece of PES support was placed between a rubber frame and a plastic plate, which was further clamped by clips. To begin with, the residual water on the support surface was first removed by air sweeping, and a dry surface was obtained thereafter. Afterwards, 50 ml aqueous solution (phase Ⅰ) containing a certain concentration of CHD was first poured onto the substrate and remained for a fixed 2 min, followed by sweeping using an air knife for approximate 2 min at an angle of 45°. Following this, 50 ml of TA solution (phase Ⅱ) at a fixed 1.0 g L-1 concentration was rapidly poured onto the CHD adsorbed substrate, allowing the reaction to last for a certain time. Then, the excessive solution was removed and the membrane was washed with running water for 15s. Finally, the fabricated membrane was stored in DI water until test or characterization. Membrane preparation was repeated for at least two times for each set of conditions to reduce error. The TA-CHD membranes prepared at different CHD concentration are labeled as TA1.0-CHDx, where X stands for the concentration of CHD. The membrane preparation process was illustrated in Scheme I.

To investigate the pH value effect on membrane formation, the pH value of the CHD phase was pre-adjusted by 0.05M HCl solution monitored by a pH meter (Mettler Toledo, FiveEasy Plus FE28). 2.3 Characterization of TA-CHD composite NF membrane Chemical composition of TA-CHD active layer was measured by attenuated total reflectance infrared spectroscopy (ATR-IR), UV-Vis and X-ray photoelectron spectroscopy (XPS). XPS characterization were carried out on an ESCALab-250Xi spectrometer (Thermo Fisher Scientific) in terms of full spectra and detailed element spectra, with raw data collected after charge correcting with C1s. Peaks for different elements are fitted using free software CasaXPS in Marquardt method. UV-Vis spectra of films were obtained using a Shimadzu UV-2700 with an integrating sphere attachment (ISR-2600) using barium sulfate (BaSO4) as reference. ATR-FTIR spectra were collected on a Nicolet iS10 spectrometer (Thermo Scientific) with an attenuated reflectance unit. Membrane morphology of the surface was observed through scanning electron microscope (SEM) and atomic force microscope (AFM). SEM images of TA-CHD composite NF membrane and PES support were recorded by field emission scanning electron microscopy (FE-SEM, Hitachi S4800, Japan) with a 5.0kV accelerating voltage. For cross section observation, the membrane samples were cut and split in liquid nitrogen to obtain a well-defined cross section, and were coated with gold prior to SEM observation. For AFM analysis, membrane sample were cut into 1cm*1cm pieces and taped on a stainless-steel specimen holder. AFM images were scanned in dynamic mode using a tuned Si-C cantilever with an area of 10*10 μm on a Shimadzu SPM-9700 AFM instrument. Surface roughness was obtained and reported in terms of arithmetical mean deviation (Ra, nm). Hydrophilicity of membrane surface in terms of water-air contact angle was measured through sessile drop method by a DSA30 drop shape analyser (KRUSS, BmbH Co, Germany). Dynamic contact angle data was collected in a period of 50s. Surface charge distribution was analyzed by means of zeta potential, and zeta potential values of the composite membrane at different pH were obtained using a streaming potential analyzer (SurPASS, Aaton Paar, Austria). Membrane samples were cut into 1cm*2cm pieces by a cutter and taped on the measuring unit. Measurements were carried out in a 1.0 mM KCl aqueous solution with a pH range from

9.0 to 3.0. Data measurements were performed for 4 cycles at each pH point. Surface zeta potential was calculated according to the Helmholtz-Smoluchowski equation. 2.4 Permeation performance test Filtration performance of the TA-CHD membrane was assessed by pure water flux and rejection of different electrolyte. Various electrolytes including NaCl, CaCl2, MgSO4 and Na2SO4, were tested at a concentration of 2,000 mg L-1, respectively. Filtration test were performed on a cross-flow filtration apparatus with an effective membrane area of 18.50 cm2. The tests were conducted under an applied pressure of 10 bar at a flow rate of 6.5 L min-1. The temperature of feed solution was maintained at (25.0 ± 1.0) °C by a water-cooling machine. All of the experiments data were collected after the membrane was pressed for 1.0 hour until steady state was reached. Water flux (Jw, L m-2 h-1) was determined by measuring the permeate volume (∆V, L) over a certain time interval (∆t, h) and was calculated by Eq. (1) (1) where

stands for the effective area (m 2) of the membrane. The salt rejection (

calculated from the electrolyte concentration of the feed ( (

m

m

) was

) and permeate

), which were measured with a conductivity meter (Mettler Toledo FiveEasy, FE38)

and calculated by Eq. (2). 1-

1

(2)

Separation performance of neutral organic solute was evaluated to study the molecular weight cut-off (MWCO) of the TA-CHD membrane. Cross-flow filtration test was performed with 0.2 g L-1 PEG at different molecular weight, i.e. 200, 400, 600, 800, 1000. Concentrations of the feed and the permeate were determined using a total organic carbon analyzer (TOC-L CPH, Shimadzu, Japan), and the rejection rates were calculated according to Eq. (2). MWCO was defined to be the lowest molecular weight in which 90% rejection of the solute was reached. The mean pore size distribution of TA-CHD NF membrane was calculated based on the work of Singh et al.[31] and Michaels[32]. It was found that when the solute rejection (R, %)

correlates with the solute diameter according to log-normal probability function, the relationship of the two could be described as

in which

where the

stands for solute diameter,

geometric standard deviation of

for geometric mean diameter of solute and

for

. When the effects of steric and hydrodynamic interaction

were left out, the geometric mean diameter of the solute and its standard deviation were regarded as the same as the mean pore size (

) and the geometric standard deviation (

membrane. From the plot of the log-normal probability, corresponding to R=50%, and

refers to the ratio of

) of the

refers to the solute diameter at 84.13% and 50%. In this way, the

pore size distribution of the membrane could be described by the probability density function as following:

in which

stands for pore size in diameter.

For PEG, the Stokes radius of solutes used could be calculated from the following equation:

where M stands for molecular weight of the solute. 2.5 Membrane fouling test Pure water permeance (PWP) was monitored to characterize the fouling behaviour with the presence of foulant. The test was conducted using the cross-flow filtration apparatus mentioned above and bovine serum albumin (BSA) was chosen as the model organic foulant. The membrane was firstly pressed with deionized water for 3h at 10bar to reach steady state, and the

BSA was pre-dissolved and added online, reaching a concentration of 100 ppm. The water flux was measured at certain time interval. 2.6 Stability test Chemical stability of the TA-CHD membrane under acidic and alkaline pH was studied. The pH values were selected referring to the cleaning instructions of commercial reverse osmosis module as suggested by major membrane manufacturers. Instead of cleaning inline, the membrane samples were cut and separately immersed in pH=2 or pH=11 solutions for up to 2 hours, following by washing with running DI water. The treated membrane pieces were then characterized of its separation performance towards a 2,000 mg L-1 Na2SO4 solution under the same operation parameters as the separation performance test. The change of surface morphology and chemical composition were characterized by AFM, UV-vis and ATR-IR spectra, respectively.

Scheme 1 Illustration of the membrane preparation process from natural product tannic acid (TA) and cyclohexane-1, 4-diamine (CHD)

3. Results and discussion 3.1 Membrane formation mechanism study

As shown in Fig. 1, three possible reactions could appear between tannic acid and cyclohexane-1, 4-diamine due to the various reactive sites when quinone derivatives were formed under weak alkaline conditions[26, 33]. To study the reaction mechanism, the TA-CHD membrane prepared at monomer mass ratio of 1.0: 2.5 (TA1.0-CHD2.5) was characterized to investigate the chemical composition by ATR-IR, UV-vis and XPS, respectively. As shown in Fig. 2, the broad peak around 3400 cm -1 was attributed to the overlap of phenol group of tannic acid and the hydrogen bond between -OH and nitrogen from the diamine. The two weak peaks at 2949 and 2875 cm-1 belonged to the -CH2- group from the diamine CHD. The peak appeared at around 1675 cm-1 in the PES substrate spectrum was most possibly attributed to the entrapped pore-forming agent polyvinylpyrrolidone (PVP) during production[34, 35]. Comparatively, the peak for the TA1.0-CHD2.5 membrane in the same range was much wider, indicating an overlap of peaks. We then fitted the peaks using Gaussian function by the Multiple Peak Fit tool from the Origin software. After fitting, the overlap was then split into two peaks, which the red dash line belonged to the C=O peak derived from the quinone derivatives and the blue dash line was attributed to the residual PVP in the substrate. The characteristic C=N bond of Schiff base, C-N bond of Michael addition were hard to identify because of the strong background peaks of the PES substrate. UV-vis spectrometry was further used to demonstrate the existence of C=N. As shown in Fig. 3a, the blue line featured the TA1.0-CHD2.5 membrane and it exhibited a wide peak from wavelength 300 to 400 nm. Since the characteristic peaks of the C=N and C=O overlapped, the data was imported and plotted, and the peaks were fitted using Multiple Peak Fit function from the Origin software. In Fig. 3b, the peaks of 1-3 belonged to the π-π* transition of the aromatic ring and attached -SO2-, whereas peak 4 and 5 were attributed to C=N (near 326 nm) and C=O (near 344 nm), respectively[36]. These two peaks indicated the existence of both Michael addition and Schiff base formation mechanism. As both reaction routes will contribute to the C=O peak intensity, the higher area ratio of C=O (4.74%) than C=N (2.10%) indicates that Michael addition accounts for a greater part. This can be explained by the difference of molar

adsorption coefficient between different chromophores, and the value of C=N[37] was generally larger than C=O[38]. According to Beer-Lambert Law (

), the lower area ratio of C=N

peak would refer to much lower concentration, and this means TA and CHD molecules were more likely to be linked via C-N bond from Michael addition. XPS spectra of the composite NF membrane and PES support were measured to further study the contribution of the two reaction routes in membrane formation. As shown in Fig. 4a, compared with the PES substrate, the emerging peak at around 400 eV belonged to N1s of TA1.0-CHD2.5 membrane. High resolution spectra of C1s and N1s core levels were fitted as well (Fig. 4b-c). The peaks centered at 288.6 eV and 286.2 eV in C1s spectrum were assigned to the C=O and C-O from TA. Two new peaks at 286.5 eV and 285.5 eV belonged to the C-N and C=N bond generated from Michael addition and Schiff base formation, whereas the corresponding N-C=C and N=C peaks appeared at around 399.7 eV and 399.0 eV in N1s spectrum, respectively[39-42]. The fitted peak areas of C-N (27.5% in C1s and 13.2% in N1s) from Michael addition were significantly higher than those of C=N (8.8% in C1s and 6.4% in N1s) from Schiff base formation, which was due to a relatively higher reaction preference for former route of catechol type polyphenols[33]. From the results of ATR-IR, UV-vis and XPS analysis, we confirmed the existence of both two reaction routes, i.e. Michael addition and Schiff base formation, and the latter route accounted for a greater portion.

Fig. 1. Possible reaction mechanisms between TA and CHD

Transmittance

-OH -NH -CH2-

C=O peak from quinone C=O peak from additive 1800

1750

1700

1650

PES support TA1.0-CHD2.5 memrbane

3500

3000

2500

2000

1500

1000

-1

Wavelength (cm )

Fig. 2. ATR-IR spectra of PES substrate and TA1.0-CHD2.5 membrane. The inset graph shows the fitted peaks in the range from 1750 to 1640 cm-1 of the spectrum of TA1.0-CHD2.5 membrane. Peaks are fitted using Gaussian function.

Fig. 3. UV-vis spectra of (a) PES support and TA1.0-CHD2.5 membrane prepared at pH 8.5 and (b) fitted peaks of TA1.0-CHD2.5 membrane in the wavelength range from 200 to 450 nm.

Fig. 4. XPS spectra of (a) full spectra of PES support and TA1.0-CHD2.5 membrane and core levels of (b) C1s peak and (c) N1s peak of TA1.0-CHD2.5 membrane.

3.2. Investigation of membrane preparation parameters As a non-organic solvent method, there was no well-defined interfacial surface existing herein. It should be noted that, with the high reactivity of tannic acid in aqueous phase, direct co-deposition with both monomers blended together will form precipitate immediately (Fig. S4). During the stage of TA immersion, the pre-absorbed CHD would act as a cross-linker to diffuse and react with TA, which consequently formed a film. The membrane preparation conditions, i.e. concentration, pH value and reaction time, were optimized by evaluating the separation performance towards a salt solution containing 2 g L -1 Na2SO4. First, the effect of monomer concentration was studied. As shown in Fig. 5a, the salt rejection first increased with CHD concentration, from 82.47 ± 0.57% (TA1.0-CHD0.5) to 96.96 ± 0.53% (TA1.0-CHD2.5), whereas higher concentration resulted in a lower rejection and higher water flux. To study this, we investigated the UV-vis spectra of the membranes prepared with various CHD concentration using the peak fitting method mentioned previously (detailed figures of all fitted peaks could be seen in Fig. S1). As summarized in Table 1, it is found that the peak intensity of C=N and C=O (represented as peak area ratio, A%) change differently as CHD concentration increased. The intensity of C=N peak increased from 1.01% (TA1.0-CHD0.5) to 2.10% (TA1.0-CHD2.5), yet the intensity further decreased from 2.07% (TA1.0-CHD5.0) to 1.62% (TA1.0-CHD10.0). By contrast, the intensity of C=O increased drastically from 1.99% (TA1.0-CHD0.5) to 4.81% (TA1.0-CHD1.0), and it changed slightly with higher concentration of CHD. This could be explained by that, at higher diamine concentration, the intra/intermolecular hydrogen bond between the monomers would compete with the rather slow formation process of quinone derivatives, impeding the following Michael addition or Schiff base reaction. The low salt rejection when [CHD]= 0.5 and 1.0 g L -1 was due to the lower overall reaction degree as indicated by the low intensity of C=O peak. Thus, the concentration of monomer CHD was kept at 2.5 g L-1 for further investigation. The pH value of the amine solution has influences on both the oxidation of pyrogallol-type phenols into quinones and the ionization state of the diamine, affecting the formation of C=N or

aryl-aryl bond. As shown in Fig. 5b, as the pH value decreased from 11.30 (pristine solution) to 8.5, the Na2SO4 rejection first went up (from 83.02 ± 0.53% to 96.96 ± 0.53%), and then dropped as the pH value reached neutral state (82.68 ± 0.93% at pH=7). To explain this, the acid dissociation constant of trans-cyclohexane-1,4-diamine was calculated to be pKa1=10.78 ± 0.7 and pKa2=8.40 ± 0.3 via website tool ACD/I-Lab[43]. When pH was lower than pK a2, both nitrogen in the CHD molecule would be protonated, losing its nucleophilic reactivity and compromising the reaction degree between TA and CHD. On the other hand, higher pH level would lead to self-aggregation of tannic acid[44], which interfered with the formation of the selective layer. Therefore, the pH value of the diamine phase was set at 8.50. The effect of reaction time between TA and CHD was further investigated. As shown in Fig. 5c, extending the reaction time in certain range would enhance the Na 2SO4 rejection, whereas further enhancement would drastically decrease the water flux. This is due to the greater cross-linking degree as the reaction time is prolonged, which usually improves the rejection rate (98.24 ± 0.34% at 60s). Overall, the reaction time was determined to be 15s. To conclude, with the aim of obtaining high salt rejection at relatively low flux decline, the fabrication parameters for TA-CHD composite NF membrane were determined as following: [CHD]=2.5 g L-1, amine solution pH=8.50, reaction time=15s. Thus, the TA1.0-CHD2.5 membrane prepared at these conditions was used for further characterization and performance study. Table 1

UV-vis fitted peaks of C=N and C=O of the TA-CHD membranes [TA] / g L-1

[CHD] / g L-1

1.0

C=N

C=O

λmax / nm

A%

λmax / nm

A%

0.5

325.95

1.01

342.81

1.99

1.0

1.0

321.83

1.26

344.13

4.81

1.0

2.5

326.14

2.10

344.13

4.74

1.0

5.0

325.91

2.07

343.65

4.61

1.0

10.0

323.21

1.62

337.93

5.01

Fig. 5. (a) Water flux and rejection of Na2SO4 of TA-CHD membranes prepared at varied CHD concentration with a fixed 1.0 g L-1/ TA; (b) effect of pH value of CHD solution and (c) effect of reaction time on membrane performance.

3.3 Membrane characterization 3.3.1 Charge distribution

Since the surface charge of nanofiltration membrane would significantly influence the separation performance of differently charged ions, we investigated the charge distribution of the PES support and TA 1.0-CHD2.5 membrane under various pH values. As a phenol-rich chemical, the -OH groups attached to the aromatic ring of the tannic acid molecule contributed to its negative charge on the membrane surface. As shown in Fig. 6a, compared with the PES support, the TA1.0-CHD2.5 membrane exhibited relatively lower zeta potential over the whole examined pH range. The isoelectric point of the TA1.0-CHD2.5 membrane was 3.57 against 4.28 of PES support. 3.3.2 Hydrophilicity The hydrophilicity/ wettability of membrane surface has significant impact on its anti-fouling performance and water permeance, thus we further measured the water-air contact angle of the TA1.0-CHD2.5 membrane. As shown in Fig. 6b, the dynamic contact angle test showed the prepared TA1.0-CHD2.5 membrane had a lower initial contact angle (27.6 ± 0.1°) than that of PES support (54.9 ± 1.0°), exhibiting a better hydrophilicity. This was mainly due to the large amount of phenol group in TA. As time prolonged, the value for support decreased to 48.6 ± 1.0°, whereas the number slightly decreased to 25.5 ± 0.1° for TA1.0-CHD2.5 membrane. This could be explained by that the ultrafiltration support possessed a higher porosity and more larger pores, which would absorb water and result in a declined contact angle.

Fig. 6. (a) Dynamic contact angle of the PES support and TA1.0-CHD2.5 membrane; (b) Surface zeta potential of TA1.0-CHD2.5 membrane and PES support. Tests were performed with 1.0 mM KCl with a pH scan range 3-9.

3.3.3 Morphology Surface morphology, thickness and roughness of the TA1.0-CHD2.5 membrane were investigated, respectively. As illustrated in Fig. 6, compared with the bare PES substrate (Fig. 7a-b), the TA1.0-CHD2.5 membrane (Fig. 7d-e) exhibited a relatively rougher surface with some round nodules at different sizes. These surface structures contributed to the high roughness (77.5 nm) against the smoother support (14.8 nm) as confirmed by AFM height images (Fig. 7c, 7f). The results were supposedly due to the non-organic solvent method, where no interface between the monomers existed, thus allowing a rapid mix-and-react process, and this would likely to lead

to self-aggregation. Thickness of the selective layer of the TA1.0-CHD2.5 membrane was measured to be approximate 126 nm by hand from the cross-section SEM images. It is worth noting that the aggregates discovered on the membrane surface (Fig. 7d) were measured to exhibit a diameter of 174.4 ± 44.5 nm using the software ImageJ (Fig. S3), which is larger than the thickness. The existence of such particles on the membrane surface may contribute to the high salt rejection rates due to the additional steric hinderance.

Fig. 7. Morphologies of PES support (left) and TA1.0-CHD2.5 membrane (right), where (a) and (d) were surface morphologies, (b) and (e) were cross-section images and (c) and (f) were atomic force microscopy height images (10*10 μm2)

3.4 Membrane performance 3.4.1 Permeation performance of TA-CHD composite NF membrane Molecular weight cut-off (MWCO) of the TA-CHD membrane was determined using neutral organic solute PEG of varied molecular weight. Rejection rates against PEG molecular weight was plotted in Fig. 8a. By definition, the MWCO was determined to be the molecular weight when 90% of the solute was retained. As for TA-CHD membrane prepared at

concentration ratio 1.0/2.5, the MWCO was observed to be approximately 600 Da, which fell in the range of conventional nanofiltration membrane[45]. Pore size distribution is another important indicator for the prediction of membrane selectivity. The pore size distribution of membranes prepared at various monomer mass ratio were shown in Fig. 8b. As the diamine concentration increased from 1 g L-1, the curve became narrower and the peak moved towards lower diameter. This trend indicated that the effective pore for solute rejection became smaller, resulting in higher salt rejection (Fig. 5a). The curve moved slightly to the right when the diamine concentration reached 10 g L -1, and the corresponding Na 2SO4 rejection of TA1.0-CHD10.0 decreased to 91.12 ± 0.42%. This was possibly due to the hydrogen bond formation between CHD and TA molecules that influenced the film formation process, and this in turn resulted in a larger mean pore size. Desalination performance of different electrolytes was summarized in Fig. 9. The TA1.0-CHD2.5 membrane exhibited relatively higher rejection towards two sulfates, with Na2SO4 at 96.96 ± 0.53% and MgSO 4 at 89.26 ± 0.20%. The rejection for NaCl was 50.70 ± 1.22%, and due to a negatively charged surface as demonstrated by streaming potential analysis, the rejection for CaCl 2 was the lowest among all, at 40.54 ± 1.87%. The rejection of all electrolytes exhibited an order of Na 2SO4 > MgSO 4 > NaCl > CaCl 2. The higher rejection rate of sulfates compared with chlorides are mainly due to the electrostatic interaction between membrane surface and ions. According to Donnan exclusion, when membrane was negatively charged, electrolytes with bivalent anions were more likely to be rejected. Affinity of a negatively charged surface towards bivalent cations lead to inferior rejection of MgSO 4 and CaCl 2 compared with identical anion counterpart. The salt water flux for varied electrolytes were of the same level (approximate 35 L m-2 h-1, 10 bar), whereas the slightly lower value of MgSO 4 was probably due to coordination of Mg 2+ with the residual phenolic group of TA molecules, which cross-linked the chain network and led to a decreased flux. Performance comparison with literature reported polyphenol-based membranes can be found in supporting information (Table S1).

Fig. 8. (a) Rejections towards PEG of TA1.0-CHD2.5 membrane; (b) pore size distribution of the TA/CHD membranes prepared at different monomer mass ratio.

Fig. 9. Separation performance of TA1.0-CHD2.5 membrane towards various electrolytes.

3.4.2 Fouling test

The fouling propensity of the prepared TA1.0-CHD2.5 membrane was evaluated by measuring the water flux of a feed solution containing BSA as foulant. As shown in Fig. 10, slight flux decline appeared during stabilization stage with pure water, which is due to the compacting effect under hydraulic pressure. Water flux decreased significantly from 38.1 to 27.4 L m-2 h-1 upon foulant adding, and declined gradually in the following measurement. A stabilized water flux was reached at around 22.6 L m -2 h-1 after the 7 hours test, with a 40.6% overall flux decline compared with the initial value. It was estimated that the flux decline was due to the rough surface morphology. As surface roughness has a significant effect on fouling behavior[46, 47], the rough surface of TA1.0-CHD2.5 membrane (Ra=77.5 nm) would allow more open sites for the accumulation of foulant molecules, therefore contribute to the relative high propensity of fouling.

Fig. 10. Fouling performance of the TA1.0-CHD2.5 membrane tested under 100ppm BSA solution at 10 bar

3.4.3 Stability test To investigate the stability of the prepared TA-CHD membrane, we tested the separation performance after chemical cleaning. The cleaning conditions were selected similar to common membrane washing procedure to evaluate the potential of application of the TA-CHD membrane, and membrane samples were immersed either in an acidic solution (pH=2) or an alkaline solution (pH=11). As shown in Fig. 11a, after immersed in acidic (pH=2) aqueous solution for 2 h, the Na2SO4 rejections of TA1.0-CHD2.5 declined from 96.96 ± 0.53% to 92.15 ± 0.67%, which

was due to the fast hydrolysis rate of Schiff base derived from aliphatic amine in acidic pH value[48]. Further UV-vis characterization (Fig. 11b) confirmed this that the peak intensity of C=N and C=O from Schiff base formation decreased drastically after cleaned in pH=2 aqueous solution for 2 h. In contrast, the Na2SO4 rejection of the TA1.0-CHD2.5 membrane treated in alkaline (pH=11) solution changed relatively little (96.09 ± 1.74%), and UV-vis spectral line in the range of 300-400 nm was similar to the pristine membrane. It should be noted that the water permeance increases notably for the alkaline treated one, which was probably due to the hydrolysis of ester group (-O-C=O) within the tannic acid molecule in high pH condition. As mentioned previously, a high tendency of hydrogen bond formation existed between the phenol rich TA and the CHD, resulting in formation of sphere-like structure as observed from AFM images. It was estimated that under either pH value (2 or 11), the hydrogen bond would break, and the dissolved TA molecules and CHD molecules will be removed by cleaning. As a result, the amount of sphere-like structures decreased. As shown in Fig. 11c-d, the AFM height images of the cleaned TA1.0-CHD2.5 membrane exhibited that the amount of the spheres had decreased in both pH conditions, leading to lower surface roughness of 21.4 nm (pH=2) and 18.9 nm (pH=11). This phenomenon was further confirmed by UV-vis and ATR-IR spectra. As shown in Fig 11b, the intensity of peak Ⅰ and Ⅱ, which were attributed to the π-π* transition of the TA aromatic ring, dropped after treatment. ATR-IR spectra (Fig. S2) of the treated membrane also exhibited that the intensity of hydrogen bond decreased after chemical cleaning.

Fig. 11. (a) Membrane performance before and after cleaning in pH=2 and pH=11; (b) UV-vis spectra of chemical cleaned membrane, pristine membrane and substrate; AFM height images (10*10 μm2) of TA1.0-CHD2.5 membranes after immersion in (c) pH=2 and (d) pH=11, surface roughness is reported in arithmetical mean deviation (Ra).

4. Conclusions In this work, we explored using a natural product tannic acid to fabricate nanofiltration membrane. The preparation process was carried out in a non-organic solvent manner enabled by the solubility and reactivity of tannic acid. The disappearance of a phase interface promotes a

fast mix-and-react process, which was made possible by the highly reactive quinone derivative of TA. The membrane formation mechanism via Michael addition and Schiff base reaction was studied and confirmed by IR, UV-vis and XPS spectra, respectively. The as-prepared TA1.0-CHD2.5 composite NF membrane exhibit a Na 2SO4 rejection of 96.96% and a NaCl rejection of 50.70% with a MWCO at 600 Da, which was in the range of nanofiltration membrane. Chemical stability test of the membrane in two extreme pH showed the membrane maintained good salt rejection after treatment, and it was relatively more stable in alkaline condition than under acidic pH. Moreover, as the aim of reducing toxicity in chemical materials production has been persistently pursued, the non-organic solvent method devised herein provide an eco-friendly way to exploit the potential of nature-derived polyphenols in water purification membrane fabrication.

Conflicts of interest Declarations of interest: none.

Acknowledgements The authors gratefully thank the financial support from the following grants and funds: Fundamental Research Funds for the Central Universities (No. 15CX02015A, 16CX05009A, 18CX05006A), the National Natural Science Foundation of China (grant no. 21502227), the Province Key Research and Development Program of Shandong (No. 2016GSF115032), Qingdao Science and Technology Plan Project (176319gxx), and Shandong Province Major Science and Technology Innovation Project (2018CXGC1002).

Appendix. A Supporting information Supplementary figures associated with this article can be found in the supporting information.

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Abstract Graphic

Nanofiltration membrane was prepared via non-organic solvent method using natural product tannic acid.

Highlights:

    

Nanofiltration membrane prepared from nature-derived tannic acid and cyclohexane-1,4-diamine Formation mechanisms, i.e. Michael addition and Schiff base formation, were confirmed The membrane has 97% and 50.7% rejections for Na2SO4 and NaCl and a water flux at 35 LMH Membrane cleaning showed stability in both alkaline and acidic pH Membrane showed fouling tendency due to high surface roughness.