Novel Graphene OxideâConfined Nanospace Directed Synthesis of

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Novel graphene oxide-confined nanospace directed synthesis of glucosebased porous carbon nanosheets with enhanced adsorption performance Atian Xie, Jiangdong Dai, Jiuyun Cui, Jihui Lang, Maobin Wei, Xiao-Hui Dai, Chunxiang Li, and Yongsheng Yan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02917 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

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ACS Sustainable Chemistry & Engineering

Novel graphene oxide-confined nanospace directed synthesis of glucose-based porous carbon nanosheets with enhanced adsorption performance

Atian Xiea, Jiangdong Daia, Jiuyun Cuib, Jihui Langc, Maobin Weic, Xiaohui Daia, Chunxiang Lia* and Yongsheng Yana* a

Institute of Green Chemistry and Chemical Technology, School of Chemistry and Chemical Engineering,

Jiangsu University, Zhenjiang 212013, China b

School of Materials Science and Engineering , Jiangsu University, Zhenjiang 212013, China

c

College of Physics, Jilin Normal University, Siping 136000, China

*Corresponding Author E-mail: [email protected], [email protected] Tel: +86 0511-88790683; Fax: +86 0511-88791800

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 by 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, 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 this graphene oxide-confined nanospace directed-KOH activated process biomass-based carbon nanosheets is highly promising as an absorbent in 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 the environmental issues, because the severe water pollution has received global scale attention.1, 2 Antibiotics (such as sulfamethazine, tetracycline etc.) are considered as potential toxicological pollutants due to their a variety of potential adverse effects. The removal of antibiotic drugs using adsorption approach is considered as a low-cost and versatile, and represents one of the most widely used methods.3 In 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 show high efficiency for the removal of organic pollutants owing to 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 contaminations. 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 result in the application in many fields, such as energy storage and conversion, oxygen reduction reaction, drug delivery, electrochemistry, and environment remediation.8 Generally, carbon allotropes could be classified 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 depend 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 increasingly researches of energy storage and conversion.10 To the best of our knowledge, few works has been demonstrated 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 mass-transfer resistance within PCNS. In addition, the aggregation or restacking not occurs in PCNS thanks to its weaker intersheet Van der Waals attraction, and thus the unique properties of individual sheets will not be significantly recession.1 Figure 1 illustrates the distribution of the effective adsorption 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


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[a3 - (a - 2x)3]. If the PCNS with a scale of 2x nm, all of adsorption sites are all effective for adsorbates. As a consequence, the binding capacity, binding kinetics, and site accessibility make PCNS an ideal absorbents for environmental remediation.

Figure 1 The distribution of the effective adsorption sites in bulk carbon materials and PCNS 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, 2D spatial confinement strategy has been developed for synthesizing 2D carbon materials.12, 13 However, the consumption, washing or etching of templates is usually time-consuming and hard sledding. Such as, Zhou et al. reported a highly crystalline MoS2 nanosheets with few layers anchored on 3D porous carbon nanosheets networks via cubic NaCl particles 2D spatial confinement strategy.14 Wang et al. prepared a 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 nano-confined template is critical for 2D spatial confinement strategy. Graphene oxide (GO) has a typical layered structure, their unique structure, high surface area, high flexibility, and accessible interface designs make GO a popular candidate nano-confined template for fabricating other 2D materials.16 Yuan et al. demonstrated the 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 GO were simultaneously converted to carbon materials.20



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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 is adopted to create a 2D-confined space to achieve the construction of carbon nanosheets, and alkali activated is conducted to generate porous structure, which offer several advantages as adsorbent: 2D porous carbon nanosheets not only enhanced the diffusion rate of adsorbate, but also improved the utilization of the overall adsorption sites. With this inspiring merits, this GPCNS demonstrates an 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 washing or etching template process, which may be very suitable for scalable and cost-efficient production of GPCNS.

Scheme 1 Schematic diagram of fabrication process of GPCNS Experimental section Preparation of GO GO was fabricated according to modified Hummers method.21 Typically, natural graphite powder (1 g), NaNO3 (2.5 g) and H2SO4 (30 mL) was added in 250 mL three neck flask under magnetic stirring and putted 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 oC for 30 min. Then, distilled water (40 mL) was dropwise added into the above mixture, and heated up to 98 oC for 40 min. After cooling, H2O2 (10 mL) was slowly added the mixture. The resultant products were alternately centrifuged and washed with HCl (10%) and deionized water several times to about neutral drying in vacuum oven at 60 oC 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 designed amount GO dispersion (2、4、12、 20 mL) were added to the glucose solution with ultrasonic treatment for 30 min. The mixed solution was dried at 80 oC to get solid mixture (G-GO). After that, right amount of the G-GO was treated at 500 °C in a tubular


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furnace for 2 h with a ramp of 5.0 oC min-1 under N2 flow, and the carbonized G-GO (CG) was obtained after naturally down to room temperature. Subsequently, CG and KOH (mass ratio of CG:KOH=1:4) were evenly grinded, which was treated at 850 °C and maintained 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 and rinsing thoroughly with deionized water to neutral, and dried at 60 °C for 12 h. The as-prepared GPCNS by different GO dosage was labeled as GPCNS-x, such as GPCNS-1, indicated that the mass ratio of glucose to graphene oxide was 100:1. For control, pure glucose porous carbon (labeled as GPCNS-0) was prepared using same parameters except that no addition of GO dispersion. Results and discussion Optimization of conditions The influences of GO dosage on adsorption capacity and specific surface area of GPCNS-x were firstly investigated, the result is shown in Figure 2. 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 diagram 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 a poor adsorption. Interestingly, carbon nanosheets greatly reduces 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, the 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 no otherwise specified, GPCNS refers to the GPCNS-1.



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

Figure 3 The SMZ adsorption diagram of carbon nanosheets and bulk carbon 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 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


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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.

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)



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Figure 5 TEM images of GPCNS-0 (a, b), GPCNS-1 (c, d) and GPCNS-5 (e, f) 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 low pressure region, which indicates the existence of large micropores. Meanwhile, the hysteresis phenomenon caused by capillary condensation is observed, which shows signal of typical mesopores.22 By BET method, the specific surface area of GPCNS-x are about 3000 m2 g-1. With the increase of GO dosage, the specific surface area increased slightly, and 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


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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 structure and phase structure 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 22-26o corresponding (002) crystal surface, shows the lower crystallinity of CG and GPCNS. The weaker peak in the range of 41-45o is correspond to diffraction peak of the (100) and (101) crystalline surface, indicated that CG and GPCNS have a certain graphitization structure.23 Differently, the background of GPCNS spectrum increases in the range of 10-39o, 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 structure. Figure 6b shows the Raman spectra of CG and GPCNS. The spectra have two distinct peaks at 1591 and 1343 cm-1 assigning 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 sub peaks located at 1163, 1332, 1499 and 1590 cm-1 by using 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 ideal graphitic sp2 carbon.25 The detailed percentage content is shown in Table S2. 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+3H2+ 2K2CO3 (T > 400 °C) K2CO3+C↔K2O+2CO (T > 700 °C) K2CO3 ↔K2O+CO (T > 700 °C) 2K+CO2↔K2O+CO (T > 700 °C) K2O+C↔2K+CO (T > 800 °C)



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Figure 6 XRD patterns (a) and Raman spectra (b) of CG and GPCNS 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 284.5 and 532.1 eV corresponding to C1s and O1s, respectively, indicating that GPCNS is mainly composed of C and O elements. Figure S2b, c show the high-resolution XPS spectra of C1s and O1s by Gauss fitting method, respectively. From Figure S2b, the C1s 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, C=O,27-29, respectively. In Figure S2c, three peaks located at 527.3, 530.05 and 533.2 eV can well fit the O1s spectrum, which correspond 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 oxygen-containing 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.


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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 adsorbents surface in the form of monolayer, while the Freundlich isotherm model is multilayer adsorption. Fundamentally, the adsorption isotherms show how the adsorbate and adsorbent interact, and thus study of isotherm model is useful for optimizing adsorbent use. The linear equations of Langmuir and Freundlich isotherm models are as follows:36, 37

Ce 1 C = + e Qe K LQm Qm

(1)

1 ln Qe = ln K F + ( ) ln Ce 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, b, the Langmuir isotherm model (R2>0.99) can better describe the adsorption behavior than Freundlich isotherm model, showed that the adsorption of SMZ onto the GPCNS surface is likely to be monolayer adsorption. The Figure 7c shows the nonlinear fitting of the Langmuir and Freundlich isotherm adsorption model. The fitting values by Langmuir are close to the experimental values, while the Freundlich fitting values deviates 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 of sulfonamide antibiotics are listed in Table 1.38-41 The results indicated that the GPCNS has excellent adsorption capacity for SMZ, which proved it has great advantages both in cost and adsorption performance and shows a good application prospect.



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Figure 7 The linear fitting of Langmuir (a) and Freundlich model (b); (c) the non-linear fitting of Langmuir and Freundlich model; (d) plot of lnK0 vs 1/T Table 1 Comparison of adsorption capacity of GPCNS and other adsorbents for sulfonamide antibiotics Adsorbents

Adsorbates

Qm(mg g-1)

T (K)

Refs.

N-TiO2/AC

Sulfamethazine

201±2

298±2

38

298

39

P-MWCNTs

24.78 Sulfamethazine

H-MWCNTs

13.31

MOPAC

Sulfamethoxazole

42.5

298

40

MACC

Sulfamethoxazole

159

298

41

820.27

298

845.27

308

895.27

318

GPCNS

Sulfamethazine

This work

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, indicated that the adsorption process of SMZ onto GPCNS is affected by temperature. The Gibbs free energy change (∆Gθ), standard enthalpy change (∆Hθ), and standard entropy change (∆Sθ) were calculated by the following formulas:42 ∆Gθ = -RTln(K0)

lnK0 =

∆Sθ Hθ R

-

RT

(3) (4)

K0 define as follow：

K0 =

as vs Qe = ae ve Ce

(5)

where, R (8.314 J mol-1·K-1) is the gas constant, and T (K) is the temperature. The values of ∆Hθ and ∆Sθ can


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be derived by using the intercept and slope of the lnK0 vs 1/T fitting graph (Fig. 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, the values of K0, ∆Hθ, ∆Sθ, ∆Gθ and the related parameters are shown in Table S3. The ∆Hθ of the adsorption process is 1.515 kJ mol-1, which means that the adsorption process is endothermic. In addition, the ∆Sθ of the adsorption process is positive value, indicated that the increase of the freedom degree of the solid-liquid interface in the system. The ∆Gθ is negative, which proves that the adsorption of SMZ onto 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-depending 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-first-order and pseudo-second-order kinetic models were used to fit the kinetic adsorption data. The liner equations are given as follows:43, 44

ln(Qe − Qt ) = lnQe - K1t

(6)

t 1 t = + 2 Qt K 2Qe Qe

(7)

here, k1 (min-1) and k2 (g mg-1 min-1) are adsorption rate constants of the pseudo-first-order, pseudo-second-order

kinetic

models,

respectively.

The

linear

fitting

by

pseudo-first-order,

pseudo-second-order kinetic models are shown in Figure 8b, 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 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 the SMZ0 around at pH = 5.46 As the SMZ- became prevalent at the pH ranging from 5 to 8, while the SMZ mainly existed in the SMZ+ at the pH ranging from 2.07 to 5. Thus, this adsorption processes were divided into three parts: (1) When pH at 2-3.7, the adsorption was inhibited due to electrostatic repulsion between positively charged GPCNS and SMZ+. (2) When pH at 3.7-5, the electrostatic interaction of negatively charged GPCNS and SMZ+ is benefit for SMZ adsorption, and GPCNS shows 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 (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 high ionic strength(> 0.12 M) is unfavorable to 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 ability of anti ion interference. Adsorption stability and recyclability To investigate the adsorption stability of GPCNS, the desorption tests were conducted. The GPCNS (4 mg)



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after adsorption in adsorption isotherm by different concentration 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 leaching solution. Notably, the leaching concentration of SMZ increases gradually with the increase of used concentration in adsorption process. In addition, the Figure S9b shows the desorption ratio for different used concentration in adsorption process. When used concentration is 200 mg L-1, the desorption ratio is just about 3%. While used concentration is 300 mg L-1, the desorption ratio is reached about 6%. This may be because GPCNS with more SMZ in adsorption system have bigger surface energy, and thus it will release SMZ molecules to minimize surface energy. Importantly, the low desorption ratio demonstrated the adsorption of SMZ onto GPCNS is stable enough. Moreover, the SEM images of after adsorption were observed. From Figure S10, the structure of GPCNS did not damage obviously and remained defined sheet structure. The observation indicated this 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, the experimental results as shown in Figure S11. The adsorption capacity of GPCNS decreased in every cycles, and the adsorption capacity can still reach 512.95 mg g-1 after 5th cycle. The excellent regenerability make 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 aromatic structure, thus GPCNS and SMZ molecules can form π-π interaction.48 Secondly, SMZ molecules exhibit various modes of existence in different solution pH, the surface of GPCNS contains oxygen-containing functional groups such as –OH, C=O, –COOH 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, –COOH onto the surface of GPCNS in water environment.34 The detailed analysis has been deeply discussed in FT-IR 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


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capacity.

Figure 9 Proposed mechanism for SMZ adsorption onto GPCNS Conclusions To get rid of SMZ from wastewater, the GPCNS was synthesized using glucose as biomass precursor, GO as structure directing agent via confined nanospace directed strategy and 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 excellent adsorption capacity of 895.27 mg g-1 for SMZ at 318 K and 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, the GPCNS also has excellent regenerability affording for long-term use. This work shines light on the potential of fabricating renewable porous carbonaceous nanosheets materials directly from biomass resources as advanced adsorbent in environmental pollution cleanup. Associated content Supporting Information. Chemicals and materials; Instruments; Batch adsorption; N2 sorption; XPS; FT-IR analysis and EDS analysis; the fitting of intra-particle diffusion model; influence of pH; Zeta potential of GPCNS; Effect of ionic strength; the UV-vis absorption spectra of leaching solution; the SEM images of GPCNS after adsorption; reusability of GPCNS; the porosity characteristics of GPCNS-x; the detailed percentage of carbon species; Langmuir and Freundlich isotherm model, adsorption thermodynamics and adsorption kinetics parameters; parameter of Intra-particle diffusion model. Acknowledgements The authors are grateful for financial supported from the National Natural Science Foundation of China (51608226, 21576111, U1510126), Natural Science Foundation of Jiangsu Province (BK20140534,



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Abstract graphic

Porous carbon nanosheets from glucose biomass were synthesized by an integrated graphene oxide-confined nanospace directed-KOH activated process for efficient removal sulfamethazine.


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