Iron Foil as an Effective and Nonfiltration Catalyst for

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Magnetite/Iron Foil as an Effective and Nonfiltration Catalyst for Heterogeneous Fenton-like Reactions under Neutral Conditions Xiaoyi Wang,† Yulong Liao,*,†,‡ Quanjun Xiang,† Huaiwu Zhang,‡ Yuanxun Li,‡ and Zhiyong Zhong‡ †

Center for Applied Chemistry and ‡State Key Laboratory of Electronic Thin Film and Integrated Devices, University of Electronic Science and Technology of China, Pixian, Chengdu 610000, Sichuan, China

Inorg. Chem. Downloaded from pubs.acs.org by QUEEN MARY UNIV OF LONDON on 03/27/19. For personal use only.

S Supporting Information *

catalyst, MIF shows a great performance for the degradation of Rhodamine B (RhB) under neutral conditions. Most importantly, the Fe3O4 nanocrystals in our work are supported by iron foils at the macroscale, and neither a magnetic separation nor a filtration procedure is required after the reaction. Procedures of traditional heterogeneous Fenton-like reactions using magnetite-based catalysts are schematically illustrated in Figure 1a. Except for the separation and filtration processes,

ABSTRACT: A magnetite/iron foil (MIF) composite was synthesized as a heterogeneous Fenton-like catalyst. The MIF catalyst effectively degraded Rhodamine B under neutral conditions (degradation efficiency = 86%), avoiding the procedure of pH adjustment. The MIF catalyst could be conveniently recycled without filtration, and the advantages of the stability and reusability of a MIF catalyst made it promising in practical wastewater treatment.

T

he advanced oxidation process (AOP) is considered to be a promising technique and has shown great potential for the remediation of wastewater.1−4 Among the various AOPs, the Fenton process is intensively studied because of its advantages such as simplicity, high efficiency, and mild reacting conditions.5 In general, the traditional homogeneous Fenton process requires acidic solutions containing ferrous ions (Fe2+) and hydrogen peroxide (H2O2).6 Unfortunately, there are some critical drawbacks.7 Before reaction, the pH of the wastewater needs to be adjusted to about 3, which is the optimal condition for the homogeneous Fenton process. After reaction, a neutralization process using sodium hydroxide (NaOH) is indispensable in order to meet discharge standards; meanwhile, some precipitates are formed [Fe(OH)3] and need to be filtered. To address this issue, much effort has been made to investigate the heterogeneous Fenton-like systems in which an oxidation reaction occurs at the solid−liquid interface and a variety of iron-based catalysts are studied.8 In particular, magnetite (Fe3O4)-based materials have attracted increasing interest because of their unique features.9,10 Recently, immobilizing Fe3O4 nanomaterials onto various supports is becoming a research hotspot because it can preserve the advantages and alleviate the aggregation effect at the same time.11,12 However, high-temperature thermal treatment is usually needed to allow the generation of Fe3O4 products, and it will undoubtedly increase the energy consumption. On the other hand, the Fe3O4 products are supported at the microscale, and the corresponding catalyst is usually in a powder form, which means a filtration process is essential after reaction.13 Therefore, a nonfiltration catalyst prepared at low temperatures would not only significantly simplify the reaction processes but also reduce the costs. Herein, for the first time, we reported the magnetite/iron foil (MIF) composite as an effective and easily recoverable catalyst of heterogeneous Fenton-like reactions. When serving as a © XXXX American Chemical Society

Figure 1. Schematic illustrations of the procedures of (a) traditional heterogeneous Fenton-like reactions using powder-form catalysts and (b) this work using a MIF catalyst.

NaOH is always employed to precipitate the residual iron ions after reaction and a second filtration is required. In contrast, the procedures using MIF as the catalyst are more convenient (see Figure 1b). First, the MIF catalyst can just be easily taken out and recycled after reaction without filtration, which will simplify the treatment procedures and reduce costs. Second, there are nearly no residual iron ions in the solution after reaction. Third, the degradation rate can be elevated by simply adding more MIFs without worrying about the aggregation phenomenon of the catalysts that are in powder form. In Figure 2a, a diffraction peak located at 44.7° is assigned to the substrate of iron foil. Except for that, all of the other peaks match well with the standard database peaks (JCPDS 19-0629), indicating the inverse spinel structure of the prepared magnetite products.14 Specifically, the characteristic peaks located at 18.3, 30.2, 35.7, 43.3, 53.5, and 57.1° could be indexed to the (111), (220), (311), (400), (422), and (511) planes of Fe3O4. In Received: February 25, 2019

A

DOI: 10.1021/acs.inorgchem.9b00546 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 2. (a) XRD patterns of the as-prepared MIF composite. (b) SEM images of the Fe3O4 layer. (c) Cross-sectional SEM image of the MIF composite. (d−f) TEM and HRTEM images of the Fe3O4 nanocrystals.

addition, no other peaks corresponding to hematite are detected, revealing that the obtained Fe3O4 layer of the MIF composite is pure. The hysteresis loop and X-ray photoelectron microscopy (XPS) spectrum of the prepared Fe3O4 products are presented in Figure S1. Besides, the energy-dispersive spectroscopy (EDS) spectrum of the MIF composite is presented in Figure S2. It can be concluded that the MIF composites were successfully synthesized and the purity of Fe3O4 is acceptable. Figure 2b shows the scanning electron microscopy (SEM) image of the Fe3O4 products. Homogeneously distributed nanocrystals are observed, the nanocrystals possess a polyhedral outline, and the average diameter is estimated to be about 35 nm. In order to get a full view of the MIF, a cross-sectional image was characterized and the thickness of the Fe3O4 layer was 3.5 μm, as shown in Figure 2c. Parts d and e of Figure 2 present the transmission electron microscopy (TEM) images of the Fe3O4 powders scraped from the MIF composite. A polyhedral feature is clearly observed, and the average diameter is about 33 nm, in accordance with the SEM images. The high-resolution TEM (HRTEM) image is also presented in Figure 2f. Continuous and clear crystal lattices are observed, indicating a high-crystallization nature of the Fe3O4 products. Additionally, the spacing of the lattice fringes is 0.485 nm, which matches well with the interplanar distance of the (111) planes of Fe3O4.15 Batch experiments using MIF as the catalyst were carried out to evaluate the degradation efficiency of RhB under different conditions. As shown in Figure 3a, RhB degrades dramatically in the presence of MIF and H2O2 (degradation efficiency = 86%), and the solution is almost colorless after 120 min of reaction (see the inset). In contrast, nearly no degradation phenomenon could be achieved when the system did not contain the MIF or H2O2 (degradation efficiency = 10%; see Figure S3). We can conclude that MIF adsorption did not contribute to the degradation and the contribution of H2O2-only conditions was negligible. Therefore, the remarkable degradation performance of the MIF + H2O2 + RhB system was ascribed to the heterogeneous Fenton-like reactions, in which HO• radicals were formed, resulting from the reaction of MIF and H2O2 (eqs 1 and 2). Moreover, the catalytic performances of the MIF catalyst under acidic and alkaline conditions were also investigated and are presented in Figure S4. The influence of the H2O2 concentration on the degradation efficiency is presented in Figure 3b. A lower H2O2 concentration leads to a

Figure 3. (a) UV−vis spectra of the RhB solution at different times of the MIF + H2O2 + RhB system (reaction temperature = 40 °C; H2O2 concentration = 0.1 mol L−1). (b) Effect of the H2O2 concentration on RhB degradation. C/C0 plots of the systems (c) with and without the existence of tert-butyl alcohol and (d) using Fe3O4 powders or MIF as the catalyst. (e) Schematic illustration of the MIF-involved heterogeneous Fenton-like reactions.

slower degradation rate but excessive H2O2 also hinders the degradation process. Regarding the latter phenomenon, it might be caused by two factors: (i) Excessive H2O2 is adsorbed on the surfaces of the Fe3O4 nanocrystals, impeding sufficient contact with RhB. (ii) Excessive H2O2 will consume HO• radicals and therefore reduces the degradation efficiency.16 B

DOI: 10.1021/acs.inorgchem.9b00546 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry tert-Butyl alcohol, the scavenger of the HO• radical, was employed to identify reactive species (see Figure 3c). The presence of tert-butyl alcohol significantly hinders the degradation process, suggesting that HO• radicals are the main species responsible for the removal of RhB. Although FeIIsur can be regenerated through the reactions, as depicted in eqs 3 and 4, the conversion of FeIIIsur to FeIIsur is quite limited. Thus, improving the reduction rate of FeIIIsur to FeIIsur is of significance for continuously generating HO• radicals. In this context, metallic iron (Fe0) has been investigated as a facile and low-cost electron donor to reduce FeIIIsur on the surface of magnetite. Wang et al. prepared a Fe0/Fe3O4 composite on reduced graphene oxide (RGO) and used it as the catalyst.17 The RGO acted as a support of the Fe0/Fe3O4 composite, and Fe0 acted as a reductant to reduce FeIIIsur. In our work, it is believed that the iron substrate serves not only as an iron source and a support but also as electron donors to generate FeIIsur. To identify the positive effect of the iron substrate, Fe3O4 powders were scraped from the MIF composite for RhB degradation while maintaining the other conditions, as shown in Figure 3d. Although the degradation speed of Fe3 O 4 powders is comparative with that of MIF in the first stage, it becomes lower with increasing time. This could be attributed to two reasons: First, the regeneration rate of FeIIsur is limited without the iron substrate, which can act as an electron donor. Second, the Fe3O4 powders may be more prone to aggregation and lead to the low degradation efficiency. As for MIF, FeIIsur can be produced/regenerated through the reduction reaction of FeIIIsur with the assistance of Fe0 (eq 5). There is no doubt that electron transfer between Fe0 and FeIIIsur will notably promote the Fenton-like reactions and accelerate the degradation rate. Figure S5 shows digital photographs of the degradation process, which can be considered as evidence of the mechanism. As for the MIF + H2O2 + RhB system, some bubbles emerge in the first stage, as shown in Figure S5a. After 30 min, we take photographs of the solutions under different conditions (see Figure S5b,c). Interestingly, numerous bubbles are observed in the solution of the MIF + H2O2 + RhB system. Meanwhile, the color is remarkably lighter compared with that of other systems. It can be inferred that the reaction in eq 2 not only causes the bubbles but also effectively degrades the RhB solution at the same time. According to the above-mentioned results, a schematic illustration of the MIF-involved heterogeneous Fenton-like reactions is shown in Figure 3e. Actually, there are many other accompanying intermediates and reactions during the degradation process, and further insight is under research. Fe II sur + H 2O2 → Fe III sur + OH− + HO•

(1)

HO• + RhB → CO2 + H 2O + others

(2)

Fe III sur + H 2O2 → Fe II sur + H+ + HOO•

(3)

Fe III sur + HOO• → Fe II sur + H 2O2

(4)

Fe 0 + Fe III sur → Fe II sur

(5)

Figure 4. (a) Digital photographs of the MIF composite before and after a 10-cycle reaction. (b) Relative efficiencies of different cycles compared with the first run. (c and d) SEM images of the sample after the 1st and 10th cycles. The insets are digital photographs of the RhB solutions after reaction.

relative efficiencies of each cycle compared with the first run, and the relative efficiency of the 10th cycle still reaches up to 93%. The good reusability of the MIF catalyst could be attributed to two factors: (i) The Fe3O4 layer is tightly integrated with the substrate, and no powders fall off during the heterogeneous reaction. (ii) The involvement of an iron substrate could supplement the iron(II) species during the reaction process. On the other hand, the microstructure and morphology of the Fe3O4 products are also important factors. As shown in Figure 4c, the SEM image of the sample after the first cycle is almost the same as that of the as-prepared products (Figure 2b). Except for the unsharp polyhedral outlines of the sample after 10 cycles, as shown in Figure 4d, no significant difference could be detected. Moreover, the corresponding X-ray diffraction (XRD) patterns and TEM image of the MIF catalyst are also presented in Figure S6, confirming the structural stability of MIF. Besides, the performances of the MIF catalyst for degrading other dyes such as methyl orange and methylene blue were also investigated, as shown in Figure S7. In the present work, a novel heterogeneous Fenton-like catalyst MIF, which is composed of an iron substrate and a robust Fe3O4 layer, has been successfully fabricated. The MIF exhibited excellent performances for degrading RhB under neutral conditions, which could be attributed to the synergistic effect of an iron substrate and a magnetite layer. In addition, the reusability of MIF was outstanding because a relative efficiency of 93% could still be obtained after 10 cycles. Considering the advantages of high efficiency, no pH adjustment, excellent reusability, and ease of recycling, MIF is very promising as an efficient and nonfiltration catalyst for heterogeneous Fenton-like reactions.

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As shown in Figure 4, the reusability and morphology changes of MIF are also studied. The MIF was recycled after the degradation process by rinsing with ethyl alcohol and then reused in the next run under the same conditions as before. Figure 4a presents digital photographs of MIF before and after a 10-cycle reaction. No obvious changes can be observed, and the Fe3O4 layer is still homogeneous and intact. Figure 4b shows the

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00546. Experimental details, XPS, EDS, TEM, and XRD results, and some digital photographs (PDF) C

DOI: 10.1021/acs.inorgchem.9b00546 Inorg. Chem. XXXX, XXX, XXX−XXX

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in Water Using Magnetic Nanocomposite (MCM-41/magnetite). J. Catal. 2014, 2014, 1. (14) Wang, X.; Liao, Y.; Zhang, D.; Wen, T.; Zhong, Z. A Review of Fe3O4 Thin Films: Synthesis, Modification and Applications. J. Mater. Sci. Technol. 2018, 34, 1259−1272. (15) Cheng, W.; Tang, K.; Qi, Y.; Sheng, J.; Liu, Z. One-Step Synthesis of Superparamagnetic Monodisperse Porous Fe3O4 Hollow and CoreShell Spheres. J. Mater. Chem. 2010, 20, 1799−1805. (16) Chen, F.; Xie, S.; Huang, X.; Qiu, X. Ionothermal Synthesis of Fe3O4 Magnetic Nanoparticles as Efficient Heterogeneous Fenton-Like Catalysts for Degradation of Organic Pollutants with H2O2. J. Hazard. Mater. 2017, 322, 152−162. (17) Wang, P.; Zhou, X.; Zhang, Y.; Yang, L.; Zhi, K.; Wang, L.; Zhang, L.; Guo, X. Unveiling the Mechanism of Electron Transfer Facilitated Regeneration of Active Fe2+ by Nano-Dispersed Iron/ Graphene Catalyst for Phenol Removal. RSC Adv. 2017, 7, 26983− 26991.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.L.). ORCID

Yulong Liao: 0000-0003-3761-7170 Quanjun Xiang: 0000-0002-4486-7429 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National R&D Program of China under Grant 2017YFA0207400, National Key Research and Development Plan under Grant 2016YFA0300801, National Natural Science Foundation of China under Grant 61571079, Fundamental Research Funds for the Central Universities under Grant ZYGX2018J029, and Science and Technology Project of Sichuan Province under Grant 2017JY0002.

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DOI: 10.1021/acs.inorgchem.9b00546 Inorg. Chem. XXXX, XXX, XXX−XXX