carboxymethyl cellulose hydrogels reinforced with graphene oxide and bentonite for enhanced adsorption of methylene blue

carboxymethyl cellulose hydrogels reinforced with graphene oxide and bentonite for enhanced adsorption of methylene blue

Accepted Manuscript Title: Eco-friendly polyvinyl alcohol/carboxymethyl cellulose hydrogels reinforced with graphene oxide and bentonite for enhanced ...

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Accepted Manuscript Title: Eco-friendly polyvinyl alcohol/carboxymethyl cellulose hydrogels reinforced with graphene oxide and bentonite for enhanced adsorption of methylene blue Authors: Hongjie Dai, Yue Huang, Huihua Huang PII: DOI: Reference:

S0144-8617(17)31490-X https://doi.org/10.1016/j.carbpol.2017.12.073 CARP 13135

To appear in: Received date: Revised date: Accepted date:

19-10-2017 10-12-2017 28-12-2017

Please cite this article as: Dai, Hongjie., Huang, Yue., & Huang, Huihua., Eco-friendly polyvinyl alcohol/carboxymethyl cellulose hydrogels reinforced with graphene oxide and bentonite for enhanced adsorption of methylene blue.Carbohydrate Polymers https://doi.org/10.1016/j.carbpol.2017.12.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

[Title Page] • Title. Eco-friendly polyvinyl alcohol/carboxymethyl cellulose hydrogels reinforced with graphene oxide and bentonite for enhanced adsorption of methylene blue • Author names and affiliations. Hongjie Dai, Yue Huang, and Huihua Huang*

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School of Food Science and Engineering, South China University of Technology, Guangzhou 510641, China

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• Corresponding author.

Huihua Huang (E-mail: [email protected]; Tel: +86 20-87112851)

School of Food Science and Engineering, South China University of Technology,

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Guangzhou 510641, China.

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• Present/permanent address.

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No.381, Wushan Road, Tianhe District, Guangzhou City, Guangdong Province,

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China.

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Graphical abstract

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Highlights

Eco-friendly PVA/CMC hydrogels reinforced with GO and bentonite were



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prepared.

Introducing GO and bentonite promoted formation of porous structure of



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hydrogels.

Improving swelling, thermal stability and MB adsorption by introducing two fillers.



MB adsorption was well fitted by Langmuir isotherm and pseudo-second-order

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model.

The hydrogels displayed good pH sensitivity and reusability for MB adsorption.

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Abstract: Eco-friendly polyvinyl alcohol/carboxymethyl cellulose (isolated from

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pineapple peel) hydrogels reinforced with graphene oxide and bentonite were prepared as efficient adsorbents for methylene blue (MB). The structure and

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morphology of the prepared hydrogels were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), X-ray diffraction (XRD), thermogravimetry (TG) and differential scanning calorimetry (DSC).

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Introducing graphene oxide and bentonite into the hydrogels evidently enhanced the thermal stability, swelling ability and MB adsorption capacity. The effects of initial concentration of MB, pH, contact time and temperature on MB adsorption capacity of the prepared hydrogels were investigated. Adsorption kinetics and equilibrium adsorption isotherm fitted pseudo-second-order kinetic model and Langmuir isotherm 3

model well, respectively. After introducing graphene oxide and bentonite into the hydrogels, the maximum adsorption capacity calculated from the Langmuir isotherm model reached 172.14 mg/g at 30 °C, obviously higher than the hydrogels prepared without these additions (83.33 mg/g). Furthermore, all the prepared hydrogels also displayed good reusability for the efficient removal of MB. Consequently, the prepared hydrogels could be served as eco-friendly, stable, efficient and reusable

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adsorbents for anionic dyes in wastewater treatment.

Keywords: pineapple peel; polyvinyl alcohol; bentonite; graphene oxide; adsorption;

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methylene blue 1. Introduction

Recently, increasing concerns and seriousness of environmental issues have

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accelerated the research for the utilization of agricultural waste materials. Pineapple

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(Ananas comosus), as one of the most popular tropic fruits in the world, is produced

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with world total production of 16 ~ 19 million tonnes annually (Dai & Huang, 2016). However, during pineapple processing, the produced wastes (peel, core, stem, and

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leaves) were discarded and generally accounted for 50% (w/w) of total pineapple

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weight (Rattanapoltee & Kaewkannetra, 2014). Among these wastes, pineapple peel, accounting for 35% of total pineapple weight, is substantially produced when pineapple is consumed as fresh fruits or processed into juice (Dai & Huang, 2017a). Besides the characteristics of abundance, renewability and biodegradability, pineapple

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peel is principally composed of cellulose, hemicellulose, lignin, and pectin, in which cellulose occupies 20 ~ 25% of the dry weight (Dai & Huang, 2016). Hence, the

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multipurpose utilization of pineapple peel is of important significance, especially in the use of cellulose. In our previous work, we synthesized a series of pineapple peel

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cellulose-based hydrogels for the applications in drug release, dye and heavy metal adsorption, and enzyme immobilization (Dai, Ou, Liu & Huang, 2017; Dai et al., 2017; Dai & Huang, 2016, 2017a, b; Hu et al., 2010; Hu, Wang & Huang, 2013; Hu, Zhao, Song & Huang, 2011). However, there is still lack of data concerning the use of pineapple peel cellulose and its hydrogels. Hydrogels are defined as the three-dimensional hydrophilic networks via chemical 4

and/or physical cross-linking with the ability to absorb and retain large amounts of water or biological fluids without dissolution (Guo, Duan, Cui & Zhu, 2015). Besides the water absorption and water holding properties, hydrogels also can be designed to respond intelligently to external stimuli such as temperature, pH, salt, light and electric field (Ahmed, 2015; Dai & Huang, 2017a). Based on these characteristics, hydrogels are intensively applied in drug and cell delivery system (Bao et al., 2017;

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Seeli & Prabaharan, 2017; Yu et al., 2016), enzyme immobilization (Dai, Ou, Liu &

Huang, 2017), absorbents (Thombare, Jha, Mishra & Siddiqui, 2017) and agriculture

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fields (Zhang et al., 2017). Compared with the hydrogels based on synthetic polymers

such as acrylic acid, acrylamide and poly(ethylene glycol), hydrogels based on natural polymer such as cellulose (Dai, Ou, Liu & Huang, 2017), starch (Van Nieuwenhove

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et al., 2017), chitosan (Sampath et al., 2017), sodium alginate (Bhutani et al., 2016)

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and gelatin (Fan et al., 2016), have captured more research interest recently due to

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their high hydrophilicity, biocompatibility, nontoxicity and biodegradability. Carboxymethyl cellulose (CMC), as one of the most important industrial

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biopolymers, is a water-soluble cellulose derivate produced by partial substitution of

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the 2, 3, and 6 hydroxyl groups of cellulose by carboxymethyl groups (Son, Rhee & Park, 2015). Due to its solubility, biocompatibility and biodegradation, CMC-based hydrogels exhibit considerable potential use in enzyme immobilization (Dai, Ou, Liu & Huang, 2017), wound healing (Oliveira et al., 2017), drug delivery (Rasoulzadeh &

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Namazi, 2017), and absorbents (Saber-Samandari et al., 2016). Polyvinyl alcohol (PVA) is a semi-crystalline polymer synthesized from the hydrolysis of polyvinyl

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acetate (PVAc). Besides non-toxicity, high hydrophilicity, and biocompatibility, PVA can easily form hydrogel by repeated freeze-thaw cycles without any chemical

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crosslinkers that may lead to toxicity (Kim et al., 2015). Especially, massive hydroxyl groups on the main chain of PVA can be a source of hydrogen bonding, and hence assist in the formation of hydrogel composites (Deshmukh et al., 2016). Recently, various fillers have been introduced into hydrogels to enhance the properties of hydrogels for various applications (Usman, Hussain, Riaz & Khan, 2016). Graphene oxide (GO), as an oxidized form of graphene sheets, contains 5

numerous reactive oxygen-containing functional groups such as epoxide, hydroxyl, and carboxylic groups on its surface (Guo, Duan, Cui & Zhu, 2015). Besides these functional groups, other superior characteristics such as high mechanical properties, large specific surface area, high hydrophilicity and good biocompatibility endow GO a good candidate as a reinforcing filler in composite materials (Guo, Duan, Cui & Zhu, 2015; Liu et al., 2016). Hydrogels loaded with clay also aroused increasing interest

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worldwide recently, due to the improvements in swelling ability, gel strength, thermal stability, and adsorption ability (Pourjavadi et al., 2016). In our previous studies, the

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incorporation of kaolin (Dai & Huang, 2017b) and carclazyte (Dai & Huang, 2017a) significantly enhanced methylene blue (MB) adsorption ability of the prepared

hydrogels. Bentonite is a kind of layered clay mineral and primarily composed of

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montmorillonite that is a 2:1 type aluminosilicate. Due to low-cost, easy availability,

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high cation exchange capacity and surface area, bentonite has been widely applied in

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dyes removal recently (Bhattacharyya & Ray, 2015).

As mentioned above, introductions of reinforcing fillers would be considered as an

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effective method to improve the performance of the hydrogels. Although there have

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been reports of individual GO or bentonite as fillers in the preparation of hydrogels (Liu et al., 2016; Sarkar & Singh, 2017), the combined introductions of these two fillers into hydrogels have not yet been reported in the literatures. Due to their possessions of abundant functional groups and unique structures, a significantly

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improved performance of the hydrogels should be produced because of the synergetic effect. Herein, the novel carboxymethyl cellulose (isolated from pineapple peel)/PVA

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hydrogels reinforced with GO and bentonite were eco-friendly synthesized by a simple repeated freeze-thaw cycles. The prepared hydrogels were characterized using

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Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry (TG) and differential scanning calorimetry (DSC). The effects of GO and bentonite on structure, swelling behavior, pH sensitivity and methylene blue adsorption of the prepared hydrogels were investigated and compared. 2. Materials and methods 6

2.1. Materials and reagents Pineapple peel was collected from a local pineapple processing factory (Guangzhou City, China). Carboxymethyl cellulose isolated from pineapple peel (PCMC) was obtained according to our previous study (Dai, Ou, Liu & Huang, 2017). PVA was supplied by Dahao Fine Chemicals Co., Ltd. (Shantou City, China). Single-layer graphene oxide (GO) was purchased from Guangzhou Feibo Co., Ltd. (Guangzhou

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City, China), with thickness of 0.6-1.0 nm and specific surface area of 1000-1217

m2/g. Bentonite was purchased from Shanghai No.4 Reagent & H.V. Chemical Co.,

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Ltd. (Shanghai City, China). Methylene blue was supplied by Guangzhou Chemical

Reagent Co., Ltd. (Guangzhou City, China). All other chemicals and solvents used in this study were of analytical grade and solutions were prepared with distilled water.

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2.2. Hydrogels preparation

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The hydrogels were prepared using an environmentally friendly freeze-thaw cycles.

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The prepared hydrogels were named based on their initial compositions (for examples, PVA/PCMC/GO/bentonite means 5.0 g PVA + 2.0 g PCMC + 50 mg GO + 1.0 g

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bentonite; PVA/PCMC/GO means 5.0 g PVA + 2.0 g PCMC + 50 mg GO;

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PVA/PCMC/bentonite means 5.0 g PVA + 2.0 g PCMC + 1.0 g bentonite; PVA/PCMC means 5.0 g PVA + 2.0 g PCMC). Taking the preparation of PVA/PCMC/GO/bentonite as an example, 5.0 g of PVA powders was dissolved in 80 mL of distilled water under magnetic stirring at 95 °C for 3.0 h. After cooling down to

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room temperature, 2.0 g of PCMC was added into the solution under stirring to form a homogeneous mixed solution. In the meantime, GO powders (50 mg) and bentonite

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(1.0 g) were mixed with 20 mL of distilled water under ultrasound sonication for 30 min to promote their complete exfoliations and uniform dispersions (Gan et al., 2015;

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Huang et al., 2015; Pourjavadi et al., 2016; Heydari et al., 2017;), and then the mixture was gradually dropped into the mixed solution of PVA and PCMC and stir-treated at 95 °C for 1.0 h. After removing the air bubbles by sonication, the mixed solution was carefully poured into a cylindrical mold (6-well culture plate), followed by three freeze-thaw cycles (freezing at -20 °C for 8 h and thawing at room temperature for 4 h) to form hydrogel. To remove the unreacted monomers, the 7

obtained hydrogel (PVA/PCMC/GO/bentonite) was soaked in distilled water for three days with water-change every 12 h. Finally, PVA/PCMC/GO/bentonite was dried in a vacuum freeze drier at -55 °C for 24 h and now was available for further study. 2.3. Characterization 2.3.1. Fourier-transformed infrared spectroscopy FTIR spectra of the samples were recorded on a FTIR spectrometer (Vector 33,

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Bruker, Germany) in the frequency range of 4000-500 cm-1 at a resolution of 4 cm-1.

Prior to measurement, the tested specimens were prepared using the standard KBr

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pressed pellet method. 2.3.2. Scanning electron micrograph

The surface morphologies of the samples were observed using a scanning electron

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microscope (S-3700N, Hitachi, Japan). Before analysis, a thin gold film was

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sputter-coated on the surface of the samples using a sputter coater (Cressington 108

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auto, Watford, UK). 2.3.3. X-ray diffraction

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The XRD patterns of the samples were measured using an X-ray diffractometer (D8

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ADVANCE, Bruker, Germany) with Cu-Ka radiation (λ = 0.15418 nm) as X-ray source at 40 kV of accelerating voltage and 40 mA of current. The scanning speed was set at 2°/min in the region of the diffraction angle (2θ) from 3° to 50°. 2.3.4. Thermal analysis

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The thermal stability of the samples was performed using a simultaneous thermal analyzer (STA449C, NETZSCH, Germany) based on the analysis of thermal

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gravimetric analysis (TG) and differential scanning calorimetry (DSC). The heating rate was set at 10 °C/min from room temperature to 500 °C under N2 atmosphere. The

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instrument was previously calibrated using several metals (In, Sn, Bi, Zn, Al and Au) as standards. 2.4. Swelling and pH sensitivity The gravimetric method was employed to measure the swelling ratio of the prepared hydrogels in distilled water and 0.9% NaCl solution. Prior to each experiment, the dried hydrogels were cut into flakes with about 2.0 mm of thickness. 8

Then, 30 mg of the hydrogels was immersed into excessive distilled water or 0.9% NaCl solution at room temperature. After swelling equilibrium, the swollen hydrogels were removed from the swelling medium, wiped with tissue paper to remove the surface water and weighed immediately. The equilibrium swelling ratio (ESR) was calculated using the following equation:

Ws  Wd ×100 Wd

(1)

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ESR (%) =

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where Ws (g) and Wd (g) are the masses of the swollen and dried hydrogels, respectively.

To evaluate the pH sensitivity of the prepared hydrogels, the ESR of the hydrogels at different pH values was investigated. The pH values were adjusted with diluted

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HCl or NaOH solutions and determined by a pH meter. The ionic strengths of all

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2.5.1. Adsorption kinetic

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2.5. Adsorption of methylene blue

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solutions were adjusted to 0.1 mol/L with NaCl solution.

For kinetic study, the dried hydrogels (30 mg) were immersed into 20 mL of MB

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aqueous solution of 200 mg/L at room temperature. At the completion of preset time intervals, the current concentration of MB solution was measured using a UV-vis spectrophotometer (UV-1800, Shimadzu, Japan) at 664 nm. The adsorption capacity

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of the hydrogels at time t (Qt, mg/g) and the equilibrium adsorption capacity (Qe, mg/g) were calculated based on the changes of the MB concentration before and after

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adsorption, according to the following equation, respectively: Qt =

(C0 - Ct )V m

Qe =

(C0  Ce )V m

(2)

(3)

where C0 and Ct (mg/L) are the concentrations of MB solution at the initial time and time t (h), respectively; Ce (mg/L) is the equilibrium concentration of MB solution; V (L) is the volume of the MB solution, and m (g) is the weight of the dried hydrogels. 9

2.5.2. Effects of pH and MB concentration The effects of pH and MB concentration on the MB adsorption capacity of the hydrogels were studied according to the method as described above. The influence of pH was evaluated by adjusting MB solutions to different pH values (2.0-10.0) with 0.1 mol/L NaOH or HCl solution. The influence of MB concentration was investigated over a MB concentration range from 50 to 250 mg/L.

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2.5.3. Reusability of the hydrogels

The MB-loaded hydrogels were stir-treated with 30 mL of 0.1 mol/L HCl for 3 h to

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desorb the MB dye. Subsequently, the hydrogels were washed with distilled water and then reused for adsorption processes again. The adsorption/desorption cycles were successively conducted four times according to the method described in 2.5.1, each

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trial with fresh solution.

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3. Results and discussion

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3.1. FTIR analysis

The FTIR spectra of the prepared hydrogels (PVA/PCMC/GO/bentonite, PVA/PCMC/bentonite

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PVA/PCMC/GO,

and

PVA/PCMC)

and

their

initial

components (PCMC, PVA, GO and bentonite) are displayed in Fig. 1a. For GO, the

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characteristic peaks at 3420, 1721 and 1047 cm-1 were corresponded to hydroxyl, benzene carboxyl and epoxy groups, respectively (Qi, Yao, Deng & Zhou, 2014). For PVA, the broad and intense peak at 3455 cm-1 was ascribed to the stretching vibration

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of O-H. The peaks at 2921, 1431 and 1073 cm-1 were associated with the vibrations of C-H and C-O-C stretching and C-H bending, respectively (Dai, Ou, Liu & Huang,

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2017). For PCMC, the peaks at 3460, 2903 and 1051 cm-1 were attributed to the stretching vibrations of O-H, C-H and C-O-C groups, respectively. The peaks at 1423

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and 1322 cm-1 resulted from the bending vibrations of C-H and O-H groups, respectively (Dai & Huang, 2017a). The very intense peaks at 1653 and 1423 cm-1 were assigned to the stretching vibration of -COO groups and its salt forms, corresponding to the typical adsorption of carboxymethyl cellulose (Yadav, Rhee & Park, 2014). For bentonite, the peaks at 3630, 3431, 1645, 1040, 790 and 524 cm-1 were due to the O-H (Al-OH) stretching, O-H stretching, O-H bending, Si-O 10

stretching, Si-O-Al stretching and Si-O-Al bending vibrations, respectively (Dai & Huang, 2017b; Sarkar & Singh, 2017). Compared these initial components and PVA/PCMC in the spectrum with the hydrogels (PVA/PCMC/GO/bentonite, PVA/PCMC/GO and PVA/PCMC/bentonite), no new absorbance peaks appeared with the incorporations of GO and/or bentonite. For PVA/PCMC, the peak at 3460 cm-1 was attributed to both inter- and intra-molecular hydrogen bonding among O-H

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groups in PVA and PCMC (Jose et al., 2015). Compared with PVA/PCMC, after the incorporations of GO and/or bentonite, the peak corresponding to O-H groups became

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broader and shifted to the lower wavenumbers, suggesting the formation of extensive

hydrogen bonds and better interaction among these four polymers (Liu et al., 2016). It also implies that GO and bentonite can serve as physical cross-linking agent to

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promote the formation of crosslinked hydrogels via hydrogen bonding interactions

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(Huang, Shen, Li, & Ye, 2015; Shi, Xiong, Li & Wang, 2016).

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3.2. XRD analysis

The XRD patterns of the prepared hydrogels (PVA/PCMC/GO/bentonite, PVA/PCMC/bentonite

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PVA/PCMC/GO,

and

PVA/PCMC)

and

their

initial

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components (PCMC, PVA, GO and bentonite) are displayed in Fig. 1b. As for the initial components of the hydrogels, PVA presented a strong diffraction peak at 2θ = 19.7° accompanied by a shoulder peak at 2θ = 22.8° and a weak peak at 2θ = 40.8°, corresponding to the characteristic crystalline peaks of PVA (Dai, Ou, Liu & Huang,

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2017; Guan et al., 2014). The peak at 2θ = 19.7° was attributed to the presences of strong inter- and intra-molecular hydrogen bonding in PVA structure (Islam, Rahaman

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& Yeum, 2015). PCMC only displayed a broad diffraction peak at 2θ = 22.1°, suggesting the low crystallinity of PCMC and the crystalline structure of cellulose II

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(Dai, Ou, Liu & Huang, 2017). GO showed a strong and sharp diffraction peak at 2θ = 10.3° with a corresponding d-spacing of 8.5 Å, corresponding to the characteristics peak of GO (Sahraei, Pour & Ghaemy, 2016). Bentonite exhibited the main peaks at 2θ = 5.8°, 19.7°, 20.8°, 26.5° and 27.8°, implying the presence of montmorillonite as a major phase (He et al., 2015; Sarkar & Singh, 2017). However, after the hydrogels formation, the characteristic peaks of GO were found disappeared in the XRD 11

patterns of PVA/PCMC/GO/bentonite and PVA/PCMC/GO, which was mainly due to the disappearance of the regular and periodic structure of GO in the hydrogels. These results agree with the reports of Zhang et al. (2017) and Wang et al. (2017), also suggesting the probably exfoliation and dispersion of GO sheets in the hydrogels. Similar results were also found in PVA/GO (Usman, Hussain, Riaz & Khan, 2016) and GO/CMC/alginate hydrogels (Yadav, Rhee & Park, 2014). Compared with the patterns

of

bentonite,

the

hydrogels

PVA/PCMC/GO/bentonite

and

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XRD

PVA/PCMC/bentonite only displayed a weak diffraction peak at 2θ = 26.5° belonged

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to the characteristic peak of bentonite, indicating the successful exfoliation and

dispersion of bentonite in the polymer matrix after forming hydrogels (Hosseinzadeh, Zoroufi & Mahdavinia, 2015). As observed from the figure, all the prepared hydrogels

intensity

was

found

in

other

three

hydrogels,

especially

in

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showed a main diffraction peak at 2θ = 19.5°. Compared with PVA/PCMC, a

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PVA/PCMC/GO/bentonite and PVA/PCMC/bentonite. Taking together with the FTIR results, it can be inferred the formation of hydrogen bonding and the interactions

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among PCMC, PVA, GO and bentonite.

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Fig. 1. FT-IR spectra (a) and X-ray diffraction patterns (b) of the prepared hydrogels (PVA/PCMC/GO/bentonite, PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC) and their

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initial components (PCMC, PVA, GO and bentonite)

3.3. SEM analysis

The SEM images of the prepared hydrogels PVA/PCMC/GO/bentonite,

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PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC are presented in Fig. 2. As shown in Fig. 2, PVA/PCMC displayed a relatively smooth and tight surface with the

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distribution of some bulges. However, after the introductions of GO and bentonite, the obtained hydrogels exhibited a relatively regular surface without obvious particle

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distribution, indicating the uniform dispersion and strong interaction among these polymers. Compared with PVA/PCMC, PVA/PCMC/GO and PVA/PCMC/bentonite showed some hole-shaped wrinkles and obviously porous crosslinked structure on their surfaces, respectively. Compared with PVA/PCMC/bentonite, the surface of PVA/PCMC/GO/bentonite appeared more and smaller pores accompanied with hole-shaped wrinkles as observed in PVA/PCMC/GO. Combined considerations 13

together with the FTIR and XRD analysis, it was deduced that the GO and bentonite might play the roles as physical cross-linking points to promote more junctions and pores in the prepared hydrogels, showing consistent with the results of published papers (Huang, Shen, Li, & Ye, 2015; Shi, Xiong, Li & Wang, 2016). The obtained porous structure provided not only the high surface area but also the large mass

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transfer channels, consequently leading to a high adsorption capacity for dyes.

Fig. 2. The SEM images of the prepared hydrogels PVA/PCMC/GO/bentonite, PVA/PCMC/GO,

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PVA/PCMC/bentonite and PVA/PCMC.

3.4. Thermal analysis

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Thermal characteristics of the prepared hydrogels PVA/PCMC/GO/bentonite,

PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC were investigated and compared according to TG, DTG and DSC curves, as depicted in Fig. 3. Based on these curves, the degradation of the prepared hydrogels could be divided roughly into three stages. The first stage of weight loss below 100 °C was ascribed to the evaporation of water associated with the polymer. The major weight loss from 200 to 14

350 °C and the further weight loss beyond 400 °C were corresponded to the decomposition of oxygen-containing groups like hydroxyl, carboxyl and/or epoxide groups (Usman, Hussain, Riaz & Khan, 2016). For example, the hydrogel PVA/PCMC/GO/bentonite could remain 94.35% of its initial weight after heating to 200 °C, however, when continuously being heated to 350 °C, only 39.42% of its initial weight remained. Finally, with increasing the temperature till 500 °C,

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PVA/PCMC/GO/bentonite, PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC remained 21.34%, 8.44%, 22.38% and 9.72% of their initial weights, respectively.

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The initial decomposition temperature of PVA/PCMC/GO/bentonite, PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC was 231, 228, 232 and 225 °C, respectively. The DSC curves of all prepared hydrogels displayed an endothermic peak at around

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80 °C, corresponding to the first stage of weight loss on the TG curves. At the further

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increased temperature (> 200 °C), a weak peak around 222 °C was observed for all

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prepared hydrogels. Above 300 °C, the DSC curves of PVA/PCMC/GO/bentonite, PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC appeared a major peak at

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317, 313, 319 and 308 °C, respectively, which was consistent with the main peak of

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DTG curves. Based on the above results, the addition of GO and/or bentonite could enhance the thermal stability of the prepared hydrogels. The enhanced crosslinked structure and hydrogen bonding interactions are responsible for the increased thermal stability of the prepared hydrogels (Guo, Zhang, Peng, & Yan, 2015). Moreover, the

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barrier effect of bentonite also contributed to the enhanced thermal stability of the hydrogels (Santiago, Mucientes, Osorio & Rivera, 2007). Usman et al. (2016)

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reported that the thermal stability of PVA/starch blends increased by the incorporation of GO due to strong physical bonding between GO layers and PVA/starch blends.

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El-Sherif and El-Masry (2011) also reported that bentonite could improve the thermal stability of acrylamide/2-acrylamido-2-methyl-1-propane sulfonic acid/chitosan superabsorbent polymers.

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PVA/PCMC/GO/bentonite

100

60 0

40

-2

-6 -8

20

-10

60

-2 0

-4

-12 200

300

400

20

-15

500

100

Temperature (°C)

100

200

300

0 500

400

200

300

100

500

200

300

Temperature (°C)

2

PVA/PCMC/bentonite

400

Temperature (°C)

-5

Temperature (°C)

2

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2 -2

-2 0 -3

-3

-4 -6

20

-8

-4

-10

-6 -9 -12 -15 -18

-12 100

200

300

400

100

500

Temperature (°C)

-5 100

200

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DTG (%/min)

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-1

DTG (%/min)

60

DSC (mW/mg)

-1

0

TG (%)

80

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1

0

300

0 500

400

200

300

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100

200

100

80

60

40

20 500

Temperature (°C)

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Temperature (°C)

-5

0 500

400

PVA/PCMC

100

1

DSC (mW/mg)

-9 -12

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100

-4

-6

-18

-5

-3

40

-3

-3

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-4

-4

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TG (%)

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TG (%)

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DTG (%/min)

80

DSC (mW/mg)

0

TG (%)

1

DTG (%/min)

DSC (mW/mg)

1

-3

PVA/PCMC/GO

100

300

400

0 500

Temperature (°C)

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Fig. 3. Thermal characteristics of the prepared hydrogels PVA/PCMC/GO/bentonite,

curves.

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PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC based on TG, DSC and DTG (sub-figure)

3.5. Swelling ability and pH sensitivity Fig. 4a shows the equilibrium swelling ratio of the prepared hydrogels in distilled and

0.9%

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water

NaCl

solution.

The

equilibrium

swelling

ratio

of

PVA/PCMC/GO/bentonite, PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC

CC

in distilled water was 35.23, 30.12, 32.12 and 25.16 g/g, respectively, significantly higher than their swelling ratios in 0.9% NaCl solution (9.23, 7.13, 8.75 and 6.71 g/g,

A

respectively). Compared with PVA/PCMC, introducing GO and/or bentonite into the network increased the swelling ability of the prepared hydrogels, which might be ascribed to the following reasons. The hydrophilic groups in GO and bentonite can facilitate the water diffusion due to the increased surface hydrophilicity of the hydrogels. Meanwhile, the additions of GO and bentonite promoted the formation of porous structure, resulting in a high surface area for enhanced swelling ability. Our 16

previous study indicated that carclazyte can effectively improve the swelling ability of carboxymethyl

cellulose g poly(acrylic

acid-co-acrylamide)

superabsorbent

hydrogel (Dai & Huang, 2017a). Other similar results were also reported by Peng et al. (2016) in the swelling study of cellulose/carboxymethyl cellulose/clay hydrogels and Li et al. (2014) in PVA/GO hydrogels. The effect of pH on the equilibrium swelling ratio of the prepared hydrogels was

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investigated in various pH solutions ranging from pH 2.0 to 12.0, and the results are depicted in Fig. 4b. It was noted that all prepared hydrogels clearly exhibited similar

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change tendency in swelling ratio with the increase of pH values. As shown in Fig. 4b, the swelling ratio of the prepared hydrogels significantly increased within pH 2.0-8.0 and decreased within pH 8.0-12.0, showing the maximum swelling ability at pH 8.0.

U

At the acidic conditions, a screening effect of the counter ions, i.e. Cl− in the swelling

N

medium, prevented an efficient repulsion and finally resulted in a remarkable

A

hydrogel collapsing (Debnath et al., 2015). Meanwhile, most carboxyl groups existed in the form of -COOH groups, inhibiting the electrostatic repulsion between -COO−

M

groups and consequently leading to the shrinkage of network structure (Wang et al.,

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2017). As the external pH increased, the electrostatic repulsion was improved due to the ionization of -COOH groups, which contributed to the expansion of the hydrogel network and swelling more. However, when pH increased to 10.0, a charge screening effect of counterions (Na+) would generate and subsequently hinder the swelling (Dai

EP

& Huang, 2017b). These evident pH-dependent swelling behaviors confirmed the pH-sensitive characteristic of the prepared hydrogels. Similarly, after introducing GO

CC

and/or bentonite into the hydrogels network, a higher swelling ability also can be

A

observed in various pH solutions.

17

Swelling in distilled water Swelling in 0.9% NaCl solution

a 36

Swelling ratio (g/g)

30 24 18 12

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6 0 PVA/PCMC/ GO/bentonite

PVA/PCMC/ GO

PVA/PCMC/ bentonite

PVA/PCMC

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b 10

PVA/PCMC/GO/bentonite PVA/PCMC/GO PVA/PCMC/bentonite PVA/PCMC

N

4

U

6

2 0 4

6

pH

8

10

M

2

A

Swelling ratio (g/g)

8

12

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Fig. 4. Swelling ratio of the prepared hydrogels (PVA/PCMC/GO/bentonite, PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC) in distilled water and 0.9% NaCl solution (a) and the effect of pH on the swelling ratio of the prepared hydrogels.

3.6. Adsorption of MB

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3.6.1 Adsorption kinetics

As adsorption kinetics can be applied to describe the adsorption rate and eventually

CC

to explore the mechanism and possible rate-controlling steps of dye adsorption, in this study, the adsorption kinetics of MB were investigated at 30, 40 and 50 °C,

A

respectively. As shown in Fig. 5a-c, it was apparent that the adsorption capacity rapidly increased at beginning and then gradually slowed down until equilibrium. This may be ascribed to the higher MB concentration and abundant free adsorption sites available at initial adsorption phase (Dai & Huang, 2017b). In addition, the adsorption capacity of the prepared hydrogels increased from 30 to 40 °C. and then decreased with further increase to 50 °C. Taking PVA/PCMC/GO/bentonite for example, after 18

adsorption equilibrium, the adsorption capacity at 30, 40 and 50 °C was 136.56, 157.50 and 128.15 mg/g, respectively. Moreover, the adsorption capacity exhibited a slight decrease with contact time at 50 °C. The possible explanation was that the high temperature increased the mobility of the large dye ions during the adsorption process, resulting in a decrease in the adsorption capacity. Similar results were also reported in chitosan-g-poly(acrylic acid)/montmorillonite superadsorbent nanocomposite (Wang,

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Wang & Zhang, 2008). As found previously, PVA/PCMC/GO/bentonite showed the highest adsorption capacity due to the introductions of GO and bentonite. Compared

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with PVA/PCMC, PVA/PCMC/GO and PVA/PCMC/bentonite also exhibited increased adsorption capacity due to the introduction of GO or bentonite. Taking adsorption

at

30

°C

for

example,

the

MB

adsorption

capacities

of

U

PVA/PCMC/GO/bentonite, PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC

N

were 136.56, 76.13, 127.23 and 65.83 mg/g, respectively.

A

To investigate the mechanism of the adsorption process, the kinetic experiment data

models (Eq. 5) as below:

M

were analyzed by the pseudo-first-order (Eq. 4) and pseudo-second-order kinetic

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Log (Qe  Qt )  LogQe 

k1t 2.303

t 1 1   t 2 Qt k2Qe Qe

(4) (5)

EP

where Qe (mg/g) and Qt (mg/g) are the MB adsorption capacity of the prepared hydrogels at equilibrium and at time t (min), respectively. k1 (min-1) and k2 (mg/g

CC

min-1) are the rate constants of the pseudo-first-order and pseudo-second-order kinetic models, respectively. The fitted plots of pseudo-first-order and pseudo-second-order

A

kinetic models are displayed in Fig. 5d-f and Fig. 5h-j, respectively. The kinetic parameters for the two models were obtained by linear regression of the plots, and the results are listed in Table 1. It was observed that the correlation coefficients (R2) obtained from the pseudo-second-order model was higher than that of the pseudo-first-order model,

indicating a better

fit

of kinetic data to the

pseudo-second-order model. Additionally, the Qe,cal from the pseudo-second-order 19

model agreed well with the experimental equilibrium amount of absorbed MB (Qe,exp), further demonstrating the domination of the pseudo-second-order model. Moreover, the rate constant k2 increased with the increase of the temperature, suggesting the faster adsorption rate at high temperature. These results suggested that the MB adsorption onto the prepared hydrogels was presumably controlled by a chemisorption

functional groups of the hydrogels (Dai & Huang, 2017b; Li et al., 2017).

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process involving exchange or sharing of electrons mainly between dye cations and

Table 1. Kinetic parameters of the pseudo-first-order and pseudo-second-order kinetic models for

Pseudo-first-order kinetic

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MB adsorption of the prepared hydrogels.

Pseudo-second-order kinetic

Qe,exp (mg/g)

Qe,cal

R2

k1

R2 -1

Qe,cal

k2

(mg/g)

(mg/g min-1)

(mg/g)

(min )

0.0226

0.9989

153.85

2.85×10-4

0.0249

0.9988

84.75

5.53×10-4

U

Hydrogels

30 °C 136.56

0.9836

106.73

PVA/PCMC/GO

76.13

0.9978

60.53

PVA/PCMC/bentonite

127.23

0.9968

109.04

0.0267

0.9984

142.86

3.21×10-4

PVA/PCMC

65.83

0.9826

46.62

0.0295

0.9986

71.43

8.39×10-4

PVA/PCMC/GO/bento

157.50

0.9885

99.31

0.0352

0.9997

166.67

6.04×10-4

PVA/PCMC/GO

104.54

0.998

72.28

0.0424

0.9981

108.70

1.22×10-3

PVA/PCMC/bentonite

135.56

0.9923

112.25

0.0426

0.9988

144.93

6.06×10-4

PVA/PCMC

93.38

0.9955

72.51

0.0392

0.9993

100.00

1.03×10-3

EP

50 °C

A

M

TE D

40 °C

N

PVA/PCMC/GO/bento

130.69

0.9433

67.16

0.0362

0.9985

136.99

9.90×10-4

PVA/PCMC/GO

76.51

0.9875

67.38

0.0744

0.9950

74.63

6.37×10-3

PVA/PCMC/bentonite

122.36

0.9949

104.91

0.0530

0.9967

129.87

9.63×10-4

PVA/PCMC

62.57

0.9979

55.72

0.0949

0.9972

64.52

4.34×10-3

A

CC

PVA/PCMC/GO/bento

20

b

PVA/PCMC/GO/bentonite PVA/PCMC/GO PVA/PCMC/bentonite PVA/PCMC

180 150 120 90 60 30

210

c

40 °C

180 150 120

0

90 60

PVA/PCMC/GO/bentonite PVA/PCMC/GO PVA/PCMC/bentonite PVA/PCMC

30

50

100

150

200

250

30 °C

PVA/PCMC/GO/bentonite PVA/PCMC/GO PVA/PCMC/bentonite PVA/PCMC

e

40

80

120

160

40 °C

2.0

PVA/PCMC/GO/bentonite PVA/PCMC/GO PVA/PCMC/bentonite PVA/PCMC

0

f

PVA/PCMC/GO/bentonite PVA/PCMC/GO PVA/PCMC/bentonite PVA/PCMC

3.0

t/Qt

1.0

0.5

0.5

120

160

200

240

0

30

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80

t (min)

120

150

PVA/PCMC/GO/bentonite PVA/PCMC/GO PVA/PCMC/bentonite PVA/PCMC

1.0

0

60

90

3.0

10

20

30

40

50

60

t (min)

50 °C

2.5

N

PVA/PCMC/GO/bentonite PVA/PCMC/GO PVA/PCMC/bentonite PVA/PCMC

0.0 40

80

j

1.5

1.0

60

40 °C

2.0

0.0

40

t (min)

2.5

1.5

0

20

A

2.0

90

0.0

0

i

50 °C

2.0

U

30 °C

2.5

60

0.5

PVA/PCMC/GO/bentonite PVA/PCMC/GO PVA/PCMC/bentonite PVA/PCMC

2.0

t/Qt

80 100 120 140 160

t (min)

3.0

30

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1.0

1.5 1.0

M

60

30

1.5

0.0 40

60

Time (min)

0.5

20

90

200

log (Qe-Qt)

log (Qe-Qt)

log (Qe-Qt)

1.0

0.0

t/Qt

120

1.5

0.5

h

150

Time (min)

1.5

0

PVA/PCMC/GO/bentonite PVA/PCMC/GO PVA/PCMC/bentonite PVA/PCMC

0 0

Time (min) 2.0

50 °C

180

0 0

d

210

IP T

30 °C

MB adsorption ability (mg/g)

210

MB adsorption ability (mg/g)

MB adsorption ability (mg/g)

a

0.5 0.0 120

150

t (min)

180

0

30

60

90

120

150

t (min)

Fig. 5. Adsorption kinetics of MB onto the prepared hydrogels at 30 °C (a), 40 °C (b) and 50 °C (c); the corresponding kinetic plots of Pseudo-first-order (d-f) and pseudo-second-order (h-j)

EP

models for the adsorption of MB at 30, 40 and 50 °C.

3.6.2. Effects of pH and initial MB concentration

CC

The pH value plays a key role in on the MB adsorption process, which can

influence the surface charge of an adsorbent and dissociation of functional groups on

A

its active sites (Auta & Hameed, 2014). Fig. 6a illustrates the effect of pH on the MB adsorption capacity of the prepared hydrogels. For all prepared hydrogels, the adsorption capacity of MB was found to increase gradually with increased pH from 2.0 to 10.0. At low pH, the surface charge of the hydrogels became positively charged due to the protonation of -OH and -COOH, thus making (H+) ions compete effectively with dye cations and decreased the adsorption capacity owing to electrostatic 21

repulsion force (Mahdavinia et al., 2017). While at high pH, more negatively charged surface of the hydrogels was available due to the deprotonation of -OH and -COOH, resulting in an enhanced electrostatic attraction and facilitating greater MB adsorption capacity (Sahraei, Pour & Ghaemy, 2016). Similar results were also found in the PVA/laponite RD hydrogels (Mahdavinia et al., 2017) and konjac glucomannan/GO hydrogels (Gan et al., 2015) for the application of MB adsorption. Fig. 6b depicts the

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effect of initial MB concentration on the MB adsorption ability of the prepared hydrogels. As can be observed from Fig. 6b, the adsorption capacity increased

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significantly with the increase of the initial MB concentration. As MB concentration increased from 50 to 250 mg/L, the adsorption capacity of PVA/PCMC/GO/bentonite, PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC increased from 33.59 to

U

165.73, 30.04 to 85.23, 32.94 to 149.67, and 26.13 to 75.67 mg/g, respectively. The

N

higher MB concentration would provide enhanced driving force to overcome the mass

A

transfer resistance of dye molecules from aqueous phase to solid phase (Pourjavadi et al., 2016). Fig. 6d shows the picture of MB solution (50 mg/L) before and after

M

adsorption by the prepared hydrogels (P/P/G/b, P/P/G, P/P/b and P/P represents

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PVA/PCMC/GO/bentonite, PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC, respectively). It can be clearly observed that the color of solution with PVA/PCMC/GO/bentonite

turned

to

transparent

significantly,

especially

in

comparation with PVA/PCMC, indicating the beneficial effects of GO and bentonite

EP

on MB adsorption. Fig. 6e depicts the color change of MB solution (50 ~ 250 mg/L) after the adsorption by PVA/PCMC/GO/bentonite. The high surface area and

CC

abundant functional groups such as carboxyl, hydroxyl and epoxy groups can be obtained by the additions of GO and bentonite during the hydrogel preparation

A

(Bhattacharyya & Ray, 2015; Gan et al., 2015), which was also confirmed in the SEM and FTIR results. The porous crosslinked structure in PVA/PCMC/GO/bentonite and PVA/PCMC/bentonite (see Fig. 2) would be beneficial for adsorption process since it increases surface area and facilitates diffusion of dyes into the adsorbent (Pourjavadi et al., 2016). 3.6.3. Reusability of the prepared hydrogels 22

Generally, besides high adsorption capacity, a promising adsorbent also requires good reusability to meet the demand of low-cost (Pourjavadi et al., 2016). As can been seen from Fig. 6c, after four cycles of the desorption-adsorption process, the prepared hydrogels still retained high adsorption capacity in comparation with the first cycle, indicating their good reusability for MB adsorption. Based on the above discussion, the prepared hydrogels can be applied as stable, low cost, and effective

200

MB adsorption ability (mg/g)

a

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adsorbent for MB removal.

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160 120 80

80 40 0

50

U

250

A

EP

CC

90

100 150 200 MB concentration (mg/L) 1th cycle 2nd cycle 3rd cycle 4th cycle

150 120

10

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MB adsorption ability (mg/g)

120

Swelling capacity (mg/g)

8

N

6 pH

PVA/PCMC/GO/bentonite PVA/PCMC/GO PVA/PCMC/bentonite PVA/PCMC

160

c

4

A

2

200

M

40 0

b

PVA/PCMC/GO/bentonite PVA/PCMC/GO PVA/PCMC/bentonite PVA/PCMC

60 30 0

PVA/PCMC/ PVA/PCMC/ PVA/PCMC/ PVA/PCMC GO/bentonite GO bentonite

Fig. 6. The effects of pH (a) and initial MB concentration (b) on the MB adsorption ability of the prepared hydrogels; the reusability of the prepared hydrogels for MB adsorption (c); the images of 50 mg/L MB solution before and after adsorption by the prepared hydrogels (d) and the images of different MB concentration solution before and after adsorption by PVA/PCMC/GO/bentonite 23

(P/P/G/b) (e).

3.6.4 Adsorption isotherm In this study, Langmuir and Freundlich isotherm models were applied to determine the MB adsorption isotherm parameters on the prepared hydrogels. Langmuir isotherm assumes that adsorption occurs as monolayer coverage of adsorbate on homogeneous adsorbent surfaces with a finite number of adsorption sites (Li et al.,

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2017), whereas Freundlich isotherm considers that adsorption occurs as multilayer coverage of adsorbate on heterogenous surfaces (Sahraei, Pour & Ghaemy, 2016).

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The nonlinear form of Freundlich and Langmuir isotherm models can be expressed as Eq. 6 and Eq. 7, respectively: Ce C 1  e  Qe Qmax K L Qmax

U

1 ln Ce n

(7)

N

ln Qe  ln K F 

(6)

A

where Qe (mg/g) and Ce (mg/L) are the equilibrium adsorption capacity and the

M

equilibrium dye concentration, respectively. Qm (mg/g) is the maximum adsorption capacity. KL (L/mg) is a constant related to the affinity of binding sites. KF ((mg/g)

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(L/mg)1/n) and 1/n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. Based on the data from Fig. 6b, the all expressions for Langmuir and Freundlich isotherm models were calculated and summarized in

EP

Table 2. By comparing the R2 values listed in Table 2, adsorption of MB onto the prepared hydrogels could be better fitted to Langmuir isotherm model than Freundlich

CC

model, confirming the monolayer adsorption of MB onto the hydrogels surface. Based on Langmuir isotherm model, the maximum MB adsorption capacity of

A

PVA/PCMC/GO/bentonite, PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC was 172.41, 89.29, 163.93 and 83.33 mg/g, respectively. For Langmuir isotherm model, the main characteristic of Langmuir isotherm model can be described by a dimensionless constant adsorption parameter (RL), and can be expressed as follows: RL 

1 1  K LC0

(8)

24

where KL is the Langmuir constant (L/mg) and C0 is the initial concentration of MB solution. The value of RL implies the adsorption to be either unfavorable (RL > 1), liner (RL = 1), favorable (0 < RL < 1) or irreversible (RL< 0) (Mahdavinia et al., 2017). Taking adsorption at 200 mg/L of initial MB concentration for example, the values of RL for PVA/PCMC/GO/bentonite, PVA/PCMC/GO, PVA/PCMC/bentonite and PVA/PCMC were 0.0393, 0.0627, 0.0877 and 0.1075, respectively, indicating that

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adsorption of MB on the prepared hydrogels was favorable.

Table 2. The parameters of Langmuir and Freundlich isotherm models for MB adsorption by the

R2

Freundich Qmax

KL(L/mg)

(mg/g) 0.9654

172.41

0.1221

PVA/PCMC/GO

0.9933

89.29

0.0697

PVA/PCMC/bentonite

0.9666

163.93

0.0520

PVA/PCMC

0.9992

83.33

A

((mg/g)

(L/mg) )

n

0.948

4.4863

59.42

0.990

3.9432

22.81

0.988

2.5050

22.52

0.940

2.8369

12.47

M

4. Conclusion

0.0415

KF

1/n

N

PVA/PCMC/GO/bentonite

R2

U

Langmuir Hydrogels

SC R

prepared hydrogels.

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In this study, novel eco-friendly PVP/PCMC/GO/bentonite hydrogels were synthesized by an easily and green freeze-thaw cycles and evaluated as a green adsorbent for the efficient removal of MB. The FTIR and XRD results implied the strong interactions among PCMC, PVA, GO and bentonite due to the formation of

EP

hydrogen bonding. The SEM analysis indicated that introducing GO and/or bentonite into hydrogel networks promoted formation of porous structure. The thermal analysis

CC

verified the positive effects of GO and bentonite on the enhancement of thermal stability of the hydrogels. The prepared hydrogels showed evident pH sensitivity and

A

swelled more due to the introductions of GO and bentonite. The MB adsorption capacity of the prepared hydrogels was significantly influenced by solution pH, initial MB concentration, contact time and temperature. The introductions of GO and bentonite considerably enhanced adsorption ability for MB. Adsorption of MB onto the prepared hydrogels could be well described by pseudo-second-order kinetic model and Langmuir isotherm model. The prepared hydrogels showed high resilience on MB 25

adsorption capacity after four adsorption-desorption cycles. Consequently, the prepared hydrogels, especially PVP/PCMC/GO/bentonite, can be applied as stable, eco-friendly, efficient and reusable adsorbents for wastewater treatment. Acknowledgements This work was supported by National Natural Science Foundation of China under grant number 31471673 and 31271978.

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