Dyes removal by composite membrane of sepiolite impregnated polysulfone coated by chemical deposition of tea polyphenols

Journal Pre-proof Dyes removal by composite membrane of sepiolite impregnated polysulfone coated by chemical deposition of tea polyphenols Zexian Zhu, Dongqing Liu, Sizhou Cai, Yankun Tan, Jing Liao, Yuanjian Fang

PII:

S0263-8762(20)30057-5

DOI:

https://doi.org/10.1016/j.cherd.2020.02.001

Reference:

CHERD 3987

To appear in:

Chemical Engineering Research and Design

Received Date:

6 November 2019

Revised Date:

24 January 2020

Accepted Date:

1 February 2020

Please cite this article as: Zhu Z, Liu D, Cai S, Tan Y, Liao J, Fang Y, Dyes removal by composite membrane of sepiolite impregnated polysulfone coated by chemical deposition of tea polyphenols, Chemical Engineering Research and Design (2020), doi: https://doi.org/10.1016/j.cherd.2020.02.001

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Dyes

removal

by

composite

membrane

of

sepiolite

impregnated polysulfone coated by chemical deposition of tea polyphenols

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Zexian Zhua, Dongqing Liu*a, Sizhou Caia, Yankun Tanb, Jing Liaoa, Yuanjian Fanga

a State Key Laboratory of Separation Membranes and Membrane Processes, School of Material Science and Engineering, Tiangong University, 300387 Tianjin, China

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b Yiwu Huading Nylon Co.Ltd., 322000 Jinhua, Zhejiang, China

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*Corresponding author at: State Key Laboratory of Separation Membranes and Membrane Processes, School of Material Science and Engineering, Tiangong University, 300387 Tianjin,

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China

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*Email address: [email protected]

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Research Highlights

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·Pure water flux of polysulfone membrane was increased greatly by sepiolite and CTAB. ·A mixture of tea polyphenols could prepare effective separative layers though chemical deposition with MTM and PEI. ·PT membrane showed more than 98.6% rejection for CR and NR under the water permeance about 3.0 L·m-2·h-1·bar-1 over 200 h at 1 bar. ·The thickness of the functional layer and the chargeability of membrane surface could be adjusted by amine monomer.

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Abstract: Two multi-amine, metformin (MTM) and polyethyleneimine (PEI), were used to prepare separative layer of composite membrane through chemical deposition with tea polyphenols (Tph), an inexpensive mixture of nature polyphenols, on a hybrid polysulfone (Psf) membrane. The supporting layer was a blend membrane of Psf, coupling agent treated sepiolite and cetyltrimethylammonium bromide (CTAB). Sepiolite and the surfactant improved surface hydrophilicity of membrane. Pure water flux (PWF) of membranes was increased from 33.97 to

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611.46 L·m-2·h-1·bar-1. The optimum mass ratio for MTM to Tph and PEI to Tph were 9.4:1 and

2.4:1 in chemical deposition reaction, respectively. MTM-Tph (MT) surface was negative charged while PEI-Tph (PT) was positive charged. Monomer reactivity determined the thickness of

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functional layer. Both composite membranes showed dyes rejection capacity at 1 bar. PT layer was

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thicker than MT, so did its rejection for dyes. The rejection of PT for 5 dyes were all higher than

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98% and remained at 99.3% and 98.6% in 200 h for Congo red (CR) and neutral red (NR). PT membrane displayed the potential in textile waste water treatment.

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Keywords: Composite membrane, Sepiolite, Chemical deposition, Tea polyphenols

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1. Introduction

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Organic chemicals contained waste waters, especially textile waste water, are conundrum in industry area recent years. Tremendous attempts were applied including adsorption, coagulation, electrochemical techniques, oxidation and biodegradation [Hannachi et al., 2019; Sun et al., 2017; Cui et al., 2017; Song et al., 2019; Fonseca et al., 2018]. Nanofiltration (NF) technology reveals a competitive separation and purification process in waste water treatment [Zhao et al., 2018; Liu et al., 2017]. 2

However, efficiency of NF membranes could be severely limited by fouling and concentration polarization [Lin et al., 2015]. Fouling is commonly caused by formation cake layer of organic compounds and pore blocking, which at last declines membrane permeability [Esfahani et al., 2019]. In addition, high salinity in textile water plays a negative role on membrane flux, considering the high osmotic pressure

the treatment of textile waste water under large flux.

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caused by salt rejection. Therefore, hydrophilic surface of NF membrane is desired in

To this purpose, a burgeoning membrane technology, loose nanofiltration (LNF) membrane, exhibits potentials in this complicated water treatment area. LNF

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membranes generally possess large pore size and charged groups, such as sulfonic

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acid group or quaternary ammonium group. Therefore, charged films can remove ionic dyes while allowing salts to pass through [Wang et al., 2018; Liu et al., 2014].

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Positive charged species have showed rejection not only to cationic dyes [Peydayesh

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et al., 2018], but also to some salts, such as CaCl2 and MgCl2, while maintaining a reasonably high permeability [Cihanoğlu et al., 2018].

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Mussel-simulated chemistry shows great potential in preparation of composite LNF. The main reaction is co-deposition between polyhydroxyl phenols and

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polyamines in aqueous media, involving Michael addition and Schiff base reactions [Qiu et al., 2018]. Co-deposition is easily operated and the obtained functional surfaces are more hydrophilic than the usual product of interfacial polymerization (IP), which is benefit for fouling resistance to the surface layer of composite membrane. The initial monomer was dopamine, which possesses an amine and two hydroxyl 3

groups in the same molecule [Lee et al., 2007]. The product of dopamine self-polymerization is polydopamine, which promoted surface hydrophilicity, water permeation flux and anti-fouling property of membrane [Yang et al., 2018; Wang et al., 2018; Zhu et al., 2018; Lv et al., 2015]. Subsequently, the couple of catechol and PEI were confirmed an effective and economical coating. Qiu [Qiu et al., 2015] co-deposited this couples on the surface of polypropylene micro-filtration membrane

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and obtained a LNF membrane to remove dyes from simulated textile waste water. Xu [Xu et al., 2016] used a more hydrophilic base, polyacrylonitrile, and higher monomer

concentration to achieve a composite nanofiltration membrane, which showed 80%

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MgCl2 rejection. Zhang [Zhang et al., 2019] employed epigallocatechin gallate

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(EGCg) and PEI to react on PES ultrafiltration membrane. The composite membrane revealed excellent rejection towards dyes of different charges, such as CR (99.0%)

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and bromocresol green (98.7%).

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Skins prepared by chemical deposition are also looser than those by IP method. Therefore, they provide higher water permeation. There are two effective strategies

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for fabricate LNF membranes beside increasing its hydrophilicity, including: i) improving water permeation of support base; ii) loosening or thinning functional layer.

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For support layer, thickness and pore structure are crucial for the water permeation. Meanwhile, membrane will achieve ultrafast solvent permeance when the thickness of skin layer is reduced below 10 nm [Karan et al., 2015]. Here, two aspects were attempted including employ inorganic particles to increase hydrophilicity of supporting membrane and chemical deposition to obtain 4

loose functional layer. Sepiolite, a magnesium silicate, had reported to adsorb hazardous heavy metal ions, such as Cd2+ and Cr3+ [Kocaoba, 2009]. As a natural inorganic material, it has potential to adjust hydrophilicity of membrane. Hence, sepiolite particles were mixed with CTAB and Psf after treated by coupling agent to prepare blend membrane. Tph is a mixture of natural polyphenols exacted from tea leaves, which is

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composed of 16 chemical compounds, mainly including flavanols, flavanones, flavonols and phenolic acids. All of them have at least a phenol group [Ai et al., 2019;

Grzesik et al., 2018], which have the potential to perform chemical deposition with

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polyamines under the mechanism of mussel-simulated chemistry. Each pure

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component in Tph is very expensive, which limited practice application of the approach. Furthermore, non-reaction component would be washed out and leave

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effective product on the supporting layer. Here, Tph was used as the main substrate to

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react with MTM and PEI on the skin of sepiolite impregnated Psf membrane to prepare composite membranes. Density of aromatic rings could be increased by

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polyphenols of Tph, which provided superiority to skin layer in removal of dyes and

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salts, especially from the economic consideration.

2. Experimental 2.1 Materials Psf was provided by Solvay Co., Ltd. 200 Mesh sepiolite powder was supplied by Hebei Hongyao Mineral products processing Co., Ltd. Congo red (CR, AR), 5

neutral red (NR, AR), acid chrome blue K (ABK, AR), crystal violet (CV, AR), methylene blue (MB, AR), Pb(NO3)2, MgCl2, MgSO4, NaCl, Na2SO4, humic acid (HA) and (NH4)2S2O8 were purchased from Tianjin Guangfu Fine Chemical Research Institute. Branched polyethylenimine (PEI, Mw=600), Tris(hydroxymethyl)amino methane (Tris) and coupling agent KH792 were supplied by J&K Scientific Ltd. N,Ndimethylacetamide (DMAc) and polyethylene glycol (PEG, 1000-6000) came from

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Tianjin Kemiou Chemical Reagent Technologies Co., Ltd. Cetyltrimethylammonium bromide (CTAB) and metformin hydrochloride (MTM) were purchased from Adamas

Co., Ltd. Tea polyphenols (Tph, Cas: 84650-60-2, purity=98.65%) was provided by

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2.2 Membrane Preparation

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supplied by Shanghai Aladdin Reagent Co.

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Shanghai Civi Chemical Technology Co., Ltd. Bovine serum albumin (BSA) was

2.2.1 Preparation of base membrane

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Sepiolite powder was immediately poured into icy water after heated at 300 ℃ for

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3 h. After 30 min violent stir, the slurry was filtered by 1 μm film and the filtrate was centrifuged at a speed of 10000 rpm. The solid layer was then dried at 60 ℃ for 6 h to

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achieve efflorescent sepiolite. Efflorescent sepiolite (2 g) was dispersed in 200 mL mixture of water and ethanol (V:V=1:1). After 30 min ultrasonic treatment, 10 g KH792 was added and refluxed for 8 h. The slurry was centrifuged at a speed of 10000 rpm and the solid layer was dried at 60 ℃ for 6 h to obtain KH792 treated sepiolite (K-sepiolite). 6

Psf 3 g, K-sepiolite 0.15 g and 16.85 g DMAc were agitated together to prepare casting dope. It was then vacuum defoamed for 12 h at room temperature and scrapped into film by 200 μm thickness coating stick. Membranes were immersed in 500 mL deionic (DI) water. The water was changed for 3 times until no UV-vis signal detected. The membrane was named Psf-sepiolite (PSE) membrane. For Psf-sepiolite-CTAB (PSEC) membrane, the recipe of casting dope was

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comprised of 0.15 g CTAB, 3 g Psf, 0.15 g K-sepiolite and 16.7 g DMAc. 2.2.2 Preparation of composite membrane

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A series of mixed solution of Tph-MTM or Tph-PEI (20 mL) were shaken in

UV-vis to determine the optimum ratio.

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capped vials at 30 ℃ of different mass ratios. The resulted mixtures were tested by

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0.3 g Tph and 2.82 g MTM were dissolved in 100 mL DI water and the pH was adjusted by Tris and HCl aqueous solution to 8.5. PSE and PSEC membranes (35

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mm×35 mm) were immersed in the mixture and shaken for 6 h at 30 ℃. Membranes were rinsed by DI water until no monomer signal was detected by UV-vis. The film

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was then soaked in DI water for use and named MT membrane.

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For Tph-PEI composite membrane, 0.3 g Tph and 0.72g PEI were dissolved in 100 mL DI water. The pH was adjusted by Tris and HCl aqueous solution to 8.5. The functional layer was fabricated according to the procedure of MT membrane. The membrane was named PT membrane.

2.3 Instrument 7

XPS analyses was performed by a Thermofisher K-alpha spectrometer using a focused mono chromatized Al Kα radiation. ATR-IR spectroscopy was measured by Nicolet6700 infrared spectrometer (Thermo Fisher, USA). FE-SEM (S-4800, Hitachi, Japan) was used to collect surface and cross-section morphology of membranes. Dynamic water contact angel (DWCA) was tested by a contact angle meter (Drop Shape Analysis 100, Kruss BmbH Co., Germany). The tensile strength of membrane

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was investigated at room temperature using a tensile tester (JBDL-200N, China) at a drafting speed of 1 mm/min. Transmission electron microscope (TEM) images were acquired through a Hitachi H7650 transmission electron microscope with an

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accelerating voltage of 100 kV. Membrane surface charge properties were

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characterized by streaming zeta potential measurements with a SurPASS electrokinetic analyzer (Anton Paar, Austria). UV-vis of aqueous solutions were

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measured by a ultraviolet spectrophotometer TU-1901 (Beijing Purkinje General

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Instrument Co., Ltd.). High performance liquid chromatography- mass spectrometry (HPLC-MS) was provided by a liquid chromatography- high resolution four-pole time

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of flight tandem mass spectrometer (miorOTOF-QII, Brooke Dalton Co., USA). Average pore sizes of blend membrane was tested by an automatic mercury

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porosimeter (Auto pore IV9500, McMurray Instrument Co., Ltd., USA). Total organic carbon analyzer (TOC-VCPH, SHIMADZU, Japan) was used to analyze the concentration of PEG solutions.

2.4 Membrane performance

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2.4.1 Dyes and inorganic salts rejection Dyes including 100 mg·L-1 CR, NR, ABK, CV, MB solutions and 1 g L-1 salt solutions of Pb(NO3)2, MgCl2, MgSO4, NaCl and Na2SO4 were chosen to operate filtration tests. The concentrations of dyes and salts solution were determined by UV-vis and conductivity meter, respectively. Permeation (J) and rejection (R) were

J=

Cp ) ×100% (2) Cf

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R= (1-

V (1) AΔt

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calculated by Eq. (1) and (2):

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Where V (L) is the permeation volume, A (m2) is the effective membrane area (7.07×10-4 m2), Δt (h) is operation time, Cp (g·L-1) and Cf (g·L-1) are the

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concentrations of permeation and feed solutions, respectively.

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2.4.2 Long-term performance test

Long-term filtration by membrane was conducted to investigate permeance

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stability using 100 mg·L-1 CR and NR and 1g·L-1 MgCl2 and Pb(NO3)2 aqueous

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solutions under total recirculation mode. Water permeation and rejection were recorded per 10 h in 200 h. 2.4.3 Facationation of NR and Na2SO4 by composite membrane Fractionation of dye and inorganic salt was conducted using NR-Na2SO4 (0.1 g·L-1-1 g·L-1) blend aqueous solution under a total recirculation mode. Water flux and solute rejection were recorded per 2 h in the first 10 h and then once per 10 h in the 9

later 190 h. 2.4.4 Anti-fouling performance Anti-fouling performance of membrane was evaluated flux recovery ratio (FRR) of membrane. FRR was evaluated by filtration test of 2 100 mg·L-1 dye solutions, MB and CV, 1 g·L-1 MgCl2 solution, 1 g·L-1 HA solution (pH=5.5) and 1 g·L-1 BSA Tris solution (pH=9.8). Filtration test of 0.1 g·L-1 (NH4)2S2O8 aqueous solution (pH=5.8)

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was used to evaluate the antioxidant capacity of PT membrane. Rejection for dye and

MgCl2 solutions was recorded every 2 h. After every 10 h filtration, the membrane

was washed by DI water for 2 h. The whole test included three filtration and washing

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cycles. BSA Tris solution was composed of 1 g BSA and 1 L 1.2 g·L-1 Tris solution.

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The membrane was initially rinsed by DI water and replaced by HA solution or BSA buffer solution. Water flux was recorded once an hour in 20 h and rinsed by DI water

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for 1 h. FRR was calculated by Equation (3):

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FRR= (

Jw.2 ) ×100% (3) Jw.1

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Where Jw.1 and Jw.2 are the initial and final pure water flux, respectively.

2. Results and discussion

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The process to prepare the LNF composite membrane was graphed in Scheme 1.

Commercialized sepiolite was weathered to prepare nanoparticle. The

efflorescent sepiolite was treated by coupling agent to improve its compatibility with polymer and then mixed with Psf and CTAB to prepare blend membrane. This membrane was used as supporting layer to fabricate composite membrane through 10

chemical deposition of Tph and MTM or PEI600. Structure of the two composite membranes was characterized and their LNF performance were evaluated in this work.

3.1 Characterization and evaluation of blend membrane

To improve the compatibility between inorganic particle and polymer, coupling

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agent KH792 was used to prepare K-sepiolite. KH792 has a primary and a secondary amine groups, which could introduce positive charges to membrane and further

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increase hydrophilicty. Characterization data of K-sepiolite were showed in Fig. S1.

Table 1 provided formula of blend membranes. K-sepiolite and cationic

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surfactant, CTAB, were investigated to improve membrane hydrophilicity. K-sepiolite

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slightly increased average pore size from 108 nm to 155 nm, but largely improved PWF from 33.97 to 513.52 L·m-2·h-1·bar-1. K-sepiolite helped to build more

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hydrophilic channels inside hydrophobic polymer matrix, which facilitated water permeation. CTAB greatly enlarged pore size to 488 nm but moderately increase PWF

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to 611.46 L·m-2·h-1·bar-1. Pore size data reflected properties of whole matrix of

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membrane, but not merely skin layer, though the later played great role on permeability. It was also observed that Si element uniformly distributed in the cross-section of a PSEC membrane by SEM-EDS (Fig. S2). The space propped up by sepiolite was hydrophilic owing to its strong polarity, which was benefit for water permeation.

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Surface properties of membranes were characterized by ATR-IR, XPS and DWCA. In ATR-IR spectra, both PSE and PSEC curves possessed tiny peaks at 1648 cm-1, which were attributed to N-C bonds in KH792 (Fig. 1a). There revealed N and Si element peaks in XPS spectra of PSE and PSEC (Fig. 1b), which came from sepiolite and KH792. PSEC showed Br element peak and the intensity of Si peak declined. CTAB decreased surface tension of Psf membrane and sank sepiolite into

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the bulk matrix. Therefore, Si peak declined. N content was further increased by

CTAB in PSEC though large amount of CTAB effused into coagulation bath during

immersion. CTAB was detected in used coagulation bath by HPLC-MS (Fig. S3).

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DWCA of surface decreased from 85.53° of pristine Psf to 70.72° of PSE and 53.67°

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of PSEC (Fig. 1c). Both PSE and PSEC membranes provided higher water permeation and solute rejection than Psf membrane in filtration of MB and Pb(NO3)2

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(Fig. 1d). Sepiolite and CTAB improved water permeation through increasing both

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surface hydrophilicity and pore sizes. CTAB is an amphiphilic surfacant. Its hydrophobic part, the hydrocarbon chain, could complex with hydrophobic part of Psf

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through hydrophobic-hydrophobic interaction in casting solution. The hydrophilic part of Psf also combined with ammonium groups at the same time. CTAB could

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adjust molecular aggregation and orientation of Psf in the solution. During phase conversion process, the hydrophilic part of CTAB tended to diffuse toward the channels of water flowing through. It also dragged Psf molecules and helped them to distribute orderly through intermolecular interaction. As a result, ammonium groups were left on the surface of membrane and pore walls were formed by solution 12

exchange. Meanwhile, the hydrocarbon chains stick inside the bulk polymer. Thus, the surface hydrophilicity was improved by addition of surfacant. Furthermore, amine of KH792 and ammonium of CTAB could also endow membranes with electrostatic repellent to cations.

The morphology of membranes was observed by SEM (Fig. 2). There are many

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sepiolite particles on PSE membrane surface. Surfaces of pristine Psf and PSEC membranes were smooth. Sepiolite particles were buried in the bulk membrane as

CTAB addition. It was agree with the small peak of Si element in XPS curve (Fig. 1b,

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in blue). The thickness of three membranes were 68 μm, 70 μm and 111 μm, respectively. Since hydrocarbon chain of KH792 was far shorter than the diameter of

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sepiolite particle, K-sepiolite could be supposed to be incompressible. The voids

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between polymer chains were increased by the insertion of K-sepiolite. Therefore, the total volume of mixture was enhanced. Film area was certain under a fixed scratching

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condition, which resulted to increase of membrane thickness. In addition, CTAB decreased surface tensile of polymer phase, which produced more surface or interface.

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The pore walls was thinned and larger voids were emerged. The skin layer thicknesses

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of Psf membrane was 0.9 μm. It was hard to observe the thickness of skin layers of PSE and PESC membranes, which were substituted by a loose sponge-like layer. Therefore, their water flux significantly increased comparing to Psf membrane. Tensile strength of PSE film was greater than Psf’s and PSEC’s (Table 1, Fig. S4). K-sepiolite and CTAB both loosened the solid structure of certain mass of polymer. The pore wall of PSEC membrane was thinner than PSE’s so its tensile strength was 13

also lower.

3.2 Characterization of composite membrane

Tph is a natural mixture of flavanones, flavonols, flavanols and phenolic acids, including 16 compounds (Table S1). Among them, there are 13 polyphenols and 3 single phenols, which have the ability to react with amines under the mechanism of

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Michael addition and Schiff base reaction. Chemical deposition between such a

mixture and polyamine could produce hydrophilic surface, which was effective for

charged molecules removal according to mussel-stimulated chemistry mechanism.

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PEI and MTM were investigated as candidates of polyamine to prepare functional

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layer of composite membrane (Scheme 1). The group of -C=NH in MTM had three resonance states as shown in Scheme 2. It hence served more than one NH2 groups to

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construct network with Tph.

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The optimum ratios of Tph/polyamines were determined by UV-vis. For MT-Tph system, the absorption at 232 nm was graphed and the peak was the mass ratio of

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9.4:1 (Fig. 3a). The inset showed that the maximum was slightly hypsochromic shift.

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The absorption at 206 nm was chosen for PT-Tph system. The apex was 2.4:1(Fig. 3b). The values were further confirmed by dye CV filtration tests by composite membranes. It could be deduced from the optimum ratios of the two series that effective positions to react with PT in Tph were more than those with MTM. The highest rejection for MT-Tph membranes was 97.5% and the lowest water permeantion was 62.39 L·m-2·h-1·bar-1 (Fig. 3c). The best rejection for PT- Tph series 14

was 99.5% and the lowest permeation was 3.81 L·m-2·h-1·bar-1(Fig. 3d). Monomer concentration had great influence on separative layer. The detail data were listed in Fig. S5. Tph concentration was finally decided as 3 g·L-1. In the filtration of MB, NR and CR, rejection of PSE composite membrane were 98.6%, 99.4% and 99.6% under the water permeation of 2.77 L·m-2·h-1·bar-1, 3.08 L·m-2·h-1·bar-1 and 2.82 L·m-2·h-1·bar-1, respectively. Rejection by PSEC composite membrane was almost the

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same, but water flux were higher than PSEs’, as 3.29 L·m-2·h-1·bar-1, 3.56 L·m-2·h-1·bar-1 and 3.22 L·m-2·h-1·bar-1 separately (Fig. S6). Hence, PSEC was chosen

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as a base membrane in the work below.

Fig. 4 showed the surface properties of composite membranes. In ATR-IR curve

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of MT, the broad weak peak at 3420 cm-1 was the overlap of NH2 and OH stretching

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(Fig. 4a) while the counterpart in PT curve was strong. Peaks at 1661 cm-1 were stretching vibration of N-C and N=C bonds. The gen-dimethyl peak of Psf at 1300

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cm-1 was shielded by coating layer. In XPS spectra (Fig. 4b), N element peaks increased by chemical deposition. The high resolution of N1s showed that there were

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peaks of NH+3 at 401.88 eV in both MT and PT membranes (Fig. 4c, d), which

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derived from protonated NH2. PT has more NH+3 than MT owing to more NH2 in PEI molecule. MT membrane had -C-N=C- peak at 399.58 eV, which was formed following the mechanism in Scheme 2. DWCA of MT decreased to zero within 160 s and PT’s within 90s (Fig. 4e). Both membranes were superhydrophilic owing to plenty of hydroxyl and amino groups, even COOH, on their surfaces. The isoelectric point of MT and PT membrane surfaces were 4.1 and 7.4, respectively (Fig. 4f). It 15

revealed that MT surface was negative charged and PT’s was positive charged. The solution pH of 3 g·L-1 Tph is 3.7 so it is acidic. MTM could not provide enough alkalinity to overcome the influence of Tph on surface electrical property. PT surface possessed enough NH+3 groups to offset the influence of phenol hydroxyl and carboxylic acid groups from Tph, which dissociated into -O- and -COO-. The result

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was agree with XPS data above.

SEM images displayed detail structure of membranes (Fig. 5). Particles on PT surface were more than MT’s, which came from uneven accumulation of cross-linked

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polymer during shaking. In cross-section images, it could be observed that chemical

deposition achieved flatter surface than base membrane, except those particles.

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Surface layer of PT was 140 nm while MT’s was 40 nm. Oscillation and deposition

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filled the ravines on the surface. The molecular weight of PEI600 is larger than MTM’s (129 Da), and meanwhile PEI is highly branched, which conduced to PT

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polymer blocking pores more easily than MT’s. Moreover, effective positions of PT to react with Tph were more than MTM’s, which meant that PT polymer should be

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denser and more easily to stick on the wall of pores. The molecular weight cutoff data

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also confirmed that PT could intercept PEG2000 and MT could reject PEG6000 (Table 2). It was confirmed that the mass of PT polymer was largely more than those of MT’s using 100 mL 3 g·L-1 Tph. Therefore, PT polymer seeped deeper inside sponge like pores of PSEC. The network fibers of PSEC were totally coated by PT layer and pores were narrowed. Therefore, PT skin was thicker than MT’s, which finally led to its higher rejection and lower water permeation. 16

3.3 Performance of composite membrane

Fig. 6 was dye filtration performance of composite membranes. The largest water permeation of MT membrane was 72.36 L·m-2·h-1·bar-1 for ABK among five dyes (Fig. S7). Its rejection was rightfully the smallest, 58.8%, though ABK was an anionic dye. Rejection to CR, CV, MB and NR were 98.2%, 97.5%, 87.0% and 69.1% under

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the flux around 60 L·m-2·h-1·bar-1, respectively. PT removed all dyes at the efficiency more than 98.5% from their solution under the flux about 3.30 L·m-2·h-1·bar-1. The difference between MT and PT membranes was mainly resulted from three points: (i)

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PT’s functional layer was thicker than MT’s, which decreased water flux and increased dyes rejection; (ii) PEI possessed longer molecular chain and more reacting

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position (-NH2 group), which made functional layer more compact; (iii) PT layer

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possessed more NH+3 than MT layer, which provided stronger repellent to cationic

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dyes.

Fig. 7 was salt filtration performance of composite membranes. The water

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permeance of MT film was at the range of 100~110 L·m-2·h-1·bar-1, which were far greater than PT’s, 3.5~4.0 L·m-2·h-1·bar-1. Its rejection to all 5 salts were lower than

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10%, which decreased according to the order of Pb(NO3)2＞Na2SO4＞MgSO4＞ MgCl2＞NaCl. This indicated that MT had an anionic charged functional layer [Bai et al., 2019]. Salt rejection of PT membrane was greatly different from MT’s, which decreased in the order of Pb(NO3)2＞MgCl2＞NaCl＞MgSO4＞Na2SO4. It was a behavior of positive charged membrane [Fang et al., 2018], which showed higher 17

repellent to divalent (Mg2+) than monovalent cations (Na+). What is more, the hydraulic diameter of two Cl- anions is greater than a SO42- group. These led to the largest rejection to MgCl2 and the lowest to Na2SO4. Pb(NO3)2 removal rate by PT film was 81.4% while by MT was 9.1%. The result further illustrated that PT was more compact and effective for the removal of hazardous substances than MT. In the later work, PT composite membrane was further investigated about longtime

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operation stability and fouling resistant ability.

Long time filtration of dye and salt solutions were operated to evaluate

membrane stability. PT membrane revealed operation stability in CR and NR filtration

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within 200 h (Fig. 8a). The molecular weight of CR was the largest and NR’s was the

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smallest among 5 dyes. The initial rejections were all 99.3% for CR and NR in the first 120 h. Then removal rate of NR tented to continuously decrease until 98.6%

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while CR rejection was stable all the time. Water flux slightly decreased at first and

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then remained at 2.85 L·m-2·h-1·bar-1 for CR and 3.36 L·m-2·h-1·bar-1 for NR. Pb(NO3)2 rejection remained at 62.9% in 24 h and then quickly dropped (Fig. 8b).

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After pure water wash, the performance was not recovered. It was believed that Pb(II) could slowly complex with N in composite layer during filtration and finally

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destroyed the surface chemistry. MgCl2 removal rate remained at 43.9% in 200 h. The water flux of MgCl2 was slightly higher than Pb(NO3)2’s. During filtration of blend solution NR-Na2SO4, rejection to NR was stable at 97.3% in 200 h (Fig. 8c) while Na2SO4 rejection dropped from 41.7% to 11.4% in 2 h and remained. PT membrane was very stable for dyes removal, but its rejection for 18

salts decreased after a period of time. It was believed that long time interaction between metal cations and PT layer led to rejection decline. PT membrane displayed the capacity to fractionate dye and inorganic salt from their solution.

.

Anti-fouling performance of PT membrane were employed using MB and MgCl2

ro of

solution. MB rejection continuously decreased from 98.6% to 86.9% in 10 h. After 2 h DI water wash, it recovered to the original value (Fig. 9a). After three circles running, the performance repeated in the same mode and the flux slightly fluctuated. It

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demonstrated that PT membrane had anti-fouling ability in dye removal. However, the

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recovery rate of MgCl2 rejection was only 87.0% after 2 rounds (Fig. 9b). It was believed that interaction between Mg2+ and OH of Tph destroyed the skin structure in

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a long period of time. The rejection to CV was stable at 98.5% in the first 10 h and

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dropped to 87.7% in the next 10 h. After 2 h water wash, the rejection surprisingly returned to 99.4% and kept, just like for CR and NR (Fig. 9c). It was believed that

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there was intermolecular interaction between CV and PT layer. CV molecules probably embedded into PT network and adjusted their orientation under the scour of

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current. As a result, π-π stacking was strengthened, which increased the rejection to CV [Hou et al., 2018]. Figure 9d provided FRR of PT membrane after a round of filtration-DI water rinse for HA and BSA. Anti-fouling ability of the super-hydrophilic surface was excellent and FRR of PT for BSA Tris solution and HA aqueous solution were 19

separately 93.1% and 90.2 %. FRR of (NH4)2S2O8 filtration was 96.5%, which illustrated that PT showed the antioxidant capacity to some extent. In a summary, sepiolite is a good candidate for improvement of membrane hydrophlicity. The combination of sepiolite and CTAB thinned the skin layer of asymmetric Psf membrane, which resulted to a large pure water flux. As a mixture of multiple phenols, Tph showed great reactivity and performance in mussel-stimulated

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chemistry. Composite membranes possessed hydrophilic skin layer, whose electrical property could be tuned by species of polyamine. The reaction of Tph and PEI

prepared a compact skin layer suitable for removal of many dyes with a high water

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permeation under 1 bar.

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4. Conclusions

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The performance of composite membrane was determined by both base

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membrane and composite layer. Supporting layer of good permeability provides composite membrane with high water flux. Functional layer structure decided the

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rejection for solutes, including dyes and salts. Sepiolite obtained better compatibility with Psf after treated by KH792 and increased water flux of Psf membrane. CTAB

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further regulated membrane structure and increased surface hydrophilicity, hence increased PWF from 33.97 L·m-2·h-1·bar-1 of pristine Psf membrane to 611.46 L·m-2·h-1·bar-1 of PSEC membrane. MT and PT membranes fabricated through Tph chemical deposition with MTM or PEI both contained super-hydrophilic surfaces. MT film has a negative charged skin layer while PT has a positive charged. MT film 20

provided higher water flux and lower rejection to dyes and salts, except CV or CR. The rejection of PT for 5 dyes were all higher than 98.5% and salts removal rates were 81.4% for Pb(NO3)2 and 60.5% for MgCl2. The salt rejection capacity was MgCl2＞NaCl＞MgSO4＞Na2SO4. Rejections to CR and NR were kept 99.3% and 98.6% in 200 h. The rejection recovery test also confirmed that PT showed anti-fouling ability. The process of chemical deposition is convenient to operate and

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Tph displays the potential in preparation of hydrophilic functional layer of low

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pressure composite membrane for dye removal from waste water.

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Acknowledgements

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There are no conflicts to declare.

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Declaration of Interest Statement

The authors acknowledge the financial support from the National Natural Science

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Foundation of China (Grant No. 51373119) and Tianjin Key Projects of New

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Materials Science and Technology (17ZXCLGX00050).

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Figure captions

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Fig. 1. ATR-IR curves (a), XPS spectra (b), DWCA (c) and performance of three membranes (d).

27

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Fig. 2. SEM images of membranes.

Fig. 3. UV-vis spectra of MTM-Tph blend solution (a) and PEI-Tph solution (b) ; Permeance and rejection of CV solution of MTM -Tph (c) and PEI-Tph (d) composite membranes.

28

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Fig. 4. ATR-IR spectra (a); XPS curves (b); N1s high resolution of MT (c) and PT(d); DWCA (e)

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and zeta potential (f) of base and composite membrane.

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Fig. 5. SEM images of composite membrane.

Fig. 6. Water permeance (a) and dye rejection (b) of membranes in dyes filtration.

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Fig. 7. Inorganic salts permeance (a) and rejection (b) of composite membrane

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Fig. 8. Long-term operation of CR and NR solution filtration (a), inorganic salts solution filtration (b), mixed solution of NR and Na2SO4 filtration (c) for PT membrane.

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Fig. 9. Rejection recovery of MB (a), MgCl2 (b), CV (c), FRR of three foulant solutions (d) for PT membrane

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Scheme captions

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Scheme 1. Preparation of composite membrane.

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Scheme 2. Resonance structures of metformin.

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Table

Table 1. Membrane formulas and properties Formula of casting dopes

Pore size

Membranes

Tensile strength PWF*

Psf (g)

K-Sepiolite (g)

CTAB (g)

DMAc (g)

(nm)

Psf

3.00

0

0

17.00

108.00

PSE

3.00

0.15

0

16.85

155.00

513.52

2.47

PSEC

3.00

0.15

0.15

16.70

488.00

611.46

1.86

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Molecular Weight Cutoff (Da)

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MT

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Table 2. Molecular weight cutoff of composite membranes

Membrane

2000

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PT

6000

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2.16

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33.97

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*PWF is pure water flux with the unit of L·m-2·h-1·bar-1.

(MPa)