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Effect of pyrolysis temperature on characteristics and aromatic contaminants adsorption behavior of magnetic biochar derived from pyrolysis oil distillation residue
Bioresource Technology 223 (2017) 20–26
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Effect of pyrolysis temperature on characteristics and aromatic contaminants adsorption behavior of magnetic biochar derived from pyrolysis oil distillation residue Hao Li, Samah Awadh Ali Mahyoub, Wenjie Liao, Shuqian Xia ⇑, Hechuan Zhao, Mengya Guo, Peisheng Ma Key Laboratory for Green Chemical Technology of State Education Ministry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, People’s Republic of China
h i g h l i g h t s Magnetic biochars were easily synthesized by pyrolyzing distillation residue. Pyrolysis temperature showed a pronounced effect on magnetic biochar properties. Adsorption of aromatic contaminants on magnetic biochars were investigated. Adsorption mechanisms associated with biochar properties and target contaminants.
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Article history: Received 16 August 2016 Received in revised form 10 October 2016 Accepted 12 October 2016 Available online 14 October 2016 Keywords: Distillation residue Magnetic biochar Pyrolysis Characterization Adsorption
a b s t r a c t The magnetic biochars were easily fabricated by thermal pyrolysis of Fe(NO3)3 and distillation residue derived from rice straw pyrolysis oil at 400, 600 and 800 °C. The effects of pyrolysis temperature on characteristics of magnetic biochars as well as adsorption capacity for aromatic contaminants (i.e., anisole, phenol and guaiacol) were investigated carefully. The degree of carbonization of magnetic biochars become higher as pyrolysis temperature increasing. The magnetic biochar reached the largest surface area and pore volume at the pyrolysis temperature of 600 °C due to pores blocking in biochar during pyrolysis at 800 °C. Based on batch adsorption experiments, the used adsorbent could be magnetically separated and the adsorption capacity of anisole on magnetic biochars was stronger than that of phenol and guaiacol. The properties of magnetic biochar, including surface area, pore volume, aromaticity, grapheme-like-structure and iron oxide (c-Fe2O3) particles, showed pronounced effects on the adsorption performance of aromatic contaminants. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The fast pyrolysis technology has been considered as a promising process since it offers a feasible method to convert waste biomass into liquid pyrolysis oil. Unfortunately, the pyrolysis oil consists of a large number of unstable oxygen-containing compounds, and it is difficult to directly apply the pyrolysis oil in existing equipment (Li et al., 2016c, 2015b). Therefore, it is very necessary to upgrade pyrolysis oil. Distillation offers a feasible and effective way to obtain the high-grade fuel and high-value chemicals from the biomass pyrolysis oil. Most researches have been conducted to the production of ⇑ Corresponding author. E-mail address: [email protected] (S. Xia). http://dx.doi.org/10.1016/j.biortech.2016.10.033 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.
the volatile fraction, in which the distillates have been achieved in various applications either as an energy or chemical source (Capunitan and Capareda, 2013; Zhang et al., 2013b). In fact, approximately 30–50 wt.% solid residue as a by-product was parallel formed after distillation of pyrolysis oil, which can be seen in the previous studies (Capunitan and Capareda, 2013; Zheng and Wei, 2011). In order to achieve comprehensive utilization of pyrolysis oil, the parallel generated distillation bottom (or distillation residue) should be concerned. Unfortunately, there were by far very few studies have focus on the application of the distillation residue derived from pyrolysis oil. Zhang et al. (2013b) made distillation residue a source of renewable chemicals using co-pyrolysis. Elkasabi et al. (2015) applied the bio-oil distillate bottoms (distillation residue) as a feedstock to further upgrade for the production of calcined coke.
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In recently published paper, the distillation residue was utilized as solid fuel to co-fire with lignite, in which the waste was recycled (Li et al., 2016b). In fact, the development of magnetic biochar from biomass (especially agricultural waste) have been attracted many researchers (Kundu et al., 2015; Thines et al., 2017). These magnetic biochar showed a remarkable application as an adsorbent for various wastewater treatments, which may be due that the magnetic biochars not only can adsorb the contaminants from aqueous solutions, but also can be easily isolated with external magnets after adsorption. Two publications reported the production of magnetic bio-char derived from palm oil empty fruit bunch as the raw material by using microwave heating technique (Mubarak et al., 2014, 2016). Thines et al. (2016) focused on the conversion of the durian’s rind into magnetic biochar in the presence of three different metallic salts by employing a novel vacuum condition in an electrical muffle furnace. Noraini et al. (2016) employed a novel method for the preparation of magnetic biochar from sugarcane bagasse by microwave-assisted pyrolysis at a microwave power of 600 W. Wei et al. (2016) obtained magnetic biochar composite from shell as raw material by using coprecipitation method in the presence of FeCl3 and FeSO4. However, there is no study on the application of pyrolysis oil distillation residue for the synthesis of magnetic biochar using for the adsorption of aromatic contaminants from aqueous solution in the literature. Various aromatic pollutants in aqueous solution were a threat to human health. And adsorption was one of the convenient method for the organic contaminants removal from aqueous solution. In this work, a novel magnetic biochar has been easily fabricated using the discarded material, the rice straw pyrolysis oil distillation residue, with impregnated Fe(NO3)3. The pyrolysis of the impregnated distillation residue was carried out at 400, 600 and 800 °C. The effect of pyrolysis temperature on the characteristics of magnetic biochars were investigated comprehensively in the study. In addition, batch adsorption experiments were conducted to investigate the adsorption capacity of the magnetic biochars to various aromatics (i.e., anisole, phenol and guaiacol). It is anticipated that the feasible method presented here can achieve the comprehensive utilization of the distillation by-product of pyrolysis oil. 2. Materials and methods 2.1. Materials The rice straw pyrolysis oil studied in this work was kindly provided by Shaanxi Yingjiliang Bio-energy Corporation, China. The bio-oil was produced in a downstream circulating fluidized bed reaction by flash pyrolysis of rice straw (500 °C/s and 500 °C for the heating rate and reaction temperature, respectively). The pyrolysis oil poured into a round-bottom flask was distillated at a certain temperature (140, 180, 220 and 250 °C) for 30 min. Then, the distillate bottom was obtained. The distillation residue was the distillate bottom. The detail experimental procedure about the distillation was described in the reported paper (Li et al., 2015a). The iron nitrate nonahydrate (Fe(NO3)39H2O), guaiacol, anisole and phenol were obtained from Aladdin-reagent, China. 2.2. Preparation of magnetic biochar In this study, the magnetic biochar was synthesized using pyrolysis oil distillation residue as raw material. The distillation residue was initially washed with DI water and ethanol to remove impurities, respectively, and dried in an oven at 80 °C for 12 h under air atmosphere. The dried distillation residue was then grinded to 200 mesh. Subsequently, the grinded distillation residue
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(5 g) was immersed into the prepared Fe(NO3)3 solution (2.02 g Fe (NO3)3 in 20 mL of DI water). After stirring for 2 h, the sample was dried at 80 °C for 12 h under air atmosphere. The obtained material was pyrolyzed for 2 h under nitrogen (N2) flow of 100 mL/min1 at a heating rate of 10 °C/min. The pyrolysis temperature were 400 °C, 600 °C and 800 °C, respectively. The obtained magnetic biochars (MB) were grinded to 200 mesh, washed with DI water, filtered, dried (60 °C, 4 h) in a vacuum oven, and sealed in a container before use. In this study, MB 400, MB 600 and MB 800 were the magnetic biochars prepared at the pyrolysis temperature of 400 °C, 600 °C and 800 °C, respectively. 2.3. Characterization Ultimate analyses were performed on the elemental analyzer (Vario micro cube, Elementar, Germany). Thermogravimetric analysis was analyzed by TG (TG 209 F3, Netzsch, Germany). For each experiment, approximately 5 mg of sample was processed and heated up to 900 °C at heating rate of 10 °C/min under nitrogen or air atmosphere. The DSC analysis of samples (MB 400, MB 600 and MB 800) were performed on the DSC analyzer (DSC 2, Mettler-Toledo, Switzerland) under nitrogen purge of 80 ml/min and at the heating rate of 10 °C/min. The surface area, pore volume, and pore size of the magnetic biochars were determined with an instrument (V-Sorb X800, Gold app instruments, China). The surface morphology of magnetic biochars were studied by scanning electron microscopy (S-4800, Hitachi, Japan). Chemical functional groups of magnetic biochars were analyzed using FTIR (FTS6000, Bio-rad, US). The crystallinity and structure of the magnetic biochars were analyzed through XRD (D8-Advance, Bruker, Germany) with Cu Ka radiation (k = 1.542 Å) at 40 kV and 40 mA and scanned from 10 to 90° at a rate of 0.5°/s. The magnetic properties of MB 400, MB 600 and MB 800 were evaluated using Vibrating Sample Magnetometer (VSM). 2.4. Batch adsorption experiments Adsorption of aromatics (anisole, phenol and guaiacol) were conducted using batch adsorption approach. The adsorption experiments were performed over the initial aromatics concentration range from 3 to 35 mg/L. About 0.0090 g magnetic biochar was weighed to each 50 mL tube and then 30 mL aromatics solution with one specific concentration was added. The mixtures were shaken for 24 h at 298.15 K on the shaker, which is sufficient for reaching equilibrium. Then, the samples were centrifuged for 5 min at 4000 rpm and filtered using a 0.22 lm nylon membrane filter. The aromatics concentrations were determined using an ultraviolet–visible (UV–vis) spectrophotometer (TU 1900, Purkinje General Instruments Co. Ltd., China) at 269 nm (for anisole), 270 nm (for phenol) and 274 nm (for guaiacol). The magnetic biochars were obtained for characterization after adsorption of phenol over the initial aromatics concentration of 35 mg/L. 3. Results and discussions 3.1. Characterization of magnetic biochar 3.1.1. Ultimate analysis The elemental composition of the magnetic biochars (MB 400, MB 600 and MB 800) were investigated and presented in Table 1. The results showed that the carbon contents of magnetic biochar decreased with the pyrolysis temperature increasing from 400 °C to 800 °C. It may be attributed that higher pyrolysis temperature led to the larger loss of volatiles in distillation residue. In addition, the H/C atomic ratio could be applied to evaluate the aromaticity of
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Table 1 The elemental composition of the magnetic biochars (MB 400, MB 600 and MB 800).
C, % H, % N, % H/C atomic ratio
MB 400
MB 600
MB 800
29.92 4.37 1.37 1.75
28.16 3.26 1.34 1.39
22.98 1.68 1.19 0.88
magnetic biochars. And the higher the H/C atomic ratio is, the lower the aromaticity of magnetic would be (Ahmad et al., 2013). Table 1 illustrated that the values of H/C atomic ratio gradually declined with pyrolysis temperatures increasing. When the pyrolysis temperature rose up 800 °C, the value of H/C atomic ratio was the minimum, which indicated the MB 800 is highly carbonized and exhibits the highest aromaticity among the magnetic biochars. 3.1.2. Thermal analysis The TG and DTG analysis of the distillation residue and magnetic biochars under nitrogen were presented in Fig. 1. As shown in TG curves under nitrogen, the first stage (<220 °C) for the distillation residue maybe attributed to the removal of moisture and other small molecule residue. The weight loss indicated the moisture and low-boiling volatiles contents of the distillation residue were approximately 10%. The thermal decomposition actively took place in 200–600 °C under nitrogen due to the progressive pyrolysis of macromolecular components (Lee et al., 2013; Xu and Chen, 2013). The mass loss of the distillation residue was greater than 55% under nitrogen at nearly 900 °C. In the DTG curve under nitrogen (shown in Fig. 1), the maximum rate of weight loss of distillation residue (1.93 wt.%/min) is much higher than that of MB 400 (0.84 wt.%/min), MB 600 (0.65 wt.%/min) and MB 800 (0.58 wt.%/ min), while the maximum weight loss temperature of distillation residue is lower than that of magnetic biochars. This phenomenal may be due that more volatile substances existed in distillation residue. No phase change was found on all the magnetic biochars at 40–300 °C, indicating decomposition did not occur at this temperature range under nitrogen. Compared to distillation residue, the magnetic biochars (MB 400, MB 600 and MB 800) showed higher thermal stability and began to decompose at a higher temperature under nitrogen. The total mass lass of MB 400, MB 600 and MB 800 in nitrogen at 900 °C were 29 wt%, 15 wt% and 7 wt %, respectively. Al-Wabel et al. (2013) used the conocarpus wastes to prepare biochars at different pyrolysis temperatures (200– 800 °C). And they found that the occurred weight loss followed
Fig. 1. TG and DTG curves of distillation residue, MB 400, MB 600 and MB 800 under nitrogen purge of 50 mL/min and heating rate of 10 °C/min.
the order: conocarpus wastes feedstock > 200 °C biochar > 400 °C biochar > 600 °C biochar > 800 °C biochar, which is consistent with the results of this work. As expected, the mass loss of magnetic biochar tended to decrease as the pyrolysis temperature rose from 400 °C to 800 °C in nitrogen. The thermal stability of magnetic biochars can be also confirmed by DSC profiles (data not shown). Compared to the DSC curves of each magnetic biochar in nitrogen, the starting reaction temperature of MB 800 was higher than that of MB 400 and MB 600, which indicated that MB 800 has the better thermal resistance. In order to investigate the thermal oxidative stability, the TG analysis for distillation residue, MB 400, MB 600 and MB 800 were carried out in air (shown in Fig. S1). When heating temperature was implemented between 40 and 285 °C, the mass of all the magnetic biochars only decreased slightly (due to water loss). And the main decomposition for MB 400, MB 600 and MB 800 under air atmosphere was in 290–600 °C. But, the decomposition temperature of MB 800 is higher than that of MB 400 and MB 600 under air atmosphere. Therefore, as pyrolysis temperature increased, the magnetic biochars became more thermal stability. 3.1.3. Morphological analysis Scanning electron microscopy (SEM) images of MB 400, MB 600 and MB 800 were applied to investigate the influence of pyrolysis temperature on the morphology of the magnetic biochars (shown in supporting information, Fig. S2). The SEM images revealed that the magnetic biochars underwent clear structural modifications as pyrolysis temperature increasing. The reaction mechanisms of biomass pyrolysis were defined in three main steps: 1st step: Biomass ? Water + Unreacted residue; 2nd step: Unreacted residue ? (Volatile + Gases)1 + (Char)1; 3rd step: (Char)1 ? (Volatile + Gases)2 + (Char)2 (Demirbas, 2004; Kim et al., 2012). The biochar primarily generated in the second step. And some chemical rearrangement of biochar occurred in the last step. The morphological changes in the magnetic biochars obtained in this work may be due to the char formation reactions. As the pyrolysis temperature rose from 400 °C to 600 °C, the degree of carbonization of magnetic biochar was enhanced. The MB 600 presented a typical char structure (Fig. S2(b)). The surface morphology of the MB 600 illustrated in Fig. S2(b) is very similar to the biochar derived from corn cob (Yu et al., 2014). Visual inspection of the images of MB 600 and the biochar derived from corn cob revealed that the external surface were full of pores, which enlarge the surface area and provide more adsorption sites. While the pyrolysis temperature was reached at 800 °C, the char’s remaining carbons were further rearranged and MB 800 presented a three-dimensional structure (Fig. S2(d)). 3.1.4. Structural analysis X-ray diffraction (XRD) was applied to investigate the magnetic biochars crystallinity and structure (shown in supporting information, Fig. S3(a)). As the pyrolysis temperature rose up 600 °C, a peak at 2h = 44.4° is noted, which was assigned to the (1 0 1) diffraction pattern of the partially graphitized carbon (Li et al., 2013). From MB 600 to MB 800, the peak (2h = 44.4°) increased in intensity, indicating that the crystalline structure of the partially graphitized carbon in MB 800 is higher than that of MB 600 and the degree of orientation of the aromatic lamellae in MB 800 becomes higher (Fu et al., 2011). In addition, five indexed planes (2 2 0), (3 1 1), (4 0 0), (5 1 1), and (4 4 0), which correspond to the diffraction peaks at 30.2°, 35.5°, 43.2°, 57.3°, and 62.9° (Zhang et al., 2013a), were detected in the biochars in the work. These diffraction peaks are the characterizations of maghemite, which was identified as the major crystalline phase in the magnetic biochar. So the X-ray diffractions indicate that the maghemite (c-Fe2O3) maybe exist in MB 400, MB 600 and MB 800. The magnetic hysteresis curves for
H. Li et al. / Bioresource Technology 223 (2017) 20–26
MB 400, MB 600 and MB 800 were presented in Fig. 2. The samples showed ferromagnetic properties, revealed by the coercive field of 2.02 Oe to 19.86 Oe and the saturation magnetization of 5.50– 18.36 emu/g for all magnetic biochars prepared in this work. The saturation magnetization of MB 800 (18.36 emu/g) is absolutely higher than that MB 400 and MB 600, which may be due to the larger existence of the iron oxide particles in MB 800. And, the magnetic biochars clearly showed a good magnate property, which could be attracted by an outer magnet (shown in Fig. S4). This property is essentially important for the convenient recycling of contaminant-laden magnetic biochar adsorbents after use. The Fourier transform infrared spectra (FTIR) analysis was used to reveal the chemical structure of magnetic biochars. The strong peak at approximately 3400 cm1 was observed in the spectra of MB 400 and MB 600 (shown in Fig. S3(b)), which attributed to O–H stretching and strong hydrogen bonding. However, the intensity of this peak declined with the increase of pyrolysis temperature. Meanwhile, the peak (at approximately 3400 cm1) was almost disappeared in the spectrum for MB 800, suggesting the acceleration of dehydration reaction in distillation residue at the increased temperature (Chen et al., 2012; Kim et al., 2012). Some appeared peaks for magnetic biochars at approximately 1600 cm1 were the aromatic C@C stretching vibrations, suggesting that aromatics existed in magnetic biochars. The absorption peaks between 1350 and 1470 cm1 can be assigned to the stretching vibration of methyl CAH, which indicate the possible presence of alkanes (Lu et al., 2008). The peaks between 630 and 850 cm1 are the representative of the presence of adjacent aromatic hydrogens in the magnetic biochars (Angın, 2013). But these peaks generally declined in the spectrum for MB 800, which suggested that polar functional groups tended to be diminished as the pyrolysis temperature increasing (Al-Wabel et al., 2013). In additional, the baseline of FTIR spectrum gradually drifted upwards as pyrolysis temperature increasing, which probably attributed to the higher degree of aromaticity for MB 800 (Angın, 2013; Hossain et al., 2011).
3.1.5. BET surface areas and pore volumes Fig. 3(a) illustrated nitrogen adsorption isotherms of MB 400, MB 600 and MB 800. MB 600 exhibited a higher adsorption capacity for nitrogen than MB 800 and MB 400, which indicated that the MB 600 has much more porous structures. The BET surface area and pore volume (total and micro) of the magnetic biochars (MB 400, MB 600 and MB 800) were gathered in Table 2. The results revealed that the surface areas and pore volumes were largely
Fig. 2. Magnetic hysteresis loop of MB 400, MB 600 and MB 800.
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dependent on the pyrolysis temperature. When the pyrolysis temperature was elevated from 400 °C to 600 °C, the values of BET surface areas and pore volumes (total and micro) increased significantly and reached maximum (50.62 m2/g, 0.0929 cm3/g and 0.0343 cm3/g for the BET surface area, total and micro pore volumes, respectively). The remarkable improvement in the surface area and pore volume of magnetic biochars may be due to significant increase with the release of volatiles (Angın, 2013). The vascular bundles structure in magnetic biochar was formed through the evaporation of volatile components and the specific surface area and pore structure of biochar were enhanced (Tan et al., 2015). As the pyrolysis temperature rose to 600 °C, distillation residue can be completely decomposed (shown in Fig. 1), which maybe enhance the generation of highly ordered aromatic structures of magnetic biochar. However, above 600 °C, the pore volume and surface area of MB 800 were decreased accordingly, which may be due to the blockage of pores in the biochar at the higher pyrolysis temperature (Fu et al., 2011). 3.2. Adsorption of aromatics by the magnetic biochars 3.2.1. Adsorption isotherm In order to investigate the adsorption capacity of the magnetic biochars toward the aromatics contaminants (i.e., anisole, phenol and guaiacol), the adsorption isotherm study was performed at 25 °C in the initial concentration range from 3 to 35 mg/L. The equilibrium adsorption capacity was obtained by Eq. (1),
Q e ¼ ðC 0 C e ÞV=M
ð1Þ
where Qe represents the equilibrium adsorption capacity of organic (mg/g); C0 and Ce represent the organic concentration of the initial and equilibrium aqueous phase (mg/L), respectively; M and V represent the mass of the adsorbent (g) and the volume of solution (L), respectively. To illustrate the adsorption characterization of the magnetic biochars for aromatics, the adsorption data were calculated by the Freundlich model, which was commonly used to describe the adsorption onto a heterogeneous surface. The Freundlich model can be written using Eq. (2):
Q e ¼ K F C e1=n
ð2Þ
where n and KF (mg/g) are Freundlich constants related to the sorption intensity and capacity, respectively. Table 3 listed the Freundlich isotherm parameters for the adsorption of aromatics from aqueous solutions on magnetic biochars. The R2 values are larger than 0.976, suggesting that the adsorption of aromatics onto the magnetic biochars were well fitted with Freundlich model. The higher the KF value is, the stronger the adsorption capacity is. The adsorption isotherms of anisole, phenol and guaiacol for the magnetic biochars were presented in Fig. 3(b), (c) and (d), respectively. The maximum equilibrium adsorption capacity for anisole, phenol and guaiacol were 70.4 mg/g, 17.2 mg/g and 23.3 mg/g, respectively. The results showed that the adsorption capacity of anisole on the magnetic biochars was stronger than that of phenol and guaiacol. Fig. 3 (b) and (c) illustrated that the adsorption ability of anisole and phenol onto the magnetic biochars followed the order: MB 600 > MB 800 > MB 400, which meant that the adsorption capacities of magnetic biochar for phenol and anisole reached the maximum at the pyrolysis temperature of 600 °C. Compared with anisole and phenol adsorption behavior on the magnetic biochars, the adsorption capacity of the magnetic biochars for guaiacol was improved as the pyrolysis temperature increasing (shown in Fig. 3 (d). The FTIR spectra of MB 600 after phenol adsorption were analyzed (data not shown). Compared with FTIR spectra of fresh MB
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Fig. 3. Adsorption isotherms of MB 400, MB 600 and MB 800. (a) Nitrogen adsorption isotherms of magnetic biocahrs. (b) Anisole adsorption isotherms; (c) Phenol adsorption isotherms; (d) Guaiacol adsorption isotherms of the magnetic biochars in aqueous solutions.
Table 2 The BET surface areas and pore volumes of MB 400, MB 600 and MB 800.
N2-BET area (m2/g) Total pore volume (cm3/g) Micropore volume (cm3/g) Pore size (nm)
MB 400
MB 600
MB 800
10.75 0.0590 0.0074 10.87
50.62 0.0929 0.0343 10.63
40.73 0.0786 0.0258 10.16
600, the intensity of the peak between 630 and 850 cm1 weakly increased for MB 600 after phenol adsorption. The result suggested that phenol was adsorbed to the surface of the magnetic biochar (MB 600). In addition, the TG analysis was carried out to investigate the stability of magnetic biochars after adsorption of aromatic contaminants (shown in supporting information, Fig. S5). The TG curves for magnetic biochars (MB 400, MB 600 and MB 800) before and after adsorption of phenol were almost coincident, which indicated that the magnetic biochars after phenol adsorption also have a stable structure.
3.2.2. Adsorption mechanisms According to the above results, the pyrolysis temperature has significant influences on the adsorption behavior of magnetic biochar towards aromatic contaminants. It may be due that the properties of magnetic biochar, including BET surface area, pore volume, aromaticity, thermal stability, grapheme-like-structure and iron oxide (c-Fe2O3), were sensitive to the thermal pyrolysis temperature. In fact, the adsorption of aromatic contaminants on magnetic biochar was controlled by multiple processes associated with both carbonaceous material and iron oxide. Some studies have reported that the prime mechanisms for the organic pollutants adsorption behavior on biochars were included in porefilling, hydrogen bonds, hydrophobic effect and electrostatic interaction (Tan et al., 2015). But, the specific mechanisms of different organic contaminants were varied with the characteristic of magnetic biochar. Meanwhile, the effect of iron oxide (c-Fe2O3) particles on the adsorption mechanism should be investigated due to the magnetic biochars used in this study. For the adsorption of anisole and phenol on MB 400, MB 600 and MB 800, the adsorption capability was correlated with the surface properties of magnetic biochar (surface area and pore vol-
Table 3 The Freundlich parameters for the adsorption of aromatics from aqueous solutions on the magnetic biochars. Aromatics
Anisole Phenol Guaiacol
MB 400
MB 600 2
KF, mg/g
1/n
R
4.8250 0.3213 0.6631
0.7614 1.0188 1.0232
0.9792 0.9867 0.9937
MB 800 2
KF, mg/g
1/n
R
KF, mg/g
1/n
R2
16.6015 9.2776 1.4554
0.5241 0.1803 0.8155
0.9893 0.9768 0.9891
11.5956 2.4511 2.1536
0.6061 0.5557 0.7241
0.9854 0.9854 0.9897
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ume). Due to the pronounced pore-filling effect, high surface areas and pore volumes of magnetic biochar generally promote anisole and phenol adsorption. While pyrolysis temperature was raised up 600 °C, the surface area and pore volume of magnetic biochar (MB 600) reached maximum and the adsorption capabilities for anisole and phenol were the largest. Some pores may be blocked when the pyrolysis temperature was elevated from 600 °C to 800 °C. So, the adsorption ability of MB 800 towards anisole and phenol was diminished. However, the adsorption capacity of guaiacol on the magnetic biochars were improved as the pyrolysis temperature increasing (shown in Fig. 3(d)), which was not coincident with the surface properties of magnetic biochars. The main mechanisms of guaiacol adsorption on magnetic biochars can be explained by the p-p electron donor–acceptor (EDA) interaction. The high pyrolysis temperature led to a high degree of carbonization of the distillation residue, thereby building a grapheme-likestructure, which confirmed by the XRD results (shown in Fig. S3 (a)). And the grapheme-like-structure in MB 800 is higher than that of MB 400 and MB 600. The magnetic biochar with the graphemelike-structure has a higher p-electron density, suggesting that the graphitic layers of the magnetic biochar could be regarded as a pelectron donor (Li et al., 2016a). Hence the p-p EDA interaction between guaiacol molecules and graphite layers of magnetic biochar would be enhanced with the increase in the content of graphite layers. According to the above analysis, the low adsorption capacity of phenol and guaiacol on MB 400 (shown in Fig. 3) may be due to the low surface area, pore volume and grapheme-likestructure in MB 400. Furthermore, the remarkable high performance of magnetic biochars in the removal of anisole may be attributed to hydrophobic effect, which may be due to the high hydrophobicity in magnetic biochars. Therefore, the adsorption of anisole on magnetic biochars is most likely controlled by porefilling and hydrophobic effect. In addition, the influence of the iron oxide (c-Fe2O3) particles on the adsorption mechanism may be involved in two aspects. Firstly, the iron oxide particles as a kind of magnetic materials was capable of changing the structure and nature of aqueous solution through resonance, such as reducing the surface tension and viscosity of solutions (Li et al., 2016a). Thus the mobility of aromatic contaminants in aqueous solution was enhanced, which was advantageous to adsorb contaminants on magnetic biochars. Secondly, organic pollutants in solutions could bond with the iron oxide particles through AOH group (Wang et al., 2015). Phenol and guaiacol contain hydroxyl, which could be causing surface complexation with the iron oxide particles. Although some researches revealed that the external magnetic field can be applied to replace the magnetic sorbents with normal sorbents for pollutants removal (Li et al., 2016a), the magnetic biochar has a great advantages which can be conveniently isolated from the aqueous solutions by using an outer magnet.
4. Conclusion The distillation residue was efficiently transformed to magnetic biochar through pyrolysis, providing a promising strategy for waste resource recovery. The physicochemical properties of magnetic biochar, such as surface structure, aromaticity, thermal stability and grapheme-like-structure, were markedly affected by the pyrolysis temperature. The adsorption capacity of anisole and phenol onto the magnetic biochars followed the order: MB 600 > MB 800 > MB 400. The adsorption mechanisms depended on magnetic biochar properties and target contaminants. The p-p electron donor–acceptor interaction was the dominant mechanism for guaiacol adsorption on magnetic biochars, while the anisole adsorption was controlled by pore-filling and hydrophobic effect.
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Acknowledgements The authors sincerely acknowledge the National Basic Research (973) special preliminary study program (2014CB260408) for the financial support.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2016.10. 033.
Reference Ahmad, M., Lee, S.S., Rajapaksha, A.U., Vithanage, M., Zhang, M., Cho, J.S., Lee, S.E., Ok, Y.S., 2013. Trichloroethylene adsorption by pine needle biochars produced at various pyrolysis temperatures. Bioresour. Technol. 143, 615–622. Al-Wabel, M.I., Al-Omran, A., El-Naggar, A.H., Nadeem, M., Usman, A.R.A., 2013. Pyrolysis temperature induced changes in characteristics and chemical composition of biochar produced from conocarpus wastes. Bioresour. Technol. 131, 374–379. Angın, D., 2013. Effect of pyrolysis temperature and heating rate on biochar obtained from pyrolysis of safflower seed press cake. Bioresour. Technol. 128, 593–597. Capunitan, J.A., Capareda, S.C., 2013. Characterization and separation of corn stover bio-oil by fractional distillation. Fuel 112, 60–73. Chen, Y., Yang, H., Wang, X., Zhang, S., Chen, H., 2012. Biomass-based pyrolytic polygeneration system on cotton stalk pyrolysis: Influence of temperature. Bioresour. Technol. 107, 411–418. Demirbas, A., 2004. Effects of temperature and particle size on bio-char yield from pyrolysis of agricultural residues. J. Anal. Appl. Pyrol. 72, 243–248. Elkasabi, Y., Boateng, A.A., Jackson, M.A., 2015. Upgrading of bio-oil distillation bottoms into biorenewable calcined coke. Biomass Bioenergy 81, 415–423. Fu, P., Yi, W., Bai, X., Li, Z., Hu, S., Xiang, J., 2011. Effect of temperature on gas composition and char structural features of pyrolyzed agricultural residues. Bioresour. Technol. 102, 8211–8219. Hossain, M.K., Strezov, V., Chan, K.Y., Ziolkowski, A., Nelson, P.F., 2011. Influence of pyrolysis temperature on production and nutrient properties of wastewater sludge biochar. J. Environ. Manage. 92, 223–228. Kim, K.H., Kim, J.Y., Cho, T.S., Choi, J.W., 2012. Influence of pyrolysis temperature on physicochemical properties of biochar obtained from the fast pyrolysis of pitch pine (Pinus rigida). Bioresour. Technol. 118, 158–162. Kundu, A., Gupta, B.S., Hashim, M.A., Sahu, J.N., Mujawar, M., Redzwan, G., 2015. Optimisation of the process variables in production of activated carbon by microwave heating. RSC Adv. 5, 35899–35908. Lee, Y., Park, J., Ryu, C., Gang, K.S., Yang, W., Park, Y.K., Jung, J., Hyun, S., 2013. Comparison of biochar properties from biomass residues produced by slow pyrolysis at 500 °C. Bioresour. Technol. 148, 196–201. Li, Y., Zhu, C., Lu, T., Guo, Z., Zhang, D., Ma, J., Zhu, S., 2013. Simple fabrication of a Fe2O3/carbon composite for use in a high-performance lithium ion battery. Carbon 52, 565–573. Li, H., Xia, S., Li, Y., Ma, P., Zhao, C., 2015a. Stability evaluation of fast pyrolysis oil from rice straw. Chem. Eng. Sci. 135, 258–265. Li, H., Xia, S., Wu, M., Ma, P., 2015b. Isobaric (vapour + liquid) equilibria of binary systems containing butyl acetate for the separation of methoxy aromatic compounds (anisole and guaiacol) from biomass fast pyrolysis oil. J. Chem. Thermodyn. 87, 141–146. Li, G., Zhu, W., Zhang, C., Zhang, S., Liu, L., Zhu, L., Zhao, W., 2016a. Effect of a magnetic field on the adsorptive removal of methylene blue onto wheat straw biochar. Bioresour. Technol. 206, 16–22. Li, H., Xia, S., Ma, P., 2016b. Thermogravimetric investigation of the co-combustion between the pyrolysis oil distillation residue and lignite. Bioresour. Technol. 218, 615–622. Li, H., Xia, S., Ma, P., 2016c. Upgrading fast pyrolysis oil: Solvent–anti-solvent extraction and blending with diesel. Energy Convers. Manage. 110, 378–385. Lu, Q., Yang, X.L., Zhu, X.F., 2008. Analysis on chemical and physical properties of bio-oil pyrolyzed from rice husk. J. Anal. Appl. Pyrol. 82, 191–198. Mubarak, N.M., Kundu, A., Sahu, J.N., Abdullah, E.C., Jayakumar, N.S., 2014. Synthesis of palm oil empty fruit bunch magnetic pyrolytic char impregnating with FeCl3 by microwave heating technique. Biomass Bioenergy 61, 265–275. Mubarak, N.M., Sahu, J.N., Abdullah, E.C., Jayakumar, N.S., 2016. Plam oil empty fruit bunch based magnetic biochar composite comparison for synthesis by microwave-assisted and conventional heating. J. Anal. Appl. Pyrol. 120, 521– 528. Noraini, M.N., Abdullah, E.C., Othman, R., Mubarak, N.M., 2016. Single-route synthesis of magnetic biochar from sugarcane bagasse by microwave-assisted pyrolysis. Mater. Lett. 184, 315–319. Tan, X., Liu, Y., Zeng, G., Wang, X., Hu, X., Gu, Y., Yang, Z., 2015. Application of biochar for the removal of pollutants from aqueous solutions. Chemosphere 125, 70–85.
26
H. Li et al. / Bioresource Technology 223 (2017) 20–26
Thines, K.R., Abdullah, E.C., Ruthiraan, M., Mubarak, N.M., Tripathi, M., 2016. A new route of magnetic biochar based polyaniline composites for supercapacitor electrode materials. J. Anal. Appl. Pyrol. 121, 240–257. Thines, K.R., Abdullah, E.C., Mubarak, N.M., Ruthiraan, M., 2017. Synthesis of magnetic biochar from agricultural waste biomass to enhancing route for waste water and polymer application: a review. Renewable Sustainable Energy Rev. 67, 257–276. Wang, S., Gao, B., Zimmerman, A.R., Li, Y., Ma, L., Harris, W.G., Migliaccio, K.W., 2015. Removal of arsenic by magnetic biochar prepared from pinewood and natural hematite. Bioresour. Technol. 175, 391–395. Wei, D., Ngo, H.H., Guo, W., Xu, W., Zhang, Y., Du, B., Wei, Q., 2016. Biosorption of effluent organic matter onto magnetic biochar composite: behavior of fluorescent components and their binding properties. Bioresour. Technol. 214, 259–265.
Xu, Y., Chen, B., 2013. Investigation of thermodynamic parameters in the pyrolysis conversion of biomass and manure to biochars using thermogravimetric analysis. Bioresour. Technol. 146, 485–493. Yu, J., Zhao, Y., Li, Y., 2014. Utilization of corn cob biochar in a direct carbon fuel cell. J. Power. Sources 270, 312–317. Zhang, M., Gao, B., Varnoosfaderani, S., Hebard, A., Yao, Y., Inyang, M., 2013a. Preparation and characterization of a novel magnetic biochar for arsenic removal. Bioresour. Technol. 130, 457–462. Zhang, X.S., Yang, G.X., Jiang, H., Liu, W.J., Ding, H.S., 2013b. Mass production of chemicals from biomass-derived oil by directly atmospheric distillation coupled with co-pyrolysis. Sci. Rep. 3, 1120. Zheng, J.L., Wei, Q., 2011. Improving the quality of fast pyrolysis bio-oil by reduced pressure distillation. Biomass Bioenergy 35, 1804–1810.
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Effect of pyrolysis temperature on characteristics and aromatic contaminants adsorption behavior of magnetic biochar derived from pyrolysis oil distillation residue Hao Li, Samah Awadh Ali Mahyoub, Wenjie Liao, Shuqian Xia ⇑, Hechuan Zhao, Mengya Guo, Peisheng Ma Key Laboratory for Green Chemical Technology of State Education Ministry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, People’s Republic of China
h i g h l i g h t s Magnetic biochars were easily synthesized by pyrolyzing distillation residue. Pyrolysis temperature showed a pronounced effect on magnetic biochar properties. Adsorption of aromatic contaminants on magnetic biochars were investigated. Adsorption mechanisms associated with biochar properties and target contaminants.
a r t i c l e
i n f o
Article history: Received 16 August 2016 Received in revised form 10 October 2016 Accepted 12 October 2016 Available online 14 October 2016 Keywords: Distillation residue Magnetic biochar Pyrolysis Characterization Adsorption
a b s t r a c t The magnetic biochars were easily fabricated by thermal pyrolysis of Fe(NO3)3 and distillation residue derived from rice straw pyrolysis oil at 400, 600 and 800 °C. The effects of pyrolysis temperature on characteristics of magnetic biochars as well as adsorption capacity for aromatic contaminants (i.e., anisole, phenol and guaiacol) were investigated carefully. The degree of carbonization of magnetic biochars become higher as pyrolysis temperature increasing. The magnetic biochar reached the largest surface area and pore volume at the pyrolysis temperature of 600 °C due to pores blocking in biochar during pyrolysis at 800 °C. Based on batch adsorption experiments, the used adsorbent could be magnetically separated and the adsorption capacity of anisole on magnetic biochars was stronger than that of phenol and guaiacol. The properties of magnetic biochar, including surface area, pore volume, aromaticity, grapheme-like-structure and iron oxide (c-Fe2O3) particles, showed pronounced effects on the adsorption performance of aromatic contaminants. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The fast pyrolysis technology has been considered as a promising process since it offers a feasible method to convert waste biomass into liquid pyrolysis oil. Unfortunately, the pyrolysis oil consists of a large number of unstable oxygen-containing compounds, and it is difficult to directly apply the pyrolysis oil in existing equipment (Li et al., 2016c, 2015b). Therefore, it is very necessary to upgrade pyrolysis oil. Distillation offers a feasible and effective way to obtain the high-grade fuel and high-value chemicals from the biomass pyrolysis oil. Most researches have been conducted to the production of ⇑ Corresponding author. E-mail address: [email protected] (S. Xia). http://dx.doi.org/10.1016/j.biortech.2016.10.033 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.
the volatile fraction, in which the distillates have been achieved in various applications either as an energy or chemical source (Capunitan and Capareda, 2013; Zhang et al., 2013b). In fact, approximately 30–50 wt.% solid residue as a by-product was parallel formed after distillation of pyrolysis oil, which can be seen in the previous studies (Capunitan and Capareda, 2013; Zheng and Wei, 2011). In order to achieve comprehensive utilization of pyrolysis oil, the parallel generated distillation bottom (or distillation residue) should be concerned. Unfortunately, there were by far very few studies have focus on the application of the distillation residue derived from pyrolysis oil. Zhang et al. (2013b) made distillation residue a source of renewable chemicals using co-pyrolysis. Elkasabi et al. (2015) applied the bio-oil distillate bottoms (distillation residue) as a feedstock to further upgrade for the production of calcined coke.
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In recently published paper, the distillation residue was utilized as solid fuel to co-fire with lignite, in which the waste was recycled (Li et al., 2016b). In fact, the development of magnetic biochar from biomass (especially agricultural waste) have been attracted many researchers (Kundu et al., 2015; Thines et al., 2017). These magnetic biochar showed a remarkable application as an adsorbent for various wastewater treatments, which may be due that the magnetic biochars not only can adsorb the contaminants from aqueous solutions, but also can be easily isolated with external magnets after adsorption. Two publications reported the production of magnetic bio-char derived from palm oil empty fruit bunch as the raw material by using microwave heating technique (Mubarak et al., 2014, 2016). Thines et al. (2016) focused on the conversion of the durian’s rind into magnetic biochar in the presence of three different metallic salts by employing a novel vacuum condition in an electrical muffle furnace. Noraini et al. (2016) employed a novel method for the preparation of magnetic biochar from sugarcane bagasse by microwave-assisted pyrolysis at a microwave power of 600 W. Wei et al. (2016) obtained magnetic biochar composite from shell as raw material by using coprecipitation method in the presence of FeCl3 and FeSO4. However, there is no study on the application of pyrolysis oil distillation residue for the synthesis of magnetic biochar using for the adsorption of aromatic contaminants from aqueous solution in the literature. Various aromatic pollutants in aqueous solution were a threat to human health. And adsorption was one of the convenient method for the organic contaminants removal from aqueous solution. In this work, a novel magnetic biochar has been easily fabricated using the discarded material, the rice straw pyrolysis oil distillation residue, with impregnated Fe(NO3)3. The pyrolysis of the impregnated distillation residue was carried out at 400, 600 and 800 °C. The effect of pyrolysis temperature on the characteristics of magnetic biochars were investigated comprehensively in the study. In addition, batch adsorption experiments were conducted to investigate the adsorption capacity of the magnetic biochars to various aromatics (i.e., anisole, phenol and guaiacol). It is anticipated that the feasible method presented here can achieve the comprehensive utilization of the distillation by-product of pyrolysis oil. 2. Materials and methods 2.1. Materials The rice straw pyrolysis oil studied in this work was kindly provided by Shaanxi Yingjiliang Bio-energy Corporation, China. The bio-oil was produced in a downstream circulating fluidized bed reaction by flash pyrolysis of rice straw (500 °C/s and 500 °C for the heating rate and reaction temperature, respectively). The pyrolysis oil poured into a round-bottom flask was distillated at a certain temperature (140, 180, 220 and 250 °C) for 30 min. Then, the distillate bottom was obtained. The distillation residue was the distillate bottom. The detail experimental procedure about the distillation was described in the reported paper (Li et al., 2015a). The iron nitrate nonahydrate (Fe(NO3)39H2O), guaiacol, anisole and phenol were obtained from Aladdin-reagent, China. 2.2. Preparation of magnetic biochar In this study, the magnetic biochar was synthesized using pyrolysis oil distillation residue as raw material. The distillation residue was initially washed with DI water and ethanol to remove impurities, respectively, and dried in an oven at 80 °C for 12 h under air atmosphere. The dried distillation residue was then grinded to 200 mesh. Subsequently, the grinded distillation residue
21
(5 g) was immersed into the prepared Fe(NO3)3 solution (2.02 g Fe (NO3)3 in 20 mL of DI water). After stirring for 2 h, the sample was dried at 80 °C for 12 h under air atmosphere. The obtained material was pyrolyzed for 2 h under nitrogen (N2) flow of 100 mL/min1 at a heating rate of 10 °C/min. The pyrolysis temperature were 400 °C, 600 °C and 800 °C, respectively. The obtained magnetic biochars (MB) were grinded to 200 mesh, washed with DI water, filtered, dried (60 °C, 4 h) in a vacuum oven, and sealed in a container before use. In this study, MB 400, MB 600 and MB 800 were the magnetic biochars prepared at the pyrolysis temperature of 400 °C, 600 °C and 800 °C, respectively. 2.3. Characterization Ultimate analyses were performed on the elemental analyzer (Vario micro cube, Elementar, Germany). Thermogravimetric analysis was analyzed by TG (TG 209 F3, Netzsch, Germany). For each experiment, approximately 5 mg of sample was processed and heated up to 900 °C at heating rate of 10 °C/min under nitrogen or air atmosphere. The DSC analysis of samples (MB 400, MB 600 and MB 800) were performed on the DSC analyzer (DSC 2, Mettler-Toledo, Switzerland) under nitrogen purge of 80 ml/min and at the heating rate of 10 °C/min. The surface area, pore volume, and pore size of the magnetic biochars were determined with an instrument (V-Sorb X800, Gold app instruments, China). The surface morphology of magnetic biochars were studied by scanning electron microscopy (S-4800, Hitachi, Japan). Chemical functional groups of magnetic biochars were analyzed using FTIR (FTS6000, Bio-rad, US). The crystallinity and structure of the magnetic biochars were analyzed through XRD (D8-Advance, Bruker, Germany) with Cu Ka radiation (k = 1.542 Å) at 40 kV and 40 mA and scanned from 10 to 90° at a rate of 0.5°/s. The magnetic properties of MB 400, MB 600 and MB 800 were evaluated using Vibrating Sample Magnetometer (VSM). 2.4. Batch adsorption experiments Adsorption of aromatics (anisole, phenol and guaiacol) were conducted using batch adsorption approach. The adsorption experiments were performed over the initial aromatics concentration range from 3 to 35 mg/L. About 0.0090 g magnetic biochar was weighed to each 50 mL tube and then 30 mL aromatics solution with one specific concentration was added. The mixtures were shaken for 24 h at 298.15 K on the shaker, which is sufficient for reaching equilibrium. Then, the samples were centrifuged for 5 min at 4000 rpm and filtered using a 0.22 lm nylon membrane filter. The aromatics concentrations were determined using an ultraviolet–visible (UV–vis) spectrophotometer (TU 1900, Purkinje General Instruments Co. Ltd., China) at 269 nm (for anisole), 270 nm (for phenol) and 274 nm (for guaiacol). The magnetic biochars were obtained for characterization after adsorption of phenol over the initial aromatics concentration of 35 mg/L. 3. Results and discussions 3.1. Characterization of magnetic biochar 3.1.1. Ultimate analysis The elemental composition of the magnetic biochars (MB 400, MB 600 and MB 800) were investigated and presented in Table 1. The results showed that the carbon contents of magnetic biochar decreased with the pyrolysis temperature increasing from 400 °C to 800 °C. It may be attributed that higher pyrolysis temperature led to the larger loss of volatiles in distillation residue. In addition, the H/C atomic ratio could be applied to evaluate the aromaticity of
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Table 1 The elemental composition of the magnetic biochars (MB 400, MB 600 and MB 800).
C, % H, % N, % H/C atomic ratio
MB 400
MB 600
MB 800
29.92 4.37 1.37 1.75
28.16 3.26 1.34 1.39
22.98 1.68 1.19 0.88
magnetic biochars. And the higher the H/C atomic ratio is, the lower the aromaticity of magnetic would be (Ahmad et al., 2013). Table 1 illustrated that the values of H/C atomic ratio gradually declined with pyrolysis temperatures increasing. When the pyrolysis temperature rose up 800 °C, the value of H/C atomic ratio was the minimum, which indicated the MB 800 is highly carbonized and exhibits the highest aromaticity among the magnetic biochars. 3.1.2. Thermal analysis The TG and DTG analysis of the distillation residue and magnetic biochars under nitrogen were presented in Fig. 1. As shown in TG curves under nitrogen, the first stage (<220 °C) for the distillation residue maybe attributed to the removal of moisture and other small molecule residue. The weight loss indicated the moisture and low-boiling volatiles contents of the distillation residue were approximately 10%. The thermal decomposition actively took place in 200–600 °C under nitrogen due to the progressive pyrolysis of macromolecular components (Lee et al., 2013; Xu and Chen, 2013). The mass loss of the distillation residue was greater than 55% under nitrogen at nearly 900 °C. In the DTG curve under nitrogen (shown in Fig. 1), the maximum rate of weight loss of distillation residue (1.93 wt.%/min) is much higher than that of MB 400 (0.84 wt.%/min), MB 600 (0.65 wt.%/min) and MB 800 (0.58 wt.%/ min), while the maximum weight loss temperature of distillation residue is lower than that of magnetic biochars. This phenomenal may be due that more volatile substances existed in distillation residue. No phase change was found on all the magnetic biochars at 40–300 °C, indicating decomposition did not occur at this temperature range under nitrogen. Compared to distillation residue, the magnetic biochars (MB 400, MB 600 and MB 800) showed higher thermal stability and began to decompose at a higher temperature under nitrogen. The total mass lass of MB 400, MB 600 and MB 800 in nitrogen at 900 °C were 29 wt%, 15 wt% and 7 wt %, respectively. Al-Wabel et al. (2013) used the conocarpus wastes to prepare biochars at different pyrolysis temperatures (200– 800 °C). And they found that the occurred weight loss followed
Fig. 1. TG and DTG curves of distillation residue, MB 400, MB 600 and MB 800 under nitrogen purge of 50 mL/min and heating rate of 10 °C/min.
the order: conocarpus wastes feedstock > 200 °C biochar > 400 °C biochar > 600 °C biochar > 800 °C biochar, which is consistent with the results of this work. As expected, the mass loss of magnetic biochar tended to decrease as the pyrolysis temperature rose from 400 °C to 800 °C in nitrogen. The thermal stability of magnetic biochars can be also confirmed by DSC profiles (data not shown). Compared to the DSC curves of each magnetic biochar in nitrogen, the starting reaction temperature of MB 800 was higher than that of MB 400 and MB 600, which indicated that MB 800 has the better thermal resistance. In order to investigate the thermal oxidative stability, the TG analysis for distillation residue, MB 400, MB 600 and MB 800 were carried out in air (shown in Fig. S1). When heating temperature was implemented between 40 and 285 °C, the mass of all the magnetic biochars only decreased slightly (due to water loss). And the main decomposition for MB 400, MB 600 and MB 800 under air atmosphere was in 290–600 °C. But, the decomposition temperature of MB 800 is higher than that of MB 400 and MB 600 under air atmosphere. Therefore, as pyrolysis temperature increased, the magnetic biochars became more thermal stability. 3.1.3. Morphological analysis Scanning electron microscopy (SEM) images of MB 400, MB 600 and MB 800 were applied to investigate the influence of pyrolysis temperature on the morphology of the magnetic biochars (shown in supporting information, Fig. S2). The SEM images revealed that the magnetic biochars underwent clear structural modifications as pyrolysis temperature increasing. The reaction mechanisms of biomass pyrolysis were defined in three main steps: 1st step: Biomass ? Water + Unreacted residue; 2nd step: Unreacted residue ? (Volatile + Gases)1 + (Char)1; 3rd step: (Char)1 ? (Volatile + Gases)2 + (Char)2 (Demirbas, 2004; Kim et al., 2012). The biochar primarily generated in the second step. And some chemical rearrangement of biochar occurred in the last step. The morphological changes in the magnetic biochars obtained in this work may be due to the char formation reactions. As the pyrolysis temperature rose from 400 °C to 600 °C, the degree of carbonization of magnetic biochar was enhanced. The MB 600 presented a typical char structure (Fig. S2(b)). The surface morphology of the MB 600 illustrated in Fig. S2(b) is very similar to the biochar derived from corn cob (Yu et al., 2014). Visual inspection of the images of MB 600 and the biochar derived from corn cob revealed that the external surface were full of pores, which enlarge the surface area and provide more adsorption sites. While the pyrolysis temperature was reached at 800 °C, the char’s remaining carbons were further rearranged and MB 800 presented a three-dimensional structure (Fig. S2(d)). 3.1.4. Structural analysis X-ray diffraction (XRD) was applied to investigate the magnetic biochars crystallinity and structure (shown in supporting information, Fig. S3(a)). As the pyrolysis temperature rose up 600 °C, a peak at 2h = 44.4° is noted, which was assigned to the (1 0 1) diffraction pattern of the partially graphitized carbon (Li et al., 2013). From MB 600 to MB 800, the peak (2h = 44.4°) increased in intensity, indicating that the crystalline structure of the partially graphitized carbon in MB 800 is higher than that of MB 600 and the degree of orientation of the aromatic lamellae in MB 800 becomes higher (Fu et al., 2011). In addition, five indexed planes (2 2 0), (3 1 1), (4 0 0), (5 1 1), and (4 4 0), which correspond to the diffraction peaks at 30.2°, 35.5°, 43.2°, 57.3°, and 62.9° (Zhang et al., 2013a), were detected in the biochars in the work. These diffraction peaks are the characterizations of maghemite, which was identified as the major crystalline phase in the magnetic biochar. So the X-ray diffractions indicate that the maghemite (c-Fe2O3) maybe exist in MB 400, MB 600 and MB 800. The magnetic hysteresis curves for
H. Li et al. / Bioresource Technology 223 (2017) 20–26
MB 400, MB 600 and MB 800 were presented in Fig. 2. The samples showed ferromagnetic properties, revealed by the coercive field of 2.02 Oe to 19.86 Oe and the saturation magnetization of 5.50– 18.36 emu/g for all magnetic biochars prepared in this work. The saturation magnetization of MB 800 (18.36 emu/g) is absolutely higher than that MB 400 and MB 600, which may be due to the larger existence of the iron oxide particles in MB 800. And, the magnetic biochars clearly showed a good magnate property, which could be attracted by an outer magnet (shown in Fig. S4). This property is essentially important for the convenient recycling of contaminant-laden magnetic biochar adsorbents after use. The Fourier transform infrared spectra (FTIR) analysis was used to reveal the chemical structure of magnetic biochars. The strong peak at approximately 3400 cm1 was observed in the spectra of MB 400 and MB 600 (shown in Fig. S3(b)), which attributed to O–H stretching and strong hydrogen bonding. However, the intensity of this peak declined with the increase of pyrolysis temperature. Meanwhile, the peak (at approximately 3400 cm1) was almost disappeared in the spectrum for MB 800, suggesting the acceleration of dehydration reaction in distillation residue at the increased temperature (Chen et al., 2012; Kim et al., 2012). Some appeared peaks for magnetic biochars at approximately 1600 cm1 were the aromatic C@C stretching vibrations, suggesting that aromatics existed in magnetic biochars. The absorption peaks between 1350 and 1470 cm1 can be assigned to the stretching vibration of methyl CAH, which indicate the possible presence of alkanes (Lu et al., 2008). The peaks between 630 and 850 cm1 are the representative of the presence of adjacent aromatic hydrogens in the magnetic biochars (Angın, 2013). But these peaks generally declined in the spectrum for MB 800, which suggested that polar functional groups tended to be diminished as the pyrolysis temperature increasing (Al-Wabel et al., 2013). In additional, the baseline of FTIR spectrum gradually drifted upwards as pyrolysis temperature increasing, which probably attributed to the higher degree of aromaticity for MB 800 (Angın, 2013; Hossain et al., 2011).
3.1.5. BET surface areas and pore volumes Fig. 3(a) illustrated nitrogen adsorption isotherms of MB 400, MB 600 and MB 800. MB 600 exhibited a higher adsorption capacity for nitrogen than MB 800 and MB 400, which indicated that the MB 600 has much more porous structures. The BET surface area and pore volume (total and micro) of the magnetic biochars (MB 400, MB 600 and MB 800) were gathered in Table 2. The results revealed that the surface areas and pore volumes were largely
Fig. 2. Magnetic hysteresis loop of MB 400, MB 600 and MB 800.
23
dependent on the pyrolysis temperature. When the pyrolysis temperature was elevated from 400 °C to 600 °C, the values of BET surface areas and pore volumes (total and micro) increased significantly and reached maximum (50.62 m2/g, 0.0929 cm3/g and 0.0343 cm3/g for the BET surface area, total and micro pore volumes, respectively). The remarkable improvement in the surface area and pore volume of magnetic biochars may be due to significant increase with the release of volatiles (Angın, 2013). The vascular bundles structure in magnetic biochar was formed through the evaporation of volatile components and the specific surface area and pore structure of biochar were enhanced (Tan et al., 2015). As the pyrolysis temperature rose to 600 °C, distillation residue can be completely decomposed (shown in Fig. 1), which maybe enhance the generation of highly ordered aromatic structures of magnetic biochar. However, above 600 °C, the pore volume and surface area of MB 800 were decreased accordingly, which may be due to the blockage of pores in the biochar at the higher pyrolysis temperature (Fu et al., 2011). 3.2. Adsorption of aromatics by the magnetic biochars 3.2.1. Adsorption isotherm In order to investigate the adsorption capacity of the magnetic biochars toward the aromatics contaminants (i.e., anisole, phenol and guaiacol), the adsorption isotherm study was performed at 25 °C in the initial concentration range from 3 to 35 mg/L. The equilibrium adsorption capacity was obtained by Eq. (1),
Q e ¼ ðC 0 C e ÞV=M
ð1Þ
where Qe represents the equilibrium adsorption capacity of organic (mg/g); C0 and Ce represent the organic concentration of the initial and equilibrium aqueous phase (mg/L), respectively; M and V represent the mass of the adsorbent (g) and the volume of solution (L), respectively. To illustrate the adsorption characterization of the magnetic biochars for aromatics, the adsorption data were calculated by the Freundlich model, which was commonly used to describe the adsorption onto a heterogeneous surface. The Freundlich model can be written using Eq. (2):
Q e ¼ K F C e1=n
ð2Þ
where n and KF (mg/g) are Freundlich constants related to the sorption intensity and capacity, respectively. Table 3 listed the Freundlich isotherm parameters for the adsorption of aromatics from aqueous solutions on magnetic biochars. The R2 values are larger than 0.976, suggesting that the adsorption of aromatics onto the magnetic biochars were well fitted with Freundlich model. The higher the KF value is, the stronger the adsorption capacity is. The adsorption isotherms of anisole, phenol and guaiacol for the magnetic biochars were presented in Fig. 3(b), (c) and (d), respectively. The maximum equilibrium adsorption capacity for anisole, phenol and guaiacol were 70.4 mg/g, 17.2 mg/g and 23.3 mg/g, respectively. The results showed that the adsorption capacity of anisole on the magnetic biochars was stronger than that of phenol and guaiacol. Fig. 3 (b) and (c) illustrated that the adsorption ability of anisole and phenol onto the magnetic biochars followed the order: MB 600 > MB 800 > MB 400, which meant that the adsorption capacities of magnetic biochar for phenol and anisole reached the maximum at the pyrolysis temperature of 600 °C. Compared with anisole and phenol adsorption behavior on the magnetic biochars, the adsorption capacity of the magnetic biochars for guaiacol was improved as the pyrolysis temperature increasing (shown in Fig. 3 (d). The FTIR spectra of MB 600 after phenol adsorption were analyzed (data not shown). Compared with FTIR spectra of fresh MB
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Fig. 3. Adsorption isotherms of MB 400, MB 600 and MB 800. (a) Nitrogen adsorption isotherms of magnetic biocahrs. (b) Anisole adsorption isotherms; (c) Phenol adsorption isotherms; (d) Guaiacol adsorption isotherms of the magnetic biochars in aqueous solutions.
Table 2 The BET surface areas and pore volumes of MB 400, MB 600 and MB 800.
N2-BET area (m2/g) Total pore volume (cm3/g) Micropore volume (cm3/g) Pore size (nm)
MB 400
MB 600
MB 800
10.75 0.0590 0.0074 10.87
50.62 0.0929 0.0343 10.63
40.73 0.0786 0.0258 10.16
600, the intensity of the peak between 630 and 850 cm1 weakly increased for MB 600 after phenol adsorption. The result suggested that phenol was adsorbed to the surface of the magnetic biochar (MB 600). In addition, the TG analysis was carried out to investigate the stability of magnetic biochars after adsorption of aromatic contaminants (shown in supporting information, Fig. S5). The TG curves for magnetic biochars (MB 400, MB 600 and MB 800) before and after adsorption of phenol were almost coincident, which indicated that the magnetic biochars after phenol adsorption also have a stable structure.
3.2.2. Adsorption mechanisms According to the above results, the pyrolysis temperature has significant influences on the adsorption behavior of magnetic biochar towards aromatic contaminants. It may be due that the properties of magnetic biochar, including BET surface area, pore volume, aromaticity, thermal stability, grapheme-like-structure and iron oxide (c-Fe2O3), were sensitive to the thermal pyrolysis temperature. In fact, the adsorption of aromatic contaminants on magnetic biochar was controlled by multiple processes associated with both carbonaceous material and iron oxide. Some studies have reported that the prime mechanisms for the organic pollutants adsorption behavior on biochars were included in porefilling, hydrogen bonds, hydrophobic effect and electrostatic interaction (Tan et al., 2015). But, the specific mechanisms of different organic contaminants were varied with the characteristic of magnetic biochar. Meanwhile, the effect of iron oxide (c-Fe2O3) particles on the adsorption mechanism should be investigated due to the magnetic biochars used in this study. For the adsorption of anisole and phenol on MB 400, MB 600 and MB 800, the adsorption capability was correlated with the surface properties of magnetic biochar (surface area and pore vol-
Table 3 The Freundlich parameters for the adsorption of aromatics from aqueous solutions on the magnetic biochars. Aromatics
Anisole Phenol Guaiacol
MB 400
MB 600 2
KF, mg/g
1/n
R
4.8250 0.3213 0.6631
0.7614 1.0188 1.0232
0.9792 0.9867 0.9937
MB 800 2
KF, mg/g
1/n
R
KF, mg/g
1/n
R2
16.6015 9.2776 1.4554
0.5241 0.1803 0.8155
0.9893 0.9768 0.9891
11.5956 2.4511 2.1536
0.6061 0.5557 0.7241
0.9854 0.9854 0.9897
H. Li et al. / Bioresource Technology 223 (2017) 20–26
ume). Due to the pronounced pore-filling effect, high surface areas and pore volumes of magnetic biochar generally promote anisole and phenol adsorption. While pyrolysis temperature was raised up 600 °C, the surface area and pore volume of magnetic biochar (MB 600) reached maximum and the adsorption capabilities for anisole and phenol were the largest. Some pores may be blocked when the pyrolysis temperature was elevated from 600 °C to 800 °C. So, the adsorption ability of MB 800 towards anisole and phenol was diminished. However, the adsorption capacity of guaiacol on the magnetic biochars were improved as the pyrolysis temperature increasing (shown in Fig. 3(d)), which was not coincident with the surface properties of magnetic biochars. The main mechanisms of guaiacol adsorption on magnetic biochars can be explained by the p-p electron donor–acceptor (EDA) interaction. The high pyrolysis temperature led to a high degree of carbonization of the distillation residue, thereby building a grapheme-likestructure, which confirmed by the XRD results (shown in Fig. S3 (a)). And the grapheme-like-structure in MB 800 is higher than that of MB 400 and MB 600. The magnetic biochar with the graphemelike-structure has a higher p-electron density, suggesting that the graphitic layers of the magnetic biochar could be regarded as a pelectron donor (Li et al., 2016a). Hence the p-p EDA interaction between guaiacol molecules and graphite layers of magnetic biochar would be enhanced with the increase in the content of graphite layers. According to the above analysis, the low adsorption capacity of phenol and guaiacol on MB 400 (shown in Fig. 3) may be due to the low surface area, pore volume and grapheme-likestructure in MB 400. Furthermore, the remarkable high performance of magnetic biochars in the removal of anisole may be attributed to hydrophobic effect, which may be due to the high hydrophobicity in magnetic biochars. Therefore, the adsorption of anisole on magnetic biochars is most likely controlled by porefilling and hydrophobic effect. In addition, the influence of the iron oxide (c-Fe2O3) particles on the adsorption mechanism may be involved in two aspects. Firstly, the iron oxide particles as a kind of magnetic materials was capable of changing the structure and nature of aqueous solution through resonance, such as reducing the surface tension and viscosity of solutions (Li et al., 2016a). Thus the mobility of aromatic contaminants in aqueous solution was enhanced, which was advantageous to adsorb contaminants on magnetic biochars. Secondly, organic pollutants in solutions could bond with the iron oxide particles through AOH group (Wang et al., 2015). Phenol and guaiacol contain hydroxyl, which could be causing surface complexation with the iron oxide particles. Although some researches revealed that the external magnetic field can be applied to replace the magnetic sorbents with normal sorbents for pollutants removal (Li et al., 2016a), the magnetic biochar has a great advantages which can be conveniently isolated from the aqueous solutions by using an outer magnet.
4. Conclusion The distillation residue was efficiently transformed to magnetic biochar through pyrolysis, providing a promising strategy for waste resource recovery. The physicochemical properties of magnetic biochar, such as surface structure, aromaticity, thermal stability and grapheme-like-structure, were markedly affected by the pyrolysis temperature. The adsorption capacity of anisole and phenol onto the magnetic biochars followed the order: MB 600 > MB 800 > MB 400. The adsorption mechanisms depended on magnetic biochar properties and target contaminants. The p-p electron donor–acceptor interaction was the dominant mechanism for guaiacol adsorption on magnetic biochars, while the anisole adsorption was controlled by pore-filling and hydrophobic effect.
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Acknowledgements The authors sincerely acknowledge the National Basic Research (973) special preliminary study program (2014CB260408) for the financial support.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2016.10. 033.
Reference Ahmad, M., Lee, S.S., Rajapaksha, A.U., Vithanage, M., Zhang, M., Cho, J.S., Lee, S.E., Ok, Y.S., 2013. Trichloroethylene adsorption by pine needle biochars produced at various pyrolysis temperatures. Bioresour. Technol. 143, 615–622. Al-Wabel, M.I., Al-Omran, A., El-Naggar, A.H., Nadeem, M., Usman, A.R.A., 2013. Pyrolysis temperature induced changes in characteristics and chemical composition of biochar produced from conocarpus wastes. Bioresour. Technol. 131, 374–379. Angın, D., 2013. Effect of pyrolysis temperature and heating rate on biochar obtained from pyrolysis of safflower seed press cake. Bioresour. Technol. 128, 593–597. Capunitan, J.A., Capareda, S.C., 2013. Characterization and separation of corn stover bio-oil by fractional distillation. Fuel 112, 60–73. Chen, Y., Yang, H., Wang, X., Zhang, S., Chen, H., 2012. Biomass-based pyrolytic polygeneration system on cotton stalk pyrolysis: Influence of temperature. Bioresour. Technol. 107, 411–418. Demirbas, A., 2004. Effects of temperature and particle size on bio-char yield from pyrolysis of agricultural residues. J. Anal. Appl. Pyrol. 72, 243–248. Elkasabi, Y., Boateng, A.A., Jackson, M.A., 2015. Upgrading of bio-oil distillation bottoms into biorenewable calcined coke. Biomass Bioenergy 81, 415–423. Fu, P., Yi, W., Bai, X., Li, Z., Hu, S., Xiang, J., 2011. Effect of temperature on gas composition and char structural features of pyrolyzed agricultural residues. Bioresour. Technol. 102, 8211–8219. Hossain, M.K., Strezov, V., Chan, K.Y., Ziolkowski, A., Nelson, P.F., 2011. Influence of pyrolysis temperature on production and nutrient properties of wastewater sludge biochar. J. Environ. Manage. 92, 223–228. Kim, K.H., Kim, J.Y., Cho, T.S., Choi, J.W., 2012. Influence of pyrolysis temperature on physicochemical properties of biochar obtained from the fast pyrolysis of pitch pine (Pinus rigida). Bioresour. Technol. 118, 158–162. Kundu, A., Gupta, B.S., Hashim, M.A., Sahu, J.N., Mujawar, M., Redzwan, G., 2015. Optimisation of the process variables in production of activated carbon by microwave heating. RSC Adv. 5, 35899–35908. Lee, Y., Park, J., Ryu, C., Gang, K.S., Yang, W., Park, Y.K., Jung, J., Hyun, S., 2013. Comparison of biochar properties from biomass residues produced by slow pyrolysis at 500 °C. Bioresour. Technol. 148, 196–201. Li, Y., Zhu, C., Lu, T., Guo, Z., Zhang, D., Ma, J., Zhu, S., 2013. Simple fabrication of a Fe2O3/carbon composite for use in a high-performance lithium ion battery. Carbon 52, 565–573. Li, H., Xia, S., Li, Y., Ma, P., Zhao, C., 2015a. Stability evaluation of fast pyrolysis oil from rice straw. Chem. Eng. Sci. 135, 258–265. Li, H., Xia, S., Wu, M., Ma, P., 2015b. Isobaric (vapour + liquid) equilibria of binary systems containing butyl acetate for the separation of methoxy aromatic compounds (anisole and guaiacol) from biomass fast pyrolysis oil. J. Chem. Thermodyn. 87, 141–146. Li, G., Zhu, W., Zhang, C., Zhang, S., Liu, L., Zhu, L., Zhao, W., 2016a. Effect of a magnetic field on the adsorptive removal of methylene blue onto wheat straw biochar. Bioresour. Technol. 206, 16–22. Li, H., Xia, S., Ma, P., 2016b. Thermogravimetric investigation of the co-combustion between the pyrolysis oil distillation residue and lignite. Bioresour. Technol. 218, 615–622. Li, H., Xia, S., Ma, P., 2016c. Upgrading fast pyrolysis oil: Solvent–anti-solvent extraction and blending with diesel. Energy Convers. Manage. 110, 378–385. Lu, Q., Yang, X.L., Zhu, X.F., 2008. Analysis on chemical and physical properties of bio-oil pyrolyzed from rice husk. J. Anal. Appl. Pyrol. 82, 191–198. Mubarak, N.M., Kundu, A., Sahu, J.N., Abdullah, E.C., Jayakumar, N.S., 2014. Synthesis of palm oil empty fruit bunch magnetic pyrolytic char impregnating with FeCl3 by microwave heating technique. Biomass Bioenergy 61, 265–275. Mubarak, N.M., Sahu, J.N., Abdullah, E.C., Jayakumar, N.S., 2016. Plam oil empty fruit bunch based magnetic biochar composite comparison for synthesis by microwave-assisted and conventional heating. J. Anal. Appl. Pyrol. 120, 521– 528. Noraini, M.N., Abdullah, E.C., Othman, R., Mubarak, N.M., 2016. Single-route synthesis of magnetic biochar from sugarcane bagasse by microwave-assisted pyrolysis. Mater. Lett. 184, 315–319. Tan, X., Liu, Y., Zeng, G., Wang, X., Hu, X., Gu, Y., Yang, Z., 2015. Application of biochar for the removal of pollutants from aqueous solutions. Chemosphere 125, 70–85.
26
H. Li et al. / Bioresource Technology 223 (2017) 20–26
Thines, K.R., Abdullah, E.C., Ruthiraan, M., Mubarak, N.M., Tripathi, M., 2016. A new route of magnetic biochar based polyaniline composites for supercapacitor electrode materials. J. Anal. Appl. Pyrol. 121, 240–257. Thines, K.R., Abdullah, E.C., Mubarak, N.M., Ruthiraan, M., 2017. Synthesis of magnetic biochar from agricultural waste biomass to enhancing route for waste water and polymer application: a review. Renewable Sustainable Energy Rev. 67, 257–276. Wang, S., Gao, B., Zimmerman, A.R., Li, Y., Ma, L., Harris, W.G., Migliaccio, K.W., 2015. Removal of arsenic by magnetic biochar prepared from pinewood and natural hematite. Bioresour. Technol. 175, 391–395. Wei, D., Ngo, H.H., Guo, W., Xu, W., Zhang, Y., Du, B., Wei, Q., 2016. Biosorption of effluent organic matter onto magnetic biochar composite: behavior of fluorescent components and their binding properties. Bioresour. Technol. 214, 259–265.
Xu, Y., Chen, B., 2013. Investigation of thermodynamic parameters in the pyrolysis conversion of biomass and manure to biochars using thermogravimetric analysis. Bioresour. Technol. 146, 485–493. Yu, J., Zhao, Y., Li, Y., 2014. Utilization of corn cob biochar in a direct carbon fuel cell. J. Power. Sources 270, 312–317. Zhang, M., Gao, B., Varnoosfaderani, S., Hebard, A., Yao, Y., Inyang, M., 2013a. Preparation and characterization of a novel magnetic biochar for arsenic removal. Bioresour. Technol. 130, 457–462. Zhang, X.S., Yang, G.X., Jiang, H., Liu, W.J., Ding, H.S., 2013b. Mass production of chemicals from biomass-derived oil by directly atmospheric distillation coupled with co-pyrolysis. Sci. Rep. 3, 1120. Zheng, J.L., Wei, Q., 2011. Improving the quality of fast pyrolysis bio-oil by reduced pressure distillation. Biomass Bioenergy 35, 1804–1810.