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Advanced overlap adsorption model of fewlayer boron nitride for aromatic organic pollutants Honghong Chang, Yanhong Chao, Jingyu Pang, Hongping Li, Linjie Lu, Minqiang He, Guangying Chen, Wenshuai Zhu, and Huaming Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05092 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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Industrial & Engineering Chemistry Research

Advanced overlap adsorption model of few-layer boron nitride for aromatic organic pollutants

Honghong Chang1, Yanhong Chao2*, Jingyu Pang1, Hongping Li1, Linjie Lu1, Minqiang He1, Guangying Chen3, Wenshuai Zhu1*, Huaming Li1

1

School of Chemistry and Chemical Engineering, Institute of Energy, Jiangsu

University, Zhenjiang, 212013, P.R. China 2

School of Pharmacy, Jiangsu University, Zhenjiang, 212013, P.R. China

3

Key Laboratory of Tropical Medicinal Plant Chemistry of Education, Hainan Normal

University, Haikou 571158, China

*Email: [email protected] (Y. H. Chao); [email protected] (W. S. Zhu);

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Designing overlap interaction adsorbents with high surface area has emerged as an intriguing method to enhance adsorption performance. Herein, a two dimensional (2D) few-layer graphene-like boron nitride (g-BN) with 556 m2 g-1 surface area was successfully prepared and employed in the adsorption of aromatic organic pollutants methylene blue (MB) and rhodamine B (RhB) via π-π overlap interaction. The adsorption contact time, temperature, pH, kinetics, isotherms, and thermodynamics were investigated. Pseudo-second-order kinetic model fitted well with experiment data, isotherm data displayed better fit with Freundlich model. The experimental results showed that the equilibrium adsorption capacity of g-BN for MB and RhB were high to 522 mg g-1 and 488 mg g-1, respectively. The thermodynamics parameters suggested that the adsorption processes were spontaneous and exothermic. Based on the density functional theory (DFT), the adsorption mechanism was probably dominated by π-π stacking interaction with overlap form, which was confirmed by the XRD, FTIR and Freundlich isotherm. These findings displayed that g-BN was a desirable adsorbent for removing aromatic organic pollutants from wastewater. Keywords: Boron nitride; Multilayer adsorption; π-π stacking interaction; Overlap adsorption model

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1. INTRODUCTION With the development of industrialization and urbanization, aromatic organic dyes have been widely used in food processing, printing, paper making, textile, and leather, etc. However, large amounts of residual organic dyes have been found in aqueous matrix, which are undesirable compounds with potential risk of cancer and toxic to human beings, microorganisms and fish species. Therefore, it is essential to develop effective measures to remove these aromatic organic pollutants from water.1-5 To date, various efficient technologies have been used to treat the wastewater such as adsorption,6-9 coagulation,10 nanofiltration,11, 12 electrochemical deposition13 and catalytic photodissociation.14,

15

Among them, adsorption is one of the most

extensively used technologies for wastewater treatment due to the low-cost, high efficiency and easy operation.16 Various adsorption materials have been developed such as activated carbon,17, 18 carbon nanotubes,19-24 silica,25 zeolite,26 metal-organic frame work27 and graphene oxides (GOs).28, 29 Among these adsorbents, GOs often present significantly higher maximum adsorption capacities than that of other adsorbents, which can be attributed to the higher adsorption energy between GOs and adsorbates by π−π stacking interaction. Recently, boron nitride (BN) nanomaterial,30, 31 a graphene-like isoelectronic species, has been widely used in adsorption fields due to the large surface area and the delocalized π bonds interaction with adsorbates. Dmitri et al32 prepared BN-based porous monoliths and employed them as adsorbents for Cd( II ) removal. Lei et al33 reported porous boron nitride nanosheets can clean oil and Congo red in water. Tang

3



et al34 fabricated an activated boron nitride to remove cationic dyes. Until now, most reported adsorption isotherms of BN nanomaterials were accorded with Langmuir model, indicating the monolayer adsorption mechanism. However, the overlapping adsorption model of BN adsorbents has not been reported. Herein, a two dimensional (2D) few-layer graphene-like BN (g-BN) was designed and used as the overlapping adsorbent for removing aromatic organic pollutants methylene blue (MB) and rhodamine b (RhB) from wastewater. This overlapping adsorption mechanism on 2D g-BN was proved by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and theoretical calculations of density functional theory (DFT). The effects of contact time, temperature and pH were explored to investigate the adsorption performance. Additionally, the adsorption kinetics, isotherms and thermodynamics were also analyzed in detail. 2. EXPERIMENTAL 2.1 Materials and Reagents All of the chemicals used in this study were of analytical grade. Methylene blue (MB, 82.0% purity) was provided by the Shanghai Macklin Biochemical Co., Ltd. Rhodamine b (RhB, AR), boron oxide, urea and 1-propanol were obtained from Sinopharm Chemical Reagent Co., Ltd. 2.2 Preparation of Sample The g-BN was prepared in a typical process: 0.34 g B2O3 and 14.4 g urea were dissolved in the mixture solution (20 mL deionized water and 20 mL 1-propanol) and heated at 50°C. When the water evaporated completely, the precursor was calcined at

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900°C in tube furnace under N2 atmosphere for 2 h. The obtained white powder products were marked as few-layered g-BN. 2.3 Characterization Method Atomic force microscopy (AFM) images were obtained by a MFP-3D. Scanning electron microscopy (SEM) was carried out using JSM-6010PLUS/LA. Transmission electron microscopy (TEM) images were obtained from a JEOL JEM-2100 TEM at an accelerating volt-age of 200 kV. FTIR spectra were recorded on a Nicolet Nexus 470 by using KBr pellets at room temperature. XRD patterns were obtained from a Shimadzu XRD-6000 X-ray diffractometer with high-intensity Cu K α (λ =1.54 Å). A Shimadzu UV-2450 spectrophotometer was used to detect the content of the pollutants in the solution. Raman spectrum was obtained from a DXR Raman microscope at room temperature. The Brunauer-Emmett-Teller (BET) method was used to calculate the surface area of the sample with nitrogen adsorption at 77.3 K by TriStar II 3020. 2.4 Adsorption Experiment To evaluate the efficiency of g-BN for MB and RhB adsorption in aqueous solution, a series of batch adsorption experiments were performed in 100 mL conical flasks. The adsorption kinetics experiment was conducted at the room temperature. A portion of 5 mg g-BN was added to 25 mL of dye solution with the concentration of 200 mg L-1 and then shook at the speed of 130 rpm. At different contact times, a certain volume of supernatant was aspirated and filtered to detect the residual concentration of adsorbates using UV-Vis spectrophotometer. Adsorption isotherm studies were measured with initial concentration of 100-300 mg L-1. Adsorption

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thermodynamic experiments were carried out at 298, 308, 318 and 328 K. For pH effect study, the solution was adjusted in the range of 2-12 using 0.1 mol L-1 HCl or NaOH. The adsorption capacity at specific time t (qt, mg g-1) and equilibrium (qe, mg g-1) were calculated by the following equations: qt = qe =

C0 -Ct V

(1)

m C0 -Ce V

(2)

m

where C0, Ct and Ce (mg L-1) are the initial concentration, residual concentration at time t (min) and residual concentration at equilibrium of the adsorbate solution, respectively. V (L) is the volume of the solution and m is the mass of g-BN (g). 3. RESULTS AND DISCUSSION 3.1 Characterization As shown in the Figure 1a, the crystalline structure of the as-prepared adsorbent is characterized by XRD. The characteristic peak at 23.4° and 42.2° are indexed to (002) and (100) plane, according with hexagonal BN (JCPDS Card No. 34-0421). The broad peak indicates that the obtained g-BN exhibits a poor crystallinity. FTIR spectrum of g-BN is shown in the Figure 1b, the strong adsorption peaks at 1391 cm-1 and 807 cm-1 belong to in-plane stretching vibration of B-N and out-of-plane bending vibration of B-N-B,35-37 respectively. The adsorption peak at 3421 cm-1 ascribe to the O-H stretching band. 38

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Transmittance (%)

(002) Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(100)

20

-1

1391 cm-1

(b)

(a) 10

807 cm

3421 cm-1

30

40

50

60

70

80

4000

2-Theta (degree)

3000 2000 1000 -1 Wavenumber (cm )

Figure 1. XRD pattern (a) and FTIR spectrum (b) of g-BN. AFM is applied to investigate the thickness and the structure of the as-prepared adsorbent. It can be seen from Figure 2a that the thickness of g-BN is about 3 nm, which indicates a few-layer structure of graphene-like BN.39 To further identify the internal structure of the g-BN, SEM and TEM are measured. SEM image of the g-BN in Figure 2b reveals a sheet-like morphology with rough surface. Meanwhile, TEM image of g-BN in Figure 2c displays an ultrathin structure.

Figure 2. AFM (a), SEM (b) and TEM (c) images of g-BN.

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Figure 3a presents the Raman spectrum of g-BN. The peak at 1374 cm-1 is assigned to B-N vibration mode (E2g).40 Compared with the bulk h-BN which shows the E2g vibration mode at 1370 cm-1, the B-N vibration of g-BN upshifted to a higher frequency, indicating that the g-BN has few-layered structure.41-43 The surface area and pore structure of g-BN were analyzed by N2 adsorption-desorption isotherm. As shown in Figure 3b, g-BN exhibits a typical type II with H3 hysteresis loop in relative pressure range of 0.4-1.0, indicating the slit-shaped porous.44 Based on calculation, the g-BN possesses a high surface area of 556 m2 g-1. The pore size distribution was analyzed by Barrett-Joyner-Halenda (BJH) model with the peak centered at 2 nm and

(a)

3 Qualitity Adsorption (cm /g)

1374 cm-1

1000

1200 1400 1600 1800 -1 Wavenumber (cm )

2000

800 600 400

dV/dlog(w) Pore Volume (cm3/g )

the pore volume is 0.18 cm3 g-1.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(b)

0.30 0.25 0.20 0.15 0.10 0.05 0.00

0

200 0 0.0

2

4

6 8 10 12 14 16 18 20 Pore Width (nm)

0.2 0.4 0.6 0.8 Relative Pressure (P/P0)

1.0

Figure 3. Raman spectrum of g-BN (a) and N2 adsorption-desorption curves of g-BN (the inset show the pore size distribution curves) (b). 3.2 Effect of Contact Time and Adsorption Kinetics The effect of contact time was shown in Figure 4a. The adsorption capacity increased sharply during the initial period from 10 to 50 min and then tended to equilibrium after 60 min. The rapid adsorption at initial period was due to the adequate vacant sorption sites on the surface of g-BN. While the adsorption rate

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attained slow with the time enhanced was probably ascribed to the adsorbent surface which were occupied by adsorbate molecules completely. Therefore, the optimum equilibrium time for MB and RhB onto g-BN was 60 min at room temperature and the adsorption capacity reached 522 and 488 mg g-1 for MB and RhB, respectively. 540

0.40

(a)

0.35 t/qt (min g/mg)

510 480 450

t

q (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


420

0

30

60

y=0.0086+0.0019x 2 R =0.9994

0.30 0.25 0.20 0.15

y=0.0068+0.0018x 2 R =0.9998

0.10

MB RhB

390

(b)

MB RhB

0.05 0.00

90 120 150 180 t (min)

0

30

60

90 120 150 180 t (min)

Figure 4. Effect of contact time (a) and the linear regression kinetic plots of pseudo-second-order model (b) for MB and RhB adsorption onto g-BN. (C0 = 200 mg L-1; T = 298 K; madsorbent = 5 mg; V = 25 mL) The adsorption kinetics was one of the most significance characters for the adsorbent. In this work, the experiment data were analyzed by pseudo-first-order model and pseudo-second-order model. These two kinetic models can be presented as following equations: lnqe -qt = lnqe -k1t t qt

=

1 k2 q2e

+

t qe

(3) (4)

where qt (mg g-1) is the adsorption capacity of g-BN at time t (min); k1 and k2 are the adsorption rate constants of pseudo-first-order model and pseudo-second-order model, respectively. The detail kinetic parameters were listed in Table 1 and the linear regression 9



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kinetic plots of pseudo-second-order model were shown in Figure 4b. The pseudo-second-order model had higher correlation coefficients (R2>0.99) than that of the pseudo-first-order model. Moreover, the values of the calculated equilibrium adsorption capacity (qe,cal ) with pseudo-second-order model were in close agreement with the experiment data (qe,exp). Therefore, the adsorption performance of MB and RhB onto g-BN can be better predicted with the pseudo-second-order kinetic model.45 Table 1. Kinetics parameters for the adsorption of MB and RhB onto g-BN Kinetic models

Pseudo-first-order

Pseudo-second-order

Parameters

MB

RhB

qe, exp (mg g-1)

522

488

qe, cal (mg g-1)

44

64

k1

0.00011

0.000052

R2

0.886

0.776

qe, cal (mg g-1)

526

500

k2

0.00053

0.00046

H

147.1

114.9

R2

0.9998

0.9994

3.3 Effect of pH The effect of pH was investigated at the range of 2-12, and the results were shown in Figure 5. The adsorption of MB and RhB onto g-BN was both dependent on pH. As shown in Figure 5, the adsorption capacity of MB was increased steadily when the pH value increased from 2 to 12, which probably attributed to the competition between

10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the cationic dyes and excess OH−/H+ ions in the solution. However, as shown in Figure 5, the removal of RhB was initially increased as pH enhanced but then decreased obviously when pH > 8. When the pH increased from 2 to 8, the adsorption capacity of RhB increased from 408 to 537 mg g-1, but as the pH continuously increased to 12, the adsorption capacity decreased to 459 mg g-1. This result can be explained from the structure of RhB, which containing the carboxylic group and can be negatively charged under higher pH condition. The electrostatic repulsion would occur between negatively charged RhB and g-BN under alkaline medium and lead to the decline of the adsorption ability. Therefore, we conclude that MB/RhB adsorbed onto BN highly influenced by solution pH.

550

qe (mg/g)

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500

450 MB RhB

400 2

4

6

8

10

12

pH

Figure 5. Effect of pH for MB and RhB adsorption onto g-BN. (C0 = 200 mg L-1; T = 298 K; madsorbent = 5 mg; V = 25 mL; t = 120 min; pH = 2 ~ 12) 3.4 Adsorption of thermodynamics The effect of temperature on the adsorption of MB and RhB onto g-BN was investigated under the temperature of 298-328 K and the results were shown in Figure 6. It was observed that the adsorption capacity decreased with the temperature advanced, suggesting lower temperature was favorable for the adsorption. 11



The thermodynamic parameters which include standard Gibbs free energy (∆G°), standard entropy (∆S°) and standard enthalpy (∆H°) were calculated by the following equations:

Kc =

qe

(5)

Ce

∆G° = − R T lnKc

lnKc =

∆S° R

−

(6)

∆H°

(7)

RT

where Kc is the distribution coefficient for the adsorption, R is the ideal gas constant (8.314 J mol-1 K-1), T is the adsorption temperature (K). As can be seen in Table 2, the negative values of ∆G° indicated that the adsorption process was spontaneous. The negative value of ∆Ho suggested that MB and RhB adsorbed onto g-BN was an exothermic process. The value of ∆S° was -30 J mol-1 K-1 for MB and -136 J mol-1 K-1 for RhB indicated that the disorder at the interface decreased during the adsorption.46

RhB MB

600 500 qe (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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400 300 200 100 0 298

308

318 T (K)

328

Figure 6. Effect of temperature for MB and RhB adsorption onto g-BN. (C0 = 200 mg L-1; madsorbent = 5 mg; V = 25 mL; t = 120 min; T = 298 K ~ 328 K)

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Table 2. Thermodynamic parameters for the adsorption of MB and RhB onto g-BN ∆Go (KJ mol-1)

∆Ho

∆So

(KJ mol-1)

(J mol-1K-1)

298 K

308 K

318 K

328 K

MB

-15.52

-30

-6.37

-6.17

-6.16

-5.37

RhB

-46.19

-136

-5.68

-4.21

-2.77

-1.64

Sorbates

3.5 Adsorption isotherms Adsorption isotherms are often used to describe the adsorption performance. In this study, Langmuir and Freundlich models were applied to explain the experimental data. The Langmuir model assumes a monolayer and uniform adsorption. It is expressed as: Ce qe

=

Ce qm

+

1 qm b

(8)

where qm (mg g-1) is the maximum adsorption capacity, b (L mg-1) is the Langmuir isotherm constant which relates to the energy of adsorption. For the removal of MB and RhB，the dimensionless constant separation factor (RL) calculated by the following equation: RL =1/(1+bCi )

(9)

where Ci (mg L -1) represents the initial concentration of dyes. The values of RL indicate the type of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0).47, 48 The Freundlich isotherm evaluates the multilayer adsorption process and

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heterogeneous surface, which can be written as: lnqe = lnKF + lnCe 1

(10)

n

where KF (mg g-1) is the Freundlich constant which means adsorption or distribution coefficient, n is the non-linear parameter which represents the heterogeneous energy for adsorption happened on the surface. The average percentage error of APE % was also used to show the proper adsorption model. The values of APE % can be calculated by the following equation: APE%= ∑Ni=1 (qe,exp- qe,cal )/qe,exp /N*100

(11)

where qe,exp and qe,cal are the adsorption capacity of experimental value and calculated value by the isotherm models, respectively. N is the number of experimental data points. The adsorption isotherms of MB and RhB onto g-BN were shown in Figure 7, and the isotherm parameters as well as the correlation coefficients of R2 and APE % were summarized in Table 3. It was shown that the correlation coefficients (R2 > 0.9020) of the two models were reasonably good for the adsorption equilibrium data of MB and RhB. However, Freundlich isotherm model showed higher correlation coefficients (R2 > 0.9508) and lower average percentage errors (APE % < 4.94) than that of Langmuir model, suggesting that Freundlich model was more likely to predict the adsorption isotherms data of MB and RhB onto g-BN, and the nature of the adsorption was proposed as multilayer adsorption onto heterogeneous surface. The possible adsorption interaction may be the π-π stacking interaction between organic dyes and g-BN.49, 50

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As shown in Table 3, all the values of 1/n of Freundlich isotherm model were in all the range of 0-1, indicating the adsorption was favorable.51, 52 The parameters of KF, which means the relevant to adsorption capacity,53 were decreased with the temperature increased, suggesting that higher temperature can’t facilitate the adsorption. This result was similar to the thermodynamic exothermic conclusion.

900

600 500

(a) 0

Langmuir

700

Freundlich

600 500

300 30

900

60 90 Ce (mg/L)

120

150

800

600 500

30

60

90 120 Ce (mg/L)

150

180

328K MB RhB Langmuir Freundlich

900 qe (mg/g)

700

(b) 0


800

700 600 500 400

400 300

RhB

400

400 300

MB

800 qe (mg/g)

qe (mg/g)

700

308K

900


800

qe (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300

(c) 0

30

60

(d) 0

90 120 150 180 Ce (mg/L)

30

60

90 120 150 180 Ce (mg/L)

Figure 7. Adsorption isotherms of MB and RhB onto g-BN at different temperatures. (C0 = 100~300 mg L-1; madsorbent = 5 mg; V = 25 mL; t = 120 min)

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Table 3. Isotherm parameters of MB and RhB adsorption onto g-BN. Langmuir

Freundlich

T Sorbates

qm

b

R2

RL

APE

KF

（%）

(L mg-1)

1/n

R2

APE

(K)

RhB

MB

(mg g-1)

(L mg-1)

298

833

0.0368

0.17- 0.59

0.942

5.32

110.43

0.39

0.9663

3.37

308

625

0.0714

0.07-0.45

0.9140

6.15

87.96

0.26

0.9806

2.39

318

625

0.0423

0.11-0.49

0.8750

8.48

63.75

0.33

0.9508

4.94

328

588

0.0217

0.19-0.56

0.9674

3.74

61.58

0.41

0.9936

1.68

298

909

0.0311

0.17- 0.60

0.9020

6.20

128.87

0.36

0.9759

3.30

308

1000

0.0290

0.19-0.63

0.9807

4.23

103.94

0.42

0.9978

0.987

318

1000

0.0305

0.20-0.62

0.9686

5.68

101.76

0.43

0.9924

2.23

328

1428

0.0094

0.45-0.78

0.9850

1.97

35.26

0.65

0.9925

1.32

（%）

3.6 Mechanism The adsorption process and the adsorption mechanism of MB and RhB onto g-BN were further analyzed by XRD, FTIR and theoretical calculations of density functional theory (DFT). Figure 8a showed the XRD patterns of g-BN before and after adsorption of MB and RhB. The (002) peak after adsorption upshifted to 25.6o (MB) and 25.9o (RhB) which was attributed to the decrease of interplanar distance. Moreover, the increasing intensity of the (002) peak was due to the multilayer adsorption of dyes with an extended/order stacking in the direction.54-56 FTIR spectra of g-BN with adsorbed MB and RhB were shown in Figure 8b, the characteristic peaks 16


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of MB and RhB (1596 cm-1, 1175 cm-1, 1600 cm-1, 1133 cm-1) also observed on g-BN after adsorption. The B-N peak of g-BN after adsorption shifted from 1390 cm-1 to 1395 cm-1 (MB) or to 1394 cm-1 (RhB) which can be attributed to the classical FTIR harmonic oscillator models,57 When the dyes were adsorbed onto g-BN, the effective restoring forces acted on the atoms increased and the phonons to be harden due to the interlayer van der Wales interactions increased which lead to the stretching vibration of B-N upshifted to high wavenumbers. Furthermore, the π-π stacking interaction could be responsible for the adsorption according to the similar structure of aromatic arings group in dye molecules and the hexagon structure in g-BN. Above all, the strong π-π stacking interaction was consistent with the harmonic oscillator model and XRD promoting multilayer adsorption and increasing adsorption capacity.

(a)

10

(b)

20

30 40 50 60 2-Theta (degree)

70

Transmitannce (%)

MB adsorbed g-BN RhB adsorbed g-BN g-BN

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

4000

RhB RhB adsorbed g-BN

MB MB adsorbed g-BN

g-BN

3000 2000 -1 1000 Wavenumber (cm )

Figure 8. XRD patterns (a) and FTIR spectra (b) of g-BN before and after adsorption of MB and RhB. In order to further understand the adsorption mechanism, dispersion corrected density functional theory (DFT) calculation was performed at B3LYP+D/6-31+G(d,p) theoretical level. The optimized structure was plotted in Figure 9a. It can be seen that the aromatic ring of MB molecule is parallel with the plane of BN, which is a typical

17



π-π stacking interaction.39 Moreover, above XRD and FTIR discussions show a multilayer adsorption phenomenon. Combined with the experimental evidences and DFT calculations, it is reasonable to propose a multilayer adsorption model to interpret the Freundlich adsorption isotherm (Figure 9b). The π-π stacking interaction in this model contains two components. The one is the π-π stacking interaction between BN and MB; the other is the π-π stacking interaction between MB molecules. The multilayer model will lead to strengthen of the layer interaction, thus results in a decreasing of interlayer distance as shown in XRD patterns and a blue shift of peaks in the FTIR spectra.

Figure 9. π-π stacking interaction predicted by DFT calculation (a) and Proposed multilayer adsorption model based on DFT calculation (b).

4. CONCLUSIONS In this study, 2D few-layer g-BN had been successfully fabricated. XRD, FTIR, AFM, SEM, TEM and BET had been characterized to investigate the structure and physico-chemical property of the g-BN. Prepared g-BN exhibited efficient removal 18


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for MB and RhB from aqueous solution with an excellent adsorption capacity of 522 mg g-1 and 488 mg g-1, respectively. The adsorption kinetics followed well with pseudo-second-order kinetic model. The adsorption was pH dependent, spontaneous and exothermic. Equilibrium adsorption data were well fitted by Freundlich isotherm model, indicating the multilayer adsorption nature of MB and RhB onto g-BN. Overlapping adsorption mechanism of π-π stacking interaction was proposed by XRD, FTIR and dispersion corrected density functional theory. All of which displayed that the prepared 2D few-layer g-BN performed a novel overlapping adsorption with high capacity for MB and RhB and demonstrated great potential application in removal of aromatic organic pollutants from wastewater.

ACKNOWLEDGEMENT This work was financially supported by the National Nature Science Foundation of China (Nos. 21506083, 21576122, 21722604), Postdoctoral Science Foundation of China (No.2017M611726), Advanced Talents of Jiangsu University (No. 15JDG176).

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The overlap adsorption model of organic dye onto BN by π-π stacking interaction.

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