One-step synthesis of SnO₂-reduced graphene oxide (SOG) composites for efficient removal of organic dyes from wastewater

Abstract

In this work, SnO₂ was used to functionalize graphene oxide (GO) in order to promote the sorption efficiencies of dyes. The texture properties of the as-synthesized SnO₂/rGO (SOG) nanocomposites were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and N₂ adsorption–desorption isotherms. The SOG composites exhibited fast adsorption rates toward multiplex dyes. In this paper, we used rhodamine B (Rh B), methylene blue (MB), methyl green (MG) and methyl red (MR) as model organic dye pollutants, and the maximum adsorption capabilities for Rh B, MB, MG, and MR reached 115.4, 72.2, 76.5 and 108.3 mg g⁻¹, respectively. Moreover, the adsorbent could be easily regenerated by washing it with ethanol. The regenerated SOG nanocomposites show a quite stable adsorption performance which can be reused for dye removal. These results demonstrated that the SOG nanocomposites could be used as a good alternative for the effective removal of organic dyes from wastewater in wide pH ranges.

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1. Introduction

Nowadays, a growing number of contaminants including heavy metals and dyes are detected in water, which poses a potential risk to human health and ecological systems.¹ Dyes are colored organic compounds which can color other substances. These substances are usually present in the effluent water of many industries, such as the textile, leather, paper, printing, and cosmetics industries. Organic dyes such as rhodamine B (Rh B), methylene blue (MB), methyl green (MG) and methyl red (MR) (Fig. 1) are the most frequently used colorants in the textile and food industries. The complex aromatic structures of dyes make them more stable and more difficult to remove from the effluents, which leads them to be discharged into water bodies directly.^2,3 The most commonly used methods for color removal are biological oxidation and chemical precipitation. However, these methods are effective and economic only in the cases where the solute concentrations are relatively high. For the sake of human health and ecological security, inexpensive, swift, effective and environmentally friendly new approaches to remove dyes from wastewater are urgently needed. Among various removal methods, adsorption technology is gaining significance in fundamental studies and industrial applications, due to its easy operation and the wide availability of adsorption materials.⁴

Fig. 1 The chemical structure of four dyes.

Many kinds of adsorbents have been reported to removal of dyes.^5–8 Carbon materials, such as graphene,^9–11 activated carbon,^12,13 and carbon nanotubes (CNTs)^14,15 with high surface area, good stability, and environmentally friendly characteristics, are considered to be ideal candidates for the adsorption of various organic pollutants. However, it is still not satisfactory for the reported removal capability of organic dyes. It is worth pointing out that most of the developed adsorbents cannot be reused and this leads to serious recontamination problems. In addition, adsorption efficiency of many adsorbents reported in literatures was affected by pH and dye types. Therefore, it is still a big challenge to find efficient and reversible dye adsorptions.¹⁶ Tin dioxide is the most thermodynamically stable phase of tin oxide, which has been extensively used in gas sensors,^17,18 energy storage^19,20 and promising adsorbents for water treatment.^21–29 However, these synthesis methods typically involve complex routes, which result in SnO₂ particles with a wide size distribution and non-defined shape. Here, SnO₂-reduced graphene oxide (SOG) nanocomposites were successfully fabricated by a hydrothermal treatment of aqueous graphene oxide in the presence of SnCl₄ and Na₂S₂O₃ and following a reducing route by using NaBH₄. The SOG nanocomposites showed good stability in solutions. We explored the as-prepared SOG nanocomposites as a sorbent for the removal of organic dyes, such as rhodamine B (Rh B), methylene blue (MB), methyl green (MG) and methyl red (MR) without involving any other complex process. Moreover, the adsorbent could be easily regenerated by washing it with ethanol. The SOG composites showed excellent recycled adsorption ability for Rh B, MB, MG and MG with a high adsorption and stability that showed no structure and performance degradation after 10 cycles. It is worth mentioning that the SOG exhibits high adsorption capacities and fast adsorption rates toward multiple organic dyes in wide pH ranges. These results suggest that the SOG can be used as a good alternative for the effective removal of organic dyes from wastewater.

2. Materials and methods

2.1 Chemicals and reagents

Rh B, MG, MB, MR were used as the adsorbates in this study, which were analytical reagent (A. R.) grade, and purchased from Beijing chemical reagent Co. Ltd. Graphite, stannic chloride, sodium thiosulfate, sodium nitrate, potassium permanganate, sulfuric acid (99%), hydrochloric acid, sodium borohydride, hydrogen peroxide (30%) were also A. R. grade, and purchased from Sinopharm Chemical Reagent Co., Ltd. All solutions were prepared by using deionized water (DI water), which obtained using a Milli-Q water purification system (Millipore, Billerica, MA, USA).

2.2 Preparation of SnO₂-reduced graphene oxide (SOG) nanocomposite

GO nanosheets were prepared from natural graphite flakes by a modified Hummer's method.^30,31 SnCl₄ solution (105 mg in 7 mL of DI water) was added slowly into GO solution (40 mg in 20 mL of DI water) with stirring for 3.5 h for ion exchange, then Na₂S₂O₃ solution (224 mg in 8 mL of DI water) was added into the above mixed solution with stirring for 2 h. And then, NaBH₄ solution (20 mg in 5 mL of DI water) was added into the above solution system. After stirring for 5 min, the mixed solution was transferred to a 50 mL sealed autoclave, and then heated at 180 °C for 24 h. The product was collected by suction filtration and washing several times with ethanol and deionized water alternately, and dried at 60 °C for 5 h under vacuum. Pure SnO₂ was also synthesized with a similar route of the SOG without adding GO.

2.3 Characterization

X-ray diffraction (XRD) (Bruker D8 advance) with Cu K_α (λ = 1.5406 Å, 40 kV × 40 mA) was applied to obtain the phase structure of the sample. X-ray photoelectron spectroscopy (XPS) was carried on a 250XI X-ray photoelectron spectrometer with Mg K_α radiation (1253.6 eV) at a base pressure of 1 × 10⁻⁸ Torr. The spectra were recorded with the pass energy of 50 eV after Ar⁺ sputtering of the surface for 15 min. All binding energy values were referenced to the C 1s peak of contaminant carbon at 284.6 eV. Raman spectra were recorded on an in Via-Reflex at an excitation wavelength of 633 nm. The morphology of the as-synthesized sample was obtained with a transmission electron microscopy (TEM, Hitachi H7650B, operating at 80 kV) and a high-resolution transmission electron microscopy (HRTEM, JEM-2010F, operating at 200 kV). N₂ adsorption/desorption isotherms were measured on Thermo Fisher Scientific Surfer Gas Adsorption Porosimeter at 77 K. Before measurement, the sample was degassed at 250 °C for 6 h. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method based on the adsorption data in the linear relative pressure (P/P₀) range of 0.05–0.35. The pore size distribution was calculated from the corresponding adsorption branch of N₂ isotherm by Barrett–Joyner–Halenda method for mesoporous. Absorption spectra were recorded on a Perkin-Elmer Lambda 35 UV/Vis spectrophotometer.

2.4 Adsorption studies

Adsorption experiments were carried out typically by stirring 50 mg of SOG in 200 mL of water solution containing 1 × 10⁻⁵ M organic dye in 500 mL glass bottle. The effects of varying both the adsorbent concentration and the pH of the solution were also studied. All the experiments were carried out at a constant speed of 200 rpm with magnetic stirring. During each run, aliquots of 0.1 mL were withdrawn from the solutions at regular intervals of time, diluted, and centrifuged for 10 min at 2000 rpm, and the absorbance of the supernatant solution was measured. The dye concentration was estimated by using a UV-vis spectrophotometer with 1 cm path-length cell to monitor the absorbance at λ_max, corresponding to the maximum absorbance of the measured dyes. On the basis of Beer–Lambert law, the absorbance range was chosen to vary between 0.1 and 1.0. Calibration curves were plotted to establish the relation between absorbance and concentration of the standard dye solutions. pH measurements were carried out using a PHS-3E pH meter.

2.5 Calculation

The removal (%) of the SOG hybrids was defined based on the following equation:

Removal (%) = (1 − C₁/C₀) × 100% (1)

where C₀ (mg L⁻¹) is the initial dye concentration before removal, C₁ (mg L⁻¹) is the concentration of dye remaining in the solution after treatment for 10 min.

The amount of adsorbed dyes by the SOG in each time interval t, and q_t was calculated by the following mass balance equation,

q_t = (C₀ − C_t)V/m (2)

where q_t is the amount of dye adsorbed per unit weight of SOG at any time t (mg g⁻¹); C₀ and C_t are the initial and liquid phase concentrations of the dye solution at any time t (mg L⁻¹), respectively; V is the volume of the dyes solution (200 mL); and m is the mass of the SOG used (50 mg). For the experiments with varying dye concentrations, the amount of the SOG was 50 mg.

3. Results and discussion

Fig. 2 presents the schematic to show the synthesis route of the SOG, via a hydrothermal method. The possible formation mechanism is that the functional groups on GO (i.e., hydroxyl, carboxyl, epoxide) can serve as the nucleation sites of SnO₂ nanoparticles.³² According to the Pearson's hard and soft acid–base (HSAB) principle,³³ the chemical routes for SnO₂ precursors are described as follows:³⁴

S₂O₃²⁻ + H₂O ↔ HS₂O₃⁻ + OH⁻

Sn⁴⁺ + 4 OH⁻ = Sn(OH)₄

Sn(OH)₄ = SnO₂ + 2H₂O

Fig. 2 Schematic figure of the preparation procedures of SOG nanocomposite.

Firstly, Sn⁴⁺ was anchored onto GO via electrostatic interaction with OH and COOH groups. During the controlled hydrolysis of S₂O₃²⁻, Sn(OH)₄ was formed on GO surface and resulting in the hierarchical SnO₂/GO nanocomposite. The SnO₂/GO nanocomposite were reduced by NaBH₄ to obtain SnO₂/rGO (SOG) nanocomposite.

The morphology and microstructure of the sample were obtained by TEM observation (Fig. 3). It can be clearly seen that GO displayed a wrinkled paper-like structure of the ultrathin sheets and stacking of sheets (Fig. 3a). As shown in Fig. 3b, the SnO₂ nanoparticles are uniformly dispersed on the graphene nanosheets. Furthermore, excess space exists amongst the SnO₂ nanoparticles. Using Nano Measurer soft, we collected 114 particles in Fig. 3b for statistics, and the results show the average diameter of SnO₂ NPs is about 3.67 nm (Fig. 3c). Meanwhile, according to HRTEM image (Fig. 3d), the SnO₂ NPs size could be measured about 3.5 nm, which is in agreement with the statistical results very well.

Fig. 3 (a) TEM image of rGO nanosheets, (b) TEM image of SOG nanocomposite, (c) size distribution of SnO₂ nanoparticles in SOG, (d and f) HRTEM image of SOG nanocomposite (e) XRD patterns of the SOG.

The phase structures of the samples were examined using XRD. As can be seen in Fig. 3e, all diffraction patterns reveal the successful formation of the tetragonal rutile SnO₂ (according to the JCPDS card no. 41-1445). The main diffraction peaks appearing at 26.6°, 33.9°, 37.9°, 51.8° and 64.7°, correspond to the (110), (101), (200), (211) and (112) planes of the tetragonal rutile SnO₂ structure, respectively.³⁵ In addition, the lattice fringe d-space of 0.353 and 0.332 nm can be attributed to the (110) plane of SnO₂ on the basis of the HRTEM image (Fig. 3f). The average crystallite sizes of the as-prepared SOG nanocomposite are about 5.09 nm based on Scherrer equation, which is consistent with the TEM observation. XPS is a powerful tool to identify the elements' states in bulk material. The XPS of survey SOG is shown in Fig. 4a.³⁶ The spectrum of elemental composition analysis show the presence of C 79.89 at%, Sn 4.77 at%, O 15.34 at% in the SOG nanocomposite. Fig. 4b displays the spectra of C 1s of the SOG composite. The C 1s region of graphite oxide gives two components at around 284.6 and 286.8 eV, respectively, which can be generally assigned to the sp² carbon in C–C bonding and the carbon in C–O bonding, indicating that most of the oxygenated functional groups on reduced graphene oxide have been removed during the hydrothermal process. Fig. 4c shows the O 1s XPS spectrum of the SOG nanocomposites, which can be fitted into two peaks. The peak at 531.1 eV is attributed to C=O group in a carbonyl group or carboxylic group, or shoulder peak of O 1s in SnO₂, and the peak at 532.5 eV is ascribed to C–OH and C–O–C groups. Fig. 4d shows two peaks at 487.2 and 495.6 eV which can be attributable to Sn 3d_5/2 and Sn 3d_3/2 spin–orbit peaks of SnO₂. These results confirm the formation of SnO₂ nanoparticles anchored on the surface of graphene sheets.

Fig. 4 (a) XPS survey scan of the SOG; (b) the XPS spectra of the C 1s peaks of the SOG; (c) the XPS spectra of the O 1s peaks of the SOG; (d) the XPS spectra of the Sn 3d peaks of the SOG.

To better understanding the structure of the SOG nanocomposites, Raman spectra of the SOG nanocomposite and GO were performed and the results are shown in Fig. 5a. Both the GO and the SOG nanocomposite exhibited two major D and G peaks, corresponding to the defect formation and the first-order scattering of the E_2g mode of sp² domains in the GO nanosheets, respectively.³⁷ Compared GO with SOG, the D band shifted from 1341 to 1331 cm⁻¹ and the G band shift from 1598 to 1588 cm⁻¹, and the results indicated that GO had been reduced into rGO.³⁸ For the GO, the intensity ratio of the D and G bands (I_D/I_G) was 1.082, while the I_D/I_G ratio of the SOG nanocomposites increased to 1.559, suggestive of reduction of the GO to reduced graphene oxide by using NaBH₄.^39–41 The good distribution and adhesion of SnO₂ nanoparticles on rGO nanosheets may be attributed to the hydrophobic basal plane and hydrophilic edges of rGO nanosheets,⁴² which probably served as a surface ligand binding to the Sn⁴⁺ and there by confined the nucleation of SnO₂ nanoseed and the growth of SnO₂ nanosheets on its surface.

Fig. 5 (a) Raman spectra of GO and SOG, (b) the nitrogen adsorption–desorption isotherms and (inset: pore-size distribution of SOG).

The N₂ adsorption–desorption isotherm is a type IV based on a significant hysteresis loop, which indicates that the SOG possesses a typical mesoporous structure (Fig. 5b). From the adsorption branch of the isotherm, the specific surface area is 390.6 m² g⁻¹ and the total pore volume of the SOG is 1.565 cm³ g⁻¹ calculated by a multi-point Brunauer–Emmett–Teller method. The distribution of pore width is concentrated in the range of a typical mesoporous structure (inset of Fig. 5b) and the most probable pore width is 19.1 nm calculated by using the Barrett–Joyner–Halenda model. The large pore volumes and high specific surface area of the as-prepared SOG should guarantee the adsorption performance of dyes such as Rh B, MB, MG and MR in water.

The adsorption behavior of the SOG was evaluated by using Rh B as a model adsorbate. Three other organic dyes, including MB, MG and MR, were used to evaluate the adsorption universality. In order to investigate the adsorption of Rh B from water onto the SOG, we immersed the SOG (50 mg) into an aqueous solution of Rh B dye (1.0 × 10⁻⁵ M, 200 mL) for a designed time. As shown in Fig. 6a, the UV-vis absorption spectra of the test solution at various times were used to estimate the adsorption process, and the typical Rh B absorption band gradually disappeared correlating with the images observed by the naked eye (inset of Fig. 6a). This result shows the concentration of Rh B decreased to zero in 10 min, which is much faster than those reported in recent literature.^43–52 In order to investigate the potential application of the SOG as an adsorbent in wastewater treatment, we carried out adsorption experiment of the SOG to MG, MR and MB, respectively. Fig. 6b–d showed that the SOG can also remove MG, MR, MB quickly. The maximum adsorption amount of the SOG for Rh B reached to 115.4, 72.2, 76.5 and 108.3 mg g⁻¹ for Rh B, MB, MR and MG, respectively.

Fig. 6 UV-vis spectra of the (a) Rh B (b) MG; (c) MR and MB solution (1.0 × 10⁻⁵ M, 200 mL) and those after treatment with SOG (50 mg) at different times.

To study the preformation of the SOG adsorption, we also chose Rh B as model dye. In comparison to SnO₂ and GO, the SOG show a best adsorption performance for Rh B, which clearly evidences that the SOG possesses more significant adsorption ability for an organic dye (Rh B) than that of pure SnO₂ and GO (Fig. 7a). As is known to all, the absorption behavior of Rh B molecules may result from physical adsorption on the rGO surface. Hydrophobic interactions (π–π stacking) could occur between the hydrophobic basal planes of the SOG nanocomposites and the aromatic rings of the dyes, which lead to strong π–π stacking interactions in neutral conditions. The interaction between the dye molecules and adsorbents are mainly responsible for the superior adsorption of the SOG composites. Additionally, inorganic SnO₂ NPs might absorb some organic dye molecules in water through hydrogen bonding, due to they have highly unusual meso/macroscopic superstructures.

Fig. 7 (a) Plot of (C_t/C₀) of Rh B against time for the SOG, GO, pure SnO₂ nanoparticles, (b) plot of ln(C/C₀) of Rh B against time for the SOG, (c) plot of (C_t/C₀) of Rh B against time for the SOG at different pH, (d) the removal efficiency of the SOG nanocomposite. (e) UV-vis spectra of the mixture dyes before and after dealing with the SOG, (f) the SOG in various cycles (the concentration of dyes is 1 × 10⁻⁵ M).

The adsorption kinetics can be described as ln(C/C₀) = ln(A/A₀) = −kt. The ratio of C_t (the initial concentration of Rh B at adsorption time t) to C₀ (the initial concentration of Rh B) is directly given from the relative intensity ratio of the respective absorbance (A_t/A₀). k is the apparent first-order rate constant (min⁻¹) and t is adsorption time. Fig. 7b shows the ln(C_t/C₀) versus time and good linear relationships are observed with the correlation coefficients (R²) value of 0.993, showing that the adsorption follows the pseudo-first-order kinetics. Calculated from the slope of the straight line, the kinetic adsorption rate constants (k) are −0.694 min⁻¹.

In general, both surface charge of the adsorbent and ionization degree of the adsorbate are strongly affected by the solution pH. Therefore, pH of the solution maybe plays an important role in controlling the interactions between the adsorbent and the adsorbate.^53–56 To assess the effect of pH value on the adsorption efficiency, the adsorption capacities of dyes onto the SOG as a function of the solution pH are shown in Fig. 7c. The present experiments have indicated that Rh B adsorption was less affected by the increasing pH from 4 to 9.18. These results suggest that the SOG have good adsorption efficiency in wide pH ranges and can meet the needs of the actual dye removal in sewage treatment.

Fig. 7d shows that the removal efficiency of Rh B, MB, MG, and MR at concentration of 1.0 × 10⁻⁵ M can reach to 98.2%, 82.3%, 95.7%, and 94.5%, respectively. These results suggest that the as-prepared SOG nanocomposites show excellent removal performance for these dyes. To investigate the practical applications of the as-prepared SOG, the Rh B, MB, MG and MR mixture solution were deal with the SOG (Fig. 7e), these results showed that SOG have an excellent removal performance for multiplex dyes.

The recycling of the SOG is also important for its industrial application. Because of the superior solubility in water, GO sheets are very difficult to separate after adsorption, which seriously reduces their application value. In this study, the used SOG adsorbent can be regenerated by washing it with ethanol solution for the next use. The recycling experiments were easily conducted by washing the SOG with ethanol (10 mL) for 10 min under magnetic stirring. The effect of the recycling times on the adsorption of Rh B, MB, MR and MG onto the SOG is shown in Fig. 7f. Using the recycled SOG as the adsorbent for removal of Rh B (1.0 × 10⁻⁵ M, 200 mL) from water, the completed removal of Rh B was obtained within 10 min, which showed that the adsorption ability of the SOG was almost retained after use and recycling. Actually, after more than ten cycles, the maximum adsorption rate of Rh B for SOG almost no change. The stability of the SOG paves the way for its practical application in sewage treatment. The excellent adsorption performances of the SOG may be ascribed to the big pore volumes, high specific surface area and for the existence of SnO₂ NPs in nanocomposite.

4. Conclusions

In summary, SnO₂ nanoparticles (∼3.67 nm in size) anchored on the rGO framework have been successfully prepared via a simple hydrothermal method. This material has been demonstrated to be highly effective for the removal of the organic dye, such as rhodamine B, methylene blue, methyl red and methyl green, from wastewater. The adsorption equilibrium was attained within times of less than 10 min. The nanostructure with excess buffering space plays a crucial role in the enhancement of absorption properties for removal of contaminants from water. The ultra-fine SnO₂ nanoparticles also directly determine that this composite had good process ability and stability, and exhibited excellent adsorption and desorption performance for organic dyes. Additionally, the experimental results show that the well-designed SOG nanocomposite have great potential to be absorption agents for removal of contaminants from water, and the as-prepared SOG composite can be reused as absorption for more than ten times without a significant loss of its adsorption capacity. Therefore, the SOG may be utilized as a promising candidate for the removal of multiplex organic dyes.

Acknowledgements

This work was jointly supported by the National Program on Key Basic Research Project (973 Program, No. 2013CB933804) and the National Natural Science Foundation of China (No. 21271112, 21231005) are acknowledged.

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