Pore-size control of chitin nanofibrous composite membrane using metal-organic frameworks

Herein, environmentally benign chitin nanofiber (ChNF) membranes were fabricated by regulating suspension behavior. The introduction of zeolitic imidazole frameworks (ZIF-8) into the composite membranes led to the domain formation of ChNF derived by coordinative interaction, resulting in pore size-tunable membranes.

Available online 16 October 2021

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Research paper

Pore-size control of chitin nanofibrous composite membrane using

metal-organic frameworks

aMaterials Architecting Research Center, Korea Institute of Science Technology, Seoul 02792, Republic of Korea

bDepartment of Organic and Nano System Engineering, Konkuk University, Seoul 05029, Republic of Korea

cDepartment of Chemical and Biological Engineering, Korea University, 5-1 Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea

dDivision of Nano & Information Technology, KIST School, Korea University of Science and Technology, Seoul 02792, Republic of Korea

eKHU-KIST Department of Converging Science and Technology, Kyung Hee University, Seoul 02447, Republic of Korea

A R T I C L E I N F O

Keywords:

Chitin nanofiber (ChNF)

Pore size control

Bioderived membrane

Water purification

Rheology

A B S T R A C T Herein, environmentally benign chitin nanofiber (ChNF) membranes were fabricated by regulating suspension behavior The introduction of zeolitic imidazole frameworks (ZIF-8) into the composite membranes led to the domain formation of ChNF derived by coordinative interaction, resulting in pore size-tunable membranes Based

on the rheological, morphological, and structural characterizations, the driving force of pore-size control was studied in the aqueous suspension of ChNF and ZIF-8 according to the relative concentration At critical con-centration, the 30-ChNF membrane presents superior water permeance (40 LMH h− 1) while maintaining a high

rejection rate (>80% for all organic dyes) Moreover, the molecular size cut-off of the composite membranes for

dyes can be controlled in the range of less than 1 nm to 2 nm The experimental results provide a simple strategy for the preparation of pore tunable ChNF membranes using MOF with high mechanical strength, good durability, high flux, dye rejection, and antifouling ability

1 Introduction

In general, nanofibers are defined as fibers with diameters less than

Xia, 2004) Material-wisely, nanofibers are interesting nanomaterials

due to their unique features from dimensional, optical, and mechanical

range of application areas such as nanocomposites, filtrations, energy

Mandal, 2018; Seo, Cho, Lee, Lee, & Baek, 2020;) Most of all, the

nanofibrous structure has been widely utilized in membrane technology

et al., 2021) Among various pollutants in wastewater, organic dyes have

raised great concerns due to their enormous usage, and lethality Not

only are the dyes toxic and carcinogenic, but even small amounts can

& Sharma, 2021; Rajabi, Mahanpoor, & Moradi, 2017; Robati et al.,

2016; Zhu, Chen, & Lou, 2012) As an effective way to remove dyes, the nanofibrous membrane presents high performances due to the three-

nanofibrous membrane was produced from a polymer solution by electrospinning and could be controlled by their diameters or

However, the electrospinning process requires a large amount of organic solvents with low productivity, causing environmental pollution With increasing environmental concerns, nanofibers from the hier-archical structures of biomass have been greatly gained attention by their environmental friendliness In nature, many types of biomasses have nanofibers in their complex hierarchical organization such as

(Isogai, Saito, & Fukuzumi, 2011; Rossi et al., 2021) This has triggered many researchers to explore the application of membrane technology using biomass-driven nanofibers The hydrophilicity of biomass-driven nanofibers can give an advantage for improving the membrane

* Corresponding authors at: Materials Architecting Research Center, Korea Institute of Science Technology, Seoul 02792, Republic of Korea

E-mail addresses: scho@kist.re.kr (S Cho), baek@kist.re.kr (K.-Y Baek)

1 These authors contributed equally

Carbohydrate Polymers

https://doi.org/10.1016/j.carbpol.2021.118754

Received 17 August 2021; Received in revised form 23 September 2021; Accepted 6 October 2021

biomass is highly required to broaden its applications: for example,

hemofiltration of blood for medical care, purification of industrial

modulation First, the fabrication of nanofibrous membranes with

smaller diameters induces dense packing of nanofiber, resulting in pore

morphology with a thinner active layer can induce opening inter-

Shi, & Fang, 2017) For instance, Buehler and coworkers (Ling, Jin,

Kaplan, & Buehler, 2016) demonstrated the control of membrane pore

size by varying active layer thickness (<500 nm) using silk nanofiber

(SNF) However, these approaches were limited to ultra-thin active layer

thickness (<500 nm) Also, the membranes require support layers and

undergo compaction against the hydraulic pressure, resulting in the flux

Tuuva, & M¨antt¨ari, 2013) Lastly, the insertion of nano-dimensional

additives could tune the membrane pore size via the regulation of

Sun et al., 2021) However, the pore-modulation of the membrane using

nanofiber interaction to directly prepare the membrane from the

aqueous dispersion has been less explored

Recently, the metal-organic frameworks (MOF) have been emerging

nanoparticles for carbohydrate-based nanofibers as a provider of

phys-ical and chemphys-ical functions Their composite showed extraordinary

features in the catalytic degradation, absorbent, and water purifications

due to high surface area, high chemical activity, and physical tolerance

(Abdelhameed, El-Shahat, & Emam, 2020; Abdelhameed & Emam,

2019; Emam, El-Shahat, & Abdelhameed, 2021; Khajeh, Oveisi,

Bar-khordar, Rakhshanipour, & Sargazi-Avval, 2021; Liu et al., 2021; Liu

et al., 2021) However, there was a lack of study on the MOF role for the

interaction changer of pore-size tuning in carbohydrate-based

mem-brane technologies In our group, we demonstrated that the zeolitic

imidazolate frameworks (ZIF-8) could induce the porous morphologies

of cellulose nanofibers for the water treatment membrane with selective

perfor-mance for purifying wastewater, the precise control of pore size could

not be achieved using the in-situ growing ZIF-8 at the nanofibers The

MOF grown at the fibers can deform the nanofiber shape due to the

strong adhesion rather than changing the fiber-to-fiber interaction

Moreover, the total amounts, particle sizes, and defect sites of MOF can

be limited by the in-situ synthesis condition of MOF in the matrix

Therefore, a novel approach is required for the simple utilization of

pore-size controllable membranes using carbohydrate-based nanofibers

In this study, we focused on the pore-size modulation of the bio-

derived nanofibrous membrane in the nanoscale It was assumed that

the regulation of fiber-to-fiber interaction can induce the formation of

domain structures, resulting in the tunable pore size of the membrane

Among bio-derived nanofibers, the chitin nanofiber (ChNF) was selected

as a platform material due to its superior hydrophilicity, anti-bacterial

Mah-moodi, 2020) As a nanofiber interaction changer, the ZIF-8 with a

2.1 Materials

fiber length, 5 nm of mean diameter) was purchased from ANPOLY (Republic of Korea) The ChNF suspension was titrated to pH 7.0 with 1

98%), 2-methylimidazole (99%), bovine serum albumin (BSA), orange G (OG, 80%), Janus Green B (JGB, 65%), bromothymol blue (BB, 95%), vitamin B12 (VB12, 98%), methyl orange (MO, 85%), and methylene blue (MB, 82%) were purchased from Sigma Aldrich All chemicals were

was used in all experiments

2.2 Hydrothermal synthesis of ZIF-8

Zeng, Zhao, & Lai, 2011) Briefly, 0.41 g of Zn(NO3)2∙6H2O (0.0013 mol), and 7.92 g of 2-methyl imidazole (0.096 mol) were separately dissolved in 15 mL of water After 10 min of sonication, the two

24 h The crude product was washed by centrifugation against water and

2.3 Preparation of ChNF/ZIF-8 composite membranes

The fabrication of ChNF/ZIF-8 composite membrane is illustrated in

Fig 1a ZIF-8 was blended with ChNF suspension for the concentrations

of ZIF-8 within the resulting composite membranes to be 10, 20, 30, 40, and 50 wt% (Table S1) The composite dispersions were filtered upon quantitative filter papers (No 6, Advantec, Japan) using a dead-end filtration apparatus (Amicon 8050, Millipore, USA) under 1 bar with a

the input amount of ZIF-8 in the suspension; for example, 10-ChNF re-fers to the ChNF composite membrane with the ZIF-8 input fraction of

10 wt%

2.4 Characterizations

The contents of ZIF-8 in the composite membranes were character-ized by thermogravimetric analysis (TGA, TA Instrument TGA Q-50)

characterize the crystal structure of the samples in the scanning degree

Å) The chemical structure of the specimens was investigated with Fourier-transform infrared spectroscopy (FT-IR, Thermo Nicolet iS10 equipped with attenuated total reflectance (ATR)) The rheological properties of the ChNF/ZIF-8 aqueous dispersion were measured by a rotational rheometer (RS-1, HAAKE) using a parallel disk type in steady mode The surface zeta-potential and the particle size of the composite

Otsuka electronics) The N2 adsorption behavior was confirmed by

BELSORP-Mini II (MicrotracBEL, Japan) under 77 K The morphologies

of the composite membranes were observed by field emission scanning

electron microscopy (FE-SEM, FEI Inspect F50) To prohibit the

SEM measurements The nano-indentation (Bruker, TI-950 with

Berko-vich Tip) was used to evaluate the mechanical properties of the

mem-branes The rejection rate of the organic dyes in the filtered water was

calculated from the absorption intensities using UV–Vis spectroscopy

(JACSO V-670)

2.5 Membrane performance

The water permeability of the membranes was measured using a

dead-end filtration (Amicon 8050, Millipore, USA) The effective area

of permeate (ΔV) with the interval time (Δt)

Pw(LMH∙bar− 1=L∙m− 2∙h− 1∙bar− 1)

=ΔV∙A− 1∙Δt− 1∙P− 1

The organic dye solution was prepared as 10 ppm, and the rejection

rate (R) for the organic dye was calculated by the absorbance of feed

)

∙Asf− 1∙100 (%)

3 Results and discussion

3.1 Microdomains formation in composite suspension

Fig 1 describes the pore-forming process during the membrane

local concentration of the composite suspension became higher in

structure of ChNF can be expected as a densely packed membrane with low water permeance owing to the strong hydrogen bond between

suitable additive could be changed by different fiber-to-fiber in-teractions, resulting in the formation of a specific domain structure and

we utilized ZIF-8 as domain former for ChNF membrane with its dual action of attractive interaction by metal-amine interaction and repulsive interaction by same surface charge with ChNFs (Interaction mechanism between ZIF-8 and ChNF was illustrated in Scheme S1)

A rheological study was performed to understand the interaction

versus shear rate for ChNF and ChNF/ZIF-8 composite aqueous

disper-sions For the ChNF dispersion, two-regime flow behavior was observed The viscosity of the ChNF dispersion gradually decreased below the

Fig 1 Schematic illustration for (a) a wet-laid process for membrane fabrication and (b) suggested pore-forming mechanism with ChNF and ZIF-8

Fig 2 (a) Viscosity-shear rate curves of ChNF and ChNF/ZIF-8 aqueous dispersion The inserted images are 30-, 40-ChNF dispersion after leaning (b) Schematic

illustration for the flow behavior of ChNF according to ZIF-8 loading amounts under shear rate

does not destroy the entanglement of nanofibers Above the shear rate

enough for the disentanglement of nanofibers, the viscosities of the

composite dispersion more rapidly decreased with the increase of shear

behavior was a similar phenomenon with the anisotropic dispersion of

Li, Revol, & Marchessault, 1996; Wissbrun, 1981) The strong metal-

amine interaction between ChNF and ZIF-8 could lead to the

forma-tion of micro-domains between ChNF and ZIF-8 with the morphology of

could be confirmed in the result of a dynamic light scattering (DLS)

analysis, exhibiting additional particles with a size around 100–200 nm

(Fig S1) The reduced number of free nanofibers gave rise to the

the friction of the nanofibers increases, which leads to the decrease of

the viscosity drop rate similar to that of the ChNF dispersion

For ChNF/ZIF-8 dispersion with the ZIF-8 contents over 40 wt%, the

right) The excessively loaded ZIF-8 would be positioned between the

domains and increase the inter-domain interaction, resulting in a gel-

In the DLS analysis, the presence of free ZIF-8 was also observed in the

dilute dispersion of ChNF/ZIF-8 with ZIF-8 contents over 40 wt%

(Fig S1) As the shear rate increased, the viscosity descended with the

contents of 0 to 30 wt% It can be interpreted that the ZIF-8 between the

et al., 2018) The pristine ChNF membrane showed three steps of weight

degradation of the acetyl groups, and the backbone of chitin,

ZIF-8 contents for each sample were 8.2, 19.8, 30.7, 38.9, and 53.0 wt% for 10-, 20-, 30-, 40-, and 50-ChNF, respectively (Table S1) The contents

of ZIF-8 in the composite were not different from the input amounts It refers that each structure of ZIF-8 and ChNF did not collapse while the ChNF holds physically the ZIF-8

Fig 3b shows the XRD patterns of the ChNF/ZIF-8 composites The

prepared ZIF-8 had sharp peaks at 2θ = 7.3, 10.4, 12.7, 14.7, 16.4, and

18◦ (Cho et al., 2018) The ChNF exhibited the typical crystalline

crystalline structures as both characteristic patterns of ZIF-8 and ChNF The peaks associated with ZIF-8 in the composite membranes became more intense with the higher content of ZIF-8 As coincident with the XRD result, the characteristic peaks of ChNF and ZIF-8 were retained in

composite membranes, the characteristic vibration peaks for both ChNF

The XRD and FT-IR results imply that the chemical structure of ChNF and ZIF-8 did not change even after blending As a result, both ChNF and ZIF-8 have excellent stability in the physical and chemical states in the composite membranes

The surface morphologies of the prepared membranes were

surface with randomly distributed nanofibers In the composite

Fig 3 (a) TGA, (b) XRD, and (c) FT-IR spectra of ChNF, ZIF-8, and the composite membranes

membranes, the surface became rougher with the higher amount of ZIF-

8 The surface of the composite membrane with lower 30 wt% ZIF-8

showed relatively smoother and no noticeable free ZIF-8 nanoparticles

due to the formation of nanofiber-wrapped ZIF-8 (ZIF-8@ChNF) as

discussed above However, the surface turned coarse distinctively at

above 40 wt% of ZIF-8 loading These results were supported by DLS and

surface zeta-potential analyses (Figs S1 and S2) For the composite

dispersions with ZIF-8 loading from 10 to 30 wt%, the surface charges

were steady at 11 to 13 mV, which was close to that of the ChNF

dispersion On the other hand, the surface charges abruptly increased in

the composite dispersion for 40-ChNF and 50-ChNF due to the free ZIF-8

on the surface After the filtration process, some free ZIF-8 nanoparticles

laid on the surface of the resulting membranes, leading to a sandpaper-

like morphology

Then, the thickness of the composite membranes using SEM analysis

was measured to study the volume of the composite membranes related

smooth surface line with a densely packed cross-section The addition of

ZIF-8 in the composite dispersion led to the thickness expansion of the

membranes The thickness of the composite membranes less than 30 wt

% of ZIF-8 was well-matched with the calculated values based on the

Jiang, Peterson, & Qin, 2018) It reveals that the composite membrane

up to 30 wt% loadings of ZIF-8 did not possess any additional pores except the pores within the ChNF/ZIF-8 domain Notably, the 40-ChNF and 50-ChNF membranes were much thicker than the calculated value This result indicates the existence of additional pores Overall, ZIF-8 played a critical role as a binder and a spacer of the nanofibers Below

30 wt% loading, most of the ZIF-8 was wrapped by ChNF (ZIF-8@ChNF),

as seen in DLS analysis (Fig S1) The composite membranes exhibited densely packed structures by the interaction between ZIF-8@ChNF domain and ChNF as like the ChNF membrane itself Above 40 wt% loading of ZIF-8, the additional free ZIF-8 nanoparticles widened the gap among ChNFs due to the repulsive interaction of the same positive surface charges for both free ZIF-8 and ChNF, which introduce pores into the membranes

The surface area and pore size of the composite membranes were

particles showed a typical type I adsorption by micro-porosity On the

Fig 4 SEM images of ChNF and ChNF/ZIF-8 composite membranes: (a) 0-ChNF, (b) 10-ChNF, (c) 20-ChNF, (d) 30-ChNF, (e) 40-ChNF, and (f) 50-ChNF (Inset

present SEM images with higher magnification)

Fig 5 Cross-sectional SEM images of the prepared membranes: (a) 0-, (b) 10-, (c) 20-, (d) 30-, (e) 40-, and (f) 50-ChNF (g) Plot of the membrane thickness vs ZIF-

8 contents

adsorption compared to the ZIF-8 The N2 adsorption behavior of the

composite membranes was dependent on the amount of ZIF-8 loaded

The surface areas of the composite membranes were calculated by the

BET method The surface area shows good linearity with ZIF-8 contents,

indicating the successful introduction of ZIF-8 into a composite (Fig S3)

Pores in the composite membranes could be formed either by ZIF-8 itself

size distribution of the prepared membranes using the MP method The

ZIF-8 had 7 Å sized pores, which is in good agreement with the previous

sized pores of 0.7, and 1.3 nm, while 40- and 50-ChNF had three sized

pores of 0.7, 1.3, and 1.8 nm In addition, the more ZIF-8 loaded within

the membranes (40-, 50-ChNF), the higher the fraction of 1.3 and 1.8 nm

pores in all composites We assume that the 1.3 nm-pores are derived in

the domain region between ChNF and ZIF-8, that is, ZIF-8@ChNF, where

ZIF-8 effectively pulls the nanofibers as a binder However, the 1.8 nm-

pores appear only in 40- and 50-ChNF The excess amounts of ZIF-8 can

be positioned between the domains, resulted in large-sized pores by

and pore volume distribution with specific diameters in the composite

membranes This trend according to the ZIF-8 contents is coincident

with the rheological and morphological studies These results suggest

that we can modulate the pore structure by controlling ZIF-8 contents

Furthermore, the homogeneous tuning of the pore structure could

enable a superior separation for the composite membranes

3.3 Membrane performance

We investigated the water treatment performance of the membranes

of the ChNF/ZIF-8 membranes was gradually improved as the amounts

of ZIF-8 increased, which is well correlated with the increase of porosity according to the ZIF-8 contents The membranes were completely compressed for 12 h against the hydraulic pressure The permeance drop

membrane The DR of the 0-, 10-, 20-, 30-, 40- and 50-ChNF were 17.8,

32.7, 31.5, 25.6, 30.2 and 46.2%, respectively Among the membranes, 30-ChNF had the lowest value of the drop rate A well-matched ratio for the strong interaction between the ChNF and the ZIF-8 could attribute to the high durability of 30-ChNF The filtration performance of the membranes was examed for organic dyes after compaction The com-posite membranes were used for the dye rejection of JGB according to

ChNF exhibited a rejection rate of over 99% For the 40-, and 50- ChNF membranes, the rejection rates were decreased This abrupt drop in rejection rate was related to the pore size of the membranes To evaluate the rejection mechanism, the organic dyes were tested with various surface charges and different molecule sizes (Table S2) The

rejection rate of 30-ChNF against the molecular size of organic dyes For

all the composite membranes, the rejection rate versus the molecular size

of dye shows in Fig S5 The rejection rate of 30-ChNF only depended on the molecular size of the dyes In detail, the dyes (JG, VB, and BBR) with molecular sizes over 1.4 nm were filtered off with a rejection rate over 98% even with a different charge The rejection rate of 30-ChNF started

to decrease with the molecular size close to 1.3 nm (MB and MO) A noticeable drop in rejection rate was observed for dyes smaller than 1.2

nm The molecular size cut-off of 1.3 nm for the rejection rate is close to

Fig 6 (a) N2 adsorption isotherm at 77 K and (b) pore size distribution of ZIF-8, ChNF membrane, and ZIF-8/ChNF composite membranes

Table 1

The summarized data for the pore structure of ChNF and ZIF-8 composite membranes

Sample name ZIF-8 contents

(wt%) Measured thickness

a

(μm) Calculated thickness

b

(μm) Total pore volume (Vp)

c

(cm 3 ) Pore volume

c (cm 3 ) 0.7 nm pores 1.3 nm pores 1.8 nm pores

10-ChNF 8.2 16.8 ± 1.7 16.8 0.0017 0.0006 0.0011 –

20-ChNF 19.8 18.2 ± 1.7 18.3 0.0020 0.0008 0.0012 –

30-ChNF 30.7 20.3 ± 2.2 20.3 0.0027 0.0012 0.0015 –

40-ChNF 38.9 27.2 ± 3.0 22.9 0.0096 0.0023 0.0023 0.0050 50-ChNF 53.0 33.7 ± 3.8 26.5 0.0153 0.0039 0.0044 0.0070

aThe thickness of the membrane was measured by SEM image

b The thickness of the membrane was calculated based on the relative volume expansion per the amount of ZIF-8 added

cTotal pore volume was calculated by N2 adsorption isotherm

the pore size of the dried 30-ChNF analyzed by the N2 absorption

behavior This result suggests that the porous structure of ChNF/ZIF-8

membranes can be precisely tuned with an inter-particle interaction in

the nanoscale for specific target molecules Moreover, the JGB rejection

(99%) of 30-ChNF was stably maintained after long-term use of 8 h

(Fig S6) This high stability of the membrane was characterized by the

FT-IR and XRD analysis (Fig S7) The chemical and crystal structure did

not change before and after the use of dye rejection Also, the 30-ChNF

showed good stabilities in the range of natural and basic conditions

(Fig S8) The acidic condition of pH 4.0 led to the flux drop, which was

2018)

In water treatment applications, mechanical strength and hardness

are key factors for the long-term use of the membrane The mechanical

properties of the prepared membranes were tested by the nano-

mem-branes were kept to that of 0-ChNF at ca 5.5 GPa up to 30-ChNF The

low porosity mainly formed within the domain of ChNF and ZIF-8 (ZIF-

8@ChNF) attributes to an equivalent modulus at these ranges However,

the reduced modulus and hardness considerably decreased along with

the contents of ZIF-8 for 40- and 50-ChNF The reduction would be

induced by the enlarged space among inter-nanofibers as described

above

The anti-fouling characteristics of the prepared membranes were

The 0-ChNF had a declined flux of ca 20% within 9 h by the protein

Then, it recovered the flux to around 92% after the hydrodynamic

rinsing As well-known, the hydrophilicity of the ChNF membrane can

suppress the attachment of BSA due to the water layer upon the

mem-brane The fouling of 30-ChNF was slightly more than that of 0-ChNF It

could be interpreted that the rougher surface and the higher surface area accelerated adhesion of BSA onto the 30-ChNF surface However, the flux of 30-ChNF has successfully recovered to 90% after hydrodynamic rinsing

4 Conclusion

In this work, we successfully developed the pore size modulated membranes by regulating of suspension behavior of ChNF using ZIF-8 The composite ratio of ChNF and ZIF-8 affected the flow behavior of ChNF/ZIF-8 suspension At the low concentration of ZIF-8, the strong amine-metal interaction formed ZIF-8@ChNF micro-domains Above the threshold amount of ZIF-8, then, the excess ZIF-8 acted as a spacer The ChNF/ZIF-8 ratio-dependent microstructure within the precursor suspension allowed homogeneous pore size tuning, resulting in superior

mo-lecular cut-off of the membrane (in range of 1 nm to 2 nm) In addition to the tunable pore size, the composite membranes had superior merits on mechanical strength, long-term usability, filtration performance, and anti-fouling These results can provide the fundamental idea for high- performance nanofibrous membranes capable of controlling the pore size using the particle-to-particle interaction

Fig 7 (a) Pure water flux vs time curves of the prepared membranes (b) Permeance and rejection rate of the prepared membranes against JGB (c) Rejection rate vs

molecular size of organic dyes for the composite membranes from the result in Table S2

Fig 8 Reduced moduli and hardnesses of the composite membranes

Fig 9 Anti-fouling properties of 0-ChNF and 30-ChNF against BSA

This work was supported by National Research Foundation of Korea

(NRF- 2020M3H4A3106354) and KIST institutional program This work

was partially supported by the Technology Innovation Program

(20008653) funded by the Ministry of Trade, Industry & Energy

(MOTIE, Korea)

Appendix A Supplementary data

org/10.1016/j.carbpol.2021.118754

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