Facile preparation of intercrossed-stacked porous carbon originated from potassium citrate and their highly effective adsorption performance for chloramphenicol

Accepted Manuscript Facile preparation of intercrossed-stacked porous carbon originated from potassium citrate and their highly effective adsorption performance for chloramphenicol Sujun Tian, Jiangdong Dai, Yinhua Jiang, Zhongshuai Chang, Atian Xie, Jinsong He, Ruilong Zhang, Yongsheng Yan PII: DOI: Reference:

S0021-9797(17)30716-6 http://dx.doi.org/10.1016/j.jcis.2017.06.062 YJCIS 22489

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

10 May 2017 15 June 2017 17 June 2017

Please cite this article as: S. Tian, J. Dai, Y. Jiang, Z. Chang, A. Xie, J. He, R. Zhang, Y. Yan, Facile preparation of intercrossed-stacked porous carbon originated from potassium citrate and their highly effective adsorption performance for chloramphenicol, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/ j.jcis.2017.06.062

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Facile preparation of intercrossed-stacked porous carbon originated from potassium citrate and their highly effective adsorption performance for chloramphenicol Sujun Tian a, Jiangdong Dai a*, Yinhua Jiang a, Zhongshuai Chang a, Atian Xie a, Jinsong He a, Ruilong Zhang b, Yongsheng Yan a*. a

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

Engineering Jiangsu University, Zhenjiang 212013, China b

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

* Corresponding author. Tel: +86-0511-88790683; fax: +86-0511-88791800 E-mail: [email protected] (Dai J.D.) and [email protected] (Yan Y.S.) Abstract Recently, antibiotics pollution has attracted more interests from many researches which causes potential risks on the ecosystem and human health. Herein, the porous carbons (PCs) was prepared by directly simultaneous carbonization/self-activation of potassium citrate at 750-900 oC for chloramphenicol (CAP) removal from aqueous solution. The batch experiments were studied, which indicated that PCs prepared at 850 oC, namely PCPCs-850, possessed excellent adsorption ability for CAP with a maximum adsorption amount of 506.1 mg g-1. Additionally, PCPCs-850 showed a large BET surface area of 2337.06 m2 g-1 and microporosity of 89.11% by N2 adsorption-desorption experiment. The Langmuir and pseudo-second-order model could more precisely describe the experimental data. And thermodynamic analysis illustrated that CAP adsorption onto PCPCs-850 was an endothermic and spontaneous process. Importantly, the adsorbent exhibited good stability and regeneration after four times cycles. Based on these excellent performance, it is potential that PCPCs-850 can be used as a promising adsorbent for treating contaminants in wastewater. Keywords: Antibiotics, Porous carbons, Excellent adsorption ability, Stability, Regeneration

1. Introduction Recently, antibiotic pollutants frequently exposed to all around the world has attracted much more attention from the society. Antibiotics, such as sulfonamides, tetracyclines, macrolide [1], quinolones [2], have been widely employed for the prevention and treatment of infectious diseases in humans and the growth inhibitors of animals. Seriously, the antibiotics injected into the organism could not be completely adsorbed, leading to a large number of parent drug are discharged into environment via the excrement [3]. Thus, antibiotics have been recognized as new pollutants in water environment, which are difficult to be biodegraded due to their high biological activity, persistence and bioaccumulation [4]. The most representative is chloramphenicol (CAP). CAP, a broad-spectrum antibiotic, is firstly synthetized to combat bacterial infections like gram-positive and gram-negative [5]. However, although serving as benevolent role for the prevention of disease, its adverse effects such as aplastic anemia [6] could not be ignored as before. Importantly, excessive use of CAP could cause the increasing gene resistance on human, making it more difficult to cure the existing or unknown disease [7]. Thus, CAP has been strictly forbidden using in livestock, poultry, food-producing animals and so on all around the world (i.e. China, USA) [8]. However, driven by the interest, CAP is still used in many developing nations due to its high-effective and low-cost, especially in China. In addition, CAP has been frequently detected in water environment [9] owing to its difficulty in metabolism and degradation, which has a potential risk on human health. This is an important subject need to solve based on the toxicity of CAP. However, it is found that many researches only focus on how to detect or extract CAP in reported literates [6, 8, 10]. There are few works published for removal of CAP. Therefore, it is highly urgent and necessary to develop a simple and effective method for removing CAP in wastewater. Traditionally, there are many methods to treat organic pollutants from wastewater, including chemical oxidation [11], photodegradation [12], adsorption [13] and so on, in which adsorption is regarded as the most effective method to eliminate pollutants due to high efficiency, low-cost and eco-friendly [14]. Thereupon, more and more adsorbents, such as porous resins, clays, porous carbons (PCs) and metal oxides or hydroxide, have been explored for the removal of contaminants in aquatic environment [15-18]. For example, Chen et. al. studied the adsorption ability of sulfamethoxazole (SMZ) and ciprofloxacin (CIP) from aqueous solution by graphene oxide [19].

The spherical mesoporous silica prepared by Liang’s group was used to adsorb quinolone antibiotics [20]. He et. al. utilized PCs derived from black liquor as adsorbents to investigate the adsorption capacity on tetracycline (TC) [21]. Among these adsorbents, PCs are recognized as the most excellent candidate for treatment of antibiotics-containing wastewater because of their large specific surface area, high porosity, economic availability, ultrahigh chemical stability and reusability [22]. As far as we know, high specific surface area and abundant pore structure are two important factors for antibiotics adsorption which could provide sufficient adsorption sites for pollutant molecule. Generally, PCs are prepared by the pyrolysis of carbonaceous materials at high temperature, following by activation via several chemical regents (i.e. KOH, H2SO4, K2CO3). However, the process is complicated, time-consuming and the chemical regents might cause corrosion to the equipment, which is contrary to the concept of sustainable development. Therefore, it is greatly desirable to find a facile and environmental-friendly approach to prepare PCs with optimal porosity. Moreover, what we have to mention that there are very few papers reported to concentrate their interest on CAP adsorption via the PCs in previous works [3, 7, 14, 23]. Even this, the processing result for CAP on PCs is still negative, further implying the necessity of developing PCs with excellent performance for CAP adsorption. In this work, PCs with large surface area and well-defined pore structure was firstly prepared by using potassium citrate as precursor without further activation process for CAP adsorption. Specially. The paper will be conducted from the three points as follows: (I) preparation of adsorbents and their adsorptive behavior; (II) the adsorption isotherms, kinetics and thermodynamic; (III) the effects of solution pH, different ion concentration and humic acid on CAP adsorption; (IV) the adsorption mechanism and regeneration performance. All these results discussed indicate that as-prepared PCs are a promising candidate for application in wastewater treatment. 2. Materials and methods 2.1. Materials and reagents All chemicals used in this study were of analytical grade without any further purification. CAP (C11H12C12N2O5, 98%) was purchased from Aladdin Industrial Corporation (Shanghai, China). Hydrochloric acid (HCl, 12mol L-1), and potassium citrate (K3C6H5O7· H2O, ≥99.5%) were supplied from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Preparation and characterization of samples The porous carbons were prepared from potassium citrate according to previous reported method [24] for different application. Briefly, 10.0 g of potassium citrate was calcined under inert atmosphere in the temperature range of 750-900 oC for 1.0 h. Then, the black powder was washed with diluted hydrochloric acid to remove inorganic impurities, such as potassium compounds. For purification, the resulting products was washed with water many times. Finally, the samples were dried and collected, denoted as PCPCs-x, x represents activation temperature (i.e. PCPCs-750, PCPCs-850). The Fourier transform infrared spectra (FT-IR) was performed by a Nexus-470 spectrophotometer (Thermo Nicolet, USA) with KBr pellets. N2 adsorption-desorption measurements were made by a Quantachrome Autosorb-iQC volumetric instrument at 77 K. Elemental analysis of the samples was conducted using an element analyzer (FLASH1112A, CE, Italy). X-ray diffraction (XRD) analysis was compiled on a powder X-ray diffractometer (SmartLab, Rigaku, Japan) using Cu Kα radiation (λ=1.5406Å, 40 kV, 40mA), with the data collected from 2θ = 10-70o at a scan rate of 5o min-1 for identifying the phase. X-ray photoelectron spectroscopy (XPS) was conducted in a Kratos Axis Ultra DLD spectrometer with X-ray excitation provided via a monochromatic Al Kα source. The textural structure and morphology was recorded using scanning electron microscopy (SEM, S-4800, Hitachi, Japan) and transmission electron microscopy (TEM, IEM-200CX, JEOL, Japan). Raman spectra were recorded with a laser Raman spectrometer (DXR, Thermo Fisher, USA). 2.3. Batch adsorption experiments The batch adsorption experiments were carried out in a series of centrifuge tubes containing 2.0 mg dose of PCPCs-x and 10 mL of CAP solutions with different initial concentrations of 30, 40, 50, 70, 90, 100, 110, 150, 170, 200, 250, 300 mg L-1, respectively. The experiments were performed in water bath at 288, 298, 308 K for 24 h, respectively, until reaching adsorption equilibrium. The kinetics studies were conducted in a series of centrifuge tubes with 2.0 mg of adsorbent and 10 mL of CAP solution (300 mg L-1) at fixed time intervals of 5, 10, 15, 30, 60, 90, 120, 180, 240, 300, 420 min, respectively. Afterward, the mixture was filtered using 0.22 µm membrane filters and the free CAP concentrations were estimated utilizing the UV-Vis spectra (Shimadzu, UV-2450PC) at the maximum wavelength of 278 nm. The adsorption capacity at

equilibrium, Qe (mg g-1) and the amount of CAP adsorbed at t time, Qt (mg g-1) was calculated as follows:

 −   1

 −  
= 2 
 =

where C0 and Ce (mg L-1) are the initial and equilibrium CAP concentration, respectively, Ct (mg L-1 ) is the CAP concentration at a certain time. V (L) and m (g) are the solution volume and mass of adsorbents, respectively. To investigate the effect of solution pH on the adsorption progress, 2.0 mg of PCPCs-850, which exhibited optimal adsorption ability, was added into 300 mg L-1 of CAP solution at pH range from 3.0-11 at 298 K. The solution pH was adjusted with 0.1 mol L-1 HCl and NaOH. To study the effects of different valent metal ions and humic acid on CAP adsorption onto adsorbent, keeping the mass of adsorbent and initial CAP concentration constant, different concentration of metal ions and humic acid were added into in a series of centrifuge tubes, respectively. For the recycling examination, first, the PCPCs-850 after adsorption was placed into 0.2 M of NaOH solution for several hours at 298 K for the purpose of desorption and then washed using the mixture of deionized water and ethanol. Finally, the adsorbent was dried at 70 oC for the next adsorption experiment. All adsorption experiments were performed in triplicate. 2.4. Adsorption isotherms The adsorption equilibrium isotherm is an effective method, which can estimate accurately the relationship between adsorbed amount and equilibrium concentration. In the present work, the Langmuir [25], Freundlich [26], and Temkin [27] adsorption isotherm models were applied to analyze the equilibrium data. The mathematical equations of Langmuir, Freundlich, and Temkin models are given below:

Langmuir

 1 1 = +  3

 
 

1 Freundlich ln
 = ln$ + ln 4 % Temkin
 = )ln* + )ln 5

where Qm (mg g-1) is the maximum adsorption amount for CAP per unit mass of carbon, KL (L mg-1), KF ((mg g-1) (L mg-1)1/n) are Langmuir and Freundlich constant, respectively, and 1/n is a

measure of the adsorption intensity, b related to the adsorption heat is the Temkin constant and KT is the equilibrium binding constant. RL is introduced to evaluate the preference of adsorbent/adsorbate system during the adsorption progress. RL is represented as follows: [7] , =

2.5. Adsorption kinetics In

order

to

investigate

1 6 1 +  

the

adsorption

mechanism,

the

pseudo-first-order

and

pseudo-second-order kinetic models were employed to analyze the kinetics data. The mathematical equations can be expressed as: [28] Pseudo-first-order model:

ln
. −
  = ln
 − / 0 7

Pseudo-second-order model:

0 1 1 = + 0 8
. 2
 2


where K1 (min-1) and K2 (g mg-1 min-1 ) are the adsorption constants of pseudo-first-order and pseudo-second-order, respectively, Qe (mg g-1) and Qt (mg g-1) is the adsorption capacity at equilibrium and t (min) time, respectively, Kp (g mg-1 min-1/2) is the intra-particle diffusion rate coefficient and Cp (mg g-1) is a constant related to the thickness of the boundary layer. The determination coefficient R2 and normalized standard deviation ∆Qe (%) were used to evaluate which model can best describe the experimental data. The ∆Qe can be calculated by equation (9): [29]
9: −
;<= 2 > 6
9: %
 = 100 7 9 ?−1 8

where Qexp and Qcal (mg g-1) are the experimental and calculated adsorption amount, respectively. 3. Results and discussion 3.1. Materials characterization Fig. 1 (a-b) shows the SEM images of the PCPCs-850. It is observed that the calcination treatment of potassium citrate directly results in the formation of carbons particles, which are composed of interconnected and stacked carbon sheets or block. This could be because that the

intercalation of potassium into graphene carbon and the generation of potassium steam and carbon oxide gas could lead to micropore structure during activation process. Similar mechanism appeared in previous literature [30]. Fig. 1 (c-d), the closer image of Fig. 1 (a-b), respectively, as marked by red rectangle, shows the relatively rough surface of carbon materials with a wrinkled carbon sheets. Additionally, Fig. 1 (e-f) displays the TEM image of PCPCs-850, an obvious sheet carbon can be clearly seen that is consistent with the SEM results and the surface of carbon materials uniformly distributes large amount of tiny pores that can provide abundant adsorption sites for target pollutant in wastewater. Above these results demonstrate that porous carbons are successfully prepared. FT-IR spectra studies of pure CAP, PCPCs-850 conducted before and after CAP adsorption were presented in Fig. 2, which identified the presence of characteristic bands corresponding to various functional groups. A broad peak at around 3400 cm-1 should be ascribed to O-H stretching vibration [31]. The band between 2830 and 2930 cm-1 reveals the existence of C-H bending vibration [32]. Moreover, the presence of C=O (1720 cm-1) [33] and C-O-C (1160-1030 cm-1) [34] functional groups suggests that oxygen-containing groups were successfully introduced into carbon materials. And the band centered 1590-1580 cm-1 is related to C=C stretching vibration [35]. The detailed elemental content of PCPCs-x samples is determined using element analyzer (FLASH1112A, CE, Italy) by combustion method and listed in Table. 1, from which it can be observed that the carbon and oxygen content of PCPCs-850 is richer than other adsorbents. What’s more, it is worth noting that the total amount of C, O and H is not 100 percent, perhaps suggesting the presence of impurities. From the spectrum of pure CAP, the four obvious peaks at 1625-1450 cm-1 could be observed, which corresponds to the skeleton vibration of benzene ring. The peak at 3340 cm-1 and 2900 cm-1 can be assigned to the O-H or N-H stretching vibration and C-H stretching vibration, respectively [31]. And the peak at 1365-1290 cm-1 is the characteristic peak of -NO2. And other characteristic peaks of CAP can also be seen which is in good with that reported in literature [23]. Clearly, the characteristic peaks of CAP could be found from the PCPC-850 after CAP adsorption, illustrating that CAP molecule is successfully adsorbed onto carbon materials. What’s more, the peak of carbonyl becomes stronger compared with that before adsorption, which could be ascribed to the superimposition effect between identical group. Fig. 3a displays the XRD analysis to study the crystalline/amorphous nature of PCPCs-x. It is

evident that a peak at around 25o and 43o corresponds to (002) facet [36] and (100) plane [37] of graphite carbon in as-prepared carbon materials, respectively. During self-activation process, the precursor (carbon source) was transformed into hybrids of defected carbon and graphite carbon. The formation of porous structure was ascribed to defected carbon while crystalline structure originated from graphite carbon. In addition, Raman spectrum of PCPCs-x were also studied, as shown in Fig. 3b. In Raman spectra, the two single peaks at approximately 1363 cm-1 (D band) and 1590 cm-1 (G band) are related to the defected carbon and E2g tangential mode (graphite carbon), respectively [38, 39]. The relative degree of defects of carbon materials are estimated by the intensity ratio (ID/IG). From PCPCs-750 to PCPCs-850, it can be seen that the value of ID/I G decreased from 1.074-0.983 with the increasing temperature, indicating high degree of graphitization. This may be caused by the removal amorphous carbon or other impurities [40]. It is worth noting that the value of ID/IG increased to 1.018 when the temperature reached 900 oC, which may be ascribed to the destruction of carbon structure owing to the excessive calcination for the raw materials. As shown in Fig. 4, the XPS spectra displays the elemental composition and bonding configurations of PCPCs-850. Two typical peaks can be observed, which corresponds to the binding energies of C1s and O1s, respectively (Fig. 4a), which reveals the presence of carbon and oxygen on the PCPCs-850. The high-resolution C1s can be divided into three peaks, locating at 284.5 eV, 284.83 eV and 285 eV [41, 42], which is assigned to C-C (48.3%), C-O (39.1%) and C=C (12.6%), respectively (Fig. 4b). On the other hand, the divided O1s has three peaks, which gives an overview of the environment around O (Fig. 4c), in which the peaks at 531.1 eV, 532.4 eV and 533.7 eV are matched with C=O (10.2%), R-OH (79.7%) and C-O-C (10.2%), respectively [29, 41, 43], suggesting that there is fewer oxygen content, which is consistent with the result in Table. 1. Fig. 5 shows the N2 adsorption-desorption isotherms to explore the porosity of porous carbon materials as well as the influence of activated temperature on pore structure. It is evident that the isotherms of PCPCs-x exhibit type I according to IUPAC classification, mainly indicating micropores structures. All the isotherms, especially the PCPCs-850 carbon materials, displayed a widening of the knee at a very low relative pressure (P/P0<0.1) in Fig. 5a, demonstrating a well development of microporosity [44], which was further identified by the pore size distribution in

Fig. 5b. The abrupt increase at higher relative pressure around 1.0 may be assigned to the N2 adsorption in macropores. The pore size distributions in Fig. 5b showed similar pore structures for PCPCs-750, PCPCs-800, PCPCs-850, PCPCs-900, whose micropores size changed from 0.4 to 1.5 nm, in which the microporosity of PCPCs-850 was optimal. In addition, the textural properties of samples are presented in Table. 2. With the activated temperature increasing, the large BET specific surface area and pore volume of PCPCs-x (x=750, 800, 850) ranged from 1457.63 to 2337.06 m2 g-1 and from 0.679 to 1.226 cm3 g-1, respectively. This could be because that more potassium steam and carbon dioxide escaped during activation process with increasing temperature. However, the large BET specific surface area of PCPCs-900 decreased, which can be ascribed to the destruction of pore structure due to ultrahigh activation temperature. Simultaneously, it is worthy of mentioning that the micropores make a great contribution to the total specific surface area as calculated with the DFT method in the N2 adsorption isotherms, whereas abundant microporosity can have a potential application for CAP adsorption. 3.2. Adsorption isotherms analysis The equilibrium adsorption isotherms are useful tools to determine the amount of needed adsorbent to adsorb a certain amount of required adsorbate and well understand the adsorption mechanism when the system is approached. The Langmuir, Freundlich and Temkin isotherm model depict monolayer physical adsorption upon a homogeneous surface, multilayer adsorption and strong electrostatic interaction between positive and negative charges, respectively. Fig. 6a shows the non-linear fitting curves of Langmuir, Freundlich and Temkin for CAP adsorption onto PCPCs-850 at different temperature, respectively. The detailed fitting parameters are listed in Table. 3. For CAP adsorption towards PCPCs-850, the determination coefficient (R2) of Freundlich and Temkin were in the range of 0.824-0.857 and 0.716-0.799, respectively. What’s more, the fitting curves seriously deviated from experimental plots. However, the determination coefficient (R2) of Langmuir model changed from 0.992 to 0.995, simultaneously, the normalized standard deviation ∆Qe (%) was much smaller than the other two models. And the fitting curves basically coincided with the experimental plots, showing that correlations of the isotherms are in the order: Langmuir>Freundlich>Temkin. Thus, the monolayer physical adsorption could play an important role in CAP adsorption by PCPCs-850. On the other hand, the fitting parameters of the other three adsorbents (PCPCs-750, PCPCs-800, PCPCs-900) by three isotherm model have also

been presented in Table. 3. It was obvious from Table. 3 that the R2 value of three adsorbents was far higher than 0.99 for Langmuir model, whereas lower than 0.99 for Freundlich and Temkin model, further verifying better fitting effect by Langmuir model. Meanwhile, KL value increased which could be associated to increasing temperature and inherent chemical properties of adsorbents. Furthermore, the maximum adsorption amount Q m of CAP on PCPCs-850 reaches 476.3, 506.1, 521.4 mg g-1 at 288, 298, 308 K, respectively, which is much higher than those reported adsorbents in the literature (see in Fig. 7) [3, 7, 14, 23]. Also, KF value increased with the increasing temperature that might be related to a decrease of heterogeneity factor. As we know, physical adsorption is exothermic and prefers lower temperature, however, the CAP adsorption amount onto PCPCs-x increases with temperature increasing. Therefore, we infer that there can exist chemical adsorption in CAP adsorption by PCPCs-x. The increasing temperature is in favor of chemical adsorption, thereby leading to higher adsorption capacity. The separation factor RL in Fig. 6b is another parameter used to evaluated when an adsorption is favorable, RL=0, irreversible, RL>1, unfavorable, or 0< RL<1, favorable. The value RL changes from 0.05-0.32, 0.02-0.07, 0.01-0.02 at 288, 298, 308 K, respectively, the adsorption process of CAP towards PCPCs-850 is favorable in the studied concentration range [45]. 3.3. Adsorption kinetic analysis The kinetic study is an important technique for understanding the adsorption dynamics and mechanism. The non-linear fitting curves of pseudo-first-order and pseudo-second-order model for PCPCs-x are shown in Fig. 8. Initially, the adsorption process is rapid in first 50 min and then becomes slow and stagnates with the contact time increasing, which may be because that lots of vacant surface sites are easily available for CAP molecule in the initial stage, and afterwards the remaining vacant sites are difficult to be occupied due to the repulsive forces between CAP molecule on PCPCs-x and the bulk phase [46]. And it can be seen from the figure that the adsorption of CAP onto PCPCs-x reaches equilibrium at around 300 min. In addition, the fitting curve of pseudo-first-order model deviates from experimental plots while pseudo-second-order model can coincide better with experimental plots. Meanwhile, the adsorption kinetic parameters of two models are summarized in Table. 4. The determination coefficient (R2) and normalized standard deviation (∆Qe, %) both indicates a poor fitting results for pseudo-first-order model. However, the R2 value of pseudo-second-order model is higher than 0.99 and ∆Qe is lower for

PCPCs-x, showing the better fitting of adsorption kinetics. Importantly, the Qe,exp value is in good agreement with Qe,cal value for CAP adsorption onto PCPCs-x, further verifying the above results. On the other hand, K2 value increased with the activation temperature increasing, indicating that PCPCs-850 possessed faster adsorption kinetic, which could be ascribed to greater porosity of adsorbent. The result is in good agreement with that in Table. 2 and Table. 3, verifying optimal adsorption ability for PCPCs-850. To identify whether the interaction between adsorbent and adsorbate is the chemical adsorption or not, the thermodynamic analysis is an approximate method, as will be discussed in the following section. 3.4. Thermodynamic studies Thermodynamics of CAP adsorption onto PCPCs-850 were carried out to determine the inherent energies changes in adsorption process. The relative thermodynamic parameters, Gibbs free energy (∆Gθ) were calculated by equation (10)-(12) in Table. 5, standard enthalpy (∆Hθ), and standard entropy (∆Sθ) were calculated by equation (13): [47, 48]

ΔB C = ΔDC − EΔF C 13

where T (K) and R (8.3145 J mol-1 K-1) is the temperature and the ideal gas constant, respectively. The ∆Hθ and ∆Sθ are evaluated from the slope and intercept of ∆Gθ vs T plot (see in Fig. 9), and the thermodynamic parameters are tabulated in Table. 5. As shown in Table. 5, the determination coefficient (R2) of equation (10) was up to 0.9959, higher than the two equations, indicating that the equation (10) was more suitable to evaluate the relative thermodynamic parameters. According to the previous literature [49], the value of ∆Gθ for physisorption was -20-0 kJ mol-1. The ∆Gθ value obtained for CAP adsorption onto PCPCs-850 was negative, changing from -0.3611 to -6.110, which demonstrated that CAP adsorption onto PCPCs-850 was a spontaneous physisorption process. Meanwhile, the ∆Gθ value decreased with increasing temperature, suggesting that high temperature was favorable for adsorption process, which was consistent with the results in Fig. 6a. The positive value ∆Sθ and ∆Hθ suggested increased randomness at solid/solution interface during the adsorption process and an endothermic adsorption in nature, respectively. And the magnitude of ∆Hθ could be used to study the involved force in the whole adsorption process. The ∆Hθ value of hydrogen bond forces, electrostatic interaction and van der Waals forces were 2-40 kJ mol-1, 20-80 kJ mol-1 and 4-10 kJ mol-1, respectively [50]. In this work, ∆Hθ was found to be 32.48 kJ mol-1, indicating that hydrogen bond

forces and electrostatic interaction were involved in adsorption process. Also, this showed highest adsorptive ability of PCPCs-850 for CAP antibiotic. Above these results suggested that PCPCs-850 is an effective adsorbent and can be applied for removal of CAP from an aqueous medium. 3.5. Effect of solution pH, ion strength and humic acid The dependency of CAP adsorption (C0=300 mg L-1) by PCPCs-850 at different pH values is depicted in Fig. 10a and the pH effect on the zeta potential of PCPCs-850 also shown in Fig. 10a (Blue line). Accordingly, the isoelectric point pHpzc is at 4.2, suggesting that the surface of PCPCs-850 is positively charged in the pH range of 3.0-4.2. It was noticed from the figure that the influence of pH (4-9) was insignificant. However, there was slight decrease at pH 3 which may be ascribed to the repulsion effect between positively charged CAP and carbon adsorbent. While the adsorption uptakes of CAP onto PCPCs-850 increased at pH 10 and 11, which could be attributed to the fact: the hydroxyls of CAP dissociated leaving positive charged CAP (see in Fig. 11) that attached to the negative charge of carbon surface, which caused a higher accumulation of solutes when solution pH>pKA=5.5 [51]. The metal ions and humic acid, generally, are contained in waste water, so it is essential to investigate the effect of different metal ions strength and the concentration of humic acid on CAP adsorption onto PCPCs-850. As shown in Fig. 10b, the adsorption capacity of CAP reduced with the addition of different valence mental ions, especially the divalent ions, Ca2+ and Ni2+ decreasing around 9.5% which may be assigned to the stronger electrostatic interaction or the existence of competitive relationship between CAP and high valence ions. On the other hand, although with the increasing ion concentration, the adsorption capacity of CAP on PCPCs-850 decreased, implying more competitive effects, the adsorption amount was still high up to 392.55 mg g-1 that suggested the potential application of as-prepared carbon materials in wastewater treatment. From the Fig. 10c, with the concentration of humic acid increasing the humic acid had an obvious influence on CAP adsorption. The result could be resulted from the competitive effect between humic acid and CAP molecule by hydrogen bonding, π-π EDA interaction (see the structure of humic acid in Fig. 11). However, the adsorption uptakes still reached about 200 mg g-1 decreasing by 59.9% although the concentration of 70 mg L-1 for humic acid which was far higher than that in realistic water environment, indicating that the adsorbents had the potential to be

employed to the antibiotics adsorption. 3.6. Cycle experiments The regeneration property of PCPCs-850 was studied by four times adsorption-desorption under the same conditions, as seen in Fig. 12. After four times cycles, the adsorption amount of CAP was only decreased by 6.69% which exhibited an excellent stability and regeneration performance. 3.7. Adsorption mechanisms Generally, antibiotics adsorption on carbonaceous materials includes physical and chemical interaction. According to above analysis results, PCPCs-850 has a large specific surface area and high microporosity that might promote the adsorption performance of PCPCs-850 for CAP. In addition, the size of CAP molecule is 0.436 nm calculated according to reported method in previous literature [52], which is smaller than the size of as-prepared PCPCs-x, indicating that the micropore-filling effect can play an important role during CAP removal. Similar mechanism has also been reported in previous work [53]. What’s more, the thermodynamic studies indicate that hydrogen bond forces and electrostatic interaction are involved in the adsorption process, demonstrating that physisorption plays an important role for CAP adsorption towards PCPCs-x. On the other hand, as-prepared adsorbent contains abundant oxygen-containing groups (O-H, C=O, C-O-C) as pronounced in FT-IR and XPS spectra, which can interact with oxygen-containing and nitrogen-containing functional groups of CAP molecule by π-π EDA interactions between oxygen-containing groups. This is beneficial for the improvement of CAP adsorption amount on PCPCs-850. Similar observations on the adsorption of norfloxacin to CNTs were also found in previous study [54]. Furthermore, the result of FT-IR after CAP adsorption further illustrates that there exists interaction of functional groups between PCPCs-850 and CAP antibiotic. And CAP molecule can dissociate into positive ion thereby adsorbing to the surface of negatively charged carbons by electrostatic interaction. Meanwhile, considering the chemical structure of CAP molecule, a benzene ring as well as ketone group (see in Fig. 11), which has strong electron-withdrawn ability and can serve as π-electron-acceptors to interact with the polarized aromatic rings of graphite structure in PCPCs-850 (as π-electron-donors) [53]. Thus, physical and chemical interaction have a synergistic effect for CAP adsorption onto PCPCs-850. 4. Conclusions

In the study, we successfully prepared interconnected and stacked microporous carbon nanosheets by self-carbonization and self-activation of potassium citrate. The as-prepared PCPCs-850 exhibited a maximum adsorption capacity of 506.1 mg g-1 at 298 K with a large specific surface area of 2337.06 m2 g-1 and microporosity of 89.11%. In addition, the adsorption data can be better fitted by the Langmuir model and the adsorbent had a fast adsorption kinetic, which was well described by the pseudo-second-order rate equation. The thermodynamic parameters revealed a spontaneous and endothermic adsorption process for CAP. Simultaneously, physical and chemical interaction was involved in the whole adsorption process, which synergistically made a great contribution to the high CAP adsorption capacity. Particularly, it was worth mentioning that the as-prepared adsorbent still remained high adsorption capacity with a good stability and regeneration although facing the complex water environment. Based on these excellent properties, it is expected that PCPCs-850 as a promising adsorbent is applied for treating the antibiotic wastewater. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 21676127, U1510126, 51608226, 21576111 and U1407123), Natural Science Foundation of Jiangsu Province (BK20140534, BK20140580, BK20160501 and BK20151350), Research Fund for the Doctoral Program of Higher Education of China (20133227110022 and 20133227110010) and Jiangsu Planned Projects for Postdoctoral Research Funds (1501067C). References: [1] Y. Ma, C. A. Wilson, J. T. Novak, R. Riffat, S. Aynur, S. Murthy, A. Pruden, Effect of Various Sludge Digestion Conditions on Sulfonamide, Macrolide, and Tetracycline Resistance Genes and Class I Integrons. Environ. Sci. Technol. 45 (2011) 7855-7861. [2] C. Blasco, Y. Picó, Development of an improved method for trace analysis of quinolones in eggs of laying hens and wildlife species using molecularly imprinted polymers. J. Agric. Food. Chem. 60 (2012) 11005-11014. [3] J. Dai, J. He, A. Xie, G. Lin, J. Pan, C. Xiang, Z. Zhou, W. Xiao, Y. Yan, Novel pitaya-inspired well-defined core–shell nanospheres with ultrathin surface imprinted nanofilm from magnetic mesoporous nanosilica for highly efficient chloramphenicol removal. Chem. Eng. J. 284 (2016) 812-822.

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Fig. 1. SEM (a-d) and TEM (e-f) image of PCPCs-850.

Fig. 2. FT-IR spectrum of pure CAP, PCPCs-850 before and after CAP adsorption.

Fig. 3. The XRD patterns and Roman spectra for PCPCs-x (x=750, 800, 850, 900).

Fig. 4. (a) XPS survey spectra; (b) C1s spectra; (c) O1s spectra of PCPCs-850.

Fig. 5. (a) N2 adsorption-desorption isotherms and (b) pore size distribution of PCPCs-x (x=750, 800, 850, 900).

Fig. 6. (a) Adsorption experimental plots and non-linear fitting curves of CAP adsorption onto PCPCs-850, (b) Effect of initial concentration of CAP and temperature on the separation factor RL of Langmuir isotherm model.

Fig. 7. The adsorption amount of CAP onto PCPCs-850 compared with the reported literature at 298 K.

Fig. 8. The adsorption kinetics of CAP onto PCPCs-x (T=298 K, x=750, 800, 850, 900).

Fig. 9. The fitting curves of ∆G plotted versus T for different Gibbs equation.

Fig. 10. (a) Effects of solution pH, (b) ion species and concentration, (c) humic acid on CAP adsorption towards PCPCs-850 (T=298 K).

Fig. 11. The dissociation of CAP molecule and structure of humic acid.

Fig. 12. Reusability of PCPCs-850 on CAP (T=298 K).

Table. 1. Elemental content of PCPCs-x (x=750, 800, 850, 900).

Elemental content (wt%) Samples

O

C

H

PCPCs-750

1.58

78.5

0.52

PCPCs-800

1.61

79.1

0.53

PCPCs-850

1.74

88.8

1.16

PCPCs-900

1.67

79.4

0.57

Table. 2. Textural properties of PCPCs-x (x=750, 800, 850, 900). SBETa

Smicrob

Vtotalc

Vmicrod

Samples

(m2 ·g-1)

(m2·g-1)

(cm3 ·g-1)

(cm3·g-1)

PCPCs-750

1457.63

1393.95

0.679

PCPCs-800

1599.68

1548.98

PCPCs-850

2337.06

PCPCs-900

2106.66

(nm)

0.531

78.27

1.863

0.692

0.591

85.42

1.730

2008.58

1.226

1.092

89.11

1.921

2034.96

1.169

1.034

88.44

2.027

SBET: BET surface area. Smicro: micropore surface area.

c

Vtotal: total pore volume.

d e

daveragee

(%)

a

b

Vmicro/Vtotal

Vmicro: micropore volume.

daverage: average pore diameter.

Table. 3. Isotherm parameters of Langmuir, Freundlich and Temkin models for the adsorption of CAP on PCPCs-x at different temperature. PCPCs-750 Models

Langmuir

Constants

PCPCs-900

298

308

288

298

308

288

298

308

288

298

308

K

K

K

K

K

K

K

K

K

K

K

K

Qm (mg g-1)

328.7

394.9

402.6

414.3

451.3

472.2

476.3

506.1

521.4

434.8

467.5

498.2

KL (L mg-1 )

0.058

0.116

0.153

0.063

0.128

0.161

0.071

0.150

0.180

0.068

0.132

0.172

R2

0.992

0.991

0.990

0.991

0.993

0.993

0.992

0.995

0.995

0.992

0.990

0.993

∆Qe (%)

0.12

0.23

0.21

0.47

0.83

0.46

0.31

0.78

0.35

0.66

0.25

0.28

105.6

147.7

144.9

112.4

156.2

190.7

117.7

184.1

200.2

114.3

173.4

196.4

1/n

0.426

0.397

0.399

0.341

0.325

0.305

0.268

0.203

0.196

0.151

0.121

0.114

R2

0.857

0.813

0.884

0.883

0.832

0.867

0.824

0.857

0.836

0.887

0.830

0.862

∆Qe (%)

3.83

4.17

4.15

5.28

3.41

6.16

5.48

7.14

3.36

5.73

3.93

4.57

KT

1.36

1.41

1.47

1.47

1.63

1.69

2.18

4.64

4.71

1.49

2.08

2.14

R2

0.742

0.794

0.769

0.783

0.864

0.786

0.782

0.799

0.716

0.753

0.709

0.892

∆Qe (%)

6.13

9.45

7.53

8.36

4.24

5.78

7.19

6.58

8.27

5.74

9.88

5.15

(L mg-1)1/n)

Temkin

PCPCs-850

288

KF((mg g-1)

Freundlich

PCPCs-800

Table. 4. Adsorption kinetic parameters of PCPCs-x (x=750, 800, 850, 900) onto CAP at 298 K. Model

Samples

Pseudo-first-order model Qe,exp

K1×10-2

Qe,cal -1

Pseudo-second-order model

-1

-1

∆Qe

(mg g )

(mg g )

(min)

(%)

PCPCs-750

386.27

341.69

6.08

8.94

PCPCs-800

413.20

382.10

5.12

PCPCs-850

485.87

451.66

PCPCs-900

420.34

390..16

2

R

K2×10-4

Qe,cal -1

-1

∆Qe -1

R2

(mg g )

(g mg min )

(%)

0.8451

391.49

1.51

0.21

0.9915

5.79

0.8715

425.26

1.78

0.19

0.9946

5.36

3.02

0.8114

492.04

2.52

0.16

0.9929

7.29

3.77

0.8508

428.76

2.31

0.18

0.9913

Table. 5. Adsorption thermodynamics parameters for CAP adsorption on PCPCs-850 according different Gibbs equation. T

Gibbs Equation form

a

∆Gθ=-RTln(K0)

b

c

∆Gθ=-RTln(Qe/Ce)

(10)

(11)

∆Gθ=-RTln(KL×Ma×103×C0) (12)

∆Gθ

∆Hθ -1

∆Sθ -1

-1

R2

(K)

(kJ mol )

(kJ mol )

(kJ mol )

288

-3.611

32.48

0.125

0.9959

298

-4.762

308

-6.110

288

-1.814

6.163

0.028

0.9622

298

-2.137

308

-2.369

288

-37.70

34.08

0.25

0.9549

298

-40.86

308

-42.70

a: K0 is the distribution coefficient, which is the intercept of fitting curve ln(Qe/Ce) vs Qe on the vertical axis; b: Qe (mg g-1) and Ce (mg L-1) are the amount of solute adsorbed at equilibrium and equilibrium liquid phase concentration; c: KL (L mg-1) from Langmuir model, Ma (g mol-1) and C0 are the molecule weight of CAP and initial CAP concentration, respectively.

Graphical abstract: