Highly Efficient Magnetic Nitrogen-Doped Porous Carbon Prepared by

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Highly efficient magnetic nitrogen-doped porous carbon prepared by one-step carbonization strategy for Hg2+ removal from water Lijin Huang, Man He, Bei-Bei Chen, Qian Cheng, and Bin Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15106 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 5, 2017

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ACS Applied Materials & Interfaces

Highly efficient magnetic nitrogen-doped porous carbon prepared by one-step carbonization strategy for Hg2+ removal from water Lijin Huang, Man He, Bei-bei Chen, Qian Cheng, Bin Hu* Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan 430072, Hubei Province, P. R. China ABSTRACT: Hydrophilic magnetic N-doped porous carbon composites (MNPCs) with high special surface areas and rich nitrogen content was prepared via simple one-step carbonization of zinc oxide nanoparticles (ZnO NPs), 2-methylimidazole (HmIm) and Fe3O4@SiO2 magnetic nanoparticles (MNPs) mixture directly. During the carbonization process, ZnO NPs directly reacts with HmIm to yield porous ZIF-8 while the MNPs are incorporated into the frameworks to generate magnetic metal-organic frameworks (MFCs), and the MFCs acts as a self-sacrificing template to prepare MNPC. The obtained MNPCs via simple one-step carbonization strategy displays

higher

adsorption

capacity

(429

mg

g-1)

for

Hg2+

ions

than

MNPC-T700-M3-T (382 mg g-1) which was obtained by two-step synthesis strategy for comparison. It also exhibits very fast adsorption dynamics (adsorption rate constant (K2) = 2.45 g mg-1 min-1) for Hg2+ and could efficiently remove 95% Hg2+ in 2 min for 20 mg L-1 Hg2+ solution. Furthermore, the prepared MNPC exhibits good chemical stability and the adsorption capacity is still more than 95% even after 10 adsorption-elution cycles. The proposed method is easy-processing and economic, which not only provides highly efficient MNPCs for metal ions capture, but also paves

the

ways

towards

various

MFCs

with

different

ligands

through

solvent/additive-free synthesis approaches. KEYWORDS: metal-organic frameworks, magnetic porous carbon, one-step carbonization, Hg2+ removal, cost-effective 1

ACS Paragon Plus Environment


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1. INTRODUCTION Over the past decade, carbon-based materials, such as activated carbon (AC), carbon nanotubes (CNTs), graphene/graphene oxide (GN/GO) and ordered mesoporous carbon, exhibit excellent biocompatibility, chemical, mechanical and thermal stability, thus have attracted increasing attention in catalysis, gas adsorption, drug delivery, solid phase extraction and other areas1-3. AC, one of the most widely used conventional porous carbon (PC) materials in the history of mankind, suffers from poor selectivity due to its scanty adsorption sites on its inert surface; the raw CNTs are insoluble and hard to disperse in all solvents, along with the purification difficulty. For preparation of GN-based sorbents, it’s necessary to exfoliate graphite oxide before the reduction, resulting in the introduction of hazardous metal ions as well as poisonous gases4; separation or regeneration of these CNT/GN-based materials is difficult. Furthermore, in order to improve their adsorption capacity or selectivity for heavy metal ions, post-modification

such

as

chemical

oxidation

to

introduce

oxygen

or

nitrogen-containing functional groups is imperative5,6. Among all the carbon-based materials with high surface areas, including PC, CNTs, GN and their functionalized forms, PC is an ideal adsorbent candidate owing to its easy preparation, low-cost, fast mass transport, excellent chemical stability and no need for a secondary functionalization7,8. Previous researches show that heteroatom doping (e.g. N, S) within the skeleton of PCs is an effective way to tune selectivity or/and capacity toward metal ions, which promote their applications greatly, especially in heavy metal ions removal9,10. However, the relative complicated preparation process and unstable functional groups via post-synthesis treatment limit its wider applicability. Therefore, the development of functionalized PC materials via a more simplified method is of great interest11. Metal-organic frameworks (MOFs) are materials with porous structure, which are constructed by combining the metal nodes and organic ligands12. Due to their chemical flexibility and high surface areas, MOFs have been proved to be a class of ideal 2


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sacrificial templates for preparing PC materials13-16. By converting the highly ordered frameworks, uniform heteroatom functionalized PC can be obtained either with or without additional carbon precursors (i.e., direct carbonization). For indirect carbonization, other carbon sources such as ethylenediamine and furfuryl alclhol would be involved via vapor phase or wet impregnation method before carbonization in an inert atmosphere; for direct carbonization, MOF is adopted as both carbon source and template, and it involves only a single calcination step and requires no additional carbon source. Comparatively, the latter is more convenient in the preparation process, and attractive for industrial-scale application.17 Since the pioneering work by Xu et al.18, a lot of PC materials derived from direct carbonization MOFs have been developed for electrocatalysis, gas storage, supercapacitors and pollutants removal.19-21 Zeolitic imidazolate frameworks (ZIFs), as a subspecies of MOFs, are porous coordination polymers constructed by N-containing ligands (e.g. imidazole and its derivatives) and metal ions (mostly Zn and Co)22-25. Due to their high N and C content, the N-doped PC (NPC) materials obtained by calcination of ZIFs showed uniform N distribution and rich-N composition, offering an attractive alternative platform for pollutants treatment, such as dyes26 and heavy metal ions27. Importantly, because N atoms in the NPCs derived from ZIFs mainly exist in the form of pyrrole and pyridine28, which can provide two unpaired electrons for metal ions complex, thus the synthesized NPCs can serve as an excellent sorbent for heavy metal ions elimination29-32. However, it should be noted that the present NPCs preparation methods based on ZIFs as precursor synthesized in advance, are relatively expensive and time-consuming due to the complex preparation process involving wet-chemistry-based crystallization and separation33. Recently, the synthesis of Zn-based ZIF precursors have been simplified by using a facile, one-step synthetic method34. However, heat-treatment at 180 °C for 18 h to prepare ZIFs precursor is still required, which is time-consuming. In addition, the general separation technologies for these NPCs sorbents from water solution, such as high-speed centrifugation or filtration, are inconvenient, tedious, and inefficient. 3



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To meet the above challenges, combining the good adsorption properties of NPC and the rapid separation ability of magnetic material to build MNPCs is of great interest in different technological fields35. Significant efforts have been devoted for the preparation

of

MNPCs,

including

template-based

synthesis,

hydrothermal/solvothermal method, chemical vapour deposition, sol-gel process, and pyrolysis procedure36. Unfortunately, there are still many considerable challenges and issues for these traditional methods, such as the difficulty in tailoring the surface properties of composites under exact control, magnetic transition metal leaching under acidic aqueous solutions, time consuming, and multi-step troublesome preparation procedures along with high-cost (e.g. repeated impregnation with metal precursors and carbon, as well as hard templates removal)30, 37-39. Therefore, developing a cheap and convenient way to fabricate MNPCs is urgently to meet demands for heavy metal ions removal from polluted water. Herein, we describe a fast, facile, one-step synthetic process to prepare MNPCs with high special surface areas, high N-content, narrow pore sizes distribution and superparamagnetic through direct carbonization of ZnO NPs, HmIm and Fe3O4@SiO2 MNPs mixture (Scheme 1). This synthesis strategy is based on the work for ZIFs preparation under solid-state by heating the mixture of ZnO and N-containing ligands directly with water as the only by-product40-42. During the carbonizing process, ZnO NPs and HmIm react directly to yield ZIF-8 under lower temperature (~160 oC) and the MNPs incorporate into the frameworks to generate MFCs, and the MFCs act as self-template and transform into MNPC under high temperature. Furthermore, it is possible to recover the excess pure ligand by using a cold trap, thereby making the synthesis procedure completely waste-free. In addition, the magnetization parameters of the MNPCs can be precisely controlled by varying the quantity of the MNPs predecessor. The carbonizing temperature and the quantity of MNPs for MNPCs preparation were fully optimized. This simple synthesis procedure shows decisive economic, environmental and technological advantages, which is practically beneficial for industrial production. What’s more, the prepared MNPCs showed excellent 4


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chemical stability and adsorption performance for Hg2+, providing an optional synthesis strategy for preparing magnetic materials which shows high selectivity and application potential in metal ions removal from polluted water.

Scheme 1. Schematic diagram of the preparation of MNPCs 2. EXPERIMENTAL 2.1 Chemicals and materials Nitric acid (HNO3), sodium hydroxide (NaOH) and concentrated hydrochloric acid (HCl) were analytical grade and used as received. ZnO NPs (15 nm) and HmIm were obtained from Aladdin (Shanghai, China). HgCl2 was used for preparation Hg2+ stocking solution (1000 mg L-1) and all the working solutions with different concentrations were prepared by stepwise dilution of the stock solution accordingly. 2.2 Synthesis of MNPCs The Fe3O4@SiO2 MNPs (~15 nm) were obtained via the previous method43. One-step Synthesis HmIm (903.1 mg, 11 mmol), Fe3O4@SiO2 MNPs and ZnO NPs (407.0 mg, 5 mmol) were mixed, grinded to uniform, and placed in a quartz tube. The tube was purged with N2 for 30 min before it was heated to the required temperature (i.e. 600 °C, 700 °C and 800 °C) with the heating rate of 3 °C min-1. And it was kept at the temperature for 1 h under a flowing N2 before it cooled to room temperature. The pyrolyzed sample was ultrasounded in HCl (1.0 M) for 2 h and immersed at room temperature for 24 h with continuous stirring. After immersion, the acid-treated sample was washed with water until it was neutral, dried at 80 oC under vacuum for 12 h. The resultant materials were denoted as MNPC-TX-MY, where the X represented 5



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temperature of carbonization 600 °C, 700 °C and 800 °C, respectively. And the Y (=1, 2, 3, 4) represented the amount of spiked MNPs: 400 mg, 200 mg, 100 mg, and 50 mg, respectively. MNPC by two-step (denoted as MNPC-T700-M3-T): The synthesis of magnetic ZIF-8 precursor was carried out according to the reported procedure with minor modification.40 MNPs (100.0 mg), HmIm (903.1 mg, 11 mmol) and ZnO NPs (407.0 mg, 5 mmol) were mixed and sealed in an autoclave under N2 atmosphere. The mixture was heated at 180 °C for 18 h and the product was obtained as Fe3O4@SiO2/ZIF-8. MNPC-T700-M3-T was prepared by weighing the precursor (600.0 mg) in a ceramic boat and fixed in quartz tube. The tube was purged with N2 for 30 min before it was heated at 700 oC with heating rate of 3 °C min-1 and maintained for 1 h under flowing N2. The pyrolyzed sample was ultrasounded in HCl (1 M) for 2 h and immersed at room temperature for 24 h with continuous stirring. After immersion, the acid-treated sample was washed to neutral, and dried at 80 oC under vacuum for 12 h. Similarly, NPC-T700 was prepared using the same procedure of one-step strategy but without MNPs. 2.3 Characterization of materials The concentration of target metals ions in solution was determined by Intrepid XSP Radial inductively coupled plasma optical emission spectrometry (ICP-OES) (Thermo, Waltham, MA, USA). PPMS-9 vibrating sample magnetometer (VSM) (QUANTOM, USA) was used to study the magnetic properties of the materials. Transmission electron micrograph (TEM) images were collected by JEM-2010 electron microscope (Tokyo, Japan). Thermodynamic analysis (TGA) was measured in nitrogen on PE diamond TG/DTA 6300 (USA) and the heating speed was 5 oC min-1. X-ray photoelectron spectroscopy (XPS) was analysed via an ESCALAB 250 XPS with Al Kα X-ray as the excitation source. The XRD patterns were identified by Bruker D8 diffractometer (Germany) with monochromatized Cu Kα radiation (40 kV, 40 mA). ASAP 2020 apparatus (Micromeritics, USA) was used to measure the Brunauer-Emmet-Teller (BET) surface area. The Fourier transform infrared spectrometer (FT-IR) spectrums of 6


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the as-prepared composites were measured by a NEXUS 870 spectroscopy (Thermo, Madison, USA). 2.4 Adsorption studies The adsorption of Hg2+ by MNPC was performed by adding 10 mg adsorbent into 10 mL Hg2+ solution. The solution pH was adjusted with 0.1 M HNO3 or NaOH. The metal ions concentration of residual solution after shaking 60 min at room temperature was detected by ICP-OES. The influence of the exposure time on the adsorption of Hg2+ onto MNPC was investigated by adding MNPC-T700-M3 to Hg2+ (20 mg L-1) solution. Supernatant was taken out at certain time intervals and the concentration of Hg2+ was detected by ICP-OES. All experiments were performed in triplicate under the same conditions. 3 RESULTS AND DISCUSSION 3.1 Materials characterization As can be seen from the XRD patterns in Fig. 1 (a) and (b), the synthesized Fe3O4@SiO2/ZIF-8 in two-step strategy has pure phases that are consistent with those of the simulated ZIF-8, suggesting that ZnO NPs had converted into ZIF-8 completely. The result also demonstrated that inserted MNPs does not destruct the crystallization of ZIFs. XRD patterns of the mixture sample (ZnO NPs+MNPs+HmIm) which was heated up to 200 oC during the one-step synthesis process revealed that ZnO NPs have reacted with HmIm directly to yield porous ZIF-8 and MNPs have been incorporated into the frameworks during the heat processing. After carbonization, the characteristic XRD peaks of ZIF-8 all lost, indicating that the samples have transformed into magnetic amorphous carbons. And, the XRD patterns showed no significant difference between MNPCs obtained by two-step carbonization strategy (MNPC-T700-M3-T) and One-step carbonization strategy due to their similar component. When the temperature of carbonization increased from 600 oC to 800 oC, the characteristic peaks related to crystalline Fe3O4 (JCPDS file, No. 19-0629) increased, corresponding to the fact that more organic species decomposed under higher temperature. As is shown, when the amount of added MNPs decreased from 400 mg (MNPC-T700-M1) to 50 mg 7



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(MNPC-T700-M4), the characteristic peaks related to crystalline Fe3O4 was weakening gradually and disappeared finally. This result indicated that the magnetization parameters of the MNPCs can be precisely controlled just by varying the MNPs predecessor. It can be verified by the results of magnetization curves specified as follows.

Fig. 1 XRD patterns of different samples (a) and (b); and FT-IR spectra of different samples (c) and (d). Note: 200 oC represent the sample of ZnO NPs+MNPs+HmIm mixture which was heated up to 200 oC during the one-step carbonization process. To confirm the formation of MNPC, FT-IR characterizations were conducted and the results are shown in Fig. 1 (c) and (d). For MNPs, the peak at 1090 cm-1 is ascribed to O-Si-O stretching vibration. For Fe3O4@SiO2/ZIF-8 and the mixture sample (ZnO NPs+MNPs+HmIm) which was obtained by heating up to 200 oC during one-step synthesis, the peaks at 3132 cm-1 and 2927 cm-1 can be ascribed to the asymmetric stretching vibration of aromatic ring C-H in HmIm. While the absorption peaks 8


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between 900-1300 cm-1 belong to the stretching and plane bending of imidazole ring, and the peaks of C=C stretching vibration on the benzene ring appeared at 1420 cm-1 and 1383 cm-1. The peak at 421 cm-1 is attributed to Zn-N stretching vibration. The results suggest that ZnO NPs have converted into ZIF-8 successfully. Moreover, the bands corresponding to ZIF-8 all lost in the spectrum of carbonized samples, due to the decomposition of ZIF-8 at high temperature. Although the proportion of chemical composition in each MNPCs are different, the samples synthesized under different pyrolysis temperatures from 600 to 800 oC show a similar FT-IR spectrum due to their similar chemical composition. The spectral peaks of C=N bonding (~1600 cm-1) and N-H bonding (~1300 cm-1) confirmed the presence of N in composites. The BET surface area and pore volume of the as-prepared MNPCs were investigated by N2 adsorption-desorption (Fig. 2 and Table 1). Compared with bare MNPs, Fe3O4@SiO2/ZIF-8 showed an increase of BET surface area and pore volume, which can be attributed to the transformation of ZnO NPs into porous MOFs (ZIF-8).

Fig. 2 N2 adsorption-desorption isotherms of Fe3O4@SiO2, Fe3O4@SiO2/ZIF-8, MNPC-T600-M3, MNPC-T700-M3, MNPC-T800-M3 (a); MNPCs prepared with different amount of spiked MNPs (b). As is shown in Table 1, the surface areas of the obtained MNPCs via different carbonization temperature (600-800 oC) are different. The low surface area of sample MNPC-T600-M3 indicated that ZIF-8 didn’t convert into NPC completely under the relatively low carbonization temperature (600 oC), and the porous structure was seriously collapsed after removal of inorganic residuals by acid washing.44 The lower surface area of sample MNPC-T800-M3 was attributed to the fact that more carbon and 9



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nitrogen species lost at high temperatures28. This result was confirmed by the results of EDX analysis as shown below. Moreover, the narrow pore size of ~ 4 nm and high surface areas (660 m2 g-1) of the obtained MNPCs at 700 oC with the use of 100 or 50 mg MNPs were both larger than that of the NPC (~ 1 nm and 520 m2 g-1) obtained by direct carbonization of a commercially available ZIF-8 without any additional carbon sources44. Table 1 Surface area, pore sizes, pore volumes and N-contents of different materials BET Surface area Material

Pore Sizes (nm) Pore volumes

(m² g-1)

(mL g-1)

Fe3O4@SiO2

70.6

14.7

0.26

Fe3O4@SiO2/ZIF-8

1050

19.6

0.76

MNPC-T600-M3

69

3.60

0.29

MNPC-T700-M1

358

3.97

0.26

MNPC-T700-M2

418

3.95

0.53

MNPC-T700-M3

653

3.97

0.48

MNPC-T700-M4

666

3.95

0.37

MNPC-T800-M3

435

3.94

0.75

MNPC-T700-M3-T

120

3.67

0.09

NPC-T700

109

3.96

0.09

520

0.27

1.06

ZIF-8-700

44

As can be seen, the surface areas of the samples exhibited a significant decrease after carbonization, and with the decrease of the spiked amount of MNPs from 400 mg to 100 mg during the preparation process, the surface areas of MNPC increased significantly from 358 m2 g-1 to 653 m2 g-1, respectively. And all the carbonized samples showed type IV isotherm curves, indicating the pores were mesopores except MNPC-T600-M3. However, further decreasing the spiked amount of MNPs showed little effect on the surface areas increase of MNPC-T700-M4, corresponding to the fact that some of NPC have not combined with MNPs and could not be separated effectively under external magnetic field. What’s interesting, the MNPC prepared by one-step synthesis strategy showed much higher surface areas than that of MNPC-T700-M3-T. 10


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The reason may be that during the one-step carbonizing process, some of ZnO NPs didn’t convert into ZIFs and the unreacted ZnO NPs would act as a second template. EDX spectra were used to determine the relative contents of elements in the MNPCs (Table 2). The results indicated that the resulting MNPCs showed a decreased N content from 17.5 to 4.79 wt% with increasing carbonization temperature from 600 to 800 oC Table 2 The relative N and Fe contents in as-prepared MNPCs Sample

N content (wt%)

Fe content (wt%)

MNPC-T600-M3

17.5

4.50

MNPC-T700-M1

4.04

29.6

MNPC-T700-M2

8.13

15.8

MNPC-T700-M3

12.8

4.46

MNPC-T700-M4

16.2

4.50

MNPC-T800-M3

4.79

12.2

due to the instability of nitrogen at high temperature44. These results demonstrated that more organic composites lost at high temperatures and the final components are tunable by varying the reaction temperature.

Fig. 3 High resolution spectra of C1s (a) and N 1s (b) for MNPC-T700-M3. The adsorption performance of MNPC not only depends on the N content, the type of doped nitrogen species also plays a very important role. The surface information of the MNPC-T700-M3 was analysed by X-ray photoelectron spectroscopy (XPS). The peaks at 284.8, 286.2 and 287.3 eV in the C1s XPS spectrum (Fig. 3(a)) corresponded to C=C, 11



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C-N and C-O, respectively52-53; while the status of N in the MNPCs (Fig. 3(b)) was summarized into three different types: pyridinic-N (398.4 ±0.2 eV), pyrrolic-N (400.0±0.2 eV), and graphitic-N (401.0 ±0.2 eV), which is similar to that of NPC obtained through direct carbonization of ZIFs46, 47. The presence of pyridinic-N and pyrrolic-N was considered to be responsible for selective adsorption performance towards Hg2+ in the previous study31, 32, 49. VSM was employed to characterize the magnetic properties of MNPCs and the corresponding hysteresis curves are shown in Fig. S1 (a). The saturation magnetization of MNPC-T700-M1, MNPC-T700-M2, MNPC-T700-M3 and MNPC-T700-M4 are 10.7, 8.5, 6.8 and 3.0 emu g-1, respectively. This not only clearly reveals that all the prepared MNPCs are superparamagnetic, but also confirms that the magnetization parameters of the MNPCs can be conveniently adjusted by varying the quantity of the added MNPs. Moreover, due to their high N-content, good hydrophilic surface and superparamagnetic, all the MNPCs dispersed well in aqueous solution and can be recovered easily through a magnet, and the image Fig. S1 (b) clearly showed the separation process of MNPC-T700-M3. Considering the easy preparation, high surface areas and proper magnetic properties, MNPC-T700-M3 was used for the subsequent adsorption experiments and characterization.

Fig. 4 TEM images of MNPs (a), MNPC-T700-M3 (b), SEM images of Fe3O4@SiO2/ZIF-8 (c), MNPC-T700-M3 (d) and elemental mapping images of MNPC-T700-M3 (e). The SEM and TEM images for MNPC-T700-M3 are shown in Fig. 4. The TEM 12


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image of MNPs (Fig. 4(a)) revealed that the size of MNPs were about 15 nm. Fig. 4(b) shows the morphology of MNPC-T700-M3, suggesting the presence of aggregated MNPs in the NPC. Nevertheless, it could be distinguished that the MNPs was surrounded by a layer of NPC. The SEM images reveal that Fe3O4@SiO2/ZIF-8 has ordered crystalline shapes with the MNPs either dispersed on the external surface, or incorporated into ZIF crystals (Fig. 4(c)). Interestingly, compared with MNPC-T700-M3-T and NPC-T700 (Fig. S2), SEM image of MNPC-T700-M3 (Fig. 4(d)) shows a more uniform rough continuous surface throughout the entire products. The element mapping results (Fig. 4(e)) show the dark-field TEM image of MNPC-T700-M3 and its corresponding elemental mapping of O, Si, Fe, C and N. The uniform N element signals indicating the N element were homogeneously distributed throughout the structure of MNPC-T700-M3. The TG curves of prepared MNPC-T700-M3 are presented in Fig. S3, along with Fe3O4@SiO2 MNPs and Fe3O4@SiO2/ZIF-8. For bare MNPs, a little weight loss is observed after 800 °C, demonstrating that the MNPs has a good stability. While Fe3O4@SiO2/ZIF-8 is stable in nitrogen up to 500 oC, and the weight loss under 550 °C, which is in accordance to that reported in literature22, could be attributed to the decomposition of organic species and conversion to ZnO/carbon composite. On the other hand, MNPC-T700-M3 displays obvious different thermal behaviours compared with their parental Fe3O4@SiO2/ZIF-8. An obvious weight loss was observed below 100 oC for MNPC-T700-M3, which could be ascribed to the removal of adsorbed water from the channels of porous composites. The weight loss at above 650 oC could be attributed to the burning of formed carbon species. The results reveal that MNPC-T700-M3 exhibits excellent thermal stability. 3.2 Adsorption of Hg2+ by MNPCs 3.2.1 Selection of sorbents The adsorption capacity for Hg2+ obtained by MNPC-T600-M3, MNPC-T700-M3, MNPC-T800-M3 and MNPC-T700-M3-T is shown in Fig. 5. As can be seen, 13



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MNPC-T700-M4 showed the highest adsorption capacity (476 mg g-1) due to its high surface areas and proper N content, which could provide more effective binding sites for Hg2+ capture. However, the poorer magnetism of MNPC-T700-M4 (3.0 emu g-1) made the phase separation time much longer than MNPC-T700-M3. Thereby, MNPC-T700-M3 with proper magnetism (6.8 emu g-1) and high adsorption capacity (429 mg g-1) was used for the subsequent adsorption investigation.

Fig. 5 The adsorption capacity of Hg2+ obtained by different MNPCs in 500 mg L-1 Hg2+ solution at pH 6. 3.2.2 Effect of pH on the adsorption The adsorption behaviour of Hg2+ was strongly influenced by the pH of the solution, so it’s necessary to consider the solution pH carefully. Fig. 6(a) displayed the adsorption behaviour of Hg2+ on MNPC-T700-M3 at different pH. It was found that the adsorption ability of bare MNPs for Hg2+ is poor and improved greatly after NPC functionalization. The removal efficiency by MNPC-T700-M3 was increased when the solution pH varied from 2 to 4, and then kept constant at pH>4. This can be attributed to the fact that the N species on MNPC tend to be protonated and compete with metal ions, resulting in a low removal efficiency. The concentration of H+ and the degree of protonation were declined as the pH increased. In addition, the interaction between the mercury species and the N functional group could be enhanced29,

49

. Taking the

hydrolysis of Hg2+ under high pH into consideration, the adsorption investigations were carried out at pH 6.

14


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Fig. 6 Effect of pH on the removal efficiency of Hg2+ (10 mg L-1) with MNPC-T700-M3 and Fe3O4@SiO2 (a); Selectivity of MNPC-T700-M3 for Hg2+ (each ion of 10 mg L-1 at pH 6) (b). 3.2.3 Selectivity of MNPC-T700-M3 for Hg2+ For the practical use, selectivity of an adsorbent is an important aspect. By using some common heavy metal ions in environmental water samples as the coexisting ions, including Cu2+, Co2+, Cd2+, Cr3+, Mn2+, Ni2+ and Pb2+, we investigated the effect of coexisting ions on the adsorption of Hg2+ by MNPC-T700-M3 from aqueous solution (Fig. 6(b)). As can be seen, the elimination of Hg2+ at pH 6 is > 99% even though the concentration of coexisting ions up to 10 mg L-1. High selectivity for Hg2+ indicates that the affinity of MNPC-T700-M3 toward Hg2+ is much stronger rather than other ions and the selectivity can be attributed to the different interaction abilities between metal ions and the sorbents50. According to the theory of Evert Nieboe51, metal ions can be separated into Class B (e.g. Hg2+, Pt2+), borderline (e.g. Pb2+, Cr3+, Cu2+, Cd2+, Co2+, Mn2+, Ni2+), and Class A (e.g. Na+, K+, Mg2+, Al3+). Among them, metal ions in Class B and A has the highest and lowest covalent index, respectively, and metal ions in Class B prefer to form stable covalent complexes32, 51. The covalent index of ions follows the order of Hg2+(4.1) > Pb2+(3.3) > Cu2+(3.0)> Cd2+(2.9) > Co2+(2.7) > Ni2+(2.5)> Cr3+(2.4)> Mn2+(2.0).51 MNPC-T700-M3 contains pyridinic-N and pyrrolic-N which has good affinity to complex with metal ions.32,49 Compared with ions that belong to borderline ions, Hg2+ belonging to Class B, would be adsorbed by MNPC-T700-M3 preferentially due to the highest covalent index. 15



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3.3 Adsorption kinetics The kinetic of Hg2+ adsorption by MNPC-T700-M3 was investigated to evaluate the mechanism of adsorption. The result (Fig. 7(a)) showed that 96% Hg2+ can be removed in 2 min for 20 mg L-1 Hg2+ solution, demonstrating that the MNPC-T700-M3 possessed fast adsorption dynamics for Hg2+ capture from aqueous solution due to its porous structure. The kinetic data can be fitted with pseudo-second-order kinetic Eq. (1), t t 1 = + Qt Qe K 2 Qe2

Eq. (1)

Fig. 7 Effect of contact time on the adsorption of Hg2+ over MNPC-T700-M3 (a), along with the pseudo-second-order kinetic plot for adsorption of Hg2+ (20 mg L-1) (b); adsorption isotherms curve of Hg2+ (c) along with the linear regression by fitting the equilibrium adsorption data with Langmuir adsorption model for Hg2+ (d). where K2 is the rate constant for pseudo-second-order model (g mg-1 min-1), and Qt and Qe are the adsorbed amount at time t and the adsorption capacity at equilibrium 16


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(mg g-1), respectively. As shown in Fig. 7(b), pseudo-second order model matches well with the experimental kinetics data for MNPC-T700-M3 with a high correlation coefficient (R2=0.999). And K2 was calculated to be 2.45 g mg-1 min-1, much higher than other carbon-based sorbents for Hg2+ capture under similar situations, e.g. MWCNTs-SH (0.027 g mg-1 min-1)52, CNT-S (0.003 g mg-1 min-1)53, polypyrrole (PPy)-RGO (3.1×10-3 g mg-1 min-1)49, polyaniline (PANI)/RGO (1.2×10-4 g mg-1 min-1)31, amino functionalized magnetic graphenes composite (2.7×10-2 g mg-1 min-1)54, and Fe3O4/M-COFs (2.3×10-3 g mg-1 min-1)55. Such a fast Hg2+ removal rate may be attributed to the high surface areas and mesoporous structure of the prepared MNPCs. The homogeneous distribution of nitrogen can remarkably improve the hydrophilicity of the MNPC and provide plenty of pyridinic-N and pyrrolic-N binding sites for Hg2+ capture. 3.4 Adsorption isotherms T Adsorption isotherm of Hg2+ on the as-prepared MNPC-T700-M3 was investigated to evaluate the adsorption capacity of MNPC-T700-M3 for Hg2+ (Fig. 7(c)). Varying the original concentration of Hg2+ from 50 to 500 mg L-1, the amount of Hg2+ adsorbed by MNPC-T700-M3 increased sharply and then reached saturated adsorption. The data under equilibrium adsorption was analysed under the help of linear equation (Eq. (2)) of Langmuir adsorption model: Ce 1 Ce = + qe qmKL qm −1

Eq. (2)

−1

where Ce (mg L ) and qe (mg g ) represent Hg2+ concentration and adsorption amount of Hg2+ at equilibrium, respectively. qm (mg g−1) is the adsorption capacity, and KL (L mg−1) represents the Langmuir constant. By fitting the experimental data, it can be concluded that the Langmuir’s adsorption model fits the data well (correlation coefficient R2>0.998) (Fig. 7(d)). And the adsorption capacity of Hg2+ on MNPC-T700-M3 was determined to be 429 mg g-1, lower than some non-magnetic carbon-based composites, such as sulfur-functionalized mesoporous carbon (435-732 mg g-1)9, PPy-RGO (980 mg g-1)49, carbonaceous material (796 mg g-1)67 and 17



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sulfur-impregnated AC (800 mg g-1)68. However, MNPC-T700-M3 shows much faster Table 3 Comparison of adsorption capacities of Hg2+ obtained by different adsorbents

Adsorbents

Capacity (mg g-1)

Reference

AC cloth Amine-modified AC SWCNT-SH AMT-OCMK-3 Sulfur-Functionalized Mesoporous Carbon S-functionalized AC fibers Sulfur/Reduced GO Nanohybrid PPy-RGO PANI/RGO Thiol-functionalized magnetite/graphene oxide (MGO) SH-MAC Thiol-functionalized CNTs/Fe3O4 Graphene oxide MNPs Amino functionalized magnetic graphenes composite Fe3O4/M-COFs Bi-I-functionalized Fe3O4@SiO2@HKUST-1 MFC-S MNPC-T600-M3 MNPC-T700-M3 MNPC-T700-M4 MNPC-T800-M3 MNPC-T700-M3-T

70 119 131 450.5 435-732

56 57 58 59 9

710 908 980 1000 289.9

60 61 49 31 62

37.6 65.5 16.6 168

63 64 65 54

97.7 264

65 43

282 297 429 476 182 382

66

This work

adsorption kinetics and can be recovered more easily during the adsorption process due to the introduction of magnetic core. As well as we know, this adsorption capacity is highest among various magnetic carbon-based adsorbents for Hg2+ capture so far (Table 3). Compared with MFCs materials that previously reported43,66, the proposed method for MNPC preparation via simple one-step carbonization strategy shows more cost efficient and time-saving as well as readily available precursors; and the as-prepared materials showed better stability, faster adsorption kinetics as well as higher adsorption capacity for Hg2+. In addition, the potential application of MNPC-T700-M3 for the removal of Hg2+ from practical water samples has been 18


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evaluated by determining the adsorption efficiency of Hg2+ in spiked tap water, Yantze River water and domestic sewage (each spiking at 50, 300 and 500 mg L−1). The results for different water samples have been listed in Table S1. As can be seen, the coexisting cations and anions in real water samples has little effect on Hg2+ removal, demonstrating the application potential of the as-prepared MNPC in real water treatment. 3.5 Adsorption mechanism To further make sense of the adsorption mechanism of Hg2+ on MNPC-T700-M3, the XPS survey spectra after adsorption (denoted as MNPC-T700-M3-Hg2+) and the high resolution XPS scans of N 1s and Hg 1s were conducted (Fig. S4). It is known that the binding energy of 4f7/2 of mercuric chloride appeared at 101.4 and 105.3 eV respectively69. Fig. S4(a) shows the Hg 4f spectrum of MNPC-T700-M3-Hg2+. Two photoelectron peaks at 103.5 eV and 107.3 eV can be assigned to Hg 4f7/2 and Hg 4f5/2, respectively. The shift of 2.1 eV towards higher binding energy of Hg2+ in MNPC-T700-M3-Hg2+ indicates the formation of a covalent bond between an empty orbital of Hg2+ and π electrons of the N species on MNPC-T700-M3.70 However, no significant changes in the C1s spectrum were observed on MNPC-T700-M3-Hg2+ (Fig. S4 (b)), indicating no interaction between Hg2+ ions and carbon atoms. The N1s peaks of MNPC-T700-M3 (Fig. S4 (c)) exhibits ~0.7 eV shift towards higher binding energy upon adsorption of Hg2+, indicating the nitrogen mainly chelate with Hg2+.32 3.6 Reusability

Fig. 8 Hg2+ removal by recycled MNPC-T700-M3 (500 mg L-1 Hg2+ solution at pH6). 19



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For the practical use of sorbents, recycling and regeneration are of great significance. 1.0 M HCl was effective for the desorption of metal ions. MNPC-T700-M3 can be regenerated and reused and the initial adsorption capacity can be still remained even after 10 adsorption/elution cycles (Fig.8). The stability of MNPC-T700-M3 after 10 adsorption/elution cycles for Hg2+ was confirmed by the PXRD patterns and N2 adsorption-desorption isotherms characterization. As shown in Fig. S5 and Fig. S6, the good preservation of its PXRD patterns and high surface areas (647 m2 g-1) after 10 adsorption/elution cycles reveal that MNPC-T700-M3 is chemically stable. 4. CONCLUSIONS Herein, MNPC with uniform heteroatom N doping was successfully synthesized by a fast and facial one-step carbonization strategy. Our approach reduces the operation process, reagent dosage and the cost of preparation, offering a robust strategy for MNPCs preparation with different ligands and metals sources. Notably, the prepared MNPCs provide high selective affinity, removal efficiency and adsorption capacity (429 mg g-1) for Hg2+ capture from aqueous solution. Besides, MNPCs possess high special surface areas, narrow pore size distributions, fast adsorption kinetic, good chemical stability and excellent reusability. All these endow the proposed method with good potential for the synthesis of heteroatom functionalized MPCs for contaminated environment remediation as well as other application.

ASSOCIATED CONTENT Supporting Information VSM magnetization curves of MNPCs; SEM images of MNPC-T700-M3-T; TG curve of different samples, High resolution Hg 4f data, C1s data and high resolution N 1s data of MNPC-T700-M3-Hg2+, the absorption efficiency for different water samples, XRD patterns and FT-IR of reused MNPC-T700-M3. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author 20


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*

Tel: 86-27-68752701; Fax: 86-27-68754067;

*

E-mail address: [email protected]

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos.:20975076). References (1) J. Lee, J. Kim, T. Hyeon, Recent Progress in the Synthesis of Porous Carbon Materials. Adv. Mater. 2006, 18, 2073-2094. (2) C. Liang, Z. Li, S. Dai, Mesoporous Carbon Materials: Synthesis and Modification. Angew. Chem. Int. Ed. 2008, 47, 3696-3717. (3) V. Georgakilas, M. Otyepka, A.B. Bourlinos, V. Chandra, N. Kim, K.C. Kemp, P. Hobza, R. Zboril and K. S. Kim, Functionalization of Graphene: Covalent and Non-Covalent Approaches, Derivatives and Applications. Chem. Rev. 2012, 112, 6156-6214. (4) L. Peng, Z. Xu, Z. Liu, Y. Wei, H. Sun, Z. Li, X. L. Zhao and C. Gao, An Iron-Based Green Approach to 1-H Production of Single-Layer Graphene Oxide. Nat. Commun. 2015, 6, 5716. (5) K. Balasubramanian, M. Burghard, Chemically Functionalized Carbon Nanotubes. Small 2005, 1, 180-192. (6) F. Perreault, A. Fonseca de Faria, M. Elimelech, Environmental Applications of Graphene-Based Nanomaterials. Chem. Soc. Rev. 2015, 44, 5861-5896. (7) D. Tang, S. Hu, F. Dai, R. Yi, M.L. Gordin, S. Chen, J. Song and D. Wang, Self Templated Synthesis of Mesoporous Carbon from Carbon Tetrachloride Precursor for Supercapacitor Electrodes. ACS Appl. Mater. Interfaces 2016, 8, 6779-6783. (8) Z. Ma, H. Zhang, Z. Yang, G. Ji, B. Yu, X. Liu, and Z. M. Liu, Mesoporous Nitrogen-Doped Carbons with High Nitrogen Contents and Ultrahigh Surface Areas: Synthesis and Applications In Catalysis. Green Chem. 2016, 18, 1976-1982. 21



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Highly Efficient Magnetic Nitrogen-Doped Porous Carbon Prepared by

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