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Characterization of biochar from fast pyrolysis and its effect on chemical properties of the tea garden soil
Journal of Analytical and Applied Pyrolysis 110 (2014) 375–381
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Characterization of biochar from fast pyrolysis and its effect on chemical properties of the tea garden soil Yan Wang a,b , Renzhan Yin a,b , Ronghou Liu a,b,∗ a Biomass Energy Engineering Research Centre, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China b Key Laboratory of Urban Agriculture (South), Ministry of Agriculture, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 14 July 2014 Accepted 14 October 2014 Available online 23 October 2014 Keywords: Rice husk Elm Pyrolysis Biochar Soil Nutrients
a b s t r a c t The characteristics and application of biochar from conventionally slow pyrolysis have been studied a lot, but biochar, as a byproduct in the bio-oil production process, produced by fast pyrolysis was rarely studied. This work assessed the characterization and utilization of biochars derived from rice husk (RH) and elm sawdust (ES) by fast pyrolysis. Incubation experiment of rice husk biochar (RHB) and acid soil in a controlled cabinet was carried out to test the effect of biochar on soil available elements. The volatile and fixed carbon was 2.2 and 1.7-fold respectively higher in elm sawdust biochar (ESB) than those in RHB, but the ash content was 4.2-fold higher in RHB than that in ESB. Although the C, H, N, and O contents were significantly varied in two biochars, the ratio H/C and O/C were nearly the same. The Fourier Transform Infrared Spectroscopy (FTIR) results revealed that RHB had more functional groups than ESB. More surface area was found in RHB (78.15 m2 g−1 ) than ESB (0.22 m2 g−1 ) by BET test. Incorporation of the biochar improved the quality of acid soil properties. The levels of soil pH, K, Ca, Mg, Na and total C and N increased while the Al and Pb contents decreased. Total carbon and potassium increased by 72% and by 6.7-fold respectively over the control at 4% of rice husk biochar adding level. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The bioenergy production by biomass resources leads the attention in many countries. Agricultural residue is a form of biomass and widely available. Thermochemical conversion is one of the convenient ways for converting biomass into energy. It includes combustion, gasification and pyrolysis progress [1]. Of these processes, pyrolysis has been receiving increasing attention as an efficient way of converting biomass into syngas, liquids, and biochar in recent years. The term ‘biochar’ is developed in conjunction with soil management and carbon sequestration issues due to the specifically stable structure [2]. Pyrolysis reactor design, biomass type, reaction parameters (temperature, heating rate, residence time, catalyst and pressure) and feedstock characteristics (particle size, shape, and structure) have strong effects on the yield and properties of biochar [3].
∗ Corresponding author at: Biomass Energy Engineering Research Centre, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China. Tel.: +86 21 34205744; fax: +86 21 34205744. E-mail address: [email protected] (R. Liu). http://dx.doi.org/10.1016/j.jaap.2014.10.006 0165-2370/© 2014 Elsevier B.V. All rights reserved.
Poor agricultural management has increased the CO2 emissions from soil and conventional inversion tillage practices in sandy soil accelerate organic residue decomposition [4]. Some methods have been used to increase the soil organic carbon such as adding crop residue and animal waste into soil, but the biomass could be degraded in several years [5]. Biochar has its unique advantages due to its special element composition and surface structure. Specially, unlike compost or other bio-waste, biochar appears to permanently sequester carbon that plants have absorbed from the atmosphere [6]. Biochar is carbon-rich organic material often produced by biomass pyrolysis. The application of biochar into soil is a promising strategy for sequestering atmospheric carbon dioxide and improving soil quality [7,8]. Generally, the biochar is beneficial for acidic and unfertilized soils. The application of biochar into soils could increase soil nutrient retention, water holding capacity, soil pH value and crop yield because the biochar contains inorganic components (e.g., Ca, K, Mg, P, etc.) that acts a liming agent and supply plant available nutrients [9–11], and decrease the bioavailability of heavy metals and organic contamination [7,8,12]. In addition, biochar could improve soil for long time by changing the microbial community [8]. Review of previous research showed a huge range of biochar application rates (0.5–135 t/ha of biochar) as well as a huge range of plant responses
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(−29 to 324%)[13]. Some research reported the carbon of biochar in the soil lost seriously due to the low quality of biochar. Moreover the quality of the biochar on soil nutrient improvement is varied based on the pyrolysis condition. Biochar made at low temperature is easy to be degraded but at high temperature is hard to be degraded [14]. In addition, biochars will not last forever in soils, and any organic char in soil is decomposed finally [15]. However, agricultural residues considered as a huge form of biomass is often burned, and pollutes the environment of China. So, a proper technique is required to utilize the agriculture waste. Rice husk is often used to produce bio-oil by Fluidized-bed reactor or other fast pyrolysis system. And biochar was produced at the same time. Although the characteristics of biochar from slow pyrolysis and its soil application have been studied for many years, soil application study of RHB from fast pyrolysis was rarely reported. Bare in this mind, the objectives of present research work were to assess characters of biochar from rice husk and elm sawdust by fast pyrolysis, and to investigate the possible application of RHB in soil for enhancing soil nutrients.
2. Materials and methods 2.1. Production of biochar and soil sample collection The fully controlled fluidized-bed reactor fast pyrolysis system with a biomass throughput of 1–5 kg/h was constructed at Shanghai Jiao Tong University, and the reactor structure was described previously [16]. RH and ES particles ranged from 0.2 to 1 mm were used as the feedstock for the biochar production by fast pyrolysis. The feedstock was heated at 105 ◦ C for 12 h to remove water before producing biochar. The reaction temperature was 550 ± 25 ◦ C and Nitrogen (99.9%) was used as carrier gas in the system, and its flow was 60 L/min. Soil samples were collected from the 0 to 20 cm surface layer in a Ling Gu tea plantation which has a long history of planting tea in Yi Xing, China. The soil sample was air dried, grounded to pass through a 2-mm sieve before use.
2.3. Methods The moisture contents of biomass and biochar (wt/wt) were determined by drying 2 g portion of each feedstock in oven at 80 ◦ C for overnight [12]. The pH of the biochar was measured (1:10 ratio of biochar solutions in de-ionized water) by a pH meter (METTLER TOLEDO, FE-20 K). The ash and volatile matter of feedstock and biochar samples were tested according to ASTM-E1755 and ASTM-E872, respectively. The contents of carbon, nitrogen, hydrogen and oxygen of biomass and biochar were analyzed by CHNO elemental analyzer (Perkin Elmer, PE2400II). In this method, the O content was determined by difference. Fourier transform infrared spectra of biomass and biochar were recorded in the 4000–400 cm−1 region with a resolution of 0.4 cm−1 by a FTIR spectrometer (EQUINOX55, Brucks). Scanning electron microscopes (SEM) of biomass and biochar samples were taken by using a FEI Nova Nano Scaning Electrone Microscope. Varying magnifications were used to compare the structure and surface characteristics of the two biomass and two biochar samples. Special surface area of biochar was measured in automated volumetric gas adsorption apparatus by ASAP 2010 using nitrogen as an adsorbent at 77 K. The available nutrients of biochar exacted by Mehlich 3 extraction (0.2 mol/L CH3 COOH + 0.25 mol/L NH4 NO3 + 0.013 mol/L HNO3 + 0.015 mol/L NH4 F + 0.001 mol/L EDTA) [17] were measured by Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES). The soil pH was determined in a ratio of 1: 2.5 soil-KCl solutions (1 mol/L) with a pH meter (METTLER TOLEDO, FE-20K). Total content of carbon and nitrogen of soil were determined by an elemental analyzer (Perkin Elmer, PE2400II). According to literature [17], soil extractable elements were extracted with Mehlich 3 extraction and determined by ICP-AES. 2.4. Statistical analysis Quantitative data are presented as mean values ± standard error (n = 3). One-way analysis of variance (ANOVA) was undertaken to determine significant differences between the treatments. Significant difference was statistically considered at the level of p <0.05. 3. Results and discussion
2.2. Incubation of biochar and acid soil Three repeated soil samples (1000 g) were mixed thoroughly with biochar to create mixture with ratios of (biochar/soil) 0, 0.5%, 1%, 2% and 4% by weight. All mixtures were placed in foamed plastic boxes. Deionized water was added to bring soil water content to about 75% of field water-holding capacity. Plastic film was used to cover the boxes and a small hole was made to allow gas exchange but minimize moisture loss. All boxes were placed in a climatic box and incubated at 25 ± 1 ◦ C for 60 days. After 60 days, soil samples were taken from the boxes, air-dried, and ground to pass through a 2-mm sieve for determination of pH, extractable Ca, Mg, K, Na, Al and Pb content. Part of soil was passed through a 0.25-mm sieve for determination of total carbon and nitrogen.
3.1. Proximate and ultimate analysis of biomass and their biochars Table 1 shows the proximate and ultimate analysis of biomass and their biochars. Results showed that the fixed carbon and the elementary carbon contents of the biomass (being biochar) increased after biomass pyrolysis at the expense of volatile matter, elemental oxygen, and hydrogen. The ash contents of biochars were also higher than the biomass samples due to the reason that the mineral matter formed ash which remained in biochars after pyrolysis. Compared to ESB, RHB contained more ash, less fixed carbon and volatile component, which was consistent with the raw materials. The contents of fixed carbon, volatile component
Table 1 Proximate and ultimate analysis of biomass and their biochars (fresh basis). Proximate analysis (wt.%)
Ultimate analysis (wt.%)
Materials
Water content
Volatile component
Fixed carbona
Ash content
C
H
N
Oa
H/C
O/C
RH RHB ES ESB
10.08 2.60 4.93 2.38
72.40 13.11 87.75 29.08
3.40 33.35 4.29 56.55
14.12 50.94 3.03 11.99
35.15 37.66 47.00 67.98
4.16 1.83 5.39 3.78
0.36 0.30 0.29 0.39
46.21 9.27 44.29 15.86
0.12 0.05 0.11 0.06
1.31 0.25 0.94 0.23
a
By difference
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Fig. 1. SEM imagines of biomass and biochar (a) rice husk (b) rice husk biochar (c) elm sawdust (d) elm sawdust biochar.
and ash of RHB were 33.35%, 13.11% and 50.94%, respectively. And those of ESB were 56.55%, 29.08% and 11.99%, respectively. In comparison, the contents of fixed carbon, volatile component and ash were 69.6%, 17.49% and 12.91% [18] and 14.0%, 78.7% and 7.3% also for rape plant biochar, 57.7%, 13.4% and 28.9% for sunflower biochar, respectively [19]. So the different feedstocks and different manufacturing conditions of biochar lead to different characteristics of biochars. The degree of carbonization could be described by the molar H/C ratio because H is primarily associated with plant organic matter [20]. The observed H/C ratios of 0.05 and 0.06 for RHB and ESB, respectively, indicate that these biochars are highly carbonized. The molar O/C ratio of biochar has been used to assess the ability of surface hydrophilicity, because it is indicative of polargroup content [21]. The RHB had a higher O/C of 0.25 than ESB with O/C of 0.23, but both of them were more hydrophilic than the cotton straw biochar which had low O/C (0.04) [22]. Biochar characteristics are affected by many factors including feedstock types, pyrolysis temperature, heating time and reaction rate. Generally, biomass pyrolysis at low temperature could produce maximum biochar [23,24]. Ash content of animal waste biochar is higher than plant residue biochar at the same pyrolysis temperature. For same feedstock, the ash and volatile content increased and carbon content decreased with increasing pyrolysis temperature [25]. 3.2. SEM imagines analysis of biomass and their biochars In the process of fast pyrolysis, cellulose, hemicellulose, and lignin were decomposed. SEM images of the biomass and biochar were taken to investigate their visual structures. Fig. 1 shows the comparison of SEM images of rice husk, elm sawdust and their biochars. It was obvious from the images that the surface morphology of the biomass (being biochar) changed after pyrolysis. There was little pore on the surface of raw materials but pores were generated after pyrolysis. The biochars were rich in pores, and most pores generated at regular position. It was related to the biomass structure (Fig. 1a and c). The shape of the fiber was not changed in biomass after pyrolysis. Hence, fast pyrolysis did not
destroy the carbon structure skeleton of the biomass. SEM analysis of powdered samples of feedstocks and biochars also indicated the porous structure on the surface of the biochars existed, but Fig. 1b and d shows that some pores filled by volatile matters and ash, would decrease the porous volume. Generally, the porosity of biochar will be increased, after the activation. Purevsuren and Avid also found the volatile in the casein biochar pores by SEM [26]. 3.3. FTIR spectroscopic analysis of biomass and biochars Table 2 presents the functional groups of biomass and biochar samples determined by the FTIR analysis. FTIR spectra were collected on powdered aliquots of each sample, from 400 to 4000 cm−1 at a resolution of 0.44 cm−1 using a Bruker EQUINOX55 spectrometer. This method is frequently used in investigations of surface chemistry of chars and active carbons. Results showed that various bonds in the spectra representing O H water, alcohol, phenols (at 3403 cm−1 ), C H alkane groups (2909 cm−1 ), N H acid amides (1631 cm−1 ), C = O amides (571 cm−1 ), C O group ethers, alcohol (1055 cm−1 ), P H organophosphorus (2353 cm−1 ), C H (2971 cm−1 , 1066 cm−1 , 798 cm−1 ). Table 2 shows that the similar functional groups were found in two types of feedstock and both the feedstocks had more functional groups than biochars. The asymmetric (2917 cm−1 ) C H stretching bands were associated with aliphatic functional groups [27]. In RHB, only the functional groups 2917 cm−1 (alkane, aliphatic) and 2353 cm−1 (organophosphorus) disappeared, and a new functional groups (C H, deformation vibration, arene) generated. The arene structure made the biochar more stable. Only the functional groups of 1631 cm−1 (acid amides) and 1397 cm−1 (olefins) were found in ESB. The aliphatic C H stretching vibration (2930–2850 cm−1 ) disappeared which suggests a loss of labile, aliphatic compounds during pyrolysis and the formation of more recalcitrant, aromatic constituents [27]. The loss of OH and aliphatic groups promoted pore formation due to a concurrent development of fused-ring structures [28]. Cao et al. (2009) attributed the effective heavy metal removal by biochar sorbents to either precipitation of lead
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Table 2 Functional groups of biomass and biochar samples determined by the FTIR analysis. Wave number/cm−1
Functional groups (vibration)
Compounds
RH 3446 2917 2353 1634 1066
O-H, stretching C H, stretching P H, stretching N H, flexural C H, flexural
Water, alcohol, phenols Alkane, aliphatic Organophosphorus Acid amides Olefins
ES 3403 2909 1631 1397 1055 571
O C N C C C
Water, alcohol, phenols Alkane, aliphatic Acid amides Olefins Ethers, alcohol Amides
H, stretching H, stretching H, flexural H, flexural O, stretching O deformation
onto the biochar surface or electrostatic interactions between lead species and negatively charged functional groups on biochar’s surface [29].
Wave number/cm−1
Functional groups (vibration)
Compounds
RHB 3446
O H, stretching
Water, alcohol, phenols
1634 1066 798 ESB
N H, flexural C H, flexural C H, deformation
acid amides olefins arene
1631 1397
N H, flexural C H, flexural
Acid amides Olefins
RHB was better than ESB to improve soil from the point of higher pH value, more functional groups and nutrients, lager surface area and large amounts of resources. Hence, RHB was chosen to carry out the following soil improvement experiment.
3.4. Specific surface area and pH analysis of biochars 3.5. Analysis of extractable elements in RHB Generally, the biochars play a role in liming because plant materials used for biochar production contain base cations, and biochars inherited these base cations during pyrolysis of the biomass and the alkalinity increased with the pyrolysis temperature [7]. The pH of 7.96 was higher in RHB than the ESB (pH 7.62) due to the high content of ash in RHB. The pH of rice husk biochar was similar to the pH values of other rice husk biochars made by slow pyrolysis which has a pH of 7.8 (unknown temperature) [30], 8.01 (350 ◦ C, 4 h) [31] and 7.99 (500 ◦ C, 2 h)[32]. However it was different from the pH (10.0) of rice husk biochar produced from slow pyrolysis at 500 ◦ C for 4 h that Wang et al. (2013) reported. Reactor type was little associated with the pH of the biochar, and temperature and residence time decided the pH for the same feedstock. Cellulose and hemicelluloses were abundant in RHB, and they could be decomposed in pyrolysis process, producing organic acids and phenolic substances which lowered the pH of the products. The pH of biochar made from animal waste is higher than most plant residue biochars due to the higher ash content [33]. Specific surface areas of RHB and ESB were 78.15 m2 g−1 and 0.22 m2 g−1 , respectively, measured in automated volumetric gas adsorption apparatus by ASAP 2010 using nitrogen as an adsorbate at 77 K. From the above results, it can be seen that biochar from rice husk fast pyrolysis had higher BET surface area than biochar made from wood fast pyrolysis. Claoston et al. (2014) reported that the BET surface areas of rice husk and empty fruit bunch biocahr made by slow pyrolysis at 500 ◦ C for 2 h were 230.91 m2 g−1 and 15.42 m2 g−1 , respectively [32], but Wang et al. (2013) reported that the BET surface areas of rice husk biochar and elm biochar from slow pyrolysis at 500 ◦ C for 4 h were 12.2 m2 g−1 and 84.3 m2 g−1 , respectively [27]. Hence, BET surface area was significantly influenced by the reactor type, pyrolysis temperature and it increases with increase of pyrolysis temperature in a range of temperature. The specific surface area of crop-residue biochars was increased from 116 m2 g−1 to 438 m2 g−1 with increase of temperature from 400 ◦ C to 600 ◦ C [21], but it decreased at 700 ◦ C. So, the specific surface area of biochar is closely related to pyrolysis condition. A relatively structure of carbon matrix with extensive surface area was observed in RHB, suggesting that it might act as a surface sorbent to play an important role in environmental pollution control. In addition, the apparent density of RHB was 418.3 kg/m3 and it indicated that it was easier to spread on the surface soil by machine.
The contents of extractable nutrients in RHB were as follows (mg/g): K 6.445, Al 0.041, Ca 0.962, Mg 0.462, Na 0.156, P 0.735. Specifically, it was rich in potassium (6.445 mg/g), while the content of extractable heavy metal was rather low (e.g., Pb 0.000717 mg/g, As 0.000356 mg/g and Cu 0.000506 mg/g). The nutrients content was varied for different biochar. Rubber wood biochar had very high contents of available P (0.747 mg/g), K (6.895 mg/g), Mg (0.908 mg/g) and Ca (9.799 mg/g) [34]. The extracted P, K and Mg contents were similar to that of rice husk biochar, but Ca content was rather high. The K content was high in peanut hull biochar (8.142 mg/g), but it was low in pine bark biochar (0.843 mg/g) [35]. Dissolved phosphorus in biochar could remove Pb2+ by forming precipitate lead by forming (e.g., (Pb)5 (PO4 )3 Cl) [36] in addition to be as soil nutrients. 3.6. Effects of the RHB on chemical properties of the acid soil The soil characteristics were as follows: pH 3.33, total C 2.1%, total N 0.22%, extractable K 55 mg/kg, Al 1327 mg/kg, Ca 68 mg/kg, Mg 10 mg/kg, Na 7 mg/kg and Pb 6.7 mg/kg. The soil chosen for the experiment was acid and low in potassium, while the aluminum content was very high. Aluminum (Al) is the most abundant metal in the earth’s crust, accounting for 7% of its mass [37]. Too much aluminum in soils could produce harmful effect to plants growth due to inhibition of root growth. 3.6.1. Effect of the RHB on pH, total carbon and nitrogen of soil Acid in the soil could change the form of nutrients and inhibit some elements absorption of the root. Overall, biochar had a small but significant effect on soil pH (a shift in pH of 0.2 units). Biochar improved the soil pH obviously with the addition of 4% biochar (p < 0.05). The reason that soil pH increased with increasing biochar addition was mainly due to alkalinity of biochar. Soil pH increased from 3.43 to 3.63 by addition of biochar from 0 to 4% (Fig. 2). The result was consistent with other previous report [9,24,38]. Fig. 3 shows effects of RHB on total carbon and nitrogen of soil. Result indicated that spiking RHB into soil could increase soil carbon content significantly and elevate nitrogen slightly. Total carbon in the soil was significantly elevated from 2.04% of the control soils to 3.51% at 4% biochar addition. The RHB contained 37.66% of carbon which was hard to degrade due to its aromatic structure. Hence, adding biochar into soil could increase carbon sequestration for a
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Fig. 2. Effect of rice husk biochar on soil pH.
long time. Biochar could be reserved for hundreds to thousands of years in forest ecosystem [39]. The soil nitrogen increased from 0.218% to 0.242% after 60 days incubation with 4% biochar addition due to reasons that RHB changed the soil humidity, aeration status, inhibited the microbial denitrification, and decreased the generation and released nitrogen oxide [40] in addition to biochar containing nitrogen. It was reported that most of the nitrogen in biochar were heterocyclic compounds which were rather stable in the soil and environment, and the rate of the nitrogen release was very slow [41]. 3.6.2. Effect of the RHB on soil extractable elements Changes of extractable elements in soil were measured at the end of the pot experiment. Biochar amendment increased soil extractable elements such as Na, K, Ca, and Mg but decreases Al and Pb (Fig. 4). Similarly, Novak (2009) has reported that addition of biochar into the Norfolk soil increased soil pH, soil organic carbon, Ca, K, Mn, P and decreased exchangeable acidity, S, and Zn [10]. The magnitudes of changes were roughly proportional to the rate of biochar application (e.g. increases or decreases with increasing rate of biochar application). There were two reasons for this. One was biochar itself contained some nutrients, and the other was that biochar was rich in pores which could adsorb the elements in soil and prevent it from leaching to the ground water. In some case, statistical difference was significant
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only in the higher rates, namely 4% of biochar application but not at low addition. Soil exchangeable cations and negative surface charge of biochar particles increased due to carboxylate groups of the biochar and exposed carboxylate groups of organic acids sorbed by the biochar, both of which increased negative surface charge of biochar particles [42]. Adding 4% of RHB into soil could increase the K content from 42 to 324 mg/kg due to high potassium content of RHB. Hence, RHB could be used as supplement of potassium to soil. The extractable Al in the soil decreased from 1304 to 805 mg/kg at addition of 4% biochar. Due to the biochar liming effect, the aluminum species were converted to Al(OH)2+ and Al(OH)2 + monomers, which were strongly adsorbed by biochar [43]. In addition, it was noted that the symptoms of severe A1 toxicity was enhanced in the field of Ca deficiency and that application of Ca alleviated [37] the above phenomenon. Calcium (0.962 mg/g) in RHB could alleviate the Al toxicity. Hence, incorporation of biochars can decrease the potential toxicity of Al to plants in acid soils. During pyrolysis, nutrients (primarily K, Ca, and Mg) of biomass formed metal oxides and mixed with the biochar. And when RHB was buried in the soil, these oxides can react with H+ and monomeric of Al species, to modify the soil pH and exchangeable acidity values. The highest level of cation exchangeable capability (CEC), P and K has been observed in acid sulfate soil treated with RHB [12]. Many reports pointed out that biochar could increase the crop yield, but it was related to type of biochar, crop and soil types [8,11,44,45]. It was reported that the soil exchangeable Na and exchangeable Ca was not significantly influenced by biochar additions in a three-year field trial, and the addition of the biochar to soil causes small and potentially transient changes in a temperate agroecosystem function [8]. Rice husk belongs to herbaceous plant and wood sawdust belongs to wood plant. It is an unique angle to analyze the biochar from the point of view of herbaceous plant and wood. Wang et al. (2013) compared the straw-based biochar and wood-based biochar. The biochars from crop straw (herbaceous plant) may be more effective for improving soil fertility and C sequestration. The three straw-based biochars (rice straw, corn straw and wheat straw) generally showed higher yield (19.0–37.6 wt%) and higher pH (9.2–11.1), and consistently exhibited far greater ash percentage (14.5– 40.3 wt%), CEC (14.1–34.8 cmol kg−1 ), and the total C, N, P, Ca, and Mg, Na, K than the two wood-based biochars (bamboo and elm) [27]. As the above reported of present research, the ash content of the rice husk (herbaceous plant) biochar was higher than that of the elm sawdust biochar.
Fig. 3. Effect of the rice husk biochar on (a) level of soil total carbon, (b) total nitrogen.
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Fig. 4. Effect of the rice husk biochar on chemical properties of soil (a) potassium (b) calcium (c) sodium (d) magnesium (e) aluminum and (f) lead, 1 ppm in the figure represents 10 mg/kg.
4. Conclusions Characteristics of biochar derived from the fast pyrolysis of RH and ES were rather different. Compared to sawdust biochar, RHB was high in ash, while low in volatile and fixed carbon content. Similar functional groups were found in RH and ES, both of which had more functional groups than their biochars. RHB contained more
function groups than that of ESB, and a new functional group (C H, deformation vibration, arene) generated in RHB, which makes the biochar more stable. Larger surface area (78.15 m2 g−1 ) was found in RHB. Incorporation of the RHB significantly improved the soil properties. Total carbon, nitrogen, pH and extractable Ca, Mg, K, and Na contents in soil increased and the available Al and Pb contents decreased.
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Acknowledgements Financial support from the National High Technology Research and Development Program (863 Program, Grant NO. 2012AA101808) of China is acknowledged. In addition, Professor Sergio Capareda from Department of Biological & Agricultural Engineering, Texas A&M University, USA is acknowledged for his valuable comments and suggestions about this paper. References [1] A. Bridgwater, D. Meier, D. Radlein, Org. Geochem. 30 (1999) 1479. [2] J. Lehmann, J. Gaunt, M. Rondon, Mitig. Adapt. Strat. Glob. Change 11 (2006) 395. [3] A. Bridgwater, J. Anal. Appl. Pyrolysis 51 (1999) 3. [4] P.J. Bauer, J.R. Frederick, J.M. Novak, P.G. Hunt, Soil Till. Res. 90 (2006) 205. [5] D. Laird, P. Fleming, B. Wang, R. Horton, D. Karlen, Impact of soil biochar applications on nutrient leaching, in: International Annual Meetings ASA CSSA SSSA, 2008. [6] M. Collison, L. Collison, R. Sakrabani, B. Tofield, Z. Wallage, Low Carbon Innovation Centre, Norwich, UEA, 2009. [7] J. Lehmann, Front. Ecol. Environ. 5 (2007) 381. [8] D.L. Jones, J. Rousk, G. Edwards-Jones, T.H. DeLuca, D.V. Murphy, Soil Biol. Biochem. 45 (2012) 113. [9] K.Y. Chan, L. Van Zwieten, I. Meszaros, A. Downie, S. Joseph, Aust. J. Soil Res. 45 (2007) 629. [10] J.M. Novak, W.J. Busscher, D.L. Laird, M. Ahmedna, D.W. Watts, M.A.S. Niandou, Soil Sci. 174 (2009) 105. [11] L. Van Zwieten, S. Kimber, S. Morris, K.Y. Chan, A. Downie, J. Rust, S. Joseph, A. Cowie, Plant Soil 327 (2009) 235. [12] A. Masulili, W.H. Utomo, M. Syechfani, J. Agric. Sci. 2 (2010) P39. [13] B. Glaser, J. Lehmann, W. Zech, Biol. Fert. Soils 35 (2002) 219. [14] M.A. Rondon, J. Lehmann, J. Ramírez, M. Hurtado, Biol. Fert. Soils 43 (2007) 699. [15] C.H. Cheng, J. Lehmann, M.H. Engelhard, Geochim. Cosmochim. Acta 72 (2008) 1598. [16] T. Chen, C. Wu, R. Liu, W. Fei, S. Liu, Bioresour. Technol. 102 (2011) 6178. [17] A. Mehlich, Commun. Soil Sci. Plant Anal. 15 (1984) 1409. [18] F. Karaosmanoglu, A. Isigigür-Ergüdenler, A. Sever, Energy Fuels 14 (2000) 336.
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Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Characterization of biochar from fast pyrolysis and its effect on chemical properties of the tea garden soil Yan Wang a,b , Renzhan Yin a,b , Ronghou Liu a,b,∗ a Biomass Energy Engineering Research Centre, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China b Key Laboratory of Urban Agriculture (South), Ministry of Agriculture, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 14 July 2014 Accepted 14 October 2014 Available online 23 October 2014 Keywords: Rice husk Elm Pyrolysis Biochar Soil Nutrients
a b s t r a c t The characteristics and application of biochar from conventionally slow pyrolysis have been studied a lot, but biochar, as a byproduct in the bio-oil production process, produced by fast pyrolysis was rarely studied. This work assessed the characterization and utilization of biochars derived from rice husk (RH) and elm sawdust (ES) by fast pyrolysis. Incubation experiment of rice husk biochar (RHB) and acid soil in a controlled cabinet was carried out to test the effect of biochar on soil available elements. The volatile and fixed carbon was 2.2 and 1.7-fold respectively higher in elm sawdust biochar (ESB) than those in RHB, but the ash content was 4.2-fold higher in RHB than that in ESB. Although the C, H, N, and O contents were significantly varied in two biochars, the ratio H/C and O/C were nearly the same. The Fourier Transform Infrared Spectroscopy (FTIR) results revealed that RHB had more functional groups than ESB. More surface area was found in RHB (78.15 m2 g−1 ) than ESB (0.22 m2 g−1 ) by BET test. Incorporation of the biochar improved the quality of acid soil properties. The levels of soil pH, K, Ca, Mg, Na and total C and N increased while the Al and Pb contents decreased. Total carbon and potassium increased by 72% and by 6.7-fold respectively over the control at 4% of rice husk biochar adding level. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The bioenergy production by biomass resources leads the attention in many countries. Agricultural residue is a form of biomass and widely available. Thermochemical conversion is one of the convenient ways for converting biomass into energy. It includes combustion, gasification and pyrolysis progress [1]. Of these processes, pyrolysis has been receiving increasing attention as an efficient way of converting biomass into syngas, liquids, and biochar in recent years. The term ‘biochar’ is developed in conjunction with soil management and carbon sequestration issues due to the specifically stable structure [2]. Pyrolysis reactor design, biomass type, reaction parameters (temperature, heating rate, residence time, catalyst and pressure) and feedstock characteristics (particle size, shape, and structure) have strong effects on the yield and properties of biochar [3].
∗ Corresponding author at: Biomass Energy Engineering Research Centre, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China. Tel.: +86 21 34205744; fax: +86 21 34205744. E-mail address: [email protected] (R. Liu). http://dx.doi.org/10.1016/j.jaap.2014.10.006 0165-2370/© 2014 Elsevier B.V. All rights reserved.
Poor agricultural management has increased the CO2 emissions from soil and conventional inversion tillage practices in sandy soil accelerate organic residue decomposition [4]. Some methods have been used to increase the soil organic carbon such as adding crop residue and animal waste into soil, but the biomass could be degraded in several years [5]. Biochar has its unique advantages due to its special element composition and surface structure. Specially, unlike compost or other bio-waste, biochar appears to permanently sequester carbon that plants have absorbed from the atmosphere [6]. Biochar is carbon-rich organic material often produced by biomass pyrolysis. The application of biochar into soil is a promising strategy for sequestering atmospheric carbon dioxide and improving soil quality [7,8]. Generally, the biochar is beneficial for acidic and unfertilized soils. The application of biochar into soils could increase soil nutrient retention, water holding capacity, soil pH value and crop yield because the biochar contains inorganic components (e.g., Ca, K, Mg, P, etc.) that acts a liming agent and supply plant available nutrients [9–11], and decrease the bioavailability of heavy metals and organic contamination [7,8,12]. In addition, biochar could improve soil for long time by changing the microbial community [8]. Review of previous research showed a huge range of biochar application rates (0.5–135 t/ha of biochar) as well as a huge range of plant responses
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(−29 to 324%)[13]. Some research reported the carbon of biochar in the soil lost seriously due to the low quality of biochar. Moreover the quality of the biochar on soil nutrient improvement is varied based on the pyrolysis condition. Biochar made at low temperature is easy to be degraded but at high temperature is hard to be degraded [14]. In addition, biochars will not last forever in soils, and any organic char in soil is decomposed finally [15]. However, agricultural residues considered as a huge form of biomass is often burned, and pollutes the environment of China. So, a proper technique is required to utilize the agriculture waste. Rice husk is often used to produce bio-oil by Fluidized-bed reactor or other fast pyrolysis system. And biochar was produced at the same time. Although the characteristics of biochar from slow pyrolysis and its soil application have been studied for many years, soil application study of RHB from fast pyrolysis was rarely reported. Bare in this mind, the objectives of present research work were to assess characters of biochar from rice husk and elm sawdust by fast pyrolysis, and to investigate the possible application of RHB in soil for enhancing soil nutrients.
2. Materials and methods 2.1. Production of biochar and soil sample collection The fully controlled fluidized-bed reactor fast pyrolysis system with a biomass throughput of 1–5 kg/h was constructed at Shanghai Jiao Tong University, and the reactor structure was described previously [16]. RH and ES particles ranged from 0.2 to 1 mm were used as the feedstock for the biochar production by fast pyrolysis. The feedstock was heated at 105 ◦ C for 12 h to remove water before producing biochar. The reaction temperature was 550 ± 25 ◦ C and Nitrogen (99.9%) was used as carrier gas in the system, and its flow was 60 L/min. Soil samples were collected from the 0 to 20 cm surface layer in a Ling Gu tea plantation which has a long history of planting tea in Yi Xing, China. The soil sample was air dried, grounded to pass through a 2-mm sieve before use.
2.3. Methods The moisture contents of biomass and biochar (wt/wt) were determined by drying 2 g portion of each feedstock in oven at 80 ◦ C for overnight [12]. The pH of the biochar was measured (1:10 ratio of biochar solutions in de-ionized water) by a pH meter (METTLER TOLEDO, FE-20 K). The ash and volatile matter of feedstock and biochar samples were tested according to ASTM-E1755 and ASTM-E872, respectively. The contents of carbon, nitrogen, hydrogen and oxygen of biomass and biochar were analyzed by CHNO elemental analyzer (Perkin Elmer, PE2400II). In this method, the O content was determined by difference. Fourier transform infrared spectra of biomass and biochar were recorded in the 4000–400 cm−1 region with a resolution of 0.4 cm−1 by a FTIR spectrometer (EQUINOX55, Brucks). Scanning electron microscopes (SEM) of biomass and biochar samples were taken by using a FEI Nova Nano Scaning Electrone Microscope. Varying magnifications were used to compare the structure and surface characteristics of the two biomass and two biochar samples. Special surface area of biochar was measured in automated volumetric gas adsorption apparatus by ASAP 2010 using nitrogen as an adsorbent at 77 K. The available nutrients of biochar exacted by Mehlich 3 extraction (0.2 mol/L CH3 COOH + 0.25 mol/L NH4 NO3 + 0.013 mol/L HNO3 + 0.015 mol/L NH4 F + 0.001 mol/L EDTA) [17] were measured by Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES). The soil pH was determined in a ratio of 1: 2.5 soil-KCl solutions (1 mol/L) with a pH meter (METTLER TOLEDO, FE-20K). Total content of carbon and nitrogen of soil were determined by an elemental analyzer (Perkin Elmer, PE2400II). According to literature [17], soil extractable elements were extracted with Mehlich 3 extraction and determined by ICP-AES. 2.4. Statistical analysis Quantitative data are presented as mean values ± standard error (n = 3). One-way analysis of variance (ANOVA) was undertaken to determine significant differences between the treatments. Significant difference was statistically considered at the level of p <0.05. 3. Results and discussion
2.2. Incubation of biochar and acid soil Three repeated soil samples (1000 g) were mixed thoroughly with biochar to create mixture with ratios of (biochar/soil) 0, 0.5%, 1%, 2% and 4% by weight. All mixtures were placed in foamed plastic boxes. Deionized water was added to bring soil water content to about 75% of field water-holding capacity. Plastic film was used to cover the boxes and a small hole was made to allow gas exchange but minimize moisture loss. All boxes were placed in a climatic box and incubated at 25 ± 1 ◦ C for 60 days. After 60 days, soil samples were taken from the boxes, air-dried, and ground to pass through a 2-mm sieve for determination of pH, extractable Ca, Mg, K, Na, Al and Pb content. Part of soil was passed through a 0.25-mm sieve for determination of total carbon and nitrogen.
3.1. Proximate and ultimate analysis of biomass and their biochars Table 1 shows the proximate and ultimate analysis of biomass and their biochars. Results showed that the fixed carbon and the elementary carbon contents of the biomass (being biochar) increased after biomass pyrolysis at the expense of volatile matter, elemental oxygen, and hydrogen. The ash contents of biochars were also higher than the biomass samples due to the reason that the mineral matter formed ash which remained in biochars after pyrolysis. Compared to ESB, RHB contained more ash, less fixed carbon and volatile component, which was consistent with the raw materials. The contents of fixed carbon, volatile component
Table 1 Proximate and ultimate analysis of biomass and their biochars (fresh basis). Proximate analysis (wt.%)
Ultimate analysis (wt.%)
Materials
Water content
Volatile component
Fixed carbona
Ash content
C
H
N
Oa
H/C
O/C
RH RHB ES ESB
10.08 2.60 4.93 2.38
72.40 13.11 87.75 29.08
3.40 33.35 4.29 56.55
14.12 50.94 3.03 11.99
35.15 37.66 47.00 67.98
4.16 1.83 5.39 3.78
0.36 0.30 0.29 0.39
46.21 9.27 44.29 15.86
0.12 0.05 0.11 0.06
1.31 0.25 0.94 0.23
a
By difference
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Fig. 1. SEM imagines of biomass and biochar (a) rice husk (b) rice husk biochar (c) elm sawdust (d) elm sawdust biochar.
and ash of RHB were 33.35%, 13.11% and 50.94%, respectively. And those of ESB were 56.55%, 29.08% and 11.99%, respectively. In comparison, the contents of fixed carbon, volatile component and ash were 69.6%, 17.49% and 12.91% [18] and 14.0%, 78.7% and 7.3% also for rape plant biochar, 57.7%, 13.4% and 28.9% for sunflower biochar, respectively [19]. So the different feedstocks and different manufacturing conditions of biochar lead to different characteristics of biochars. The degree of carbonization could be described by the molar H/C ratio because H is primarily associated with plant organic matter [20]. The observed H/C ratios of 0.05 and 0.06 for RHB and ESB, respectively, indicate that these biochars are highly carbonized. The molar O/C ratio of biochar has been used to assess the ability of surface hydrophilicity, because it is indicative of polargroup content [21]. The RHB had a higher O/C of 0.25 than ESB with O/C of 0.23, but both of them were more hydrophilic than the cotton straw biochar which had low O/C (0.04) [22]. Biochar characteristics are affected by many factors including feedstock types, pyrolysis temperature, heating time and reaction rate. Generally, biomass pyrolysis at low temperature could produce maximum biochar [23,24]. Ash content of animal waste biochar is higher than plant residue biochar at the same pyrolysis temperature. For same feedstock, the ash and volatile content increased and carbon content decreased with increasing pyrolysis temperature [25]. 3.2. SEM imagines analysis of biomass and their biochars In the process of fast pyrolysis, cellulose, hemicellulose, and lignin were decomposed. SEM images of the biomass and biochar were taken to investigate their visual structures. Fig. 1 shows the comparison of SEM images of rice husk, elm sawdust and their biochars. It was obvious from the images that the surface morphology of the biomass (being biochar) changed after pyrolysis. There was little pore on the surface of raw materials but pores were generated after pyrolysis. The biochars were rich in pores, and most pores generated at regular position. It was related to the biomass structure (Fig. 1a and c). The shape of the fiber was not changed in biomass after pyrolysis. Hence, fast pyrolysis did not
destroy the carbon structure skeleton of the biomass. SEM analysis of powdered samples of feedstocks and biochars also indicated the porous structure on the surface of the biochars existed, but Fig. 1b and d shows that some pores filled by volatile matters and ash, would decrease the porous volume. Generally, the porosity of biochar will be increased, after the activation. Purevsuren and Avid also found the volatile in the casein biochar pores by SEM [26]. 3.3. FTIR spectroscopic analysis of biomass and biochars Table 2 presents the functional groups of biomass and biochar samples determined by the FTIR analysis. FTIR spectra were collected on powdered aliquots of each sample, from 400 to 4000 cm−1 at a resolution of 0.44 cm−1 using a Bruker EQUINOX55 spectrometer. This method is frequently used in investigations of surface chemistry of chars and active carbons. Results showed that various bonds in the spectra representing O H water, alcohol, phenols (at 3403 cm−1 ), C H alkane groups (2909 cm−1 ), N H acid amides (1631 cm−1 ), C = O amides (571 cm−1 ), C O group ethers, alcohol (1055 cm−1 ), P H organophosphorus (2353 cm−1 ), C H (2971 cm−1 , 1066 cm−1 , 798 cm−1 ). Table 2 shows that the similar functional groups were found in two types of feedstock and both the feedstocks had more functional groups than biochars. The asymmetric (2917 cm−1 ) C H stretching bands were associated with aliphatic functional groups [27]. In RHB, only the functional groups 2917 cm−1 (alkane, aliphatic) and 2353 cm−1 (organophosphorus) disappeared, and a new functional groups (C H, deformation vibration, arene) generated. The arene structure made the biochar more stable. Only the functional groups of 1631 cm−1 (acid amides) and 1397 cm−1 (olefins) were found in ESB. The aliphatic C H stretching vibration (2930–2850 cm−1 ) disappeared which suggests a loss of labile, aliphatic compounds during pyrolysis and the formation of more recalcitrant, aromatic constituents [27]. The loss of OH and aliphatic groups promoted pore formation due to a concurrent development of fused-ring structures [28]. Cao et al. (2009) attributed the effective heavy metal removal by biochar sorbents to either precipitation of lead
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Table 2 Functional groups of biomass and biochar samples determined by the FTIR analysis. Wave number/cm−1
Functional groups (vibration)
Compounds
RH 3446 2917 2353 1634 1066
O-H, stretching C H, stretching P H, stretching N H, flexural C H, flexural
Water, alcohol, phenols Alkane, aliphatic Organophosphorus Acid amides Olefins
ES 3403 2909 1631 1397 1055 571
O C N C C C
Water, alcohol, phenols Alkane, aliphatic Acid amides Olefins Ethers, alcohol Amides
H, stretching H, stretching H, flexural H, flexural O, stretching O deformation
onto the biochar surface or electrostatic interactions between lead species and negatively charged functional groups on biochar’s surface [29].
Wave number/cm−1
Functional groups (vibration)
Compounds
RHB 3446
O H, stretching
Water, alcohol, phenols
1634 1066 798 ESB
N H, flexural C H, flexural C H, deformation
acid amides olefins arene
1631 1397
N H, flexural C H, flexural
Acid amides Olefins
RHB was better than ESB to improve soil from the point of higher pH value, more functional groups and nutrients, lager surface area and large amounts of resources. Hence, RHB was chosen to carry out the following soil improvement experiment.
3.4. Specific surface area and pH analysis of biochars 3.5. Analysis of extractable elements in RHB Generally, the biochars play a role in liming because plant materials used for biochar production contain base cations, and biochars inherited these base cations during pyrolysis of the biomass and the alkalinity increased with the pyrolysis temperature [7]. The pH of 7.96 was higher in RHB than the ESB (pH 7.62) due to the high content of ash in RHB. The pH of rice husk biochar was similar to the pH values of other rice husk biochars made by slow pyrolysis which has a pH of 7.8 (unknown temperature) [30], 8.01 (350 ◦ C, 4 h) [31] and 7.99 (500 ◦ C, 2 h)[32]. However it was different from the pH (10.0) of rice husk biochar produced from slow pyrolysis at 500 ◦ C for 4 h that Wang et al. (2013) reported. Reactor type was little associated with the pH of the biochar, and temperature and residence time decided the pH for the same feedstock. Cellulose and hemicelluloses were abundant in RHB, and they could be decomposed in pyrolysis process, producing organic acids and phenolic substances which lowered the pH of the products. The pH of biochar made from animal waste is higher than most plant residue biochars due to the higher ash content [33]. Specific surface areas of RHB and ESB were 78.15 m2 g−1 and 0.22 m2 g−1 , respectively, measured in automated volumetric gas adsorption apparatus by ASAP 2010 using nitrogen as an adsorbate at 77 K. From the above results, it can be seen that biochar from rice husk fast pyrolysis had higher BET surface area than biochar made from wood fast pyrolysis. Claoston et al. (2014) reported that the BET surface areas of rice husk and empty fruit bunch biocahr made by slow pyrolysis at 500 ◦ C for 2 h were 230.91 m2 g−1 and 15.42 m2 g−1 , respectively [32], but Wang et al. (2013) reported that the BET surface areas of rice husk biochar and elm biochar from slow pyrolysis at 500 ◦ C for 4 h were 12.2 m2 g−1 and 84.3 m2 g−1 , respectively [27]. Hence, BET surface area was significantly influenced by the reactor type, pyrolysis temperature and it increases with increase of pyrolysis temperature in a range of temperature. The specific surface area of crop-residue biochars was increased from 116 m2 g−1 to 438 m2 g−1 with increase of temperature from 400 ◦ C to 600 ◦ C [21], but it decreased at 700 ◦ C. So, the specific surface area of biochar is closely related to pyrolysis condition. A relatively structure of carbon matrix with extensive surface area was observed in RHB, suggesting that it might act as a surface sorbent to play an important role in environmental pollution control. In addition, the apparent density of RHB was 418.3 kg/m3 and it indicated that it was easier to spread on the surface soil by machine.
The contents of extractable nutrients in RHB were as follows (mg/g): K 6.445, Al 0.041, Ca 0.962, Mg 0.462, Na 0.156, P 0.735. Specifically, it was rich in potassium (6.445 mg/g), while the content of extractable heavy metal was rather low (e.g., Pb 0.000717 mg/g, As 0.000356 mg/g and Cu 0.000506 mg/g). The nutrients content was varied for different biochar. Rubber wood biochar had very high contents of available P (0.747 mg/g), K (6.895 mg/g), Mg (0.908 mg/g) and Ca (9.799 mg/g) [34]. The extracted P, K and Mg contents were similar to that of rice husk biochar, but Ca content was rather high. The K content was high in peanut hull biochar (8.142 mg/g), but it was low in pine bark biochar (0.843 mg/g) [35]. Dissolved phosphorus in biochar could remove Pb2+ by forming precipitate lead by forming (e.g., (Pb)5 (PO4 )3 Cl) [36] in addition to be as soil nutrients. 3.6. Effects of the RHB on chemical properties of the acid soil The soil characteristics were as follows: pH 3.33, total C 2.1%, total N 0.22%, extractable K 55 mg/kg, Al 1327 mg/kg, Ca 68 mg/kg, Mg 10 mg/kg, Na 7 mg/kg and Pb 6.7 mg/kg. The soil chosen for the experiment was acid and low in potassium, while the aluminum content was very high. Aluminum (Al) is the most abundant metal in the earth’s crust, accounting for 7% of its mass [37]. Too much aluminum in soils could produce harmful effect to plants growth due to inhibition of root growth. 3.6.1. Effect of the RHB on pH, total carbon and nitrogen of soil Acid in the soil could change the form of nutrients and inhibit some elements absorption of the root. Overall, biochar had a small but significant effect on soil pH (a shift in pH of 0.2 units). Biochar improved the soil pH obviously with the addition of 4% biochar (p < 0.05). The reason that soil pH increased with increasing biochar addition was mainly due to alkalinity of biochar. Soil pH increased from 3.43 to 3.63 by addition of biochar from 0 to 4% (Fig. 2). The result was consistent with other previous report [9,24,38]. Fig. 3 shows effects of RHB on total carbon and nitrogen of soil. Result indicated that spiking RHB into soil could increase soil carbon content significantly and elevate nitrogen slightly. Total carbon in the soil was significantly elevated from 2.04% of the control soils to 3.51% at 4% biochar addition. The RHB contained 37.66% of carbon which was hard to degrade due to its aromatic structure. Hence, adding biochar into soil could increase carbon sequestration for a
Y. Wang et al. / Journal of Analytical and Applied Pyrolysis 110 (2014) 375–381
Fig. 2. Effect of rice husk biochar on soil pH.
long time. Biochar could be reserved for hundreds to thousands of years in forest ecosystem [39]. The soil nitrogen increased from 0.218% to 0.242% after 60 days incubation with 4% biochar addition due to reasons that RHB changed the soil humidity, aeration status, inhibited the microbial denitrification, and decreased the generation and released nitrogen oxide [40] in addition to biochar containing nitrogen. It was reported that most of the nitrogen in biochar were heterocyclic compounds which were rather stable in the soil and environment, and the rate of the nitrogen release was very slow [41]. 3.6.2. Effect of the RHB on soil extractable elements Changes of extractable elements in soil were measured at the end of the pot experiment. Biochar amendment increased soil extractable elements such as Na, K, Ca, and Mg but decreases Al and Pb (Fig. 4). Similarly, Novak (2009) has reported that addition of biochar into the Norfolk soil increased soil pH, soil organic carbon, Ca, K, Mn, P and decreased exchangeable acidity, S, and Zn [10]. The magnitudes of changes were roughly proportional to the rate of biochar application (e.g. increases or decreases with increasing rate of biochar application). There were two reasons for this. One was biochar itself contained some nutrients, and the other was that biochar was rich in pores which could adsorb the elements in soil and prevent it from leaching to the ground water. In some case, statistical difference was significant
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only in the higher rates, namely 4% of biochar application but not at low addition. Soil exchangeable cations and negative surface charge of biochar particles increased due to carboxylate groups of the biochar and exposed carboxylate groups of organic acids sorbed by the biochar, both of which increased negative surface charge of biochar particles [42]. Adding 4% of RHB into soil could increase the K content from 42 to 324 mg/kg due to high potassium content of RHB. Hence, RHB could be used as supplement of potassium to soil. The extractable Al in the soil decreased from 1304 to 805 mg/kg at addition of 4% biochar. Due to the biochar liming effect, the aluminum species were converted to Al(OH)2+ and Al(OH)2 + monomers, which were strongly adsorbed by biochar [43]. In addition, it was noted that the symptoms of severe A1 toxicity was enhanced in the field of Ca deficiency and that application of Ca alleviated [37] the above phenomenon. Calcium (0.962 mg/g) in RHB could alleviate the Al toxicity. Hence, incorporation of biochars can decrease the potential toxicity of Al to plants in acid soils. During pyrolysis, nutrients (primarily K, Ca, and Mg) of biomass formed metal oxides and mixed with the biochar. And when RHB was buried in the soil, these oxides can react with H+ and monomeric of Al species, to modify the soil pH and exchangeable acidity values. The highest level of cation exchangeable capability (CEC), P and K has been observed in acid sulfate soil treated with RHB [12]. Many reports pointed out that biochar could increase the crop yield, but it was related to type of biochar, crop and soil types [8,11,44,45]. It was reported that the soil exchangeable Na and exchangeable Ca was not significantly influenced by biochar additions in a three-year field trial, and the addition of the biochar to soil causes small and potentially transient changes in a temperate agroecosystem function [8]. Rice husk belongs to herbaceous plant and wood sawdust belongs to wood plant. It is an unique angle to analyze the biochar from the point of view of herbaceous plant and wood. Wang et al. (2013) compared the straw-based biochar and wood-based biochar. The biochars from crop straw (herbaceous plant) may be more effective for improving soil fertility and C sequestration. The three straw-based biochars (rice straw, corn straw and wheat straw) generally showed higher yield (19.0–37.6 wt%) and higher pH (9.2–11.1), and consistently exhibited far greater ash percentage (14.5– 40.3 wt%), CEC (14.1–34.8 cmol kg−1 ), and the total C, N, P, Ca, and Mg, Na, K than the two wood-based biochars (bamboo and elm) [27]. As the above reported of present research, the ash content of the rice husk (herbaceous plant) biochar was higher than that of the elm sawdust biochar.
Fig. 3. Effect of the rice husk biochar on (a) level of soil total carbon, (b) total nitrogen.
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Fig. 4. Effect of the rice husk biochar on chemical properties of soil (a) potassium (b) calcium (c) sodium (d) magnesium (e) aluminum and (f) lead, 1 ppm in the figure represents 10 mg/kg.
4. Conclusions Characteristics of biochar derived from the fast pyrolysis of RH and ES were rather different. Compared to sawdust biochar, RHB was high in ash, while low in volatile and fixed carbon content. Similar functional groups were found in RH and ES, both of which had more functional groups than their biochars. RHB contained more
function groups than that of ESB, and a new functional group (C H, deformation vibration, arene) generated in RHB, which makes the biochar more stable. Larger surface area (78.15 m2 g−1 ) was found in RHB. Incorporation of the RHB significantly improved the soil properties. Total carbon, nitrogen, pH and extractable Ca, Mg, K, and Na contents in soil increased and the available Al and Pb contents decreased.
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Acknowledgements Financial support from the National High Technology Research and Development Program (863 Program, Grant NO. 2012AA101808) of China is acknowledged. In addition, Professor Sergio Capareda from Department of Biological & Agricultural Engineering, Texas A&M University, USA is acknowledged for his valuable comments and suggestions about this paper. References [1] A. Bridgwater, D. Meier, D. Radlein, Org. Geochem. 30 (1999) 1479. [2] J. Lehmann, J. Gaunt, M. Rondon, Mitig. Adapt. Strat. Glob. Change 11 (2006) 395. [3] A. Bridgwater, J. Anal. Appl. Pyrolysis 51 (1999) 3. [4] P.J. Bauer, J.R. Frederick, J.M. Novak, P.G. Hunt, Soil Till. Res. 90 (2006) 205. [5] D. Laird, P. Fleming, B. Wang, R. Horton, D. Karlen, Impact of soil biochar applications on nutrient leaching, in: International Annual Meetings ASA CSSA SSSA, 2008. [6] M. Collison, L. Collison, R. Sakrabani, B. Tofield, Z. Wallage, Low Carbon Innovation Centre, Norwich, UEA, 2009. [7] J. Lehmann, Front. Ecol. Environ. 5 (2007) 381. [8] D.L. Jones, J. Rousk, G. Edwards-Jones, T.H. DeLuca, D.V. Murphy, Soil Biol. Biochem. 45 (2012) 113. [9] K.Y. Chan, L. Van Zwieten, I. Meszaros, A. Downie, S. Joseph, Aust. J. Soil Res. 45 (2007) 629. [10] J.M. Novak, W.J. Busscher, D.L. Laird, M. Ahmedna, D.W. Watts, M.A.S. Niandou, Soil Sci. 174 (2009) 105. [11] L. Van Zwieten, S. Kimber, S. Morris, K.Y. Chan, A. Downie, J. Rust, S. Joseph, A. Cowie, Plant Soil 327 (2009) 235. [12] A. Masulili, W.H. Utomo, M. Syechfani, J. Agric. Sci. 2 (2010) P39. [13] B. Glaser, J. Lehmann, W. Zech, Biol. Fert. Soils 35 (2002) 219. [14] M.A. Rondon, J. Lehmann, J. Ramírez, M. Hurtado, Biol. Fert. Soils 43 (2007) 699. [15] C.H. Cheng, J. Lehmann, M.H. Engelhard, Geochim. Cosmochim. Acta 72 (2008) 1598. [16] T. Chen, C. Wu, R. Liu, W. Fei, S. Liu, Bioresour. Technol. 102 (2011) 6178. [17] A. Mehlich, Commun. Soil Sci. Plant Anal. 15 (1984) 1409. [18] F. Karaosmanoglu, A. Isigigür-Ergüdenler, A. Sever, Energy Fuels 14 (2000) 336.
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