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Porous material from cellulose nanofibrils coated with aluminum hydroxyde as an effective adsorbent for fluoride
Journal Pre-proof Porous material from cellulose nanofibrils coated with aluminum hydroxyde as an effective adsorbent for fluoride Norhene Mahfoudhi, Sami Boufi
PII:
S2213-3437(20)30127-5
DOI:
https://doi.org/10.1016/j.jece.2020.103779
Reference:
JECE 103779
To appear in:
Journal of Environmental Chemical Engineering
Received Date:
2 December 2019
Revised Date:
17 January 2020
Accepted Date:
11 February 2020
Please cite this article as: Mahfoudhi N, Boufi S, Porous material from cellulose nanofibrils coated with aluminum hydroxyde as an effective adsorbent for fluoride, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2020.103779
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Porous material from cellulose nanofibrils coated with aluminum hydroxyde as an effective adsorbent for fluoride
Norhene Mahfoudhi* and Sami Boufi* University of Sfax-Faculty of Science - LMSE- BP 1171 3000 - Sfax -Tunisia Corresponding author at: Faculty of Science - LMSE-University of Sfax BP 1171 3000 -
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Sfax -Tunisia * Tel: 216 74274400 * Fax: 216 74274437
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*E-mail:[email protected] /[email protected]
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Highlights
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Porous cellulose aerogel modified with aluminium hydroxide was prepared and characterized The aerogel was used as an adsorbent for the removal of fluoride from aqueous solution The hybrid aerogel showed a great promise to be applied as a reusable adsorbent for fluoride
Abstract
A novel aluminium hydroxide modified porous cellulose aerogel was prepared and used
as an effective adsorbent for the removal of fluoride from aqueous solution. The adsorbent was prepared by freeze-drying of cellulose nanofibrils gel followed by impregnation with
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AlCl3 solution and washing with water. The ensuing sorbent was characterized by SEM
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observation, XPS, TGA and BET measurement for specific-surface evaluation. The CNFAl(OH)3 modified aerogel was shown to be an effective adsorbent for fluoride in water, with an adsorption capacity ranging from 20to 35 mg.g-1, depending on the Al(OH)3 loading. The spent CNF-Al adsorbent was regenerable by washing with NaOH solution and could be reused up to five adsorption-desorption cycles without significant loss of its adsorption capacity. The CNF –Al aerogel showed a great promise to be applied as a reusable adsorbent from a renewable resource for defluorination.
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Key words: cellulose nanofibrils, fluoride, aerogel, adsorption.
1. Introduction
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The problem of water defluorination is of a major importance, nowadays due to chronic disease such as fluorosis, neurological damage and softening of bones imparted by
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contamination of drinking water by fluoride with content exceeding 1.5 mg.L-1 [1]. In fact, many actual methods are being used to overcome this problem [2]. The present defluorination
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methods include precipitation, membrane separation, ion exchange and adsorption [3]. Among them, adsorption is a new approach with promising results. Researchers consider it as the most promising method due to its effectiveness, convenience, easy operation, simplicity of
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design, economic and environmental aspects [4].
In this context, the adsorption of fluoride attracted the attention of many studies, dealing
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with natural biobased adsorbents instead of conventional synthetic adsorbents. For instance, synthetic apatites [5], bentonite clay [6] and modified chitosan beads [7] proved to be good
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adsorbents for fluoride. Metal-binding biopolymers were tested to remove water contaminants [8].Yet, such biomaterials are still faced with problems such as relatively low adsorption ability or large decline in reused performance. Adsorption [9] of fluoride on natural polymers obtained from agricultural waste products has also emerged as an excellent alternative to the traditional techniques of reverse osmosis and membrane filtration, decreasing the energy consumption [10]. Because of their low cost and the presence of a variety of functional groups for modification, the uses of agro-waste material has attracted increasing interest in this field
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[11]. Fluoride removal has been investigated by various biobased materials [12], including pine sawdust [13], tea [14], cellulose [15], chitin [16], orange waste [17], chitosan [18], etc.. During the last two decades, nanosized cellulose fibrils have emerged as one of the most promising nanomaterials in water remediation. Their nanoscale dimensions, biodegradable character, strong reinforcing effect, high aspect ratio, lightweight, and sustainability are examples of positive properties driving toward the wide interest in nanocelluloses [19]. Furthermore, nanocellulose is classified as a non-toxic material [20] completely biodegradable and without adverse effects on the environment [21].
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Taking advantage of the mentioned biopolymer, native cellulose fibers were surface modified by poly (N,N-dimethyl aminoethyl methacrylate) (PDMAEMA) to generate an
anion adsorbent. This adsorbent had high efficiency in removing F− from aqueous solutions, even at low initial concentrations. Adsorption kinetics showed that the adsorption equilibrium was reached within 1 min attaining a maximum fluoride adsorption [22] of 8.5 mg.g-1 . In
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a maximum fluoride adsorption [23] of 0.6 mmol.g-1.
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another study, cationic CNF with a maximum cationic charge content of 1.2 mmol.g-1 showed
Furthermore, metal deposition of Cerium onto cellulose such as cerium loaded cellulose
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nanocomposite beads or iron oxide deposition onto cellulose were tested for fluoride removal. Fluoride adsorption on cerium loaded nanocellulose proved that almost 90% of adsorbed fluoride can be eluted with 0.01N NaOH and regenerated for at least three times [24].
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Aluminium coated Chitosan-Fe(III) hydrogel was also tested for fluoride removal and showed a relatively high adsorption capacity of 31.16 mg.g-1. After regeneration, the
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adsorbent performed higher defluoridation efficiency than before. The mechanism of adsorption was described by the ligand and ions exchange that happened on the active sites
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[25].
However, the use of nanoscale cellulose as a support material for fluoride adsorption has
not yet been widely investigated up to our knowledge. For this reason, in this work, porous nanocellulose based aerogel coated with aluminium hydroxide was prepared and tested for fluoride removal from aqueous solutions. The study tackles the kinetic aspect of the defluorination process in batch and continuous modes. The regeneration of CNF-Al aerogel
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was successfully obtained with an alkaline treatment (0.1 M of NaOH), showing the possible reuse of this adsorbent for multiple cycles.
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2. Materials Commercial bleached Eucalyptus pulp (Eucalyptus globulus) from Ence-CelulosasAustria in the form of dry sheets was used as a starting material for the preparation of CNF. Aluminium Chloride (AlCl3), Hydrogen Chloride (HCl) and Sodium Hydroxide (NaOH) were purchased from Aldrich. 2.1. Adsorbents Fibres were first pre-treated by TEMPO-mediated oxidation to facilitate the disintegration process and produce CNF with high yield. The oxidation was carried out at pH 10 following
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the method reported in literature [26]. A detailed description of the process was reported in
our previous work [27]. The oxidation degree of fibres was evaluated as 1100 μmol/g. Then, the oxidized fibres
at a consistency of 1.5% were homogenized in a high pressure
homogenizer (NS1001L PANDA 2 K-GEA, Italy) applying six shear cycles (3 x 300 bar; 3 x
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600 bar). A thick transparent CNF gel was obtained after six passes. AFM images of the CNFs gel are given in Fig. 1S.
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The CNF gel was placed in glass cylindrical tubes (h/D: 15/ 15mm) and freezed at -20°C
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for about 24 h. Thefrozen sample was then freeze dried until complete water sublimation. To immobilize the Al(OH)3 layer, CNF aerogel cylinder, with a dried mass of 1g each, were immersed in 25 mL of AlCl3 solutions with various concentrations (C1=0.15 M,
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C2=0.075 M and C3=0.037 M) for 3 h. After that, the modified CNF aerogel (Al-CNF) was transferred in distilled water and pH was stabilized at 7 with NaOH (0.1 M). An additional washing of the material was introduced to eliminate any trace of chloride or unreacted
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aluminium.
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Three samples labelled CNF1-Al(OH)3, CNF2- Al(OH)3 and CNF3- Al(OH)3 were produced, using respective concentrations of AlCl3 equal to C1=0.15 M, C2=0.075 M and C3=0.037 M.
2.2. Batch adsorption experiments 50 mL of F- (concentration ranging from 1 to 100 mg. L-1) solution was prepared in 100
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mL beaker flasks at room temperature. The CNF-Al(OH)3 aerogel was immersed in this solution for 2h for kinetic studies. Then, a spot of sample solution was taken out at desired
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time intervals to analyse the residual F-. The initial pH values ranged from 3 to 12, and were adjusted by 0.1 M HCl or 0.1 M NaOH solutions. The fluoride concentration was measured by a fluoride specific ion electrode ISE with the help of total ionic strength adjustment buffer (TISAB II) solution (14.5 g NaCl, 2.0 g sodium citrate and 14.25 ml glacial acetic acid and an amount of 40% NaOH in a volume of 250 ml, the pH~5.5). 10mL of a Total Ionic Strength Adjustment Buffer (TISAB II) was added to 10 mL of water sample (withdrawn from each experiment) and standards. All measurements were carried out at 25°C. The adsorption capacity at time t, qt (mg.g-1) was calculated by Eq.
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(1):
qt (C0 Ct )Vm (1) Where
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C0 and Ct (mg.L-1) stand for the initial concentration and the concentration at time t,
respectively. V (L) is the volume of the solution, and m (g) is the mass of the sorbent used.
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2.3. Column study
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A continuous batch adsorption experiment was carried out using CNF-Al(OH)3 aerogel packed column. The glass column was of 1 cm diameter and 15 cm length. Carrying 0.2 g of CNF-Al(OH)3 aerogel pellets, the effluent solutions were percolated from the bottom to the
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top using a peristaltic pump at a flow rate of 5 ml.min-1. Effluent samples were collected at regular intervals to determine their concentrations.
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2.4. Regeneration of the adsorbent After adsorption experiments, the CNF–Al aerogel pellets were removed, washed with
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deionized water, and placed in 0.1 M NaOH solution for 3 h and then washed with 0.1 M HCl. Finally, the pellets were washed again with deionized water for several times and prepared for the next cycle. 3. Methods 3.1. SEM Observation
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The observation of the CNF-Al(OH)3 aerogel morphology was performed by scanning electron microscopy (SEM) using a (JEOL JSM-6390LV) operating at 5–10 kV. 3.2. XPS Samples were characterized by X-Ray Photoelectron spectroscopy (XPS), using a XSAM 800 spectrometer from Kratos, emitting non monochromatic Al Kα X- radiation. The analysis was performed in FAT mode, with pass energy of 20 eV and setting the emission source with 12 KV and 10 mA. The estimated pressure was of ̴ 10-7 Pa and the take-off angle (TOA) was equal to 45°. A Shirley background was subtracted and curve fitting for component peaks was
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carried out using Gaussian-Lorentzian profiles. No charge compensation was used. Binding energies were corrected using as reference the binding energy of aliphatic carbon at 285 eV. Surface composition was determined using the manufacturer’s sensitivity factors.
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3.3. TGA analysis
The thermogravimetric analysis (TGA) was performed with a thermogravimetric analyser (TGA 400 Perkin Elmer). Samples were about 10 mg and the heating cycle was from 20 to
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3.4. Brunauer– Emmett–Teller (BET)
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800°C at a heating rate of 10 °C/min under air flow.
The surface area and porosity of the fibers were estimated by adsorption/desorption
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isotherms of nitrogen gas (N2) at 77 K using the volumetric gas adsorption analyzer Quantachrome NovaWin (NovaWin, version 10.01, Boynton Beach, FL, USA). Based on the curves generated by the pressure variation of the adsorption and desorption isotherms, the
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specific surface area was estimated by the Brunauer-Emmett-Teller (BET) method.
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4. Results and discussions The porous sponge-like aerogel was obtained by freeze-drying of a CNF suspension at 1%
solid content. The CNFs suspension in the form of a transparent gel was obtained by a high pressure homogenization and was composed of more than 95% of nanosized fibrils. After complete water removal through freeze-drying at -20°C, a lightweight sponge-like aerogel was produced. Then the aerogel was immersed in a solution of AlCl3 for 1h, followed by abundant rinsing with water until neutral pH. This treatment did not alter neither the shape of
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the aerogel nor its integrity. It also increased the aerogel resistance to water disintegration,
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even after multiple use and prolonged immersion in water. This phenomenon could be explained by the strong capacity of Al3+ ions to chelate with the carboxyl groups of CNF surface, resulting in the irreversible chemical crosslinking of the CNF network. Then, after the removal of the aerogel from AlCl3 solution and its immersion in w ater bath at pH 7, the adsorbed Al3+ ions will be converted into aluminium hydroxide (Al(OH)2+, Al(OH)3 ) that aggregate to form aluminium hydroxide nanoparticles firmly bound to the cell wall of the cellulose aerogel. Given the insolubility of aluminium hydroxide within pH domains between 4 to 11 (pks=33), the generated Al(OH)3 will remain encrusted within the cellulose aerogel. SEM observations of the CNF-(Al(OH)3 aerogel sample revealed a sponge-like material
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formed by heterogeneously shaped pores with sizes from 50 to 300 μm forming open
channels with a thin cell of 100 to 200 nm thickness. The surface of the pore’s wall looks
rough, revealing spherical nodules with 50 to 300 nm in diameter, lining the surface which presumably corresponds to the Al(OH)3 particles. No meaningful evolution of the
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morphology of the aerogel could be noted after the treatment with AlCl3 solution as inferred from comparison of the aspect of the aerogel prior (Fig. 1A) and after immersion in AlCl3
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solution (Fig 1B).
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Figure1: FE-SEM images of the cryofractured (A, B) neat CNF and (C,D)CNF3-Al(OH)3 aerogel. (B and D) correspond to cross-section of the cell wall of the channel of theaerogel.
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(The insert shows the photo of the neat (A) and Al(OH)3 functionalized aerogel (C))
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XPS characterization of CNF-Al(OH)3 confirmed the presence of Al through its typical photopic around 74.45 eV, with atomic surface concentration of about 2%. The ratio Al/Cl was
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found to be around 10, meaning that most of the grafted AlCl3 is in the hydrolyzed form.
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A
Name O1s C1s Al2p Cl2p Na1s
Peak BE Atomic % 532.4 35.21 285.96 62.29 74.45 2.05 199.81 0.21 1071.28 0.24
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C
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Figure 2: (A) XPS survey of neat CNF2-Al(OH)3 sample (H) and the high resolution of (B) C1s, (C) O1 s and (D) Al2p regions.
The amount of loaded Al(OH)3 on the cellulose aerogel was estimated from TGA after the
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total thermal degradation of the cellulose. The final residual mass, after heating up to 700 °C was around 1%, 6% and 10% for neat CNF and CNF2-Al(OH)3 and CNF3-Al(OH)3, respectively.
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Assuming that Al(OH)3 has undergone a dehydratation at a temperature around 400°C with a release of 1.5 mole of water per a mole of Al(OH)3, then the amount of loaded Al(OH)3
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could be estimated to about 10% and 16% for CNF2-Al(OH)3 and CNF3-Al(OH)3,, respectively. Then, it is possible to control the amount of loaded Al(OH)3 on the CNFaerogel through the initial concentration of theAlCl3 precursor (Figure 3). The neat CNF aerogel has a 12.5 m2.g-1 Brunauer– Emmett–Teller (BET) specific surface area and a density of 0.03 g.cm-3. After the treatment with AlCl3, the specific surface area strongly decreased to about 4.7 and 3.2 m2.g-1, and the density grows to 0.065 and 0. 087
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g.cm-3 for CNF2-Al(OH)3 and CNF3-Al(OH)3,, respectively. This evolution might be
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explained by the accumulation of Al(OH)3 within the fine pores of the aerogel forming a layer coating the cell wall, clogging the access to the tiny pores of the aerogel (Table 1).
100
70 60 50 40 30
Nneat CNF CNF-Al1 CNF-Al2 CNF-Al3
20 10 0 50
150
250
350
450
550
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Temperature (°C)
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Weight (%)
90 80
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Figure3: TGA of neat CNF and Al-CNF aerogel with two Al contents
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Table 1: Comparison between neat CNF /Al-CNF aerogel properties
CNF1-Al(OH)3
CNF2-Al(OH)3
CNF3-Al(OH)3
0.062
0.065
0.087
Specific surface (m2.g-1) 12.5
4.1
4.7
3.2
Residue at 700°C (%)
6
9
10.5
0.03
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Density (g.cm-3)
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Neat CNF
To assess how the presence of Al(OH)3 improved the removal of fluoride ions, the
adsorption performance of the CNF-Al aerogels with different contents of Al(OH)3 was studied by measuring the adsorption amount of fluoride at different initial concentrations Fand constant weight of the sorbent. Results of this study were reported in the form of an adsorption isotherm.
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The first evidence of the effectiveness of CNF-Al aerogel as an adsorbent for F- is the strong increase in the adsorption amount after the modification of the CNF aerogel with aluminium hydroxide. Moreover, the rise of the adsorption isotherm as the loading of Al(OH)3 is getting up, along with the shift of the adsorption capacity to higher value, is indicative of a strong correlation between the effectiveness of F- adsorption and the content in Al(OH)3. The isotherm model of Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich (D– R) were used to fit the experimental data.
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* The Langmuir model The Langmuir model describes the monolayer adsorption on a structurally homogeneous adsorbent, considering the active sites as energetically equivalent and independent Eq. (2): Qmax .k .Ce 1 k .Ce
(2)
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qe
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Where qe : the adsorption capacity at equilibrium
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Ce : the equilibrium concentration of fluoride
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Qmax (mg.g−1) : the maximum adsorption capacity K: the Langmuir constant related to the energy of adsorption. A dimensionless constant called the equilibrium parameter (RL) can be used to express
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the important characteristics of the isotherm as follows (Eq. (3)):
The value of RL is indicative whether the adsorption process is favourable. The high regression coefficient exceeding 0.98, especially for CNF2-Al(OH)3 and CNF3Al(OH)3, indicated that Langmuir model fit reasonably well with the fluoride adsorption. Both 1
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of the maximum adsorption capacity (Qmax) and K increased with the Al(OH)3 loading,
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confirming its key role in the adsorption process. For CNF3-Al(OH)3, the Qmax and K were respectively around 38 mg.g-1 and 2.3 L.mg-1, which is satisfactory for the effective retention of fluoride in aqueous solution. * The Freundlich model Freundlich model is an empirical equation widely applied to depict adsorption on heterogeneous surfaces. Freundlich model can be expressed by Eq. (4): qe K f .Ce1/ n
(4)
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Where
Kf (L.mol−1): Freundlich constants related to the adsorption capacity n: the adsorption efficiency of the sorbent
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The values of n and Kf were determined from the plot of log (qe) versus log(Ce) and
reported in Table 2. Although, the Freundlich equation has lower R2 than Langmuir, which
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would favour Langmuir model to Freundlich, the value of n exceeding 1 and increasing with
* The Tempkin model
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the Al(OH)3 loading is indicative of the favourable adsorption of F- onto CNF-Al aerogel.
Where B=
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It is related to monolayer adsorption on heterogeneous surfaces. (Eq. 5).
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ln qe ln qm 2
(6)
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Where qm is the maximum adsorption capacity and the parameter ß is a constant related to Ɛ (Polanyi potential) given by Eq (7):
1 RT ln 1 Ce
(7)
The plot of lnqe against Ɛ2 gives the respective D–R isotherm parameters.
1 2
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The suitability of an isotherm that best correlates the experimental data can be assessed using the chi square test given by Eq (8):
2
(qe exp qe cal )2 qe cal
(8)
Referring to the chi square values 2, the adsorption of F- onto CNF-Al(OH)3 adsorbent follows the order, D–R > Langmuir> Freundlich > Temkin model. The experimental equilibrium adsorption capacity (qe) was found at different initial
particular isotherm model is close to the experimental value.
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fluoride ion concentrations. The value of 2 would be less when the data obtained from a
The plot of qe against Ce highlights that the experimental equilibrium adsorption capacity data suits the Freundlich isotherm and D-R isotherm as well (Fig.4).
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It can be seen that the percentage of Fluoride adsorption from water at different initial concentrations ranging from 1 to 100 ppm. It is obvious that neat CNF has a negligible
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adsorption yield, whereas aluminium doped aerogels succeeded to remove important amounts of Fluoride ions reaching 100%. CNF2-Al(OH)3 and CNF3-Al(OH)3 showed similar results and
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were better than CNF3-Al(OH)3. This means that exceeding a certain amount of aluminium grafting would rather decrease the adsorption efficiency of the hybrid aerogel (Fig.5). This phenomenon has been investigated in other studies and confirmed the saturation of the active
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adsorption sites [28].
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Q (mg/g)
45 40
Neat CNF
35
CNF3-Al(OH)3
30
CNF2-Al(OH)3
25
CNF1-Al(OH)3
20 15 10 5 0 0
20
40
60
80
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Ce (mg/L)
100
Figure 4: Adsorption Isotherms of F- on the different sample of CNF-Al(OH)3 and on
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100 90 80 70 60 50 40 30 20 10 0
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Fluoride adsorption %
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neat CNF
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1
10
20
30
60
100
C0 (ppm)
Figure 5: Fluoride adsorption efficiency according to the initial concentration of fluoride and CNF sorbent used. 1 4
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Table 2: Isotherm parameters relative to Langmuir, Freundlich, Tempkin & D-R models Isotherm
Parameters Qmax (mg.g-1) K (L.mg-1)
Langmuir CNF1-Al(OH)3 CNF2-Al(OH)3 CNF3-Al(OH)3 Neat CNF
37,453 33,113 25,707 0,623
R2
RL
2,302 1,325 0,041 0,142
0,303 0,431 0,961 0,875
X2
0,999 0,9984 0,9918 0,989
0,0114 0,0113 1,856 0,00085
KF (mgl-l/n g-1Ll/n)
N
R2
X2
CNF1-Al(OH)3 CNF2-Al(OH)3 CNF3-Al(OH)3 Neat CNF
19,311 15,158 1,654 0,0212
3,935 4,575 1,652 1,204
0,9293 0,9433 0,9887 0,9421
0,029 0,0143 0,096 0,136
Tempkin
KT (L.mg-1)
B
R2
X2
CNF1-Al(OH)3 CNF2-Al(OH)3 CNF3-Al(OH)3 Neat CNF
222,77 16,575 2,324 0,992
4,1646 5,3758 5,4433 0,1403
0,9477 0,9419 0,9797 0,8209
0,0118 0,0852 1,268 0,0029
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Freundlich
qm(mg.g-1)
Β (mol2.kJ-2)
E (kJ.mol-1)
R2
X2
CNF1-Al(OH)3 CNF2-Al(OH)3 CNF3-Al(OH)3 Neat CNF
37,169 31,056 17,65 0,641
0,1243 0,3428 22,108 22,082
2,0056 1,2077 0,151 0,1504
0,9998 0,9984 0,9768 0,9955
0,00366 0,0672 0,0749 0,00262
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DubininRadushkevich
The thermodynamic parameters, free energy of adsorption ΔG˚, entropy ΔS˚, and enthalpy
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ΔH˚ for the adsorption process by the Al-CNF aerogel were determined using Van’t Hoff
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equation (Eq.09): LnKc Ln
qe S H Ce R RT
(9)
Where Kc is the equilibrium distribution coefficients (Kc =qe/Ce), qe and Ce the amounts adsorbed at equilibrium and the equilibrium concentration, respectively, R the gas constant (8.314 J mol-1 K-1) and T the temperature (K).
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The enthalpy change was obtained by calculating the slope of a plot of ln Kc versus 1/T, while choosing three different temperatures (298, 303, 308° K). Additional details for the calculation were given in supplementary material. The thermodynamic results are listed in Table 3. The Arrhenius equation to calculate adsorption activation energy is given in the following equation: ln K2 = lnA -Ea / RT , where
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K2 is the pseudo-second-order kinetic rate [27].
Table 3: Thermodynamic parameters ΔH°, ΔS° and ΔG°, and activation energy Ea ΔH° (KJ.mol-1)
ΔS° (KJ.mol-1)
Ea (KJ.mol-1)
25.02
120.12
27.94
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ΔG° (KJ.mol-1) -10.32 -10.92 -11.52
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Temperature (K) 298 303 308
The adsorption free energy (ΔG˚) was negative under all conditions, which reflects the
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spontaneity of the process. A positive enthalpy of adsorption was found, implying that the adsorption process was endothermic. The positive value of ΔH˚ might be explained by the adsorption process involving ligand exchange process, which requires energy. The ΔS˚ was
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also positive indicating an increased randomness at the solid/solution interface during the adsorption process, even though F- ions were immobilized on the surface of the adsorbent.
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The positive value might be explained by the increased mobility gained by released OH- ions following the F- adsorption.
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Adsorption of fluoride by the adsorbents was tested at different pH. Results reported in
Fig. 6 show that at pH between 5-10, the adsorption capacity of CNF-Al(OH)3 remained nearly the same and strongly decreased at pH lower than 4 or over 12. This evolution might be explained by the chemistry of the aluminium hydroxide and the mechanism of adsorption of F- on CNF-Al(OH)3. 1 6
In fact, the key element of CNF-Al(OH)3 governing the adsorption is Al(OH)3, while the CNF aerogel provided the porous frame with high specific surface on which Al(OH)3 will be
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formed. Al(OH)3 is known to interact with F- by ion exchange with hydroxide group and through electrostatic interaction [29]. At pH lower than 3, the decrease in the F- adsorption might be explained by the protonation of F- into HF (pKa=3.5) with low possibility of interaction through electrostatic attraction with Al3+. At pH between 4 and 10, the adsorption is driven by ion exchange mechanism. Over pH 12, the decrease in the adsorption might be the consequence of the
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conversion of Al(OH)3 into Al(OH)4- that would repel F- ions.
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4 3
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Q(mg/g)
5
CNF1-Al(OH)3 2 CNF23-Al(OH)3
0 4
6
8
10
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2
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CNF2-Al(OH)3 1
12
pH
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Figure 6: pH sensitivity of CNF-Al(OH)3 aerogel towards fluoride adsorption
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To better understand the adsorption mechanism, a zeta-potential analysis of CNF and CNF-Al(OH)3 aerogels in the absence and in the presence of F- was investigated. The changes
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in the ζ- potential) with pH for the neat CNF and CNF1-Al(OH)3 (Figure 7) reflected the changes in surface charges, according to the pH. The neat CNF had an isoelectronic point (IEP) around 3.5 and became negative above pH 4. These charges arose from the carboxylic groups on the CNF surface stemming from the oxidation treatment of the cellulose fibers to facilitate their disintegration into CNFs. CNF1-Al(OH)3 exhibited different properties and a IEP being around 8 (IEP=8). At pH lower than 7 the surface is positively charged and over pH 1
12, CNF1-Al(OH)3 is negatively charged. This behaviour is explained by the chemistry of 7 Al(OH)3 hydroxide with multiple possibility of coordination with hydroxide according to the
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pH. At pH lower than 6, the hydroxide is mainly in the form of Al3+, Al(OH)2+ and at pH between 6 to 10 the main dominant species is Al(OH)3. Over pH 10, Al(OH)3 is in the form
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Al(OH) 4- which accounts for the negative charges of CNF-Al(OH)3 over this pH.
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Figure 7: zeta potential of neat CNF aerogel and CNF-Al aerogel
The change in the adsorbed amount of F- with time was studied to assess the adsorption
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kinetics. From Fig.8, it can be seen that the adsorption process was quite fast with about 80% of the adsorption capacity reached within 5 min, almost 90% within 20 min. The equilibrium adsorption was reached after 30 min, which is quite rapid adsorption process.
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In order to describe the kinetic behaviour, the experiment data were fitted with the pseudo-first- order model (Eq.10) and pseudo-second-order model (Eq.11), their equations
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are expressed as:
qt qe 1 exp(k1t )
qt
q e2 k2t 1 k2 qet
(10)
(11) 1
Where qt (mg.g-1) and qe (mg.g-1) are the amount of fluoride absorbed on the adsorbent8at time t and at equilibrium; k1 (min-1) and k2 (g.mg-1 min-1) are the rate constants of the pseudo-
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first- order and pseudo-second-order model. The parameters and the corresponding information are displayed in Table 4. According to the values of R2 and qe, the pseudosecond-order model provides the best correlation of the data compared to the first order kinetic model, which indicates that the rate-limiting may be chemical exchange process.
t shows that the adsorption kinetic can be fitted with two linear
The plots of qt vs
segments. The presence of two linear portions is indicative of a multi-step adsorption process with intraparticle diffusion (Fig.8) playing a significant role in the adsorption of F-. The first linear portion is associated with the transport of fluoride from boundary film to the exterior surface of adsorbent (film diffusion). The second portion is associated to the transfer of
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b
a
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5
5
4
4
Q (mg/g)
3
3
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Q (mg/g)
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fluoride from the surface to the pores of adsorbent (intra-particle diffusion).
2
2
C1-Al-CNF
1
C2-Al-CNF
1
C1-Al-CNF C2-Al-CNF
C3-Al-CNF
C3-Al-CNF C4-Al-CNF
0
5
10
15
20
25
30
35
t (min)
40
45
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0
0
50
55
60
0
65
2
4
t 1/2 (min)
6
8
10
25
10
20
-2
-6 -8
C1-Al-CNF
C2-Al-CNF
50
60
c 70
d
20
15
10 C1-Al-CNF C2-Al-CNF
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C3-Al-CNF
C3-Al-CNF
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-10
40
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ln(qe-qt)
-4
30
t/qt (min/mg/g)
0
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0
C4-Al-CNF
0
C4-Al-CNF
-12
0
t (min)
10
20
30
40
50
60
70
t (min)
Figure 8: Kinetic study of 10 mg/L Fluoride removal (a) modelled with pore diffusion model (b), pseudo first order model (c), and pseudo second order model (d).
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Table 4: kinetic parameters relative to pseudo first order, pseudo second order and pore diffusion models C0(mg.L-1) qeexp(mg.g-1) qecal ( mg.g-1)
Pseudo-first-order K1 (min-1)
R2
X2
Pseudo-second-order qecal (mg.g-1)
K1 R2 (g.mg-1min-1)
X2
Pore diffusion model Kd R2 (mg.g-1.min-0,5)
X2
CNF1-Al(OH)3
10
4,998
3,278
0,193
0,988
0,902
4,97
0,426
0,999
0,001
0,869
0,914
0,447
CNF1-Al(OH)3
10
4,835
4,261
0,147
0,992
0,077
4,17
0,117
0,998
0,106
0,663
0,94
0,003
CNF1-Al(OH)3
10
3,48
2,382
0,1
0,974
0,506
3,58
0,132
0,997
0,003
0,486
0,958
0,021
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After adsorption experiments, the CNF-Al(OH)3 aerogel pellets were removed, washed with deionized water, and placed in 0.1 M NaOH solution for 3 h and then washed with 0.1 M HCl. Finally, the pellets were washed again with deionized water for several times and prepared for the next cycle of defluorination.
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In addition to a high adsorption capacity and rapid kinetic uptake, it is also important that the loaded adsorbates could be easily desorbed and reused for multiple cycles. Desorption of
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the adsorbed fluoride ions from the CNF-Al(OH)3 aerogels were successfully made with an alkaline treatment for about 3h in NaOH (0.1 M) solution. Results showed (Fig. 2S) that
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desorption efficiencies approached 95% and that the adsorbent can be used for more than 5 cycles without appreciable depression of its adsorption capacity. Once reaching the 6th regeneration cycle, the adsorbent starts to weaken and a certain mass loss is witnessed. The
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possibility to desorb fluoride by NaOH solution and regenerate the CNF-Al(OH)3 sorbent might be explained by the predominance of Al(OH)4- form of Al(OH)3 at pH over 12. Al-F and Al-OF will release F- with the formation of Al(OH)4-.
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The results obtained from batch adsorption experiments showed the effective use of CNFAl(OH)3 aerogel to remove metal ions with an effectiveness exceeding 99%. To explore the
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adsorption capacity during continuous operation, a laboratory column filled with CNF1Al(OH)3 was designed. From the economic point of view, a fixed bed column is one of the most effective configurations for cyclic adsorption–desorption, allowing a more efficient use of the adsorbent. A test was conducted on 10 mg/L fluoride solution at pH 7 and a flow rate 5 ml/min using a glass column with 15 cm height (Fig.9). The ratio (Cs/C0) of the effluent concentration Cs to the input concentration C0 was plotted against the effluent volume to
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obtain the breakthrough curve at a constant flow rate. The breakthrough volume, defined as
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the point when the concentration of the effluent reaches 5% of the input concentration, was found to be around 180 ml, meaning that for this treated volume, more than 95% of fluoride present is adsorbed. Over this volume, the adsorption efficiency decreased progressively until the complete saturation of the adsorbent. The total adsorption amount of fluoride under dynamic condition at saturation was found to be around 19 mg/g, which is about 75% of the adsorption capacity found under batch condition. The difference in the adsorption capacity might be explained by kinetic consideration, considering that under dynamic flow the contact time of the effluent is not enough to reach the equilibrium adsorption condition. Interestingly, the column could be regenerated following the same procedure and reused for multiple
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adsorption cycle (results not shown). Comparing the results of CNF-Al(OH)3 adsorption with previous studies, made us
appreciate the high adsorption capacity brought by the inclusion of nanocellulose within the aerogel structure (Table 5).
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Two questions might be raised; (1) Are CNF-Al(OH)3 based adsorbents as efficient as
current adsorbents? , and (2) what will be the utility of developing new adsorbents for fluoride
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removal. In fact when we compare the CNF-Al(OH)3 to some of the fluoride adsorbents reported in the literature (Table 5), it can be seen that CNF-Al(OH)3 porous adsorbent has a
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good ability for fluoride removal with an adsorption capacity which is competitive with current adsorbents. Other merits of CNF-Al(OH)3 adsorbents are as follows; (i) the high porosity facilitated the use of sorbent by continuous filtration through columns, avoiding the
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multi-steps recovering of the sorbent, (ii) the CNF-Al(OH)3 sorbents are not complicated to produce, and (iii) the CNF-Al(OH)3 could be regenerated easily by simple washing with NaOH solution. Work is ongoing to study how the presence of other anions such as Cl-, NO3-,
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sulphate is likely to affect the adsorption selectivity and efficiency for fluoride.
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Figure 9: Column study of Fluoride adsorption by CNF-Al(OH)3 aerogel
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Table5: Comparative data about adsorption capacity of current fluoride adsorbent Adsorption capacity (mg.g-1)
Adsorbent
16.9
[30]
23.7
[31]
La(III)-andY(III)-impregnated Al2O3
8
[32]
Granular ferric hydroxide
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Activated Al2O3
Reference
7
[33]
91
[34]
Al2O3/Carbon nanotubes
28.7
[35]
Basic aluminium sulfate@graphene
33.4
[36]
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Al–Ce oxide
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Alumina hydroxide
4. Conclusion
To conclude, this work deals with the preparation of a hybrid nanocomposite based on nanfibrillated cellulose and investigated its application in water defluorination. The nanofibrillated cellulose based aerogel was impregnated with aluminium hydroxide which, bounded to the hydroxyl ions of the TEMPO oxidized nanofibers, showed an enhanced
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removal capacity of fluoride ions from water. Native unmodified surfaces made of
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nanocellulose a poor sorbent of fluoride from water, but could provide stable supports for achieving high dispersion of Aluminium Hydroxide, which are powerful fluoride sorbents. This proves that the incorporation of some metal ions like Aluminium (III) into CNF surfaces can significantly increase fluoride adsorption capacity. In fact, when the CNF material is impregnated with Aluminium (II) and its complexes, the adsorption capacity has improved by a factor of 20. Since the control over CNF aerogel surface areas and the size distribution of the loaded metal oxide are key factors to understand the improvement of fluoride adsorption capacity increase, the aerogel was characterized using TGA, XPS and SEM analysis. The mechanism
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of fluoride adsorption was confirmed using zeta potential measurement. A column study of a continuous removal processing was conducted showing the reusability of the CNF-Al(OH)3 aerogel.
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Norhene Mhafoudhi has carried out the experimental work and contributed to the writing of the manuscript Sami boufi has supervised the work and write the manuscript
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CRediT author statement
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Chaabane, Low-cost fluoride adsorbents prepared from a renewable biowaste: Syntheses, characterization and modeling studies, Arabian Journal of Chemistry, (2015). [13] A. Vázquez-Guerrero, R. Alfaro-Cuevas-Villanueva, J.G. Rutiaga-Quiñones, R. Cortés-Martínez, Fluoride removal by aluminum-modified pine sawdust: Effect of competitive ions, Ecological Engineering, 94 (2016) 365-379. [14] H. Cai, L. Xu, G. Chen, C. Peng, F. Ke, Z. Liu, D. Li, Z. Zhang, X. Wan, Removal of fluoride from drinking water using modified ultrafine tea powder processed using a ball-mill, Applied Surface Science, 375 (2016) 74-84. [15] Y. Wang, S. Yadav, T. Heinlein, V. Konjik, H. Breitzke, G. Buntkowsky, J.J. Schneider, K. Zhang, Ultra-light nanocomposite aerogels of bacterial cellulose and reduced graphene oxide for specific absorption and separation of organic liquids, RSC Advances, 4 (2014) 21553–21558. [16] G. Jayapriya, R. Ramya, X.R. Rathinam, P.N. Sudha, Equilibrium and kinetic studies of fluoride adsorption by chitin/cellulose composite, Arch Appl Sci Res, 3 (2011) 415-423. [17] H. Paudyal, B. Pangeni, K. Inoue, H. Kawakita, K. Ohto, H. Harada, S. Alam, Adsorptive removal of fluoride from aqueous solution using orange waste loaded with multi-valent metal ions, Journal of Hazardous Materials, 192 (2011) 676-682. [18] S.S. Waghmare, T. Arfin, Fluoride removal from water by various techniques, Int J Innov Sci Eng Technol, 2 (2015) 560-571. [19] H.P.S. Abdul Khalil, Y. Davoudpour, M.N. Islam, A. Mustapha, K. Sudesh, R. Dungani, M. Jawaid, Production and modification of nanofibrillated cellulose using various mechanical processes: A review, Carbohydrate Polymers, 99 (2014) 649-665. [20] J. Vartiainen, T. Pöhler, K. Sirola, L. Pylkkänen, H. Alenius, J. Hokkinen, U. Tapper, P. Lahtinen, A. Kapanen, K. Putkisto, P. Hiekkataipale, P. Eronen, J. Ruokolainen, A. Laukkanen, Health and environmental safety aspects of friction grinding and spray drying of microfibrillated cellulose, Cellulose, 18 (2011) 775-786. [21] M. Vikman, J. Vartiainen, I. Tsitko, P. Korhonen, Biodegradability and Compostability of Nanofibrillar Cellulose-Based Products, Journal of Polymers and the Environment, 23 (2015) 206-215. [22] Y. Tian, M. Wu, R. Liu, D. Wang, X. Lin, W. Liu, L. Ma, Y. Li, Y. Huang, Modified native cellulose fibers—A novel efficient adsorbent for both fluoride and arsenic, Journal of Hazardous Materials, 185 (2011) 93100. [23] H. Sehaqui, U.P. de Larraya, P. Liu, N. Pfenninger, A.P. Mathew, T. Zimmermann, P. Tingaut, Enhancing adsorption of heavy metal ions onto biobased nanofibers from waste pulp residues for application in wastewater treatment, Cellulose, 21 (2014) 2831-2844. [24] Y. Zhou, S. Fu, L. Zhang, H. Zhan, M.V. Levit, Use of carboxylated cellulose nanofibrils-filled magnetic chitosan hydrogel beads as adsorbents for Pb(II), Carbohydrate Polymers, 101 (2014) 75-82. [25] N. Chen, C. Feng, M. Li, Fluoride removal on Fe–Al-impregnated granular ceramic adsorbent from aqueous solution, Clean Technologies and Environmental Policy, 16 (2014) 609-617. [26] A. Isogai, T. Saito, H. Fukuzumi, TEMPO-oxidized cellulose nanofibers, Nanoscale, 3 (2011) 71-85. [27] N. Mahfoudhi, S. Boufi, Poly (acrylic acid-co-acrylamide)/cellulose nanofibrils nanocomposite hydrogels: effects of CNFs content on the hydrogel properties, Cellulose, 23 (2016) 3691-3701. [28] M. Barathi, A.S. Krishna Kumar, C.U. Kumar, N. Rajesh, Graphene oxide–aluminium oxyhydroxide interaction and its application for the effective adsorption of fluoride, RSC Advances, 4 (2014) 53711-53721. [29] S. Tokunaga, M.J. Haron, S.A. Wasay, K.F. Wong, K. Laosangthum, A. Uchiumi, Removal of fluoride ions from aqueous solutions by multivalent metal compounds, International Journal of Environmental Studies, 48 (1995) 17-28. [30] Y. Ku, H.-M. Chiou, The Adsorption of Fluoride Ion from Aqueous Solution by Activated Alumina, Water, Air, and Soil Pollution, 133 (2002) 349-361. [31] B. Shimelis, F. Zewge, B.S. Chandravanshi, REMOVAL OF EXCESS FLUORIDE FROM WATER 2 BY ALUMINUM HYDROXIDE, Bulletin of the Chemical Society of Ethiopia, 20 (2006) 17–34. 4 [32] S.A. Wasay, S. Tokunaga, S.-W. Park, Removal of Hazardous Anions from Aqueous Solutions by La(lll)- and Y(lll)-lmpregnated Alumina, Separation Science and Technology, 31 (1996) 1501-1514.
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PII:
S2213-3437(20)30127-5
DOI:
https://doi.org/10.1016/j.jece.2020.103779
Reference:
JECE 103779
To appear in:
Journal of Environmental Chemical Engineering
Received Date:
2 December 2019
Revised Date:
17 January 2020
Accepted Date:
11 February 2020
Please cite this article as: Mahfoudhi N, Boufi S, Porous material from cellulose nanofibrils coated with aluminum hydroxyde as an effective adsorbent for fluoride, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2020.103779
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Porous material from cellulose nanofibrils coated with aluminum hydroxyde as an effective adsorbent for fluoride
Norhene Mahfoudhi* and Sami Boufi* University of Sfax-Faculty of Science - LMSE- BP 1171 3000 - Sfax -Tunisia Corresponding author at: Faculty of Science - LMSE-University of Sfax BP 1171 3000 -
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Sfax -Tunisia * Tel: 216 74274400 * Fax: 216 74274437
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*E-mail:[email protected] /[email protected]
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Highlights
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Porous cellulose aerogel modified with aluminium hydroxide was prepared and characterized The aerogel was used as an adsorbent for the removal of fluoride from aqueous solution The hybrid aerogel showed a great promise to be applied as a reusable adsorbent for fluoride
Abstract
A novel aluminium hydroxide modified porous cellulose aerogel was prepared and used
as an effective adsorbent for the removal of fluoride from aqueous solution. The adsorbent was prepared by freeze-drying of cellulose nanofibrils gel followed by impregnation with
1
AlCl3 solution and washing with water. The ensuing sorbent was characterized by SEM
1
observation, XPS, TGA and BET measurement for specific-surface evaluation. The CNFAl(OH)3 modified aerogel was shown to be an effective adsorbent for fluoride in water, with an adsorption capacity ranging from 20to 35 mg.g-1, depending on the Al(OH)3 loading. The spent CNF-Al adsorbent was regenerable by washing with NaOH solution and could be reused up to five adsorption-desorption cycles without significant loss of its adsorption capacity. The CNF –Al aerogel showed a great promise to be applied as a reusable adsorbent from a renewable resource for defluorination.
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Key words: cellulose nanofibrils, fluoride, aerogel, adsorption.
1. Introduction
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The problem of water defluorination is of a major importance, nowadays due to chronic disease such as fluorosis, neurological damage and softening of bones imparted by
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contamination of drinking water by fluoride with content exceeding 1.5 mg.L-1 [1]. In fact, many actual methods are being used to overcome this problem [2]. The present defluorination
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methods include precipitation, membrane separation, ion exchange and adsorption [3]. Among them, adsorption is a new approach with promising results. Researchers consider it as the most promising method due to its effectiveness, convenience, easy operation, simplicity of
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design, economic and environmental aspects [4].
In this context, the adsorption of fluoride attracted the attention of many studies, dealing
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with natural biobased adsorbents instead of conventional synthetic adsorbents. For instance, synthetic apatites [5], bentonite clay [6] and modified chitosan beads [7] proved to be good
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adsorbents for fluoride. Metal-binding biopolymers were tested to remove water contaminants [8].Yet, such biomaterials are still faced with problems such as relatively low adsorption ability or large decline in reused performance. Adsorption [9] of fluoride on natural polymers obtained from agricultural waste products has also emerged as an excellent alternative to the traditional techniques of reverse osmosis and membrane filtration, decreasing the energy consumption [10]. Because of their low cost and the presence of a variety of functional groups for modification, the uses of agro-waste material has attracted increasing interest in this field
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[11]. Fluoride removal has been investigated by various biobased materials [12], including pine sawdust [13], tea [14], cellulose [15], chitin [16], orange waste [17], chitosan [18], etc.. During the last two decades, nanosized cellulose fibrils have emerged as one of the most promising nanomaterials in water remediation. Their nanoscale dimensions, biodegradable character, strong reinforcing effect, high aspect ratio, lightweight, and sustainability are examples of positive properties driving toward the wide interest in nanocelluloses [19]. Furthermore, nanocellulose is classified as a non-toxic material [20] completely biodegradable and without adverse effects on the environment [21].
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Taking advantage of the mentioned biopolymer, native cellulose fibers were surface modified by poly (N,N-dimethyl aminoethyl methacrylate) (PDMAEMA) to generate an
anion adsorbent. This adsorbent had high efficiency in removing F− from aqueous solutions, even at low initial concentrations. Adsorption kinetics showed that the adsorption equilibrium was reached within 1 min attaining a maximum fluoride adsorption [22] of 8.5 mg.g-1 . In
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a maximum fluoride adsorption [23] of 0.6 mmol.g-1.
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another study, cationic CNF with a maximum cationic charge content of 1.2 mmol.g-1 showed
Furthermore, metal deposition of Cerium onto cellulose such as cerium loaded cellulose
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nanocomposite beads or iron oxide deposition onto cellulose were tested for fluoride removal. Fluoride adsorption on cerium loaded nanocellulose proved that almost 90% of adsorbed fluoride can be eluted with 0.01N NaOH and regenerated for at least three times [24].
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Aluminium coated Chitosan-Fe(III) hydrogel was also tested for fluoride removal and showed a relatively high adsorption capacity of 31.16 mg.g-1. After regeneration, the
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adsorbent performed higher defluoridation efficiency than before. The mechanism of adsorption was described by the ligand and ions exchange that happened on the active sites
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[25].
However, the use of nanoscale cellulose as a support material for fluoride adsorption has
not yet been widely investigated up to our knowledge. For this reason, in this work, porous nanocellulose based aerogel coated with aluminium hydroxide was prepared and tested for fluoride removal from aqueous solutions. The study tackles the kinetic aspect of the defluorination process in batch and continuous modes. The regeneration of CNF-Al aerogel
3
was successfully obtained with an alkaline treatment (0.1 M of NaOH), showing the possible reuse of this adsorbent for multiple cycles.
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2. Materials Commercial bleached Eucalyptus pulp (Eucalyptus globulus) from Ence-CelulosasAustria in the form of dry sheets was used as a starting material for the preparation of CNF. Aluminium Chloride (AlCl3), Hydrogen Chloride (HCl) and Sodium Hydroxide (NaOH) were purchased from Aldrich. 2.1. Adsorbents Fibres were first pre-treated by TEMPO-mediated oxidation to facilitate the disintegration process and produce CNF with high yield. The oxidation was carried out at pH 10 following
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the method reported in literature [26]. A detailed description of the process was reported in
our previous work [27]. The oxidation degree of fibres was evaluated as 1100 μmol/g. Then, the oxidized fibres
at a consistency of 1.5% were homogenized in a high pressure
homogenizer (NS1001L PANDA 2 K-GEA, Italy) applying six shear cycles (3 x 300 bar; 3 x
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600 bar). A thick transparent CNF gel was obtained after six passes. AFM images of the CNFs gel are given in Fig. 1S.
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The CNF gel was placed in glass cylindrical tubes (h/D: 15/ 15mm) and freezed at -20°C
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for about 24 h. Thefrozen sample was then freeze dried until complete water sublimation. To immobilize the Al(OH)3 layer, CNF aerogel cylinder, with a dried mass of 1g each, were immersed in 25 mL of AlCl3 solutions with various concentrations (C1=0.15 M,
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C2=0.075 M and C3=0.037 M) for 3 h. After that, the modified CNF aerogel (Al-CNF) was transferred in distilled water and pH was stabilized at 7 with NaOH (0.1 M). An additional washing of the material was introduced to eliminate any trace of chloride or unreacted
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aluminium.
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Three samples labelled CNF1-Al(OH)3, CNF2- Al(OH)3 and CNF3- Al(OH)3 were produced, using respective concentrations of AlCl3 equal to C1=0.15 M, C2=0.075 M and C3=0.037 M.
2.2. Batch adsorption experiments 50 mL of F- (concentration ranging from 1 to 100 mg. L-1) solution was prepared in 100
4
mL beaker flasks at room temperature. The CNF-Al(OH)3 aerogel was immersed in this solution for 2h for kinetic studies. Then, a spot of sample solution was taken out at desired
4
time intervals to analyse the residual F-. The initial pH values ranged from 3 to 12, and were adjusted by 0.1 M HCl or 0.1 M NaOH solutions. The fluoride concentration was measured by a fluoride specific ion electrode ISE with the help of total ionic strength adjustment buffer (TISAB II) solution (14.5 g NaCl, 2.0 g sodium citrate and 14.25 ml glacial acetic acid and an amount of 40% NaOH in a volume of 250 ml, the pH~5.5). 10mL of a Total Ionic Strength Adjustment Buffer (TISAB II) was added to 10 mL of water sample (withdrawn from each experiment) and standards. All measurements were carried out at 25°C. The adsorption capacity at time t, qt (mg.g-1) was calculated by Eq.
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(1):
qt (C0 Ct )Vm (1) Where
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C0 and Ct (mg.L-1) stand for the initial concentration and the concentration at time t,
respectively. V (L) is the volume of the solution, and m (g) is the mass of the sorbent used.
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2.3. Column study
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A continuous batch adsorption experiment was carried out using CNF-Al(OH)3 aerogel packed column. The glass column was of 1 cm diameter and 15 cm length. Carrying 0.2 g of CNF-Al(OH)3 aerogel pellets, the effluent solutions were percolated from the bottom to the
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top using a peristaltic pump at a flow rate of 5 ml.min-1. Effluent samples were collected at regular intervals to determine their concentrations.
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2.4. Regeneration of the adsorbent After adsorption experiments, the CNF–Al aerogel pellets were removed, washed with
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deionized water, and placed in 0.1 M NaOH solution for 3 h and then washed with 0.1 M HCl. Finally, the pellets were washed again with deionized water for several times and prepared for the next cycle. 3. Methods 3.1. SEM Observation
5
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The observation of the CNF-Al(OH)3 aerogel morphology was performed by scanning electron microscopy (SEM) using a (JEOL JSM-6390LV) operating at 5–10 kV. 3.2. XPS Samples were characterized by X-Ray Photoelectron spectroscopy (XPS), using a XSAM 800 spectrometer from Kratos, emitting non monochromatic Al Kα X- radiation. The analysis was performed in FAT mode, with pass energy of 20 eV and setting the emission source with 12 KV and 10 mA. The estimated pressure was of ̴ 10-7 Pa and the take-off angle (TOA) was equal to 45°. A Shirley background was subtracted and curve fitting for component peaks was
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carried out using Gaussian-Lorentzian profiles. No charge compensation was used. Binding energies were corrected using as reference the binding energy of aliphatic carbon at 285 eV. Surface composition was determined using the manufacturer’s sensitivity factors.
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3.3. TGA analysis
The thermogravimetric analysis (TGA) was performed with a thermogravimetric analyser (TGA 400 Perkin Elmer). Samples were about 10 mg and the heating cycle was from 20 to
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3.4. Brunauer– Emmett–Teller (BET)
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800°C at a heating rate of 10 °C/min under air flow.
The surface area and porosity of the fibers were estimated by adsorption/desorption
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isotherms of nitrogen gas (N2) at 77 K using the volumetric gas adsorption analyzer Quantachrome NovaWin (NovaWin, version 10.01, Boynton Beach, FL, USA). Based on the curves generated by the pressure variation of the adsorption and desorption isotherms, the
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specific surface area was estimated by the Brunauer-Emmett-Teller (BET) method.
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4. Results and discussions The porous sponge-like aerogel was obtained by freeze-drying of a CNF suspension at 1%
solid content. The CNFs suspension in the form of a transparent gel was obtained by a high pressure homogenization and was composed of more than 95% of nanosized fibrils. After complete water removal through freeze-drying at -20°C, a lightweight sponge-like aerogel was produced. Then the aerogel was immersed in a solution of AlCl3 for 1h, followed by abundant rinsing with water until neutral pH. This treatment did not alter neither the shape of
6
the aerogel nor its integrity. It also increased the aerogel resistance to water disintegration,
6
even after multiple use and prolonged immersion in water. This phenomenon could be explained by the strong capacity of Al3+ ions to chelate with the carboxyl groups of CNF surface, resulting in the irreversible chemical crosslinking of the CNF network. Then, after the removal of the aerogel from AlCl3 solution and its immersion in w ater bath at pH 7, the adsorbed Al3+ ions will be converted into aluminium hydroxide (Al(OH)2+, Al(OH)3 ) that aggregate to form aluminium hydroxide nanoparticles firmly bound to the cell wall of the cellulose aerogel. Given the insolubility of aluminium hydroxide within pH domains between 4 to 11 (pks=33), the generated Al(OH)3 will remain encrusted within the cellulose aerogel. SEM observations of the CNF-(Al(OH)3 aerogel sample revealed a sponge-like material
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formed by heterogeneously shaped pores with sizes from 50 to 300 μm forming open
channels with a thin cell of 100 to 200 nm thickness. The surface of the pore’s wall looks
rough, revealing spherical nodules with 50 to 300 nm in diameter, lining the surface which presumably corresponds to the Al(OH)3 particles. No meaningful evolution of the
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morphology of the aerogel could be noted after the treatment with AlCl3 solution as inferred from comparison of the aspect of the aerogel prior (Fig. 1A) and after immersion in AlCl3
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solution (Fig 1B).
7
7
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Figure1: FE-SEM images of the cryofractured (A, B) neat CNF and (C,D)CNF3-Al(OH)3 aerogel. (B and D) correspond to cross-section of the cell wall of the channel of theaerogel.
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(The insert shows the photo of the neat (A) and Al(OH)3 functionalized aerogel (C))
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XPS characterization of CNF-Al(OH)3 confirmed the presence of Al through its typical photopic around 74.45 eV, with atomic surface concentration of about 2%. The ratio Al/Cl was
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found to be around 10, meaning that most of the grafted AlCl3 is in the hydrolyzed form.
8
8
A
Name O1s C1s Al2p Cl2p Na1s
Peak BE Atomic % 532.4 35.21 285.96 62.29 74.45 2.05 199.81 0.21 1071.28 0.24
D
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C
B
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Figure 2: (A) XPS survey of neat CNF2-Al(OH)3 sample (H) and the high resolution of (B) C1s, (C) O1 s and (D) Al2p regions.
The amount of loaded Al(OH)3 on the cellulose aerogel was estimated from TGA after the
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total thermal degradation of the cellulose. The final residual mass, after heating up to 700 °C was around 1%, 6% and 10% for neat CNF and CNF2-Al(OH)3 and CNF3-Al(OH)3, respectively.
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Assuming that Al(OH)3 has undergone a dehydratation at a temperature around 400°C with a release of 1.5 mole of water per a mole of Al(OH)3, then the amount of loaded Al(OH)3
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could be estimated to about 10% and 16% for CNF2-Al(OH)3 and CNF3-Al(OH)3,, respectively. Then, it is possible to control the amount of loaded Al(OH)3 on the CNFaerogel through the initial concentration of theAlCl3 precursor (Figure 3). The neat CNF aerogel has a 12.5 m2.g-1 Brunauer– Emmett–Teller (BET) specific surface area and a density of 0.03 g.cm-3. After the treatment with AlCl3, the specific surface area strongly decreased to about 4.7 and 3.2 m2.g-1, and the density grows to 0.065 and 0. 087
9
g.cm-3 for CNF2-Al(OH)3 and CNF3-Al(OH)3,, respectively. This evolution might be
9
explained by the accumulation of Al(OH)3 within the fine pores of the aerogel forming a layer coating the cell wall, clogging the access to the tiny pores of the aerogel (Table 1).
100
70 60 50 40 30
Nneat CNF CNF-Al1 CNF-Al2 CNF-Al3
20 10 0 50
150
250
350
450
550
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Temperature (°C)
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Weight (%)
90 80
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Figure3: TGA of neat CNF and Al-CNF aerogel with two Al contents
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Table 1: Comparison between neat CNF /Al-CNF aerogel properties
CNF1-Al(OH)3
CNF2-Al(OH)3
CNF3-Al(OH)3
0.062
0.065
0.087
Specific surface (m2.g-1) 12.5
4.1
4.7
3.2
Residue at 700°C (%)
6
9
10.5
0.03
1
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Density (g.cm-3)
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Neat CNF
To assess how the presence of Al(OH)3 improved the removal of fluoride ions, the
adsorption performance of the CNF-Al aerogels with different contents of Al(OH)3 was studied by measuring the adsorption amount of fluoride at different initial concentrations Fand constant weight of the sorbent. Results of this study were reported in the form of an adsorption isotherm.
1 0
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The first evidence of the effectiveness of CNF-Al aerogel as an adsorbent for F- is the strong increase in the adsorption amount after the modification of the CNF aerogel with aluminium hydroxide. Moreover, the rise of the adsorption isotherm as the loading of Al(OH)3 is getting up, along with the shift of the adsorption capacity to higher value, is indicative of a strong correlation between the effectiveness of F- adsorption and the content in Al(OH)3. The isotherm model of Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich (D– R) were used to fit the experimental data.
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* The Langmuir model The Langmuir model describes the monolayer adsorption on a structurally homogeneous adsorbent, considering the active sites as energetically equivalent and independent Eq. (2): Qmax .k .Ce 1 k .Ce
(2)
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qe
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Where qe : the adsorption capacity at equilibrium
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Ce : the equilibrium concentration of fluoride
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Qmax (mg.g−1) : the maximum adsorption capacity K: the Langmuir constant related to the energy of adsorption. A dimensionless constant called the equilibrium parameter (RL) can be used to express
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the important characteristics of the isotherm as follows (Eq. (3)):
The value of RL is indicative whether the adsorption process is favourable. The high regression coefficient exceeding 0.98, especially for CNF2-Al(OH)3 and CNF3Al(OH)3, indicated that Langmuir model fit reasonably well with the fluoride adsorption. Both 1
1
of the maximum adsorption capacity (Qmax) and K increased with the Al(OH)3 loading,
11
confirming its key role in the adsorption process. For CNF3-Al(OH)3, the Qmax and K were respectively around 38 mg.g-1 and 2.3 L.mg-1, which is satisfactory for the effective retention of fluoride in aqueous solution. * The Freundlich model Freundlich model is an empirical equation widely applied to depict adsorption on heterogeneous surfaces. Freundlich model can be expressed by Eq. (4): qe K f .Ce1/ n
(4)
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Where
Kf (L.mol−1): Freundlich constants related to the adsorption capacity n: the adsorption efficiency of the sorbent
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The values of n and Kf were determined from the plot of log (qe) versus log(Ce) and
reported in Table 2. Although, the Freundlich equation has lower R2 than Langmuir, which
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would favour Langmuir model to Freundlich, the value of n exceeding 1 and increasing with
* The Tempkin model
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the Al(OH)3 loading is indicative of the favourable adsorption of F- onto CNF-Al aerogel.
Where B=
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It is related to monolayer adsorption on heterogeneous surfaces. (Eq. 5).
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ln qe ln qm 2
(6)
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Where qm is the maximum adsorption capacity and the parameter ß is a constant related to Ɛ (Polanyi potential) given by Eq (7):
1 RT ln 1 Ce
(7)
The plot of lnqe against Ɛ2 gives the respective D–R isotherm parameters.
1 2
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The suitability of an isotherm that best correlates the experimental data can be assessed using the chi square test given by Eq (8):
2
(qe exp qe cal )2 qe cal
(8)
Referring to the chi square values 2, the adsorption of F- onto CNF-Al(OH)3 adsorbent follows the order, D–R > Langmuir> Freundlich > Temkin model. The experimental equilibrium adsorption capacity (qe) was found at different initial
particular isotherm model is close to the experimental value.
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fluoride ion concentrations. The value of 2 would be less when the data obtained from a
The plot of qe against Ce highlights that the experimental equilibrium adsorption capacity data suits the Freundlich isotherm and D-R isotherm as well (Fig.4).
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It can be seen that the percentage of Fluoride adsorption from water at different initial concentrations ranging from 1 to 100 ppm. It is obvious that neat CNF has a negligible
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adsorption yield, whereas aluminium doped aerogels succeeded to remove important amounts of Fluoride ions reaching 100%. CNF2-Al(OH)3 and CNF3-Al(OH)3 showed similar results and
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were better than CNF3-Al(OH)3. This means that exceeding a certain amount of aluminium grafting would rather decrease the adsorption efficiency of the hybrid aerogel (Fig.5). This phenomenon has been investigated in other studies and confirmed the saturation of the active
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adsorption sites [28].
1 3
13
Q (mg/g)
45 40
Neat CNF
35
CNF3-Al(OH)3
30
CNF2-Al(OH)3
25
CNF1-Al(OH)3
20 15 10 5 0 0
20
40
60
80
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Ce (mg/L)
100
Figure 4: Adsorption Isotherms of F- on the different sample of CNF-Al(OH)3 and on
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100 90 80 70 60 50 40 30 20 10 0
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Fluoride adsorption %
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neat CNF
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1
10
20
30
60
100
C0 (ppm)
Figure 5: Fluoride adsorption efficiency according to the initial concentration of fluoride and CNF sorbent used. 1 4
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Table 2: Isotherm parameters relative to Langmuir, Freundlich, Tempkin & D-R models Isotherm
Parameters Qmax (mg.g-1) K (L.mg-1)
Langmuir CNF1-Al(OH)3 CNF2-Al(OH)3 CNF3-Al(OH)3 Neat CNF
37,453 33,113 25,707 0,623
R2
RL
2,302 1,325 0,041 0,142
0,303 0,431 0,961 0,875
X2
0,999 0,9984 0,9918 0,989
0,0114 0,0113 1,856 0,00085
KF (mgl-l/n g-1Ll/n)
N
R2
X2
CNF1-Al(OH)3 CNF2-Al(OH)3 CNF3-Al(OH)3 Neat CNF
19,311 15,158 1,654 0,0212
3,935 4,575 1,652 1,204
0,9293 0,9433 0,9887 0,9421
0,029 0,0143 0,096 0,136
Tempkin
KT (L.mg-1)
B
R2
X2
CNF1-Al(OH)3 CNF2-Al(OH)3 CNF3-Al(OH)3 Neat CNF
222,77 16,575 2,324 0,992
4,1646 5,3758 5,4433 0,1403
0,9477 0,9419 0,9797 0,8209
0,0118 0,0852 1,268 0,0029
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Freundlich
qm(mg.g-1)
Β (mol2.kJ-2)
E (kJ.mol-1)
R2
X2
CNF1-Al(OH)3 CNF2-Al(OH)3 CNF3-Al(OH)3 Neat CNF
37,169 31,056 17,65 0,641
0,1243 0,3428 22,108 22,082
2,0056 1,2077 0,151 0,1504
0,9998 0,9984 0,9768 0,9955
0,00366 0,0672 0,0749 0,00262
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DubininRadushkevich
The thermodynamic parameters, free energy of adsorption ΔG˚, entropy ΔS˚, and enthalpy
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ΔH˚ for the adsorption process by the Al-CNF aerogel were determined using Van’t Hoff
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equation (Eq.09): LnKc Ln
qe S H Ce R RT
(9)
Where Kc is the equilibrium distribution coefficients (Kc =qe/Ce), qe and Ce the amounts adsorbed at equilibrium and the equilibrium concentration, respectively, R the gas constant (8.314 J mol-1 K-1) and T the temperature (K).
1 5
15
The enthalpy change was obtained by calculating the slope of a plot of ln Kc versus 1/T, while choosing three different temperatures (298, 303, 308° K). Additional details for the calculation were given in supplementary material. The thermodynamic results are listed in Table 3. The Arrhenius equation to calculate adsorption activation energy is given in the following equation: ln K2 = lnA -Ea / RT , where
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K2 is the pseudo-second-order kinetic rate [27].
Table 3: Thermodynamic parameters ΔH°, ΔS° and ΔG°, and activation energy Ea ΔH° (KJ.mol-1)
ΔS° (KJ.mol-1)
Ea (KJ.mol-1)
25.02
120.12
27.94
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ΔG° (KJ.mol-1) -10.32 -10.92 -11.52
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Temperature (K) 298 303 308
The adsorption free energy (ΔG˚) was negative under all conditions, which reflects the
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spontaneity of the process. A positive enthalpy of adsorption was found, implying that the adsorption process was endothermic. The positive value of ΔH˚ might be explained by the adsorption process involving ligand exchange process, which requires energy. The ΔS˚ was
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also positive indicating an increased randomness at the solid/solution interface during the adsorption process, even though F- ions were immobilized on the surface of the adsorbent.
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The positive value might be explained by the increased mobility gained by released OH- ions following the F- adsorption.
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Adsorption of fluoride by the adsorbents was tested at different pH. Results reported in
Fig. 6 show that at pH between 5-10, the adsorption capacity of CNF-Al(OH)3 remained nearly the same and strongly decreased at pH lower than 4 or over 12. This evolution might be explained by the chemistry of the aluminium hydroxide and the mechanism of adsorption of F- on CNF-Al(OH)3. 1 6
In fact, the key element of CNF-Al(OH)3 governing the adsorption is Al(OH)3, while the CNF aerogel provided the porous frame with high specific surface on which Al(OH)3 will be
16
formed. Al(OH)3 is known to interact with F- by ion exchange with hydroxide group and through electrostatic interaction [29]. At pH lower than 3, the decrease in the F- adsorption might be explained by the protonation of F- into HF (pKa=3.5) with low possibility of interaction through electrostatic attraction with Al3+. At pH between 4 and 10, the adsorption is driven by ion exchange mechanism. Over pH 12, the decrease in the adsorption might be the consequence of the
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conversion of Al(OH)3 into Al(OH)4- that would repel F- ions.
6
4 3
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Q(mg/g)
5
CNF1-Al(OH)3 2 CNF23-Al(OH)3
0 4
6
8
10
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2
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CNF2-Al(OH)3 1
12
pH
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Figure 6: pH sensitivity of CNF-Al(OH)3 aerogel towards fluoride adsorption
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To better understand the adsorption mechanism, a zeta-potential analysis of CNF and CNF-Al(OH)3 aerogels in the absence and in the presence of F- was investigated. The changes
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in the ζ- potential) with pH for the neat CNF and CNF1-Al(OH)3 (Figure 7) reflected the changes in surface charges, according to the pH. The neat CNF had an isoelectronic point (IEP) around 3.5 and became negative above pH 4. These charges arose from the carboxylic groups on the CNF surface stemming from the oxidation treatment of the cellulose fibers to facilitate their disintegration into CNFs. CNF1-Al(OH)3 exhibited different properties and a IEP being around 8 (IEP=8). At pH lower than 7 the surface is positively charged and over pH 1
12, CNF1-Al(OH)3 is negatively charged. This behaviour is explained by the chemistry of 7 Al(OH)3 hydroxide with multiple possibility of coordination with hydroxide according to the
17
pH. At pH lower than 6, the hydroxide is mainly in the form of Al3+, Al(OH)2+ and at pH between 6 to 10 the main dominant species is Al(OH)3. Over pH 10, Al(OH)3 is in the form
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Al(OH) 4- which accounts for the negative charges of CNF-Al(OH)3 over this pH.
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Figure 7: zeta potential of neat CNF aerogel and CNF-Al aerogel
The change in the adsorbed amount of F- with time was studied to assess the adsorption
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kinetics. From Fig.8, it can be seen that the adsorption process was quite fast with about 80% of the adsorption capacity reached within 5 min, almost 90% within 20 min. The equilibrium adsorption was reached after 30 min, which is quite rapid adsorption process.
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In order to describe the kinetic behaviour, the experiment data were fitted with the pseudo-first- order model (Eq.10) and pseudo-second-order model (Eq.11), their equations
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are expressed as:
qt qe 1 exp(k1t )
qt
q e2 k2t 1 k2 qet
(10)
(11) 1
Where qt (mg.g-1) and qe (mg.g-1) are the amount of fluoride absorbed on the adsorbent8at time t and at equilibrium; k1 (min-1) and k2 (g.mg-1 min-1) are the rate constants of the pseudo-
18
first- order and pseudo-second-order model. The parameters and the corresponding information are displayed in Table 4. According to the values of R2 and qe, the pseudosecond-order model provides the best correlation of the data compared to the first order kinetic model, which indicates that the rate-limiting may be chemical exchange process.
t shows that the adsorption kinetic can be fitted with two linear
The plots of qt vs
segments. The presence of two linear portions is indicative of a multi-step adsorption process with intraparticle diffusion (Fig.8) playing a significant role in the adsorption of F-. The first linear portion is associated with the transport of fluoride from boundary film to the exterior surface of adsorbent (film diffusion). The second portion is associated to the transfer of
6
6
b
a
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5
5
4
4
Q (mg/g)
3
3
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Q (mg/g)
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fluoride from the surface to the pores of adsorbent (intra-particle diffusion).
2
2
C1-Al-CNF
1
C2-Al-CNF
1
C1-Al-CNF C2-Al-CNF
C3-Al-CNF
C3-Al-CNF C4-Al-CNF
0
5
10
15
20
25
30
35
t (min)
40
45
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0
0
50
55
60
0
65
2
4
t 1/2 (min)
6
8
10
25
10
20
-2
-6 -8
C1-Al-CNF
C2-Al-CNF
50
60
c 70
d
20
15
10 C1-Al-CNF C2-Al-CNF
5
C3-Al-CNF
C3-Al-CNF
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-10
40
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ln(qe-qt)
-4
30
t/qt (min/mg/g)
0
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0
C4-Al-CNF
0
C4-Al-CNF
-12
0
t (min)
10
20
30
40
50
60
70
t (min)
Figure 8: Kinetic study of 10 mg/L Fluoride removal (a) modelled with pore diffusion model (b), pseudo first order model (c), and pseudo second order model (d).
1 9
19
Table 4: kinetic parameters relative to pseudo first order, pseudo second order and pore diffusion models C0(mg.L-1) qeexp(mg.g-1) qecal ( mg.g-1)
Pseudo-first-order K1 (min-1)
R2
X2
Pseudo-second-order qecal (mg.g-1)
K1 R2 (g.mg-1min-1)
X2
Pore diffusion model Kd R2 (mg.g-1.min-0,5)
X2
CNF1-Al(OH)3
10
4,998
3,278
0,193
0,988
0,902
4,97
0,426
0,999
0,001
0,869
0,914
0,447
CNF1-Al(OH)3
10
4,835
4,261
0,147
0,992
0,077
4,17
0,117
0,998
0,106
0,663
0,94
0,003
CNF1-Al(OH)3
10
3,48
2,382
0,1
0,974
0,506
3,58
0,132
0,997
0,003
0,486
0,958
0,021
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After adsorption experiments, the CNF-Al(OH)3 aerogel pellets were removed, washed with deionized water, and placed in 0.1 M NaOH solution for 3 h and then washed with 0.1 M HCl. Finally, the pellets were washed again with deionized water for several times and prepared for the next cycle of defluorination.
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In addition to a high adsorption capacity and rapid kinetic uptake, it is also important that the loaded adsorbates could be easily desorbed and reused for multiple cycles. Desorption of
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the adsorbed fluoride ions from the CNF-Al(OH)3 aerogels were successfully made with an alkaline treatment for about 3h in NaOH (0.1 M) solution. Results showed (Fig. 2S) that
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desorption efficiencies approached 95% and that the adsorbent can be used for more than 5 cycles without appreciable depression of its adsorption capacity. Once reaching the 6th regeneration cycle, the adsorbent starts to weaken and a certain mass loss is witnessed. The
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possibility to desorb fluoride by NaOH solution and regenerate the CNF-Al(OH)3 sorbent might be explained by the predominance of Al(OH)4- form of Al(OH)3 at pH over 12. Al-F and Al-OF will release F- with the formation of Al(OH)4-.
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The results obtained from batch adsorption experiments showed the effective use of CNFAl(OH)3 aerogel to remove metal ions with an effectiveness exceeding 99%. To explore the
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adsorption capacity during continuous operation, a laboratory column filled with CNF1Al(OH)3 was designed. From the economic point of view, a fixed bed column is one of the most effective configurations for cyclic adsorption–desorption, allowing a more efficient use of the adsorbent. A test was conducted on 10 mg/L fluoride solution at pH 7 and a flow rate 5 ml/min using a glass column with 15 cm height (Fig.9). The ratio (Cs/C0) of the effluent concentration Cs to the input concentration C0 was plotted against the effluent volume to
2 0
obtain the breakthrough curve at a constant flow rate. The breakthrough volume, defined as
20
the point when the concentration of the effluent reaches 5% of the input concentration, was found to be around 180 ml, meaning that for this treated volume, more than 95% of fluoride present is adsorbed. Over this volume, the adsorption efficiency decreased progressively until the complete saturation of the adsorbent. The total adsorption amount of fluoride under dynamic condition at saturation was found to be around 19 mg/g, which is about 75% of the adsorption capacity found under batch condition. The difference in the adsorption capacity might be explained by kinetic consideration, considering that under dynamic flow the contact time of the effluent is not enough to reach the equilibrium adsorption condition. Interestingly, the column could be regenerated following the same procedure and reused for multiple
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adsorption cycle (results not shown). Comparing the results of CNF-Al(OH)3 adsorption with previous studies, made us
appreciate the high adsorption capacity brought by the inclusion of nanocellulose within the aerogel structure (Table 5).
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Two questions might be raised; (1) Are CNF-Al(OH)3 based adsorbents as efficient as
current adsorbents? , and (2) what will be the utility of developing new adsorbents for fluoride
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removal. In fact when we compare the CNF-Al(OH)3 to some of the fluoride adsorbents reported in the literature (Table 5), it can be seen that CNF-Al(OH)3 porous adsorbent has a
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good ability for fluoride removal with an adsorption capacity which is competitive with current adsorbents. Other merits of CNF-Al(OH)3 adsorbents are as follows; (i) the high porosity facilitated the use of sorbent by continuous filtration through columns, avoiding the
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multi-steps recovering of the sorbent, (ii) the CNF-Al(OH)3 sorbents are not complicated to produce, and (iii) the CNF-Al(OH)3 could be regenerated easily by simple washing with NaOH solution. Work is ongoing to study how the presence of other anions such as Cl-, NO3-,
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sulphate is likely to affect the adsorption selectivity and efficiency for fluoride.
2 1
21
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Figure 9: Column study of Fluoride adsorption by CNF-Al(OH)3 aerogel
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Table5: Comparative data about adsorption capacity of current fluoride adsorbent Adsorption capacity (mg.g-1)
Adsorbent
16.9
[30]
23.7
[31]
La(III)-andY(III)-impregnated Al2O3
8
[32]
Granular ferric hydroxide
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Activated Al2O3
Reference
7
[33]
91
[34]
Al2O3/Carbon nanotubes
28.7
[35]
Basic aluminium sulfate@graphene
33.4
[36]
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Al–Ce oxide
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Alumina hydroxide
4. Conclusion
To conclude, this work deals with the preparation of a hybrid nanocomposite based on nanfibrillated cellulose and investigated its application in water defluorination. The nanofibrillated cellulose based aerogel was impregnated with aluminium hydroxide which, bounded to the hydroxyl ions of the TEMPO oxidized nanofibers, showed an enhanced
2
2
removal capacity of fluoride ions from water. Native unmodified surfaces made of
22
nanocellulose a poor sorbent of fluoride from water, but could provide stable supports for achieving high dispersion of Aluminium Hydroxide, which are powerful fluoride sorbents. This proves that the incorporation of some metal ions like Aluminium (III) into CNF surfaces can significantly increase fluoride adsorption capacity. In fact, when the CNF material is impregnated with Aluminium (II) and its complexes, the adsorption capacity has improved by a factor of 20. Since the control over CNF aerogel surface areas and the size distribution of the loaded metal oxide are key factors to understand the improvement of fluoride adsorption capacity increase, the aerogel was characterized using TGA, XPS and SEM analysis. The mechanism
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of fluoride adsorption was confirmed using zeta potential measurement. A column study of a continuous removal processing was conducted showing the reusability of the CNF-Al(OH)3 aerogel.
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Norhene Mhafoudhi has carried out the experimental work and contributed to the writing of the manuscript Sami boufi has supervised the work and write the manuscript
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CRediT author statement
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