Thiol-functionalized cellulose nanofiber membranes for the effective adsorption of heavy metal ions in water

Thiol-functionalized cellulose nanofiber membranes for the effective adsorption of heavy metal ions in water

Journal Pre-proof Thiol-Functionalized Cellulose Nanofiber Membranes for the Effective Adsorption of Heavy Metal Ions in Water Hyeong Yeol Choi, Jong H...

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Journal Pre-proof Thiol-Functionalized Cellulose Nanofiber Membranes for the Effective Adsorption of Heavy Metal Ions in Water Hyeong Yeol Choi, Jong Hyuk Bae, Yohei Hasegawa, Sol An, Ick Soo Kim, Hoik Lee, Myungwoong Kim

PII:

S0144-8617(20)30055-2

DOI:

https://doi.org/10.1016/j.carbpol.2020.115881

Reference:

CARP 115881

To appear in:

Carbohydrate Polymers

Received Date:

19 November 2019

Revised Date:

1 January 2020

Accepted Date:

13 January 2020

Please cite this article as: Choi HY, Bae JH, Hasegawa Y, An S, Kim IS, Lee H, Kim M, Thiol-Functionalized Cellulose Nanofiber Membranes for the Effective Adsorption of Heavy Metal Ions in Water, Carbohydrate Polymers (2020), doi: https://doi.org/10.1016/j.carbpol.2020.115881

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Thiol-Functionalized

Cellulose

Nanofiber

Membranes for the Effective Adsorption of Heavy

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Metal Ions in Water

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Hyeong Yeol Choi,1, 2 Jong Hyuk Bae,1, 3 Yohei Hasegawa,4 Sol An,5 Ick Soo Kim,4,* Hoik Lee,1,*

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Myungwoong Kim,5,*

Research Institute of Industrial Technology Convergence, Smart Textiles R&D Groups, Korea

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Institute of Industrial Technology, Ansan 15588, Korea

Department of Advanced Organic Materials and Textile System Engineering, Chungnam National

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University, Daejeon 34134, Korea

Department of Organic and Nano Engineering, Hanyang University, Seoul 04763, Korea

4

Nano Fusion Technology Research Group, Division of Frontier Fibers, Institute for Fiber

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3

Engineering (IFES), Interdisciplinary Cluster for Cutting Edge Research (ICCER), Shinshu University, Tokida 3-15-1, Ueda, Nagano 386-8567, Japan. Department of Chemistry and Chemical Engineering, Inha University, Incheon 22212, Korea.

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*Corresponding

authors:

Myungwoong

Kim

([email protected]),

Hoik

Lee

([email protected]), Ick Soo Kim ([email protected]) Keyword: Cellulose; Thiol functionality; Nanofiber; Metal ion; Adsorption Isotherm; Adsorption kinetics

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Highlights:  

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 

Thiol functionalization on cellulose nanofiber surface imparting ability to adsorb met al ions Adsorption occurring only on the surface with homogeneously distributed adsorption energy Kinetic studies revealing the role of surface thiol in metal ion adsorption mechanism Expandability of cellulose for biocompatible, nontoxic, and sustainable water purifica tion membrane applications

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Abstract This work reports the fabrication of a thiol-functionalized cellulose nanofiber membrane that can effectively adsorb heavy metal ions. Thiol was incorporated onto the surface of cellulose nanofibers, which were fabricated by the deacetylation of electrospun cellulose acetate nanofibers and subsequent esterification of a thiol precursor molecule. Adsorption mechanism was investigated using adsorption isotherms. Adsorption capacity as a function of adsorbate

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concentration was described well with Langmuir isotherm, suggesting that metal ions form a

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surface monolayer with a homogenously distributed adsorption energy. maximum adsorption capacities in the Langmuir isotherm for Cu(II), Cd(II), and Pb(II) ions were 49.0, 45.9, and 22.0

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mg·g-1, respectively. The time-dependent adsorption capacities followed a pseudo-second-order

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kinetic model, suggesting that chemisorption of each doubly charged metal ion occurs with two thiol groups on the surface. These results highlight the significance of surface functionality on

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biocompatible, nontoxic, and sustainable cellulose materials to expand their potential and applicability towards water remediation applications.

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Keywords: Thiol functionality; Nanofiber; Metal ion; Adsorption Isotherm; Adsorption kinetics

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Introduction As the quantity of industrial wastewater increases, water pollution and contamination by heavy metal ions in wastewater have become serious environmental issues. Heavy metal ions, such as Hg(II), Cr(IV), Cu(II), Cd(II), and Pb(II), have adverse effects on the aquatic ecosystem, and eventually humans by their accumulation in tissues, resulting in serious health issues. Over the last

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few decades, a number of strategies, including chemical precipitation,(D. Feng, Aldrich, & Tan, 2000) ion exchange,(Da̧browski, Hubicki, Podkościelny, & Robens, 2004) coagulation

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sedimentation,(Charerntanyarak, 1999; El Samrani, Lartiges, & Villiéras, 2008) and membrane

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separation,(Ortiz et al., 1992) have been extensively studied to eliminate the ions in aqueous effluents from a range of sources. Among these approaches, the use of a membrane to adsorb metal

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ions has been considered as an effective method for water remediation because it can remove relatively large quantities of metal ions by the large surface area of the membrane.(Ki, Gang, Um,

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& Park, 2007) Particularly, electrospun nanofiber membranes have great potential as an ideal filter for water remediation owing to their unique characteristics, such as the high surface-to-mass ratio,

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high porosity with excellent pore interconnectivity, flexibility with reasonable strength, submicron pore size, and large area to volume ratio (Lee & Kim, 2018). Cellulose is a naturally abundant renewable natural polymer that is a highly attractive owing

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to its unique properties and characteristics, such as excellent thermal stability, chemical resistance, and biodegradability.(Atila, Keskin, & Tezcaner, 2015) The attractive material has been utilized recently as an alternative of synthetic polymers to solve environmental issues.(Frenot & Chronakis, 2003; Rezaei, Nasirpour, & Fathi, 2015) Cellulose consists of glucose repeating units with abundant hydroxyl groups, which not only enables extensive hydrogen bonding, but also provides excellent reactive sites to incorporate a range of chemical functionalities to achieve

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desirable properties.(d’Halluin et al., 2017) The fibrous structure of cellulose on the nanometer scale also should share the advantages of nanofibrous polymeric materials with a high surface area, controllable porous structure, and exceptional mechanical properties, which facilitate its applications in composite materials, biomedical applications, and protecting clothing.(Lee, Nishino, Sohn, Lee, & Kim, 2018) On the other hand, making cellulose into desired structures, including nanofibers, is challenging because of its strong hydrogen bonding, which leads to

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extremely poor solubility in a number of common solvents.(Xu et al., 2008; C. Zhang et al., 2014)

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More importantly, although its physicochemical structure and properties are suitable for filtration membrane applications, it exhibits poor adsorption behavior for metal ions. For the applicability

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of cellulose for metal removal, blending with other adsorbing materials, such as chitosan, has been

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proposed; however, cellulose does not act as an adsorbent, but as a supporter to improve the mechanical properties.(Phan, Lee, Huang, Mukai, & Kim, 2019) This poor adsorption capability

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limits the potential of cellulose membrane applications for water remediation processes. Although some functionalities such as primary amine and carboxylic acid on cellulose nanofibers have been

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exploited for metal adsorption;(C. Zhang et al., 2017; K. Zhang et al., 2019) one of the most metalinteractive functional group, thiol, has not been extensively investigated as a surface functionality for metal ion removal in cellulose nanofiber membrane.

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To realize thiol-functionalized cellulose as a metal adsorption membrane material, we demonstrate the use of an electrospun cellulose nanofiber membrane functionalized with thiol functionality using a post-electrospinning modification strategy. We recently reported the use of surface thiol functionality to enable an effective pathway to introduce a variety of chemical functionalities through efficient chemical reactions.(An et al., 2019) Because cellulose exhibits poor solubility in many common solvents, cellulose acetate (CA), which is soluble and can be

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processed in a range of solvents, was first used to fabricate nanofibers through electrospinning. Subsequently, a deacetylation process was carried out on the CA nanofibers, leading to cellulose nanofibers.(Liu & Hsieh, 2002, 2003; Ma, Kotaki, & Ramakrishna, 2005; Son, Youk, Lee, & Park, 2004) The hydroxyl groups on cellulose nanofiber surface were implemented to functionalize with thiol group, i.e. esterification with 3,3’-dithiodipropionic acid, and further reductive cleavage of the disulfide bond (Figure 1). (Li, Wang, Yang, & Zhang, 2014) This thiol group was versatile to

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bring desired functionalities toward target properties and applications via highly efficient and mild

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surface chemical reactions.(An et al., 2019) The thiol group is a well-known functionality exhibiting exceptional ability to interact and capture metal ions through chelation.(Lagadic,

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Mitchell, & Payne, 2001; Xue & Li, 2008; Yang et al., 2014) Therefore, our strategy to fabricate

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thiol-functionalized cellulose (TC) nanofibers is expected to be effective to impart adsorption capability for heavy metal ions as the large surface area of the nanofiber structure can maximize

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the number of adsorption sites that can interact with heavy metals. The fabricated TC nanofiber membrane was closely examined using a variety of characterization tools including Fourier

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Transform Infrared and X-ray photoemission spectroscopies, evidencing validity of characterization methods and solid confirmation of successful functionalization. The heavy metal adsorption ability of the TC nanofiber membrane was investigated thoroughly as functions of the concentration and contact time for Cu(II), Cd(II), and Pb(II) ions. Further adsorption kinetic

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studies and an interpretation with isotherm and kinetic models clearly revealed the adsorption mechanism of doubly charged metal ions on the surface of TC nanofibers. Moreover, current studies have shown that the modification of cellulose to incorporate thiol-functionality has potential for a sustainable cellulose-based water remediation material, by maximizing metal adsorption capacity with morphology control techniques to increase surface area,(Lee et al., 2018;

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K. Zhang et al., 2019) which can eventually be an important alternative to unsustainable synthetic

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polymers.

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Figure 1. Schematic illustration depicting fabrication process for thiol-functionalized cellulose

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2. Experimental

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nanofiber membrane.

2.1 Materials

Cellulose acetate (CA, Mn ~ 30,000 g/mol), 3,3'-dithiodipropionic acid (DTDPA, 99%), 1,1-

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carbonyldiimidazole (CDI, >90%), and ammonium thioglycolate aqueous solution (AmTG in H2O, 60%) were purchased from Sigma-Aldrich. N,N-dimethylformamide (DMF, 98%), acetone (99.5%), N,N-dimethylacetamide (DMAc, 98%), ethanol (99.5%), phosphate-buffered saline (PBS) solution (pH 7.4), and sodium hydroxide (NaOH, 97%) were obtained from Pharmaceutical Industry Co., Ltd. Copper(II) sulfate pentahydrate (CuSO4·5H2O, 98%), cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O, 98%), and lead(II) nitrate (Pb(NO3)2, 99%) for the metal ion

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adsorption experiments were supplied by Sigma-Aldrich. All reagents were used as received unless otherwise noted.

2.2 Characterization The nanofiber morphologies were examined by scanning electron microscopy (SEM, JSM6010LA SEM, JEOL, Japan). All SEM specimens were coated with platinum using a JFC-1200

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fin coater (JEOL, Japan) for 60 sec prior to SEM imaging for the conductivity of the samples.

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Fourier transform infrared (FT-IR, IR Prestige-21, Shimadzu Co., Japan) spectroscopy was carried out at room temperature between 4000 cm-1 and 600 cm-1 with a resolution of 4 cm-1. X-ray

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photoelectron spectroscopy (XPS, Shimadzu-Kratos AXIS-ULTRA HAS SV, Shimadzu Co., Japan)

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was conducted using an Al X-ray source. The C(1s) peak at 284.6 eV was used as the reference for binding energy calibration. The metal ion concentrations in the solutions before and after the

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adsorption tests were measured using inductively coupled plasma-optical emission spectrometry

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(ICP-OES, SPS 3100, Hitachi High-Tech Science Corporation, Tokyo, Japan).

2.2 3 Fabrication of thiol-functionalized cellulose nanofiber To incorporate thiol groups onto the cellulose nanofibers, the CA nanofibers were first fabricated from a CA solution by electrospinning. The electrospinning apparatus equipped with a

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high voltage power supply (Har-100*12, Matsusada Co. Tokyo, Japan) as a source of the electric field, a plastic syringe containing metallic needle (0.6 mm), and cylinder type metallic drum as a collector was used. A 19 wt% CA solution in a mixed solvent of DMF and acetone (4/6, v/v) was prepared by vigorous stirring at room temperature for 24 hours, which was then injected into the syringe. The electrospinning parameters were fixed to a voltage of 11.5 kV, a tip-to-collector

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distance of 15 cm, and a flow rate of 15 L/min. All electrospinning experiments were conducted at room temperature with a humidity of ~40%. The obtained CA nanofiber was immersed in a 0.05M NaOH aqueous solution at room temperature for 24 hours, followed by thorough washing with ethanol, acetone, and water. The resulting nanofiber mat was dried under vacuum to remove the residual solvents. The cellulose nanofiber mat produced was subjected to an esterification reaction with DTDPA by CDI coupling. DTDPA (0.13 g, 0.62 mmol) and CDI (0.05g, 0.31 mmol)

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were dissolved in 10 mL of DMAc, and the deacetylated cellulose nanofiber mat (0.01 g) was then

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immersed in the mixture, followed by vigorous stirring at 80 °C for 22 hours. The nanofiber mat was then washed with ethanol and acetone and dried at room temperature. Subsequently, to cleave

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the disulfide bond of DPTDA, the DTDPA-incorporated cellulose nanofiber was further immersed

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in 5 g of an aqueous solution of AmTG at room temperature for three hours. The TC nanofiber

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temperature.

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sample was obtained by thorough washing with ethanol and acetone, followed by drying at room

2.4 Adsorption experiments

The metal ion adsorption behavior of the TC nanofibers was studied using aqueous solutions

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of Cu(II), Cd(II), and Pb(II). Batch adsorption experiments were conducted with variations of different parameters, such as the pH, metal ion concentration, and contact time. To accomplish this, the TC nanofiber (0.01g) was soaked and stirred in 50 mL of a metal ion solution under ambient conditions. The pH, metal ion concentration, and contact time were varied from 2.0 to 7.0 using PBS, 20 ppm to 400 ppm, and 10 minutes to 24 hours, respectively. Unless specified otherwise, the pH, concentration, and contact time were fixed to 4.0, 200 ppm, and 10 hours, respectively.

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The metal ion concentration upon each adsorption experiment was determined by ICP-OES, which was then was used for isotherm and kinetic model analyses. The adsorption capacity at equilibrium (qe) was calculated using equation (1) given below:

𝑞𝑒 =

𝑉(𝐶𝑜 −𝐶𝑓 )

(1)

𝑚

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where V is the solution volume; Co and Cf are the initial and equilibrium metal ion concentrations

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in solution (mg·L-1); and m is the mass (g) of the adsorbent.

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2.5 Studies on the adsorption behaviors

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2.5.1 Adsorption isotherm: Langmuir and Freundlich models

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The adsorption isotherm is defined as a relationship between the adsorbate concentration in the solution and the amount adsorbed at the interface at constant temperature. The isotherm models

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used widely are Langmuir isotherm and Freundlich isotherm, which can reveal the maximum adsorption capacity and binding affinity. In the Langmuir isotherm, it is assumed that every adsorption site and the binding ability of the adsorbate are equivalent and independent of whether the adjacent sites are occupied or not.(Bulut & Baysal, 2006) This assumption suggests that

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adsorption occurs to form a monolayer on a substrate surface, having equally distributed the adsorption energy over the entire surface (Mall, Srivastava, & Agarwal, 2006). The general form of the Langmuir isotherm is given by the following: 𝐶𝑒 𝑞𝑒

=

𝐶𝑒 𝑞𝑚𝑎𝑥

+

1 𝐾𝐿 𝑞𝑚𝑎𝑥

(2)

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where qe is the amount of an adsorbed molecule at equilibrium (mg·g-1); qmax is the maximum adsorption capacity (mg·g-1); Ce is the equilibrium concentration of free adsorbate molecule (mg·L-1) in solution, and KL (L·mg-1) is the Langmuir constants related to free energy of adsorption. In particular, KL is an essential parameter to predict the affinity between the adsorbate and adsorbent. This value was used to calculate RL called the dimensionless equilibrium parameter or

𝑅𝐿 =

1

(3)

1+𝐾𝐿 𝐶𝑜

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which is given by the following equation:(Rahman & Islam, 2009)

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separation factor. The value of RL provides important information on the adsorption strength,

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where Co (mg·L-1) is the initial concentration of the adsorbate. The value of RL represents the

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affinity between the adsorbate and adsorbent, i.e., irreversible (RL = 0), favorable (0 < RL < 1),

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unfavorable (RL > 1), and linear (RL = 1) adsorption.

The Freundlich model is an empirical equation describing the adsorption behavior of adsorbates on a heterogeneous surface with active sites having non-uniform energies.(Freundlich,

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1907) Equations for the model is given by the following: 1

ln 𝑞𝑒 = ln 𝐾𝑓 + 𝑛 ln 𝐶𝑒

(4)

1/𝑛

(5)

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𝑞𝑒 = 𝐾𝐹 𝐶𝑒

where Kf is the Freundlich capacity constant and n is the adsorption intensity. The qe and Ce are the same quantities used in the Langmuir isotherm.(Ma et al., 2005) This model is typically employed to describe multilayer adsorption on a heterogeneous surface.

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2.5.2 Adsorption kinetics: Pseudo-first-order and pseudo-second-order adsorption models Pseudo-first-order and pseudo-second-order kinetics are typically employed to describe the kinetics of the adsorption behavior.(Min et al., 2019) A pseudo-first-order kinetic model is based on the relationship between the adsorption rate and number of unoccupied sites. In the model, the rate of occupation of adsorption sites is proportional to the number of the unoccupied adsorption

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sites.(Chen, Lin, & Hsu, 2008; Monier, Ayad, & Sarhan, 2010) On the other hand, in the pseudosecond-order kinetic model, the adsorption rate is related to the square of the product of the number

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of unoccupied sites and number of occupied sites.(Kampalanonwat & Supaphol, 2010) Adsorption

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was assumed to occur in the square-shaped sites, and the number of available adsorption sites on the absorbent at equilibrium is related to that of the occupied sites.(Chen et al., 2008; Monier et

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al., 2010) The pseudo-first-order model and the pseudo-second-order model are given by equations

𝑡

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(6) and (7), respectively: log(𝑞𝑒 − 𝑞𝑡 ) = log 𝑞𝑒 − 𝑘1 2.303 =𝑘

1

2 2 𝑞𝑒

𝑡

+𝑞

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𝑡 𝑞𝑡

𝑒

(6)

(7)

where qe (mg·g-1) is the adsorption capacity at time t (min), qt (mg·g-1) is the adsorption capacity at t, and k1 (min-1) and k2 (g·mg-1·min-1) are the rate constants for the pseudo-first-order and

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pseudo-second-order kinetic models, respectively.

3. Results and discussion 3.1 Fabrication of thiol-functionalized cellulose nanofibers

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The fabrication of cellulose nanofibers by electrospinning is challenging because of strong inter- and an intra-molecular hydrogen bonding, making it insoluble in various solvents. Few reports on cellulose nanofibers formed by electrospinning were found, however, harsh chemicals are typically used in the process and byproducts harmful to the environment could be released.(C. Zhang et al., 2014) To overcome this issue, cellulose nanofibers can be formed through the deacetylation process of electrospun CA nanofiber. First, CA nanofibers were fabricated

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successfully via an electrospinning process, as shown in Figure 2a. The presented CA nanofibers

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exhibited a smooth surface, randomly oriented, and bead-free structure with a uniform size, 379 ± 90 nm in diameter. The CA nanofibers were treated with an aqueous NaOH solution, resulting in

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complete deprotection of acetyl group in the CA nanofiber. Figure 2b presents the structure of

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cellulose nanofibers obtained from CA nanofibers, also showed a straight and bead free structure with 388 ± 109 nm. The morphology of the resulting nanofibers is well preserved upon a

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deacetylation process. The resulting cellulose nanofibers were modified further with DTDPA by CDI esterification to incorporate thiol groups into the cellulose chains.(An et al., 2019) DTDPA

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reacts with OH groups on the cellulose backbone by esterification with the activation agent, CDI.(Liebert & Heinze, 2005) The morphology of the TC nanofiber did not show any significant changes while its diameter, 418 ± 68 nm, was slightly larger than that of CA and cellulose

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nanofibers. (Figure 2c)

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Figure 2. SEM images showing the morphologies of (a) CA nanofibers, (b) cellulose nanofibers,

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and (c) TC nanofibers.

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The cellulose thiol-functionalization process was confirmed by FT-IR spectroscopy and XPS

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(Figure 3). CA nanofiber showed two intense peaks at 1720 cm-1 and 1230 cm-1, which were assigned to C=O and C-O bonds in the acetate group, respectively.(Filho et al., 2008; Tian et al.,

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2011) Upon deacetylation, a broad peak in the region between 3100 and 3600 cm-1, which is a characteristic vibrational mode of the O-H bond, was observed. Two intense peaks for the acetate

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group disappeared, indicating the successful fabrication of cellulose nanofibers. Upon a reaction of the OH group with DTDPA, the disulfide of DTDPA was cleaved to form thiol functionality on the cellulose nanofiber. As a consequence, the two peaks due to an ester at 1720 cm-1 and 1230 cm-1 re-emerged, and simultaneously, the intensity of the hydroxyl peak was decreased

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dramatically.(An et al., 2019) On the other hand, the characteristic peak of thiol, which typically appears at 2560 cm-1, was not observed in the FT-IR spectrum in Figure 3a, possibly due to the low concentration of thiol on the nanofiber surface.(Kanoth, Claudino, Johansson, Berglund, & Zhou, 2015) The sample was characterized further by XPS spectroscopy, which is a surfacesensitive characterization tool with a sampling depth of ~10 nm, as shown in Figure 3b-c. A peak at ~227 eV assigned to S2s was clearly observed, indicating the successful incorporation of thiol

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onto the cellulose nanofiber.(X.-N. Zhang, Hollimon, & Brodus, 2016) The C1s peak at ~285 eV was deconvoluted to five different peaks: C-C at 284.7 eV, C-O at 286.4 eV, O-C-O at 287.7 eV, O-C=O at 288.9 eV and C-S at 285.2 eV (Figure 3b).(Cai, Wang, Neoh, & Kang, 2011) The peak for S2s was deconvoluted into two peaks: 228.0 eV for sulfur in C-S-H and 231.5 eV for C-S-SC.(Raevskaya et al., 2018) Quantitatively, the integrated intensity ratio of C1s and S2s was calculated to be 16.2, whereas the theoretical value of [C]/[S] was 15 when one thiol group was

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incorporated in a repeating unit of cellulose. Therefore, TC nanofibers have 0.92 thiol groups per

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repeating unit of cellulose as a consequence of the chemical modification. It should be noted that the analysis with S2s signal is not only quantitatively comparable to the analysis with S2p signal

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which was shown in the previous report,(An et al., 2019) but it also provides clearer understanding

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on the surface chemical composition including unreacted residual disulfide bond upon the cleavage

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reaction.

Figure 3. (a) FT-IR spectra of CA (bottom, black), cellulose (middle, red), and TC nanofiber (top, blue), and multiplex XPS spectra of TC nanofiber for (b) C1s and (c) S2s peaks.

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3.2 pH effect on the metal ion adsorption behaviors The pH is typically considered the very first parameter when examining metal ion adsorption behavior because the pH of the medium can affect the solubility of the adsorbate, concentration of counter ions on the adsorbent, and the degree of ionization of the adsorbate. The effect of the pH of the initial aqueous solution on the heavy metal ion adsorption behaviors was investigated over

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the pH range of 2 to 7, which was selected to prevent the precipitation of metal particles, which generally formed in a basic medium. The experiments were conducted with 0.01 g of TC

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nanofibers soaked in 50 mL of three 200 ppm metal ion solutions of Cu(II), Cd(II), and Pb(II), for

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12 hours.

Figure 4 show the adsorption behaviors of Cu(II), Cd(II), and Pb(II) ions onto TC nanofibers

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at different pH. At low pH, the amounts of adsorbed metal ions are negligible for all species, which

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is due to the affinity of a proton to the adsorption sites. Therefore, metal ions should compete with protons to occupy the adsorption sites. Therefore, the adsorption efficiency is very low at pH 2,

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and a slight increase was observed at pH 3. This behavior is useful for removing adsorbed metal ions from the adsorbents into a solution.(Boudrahem, Aissani-Benissad, & Soualah, 2011) At higher pH than 3, the adsorption behavior of the TC nanofibers increased dramatically; the maximum adsorption was achieved at pH 4 for all metal ions. The amounts of adsorbed Cu(II),

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Cd(II), and Pb(II) at pH 4 were determined to be 38.64, 30.41, and 19.2 mg/g, respectively. A further increase in pH to 7 led to a decrease of the amounts of metal ions adsorbed. This is likely attributed to the formation of disulfide bond by disulfide interchange or thiol oxidation reactions. Monahan et al. reported that disulfide interchange reactions are favored at high pH, but can be suppressed at low pH.(Monahan, German, & Kinsella, 1995) Therefore, disulfide bond formation

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or the oxidation of thiol possibly occurs at pH 5 to 7, resulting a loss of the adsorption sites on the

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Copper Cadmium Lead

30

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10

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4

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Adsorption capacity (mg/g)

TC nanofiber surface.

5

6

7

pH

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Figure 4. Adsorption behaviors of TC nanofiber for the three types of heavy metal ions, Cu(II), Cd(II), and Pb(II), as a function of pH of the medium.

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3.3 Adsorption isotherms and kinetics Adsorption behaviors are typically investigated using two isotherm models: Langmuir

isotherm and Freundlich isotherm models, which are expressed by the equations (2) and (4), respectively. Langmuir isotherms were constructed by measuring qe and Ce with 0.01 g of TC nanofiber, which was immersed in 50 mL of aqueous solutions at pH 4.0 with varied metal ion

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concentrations for 10 hours. Figure 5 shows the adsorption capacity (qe, mg·g-1) for various metal ions as a function of the initial ion concentration (C0), as well as the fitting results with the models. A linear fit was carried out using equation (2) for the Langmuir isotherm and equation (4) for the Freundlich isotherm, and the results are shown in Figure S1 and S2, and all resulting parameters were tabulated in Table 1. A comparison of the two models revealed R2 values by a fit with the Langmuir model for all ions in the range of 0.994-0.999, whereas those of the Freundlich model

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fall in the range of 0.822-0.935, strongly suggesting that the Langmuir model well-describes the

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adsorption behaviors of metal ions onto the TC nanofiber. The Langmuir isotherm suggests that (i) the adsorption occurs only in monolayer coverage; (ii) all adsorption sites are equivalent with

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uniform binding affinity and the surface is uniform; (iii) the adsorption capability of adsorbates at

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the given sites is independent of neighboring adsorption sites, i.e., no interaction between adsorbates and a homogeneously distributed adsorption energy over the entire coverage surface.

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Therefore, the adsorption mechanism has been clearly elucidated. The distribution of thiol on the TC nanofiber was distributed homogeneously, in which the thiol on the surface only leads to the

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uniform adsorption of metal ions to form a monolayer coverage. The maximum adsorption capacities (qmax) of the TC nanofiber for Cu(II), Cd(II), and Pb(II) were 49.0, 45.9, and 22.0 mg·g, respectively, which is in good agreement of the trend in the size of given ions.

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Figure 5. Isotherms showing the adsorption of (a) Cu(II), (b) Cd(II), and (c) Pb(II) ions onto TC nanofibers and fitting results with Langmuir (black solid line) and Freundlich (red solid line)

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isotherm models.

Langmuir isotherm

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Metal ion

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Table 1. Fitting parameters obtained with Langmuir and Freundlich isotherm models

Freundlich isotherm

KL (L·mg-1)

R2

KF (mg·g-1)

n-1

R2

Cu(II)

49.0

0.016

0.999

7.51

0.5284

0.926

Cd(II)

45.9

0.011

0.995

2.17

0.6023

0.935

22.0

0.052

0.994

2.68

0.3937

0.822

Pb(II)

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qmax (mg·g-1)

The pseudo-first-order model (equation (6)), and pseudo-second-order model (equation (7))

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are models typically employed to describe the kinetic behaviors of adsorption. These two models were used to describe the kinetics of the adsorption of metal ions onto TC nanofibers with the time-dependent adsorption experiments with a metal ion initial concentration at 200 ppm and a pH of 4.0. Figure 6a and 6b show the adsorption capacity as a function of the contact time, and the corresponding fitting results with a pseudo-second-order model, equation (7). Figure S3 shows

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the fitting results with the pseudo-first-order model. Table 2 lists the resulting kinetic parameters obtained from both models. The pseudo-second-order kinetic model (R2 = 0.999) better described time-dependent adsorption behavior than the pseudo-first-order kinetic model, as confirmed by the much higher correlation coefficient (R2). The second-order kinetic model was derived under the assumption of adsorption; two polar sites are involved for a single metal ion with a double charge to be adsorbed.(Ho & McKay, 1999) Therefore, these results strongly suggest that the adsorption

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of single Cu(II), Cd(II), and Pd(II) ions onto TC nanofibers occurs via two thiol groups, possibly

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by chemical adsorption by electron sharing and exchange between the metal ions and the thiol or thiolate group on the TC nanofiber. The experimentally determined equilibrium adsorption

-p

capacity (qe) also confirmed the chemisorption of metal ions on the TC nanofiber. According to

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the kinetic adsorption behavior curve in Figure 6a, the saturated adsorption of Cu(II), Cd(II), and Pb(II) metal ions onto TC nanofibers were estimated to be approximately 39.6, 30.0, and 19.6

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mg·g-1, respectively. These were similar to the qe values calculated with the pseudo-second-order kinetic model, confirming that the model is suitable for the observed adsorption behavior.(N. Feng,

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Guo, & Liang, 2009)

There have been few reports on metal ion adsorption behaviors of monolayer surface functionalities on various cellulose surfaces in aqueous media. For example, it was demonstrated

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that surface thiol monolayer on cellulose having rough and uneven surface utilized for Hg(II) in wastewater with maximum adsorption capacity of 11.0 mg·g -1, estimated with Langmuir model. (Z. Zhang, Cao, Chen, & Huang, 2017) Another surface functionality in a monolayer form, -COONa+ group on carboxymethylated cellulose fibers, was quite effective to adsorb Cu2+ with the maximum capacity of 22.8 mg·g -1, obtained with Langmuir model, showing 130 times increase capacity compared to unmodified cellulose fibers.(Wang, Liu, Duan, Sun, & Xu, 2019) The

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capacity can be improved by increasing the concentration of surface adsorbing functional group by grafting of polymer chains which exhibit metal ion adsorbing functional groups on cellulose surfaces. Anbia et al. reported that the maximum adsorption capacity for Pt(IV) of thiol and primary amine groups on poly(glycidyl metharylate) grafted magnetic cellulose was 40.5 mg·g -1, determined with Langmuir model. (Anbia & Rahimi, 2017) The capacity can be further improved by pushing the size limit to sub-100 nm dimension; Yang et al. showed that polycysteine, offering

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high thiol concentration in unit volume, grafted ultra-fine cellulose nanofiber with extremely small

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diameter about 5 nm was highly effective to adsorb Cu(II) with the capacity of 131 mg·g-1

-p

(Langmuir model).

The TC nanofibers in this study exhibit relatively high maximum capacity of 49.0, 45.9, and

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22.0 mg·g-1 for Cu(II), Cd(II), and Pb(II), respectively, despite its monolayer structure and modest size of ~0.4 µm. These results clearly elucidate potential utilization of cellulose, which typically

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exhibits extremely poor metal adsorption capability,(Phan et al., 2019; Wang et al., 2019) by effective surface modification to introduce tailored surface chemical functionality. Based on

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current results, the adsorption performance of cellulose can be further improved by combining other technique, for example, controlling the morphology of cellulose nanofibers to increase surface area.(Lee et al., 2018; K. Zhang et al., 2019) Ultimately, underlying principles in

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fabrication and modification approaches to graft thiol groups on the surface of the nanofibers will be considered as effective fundamentals towards high-performance adsorbents for metal ions in water.

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of ro -p re lP

Figure 6. (a) Adsorption capacities of Cu(II), Cd(II), and Pb(II) onto TC nanofibers as a function

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of time and (b) fitting results with the pseudo-second-order kinetic model.

Table 2. Kinetics parameters based on the pseudo-first-and the pseudo-second-order kinetic for

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describing the adsorption of Cu(II), Cd(II), and Pb(II) ions onto TC nanofibers.

Metal ion

qe, exp (mg·g-1)

Cu(II)

pseudo-first-order

pseudo-second-order R2

k2 (min-1)

qe, cal (mg·g1 )

R2

39.3

0.941

1.09 × 10-3

39.8

0.999

0.002

27.8

0.915

6.16 × 10-4

31.6

0.999

0.001

24.5

0.910

5.33 × 10-4

20.4

0.999

k1 (min-1)

qe, cal (mg·g 1 )

39.6

0.003

Cd(II)

30.0

Pb(II)

19.6

-

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4. Conclusion We demonstrated the fabrication of the thiol-functionalized cellulose nanofiber membrane and its capability to adsorb heavy metal ions, i.e. Cu(II), Cd(II), and Pb(II), in water. Cellulose nanofibers were fabricated by electrospinning cellulose acetate nanofibers and subsequent

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deacetylation process. The cellulose nanofiber was modified further to successfully incorporate

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surface thiol functionality by the esterification of a hydroxyl group with 3,3’-dithiodipropionic acid and further cleavage of the disulfide bond. The adsorption behaviors of the resulting TC

-p

nanofiber membrane were evaluated thoroughly for Cu(II), Cd(II), and Pb(II) ions. The TC nanofiber membrane exhibited effective and rapid adsorption behavior toward heavy metal ions in

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aqueous solutions, where the optimal initial pH was 4.0. The adsorption mechanism was revealed

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by observing the adsorption isotherm and adsorption kinetics. The equilibrated adsorption capacity as a function of the adsorbate concentration was described precisely using the Langmuir isotherm

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model, strongly suggesting that the adsorbates form a monolayer with a homogenously distributed surface adsorption energy on the TC nanofiber surface. The maximal adsorption capacities obtained from the model were 49.0, 45.9, and 22.0 mg·g-1 for Cu(II), Cd(II), and Pb(II) ions, respectively. Kinetic studies with a pseudo-second-order kinetic model clearly showed that the

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chemisorption of single doubly charged metal ion onto TC nanofiber occurs via two thiol groups. These results highlight the impact of the chemical surface functionalization of cellulose nanofiber by post-electrospinning modification, expanding the potential and applicability of cellulose materials towards water remediation processes.

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Acknowledgment We gratefully acknowledge the National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIP) (Grant No. 2018R1D1A1B07044345).

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