Tunable synthesis of SiO2-encapsulated zero-valent iron nanoparticles for degradation of organic dyes
© Mao et al.; licensee Springer. 2014
Received: 15 August 2014
Accepted: 9 September 2014
Published: 16 September 2014
A series of nanocomposites consisting of zero-valent iron nanoparticles (ZVI NPs) encapsulated in SiO2 microspheres were successfully synthesized through a successive two-step method, i.e., the wet chemical reduction by borohydride followed by a modified Stöber method. The as-synthesized nanocomposites were characterized using X-ray diffraction, field emission scanning electron microscopy, vibrating sample magnetometer, and inductively coupled plasma-atomic emission spectrometer. The catalytic performance of SiO2-encapsulated ZVI nanocomposites for the degradation of organic dyes was investigated using methylene blue (MB) as the model dye in the presence of H2O2. The results showed that the degradation efficiency and apparent rate constant of the degradation reaction were significantly enhanced with increased ZVI NPs encapsulated in SiO2 microspheres, whereas the dosage of H2O2 remarkably promoted degradation rate without affecting degradation efficiency. The content-dependent magnetic property ensured the excellent magnetic separation of degradation products under an external magnet. This strategy for the synthesis of SiO2-encapsulated ZVI NPs nanocomposites was low cost and easy to scale-up for industrial production, thereby enabling promising applications in environmental remediation.
Organic dyes are used in numerous industries including textile, cosmetics, food, and pharmaceutical because of their fascinating properties, such as high wet fastness profile, brilliant shades, and relatively low cost. However, the compulsive utilization of organic dyes has caused serious environmental pollution and posed public health risks [1–3]. Most organic dyes are resistant to decompose in a natural environment and can cause serious diseases because of their transformation into genotoxic and carcinogenic species. Existing strategies focus on the removal and degradation of organic contaminants from effluents, including adsorption, coagulation, chemical oxidation, electrochemical degradation, and biological degradation, among others [4–9]. The major disadvantage of these physical methods is the dye molecules that are transferred to another phase rather than destroyed . Additionally, biological degradation of organic contaminants suffers from low degradation efficiency, high cost, and rigorous degradation conditions . Conversely, chemical methods display promising potential in the degradation of organic contaminants, although they require various high-performance catalysts [12–14]. The disposal of chemicals containing sludge at the end of degradation also entails complicated posttreatment processes and further use of chemicals . Therefore, it is indispensable and emergent to explore the novel strategies for the degradation of organic contaminants with high efficiency and low cost.
Over the past decades, zero-valent iron (ZVI) nanoparticles (NPs) have been considered to possess promising potential in environmental remediation because of their low cost, high reactivity as a reducing agent, and ability to generating reactive oxygen species (ROS) through the Fenton reaction [16, 17]. In a typical Fenton reaction, Fe2+/Fe3+ reacts with H2O2 and generates the hydroxyl radical (·OH), which is a very strong oxidant capable of decomposing various organic contaminants [18, 19]. The use of ZVI powder instead of iron salts avoids the introduction of counter anions into aquatic systems. In addition, the concentration of Fe2+/Fe3+ in wastewater treated by ZVI is significantly lower than those treated with iron salts . Moreover, the excess catalyst contained in the sludge could be easily recycled by magnets. However, the agglomeration of ZVI NPs is one of the most fatal shortages of this process, which results in the rapid inactivation of chemical reactivity. Therefore, modification of the surface of ZVI NPs to reduce agglomeration and prevent oxidation of ZVI NPs is necessary. Among various stabilizers, SiO2 coating has attracted considerable attention because of its low cost and environmental friendliness. Various strategies have been developed for the synthesis of Fe/SiO2 nanocomposites, including sol-gel [21–26], mechanochemical billing [27, 28], spray drying , arc discharge , wet chemical route , and ion implantation . Thus far, the low-cost and large-scale synthesis of ZVI NPs with high reactivity and mobility remains a great challenge.
In this work, a facile two-step strategy was developed to synthesize SiO2-encapsulated ZVI NPs. The ZVI NPs were first synthesized through wet chemical reduction with NaBH4 and subsequently encapsulated by SiO2 through the classical Stöber process. The content of ZVI NPs encapsulated in SiO2 was adjusted by changing the reaction parameters. The products were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), vibrating sample magnetometer (VSM), and inductively coupled plasma-atomic emission spectrometer (ICP-AES). Moreover, the catalytic degradation of organic contaminants by SiO2-encapsulated ZVI NPs nanocomposites was evaluated using methylene blue (MB) as the model dye by monitoring the changes of UV-vis spectra at different time intervals at room temperature.
FeCl2 · 4H2O, NaBH4, tetraethoxysilane (TEOS), ethanol, and H2O2 were of analytic grade (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and used without further purification. Deionized water (16 M Ω·cm) was obtained from a Nanopure Water Systems (Thomas Scientific, Swedesboro, NJ, USA).
Synthesis of SiO2-encapsulated ZVI nanocomposites
The initial concentrations of iron salt used in the synthesis
The initial concentration of Fe2+(mmol)
Characterization of SiO2-encapsulated ZVI nanocomposites
The phase structure of the samples was characterized on a power XRD (D8 Advance, Bruker Corp., Karlsruhe, Germany) using Cu Kα radiation (λ) 1.5406 Å. The morphology of the samples was observed using field emission scanning electron microscope (FESEM; S4800, Hitachi Corp., Chiyoda-ku, Japan). The elemental composition was measured with an Optima 4300DV ICP-AES (Optima 4300DV, PerkinElmer Corp., Yokohama, Kanagawa, Japan). Magnetic property was measured on VSM (JDAW-2000D, Yingpu Corp., Hangzhou, China). Nitrogen (N2) adsorption-desorption isotherms were measured with a Micromeritics apparatus (TriStar II 3020, Micromeritics Instrument Corp., Norcross, GA, USA). UV-vis spectra were recorded on a UV-vis spectrophotometer (UV-2550 PC, Shimadzu Corp., Kyoto, Japan).
Catalytic degradation of MB by SiO2-encapsulated ZVI nanocomposites
Batch experiments were carried out to evaluate the catalytic performance of SiO2-encapsulated ZVI NPs nanocomposites using MB as the model at room temperature. In a typical experiment, the sample (20 mg) and H2O2 (1 mL) was added into 50 mL of MB (10 mg/L) aqueous solution. The suspension was continuously stirred at room temperature under visible light irradiation. The supernatant was collected at designated time intervals for UV-vis measurement after centrifugation (10,000 rpm, 3 min). The catalytic degradation of MB was evaluated by monitoring the changes of UV-vis spectra.
Results and discussion
Characterization of SiO2-encapsulated ZVI nanocomposites
The saturation magnetization, remanence, and coercivity of as-synthesized nanocomposites
M s (emu/g)
Degradation assessment of MB treated with SiO2-encapsulated ZVI NPs
where A and A0 were the absorbance at the maximum absorption peak of ca. 664 nm of MB at different time intervals and initial time, respectively.
In general, ZVI in the presence of H2O2 produces a Fenton-like reaction through generating numerous · OH, which can destroy various organic contaminants (such as halogenated hydrocarbons, aromatic compounds, detergents and pesticides, etc.) [16–19]. In the present study, ZVI NPs were encapsulated in SiO2 nanosphere, preventing from the surface oxidation of ZVI NPs. Meanwhile, ZVI NPs acted as Fe reservoir releasing iron ions (as shown in Figure 4), which catalytically broke down the H2O2 molecules into · OH. It was suggested that · OH can attack the C-S+ = C and C-N = C group in MB molecules, resulting in the split of the S+ = C and N = C double bond [43, 44]. The further attack of · OH on S-Cl, C-NH2, and C-SO3H containing in various intermediates finally generated inorganic molecules or structures, such as H2O, CO2, Cl−, NO3−, and SO42−.
In summary, SiO2-encapsulated ZVI NPs nanocomposites with tunable Fe content were synthesized using a two-step strategy. The results demonstrated that the catalytic degradation of MB depended on the content of ZVI NPs encapsulated in SiO2 microspheres. The dosage of H2O2 significantly promoted degradation rate without changing degradation efficiency. Furthermore, the content-dependent magnetic property of ZVI NPs enabled easy separation of the degradation products from the coagulated sludge using an external magnet. This strategy was facile and low cost for the industrial production of ZVI NPs for applications in environmental remediation.
This work was financially supported by the Natural Science Foundation of China (No. 30800256), the basic research project of Wuhan Science and Technology Bureau (No. 2014060101010041), and the Self-Determined and Innovative Research Funds of WUT (2014-CL-B1-13).
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