polyacrylamide (PAM) composite hydrogels as efficient cationic dye adsorbents

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Graphene oxide (GO)/polyacrylamide (PAM) composite hydrogels as efficient cationic dye adsorbents Yuyan Yang, Shasha Song ∗ , Zengdian Zhao ∗ School of Chemical Engineering, Shandong University of Technology, Zibo, 255000, PR China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• PAM/GO composite hydrogels were obtained by mixing appropriate amounts of PAM and GO in water. • PAM/GO hydrogels were formed driven by various kinds of noncovalent interaction. • The composite hydrogels exhibited high adsorption efficiency and capability for the cationic dyes.

a r t i c l e

i n f o

Article history: Received 19 August 2016 Received in revised form 8 October 2016 Accepted 27 October 2016 Available online xxx Keywords: Graphene composite hydrogels Porous networks Rheological properties Cationic dye adsorbents

a b s t r a c t Composite hydrogels were prepared in the mixtures of graphene oxide (GO) and polyacrylamide (PAM) in water. The gelation behavior of the hydrogels was studied in detail. The formation of GO/PAM composite hydrogels were induced by various kinds of noncovalent interactions between GO sheets and PAM, including hydrogen bonding, electrostatic interaction, hydrophobic interaction, van der Waals force, the ␲-␲ stacking of GO sheets, and etc. Microstructures determined by TEM and SEM demonstrated that GO/PAM hydrogels displayed three-dimensional network structures, and the network of the gels gradually changed regular with the increasing concentration of GO at a fixed concentration of PAM (cPAM ), conversely, with the increase in PAM concentration at a fixed concentration of GO (cGO ), the hole-structures of gels become irregular. Similarly, at the fixed molar ratio of GO to PAM of 1:1, with the increasing total concentration, the hole-structures of gels also become irregular. The cationic dye molecules, i.e., Methylene Blue (MB) and Rhodamine 6G were efficiently entrapped to the xerogels within 20 and 60 min with the maximum adsorption value of 292.84 and 288 mg g−1 , respectively. The xerogels exhibited excellent adsorption efficiency and capability for the cationic dyes and were promising to act as toxic materials adsorbents. The adsorption kinetics of dye followed the pseudo-second order model and the dye adsorption isotherm follows both Langmuir and Freundlich adsorption isotherms model. © 2016 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding authors. E-mail addresses: [email protected] (S. Song), [email protected] (Z. Zhao).

As a kind of novel polymer materials, polymer hydrogels have been widely applied in drug delivery systems, biosensors, sustained drug-release, and etc. [1–5] However, its poor mechanical strength (toughness and crack resistance) greatly limited their applications [6–9]. In the past decade, many investigators have been devoted to improving the mechanical performance of the hydrogels. Added

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nanofillers (such as silica, carbon nanotubes) to the hydrogels is considered to be an effective way to enhance the mechanical strength of the hydrogels. Since the graphene was first reported in 2004, it has been widely investigated due to its excellent electrical conductivity, thermal stability, mechanical strength and adsorption capacity. Thus, grapheme should be an excellent additive to enhance the mechanical strength of the hydrogels [10–13]. For traditional hydrogel, three-dimensional (3D) network was usually formed by one-dimensional (1D) molecules aggregation (e.g. nanofibers and tubes). However, the framework of graphene-based hydrogels was different from traditional hydrogel since to the two-dimensional (2D) lattice structure of GO, and the various oxygen-containing groups (hydroxyl, epoxide groups and carboxyl groups etc.) on the basal planes and edges of modified graphene. Meanwhile, these oxygen-containing groups impart GO strong interaction with polar small molecules or polymers to form hydrogels. Therefore, the graphene composite hydrogel was always synthesized by the crosslinking of polymer chains driven by various kinds of noncovalent bonds (i.e., hydrogen bonding) [14–16]. Compared to traditional hydrogels, the graphene composite hydrogels not only have high mechanical strength and toughness, but also possess outstanding electrical and thermal performance, which immensely broaden the hydrogels applications [17–20]. Li et al. used GO as 2D macromolecules prepared GO/PAM composite hydrogels. They demonstrated that the mechanical properties of the GO/PAM hydrogels were significantly improved [21]. Among the dye removal techniques from aqueous solutions, adsorption has received much attention in industial applications. The adsorption of dyes was studied previously using various adsorbents such as sepiolite and expanded vermiculite [22–25]. Due to low adsorption performances of these adsorbents, researches have been continued for effective adsorbents having reasonable adsorption efficiencies. Large specific surface and oxygen-containing functional groups impart GO a bright application in sewage treatment. In this article, GO/polyacrylamide (PAM) composite hydrogel was prepared, in which the PAM acted as the cross-linker and the main driven force was various noncovalent interactions. The effect of the concentration of PAM and GO to the gelation were studied in detail. This work demonstrated that GO/PAM hydrogels exhibited excellent mechanical strength and rapid dye adsorption capability. It can be expected that GO/PAM hydrogels with excellent mechanical properties could play more important role in electrochemical field.

GO dispersion solution preparation: GO was prepared from natural flake graphite by a modified Hummers method [26,27] and purified by dialysis for one week to remove the impurities. Then the GO was exfoliated by sonication for 4 h and dispersed into aqueous solution. Thus, different concentrations of GO sheets solution were obtained. GO/PAM composite hydrogels preparation: PAM solution and GO dispersion at a certain ratio were mixed together and shaken violently on the vortex oscillator to forminghomogeneous GO/PAM hydrogels. The composite gels were kept at room temperature for about 2 weeks before the characterization. 2.3. Scanning electron microscopy (SEM) observations A drop of gel sample was placed on a silica wafer, and most of the colloid gel was removed using small forceps to form a thin film. The wafer was freeze-dried in a vacuum extractor at −40 ◦ C for few days and was observed on FEI Sirion 200 (U.S.) SEM at 10.0 kV. 2.4. Transmission electron microscopy (TEM) observations For transmission electron microscopy (TEM) observations, a small volume of gel sample was placed on a TEM grid and the excess solution was wicked away with filter paper.Then the copper grid was observed on a Tecnai G2 F20 S-TWIN (U.S.) TEM operating at 120 kV. 2.5. The X-ray photoelectron spectroscopy (XPS) The X-ray photoelectron spectrum (XPS) was conducted on an X-ray photoelectron spectrometer using an Mg−K␣ radiation exciting source (AXIS ULTRA DLD, Kratos). 2.6. Fourier transform infrared (FT-IR) spectroscopy Fourier transform infrared (FT-IR) spectrums of samples were obtained from a Nicolet 5700 FT-IR spectrometer (Theermo Electron, America). The gel samples were mixed with KBr and pressed into a plate for the measurements. 2.7. X-ray diffraction (XRD) measurements X-ray diffraction (XRD) patterns were recorded on a Bruker AXS D8 Advance (Germany) X-ray diffractometer with Cu K␣ radiation (␭ = 0.15418 nm) and a graphite monochromator. 2.8. Rheological measurements

2. Experimental sections 2.1. Chemicals and materials Natural flake graphite (99%) with average particle sizes of 300 mesh was purchased from Qingdao Tianheda Graphite Co., Ltd. Sulfuric acid (98 wt%) and hydrogen peroxide (30 wt%) were obtained from Zhiyuan (Tianjin) chemical reagent Co., Ltd. Sodium carbonate (99.8%) and polyacrylamide (PAM) with molar mass more than 5 × 105 g mol−1 were purchased from Kermel (Tianjin) Chemical reagent Co., Ltd. Ultrapure water (18.25 M cm) was obtained using a UPH-IV ultrapure water purifier (China).

The rheological measurements of hydrogels were performed on a HAAKE RS6000 rheometer with a cone-plate system (C35/1◦ Ti L07116 at 25.0 ± 0.1 ◦ C, diameter 35 mm; cone angle 1◦ ). The viscoelastic properties of the hydrogels were determined by using oscillatory measurements with an amplitude sweep in the frequency range of 0.01–10 Hz. Prior to the frequency sweep, the linear viscoelastic region was determined by stress-sweep measurements. The thixotropic property of the hydrogels was performed by steady shear experiment with shear rate range of 0.001–1000 s−1 , and then the shear rate decrease from 1000 to 0.001 s−1 . 2.9. Dye adsorption tests

2.2. Samples preparation PAM solution preparation: a certain amount of PAM was added to water, and stirred continuously at room temperature until dissolved completely.

For MB adsorption process, 0.044 g xerogels was submerged in 200 mL 72.24 mg L−1 MB solution and kept shocking. A series of concentration variation of dye within 20 min was monitored by UV–vis spectra. Similarly, 0.005 g xerogels was submerged in 200 mL 8.6 mg L−1 Rhodamine 6G solution and kept shocking. With

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the change of time, a series of concentration variation of dye within 60 min was monitored by UV–vis spectra. The whole adsorption process was conducted at room temperature. 2.10. UV–vis spectra measurements The UV–vis spectra measurements were performed on a HITACHIU-4100 spectrophotometer. The scan rate for each measurement was 200 nm min−1 . 3. Results and discussion 3.1. Gelation behavior Graphene oxide (GO) have plenty of oxygen-containing groups, which can be used as a gelator to prepare composite hydrogel. Polyacrylamide (PAM), a soluble polymer, can be used as a crosslinker to form composite hydrogels with other gelator. In this paper, GO/PAM composite hydrogels were obtained by mixing appropriate amounts of PAM and GO solution in water, and the photo of the sample was shown in Fig. 1. For hydrogels, the gelation capability can be expressed by “critical gelation concentration (CGC)”, which can be defined as either the minimum amounts of gelators required to gelate 1 mL water or the minimum mass fraction of the gelators for the gel formation [28]. In this paper, critical gelation concentration (CGC) was 1 mg mL−1 GO/2 mg mL−1 PAM (Fig. 1a), which displayed superior gelation capability. For further detailed study, typical samples were selected. One was at a fixed PAM (GO) concentration with different amounts of GO (PAM), whereas another is at the fixed molar ratio of GO to PAM of 1:1 with various total concentration. 3.2. Characterization of GO As shown in Fig. 2, the obtained GO was single or several layers, and some of which fold to induce wrinkles. The oxygen-containing groups (hydroxyl, epoxide groups, carboxyl groups, etc.) on the GO could alter the van der Waals force between the layers, which impart it good water solubility. Various hydrophilic oxygenated functional groups rendered GO could disperse in water stably for several months without precipitation appeared [29–31]. The amounts of functional groups of GO was examined by X-ray photoelectron spectroscopy (XPS). The peaks of C 1s and O 1s were observed in GO (Fig. 3a). The C 1s XPS signal in GO sample clearly indicated the presence of the nonoxygenated ring C (∼284.6 eV), the C atom in C O bond (hydroxyl and epoxy, ∼286.2 eV), the carbonyl C (∼287.8 eV), and the carboxylate carbon (O C O) (∼289.0 eV) (Fig. 3b). 3.3. Microstructures of the GO/PAM hydrogels GO sheets were randomly orientated in solution, when PAM was added, PAM induced the GO sheets to aggregate in an energetically favorable layer-by-layer manner to form hydrogels. The SEM images (Fig. 4) of the GO/PAM composite hydrogels reveal that the hydrogels are composed of 3D porous networks. At a fixed cPAM = 2 mg mL−1 , the GO concentrations increased from 1 mg mL−1 to 4 mg mL−1 , the holes of the network of the gels gradually changed regular (Fig. 4a–d). With increasing PAM concentration from 2 mg mL−1 to 4 mg mL−1 at constant GO concentration of 4 mg mL−1 (Fig. 4d–f), the hole-structures of the gels become irregular, which may be due to the weak repulsion forces among GO layers. At the fixed molar ratio of GO to PAM of 1:1, with the total concentration variation from 2 mg mL−1 to 4 mg mL−1 , the holestructures of the gels also become irregular (Fig. 4b and f).

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3.4. Rheological properties of GO/PAM hydrogels Rheological property is important for the applications of gels, which can be evaluated by measuring the viscoelasticity and the mechanical strength of the gels. The solid-like network structure of the gels that are sheared under an increasing stress will break suddenly at a critical shear stress, ␶c , which is the so-called “yield stress”. The yield stress could reflect the strength of the microstructure of the gels. The influence of gelator concentration on the mechanical strength of the hydrogels were studied (Fig. 5). From Fig. 5a and d, one can see that at a fixed cGO : cPAM = 1:1, the yield stress increased gradually, ranging from 17 to 114.2 Pa with the increase of total concentration from 2 to 5 mg mL−1 , revealing the improvement in mechanical strength. The mechanical strength variation of the hydrogels with the increase of GO concentration at a fixed cPAM = 2 mg mL−1 was shown in Fig. 5b and e. The yield stress elevated from 14.35 to 187.5 Pa with GO concentrations increased from 1 mg mL−1 to 5 mg mL−1 , indicating the increased GO amounts effectively enhanced mechanical strength of gels. As shown in Fig. 5c, at a fixed GO concentration of 1 mg mL−1 , when cPAM = 2 mg mL−1 , the yield stress of the gel was 16.5 Pa, which was much higher than that at cPAM = 4 mg mL−1 (10.1 Pa), implying that much PAM was unfavorable to enhance mechanical strength. Oscillation frequency was utilized to determine the viscoelasticity of the hydrogels. As can be seen in Fig. 6, the elastic modulus (G’) is much higher than viscous modulus (G”) over the studied frequency region, implying that elastic property is dominant [32,33]. As shown in Fig. 6a and 6c, at the concentration of 2 mg mL−1 GO/2 mg mL−1 PAM, the value of G’ and G” was 80 Pa and 13 Pa, implying theweak viscoelasticity. With the gradually increase of total concentration (GO:PAM = 1:1), G’ and G” reached about 7400 Pa and 700 Pa, respectively, at 5 mg mL−1 GO/5 mg mL−1 PAM, implying synergistic effect of the more rigid network structure [34,35]. At a fixed cPAM = 2 mg mL−1 , G’ and G” was about 30 and 5 Pa at cGO = 1 mg mL−1 , respectively, when cGO increased to 5 mg mL−1 , G’ and G” increased to about 15,000 and 1130 Pa, respectively, being hundreds of times those at cGO = 1 mg mL−1 , meaning the greatly enhancement of the mechanical strength of the gels. Fig. 6e shows the similar rheological behavior of two selected gels at a fixed cGO = 1 mg mL−1 . At at cPAM = 2 mg mL−1 G’ and G” reached about 30 Pa and 5 Pa, respectively, implying the rigid network structure. With PAM concentration increasing to 4 mg mL−1 , the more compact network structure induces the increase of G’ and G”, reaching about 80 Pa and 10 Pa, respectively. The thixotropic property of gels is one of the important research content in the field of rheology [36–38]. In order to explore the thixotropic property of the hydrogels, steady shear experiment was performed. From Fig. 7, one can see the obvious hysteresis loop, which indicates that GO/PAM composite hydrogels have mechanical responsiveness property. The viscosity of the hydrogel gradually declined with the increase of shear rate, and which could gradually restore the original state again when the shear rate decreased gradually (Fig. 7). A probable reason may be that the introduction of extra energy (shear force) broke the balance of the multiple noncovalent interactions in hydrogels, leading to the destroying of the network structure of gels. When energy decreased, PAM and GO formed gels again due to restore of the interactions between PAM and GO. It can be concluded that the prepared GO/PAM hydrogel was a mechanical-responsive composite hydrogels, which supply promising opportunities for all biological systems down to the level of tissues and cells [39]. 3.5. Formation mechanism of GO/PAM hydrogels FT-IR spectrum is a powerful method to explore the gel formation mechanism [40]. Fig. 8a (curve 1) shows the FT-IR spectrum of

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Fig. 1. Photos of typical hydrogels samples: (a) 1 mg mL−1 GO/2 mg mL−1 PAM; (b) 2 mg mL−1 GO/2 mg mL−1 PAM; (c) 3 mg mL−1 GO/2 mg mL−1 PAM; (d) 4 mg mL−1 GO/2 mg mL−1 PAM.

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Fig. 2. TEM image of GO.

Fig. 3. (a) XPS survey spectra of GO. (b) High-resolution XPS spectra of C 1s peaks for GO.

Fig. 4. SEM images of freeze-dried samples: (a) 1 mg mL−1 GO/2 mg mL−1 PAM; (b) 2 mg mL−1 GO/2 mg mL−1 PAM; (c) 3 mg mL−1 GO/2 mg mL−1 PAM; (d) 4 mg mL−1 GO/2 mg mL−1 PAM; (e) 4 mg mL−1 GO/3 mg mL−1 PAM; and (f) 4 mg mL−1 GO/4 mg mL−1 PAM.

GO sheets, in which the evident bands at 3420, 1730, 1630, 1395.5 and 1069.7 cm−1 was attributed to the symmetric and asymmetric stretching vibration of O H, the stretching vibration of carbonyl group, the deformation vibration of O H, the bending vibration of O H and the stretching vibration of the C O bond in epoxide group, respectively [41,42]. For PAM (Fig. 8a curve 5), the strongest peaks at 3424.3 and 1700 cm−1 were ascribed to N−H stretching and C O carbonyl stretching, respectively. The vibrational bands

of amide I and amide II modes appeared at 1650 and 1548 cm−1 [43]. For PAM/GO gels (Fig. 8a curve 2,3,4), the carbonyl stretching peaks of GO at 1730 cm−1 disappeared and a new peak at 1660 cm−1 appeared, which clearly indicated that the COOH of GO was converted into COO− with NH3 + of PAM as the counterions. In addition to electrostatic interaction, hydrogen bonds between the OH, the epoxy groups of GO and the −NH2 and C O of PAM also existed in the composite hydrogels. In Fig. 8a curve

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Fig. 5. Stress sweeps: Elastic modulus (G’) of gels (a) at cGO : cPAM = 1:1 with different total concentration (mg mL−1 ), (b) at cPAM = 2 mg mL−1 with different cGO (mg mL−1 ), and (c) at cGO = 1 mg mL−1 with different cPAM (mg mL−1 ). (d) ␶c as the function of gel concentration at cGO : cPAM = 1:1, and (e) as the function of GO concentration at cPAM = 2 mg mL−1 .

Fig. 6. Oscillation frequency scanning: the elastic modulus (G’) of hydrogels (a) at cGO : cPAM = 1:1 with different total concentration (mg mL−1 ), and (b) at cPAM = 2 mg mL−1 with different cGO (mg mL−1 ); the viscous modulus (G”) of the hydrogels (c) at cGO : cPAM = 1:1 with different total concentration (mg mL−1 ), and (d) at cPAM = 2 mg mL−1 with different cGO (mg mL−1 ); (e) G’ (solid symbols) and G” (open symbols) as a function of hydrogels: 1 mg mL−1 GO/2 mg mL−1 PAM (square symbols) and 1 mg mL−1 GO/4 mg mL−1 PAM (round symbols).

2,3,4, at fixed cPAM = 2 mg mL−1 with cGO from 2 to 4 mg mL−1 , the C−O stretching (1069.7 cm−1 ) in epoxide group gradually became weaker and the O H bending (1395.5 cm−1 ) of GO shifted to the higher wavenumber, which suggested the hydrogen bonds formation.

In order to further explore the formation mechanism of GO/PAM hydrogels, the X-ray diffraction (XRD) experiment was carried out. As shown in Fig. 8b curve 2,3,4, GO/PAM hydrogels had a peak at the angle range of 5◦ –12◦ . According to Bragg’s law, the peak of pure GO at 2␪ = 10.9◦ was calculated to about 1.62 nm, which might be

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Fig. 7. Steady shear: viscosity as a function of shear rate of (a) 4 mg mL−1 GO/4 mg mL−1 PAM, (b) 5 mg mL−1 GO/5 mg mL−1 PAM.

Fig. 8. (a) FT–IR spectra of freeze-dried samples: curve 1) GO, curve 2) 2 mg mL−1 GO/2 mg mL−1 PAM, curve 3) 4 mg mL−1 GO/2 mg mL−1 PAM, curve 4) 4 mg mL−1 GO/4 mg mL−1 PAM, and curve 5) PAM; (b) XRD patterns: curve 1) GO, curve2) 2 mg mL−1 GO/2 mg mL−1 PAM, curve3) 4 mg mL−1 GO/2 mg mL−1 PAM, and curve4) 4 mg mL−1 GO/4 mg mL−1 PAM.

ascribed to the interplanar distance of GO sheets [44]. The reflection peak of GO/PAM hydrogels shifted to the smaller angle (the interplanar distance of GO sheets in the GO/PAM hydrogels increased), which might be attributed to the intercalation and the synergistic effect between PAM chains and GO sheets, leading to the GO/PAM hydrogels formation. The XRD results indicated that the stacking of GO sheets was partly prevented by the insert of PAM chains. Based on the FT-IR and XRD results, a reasonable formation mechanism of GO/PAM composite hydrogels was proposed and shown in Scheme 1. The various oxygen-containing groups (hydroxyl, epoxide groups and carboxyl groups etc.) were on the basal planes and edges of modified graphene. When PAM was added, the PAM molecules insert in the layers of GO, which partly prevented the stacking of GO sheets (XRD had explained). As shown in Scheme 1, the COOH group of GO can protonate −NH2 group of PAM to form NH3 + ···COO− ion pairs. Furthermore, besides the electrostatic interaction, there are three kinds of hydrogen bonds existed in the hydrogel. One is between the hydroxyl groups of GO and the carboxyl groups of PAM, and the other two exists between the protonated NH3 + of PAM and the carboxyl groups and the epoxy groups of GO, which has been proved by FT–IR. Combined with the ␲−␲ stacking of GO sheets and van der Waals force between PAM and GO, the 3D network of gels was constructed. 3.6. Dye adsorption studies The dye molecules were efficiently entrapped to the xerogels within 20 min and the bluish solution became nearly clear, as shown in Fig. 9b. From Fig. 9a and b, one can see that cMB decreased sharply after the addition of xerogels initially, and then decreases gradually, reaching an unchangeable value after 20 min (Fig. 9a). Combining the calibration curve of MB (Fig. S2), we can calculate the maximum adsorption value of GO/PAM xerogels was

292.84 mg g−1 , which is a better result compared with highest MB adsorption capacity (162.96 mg g−1 ) of the mixed oxide nanostructures [45]. The adsorption of Rhodamine 6G by the xerogels was also studied. Similarly, 0.005 g xerogels was submerged in 200 mL 8.6 mg L−1 Rhodamine 6G solution and kept shocking. From the UV–vis spectra (Fig. 9c), it is found that the adsorption efficiency is much lower than that of MB. Combined with the calibration curve of Rhodamine 6G (Fig. S2), we can conclude that almost all the dye content can be adsorbed within 60 min with a dye adsorption value of 288 mg g−1 , compared to 217.39 mg g−1 of titanium phosphate [46]. It could be concluded that the efficient adsorption of the cationic toxic dye molecules (MB and Rhodamine 6G) endowed GO/PAM hydrogels become an environmentally friendly water-purifying agent. Methylene blue (MB) and Rhodamine 6G are aromatic heterocyclic cationic dyes, and the chemical structures of them are shown in Scheme 2. The hydrogels could adsorb the dyes were mainly driven by hydrogen bonding, electrostatic interaction, the ␲–␲ stacking and Van der Waals force between dye and the GO/PAM hydrogel. It could be conclude that the efficient adsorption of the toxic dye molecules endowed the GO/PAM hydrogels become an environmentally friendly water-purifying agent. Lagergren’s pseudo-first order kinetic model is generally k1 t, where k1 is the rate expressed as: log(qe − qt ) = log qe − 2.303 constant of pseudo-first order adsorption (min−1 ) and qe and qt are the amount of dye adsorbed on adsorbent at equilibrium and at time t (mg g−1 ). The experimental adsorption data for MB and Rh 6G over the GO/PAM hydrogel using the Lagergren’s pseudofirst order rate equation in the linear form were shown in Fig. 10a and 10c. k1 were determined by fitting the Lagergren’s pseudosecond order rate equation. The Ho’s pseudo-second order kinetic model is expressed as: qtt = 1 2 + qte , where k2 is the rate conk2 qe

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Scheme 1. Schematic representation of the network structure of GO/PAM hydrogel.

Fig. 9. UV–vis spectra of aqueous solutions with GO/PAM gels at different times: (a) MB and (c) Rhodamine 6G. The concentration Variations at different times: (b) MB and (d) Rhodamine 6G.

CH3

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H3C N

S

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Scheme 2. The chemical Structures of Methylene Blue (MB) and Rhodamine 6G.

stant of pseudo-second order adsorption (g (mg min)−1 ) and qe and qt are the amounts of dye adsorbed on adsorbent at equilibrium, and at time t (mg g−1 ).The experimental adsorption data of the linear fits were shown in Fig. 10b and d. The slope and intercept of the linear fits were used to determine k2 . As can be seen in Fig. 10, the pseudo-first order model (Fig. 10a and c) does not fit with our experimental data and the pseudo-second order model (Fig. 10b and d) is more appropriate for describing the adsorption behaviors of the GO/PAM composite hydrogels prepared in present work.

The isotherm models of Langmuir and Freundlich were used to describe the equilibrium adsorption isotherm. The linearized Langmuir and Freundlich isotherm are shown: q1e = q K1 C + q1m m a e

and log qe = log Kf + lognCe . Where Ce (mg L−1 ) is the equilibrium concentration, qe (mg g−1 ) is the amount of dye adsorbed at equilibrium, and qm (mg g−1 ) and Ka (L mg−1 ) are Langmuir constants related to adsorption capacity and energy of adsorption, respectively. Kf (mg g−1 ) (dm3 mg−1 )1/n is the Freundlich adsorption constant and 1/n is a measure of the adsorption intensity. The Langmuir and Freundlich adsorption isotherms of MB and Rh6G are

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0.24

1/Ce

0.32

0.40

2.5

c 2.4

2.44

d

log qe

log qe

2.3 2.40

2.2

2.36

2.1 2.0

2.32 1.65

1.68

1.71

1.74

log Ce

1.77

1.80

0.4

0.5

0.6

log Ce

0.7

0.8

0.9

Fig. 11. The linearized Langmuir isotherm for (a) MB and (c) Rh 6G adsorption. The linearized Freundlich isotherm for (b) MB and (d) Rh 6G adsorption.

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shown in Fig. 11, which indicated both Langmuir and Freundlich adsorption isotherms showed good correlation. As already demonstrated by others [47], The MB and Rh6G adsorption isotherm follows both Langmuir and Freundlich adsorption isotherms model. 4. Conclusions In summary, composite hydrogel was obtained by the mixtures of GO and PAM in water. The main driving forces for the gels formation were hydrogen bonding, electrostatic interaction, van der Waals force, the ␲–␲ stacking and etc. SEM images showed that the hydrogels were composed of porous 3D network structure. The increase amounts of GO (PAM) at a fixed PAM (GO) concentration, or the increase of total concentration at the fixed molar ratio of GO to PAM of 1:1 significantly improved the viscoelastic of GO/PAM hydrogels. The adsorption kinetics of dye followed the pseudosecond order model and the dye adsorption isotherm follows both Langmuir and Freundlich adsorption isotherms model. The efficient adsorption of the toxic dyes demonstrated that GO/PAM hydrogel system is an environmentally friendly water-purifying agent. Acknowledgements This work is financially supported by the NSFC (21603123), the Opening Project of Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province (KF201405). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2016.10. 060. References [1] A. Wang, Y. Cui, J. Li, J. Hest, Fabrication of gelatin microgels by a cast strategy for controlled drug release, Adv. Funct. Mater. 22 (2012) 2673–2681. [2] Q. Zou, L. Zhang, X. Yan, A. Wang, G. Ma, J. Li, H. Möhwald, S. Mann, Multifunctional porous microspheres based on peptide–porphyrin hierarchical co-assembly, Angew. Chem. Int. Ed. 53 (2014) 2366–2370. [3] M. Du, W. Song, Y. Cui, Y. Yang, J. Li, Fabrication and biological application of nano-hydroxyapatite (nHA)/alginate (ALG) hydrogel as scaffolds, J. Mater. Chem. 21 (2011) 2228–2236. [4] P. Zhu, X. Yan, Y. Su, Y. Yang, J. Li, Solvent-induced structural transition of self-assembled dipeptide: from organogels to microcrystals, Chem. Eur. J. 16 (2010) 3176–3183. [5] X. Yan, Y. Cui, Q. He, K. Wang, J. Li, Organogels based on self-assembly of diphenylalanine peptide and their application to immobilize quantum dots, Chem. Mater. 20 (2008) 1522–1526. [6] J. Matson, C. Newcomb, R. Bitton, S. Stupp, Nanostructure-templated control of drug release from peptide amphiphile nanofiber gels, Soft Matter 8 (2013) 3586–3595. [7] R. Matten, T. Hoare, Injectable, in situ gelling, cyclodextrin-dextran hydrogels for the partitioning-driven release of hydrophobic drugs, J. Mater. Chem. B 2 (2014) 5157–5167. [8] E. Moysan, Y. González-Fernández, N. Lautram, J. Béjaud, G. Bastiat, J. Benoit, An innovative hydrogel of gemcitabine-loaded lipid nanocapsules: when the drug is a key player of the nanomedicine structure, Soft Matter 10 (2014) 1767–1777. ˜ [9] M. Rodrigue, A. Calpena, D. Amabilino, M. Garduno-Ramírez, L. Pérez-García, Supramolecular gels based on a gemini imidazolium amphiphile as molecular material for drug delivery, J. Mater. Chem. B 2 (2014) 5419–5429. [10] Y. Su, Y. Lv, Cheminform abstract: graphene and graphene oxides: recent advances in chemiluminescence and electrochemiluminescence, Cheminform 45 (2014) 29324–29339. [11] H. Cong, X. Ren, P. Wang, S. Yu, Macroscopic multifunctional graphene-based hydrogels and aerogels by a metal ion induced self-assembly process, ACS Nano 6 (2012) 2693–2703. [12] W. Hou, B. Tang, L. Lu, J. Sun, J. Wang, C. Qin, Preparation and physico-mechanical properties of amine-functionalized graphene/polyamide 6 nanocomposite fiber as a high performance material, RSC Adv. 4 (2014) 4848–4855. [13] F. Perreault, A. Faria, M. Elimelesh, Environmental applications of graphene-based nanomaterials, Cheminform 46 (2015) 5861–5896.

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