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Characterization and adsorption performance of biochars derived from three key biomass constituents
Fuel 269 (2020) 117142
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Full Length Article
Characterization and adsorption performance of biochars derived from three key biomass constituents
T
Jiang Wana, Lin Liua,b, , Khurram Shahzad Ayuba,c, Wei Zhanga,b, , Genxiang Shend, Shuangqing Hud, Xiaoyong Qiand ⁎
⁎
a
State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, School of Resource and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China b Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China c Department of Chemical Engineering, University of Gujrat, Gujrat, Pakistan d Shanghai Academy of Environmental Sciences, Shanghai 200233, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Biochar Biomass constituents Characterization Sulfadimidine Sorption
The effects of biomass constituents on biochar adsorption performance are still unclear. In this work, three main biomass constituents, lignin, cellulose and xylan (hemicellulose), were converted into biochar. These biochars showed obvious differences, which were observed by physicochemical characteristics analysis. Based on the acid-base titration, the contents of functional groups on different biochars decreased as lignin > hemicellulose > cellulose. The findings from elemental analyses showed that the degree of lignin pyrolysis was lower than cellulose and hemicellulose; however, lignin biochar had the highest yield and ash content. The scanning electron micrographs demonstrated that both lignin and xylan biochars were similarly spherical, while cellulose biochar was fibrous. Furthermore, the sulfadimidine (SMT) adsorption experiments revealed that physicochemical characteristics of biochars obviously affected the adsorption behavior. Cellulose and xylan biochars indicated similar pH-dependent sorption patterns, but the latter exhibited higher adsorption capacity. In contrast, lignin biochar had no adsorption capacity due to the high ash content. The adsorption was
⁎ Corresponding authors at: State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, School of Resource and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China. E-mail addresses: [email protected] (W. Zhang), [email protected] (L. Liu).
https://doi.org/10.1016/j.fuel.2020.117142 Received 10 October 2019; Received in revised form 15 January 2020; Accepted 18 January 2020 0016-2361/ © 2020 Published by Elsevier Ltd.
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dominated by the electrostatic repulsion, EDA interactions, hydrogen bonds and negative charge-assisted hydrogen-bond. Overall, these observations are of significant importance for guiding in the selection of feedstock used for biochar preparation and predicting the sorption behavior of SMT on biochars.
1. Introduction
chemical properties of biochars produced by the three key biomass constituents, especially for the hemicellulose. Most scholars have divided the raw materials into herbs and wood, and even then, there has been little comparison of the feedstock themselves. Few reports have linked the biomass components to the resulting biochars [28,29]. Hence, the deeper physicochemical characteristics and sorption performance of biochars produced from different biomass constituents should be concerned. In the present study, the biochars from biomass constituents were first prepared. Then their physicochemical characteristics were analyzed by using various techniques, such as acid-base titration method [30] was used to observe functional groups on biochar surface. For further comparison, the biochars were used to adsorb sulfadimidine in a series of solutions. Finally, the characteristics of biochars were determined by observing the different adsorption effects. Additionally, the sorption mechanisms of SMT on biochars were probed in detail.
Biochar is the carbon-rich solid produced by pyrolysis of biomass under limited oxygen in a controlled environment [1]. In the past few decades, biochar has been extensively studied in various fields such as agriculture [2], carbon sequestration [3], biochemical engineering [4] and environment [5]. Biochar has the ability to enhance soil fertility by stimulating soil microbial communities and increasing the durability of fertilizers [6], capture carbon to reduce emissions of greenhouse gases (CO2, CH4, and N2O) into the atmosphere [7] and process into biomass energy [8] or catalyst support [9] in chemical production. In the field of environment, many studies have shown that biochar has a good performance in immobilizing organic and inorganic pollutants [10,11]. Many of these pollutants are not biodegradable and can easily accumulate in organisms through the food chain. Besides, some pollutants may turn into other forms in the environment and cause more complicated effects [12]. Biochar can separate pollutants from the water or solid phase and immobilize them. Biochar comes from a wide range of biomass such as algae, manure, agriculture residues, energy crops, activated sludge and forest residues [13,14]. Especially, wide varieties of plant residues have been used to produce biochar. Plant residues have a porous and loose structure. Moreover, their surface contains abundant carboxyl, hydroxyl and other reactive groups. Hence, a variety of plant residues, such as walnut shell [15], banana peel [16], olive powder [17], bagasse [18], have been used in the production of biochar and which acts as adsorbent. In general, plant residues consist of inorganic and organic phases. Inorganic phase consists of silicate, sulfate, hydroxide or other inorganic salts. The organic phase is of great importance for the plant composition. It is mainly consisted of lignin, cellulose and hemicellulose. Lignin is a natural aromatic biopolymer with phenolic character, which is helical and contains ether and carbon–carbon linkages. Lignin is the main adhesive for the fibrous components in plants [19], which accounts for 16–33% of the mass of wood [20,21]. Cellulose is a glucose (a six sugar linked by β (1–4) glycoside bonds) polymer. Cellulose has fibrous structure with smooth surface and provides strength to the wood, which accounts for 40–50% of the mass of dry wood [20]. Hemicellulose is a highly branched macromolecular substance of different sugars (e.g. xylan, pentosan or polyose) [21]. Hemicellulose has a random and amorphous structure, which accounts for 25–35% of the mass of dry wood [20]. Many efforts have been made to explore the pyrolysis process of lignin, cellulose and hemicellulose. The pyrolysis visualization study [22], morphological and surface area evaluation [23] were carried out on different compositions of biomass. Moreover, several experiments have been carried out to find the sorption performance of biochar produced from these constituents. Lignin and cellulose were pyrolyzed at different temperatures in order to produce biochar, and a variety of feedstock (e.g., Wood chips and rice straw and chitin) were used for comparison [23,24]. The lignin biochar is considered having good performance in aromatic pollutants removal. Moreover, the sorption efficiency of various biochars are also very different, which are related to the surface characteristics of biochars. Sulfonamide antibiotics as an environmental risk were found to be pH-dependent in sorption [25]. The electrostatic repulsion, EDA interactions, hydrogen bonds and negative charge-assisted hydrogen-bond [(-)CAHB] mechanism have been extensively reported [26]. The sorption mechanism and capacity of different biochars are always slightly different in different experiments, which may be due to the difference in feedstock composition of biochars [27]. However, few studies have focused on the physical and
2. Materials and methods 2.1. Materials Cellulose (CAS: 9004-34-6) and lignin (Dealkaline, CAS: 9005-53-2) were purchased from Macklin Inc. Cellulose is white fibrous powder whereas the lignin is brownish black powder. The hemicellulose is difficult to find because it is less available commercially. Hence, we replaced it with xylan, which has been widely used as a substitute for hemicellulose [31]. Xylan (≥85%, CAS: 9014-63-5) is yellow powder extracted from bagasse and bought from Shanghai Yuanju Biotechnology Co., Ltd. Sulfamethazine (SMT, 99%) was of analytical grade and purchased from Aladdin Chemistry Co., Ltd. The physical and chemical properties of SMT are listed in Table S1. The dissociation constants (pKa1, pKa2,) for SMT are 2.28 and 7.42 [32], respectively. All other chemicals used in this study were analytical reagent and all solutions were prepared with deionized water. 2.2. Biochars Biochars were prepared in a tubular furnace having temperatureprogramming function. An appropriate amount of materials was placed into a quartz boat, and then fed to a tube furnace. Before starting the heat treatment, air was purged with continuous nitrogen flow. The temperature of the tube furnace was increased from room temperature to 100 ℃ in 5 min. After that, the temperature was increased till 700 ℃ at a rate of 10 ℃ min−1 and maintained it for 120 min after which tube furnace was finally cooled to room temperature. In order to remove the ash, the obtained biochars were ground and placed in a conical flask with acid (1 g biochar with 25 mL acid; 1 mol L−1 HF/HCl = 1:1), and then placed in a shaker (120 rpm, 25 ℃) for 12 h. Then, biochars were washed with deionized water to remove residual acid from last step. Finally, washed biochars were dried in an oven at 105 ℃. During the washing step, the biochars were separated by using filtration process with the help of 5 μm PTFE (polytetrafluoroethylene) filter membrane. Finally, the oven-dried biochars were ground and passed through a 0.28 mm sieve. Biochars prepared from lignin, cellulose and xylan at 700 ℃ were accordingly called as L700, C700 and X700. 2.3. Characterizations of biochars The physicochemical characteristics of biochars were examined 2
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using Fourier transform infrared spectroscopy and Scanning electron microscope (SEM). The surface morphology of biochars was observed by Scanning electron microscope (15 kV, Hitachi S-3400 N). The yield of products was calculated based on the dried weight of raw materials. Ash contents in biochars were measured by heating biochars in the air at 800 ℃ for 4 h. Elemental (C, H, and N) analyses were conducted using a vario EL cube (Elementar), and elemental oxygen (O) content was calculated by mass balance. These compositions were on an ash-free basis. The chemical characteristics of biochars’ surface were determined by acid-base titration [30]. Briefly, the appropriate amounts of biochars were added to the NaOH solution and mixed well until the pH was stable. Then the solution was titrated with hydrochloric acid and the change in pH was recorded to obtain the titration curve. Then the proton absorption of biochar was obtained through proton mass-balance. Finally, the proton absorption curve was obtained. Which was used to determine the pKa value and content of the surface functional groups of biochar. More details are provided in Supplementary material.
composed of polysaccharides, which are relatively high in oxygen and easy to be pyrolyzed [35,36]. These results were similar to the TG study [37]. Ash contents were very low in C700 (0.25%) and X700 (3.95%), which meant that they had a lower proportion of the inorganic mineral. While, the higher ash content of L700 (12.86%) was due to the impurity introduced in the production process (Text S3) [24]. The degree of carbonization can be described by the molar H/C ratio, because H is primarily associated with plant organic matter. The observed H/C ratio were very low (≪1%), indicating that all biochars were greatly carbonized and exhibited highly aromatic structure. Moreover, different H/C ratios indicate that the degree of pyrolysis of various materials was different [31]. The degree of cellulose pyrolysis was the highest, followed by xylan and lignin pyrolysis was the least. The polar groups on the char surfaces act as water adsorption centers and facilitate the formation of water clusters on the carbon surfaces. The molar O/C ratios can be used to measure the polarity of biochar (Chen et al., 2008). Relatively lower O/C values of C700 (0.066) than L700 (0.139) and X700 (0.107) indicated that the C700 surface may have less polar functional groups. Thus, the C700 showed higher hydrophobicity compared with the others. In conclusion, using raw materials with high lignin content may increase the yield, because the lignin has better thermal resistance. In addition, the lignin biochar with a high ash content could be better used as soil amendments to improve fertility [38]. Biochar with a high carbon content can be obtained by selecting materials rich in cellulose, which may function in carbon fixation [39].
2.4. Sorption experiments The SMT stock solution (1 g L−1) was prepared by dissolving 0.1 g SMT powder in 100 mL methanol and stored at −22 °C. The sorption isotherms were determined by batch equilibrium of biochar samples (100 mg) in SMT aqueous solutions (100 mL, C0 = 500 μg) of various pH values, which ranged from 2.5 to 10.5. The pH of batch solutions were adjusted with HCl or NaOH solutions. Moreover, 0.01 M NaCl was used to maintain a stable ionic strength [33,34]. The sorption experiment was carried out in dark in a thermostatic water bath shaker (200 rpm) at 25 ± 0.1 ℃ for 4 d so that the adsorption of biochars to SMT reached the equilibrium end. Finally, 1 mL of supernatant was taken and filtered with 0.22 μm water phase needle filter (ANPEL) for further analysis. The concentration of SMT was detected by using Ultra high performance liquid chromatograph (UHPLC) (Nexera UHPLC, Shimadzu) with MS/MS (LCMS-8050, Shimadzu). More details are provided in Supplementary material (Table S2). The apparent sorption distribution ratio (Kd, L kg−1) was defined as:
Kd =
Qe C0 Ce Vw = Ce Ce Ms
3.2. Surface morphology of biochars The pyrolysis of biochar is a process in which raw materials are heated into a molten state and then condensed. This process is accompanied by a series of reactions caused by thermal decomposition of raw materials, including the release and precipitation of volatile gases, the formation and rupture of vesicles, and the melting and recrystallization of salt [22]. Through scanning electron micrographs, the condensed products after pyrolysis can be observed intuitively. Different raw materials have different pyrolysis processes displaying different surface morphology (Fig. 1). L700 is like many broken ball fragments. Moreover, many circular holes of varying sizes were observed. This regular round hole might be caused by the expansion of decomposing gases when the material is molten [40]. Lignin biochar has been revealed spherical in other reports. The fragmentation of the carbon in this study may be due to grinding and pickling. In addition, as observed, there are many granular substances on the surface of L700, which may be a secondary production of volatile gases precipitation [41]. Hence, the acid demineralization process could remove most of the ash and mineral salts [42], but it didn’t work very well on these precipitation. Cellulose biochar remained short fibrous structure, with a slight tendency to agglomerate. This is because the method used in the experiment is slow pyrolysis (10 ℃ min−1). During the slow pyrolysis period, the external structure of cellulose will absorb a lot of heat and melt quickly, while the internal structure will crosslink and polymerize as a core before it becomes completely molten [23]. Additionally, it is further observed that the surface of C700 had many wrinkled lines and pores. In contrast, SEM images of X700 made from xylan looked like carbon particles in the
(1) −1
where Qe is the concentration of SMT on biochar (mg Kg ), C0 is the initial concentration of SMT without biochar (mg L−1), Ce is the equilibrium concentration of SMT after sorption (mg L−1), V is the solution volume (L), and M is the biochar mass (Kg). 3. Results and discussion 3.1. Compositions of biochars The elemental compositions and yield are illustrated in Table 1. The yields of L700, C700 and X700 were 53.19%, 20.23% and 19.64%, respectively. The highest yield of L700 might be due to the highest thermal stability of lignin, which is consisted of three kinds of benzenepropane and is highly cross-linked. However, cellulose and xylan are Table 1 Yields and elemental compositions of L700, C700 and X700. Biochar
L700 C700 X700
Yield (%)
53.19 ± 2.10 20.23 ± 0.96 19.64 ± 0.91
Ash (%)
12.86 ± 0.47 0.25 ± 0.11 3.95 ± 0.71
Elemental compositions C (%)
H (%)
O (%)
N (%)
O/C
H/C
85.89 ± 1.52 92.00 ± 1.44 88.52 ± 2.12
1.73 ± 0.35 1.48 ± 0.15 1.62 ± 0.16
11.93 ± 1.32 6.04 ± 1.38 9.45 ± 2.05
0.46 ± 0.14 0.48 ± 0.08 0.41 ± 0.08
0.139 0.066 0.107
0.020 0.016 0.018
3
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Fig. 1. SEM of L700, C700 and X700.
shape of platelets. It has been reported that there were submicron pellet structures in xylan biochar [23]. However, in our experiment, many biochar particles with a diameter of 1–20 µm appeared. This may be due to the differences in raw materials such as biomass, processing and purity. More details about SEM are provided in Supplementary material (Fig. S1). From the surface morphology analysis, it could be inferred that the cellulose may act as a skeleton during the pyrolysis process of biochar. Because of cellulose’s low pyrolysis temperature, it can form the core of biochar first, and it is not like lignin and xylan to condense into spherical particles, but long strips. Hence, if there is a demand for the shape of the biochar in the production, it may be necessary to pay more attention to the cellulose in the raw materials.
four categories: hydrogen bonded carboxyl, carboxyl, lactones and phenolic eOH. Table 2 lists the pKa of main functional groups (n = 4) on biochar surfaces obtained by fitting. The pKa values of the first functional groups 3.01 (L700), 3.60 (C700) and 3.57 (X700), were close to the pKa value of 2-hydroxybenzoic acid (2.91) [30]. They had a lower pKa value than normal carboxylic acids because of the formation of hydrogen bond. Therefore, the first group is called hydrogen-bonded carboxyl. Moreover, the pKa values of the second functional groups were 5.73 (L700), 5.28 (C700) and 5.93 (X700), which are close to the pKa values of propanoic acid (4.88) and adipic acid (4.42, 5.41) [30]. The pKa values of the third functional groups were 7.01 (L700), 7.34 (C700) and 7.37 (X700), which are lactones. Although lactone groups are not normally acidic or basic groups, they can be cleaved under alkaline conditions. The pKa values of the fourth functional groups were 10.09 (L700), 9.85 (C700) and 9.65 (X700), which are close to the pKa value of phenol (9.86). These data suggested that all biochars are indistinguishable in the types of functional groups. By contrast, some differences can be observed from the number of functional groups. The total number of functional groups on C700 (0.188 mmol g−1) was significantly lower than that of the other two groups (0.350 and 0.427 mmol g−1). This was consistent with the characterization results of elemental analysis, which is that C700 has the lowest O/C value. Z. Chen et al. measured the number of functional groups in the actual biomass by acid-base titration, which was higher than the number in this study [30].
3.3. Chemical surface characterization Acid-base titration was implemented, and the proton-consumed curves of different biochars are displayed in Fig. 2. There are different best fitting methods in the process of acid-base titration, that is, the number of functional groups (the value of n) and the types. The best fitting method of acid-base titration of a class F coal fly was to set as n = 3 [43]. However, the best value for activated sludge was n = 1 [44], which meant only carboxyl groups (eCOOH) on the sludge. For biochar, the best value was to set n = 4 [30]. According to their results, functional groups in this experiment are classified into the following
Fig. 2. Proton-consumed curves of L700, C700 and X700.
4
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Table 2 The pKa values and content of surface oxygen-containing groups of L700, C700 and X700. Biochars
L700 C700 X700
Hydrogen bonded carboxyl
Carboxyl
Lactones
Phenolic eOH
pKa(1)
Content (mmol/g)
pKa(2)
Content (mmol/g)
pKa(3)
Content (mmol/g)
pKa(4)
Content (mmol/g)
3.01 ± 0.40 3.60 ± 0.78 3.57 ± 0.35
0.197 ± 0.061 0.046 ± 0.048 0.140 ± 0.035
5.73 ± 0.14 5.28 ± 0.78 5.93 ± 0.34
0.042 ± 0.013 0.057 ± 0.019 0.084 ± 0.019
7.01 ± 0.26 7.34 ± 0.81 7.37 ± 0.80
0.067 ± 0.011 0.045 ± 0.024 0.084 ± 0.028
10.09 ± 0.20 9.85 ± 0.46 9.65 ± 0.35
0.045 ± 0.019 0.040 ± 0.024 0.118 ± 0.07
Total content (mmol/g) 0.350 ± 0.018 0.188 ± 0.115 0.427 ± 0.011
hydration and tautomerization. The SMT0 can convert to a zwitterion state (SMT ± ), and the adsorption can stabilize the zwitterion formation [25]. The pH range between 4 and 5, the maximum Kd values for SMT on X700 were determined. The SMT primarily existed as a neutral state (SMT0), and SMT0 tended to be converted to a zwitterion state (SMT ± ), i.e. the amino groups were protonated (NH3+) and the sulfamido were deprotonated (eSO2N−e) on SMT ± . These groups could formed strong hydrogen bonds with the carboxyl group on the biochars, and the EDA interactions might assist in this process [26]. Hence, the hydrogen bonding interaction may dominate the sorption. Simultaneously, the impacts of electrostatic repulsion were reduced in this pH region, thus, the maximum Kd values were observed. However, compared with X700, the optimum adsorption for SMT on C700 occurred at pH ~ 6.5, which might be due to the differences in functional group contents and pKa (especially carboxyl groups) [34]. As the pH of solution increased further, the adsorption decreased accordingly. Because the SMT was converted to SMT0 or SMT− (pKa2, SMT = 7.42), the hydrogen bonding interaction was decreased and the electrostatic repulsion was increased. However, another small increase of Kd values for SMT on all biochars were visible at pH ~ 8. In this pH region, the deprotonation of the amino group and sulfamido on SMT were enhanced, i.e. SMT ± or SMT0 were mostly converted to an anion state (SMT−). Moreover, proposed SMT− could get proton by proton exchange with water molecule [25], then a strong negative charge-assisted H-bond, (-)CAHB, formed between natural molecules of SMT and carboxylate or phenolate on the biochars surface, resulting in adsorption a rise in Kd value. Finally, at the higher pH region, the electrostatic repulsion was increased while the sorption was decreased. According to the above experiments, if the biochar is used for adsorption, it is necessary to consider the content of lignin in the raw materials and the removal of ash. In previous studies, the researchers mainly focused on the lignin and cellulose contents of the raw
The biochar from the lignin with a high thermal resistance has more surface functional groups, because the number of functional groups will decline with the increase of pyrolysis degree [30]. Hence, a high surface functional groups content of lignin may be desirable for the biochar destined to be used as a catalyst, because it may provide more active sites for catalysis. 3.4. Adsorption performance and mechanism of biochars to SMT The adsorption performance of all biochars to SMT under different pH conditions were explored, and the sorption coefficients (Kd) of SMT on these biochars have been indicated in Fig. 3. Simultaneously, the degradation of antibiotics under different conditions has also been taken into account. L700 barely have adsorption effect, mainly because the content of ash produced by lignin is too high (Table 1). In addition, as observed from the SEM, L700′s surface had many large hole caused by pyrolysis gases (Fig. 1), while small holes that were conducive to adsorption had not been found. Furthermore, L700 had many particles formed by gas precipitation on its surface (Fig. 1), and they occupied many adsorption sites. By contrast, the adsorption capacity of C700 and X700 were different, but the trend of adsorption was the same as that of other report [45]. At pH < 3, all biochars had lower Kd values. The maximum Kd values were observed at pH ~ 3–7 for SMT on X700 (1497.7 L kg−1) and C700 (926.1 L kg−1), and then the adsorption was decreased. Subsequently, another small sharp increase of Kd values were observed at pH ~ 8–9. Finally, lower adsorptions were found at pH > 9. M.B. Ahmed et al. [34] and M. Xie et al. [27] have reported similar adsorption patterns. The adsorption performance changed clearly with the pH of the reaction solution. As many scholars have reported, sulfonamides sorption on carbonaceous materials (e.g., biochar, graphene) revealed pronounced pH dependence [25,26,34]. It is also closely related to the properties of adsorbate and adsorbent, such as the surface morphology and functional groups of adsorbent and the pKa of sulfonamides [25]. The proposed sorption mechanism for SMT on biochars are displayed in Fig. 4. According to acid-base titration, when the pH < 3, most carboxyl groups on the biochars surface were protonated (pKa1, BC < 3), so biochars surface was positively charged. Simultaneously, a significant portion of SMT in the solution was in a positive state (SMT+) (Fig. S4). Hence, it was expected that the electrostatic repulsion may cause the lower sorption. Even so, still there was some sorption which may be dominated by the π+-π electron donor–acceptor interaction (π+-π EDA) [25], which caused by protonated aniline ring on SMT (acceptor) and the biochar surface (donor). SMT+ was the acceptor because of the strong electronegativity of the -SO2NH- (contribution to electronic resonance) and eNH3+ (donate loan pair electrons to the benzene ring) groups [34]. In addition, usually the biochars surface were enriched with C]C, eOH functional groups. Therefore, the biochar surface was π-electron-donor. Then, as the pH goes up, the hydrogen-bonded carboxyl on biochars surface gradually deprotonated into neutral state, so the effect of electrostatic repulsion declines. Simultaneously, the role of hydrogen bonding interaction began to appear. In acidic region, hydrogen bonding interaction primarily caused by the amino group (protonated) on SMT and the carboxyl group (deprotonated by hydration). The amino group can be protonated by
Fig. 3. Effect of pH on the distribution coefficient (Kd) for SMT on L700, C700 and X700. Vertical dash lines represent the two pKa values of SMT, and vertical dot lines represent the two pKa values (hydrogen bonded carboxyl and phenolic eOH) of oxygen-containing groups of X700. Natural degradation of the SMT at different pH values has also been considered. 5
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Fig. 4. Proposed sorption mechanism for SMT on biochars with the pH change. Vertical dot lines represent the two pKa values of SMT, and vertical dash lines represent the two pKa values (hydrogen bonded carboxyl and phenolic eOH) of oxygencontaining groups of biochar. Blue arrows represent the direction and intensity of the π electron flow. The blue band at the bottom represent changes in electrostatic repulsion. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Acknowledgements
materials, or divided the materials into herbs and wood. However, the biochar with the best adsorption performance in this experiment was made from xylan, a substitute for hemicellulose. Hence, the hemicellulose in raw materials should not be ignored.
This research was supported by projects of the National Water Pollution Control and Treatment Science and Technology Major Project (2017ZX07207002); the National Natural Science Foundation of China (41877124, 21737005).
4. Conclusions
Appendix A. Supplementary data
Biochar performance is highly related to the feedstock of pyrolysis. In this work, three main biomass constituents, lignin, cellulose and xylan (hemicellulose), were converted into biochars. These biochars showed obvious differences, which were observed by physicochemical characteristics analyses. The biochar yield of lignin was the highest due to its excellent thermal resistance. Lignin biochar may have a good performance for soil improvement (catalysis) because of its high ash (surface functional group) content. According to the surface morphology analysis, the cellulose with poor thermal resistance may act as the skeleton of biochar during the pyrolysis process. Because of a high carbon ratio of cellulose biochar, its surface functional groups content was the least, which may function in carbon fixation. The adsorption of sulfamethazine by different pH conditions showed that the biochar from hemicellulose (xylan), which was usually neglected, had the best adsorption effect. Finally, the adsorption mechanism was described in detail in combination with biochar characterization results. Overall, the work sheds some light on the relative importance of different biochar feedstock on the final biochar product, which is of significant importance for guiding in the selection of feedstock used for biochar preparation and an important step towards the “designed” biochar.
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CRediT authorship contribution statement Jiang Wan: Conceptualization, Methodology, Formal analysis, Writing - original draft, Investigation, Writing - review & editing. Lin Liu: Validation, Data curation, Supervision, Investigation. : . Khurram Shahzad Ayub: Writing - review & editing, Writing - original draft. Wei Zhang: Validation, Resources, Visualization, Funding acquisition, Data curation. Genxiang Shen: Project administration, Funding acquisition. Shuangqing Hu: Project administration, Funding acquisition. Xiaoyong Qian: Writing - review & editing. Declaration of Competing Interest 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. 6
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Full Length Article
Characterization and adsorption performance of biochars derived from three key biomass constituents
T
Jiang Wana, Lin Liua,b, , Khurram Shahzad Ayuba,c, Wei Zhanga,b, , Genxiang Shend, Shuangqing Hud, Xiaoyong Qiand ⁎
⁎
a
State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, School of Resource and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China b Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China c Department of Chemical Engineering, University of Gujrat, Gujrat, Pakistan d Shanghai Academy of Environmental Sciences, Shanghai 200233, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Biochar Biomass constituents Characterization Sulfadimidine Sorption
The effects of biomass constituents on biochar adsorption performance are still unclear. In this work, three main biomass constituents, lignin, cellulose and xylan (hemicellulose), were converted into biochar. These biochars showed obvious differences, which were observed by physicochemical characteristics analysis. Based on the acid-base titration, the contents of functional groups on different biochars decreased as lignin > hemicellulose > cellulose. The findings from elemental analyses showed that the degree of lignin pyrolysis was lower than cellulose and hemicellulose; however, lignin biochar had the highest yield and ash content. The scanning electron micrographs demonstrated that both lignin and xylan biochars were similarly spherical, while cellulose biochar was fibrous. Furthermore, the sulfadimidine (SMT) adsorption experiments revealed that physicochemical characteristics of biochars obviously affected the adsorption behavior. Cellulose and xylan biochars indicated similar pH-dependent sorption patterns, but the latter exhibited higher adsorption capacity. In contrast, lignin biochar had no adsorption capacity due to the high ash content. The adsorption was
⁎ Corresponding authors at: State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, School of Resource and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China. E-mail addresses: [email protected] (W. Zhang), [email protected] (L. Liu).
https://doi.org/10.1016/j.fuel.2020.117142 Received 10 October 2019; Received in revised form 15 January 2020; Accepted 18 January 2020 0016-2361/ © 2020 Published by Elsevier Ltd.
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dominated by the electrostatic repulsion, EDA interactions, hydrogen bonds and negative charge-assisted hydrogen-bond. Overall, these observations are of significant importance for guiding in the selection of feedstock used for biochar preparation and predicting the sorption behavior of SMT on biochars.
1. Introduction
chemical properties of biochars produced by the three key biomass constituents, especially for the hemicellulose. Most scholars have divided the raw materials into herbs and wood, and even then, there has been little comparison of the feedstock themselves. Few reports have linked the biomass components to the resulting biochars [28,29]. Hence, the deeper physicochemical characteristics and sorption performance of biochars produced from different biomass constituents should be concerned. In the present study, the biochars from biomass constituents were first prepared. Then their physicochemical characteristics were analyzed by using various techniques, such as acid-base titration method [30] was used to observe functional groups on biochar surface. For further comparison, the biochars were used to adsorb sulfadimidine in a series of solutions. Finally, the characteristics of biochars were determined by observing the different adsorption effects. Additionally, the sorption mechanisms of SMT on biochars were probed in detail.
Biochar is the carbon-rich solid produced by pyrolysis of biomass under limited oxygen in a controlled environment [1]. In the past few decades, biochar has been extensively studied in various fields such as agriculture [2], carbon sequestration [3], biochemical engineering [4] and environment [5]. Biochar has the ability to enhance soil fertility by stimulating soil microbial communities and increasing the durability of fertilizers [6], capture carbon to reduce emissions of greenhouse gases (CO2, CH4, and N2O) into the atmosphere [7] and process into biomass energy [8] or catalyst support [9] in chemical production. In the field of environment, many studies have shown that biochar has a good performance in immobilizing organic and inorganic pollutants [10,11]. Many of these pollutants are not biodegradable and can easily accumulate in organisms through the food chain. Besides, some pollutants may turn into other forms in the environment and cause more complicated effects [12]. Biochar can separate pollutants from the water or solid phase and immobilize them. Biochar comes from a wide range of biomass such as algae, manure, agriculture residues, energy crops, activated sludge and forest residues [13,14]. Especially, wide varieties of plant residues have been used to produce biochar. Plant residues have a porous and loose structure. Moreover, their surface contains abundant carboxyl, hydroxyl and other reactive groups. Hence, a variety of plant residues, such as walnut shell [15], banana peel [16], olive powder [17], bagasse [18], have been used in the production of biochar and which acts as adsorbent. In general, plant residues consist of inorganic and organic phases. Inorganic phase consists of silicate, sulfate, hydroxide or other inorganic salts. The organic phase is of great importance for the plant composition. It is mainly consisted of lignin, cellulose and hemicellulose. Lignin is a natural aromatic biopolymer with phenolic character, which is helical and contains ether and carbon–carbon linkages. Lignin is the main adhesive for the fibrous components in plants [19], which accounts for 16–33% of the mass of wood [20,21]. Cellulose is a glucose (a six sugar linked by β (1–4) glycoside bonds) polymer. Cellulose has fibrous structure with smooth surface and provides strength to the wood, which accounts for 40–50% of the mass of dry wood [20]. Hemicellulose is a highly branched macromolecular substance of different sugars (e.g. xylan, pentosan or polyose) [21]. Hemicellulose has a random and amorphous structure, which accounts for 25–35% of the mass of dry wood [20]. Many efforts have been made to explore the pyrolysis process of lignin, cellulose and hemicellulose. The pyrolysis visualization study [22], morphological and surface area evaluation [23] were carried out on different compositions of biomass. Moreover, several experiments have been carried out to find the sorption performance of biochar produced from these constituents. Lignin and cellulose were pyrolyzed at different temperatures in order to produce biochar, and a variety of feedstock (e.g., Wood chips and rice straw and chitin) were used for comparison [23,24]. The lignin biochar is considered having good performance in aromatic pollutants removal. Moreover, the sorption efficiency of various biochars are also very different, which are related to the surface characteristics of biochars. Sulfonamide antibiotics as an environmental risk were found to be pH-dependent in sorption [25]. The electrostatic repulsion, EDA interactions, hydrogen bonds and negative charge-assisted hydrogen-bond [(-)CAHB] mechanism have been extensively reported [26]. The sorption mechanism and capacity of different biochars are always slightly different in different experiments, which may be due to the difference in feedstock composition of biochars [27]. However, few studies have focused on the physical and
2. Materials and methods 2.1. Materials Cellulose (CAS: 9004-34-6) and lignin (Dealkaline, CAS: 9005-53-2) were purchased from Macklin Inc. Cellulose is white fibrous powder whereas the lignin is brownish black powder. The hemicellulose is difficult to find because it is less available commercially. Hence, we replaced it with xylan, which has been widely used as a substitute for hemicellulose [31]. Xylan (≥85%, CAS: 9014-63-5) is yellow powder extracted from bagasse and bought from Shanghai Yuanju Biotechnology Co., Ltd. Sulfamethazine (SMT, 99%) was of analytical grade and purchased from Aladdin Chemistry Co., Ltd. The physical and chemical properties of SMT are listed in Table S1. The dissociation constants (pKa1, pKa2,) for SMT are 2.28 and 7.42 [32], respectively. All other chemicals used in this study were analytical reagent and all solutions were prepared with deionized water. 2.2. Biochars Biochars were prepared in a tubular furnace having temperatureprogramming function. An appropriate amount of materials was placed into a quartz boat, and then fed to a tube furnace. Before starting the heat treatment, air was purged with continuous nitrogen flow. The temperature of the tube furnace was increased from room temperature to 100 ℃ in 5 min. After that, the temperature was increased till 700 ℃ at a rate of 10 ℃ min−1 and maintained it for 120 min after which tube furnace was finally cooled to room temperature. In order to remove the ash, the obtained biochars were ground and placed in a conical flask with acid (1 g biochar with 25 mL acid; 1 mol L−1 HF/HCl = 1:1), and then placed in a shaker (120 rpm, 25 ℃) for 12 h. Then, biochars were washed with deionized water to remove residual acid from last step. Finally, washed biochars were dried in an oven at 105 ℃. During the washing step, the biochars were separated by using filtration process with the help of 5 μm PTFE (polytetrafluoroethylene) filter membrane. Finally, the oven-dried biochars were ground and passed through a 0.28 mm sieve. Biochars prepared from lignin, cellulose and xylan at 700 ℃ were accordingly called as L700, C700 and X700. 2.3. Characterizations of biochars The physicochemical characteristics of biochars were examined 2
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using Fourier transform infrared spectroscopy and Scanning electron microscope (SEM). The surface morphology of biochars was observed by Scanning electron microscope (15 kV, Hitachi S-3400 N). The yield of products was calculated based on the dried weight of raw materials. Ash contents in biochars were measured by heating biochars in the air at 800 ℃ for 4 h. Elemental (C, H, and N) analyses were conducted using a vario EL cube (Elementar), and elemental oxygen (O) content was calculated by mass balance. These compositions were on an ash-free basis. The chemical characteristics of biochars’ surface were determined by acid-base titration [30]. Briefly, the appropriate amounts of biochars were added to the NaOH solution and mixed well until the pH was stable. Then the solution was titrated with hydrochloric acid and the change in pH was recorded to obtain the titration curve. Then the proton absorption of biochar was obtained through proton mass-balance. Finally, the proton absorption curve was obtained. Which was used to determine the pKa value and content of the surface functional groups of biochar. More details are provided in Supplementary material.
composed of polysaccharides, which are relatively high in oxygen and easy to be pyrolyzed [35,36]. These results were similar to the TG study [37]. Ash contents were very low in C700 (0.25%) and X700 (3.95%), which meant that they had a lower proportion of the inorganic mineral. While, the higher ash content of L700 (12.86%) was due to the impurity introduced in the production process (Text S3) [24]. The degree of carbonization can be described by the molar H/C ratio, because H is primarily associated with plant organic matter. The observed H/C ratio were very low (≪1%), indicating that all biochars were greatly carbonized and exhibited highly aromatic structure. Moreover, different H/C ratios indicate that the degree of pyrolysis of various materials was different [31]. The degree of cellulose pyrolysis was the highest, followed by xylan and lignin pyrolysis was the least. The polar groups on the char surfaces act as water adsorption centers and facilitate the formation of water clusters on the carbon surfaces. The molar O/C ratios can be used to measure the polarity of biochar (Chen et al., 2008). Relatively lower O/C values of C700 (0.066) than L700 (0.139) and X700 (0.107) indicated that the C700 surface may have less polar functional groups. Thus, the C700 showed higher hydrophobicity compared with the others. In conclusion, using raw materials with high lignin content may increase the yield, because the lignin has better thermal resistance. In addition, the lignin biochar with a high ash content could be better used as soil amendments to improve fertility [38]. Biochar with a high carbon content can be obtained by selecting materials rich in cellulose, which may function in carbon fixation [39].
2.4. Sorption experiments The SMT stock solution (1 g L−1) was prepared by dissolving 0.1 g SMT powder in 100 mL methanol and stored at −22 °C. The sorption isotherms were determined by batch equilibrium of biochar samples (100 mg) in SMT aqueous solutions (100 mL, C0 = 500 μg) of various pH values, which ranged from 2.5 to 10.5. The pH of batch solutions were adjusted with HCl or NaOH solutions. Moreover, 0.01 M NaCl was used to maintain a stable ionic strength [33,34]. The sorption experiment was carried out in dark in a thermostatic water bath shaker (200 rpm) at 25 ± 0.1 ℃ for 4 d so that the adsorption of biochars to SMT reached the equilibrium end. Finally, 1 mL of supernatant was taken and filtered with 0.22 μm water phase needle filter (ANPEL) for further analysis. The concentration of SMT was detected by using Ultra high performance liquid chromatograph (UHPLC) (Nexera UHPLC, Shimadzu) with MS/MS (LCMS-8050, Shimadzu). More details are provided in Supplementary material (Table S2). The apparent sorption distribution ratio (Kd, L kg−1) was defined as:
Kd =
Qe C0 Ce Vw = Ce Ce Ms
3.2. Surface morphology of biochars The pyrolysis of biochar is a process in which raw materials are heated into a molten state and then condensed. This process is accompanied by a series of reactions caused by thermal decomposition of raw materials, including the release and precipitation of volatile gases, the formation and rupture of vesicles, and the melting and recrystallization of salt [22]. Through scanning electron micrographs, the condensed products after pyrolysis can be observed intuitively. Different raw materials have different pyrolysis processes displaying different surface morphology (Fig. 1). L700 is like many broken ball fragments. Moreover, many circular holes of varying sizes were observed. This regular round hole might be caused by the expansion of decomposing gases when the material is molten [40]. Lignin biochar has been revealed spherical in other reports. The fragmentation of the carbon in this study may be due to grinding and pickling. In addition, as observed, there are many granular substances on the surface of L700, which may be a secondary production of volatile gases precipitation [41]. Hence, the acid demineralization process could remove most of the ash and mineral salts [42], but it didn’t work very well on these precipitation. Cellulose biochar remained short fibrous structure, with a slight tendency to agglomerate. This is because the method used in the experiment is slow pyrolysis (10 ℃ min−1). During the slow pyrolysis period, the external structure of cellulose will absorb a lot of heat and melt quickly, while the internal structure will crosslink and polymerize as a core before it becomes completely molten [23]. Additionally, it is further observed that the surface of C700 had many wrinkled lines and pores. In contrast, SEM images of X700 made from xylan looked like carbon particles in the
(1) −1
where Qe is the concentration of SMT on biochar (mg Kg ), C0 is the initial concentration of SMT without biochar (mg L−1), Ce is the equilibrium concentration of SMT after sorption (mg L−1), V is the solution volume (L), and M is the biochar mass (Kg). 3. Results and discussion 3.1. Compositions of biochars The elemental compositions and yield are illustrated in Table 1. The yields of L700, C700 and X700 were 53.19%, 20.23% and 19.64%, respectively. The highest yield of L700 might be due to the highest thermal stability of lignin, which is consisted of three kinds of benzenepropane and is highly cross-linked. However, cellulose and xylan are Table 1 Yields and elemental compositions of L700, C700 and X700. Biochar
L700 C700 X700
Yield (%)
53.19 ± 2.10 20.23 ± 0.96 19.64 ± 0.91
Ash (%)
12.86 ± 0.47 0.25 ± 0.11 3.95 ± 0.71
Elemental compositions C (%)
H (%)
O (%)
N (%)
O/C
H/C
85.89 ± 1.52 92.00 ± 1.44 88.52 ± 2.12
1.73 ± 0.35 1.48 ± 0.15 1.62 ± 0.16
11.93 ± 1.32 6.04 ± 1.38 9.45 ± 2.05
0.46 ± 0.14 0.48 ± 0.08 0.41 ± 0.08
0.139 0.066 0.107
0.020 0.016 0.018
3
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Fig. 1. SEM of L700, C700 and X700.
shape of platelets. It has been reported that there were submicron pellet structures in xylan biochar [23]. However, in our experiment, many biochar particles with a diameter of 1–20 µm appeared. This may be due to the differences in raw materials such as biomass, processing and purity. More details about SEM are provided in Supplementary material (Fig. S1). From the surface morphology analysis, it could be inferred that the cellulose may act as a skeleton during the pyrolysis process of biochar. Because of cellulose’s low pyrolysis temperature, it can form the core of biochar first, and it is not like lignin and xylan to condense into spherical particles, but long strips. Hence, if there is a demand for the shape of the biochar in the production, it may be necessary to pay more attention to the cellulose in the raw materials.
four categories: hydrogen bonded carboxyl, carboxyl, lactones and phenolic eOH. Table 2 lists the pKa of main functional groups (n = 4) on biochar surfaces obtained by fitting. The pKa values of the first functional groups 3.01 (L700), 3.60 (C700) and 3.57 (X700), were close to the pKa value of 2-hydroxybenzoic acid (2.91) [30]. They had a lower pKa value than normal carboxylic acids because of the formation of hydrogen bond. Therefore, the first group is called hydrogen-bonded carboxyl. Moreover, the pKa values of the second functional groups were 5.73 (L700), 5.28 (C700) and 5.93 (X700), which are close to the pKa values of propanoic acid (4.88) and adipic acid (4.42, 5.41) [30]. The pKa values of the third functional groups were 7.01 (L700), 7.34 (C700) and 7.37 (X700), which are lactones. Although lactone groups are not normally acidic or basic groups, they can be cleaved under alkaline conditions. The pKa values of the fourth functional groups were 10.09 (L700), 9.85 (C700) and 9.65 (X700), which are close to the pKa value of phenol (9.86). These data suggested that all biochars are indistinguishable in the types of functional groups. By contrast, some differences can be observed from the number of functional groups. The total number of functional groups on C700 (0.188 mmol g−1) was significantly lower than that of the other two groups (0.350 and 0.427 mmol g−1). This was consistent with the characterization results of elemental analysis, which is that C700 has the lowest O/C value. Z. Chen et al. measured the number of functional groups in the actual biomass by acid-base titration, which was higher than the number in this study [30].
3.3. Chemical surface characterization Acid-base titration was implemented, and the proton-consumed curves of different biochars are displayed in Fig. 2. There are different best fitting methods in the process of acid-base titration, that is, the number of functional groups (the value of n) and the types. The best fitting method of acid-base titration of a class F coal fly was to set as n = 3 [43]. However, the best value for activated sludge was n = 1 [44], which meant only carboxyl groups (eCOOH) on the sludge. For biochar, the best value was to set n = 4 [30]. According to their results, functional groups in this experiment are classified into the following
Fig. 2. Proton-consumed curves of L700, C700 and X700.
4
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Table 2 The pKa values and content of surface oxygen-containing groups of L700, C700 and X700. Biochars
L700 C700 X700
Hydrogen bonded carboxyl
Carboxyl
Lactones
Phenolic eOH
pKa(1)
Content (mmol/g)
pKa(2)
Content (mmol/g)
pKa(3)
Content (mmol/g)
pKa(4)
Content (mmol/g)
3.01 ± 0.40 3.60 ± 0.78 3.57 ± 0.35
0.197 ± 0.061 0.046 ± 0.048 0.140 ± 0.035
5.73 ± 0.14 5.28 ± 0.78 5.93 ± 0.34
0.042 ± 0.013 0.057 ± 0.019 0.084 ± 0.019
7.01 ± 0.26 7.34 ± 0.81 7.37 ± 0.80
0.067 ± 0.011 0.045 ± 0.024 0.084 ± 0.028
10.09 ± 0.20 9.85 ± 0.46 9.65 ± 0.35
0.045 ± 0.019 0.040 ± 0.024 0.118 ± 0.07
Total content (mmol/g) 0.350 ± 0.018 0.188 ± 0.115 0.427 ± 0.011
hydration and tautomerization. The SMT0 can convert to a zwitterion state (SMT ± ), and the adsorption can stabilize the zwitterion formation [25]. The pH range between 4 and 5, the maximum Kd values for SMT on X700 were determined. The SMT primarily existed as a neutral state (SMT0), and SMT0 tended to be converted to a zwitterion state (SMT ± ), i.e. the amino groups were protonated (NH3+) and the sulfamido were deprotonated (eSO2N−e) on SMT ± . These groups could formed strong hydrogen bonds with the carboxyl group on the biochars, and the EDA interactions might assist in this process [26]. Hence, the hydrogen bonding interaction may dominate the sorption. Simultaneously, the impacts of electrostatic repulsion were reduced in this pH region, thus, the maximum Kd values were observed. However, compared with X700, the optimum adsorption for SMT on C700 occurred at pH ~ 6.5, which might be due to the differences in functional group contents and pKa (especially carboxyl groups) [34]. As the pH of solution increased further, the adsorption decreased accordingly. Because the SMT was converted to SMT0 or SMT− (pKa2, SMT = 7.42), the hydrogen bonding interaction was decreased and the electrostatic repulsion was increased. However, another small increase of Kd values for SMT on all biochars were visible at pH ~ 8. In this pH region, the deprotonation of the amino group and sulfamido on SMT were enhanced, i.e. SMT ± or SMT0 were mostly converted to an anion state (SMT−). Moreover, proposed SMT− could get proton by proton exchange with water molecule [25], then a strong negative charge-assisted H-bond, (-)CAHB, formed between natural molecules of SMT and carboxylate or phenolate on the biochars surface, resulting in adsorption a rise in Kd value. Finally, at the higher pH region, the electrostatic repulsion was increased while the sorption was decreased. According to the above experiments, if the biochar is used for adsorption, it is necessary to consider the content of lignin in the raw materials and the removal of ash. In previous studies, the researchers mainly focused on the lignin and cellulose contents of the raw
The biochar from the lignin with a high thermal resistance has more surface functional groups, because the number of functional groups will decline with the increase of pyrolysis degree [30]. Hence, a high surface functional groups content of lignin may be desirable for the biochar destined to be used as a catalyst, because it may provide more active sites for catalysis. 3.4. Adsorption performance and mechanism of biochars to SMT The adsorption performance of all biochars to SMT under different pH conditions were explored, and the sorption coefficients (Kd) of SMT on these biochars have been indicated in Fig. 3. Simultaneously, the degradation of antibiotics under different conditions has also been taken into account. L700 barely have adsorption effect, mainly because the content of ash produced by lignin is too high (Table 1). In addition, as observed from the SEM, L700′s surface had many large hole caused by pyrolysis gases (Fig. 1), while small holes that were conducive to adsorption had not been found. Furthermore, L700 had many particles formed by gas precipitation on its surface (Fig. 1), and they occupied many adsorption sites. By contrast, the adsorption capacity of C700 and X700 were different, but the trend of adsorption was the same as that of other report [45]. At pH < 3, all biochars had lower Kd values. The maximum Kd values were observed at pH ~ 3–7 for SMT on X700 (1497.7 L kg−1) and C700 (926.1 L kg−1), and then the adsorption was decreased. Subsequently, another small sharp increase of Kd values were observed at pH ~ 8–9. Finally, lower adsorptions were found at pH > 9. M.B. Ahmed et al. [34] and M. Xie et al. [27] have reported similar adsorption patterns. The adsorption performance changed clearly with the pH of the reaction solution. As many scholars have reported, sulfonamides sorption on carbonaceous materials (e.g., biochar, graphene) revealed pronounced pH dependence [25,26,34]. It is also closely related to the properties of adsorbate and adsorbent, such as the surface morphology and functional groups of adsorbent and the pKa of sulfonamides [25]. The proposed sorption mechanism for SMT on biochars are displayed in Fig. 4. According to acid-base titration, when the pH < 3, most carboxyl groups on the biochars surface were protonated (pKa1, BC < 3), so biochars surface was positively charged. Simultaneously, a significant portion of SMT in the solution was in a positive state (SMT+) (Fig. S4). Hence, it was expected that the electrostatic repulsion may cause the lower sorption. Even so, still there was some sorption which may be dominated by the π+-π electron donor–acceptor interaction (π+-π EDA) [25], which caused by protonated aniline ring on SMT (acceptor) and the biochar surface (donor). SMT+ was the acceptor because of the strong electronegativity of the -SO2NH- (contribution to electronic resonance) and eNH3+ (donate loan pair electrons to the benzene ring) groups [34]. In addition, usually the biochars surface were enriched with C]C, eOH functional groups. Therefore, the biochar surface was π-electron-donor. Then, as the pH goes up, the hydrogen-bonded carboxyl on biochars surface gradually deprotonated into neutral state, so the effect of electrostatic repulsion declines. Simultaneously, the role of hydrogen bonding interaction began to appear. In acidic region, hydrogen bonding interaction primarily caused by the amino group (protonated) on SMT and the carboxyl group (deprotonated by hydration). The amino group can be protonated by
Fig. 3. Effect of pH on the distribution coefficient (Kd) for SMT on L700, C700 and X700. Vertical dash lines represent the two pKa values of SMT, and vertical dot lines represent the two pKa values (hydrogen bonded carboxyl and phenolic eOH) of oxygen-containing groups of X700. Natural degradation of the SMT at different pH values has also been considered. 5
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Fig. 4. Proposed sorption mechanism for SMT on biochars with the pH change. Vertical dot lines represent the two pKa values of SMT, and vertical dash lines represent the two pKa values (hydrogen bonded carboxyl and phenolic eOH) of oxygencontaining groups of biochar. Blue arrows represent the direction and intensity of the π electron flow. The blue band at the bottom represent changes in electrostatic repulsion. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Acknowledgements
materials, or divided the materials into herbs and wood. However, the biochar with the best adsorption performance in this experiment was made from xylan, a substitute for hemicellulose. Hence, the hemicellulose in raw materials should not be ignored.
This research was supported by projects of the National Water Pollution Control and Treatment Science and Technology Major Project (2017ZX07207002); the National Natural Science Foundation of China (41877124, 21737005).
4. Conclusions
Appendix A. Supplementary data
Biochar performance is highly related to the feedstock of pyrolysis. In this work, three main biomass constituents, lignin, cellulose and xylan (hemicellulose), were converted into biochars. These biochars showed obvious differences, which were observed by physicochemical characteristics analyses. The biochar yield of lignin was the highest due to its excellent thermal resistance. Lignin biochar may have a good performance for soil improvement (catalysis) because of its high ash (surface functional group) content. According to the surface morphology analysis, the cellulose with poor thermal resistance may act as the skeleton of biochar during the pyrolysis process. Because of a high carbon ratio of cellulose biochar, its surface functional groups content was the least, which may function in carbon fixation. The adsorption of sulfamethazine by different pH conditions showed that the biochar from hemicellulose (xylan), which was usually neglected, had the best adsorption effect. Finally, the adsorption mechanism was described in detail in combination with biochar characterization results. Overall, the work sheds some light on the relative importance of different biochar feedstock on the final biochar product, which is of significant importance for guiding in the selection of feedstock used for biochar preparation and an important step towards the “designed” biochar.
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CRediT authorship contribution statement Jiang Wan: Conceptualization, Methodology, Formal analysis, Writing - original draft, Investigation, Writing - review & editing. Lin Liu: Validation, Data curation, Supervision, Investigation. : . Khurram Shahzad Ayub: Writing - review & editing, Writing - original draft. Wei Zhang: Validation, Resources, Visualization, Funding acquisition, Data curation. Genxiang Shen: Project administration, Funding acquisition. Shuangqing Hu: Project administration, Funding acquisition. Xiaoyong Qian: Writing - review & editing. Declaration of Competing Interest 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. 6
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