Novel Graphene OxideâConfined Nanospace Directed Synthesis of

Research Article pubs.acs.org/journal/ascecg

Novel Graphene Oxide−Confined Nanospace Directed Synthesis of Glucose-Based Porous Carbon Nanosheets with Enhanced Adsorption Performance Atian Xie,† Jiangdong Dai,† Jiuyun Cui,‡ Jihui Lang,§ Maobin Wei,§ Xiaohui Dai,† Chunxiang Li,*,† and Yongsheng Yan*,† †

Institute of Green Chemistry and Chemical Technology, School of Chemistry and Chemical Engineering, and ‡School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China § College of Physics, Jilin Normal University, Siping 136000, China S Supporting Information *

ABSTRACT: Glucose-based porous carbon nanosheets (GPCNS) were synthesized by an integrated graphene oxide−confined nanospace directed KOH-activated process and were applied as adsorbent for efficient removal of sulfamethazine (SMZ). The effects of GO dosage on the structure, specific surface area, and adsorption capacity of GPCNS-x were investigated. The highest SMZ uptake of 820.27 mg g−1 (298 K) was achieved in glucose-based porous carbon nanosheets inherited from using 1% GO relative to glucose (GPCNS-1). Also, the adsorption isotherms, thermodynamics, and kinetics of SMZ onto GPCNS-1 were studied in detail. In addition, the effects of ionic strength and solution pH on the adsorption capacity of GPCNS-1 were also investigated, indicating good environmental tolerance of GPCNS-1. Furthermore, regeneration experiments showed that GPCNS-1 has good reproducibility and durability. We believe that these graphene oxide−confined nanospace directed KOH-activated process biomass-based carbon nanosheets are highly promising as absorbents in the field of environmental protection. KEYWORDS: Graphene oxide, Confined nanospace directed synthesis, Porous carbon nanosheets, Sulfamethazine, KOH activation

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INTRODUCTION The development of robust absorbent materials for the elimination and separation of pollutants from wastewater is of great significance to solve environmental issues, because the problem of severe water pollution has received global-scale attention.1,2 Antibiotics (such as sulfamethazine, tetracycline, etc.) are considered as toxic pollutants due to their variety of potentially adverse effects. The removal of antibiotic drugs using an adsorption approach is considered to be low cost and versatile and represents one of the most widely used methods.3 In recent decades, all kinds of absorbents including carbon nanotubes (CNTs),4 activated carbon,5 halloysite nanotubes,6 and mesoporous silica7 have been used in the adsorption of antibiotic drugs from wastewater. Especially, carbon materials have shown high efficiency for the removal of organic pollutants owing to their large surface area and chemical stability and, thus, have been extensively applied in water purification. However, these carbon materials show some drawbacks of sluggish adsorption kinetics and limited adsorption capacity for the removal of organic contaminants. Recently, two-dimensional carbon nanosheets (CNS) have attracted tremendous research interest for their large surface area, developed porous structure, excellent chemical reactivity, and good stability resulting in application in many fields, such © 2017 American Chemical Society

as energy storage and conversion, oxygen reduction reaction, drug delivery, electrochemistry, and environmental remediation.8 Generally, carbon allotropes could be classified as 0D (carbon dots), 1D (carbon nanotubes, carbon fibers), 2D (carbon nanosheets and graphene oxides), and 3D (carbon aerogels) materials according to their structure.9 As we known, the application of carbon materials mainly depends on the structure and surface chemical composition properties. Notably, 2D carbon materials, especially porous carbon nanosheets (PCNS), have considerable advantages such as high surface-to-volume ratio and excellent electronic transportation properties, thus resulting in increasing research into energy storage and conversion.10 To the best of our knowledge, few works have demonstrated the advantages of PCNS in adsorption of antibiotics. Bulk carbon materials often exhibit high specific surface area but low adsorption capacity and poor site accessibility to adsorbates.11 However, the much smaller thickness of PCNS endows fast adsorption kinetics and a high utilization efficiency of the overall adsorption sites and specific surface area due to low masstransfer resistance within PCNS. In addition, aggregation or Received: August 22, 2017 Revised: September 23, 2017 Published: October 25, 2017 11566

DOI: 10.1021/acssuschemeng.7b02917 ACS Sustainable Chem. Eng. 2017, 5, 11566−11576

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ACS Sustainable Chemistry & Engineering

nanoconfined template for fabricating other 2D materials.16 Yuan et al. demonstrated that functionalized graphenes are promising templates for the construction of 2D composite materials.17−19 Interestingly, it is not necessary to remove GO from the carbon product because it is simultaneously converted to carbon materials.20 Herein, we demonstrated a novel graphene oxide−confined nanospace directed synthesis of glucose-based porous carbon nanosheets (GPCNS), as shown in Scheme 1. In this synthesis strategy, biomass glucose is used as carbon precursor, GO nanosheets are adopted to create a 2D-confined space to achieve the construction of carbon nanosheets, and alkali activation is conducted to generate a porous structure, which offer several advantages as an adsorbent: 2D porous carbon nanosheets not only enhance the diffusion rate of adsorbate, but also improve the utilization of the overall adsorption sites. With these inspiring merits, this GPCNS demonstrates outstanding adsorption kinetics and excellent adsorption compared to bulk porous carbon. Moreover, this facile fabrication strategy has been achieved using tiny GO dosage and without a washing or etching template process, which may be very suitable for scalable and cost-efficient production of GPCNS.

restacking does not occur in PCNS thanks to its weaker intersheet van der Waals attraction, and thus, the unique properties of individual sheets will not be significantly affected.1 Figure 1 illustrates the distribution of the effective adsorption

Figure 1. Distribution of the effective adsorption sites in bulk carbon materials and PCNS.

sites in bulk carbon and PCNS, respectively. We assume that these adsorbates can reach within x nm in the bulk carbon materials with a scale of a. Thus, the effective volume of bulk carbon materials that can adsorbed adsorbates is [a3 − (a − 2x)3]. If the PCNS has a scale of 2x nm, all of the adsorption sites are all effective for adsorbates. As a consequence, the binding capacity, binding kinetics, and site accessibility make PCNS an ideal absorbent for environmental remediation. On the other hand, many 3D porous carbon materials such as activated carbons were easily obtained via simple pyrolysis and activation of biomass precursor; depressingly, the 2D carbon materials were scarcely derived from biomass, and the expensive or complicated synthesis procedure of PCNS seriously limited its large-scale applications.9 Thus, the development of simple and versatile synthesis approach of PCNS using bioresources is highly desired. Recently, a 2D spatial confinement strategy has been developed for synthesizing 2D carbon materials.12,13 However, the consumption, washing, or etching of templates is usually timeconsuming and hard sledding. For instance, Zhou et al. reported highly crystalline MoS2 nanosheets with a few layers anchored on 3D porous carbon nanosheets networks via a cubic NaCl particle 2D spatial confinement strategy.14 Wang et al. prepared 3D porous carbon nanosheets with oriented and interconnected nanostructure using zinc layered hydroxide nitrate as a layered template and providing a nanospace to confine the carbonization process of gallic acid.15 Consequently, the nanoconfined template is critical for the 2D spatial confinement strategy. Graphene oxide (GO) has a typical layered structure, and its unique structure, high surface area, high flexibility, and accessible interface designs make GO a popular candidate for a

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EXPERIMENTAL SECTION

Preparation of GO. GO was fabricated according to a modified Hummers method.21 Typically, natural graphite powder (1 g), NaNO3 (2.5 g), and H2SO4 (30 mL) were added in a 250 mL three neck flask under magnetic stirring and placed into ice water for 20 min. Then, KMnO4 (4 g) was slowly added in the above mixture. After stirring for 120 min, the mixture was heated up to 35 °C for 30 min. Then, distilled water (40 mL) was added dropwise into the above mixture, and it was heated up to 98 °C for 40 min. After cooling, H2O2 (10 mL) was slowly added to the mixture. The resultant products were alternately centrifuged and washed with HCl (10%) and deionized water several times to about neutral drying in a vacuum oven at 60 °C for 24 h. Then, the appropriate amount of GO was added into deionized water to prepare the 5 mg mL−1 dispersion for further use. Preparation of GPCNS. Glucose (2 g) was added in deionized water (10 mL), and then, the designed amount of GO dispersion (2, 4, 12, 20 mL) was added to the glucose solution with ultrasonic treatment for 30 min. The mixed solution was dried at 80 °C to get solid mixture (G-GO). After that, right amount of the G-GO was treated at 500 °C in a tubular furnace for 2 h with a ramp of 5.0 °C min−1 under N2 flow, and the carbonized G-GO (CG) was obtained after cooling naturally down to room temperature. Subsequently, CG and KOH (mass ratio of CG:KOH = 1:4) were evenly ground, which was treated at 850 °C and maintained for 1 h with a ramp of 5.0 °C min−1 under N2 flow. Then, the resultant was washed with about 10 wt % HCl. The glucose-based porous carbon nanosheets (GPCNS) was obtained after vacuum filtrating, rinsing thoroughly with deionized water to neutral, and dring at 60 °C for 12 h. The as-prepared GPCNS by different GO dosages were labeled as GPCNS-x, such as GPCNS-1, which indicated that the mass ratio of glucose to graphene oxide was 100:1. For control, pure glucose porous carbon (labeled as GPCNS-0)

Scheme 1. Schematic Diagram of the Fabrication Process of GPCNS

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ACS Sustainable Chemistry & Engineering was prepared using same parameters except that no GO dispersion was added.

bulk structure, the particle size is up to tens of micrometers. The morphology of GPCNS-0.5 has obviously changes that the bulk structure turned into sheet structure. As the GO dosage increases to 1%, GPCNS-1 shows a lamellar structure with thinner thickness. However, there are no obvious changes in sheet structure when the GO dosage continues to increase. The results show that the addition of a small amount of GO can effectively induce the synthesis of carbon nanosheets, which is more conducive to the activation of KOH and the adsorption of SMZ. The microstructure of GPCNS-0, GPCNS-1, and GPCNS-5 was further investigated by TEM, and the results are shown in Figure 5. The GPCNS-0 (Figure 5a) shows bulk structure, and the higher magnification indicated the presence of porous structures. Figure 5c shows that GPCNS-1 is plate-like structure, consistent with SEM observations. From the enlarged view (Figure 5d), the GPCNS-1 shows thin and porous structures, which proves that the pore structure was formed during the activation of KOH. The GPCNS-5 (Figure 5e, f) also shows a sheet and porous structures similar to GPCNS-1. The observation from TEM images further confirmed the successful preparation of porous carbon nanosheets by graphene oxide−confined nanospace directed KOH-activated process. This unique 2D porous structures are beneficial for the adsorption of SMZ molecules onto GPCNS. The N2 adsorption−desorption isotherm test was used to further analyze the porosity characteristics of GPCNS-x, and the results are shown in Figure S1a. The GPCNS-x have higher adsorption capacity in the low pressure region, which indicates the existence of large micropores. Meanwhile, the hysteresis phenomenon caused by capillary condensation is observed, which shows the signal of typical mesopores.22 By the BET method, the specific surface area of GPCNS-x is about 3000 m2 g−1. With the increase of GO dosage, the specific surface area increased slightly, GPCNS-1 has maximum of 3145.7 m2 g−1, and then decreased gradually. Figure S1b shows the pore size distribution analysis by DFT method. The pore size is centered within 5 nm, which are mainly micropores and small mesopores. Notably, the pore size do not differ significantly with the increase of GO dosage, and the average pore size is about 2.2 nm. Detailed pore characteristic parameters are listed in Table S1. The GPCNS-1 has a larger total pore volume of 1.7536 cm3 g−1, which facilitates the diffusion of SMZ molecules greatly accelerating the adsorption rate. The lattice and phase structures of CG and GPCNS were analyzed by XRD and Raman, as shown in Figure 6. From the XRD spectra, there is the peak in the range of 22−26° corresponding to the (002) crystal surface, which shows the lower crystallinity of CG and GPCNS. The weaker peak in the range of 41−45° corresponds to the diffraction peak of the (100) and (101) crystalline surface, indicated that CG and GPCNS have a

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RESULTS AND DISCUSSION Optimization of Conditions. The influences of GO dosage on adsorption capacity and specific surface area of GPCNS-x were first investigated, and the results are shown in Figure 2.

Figure 2. Effects of GO addition on equilibrium adsorption capacity and specific surface area of GPCNS-x (C0 = 150 mg L−1, T = 298 K).

Interestingly, the addition of GO has a trivial impact on specific surface area but has a significant impact on adsorption capacity. Notably, GPCNS-x with high specific surface area show slightly higher adsorption capacity, indicated specific surface area has certain enhancement effect to the adsorption due to its more adsorption sites. Differently, the GPCNS-x by using graphene oxide−confined nanospace directed synthesis display significant enhanced in adsorption capacity. This is because more available adsorption sites were exposed in carbon nanosheets than bulk carbon. The SMZ adsorption diagrams of carbon nanosheets and bulk carbon are shown in Figure 3. The SMZ molecules are difficult to diffuse into the adsorption sites within bulk carbon adsorbent resulting poor adsorption. Interestingly, carbon nanosheets greatly reduce the mass transfer resistance of SMZ because of their unique 2D structure. All the adsorption sites within carbon nanosheets are almost available leading to an excellent adsorption capacity. Here, GPCNS-1 shows optimal adsorption capacity; therefore, we chose GPCNS-1 as the research model in the batch adsorption experiments, and their physical and chemical properties were studied in detail. Notes: If noy otherwise specified, GPCNS refers to the GPCNS-1. Characterizations. The morphology evolution of GPCNS-x was observed by SEM. Figure 4 shows the SEM images of GPCNS-0 (a, b), GPCNS-0.5 (c, d), GPCNS-1 (e, f), GPCNS-3 (g, h), and GPCNS-5 (i, j). The GPCNS-0 shows irregular

Figure 3. SMZ adsorption diagram of carbon nanosheets and bulk carbon. 11568


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Figure 4. SEM images of GPCNS-0 (a, b), GPCNS-0.5 (c, d), GPCNS-1 (e, f), GPCNS-3 (g, h), and GPCNS-5 (i, j).

certain graphitization structure.23 Differently, the background of the GPCNS spectrum increases in the range of 10−39°, which may be caused by the scattering of X-ray by a large number of pore structures.24 The above results demonstrated that GPCNS has a certain graphitization structure and a large number of pore structures. Figure 6b shows the Raman spectra of CG and GPCNS. The spectra have two distinct peaks at 1591 and 1343 cm−1 assigned to G and D peaks, respectively. The intensity ratio of G and D

peak (IG/ID) can assess the graphitization degree of carbon materials, and the higher values mean the higher graphitization degree. In order to better study the structural properties of CG and GPCNS, the Raman spectra are divided into four subpeaks located at 1163, 1332, 1499, and 1590 cm−1 using a Gauss fitting method, namely, D4, D1, D3, and G peaks. Wherein, D1 and D4 represent disordered graphitic carbon, D3 represents amorphous carbon, and G represents an ideal graphitic sp2 carbon.25 The detailed percentage content is shown in Table S2. 11569


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Figure 5. TEM images of GPCNS-0 (a, b), GPCNS-1 (c, d), and GPCNS-5 (e, f).

Figure 6. XRD patterns (a) and Raman spectra (b) of CG and GPCNS.

After activation, the content of amorphous carbon is reduced from 40.72% to 24.89%, and the disordered graphitic carbon increases significantly, while the ideal graphitic sp2 carbon decreases slightly. The reason is that, during the activation reaction, KOH was reacting with amorphous carbon to produce gases (K steam, H2, CO, etc.), which increases the defect degree of the ideal graphitic sp2 carbon. The detailed reaction process is as follows:26 6KOH + C ↔ 2K + 3H 2 + 2K 2CO3

K 2CO3 + C ↔ K 2O + 2CO

K 2CO3 ↔ K 2O + CO

(T > 700 °C)

2K + CO2 ↔ K 2O + CO

K 2O + C ↔ 2K + CO

(T > 700 °C)

(T > 700 °C)

(T > 800 °C)

The surface chemical composition of GPCNS was analyzed by XPS, and the results are shown in Figure S2. The survey scans spectrum of GPCNS (Figure S2a) shows two peaks at

(T > 400 °C) 11570


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Figure 7. Linear fitting of Langmuir (a) and Freundlich model (b). (c) Nonlinear fitting of Langmuir and Freundlich model. (d) Plot of ln K0 vs 1/T.

Adsorption Isotherms. The adsorption isotherm mainly describes the distribution of the solid and liquid phases under the adsorption−desorption equilibrium. The Langmuir isotherm model assumed that the adsorbate is adsorbed on the adsorbent surface in the form of a monolayer, while the Freundlich isotherm model is multilayer adsorption. Fundamentally, the adsorption isotherm shows how the adsorbate and adsorbent interact, and thus, study of the isotherm model is useful for optimizing adsorbent use. The linear equations of Langmuir and Freundlich isotherm models are as follows:36,37

284.5 and 532.1 eV corresponding to C 1s and O 1s, respectively, indicating that GPCNS is mainly composed of C and O elements. Figure S2b and c shows the high-resolution XPS spectra of C 1s and O 1s by the Gauss fitting method, respectively. From Figure S2b, the C 1s spectrum can be divided into three sub peaks at 283.4, 284.73, and 286.3 eV, corresponding to C−C and CC, C−O, and CO,27−29 respectively. In Figure S2c, three peaks located at 527.3, 530.05, and 533.2 eV can fit the O 1s spectrum well, which corresponds to CO, H−O, and C−O−C,30,31 respectively. XPS analysis indicates that the surface of GPCNS contains a certain amount of oxygencontaining functional groups, which is more conducive to the adsorption and binding of SMZ molecules. To detect the functional groups on the GPCNS before and after adsorption of SMZ, the FT-IR analysis was performed. From Figure S3, the obvious peaks at around 1645 and 3430 cm−1 were assigned to the stretching vibration of CO and −OH, and the adsorption peaks around 1186 and 1036 cm−1 were attributed to the stretching vibrations of C−O.32 After SMZ sorption, The stretching vibrations of −OH and C−O were weakened, indicating the hydrogen bonding interaction might be formed between the −NH and −NH2 on target molecules and O-containing groups on GPCNS.33,34 Moreover, the weak peaks at around 1468 and 1580 cm−1 attributed to the skeletal vibration of aromatic CC bonds were disappeared after adsorption of SMZ, which may be attributed to the π−π interaction between the benzene rings of SMZ and the hexagonal skeleton of GPCNS.35 The signals from the FT-IR results demonstrated the hydrogen bonding between −OH and −NH or −NH2. Moreover, the π−π interaction between SMZ and the benzene rings within the GPCNS. Additionally, the EDS analysis of GPCNS before and after adsorption of SMZ are shown in Figure S4. The presence of N and S element after adsorption further confirmed the SMZ were really adsorbed onto the surface of GPCNS.

Ce C 1 = + e Qe KLQ m Qm

ln Q e = ln KF +

(1)

⎛1⎞ ⎜ ⎟ln C e ⎝n⎠

(2)

here, Qm (mg g−1) represents maximum monolayer adsorption capacity, and KL (mL mg−1) is the Langmuir constant. KF [(mg g−1)(L mg−1)1/n] and n are the constants of the Freundlich isotherm model. The adsorption isotherm data are analyzed by Langmuir and Freundlich isotherm models. The fitting lines and parameters are presented in Figure 7a−c and Table S3. Clearly, from Figure 7a and b, the Langmuir isotherm model (R2 > 0.99) can better describe the adsorption behavior than the Freundlich isotherm model, which showed that the adsorption of SMZ onto the GPCNS surface is likely to be monolayer adsorption. Figure 7c shows the nonlinear fitting of the Langmuir and Freundlich isotherm adsorption models. The fitting values by Langmuir are close to the experimental values, while the Freundlich fitting values deviate from the experimental values. Adsorption isotherm experimental results show that the maximum adsorption capacity of GPCNS to SMZ is up to 820.27 mg g−1 at 298 K, and the comparison of previously reports on adsorbents for the adsorption 11571


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where, R (8.314 J mol−1 K−1) is the gas constant, and T (K) is the temperature. The values of ΔHθ and ΔSθ can be derived by using the intercept and slope of the ln K0 vs 1/T graph (Figure 8d). When the concentration of SMZ in solution gradually approaches 0, the values of K0 can be obtained by fitting ln(qs/Cs) vs qs. Wherein, qs (mmol g−1) is the adsorption capacity of the SMZ per unit gram of the GPCNS, and Cs (mmol mL−1) is the equilibrium concentration of SMZ. The thermodynamic fitting diagram is shown in Figure 7d, and the values of K0, ΔHθ, ΔSθ, ΔGθ, and the related parameters are shown in Table S3. The ΔHθ of the adsorption process is 1.515 kJ mol−1, which means that the adsorption process is endothermic. In addition, the ΔSθ of the adsorption process is a positive value, which indicates the increase of the freedom degree of the solid−liquid interface in the system. The ΔGθ is negative, which proves that the adsorption of SMZ onto the GPCNS surface is spontaneous at the studied temperature. Adsorption Kinetics. The adsorption kinetic properties of SMZ adsorbed onto GPCNS were also investigated. The kinetics curves of time dependence are shown in Figure 8a; the adsorption rate is faster at the initial stage (the first 10 min) and then decreases gradually. At last, the adsorption reaches equilibrium at about 60 min demonstrating excellent kinetic performance. In order to further study the kinetic properties, the pseudo-firstorder and pseudo-second-order kinetic models were used to fit the kinetic adsorption data. The liner equations are given as follows:43,44

Table 1. Comparison of Adsorption Capacity of GPCNS and Other Adsorbents for Sulfonamide Antibiotics adsorbents

adsorbates

N-TiO2/AC

sulfamethazine

P-MWCNTs

sulfamethazine

H-MWCNTs

Qm (mg g−1) 201 ± 2 24.78

T (K)

refs

298 ± 2

38

298

39

13.31

MOPAC

sulfamethoxazole

298

40

MACC

sulfamethoxazole

159

42.5

298

41

GPCNS

sulfamethazine

820.27 845.27 895.27

298 308 318

this work

of sulfonamide antibiotics is shown in Table 1.38−41 The results indicated that the GPCNS has excellent adsorption capacity for SMZ, which proved that it has great advantages both in cost and adsorption performance and shows good application prospects. Adsorption Thermodynamics. In order to evaluate the adsorption thermodynamic properties, adsorption tests were carried out at different temperatures. Figure 7c shows that the adsorption amount increases from 820.27 to 895.27 mg g−1 with the increases of temperature, indicating that the adsorption process of SMZ onto GPCNS is affected by temperature. The Gibbs free energy change (ΔGθ), standard enthalpy change (ΔHθ), and standard entropy change (ΔSθ) were calculated by the following formulas:42

ΔG θ = −RT ln(K 0) θ

as v Q = s e ae ve Ce

ln(Q e − Q t) = ln Q e − K1t

θ

ΔS H ln K 0 = − R RT K0 is defined as follows: K0 =

(3)

(6)

t 1 t = + Qt Qe K 2Q e2

(4)

−1

(7) −1

−1

here, k1 (min ) and k2 (g mg min ) are adsorption rate constants of the pseudo-first-order and pseudo-second-order kinetic models, respectively. The linear fitting by pseudo-first-order

(5)

Figure 8. Adsorption kinetics (a) and the linear-fitting kinetics by pseudo-first-order (b) and pseudo-second-order (c) rate model. (d) UV−vis absorption spectra of SMZ solution with 50 mg L−1. 11572


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(150 mg L−1) to study the effect on adsorption capacity of GPCNS. From Figure S8a, the adsorption capacity of GPCNS to SMZ is almost unchanged at low concentration of NaCl solution (0.05 and 0.1 M). While the adsorption amount of GPCNS to SMZ decreases gradually at high ionic strength (>0.1 M). Presumably, the addition of other ions can disturb the interaction between GPCNS and SMZ, leading to a decrease in the adsorption capacity. To illustrate this, the ion strength effect on the zeta potential of GPCNS was measured. As shown Figure S8b, the surface of GPCNS is positive when the concentration of NaCl solution is low 0.12 M, and the electronegativity increases with increasing concentration of NaCl solution. In addition, the SMZ is negatively charged in natural solution medium (pH is about 5.8). Thus, the results demonstrated that high ionic strength (>0.12 M) is unfavorable to the adsorption process due to electrostatic repulsion. Notably, although the ionic strength has a certain effect on the adsorption of SMZ onto GPCNS, the adsorption capacity of GPCNS still reaches 557.05 mg g−1 at higher ionic strength. The results show that GPCNS has certain abilities of anti-ion interference. Adsorption Stability and Recyclability. To investigate the adsorption stability of GPCNS, the desorption tests were conducted. The GPCNS (4 mg) after adsorption in adsorption isotherm by different concentrations of SMZ (50−300 mg L−1) were immersed in deionized water (20 mL) for 12 h at 298 K, respectively. The Figure S9a shows the UV−vis absorption spectra of the leaching solution. Notably, the leaching concentration of SMZ increases gradually with the increase of used concentration in adsorption processes. In addition, Figure S9b shows the desorption ratio for different concentrations used in the adsorption process. When the concentration used is 200 mg L−1, the desorption ratio is just about 3%. When the concentration used is 300 mg L−1, the desorption ratio is about 6%. This may be because GPCNS with more SMZ in the adsorption system has bigger surface energy, and thus, it will release SMZ molecules to minimize surface energy. Importantly, the low desorption ratio demonstrated that the adsorption of SMZ onto GPCNS is stable enough. Moreover, the SEM images after adsorption were observed. From Figure S10, the structure of GPCNS was not obviously damaged and remained in the defined sheet structure. This observation indicated that the robust sheet structure of GPCNS enables a long lifespan. The saturated GPCNS was immersed in 0.2 M NaOH solution to release SMZ molecules from the surface of the GPCNS. The GPCNS after desorption was reused to evaluate the regeneration performance of GPCNS. The adsorption−desorption experiments were carried out 5 cycles, and the experimental results are shown in Figure S11. The adsorption capacity of GPCNS decreased in every cycle, and the adsorption capacity still reached 512.95 mg g−1 after the fifth cycle. The excellent regenerability makes GPCNS a promising adsorbent for wastewater treatment. Proposed Adsorption Mechanisms. The surface functional groups, pore structure, or the mass transfer process of solute in the solution can affect the adsorption process. Figure 9 shows the schematic diagram of adsorption mechanism for GPCNS to SMZ. First of all, the surface of GPCNS has a certain graphitization structure (confirmed by XRD and Raman results), and the structure of SMZ contains an aromatic structure; thus, GPCNS and SMZ molecules can form π−π interactions.48 Second, SMZ molecules exhibit various modes of existence at different solution pH, the surface of GPCNS contains oxygencontaining functional groups such as −OH, CO, −COOH,

and pseudo-second-order kinetic models is shown in Figure 8b and c, and the related fitting parameters are shown in Table S3. The results indicated that the pseudo-second-order kinetic model can describe the overall adsorption behavior (R2 > 0.99), and the linearity of pseudo-first-order equation (R2 < 0.98) is not ideal enough. Therefore, the results show that the adsorption rate is mainly controlled by chemisorption. Figure 8d shows UV−vis absorption spectra of 50 mg L−1 SMZ solution at different adsorption times. The concentration of SMZ decreases gradually with the increase of time, and the residual concentration of SMZ is almost zero after the adsorption lasts 120 min. In order to better study the kinetic properties of the adsorption process, kinetic adsorption data were further analyzed by intraparticle diffusion model. The equation is as follows:45

Q t = K i1/2 + C

(8)

here, Ki (mg g−1 min−1/2) and C are the intraparticle diffusion coefficient and intercept. The fitting lines and related fitting parameters of the intraparticle diffusion model are shown in Figure S5 and Table S4, respectively. According to the intraparticle diffusion model, if the Qt vs t1/2 is a straight line through the origin, intraparticle diffusion is the only rate-controling step. Instead, if it is a multilinear relationship, this indicates that there are multiple rate-controling steps. As shown in Figure S5, the fitting lines are multilinear (three), and the slope decreases gradually, which indicates that this adsorption process includes three rate-controling steps: first, the SMZ molecules diffuse rapidly from the bulk solution to the GPCNS surface; second, SMZ molecules diffuse from the adsorbent surface into the micropores and mesopores channels; third, adsorption tends to reach an equilibrium. The porous and unique 2D structure of GPCNS greatly reduces the mass transfer resistance of SMZ molecules resulting in a rapid adsorption rate. Effect of Solution pH. The adsorbent and adsorbate exhibits different matter forms in different pH solutions, thus the solution pH has a great influence on the adsorption process. As shown in Figure S6a, the forms of SMZ in pH < 2.07, 2.07 < pH < 7.49, and pH > 7.49 are mainly SMZ+, SMZ0, and SMZ−, respectively.46 Figure S6b shows the adsorption capacity of GPCNS increased as the pH increased from 2 to 4; however, further increasing pH led to a continuous decrease of adsorption capacity. The pH effect on the zeta potential of GPCNS is exhibited in Figure S7, the point of zero charge (pHpzc) of GPCNS is about 3.7, thus the surface of GPCNS was negatively charged in the pH range of 3.7−8 and positively charged in the pH range of 2−3.7.47 Also, the SMZ mainly existed in SMZ0 at pH = 5.46 As SMZ− became prevalent at pH ranging from 5 to 8, while the SMZ mainly existed in the SMZ+ at the pH range from 2.07 to 5. Thus, these adsorption processes were divided into three parts: (1) When pH was at 2−3.7, the adsorption was inhibited due to electrostatic repulsion between positively charged GPCNS and SMZ+. (2) When pH was at 3.7−5, the electrostatic interaction of negatively charged GPCNS and SMZ+ is beneficial for SMZ adsorption, and GPCNS shows the best adsorption capacity at pH = 4. (3) When pH increased to 8, the electrostatic repulsion of positively charged GPCNS and SMZ− will inhibit adsorption leading to decline of adsorption capacity. The above results show that the adsorption process of GPCNS onto SMZ involves electrostatic interaction. Effect of Ionic Strength. The ionic strength of solution also affects the adsorption process. Experiments were carried out using mixed solutions of NaCl (0.05−0.5 M) and SMZ 11573


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Figure 9. Proposed mechanism for SMZ adsorption onto GPCNS.

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and so on (confirmed by XPS and FT-IR results), and electrostatic interaction may be involved in the adsorption process. In addition, the functional groups of amino, sulfonamide and pyrimidine rings in the molecular structure of SMZ can interact with −OH, CO, and −COOH onto the surface of GPCNS in a water environment.34 The detailed analysis has been deeply discussed in the Characterization and Effect of Solution pH sections. It should be noted that the high specific surface area and sheet structure of GPCNS provides a large number of available adsorption active sites for the above SMZ adsorption, which is the decisive factor of the adsorbent adsorption capacity.

Corresponding Authors

*E-mail: [email protected] (C.L.). *E-mail: [email protected]. Tel.: +86 0511-88790683. Fax: +86 0511-88791800 (Y.Y.). ORCID

Jiangdong Dai: 0000-0001-5268-0755 Chunxiang Li: 0000-0003-3775-4167 Yongsheng Yan: 0000-0001-7083-3292 Notes

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The authors declare no competing financial interest.

CONCLUSIONS To get rid of SMZ from wastewater, the GPCNS was synthesized using glucose as a biomass precursor, GO as structure directing agent via confined nanospace directed strategy, and a KOH activation method. The results show that the addition of a small amount of GO can effectively induce the synthesis of carbon nanosheets from bulk carbon. The GPCNS displays an excellent adsorption capacity of 895.27 mg g−1 for SMZ at 318 K, and a fast adsorption rate attribute to porous and flaky structure. In addition, GPCNS has a certain ability to resist environmental interference such as ionic strength and solution pH. Importantly, GPCNS also has excellent regenerability affording for long-term use. This work shines light on the potential of fabricating renewable porous carbonaceous nanosheet materials directly from biomass resources as an advanced adsorbent in environmental pollution cleanup.

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AUTHOR INFORMATION

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ACKNOWLEDGMENTS

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REFERENCES

The authors are grateful for financial support from the National Natural Science Foundation of China (51608226, 21576111, U1510126) and Natural Science Foundation of Jiangsu Province (BK20140534, BK20160501 and BK20170532).

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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.7b02917. Chemicals and materials; instruments; batch adsorption; N2 sorption; XPS; FT-IR analysis and EDS analysis; fitting of the intraparticle diffusion model; influence of pH; zeta potential of GPCNS; effect of ionic strength; UV− vis absorption spectra of leaching solution; SEM images of GPCNS after adsorption; reusability of GPCNS; porosity characteristics of GPCNS-x; detailed percentage of carbon species; Langmuir and Freundlich isotherm model, adsorption thermodynamics and adsorption kinetics parameters; parameters of intraparticle diffusion model (PDF) 11574


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