Novel method of preparation of tricarboxylic cellulose nanofiber for efficient removal of heavy metal ions from aqueous solution

Novel method of preparation of tricarboxylic cellulose nanofiber for efficient removal of heavy metal ions from aqueous solution

Accepted Manuscript Novel method of preparation of tricarboxylic cellulose nanofiber for efficient removal of heavy metal ions from aqueous solution ...

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Accepted Manuscript Novel method of preparation of tricarboxylic cellulose nanofiber for efficient removal of heavy metal ions from aqueous solution

Ragab E. Abou-Zeid, Sawsan Dakrory, Korany A. Ali, Samir Kamel PII: DOI: Reference:

S0141-8130(18)31124-3 doi:10.1016/j.ijbiomac.2018.07.127 BIOMAC 10171

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

8 March 2018 17 July 2018 20 July 2018

Please cite this article as: Ragab E. Abou-Zeid, Sawsan Dakrory, Korany A. Ali, Samir Kamel , Novel method of preparation of tricarboxylic cellulose nanofiber for efficient removal of heavy metal ions from aqueous solution. Biomac (2018), doi:10.1016/ j.ijbiomac.2018.07.127

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ACCEPTED MANUSCRIPT Novel method of preparation of tricarboxylic cellulose nanofiber for efficient removal of heavy metal ions from aqueous solution Ragab E. Abou-Zeid a, b,*, Sawsan Dakrorya, Korany A. Alic, d, Samir Kamela a

Cellulose and Paper Department, National Research Centre, 12622 Dokki, Giza, Egypt.

b

Univ. Grenoble Alpes, CNRS, Grenoble INP, LGP2, F-38000 Grenoble, France

Center of Excellence, advanced material & Nanotechnology Group, National Research

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c

d

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Centre, 12622 Dokki, Giza, Egypt

Applied Organic Chemistry Department, National Research Centre, 12622 Dokki, Giza, Egypt

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*

Corresponding author; Ragab Abouzeid ([email protected])

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Cellulose and Paper Department, National Research Centre, 33 El Bohouth St., Dokki, Giza, Egypt, P.O. 12622

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Tel/ Fax, (202) 33322418 (202) 33370931 ABSTRACT

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2,3,6 tricarboxy cellulose nanofiber (TPC-CNFs) was prepared by 2,2,6,6-tetramethylpiperidine-

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1-oxyl (TEMPO) oxidation of dissolving cellulose pulp (selective at C-6)

followed by

periodate-chlorite oxidation (selective on C-2 and C-3). Characterization of the prepared

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samples was carried out using, atomic force microscope (AFM), carboxylate content determination, FTIR spectroscopy, X - Ray diffraction and light transmittance spectra. Also the

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mechanical properties of TEMPO oxidized of cellulose nanofiber (T-CNFs) and TPC-CNFs with and without polyamide-amine-epichlorohydrin crosslinker (PAE) films were determined which the tensile strength were 8.19, 12.43 and 20.5 MPa and elastic moduli of 1814, 1097 and 1150 MPa respectively. The tricaboxy cellulose nanofiber was developed as a novel adsorbent of heavy metal ions. Removal of heavy metals such as Cu2+, Ca2+ and Pb2+ from aqueous solution was carried out and the adsorption efficiencies were analyzed. On the other hand, effect

ACCEPTED MANUSCRIPT of addition of crosslinking agent to CNFs and the carboxylate contents of CNFs was investigated. Key words; Tricarboxy cellulose nanofiber; TEMPO/Periodate oxidation; Heavy

metal

removal

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1. Introduction

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Cellulose, widely available and renewable polymers in nature, has been used for thousands of

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years for human basic needs, for example, as construction material, clothing production and as an energy source. Today, due to its chemical modifications, has found new and interesting

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applications in the food industry, medicine, cosmetics, electronic devices, water treatment and many other applications [1]. The oxidation of dissolving cellulose pulp is one of the most

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important and common modification methods to convert the cellulose into value-added derivatives containing different carboxyl contents. Oxidized cellulose is used in biomedical

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applications such as absorbable haemostats or scaffolds for tissue engineering because it is

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easily degradable in the human body under physical conditions [2]. Furthermore, it can be used as carrier material for agricultural, cosmetic and pharmaceutical applications [3]. Calvini et al.

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demonstrated that the oxidation of regenerated cellulose using mixtures of strong acids such as HNO3/H3PO4 with /without sodium nitrite and different carboxyl contents and degrees of

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crystallinity of oxidized cellulose were widely studied using nitrogen dioxide in chloroform [4]. Recently, TEMPO-mediated oxidation one of the most used chemical methods for oxidation and surface modification of cellulose to selectively convert of the primary hydroxyl groups (C6) to carboxylic (–COOH) groups [5–11]. The preparation of TEMPO oxidized cellulose nanofibers from Prunus amygdalus stem were studied by Khiari 2017, it was found that the nanofibers have good morphological properties with diameters varied from 3 up to 18 nm. These CNFs were used as new nanofiller to prepare latex nanocomposites with high thermal and

ACCEPTED MANUSCRIPT mechanical properties [12]. Bettaieb et al. reported the preparation of different grade of cellulose nanofiber from posidonia oceanica balls and leaves using TEMPO mediated oxidation. It was found that the stronger fibrous network structures were formed by increasing the oxidant concentration [13,14]. Oxidation of microcrystalline cellulose was applied to get 6-carboxy cellulose using two-step. First oxidation was planned to prepare water-soluble β-1, 4-linked

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glucuronic acid in high yield followed by TEMPO oxidation. The water soluble product has low

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degree of polymerization of only 38-79 as compared to 220-680 of the starting material [15].

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Periodate oxidation of cellulose cleaved C2-C3 bonds of glucopyranose ring and selectively oxidize C-2 and C-3 hydroxyl groups to form 2, 3-dialdehyde groups along the chains of

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cellulose [16]. Sergiu et al. prepared water soluble carboxyl functionalized cellulosic by combining of two nitroxyl mediated reaction and periodate oxidation, in a one-shot reaction with

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carboxylic content of 3110 mM kg-1 [15]. Another method for preparation of tricarboxy cellulose is initially synthesized in a three step reaction of cellulose with N2O4, NaIO4, and

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HClO2, these three step reactions were used N2O4, NaIO4, and chloroform which are toxic and

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explosive reagents [17]. Another report were used two step to prepare tricarboxylic cellulose NaIO4 and N2O4 and was then announced as an asset for the arrangement of tartaric acid, a The removal of heavy metals from

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hemostatic agent and a chelator of metallicions [18].

wastewater are one of the most significant studies due to the harmful effect of these heavy metal

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due to their toxic nature, not biodegradable and easily accumulate in living organisms causing various diseases. The oxidized cellulose nanomaterials are a promising alternative absorbent material for heavy metals removal due to its high surface area, biodegradable, low cost and high carboxylate contents, and its potential use in water treatment has gained increasing attention. It was reported that the surface area of nanocellulose was in the range from 300–400 m2/g, which is very valuable in the adsorption capacity for heavy metal removal [19,20]. The advantage of incorporating carboxylate groups has also been revealed on cellulose CNFs. Srivastava et al.

ACCEPTED MANUSCRIPT showed the ability of COO- modified cellulose nanofibers to adsorb Ni2+ and Cr3+ in addition of Cd2+ and Pb2+ with efficiencies 3–10% compared to unmodified CNFs [21,22]. The utilization of TEMPO oxidized nanofiber for removal of radioactive uranylions (UO22+) from solution was studied with the efficiency of 2–3 times greater than that achieved with traditional adsorbent such as hydrogels, montmorillonite and silica particles.

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Zheng and co-workers were reported that the carboxylated CNFs incorporated PVA hybrid

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aerogels, which prepared by freeze drying the crosslinked carboxylated CNFs/PVA composite

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gel, having good metal ions adsorption capability [23]. The modification of TEMPO oxidized CNFs with polyethylenimine (PEI) for the adsorptive removal of Cu2+ was studied by Zhang et

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al. it was found that the adsorption capacity was increased by introducing abundant carboxyl and amino functional groups on CNFs which act as active sites for the binding of Cu 2+ ions and the

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qmax was estimated to be 0.82 mmol/g [24]. Hassan el al. studied the preparation of new Cuterpyridine-modified oxidized CNFs membranes to recover water from wastewater paper

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industry [25]. Also, oxidized cellulose nanofibers were used as ultra-thin films membranes for

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wastewater treatment. Structured multi-layered thin film membranes of CNFs and TEMPOoxidized CNFs/Activated carbon for water purification of E. coli bacteria in water were prepared

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[25]. To the best of our knowledge, no reports are available on the preparation of 2, 3, 6 tricarboxylic cellulose using TEMPO oxidation followed by periodate oxidation methods and

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studying of its adsorption of heavy metal ions from wastewater. In the present study, 2, 3, 6 tricarboxylic cellulose nanofibers (TPC-CNFs) was first performed by TEMPO mediated oxidation of dissolving bagasse pulp. The TEMPO oxidized cellulose were mechanically defibrillation using high shear homogenizer and re-oxidized by periodate/chlorite method to obtain TPC-CNFs. Afterward, the preparation of cellulose nanofibers films were performed to remove the Ca2+, Cu2+ and Pb2+ from aqueous solution. 2. Materials and Method

ACCEPTED MANUSCRIPT 2.1. Materials Bagasse raw material was supplied from Quena Company of Paper Industry, Egypt. The chemical composition of the bagasse raw material was determined according to Tappi standards (Klason lignin, Pentosan, alpha cellulose and ash content) and the results are 22, 44.4, 28.1 and 1.4 respectively. Dissolving bagasse pulp was prepared according to Marcela et al [26]. The

low

content

of

lignin.

Sodium

metaperiodite

(NaIO4),

NaBr

and

2,2,6,6-

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very

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chemical composition of the prepared pulp was cellulose content (96%), hemicellulose (3%) and

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tetramethylpiperidine-1-oxyl (TEMPO) were purchased from Sigma Aldrich. Polyamide-amineepichlorohydrin (PAE) was commercial grade (~33%, w/w solid content, Solines, Wilmington,

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DE, USA). PAE solutions were diluted to 1 wt % with distilled water before addition. Additional chemicals needed for the different analytical methods were bought from Sigma-

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Aldrich. All chemical reagents were used with no further purification. 2.2. TEMPO oxidation of cellulose

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The method used was based on that of Saito et al. [27]. Dissolving cellulose pulp (5 g) was

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dispersed in distilled water (500 ml) with TEMPO (0.08 g, 0.5 mmol) and sodium bromide (0.8 g, 8 mmol). Then 50 ml of sodium hypochlorite solution (10 %) is then added with stirring and

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the pH was adjusted to 10. At the end of reaction the pH is adjusted to 7 and the product was centrifuged at 7000 rpm. The product was further purified by repeated adding water, dispersion,

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and centrifugation. Finally the product was purified by dialysis for 1 week against deonized water. TEMPO-oxidized cellulose was disintegrated by high-shear homogenizer (T-T18 ULTRA TURRAX) at 10000 rpm using 2 % pulp concentration. 2.3. Periodate-chlorite oxidation of TEMPO-oxidized cellulose Periodate-chlorite oxidation steps were carried out according to a procedure described previously [28]. 12 g of T-CNFs diluted to 1 % consistency in distilled water was heated to 60 °C in a water bath, 46 mmol of sodium metaperiodite (NaIO4) was added and the reaction

ACCEPTED MANUSCRIPT container was covered with aluminium foil to avoid photo-induced decomposition of the periodate. The reaction was stopped after 3h by washing the resulting dialdehyde cellulose nanofiber with distilled water and filtering it in a funnel. In the second step, 4.5 g of dialdehyde CNFs was diluted to a consistency of 4.5 % in distilled water, and 50 mmol of sodium chlorite (NaClO2) was dissolving in 40 g distilled water in a separate beaker, after which 60 ml of 20 %

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acetic acid was added slowly so that a yellowish colour was obtained. Finally the two

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suspensions of diluted dialdehyde CNFs and chemicals were mixed together and stirred for 48 h

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at room temperature. Again the resulting product (tricarboxyl acid CNFs) was washed with deionized water and filtered. Also the yield of the prepared tricarboxylic acid was determined

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and it has been in the range from of 80-85 %. 2.4. Determination of carboxylate content

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The carboxylate content of T-CNFs and TPC-CNFs were determined by electric conductivity titration method [7,29]. A dried sample (∼50 mg) was thoroughly mixed with 0.01 M HCl (15

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mL) and deionized water (20 ml), and the mixture was vigorously stirred to achieve a well-

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dispersed suspension. Then, the mixture was titrated with 0.01M sodium hydroxide (NaOH) solution. The carboxylate content of T-CNFs and TPC-CNFs were determined from the sudden

Eq. (1):

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change in conductivity. The content of carboxylate groups, C (mmol/g), was calculated using

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C = ((V1 − V0) × CNaOH) / m

Eq. 1

where V1and V0 are the volumes of standard NaOH solution before and after titration, CNaOH is the concentration of standard NaOH solution, and m is the weight of the dried sample. 2.5. Preparation of oxidized cellulose nanofibril films Cellulose nanofiber films were prepared by casting the CNFs dispersion with/without 4 % polyamide-amine-epichlorohydrin (based on oven-dry weight of CNFs) as crosslinker onto a petri dish and drying at 40 °C for 3 days after that the temperature of the oven was increased to

ACCEPTED MANUSCRIPT 105° C for 30 min. The cast films with about 10 μm thickness were easily detached from the petri dish after drying. 2.6. Characterization of T-CNFs and TPC-CNFs 2.6.1. AFM The prepared CNF were characterized by AFM (atomic force microscope) using AFM

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Multimode (DI, Veeco, Instrumentation Group) in trapping mode with multi 130 tips.

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2.6.2. FTIR analysis

spectrometer (Unicam, UK) by KBr technique.

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2.6.3. X-ray diffraction pattern (XRD)

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Fourier transform infrared (FTIR) spectroscopy was carried out using a Mattson 5000

XRD were recorded using X-ray diffractometer (PANa-lytical, Netherlands) at room

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temperature with amonochromatic Cu K𝛼 radiation source (𝜆 = 0.154 nm) in step-scan mode with a 2𝜃 angle ranging from 5° to 80° with a step of 0.04 and a scanning time of 5.0 min. The

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crystallinity index was calculated from the height of the (200) peak (I200 2θ=22°) and the

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intensity minimum between the (200) and 110 peaks (Iam 2θ = 18°) using the Segal method as shown in Eq. 2. I200 represents both crystalline and Iam represents the amorphous material [30]. Eq. 2

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Crystalinity % = 1- (Iam/I200) ×100 2.6.4. Optical properties - UV-visible spectroscopy analysis

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The light transmittance spectra of the T-CNFs and TPC-CNFs/water dispersions were measured from 200 to 800 nm with a Shimadzu UV-1800 spectrometer. The solid content of TEMPOCNFs suspension was 0.3% at room temperature. 2.7. Mechanical properties. The stress strain curve of the resulted films was determined using a Lloyd instrument (Lloyd Instruments, West Sussex, United King-dom) with a 100-N load cell. 2.8. Adsorption of heavy metal ions

ACCEPTED MANUSCRIPT The adsorption experiments were performed on a platform shaker at 200 r/min and (25 ± 2) °C using 150 mL shaker flasks. In order to avoid the formation of insoluble metal hydroxides, the pH of adsorption kinetics experiments was kept at 6.0 for Cd2+ and 5.5 for Pb2+ [31]. Either 0.1 mol/L HCl or 0.1 mol/L NaOH solutions were used to adjust the pH values during the adsorption experiments. The metal ion concentration was analyzed using an inductively coupled

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plasma optical emission spectrometer (ICP-OES, Optima 2000, Perkin Elmer, USA). The

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adsorption capacity qe (mg/g) was calculated as described by the following equation:

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qe = (C0 − Ce)V / m

where, C0 (mg/L) is the initial metal ion concentration, Ce (mg/L) is the metal ion equilibrium

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concentration, V (L) is the volume of the metal ion solution and m (g) is the mass of adsorbent. The adsorbent dose was kept at 1 g/L for all the adsorption experiments.

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3. Results and Discussion

3.1. Proposed mechanism of oxidation

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In the first stage of the reaction, the stable nitroxyl radical TEMPO acts as a mediator in the

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presence of sodium bromide and sodium hypochlorite to selectively oxidation the primary hydroxyl groups (C6) which considered more reactive than hydroxyl in C2 and C3. The second

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stage for the selective oxidation of hydroxyl groups in C2 and C3 uses periodates, resulting in two aldehyde groups with breaking of the C2-C3 linkage, which further oxidized to generate

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carboxylic cellulose (Scheme1). Scheme1.

The oxidation of dissolving cellulose using TEMPO and periodate/chlorite method were confirmed by FTIR, the spectra of dissolving cellulose pulp, T-CNF and TPC-CNFs are shown in Figure 1. All the characteristic cellulose peak, such as a broad peak at around 3400 cm −1 is attributed to the stretching vibration of OH groups and the peak at 2900 cm -1 for sp3 hybridized C–H stretching, are present in all the samples. The spectral bands observed in all cellulose, T-

ACCEPTED MANUSCRIPT CNFs and TPC-CNFs spectra in the region of 1632 cm−1are due to OH bending of adsorbed water. The spectral bands observed in the region of 1008 -1048 cm−1 are due to stretching vibration of C-O-C of pyranose ring. After oxidation, the new absorption peaks at 1740 cm−1 and 1742 cm−1 for T-CNF and TPC-CNF respectively are attributed to carbonyl groups in the free COOH

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group [32]. As the PAE crosslinked TPC-CNFs occurs, a peak around 1632 cm-1 which is

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corresponding to the amide group belonging to PAE. Ref Also the intensity of the beak sat 1742

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cm-1 crossponding to the C=O vibrations were increased due to the ester formation functions in the crosslinking network.

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Fig. 1.

3.2. Surface charge and Determination of the carboxylic group content

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Surface charge of the fibers can play an important role in their properties. Anionic sites estimation for TPC-CNFs showed higher negative surface charge due to formation of

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tricarboxylic groups onto their surface as a result of the TEMPO oxidation followed by

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periodate-chlorite oxidation. The direct conductometric titration of T-CNFs and TPC-CNFs were used to determine the acidic group content and the carboxyl contents of the T-CNF and

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TPC-CNFs were 0.7 ±0.1 and 3 ±0.3 mmol/g, respectively. This part was added to the revised manuscript in red colour. Also the carboxyl content of the crosslinked TPC-CNFs using PAE

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was determined to be 1.9 ±0.2 mmol/g. This decrease in the carboxyl content was due to the conversion of the surface carboxyl groups of the TPC-CNFs into ester bonds with the azetidinium groups of PAE. 3.3. The morphology of oxidized cellulose using AFM Figure 2 shows AFM images of T-CNFs and TPC-CNFs dispersions with different carboxyl content. It is shown that all the dispersions display the web-like structure consisting of many randomly entangled and well-individualized fibers with the nanosized dimension. This suggests

ACCEPTED MANUSCRIPT that dissolving bagasse pulp fibers are successfully nanofibrillated through the TEMPOoxidation pretreatment and TEMPO- oxidation followed by periodate-chlorite oxidation. The AFM images shows that by increasing the carboxylate content, from 0.7 to 3 mmol/g for TCNFs and TPC-CNFs respectively, the nanofibers become finer in width and the length which is attributed to loosen adhesion between microfibrils and electrostatic repulsions between the

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surface charges.

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Fig. 2.

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3.4. X-ray diffraction analysis of T-CNFs and TPC-CNFs

X-ray diffraction patterns of dissolving cellulose pulp, T-CNFs, and TPC-CNFs were

peaks at 2θ from 15-17°, 22.5°,

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investigated to study the effect of oxidation on the crystallinity change of pulp (Figure 3). The and 34°,

corresponds to the (110), (200) and (004)

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crystallographic planes respectively [33]. The crystallinity of dissolving cellulose pulp, T-CNFs, and TPC-CNFs are 59.7, 76.9, and 55 respectively. It can be seen that crystallinity increases

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from 59.7 to 76.9 % upon TEMPO oxidation of cellulose pulp, this is due to the degradation in

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the amorphous regions would occur during the oxidation reaction, leading to their loss in the water-soluble fraction, thus increasing the overall crystallinity percentage. However, the

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crystalline index of T-CNF was decreased according to the oxidation level by periodate and

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chlorite to 55%.

Fig. 3.

3.5. Optical properties - UV-visible spectroscopy analysis Figure 4 gives UV–VIS transmittance spectra of T-CNFs and TPC-CNFs dispersions with different carboxylate contents. It can be seen that the transmittance of T-CNF dispersions is progressively increased when charge content is increased from 0.7 to 3 mmol/g. In the case of TPC-CNF, the light transmittance at 600 nm (optical transmittance values were taken at 600 nm), which is the approximate average wavelength of the visible light region is as high as 75%,

ACCEPTED MANUSCRIPT which is approximately 3 folds that of T-CNFs (T= 25%). It is known that the state of TPCCNFs dispersions depends largely on its surface charge. The increased in the transmittance were due to that the oxidized cellulose nanofiber become shorter in length which gives more homogeneity of the suspension. Fig. 4.

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3.6. Mechanical properties of CNFs films

In order to further enhance the mechanical properties of the CNFs films to be used in wastewater

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treatment application, PAE was employed to crosslink the CNFs. The mechanism of the

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crosslinked was shown in scheme 2.

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Scheme 2.

The T-CNFs, TPC-CNFs and TPC-CNFs/PAE films were prepared by casting method and

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characterizations of the films revealed that films have promising properties with potential applications. The tensile strength was 8.19, 12.43, and 20.51(MPa) while Young's Modulus was

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1814.94, 1097.23, and 1150.00 (MPa) for prepared films from T-CNFs, TPC-CNFs, and TPC-

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CNFs/PAE respectively. Stress strain curve was shown in Figure 5. The results indicate that tensile properties and Young's Modulus of TPC-CNFs was higher than of T-CNFs.

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Fig. 5.

3.7. Applications of prepared CNFs for heavy metal removal

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The mechanisms of the sorption process inside cellulose nanofiber are schematically shown in Fig. 7. The positively charged metal ions interact specially with the negative charged species of functional groups due to electrostatic attraction.

Fig. 6.

The different types of cellulose nanofibers (T-CNFs, TPC-CNFs and crosslinked TPC-CNFs) were tested for the removal of heavy metal ions of Cu2+ , Pb2+and Ca2+ using metal ions concentration of 250 ppm and contact time of 2 hours. The concentrations of metal cations were

ACCEPTED MANUSCRIPT measured before and after the treatment process. The results are represented in figure 7. It is appear from the figure that the affinity of three types of oxidized CNF toward the metal ions was different. Copper and lead metal ions were the highest adsorbed ions 92.23, 97.34 and 82.19 mg/g for T CNFs, TPC-CNFs and crosslinked TPC-CNFs respectively. T-CNFs shows the lowest adsorption capacity towards Ca2+. The effect of crosslinking agent on the adsorption was

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studied by adding other 5% of PAE crosslinker to the TPC-CNFs films. It can be seen that the

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absorption of metal ions (Cu2+, Pb2+, and Ca2+) were decreased by using the crosslinking agent

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[34]. This decreasing in the absorption of metal ions were due to the conversion surface carboxyl groups into ester bonds with the azetidinium groups of PAE as seen in scheme 2. The

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high adsorption capacity is comparable with other reported CNF adsorbent for Cu2+. It was reported that the adsorption capacity of Cu2+ of CNFs was 13 mg/g while with the value

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obtained for oxidized CNFs was 112 mg/g [35,36].

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4.Conclusion

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Fig. 7.

The preparation of tricarboxylic cellulose has been successfully prepared by a combination of

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two steps, TEMPO oxidation (selective at C-6) followed by periodate-chlorite oxidation (selective on C-2 and C-3). The carboxyl content of TEMPO oxidized cellulose nanofiber (T-

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CNFs) and TEMPO-periodate oxidation (TPC-CNFs) was found to be 0.7 and 3 mmol/g with further decrease in the crystallinity from 76.9 to 55%. Also the morphology of the T-CNFs and TPC-CNFs was confirmed with AFM without further degradation of the CNFs. T-CNF and TPC-CNFs were as a new bio absorbent nanomaterial of heavy metal ions in aqueous such as Cu2+, Pb2+, and Ca2+. This work has demonstrated that tricaroxylic nanocellulose are highly promising biosorbents for scavenging metal ions from water and may enable next-generation of water purification technologies due to high surface area and high surface charge.

ACCEPTED MANUSCRIPT Conflict of Interests The authors declare that they have no conflict to interests. Acknowledgement The authors acknowledge the Science and Technology Development Fund (STDF), Egypt for

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financial support of the research activities related to project; Project ID 15203.

D. Klemm, F. Kramer, S. Moritz, T. Lindström, M. Ankerfors, D. Gray, A. Dorris,

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[1]

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Captions Scheme1. Illustrate the full oxidation of cellulose. Fig.1. FTIR spectra for (a) cellulose (b) T-CNFs, (c) TPC-CNFs and (d) crosslinked TPC-CNFs

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Fig. 2. AFM images T-CNFs (right) and TPC-CNFs (left) with 10µm scale. Fig. 3. XRD diffraction patterns of dissolving cellulose pulp, T-CNFs and TPC-CNFs

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Fig. 4. UV–VIS transmittance spectra of aqueous T-CNFs and TPC-CNFs dispersions with

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varying carboxylate contents.

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Scheme 2. Crosslinking reaction between CNFs and PAE Fig. 5. Stress /strain curve of T-CNFs and TPC-CNFs

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Fig. 6. Schematics of metal ion adsorption mechanism.

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Fig.7. Heavy metal adsorption by tricarboxylic cellulose nanofibers

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