Prussian blue-embedded carboxymethyl cellulose nanofibril membranes for removing radioactive cesium from aqueous solution

Journal Pre-proof Prussian blue-embedded carboxymethyl cellulose nanofibril membranes for removing radioactive cesium from aqueous solution Semin Eun (Conceptualization) (Methodology) (Software), Hye-Jin Hong (Data curation) (Writing - original draft), Hyuncheol Kim (Visualization) (Investigation), Hyeon Su Jeong (Data curation) (Visualization), Soonhyun Kim (Software) (Validation), Jongwon Jung (Data curation) (Supervision), Jungho Ryu (Supervision) (Writing - review and editing)

PII:

S0144-8617(20)30158-2

DOI:

https://doi.org/10.1016/j.carbpol.2020.115984

Reference:

CARP 115984

To appear in:

Carbohydrate Polymers

Received Date:

27 November 2019

Revised Date:

16 January 2020

Accepted Date:

10 February 2020

Please cite this article as: Eun S, Hong H-Jin, Kim H, Jeong HS, Kim S, Jung J, Ryu J, Prussian blue-embedded carboxymethyl cellulose nanofibril membranes for removing radioactive cesium from aqueous solution, Carbohydrate Polymers (2020), doi: https://doi.org/10.1016/j.carbpol.2020.115984

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Prussian

blue-embedded

carboxymethyl

cellulose

nanofibril membranes for removing radioactive cesium from aqueous solution Semin Eun†, ┴ , Hye-Jin Hong § , ┴ , Hyuncheol Kim¶, Hyeon Su Jeongǁ, Soonhyun Kimǁǁ,

†

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Jongwon Jung†, Jungho Ryu‡,*

School of Civil Engineering, ChungBuk National University (CBNU), Cheongju, Chungbuk

28644, Korea (S. Eun : [email protected], J. Jung : [email protected]) ‡

Geologic Environment Research Division, Korea Institute of Geoscience and Mineral

§

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Resources (KIGAM), Daejeon 34132, Korea (J. Ryu : [email protected])

Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources

(KIGAM), Daejeon 34132, Korea (H.-J. Hong : [email protected]) ¶

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Nuclear Emergency and Environmental Protection Division, Korea Atomic Energy Research

Institute (KAERI), Daejeon 34057, Korea (H. Kim : [email protected]) ǁ

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Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST),

Wanju, Jeonbuk 55324, Korea (H.S. Jung : [email protected]) ǁǁ

Division of Energy Technology, Daegu Gyeongbuk Institute of Science and Technology

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(DGIST), Daegu 42988, Korea (S. Kim : [email protected]) *Corresponding author

These authors contributed equally to this work.

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┴

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(J. Ryu) Tel: +82-42-868-3943; E-mail: [email protected]

Highlights 

A Prussian blue (PB)-embedded carboxymethylcellulose nanofibril (CMCNF) was prepared.

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

PB nanoparticles were formed in situ at the Fe3+ sites of a CMCNF framework.



The PB-CMCNF exhibited a 2.5-fold-higher Cs adsorption capacity than commercial PB.



137

Cs removal from seawater was demonstrated by using a sequential filtration method.

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Graphical abstract

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Abstract

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In this study, we synthesized a Prussian blue (PB)-embedded macroporous carboxymethyl cellulose nanofibril (CMCNF) membrane for facile cesium (Cs) removal. The PB was formed

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in situ at Fe3+ sites on a CMCNF framework cross-linked using FeCl3 as a cross-linking agent. Cubic PB particles of size 5–20 nm were observed on the macroporous CMCNF membrane

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surface. The PB-CMCNF membrane showed 2.5-fold greater Cs adsorption capacity (130

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mg/gPB-CMCNF) than commercial PB nanoparticles, even though the PB loading of the PBCMCNF membrane was less than 100 mg/gPB-CMCNF. The macroporous structure of the CMCNF membrane led to improved diffusion in the solution, thereby increasing the Cs adsorption capacity. The Cs adsorption behavior was systematically investigated in different solution chemistry. Finally,

137

Cs removal using a semicontinuous adsorption module was 2

demonstrated in real seawater. The results showed that the PB-CMCNF membrane is a highly effective, practical material for the removal of 137Cs from aqueous environments.

Keywords: Carboxymethyl cellulose nano-fibril, Prussian blue (PB), PB-membrane,

137

Cs,

seawater

1. Introduction

mainly 131I,

137

Cs,

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Cs, 90Sr, and

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The Fukushima nuclear power plant accident in 2011 caused a large amount of radionuclides, Zr, to be released into the environment (Buesseler et al.,

2012; Kinoshita et al., 2011). Among the released radioactive nuclides, the Cs isotopes ( 137Cs, Cs) have attracted particular concern not only because they were discharged in huge

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quantities but also because of their specific properties. Cs is a strong emitter of gamma rays,

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which are very harmful to humans; has a high fission yield with a long half-life (137Cs T1/2 = 30

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years); and exhibits high solubility, resulting in easy migration through the environment (Calabrese, 2011; Delchet et al., 2012; D. Ding, Lei, Yang, Feng, & Zhang, 2013; Yasunari et

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al., 2011) .

Numerous methods have been proposed for removing Cs from the environment, including

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chemical precipitation (Beheir, Benyamin, & Mekhail, 1998; Rogers, Bowers, & GatesAnderson, 2012), solvent extraction (Xu, Wang, & Chen, 2012), adsorption (X. Liu et al., 2014;

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Mihara et al., 2016) and coagulation (Kamaraj & Vasudevan, 2015). Among these methods, adsorption has attracted broad attention as an approach that can selectively remove radionuclides, thus minimizing the volume of radioactive waste. Among the various adsorbents that have been investigated for the removal of Cs, Prussian blue (PB) has been reported to be a promising material because of its outstanding adsorption capacity and high selectivity toward 3

Cs (Hu, Fugetsu, Yu, & Abe, 2012; Vipin, Hu, & Fugetsu, 2013; Wang, Zhuang, & Liu, 2018). PB consists of a cubic lattice of Fe2+ and Fe3+ with CN ligands, along with K+ counter ions (Buser, Schwarzenbach, Petter, & Ludi, 1977; Keggin & Miles, 1936; Pyrasch, Toutianoush, Jin, Schnepf, & Tieke, 2003). Because the cavity size of PB is limited, Cs ions can be selectively adsorbed onto PB via ion exchange with K+ (Faustino et al., 2008; Manivannan, Kang, & Kim, 2016). Although PB is a promising material for the removal of Cs, the use of PB nanoparticles to remove Cs from a radioactive waste solution is limited by separation

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problems. Numerous studies have sought to use granulated particles comprised of PB and a binding material for facile removal of radionuclides from radioactive wastewater. Alginate (Jang & Lee, 2016a; Mihara et al., 2016; Vipin et al., 2013), cellulose (Kim et al., 2019; Vipin

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et al., 2016), polyvinyl alcohol (Jang & Lee, 2016a), poly (acrylic acid) (Yang, Li, Zhai, & Yu,

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2015), chitin (Vincent et al., 2015), chitosan (Dechojarassri et al., 2017; Fujisaki, Kashima, Hagiri, & Imai, 2019) and magnetite (Jang & Lee, 2016b; Thammawong, Opaprakasit,

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Tangboriboonrat, & Sreearunothai, 2013) have been used as binding materials for PB. However, these binding materials blocked the diffusion of solute into PB, thereby interfering

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with the adsorption of Cs and resulting in a severe decrease in adsorption capacity. Accordingly, a method for using PB nanoparticles without a binding material is needed to

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achieve the required Cs-adsorption performance of PB. Herein, we investigated in situ immobilization of PB onto carboxymethyl cellulose nanofibril

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(CMCNF) membranes without the use of a binding material. CMCNFs have been reported as a super-adsorbent material because of their abundant hydroxyl and carboxyl groups and excellent stability. In addition, CMCNFs can be easily cross-linked by reaction with Fe3+ and transformed into an insoluble structure (Barkhordari, Yadollahi, & Namazi, 2014). Because Fe3+ is also a precursor in PB synthesis, we strategically used Fe3+ ions to cross-link CMCNF 4

membranes as a precursor for preparing PB in situ. First, the macroporous CMCNF membrane structure was prepared by a freeze-drying technique and subsequently cross-linked with FeCl3 solution to reinforce its chemical structure. PB nanoparticles were formed in situ and immobilized at the Fe3+ sites of a CMCNF membrane by reaction with hexacyanoferrate (HCF). Physical and chemical characterizations of the CMCNF membrane with immobilized PB (PB-CMCNF) were performed, and its adsorption behavior was evaluated in terms of the isotherm, kinetics, and solution chemistry studies. Finally, semicontinuous Cs removal

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experiments were conducted in seawater medium to demonstrate the practical application of the PB-CMCNF membrane to the removal of radioactive 137Cs.

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2. Experimental Section

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2.1 Materials

For the synthesis of the CMCNF membrane, viscous sodium Na-CMCNF solution in DI

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water (3 wt%) was provided by Asia Nanocellulose. The degree of substitution (the number of carboxymethyl groups attached to each anhydroglucose unit) of Na-CMCNF was 0.45–0.5,

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both in the dissolved form and in the dispersed fibrous form in the aqueous state. The CMCNFs exhibited a fibrous shape with a diameter of ~20 nm. Iron(III) chloride (FeCl3, ≥97.0%, Sigma-

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Aldrich) was used as a cross-linking reagent. Potassium hexacyanoferrate( Ⅱ ) trihydrate

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(K4Fe(CN)6·3H2O, ≥98.5%, Sigma-Aldrich) was used as a precursor for PB synthesis on cross-linked CMCNF. Cesium chloride (CsCl, ≥ 99.9%, Sigma-Aldrich) was used in all adsorption tests as a Cs source. Sodium chloride (NaCl, ≥ 99.0 % Sigma-Aldrich), potassium chloride (KCl, ≥99.0%, Sigma-Aldrich), magnesium chloride hexahydrate (MgCl2·6H2O, ≥ 99.0%, Sigma-Aldrich), and calcium chloride dihydrate (CaCl2·2H2O, ≥ 99.0%, Sigma5

Aldrich) were used in the competitive adsorption tests. All reagents and solvents were reagent grade and used without further treatment.

2.2 Synthesis of PB-CMCNF membrane Figure S1 shows the procedure for synthesizing a PB-CMCNF membrane. First, 2 wt% of Na-CMCNF was dissolved homogeneously in DI water. Na-CMCNF solution (5 g) was placed in a Petri dish (60 × 15 mm2, polystyrene, SPL), frozen for 6 h, and quickly transferred to a

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freeze-dryer (Ilshin, FDS8508). After freeze-drying for 12 h, the prepared CMCNF membrane was immersed in a 0.1 M FeCl3 solution for cross-linking, and unreacted Fe3+ ions were removed by washing with distilled water several times. The Fe 3+ cross-linked CMCNF

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membrane was then reacted with 0.1 or 0.5 M potassium hexacyanoferrate II (HCF) to

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synthesize PB. The reaction time with the HCF solution was approximately 10 s, which was sufficient to synthesize PB. The prepared PB-CMCNF membrane was rinsed with DI water

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several times to remove unreacted HCF and to stabilize the PB. The samples were stored in DI water and used after the water was squeezed out. The samples are hereinafter denoted as 0.1M-

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PB-CMCNF and 0.5M-PB-CMCNF membranes according to the concentration of the HCF

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solution.

2.3 Characterization of PB-CMCNF membranes

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The morphology of the prepared PB-CMCNF membranes was examined by scanning

electron microscopy (SEM, Verios 460 L, FEI, USA). The variety of functional groups of CMCNF membranes and synthesized PB were characterized by Fourier transform infrared spectroscopy (FTIR, Cary 630, Agilent). Samples were analyzed by X-ray diffraction (XRD, X-Pert PRO MPD, Rigaku) to confirm the crystal structure of synthesized PB and by 6

thermogravimetric analysis (TGA, DTG-60H, Shimadzu) to determine the amount of PB loaded onto the PB-CMCNF membranes. The porosity and pore size distribution were measured using mercury intrusion porosimetry (AutoPore, V9610, Micromeritics).

2.4 Batch Cs adsorption experiments

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Batch-type Cs adsorption experiments were conducted by adding a PB-CMCNF membrane to 50 mL of a Cs-spiked solution. The average weight of the PB-CMCNF membrane samples was 1.3 g. To evaluate the Cs adsorption kinetics for the PB-CMCNF membranes, 200 mg/L

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of Cs-spiked solution was used and sample aliquots were intermittently withdrawn for analysis

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of the Cs concentration. The adsorption isotherm was conducted with several Cs concentration levels for 6 h of contact time. In the Cs adsorption with competitive ions, the major cations

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(Na, K, Ca, Mg) of seawater were introduced and tested at the same concentration as Cs, 100 mg/L. The concentration of Cs in all experiments and the concentrations of the major cations

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were measured by ion chromatography (IC, 881 Compact IC Pro, Metrohm) and inductively

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coupled plasma atomic emission spectroscopy (ICP-AES, ULTIMA 2, Horiba), respectively.

2.5 Semicontinuous 137Cs adsorption experiments

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A 100 Bq/L solution of 137Cs was prepared by diluting 137Cs stock solution (Eckert & Ziegler)

with DI water and seawater. For the practical application of the PB-CMCNF membrane, a syringe filter holder (Advantec MFS 43303010, polypropylene) was used. A PB-CMCNF membrane was placed in the syringe filter holder and

137

Cs-containing water was passed

through the membrane using a syringe. The syringe was filled with 50 mL of 7

137

Cs solution,

and the solution was rapidly passed through the filter to remove

137

Cs from DI water and

seawater. The contact time with the PB-CMCNF membrane was approximately 10 s. The filtered solution was placed in a plastic vial, and the activity concentration of

137

Cs was

analyzed by liquid scintillation counting (LSC, Quantulus 1220, Perkin Elmer).

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3. Results and Discussion 3.1 Characterization of the CMCNF membrane FTIR analysis

The CMCNF membrane was prepared using the freeze-drying method. The freeze-dried neat

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CMCNF membrane was subsequently cross-linked with FeCl3 solution to produce an insoluble

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structure. Figure 1a shows photos of the neat and cross-linked CMCNF membranes with different concentrations of FeCl3. Whereas the neat CMCNF was white, the CMCNF after

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reaction with FeCl3 solutions was brownish-red, the typical color of Fe3+ ions. Notably, however, the brown color faded when the FeCl3 concentration was increased to greater than

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0.1 M. Figure 1b shows the FTIR spectra of the neat and cross-linked CMCNF membranes as a function of the FeCl3 concentration. After reaction with FeCl3 solution, a new peak appeared

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at 1740 cm−1 and its intensity also increased in proportion to the FeCl3 concentration. The 1740 cm−1 peak represents the stretching vibration of carboxylic groups (COO−) interacting

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electrostatically with Fe3+ ions (Shi et al., 2005). Oppositely, the band at 1569.5 cm−1 representing free carboxylic groups (COO−) decreased with increasing Fe3+ concentration. These results can be explained by the hypothesis that the increase in Fe3+ concentration accelerated the bonding between Fe3+ and COO−, thereby reducing the number of free carboxylic groups of the CMCNF. It is also consistent with the observation that the brown color 8

of the CMCNF membrane became lighter with increasing FeCl3 concentration. The presence of more Fe3+ ions in the higher-concentration FeCl3 solutions led to a more favorable reaction between Fe3+ and the carboxylic groups, resulting in more rigid cross-linking and the disappearance of properties associated with Fe3+. Therefore, the color intensity of the CMCNF

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membrane was inversely proportional to the FeCl3 concentration.

Figure 1. (a) Photographs, (b) FTIR spectra of CMCNF membranes before (neat CMCNF) and

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after cross-linking with 0.1, 0.5, and 1 M FeCl3 solutions, and (c) SEM images.

SEM analysis

Figure 1c shows SEM images of the CMCNF membranes. The neat CMCNF membrane displays a flat and smooth surface. However, the CMCNF membrane cross-linked with 0.1 M 9

FeCl3 shows irregular wrinkles on its surface. Interestingly, upon further increase of the FeCl 3 concentration the surface became flat and smooth again. Under the 0.1 M FeCl3 condition, the concentration of Fe3+ was relatively lower than that of hydroxyl groups and carboxylic groups in the CMCNF membrane; thus, more CMCNFs were gathered around a single Fe 3+ ion. This arrangement likely caused the observed wrinkled texture of the CMCNF membrane surface. By contrast, the sufficient amount of Fe3+ in the 0.5 M and 1 M FeCl3 solutions increased the cross-linking density, making the surface flat and smooth like the surface of the neat CMCNF

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membrane. This behavior was closely related to changes in the pore structure. Although the CMCNF membrane exhibited a highly porous structure consisting of 100–600 m large macropores, the dominant macropore diameter decreased to 60 m after cross-linking with

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FeCl3. The porosity also decreased from 98.5% to 93.3% after cross-linking, but this change

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3.2 PB-embedded CMCNF membrane

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was not significant (Figure S2).

3.2.1 Optimization of PB-CMCNF membrane synthesis

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PB nanoparticles are easily synthesized via the homogeneous solution-based reaction between Fe3+ ions and potassium hexacyanoferrateII (HCF). In this study, we used Fe3+ bound

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to a CMCNF membrane framework as a Fe3+ precursor to synthesize PB in situ. Because the Fe3+-cross-linked CMCNF membrane contains Fe3+ ions bound to its structure, the simple

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immersion of CMCNF samples into a HCF solution enabled PB formation. As soon as the CMCNF membranes were dipped into a HCF solution, their color changed from brown to darkgreen, the typical color of PB (Figure 2a). The FTIR spectra of the treated CMCNF membrane showed a peak at 2085.5 cm−1, which corresponds to the stretching vibration of the –C≡N– groups of PB (Figure 2b).(N. Ding & 10

Kanatzidis, 2010; Kulesza, Malik, Denca, & Strojek, 1996; Lejeune, Brubach, Roy, & Bleuzen, 2014) These results demonstrate that PB was successfully synthesized on the Fe3+-cross-linked CMCNF membrane by reaction with HCF. However, in the spectra of the CMCNF samples cross-linked with greater concentrations of Fe3+ (0.5 and 1 M), the intensity of the band at 2085.5 cm−1 decreased and no dramatic color change of the sample was observed before and after reaction with HCF. This result reveals that in situ PB formation is hindered on CMCNF membranes treated with an Fe3+ solution with a concentration greater than 0.1 M. This

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difficulty might be due to the higher concentration of Fe 3+ ions causing irreversible bonding between Fe3+ and the carboxylic groups of CMCNF, leading to a more rigid cross-linked structure that prevents the Fe3+ ions from participating in the PB-forming reaction. Because the

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high concentration (>0.5 M) of Fe3+ is inappropriate for preparing a PB-CMCNF sample, the

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cross-linking conditions were optimized for 0.1 M FeCl3 solution in subsequent experiments.

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Figure 2. (a) Photographs and (b) FTIR spectra of PB-CMCNF samples prepared using

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CMCNF membranes cross-linked with 0.1, 0.5, and 1.0 M FeCl3.

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3.2.2 Characterization of the PB-CMCNF membrane XRD analysis

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PB-embedded CMCNF samples prepared using 0.1 and 0.5 M HCF solutions (i.e., 0.1M-PB-

CMCNF and 0.5M-PB-CMCNF, respectively) were systematically characterized. Figure 3a shows the XRD patterns of the PB-CMCNF membranes. The XRD pattern of the soluble PB nanoparticles shows diffraction peaks at 17.4°, 24.8°, 35.3°, 39.5°, 43.0°, 51.2°, 53.9°, 57.2°, 66.2°, and 68.1°, consistent with the characteristic peaks of PB (Li-Hua, Qin, Hai-Yan, & Xiao12

Ya, 2007; Li et al., 2004; S.-Q. Liu et al., 2011). The XRD patterns of the 0.1M-PB-CMCNF and 0.5M-PB-CMCNF membranes were both similar to the pattern observed for PB, although their peak intensities were very low because of the relatively small amount of PB loaded onto the membranes. These results confirm the presence of PB. The pattern of the 0.5M-PBCMCNF exhibited slightly sharper main diffraction peaks than the pattern of the 0.1M-PBCMCNF, consistent with more PB being loaded onto the 0.5M-PB-CMCNF membrane. The inset in Figure 3a shows photos of the PB-CMCNF samples. The blue color of the

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synthesized PB-CMCNF membrane clearly becomes darker with increasing concentration of the HCF solution used for the treatment, providing strong evidence of the formation of a greater

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amount of PB in the membrane treated with a more concentrated HCF solution.

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FTIR & SEM analysis

The FTIR spectra of the PB-CMCNF membranes are shown in Figure 3b. The spectrum of

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the 0.5M-PB-CMCNF membrane shows a more intense band at 2085 cm−1, which corresponds to an Fe–CN stretching vibration of PB, than the spectrum of the 0.1M-PB-CMCNF membrane

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(N. Ding & Kanatzidis, 2010; Kulesza et al., 1996; Lejeune et al., 2014). Figure 3c shows SEM images of the PB-CMCNF membranes prepared with HCF solutions with different

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concentrations. The images show nanometer-sized PB particles (5–20 nm) covering the surface of the CMCNF membranes. The PB nanoparticles, which exhibit an irregular morphology, are

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sparsely distributed on the surface of the 0.1M-PB-CMCNF sample, whereas PB particles densely cover the surface of the 0.5M-PB-CMCNF sample. In addition, the PB particles on the 0.5M-PB-CMCNF membrane exhibit a cubic morphology, which is a typical feature of wellcrystallized PB (Ming, Torad, Chiang, Wu, & Yamauchi, 2012; Prabakar, Jeong, & Pyo, 2015).

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From these results, we concluded that increasing the HCF concentration leads to enhanced PB

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formation on the CMCNF membrane surface and to greater crystallinity of the PB.

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Figure 3. (a) XRD patterns, (b) FTIR spectra, (c) SEM images, and (d) TGA curves of PB-

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CMCNF membranes. TG analysis

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Figure 3d shows the weight change of different CMCNF membrane samples as a function of temperature. In the case of the neat CMCNF membrane, approximately 20% weight loss was observed at 100 ℃ because of the evaporation of water. A second degradation was observed at 280 ℃, which can be attributed to the decomposition of carboxylic acid groups to CO 2. Finally, degradation of the remaining carbon was observed at 615 ℃ (Biswal & Singh, 2004; 14

Rani, Rudhziah, Ahmad, & Mohamed, 2014). After the cross-linking of neat CMCNF with Fe3+, the weight loss at 280 ℃ disappeared because the carboxylic groups were bound to Fe3+ ions. In the case of the PB-CMCNF membranes, a slight weight loss at 280 ℃ was again observed. The re-emergence of this weight loss is attributed to the conversion of a portion of the cross-linked carboxylate groups into free carboxylic groups as a result of the reaction with HCF to synthesize PB, which consumes the Fe3+ ions used for cross-linking of the CMCNF membrane. However, pure PB was rapidly degraded at 25–300 ℃ and completely degraded at

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350 ℃. The residual content of PB was as high as 50.8%, and no further weight loss occurred until 1500 ℃. Therefore, the amount of PB loaded onto the CMCNF membrane could be determined by calculating the remaining weight differences. The PB contents of the 0.1M-PB-

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CMCNF and 0.5M-PB-CMCNF membranes were estimated to be 37.1 mg/gPB-CMCNF and 71.4

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3.3 Batch-type Cs adsorption study

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mg/gPB-CMCNF, respectively.

To elucidate the adsorption behavior of Cs on the prepared PB-CMCNF membranes,

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adsorption isotherms and kinetic studies were performed. Figure 4a shows the Cs adsorption isotherm of PB-CMCNF membranes and commercial soluble PB particles. Compared with

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commercial PB particles, the PB-CMCNF membranes showed greater Cs adsorption capacities, with the 0.5M-PB-CMCNF membrane exhibiting a greater Cs adsorption capacity

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than the 0.1M-PB-CMCNF membrane. Notably, a much higher Cs adsorption capacity was achieved for the PB-CMCNF membranes even though their PB content was less than 100 mg/gPB-CMCNF. This enhanced adsorption capacity demonstrates that the Cs adsorption capacity of PB can not only be maintained but also improved by immobilization onto the support material The remarkable increase in Cs adsorption capacity of PB-CMCNF is attributed to i) 15

the prevention of PB nanoparticle aggregation by the surface immobilization of the nanosized PB particles on the CMCNF framework; and ii) the large surface area and highly porous structure of the CMCNF membrane, which promoted solution contact and diffusion of Cs to

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the PB.

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Figure 4. (a) Adsorption isotherms and (b) adsorption kinetics of Cs on PB-CMCNF

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membranes

Two model isotherms, i.e., the Langmuir and Freundlich models (see SI), were applied to fit

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the experimental data. The Cs adsorption on commercial PB was found to be in conformity with the Langmuir model (Figure S3), whereas the Cs adsorption onto PB-CMCNF membranes

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was better fit by the Freundlich model (Table 1). This result means that Cs adsorption occurs heterogeneously on the PB-CMCNF membrane surface to form a multilayer of Cs adsorbate

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(Hong et al., 2018; Hong et al., 2016).

Table 1. Cs adsorption isotherm model constants on PB-CMCNF membranes Langmuir

Freundlich

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KL (L/mg)

Qmax (mg/g)

R2

KF (L/mg)

n

R2

0.018

55.24

0.975

18.59

7.99

0.787

0.1M-PB-CMCNF 0.0069

81.30

0.984

11.87

3.45

0.999

0.5M-PB-CMCNF 0.0027

131.58

0.937

18.32

4.16

0.999

Commercial PB

As shown in the SEM images (Figure 3c), the PB was not uniformly distributed on the CMCNF membrane surface and even formed a multilayer on the 0.5M-PB-CMCMNF membrane.

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Because the PB is an active material for Cs adsorption and the distribution of PB is not uniform on the surface, the observation of heterogeneous Cs adsorption onto the PB-CMCNF membranes is reasonable. The 0.1M-PB-CMCNF and 0.5M-PB-CMCNF membranes

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exhibited excellent Cs uptakes of approximately 80 mg/g PB-CMCNF and 130 mg/gPB-CMCNF,

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respectively. The Cs adsorption capacities of other PB-based adsorbents are summarized in Table S1. Although several PB-based adsorbents exhibited Cs adsorption capacities

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comparable to that of the PB-CMCNF membrane, the amount of PB embedded on the PBCMCNF membrane is low (71.4 mg/gPB-CMCNF) compared to the other adsorbents, indicating

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that the PB-CMCNF membrane is an effective composite material to maximize the Cs adsorption performance of PB particles.

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Figure 4b shows time-dependent Cs adsorption profiles of the PB-CMCNF membranes. Both of the PB-CMCNF membranes displayed very high adsorption rates: 60% Cs uptake was

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achieved within 1 min and reached adsorption equilibrium within 1 h. The adsorption kinetics was analyzed by introducing two model equations (see SI). The resultant curve-fitting indicated that the Cs adsorption onto the PB-CMCNF membrane followed a pseudo-second-order model. This result suggests that the rate of Cs adsorption onto PB-CMCNF is determined by the chemisorption rate rather than by intraparticle diffusion, which is consistent with good 17

availability of PB on the highly porous CMCNF membrane surface, causing the chemisorption process to be the rate limiting step rather than diffusion of Cs through the solution to the PB particles. Rapid Cs adsorption was also observed in seawater medium (Figure S4). Although the amount of Cs uptake was decreased by interference from other cations in seawater, the adsorption of Cs was completed within 10 min. To elucidate the Cs adsorption mechanism of PB-CMCNF, K release was monitored during the course of Cs adsorption (Figure 5a). K release was found to be linearly proportional and

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stoichiometrically balanced with Cs uptake for both the 0.1M-PB-CMCNF and 0.5M-PBCMCNF membranes. This result clearly indicates that adsorption onto the PB-CMCNF membrane occurs via an ion-exchange reaction between Cs and K. Previous results have

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suggested two major mechanisms of Cs adsorption onto PB: physical adsorption, implying Cs

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capture in the PB lattice; and chemical adsorption mainly via ion-exchange (Ishizaki et al., 2013). However, we confirmed that Cs adsorption onto PB-CMCNF prepared in the present

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study proceeded mainly through the ion-exchange mechanism. Figure 5b shows the Cs adsorption efficiency of commercial PB and the PB-CMCNF membranes in the presence of

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Na, K, Mg, and Ca. Because seawater is regarded as the most complex medium among various natural aqueous media contaminated with Cs, understanding the effect of major coexisting

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cations in seawater on Cs removal is important for estimating the effectiveness of PB-CMCNF from a practical standpoint. The experiments were conducted in Cs/Na, Cs/K, Cs/Mg, and

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Cs/Ca binary component systems to determine the influence of each cation on Cs adsorption.

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Figure 5. (a) Ion exchange between Cs and K in the course of Cs adsorption onto PB-CMCNF

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and (b) the effect of competing ions on Cs adsorption onto PB-CMCNF.

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In the presence of Cs only, the adsorption performance order was 0.5M-PB-CMCNF > 0.1MPB-CMCNF >> commercial PB, consistent with the adsorption isotherm results. In the binary

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component system of Cs and each cation (Na, K, Mg, and Ca), all of the cations were found to hinder Cs adsorption because of competitive adsorption effects, although the Cs adsorption

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capacity exhibited a similar tendency to that in the Cs-only system. Among the investigated cations, the monovalent cations (Na and K) hindered Cs uptake less than the divalent cations

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(Mg and Ca). This result is attributed to the divalent cations having a greater ionic valence and stronger electrostatic attraction force than the monovalent cations (Hong et al., 2016). Although

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the Cs adsorption capacity was negatively affected by the presence of competitive ions, the 0.5M-PB-CMCNF membrane still demonstrated greater than 60% Cs removal efficiency. Given that Cs is a monovalent cation, the 0.5M-PB-CMCNF membrane shows excellent selectivity toward Cs.

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To further assess the effect of co-existing cations, the distribution coefficient (Kd) for Cs adsorption on 0.5M-PB-CMCNF membrane was determined in the presence of Na, K, Mg, and Ca (Figure S5). The log Kd value was 3.06 for the Cs-only system and decreased to 2.0~2.5 in the presence of K, Mg, and Ca due to competitive adsorption. The divalent cations (Mg and Ca) displayed a substantially lower log Kd value of around 2.0 due to their high charge density. The presence of K adversely affected Cs adsorption, with the log Kd value decreasing to 2.1 with increasing K concentration. This can be ascribed to ion exchange between K and Cs.

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Meanwhile, the effect of Na was negligible for Na concentrations up to 4.5 mM. Further increase of the Na concentration interfered with Cs adsorption onto the PB-CMCNF membranes. These results indicate that the order of the interference effect of cations against Cs

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adsorption on PB-CMCNF membranes is Mg ≈ Ca > K >> Na.

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Next, we tested Cs removal from seawater medium and compared the adsorption performance of commercial PB and the PB-CMCNF membrane. As shown in Table S2, the Cs uptake

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capacities of commercial PB and the 0.5M-PB-CMCNF membrane were observed to be 23.06 mg/g and 27.36 mg/gPB-CMCNF, respectively. These findings indicate that Cs removal could be

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achieved effectively even in seawater, with the Cs uptake shown by PB-CMCNF being slightly higher than that observed for commercial PB. Furthermore, it should be noted that the actual

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amount of PB applied in adsorption tests using commercial PB powder was about 10 times greater than using the PB-CMCNF membrane (0.1 g vs 0.011 g, respectively), demonstrating

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that the PB-CMCNF membrane is a promising material to maximize the Cs adsorption performance of PB.

3.4 Semicontinuous 137Cs adsorption study

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To demonstrate the practical application of the 0.5M-PB-CMCNF membrane to Cscontaminated aqueous media, we conducted semicontinuous Cs removal experiments using a sequential filtration module (Figure 6a). The 0.5M-PB-CMCNF membrane was placed in a syringe filtering unit and Cs-spiked water was forced through the filtering unit by syringe. The contact time was very short because of the macroporous structure of the PB-CMCNF (5 mL/s). The filtered solution could be sequentially recirculated to a 0.5M-PB-CMCNF-membranepacked filtration module to complete Cs removal. The removal efficiency at the first filtration

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was approximately 90% and 85% for Cs-spiked solutions with concentrations of 50 and 100

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mg/L, respectively.

Figure 6. (a) Schematic of the semicontinuous Cs adsorption apparatus and (b) Cs removal

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efficiency in the course of recirculating operation.

The removal of only part of the Cs during a single filtration is attributed to the very fast flow rate resulting from the macroporous structure of the PB-CMCNF membrane, which consists mainly of 60 μm pores, not allowing sufficient contact time. With increasing filtration number, the removal efficiency increased and finally reached 100% at the third filtration (Figure 6b). 21

In addition, a semicontinuous Cs adsorption apparatus was used to remove radioactive Cs (137Cs) from DI water and natural seawater. The experimental conditions and results are listed in Table 2.

Table 2. The experimental conditions and results of semicontinuous 137Cs removal. [137Cs]0 (Bq/L)

Volume (mL)

Flow (mL/s)

DI water

100

50

5

Seawater

100

50

5

137

93 68

Cs was 93% in DI water but decreased to 68% in seawater

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The removal efficiency of

rate Removal efficiency (%)

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Medium

medium. As noted in the discussion of the effect of competing cations, seawater contains a

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complex matrix of ions such as Na, K, Ca, and Mg that substantially influence Cs adsorption, resulting in a 25% reduction of the Cs removal efficiency. Nonetheless, this result demonstrates

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the feasibility of the PB-CMCNF membrane for the removal of 137Cs in aquatic environments

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4. Conclusions

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including seawater.

The release of radioactive

137

Cs into the environment following the accident in Fukushima

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in 2011 has been a serious problem. PB has been reported as a promising adsorbent to remove Cs from aqueous media. However, the direct application of PB in nanoparticulate form to Cs treatment was complicated by separation issues. To overcome the separation-related limitations while maintaining the adsorption performance of PB nanoparticles, we prepared PB-CMCNF composite filters by embedding PB in situ onto a macroporous CMCNF membrane. The 22

macroporous CMCNF membrane structure was obtained by freeze-drying followed by Fe3+ crosslinking. The use of a Fe3+ CMCNF framework allowed PB to be synthesized in situ via reaction with HCF on the macroporous CMCNF surface. Subsequent physicochemical characterization confirmed that cube-shaped PB particles with sizes of 2–50 nm were formed and that the amount of PB loaded onto the CMCNF membrane was proportional to the HCF concentration. The PB-CMCNF membrane exhibited a 2.5-fold-higher Cs adsorption capacity (130 mg/gPB-CMCNF) than commercial soluble PB, despite the relatively low content of PB (<

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100 mg/gPB-CMCNF) on the membrane; this performance is attributed to the surface coverage of nanosized PB and to the macroporous structure of the CMCNF membrane, which minimizes diffusion resistance. In accordance with the structural features of the PB-CMCNF membrane,

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Cs adsorption on PB-CMCNF proceeded rapidly within 1 h following a pseudo-second-order

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kinetic model and was fitted well with the Freundlich isotherm model. The Cs adsorption mechanism was found to be based on ion exchange between Cs and K. The presence of other

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cations in solution adversely affected Cs adsorption, with divalent cations such as Mg and Ca having a greater effect than monovalent cations. Finally, we demonstrated radioactive

137

Cs

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removal from natural seawater using a sequential filtration method, observing ca. 25% lower removal efficiency in seawater compared to an aqueous solution containing the same

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concentration of 137Cs. These results demonstrate the practical effectiveness of the PB-CMCNF

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membrane to treat 137Cs-contaminated water.

CRediT Semin Eun: Conceptualization, Methodology, Software. 23

Hye-Jin Hong: Data curation, Writing- Original draft preparation. Hyuncheol Kim: Visualization, Investigation. Hyeon Su Jeong: Data curation, Visualization. Soonhyun Kim: Software, Validation. Jongwon Jung: Data curation, Supervision. Jungho Ryu: Supervision, Writing- Reviewing and Editing.

Acknowledgement

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This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (project 19-3413) funded by the Ministry of Science and ICT and research project titled "Development of valuable metal recycling and reuse technology by using

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highly metal selective nano-cellulose base material (NRF-2018R1C1B6001081)" funded by National Research Foundation of Korea. This work was also supported by the Basic Science

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Research Program through the National Research Foundation of Korea (NRF), funded by the

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Ministry of Science, ICT & Future Planning (2017R1A2B4003919).

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