Environmental Pollution 195 (2014) 84e90

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Characterization of nitrogen-rich biomaterial-derived biochars and their sorption for aromatic compounds Meng Zhang a, Liang Shu a, Xiaofang Shen a, Xiaoying Guo a, Shu Tao a, Baoshan Xing b, Xilong Wang a, * a b

Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 May 2014 Received in revised form 5 August 2014 Accepted 13 August 2014 Available online 6 September 2014

Biochars from nitrogen-rich biomaterials (i.e., a-amylase, chitin and zein) were produced at different temperatures (i.e. 170, 250, 350 and 450
C) and characterized, and their sorption for phenanthrene, naphthalene and 1-naphthol was investigated. The organic carbon content normalized-sorption coefficient (Koc) of the tested compounds by biochars increased with increasing charring temperature, attributed to the reduction of O-containing polar moieties especially the O-alkyl components, and the newly created aromatic carbon domains. The N-heterocyclic ring structure formed during charring process may enhance pep interactions between aromatics and the aromatic components in the resulting biochars. However, pep interactions did not dominate sorption of aromatics by N-rich biochars. Sorption of the tested compounds by N-rich biochars was predominantly controlled by the hydrophobic interactions between these chemicals and the aromatic components in biochars. Both N- and O-containing polar moieties at the biochar surfaces negatively affected their sorption for aromatics. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Sorption N-rich biochar Aromatic compound Interaction mechanism

1. Introduction Biochar is a type of carbon-rich material originating from pyrolysis or incomplete combustion of biomass that has extensive applications to soils (Bridgwater, 2003). It is not only used to improve soil quality and increase crop yield (Renner, 2007), but also contributes to reducing greenhouse gas emissions to the atmosphere (Lehmann, 2007). Importantly, biochar serves as a potential sorbent to remove organic contaminants and heavy metals, thus influencing their transport and biodegradation (Cao et al., 2009). Beesley et al. (2010) reported more than 50% reduction in freely dissolved concentrations of polycyclic aromatic hydrocarbons with 4e5 aromatic rings and over 40% reduction of those with 2e3 rings in a contaminated soil amended with hardwood-derived biochar over 60 d field-exposure compared to non-amended soils. Sorption enhancement of other chemicals such as volatile petroleum hydrocarbons and herbicides by soils and sediments after amending with biochar was also reported (Bushnaf et al., 2011; Spokas et al., 2009; Wang et al., 2010a). Sheng et al. (2005) found that 1% (wt) wheat-biochar contributed 80e86% of the overall sorption of

* Corresponding author. E-mail addresses: [email protected], [email protected] (X. Wang). http://dx.doi.org/10.1016/j.envpol.2014.08.018 0269-7491/© 2014 Elsevier Ltd. All rights reserved.

diuron and bromoxynil and 70% of ametryne by biochar-amended soil. Biochars generally have much higher sorption for hydrophobic organic contaminants (HOCs) than non-carbonaceous sorbents. Wang and Xing (2007) reported that the organic carbon contentnormalized sorption coefficient (Koc) of naphthalene by a cellulose-derived biochar generated at 250
C at 0.03 times of its water solubility reached 53,400 L/kg, whereas the corresponding value by cellulose was only 19 L/kg. Because of the high sorption of HOCs by biochars, such a process may significantly change their environmental behaviors. Therefore, a better understanding of the underlying mechanisms controlling sorption of HOCs to biochars is of great importance for predicting their environmental behaviors. A previous study showed that sorption of bisphenol A and 17aethinyl estradiol by several thermally and hydrothermally produced biochars was controlled by their aliphatic carbon contents (Sun et al., 2011a). Sun et al. (2011b) proposed that amorphous carbons were predominant sorption domains for sorption of fluridone and norflurazon by wood- and grass-origin biochars. Furthermore, it was documented that multiple interaction mechanisms such as hydrophobic interactions (Zhang et al., 2010), pep interactions (Wang et al., 2010b), hydrogen bonding (Lin and Xing, 2008) and electrostatic interactions (Chen et al., 2011) were responsible for HOC sorption by the carbonaceous sorbents such as activated carbons, carbon nanomaterials and black carbons. As for

M. Zhang et al. / Environmental Pollution 195 (2014) 84e90

sorption of HOCs to biochars, almost all previous studies focused on the biochars derived from biomaterials such as whole wood chips or carbon-rich biopolymers (Chun et al., 2004; Zhu et al., 2005). Nitrogen-rich biomass (e.g., amino acids and protein), as a very important component of plants and animal tissues, is ubiquitous in the environment. Because of the incidental or accidental forest fires or anthropogenic factors, they are often charred. However, to date, studies on sorption of HOCs by nitrogen-rich biomaterial-derived biochars are scarce. It is possible that the nitrogen-rich biomaterialderived biochars have different SA and porosity, as well as bulk and surface chemical composition with the regular ones due to their original feedstock difference (Fernandes et al., 2003), hence their sorption characteristics for HOCs and the mechanisms could be different. To help systematically understand sorption mechanisms of HOCs by biochars originating from diverse source biomaterials, it is indispensable to gain insight into the interaction mechanisms between HOCs and N-rich biomaterial-derived biochars. As a first attempt to address sorption mechanisms of HOCs by Nrich biochars, the key objectives of this work are: (1) to examine sorption behaviors of phenanthrene (Phen), naphthalene (Naph) and 1-naphthol (1-Naph) by biochars originating from N-rich biomaterials (i.e., a-amylase, chitin and zein) generated at four charring temperatures; (2) to explore influences of SA, polarity and chemical composition of aforementioned biochars as well as physicochemical properties of three aromatics on their interactions and the associated mechanisms; and (3) to identify the key mechanisms regulating sorption of aromatics to N-rich biochars. 2. Materials and methods 2.1. Sorbates and sorbents Phen, Naph and 1-Naph were used as sorbates. The reasons for choosing these chemicals are described in the Supplementary data. The 14C labeled and non-labeled of them were purchased from SigmaeAldrich Chemical Co., Ltd. Selected properties of these chemicals are listed in Table S1 in the Supplementary data. The a-amylase, chitin and zein were obtained from Tokyo Chemical Industries (Japan) and used as the original biomaterials. They were packed into the porcelain crucible covered with lid to limit the air supply and heated in a muffle furnace at different temperatures for 8 h to produce biochars. The biochars derived from a-amylase at 170, 250, 350 and 450
C were labeled as A170, A250, A350 and A450, respectively. Those derived from chitin and zein at the corresponding charring temperatures were marked as C170, C250, C350 and C450, as well as Z170, Z250, Z350 and Z450. 2.2. Sorbent characterization The bulk carbon, hydrogen and nitrogen contents of biochars were measured using a Vario EL Elemental Analyzer (Hanau, Germany) and oxygen content was calculated by mass balance. The carbon, nitrogen and oxygen contents as well as the abundance of carbon-based functionalities at biochar surfaces were determined with an Axis Ultra Imaging X-ray Photoelectron Spectrometer (XPS) (Kratos Analytical Ltd., UK). The assignments of specific functionalities are summarized in Table S2. Solid-state cross-polarization magic-angle-spinning and total-sidebandsuppression 13C NMR spectra of biochars were obtained using a Bruker Avance III 400 MHz NMR spectrometer (Germany) to collect their structural information. The NMR running parameters and chemical shift assignments are depicted elsewhere (Wang et al., 2007). To detect surface hydrophobicity of biochars in aqueous phase, their contact angles were measured with a contact angle measuring system (OCA20, Dataphysics, Germany). Details of contact angle measurement are summarized in the Supplementary data. The surface and pore characteristics of biochars were analyzed using an Autosorb-1-MP Surface Area Analyzer (Quantachrome, USA). Methods for SA and porosity determination are described in the Supplementary data. Fourier transform infrared (FTIR) spectra of all samples were obtained using KBr pellets with a VECTOR22 Spectrometer (Bruker, Germany) in the wave number ranging from 400 to 4000 cm1. 2.3. Batch sorption experiments Sorption isotherms were obtained using a batch equilibration technique. Stock solutions of 14C-labeled Phen, Naph and 1-Naph and non-labeled Phen and Naph were prepared with methanol, whereas the non-labeled 1-Naph was made with deionized water. Test solutions of various sorbates at different concentrations (0.05e0.9 times of water solubility of each compound) were prepared with their 14Clabeled and non-labeled stock solutions and the background solution which contained 0.01 M CaCl2 to maintain a constant ionic strength and 200 mg/L NaN3 to

85

inhibit bioactivity. They were shaken for approximately 1 h, and then added to the screw cap vials which contained appropriate amount of preweighed biochars until a minimum headspace was reached. The solid to solution ratio was adjusted to have 20e80% sorbate uptake at equilibrium. All samples and blanks were run in duplicate. To minimize the cosolvent effect, the methanol content introduced to the sorption systems was controlled to be less than 0.1% (v/v). All vials were shaken on a rotary shaker for 9 days, and such a period was demonstrated in our preliminary tests to be long enough to reach sorption equilibrium (Fig. S1 in the Supplementary data). Afterward, the vials were centrifuged at 4500 rpm for 30 min, and 2 mL supernatant was sampled and added to 4 mL cocktail to detect the 14C-activity of sorbate using a liquid scintillation counter (LS 6500, Beckman Coulter, USA). Since the mass loss of sorbate was negligible, its sorbed amount to the biochars was calculated by mass balance. Low mass loss of Naph during sorption experiments has been proved in previous studies (Piatt et al., 1996; Chen and Chen, 2009), due mainly to the very low headspace. To examine whether 1-Naph was ionized or not, the pH values in the test solution were measured, with details in the Supplementary data, and the results are presented in Table S3. The dissolved organic carbon (DOC) contents in the sorption systems were determined with a total organic carbon analyzer (TOC-VCPH, Shimadzu, Japan). Details for DOC determination are presented in the Supplementary data, and the results are summarized in Table S4. 2.4. Data analysis Sorption isotherm data were fitted with Polanyi, and logarithmic form of Freundlich models. Logarithmic form of Freundlich model: log Q ¼ n log Ce þ log Kf where Q (mg/kg) and Ce (mg/L) are, respectively, the equilibrium concentrations of sorbate in sorbed and liquid phases, Kf [(mg/kg)/(mg/L)n] is the sorption coefficient, and n is the nonlinearity index. Polanyi model:   log Q ¼ log Q0 r þ aðεsw =Vs Þb where Q0 is the maximum volume sorption capacity of sorbate (mm3/kg), r is the density of sorbate (g/mL); εsw ¼ RTln(Sw/Ce) is the sorption potential (J/mol), and Vs (mL/mol) is the molar volume of sorbate. R (8.314 J/mol K) is the universal gas constant, T (K) is absolute temperature, and Sw is the water solubility of sorbate (mg/ L). Both a and b are empirical parameters for isotherm data fitting.

3. Results and discussion 3.1. Sorption isotherms Sorption isotherms of Phen, Naph and 1-Naph by N-rich biochars are presented in Fig. 1. Isotherm data were well fitted with both Freundlich and Polanyi models as indicated by high R2 and low mean-weighted-square-error (MWSE) values (Tables S5 and S6). The equilibrium pH values in solution with various biochars ranged in 6.3e8.4, showing that 1-Naph was mostly present in the molecular form in the sorption systems (Table S3). The DOC content released from unit organic carbon mass of biochars during sorption process was less than 3% (Table S4). This implied that the released DOC to aqueous phase had little effect on sorption of the tested compounds by the biochars. The logKoc values of Phen, Naph and 1Naph by biochars were derived from Ce ¼ 0.3 Sw using Freundlich model and used for discussion. The logKoc values of the tested compounds by all biochars sharply increased as the charring temperature was increased from 170 to 250
C (Fig. S2 and Table S5). This was because the biochars produced at relatively low temperature (170
C) still largely retained the original organic structures with abundant hydrophilic functionalities. Much higher H/C ratios and lower aromatic carbon contents of the biochars produced at low temperature (e.g., 170
C) relative to those at high temperatures (e.g., 350 and 450
C) demonstrated that they were less carbonized and had lower aromatic components (Tables 1 and 2) (Chun et al., 2004; Chen et al., 2008). In addition, the biochars produced at 170
C had relatively low porosity and SAs, thus low sorption affinity for these compounds (Table 1). With increasing charring temperature, the H/C ratio decreased, indicating carbonization

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M. Zhang et al. / Environmental Pollution 195 (2014) 84e90

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▵

Fig. 1. Sorption isotherms of Phen (A), Naph (B) and 1-Naph (C) by N-rich biochars. The charring temperature: 170
C ( ), 250
C (◊), 350
C ( ), and 450
C (B). Blue symbols are for a-amylase-derived biochars, red ones are for chitin-derived biochars and black ones are for zein-derived biochars. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

enhancement of the biochars. Substantial pores and SAs were also created to provide new accessible sites for HOC sorption. Further analysis showed that logKoc values of Phen, Naph and 1-Naph by the biochars were positively correlated with their SAs and total porosity, suggesting that their sorption to the tested biochars was a surface-adsorption (both external and pore surfaces) dominant process (Figs. S3 and S4). To test if SA and porosity of biochars dominated sorption of the tested aromatics, the SA- and total porosity-normalized Koc values of a given compound by all biochars were calculated. It was found that the SA-normalized Koc values of Phen, Naph and 1-Naph by all sorbents varied by 1.90, 1.81 and 1.73 log units, respectively. Similarly, the total porosity-normalized Koc values of the corresponding chemicals by all biochars also varied by 2.65, 2.42, and 1.94 log units, respectively. This indicated that except for SA and porosity, other factors such as chemical composition and polarity of the biochars could jointly affect sorption of these chemicals. Otherwise, the SA or total porosity-normalized Koc values of a specific compound by all biochars should have been similar, inconsistent with our observation. For a-amylase- and zein-derived biochars, a sharp increase in aromatic carbon content along with a significant decrease in polar carbon content occurred from A170 to A250 as well as from Z170 to Z250, illustrating considerable hydrophobicity enhancement (Table 2). Sorption enhancement of Phen, Naph, and 1-Naph by the zein- and a-amylase-origin biochars occurred at the same temperature interval ranging from 170 to 250
C (Fig. S2). As the charring temperature was over 250
C, the logKoc values of a given

compound by these biochars did not change very much, mostly because their chemical composition only slightly altered over 250
C (Fig. S2 and Table 2). In comparison, the logKoc values of Phen, Naph and 1-Naph by chitin-derived biochars increased with increasing charring temperature from 170 to 350
C, and then slightly dropped as the temperature reached 450
C. This was because the chemical composition of chitin-derived biochars still exhibited evident changes after the charring temperature was increased from 250 to 350
C. 3.2. Roles of chemical composition of biochars in HOC sorption The biochars derived from the same biomaterial had substantial differences in 13C NMR spectra, which were highly charring temperature-dependent (Fig. S5). The NMR spectra of biochars derived at high temperatures (350 and 450
C) were dominated by a large aryl carbon resonance peak centered at 128 ppm, demonstrating formation of large quantities of aromatic structures. The aromatic carbon content of biochars originated from a given N-rich biomaterial increased and their H/C ratio decreased with increasing charring temperature (Tables 1 and 2), indicating the aromaticity enhancement. Such a trend was consistent with that of other biopolymer-derived biochars (Wang and Xing, 2007). The pep interactions between aromatics and the aromatic components in graphene sheets of wood biochars and biopolymer-derived biochars have been elucidated in previous studies (Wang and Xing, 2007; Zhu et al., 2005). For our case, significantly positive

Table 1 Bulk elemental composition, atomic ratio, surface area and porosity of the biochars. Biochar

A170 A250 A350 A450 C170 C250 C350 C450 Z170 Z250 Z350 Z450 a b c d

C (%)

43.3 50.0 48.7 51.3 43.9 47.0 72.3 67.0 52.8 62.7 61.8 68.9

H (%)

5.69 4.81 3.93 1.88 6.67 6.18 3.94 2.98 7.34 6.69 4.06 3.27

N (%)

7.00 7.52 7.52 6.76 6.46 7.19 10.59 12.22 14.96 14.84 12.55 13.12

O (%)

44.0 37.7 39.8 40.0 43.0 39.7 13.2 17.8 24.9 15.8 21.6 14.7

H/C

1.58 1.15 0.96 0.44 1.81 1.57 0.65 0.53 1.66 1.27 0.78 0.57

H/N

11.38 8.96 7.32 3.89 14.46 12.03 5.21 3.41 6.87 6.31 4.53 3.49

O/C

0.76 0.57 0.61 0.58 0.74 0.63 0.14 0.20 0.35 0.19 0.26 0.16

N/C

0.14 0.13 0.13 0.14 0.13 0.13 0.13 0.16 0.24 0.20 0.17 0.16

(O þ N)/C

0.90 0.69 0.75 0.70 0.86 0.76 0.26 0.36 0.60 0.39 0.44 0.32

Micropore volume determined by N2 sorption isotherm. Sum of the meso- and macropore volumes determined by N2 sorption isotherm. Total porosity, Vmic þ Vmes þ Vmac. Micropore volume determined by CO2 sorption isotherm.

SA (m2/g)

1.36 37.97 15.88 16.23 1.92 12.57 117.2 79.51 1.70 15.37 12.16 25.70

N2 isotherm at 77 K

CO2 isotherm at 273 K

Pore volume (cm3/g)

Cumulative pore volumed (cm3/g)

Vmica

Vmes þ Vmacb

Totalc

0 0.002 0 0.001 0.001 0.001 0.012 0.011 0.001 0 0 0.003

0.003 0.052 0.021 0.023 0.006 0.018 0.187 0.121 0.004 0.020 0.016 0.060

0.003 0.054 0.021 0.024 0.007 0.019 0.199 0.132 0.005 0.020 0.016 0.063

0.005 0.025 0.027 0.048 0.015 0.023 0.076 0.075 0.006 0.018 0.028 0.068

M. Zhang et al. / Environmental Pollution 195 (2014) 84e90 Table 2 Solid-state Biochar

A170 A250 A350 A450 C170 C250 C350 C450 Z170 Z250 Z350 Z450 a b c

13

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C NMR spectra data of the biochars. Distribution of C chemical shift (ppm) (%) 0e50

50e61

61e96

96e109

109e145

145e163

163e190

190e220

31.3 35.7 32.7 3.7 13.9 13.9 12.2 3.7 46.5 47.6 16.6 7.7

8.6 3.5 0.2 0.5 16.4 17.4 0.6 0.1 7.2 5.6 0.7 0.2

27.2 4.2 1.1 1.8 45.8 43.5 1 0.9 15.5 1.7 0.5 1.3

4.2 2.2 4.6 4.9 12.0 11.7 3.2 3.9 3.4 1.1 2.7 4.8

11.6 33.8 47.3 73.9 0.5 0.7 60.9 72.8 0.2 23.6 55.4 72.1

3.1 8.9 9.2 10.8 0.1 0.6 13.2 12.6 2.8 6.3 13.7 10.9

13.6 9.5 4.0 3.5 11.1 12.0 6.7 4.9 19.9 13.3 8.3 2.4

0.5 2.1 0.8 0.8 0.2 0.1 2.3 1.1 4.4 0.8 2.2 0.5

Hydrophobic Ca (%)

O-alkyl Cb (%)

Polar Cc (%)

42.9 69.5 80.0 77.6 14.4 14.6 73.1 76.5 46.7 71.2 72.0 79.8

40.0 9.9 5.9 7.2 74.2 72.6 4.8 4.9 26.1 8.4 3.9 6.3

57.1 30.5 20.0 22.4 85.6 85.4 26.9 23.5 53.3 28.8 28.0 20.2

Alkyl C (0e50 ppm) þ aromatic C (109e145 ppm). C region of 50e109 ppm. C region of O-alkyl C (50e109 ppm) and 145e220 ppm.

correlation between logKoc values of Phen, Naph and 1-Naph by all biochars and their aromatic carbon contents was observed (Fig. 2). This was because Phen, Naph and 1-Naph, as electron donors, were able to interact with the aromatic moieties in biochars through pep interactions where they worked as electron acceptors (Zhu and Pignatello, 2005; Wijnja et al., 2004). 1-Naph had an electron-donating group eOH in its structure, which may push electrons to the aromatic rings, leading to its p electron density on aromatic rings being the highest and that of Phen being the lowest among all tested aromatics. The H/N ratio of biochars derived from a given biomaterial decreased with increasing charring temperature, illustrating that the N-heterocyclic ring structure was possibly created during the charring process (Table 1). This can be further supported by the appearance of CeN stretching band at 1271 and 1383 cm1 for heterocyclic ring as shown in FTIR spectra of the sorbents (Fig. S6). Such a structure was able to interact with Phen, Naph and 1-Naph through pep interactions, resulting in significantly negative correlation between logKoc values of these compounds by all biochars and their H/N ratios (Fig. S7). If pep interactions dominated sorption of the tested compounds by the biochars, the logKoc values of these compounds by a given biochar should have followed the order of 1-Naph > Naph > Phen, which was exactly opposite to our experimental observation (Fig. S2). However, the logKoc values of Phen, Naph and 1-Naph by a given biochar followed the same order of their logKow values (Tables S1 and S5), noting that hydrophobic (van der Waals) interactions played a more important role in sorption of these compounds by Nrich biochars used in this work relative to the pep interactions. As shown in NMR spectra (Fig. S5), the signal intensity at 22 ppm for a-amylase-derived biochars was preserved at 170e350
C and vanished at 450
C, indicating the presence of long

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▵

methylene-chains during the charring process. For chitin- and zeinorigin biochars, the peak at 22 ppm was preserved when the charring temperature was increased from 170 to 250
C and sharply shrank to 350
C and almost disappeared until the temperature reached 450
C. A recent study emphasized the positive roles of alkyl components in sorption of phthalic acid esters to biochars derived at low heating temperature (300e400
C) (Sun et al., 2012a). For our case, the logKoc values of Phen, Naph and 1-Naph by all biochars were not closely correlated with their alkyl carbon contents (Fig. 2). For a-amylase- and zein-derived biochars, the 170
C and 250
C biochars had comparable alkyl carbon content, while the latter one had much higher logKoc values for Phen, Naph and 1-Naph (Tables 2 and S5). The logKoc values of these compounds by C350 were much higher than C250 and C170 although these three biochars had comparable alkyl carbon content. These observations revealed that sorption of Phen, Naph and 1-Naph by the tested biochars was not dominantly regulated by their alkyl carbon contents. Further analysis showed that A170, C170, C250 and Z170 also had higher O-alkyl carbon contents, the alkyl carbon moieties in these sorbents were most likely blocked by the surrounding O-alkyl components, thus decreasing the accessibility of aromatics to the sorption sites in alkyl carbon domains. Furthermore, the logKoc values of Phen, Naph and 1-Naph by all biochars were significantly and positively correlated with a sum of their alkyl and aryl carbon contents (Fig. 2). A comparison of the roles of alkyl carbon, aromatic carbon and a sum of alkyl and aromatic carbon domains in sorption of Phen, Naph and 1-Naph by the biochars indicated that, relative to the alkyl carbon domains, hydrophobic interactions between these chemicals and the aromatic components in the biochars played a more important role in their sorption.

Fig. 2. The relationships between logKoc values of Phen ( ), Naph ( ), 1-Naph (B) and the bulk specific parameters of biochars including aromatic carbon, alkyl carbon and alkyl þ aryl carbon content. The p value less than 0.01 or 0.05 indicates significant correlation. Here, S is the slope of the trend line.

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M. Zhang et al. / Environmental Pollution 195 (2014) 84e90

3.3. Roles of biochar polarity in HOC sorption The total polar carbon content of a-amylase- and zein-origin biochars declined as the charring temperature was increased from 170 to 250
C (Table 2). However, that of chitin-derived biochars did not change until the temperature was increased to 250
C and then steeply reduced to 26.9% at 350
C and to 23.5% at 450
C. This can be attributed to the higher activation energy of heatinduced alteration of chitin relative to a-amylase and zein. Zein and a-amylase are common proteins with higher contents of thermally labile amino acids and polypeptides (Wang et al., 2003). However, the acetylated side-chains in chitin had higher thermal stability and chitin consisted of more stable amino sugar polymer structure, which was illustrated by the peak at 55 ppm in NMR spectra (Tang et al., 2005). The total polar carbon content of all biochars was positively correlated with their O-alkyl carbon contents (p < 0.01), and both of these parameters generally decreased with increasing charring temperature (Fig. S8 and Table 2). This suggested that loss of O-containing moieties in the N-rich biomaterial-derived biochars during charring process was contributed mainly from the O-alkyl functionalities, and the bulk biochars tended to be more hydrophobic with increasing charring temperature. The O-containing polar moieties had negative influence on sorption of Phen, Naph and 1-Naph by the tested biochars, as indicated by the fact that the logKoc values of these compounds by all sorbents were inversely correlated with their total polar carbon and O-alkyl carbon contents (Fig. 3). This can be a result that the biochars with abundant O-containing functionalities were able to provide plenty of sites for water cluster formation via H-bonding, thus reducing accessibility of sorbate molecules to the hydrophobic sorption domains. The molecular simulation results further revealed that a slight increase in O-containing functional groups remarkably facilitated water cluster formation (Muller and Gubbins, 1998). Another point was that, as the O-containing moieties in the biochars increased, water molecules would more strongly compete for sorption sites, making biochars energetically less favorable for HOC sorption. Similarly, a negative correlation between the abundance of O-containing moieties and sorption of 2-methyl isoborneol by activated carbon was observed (Pendleton et al., 1997). However, Sun et al. (2012b) reported that the Koc values of fluridone and norflurazon increased with increasing polar carbon contents in biochars, which was because these two herbicides had many polar sites to form H-bonding with the O-containing moieties in biochars. Also, no correlation between logKoc values of Phen, Naph and 1-Naph by the biochars and their abundance of other polar functionalities (145e220 ppm in NMR spectra) was observed (Fig. 3). Such an observation demonstrated that the O-alkyl components of N-rich biochars had greater negative effect

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on accessibility of these chemicals to their hydrophobic domains in comparison with the phenolic, carboxylic and carbonyl groups. The bulk polarity index expressed as (O þ N)/C of the biochars generally reduced with increasing charring temperature, in agreement with the trend of their polar carbon content (Tables 1 and 2). The polarity decline during charring process resulted mainly from progressive loss of O-containing functionalities through dehydratation, decarboxylation and decarbonylation (Keiluweit et al., 2010). The a-amylase-origin biochars had relatively higher bulk (O þ N)/C values, however the logKoc values of Phen, Naph and 1Naph by these sorbents were higher than those derived from chitin and zein, making logKoc values of these compounds by all biochars did not significantly correlate with their (O þ N)/C values (Fig. S9). This was inconsistent with the significantly negative correlation between logKoc values of these compounds by the biochars and their total polar carbon contents (Fig. 3). Such a difference was not expected to be a result of the incorporation of Ncontaining moieties in polarity index calculation, because the bulk N/C ratios of all biochars were comparable (Table 1). Furthermore, the logKoc values of Phen, Naph and 1-Naph by all biochars were not correlated with their bulk O/C ratios (p ¼ 0.360, 0.205 and 0.243, respectively) (Fig. S9). One possible reason for the poor correlation was that the orientation of the CeO and CeN bonds in biochars was not considered when calculating their polarity index (O þ N)/C. Due to molecular geometry of the organic components or fragments in biochars, some dipole moment from CeO and CeN bonds at opposite directions would be offset. As a result, the (O þ N)/C value may fail to describe the real polarity of biochars in some circumstances. Such an effect was more pronounced for the a-amylaseorigin biochars relative to those from both chitin and zein. Based upon this, a-amylase-derived biochars were not included in the following discussion. Sorption data of Phen and Naph by celluloseorigin nitrogen free biochars derived under similar temperatures (i.e. 250, 320, and 400
C) through similar charring method from Wang and Xing (2007) was included for comparison. It was found that the logKoc values of Phen and Naph by the cellulose-origin biochars were higher than those derived from chitin and zein although they had comparable O/C values (Fig. S10). However, if we put these biochars together, the logKoc values of Phen and Naph by cellulose-, chitin- and zein-derived biochars were significantly and negatively correlated with their (O þ N)/C values (p < 0.01) and polar carbon contents (p < 0.01) (Fig. S10). This implied that both polar carbon content and the (O þ N)/C value of cellulose-, chitinand zein-derived biochars can be used as an index to describe their polarity although the cellulose-origin biochars were free of nitrogen. This also indicated that the inhibiting roles of N-containing polar functionalities in N-rich biochars in HOC sorption cannot be ignored.

Fig. 3. The relationships between logKoc values of Phen ( ), Naph ( ), 1-Naph (B) and the total polar carbon, O-alkyl carbon and other polar carbon content (145e220 ppm in 13C NMR spectra) of the biochars.

M. Zhang et al. / Environmental Pollution 195 (2014) 84e90

89

greater portion of hydrophobic domains would be at the surfaces of biochars and exposed to the aromatics in the sorption systems, thereby facilitating their sorption through hydrophobic and pep interaction mechanisms.

3.4. Influence of biochar surface properties on HOC sorption The XPS data showed that surface polarity index (O þ N)/C of all biochars was much lower than their respective bulk values (Tables 1 and 3), indicating their heterogeneous composition. Two possible reasons were proposed for the polarity difference between the surface and bulky particle: (i) volatile components produced during charring were condensed and trapped in pores (Bourke et al., 2007); and (ii) the polar moieties could be fixed in newly created aromatic domains and located at the interior part of biochars due to the random arrangement of aromatic components (Keiluweit et al., 2010). Based upon their heterogeneous composition, influence of the surface properties of biochars on Phen, Naph and 1-Naph sorption was examined. It was evident that the relative polarity index discrepancy between the bulk and surface of the aamylase-derived biochars was more pronounced than those derived from chitin and zein (Table 3). Substantial polar moieties in the inner space of a-amylase biochars exhibited relatively higher bulk polarity but less suppressed HOC sorption (Fig. S9). Significantly negative correlations between logKoc values of the tested compounds and the surface polarity index of all biochars were observed, while those for the bulk polarity index were much poorer (Fig. 4 and Fig. S9), suggesting that surface polar functionalities would more effectively block high-energy sites in the biochars and suppress sorption of aromatics. Furthermore, the N-, and O-containing polar moieties at the biochar surfaces were equally important for suppressing sorption of Phen, Naph and 1-Naph, as indicated by the significantly negative correlations between logKoc values of these compounds by all biochars and their surface O/C and N/C ratios (Fig. 4). The abundance of alkyl carbon components at the surfaces of all biochars was much higher than that of the bulky particles as shown by XPS and NMR data (Tables 2 and S2), implying that the alkyl carbon domains in biochars mostly resided at their surfaces, such that they were highly accessible to the sorbate molecules. Therefore, the logKoc values of Phen, Naph and 1-Naph by all biochars were significantly and positively correlated with their surface alkyl carbon contents, whereas they were poorly correlated with the bulk alkyl carbon content of these samples as stated before (Fig. 2 and Fig. S11). These observations clearly noted the roles of surface chemical composition of biochars in sorption of Phen, Naph and 1-Naph. The logKoc values of Phen, Naph and 1-Naph by all biochars were significantly and positively correlated with their contact angles (Fig. S12). Since contact angle is an index for describing the surface hydrophobicity of biochars at the solideliquid interfaces, an increase in contact angle implied that a

3.5. Sorption isotherm nonlinearity The Freundlich n is often used as an indicator to describe the sorption isotherm nonlinearity. The sorption isotherms of Phen, Naph and 1-Naph by N-rich biochars tended to be more nonlinear with increasing charring temperature, as evidenced by the reduction of Freundlich n values (Table S5), which can be attributed to the heterogeneous sorption sites in these biochars. Significant correlation between Freundlich n values and the microporosity of the sorbents indicated that the micropores produced during charring process were partially responsible for the isotherm nonlinearity (Fig. S13). As reported, the micropore could possess overlapping potential fields from neighboring pore walls, resulting in increased energies of interaction between sorbate and sorbent (Gregg and Sing, 1982). Sorption of a given compound in pores would thus give nonlinear isotherm. Qiu et al. (2014) also reported that abundant nanopores in the biochars that were derived at high charring temperature were responsible for strong nonlinear sorption of Phen. It was reported that the aromatic moieties in organic matter were responsible for the nonlinearity of sorption isotherm (Chefetz and Xing, 2009). In this study, the Freundlich n values were observed to be significantly and negatively correlated with the aromatic carbon content of N-rich biochars (Fig. S14). This can be a result of the increasing condensation degree of aromatic components during charring process (Keiluweit et al., 2010), as supported by the fact that the sorption isotherm nonlinearity of Naph and 1Naph by biochars derived from orange peels was positively correlated with their aromaticity expressed as H/C ratio (Chen and Chen, 2009). 4. Conclusions Given that N-rich biomass is ubiquitous in the environment and it would be charred due to forest fires or other processes. A better understanding of the physicochemical property changes of compositionally different N-rich biomaterials induced by charring and the associated impact on HOC sorption is important for elucidating their environmental processes as affected by interactions with N-rich biochars. Our observation showed that N-heterocyclic ring structure was formed during charring process of the N-rich biomaterials, which may enhance pep interactions between the

Table 3 Surface elemental composition, atomic ratio and contact angle of the biochars. Biochar

C (%)

N (%)

O (%)

N/C

O/C

(O þ N)/C

D (O þ N)/Ca

Contact angle (
) Mean

Std.

A170 A250 A350 A450 C170 C250 C350 C450 Z170 Z250 Z350 Z450

70.76 84.67 89.67 68.53 54.67 65.46 75.68 80.93 74.15 70.18 81.81 91.67

8.60 3.02 1.39 2.30 8.51 10.13 8.28 2.86 14.92 10.46 10.27 2.50

12.25 11.54 6.76 12.37 34.43 13.26 11.33 15.65 10.45 9.60 6.68 5.53

0.10 0.03 0.01 0.03 0.13 0.13 0.09 0.03 0.17 0.13 0.11 0.02

0.13 0.10 0.06 0.14 0.47 0.15 0.11 0.15 0.11 0.10 0.06 0.05

0.23 0.13 0.07 0.16 0.61 0.28 0.21 0.18 0.28 0.23 0.17 0.07

0.74 0.81 0.91 0.84 0.30 0.63 0.21 0.51 0.53 0.41 0.61 0.79

53.27 140.68 138.59 127.31 NDb 84.00 140.55 130.03 123.48 114.43 141.39 123.07

2.65 1.95 1.98 1.89

a b

D (O þ N)/C ¼ (bulk  surface)/bulk. Not detected.

2.72 2.09 3.74 2.18 4.50 2.78 3.37

90

M. Zhang et al. / Environmental Pollution 195 (2014) 84e90

▫

▵

Fig. 4. The relationships between logKoc values of Phen ( ), Naph ( ), 1-Naph (B) by all biochars and their surface polarity index including surface (O þ N)/C, O/C and N/C ratio.

aromatics and aromatic components in the resulting biochars. However, the pep interactions were not a dominant mechanism controlling sorption of Phen, Naph and 1-Naph by the tested biochars. Relatively more important role of aromatic carbon components, compared to the alkyl carbon ones in the N-rich biochars in sorption of aromatics through hydrophobic interactions may lead to a better understanding of their underlying interaction mechanisms. Results of this study are of great significance for predicting the environmental behaviors of aromatics and their remediation in polluted soils. Acknowledgments This study was supported by the National Natural Science Foundation of China (41271461, 41328003, 41390240 and 41130754), 973 Program (2014CB441101), 111 Program (B14001) and the Startup Fund for the Peking University 100-Talent Program. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.envpol.2014.08.018. References nez, E., Gomez-Eyles, J.L., 2010. Effects of biochar and Beesley, L., Moreno-Jime greenwaste compost amendments on mobility, bioavailability and toxicity of inorganic and organic contaminants in a multi-element polluted soil. Environ. Pollut. 158, 2282e2287. Bourke, J., Manley-Harris, M., Fushimi, C., Dowaki, K., Nunoura, T., Antal, M.J., 2007. Do all carbonized charcoals have the same chemical structure? 2. A model of the chemical structure of carbonized charcoal. Ind. Eng. Chem. Res. 46, 5954e5967. Bridgwater, A.V., 2003. Renewable fuels and chemicals by thermal processing of biomass. Chem. Eng. J. 91, 87e102. Bushnaf, K.M., Puricelli, S., Saponaro, S., Werner, D., 2011. Effect of biochar on the fate of volatile petroleum hydrocarbons in an aerobic sandy soil. J. Contam. hydrol. 126, 208e215. Cao, X.D., Ma, L.N., Gao, B., Harris, W., 2009. Dairy-manure derived biochar effectively sorbs lead and atrazine. Environ. Sci. Technol. 43, 3285e3291. Chefetz, B., Xing, B.S., 2009. Relative role of aliphatic and aromatic moieties as sorption domains for organic compounds: a review. Environ. Sci. Technol. 43, 1680e1688. Chen, B.L., Chen, Z.M., 2009. Sorption of naphthalene and 1-naphthol by biochars of orange peels with different pyrolytic temperatures. Chemosphere 76, 127e133. Chen, B., Zhou, D., Zhu, L., 2008. Transitional adsorption and partition of nanpolar and polar aromatic contaminants by biochars of pine needles with different pyrolytic temperatures. Environ. Sci. Technol. 42, 5137e5143. Chen, X., Xia, X.H., Wang, X.L., Qiao, J.P., Chen, H.T., 2011. A comparative study on sorption of perfluorooctane sulfonate (PFOS) by chars, ash and carbon nanotubes. Chemosphere 83, 1313e1319. Chun, Y., Sheng, G.Y., Chiou, C.T., Xing, B.S., 2004. Compositions and sorptive properties of crop residue-derived chars. Environ. Sci. Technol. 38, 4649e4655. 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. Gregg, S.J., Sing, K.S.W., 1982. Adsorption, Surface Area, and Porosity, second ed. Academic Press, New York.

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