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Combining bulk characterization and benzene polycarboxylic acid molecular markers to describe biochar properties
Chemosphere 227 (2019) 381e388
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Combining bulk characterization and benzene polycarboxylic acid molecular markers to describe biochar properties Zhaofeng Chang a, b, Luping Tian a, c, Jun Zhang a, b, Qing Zhao d, Fangfang Li a, b, Min Wu a, b, *, Bo Pan a, b a
Faculty of Environmental Science & Engineering, Kunming University of Science & Technology, Kunming, 650500, China Yunnan Provincial Key Laboratory of Soil Carbon Sequestration and Pollution Control, Kunming, 650500, Yunnan, China Yunnan Institute of Environmental Science, Kunming, 650500, China d Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China b c
h i g h l i g h t s The B6CA/BPCAs ratios increase with the pyrolysis temperature. The polar functional groups formed by oxidation block biochar pore structures. HNO3/H2SO4 oxidation decreases BPCAs contents but increases B6CA/BPCAs. The nonlinear factors are positively related to B5CA/B6CA for all biochars. N and log Kd could be predicted by combining (O þ N)/C and BPCA parameters.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 February 2019 Received in revised form 25 March 2019 Accepted 26 March 2019 Available online 10 April 2019
The physicochemical properties of biochar determined its sorption of organic contaminations, and the environmental aging process changed the biochar properties. However, the correlation between biochar heterogeneous properties and their sorption characteristics is unclear. In this study, peanut shell biochars were produced at 200e700 C, and HNO3/H2SO4 was used to oxidize 400 C biochar for 2e10 h to simulate the enhanced aging process of biochar in the environment. Benzene polycarboxylic acid (BPCA) molecular markers, and bulk characterization were analyzed to describe biochar physicochemical properties and to further predict the sorption characteristics to bisphenol A (BPA). For pristine biochars, the mellitic acid/BPCAs (B6CA/BPCAs) increased with the raise of pyrolysis temperature and the H/C atomic ratio was positively correlated with benzenepentacarboxylic acid/B6CA (B5CA/B6CA) (P < 0.01), which indicated the increased aromatic condensation. After HNO3/H2SO4 treatment, the aromaticity (H/C ratio) decreased while the highly condensed components in biochars were enriched (increased B6CA/ BPCAs values). Multiple regression models were adopted to establish a quantitative relationship between biochar heterogeneous properties and their sorption of BPA. Both nonlinearity coefficient N values (N ¼ 0.08 þ 0.103 B5CA/B6CA þ 0.721 (O þ N)/C, R2 ¼ 0.985) and single-point sorption coefficients log Kd (log Kd ¼ 1.236 þ 0.006 BPCAs þ 1.449 (O þ N)/C, R2 ¼ 0.936) could be estimated combining molecular markers and polarity parameters for biochars. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Patryk Oleszczuk Keywords: Oxidized biochars Benzene polycarboxylic acids Physicochemical properties Sorption Organic contaminants
1. Introduction Biochar is a carbon-rich substance that is produced in the course of pyrolysis of organic feedstocks under oxygen-limited or oxygen-
* Corresponding author. Faculty of Environmental Science & Engineering, Kunming University of Science & Technology, Kunming, 650500, China. . E-mail address: [email protected] (M. Wu). https://doi.org/10.1016/j.chemosphere.2019.03.164 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
free conditions (Yang et al., 2016). Due to the wide sources of biomass and the simple preparation procedure, biochar has been widely used in production practices, such as waste recycling, carbon sequestration, soil improvement, and contamination control (Ahmad et al., 2012; Ok et al., 2015). With biochar applied to the environment, biotic and/or abiotic processes (aging) could alter the biochar properties. Nguyen et al. (2008) reported that the O/C ratios of aged biochar increased by more than 133 and 192% after 10 and
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30 years of cultivation, respectively. Recently, the changing chemical and physical properties, e.g., the particle size, surface functional groups and aromatic structures, of aging biochar have attracted extensive interest as potential control factors for binding organic contaminations and thus reducing their mobility. The environmental aging of biochar is a long-term procedure that lasts from several years to thousands of years. In laboratory simulations, chemical reagents such as H2O2, HNO3 and the mixture of HNO3/H2SO4 were usually used to oxidize biochar to simulate the environmental aging of biochar (Ghaffar et al., 2015; Jin et al., 2017). Thus, the effect of oxidation on the sorption of organic contaminations on aged biochar could be studied. For instance, HNO3 treatment increased aromaticity, decreased alkyl C and enriched O, which improved the sorption of PHE on grass straw biochar (Jin et al., 2017). Cheng et al. (2014) found that increased surface functional groups of aged biochar reduced the sorption of diuron and atrazine. The enhanced sorption capacity of phthalate esters was also found for HNO3/H2SO4 oxidized biochar (Ghaffar et al., 2015). Obviously, changes in biochar properties affect its environmental behavior, and researchers have tried to establish a link between the biochar properties and sorption properties of organic ska and Oleszczuk, 2015). contaminations (Sun et al., 2013; Zielin However, various biochar properties have been reported to correlate with their sorption characteristics, and each was provided with an explanation (Li et al., 2017; Wang et al., 2016). Han et al. (2014) suggested that the nanopore-filling mechanism dominated biochar sorption of phenanthrene because of the significant correlation between the surface area and Koc. Hydrogen bond and p-p EDA interactions may facilitate bisphenol A adsorption in lowtemperature biochar due to its amount of oxygen-containing functional groups (Chu et al., 2017; Li et al., 2017). Various adsorption mechanisms may be derived from the heterogeneous structure of biochar. Therefore, although various studies have been reported to correlate biochar properties with their sorption characteristics, no applicable equation has been proposed. Previous studies have suggested that benzene polycarboxylic acid (BPCA) molecular markers provide a useful method for describing biochar heterogeneous properties (Chang et al., 2018). The distribution of individual BPCA molecular biomarkers allowed us to analyze the size of the aromatic clusters of biochars. For the same type of biochars, BPCA parameters were significantly correlated with the nonlinear sorption behavior of organic contaminations (Chang et al., 2018). These results indicated the potential application of molecular markers in predicting contamination behavior. However, this method was applied in only one type of biochar. The applicability of molecular markers in describing the properties of different types of biochars, such as biochars of different aging stages, was not tested. In this study, biochars were prepared using peanut shells at 200e700 C. In addition, an additional set of biochars was prepared after oxidizing 400 C biochar with an HNO3/H2SO4 mixture for various durations. This oxidation also simulates the accelerated aging process of biochar in the environment. Both BPCA molecular markers and common bulk chemical analysis, including elemental compositions, functional groups, specific surface areas (SA), pore size distributions and total pore volumes (TPV), were used to describe the biochar properties. In addition, the sorption characteristics to bisphenol A (BPA) were also investigated, with attempts to correlate them to the biochar properties. 2. Materials and methods 2.1. Preparation and oxidation treatment of biochars The peanut shells were collected from a farmer's market in
Chenggong district, Kunming city. When dried in an oven at 60 C for 7 days, they were ground and crushed through a 250-mm sieve. The treated feedstocks were pyrolyzed at 200e700 C for 4 h under a N2 atmosphere in a muffle furnace. Then, the bulk biochars were ground gently and sieved through a 150-mm sieve. The prepared biochars were marked as P2, P3, P4, P5, P6, and P7, with P referring to peanut shell feedstocks and 2e7 referring to pyrolysis temperatures of 200, 300, 400, 500, 600 and 700 C. A mixture of concentrated HNO3/H2SO4 (volume ratio 1:3) (Qian and Chen, 2014) was used to oxidize 400 C biochar (close to the temperature of black carbon formed by natural fire at 450 C) under different treatment times to simulate various stages of biochar in the enhanced environment aging process. Specifically, 5 g of biochar was immersed in 400 mL of HNO3/H2SO4 mixture and maintained at 70 C for 2, 4, 6, 8, and 10 h. Then, the mixture was filtered through a 0.45-mm membrane and repeatedly rinsed with deionized water until the pH value remained unchanged. The treated biochars were dried in an oven at 60 C for 7 days and denoted by P4-2, P4-4, P4-6, P4-8 and P4-10, where 2e10 indicates the oxidation duration. 2.2. Common physicochemical properties of pristine and oxidized 400 C biochars All sample elemental analyses were conducted by various MICRO cube elemental analyzers at 850 C to measure the C, H, N, and S and at 1150 C for measuring O. The specific surface area, pore size distribution and pore volume were determined by N2 sorption at 77 K (Autosorbe1C, Quantachrome). Functional groups were also characterized by a Fourier transform infrared spectrometer (FTIR, 640eIR, Varian): biochar samples were mixed evenly with KBr (mass ratio, 1:800), then ground and pressed into a sheet. The scanning range of the spectrometer was 4000e400 cm1 with a resolution of 2 cm1. 2.3. Benzene polycarboxylic acid molecular markers analysis All sample BPCA characterizations were analyzed according to Brodowski et al. (2005). Briefly, to eliminate the polyvalent metals, less than 5 mg of OC of the biochars was digested in 10 mL of 4 M trifluoroacetic acid (TFA) at 105 C for 4 h. After cooling, the residue was rinsed several times with deionized water by filtration through a glass fiber filter (Whatman GF/A 1.6 mm) and dried at 40 C for 3 h. The residue was then transferred to Teflon-lined bombs, to which 2 mL of 65% HNO3 was added and reacted at 170 C for 8 h in a high pressure digestion apparatus. The mixture was filtered with an ashless cellulose filter. Then, 2 mL of digestion solution was diluted 5 times with 10 mL of deionized water, and 100 mL of citric acid as the first internal standard was added. The solution was treated using cation exchange resin (Dowex 50 WX8, 200e400 mesh). The treated aqueous samples were freeze-dried and re-dissolved in methanol, and 100 mL of biphenyl-2,20 -dicarboxylic acid in methanol was added as the second internal standard. After drying by passing nitrogen gas, the samples were derivatized to trimethylsilyl-derivatives for GC-MS analyses (Agilent, 7890A GC equipped with a 5975C quadrupole mass selective detector). The GC-MS was equipped with an FID detector and HP-5MS fused silica capillary column (30 m 0.25 mm 0.25 mm). High purity N2 was used as a carrier gas at a flow rate of 3 mL/min and a split ratio of 10:1. The temperatures of the detector and injector were set at 300 C. The heating program was as follows: 100 C held for 2 min, then increased to 250 C by 20 C min1 and held for 12 min. Following that, the temperature was increased to 300 C by 10 C min1 and held for 5 min.
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2.4. Batch sorption experiments The adsorbate, BPA were purchased from Sigma-Aldrich. The physicochemical properties are listed in Table S1. Batch sorption experiments were conducted for BPA on pristine biochars and oxidized 400 C biochars using 4-mL vials with Teflon-lined screw caps. The stock solutions of BPA (64 mg/L) were diluted to 1e64 mg/ L with a background solution containing 0.02 M NaCl and 200 mg/L NaN3. A certain amount of biochar was added to the vials, and two parallel samples were set for each concentration point. Then, all of the vials were shaken for 7 days on an air-bath shaker in the dark at 25 C to achieve sorption equilibrium. After sorption equilibrium, all samples were centrifuged at 10,000 g for 15 min. For adsorbate quantification, 1 mL of supernatant was transferred to the liquid phase vial and then detected by high-performance liquid chromatography (HPLC). The concentration difference between the initial concentration and the equilibrium aqueous-phase concentration was used to calculate the solid-phase concentrations. The HPLC (Agilent Technologies 1200) was equipped with a reversed-phase C18 column (5 mm, 4.6 150 mm), and the temperature of the column was set at 30 C. The BPA was detected at 280 nm with a UV detector and the injection volume was 10 mL. The mobile phase consisted of acetonitrile and deionized water of 40:60 (V:V) with a flow rate of 1 mL/min. 2.5. Data analysis The Freundlich model (logarithmic form) was selected to fit sorption isotherms by SigmaPlot 10.0.
log Q e ¼ log KF þ N log Ce
(1)
where Qe (mg/kg) is the equilibrium solid phase concentration and Ce (mg/L) is the equilibrium liquid phase concentration. KF [(mg/ kg)/(mg/L)N] is the Freundlich sorption coefficient, and N is the nonlinear coefficient. The single-point sorption coefficient (Kd) were calculated:
Kd ¼ Q e = C e
(2)
The Kd was calculated at Ce ¼ 0.01 Cs and Ce ¼ 0.1 Cs, where Cs (mg/L) is the water solubility, 380 mg/L for BPA. All correlation analyses were conducted using Pearson's correlation analysis, and a significance test was conducted using a oneway t-test (a ¼ 0.05) by SPSS 19.0. 3. Results and discussion 3.1. Physicochemical properties of pristine and oxidized biochars As listed in Table 1, for pristine biochars, the C content increased while the O and H contents decreased with the raising pyrolysis temperature, which is consistent with the data of previous researches (Han et al., 2014; Keiluweit et al., 2010). To be more specific, for biochar prepared at a lower temperature ranging from 200 to 400 C, the C content increased from 54.3% to 73.5%, and the O content decreased from 35.6% to 15.1%, while when pyrolysis was conducted at a higher temperature between 500 and 700 C, it resulted in the C content increasing from 77.4% to 82.8%, and the O content decreasing from 15.1% to 7.8% (Table 1), which suggested that substantial changes in the C and O contents occurred when the temperature range was 200e400 C. Keiluweit et al. (2010) and Schneider et al. (2010) described this phase of biochar, with a dramatic reduction of the H and O contents, as “transition charcoals”. Compared with lower temperature biochar
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(P2eP4), the total pore volume, micropore volume (MPV) and specific SA of biochar prepared at high temperature (P5eP7) were significantly elevated, which showed that the pore structure of biochar was developed obviously under high temperature pyrolysis (Table 1). As was reported in previous studies, high temperature heating expanded the pores and rearranged the graphite sheets and caused an increase in TPV and SA (Wang et al., 2006). For oxidized 400 C biochar, HNO3/H2SO4 treatment greatly altered its physicochemical properties. Oxidation decreased the C content dramatically while increasing the O content to varying degrees depending on the oxidation time. The elevated O content probably resulted from the formation of oxygen-containing functional groups on oxidized biochars, e.g., phenolic, nitro and carboxylic functional groups. The introduction of these oxygencontaining functional groups increased the polarity of oxidized biochars with the increase in the O/C and (N þ O)/C atomic ratios (Table 1). It should be noted that the properties of oxidized biochars at different oxidation times vary widely. For instance, as the oxidation time was extended, the contents of C, H and O decreased, especially for C reduced by 55%, while the H/C atomic ratio increased from 0.55 to 0.70 (Table 1, Fig. S1). To better understand the intrinsic links between biochar properties in different oxidation stages, we analyzed the relationships between the properties of oxidized biochars (Fig. 1). A significant negative correlation between (O þ N)/C and TPV or MPV was observed (r ¼ 0.885, P < 0.01 for TPV, Fig. 1a; r ¼ 0.976, P < 0.01 for MPV, Fig. 1b). The formation of polar functional groups may decrease the pore volumes, especially the micropore volume, which also resulted in a reduced specific surface area (r ¼ 0.998, P < 0.01, Fig. 1c). Considering the significant positive correlation between MPV and SA (r ¼ 0.975, P < 0.01, Fig. 1d), the reduction of the specific surface area may have resulted from pore blocking by polar functional groups of oxidized biochars. FTIR spectra also confirmed the change in the surface functional groups of biochars based on the pyrolysis temperature and oxidation time. As shown in Fig. 2a, with the raise in pyrolysis temperature, the intensities of bands for OeH (3455 cm1), aliphatic CeH (2923 and 2852 cm1), carboxyl C]O (1716 cm1), and carboxylate eCOOe (1070 cm1) decreased, which suggested the decrease in polar functional groups. Fig. 2b displays that the HNO3/H2SO4 treatment changed the structure of 400 C biochars. After oxidation, the intensity of C]O absorption at 1716 cm1 enhanced, which indicated the increase in carboxyl functional groups (Uchimiya et al., 2012). The peak strength decreased with the extension in oxidation time, which is consistent with the decreasing trend of O content (Table 1). The intensity of the aromatic C]C absorption peak at 1623 cm1 diminished for oxidized biochars, which was due to a reduction of the nonpolar groups (Chen et al., 2016). The absorption peaks at 1376 and 1540 cm1 could be attributed to the stretching vibration of symmetrical NO2 and asymmetric NO2 (Seredych et al., 2008). The trend of increased intensity is consistent with changing the N content, which decreases with the increasing oxidation time (Table 1). Previous studies also reported the existence of nitrate and nitro groups by HNO3 oxidation (Chingombe et al., 2005; Qian and Chen, 2014). The peak of 875 cm1 is attributed to aromatic CeH (Sun et al., 2011), and the intensities were slightly stronger for P4-8 and P4-10 than that of the original biochar, which indicated that the aromatic ring was enriched during the oxidation process. The intensity of carboxylate eCOOe (1070 cm1) (Ghaffar et al., 2015) also significantly enhanced after oxidation. These findings are consistent with the results of the elemental analysis, which indicated that a large number of oxygen-containing functional groups were generated after HNO3/H2SO4 treatment.
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Table 1 Bulk analysis of the pristine and oxidized biochars prepared from peanut shells. Biochars
P P2 P3 P4 P5 P6 P7 P4e2 P4e4 P4e6 P4e8 P4e10
Elemental composition (%)
Atomic ratio
BET-N2
C
H
O
N
S
H/C
O/C
(N þ O)/C
TPVa (cm3/g)
MPVa (cm3/g)
APDa (nm)
SAa (m2/g)
44.4 54.3 66.9 73.5 77.4 80.1 82.8 55.0 49.7 45.5 34.4 32.8
5.2 5.2 3.6 3.3 2.3 1.7 1.1 2.5 2.4 2.3 2.0 1.9
37.9 35.6 20.9 15.1 11.2 9.4 7.8 33.8 35.8 34.0 25.5 23.7
1.5 0.9 1.1 1.3 1.0 0.8 0.7 3.7 3.5 3.2 2.2 2.0
0.18 0.09 0.18 0.40 0.20 0.17 0.17 0.15 0.06 0.06 0 0.58
1.41 1.14 0.64 0.54 0.36 0.25 0.16 0.55 0.59 0.60 0.67 0.70
0.64 0.49 0.23 0.15 0.11 0.09 0.07 0.46 0.54 0.56 0.56 0.54
0.67 0.51 0.25 0.17 0.12 0.10 0.08 0.52 0.60 0.62 0.61 0.59
0.024 0.043 0.062 0.027 0.072 1.171 1.768 0.071 0.04 0.023 0.049 0.049
0.0010 0.0015 0.0021 0.0013 0.0350 0.1198 0.0917 0.0063 0.0027 0.0011 0.0023 0.0037
16.88 17.93 11.23 9.10 3.811 18.94 32.5 4.36 7.54 13.93 16.43 8.16
5.66 9.67 22.00 11.68 75.59 247.31 217.59 64.89 21.06 6.72 11.85 24.27
P: peanut shell feedstocks; P2eP7: biochars produced at 200e700 C. P4-2 to P4-10: the oxidation of 400 C biochar for 2e10 h by HNO3/H2SO4 treatment. a TPV: total pore volume; MPV: micropore volume; APD: average pore diameter; SA: specific surface area.
Fig. 1. The correlations between different physicochemical properties of 400 C biochar oxidized at different durations. Significant difference at P < 0.01. TPV: total pore volume; MPV: micropore volume; SA: specific surface area.
3.2. BPCA characteristics of biochars produced at different temperatures The BPCA contents increased from 2.3 mg/g biochar (P) to 224.3 mg/g biochar (P5) with the raise in pyrolysis temperature. A dramatic rise in the BPCA content occurred from 4.6 mg/g biochar (8.7 mg/g C) to 143.2 mg/g biochar (213.9 mg/g C) in the 200e300 C temperature range (Fig. 3a), indicating that this temperature range is key for the formation of aromatic carbon, which is consistent with the H/C atomic ratio (which decreased from 1.14 to 0.64). When the temperature was higher than 500 C, the BPCA content did not change significantly (P5eP7), while mellitic acid (B6CA) increased continuously from 144.5 mg/g biochar to
248.5 mg/g biochar. The continuously increased B6CA content may have resulted from the highly condensed aromatic clusters, which are the precursors to the formation of B6CA (Brodowski et al., 2005; Glaser et al., 1998; Llorente et al., 2017). The organic carbon normalized BPCA content showed the same phenomenon (Fig. 3a). Researchers often infer biochar properties, especially the aromatic condensation/aromaticity, based on the individual BPCA distribution patterns (Lehndorff et al., 2014; Wolf et al., 2013). We plotted the ratio of B5CA/B6CA over the H/C atomic ratio, and a significant positive correlation was observed (r ¼ 0.981, P < 0.01, Fig. 4b). The ratios of B6CA/BPCAs, B5CA/B6CA and B6CA/B4CAs are usually used to estimate the pyrolysis temperature and to trace the sources of black carbon (Acksel et al., 2016; Boot et al., 2015;
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Fig. 2. FTIR spectra of the pristine (a) and oxidized biochars (b). P: peanut shell feedstocks; P2 to P7: biochars produced at 200e700 C. P4-2 to P4-10: oxidized 400 C biochar for 2e10 h by HNO3/H2SO4 treatment.
Fig. 3. BPCA contents (a) and individual BPCA distribution patterns (b) of biochars produced at different temperatures. Different letters on the total of the columns suggest a significant difference (P < 0.01). B6CA: mellitic acid; B5CA: benzenepentacarboxylic acid; B4CAs: sum of mellophanic acid, prehnitic acid and pyromellitic acid; B3CAs: sum of trimellitic acid, hemimellitic acid and trimesic acid.
Wiedemeier et al., 2015). In our study, B6CA/BPCAs increased with the increased pyrolysis temperature and exceeded 69% at 600 C and 700 C (Fig. 3b), which suggested the development of graphene-like structures due to the condensation of benzene rings at a high temperature. Schneider et al. (2010) reported that the condensation reaction showed absolute superiority at 600e1000 C. 3.3. BPCA characteristics of biochars oxidized by HNO3/H2SO4 To compare the changes in the BPCA content before and after HNO3/H2SO4 treatment, the individual BPCA contents were calculated per gram of the original biochar and were referred to as the absolute BPCA content. The results are shown in Fig. 5. The HNO3/ H2SO4 treatment caused a significant reduction of the absolute BPCA content, from 113.7 mg/g to 0.6 mg/g biochar. The reduction of the BPCA content was no longer dramatic when the oxidation time exceeded 4 h (Fig. 5a). In the strong oxidation condition of the HNO3/H2SO4 mixture, biochar oxidation not only occurred on the surface but also in the inner aromatic clusters. Nevertheless, the mass loss rate of B6CA is lower than that of B5CA, B4CAs and B3CAs (Fig. 5b), which indicated that the aromatic structure with high condensation has a stronger oxidation resistance than that of the part with low condensation. In other words, once the biochars are
applied to soils or other natural environments, the fractions with high condensation will be enriched after a long period under biological or physical degradation. Fig. 5c further demonstrates this point. As the oxidation time was extended, the relative content of B3CA and B4CA decreased, while B6CA increased from 32.0% to 42.4%. We also conducted an analysis on the relationship between B5CA/B6CA to H/C as for the pristine biochars. However, the B5CA/ B6CA and H/C atomic ratios showed a negative relationship (r ¼ 0.996, P < 0.01, Fig. 4d) for oxidized biochars, which is opposite to what was shown for pristine biochars. This phenomenon is related to the heterogeneous properties of the biochars. During oxidation, the aliphatic carbon linked to aromatic clusters was first decomposed, which led to the exposure of aromatic clusters. The benzene rings located at the edge of the aromatic structures were then decomposed faster than the rings located inside the core. Consequently, oxygen-containing functional groups formed at the bond-breaking sites. These labile aromatic structures are precursors of B3CAs and B4CAs. With progressive aging, components with larger aromatic clusters were then broken into smaller aromatic components, which are more likely to produce B5CA and B6CA. This process reduced the biochar aromaticity, but the structure of the condensed carbon structure became more uniform, mostly in the presence of six conjugated benzene rings (Mao et al.,
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Fig. 4. Correlation between the B5CA/B6CA ratio and H/C atomic ratio of the pristine biochars (P2eP7) b (a, b) and oxidized biochars (c, d). No B5CA or B6CA was detected in the original feedstock.
Fig. 5. The absolute BPCA contents (based on the original mass of biochar) (a), mass loss ratios (b) and the individual BPCA distribution patterns (c) of oxidized 400 C biochars during different times.
2012). These highly heterogeneous structural changes could not be identified by using the common bulk chemical analysis. Clearly, when the bulk chemical analysis and molecular biomarker method (BPCAs) were combined, the heterogeneous properties of biochars and their environmental implications could be described better. 3.4. Sorption of BPA on pristine and modified biochars The Freundlich model was employed to fit the sorption isotherms (Fig. S2) of BPA on biochars, and the fitting results are shown in Table S2. For all samples, the R2adj values were in the range of 0.952e0.996. The nonlinearity coefficient N values decreased with raising pyrolysis temperature from 200 to 700 C. A positive correlation between the H/C atomic ratio and the N values
was found in this research (Fig. 6a), which is consistent with the studies of other researchers (Chang et al., 2018; Li et al., 2017; Xiao et al., 2016). In addition, the N values decreased with the increasing MPV and SA (Fig. S3), which suggested that the higher microporosity and SA may contribute to the site heterogeneity. After HNO3/ H2SO4 treatment, the N values of BPA sorption on 400 C biochar increased from 0.35 to 0.65, while the N values decreased with increasing oxidation time (Table 2). Obviously, oxidized biochars showed higher N values (0.58e0.65) than did the original biochar, which indicated that the heterogeneous sorption sites were partly destroyed by HNO3/H2SO4 treatment. However, a negative correlation between the H/C atomic ratios and N values was observed for the oxidized biochars, which is opposite to that of the pristine biochars (Fig. 6a). No relationship was noted between the N values
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Fig. 6. The correlations between sorption characteristics and biochar physicochemical properties. The relationship of nonlinearity coefficient N values and H/C atomic ratios (a), B5CA/B6CA (b); The comparison of predicted and detected N values using multiple regression analysis. N values could be estimated using B5CA/B6CA and (O þ N)/C (c), while log Kd values could be estimated using BPCAs and (O þ N)/C. Kd values presented here were calculated at Ce ¼ 0.01Cs (d).
and MPV or SA for the oxidized biochars. On the other hand, significant positive correlations were found between B5CA/B6CA and N values for both kinds of biochars (Fig. 6b). The smaller B5CA/B6CA with the smaller N values may be due to the enrichment of condensed aromatic structures during pyrolysis or oxidation. However, these positive correlations for the pristine and oxidized biochars could not be described using a uniform equation because of the two separate sets of data, as presented in Fig. 6b. Here, we call the readers’ attention to the correlation analysis, which should be carefully carried out when discussion the sorption mechanisms, because the apparent sorption characteristics may be associated with several physiochemical properties. Even for an empirical mathematic expression, it is risky to apply it to biochars of different origins. Apparently, using only one biochar property could not estimate the sorption characteristics (N values as discussed above) when grouping the pristine and oxidized biochars together. A multiple regression analysis was made between the N values and several biochar properties, including both common physicochemical properties and the BPCA parameters. An acceptable estimation of N values using B5CA/B6CA and (O þ N)/C was obtained as N ¼ 0.08 þ 0.103 B5CA/B6CA þ 0.721 (O þ N)/C (R2 ¼ 0.985, Fig. 6c). A similar multiple regression analysis was also conducted for the log Kd values, which could be estimated by combining BPCAs and (O þ N)/C (R2 ¼ 0.936, Fig. 6d). Although the application of these equations should be tested for various biochar samples, this type of multiple regression analysis that combines common physicochemical properties and biomarker parameters may provide a new
idea for sorption characteristics estimation. 4. Conclusions This study combined bulk chemical analysis and BPCA molecular makers to describe the biochar heterogeneous properties and to characterize their sorption of BPA (nonlinearity coefficient N, single-point sorption coefficients log Kd). When describing the pristine biochars, the H/C ratio and BPCA parameters showed the same result: the aromaticity or aromatic condensation increased with the raise in pyrolysis temperature. After being oxidized by HNO3/H2SO4, the aromaticity (H/C ratio) decreased while the B6CA/ BPCAs increased. In another word, although the overall aromaticity decreased after the oxidation, the content of large aromatic clusters was higher, suggesting the heterogeneous properties of biochars. The unsaturated C bonds in the less condensed structures may be oxidized or saturated, while the real condensed structures were enriched because of the release of broken small molecules and the oxidation of the less condensed structures (such as B3CAs and B4CAs). Therefore, combining both methods can better describe the heterogeneous properties of biochars regarding their physicochemical properties and sorption characteristics. Acknowledgments This work was supported by The National Key Research and Development Program of China (2017YFD0801000), the National Natural Science Foundation of China (41725016 and 41473116), and
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the Yunnan applied basic research project (2016FA040). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.03.164. References Acksel, A., Amelung, W., Kühn, P., Gehrt, E., Regier, T., Leinweber, P., 2016. Soil organic matter characteristics as indicator of Chernozem genesis in the Baltic Sea region. Geoderma Reg 7 (2), 187e200. Ahmad, M., Lee, S.S., Dou, X., Mohan, D., Sung, J.-K., Yang, J.E., Ok, Y.S., 2012. Effects of pyrolysis temperature on soybean stover-and peanut shell-derived biochar properties and TCE adsorption in water. Bioresour. Technol. 118, 536e544. Boot, C.M., Haddix, M., Paustian, K., Cotrufo, M.F., 2015. Distribution of black carbon in ponderosa pine forest floor and soils following the High Park wildfire. Biogeosciences 12 (10), 3029e3039. Brodowski, S., Rodionov, A., Haumaier, L., Glaser, B., Amelung, W., 2005. Revised black carbon assessment using benzene polycarboxylic acids. Org. Geochem. 36 (9), 1299e1310. Chang, Z., Tian, L., Wu, M., Dong, X., Peng, J., Pan, B., 2018. Molecular markers of benzene polycarboxylic acids in describing biochar physiochemical properties and sorption characteristics. Environ. Pollut. 237, 541e548. Chen, Z., Chen, B., Chiou, C.T., 2016. Fast and slow rates of naphthalene sorption to biochars produced at different temperatures. Environ. Sci. Technol. 46 (20), 11104e11111. Cheng, C.H., Lin, T.P., Lehmann, J., Fang, L.J., Yang, Y.W., Menyailo, O.V., Chang, K.H., Lai, J.S., 2014. Sorption properties for black carbon (wood char) after long term exposure in soils. Org. Geochem. 70 (5), 53e61. Chingombe, P., Saha, B., Wakeman, R.J., 2005. Surface modification and characterisation of a coal-based activated carbon. Carbon 43 (15), 3132e3143. Chu, G., Zhao, J., Chen, F., Dong, X., Zhou, D., Liang, N., Wu, M., Pan, B., Steinberg, C.E., 2017. Physi-chemical and sorption properties of biochars prepared from peanut shell using thermal pyrolysis and microwave irradiation. Environ. Pollut. 227, 372e379. Ghaffar, A., Ghosh, S., Li, F., Dong, X., Zhang, D., Wu, M., Li, H., Pan, B., 2015. Effect of biochar aging on surface characteristics and adsorption behavior of dialkyl phthalates. Environ. Pollut. 206, 502e509. Glaser, B., Haumaier, L., Guggenberger, G., Zech, W., 1998. Black carbon in soils: the use of benzenecarboxylic acids as specific markers. Org. Geochem. 29 (4), 811e819. Han, L., Sun, K., Jin, J., Wei, X., Xia, X., Wu, F., Gao, B., Xing, B., 2014. Role of structure and microporosity in phenanthrene sorption by natural and engineered organic matter. Environ. Sci. Technol. 48 (19), 11227e11234. Jin, J., Sun, K., Wang, Z., Han, L., Du, P., Wang, X., Xing, B., 2017. Effects of chemical oxidation on phenanthrene sorption by grass- and manure-derived biochars. Sci. Total Environ. 598, 789e796. Keiluweit, M., Nico, P.S., Johnson, M.G., Kleber, M., 2010. Dynamic molecular structure of plant biomass-derived black carbon (biochar). Environ. Sci. Technol. 44 (4), 1247e1253. Lehndorff, E., Roth, P.J., Cao, Z.H., Amelung, W., 2014. Black carbon accrual during 2000 years of paddy-rice and non-paddy cropping in the Yangtze River Delta, China. Glob. Chang. Biol. 20 (6), 1968e1978. Li, J., Liang, N., Jin, X., Zhou, D., Li, H., Wu, M., Pan, B., 2017. The role of ash content on
bisphenol A sorption to biochars derived from different agricultural wastes. Chemosphere 171, 66e73. n, M.B., Glaser, B., 2017. Rapid and economical quantification of Llorente, M., Turrio black carbon in soils using a modified benzene polycarboxylic acids (BPCA) method. Org. Geochem. 115, 197e204. Mao, J.D., Johnson, R.L., Lehmann, J., Olk, D.C., Neves, E.G., Thompson, M.L., Schmidtrohr, K., 2012. Abundant and stable char residues in soils: implications for soil fertility and carbon sequestration. Environ. Sci. Technol. 46 (17), 9571e9576. Nguyen, B.T., Lehmann, J., Kinyangi, J., Smernik, R., Riha, S.J., Engelhard, M.H., Swanston, C., Smernik, R., 2008. Long-term black carbon dynamics in cultivated soil. Biogeochemistry 89 (3), 295e308. Ok, Y.S., Uchimiya, S.M., Chang, S.X., Bolan, N., 2015. Biochar: Production, Characterization, and Applications. CRC Press. Qian, L., Chen, B., 2014. Interactions of aluminum with biochars and oxidized biochars: implications for the biochar aging process. J. Agric. Food Chem. 62 (2), 373e380. Schneider, M.P.W., Hilf, M., Vogt, U.F., Schmidt, M.W.I., 2010. The benzene polycarboxylic acid (BPCA) pattern of wood pyrolyzed between 200 C and 1000 C. Org. Geochem. 41 (10), 1082e1088. Seredych, M., Hulicova-Jurcakova, D., Gao, Q.L., Bandosz, T.J., 2008. Surface functional groups of carbons and the effects of their chemical character, density and accessibility to ions on electrochemical performance. Carbon 46 (11), 1475e1488. Sun, K., Kang, M., Zhang, Z., Jin, J., Wang, Z., Pan, Z., Xu, D., Wu, F., Xing, B., 2013. Impact of deashing treatment on biochar structural properties and potential sorption mechanisms of phenanthrene. Environ. Sci. Technol. 47 (20), 11473e11481. Sun, K., Ro, K., Guo, M., Novak, J., Mashayekhi, H., Xing, B., 2011. Sorption of bisphenol A, 17a-ethinyl estradiol and phenanthrene on thermally and hydrothermally produced biochars. Bioresour. Technol. 102 (10), 5757e5763. Uchimiya, M., Bannon, D.I., Wartelle, L.H., 2012. Retention of heavy metals by carboxyl functional groups of biochars in small arms range soil. J. Agric. Food Chem. 60 (7), 1798e1809. Wang, X., Sato, T., Xing, B., 2006. Competitive sorption of pyrene on wood chars. Environ. Sci. Technol. 40 (10), 3267. Wang, Z., Han, L., Sun, K., Jin, J., Ro, K.S., Libra, J.A., Liu, X., Xing, B., 2016. Sorption of four hydrophobic organic contaminants by biochars derived from maize straw, wood dust and swine manure at different pyrolytic temperatures. Chemosphere 144, 285e291. Wiedemeier, D.B., Abiven, S., Hockaday, W.C., Keiluweit, M., Kleber, M., Masiello, C.A., Mcbeath, A.V., Nico, P.S., Pyle, L.A., Schneider, M.P.W., 2015. Aromaticity and degree of aromatic condensation of char. Org. Geochem. 78, 135e143. Wolf, M., Lehndorff, E., Wiesenberg, G.L.B., Stockhausen, M., Schwark, L., Amelung, W., 2013. Towards reconstruction of past fire regimes from geochemical analysis of charcoal. Org. Geochem. 55 (1), 11e21. Xiao, X., Chen, Z., Chen, B., 2016. H/C atomic ratio as a smart linkage between pyrolytic temperatures, aromatic clusters and sorption properties of biochars derived from diverse precursory materials. Sci. Rep. 6, 22644. Yang, K., Yang, J., Yuan, J., Wu, W., Lin, D., 2016. Correlations and adsorption mechanisms of aromatic compounds on a high heat temperature treated bamboo biochar. Environ. Pollut. 210, 57e64. ska, A., Oleszczuk, P., 2015. Evaluation of sewage sludge and slow pyrolyzed Zielin sewage sludge-derived biochar for adsorption of phenanthrene and pyrene. Bioresour. Technol. 192, 618e626.
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Combining bulk characterization and benzene polycarboxylic acid molecular markers to describe biochar properties Zhaofeng Chang a, b, Luping Tian a, c, Jun Zhang a, b, Qing Zhao d, Fangfang Li a, b, Min Wu a, b, *, Bo Pan a, b a
Faculty of Environmental Science & Engineering, Kunming University of Science & Technology, Kunming, 650500, China Yunnan Provincial Key Laboratory of Soil Carbon Sequestration and Pollution Control, Kunming, 650500, Yunnan, China Yunnan Institute of Environmental Science, Kunming, 650500, China d Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China b c
h i g h l i g h t s The B6CA/BPCAs ratios increase with the pyrolysis temperature. The polar functional groups formed by oxidation block biochar pore structures. HNO3/H2SO4 oxidation decreases BPCAs contents but increases B6CA/BPCAs. The nonlinear factors are positively related to B5CA/B6CA for all biochars. N and log Kd could be predicted by combining (O þ N)/C and BPCA parameters.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 February 2019 Received in revised form 25 March 2019 Accepted 26 March 2019 Available online 10 April 2019
The physicochemical properties of biochar determined its sorption of organic contaminations, and the environmental aging process changed the biochar properties. However, the correlation between biochar heterogeneous properties and their sorption characteristics is unclear. In this study, peanut shell biochars were produced at 200e700 C, and HNO3/H2SO4 was used to oxidize 400 C biochar for 2e10 h to simulate the enhanced aging process of biochar in the environment. Benzene polycarboxylic acid (BPCA) molecular markers, and bulk characterization were analyzed to describe biochar physicochemical properties and to further predict the sorption characteristics to bisphenol A (BPA). For pristine biochars, the mellitic acid/BPCAs (B6CA/BPCAs) increased with the raise of pyrolysis temperature and the H/C atomic ratio was positively correlated with benzenepentacarboxylic acid/B6CA (B5CA/B6CA) (P < 0.01), which indicated the increased aromatic condensation. After HNO3/H2SO4 treatment, the aromaticity (H/C ratio) decreased while the highly condensed components in biochars were enriched (increased B6CA/ BPCAs values). Multiple regression models were adopted to establish a quantitative relationship between biochar heterogeneous properties and their sorption of BPA. Both nonlinearity coefficient N values (N ¼ 0.08 þ 0.103 B5CA/B6CA þ 0.721 (O þ N)/C, R2 ¼ 0.985) and single-point sorption coefficients log Kd (log Kd ¼ 1.236 þ 0.006 BPCAs þ 1.449 (O þ N)/C, R2 ¼ 0.936) could be estimated combining molecular markers and polarity parameters for biochars. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Patryk Oleszczuk Keywords: Oxidized biochars Benzene polycarboxylic acids Physicochemical properties Sorption Organic contaminants
1. Introduction Biochar is a carbon-rich substance that is produced in the course of pyrolysis of organic feedstocks under oxygen-limited or oxygen-
* Corresponding author. Faculty of Environmental Science & Engineering, Kunming University of Science & Technology, Kunming, 650500, China. . E-mail address: [email protected] (M. Wu). https://doi.org/10.1016/j.chemosphere.2019.03.164 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
free conditions (Yang et al., 2016). Due to the wide sources of biomass and the simple preparation procedure, biochar has been widely used in production practices, such as waste recycling, carbon sequestration, soil improvement, and contamination control (Ahmad et al., 2012; Ok et al., 2015). With biochar applied to the environment, biotic and/or abiotic processes (aging) could alter the biochar properties. Nguyen et al. (2008) reported that the O/C ratios of aged biochar increased by more than 133 and 192% after 10 and
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30 years of cultivation, respectively. Recently, the changing chemical and physical properties, e.g., the particle size, surface functional groups and aromatic structures, of aging biochar have attracted extensive interest as potential control factors for binding organic contaminations and thus reducing their mobility. The environmental aging of biochar is a long-term procedure that lasts from several years to thousands of years. In laboratory simulations, chemical reagents such as H2O2, HNO3 and the mixture of HNO3/H2SO4 were usually used to oxidize biochar to simulate the environmental aging of biochar (Ghaffar et al., 2015; Jin et al., 2017). Thus, the effect of oxidation on the sorption of organic contaminations on aged biochar could be studied. For instance, HNO3 treatment increased aromaticity, decreased alkyl C and enriched O, which improved the sorption of PHE on grass straw biochar (Jin et al., 2017). Cheng et al. (2014) found that increased surface functional groups of aged biochar reduced the sorption of diuron and atrazine. The enhanced sorption capacity of phthalate esters was also found for HNO3/H2SO4 oxidized biochar (Ghaffar et al., 2015). Obviously, changes in biochar properties affect its environmental behavior, and researchers have tried to establish a link between the biochar properties and sorption properties of organic ska and Oleszczuk, 2015). contaminations (Sun et al., 2013; Zielin However, various biochar properties have been reported to correlate with their sorption characteristics, and each was provided with an explanation (Li et al., 2017; Wang et al., 2016). Han et al. (2014) suggested that the nanopore-filling mechanism dominated biochar sorption of phenanthrene because of the significant correlation between the surface area and Koc. Hydrogen bond and p-p EDA interactions may facilitate bisphenol A adsorption in lowtemperature biochar due to its amount of oxygen-containing functional groups (Chu et al., 2017; Li et al., 2017). Various adsorption mechanisms may be derived from the heterogeneous structure of biochar. Therefore, although various studies have been reported to correlate biochar properties with their sorption characteristics, no applicable equation has been proposed. Previous studies have suggested that benzene polycarboxylic acid (BPCA) molecular markers provide a useful method for describing biochar heterogeneous properties (Chang et al., 2018). The distribution of individual BPCA molecular biomarkers allowed us to analyze the size of the aromatic clusters of biochars. For the same type of biochars, BPCA parameters were significantly correlated with the nonlinear sorption behavior of organic contaminations (Chang et al., 2018). These results indicated the potential application of molecular markers in predicting contamination behavior. However, this method was applied in only one type of biochar. The applicability of molecular markers in describing the properties of different types of biochars, such as biochars of different aging stages, was not tested. In this study, biochars were prepared using peanut shells at 200e700 C. In addition, an additional set of biochars was prepared after oxidizing 400 C biochar with an HNO3/H2SO4 mixture for various durations. This oxidation also simulates the accelerated aging process of biochar in the environment. Both BPCA molecular markers and common bulk chemical analysis, including elemental compositions, functional groups, specific surface areas (SA), pore size distributions and total pore volumes (TPV), were used to describe the biochar properties. In addition, the sorption characteristics to bisphenol A (BPA) were also investigated, with attempts to correlate them to the biochar properties. 2. Materials and methods 2.1. Preparation and oxidation treatment of biochars The peanut shells were collected from a farmer's market in
Chenggong district, Kunming city. When dried in an oven at 60 C for 7 days, they were ground and crushed through a 250-mm sieve. The treated feedstocks were pyrolyzed at 200e700 C for 4 h under a N2 atmosphere in a muffle furnace. Then, the bulk biochars were ground gently and sieved through a 150-mm sieve. The prepared biochars were marked as P2, P3, P4, P5, P6, and P7, with P referring to peanut shell feedstocks and 2e7 referring to pyrolysis temperatures of 200, 300, 400, 500, 600 and 700 C. A mixture of concentrated HNO3/H2SO4 (volume ratio 1:3) (Qian and Chen, 2014) was used to oxidize 400 C biochar (close to the temperature of black carbon formed by natural fire at 450 C) under different treatment times to simulate various stages of biochar in the enhanced environment aging process. Specifically, 5 g of biochar was immersed in 400 mL of HNO3/H2SO4 mixture and maintained at 70 C for 2, 4, 6, 8, and 10 h. Then, the mixture was filtered through a 0.45-mm membrane and repeatedly rinsed with deionized water until the pH value remained unchanged. The treated biochars were dried in an oven at 60 C for 7 days and denoted by P4-2, P4-4, P4-6, P4-8 and P4-10, where 2e10 indicates the oxidation duration. 2.2. Common physicochemical properties of pristine and oxidized 400 C biochars All sample elemental analyses were conducted by various MICRO cube elemental analyzers at 850 C to measure the C, H, N, and S and at 1150 C for measuring O. The specific surface area, pore size distribution and pore volume were determined by N2 sorption at 77 K (Autosorbe1C, Quantachrome). Functional groups were also characterized by a Fourier transform infrared spectrometer (FTIR, 640eIR, Varian): biochar samples were mixed evenly with KBr (mass ratio, 1:800), then ground and pressed into a sheet. The scanning range of the spectrometer was 4000e400 cm1 with a resolution of 2 cm1. 2.3. Benzene polycarboxylic acid molecular markers analysis All sample BPCA characterizations were analyzed according to Brodowski et al. (2005). Briefly, to eliminate the polyvalent metals, less than 5 mg of OC of the biochars was digested in 10 mL of 4 M trifluoroacetic acid (TFA) at 105 C for 4 h. After cooling, the residue was rinsed several times with deionized water by filtration through a glass fiber filter (Whatman GF/A 1.6 mm) and dried at 40 C for 3 h. The residue was then transferred to Teflon-lined bombs, to which 2 mL of 65% HNO3 was added and reacted at 170 C for 8 h in a high pressure digestion apparatus. The mixture was filtered with an ashless cellulose filter. Then, 2 mL of digestion solution was diluted 5 times with 10 mL of deionized water, and 100 mL of citric acid as the first internal standard was added. The solution was treated using cation exchange resin (Dowex 50 WX8, 200e400 mesh). The treated aqueous samples were freeze-dried and re-dissolved in methanol, and 100 mL of biphenyl-2,20 -dicarboxylic acid in methanol was added as the second internal standard. After drying by passing nitrogen gas, the samples were derivatized to trimethylsilyl-derivatives for GC-MS analyses (Agilent, 7890A GC equipped with a 5975C quadrupole mass selective detector). The GC-MS was equipped with an FID detector and HP-5MS fused silica capillary column (30 m 0.25 mm 0.25 mm). High purity N2 was used as a carrier gas at a flow rate of 3 mL/min and a split ratio of 10:1. The temperatures of the detector and injector were set at 300 C. The heating program was as follows: 100 C held for 2 min, then increased to 250 C by 20 C min1 and held for 12 min. Following that, the temperature was increased to 300 C by 10 C min1 and held for 5 min.
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2.4. Batch sorption experiments The adsorbate, BPA were purchased from Sigma-Aldrich. The physicochemical properties are listed in Table S1. Batch sorption experiments were conducted for BPA on pristine biochars and oxidized 400 C biochars using 4-mL vials with Teflon-lined screw caps. The stock solutions of BPA (64 mg/L) were diluted to 1e64 mg/ L with a background solution containing 0.02 M NaCl and 200 mg/L NaN3. A certain amount of biochar was added to the vials, and two parallel samples were set for each concentration point. Then, all of the vials were shaken for 7 days on an air-bath shaker in the dark at 25 C to achieve sorption equilibrium. After sorption equilibrium, all samples were centrifuged at 10,000 g for 15 min. For adsorbate quantification, 1 mL of supernatant was transferred to the liquid phase vial and then detected by high-performance liquid chromatography (HPLC). The concentration difference between the initial concentration and the equilibrium aqueous-phase concentration was used to calculate the solid-phase concentrations. The HPLC (Agilent Technologies 1200) was equipped with a reversed-phase C18 column (5 mm, 4.6 150 mm), and the temperature of the column was set at 30 C. The BPA was detected at 280 nm with a UV detector and the injection volume was 10 mL. The mobile phase consisted of acetonitrile and deionized water of 40:60 (V:V) with a flow rate of 1 mL/min. 2.5. Data analysis The Freundlich model (logarithmic form) was selected to fit sorption isotherms by SigmaPlot 10.0.
log Q e ¼ log KF þ N log Ce
(1)
where Qe (mg/kg) is the equilibrium solid phase concentration and Ce (mg/L) is the equilibrium liquid phase concentration. KF [(mg/ kg)/(mg/L)N] is the Freundlich sorption coefficient, and N is the nonlinear coefficient. The single-point sorption coefficient (Kd) were calculated:
Kd ¼ Q e = C e
(2)
The Kd was calculated at Ce ¼ 0.01 Cs and Ce ¼ 0.1 Cs, where Cs (mg/L) is the water solubility, 380 mg/L for BPA. All correlation analyses were conducted using Pearson's correlation analysis, and a significance test was conducted using a oneway t-test (a ¼ 0.05) by SPSS 19.0. 3. Results and discussion 3.1. Physicochemical properties of pristine and oxidized biochars As listed in Table 1, for pristine biochars, the C content increased while the O and H contents decreased with the raising pyrolysis temperature, which is consistent with the data of previous researches (Han et al., 2014; Keiluweit et al., 2010). To be more specific, for biochar prepared at a lower temperature ranging from 200 to 400 C, the C content increased from 54.3% to 73.5%, and the O content decreased from 35.6% to 15.1%, while when pyrolysis was conducted at a higher temperature between 500 and 700 C, it resulted in the C content increasing from 77.4% to 82.8%, and the O content decreasing from 15.1% to 7.8% (Table 1), which suggested that substantial changes in the C and O contents occurred when the temperature range was 200e400 C. Keiluweit et al. (2010) and Schneider et al. (2010) described this phase of biochar, with a dramatic reduction of the H and O contents, as “transition charcoals”. Compared with lower temperature biochar
383
(P2eP4), the total pore volume, micropore volume (MPV) and specific SA of biochar prepared at high temperature (P5eP7) were significantly elevated, which showed that the pore structure of biochar was developed obviously under high temperature pyrolysis (Table 1). As was reported in previous studies, high temperature heating expanded the pores and rearranged the graphite sheets and caused an increase in TPV and SA (Wang et al., 2006). For oxidized 400 C biochar, HNO3/H2SO4 treatment greatly altered its physicochemical properties. Oxidation decreased the C content dramatically while increasing the O content to varying degrees depending on the oxidation time. The elevated O content probably resulted from the formation of oxygen-containing functional groups on oxidized biochars, e.g., phenolic, nitro and carboxylic functional groups. The introduction of these oxygencontaining functional groups increased the polarity of oxidized biochars with the increase in the O/C and (N þ O)/C atomic ratios (Table 1). It should be noted that the properties of oxidized biochars at different oxidation times vary widely. For instance, as the oxidation time was extended, the contents of C, H and O decreased, especially for C reduced by 55%, while the H/C atomic ratio increased from 0.55 to 0.70 (Table 1, Fig. S1). To better understand the intrinsic links between biochar properties in different oxidation stages, we analyzed the relationships between the properties of oxidized biochars (Fig. 1). A significant negative correlation between (O þ N)/C and TPV or MPV was observed (r ¼ 0.885, P < 0.01 for TPV, Fig. 1a; r ¼ 0.976, P < 0.01 for MPV, Fig. 1b). The formation of polar functional groups may decrease the pore volumes, especially the micropore volume, which also resulted in a reduced specific surface area (r ¼ 0.998, P < 0.01, Fig. 1c). Considering the significant positive correlation between MPV and SA (r ¼ 0.975, P < 0.01, Fig. 1d), the reduction of the specific surface area may have resulted from pore blocking by polar functional groups of oxidized biochars. FTIR spectra also confirmed the change in the surface functional groups of biochars based on the pyrolysis temperature and oxidation time. As shown in Fig. 2a, with the raise in pyrolysis temperature, the intensities of bands for OeH (3455 cm1), aliphatic CeH (2923 and 2852 cm1), carboxyl C]O (1716 cm1), and carboxylate eCOOe (1070 cm1) decreased, which suggested the decrease in polar functional groups. Fig. 2b displays that the HNO3/H2SO4 treatment changed the structure of 400 C biochars. After oxidation, the intensity of C]O absorption at 1716 cm1 enhanced, which indicated the increase in carboxyl functional groups (Uchimiya et al., 2012). The peak strength decreased with the extension in oxidation time, which is consistent with the decreasing trend of O content (Table 1). The intensity of the aromatic C]C absorption peak at 1623 cm1 diminished for oxidized biochars, which was due to a reduction of the nonpolar groups (Chen et al., 2016). The absorption peaks at 1376 and 1540 cm1 could be attributed to the stretching vibration of symmetrical NO2 and asymmetric NO2 (Seredych et al., 2008). The trend of increased intensity is consistent with changing the N content, which decreases with the increasing oxidation time (Table 1). Previous studies also reported the existence of nitrate and nitro groups by HNO3 oxidation (Chingombe et al., 2005; Qian and Chen, 2014). The peak of 875 cm1 is attributed to aromatic CeH (Sun et al., 2011), and the intensities were slightly stronger for P4-8 and P4-10 than that of the original biochar, which indicated that the aromatic ring was enriched during the oxidation process. The intensity of carboxylate eCOOe (1070 cm1) (Ghaffar et al., 2015) also significantly enhanced after oxidation. These findings are consistent with the results of the elemental analysis, which indicated that a large number of oxygen-containing functional groups were generated after HNO3/H2SO4 treatment.
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Table 1 Bulk analysis of the pristine and oxidized biochars prepared from peanut shells. Biochars
P P2 P3 P4 P5 P6 P7 P4e2 P4e4 P4e6 P4e8 P4e10
Elemental composition (%)
Atomic ratio
BET-N2
C
H
O
N
S
H/C
O/C
(N þ O)/C
TPVa (cm3/g)
MPVa (cm3/g)
APDa (nm)
SAa (m2/g)
44.4 54.3 66.9 73.5 77.4 80.1 82.8 55.0 49.7 45.5 34.4 32.8
5.2 5.2 3.6 3.3 2.3 1.7 1.1 2.5 2.4 2.3 2.0 1.9
37.9 35.6 20.9 15.1 11.2 9.4 7.8 33.8 35.8 34.0 25.5 23.7
1.5 0.9 1.1 1.3 1.0 0.8 0.7 3.7 3.5 3.2 2.2 2.0
0.18 0.09 0.18 0.40 0.20 0.17 0.17 0.15 0.06 0.06 0 0.58
1.41 1.14 0.64 0.54 0.36 0.25 0.16 0.55 0.59 0.60 0.67 0.70
0.64 0.49 0.23 0.15 0.11 0.09 0.07 0.46 0.54 0.56 0.56 0.54
0.67 0.51 0.25 0.17 0.12 0.10 0.08 0.52 0.60 0.62 0.61 0.59
0.024 0.043 0.062 0.027 0.072 1.171 1.768 0.071 0.04 0.023 0.049 0.049
0.0010 0.0015 0.0021 0.0013 0.0350 0.1198 0.0917 0.0063 0.0027 0.0011 0.0023 0.0037
16.88 17.93 11.23 9.10 3.811 18.94 32.5 4.36 7.54 13.93 16.43 8.16
5.66 9.67 22.00 11.68 75.59 247.31 217.59 64.89 21.06 6.72 11.85 24.27
P: peanut shell feedstocks; P2eP7: biochars produced at 200e700 C. P4-2 to P4-10: the oxidation of 400 C biochar for 2e10 h by HNO3/H2SO4 treatment. a TPV: total pore volume; MPV: micropore volume; APD: average pore diameter; SA: specific surface area.
Fig. 1. The correlations between different physicochemical properties of 400 C biochar oxidized at different durations. Significant difference at P < 0.01. TPV: total pore volume; MPV: micropore volume; SA: specific surface area.
3.2. BPCA characteristics of biochars produced at different temperatures The BPCA contents increased from 2.3 mg/g biochar (P) to 224.3 mg/g biochar (P5) with the raise in pyrolysis temperature. A dramatic rise in the BPCA content occurred from 4.6 mg/g biochar (8.7 mg/g C) to 143.2 mg/g biochar (213.9 mg/g C) in the 200e300 C temperature range (Fig. 3a), indicating that this temperature range is key for the formation of aromatic carbon, which is consistent with the H/C atomic ratio (which decreased from 1.14 to 0.64). When the temperature was higher than 500 C, the BPCA content did not change significantly (P5eP7), while mellitic acid (B6CA) increased continuously from 144.5 mg/g biochar to
248.5 mg/g biochar. The continuously increased B6CA content may have resulted from the highly condensed aromatic clusters, which are the precursors to the formation of B6CA (Brodowski et al., 2005; Glaser et al., 1998; Llorente et al., 2017). The organic carbon normalized BPCA content showed the same phenomenon (Fig. 3a). Researchers often infer biochar properties, especially the aromatic condensation/aromaticity, based on the individual BPCA distribution patterns (Lehndorff et al., 2014; Wolf et al., 2013). We plotted the ratio of B5CA/B6CA over the H/C atomic ratio, and a significant positive correlation was observed (r ¼ 0.981, P < 0.01, Fig. 4b). The ratios of B6CA/BPCAs, B5CA/B6CA and B6CA/B4CAs are usually used to estimate the pyrolysis temperature and to trace the sources of black carbon (Acksel et al., 2016; Boot et al., 2015;
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385
Fig. 2. FTIR spectra of the pristine (a) and oxidized biochars (b). P: peanut shell feedstocks; P2 to P7: biochars produced at 200e700 C. P4-2 to P4-10: oxidized 400 C biochar for 2e10 h by HNO3/H2SO4 treatment.
Fig. 3. BPCA contents (a) and individual BPCA distribution patterns (b) of biochars produced at different temperatures. Different letters on the total of the columns suggest a significant difference (P < 0.01). B6CA: mellitic acid; B5CA: benzenepentacarboxylic acid; B4CAs: sum of mellophanic acid, prehnitic acid and pyromellitic acid; B3CAs: sum of trimellitic acid, hemimellitic acid and trimesic acid.
Wiedemeier et al., 2015). In our study, B6CA/BPCAs increased with the increased pyrolysis temperature and exceeded 69% at 600 C and 700 C (Fig. 3b), which suggested the development of graphene-like structures due to the condensation of benzene rings at a high temperature. Schneider et al. (2010) reported that the condensation reaction showed absolute superiority at 600e1000 C. 3.3. BPCA characteristics of biochars oxidized by HNO3/H2SO4 To compare the changes in the BPCA content before and after HNO3/H2SO4 treatment, the individual BPCA contents were calculated per gram of the original biochar and were referred to as the absolute BPCA content. The results are shown in Fig. 5. The HNO3/ H2SO4 treatment caused a significant reduction of the absolute BPCA content, from 113.7 mg/g to 0.6 mg/g biochar. The reduction of the BPCA content was no longer dramatic when the oxidation time exceeded 4 h (Fig. 5a). In the strong oxidation condition of the HNO3/H2SO4 mixture, biochar oxidation not only occurred on the surface but also in the inner aromatic clusters. Nevertheless, the mass loss rate of B6CA is lower than that of B5CA, B4CAs and B3CAs (Fig. 5b), which indicated that the aromatic structure with high condensation has a stronger oxidation resistance than that of the part with low condensation. In other words, once the biochars are
applied to soils or other natural environments, the fractions with high condensation will be enriched after a long period under biological or physical degradation. Fig. 5c further demonstrates this point. As the oxidation time was extended, the relative content of B3CA and B4CA decreased, while B6CA increased from 32.0% to 42.4%. We also conducted an analysis on the relationship between B5CA/B6CA to H/C as for the pristine biochars. However, the B5CA/ B6CA and H/C atomic ratios showed a negative relationship (r ¼ 0.996, P < 0.01, Fig. 4d) for oxidized biochars, which is opposite to what was shown for pristine biochars. This phenomenon is related to the heterogeneous properties of the biochars. During oxidation, the aliphatic carbon linked to aromatic clusters was first decomposed, which led to the exposure of aromatic clusters. The benzene rings located at the edge of the aromatic structures were then decomposed faster than the rings located inside the core. Consequently, oxygen-containing functional groups formed at the bond-breaking sites. These labile aromatic structures are precursors of B3CAs and B4CAs. With progressive aging, components with larger aromatic clusters were then broken into smaller aromatic components, which are more likely to produce B5CA and B6CA. This process reduced the biochar aromaticity, but the structure of the condensed carbon structure became more uniform, mostly in the presence of six conjugated benzene rings (Mao et al.,
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Fig. 4. Correlation between the B5CA/B6CA ratio and H/C atomic ratio of the pristine biochars (P2eP7) b (a, b) and oxidized biochars (c, d). No B5CA or B6CA was detected in the original feedstock.
Fig. 5. The absolute BPCA contents (based on the original mass of biochar) (a), mass loss ratios (b) and the individual BPCA distribution patterns (c) of oxidized 400 C biochars during different times.
2012). These highly heterogeneous structural changes could not be identified by using the common bulk chemical analysis. Clearly, when the bulk chemical analysis and molecular biomarker method (BPCAs) were combined, the heterogeneous properties of biochars and their environmental implications could be described better. 3.4. Sorption of BPA on pristine and modified biochars The Freundlich model was employed to fit the sorption isotherms (Fig. S2) of BPA on biochars, and the fitting results are shown in Table S2. For all samples, the R2adj values were in the range of 0.952e0.996. The nonlinearity coefficient N values decreased with raising pyrolysis temperature from 200 to 700 C. A positive correlation between the H/C atomic ratio and the N values
was found in this research (Fig. 6a), which is consistent with the studies of other researchers (Chang et al., 2018; Li et al., 2017; Xiao et al., 2016). In addition, the N values decreased with the increasing MPV and SA (Fig. S3), which suggested that the higher microporosity and SA may contribute to the site heterogeneity. After HNO3/ H2SO4 treatment, the N values of BPA sorption on 400 C biochar increased from 0.35 to 0.65, while the N values decreased with increasing oxidation time (Table 2). Obviously, oxidized biochars showed higher N values (0.58e0.65) than did the original biochar, which indicated that the heterogeneous sorption sites were partly destroyed by HNO3/H2SO4 treatment. However, a negative correlation between the H/C atomic ratios and N values was observed for the oxidized biochars, which is opposite to that of the pristine biochars (Fig. 6a). No relationship was noted between the N values
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Fig. 6. The correlations between sorption characteristics and biochar physicochemical properties. The relationship of nonlinearity coefficient N values and H/C atomic ratios (a), B5CA/B6CA (b); The comparison of predicted and detected N values using multiple regression analysis. N values could be estimated using B5CA/B6CA and (O þ N)/C (c), while log Kd values could be estimated using BPCAs and (O þ N)/C. Kd values presented here were calculated at Ce ¼ 0.01Cs (d).
and MPV or SA for the oxidized biochars. On the other hand, significant positive correlations were found between B5CA/B6CA and N values for both kinds of biochars (Fig. 6b). The smaller B5CA/B6CA with the smaller N values may be due to the enrichment of condensed aromatic structures during pyrolysis or oxidation. However, these positive correlations for the pristine and oxidized biochars could not be described using a uniform equation because of the two separate sets of data, as presented in Fig. 6b. Here, we call the readers’ attention to the correlation analysis, which should be carefully carried out when discussion the sorption mechanisms, because the apparent sorption characteristics may be associated with several physiochemical properties. Even for an empirical mathematic expression, it is risky to apply it to biochars of different origins. Apparently, using only one biochar property could not estimate the sorption characteristics (N values as discussed above) when grouping the pristine and oxidized biochars together. A multiple regression analysis was made between the N values and several biochar properties, including both common physicochemical properties and the BPCA parameters. An acceptable estimation of N values using B5CA/B6CA and (O þ N)/C was obtained as N ¼ 0.08 þ 0.103 B5CA/B6CA þ 0.721 (O þ N)/C (R2 ¼ 0.985, Fig. 6c). A similar multiple regression analysis was also conducted for the log Kd values, which could be estimated by combining BPCAs and (O þ N)/C (R2 ¼ 0.936, Fig. 6d). Although the application of these equations should be tested for various biochar samples, this type of multiple regression analysis that combines common physicochemical properties and biomarker parameters may provide a new
idea for sorption characteristics estimation. 4. Conclusions This study combined bulk chemical analysis and BPCA molecular makers to describe the biochar heterogeneous properties and to characterize their sorption of BPA (nonlinearity coefficient N, single-point sorption coefficients log Kd). When describing the pristine biochars, the H/C ratio and BPCA parameters showed the same result: the aromaticity or aromatic condensation increased with the raise in pyrolysis temperature. After being oxidized by HNO3/H2SO4, the aromaticity (H/C ratio) decreased while the B6CA/ BPCAs increased. In another word, although the overall aromaticity decreased after the oxidation, the content of large aromatic clusters was higher, suggesting the heterogeneous properties of biochars. The unsaturated C bonds in the less condensed structures may be oxidized or saturated, while the real condensed structures were enriched because of the release of broken small molecules and the oxidation of the less condensed structures (such as B3CAs and B4CAs). Therefore, combining both methods can better describe the heterogeneous properties of biochars regarding their physicochemical properties and sorption characteristics. Acknowledgments This work was supported by The National Key Research and Development Program of China (2017YFD0801000), the National Natural Science Foundation of China (41725016 and 41473116), and
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the Yunnan applied basic research project (2016FA040). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.03.164. References Acksel, A., Amelung, W., Kühn, P., Gehrt, E., Regier, T., Leinweber, P., 2016. Soil organic matter characteristics as indicator of Chernozem genesis in the Baltic Sea region. Geoderma Reg 7 (2), 187e200. Ahmad, M., Lee, S.S., Dou, X., Mohan, D., Sung, J.-K., Yang, J.E., Ok, Y.S., 2012. Effects of pyrolysis temperature on soybean stover-and peanut shell-derived biochar properties and TCE adsorption in water. Bioresour. Technol. 118, 536e544. Boot, C.M., Haddix, M., Paustian, K., Cotrufo, M.F., 2015. Distribution of black carbon in ponderosa pine forest floor and soils following the High Park wildfire. Biogeosciences 12 (10), 3029e3039. Brodowski, S., Rodionov, A., Haumaier, L., Glaser, B., Amelung, W., 2005. Revised black carbon assessment using benzene polycarboxylic acids. Org. Geochem. 36 (9), 1299e1310. Chang, Z., Tian, L., Wu, M., Dong, X., Peng, J., Pan, B., 2018. Molecular markers of benzene polycarboxylic acids in describing biochar physiochemical properties and sorption characteristics. Environ. Pollut. 237, 541e548. Chen, Z., Chen, B., Chiou, C.T., 2016. Fast and slow rates of naphthalene sorption to biochars produced at different temperatures. Environ. Sci. Technol. 46 (20), 11104e11111. Cheng, C.H., Lin, T.P., Lehmann, J., Fang, L.J., Yang, Y.W., Menyailo, O.V., Chang, K.H., Lai, J.S., 2014. Sorption properties for black carbon (wood char) after long term exposure in soils. Org. Geochem. 70 (5), 53e61. Chingombe, P., Saha, B., Wakeman, R.J., 2005. Surface modification and characterisation of a coal-based activated carbon. Carbon 43 (15), 3132e3143. Chu, G., Zhao, J., Chen, F., Dong, X., Zhou, D., Liang, N., Wu, M., Pan, B., Steinberg, C.E., 2017. Physi-chemical and sorption properties of biochars prepared from peanut shell using thermal pyrolysis and microwave irradiation. Environ. Pollut. 227, 372e379. Ghaffar, A., Ghosh, S., Li, F., Dong, X., Zhang, D., Wu, M., Li, H., Pan, B., 2015. Effect of biochar aging on surface characteristics and adsorption behavior of dialkyl phthalates. Environ. Pollut. 206, 502e509. Glaser, B., Haumaier, L., Guggenberger, G., Zech, W., 1998. Black carbon in soils: the use of benzenecarboxylic acids as specific markers. Org. Geochem. 29 (4), 811e819. Han, L., Sun, K., Jin, J., Wei, X., Xia, X., Wu, F., Gao, B., Xing, B., 2014. Role of structure and microporosity in phenanthrene sorption by natural and engineered organic matter. Environ. Sci. Technol. 48 (19), 11227e11234. Jin, J., Sun, K., Wang, Z., Han, L., Du, P., Wang, X., Xing, B., 2017. Effects of chemical oxidation on phenanthrene sorption by grass- and manure-derived biochars. Sci. Total Environ. 598, 789e796. Keiluweit, M., Nico, P.S., Johnson, M.G., Kleber, M., 2010. Dynamic molecular structure of plant biomass-derived black carbon (biochar). Environ. Sci. Technol. 44 (4), 1247e1253. Lehndorff, E., Roth, P.J., Cao, Z.H., Amelung, W., 2014. Black carbon accrual during 2000 years of paddy-rice and non-paddy cropping in the Yangtze River Delta, China. Glob. Chang. Biol. 20 (6), 1968e1978. Li, J., Liang, N., Jin, X., Zhou, D., Li, H., Wu, M., Pan, B., 2017. The role of ash content on
bisphenol A sorption to biochars derived from different agricultural wastes. Chemosphere 171, 66e73. n, M.B., Glaser, B., 2017. Rapid and economical quantification of Llorente, M., Turrio black carbon in soils using a modified benzene polycarboxylic acids (BPCA) method. Org. Geochem. 115, 197e204. Mao, J.D., Johnson, R.L., Lehmann, J., Olk, D.C., Neves, E.G., Thompson, M.L., Schmidtrohr, K., 2012. Abundant and stable char residues in soils: implications for soil fertility and carbon sequestration. Environ. Sci. Technol. 46 (17), 9571e9576. Nguyen, B.T., Lehmann, J., Kinyangi, J., Smernik, R., Riha, S.J., Engelhard, M.H., Swanston, C., Smernik, R., 2008. Long-term black carbon dynamics in cultivated soil. Biogeochemistry 89 (3), 295e308. Ok, Y.S., Uchimiya, S.M., Chang, S.X., Bolan, N., 2015. Biochar: Production, Characterization, and Applications. CRC Press. Qian, L., Chen, B., 2014. Interactions of aluminum with biochars and oxidized biochars: implications for the biochar aging process. J. Agric. Food Chem. 62 (2), 373e380. Schneider, M.P.W., Hilf, M., Vogt, U.F., Schmidt, M.W.I., 2010. The benzene polycarboxylic acid (BPCA) pattern of wood pyrolyzed between 200 C and 1000 C. Org. Geochem. 41 (10), 1082e1088. Seredych, M., Hulicova-Jurcakova, D., Gao, Q.L., Bandosz, T.J., 2008. Surface functional groups of carbons and the effects of their chemical character, density and accessibility to ions on electrochemical performance. Carbon 46 (11), 1475e1488. Sun, K., Kang, M., Zhang, Z., Jin, J., Wang, Z., Pan, Z., Xu, D., Wu, F., Xing, B., 2013. Impact of deashing treatment on biochar structural properties and potential sorption mechanisms of phenanthrene. Environ. Sci. Technol. 47 (20), 11473e11481. Sun, K., Ro, K., Guo, M., Novak, J., Mashayekhi, H., Xing, B., 2011. Sorption of bisphenol A, 17a-ethinyl estradiol and phenanthrene on thermally and hydrothermally produced biochars. Bioresour. Technol. 102 (10), 5757e5763. Uchimiya, M., Bannon, D.I., Wartelle, L.H., 2012. Retention of heavy metals by carboxyl functional groups of biochars in small arms range soil. J. Agric. Food Chem. 60 (7), 1798e1809. Wang, X., Sato, T., Xing, B., 2006. Competitive sorption of pyrene on wood chars. Environ. Sci. Technol. 40 (10), 3267. Wang, Z., Han, L., Sun, K., Jin, J., Ro, K.S., Libra, J.A., Liu, X., Xing, B., 2016. Sorption of four hydrophobic organic contaminants by biochars derived from maize straw, wood dust and swine manure at different pyrolytic temperatures. Chemosphere 144, 285e291. Wiedemeier, D.B., Abiven, S., Hockaday, W.C., Keiluweit, M., Kleber, M., Masiello, C.A., Mcbeath, A.V., Nico, P.S., Pyle, L.A., Schneider, M.P.W., 2015. Aromaticity and degree of aromatic condensation of char. Org. Geochem. 78, 135e143. Wolf, M., Lehndorff, E., Wiesenberg, G.L.B., Stockhausen, M., Schwark, L., Amelung, W., 2013. Towards reconstruction of past fire regimes from geochemical analysis of charcoal. Org. Geochem. 55 (1), 11e21. Xiao, X., Chen, Z., Chen, B., 2016. H/C atomic ratio as a smart linkage between pyrolytic temperatures, aromatic clusters and sorption properties of biochars derived from diverse precursory materials. Sci. Rep. 6, 22644. Yang, K., Yang, J., Yuan, J., Wu, W., Lin, D., 2016. Correlations and adsorption mechanisms of aromatic compounds on a high heat temperature treated bamboo biochar. Environ. Pollut. 210, 57e64. ska, A., Oleszczuk, P., 2015. Evaluation of sewage sludge and slow pyrolyzed Zielin sewage sludge-derived biochar for adsorption of phenanthrene and pyrene. Bioresour. Technol. 192, 618e626.