Isolation and characterization of biochar-derived organic matter fractions and their phenanthrene sorption

Environmental Pollution 236 (2018) 745e753

Contents lists available at ScienceDirect

Environmental Pollution journal homepage: www.elsevier.com/locate/envpol

Isolation and characterization of biochar-derived organic matter fractions and their phenanthrene sorption* Jie Jin a, b, Ke Sun b, *, Wei Liu a, Shiwei Li a, Xianqiang Peng a, Yan Yang b, Lanfang Han b, Ziwen Du c, Xiangke Wang a a b c

College of Environmental Science and Engineering, North China Electric Power University, Beijing, 102206, China State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing, 100875, China College of Environmental Science and Engineering, Beijing Forestry University, Beijing, 100083, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 November 2017 Received in revised form 26 January 2018 Accepted 5 February 2018

Chemical composition and pollutant sorption of biochar-derived organic matter fractions (BDOMs) are critical for understanding the long-term environmental significance of biochar. Phenanthrene (PHE) sorption by the humic acid-like (HAL) fractions isolated from plant straw- (PLABs) and animal manurebased (ANIBs) biochars, and the residue materials (RES) after HAL extraction was investigated. The HAL fraction comprised approximately 50% of organic carbon (OC) of the original biochars. Results of XPS and 13 C NMR demonstrated that the biochar-derived HAL fractions mainly consisted of aromatic clusters substituted by carboxylic groups. The CO2 cumulative surface area of BDOMs excluding PLAB-derived RES fractions was obviously lower than that of corresponding biochars. The sorption nonlinearity of PHE by the fresh biochars was significantly stronger than that of the BDOM fractions, implying that the BDOM fractions were more chemically homogeneous. The BDOMs generally exhibited comparable or higher OCnormalized distribution coefficients (Koc) of PHE than the original biochars. The PHE logKoc values of the fresh biochars correlated negatively with the micropore volumes due to steric hindrance effect. In contrast, a positive relationship between the sorption coefficients (Kd) of BDOMs and the micropore volumes was observed in this study, suggesting that pore filling could dominate PHE sorption by the BDOMs. The positive correlation between the PHE logKoc values of the HAL fractions and the aromatic C contents indicates that PHE sorption by the HAL fractions was regulated by aromatic domains. The findings of this study improve our knowledge of the evolution of biochar properties after application and its potential environmental impacts. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Biochar Biochar-derived organic matter Sorption Phenanthrene

1. Introduction Recently, soil application of biochar has attracted great attention in agricultural practices to improve soil fertility and to mitigate climate change through soil C sequestration (Lehmann, 2007; Reisser et al., 2016; Rombol
a et al., 2015; Yoshizawa, 2016). With the increasing recognition of biochar as a potential means for soil remediation, there is an urgent need to examine the fate of biochar after application, as it would play vital roles in the biogeochemical cycle of soil organic carbon (OC) as well as the environmental behaviors of nutrients and contaminants in soils and aquatic systems

*

This paper has been recommended for acceptance by B. Nowack. * Corresponding author. E-mail address: [email protected] (K. Sun).

https://doi.org/10.1016/j.envpol.2018.02.015 0269-7491/© 2018 Elsevier Ltd. All rights reserved.

(Gibson et al., 2016; Harvey et al., 2016; Jin et al., 2017b; Qu et al., 2016). In evaluating the long-term environmental significance of biochar in soil, it is crucial to have an understanding of the evolution of its physicochemical properties after application (Harvey et al., 2016). There is a general consensus that physicochemical heterogeneity of fresh biochar is mainly controlled by source material and the production conditions (Ahmad et al., 2014; Jin et al., 2014; Lehmann and Joseph, 2015). Once it has been applied into soils, biochar would undergo alterations in physicochemical properties with time (Hale et al., 2011; Rechberger et al., 2017). Many previous studies have detected such alterations of biochar, such as fragmentation to smaller particles, deposition of minerals and natural organic matter (NOM), decrease in pore availability and polyaromatic ring size, and concentration of polar groups (Cheng et al., 2008; Ghaffar et al., 2015; Rechberger et al., 2017; Shi et al., 2015;

746

J. Jin et al. / Environmental Pollution 236 (2018) 745e753

Singh et al., 2012). These studies examined the evolution of biochar's physicochemical properties by considering biochar materials as integral particles. However, with weathering, biochar will release fulvic acid-like (FAL) as well as poly aromatic humic acidlike (HAL) molecules (Hiemstra et al., 2013; Mao et al., 2012; Shindo and Nishimura, 2016). This indicates that biochar materials can be actively involved in the pedogenic process and can eventually become components of soil organic matter (SOM) (Knicker, 2011). Using the classic fractionation method of SOM recommended by the International Humic Substances Society (IHSS) to obtain fulvic acid (FA) and humic acid (HA) fractions, not only geologically formed NOM fractions but also biochar-derived organic matter (BDOM) fractions can be isolated from soils (Hiemstra et al., 2013; Mao et al., 2012). Specifically, in Japanese Andosols, the contribution of pyrogenic HA derived from charred plant residues to HA in the whole soils was in the range of 12e44% (Shindo and Nishimura, 2016). Any consideration of the environmental behaviors and impacts of biochar must consider the physicochemical properties of these BDOM fractions, which, however, are still largely unknown. Moreover, biochar is well known as an effective environmental sorbent to retain hydrophobic organic compounds (HOCs) (Cornelissen et al., 2005; Hale et al., 2011; Jin et al., 2017a). The aforementioned incorporation of BDOMs into soils requires insight into the BDOMs-HOC interactions. Derived from biochar, BDOM fractions may show sorption properties for HOCs similar to biochar. It is generally accepted that biochar's very high affinity and capacity for sorbing HOCs is primarily due to its hydrophobic and nanoporous character (Ahmad et al., 2014; Lehmann and Joseph, 2015). Previous studies also found that the aromatic moieties of biochar made significant contributions to the overall sorption of HOCs (Ahmad et al., 2014; Sander and Pignatello, 2005; Zhu and Pignatello, 2005). Therefore, it is reasonable to assume that the micropores and aromatic components of BDOM fractions will regulate their sorption of HOCs. In the present study, HAL fractions were extracted from biochars produced from plant straws and animal manures at different temperatures. The physicochemical properties of the HAL fractions and the residue (RES) materials after HAL extraction were investigated. The sorption of phenanthrene (PHE), a typical representative of HOCs, by these BDOM fractions were examined. The major objectives of this study were to (1) determine the physicochemical properties of biochar-derived HAL and RES fractions, (2) quantify the sorption coefficients of PHE to these BDOMs, and (3) investigate the underlying sorption mechanisms of BDOMs. 2. Material and methods 2.1. Chemicals and materials Pure analytical grade (>98%) standards and radiochemicals (isotopic purity ¼ 98%) of PHE were obtained from Sigma-Aldrich Chemical Co. (USA). Methanol was purchased from Merck Co. (Germany) in HPLC grade. All other reagents were obtained from Beijing Chemical Reagent Co. (China) in analytical grade. Five biomass input materials were selected to produce biochars: wheat straw, maize straw, swine manure, cow manure, and chicken manure. The feedstocks were oven-dried at 80  C and ground to <1 mm. Subsequently, the ground feedstocks were placed in ceramic pots, covered with lids, and carbonized at specified temperatures (i.e., 300, 450, and 600  C) for 1 h using a muffle furnace at 10  C/min under oxygen-limited conditions. After cooling to

room temperature inside the furnace, the biochars were treated with 0.1 M of HCl to remove some soluble salts, carbonate, and dissolved organic matter. Next, the biochars were flushed by deionized water, freeze-dried, gently ground, and passed through a 0.25-mm sieve. Chemical oxidation, which can provide an indication of hundreds to thousands of years of natural aging processes in soils (Hale et al., 2011), was used to artificially age biochars. The biochars were suspended in 25% HNO3 (1:30, m/v) at a 1:30 solid/liquid ratio. The flasks containing the mixtures were heated under reflux at approximately 90  C for 4 h (Shindo and Honma, 1998). After cooling, the HNO3-treated residues of biochar were collected by centrifugation, washed thoroughly until neutral pH was attained, and freeze-dried for subsequent use. Next, the residues were mixed with 0.1 M of NaOH solution at a 1:10 solid/liquid ratio in a sealed flask under ambient conditions. After each flask had been shaken for 24 h, the suspension was centrifuged at 3000 g for 30 min. The supernatant was acidified (pH ¼ 2 with 6 M of HCl) to separate the pyrogenic HAL precipitate from the soluble FAL fraction. The retained material after NaOH extraction was collected as the RES fraction. The isolated BDOM fractions were flushed five times with deionized water, freeze-dried, gently ground to pass through a 0.25-mm sieve, and stored until further use. The original biochars are hereafter referred to as SWH, SMA, MSW, MCW, and MCH according to the feedstock sources (wheat straw, maize straw, swine manure, cow manure, and chicken manure, respectively). Their BDOM fractions were named as SWH-HAL and SWH-RES, SMA-HAL and SMA-RES, MSW-HAL and MSW-RES, MCW-HAL and MCW-RES, and MCH-HAL and MCH-RES, respectively. The biochars produced from plant straws and animal manures were named as PLAB and ANIB, respectively. 2.2. Sorbent characterization Bulk elemental C, N, H, and O abundances of the biochars and their BDOM fractions were determined using an Elemental Analyzer (Elementar Vario ELIII, Germany). Ash contents were measured by combustion at 750  C for 4 h. X-ray photoelectron spectroscopy (XPS) was conducted using an ESCALAB 250 XPS (Thermo Scientific, USA) spectrometer with a monochromatic Al Ka radiation source. Peak deconvolutions of high resolution C1s spectra were analyzed using the GaussianeLorentzian sum function (Avantage software, Thermo Scientific), and the resolved peaks were C-C at 284.9 eV, C-O at 286.5 eV, C¼O at 287.9 eV, and COO at 289.4 eV. The contents of the surface acidic functional groups of the BDOM fractions were determined using Boehm's titration. Briefly, 0.1 g of BDOM sample was immersed into 20 mL 0.01 M NaHCO3, Na2CO3, and NaOH solutions, respectively. The mixtures were shaken for 24 h to reach the equilibration and then carefully filtered. Next, 10 mL of aliquot from each filtrate was titrated with 0.01 M HCl solution to determine the contents of various acidic groups, including carboxylic, lactonic, and phenolic groups. The 13C nuclear magnetic resonance (NMR) spectra of the samples were obtained with an AVANCE 300 (Bruker, Germany) using the crosspolarization magic angle spinning (CP/MAS) technique, operated at a spin rate of 12 kHz and a 13C frequency of 75 MHz. For organic matter containing pores smaller than 0.5 nm, the surface area (SA) can be underestimated by the traditionally recommended N2 adsorption techniques (Lattao et al., 2014; Ravikovitch et al., 2005; Xing and Pignatello, 1997). Accordingly, CO2 at 273 K, which can enter the micropores (0e1.4 nm), has been used as an alternative gas for these materials (Xing and Pignatello, 1997). In this study,

J. Jin et al. / Environmental Pollution 236 (2018) 745e753

physisorption isotherms of CO2 for all materials were measured on an Autosorb-iQ gas analyzer (Quantachrome Instrument Corp., USA) at 273 K after the materials had been degassed for 8 h under a vacuum at 105  C. The cumulative micropore volume, cumulative micropore SA (CO2-SA), and micropore size distribution (for pores between 0.3 and 1.5 nm in width) of the materials were calculated using a built-in density functional theory (DFT) simulation. The zeta potentials of the BDOM samples at various pH values were measured using a Delsa™ Nano C particle analyzer (Beckman Coulter, USA).

2.3. Sorption experiment Batch PHE sorption experiments were conducted in duplicate in 40-mL glass vials. The background solution (pH ¼ 6.5) consisted of 0.01 M of CaCl2 to maintain a stable ionic concentration and 200 mg/L of NaN3 to prevent PHE biodegradation. The 14C labeled and unlabeled stock solutions of PHE were prepared in methanol before being added to the background solution. Initial concentrations of PHE ranged from 2 to 1100 mg/L. The total amount of methanol in the test solutions was kept below 0.1% by volume. The appropriate amount of sorbents was added into the vials to obtain 20e80% sorption of PHE at apparent equilibrium. Next, all vials were sealed with Teflon screw caps and shaken in the dark for 10 d at 23 ± 1  C. On the basis of the adsorption kinetic results (Fig. S1), apparent sorption equilibrium was reached around 10 d. After equilibration, the vials were centrifuged at 3000 g for 20 min. Subsequently, 1.5 mL of the supernatant was withdrawn and added to Scintiverse cocktail (4 mL, Fisher Scientific, USA) for liquid scintillation counting (Beckman Counter LS6500, USA). The counting efficiency factor is 78.7% for 14C (Yaghoubi et al., 2007), which was used to calibrate the scintillation counter. The pH values of the supernatant ranged from 6.4 to 6.8. Blank results revealed that the degradation or sorptive losses of PHE by the glass vials were negligible. Hence, sorbed PHE concentrations of biochars and their BDOM fractions were calculated by mass difference.

2.4. Data analysis Sorption data of PHE by the biochars and their BDOM fractions were fitted to the Freundlich model: qe ¼ KF Cne

(1)

Kd ¼ qe/Ce

(2)

Koc ¼ Kd/foc

(3)

where qe (mg/g) represents the amount of PHE sorbed per gram of sorbent, Ce (mg/L) is the concentration of PHE remaining in solution, KF ((mg/g)/(mg/L)n) is the Freundlich constant, and n describes the nonlinearity of the sorption isotherm. Sorption distribution coefficients (Kd) of PHE were calculated at Ce ¼ 0.01, 0.1 and 1 Sw (water solubility, 1.12 mg/L at 22  C) from the slope of the sorption isotherms. The OC-normalized sorption coefficients (Koc) were then calculated by dividing Kd values by the corresponding OC content (foc). The relationships among properties of the samples and their sorption coefficients for PHE were investigated by Pearson correlation analysis using IBM SPSS Statistics 20 software (SPSS Inc., USA). The regression was carried out with SigmaPlot (SigmaPlot 10.0, USA).

747

3. Results and discussion 3.1. Yield of BDOM fractions The yields of BDOM fractions varied with the charring temperatures (Table S1). The biochars produced at 300  C were easily oxidized by HNO3 treatment due to their insufficient carbonization and provided very low HAL and RES yields. The increase of the heat treatment temperature to 450  C led to higher resistance to HNO3 oxidation, increasing the yields of HAL and RES. The biochars obtained at 600  C possessed highly condensed structures, producing minimal HAL (0.4e3.3 wt%) and large quantities of RES. The HAL fractions extracted from the biochars produced at 300 and 600  C were not sufficient for further use. Therefore, only the biochars charred at 450  C and their BDOM fractions were used in the following research. The mass recovery of HAL and RES extracted from the biochars produced at 450  C varied with the source materials (Fig. 1a and Table S1). PLABs gave higher weight yields of HAL than ANIBs, which were in the ranges of 61.7e63.1% and 4.4e45.0%, respectively. Notably, the mass recovery of HAL correlated positively and significantly with the OC content of the fresh biochar, while that of RES was inversely related to the biochar OC content (Fig. S2). This implies that with weathering, the majority of biochar's organic matter was more inclined to be present in the form of HAL materials. Consistently, except for MCH-derived organic matter fractions, the OC recovery of HAL was found to account for 45.5e69.3% of the total OC in the original biochars, obviously higher than that of RES (1.6e11.7%) (Fig. 1b and Table S1). It is thus expected that the large-scale use of biochar in soil remediation and the subsequent weathering would make a great contribution to HA content in soils. The remaining 29.1e68.1% could be attributed to the uncollected FAL fraction. 3.2. Chemical composition of the biochars and BDOM fractions The bulk and surface elemental composition of the original biochars and the BDOM fractions are given in Table 1. The OC content (31.0e53.6%) of the HAL fractions did not show remarkable differences among the feedstocks (Table 1) and was comparable to that of the soil-derived HA fractions (Kang and Xing, 2005). In contrast, obvious differences were found in the OC content of the RES fractions. The OC content of the RES fractions extracted from PLABs was more than 10 times that of RES derived from ANIBs (Table 1). Fig. 2 shows a van Krevelen diagram based on the samples in Table 1 and illustrates the compositional fields occupied by biomass, carboxyl-rich alicyclics, and separate plant components. The position of the HAL fractions was generally concentrated within the carboxyl-rich alicyclics zone. A previous study (Mao et al., 2012) also showed that HA molecules extracted from Amazonia Dark Earths, which have been formed from ancient inputs of biochar, are composed of ~6 fused aromatic rings substituted by COO groups. The changes in atomic H/C and O/C ratios (Fig. 2) indicate that with weathering, part of the biochar will be subject to oxidation and hydration and transformed into HAL materials. In contrast, the RES fractions occupied broad compositional fields in the diagram. The RES fractions of biochars are actually chunks of highly graphitized carbons that retained after HNO3 oxidation and HAL extraction. Accordingly, the chemical composition of RES is highly dependent on the chemical composition and stability of the pristine biochar. Therefore, the broad compositional fields of the RES fractions in the van Krevelen diagram could be attributed to their different parent biochars, which had great difference in chemical composition (Fig. 2) and stability as affected by the feedstock materials (Mia et al., 2017). In addition, it is noted that the

748

J. Jin et al. / Environmental Pollution 236 (2018) 745e753

Fig. 1. Mass recovery (a), organic carbon (OC) recovery (b), bulk (c) and surface (d) polarity, surface COO content measured using XPS (e), bulk COO content determined by 13C NMR (f), CO2-derived calculative surface area (CO2-SA) (g), and OC-normalized surface area (CO2-SA/OC) (h) of biochars, humic acid-like (HAL) fractions, and the residue (RES) materials after HAL extraction. Note that SWH, SMA, MSW, MCW, and MCH represent the biochars obtained from wheat straw, maize straw, and manures of swine, cow, and chicken, respectively.

HAL fractions contained more elemental N than the original biochars, which could be attributed to the formation of eNO2 groups introduced by HNO3 treatment (Mia et al., 2017). It is not unexpected that the HAL and RES fractions, except for RES fractions obtained from ANIBs, had remarkably higher bulk polarity ((O þ N)/C) than the original biochars (Fig. 1c). It is widely assumed that polarity of geosorbents can weaken their sorption for HOCs (Chefetz and Xing, 2009). Therefore, it is expected that the BDOM fractions would have much lower PHE sorption than the corresponding original biochars. Moreover, the bulk and surface polarity of the HAL fractions was found to be consistently higher than that of the corresponding RES fractions (Fig. 1c and d). Additionally, the fitting results of the high-resolution XPS C1s spectra (Table S2) neatly indicate that the oxidization of biochar and the subsequent alkali extraction concomitantly facilitate the formation of O-containing functional groups, particularly of surface COO groups. Moreover, the surface COO contents of the HAL fractions were estimated to be 4.6e9.8%, higher than or comparable with those of the RES fractions (2.0e5.5%) (Fig. 1e and Table S2). The high

surface COO content of the BDOMs would contribute greatly to cation exchange capacity (CEC) of biochar-treated soil. While other studies have previously proposed that the large CEC in Amazonian Dark Earths (Novotny et al., 2007) and Iowa Mollisols (Mao et al., 2012) is related to the high density of COO groups on the biochar residues, our work further indicates the predominant contribution of HAL to COO contents, and thus to the CEC of biochar-rich soils. Fig. S3 shows the quantitative 13C NMR spectra of the OC from the biochars and BDOM fractions. The spectra of the original biochars produced at 450  C are dominated by aromatic C peaks near 130 ppm (Fig. S3). It is well known that with increasing charring temperature, aromatic compounds become prevalent, while Ocontaining and aliphatic groups are lost (Keiluweit et al., 2010). The aromaticity of the HAL fractions was approximately 96.0%, slightly higher than that of the original biochars and RES fractions (Table S3). Moreover, in comparison with the original biochars, particularly noteworthy of the spectra of HAL and RES fractions is the signals (165e190 ppm) of carboxyl groups (Figs. 1f and S3), indicating a high degree of oxidation of the samples. The carboxyl

J. Jin et al. / Environmental Pollution 236 (2018) 745e753

749

Table 1 Bulk and surface elemental composition, surface area, and pore volume of the biochars, humic acid-like (HAL) fractions, and the residue (RES) materials after HAL extraction. Samples

SWH SMA MSW MCW MCH SWH-HAL SMA-HAL MSW-HAL MCW-HAL MCH-HAL SWH-RES SMA-RES MSW-RES MCW-RES MCH-RES

Bulk elemental composition

Ash (%)

Ca (%)

O (%)

N (%)

H (%)

C/N

O/C H/C (O þ N)/ C

70.2 74.4 33.7 29.5 9.8 53.2 53.6 51.9 52.6 31.0 52.0 52.1 4.7 1.4 2.8

12.9 11.8 10.2 4.1 3.6 31.8 31.5 30.0 28.9 20.5 27.9 29.0 1.4 0.0 0.1

0.5 1.0 2.6 1.4 0.5 3.2 3.5 6.6 4.9 3.6 2.6 3.0 0.4 0.1 0.0

4.3 3.8 2.6 1.0 0.9 3.2 3.1 3.3 3.0 2.6 3.1 3.2 0.3 0.2 0.1

178.0 85.9 15.3 24.8 21.6 19.6 17.8 9.1 12.5 10.1 23.5 20.2 13.0 19.4 94.8

0.14 0.12 0.23 0.10 0.28 0.45 0.44 0.43 0.41 0.50 0.40 0.42 0.22 0.01 0.04

0.73 0.61 0.91 0.39 1.12 0.73 0.69 0.76 0.67 1.02 0.72 0.74 0.90 1.38 0.64

0.14 0.13 0.29 0.15 0.33 0.50 0.50 0.54 0.49 0.60 0.44 0.47 0.30 0.07 0.05

12.2 9.1 50.9 68.1 85.2 8.6 8.2 8.2 10.7 42.2 14.4 12.7 93.2 98.3 96.9

Surface elemental composition (XPS) C (%)

O (%)

N (%)

Si (%)

(O þ N)/ Cb

68.7 73.7 48.5 49.0 40.2 63.6 64.7 73.2 62.9 34.4 63.2 65.6 10.1 7.0 9.4

17.7 16.0 25.7 24.6 36.0 27.1 28.5 25.7 29.6 36.6 26.6 26.0 45.8 48.2 47.1

2.3 2.1 4.6 6.6 3.5 3.0 4.0 0.0 0.0 4.1 3.3 3.5 0.0 0.0 0.0

11.4 8.3 12.3 14.1 20.3 1.0 1.2 1.1 3.0 13.7 1.9 0.8 30.0 32.6 31.0

0.09 0.17 0.14 0.17 0.07 0.26 0.30 0.17 0.24 0.32 0.22 0.12 0.13 0.14 0.13

CO2-SA (m2/g)c

CO2-SA/OC (m2/g)d

Micropore volume (cm3/g)

Micropore width (nm)

349.7 388.3 162.0 69.9 33.2 39.2 57.4 70.7 46.8 57.3 314.6 331.2 26.5 14.4 8.9

498.1 521.9 480.7 236.9 338.8 73.7 107.1 136.2 89.0 184.8 605.0 635.7 563.8 1028.6 317.9

0.101 0.111 0.046 0.019 0.010 0.015 0.020 0.025 0.017 0.022 0.082 0.087 0.009 0.005 0.003

0.418 0.418 0.479 0.480 0.573 0.573 0.822 0.548 0.822 0.822 0.479 0.479 0.479 0.822 0.822

Note that SWH, SMA, MSW, MCW, and MCH represent the biochars obtained from wheat straw, maize straw, and manures of swine, cow, and chicken, respectively. a Organic carbon content. b The surface (O þ N)/C ratios were estimated using the XPS data of surface functional groups listed in Table S2. c Surface area was determined using CO2 adsorption. d Organic carbon (OC)-normalized surface area.

signal centered at 168 ppm was thought to be attributed to the carboxyl groups attached to condensed aromatic rings (Mao et al., 2012). The carboxyl groups of HA extracted from soils were mainly bound to aliphatic and amide groups and resonated at 173 ppm (Fernandes et al., 2003). Furthermore, the HAL extracts consisted of 3.6% aliphatic C (Table S3). The narrow aliphatic C peak of HAL, centered at 20 ppm, was attributed to short-chain alkyl substituents on pyrogenic aromatic rings (Trompowsky et al., 2005). The relative poor spectral quality of the RES fractions isolated from ANIBs was due to the interference by the abundant minerals (Table 1). In this study, the 13C NMR spectra of the BDOM fractions were both characterized by aromatic and carboxyl structure (Fig. S3). It is noted that the biochars used in this study were produced at 450  C and mainly consisted of aromatic structure. Biochars produced at lower temperatures would contain large quantities of aliphatic C (Qiu et al., 2014). The HAL and RES fractions extracted from these biochars may differ greatly in

chemical composition, which needs further examination. The Boehm titration results of the RES fractions are listed in Table S4. Considering that the HAL fractions can dissolve in basic solutions, the amounts of acidic functional groups of the HAL fractions were not measured by Boehm titration. The contents of the surface carboxylic and phenolic group of the RES fractions were in the range of 0.21e0.38 and 0.18e0.22 mmol/g, respectively. It can be concluded that the surface carboxylic groups are present in a higher proportion in RES fractions. The surface acidic group content of the RES fractions was comparable to that of wood biochar (0.23 ± 0.003 mmol/g), which was produced at 450  C and pretreated with NaOH, and then with HCl (Tsechansky and Graber, 2014). In addition, the zeta potential results demonstrate that the BDOM fractions were negatively charged within the pH range of 5.0e7.5 (Fig. S4), implying that most of the surface carboxylic and phenolic groups were speciated in de-protonated forms. 3.3. Surface area and micropore volume of the biochars and BDOM fractions

Fig. 2. Van-Krevelen plot showing atomic H/C and O/C of biochars, humic acid-like (HAL) fractions, and the residue (RES) materials after HAL extraction.

Fig. S5 shows the CO2 adsorption/desorption isotherms of the biochars and their HAL and RES fractions. The pore-size distribution (Fig. S6) obtained from the isotherm indicates the presence of micropores smaller than 1.5 nm in the samples. The cumulative CO2SA and micropore volume of the original biochars are listed in Table 1 with their ranges of 33.2e388.3 m2/g and 0.010e0.111 cm3/ g, respectively. The extremely high CO2-SA and large total micropore volume strongly support the fact that the biochars have a nanoporous structure (Lehmann and Joseph, 2015). Moreover, OC was suggested to be a major contributor to CO2-SA of organic materials (Ahmad et al., 2014; Jin et al., 2015). The remarkably larger CO2-SA values of PLABs than those of ANIBs (Fig. 1g and Table 1) could be attributed to the higher OC contents of PLABs. Consistently, PLABs and ANIBs demonstrated comparable OC-normalized CO2-SA (CO2-SA/OC, Fig. 1h). The CO2-SA values of the PLAB-derived RES fractions were comparable to those of the original biochars (Table 1). However, the CO2-SA values of the PLAB-derived HAL samples were one order of magnitude lower than those of the original biochars (Table 1). It can be concluded that the micropores

750

J. Jin et al. / Environmental Pollution 236 (2018) 745e753

of the oxidized PLAB materials are mainly derived from the RES fractions. For ANIBs, their HAL and RES fractions both exhibited obviously lower CO2-SA values than the corresponding biochars (Table 1). HNO3 treatment may destruct micropores of biochar (Yakout et al., 2015), which could be responsible for the decreased CO2-SA values of the BDOM fractions. Inconsistently, previous € tter and Marschner, 2015; Jin et al., 2016) found that studies (Heitko the surface areas of biochars increased after incubation with soils. The different change trends of biochar surface areas may be attributed to the different aging methods used in the previous and present studies.

Moreover, the comparison of logKoc values (Ce ¼ 0.01Sw) of the biochars and BDOM fractions revealed that the BDOM fractions excluding MCW-HAL, MCH-HAL, and MCH-RES showed comparable or even higher logKoc values than the corresponding fresh biochars (Fig. S8). It must also be mentioned that the BDOMs generally exhibited superior sorption for PHE than the soil derived organic matters (Table S5) (Jin et al., 2017b; Sun et al., 2013a). The results not only indicate the promising future of biochar application in remediating PHE contaminated soil, but also imply that biochar addition and subsequent weathering could greatly affect the fate of PHE in biochar-rich soils.

3.4. Comparison of PHE sorption by biochars and BDOM fractions

3.5. The sorption mechanisms of PHE by biochars and BDOM fractions

PHE sorption isotherms for the biochars and BDOM fractions are presented in Fig. S7, and the sorption coefficients are listed in Table 2. All sorption isotherms of the biochars and BDOM fractions were highly nonlinear and were well fitted by the Freundlich model with R2 values of 0.96e1.00 (Table 2). The nonlinear coefficients (n) of the original biochars ranged from 0.43 to 0.53 (Table 2), while the logKoc values (Ce ¼ 0.01Sw) were in the range of 5.16e5.56 mL/g (Table 2). The PHE sorption coefficients of the tested biochars were comparable to those of the fresh and aged biochars reported in literature (Table S5) (Han et al., 2014; Ren et al., 2016; Sun et al.,  ska and Oleszczuk, 2015). 2011; Wu et al., 2013; Zielin The n values of the HAL and RES fractions were in the range of 0.45e0.67 and 0.46e0.60, respectively (Table 2). The data presented in Fig. S8 indicate that the fresh biochars exhibited significantly stronger sorption nonlinearity of PHE than the BDOM fractions, with the exceptions of SWH-RES, MSW-HAL, and MCWRES, implying that the BDOM fractions are more chemically homogeneous. This result was consistent with a previous study (Hale et al., 2011), which showed that aging treatment obviously weakened the sorption nonlinearity of pyrene by biochars. Additionally, the ANIBs exhibited significantly higher PHE logKoc values (Ce ¼ 0.01Sw) than the PLABs (Table 2 and Fig. S8). The surface C contents of ANIBs were higher than the corresponding bulk C contents (Table 1), indicating that the OM is likely concentrated on the surface of ANIB particles. This configuration would enhance the accessibility of ANIB sorption domains for PHE molecules, which could be responsible for to the higher PHE sorption of ANIBs.

The BDOM fractions generally contained more O-containing functional groups than the fresh biochars as evidenced by 13C NMR analysis (Table S3). These hydrophilic groups can easily attract water molecules to sorbent surfaces via H-bonding interactions, thereby hindering hydrophobic PHE molecules from approaching the sorption sites on sorbent surfaces (Sun et al., 2013b; Zhu and Pignatello, 2005). Therefore, it was expected that the BDOM fractions would have lower PHE sorption than the fresh biochars. Apparently, the bulk polar groups played minimal roles in the undiminished PHE sorption of BDOM fractions relative to fresh biochars. Notably, PHE logKoc values (Ce ¼ 0.01Sw) of all tested samples were positively correlated with their surface hydrophobic C content (C-C content measured using XPS, Fig. S9). Except for MCW-HAL and MCH-HAL, the BDOM fractions had comparable or higher surface C-C content than their corresponding fresh biochars (Table S2), which could account for the unexpected high PHE sorption by the BDOM fractions. This also implies that PHE sorption by the biochars and their BDOM fractions is dependent on their abundance of surface C-C domains. It has been proposed previously that nanopore-filling was the dominant mechanism for PHE sorption by biochars (Han et al., 2014; Yang et al., 2016). Contrary to expectation, a negative relationship between the PHE logKoc values of the fresh biochars and the micropore volumes was observed in this study (Fig. 3a). This negative relationship could be caused by the size exclusion effect, i.e., the PHE molecules were sterically excluded from a large portion

Table 2 Freundlich isotherm parameters and concentration-dependent distribution coefficients (logKd and logKoc) for phenanthrene (PHE) on the biochars, humic acid-like (HAL) fractions, and the residue (RES) materials after HAL extraction. Samples

KF

logKF

n

Na

R2

logKd (mL/g) Ce ¼ 0.01Sw

Ce ¼ 0.1Sw

Ce ¼ 1Sw

Ce ¼ 0.01Sw

Ce ¼ 0.1Sw

Ce ¼ 1Sw

SWH SMA MSW MCW MCH SWH-HAL SMA-HAL MSW-HAL MCW-HAL MCH-HAL SWH-RES SMA-RES MSW-RES MCW-RES MCH-RES

370.5 ± 33.0 c 424.1 ± 34.7 314.4 ± 42.7 380.0 ± 38.9 89.3 ± 9.1 181.8 ± 19.2 229.0 ± 31.3 807.3 ± 142.0 334.2 ± 64.7 121.8 ± 14.6 1100.3 ± 122.9 966.4 ± 119.5 52.1 ± 11.5 25.6 ± 4.6 2.3 ± 0.4

2.57 2.63 2.50 2.58 1.95 2.26 2.36 2.91 2.52 2.09 3.04 2.99 1.72 1.41 0.36

0.51 ± 0.015 0.43 ± 0.015 0.49 ± 0.024 0.48 ± 0.018 0.53 ± 0.017 0.65 ± 0.018 0.64 ± 0.022 0.45 ± 0.028 0.60 ± 0.031 0.67 ± 0.019 0.46 ± 0.018 0.49 ± 0.020 0.55 ± 0.039 0.47 ± 0.029 0.60 ± 0.030

20 20 20 20 20 18 17 16 18 20 19 19 18 20 18

0.99 0.99 0.98 0.99 0.99 1.00 0.99 0.97 0.98 0.99 0.99 0.99 0.96 0.97 0.98

5.06 5.03 4.97 5.03 4.45 4.89 4.98 5.33 5.11 4.74 5.47 5.45 4.25 3.85 2.94

4.57 4.46 4.46 4.51 3.98 4.53 4.63 4.78 4.71 4.41 4.93 4.93 3.80 3.32 2.54

4.08 3.89 3.96 3.99 3.51 4.18 4.27 4.22 4.31 4.09 4.39 4.42 3.36 2.79 2.15

5.21 5.16 5.44 5.56 5.47 5.16 5.25 5.61 5.39 5.25 5.76 5.73 5.58 5.70 4.50

4.72 4.59 4.93 5.04 4.99 4.81 4.90 5.06 4.99 4.92 5.21 5.21 5.13 5.16 4.10

4.23 4.02 4.43 4.52 4.52 4.45 4.54 4.51 4.59 4.59 4.67 4.70 4.69 4.63 3.70

logKocb (mL/g)

Note that SWH, SMA, MSW, MCW, and MCH represent the biochars obtained from wheat straw, maize straw, and manures of swine, cow, and chicken, respectively. a Number of data. b Koc is the organic carbon (OC)-normalized sorption distributed coefficient (Kd). c Standard deviation.

J. Jin et al. / Environmental Pollution 236 (2018) 745e753

of the biochar micropores, considering that the average micropore widths of the fresh biochars (0.418e0.573 nm, Table 1) are smaller than PHE molecular diameters (0.58 nm width and 0.78 nm length). The steric hindrance effect on biochar sorption has been suggested widely in previous studies (Yang et al., 2016; Zhu et al., 2005). On the other hand, the micropores (0.479e0.822 nm width) of the BDOM fractions were widened after HNO3 oxidation (Table 1 and Fig. S6), likely due to the decomposition of volatile organic fractions from carbon skeleton (Lian and Xing, 2017). As a result, the micropores of the BDOM fractions were more accessible to PHE molecules. Consistently, the PHE logKd values of the BDOM fractions were found to correlate positively with the micropore volumes (Fig. 3b). Additionally, the negative correlation of the logKoc values (Ce ¼ 0.01Sw) of PHE by the original biochars to their aromatic C contents was observed (Fig. 3c). This is a somewhat surprising finding, since one would expect at first view that, the biochar with a higher content of aromatic C should be better sorbent for PHE, due to more favorable hydrophobic and electron donor-acceptor (EDA) interactions (Park et al., 2013). The parameter of aromatic C in biochar sorption might be entangled with other structural or pore parameters that might reversely affect adsorption (Zhu and Pignatello, 2005). As mentioned above, the PHE molecules were excluded sterically from the biochar micropores due to steric hindrance effect. This may restrict PHE molecules to access the aromatic sorption domains at the same time, given that the micropores of biochars produced at 450  C were primarily contributed by the aromatic structures (Lian and Xing, 2017). In contrast, the HAL micropores with larger pore size were more accessible to PHE molecules. Therefore, PHE molecules can be more easily captured by the aromatic sorption domains of the HAL fraction. As expected, the PHE logKoc values of the HAL fractions correlated positively with the aromatic C contents (Fig. 3d), indicating that p-p EDA interactions regulated PHE sorption by the HAL fractions. Furthermore, the aforementioned dissociated carboxyl groups can donate

751

electrons to the attached benzoic rings. As a result, the p-p EDA interactions between PHE (p-donor) and the aromatic rings closer to the edges (p-donor) of HAL fractions are unlikely. PHE may be attracted to the electron-depleted regions of HAL fractions further away (Zhu and Pignatello, 2005). In addition, a previous study (Kang and Xing, 2005) showed the dominant role of aliphatic domains in PHE sorption by the soil-derived HA fractions. The difference between the two studies could be that the tested HA fractions were extracted from different sources. Consequently, the HA fractions used in the two studies had quite different chemical compositions. Soil-derived HA contained abundant aliphatic C (Kang and Xing, 2005), while the signals of aliphatic C of biocharderived HAL were negligible in the 13C NMR spectra (Fig. S3). 4. Conclusions In this study, the physicochemical properties and PHE sorption by the BDOM fractions isolated from oxidized biochars produced from various feedstocks were examined. Our results show that after chemical oxidation, large quantities of biochar organic matter were more inclined to be present in the form of HAL materials. It is thus expected that the large-scale use of biochar in soil remediation and the subsequent weathering would make a great contribution to HA content in soils. In addition, the BDOM fractions generally demonstrated comparable or even higher PHE sorption (logKoc) than the original biochars. The role of aromatic domains in biochar sorption was reversely affected by the micropore structure of the biochars. In contrast, aromatic domains regulated PHE sorption by the HAL fractions as indicated by the positive correlation between the PHE logKoc values and the aromatic C contents. These findings shed new light on the environmental behavior of HOCs in biocharamended soil. It should be pointed out that after being added into soils, biochar may be degraded through processes of photooxidation, solubilization, and biological utilization (Hale et al., 2011), which are hardly generated by the conditions provided by one-step

Fig. 3. Correlation between the logKoc values of phenanthrene (PHE) by biochars and the micropore volumes (a); correlation between the logKd values of PHE by the biocharderived organic matter (BDOM) fractions except for SWH-RES and SMA-RES and the micropore volumes (b); correlation between the logKoc values of original biochars excluding MSW and the aromatic C contents (c); and correlation between the logKoc values of humic acid-like (HAL) fractions and the aromatic C contents (d). Note that SWH, SMA, and MSW represent the biochars obtained from wheat straw, maize straw, and swine manure, respectively. RES denotes the residue materials after HAL extraction.

752

J. Jin et al. / Environmental Pollution 236 (2018) 745e753

HNO3 oxidation. The characteristics of the BDOM fractions formed during these processes remain to be explored in future research. Acknowledgements This research was supported by National Natural Science Foundation–Outstanding Youth Foundation (41522303), National Natural Science Foundation of China (41473087 and 41703097), China Postdoctoral Science Foundation (2016M600070), the Interdiscipline ResearcheFunds of Beijing Normal University, and the Fundamental Research Funds for the Central Universities (2017MS047). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.envpol.2018.02.015. References Ahmad, M., Rajapaksha, A.U., Lim, J.E., Zhang, M., Bolan, N., Mohan, D., Vithanage, M., Lee, S.S., Ok, Y.S., 2014. Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere 99, 19e33. Chefetz, B., Xing, B., 2009. Relative role of aliphatic and aromatic moieties as sorption domains for organic compounds: a review. Environ. Sci. Technol. 43, 1680e1688. Cheng, C.-H., Lehmann, J., Engelhard, M.H., 2008. Natural oxidation of black carbon in soils: changes in molecular form and surface charge along a climosequence. Geochem. Cosmochim. Acta 72, 1598e1610. € Bucheli, T.D., Jonker, M.T.O., Koelmans, A.A., van Cornelissen, G., Gustafsson, O., Noort, P.C.M., 2005. Extensive sorption of organic compounds to black carbon, coal, and kerogen in sediments and soils: mechanisms and consequences for distribution, bioaccumulation, and biodegradation. Environ. Sci. Technol. 39, 6881e6895. Fernandes, M.B., Skjemstad, J.O., Johnson, B.B., Wells, J.D., Brooks, P., 2003. Characterization of carbonaceous combustion residues. I. Morphological, elemental and spectroscopic features. Chemosphere 51, 785e795. 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. Gibson, C., Berry, T.D., Wang, R., Spencer, J.A., Johnston, C.T., Jiang, Y., Bird, J.A., Filley, T.R., 2016. Weathering of pyrogenic organic matter induces fungal oxidative enzyme response in single culture inoculation experiments. Org. Geochem. 92, 32e41. Hale, S.E., Hanley, K., Lehmann, J., Zimmerman, A., Cornelissen, G., 2011. Effects of chemical, biological, and physical aging as well as soil addition on the sorption of pyrene to activated carbon and biochar. Environ. Sci. Technol. 45, 10445e10453. 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, 11227e11234. Harvey, O.R., Myers-Pigg, A.N., Kuo, L.-J., Singh, B.P., Kuehn, K.A., Louchouarn, P., 2016. Discrimination in degradability of soil pyrogenic organic matter follows a return-on-energy-investment principle. Environ. Sci. Technol. 50, 8578e8585. €tter, J., Marschner, B., 2015. Interactive effects of biochar ageing in soils Heitko related to feedstock, pyrolysis temperature, and historic charcoal production. Geoderma 245e246, 56e64. Hiemstra, T., Mia, S., Duhaut, P.-B., Molleman, B., 2013. Natural and pyrogenic humic acids at goethite and natural oxide surfaces interacting with phosphate. Environ. Sci. Technol. 47, 9182e9189. Jin, J., Kang, M., Sun, K., Pan, Z., Wu, F., Xing, B., 2016. Properties of biochar-amended soils and their sorption of imidacloprid, isoproturon, and atrazine. Sci. Total Environ. 550, 504e513. Jin, J., Sun, K., Wang, Z., Han, L., Du, P., Wang, X., Xing, B., 2017a. Effects of chemical oxidation on phenanthrene sorption by grass-and manure-derived biochars. Sci. Total Environ. 598, 789e796. Jin, J., Sun, K., Wang, Z., Han, L., Pan, Z., Wu, F., Liu, X., Zhao, Y., Xing, B., 2015. Characterization and phthalate esters sorption of organic matter fractions isolated from soils and sediments. Environ. Pollut. 206, 24e31. Jin, J., Sun, K., Wang, Z., Yang, Y., Han, L., Xing, B., 2017b. Characterization and phenanthrene sorption of natural and pyrogenic organic matter fractions. Environ. Sci. Technol. 51, 2635e2642. Jin, J., Sun, K., Wu, F., Gao, B., Wang, Z., Kang, M., Bai, Y., Zhao, Y., Liu, X., Xing, B., 2014. Single-solute and bi-solute sorption of phenanthrene and dibutyl phthalate by plant-and manure-derived biochars. Sci. Total Environ. 473, 308e316. Kang, S., Xing, B., 2005. Phenanthrene sorption to sequentially extracted soil humic acids and humins. Environ. Sci. Technol. 39, 134e140. 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, 1247e1253. Knicker, H., 2011. Pyrogenic organic matter in soil: its origin and occurrence, its chemistry and survival in soil environments. Quat. Int. 243, 251e263. Lattao, C., Cao, X., Mao, J., Schmidt-Rohr, K., Pignatello, J.J., 2014. Influence of molecular structure and adsorbent properties on sorption of organic compounds to a temperature series of wood chars. Environ. Sci. Technol. 48, 4790e4798. Lehmann, J., 2007. A handful of carbon. Nature 447, 143e144. Lehmann, J., Joseph, S., 2015. Biochar for Environmental Management: Science, Technology and Implementation. Routledge. Lian, F., Xing, B., 2017. Black carbon (biochar) in water/soil environments: molecular structure, sorption, stability, and potential risk. Environ. Sci. Technol. 51, 13517e13532. Mao, J.-D., Johnson, R., Lehmann, J., Olk, D., Neves, E., Thompson, M., SchmidtRohr, K., 2012. Abundant and stable char residues in soils: implications for soil fertility and carbon sequestration. Environ. Sci. Technol. 46, 9571e9576. Mia, S., Dijkstra, F.A., Singh, B., 2017. Long-term aging of biochar: a molecular understanding with agricultural and environmental implications. Adv. Agron. 141, 1e51. Novotny, E.H., deAzevedo, E.R., Bonagamba, T.J., Cunha, T.J., Madari, B.E., Benites, V.d.M., Hayes, M.H., 2007. Studies of the compositions of humic acids from Amazonian dark earth soils. Environ. Sci. Technol. 41, 400e405. Park, J., Hung, I., Gan, Z., Rojas, O.J., Lim, K.H., Park, S., 2013. Activated carbon from biochar: influence of its physicochemical properties on the sorption characteristics of phenanthrene. Bioresour. Technol. 149, 383e389. Qiu, M., Sun, K., Jin, J., Gao, B., Yan, Y., Han, L., Wu, F., Xing, B., 2014. Properties of the plant- and manure-derived biochars and their sorption of dibutyl phthalate and phenanthrene. Sci. Rep. 4. https://doi.org/10.1038/srep05295. Qu, X., Fu, H., Mao, J., Ran, Y., Zhang, D., Zhu, D., 2016. Chemical and structural properties of dissolved black carbon released from biochars. Carbon 96, 759e767. Ravikovitch, P.I., Bogan, B.W., Neimark, A.V., 2005. Nitrogen and carbon dioxide adsorption by soils. Environ. Sci. Technol. 39, 4990e4995. Rechberger, M.V., Kloss, S., Rennhofer, H., Tintner, J., Watzinger, A., Soja, G., Lichtenegger, H., Zehetner, F., 2017. Changes in biochar physical and chemical properties: accelerated biochar aging in an acidic soil. Carbon 115, 209e219. Reisser, M., Purves, R.S., Schmidt, M.W.I., Abiven, S., 2016. Pyrogenic carbon in soils: a literature-based inventory and a global estimation of its content in soil organic carbon and stocks. Front. Earth Sci. 4, 1e14. Ren, X., Sun, H., Wang, F., Cao, F., 2016. The changes in biochar properties and sorption capacities after being cultured with wheat for 3 months. Chemosphere 144, 2257e2263. Rombol
a, A.G., Meredith, W., Snape, C.E., Baronti, S., Genesio, L., Vaccari, F.P., Miglietta, F., Fabbri, D., 2015. Fate of soil organic carbon and polycyclic aromatic hydrocarbons in a vineyard soil treated with biochar. Environ. Sci. Technol. 49, 11037e11044. Sander, M., Pignatello, J.J., 2005. Characterization of charcoal adsorption sites for aromatic compounds: insights drawn from single-solute and bi-solute competitive experiments. Environ. Sci. Technol. 39, 1606e1615. Shi, K., Xie, Y., Qiu, Y., 2015. Natural oxidation of a temperature series of biochars: opposite effect on the sorption of aromatic cationic herbicides. Ecotoxicol. Environ. Saf. 114, 102e108. Shindo, H., Honma, H., 1998. Comparison of humus composition of charred Susuki (Eulalia, Miscanthus sinensis) plants before and after HNO3 treatment. Soil Sci. Plant Nutr. 44, 675e678. Shindo, H., Nishimura, S., 2016. Pyrogenic organic matter in Japanese andosols: occurrence, transformation, and function. In: Guo, M., He, Z., Uchimiya, M. (Eds.), Agricultural and Environmental Applications of Biochar: Advances and Barriers, SSSA Special Publication 63. SSSA, Madison, WI, pp. 29e62. https:// doi.org/10.2136/sssaspec-pub63.2014.0036.5. Singh, B.P., Cowie, A.L., Smernik, R.J., 2012. Biochar carbon stability in a clayey soil as a function of feedstock and pyrolysis temperature. Environ. Sci. Technol. 46, 11770e11778. Sun, K., Jin, J., Kang, M., Zhang, Z., Pan, Z., Wang, Z., Wu, F., Xing, B., 2013a. Isolation and characterization of different organic matter fractions from a same soil source and their phenanthrene sorption. Environ. Sci. Technol. 47, 5138e5145. Sun, K., Kang, M., Zhang, Z., Jin, J., Wang, Z., Pan, Z., Xu, D., Wu, F., Xing, B., 2013b. Impact of deashing treatment on biochar structural properties and potential sorption mechanisms of phenanthrene. Environ. Sci. Technol. 47, 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, 5757e5763. Trompowsky, P.M., de Melo Benites, V., Madari, B.E., Pimenta, A.S., Hockaday, W.C., Hatcher, P.G., 2005. Characterization of humic like substances obtained by chemical oxidation of eucalyptus charcoal. Org. Geochem. 36, 1480e1489. Tsechansky, L., Graber, E.R., 2014. Methodological limitations to determining acidic groups at biochar surfaces via the Boehm titration. Carbon 66, 730e733. Wu, M., Pan, B., Zhang, D., Xiao, D., Li, H., Wang, C., Ning, P., 2013. The sorption of organic contaminants on biochars derived from sediments with high organic carbon content. Chemosphere 90, 782e788. Xing, B., Pignatello, J.J., 1997. Dual-mode sorption of low-polarity compounds in glassy poly (vinyl chloride) and soil organic matter. Environ. Sci. Technol. 31, 792e799. Yaghoubi, S.S., Berger, F., Gambhir, S.S., 2007. Studying the biodistribution of positron emission tomography reporter probes in mice. Nat. Protoc. 2, 1752e1755.

J. Jin et al. / Environmental Pollution 236 (2018) 745e753 Yakout, S.M., Daifullah, A.E.H.M., El-Reefy, S.A., 2015. Pore structure characterization of chemically modified biochar derived from rice straw. Environ. Eng. Manag. J. 14, 473e480. Yang, K., Yang, J., Jiang, Y., Wu, W., Lin, D., 2016. Correlations and adsorption mechanisms of aromatic compounds on a high heat temperature treated bamboo biochar. Environ. Pollut. 210, 57e64. Yoshizawa, S., 2016. Biochar for carbon storage in the soil and for soil improvement. Carbon 104, 263e264. Zhu, D., Kwon, S., Pignatello, J.J., 2005. Adsorption of single-ring organic compounds

753

to wood charcoals prepared under different thermochemical conditions. Environ. Sci. Technol. 39, 3990e3998. Zhu, D., Pignatello, J.J., 2005. Characterization of aromatic compound sorptive interactions with black carbon (charcoal) assisted by graphite as a model. Environ. Sci. Technol. 39, 2033e2041.  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.