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Research Article pubs.acs.org/journal/ascecg

Self-Assembled Sponge-like Chitosan/Reduced Graphene Oxide/ Montmorillonite Composite Hydrogels without Cross-Linking of Chitosan for Effective Cr(VI) Sorption Peng Yu, Han-Qing Wang, Rui-Ying Bao, Zhengying Liu, Wei Yang,* Bang-Hu Xie, and Ming-Bo Yang State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, China S Supporting Information *

ABSTRACT: Sponge-like chitosan (CS)/reduced graphene oxide (rGO)/montmorillonite (MT) porous composite hydrogels were synthesized with a facile strategy of in situ reduction of graphene oxide (GO) without cross-linking of CS. Integral pore structure can be formed in the hydrogels when proper amount of MT was introduced. The composite hydrogel can restore to the original shape and dimension after compressive deformation like a sponge and show good stability in acid condition. Batch equilibrium measurements on the composite porous hydrogel were carried out to optimize the parameters for the removal of hexavalent chromium ions (Cr(VI)). The results reveal that the Cr(VI) ion sorption of the composite hydrogels is highly pH dependent and is the most effective at pH = 2. The sorption capacity increases with increasing temperature and the maximum Cr(VI) uptake of the composite hydrogel is 87.03 mg/g at 288 K. The sorption follows pseudo-second-order kinetic model and Langmuir isotherm. The composite porous hydrogels can be repeatedly used as adsorbent and a high sorption capacity after repeated usage can be maintained. Such an in situ GO reduction strategy can also be used to fabricate a variety of porous 3D polymer/rGO-based nanocomposites for biomaterials, energy storage materials, and adsorbent materials. KEYWORDS: Chitosan/reduced graphene oxide/montmorillonite composite hydrogel, Cr(VI) sorption, In situ reduction of graphene oxide strategy, Porous structure, Repeated usage

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acid condition.7 Cross-linking CS with epichlorohydrin and glutaraldehyde can overcome this problem, but owing to the consumption of amine groups and hydroxyl groups during cross-linking, the sorption capacity of cross-linked CS has been found to be greatly reduced. So, the modification of CS with some other substances to obtain efficient composite adsorbent is still needed.8,9 Graphene, with high specific surface area, electrical conductivity, and other particular features, has received much attention and is widely utilized in electrically conductive materials, biological applications, supercapacitors, and optical materials.10,11 In the treatment of wastewater containing heavy metal ions, its high specific surface area has been combined with the excellent sorption capacity of CS to increase the specific surface area of the composite adsorbent and the sorption amount.8,12−14 Hydrogel prepared from reduced graphene oxide (rGO) has also caught much attention owing to the three-dimensional structure, high specific surface area,

INTRODUCTION As a heavy metal element, chromium ions are widely presented in industrial processes, such as leather tanning, paint manufacturing, and textile coloring and so on,1,2 leading to great threat to human health and the environment. So, it is of vital importance to develop cost-effective and durable methods and materials to efficiently treat the industrial wastewater containing chromium ions. Till now, chemical precipitation, electrolysis, membrane separation, ion exchange, and sorption have been widely studied and applied in the treatment of water containing chromium ions. In all these methods, sorption has received much attention because of its many advantages, including being fast and efficient and environmentally friendly.3,4 Chitosan (CS) is an environmentally friendly natural polymer material with an amount only after cellulose in nature. The amine and hydroxyl groups on CS chains can serve as the chelating and reaction sites, so CS can entrap various metal ions, chemically or physically. Amine and hydroxyl groups show strong functions of electrostatic sorption and chelation effect toward heavy metal ions,5,6 so CS is a very promising sorption material and has been widely studied. However, CS dissolves in © 2016 American Chemical Society

Received: September 19, 2016 Revised: December 19, 2016 Published: December 29, 2016 1557

DOI: 10.1021/acssuschemeng.6b02254 ACS Sustainable Chem. Eng. 2017, 5, 1557−1566

Research Article

ACS Sustainable Chemistry & Engineering and great potential to be widely applied in the field of supercapacitors, catalysis, biological applications, sorption and so on. Montmorillonite (MT), a cheap, and sheet-like inorganic material with high specific surface area, is a traditional adsorbent material,15 but its sorption capacity is low when used as an adsorbent material individually. Although modification of MT can increase its sorption capacity for heavy metal ions to some extent, the amount of sorption is still quite low.16 However, it can be used as filler to increase the mechanical strength of absorbent. Because of CS, graphene, and its derivatives, such as rGO, montmorillonite were applicable in the sorption of heavy metal ions, various composite materials prepared with them by combining their advantageous characteristics through different ways have been reported.8,12 Most traditional sorption materials are in the form of powder to increase the contact area of the adsorbent and the solution, which clearly leads to the difficulty of collecting the adsorbent after sorption.17 So many kinds of absorbents with pore structures have been developed, such as cellulose composites, chitosan composites, graphene composites and so on.13,18,19 Here, a facile method to prepare CS/rGO/MT ternary composite porous hydrogel, which does not dissolve in the acid solution even without cross-linking of CS, is reported for the first time. The composite hydrogels show large surface areas, and the interconnected pores allow the adsorbate to diffuse easily into the absorbent, leading to a large sorption capacity toward chromium ion. Moreover, the composite hydrogel shows high mechanical properties and behaves like a sponge, so the composite hydrogel can be collected easily after usage. The sorption capacity for repeated use of the composite hydrogel is also pretty good.

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H2O2 was added in and the mixture was left overnight. Thereafter the clear supernatant was filtered. The filtration was washed with 5% HCl aqueous solution to remove metal ions and was washed with distilled water to remove the acid. At last, the black-brown viscous liquid was lyophilized to be solid for further use. Preparation of CS/rGO/MT Composite Hydrogels. The typical hydrogel preparation procedure is as follows. First, different amount of MT powder (0, 0.1, 0.2, 0.4, 0.8 g) was added to 25 mL beakers containing 20 mL 1 wt% GO solution under magnetic stirring until MT uniformly dispersed in the suspension and the whole system was sonicated for another 0.5 h. Second, 0.4 mL glacial acetic acid was added into the solution, and then 0.6 g CS powder was added in. Magnetic stirring continued for 2 h to get a viscous homogeneous liquid and then 1 g NaVC was added, and the whole system was stirred for 10 min. Third, the beaker was sealed with a plastic wrap and kept at 50 °C for 12 h to allow for the mixture to self-assemble into CS/rGO/ MT composite hydrogel. After freezing, the hydrogels were lyophilized under condition of −50 °C and 18 Pa, and the lyophilized gels were washed repeatedly with distill water to remove extra NaVC and glacial acetic acid. Finally, the washed hydrogels were lyophilized again for further test. The hydrogels were named as MT-0, MT-0.1, MT-0.2, MT-0.4, and MT-0.8, respectively. Sorption Methods. The sorption measurements were performed in a 100 mL break with magnetic stirring. It should be noted that the batch sorption measurements were performed in solutions at a pH of 2. For instance, 50 mg of sorbent was added into Cr(VI) ions solution (50 mL and 100 mg/L) and the sorption was performed at 15 °C for 3 h. After sorption, the mixture was filtered and the filtrate was analyzed by UV−vis spectrophotometer (model UV-3600, Shimadzu, Japan) to determine the concentration of Cr(VI) ions with 1,5-diphenyl carbazide method at 540 nm.8,9,17 The sorption capacities were then calculated with eq 1:

qe =

(c0 − ce)V m

(1)

in which qe is the equilibrium sorption capacity (mg/g), c0 and ce are the initial and equilibrium Cr(VI) concentration in the liquid phase (mg/L), respectively, V is the solution volume (L), and m is the adsorbent mass (g). The initial solution pH was adjusted by either 0.1 mol/L NaOH or 0.1 mol/L HCl. The sorption thermodynamics was determined at 15, 30, 45, 60, and 75 °C. Effect of pH on the sorption was examined in the pH range of 1−6. For the determination of the sorption kinetics, the initial test solution at a pH of 2 was sampled at various time intervals. For the characterization of regeneration and reuse of the composite hydrogels, the sorption was performed in Cr(VI) solution (50 mL and 100 mg/L) at a pH of 2 with 50 mg of MT-0.2 at 15 °C under magnetic stirring for 3 h. After filtration, the samples were immersed NaOH aqueous solution (100 mL and 1 mol/L) and agitated at 15 °C for 1 h for 3 times. Then, the samples were washed with distilled water. The adsorbent was then lyophilized for the next cycle. The sorptionregeneration-reuse cycles for Cr (VI) uptake analysis were repeated for three times. Characterization. Thermogravimetric Analyzer (TGA, Q600, TA Instruments, USA) was used to provide evidence for the complete removal of NaVC in the composite at a 10 °C/min heating rate from 50 to 800 °C in a nitrogen atmosphere with a 20 mL/min flow rate. Figure S2 gives the DTG curves for the raw materials and the composite. We can see that there is no weight loss peak of NaVC on the DTG curve of the composite. So, NaVC was washed completely, which excludes the influence of NaVC in the subsequent tests. Fourier transform infrared (FTIR) spectra of CS, GO, MT, MT-0.2 were obtained with a Thermo Nicolet 6700 FTIR spectrometer (Madison, WI, USA) at a resolution of 4 cm−1 in transmission mode. Wide angle X-ray diffraction (WAXD) test was performed with a DX1000 X-ray diffractometer (Dandong Fanyuan Instrument Co. LTD, China) at room temperature. After freeze-drying, the sample powders were scanned in diffraction angle 2θ = 3−80° at 2°/min using CuKα radiation (λ= 0.154056 nm) with a filament voltage of 40 kV and a

EXPERIMENTAL SECTION

Materials. Natural graphite flakes (average particle size: 200 meshes, purity ≥99.9%) were bought from Shenghua Research Institute (Changsha, China) and were used without further purification. Montmorillonite (the specific surface area is 20−40 m2/ g, and the SEM image is shown in Figure S1) and sodium ascorbate (NaVC, 99%) were purchased from Aladdin. CS (with an deacetylation degree of 90%, a vicosity of 50 mpa.s) was a product of Jinan Haidebei Marine Bioengineering Co. Ltd. (Jinan China). Potassium permanganate (KMnO4), concentrated sulfuric acid (H2 SO4 ), potassium persulfate (K 2S 2 O8 ), hydrogen peroxide (H2O2), and phosphorus pentoxide (P2O5) were purchased from Haihong Chemical Reagents Company (Chengdu, China), and were of analytical grade. Preparation of Graphene Oxide (GO). GO was synthesized with a two-step oxidation procedure.20Concentrated H2SO4 (150 mL) was added in a 250 mL three-necked, round-bottomed flask and heated to 90 °C. Twenty grams of K2S2O8 and 20 g P2O5 were then added into the solution under continuous stirring until all the substances were completely dissolved. Then, the transparent solution was cooled down to 80 °C. Twenty-four grams of natural graphite flakes was then added in and the mixture was kept at 80 °C for 4.5 h. After that the mixture was poured into 1 L distilled water slowly and left overnight. Then the mixture was filtered and washed with distilled water to remove all the soluble substances, after which the pretreated graphite was dried in vacuum oven under 40 °C. Next, 6 g pretreated graphite was added to a 1 L flask containing 230 mL concentrated H2SO4, and under continuous stirring in an ice bath 30 g KMnO4 was added in slowly. Then, it was kept at 35 °C for 2 h. After that, 460 mL distilled water was added in slowly in order to avoid the temperature to be over 70 °C. When the temperature remained constant, the mixture was added into 1.4 L distilled water and stirred for another 2 h. Then 25 mL 30% 1558


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Figure 1. (a) The FI-IR of GO, CS, MT, and MT-0.2. (b) WAXD spectra of GO, CS, MT, MT-0.2, and NaVC. (c) 1H NMR spectra of GO, CS, CS/GO composite. (d) Raman spectra of MT, CS, GO, CS/GO/MT, and MT-0.2.

289.0 eV).21,22 The abundant hydrophilic oxygen containing functional groups made GO highly soluble in aqueous solution. Characterization of Composite Hydrogels. The FTIR spectra of MT, CS, GO, and MT-0.2 are shown in Figure 1a. We can see that the absorption band at 1727 cm−1 (CO stretching vibrations) and 1224 cm−1 (epoxy groups) in the curve of GO disappear in the curve of MT-0.2, indicating that GO was reduced by NaVC.23,24 The characteristic absorption bands of CS at 1650 and 1601 cm−1 (CO stretching vibration of amide I and N−H bending vibration of amide) also disappear in the curve of MT-0.2, showing that chemical reaction of CS occurred in the sample preparation process. In CS, the absorption band corresponding to NH2 vibration is at 1601 cm−1 and in GO, the absorption band corresponding to CO stretch of carboxylic group is at 1727 cm−1. In MT-0.2, both of these two absorption bands disappear, which shows that hydroxyl and amino groups on CS molecular chain reacted with the carboxyl groups on GO sheets.25,26 Characteristic absorption band of MT at 1035 cm−1 (asymmetrical bond stretching vibration of Si−O−Si) appears in the curve of MT-0.2,27 indicating that MT was loaded into the composite hydrogel. The presence of MT in the composite hydrogel is also evident in WAXD characterization (Figure 1b), in which the characteristic diffraction peaks of MT (indexed on the spectrum) appear in the curve of MT-0.2.28 In the enlarged view of the curve of MT and MT-0.2, the d(001) peak shifts to a lower angle. The interlayer distance d (001) of raw montmorillonite can be calculated by Bragg equation, and the result is 1.37 nm. The interlayer distance d (001) of the clay in the composite is 1.68 nm. The smaller distance of interlayer can be attributed to an intercalated structure of the MT layers in the composite.29 Therefore, we can conclude that the CS molecular chains or chain ends were intercalated into the MT gallery and an intercalated structure formed in the compo-

current of 40 mA. Proton nuclear magnetic resonance (1H NMR) spectra were obtained on a BRUKER (AV II-600 MHz) spectrometer. Raman spectra were recorded with a Labram HR spectrometer (HORIBA Jobin Yvon) utilizing 532 nm laser excitation. The morphology of the samples was characterized using a JEOL JSM5900LV field-emission scanning electron microscope (FESEM, Japan) with an accelerating voltage of 20 kV and a Tecnai G2 F20 S-TWIN high resolution transmission electron microscope (TEM, FEI Company, USA) with an accelerating voltage of 200 kV. The copper net method was used to prepare the TEM samples. Briefly, proper amount of composite was milled into powder and sonicated for 2 h in proper amount of distilled water, then one drop of the dispersion was transferred onto the copper net and was dried before TEM observation. X-ray photoelectron spectroscopy (XPS) characterization was carried out using an XSAM800 XPS (Kratos Company, UK) with AlKα radiation (hv =1486.6 eV). Dynamic rheological tests on a stresscontrolled rotational rheometer (AR2000EX, TA Instruments, USA) with a parallel-plate geometry (25 mm in diameter) at 25 °C, were used to examine the mechanical responses of the composite hydrogels. The wet hydrogels were transferred onto the plate carefully and slowly, and then a steady preshear was applied to eliminate the effect of any previous shear history before each test. Viscosity is also tested on the same rotational rheometer, but with 20 mm 2° steel cone geometry under steady state flow. Specific surface area (SSA) of the composite hydrogels were achieved by analyzing N2 adsorption/desorption at 77 K through a Brunner−Emmet−Teller (BET) measurement (Autosorb iQ/ASiQ, Quantachrome, USA). The outgas time is 4 h, and the outgas temperature is 80 °C.

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RESULTS AND DISCUSSION Chemical Composition of GO. The chemical compositions of GO were examined by XPS and the results were given in Figure S3. In Figure S3, oxygen containing groups were found to be connected to graphene sheets, and the XPS spectra of GO can be fitted into four peaks corresponding to sp2 carbon (CC, 284.5 eV), carbonyl (CO, 287.4 eV), epoxy/ hydroxyls (C−O, 285.8 eV), and carboxylates (OC−O, 1559


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Figure 2. SEM images of (a) MT-0, (b) MT-0.1, (c) MT-0.2, (d) MT-0.4, (e) MT-0.8, (f) Enlarged view of MT-0.2 and (g) TEM graph of MT-0.2.

Figure 3. Schematic diagram of formation mechanism of pore structure in composite hydrogels.

sites.30 In addition, the WAXD pattern of GO shows typical peaks at 10.5° and 42.2° while in the pattern of the composite, typical peaks of GO disappear, also confirming the reduction of GO during the sample preparation process. It is worth mentioning that the characteristic absorption bands of NaVC cannot be found in the curve of the composite (Figure S4), which proves that NaVC was cleared completely. The reaction between GO and CS can also be proven by 1H NMR spectra and Raman spectra. The results are shown in Figure 1c,d. Both GO and CS have active functional groups, such as amino groups of CS and carboxyl group of GO. In order to make clear the possible reactions between GO and CS, proper amount of CS, GO, and GO/CS were dissolved in 2 wt % deuterated acetic acid solution. The 1H NMR spectra of GO, CS, and GO/CS given in Figure 1c show that the characteristic chemical shifts of CS at about 3.1, and 3.5−4.0 ppm can be distinguished clearly. It is worth noting that the diamagnetic ring currents in graphene can induce significant upfield shifting of the proton signals in the glucosamine backbone of CS,31 which can also be seen in Figure 1c. The Raman spectra of GO, CS, MT, MT-0.2, and CS/GO/ MT are shown in Figure 1d. The preparation method of CS/ GO/MT is described in Supporting Information (SI-1). Briefly, the preparation method of CS/GO/MT is the same as MT-0.2 but without adding NaVC. From Figure 1d, CS and MT do not show any peaks. In GO, the peaks at around 1588 and 1345 cm−1 can be attributed to G band (E2g mode of sp2 carbon atoms) and D band (symmetry A1g mode), respectively. In CS/GO/MT, the peak of G band upshifts compared with that of GO, which is owing to the changes of electronic structure of GO with the interaction with CS.32 The G band of MT-0.2 occurs at 1569 cm−1, which corresponds to the recovery of the hexagonal network of carbon atoms with defects.33 Moreover, the increased D/G intensity ratio of sample MT-0.2 compared

to that in GO and CS/GO/MT demonstrates the removal of oxygen groups and restoration of the sp2 network during the reduction process.34 Morphology and Pore Structure of the Composite Hydrogels. The microstructures of the composite hydrogels were observed with SEM and TEM, as shown in Figure 2. For MT-0 (Figure 2a), without the presence of MT, thick and continuous walls of GO and CS was formed, because of the π−π interactions between GO sheets after the oxygencontaining groups on GO sheets were removed by the reducing effect of NaVC and the connection of GO sheets by the CS molecular chains. Without MT to reinforce the cell walls, the walls are so soft that they stack together (as shown in the cycles) and the pore structure was not so integral. When proper amount of MT is introduced in the composite, the walls become strong and integral pore structure is formed (Figure 2c, d,e). The thickness of the wall increases with increasing amount of MT. In the enlarged view of MT-0.2 (Figure 2f), the details of the wall revealed that CS mixed with MT is attached on the surface of reduced graphene sheets. In the red cycle of Figure 2f, we can see the rGO sheets attached to each other because of the π−π interactions after the reduction of GO to form a layered structure to build the wall of the pore structure. Form the TEM graph (Figure 2g), MT layers scattered in the wall of the pore structure. So, we can conclude that only when proper amount of MT is introduced into the composite, integral pore structure can be formed. To reveal the effect of reduction of GO on the structure of the composite hydrogel, the SEM image of CS/GO/MT was also shown in Figure S5. We can see that by a simple compounding process, the pore structure formed is quite different from that for MT-0.2. The pores formed in Figure S5 is related to the crystallization of water. So, the reduction of GO is a very important process in the preparation of the porous composite adsorbent. 1560


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Figure 4. (a) Storage moduli of MT-0, MT-0.1, MT-0.2, MT-0.4, and MT-0.8; (b) viscosity and (c) shear stress as functions of shear rate showing the acid resistance of pure CS and MT-0.2.

Formation Mechanism of the Integral Pore Structure. According to microstructures revealed in the morphology characterization by SEM and TEM, the morphology formation mechanism of the pore structure is schematically given in Figure 3. First, MT particles uniformly dispersed in the GO solution during ultrasonic treatment. When CS was added into the solution, the hydroxyl groups and amino groups on the CS chains reacted with the carboxyl groups on GO sheets, and there was also strong hydrogen bonding between the oxygen containing groups on GO sheets and amino, hydroxyl groups on CS chains.25,26 As a result, CS molecular chains attached onto GO sheets and some of the CS molecular chains acted as the bridge of adjacent GO sheets. At the same time, a lot of MT particles were intercalated by CS molecular chains or chain ends adhered to the surface of GO sheets because of the strong hydrogen bonding interaction of CS and adhesion with CS. When NaVC was added, GO sheets were reduced. The π−π interactions between rGO sheets were formed and rGO sheets attached with CS and MT stacked together. Thus, owing to the connection of GO sheets by the CS molecular chains, the pore structure is formed. In this process, MT acts as a supporting filler to build the pore structure, and when proper amount of MT is introduced, integral pore structure can be formed. After the integral pore structure is formed, the thickness of cell wall increases with increasing amount of MT. Mechanical Behavior and Stability of the Composite Hydrogels under Acidic Condition. The mechanical performance of the composite hydrogels was characterized by a dynamic rheometer. It is seen in Figure 4a that the storage modulus of the composite hydrogels maintain constant values with increasing frequency, which clearly indicated that stable network structures were formed in the hydrogels.35 The storage modulus increases with increasing loading of MT and the storage modulus of MT-0.2 show the greatest increase, possibly due to the beginning of the formation of integral pore structure when 0.2 g MT was introduced. What’s more, the composite hydrogels which have integral pore structure show a great resilience. From the inset digital photos of Figure 4, it can be seen that MT-0.2 can restore to the original dimensions after compressive deformation like a sponge, which can ensure the integration of the hydrogels under stirring during the sorption process. The swelling degree of composite was also investigated, and the CS hydrogel and MT-0.2 were wetted by the Cr(VI) solution (pH = 2). The swelling condition was shown in Figure S6. As can be seen, MT-0.2 were not swelled by acid solution while CS hydrogel was greatly swelled. This also shows the stability of composite material. When the amount of MT is up to 0.8 g, although integral pore structure can also be formed in the composite hydrogel and the storage

moduli are high, the hydrogels are brittle and are damaged easily under pressure. Because most of Cr(VI) ion containing industrial wastewater is acidic and pure CS dissolves in acid condition, so the ability of acid resistance of the composite hydrogel is also investigated. 0.1 g CS and 0.167g MT-0.2 which contain the same amount of CS were submerged in 5 mL Cr(VI) solution (100 mg/L, pH = 2) under the same stirring for 2 h, and then the viscosity of the solutions was measured. From Figure 4b, the CS solution exhibits shear thinning behavior and the viscosity is higher than that of Cr(VI) solution (pH = 2), meaning that CS has dissolved in the acid. Surprisingly, the viscosity curve of sample MT-0.2 is similar to that of Cr (VI) solution (pH = 2) and the viscosity of solution is not changed with increasing shear rate, clearly showing that CS in the composite is not dissolved. This point can also be proven by the shear stress-shear rate curves (Figure 4c). The shear stress of CS solution increases much more than that of Cr (VI) solution (pH = 2) and MT-0.2 suspension with increasing shear rate, which also shows that the solution of CS is more viscous than MT-0.2 suspension. So, the acid resistance of MT-0.2 is much better than pure CS.7,36 Actually, that CS solution is more viscous than MT-0.2 suspension and Cr(VI) solution (pH = 2) can be directly distinguished by naked eye (inset pictures in Figure 4b). It can be explained that CS dissolves in acidic media becomes a polyelectrolyte because of the protonation of the amino groups.37 After the amino groups of CS molecular chains react with the carboxyl groups on GO sheets, the amount of amino groups decreases, and as a result, the solubility of CS from the composite decreases in acid solution. Specific Surface Areas. The specific surface area is an important parameter for absorbents in practical use. Generally speaking, the sorption capacity is higher and the sorption rate is faster when the specific surface area of the absorbent is larger.12 So, most absorbents are in the form of powder. When the specific surface area increases, the contact area between absorbent and wastewater increases, and the electrostatic sorption and chelation effect can function more adequately. Here, qualitative and quantitative analysis of the specific surface area of the composite hydrogels was performed by SEM and BET, respectively. The SEM images of CS hydrogel, CS/MT hydrogel, rGO hydrogel (with the detailed preparation procedure described in SI-2, SI-3, SI-4 in the Supporting Information), and MT-0.2 are shown in Figure S7. From the enlarged views, it can be seen that the inner surface CS hydrogel is very smooth, but after the addition of MT, the surface becomes very rough (Figure S7b). It is widely known that rGO has a very high specific surface area, as also can be seen in Figure S7c, so when rGO is 1561


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kept by the increased contact area owing to the too much replacement of CS and decreases. The sorption capacity of the active substance, CS, in the composites shown with the purple bars, increases with increasing amount of MT, owing to that MT increases the surface area of the active substance. To compare the difference of sorption capacity between raw material and composite, we still tried to measure the sorption capacity of raw materials. The results were shown in Table 2.

introduced, the surface of the pores formed in MT-0.2 is very rough (Figure S7d), which can increase the contact area between absorbent and wastewater vastly, and the sorption capacity and sorption rate are hopeful to be greatly enhanced. The surface area data of CS hydrogel, CS/MT hydrogel, rGO hydrogel, MT, MT-0, and MT-0.2 were summarized in Table 1. Table 1. Surface Area of CS Hydrogel, CS/MT Hydrogel, rGO Hydrogel, MT, MT-0, and MT-0.2 sample name surface area (m2/g)

CS hydrogel

CS/MT hydrogel

rGO hydrogel

MT

3.4

3.6

7.1

26.7

Table 2. Sorption Capacities of CS Powder, MT Powder, and rGO Hydrogel

MT-0 MT-0.2 1.3

4.1

The surface areas of MT powder and rGO hydrogel are relatively high. So, after the introduction of MT and GO, the surface area of the composite hydrogels becomes higher. But the surface area of MT-0 is quite low because of the formation of the thick walls (Figure 2a). Cr (VI) Sorption Capacity. Cr (VI) sorption of was performed by placing 50 mg of adsorbent in a 100 mL beaker containing 50 mL 100 mg/L Cr(VI) solutions at pH = 2 and 15 °C. The sorption of Cr (VI) onto MT-0.2 composite was verified with XPS. The XPS spectra of MT-0.2 composite before and after treatment are shown in Figure 5a. The Cr 2p

raw material

CS powder

MT powder

rGO hydrogel

sorption capacity(mg/g)

54.72

7.93

11.08

Considering the proportion of raw materials in the composite, the sum sorption capacity of individual materials is calculated to be 36.63 mg/g. However, the sorption capacity of MT-0.2 is 64.69 mg/g, showing an increase of about 76.6%. The reason is that CS will be more efficiently used and stabilized in the process of sorption in the prepared composite adsorbent. Effect of pH on the Sorption Capacity. The pH value of the Cr(VI) solution is one of the most important variable influencing the sorption capacity. The sorption process of metal ions is sensitive to pH. So, the effect of pH on the sorption capacity was examined over the pH range from 1.0 to 6.0, and the results are shown in Figure 6. The maximum sorption value

Figure 5. (a) XPS spectra of MT-0.2 composite before and after sorption of Cr (VI) and (b) chromium sorption capacities of MT-0, MT-0.1, MT-0.2, MT-0.4, and MT-0.8 based on the quality of composite (red bar) and the active substance, CS, in the composite (purple bar) .

peak in the XPS spectrum of chromium-sorbed MT-0.2 composite clearly confirms the sorption of Cr(VI). The uptake capacities of composites were shown in Figure 5b. The sorption capacity of the composites (red bar) does not change much with increasing content of MT at relatively low content, but deceases obviously when the amount of MT is high. The reasons for the change of sorption capacity of the composites with different MT loadings are as follows. When there is no MT, the pore walls consisting of CS and GO are thick (Figure 2a) and the thick walls will decrease the contact area between adsorbent and heavy metal ion solution and also decreases the electrostatic sorption of the adsorbent and the chelation effect toward the heavy metal ion. After the addition of MT, a certain amount of CS is replaced, but the wall will not stack together to be so thick and an integral pore structure forms which increases the contact areas between the adsorbent and heavy metal ions in solution and also increases the electrostatic sorption and the chelation effect of the adsorbent toward heavy metal ions. So, although the amount of active substance, CS, in the composites declines, the sorption capacity remains at a high level. When the amount of MT is too much, the sorption capacity cannot be

Figure 6. Effect of pH of Cr(VI) solution on the Cr(VI) sorption capacity of MT-0.2 composite hydrogel.

for Cr(VI) onto MT-0.2 was 64.44 mg/g at pH = 2, which can be explained as follows. Cr(VI) exists mainly in the anionic form,38 and at a lower pH, the amine groups can be easily protonated, and the protonation extent of amine group will be reduced with increasing pH value. The amine group cations in the absorbent will tend to form complexes with Cr(VI) anions owing to electrostatic attraction. Thus, the sorption of Cr(VI) on MT-0.2 reaches the maximal value at pH= 2. Sorption Thermodynamics. Figure 7 gives the Cr(VI) sorption of the MT-0.2 composite hydrogel at various temperatures. As we can see, the sorption capacity increases with increasing temperature in the tested temperature range. The thermodynamic parameters including ΔG, ΔH, and ΔS, can be calculated with the temperature-dependent sorption equation and Van’t Hoff equation: 1562


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Figure 8. Effects of contact time on Cr(VI) sorption efficiency of MT0.2 at 15 °C.

Figure 7. Effect of temperature on the Cr(VI) sorption of of MT-0.2.

⎛ q ⎞ ΔS ΔH 1 ln⎜ e ⎟ = − R R T ⎝ ce ⎠

(2)

ΔG = ΔH − T ΔS

(3)

Table 3. Pseudo-First-Order and Pseudo-Second-Order Kinetic Model Parameters for Cr(VI) Sorption of MT-0.2 kinetic model

where ΔG is the change of Gibbs free energy (kJ/mol), ΔH (kJ/mol) is the enthalpy change, ΔS (J/(mol K)) is the change of entropy, K = (qe/ce) is the sorption equilibrium constant, R is the universal gas constant (8.314 J/(mol k)). Thus, ΔG, ΔH, and ΔS can be calculated from the linear fitting of ln(qe/ce) versus 1/T and Van’t Hoff equations. The values of ΔH and ΔS were 16.09 kJ/mol and 60.33 J/ (mol K). ΔH shows an endothermic nature for the sorption process. The positive value of ΔS shows good affinity of Cr(VI) toward the adsorbent. Furthermore, the value of ΔG is always negative at various temperature, indicating the spontaneity and chemisorption of sorption process for Cr(VI) on the absorbent. The value of ΔH gives information about the type of sorption. The magnitude of ΔH for physisorption is 2.1−20.9 kJ/mol and for chemisorption is 20.9−418.4 kJ/mol.9 The ΔH here was found to be 16.09 kJ/mol, indicating that the sorption process was more inclined to physical sorption process. Effect of Contact Time and Kinetic Studies. The sorption kinetics is one of the main point to evaluate the quality of an absorbent. Rapid interaction is very significant for water treatment in modern industry. The results of the kinetics of Cr(VI) removal is shown in Figure 8. The sorption capacity rises significantly in the first 2 h, contributing 87.5% of the ultimate sorption capacity, and then reaches the sorption equilibrium in 3 h. The ultimate sorption capacity is 66.75 mg/ g. The pseudo-first-order model and also the pseudo-secondmodel were adopted to elucidate the sorption mechanism and evaluate the sorption kinetic process. The linear forms of the two model equations can be described as eqs 4 and 5: ln(qe − qt) = ln qe − k1t

(4)

t /qt = 1/v 0+t /qe

(5)

pseudo-first-order model pseudo-secondorder model

qe,exp (mg/g)

V0 (mg/(g min))

66.75 66.75

K (L/min)

qe,cal (mg/g)

R2

0.0165

49.12

98.99

70.93

99.89

3.189

and qe,exp is the experimental value. v0 and qe,cal were calculated from the linear fitting of t/qt versus t. The correlation coefficients showed a better fitting of the pseudo-second-order model and the qe,cal was closer to experimental value, indicating that the sorption kinetics follows the pseudo-second-order model and the rate-limiting step may include the chemical sorption involving valency forces through sharing or exchanging of electrons between sorbent and sorbate.28,39 Effect of Initial Cr(VI) Concentration and Sorption Isotherm. Figure 9 shows the Cr(VI) sorption curves and isotherms on MT-0.2 at initial concentrations of Cr(VI) from 0 to 240 mg/L (pH = 2.0). Obviously the Cr(VI) sorption capacity increases rapidly with the increase of Cr(VI) concentration in the low initial concentration range and then continues to increase slowly at high initial concentration.

where qe (mg/g) and qt (mg/g) are the sorption capacity at equilibrium and at time t, K1 is the pseudo-first-order sorption rate constant, v0 denotes the initial sorption rate (mg/(g min)), and R2 is the correlation coefficients. The results are listed in Table 3. qe,cal (mg/g) and K1 was calculated from the slope and intercept according to the fitting line of ln(qe,exp − qt) versus t

Figure 9. Sorption isotherm for Cr(VI) on MT-0.2. 1563


Research Article


Performance of the Composite Hydrogel after Repeated Use. To evaluate the cost-effectiveness of the sorption process, the regeneration and reusability of the composite were investigated. One mol/L NaOH was used as a solvent for Cr(VI) desorption to evaluate the reusability of the composite MT-0.2, and the results are shown in Figure 10a. Clearly, even after four cycles, the Cr(VI) removal efficiency of the composite MT-0.2 still remains almost 98.9% of the initial uptake. Moreover, the storage modulus of the composite after four cycles is also almost the same (Figure 10b), strongly indicating that MT-0.2 can be repeatedly used for the removal of Cr(VI) in wastewater effectively.

Langmuir (eq 6) and Freundlich (eq 7) models were adopted further understand the interactions of Cr(VI) and the adsorbent. 1/qe = 1/qmax + (1/qmax b)(1/ce)

(6)

log qe = log k + (1/n)log ce

(7)

where qe (mg/g) and qmax (mg/g) are the equilibrium sorption capacity and the maximum sorption capacity, respectively, ce (mg/L) is the equilibrium concentration of Cr(VI), b (L/g) is the Langmuir constant, n and k are the Freundlich constant, R2 is the correlation coefficient. The parameters calculated were collected in Table 4. Obviously the Langmuir model gives Table 4. Constants of Isotherm Models for Cr(VI) Sorption on MT-0.2 Langmuir b

R2

n

k

R2

87.03

0.07086

0.9987

2.434

12.08

0.9538

better fitting of the experimental data with R2 = 0.9987, indicating that the Cr(VI) sorption of MT-0.2 is a monolayer sorption. qmax(mg/g) was 87.03 mg/g, higher than those of the recently reported adsorbents (listed in Table 5). Table 5. Comparison of Sorption Capacities of MT-0.2 and Reported Values on Various Adsorbents for Cr(VI) adsorbent triethylenetetramine modified graphene oxide/ chitosan chitosan cross-linked chitosan chitosan/montmorillonite chitosan/cellulose chitosan cross-linked with epichlorohydrin magnetic cyclodextrin−chitosan/graphene oxide composite graphene nanosheets cyclodextrin-chitosan modified GO ethylenediamine-modified cross-linked magnetic chitosan protonated cross-linked chitosan β-cyclodextrin/ethylenediamine/magnetic graphene oxide MT-0.2

qmax (mg/g)

reference

219.5

8

22.09 86.81 41.67 13.05 52.3 67.66

40 41 42 43 44 45

43 61.31 51.813

46 47 48

189.3 68.41

49 50

87.03

this work

CONCLUSIONS

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ASSOCIATED CONTENT

Chitosan/reduced graphene oxide/montmorillonite composite porous hydrogel was prepared via a strategy of in situ reduction of graphene oxide. Integral pore structure was formed in the hydrogel when proper amount of MT was introduced. The storage modulus increased a lot when the integral pore structure was formed. The integral pore structure makes the hydrogels behave like sponges and can stabilize the hydrogels under stirring during sorption process. The sorption of Cr(VI) on the composites were examined. The maximum uptake of Cr(VI) of composite was 87.03 mg/g at 288 K and the sorption followed the pseudo-second-order model and Langmuir isotherm. The Cr(VI) sorption capacity increased with increasing temperature. The sorption was found to be an endothermic and spontaneous chemical process which included electrostatic interaction. The composite hydrogels can be recycled with 1 mol/L NaOH. Thus, the composite hydrogels are potentially to be used for the removal of Cr(VI) from the wastewater.

Freundlich

qmax(mg/g)

■

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02254. Preparation methods for the control samples CS/GO/ MT, CS hydrogels, CS/MT hydrogels, and rGO hydrogels and some more detailed characterization results for the composite hydrogels and the control samples (PDF)

Figure 10. (a) Desorption-regeneration cycles. (b) Storage modulus of the composite MT-0.2 and MT-0.2 after four cycles. 1564


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AUTHOR INFORMATION

Corresponding Author

*Telephone: + 86 28 8546 0130; Fax: + 86 28 8546 0130; Email: [email protected]. ORCID

Wei Yang: 0000-0003-0198-1632 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

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REFERENCES

The financial support from National Natural Science Foundation of China (51422305 and 51421061), Sichuan Provincial Science Fund for Distinguished Young Scholars (2015JQO003), and the Innovation Team Program of Science & Technology Department of Sichuan Province (Grant No. 2014TD0002) was acknowledged.

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