Impact of Deashing Treatment on Biochar Structural Properties and

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Impact of Deashing Treatment on Biochar Structural Properties and Potential Sorption Mechanisms of Phenanthrene Ke Sun,*,† Mingjie Kang,† Zheyun Zhang,‡ Jie Jin,† Ziying Wang,† Zezhen Pan,† Dongyu Xu,† Fengchang Wu,§ and Baoshan Xing∥ †

State Key Laboratory of Water Simulation, School of Environment, Beijing Normal University, Beijing 100875, China Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08544, United States § State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China ∥ Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States ‡

S Supporting Information *

ABSTRACT: Knowledge of the mineral effects of biochars on their sorption of hydrophobic organic contaminants (HOCs) is limited. Sorption of phenanthrene (PHE) by plant-residue derived biochars (PLABs) and animal waste-derived biochars (ANIBs) obtained at two heating treatment temperatures (HTTs) (450 and 600 °C) and their corresponding deashed biochars was investigated. The decreased surface polarity and increased bulk polarity of biochars after deashing treatment indicated that abundant minerals of biochars benefit external exposure of polar groups associated organic matter (OM). Organic carbon (OC)normalized distribution coefficients (Koc) of PHE by biochars generally increased after deashing, likely due to enhancement of favorable and hydrophobic sorption sites caused by mineral removal. Positive correlation between PHE log Koc by PLABs and bulk polarity combined with negative correlation between PHE log Koc values by ANIBs and surface polarity suggested PLABs and ANIBs have different sorption mechanisms, probably attributed to their large variation of ash content because minerals influenced OM spatial arrangement within biochars. Results of this work could help us better understand the impact of minerals, bulk/surface polarity, and sorption domain arrangement of biochars on their HOCs sorption and predict the fate of HOCs in soils after biochar application.

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nonlinear adsorption of HOCs.12,14,17 Though sorption characteristics of biochars have been addressed in these elegant studies, it is worth noting that most sorption studies for HOCs, to date, have used low-mineral biochars produced from relatively pure plant residues. Even though some biochars contain considerable ash (e.g., grass biochars), the sorption properties of the ash component have been little considered.5,14 For sorption of HOCs, it is reasonable to assume that the organic matter (OM) would still dominate the sorption of mineral-rich biochars; however, the presence of minerals is likely to have at least a secondary influence on the sorption of HOCs.5 With the addition of biochar to soil, it is expected that biochars interact with minerals of soil and the interaction should also affect the sorption of HOCs. Moreover, several previous studies have reported that plentiful portions of biochar have been found in the organo-mineral fraction of soil,19−21

INTRODUCTION Biochar is produced as a soil amendment for agricultural and environmental gain.1 Sorption is a vital process controlling the mobility and bioavailability of hydrophobic organic compounds (HOCs) in soil amended with biochar.2−7 Therefore, the sorption properties of biochars have been studied extensively in the past decade in response to the growing awareness of the biochar importance to the overall sorption properties of soils6,8−11 Recently, several studies focused on how variations in the bulk properties of plant-derived biochars influence their sorptive characteristics for HOCs.12−14 The sorption strength of biochars is reported to be as a function of their physical and chemical properties,12,14,15 which vary dramatically with pyrolysis condition, heat treatment temperature (HTT), and feedstock sources.12,14,16,17 HTT is a key factor that greatly impacts the properties of biochar (development of surface area and aromaticity resulting from graphitization of the source material)12,17 and its sorption properties for HOCs.4,12,14,18 Biochars produced at low HTTs are not all carbonized while those obtained at relative high HTTs are well-carbonized and provide large surface area (SA) and considerable micropores for © 2013 American Chemical Society

Received: Revised: Accepted: Published: 11473

December 11, 2012 September 7, 2013 September 11, 2013 September 11, 2013 dx.doi.org/10.1021/es4026744 | Environ. Sci. Technol. 2013, 47, 11473−11481

Environmental Science & Technology

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materials, including three plants straws of rice, wheat, and maize and three animal manures of chicken, swine, and cow, were selected to obtain PLABs and ANIBs, respectively. After washing and grinding to obtain a particle size of less than 1.5 mm, these feedstocks were charred at 450 and 600 °C, respectively, for 1 h in a closed container under oxygen-limited conditions in a muffle furnace. The HTTs were raised to the desired values (450 and 600 °C) at a ramp rate of 10 °C/min. Then the biochars were washed with 0.1 M HCl followed by ionized water flushing till neutral pH,33 subsequently ovendried at 105 °C, and gently milled to pass a 0.25 mm sieve (60 mesh) prior to further analysis as the original biochars. A 0.1 M HCl was selected to wash biochars in order to decrease pH values of biochars and also remove some nutrients (soluble salts and potassium compounds), carbonates, and dissolved organic matter (DOM), which could prevent other factors mainly including pH values and DOM of biochars from impacting PHE sorption by biochars and better focus on investigating the influence of mineral within biochars on PHE sorption. To obtain the deashed biochars and investigate the effect of minerals on sorption for HOCs, all biochars were demineralized by adding 1 M HCl and 10% (v/v) HF at a 1:5 solid/liquid ratio and shaking at 40 °C and 140 rpm for 5 d.31 Then, the supernatant was removed after centrifugation at 4500 rpm for 30 min. The same treatment was repeated for six times to get deashed samples with adequate content of organic carbon (OC). Then, the deashed samples were freeze-dried. Original biochar samples were hereafter abbreviated and referred as to their individual two initial capitals of feedstock source (plant residues: rice straw, wheat straw, and maize stalk; animal-waste: manures of chicken, swine, and cow) (i.e., RI, WH, MA, CH, SW, and CO) and HTTs (450 and 600 °C) (i.e., RI450, RI600, WH450, WH600, MA450, MA600, CH450, CH600, SW450, SW600, CO450, and CO600). Their deashed biochars were accordingly named as DRI450, DRI600, DWH450, DWH600, DMA450, DMA600, DCH450, DCH600, DSW450, DSW600, DCO450, and DCO600, respectively. The biochars produced at low and high temperature (450 and 600 °C) were named as LTBs and HTBs, respectively, and their deashed counterparts were denoted as DLTBs and DHTBs accordingly. Characterization of Biochars. The bulk C, H, N, and O content of the biochars was determined by an Elementar Vario ELIII elemental analyzer via complete combustion. Therefore, bulk elemental composition obtained from elemental analyzer represented the C, H, N, and O content per mass of biochar, which was measured in duplicate to obtain the average data. Ash content of the biochars was measured by heating samples at 750 °C for 4 h.34 Fourier transform infrared (FTIR) spectroscopy spectra were recorded by a Nexus 670 FTIR spectrophotometer (Thermo Nicolet Corporation, US). The detailed characterization procedure was available in the Supporting Information. The soild-state cross-polarization magic-angle-spinning 13C nuclear magnetic resonance (13C NMR) spectra of original biochars after being ground were obtained using a Bruker Avance 300 NMR spectrometer (Karlsruhe, Germany) for structural composition. The NMR running parameters are available in the Supporting Information and chemical shift assignments are depicted elsewhere.35 X-ray diffraction (XRD) patterns were recorded on a X’Pert PRO MPD diffractometer (PAN-alytical, Holland) using Cu Kα radiation. The particle structure and surface topography of biochars were investigated using scanning electron microscope (SEM) imaging analysis16 with a Hitachi S4800 scanning

suggesting that biochars may indeed have interactions with minerals. We hypothesize minerals (or ash) within high-mineral biochars affect the sorption of HOCs in addition to the dominant role of OM. Sorption behavior of high-mineral biochars in comparison to that of low-mineral biochars, which have been more commonly investigated, is an important topic of the ongoing research to determine the role of mineral in the sorption of HOCs by biochars. The effect of OM−mineral interactions on sorption of HOCs by soil and sediment and their associated mechanisms have been investigated extensively.22−25 Wang and Xing26 observed that the Koc of adsorbed humic acids (HA) for phenanthrene (PHE) on the clay−HA complexes could be up to several times higher than that of the source HA, which was inconsistent with the previous result that the Koc of PHE by the dissolved HAs was several orders of magnitude higher than that to the clayassociated HA.22 Yang et al.25 reported different changes in surface polarity of the original and deashed HAs and argued that humins (HMs) can be ascribed to the distinct interaction between OM and minerals. Additionally, conformation change of OM resulted from the interactions with minerals was observed to be another factor governing HOCs sorption.27−29 Also, the microscaled domain arrangement of OM was potentially altered after interacting with mineral or when minerals were removed.25,30 However, the effects of minerals within biochars and the spatial arrangement of OM affected by the presence of minerals on the sorption of HOCs are not clear. Additionally, the difference of bulk and surface elemental contents (e.g., C, O, and N) of SOM samples has been reported.25,31,32 Further, the considerable influence of the surface polarity of soil OM on the sorption of PHE has been identified.25 To our knowledge, surface compositions of biochars have not been systematically investigated. The influence of mineral on the surface properties of biochars and the roles of the surface polarity in HOCs sorption are also poorly understood. Therefore, it is essential to investigate the surface and bulk compositions of biochars simultaneously in order to reveal the mechanisms of HOCs sorption by biochar. Therefore, the major objectives of this study are to investigate the impact of minerals on sorption of PHE by biochars and identify the importance of surface/bulk polarity and spatial arrangement of OM of biochars as well as their other structural components (e.g., aromaticity or aliphaticity) in the PHE sorption in order to understand the sorption mechanisms. To meet these objectives, high-mineral biochars from animal-waste (ANIBs) and low-mineral biochars from plant-residue (PLABs) are produced at low and high HTTs (450 and 600 °C, respectively). The influence of mineral on sorption of HOCs by biochars will be understood through investigating the change of PHE sorption properties of lowmineral PLABs and high-mineral ANIBs after deashing treatment. The bulk and surface structural characteristics (polarity, aromaticity, SA or porosity) of PLABs and ANIBs along with their deashed counterparts are analyzed and to interpret how minerals affect the sorption of HOCs by biochars and present the underlying mechanisms.

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MATERIALS AND METHODS Chemicals and Sorbents. PHE, a typical representative of PAHs, was purchased from Sigma-Aldrich Chemical Co. The flowchart of producing the original biochars and their corresponding deashed biochars is shown in Supporting Information Figure S1. Basically, two kinds of feedstock 11474

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Table 1. Bulk and Surface Elemental Composition, Atomic Ratio, Ash Content, and CO2 Surface Area (CO2-SA) samples

N (%)

C (%)

H (%)

O (%)

H/C

(O + N)/Ca

ash (%)

CO2-SA

C (%)c

O (%)c

N (%)c

Si(%)c

(O + N)/Cc

RI450 RI600 DRI450 DRI600 WH450 WH600 DWH450 DWH600 MA450 MA600 DMA450 DMA600 CH450 CH600 DCH450 DCH600 SW450 SW600 DSW450 DSW600 CO450 CO600 DCO450 DCO600

0.83 0.83 1.17 1.14 0.46 0.42 0.66 0.45 1.01 0.99 1.29 1.16 0.53 0.33 1.40 0.97 2.57 2.46 3.53 2.76 1.39 1.11 2.25 1.90

57.9 59.3 72.5 80.4 70.2 77.8 74.1 83.9 74.4 79.9 78.8 84.2 9.8 8.7 26.1 22.6 33.7 35.6 43.4 42.8 29.5 30.7 51.4 50.5

3.31 2.34 3.71 2.53 4.28 3.08 3.51 2.65 3.81 3.71 3.31 2.36 0.91 0.50 2.24 1.42 2.55 1.79 2.77 2.44 0.95 0.46 2.76 1.93

11.8 5.5 15.4 7.4 12.9 5.3 15.2 5.9 11.8 5.6 14.0 6.9 3.6 1.5 15.9 12.0 10.2 7.9 9.9 6.8 4.1 1.2 14.6 10.0

0.69 0.47 0.61 0.38 0.73 0.48 0.57 0.38 0.61 0.56 0.50 0.34 1.12 0.68 1.03 0.76 0.91 0.60 0.77 0.69 0.39 0.18 0.64 0.46

0.16 0.08 0.17 0.08 0.14 0.06 0.16 0.06 0.13 0.06 0.15 0.07 0.33 0.16 0.50 0.44 0.29 0.23 0.24 0.17 0.15 0.06 0.25 0.18

26.2 32.0 7.2 8.6 12.2 13.4 6.6 7.2 9.1 9. 2.6 5.4 85.2 89.0 54.5 63.1 50.9 52.3 40.4 45.2 68.1 71.2 28.9 35.7

293.4 390.6 ndb nd 349.7 499.2 nd nd 388.3 513.4 nd nd 33.2 38.4 nd nd 162.0 206.1 nd nd nd nd nd nd

63.4 66.1 74.8 82.5 68.7 77.3 79.3 82.3 73.7 74.7 79.2 82.7 40.2 39.7 26.5 32.0 48.5 48.8 36.4 36.2 49.0 49.5 44.2 51.3

21.5 21.1 17.2 14.2 17.7 16.7 18.2 13.6 16.0 18.1 17.2 14.3 36.0 36.1 11.6 11.1 25.7 26.1 10.9 10.0 24.6 25.6 13.3 8.9

3.14 1.17 2.83 1.63 2.27 1.13 1.00 1.50 2.06 1.46 1.56 1.43 3.50 2.15 2.02 2.41 4.55 3.11 2.70 2.18 6.55 2.44 2.26 2.40

11.96 11.63 2.40 1.67 11.35 4.88 1.59 1.55 8.28 5.79 2.00 1.58 20.32 16.78 4.23 4.08 12.32 4.20 2.27 1.99 14.05 12.27 3.79 3.61

0.30 0.25 0.21 0.15 0.22 0.17 0.18 0.14 0.19 0.20 0.18 0.14 0.74 0.73 0.39 0.33 0.48 0.46 0.29 0.26 0.49 0.43 0.27 0.17

a

Polarity index. bNot detected. cSurface. Note that RI, WH, MA, CH, SW, and CO represent the biochars obtained from rice, wheat and maize, and manures of chicken, swine, and cow, respectively. D denotes deashing treatment for the biochars; The bulk elemental composition of the biochars were determined with elemental analyzer and the surface elemental composition of the biochars were measured with XPS; CO2-SA, surface area obtained from CO2 adsorption isotherm. The polarity index ((O + N)/C) of organic matter in the bulk and at the surface of individual samples was calculated from the atomic ratio of (N + O) and C.

mg/L NaN3 as a biocide. The initial concentrations ranged from 2 to 1100 μg/L, which was chosen to cover the range between detection limit and aqueous solubility (1.12 mg/L). The volume ratio of methanol to water was controlled below 0.001 to avoid the cosolvent effect. The test solutions were homogenized by shaking at 100 rpm for approximately 1 h. Finally, they were added to the vials with appropriate amount of sorbents (0.1−8 mg), which was controlled to achieve 20− 80% uptake of initially PHE at equilibrium. Headspace of vials was kept minimal to reduce solute vapor loss. Control experiments were run concurrently. On the basis of the preliminary tests, apparent sorption equilibrium was reached around 10 d according to the sorption kinetics of PHE (Supporting Information Figure S2). After being shaken on the rotary shaker for 10 d at room temperature, all vials were placed upright for 24 h. About 2 mL supernatant was withdrawn from each sample and added to the 2 mL vial for analyzing the sorbate concentration in solution phase with HPLC (1260 Series, Agilent Technolohies, Santa Clara, CA) equipped with a reversed-phase C18 analytical column (5 μm, 4.6 mm × 250 mm) with a diode array detector for concentrations ranging from 50 to 1100 μg/L and a fluorescence detector for concentrations from approximately 0.5 to 50 μg/L because the fluorescence detector was unable to measure the high concentrations of PHE accurately.40 The mobile phase was 90:10 (v:v) of methanol and deionized water. All samples, along with blanks, were measured in duplicate. Because sorption by the vials was insignificant as shown from the analysis of blank experiments, uptake of sorbate by the biochar was calculated by mass balance assuming no other losses.

microscope (Japan), and surface elemental analysis was also conducted simultaneously with the SEM at the same surface locations using energy dispersive X-ray spectroscopy (EDS, Horiba7953-H EDS spectrometer), which provides semiquantitative analysis of the distribution of elements with a sampling depth of 1−2 μm.36 The detailed procedures of XRD and SEM-EDS as well as the result of characterization were described in the Supporting Information. Specific elemental composition (e.g., C, O, N, and Si) of the top surface layer for biochars (depth: 3−5 nm)37 was examined using X-ray photoelectron spectroscopy (XPS) with a Kratos Axis Ultra electron spectrometer using monochromated Al Kα source operated at 225 W, and more detailed information of characterization is described elsewhere.25,32 The CO2-SA of the original biochars was obtained by gas adsorption using an Autosorb-1 gas analyzer (Quantachrome Instrument Corp., Boynton Beach, FL) using CO2 isotherm at 273 K because previous studies show that N2 at 77 K was unable to detect black carbon (BC) microporosity while CO2 at 273 K can enter the micropores (0−1.4 nm).38,39 The CO2-SA of micropores was calculated using nonlocal density functional theory (NLDFT) and grand canonical Monte Carlo simulation (GCMC).11 Sorption Experiments. The batch sorption experiments were conducted using 40 and 60 mL glass vials capped with Teflon-lined screw. PHE was dissolved in methanol as stock solution and stored at 4 °C in the dark. Then, the stock solution was diluted sequentially to 10 concentrations distributed evenly on a log scale using solution containing 0.01 M CaCl2 to maintain a constant ionic strength and 200 11475

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Data Analysis. The sorption data were fitted with Freundlich model (logarithmic form): log qe = log KF + n log Ce

(1)

where qe is the solid-phase concentration (μg/g), Ce is the solution phase concentration (μg/L), KF is the sorption affinityrelated parameter ((μg/g)/(μg/L)n), and n is the nonlinear coefficient. The sorption distribution coefficient (Kd) and the OCnormalized Kd (Koc) were calculated by the eqs 2 and 3.

Kd = qe /Ce

(2)

Koc = Kd /foc

(3)

where foc is OC content. The log Kd and log Koc values (Ce = 0.01, 0.1, and 1Sw, water solubility of PHE) of original and deashed biochars were calculated according the above equations. The investigated correlations among properties of sorbents as well as their sorption coefficients of PHE (Pearson correlation coefficients: P and r values) were obtained from the Pearson correlation analysis by SPSS 16.0 software (SPSS Inc., USA).

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Figure 1. Comparison of bulk or surface C (a) and O (b) and polarity (c) between these original biochars (plant residue-derived biochars (PLABs): RI450, RI460, WH450, WH600, MA450, and MA600 and animal waste-derived biochars (ANIBs): CH450, CH600, SW450, SW600, CO450, and CO600) and their corresponding deashed biochars. (RI, WH, MA, CH, SW, and CO refer to the biochars produced from rice, wheat, maize, chicken, swine, and cow, and 450 and 600 °C represents pyrolysis or heat treatment temperature (HTT) to obtain the low- and high-temperature biochars.).

RESULTS AND DISCUSSION Characterization of Original Biochars. The elemental composition, ash content, atomic ratio, and surface area of original and deashed PLABs and ANIBs at two different HTTs (i.e., 450 and 600 °C) are listed in Table 1. The C content of the biochar increased with increasing HTT, while H and O content decreased with increasing HTT, indicating an increasing degree of carbonization of chars following dehydration, decarboxylation, and decarbonylation during pyrolysis.17 The ash content of biochar increased with increasing its HTT (Table 1), which is consistent with the data of previous research.17 Generally, the ash content of ANIBs was higher than that of PLABs,41 which was attributed to the high ash contents of feedstocks for ANIBs relative to those of PLABs. Additionally, the CO2-SA of PLABs (293.4− 513.4 m2/g) was remarkably higher than that of ANIBs (33.2− 206.1 m2/g) (Table 1), which could be explained by a significantly positive relationship between CO2-SA of these biochars and their OC contents (r = 0.97, p < 0.01, Supporting Information Figure S3a) combined with a good negative correlation between CO2-SA of these biochars and their ash contents (r = −0.94, p < 0.01, Figure S3b), suggesting that OC is a major contributor to CO2-SA of these biochars. If OC of biochars dominated CO2-SA of biochars, OC-normalized CO2SA (CO2-SA/OC) of all biochars should be comparable. However, HTBs exhibited higher CO2-SA/OC relative to LTBs (Figure S3c), supported by previous observations showing that high HTT greatly enlarges the SA of biochars.12,17 The sharp enhancement of the CO2-SA/OC of HTBs could be attributed to the complete destruction of aliphatic alkyl and ester CO groups and the further removal of aromatic CO and phenolicOH linked to aromatic cores at higher HTT (Supporting Information Figure S4 and Table S1), thus increasing SA.12 Effect of Deashing on Properties of Biochars. Deashing greatly influenced the composition of the biochars. Bulk C and O contents of biochars generally increased after deashing treatment except for SW biochars (Table 1 and Figure 1a and

b) and a significantly positive correlation of increased C to the decreased ash contents of biochars after deashing treatment was observed (r = 0.93, p < 0.01, Supporting Information Figure S3d), indicating a significant removal of minerals from the original biochars especially for those minerals-rich biochars, which could be further reflected by the large reduction of minerals as indicated by XRD spectra after deashing treatment ( Supporting Information Figure S5). Moreover, after deashing, the bulk polarity of biochars based on their elemental ratios of (N + O)/C generally increased except for SW biochars (Table 1 and Figure 1c). Some polar groups associated with minerals of the SW biochars were likely removed during the process of mineral removal. Surface chemical composition (C, O, and Si) of biochars as determined by XPS also greatly changed after deashing (Table 1). The surface C contents of PLABs increased after deashing while those of ANIBs reduced although the corresponding bulk C contents of PLABs and ANIBs both increased after deashing (Table 1 and Figure 1a), indicating that OM of the deashed ANIBs were partly covered by minerals due to the spatial rearrangement of OM during the process of the mineral removal. Surface C of original ANIBs was obviously higher than their corresponding bulk C (Table 1 and Figure 1a), suggesting the minerals of these original ANIBs were likely covered by OM as well as the heterogeneous spatial arrangement of composition within biochars, which is also supported by the results of their SEM-EDS spectra (Supporting Information Figure S6). Some studies reported that soil minerals have been shown to be mainly covered by OM.42,43 However, OM was 11476

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Table 2. Freundlich Isotherm Parameters and Concentration-Dependent Distribution Coefficients (Koc) for the Samples log Koc (mL/g) samples RI450 RI600 DRI450 DRI600 WH450 WH600 DWH450 DWH600 MA450 MA600 DMA450 DMA600 CH450 CH600 DCH450 DCH600 SW450 SW600 DSW450 DSW600 CO450 CO600 DCO450 DCO600

log KFa 2.73 3.12 3.06 3.54 2.67 2.87 2.97 2.92 2.64 3.11 2.73 3.21 2.06 2.31 2.86 2.56 2.54 3.29 3.07 3.21 2.40 2.82 3.20 3.40

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.46c 2.19 2.06 2.64 1.38 1.82 2.19 2.05 1.48 2.19 1.57 2.10 1.04 1.55 2.23 1.97 1.69 2.15 1.73 1.81 1.30 2.11 1.73 2.01

n 0.43 0.38 0.50 0.40 0.48 0.41 0.47 0.49 0.43 0.54 0.55 0.46 0.49 0.42 0.48 0.70 0.48 0.29 0.54 0.44 0.55 0.35 0.53 0.40

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.01c 0.02 0.02 0.02 0.01 0.02 0.03 0.02 0.01 0.02 0.01 0.01 0.02 0.03 0.04 0.04 0.02 0.01 0.01 0.01 0.01 0.03 0.01 0.01

log Kd

Nb

R2

Ce = 0.01Sw

Ce = 0.1Sw

Ce = 1Sw

Ce = 0.01Sw

18 20 18 20 20 18 18 18 18 19 19 18 20 20 20 19 20 18 18 18 20 19 18 18

0.996 0.971 0.991 0.978 0.997 0.988 0.970 0.984 0.992 0.989 0.996 0.993 0.990 0.955 0.948 0.971 0.977 0.974 0.998 0.998 0.995 0.911 0.999 0.997

5.38 5.69 5.67 6.01 5.28 5.37 5.55 5.46 5.17 5.73 5.36 5.72 5.53 5.76 5.91 5.89 5.47 6.00 5.94 5.98 5.45 5.65 6.00 6.07

4.81 5.06 5.17 5.41 4.76 4.78 5.01 4.96 4.60 5.26 4.91 5.19 5.02 5.18 5.39 5.58 4.94 5.29 5.48 5.42 5.00 5.01 5.53 5.46

4.24 4.44 4.67 4.82 4.24 4.19 4.48 4.45 4.03 4.80 4.46 4.65 4.50 4.59 4.87 5.28 4.42 4.58 5.01 4.85 4.55 4.36 5.06 4.86

3.12 3.37 4.11 4.83 3.71 4.18 4.11 4.58 3.84 4.58 4.22 4.82 0.54 0.50 1.54 1.33 1.84 2.13 2.58 2.56 1.61 1.73 3.09 3.06

a Sorption affinity coefficient with units of (μg/g)/(μg/L)n. bNumber of data. Koc is the organic carbon (OC)-normalized sorption distribution coefficient (Kd). cStandard deviation. Note that RI, WH, MA, CH, SW, and CO represent the biochars obtained from rice, wheat, and maize, and manures of chicken, swine, and cow, respectively. D denotes deashing treatment for the biochars. 450 and 600 represent the heating treatment temperatures.

suggesting that the O atoms bound to minerals could be removed during deashing, thus some O atoms of SW biochars were likely involved in their interaction with minerals. Effect of Both Heating Temperature and Deashing on Nonlinearity n Values. The Freundlich isotherms are shown in Supporting Information Figure S7, and the fitting parameters are listed in Table 2. All sorption isotherms of PHE were highly nonlinear and well fitted with the Freundlich model with nonlinear coefficient (n) being in the range of 0.29−0.70, supported by the previous result that the isotherm of the black carbon remaining after the sediment combustion was highly nonlinear.45 For most original and deashed biochars, the values of n decreased with increasing HTT, which presumably corresponded to their rising aromaticity (Supporting Information Table S2).4,12,46 In addition, the HTBs had larger CO2-SA than the LTBs (Table 1), which could account for why PHE sorption isotherms generally became more nonlinear with increasing HTT. Larger CO2-SA could reflect the more micropores in biochars because it represents the SA of micoropores (0−1.4 nm),38,39 which should provide more available adsorption sites. The microporosity gives rise to isotherm nonlinearity, which is indicative of a distribution of site energies.47 Zhu et al.48 reported that aromatic domains contributed to the nonlinearity, which was potentially due to the nanovoids in the condensed aromatic domains. Moreover, deashing treatment generally exerted little influence on the nonlinearity of PHE isotherms of biochars with the exception of CH600 and SW600 (Table 2). This result was consistent with a previous study that deashing treatment slightly affected

not concentrated on the surface of original PLABs, which was possibly supported by abundant evidence that substantial parts of mineral surfaces are not covered by OM as reviewed by Kleber et al.44 The increased surface C amounts of the PLABs after deashing was consistent with their increased bulk C amounts (Table 1 and Figure 1a); however, the surface C content of ANIBs decreased with their increasing total C (Table 1), suggesting that minerals of these PLABs had no obvious effect on the spatial distribution of OM while minerals of ANIBs impacted their spatial distribution of OM greatly. Conversely, the increasing bulk O content and decreasing surface O content for the investigated biochars except for SW biochars after deashing (Figure 1b) indicated that minerals of biochars affected the spatial distribution of polar groups associated with PLABs and ANIBs, which was also supported by better correlation between surface polarity ((O + N)/C) of biochars and their ash content (r = 0.95, p < 0.01,Supporting Information Figure S3e,) than the correlation between their corresponding bulk polarity and ash content (r = 0.49, p = 0.10, Figure S3e). Also, in spite of general increasing bulk polarity as indicted by their index values of (O + N)/C except for SW biochars, their surface polarity of both PLABs and ANIBs decreased to varying degrees after deashing (Figure 1c), indicating that abundant minerals of biochars would benefit the exposure of polar groups on the surface of the tested biochars, which could account for higher surface polarity of original biochars than their corresponding deashed ones (Table 1 and Figure 1c). For SW biochars, deashing treatment slightly decreased the total O content of SW450 and SW600 from 10.2% and 9.9% to 7.9% and 6.8%, respectively (Table 1), 11477

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isotherm nonlinearity of PHE sorption by humic acids and humins.25 Impact of Heating Temperature and Deashing Treatment on PHE Sorption Capacity (logKoc). The log Koc values of each sorbent varied with three equilibrium concentrations (i.e., Ce = 0.01, 0.1, 1SW) due to the nonlinearity of sorption isotherms and decreased with increasing concentrations of PHE (Table 2), reflecting that PHE was initially preferentially captured by the sorption sites with high energy.47 Second, it has been reported that HTBs are more effective in sorption and sequestration of organic contaminants in soils,4,12 which was consistent with the observation of this study that logKoc values (Ce = 0.01Sw) of PHE sorption by biochars increased with increasing HTT (Table 2). However, this result was different from sorption of other types of HOCs including phthalic acid esters (PAEs), and herbicides by biochars in our previous studies,14,15 which could be ascribed to LTBs, which generally contained more polar groups relative to HTBs, could be involved in more interactions via specific adsrotpion including H-bonding and π−π interacions between LTBs and PAEs or herbicides containing polar groups as well. The log Koc values of PHE (Ce = 0.01Sw) by the biochars increased after deashing treatment except that log Koc values of MA600 and SW600 basically remained the same (Table 2). Similarly, it was previously determined that PHE Koc values by the dissolved HA was several orders of magnitude higher than that of the HA associated with minerals.22 It has been reported that HOC sorption by OM−clay complexes was affected by minerals via interactions of hydroxyl sites on mineral surfaces with the carboxyl groups on humic substances, which could determine the interfacial configuration of humic coatings and further alter the size or accessibility of hydrophobic domains of sorbents.49 Similarly, these interactions between the hydroxyl sites on mineral surfaces and the carboxyl groups of these investigated biochars in this study may occur. On the basis of the XPS results (Table 1), the surface O content of most biochars decreased after deashing treatment especially for ANIBs, although their bulk O concentration generally increased (Figure 1b). Hence, after deashing, biochars should possess more hydrophobic domains and less polar functional groups, resulting in the increase of favorable and hydrophobic sorption sites of OM, which could enhance PHE sorption. Moreover, the impact of minerals on sorption was also confirmed by the positive correlation between the ash content of LTBs (except CH450) and their rise-values of PHE log Koc of biochars (r = 0.98, p < 0.01) (Figure 2a), suggesting that the amounts of minerals might account for how much of sorption capacity (Koc) of the LTBs would increase after deashing. On the whole, minerals should have little influence on sorption of HTBs compared to that on sorption of LTBs because of weak interaction between minerals and HTBs, which contained their negligible carbonyl and phenolic functional groups and substantial amounts of aromatic components (Supporting Information Table S2 and Figure S4). Kang and Xing50 reported that aliphatic C and alkyl O were more inclined to bind to the mineral than aromatic components and Yang et al.25 implied that the carbonyl and phenolic components could be the predominant component responsible for the interactions between OM and minerals, which both could explain that the removal of minerals had relative little influence on PHE sorption of HTBs due to their negligible polar groups (Supporting Information Table S2 and Figure S4). Moreover, a significant and positive correlation between the ash contents

Figure 2. Correlation between the ash content of low temperature biochars (LTBs) (produced from chicken waste at 450 °C) except for CH450 (▲) and their increased of log Koc value of phenanthrene (PHE) (Ce = 0.01Sw) from these original LTBs to their deashed LTBs after deashing treatment (the change value between their two log Koc values) (a); correlation between log Koc values of PHE (Ce = 0.01Sw) by LTBs and their ash content or the surface Si content (b); and relationships of the PHE log Kd values (Ce = 0.01Sw) of all biochars (original and deashed biochars) to bulk or surface C content (c), ash or Si content (d), surface polarity ((N + O)/C) as indicated by XPS data (e), and bulk polarity ((N + O)/C) according to the elemental analysis data (f).

and PHE log Koc values was observed only for the LTBs in this study (r = 0.92, p < 0.01) (Figure 2b). Therefore, the minerals of LTBs should play a more important role in the sorption of PHE compared to the minerals of the HTBs, which was further confirmed by the positive relationship between PHE log Koc values of the original LTBs and their surface Si contents (r = 0.85, p < 0.05) (Figure 2b). Wang and Xing26 observed that Koc of PHE by humic acids increased up to several times after they were loaded to minerals (montmorillonite and kaolinite). Correlations between log Kd or log Koc Values and Biochar Properties. Correlations between the property parameters of the original and deashed biochars and their log Kd or log Koc values of PHE were evaluated to determine their individual influence on PHE sorption. First, a significant and positive correlation between log Kd of PHE by all tested sorbents and their bulk C contents (r = 0.99, p < 0.01) (Figure 2c) along with a significant and negative correlation of log Kd of PHE to their ash content (r = −0.97, p < 0.01, Figure 2d) were observed in this study, consistent with that OM has been identified as a predominant sorbent for HOCs in soils and sediments as long as the total OC is >0.1%.46 Additionally, the surface polarity ((N + O)/C) as indicated by XPS of both original and deashed biochars had significant influence on their 11478

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enhanced by electron-withdrawing substituents.51 Moreover, previous studies showed the minimal role of H-bonding in aromatic compound sorption by chars.10 Therefore, π−π interactions should be one of major sorption mechanism of PHE by PLABs. On the other hand, the log Koc values of PHE by ANIBs produced at 450 °C were negatively correlated with their surface polarity index ((O + N)/C) excluding CH450 (r = −0.93, p < 0.05) (Figure 3b). These data suggested that the surface polarity of ANIBs played an important role in controlling sorption of PHE compared to the bulk polarity of ANIBs. The difference in the role played by the polarity between PLABs and ANIBs implied the different sorption mechanism of PHE by PLABs and ANIBs, which was likely attributed to the large variation of the ash content between PLABs and ANIBs because minerals influenced the spatial arrangement of OM within biochars as mentioned above. In addition, with the exception of CH600, log Koc (Ce = 0.1Sw) of the original HTBs (600 °C) was positively correlated with aromatic C (r = 0.99, p < 0.05) and negatively related with their aliphatic C content (r = −0.99, p < 0.05) (Figure 3c and d), indicating that condensed aromatic C-rich biochars have high binding affinity to PHE and are important in sorption of PHE by HTBs, supported by previous reports.10,12,14 It has been documented that both alkyl and aromatic domains as hydrophobic carbon components may interact with PHE via hydrophobic interaction (van der Waals force), but PHE could also be sorbed to aromatic domains via π−π interactions. Previous studies showed that π−π interactions were much stronger than van der Waals force.52 The PHE log Koc values of original HTBs (600 °C) increased with their increasing aromatic carbon contents in this study (Figure 3c and d), suggesting that π−π interaction between PHE and biochars could dominate. While the negative relationship between PHE log Koc of original LTBs (450 °C) and their aromatic carbon content (r = −0.95, p < 0.05), along with the significantly negative correlation between log Koc and the surface C content (r = −0.96, p < 0.05) (Figure 3e), suggests that polar and aliphatic domains or other structural components regulate sorption of PHE to LTBs, which was further supported by the positive correlation between log Koc and surface (O + N)/C values (Figure 3f). Environmental Implications. In this study we used lowand high-mineral biochars to investigate the influence of minerals on the sorption of PHE by these biochars and their deashed counterparts. The results suggest that the minerals within the biochars have distinct effects on sorption of HOCs by the low- and high-mineral biochars and the presence of abundant minerals alters the physicochemical properties of biochars and their spatial arrangement of OM, which in turn affects their sorption of PHE. It has also been shown that the sorption affinity of soils which contain large amounts of biochar is not as high as what would be expected based on the sorption properties of fresh biochar.5 Elucidating the influence of minerals within biochar on the overall sorption of HOCs would help to accurately predict sorption properties of biocharamended soils and better understand the mobility and bioavailability of HOCs in soil amended with biochar. This study indicates that the influence of minerals on sorptive properties should be considered when designing biochars as sorbents for HOCs.

sorption of PHE compared to the bulk polarity according to the comparison of the two correlations coefficients between the surface polarity and the log Kd values of PHE (r = 0.90, p < 0.01) (Figure 2e) and between bulk polarity and the log Kd values of PHE (r = 0.63, p < 0.01) (Figure 2f). On the other hand, in order to investigate the effect of physicochemical characteristics of OM within biochars on their sorption of PHE, their log Koc values were related to structural properties of biochars. For original and deashed PLABs produced at 450 °C, there was a positive correlation between the log Koc values of PHE and bulk (O + N)/C ratios of biochars (r = 0.91, p < 0.05), and the similar trend occurred for original and deashed PLABs obtained at 600 °C (r = 0.81, p < 0.05) (Figure 3a),

Figure 3. Correlations between log Koc values (mL/g) of PHE by plant-residue or animal-waste derived biochars (i.e., PLABs and ANIBs) except for CH450 (▲) and the atomic ratio of bulk or surface polarity ((O + N)/C) as indicated by elemental and XPS data, respectively (a and b); Correlations between logKoc values of PHE by HTBs except for CH600 (Δ) and their aromatic C or aliphatic C contents (c and d); Relationships between log Koc values of PHE by LTBs except for SW450 (▲) and their aromatic C or surface C content (e), and the surface polarity (f) (HTBs, high-temperature biochars; LTBs, low-temperature biochars).

thus, the polar groups of PLABs may play a dominant role in sorption of PHE. This positive correlation between the log Koc values and bulk (O + N)/C ratios of PLABs biochars (Figure 3a) suggest specific interactions, including H-bonding interactions or π−π interactions between electron-rich (π-donor) PHE with π-acceptor sites in OM within the biochars, possibly also play an important role in the PHE sorption besides hydrophobic interactions. It has been reported that the aromatic rings substituted with two or more electronwithdrawing groups such as carbonyl (ketone, aldehyde) and carboxyl (Ar−CO2H, Ar−CO2R, Ar−CONR2) was one of πacceptor sites in OM, and the acceptor ability of OM is 11479

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ASSOCIATED CONTENT

S Supporting Information *

Table of surface carbon functional group composition and functional groups from the 13C NMR spectra; figure of the kinetics of phenanthrene sorption by the selected biochars as well as their corresponding deashed biochars; figure of correlations among the properties of original and deashed biochars including CO2-surace area, organic carbon (OC), ash content, and bulk/surface polarity; figure of 13C NMR and FTIR spectra of original PLABs and ANIBs; figures of XRD spectra of the original and deashed biochars; figure of SEMEDS of the selected CH600 and RI600 samples; figure of sorption isotherms of PHE by the all tested biochars. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-10-58807493. Fax: 86-10-58807493. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by National Natural Science Foundation of China (41273106), USDA Hatch Program (MAS 00982), Program for New Century Excellent Talents in University (NCET-09-0233), and Fundamental Research Funds for the Central Universities (2009SD-8). We would like to thank all the reviewers for their helpful suggestions to improve the manuscript. We also thank Dr. Alyse Enger from Princeton University for useful comments.

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