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Oct 5, 2016 - Nanostructures Derived from Porous CaO Network. Weijie You,. †,‡ ... Xiu Wang,. †,‡ ... ABSTRACT: Using the porous framework of CaO as.
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Synthesis and Adsorption Properties of Hierarchically Ordered Nanostructures Derived from Porous CaO Network Weijie You,†,‡ Yali Weng,†,‡ Xiu Wang,†,‡ Zanyong Zhuang,*,†,‡ and Yan Yu*,†,‡ †

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

ABSTRACT: Using the porous framework of CaO as templates and reagents, we explored a surfactant-free and economical method for preparing calcium silicate hydrate (CSH) hierarchically ordered nanostructures. Incorporation of SiO2 nanoparticles into the CaO framework, followed by a reaction assisted by hydrothermal treatment, resulted in the formation of CSH with well-defined morphologies. The structural features of CSH were characterized by 3-D hierarchical networks, wherein nanofibers assembled to form nanosheets, and nanosheets assembled to form hierarchically ordered structures. Investigation of the crystal growth mechanism indicated that the key to forming the CSH ordered assembly structure was confining the Ca/Si ratio within a small range. Nonclassic oriented aggregation mechanism was used to describe the crystal growth of nanosheets, while the porous CaO framework served as template/reagents responsible for the formation of hierarchical structures. The resulting CSH adsorbent exhibited better performance in removing Pb(II) compared with other types of random CSH adsorbents. Additionally, the hierarchical structure of CSH provided more pores and active sites as support for other active functional materials such as zerovalent iron (Fe0). As-produced CSH@Fe nanocomposite with self-supported structures displayed high capacities for removal of Pb(II) after five adsorption−desorption cycles, and high capacities for other heavy metal ions (Cu2+, Cd2+, and Cr2O72−) and organic contaminants. KEYWORDS: Hierarchically ordered structure, oriented aggregation, porous CaO framework, surfactant-free, calcium silicate hydrates



INTRODUCTION Low-dimensional nanomaterials such as nanowires and nanosheets have a variety of applications in catalysis, energy conversion/storage, and environmental remediation.1−3 Nevertheless, tight agglomeration and stacking of these nanounits driven by high surface energies always lead to fast deterioration of their physicochemical properties. To overcome this problem, well-defined 3-D nanomaterials have been fabricated using surfactants such as tetrabutylammonium bromide (TBAB), polyvinylpyrrolidone (PVP), and block copolymers P123 as soft templates for the hierarchical assembly of nanomaterials.4−6 However, it is difficult to choose a suitable template for a specific application, and removal of the template after synthesis can be time-consuming or incomplete, which produces potentially toxic contaminants. As an alternative, hard sacrificial templates have received much attention for the preparation of functional porous materials.7−10 In many studies, SiO2 or carbon microspheres serve as mechanical supports to fabricate hollow or porous nanostructures.9,10 More recently, many researchers shifted their attention to calcium carbonate (CaCO3),11,12 which is an inexpensive, eco-friendly, and naturally abundant material. However, CaCO3 cannot be used as a template due to the tight packing of its surface as well as its size, which is typically © 2016 American Chemical Society

over several microns. To overcome these disadvantages, Akiyama et al. reported the coprecipitation and subsequent decomposition of Mn−Ca-carbonates to synthesize manganese oxide hollow structures.11 Shi et al. also showed that calcination of CaCO3, which releases CO2, could form a loose and porous framework of CaO, rendering it a suitable template for creating 3-D graphene foams.13 Despite the improvements of these new strategies, acid etching was usually required to remove the templates, increasing the cost and possibility of environmental pollution. It is still highly desirable to rationally design and synthesize 3-D nanoarchitectures using a green and facile approach. Over the past decade, hierarchical materials based on calcium silicate hydrate (CSH) have drawn growing attention for their potential applications in bone tissue engineering, drug delivery, and heavy metals extraction.14−18 Nevertheless, there are few reports of the surfactant-free synthesis of CSH hierarchical materials, since this kind of CSH nanomaterials is usually difficult to prepare using most known methods such as coprecipitation, sol−gel, and hydrothermal method. Previously, Received: September 13, 2016 Accepted: October 5, 2016 Published: October 5, 2016 33656

DOI: 10.1021/acsami.6b11633 ACS Appl. Mater. Interfaces 2016, 8, 33656−33665

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) XRD patterns of CaCO3, CaO@SiO2, CSH1, CSH2, CSH3, and CSH4; (b) TGA curves of CSH1, CSH2, CSH3, and CSH4. 35 mL of 0.1 mol/L Fe(II) solution in a 500-mL three-neck roundbottom flask. Subsequently, NaBH4 solution (70 mL, 0.25 mol/L) was added dropwise to the flask at a rate of 3 mL/min. The solution was stirred for another 30 min under room temperature. Before and throughout the procedure, N2 was introduced to expel the dissolved oxygen and maintain an anaerobic environment. Finally, the black metal particles were collected, washed with ultrapure water and ethanol, and dried under vacuum. The content of doped Fe0 was assayed by dissolving each sample of CSH@Fe in 5% nitric acid, followed by ICP analysis of Fe. Unsupported Fe0 was prepared similarly without CSH as the control. Pb(II) Removal by CSH and CSH@Fe. An amount of 30 mg of each adsorbent (CSH@Fe, CSH, or Fe0) was added into Pb(II) solution (50 mL, 1000 mg/L). The reaction proceeded at room temperature for 6 h. At given time intervals, 1 mL of solution was retrieved for analysis. Additionally, a series of simulated wastewater samples containing only one target contaminant, i.e. Cu2+, Cr2O72−, Cd2+, CR, or MO, were prepared to test the removal capacities of CSH@Fe. All tests were conducted in triplicate and the average value was used for calculation. The removal capacity (qe, mg/g) of all samples was calculated according to eq 13

we reported the transformation of oyster shells, mainly CaCO3, into CHS by calcination.18,19 The as-synthesized CHS material showed high adsorption capacities toward organic pollutants and heavy metal ions including Cr, Cd, Pb, and Mn.18,19 However, little is known regarding the underlying mechanism of this new approach of fabricating hierarchically ordered CSH. In this work, we report a surfactant-free and economical method for preparing CSH, utilizing the porous framework of CaO resulted from the decomposition of CaCO3 as templates. Incorporation of SiO2 into the CaO framework helped create the 3D hierarchical CSH networks constructed by nanosheet building blocks. It was found that the key to forming ordered self-assembled structures was a confined Ca/Si ratio within a small range. The composite of CSH@Fe, i.e. nanoscale zerovalent iron (Fe0) supported by the hierarchical CSH structures, displayed outstanding performance for the removal of heavy metal ions including Pb2+, Cu2+, Cd2+, and Cr2O72−, as well as organic pollutants. The underlying mechanism of forming hierarchical structure was proposed to be nonclassic oriented aggregation pathways of crystal growth of nanosheets, with CaO framework serving as template/support. We anticipate the finding of this work to enrich the understanding and pathway of functional materials design.



qe =

EXPERIMENTAL SECTION

(CO − Ce)· V W

(1)

where Co and Ce are the initial and equilibrium concentration (mg/L) of the target contaminant, respectively. V is the volume of the solution (L), and W is the weight of the adsorbent (g). Characterization. X-ray diffraction (XRD) analysis of the samples was recorded on a Philips X’pert-MPDX-ray diffractometer. Fourier transform infrared spectra (FTIR) of CSHs were obtained on a TJ27030A spectrophotometer. Thermogravimetric analysis (TGA) was conducted on a TGA-Q600 (Switzerland) at a heating rate of 5 °C min−1 from 25 to 1000 °C. The morphologies of the CSH and CSH@ Fe were collected by a scanning electron microscope (SEM, Philips XL30) and transmission electron microscopy (TEM, FEI Tecnai G2 F20) operating at an accelerating voltage of 200 kV. BET surface area of samples was measured by a quantachrome autosorb-1-C-TCD automated gas sorption analyzer. The zeta potentials of samples were measured by a Zetasizer Nano ZS-90. The chemical composition of CSH and CSH@Fe before and after Pb(II) adsorption was determined by an X-ray photoelectron spectrometer (XPS, PHI 5000 Versa Probe). The concentrations of contaminants in solution were determined by inductively coupled plasma atomic emission spectrometry (ICP-MS, Elan-9000, PE) and a Shimadzu UV-1800 spectrophotometer. Reusability Study. The desorption of metals from CSH3 and CSH3@Fe was performed by putting the spent adsorbent into 0.1 M HCl solution at room temperature. The regenerated CSH3 was washed with ultrapure water, and imbedded with similar amount of Fe0

Materials. Fumed silica was purchased from XIBEI Iron Alloy Company in China. Calcium carbonate (CaCO3), sodium borohydride (NaBH4), ferrous sulfate (FeSO4·7H2O), ethanol (>99%), copper sulfate pentahydrate (CuSO 4 ·5H 2 O), potassium dichromate (K2Cr2O7), lead nitrate (Pb(NO3)2), cadmium nitrate (Cd(NO3)2), methyl orange (MO), and Congo red (CR) were provided by Sinopharm Chemical Reagents Co., Ltd. China. All reagents were of analytical grade, and used as received without further purification. Ultrapure water (18.0 MΩ cm−1) in the experiments was prepared by using an ultrapure purification system. Synthesis of Calcium Silicate Hydrates (CSHs). CSHs were prepared via calcination and hydrothermal method. Briefly, different molar ratios of calcium carbonate and fumed silica (Ca/Si = 1:1, 1:1.1, 1:1.2, and 1:1.3) were mixed. The mixtures were calcined at 800 °C for 2 h, followed by a hydrothermal treatment at 170 °C for 4 h to obtain a series of products. The as-synthesized CSHs with Ca/Si = 1:1, 1:1.1, 1:1.2, and 1:1.3 are designated as CSH1, CSH2, CSH3, and CSH4, respectively. Finally, the CSH1, CSH2, CSH3, and CSH4 were collected by centrifugation, washed with ultrapure water and ethanol several times, and then dried at 50 °C. Preparation of Fe0 and CSH@Fe Composite. The composite CSH-supported Fe0 (CSH@Fe) was prepared via a chemical reduction procedure based on a previously reported method.20 Briefly, CSH (0.406 g) was dispersed in 35 mL of ethanol, followed by mixing with 33657

DOI: 10.1021/acsami.6b11633 ACS Appl. Mater. Interfaces 2016, 8, 33656−33665

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Figure 2. SEM images of (a−c) CaCO3; (d−f) CaO; (g−i) CaO@SiO2; (j−l) CSH1; (m−o) CSH2; (p−r) CSH3, and (s−u) CSH4. to form CSH3@Fe as described above. The regenerated CSH3 and CSH3@Fe were used in the next cycle of adsorption experiments.



attributable to CSH can be found, indicating few CSHs were formed. Subsequently, hydrothermal treatment of CaO@SiO2 in water at 170 °C for 4 h was performed. As shown in Figure 1a, new diffraction peaks at 28.8°, 29.3° 29.9°, and 31.8° attributable to the CSH15,16 replaced the original diffraction peaks from CaO@SiO2, indicating the complete transformation of CaO@SiO2 into the CSH. No significant change in the diffraction pattern can be observed for CSH1, CSH2, CSH3, and CSH4, which suggests that different Ca:Si ratios did not cause a phase change. The FTIR spectra of CSHs (Figure S1 in the Supporting Information, SI) confirm the XRD analysis that

RESULTS AND DISCUSSION

Composition and Structure of CSHs Materials. The structure and phase of each sample can be determined from the XRD patterns shown in Figure 1a. The starting material CaCO3 can be well indexed to calcite,21 the most stable phase of naturally occurring CaCO3. After annealing of the mixed CaCO3 and fumed silica (Ca:Si = 1:1) at 800 °C for 2 h, the calcite decomposed to form CaO.16 No obvious peaks 33658

DOI: 10.1021/acsami.6b11633 ACS Appl. Mater. Interfaces 2016, 8, 33656−33665

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ACS Applied Materials & Interfaces Table 1. Parameters of Porous Structure of CSH1, CSH2, CSH3, and CSH4 sample

specific surface area (m2·g−1)

volume of pores (cm3·g−1)

average pore size (nm)

CSH1 CSH2 CSH3 CSH4

60.4 87.3 107 158

0.280 0.295 0.404 0.598

5.53 4.36 4.62 4.72

CSHs is in domain of 4.36−5.53 nm, indicating the obtained materials can be classified as mesoporous materials. Underlying Mechanism for the Formation of Hierarchically Ordered Structures. SEM images of samples collected from different stages help to probe the underlying mechanism for forming CSH3. Figure 3a shows that, after

all the samples have similar structure. The slight difference in relative intensities of the diffraction peaks among these samples may be associated with the lattice orientation of nanocrystal growth. As shown in Figure 1b, the TGA curves of fresh CSH1, CSH2, CSH3, and CSH4 all show a similar two-stage weight loss between 25 and 1000 °C. The first 7−9 wt % loss took place below 200 °C, assigned to the loss of physically adsorbed water. A gradual loss of 10−12 wt % between 200 and 700 °C can be attributed to the thermal decomposition of crystal water from CSH.22 In general, pure calcite decomposes above 640 °C (with TGA curve shown in Figure S2). As there is basically no weight loss for all the samples above 700 °C (Figure 1b), it confirms the XRD pattern that the calcite completely decomposed into CaO, and as-produced samples contained only CSH. It can also be deduced from the weight loss of the structure H2O that the chemical composition of the samples is (CaSiO3)·H2O. Morphologies of CSHs. Figure 2 presents the SEM images of CaCO3, CaO, CaO@SiO2, and CSH1−CSH4. All these samples contain aggregates with diameters of 20−30 μm. The pure calcite particles have smooth surfaces (Figure 2a−c). Annealing of calcite at 800 °C for 2 h resulted in a 3-D interconnected porous CaO framework (Figure 2d−f). After further annealing of CaO framework with SiO2 at 800 °C for 2 h, silica particles were attached onto CaO, generating CaO@ SiO2 (Figure 2g−i). After hydrothermal treatment of CaO@ SiO2, the CSH1−CSH4 with different morphologies were fabricated. At the Ca:Si molar ratio of 1:1, the individual CSH particles are in the form of nanofibers with an average length of 1−2 μm and an average diameter of dozens of nanometers. These CSH nanofibers are arranged in the form of tightly packed bundles (Figure 2j−l). As shown in Figure 2m−r, a slight change of Ca:Si molar ratio to 1:1.1 or 1.1.2 resulted in orderly arrangement of CSH nanofibers to form small pieces of nanosheets. Several pieces of nanosheets assemble to form folded sheets, which further aggregate into hierarchically ordered walnut-shaped microspheres with an average diameter of 2−3 μm. Nanosheets are arranged in good order to form walnut-shaped microspheres, indicating the formation of ordered nanostructure. At the Ca:Si ratio of 1:1.3, randomly aggregated micron-sized CSH sheets start to form CSH4 as shown in Figure 2s−u. A small fraction of CSH4 containing both the stacked large-sized sheets and nanofibers can be observed as well (Figure S3). As shown in Figure S4, the nitrogen adsorption/desorption isotherms of all samples can all be classified as a type H3 hysteresis loop,23,24 with the adsorption occurring mainly in the medium- and high-pressure regions (0.4 < p/p0 < 1). Table 1 summarizes the specific surface areas and pore parameters of CSHs. With the Ca:Si ratio from 1:1 to 1:4 is the rise of specific surface area from 60.4 to 158 cm3/g, and the rise of pore volume from 0.280 to 0.598 cm3·g−1. The mean pore size of

Figure 3. (a) SEM image of CaO@SiO2 and (b−d) corresponding elemental mapping images of Ca, O, and Si.

mixing and calcination of CaO and SiO2 at 800 °C for 2 h (Ca:Si ratio = 1:1.2), the SiO2 particles (dozens of nanometers) were attached onto CaO, generating CaO@SiO2. Further elemental mapping (Figure 3b−d) indicates that the CaO@ SiO2 contains uniformly distributed Ca (from CaO), Si (from SiO2), and O, implying that SiO2 has been incorporated into the CaO framework. The newly formed CSHs are of an average size of 20−30 μm, close to the original CaO@SiO2 aggregates with size of 20−30 μm (Figure 2). This supports the standpoint that the CaO@ SiO2 aggregates are the precursor of the CSHs. To verify this, the morphologies evolution of CSH as a function of coarsening time (0.5, 1, and 2 h) are recorded in Figure 4 a−f. At 0.5 h, although most of the CaO@SiO2 were still shaped as original spherical nanocrystals, a few CSH nanosheets appeared (Figure 4a, d). At 1 h, more and more CSH nanosheets can be found. Moreover, CSH nanosheets started to assemble to form hierarchical porous structures (Figure 4b, e). By 2 h, the surfaces of original microspheres were covered by a layer of CSH sheets, similar to 3-D walnut-shaped nanostructures shown in Figure 4c, f. The morphological evolution from Figure 4a to 4f confirmed the above speculation that the porous CaO served as a substrate for CaO@SiO2, and CaO@SiO2 served as a precursor for oriented assembled structures. The self-assembly mechanism is proposed as follows. Figure 4g shows the TEM images of CSH3, which indicates the spontaneous side-by-side attachment of CSH nanofibers to form nanosheets. Selected Area Electron Diffraction (SAED) patterns of nanofibers aggregates show characteristics of a single crystal, suggesting a precise arrangement of nanofibers with parallel crystallographic alignment. The growth feature is 33659

DOI: 10.1021/acsami.6b11633 ACS Appl. Mater. Interfaces 2016, 8, 33656−33665

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Figure 4. SEM images of (a−c) CSH3 after coarsened for 0.5, 1, and 2 h, respectively, (d−f) enlarged images of planes a−c, respectively, (g) TEM images of CSH3, and (h) scheme depicting the self-assembly mechanism of CSH3.

Figure 5. (a) Pb(II) removal capacities by CSH1, CSH2, CSH3, and CSH4 (dose 30 mg, stirring speed 250 r/min, initial concentration 1000 mg/L, initial pH 6, temperature 25 °C) and (b) Zeta potential of CSH1, CSH2, CSH3, and CSH4.

76−110 mg/g for the four types of CSHs. The equilibrium capacity (Qe) increases in the order of CSH4 (76.3 mg/g), CSH1 (87.8 mg/g), CSH 2 (107 mg/g), and CSH3 (116 mg/ g). Actually, the XRD (Figure 1), FTIR (Figure S1), and Zeta potential analysis (Figure 5b) confirm that all four kinds of CSHs have similar structure and surface charge. Although CSH4 has the largest specific surface area, it has the lowest adsorption capacity for Pb(II). In comparison, the hierarchically assembled CSH3 exhibit the highest adsorption capacity, as it provides more active sites for contaminants adsorption. Hierarchically Ordered CSH as a Support for Fe0. Although CSH exhibited relatively high adsorption performance, the application of CSHs was still limited by a single type of functional group, i.e. the hydroxyl groups, and low density of active adsorption sites.28 Hence, in the following, the CSH served as a support for Fe0, an adsorbent with large specific

in line with nonclassic oriented aggregation pathways of crystal growth as described in recent literature.25−27 Notably in our experiment, extra silicon atoms were found to facilitate the formation of nanosheets. As depicted in Figure 4h, it can be speculated that more silicon could serve as a “bridge” to coordinate with calcium cations in the reaction process, which drives the assembly of CSH nanosheets in order. Larger or smaller Ca/Si ratios cannot achieve the ordered assembly structure of CSH. With a lower Si content, fewer nanosheets can be found (Figure 2j−l), while with a relatively higher Si content, large-sized CSH sheets will be formed (Figure 2s−u). Application of Hierarchically Ordered CSH for Environmental Remediation. The adsorption abilities of each sample were first studied in simulated wastewater containing Pb(II). Figure 5a compares the Pb(II) adsorption capacities of CSH1 to CSH4. It can be seen that Pb(II) adsorption reached equilibrium at around 3 h, maximizing at 33660

DOI: 10.1021/acsami.6b11633 ACS Appl. Mater. Interfaces 2016, 8, 33656−33665

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Figure 6. (a) XRD patterns of CSH1@Fe, CSH2@Fe, CSH3@Fe, and CSH4@Fe, (b) XPS survey scans of CSH3 and CSH3@Fe with inset showing high resolution of Fe XPS of CSH3@Fe.

Figure 7. SEM images of (a) CSH3, (b−c) CSH3@Fe with different magnification, (d) TEM image of CSH3@Fe, and (e−i) SEM images of CSH3@Fe and corresponding elemental mapping images of Ca, Si, O, and Fe.

Figure 8. (a) Pb(II) removal capacities and efficiencies by different types of CSH@Fe (dose 30 mg, stirring speed 250 r/min, initial concentration 1000 mg/L, initial pH 6, temperature 25 °C), (b) XPS survey scans of CSH3@Fe before and after adsorption of Pb(II) ions.

surface area, high reactivity, and strong reducing power.29,30 Generally, Fe0 nanoparticles tend to aggregate due to the magnetic force or van der Waals’ force, leading to sharp deterioration of activity and efficiency as adsorbents.30,31 We speculate that the high chemical stability of CSH materials15−19 make them ideal candidates as support for Fe0.

Herein, the composite CSH-supported Fe0 (CSH@Fe) was prepared by a liquid-phase sodium borohydride reduction method.20 As shown in Figure 6a, after the hybridization of Fe, new diffraction peaks at 44.8° appeared, which can be assigned to the diffraction of Fe0.20,30 There is no diffraction peak related to other forms of Fe in the XRD patterns. This indicates the successful incorporation of Fe0 on the surface of CSH. Figure 33661

DOI: 10.1021/acsami.6b11633 ACS Appl. Mater. Interfaces 2016, 8, 33656−33665

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Figure 9. (a) XPS spectra of Pb 4f of the CSH3@Fe after adsorption of Pb(II), (b) XRD patterns of CSH3@Fe before and after the uptake of Pb(II) ions.

Figure 10. (a) Pb(II) adsorption performances by CSH3@Fe, pure Fe0, and mechanically mixed CSH3 and Fe0, (b) adsorption of different pollutants onto CSH3@Fe.

Pb2 + + Fe 0(s) = Pb0(s) + Fe 2 +

6b presents the XPS spectra of CSH3 and CSH3@Fe. In comparison with the native CSH3, XPS spectra of CSH3@Fe have new peaks at 711 and 725 eV attributable to Fe 2p, demonstrating Fe was successful immobilized on CSH3.30 The high-resolution XPS pattern displays a weak peak at 706.7 eV, again suggesting the presence of Fe0.30,32 The SEM (Figure 7b,c) and TEM images (Figure 7d) of CSH3@Fe clearly show that Fe0 particles with an average diameter around 20 nm are well dispersed on the surface of CSH3 as compared to the native smooth CSH3 (Figure 7a). The corresponding elemental mapping analysis of Ca, Si, O, and Fe (Figure 7e−i) confirms the well dispersing of Fe on CSH. Therefore, CSH3 as the support successfully avoided the agglomeration of Fe0 nanoparticles. Figure 8a shows the kinetic adsorption capacities of the four types of CSH@Fe for Pb(II). For all four types of adsorbents, the uptake amounts see a steep rise within 30 min, followed by an equilibrium state at around 2 h. Compared with the native CSH, the CSH@Fe reached equilibrium faster with a 4-fold increase of the adsorption capacity for Pb(II). The Pb(II) adsorption capacities of CSH1@Fe, CSH2@Fe, CSH3@Fe, and CSH4@Fe are calculated to be 384, 432, 458, and 376 mg/ g, respectively. Similar to the pure CSH, CSH3@Fe has the highest capacity. It is worthy to mentioned that the reduction potential of Pb(II)/Pb (E0= −0.126 V vs NHE) is more positive than that of Fe(II)/Fe (E0= −0.44 vs NHE). Hence, in theory, the Fe0 can reduce the Pb(II) into Pb0 according to eq 2.

(2)

Figure 8b displays the elemental composition of CSH3 and CSH3@Fe before and after the Pb(II) adsorption analyzed by XPS. The appearance of Pb 4f peak at 136 eV after adsorption suggests the attachment of Pb(II) to [email protected] Moreover, the high-resolution XPS spectrum of Pb on CSH3@Fe (Figure 9a) can be resolved into two peaks. One peak at 136.7 eV fits the appearance of metallic Pb0,20,34 and the other at 138.2 eV can be assigned to PbO.20,34 However, the appearance of PbO could possibly stem from the ready oxidation of metallic lead and the limited penetration depth of the XPS method.20,35 Hence, the XRD analysis was used for the phase determination of Pb. Figure 9b shows that after the adsorption of Pb(II) on CSH3@Fe, new diffraction peaks at 35° and 62° attributable to Pb0 appeared,36 while no obvious diffraction of PbO was found. This suggests that Pb(II) was reduced by Fe0 to form Pb0. The high activity of Fe0 on CSH3@Fe contributes to the 4-fold increase of the capacity for Pb(II) over that of the native CSH sample. We wondered whether the differences of Pb(II) adsorption capacities resulted from the differences in size or actual content of loaded Fe0. As shown in Figure 6a, first, there was no obvious difference in the size of Fe0 because of little change of peak width of Fe0. However, analysis of the content of doped Fe0 shows that CSH3 has the highest Fe0 content (40.6%), higher than that of CSH4 (30.2%). Correspondingly, the Pb(II) adsorption capacity of CSH3@Fe increased by 21.6% as compared to CSH4@Fe. It is therefore concluded that the self33662

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Figure 11, the equilibrium capacity of Pb(II) by CSH3 almost remained unchanged after five adsorption−desorption cycles, with Qe slightly decreasing from 108 mg/g in first cycle to 96.6 mg/g in fifth cycle. The refreshed CSH3@Fe also maintains high efficiency, with Qe only slightly decreasing from 435 mg/g in first cycle to 418 in fifth cycle. This characteristic could help to support long-term application of CSH3 and CSH3@Fe in water treatment.

assembled ordered structure in CSH3@Fe can enhance Pb(II) adsorption capacity by providing more active Fe0 loading sites. In addition, two control samples were prepared and evaluated, including a mechanically mixed sample of the equal amounts (by mass) of CSH3 and Fe0, and a sample of pure Fe0. As shown in Figure 10a, the CSH3@Fe has the highest capacity and efficiency in Pb(II) removal, confirming the conclusion that the hierarchically ordered structure of CSH3 facilitates the dispersing and improves the activity of Fe0 particles. In this work, the CSH3@Fe was also used to treat various kinds of heavy metal ions and organic pollutants. As shown in Figure 10b, the CSH3@Fe exhibited excellent adsorption capacities toward Pb2+, Cu2+, Cd2+, Cr2O72−, MO, and CR, with the capacities up to 458, 547, 397, 331, 425, and 568 mg/g, respectively. Table 2 summarizes the adsorption capacities of



CONCLUSION In this work, we developed a surfactant-free and economical pathway to prepare the calcium silicate hydrate (CSH) hierarchically ordered nanostructure. Porous framework of CaO was first obtained by the decomposition of CaCO3. SiO2 nanoparticles were then incorporated into the CaO framework, followed by a reaction assisted by hydrothermal treatment to form CSH. CSH hierarchically ordered nanostructure was obtained by confining the Ca/Si ratio within a small range. The structural features of CSH were characterized by 3D hierarchical networks. Nanofibers orderly assembled to form nanosheets, and nanosheets assembled to well-defined hierarchical structure. Nonclassic oriented aggregation pathways of crystal growth were used to describe the crystal growth of nanosheets. It was proposed that CaO framework served as both template and reagent to create the oriented assembly structure. The CSH with ordered structure exhibited better performance in removing Pb(II) compared with other types of CSH. Additionally, the hierarchical structure of CSH can provide more pores and active sites as support for nanoscale Fe0. The composite of CSH@Fe displayed higher capacity and good reusability for the removal of both heavy metal ions (Pb2+, Cu2+, Cd2+, Cr2O72−) and organic pollutants, with capacities up to 350−560 mg/g. We anticipate the finding of this work to shed new light on the synthesis of complex nanomaterials with novel properties.

Table 2. Comparison of Maximum Adsorption Capacities of Various Sorbents As Reported in the Literature for Pb(II), Cu(II), Cd(II), and Cr(VI) Qm (mg/g) adsorbent

Pb(II)

Cu(VI)

Cd(II)

Cr(VI)

ref

CSH3@Fe kaolin-supported nZVI Mt-nZVI zeolite-nZVI sineguelas-supported nZVI sepiolite-supported nZVI bentonite-supported nZVI Al-bent@nZVI walnut-nZVI nZVI Au-nZVI S-nZVI

458 440 126 348 225 331

547

397

331

this work 37 38 36 39 34 40 41 42 43 44 45

256 100 180 458 182

152 188 85



different kinds of adsorbents (Fe0-based nanomaterials) for Pb2+, Cu2+, Cd2+, and Cr2O72− reported in recent years. The CSH3@Fe has a higher capacity than those of many other types of Fe0-based adsorbents. Desorption and Regeneration of CSH and CSH@Fe. Figure 11 shows the Pb(II) adsorption by using CSH3 and CSH3@Fe over five repeated cycles, respectively. As shown in

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11633. FTIR spectra of CSHs, TGA analysis of CaCO3, additional SEM images of CSH4, and N2 adsorption/ desorption isotherms and pore size distribution curves of CSHs (PDF)



AUTHOR INFORMATION

Corresponding Authors

*Fax: +86 591 22866534. E-mail: [email protected]. *Fax: +86 591 22866534. E-mail: [email protected]. Notes

The authors declare no competing financial interest.



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



Figure 11. Repeated adsorption of Pb(II) by CSH3 and CSH3@Fe. (adsorption conditions: dose 30 mg, volume of solution 50 mL, initial concentration 1000 mg/L Pb(II), contact time 90 min, stirring speed 250 r/min, temperature 25 °C).

REFERENCES

(1) Dou, J.; Zeng, H. C. Integrated Networks of Mesoporous Silica Nanowires and Their Bifunctional Catalysis-Sorption Application for Oxidative Desulfurization. ACS Catal. 2014, 4 (2), 566−576.

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DOI: 10.1021/acsami.6b11633 ACS Appl. Mater. Interfaces 2016, 8, 33656−33665

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DOI: 10.1021/acsami.6b11633 ACS Appl. Mater. Interfaces 2016, 8, 33656−33665