Preparation and Characterization of Cellulose-Based Nanofiltration

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Preparation and characterization of cellulose-based nanofiltration membranes by interfacial polymerization with piperazine and trimesoyl chloride Shi Li, Shengnan Liu, Fang Huang, Shan Lin, Hui Zhang, Shilin Cao, Lihui Chen, Zhibin He, Ryan Lutes, junhui Yang, Yonghao Ni, and Liulian Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02720 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 11, 2018

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Preparation

and

characterization

of

cellulose-based

nanofiltration membranes by interfacial polymerization with piperazine and trimesoyl chloride Shi Li†, Shengnan Liu†, Fang Huang†, Shan Lin†, Hui Zhang†, Shilin Cao†, Lihui Chen†, Zhibin He‡, Ryan Lutes‡, Junhui Yang†, Yonghao Ni†‡*, Liulian Huang†* †

College of Materials Engineering, Fujian Agriculture and Forestry University, Fuzhou 350002, China; ‡ Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3

GRAPHICAL ABSTRACT This work presents the development of a green, sustainable and biodegradable bamboo cellulose composite nanofiltration membrane based on the interfacial polymerization of piperazine and 1,3,5-trimesoyl chloride. Nonwovens HN

NH

COCl 1

1

0

TE M PE RA T UR E O N/OFF

Cellulose made from bamboo

COCl

ClOC

Cellulose dissolution Casting to form bamboo cellulose membrane in NMMO/H2O

N N

O N C

O C

N

O N C

O C

m COOH

N C O

n C O

HOOC C O m

N N C O

Na+

SO42-

Forming bamboo cellulose nanofiltration membrane

Cl- H2O Permeation paths

C O C O n

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

ABSTRACT: A hydrophilic bamboo cellulose nanofiltration membrane (IP-NF-BCM) was prepared through interfacial polymerization (IP) of amino-functional piperazine (PIP) and 1,3,5-trimesoyl chloride (TMC) on cellulose surface. The in-situ formation of polyamide into the mesoporous structure of the regenerated cellulose film, created a uniform microporous membrane, which can be used for water softening by nanofiltration.

The interfacial polymerization reaction conditions were optimized in

terms of the performance of resultant nanofiltration membranes.

The chemical

structure, morphology and surface charge of the composite membranes were

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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characterized based on thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), nuclear magnetic resonance (NMR), and Brunner-Emmet-Teller (BET) nitrogen absorption. The water permeation and salt rejection capability of the bamboo cellulose thin-film-composite nanofiltration membranes were evaluated using 500 ppm salt solutions at 0.5 MPa pressure. Results show that the rejection rate for NaCl reached 40% and water flux reached 15.64 L/(m2·h). The average pore size of the bamboo cellulose thin-film-composite membranes was 1.0 nm. KEYWORDS: Cellulose Thin, film composite (TFC) membranes, Interfacial polymerization, Desalination INTRODUCTION Water purification is in high demand with the rapid improvement of living standards in China and membrane based technology plays a key role

1-3

. Different

membrane methods have been used for water treatment, including microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO) and membrane distillation (MD)

4-5

. Advanced water treatment with nanofiltration membrane is

superior in many aspects, including simultaneous removal of inorganic pollutants such as nitrate, arsenic, fluoride, heavy metals and pesticide residues, and has been considered to be a good option for water desalination treatment6-8. Natural fibers can be advantageously utilized for the development of many valueadded products, including biological degradable composites with good physical properties 9-11. Plant biomass is a sustainable source of fuels and materials available to humanity. Cellulosic materials are particularly attractive because of their relatively low cost and plentiful supply. Cellulose (poly-(1,4)-d-glucose) which is widely used in the fields of biotechnology, cosmetics, waste water treatment, membrane separation, food packaging, pharmaceuticals and drug carriers, has attracted attention due to the abundance of resources, excellent physical, chemical and biological properties, and eco-friendly characteristic of biomass 11-16. N-methylmorpholine-N-oxide (NMMO) monohydrate is an excellent cellulose solvent, thanks to the strong N–O dipole of NMMO 17-19. NMMO is used as a solvent for direct dissolution of cellulose in industrial scale (the so called Lyocell process). Li et al. have developed a hydrophilic hollow fiber ultrafiltration (UF) membrane for oil–water separation

20

. Kabir et al. and Zhang et al. investigated the effects of

N-methylmorpholine -N-oxide (NMMO) pretreatment, and they found that when the


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concentration of NMMO in aqueous solution is 85%, cellulose could affect the mechanical properties, color strength and crystallinity of lyocell fibers21-22. Fang et al. have successfully developed polyamide-based composite nanofiltration (NF) hollow fiber membranes that were suitable for water softening applications, the resultant membrane possessed a molecular weight cut-off of 380 Da, and showed high water permeability and salt rejection

23

. This thin film composite nanofiltration

membrane was fabricated by coating a water-soluble disulfonated poly (arylene ether sulfone) which contained pendant amine groups (SPES-NH2) onto a polysulfone support membrane, the resultant membrane exhibited a higher permeation flux and high levels of Na2SO4 rejection at an operating pressure of 4 bars. Zarrabi et al. have prepared modified thin film composite nanofiltration (TFC/NF) membranes using interfacial polymerization of piperazine and trimesoyl chloride monomers on polysulfone substrate

24

. Wang et al. added the TiO2 sol to the polysulfone (PSF)

membrane casting solution in order to prepare polysulfone/titania hybrid membranes, and the rejection rate for Na2SO4 reached 96.94% while water flux reached 12.84 L·m−2·h−1 25. To date there are no publications regarding cellulose based NF membranes prepared using the interface polymerization technique. In this paper, we reported a cellulose-based NF membrane based on the interfacial polymerization (IP) of amino-functional piperazine (PIP) and 1,3,5-trimesoyl chloride (TMC). Bamboo cellulose was first dissolved in NMMO and then regenerated to have a porous cellulose membrane which was soaked in PIP and TMC solutions sequentially to form a thin PIP-TMC layer on top of the cellulose support membrane by interfacial polymerization. The PIP-TMC layer effectively reduced the pore size of the cellulose film from about 6 nm to 1 nm, which imparted the cellulose composite membrane with good salt rejection and water flux. EXPERIMENTAL SECTION Materials. Bamboo cellulose was provided by a pulp mill in Southern China, and N-methylmorpholine-N-oxide (NMMO) was obtained from Tianjin Hai Nachuan Technology Development Co., Ltd. Gallic acid (PG, Sinopharm Group Chemical Reagent Co., Ltd.), Piperazine (PIP, 99% , Chengdu Yi Keda Chemical Reagent Co., Ltd.) and 1,3,5-trimesoyl chloride (TMC, 98%, Chengdu Yi Keda Chemical Reagent Co., Ltd.) were used as the monomers to establish the selective layer on cellulose recycled film substrate by Non-woven bottom. n-Hexane (99%, Sinopharm Group



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Chemical Reagent Co., Ltd.) was used to dissolve TMC. Inorganic salts including sodium sulfate, sodium chloride, magnesium sulfate magnesium chloride, and calcium chloride were purchased from the Sinopharm Group Chemical Reagent Co., Ltd., and used as the charged solutes to determine the de-salination performance of resultant membranes. Membrane of regenerated film. Bamboo cellulose was first dissolved with NMMO solvent to have a viscous cellulose solution, based on the method reported in the literature20, 27-28. Regenerated cellulose films were then prepared using the Immersion Gel Method29-30. First, NMMO (80 g) was mixed with 12.3 g deionized water in a three-necked flask which was heated in an oil bath at 90℃. Once the NMMO was dissolved, n-propyl gallate (2-3 wt‰, based on NMMO) was added into the solution as an antioxidant. After that, 4.0 g of bamboo cellulose pulp was added to the solution, the temperature of the oil bath was raised to 110℃, and agitation continued for 2~3 h until the cellulose was completely dissolved. Then the temperature was decreased to 90℃, and the mixer was turned off to let the solution stand still for 5 h to get rid of gas bubbles. Vacuum suction was applied to accelerate the removal of gas bubbles from the solution. The bamboo cellulose NMMO solution was uniformly applied to a nonwoven fabric surface to obtain a cellulose primary film, which was soaked in deionized water for 24～48 h to form a coagulation film. The film was then dried in the air in a fume hood on a flat plate to have a porous regenerated cellulose film. Preparation of Cellulose based Nanofiltration Membranes Nonwovens

1

1

0

TE MP ER AT URE ON/O FF

Cellulose made from bamboo

Cellulose dissolution in NMMO/H2O

Casting to form bamboo cellulose membrane

COCl

ClOC

COCl

COCl

COCl

HN

Air drying to form bamboo cellulose nanofiltration membrane (room temperature, 24 h)

ClOC

COCl

ClOC

NH

COCl

Cellulose membrane immersed in TMC solution

Air drying (room temperature, 24 h)

Cellulose membrane immersed in amine solution

Fig.1. Experimental procedure for the preparation of cellulose nanofiltration thin-film - composites (TFC) based on the interfacial polymerization of PIP and TMC


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As shown in Fig. 1, at the interface of the two immiscible solvent systems, anhydrous piperazine in the water phase and the 1,3,5-trimesoyl chloride in the organic phase will react to form a poly amide polymer (Fig. 1). The interfacial polymerization of PIP and TMC reaction has been described in detail in the literature 30-33 . The interfacial polymerization involved the reaction of di-, tri- or multifunctionalized monomers. The electrophilic TMC monomer is susceptible to the reaction with water, so it is dissolved in the organic phase. The reaction of the two active monomers resulted in formation of linear polymer chains 34. The aqueous solutions of 2.0wt% PIP and the n-hexane organic phase solution containing 0.15wt% trimesoyl chloride (TMC) were prepared. The prepared porous regenerated film was first immersed in the PIP aqueous solution for 30 min and air dried. After that it was immersed in the TMC solution for 3 min and then air dried again to obtain a regenerated cellulose nano-filtration membrane. CHARACTERICATION SEM observation. Regenerated cellulose films and cellulose based nanofiltration membranes were observed on a scanning electron microscope (SEM, Japan Electronics Co., Ltd.). A fresh cross sectional surface was prepared for observation by fracturing the membranes in liquid nitrogen. The samples were coated with a thin layer of gold under vacuum, and SEM images were taken at 1000 and 2000 magnifications with 3 kV accelerating voltage. XRD and crystallinity index. The crystallinity of the BCM and IP-NF-BCM were determined via XRD using an X’Pert Pro diffraction system (Switzerland, Bruker AVANCE Ⅲ 600MHz spectrometer). Freeze-dried BCM and IP-NF-BCM, were put on glass plates, which were fixed on the MDR cradle. The diffraction spectrum was taken at 2.4°/min for a 2θ range of 5°–80°with a step size of 0.02. Cu KR radiation (k = 1.54 Å) was generated at 40 kV and 40 mA. The crystallinity was expressed as the crystallinity index (CrI), which was calculated with the intensities of 2θ= 18°and 2θ= 22°. FT-IR analysis. Pretreated BCM and IP-NF-BCM were cut into tiny chips and ground to fine powders, then the samples were prepared with the potassium bromide (KBr) pellet method. Infrared spectra in 4000-400 cm-1 range were recorded with an FT-IR instrument (Nicolet 5700, USA) at room temperature. TGA analysis. Thermo gravimetric analysis (TGA) is a useful technique for the study of polymeric materials 31. The mass change of the membrane material as the function of temperature was recorded with a TG-DTA instrument (Germany Chi Chi Instrument Manufacturing Co., Ltd. Shanghai Representative Office, Netzsch STA



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449 F3) at a heating rate of 10 ℃/min and a nitrogen flow rate of 20 mL/min. Approximately 2~3 mg of sample was weighed and heated from 25 ℃to 900 ℃. The TG–DTA data were collected at a rate of 1 Hz 35. 13

C-NMR analysis. Cross polarization/magic angle spinning (CP/MAS)

13

C

solid-state NMR spectra were recorded on a Chemagnetics CMX100 spectrometer operating under a static field strength of 2.3 T (100 mHz 1H) at 25°C. The contact time for CP was 1 ms with a proton 90° pulse of 5.5 µs and decoupling power of 45 kHz. The MAS speed was 3 kHz. The delay time after the acquisition of the FID signal was 2 s. The chemical shifts were calibrated using an external hexamethylbenzene standard methyl resonance at 17.3 ppm. On the same instrument, Bloch decay mode (Single Pulse Excitation) was operated with a proton pulse of 5.5 µs and decoupling power of 45 kHz. The MAS speed was 3 kHz. The delay time after the acquisition of the FID signal was 30 s. BET analysis. The regenerated cellulose membranes and the cellulose composite nanofiltration membranes were freeze-dried and cut into a 0.2 x 0.2 cm piece. The surface area, pore volume and pore size distribution of the BCM and IP-NF-BCM were measured using the nitrogen adsorption method. The sample tube was charged and subjected to a BET test (Beijing Beside Instrument Technology Co., Ltd, 3H-2000 BET-A) to analyze the specific surface area and average pore diameter. AFM analysis. The surface morphology and roughness of the regenerated cellulose membranes were examined with an AFM(Changsha Branch US Analytical Instrument Co., Ltd., Hitachi AFM5100). Desalination performance of NF membranes The permeate flux was calculated by Eq. (1): V （1） J= A⋅t Where J is the permeate flux (L·m−2·h−1), V is the volume of permeate (L), A is the effective area of the membrane (m2), and t is the permeate collection time (h). R is the rejection of NaCl, defined by Eq. (2):

R=

C − Cp

C

× 100%

（2）

Where Cp is the ionic conductivity of the permeate (µs/cm) and C is the ionic conductivity of the retentate (µs/cm), measured with ionic conductivity meter (STARTER 3100C, Ohaus, Parsippany, NJ, USA).


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RESULTS AND DISCUSSION Concept of developing cellulose-based TFC NF membrane based on the interfacial polymerization of PIP and TMC

Larger average pore size

Obvious surface complexes Forming regenerated cellulose membrane

Interfacial polymerization of PIP and TMC to form cellulose nanofitration membrane

interfacial polymer layers

Enhance NaCl rejection

Regenerated cellulose membrane

Na+

SO42Cl- H2O Permeation paths

Fig. 2. Process concept for the fabrication of cellulose based nanofiltration membranes. As shown in Fig. 2, the idea was to fabricate a renewable and biodegradable cellulose-based nanofiltration NF membrane by preparing a mesoporous (pore size between 2 to 50 nm) cellulose membrane as the support substrate for a microporous (pore size from 0.4 to 2 nm) thin layer of polyamide. As discussed in the Experimental section, first, bamboo cellulose was dissolved in NMMO solvent to have a cellulose solution. Then the solution was cast on a nonwoven fabric on a glass sheet which was submerged into a water bath. With the NNMO solvent being diluted with water, the dissolved cellulose coagulated to form a gel membrane. The gel membrane was air dried at controlled temperature and humidity to yield a porous cellulose support membrane with good water flux. The NMMO solvent can be recovered and reused for the dissolution and regenerating process. Subsequently, to form a thin layer of polyamide on top of the support substrate, the cellulose membrane was submerged in PIP and TMC solutions sequentially to induce interfacial polymerization. The PIP-TMC thin-film layer effectively reduced the pore size of the composite membrane from about 6 nm to 1 nm, which was of critical importance for nanofiltration processes. The poly(piperazineamide) has carboxylic acid groups due to partial hydrolysis of the acyl chloride unit of TMC during the



interfacial polymerization. The carboxylic acid groups influenced the water flux, as well as ion rejection due to electrostatic repulsion between the charged membrane surface and ions. Effect of interfacial polymerization conditions on NF performance ·

40

30

40

30

36

25

32

25

32

20

24

20

28

15

16

15

24

8

10

10 0.5

1.0

1.5

2.0

2.5

-2

Flux, Lm h

20

3.0

0.05

0.10

0.15

0.20

0.25

0.30

CTMC, %

CPIP, % (c)

(d) 55

32

54

20

50

28

45

18

45

24

36

16

40

20

27

14

35

16

18

12

30

9

10

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

-2

Flux, Lm h

-2

12

-1

22

Rejection, %

63

-1

36

25 0

5

TTMC, min

10

15

20

25

30

Operating time, h

Fig. 3. Effect of interfacial polymerization conditions on the performance of the cellulose composite nanofiltration membranes (a: concentration of PIP, same for the others；b: concentration of TMC；c: reaction time；d: running time of composite nanofiltration membrane) Fig. 3 shows the process parameters of the interfacial polymerization on the nanofiltration performance of resulting cellulose composite membranes. In Fig. 3a, the concentration of the piperazine solution increased from 0.50 wt% to 3.0 wt%, while the TMC concentration and reaction temperature and time remained constant. When increasing PIP concentration from 0.5 to 2%, the water flux decreased while the rejection of the resultant membranes to NaCl increased, due to increased thickness and crosslinking of the polyamide thin film layer. However, when the PIP concentration was higher than 2.0% the rejection rate started to decrease. At a high concentration of polypiperazine, the polymerization rate was too fast, resulting in structure non-uniformity of the polyamide film. Furthermore, excessive amounts of PIP can diffuse into the oil phase to react with TMC, decreasing the crosslinking of


Rejection, %

-2

Flux, Lm h ·

-1

35

Rejection, %

48

-1

35

Rejection, %

(b)

(a)

Flux, Lm h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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the polyamide film 36. Slightly increased water flux when the PIP concentration was higher than 2% might also indicate structure imperfection of the polyamide functional thin film at a higher PILP concentration. Similarly, in Fig. 3b, the concentration of TMC increased from 0.05 wt% to 0.3 wt%, while the PIP concentration, reaction temperature and time were held constant. The rejection rate of the resultant composite membranes first increased and then decreased, while the water flux decreased first and then increased, with a maximum reject and minimum water flux at 0.15% TMC concentration. The increase in salt rejection rate and decrease in water flux with increased TMC concentration were due to the growth of the polyamide functional film. However, when the concentration was more than 0.15wt%, as the polypiperazine was depleted, the excess of TMC hydrolyzed and caused imperfection to the network structure of the polyamide functional thin film layer. As a result, the rejection rate decreased, and the water flux increased 25, 37-39.

Similar effects of TMC concentration has been reported by others

on the nanofiltration performance of polysulfone based composite membranes. The reaction kinetics of the interfacial polymerization of PIP and TMC was further studied and the results are shown in Fig. 3c. It shows that the polymerization reaction had fast kinetics, and the reaction seemed to complete in about 3 minutes. The changes of the rejection rate and permeate flux were minimal after 3 minutes of reaction. Similar fast polymerization reaction kinetics have been reported in the literature. For example, An et al. observed that a reaction time of 60 s was sufficient in completing the PIP/TMC polymerization reactions under similar conditions 52. Fig. 3c also shows that by increasing reaction time, the salt rejection rate of the composite membrane increased, and the water flux decreased, which can be attributed to the growth of the polyamide functional film in thickness and structure perfection. When the reaction time was 3.0 min, the water flux was 15.68 L/ (m2 • h), and the salt rejection was 40.12%. In Fig. 3d, to evaluate the performance stability, the cellulose composite membrane was subjected to 30 cycles of nanofiltration. Each cycle consisted of 10 hours of nano-filtration of 500 mg/L NaCl water solution under 0.5 MPa pressure and one hour of soaking in 500 ml deionized water to refresh the membrane. The results in Fig. 3d indicate that as the number of cycles increased, the slat rejection rate increased, and water flux decreased gradually. After 30 cycles of use, the salt rejection rate increased by 3.17%, while the water flux decreased by 13.34%. Continuous working under high pressure might cause irreversible compression of the membrane, and salt ions might



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also get trapped in the membrane and plug the pores, all of which in turn decreases the permeability of the membrane. Table 1 Different salt ion rejection and water flux of the resultant IP-NF-BCM cellulose composite nanofiltration membranes Type of Inorganic salt Rejection, % Flux, L/(m2·h)

Na2SO4

NaCl

MgSO4

MgCl2

CaCl2

71.23

40.12

62.33

24.66

29.13

13.04

15.78

14.04

11.94

21.17

Table 1 shows the performance of the IP-NF-BCM cellulose composite nanofiltration membrane in treating different salt solutions under the standard operating conditions (500 mg/L initial slat concentration, 0.5 MPa pressure). The highest rejection rate was observed for Na2SO4, and the lowest for MgCl2. Among all the five salts investigated in the current study, the rejection rate of the cellulose composite membrane to a salt decreased in the order of Na2SO4> MgSO4> NaCl> CaCl2> MgCl2, and the aqueous flux decreased in the order of MgCl2> NaCl> MgSO4 > Na2SO4> CaCl2.

Zarrabi et al also found that the rejection rate of a thin

film composite nanofiltration membrane modified with amine functionalized MWCNT (NH2-MWCNT) was higher to Na2SO4 than to that of NaCl. However, the order of flux was different between the two studies. The flux of Na2SO4 solution was higher than that of NaCl solution in their system, while the opposite was found in our system. The discrepancy was probably due to the difference in chemical composition of the functional thin film layer. FT-IR, 13C-NNR, TGA and XRD analyses


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Absorbance (%)

a

O N C

N

O C

N

N

O C

CO-polymers/IP-NF-BCM

b

O C

m

n

COOH

C

O

CO-polymers 1630 1446 1255

Co-ploymers

C1 IP-NF-BCM

C2,3,5 C4 C2,3,5

IP-NF-BCM BCM

C1 BCM

C6

C6

C4

BC

4000

3500

3000

2500

2000

1500

1000

500

200 180 160 140 120 100

Wavenumber (cm-1) 110

c

298.12

90 80 70 60 50 40 30 20

22.72

16.76

Intensity（ a.u.）

270.73

80

60

40

20

0

δ(ppm)

BC BCM IP-NF-BCM

285.67

100

TG(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17.36 12.48

d

BC BCM IP-NF-BCM

22.74

22.74 16.26

10 0 100

200

300

400

500

600

700

10

20

30

Temperature(°C)

40

50

2θ（ °）

60

70

80

Fig.4. (a) FT-IR spectra of bamboo cellulose (BC), bamboo cellulose support membrane (BCM), bamboo cellulose nanofiltration membrane by interfacial polymerization (IP-NF-BCM); (b)13C-NMR of the BCM and IP-NF-BCM; (c) TG of BC, BCM, IP-NF-BCM; (d) XRD of BC, BCM, IP-NF-BCM. From Fig. 4(a), the conjugation effect of the carbonyl group with the benzene ring of the polyphenazine trimesteryl chloride caused a characteristic absorption peak at near 1630 cm-1, indicating the formation of polyamide by interfacial polymerization. The cellulose nanofiltration membrane compounded the polyphenanthroline trimethyl forming the thin functional layer. The changes in cellulose crystallinity of the BCM and IP-NF-BCM were investigated with a solid state

13

C CP MAS NMR spectrometer (Fig. 4(b)). The

typical peaks for NMMO regenerated cellulose did not appear in the 13C NMR spectra 41

. The NMMO signal at 60.4 ppm that almost disappeared in the CP/MAS mode was

recovered in the Bloch decay mode. The cellulose C1, C2, C3, and C5 peak signals resolutions in the Bloch decay mode were weaker than those in the CP/MAS mode because of poorer signal to noise ratio in the Bloch decay mode. Partial overlap between the cellulose C6 peak and the NMMO peak made it difficult to observe changes in the C6 peak. Significant differences were observed in the C4 peak between the CP/MAS and Bloch decay mode. The strong C4 peak of crystalline cellulose between 86 and 92 ppm disappeared in the Bloch decay mode

42-43

and the strong

-CH2-N peak of crystalline cellulose (48 ppm and 43 ppm) appeared, the strong -C=O-NH- peak of crystalline cellulose (170 ppm) appeared, and the aromatic ring



peak for IP-NF-BCM (136 and 127ppm) appeared. The thermal behavior of natural fiber rubber composites was closely related to its constituents 44-45. TGA showed that the dehydration as well as the degradation of fibers which did not significantly contribute to the weight loss of the composites occurred in the temperature range from 50 to 200°C, but the severe weight loss that appeared in the range of 200-400°C was due to the major decomposition of cellulose, hemicellulose and other organic materials 44. The thermal stability of BC, BCM and IP-NF-BCM films was tested in a nitrogen atmosphere by means of a TG-DTA analyzer. The initial decomposition temperatures of BC, BCM and IP-NF-BCM were 285.67°C, 270.73°C and 298.12°C, respectively, and the corresponding maximum decomposition temperatures were 370.42°C, 342.99°C and 350.48°C, respectively. The results show that compared with the original bamboo cellulose (BC), the thermal stability of the regenerated cellulose (BCM) was slightly lower, which might be caused by cellulose degradation in the dissolution and regeneration processes 17, 46-47. However, it is interesting to note that the IP-NF-BCM had improved thermal stability compared to BCM, probably due to the strong interaction between the BCM support layer and the polyamide layer. Fig. 4d shows the XRD spectra of BC, BCM and IP-NF-BCM. The change of the intensity of the two major diffraction peaks at 2θ indicated that significant decrease in cellulose crystallinity in the dissolution and regeneration process 48, as well as in the interfacial polymerization process.

15

Quantity Adsorbed(cm3/g STP)

BET analysis 3 Quantity Adsorbed(cm /g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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a

12 9 6 3 0 -3 0.0

0.2

0.4

0.6

(

0.8

1.0

18

b

16 14 12 10 8 6 4 2 0 -2 0.0

)

0.2

0.4

0.6

0.8

Relative Pressure(P/P0)

Relative Pressure P/P0


1.0

Page 13 of 20

2.0

C -3

IP-NF-BCM

1.5

1.0

3

dV/dD (cm /g/nm)*10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5

0.0 0

10

20

30

40

50

Pore Diameter (nm)

Fig.5. BET analyses of the cellulose membranes: a. BCM substrate membrane; b. IP-NF-BCM membrane; c. pore size distribution of the IP-NF-BCM membrane. Figure 5 shows the BET nitrogen absorption of the cellulose membranes. It shows that, by increasing relative pressure, the regeneration of the adsorption capacity of the film decreased slightly, followed by a sharp rise. This was a typical type of isotherm for porous materials

46, 49

. As shown in Table 2, the average pore size of the

regenerated cellulose membranes was 6.2 nm and the BET surface area was 11 m2/g. For the cellulose based nanofiltration membranes, the BET surface area was 32 m2/g, and the average pore size was 1.0 nm. These results indicate that forming a thin layer of polyamide film on top of the cellulose support membrane by the interfacial polymerization filled up bigger pores and created many nano pores at the same time. As a result, the average pore size decreased, and the specific surface area increased. Table 2 Specific surface area, pore size and pore volume of the BCM and IP-NF-BCM membranes Type of membrane

BET Surface Area 2

-1

/(m ·g )

Pore Volume 3

Pore Size

-1

/nm

-2

6.2 1.0

/(cm ·g )

BCM

1.1×10

1.7×10

IP-NF-BCM

3.2×10

2.4×10-2

Morphology



Fig.6. SEM images of the BCM and IP-NF-BCM membranes. a. BCM top surface; b. IP-NF-BCM top surface; c. IP-NF-BCM cross section；d. AFM image of BCM top surface; e. AFM image of IP-NF-BCM top surface. As discussed earlier, it is important to prepare a porous regenerated cellulose membrane with a good water flux as the supporting substrate for the polyamide functional thin film. The porous morphology of the BCM regenerated cellulose support membrane is rather evident (Fig 6 a), its average pore size is 6.2 nm (Table 2). After interfacial polymerization of PIP and TMC, a highly porous polyamide film was formed on top of the BCM membrane (Fig. 6 b), with the average pore size decreased to 1.0 nm (Table 2). The topology of both the BCM regenerated cellulose support membrane and the IP-NF-BCM membrane was further studied by AFM, and the results in Fig. 6 (d and e) demonstrate that the surface roughness of the BCM membrane increased from 0.018 um to 0.105 um after the interfacial polymerization. In the interfacial polymerization process, the membrane growth was controlled by the partition of diamine monomer through nascent film. The membrane growth was controlled by partition of diamine monomer through the nascent film 50-51. The above results are consistent with those in a previous study 52, in which the RMS roughness value of a membrane was found to increase after the interfacial polymerization. The SEM image of the cross- section of the IP-NF-BCM membrane was shown in Fig. 6c. It is evident that the PIP/TMC polyamide layer (about 10 µm in thickness) is on top of the cellulose supporting film, which has an average thickness of 44±2 µm. The clear border line between the top layer and the support layer indicates that the


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diffusion of PIP monomer into the substrate was limited to a very short depth from the interface. This may be attributed to the small pore size of the regenerated cellulose films. The SEM image in Fig 6c also demonstrates that the in-situ polymerization of PIP and TMC resulted in a microporous layer that markedly decreased in the pore size of the membrane from 6.2 nm down to 1.0 nm (Table 2).

Fig. 7 Shape diagram of drip film surface (a: BCM; b: IP-NF-BCM) Contact angle analysis was performed to determine the hydrophilicity of the original cellulose supporting films and the IP-NF-BCM membranes, and the results are included in Fig 7. The contact angle for the BCM films was 35.9±3.9°, and 56.1±2.7°for the IP-NF-BCM membranes, indicating that the surface hydrophilicity decreased after the interfacial polymerization. Cellulose substrate itself is highly hydrophilic due to abundance of hydroxyl groups. After the interface modification, the formation of a polyamide layer on the surface covered up the hydroxyl groups and reduced the surface hydrophilicity. CONCLUSION A novel cellulose based composite nanofiltration membrane (IP-NF-BCM) was successfully fabricated through interfacial polymerization of amino-functional piperazine and 1,3,5-trimesoyl chloride on top of a porous bamboo cellulose support membrane. The average pore size of the bamboo cellulose support membrane was about 6 nm, which was reduced to about 1.0 nm after the deposition of a functional polyamide film on top of the cellulose membrane by interfacial polymerization. Nanofiltration performance tests show that the rejection rate of the IP-NF-BCM membranes to NaCl was about 40% with a saline aqueous flux of about 16 L/(m2·h), under the standard operating conditions (500 mg/L initial saline concentration, 0.5 MPa pressure).

AUTHOR INFORMATION Corresponding author: [email protected](Y.N.), [email protected] (L.H.)



Notes The authors declare no competing financial interest.

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Preparation and Characterization of Cellulose-Based Nanofiltration

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