Hierarchical core–shell structured Fe₃O₄@NiSiO₃ magnetic microspheres: preparation, characterization, and adsorption of methylene blue from aqueous solution

Abstract

Hierarchical core–shell structured Fe₃O₄@NiSiO₃ magnetic microspheres have been synthesized using a modified Stöber sol–gel process and solvothermal methods. The prepared composites were investigated by SEM-EDS, TEM, XRD, FTIR, SQUID magnetometry, and nitrogen adsorption/desorption measurements. The microspheres possess high porosity and magnetic properties, which allow the Fe₃O₄@NiSiO₃ microspheres to exhibit efficient adsorption capacity and convenient separation. The adsorption isotherm and adsorption kinetics for the adsorption of methylene blue (MB) on the Fe₃O₄@NiSiO₃ microspheres were analyzed. The adsorption isotherm is well fitted by the Langmuir isotherm model, which is valid for monolayer adsorption of MB on the surface of Fe₃O₄@NiSiO₃ magnetic microspheres. The pseudo-second-order model accurately describes the adsorption kinetics process for the adsorption of MB on the Fe₃O₄@NiSiO₃ microspheres, suggesting a chemical adsorption process. MB removal of the Fe₃O₄@NiSiO₃ magnetic microspheres can reach 87% after the fifth adsorption, indicating good regeneration capacity and reusability.

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1 Introduction

In recent years, water pollution has been a very big problem because of the inappropriate treatment of household and industrial wastes before they are drained into rivers.¹ Dye producing and consuming industries, such as printing, food, paint, plastics, and pharmaceutical, trigger a large volume of toxic wastewater.² MB is one of the basic dyes, which is widely employed in coloring paper, dyeing cottons, wools, coating paper and some medical uses. Massive exposure to MB causes increased heart rate, vomiting, shock, Heinz body formation, cyanosis, jaundice, quadriplegia and tissue necrosis in humans.^3,4 Due to the molecular structures of dyes becoming more complicated and stable, it is difficult to treat the wastewater. Hence, removal of the dye from wastewater is of utmost importance.

A great number of physical and chemical methods such as precipitation, coagulation,⁵ reverse osmosis, ozonation,^6,7 filtration, adsorption,^8–11 and advanced oxidation processes,^12,13 etc. have used for removal of dyes. Among these methods, adsorption takes advantages of its ease of operation, high efficiency, low energy consumption, low operating cost and low sensitivity to toxic environments, generally considered as the preferred method for removing organic dyes from aqueous solution.^14,15 With the rapid advance of nanotechnology, there has brought various nanomaterials in the respect of treatment of dyeing aqueous solution. Different morphologies of micro/nanostructures have been used as adsorbents for the adsorption of organic dyes in aqueous solution, which exhibit remarkably enhanced sorption capacity. Among the various morphologies, the fabrication of three-dimensional (3D) hierarchical inorganic micro/nanomaterials by using nanoparticles (0D), nanorods (1D), and nanoplates (2D) as building blocks, has attracted considerable attention due to their large surface area, abundant surface functional groups, developed porous structure, good chemical stabilities, and their potential applications in catalysis, energy conversion and storage, environmental abatement, and sensing.¹⁶ To date, lots of micro/nanomaterials have been used as adsorbents for water decontamination.^15,17–23 However, these adsorbents still suffer from issues involving separation inconvenience from a large volume of water, which limits the application of them in wastewater treatment. Thus, the development of new micro/nanomaterials as adsorbents with a facile separation property is great interest.^24–28

It is worth mentioning that iron oxide nanoparticles, in particular magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃), have been demonstrated a rapid response to the external magnetic field which can be extensively applied in designing nanocomposite adsorbents. Recently, considerable efforts have been devoted to the functionalization magnetic nanoparticles by coating them with other materials, such as inorganic materials, polymers, etc.^29–33 Surface functionalization has been found to enhance the stability and sorption capacity of magnetic nanocomposites. Thus, it is highly essential to explore new functional composites with efficiently magnetic separation and absorptive capacity while exhibit a low-cost and environmentally benign nature.

Nickel-based materials have an important function, and are employed in various fields, such as catalysis,³⁴ biomolecule separation,³⁵ and adsorption³⁶ because of its high chemical and thermal stability. As a nickel-based material, nickel silicate with high surface area could enhance the accessibility of adsorbates to reactive sites. In the past years, researches have been conducted to functionalize the nickel silicate porous shell with magnetic nanoparticles core to apply for the affinity purification of his-tagged proteins.^35,37 To the best of our knowledge, there have been few reports on the application of Fe₃O₄@NiSiO₃ particles in adsorption of dyes from aqueous solution. Herein, we report a combination of Fe₃O₄ with NiSiO₃ for synthesis of hierarchical core–shell structured Fe₃O₄@NiSiO₃ magnetic microspheres (Scheme 1). In the synthetic process, the SiO₂ as linker shell was coated on the Fe₃O₄ by modified Stöber sol–gel process and then the SiO₂ shell was broken by hydroxide ions into silicate ions which reacted with nickel–ammonia complexes to form NiSiO₃ shell on the Fe₃O₄. The synthetic Fe₃O₄@NiSiO₃ magnetic microspheres with hierarchical shell and magnetic core possess high porosity and magnetic property, which allow the components to exhibit adsorption property and separation property in aqueous solution.

Scheme 1 Schematic illustration of synthesis of hierarchical core–shell structured Fe₃O₄@NiSiO₃ magnetic microspheres.

2 Experimental

Materials

FeCl₃·6H₂O, NiCl₂·6H₂O, NH₄Cl, ethylene glycol (EG) and aqueous ammonia (25–28%) were purchased from Beijing Chemical Reagent Co. Na₃Cit and anhydrous NaOAc were purchased from Beijing Yili Fine Chemicals Co., Ltd. Tetraethyl orthosilicate (TEOS) was purchased from Sinopharm Chemical Reagent Co., Ltd. All of this chemicals were of analytical grade reagents and used directly without further purification.

Synthesis of Fe₃O₄ microspheres

Fe₃O₄ microspheres were prepared by the solvothermal method. FeCl₃·6H₂O (0.54 g) was first mixed with EG (10 mL) under magnetic stirring to form a clear solution. Then, Na₃Cit (0.20 g) was added, stirring for a period of time to form a homogeneous solution, followed by the addition of NaOAc (0.90 g). The homogeneous mixture was transferred into a Teflon-lined autoclave and sealed to heat at 230 °C for 10 h. The synthesized mixture was then allowed to cool naturally to room temperature. The black products were washed with ethanol and distilled water for three times respectively. At last, the Fe₃O₄ microspheres were redispersed in ethanol for farther use.

Synthesis of Fe₃O₄@SiO₂ core–shell microspheres

In a classical procedure, above-mentioned Fe₃O₄ microspheres were dispersed in the mixture solution of H₂O (20 mL) and ethanol (136 mL). The mixture solution was sonicated for 15 min. After that, concentrated NH₄OH solution (25 wt%, 6.0 mL) mixed with H₂O (20 mL) was added dropwise to the solution under sonication. After sonication for about 15 min, TEOS (1.0 mL) dispersed in ethanol (24 mL) was then added drop by drop to the solution under ultrasonication. The resulting solution was sonicated for 90 min at room temperature. The obtained particles were washed with ethanol and distilled water for three times respectively. At last, the Fe₃O₄@SiO₂ microspheres were redispersed in ethanol for farther use.

Synthesis of Fe₃O₄@NiSiO₃ core–shell microspheres

Fe₃O₄@NiSiO₃ microspheres were prepared by the solvothermal method. The synthetic Fe₃O₄@SiO₂ (40 mg) dispersed in ethanol (40 mL) was under ultrasonication for 30 min. NiCl₂·6H₂O (266.6 mg) and NH₄Cl (553.0 mg) were dissolved in a mixture solution containing deionized water (20 mL), ethanol (20 mL) and ammonia solution (2.5 mL, 28%). These two solutions were mixed with each other and transferred into a Teflon-lined autoclave (100 mL) and sealed to heat at 160 °C for 12 h. After the reaction, the autoclave was cooled to the room temperature. The aurantius precipitate (Fe₃O₄@NiSiO₃ microspheres) was collected by centrifugation and washed with ethanol and distilled water for three times respectively and then dried in a vacuum at 40 °C overnight. In addition, Ni element was replaced by Co, Cu and Zn element respectively (Fig. S2†). The procedure was similar to the synthesis of Fe₃O₄@NiSiO₃ except that MCl₂ (M = Co, Cu and Zn) was used in place of NiCl₂.

Dye adsorption and adsorbent regeneration experiment

The desired amount of the Fe₃O₄@NiSiO₃ microspheres as adsorbent in the suspension was mixed with the aqueous solutions of MB. After ultrasonication for a certain time, the Fe₃O₄@NiSiO₃ adsorbent was separated by a magnet and the supernatant solution was analyzed with UV-vis spectrometer at 664 nm to determine the concentrations of dyes in the solution. To study the change of the solution pH before and after adsorption, we have measured the solution pH of the addition of the adsorbents and the solution pH before and after the adsorption of MB by the pH test paper (Fig. S3†). The solution pH kept at 7 (Fig. S3†) during the adsorption of MB, which indicated that there was little effect on the solution pH. To estimate the adsorption capacity, the initial concentrations of MB were varied in the range of 1–200 mg L⁻¹, and the dosage of Fe₃O₄@NiSiO₃ adsorbent was kept at 5 mg. The mixture was sonicated at room temperature (300 K) for a certain time and the concentration of MB in the final solution was measured with UV-vis spectroscopy after the separation of adsorbent by a magnet. At the initial MB concentration of 18.11 mg L⁻¹, the adsorption kinetic of Fe₃O₄@NiSiO₃ adsorbent was obtained by monitoring the concentration of MB at various time intervals. In order to research the regeneration of Fe₃O₄@NiSiO₃ adsorbent, 5 mg of Fe₃O₄@NiSiO₃ adsorbent was added to 5 mL of dye solution with a concentration of 18.11 mg L⁻¹, and the mixture was carefully sonicated at ambient temperature for 210 min. After magnetic separation, the concentration of the supernatant residual dye solution was monitored with UV-vis spectrometer. The MB-adsorbed Fe₃O₄@NiSiO₃ adsorbent was washed using ethylene glycol and ethanol for several times, which could realize the regeneration of the adsorbent. The regeneration experiment was successively repeated five times.

Measurements and characterizations

A scanning electron microscope (SEM, S4800, Hitachi) equipped with an energy-dispersive X-ray spectrum was applied to determine the morphology and composition of the as-prepared samples. The SEM samples were prepared by depositing a dilute aqueous dispersion of the as-prepared samples onto a silicon wafer. Transmission electron microscope (TEM) measurements were carried out on a JEOL JEM-2010EX transmission electron microscope with a tungsten filament at an accelerating voltage of 200 kV. The samples were prepared by placing a drop of prepared solution on the surface of a copper grid and dried at room temperature. X-ray powder diffraction (XRD) measurements were performed on a D8 Focus diffractometer (Bruker) at a scanning rate of 2.5° min⁻¹ in the 2θ range from 20° to 70°, with using of Cu-Kα radiation (λ = 0.15405 nm). FTIR spectra was measured using a Nicolet 6700 IR Fourier spectrometer equipped with smart iTR FTIR attachment in the range from 500 to 4000 cm⁻¹. The magnetization curves of the products were recorded on a Quantum Design MPMS-XL7 superconducting quantum interference device (SQUID) magnetometer at 300 K. N₂ adsorption–desorption isotherms were recorded on a Micromeritics ASAP 2020M automated sorption analyzer. UV-vis spectroscopy was carried out with a JASCO V-550 UV-vis spectrometer.

3 Results and discussion

The size and morphologies of the three as-synthesized samples were investigated by SEM and TEM (Fig. 1). Fig. 1a shows the SEM images of the highly water-dispersible Fe₃O₄ microspheres which synthesized by one-step solvothermal strategy. It can be seen that the Fe₃O₄ microspheres with rough surface have a relatively uniform diameter of about 350 nm. A typical TEM image (Fig. 1d) of the Fe₃O₄ microsphere shows that the microspheres are composed of a large quantity of small nanoparticles of about 10–15 nm, which could explain why the surface of Fe₃O₄ is rough. After coating the Fe₃O₄ microsphere with SiO₂ via a typical modified Stöber sol–gel reaction, a SiO₂-shell–Fe₃O₄-core structure was obtained, whose surface is much smoother than Fe₃O₄ (Fig. 1b). The Fe₃O₄ cores are black spheres, the silica shells of Fe₃O₄@SiO₂ are a gray color with an average thickness of about 100 nm (Fig. 1e). The SEM image in Fig. 1c shows that the Fe₃O₄@NiSiO₃ microspheres present a flowerlike hierarchical surface with a relatively average diameter of about 550 nm. The detailed morphologies of the as-synthesized Fe₃O₄@NiSiO₃ microspheres are shown in Fig. 1f and S1,† which reveal each NiSiO₃ nanoflake perpendicularly grafted to the solid core so that the external surface of the microspheres is composed of the edges of the flakes. The dark regions represent the Fe₃O₄ microsphere cores and the bright regions represent the NiSiO₃ nanoflake shells, which clearly displays that the Fe₃O₄ core is well wrapped by the coating NiSiO₃ flowerlike nanoflake layers.

Fig. 1 SEM image of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, (c) Fe₃O₄@NiSiO₃; TEM image of (d) Fe₃O₄, (e) Fe₃O₄@SiO₂, (f) Fe₃O₄@NiSiO₃.

A possible formation process of hierarchical NiSiO₃ shell is due to the fact that the Si–O bonds were broken to form silicate ions under an alkaline condition, and the generated silicate ions continually reacted with nickel–ammonia complexes to form NiSiO₃ nanoparticles under high temperature. Then these nanoparticles grow along the 2D direction, thereby resulting in the formation of nanoflakes. Third, the nanoflakes grow until all the nanoparticles are consumed, accompanied by their self-organization into the flowerlike hierarchical structure on the surfaces of Fe₃O₄ microspheres. Finally, the flowerlike Fe₃O₄@NiSiO₃ hierarchical microspheres are obtained. In the experiments, the morphology of the as-synthesized NiSiO₃ was a porous structure composed of flowerlike hierarchical structure. When Ni element was replaced by Co and Cu, there was only nanoparticles heterogeneous coating on the Fe₃O₄ microspheres, which was absent of porous flowerlike hierarchical structure (Fig. S2A and B†). Furthermore, when Ni element was replaced by Zn, there was countable nanoflake perpendicularly grafted to the Fe₃O₄ microspheres, which was far from porous structure (Fig. S2C†). The results indicate that the Ni element plays an important role in the formation of the flowerlike hierarchical structures.

In order to prevent the deposition of nickel hydrate during the process, NH₃–NH₄Cl buffer system was introduced to maintain the pH value throughout the experiments, which would be further confirmed by the XRD analysis later in the paper. The above results indicate that the synthetic composites are hierarchical core–shell structured microspheres.

The core–shell structure and the composition of the composites were father ascertained by using the energy dispersive X-ray (EDS) analyses. The EDS spectrum confirms the presence of Fe, Ni, Si, and O in the as-prepared Fe₃O₄@NiSiO₃ microspheres (Fig. 2a). The sample was on the silicon wafer during the EDS analyses, which resulted in the strong peak of Si in the EDS spectrum. To further investigate their microstructure, elemental mapping is employed to investigate the elemental distributions in the core–shell structure. EDS mapping images (Fig. 2c–f) correspond to the elemental distribution of Fe, O, and Ni. The Fe element stays in the core region, and the Ni element is detected in the shell region, while the O element can be observed in both regions. The EDS line scanning data (Fig. 2b) is also characteristic of a core–shell type structure. Fe element as a component of the core part is dominant at the center of microsphere, and Ni element as a component of the microsphere shell is abundant at the edge of the microsphere. These results further confirm the core–shell structure of Fe₃O₄@NiSiO₃ microspheres.

Fig. 2 Energy-dispersive X-ray spectroscopy (EDS) spectrum (a), line scanning data (b) and mapping images(c–f): (d) Fe element, (e) Ni element, (f) O element of Fe₃O₄@NiSiO₃ microsphere.

Fig. 3A shows the X-ray diffraction patterns of Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@ NiSiO₃. The sharp diffraction peaks at 30.2°, 35.5°, 43.5°, 53.5°, 57.3°, and 62.8° are indexed as the (220), (311), (400), (422), (511), and (440) lattice planes of standard Fe₃O₄ (JCPDS no. 19-0629).^38,39 No other peaks are observed in the pattern of Fe₃O₄, indicating that the sample is pure Fe₃O₄ crystalline phase. The reflection characteristic of amorphous SiO₂ is appeared in the pattern of Fe₃O₄@SiO₂. The presence of diffraction peaks at 24.6°, 34.6°, 40.6°, 53.7°, and 60.5° in the pattern of Fe₃O₄@NiSiO₃ correspond to the (103), (110), (200), (210), (300) planes, respectively, of nickel silicate crystal (JCPDS 43-0664).^40,41 Furthermore, no other peaks for other phases can be detected in the pattern of Fe₃O₄@NiSiO₃, indicating that no other reaction occurred during the hierarchical NiSiO₃ shell formation process.

Fig. 3 (A) XRD patterns of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, and (c) Fe₃O₄@NiSiO₃; (B) FTIR spectra of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, and (c) Fe₃O₄@NiSiO₃.

Fig. 3B presents the FTIR spectra of Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@NiSiO₃ nanocomposites. The absorption peak at 546 cm⁻¹ corresponded to the Fe–O vibration related to the magnetite phase (Fig. 3Ba). The absorption bands at 1552 cm⁻¹ and 1381 cm⁻¹ are assigned to the stretching vibrations of carboxyl salt, which is because of that Na₃Cit was added in the synthesis of Fe₃O₄. Compared with Fe₃O₄, the Fig. 3Bb presents an intense adsorption peak at 1066 cm⁻¹ and two weak peaks at 953 cm⁻¹ and 796 cm⁻¹, which could be ascribed to the vibrations of Si–O–Si, Si–OH, and Si–O groups in the SiO₂ shell. In Fig. 3Bc, two adsorption peaks at 953 cm⁻¹ and 796 cm⁻¹ disappear, and the peak at 1066 cm⁻¹ shifts to 980 cm⁻¹, which might be ascribed to the formation of Si–O–Ni bonds.⁴¹ Based on all above characterization results, the hierarchical core–shell microspheres with magnetic Fe₃O₄ cores and hierarchical NiSiO₃ shells have been successfully synthesized via a facile and simple approach.

Fig. 4A shown that the magnetic hysteresis loops of Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@NiSiO₃ at room temperature (T = 300 K) and the saturation magnetization values are 70.5 emu g⁻¹, 38.4 emu g⁻¹, and 21.0 emu g⁻¹, respectively. All the samples have strong magnetism with negligible coercivity and remanence at room temperature. No hysteresis loop can be observed, which shows a superparamagnetic characteristic. The saturation magnetization of Fe₃O₄@NiSiO₃ decreases evidently compared with Fe₃O₄. However, the value is strong enough to achieve a facile magnetic separation. Fe₃O₄@NiSiO₃ could be fast aggregated by an external magnetic field from their homogeneous dispersion. After removing of the magnetic field, the redispersion of the nanocomposites occurred quickly with a slight shaking (inset of Fig. 4A). The magnetic property is essentially important for the separation and reuse of the synthetic Fe₃O₄@NiSiO₃ microspheres as an adsorbent in aqueous solution.

Fig. 4 (A) Room-temperature (300 K) magnetic hysteresis loops of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, (c) Fe₃O₄@NiSiO₃ and photograph of magnetic separation and redispersion process (inset) of Fe₃O₄@NiSiO₃ in water solution; (B) N₂ sorption isotherms and pore size distribution (inset) of Fe₃O₄@NiSiO₃.

The N₂ adsorption and desorption analysis was then introduced to investigate the specific surface area and porosity of the core–shell structured Fe₃O₄@NiSiO₃ microspheres. The typical N₂ adsorption–desorption isotherm for the Fe₃O₄@NiSiO₃ microspheres and the corresponding pore size distribution are shown in Fig. 4B. This isotherm can be categorized as type IV with a hysteresis loop observed at a relative pressure of 0.01–1.0. At low relative pressure (P/P₀ < 0.45), the adsorption and desorption curve coincide because of reversible monolayer adsorption. At a higher relative pressure region (0.45 < P/P₀ < 1.0), the isotherm has significant hysteresis, which can be ascribed to the presence of a mesoporous structure in the interleaving nanoplates.^42,43 The BET surface area and the total pore volume of the as-prepared Fe₃O₄@NiSiO₃ microspheres are calculated to be 121.7 m² g⁻¹ and 0.23 cm³ g⁻¹, respectively. The pore-size distribution plot (inset of Fig. 4B) confirms that the Fe₃O₄@NiSiO₃ microspheres have well-developed mesopore with a diameter of 3.73 nm. The large specific surface area and high porosity of the core–shell structured Fe₃O₄@NiSiO₃ microspheres make them very promising candidates for the adsorption of dyes in aqueous solution.

Nanomaterial adsorbents showed higher adsorption properties than bulk materials because of the nanoscale effects, which may provide an efficient way to adsorb of dyes from aqueous solution. Porous materials with high specific surface area are usually employed for adsorption of dyes from aqueous solution because of their capability to adsorb a large quantity of pollutants, and high efficiency in degrading the unwanted species. The as-prepared hierarchical core–shell structured Fe₃O₄@NiSiO₃ microspheres with high surface area and large total pore volume can be used as adsorbent to remove organic dyes with a simple and rapid magnetic separation via a magnetic field. The adsorption of MB for Fe₃O₄@NiSiO₃ microspheres was studied by UV-vis spectrophotometer at 664 nm. UV-vis absorption spectroscopy measurements were performed to determine the concentration of MB before and after adsorption experiments.

In the past years, there has brought various materials for adsorption of MB from aqueous solution.^15,44

The adsorption isotherm is an effective method of investigating the adsorption ability of the adsorbent and understanding the interactions between the adsorbate and adsorbent. Fig. 5(a) shows the adsorption isotherm of MB on the Fe₃O₄@NiSiO₃ microspheres. The amount of adsorbed MB dramatically increased at a lower final solution concentration, suggesting a high affinity between MB molecules and the Fe₃O₄@NiSiO₃ microspheres surface. The adsorbed amount then reached a plateau at a higher equilibrium solution concentration (18.11 mg g⁻¹), reflecting the saturated adsorption. Although the adsorption capacity of the Fe₃O₄@NiSiO₃ is not extremely high compared with those materials obtained in the reported literature^15,21,44,45 for the absorption of dye in aqueous solution, the magnetic absorber Fe₃O₄@NiSiO₃ could be separated conveniently from a large volume of aqueous solution which could avoid the secondary pollution. In this study, Langmuir, Freundlich and Temkin isotherm models were used to describe the equilibrium adsorption. The fitting isotherms and corresponding parameters are shown in Fig. 5(b–d) and summarized in Table S1.† The correlation coefficients (R²) indicate that the Langmuir isotherm model fits the experimental data best (R_L² = 0.9924), showing that MB adsorption on Fe₃O₄@NiSiO₃ microspheres could be described by the Langmuir isotherm model, in accordance with monolayer adsorption. Also, the maximum adsorption capacity of Langmuir isotherm model is 19.14 mg g⁻¹ (Table S1†), which is close to the experimental value (18.11 mg g⁻¹) than other two isotherm models. The Langmuir model is valid for monolayer adsorption on a surface containing a finite number of identical sites. There are no interactions among the adsorbate molecules and no further adsorption can take place at the sites that have been occupied by dye molecules.⁴⁶ The result demonstrates the homogeneous nature of the sample surface and the formation of monolayer coverage of MB molecules on the adsorbent.

Fig. 5 (a) Adsorption isotherm, (b) Langmuir, (c) Freundlich, (d) Temkin isotherm models for the adsorption of MB on the Fe₃O₄@NiSiO₃ microspheres.

The adsorption kinetic behavior of the as-prepared Fe₃O₄@NiSiO₃ microspheres for MB adsorption from aqueous solutions was investigated, which provided important data for understanding the dynamic of sorption reaction. As shown in Fig. 6, the composites exhibited a continuous adsorption process, with equilibrium time of approximately 210 min for MB adsorption. The pseudo-first-order and pseudo-second-order reaction rate equations are the most commonly applied models to examine the adsorption mechanism based on the empirical data.

Fig. 6 Adsorption kinetic curve for the adsorption of MB on the Fe₃O₄@NiSiO₃ microspheres.

The pseudo-first-order model:

The pseudo-second-order model:

where q_e and q_t represent the amount of dye adsorbed (mg g⁻¹) at equilibrium and time t, respectively, and the k₁ (min⁻¹) or k₂ (g mg⁻¹ min⁻¹) values are the kinetic rate constants.^47,48

The Fig. 7(a) and (b) illustrate the straight line plot of ln(q_e − q_t) against time for the pseudo-first order reaction, as well as the t/q_t plot against time for the pseudo-second-order reaction in the adsorption of MB dye onto Fe₃O₄@NiSiO₃ microspheres. The kinetic parameters (Table S2†) indicate that the pseudo-second-order model with a high correlation coefficient (R₂² = 0.9996) seemed better to accurately describe present adsorption process than the pseudo-first-order model. At the same time, the theoretical q_e value, estimated from the pseudo-second order kinetic model, is very close to the experimental q_e value. Therefore, the adsorption kinetic follows the pseudo-second-order model, suggesting a chemical adsorption process,⁴⁹ which is as the rate-limiting process.⁵⁰ The overall rate of adsorption can be described by the following three steps: (1) surface diffusion where the sorbate is transported from the bulk solution to the external surface of sorbent, (2) intraparticle or pore diffusion, where sorbate molecules move into the interior of sorbent particles, and (3) adsorption on the interior sites of the sorbent.

Fig. 7 (a) pseudo-first-order kinetic plot and (b) pseudo-second-order kinetic plot for the adsorption of MB on the Fe₃O₄@NiSiO₃ microspheres.

The regeneration ability of the adsorbent is crucial for its practical application and economic necessity. Fig. 8 shows the correlation between the adsorption efficiency of MB and the cycle number, confirming that the MB removal can reach 87% after the fifth adsorption, indicating good regeneration capacity and reusability. The decrease in the adsorption capacity was attributed to the incomplete desorption of dyes from the surface of Fe₃O₄@NiSiO₃ microspheres.

Fig. 8 The percentage MB removal by adsorption using the Fe₃O₄@NiSiO₃ microspheres in recycle runs.

All in all, the prepared hierarchical core–shell structured Fe₃O₄@NiSiO₃ magnetic microspheres as an adsorbent can be easily recycled and reused several times, which could make the adsorbent long-term use in adsorption of MB from aqueous solution.

4 Conclusions

In conclusion, hierarchical core–shell structured Fe₃O₄@NiSiO₃ magnetic microspheres have been synthesized by modified Stöber sol–gel process and solvothermal method. The outer hierarchical NiSiO₃ shell could support the Fe₃O₄@NiSiO₃ microspheres exhibit excellent performance in the adsorption of MB, and the magnetic Fe₃O₄ core allows the composites to be conveniently separated by using a magnet. The adsorption isotherm demonstrates the monolayer adsorption of MB on the surface of Fe₃O₄@NiSiO₃ magnetic microspheres and the adsorption kinetic shows a chemical adsorption process. In addition, the removal capacities of Fe₃O₄@NiSiO₃ microspheres for MB were maintained well after several cycles, which could make the adsorbent long-term use in adsorption of MB from aqueous solution.

Acknowledgements

This work was financially supported the National Natural Science Foundation of China (Grants 21201160).

Notes and references

1. Y.-F. Lin, H.-W. Chen, P.-S. Chien, C.-S. Chiou and C.-C. Liu, J. Hazard. Mater., 2011, 185, 1124–1130.
2. S. A. Saad, K. M. Isa and R. Bahari, Desalination, 2010, 264, 123–128.
3. V. Vadivelan and K. V. Kumar, J. Colloid Interface Sci., 2005, 286, 90–100.
4. K. V. Kumar, V. Ramamurthi and S. Sivanesan, J. Colloid Interface Sci., 2005, 284, 14–21.
5. J.-W. Lee, S.-P. Choi, R. Thiruvenkatachari, W.-G. Shim and H. Moon, Water Res., 2006, 40, 435–444.
6. M. Muthukumar, D. Sargunamani, N. Selvakumar and J. Venkata Rao, Dyes Pigm., 2004, 63, 127–134.
7. H. Selcuk, Dyes Pigm., 2005, 64, 217–222.
8. V. Janaki, K. Vijayaraghavan, B.-T. Oh, K.-J. Lee, K. Muthuchelian, A. K. Ramasamy and S. Kamala-Kannan, Carbohydr. Polym., 2012, 90, 1437–1444.
9. S. Chatterjee, M. W. Lee and S. H. Woo, Bioresour. Technol., 2010, 101, 1800–1806.
10. L. S. Silva, L. C. B. Lima, F. C. Silva, J. M. E. Matos, M. R. M. C. Santos, L. S. Santos Júnior, K. S. Sousa and E. C. da Silva Filho, Chem. Eng. J., 2013, 218, 89–98.
11. J. J. Lunhong Ai and J. Tang, Chin. J. Appl. Chem., 2010, 6, 710–715.
12. T.-C. Hsu, Fuel, 2008, 87, 3040–3045.
13. Q. F. Alsalhy, T. M. Albyati and M. A. Zablouk, Chem. Eng. Commun., 2013, 200, 1–19.
14. T. M. Al-Bayati, Part. Sci. Technol., 2014, 32, 616–623.
15. J. Gong, J. Liu, X. Chen, Z. Jiang, X. Wen, E. Mijowska and T. Tang, J. Mater. Chem. A, 2015, 3, 341–351.
16. Q. Hao, S. Liu, X. Yin, Z. Du, M. Zhang, L. Li, Y. Wang, T. Wang and Q. Li, CrystEngComm, 2011, 13, 806–812.
17. R. Rostamian, M. Najafi and A. A. Rafati, Chem. Eng. J., 2011, 171, 1004–1011.
18. Z. Gan, A. Zhao, Q. Gao, M. Zhang, D. Wang, H. Guo, W. Tao, D. Li, E. Liu and R. Mao, RSC Adv., 2012, 2, 8681–8688.
19. S. Mahdavi, M. Jalali and A. Afkhami, Chem. Eng. Commun., 2013, 200, 448–470.
20. X. Wang, W. Cai, G. Wang, Z. Wu and H. Zhao, CrystEngComm, 2013, 15, 2956–2965.
21. J. Gong, J. Liu, Z. Jiang, X. Wen, E. Mijowska, T. Tang and X. Chen, J. Colloid Interface Sci., 2015, 445, 195–204.
22. F. L. Yuli Yin, W. Rao, Z. h. Zhang and L. Yan, Chin. J. Appl. Chem., 2015, 32, 472–480.
23. W. Z. Zhongzheng Wu, D. Wang, L. Cun and Z. Wu, Chin. J. Appl. Chem., 2014, 9, 1089–1095.
24. M. N. Nadagouda, C. Bennett-Stamper, C. White and D. Lytle, RSC Adv., 2012, 2, 4198–4204.
25. V. Yathindranath, M. Worden, Z. Sun, D. W. Miller and T. Hegmann, RSC Adv., 2013, 3, 23722–23729.
26. J. C. Xu, P. H. Xin, Y. B. Han, P. F. Wang, H. X. Jin, D. F. Jin, X. L. Peng, B. Hong, J. Li, H. L. Ge, Z. W. Zhu and X. Q. Wang, J. Alloys Compd., 2014, 617, 622–626.
27. D. Guo, W. Ren, Z. Chen, M. Mao, Q. Li and T. Wang, RSC Adv., 2015, 5, 10681–10687.
28. H. Y. Zhu, R. Jiang, S. H. Huang, J. Yao, F. Q. Fu and J. B. Li, Ceram. Int., 2015, 41, 11625–11631.
29. C. Chen, P. Gunawan and R. Xu, J. Mater. Chem., 2011, 21, 1218–1225.
30. K. Mandel, F. Hutter, C. Gellermann and G. Sextl, ACS Appl. Mater. Interfaces, 2012, 4, 5633–5642.
31. A. Farrukh, A. Akram, A. Ghaffar, S. Hanif, A. Hamid, H. Duran and B. Yameen, ACS Appl. Mater. Interfaces, 2013, 5, 3784–3793.
32. X. Zhang, M. Lin, X. Lin, C. Zhang, H. Wei, H. Zhang and B. Yang, ACS Appl. Mater. Interfaces, 2014, 6, 450–458.
33. Y. Xu, J. Jin, X. Li, Y. Han, H. Meng, C. Song and X. Zhang, Microchim. Acta, 2015, 182, 2313–2320.
34. X. Song and L. Gao, J. Phys. Chem. C, 2008, 112, 15299–15305.
35. Z. Liu, M. Li, F. Pu, J. Ren, X. Yang and X. Qu, J. Mater. Chem., 2012, 22, 2935–2942.
36. L. Tan, X. Zhang, Q. Liu, J. Wang, Y. Sun, X. Jing, J. Liu, D. Song and L. Liu, Dalton Trans., 2015, 44, 6909–6917.
37. Y. Wang, G. Wang, Y. Xiao, Y. Yang and R. Tang, ACS Appl. Mater. Interfaces, 2014, 6, 19092–19099.
38. Y. Wang, Y. Shen, A. Xie, S. Li, X. Wang and Y. Cai, J. Phys. Chem. C, 2010, 114, 4297–4301.
39. X. Hou, W. Zhang, X. Wang, S. Hu and C. Li, Science Bulletin, 2015, 60, 884–891.
40. Z. Liu, M. Li, X. Yang, M. Yin, J. Ren and X. Qu, Biomaterials, 2011, 32, 4683–4690.
41. Y. Wu, G. Chang, Y. Zhao and Y. Zhang, Dalton Trans., 2014, 43, 779–783.
42. Y. Meng, D. Chen and X. Jiao, J. Phys. Chem. B, 2006, 110, 15212–15217.
43. Y. Tao, H. Kanoh, L. Abrams and K. Kaneko, Chem. Rev., 2006, 106, 896–910.
44. Y. Wang, G. Wang, H. Wang, C. Liang, W. Cai and L. Zhang, Chem.–Eur. J., 2010, 16, 3497–3503.
45. Y. Lin, Z. Zeng, J. Zhu, Y. Wei, S. Chen, X. Yuan and L. Liu, Mater. Lett., 2015, 156, 169–172.
46. I. Langmuir, J. Am. Chem. Soc., 1918, 40, 1361–1403.
47. Q. Zhang, Z. Zhang, J. Teng, H. Huang, Q. Peng, T. Jiao, L. Hou and B. Li, Ind. Eng. Chem. Res., 2015, 54, 2940–2949.
48. R. Chen, J. Yu and W. Xiao, J. Mater. Chem. A, 2013, 1, 11682–11690.
49. T. Lu, T. Xiang, X.-L. Huang, C. Li, W.-F. Zhao, Q. Zhang and C.-S. Zhao, Carbohydr. Polym., 2015, 133, 587–595.
50. H. K. Boparai, M. Joseph and D. M. O'Carroll, J. Hazard. Mater., 2011, 186, 458–465.

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Footnote

† Electronic supplementary information (ESI) available: TEM image of Fe₃O₄@NiSiO₃ magnetic microspheres in Fig. S1. SEM images after replaced Ni element by Co (A), Cu (B) and Zn (C) element in Fig. S2. The solution pH of the addition of the adsorbents, (A) before (B) and after the adsorption of MB (C) in Fig. S3. Langmuir, Freundlich and Temkin model parameters for adsorption of MB onto the Fe₃O₄@NiSiO₃ microspheres in Table S1. Kinetic adsorption parameters for MB adsorbed onto the Fe₃O₄@NiSiO₃ microspheres obtained by using pseudo-first-order and pseudo-second-order models in Table S2. See DOI: 10.1039/c6ra01142j

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