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Simulated degradation of biochar and its potential environmental implications
Environmental Pollution 179 (2013) 146e152
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Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Simulated degradation of biochar and its potential environmental implications Zhaoyun Liu a, Walelign Demisie a, b, Mingkui Zhang a, * a
Zhejiang Provincial Key Laboratory of Subtropical Soil and Plant Nutrition, College of Environmental & Resource Sciences, Zhejiang University, Hangzhou 310058, China b Department of Dry Land Crop Science, Jijiga University, Jijiga, Ethiopia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2012 Received in revised form 8 April 2013 Accepted 15 April 2013
A simulated oxidation technique was used to examine the impacts of degradation on the surface properties of biochar and the potential implications of the changes in biochar properties were discussed. To simulate the short- and long-term environmental degradation, mild and harsh degradation were employed. Results showed that after mild degradation, the biochar samples showed significant reductions in surface area and pore volumes. After harsh degradation, the biochar samples revealed dramatic variations in their surface chemistry, surface area, pore volumes, morphology and adsorption properties. The results clearly indicate that changes of biochar surface properties were affected by biochar types and oxidative conditions. It is suggested that biochar surface properties are likely to be gradually altered during environmental exposure. This implies that these changes have potential effects for altering the physicochemical properties of biochar amended soils. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Biochar Simulated degradation Surface properties Environmental implications
1. Introduction Biochar is a form of black carbon (C) produced by heating biomass in a low or zero oxygen environment. There is no clear-cut boundary between black C and biochar, the term ‘black carbon’ is often used synonymously with the term ‘biochar’ in the literature (Nguyen and Lehmann, 2009; Keiluweit et al., 2010; Zimmerman, 2010). Black C is the term for the continuum of C forms, regardless of the production purpose (e.g., naturally produced or deliberately manufactured for energy, fuel or C sequestration) or source of material (e.g., fossil fuels or biomass) (Goldberg, 1985; Schmidt and Noack, 2000). On the other hand, biochar is pyrolysed organic matter created specifically for C sequestration or soil quality improvements (Lehmann et al., 2006). For the purpose of this paper, we will use biochar to describe the solid residues producing by thermal decomposition of biomass under oxygen-limit conditions. Biochar has received widespread attention from environmental chemists for its ability to enhance soil fertility, adsorb contaminants and sequester atmospheric C in terrestrial systems to offset C emissions and combat global climate change (Yang and Sheng,
* Corresponding author. E-mail address: [email protected] (M. Zhang). 0269-7491/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2013.04.030
2003; Lehmann et al., 2003, 2006,2008; Nguyen and Lehmann, 2009). However, there is a little information on the stability of biochar in the environment, which is fundamental if we are to understand the role that biochar may play in reducing environmental pollution or how it can be used to improve soil fertility. Due to its high aromaticity, biochar is highly stable and can persist in the environment over long periods of time (Goldberg, 1985; Schmidt and Noack, 2000; Skjemstad et al., 2002; Lehmann, 2007). Accordingly, the 14C ages of biochar (referred as black carbon in the literature) were found to lie between 1160 and 5040 years (Schmidt et al., 2002). Even though biochar appears to be relatively recalcitrant, it must ultimately mineralize. Otherwise, soil organic C would be dominated by accumulated biochar over geological time scales (Goldberg, 1985). Furthermore, an increasing number of observations suggest that biochar can be degraded, by both biotic and abiotic processes (Hamer et al., 2004; Cheng et al., 2006, 2008; Guggenberger et al., 2008). Biological mineralization of biochar has been investigated through incubating biochar with sand, soil, inoculum solution, or nutrients (Hamer et al., 2004; Cheng et al., 2006; Cheng and Lehmann, 2009; Hilscher et al., 2009; Nguyen and Lehmann, 2009; Zimmerman, 2010). On the other hand, to investigate the degradation of biochar, many researchers have used different chemical oxidants, such as oxygen (Toles et al., 1999), acidified potassium dichromate (Ascough et al., 2011), nitric acid
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(Kamegawa et al., 1998; Moreno-castilla et al., 2000), ozone (Kawamoto et al., 2005) or air alone (Cheng et al., 2006). These studies focused on different aspects biochar stability. For example, biotic and abiotic oxidation processes were compared (Cheng et al., 2006), and half-life or susceptibility of biochar have been investigated (Kawamoto et al., 2005; Zimmerman, 2010; Ascough et al., 2011). These previous studies showed that incubations of biochar at around 30 C produce mild degradation, which could represent the short-term environmental degradation. Conversely, strong chemical oxidants (e.g., concentrated nitric acid) can harshly oxidize biochar. This kind of oxidation may not occur in the initial phase of biochar environmental exposure, but the changes in the biochar properties provide a basis for investigating the long-term stability of biochar, since it is a challenge to study the long-term environmental degradation of biochar directly. There remains little information on the effects of environmental degradation of biochar over time, with respect to surface structural characteristics and adsorption properties, in particular. To investigate the variation in biochar surface properties, we used an aerobic incubation at 30 C (mild degradation) and concentrated nitric acid (harsh degradation) to simulate the short- and long-term environmental degradation, respectively. Therefore, our objectives were to (1) examine the impacts of the simulated short- and longterm environmental degradation on biochar surface properties, and (2) discuss the potential environmental implications of the changes in the biochar properties caused by the simulated degradation. 2. Materials and methods 2.1. Production of biochar Biochar was produced form three biomass types: the living branch portion of ten-year-old oak (Quercus phillyraeoides), three-year-old bamboo (Phyllostachy edulis) and the harvested rice (Oryza stativa) straw residues obtained from Huajiachi campus, Zhejiang University, Hangzhou, China. The branches of oak and bamboo were collected in the fall in Tianmu Mountain, Linan, Zhejiang Province, China. The straw and branch were air-dried at room temperature and cut into <2 cm pieces. These materials were then placed in ceramic pots, each covered with a suitable lid, and pyrolysed under oxygen-limited conditions in a muffle furnace. The furnace was set to a heating rate of approximately 10 C min1, and then held at 350 C for rice straw and 600 C for oak and bamboo for 2 h. After pyrolysing the biochar samples were allowed to cool to room temperature. These conditions were chosen to represent natural fires, in which the litter layer temperatures are often around 300 C, elsewhere temperatures might reach 600 C (Pyne et al., 1996). The charred biomass materials were milled to pass a 0.15-mm sieve and stored in a desiccator. The resulting samples are referred to as oak-C, bamboo-C and straw-C, respectively. 2.2. Basic properties of biochar The basic physical and chemical properties of biochars were displayed in Table S1. It revealed that yield, ash, N%, H%, O% and water holding capacity in ricestraw derived biochar were highest, while pH, C% and surface area were lowest. 2.3. Experimental design 2.3.1. Mild degradation Mild degradation of biochar was carried out in an aerobic incubation experiment. For each treatment, 10 g of biochar were placed in a 500-mL conical flask. To maintain 50% water holding capacity (Cheng et al., 2006), 2.7, 3.5 and 4 mL sterilized water were added to oak-C, bamboo-C and straw-C, respectively. After adding water, the flasks were incubated at 30 C. To aerate and keep the water content constant, each flask was opened and its moisture was adjusted every 5 days during the first 100 days and then every 10 days for the duration of the incubation period. All bottles with the materials to be incubated, were sterilized by high temperature (121 C) and pressure (200 kPa). After 374 of days incubation, the flasks were removed and dried at 70 C for 48 h. The mild degradation was conducted in triplicate. The mineralized biochar samples are hereafter referred to as oak-CM, bamboo-CM and straw-CM, respectively. 2.3.2. Harsh degradation Harsh degradation of biochar was conducted with modifications to methods outlined by Moreno-Castilla et al. (1995). The biochar was mixed in a 1:10 weight ratio with concentrated HNO3 (65%) in flasks. The flasks were heated at 80 C and
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shaken at 120 rpm in a water-bath at constant temperature for 48 h. After cooling down to room-temperature, the residues were washed with deionized water to remove nitrates. The harsh degradation was conducted in triplicate. The oxidized samples will be referred to as oak-CH, bamboo-CH and straw-CH, respectively. 2.4. Characterization of biochar The FTIR spectra of biochar samples were obtained by using a Nicolet Ava Tar370 FTIR spectrometer (Nicolet Instrument Corporation). Surface analysis of biochar was conducted with a VG ESCALAB MARKⅡ electron spectrometer (VG Scientific, East Grinstead, Sussex, U.K.) employing monochromatic Mg Ka X-ray source (1253.6 eV). The surface area and pore volumes of biochar samples were measured by nitrogen adsorption using a TriStarⅡ3020 Micrometritics surface area analyzer. Morphology of the biochar particles were investigated using a Hitachi S-4800 scanning electron microscopy (SEM) at ambient temperature and 5.0 kV, after coating the particles with gold. Specific details of these measurements are found in the Supplementary material. 2.5. Adsorption experiment To better understand the influence of degradation on the adsorption capability of biochar, nitrobenzene uptake by biochar was conducted in aqueous solution. Nitrobenzene was used as an example, because it is a common contaminant in natural environments, such as air, sediments and surface water (Gatermann et al., 1995; Li et al., 2003; He et al., 2006) and widely used in adsorption studies (Chun et al., 2004; Chen et al., 2008). Biochars (50 mg) and 10 mL of solution containing 50e1500 mg L1 nitrobenzene were placed in 15-mL amber screw cap glass tubes. The tubes were immediately closed with polytetrafluoroethylene-lined screw caps and placed on a rotating shaker and agitated (120 rpm) at room temperature (25 C) for 24 h. A preliminary study indicated that 24 h was sufficient to reach the apparent equilibrium for nitrobenzene adsorption by biochar. After the establishment of sorption equilibrium, the aqueous solution were filtered through a 0.45 mm cellulose acetate membrane filter paper. Then, the concentration of nitrobenzene in the filtrate was analysed by a UVevis spectrophotometer at a wavelength of 268 nm. The quantitative amount of adsorbed nitrobenzene was calculated as the difference between the added amount and the amount remaining in the final solution. Each experiment was performed in triplicate under identical conditions, and the average data were reported. Isotherms were expressed as the average amount of nitrobenzene adsorbed by biochar (mg kg1) at the equilibrium concentration and fitted according to the Freundlich equation (qe ¼ KFCne , Where qe is the amount of adsorbate adsorbed per unit mass of biochar at equilibrium in mg kg1; KF (mg1 n Ln kg1) is the Freundlich affinity coefficient; n (dimensionless) is the Freundlich linearity index; Ce (mg L1) is the concentration at equilibrium) (Freundlich, 1928). Blanks containing no sorbents were carried out to ensure that adsorption to glass tubes and degradation of nitrobenzene were negligible. 2.6. Statistical analysis All experiments were conducted with three replicates. Statistical difference between means were determined according to Tukey’s test, and statistical significance was assigned at the p < 0.05 level. Statistical analyses was performed using SPSS v.18.
3. Results 3.1. FTIR spectroscopy The broad peak at 3440 cm1 represented the eOH stretching vibration (Fig. 1) (Guo and Bustin, 1998; Cheng et al., 2006). The bands in the 800e870 cm1 region were attributed to CeH out-of plane deformation (Biniak et al., 1997; Moreno-castilla et al., 2000; Ascough et al., 2011). The peaks at 1060e1100 cm1 region could be assigned to aliphatic ether (CeO) symmetrical stretching (Biniak et al., 1997). The band at 1241 cm1 was due to the aromatic CeO-structures and phenolic eOH stretching (Guo and Bustin, 1998). The band at 1380 cm1 was probably due to the stretching vibrations of eNO2 (Akhter et al., 1984). The 1435 cm1 could be attributed to aliphatic C-Hx bending vibrations (Moreno-castilla et al., 2000; Ascough et al., 2011). The aromatic groups C]C detected at 1600 cm1 (Cheng et al., 2006). The peaks at 1538 and 1718 cm1 represented aromatic ring stretching coupled to highly conjugated carbonyl groups and carboxyl C]O stretching vibration, respectively (Moreno-castilla et al., 2000; Cheng et al., 2006; Ascough et al., 2011).
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of biochar. It is interesting to note that the hydroxyl/ether groups (CeO) increased significantly for all of the three types of biochar after mild degradation (Table 1). Whereas, the carbonyl (C]O) and carboxylic (COOR) groups remained unchanged for oak-C and bamboo-C after mild degradation. The Oc/C ratios (Oc is oxygen bound to carbon) did not significantly increased for oak-CM and bamboo-CM with respect to the corresponding original oak-C and bamboo-C. The carboxylic groups (COOR) and the Oc/C ratio of straw-C increased significantly after mild degradation. The most pronounced changes were observed after harsh degradation, which caused both aliphatic/aromatic carbon groups (CeC, C]C or CeH) and hydroxyl/ether groups (CeO) to decrease significantly, while carbonyl (C]O), carboxylic (COOR) groups and the Oc/C ratios significantly increased for all of the three types of biochars. 3.3. Surface area and pore volumes According to the International Union of Pure and Applied Chemistry (IUPAC) recommendations, pores may be classified in different classes depending on their size: micropores (size <2 nm), mesopores (2 nm < size<50 nm) and macropores (size >50 nm). Table 2 provides the results for the surface area and pore volumes of micropores and mesopores measured by quantitative analysis of nitrogen adsorption isotherms at 77 K in both the original and mildly or harshly degraded biochar samples. Oak-C and bamboo-C have greater surface area and micropore volumes than straw-C. Mild degradation showed a significant effect in reducing the surface area and micropore volumes in the three types of biochar. In contrast, harsh degradation caused significant increase in surface area and total pore volumes of oak-C and straw-C. However, harsh degradation resulted in a significant reduction in surface area and pore volumes in bamboo-C. 3.4. Morphology of biochar
Fig. 1. FTIR spectra of the original biochar (Oak-C, Bamboo-C, and Straw-C) and the degraded biochar (Oak-CM, Oak-CH, Bamboo-CM, Bamboo-CH, Straw-CM, and Straw-CH).
3.2. XPS analysis Deconvolution was conducted on high-resolution XPS spectra of C1s and O1s to quantify different C and O forms at the outer surface
It is evident that micropores dominated the oak-C, oak-CM and oak-CH surface (Fig. 2a,b,c). Further, after mild degradation, the micropores were slightly blocked by the mobile components of oak-C. However, more micropores appeared after harsh degradation. The surface morphology of bamboo-C was similar to oak-C, however, the SEM micrograph of bamboo-CM revealed obvious blockage of pores (Fig. 2d,e). Afterwards, the SEM imaging of bamboo-CH showed severe destruction of micropores (Fig. 2f). The plant cell structure of the original biomass material was clearly visible in straw-C particles, and was slightly damaged by mild degradation (Fig. 2g,h). Whereas, we found destroyed wall structures of straw-CH, and the particles became much smaller and
Table 1 Chemical composition of carbon (C1s) and oxygen (O1s) from high-energy resolution XPS spectra of the original biochar (Oak-C, Bamboo-C, and Straw-C) and the degraded biochar (Oak-CM, Oak-CH, Bamboo-CM, Bamboo-CH, Straw-CM, and Straw-CH) (n ¼ 3). Samples
CeC, C]C, or CeH Oak-C Oak-CM Oak-CH Bamboo-C Bamboo-CM Bamboo-CH Straw-C Straw-CM Straw-CH
Oca/C
C1s composition (%)
65.1d 62.6c 54.6a 65.9d 64.5d 56.9b 82.5g 74.3f 71.6e
1.5 1.8 1.1 1.2 1.4 0.8 2.5 1.8 1.7
CeO 14.0c 16.5d 15.5d 20.1e 21.6f 16.0d 11.9b 19.3e 4.4a
C]O
0.4 0.8 0.5 0.9 0.7 0.6 0.3 0.8 0.1
13.9c 13.9c 16.0e 9.5b 9.5b 14.9d ND ND 7.1a
O1s composition (%)
COOR
0.4 0.5 0.7 0.2 0.1 0.6
0.2
7.0d 7.0d 14.0f 4.5a 4.5a 12.2e 5.6b 6.4c 17.0g
O]C
0.1 0.2 0.5 0.1 0.1 0.3 0.1 0.1 0.6
0.22a 0.23a 0.29bc 0.22a 0.24a 0.31c 0.23a 0.27b 0.51d
0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.00 0.01
30.8a 31.2a 43.3b 29.5a 31.0a 56.0c 43.5b 45.9b 87.8d
Each value represents means standard deviation, and the different letters within a column indicate a significant difference at p < 0.05. ND, not detectable. a Oc: oxygen bound to carbon.
OHeC or CeOeC
0.8 0.6 1.1 0.5 0.7 1.8 0.9 1.1 4.2
69.2d 68.8d 56.7c 70.5d 69.0d 44.0b 56.5c 54.1c 12.2a
1.4 0.9 1.1 2.0 1.3 0.8 1.2 0.6 0.2
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Table 2 Surface area and pore volume parameters of the original biochar (Oak-C, Bamboo-C, and Straw-C) and the degraded biochar (Oak-CM, Oak-CH, Bamboo-CM, Bamboo-CH, Straw-CM, and Straw-CH) (n ¼ 3). Samples
Surface areaa m2 g1
Oak-C Oak-CM Oak-CH Bamboo-C Bamboo-CM Bamboo-CH Straw-C Straw-CM Straw-CH
154.6f 116.9d 282.4g 137.7e 14.5b 1.2a 18.0b 5.7a 74.7c
2.0 2.0 7.3 2.5 0.4 0.0 0.3 0.1 0.9
Sextb m2 g1 30.0d 24.5c 46.8e 23.5c 0.2a 0.1a 4.0b 2.9b 70.0f
1.1 0.8 1.1 0.7 0.0 0.0 0.1 0.1 1.1
Vtc cm3 g1 0.081f 0.061d 0.141g 0.067e 0.008b 0.002a 0.013c 0.008b 0.166h
0.001 0.001 0.001 0.001 0.000 0.000 0.000 0.000 0.004
Vmicd cm3 g1 0.058f 0.043d 0.109g 0.053e 0.007c 0.001a 0.006b 0.001a 0.001a
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Vmese cm3 g1 0.023e 0.018d 0.032f 0.014c 0.001a 0.001a 0.007b 0.007b 0.165g
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.004
Each value represents means standard deviation, and the different letters within a column indicate a significant difference at p < 0.05. a Determined by N2 adsorption using the BrunauereEmmetteTeller (BET) method. b External surface area, calculated by t-plot method. c Total pore volume, determined at P/P0 ¼ 0.97. d Micropore volume, calculated using t-plot method. e Mesopore volume, calculated by Vt Vmic method.
Fig. 2. Scanning electron microscopy (SEM) images of the nine biochar samples: (a) Oak-C, 100KX; (b) Oak-CM, 100KX; (c) Oak-CH, 100KX; (d) Bamboo-C, 100KX; (e) Bamboo-CM, 100KX; (f) Bamboo-CH, 100KX; (g) Straw-C, 5000X; (h) Straw-CM, 3000X; (i) Straw-CH, 5000.
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retained less evidence of original plant tissue structure, under harsh oxidation (Fig. 2i).
adsorption between straw-C and straw-CM, while a considerable reduction was observed for straw-CH.
3.5. Adsorption properties
4. Discussion
Adsorption isotherms of nitrobenzene to biochar are presented in Fig. 3. The fitting parameters are summarized in Table S2. The Freundlich model fitted all adsorption data well (R2 > 0.97). It is evident that straw-C shows much less curvilinear nitrobenzene uptake than oak-C and bamboo-C, as reflected by the Freundlich n values. Additionally, the Freundlich n values increased more rapidly after harsh degradation than mild degradation for all of the three biochar samples (Table S2). Moreover, the nitrobenzene uptake followed the decreasing order of oak-C > oak-CM > oak-CH at lower equilibrium concentration (<200 mg L1). However, oak-CH showed a dramatic increase in adsorption affinity at higher equilibrium concentration. Bamboo-CM and bamboo-CH showed greater reduction in adsorption capacity for nitrobenzene compared to bamboo-C. It was observed that there were no difference in nitrobenzene
4.1. Effects of mild and harsh degradation on biochar’s surface chemistry Based on the FTIR spectra and XPS analysis, mild degradation had no significant effect on the formation of oxygen-containing functional groups of oak-C and bamboo-C. However, the carboxylic groups (COOR) and surface Oc/C ratios of the straw-C increased significantly after mild degradation. The reason could be that the biochars exposed to higher pyrolysis temperatures (oak-C and bamboo-C) were considerably more stable than the biochar produced at lower temperatures. Similarly, Ascough et al. (2011) observed that biochar produced at 300 C contained C with more labile fractions than biochar produced at 400 C. Harsh degradation produced more carboxylic groups on the surfaces of all three biochar types, resulting in a significant increase in Oc/C ratios. These results are due to the reaction of the biochar with the strong chemical oxidant. Similar results have been obtained by other authors (Moreno-Castilla et al., 1995, 2000; Kawamoto et al., 2005; Ascough et al., 2011). Our results are also in agreement with the findings of Glaser et al. (2001) and Cheng et al. (2008) who found significant formation of carboxylic groups on the edges of aromatic cores of biochar following centuries or millennia under environmental oxidation. Thus the strong chemical oxidants used here can simulate the long-term environmental degradation of biochar insitu. The presence of oxidized functional groups on the surface of biochar particles has been shown to improve soil fertility by increasing cation exchange capacity (Glaser et al., 2001; Liang et al., 2006). Furthermore, it is proposed that the increase of oxygencontaining functional groups in harshly oxidized biochars could act as water adsorption centres to reduce the surface area accessible to solute (Foley et al., 1997). This may explain the observed decrease in nitrobenzene adsorption (Fig. 3). Cheng and Lehmann (2009) found after 130 year of exposure in soils, the historical biochar sample showed an evident decrease in adsorption capacity for hydroquinone. In addition, Cheng et al. (2008) reported this historical biochar samples contained a lot of oxygen-containing functional groups, which is similar to the harshly oxidized biochar samples of the current study. These observations imply that long-term environmental degradation of biochar causes an increase in oxygen-containing functional groups, leading to enhancing soil fertility, and decreased soil adsorption capacity for polar organic pollutants. 4.2. Effects of mild and harsh degradation on the surface structural characteristics of biochar
Fig. 3. Adsorption isotherms of nitrobenzene to the original biochar (Oak-C, BambooC, and Straw-C) and the degraded biochar (Oak-CM, Oak-CH, Bamboo-CM, Bamboo-CH, Straw-CM, and Straw-CH).
As a result of mild degradation, the more labile fractions of the original biochar samples were oxidized, and as a consequence the mobile components produced could occupy the pore spaces of the biochar samples (Fig. 2), resulting in a significant decrease in the surface area and pore volumes (Table 2). This finding indicates that the changes of biochar surface properties could occur through air oxidation alone, even without interactions between biochar and soil components. Surface area and micropores are the key factors for adsorption process (Chun et al., 2004; Chen et al., 2008), the changes in surface area and porosity of oak-C and bamboo-C had effect on their adsorption capacity for nitrobenzene (Fig. 3). While, for the straw derived biochar samples, the adsorption capacity is irrelevant with surface area, because the partitioning is the
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dominate mechanism for this non-microporous material (Chun et al., 2004; Chen et al., 2008). Besides sorption of organic contaminants, many previous studies have concluded that biochar might contribute to significant changes in soil physical properties, such as soil water holding capacity and nutrient retention capacity, because of the high specific surface area and porosity of biochar (Glaser et al., 2002a, 2002b; Cohen-Ofri et al., 2006). Therefore, the mildly degraded biochar samples will be less effective in improving soil water holding capacity, nutrient retention capacity and adsorption capacity for organic contaminants compared to the original biochar samples. However, unlike mild degradation, the changes of biochar surface area and pore structures caused by harsh degradation, were highly related with biochar types. In the case of oak-C and straw-C, their surface area and porosity increased significantly after harsh degradation. While, for bamboo-C, significant decreases in surface area and pore volumes were observed. This means that the stability of biochar depends on both feedstock types and pyrolysis temperature. It can be seen that harsh degradation could prominently develop the porosity of oak-C, make smaller size particles for strawC, and destroy the pore walls of bamboo-C (Fig. 2). Thus in order to improve soil surface area and porosity, our results indicate that biochar produced from woody biomass at higher temperature and oxidized by harsh degradation might perform more effectively than biochar produced from the herbaceous biomass at lower temperature, with respect to simulating long term effects. 5. Conclusions The findings of this study showed that changes in biochar properties, obtained from laboratory simulations, provide evidence for short- or long-term environmental degradation of biochar. The changes of biochar surface properties mainly depend on the type of biochar (e.g., original feedstock and pyrolysis temperature) and oxidative conditions. It is suggested that the changes of biochar properties will, in turn, affect the properties of biochar amended soils. The actual influence of synthetically degraded biochars on soil properties remains to be studied. Future work should focus on the practical implications of oxidized biochar on soil properties. Acknowledgements This research was supported by Natural Science Foundation of China (No. 40771090; 40471064). We thank Ms. Lijuan Mao, the technician of 985-Institute of Agrobiology and Environmental Sciences of Zhejiang University, for the assistance in surface area analyzer. We also thank the anonymous reviewers for their warm work and useful comments. Appendix A. Supplementary material Supplementary material related to this article can be found at http://dx.doi.org/10.1016/j.envpol.2013.04.030. References Akhter, M.S., Chughtai, A.R., Smith, D.M., 1984. Reaction of hexane soot with NO2/ N2O4. The Journal of Physical Chemical 88, 5334e5342. Ascough, P.L., Bird, M.I., Francis, S.M., Thornton, B., Midwood, A.J., Scott, A.C., Apperley, D., 2011. Variability in oxidative degradation of charcoal: influence of production conditions and environmental exposure. Geochimica et Cosmochimica Acta 75, 2361e2378. ski, G., Siedlewski, J., Swiatkowski, Biniak, S., Szyman A., 1997. The characterization of activated carbons with oxygen and nitrogen surface groups. Carbon 35, 1799e1810. Chen, B.L., Zhou, D.D., Zhu, L.Z., 2008. Transitional adsorption and partition of nonpolar and polar aromatic contaminants by biochars of pine needles with
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Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Simulated degradation of biochar and its potential environmental implications Zhaoyun Liu a, Walelign Demisie a, b, Mingkui Zhang a, * a
Zhejiang Provincial Key Laboratory of Subtropical Soil and Plant Nutrition, College of Environmental & Resource Sciences, Zhejiang University, Hangzhou 310058, China b Department of Dry Land Crop Science, Jijiga University, Jijiga, Ethiopia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2012 Received in revised form 8 April 2013 Accepted 15 April 2013
A simulated oxidation technique was used to examine the impacts of degradation on the surface properties of biochar and the potential implications of the changes in biochar properties were discussed. To simulate the short- and long-term environmental degradation, mild and harsh degradation were employed. Results showed that after mild degradation, the biochar samples showed significant reductions in surface area and pore volumes. After harsh degradation, the biochar samples revealed dramatic variations in their surface chemistry, surface area, pore volumes, morphology and adsorption properties. The results clearly indicate that changes of biochar surface properties were affected by biochar types and oxidative conditions. It is suggested that biochar surface properties are likely to be gradually altered during environmental exposure. This implies that these changes have potential effects for altering the physicochemical properties of biochar amended soils. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Biochar Simulated degradation Surface properties Environmental implications
1. Introduction Biochar is a form of black carbon (C) produced by heating biomass in a low or zero oxygen environment. There is no clear-cut boundary between black C and biochar, the term ‘black carbon’ is often used synonymously with the term ‘biochar’ in the literature (Nguyen and Lehmann, 2009; Keiluweit et al., 2010; Zimmerman, 2010). Black C is the term for the continuum of C forms, regardless of the production purpose (e.g., naturally produced or deliberately manufactured for energy, fuel or C sequestration) or source of material (e.g., fossil fuels or biomass) (Goldberg, 1985; Schmidt and Noack, 2000). On the other hand, biochar is pyrolysed organic matter created specifically for C sequestration or soil quality improvements (Lehmann et al., 2006). For the purpose of this paper, we will use biochar to describe the solid residues producing by thermal decomposition of biomass under oxygen-limit conditions. Biochar has received widespread attention from environmental chemists for its ability to enhance soil fertility, adsorb contaminants and sequester atmospheric C in terrestrial systems to offset C emissions and combat global climate change (Yang and Sheng,
* Corresponding author. E-mail address: [email protected] (M. Zhang). 0269-7491/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2013.04.030
2003; Lehmann et al., 2003, 2006,2008; Nguyen and Lehmann, 2009). However, there is a little information on the stability of biochar in the environment, which is fundamental if we are to understand the role that biochar may play in reducing environmental pollution or how it can be used to improve soil fertility. Due to its high aromaticity, biochar is highly stable and can persist in the environment over long periods of time (Goldberg, 1985; Schmidt and Noack, 2000; Skjemstad et al., 2002; Lehmann, 2007). Accordingly, the 14C ages of biochar (referred as black carbon in the literature) were found to lie between 1160 and 5040 years (Schmidt et al., 2002). Even though biochar appears to be relatively recalcitrant, it must ultimately mineralize. Otherwise, soil organic C would be dominated by accumulated biochar over geological time scales (Goldberg, 1985). Furthermore, an increasing number of observations suggest that biochar can be degraded, by both biotic and abiotic processes (Hamer et al., 2004; Cheng et al., 2006, 2008; Guggenberger et al., 2008). Biological mineralization of biochar has been investigated through incubating biochar with sand, soil, inoculum solution, or nutrients (Hamer et al., 2004; Cheng et al., 2006; Cheng and Lehmann, 2009; Hilscher et al., 2009; Nguyen and Lehmann, 2009; Zimmerman, 2010). On the other hand, to investigate the degradation of biochar, many researchers have used different chemical oxidants, such as oxygen (Toles et al., 1999), acidified potassium dichromate (Ascough et al., 2011), nitric acid
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(Kamegawa et al., 1998; Moreno-castilla et al., 2000), ozone (Kawamoto et al., 2005) or air alone (Cheng et al., 2006). These studies focused on different aspects biochar stability. For example, biotic and abiotic oxidation processes were compared (Cheng et al., 2006), and half-life or susceptibility of biochar have been investigated (Kawamoto et al., 2005; Zimmerman, 2010; Ascough et al., 2011). These previous studies showed that incubations of biochar at around 30 C produce mild degradation, which could represent the short-term environmental degradation. Conversely, strong chemical oxidants (e.g., concentrated nitric acid) can harshly oxidize biochar. This kind of oxidation may not occur in the initial phase of biochar environmental exposure, but the changes in the biochar properties provide a basis for investigating the long-term stability of biochar, since it is a challenge to study the long-term environmental degradation of biochar directly. There remains little information on the effects of environmental degradation of biochar over time, with respect to surface structural characteristics and adsorption properties, in particular. To investigate the variation in biochar surface properties, we used an aerobic incubation at 30 C (mild degradation) and concentrated nitric acid (harsh degradation) to simulate the short- and long-term environmental degradation, respectively. Therefore, our objectives were to (1) examine the impacts of the simulated short- and longterm environmental degradation on biochar surface properties, and (2) discuss the potential environmental implications of the changes in the biochar properties caused by the simulated degradation. 2. Materials and methods 2.1. Production of biochar Biochar was produced form three biomass types: the living branch portion of ten-year-old oak (Quercus phillyraeoides), three-year-old bamboo (Phyllostachy edulis) and the harvested rice (Oryza stativa) straw residues obtained from Huajiachi campus, Zhejiang University, Hangzhou, China. The branches of oak and bamboo were collected in the fall in Tianmu Mountain, Linan, Zhejiang Province, China. The straw and branch were air-dried at room temperature and cut into <2 cm pieces. These materials were then placed in ceramic pots, each covered with a suitable lid, and pyrolysed under oxygen-limited conditions in a muffle furnace. The furnace was set to a heating rate of approximately 10 C min1, and then held at 350 C for rice straw and 600 C for oak and bamboo for 2 h. After pyrolysing the biochar samples were allowed to cool to room temperature. These conditions were chosen to represent natural fires, in which the litter layer temperatures are often around 300 C, elsewhere temperatures might reach 600 C (Pyne et al., 1996). The charred biomass materials were milled to pass a 0.15-mm sieve and stored in a desiccator. The resulting samples are referred to as oak-C, bamboo-C and straw-C, respectively. 2.2. Basic properties of biochar The basic physical and chemical properties of biochars were displayed in Table S1. It revealed that yield, ash, N%, H%, O% and water holding capacity in ricestraw derived biochar were highest, while pH, C% and surface area were lowest. 2.3. Experimental design 2.3.1. Mild degradation Mild degradation of biochar was carried out in an aerobic incubation experiment. For each treatment, 10 g of biochar were placed in a 500-mL conical flask. To maintain 50% water holding capacity (Cheng et al., 2006), 2.7, 3.5 and 4 mL sterilized water were added to oak-C, bamboo-C and straw-C, respectively. After adding water, the flasks were incubated at 30 C. To aerate and keep the water content constant, each flask was opened and its moisture was adjusted every 5 days during the first 100 days and then every 10 days for the duration of the incubation period. All bottles with the materials to be incubated, were sterilized by high temperature (121 C) and pressure (200 kPa). After 374 of days incubation, the flasks were removed and dried at 70 C for 48 h. The mild degradation was conducted in triplicate. The mineralized biochar samples are hereafter referred to as oak-CM, bamboo-CM and straw-CM, respectively. 2.3.2. Harsh degradation Harsh degradation of biochar was conducted with modifications to methods outlined by Moreno-Castilla et al. (1995). The biochar was mixed in a 1:10 weight ratio with concentrated HNO3 (65%) in flasks. The flasks were heated at 80 C and
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shaken at 120 rpm in a water-bath at constant temperature for 48 h. After cooling down to room-temperature, the residues were washed with deionized water to remove nitrates. The harsh degradation was conducted in triplicate. The oxidized samples will be referred to as oak-CH, bamboo-CH and straw-CH, respectively. 2.4. Characterization of biochar The FTIR spectra of biochar samples were obtained by using a Nicolet Ava Tar370 FTIR spectrometer (Nicolet Instrument Corporation). Surface analysis of biochar was conducted with a VG ESCALAB MARKⅡ electron spectrometer (VG Scientific, East Grinstead, Sussex, U.K.) employing monochromatic Mg Ka X-ray source (1253.6 eV). The surface area and pore volumes of biochar samples were measured by nitrogen adsorption using a TriStarⅡ3020 Micrometritics surface area analyzer. Morphology of the biochar particles were investigated using a Hitachi S-4800 scanning electron microscopy (SEM) at ambient temperature and 5.0 kV, after coating the particles with gold. Specific details of these measurements are found in the Supplementary material. 2.5. Adsorption experiment To better understand the influence of degradation on the adsorption capability of biochar, nitrobenzene uptake by biochar was conducted in aqueous solution. Nitrobenzene was used as an example, because it is a common contaminant in natural environments, such as air, sediments and surface water (Gatermann et al., 1995; Li et al., 2003; He et al., 2006) and widely used in adsorption studies (Chun et al., 2004; Chen et al., 2008). Biochars (50 mg) and 10 mL of solution containing 50e1500 mg L1 nitrobenzene were placed in 15-mL amber screw cap glass tubes. The tubes were immediately closed with polytetrafluoroethylene-lined screw caps and placed on a rotating shaker and agitated (120 rpm) at room temperature (25 C) for 24 h. A preliminary study indicated that 24 h was sufficient to reach the apparent equilibrium for nitrobenzene adsorption by biochar. After the establishment of sorption equilibrium, the aqueous solution were filtered through a 0.45 mm cellulose acetate membrane filter paper. Then, the concentration of nitrobenzene in the filtrate was analysed by a UVevis spectrophotometer at a wavelength of 268 nm. The quantitative amount of adsorbed nitrobenzene was calculated as the difference between the added amount and the amount remaining in the final solution. Each experiment was performed in triplicate under identical conditions, and the average data were reported. Isotherms were expressed as the average amount of nitrobenzene adsorbed by biochar (mg kg1) at the equilibrium concentration and fitted according to the Freundlich equation (qe ¼ KFCne , Where qe is the amount of adsorbate adsorbed per unit mass of biochar at equilibrium in mg kg1; KF (mg1 n Ln kg1) is the Freundlich affinity coefficient; n (dimensionless) is the Freundlich linearity index; Ce (mg L1) is the concentration at equilibrium) (Freundlich, 1928). Blanks containing no sorbents were carried out to ensure that adsorption to glass tubes and degradation of nitrobenzene were negligible. 2.6. Statistical analysis All experiments were conducted with three replicates. Statistical difference between means were determined according to Tukey’s test, and statistical significance was assigned at the p < 0.05 level. Statistical analyses was performed using SPSS v.18.
3. Results 3.1. FTIR spectroscopy The broad peak at 3440 cm1 represented the eOH stretching vibration (Fig. 1) (Guo and Bustin, 1998; Cheng et al., 2006). The bands in the 800e870 cm1 region were attributed to CeH out-of plane deformation (Biniak et al., 1997; Moreno-castilla et al., 2000; Ascough et al., 2011). The peaks at 1060e1100 cm1 region could be assigned to aliphatic ether (CeO) symmetrical stretching (Biniak et al., 1997). The band at 1241 cm1 was due to the aromatic CeO-structures and phenolic eOH stretching (Guo and Bustin, 1998). The band at 1380 cm1 was probably due to the stretching vibrations of eNO2 (Akhter et al., 1984). The 1435 cm1 could be attributed to aliphatic C-Hx bending vibrations (Moreno-castilla et al., 2000; Ascough et al., 2011). The aromatic groups C]C detected at 1600 cm1 (Cheng et al., 2006). The peaks at 1538 and 1718 cm1 represented aromatic ring stretching coupled to highly conjugated carbonyl groups and carboxyl C]O stretching vibration, respectively (Moreno-castilla et al., 2000; Cheng et al., 2006; Ascough et al., 2011).
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of biochar. It is interesting to note that the hydroxyl/ether groups (CeO) increased significantly for all of the three types of biochar after mild degradation (Table 1). Whereas, the carbonyl (C]O) and carboxylic (COOR) groups remained unchanged for oak-C and bamboo-C after mild degradation. The Oc/C ratios (Oc is oxygen bound to carbon) did not significantly increased for oak-CM and bamboo-CM with respect to the corresponding original oak-C and bamboo-C. The carboxylic groups (COOR) and the Oc/C ratio of straw-C increased significantly after mild degradation. The most pronounced changes were observed after harsh degradation, which caused both aliphatic/aromatic carbon groups (CeC, C]C or CeH) and hydroxyl/ether groups (CeO) to decrease significantly, while carbonyl (C]O), carboxylic (COOR) groups and the Oc/C ratios significantly increased for all of the three types of biochars. 3.3. Surface area and pore volumes According to the International Union of Pure and Applied Chemistry (IUPAC) recommendations, pores may be classified in different classes depending on their size: micropores (size <2 nm), mesopores (2 nm < size<50 nm) and macropores (size >50 nm). Table 2 provides the results for the surface area and pore volumes of micropores and mesopores measured by quantitative analysis of nitrogen adsorption isotherms at 77 K in both the original and mildly or harshly degraded biochar samples. Oak-C and bamboo-C have greater surface area and micropore volumes than straw-C. Mild degradation showed a significant effect in reducing the surface area and micropore volumes in the three types of biochar. In contrast, harsh degradation caused significant increase in surface area and total pore volumes of oak-C and straw-C. However, harsh degradation resulted in a significant reduction in surface area and pore volumes in bamboo-C. 3.4. Morphology of biochar
Fig. 1. FTIR spectra of the original biochar (Oak-C, Bamboo-C, and Straw-C) and the degraded biochar (Oak-CM, Oak-CH, Bamboo-CM, Bamboo-CH, Straw-CM, and Straw-CH).
3.2. XPS analysis Deconvolution was conducted on high-resolution XPS spectra of C1s and O1s to quantify different C and O forms at the outer surface
It is evident that micropores dominated the oak-C, oak-CM and oak-CH surface (Fig. 2a,b,c). Further, after mild degradation, the micropores were slightly blocked by the mobile components of oak-C. However, more micropores appeared after harsh degradation. The surface morphology of bamboo-C was similar to oak-C, however, the SEM micrograph of bamboo-CM revealed obvious blockage of pores (Fig. 2d,e). Afterwards, the SEM imaging of bamboo-CH showed severe destruction of micropores (Fig. 2f). The plant cell structure of the original biomass material was clearly visible in straw-C particles, and was slightly damaged by mild degradation (Fig. 2g,h). Whereas, we found destroyed wall structures of straw-CH, and the particles became much smaller and
Table 1 Chemical composition of carbon (C1s) and oxygen (O1s) from high-energy resolution XPS spectra of the original biochar (Oak-C, Bamboo-C, and Straw-C) and the degraded biochar (Oak-CM, Oak-CH, Bamboo-CM, Bamboo-CH, Straw-CM, and Straw-CH) (n ¼ 3). Samples
CeC, C]C, or CeH Oak-C Oak-CM Oak-CH Bamboo-C Bamboo-CM Bamboo-CH Straw-C Straw-CM Straw-CH
Oca/C
C1s composition (%)
65.1d 62.6c 54.6a 65.9d 64.5d 56.9b 82.5g 74.3f 71.6e
1.5 1.8 1.1 1.2 1.4 0.8 2.5 1.8 1.7
CeO 14.0c 16.5d 15.5d 20.1e 21.6f 16.0d 11.9b 19.3e 4.4a
C]O
0.4 0.8 0.5 0.9 0.7 0.6 0.3 0.8 0.1
13.9c 13.9c 16.0e 9.5b 9.5b 14.9d ND ND 7.1a
O1s composition (%)
COOR
0.4 0.5 0.7 0.2 0.1 0.6
0.2
7.0d 7.0d 14.0f 4.5a 4.5a 12.2e 5.6b 6.4c 17.0g
O]C
0.1 0.2 0.5 0.1 0.1 0.3 0.1 0.1 0.6
0.22a 0.23a 0.29bc 0.22a 0.24a 0.31c 0.23a 0.27b 0.51d
0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.00 0.01
30.8a 31.2a 43.3b 29.5a 31.0a 56.0c 43.5b 45.9b 87.8d
Each value represents means standard deviation, and the different letters within a column indicate a significant difference at p < 0.05. ND, not detectable. a Oc: oxygen bound to carbon.
OHeC or CeOeC
0.8 0.6 1.1 0.5 0.7 1.8 0.9 1.1 4.2
69.2d 68.8d 56.7c 70.5d 69.0d 44.0b 56.5c 54.1c 12.2a
1.4 0.9 1.1 2.0 1.3 0.8 1.2 0.6 0.2
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Table 2 Surface area and pore volume parameters of the original biochar (Oak-C, Bamboo-C, and Straw-C) and the degraded biochar (Oak-CM, Oak-CH, Bamboo-CM, Bamboo-CH, Straw-CM, and Straw-CH) (n ¼ 3). Samples
Surface areaa m2 g1
Oak-C Oak-CM Oak-CH Bamboo-C Bamboo-CM Bamboo-CH Straw-C Straw-CM Straw-CH
154.6f 116.9d 282.4g 137.7e 14.5b 1.2a 18.0b 5.7a 74.7c
2.0 2.0 7.3 2.5 0.4 0.0 0.3 0.1 0.9
Sextb m2 g1 30.0d 24.5c 46.8e 23.5c 0.2a 0.1a 4.0b 2.9b 70.0f
1.1 0.8 1.1 0.7 0.0 0.0 0.1 0.1 1.1
Vtc cm3 g1 0.081f 0.061d 0.141g 0.067e 0.008b 0.002a 0.013c 0.008b 0.166h
0.001 0.001 0.001 0.001 0.000 0.000 0.000 0.000 0.004
Vmicd cm3 g1 0.058f 0.043d 0.109g 0.053e 0.007c 0.001a 0.006b 0.001a 0.001a
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Vmese cm3 g1 0.023e 0.018d 0.032f 0.014c 0.001a 0.001a 0.007b 0.007b 0.165g
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.004
Each value represents means standard deviation, and the different letters within a column indicate a significant difference at p < 0.05. a Determined by N2 adsorption using the BrunauereEmmetteTeller (BET) method. b External surface area, calculated by t-plot method. c Total pore volume, determined at P/P0 ¼ 0.97. d Micropore volume, calculated using t-plot method. e Mesopore volume, calculated by Vt Vmic method.
Fig. 2. Scanning electron microscopy (SEM) images of the nine biochar samples: (a) Oak-C, 100KX; (b) Oak-CM, 100KX; (c) Oak-CH, 100KX; (d) Bamboo-C, 100KX; (e) Bamboo-CM, 100KX; (f) Bamboo-CH, 100KX; (g) Straw-C, 5000X; (h) Straw-CM, 3000X; (i) Straw-CH, 5000.
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retained less evidence of original plant tissue structure, under harsh oxidation (Fig. 2i).
adsorption between straw-C and straw-CM, while a considerable reduction was observed for straw-CH.
3.5. Adsorption properties
4. Discussion
Adsorption isotherms of nitrobenzene to biochar are presented in Fig. 3. The fitting parameters are summarized in Table S2. The Freundlich model fitted all adsorption data well (R2 > 0.97). It is evident that straw-C shows much less curvilinear nitrobenzene uptake than oak-C and bamboo-C, as reflected by the Freundlich n values. Additionally, the Freundlich n values increased more rapidly after harsh degradation than mild degradation for all of the three biochar samples (Table S2). Moreover, the nitrobenzene uptake followed the decreasing order of oak-C > oak-CM > oak-CH at lower equilibrium concentration (<200 mg L1). However, oak-CH showed a dramatic increase in adsorption affinity at higher equilibrium concentration. Bamboo-CM and bamboo-CH showed greater reduction in adsorption capacity for nitrobenzene compared to bamboo-C. It was observed that there were no difference in nitrobenzene
4.1. Effects of mild and harsh degradation on biochar’s surface chemistry Based on the FTIR spectra and XPS analysis, mild degradation had no significant effect on the formation of oxygen-containing functional groups of oak-C and bamboo-C. However, the carboxylic groups (COOR) and surface Oc/C ratios of the straw-C increased significantly after mild degradation. The reason could be that the biochars exposed to higher pyrolysis temperatures (oak-C and bamboo-C) were considerably more stable than the biochar produced at lower temperatures. Similarly, Ascough et al. (2011) observed that biochar produced at 300 C contained C with more labile fractions than biochar produced at 400 C. Harsh degradation produced more carboxylic groups on the surfaces of all three biochar types, resulting in a significant increase in Oc/C ratios. These results are due to the reaction of the biochar with the strong chemical oxidant. Similar results have been obtained by other authors (Moreno-Castilla et al., 1995, 2000; Kawamoto et al., 2005; Ascough et al., 2011). Our results are also in agreement with the findings of Glaser et al. (2001) and Cheng et al. (2008) who found significant formation of carboxylic groups on the edges of aromatic cores of biochar following centuries or millennia under environmental oxidation. Thus the strong chemical oxidants used here can simulate the long-term environmental degradation of biochar insitu. The presence of oxidized functional groups on the surface of biochar particles has been shown to improve soil fertility by increasing cation exchange capacity (Glaser et al., 2001; Liang et al., 2006). Furthermore, it is proposed that the increase of oxygencontaining functional groups in harshly oxidized biochars could act as water adsorption centres to reduce the surface area accessible to solute (Foley et al., 1997). This may explain the observed decrease in nitrobenzene adsorption (Fig. 3). Cheng and Lehmann (2009) found after 130 year of exposure in soils, the historical biochar sample showed an evident decrease in adsorption capacity for hydroquinone. In addition, Cheng et al. (2008) reported this historical biochar samples contained a lot of oxygen-containing functional groups, which is similar to the harshly oxidized biochar samples of the current study. These observations imply that long-term environmental degradation of biochar causes an increase in oxygen-containing functional groups, leading to enhancing soil fertility, and decreased soil adsorption capacity for polar organic pollutants. 4.2. Effects of mild and harsh degradation on the surface structural characteristics of biochar
Fig. 3. Adsorption isotherms of nitrobenzene to the original biochar (Oak-C, BambooC, and Straw-C) and the degraded biochar (Oak-CM, Oak-CH, Bamboo-CM, Bamboo-CH, Straw-CM, and Straw-CH).
As a result of mild degradation, the more labile fractions of the original biochar samples were oxidized, and as a consequence the mobile components produced could occupy the pore spaces of the biochar samples (Fig. 2), resulting in a significant decrease in the surface area and pore volumes (Table 2). This finding indicates that the changes of biochar surface properties could occur through air oxidation alone, even without interactions between biochar and soil components. Surface area and micropores are the key factors for adsorption process (Chun et al., 2004; Chen et al., 2008), the changes in surface area and porosity of oak-C and bamboo-C had effect on their adsorption capacity for nitrobenzene (Fig. 3). While, for the straw derived biochar samples, the adsorption capacity is irrelevant with surface area, because the partitioning is the
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dominate mechanism for this non-microporous material (Chun et al., 2004; Chen et al., 2008). Besides sorption of organic contaminants, many previous studies have concluded that biochar might contribute to significant changes in soil physical properties, such as soil water holding capacity and nutrient retention capacity, because of the high specific surface area and porosity of biochar (Glaser et al., 2002a, 2002b; Cohen-Ofri et al., 2006). Therefore, the mildly degraded biochar samples will be less effective in improving soil water holding capacity, nutrient retention capacity and adsorption capacity for organic contaminants compared to the original biochar samples. However, unlike mild degradation, the changes of biochar surface area and pore structures caused by harsh degradation, were highly related with biochar types. In the case of oak-C and straw-C, their surface area and porosity increased significantly after harsh degradation. While, for bamboo-C, significant decreases in surface area and pore volumes were observed. This means that the stability of biochar depends on both feedstock types and pyrolysis temperature. It can be seen that harsh degradation could prominently develop the porosity of oak-C, make smaller size particles for strawC, and destroy the pore walls of bamboo-C (Fig. 2). Thus in order to improve soil surface area and porosity, our results indicate that biochar produced from woody biomass at higher temperature and oxidized by harsh degradation might perform more effectively than biochar produced from the herbaceous biomass at lower temperature, with respect to simulating long term effects. 5. Conclusions The findings of this study showed that changes in biochar properties, obtained from laboratory simulations, provide evidence for short- or long-term environmental degradation of biochar. The changes of biochar surface properties mainly depend on the type of biochar (e.g., original feedstock and pyrolysis temperature) and oxidative conditions. It is suggested that the changes of biochar properties will, in turn, affect the properties of biochar amended soils. The actual influence of synthetically degraded biochars on soil properties remains to be studied. Future work should focus on the practical implications of oxidized biochar on soil properties. Acknowledgements This research was supported by Natural Science Foundation of China (No. 40771090; 40471064). We thank Ms. Lijuan Mao, the technician of 985-Institute of Agrobiology and Environmental Sciences of Zhejiang University, for the assistance in surface area analyzer. We also thank the anonymous reviewers for their warm work and useful comments. Appendix A. Supplementary material Supplementary material related to this article can be found at http://dx.doi.org/10.1016/j.envpol.2013.04.030. References Akhter, M.S., Chughtai, A.R., Smith, D.M., 1984. Reaction of hexane soot with NO2/ N2O4. The Journal of Physical Chemical 88, 5334e5342. Ascough, P.L., Bird, M.I., Francis, S.M., Thornton, B., Midwood, A.J., Scott, A.C., Apperley, D., 2011. Variability in oxidative degradation of charcoal: influence of production conditions and environmental exposure. Geochimica et Cosmochimica Acta 75, 2361e2378. ski, G., Siedlewski, J., Swiatkowski, Biniak, S., Szyman A., 1997. The characterization of activated carbons with oxygen and nitrogen surface groups. Carbon 35, 1799e1810. Chen, B.L., Zhou, D.D., Zhu, L.Z., 2008. Transitional adsorption and partition of nonpolar and polar aromatic contaminants by biochars of pine needles with
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