Template-free Synthesis of Large-Pore-Size Porous Magnesium

Research Article pubs.acs.org/journal/ascecg

Template-free Synthesis of Large-Pore-Size Porous Magnesium Silicate Hierarchical Nanostructures for High-Efficiency Removal of Heavy Metal Ions Renyao Huang,† Minjie Wu,† Tao Zhang,‡ Dianqing Li,† Pinggui Tang,† and Yongjun Feng*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, No. 15 Beisanhuan East Road, Beijing, 100029, China ‡ Beijing Center for Physical & Chemical Analysis, No. 27 Xisanhuan North Road, Beijing, 100089, China S Supporting Information *

ABSTRACT: It remains a big challenge to develop high-efficiency and low-cost adsorption materials to remove toxic heavy metal ions in water. Here, we developed a template-free synthesis method to fabricate high surface area and large pore size magnesium silicate hierarchical nanostructures in a mixed solvent of ethanol and water and carefully investigated the corresponding adsorption behavior for Pb2+, Zn2+, and Cu2+ in aqueous solution. The results reveal that the ethanol volume fraction in the solvent plays an important role to optimize the pore structure, which directly determines the adsorption capacity and the adsorption rate for heavy metal ions. When the ethanol volume fraction is beyond 50%, the obtained magnesium silicate presents a flowerlike structure with a hierarchical pore distribution: 0.5−2, 2−30, and 30−200 nm. When the ethanol volume faction is 90%, for example, the sample exhibits a maximum adsorption capacity of 436.68, 78.86, and 52.30 mg/g for Pb2+, Zn2+, and Cu2+ ions, which has a BET surface area of 650.50 m2/g and an average pore diameter of 6.89 nm, respectively. Also, the sample presents excellent repeated adsorption performance after three elutions. The obtained materials show widely promising and practical applications in water treatment in a wide pH range from 2.8 to 5.8. KEYWORDS: Template-free, Magnesium silicate, Adsorption, Heavy metal ions, Recyclability

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agents.1,11,19,25,26 Generally, yet, the template-assisted synthesis process and the removal of the templates are complicated, energy-consuming, and environmentally unfriendly. Therefore, it is of great interest to develop one-step template-free ways to prepare PMS with high surface area and suitable pore size distribution. In this work, we fabricated an ethanol-assisted solvothermal method to prepare high-surface-area and large-pore-diameter flowerlike PMS hierarchical nanostructures without any template or surfactant, carefully investigated the corresponding adsorption behavior for Pb2+, Cu2+, and Zn2+ ions, and further examined the relationship between the pore structure and the adsorption behavior of the PMS. Also, the recyclability of the adsorbent materials was examined using disodium EDTA as an eluent. The growth process and the recyclability of the PMS samples is presented in Scheme 1.

INTRODUCTION With fast economic development and increasing industrialization, large quantities of wastewater contaminated with heavy metals have been produced and caused serious health and environment problems1−4 because of their high toxicity, nonbiodegradation, and persistence.5−7 Lots of techniques have been developed to treat these wastewaters,8 for example, chemical treatment,9,10 adsorption,11,12 and membrane separation.13 Among them, the adsorption method has attracted increasing interest due to its low cost, easy operation, and environmental friendliness.1 Meanwhile, various high-performance adsorbents have been explored such as porous carbon,14,15 metal oxide,16 metal hydroxides,17,18 and silicates.19−24 Porous magnesium silicate (PMS), as a typical twodimension layered silicate compound, has shown promising adsorption capacity of heavy metal cations in wastewater, for instance, hollow/core−shell spheres,1,19,25 core−shell nanorods,11 nanotubes,19 and ordered mesoporous magnesium silicate.26 Furthermore, the corresponding adsorption capacity of PMS depends on surface area and pore size distribution,27,28 which is up to controllable synthesis method. To date, the various PMS with high surface area were mostly prepared using different hard or soft templates including directing © XXXX American Chemical Society

Received: January 14, 2017 Revised: January 26, 2017 Published: February 6, 2017 A

DOI: 10.1021/acssuschemeng.7b00140 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

amount of Pb2+ adsorbed by the sample was calculated using the following equation:

Scheme 1. Proposed Synthesis and Recycle Mechanism of PMS

qt =

(C0 − Ct )V m

(1) 2+

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where Ct (mg/L) is the concentration of Pb at time t, qt (mg/g) is the corresponding amount of Pb2+ adsorbed by sample, C0 (mg/L) is the initial concentrations of Pb2+, V (L) and m (g) are the volume of Pb2+ solution and the mass of sample, respectively. The adsorption isotherm experiments were performed in 100 mL flasks by dispersing as-prepared PMS powder (17 mg) into 50 mL of different concentration Pb2+ solution (0−265 mg/L, pH = 5.8). These flasks were shaken in a thermostatic shaker for 24 h. The other operation procedures are the same as above. The performance of sample in acid solution were studied in 100 mL flasks by dispersing as-prepared PMS powder (17 mg) into 50 mL of different concentration metal ions solution at pH = 2.8 (using dilute HNO3 solution to adjust the pH value) in the system: 0−265 mg/L for Pb2+, 0−60 mg/L for Zn2+, and 0−50 mg/L for Cu2+. These flasks were shaken in a thermostatic shaker for 24 h. The other operation procedures are the same as above The recyclability was studied in 100 mL flask by dispersing 17 mg 90-PMS powder into 50 mL Pb2+ solution with the Pb2+concentration of 105 mg/L (pH = 5.8). These flask were shaken in a thermostatic shaker for 24 h at 25 °C (160 rpm). Then, the adsorbent was separated by centrifugation (3900 rpm, 5 min), the concentration of Pb2+ remaining in the solution was immediately determined using ICP-AES. The adsorbent was eluted by 1 mM of disodium EDTA solution (10 mL) for 2 h. Finally, the 90-PMS powder was collected for the next cycle after washing with water, centrifugation, and drying at 60 °C for 24 h. The recycle experiments were repeated three times following the same procedure. All of these adsorption experiments were carried out three times, and the average values were used to prepare the graphs in this work.

EXPERIMENTAL SECTION

Materials. Sodium silicate nonahydrate (Na2SiO3·9H2O), magnesium nitrate hexahydrate (Mg(NO3)2·6H2O), absolute ethanol (C2H5OH), nitric acid (HNO3, 28 wt %), lead nitrate (Pb(NO3)2), copper nitrate (Cu(NO3)2), and zinc nitrate (Zn(NO3)2) were all of analytical grade and used without further purification. The water used in all experiments was deionized water. Preparation of PMS. A series of PMS adsorption materials were prepared in a one-step solvothermal route using magnesium nitrate hexahydrate and sodium silicate nonahydrate as the sources of Mg and Si, and the mixture of water and ethanol as the reaction solvent. Typically, 2.1 g (8.2 mmol) Mg(NO3)2·6H2O was dissolved in 110 mL of water and ethanol mixed solvent to form solution A, and 2.3 g (8.1 mmol) Na2SiO3·9H2O was done in 10 mL water to form solution B. Subsequently, both solutions were rapidly mixed under vigorous stirring to produce a white suspension and kept stirring for 5 min. Then the resulting suspension was transferred into a Teflon-lined autoclave (200 mL), kept in an oven at 170 °C for 24 h under vigorous stirring, and then cooled down to room temperature. Finally, the PMS powder was collected after washing with water until pH = 7, centrifuged and dried at 60 °C for 24 h. Five PMS samples were individually prepared in the mixed solvent with different volume fraction of ethanol: 0% (0-PMS), 25% (25-PMS), 50% (50-PMS), 75% (75-PMS), and 90% (90-PMS) under the same procedures. Characterization. Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 Advance X-ray powder diffractometer (Cu Ka radiation, λ = 0.15406 nm) in the range of 9−75°/2θ at a scan speed of 10° min−1. Morphologies were examined with a Hitachi S4700 scanning electron microscope (SEM) at 30 kV and a JEOL JEM2100F high-resolution transmission electron microscope (HRTEM) with a line resolution of 0.19 nm. Low-temperature nitrogen adsorption−desorption experiments were performed on an ASAP 2460 analyzer. The specific surface area was calculated according to the Brunauer−Emmett−Teller (BET) method based on the adsorption isotherm. The density functional theory (DFT) method was used to calculate the pore volume and the pore size distribution. Adsorption Studies. The adsorption experiments were carried out in 100 mL conical flasks by dispersing 17 mg of as-prepared PMS powder into 50 mL Pb2+ solution with the Pb2+ concentration of 105 mg/L (Pb2+ began to form precipitation at pH = 7,29,30 and the pH of our adsorption experiments is controlled at 5.8) to obtain a suspension. Then, these flasks were sealed with plastic wrap and shaken in a thermostatic shaker for 0−24 h at 25 °C). After a certain time, the adsorbent was separated by centrifugation (3900 rpm, 3 min), and the concentration of Pb2+ remaining in solution was immediately determined using a Shimadzu ICPS-75000 inductively coupled plasma atomic emission spectrometer (ICP-AES). The

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RESULTS AND DISCUSSION Structure and Morphologies of PMS. Figure 1 presents the PXRD patterns of five PMS samples prepared in the mixed

Figure 1. XRD of the porous magnesium silicate samples prepared with different ethanol volume fractions in the mixed solvent.

ethanol and water solvent with different volume fraction of ethanol. The samples show a series of typical Bragg diffraction peaks of Mg3Si2O5(OH)4 at 2θ = 12.45°, 35.90°, 59.94° for (002), (112), and (300), respectively, which well match the JCPDS card no. 22-1155 although the corresponding intensities of diffraction peaks are slightly low as reported by Zhang et al.31 Furthermore, these results suggest that the amount of ethanol in the mixed solvent has little influence on the phase and the crystallite structure of PMS samples. Figure 2 shows the SEM images of five prepared PMS samples with different volume fraction of ethanol from 0 to B


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Figure 2. SEM and HRTEM images of the porous magnesium silicate samples prepared with different volume fraction of ethanol in the mixed solvent. (inset) High magnification for clear observation.

90% in the mixed solvent. One observes that the morphology is greatly varied with increase of ethanol from disordered platelike structure for 0-PMS sample to three-dimensional flowerlike structure for 90-PMS sample. For example, when the volume fraction of ethanol is 0%, the sample exhibits a disordered platelike structure; when the fraction is increased beyond 50%, the flowerlike PMS was formed by self-assembling with an average diameter of ca. 100 nm. Furthermore, the size of the flowerlike PMS is enlarged from ca. 100 nm for 50-PMS to ca. 250 nm for 90-PMS. It is worthy to note that sodium silicate nonahydrate cannot completely dissolve in anhydrous ethanol; therefore, the ethanol volume fraction cannot reach 100%. As observed in the inset graph in Figure 2 (HRTEM), the asprepared PMS is like a bird’s nest composed by lots of interlaced nanosheets with an average thickness about 5 nm. The space among these interlaced nanosheets results in the formation of large pores. That is to say, the pore size is larger and the pore number is more when the nanosheet is longer as prepared in the higher volume fraction of ethanol. Undoubtedly, high volume fraction of ethanol will favor to obtain large pore size and high surface area, which is in good agreement with the results as determined by bulk density: 1.35 g/cm3 for 0-PMS and 0.45 g/cm3 for 90-PMS. The former value is ca. 3 times higher than the latter. Pore Structure and Properties of PMS. Figure 3 illustrates the N2 adsorption/desorption isotherm at 77 K and the pore-size distribution of five prepared samples. In Figure 3a and b, 0-, 25-, and 50-PMS samples exhibit a typical Type H2 hysteresis loop, often referred to as “ink bottle” pores,32 and 75- or 90-PMS samples show a typical Type H3 one, often observed with loosely coherent aggregates of platelike particles.32 These results are consistent with those observed by SEM and HRTEM in Figure 2. In Figure 3c, the pore size distribution shows three peaks for all of the five samples: (1) 0.5−1, (2) 1−2, and (3) 2−200 nm. Furthermore, all the samples exhibit a similar distribution below 2 nm and a big difference between 2−30 and 30−200 nm as shown in Figure 3d. With the increase of ethanol volume fraction, the pore diameter remarkably turns larger, especially when the ethanol is beyond 50%. Compared with the 90-PMS sample, interestingly, the 75-PMS sample has more pore between 2−30 nm and less between 30−200 nm. Also, Table 1 summarizes all the BET data of five as-prepared samples. The surface area is increased from 529.53 to 708.64 m2/g due to a greater number

Figure 3. (a) N2 adsorption−desorption isotherms of 0-PMS, 25PMS, and 50-PMS; (b) 75-PMS and 90-PMS samples; (c) pore-size distribution of all samples; (d) pore volume distribution in the different pore size of all the five samples.

Table 1. BET Analysis of the Samples with Different Ethanol Content sample

surface area (m2/g)

pore volume (cm3/g)

average pore diameter (nm)

0-PMS 25-PMS 50-PMS 75-PMS 90-PMS

529.53 588.82 659.91 708.64 650.50

0.308 0.333 0.501 1.114 1.121

2.32 2.26 3.04 6.29 6.89

of pores following the increase of ethanol volume fraction from 0 to 75% and, then, decreased to 650.50 m2/g corresponding to the ethanol volume of 90% due to having large pores. The results show that ethanol plays a key role in the growth process of PMS. The surface area of 708.64 m2/g, and the average pore diameter of 6.29 nm are the highest values for the PMS in the literature, suggesting that the ethanol-assisted solvothermal method is an viable way to prepare high-surface-area and largepore-size PMS without any template and surfactant. The highC


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4),38,39 and show the corresponding results in Figure 4b−d and Table 2.

surface-area and the large-pore-size favor to enhance adsorption capacity and accelerate adsorption rate of the PMS for remove of heavy metal ions.33,34 Adsorption Behavior of PMS for Heavy Metal Ions. Figure 4 demonstrates the adsorption behavior and three

Psuedo-first-order kinetics log(qe − qt ) = log(qe) −

k1 t 2.303

(2)

Pseudo-second-order kinetics t 1 t = + 2 qt qe k 2qe

(3)

Intraparticle diffusion qt = kdif t + c

(4)

where qe (mg/g) and qt (mg/g) are the adsorbed quantity for Pb2+ at equilibrium and at adsorbed time t (h), respectively; k1 (1/h), k2 (g/(mg h)), and kdif (mg/(g h1/2)) are the rate constants for the pseudo-first-order kinetic model, the pseudosecond-order kinetic model, and the intraparticle diffusion model, and C is the intercept. Through the linear relationship, the parameters such as k1, k2, kdif, C, and qe, can be determined from the slope and intercept. For all five samples, the adsorption behavior matches well the pseudo-second-order model with an R2 value much close to 1 in Table 2, and also the calculated qe values based on the pseudosecond-order model are in good agreement with the experimental data. These results suggest that the chemisorption plays a main role during the adsorption process of the PMS for Pb2+.36,38 Additionally, Figure 4d demonstrates that the behavior of all five samples follows up the intradiffusion model with three steps: the external surface adsorption or the boundary layer diffusion, the intraparticle diffusion, and the final equilibrium stage.40,41 Here one can observe the following: (1) the intercept C value of the 75-PMS is the largest one due to the highest surface area among the five samples;38 (2) both of 75- and 90PMS have much shorter intraparticle diffusion processes than the other three samples because of the related larger pore size; (3) for all of 0-, 25-, and 50-PMS, the intraparticle diffusion process is separated into two step with first diffusion of the Pb2+ cations into the mesopores and then into the micropores with a slow rate.40 Figures 5 and S1 and Table S1 further show the adsorption behavior of PMS samples for Pb2+ ions by comparison in pore size between before and after adsorption. For 25-PMS, the adsorption of Pb2+ cause the more reduction of microspore in 0.5−2 nm than that of mesopores in 2−30 nm; for 90-PMS, the large pores between 2−30 and 30−200 nm are significantly reduced resulting from the adsorption of Pb2+, other than those in 0.5−2 nm. The microspores are formed from the layered structure of PMS, and the large pores are formed from the interlaced nanosheets. For the adsorption for small species such

Figure 4. (a) Variation of adsorption capacity with time, (b) pseudofirst-order kinetics, (c) pseudo-second-order kinetics, and (d) intraparticle diffusion kinetics for Pb2+ by the different samples.

corresponding kinetic models of the five samples for Pb2+ at C0 = 105 mg L−1 in aqueous solution at 25 °C. In Figure 4a, 75and 90-PMS samples reach the adsorbed equilibrium much faster than another three samples resulting from larger pore size; 50-, 75-, and 90-PSM samples have much higher the equilibrium quantity (qe) related to another two samples due to higher surface area, as observed in Figures 2 and 3. On one hand, the 90-PMS shows the faster adsorption rate before reaching the equilibrium, the shorter equilibrium time, and the same qe related to the 75-PMS although the 90-PMS has less surface area than the 75-PMS; on the other hand, the 90-PMS exhibits much faster adsorption rate and much higher qe related to the 50-PMS although both samples have the similar surface area. These results suggests that the adsorption rate mainly depends on the pore size because the 90-PMS has larger pore size than both of 50- and 75-PMS and the equilibrium quantity does on the surface area. Furthermore, the pore size plays a more important role under our investigated conditions, indicating that the adsorption of PMS for the Pb2+ is probably determined by the intradiffusion in the pore. Here, we further examine the adsorption mechanism of PMS for Pb2+ using three typical kinetic models, the pseudo-firstorder kinetic model (eq 2),35 the pseudo-second-order kinetic model (eq 3),36,37 and the intraparticle diffusion model (eq

Table 2. Pseudo-First-Order and Pseudo-Second-Order Adsorption Kinetic Constants for Pb2+ Adsorption pseudo-first-order model

pseudo-second-order model 2

sample

qe,exp (mg/g)

qe,cal (mg/g)

k1 (1/h)

R

0-PMS 25-PMS 50-PMS 75-PMS 90-PMS

307.55 306.74 292.15 247.88 229.88

106.55 122.57 178.21 152.42 161.98

0.223 0.251 0.258 0.460 0.813

0.979 0.983 0.985 0.918 0.950 D

qe,cal (mg/g)

k2 (mg/(g h))

R2

228.71 248.87 298.80 317.13 317.23

0.797 0.699 0.400 0.706 1.036

0.999 0.997 0.993 0.995 0.996


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Table 3. Langmuir Isotherm Parameters for Pb2+ Adsorption sample

KL (L/mg)

qm (mg/g)

R2

0 -PMS 25-PMS 50-PMS 75-PMS 90-PMS

0.636 0.915 1.461 1.156 1.388

243.91 265.96 315.46 421.94 436.68

0.999 0.999 0.999 0.999 0.999

436.68 mg/g for 90-PMS, following the increase of surface area and pore size as described in Figure 3. The qm value of 436.68 mg/g for 90-PMS is the highest among the reported data for PMS in the literature, which are summarized in Table 4. These results confirm that this ethanol-assisted method is an active way to prepare enhanced performance PMS for remove of Pb2+ cations in water.

Figure 5. (a) Pore-size distribution of 25-PMS, 50-PMS, and 90-PMS; (b) pore volume distribution of 25-PMS, 50-PMS, and 90-PMS after adsorption of Pb2+.

as Pb2+ (0.1−0.2 nm), the microspores are available to adsorb the Pb2+, and the large pores enlarge the exposure of the microspores for the more adsorption. These results further suggest that the pore size and the surface area of the adsorbents play an important role for the effective remove of Pb2+ in water. Therefore, it is important to develop available synthesis methods to prepare high-surface-area and large-pore-size adsorbent and then improve the adsorption capacity. Figure 6a shows the equilibrium adsorption data of the five samples for the Pb2+ in water, and the corresponding results are

Table 4. Comparison of Various Adsorbents and the Adsorption Ability of Pb2+ category magnesium silicate

others

Figure 6. (a) Curve and (b) liner fitting of Langmuir isotherm for Pb2+ onto five samples with a pH value of 5.8; (c) Langmuir isotherm of 90PMS for Cu2+, Zn2+, and Pb2+ with a pH value of 2.8; (d) recyclability of 90-PMS using disodium EDTA as eluent.

expressed in the linear form of the Langmuir adsorption model (eq 5)42 in Figure 6b to evaluate the maximum adsorption capacity of each sample under the investigated conditions as listed in Table 3. Ce C 1 = + e qe KLqm qm

adsorbents

BET area (m2/g)

magnesium silicate hollow spheres-1 magnesium silicate hollow spheres-2 magnesium silicate hollow spheres-3 flowerlike zinc silicates α-FeOOH hollow spheres MoS4−LDH GO−chitosan cellulose/manganese oxide amino siloxane oligomer/GO polyhedral oligomeric silsesquioxanes/ Fe3O4 β-cyclodextrin stabilized Fe3S4 carbon dot modified mesoporous organosilica GO/chitosan− poly(acrylic acid) 75-PMS

521

708.64

90-PMS

650.50

pore size (nm)

ref

300

1

147

25

355

65

19

236

210

43

97

80

44

290 95 80.1

45 46 47

312.5

48

90.9

49

470

3−5

qm (mg/g)

2.8

53.7 653.59

468.46

4.56

256

50

140.25

51

138.89

52

6.29

421.94

6.89

436.68

this work this work

5.50

Also, we have extended this study to examine the adsorption capacity of the prepared 90-PMS samples for Cu2+ and Zn2+ cations, which are another two common high toxic heavy metals in the environment, at pH = 2.8. Also, the Pb2+ cations was investigated as the reference. In Figures 6c and 2S and Table 2S, the maximum adsorption capacity for Pb2+, Cu2+, and Zn2+ at pH = 2.8 is 393.70, 52.30, and 78.86 mg/g, respectively, which are comparable with those reported in the literature.12 These results show that the 90-PMS sample has high adsorption performance for Pb2+, Cu2+, and Zn2+at the pH value of 2.8. Here, the adsorption capacity for Pb2+ at pH = 2.8 is lower than that at pH = 5.8 due to the competing adsorption with H+.24 Additionally, Figure 3S demonstrates that 90-PMS

(5)

where qe (mg/g) and qm (mg/g) are the equilibrium adsorption quantity and the maximum adsorption capacity, respectively; Ce is the equilibrium concentration; KL is the equilibrium constant (L/mg). Through the linear relationship between Ce/qe and Ce, the other parameters, KL and qm, can be determined from the slope and intercept. One observes that the qm value remarkably increase about two times from 243.91 mg/g for 0-PMS to E


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has a good adsorption capacity in a wide pH range from 2.8 to 5.8. Furthermore, the recyclability of the adsorbent materials is another important parameter for practical applications for remove of heavy metals. Figure 6d displays the recyclability of the 90-PMS sample after three cycles using 1 mM disodium EDTA as an eluent. The 90-PMS sample exhibits a good adsorption ability with removal efficiency of 85% which is promising for practical applications.

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CONCLUSIONS In this work, we successfully fabricated high-surface-area and large-pore-size porous magnesium silicate with the flower-like structure as one of high-performance adsorbent materials for remove of heavy metals in water by the ethanol-assisted method as one of template-free ones. The ethanol volume fraction in the mixed solvent significantly influences the morphologies, the surface area, the pore size, and the pore size distribution, and then varies the adsorption behavior of the prepared samples for heavy metals. The high surface area of 708.64 m2/g and the large pore size of 6.89 nm have been obtained for the samples when the ethanol is 75% and 90%, respectively. The samples with high-surface-area and large-poresize show much higher adsorption performance for Pb2+, Cu2+, and Zn2+ under the investigated conditions, compared with those reported in the literature. Therefore, the porous magnesium silicate as one of high-performance and low-cost adsorption materials has widely promising applications in water treatment and in situ soil remediation for high-efficiency remove of heavy metal ions.

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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.7b00140. Pore size and BET data after adsorption for Pb2+, Langmuir isotherms for Pb2+, Zn2+, and Cu2+ at pH = 2.8, and effect of pH values on adsorption capacity of Pb2+ (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Yongjun Feng: 0000-0001-9254-6219 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China, National Basic Research Program of China (973 program, 2014CB932104), Beijing Engineering Center for Hierarchical Catalysts, and Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1205).

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REFERENCES

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Template-free Synthesis of Large-Pore-Size Porous Magnesium

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