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zinc oxide nanocomposite as adsorbent for dye removal
Accepted Manuscript New chitosan/silica/zinc oxide nanocomposite as adsorbent for dye removal
Hazem Hassan, Ahmed Salama, Ahmed K. El-ziaty, Mohamed El-Sakhawy PII: DOI: Reference:
S0141-8130(18)35840-9 https://doi.org/10.1016/j.ijbiomac.2019.03.087 BIOMAC 11917
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
29 October 2018 13 March 2019 13 March 2019
Please cite this article as: H. Hassan, A. Salama, A.K. El-ziaty, et al., New chitosan/ silica/zinc oxide nanocomposite as adsorbent for dye removal, International Journal of Biological Macromolecules, https://doi.org/10.1016/j.ijbiomac.2019.03.087
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ACCEPTED MANUSCRIPT New chitosan/silica/zinc oxide nanocomposite as adsorbent for dye removal Hazem Hassana, Ahmed Salamab, Ahmed K. El-ziatya and Mohamed El-Sakhawyb a
Chemistry Department, Faculty of Science, Ain Shams University Abbassia, Cairo, Egypt.
b
Cellulose and Paper Department, National Research Centre, 33 El-Bohouth st., Dokki, P.O. 12622, Giza, Egypt
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Correspondence should be addressed to Mohamed El-Sakhawy; [email protected]
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Abstract
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Chitosan/silica hybrid was used for zinc oxide (ZnO) nanoparticles immobilization to form chitosan/silica/zinc oxide nanocomposite. This nanocomposite was utilized to eliminate methylene blue (MB) from wastewater. The effect of the ZnO configuration on the adsorption
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properties of the nanocomposite was studied in details. The best interpretation for the equilibrium data was given by Langmuir isotherm, and the highest adsorption capacity of MB
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reached to 293.3 mg/g in slight basic medium. As an effective and low-cost adsorbent, chitosan/silica/zinc oxide nanocomposite is expected to have a promising future for
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adsorption of organic dyes from its aqueous solutions.
1.
Introduction
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Keywords: Chitosan; Silica hybrid; Methylene blue (MB); Zinc oxide (ZnO); Nanocomposite
Several industries such as textiles, printing and painting represents the main sources of
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contaminated water with dyes [1]. Due to its environmental impact, contaminated wastewater with dyes should be subjected to an adequate treatment before discharging. Several treatment
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methods such as flocculation, chemical oxidation, and membrane separation have been used to reduce total dyes level in industrials waste water [2]. However, these technologies show low capacity of dye extraction, slow kinetics of dye removal and are poorly reusable. Recently, adsorption technique has emergent as an economic and efficient technique for dyes and heavy metals removal from contaminated aqueous solutions, particularly with low concentration solutions [3,4]. The performance of different adsorbents as activated carbon [5], hydrogels [6,7], carbon nanotubes [8] and hybrid materials [9–11] were recently investigated.
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ACCEPTED MANUSCRIPT Natural polymers have attracted an increasing interest as promising candidates in waste water purifications. For example, T. Budnyak et. al. showed that the adsorption capacity of technical lignins/silica hybrid materials such as LignoBoost/silica and CleanFlowBlack/silica for MB adsorption are 42.2 and 59.9 mg/g, respectively [4]. Due to their limitations as adsorbents, several techniques have been considered to increase the performance of polysaccharides as adsorbents [12]. Moreover, polysaccharides based hybrid materials have
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recently utilized a an economic and efficient adsorbent for waste water purification [13,14]. For example, Hassan et. al. prepared novel ZnO cellulose acetate composite nanofiber via electrospinning technique. Phenol sorption process by the nanofibers was studied [15]. A.
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Salama investigated the capability of cellulose grafted soy protein isolate/calcium phosphate
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hybrid as adsorbent material for organic dyes elimination from solutions [9]. Also, chitosan/silica nanocomposite was prepared and investigated for metals removal [16].
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Chitosan has been investigated as new adsorbent for organic dyes and heavy metals purification. For improving its adsorption capacity, different chemical modifications are
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necessary for increasing its adsorption capacity. Chitosan was functionalized with various reagents such as thiourea for enhancing its adsorption capacity [17]. Various chitosan composites such as chitosan/TiO2 [18], chitosan/cuprous oxide [19] and chitosan/CdS [20]
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were examined and investigated to remove pollutants. Literature review exhibited that only
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chitosan/ZnO nanoparticle was synthesized via zinc oxides nanoparticles immobilization onto chitosan. Chitosan/ ZnO nanoparticles was applied as dye adsorbent for textile dyes [21]. In view of our interest in exploring the adsorption capacity of polysaccharides based hybrid
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materials, in this work, we explored the removal efficiency of MB dye with new modified chitosan/silica/zinc oxide nanocomposite. The rate of the organic dye removal was
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determined through a series of batch experiments. Chemical Oxygen Demand (COD) which considered as an important parameter for control of wastewater treatment process and for wastewater studies was also investigated. This study provides the suggested mechanism of organic pollutants removal by chitosan/silica/zinc oxide nanocomposite and thus lays an important theoretical basis for the utilization of nanocomposite in the water purification.
2.
Experimental
2.1.
Materials.
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ACCEPTED MANUSCRIPT Low viscous chitosan from shrimp shells and tetraethyl orthosilicate (TEOS), (99.9%) was purchased from Sigma Aldrich. The other chemicals such as zinc acetate, methylene blue and sodium hydroxide were of analytical grade and used as received without further purification. 2.2.
Chitosan/silica hybrid preparation.
Chitosan (2g) was dissolved in 100 mL 2% acetic acid solution. After completely dissolve of chitosan, 0.5 mL distilled water, 1mL ethanol, 0.5 mL HCl and 2mL TEOS were shacked
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well with 6 mL chitosan solution for 2 hours at 60 oC. Then, sodium hydroxide, 0.1 molar solutions was added dropwise to attain slight alkaline solution and leaved for 48 hours until
2.3.
Chitosan/silica/zinc oxide hybrid preparation.
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complete gelation. The formed gel was washed with demineralized water until neutrality.
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The formed chitosan/silica hybrid was immersed in 50 mL zinc acetate solution (6%). Then NaOH solution (1 M) was dropped to the solution until the pH reach to 7 and stirred for 2
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hours at 70oC. The chitosan/silica zinc oxide formed was sieved to separate the only zinc oxide nanoparticles and washed by distilled water 3 times and dried in 80oC for 48 hours and stored in glass vial. Adsorption study.
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2.4.
The adsorption studies were performed by batch method technique. In brief, 0.05 g of
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adsorbent was soaked in 50 mL dye solutions. At the end of the adsorption process, the solution was separated from the adsorbents with a syringe filter and the dye concentration
2.5.
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was measured using UNICO UV-2000 spectrophotometer at 630 nm. Determination of COD reduction in wastewater.
The COD determination evaluates the oxygen equivalent in the organic matter that is
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subjected to oxidation by a strong chemical oxidant. The dichromate reflux technique was used for the COD estimation because it has advantages compared with other oxidants as
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oxidizability, applicability to various samples and ease of handling. Assessing was done using standard method by reflux, titrimetric method. Samples were prepared from dye stock of 1000ppm in dark glass bottles. The experiments were carried out at ambient temperature (25°C) in batch mode for measurement of adsorption capacities. The batch experiments were run in flasks of 100 mL capacity using a flask shaker at 800 rpm speed during the experiment. Different buffer amount was added to the samples to obtain different pH values.
The
samples were withdrawn from the shaker at desired time , filtered and analyzed for the COD.
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ACCEPTED MANUSCRIPT 2.6.
Characterization.
Fourier transform infrared spectroscopy (FT-IR) was done on a FTIR (Mattson 5000 FTIR spectrometer) using KBr discs in the range of 4000–500 cm−1. Scanning electron microscopy (SEM) was done on Model Quanta 250 FEG (Field Emission Gun) attached with EDX Unit (Energy Dispersive X-ray Analyses), with accelerating voltage 30 K. Transmission electron microscope (TEM) images were done with a JEOL JEM-2100 electron microscopy at 100k×
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magnification, with an acceleration voltage of 120 kV. X-ray diffractometer equipped with an automatic divergent slit Philips diffractometer (type PW 3710) was used. The patterns were run with Ni-filtered copper radiation (λ = 1.5404 Å) at 30 kV and 10 mA with a scanning
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speed of 2θ =2.5º/min. Thermogravimetric and differential thermal analyses (TGA/DTGA)
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were carried out using Shimadzu-50 thermal analyzer units. Sample was heated from 303K to
3.
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1273 K at a heating rate of 283K/min in a current of N2 flowing at a rate of 30 mL/min
Results and discussion
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Scheme 1 shows the preparation of chitosan/silica/ZnO nanocomposite. Neat chitosan/silica hybrid was formed through the hydrolysis of TEOS to the corresponding tetrahydroxysilane. This step was followed by the condensation of silanol groups to afford silica gel network
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containing chitosan chains via a sol-gel process. After that, the ZnO nanoparticles were
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precipitated inside the network structure of chitosan/silica hybrid. The amino and hydroxyl groups of chitosan and the residual hydroxyl groups in silica network may have an important
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role for immobilization of ZnO nanoparticles.
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3.1.
IR spectroscopy.
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Scheme 1: Preparation of chitosan/silica/ZnO nanocomposite as adsorbent.
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The functional groups present in the chitosan, chitosan/silica and chitosan/silica/zinc oxide hybrid materials were monitored by FT-IR spectroscopy as shown in Figure 1. Chitosan exhibits characteristic band at 3451 cm−1 which assigned to NH, OH symmetric stretching
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vibration. In addition, characteristic signals at 1656 and 1580 cm−1 may attributed to C=O stretching (amide I) and –NH stretching (amide II), respectively [22]. The most intense band
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in the chitosan/silica hybrid was found between 1090 and 1030 cm-1 and can be associated with the Si– O– Si and Si –O – C vibrations. This band confirms that the hybridization went well, together with the peak at 956 cm-1, which occurs because of the Si– OH stretch that is shifted from 950 cm1 by hydrogen-bonding interactions. New peak at 529 cm-1 in the chitosan/silica/zinc oxide nanocomposite was assigned to the stretching vibration of Zn-O [23].
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ACCEPTED MANUSCRIPT C 2376 782
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798 965
1438 1643
464
1086
3451
A
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464
1086
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798 965
1438 1643
3451
4000 3500 3000 2500 2000 1500 1000 -1
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Transmittanc e
529 436
1025
1401 1635
3426
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2928
3745
B
400
The
FT-IR
chitosan/silica/ZnO (C).
of
chitosan
(A),
chitosan/silica
hybrid
(B)
and
SEM and TEM analysis.
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3.2.
spectra
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Figure1:
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Wave number/cm
The surface morphology of chitosan/silica/zinc oxide nanocomposite was investigated using
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SEM analysis. Figure 2A shows that the surface morphology of the blend is relatively homogenous. The formed zinc oxide layer shows the existence of microplates with ~ 1 µm length. These plates consist of ZnO nanoparticles with diameter ~50 nm as showed in Figure 2C. ZnO nanoparticles appear to be more distinct and uniform, due to the communication between the chitosan/silica nanocomposite blend and the formed nanoparticles that decreases the attractive forces between nanoparticles and reducing their aggregation tendency. Direct proof of the chitosan/silica/zinc oxide nanocomposite configuration is given by transmission electron microscopy (TEM) as shown in Figure 2B, C and D. The nanocomposite is homogeneous and consists of densely packed nanosphere with ~10 nm mean diameter. High resolution transmission electron microscopy showed 2D lattice fringes 6
ACCEPTED MANUSCRIPT of the nanocrystal particles with a ~ 0.25 nm distance between the adjacent lattice consistent with (100) reflection of ZnO nanoparticles (JCPDS no. 36-1415). Moreover, the selected area electron diffraction (SAED), Figure 2 D, showed high crystallinity degree of the ZnO
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nanoparticle.
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FIGURE 2: SEM of chitosan/silica/zinc oxide nanocomposite (A).TEM image (B), HRTEM
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B
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Intensity
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image (C) and SAED pattern of chitosan/silica/zinc oxide nanocomposite (D).
A
10
20
30
40
50
60
70
2-Theta, degree
Figure 3: XRD patterns of chitosan/silica (A) and chitosan/silica/zinc oxide nanocomposite (B).
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ACCEPTED MANUSCRIPT 3.3.
XRD.
XRD patterns of chitosan/silica and chitosan/silica/ZnO nanocomposite were showed in Figure 3. The XRD pattern of chitosan/silica showed no distinct peaks except the broad one centered at 23o
which normally accompanied the polysaccharides diffraction [24].
Chitosan/silica/ZnO nanocomposite exhibits peaks at 2θ values of 31.8o, 34.5o, 36.2o, 47.4o, 56.7o, and 63o assigned to the ZnO crystal planes of (100), (002), (101), (102), (110), and
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(103), respectively [21].The results suggested the successful formation of crystalline zinc
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oxide on chitosan/silica hybrid.
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100 Chitosan – Silica / Zn
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90
80
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90 80
Chitosan - Silica
70
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Weight %
100
80 60 40 20
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100
Chitosan
0 100 200 300 400 500 600 700 800 900 1000
Temp. °C
Figure 4: The TGA curves of Chitosan, Chitosan/silica and Chitosan/silica/ZnO nanocomposite.
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ACCEPTED MANUSCRIPT 3.4.
TGA analysis.
Thermal stabilities of the prepared nanocomposites were investigated. From the TGA curves thermal decomposition parameters were determined. Chitosan, Chitosan/silica and Chitosan/silica/Zinc oxide nanocomposite data at a heating rate of 10C/min from 50-1000 °C are shown in Figure 4. Generally, all nanocomposite samples undergo two stage of degradation: (i) loss of water molecules and (ii) decomposition of organic polymer material.
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Chitosan exhibits two stages weight loss in the range of 50-630°C. The first weight losses (7%) for chitosan polymer occurred at 50-155°C due to evaporation of water molecules
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through dehydration reaction of chitosan. The second step of the weight loss (93%) at 211-
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630C is attributed to the decomposition of polymer chains. The thermal analysis curve of chitosan/silica shows two stages of decomposition .The first decomposition step is started at
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50-140°C with mass loss of 25% corresponding to loss of hydrated water molecules. The second step of the weight loss (33%), in the temperature range 165-630°C, is referred to the decomposition of polymeric network. This signifies that the chitosan/silica hybrid completely
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involves in crosslinking and obviously the results exhibited lower weight loss compared to neat chitosan. However, the chitosan/silica/zinc oxide was thermally decomposed in two
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successive decomposition steps. The first predicted mass loss of 12% was recorded in the temperature range 50-250C. The second step occurs within the temperature range 290-
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630°C with total mass loss 23% and leaving 77% silica and zinc oxide residues. The thermodynamic activation parameters of the structural properties of the chitosan,
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chitosan/silica and chitosan/silica/zinc oxide materials on the thermal behavior, namely activation energy (Ea), enthalpy (H*), entropy (S*), Gibbs free energy change of the
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decomposition (G*) and the order, n, were predicted from the TG themograms by employing the Coast–Redfern [25] and Horowitz–Metzger equations [26]. The kinetic parameters are listed in Tables 1-2. The following remarks can be pointed out: (i) The negative activation entropies values S* indicate a more ordered activated hybrid polymer than the reactants and/or the reactions are slow. (ii) There is no obvious trend in the heat of activation Ea or the activation enthalpies H* values.
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ACCEPTED MANUSCRIPT Table 1: The Kinetic parameters of chitosan, chitosan/silica and chitosan/silica/zinc oxide by Coats Redfern program. Samples Step
T/K
E / KJ
A/ S-1
mol
-1
Order R2
H*
/ S*
/ G*
/
KJ mol- KJ mol- KJ mol(n)
1
1
16.872
- 0.115
K-1
1
First
358
7.1x103
19.85
0.95
1.0
Second
511
4.09x109
141.29
0.99
1.0
137.041 - 0.02
138.08
Chitosan-
First
343
1.2x103
26.499
0.94
1.0
23.646
- 0.129
68.064
Silica
Second
592
1.9 x106
3.922
0.97
1.0
1.003
- 0.307
180.80
Chitosan-
First
603
3.6 x105
4.810
Silica/Zn
Second
679
6.7 x105
23.276
0.90
1.0
0.206
- 0.086
52.171
0.91
1.0
17.626
- 0.281
208.44
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58.136
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Chitosan
TABLE 2: Kinetic Parameters of Chitosan, Chitosan -Silica and Chitosan-silica/ZnO by
Samples
Order H* / KJ S* / KJ G* (n)
mol-1K-1
KJ mol-1
- 0.258
100.269
1.6x103
65.796
0.99 1.0
61.548
- 0.158
142.342
343
1.3x10-2
13.078
0.93 1.0
10.227
- 0.238
91.983
Second 592
5.2 x10-5
5.246
0.98 1.0
0.324
- 0.293
174.038
603
3.5 x10-4
11.298
0.96 1.0
6.285
- 0.265
165.984
Second 679
3.8 x10-4
13.477
0.91 1.0
7.831
- 0.284
201.186
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Chitosan- First
However, the activation energy values G* elevated for the successive decomposition stages of a given hybrid composite. This is caused by the considerable increase in the TS* values
10
/
mol-1 7.757
Chitosan- First
Silica/Zn
3.7x10-3
mol-1
R2
0.95 1.0
Second 511
Silica
E / KJ
10.734
Chitosan
358
A/ S-1
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First
T/K
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Step
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Horowitz–Metzger program.
ACCEPTED MANUSCRIPT from one step to another which supersede the values of H*. Increasing the values of G* for the following steps of a given hybrid composites reflects that the removal rate of the subsequent hybrid composite will be lower than that of the precedent chitosan. This may be ascribed to the structural and it also mean the thermal stability of the composites sequence is chitosan
Adsorption properties of MB by chitosan/silica/ZnO nanocomposite.
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The adsorption performance of chitosan/silica and chitosan/silica/ZnO nanocomposite was calculated for removal of MB from aqueous solutions. It is known from other studies that the
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creation of a silica network in the presence of polymer results in an increase in its porosity,
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which increase the specific surface area of the obtained materials [27]. Various parameters including solution pH, contact time and the initial MB concentration were studied to evaluate
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the adsorption properties.
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100 90
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80
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60 50 40 30
A
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20
B
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qe, mg/g
70
10
0
1
2
3
4
5
6
7
8
9
10
pH FIGURE 5: Effect of the pH values on the adsorption capacities of chitosan/silica (A) and chitosan/silica/ZnO nanocomposite (B) for MB. (Dye concentration 100 mg/L; sample amount 0.05 g/50 mL and equilibrium time 70 min).
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ACCEPTED MANUSCRIPT The pH of the adsorption medium is a crucial factor that effects the dye adsorption [6]. The pH dependence of the MB adsorption was studied in the pH range 2-9 for chitosan/silica and chitosan/silica/ZnO nanocomposite as shown in Figure 5. For both samples, the adsorption rate was gradually increased to reach the optimum adsorption at pH7 and reach to 62 and 83 mg/g for chitosan/silica and chitosan/silica/ZnO nanocomposite, respectively. Under the same experimental conditions, the adsorption capacity was significantly high for
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chitosan/silica/ZnO nanocomposite comparing with chitosan/silica hybrid. This behavior can be explained on the basis of electrostatic interaction between the negative charges on
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chitosan/silica/zinc oxide surface and the cationic MB molecules.
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3.5.1. Effect of contact time
It has been reported that the physicochemical process rate of MB molecules uptake by the adsorbent surface depends on the contact time between the dye solution and the solid
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adsorbent and also on the diffusion process [8]. The surface structure of the chitosan/silica and chitosan/silica/ZnO nanocomposite is expected to have a strong influence on its
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adsorption selectivity and capacity.
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100
80 70 60
A
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qe, mg/g
B
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90
50
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40 30 20 10 0
20
40
60
80
100
120
140
160
180
Time, min. Figur 6: Effect of the adsorption time on adsorption capacity of chitosan/silica (A) and chitosan/silica/ZnO nanocomposite (B). (MB concentration 100 mg/L; sample amount 0.05 g/50 mL and pH 7.0).
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ACCEPTED MANUSCRIPT The adsorption of MB on chitosan/silica and chitosan/silica/ZnO nanocomposite was rapid for the first 60 minutes after which it slowed down and flattened off as contact time increased (~120 to ~160min). This may be due to the configuration of monolayer of dye molecules at outer surface of the nanocomposite. It finally reached a stability area after 120 min as shown in Figure 6. This phenomenon is assigned to the high number of available adsorption sites which is accessible at the initial stage for adsorption, but after going of time, the remaining
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available adsorption sites are more difficultly accessible. Moreover, the micro porous structure of the chitosan/silica/ZnO nanocomposite facilitates the diffusion process.
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3.5.2. Adsorption kinetics:
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Figure 6 shows the effect of contact time on MB removal by adsorbent. Pseudo first and second-order kinetic models were used to fit the experimental data to investigate the prospective rate-controlling steps implicate the adsorption process. Linear forms of Pseudo
𝑡
(1)
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𝐾1
log (qe − qt) = log (qe) – 2.303 𝑡
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first and second-order are given in equations 1 and 2:
𝑞𝑡
𝑡
1
= 𝑞𝑒 + 𝑘2𝑞𝑒2
(2)
where qt(mg/g) is the adsorption capacity at time t (min), q e(mg/g) is the adsorption capacity
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at equilibrium, and k1(min−1) and k2(g (mg min−1)) are the kinetic rate constants for the
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Pseudo-first-order and Pseudo-second-order models, respectively. TABLE 3: Kinetic parameters for MB adsorption by chitosan/silica/ZnO nanocomposite. Pseudo first-order model
94.8
AC
CE
qe.exp(mg/g) qe.cal(mg/g) K1(min−1)
74
0.023
Pseudo second-order model R2
qe.cal(mg/g)
K2(g(mg
R2
min)−1)) 0.98
107.9
3.9 x 10-4
0.997
The linear plots of log (qe − qt) vs. t (for the Pseudo-first order) and (t/q) vs. t (for the Pseudo-second-order) models are drawn. Rate constants (k1 and k2), correlation coefficients and the calculated qe,cal for the two kinetic models were calculated. Based on the obtained correlation coefficients (R2) it is clear, from Table 3, that the adsorption followed the Pseudosecond-order model as the adsorption of MB on lignin silica hybrid [28]. This result proves
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ACCEPTED MANUSCRIPT that the chemical interaction between the nanocomposite surfaces and the MB is dominated the adsorption process. 3.5.3. Effect of MB concentration. Figure 7 demonstrates the effect of MB concentration on the adsorption capacity of chitosan/silica and chitosan/silica/ZnO nanocomposite. It is obvious that MB removal increases from 198 to 274 mg/g at MB concentration 550 ppm for both chitosan/silica and
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chitosan/silica/ZnO nanocomposite, respectively, then tend to levels off with higher concentrations. However, the leveling off for the adsorption process after 550 ppm has been
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observed because of the saturation of active adsorption sites within nanocomposite.
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250
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150
100
CE
50
0
A
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qe, mg/g
200
B
50
AC
0
100 150 200 250 300 350 400 450 500 550 600
MB Conc., ppm
FIGURE 7: Effect of the primary MB concentration on adsorption capacity of chitosan/silica (A) and chitosan/silica/ZnO nanocomposite (B). (Sample amount 0.05 g/50 mL; pH 7 and contact time 24 hours). Langmuir and Freundlich isotherm equations were checked for describing the interaction mechanism between MB and the nanocomposite surface, with the experimentally obtained equilibrium data. Langmuir isotherm model supposes that adsorption takes place at specific
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ACCEPTED MANUSCRIPT homogeneous sites within the adsorbent. The linear form of this isotherm equation can be written as follows: [29]
Ce K C s e qe qmax qmax
(4)
where qe is the MB adsorption amounts at equilibrium on the nanocomposite (mg/g), Ce is the
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MB concentrations at equilibrium in the solution (mg/l), qmax represents the maximum amount of MB that could be adsorbed on the nanocomposite (mg/g), and Ks is the Langmuir model constant (mg/l). The plot of the experimental Ce/qe vs. Ce for the experimental data
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showed elevated correlation coefficient (R2 > 0.98) of the linearized Langmuir equation
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indicates that the Langmuir model can illustrate the adsorption of MB on chitosan/silica/ZnO nanocomposite (Table 4). From the slope and the intercept of the straight line, the values of
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qmax and Ks are estimated to be 293.3 mg/g and 16.2 mg/l, respectively. The Langmuir isotherm model supposes the monolayer adsorption of the MB on the chitosan/silica/ZnO
adsorption.
1 log Ce log P n
(5)
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log qe
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The Freundlich equation is given by:
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nanocomposite surface and all adsorption sites have equal energies and enthalpies of
where P is a constant representing the adsorption capacity (mg/g) and n is a constant
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represents the adsorption intensity (dimensionless). Freundlich parameters prove that the linear coefficient was 0.724. Moreover, the Freundlich model constants P and n values are 42.2 and 2.66 respectively (Table 4). These results signify that this model not illustrate the
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adsorption processes for MB adsorption on the nanocomposite.
Table 4: Parameters for MB adsorption by chitosan/silica/ZnO nanocomposite according to different equilibrium models. Langmuir isotherm constants
Freundlich isotherm constants
Ks (mg/L)
qm(mg/g)
R2
P(mg/g)
n
R2
16.2
293.3
0.988
42.2
2.66
0.724
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Feasibility of MB adsorption was calculated using separation factor (R L) which is described by the following equation:
RL
1 1 KsCO
(6)
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where Co is the initial concentration of MB (ppm). This factor is used to expect if adsorption system is favorable or unfavorable. The calculated results exposed that the RL values for the
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adsorption of different studied initial concentrations of MB fall between 0 and 1 range. This finding exhibits that the MB adsorption on chitosan/silica/ZnO nanocomposite is favorable. COD studies.
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3.6.
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3.6.1. Effect of pH on adsorption.
The residual COD concentration was measured after the solutions treatment with chitosan/silica and chitosan/silica/ZnO nancomposite. The contribution of pH to the
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COD reduction was investigated starting with 1086 mg/l which is the blank COD concentration. The experiments performed for determining the percent reduction in COD by
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varying pH values, Figure 8, showed that COD reduction recorded maximum values of 56%
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and 76% at pH 7 for chitosan/silica and chitosan/silica/ZnO nanocomposite, respectively. 3.6.2. Effect of contact time on COD values. The investigation of contact time was done on adsorbents at pH 7 with the contact time
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range from 0 to 160 minutes, under constant shaking speed. Figure 9 shows that increasing the contact time above 120 minutes do not affect the COD values. The maximum results were
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achieved by applying contact time 120 min. When comparing the results achieved using two adsorbents, it was found that the measured COD values were attained to 72 and 95 for chitosan/silica and chitosan/silica/ZnO, respectively. This result is in a harmony with other studies which stated that the efficiency of the sorption process depends on the pH of solution and the phase contact time [4].
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70
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56
B A
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42
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28 14 0 1
2
3
4
5
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Arabic(COD reduction)
84
6
7
8
9
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pH
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Figure 8: Effect of pH on COD of chitosan/silica (A) and chitosan/silica/ZnO (B).
90 80
A
CE
70 60 50 40
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COD reduction
B
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100
30 20 10
0 0
20
40
60
80
100
120
140
160
Contact time, min.
Figure 9: Effect of contact time on COD of chitosan/silica (a) and chitosan/silica/ZnO nanocomposite (B). 17
ACCEPTED MANUSCRIPT 4.
Conclusion
New chitosan/silica and chitosan/silica zinc oxide hybrids were designed to be efficient adsorbents for dye-polluted water purification. The study also compared between the chitosan/silica and chitosan/silica/zinc oxide hybrids adsorption capacity. The existence of ZnO nanoparticles in the hybrid enhances the adsorption capacity and antibacterial activity of the adsorbent. MB adsorption was studied from the aqueous solutions, and adsorption was
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found to be efficient at pH 7. The adsorption follows Langmuir isotherm and was observed to be a feasible process. The candidate materials exhibited a high maximum removal capacity
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and therefore, these hybrid materials behave as good candidates for MB removal in water
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purification
Acknowledgements
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The authors wish to acknowledge the financial support, motivations and valuable supports by National Research Centre, Egypt.
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ACCEPTED MANUSCRIPT Highlights 1- A novel chitosan/silica hybrid was prepared by sol-gel technique. 2- ZnO2 nanoparticles were formed in the presence of the hybrid materials. 3- Chitosan/silica/ZnO2 showed antimicrobial properties and applied as efficient adsorbent for MB.
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4- The adsorption kinetics followed Pseudo second order model and adsorption isotherm fit with Langmuir model.
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Hazem Hassan, Ahmed Salama, Ahmed K. El-ziaty, Mohamed El-Sakhawy PII: DOI: Reference:
S0141-8130(18)35840-9 https://doi.org/10.1016/j.ijbiomac.2019.03.087 BIOMAC 11917
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
29 October 2018 13 March 2019 13 March 2019
Please cite this article as: H. Hassan, A. Salama, A.K. El-ziaty, et al., New chitosan/ silica/zinc oxide nanocomposite as adsorbent for dye removal, International Journal of Biological Macromolecules, https://doi.org/10.1016/j.ijbiomac.2019.03.087
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ACCEPTED MANUSCRIPT New chitosan/silica/zinc oxide nanocomposite as adsorbent for dye removal Hazem Hassana, Ahmed Salamab, Ahmed K. El-ziatya and Mohamed El-Sakhawyb a
Chemistry Department, Faculty of Science, Ain Shams University Abbassia, Cairo, Egypt.
b
Cellulose and Paper Department, National Research Centre, 33 El-Bohouth st., Dokki, P.O. 12622, Giza, Egypt
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Correspondence should be addressed to Mohamed El-Sakhawy; [email protected]
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Abstract
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Chitosan/silica hybrid was used for zinc oxide (ZnO) nanoparticles immobilization to form chitosan/silica/zinc oxide nanocomposite. This nanocomposite was utilized to eliminate methylene blue (MB) from wastewater. The effect of the ZnO configuration on the adsorption
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properties of the nanocomposite was studied in details. The best interpretation for the equilibrium data was given by Langmuir isotherm, and the highest adsorption capacity of MB
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reached to 293.3 mg/g in slight basic medium. As an effective and low-cost adsorbent, chitosan/silica/zinc oxide nanocomposite is expected to have a promising future for
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adsorption of organic dyes from its aqueous solutions.
1.
Introduction
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Keywords: Chitosan; Silica hybrid; Methylene blue (MB); Zinc oxide (ZnO); Nanocomposite
Several industries such as textiles, printing and painting represents the main sources of
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contaminated water with dyes [1]. Due to its environmental impact, contaminated wastewater with dyes should be subjected to an adequate treatment before discharging. Several treatment
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methods such as flocculation, chemical oxidation, and membrane separation have been used to reduce total dyes level in industrials waste water [2]. However, these technologies show low capacity of dye extraction, slow kinetics of dye removal and are poorly reusable. Recently, adsorption technique has emergent as an economic and efficient technique for dyes and heavy metals removal from contaminated aqueous solutions, particularly with low concentration solutions [3,4]. The performance of different adsorbents as activated carbon [5], hydrogels [6,7], carbon nanotubes [8] and hybrid materials [9–11] were recently investigated.
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ACCEPTED MANUSCRIPT Natural polymers have attracted an increasing interest as promising candidates in waste water purifications. For example, T. Budnyak et. al. showed that the adsorption capacity of technical lignins/silica hybrid materials such as LignoBoost/silica and CleanFlowBlack/silica for MB adsorption are 42.2 and 59.9 mg/g, respectively [4]. Due to their limitations as adsorbents, several techniques have been considered to increase the performance of polysaccharides as adsorbents [12]. Moreover, polysaccharides based hybrid materials have
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recently utilized a an economic and efficient adsorbent for waste water purification [13,14]. For example, Hassan et. al. prepared novel ZnO cellulose acetate composite nanofiber via electrospinning technique. Phenol sorption process by the nanofibers was studied [15]. A.
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Salama investigated the capability of cellulose grafted soy protein isolate/calcium phosphate
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hybrid as adsorbent material for organic dyes elimination from solutions [9]. Also, chitosan/silica nanocomposite was prepared and investigated for metals removal [16].
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Chitosan has been investigated as new adsorbent for organic dyes and heavy metals purification. For improving its adsorption capacity, different chemical modifications are
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necessary for increasing its adsorption capacity. Chitosan was functionalized with various reagents such as thiourea for enhancing its adsorption capacity [17]. Various chitosan composites such as chitosan/TiO2 [18], chitosan/cuprous oxide [19] and chitosan/CdS [20]
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were examined and investigated to remove pollutants. Literature review exhibited that only
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chitosan/ZnO nanoparticle was synthesized via zinc oxides nanoparticles immobilization onto chitosan. Chitosan/ ZnO nanoparticles was applied as dye adsorbent for textile dyes [21]. In view of our interest in exploring the adsorption capacity of polysaccharides based hybrid
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materials, in this work, we explored the removal efficiency of MB dye with new modified chitosan/silica/zinc oxide nanocomposite. The rate of the organic dye removal was
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determined through a series of batch experiments. Chemical Oxygen Demand (COD) which considered as an important parameter for control of wastewater treatment process and for wastewater studies was also investigated. This study provides the suggested mechanism of organic pollutants removal by chitosan/silica/zinc oxide nanocomposite and thus lays an important theoretical basis for the utilization of nanocomposite in the water purification.
2.
Experimental
2.1.
Materials.
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ACCEPTED MANUSCRIPT Low viscous chitosan from shrimp shells and tetraethyl orthosilicate (TEOS), (99.9%) was purchased from Sigma Aldrich. The other chemicals such as zinc acetate, methylene blue and sodium hydroxide were of analytical grade and used as received without further purification. 2.2.
Chitosan/silica hybrid preparation.
Chitosan (2g) was dissolved in 100 mL 2% acetic acid solution. After completely dissolve of chitosan, 0.5 mL distilled water, 1mL ethanol, 0.5 mL HCl and 2mL TEOS were shacked
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well with 6 mL chitosan solution for 2 hours at 60 oC. Then, sodium hydroxide, 0.1 molar solutions was added dropwise to attain slight alkaline solution and leaved for 48 hours until
2.3.
Chitosan/silica/zinc oxide hybrid preparation.
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complete gelation. The formed gel was washed with demineralized water until neutrality.
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The formed chitosan/silica hybrid was immersed in 50 mL zinc acetate solution (6%). Then NaOH solution (1 M) was dropped to the solution until the pH reach to 7 and stirred for 2
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hours at 70oC. The chitosan/silica zinc oxide formed was sieved to separate the only zinc oxide nanoparticles and washed by distilled water 3 times and dried in 80oC for 48 hours and stored in glass vial. Adsorption study.
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2.4.
The adsorption studies were performed by batch method technique. In brief, 0.05 g of
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adsorbent was soaked in 50 mL dye solutions. At the end of the adsorption process, the solution was separated from the adsorbents with a syringe filter and the dye concentration
2.5.
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was measured using UNICO UV-2000 spectrophotometer at 630 nm. Determination of COD reduction in wastewater.
The COD determination evaluates the oxygen equivalent in the organic matter that is
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subjected to oxidation by a strong chemical oxidant. The dichromate reflux technique was used for the COD estimation because it has advantages compared with other oxidants as
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oxidizability, applicability to various samples and ease of handling. Assessing was done using standard method by reflux, titrimetric method. Samples were prepared from dye stock of 1000ppm in dark glass bottles. The experiments were carried out at ambient temperature (25°C) in batch mode for measurement of adsorption capacities. The batch experiments were run in flasks of 100 mL capacity using a flask shaker at 800 rpm speed during the experiment. Different buffer amount was added to the samples to obtain different pH values.
The
samples were withdrawn from the shaker at desired time , filtered and analyzed for the COD.
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ACCEPTED MANUSCRIPT 2.6.
Characterization.
Fourier transform infrared spectroscopy (FT-IR) was done on a FTIR (Mattson 5000 FTIR spectrometer) using KBr discs in the range of 4000–500 cm−1. Scanning electron microscopy (SEM) was done on Model Quanta 250 FEG (Field Emission Gun) attached with EDX Unit (Energy Dispersive X-ray Analyses), with accelerating voltage 30 K. Transmission electron microscope (TEM) images were done with a JEOL JEM-2100 electron microscopy at 100k×
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magnification, with an acceleration voltage of 120 kV. X-ray diffractometer equipped with an automatic divergent slit Philips diffractometer (type PW 3710) was used. The patterns were run with Ni-filtered copper radiation (λ = 1.5404 Å) at 30 kV and 10 mA with a scanning
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speed of 2θ =2.5º/min. Thermogravimetric and differential thermal analyses (TGA/DTGA)
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were carried out using Shimadzu-50 thermal analyzer units. Sample was heated from 303K to
3.
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1273 K at a heating rate of 283K/min in a current of N2 flowing at a rate of 30 mL/min
Results and discussion
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Scheme 1 shows the preparation of chitosan/silica/ZnO nanocomposite. Neat chitosan/silica hybrid was formed through the hydrolysis of TEOS to the corresponding tetrahydroxysilane. This step was followed by the condensation of silanol groups to afford silica gel network
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containing chitosan chains via a sol-gel process. After that, the ZnO nanoparticles were
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precipitated inside the network structure of chitosan/silica hybrid. The amino and hydroxyl groups of chitosan and the residual hydroxyl groups in silica network may have an important
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role for immobilization of ZnO nanoparticles.
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3.1.
IR spectroscopy.
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Scheme 1: Preparation of chitosan/silica/ZnO nanocomposite as adsorbent.
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The functional groups present in the chitosan, chitosan/silica and chitosan/silica/zinc oxide hybrid materials were monitored by FT-IR spectroscopy as shown in Figure 1. Chitosan exhibits characteristic band at 3451 cm−1 which assigned to NH, OH symmetric stretching
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vibration. In addition, characteristic signals at 1656 and 1580 cm−1 may attributed to C=O stretching (amide I) and –NH stretching (amide II), respectively [22]. The most intense band
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in the chitosan/silica hybrid was found between 1090 and 1030 cm-1 and can be associated with the Si– O– Si and Si –O – C vibrations. This band confirms that the hybridization went well, together with the peak at 956 cm-1, which occurs because of the Si– OH stretch that is shifted from 950 cm1 by hydrogen-bonding interactions. New peak at 529 cm-1 in the chitosan/silica/zinc oxide nanocomposite was assigned to the stretching vibration of Zn-O [23].
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ACCEPTED MANUSCRIPT C 2376 782
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798 965
1438 1643
464
1086
3451
A
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464
1086
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798 965
1438 1643
3451
4000 3500 3000 2500 2000 1500 1000 -1
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Transmittanc e
529 436
1025
1401 1635
3426
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2928
3745
B
400
The
FT-IR
chitosan/silica/ZnO (C).
of
chitosan
(A),
chitosan/silica
hybrid
(B)
and
SEM and TEM analysis.
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3.2.
spectra
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Figure1:
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Wave number/cm
The surface morphology of chitosan/silica/zinc oxide nanocomposite was investigated using
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SEM analysis. Figure 2A shows that the surface morphology of the blend is relatively homogenous. The formed zinc oxide layer shows the existence of microplates with ~ 1 µm length. These plates consist of ZnO nanoparticles with diameter ~50 nm as showed in Figure 2C. ZnO nanoparticles appear to be more distinct and uniform, due to the communication between the chitosan/silica nanocomposite blend and the formed nanoparticles that decreases the attractive forces between nanoparticles and reducing their aggregation tendency. Direct proof of the chitosan/silica/zinc oxide nanocomposite configuration is given by transmission electron microscopy (TEM) as shown in Figure 2B, C and D. The nanocomposite is homogeneous and consists of densely packed nanosphere with ~10 nm mean diameter. High resolution transmission electron microscopy showed 2D lattice fringes 6
ACCEPTED MANUSCRIPT of the nanocrystal particles with a ~ 0.25 nm distance between the adjacent lattice consistent with (100) reflection of ZnO nanoparticles (JCPDS no. 36-1415). Moreover, the selected area electron diffraction (SAED), Figure 2 D, showed high crystallinity degree of the ZnO
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nanoparticle.
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FIGURE 2: SEM of chitosan/silica/zinc oxide nanocomposite (A).TEM image (B), HRTEM
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B
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Intensity
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image (C) and SAED pattern of chitosan/silica/zinc oxide nanocomposite (D).
A
10
20
30
40
50
60
70
2-Theta, degree
Figure 3: XRD patterns of chitosan/silica (A) and chitosan/silica/zinc oxide nanocomposite (B).
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XRD.
XRD patterns of chitosan/silica and chitosan/silica/ZnO nanocomposite were showed in Figure 3. The XRD pattern of chitosan/silica showed no distinct peaks except the broad one centered at 23o
which normally accompanied the polysaccharides diffraction [24].
Chitosan/silica/ZnO nanocomposite exhibits peaks at 2θ values of 31.8o, 34.5o, 36.2o, 47.4o, 56.7o, and 63o assigned to the ZnO crystal planes of (100), (002), (101), (102), (110), and
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(103), respectively [21].The results suggested the successful formation of crystalline zinc
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oxide on chitosan/silica hybrid.
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100 Chitosan – Silica / Zn
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80
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90 80
Chitosan - Silica
70
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Weight %
100
80 60 40 20
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100
Chitosan
0 100 200 300 400 500 600 700 800 900 1000
Temp. °C
Figure 4: The TGA curves of Chitosan, Chitosan/silica and Chitosan/silica/ZnO nanocomposite.
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TGA analysis.
Thermal stabilities of the prepared nanocomposites were investigated. From the TGA curves thermal decomposition parameters were determined. Chitosan, Chitosan/silica and Chitosan/silica/Zinc oxide nanocomposite data at a heating rate of 10C/min from 50-1000 °C are shown in Figure 4. Generally, all nanocomposite samples undergo two stage of degradation: (i) loss of water molecules and (ii) decomposition of organic polymer material.
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Chitosan exhibits two stages weight loss in the range of 50-630°C. The first weight losses (7%) for chitosan polymer occurred at 50-155°C due to evaporation of water molecules
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through dehydration reaction of chitosan. The second step of the weight loss (93%) at 211-
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630C is attributed to the decomposition of polymer chains. The thermal analysis curve of chitosan/silica shows two stages of decomposition .The first decomposition step is started at
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50-140°C with mass loss of 25% corresponding to loss of hydrated water molecules. The second step of the weight loss (33%), in the temperature range 165-630°C, is referred to the decomposition of polymeric network. This signifies that the chitosan/silica hybrid completely
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involves in crosslinking and obviously the results exhibited lower weight loss compared to neat chitosan. However, the chitosan/silica/zinc oxide was thermally decomposed in two
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successive decomposition steps. The first predicted mass loss of 12% was recorded in the temperature range 50-250C. The second step occurs within the temperature range 290-
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630°C with total mass loss 23% and leaving 77% silica and zinc oxide residues. The thermodynamic activation parameters of the structural properties of the chitosan,
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chitosan/silica and chitosan/silica/zinc oxide materials on the thermal behavior, namely activation energy (Ea), enthalpy (H*), entropy (S*), Gibbs free energy change of the
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decomposition (G*) and the order, n, were predicted from the TG themograms by employing the Coast–Redfern [25] and Horowitz–Metzger equations [26]. The kinetic parameters are listed in Tables 1-2. The following remarks can be pointed out: (i) The negative activation entropies values S* indicate a more ordered activated hybrid polymer than the reactants and/or the reactions are slow. (ii) There is no obvious trend in the heat of activation Ea or the activation enthalpies H* values.
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ACCEPTED MANUSCRIPT Table 1: The Kinetic parameters of chitosan, chitosan/silica and chitosan/silica/zinc oxide by Coats Redfern program. Samples Step
T/K
E / KJ
A/ S-1
mol
-1
Order R2
H*
/ S*
/ G*
/
KJ mol- KJ mol- KJ mol(n)
1
1
16.872
- 0.115
K-1
1
First
358
7.1x103
19.85
0.95
1.0
Second
511
4.09x109
141.29
0.99
1.0
137.041 - 0.02
138.08
Chitosan-
First
343
1.2x103
26.499
0.94
1.0
23.646
- 0.129
68.064
Silica
Second
592
1.9 x106
3.922
0.97
1.0
1.003
- 0.307
180.80
Chitosan-
First
603
3.6 x105
4.810
Silica/Zn
Second
679
6.7 x105
23.276
0.90
1.0
0.206
- 0.086
52.171
0.91
1.0
17.626
- 0.281
208.44
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58.136
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Chitosan
TABLE 2: Kinetic Parameters of Chitosan, Chitosan -Silica and Chitosan-silica/ZnO by
Samples
Order H* / KJ S* / KJ G* (n)
mol-1K-1
KJ mol-1
- 0.258
100.269
1.6x103
65.796
0.99 1.0
61.548
- 0.158
142.342
343
1.3x10-2
13.078
0.93 1.0
10.227
- 0.238
91.983
Second 592
5.2 x10-5
5.246
0.98 1.0
0.324
- 0.293
174.038
603
3.5 x10-4
11.298
0.96 1.0
6.285
- 0.265
165.984
Second 679
3.8 x10-4
13.477
0.91 1.0
7.831
- 0.284
201.186
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Chitosan- First
However, the activation energy values G* elevated for the successive decomposition stages of a given hybrid composite. This is caused by the considerable increase in the TS* values
10
/
mol-1 7.757
Chitosan- First
Silica/Zn
3.7x10-3
mol-1
R2
0.95 1.0
Second 511
Silica
E / KJ
10.734
Chitosan
358
A/ S-1
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First
T/K
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Step
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Horowitz–Metzger program.
ACCEPTED MANUSCRIPT from one step to another which supersede the values of H*. Increasing the values of G* for the following steps of a given hybrid composites reflects that the removal rate of the subsequent hybrid composite will be lower than that of the precedent chitosan. This may be ascribed to the structural and it also mean the thermal stability of the composites sequence is chitosan
Adsorption properties of MB by chitosan/silica/ZnO nanocomposite.
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The adsorption performance of chitosan/silica and chitosan/silica/ZnO nanocomposite was calculated for removal of MB from aqueous solutions. It is known from other studies that the
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creation of a silica network in the presence of polymer results in an increase in its porosity,
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which increase the specific surface area of the obtained materials [27]. Various parameters including solution pH, contact time and the initial MB concentration were studied to evaluate
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the adsorption properties.
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100 90
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80
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60 50 40 30
A
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20
B
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qe, mg/g
70
10
0
1
2
3
4
5
6
7
8
9
10
pH FIGURE 5: Effect of the pH values on the adsorption capacities of chitosan/silica (A) and chitosan/silica/ZnO nanocomposite (B) for MB. (Dye concentration 100 mg/L; sample amount 0.05 g/50 mL and equilibrium time 70 min).
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ACCEPTED MANUSCRIPT The pH of the adsorption medium is a crucial factor that effects the dye adsorption [6]. The pH dependence of the MB adsorption was studied in the pH range 2-9 for chitosan/silica and chitosan/silica/ZnO nanocomposite as shown in Figure 5. For both samples, the adsorption rate was gradually increased to reach the optimum adsorption at pH7 and reach to 62 and 83 mg/g for chitosan/silica and chitosan/silica/ZnO nanocomposite, respectively. Under the same experimental conditions, the adsorption capacity was significantly high for
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chitosan/silica/ZnO nanocomposite comparing with chitosan/silica hybrid. This behavior can be explained on the basis of electrostatic interaction between the negative charges on
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chitosan/silica/zinc oxide surface and the cationic MB molecules.
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3.5.1. Effect of contact time
It has been reported that the physicochemical process rate of MB molecules uptake by the adsorbent surface depends on the contact time between the dye solution and the solid
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adsorbent and also on the diffusion process [8]. The surface structure of the chitosan/silica and chitosan/silica/ZnO nanocomposite is expected to have a strong influence on its
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adsorption selectivity and capacity.
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100
80 70 60
A
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qe, mg/g
B
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90
50
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40 30 20 10 0
20
40
60
80
100
120
140
160
180
Time, min. Figur 6: Effect of the adsorption time on adsorption capacity of chitosan/silica (A) and chitosan/silica/ZnO nanocomposite (B). (MB concentration 100 mg/L; sample amount 0.05 g/50 mL and pH 7.0).
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ACCEPTED MANUSCRIPT The adsorption of MB on chitosan/silica and chitosan/silica/ZnO nanocomposite was rapid for the first 60 minutes after which it slowed down and flattened off as contact time increased (~120 to ~160min). This may be due to the configuration of monolayer of dye molecules at outer surface of the nanocomposite. It finally reached a stability area after 120 min as shown in Figure 6. This phenomenon is assigned to the high number of available adsorption sites which is accessible at the initial stage for adsorption, but after going of time, the remaining
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available adsorption sites are more difficultly accessible. Moreover, the micro porous structure of the chitosan/silica/ZnO nanocomposite facilitates the diffusion process.
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3.5.2. Adsorption kinetics:
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Figure 6 shows the effect of contact time on MB removal by adsorbent. Pseudo first and second-order kinetic models were used to fit the experimental data to investigate the prospective rate-controlling steps implicate the adsorption process. Linear forms of Pseudo
𝑡
(1)
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𝐾1
log (qe − qt) = log (qe) – 2.303 𝑡
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first and second-order are given in equations 1 and 2:
𝑞𝑡
𝑡
1
= 𝑞𝑒 + 𝑘2𝑞𝑒2
(2)
where qt(mg/g) is the adsorption capacity at time t (min), q e(mg/g) is the adsorption capacity
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at equilibrium, and k1(min−1) and k2(g (mg min−1)) are the kinetic rate constants for the
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Pseudo-first-order and Pseudo-second-order models, respectively. TABLE 3: Kinetic parameters for MB adsorption by chitosan/silica/ZnO nanocomposite. Pseudo first-order model
94.8
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qe.exp(mg/g) qe.cal(mg/g) K1(min−1)
74
0.023
Pseudo second-order model R2
qe.cal(mg/g)
K2(g(mg
R2
min)−1)) 0.98
107.9
3.9 x 10-4
0.997
The linear plots of log (qe − qt) vs. t (for the Pseudo-first order) and (t/q) vs. t (for the Pseudo-second-order) models are drawn. Rate constants (k1 and k2), correlation coefficients and the calculated qe,cal for the two kinetic models were calculated. Based on the obtained correlation coefficients (R2) it is clear, from Table 3, that the adsorption followed the Pseudosecond-order model as the adsorption of MB on lignin silica hybrid [28]. This result proves
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ACCEPTED MANUSCRIPT that the chemical interaction between the nanocomposite surfaces and the MB is dominated the adsorption process. 3.5.3. Effect of MB concentration. Figure 7 demonstrates the effect of MB concentration on the adsorption capacity of chitosan/silica and chitosan/silica/ZnO nanocomposite. It is obvious that MB removal increases from 198 to 274 mg/g at MB concentration 550 ppm for both chitosan/silica and
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chitosan/silica/ZnO nanocomposite, respectively, then tend to levels off with higher concentrations. However, the leveling off for the adsorption process after 550 ppm has been
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observed because of the saturation of active adsorption sites within nanocomposite.
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150
100
CE
50
0
A
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qe, mg/g
200
B
50
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0
100 150 200 250 300 350 400 450 500 550 600
MB Conc., ppm
FIGURE 7: Effect of the primary MB concentration on adsorption capacity of chitosan/silica (A) and chitosan/silica/ZnO nanocomposite (B). (Sample amount 0.05 g/50 mL; pH 7 and contact time 24 hours). Langmuir and Freundlich isotherm equations were checked for describing the interaction mechanism between MB and the nanocomposite surface, with the experimentally obtained equilibrium data. Langmuir isotherm model supposes that adsorption takes place at specific
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Ce K C s e qe qmax qmax
(4)
where qe is the MB adsorption amounts at equilibrium on the nanocomposite (mg/g), Ce is the
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MB concentrations at equilibrium in the solution (mg/l), qmax represents the maximum amount of MB that could be adsorbed on the nanocomposite (mg/g), and Ks is the Langmuir model constant (mg/l). The plot of the experimental Ce/qe vs. Ce for the experimental data
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showed elevated correlation coefficient (R2 > 0.98) of the linearized Langmuir equation
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indicates that the Langmuir model can illustrate the adsorption of MB on chitosan/silica/ZnO nanocomposite (Table 4). From the slope and the intercept of the straight line, the values of
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qmax and Ks are estimated to be 293.3 mg/g and 16.2 mg/l, respectively. The Langmuir isotherm model supposes the monolayer adsorption of the MB on the chitosan/silica/ZnO
adsorption.
1 log Ce log P n
(5)
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log qe
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The Freundlich equation is given by:
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nanocomposite surface and all adsorption sites have equal energies and enthalpies of
where P is a constant representing the adsorption capacity (mg/g) and n is a constant
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represents the adsorption intensity (dimensionless). Freundlich parameters prove that the linear coefficient was 0.724. Moreover, the Freundlich model constants P and n values are 42.2 and 2.66 respectively (Table 4). These results signify that this model not illustrate the
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adsorption processes for MB adsorption on the nanocomposite.
Table 4: Parameters for MB adsorption by chitosan/silica/ZnO nanocomposite according to different equilibrium models. Langmuir isotherm constants
Freundlich isotherm constants
Ks (mg/L)
qm(mg/g)
R2
P(mg/g)
n
R2
16.2
293.3
0.988
42.2
2.66
0.724
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Feasibility of MB adsorption was calculated using separation factor (R L) which is described by the following equation:
RL
1 1 KsCO
(6)
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where Co is the initial concentration of MB (ppm). This factor is used to expect if adsorption system is favorable or unfavorable. The calculated results exposed that the RL values for the
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adsorption of different studied initial concentrations of MB fall between 0 and 1 range. This finding exhibits that the MB adsorption on chitosan/silica/ZnO nanocomposite is favorable. COD studies.
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3.6.
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3.6.1. Effect of pH on adsorption.
The residual COD concentration was measured after the solutions treatment with chitosan/silica and chitosan/silica/ZnO nancomposite. The contribution of pH to the
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COD reduction was investigated starting with 1086 mg/l which is the blank COD concentration. The experiments performed for determining the percent reduction in COD by
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varying pH values, Figure 8, showed that COD reduction recorded maximum values of 56%
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and 76% at pH 7 for chitosan/silica and chitosan/silica/ZnO nanocomposite, respectively. 3.6.2. Effect of contact time on COD values. The investigation of contact time was done on adsorbents at pH 7 with the contact time
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range from 0 to 160 minutes, under constant shaking speed. Figure 9 shows that increasing the contact time above 120 minutes do not affect the COD values. The maximum results were
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achieved by applying contact time 120 min. When comparing the results achieved using two adsorbents, it was found that the measured COD values were attained to 72 and 95 for chitosan/silica and chitosan/silica/ZnO, respectively. This result is in a harmony with other studies which stated that the efficiency of the sorption process depends on the pH of solution and the phase contact time [4].
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98
70
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56
B A
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42
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28 14 0 1
2
3
4
5
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Arabic(COD reduction)
84
6
7
8
9
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pH
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Figure 8: Effect of pH on COD of chitosan/silica (A) and chitosan/silica/ZnO (B).
90 80
A
CE
70 60 50 40
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COD reduction
B
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100
30 20 10
0 0
20
40
60
80
100
120
140
160
Contact time, min.
Figure 9: Effect of contact time on COD of chitosan/silica (a) and chitosan/silica/ZnO nanocomposite (B). 17
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Conclusion
New chitosan/silica and chitosan/silica zinc oxide hybrids were designed to be efficient adsorbents for dye-polluted water purification. The study also compared between the chitosan/silica and chitosan/silica/zinc oxide hybrids adsorption capacity. The existence of ZnO nanoparticles in the hybrid enhances the adsorption capacity and antibacterial activity of the adsorbent. MB adsorption was studied from the aqueous solutions, and adsorption was
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found to be efficient at pH 7. The adsorption follows Langmuir isotherm and was observed to be a feasible process. The candidate materials exhibited a high maximum removal capacity
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and therefore, these hybrid materials behave as good candidates for MB removal in water
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purification
Acknowledgements
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The authors wish to acknowledge the financial support, motivations and valuable supports by National Research Centre, Egypt.
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ACCEPTED MANUSCRIPT Highlights 1- A novel chitosan/silica hybrid was prepared by sol-gel technique. 2- ZnO2 nanoparticles were formed in the presence of the hybrid materials. 3- Chitosan/silica/ZnO2 showed antimicrobial properties and applied as efficient adsorbent for MB.
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4- The adsorption kinetics followed Pseudo second order model and adsorption isotherm fit with Langmuir model.
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