Highly Efficient Adsorption of Oils and Pollutants by Porous Ultrathin

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Highly Efficient Adsorption of Oils and Pollutants by Porous Ultrathin Oxygen-Modified BCN Nanosheets Quanguo Hao,† Yanhua Song,*,‡ Zhao Mo,† Siddharth Mishra,§ Jingyu Pang,∥ Yangxian Liu,† Jiabiao Lian,† Jingjie Wu,⊥ Shouqi Yuan,† Hui Xu,*,† and Huaming Li†

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†

School of the Environment and Safety Engineering, Institute for Energy Research, Jiangsu University, Zhenjiang 212013, P. R. China ‡ School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, P. R. China § Department of Material Science and Engineering, University of Cincinnati, Cincinnati, Ohio 45221, United States ∥ Henan Engineering Research Center of Industrial Circulating Water Treatment, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, P. R. China ⊥ Department of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221, United States S Supporting Information *

ABSTRACT: The effective removal of oils and pollutants by adsorption plays a crucial role in the water purification and treatment process. Various nanostructured adsorbents have been reported in the past, while a majority of these lack in some critical chemical and structural characteristics which limit their practical applications. To enhance the adsorption process, there is a need for the development of advanced adsorbents with excellent adsorption capacity. Herein, porous ultrathin, recyclable, and large-scale oxygen-modified borocarbonitride (O-BCN) nanosheets with the large specific surface area and hightemperature oxidation resistance have been synthesized, which possess a remarkable adsorption performance for different oils ranging from 2360 to 4370 wt %. In the case of crude oil, it showed the maximum adsorption capacity equal to 43.7 times its own weight. Moreover, the saturation adsorption for Rhodamine B (RhB) dye reached 300.8 mg/g which denotes more than 98% removal efficiency. Kinetics studies reveal the pseudo-second-order model which follows the Freundlich isotherm suggesting multilayer heterogeneous adsorption on the surface of the ultrathin O-BCN nanosheets. This novel ultrathin O-BCN material is a low-cost, highly efficient, and ecofriendly alternative to expensive, inefficient, and toxic chemicals used for wastewater treatment. KEYWORDS: Porous ultrathin O-BCN nanosheets, Adsorption, Wastewater treatment, Surface chemistry

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INTRODUCTION Over the past decade, oil spills and the release of toxic materials have made a serious negative impact on the world’s water environment.1 Crude oil and chemical leakage accidents have happened frequently during crude oil exploration and transportation processes which has resulted in a serious impact on the ecological system. One of the consequences of this issue is the extinction of different aquatic plants and animal species (e.g., Rhine river pollution event, CNOOC oil spill in Bohai Bay, and the Mexican crude oil spill).2−6 Usually burning, isolation, and collection for postprocessing are the three basic © XXXX American Chemical Society

approaches for removal of oil from water, but to provide a long-term solution to the frequent oil spill accidents, it is highly imperative to develop efficient technologies and use advanced materials to clean up water pollutants for minimizing the damage to aquatic flora and fauna. Until now, many technologies have been developed for the removal of oils and chemicals which include chemical methods Received: October 8, 2018 Revised: November 30, 2018 Published: December 31, 2018 A

DOI: 10.1021/acssuschemeng.8b05173 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Sinopharm Chemical Reagent Co. Ltd. The cooking oil, lubricating oil, pump oil, crude oil, and diesel oil were purchased from the SINOPEC (the China Petrochemical Corporation). Synthesis of Ultrathin CN. A 2 g portion of melamine was put into an alumina crucible and calcined at 550 °C for 4 h at 2 °C/min in the muffle furnace. The obtained sample was converted into a powder and calcined again at 550 °C. The final obtained sample was denoted as ultrathin CN. Synthesis of 2D Porous Ultrathin O-BCN Nanosheets. A series of 2D porous ultrathin O-BCN nanosheets were prepared at different temperatures. Different amounts of H3BO3 were dissolved in 35 mL of ethanol, and 0.1 g of ultrathin CN was added followed by mechanical agitation processing to mix H3BO3 and ultrathin CN evenly. The suspending liquid was evaporated slowly. Next, the obtained solid was converted into a powder and calcined in a tube furnace at 400, 600, 700, 800, and 900 °C at 5 °C/min for 3 h under nitrogen atmosphere, separately. The obtained samples were denoted as O-BCN-400, O-BCN-600, O-BCN-700, O-BCN-800, and O-BCN900, respectively. Different compositions of ultrathin O-BCN nanosheets [1:3, 1:2, 1:1, 2:1 (H3BO3 to ultrathin CN)] were also prepared at 800 °C at 5 °C/min for 3 h under nitrogen atmosphere. The obtained samples were denoted as O-BCN-1, O-BCN-2, OBCN-3, and O-BCN-4, respectively. Oil adsorption tests. Five different kinds of oils with different densities, including cooking oil, lubricating oil, pump oil, crude oil, and diesel oil, were tested. In detail, the exact chemical compositions of the different oils are shown in the Supporting Information. The capacity was assessed by measuring the mass of the dry ultrathin OBCN nanosheets before and after oil adsorption, and then the ratio was obtained (W wt %, the latter to the former). The samples were dipped in the oils for overnight to ensure full saturation before weighing. The weight measurements need to be completed rapidly to avoid volatilization of the oils. The commercial BN and ultrathin CN were also tested in the same way to compare with the ultrathin OBCN nanosheets. Oil Adsorption Cycling Tests. The adsorption cycling tests were performed similarly to the above method five times. The porous ultrathin O-BCN nanosheets saturated with crude oil were recycled by cleaning three times with organic solvent (methylbenzene), and then calcining in the muffle furnace at 400 °C at 5 °C/min for 2 h. Pollutants Adsorption Tests. The RhB was used as a representative pollutant. The RhB dye solutions with different concentrations were prepared by dissolving different amounts of RhB into the deionized water. In a typical adsorption test for RhB, 5 mg of O-BCN was added into 10 mL of RhB solution (100 mg/L) under constant stirring at 298 K. The remaining RhB after adsorption at different time intervals was determined by UV−vis spectroscopy at the maximum absorption wavelength of 553 nm. The adsorption capacity qt was calculated in this following equation: qt = V(C0 − Ct)/ m, where V (L) is the volume of the solution, C0 (mg/L) is the initial concentration, Ct is the concentration at different time intervals of RhB, and m (g) is the mass of the adsorbent. To evaluate the effect of the temperature on the adsorption process, 5 mg of O-BCN-2 was added into 10 mL of RhB solution with the initial concentration varying from 100 to 600 mg/L at 298, 308, and 318 K in a water bath until adsorption equilibrium. Adsorption Kinetics Fitting. The linear form of two kinetics model equations can be expressed in the following equation:

(solidifiers, coagulation, photocatalytic degradation), biodegradation, physical methods (skimmers, booms, adsorption, separation, membrane filtration), and in situ burning.7−9 Among these methods, adsorption has been widely used to remove harmful pollutants from the wastewater owing to its simple operation, effectiveness, low cost, and ecofriendliness.10,11 The development of novel efficient adsorbents by designing a new structure to accelerate the transport rate (adsorption kinetics) and enhance adsorption capacity has attracted great attention.12−14 Consequently, various sorbents have been developed for the practical application, including activated carbons, activated alumina, aluminosilicates, nanocomposites, clays, and resins.15−18 However, these traditional adsorbents due to their size-exclusion and pore-stoppage effect have poor performance especially in the case of oils.19 Recently, carbon materials with three-dimensional (3D) aerogel and foam structure have been shown to efficiently increase the adsorption of oil from wastewater.2−5 These materials achieve an appreciable performance by the development of a hydrophobic or oleophilic surface, yet the low adsorption rate, limited surface area, and requirement for complex preparation methods hinder their practical application. Two-dimensional (2D) materials have excellent adsorption performance and fast kinetics due to their characteristic advantages of designability, adjustability, and large specific surface area.20 Meanwhile, these materials also address the problems of easy pore-stoppage and size confinement effects of 3D materials. Boron nitride (BN), also known as “white graphene”, is a typical 2D material which shows high adsorption of different compounds ranging from organic contaminants to hydrogen gas.21,22 The borocarbonitride (BCN) remedies the shortcomings of BN, enabling a rich variation in surface structure and components, adsorption sites, and applications.23−25 The key advantage in the case of BCN nanosheets is that it can be critically adjusted by variation of its chemical constituent to engender more adsorption sites and surface functional groups for extensive application.26−28 The ultrathin structure of BCN brings in more active sites and higher specific surface area as compared to normal BCN or BN sheets which makes ultrathin BCN nanosheets an ideal material with highly efficient adsorption rate and capacity. Herein, a conceptual design of porous ultrathin O-BCN nanosheets on a large scale (about 5 μm wide) using CN nanosheets as a template with the high yield of 30 wt % is proposed. For the different oils, the weight adsorption capacity ranged from 2360 to 4370 wt %. The saturation adsorption capacity for Rhodamine B (RhB) dye reached 300.8 mg/g. This proof-of-concept demonstration is based on the practical use of the metal-free ultrathin O-BCN nanosheets as adsorbent due to its low cost and high adsorption properties. The porous ultrathin O-BCN nanosheets with the high specific surface area (861.34 m2/g) and rich surface chemistry show rapid adsorption kinetics and high saturation capacity. The simple operation and highly efficient adsorption properties indicate that the 2D porous ultrathin O-BCN nanosheets can be used as a promising adsorbent for industrial effluent treatment and crude oil spills.

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Pseudo-first-order kinetics

log(qe − qt ) = log qe − k1t

(1)

Pseudo-second-order kinetics

t /qt = 1/k 2qe 2 + t /qe

EXPERIMENTS

Materials. Melamine (>99.0%, CAS 108-78-1), ethanol (99.7%, CAS 64-17-5), boric acid (H3BO3) (95.0−98.0%, CAS 10043-35-3), and commercial BN (99.5%, CAS 10043-11-5) were purchased from

(2)

where qt is the adsorption capacity at time t and qe (mg/g) the equilibrium adsorption capacity. k1 (min−1) and k2 (g/(mg min)) are the rate constants of the two models, respectively. B


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Figure 1. Oil adsorption properties: (a) Adsorption capacities of the ultrathin O-BCN nanosheets for five oils. (b) Contrast of the adsorption capacities of the ultrathin O-BCN nanosheets, commercial bulk BN particles, and ultrathin CN nanosheets. (c) Picture of the equipment for oil adsorption tests with ultrathin O-BCN nanosheets. (d) After 3 min, picture of oil adsorption of ultrathin O-BCN nanosheets, inset showing the adsorption process after 30 s. (e) Picture of calcining the saturated O-BCN nanosheets in air for recycling, inset showing the sample after calcining. (f) After 3 min, picture of second oil adsorption test of the recycle nanosheets, inset showing the adsorption process after 30 s. Adsorption Thermodynamics. The van’t Hoff analysis can illustrate whether the adsorption process is spontaneous, and the various thermodynamic parameters were calculated as follows:

Adsorption Isotherm Fitting. The Langmuir isotherm model as a hypothetical model can be used to describe monolayer adsorption process. The model can be expressed in the equation:

Ce/qe = Ce/qm + 1/qmKL

(3)

where KL is the Langmuir constant. Ce (mg/L) is the concentration of RhB at the equilibrium point, and qm (mg/g) is the maximum adsorption capacity. The Freundlich isotherm model can illustrate the homogeneous adsorption and heterogeneous adsorption process. The process can be described in the equation qe = KFCe1/ n

(4)

i=1

ln Kc = ΔS /R − ΔH /RT

(7)

ΔS = (ΔH − ΔG)/T

(8)

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RESULTS AND DISCUSSION Removal of Oil. To evaluate the adsorption properties of porous ultrathin O-BCN nanosheets, five types of oils (cooking oil, lubricating oil, pump oil, crude oil, and diesel oil) were chosen as shown in Figure 1a. The 2D porous ultrathin OBCN nanosheets exhibit excellent adsorption capacities ranging from 2360 to 4370 wt %. The adsorption capacity of the porous ultrathin O-BCN is up to 30 and 43.7 times its own

N

∑ |(qe,exp − qe,cal)/qe,exp|/N × 100%

(6)

where T is the temperature (K), ln Kc = qe/Ce, ΔG is Gibbs free energy change (J/mol), ΔH is enthalpy change (J/mol), ΔS is entropy change (J/(mol K)), and R is the gas constant (8.314 J/(K mol)).

where KF is the Freundlich constant, and n is adsorption intensity. Both Ce and qe are as defined above. The average percentage error (APE) can calculate the optimal model to describe the adsorption equilibrium process. APE can be expressed in the equation APE (%) =

ΔG = − RT ln Kc

(5)

where N is the number of experimental data points. C


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Figure 2. (a) Structure of RhB. (b) Kinetics curves of adsorption of O-BCN samples at 298 K. (c) Adsorption isotherms of RhB on O-BCN samples at 298 K. (b) Adsorption isotherms of RhB on O-BCN-2 at different temperatures.

the solution color changed to light red quickly; after some time, it became a colorless solution on filtration (Figure S3b). The removal rate for RhB is higher than 98% in this adsorption process. The optimized O-BCN nanosheets exhibited high adsorption capacity up to 300.8 mg/g, which is almost an order of magnitude superior to the ultrathin CN. To further explore the adsorption kinetics of RhB by ultrathin O-BCN nanosheets, two widely accepted kinetic models, pseudo-first-order and pseudo-second-order, were used to fit the experimental data for the adsorption of RhB. The kinetic parameters are shown in Table S2. The correlation coefficients (R2) were calculated. The data indicated that the adsorption process of RhB onto O-BCN samples can be better described by the pseudo-second-order model (Figure S4a,b). Therefore, it can be concluded that the process is mainly controlled by chemical adsorption which involves electrostatic attraction between O-BCN and RhB substrates.35 The adsorption isotherms of RhB on O-BCN were initially studied at 298 K, as shown in Figure 2c. Initially, with the increase in the concentration of RhB, the adsorption capacity of O-BCN gradually increased; however, when the concentration reached a certain value, the adsorption process reached equilibrium with saturation adsorption capacity. The equilibrium adsorption capacity of O-BCN-2 reached 300.8 mg/g which is greater than BN hollow spheres and BN nanocarpets.36,37 The adsorption isotherms were fitted in Langmuir and Freundlich models which corresponded to homogeneous adsorption and heterogeneous adsorption,38 respectively. The fitting parameters are shown in Table S2. The R2 and average percentage error (APE) were compared, and it was found that the Freundlich isotherm fitted better for the samples (Figure S4c,d). Hence, the above isothermal adsorption should be understood as a heterogeneous multilayer adsorption. This could be attributed to inhomogeneous hydroxylation on the surface of the O-BCN which leads to unused heterogeneous adsorption sites. Fortunately, the optimum adsorption sites could be obtained by regulating H3BO3 proportion and hydroxylation degrees.

weight for pump oil and crude oil, respectively. The various adsorption capacities for the porous ultrathin O-BCN are due to different densities, and are different. Compared to those of the commercial bulk BN particles and ultrathin CN (Figure 1b), as well as the other materials, including the bixbyite nanowires, the graphene/FeOOH aerogel, and the hydrophobic carbon nanotubes,29−32 the adsorption capacity of porous ultrathin O-BCN is much higher, demonstrating that the substitution of carbons with boron atoms and hydroxylation lead to an improvement in its adsorption performance. The adsorption process of crude oil is shown in Figure 1c,d. When the O-BCN was dropped on the oil−water surface, its color changed to dark brown quickly indicating the adsorption rate is very fast (Figure 1d, inset at 30 s). After 3 min, the process was completed with a capacity of 4370 wt %. The saturated O-BCN nanosheets floated on the surface of the water and could be removed easily. The O-BCN nanosheets were recycled after calcining in the muffle furnace owing to its high oxidation resistance. The regenerated O-BCN preserved the same structure as the fresh material (shown in Figure 1e and Figure S1). The second oil adsorption test of cleaned OBCN nanosheets is shown in Figure 1f. The crude oil was adsorbed rapidly with only a slight decrease in the adsorption capacity (to 4020 wt %) due to the harsh conditions or some carbonaceous matter that was left. Removal of Dye from Water. Rhodamine B (RhB), an artificial dye, is a potential organic pollutant which can cause cancer in humans.33,34 RhB contains numerous π bonds in its structure, as shown in Figure 2a. The porous ultrathin O-BCN nanosheets were used to remove RhB from solution, and UV− vis spectroscopy was used to evaluate the removal rate and the adsorption capacity. For comparison, the precursor of ultrathin CN was also evaluated under the same conditions, as shown in Figure 2b and Figure S3a. The ultrathin CN showed low adsorption kinetics. The introduction of B atoms into the ultrathin CN leads to the drastic improvement of the adsorption capacity. When the O-BCN nanosheets were released into the red RhB solution and mixed to uniformity, D


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and microstructure of porous ultrathin O-BCN nanosheets were investigated using scanning and transmission electron microscopy, as shown in Figure 3. The high-magnification image in Figure 3a confirms that the sample showed a twodimensional supersize nanosheet (about 5 μm wide). The literature survey indicates that this is the largest nanoscale structure for O-BCN materials reported so far.2,32 The transverse scale sheet tends to bend, and its edges seem frizzled due to the low surface energy of the nanosheets,40 which contributes to the fast adsorption kinetics and high adsorption capacity for oil removal.5 The low-magnification TEM image in Figure 3b shows that the supersize O-BCN nanosheets are crumpled and contain entangled wrinkles. A flakelike morphology similar to few-layer graphene confirms that the folded O-BCN nanosheet layers constitute the large nanoscale structure. In particular, the porous structures could also be observed distinctly from Figure 3c−f, which provides the high surface area to accommodate organic pollutants. The porous structures originated from the process of the B atom replacing a carbon atom (the detailed description is shown in the Supporting Information). From the high-resolution TEM (HRTEM) (Figure 3g), an interlayer crystal lattice spacing of 0.33 nm can be observed, corresponding to the (002) plane, and the nanosheets are made up of almost 5 stacked layers with an overall thickness of 1.5−3 nm. From the AFM profile, the thickness of the O-BCN nanosheet structure was estimated to be ∼3 nm (Figure 3h). Such a two-dimensional ultrathin nanosheet thickness below 10 nm is desirable for enhanced adsorption capacity as the smaller thicknesses lead to the larger

The temperature influences the adsorption process which offers the basis for energy change of the process. As shown in Figure 2d and Figure S6, the adsorption capacity decreased as the temperature increased. There can be two probable reasons for this observation. First, increasing mobility of RhB with the increase of temperature covers the adsorption sites thereby preventing the adsorbed molecule from further diffusing in the internal space of O-BCN. Second, the increasing temperature destroys the surface hydroxyl structure which weakens the electrostatic attraction between O-BCN samples and RhB. The thermodynamic parameters were further estimated using the vant Hoff analysis.39 As shown in Table 1, it was concluded Table 1. Thermodynamic Parameters for the RhB Adsorption onto O-BCN-2 T (K)

ΔG (kJ/mol)

ΔH (kJ/mol)

ΔS (J/(mol K))

R2

298 308 318

−0.33 −0.74 −1.17

−1.46

−40.4

0.99

that the adsorption process was exothermal, feasible, and spontaneous. Moreover, the negative value of ΔS (−40.4 J/ (mol K)) indicates that the degrees of freedom decrease at the solid/liquid interface.19 Sorbent Structure Characterization. The ultrathin OBCN nanosheets show excellent adsorption performance for different oils and dyes which is primarily determined by their physical property and rich chemical structure. The morphology

Figure 3. (a)Scanning electron microscope (SEM) pattern. (b−f) Transmission electron microscopy (TEM) patterns. (g) HRTEM image, inset is the fast Fourier transform image. (h) Atomic force microscopy (AFM) pattern, inset showing the thickness. (i) Elemental mapping images of B, C, N, and O of O-BCN-2. E


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Figure 4. (a) X-ray diffraction (XRD) patterns. (b) Fourier transform infrared (FT-IR) spectra. X-ray photoelectron spectroscopic (XPS) characterizations of the O-BCN-2 (c) survey spectrum, (d) B 1s, (e) C 1s, and (f) N 1s.

Figure 5. (a) UV−vis absorption spectra for O-BCN samples. (b) Thermogravimetric analysis (TGA) for O-BCN-2, ultrathin CN in air from 0 to 800 °C. (c) Nitrogen adsorption−desorption isotherms. (d) Corresponding pore size distribution for O-BCN samples.

surface area at a similar or lower weight.41 The XRD patterns of ultrathin CN and a series of O-BCN nanosheets are shown in Figure 4a. Two broad peaks can be attributed to the previously reported O-BCN materials.42 A strong characteristic peak of (002) and (100) planes can be seen, respectively.23 The patterns are similar to that of ultrathin CN at 27.7°, which evidently shifts toward a lower angle, which corresponds to the layered structure stacking.43−45 It also suggests that the OBCN nanosheets have a similar layered structure like graphite. The FT-IR spectroscopy was also carried out to characterize the chemical structure of O-BCN nanosheets (Figure 4b). The two strong peaks at 780 and 1380 cm−1 correspond to the out of plain B−N−B bending vibration and the in plain B−N

transverse stretching vibration, respectively.23 The peak of B− C bonds at 1100 cm−1 could not be detected due to the influence of strong B−N vibrations.46 A small peak at 1623 cm−1 could be seen which corresponds to the sp2 C−N bond vibration.46 The FT-IR spectra confirm that the material has a BCN structure. XPS analysis was done to further investigate the structure of O-BCN nanosheets. As shown in Figure 4c, distinctly, the OBCN nanosheets are constituted by B, C, N, and the high content of O elements. Simultaneously, the element mapping of O-BCN-2 (Figure 3i) suggests that there was a uniform distribution of B, N, C, and O atoms throughout the whole selected area, also proving homogeneity of the hybridized OF


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ACS Sustainable Chemistry & Engineering BCN. The high-resolution XPS spectra of B, C, and N are shown in Figure 4d−f. The high resolution of the B 1s spectrum was deconvoluted into three peaks at 192.0, 190.6, and 189.8 eV which corresponded to B−O, B−N, and B−C bonds, respectively.41 The percentage of these bonds calculated from the B 1s spectrum was 52%, 45%, and 3%, respectively. The above results also indicate the chemical bonding of −OH groups on O-BCN surfaces to form the ultrathin O-BCN. The high hydroxylation degree increases the hydrophilicity of the sample making it conducive to the removal of water-soluble pollutants.47 In the high-resolution N 1s spectrum, the percentage of N−B bonds located at 398.2 eV was 83.7%, while the N−C bond located at 400.3 eV was 16.3%. Thus, the N−B bond was the main covalent link which leads to the hybridized O-BCN material. It also indicates an extensive existence of BN domains in the prepared samples. In the case of C 1s, the high-resolution spectrum was deconvoluted into four peaks at around 288.8, 286.4, 284.7, and 284.1 eV, which can be attributed to the carbonyl,48 C− N,49 C−C, and C−B50 bonding structures, respectively. Both C−N and C−B bonds were made up of nitrogen and boron atoms coupled with carbon atoms on the edge. In the C 1s spectrum, the percentage of the C−C bond was the maximum. Some B atoms replace carbon atoms successfully to form a C− B bond while most C domains embed in the h-BN matrix to form the ultrathin O-BCN atomic layers.46 From the elemental analysis (Table S4), with increasing H3BO3 content, the carbon content gradually decreased (from 1.68 to 0.16), indicating that the carbon atoms were substituted by boron atoms successfully. All the O-BCN samples show similar UV−vis absorption spectra (Figure 5a). Compared with ultrathin CN, the O-BCN samples generated the blue shift due to the substitution of carbon with boron which is indicative of the formation of the O-BCN structure. In addition, from the thermogravimetric analysis of O-BCN-2 and ultrathin CN in the air up to 800 °C (Figure 5b), it was concluded that the extent of oxidation inhibition effect of the O-BCN-2 was superior to the ultrathin CN. It also turns out that the chemical property changed notably from ultrathin CN to ultrathin O-BCN nanosheets. To further confirm the specific surface area of O-BCN, the N2 adsorption−desorption was performed. As shown in Figure 5c, these O-BCN samples show similar type-IV isotherms with a hysteresis loop, suggesting the typical mesoporous structure. The typical mesoporous structure did not change for any OBCN sample although the Brunauer−Emmett−Teller (BET) specific surface area (SBET) first increases (in the case of OBCN-2) and then declines (for O-BCN-3 onward). The adsorptive activity reduces with increase in H3BO3 (synthesis precursor) but is still higher than the pure ultrathin CN (264.7 m2/g).50 It can be concluded that the content of H3BO3 strongly affects the SBET and adsorptive activity. The synthesized O-BCN-2 material exhibits a large specific surface area of 861.34 m2/g, higher than those of CNF aerogel (547 m2/g) and monolithic graphene aerogels (249 m2/g) (as shown in Figure S13).4,51 The detailed structure properties of O-BCN samples are shown in Table 2. The pore diameter of O-BCN calculated from the Barrett−Joyner−Halenda (BJH) desorption branch was approximately 10 nm as shown in Figure 5d. Among others, the O-BCN-2 shows the maximum pore volume. Therefore, it can be asserted that a highly developed mesoporous structure advantageous for the adsorption of dye molecules or oils was fabricated successfully.

Table 2. Structure Properties of O-BCN Samples sample O-BCN-1 O-BCN-2 O-BCN-3 O-BCN-4

SBETa (m2/g) 772.48 861.34 282.82 141.46

± ± ± ±

6 6 6 6

Vtb (cm3/g)

Dpc (nm)

2.60 2.50 0.71 0.26

13.44 11.62 10.05 7.22

a

BET specific surface area. bTotal pore volume. cThe maximum pore diameter of the O-BCN samples calculated by the BJH method.

Relationship between Structure and Adsorption Performance. From the above results, it can be stated without any doubt that porous ultrathin O-BCN nanosheets show excellent adsorption capacity for oils and dyes. For the different kinds of oils especially crude oil, the adsorption capacity of the ultrathin O-BCN nanosheets was much higher than that of the commercial bulk BN particles and ultrathin CN. The salient features which lead to superior performance are discussed below: First, nanosheet swelling and its frizzled edges (Figure 3a) due to the low surface energy promoted the enhanced adsorption of oil.41 Compared with the XRD spectra of ultrathin CN (Figure 4a), a slight skewing of (002) toward a lower angle increases in the interlayer distance, which contributes to additional capacity and faster molecular diffusion for oil adsorption.52 On the other hand, the ultrathin O-BCN nanosheets with a porous structure and enormous specific surface area offer many active sites and sufficient space for attachment of oil molecules. Ample attachment sites on the surface increase adsorption kinetics of the adsorbate on OBCN. Compared with the FT-IR spectra of BCN-2 before and after the absorption test (Figure S1b), we cannot see any change. Therefore, it can be deduced that the physical interactions between O-BCN and the absorbed oil play a leading role. Lastly, the high hydroxylation degree on the surface of the sample increases the surface electronegativity and hydrophilicity, thus conducive to the removal of oil. The hydroxylation is essentially a result of the addition of the boron atoms into the ultrathin CN nanosheets (Scheme S1). Compared with oils adsorption, dye adsorption is easier to characterize. From the adsorption rates, kinetics curves, and adsorption isotherms of RhB on O-BCN-2, it can be concluded that the O-BCN sample exhibits excellent adsorption efficiency for dye removal. The reason for this result is similar to oil adsorption, but the role played by surface hydroxylation is very prominent. Because of the hydrophilicity of the dye, abundant hydroxylation onto the surface of the O-BCN allows easier adsorption for it than for oils. Moreover, the high degree of hydroxylation can form stable water solutions leading to the better contact area, and numerous porous nanosheets offer large space for RhB molecule attachment. Apart from those, some nitrogen vacancies existing at the edge of the nanosheets maybe offer additional adsorption sites for the dye removal (Figure S17).53,54 Last but not the least, the large specific surface area of O-BCN provides storage space for the RhB molecule.

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CONCLUSION An effective and simple way to synthesize the two-dimensional ultrathin O-BCN nanosheets with high adsorption capacity has been presented. The O-BCN nanosheets are constituted by light, abundant low-cost nonmetallic elements which possess a high specific surface area, and surface hydroxylation degrees resulting in more adsorption sites to facilitate the mass and G


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heat transfer of the adsorption process. These properties have been exploited to obtain high adsorption of oils and dyes in the wastewater. In the case of crude oil, the weight adsorption capacity reached up to 4370 wt %. The removal rate for RhB dye (100 ppm) was higher than 98% with saturation adsorption capacity of 300.8 mg/g. The adsorption process can be described by the pseudo-second-order model which followed the Freundlich isotherm suggesting multilayer heterogeneous adsorption on the surface of the ultrathin OBCN nanosheets. The synthesis of crimped nanosheets with low surface energy, tunable degree of surface hydroxylation, and molecular structure of O-BCN provides a promising method for surface modification and hybridization of parent materials, which can be further used to improve the similar type of adsorbents for water purification and treatment.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b05173. Additional characterization details and figures including cycling runs, FT-IR spectra, XRD, TEM, XPS, adsorption rates, color change, fitted kinetics models, adsorption isotherms, elemental composition, schematics, SEM patterns, UV−Raman spectra, contact angles, and EPR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone:+86-0511-88799500. Fax: +86-0511-88799500. Email: [email protected]. *E-mail: [email protected]. ORCID

Siddharth Mishra: 0000-0002-4028-0671 Yangxian Liu: 0000-0001-9069-4007 Hui Xu: 0000-0002-2823-5915 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Nature Science Foundation of China (21776118, 21476097), Key Laboratory of Tropical Medicinal Plant Chemistry of Ministry of Education-Hainan Normal University (rdyw2018002), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Hightech Research Key laboratory of Zhenjiang (SS 2018002). This study was supported by the high-performance computing platform of Jiangsu University.

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REFERENCES

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Highly Efficient Adsorption of Oils and Pollutants by Porous Ultrathin

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