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Steam explosion of crop straws improves the characteristics of biochar as a soil amendment
Journal of Integrative Agriculture 2019, 18(7): 1486–1495 Available online at www.sciencedirect.com
ScienceDirect
RESEARCH ARTICLE
Steam explosion of crop straws improves the characteristics of biochar as a soil amendment CHEN Xue-jiao1, LIN Qi-mei1, 2, Muhammad Rizwan1, ZHAO Xiao-rong1, 2, LI Gui-tong1, 2 1 2
College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, P.R.China Key Laboratory of Arable Land Conservation (North China), Ministry of Agriculture/Key Laboratory of Plant-Soil Interactions, Ministry of Education, Beijing 100193, P.R.China
Abstract Five crop straws (wheat, rice, maize, oil-rape, and cotton) were first steam-exploded for 2 min at 210°C, 2.5 MPa and then pyrolyzed at 500°C for 2 h. Steam explosion (SE) induced 47–95% and 5–16% reduction of hemicellulose and cellulose, respectively, in the crop straws. The biochars derived from SE-treated feedstocks had a lower specific surface area (SSA) and pore volume, compared to those from pristine feedstocks, with one exception that SE enhanced SSA of oil-rape straw biochar by approximately 16 times. After SE, biochars had significant higher anion exchange capacity (AEC) (6.88–11.44 cmol kg–1) and point of zero net charges (PZNC) (pH 3.61–5.32) values. It can thus be speculated that these biochars may have higher potential for anions adsorption. In addition, oil-rape straw might be suitable to SE pretreatment for preparing biochar as a soil amendment and sorbent as well. Further work is required for testing its application in soil. Keywords: crop straws, steam explosion, biochar, characterization
(Schimmelpfennig and Glaser 2012; Kambo and Dutta
1. Introduction Biochar has been recently considered as a promising soil conditioner (Luo et al. 2014; Agegnehu et al. 2016) and suitable biosorbent as well for removing pollutants from wastewater (Trakal et al. 2014). These functions of biochar are greatly dependent on its nature, such as recalcitrance to chemical and microbiological decomposition (Naisse et al. 2013), porous structure and enormous surface area
2015), abundant surface functional groups (Chen et al. 2017), and positive and negative charges (Zhao et al. 2015). Of these characters, pore structure, ash, minerals, and surface charge are predominately controlled by the feedstocks (Zhao et al. 2013). For example, under the same pyrolysis conditions, biochars originated from woody biomass are predominated by micropores and have a larger specific surface area, compared with those from crop straws and manure which have more mesopores (Zhao et al. 2014). However, biochars derived from manure usually have higher adsorption capacities of heavy metal ions than those from
Received 30 May, 2018 Accepted 12 November, 2018 CHEN Xue-jiao, E-mail: [email protected]; Correspondence LIN Qi-mei, Tel: +86-10-62732502, E-mail: [email protected]
plant residues, because the former is usually abundant of
© 2019 CAAS. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). doi: 10.1016/S2095-3119(19)62573-6
in China, are usually used as the feedstock for producing
ash and minerals (Cao et al. 2009; Luo et al. 2014). Crop straws, one of the richest renewable resources biochar.
However, because of the highly compact
architecture of the lignocellulosic structure, crop straws need
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to be pretreated through grinding, hydrolysis, irradiation, torrefaction, etc., prior to the further utilization, for a higher efficiency. Steam explosion (SE) is one of the most common methods used for crop straw pretreatment, because of its less environmental impact, lower cost and energy demand, compared to other treatments (Kan et al. 2016). During the SE process, biomass is subjected to high temperature (160–260°C) and pressure (0.69–4.83 MPa) followed by a sudden decompression. As a result, the fiber material is scattered and the covalent bonds between hemicellulose and lignin are broken as well (Li et al. 2007; Su et al. 2015). It is reported that SE pretreatment could increase the digestibility and saccharification of biomass (Sarkar et al. 2012). Deepa et al. (2011) found that SE led to a reduction in the diameter of banana fibers but increase in crystallinity degree and thermal stability of cellulose. Biswas et al. (2011) measured increases in thermal stability of both lignin and cellulose, and decomposition rate of hemicellulose in salix wood as well following SE pretreatment. Das and Sarmah (2015) reported that the steam treatment of fir wood at 121°C for 30 min led to much lower contents of xylose and mannose in the bio-oil, whereas little change in the yields of both bio-oil and biochar. They suggested that steam could efficiently decompose the hemicellulose. Wang et al. (2011) also observed a similar change with regards to the hemicellulose in SE-treated pinewood but a lower bio-oil yield in the further pyrolysis. However, Li et al. (2007) detected some “unexploded” materials and therefore argued that SE pretreatment might not be suitable for all feedstocks because the products were highly heterogeneous. It is known that the crop residues have distinct fiber structure and thus may be suitable for different pretreatment methods (Buranov and Mazza 2008). Wheat, rice, maize, oil-rape, and cotton are the common crops and widely cultivated in China, which therefore generates a large number of residues especially in the harvest seasons.
According to the CAYEC (2014), the annual total production of these crop straws were more than 8×108 tons. Apart from returning to the field, the remaining crop straws are still abundant and need to be utilized properly. To date, little information is available regarding whether SE can pretreat the crop straws for the purpose of achieving proper biochars as a soil amendment. Therefore, in this study, five crop straws were subjected to SE pretreatment and then slowly pyrolyzed in a muffle furnace. The obtained biochars were analyzed for their properties. We hypothesized that the changes in crop straws following SE pretreatment could further influence their biochars, and the characteristics of biochar might vary greatly with crop straws. The goals were: (1) to compare the impacts of SE pretreatment on different crop straws and their biochars; (2) to measure the properties of biochar derived from steam-exploded crop straws; and (3) to evaluate their potential values as soil amendments or for other uses.
2. Materials and methods 2.1. Crop straws The tested crop straws included wheat (WS), rice (RS), maize (MS), oil-rape (ORS), and cotton (CS) straws which were collected from Shangzhuang Experimental Station (40.14°N, 116.18°E) , China Agricultural University, Beijingand Linquan County (33.08°N, 115.23°E), Anhui Province, China, respectively. After air-drying, they were divided into three subsamples randomly and smashed into 1–2 cm segments. The basic properties are shown in Table 1 and Fig. 1.
2.2. Steam explosion The crop straws of WS, RS, MS, ORS, and CS were steam-
Table 1 The yield and chemical composition of pristine and steam-exploded feedstocks (FS) and their biochars (BC) Crop straws1) Wheat SE-wheat Rice SE-rice Maize SE-maize Oil-rape SE-oil-rape Cotton SE-cotton 1)
Yield (%) BC 39.53 b 43.07 a 37.46 b 40.69 a 37.07 b 41.92 a 35.60 b 40.52 a 31.95 b 34.16 a
C (%) FS BC 36.80 b 76.62 a 40.78 a 65.64 b 39.09 b 74.91 a 41.23 a 67.24 b 40.70 b 69.89 b 46.10 a 70.81 a 41.28 b 75.26 a 44.57 a 73.14 b 44.81 b 77.33 a 45.21 a 75.07 b
H (%) FS BC 5.33 a 3.54 a 5.31 a 3.35 b 5.73 a 3.72 a 5.40 b 3.11 b 5.74 a 3.71 a 5.61 b 3.60 b 5.91 a 3.65 a 5.60 b 3.68 a 6.12 a 3.51 b 6.03 b 3.65 a
O (%) FS BC 42.46 a 18.59 b 37.22 b 29.70 a 40.87 a 20.12 b 37.39 b 28.43 a 41.30 a 24.68 a 36.21 b 23.97 b 41.39 a 19.79 b 37.17 b 21.73 a 44.28 a 18.16 b 43.41 b 20.38 a
N (%) FS BC 0.90 a 1.25 b 0.83 b 1.31 a 0.71 b 1.25 a 0.76 a 1.22 b 1.21 a 1.72 a 1.05 b 1.62 b 1.11 b 1.30 b 1.19 a 1.45 a 0.66 a 1.00 a 0.66 a 0.90 b
Atom H/C FS BC 1.74 a 0.55 b 1.56 b 0.61 a 1.76 a 0.60 a 1.57 b 0.56 b 1.69 a 0.64 a 1.46 b 0.61 b 1.72 a 0.58 b 1.51 b 0.60 a 1.64 a 0.54 b 1.60 b 0.58 a
Atom O/C FS BC 0.87 a 0.18 b 0.68 b 0.34 a 0.78 a 0.20 b 0.68 b 0.32 a 0.76 a 0.26 a 0.59 b 0.25 b 0.75 a 0.20 b 0.63 b 0.22 a 0.74 a 0.18 b 0.72 b 0.20 a
SE-wheat, SE-rice, SE-maize, SE-oil-rape, and SE-cotton represent the steam-exploded wheat, rice, maize, oil-rape, and cotton straws, respectively. Different letters in same crop straw or biochar indicate a significant change (P<0.05) before and after the steam explosion.
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RM
and functional groups.
SE
60
Cellulose (%)
40
2.3. Biochar production
a
50 ab
a
b
a b
a
30
b
b
20 10 0
B
Hemicellulose (%)
Rice Maize Oil-rape Cotton Crop straws
35 30
a
a
a
25
2.4. Assay methods
20
a
a
15
b
10 5 0
C
Wheat
b Wheat
b
b
b
Rice Maize Oil-rape Cotton Crop straws
25 aa
Lignin (%)
20 15 aa 10 5 0
Three portions of each pristine crop straw (WS, RS, MS, ORS, and CS) and each steam-exploded straw (WSSE, RSSE, MSSE, ORSSE, and CSSE), each portion 100 g, were separately placed into stainless steel cylinders (diameter=7.5 cm, height=11 cm) with screw lids at both ends in which one end was connected to a steel tube for N2 gas flow. All the cylinders were placed in a muffle furnace (SX4-10; Shuli, China), heated to 500°C at an increasing rate of 20°C min–1, and then maintained for 2 h under an N2 gas atmosphere (10 psi). The collected biochars were defined as BWS, BRS, BMS, BORS, BCS and BWSSE, BRSSE, BMSSE, BORSSE, BCSSE, respectively, and weighed for calculating the yields.
a b a
b a
Wheat
Rice
b
Maize Oil-rape Cotton Crop straws
Fig. 1 Cellulose (A), hemicellulose (B), and lignin (C) in the pristine (RM) and steam-exploded (SE) crop straws. Different letters in the same crop straw indicate a significant difference at the P<0.05 level. The bars in each column represent standard deviation of the means.
exploded for 2 min at 210°C and 2.5 MPa (Su et al. 2015) using a QB-200 platform in Hebi Heavy-Duty Mechanical Factory, China. The resulting products, recorded as WSSE, RSSE, MSSE, ORSSE, and CSSE, respectively, were oven-dried at 85°C for biochar production and measured for cellulose, hemicellulose, lignin, carbon (C), hydrogen (H), nitrogen (N), silicon (Si), morphological characteristics, pore structure,
The biochars are known to contain a high content of ash and soluble organic components, which are readily dissolved over time and thus play short-term roles in soil (Pereira et al. 2015). Therefore, in order to determine the intrinsical surface properties, as well as the long-term effect, biochars were usually demineralized with acids prior to assay (Chun et al. 2004; Li et al. 2017; Yue et al. 2017). Similar to the previous studies, the bulk of the biochars was thoroughly mixed with 1 mol L–1 HCl (1:10, w/v) at room temperature for 30 min, and then centrifuged in this study. Following three rounds of this procedure, the biochars were further washed with a mixture solution of 1 mol L–1 HCl and 1 mol L–1 HF. At the last step, the biochars were rinsed with deionized water until the pH values of the leachates were neutral, and then oven-dried at 60°C for assaying C, H, N, morphological characteristics, pore structure, surface charge, and functional groups. Cellulose, hemicellulose, and lignin were measured using the Van Soest method (Van Soest et al. 1991). Total C, H, and N were examined by an element analyzer (Vario EL III, Elementar Analysensysteme GmbH, Germany), and oxygen (O) content was calculated as the difference between 100% and the sum of ash, C, H, and N. Total Si content was measured by the molybdenum blue colorimetric method following NaOH digestion (Elliott and Snyder 1991). Morphological characteristics and functional groups were investigated using a scanning electronic microscope (SEM; S-3400N, Hitachi, Japan) and a Fourier transform infrared spectrometer (FTIR; Shimadzu, Japan), respectively. Pore structure was determined by a V-sorb 2008P Pore Analyzer (Gold APP, China). The specific surface area was obtained based on the multi-point Brunauer-Emmett-Teller (BET) adsorption isotherm. The analysis of both mesopores
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and macropores was implemented by the BJH (BarrettJoyner-Halenda (BJH) method, while that of micropores by the SF (Saito-Foley) method. Surface charges were assessed by the ion adsorption method (Cheng et al. 2008). The point of zero net charges (PZNC), anion exchange capacity (AEC), and cation exchange capacity (CEC) were calculated, respectively, as the pH where the amount of positive charge is equal to negative charge, the amount of net surface positive charge at pH 3.5, and the amount of surface negative charge at pH 7. The detailed procedures could be found in Appendix A.
2.5. Statistical analysis Apart from SEM and FTIR, all data were the means of triplicates and based on the oven-dry mass. An analysis of variance (ANOVA) was carried out in PASW Statistics18 Software. The significant differences were revealed by the least significant difference (LSD) test at the P<0.05 level.
3. Results and discussion 3.1. Changes of cellulose, hemicellulose, and lignin in crop straws The pristine crop straws contained 34–49% cellulose, 10–29% hemicellulose, and 4–26% lignin. Wheat, rice, and maize straws had higher hemicellulose, while oil-rape and cotton straws had higher lignin (Fig. 1). Hemicellulose significantly decreased by 47–95% following the SE pretreatment, which was similar to the result obtained by Bauer et al. (2014) and Huang et al. (2015). Cellulose decreased by 5–16%, much lower than hemicellulose, because the acetyl groups in the hemicellulose were hydrolyzed once the temperature exceeded 150°C, whereas the hydrolysis of cellulose started at a higher temperature of 200°C (Hendriks and Zeeman 2009). The decrease of hemicellulose was greater in maize straw and lower in cotton straw (Fig. 1-B). But for cellulose, the decrease was greater in cotton straw and lower in wheat straw (Fig. 1-A). These variations may largely depend on the structural characteristics of the tested crop straws (Sarkar et al. 2012). The cotton straw has a similar lignin structure to hardwood which is considerably different from that of other herbaceous crop straws, such as maize, wheat, and rice straws. The covalent bonds between hemicellulose and lignin in cotton straw might be more stable (Buranov and Mazza 2008; Wu et al. 2015). Therefore, the reduction of hemicellulose was much lower for cotton straw (47%), in contrast to other straws (84–95%). The great decrease in cellulose for cotton straw might be the consequence of the weak hydrogen bonds between hemicellulose and cellulose (Huang et al.
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2015). However, the slight decrease of cellulose in wheat straw (5%) might be largely related to the high degree of polymerization (Zhao et al. 2016). The SE pretreatment induced lignin to increase by 8–18% in wheat and rice straws, whereas decrease by 29% in maize straw, and change little in oil-rape and cotton straws (Fig. 1-C), which indicated that the change of lignin was highly dependent on the type of biomass. During SE process, hydrolysis of acetyl groups in hemicellulose can lead to an acidic environment and formation of the carbonium ion by the proton-induced elimination of ROH (water, alcohol, or acid) from the benzylic position. The carbonium ion might react in a β-O-4’ inter-unit connection structure (Li et al. 2009). The ether bonds between lignin and hemicellulose in maize straw could be effectively destroyed by SE pretreatment (Buranov and Mazza 2008). With more hemicellulose losing, the bond linking between lignin and hemicellulose would be affected and the lignin must be destroyed in a certain extent. Little change of lignin in oil-rape and cotton straws is probably because of lower reduction of hemicellulose, and relatively stable linkages between hemicellulose and lignin. Huang et al. (2015) reported that depolymerization and condensation of lignin coexisted during the SE process. The monosaccharides degraded from polysaccharides and electron-rich C atoms in guaiacyl and syringyl rings from lignin might be conducive to polymerization of the small decomposed molecules (Bauer et al. 2014), and therefore induced the increase of lignin in wheat and rice straws.
3.2. Changes of biochar yield and chemical composition The biochar yields of pristine crop straws were 31.95– 39.53%, the lowest in cotton straw and the highest in wheat straw. The SE-treated feedstocks had 7–14% higher biochar yields than those of pristine feedstocks (Table 1), which might be attributed to the reduction of hemicellulose and cellulose (Tripathi et al. 2016). Among the tested crop straws, cotton straw had relatively higher C and H, but lower N and wheat straw had relatively lower C and H (Table 1). Both wheat and rice straws had more than 14.16% Si, while others contained less than 4.59% Si (data not shown). After SE pretreatment, C content in crop straws increased by 1–13%, which might be primarily due to changes in the epidermis (Su et al. 2015) and induced by the formation of stable C-C structures following the condensation of monosaccharides and lignin (Li et al. 2007). Moreover, the O content decreased by 2–12%, which was owing to the dehydration during condensation and the hydrolysis of hydroxyl groups (-OH) in hemicellulose. The content of H, N, and Si was only slightly changed, but H/C and O/C ratios
2)
1)
Wheat SE-wheat Rice SE-rice Maize SE-maize Oil-rape SE-oil-rape Cotton SE-cotton
FS 2.69 a 2.54 b 1.69 b 2.61 a 6.07 a 2.67 b 1.74 b 6.63 a 1.95 a 0.96 b
BC 434.09 a 38.61 b 182.59 a 102.62 b 18.08 a 7.01 b 15.16 b 242.06 a 201.29 a 56.45 b
FS 0.018 b 0.020 a 0.024 a 0.016 b 0.034 a 0.028 b 0.010 b 0.028 a 0.012 a 0.005 b
BC 0.289 a 0.092 b 0.202 a 0.178 b 0.041 a 0.028 b 0.049 b 0.197 a 0.136 a 0.071 b
FS 26.97 b 31.17 a 57.61 a 23.96 b 22.32 b 41.12 a 22.80 a 16.96 b 23.63 a 21.85 b
BC 2.66 b 9.49 a 4.42 b 6.93 a 9.16 b 16.11 a 13.00 a 3.25 b 2.70 b 5.02 a
Micro-pore MPS (nm) PV (cm3 g–1) FS BC FS BC 0.0007 a 0.170 a 1.33 b 0.57 a 0.0007 a 0.012 b 1.66 a 0.54 b 0.0005 a 0.069 a 1.10 b 0.79 a 0.0005 a 0.035 b 2.37 a 0.48 b 0.0009 a 0.006 a 1.69 a 1.10 b 0.0006 b 0.003 b 1.68 a 1.33 a 0.0005 b 0.003 b 0.88 b 1.76 a 0.0011 a 0.090 a 0.96 a 1.24 b 0.0005 a 0.075 a 1.23 b 1.04 a 0.0002 b 0.022 b 2.03 a 0.74 b APS (nm) PV (cm3 g–1) SSA (m2 g–1)
Apart from turning dark brown, all crop straws were broken, split, and even smashed after SE pretreatment, which had a significant influence on the structure of the corresponding biochars. For example, the surface of biochars derived from SE-treated feedstocks became rougher, compared to the smooth surface, clear anatomy, and distinct pore structure in the biochars derived from the pristine feedstocks (Appendices B and C). The darkened color indicated slight carbonization of the feedstock and could probably be attributed to the formation of chromophoric groups, such as carboxyl, phenol, and other micromolecules, which were degraded from cellulose, hemicellulose, and lignin (Tooyserkani et al. 2013). In addition, the destruction of xylem vessels, parenchyma cells, and fibrous structures of plant tissues during the explosion might be conducive to the heat transmission and the formation of charcoal in the following pyrolysis (Carpenter et al. 2014). The pristine crop straws had an SSA of 1.69–6.07 m2 g–1 and pore volume (PV) of 0.010–0.034 cm3 g–1. The highest SSA was found in maize straw which was dominated by 2–4 nm mesopores. The SE pretreatment reduced the average pore sizes (APS) of oil-rape and rice straws by 26 and 58%, accordingly, increased SSA by 281 and 54%, respectively. Moreover, the PV of oil-rape straw increased by 184%, which were closely related to the augmentation of pores, especially the micropores (Table 2; Fig. 2-A). However, the PV of rice straw decreased by 36%, which was probably due to the shrinkage of meso/macropores (Fig. 2-B) and pore collapse during SE (Liang et al. 2016). The APS of maize and wheat straws was enhanced by 84 and 16% after SE, accordingly, the SSA values were decreased by 56 and 6%, respectively, the PV was also decreased by 18% in maize straw. The SE-treated cotton straw had 51% lower SSA and 55% lower PV, but 8% higher APS compared to the pristine cotton straw, which was attributed to the great decrease of both micropores and meso/macropores (Table 2; Appendix D). The obtained biochars had much smaller APS (43–92%) and
Table 2 Specific surface area and pore structure of the pristine and steam-exploded feedstocks (FS) and their biochars (BC)1）
3.3. Morphological characteristics and pore structure of crop straws and biochars
Meso/Macro-pore PV (cm3 g–1) MPS (nm) FS BC FS BC 0.019 b 0.145 a 2.03 a 2.24 a 0.020 a 0.091 b 2.03 a 2.05 b 0.025 a 0.164 b 3.14 a 2.05 a 0.016 b 0.167 a 2.02 b 2.03 b 0.036 a 0.040 a 2.24 b 3.55 a 0.028 b 0.028 b 2.54 a 2.03 b 0.010 b 0.052 b 2.79 a 3.13 a 0.030 a 0.130 a 2.04 b 2.02 b 0.012 a 0.078 a 4.41 a 2.05 a 0.006 b 0.060 b 3.56 b 2.02 b
were significantly decreased (Table 1). It indicated that carbonization occurred during the SE process (Tooyserkani et al. 2013). All the biochars had similar contents of C, H, O, and N. The C content, ranging from 65.64 to 77.33%, was 54–108% as high as that of the pristine feedstocks. Both H and O contents, ranging from 3.11 to 3.72% and from 18.16 to 29.70%, respectively, decreased by 34–43 and 20–59% compared with those of feedstocks. Accordingly, the ratios of H/C and O/C were largely reduced to around 0.6 and 0.2, which indicated more condensed aromatic structures and a higher degree of carbonization. The biochars derived from SE-treated feedstocks had 3–14% lower C but 10–60% higher O, accordingly, 10–89% higher O/C ratios compared with those from pristine feedstocks (Table 1). It may be related to the changes in ash and relative proportions of cellulose and lignin in the SE-treated feedstocks (Robert and Nys 2016).
SSA, specific surface area; PV, pore volume; APS, average pore size; MPS, median pore size. SE-wheat, SE-rice, SE-maize, SE-oil-rape, and SE-cotton represent the steam-exploded wheat, rice, maize, oil-rape, and cotton straws, respectively. Different letters in same crop straw or biochar indicate a significant difference (P<0.05) before and after the steam explosion.
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Crop straw2)
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0.160
0.120
dv/dw (cm3 g–1 nm-1)
0.0030
BORS BORS-SE
dv/dw (cm3 g–1 nm-1)
A
0.080
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ORS ORS-SE
0.0025 0.0020 0.0015 0.0010 0.0005 0.0000
1
10 100 Pore width (nm)
1 000
0.040
B 0.200
1
10
BRS BRS-SE
dv/dw (cm3 g–1 nm-1)
0.150
0.100
Pore width (nm)
100
0.0016
dv/dw (cm3 g–1 nm-1)
0.000
1 000
RS RS-SE
0.0012 0.0008 0.0004 0.0000
1
10 100 Pore width (nm)
1 000
0.050
0.000
1
10
100
1 000
Pore width (nm)
Fig. 2 Pore size distribution of pristine and steamed-exploded (SE) oil-rape straw (A), rice straw (B) and their biochars. BORS, biochars derived from oil-rape straw; BORS-SE, biochars derived from SE-treated oil-rape straw; ORS, oil-rape straw; ORS-SE, SE-treated oil-rape straw; BRS, biochars derived from rice straw; BRS-SE, biochars derived from SE-treated rice straw; RS, rice straw; RS-SE, SE-treated rice straw. dv/dw indicates the changes of pore volume with pore size.
median pore size (MPS) (15–57%) and therefore 3–161 times and 1–16 times higher SSA and PV respectively, compared to those of feedstocks (Table 2). The volatilization of organic components in plant tissues during pyrolysis could contribute to the changes in pores and increases of SSA and PV. The feedstocks with well-developed anatomy and small cells with highly volatile components are the main reasons why biochar has a rich porous structure (Zhao
et al. 2013, 2014). The wheat, rice, and cotton straws had a well-organized structure and readily formed small pores during pyrolysis. Their biochars thus had higher SSA and PV values. Meanwhile, maize and oil-rape straws had relatively loose tissues, so the resulting biochars were predominantly comprised of large pores and had lower SSA and PV values. Generally, SE pretreatment had a negative effect on the pore structure of biochars because of the lower
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SSA (44–91%) and PV (12–68%) values in SE-treated wheat, rice, maize, and cotton straws. But for the biochar derived from SE-treated oil-rape straw, it had 16 and 4 times higher SSA and PV, respectively, than those from the pristine feedstock, which may be related to the formation of small pores. Moreover, the destruction of anatomy structure and hemicellulose hydrolysis, lignin softening during the SE process might take influence on the volatilization of organic components in the subsequent pyrolysis, furthermore, favor the formation of large pores (Liang et al. 2016). That could explain why the biochars following SE pretreatment had lower SSA and PV values.
3.4. Surface functional groups of crop straws and biochars The tested crop straws had similar FTIR spectra which were significantly altered by the SE pretreatment (Fig. 3). After the SE, the peak intensity of 3 300–3 500 cm–1 from stretching vibration of hydroxyl groups (–OH) in cellulose or hemicellulose decreased, which suggested the decomposition of hemicellulose or removal of –OH. The peak at 1 738 cm–1 from –C=O that was the characteristic peak of hemicellulose (Zhang et al. 2008) nearly disappeared, which could be ascribed to deacetylation (Li et al. 2009). The peak at 2 923 cm–1 was the antisymmetric stretching vibration of –CH2– derived from alkane and associated with methyl and methylene, while peaks at 1 510 and 1 043 cm–1 represented the stretching vibrations of benzene rings and symmetric stretching vibrations of aromatic esters (C–O–C), respectively, which were all the characteristics of 100
Transmittance (%)
80
–CH2– –OH–
RM
Benzene ring –C=O C–O–C
SE 60 BC 40
BC-SE Aromatic C–H
20 0 3 500
Aromatic C=N/C=C 3 000 2 500 2 000 1 500 Wavenumber (cm–1)
1 000
500
Fig. 3 Fourier-transform infrared spectroscopy (FTIR) spectra of pristine and steam-exploded maize straws and their biochars. RM, pristine maize straw; SE, steamed-exploded maize straw; BC, biochar derived from pristine maize straw; BC-SE, biochar derived from steamed-exploded maize straw. The spectra of other crop straws and biochars are not shown due to their similarity.
lignin (Wood et al. 2014). Little change was observed in 2 923 and 1 043 cm–1, revealing the stable structure of lignin, but the intensity increase at 1 510 cm–1 could imply the enhancement of aromatic structures during the SE process (Zhang et al. 2008). Compared to the pristine feedstocks, the newly-formed peaks at 1 607–1 618 and 1 424 cm–1 in the SE-treated feedstocks represented the stretching vibrations of aromatic C=N/C=C and C-N, respectively, within primary amides. In addition, the emerging peak at 1 111 cm–1 indicating the stretching vibrations of acyclic aliphatic anhydride (C–O–C) might be the result of dehydration and condensation among hydrolysates, under high temperature and pressure during the SE process. Generally, the surface functional groups in the biochars didn’t show much difference regardless of the SE pretreatment but had changed a lot (Fig. 3) owing to a series of chemical reactions during pyrolysis, compared to that in the feedstocks. For example, the large intensity reduction of 3 300–3 500 cm–1 may reveal severe dehydration and chemical bond cleavage. The disappearance of the peak at 2 923 cm–1 might indicate the breakage of methoxyl groups in lignin and formation of CH4 (Yang et al. 2007). In addition, the peaks at 798 and 1 607 cm–1 related to the stretching vibrations of aromatic C–H and C=C in the carbon skeleton, respectively, indicated higher aromaticity of biochars than feedstocks.
3.5. Surface charge of biochars The obtained biochars carried a large number of surface charges (Fig. 4; Appendix E). Three types of functional groups, i.e., oxonium heterocycles, N heterocycles, and condensed aromatic carbon which attract protons from aqueous solutions, can provide positive charge on biochar surface (Lawrinenko et al. 2017). The positive charge decreased rapidly with increasing solution pH values in this study, which indicated that they were mainly originated from N heterocycles and condensed aromatic carbon that were found to be the primary sources of pH-dependent positive charge (Lawrinenko et al. 2017). The anion exchange capacity (AEC) values ranged from 2.13 to 4.87 cmol kg–1, higher in biochars derived from maize straw but lower in those from wheat straw, which might be related to the functional groups in different feedstocks. With the increasing pH values, the negative charge had a trend of increase (Fig. 4; Appendix E), which has been widely recognized to be attributed to the deprotonation of acid oxygen-containing functional groups, such as carboxyl, hydroxyl, and carbonyl. The minerals on biochar could also provide the permanent negative charge (Zhao et al. 2015). However, acid washing could efficiently remove the minerals (Chun et al. 2004), so the CEC values of the biochars in this
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Positive charge
B 120
120
Surface charge (cmol kg–1)
Surface charge (cmol kg–1)
A 140
100 80 60 40 20 0
Negative charge
2
4
pH
6
8
10
100 80 60 40 20 0
0
2
4
6
pH
8
10
12
14
Fig. 4 Changes in surface positive and negative charges with medium pH values for the biochars derived from pristine (A) and steam-exploded (B) oil-rape straw. Table 3 Point of zero net charge (PZNC), anion exchange capacity (AEC), and cation exchange capacity (CEC) of biochars derived from pristine and steam-exploded crop straws Biochar1) Wheat straw SE-wheat straw Rice straw SE-rice straw Maize straw SE-maize straw Oil-rape straw SE-oil-rape straw Cotton straw SE-cotton straw 1)
PZNC 2.88 4.12 2.83 4.59 2.70 5.32 2.96 4.11 2.87 3.61
AEC (cmol kg–1) (pH3.5) 2.13 6.88 2.51 7.15 4.87 11.44 4.27 7.94 2.62 7.45
CEC (cmol kg–1) (pH7) 10.80 14.07 13.21 8.11 14.43 9.78 15.03 24.25 10.48 16.54
SE-wheat straw, SE-rice straw, SE-maize straw, SE-oil-rape straw, and SE-cotton straw represent the biochars derived from steam-exploded wheat, rice, maize, oil-rape, and cotton straws, respectively.
study, ranging from 10.48 to 15.03 cmol kg–1, were much lower than those reported in the literature (10–540 cmol kg–1) (Zhao et al. 2015). It is worth noting that the AEC values of biochars derived from steam-exploded feedstocks (6.88–11.44 cmol kg–1) were approximately 2–3 times as high as that of the biochars from pristine feedstocks (Table 3), which suggested the increase of positive charge. On the one hand, it may be closely related to the N-containing functional groups, such as aromatic C=N and C–N (Fig. 3) within primary amides or some N heterocycles. Moreover, SE pretreatment induced slight carbonization of the feedstock, which indicated more condensed aromatic carbon that can attract protons and provide positive charge (Lawrinenko and Laird 2015). The PZNC values of all the biochars derived from SE-treated feedstocks significantly increased from pH 2.70–2.96 to pH 3.61–5.32, but SE pretreatment led to inconsistent changes
as to the CEC values. For instance, SE pretreatment induced a reduction of the negative charge in biochars derived from rice and maize straws, their CEC values were 8.11 and 9.78 cmol kg–1, 39 and 32%, respectively, lower than those of pristine feedstocks. On the contrary, the CEC values of biochars derived from SE-treated wheat, oil-rape, and cotton straw, 14.07, 24.25, and 16.54 cmol kg–1, respectively, were increased by 30, 61, and 58%, compared to that of biochars from pristine crop straws. All these variations were likely to be related to the acid oxygen-containing groups, but the exact mechanisms are still needed for the investigation.
3.6. Pedological and environmental implications Biochar has demonstrated clear potential as a soil amendment, even though the results are contradictory depending on different feedstocks, pyrolysis conditions, and the types of soil (Jeffery et al. 2011; He et al. 2017). The biochars derived from wheat, rice, and cotton straws in this study exhibited large specific surface area (>180 m2 g–1), which met the requirement that the surface area of biochars should be >100 m2 g–1 for a good performance in the soil (Schimmelpfennig and Glaser 2012). After SE pretreatment, the biochar derived from oil-rape straw owned a surface area of 242.06 m2 g–1 that was much larger than the biochars produced at a similar pyrolysis condition (Kambo and Dutta 2015). The highly porous structure of these biochars may imply their capacity to improve soil aeration and restore plant available water (0.2–50 μm), especially in sandy soils (Abel et al. 2013). Moreover, it could provide an asylum to the beneficial soil organisms such as arbuscular mycorrhiza (AM) fungi and plant growth promoting rhizobacteria (PGPR), which would further aid the supply of water and nutrients to the plants (Kavitha et al. 2018).
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Since the environmental issues, such as inorganic and organic pollutants contamination have attracted an increasing attention, biochar is representing a kind of suitable candidates for soil remediation, which is largely ascribed to its surface electrochemistry. A few numbers of researches have focused on the CEC of biochars which could enhance the nutrient retention capacity of the soil, and contribute to the adsorption of cationic contaminant (Zhao et al. 2015). Besides the CEC, biochars were detected to have a certain amount of AEC (6.88–11.44 cmol kg–1) in this study. The SE pretreatment significantly increased the AEC values of biochars regardless of the type of feedstocks. The enhanced AEC may reduce leaching of anionic nutrients in the soil, or be conducive to the adsorption of anionic pollutants, such as phosphate and arsenate.
4. Conclusion The SE pretreatment on crop straws induced significant changes in their lignocellulosic structure and could be a possible method to improve the surface property of biochars. The significant levels of positive charges and AEC values in biochars derived from SE-treated crop straws showed a potential for soil amendment, with regards to the retention of anionic nutrients as well as adsorption of pollutants. Oil-rape straw might be a suitable candidate for SE pretreatment, owing to the remarkable increase of SSA in both the feedstock and biochar, which indicated a high potential for water and nutrients retention.
Acknowledgements This work was funded by the National Key Technology R&D Program of China (2015BAD05B03). Appendices associated with this paper can be available on http://www.ChinaAgriSci.com/V2/En/appendix.htm
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Exe cu tive Editor-in-Chief ZHANG Wei-li Managing editor SUN Lu-juan
ScienceDirect
RESEARCH ARTICLE
Steam explosion of crop straws improves the characteristics of biochar as a soil amendment CHEN Xue-jiao1, LIN Qi-mei1, 2, Muhammad Rizwan1, ZHAO Xiao-rong1, 2, LI Gui-tong1, 2 1 2
College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, P.R.China Key Laboratory of Arable Land Conservation (North China), Ministry of Agriculture/Key Laboratory of Plant-Soil Interactions, Ministry of Education, Beijing 100193, P.R.China
Abstract Five crop straws (wheat, rice, maize, oil-rape, and cotton) were first steam-exploded for 2 min at 210°C, 2.5 MPa and then pyrolyzed at 500°C for 2 h. Steam explosion (SE) induced 47–95% and 5–16% reduction of hemicellulose and cellulose, respectively, in the crop straws. The biochars derived from SE-treated feedstocks had a lower specific surface area (SSA) and pore volume, compared to those from pristine feedstocks, with one exception that SE enhanced SSA of oil-rape straw biochar by approximately 16 times. After SE, biochars had significant higher anion exchange capacity (AEC) (6.88–11.44 cmol kg–1) and point of zero net charges (PZNC) (pH 3.61–5.32) values. It can thus be speculated that these biochars may have higher potential for anions adsorption. In addition, oil-rape straw might be suitable to SE pretreatment for preparing biochar as a soil amendment and sorbent as well. Further work is required for testing its application in soil. Keywords: crop straws, steam explosion, biochar, characterization
(Schimmelpfennig and Glaser 2012; Kambo and Dutta
1. Introduction Biochar has been recently considered as a promising soil conditioner (Luo et al. 2014; Agegnehu et al. 2016) and suitable biosorbent as well for removing pollutants from wastewater (Trakal et al. 2014). These functions of biochar are greatly dependent on its nature, such as recalcitrance to chemical and microbiological decomposition (Naisse et al. 2013), porous structure and enormous surface area
2015), abundant surface functional groups (Chen et al. 2017), and positive and negative charges (Zhao et al. 2015). Of these characters, pore structure, ash, minerals, and surface charge are predominately controlled by the feedstocks (Zhao et al. 2013). For example, under the same pyrolysis conditions, biochars originated from woody biomass are predominated by micropores and have a larger specific surface area, compared with those from crop straws and manure which have more mesopores (Zhao et al. 2014). However, biochars derived from manure usually have higher adsorption capacities of heavy metal ions than those from
Received 30 May, 2018 Accepted 12 November, 2018 CHEN Xue-jiao, E-mail: [email protected]; Correspondence LIN Qi-mei, Tel: +86-10-62732502, E-mail: [email protected]
plant residues, because the former is usually abundant of
© 2019 CAAS. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). doi: 10.1016/S2095-3119(19)62573-6
in China, are usually used as the feedstock for producing
ash and minerals (Cao et al. 2009; Luo et al. 2014). Crop straws, one of the richest renewable resources biochar.
However, because of the highly compact
architecture of the lignocellulosic structure, crop straws need
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to be pretreated through grinding, hydrolysis, irradiation, torrefaction, etc., prior to the further utilization, for a higher efficiency. Steam explosion (SE) is one of the most common methods used for crop straw pretreatment, because of its less environmental impact, lower cost and energy demand, compared to other treatments (Kan et al. 2016). During the SE process, biomass is subjected to high temperature (160–260°C) and pressure (0.69–4.83 MPa) followed by a sudden decompression. As a result, the fiber material is scattered and the covalent bonds between hemicellulose and lignin are broken as well (Li et al. 2007; Su et al. 2015). It is reported that SE pretreatment could increase the digestibility and saccharification of biomass (Sarkar et al. 2012). Deepa et al. (2011) found that SE led to a reduction in the diameter of banana fibers but increase in crystallinity degree and thermal stability of cellulose. Biswas et al. (2011) measured increases in thermal stability of both lignin and cellulose, and decomposition rate of hemicellulose in salix wood as well following SE pretreatment. Das and Sarmah (2015) reported that the steam treatment of fir wood at 121°C for 30 min led to much lower contents of xylose and mannose in the bio-oil, whereas little change in the yields of both bio-oil and biochar. They suggested that steam could efficiently decompose the hemicellulose. Wang et al. (2011) also observed a similar change with regards to the hemicellulose in SE-treated pinewood but a lower bio-oil yield in the further pyrolysis. However, Li et al. (2007) detected some “unexploded” materials and therefore argued that SE pretreatment might not be suitable for all feedstocks because the products were highly heterogeneous. It is known that the crop residues have distinct fiber structure and thus may be suitable for different pretreatment methods (Buranov and Mazza 2008). Wheat, rice, maize, oil-rape, and cotton are the common crops and widely cultivated in China, which therefore generates a large number of residues especially in the harvest seasons.
According to the CAYEC (2014), the annual total production of these crop straws were more than 8×108 tons. Apart from returning to the field, the remaining crop straws are still abundant and need to be utilized properly. To date, little information is available regarding whether SE can pretreat the crop straws for the purpose of achieving proper biochars as a soil amendment. Therefore, in this study, five crop straws were subjected to SE pretreatment and then slowly pyrolyzed in a muffle furnace. The obtained biochars were analyzed for their properties. We hypothesized that the changes in crop straws following SE pretreatment could further influence their biochars, and the characteristics of biochar might vary greatly with crop straws. The goals were: (1) to compare the impacts of SE pretreatment on different crop straws and their biochars; (2) to measure the properties of biochar derived from steam-exploded crop straws; and (3) to evaluate their potential values as soil amendments or for other uses.
2. Materials and methods 2.1. Crop straws The tested crop straws included wheat (WS), rice (RS), maize (MS), oil-rape (ORS), and cotton (CS) straws which were collected from Shangzhuang Experimental Station (40.14°N, 116.18°E) , China Agricultural University, Beijingand Linquan County (33.08°N, 115.23°E), Anhui Province, China, respectively. After air-drying, they were divided into three subsamples randomly and smashed into 1–2 cm segments. The basic properties are shown in Table 1 and Fig. 1.
2.2. Steam explosion The crop straws of WS, RS, MS, ORS, and CS were steam-
Table 1 The yield and chemical composition of pristine and steam-exploded feedstocks (FS) and their biochars (BC) Crop straws1) Wheat SE-wheat Rice SE-rice Maize SE-maize Oil-rape SE-oil-rape Cotton SE-cotton 1)
Yield (%) BC 39.53 b 43.07 a 37.46 b 40.69 a 37.07 b 41.92 a 35.60 b 40.52 a 31.95 b 34.16 a
C (%) FS BC 36.80 b 76.62 a 40.78 a 65.64 b 39.09 b 74.91 a 41.23 a 67.24 b 40.70 b 69.89 b 46.10 a 70.81 a 41.28 b 75.26 a 44.57 a 73.14 b 44.81 b 77.33 a 45.21 a 75.07 b
H (%) FS BC 5.33 a 3.54 a 5.31 a 3.35 b 5.73 a 3.72 a 5.40 b 3.11 b 5.74 a 3.71 a 5.61 b 3.60 b 5.91 a 3.65 a 5.60 b 3.68 a 6.12 a 3.51 b 6.03 b 3.65 a
O (%) FS BC 42.46 a 18.59 b 37.22 b 29.70 a 40.87 a 20.12 b 37.39 b 28.43 a 41.30 a 24.68 a 36.21 b 23.97 b 41.39 a 19.79 b 37.17 b 21.73 a 44.28 a 18.16 b 43.41 b 20.38 a
N (%) FS BC 0.90 a 1.25 b 0.83 b 1.31 a 0.71 b 1.25 a 0.76 a 1.22 b 1.21 a 1.72 a 1.05 b 1.62 b 1.11 b 1.30 b 1.19 a 1.45 a 0.66 a 1.00 a 0.66 a 0.90 b
Atom H/C FS BC 1.74 a 0.55 b 1.56 b 0.61 a 1.76 a 0.60 a 1.57 b 0.56 b 1.69 a 0.64 a 1.46 b 0.61 b 1.72 a 0.58 b 1.51 b 0.60 a 1.64 a 0.54 b 1.60 b 0.58 a
Atom O/C FS BC 0.87 a 0.18 b 0.68 b 0.34 a 0.78 a 0.20 b 0.68 b 0.32 a 0.76 a 0.26 a 0.59 b 0.25 b 0.75 a 0.20 b 0.63 b 0.22 a 0.74 a 0.18 b 0.72 b 0.20 a
SE-wheat, SE-rice, SE-maize, SE-oil-rape, and SE-cotton represent the steam-exploded wheat, rice, maize, oil-rape, and cotton straws, respectively. Different letters in same crop straw or biochar indicate a significant change (P<0.05) before and after the steam explosion.
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RM
and functional groups.
SE
60
Cellulose (%)
40
2.3. Biochar production
a
50 ab
a
b
a b
a
30
b
b
20 10 0
B
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a
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a
25
2.4. Assay methods
20
a
a
15
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Wheat
b Wheat
b
b
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Three portions of each pristine crop straw (WS, RS, MS, ORS, and CS) and each steam-exploded straw (WSSE, RSSE, MSSE, ORSSE, and CSSE), each portion 100 g, were separately placed into stainless steel cylinders (diameter=7.5 cm, height=11 cm) with screw lids at both ends in which one end was connected to a steel tube for N2 gas flow. All the cylinders were placed in a muffle furnace (SX4-10; Shuli, China), heated to 500°C at an increasing rate of 20°C min–1, and then maintained for 2 h under an N2 gas atmosphere (10 psi). The collected biochars were defined as BWS, BRS, BMS, BORS, BCS and BWSSE, BRSSE, BMSSE, BORSSE, BCSSE, respectively, and weighed for calculating the yields.
a b a
b a
Wheat
Rice
b
Maize Oil-rape Cotton Crop straws
Fig. 1 Cellulose (A), hemicellulose (B), and lignin (C) in the pristine (RM) and steam-exploded (SE) crop straws. Different letters in the same crop straw indicate a significant difference at the P<0.05 level. The bars in each column represent standard deviation of the means.
exploded for 2 min at 210°C and 2.5 MPa (Su et al. 2015) using a QB-200 platform in Hebi Heavy-Duty Mechanical Factory, China. The resulting products, recorded as WSSE, RSSE, MSSE, ORSSE, and CSSE, respectively, were oven-dried at 85°C for biochar production and measured for cellulose, hemicellulose, lignin, carbon (C), hydrogen (H), nitrogen (N), silicon (Si), morphological characteristics, pore structure,
The biochars are known to contain a high content of ash and soluble organic components, which are readily dissolved over time and thus play short-term roles in soil (Pereira et al. 2015). Therefore, in order to determine the intrinsical surface properties, as well as the long-term effect, biochars were usually demineralized with acids prior to assay (Chun et al. 2004; Li et al. 2017; Yue et al. 2017). Similar to the previous studies, the bulk of the biochars was thoroughly mixed with 1 mol L–1 HCl (1:10, w/v) at room temperature for 30 min, and then centrifuged in this study. Following three rounds of this procedure, the biochars were further washed with a mixture solution of 1 mol L–1 HCl and 1 mol L–1 HF. At the last step, the biochars were rinsed with deionized water until the pH values of the leachates were neutral, and then oven-dried at 60°C for assaying C, H, N, morphological characteristics, pore structure, surface charge, and functional groups. Cellulose, hemicellulose, and lignin were measured using the Van Soest method (Van Soest et al. 1991). Total C, H, and N were examined by an element analyzer (Vario EL III, Elementar Analysensysteme GmbH, Germany), and oxygen (O) content was calculated as the difference between 100% and the sum of ash, C, H, and N. Total Si content was measured by the molybdenum blue colorimetric method following NaOH digestion (Elliott and Snyder 1991). Morphological characteristics and functional groups were investigated using a scanning electronic microscope (SEM; S-3400N, Hitachi, Japan) and a Fourier transform infrared spectrometer (FTIR; Shimadzu, Japan), respectively. Pore structure was determined by a V-sorb 2008P Pore Analyzer (Gold APP, China). The specific surface area was obtained based on the multi-point Brunauer-Emmett-Teller (BET) adsorption isotherm. The analysis of both mesopores
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and macropores was implemented by the BJH (BarrettJoyner-Halenda (BJH) method, while that of micropores by the SF (Saito-Foley) method. Surface charges were assessed by the ion adsorption method (Cheng et al. 2008). The point of zero net charges (PZNC), anion exchange capacity (AEC), and cation exchange capacity (CEC) were calculated, respectively, as the pH where the amount of positive charge is equal to negative charge, the amount of net surface positive charge at pH 3.5, and the amount of surface negative charge at pH 7. The detailed procedures could be found in Appendix A.
2.5. Statistical analysis Apart from SEM and FTIR, all data were the means of triplicates and based on the oven-dry mass. An analysis of variance (ANOVA) was carried out in PASW Statistics18 Software. The significant differences were revealed by the least significant difference (LSD) test at the P<0.05 level.
3. Results and discussion 3.1. Changes of cellulose, hemicellulose, and lignin in crop straws The pristine crop straws contained 34–49% cellulose, 10–29% hemicellulose, and 4–26% lignin. Wheat, rice, and maize straws had higher hemicellulose, while oil-rape and cotton straws had higher lignin (Fig. 1). Hemicellulose significantly decreased by 47–95% following the SE pretreatment, which was similar to the result obtained by Bauer et al. (2014) and Huang et al. (2015). Cellulose decreased by 5–16%, much lower than hemicellulose, because the acetyl groups in the hemicellulose were hydrolyzed once the temperature exceeded 150°C, whereas the hydrolysis of cellulose started at a higher temperature of 200°C (Hendriks and Zeeman 2009). The decrease of hemicellulose was greater in maize straw and lower in cotton straw (Fig. 1-B). But for cellulose, the decrease was greater in cotton straw and lower in wheat straw (Fig. 1-A). These variations may largely depend on the structural characteristics of the tested crop straws (Sarkar et al. 2012). The cotton straw has a similar lignin structure to hardwood which is considerably different from that of other herbaceous crop straws, such as maize, wheat, and rice straws. The covalent bonds between hemicellulose and lignin in cotton straw might be more stable (Buranov and Mazza 2008; Wu et al. 2015). Therefore, the reduction of hemicellulose was much lower for cotton straw (47%), in contrast to other straws (84–95%). The great decrease in cellulose for cotton straw might be the consequence of the weak hydrogen bonds between hemicellulose and cellulose (Huang et al.
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2015). However, the slight decrease of cellulose in wheat straw (5%) might be largely related to the high degree of polymerization (Zhao et al. 2016). The SE pretreatment induced lignin to increase by 8–18% in wheat and rice straws, whereas decrease by 29% in maize straw, and change little in oil-rape and cotton straws (Fig. 1-C), which indicated that the change of lignin was highly dependent on the type of biomass. During SE process, hydrolysis of acetyl groups in hemicellulose can lead to an acidic environment and formation of the carbonium ion by the proton-induced elimination of ROH (water, alcohol, or acid) from the benzylic position. The carbonium ion might react in a β-O-4’ inter-unit connection structure (Li et al. 2009). The ether bonds between lignin and hemicellulose in maize straw could be effectively destroyed by SE pretreatment (Buranov and Mazza 2008). With more hemicellulose losing, the bond linking between lignin and hemicellulose would be affected and the lignin must be destroyed in a certain extent. Little change of lignin in oil-rape and cotton straws is probably because of lower reduction of hemicellulose, and relatively stable linkages between hemicellulose and lignin. Huang et al. (2015) reported that depolymerization and condensation of lignin coexisted during the SE process. The monosaccharides degraded from polysaccharides and electron-rich C atoms in guaiacyl and syringyl rings from lignin might be conducive to polymerization of the small decomposed molecules (Bauer et al. 2014), and therefore induced the increase of lignin in wheat and rice straws.
3.2. Changes of biochar yield and chemical composition The biochar yields of pristine crop straws were 31.95– 39.53%, the lowest in cotton straw and the highest in wheat straw. The SE-treated feedstocks had 7–14% higher biochar yields than those of pristine feedstocks (Table 1), which might be attributed to the reduction of hemicellulose and cellulose (Tripathi et al. 2016). Among the tested crop straws, cotton straw had relatively higher C and H, but lower N and wheat straw had relatively lower C and H (Table 1). Both wheat and rice straws had more than 14.16% Si, while others contained less than 4.59% Si (data not shown). After SE pretreatment, C content in crop straws increased by 1–13%, which might be primarily due to changes in the epidermis (Su et al. 2015) and induced by the formation of stable C-C structures following the condensation of monosaccharides and lignin (Li et al. 2007). Moreover, the O content decreased by 2–12%, which was owing to the dehydration during condensation and the hydrolysis of hydroxyl groups (-OH) in hemicellulose. The content of H, N, and Si was only slightly changed, but H/C and O/C ratios
2)
1)
Wheat SE-wheat Rice SE-rice Maize SE-maize Oil-rape SE-oil-rape Cotton SE-cotton
FS 2.69 a 2.54 b 1.69 b 2.61 a 6.07 a 2.67 b 1.74 b 6.63 a 1.95 a 0.96 b
BC 434.09 a 38.61 b 182.59 a 102.62 b 18.08 a 7.01 b 15.16 b 242.06 a 201.29 a 56.45 b
FS 0.018 b 0.020 a 0.024 a 0.016 b 0.034 a 0.028 b 0.010 b 0.028 a 0.012 a 0.005 b
BC 0.289 a 0.092 b 0.202 a 0.178 b 0.041 a 0.028 b 0.049 b 0.197 a 0.136 a 0.071 b
FS 26.97 b 31.17 a 57.61 a 23.96 b 22.32 b 41.12 a 22.80 a 16.96 b 23.63 a 21.85 b
BC 2.66 b 9.49 a 4.42 b 6.93 a 9.16 b 16.11 a 13.00 a 3.25 b 2.70 b 5.02 a
Micro-pore MPS (nm) PV (cm3 g–1) FS BC FS BC 0.0007 a 0.170 a 1.33 b 0.57 a 0.0007 a 0.012 b 1.66 a 0.54 b 0.0005 a 0.069 a 1.10 b 0.79 a 0.0005 a 0.035 b 2.37 a 0.48 b 0.0009 a 0.006 a 1.69 a 1.10 b 0.0006 b 0.003 b 1.68 a 1.33 a 0.0005 b 0.003 b 0.88 b 1.76 a 0.0011 a 0.090 a 0.96 a 1.24 b 0.0005 a 0.075 a 1.23 b 1.04 a 0.0002 b 0.022 b 2.03 a 0.74 b APS (nm) PV (cm3 g–1) SSA (m2 g–1)
Apart from turning dark brown, all crop straws were broken, split, and even smashed after SE pretreatment, which had a significant influence on the structure of the corresponding biochars. For example, the surface of biochars derived from SE-treated feedstocks became rougher, compared to the smooth surface, clear anatomy, and distinct pore structure in the biochars derived from the pristine feedstocks (Appendices B and C). The darkened color indicated slight carbonization of the feedstock and could probably be attributed to the formation of chromophoric groups, such as carboxyl, phenol, and other micromolecules, which were degraded from cellulose, hemicellulose, and lignin (Tooyserkani et al. 2013). In addition, the destruction of xylem vessels, parenchyma cells, and fibrous structures of plant tissues during the explosion might be conducive to the heat transmission and the formation of charcoal in the following pyrolysis (Carpenter et al. 2014). The pristine crop straws had an SSA of 1.69–6.07 m2 g–1 and pore volume (PV) of 0.010–0.034 cm3 g–1. The highest SSA was found in maize straw which was dominated by 2–4 nm mesopores. The SE pretreatment reduced the average pore sizes (APS) of oil-rape and rice straws by 26 and 58%, accordingly, increased SSA by 281 and 54%, respectively. Moreover, the PV of oil-rape straw increased by 184%, which were closely related to the augmentation of pores, especially the micropores (Table 2; Fig. 2-A). However, the PV of rice straw decreased by 36%, which was probably due to the shrinkage of meso/macropores (Fig. 2-B) and pore collapse during SE (Liang et al. 2016). The APS of maize and wheat straws was enhanced by 84 and 16% after SE, accordingly, the SSA values were decreased by 56 and 6%, respectively, the PV was also decreased by 18% in maize straw. The SE-treated cotton straw had 51% lower SSA and 55% lower PV, but 8% higher APS compared to the pristine cotton straw, which was attributed to the great decrease of both micropores and meso/macropores (Table 2; Appendix D). The obtained biochars had much smaller APS (43–92%) and
Table 2 Specific surface area and pore structure of the pristine and steam-exploded feedstocks (FS) and their biochars (BC)1）
3.3. Morphological characteristics and pore structure of crop straws and biochars
Meso/Macro-pore PV (cm3 g–1) MPS (nm) FS BC FS BC 0.019 b 0.145 a 2.03 a 2.24 a 0.020 a 0.091 b 2.03 a 2.05 b 0.025 a 0.164 b 3.14 a 2.05 a 0.016 b 0.167 a 2.02 b 2.03 b 0.036 a 0.040 a 2.24 b 3.55 a 0.028 b 0.028 b 2.54 a 2.03 b 0.010 b 0.052 b 2.79 a 3.13 a 0.030 a 0.130 a 2.04 b 2.02 b 0.012 a 0.078 a 4.41 a 2.05 a 0.006 b 0.060 b 3.56 b 2.02 b
were significantly decreased (Table 1). It indicated that carbonization occurred during the SE process (Tooyserkani et al. 2013). All the biochars had similar contents of C, H, O, and N. The C content, ranging from 65.64 to 77.33%, was 54–108% as high as that of the pristine feedstocks. Both H and O contents, ranging from 3.11 to 3.72% and from 18.16 to 29.70%, respectively, decreased by 34–43 and 20–59% compared with those of feedstocks. Accordingly, the ratios of H/C and O/C were largely reduced to around 0.6 and 0.2, which indicated more condensed aromatic structures and a higher degree of carbonization. The biochars derived from SE-treated feedstocks had 3–14% lower C but 10–60% higher O, accordingly, 10–89% higher O/C ratios compared with those from pristine feedstocks (Table 1). It may be related to the changes in ash and relative proportions of cellulose and lignin in the SE-treated feedstocks (Robert and Nys 2016).
SSA, specific surface area; PV, pore volume; APS, average pore size; MPS, median pore size. SE-wheat, SE-rice, SE-maize, SE-oil-rape, and SE-cotton represent the steam-exploded wheat, rice, maize, oil-rape, and cotton straws, respectively. Different letters in same crop straw or biochar indicate a significant difference (P<0.05) before and after the steam explosion.
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Crop straw2)
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0.160
0.120
dv/dw (cm3 g–1 nm-1)
0.0030
BORS BORS-SE
dv/dw (cm3 g–1 nm-1)
A
0.080
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ORS ORS-SE
0.0025 0.0020 0.0015 0.0010 0.0005 0.0000
1
10 100 Pore width (nm)
1 000
0.040
B 0.200
1
10
BRS BRS-SE
dv/dw (cm3 g–1 nm-1)
0.150
0.100
Pore width (nm)
100
0.0016
dv/dw (cm3 g–1 nm-1)
0.000
1 000
RS RS-SE
0.0012 0.0008 0.0004 0.0000
1
10 100 Pore width (nm)
1 000
0.050
0.000
1
10
100
1 000
Pore width (nm)
Fig. 2 Pore size distribution of pristine and steamed-exploded (SE) oil-rape straw (A), rice straw (B) and their biochars. BORS, biochars derived from oil-rape straw; BORS-SE, biochars derived from SE-treated oil-rape straw; ORS, oil-rape straw; ORS-SE, SE-treated oil-rape straw; BRS, biochars derived from rice straw; BRS-SE, biochars derived from SE-treated rice straw; RS, rice straw; RS-SE, SE-treated rice straw. dv/dw indicates the changes of pore volume with pore size.
median pore size (MPS) (15–57%) and therefore 3–161 times and 1–16 times higher SSA and PV respectively, compared to those of feedstocks (Table 2). The volatilization of organic components in plant tissues during pyrolysis could contribute to the changes in pores and increases of SSA and PV. The feedstocks with well-developed anatomy and small cells with highly volatile components are the main reasons why biochar has a rich porous structure (Zhao
et al. 2013, 2014). The wheat, rice, and cotton straws had a well-organized structure and readily formed small pores during pyrolysis. Their biochars thus had higher SSA and PV values. Meanwhile, maize and oil-rape straws had relatively loose tissues, so the resulting biochars were predominantly comprised of large pores and had lower SSA and PV values. Generally, SE pretreatment had a negative effect on the pore structure of biochars because of the lower
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SSA (44–91%) and PV (12–68%) values in SE-treated wheat, rice, maize, and cotton straws. But for the biochar derived from SE-treated oil-rape straw, it had 16 and 4 times higher SSA and PV, respectively, than those from the pristine feedstock, which may be related to the formation of small pores. Moreover, the destruction of anatomy structure and hemicellulose hydrolysis, lignin softening during the SE process might take influence on the volatilization of organic components in the subsequent pyrolysis, furthermore, favor the formation of large pores (Liang et al. 2016). That could explain why the biochars following SE pretreatment had lower SSA and PV values.
3.4. Surface functional groups of crop straws and biochars The tested crop straws had similar FTIR spectra which were significantly altered by the SE pretreatment (Fig. 3). After the SE, the peak intensity of 3 300–3 500 cm–1 from stretching vibration of hydroxyl groups (–OH) in cellulose or hemicellulose decreased, which suggested the decomposition of hemicellulose or removal of –OH. The peak at 1 738 cm–1 from –C=O that was the characteristic peak of hemicellulose (Zhang et al. 2008) nearly disappeared, which could be ascribed to deacetylation (Li et al. 2009). The peak at 2 923 cm–1 was the antisymmetric stretching vibration of –CH2– derived from alkane and associated with methyl and methylene, while peaks at 1 510 and 1 043 cm–1 represented the stretching vibrations of benzene rings and symmetric stretching vibrations of aromatic esters (C–O–C), respectively, which were all the characteristics of 100
Transmittance (%)
80
–CH2– –OH–
RM
Benzene ring –C=O C–O–C
SE 60 BC 40
BC-SE Aromatic C–H
20 0 3 500
Aromatic C=N/C=C 3 000 2 500 2 000 1 500 Wavenumber (cm–1)
1 000
500
Fig. 3 Fourier-transform infrared spectroscopy (FTIR) spectra of pristine and steam-exploded maize straws and their biochars. RM, pristine maize straw; SE, steamed-exploded maize straw; BC, biochar derived from pristine maize straw; BC-SE, biochar derived from steamed-exploded maize straw. The spectra of other crop straws and biochars are not shown due to their similarity.
lignin (Wood et al. 2014). Little change was observed in 2 923 and 1 043 cm–1, revealing the stable structure of lignin, but the intensity increase at 1 510 cm–1 could imply the enhancement of aromatic structures during the SE process (Zhang et al. 2008). Compared to the pristine feedstocks, the newly-formed peaks at 1 607–1 618 and 1 424 cm–1 in the SE-treated feedstocks represented the stretching vibrations of aromatic C=N/C=C and C-N, respectively, within primary amides. In addition, the emerging peak at 1 111 cm–1 indicating the stretching vibrations of acyclic aliphatic anhydride (C–O–C) might be the result of dehydration and condensation among hydrolysates, under high temperature and pressure during the SE process. Generally, the surface functional groups in the biochars didn’t show much difference regardless of the SE pretreatment but had changed a lot (Fig. 3) owing to a series of chemical reactions during pyrolysis, compared to that in the feedstocks. For example, the large intensity reduction of 3 300–3 500 cm–1 may reveal severe dehydration and chemical bond cleavage. The disappearance of the peak at 2 923 cm–1 might indicate the breakage of methoxyl groups in lignin and formation of CH4 (Yang et al. 2007). In addition, the peaks at 798 and 1 607 cm–1 related to the stretching vibrations of aromatic C–H and C=C in the carbon skeleton, respectively, indicated higher aromaticity of biochars than feedstocks.
3.5. Surface charge of biochars The obtained biochars carried a large number of surface charges (Fig. 4; Appendix E). Three types of functional groups, i.e., oxonium heterocycles, N heterocycles, and condensed aromatic carbon which attract protons from aqueous solutions, can provide positive charge on biochar surface (Lawrinenko et al. 2017). The positive charge decreased rapidly with increasing solution pH values in this study, which indicated that they were mainly originated from N heterocycles and condensed aromatic carbon that were found to be the primary sources of pH-dependent positive charge (Lawrinenko et al. 2017). The anion exchange capacity (AEC) values ranged from 2.13 to 4.87 cmol kg–1, higher in biochars derived from maize straw but lower in those from wheat straw, which might be related to the functional groups in different feedstocks. With the increasing pH values, the negative charge had a trend of increase (Fig. 4; Appendix E), which has been widely recognized to be attributed to the deprotonation of acid oxygen-containing functional groups, such as carboxyl, hydroxyl, and carbonyl. The minerals on biochar could also provide the permanent negative charge (Zhao et al. 2015). However, acid washing could efficiently remove the minerals (Chun et al. 2004), so the CEC values of the biochars in this
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Positive charge
B 120
120
Surface charge (cmol kg–1)
Surface charge (cmol kg–1)
A 140
100 80 60 40 20 0
Negative charge
2
4
pH
6
8
10
100 80 60 40 20 0
0
2
4
6
pH
8
10
12
14
Fig. 4 Changes in surface positive and negative charges with medium pH values for the biochars derived from pristine (A) and steam-exploded (B) oil-rape straw. Table 3 Point of zero net charge (PZNC), anion exchange capacity (AEC), and cation exchange capacity (CEC) of biochars derived from pristine and steam-exploded crop straws Biochar1) Wheat straw SE-wheat straw Rice straw SE-rice straw Maize straw SE-maize straw Oil-rape straw SE-oil-rape straw Cotton straw SE-cotton straw 1)
PZNC 2.88 4.12 2.83 4.59 2.70 5.32 2.96 4.11 2.87 3.61
AEC (cmol kg–1) (pH3.5) 2.13 6.88 2.51 7.15 4.87 11.44 4.27 7.94 2.62 7.45
CEC (cmol kg–1) (pH7) 10.80 14.07 13.21 8.11 14.43 9.78 15.03 24.25 10.48 16.54
SE-wheat straw, SE-rice straw, SE-maize straw, SE-oil-rape straw, and SE-cotton straw represent the biochars derived from steam-exploded wheat, rice, maize, oil-rape, and cotton straws, respectively.
study, ranging from 10.48 to 15.03 cmol kg–1, were much lower than those reported in the literature (10–540 cmol kg–1) (Zhao et al. 2015). It is worth noting that the AEC values of biochars derived from steam-exploded feedstocks (6.88–11.44 cmol kg–1) were approximately 2–3 times as high as that of the biochars from pristine feedstocks (Table 3), which suggested the increase of positive charge. On the one hand, it may be closely related to the N-containing functional groups, such as aromatic C=N and C–N (Fig. 3) within primary amides or some N heterocycles. Moreover, SE pretreatment induced slight carbonization of the feedstock, which indicated more condensed aromatic carbon that can attract protons and provide positive charge (Lawrinenko and Laird 2015). The PZNC values of all the biochars derived from SE-treated feedstocks significantly increased from pH 2.70–2.96 to pH 3.61–5.32, but SE pretreatment led to inconsistent changes
as to the CEC values. For instance, SE pretreatment induced a reduction of the negative charge in biochars derived from rice and maize straws, their CEC values were 8.11 and 9.78 cmol kg–1, 39 and 32%, respectively, lower than those of pristine feedstocks. On the contrary, the CEC values of biochars derived from SE-treated wheat, oil-rape, and cotton straw, 14.07, 24.25, and 16.54 cmol kg–1, respectively, were increased by 30, 61, and 58%, compared to that of biochars from pristine crop straws. All these variations were likely to be related to the acid oxygen-containing groups, but the exact mechanisms are still needed for the investigation.
3.6. Pedological and environmental implications Biochar has demonstrated clear potential as a soil amendment, even though the results are contradictory depending on different feedstocks, pyrolysis conditions, and the types of soil (Jeffery et al. 2011; He et al. 2017). The biochars derived from wheat, rice, and cotton straws in this study exhibited large specific surface area (>180 m2 g–1), which met the requirement that the surface area of biochars should be >100 m2 g–1 for a good performance in the soil (Schimmelpfennig and Glaser 2012). After SE pretreatment, the biochar derived from oil-rape straw owned a surface area of 242.06 m2 g–1 that was much larger than the biochars produced at a similar pyrolysis condition (Kambo and Dutta 2015). The highly porous structure of these biochars may imply their capacity to improve soil aeration and restore plant available water (0.2–50 μm), especially in sandy soils (Abel et al. 2013). Moreover, it could provide an asylum to the beneficial soil organisms such as arbuscular mycorrhiza (AM) fungi and plant growth promoting rhizobacteria (PGPR), which would further aid the supply of water and nutrients to the plants (Kavitha et al. 2018).
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Since the environmental issues, such as inorganic and organic pollutants contamination have attracted an increasing attention, biochar is representing a kind of suitable candidates for soil remediation, which is largely ascribed to its surface electrochemistry. A few numbers of researches have focused on the CEC of biochars which could enhance the nutrient retention capacity of the soil, and contribute to the adsorption of cationic contaminant (Zhao et al. 2015). Besides the CEC, biochars were detected to have a certain amount of AEC (6.88–11.44 cmol kg–1) in this study. The SE pretreatment significantly increased the AEC values of biochars regardless of the type of feedstocks. The enhanced AEC may reduce leaching of anionic nutrients in the soil, or be conducive to the adsorption of anionic pollutants, such as phosphate and arsenate.
4. Conclusion The SE pretreatment on crop straws induced significant changes in their lignocellulosic structure and could be a possible method to improve the surface property of biochars. The significant levels of positive charges and AEC values in biochars derived from SE-treated crop straws showed a potential for soil amendment, with regards to the retention of anionic nutrients as well as adsorption of pollutants. Oil-rape straw might be a suitable candidate for SE pretreatment, owing to the remarkable increase of SSA in both the feedstock and biochar, which indicated a high potential for water and nutrients retention.
Acknowledgements This work was funded by the National Key Technology R&D Program of China (2015BAD05B03). Appendices associated with this paper can be available on http://www.ChinaAgriSci.com/V2/En/appendix.htm
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Exe cu tive Editor-in-Chief ZHANG Wei-li Managing editor SUN Lu-juan