An adsorption–reduction synergistic effect of mesoporous Fe/SiO₂–NH₂ hollow spheres for the removal of Cr(VI) ions

Abstract

In this study, mesoporous amino-group functionalised iron/silica hollow spheres (Fe/SiO₂–NH₂ HSs) were successfully developed as a magnetic absorbent for the highly effective removal of Cr(VI) ions. The Fe/SiO₂–NH₂ HSs were synthesized using monodispersed silica colloids as a chemical template, followed by a one-pot hydrothermal treatment, hydrogen reduction and surface modification with an amino silane coupling agent. The Fe/SiO₂–NH₂ HSs have a diameter of ca. 950 nm and a shell thickness of ca. 60 nm, and the surfaces were composed of Fe and SiO₂ units with amino functional groups. Meanwhile, the ferromagnetic property endowed them with easy recyclability for practical applications. The Fe/SiO₂–NH₂ HSs exhibited significantly improved ability to remove pollutant Cr(VI) and methyl orange. The removal percentage of Cr(VI) (8 mg L⁻¹) could reach 98.3% in just 5 min using Fe/SiO₂–NH₂ HSs; however, only 10% of Cr(VI) were removed using the unmodified samples. XPS analysis suggested that the removal of Cr(VI) was attributed to the adsorption and reduction synergistic process of the Fe/SiO₂–NH₂ HSs. During the process, an electrostatic attraction occurred between the positively charged amino-groups and the negatively charged pollutant species in the aqueous solution, and adsorbed Cr(VI) ions were reduced to Cr(III) species by the iron of Fe/SiO₂–NH₂ HSs.

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Introduction

Currently, environmental pollution, particularly water pollution,¹ is one of the most urgent issues that need to be addressed earnestly.² Nanomaterials and nanotechnology have demonstrated a promising prospect to meet this challenge.^3,4 Owing to their high surface areas and active adsorption sites, nanoscale adsorbents have exhibited excellent adsorption performance towards the environmental pollutants, such as heavy metals and organic dyes. For example, magnetic nanomaterials have attracted considerable attention in the last decade.⁵ In particular, zero-valent iron nanoparticles (Fe NPs) have been used for the treatment of toxic contaminants in wastewater and groundwater because Fe NPs, as a reductive material with standard redox potential E⁰ = −0.440 V, is convenient and non-toxic.^6,7 The Fe NPs possess the advantage of having a large reactive surface area and high surface reactivity, and thus high chemical reaction efficiency for heavy metal removal. They would, however, tend to aggregate and it is difficult for them to be separated and recycled due to their strong attractive interparticle forces at the nanoscale. To solve this problem, Xiu and co-workers found a one-step method to fabricate mono-dispersed Fe NPs coated with a SiO₂ shell for Cr(VI) reduction.^8,9 Li and co-workers have synthesized nano zero-valent iron on silica fume to remove Cr(VI) in soil and groundwater.¹⁰ However, these works still have some deficiencies, such as the slow adsorption rate of the Cr(VI) and the adsorbent is hard to recycle due to the Fe NPs being coated on the inside of the SiO₂ shell. To overcome the critical obstacle, we synthesized mesoporous amino-group functionalised iron/silica hollow spheres (Fe/SiO₂–NH₂ HSs) which have the amino group to improve the adsorption rate. The ferromagnetic property of the Fe unit enabled the nanocomposites to be easily segregated from solution.

Hollow nanostructures are of great interest in both fundamental research and applications. The most salient feature of hollow spheres is their large fraction of void space and high surface areas, which is advantageous for applications in biomedical materials,^11–13 lithium ion batteries,^14–16 catalysis,^17–19 gas sensors,^20,21 and wastewater treatment.²² Typically, three major routes are employed to fabricate hollow nanostructures, i.e., hard template strategy, soft template strategy and template-free strategy.²³ Recently, a simple and environmentally benign way assisted by glycerol to synthesize urchin-like α-FeOOH hollow spheres was reported by Lou et al., which were very promising for wastewater treatment.²⁴ Moreover, our group has also prepared nanostructured Fe₃O₄ micron-spheres by annealing FeCO₃ spheres that could be used for the removal of Cr(VI) ions.²⁵ Although many methods have been proposed and developed to construct hollow spheres, fabricating mesoporous Fe/silica hollow nanostructures, which integrate magnetic property with high accessible void space, remains a challenge.²⁶

Herein, we fabricated Fe/SiO₂–NH₂ HSs for fast removal of Cr(VI) ions and methyl orange in wastewater. The synthetic strategy is shown in Fig. 1. Firstly, we synthesized iron silicate hollow spheres (ISHSs) via a facile chemical-template hydrothermal route;²⁷ then, the as-synthesized ISHSs were in situ transformed into iron/silica hollow spheres (Fe/SiO₂ HSs) through hydrogen reduction; finally, the Fe/SiO₂ HSs were functionalized with amino-groups utilizing silane chemistry. We found that the as-prepared Fe/SiO₂–NH₂ HSs were able to almost completely remove Cr(VI) ions (8 mg L⁻¹) from wastewater in 5 min, and could be easily separated and recovered by using an external magnet and NaOH solution, respectively, demonstrating brilliant potential application for fast removal of Cr(VI) ions.

Fig. 1 Schematic illustration for the synthesis of Fe/SiO₂–NH₂ HSs.

Experimental section

Materials

Urea, absolute ethanol, acetone, toluene, tetraethyl or thosilicate (TEOS), concentrated hydrochloric acid (HCl, 36.0–38.0%), concentrated sulfuric acid (H₂SO₄, 98%), ammonia solution (NH₃·H₂O, 25.0–28.0%), NaOH, and Pb(NO₃)₂ were purchased from Sinopharm Chemical Reagent Co. Ltd, Shanghai, China. Diphenylcarbazide (DPC) was obtained from Guangfu Fine Chemical Research Institute, Tianjin, China. Potassium dichromate (K₂Cr₂O₇, 99%) was supplied by Jinbei Fine Chemical Co. Ltd, Tianjin, China. Iron(III) acetylacetonate (Fe(acac)₃, 98%) and (3-aminopropyl)triethoxysilane (APTES, 99%) were purchased from Aladdin Chemistry Co. Ltd, Shanghai, China. All these chemicals were of analytical grade and used without further purification. Deionized water was used throughout the experiments.

Synthesis of ISHSs

In a typically process, 0.1 g silica colloidal spheres synthesized via the Stöber method²⁸ were dispersed in 20 mL of deionized water under ultrasonication; 0.5 mmol Fe(acac)₃ and 8 mmol urea were dispersed in the mixed solution of 5 mL of absolute ethanol and 25 mL deionized water under ultrasonication. The above two solutions were mixed under magnetic stirring, which was then transferred into a Teflon autoclave (70 mL) and heated at 180 °C for 12 h in an electric oven. After that the autoclave was cooled down to room temperature. The precipitate was collected by centrifugation and washed with deionized water and ethanol several times, then dried at 60 °C overnight.

Synthesis of Fe/SiO₂ HSs

0.3 g ISHSs in a ceramic boat was placed in the middle of a horizontal tube furnace and heated at 500 °C for different durations (0.5, 1, 2 and 4 h) under an atmosphere of hydrogen gas (20 mL min⁻¹). After cooling down to room temperature, Fe/SiO₂ HSs were obtained.

Synthesis of Fe/SiO₂–NH₂ HSs

0.1 g Fe/SiO₂ HSs was dispersed in 50 mL toluene under vigorous stirring, and then 0.6 mL APTES was added and the solution was refluxed in a three-necked flask at 90 °C for 10 h under nitrogen gas protection. The products were collected using a magnet and washed with ethanol and acetone several times before drying in a vacuum oven at 60 °C for 12 h.

Characterization

The phase of the products was analyzed using X-ray diffraction (XRD) using Cu_Kα radiation (XRD, Philips, X’Pert-PRO, Netherlands). The morphology of the samples was studied using field-emission scanning electron microscopy (FEI, Quanta 200 FEG, USA) and transmission electron microscopy (JEOL, JEM 2010, Japan) with an energy-dispersive X-ray spectrometer, operated at an acceleration voltage of 200 kV. The surface area of the samples was determined using nitrogen adsorption at 77 K (Micrometrics, ASAP 2020M, USA). Magnetic measurements were performed with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, MPMS XL, USA). The isoelectric points of the samples were determined using a zeta-potential analyzer (Malvern Instruments, Zetasizer 3000HSa, UK). Fourier-transform infrared spectroscopy (FTIR) was performed using a JASCOFTIR 410 spectrophotometer. X-ray photoelectron spectroscopy (XPS) of the samples was performed using a Thermo ESCALAB 250 photoelectron spectrometer with Al_Kα X-rays as the excitation source.

Evaluation of the removal of pollutants

For the adsorption of Cr(VI) ions, K₂Cr₂O₇ was used as the source of the Cr(VI) ions. Different concentrations of Cr(VI) ions (8, 15, 30, 50, 70 and 90 mg L⁻¹) were prepared and the pH value (3, 4, 5, 6, 7, 8, 9, 10) was adjusted by using HCl and NaOH solutions. The pH value was determined using a pH meter (Mettler Toledo SG2-ELK). For each sample solution, 10 mg of Fe/SiO₂ HSs or Fe/SiO₂–NH₂ HSs was dispersed in 10 mL of Cr(VI) ions solutions under ultrasonication at room temperature. After 5 min, the magnetic adsorbents were separated using a magnet. The final Cr(VI) ions in the supernatant were determined via the 1,5-diphenylcarbazide colorimetric method.²⁹ Specifically, in the case of initial concentration of 8 mg L⁻¹ Cr(VI) ions, 3 mL of the supernatant mentioned above was added with 0.25 mL of H₂SO₄ (5 M) and 0.25 mL of DPC acetone solution (10 g L⁻¹). The solutions were left for 10 min for color development, and then the concentration of the Cr(VI) ions were determined using the absorbance of the solutions at a 542 nm wavelength using a UV-vis spectrophotometer (Shimadzu, UV-2700, Japan). In the case of initial concentrations higher than 8 mg L⁻¹, the Cr(VI) ion solutions were diluted using deionized water before measuring the absorbance. The amount of Cr(VI) ions adsorbed at equilibrium (Q_e, mg g⁻¹) was calculated according to the following equation:in which V is the solution volume (mL), m is the amount of adsorbent (mg), and C₀ and C_e (mg L⁻¹) are the initial and final Cr(VI) ion concentrations, respectively.

The removal percentage (R%) of the Cr(VI) ions was calculated according to the following equation:

in which C₀ and C_e (mg L⁻¹) are the initial and final Cr(VI) ion concentrations, respectively.

The absorbed Cr(VI) ions on the surface of the Fe/SiO₂–NH₂ HSs can be easily eluted using NaOH solution. The desorption process was as follow: 10 mg of Fe/SiO₂–NH₂ HSs was dispersed in 10 mL of Cr(VI) ion solution (8 mg L⁻¹) under ultrasonication for 5 min. Then the adsorbents were separated and collected, and 10 mL of NaOH aqueous solution (10⁻³ M) was used to elute the Cr(VI) ions adsorbed on the magnetic adsorbents. The elution performance could be described by the elution efficiency (D%) which is defined by the following equation:

in which C_s is the absorbed Cr(VI) concentration on the Fe/SiO₂–NH₂ HSs, and C_d is the Cr(VI) concentration in the eluted solution.

Results and discussion

Characterization of Fe/SiO₂ HSs

The Fe/SiO₂ HSs were synthesized using monodispersed silica colloids as a chemical template, followed by a one-pot hydrothermal process, and surface modification with an amino silane coupling agent. Firstly, the synthesis of ISHSs exploits a chemical-template method developed by our previous research work.³⁰ The general process is as follows: first, silica colloidal spheres, as the chemical template, were etched using an alkaline solution and silicon–oxygen bonds were broken, generating silicate ions on the surface of the silica spheres; then, silicate ions would encounter the metal ions that exist in the solution; as a result, metal silicate would nucleate and grow in situ through the reaction between the metal ions and the silicate ions until the metal ions in the solution had been exhausted; finally, the remaining silica core could be further etched out under alkaline conditions, and the hollow structure would be obtained.

Electron microscopy was used to study the morphology of the ISHSs. Fig. 2a and b show the SEM images of silica colloidal spheres and ISHSs, respectively. We found that the silica spheres with smooth surfaces have a uniform size of ca. 500 nm. By contrast, after hydrothermal reaction, the surfaces of the products became coarse and were distributed with many lamellas, with a diameter of ca. 600 nm, which could be clearly observed from the enlarged image in Fig. 2c. The well-defined hollow structure derived from the silica template could also be identified through the TEM image in Fig. 2d. It observed that the diameter of the ISHSs is about 800 nm, and the thickness of the hollow shell is 30 nm. In order to further investigate the elemental distribution and structure of the ISHSs, elemental mapping was performed using HRTEM at HAADF (high angle annular dark field). The elemental mapping images (Fig. 2e) of the as-prepared ISHSs demonstrated that iron, silicon and oxygen elements were homogeneously distributed on the shell of the hollow spheres, further confirming the effective synthesis of the ISHSs. XRD results were used to investigate the phase structure of the silica colloidal spheres and the ISHSs (Fig. S1†). The peaks in Fig. S1b† were identified as iron silicate hydroxide (JCPDS 42-0596). The nanoscale size of the units that constituted the as-obtained products might be responsible for the broadening of these peaks.³¹

Fig. 2 SEM images of (a) silica colloidal spheres and (b) ISHSs; (c) the magnified SEM image of ISHSs; (d) TEM image of ISHSs; (e) the elemental maps of Fe, Si and O for ISHSs.

Herein, we designed a method for constructing Fe/SiO₂ nanostructures with hollow interiors by hydrogen reduction of the as-prepared ISHSs. We found that the ISHSs were in situ transformed to Fe/SiO₂ hollow spheres after annealing under an atmosphere of hydrogen gas. The transformation process was studied using XRD analysis, as shown in Fig. 3. After 0.5 h of hydrogen reduction, the peaks of the products that belonged to Fe₃O₄ (Magnetite, JCPDS 75-0449) appeared, and the peaks of Fe (Iron, JCPDS 87-0722) also coexisted after reducing for 1 h.³² The intensity of the peaks of bcc-Fe became stronger after reducing for 2 h; however, the intensity of the peaks of the magnetite started to decrease. It was found that the peaks of the magnetite almost disappeared and only the sharp peaks of the iron particles remained after 4 h of reduction. This phenomenon can be explained by the process proposed by Zielinski et al.,³³ that is, the reduction of Fe₂O₃ at a low H₂O/H₂ ratio proceeded in two steps, i.e. Fe₂O₃ → Fe₃O₄ → Fe.

Fig. 3 XRD patterns of ISHSs after hydrogen reduction for different durations at 500 °C.

The morphology of the reduction products was characterized using SEM and TEM. As shown in Fig. 4a and c, the size of the Fe/SiO₂ HSs was similar to that of the ISHSs, but the size of the lamellae on the surface shrank in length and width and became thicker. We can observe that the diameter of the Fe/SiO₂ HSs is about 930 nm, and the thickness of the hollow shell is 40 nm. This was due to the reduction of iron silicate of the ISHSs to Fe NPs, leading to the collapse of the lamellae. However, from the TEM images, as shown in Fig. 4b and d, it was found that the hollow structures remained unchanged after annealing and the shadows of the Fe NPs could be observed on the shell from Fig. 4d. As shown in the magnified TEM image of the Fe/SiO₂ HSs (Fig. 4e), it was found that the Fe NPs were homogeneously dispersed in the silica matrix. The fringe of iron particles corresponding to an interplanar distance of 0.203 nm agreed well with the lattice spacing of the (110) plane of bcc-Fe, as shown in Fig. 4f.

Fig. 4 Fe/SiO₂ HSs images: (a) SEM image; (b) TEM image; (c) a magnified SEM image; (d) magnified TEM image; Fe NPs in the Fe/SiO₂ HSs: (e) a magnified TEM image; (f) HRTEM image.

The magnetic hysteresis of the Fe/SiO₂ HSs was measured using a SQUID magnetometer at room temperature (Fig. 5). The applied field was swept between −45 and 45 kOe in the measurement. The hysteresis in the low field regime indicated that the Fe/SiO₂ HSs exhibited ferromagnetic behavior at room temperature. The saturation magnetization (M_s) was about 45.5 emu g⁻¹, and the remnant magnetization (M_r) and coercivity (H_c) were 13.3 emu g⁻¹ and 570 Oe, respectively. The Fe/SiO₂ HSs could be separated and collected within seconds by using a magnet owing to their ferromagnetic behavior, as shown in the inset of Fig. 5.

Fig. 5 Hysteresis loop of Fe/SiO₂ HSs measured at 300 K with the applied field between −45 and 45 kOe, the enlarged magnetization curve between −800 and 800 Oe (inset in upper left), and photographs of magnetic separation (inset in lower left).

Removal capability of Fe/SiO₂–NH₂ HSs for Cr(VI)

The abundant surface chemistry of silica enables the as-prepared Fe/SiO₂ HSs to be modified and functionalized to alter their interface properties. In this work, we modified the surface of the Fe/SiO₂ HSs with amino-groups as a case study and examined their capability to remove Cr(VI) ions from polluted water. The TEM image of the Fe/SiO₂–NH₂ HSs is shown in Fig. 6a. We found that the morphology of the hollow spheres was maintained except that the shell became thicker (60 nm) after surface modification. Fig. 6b shows the FTIR spectra of the Fe/SiO₂ HSs and Fe/SiO₂–NH₂ HSs. The absorption bands at 1208, 1102, 808, and 475 cm⁻¹ observed from both spectra can be ascribed to the stretching and deformation of vibrations of silica. After functionalizations, three new absorption bands, i.e. 1564, 1485, and 692 cm⁻¹ (Fig. 6b(ii)), showed up in addition to those belonging to the silica, which could be ascribed to the stretching and bending vibrations of the amino-groups. This confirmed that the amino-groups were successfully grafted onto the Fe/SiO₂ HSs. Moreover, the appearance of a N 1s peaks in the XPS spectra of hollow spheres after functionalization (Fig. 8b) further supported this conclusion.

Fig. 6 (a) TEM image of Fe/SiO₂–NH₂ HSs; (b) FTIR spectra of (i) Fe/SiO₂ HSs and (ii) Fe/SiO₂–NH₂ HSs; (c) nitrogen adsorption/desorption isotherm and pore size distribution curve (insert) of Fe/SiO₂–NH₂ HSs; (d) zeta potential versus pH of Fe/SiO₂–NH₂ HSs (the concentration of Fe/SiO₂–NH₂ HSs is 0.2 g L⁻¹).

The corresponding N₂ adsorption–desorption isotherm and pore size distributions of Fe/SiO₂–NH₂ HSs nanocomposites were characterized using N₂ physisorption experiments, as shown in Fig. 6c. It revealed that the Fe/SiO₂–NH₂ HSs nanocomposites have type III isotherms and type H₃ hysteresis loops, indicating the existence of slit-shape pores according to the IUPAC classification. The surface area as measured by the multi-point BET method from the adsorption branch was 58.3 m² g⁻¹. The pore size distribution of the sample as determined using the BJH method from the adsorption branch of the isotherm is shown in Fig. 6c, which suggested an average pore diameter of 1.9 nm. The surface charge was investigated by measuring the zeta potential of the Fe/SiO₂–NH₂ HSs at different pH values. As shown in Fig. 6d, the surface zeta potential of the Fe/SiO₂–NH₂ HSs was positive over a wide range of pH values (pH < 7). As we know, the Cr(VI) ions exist in a chemical equilibrium of three different anion species in aqueous solution, CrO₄²⁻, HCrO₄⁻ and Cr₂O₇²⁻, and the positive charge of the Fe/SiO₂–NH₂ HSs was conducive to the adsorption of Cr(VI) ions.³⁴

It is well known that Cr(VI) ions are highly toxic^35,36 and can be commonly found in wastewater as a result of the extensive use of chromium in industrial activities.³⁷ It is extremely urgent to develop effective ways to remove these ions. The adsorption method is believed to be one of the most promising approaches to meet the requirement due to its efficiency and simple process.^38,39 Thus, we applied the as-obtained Fe/SiO₂–NH₂ HSs to the removal of Cr(VI) from wastewater. The concentration of the Cr(VI) ions in the solution was determined via the colorimetric method based on the reaction of Cr(VI) ions with diphenylcarbazide.⁴⁰ A pink or purple colored chelate of chromium with diphenylcarbazone would form when the Cr(VI) ions were mixed with diphenylcarbazide in acid solution. The characteristic peak at 542 nm was measured using a UV-vis spectrometer and the stronger the intensity of the peak, the higher the concentration of the Cr(VI) ions in the sample solution. In this work, the calibration curve was established by measuring the absorbance at 542 nm of different standard solutions of Cr(VI) ions with concentrations ranging from 0 to 8 mg L⁻¹. A linear relationship between the absorbance at 542 nm and the concentration of Cr(VI) ions could be obtained through a linear fitting, as shown in Fig. S2.† The exact concentration of the Cr(VI) ions can be calculated using the linear equation after measuring the absorbance at 542 nm.

The removal performance of the Fe/SiO₂ HSs and the Fe/SiO₂–NH₂ HSs towards Cr(VI) ions with an initial concentration of 8 mg L⁻¹ was investigated and compared at various pH values within a treatment duration of 5 min. From Fig. 7a, it was found that the removal performance of Cr(VI) by the Fe/SiO₂–NH₂ HSs was significantly higher than that of unmodified Fe/SiO₂ HSs even in a low pH range, and demonstrated a nearly linear dependence with the pH value, that is, the lower the pH value of the solution, the higher removal percentage of Cr(VI) ions. Particularly, when the solution pH was 3, the removal percentage of Cr(VI) ions could reach 98.3% for the Fe/SiO₂–NH₂ HSs; however, only 10% can be removed by the Fe/SiO₂ HSs. The corresponding UV-vis absorption of the Cr(VI) ions solution before and after treatment with Fe/SiO₂–NH₂ HSs under different pH values is shown in Fig. 7b. Furthermore, Fig. S3† shows the relationship between Fe/SiO₂–NH₂ HSs removal ability of the Cr(VI) ions and the equilibrium Cr(VI) concentration of the solutions. The experimental data were simulated with the Langmuir and Freundlich isotherm models and the parameters calculated from these two models were listed in Table S1.† Judging from the correlation coefficients (R²), we found that the Freundlich model (R² = 0.985) would be a better model to fit the adsorption data than the Langmuir model (R² = 0.836). Comparing the maximum capacity of Cr(VI) ion removal with the previous study (shown in Table S2†),^41–55 the Fe/SiO₂–NH₂ HSs were better than many other reported iron-based adsorbents. Moreover, the amino modified Fe/SiO₂ HSs show a high adsorption rate, which was able to nearly completely remove Cr(VI) ions (8 mg L⁻¹) during 5 min from wastewater. It revealed that the Fe/SiO₂–NH₂ HSs could be a promising adsorbent for the removal of Cr(VI).

Fig. 7 (a) The comparison of Cr(VI) removal percentage between Fe/SiO₂ HSs and Fe/SiO₂–NH₂ HSs at various pH values within a treatment duration of 5 min (the initial concentration of Cr(VI) is 8 ppm); (b) the UV-vis spectra of Cr(VI) solutions after being treated by Fe/SiO₂–NH₂ HSs at different pH values (before treatment the initial concentration of Cr(VI) is C₀ = 8 ppm), all experiments were performed in triplicate.

The recyclability of a magnetic adsorbent is very important in view of maximizing their exploitation. We compared the ability of deionized water (DI water) to elute Cr(VI) ions absorbed on the surface of the Fe/SiO₂–NH₂ HSs with that of NaOH (10⁻³ M) solution. As shown in Fig. S4,† the elution efficiency of NaOH solution (88%) was higher than that of DI water (18%), suggesting that NaOH solution was a better choice as the eluent for Cr(VI) ion elution. Note that ∼12% of the adsorbed Cr(VI) is not removed from the Fe/SiO₂–NH₂ HSs, indicating that some of the reactive surface area of the Fe/SiO₂–NH₂ HSs was consumed. Finally, we used NaOH solution (10⁻³ M) to elute the adsorbed Cr(VI) ions and reuse the regenerated adsorbent for the removal of Cr(VI). Therefore, the as-prepared Fe/SiO₂–NH₂ HSs could be regenerated and recycled multiple times for wastewater treatment.

Although Fe/SiO₂–NH₂ HSs were able to remove Cr(VI) ions from wastewater very fast, the removal process needed to be studied. We utilized the elemental mappings of HRTEM to analyze the spatial distribution of different elements of the hollow spheres after being used for Cr(VI) removal. Cr element could be clearly identified in the mapping images besides the Fe, Si and O elements, as shown in Fig. 8a, suggesting that Fe/SiO₂–NH₂ HSs could effectively adsorb Cr(VI) ions onto their surfaces. In the above adsorption–desorption recycle experiments, when using NaOH solution as the eluent, the elution efficiency was 88%, implying that at least 88% of Cr species adsorbed on the surfaces of hollow spheres existed in the form of a hexavalent species taking into account that the elution efficiency was unable to achieve 100%. The as-prepared Fe/SiO₂–NH₂ HSs exhibited both adsorbability and the magnetic properties. Fig. S5† shows the magnetically recyclable adsorption of Cr(VI) ions, suggesting that the Fe/SiO₂–NH₂ HSs can be successfully recycled and reused for four successive cycles with a stable adsorption efficiency of more than 70%.

Fig. 8 (a) The elemental maps of Fe, Si, O and Cr of Fe/SiO₂–NH₂ HSs after treating Cr(VI) solutions; (b) XPS survey for (i) Fe/SiO₂ HSs, (ii) Fe/SiO₂–NH₂ HSs and (iii) Fe/SiO₂–NH₂ HSs after treating Cr(VI) solutions; (c) Cr 2p XPS spectra of Fe/SiO₂–NH₂ HSs after treating Cr(VI) solutions; (d) the existing forms of the Cr(VI) species in aqueous solution,³⁴ and the fast Cr(VI) removal mechanism of Fe/SiO₂–NH₂ HSs (the right part).

Moreover, we characterized the chemical compositions and valences of the Cr species absorbed on the surfaces of hollow spheres using XPS analysis. The observation of Cr 2p peaks was expected in the XPS survey of Cr(VI) treated hollow spheres (curve iii of Fig. 8b). Detailed analysis of the XPS spectrum of Cr 2p (Fig. 8c) found that Cr 2p_3/2 could be fitted into six peaks. The peaks at 578.3 eV, 578.8 eV and 579.4 eV were similar to the XPS spectra of Cr(VI) ions, but the other three peaks at 576.2 eV, 576.8 eV and 577.5 eV could be attributed to the characteristic binding energy of Cr₂O₃ or Cr(OH)₃,^56,57 suggesting that a portion of Cr(VI) ions adsorbed on the surface of the hollow spheres were reduced to Cr(III) species by the iron of the Fe/SiO₂–NH₂ HSs.

According to the discussion above, the Cr(VI) removal mechanism of Fe/SiO₂–NH₂ HSs can be explained as follows. First of all, it is well known that amino-groups are easily protonated under an acidic environment and then form positively charged groups; second, Cr(VI) ions exist in a chemical equilibrium of three different anion species in aqueous solution, CrO₄²⁻, HCrO₄⁻ and Cr₂O₇²⁻, according to the pH value and pCr (the left section of Fig. 8d); therefore, protonated amino-groups on the surface of adsorbents can quickly combine with negatively charged Cr(VI) species via electrostatic interaction, and adsorbed Cr(VI) ions were subsequently reduced to Cr(III) species by the iron of Fe/SiO₂–NH₂ HSs, leading to the immobilization and reduction of Cr(VI) by the magnetic hollow spheres. But under an alkaline environment, the amino-group would deprotonate and the bond between negatively charged Cr(VI) species and positively charged amino-groups would be broken, inducing the elution of Cr(VI) species and making the recycling of adsorbents possible. The regeneration process is illustrated in Fig. 8d (the right section). Therefore, an adsorption and reduction synergistic process can be concluded to illustrate the Cr(VI) removal mechanism of Fe/SiO₂–NH₂ HSs. The unmodified Fe/SiO₂ HSs fail to remove Cr(VI) ions due to the isoelectric point of Fe/SiO₂ HSs being at pH = 2 (similar to that of silica), and when pH > 2 the surface charge of Fe/SiO₂ HSs is negative, making them unfavorable for adsorbing anion pollutant species.

Furthermore, the unmodified and amino-modified Fe/SiO₂ HSs were employed to remove organic dyes, such as methyl orange (MO). We found that the Fe/SiO₂–NH₂ HSs could completely remove MO in 5 min when the pH value of the solution was 3; however, unmodified Fe/SiO₂ HSs demonstrated little adsorption capacity for MO. The corresponding UV-vis absorption of MO solution after being treated with adsorbents is shown in Fig. S6,† and the corresponding color change photos could be observed from Fig. S7.† The removal trend was similar to the case of Cr(VI) removal, that is, in the same pH value, the removal performance of Fe/SiO₂–NH₂ HSs was better than that of unmodified Fe/SiO₂ HSs, and the lower the pH value of the solution, the better the removal capacity of the Fe/SiO₂–NH₂ HSs. It suggested that the adsorption mechanisms of the Fe/SiO₂–NH₂ HSs towards heavy metal anion species and negatively charged organic dyes were the same. The electrostatic interaction endowed the Fe/SiO₂–NH₂ HSs with the ability to fast and effectively remove anion pollutant species.

Conclusions

In summary, iron/silica hollow spheres were fabricated through hydrogen reduction of mesoporous iron silicate hollow spheres synthesized via a facile one-pot hydrothermal method. The iron NPs were embedded in the silica matrix on the shell and endowed the hollow spheres with reducing properties and strong ferromagnetism, facilitating their application for wastewater treatment. The surfaces of iron/silica hollow spheres were then modified with amino-groups (Fe/SiO₂–NH₂ HSs), and their ability to remove anion pollutant species, such as Cr(VI) and MO, was significantly enhanced as compared to the unmodified ones. The enhanced adsorption mechanism was illustrated by the electrostatic attraction between the positively charged amino-groups and the negatively charged pollutant species. We believe that the abundant toolbox of chemistry will allow us to arbitrarily tailor the surface functionalities of the as-prepared iron/silica hollow spheres, which in combination with the unique hollow structures and the magnetic properties make them ideal candidates for a host of applications, including but not limited to nanomedicine, sensors, catalysis and wastewater treatment.

Acknowledgements

This work was supported by the National Basic Research Program of China (Grant No. 2013CB934302), the Natural Science Foundation of China (Grant No. 51472246, 21177132 and 51272255), and the Anhui Provincial Natural Science Foundation (No. 1408085MB39).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03172b

‡ These authors contributed equally to this work.

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