One-step approach for the synthesis of CoFe₂O₄@rGO core-shell nanocomposites as efficient adsorbent for removal of organic pollutants

In recent years, graphene/magnetic oxide (GM) nanocomposites have drawn tremendous scientific interest for their distinguished properties such as strong superparamagnetism, large surface area and excellent extraction ability. GM composites have been successfully applied in drug carriers (Yang et al. 2009), catalyst (Fu et al. 2012; Yao et al. 2013), lithium storage (Yang et al. 2010; Sun et al. 2014) and other fields (Liu et al. 2013; Xue et al. 2014). In addition, GM composites are excellent adsorbents in the field of water treatment. Until now, the adsorptive removal of dyes and heavy metal ions by this type of nanocomposites has been demonstrated. Li et al. (2011) synthesized magnetic CoFe₂O₄-functionalized graphene sheets (FGS) nanocomposites by hydrothermal treatment, and discovered that the cobalt ferrite nanoparticles with diameters of 10–40 nm were uniformly distributed on FGS, and the adsorption capacity for removing methyl orange (MO) is 71.54 mg.g^–1. Qi et al. (2015) found that magnetite/reduced graphene oxide (MRGO) nanocomposites obtained by different methods have different characteristics. The MRGO prepared by the co-precipitation method showed special adsorption ability to negative ions, but those synthesized by the solvothermal method had the best extraction ability and reusability to metal ions. Nevertheless, these reported magnetic nanoparticles are loaded on the surface of graphene sheets (Xie et al. 2012; Li et al. 2014), and may have the defects of detachment and aggregation of magnetic metal oxides from graphene sheets, which decreased their saturation magnetization (Fu et al. 2013; Zhan et al. 2015), and reduced their efficacy in practical applications. One of the most promising strategies to tackle the aggregation problem of metal oxides is to enwrap them with a polymer or silica shell (Dong et al. 2006; Luo et al. 2009). However, the reported materials enwrapped with the polymer or silica shell showed low saturation magnetization. Therefore, the fabrication of core-shell magnetic nanocomposites with both strong magnetic responsivity and high absorption capacity remains challenging. Core-shell structure Fe₃O₄@graphene oxide (GO) submicron particles have been prepared via the two-step electrostatic self-assembly process and exhibited large adsorption capacity for bovine serum albumin (BSA) (Wei et al. 2012).

In this paper, we develop a simple solvothermal route to prepare the core-shell structure CoFe₂O₄@rGO (abbreviated as CFG) nanocomposites, in which the reduction of GO and the crystallization of CoFe₂O₄ crystals happened in one step without adding any reducing agent. The formation mechanism of the core-shell structure was initially discussed. The adsorption properties were also measured using MO as model pollutant in aqueous solutions. The core-shell nanocomposites can suppress the aggregation of oxide nanoparticles, maintain a high saturation magnetization and give rise to a high adsorption capacity. The obtained CFG can be recyclable, and thus has great potential for the removal of toxic pollutants from wastewater.

Graphite powder, ethylene glycol (EG), polyethylene glycol 2000 (PEG 2000), anhydrous ferric chloride (FeCl₃), cobalt (II) chloride hexahydrate (CoCl₂·6H₂O), methylene blue (MB), MO, Congo red (CR), Rhodamine B (RhB) and other reagents were purchased from Sinopharm Chemical Reagent Co. (Hefei, China). All chemicals used here are analytical grade without further purification.

GO was synthesized by the modified Hummers method (Marcano et al. 2010). CFG were prepared by solvothermal method with minor modifications (Xue et al. 2014). In a typical synthesis, the as-prepared GO (0.291 g) was exfoliated by ultrasonication in 70 mL of EG and 2.5 g of polyethylene 2000 (PEG 2000) mixture at 75 °C, followed by the addition of FeCl₃ (2 g) and CoCl₂·6H₂O (1.42 g) to form a brown mixture solution. 30 mL of EG solution containing 9 g of NaAc and 1 g of NaOH were slowly added into the above brown solution, followed by vigorous stirring for 2 h. The whole process was kept ultrasonic to avoid the aggregation of the GO in EG. After that, the resulting mixture was sealed in a Teflon-lined stainless-steel autoclave, maintained at 453 K for 24 h, then cooled naturally to room temperature. The black products were obtained by magnetic separation, washed several times with absolute ethanol and deionized water and dried at 333 K for 12 h.

The powder X-ray diffraction (XRD) patterns of the samples were performed using a Rigaku D/max 2500 V X-ray diffractometer with CuKα radiation (λ = 1.5418 Å). The morphology and structure of the samples were characterized by a field emission scanning electron microscope (FESEM, Hitachi SU8020) and high-resolution transmission electron microscope (HRTEM, JEM-2100F). Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Perkin-Elmer Spectrum 100. X-ray photoelectron spectroscopy (XPS) was conducted using an ESCALAB 250 spectrometer. The magnetic properties of samples were studied using a vibrating sample magnetometer (VSM, PPMS-14T) at a temperature of 300 K. The zeta potential was measured by the ZETASIZER Nano-ZS from Malvern Instruments.

where q_e (mg g⁻¹) represents the adsorption capacity at equilibrium, C₀ and C_e (mg L^–1) are the initial and the equilibrium concentration of the dye remaining in the solution. V (L) is the volume of the aqueous solution, and m (g) is the dry weight of composites.

CFG (0.1 g) was added to 50 mL of MO solution (100 mg L^–1) and then the mixture was shaken for 1.5 h at room temperature. After separation of the nanocomposites by an external magnet, the supernatant solution was analyzed. The nanocomposites' adsorbent with MO was washed with absolute ethanol solution several times, collected by a magnet and reused for the next adsorption experiment. The reusability experiments were performed six times.

In the present work, it is worth noting that the magnetic CoFe₂O₄ nanoparticles decorated onto graphene sheets are obtained under the same conditions only by decreasing the mass ratio of CoFe₂O₄ to GO to 3. According to previous reports (Sharifi et al. 2013), there are several factors that may contribute to the formation of the core-shell CFG. First, the existence of the van der Waals forces between the adjacent magnetic CoFe₂O₄ nanoparticles make them move towards each other and GO sheets are aggregated simultaneously. Second, the graphene sheet can roll itself up into a scroll structure under ultrasonic effect. Third, the nanoparticles attached at the edge of the GO sheets may be attracted by the inner part of the GO sheets due to sparsely distributed oxygen-containing functional groups preceding the rolling (Xue et al. 2014). Although the detailed mechanism of the growth of graphene-CoFe₂O₄ core-shell nanocomposites is not very clear, it is obvious that the mass ratio of CoFe₂O₄ to GO is a key factor for the formation of core-shell hybrids.

Zeta potential of adsorbent is a key factor to influence its adsorption capacity, and thus is measured to understand further why CFG can remove anionic dye more effectively than cationic dye. The zeta potential value of CFG synthesized using EG as solvent is about 30.3 mV, indicating that the surface of CFG is positively charged. The high absolute zeta potential value also proves that the adsorbent will be stable in aqueous solution (Fan et al. 2013; Qi et al. 2015). In addition, we find that the zeta potential value of CFG synthesized with EG/H₂O mixed solution as solvent will drop to –25.3 mV, and thus the adsorption capacity for anionic dyes is obviously decreased.

Based on the above, the mechanism of the adsorption of organic dyes by CFG is probably due to electrostatic interaction. For instance, MO is an anionic azo dye which contains the sulfonic acid groups (R-SO₃Na). In aqueous solution, dye dissociates to the sulfonate anions (R-SO₃⁻) and the sodium ions (Na⁺). Therefore, R-SO₃⁻ ions were easily absorbed to the surface of CFG by electrostatic force. Although the detailed formation mechanism of CFG surface charge was not very clear, it is certain that the type of reaction media caused some surface modifications onto CFG owing to the difference in the surface tension and dielectric constant (Ramesha et al. 2011; Hayyan et al. 2015).

The related parameters of two models are calculated and displayed in Table 2. From the linear correlation coefficients (R²) at different temperatures, it can be found that the Langmuir model is more suitable for describing the adsorption of MO on CFG compared to the Freundlich model. In addition, the F values obtained from the Langmuir equation are far higher than those from the Freundlich equation. The results suggest that monolayer adsorption of MO on CFG is the main mechanism (Li et al. 2012). Besides, the values of q_max calculated from the Langmuir model are the highest at T = 298 K and the lowest at T = 318 K, which indicates that increasing temperature is unfavorable to the adsorption and the adsorption process is exothermic.

Calculated from the Langmuir isotherm model, the maximum equilibrium adsorption capacity (q_max) of MO on CFG is 263 mg g^–1 at 298 K. A comparison of q_max for MO uptake between CFG and different magnetic adsorbents reported in the literature can be seen in Table 3. It is obvious that CFG synthesized in this work exhibits quite a high q_max, which suggests that the as-synthesized CFG can be considered as an effective adsorbent for the removal of MO dye from wastewater.

The thermodynamic data calculated by Equations (5) and (6) are shown in Table 4. The value of ΔH during the adsorption process is calculated to be –34.8 kJ mol^–1, which indicates that the adsorption process is exothermic. Meanwhile, the ΔS values are all positive at different temperatures, which illustrate the increasing randomness at the solid-solution interface during fixation of MO onto the surface of CFG. The negative values of ΔG demonstrate a spontaneous and feasible process for the adsorption. The value of ΔG becomes more negative with the decrease of temperature, which indicates that the adsorption is suitable at low temperature.

In summary, we have synthesized a novel type of core-shell structure CoFe₂O₄@rGO nanocomposites (CFG) via a facile one-step solvothermal route. The as-synthesized CFG have a uniform core-shell structure with an average diameter of about 75 nm, where the CoFe₂O₄ nanoparticles are coated by a layer shell of GO with a thickness of approximately 15–20 nm. The mass ratio of CoFe₂O₄ to GO is a key factor for the formation of core-shell hybrids. CFG displays much higher adsorption capacity for anionic dyes owing to electrostatic interaction between CFG and dyes. Compared with previous literature, CFG exhibits a very high absorption capacity of 263 mg g^–1 for MO at 298 K, and the prepared CFG nanocomposites can be easily separated by an external magnetic field after adsorption. After six cycles of the adsorption–desorption process, their adsorption capacity decreased slightly. This work indicates that CFG have great potential for organic pollutants wastewater treatment.

This work was supported by the National Natural Science Foundation of China (No. 51372062).