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One-pot Synthesis of Boron Carbon Nitride Nanosheets for Facial and Efficiently Heavy Metal Ions Removal Dong Peng, Wei Jiang, Fang-Fang Li, Li Zhang, Ru-Ping Liang, and Jian-Ding Qiu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01951 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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ACS Sustainable Chemistry & Engineering

One-pot

Synthesis

of

Boron

Carbon

Nitride

Nanosheets for Facial and Efficiently Heavy Metal Ions Removal Dong Peng,a Wei Jiang,a Fang-Fang Li,a Li Zhang,a Ru-Ping Liang,a Jian-Ding Qiua,b* a

College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031,

Jiangxi, China b

Environmental Protection Materials and Equipment Engineering Technology Center

of Jiangxi, Department of Materials and Chemical Engineering, Pingxiang University, 211 Pingan North Road, Pingxiang 337055, Jiangxi, China

*Corresponding authors. Tel/Fax: +86-791-83969518. E-mail: [email protected].

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Abstract Mercury ions (Hg2+) and lead ions (Pb2+) are two of the most toxic heavy metal ions, thus the efficient and effective removal of them is of great significant. Herein, a one-pot synthesis protocol has been proposed to synthesize boron carbon nitride nanosheets (BCN NSs) via the pyrolysis of the mixture of melamine and boric acid. To optimize the synthesis conditions, a series of products were made by varying the molar ratio and heating temperature. Finally, the BCN NSs obtained from the mixture of melamine and boric acid with the molar ratio of 3:3 at 550 °C was selected for the best Hg2+ and Pb2+ removal efficiency. Due to the successfully introduction of abundant function groups, enhanced specific surface area and hydrophilicity and electrostatic attraction ability, BCN NSs exhibit excellent adsorption performance toward Hg2+ and Pb2+ with the maximum adsorption capacity of 625.0 and 210.97 mg g-1, respectively. The sorption isotherms fit well with the Langmuir adsorption model which suggesting the monolayer adsorption behavior, and the pseudo-second-order kinetic adsorption processes indicate the chemisorption mechanism. Besides, BCN NSs show excellent chemical stability and the adsorption capacities remain more than 95% even after 6 adsorption/desorption cycles. By virtue of the cost-effective and facile production methodology, combining with the excellent absorption performance, the BCN NSs show great potential for practical application in the field of wastewater purification. KEYWORDS: boron carbon nitride nanosheets, one-pot synthesis, adsorbent, heavy metal ions, water cleaning


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INTRODUCTION As we know, water is the source of life, and all known forms of life can’t live without it. However, with the population explosion and fast development of industries, the discharge of wastewater containing toxic heavy metal ions dramatically exacerbate the water crisis as well as threaten human health. Lead ions (Pb2+) and mercury ions (Hg2+) are two of the most toxic ions, which have the extremely potential to cause harm to human bodies even at their low concentrations.1-2 Hg2+ exhibits high affinity to the thiol groups of enzymes and proteins, causing adverse effects on human organs such as heart, kidney, nervous system, brain, and liver.3-5 Pb2+ can mimic the biological metal ions like Zn2+, Ca2+ and Fe3+, leading to various neurotoxin effects, muscle paralysis, and anemia.6-8 Thus, the efficient and effective uptake of these heavy metal ions from the contaminated water has evoked widespread concerns relating to the human existence and long-term development. Among the most reported methods for water purifying, adsorption is one of the most efficient and cost-effective processes.9-11 However, most of the reported absorbents are usually suffering from the problems of the complicated preparation processes, high energy consumption, high cost, and secondary pollution. The inorganic nonmetallic nano-materials, which are mainly composed of carbon, nitrogen, or boron elements, have been widely applied for the adsorption of hazardous pollutants with the advantages of ready availability of raw materials, low cost, high adsorptive capability, and less secondary 3 ACS Paragon Plus Environment


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pollution.12-18 Among them, graphene and its oxidized derivative graphene oxide (GO) are promising adsorbent candidates for wastewater treatment owing to the large specific surface area, extraordinary mechanical strength, and flat structure.16,19-20 Owning to the π-π stacking and/or electrostatic interactions, graphene and GO show excellent adsorption capability to soluble aromatic or cationic pollutants.21-23 Surface functionalization of graphene can also greatly improve its adsorption efficiency and selectivity.23-27 However, the fabrication of graphene and GO usually requires special equipment or strong oxidants.28-30 Boron nitride (BN) nanosheets, a kind of graphene-like nanomaterials composed of alternating B and N atoms, exhibits prominent sorption performances toward various organic pollutants including dyes, solvents, aromatic molecules, and oils.31-36 However, the BN nanosheets show poor adsorption performance toward the toxic metal ions due to the hydrophobicity and less functional groups in their surface. Besides, most of the BN nanosheets are obtained from heating at high temperature or exfoliation with organic solvents.31,32,37,38 The graphitic-carbon nitride (CN) analogues have also been widely studied for the potential application in sustainable energy and biosensors.39-41 But the utilization of CN nanomatrials in water cleaning is rarely studied for the poor interfacial properties including the insolubility in water, small specific surface area and few active groups around the edges.41-42 The two-dimensional covalent organic frameworks (COFs) and some other porous organic polymers with abundant coordination sites and porous structures 4 ACS Paragon Plus Environment

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have also evoked peoples widely interests owing to their superior adsorption capacities.10,43-46 However, the requirement of mass organic solvents and well-designed precursors can also cause some environmental problems. Therefore, the fabrication of a newly kind of adsorbent which can combine with the advantages of graphene, BN nanosheets and graphitic-CN analogues is of great significant. Melamine is a kind of important industrial raw materials, and has been widely applied in paints, plastics, clothing and some other industries. China is the world’s largest producer of melamine with an annual output of several millions of tons, which are serious surplus. Developing the melamine-based materials

for

environmental

applications

is

of

great

economic

and

environmental benefit. Herein, a facile and effective protocol has been proposed to synthesize porous boron carbon nitride nanosheets (BCN NSs). The BCN NSs were prepared by direct pyrolysis of the mixture of boric acid and melamine at a relatively low temperature. This is a green, facile, scalable, and inexpensive fabrication scheme for further practical application. Due to the successfully introduction of abundant function groups, enhanced specific surface area and hydrophilicity and electrostatic attraction ability, BCN NSs exhibit excellent adsorption performance to Hg2+ and Pb2+ with the maximum adsorption capacities of 625.0 and 210.97 mg g-1, respectively. The sorption isotherms fit greatly with the Langmuir adsorption model which suggesting the monolayer adsorption behavior, and the pseudo-second-order kinetic adsorption 5 ACS Paragon Plus Environment


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processes indicate the chemisorption mechanism. Besides, BCN NSs show excellent chemical stability and the adsorption capacities remain more than 95% even after 6 adsorption/desorption cycles. By virtue of the cost-effective and facile production methodology, combining with the excellent absorption performance, the BCN NSs show great potential for practical application in the field of practical wastewater purification.

EXPERIMENTAL SECTION Preparation of boron carbon nitride nanosheets. The BCN NSs were prepared by direct pyrolysis of the mixture of boric acid and melamine in a semi-closed system similar to that for the fabrication of bulk g-C3N4.39,40 In a typical approach, 3 g of melamine and 1.47 g of H3BO3 (with a molar ratio of 3:3) were dispersed in H2O and mixed to form a uniform white mixture. The well mixture was dried at 60 °C and then grinded into powders with an agate mortar. The obtained mixture was placed in a crucible with a cover and then heated at 550 °C in air for 4 h with a heating rate of 5 °C min-1. The obtained products were further rinsed with water to neutral and died at 60 °C. Finally, the pale yellow powder was collected for further application. Similarly, to optimized the molar ratio of melamine to H3BO3 and the thermolysis temperature, the mixtures containing the molar ratio vary from 3:0 to 3:5 were calcinated at various temperatures (500-650 °C) for 4 h with a heating rate of 5 °C min-1. These products were denoted as BCN-TX-Y, where X represented the calcination temperatures, and Y (=0, 1, 2, 3, 4, 5) represented the 6 ACS Paragon Plus Environment

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molar ratio of melamine to H3BO3 ranging from 3:0 to 3:5. Herein, BCN–T550-3 represents the product obtained from the mixture of melamine and boric acid with the molar ratio of 3:3 at 550 °C, which is BCN NSs unless otherwise specified.

Characterization. The size and morphology were performed by a Quanta 200 scanning electron microscopy (SEM, USA.) and a JEOL JEM-2010 transmission electron microscope (TEM, Japan). X-ray photoelectron spectroscopy (XPS) measurements were conducted on a VG Multilab 2000X instrument (Thermal Electron, USA). The X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance instrument with Cu Kα radiation. Fourier transform infrared (FTIR) spectra were recorded using a Bruker ALPHA FTIR spectrometer in attenuated total reflectance (ATR) mode in a range of 4000−600 cm−1. The exact amount of C, N, H, and O elements were obtained by the Elemental Analyzer (Vario EL III, Elementar, Germany), and the amount of B element was measured by the inductively coupled plasma-optical emission spectrometry (ICP-OES, Optima 5300DV, PerkinElmer, USA). The concentrations of mental ions were obtained on the inductively coupled plasma-mass spectrometry (ICP-MS, Thermo Fisher Scientific, USA).

Hg2+ and Pb2+ adsorption experiments. Hg2+ and Pb2+ adsorption experiments on BCN NSs were carried out as follows. For the kinetic studies, 10 mg BCN NSs were immersed in the glass bottle containing 25 mL 307.8 mg L-1 Hg2+ solution (for Pb2+ adsorption, the initial concentration was



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108.0 mg L-1). After being shaken in the thermostatic bath for different time intervals, the BCN NSs were immediately separated out by filtering, and the concentrations of the remaining Hg2+ (or Pb2+) were further determined. For the adsorption isotherm studies, 10 mg BCN NSs were dispersed in 25 mL Hg2+ (or Pb2+) solutions with different initial concentrations. To select the best adsorbent, 10 mg of different products were dispersed in 25 mL 100.0 mg L-1 Hg2+ solution under mild oscillation. 10 mg BCN NSs were dispersed in certain concentrations (50 or 100 mg L-1) of different metal ions solution (25 mL) under mild oscillation to study the selectivity of BCN NSs toward different metal ions. To investigate the effect of pH on the adsorption, solutions of Hg2+/Pb2+ with different pH value ranging from 3-8 were prepared. The pH was adjusted with NaOH or HNO3, and the initial concentrations of Hg2+ and Pb2+ were 307.8 and 108.0 mg L-1, respectively. For the regeneration experiments, the spent adsorbents were desorbed with 1.0 M HCl solution for 60 min, and then washed with deionized water for several times. After that, the adsorbent was separated for the next adsorption/desorption cycle. Unless otherwise specified, the shaking time was set as 24 hours to ensure adsorption equilibrium, the initial pH was adjusted to 7.0, and the adsorption temperature was 25 °C. The adsorption capacities of BCN NSs at time t (qt) and at equilibrium (qe) were calculated according to the following equations: ‫ݍ‬௧ = ‫ݍ‬௘ =

ሺ஼బ ି஼೟ ሻ௏ ௠ ሺ஼బ ି஼೐ ሻ௏ ௠

(1) (2)


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where C0 and Ce represent the initial and equilibrium concentrations of the metal ions (mg L-1), respectively, Ct represents the concentration at time t (mg L-1), V is the volume of the solutions (L), and m is the mass of the adsorbent (g). The Langmuir adsorption (eqn (3)) and Freundlich adsorption (eqn (4)) models were utilized to fit the adsorption isotherms: ஼೐ ௤೐

=

஼೐ ௤೘

+௞

ଵ ಽ ௤೘

(3)

ଵ

ln ‫ݍ‬௘ = ln ݇ி + ln ‫ܥ‬௘ ௡

(4)

where KL (L g-1) is the Langmuir constant, and qm (mg g-1) represents the maximum adsorption capacity of the adsorbent. KF is the Freundlich isothermal constant (mg g-1) (L mg-1)1/n, and n is the factor relating to the adsorption intensity. For the adsorption kinetic study, the pseudo second-order model was utilized (eqn (5)): ଵ ௤೐ ି௤೟

=

ଵ ௤೐

+ ݇‫ݐ‬

(5)

herein k is the rate constant of the pseudo second-order model of adsorption (g mg-1·min-1).

RESULTS AND DISCUSSION Characterizations of BCN NSs. Herein, the BCN NSs were synthesis by using melamine as carbon and nitrogen source and boric acid as the boron source with a molar ratio of 3:3. After pyrolysis at 550 °C for 4 h, the product with a pale yellow color has been obtained (as shown on Figure 1a), which is evidently different from the dark yellow color of bulk g-C3N4.39 The heating temperature for the preparation of BCN NSs is much lower than that for 9 ACS Paragon Plus Environment


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BN nanosheets fabrication.31-32 Figure S1 shows the scanning electron microscope (SEM) images of the BCN NSs with different magnifications. The obtained samples constitute of thin nanosheets, which finally form porous flower-like structures. As shown in the transmission electron microscope (TEM) image (Figure 1b) and AFM image (Figure 1c), the BCN NSs exhibit irregular shapes with the heights range from 2.2 to 3.4 nm, verifying its thin-layered characters. Different from the typical slate-like texture of bulk g-C3N4,47 the layered structure of BCN NSs is much similar to that of the exfoliated graphitic carbon nitride nanosheets.48 Compared with bulk g-C3N4, the X-ray diffraction (XRD) pattern of BCN NSs shows the characteristic peak at around 26.8°, which was intermediate between the characteristic XRD peak of g-C3N4 and h-BN (Figure 1d), suggesting the formation of B-N bonds.49-50 Besides, the significant decrease of the (002) peak intensity might be ascribed to the thin nanosheets structure of the sheets, irregular height distributions and the existence of defects in the surface. As shown in Figure 1e, except the broad adsorption band at around 3224 cm-1 originating from the N–H stretches,51 appearances of the additional O-H stretching at 3340 cm-1 and B-O deformation at 1190 cm-1 suggest the presence of B-OH groups in BCN NSs,32,52 which may come from the precursor of boric acid. The peaks located at 1384 cm-1 and 1066 cm-1 can be assigned to the B-N stretching and the cubical B-C vibrations,52-53 revealing the successfully preparation of BCN NSs. Similar to that of bulk g-C3N4, the peaks of C=N stretches of BCN NSs appeare at 1638 and 1560 cm-1,54-55 and the adsorption peaks at 810 cm-1 of BCN NSs


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represent the characteristic vibration of the triazine ring,54-56 suggesting the formation of the CN matrix in BCN NSs.

Figure 1. (a) Illustration of the preparation procedures of BCN NSs from melamine and boric acid. (b) TEM image and (c) AFM image of BCN NSs. (d) XRD and (e) FTIR spectra of bulk g-C3N4 and BCN NSs.

The chemical structure of BCN NSs was further confirmed by X-ray photoelectron spectroscopy (XPS) characterization. As shown in Figure 2a, a newly B 1s peak appears in the XPS survey spectrum of BCN NSs, and the O 1s peak is much more 11 ACS Paragon Plus Environment


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prominent compared with that of the bulk g-C3N4. The high resolution B 1s spectrum in Figure 2b can be deconvoluted into three different signals at around 189, 190.8 and 192 eV, corresponding to the B-C, B-N and B-O bonds, respectively.50,52 The C 1s spectrum (Figure 2c) is separated into four peaks centered at 288.3, 286.2 284.7, and 283.7 eV, attributing to the N-C=N, C-OH, graphitic C, and C-B bonds, respectively.50,52,56-57 The N 1s spectrum in Figure 2d exhibits the graphitic N (～ 400.2 eV), pyrrolic N (～399.6 eV), pyridinic N (～398.4 eV) and N-H bonds (401.2), similarly to that of the bulk-g-C3N4.48,55-56 In addition, the other N 1s peak at 397.7 eV corresponds to N-B.58-59 The XPS results are good agreement with that of the FTIR data, confirming that the B atom is successfully doped into the carbon nitride composites.

Figure 2. XPS spectra of BCN NSs. (a) Survey scans, (b) B 1s, (c) C 1s, and (d) N 1s. 12 ACS Paragon Plus Environment

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Adsorption studies. Selection of adsorbents. To investigate the heavy metal ions adsorption performance of BCN NSs, a series of products were synthesized via the one-step pyrolysis of the mixture of melamine and boric acid with different molar ratios under various calcination temperatures. As shown in Figure 3a, the colors of the products are gradually changed from yellow to white with increasing the proportion of boric acid, which may mainly attribute to the boron and oxygen atoms doping. Similarly, with increasing the heating temperature, the color of the products is gradually bleached and the yield is subsequently declined because of the instability of C-N bonds at high temperatures. In order to reveal the relationships between the molar ratios of the precursors and the elemental compositions of the products, the resultants obtained from the pyrolysis at 550 °C were further studied. Elemental analysis and ICP measurement results reveal the exact amount of these elements (Table S1). The contents of oxygen and boron significantly increase with the addition of H3BO3 into the raw materials. Accordingly, the new B 1s peaks appear in the XPS spectra as shown in Figure 3b. Further deconvolution of the B 1s and N 1s peaks reveals that the B-O and N-B bonds increase with the proportion of H3BO3 in the raw materials (shown in Figure S2). Meanwhile, the contents of hydroxyl and carboxyl groups are significantly increased with the addition of H3BO3 (Figure 3c), and the products obtained with the molar ratio of 3:3 (BCN NSs) own the most amino groups (-NH2) (Figure S2b). However, the adsorption bands at 810 cm-1 assigned to the vibration of triazine ring significantly decrease with increasing the proportion of H3BO3 in the raw 13 ACS Paragon Plus Environment


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materials, and no obvious adsorption bands can be observed in the FTIR spectra of BCN-T550-4 and BCN-T550-5 (Figure 3c). Therefore, we can deduce that the proportion of H3BO3 in the raw materials can significantly influence the formation of the CN matrix.

Figure 3. (a) Photos of the products obtained from the pyrolysis of the mixtures of melamine and boric acid with different molar ratio at different temperatures. XPS spectra (b) and FTIR spectra (c) of the products obtained from the pyrolysis of the mixtures of melamine and boric acid with different molar ratio at 550 °C.

To select the best adsorbent, the adsorption capacities of these products toward Hg2+ have been well studied. The adsorption efficiencies for Hg2+ obtained by the products are shown in Figure 4a. As can be seen, the bulk g-C3N4 derived from the 14 ACS Paragon Plus Environment

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pyrolysis of melamine shows poor adsorption performance toward Hg2+, which might be mainly attributed to their poor interfacial properties.41-42 With the addition of boric acid into the raw materials, the adsorption capacities of the products are significantly improved, and the products obtained from the mixture of melamine and boric acid with the molar ratio of 3:3 exhibit the superior adsorption performance. As suggested by the XPS and FT-IR characterizations (Figure S2 and Figure 3c), abundant hydroxyl and carboxyl groups and boron-related bonds are introduced around the nanosheets, which can efficiently improving the hydrophilicities and electrostatic attraction abilities. Furthermore, be different from the hydrophobicity and less functional groups in the surface of BN nanomaterials, the rich B-O and boron atom vacancies can also increase the binding performance and adsorption rate of metallic ions.60 Thus, with the coexistence of abundant nitrogen and oxygen related groups as well as the boron-containing groups, the BCN NSs show better adsorption performance compared with the CN and BN nanomaterials. Over all, the product obtained from the mixture of melamine and boric acid with the molar ratio of 3:3 at 550 °C owes the best Hg2+ adsorption capacity, which was applied for further investigations.



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Figure 4. (a) Sorption efficiency (%) of Hg2+ on the products obtained by different molar ratio of melamine and boric acid and the pyrolysis temperature. The initial concentration of Hg2+ was 100 mg L-1. (b) Sorption efficiency (%) of BCN NSs for different common metal ions at their initial concentrations of 50 and 100 mg L-1.

Selectivity of BCN NSs for heavy metal ions adsorption. To further investigate the adsorption performance of BCN NSs for the common heavy metal ions in environmental water samples, Hg2+, Pb2+, Cu2+, Cd2+, and Cr3+ were selected as the adsorbates at the initial concentrations of 50 and 100 mg L-1. As shown in Figure 4b, BCN NSs exhibit different adsorption capacities for different metal ions, and follow the order of Hg2+ > Pb2+ > Cu2+ > Cd2+ > Cr3+. The existence of abundant –NH2 and other nitrogen and oxygen related groups as well as the boron-containing groups in the surface of BCN NSs can efficiently complex with metal ions.60-62 According to the theory of Evert Nieboe, 63 Hg2+ belongs to Class B (nitrogen/Sulphur-seeking) and the others belong to borderline (or intermediate) ions, and the complication ability of Pb2+ is most similar to that of Class B. The superior adsorption performance for Hg2+ verifies the stronger affinity of BCN NSs toward Hg2+ rather than other ions. Furthermore, BCN NSs also show good adsorption performance toward Pb2+, and the adsorption efficiency is up to 99% at the initial concentrations of 50 mg L-1. Thus, we supposed that BCN NSs can be applied for selectively adsorption of Hg2+ and Pb2+.

Effect of pH on the Adsorption. The adsorption capacities toward the metal ions can be strongly influenced by the pH value since it not only affects the ion forms in solution but also the protonation degree of functional groups on BCN NSs. Figure S1 16 ACS Paragon Plus Environment

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displays the adsorption behavior of Hg2+ and Pb2+ on BCN NSs at different pH. It can be find that the removal efficiency by BCN NSs is relatively poor at acid conditions, and gradually increases as the pH increasing from 3.0 to 7.0. The pH dependence behavior of the BCN NSs can mainly attribute to the protonation of the functional groups in the surfaces (including amino groups, carboxyl groups, and boron related groups). Thus, all the adsorption studies are carried out in the neutral conditions.

Adsorption Isotherms study. The adsorption isotherms of BCN NSs for Hg2+ and Pb2+ were obtained with different initial concentrations. As shown in Figure 5a-b, it is obviously that the adsorption capacities of Hg2+ and Pb2+ increase rapidly with increasing their initial concentrations, and then gradually reach the adsorption equilibrium. The adsorption data of Hg2+ and Pb2+ were fitted by Freundlich and Langmuir isotherm models. The Langmuir isotherm model assumes that monolayer adsorption occurs on the active sites of the uniform surface which can reach the saturation.8,25 However, the Freundlich isotherm adsorption model suggests that adsorption occurs on heterogeneous surfaces through multilayer adsorption. The adsorption isotherms and the quantitative parameters are summarized in Figure 5a-b and Table S1. It shows that the fitting curves by the Langmuir isotherm model match better with the experimental adsorption data, indicating that this adsorption behavior can be better described by the Langmuir model, suggesting the monolayer coverage of the metal ions on the adsorbent. All correlation coefficient (R2) values of the Langmuir isotherm model are better than 0.99 and the value of Ce/qe is excellently linear with Ce (Figure 4b), which further supports our conclusion. Fitting the 17 ACS Paragon Plus Environment


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experimental adsorption data using the Langmuir model gives the maximum adsorption capacities of 625.0 and 210.97 mg g-1 for Hg2+ and Pb2+, respectively. Especially, the fabricated BCN NSs exhibit excellent adsorption toward Hg2+ and Pb2+, which is comparable with the reported ones (Table 1 and Table S3).

Figure 5. The adsorption isotherms of Hg2+ (a) and Pb2+ (b) onto BCN NSs. The adsorption data were fitted by the Freundlich (dashed line) and Langmuir (solid line) models. The insert curves are the linear regression by fitting the equilibrium adsorption data with Langmuir adsorption model for Hg2+ and Pb2+, respectively. The adsorption kinetics of Hg2+ (c) and Pb2+ (d) onto BCN NSs. The adsorption data were fitted with pseudo-second-order kinetic model, and the inserted curves are the kinetic plots of Hg2+ and Pb2+ on BCN NSs, respectively. Table 1. Comparison of the adsorption capacities of Hg2+ with various adsorbents. 18 ACS Paragon Plus Environment

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adsorbents

capacity (mg/g)

reference

activated carbon fibres

290-710

[64]

sulfur-functionalized mesoporous carbon

435-732

[65]

amine-modified activated carbon

119

[66]

thiol-derivatized single walled carbon nanotube

131

[67]

thiol-functionalized magnetite/graphene oxide

289.9

[68]

graphenes magnetic composite nanoparticles

23.03

[69]

graphene oxide-thymine composite

128

[26]

2-imino-4-thiobiuret−partially reduced graphene oxide

624

[24]

magnetic Nitrogen-Doped Porous Carbon

297-476

[70]

CBAP-1(AET)

232

[10]

COF-S-SH

1350

[43]

BCN NSs

625

This work

Adsorption Kinetic Study. The adsorption kinetics was further studied to analyze the adsorption performance of BCN NSs. The kinetics of Hg2+ and Pb2+ on BCN NSs were investigated at the initial concentrations of 307.8 and 108.0mg L-1, respectively. As shown in Figures 5c-d, the amount of Hg2+ and Pb2+ removal increases rapidly at the initial 5 min, then roses slowly and reaches their adsorption equilibrium in 40 min. Apparently, BCN NSs show the excellent potential for fast heavy metal ions removal. Further, the pseudo-second-order kinetic model (eqn (5)) was employed to study the adsorption kinetics, and the fitting results of adsorption kinetics are presented in Table S3. As shown in the inserted curves of Figure 5c and d, the plots of t/qt versus t show excellent linear relationships (the correlation coefficient R2 are more than 0.99), 19 ACS Paragon Plus Environment


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suggesting that the adsorption rates of Pb2+ and Hg2+ fitted perfectly with the pseudo-second-order model. The fitting curves by the pseudo-second-order model match well with the experimental adsorption data, which confirms the chemisorption of the metal ions onto the active adsorption sites is the rate-controlling step.

Adsorption Mechanism. To investigate the interaction mechanism of BCN NSs for Hg2+ and Pb2+, the zeta potential and specific surface area of BCN NSs and g-C3N4 were recorded (Figure S4). Owing to the introduction of abundant nitrogen and oxygen related groups as well as the boron-containing groups on the nanosheets surfaces, BCN NSs exhibit a much more negative surface charge of about -26.5 mV, whereas the bulk g-C3N4 is almost uncharged (Figure S4a). The negative-charged BCN NSs can efficiently attract these cationic metal ions from solution to its surfaces

via electrostatic, complexation, or hydrogen-bonding interactions.71-72 Meanwhile, the hydrophilicity of the BCN NSs could be significantly improved, and enhancing its dispersion in water. Furthermore, compared with that of the bulk g-C3N4 (~ 12.8 m2 g-1), the bigger specific surface area of the BCN NSs (~ 55.7 m2 g-1) can also significantly improve the adsorption efficiency and capacities (Figure S4b). The XPS spectra analyses of the adsorbent after metal ions adsorption have also been conducted to further study the adsorption mechanism (Figures 6). The presence of the Hg 4f and Pb 4f peaks further confirms the adsorption of Hg2+ and Pb2+ on BCN NSs (Figures 6a and b). Figure 6a presents the Hg 4f spectrum of Hg2+ on BCN NSs, the peaks at 101.9 and 106.0 eV are corresponding to the Hg 4f5/2 and Hg 4f7/2, respectively. The little shift toward higher binding energy compared with that of the 20 ACS Paragon Plus Environment

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free Hg2+ can be attributed to the interaction between Hg2+ and the adsorbent.24,70 Similarly, the Pb 4f peaks in Figure 6b are centered at 139.7 eV for Pb 4f7/2 and 144.5 eV for Pb 4f5/2, exhibiting the shift of 0.4 eV toward higher bingding energy.73 The deconvolution of the N 1s before and after Hg2+ or Pb2+ adsorption are shown in Figures 6c. Obviously, the peaks of graphitic N and N-H are significantly decreased, which could be assigned to the complexation of nitrogen and metal ions. Compared with the C-C and C-N bonds, the polar B-N and B-O bonds in the BCN NSs are more favorable for metal ions adsorption. As previously reported, the ionic B-N bonds show the “lop-side” densities properties and the poly-electron nitride could transfer more electrons to the heavy metal ions.72,74 Besides, the negatively charged oxygen atoms in B-O bonds can attract metal ions via donating their excess electrons.60 To verify the interaction between the boron related groups and metal ions, the B 1s spectra of BCN NSs before and after metal ions adsorption have been further deconvoluted (Figure 6b). The intensities of B-O peaks after Hg2+ or Pb2+ adsorption significantly decrease, whereas no obvious changes in the B-N and B-C are observed. It indicates that the B-O bonds exhibit stronger interaction with metal ions compared with that of the B-N bonds, which may due to the stronger polarity of B-O bonds. Therefore, the superior metal ions adsorption properties are mainly ascribed to (1) the more negative charges carried by BCN NSs can efficiently attract the positively charged metal ions via the electrostatic interactions; (2) the bigger specific surface area of the BCN NSs can also significantly improve the adsorption efficiency and capacities; (3) the additional B-O and boron atom vacancies can attract metal ions by 21 ACS Paragon Plus Environment


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the strong molecular interactions; (4) the nitrogenic groups and the other active sites can be chelated with metal ions and further immobilize them to the surface of BCN NSs.

Figure 6. (a) XPS spectrum of Hg 4f adsorbed on BCN NSs; (b) deconvolution analysis of N 1s after Hg2+ adsorption; (c) XPS spectrum of Pb 4f adsorbed on BCN NSs; (d) deconvolution analysis of N 1s after Pb2+ adsorption.

Adsorbent Reusability. The reusability and regeneration of the adsorbents are of great significance for the practical utilization. As proved by the pH investigation, 22 ACS Paragon Plus Environment

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BCN NSs show poor adsorption performance at the acid conditions, which can mainly attribute to the protonation of the functional groups in the surface of BCN NSs under acid conditions. Herein, HCl (1.0 M) was applied for the elution of Hg2+ and Pb2+. As shown in Figure 7, more than 90% of the initial adsorption capacities can be retained even after 6 adsorption/desorption cycles. In order to identify the desorption efficiency, the XPS spectra of BCN NSs before and after metal ions desorption are presented in Figure S5. Apparently, the XPS peaks of Hg 4f and Pb 4f disappear after desorption, indicating that most of the heavy metal ions can be efficiently desorbed from the adsorbent. The regeneration performance indicates the excellent reusability of BCN NSs which can feasibly be applied to uptake Hg2+ and Pb

2+

from wastewater.

Figure 7. Hg2+ (a) and Pb2+ (b) removal by the recycled BCN NSs. The initial concentrations of Hg2+ and Pb2+ are 307.8 and 108.0mg L-1, respectively.

CONCLUSIONS In summary, a facile, scalable and cost-effective protocol has been proposed to synthesize BCN NSs via pyrolysis of the mixture of melamine and H3BO3 with the molar ratio of 3:3 at 550 °C. Due to the successful introduction of abundant function groups, enhanced specific surface area and hydrophilicity, BCN NSs exhibit excellent 23 ACS Paragon Plus Environment


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adsorption performance to Hg2+ and Pb2+ with the maximum adsorption capacities of 625.0 and 210.97 mg g-1, respectively. The sorption isotherms fitted greatly with the Langmuir adsorption model which suggesting the monolayer adsorption behavior, and the pseudo-second-order kinetic adsorption process indicates the chemisorption mechanism. Besides, BCN NSs show excellent chemical stability and the adsorption capacities remain over 95% even after 6 adsorption/desorption cycles. By virtue of the cost-effective and facile production methodology, combining with the excellent absorption performance, the BCN NSs have great potential for practical application in wastewater purification.

ASSOCIATED CONTENT Supporting Information The SEM images of BCN NSs with different magnifications; the exact amount of the elements of different products; the XPS B 1s and N 1s peaks of different products; effect of pH on the adsorption capacity of Hg2+ and Pb2+ onto BCN NSs; summary of the Langmuir and Freundlich isotherm fitting parameters for the adsorption of pollutants on BCN NSs; comparison of the adsorption capacities of Pb2+ with various adsorbents; summary of the pseudo-second-order adsorption kinetic constants of BCN NSs; zeta potential and N2 adsorption and desorption isotherms of bulk g-C3N4 and BCN NSs; XPS spectra of BCN NSs before and after desorption with Hg2+ and Pb2+.

ACKNOWLEDGMENTS


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We gratefully acknowledge the supports from the National Natural Science Foundation of China (21675078), and the Jiangxi Province Natural Science Foundation (20165BCB18022).

Notes The authors declare no competing financial interest.

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Table of Contents Graphic: Synopsis: The one-pot synthesized boron carbon nitride nanosheets can efficiently uptake Hg2+ and Pb2+ from waste water.


One-pot Synthesis of Boron Carbon Nitride ... - ACS Publications

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