DTiO2-PDA-PEI composite nanofiltration membrane for highly pure dye separation

Journal Pre-proof Preparation of molecular selective GO/DTiO2-PDA-PEI composite nanofiltration membrane for highly pure dye separation Yanqing Xu, Guibin Peng, Junbin Liao, Jiangnan Shen, Congjie Gao PII:

S0376-7388(19)33036-4

DOI:

https://doi.org/10.1016/j.memsci.2019.117727

Reference:

MEMSCI 117727

To appear in:

Journal of Membrane Science

Received Date: 29 September 2019 Revised Date:

5 December 2019

Accepted Date: 5 December 2019

Please cite this article as: Y. Xu, G. Peng, J. Liao, J. Shen, C. Gao, Preparation of molecular selective GO/DTiO2-PDA-PEI composite nanofiltration membrane for highly pure dye separation, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2019.117727. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

CRediT author statement Yanqing Xu: Investigation, Methodology, Writing - Original Draft Guibin Peng: Investigation Junbin Liao: Data curation, Writing - Review & Editing Jiangnan Shen Resources, Writing - Review & Editing, Supervision. Congjie Gao: Supervision, Funding acquisition.

Preparation of molecular selective GO/DTiO2-PDA-PEI composite nanofiltration membrane for highly pure dye separation

Yanqing Xu a, b, Guibin Peng b, Junbin Liao b, Jiangnan Shen a, b, *, Congjie Gao a, b a

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China b Center for Membrane and Water Science & Technology, Zhejiang University of Technology, Hangzhou 310014, China * Correspondence to: J. Shen (E−mail: [email protected])

Preparation of molecular selective GO/DTiO2-PDA-PEI composite nanofiltration membrane for highly pure dye separation Yanqing Xua,b, Guibin Pengb, Junbin Liaob, Jiangnan Shena,b*, Congjie Gaoa,b a

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China b Center for Membrane and Water Science & Technology, Zhejiang University of Technology, Hangzhou 310014, China

*

Corresponding Author: Jiangnan Shen (E–mail: [email protected])

Abstract： ： The state-of-the-art graphene oxide (GO) has been wildly used in water purification due to its tunable nanoscale interlayer between the two-dimensional nanosheets. However, the structural instability and flux decline performances significantly limit the practical application of GO-based membranes. Herein, a novel GO/DTiO2-PDA-PEI (dopamine modified TiO2) composite nanofiltration membrane has been prepared via a multi-coupled strategy, combining with the self-assembly, copolymerization and surface grafting. Tuning the nanoscale interspace between the GO nanosheets, and constructing the membrane with positively charged surface and negatively charged subsurface, the loose GO/DTiO2-PDA-PEI composite nanofiltration membrane (GDP1P2-0.6 membrane with a mean pore size = 0.87 nm) show the filtration performance with water permeance as high as 41.6 L m–2 h–1 bar–1 and dye rejection around 99.9%, and the molecular selective factor (Eriochrome black T/Na2SO4) of α = 47.6. Long-term performance and stimulation have been investigated to further analyze the dye

concentration

and

salt

removing.

The

investigations

demonstrate

that

the

GO/DTiO2-PDA-PEI composite nanofiltration membrane enriches 14.9-fold Eriochrome black T (ET) and reduces Na2SO4 to 0.36% in the term of the concentration factor 15 and batch diafiltration factor 5. Keywords: molecular selective, graphene oxide, TiO2, dye and salts fractionation, batch diafiltration

1. Introduction

The nanofiltration (NF) technology plays an extraordinary role in modern industries, including desalination [1,2], water purification [3], pharmaceuticals [4], biotechnology [5] and textile industries [6,7] etc. Generally, empirical-based NF membrane fabricated by interfacial polymerization retains the almost all of the inorganic salts and small organic molecules except monovalent ions under the low permeability [8,9]. However, the industrialized application triggers higher requirements for membrane separation as the development of new materials and synthesis technology, especially the demand for precise and rapid separation of molecules [10,11].

Graphene oxide (GO), a two-dimensional (2D) material, can be easily assembled into layered stacks with a well-defined interlayer distance offering new possibilities for membrane separation [12–15]. Recently, several significant signs of progress have been achieved [16– 19], however, the structural instability and flux decline of GO membranes are still the obstacles to the practical application [20]. The weak intermolecular forces and electrostatic repulsion of GO nanosheets in aqueous medium delaminate the unique lamellar structure. It has been reported that the stability of GO multilayer structure in water can be enhanced by using cross-linking reagents like diamines, polydopamine (PDA), and metal ions. But those GO membranes have to sacrifice the water permeance [21,22] or need sophisticated nanoscale strategy [23]. Moreover, the wrinkles in GO membranes, which act as water channels, compact under the influence of hydraulic pressure leading the dramatical attenuation of permeance [24,25].

Nano-sized materials such as small molecules [26], nanoparticle [27,28], nanotubes [29,30], nanoribbons [31], or other 2D materials [32] have become an alternative solution to tuning the inters-pacing. Thermo- and pH-responsive nano-hydrogels embedded between the GO sheets adjusted the water channels by pH or template variation [28]. Chen et al. intercalated reduced graphene oxide with carbon nanotubes improving the water permeance to 2–3 fold [30]. Zhang et al. introduced the MoS2 nano-supporting spacer into the GO layers obtaining the water permeance about 10.2 ± 1.68 L m–2 h–1 bar–1 at low pressure [32]. However, those

nano-sized materials have fragile interaction with GO nanosheets which were only employed on a dead-end filtration setup. Titanium dioxide (TiO2) has been investigated for membrane modification due to its high stability, antibacterial property, photocatalytic activity etc. Abadikhah et al. fabricated the polyamide active layer by the incorporation of the reduced graphene oxide@TiO2@Ag nanocomposite for desalination (96% for Na2SO4), dye retention (98% for rose bengal), and antibacterial properties (90% reduction for Escherichia coli) [33]. Suriani et al synthesized the sodium dodecyl sulphate/GO/TiO2 and incorporated into polyvinylidene fluoride (PVDF) nanofiltration membrane for dye rejection [34]. Thus, TiO2-GO nanocomposites are a great candidate for mixed matrix membrane and usually increased the rejection of SO42–. However, GO-based nanofiltration membrane with inters-pacing tuning and modification for dye desalination is rarely considered.

In this work, a novel GO/DTiO2–PDA-PEI composite membrane with strong and enlarged interlayer distance as well as electric double layer has been fabricated by a multi-coupled strategy, combining with the self-assembly, copolymerization and surface grafting. The dopamine modified TiO2 (DTiO2) assembled into GO nanosheets tuned the layer spacing and acted as the backbone for GO wrinkle, rendering a high and sustainable flux. In addition, the copolymerization between dopamine (DA), GO and DTiO2 was used to reinforce the lamellas interaction. In addition, anion transportation was accelerated by the membrane surface coating with hyperbranched PEI. Therefore, the multi-coupled strategy of GO/DTiO2-PDA-PEI composite nanofiltration membrane preparation was demonstrated by the characterization and performance. The GO/DTiO2-PDA-PEI composite nanofiltration membrane shows the good desalination and concentration of dye, confirmed by the long-term operation and diafiltration stimulation.

2. Experimental 2.1. Chemicals and materials

A homogeneous aqueous self-synthesized GO solution with 0.25 mg/L was prepared according to our previous work [23]. Dopamine hydrochloride, tris (hydroxymethyl) aminomethane (Tris, 99.8%), ethylene imine polymer (PEI, Mw = 70 k, 50 wt.%), and TiO2

(anatase) with the average particle sizes of 10–25 nm were received from Aladdin Industrial Corporation (Shanghai, China). Polysulfone ultrafiltration support membranes (PSF, MWCO = 35 kDa) were provided by the Development Centre of Water Treatment Technology (Hangzhou, China). NaCl (99%), Na2SO4 (99%), MgCl2 (98%), MgSO4 (99%) and all other chemicals were obtained from Lingfeng Chemical Reagent Co. Ltd. (Guangdong, China). SafranineT (ST), Alizarin yellow GG (AG), Eriochome black T (ET) and Crystal violet (CV), used as model dyes at 100 ppm, were received from Aladdin Industrial Corporation (Shanghai, China), and the details chemical information were list in Table S1. Polyethyleneglycols (PEGs) with different molecular weights (400 Da, 600 Da, 800 Da, 1000 Da, and 2000 Da) were supplied by Sinopharm Chemical Reagent Co., Ltd (Hangzhou, China).

2.2 Synthesis of DTiO2 nanoparticles

The surface modification of TiO2 nanoparticles was performed according to the published work [35,36] and the procedure was described as follows: 1.0 g TiO2 was added to 150 mL ethanol and evenly dispersed by ultrasonication. Then, 0.2 g dopamine hydrochloride in 50 mL ethanol was added dropwise into the TiO2 solution under nitrogen atmosphere. The mixture was kept in oil bath at 60oC and stirred at 500 rpm for 6 h. Thereafter, the supernatant was removed, and the concentrated particles were washed with deionized water and ethanol for two times. Finally, the resultant solid was collected and dried in a vacuum oven at 60 oC for 8 h, rendering a light-yellow solid powder (marked as DTiO2). Fig. S1 shows the possible chemical structure of DTiO2. The evenly-dispersed DTiO2 aqueous dispersion (0.25 mg/L) was prepared by the ultrasonic dispersion (40 kHz) for 30 min.

2.3 Fabrication of GO/DTiO2-PDA-PEI composite membrane

The preparation of GO/DTiO2-PDA-PEI composite nanofiltration membrane is schematically illustrated in Fig. 1. 5 mL GO aqueous solution (0.25 mg/L) was mixed with the as-prepared aqueous DTiO2 (0–5 mL). Then, the mixed solution was ultrasonicated for 30 min to obtain a uniform solution before filtering on a PSF support membrane by vacuum filtration. After stabilization in room temperature for 1 h, the GO/DTiO2 substrate membrane was filtered

with 50 mL solution by dissolving dopamine hydrochloride (2.0 g/L) in Tris buffer solution (pH = 8.5 and 50.0 mM). Thereafter, the membrane was taken out and rinsed thoroughly with deionized water. Then, the as-fabricated membranes were further modified by PEI (1.0 wt.%) via immersing them in an aqueous solution for 20 min. Table 1 lists the preparation conditions of composite nanofiltration membranes. In this work, the composite membranes were named as GDP1P2-x, where G, D, P1, and P2 representing GO, DTiO2, PDA and PEI, respectively and x referring to the weight percent of DTiO2 to GO.

Fig. 1 Schematic illustration of the preparation of GO/DTiO2-PDA-PEI composite nanofiltration membranes via multi-coupled strategy. Table1. The preparation conditions of various composite nanofiltration membranes Composite membrane G-0 GD-0.6 GP1P2-0 GDP1-0.6 GDP2-0.6 GDP1P2-0.2 GDP1P2-0.4 GDP1P2-0.6 GDP1P2-0.8 GDP1P2-1

GO loading solution (mL) 5 5 5 5 5 5 5 5 5 5

DTiO2 loading solution (mL)

PDA concentration (g L–1)

PEI concentration (wt.%)

2 2

1

3 3 3 1 2 3 4 5

2 2 2 2 2

1 1 1 1 1 1

2.4 Characterization

Fourier transform infrared spectra (FTIR, Nicolet iS50, Thermo, America) was obtained to characterize the functional groups of composite membranes. X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD, Japan) was used to analyze the chemical elements of membranes. The morphologies and structures were characterized using an atomic force microscope (AFM, Nanoscope V, Bruker Dimension, America) and scanning electron microscope (SEM, SU8010, Hitachi, Japan). Zeta potential measurement was undertaken with surface analysis (Anton Paar surpass 3, GmbH, Austria), where the pH of the testing solution was automatically adjusted by NaOH (0.05 mol/L) and HCl (0.05 mol/L) solutions. The hydrophilicity of the surface was measured by conducting a static contact angle (CA) measurement (Dataphysics, OCA15EC, Germany).

2.5 Performance of GO/DTiO2-PDA-PEI composite membrane

The permeance and solute rejection of GO/DTiO2-PDA-PEI composite membranes were measured by using a homemade cross-flow filtration device (Fig. S2). The effective area of membrane was 7.0 cm2 and the constant flow rate was 4.0 L min−1 at 7.0 bar. The solute concentration of salts and dyes were performed at 2000 ppm and 100 ppm, respectively, which is usually used in textile industries. The permeate was collected after reaching the steady state under pre-pressure at 7.5 bar. The permeance J and rejection R (%) were calculated according to the Eq. (1) and Eq. (2), respectively.

J=

(1)

× ×

where V (L) is the total volume of permeate collected on a time t (h) under the operation pressure p (bar), A is the effective area of membrane (m2).

R(%) = 1 −

× 100

(2)

where Cp and Cf are the concentrations of solute in permeate and feed, respectively. UV–vis

spectrophotometry (TU-1810PC) and the electrical conductivity meter (DDS-307A, the measurement range is 0-2000 mS/cm, conductivity accuracy is 0.01 µS/cm) were used to analyze the concentrations of dyes and salts, respectively.

The molecular selectivity, identified by the separation factor of dye to salt (α), was calculated by the Eq. (3):

α=

（3）

The pore size and its distribution for GO/DTiO2-PDA-PEI composite membranes were estimated using the solute transport method [37, 38]. Steric-hindrance and hydraulic resistance were assumed to have no effects to the solute rejection. The relationship between solute separation correlates with solute diameter can be expressed as: (! ) !

=!

"

#$ % √'(

exp ,−

(#$ ! -#$ . ) '(#$ % )

/

(4)

where dp is the pore size. µp refers to the solute mean size and σp is the solute geometric standard deviation, which were determined based on the rejections of PEGs with different Stokes diameter. More detail information about the pore size and its distribution could be found in Support Information 1.2.

Antifouling performance of GO/DTiO2-PDA-PEI composite membranes was investigated using BSA (1.0 g/L aqueous solution) as the fouling model. After pure water filtrated through the membrane for 60 min and recorded the permeance as Jw,1, the BSA solution was rapidly filled and kept for another 60 min and the permeance for BSA solution was measured as Jw,2. The antifouling performance evaluation for each membrane was operated with three cycles. The flux recovery ratio (FRR) calculated as follow:

011 =

23,

23,5

× 100%

(5)

Fig. 2 shows the batch diafiltration procedure integrated with pre-concentration and post-concentration. The simulation for the process can be easily split into concentration part and diluent part and ignored the influence of concentration variation on rejection. Given our previous work [23], the concentration steps were performed by NF process and the concentration was calculated as per Eq. (6):

6

,7(8)

=

9(:)

;<=(:)

(:)

∙6

,7?(8)

(6)

where Robs is the constant value the concentration of salt and dye in the feed (Cf,i(c)) and permeate (Cp,i(c)), A?(8) and A (8) are the volume of feed solution before and after concentration, respectively.

The modeling batch processes for dilute could consider as the Eq. (7).

6

where 6

,7(?) is

,7(!)

=

9( )

9( ) B 3( )

∙6

,7(?)

(7)

the initial concentration of solute before the dilture; V0(d) is the initial volume

of feed solution; Vw(d) is the volume of pure water which is added during diafiltration.

Fig. 2 The flowchart of the whole diafiltration process.

3 Result and discussion 3.1 Characterization of DTiO2

The characteristic peaks of DA modify TiO2 nanoparticles of FTIR spectra (seen in Fig. 3a) are consistent with the results from literature reports [35]. The appearance peaks of the stretching vibration at 3400–3600 cm–1 and bending vibration at 1651cm–1 are corresponding to the N–H groups for the DTiO2 [39]. The broad band peak in the region of 700–450 cm–1 was assigned to Ti–O stretching peak [40,41]. Inset in Fig. 3(a) shows the powder of the white TiO2 and light-yellow DTiO2. The color change can be ascribed to the charge transfer interaction between DA and surface Ti atoms [42]. X-ray diffraction (XRD) characterization of the DTiO2 disclose almost identical patterns of anatase TiO2, the typical peaks at the 2θ values of 25.3°, 37.7°, 47.8°, 54.1°, 54.8° and 62.7° are well in line with the planes of (101), (004), (200), (105), (211), (204) [43,44], revealing that the DA modification of TiO2 have no effect on the crystal structure of nanoparticles.

Fig. 3 (a) The FTIR spectra of DA, TiO2 and DTiO2 (see the appearance of TiO2 and DTiO2 in the inset); (b) The XRD patterns of TiO2 and DTiO2.

3.2 Membrane characterization

FTIR spectra in Fig 4a were used to confirm the functional groups of as-prepared composite membranes. Compared with the original PSF substrate, a new peak at 1522 cm−1 corresponding to N–H shearing vibrations appears on curves of the GO/DTiO2-PDA

(GDP1-0.6 membrane) and GO/DTiO2-PDA-PEI (GDP1P2-0.6 membrane) [45]. Additional peak at 1645 cm–1 in GDP1P2-0.6 membrane is assigned to the –NH bond of secondary amines, indicating the successful graft of PEI with amine and imine groups. Besides the characteristic peaks of substrate, the broad band peak in the region of 700–450 cm–1 is assigned to Ti–O stretching peak [46], which can be found in GD-0.6, GDP1-0.6 and GDP1P2-0.6 membranes, confirming the existence of DTiO2. Raman spectroscopy (Fig. 4b) was employed to further confirm the interactions of the composite membranes. The peaks at 1340 cm−1 and 1600 cm−1 of GO membrane (G-0) corresponding to the disorder D peak and G peak, are obviously weakened without any shift in the spectrum of GD-0.6, GDP1-0.6 and GDP1P2-0.6 membranes. The newly emerged peaks at 530 cm−1 and 640 cm−1 of GDP1-0.6 and GDP1P2-0.6 membranes coordinate to the oxygen of the catechol group [47]. In addition, peaks appeared at 400 cm−1 are assigned to the TiO2 [48].

Fig. 4 The FTIR spectra (a) and Raman spectra(b) of the composite membranes.

The surface chemical elements of composite membranes were analyzed by XPS (Fig. 5 and Table 2). The peaks locate at 530.9, 399.6, and 284.5 eV in XPS spectra represent the elements O 1s, N 1s and C 1s [49–51], respectively. An intense signal of Ti 2p peak at 458.2 eV can be observed in GD-0.6 membranes and the atomic percentage of Ti is 3.34%. Because of the average scanning depth of XPS analysis is 10 nm., the Ti peak was absence in GDP1-0.6 and GDP1P2-0.6 membranes after PDA and PEI modification. Meanwhile, the atomic percentage of N 1s is 1.02% in GD-0.6, attributing to the nitrogen element of the DTiO2. In GDP1-0.6 and GDP1P2-0.6 membranes, the main peak of N 1s is corresponding to

the nitrogen from PDA and PEI. Besides, GDP1P2-0.6 membrane has a higher N/O atomic ratio than GDP1-0.6 membrane, resulting from successful anchoring of PEI and increasing the density of amine groups.

Fig. 5 XPS spectrum of the composite nanofiltration membranes.

De-convolution of C1s, N1s and O1s peaks are shown in Fig. S3–S5. The C 1s curve of GO have four peaks 284.5, 286.4, 287.4 and 288.9 eV, which are assigned to C–C, C–O, C=O and O–C=O [52]. The new peak of GD-0.6 membrane in 288.5 eV ascribed to N–C=O shows the DTiO2 self-assembled with GO [53]. O 1s of GO membrane (G-0) is resolved into C=O/O– C=O, C–O species at binding energy of 531.6, and 533 eV, respectively [53]. The new peaks of GD-0.6 membrane with binding energies at 529.6 and 529.1 eV are corresponding with N– C=O and O–Ti [54]. Only two peaks are shown at 529.6 and 531.6 eV ascribed to N–C=O and C=O for the PEI coating in GDP1P2-0.6 membrane. N1s spectra of GD-0.6 and GDP1-0.6 membrane fit into three peaks components and the binding energies at 398.8, 399.6, and 401.7 eV are corresponding to the N–R, N–R2, and N–C=O species after DA and PDA modification [55], respectively. A new peak of 401.3 eV is ascribed to R≡N inferring the PEI deposition [56].

Table 2 Surface elemental composition of composite nanofiltration membrane Surface elemental composition (at.%) Membrane G-0

C

O

N

Ti

69.76

30.24

–

–

GD-0.6

63.35

32.3

1.02

3.34

GDP1-0.6

69.12

20.77

10.11

–

GDP1P2-0.6

70.24

13.1

16.66

–

The morphologies of the composite membranes are shown in Fig. 6. The surface of the pristine surface GO membrane (G-0) shows many wave-like ripples and the roughness in AFM image is 8.8 nm. After assembled with the DTiO2 into the GO lamellar (GD-0.6 membrane), DTiO2 appeared on the wrinkled area of the GO sheet rather than on the smooth area, forming a bumpy surface morphology. In comparison with the GD-0.6 membrane, the roughness increases from 8.8 nm to 26.7 nm. This might due to the full of hydroxyl and carboxyl functional groups at the edge of the GO nanosheets would attract the –NH2 of DTiO2 [57]. The presence of a new N 1s peek for GD-0.6 in Fig. 5 also indicates it. The size of the particles decorated into GO layers are from 60 to 200 nm, which infer that nanoparticle were aggregated because of amidation [21] and higher free energy of nanoparticles [58]. A relatively smooth surface morphology with characteristic PDA particles can be found in GDP1-0.6 membrane after DA polymerization, which cover the wave/nodule-like ripples and let to decrease in the roughness to 18.2 nm. After grafting the PEI with flexible long chain, GDP1P2-0.6 membrane became flat and only a few small protrusions could be observed (Ra = 6.2).

Fig. 6 SEM surface images with 10 K and 60 K (upper right) magnifications(a) and AFM images with roughness(b) of the composite nanofiltration membranes.

Water contact angle (CA) measurement had been undertaken to evaluate the surface

hydrophilicity of the composite membranes. As shown in Fig. 7(a), GO pristine membrane (G-0) exhibits the highest CA of 46.8°. With the addition of hydrophilic DTiO2, the value of the CA decreases to 33.6°, disclosing the effect of hydrophilic group on the surface of GD-0.6 membranes in the terms of –OH and –NH2 [59,60]. The GDP1-0.6 membranes modified with PDA show the increased water contact angle (CA = 36.9°). This is possibly related to the PDA particles on the membrane surface or reduced the hydroxyl and epoxy groups resulting from the –NH2 attached on the basal planes [61]. Regarding for GDP1P2-0.6 membrane, it shows the significantly decreased contact angle with the value of 17.5°, is indicative of the strongly enhanced surface hydrophilicity by PEI grafting.

In the NF process, the Donnan effect was mainly influenced by the surface charge. As show in Fig. 7(b), the G-0 membrane has an isoelectric point around pH 5.0, which is consist with the result of reference, owing to the negative hydroxyl and carboxyl groups [62]. Amidation reaction was taken between the carboxyl groups and amine group in GD-0.6 when DTiO2 self-assembled with GO, which decreased the negative Zeta potential of GD-0.6 membrane. PDA coating membrane (GDP1-0.6) also possesses negatively charged on the top surfaces with the Zeta potential of –12.5 mV at pH 7.0, but isoelectric points of the GDP1-0.6 membranes shifts to the higher pH values on account of amino groups in PDA. After modifying with PEI, a large number of amino groups were introduced onto the membrane (GDP1P2-0.6) surface, leading to a dramatic increase in isoelectric point (IEP, approximately pH 9.0). The value of Zeta potential at pH 7.0 is close to the other PEI based nanofiltration membranes [4].

Fig. 7 Water contact angles (a) and Zeta potential (b) of composite membranes. 3.3. Membrane performance

In order to analyze the function of multi-coupled strategy on GO/DTiO2-PDA-PEI composite membranes, the permeance and rejection of the GP1P2-0.6, GDP1-0.6, GDP2-0.6 and GDP1P2-0.6 membranes with different preparation conditions have been evaluated. From Fig. 8, it can be seen that the GP1P2-0.6 has the lowest permeance (3.2 L m–2 h–1 bar–1) and highest salt rejection (61.9%~41.1%), which are attributed to the highly ordered and densely packed of GO nanosheets, which is similar to the reported data [63]. Compared with GDP1P2-0.6, the DTiO2 dispersion increases the permeance (41.2 L m–2 h–1 bar–1) by nearly 13-fold and decreases the rejection of salts to less than 10%, as MgCl2, MgSO4, NaCl and Na2SO4 were 6.0%, 4.4%, 3.3% and 2.5%, respectively. DTiO2 self-assembled with GO nanosheets broaden the GO nanosheets and facilitate the high-speed passage of water molecular. Other work has also verified the addition nanotubes in GO nanofiltration membrane can improve the permeability, like 2–3-fold [30]. The pore size and pore size distribution of composite membrane using solute transport measurement are depicted in Fig. 9 and Table S2. The mean pore size (µp) increases from 0.68 nm to 0.87 nm after DTiO2 introduced, and the MWCO increases to 978.8 Da for GDP1P2-0.6 membrane. The salt rejections of GDP1P2-0.6 are in the order of MgSO4 > MgCl2 > NaCl > Na2SO4, which can be explained by the Donnan exclusion principle. In contrary, the negative GDP1-0.6 membrane without the PEI grafting has the order of salt rejections as MgCl2 > MgSO4 > Na2SO4 > NaCl. Meanwhile, the dye retentions of loose GDP1P2-0.6 membrane still achieves the completely dyes rejection for positive crystal

violet (CV, MW = 408, 99.9%), positive safranine T (ST, MW=351, 96%) and negative eriochome black T (ET, MW = 461, 99.9%), but low rejection for the negative dye with small molecular weight, like Alizarin yellow GG (AG, MW = 309, 89.4%). It may be attributed to a combined effect of steric, Donnan and electrostatic interactions.

Without PDA copolymerization, GDP2-0.6 membranes have the lowest retentions and highest permeance [64]. Owing to the absence of PDA, the shearing force from the cross flow weakened the intermolecular forces and destroyed the membranes structure. Comparing with the rejection of GDP1P2-0.6 membrane, it reconfirms our previous work that DA copolymerization with DTiO2 and GO layers reinforce the interaction of lamellar structure.

Fig. 8 (a) Permeance and salts rejection of four different composite membranes; (b) rejection performance of four different dyes.

Fig. 9 Pore size distribution and pore structure parameters of the composite membranes. To optimize the preparation conditions, the water permeance, salts and dyes rejection of

GO/DTiO2-PDA-PEI composite nanofiltration membranes as function of DTiO2 loading have been tested and the results are showed in Fig. 10 The permeance of GO/DTiO2-PDA-PEI composite membranes increases with an increase of DTiO2 addition, from 12.3 L m–2 h–1 bar–1 of GDP1P2-0.2 membrane to 75.5 L m–2 h–1 bar–1 of GDP1P2-1 membrane, indicating that the introduction of DTiO2 leads to the significant enhancement in water permeation. It is observed that the tightly stacked GO structures becoming partial loose structure in the SEM cross-section image (Fig. 11). In addition, the effective pore size of the GO/DTiO2-PDA-PEI composite membrane increases as the increased DTiO2 loading (Fig. 9). The rejections of salts and dyes dramatically decreases with the DTiO2 addition, conforming to the trade-off effect of NF membrane.

Fig. 10 The function of DTiO2 loading on (a) permeance and salts rejections and (b) dyes rejection.

Fig. 11 SEM images of cross section of GO/DTiO2-PDA-PEI composite nanofiltration membranes with different DTiO2 loadings. The main separation mechanisms of nanofiltration membrane are the combined effects of sieving exclusion, dissolution diffusion and charge repulsion [8]. In this work, the multi-coupled strategy was used to obtain the GO/DTiO2-PDA-PEI composite membrane with effective molecular selective and high water permeance (as shown in Fig. 12). The nanoparticle DTiO2 was chosen as the backbone not only to avoid the wrinkles of GO nanosheet collapse, but also to broaden the interlamellar spacing of GO layer. The pore size

cut off of GO/DTiO2-PDA-PEI composite membranes ranges from 0.77 nm to 1.12 nm (Fig. 9), which is larger than the hydrated radius of SO42– (0.37 nm) and Cl– (0.33 nm) [65]. As a result, the loose GO/DTiO2-PDA-PEI composite nanofiltration membranes procure a significant improvement in water permeance and salts transport. Although steric exclusion is the one of major separation mechanism, charge repulsion also plays a distinct role on molecule retention [66]. The graft of PEI on the membrane surface makes the membranes composing a single positive surface layer and negative multilayers. Multiple cationic branches of PEI macromolecular acted as comb-like to accelerate the anion passage, especially for the divalent anion (SO42–). The salts permeance order was based on the principle of the Donnan equilibrium (Fig.8(a)). Tang et al. validated that the salt rejection in a positively charged NF membrane was dependent not only on pore size of membrane but also on the static electric action of membrane surface [67]. In addition, results in Fig. 8(b) indicate that high effective charge of composite membranes makes a larger contribution to dyes rejection. As shown in Fig. 10, the positive dyes like CV and ST can be easily rejected by the charge repulsion of amino groups of GDP1P2-0.6 compared to the GDP1-0.6 membranes with typical negative charge. The rejection of negative dyes like AG and ET occurred in the inner layer. Dye with larger molecular weights (ET) can be rapped and excluded by the functional groups (aromatic, catechol and carboxyl) in the matrix while small dye molecules (AG) would be adsorbed by the DTiO2[68, 69]. Combined the steric effect and charge repulsion, GO/DTiO2-PDA-PEI composite membranes show a good dye desalination capability.

The permeance and molecular selectivity of the GO/DTiO2-PDA-PEI composite with other NF membranes have been plotted in Fig. 13 and details are listed in Table S3. The separation factor (α) is defined as the ratio of dye retention rate to the salt retention rate. Compared with recent reports and commercial membranes, the GO/DTiO2-PDA-PEI composite elucidates an excellent selectivity for the dye/Na2SO4. The molecular selective factor (Eriochrome black T / Na2SO4) is α = 47.6.

Fig. 12 Schematic diagram of separation of salts and dyes for the GO/DTiO2-PDA-PEI composite membrane.

Fig. 13 Correlation between separation factor α and permeance for nanofiltration membranes in previous literature and this work.

3.4 Dye desalination Fig. 14(a) shows the long-term performance of GO/DTiO2-PDA-PEI composite membrane (GDP1P2-0.6). Herein, we obtained the durable composite membranes in cross-flow module by incorporation of “sticky” PDA into the GO/DTiO2-PDA-PEI nanosheet structure. There is no obvious decline of water permeance at the beginning, confirming the existence of DTiO2 nanoparticles avoiding the GO lamella compaction. Over time, the water permeance declines slightly, which is possibly attributed to dye aggregation and concentration polarization at

higher dye concentrations [81]. During the continuous NF process, the rejection of Na2SO4 is relatively low (2.1%∼3.2%) while the ET is high and constant (99.9% to 99.4%). The explanation for the slightly decrease of dye retention is the dye concentration polarization on the membrane surface promoted the penetration of dye molecules through the membrane. In the meantime, the dye concentration also weakens the Na2SO4 transport by preventing the contact between the salt and PEI. However, the undesirable effect of concentration polarization is weak, which is already minimized by a high flow rate (4 L min–1) of the cross flow. Meanwhile, the antifouling performance with BSA solution of GDP1P2-0.6 membrane is also taken in Fig. S6, the normalized flux recovers to a stable high-level (＞80%) for the composite membranes after protein pollution in three cycles. Therefore, concentration ratio of dye or salt (Ci/C0) in the feed solution under the concentration factor (V0/Vf) is almost consistent with the simulation date (Fig. 14(b)).

Fig. 14 Long-time separation performance (a) (inset: picture of feed and permeate collection in every 6h) and compared with simulation as concentration step (b) of GO/DTiO2-PDA-PEI composite membranes for fractionation of ET and Na2SO4.

Based on well matched-degree of measured and calculated, the simulation for the whole diafiltration process of the GO/DTiO2-PDA-PEI composite membranes are shown in Fig 15 and Table 3. The composite membranes with different DTiO2 loading including GP1P2-0, GDP1P2-0.6 and GDP1P2-1 were chosen to study the ET/Na2SO4 selectivity under the different retention. The individual retention was assumed to remain constant during the whole process.

The

dye

concentration

of

GO composite

membranes

(GP1P2-0)

and

GO/DTiO2-PDA-PEI composite membranes (GDP1P2-0.6) are 14.9-fold of the initial concentration after 15-time enrichment, while the GDP1P2-1 membrane only gets 8.08-fold,

and lost 46.1% dye. Among them, 21.6 % was lost in the concentration stage and 24.5 % in the batch diafiltration stage. The salt concentration of GO composite membranes (GP1P2-0) is nearly 75-times higher than the GDP1P2-0.6 and GDP1P2-1 membranes after the diafiltration process. During the batch-diafiltration, the variation of CEF

GH

declines as zigzag in

GP1P2-0, implying that salt is partially concentrated during the dye desalting, while the GO/DTiO2-PDA-PEI composite membranes (GDP1P2-0.6 and GDP1P2-1) are stepwise descent. It should be noted that the permeance of GDP1P2-0.6 composite membrane is 13 times faster than GP1P2-0 membrane. In summary, the GO/DTiO2-PDA-PEI composite membranes, especially GDP1P2-0.6 membrane shows a high capability for its application in purification of dyes.

Fig. 15 Simulation of dye desalting in the overall diafiltration process by the GO/DTiO2-PDA-PEI composite membranes. Table 3 Simulation parameters and computed GO/DTiO2-PDA-PEI composite membranes.

result

of

diafiltration

for

the

RNa2SO4 (%)

RET (%)

α

Ci/C0(salt)

Ci/C0(dye)

GP1P2-0

41.5

99.9

2.41

2.7 10–1

14.9

GDP1P2-0.6

2.1

99.9

47.57

3.6 10–3

14.9

GDP1P2-1

1.4

91.0

65

3.4 10–3

8.08

4. Conclusions

In this work, a novel GO/DTiO2-PDA-PEI composite nanofiltration membrane has been fabricated by using a multi-coupled strategy combined with self-assembly, copolymerization and surface grafting. The effect of DTiO2 loading on permeance, retention and dye/Na2SO4 selectivity has been systematically investigated. GDP1P2-0.6 membrane shows the high permeance (41.6 L m−2 h−1 bar−1), high ET rejection (99.9%) and low Na2SO4 reserve (2.1%). It also enriches 14.9-fold ET and reduce Na2SO4 to 0.36% after the whole diafiltration process. The long-term performance and stimulation confirm the superior dye concentration and salt removing.

Acknowledgments This work was funded by the Natural Science Foundation of Zhejiang Province (No. Q19B060013) and the Project for Assistance of Qinghai from Science and Technology Department of Zhejiang Province (No. 2018C26004). Reference

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Research highlights A novel GO/DTiO2-PDA-PEI composite nanofiltration membrane is synthesized via a multi-coupled strategy, combining with self-assembly, copolymerization and surface grafting. Tuning the nanoscale interspace between the GO nanosheets and constructing the membrane with positively charged surface resulted in high permeation of pure water and ions. The GO/DTiO2-PDA-PEI composite nanofiltration membrane (GDP1P2-0.6) shows the satisfied filtration performance with effective molecular selectivity. GO/DTiO2-PDA-PEI composite nanofiltration membrane could enrich 14.9-fold Eriochrome black T (ET) and reduce Na2SO4 to 0.36% after the diafiltration process.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: