Superabsorbent Cellulose–Clay Nanocomposite Hydrogels for Highly

Oct 5, 2016 - Superabsorbent Cellulose−Clay Nanocomposite Hydrogels for. Highly Efficient Removal of Dye in Water. Na Peng,. †,‡. Danning Hu,. â...
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Research Article pubs.acs.org/journal/ascecg

Superabsorbent Cellulose−Clay Nanocomposite Hydrogels for Highly Efficient Removal of Dye in Water Na Peng,†,‡ Danning Hu,† Jian Zeng,‡ Yu Li,‡ Lei Liang,‡ and Chunyu Chang*,†,‡ †

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China Guangzhou Sugarcane Industry Research Institute, Guangzhou 510316, China

‡

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S Supporting Information *

ABSTRACT: Toxic dyes have threatened human health through the consumption of polluted water, so removal of dyes from wastewater has become a hot topic in both academic and industrial fields. Herein, we reported a kind of cellulose−clay hydrogel with superabsorbent properties, superior mechanical performance, and high dye removal efficiency. The main strategy for the preparation of superabsorbent hydrogels was chemical crosslinking of cellulose, carboxymethyl cellulose (CMC), and the intercalated clay in NaOH/urea aqueous solution. The as-prepared hydrogels exhibited high absorption capacity for methylene blue (MB) solution through a spontaneous physic-sorption process which fitted well with pseudo-second-order and Langmuir isotherm models. The maximum removal efficiencies of hydrogel samples for MB solutions with concentrations of 10 mg L−1 and 100 mg L−1 were 96.6% and 98%, respectively. These results demonstrated that cellulose−clay nanocomposite hydrogels were effective adsorbents for removal of MB dyes, which would provide a new platform for dye decontamination. KEYWORDS: Cellulose, Clay, Nanocomposite, Superabsorbent hydrogels, Dye removal

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INTRODUCTION Over 10,000 commercially available dyes are inevitably used in many industries, such as dyestuffs, textile, paper, plastics, rubber, tannery, paints, and cosmetics.1 More than 7 × 105 tons of dyes are produced annually worldwide, and approximately 12% of dyes are lost during manufacturing and processing operations, where about 20% of these lost dyes enter industrial wastewaters and cause environmental pollution problems.2 The removal of dye pollutants from wastewater has attracted considerable attention in recent years, since many dyes are toxic, carcinogenic, mutagenic, and hazardous to aquatic living organisms,3 which also brings health risks to humans through the consumption of polluted water.4 Owing to dyes being recalcitrant, resistant to aerobic digestion, stable to oxidizing agents, and unsolvable with low concentration, wastewater containing dyes is very difficult to treat,5 so various techniques, including coagulation, flocculation, chemical oxidation, membrane separation, catalytic degradation, biodegradation, and activation carbon adsorption, are proposed to removal dyes from contaminated media.6,7 Among the numerous methods, adsorption gives the best results, as it is inexpensive, easy to perform, insensitive to toxic substances, and effective for different types of dyes.8 After activated carbon was commonly used as sorbent to remove dyes, metal−organic framework (MOF),9 graphene oxide,10 porous carbon materials,11 magnetic nanoparticles,12 and their composites were developed to remove dye from the effluent, which usually exhibited high adsorption capacity and efficiency. However, the practical © 2016 American Chemical Society

application of these high performance adsorbents was limited by their high cost. So it is still a major challenge to develop simple, cheap, and efficient adsorbents. Montmorillonite clay possesses a layered structure with a large surface area and high cationic exchange capacity, whose current market price is considered to be 20 times cheaper than that of activated carbon.13 Clay exhibited a strong affinity for both cationic and anionic dyes, whereas the sorption capacity for basic dye was much higher than that for acidic dye because of the negative charge on the structure of clay.1 On the other hand, clay could participate in the design of polymeric hydrogels through in situ polymerization, which displayed excellent mechanical properties and optical transparency.14−16 In contrast, great attention has been paid toward the hydrogels composed of clay and acrylic acid-/acrylamide-based polymers or copolymers, which exhibited improved properties, such as oxygen barrier, flame retardant, thermal stability, and adsorption capacity for industry, healthcare, and agriculture applications.17−20 However, these polyolefin based hydrogels were usually nonbiodegradable, which may also cause the environmental problem and resulted in “white pollution”. Cellulose is a kind of abundant, low cost, renewable, and biodegradable natural polymer, which was widely used to construct hydrogels.21,22 Due to combination of the distinguishing features of clay and cellulose, it is feasible to design Received: September 10, 2016 Published: October 5, 2016 7217

DOI: 10.1021/acssuschemeng.6b02178 ACS Sustainable Chem. Eng. 2016, 4, 7217−7224

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on an HR-2 hybrid rheometer (TA Instruments, USA) under the cone and plate model with a cone diameter of 40 mm and truncation of 24 μm. The mixture of cellulose and CMC solution, clay, and epichlorohydrin was stirred to form a homogeneous solution, which was quickly transferred into the rheometer for testing. By using a deformation of 10% (in the linear viscoelastic zone) and a constant shear frequency (1 Hz), a time sweep was conducted on each sample at 60 °C to record the elastic stored modulus (G′) and viscous loss modulus (G″) for the investigation of the hydrogel samples. The compressive strengths of hydrogels were measured on a universal testing machine (CMT6503, Shenzhen SANS Test Machine Co. Ltd., China) with a compression clamp under a speed of 1 mm min−1. The disk shaped samples were 2 cm−1 × 1 cm−1 (diameter × height). The gravimetric method was employed to measure the equilibrium swelling ratio of the hydrogels in the distilled water at 25 °C. The equilibrium swelling ratio was calculated according to eq 1.

superabsorbent cellulose/clay hydrogels for dye removal, which contain high specific area networks plugged by a lot of water. In our previous work, we reported a class of superabsorbent hydrogels which were prepared by chemical cross-linking cellulose and carboxyl cellulose in NaOH/urea aqueous solution, which was a “green” solvent for cellulose.23 The characteristics of the superabsorbent hydrogels were high swelling ratio in water (1000 g/g,) and low organic content (∼0.1 wt %), which could achieve controllable release of bovine serum albumin. Despite the fact that the superabsorbent hydrogels were self-standing because of the introduction of unsubstituted cellulose in the hydrogel networks, their mechanical properties (such as compressive strength) were still very low, which limited their further application. To enhance the mechanical performance of these hydrogels, we incorporated clay nanosheets into cellulose networks in this work. These novel superabsorbent hydrogels from low cost cellulose and intercalated clay were also applied for the removal of methylene blue in the water. Moreover, we characterized the structure and properties of cellulose/clay composite hydrogels, discussed the influence of the state and content of clay on the properties of hydrogels, investigated the effect of time, temperature, and pH on the adsorption capacity of hydrogels for methylene blue (MB), and clarified the mechanism of MB adsorption onto hydrogels.

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Swelling Ratio =

Ws Wd

(1)

where Ws (g) was the weight of swollen gel after equilibrium at 25 °C, and Wd (g) was the weight of dried gel. The freeze-dried hydrogels and methylene blue trihydrate were ground into powder and analyzed in KBr disks with FTIR (PerkinElmer Specturm one, USA) in the region of 400−4000 cm−1. The methylene blue solution before and after absorption was detected ESI-Q-TOF(III)-MS (Bruker, Germany). The parameters for the measurement were as follows: positive mode, capillary voltage of 4500 V, TOF voltage of 2032.5 V. Dye Adsorption. The efficiency of the adsorption of methylene blue (MB) on samples was performed by batch methods. Kinetic experiments were measured with an initial concentration of MB of 10 mg L−1 at room temperature. At desired time intervals, the remaining amount of dye in the aqueous solution was then determined by UV− vis measurement. To evaluate the thermodynamic properties, adsorption isotherms were determined by varying the concentration of MB from 10 to 200 mg L−1 at 303, 313, and 323 K, respectively. The effect of pH for adsorption of MB was investigated by a series of experiments where the initial concentration of MB was maintained constant (100 mg L−1) at different pH (1−11). All tests were conducted in 10 mL of polyethylene tubes by taking about 0.5 g of hydrogel sample with 8 mL of MB solution, where tubes were fixed in a temperature controlled shaker at 100 rpm for the desired time (except the sample for photograph). The concentration of MB in aqueous solution was determined by UV−vis spectrophotometer at 664 nm, and the hydrogel samples did not exhibit any absorbance at this wavelength. The calibration curve was reproducible and linear over the concentration range used in this work. The amount of MB adsorbed on the sample was described as the adsorption capacity (qe) and calculated by eq 2. c − ce qe = 0 ×V (2) W

EXPERIMENTAL SECTION

Materials. The cellulose samples (cotton linter pulps) were supplied by Hubei Chemical Fiber Co. Ltd. (Xiangfan, China). Its weight-average molecular weight (Mw) was determined by static laser light scattering (DAWN DSP, Wyatt Technology Co.) to be 9.2 × 104. Sodium carboxymethylcellulose (CMC, 2.4 × 104) was analyticalgrade reagent purchased from Shanghai Chemical Agents Co. Ltd. The degree of carboxymethyl substitution (DS) is 0.7, which is the number of substituents per sugar ring. Inorganic clay (Montmorillonite) (cation exchange capacity = 100 mequiv/100 g) was supplied by Hongyu Clay Company (Zhejiang, China). Epichlorohydrin (ECH) (1.18 g/mL) and methylene blue trihydrate (C16H18ClN3S·3H2O) were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Preparation of Cellulose/Clay Nanocomposite Hydrogels. The 3 wt % cellulose solution was prepared as follows: 3 g of cellulose was dissolved into 97 g of 7 wt % NaOH/12 wt % urea/81 wt % water mixture at −12 °C, while CMC was dissolved in the same solvent at room temperature to obtain a 3 wt % polymer concentration. A desired amount of modified or unmodified clay was added into the mixture of CMC and cellulose solutions with weight ratio of 9:1 under stirring. ECH was dropped into the mixture as cross-linker with stirring at 0 °C until formation of a homogeneous solution, and then the reaction was conducted at 60 °C for 12 h to obtain gels which were finally dialyzed in distilled water to remove NaOH, urea, and unreacted ECH. For the modification of clay, 2,3-epoxypropyltrimethylammonium chloride (EPTAC) was used to exchange the cation of clay. Clay (1.0 g) was dispersed into 99 g of distilled water, and excess EPTAC was added into the clay suspension. The mixture was stirred for 2 h at 80 °C under nitrogen atmosphere. Afterward, the clay was filtrated, washed with distilled water, and dried. The above procedure was repeated until the weight of clay was constant, which indicated clay was completely modified. The hydrogels prepared with modified clay were named as Gel M-5, Gel M-10, and Gel M-15, according to the content of clay, while the hydrogels composed of unmodified clay were coded as Gel U-5, Gel U-10, and Gel U-15, respectively. Characterization. TEM observation of cellulose−clay nanocomposite hydrogels was carried out on a JEM-2010 FEF (UHR) transmission electron microscope (JEOL TEM, Japan). Samples were embedded into epoxy resin and sliced into ultrathin sections before measurement. Rheology experiments of the samples were performed

where c0 (mg L−1) and ce (mg L−1) are the initial and equilibrium concentrations of MB, respectively, W (g) is the weight of dried sample, and V (L) is the volume of solution. Parallel studies of adsorption were carried out three times, and the mean value was used for calculation. The removal efficiency of hydrogel samples for MB aqueous solution was carried out by immersing 1.5 g hydrogels in 8 mL of MB aqueous solution with concentrations of 10 mg L−1 and 100 mg L−1, respectively. The removal efficiency (%) was calculated by eq 3. Removal (%) =

W0 − We W0

(3)

where W0 (g) was the initial weight of MB in the aqueous solution, and We (g) was the weight of MB in the aqueous solution after adsorption equilibrium. Besides UV−vis measurement, the removal efficiency was also determined by the total organic carbon analyzer (Vario TOC, Elementar, Germany). Here, we emphasized that the 7218

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Figure 1. Synthesis schemes of cellulose−clay nanocomposite hydrogels. (a) Hydrogel network containing unmodified clay. (b) Epoxidized clay (modified clay) cross-linked in the hydrogel networks.

Figure 2. Morphology of cellulose−clay nanocomposite hydrogels: TEM images of Gel U-15 (a, c) and Gel M-15 (b, d) with different magnifications. samples used in all dye absorption experiments were swollen hydrogel, which reached swelling equilibrium in distilled water.

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epichlorohydrin (Figure 1b). In this case, cellulose or CMC chains entered the galleries, but the clay nanosheets remained stacked.24 To confirm that clay was successfully modified by EPTAC, we compared the TGA curves of clay samples (Figure S1). The mass loss around 300 °C of modified clay, which was absent in the curve of unmodified clay, could be attributed to the loss of EPTAC, indicating that EPTAC has been introduced into clay layers by cationic exchange. Furthermore, the XRD patterns of Gel U-10 and Gel M-10 (Figure S2) showed the maximum peak of Gel U-10 could be observed at 2θ = 7.1°, whereas the obvious diffraction peak of Gel M-10 was detected at 6.3°, which was assigned to the crystal structure of clays in the hydrogel networks. The changes of peak positions indicated that the clays modified by EPTAC altered their structure, suggesting that the increase of the distance between the tetrahedral sheets led to the intercalated structure of clays.25 For direct observation of the architecture of clays in hydrogel networks, the TEM images of nanocomposite hydrogels are shown in Figure 2, where the platelets of the montmorillonite

RESULTS AND DISCUSSION

Preparation and Structure of Cellulose−Clay Nanocomposite Hydrogels. Cellulose−clay nanocomposite hydrogels were fabricated by chemical cross-linking cellulose and carboxyl cellulose (CMC) with epichlorohydrin in the presence of clay nanosheets. The schematic representations of cellulose− clay hydrogel networks containing unmodified clays and intercalated clays are shown in Figure 1. To prepare conventional hydrogels, clay without any modification was incorporated into cellulose hydrogel networks, where cellulose or CMC chains could not enter the galleries of clays (Figure 1a). For intercalated nanocomposite hydrogels, clays were first modified by 2,3-epoxypropyltrimethylammonium chloride (EPTAC) through cationic exchange. Then, the modified clays took part in the formation of hydrogel networks, where the epoxy group of EPTAC between the clay layers could react with the hydroxyl groups of cellulose and CMC as well as 7219

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Figure 3. Properties of cellulose−clay nanocomposite hydrogels. (a) Time dependence of storage modulus (G′) and loss modulus (G″) for different gelation systems at 60 °C. The data are shifted along the y axis to avoid overlapping. (b) Gelation time. (c) Storage modulus at 1 h. (d) Compressive-strain curves. (e) Photographs of M-5. (f) Swelling ratio in distilled water.

significantly shortened with the increasing of clay content (Figure 3b), revealing that clay might destroy the stability of the hybrid system and promote the formation of hydrogel networks. Moreover, the hybrid system containing clay exchanged with EPTAC showed shorter gelation time than that containing unmodified clay with the same content, such as 20.9 min for M-5 and 27.7 min for U-5. These results indicated that modified clay introduced epoxy groups into the system which greatly enhanced the opportunity of the chemical crosslinking reaction, resulting in fast formation of hydrogel networks. The storage modulus of the hybrid system at 60 min for full development of cross-linked hydrogel networks was selected to evaluate the mechanical properties of hydrogels. As shown in Figure 3c, the G′ of M-5 (11.3 Pa) was higher than that of U-5 (6.5 Pa), and the same rule could also be observed in the system with clay content of 10 or 15 wt %, G′(M-10) > G′(U10), and G′(M-15) > G′(U-15). This tendency suggested that the modified clay containing EPTAC could increase the crosslinking density of hydrogel networks, leading to higher G′ in comparison with the unmodified clay hybrid system. On the other hand, the G′ value increased as clay contents changed from 5 wt % to 15 wt %, which was consistent with the finding of Haraguchi et al. in poly(N-isopropylacrylamide)−clay

clays appeared as the dark and thin lines. For Gel U-15 (Figure 2a), the aggregated clay layers could be observed in the cellulose matrix, due to the high content of clays (15%) in the hydrogel. After the clays were modified by EPTAC, their dispersity in the hydrogel networks was greatly improved, and the clay nanosheets could be homogeneously dispersed in the matrix of Gel M-15 (Figure 2b). We should emphasize that intercalation and homogeneous dispersion of clays by EPTAC in the hydrogels were essential for an increase of the effective surface area, which might benefit the removal efficiency of dyes. In the TEM images with higher magnification, we can find the original clay structure in Gel U-15 (Figure 2c), and intercalated clay in Gel M-15 (Figure 2d). Properties of Cellulose−Clay Nanocomposite Hydrogels. The gelation times of cellulose−clay hybrid systems were monitored by rheological technology. Figure 3a presents the time sweep measurement for the viscoelastic properties of each system at 60 °C. All hybrid systems behaved as viscous liquid at initial time (G′ < G″), then G′ increased more rapidly than G″ with prolonged time, and finally G′ exceeded G″ where the system behaved as a gel state. The crossover (G′ = G″) of two curves was defined as the gel point (t = tgel), indicating the sol− gel translation of such a system and formation of the crosslinked networks.26 The gelation times of different systems 7220

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Figure 4. Adsorption of MB on cellulose−clay nanocomposite hydrogels. (a) Effect of contact time on the MB adsorption into Gel M-10 at room temperature. (b) Adsorption isotherms of MB onto Gel M-10 at 303, 313, and 323 K. (c) Effect of solution pH on the MB adsorption onto Gel 0, Gel U-10, and Gel M-10 at room temperature. (d) Efficiency of gels for removing MB from aqueous solution with concentrations of 10 and 100 mg L−1.

nanocomposite hydrogels.27 The unexpected result that G′(U5) > G′(U-10) could be attributed to the error of measurement where the hydrogel network was considered as complete formation. To further understand the mechanical properties of nanocothemposite hydrogels, the compressive stress−strain curves are shown in Figure 3d. The compressive strength of hydrogels increased markedly with increasing clay contents, which did not sacrifice the extensibility of the samples. It should be noted that the nanocomposite hydrogels with modified clay not only showed much superior compressive strength, but also exhibited larger fracture strain compared to the hydrogel samples containing unmodified clay. For example, the compressive strength of M-5 (4.7 KPa) was approximate 16 times as high as that of U-5 (0.3 KPa), while the fracture strain of M-5 (51%) was still larger than that of U-5 (43%). These results revealed that the modified clay in the hydrogel networks formed an intercalated architecture and more cross-linking points, resulting in higher toughness. The photographs of M-5 in as-prepared state and after reaching swelling equilibrium in distilled water are presented in Figure 3e. From the outstanding change in volume of M-5 after swelling in water, it can be seen that the cellulose−clay nanocomposite hydrogel exhibited very high equilibrium swelling ratio in distilled water. Moreover, the swelling ratio of all hydrogel samples was determined to be 691 to 1443 g/g, indicating their superabsorbent properties (Figure 3f). This result was related our previous findings that the electrostatic repulsion of negative charges (ionization of carboxyl groups of CMC) expanded the hydrogel networks, leading to high swelling ratio.28 When a small amount of clay (5%) was added into hydrogel networks, clay nanosheets could be uniform

dispersed, which also participated in electrostatic interactions owing to their surface contained negative charges. So the swelling ratio of U-5 were higher than that of cellulose-based superabsorbent hydrogel (∼1000 g/g).23 However, the swelling ratio of hydrogels decreased as the clay contents increased from 5% to 15%, indicating that the clay nanosheets aggregated in the conventional hydrogel networks and high cross-linking density formed in the intercalated hydrogel networks, which has been observed in the TEM images above. The hydrogels prepared by using modified clays showed lower swelling ratio in comparison with samples prepared with unmodified clays, suggesting that the cationic exchange of clays enhanced the numbers of functional groups (epoxy groups) which could react with cross-linker and the cross-linking density of hydrogel networks was significantly raised, leading to the decrease of swelling ratio. Adsorption of MB on Cellulose−Clay Nanocomposite Hydrogels. Removal of dye was performed by batch experiments to investigate the effects of various parameters, such as contact time, temperature, solution pH, and initial dye concentration, on the adsorption of dye MB onto superabsorbent hydrogels. The adsorption amount of MB on hydrogels as a function of time and the photographs of the adsorption process of the dyes are shown in Figure 4a. To accurately understand the adsorption kinetics of MB onto hydrogels, we carried out the experiment by using a low concentration of MB (10 mg L−1). The initial appearance of dye solution was blue, and Gel M-10 in the solution was almost invisible. The adsorption of MB was fast in the beginning and then reached equilibrium after 40 h. Finally, the blue color of MB almost faded from the solution, and the transparent 7221

DOI: 10.1021/acssuschemeng.6b02178 ACS Sustainable Chem. Eng. 2016, 4, 7217−7224

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ACS Sustainable Chemistry & Engineering hydrogel became blue due to the adsorption of MB. The kinetics of adsorption of MB onto Gel M-10 were analyzed by the pseudo-second-order kinetic models as expressed in eq 4. t 1 t = + 2 qt qe k 2qe

Table 1. Maximum Absorption Capability of Relevant Materials Obtained by Calculation and Experimenta qmax (mg g−1) Hydrogels

(4)

where qe (mg g−1) and qt (mg g−1) were the amount of adsorption at equilibrium and time t, respectively, and k2 (g mg−1 h−1) was the equilibrium rate constant of pseudo-secondorder adsorption, which can be calculated by the plot of t/qt versus t as shown in Figure S3. The calculated qe (50.02 mg g−1) was close to the experimental qe (47.39 mg g−1), and the correlation coefficient (R22) of the pseudo-second-order kinetics was approximately 0.9981, demonstrating that the kinetic data fitted well with the pseudo-second-order model. Similar results have been reported for absorption of MB on cellulose beads29 and chitin hydrogels.30 Figure 4b shows the adsorption isotherms of MB onto Gel M-10 at different temperatures (303, 313, and 323 K). qe values increased with an increase of the equilibrium concentration of the dye solution (ce). To quantify the effect of temperature on the sorption of MB on hydrogel samples, both Langmuir and Freundlich isotherm models were adopted to describe the relationships between adsorption amount of MB onto hydrogels and its equilibrium concentration in water at 303, 313, and 323 K, respectively. The Langmuir isotherm adsorption equation was expressed as eq 5. ce c 1 = e + qe qmax qmax b

1 ln ce n

Cal./Exp.

Refs

Methylene blue Basic yellow 28

192.3/215 180/--

31 32

Nitrazine Yellow Malachite green Rhodamine B

Chitosan/polyacrylamide

Erichrome black T Reactive blue 2

Cellulose nanocrystal/alginate Collagen-g-poly(acrylamideco-maleic anhydride)/MMT

Methylene blue Crystal Violet Methylene green

Poly(sodium acrylate-coHEMA)/sodium alginate

Congo red Methyl violet

Gum karaya-polyacrylamide/ NiS/Ni3S4 nanoparticles Cellulose/MMT

Rhodamine 6G

259/144.4 757.6/-497.5/---/520 --/407 256.4/-439/-155/-172/-120/-1244.7/--

33 34 34 35 35 36 37 37 38 38 39

Methylene blue

1065/782.9

This work

a

AA: acrylic acid; AMPS: 2-acryamido-2-methylpropanesulfonic acid; MMT: montmorillonite; HEMA: hydroxyethyl methacrylate.

ΔS° ΔH ° − R RT

(7)

ΔG° = ΔH ° − T ΔS°

(8)

ln Kd =

where Kd, the distribution coefficient of absorbent at temperature T (K), was equal to qe/ce, and R (8.314 J mol−1 K−1) was the universal gas constant. ΔH° and ΔS° were determined by the slope and intercept of the linear plot of ln Kd versus 1/T (Figure A4) to be 14.41 kJ mol−1 and 75.17 J mol−1 K−1, respectively. The positive values of ΔH° and ΔS° revealed that the adsorption was endothermic and random at the solid− solution surface, whereas the value of ΔH° was less than 40 kJ mol−1, showing that the adsorption was a physic-sorption feature.40 Next, the negative values of the Gibbs energy changes for adsorption of MB on hydrogels (ΔG° was −8.366, −9.118, and −9.870 kJ mol−1, at 303, 313, and 323 K, respectively) were obtained, suggesting that adsorption of MB on hydrogel samples was spontaneous. The increase of the absolute values of ΔG° as a function of temperature indicated that the adsorption was favorable at high temperature. The influence of solution pH on the removal of MB by hydrogels was investigated to gain further insight into the adsorption process. Figure 4c shows the qe of Gel 0, Gel U-10, and Gel M-10 in various pH solutions. The absorption capacity of Gel 0 for MB was much lower than that of composite hydrogels (Gel U-10 and Gel M-10), reflecting that clay in the hydrogel networks could promote the absorption of hydrogels for MB. While the qe values of Gel M-10 were greater than those of Gel U-10 in the range pH 1−11, indicating that the surface of intercalated clays exposed more negative charges in the hydrogels networks, which benefited their electrostatic attraction with the cationic species of MB, leading to higher qe values. On the other hand, the qe values of hydrogels slightly increased with the increase of solution pH, which also could be explained by the electrostatic interaction of positive charges of MB with the negative charges of the hydrogel network (including the clay and CMC), which were enhanced at higher pH values, leading to greater qe values. The adsorption was not

(5)

where qe (mg g−1) was the amount of adsorption at equilibrium, ce (mg L−1) was the equilibrium MB concentration, qmax (mg g−1) was the maximum adsorption at monolayer coverage, and b (L mg−1) was the Langmuir adsorption equilibrium constant related to the free energy of adsorption, respectively. The Freundlich isotherm model was applicable to heterogeneous adsorption surfaces with mutilayer adsorption as shown in eq 6.

ln qe = ln k +

Dye

Poly(AA-co-AMPS)/MMT Poly(methacrylic acid)/ Zeolite Chitosan-MMT Gum karaya-poly(acrylic acid)/SiC nanoparticles

(6)

where k (mg g−1) was the Freundlich constant, indicating the adsorption capacity, and n was the dimensionless exponent of the Freundlich equation. The MB adsorption on hydrogels were fitted well by the Langmuir isotherm model with large R22 values (>0.99) compared to the Freundlich isotherm model (Table S1), indicating that MB was adsorbed on hydrogel as a monolayer adsorption. The qmax values obtained by calculation and experiment were 1065 and 782.9 mg g−1, respectively. Table 1 lists the qmax values of relevant materials for various dyes, where all hydrogels showed high absorption capability for dyes. The qmax value of the cellulose−clay nanocomposite hydrogel was higher than that of most hydrogels31−38and comparable with that of nickel sulfide nanoparticles incorporated gum karaya-polyacrylamide composite hydrogel,39 revealing that the incorporation of intercalated clay in the cellulose hydrogel networks led to their superhigh adsorption capacity for MB in the aqueous solution. Furthermore, the standard enthalpy (ΔH°), standard entropy (ΔS°), and Gibbs free energy change (ΔG°) for adsorption of MB on hydrogels were calculated by eqs 7 and 8. 7222

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CONCLUSION We have fabricated a series of novel cellulose−clay nanocomposite hydrogels by chemical cross-linking of natural polymers (cellulose and carboxymethyl cellulose) and clay (epoxidized montmorillonite) with epichlorohydrin in NaOH/ urea aqueous solution. The modified clays participated in the formation of hydrogel networks, leading to their homogeneous dispersion in the cellulose matrix. These hydrogels exhibited superabsorbent properties in distilled water, superior mechanical performance compared to the hydrogel containing unmodified clays, and great ability for removal of methylene blue (MB) from wastewater. The adsorption of MB onto hydrogels was spontaneous monolayer adsorption through a physical process, which could be well described by pseudosecond-order and Langmuir isotherm models. With these findings, this work may promote the development of new absorbents for the removal of dye from wastewater.

drastically affected by pH, suggesting the presence of other interactions (such as hydrophobic interaction) between MB and hydrogels. Since the superabsorbent hydrogel incorporated with intercalated clays exhibited superior MB removal capacity, the effect of clay contents in the Gel M series on the removal efficiency of MB was also studied (Figure 4d). Compared to Gel0, hydrogel samples containing intercalated clays showed high removal efficiency for MB aqueous solution with concentrations of 10 and 100 mg L−1. And it could be seen that the removal efficiency for MB increased with the clay contents of hydrogel networks, indicating that intercalated clay was an important component for the absorption of MB. Gel M15 exhibited high removal efficiency for MB with different concentrations; in particular, its removal efficiency for 10 mg L−1 MB was still as high as 97%. The above results calculated by the concentration of MB obtained from UV−vis measurement were also verified by the total organic carbon content method. Furthermore, the results of ESI-Q-TOF MS indicated no degradation during the treatment of MB solution with hdyrogels (Figure S5). To understand the mechanism of the interaction between hydrogel and dye, FT-IR spectra of samples are shown in Figure 5. The sharp absorption peaks at about 3450 cm−1 were

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02178. Detail of TGA and XRD measurements in this work, TGA and XRD results, plot of t/qt versus t for the adsorption kinetics, Langmuir and Freundlich isotherm parameters, and plot of ln Kd versus 1/T in isothermal adsorptions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Pearl River S&T Nova Program of Guangzhou (201506010101), Guangzhou science and technology project (201510010221), Hubei Province Science Foundation for Youths (2015CFB499), the National Natural Science Foundation of China (21304021), and Jiangsu Province Science Foundation for Youths (BK20150382). The authors also thank Prof. Bin Hu and Dr. Beibei Chen for their kindly supporting with the ESI-Q-TOF MS measurements.

Figure 5. FT-IR spectra of cellulose−clay nanocomposite hydrogels before (a) and after (b) MB dye absorption, and (c) of MB dye.

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assigned to the O−H bond stretching vibration of the hydroxyl group of hydrogel networks and bound water of MB. Some absorption peaks around 2932 cm−1 of the hydrogel sample before absorbing MB could be attributed to the C−H stretching vibration of methylene groups in the hydrogel networks, whereas the peaks around 2921 cm−1 observed in the spectrum of MB were assigned to the C−H stretching vibration of methyl groups. However, the corresponding absorption peaks in the spectrum of hydrogel absorbed MB almost disappeared, indicating that the presence of strong hydrophobic interaction between the hydrogel network and MB limited the mobility of methylene groups of the hydrogel and methyl groups of MB. Therefore, our finding revealed that hydrophobic interaction was also a main driving force for MB absorption in this work, besides electrostatic attraction.

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DOI: 10.1021/acssuschemeng.6b02178 ACS Sustainable Chem. Eng. 2016, 4, 7217−7224