Hollow α-Fe2O3 Nanoboxes Derived from Metal–Organic Frameworks

Publication Date (Web): December 12, 2016 ... Mn-incorporated metal organic framework are for sustainably fast extraction and magnetic separation of t...
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Hollow α‑Fe2O3 Nanoboxes Derived from Metal−Organic Frameworks and Their Superior Ability for Fast Extraction and Magnetic Separation of Trace Pb2+ Qiaoling Mo,†,‡ Jinxin Wei,†,‡ Keyi Jiang,†,‡ Zanyong Zhuang,†,‡,* and Yan Yu†,‡,* †

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Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), Fujian Province University, Fuzhou, Fujian 350108, China ‡ College of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350108, China S Supporting Information *

ABSTRACT: Hierarchical metals oxide nanostructure derived from annealing of metal−organic frameworks (MOFs) usually have large particle size and low specific surface area and, as a result their activities, become limited. In this work, we incorporated KMnO4 into Prussian blue (PB) microcubes and obtained Mn-doped α-Fe2O3 nanoboxes by annealing the complex. We found that KMnO4 stayed inside the pores of the PB framework and restricted the crystal growth of α-Fe2O3 during annealing. Consequently, the Mn-doped Fe2O3 nanoboxes have small particle size, large specific area (452 m2/g), and a significant amount of adsorbed oxygen in the form of OH− on the surface as determined by X-ray photoelectron spectroscopy. It could serve as an adsorbent to quickly remove the trace-level (40 mg/L) Pb2+ from water. Within 1 min, this nanoadsorbent (0.2 g/L) extracted >70% Pb2+ in the solution, and >91.6% in 15 min. Besides, it also selectively captures Pb2+ (40 mg/L) from a synthetic Pb/Zn mining wastewater containing Zn2+ (40 mg/L) and various kinds of interfering ions (Na+, K+, Mg2+, Ca2+, SO42−, NO3−, Cl−). The maximizing capacity of Pb2+ reaches to 900 mg/g when treating concentrated Pb2+ solution (1g/L). The spent adsorbent could be easily retrieved from the solution by magnetic separation. We anticipate the findings here will help to inspire the design of other novel MOFs-derived nanomaterials. KEYWORDS: Hierarchical nanomaterials, Metal−organic frameworks, Prussian blue, Magnetic separation, Selective extraction

1. INTRODUCTION Hierarchical nanomaterials with 3D architecture have gained increasing attentions for their potential applications in various fields such as energy storage/conversion,1−3 catalysis,4,5 and environment remediation.6−11 Conventional methods to make hierarchical nanostructures via templates usually require the use of organic surfactants,12 block copolymers,13 or SiO2 as the support.14 However, template removal after the synthesis can be cumbersome and may also lead to the collapse of the nanostructure. Over the past years, solid-state decomposition of solid precursors has become a particularly interesting approach to fabricate materials with porous or hollow structures.15 Recently, to prepare complex hierarchical structures, simple metal carbonates or hydroxides were replaced with metal−organic frameworks (MOFs).16−18 The latter has been considered as more promising precursors because they are readily tunable in their framework, composition, and pore structure. In a typical case reported by Lou et al.,1 annealing of Prussian blue (PB) microcubes generated hollow iron oxide with shell architecture. Based on the reaction of PB microcubes with conjugate bases (e.g., NaOH), modified strategies enable to fabricate several kinds of nanoboxes (Fe2O3/SnO2, Fe2O3/SiO2, Fe2O3/GeO2, Fe2O3/Al2O3, and Fe2O3/B2O3) with varying composition.19 © 2016 American Chemical Society

Furthermore, a series of Prussian blue analogues (PBAs) with different chemical compositions also serves as precursors to construct the morphology-inherited nanostructures.20−22 Despite current progress, it still remains challenging to finely control the size and shape of the materials and thus delicately customize their physicochemical properties. Unfortunately, since nanocrystals are thermodynamically unstable and tend to grow into bulk crystal (particularly at high temperature),23 the metals oxides prepared solely through annealing usually have a large particle size (in hundreds of nanometers) and a low specific surface area (in tens of m2/g), and their activities and applications are hence seriously restricted.24 New methods are highly wanted to generate hierarchical metals oxide nanostructures with controlled morphology, small size, and large surface area. Doping is often an effective approach to tailor the size and structure of nanocrystals. For instance, SnO2 nanocrystals doped with metal ion are small in size, which contributes to good gas-sensing properties.25 Manganese (both the cation and its compounds) is recognized for its wide applications in Received: August 28, 2016 Revised: November 23, 2016 Published: December 12, 2016 1476

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ACS Sustainable Chemistry & Engineering Table 1. Precursor and Annealing Product precursor

annealing product

code

Fe/Mn

KMnO4 source

code

K-PB-10-L K-PB-1-L K-PB-2-S K-PB-1-S K-PB-0.5-S

10 1 2 1 0.5

solution solution solid solid solid

FM-10-L FM-1-L FM-2-S FM-1-S FM-0.5-S

catalysis,26 toxic waste remediation,27 and battery technology.28 It is also cheap and environmentally friendly. Besides, Mn and Fe have similar radius and valences, and the incorporation of Mn thus appears favorable in tailoring the size, shape, and properties of Fe2O3 nanostructures.29 In this work, we reported that the particle size of Fe2O3 nanoboxes derived from PB microcubes can be dramatically reduced by doping with KMnO4. The obtained Fe2O3 complex also has remarkably high surface-area-to-volume ratio and thus can effectively sequester Pb2+ from a synthetic wastewater by magnetic separation. The underlying mechanism of this successful doping strategy was discussed as well. The findings in this work will help to understand MOFs-derived nanomaterials and inspire further materials designs.

Mn wt %

(104)

(110)

± ± ± ± ±

35.3 32.5 29.8 26.7 17.6

76.0 50.0 47.8 30.3 27.0

1.5 7.1 34.3 40.1 62.9

0.1% 0.4% 1.5% 2% 3%

and Fe/Mn molar ratio appended. The key samples to be discussed include FM-10-L, FM-1-L, FM-2-S, FM-1-S, and FM-0.5-S, in which the number represents the Fe/Mn molar ratio, L indicates the synthesis used KMnO4 solution, and S indicates the synthesis used KMnO4 solid. 2.4. Adsorption Experiments. Each aqueous solution of Pb(II), Ag(I), Cd(II), Zn(II), and Cu(II) was prepared by dissolving the corresponding heavy metal salt in deionized water to arrive at a concentration of 40 mg/L. A synthetic wastewater containing 40 mg/L Pb2+ and 40 mg/L Zn2+ was also prepared, which contained significant amount of interfering ions including Na+ (68.3 mg/L), K+ (6.44 mg/ L), Mg2+ (11.8 mg/L), Ca2+ (34.4 mg/L), and significant amount of SO42−, NO3−, Cl−. This synthetic wastewater resembled the real tailing wastewater discharged from lead−zinc mine, and the interfering ions represented the ion content of tap water. Adsorption tests were carried out using 10 mg adsorbent in 50 mL solution unless noted otherwise. Batch experiments of adsorption were carried out in glass beakers with stirring and under ambient condition. At prescribed time intervals, an aliquot was retrieved from the solution and centrifuged. The concentration of the heavy metal in the supernatant was determined by atomic absorption spectrophotometry (SHIMADZU AA-6880). All adsorption tests were performed in triplicate. The removal capacity (Qe, mg/g) of all samples were calculated according to eq 130

2. EXPERIMENTAL SECTION 2.1. Chemicals. Potassium hexacyanoferrate(II) trihydrate (K4Fe(CN)6·3H2O), hydrochloric acid (HCl), potassium permanganate (KMnO4), lead(II) nitrate (Pb(NO3)2), silver nitrate (AgNO3), zinc nitrate (Zn(NO3)2), cadmium nitrate (Cd(NO3)2), copper nitrate trihydrate (Cu(NO3)2·3H2O), calcium nitrate(Ca(NO3)2), sodium sulfate (Na2SO4), magnesium chloride hexahydrate (MgCl2·6H2O), and potassium chloride (KCl) were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyvinylpyrrolidone (PVP, K30, MW ∼ 40 000) was purchased from Sigma-Aldrich. All chemicals used in the experiments were of analytical grade and used without further purification. Deionized (DI) water was used for all experiments. 2.2. Synthesis of Prussian Blue Microcubes and α-Fe2O3 Nanoboxes. In a typical procedure, PVP (3.8 g) and K4Fe(CN)6· 3H2O (0.11 g) were successively added into aqueous HCl (0.1 M, 50 mL) under magnetic stirring. The mixture was first stirred for 30 min, then further ultrasonicated for another 30 min, and finally heated at 80 °C for 24 h. The precipitated blue PB crystals (i.e., Fe4[Fe(CN)6]3) were centrifuged, washed first with deionized water and then with absolute ethanol, and finally dried in a vacuum oven at room temperature. The α-Fe2O3 nanoboxes were prepared by annealing the PB crystals at 650 °C with a slow heating rate of 0.5 °C/min. 2.3. Synthesis of α-Fe2O3/KMn8O16 Nanoboxes. A scalable route to make the Mn-doped α-Fe2O3 nanoboxes is as follows. Typically, the PB microcubes (0.1 g) were suspended in a mixture of ethanol (10 mL) and deionized water (17 mL), and the KMnO4 solution (1 g/L, volume varied according to the desired composition of the product) was slowly added into the PB solution dropwise. The mixture was stirred for 1 h at room temperature. The precipitates (denoted as K-PB) were centrifuged, washed with deionized water and absolute ethanol, and dried in a vacuum oven at room temperature. The Mn-doped α-Fe2O3 nanoboxes (denoted as FM-L) were then prepared by annealing the Mn-doped PB crystals at 650 °C with a slow heating rate of 0.5 °C/min. In a modified method, KMnO4 solid instead of aqueous KMnO4 solution was added into the PB suspension. The mixture was stirred for 5 min, and the precipitate was collected and purified similarly. Hollow α-Fe2O3/KMn8O16 nanoboxes (denoted as FM-S) were obtained by annealing this precipitate at 650 °C with a slow heating rate of 0.5 °C/min. In the following, as summarized in Table 1, the Mn-doped α-Fe2O3 nanoboxes are denoted as FM with the code for their source material

Q e = (C0 − Ce)*V /M

(1)

where Qe (mg/g) is the equilibrium adsorption amount, V (mL) is the volume of the solution, M (mg) is the mass of the adsorbent, and C0 and Ce are the initial and equilibrium concentrations of the adsorbate (mg/L), respectively. 2.5. Characterization. The phases of samples were determined on a PANalytical X’pert MPD X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5406 Å) at a voltage of 36 kV and a current of 30 mA in the continuous scanning mode. The scanned 2θ angle varied from 10° to 80° at 5°/min with a scanning step of 0.01°. The morphology and size of the samples were observed using a Philips XL30 scanning electron microscope (SEM) and FEI Tecnai G2 F30 transmission electron microscope (TEM). The X-ray photoelectron spectrum (XPS) was recorded on a PHI 5000 Versa Probe spectrometer. A Shimadzu AA-6880 atomic absorption spectrophotometer (AAS) was used to determine the concentration of heavy metal ions in the solution. The optical properties of the α-Fe2O3/KMn8O16 nanoboxes were measured on a Hitachi U-3010 UV−vis spectrophotometer (Tokyo, Japan). Specific surface area was calculated according to the Brunauer−Emmett−Teller (BET) method.

3. RESULTS AND DISCUSSIONS 3.1. Incorporation of Mn into α-Fe2O3 Nanoboxes. The crystallographic structure and phase purity of the samples were analyzed by XRD. As shown in Figure 1a, all diffraction peaks of as-prepared PB sample could be indexed to cubic phase Fe4[Fe(CN)6]3 (JCPDS card 73-0687). The PB was essentially free of impurities because no additional diffraction peaks were detected. The SEM observation (Figure 2a) demonstrates that these PB particles were highly uniform microcubes with an average size of 500 nm. These PB particles had very smooth surfaces (Figure 2e) and appeared like single crystals. 1477

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size of α-Fe2O3 in (104) and (110) direction also diminished to 35.3 and 76 nm, respectively (Table 1). The incorporation of Mn appears to have reduced the size of the α-Fe2O3 unit. When Mn doping was increase to an initial Fe/Mn ratio of 1:1 in K-PB-1-L, after annealing, the resulting particle size of the α-Fe2O3 in the (104) and (110) direction dropped to 32.5 and 50.1 nm, respectively. The energy dispersive spectrometry data indicated Mn was doped substantially into α-Fe2O3. Besides the peaks of α-Fe2O3, a series of weak diffraction peaks attributable to KMn8O16 (JCPDS card 04−0603) appeared as well (Figure 1, profile d). The relative concentration of α-Fe2O3 to KMn8O16 in sample is about 13:1, as determined by the ratio of Fe/Mn from the EDS analysis (Table 1). Therefore, the αFe2O3 is still the dominant phase. The SEM confirms that the size of α-Fe2O3 was indeed reduced (Figure 2c, g). The nanosheets are thinner in Mn-doped α-Fe2O3 than in pure αFe2O3. In particular, the incorporation of Mn did not destroy the hierarchical nanobox architecture. Further changes were observed when an equivalent amount of KMnO4 solid was used to replace the KMnO4 solution in the doping procedure. Figure 1d shows that FM-1-S was composed of both the α-Fe2O3 phase and the KMn8O16 phase. Nevertheless, the α-Fe2O3 was still the dominant phase, and EDS analysis determined that the relative concentration of αFe2O3 to KMn8O16 was roughly 2:1. The size of α-Fe2O3 along the (110) and (104) direction reduced dramatically to 30.3 and 27.0 nm, respectively. As shown in Figure 4, both the SEM and TEM images confirm the hollow structure of FM-1-S nanoboxes, which are composed of nanoparticles with a domain size of tens of nanometers. Besides, as shown in Figure 3d−f, incorporation of Mn by the direct addition of KMnO4 solid did not destroy the nanobox architecture of the sample either. Further elemental mapping (Figure 3h−l) of individual nanoboxes indicates that both Fe and Mn were uniformly dispersed in each microcube. Hence, the α-Fe2O3 and KMn8O16 were not simple disordered mixture but grew together symmetrically out of the microcube of the K-PB precursor.

Figure 1. (left) XRD patterns of (a) synthesized PB; (b) α-Fe2O3; (c) FM-10-L; (d) FM-1-L; (e) FM-1-S; and (f) FM-0.5-S. (right) XRD patterns of samples b−f with 2θ ranging from 31° to 37°.

Calcination at 650 °C decomposed the PB microcubes and transformed them into rhombohedral α-Fe2O3 (JCPDS card 33-0664). According to the Scherrer equation, the average size of α-Fe2O3 in the (104) and (110) direction was 60.7 and 90.7 nm, respectively. The SEM images (Figure 2b, f) show that the α-Fe2O3 had a well-defined hierarchically structured shell consisting of Fe2O3 nanosheets. Suspending PB crystals (0.05 g) in aqueous ethanol (27 mL) and adding KMnO4 solution (1 g/L, 6.4 mL) then furnished the KMnO4-treated PB samples (K-PB-10-L). The XRD pattern of K-PB-10-L in Figure 3a shows that incorporation of Mn did not change the PB phase. The SEM images (Figure 3b, c) also reveal that the K-PB-10-L was still similar to the source PB crystal and remained to be microcubes resembling single crystals. Furthermore, EDS analysis indicates the presence of trace Mn (below 1.5 wt %) in the newly formed K-PB-10-L crystals. The subsequent thermal decomposition then yielded FM-10-L, which still contained α-Fe2O3 but exhibited no diffractions of Mn-based materials (Figure 1, profile c), possibly owing to the low content of Mn. Even though no phase change was detected in FM-10-L, this Mn-doped α-Fe2O3 still shows broader diffraction than pure αFe2O3 (left panel in Figure 1). Besides, in FM-10-L, the particle

Figure 2. SEM images. (a, e) PB microcubes. (b, f) α-Fe2O3. (c, g) FM-1-L. (d, h) FM-1-S. (i) SEM image typical of FM-1-L. (j−l) EDS elemental mapping analyses corresponding to part i. 1478

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Figure 3. (a−c) XRD and SEM patterns of K-PB-10-L. (d−f) XRD and SEM patterns of K-PB-1-S. (g) HRSEM image of K-PB-1-S. (h−l) Elemental mapping images of Fe, C, N, Mn, and O corresponding to part g.

shown in Figure S1, the FM-1-S showed a moderate Brunauer− Emmett−Teller (BET) specific surface area of 452 m2/g and the mean pores size of 2.5 nm, indicating the obtained materials to be classified as mesoporous materials. In contrast, the pristine hierarchical α-Fe2O3 nanoboxes only had a much lower BET specific surface area of 25.4 m2/g.1 And the specific surface area of FM-1-S is much higher than that other adsorbents such as hollow Mn2O3 ellipsoids (24 m2/g),31 functionalized flowerlike magnetic adsorbents (FMAs; 72 m2/g),32 etc. 3.2. Underlying Mechanism of the Structural Change through Mn Incorporation. The incorporation of Mn never destroyed the microcube structure of PB. Even when the amount of Mn reached up to 40.1 wt %, the framework of PB was still maintained (Figure 3e and f). Furthermore, elemental mapping indicated that Mn always dispersed uniformly in PB crystals (Figure 3k), and no XRD diffraction of KMnO4 could

Figure 4. (a) SEM and (b) TEM images of hollow FM-1-S nanoboxes.

Smaller size is generally desirable for nanomaterials because it always contributes to a larger specific surface area and sometimes gives rise to new physicochemical properties. As

Figure 5. Development of the α-Fe2O3/KMn8O16 nanoboxes. 1479

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Figure 6. Overview (a) and the corresponding high-resolution XPS spectra (b) Fe 2p, (c) Mn 2p, and (d) O 1s of (I) FM-10-L and (II) FM-2-S.

Figure 7. (a) Absorption capacity of five kinds of heavy metal ions in the presence of the α-Fe2O3/KMn8O16 nanoboxes with different initial of Fe/ Mn. Adsorption isotherms of (b) Pb(II), (c) Cd(II), (d) Zn(II), and (e) Cu(II) using the α-Fe2O3/KMn8O16 nanoboxes.

some small molecules. Moreover, because the iron species allow for charge balance, redox reactions can also occur inside this space. We speculate that KMnO4 easily entered the cavity in the PB crystals (Figure 5) and worked as a placeholder to restrict the growth of Fe2O3 during annealing. Consequently,

be detected (Figure 3d). It is thus reasonable to assume that the Mn entered the lattice of PB and stayed inside. Owing to the asymmetric C≡N anion, the PB crystals (i.e., Fe4[Fe(CN)6]3) have perfect three-dimensional polymeric frameworks with a large cell dimension of 1.013 nm × 1.013 nm × 1.013 nm. This large open space can accommodate transition-metal ions and 1480

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Figure 8. (a) XRD, (b) SEM, and (c) HRSEM images of FM-1-S after adsorption of Pb2+. (d) HRSEM image. (e−h) EDS elemental mapping analyses corresponding to part d.

Figure 9. (a) Overview. (b) Corresponding high-resolution XPS spectra Pb 4f of the FM-1-S after adsorption of Pb2+, respectively.

oxygen atoms on the surface, accompanied by the decreased amount of the internal lattice oxygen. (2) On the other hand, the “absence” of lattice oxygen could also be ascribed to the limited penetration depth of the XPS method. 3.3. Adsorption and Catalytic Properties of MnDoped α-Fe2O3 Nanoboxes. In general, the adsorption behavior of materials largely depends on the amount of hydroxyl groups binding on the surface. FM-1-S has plenty of surface adsorbed oxygen, as well as a large specific surface area, and hence show outstanding adsorption property. Figure 7a and b compares the adsorption capacities of adsorbents, using C0 = 40 mg/L Pb(II) and 0.2 g/L adsorbents. It can be seen that Pb(II) adsorption reached equilibrium at around 30 min (Figure 7b). The equilibrium capacity (Qe) increases (Figure 7a) in the order of α-Fe2O3 (104.1 mg/g), FM-10-L (129.8 mg/g), FM-1-L (108.5 mg/g), and FM-1-S (178.8 mg/g). Figure 7b shows that Pb (II) can be quickly removed from water. The nanoadsorbents picked up >70% Pb2+ within 1 min. The removal reached 91.6% within 15 min, leaving less than 1 ppm Pb2+ remaining in the aqueous phase. The resulting water quality regarding Pb could comply with the standard of tap water. When the dose of the adsorbent was slightly increased to 0.25 g/L, within 10 min the Pb2+ concentration could decrease from the initial original 40 mg/L to 0.76 mg/L, which is below the required threshold (1 mg/L Pb) in the national standard for wastewater discharge (GB8978-1996). 3.4. Underlying Mechanism of Metal Extraction. After the α-Fe2O3/KMn8O16 nanoboxes adsorbed Pb(II), new characteristic peaks attributable to PbO (JCPDS 35-1482) could be observed in the XRD (Figure 8a). No additional

higher Mn content in the PB crystals resulted in the smaller size of the α-Fe2O3 (Table 1). The small size and the hierarchical structure of α-Fe2O3 contribute to its large specific surface area. The smaller size of α-Fe2O3 also makes it more magnetic. Among the examined samples, FM-1-S was the most magnetic tested by magnet. A control sample containing more KMn8O16 phase was much less magnetic (Figure S2). Therefore, the stronger magnetism of the α-Fe2O3/KMn8O16 nanoboxes should be attributed to the decrease in the size of α-Fe2O3 rather than to the KMn8O16 phase. XPS analysis also revealed structural changes. The XPS spectra in Figure 6 show compared signals of Mn 2p, Fe 2p, O 1s and K 2p for FM-10-L and FM-2-S. In FM-10-L (Figure 6b, profile I), the binding energy of Fe 2p3/2 and Fe 2p1/2 are 711.7 and 724.4 eV, respectively, which indicates the presence of αFe2O3.33,34 Besides, the binding energy of Mn 2p3/2 and Mn 2p1/2 are centered at 642.8 and 653.5 eV (Figure 6c, profile I), which indicates the presence of both Mn4+ and Mn3+ and can be ascribed to KMn8O16. In O 1s spectrum (Figure 6d, profile I), the low binding energy peaks at 530.4 and 531.8 eV can be assigned to lattice oxygen and surface adsorbed oxygen in the form of OH−, respectively.30 In FM-2-S, the Mn doping is higher and the Fe content is lower, which results in the weaker intensity of Fe 2p peaks (Figure 6b). Besides, a significant change can be found in the O 1s signal. Only one O 1s peak at 531.8 eV can be observed in FM-2-S. The “absence” of the lattice oxygen in XPS spectrum of FM-2-S is possibly attributed to two factors: (1) The FM-2-S composed of nanoparticles has large specific surface area and thus great amount of hydroxyl 1481

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lead−zinc mine mainly contained Pb2+ and Zn2+, and in practice the recovery test of Pb2+ from wastewater was conducted to avoid environmental pollution. To simulate the extraction of Pb which typically presents in the tailing water at mining sites, a mixed solution of 40 mg/L Pb2+ and 40 mg/L Zn2+ was prepared. This solution also contained a significant amount of Na+ (68.3 mg/L), K+ (6.44 mg/L), Mg2+ (11.8 mg/ L), Ca2+ (34.4 mg/L), SO42−, NO3−, and Cl−, such that the content of these interfering species resembled the ion composition of tap water. All tests used an adsorbent dose of 0.2 g/L. As expected, the adsorption capacity of FM-1-S toward Pb (180 mg/g) was higher than all other samples including the pristine α-Fe2O3 (Figure 10a−c). The maximizing capacity of

diffraction peaks from elemental Pb could be detected. The occurrence of PbO after adsorption can also be found in other reports in literatures.30 As the valence of Pb remained unchanged (being confirmed by XPS analysis in the following), the extraction of Pb(II) must have been driven by adsorption rather than by redox reaction. Elemental mappings of Fe, Mn, Pb, K, and O in a single nanobox show the distinctive patterns of Pb distributions (Figure 8d−i). In the sample, the uniform nanobox structures had an average size of about 1 μm, and they were stable enough to remain intact after the adsorption (Figure 8b and c). The mechanism of Pb (II) adsorption by the FM-1-S was further studied through XPS (Figure 9). Upon Pb (II) adsorption, the ratio of Fe to Mn basically remained unchanged after adsorption of Pb2+. New peaks appear at the binding energy of 138.1 and 142.9 eV, which can be ascribed to PbO as described in the literature.30 Therefore, the adsorbed Pb ions did not undergo valence change but remained bivalent, which indicates the extraction of Pb(II) was driven by adsorption rather than by redox reaction. The occurrence of PbO could lie on that, there have abundant amount of hydroxyl groups bonded on FM-1-S surface as confirmed by XPS analysis. The adsorption of Pb2+ via interaction of hydroxyl groups and Pb2+ could form the lead(II) hydroxide (Pb(OH)2);35 subsequently, the dehydration of unstable lead(II) hydroxide generates PbO. Notice that in the adsorption of Pb(II), FM-1-S showed a much higher Qe (178.8 mg/g) than many other nanomaterials reported in the literature (Table 2), such as hierarchical MnO2 Table 2. Comparison of the Adsorption Capacity of Various Adsorbents for Pb(II) adsorbent

adsorption capacity (mg/g)

ref

α-Fe2O3/KMn8O16 nanoboxes MnFe2O4 functionalized magnetic adsorbents α-FeOOH hollow sphere Fe3O4 nanoparticles m-PAA-Na-coated MNPs Fe3O4/Fe@ZnO nanospheres Fe3O4/MnO2 MnO2/CNTs hierarchical MnO2 microspheres Mn3O4/AC

179 131 26.4 80 36 40.0 163 9.81 78.7 149 49.8

this study 29 32 37 38 39 40 41 42 36 43

Figure 10. (a) Absorption efficiencies of α-Fe2O3/KMn8O16 nanoboxes in mixture solution of 40 mg/L Pb(II) and 40 mg/L Zn(II). (b) Macroscopic phenomena of magnetic responsiveness of the FM-1-S. (c) Competition absorption properties of the mixture solution (Na+, K+, Mg2+, Ca2+, SO42−, NO3−, Cl−) using the α-Fe2O3/KMn8O16 nanoboxes and commercial α-Fe2O3 powder, respectively. (d) Absorption capacity of the Pb(II) solution (1 g/L) using the FM-1-S.

Pb2+ reaches 900 mg/g when treating 1 g/L Pb2+ solution (Figure 10d). Moreover, α-Fe2O3/KMn8O16 nanoboxes exhibited high selectivity toward Pb(II) since they picked up much less Zn (21.5 mg/g). This made it possible to selectively separate Pb(II) from Zn(II) in a mixture. In practical applications, the retrieval of spent adsorbents by normal solid−liquid separation can be an obstacle. Fortunately, the α-Fe2O3/KMn8O16 nanoboxes can be efficiently separated from the adsorption system by an external magnetic field (Figure 10b).

microspheres (∼149 mg/g)36 and urchin-like α-FeOOH hollow spheres (∼80 mg/g)37 (Table 2). This underlines the strong potential of α-Fe2O3/KMn8O16 nanoboxes as a highly effective material to decontaminate water that has been polluted with Pb(II). 3.5. Adsorption Ability of FM-1-S toward Various Kinds of Heavy Metals. The adsorption ability of FM-1-S toward different kinds of heavy metals (Cu, Zn, and Cd) was also evaluated. All tested samples showed much lower Qe for Cd(II), Cu(II) and Zn(II) (Figure 7c−e). Presumably, as indicated in the literature,27,30,44 Pb2+could enter the metals oxide structure and occupy both interlayer and surface edge sites, whereas many other metals (Cu2+, Zn2+, and Cd2+) could only occupy the interlayer sites. The observed high adsorption capacities for Pb meant that the material could selectively extract the Pb from Cd(II), Cu(II), and Zn(II). We noticed that the tailing wastewater of

4. CONCLUSIONS In this work, we prepared α-Fe2O3/KMn8O16 nanoboxes and measured their adsorption properties. The incorporation of KMnO4 dramatically reduced the particle size of Fe2O3 in the nanoboxes derived from Prussian blue (PB) microcubes, because KMnO4 entered the PB framework, stayed within the pores, and consequently restricted the crystal growth of αFe2O3 during annealing. The Mn-doped α-Fe2O3 nanoboxes have small particle size, large specific area (452 m2/g), and significant amount of adsorbed oxygen in the form of OH− on 1482

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ACS Sustainable Chemistry & Engineering the surface. The Mn-doped α-Fe2O3 nanoboxes can serve as an adsorbent to quickly pick up trace level (40 mg/L) Pb2+ from water. Within 1 min, the nanoadsorbent (0.2 g/L) already extracted >70% Pb2+ in the solution, and >91.6% in 15 min. Furthermore, the α-Fe2O3/KMn8O16 nanoboxes showed excellent capability for removing heavy metal ions from wastewater. The maximum adsorption capacities were 180, 41.0, 37.9, and 30.3 mg/g for Pb2+, Cu2+, Zn2+, and Cd2+, respectively. The tested adsorbent could also selectively capture the Pb2+ (40 mg/L) from a synthetic Pb/Zn mining wastewater containing Zn2+ (40 mg/L) and various kinds of interfering ions (Na+, K+, Mg2+, Ca2+, SO42−, NO3−, Cl−). The maximizing capacity of Pb2+ reaches to 900 mg/g when treating 1 g/L Pb2+ solution. The adsorbent could be readily recovered from the solution by magnetic separation because of the magnetic property of α-Fe2O3.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02064. Additional information, including the nitrogen adsorption−desorption isotherms distribution curves of FM-1S, and additional SEM images of FM-2-S and FM-0.5-S (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +86-591-22866534. E-mail address: [email protected]. cn *E-mail: [email protected]. ORCID

Yan Yu: 0000-0001-6748-2620 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 51472050, 51402295, and 51672046).

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