Article pubs.acs.org/IECR
Methylene Blue Adsorption from Aqueous Solution by Magnetic Cellulose/Graphene Oxide Composite: Equilibrium, Kinetics, and Thermodynamics Haochun Shi, Weisong Li, Lei Zhong, and Chunjian Xu* State Key Laboratory of Chemical Engineering, Chemical Engineering Research Center, and School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ABSTRACT: In the present study, magnetic cellulose/graphene oxide composite (MCGO) was prepared as a novel adsorbent to dispose of dye wastewater. The morphology and chemical structure of the MCGO composite were characterized by the Fourier transform infrared spectrometer (FT-IR), X-ray diffraction (XRD), and scanning electronic microscope (SEM). The adsorption of methylene blue (MB) onto MCGO was studied in relation to initial concentration of MB, contact time, adsorbent dose, and pH value of solution. Adsorption kinetics and the equilibrium adsorption isotherm were fitted by a pseudo-secondorder kinetic model and Langmuir isotherm, respectively. The thermodynamic parameters indicated that the adsorption was spontaneous, favorable, and exothermic in nature. Furthermore, MCGO was very stable and can easily be recycled. The adsorption efficiency of MCGO was still over 89% after recycling for five times.
1. INTRODUCTION Dye wastewater is mainly from textile, leather, paper, rubber, plastics, cosmetics, pharmaceutical, and food industries. Because dye wastewater always comes as large quantities, complex composition, color depth, and is of high toxicity, it causes severe environmental pollution and human health hazards if it is not treated properly before discharging into the natural water. Current treatment methods for dye wastewater include physical, chemical, and biological methods, and so on. Various removal methods have been studied by adsorption,1−3 chemical coagulation,4 liquid membrane separation,5 electrolysis,6 biological treatments,7 oxidation,8 and other processes.9 However, these processes vary in their effectiveness, costs, and environmental impacts.10 Among these processes, the adsorption process is much more competitive than other methods for its ready availability, lower cost, and wider range of applications. It is of vital importance to search for adsorbents which meet the requirements and standards of the water treatment industry and also are environmentally friendly, highly effective, low cost, and being available in tonnage quantities. Biomass adsorbent has attracted more and more attention not only because of its excellent properties but also of its biodegradability, biocompatibility, and renewability.13,14 Cellulose is a natural polysaccharide which has the widest distribution and most abundance on earth. Above all, they can be regenerated. The beads, films, and resins made from natural cellulose were used for heavy metals12−14 and hazardous azo dyes adsorptions.11,15 High adsorption enthalpies generated by dyes allow as much aromatic cores to come close to the cellulose surfaces as possible.16 One disadvantage of conventional bioadsorbents based on cellulose is the difficulty to separate and recover except by using high-speed centrifugation or filtering. In addition, their relatively low adsorption capacities toward azo dyes limit the practical application of the bioadsorbents. © 2013 American Chemical Society
Graphene oxide (GO), an oxidation product of graphene, is a single sheet from graphite and has the ideal two-dimensional (2D) structure with a monolayer of carbon atoms packed into a honeycomb crystal plane. Considerable attention has been drawn to both graphene and the oxide form over the past several years because of their unique physical and chemical properties and potential applications in various research and industrial fields. Recent studies showed that the graphene oxide is ideal for adsorption of dyes and good for acting as a catalyst carrier substance due to its good mechanical strength and large surface area.9,20 Therefore, based on the cellulose adsorption properties and the intrinsic properties of graphene oxide, we combined them and introduced the magnetic iron oxide to make the separation more efficient. The aims of this work are to prepare magnetic cellulose/ graphene oxide composite (MCGO) bioadsorbent and explore the optimal conditions by using single-factor and response surface methodology (RSM). Methylene blue (MB) with large and complicated structures was selected as a model pollutant to evaluate the adsorption characteristics of MCGO under laboratory conditions. This information will be useful for further research and other practical applications of the novel bioadsorbent in dye wastewater treatment.
2. MATERIALS AND METHODS 2.1. Materials. Microcrystalline cellulose was purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). FeCl2·4H2O and FeCl3·6H2O were obtained from Damao Chemical Agent Company (Tianjin, China). Methylene Blue (MB in short, molecular formula C16H18ClN3S·3H2O) was selected as the model dye. Urea, sulfocarbamide, and other Received: Revised: Accepted: Published: 1108
August 19, 2013 December 19, 2013 December 24, 2013 December 24, 2013 dx.doi.org/10.1021/ie4027154 | Ind. Eng. Chem. Res. 2014, 53, 1108−1118
Industrial & Engineering Chemistry Research
Article
surface area of MCGO was determined from the N2 adsorption isotherm by using the 3H-2000 automatic specific surface area analyzer (Beijing, China). The surface topographies of samples were investigated using a Nanosem 430 scanning electron microscope (SEM) (FEI, U.S.A.). 2.5. Adsorption Experiments. Batch adsorption experiments were carried out by using a model SHZ-B shaking bath (Shanghai, China) with a shaking speed 150 rpm when the system achieved equilibrium. Typically, 0.05 g of MCGO and 50 mL of dye solution with desired concentration were added into 100 mL glass flasks and then shook under 25 ± 0.5 °C. At predetermined time intervals, the dispersion was drawn and separated immediately by adscititious magnet to separate and bioadsorbents were collected. Residual MB concentration in the supernatant was measured using a UV−vis spectrophotometer (UV765, Shanghai Precision & Scientific Instrument Co., Ltd., China). The absorbance of MB is λmax = 665 nm. The adsorption amount and adsorption rate are calculated based on the difference in the MB concentration in the aqueous solution before and after adsorption, according to the following equation:
chemical reagents were of analytical-reagent grade from Guangfu Fine Chemical Research Institute (Tianjin, China). 2.2. Preparation of MCGO. 2.2.1. Synthesis of Fe3O4 Nanoparticles and Graphene Oxide. Fe3O4 nanoparticles were synthesized by coprecipitation of ferric and ferrous salts under the presence of N2 gas. Amounts of 4.20 g of FeCl2· 4H2O and 11.42 g of FeCl3·6H2O were dissolved into 50 mL of deoxygenated distilled water. After stirring for 30 min, precipitation was observed under vigorous stirring by adding 45 mL of NH3·H2O solution (28%, v/v) under N2, followed by a further stirring for 2 h. The precipitates were separated by an adscititious magnet and washed with ethanol and deoxygenated distilled water for three times, respectively. Graphene oxide was prepared from natural flake graphite by Hummers method.17 Finally, all of the products were dried in an oven at 60 °C until constant weight was obtained. 2.2.2. Preparation of Magnetic Cellulose/Graphene Oxide Composites. Magnetic cellulose/graphene oxide composites were prepared via a coprecipitation method modified from Luo’s research. 18 First, 8.0 wt % NaOH/8.0 wt % sulfocarbamide/6.5 wt % urea/77.5 wt % water mixed aqueous solution was precooled to −10 °C in a ice−salt bath. Raw cellulose was immediately dispersed into the mixed aqueous solution under 16 000 rpm/min stirring for 5 min at ambient temperature to obtain a transparent cellulose solution with 6% concentration. The next step, Fe3O4 nanoparticles and GO were mixed and dispersed by ultrasound. After ultrasonic dispersion, the mixture was added into cellulose solution under mechanical agitation for 1 h. Then, the resulting suspension was poured into a coagulation bath containing sodium chloride (15 wt %) under vigorous stirring. The resulting products continued to stir for 12 h. After aging, 5 mL of epichlorohydrin was slowly added to the above mixture, and then it was heated to 70 °C with stirring for 150 min to obtain MCGO. The MCGO was separated by using magnets and washed three times with doubly distilled water and ethanol, respectively. Finally, the product was dried at 60 °C until a constant weight was obtained. 2.3. Experimental Design and Evaluation for RSM. RSM study was carried out to optimize the synthesis conditions of MCGO. Combined effects of the test variables on the response were obtained by the application of Box−Behnken design (BBD) using the software Design-Expert 7.1.6. In this study, the mass ratio of each component in the composite that predominantly affected the extent of adsorption by MCGO was identified as independent test variables. The levels and independent variables are shown in Table 1.
Q=
(C0 − Ce)V , W
E=
(C0 − Ce) × 100% C0
where C0 (mg L−1) is the initial MB concentration and Ce (mg L−1) is the MB equilibrium concentration at time t (min), V (L) is volume of solution, and W (g) is the weight of bioadsorbents. 2.6. Replication of Batch Experiments. Every adsorption experiment was carried out in triplicate to obtain results with error F” less than 0.0500 indicate model terms are significant. For color removal efficiency, m(GO) was found to have the greatest effect on the response, with p-value (prob > F) less than 0.0001 (Table 2), while both the m(cellulose) and m(Fe3O4) exhibited less effect regarding the color removal efficiency. Furthermore, the interaction coefficients (AB, BC, and AC) were insignificant model terms. Three-dimensional plots (Figure 2A−C) were drawn to investigate the relationship between the experimental variables and the response which further get the optimal range of the ratio. In Figure 2 the responses were functions of two variables at the central level of the third one. From the three-dimensional plots, the optimal values (mass of cellulose, 5.01 g; mass of Fe3O4, 2.29 g; mass of GO, 0.28 g) of the variables can be obtained. The three experimental conditions including optimal values were selected as shown in Table 3 to verify the adequacy of the experiment model. It can be seen from the table that the experimental data were in good agreement with the predicted values and the error was less than 5.0%. In order to facilitate the experiments, the ratio of 16:8:1 (mass ratio) which also located in the optimal region was selected. 3.2. Characterization of the Samples. 3.2.1. XRD Analysis. Figure 3 shows the XRD patterns of pure cellulose, GO, Fe3O4, and MCGO. Microcrystalline cellulose exhibited two main peaks at 2θ = 14.86° and 22.74°, corresponding to (110) and (200) planes, respectively.19 The XRD pattern of original GO exhibits a sharp peak at 2θ = 10.80°, which can be ascribed to the (001) plane. For the XRD pattern of pure Fe3O4, the peaks of 30.22°, 35.66°, 43.23°, 53.70°, and 62.78° corresponded to (220), (311), (400), (422), (511), and (440) planes.21 In the XRD pattern of MCGO, there were six obvious peaks, (220), (311), (400), (422), (511), and (440), indicating that Fe3O4 has been introduced into MCGO. However, the diffraction peak of GO has not been observed, suggesting that the layer stacking of GO sheets was destroyed by the loading Fe3O4.23 At the same time, diffraction peaks of the cellulose could not be found in the XRD pattern of the MCGO composites, indicating that the structure of cellulose has been changed during preparation. 3.2.2. FT-IR Analysis. The FT-IR spectra of GO and MCGO before and after adsorption are shown in Figure 4. The peaks at 1090, 1380, and 1716 cm−1 (Figure 4A) correspond to C−O− C stretching vibrations, the C−O−H deformation vibrations, and the CO stretching vibrations of the −COOH groups, respectively.22 The broad and intense band observed at 3402 cm−1 can be ascribed to the stretching vibrations of −OH, while the peak at 1618 cm−1 corresponds to aromatic skeletal CC stretching vibrations of the unoxidized graphitic domains.23 As shown in Figure 4B, there are several characteristic absorption peaks which correspond to GO, indicating that GO was introduced into MCGO composites. Furthermore, there was one characteristic absorbance band located at 2919 cm−1, which corresponds to the stretching vibrations of the −C−H, which are methyl, methylene, and methane of the cellulose. In addition, the −COOH absorbance band has shifted to a lower value, which proves that −COOH on the GO formed hydrogen bonds with the −OH groups of cellulose. Compared with the GO, a new prominent absorbance band appeared at 580 cm−1, which corresponds to the stretching mode of Fe−O.24 The FT-IR spectrum of MCGO after adsorption presents a decrease in the carboxyl group peak at 1716 cm−1, and new
Figure 1. Effects of different qualities of each component on the color removal: (A) m(Fe3O4) = 2.0 g, m(GO) = 0.3 g; (B) m(cellulose) = 3.0 g, m(GO) = 0.3 g; (C) m(cellulose) = 3.0 g, m(Fe3O4) = 2.0 g.
experimental conditions. Furthermore, the model F-value of 129.45 implied the model is significant. Meanwhile, the lack of fit p-value (prob > F) of 0.3747 demonstrated that lack of fit is not significant, which indicates the model was adequate to describe the relationship between variables and the response. 1110
dx.doi.org/10.1021/ie4027154 | Ind. Eng. Chem. Res. 2014, 53, 1108−1118
Industrial & Engineering Chemistry Research
Article
Table 2. ANOVA Results of the Response Surface Quadratic Modela source
sum of squares
degree of freedom
mean square
F-value
p-value prob > F
model A B C AB AC BC A2 B2 C2 residual lack of fit pure error
795.82 0.00101 0.050 81.92 0.014 0.23 1.07 223.98 60.59 362.68 4.78 2.41 2.37
9 1 1 1 1 1 1 1 1 1 7 3 4
88.42 0.00101 0.086 81.92 0.014 0.23 1.07 223.98 60.59 362.68 0.68 0.80 0.59
129.45 0.0015 0.13 119.93 0.021 0.33 1.57 327.90 88.70 530.95
