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Facile synthesis of hierarchical porous carbon material by potassium tartrate activation for chloramphenicol removal
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Facile synthesis of hierarchical porous carbon material by potassium tartrate activation for chloramphenicol removal Xiuzhen Zhu a, Yuan Gao b, Qinyan Yue a,∗, Yan Song a, Baoyu Gao a,∗, Xing Xu a a
Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, PR China b Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, PR China
a r t i c l e
i n f o
Article history: Received 10 August 2017 Revised 14 January 2018 Accepted 19 January 2018 Available online xxx Keywords: Potassium tartrate Porous carbon Self-activation Solid wastes Chloramphenicol removal
a b s t r a c t Potassium tartrate as activator was used to produce porous carbon materials (PCMs) by activating petroleum cokes (PC), fallen leaves (FL), chicken feathers (CF) and Enteromorpha prolifera (EP) under the identical conditions (activation temperature of 750 °C, impregnation ratio of 2.5:1 and activation time of 1 h). Potassium tartrate can also be self-activated to facilely synthesize PCMs simultaneously. The PCMs synthesized through various precursors showed significant discrepancies in morphologies and the porous structure was characterized by SEM and N2 adsorption–desorption isotherm. The BET surface areas of FL-PCM (1721 m2 /g), CF-PCM (1819 m2 /g), and EP-PCM (2151 m2 /g) were larger than that of PC-PCM (256 m2 /g) and the microporosity of EP-PCM (48%) was much lower than those of others. In addition, the adsorption properties of the four PCMs under various conditions were detected by selecting chloramphenicol (CAP) as a target adsorbate. The kinetics and isotherms of adsorption processes were well fitted by pseudo-second-order and Langmuir model for four PCMs. Meanwhile, EP-PCM exhibited high adsorption capacity of 892.86 mg/g. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Porous carbon is a classical material applied as high efficient adsorbent, electrode and catalyst carrier et al. The main characteristics of PCM are physical properties (surface area, pore volume and pore-size) and chemical properties (functional groups), which are principally affected by precursor, activator and activated condition [1]. There are various precursors to produce PCMs mainly containing petrochemicals [2], lignocellulosic biomass [3], keratoprotein [4] and algaes [5] et al. The representatives of these categories are petroleum cokes (PC), fallen leaves (FL), chicken feathers (CF) and Enteromorpha prolifera (EP), respectively. PC is an unavoidable by-product with high carbon content (about 90 wt %) and low ash [2,6]. PC is a traditional material for producing PCM with low-cost. FL falls to the road especially in autumn and is traded as rubbish. FL contains various carbonaceous compounds and is almost free, accessible and renewable [7]. As a staring material, it can realize resource utilization and endow FL with addition value. CF is mainly treated as waste by landfill without sufficient use [4]. The principal ingredient of CF is keratin which is an excellent precursor to
∗
Corresponding authors. E-mail addresses: [email protected] (Q. Yue), [email protected] (B. Gao).
prepare N-doped PCMs [4,8]. CF has been proved to be a fine material to prepare PCMs in previous studies [4]. EP, a type of green alga, is a product due to marine eutrophication, and composed of polysaccharose, protein and fat et al, which are easily pyrolyzed [9]. Hence, choosing these stuffs as the typical precursors can compare their effects for preparing PCMs. KOH is a high-efficiency activator to produce PCMs with superlarge surface area, but there still exists some deficiencies during the activating while the most prominent one is high corrosive level [7]. Hence, exploiting sylvite as activator is imperative to lower the deficiencies. Potassium tartrate is an organic sylvite which has been used to produce porous carbons with 1018 m2 /g surface area by immediate carbonization previously [10]. Therefore, potassium tartrate could activate precursors and be self-activated to synthesize PCMs with high surface area. In addition, the secondary pollution caused by potassium tartrate was lighter than that of KOH, because the anion (tartrate anion) could work as carbon sources and it’s consumed to synthesize the porous carbon. Based on the above, potassium tartrate was used to activate PC, FL, CF and EP, and self-activate to produce PCMs in this study. With the vast usage of antibiotics, the harm to the environment caused by antibiotics has drawn more attention in recent years. Antibiotics are overused in stock farming due to lack in establishing relevant regulations [11]. Approximately 75% of administered
https://doi.org/10.1016/j.jtice.2018.01.025 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: X. Zhu et al., Facile synthesis of hierarchical porous carbon material by potassium tartrate activation for chloramphenicol removal, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.01.025
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drugs by animals is not assimilated and excreted through feces or urines into the environment [12]. Bacteria resistance for antibiotics has been an intractable puzzle and has caused enormous threat to human health [13]. Therefore, many technologies have been applied to remove antibiotics, such as photocatalysis [14], advanced oxidation [15] and PCMs adsorption [16]. Due to the high adsorption capacity, low cost and eco-friendly et al, PCMs adsorption is a valid method to remove the antibiotic wastewater [17]. Combined with the above viewpoints, primary targets of this study were to probe into the feasibility that potassium tartrate could activate precursors (PC, FL, CF and EP) and be self-activated to prepare PCMs, and to investigate the differences between PCPCM, FL-PCM, CF-PCM and EP-PCM in characteristics of pore structure and morphology. Meanwhile, the activated mechanism of potassium tartrate involved in the preparation process was explored. In addition, the adsorption contrasts of CAP as a model antibiotic were investigated as a function of adsorption time, initial concentration and solution pH. 2. Materials and methods
sample, which was shaken in a constant temperature bath oscillator with a speed of 150 rpm. In kinetics studies, the samples owed 10 0 0 mg/L initial concentration of CAP and 0.050 g PCM, were taken out according to a pre-set time interval. The residual CAP was segregated with PCMs by membrane-filter and its concentration was measured at 277 nm (maximum absorption wavelength of CAP) by an apparatus UV–vis spectrophotometer (UV754, Shanghai). In thermodynamics experiments, the different initial concentrations (30 0–10 0 0 mg/L) of CAP were adsorbed by the four PCMs under 293 K and natural pH for 24 h. To study the impact of pH, the pH of CAP solution (800 mg/L) was adjusted from 2 to 12 by adding the 0.1 mol/L HCl or NaOH solutions. The quantity of adsorbed CAP at adsorption equilibrium, qe (mg/g) was calculated via the following equation:
qe =
(C0 − Ce )v w
(1)
where C0 and Ce (mg/L) are the original and equilibrium CAP concentration, respectively. v(L) is the CAP solution volume and w (g) is the PCMs weight. 3. Results and discussion
2.1. Materials 3.1. Thermal analysis All the chemical reagents were analytically pure and distilled water was used in the whole experiment. Potassium tartrate and chloramphenicol (CAP) were provided by Aladdin (Shanghai, China). The four waste materials were obtained locally, washed and smashed to powder. 2.2. Preparation of PCMs The four raw materials were carbonized under 500 °C for 1.5 h using a carbonization furnace (KSY-4D-16) with an enclosed chamber to get char. 10 g char was mixed with 25 g potassium tartrate (an impregnation ratio of 2.5:1). Then the mixtures were put into a tubular resistance furnace (SKQ-3-10) where samples were heated to a requested temperature 750 °C with a constant heating rate of 10 °C/min in the existence of N2 and then maintained for 60 min. When cooled down to room temperature, the products were washed by HCl, and distilled water until the value of pH kept constant. Finally, the samples were dried, ground and sieved to 10 0–20 0 mesh (0.074–0.15 mm). 2.3. Characterization methods To understand the weight loss of different precursors during activation process, the four kinds of mixtures (potassium tartrate and char) were characterized by thermogravimetric analysis (TGA), (SHI-MADZU, TGA-50) and heated from room temperature to 750 °C with a rate of 10 °C/min and then kept at 750 °C for 60 min under N2 atmosphere. The pore structure properties of PCMs were measured by N2 adsorption–desorption isotherms under 77 K using a surface area instrument (JW-BK 122W, Beijing JWGB Sci. & Tech. Co., Ltd., China). Meanwhile, the surface morphologies of the four PCMs were characterized by SEM (FEI, NOVA NANOSEM450, USA). X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was used to investigate the surface chemical properties of PCM. X-ray diffraction (XRD, Rigaku D/MAX-YA) was employed in the 2θ range of 10–80°. 2.4. Adsorption experiments In adsorption experiments, the influencing parameters of adsorption time, concentration and pH were explored for the four PCMs. For each sample, 0.050 g of PCM accurately weighed was added to 50 mL solution of CAP in 250 mL triangle bottle for each
By TGA and DTG (differential TGA), the pyrolysis processes of four mixtures (potassium tartrate: char of precursor = 2.5:1) are described in Fig. 1. The weight-loss showed the analogous tendency in the four compounds during pyrolysis from Fig. 1(a), which can be divided into three stages. The first section was before 300 °C, and the mass losses of PC-PCM, FL-PCM, CF-PCM and EPPCM were 16.59%, 21.77% 20.81% and 23.17%, respectively, which was ascribed to the moisture losses (adsorbed and bound water) and degradation of low-boiling compounds [18]. When the temperature reached 750 °C, there was a section of weight losses for each sample (17.46% of PC-PCM, 15.78% of FL-PCM, 19.48% of CFPCM and 17.04% of EP-PCM). There were numerous degradation reactions between four crude materials and potassium tartrate forming the cross-linked structure. In the third portion, the qualitylosses after activating 60 min at 750 °C were 11.15%, 25.26%, 19.18% and 23.02% for PC-PCM, FL-PCM, CF-PCM and EP-PCM, respectively, which produced more porous structures by the catalysis of potassium tartrate. The total weight losses were 45.2%, 62.81%, 59.47% and 63.23% for PC-PCM, FL-PCM, CF-PCM and EP-PCM, respectively. The weight loss of PC-PCM was much less than those of others, which may be caused by the firm construction of PC and difficult degradation even in the existence of activation agent [1]. Meanwhile, the DTG curves have a similar trend in Fig. 1(b) except that a rapid weight loss appeared around 50 °C and no peak occurred in 227 °C for EP-PCM. The synthesis mechanism of potassium tartrate activating PC, FL, CF and EP exhibited certain similarities with the activated process of directly carbonized organic salt or KOH chemical activation with comparison and analogy method [19,20]. When the temperature reached about 450–650 °C, the organic fraction of precursors was degraded and K2 CO3 was generated, meanwhile, the organic moiety of activated agent (tartrate) worked as carbon source. The existence of K2 CO3 was proved by XRD texting when potassium tartrate was directly carbonized at intermediate temperature of 650 °C (Fig. S1). Above 700 °C, K2 CO3 were transformed into K2 O and CO2 , and the gas of CO2 reacted with C to form CO. This reaction can contribute to generate more pores via C gasification. Meanwhile, a vital reaction could generate K over 700 °C between K2 O and C. K in the form of vapor possessed a significant function to insert into carbon matrix to form more porous structure by enlarging microstructure. Referred chemical reactions were listed as equations as followings:
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Fig. 1. TGA (a) and DTG (b) profiles for potassium tartrate impregnated char from PC, FL, CF and EP at the rate of 2.5: 1.
Fig. 2. (a) N2 adsorption–desorption isotherms (b) pore and (inset) micropore size distribution of PC-PCM, FL-PCM, CF-PCM and EP-PCM.
K2 CO3 →K2 O+CO2 CO2 +C→2CO K2 O+C→2K+CO The inorganic impurities (such as K2 CO3 , K) were removed by washing used diluted HCl, and the occupied interspaces were left. Finally, four kinds of porous carbon were obtained. 3.2. Textural characteristics Surface area, pore volume and size are vital performance indexes of multi-porous carbon working the adsorption characteristics of contaminants. The N2 adsorption-desorption isotherm and pore-size distribution of PCMs are displayed in Fig. 2. The isotherm of EP-PCM owned an emblematic hysteresis loop under high p/p0 , which was the typical feature of type I and IV according to the category of adsorption–desorption isotherm (IUPAC). This isotherm manifested EP-PCM possessed well microporous and mesoporous
structure. The semblable isotherms of PC-PCM, FL-PCM and CFPCM were found in Fig. 2(a) which were typical I. The curves firstly hoiked and then tended to equilibrium, demonstrating the main pores were microporous. This result was in sympathy with the distribution of pore size in Fig. 2(b). The concentrated distribution of pore size was nearby 2 nm, especially for FL-PCM, CF-PCM and EPPCM. There were several peaks at around 0.62, 1.0 and 1.4 nm for FL-PCM, CF-PCM and EP-PCM in inset Fig. 2(b), suggesting that the FL-PCM, CF-PCM and EP-PCM owned hierarchical micropore structure. In addition, CF-PCM also possessed a peak at around 0.37 nm and there was no obvious peak for PC-PCM. Variously significance parameters of four PCMs are summarized in Table 1. The BET surface area (SBET ) of EP-PCM (2152 m2 /g) was the larger than those of PC-PCM (256 m2 /g) FL-PCM (1721 m2 /g) and CF-PCM (1819 m2 /g). The surface areas of PCMs derived from FL, CF and EP were over 6.7 times than that derived from PC at uniform activation conditions, which was due to the firm structure of PC, leading to difficult pyrolysis under the assistance of potassium tartrate. Meanwhile, the main ingredients of EP (polysaccharose, hemi-cellulose and protein), FL (cellulose, xylogen and
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X. Zhu et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2018) 1–8 Table 1 Pore structure parameters of PC-PCM, FL-PCM, CF-PCM and EP-PCM. Sample
SBET (m2 /g)
Smic (m2 /g)
Sext (m2 /g)
Vtot (cm3 /g)
Vmic (cm3 /g)
Vmic /Vtot (%)
1−Vmic /Vtot (%)
Dp (nm)
PC-PCM FL-PCM CF-PCM EP-PCM
256 1721 1819 2152
248 1491 1649 1585
8 230 170 567
0.125 0.898 0.853 1.509
0.100 0.639 0.693 0.728
80 71 81 48
20 29 19 52
1.9 2.1 1.9 2.8
SBET : BET surface area; Smic : micropore surface area; Sext : external surface area; Vtot : total pore volume; Vmic : micropore volume; Dp : mean pore size.
Fig. 3. SEM micro-graph of (a) PC-PCM, (b) FL-PCM, (c) CF-PCM and (d) EP-PCM at 50 0 0×magnification.
carbohydrate) and CF (helix keratin) were easily to be pyrolyzed [1,4,21]. In addition, the microporosity of EP-PCM (48%) was much less than those of others and the mean pore size of EP-PCM (2.8 nm) was maximum among four PCMs, which were in accordance with the isotherm and pore-size distribution. From the above results, it’s demonstrated that FL, CF and EP were suitable materials via potassium tartrate activation to produce PCMs and potassium tartrate was an effective activation agent. Meanwhile, the degree of the four precursor activated by potassium tartrate followed the order of EP > CF > FL > PC. The porous structure of EP-PCM was contrasted to some EPbased porous carbon derived from familiar activators (KOH, H3 PO4 , ZnCl2 ) in Table S1. The surface area of EP-PCM activated by potassium tartrate was larger than that activated by H3 PO4 [22], ZnCl2 [23], and slightly lower than that activated by KOH [24]. Potassium tartrate had other innovations compared with KOH, which were low corrosive, and could act as carbon source to produce PCM through self-activation. 3.3. SEM The morphologies of four PCMs surface were characterized by SEM in Fig. 3 at identical magnification (50 0 0×). The four types of PCMs showed dramatic difference in porous structure attributed to diverse essences of raw materials. PC-PCM owned the faveolate microstructure in Fig. 3(a), but pore walls were smoother than others, which caused the structure of PC was hard to be entered to build more loose pores in the context of potassium tartrate acti-
vating. FL-PCM possessed unconsolidated configuration in Fig. 3(b). CF-PCM had many small pores in Fig. 3(c). There was deep porous structure for EP-PCM. Compared with precursor EP micrograph in others’ study [5], it seems that activating agent and EP-char via self-template formed dense porous structure without destroying the channel-like wall [25]. The function of potassium tartrate was to occupy volumes of carbonaceous materials under a certain temperature. Then, the various pores were left by pickling and water washing.
3.4. Surface chemistry analysis The elemental composition and surface chemical groups of the four porous carbons were characterized by XPS. The highresolution C1s spectrums are displayed in Fig. 4. The complicated envelopes of C1s could be de-convoluted into four peaks. The main peak (C1) located in 284.7 eV was corresponded to sp2 carbon, and other peaks located near to 285.2 eV (C2), 286.6 eV (C3), 289.6 eV (C4) were belonged to C–O (&C–N), C=O and O–C=O, respectively [26,27]. These four PCMs shared the same functional groups with different relative amounts. The relative percentage of each element (C, O, N) is illustrated in Table S2. The O content increased in the order of PC-PCM (6.98%), FL-PCM (7.23%), CF-PCM (8.54%), EPPCM (12.55%). Meanwhile, CF-PCM contained the highest N content (1.20%). The O-containing and N-containing functional groups were of benefit to the adsorption capacity, which were participated in the adsorption process of CAP on the four porous carbons.
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Fig. 4. High-resolution XPS spectra of C1 for (a) PC-PCM, (b) FL-PCM, (c) CF-PCM and (d) EP-PCM.
3.5. Effect of contact time The effect of contact time on CAP absorption by the four kinds of PCMs is depicted in Fig. 5. The analogous adsorption trends were obtained with different adsorption time. The adsorbance of CAP sharply enhanced at first, especially in the original 5 min, then gradually minished over time and gradually reached the equilibrium. Lots of empty adsorption sites and high driving force between the adsorbents and adsorbates made adsorption rapider in the beginning. As the contact time increased, the effective adsorption sites were gradually occupied by CAP and driving force gradually shrunken with the decrease of CAP concentration and active sites of PCMs. Meanwhile, the absorption resistance increased between CAP in the solution and which absorbed by PCMs. The values of time and adsorption capacities at equilibrium were (60 min, 147 mg/g), (60 min, 615 mg/g), (460 min and 703 mg/g) and (20 min, 912 mg/g) for PC-PCM, FL-PCM, CF-PCM and EP-PCM, respectively. The BET surface area of CF-PCM (1819 m2 /g) was a bit higher than that of FL-PCM (1721 m2 /g), but the time needed till the equilibrium was more than FL-PCM. This phenomenon mainly caused by steric effect which restricted the rate of conveying the CAP to the pore interior of PCMs [28]. The microporosity of CF-PCM (81%) was higher than that of FL-PCM (71%), so mesopore was beneficial to adsorb CAP. In general, EP-PCM exhibited high adsorption capacity and rate towards CAP.
Fig. 5. The influence of contact time on CAP adsorbed by PC-PCM, FL-PCM, CF-PCM and EP-PCM (nature pH, temperature 25 °C, dosage 1 g/L).
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X. Zhu et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2018) 1–8 Table 2 The coefficients and R2 of pseudo-first-order, pseudo-second-order and intra-particle diffusion models for CAP adsorbed by PC-PCM, FL-PCM, CF-PCM and EP-PCM. Kinetic models
Pseudo-first-order
Pseudo-second-order
Intra-particle diffusion
Parameters
qe ,exp (mg/g) k1 (min−1 ) qe,cal (mg/g) R2 k2 (g/mg min) qe,cal (mg/g) h (mg/g min) R2 kp1 (mg/g min1/2 ) C R2 kp2 (mg/g min1/2 ) C R2
Absorbents PC-PCM
FL-PCM
CF-PCM
EP-PCM
146.65 0.0934 83.05 0.6596 2.45 × 10−2 137.17 460.98 0.9980 44.40 8.72 0.8925 5.071 92.66 0.9866
614.63 0.1055 219.66 0.8339 6.85 × 10−4 613.50 257.82 0.9999 100.42 284.71 0.9933 13.20 489.13 0.9110
703.49 0.0612 450.67 0.9316 1.41 × 10−4 704.23 69.93 0.9992 49.82 259.21 0.9005 8.326 517.77 0.9459
912.33 0.2734 248.72 0.9667 5.65 × 10−3 909.09 4669.41 1.0 0 0 0 109.96 600.45 0.9661 8.228 868.09 0.9620
3.6. Kinetics study of CAP To get more information of adsorption rate, the models of pseudo-first-order (Eq. (2)), pseudo-second-order (Eq. (3)) and intra-particle diffusion (Eq. (4)) are employed to analyze data.
ln (qe − qt ) = ln qe − k1 t
(2)
t 1 1 = + t qt qe k2 q2e
(3)
qt = kpi t1/2 + C
h = k2 qe 2
(4)
where qe and qt (mg/g) are the quantity of CAP absorbed on the PCMs at equilibrium and setting time t. k1 (min−1 ), k2 (g/(mg min)) and kpi (mg/(g min1/2 )) is the pseudo-first-order, pseudosecond-order and intra-particle diffusion rate constant, respectively. h (mg/(g min)) is the initial adsorption rate. C is a constant of Eq. (4). The fitting results of adsorption data are listed in the Table 2. The pseudo-second-order model owed a higher consistent degree compared with pseudo-first-order models, whose R2 was greater than or equal to 0.9980. Meanwhile, the values of qe,cal calculated by pseudo-second-order formula were nearer to the experimental value, unlike the qe,cal obtained from pseudo-first-order models, which was far less than the experimental values. This result manifested that the chemisorption was a factor to influence adsorption rate between CAP and surface of PCMs [18]. According to the results of XPS analysis and the structure of CAP (Fig. S2), the electrostatic interactions were involved in the adsorption process as follows: 1. EDA interaction, the strong electron withdrawing groups (nitro, ketone, benzene) in CAP acted as π -electron-accepters. PCM acted as the π -electron-donors which had the sp2 carbon along with O-containing groups. The π -electron-accepters could bind to the π -electron-donors through π –π conjugation in the adsorption of CAP which was called electro–donor–acceptor (EDA) interaction [29]. 2. H-bond interaction, the O-containing groups of the four porous carbons could form the H-bond with the N–H, –OH structure of CAP to enrich the adsorption. The intra-particle diffusion model was employed to know the adsorption rate-determining step [30]. There involved two steps in the process of adsorption CAP by the four PCMs (Fig. S3). The external surface or momentary adsorption appearing was in the first stage, where the main rate control factor was boundary layer diffusion [31]. The rate control factor of second part was intra-particle diffusion and this stage was called gradual adsorption phase [23]. The rates of momentary adsorption (stage 1) were much higher attributed to abundantly
available surface area and active sites. There were greatly reduction in rates of stage 2, which was caused by less active sites, the lager electrostatic resistance between PCMs surface and CAP and the smaller driving force due to the decrease in CAP concentration. 3.7. Adsorption isotherm Adsorption isotherm is beneficial to understand the interaction between carbon-based adsorbent and absorbate, and predict the adsorption capacity of carbon-based adsorbent. Data of adsorption isotherm was analyzed by being fitted with two classic models (Langmuir and Freundlich equation) and the equations of Langmuir (Eqs. (5) and (6)) and Freundlich (Eq. (7)) were listed as follows:
Ce 1 1 = + Ce qe q0 b q0
(5)
1 1 + bC0
(6)
RL =
ln qe = ln Kf +
1 ln Ce n
(7)
where Ce (mg/L) is the liquid phase concentration of CAP at equilibrium; b (L/mg) represents the adsorption constant of Langmuir; q0 (mg/L) is the maximum capacity of CAP according to Langmuir model; C0 is the maximal initial concentration of CAP in isotherm study; RL is the non-dimensional constant and shows the feature of Langmuir model; Kf (mg/g(L/mg)1/n ) delegates the Freundlich adsorption constant; n is dimensionless parameter denoting the adsorption intensity for Freundlich. As shown in Table 3, the higher degree of fitting occurred in Langmuir models indicating Langmuir was more proper to explain the adsorption process which was monolayer adsorption for the four carbons. The values of RL were all between 0 to1 indicating that adsorption CAP was advantageous by the four PCMs [32], which can be proved by 1/n that all were less than 1 [33]. Meanwhile, the maximum adsorption capacities of PC-PCM, FL-PCM, CFPCM and EP-PCM were 159.98, 621.12, 740.74 and 892.86 mg/g and were consistent in order of the BET surface area. There were various adsorption capacities of CAP by PC-PCM, FL-PCM, CF-PCM, FPPCM and other absorbents in Table 4. Though, the preparation conditions of these carbons were not the same, there still existed a necessity to contrast the adsorption capacities. EP-PCM exhibited more gorgeous adsorption capacity of CAP than other adsorbents [16,29,34]. In addition, FL-PCM, CF-PCM and EP-PCM all owned a well potential for removal of CAP.
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Table 3 R2 and coefficients of Langmuir and Freundlich models of adsorption CAP onto PC-APCM, FL-PCM, CFPCM and EP-PCM. Isotherm models
Langmuir
Freundlich
Parameters
Absorbents
q0 (mg/g) Kl (L/mg) RL R2 Kf (mg/g(L/mg)1/n ) 1/n R2
PC-PCM
FL-PCM
CF-PCM
EP-PCM
159.98 8.928 × 10−3 9.73 × 10−2 0.9912 43.38 0.07132 0.9465
621.12 0.1655 5.78 × 10−3 0.9981 344.73 0.1732 0.9093
740.74 0.3470 2.76 × 10−3 0.9991 393.67 0.1235 0.9029
892.86 0.4686 2.05 × 10−3 0.9989 433.03 0.1640 0.8864
Table 4 Adsorption capacities of various adsorbents for CAP. Adsorbents
Adsorption capacity (mg/g)
Reference
Functionalized biochar Ordered mesoporous carbon N-doped porous carbon PC-PCM FL-PCM CF-PCM EP-PCM
21 210 742 160 621 741 893
[30] [16] [29] This This This This
study study study study
respectively, and were much higher than that of PC-PCM (256 m2 /g). The degree of raw material activated by potassium tartrate was in the order of EP > CF > FL > PC. The ratios of Vmic /Vtot were in an order of EP-PCM < FL-PCM < PC-PCM < CF-PCM. Meanwhile, the four PCMs showed the distinguishing contrast in adsorption capacities of 159.98 mg/g (PC-PCM); 621.12 mg/g (FL-PCM); 740.74 mg/g (CF-PCM) and 892.86 mg/g (EP-PCM). The solution pH had slight influence on the adsorption abilities of CAP by the four PCMs. Pseudo-second-order and Langmuir model severally exhibited high-consistency with kinetics and isotherm data. FL-PCM, CFPCM and EP-PCM all were gorgeous choices for removal CAP. Acknowledgments This work was supported by Tai Shan Scholar Foundation [No. ts201511003]. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2018.01.025. References
Fig. 6. The influence of initial pH on adsorption capacities of CAP by PC-PCM, FLPCM, CF-PCM and EP-PCM (temperature 25 °C, dosage 1 g/L, adsorption time 24 h).
3.8. Effect of pH The influence of pH on CAP adsorbed by four PCMs was investigated at a range from 2 to 12 at initial concentration 800 mg/g (Fig. 6). There was slight effect of pH on the adsorption by the four PCMs, but there was a little decrease at pH 6–8, once again indicating the adsorption mechanism of CAP also related to electrostatic interaction between adsorbent and CAP. Some similar conclusions can be found in previous literatures [29,35]. 4. Conclusion Potassium tartrate was proved to be an effective activating agent to produce PCMs with high surface area, especially for FL, CF and EP as staring materials, thus it can provide a reference for further research and practical application. FL-PCM, CF-PCM and EPPCM owned high BET surface area of 1721, 1819 and 2152 m2 /g,
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2018) 1–8
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Facile synthesis of hierarchical porous carbon material by potassium tartrate activation for chloramphenicol removal Xiuzhen Zhu a, Yuan Gao b, Qinyan Yue a,∗, Yan Song a, Baoyu Gao a,∗, Xing Xu a a
Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, PR China b Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, PR China
a r t i c l e
i n f o
Article history: Received 10 August 2017 Revised 14 January 2018 Accepted 19 January 2018 Available online xxx Keywords: Potassium tartrate Porous carbon Self-activation Solid wastes Chloramphenicol removal
a b s t r a c t Potassium tartrate as activator was used to produce porous carbon materials (PCMs) by activating petroleum cokes (PC), fallen leaves (FL), chicken feathers (CF) and Enteromorpha prolifera (EP) under the identical conditions (activation temperature of 750 °C, impregnation ratio of 2.5:1 and activation time of 1 h). Potassium tartrate can also be self-activated to facilely synthesize PCMs simultaneously. The PCMs synthesized through various precursors showed significant discrepancies in morphologies and the porous structure was characterized by SEM and N2 adsorption–desorption isotherm. The BET surface areas of FL-PCM (1721 m2 /g), CF-PCM (1819 m2 /g), and EP-PCM (2151 m2 /g) were larger than that of PC-PCM (256 m2 /g) and the microporosity of EP-PCM (48%) was much lower than those of others. In addition, the adsorption properties of the four PCMs under various conditions were detected by selecting chloramphenicol (CAP) as a target adsorbate. The kinetics and isotherms of adsorption processes were well fitted by pseudo-second-order and Langmuir model for four PCMs. Meanwhile, EP-PCM exhibited high adsorption capacity of 892.86 mg/g. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Porous carbon is a classical material applied as high efficient adsorbent, electrode and catalyst carrier et al. The main characteristics of PCM are physical properties (surface area, pore volume and pore-size) and chemical properties (functional groups), which are principally affected by precursor, activator and activated condition [1]. There are various precursors to produce PCMs mainly containing petrochemicals [2], lignocellulosic biomass [3], keratoprotein [4] and algaes [5] et al. The representatives of these categories are petroleum cokes (PC), fallen leaves (FL), chicken feathers (CF) and Enteromorpha prolifera (EP), respectively. PC is an unavoidable by-product with high carbon content (about 90 wt %) and low ash [2,6]. PC is a traditional material for producing PCM with low-cost. FL falls to the road especially in autumn and is traded as rubbish. FL contains various carbonaceous compounds and is almost free, accessible and renewable [7]. As a staring material, it can realize resource utilization and endow FL with addition value. CF is mainly treated as waste by landfill without sufficient use [4]. The principal ingredient of CF is keratin which is an excellent precursor to
∗
Corresponding authors. E-mail addresses: [email protected] (Q. Yue), [email protected] (B. Gao).
prepare N-doped PCMs [4,8]. CF has been proved to be a fine material to prepare PCMs in previous studies [4]. EP, a type of green alga, is a product due to marine eutrophication, and composed of polysaccharose, protein and fat et al, which are easily pyrolyzed [9]. Hence, choosing these stuffs as the typical precursors can compare their effects for preparing PCMs. KOH is a high-efficiency activator to produce PCMs with superlarge surface area, but there still exists some deficiencies during the activating while the most prominent one is high corrosive level [7]. Hence, exploiting sylvite as activator is imperative to lower the deficiencies. Potassium tartrate is an organic sylvite which has been used to produce porous carbons with 1018 m2 /g surface area by immediate carbonization previously [10]. Therefore, potassium tartrate could activate precursors and be self-activated to synthesize PCMs with high surface area. In addition, the secondary pollution caused by potassium tartrate was lighter than that of KOH, because the anion (tartrate anion) could work as carbon sources and it’s consumed to synthesize the porous carbon. Based on the above, potassium tartrate was used to activate PC, FL, CF and EP, and self-activate to produce PCMs in this study. With the vast usage of antibiotics, the harm to the environment caused by antibiotics has drawn more attention in recent years. Antibiotics are overused in stock farming due to lack in establishing relevant regulations [11]. Approximately 75% of administered
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drugs by animals is not assimilated and excreted through feces or urines into the environment [12]. Bacteria resistance for antibiotics has been an intractable puzzle and has caused enormous threat to human health [13]. Therefore, many technologies have been applied to remove antibiotics, such as photocatalysis [14], advanced oxidation [15] and PCMs adsorption [16]. Due to the high adsorption capacity, low cost and eco-friendly et al, PCMs adsorption is a valid method to remove the antibiotic wastewater [17]. Combined with the above viewpoints, primary targets of this study were to probe into the feasibility that potassium tartrate could activate precursors (PC, FL, CF and EP) and be self-activated to prepare PCMs, and to investigate the differences between PCPCM, FL-PCM, CF-PCM and EP-PCM in characteristics of pore structure and morphology. Meanwhile, the activated mechanism of potassium tartrate involved in the preparation process was explored. In addition, the adsorption contrasts of CAP as a model antibiotic were investigated as a function of adsorption time, initial concentration and solution pH. 2. Materials and methods
sample, which was shaken in a constant temperature bath oscillator with a speed of 150 rpm. In kinetics studies, the samples owed 10 0 0 mg/L initial concentration of CAP and 0.050 g PCM, were taken out according to a pre-set time interval. The residual CAP was segregated with PCMs by membrane-filter and its concentration was measured at 277 nm (maximum absorption wavelength of CAP) by an apparatus UV–vis spectrophotometer (UV754, Shanghai). In thermodynamics experiments, the different initial concentrations (30 0–10 0 0 mg/L) of CAP were adsorbed by the four PCMs under 293 K and natural pH for 24 h. To study the impact of pH, the pH of CAP solution (800 mg/L) was adjusted from 2 to 12 by adding the 0.1 mol/L HCl or NaOH solutions. The quantity of adsorbed CAP at adsorption equilibrium, qe (mg/g) was calculated via the following equation:
qe =
(C0 − Ce )v w
(1)
where C0 and Ce (mg/L) are the original and equilibrium CAP concentration, respectively. v(L) is the CAP solution volume and w (g) is the PCMs weight. 3. Results and discussion
2.1. Materials 3.1. Thermal analysis All the chemical reagents were analytically pure and distilled water was used in the whole experiment. Potassium tartrate and chloramphenicol (CAP) were provided by Aladdin (Shanghai, China). The four waste materials were obtained locally, washed and smashed to powder. 2.2. Preparation of PCMs The four raw materials were carbonized under 500 °C for 1.5 h using a carbonization furnace (KSY-4D-16) with an enclosed chamber to get char. 10 g char was mixed with 25 g potassium tartrate (an impregnation ratio of 2.5:1). Then the mixtures were put into a tubular resistance furnace (SKQ-3-10) where samples were heated to a requested temperature 750 °C with a constant heating rate of 10 °C/min in the existence of N2 and then maintained for 60 min. When cooled down to room temperature, the products were washed by HCl, and distilled water until the value of pH kept constant. Finally, the samples were dried, ground and sieved to 10 0–20 0 mesh (0.074–0.15 mm). 2.3. Characterization methods To understand the weight loss of different precursors during activation process, the four kinds of mixtures (potassium tartrate and char) were characterized by thermogravimetric analysis (TGA), (SHI-MADZU, TGA-50) and heated from room temperature to 750 °C with a rate of 10 °C/min and then kept at 750 °C for 60 min under N2 atmosphere. The pore structure properties of PCMs were measured by N2 adsorption–desorption isotherms under 77 K using a surface area instrument (JW-BK 122W, Beijing JWGB Sci. & Tech. Co., Ltd., China). Meanwhile, the surface morphologies of the four PCMs were characterized by SEM (FEI, NOVA NANOSEM450, USA). X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was used to investigate the surface chemical properties of PCM. X-ray diffraction (XRD, Rigaku D/MAX-YA) was employed in the 2θ range of 10–80°. 2.4. Adsorption experiments In adsorption experiments, the influencing parameters of adsorption time, concentration and pH were explored for the four PCMs. For each sample, 0.050 g of PCM accurately weighed was added to 50 mL solution of CAP in 250 mL triangle bottle for each
By TGA and DTG (differential TGA), the pyrolysis processes of four mixtures (potassium tartrate: char of precursor = 2.5:1) are described in Fig. 1. The weight-loss showed the analogous tendency in the four compounds during pyrolysis from Fig. 1(a), which can be divided into three stages. The first section was before 300 °C, and the mass losses of PC-PCM, FL-PCM, CF-PCM and EPPCM were 16.59%, 21.77% 20.81% and 23.17%, respectively, which was ascribed to the moisture losses (adsorbed and bound water) and degradation of low-boiling compounds [18]. When the temperature reached 750 °C, there was a section of weight losses for each sample (17.46% of PC-PCM, 15.78% of FL-PCM, 19.48% of CFPCM and 17.04% of EP-PCM). There were numerous degradation reactions between four crude materials and potassium tartrate forming the cross-linked structure. In the third portion, the qualitylosses after activating 60 min at 750 °C were 11.15%, 25.26%, 19.18% and 23.02% for PC-PCM, FL-PCM, CF-PCM and EP-PCM, respectively, which produced more porous structures by the catalysis of potassium tartrate. The total weight losses were 45.2%, 62.81%, 59.47% and 63.23% for PC-PCM, FL-PCM, CF-PCM and EP-PCM, respectively. The weight loss of PC-PCM was much less than those of others, which may be caused by the firm construction of PC and difficult degradation even in the existence of activation agent [1]. Meanwhile, the DTG curves have a similar trend in Fig. 1(b) except that a rapid weight loss appeared around 50 °C and no peak occurred in 227 °C for EP-PCM. The synthesis mechanism of potassium tartrate activating PC, FL, CF and EP exhibited certain similarities with the activated process of directly carbonized organic salt or KOH chemical activation with comparison and analogy method [19,20]. When the temperature reached about 450–650 °C, the organic fraction of precursors was degraded and K2 CO3 was generated, meanwhile, the organic moiety of activated agent (tartrate) worked as carbon source. The existence of K2 CO3 was proved by XRD texting when potassium tartrate was directly carbonized at intermediate temperature of 650 °C (Fig. S1). Above 700 °C, K2 CO3 were transformed into K2 O and CO2 , and the gas of CO2 reacted with C to form CO. This reaction can contribute to generate more pores via C gasification. Meanwhile, a vital reaction could generate K over 700 °C between K2 O and C. K in the form of vapor possessed a significant function to insert into carbon matrix to form more porous structure by enlarging microstructure. Referred chemical reactions were listed as equations as followings:
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Fig. 1. TGA (a) and DTG (b) profiles for potassium tartrate impregnated char from PC, FL, CF and EP at the rate of 2.5: 1.
Fig. 2. (a) N2 adsorption–desorption isotherms (b) pore and (inset) micropore size distribution of PC-PCM, FL-PCM, CF-PCM and EP-PCM.
K2 CO3 →K2 O+CO2 CO2 +C→2CO K2 O+C→2K+CO The inorganic impurities (such as K2 CO3 , K) were removed by washing used diluted HCl, and the occupied interspaces were left. Finally, four kinds of porous carbon were obtained. 3.2. Textural characteristics Surface area, pore volume and size are vital performance indexes of multi-porous carbon working the adsorption characteristics of contaminants. The N2 adsorption-desorption isotherm and pore-size distribution of PCMs are displayed in Fig. 2. The isotherm of EP-PCM owned an emblematic hysteresis loop under high p/p0 , which was the typical feature of type I and IV according to the category of adsorption–desorption isotherm (IUPAC). This isotherm manifested EP-PCM possessed well microporous and mesoporous
structure. The semblable isotherms of PC-PCM, FL-PCM and CFPCM were found in Fig. 2(a) which were typical I. The curves firstly hoiked and then tended to equilibrium, demonstrating the main pores were microporous. This result was in sympathy with the distribution of pore size in Fig. 2(b). The concentrated distribution of pore size was nearby 2 nm, especially for FL-PCM, CF-PCM and EPPCM. There were several peaks at around 0.62, 1.0 and 1.4 nm for FL-PCM, CF-PCM and EP-PCM in inset Fig. 2(b), suggesting that the FL-PCM, CF-PCM and EP-PCM owned hierarchical micropore structure. In addition, CF-PCM also possessed a peak at around 0.37 nm and there was no obvious peak for PC-PCM. Variously significance parameters of four PCMs are summarized in Table 1. The BET surface area (SBET ) of EP-PCM (2152 m2 /g) was the larger than those of PC-PCM (256 m2 /g) FL-PCM (1721 m2 /g) and CF-PCM (1819 m2 /g). The surface areas of PCMs derived from FL, CF and EP were over 6.7 times than that derived from PC at uniform activation conditions, which was due to the firm structure of PC, leading to difficult pyrolysis under the assistance of potassium tartrate. Meanwhile, the main ingredients of EP (polysaccharose, hemi-cellulose and protein), FL (cellulose, xylogen and
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X. Zhu et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2018) 1–8 Table 1 Pore structure parameters of PC-PCM, FL-PCM, CF-PCM and EP-PCM. Sample
SBET (m2 /g)
Smic (m2 /g)
Sext (m2 /g)
Vtot (cm3 /g)
Vmic (cm3 /g)
Vmic /Vtot (%)
1−Vmic /Vtot (%)
Dp (nm)
PC-PCM FL-PCM CF-PCM EP-PCM
256 1721 1819 2152
248 1491 1649 1585
8 230 170 567
0.125 0.898 0.853 1.509
0.100 0.639 0.693 0.728
80 71 81 48
20 29 19 52
1.9 2.1 1.9 2.8
SBET : BET surface area; Smic : micropore surface area; Sext : external surface area; Vtot : total pore volume; Vmic : micropore volume; Dp : mean pore size.
Fig. 3. SEM micro-graph of (a) PC-PCM, (b) FL-PCM, (c) CF-PCM and (d) EP-PCM at 50 0 0×magnification.
carbohydrate) and CF (helix keratin) were easily to be pyrolyzed [1,4,21]. In addition, the microporosity of EP-PCM (48%) was much less than those of others and the mean pore size of EP-PCM (2.8 nm) was maximum among four PCMs, which were in accordance with the isotherm and pore-size distribution. From the above results, it’s demonstrated that FL, CF and EP were suitable materials via potassium tartrate activation to produce PCMs and potassium tartrate was an effective activation agent. Meanwhile, the degree of the four precursor activated by potassium tartrate followed the order of EP > CF > FL > PC. The porous structure of EP-PCM was contrasted to some EPbased porous carbon derived from familiar activators (KOH, H3 PO4 , ZnCl2 ) in Table S1. The surface area of EP-PCM activated by potassium tartrate was larger than that activated by H3 PO4 [22], ZnCl2 [23], and slightly lower than that activated by KOH [24]. Potassium tartrate had other innovations compared with KOH, which were low corrosive, and could act as carbon source to produce PCM through self-activation. 3.3. SEM The morphologies of four PCMs surface were characterized by SEM in Fig. 3 at identical magnification (50 0 0×). The four types of PCMs showed dramatic difference in porous structure attributed to diverse essences of raw materials. PC-PCM owned the faveolate microstructure in Fig. 3(a), but pore walls were smoother than others, which caused the structure of PC was hard to be entered to build more loose pores in the context of potassium tartrate acti-
vating. FL-PCM possessed unconsolidated configuration in Fig. 3(b). CF-PCM had many small pores in Fig. 3(c). There was deep porous structure for EP-PCM. Compared with precursor EP micrograph in others’ study [5], it seems that activating agent and EP-char via self-template formed dense porous structure without destroying the channel-like wall [25]. The function of potassium tartrate was to occupy volumes of carbonaceous materials under a certain temperature. Then, the various pores were left by pickling and water washing.
3.4. Surface chemistry analysis The elemental composition and surface chemical groups of the four porous carbons were characterized by XPS. The highresolution C1s spectrums are displayed in Fig. 4. The complicated envelopes of C1s could be de-convoluted into four peaks. The main peak (C1) located in 284.7 eV was corresponded to sp2 carbon, and other peaks located near to 285.2 eV (C2), 286.6 eV (C3), 289.6 eV (C4) were belonged to C–O (&C–N), C=O and O–C=O, respectively [26,27]. These four PCMs shared the same functional groups with different relative amounts. The relative percentage of each element (C, O, N) is illustrated in Table S2. The O content increased in the order of PC-PCM (6.98%), FL-PCM (7.23%), CF-PCM (8.54%), EPPCM (12.55%). Meanwhile, CF-PCM contained the highest N content (1.20%). The O-containing and N-containing functional groups were of benefit to the adsorption capacity, which were participated in the adsorption process of CAP on the four porous carbons.
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Fig. 4. High-resolution XPS spectra of C1 for (a) PC-PCM, (b) FL-PCM, (c) CF-PCM and (d) EP-PCM.
3.5. Effect of contact time The effect of contact time on CAP absorption by the four kinds of PCMs is depicted in Fig. 5. The analogous adsorption trends were obtained with different adsorption time. The adsorbance of CAP sharply enhanced at first, especially in the original 5 min, then gradually minished over time and gradually reached the equilibrium. Lots of empty adsorption sites and high driving force between the adsorbents and adsorbates made adsorption rapider in the beginning. As the contact time increased, the effective adsorption sites were gradually occupied by CAP and driving force gradually shrunken with the decrease of CAP concentration and active sites of PCMs. Meanwhile, the absorption resistance increased between CAP in the solution and which absorbed by PCMs. The values of time and adsorption capacities at equilibrium were (60 min, 147 mg/g), (60 min, 615 mg/g), (460 min and 703 mg/g) and (20 min, 912 mg/g) for PC-PCM, FL-PCM, CF-PCM and EP-PCM, respectively. The BET surface area of CF-PCM (1819 m2 /g) was a bit higher than that of FL-PCM (1721 m2 /g), but the time needed till the equilibrium was more than FL-PCM. This phenomenon mainly caused by steric effect which restricted the rate of conveying the CAP to the pore interior of PCMs [28]. The microporosity of CF-PCM (81%) was higher than that of FL-PCM (71%), so mesopore was beneficial to adsorb CAP. In general, EP-PCM exhibited high adsorption capacity and rate towards CAP.
Fig. 5. The influence of contact time on CAP adsorbed by PC-PCM, FL-PCM, CF-PCM and EP-PCM (nature pH, temperature 25 °C, dosage 1 g/L).
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X. Zhu et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2018) 1–8 Table 2 The coefficients and R2 of pseudo-first-order, pseudo-second-order and intra-particle diffusion models for CAP adsorbed by PC-PCM, FL-PCM, CF-PCM and EP-PCM. Kinetic models
Pseudo-first-order
Pseudo-second-order
Intra-particle diffusion
Parameters
qe ,exp (mg/g) k1 (min−1 ) qe,cal (mg/g) R2 k2 (g/mg min) qe,cal (mg/g) h (mg/g min) R2 kp1 (mg/g min1/2 ) C R2 kp2 (mg/g min1/2 ) C R2
Absorbents PC-PCM
FL-PCM
CF-PCM
EP-PCM
146.65 0.0934 83.05 0.6596 2.45 × 10−2 137.17 460.98 0.9980 44.40 8.72 0.8925 5.071 92.66 0.9866
614.63 0.1055 219.66 0.8339 6.85 × 10−4 613.50 257.82 0.9999 100.42 284.71 0.9933 13.20 489.13 0.9110
703.49 0.0612 450.67 0.9316 1.41 × 10−4 704.23 69.93 0.9992 49.82 259.21 0.9005 8.326 517.77 0.9459
912.33 0.2734 248.72 0.9667 5.65 × 10−3 909.09 4669.41 1.0 0 0 0 109.96 600.45 0.9661 8.228 868.09 0.9620
3.6. Kinetics study of CAP To get more information of adsorption rate, the models of pseudo-first-order (Eq. (2)), pseudo-second-order (Eq. (3)) and intra-particle diffusion (Eq. (4)) are employed to analyze data.
ln (qe − qt ) = ln qe − k1 t
(2)
t 1 1 = + t qt qe k2 q2e
(3)
qt = kpi t1/2 + C
h = k2 qe 2
(4)
where qe and qt (mg/g) are the quantity of CAP absorbed on the PCMs at equilibrium and setting time t. k1 (min−1 ), k2 (g/(mg min)) and kpi (mg/(g min1/2 )) is the pseudo-first-order, pseudosecond-order and intra-particle diffusion rate constant, respectively. h (mg/(g min)) is the initial adsorption rate. C is a constant of Eq. (4). The fitting results of adsorption data are listed in the Table 2. The pseudo-second-order model owed a higher consistent degree compared with pseudo-first-order models, whose R2 was greater than or equal to 0.9980. Meanwhile, the values of qe,cal calculated by pseudo-second-order formula were nearer to the experimental value, unlike the qe,cal obtained from pseudo-first-order models, which was far less than the experimental values. This result manifested that the chemisorption was a factor to influence adsorption rate between CAP and surface of PCMs [18]. According to the results of XPS analysis and the structure of CAP (Fig. S2), the electrostatic interactions were involved in the adsorption process as follows: 1. EDA interaction, the strong electron withdrawing groups (nitro, ketone, benzene) in CAP acted as π -electron-accepters. PCM acted as the π -electron-donors which had the sp2 carbon along with O-containing groups. The π -electron-accepters could bind to the π -electron-donors through π –π conjugation in the adsorption of CAP which was called electro–donor–acceptor (EDA) interaction [29]. 2. H-bond interaction, the O-containing groups of the four porous carbons could form the H-bond with the N–H, –OH structure of CAP to enrich the adsorption. The intra-particle diffusion model was employed to know the adsorption rate-determining step [30]. There involved two steps in the process of adsorption CAP by the four PCMs (Fig. S3). The external surface or momentary adsorption appearing was in the first stage, where the main rate control factor was boundary layer diffusion [31]. The rate control factor of second part was intra-particle diffusion and this stage was called gradual adsorption phase [23]. The rates of momentary adsorption (stage 1) were much higher attributed to abundantly
available surface area and active sites. There were greatly reduction in rates of stage 2, which was caused by less active sites, the lager electrostatic resistance between PCMs surface and CAP and the smaller driving force due to the decrease in CAP concentration. 3.7. Adsorption isotherm Adsorption isotherm is beneficial to understand the interaction between carbon-based adsorbent and absorbate, and predict the adsorption capacity of carbon-based adsorbent. Data of adsorption isotherm was analyzed by being fitted with two classic models (Langmuir and Freundlich equation) and the equations of Langmuir (Eqs. (5) and (6)) and Freundlich (Eq. (7)) were listed as follows:
Ce 1 1 = + Ce qe q0 b q0
(5)
1 1 + bC0
(6)
RL =
ln qe = ln Kf +
1 ln Ce n
(7)
where Ce (mg/L) is the liquid phase concentration of CAP at equilibrium; b (L/mg) represents the adsorption constant of Langmuir; q0 (mg/L) is the maximum capacity of CAP according to Langmuir model; C0 is the maximal initial concentration of CAP in isotherm study; RL is the non-dimensional constant and shows the feature of Langmuir model; Kf (mg/g(L/mg)1/n ) delegates the Freundlich adsorption constant; n is dimensionless parameter denoting the adsorption intensity for Freundlich. As shown in Table 3, the higher degree of fitting occurred in Langmuir models indicating Langmuir was more proper to explain the adsorption process which was monolayer adsorption for the four carbons. The values of RL were all between 0 to1 indicating that adsorption CAP was advantageous by the four PCMs [32], which can be proved by 1/n that all were less than 1 [33]. Meanwhile, the maximum adsorption capacities of PC-PCM, FL-PCM, CFPCM and EP-PCM were 159.98, 621.12, 740.74 and 892.86 mg/g and were consistent in order of the BET surface area. There were various adsorption capacities of CAP by PC-PCM, FL-PCM, CF-PCM, FPPCM and other absorbents in Table 4. Though, the preparation conditions of these carbons were not the same, there still existed a necessity to contrast the adsorption capacities. EP-PCM exhibited more gorgeous adsorption capacity of CAP than other adsorbents [16,29,34]. In addition, FL-PCM, CF-PCM and EP-PCM all owned a well potential for removal of CAP.
Please cite this article as: X. Zhu et al., Facile synthesis of hierarchical porous carbon material by potassium tartrate activation for chloramphenicol removal, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.01.025
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Table 3 R2 and coefficients of Langmuir and Freundlich models of adsorption CAP onto PC-APCM, FL-PCM, CFPCM and EP-PCM. Isotherm models
Langmuir
Freundlich
Parameters
Absorbents
q0 (mg/g) Kl (L/mg) RL R2 Kf (mg/g(L/mg)1/n ) 1/n R2
PC-PCM
FL-PCM
CF-PCM
EP-PCM
159.98 8.928 × 10−3 9.73 × 10−2 0.9912 43.38 0.07132 0.9465
621.12 0.1655 5.78 × 10−3 0.9981 344.73 0.1732 0.9093
740.74 0.3470 2.76 × 10−3 0.9991 393.67 0.1235 0.9029
892.86 0.4686 2.05 × 10−3 0.9989 433.03 0.1640 0.8864
Table 4 Adsorption capacities of various adsorbents for CAP. Adsorbents
Adsorption capacity (mg/g)
Reference
Functionalized biochar Ordered mesoporous carbon N-doped porous carbon PC-PCM FL-PCM CF-PCM EP-PCM
21 210 742 160 621 741 893
[30] [16] [29] This This This This
study study study study
respectively, and were much higher than that of PC-PCM (256 m2 /g). The degree of raw material activated by potassium tartrate was in the order of EP > CF > FL > PC. The ratios of Vmic /Vtot were in an order of EP-PCM < FL-PCM < PC-PCM < CF-PCM. Meanwhile, the four PCMs showed the distinguishing contrast in adsorption capacities of 159.98 mg/g (PC-PCM); 621.12 mg/g (FL-PCM); 740.74 mg/g (CF-PCM) and 892.86 mg/g (EP-PCM). The solution pH had slight influence on the adsorption abilities of CAP by the four PCMs. Pseudo-second-order and Langmuir model severally exhibited high-consistency with kinetics and isotherm data. FL-PCM, CFPCM and EP-PCM all were gorgeous choices for removal CAP. Acknowledgments This work was supported by Tai Shan Scholar Foundation [No. ts201511003]. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2018.01.025. References
Fig. 6. The influence of initial pH on adsorption capacities of CAP by PC-PCM, FLPCM, CF-PCM and EP-PCM (temperature 25 °C, dosage 1 g/L, adsorption time 24 h).
3.8. Effect of pH The influence of pH on CAP adsorbed by four PCMs was investigated at a range from 2 to 12 at initial concentration 800 mg/g (Fig. 6). There was slight effect of pH on the adsorption by the four PCMs, but there was a little decrease at pH 6–8, once again indicating the adsorption mechanism of CAP also related to electrostatic interaction between adsorbent and CAP. Some similar conclusions can be found in previous literatures [29,35]. 4. Conclusion Potassium tartrate was proved to be an effective activating agent to produce PCMs with high surface area, especially for FL, CF and EP as staring materials, thus it can provide a reference for further research and practical application. FL-PCM, CF-PCM and EPPCM owned high BET surface area of 1721, 1819 and 2152 m2 /g,
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Please cite this article as: X. Zhu et al., Facile synthesis of hierarchical porous carbon material by potassium tartrate activation for chloramphenicol removal, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.01.025