Preparation of Graphene Oxide-Based Hydrogels as Efficient Dye Adsorbents for Wastewater Treatment
© Guo et al. 2015
Received: 23 February 2015
Accepted: 11 May 2015
Published: 27 June 2015
Graphene oxide (GO) sheets exhibit superior adsorption capacity for removing organic dye pollutants from an aqueous environment. In this paper, the facile preparation of GO/polyethylenimine (PEI) hydrogels as efficient dye adsorbents has been reported. The GO/PEI hydrogels were achieved through both hydrogen bonding and electrostatic interactions between amine-rich PEI and GO sheets. For both methylene blue (MB) and rhodamine B (RhB), the as-prepared hydrogels exhibit removal rates within about 4 h in accordance with the pseudo-second-order model. The dye adsorption capacity of the hydrogel is mainly attributed to the GO sheets, whereas the PEI was incorporated to facilitate the gelation process of GO sheets. More importantly, the dye-adsorbed hydrogels can be conveniently separated from an aqueous environment, suggesting potential large-scale applications of the GO-based hydrogels for organic dye removal and wastewater treatment.
Nowadays, harmful chemical compounds have become the main cause of water pollution. Water pollution exerts negative effects not only on species living in the water but also on the broader biological community. For instance, organic dyes are often discharged with wastewater into the local environment without adequate treatment. Rapid and convenient removal of organic dyes from wastewater has been a challenging issue faced by scientists [1–6]. For example, Kim’s groups achieved excellent systematic works in the relative fields of water remediation by various nanocomposites [1, 2]. In particular, large-scale application requires the potential dye adsorbents to exhibit a high dye removal rate within a relatively short period of time and to be environmentally friendly. For the latter, the adsorbents must be able to be properly separated from an aqueous environment after adsorbing waste dyes. In the past years, graphene oxide (GO) sheets have attracted broad attention as potential dye adsorbents because of their unique conjugated, two-dimensional (2D) structure, which exhibits superior adsorption capacity for various dye molecules through π-π stacking interactions [7–12]. In addition, the negative charges in the GO sheets due to various oxygen-rich functional groups (i.e., carboxy, carbonyl, hydroxyl groups) allow additional strong electrostatic interactions with cationic dye molecules [13–18]. However, GO sheets exhibit a high dispersibility in water, which prevents the efficient separation of dye-adsorbed GO sheets from an aqueous environment. Therefore, various GO-based adsorbent materials have been developed to facilitate the separation of dye-adsorbed GO sheets from aqueous solutions [19–21]. For example, Akhavan et al. successfully reported the preparation and magnetic separation application of superparamagnetic ZnFe2O4/reduced graphene oxide (rGO) composites by hydrothermal reaction method . In addition, their group has also investigated some bacteria bioactivity and interaction with the environment by aggregated graphene nanosheets as an encapsulating material and effective photothermal agent . In addition, depositing magnetic Fe3O4 nanoparticles on GO sheets can allow facile separation of dye-adsorbed composites by applying an external magnetic field . GO-based porous materials have also been used to adsorb organic waste dyes . Alternatively, GO-based hydrogels provide an effective solution for the easy separation of dye-adsorbed materials from water .
In this work, the facile preparation of GO/polyethylenimine (PEI) hydrogels as efficient dye adsorbents for wastewater treatment was reported. The GO/PEI hydrogels were obtained through both hydrogen bonding and electrostatic interactions between amine-rich PEI and GO sheets. PEI was incorporated to facilitate the gelation process of GO sheets, and the dye adsorption capacity of the hydrogel is mainly attributed to the GO sheets. For both methylene blue (MB) and rhodamine B (RhB), the as-prepared hydrogels exhibit removal rates within 4 h in accordance with the pseudo-second-order model. More importantly, the dye-adsorbed hydrogels can be conveniently separated from an aqueous environment, suggesting potential large-scale applications of the GO-based hydrogels for organic dye removal and wastewater treatment.
The presently used different xerogels were obtained at −50 °C via a lyophilizer (FD-1C-50, Beijing Boyikang Experimental Instrument Co., Ltd., China) to completely remove water over 2–3 days. The morphology of GO and lyophilized GO/PEI hydrogels was characterized by using both field-emission scanning electron microscopy (FE-SEM, S-4800II, Hitachi, Japan) with an accelerating voltage of 5–15 kV and transmission electron microscopy (TEM, HT7700, Hitachi High-Technologies Corporation) with commercial 300-mesh copper grids. Before SEM investigations, the prepared samples were coated with copper foil fixed by a conductive adhesive tape and covered with gold nanoparticles to make them more conductive. X-ray diffraction study was carried out by using an X-ray diffractometer (SmartLab, Rigaku, Japan) equipped with a conventional Cu Kα X-ray radiation (λ = 1.54 Å) source and a Bragg diffraction setup. Transmission Fourier transform infrared (FT-IR) spectra were obtained using a Nicolet iS10 FT-IR spectrophotometer from Thermo Fisher Scientific Inc. (Waltham, MA, USA) with an average of 16 scans and at a resolution of 4 cm−1 by the conventional KBr disk tablet method. Thermogravimetry-differential scanning calorimetry (TG-DSC) analyses of the samples were conducted in air condition by using a Netzsch STA 409 PC Luxx simultaneous thermal analyzer (Netzsch Instruments Manufacturing Co., Ltd., Germany). Raman spectroscopy was performed using a Horiba Jobin Yvon Xplora PLUS confocal Raman microscope equipped with a motorized sample stage. The wavelength of the excitation laser was 532 nm, and the power of the laser was kept below 1 mW without noticeable sample heating. The intensity of a Raman peak was extracted from the maximum value after baseline subtraction over the corresponding spectral range. X-ray photoelectron spectroscopy (XPS) was performed on Thermo Scientific ESCALAB 250Xi using 200-W monochromated Al Kα radiation. The 500-μm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was about 3 × 10−10 mbar. Typically, the hydrocarbon C(1s) line at 284.8 eV from adventitious carbon is used for energy referencing. Both survey scan and individual high-resolution scan peaks were recorded.
The adsorption experiments were designed and modified according to the previous reports [27, 28]. In adsorption experiments, about 1 mL of GO/PEI hydrogel (without lyophilizing) was added to 100 mL of either MB (10 mg·L−1) or RhB (4 mg·L−1) solutions. The dye solutions containing gel adsorbents were stirred slowly and continuously at room temperature in a dark condition. The gel samples were then separated by centrifugation at different time intervals, and the supernatant liquid was collected for subsequent analysis using an UV-vis spectrometer (752, Sunny Hengping, Shanghai, China). The absorbance at 662 nm (MB) and 554 nm (RhB) was used to determine the concentration of residual dyes in the supernatant liquid.
Results and discussion
Kinetic parameters of GO/PEI hydrogel (#2) for MB and RhB adsorptions at 298 K (experimental data from Fig. 7)
GO/PEI hydrogel (#2)
q e (mg/g)
K 1 (min−1)
q e (mg/g)
K 2 (g/min·h)
Considering the obtained experimental results described above, some important points should be proposed and discussed. Firstly, in our recent works about some organogel systems based on organic compounds [47–51], functionalized imide derivatives, with the different substituent groups (such as cholesteryl, azobenzene, or luminol), molecular skeletons, or spacers, can have a profound effect on the gelation abilities and the as-formed nanostructures. In another organogel system based on cationic amphiphile-GO nanocomposites, the headgroups in amphiphiles play a crucial role in the gelation behaviors in various organic solvents . For the present GO/PEI hydrogels, the self-assembly and regular stacking of GO sheets were significantly altered by formulation of PEI attached on the surface of GO sheets. In addition, it should be noted that the present GO/PEI hydrogels are more environmentally friendly than organogels from different organic solvents. Now the drug release behaviors and preparation of nanoparticle-containing hybrid hydrogels generated by the present supramolecular gels are under investigation to display the relationship between the as-formed nanostructures and their applications.
In summary, the facile preparation and dye adsorption capacity of GO/PEI hydrogels have been investigated. PEI was chosen for its abundant amine groups that can form hydrogen bonds with GO. Both the SEM and XRD studies clearly show that the GO sheets were successfully cross-linked in the PEI network. Meanwhile, the Raman spectra suggest that the structural features of GO sheets remain largely unchanged pre- and post-gelation. The as-prepared GO/PEI hydrogels exhibited good removal rates for both MB and RhB in accordance with the pseudo-second-order model. The current research work provides further insight into the applications of GO-based polymer-containing hydrogels as dye adsorbents for wastewater treatment.
This work was financially supported by the National Natural Science Foundation of China (grant nos. 21473153 and 21207112), the Natural Science Foundation of Hebei Province (grant no. B2013203108), the Science Foundation for the Excellent Youth Scholars from Universities and Colleges of Hebei Province (grant nos. Y2011113 and YQ2013026), the Support Program for the Top Young Talents of Hebei Province, and the Open Foundation of National Key Laboratory of Biochemical Engineering (Institute of Process Engineering, Chinese Academy of Sciences).
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