The forms of alkalis in the biochar produced from crop residues at

Bioresource Technology 102 (2011) 3488–3497

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The forms of alkalis in the biochar produced from crop residues at different temperatures Jin-Hua Yuan a,c, Ren-Kou Xu a,⇑, Hong Zhang b a

State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, P.O. Box 821, Nanjing, China Department of Chemistry, Tennessee Technological University, Cookeville, TN 38505-0001, USA c Graduate University of the Chinese Academy of Sciences, Beijing 100049, China b

a r t i c l e

i n f o

Article history: Received 11 August 2010 Received in revised form 2 November 2010 Accepted 3 November 2010 Available online 12 November 2010 Keywords: Alkalinity Biochar Carbonates Crop residues Amendment of acid soils

a b s t r a c t The forms of alkalis of the biochars produced from the straws of canola, corn, soybean and peanut at different temperatures (300, 500 and 700 °C) were studied by means of oxygen-limited pyrolysis. The alkalinity and pH of the biochars increased with increased pyrolysis temperature. The X-ray diffraction spectra and the content of carbonates of the biochars suggested that carbonates were the major alkaline components in the biochars generated at the high temperature; they were also responsible for the strong buffer plateau-regions on the acid–base titration curves at 500 and 700 °C. The data of FTIR–PAS and zeta potentials indicated that the functional groups such as –COO (–COOH) and –O (–OH) contained by the biochars contributed greatly to the alkalinity of the biochar samples tested, especially for those generated at the lower temperature. These functional groups were also responsible for the negative charges of the biochars. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction In the partial or total absence of oxygen, thermal decomposition of plant-derived biomass (oxygen-limited pyrolysis) can be manipulated to yield a solid carbon-rich residue generically referred to as biochar, in addition to the gaseous components including carbon dioxide (CO2), combustible gases, volatile oils and tarry vapors (Sohi et al., 2010). Feedstock of biochar currently used at commercial and research facilities includes wood materials (wood chip, wood pellets, and tree bark), crop residues (straw, nutshells, and rice hulls), switch grass, organic wastes (paper sludge, sugarcane bagasse, distillers grain, olive waste), chicken litter, dairy manure, and sewage sludge (Sohi et al., 2010). Conversion of the agricultural residues by pyrolysis to produce biochar can reduce farm wastes and substitute renewable energy sources for fossil-derived fuels (Demirbas et al., 2006; Laird, 2008; McHenry, 2009). Biochar has received increasing interest as an approach for nearly permanent locking of atmospheric carbon in soils through a carbonnegative process (Glaser et al., 2009). Compared to feedstock, as its derived product, biochar can lead to a lower emission of greenhouse gases such as CO2, methane and nitrous oxide (Lehmann et al., 2006; Spokas and Reicosky, 2009). Application of biochar to soils has recently been proposed as a novel approach for creating a significant, long-term sink for atmospheric CO2 in terrestrial eco⇑ Corresponding author. Tel.: +86 25 86881183; fax: +86 25 86881000.

E-mail address: rkxu@issas.ac.cn (R.-K. Xu).

0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.11.018

systems (Lehmann, 2007; Kuzyakov et al., 2009). Biochar is also beneficial to soils in many ways: it can improve soil physical properties, promote development and bioactivity of soil micro-flora, reduce nutrient leaching, recycle soil nutrients, and increase soil organic carbon and thus plant growth (Marris, 2006; Sánchez et al., 2009; Steinbeiss et al., 2009; Gaskin et al., 2010).

Acidic soils occupy approximately 30% of the total arable land on the earth. Soil acidification can result in toxicity of aluminium and manganese to plants and deficiencies of phosphorus, molybdenum, calcium, and magnesium in soils, and thus can limit crop growth and reduce crop yield. Biochar is commonly alkaline, and thus can be used as a soil amendment to neutralize soil acidity and increase soil pH (Chan et al., 2007; Novak et al., 2009). The biochar produced from anaerobically digested bagasse residue was found to have a higher pH, surface area and CEC as well as a more negative surface charge as compared to the undigested bagasse biochar and thus can be efficiently used as a soil amendment to improve soil quality (Inyang et al., 2010). A linear relationship was found between soil pH and the alkalinity of the biochars produced from nine crop residues; the incorporation of the biochars in an acidic Ultisol was shown to decrease soil exchangeable acidity (Yuan and Xu, 2010). The biochars also increased the exchangeable base cations and the effective cation exchange capacity (ECEC) of the soil (Yuan and Xu, 2010). However, a mechanistic understanding of the amelioration of the acid soils by biochar has remained limited (Gaskin et al., 2008). The forms of the alkalis of biochar, in particular, warrant an investigation.

J.-H. Yuan et al. / Bioresource Technology 102 (2011) 3488–3497

In this study, a variety of biochars from the straws of canola, corn, soybean and peanut were prepared at different temperatures (300, 500 and 700 °C) by mean of oxygen-limited pyrolysis. The objectives of this study were to compare the physicochemical properties of the biochar samples and investigate the forms of the alkalis of these samples as influenced by the pyrolysis temperature and feedstock type. The mechanistic understanding of the function of biochars as useful agents to ameliorate acid soils was also discussed.

2. Methods 2.1. Preparation of biochar The straws of canola (Brassica campestris L.), corn (Zea mays L.), soybean (Glycine max L.) and peanut (Arachis hypogaea L.) were collected from cropland in a suburb of Nanjing, China. The straws were air-dried at room temperature and ground to pass a 1-mm sieve. The ground straws were then placed in ceramic crucibles, each covered with a fitting lid, and pyrolyzed under oxygen-limited conditions in a muffle furnace (Shanghai Yi Zhong Electricity Furnace Inc, Shanghai, China). The pyrolysis temperature was raised to the selected values of 300, 500 and 700 °C at a rate of approximately 20 °C min 1 and held constant for 4 h (Chun et al., 2004). After heating for 4 h, the biochar samples were allowed to cool to room temperature. Three replicates were done for each crop straw during the biochar-generating process. All biochar samples were then examined for their physical and chemical characteristics without further treatment.

2.2. Physical and chemical properties of the biochar To analyze the biochar samples, 0.2000 g of the ground biochar (1-mm sieve) and 0.5000 g of the ground straws (1-mm sieve) were heated at 200 °C for 1 h and then at 500 °C for an additional 4 h in a muffle furnace (Slattery et al., 1991). After the furnace was cooled to room temperature, the samples were withdrawn and weighed, and these were used as the ash-mass of the biochars and straws. The ash content of the biochars and straws was calculated using the following equation: Ash content (%) = (MAsh/ MBiochar or Straw) 100, where MAsh, MBiochar and MStraw are the mass of the ash, biochars and straws, respectively. The biochar production rate was calculated using the following equation: Production rate (%) = (MBiochar/MStraw) 100, where MBiochar and MStraw are the mass of the biochar and its feedstock, respectively.

The ash alkalinity of the crop straws was determined using a modified titration method (Slattery et al., 1991). The ash was dissolved in 25 mL of 1.0 M HCl, and 5 mL of the solution was taken and titrated against a standard 0.25 M NaOH solution to determine the ash alkalinity. Of this solution, 1 mL was taken to analyze chemical composition. The contents of K+ and Na+ in the solution were determined by flame photometry, and Ca2+ and Mg2+ by atomic absorption spectrometry. The total P in the solution was determined using ascorbic acid–NH4–molybdate blue colorimetry at 700 nm (Wang et al., 2009). The total C and N contents of the biochar and straws were determined using a Leco CN-2000 analyzer (Leco Corp., St. Joseph, MI, USA) at 1200 °C. The pH of the biochars was measured in deionized water at the ratio of 1:5 wt/ wt (Gaskin et al., 2008). The biochar samples were each thoroughly mixed and allowed to equilibrate for 1 h; the pH was then measured using an Orion 720 pH meter with a combination electrode.

The CEC of the biochars was measured by a modified NH4– acetate compulsory displacement method (Gaskin et al., 2008).

Of the biochar, 0.2000 g was leached with 20 mL of deionized water five times, and the leachates were collected together. The

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K+, Na+, Ca2+ and Mg2+ in the leachate were determined as the soluble base cations of the biochar. After the fifth run of the leaching with deionized water, the biochar was leached with 20 mL of 1 M Na–acetate (pH 7) five times, and the leachates collected together.

The K+, Ca2+ and Mg2+ in the leachates were determined as the exchangeable base cations. The biochar samples were then washed with 20 mL of ethanol five times to remove the excessive Na+.

Afterwards, the Na+ on the exchangeable sites of the biochar was displaced by 20 mL of 1 M NH4–acetate (pH 7) five times, and the CEC of the biochar was calculated from the Na+ displaced by 2+ 2+ + + NHþ 4 . The Ca , Mg , K and Na in the leachates were determined using the same methods described above.

The amount of carbonates was determined by volumetric analysis of the CO2 liberated through adding 4 M HCl solution to the biochar samples (Pansu and Gautheyrou, 2006). About 5 g of CaCO3 was dried at 104 °C and then 0.0000, 0.1000, 0.2000 and 0.3000 g of the dried CaCO3 were each weighed as standard substances. Of the prepared biochar samples (0.154-mm sieve), 2.00 g was weighed in three replicates to determine the carbonate content.

In the acid–base titrations, 0.5000 g of the biochar samples (1-mm sieve) was placed in 50-mL plastic bottles. Then, 20 mL of deionized water was added to each bottle, and each of the bottles was stirred on a magnetic stirrer for 2 h at 25 °C. The samples were then titrated with 0.1 M HCl at 25 °C to the end point at pH 2.0 using an automatic titration instrument (TIM 854, Radiometer) with magnetic stirring. The titration rate was kept at 0.5 mL min 1, with data collected every 6 s.

The X-ray diffraction (XRD) analysis was conducted using a computer-controlled diffractometer (Rigaku, D/max-III C). This was operated at 40 kV and 20 mA and the data collected over a 2h range of 0–50° using the Cu-Ka radiation at a scan rate of 2° min 1. The phase peaks were identified by comparing the observed XRD patterns to the standards compiled by the Joint Committee on Powder Diffraction and Standards (PDF22004).

The FTIR–PAS (Fourier transform mid-infrared photoacoustic spectroscopy) spectra were recorded for all the biochar samples tested using a Nicolet 380 spectrophotometer (Thermo Fisher Scientific, USA) equipped with a photoacoustic cell (Model 300, MTEC, USA) (Du et al., 2009). After a sample was placed in the cell-holding cup (diameter: 5 mm, height: 3 mm) and the cell was purged with dry helium at 10 mL min 1 for 10 s, the sample was scanned from 4000 to 500 cm 1 with a resolution of 8 cm 1 and a mirror velocity of 0.48 cm s 1.

To determine the zeta potential of the biochar, 0.0450 g of the sample (0.054-mm sieve) was placed in a 250-mL conical flask, and 180 mL of 0.1 mM NaCl solution was then added to each flask.

The suspension pH was adjusted within a range 3.0–7.0 with NaOH or HCl. The suspensions were dispersed ultrasonically for 1 h at 25 ± 1 °C in a bath-type sonicator at a frequency of 40 kHz and a power of 300 W. After the sample was allowed to stand for 3 days, its electrophoresis mobility was measured using a JS94G + microelectrophoresis apparatus (Shanghai Zhongchen Digital Technique Equipment Ltd. Co., Shanghai, China) and the zeta potential values were calculated using a specific software (Hou et al., 2007). The suspension pH was determined and the pH values were used to construct graphs of the relationship between zeta potential of the biochar and pH. 2.3. Incubation experiments Incubation experiments were conducted to investigate the amelioration of an acidic Ultisol (US Soil Taxonomy) by the biochars.

The soil sample was taken from the topsoil (0–10 cm) in Anhui Province, China (119°80 E, 31°60 N). Air-dried soil samples of 150 g were placed in plastic cups to which the biochar was added each at the level of 10 g kg 1. Soil and biochar were mixed thoroughly,

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J.-H. Yuan et al. / Bioresource Technology 102 (2011) 3488–3497

et al., 2008). The present study also shows a similarity between the elemental concentration of the feedstock (Table 2) and the chemical composition of the biochars (Table 1). The straws of peanut and corn had higher contents of base cations than those of canola and soybean (Table 2), and the biochars produced from the former two straws also had higher contents of the total base cations than the biochars from the latter two straws (Table 1). The higher content of P in the peanut biochar compared with the biochars generated from other three straws (Table 1) can be attributed to the much higher content of P of the peanut straw as the feedstock (Table 2). The higher content of base cations and total P in the biochars compared with their feedstock suggests that the relevant chemical components were concentrated in the biochars during the pyrolysis of crop straws.

The contents of carbonates for the canola straw biochar prepared at 300, 500 and 700 °C were close to those for the peanut straw biochar prepared at the same temperatures. The carbonate contents of the biochar produced from soybean straw at 500 and 700 °C were lower than those from canola and peanut straws at the same temperatures. Biochar produced from corn straw at 300 °C had a higher carbonate content than those from the other three straws at the same temperature, and these three all had similar carbonate contents (Table 1).

The contents of total N for the straws of canola, corn, peanut and soybean were 1.9, 10.8, 15.0 and 23.8 g kg 1, respectively (Table 2). Compared with their feedstock, the biochars had lower contents of total N, which decreased with increased pyrolysis temperature – indicating an ignition loss of N during pyrolysis of the straws. The C/N ratio for the biochar produced from straws of canola and soybean was much higher than that for their feedstock (235 and 18 for the canola and soybean straws, respectively) due to the loss of N during pyrolysis. The C/N ratio for the corn straw biochar was also higher than for the feedstock,

and then wetted with deionized water to 70% of field waterholding capacity of the soil. All cups were covered with plastic film with a small hole to allow for required gaseous exchange as well as minimized moisture loss. The cups with various treatments were then incubated at a constant 25 °C and weighted every 3 days; water was supplied to maintain a constant moisture content throughout the experimental period. There were three replicates for each treatment with controls having no incorporated biochar.

After 60-day incubation, soil pH was determined using an Orion 720 pH meter with a combination electrode.

2.4. Statistical analysis SPSS 15.0 (SPSS Inc., Chicago, IL, USA) was used to perform the statistical analysis of the data. Correlation analyses were conducted between the alkalinity and the content of carbonates of the biochar samples tested. All results are the average of three replicates with standard errors provided.

3. Results and discussion 3.1. Chemical composition and properties of the biochar The chemical compositions of the biochar samples tested were determined (Table 1). The contents of the total base cations, soluble base cations, exchangeable base cations, carbonates and total P increased with increased pyrolysis temperature for the biochar generated from the four crop straws (P < 0.05). The content of the total P for the biochar samples was in the following order: peanut straw biochar > soybean straw biochar > corn straw biochar > canola straw biochar (P < 0.05).

The total elemental concentration of feedstock had the strongest influence on the chemical composition of biochar (Gaskin

Table 1 The pH, CEC, chemical compositions, ash contents, and production rates of the biochars produced from the straws of canola, corn, soybean, and peanut at different pyrolysis temperatures.

Soluble CEC Exchangeable Carbonate Total base Total P Total N Total C C/N base (cmol kg 1) (g kg 1) (g kg 1) (g kg 1) (cmol kg 1) cations base cations 1 1 (cmol kg ) (cmol kg ) cations (cmol kg 1)

Biochar

Temperature pH (°C)

Canola straw biochar

300 500 700

6.48k 199h 9.39g 210g 10.76f 179j

133l 222j 341e

48j 104g 186d

77i 110e 134b

15.3e 87.1cd 165.2a

1.6i 2.7h 4.8f

1.9h 0.4j 0.4j

616d 634a 549f

325d 24.43d 1610a 14.40i 1584b 9.24l

10.70l 18.40i 28.55f

Corn straw biochar

300 500 700

9.37h 183i 10.77f 304a 11.32a 210g

235f 351d 524a

131f 225b 330a

78i 106g 122c

30.5de 58.0d 94.8bc

2.5h 4.0g 6.8c

14.4c 9.0e 7.8f

536g 419j 245k

37g 32.07b 46f 20.09f 31h 12.69k

30.15e 50.70b 73.30a

Soybean straw biochar

300 500 700

7.66g 279b 10.92d 216f 11.10c 222e

157k 248g 311h

51j 93i 158e

98h 108f 123c

15.8e 74.8cd 124.4b

2.7h 4.4f 5.8e

12.7d 3.7g 1.0i

576e 626b 579d

45f 30.27c 171e 18.07g 574c 13.69j

11.15k 17.85j 23.70g

Peanut straw biochar

300 500 700

8.60i 229d 10.86e 230d 11.15b 254c

228i 366c 424b

100h 153e 204c

120d 123c 138a

13.2e 89.4c 165.8a

6.3d 9.5b 11.6a

26.0a 15.1b 15.1b

537g 485h 470i

21i 34.89a 32h 20.37e 31h 16.63h

20.10h 32.50d 38.50c

Production Ash rate (%) content (%)

Total base cations are the sum of total Ca2+, Mg2+, K+ and Na+. Soluble base cations are sum of soluble Ca2+, Mg2+, K+ and Na+. Exchangeable base cations are sum of exchangeable Ca2+, Mg2+ and K+. Means followed by the same letter within a row are not significantly different at the 5% level.

Table 2 The chemical compositions for the feedstock of the biochars, canola straw, corn straw, soybean straw, and peanut straw.

Crop straw

Ash alkalinity (cmol kg 1)

Total base cations (cmol kg 1)

Total P (g kg 1)

Total N (g kg 1)

Total C (g kg 1)

C/N ratio

Canola straw Corn straw Soybean straw Peanut straw

43.3d 50.9c 63.9b 80.9a

35.0d 81.5a 49.9c 79.4b

1.1b 0.9c 0.9c 2.4a

1.9d 10.8c 23.8a 15.0b

457a 427d 441b 429c

235a 40b 18d 28c

The total base cations are the sum of total Ca2+, Mg2+, K+ and Na+. Means followed by the same letter within a row are not significantly different at the 5% level.

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J.-H. Yuan et al. / Bioresource Technology 102 (2011) 3488–3497 Table 3 The alkalinity of the biochars produced from the straws of canola, corn, soybean, and peanut at different pyrolysis temperatures at pH 7.0, 5.0, and 2.0 (cmol kg 1). pH

7.0 5.0 2.0

Canola straw biochar

Corn straw biochar

Soybean straw biochar

Peanut straw biochar

300 °C

500 °C

700 °C

300 °C

500 °C

700 °C

300 °C

500 °C

700 °C

300 °C

500 °C

700 °C

1l 49l 174k

54h 212g 343h

115e 442b 604b

30i 100j 203j

128d 240f 369f

281a 415c 576d

4k 56k 174k

85g 197h 364g

212c 413d 594c

16j 108i 288i

111f 278e 487e

220b 462a 767a

Means followed by the same letter within a line are not significantly different at the 5% level.

while the C/N ratio for the peanut straw changed less during its pyrolysis (Table 1).

The biochar pH increased with increased temperature (P < 0.05), suggesting that higher pyrolysis temperature led to the higher pH of the biochars (Table 1). The CEC of the biochar samples ranged from 179 to 304 cmol kg 1. The CEC of biochar prepared from soybean straw at 300 °C was higher than that prepared at 500 and 700 °C; CEC of biochar of canola and corn prepared at 500 °C was higher than that at 300 and 700 °C; and CEC of biochar of peanut prepared at 700 °C was higher than that at 300 and 500 °C. The biochar yield varied, with range 9.24–34.89%, and decreased with increased temperature (P < 0.05); while the ash content of the biochar increased with increased temperature (P < 0.05) (Table 1).

3.2. Acid–base titration curves of the biochar The titration data (Table 3) indicated that the biochar samples were alkaline, and that their alkalinity increased with increased pyrolysis temperature. A constant pH automatic potentiometric titration was conducted to obtain the acid–base titration curves

for the biochar samples (Fig. 1), to probe the origin of the biochar alkalinity.

The acid–base titration curves (Fig. 1) of the biochars were similar to those for weak alkaline substances. The biochars derived from different straws had different buffer capacities for acids at different pH ranges. The titration curves for the biochars from corn straw at 300, 500 and 700 °C were straight lines, as were the titration curves for the biochars produced from straws of canola, peanut and soybean at 300 °C. There was a plateau region for the titration curves for the biochars produced from straws of canola, soybean and peanut at 500 and 700 °C (Fig. 1). This suggests that these biochar samples had a relatively higher buffer capacity for acids in the pH range at which the titration curve plateau occurred. For example, biochar produced from canola straw at 700 °C had a plateau at pH 5.5–7.5 (Fig. 1, canola) and a total of 15.87 mL of HCl was consumed in this pH range (7.94 mL of HCl was consumed to decrease pH by one unit). The plateau was at pH 5.5–8.0 for soybean straw biochar (Fig. 1, soybean), and the HCl consumed in this pH range was 11.84 mL (3.95 mL of HCl to decrease pH by 1 unit). The plateau was at pH 5.6–7.2 for peanut straw biochar (Fig. 1, peanut), and the HCl 12

12 11 9

9

8

8

7

7

6

6

5

5

4 3 300°C 2

0

5

10

4

700°C

500°C 15

20

25

Corn straw biochar

10

pH

pH

11

Canola straw biochar

10

300°C

3 30

2

35

0

5

10

Volume of HCl (mL)

20

25

30

35

12

11

11

Soybean straw biochar

10 9

9

8

8

7

7

6

6

5

5

4

300°C

3 0

5

500°C 10

4

700°C 15

20

Peanut straw biochar

10

pH

pH

15

Volume of HCl (mL)

12

2

700°C

500°C

25

Volume of HCl (mL)

30

35

2

0

5

700° C

500°C

300° C

3

10

15

20

25

30

35

40

Volume of HCl (mL)

Fig. 1. The acid–base titration curves of the biochars produced from the straws of canola, corn, soybean, and peanut at different pyrolysis temperatures.

J.-H. Yuan et al. / Bioresource Technology 102 (2011) 3488–3497

3.3. The XRD analyses of biochar

consumed in this pH range was 10.85 mL (6.78 mL of HCl to decrease pH by one unit). For the biochars produced from the straws of canola, soybean and peanut at 500 °C, the plateau regions were found at pH 6.0–7.5, 6.0–7.3 and 6.0–7.2, respectively (Fig. 1), and the volumes of HCl consumed to decrease pH by one unit in these pH ranges were 2.45, 2.79 and 4.40 mL, respectively.

The alkalinity of the biochars increased with increased pyrolysis temperature (P < 0.05), and thus the consumed HCl also increased (P < 0.05) (Table 3) when the biochar samples were titrated to the desired pH values. The amount of alkali released from the biochar when titrated to pH 7.0, 5.0 and 2.0 by HCl (Table 3) can be calculated from Fig. 1. The amount of alkali released from the biochars of the various straws produced at 300, 500 and 700 °C when titrated to pH 7.0 followed the order: corn straw biochar > peanut straw biochar > soybean straw biochar > canola straw biochar (P < 0.05); the order for biochar produced at 300 and 500 °C when titrated to pH 5.0 and 2.0 was peanut straw biochar > corn straw biochar > soybean straw biochar and canola straw biochar (P < 0.05); and the alkalinity order for biochar produced at 700 °C when titrated to pH 5.0 and 2.0 was peanut straw biochar > canola straw biochar > soybean straw biochar and corn straw biochar (P < 0.05) (Table 3). These results suggest that peanut straw biochar had the highest alkalinity among the biochar samples tested and that the alkaline components in the corn and peanut straw biochars were probably easier to release than those in the other biochars. The feedstock alkalinity also affected alkalinity of the biochar. The ash alkalinity of peanut straw was much higher than that for straws of canola, soybean and corn (Table 2), and alkalinity of the biochar produced from peanut was higher than of the other three biochars (Table 3).

The XRD spectra of the biochars produced from straws of canola, corn, soybean and peanut at 300, 500 and 700 °C are shown in Fig. 2. For the canola straw biochar (Fig. 2, canola) produced at 300 °C, only the presence of sylvite (KCl) was confirmed by peaks at 3.15 and 2.22 Å. However, when pyrolysis temperature was increased to 500 °C, in addition to sylvite, calcite (CaCO3) was also found to form in the canola straw biochar, as confirmed by peaks at 3.03, 2.49, 2.28 and 2.09 Å. The peak at 2.90 Å was attributed to the presence of dolomite, CaMg(CO3)2. When the pyrolysis temperature was increased to 700 °C, the presence of calcite was confirmed by peaks at 3.86, 3.03, 2.49, 2.28, 2.09, 1.92 and 1.87 Å; the presence of dolomite was confirmed by peaks at 2.90 and 2.69 Å; and sylvite by peaks at 3.15 and 2.22 Å. The intense sharp peak at 3.03 Å indicated that calcite was well crystallized. The increase in the peak intensity at 3.03 Å with the increase in pyrolysis temperature from 500 to 700 °C also suggested that the calcite content of the biochar increased with increased pyrolysis temperature, consistent with the carbonate content in the biochar (Table 1).

No crystal substances were observed in the XRD spectra for the biochar produced from the corn straw (Fig. 2, corn) at 300 °C. When the pyrolysis temperature was increased to 500 °C, calcite (3.03 Å), dolomite (2.90 Å) and sylvite (3.15 Å) was found to form.

When the pyrolysis temperature was increased to 700 °C, the presence of calcite was confirmed by the peaks at 3.86, 3.03, 2.49, 2.28, 2.09, 1.92 and 1.87 Å; while the presence of dolomite was confirmed at 2.90 and 2.69 Å; and sylvite at 3.15 and 2.22 Å. The intense sharp peak at 3.03 Å indicated that calcite was well crystallized compared with the XRD spectra of the biochar

Corn straw biochar

40

45

50

20

25

30

°2θ (Cu Kα )

1.92 1.87

2.28 2.22

2.49

2.09

45

50

2.28

2.14 2.09

2.28

2.14

3.03

500°C

2.49

3.03

3.34

1.92 1.87

700°C

4.26

2.09 2.09

1.92 1.87

2.28 2.28

3.03

2.49

40

Peanut straw biochar

3.03

500°C

35

°2θ (Cu Kα )

Soybean straw biochar

700° C

2.26

3.15

3.20 3.03 2.90

300°C

2.49

35

3.86

2.09 2.09

1.92 1.87

2.28 2.22 2.28 2.22

500°C

2.22

30

700°C

2.71

25

2.69

2.90

20

2.49

2.90

3.15

300°C

700°C

2.49

3.20

500°C

3.15

3.86

3.03

3.03

Canola straw biochar

2.90

3492

300° C

300°C

20

25

30

35

°2θ (Cu Kα )

40

45

50 15

20

25

30

35

40

45

°2θ (Cu Kα ) 0

Fig. 2. The X-ray diffraction spectra for the biochars produced from the straws of canola, corn, soybean, and peanut at different pyrolysis temperatures (d values given in A Å).

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J.-H. Yuan et al. / Bioresource Technology 102 (2011) 3488–3497

produced at 500 °C. This was similar to the change of the XRD spectra for the canola straw biochar with increased pyrolysis temperature (Fig. 2, canola).

Similar to the XRD spectra of the biochar produced from corn straw at 300 °C, there were no crystal substances observed in the XRD spectra for the biochar produced from soybean straw (Fig. 2, soybean) at 300 °C. When the pyrolysis temperature was increased from 300 to 500 °C, calcite formed, as confirmed by peaks at 3.03, 2.28, 2.09, 1.92 and 1.87 Å. When the pyrolysis temperature was increased to 700 °C, still only calcite was found; however, the intense sharp peak at 3.03 Å suggests that crystallinity and content of calcite increased compared with biochar prepared at 500 °C. The presence of calcite was also confirmed by the peaks at 3.03, 2.49 and 2.28 Å in the biochars produced from peanut straw (Fig. 2, peanut) at 500 and 700 °C, similar to XRD spectra for the biochars produced from canola, corn and soybean at 500 and 700 °C (Fig. 2).

However, quartz (SiO2) was confirmed in the biochars produced from peanut straw at 300, 500 and 700 °C by peaks at 3.34 and 4.26 Å, consistent with the FTIR–PAS results for the biochars (Fig. S1, Supplementary data). The intense sharp peak at 3.34 Å suggests that the quartz was well crystallized and that its content was high in the biochar; however, quartz was not observed in the biochars from soybean, canola and corn (Fig. 2).

3.4. FTIR–PAS of the biochar FTIR–PAS is a fast and convenient method to follow the progress of pyrolysis (Brewer et al., 2009). The FTIR–PAS spectra of the biochar samples (Fig. S1, Supplementary data) show the changes of the functional groups on the surfaces of the biochars produced at

300, 500 and 700 °C. The analyses of these spectra were based on previously published data.

The absorption peaks at 780 and 1400 cm 1 in the spectra of canola biochar produced at 300 °C (Fig. S1, Supplementary data) were assigned to the carboxylate (–COO ) deviational vibration and symmetric stretching (Cheng and Lehmann 2009; Fu et al., 2009; Lammers et al., 2009). When the pyrolysis temperature was increased to 500 and 700 °C, these two peaks disappeared (Fig. S1, Supplementary data), showing an ignition loss of –COO from canola straw biochar with increased temperature.

The peaks at 1590 cm 1 (Fig. S1, Supplementary data) for the biochar samples were assigned to –COO anti-symmetric stretching (Qiu et al., 2008), and the intensity of these peaks decreased when pyrolysis temperature was increased from 300 to 500 °C and completely disappeared at 700 °C (Fig. S1, Supplementary data). These data suggest that the –COO content decreased with increased pyrolysis temperature and that there was an ignition loss of –COO at 700 °C. The peaks at 860 and 1100 cm 1 (Fig. S1, Supplementary data) were assigned to the bands of the out-of-plane bending for carbonates (CO2 3 ) (Lammers et al., 2009; Michel et al., 2009; Nguyen et al., 2009). These peaks appeared in the spectra of the canola straw biochars produced at 500 and 700 °C, but not at 300 °C (Fig. S1, Supplementary data). The change of the peaks with the pyrolysis temperature was consistent with the carbonate content in the biochars (Table 1), which increased with increased pyrolysis temperature. The peaks at 1430 cm 1 (Fig. S1, Supplementary data) were assigned to the in-plane bending of carbonyl (–COH) (Cheng et al., 2008), and the intensity of these peaks increased with increased pyrolysis temperatures. This suggests that a new functional group was formed at the higher temperature.

-10

-10

Corn straw biochar

-20

Zeta potential (mV)

Zeta potential (mV)

Canola straw biochar

-30

300°C 500°C 700°C

-40

-50

2

3

4

5

6

7

-20

-30

-50

8

300°C 500°C 700°C

-40

2

3

4

pH -10

-10

-20

Zeta potential (mV)

Zeta potential (mV)

Soybean straw biochar

-30

300°C 500°C 700°C

-40

-50

2

3

4

5

pH

5

6

7

8

pH

6

7

8

Peanut straw biochar

-20

-30

-40

-50

300°C 500°C 700°C 2

3

4

5

6

7

8

pH

Fig. 3. The zeta potential–pH curves for the biochars produced from the straws of canola, corn, soybean, and peanut at different pyrolysis temperatures.

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J.-H. Yuan et al. / Bioresource Technology 102 (2011) 3488–3497

The peaks at 3400 cm 1 (Fig. S1, Supplementary data) were assigned to the hydroxyl (–OH) stretching (Yaman, 2004; Özçimen and Ersoy-Meriçboyu, 2010), and the peak intensity decreased with increased pyrolysis temperature, suggesting an ignition loss of OH at the increased temperature. The changes of the peaks and their intensities for the corn and soybean straw biochars with increased temperature were similar to those for canola biochar (Fig. S1, Supplementary data), indicating that functional groups on the biochars from soybean and corn were similar to those from canola.

The peaks at 1430, 1590 and 3400 cm 1 in the spectra of the peanut straw biochar were similar to those for the biochars of canola, corn and soybean (Fig. S1, Supplementary data). There was a peak at 780 cm 1 in the spectra of the biochar from peanut straw both at 300 and 500 °C, but was absent at 700 °C (Fig. S1, Supplementary data). This differs from the other biochar samples from canola, soybean and corn. It was worth noting that this peak occurred only in these biochars when produced at 300 °C. A notable difference for the peanut straw biochar, compared to the other biochars, was a peak at 1030 cm 1; this was assigned to Si–O stretching, indicating the presence of SiO2 in the peanut biochar. XRD results also demonstrate that crystal SiO2 was present in peanut straw biochar (Fig. S1, Supplementary data). The broad peak at 1030 cm 1 masked the peaks at 1100 and 860 cm 1 for peanut straw biochar produced at 700 °C, and consequently the latter two peaks were not visible in the corresponding spectrum.

The functional groups on the biochars can be related to the surface properties of their parent feedstock. The FTIR–PAS spectra for the straws of canola, corn, soybean and peanut as feedstock differed from those for their respective biochars (Table 4). The FTIR–PAS spectra for the straws exhibited peaks at 1060, 1600 and 2930 cm 1 (Table 4). These peaks were assigned to the straight chain C–C stretching (or sugar C–OH stretching), –COO antisymmetric stretching of amino acids, and alkane –CH2 symmetric stretching, respectively (Lammers et al., 2009). These results suggest that some functional groups of the crop straws changed to other groups or disappeared when straws were pyrolyzed.

The intensity of the peak at 3400 cm 1 for the feedstock was much higher than that for their corresponding biochars (Figs. S1 and S2, Supplementary data), and the intensity of the peak for the biochars decreased with increased pyrolysis temperature.

These results indicate that the transformation of the crop straws to biochars reduced the quantity of hydroxyl groups, and more so at higher pyrolysis temperatures. In their study on the functional groups present in biochar generated from wheat residues, Chun et al. (2004) observed that the acidic groups such as –COOH

decreased quantitatively with increased pyrolysis temperature and disappeared at high temperatures. This is consistent with the results of the present study. The peak at 1400 cm 1 occurred for the biochars produced at 300 °C as well as for their feedstock. This suggests that the biochars produced at the lower temperature retained some characteristics of their feedstock.

3.5. Zeta potentials of the biochar The zeta potential is the potential in the sliding plane of colloidal particles, and its value and sign are related to surface charge of the particles (Hunter, 1981). Protonation and deprotonation of functional groups can create a net charge on the surfaces of the solid particles, which can form an electrical double-layer in the solution phase near the surfaces (Hunter, 1981). Zeta potential values were measured as a function of solution pH for each of the biochar samples (Fig. 3). The zeta potential of the biochar samples at pH 3.0–7.0 ranged from 10 to 46 mV. The zeta potential values for all biochar samples were negative in the pH range studied, indicating that the biochar particles carried negative charges on their surfaces. The zeta potential of the biochars became more negative with increased pH, suggesting that the amount of negative charge on the biochars increased with increased pH. The zeta potential of the biochar decreased with increased pyrolysis temperature, therefore the biochars produced at the lower temperatures carried more negative surface charges than those produced at the higher temperatures.

The FTIR–PAS spectra indicated that there were ample amounts of oxygen-containing functional groups (e.g. –COO , –COH, –OH

and CO2 3 ) on the biochars. Among them, –COO and –OH contributed considerably to surface charge of the biochars. Additionally, the zeta potentials of the biochars support this interpretation. With the decline of pH, the zeta potentials of the biochar samples became less negative (Fig. 3), indicating that the association of –COO and –O with H+ may have reduced the negative charge of the biochars. The high pyrolysis temperature resulted in the decrease or even disappearance of these groups on the biochars (Fig. S1, Supplementary data). This led to a shift of the zeta potentials in the zeta-potential–pH curves for the biochars to the positive values (Fig. 3) and thus resulted in less-negative surface charges.

3.6. Forms of alkalis of the biochar in relation to acids and bases The acid–base titration curves of the biochars (Fig. 1) show an evident buffer region at pH 5.0–8.0 for the biochars generated from straws of canola, soybean and peanut at 500 and 700 °C. A previous study showed that carbonates in soils could buffer acid addition at

Table 4 The functional groups present on the biochars and their feedstock inferred from FTIR–PAS. Biochar

Temperature (°C)

–COOH (cm 1)

–OH (cm 1)

–COH (cm 1)

1 CO2 3 (cm )

Si–O (cm 1)

CH2 C–C (cm 1)

Canola straw Canola straw biochar

– 300 500 700 – 300 500 700 – 300 500 700 – 300 500 700

1400, 1600 780, 1400, 1590 1400, 1590 – 1400, 1600 780, 1400, 1590 1400, 1590 – 1400, 1600 780, 1400, 1590 1400, 1590 – 1400, 1600 780, 1400, 1590 780, 1400, 1590 –

3400 3400 3400 3400 3400 3400 3400 3400 3400 3400 3400 3400 3400 3400 3400 3400

– 1430 1430 1430 – 1430 1430 1430 – 1430 1430 1430 – 1430 1430 1430

– – 860, 1100 860, 1100 –

– – – – – – – – – – – – – 1030 1030 1030

1060, – – – 1060, – – – 1060, – – – 1060, – – –

Corn straw Corn straw biochar

Soybean straw Soybean straw biochar

Peanut straw Peanut straw biochar

860, 1100 860, 1100 – 860, 1100 860, 1100 – – – –

2930

2930

2930

2930

331.6 43 26.4 9 248.8 42

178.8 37

700 °C 300 °C

Peanut straw biochar

700 °C

500 °C

189.6 33 61.0 30

116.0 31

300 °C

330.4 55

Corn straw biochar

700 °C

3495

pH > 6.2, and when pH < 6.2 the solid carbonates disappeared from the soils (Ulrich, 1986). Therefore, it can be inferred that the buffer plateau region (Fig. 1) was mainly due to the existence of carbonates. The carbonate content (Table 1) and the XRD (Fig. 2) data support this explanation. The stronger buffering capacity of the biochars generated at higher temperatures can be attributed to increased carbonate content of the biochars with increased pyrolysis temperature.

There was no clear buffer plateau region in the titration curves for corn straw biochar (Fig. 1). This is probably because the amount of readily-released alkaline substances in the corn biochar was higher than that for the other biochars, and these alkalis masked the acid–buffer effect of the carbonates at this pH range (Table 3 and Fig. 1). The presence of CaCO3 has previously been reported previously in dairy-manure-derived biochar (Cao et al., 2009; Cao and Harris, 2010), consistent with the observations in the present study.

The correlation analysis between the alkalinity (pH 2.0) (y) and carbonate content of the biochars (x) showed a Pearson’s correlation coefficient of 0.934 (P < 0.0001), indicating a significant positive correlation (y = 154.1 + 3.3x). Hence, the carbonates in the biochars were considered to be the main alkaline substances, especially in the biochars generated at the higher temperatures.

The –COO and –O groups should also buffer the acid addition and contribute to alkalinity of the biochars through association of these groups with H+. Organic anions on the surface of biochar are also an important source of alkalinity, especially in the biochars generated at lower pyrolysis temperatures, as these groups increase quantitatively with decreased temperature (Fig. S1, Supplementary data). This is another important feature responsible for the alkaline properties of the biochars. The changes in the zeta potential values of the biochars with pH and temperature are the evidence that supports the above interpretation.

149.6 41 31.6 18 Contribution (cmol kg ) Relative contribution (%)

1

500 °C Soybean straw biochar

300 °C

Contribution of carbonates

174.2 51 30.6 18 Contribution (cmol kg 1) Relative contribution (%)

Canola straw biochar

300 °C

500 °C

3.7. Effect of pyrolysis temperature on the characteristics of the biochar and its buffering capability

Contribution of carbonates

Table 5 The contribution of carbonates to the alkalinity of the biochars produced from the straws of canola, corn, soybean, and peanut at different pyrolysis temperatures.

500 °C

700 °C

J.-H. Yuan et al. / Bioresource Technology 102 (2011) 3488–3497

Both the alkalinity and the form of alkalis for the biochars changed with pyrolysis temperature. The alkalinity and carbonate content of the biochars increased with increased pyrolysis temperature, while the biochar production rate was reduced with increased temperature (Table 1). The contribution of the organic anions to the alkalinity of the biochars decreased with increased pyrolysis temperature (Fig. S1, Supplementary data). Organic anions can quickly neutralize soil acidity when biochar is incorporated into acid soils, since the association of the organic anions with H+ is a rapid reaction; while carbonates in the biochars, especially CaCO3 and MgCO3, react slowly with H+ (Fig. 1). Therefore, the amelioration effect of the carbonates in biochars for acid soils can last long.

Biochars derived from crop straws contains alkaline substances and thus can be used as amendments for acid soils. In this study, both carbonates and organic anions contributed to the alkalinity of the biochars and were the main alkaline components in the biochars. Based on the carbonate content (Table 1) and the alkalinity of the biochars at pH 2.0 (Table 3), the contribution of carbonates to the alkalinity of the biochar samples was calculated (Table 5). The results indicated that the absolute contributions of the carbonates to the alkalinity increased sharply with the increase of pyrolysis temperature from 300 to 500 °C and then to 700 °C for biochars produced from the four different crop straws. However, the relative contribution of carbonates to the alkalinity (percentage of carbonate contribution in alkalinity, %) for the biochars produced from the straws of canola, soybean and peanut increased sharply with the increase of pyrolysis temperature from 300 to 500 °C and then changed only slightly from 500 to 700 °C. The relative contribution of the carbonates to the alkalinity (%) for

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Table 6 The effect of biochar incorporation on the pH of the acidic Ultisol tested.

Treatment

Temperature (°C)

Soil pH

DpH

Control

–

4.04 ± 0.01j

Canola straw biochar

300 500 700

4.37 ± 0.01e 4.63 ± 0.00c 5.25 ± 0.01g

0.32 0.58 1.21

Corn straw biochar

300 500 700

4.42 ± 0.00i 4.58 ± 0.01f 4.93 ± 0.03a

0.37 0.54 0.89

Soybean straw biochar

300 500 700

4.47 ± 0.00b 4.85 ± 0.01g 5.33 ± 0.00c

0.43 0.80 1.28

Peanut straw biochar

300 500 700

4.48 ± 0.01h 4.92 ± 0.02d 5.31 ± 0.02a

0.44 0.87 1.26

Means followed by the same letter within a line are not significantly different at the 5% level.

the corn straw biochar change slightly with pyrolysis temperature.

Thus, carbonates are the main alkaline components of the biochars produced from most crop straws at relatively high temperatures.

Organic anions are alkaline components in the biochars, especially for the biochars produced at low temperatures. However, it is difficult to quantify the contribution of organic anions to the alkalinity of the biochars in the present study. Some alkaline substances remain to be identified in the biochars produced at the relatively high temperatures in the present investigation, because carbonates only accounted for 40–60% of the alkalinity for most of the biochars produced at 500 and 700 °C (Table 5).

As shown in the present study, all the biochar samples carried negative surface charges (Fig. 3) and had high CEC (Table 1). Hence, biochar can enhance the ability of a soil to adsorb and retain cations (e.g. Ca2+, Mg2+, K+ and NHþ 4 ) and thus inhibit their loss by leaching from acid soils (Steiner et al., 2008; Novak et al., 2009; Sohi et al., 2010). Hence, incorporation of biochar amendments in acid soils can also improve soil fertility.

3.8. Effect of the biochar on soil pH The incubation experiments were performed to examine the effect of the biochar incorporation at rate of 10 g kg 1 on pH of an acidic Ultisol. The results of this test were presented in Table 6.

The incorporation of the biochars each increased soil pH (P < 0.05). The extent of the liming effects of the biochars produced from four crop straws increased with the rise of the pyrolysis temperature, which is consistent with the change of the alkalinity of the biochars with pyrolysis temperature (Table 1). The amelioration effects of the biochars produced from the straws of peanut, soybean and canola at 700 °C on soil pH were similar but greater than that of the biochar from corn straw at the same temperature.

The biochars from two legume straws at 500 °C showed similar amelioration effects on soil pH, and the same was true for the biochars from non-legume straws at the same temperature. The former was greater than the latter. Similar trends were observed for the biochars generated at 300 °C. These results suggest that the amelioration effects in acid soils depend on not only the alkalinity of the biochars but also on their alkali forms.

The biochar characteristics combined with their amelioration effects on soil pH suggest that the optimum pyrolysis temperature should be 500 °C for producing biochar amendments from crop straws for acid soils. At this temperature, both organic anions and carbonates can contribute significantly to the alkalinity of the biochars, and the biochars have greater amelioration effects

on soil acidity compared with the biochars produced at 300 °C. In addition, at this temperature a medium biochar production rate can be achieved.

4. Conclusions The results of this study indicate that all the biochar samples were basic, and their alkalinity increased with increased pyrolysis temperature. Inorganic carbonates were the major alkaline components of the biochars generated at the high temperatures tested.

Organic anions also contributed to alkalinity of the biochars, especially for that generated at low temperature. The biochar produced from crop straws can be used as amendments for acid soils. This study suggests that the pyrolysis at 500 °C is optimum for producing biochar for this purpose.

Acknowledgements This study was supported by the National Key Technology R&D Program of China (2009BADC6B02) and the National Natural Science Foundation of China (30821140538). We are greatly grateful to Dr. Changwen Du of the Institute of Soil Science, the Chinese Academy of Sciences for his help with collecting the FTIR–PAS spectra of the biochars tested.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2010.11.018.

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