Journal Pre-proofs Templating synthesis of hierarchical porous carbon from heavy residue of tire pyrolysis oil for methylene blue removal Yulin Zhang, Guozhao Ji, Changjing Li, Xuexue Wang, Aimin Li PII: DOI: Reference:
S1385-8947(20)30389-2 https://doi.org/10.1016/j.cej.2020.124398 CEJ 124398
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Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
12 December 2019 6 February 2020 7 February 2020
Please cite this article as: Y. Zhang, G. Ji, C. Li, X. Wang, A. Li, Templating synthesis of hierarchical porous carbon from heavy residue of tire pyrolysis oil for methylene blue removal, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124398
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Templating synthesis of hierarchical porous carbon from heavy residue of tire pyrolysis oil for methylene blue removal Yulin Zhang, Guozhao Ji, Changjing Li, Xuexue Wang, Aimin Li* Key Laboratory of Industrial Ecology and Environmental Engineering, School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, PR China *Corresponding author. Tel.: +86 411 8470 7448 (A. Li); E-mail:
[email protected] (Y. Zhang);
[email protected] (A. Li)
Abstract A simple and regulatable strategy was proposed to synthesis hierarchical porous carbon from heavy residue of waste tire derived pyrolytic oil (TPO). Magnesium acetate powder was adopted as a pore creator and adjustor which was mixed with heavy residue and tetrahydrofuran (THF) by grinding. Subsequently, the various pore structure carbons were obtained after evaporation, carbonization and MgO remove. The change in pore size was significantly observed with the magnesium acetate dosage, which was mainly attributed to the interaction between the magnesium acetate particles. In this method, the addition of magnesium acetate could result in higher carbon yield. Remarkably, the carbon prepared at optimum condition exhibited higher specific surface area and micro-mesopore structure leading to an excellent 1
performance towards dye contaminant removal. The prepared porous carbon sample exhibited best adsorption capacity (843.5 mg/g) at 298 K for methylene blue (MB). Furthermore, the effect of pH and temperature on adsorption capacity of prepared porous carbon were also investigated. The adsorption process of MB onto porous carbon was favorable according to the Freundlich model parameter (1/n < 0.5). This study highlighted a great potential of using the heavy residue of TPO as a low-cost precursor for porous carbon and simultaneously offered a value-added way to treat the heavy residue of waste tire derived pyrolytic oil. Keywords: hierarchical porous carbon, heavy residue, tire pyrolysis oil, methylene blue, adsorption
1.
Introduction With rapid growth of automobile sales, a great number of tires are being produced
to meet the demands. According to a recent report, in 2017, about 25 million tons tires were produced and 13 million tons tires were abandoned as wastes in China [1]. The improper disposal methods of waste tires (i.e. landfill and incineration) lead to environmental and public health concerns such as pollutions emission (NOx, SOx), land resources waste, mosquito breeding. Pyrolysis is a thermochemical conversion process at high temperatures (573-1073K) which decomposes and breaks polymer organics into small molecule compounds under inert conditions [2]. The main products of thermal decomposition are liquid oil, gases and solid [3]. Pyrolysis has been regarded as the 2
most effective technology to convert waste tires into valuable oil, gas and char products [4-6]. The tire pyrolysis oil (TPO) has been considered as an energy or chemical raw material with prospect of wide development and utilization [6, 7]. However, pollutants such as NOx, SOx could be released into the environment when TPO is directly burned and endangering the environment and human health [8-10]. While, TPO refining based on the difference in boiling point and volatility is an optimal way to obtain high quality fuel and chemicals (such as, BTX and d,l-limonene) [11]. In fact, the heavy residue fraction, a bottom product of this process, has the characteristics of high-carbon content, low-ash and fluidity. The high initial viscosity of heavy residue fraction restricts its application as fuel for energy generation. The heavy residue of TPO also contains high amount of N and S, which could form NOx and SOx upon combustion. Especially, the heavy residue of TPO has polycyclic aromatic hydrocarbons (PAHs), which is carcinogenicity and promotes the formation of dioxins [12]. Currently, traditional methods of treating heavy residue are coke and modified asphalt formation [13]. However, the heavy residue of TPO can be not used directly to make asphalt but as an asphalt additive to modified asphalt [13]. For coke produced by heavy residue of TPO, the value and quality of the coke is low, which was attribute to sulfur containing in heavy residue. Hence, it’s necessary to search a method to dispose the heavy residue of TPO with high value. Considering the carbon-rich nature of heavy residue, it could be used as an inexpensive feedstock to prepare porous carbon, which convert toxic PAHs into harmless and valuable carbon materials. This strategy could provide a win-win 3
benefit: an environmental reward with alterative waste disposal method and an economic reward that provide a novel, renewable and low-cost precursor for porous carbon. The porous carbon materials have a wide range of applications, and can be used as adsorbent [14-16], electrode [17, 18] and catalyst support [19] due to their high specific surface area and favorable pore structure. Therefore, many studies have been devoted to designing various porous carbon suitable for different fields. Chen et al. fabricated mesoporous carbon using P123 as a structure adjusting agent for efficient removal of Di-n-butyl phthalate efficiently [20]. Wang et al. prepared hierarchical porous carbon for supercapacitors by nano-ZnO template method [21]. The prepared carbon exhibited high specific capacitance of 127 F/g at 0.1A/g and high retention rate of 84% at 100A/g. The porous carbon could be prepared by several method such as pyrolysis carbonization [22, 23], activation [24, 25] and template method [21, 26]. The template method is favored by many researchers as it has great regulatory effect on the pore structure [27]. Many porous carbons with micropores, mesopores and macropores that was called hierarchical porous carbon have been synthesized using various organic or inorganic templates [28]. The micro-mesopores structure of hierarchical porous carbon is conducive to exceptional adsorption performance which attribute to the synergy of micropores and mesopores. Compared with single microporous structure, the mesopores added provide free diffusion paths for adsorbate into interior surface to facilitate mass transfer [29]. Recently, metal oxides with 4
different morphologies were often used as template to prepare porous carbon materials [30-32]. On the other hand, low cost and environmentally-friendly carbon precursors have been widely acknowledged in many fields, such as coal tar pitch [33], β-cyclodextrin polymers [14], cotton [34], waste plastic bag [30] and others. Nevertheless, to the best of our knowledge, little research has been undertaken on employing heavy residue of TPO as a precursor for porous carbon production through template method. In this work, the preparation of porous carbon derived from heavy residue was proposed. The hierarchical porous carbon was successfully synthesized using heavy residue of TPO as precursor and magnesium acetate as template. In the carbonization process, the magnesium acetate decomposed into magnesium oxide, while the heavy residue formed into carbon layer on the surface of magnesium oxide. The dosage of magnesium acetate has a direct effect on the pore size and specific surface area of the as-prepared hierarchical porous carbons. The introduce of magnesium acetate not only provide the porous structure but also reduce the heat loss during pyrolysis and carbonization. Furthermore, the porous carbon prepared exhibited high MB removal ability in aqueous solution (843.5 mg/g).
2.
Experiment
2.1
Preparation of samples
The heavy residue of TPO was used as the carbon precursor. The proximate analysis 5
data on air dried basis and elemental analysis of heavy residue are shown in Table 1. The porous carbon materials were synthesized by MgO template-method using magnesium acetate [Mg(CH3COO)2·4H2O] as template. Firstly, magnesium acetate was milled into powder by ball mill. Then, the heavy residue, tetrahydrofuran and magnesium acetate powder were mixed and ground by mortar and pestle. The tetrahydrofuran was added for promoting the mixing. The mixture was dried in fume hood over night to remove tetrahydrofuran at room temperature. After drying, the mixture was removed into an alundum boat and carbonized in a tubular furnace at 973 K for 2 h with a heating rate of 5 K/min under nitrogen flow of 100 mL/min. Finally, the sintered composite power was washed with 1 M HCl for 12 h to remove template and then washed with deionized water for several times to achieve neutral pH. The obtained product was dried at 378 K for 24 h and denoted as MOxy (M represents magnesium acetate and O represents the heavy residue). The value of x/y represents mass ratio of MgO to heavy residue (x/y = 1, 2, 3).
2.2 Characterization of samples (MOxy) The thermogravimetry (TG) analysis was conducted in EXSTAR SII TG/DTA 6300. Approximately 5 mg of sample was added into the cauldron initially and then heated from room temperature to 1073 K at a heating rate of 5 K/min under the nitrogen condition with a flow rate of 200 mL/min. Scanning electron microscopy (SEM, NOVA NanoSEM 450) and transmission electron microscopy (TEM, Tecnai G2F30 STWIN) were used to observe micro-morphology. The powder sample was treated by 6
gold-spray before SEM test. For the TEM test, the powder sample was dispersed into ethanol solution with ultrasonic and dripped onto the copper net. The micro-structure could be observed after ethanol volatilization. The X-ray diffraction patterns were performed by a Shimadzu diffractometer (Lab XRD-7000s) with CuKα in the 10° < 2θ < 80° range and at a scanning speed of 4°/min. The Fourier transforms infrared spectrometer (FTIR) was used to investigate the surface function groups of prepared carbon by a Thermo Fisher spectrometer 6700 in the range of 4000-400 cm-1. A small amount of powder sample was well mixed with KBr by grinding. The mixture was pressed to tablet and tested. The pore structure parameters were determined from N2 adsorption-desorption isotherm which was carried out by Quantachrome analyzer (AS-1-MP-11) at 77 K. The samples were degassed at 473 K for 4 h before N2 adsorption measurements. The BET specific surface area (St) and the micropores area (Smic) were calculated using BET method and t-plot method, respectively. The total pore volume (Vt) was estimated at a relative of 0.99. The micropore volume (Vmic) was calculated by t-plot method and the external volume (Vex) was determined by difference between Vt and Vmic. The pore size distribution was determined by BJH method and HK method. The pHzpc of porous carbon refers to the pH value of the solution when the charge on the surface of the adsorbent (carbon prepared) is zero, and can be used to analyze the surface properties and adsorption mechanism between the adsorbent and adsorbate. For pHzpc value measurement, 0.01 g of MOxy was dispersed in 25 mL NaCl solution (0.1 M) in centrifuge tube with pH from 2.0-10.0 7
adjusted by HCl (0.1 M) solution and NaOH solution (0.1 M). Then the pH value was measured after 24 h shaking. The graphic representation of pHfinal versus pHinitial was plotted and the pHzpc was obtained at ΔpH =0[35]. 2.3
Adsorption measurements
The methylene blue (MB) was selected as adsorbate in order to investigate the adsorption performance of MOxy. A stock MB solution of concentration 1000 mg/L was prepared for the adsorption studies, and then diluted to various concentrations as per requirement of experiments. For isotherms and temperature effect studies, 0.005 g MO21 sample was added into 25 mL MB solution with various concentrations (from 100 mg/L to 350 mg/L) at various setting temperatures (298 K, 308 K, 318 K). Later the mixture was shaken for 24 h before sampling and analysis. To investigate the sorption kinetics, 0.04 g MOxy was dispersed in 200 mL MB solution with a concentration of 200 mg/L in a 250 mL flask. All flasks were placed in the thermostatic water batch and shaken at a speed of 160 rpm for 12 h. During the adsorption process, 2 mL sample was taken and filtered through 0.22 μm membrane filter at preset time intervals. The MB concentration was determined by UV–Vis spectrophotometer (DR 5000, HACH) at a wavelength of 665 nm. For pH effect study, 0.005 g MO21 was added into 25 mL MB solution with a concentration of 200 mg/L. The solution pH value was adjusted using 0.1 M NaOH or 0.1 M HCl at range of 2.0-10.0. The maximum amount of MB adsorbed at equilibrium was calculated using the following equation (1): 8
(1)
𝑞e = (𝐶e ― 𝐶0) × 𝑉/𝑚 where Ce and C0 (mg/L) are the equilibrium and initial concentration of MB
solution, respectively. V (L) is the volume of the MB solution and m (g) is the mass of adsorbent (carbon prepared).
3. Results and discussion 3.1 Formation of porous carbon MOs Fig.1 shows the TG analysis of heavy residue, magnesium acetate and their mixture in N2. For the heavy oil, the weight loss was recorded severe at around 523 K, which was mainly attributed to polymerization and decomposition of the hydrocarbon compound [3, 36]. The TG curve of magnesium acetate could be divided into two stage. In the initial stage, the mass loss was computed at 34.29 % which corresponded to the crystal water loss (33.5 %) of Mg(CH3COO)2·4H2O. And the final yield obtained from TG curve was 18.6%. This value approximately equals to the mass ratio of MgO/ Mg(CH3COO)2·4H2O, which indicates magnesium acetate is almost completely decomposed into MgO. The TG curve of the mixture was almost the same as that of magnesium acetate. Compared with pure heavy residue, the mixture with addition of magnesium acetate shows higher carbon yield (from 8 % to 22 %) based on the experimental data, which was attributed to the inhibition of magnesium acetate over decomposing of heavy residue. The magnesium acetate could decompose into MgO which was confirmed by the XRD patterns (Fig.2a). The diffraction peaks of 9
Mg(CH3COO)2·4H2O can be observed at 373 K. This indicated that almost no reaction occurred below 373 K. The diffraction peaks of Mg(CH3COO)2 appeared at 473 K implying that crystal water loss occurred during the decomposition of magnesium acetate, which was consistent with TG curve (Fig. 1). Finally, the appearance of MgO diffraction peaks at 973 K indicated the formation of MgO after the thermal treatment. The MgO particles acted as template to produce cavities after removal [37]. In addition, gaseous products CO2 and H2O were also released during the decomposition of magnesium acetate according to : Mg(CH3COO)2·4H2O → 4H2O + MgO + CO2 + CH3COCH3 [38, 39]. The evolved gaseous products could create some channels and pores. The acetone could be cyclized and the cyclization products could be assembled to form carbon brings, which could also increase the carbon yield [39]. Furthermore, the magnesium acetate absorbs heat as it released H2O and CO2. This process is endothermic and might delay heat transfer. The magnesium acetate might play a role of fire retardant, which could reduce the rate of vaporization and decomposition of polymer [40]. Hence, the yield of carbon was improved with magnesium acetate addition. The procedure of MOxy preparation is demonstrated in Fig. 3. The heavy residue was viscous and could be coated on the surface of magnesium acetate particle after mixing treatment, which could lead to a three-dimensional configuration after carbonization. The process of decomposing magnesium acetate into MgO took place 10
simultaneously with the carbonization of heavy residue. After carbonization, the carbon layer was formed on the surface of MgO particles, which was clearly observed from the TEM image (Fig. 5a) of MO11 before etching. From Fig. 5a, it could be seen that the metal grid appeared in TEM image. Subsequently, the sample was washed with HCl to remove MgO particles and 3D hierarchical porous carbon with high specific surface area and micro/meso/macropores was obtained. The XRD patterns of MO11 before and after etching are shown in Fig. 2b. The diffraction peak of MgO disappeared after etching, which indicated that the MgO was almost completely removed by HCl. The broadness of peak around 25o suggested the presence of amorphous carbon phase within MO11. The weak peak at 43o indicated the turbostratic structure [41]. 3.2
Characterization of the prepared MOxy
The microscopic morphologies and pore structures of samples are important for the performance of porous carbon. Fig. 4 shows the SEM images of MO11-no (without washing) and MO11 (washed with HCl). As revealed by SEM images, MO11 shows the structure of spherical-like particles that were cross-linked with each other. After washing with 1 M HCl, the pore structure became more evident, which was further confirmed by the TEM image (Fig. 5). This result indicated that the magnesium acetate played a site-occupying role in creating the cavities. In order to demonstrate 11
the role of magnesium acetate for adjusting pore size, the mass ratio of magnesium acetate to heavy residue was adjusted from x/y = 1 to x/y = 3. The TEM images of MOxy (x/y = 1, 2, 3) are also shown in Fig. 5. The surface functional groups of MOxy were determined by FTIR analysis. As shown in Fig. 6, the FTIR spectra of all samples i.e. MO11, MO21 and MO31 showed nearly identical functional groups. The peak at around 870 cm-1 corresponded to C-H stretching out of plane of benzene ring. The peak at around 1250 cm-1 was attributed to C-H stretching in plane of benzene ring. The bands at around 1450 cm-1 and 1580 cm-1 aroused from C=C stretching vibration in benzene ring [42]. These surface functional groups suggested the presences of aromatic hydrocarbons were present in samples. The peaks observed at around 2958-2850 cm-1 were assigned to C-H stretching in methyl and methylene groups. The band at around 3400 cm-1 corresponds to the O-H stretching [35]. The pore structure characteristics such as specific surface area, pore size distribution and pore volume of the carbon prepared in this work were further investigated by N2 adsorption-desorption tests. The N2 adsorption and desorption isotherms and pore distribution of samples are shown in Fig. 7. It could be seen from Fig. 7a that the isotherms of all MOxy relate to type IV with a hysteresis loop, 12
according to the IUPAC classification. And the steep uptake at low relative pressure range indicated the presence of micropores in all samples. The marked hysteresis loop could be seen from 0.4 to 0.9 P/P0 for MO11 indicated the abundance of meso-pores in the sample. Furthermore, a steep uptake occurred at high relative pressure (P/P0 > 0.9) was attributed to non-rigid aggregates of plate-like particles or macropores [43]. With the increase of mass ratio of magnesium acetate to heavy residue, the hysteresis loops became narrower, which suggested the decrease in meso-pores. The pore size distribution was calculated based on BJH method (for mesopores in Fig. 7b) and HK method (for micropores in Fig. 7b inside). Both micropores and mesopores were presented in samples to form a hierarchical structure, which was consistent with the isotherms results as discussed above. The formation of meso-pores and macro-pores was attributed to the removal of MgO particles after HCl etching, while the micropores were ascribed to the CO2 and H2O released during magnesium acetate decomposition [38]. It was worth noting that CO2 and H2O were released from the inside out, which could produce open micro-pores for linking mesopores and avoid end-died pores. The generated micro-mesopores network could remarkably enhance the mass transport and adsorption performance [44]. The pore size distribution of micro-pores for all samples were almost the same and mainly distributed at 0.5 nm and 1.1 nm based on HK method. Additionally, the mesoporous pore size of MO11, MO21, MO31 were mainly concentrated at 7.7 nm, 4.8 nm and 3.0 nm based on BJH method, respectively, which showed a decreasing trend with an increase in 13
magnesium acetate amount. Previous studies have shown that the size of MgO particles were related to the restriction of carbon layer on the surface of MgO. The growth of MgO particles occurred during heating from 473 K to 673 K, and would be confined under the effect of carbon layer [37, 45]. The excessive inorganic particles lead to dense structure [46]. The dense structure is not conducive to MgO particle growth, which could result in the generation of micro-pores. In addition, oil could not be evenly coated on the surface of magnesium acetate particles with excessive magnesium acetate. Hence, the oil layer on some surface of magnesium acetate particles is not present and the particles could grow without the oil layer limitation resulting the macropores. Therefore, the porous carbon prepared with excessive magnesium acetate exhibited small pore size and specific surface area (like MO31). The pore structure parameters of all samples are summarized in Table 2. The BET special surface area (St) value of MO11, MO21 and MO31 were 1005.2 m2/g, 868.3 m2/g and 797.8 m2/g, respectively, which showed a decreasing trend with an increase in the mass ratio of magnesium acetate to heavy residue. It appeared that MO11 has the largest specific surface area and pore volume (2.1 cm3/g). Compared with MO11 and MO21, MO31 had more micro-pores. The average pore size (APS) changed from 8.4 nm to 2.9 nm when x/y was increased from 1 to 3. In view of the large pore size and volume of samples, the hierarchical porous carbon prepared in this work might be favorable for MB removal in an aqueous solution, since the size of a 14
MB molecule is 1.7 nm×0.76 nm×0.325 nm. The micro-meso-pores network of samples could facilitate the transportation of adsorbate molecule to the internal surface of the adsorbent. 3.3
Adsorption of methylene blue (MB) In view of the favorable pore structure of the obtained hierarchical porous
carbon, the methylene blue was selected as adsorbate in order to investigate the adsorption performance of porous carbon samples. 3.3.1
Adsorption kinetics
The adsorption kinetics were studied to investigate the process of adsorption. The pseudo-first order (PFO) [47] and pseudo-second order (PSO) [48] models were applied to analyze the sorption data of MB onto MOxy. The equations of PFO and PSO are shown as: 𝑞t = 𝑞e(1 ― 𝑒 ― 𝑘1𝑡)
(2)
𝑞2e𝑘2𝑡
(3)
𝑞t = 1 + 𝑘2𝑞e𝑡 where qt (mg/g) and qe (mg/g) are the quantity of MB adsorbed at time (t) and equilibrium, respectively, k1 (1/min) is the rate constant for PFO equation and k2 (g/mg min) is the rate constant for PSO equation [49].
Fig. 8 shows the kinetic experimental data of adsorption process. The MOxy had high adsorption capacity of MB (642 mg/g for MO11, 704.95 mg/g for MO21 and 700.45 mg/g for MO31) that were higher than that of other adsorbents, such as porous 15
biochar (512.67 mg/g) [50], shell-waste based active carbon (163.3 mg/g) [51] and ball-milled biochar (354 mg/g) [52]. This high capacity was not only attributed to high specific surface area but also from large pore volume. The time to reach equilibrium in these three systems increased from 120 min for MO11 to 600 min for MO31. Especially, in the initial stage of adsorption process, the adsorption rate and capacity in these three systems showed a downward trend from MO11 to MO31, which was attributed to the various pore sizes of the samples. The large pore volume of MO11 (2.1 cm3/g) could reduce the diffusion resistance for MB molecules, which resulted in the fast adsorption rate. In contrast, the excessive pore size of MO11 (7.7nm) could make the MB molecules more easily desorbed, which lead to a low adsorption capacity. The kinetic fitting studies were performed in order to investigate the adsorption process of MB onto MOxy. As shown in Fig. 8, the experimental data were fitted with PFO and PSO model and the corresponding kinetic parameters are shown in Table 3. It could be seen that the PSO model was better fitted to the experiment data compared to PFO model. The higher determination coefficient values (R2, 0.9954-0.9273) from PSO model also confirmed that PSO model was suitable to explain the adsorption process of MB onto MOxy. 3.3.2
Adsorption isotherms
In this work, Langmuir and Freundlich isotherm models were used for understanding 16
the characteristics of adsorption at equilibrium. The nonlinear forms of the Langmuir model [53] and Freundlich model [54] are described by following equations Eq (4) and Eq (5). 𝑞m𝐾L𝐶e
𝑞e = 1 + 𝐾L𝐶e
(4)
𝑞𝑒 = 𝐾𝐹𝐶1/𝑛 𝑒
(5)
where qe (mg/g) and qm (mg/g) are equilibrium adsorption capacity and maximum adsorption capacity, respectively. KL (L/mg) and KF ((mg/g(L/mg)1/n) are constants of Langmuir model and Freundlich model, respectively. 1/n is the intensity parameter of Freundlich model [49]. The fitting curves of tested data for different samples are shown in Fig. 9 (a), and the derived parameters are summarized in Table 4. The Langmuir model was not applicable for the all adsorption process, but the Freundlich model was suitable to explain the adsorption process. The high values of R2 were obtained for all samples by using Freundlich model which indicated that the Freundlich model fitted best to the adsorption data. In addition, the isotherm constants can be a criterion to judge the favorability of adsorption process. In the Freundlich model, the 1/n represents the magnitude of the adsorption driving force or the surface heterogeneity [49], which reflects the feasibility of process and the degree of the surface heterogeneity [55]. For the adsorption systems of all samples, the values of exponent 1/n (0.0847, 0.1134 and 0.0788, respectively) were computed less than 0.5, which indicated that the adsorption process for MB onto MOxy more easily occurred. 17
From the analysis of kinetics studies and adsorption isotherms, MO21 showed the favorable adsorption performance for MB, which not only had larger uptakes compared with MO11 but also achieved adsorption equilibrium faster than MO31. The suitable pore size of MO21 provided diffusion paths for MB. Therefore, MO21 was selected to study the effect of temperature and pH on adsorption of MB. The effect of temperature of MB on MO21 is shown in Fig.9 (b). With the increasing of temperature, the MB uptakes decreased slightly, suggesting that the adsorption was an exothermic process. 3.3.3
The effect of pH
Besides the textual properties, the pH of the MB solution also showed a significant effect on adsorption performance. The pH value of the solution not only affects the surface properties of the adsorbent but also alters the state of the adsorbate in the solution. The adsorption of MB onto MO21 increased strikingly (from 599.7 mg/g to 787.1 mg/g) as the pH value changed from 2 to 9, and then became moderate as the pH continued to rise. The adsorption capacity reached the highest value of 789.4 mg/g at pH = 10. The MB adsorption amount varied significantly with pH of solution implying that the electrostatic interaction between MO21 and MB molecule was present. MB is a cationic macromolecular dye, and MO21 surface is negative at higher pH values (pH > pHzpc). The pHzpc of MO21 was determined as 6.5 from Fig. 18
10(a). Hence, the adsorption capacity enhanced due to the electrostatic attraction between the MO21 and MB. Gradually, the adsorption sites in MO21 was positively charged with the increase in pH. This exhibited the electrostatic repulsion with dye cations, leading to the decrease in adsorption capacity. Furthermore, the excess H+ can also compete with positive MB ions for adsorption sites resulting in low adsorption capacity at lower pH. On the other hand, MO21 still performed high MB adsorption capacity in spite of the low pH (pH = 2), which was ascribed to pore filling and π–π interactions between MO21 and MB molecules [52]. To detect the surface function groups of samples after adsorption, FTIR analysis was carried out. As shown in Fig. 11, the function groups of MB contain benzene rings, symmetric methyl and C-N, etc. The corresponding peaks (at 1224 cm-1, 1384 cm-1 and 1321 cm-1) were observed in FTIR spectrum after adsorption (Fig. 11), which manifested that the adsorption of MB onto MO21occured. The pore filling mainly occurred during the adsorption process and the molecular structure of MB was not destroyed. In addition, the stretching vibration of C-H and C=C in benzene ring was enhanced (around 1250 and 1580 cm-1) after adsorption, which also indicated the presence of π–π interactions between MO21 and MB molecules. 3.3.4
Adsorption thermodynamics 19
The thermodynamic parameters can also be used to understand the type of adsorption process. Thermodynamic parameters include the Gibbs energy change (ΔGo, kJ/mol), the enthalpy change (ΔHo, kJ/mol) and the entropy change (ΔSo, J/mol/K) and can be calculated using the following equations: ∆𝐺o = ― R𝑇ln 𝐾C
(6)
∆𝐺o = ∆𝐻o ―𝑇∆𝑆o
(7)
ln 𝐾C =
―∆𝐻o R
1
×𝑇+
∆𝑆o R
(8)
where R is the universal gas constant (8.314 J/mol/K), T (K) is the temperature, KC is the thermodynamic equilibrium constant. According to the adsorption isotherm studies, KC can be derived from the constant of Freundlich model (KF, mg/g(L/mg)1/n) [56]. The formula is as follow: 1
𝐾C =
106 (1 ― 𝑛) ( 1000 𝜌 ) 𝐾F𝜌
(9)
where ρ is the density of pure water (assumed as ~1.0 g/mL) [49]. The value of ΔGo, ΔHo and ΔSo are summarized in Table 5. The Gibbs energy change (ΔGo) was negative, which indicated the adsorption process was spontaneous. The enthalpy change (ΔHo) was also negative suggesting an exothermic process of MB adsorption onto MO21, which is in consisted with the previous result. The positive value of ΔSo suggesting an entropy increase of process, which indicated an increase in disorder after adsorption. 3.3.5
Adsorption mechanism 20
As shown as Fig. 12, the adsorption mechanism was summarized based on the adsorption experiments results above. The excellent adsorption performance of MO21 was attribute to the appropriate pore size distribution. The abundant mesoporous of MO21 could not only provide more adsorption sites but also reduce the hindrance effect to promote the adsorption process. Furthermore, the surface charge and function groups of MO21 could also affect the interaction between MO21 and MB and affect the adsorption performance further. The existence of aromatic structure of MO21 and MB formed made the π-π interaction formed which was favorable for MB remove [57]. 4. Conclusion The hierarchical porous carbons with micro-mesopores were successfully synthesized derived from heavy residue of TPO using template strategy, which provided a new strategy for treating the heavy residue of TPO. Magnesium acetate not only played a template but also as a self-activation role for producing micro-mesopores structure. The obtained porous carbons had remarkable high specific surface area (1005.2-797.8 m2/g) and high value of Se/St (89.9-40.8%), which could benefit for the large dye molecules transfer and diffusion and provide abundant sites for adsorption. Notably, the dosage of magnesium acetate could adjust the pore structure and then the mechanism for that was demonstrated in this paper. As adsorbent materials, the asprepared porous carbons exhibited favorable adsorption performance for MB in aqueous solution. The maximum adsorption capacity of 843.5 mg/g for tested data 21
was achieved at 298K for MO21. The adsorption process could be explained well with pseudo-second-order kinetic model, and the adsorption equilibrium data was fitted well on Freundlich isotherm model. Meanwhile, the adsorption process was found spontaneous and exothermic in nature. In summary, this work delivered a facile way to dispose of the waste tire derived heavy residue into function carbon materials for potential high value-added utilization.
Acknowledgement This work was financially supported by the Open Foundation of Key Laboratory of Industrial Ecology and Environment Engineering, MOE (KLIEEE-19-01).
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Fig. 7. N2 adsorption-desorption isotherm (a) and pore size distribution (b) of samples. Fig. 8. Adsorption kinetic diagram (time = 12 h, MOxy dosage = 0.2 g/L, MB dosage = 200 mg/L, t = 298 K). Fig. 9. (a) Langmuir model (dotted line) and Freundlich model isotherms (solid line) for MB adsorption onto MOxy at 298 K (time = 24 h, MOxy dosage = 0.2 g/L). (b) effect of temperature on the adsorption of MB on MO21 (time = 24 h, MOxy dosage = 0.2 g/L). Fig. 10. The effect of solution pH on adsorption capacity of MO21. Fig. 11. FTIR spectrum of MO21 after adsorption (pH = 5.5) and chemical structure of MB. Fig. 12. MB adsorption mechanism diagram by MO21.
33
Fig. 1
34
a
b
Fig. 2
35
Fig. 3
36
Fig. 4
37
Fig. 5
38
Fig. 6
39
a
b
Fig. 7
40
41
Fig. 8
42
(a)
(b)
Fig. 9
43
Fig. 10
44
Fig. 11
45
Fig. 12
Table 1 The proximate analysis and elemental analysis data of heavy residue of TPO. Proximate analysis
Mean (wt%)
Ultimate analysis
Mean （wt%）
Mad
0
C
88.34
Vad
85.34
H
9.37
FCad
14.69
N
1.12
Aad
0
S
1.08
O
0.09
M: moisture content, V: volatile content, FC: fixed carbon content, A: ash content. ad
Air drying basis.
Table 2 The specific surface area, pore volume and average pore size of MOxy samples Samples
St
Smicro
（m2/g）
（m2/g）
Se（m2/g）
Se / St (%) 46
Vt
Vmicro
Vex
APS
（cm3/g）
（cm3/g）
（cm3/g）
（nm）
MO11 MO21 MO31
1005.2 868.3 797.8
100.9 188.2 472.4
904.3 680.1 325.5
89.9 78.3 40.8
2.10 1.34 0.57
0.026 0.1 0.23
2.07 1.24 0.34
Table 3 The kinetic structure parameters Pseudo-first-order Samples
qexp (mg/g)
MO11 MO21 MO31
642 704.95 700.45
Pseudo-second-order
qcal (mg/g)
R2
k1 (1/min)
qcal (mg/g)
R2
k2 (g/mg min)
612.61 655.01 623.37
0.9721 0.8919 0.8311
0.33 0.12 0.031
626.97 683.69 682.35
0.9954 0.9683 0.9273
0.0012 0.00029 0.00006
Table 4 Adsorption isotherm parameters Samples MO11 MO21 MO31
Langmuir KL qm (mg/g) (L/mg) 606.25 0.5680 759.81 1.4070 659.55 0.9913
Freundlich R2 0.5828 0.7451 0.7774
KF (mg/g(L/mg)1/n) 398.38 468.81 451.62
1/n
R2
0.0847 0.1069 0.0788
0.9880 0.9831 0.9942
Table 5 Thermodynamic parameters for MB adsorption onto MO21 Temperature (K) 298 308 318
ΔGo (KJ/mol) -28.69 -29.45 -30.28
ΔHo (KJ/mol)
ΔSo (J/mol/K)
-4.99
79.50
Graphic abstract
47
8.4 6.1 2.9
Highlights The heavy residue can be used as a novel precursor for preparing porous carbon; The pore structure can be adjusted by magnesium acetate dosage. The carbon prepared showed maximum methylene blue adsorption capacity of 843.5 mg/g at 298K.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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