Investigation on the Changes of Main Compositions and Extractives of

Mar 5, 2009 - hemicellulose (LCH) are the main compositions of rice straw, which provide the main carbon source for anaerobic microor- ganisms...
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Energy & Fuels 2009, 23, 2220–2224

Investigation on the Changes of Main Compositions and Extractives of Rice Straw Pretreated with Sodium Hydroxide for Biogas Production Yanfeng He,† Yunzhi Pang,‡ Xiujin Li,*,† Yanping Liu,† Rongping Li,† and Mingxia Zheng† Department of EnVironmental Science and Engineering, and Center for Resources and EnVironmental Research, Beijing UniVersity of Chemical Technology, Beijing 100029, People’s Republic of China ReceiVed September 6, 2008. ReVised Manuscript ReceiVed January 20, 2009

This study was conducted to investigate the changes of main compositions and extractives and their effects on biogas yield enhancement. Four NaOH doses (4%, 6%, 8%, and 10%) and four loading rates (35, 50, 65, and 80 g/L) were used. The rice straw was first pretreated by NaOH in solid-state conditions and anaerobically digested. The main compositions and extractives were then analyzed. The results showed that, compared to the untreated rice straw, 3.2%-58.1% more biogas yields were obtained with 4%-10% NaOH-treated rice straws. Hemicellulose, cellulose, and lignin were decomposed by 35.2%-54.2%, 14.2%-16.4%, and 8.0%-44.5%, respectively, for 4%, 6%, 8%, and 10% NaOH-treated rice straws. Considerable fractions of them were converted to relatively readily biodegradable substances, as indicated by increases of 80.3%-173.6% cold-water extractives and 80.4%-152.8% hot-water extractives. Some irresistible substances were removed, as represented by a 30.9%-51.8% decrease of benzene-ethanol extractives. The chemical structures of hotwater and benzene-ethanol extractives were also changed obviously. It was also found that the soluble sugar contents in the 6% NaOH-treated rice straw were twice that of the untreated one. The results specified that NaOH pretreatment was one of efficient methods to enhance biogas production of rice straw, and the changes of main compositions and extractives made important contributions to the enhancement.

1. Introduction The use of lignocellulosic waste materials from agriculture, agro-industrial processing, and forestry, as a source of chemicals and bioenergy, has received considerable interest recently, because of their abundance and renewability.1-3 Their degradation and subsequent utilization play an important role in the circulation of global carbon.4 Rice straw is one of the most abundant lignocellulosic crop residues in China, with an annual production of 230 million tons.5 Traditionally, it has been used as animal feed, fuels for cooking, feedstock for paper industry, and organic fertilizer. Recently, the bioconversion of lignocellulosic materials to ethanol has received considerable interest.6,7 The process contains two steps: the hydrolysis of cellulose to reducing sugars, and the following fermentation by yeast or bacteria, to convert the fermentable sugars to ethanol.8 The cost of ethanol production from lignocellulosic materials is relatively * Author to whom correspondence should be addressed. Tel.: +86-1064434743. E-mail: [email protected]. † Department of Environmental Science and Engineering. ‡ Center for Resources and Environmental Research. ¨ hman, M.; Boman, C.; Hedman, H.; Eklund, R. Energy Fuels 2006, (1) O 20, 1298–1304. (2) Kleinert, M.; Barth, T. Energy Fuels 2008, 22, 1371–1379. (3) Van Walsum, G. P.; Shi, H. Bioresour. Technol. 2004, 93, 217– 226. (4) Niranjane, A. P.; Madhou, P.; Stevenson, T. W. Enzy. Micro. Technol. 2007, 40, 1464–1468. (5) Ren, Z. J.; Gu, M. D. J. Anhui Agric. Sci. 2005, 33 (11), 2105– 2106 (in Chin.). (6) Bak, J. S.; Ko, J. K.; Han, Y. H.; Lee, B. C.; Choi, I. G.; Kim, K. H. Bioresour. Technol. 2009, 100, 1285–1290. (7) Ma, H.; Liu, W. W.; Chen, X.; Wu, Y. J.; Yu, Z. L. Bioresour. Technol. 2009, 100, 1279–1284. (8) Sun, Y.; Cheng, J. J. Bioresour. Technol. 2005, 96, 1599–1606.

high, based on currently available technologies, and the main challenges are the low yield and high cost of the hydrolysis process.9 Therefore, industrialization of ethanol production from lignocellulosic materials such as rice straw is not economically feasible in the foreseeable future. Conversion of rice straw to biogas through anaerobic digestion (AD) technology could be one of the promising alternatives. AD is not a new technology: it has been widely applied for the conversion of various organic wastes (such as sewage, animal manure, food waste, and municipal solid waste) to biogas.10-12 During the process, the biomass is transformed to biogas, which is a mixture of methane and carbon dioxide. In current stage, biogas production with AD has some advantages over ethanol production from lignocellulosic materials: (1) AD is not required to decompose carbohydrate polymers to reducing sugars through the hydrolysis process; this could break the major barrel for ethanol production from lignocellulosic materials, and, obviously, could also possibly reduce cost; (2) The production conditions and processes with AD are milder and simpler, making biogas production more practical; (3) AD is more environmentally friendly, because the residues from AD can be completely recycled and reused as fertilizer. However, the recalcitrant nature of the lignocellulosic substrate limits the effective conversion of rice straw to biogas. As is well-known, cellulose is a polymer that is composed of (9) Sun, Y.; Cheng, J. Y. Bioresour. Technol. 2002, 83, 1–11. (10) Sekiguchi, Y.; Kamagata, Y.; Harada, H. Curr. Opin. Biotechnol. 2001, 12, 277–282. (11) Sosnowski, P.; Wieczorek, A.; Ledakowicz, S. AdV. EnViron. Res. 2003, 7, 609–616. (12) Hamzawi, N.; Kennedy, K. J.; Mclean, D. D. Water Sci. Technol. 1998, 38 (2), 127–132.

10.1021/ef8007486 CCC: $40.75  2009 American Chemical Society Published on Web 03/05/2009

Rice Straw Pretreated with NaOH To Produce Biogas

anhydroglucose monomer units. In plants, cellulose is present mostly as lignocellulose in complex association with lignin. The distinctive structural characteristics of lignocellulose make them resistant to attack by anaerobic microorganisms.13 Thus, pretreatment of the rice straw is normally required. The pretreatment process can remove lignin and hemicellulose, reduce cellulose crystallinity, and increase the porosity of materials.14 Various physical, chemical, and biological pretreatment methods such as steam explosion, ammonization, and fungi biodegradation have been extensively investigated.9,15 Among them, sodium hydroxide (NaOH) has been proven to be capable of releasing digestible material from the cell wall and is suitable for upgrading lignocellulosic materials for AD processes and animal feeding.16,17 However, currently, most NaOH pretreatments use a large amount of chemical solution or water to soak substrates, which require recycling of the chemicals and disposal of the waste solution, and could result in potential environmental pollution.16,18 A new solid-state NaOH pretreatment method was developed in our study, which used a limited amount of water and did not generate waste chemical solution. This method avoided the problems that have been encountered with the conventional methods and also was proven to be effective in enhancing biodegradability and biogas yield.19,20 Three biogas plants that use this pretreatment method have been built in China to produce biogas from rice straw with digester volumes of up to 1000 m3. Our previous study has explored the changes of chemical structures and physical characteristics of cellulose, hemicellulose, and lignin during NaOH solid-state pretreatment, and the effects of the changes on biodegradation and biogas production of rice straw.21 However, the changes of main compositions and extractives during NaOH pretreatment were not investigated. The work presented here is a continuation of our previous research. The objective was to investigate the changes of main compositions and structural changes of extractives during solidstate NaOH pretreatment and the effects of the changes on biodegradation and biogas production of rice straw. 2. Experimental Section 2.1. Materials. The rice straw used in the study was collected from Tong County (Beijing City, PRC). It was chopped by a paper chopper (PC500, Staida Co., Tianjing, PRC) and then ground into 5-10-mm particles by a hammer mill (FE130, Staida Co., Tianjing, PRC). The activated sludge was taken from a mesophilic anaerobic digester in Gaobeidian Wastewater Treatment Work (Beijing, PRC). It contained 21.9 g/L total solids (TS), 10.4 g/L volatile solids (VS), and 20.7 g/L mixed liquor suspended solids (MLSS). 2.2. NaOH Solid-State Pretreatment. Rice straw was pretreated by NaOH in solid-state conditions before AD. Four NaOH doses of 4%, 6%, 8%, and 10%, based on the dry matter of rice straw, were used, and, therefore, the pretreatment experiment was classified (13) Imai, M.; Kohei, I.; Suzuki, I. Biochem. Eng. J. 2004, 17, 79–83. (14) Zhu, S. D.; Wu, Y. X.; Yu, Z. N.; Chen, Q. M.; Wu, G. Y.; Yu, F. Q.; Wang, C. W.; Jin, S. W. Biosys. Eng. 2006, 94, 437–442. (15) Charles, E. W.; Bruce, E. D.; Richard, T. E.; Mark, H.; Michael, R. L.; Lee, Y. Y. Bioresour. Technol. 2005, 96, 1959–1966. (16) Wu, J.; Xu, L. J.; Xie, J. L. Acta Sci. Circumstantiae 2006, 26 (2), 252–255 (in Chin.). (17) Dar, G. H.; Tandon, S. M. Biol. Wastes 1987, 21 (2), 75–83. (18) Zhang, R. H. ; Zhang, Z. Q. Bioresour. Technol. 1999, 68 (3), 235– 245. (19) Luo, Q. M.; Li, X. J. Trans. CSAE 2005, 21 (2), 111–115. (20) Pang, Y. Z.; Liu, Y. P.; Li, X. J.; Wang, K. S.; Yuan, H. R. Energy Fuels 2008, 22 (4), 2761–2766. (21) He, Y. F.; Pang, Y. Z.; Liu, Y. P.; Li, X. J.; Wang, K. S. Energy Fuels 2008, 22, 2775–2781.

Energy & Fuels, Vol. 23, 2009 2221 into four groups accordingly. For each group, 26 beakers (capacity of 1 L) were prepared. First, 100 g of dry rice straw was placed into each beaker. The required amount of NaOH, depending on the dose used, was then added into each beaker, followed by the addition of 80 g of distilled water to bring the moisture content to 80% (dry basis). The amount of NaOH and the moisture content were determined by previous tests that have been conducted in our laboratory.19 Finally, all the prepared beakers were covered with plastic films, closed with plastic rings, and placed in the laboratory at ambient temperature (20 ( 2 °C) for 3 weeks. At the end of the chemical pretreatment, the NaOH-pretreated rice straw was dried in an oven (HDG-9240A, Jinghong Co., Shanghai, PRC) at 60 °C for 48 h and then kept in a refrigerator for chemical analyses and AD experiments. 2.3. AD Experiment. The untreated (raw) and NaOH-treated rice straws were digested in batch anaerobic digesters. The volume of each digester was 2 L, with a working volume of 1.5 L. The required amount of untreated and 4%, 6%, 8%, and 10% NaOHtreated rice straws was placed into each digester, making four loading rates (LR) of 35, 50, 65, and 80 g/L, respectively. The LR was defined as the dry weight of rice straw loaded per liter of effective volume of digester (g TS/L). The digestion experiment for each loading rate was duplicated. Each digester was seeded to maintain the activated sludge MLSS in the digester at 15 g/L, which was based on the research results from Zhang.18 The required amount of ammonia chloride (NH4Cl) was added to each digester to adjust the carbon-to-nitrogen ratio (C/N) to 25, which is believed to be optimal for anaerobic bacteria growth.22 The prepared digesters were then placed in the shakers for AD tests at mesophilic temperature (35 °C) and a shaking speed of 120 rpm. 2.4. Sampling and Analytical Methods. The daily biogas production for each anaerobic digester was recorded using the water displacement method, and the corresponding cumulative biogas volume was calculated. The untreated and NaOH-treated rice straw samples were analyzed for TS and VS, according to the APHA standard methods.23 The total carbon (TC) and total nitrogen (TN) were analyzed with the TC analyzer (Skalar Primacsslc, The Netherlands) and the total Kjeldah nitrogen analyzer (Model KDN2C, Shanghai, PRC). The content of cellulose, hemicellulose, and lignin was analyzed according to the method of Van Soest.24 The extractives in cold-water, hot-water, and benzene-ethanol solvents were determined according to the standard methods.25 Fourier transform infrared (FTIR) spectra were obtained on an FTIR spectrophotometer (Nicolet Model 5DXC) using a KBr disk that contained 1% finely ground samples. A total of 30 scans were taken with a resolution of 4 cm-1. The analysis of soluble sugars in hotwater extractives of rice straw was performed on a Waters highperformance liquid chromatography (HPLC) system that was equipped with a refractive index detector, an autosampler, and a Sugar-Pak-1 column (6.5 mm i.d. × 300 mm). Elution was conducted at a flow rate of 0.6 mL/min at 70 °C. The mobile phase was water.

3. Results and Discussion 3.1. Biogas Production. The untreated and NaOH-treated rice straws were anaerobically digested, and the daily biogas production for each run was recorded. The biogas yield, which was defined as the biogas production per gram of VS loaded (B/VS), was calculated and used to evaluate the effectiveness of the NaOH pretreatment. The results in Figure 1 showed that the 4%, 6%, 8%, and 10% NaOH-treated rice straws achieved, respectively, 3.2%-28.6%, 27.3%-64.5%, 30.6%-57.1%, and (22) Yen, H. W.; Brune, D. E. Bioresour. Technol. 2007, 98, 130–134. (23) American Public Health Association (APHA) Standard Methods for the Examination of Water and Wastewater, 20th Edition; American Public Health Association (APHA): Washington, DC, 1998. (24) Van Soest, P. J.; Robertson, J. B.; Lewis, B. A. J. Dairy Sci. 1991, 74, 3583–3597. (25) Cui, Y. Z.; Fang, G. Z.; Jin, Z. L. J. Northeast For. UniV. 1997, 25 (1), 38–40 (in Chin.).

2222 Energy & Fuels, Vol. 23, 2009

He et al. Table 1. Changes of Main Compositions of Rice Straw after NaOH Pretreatment at Various Doses Value

Figure 1. Comparison on the biogas yields of the untreated and NaOHtreated rice straws.

15.2%-58.1% more biogas yield than that of the untreated one at the corresponding loading rates. The highest biogas yields of 0.36, 0.39, 0.52, 0.47, and 0.50 L/(g of VS) were obtained for the untreated, 4%, 6%, 8%, and 10% NaOH-treated rice straws, respectively, at a loading rate of 50 g/L. This result showed that NaOH pretreatment could obviously enhance the biogas production of rice straw. The maximum possible yield (MPY) was also analyzed. According to the calculation method proposed by Bushwell,26 the MPY of rice straw is 0.70 L/(g of VS). The highest biogas yields of the untreated, 4%, 6%, 8%, and 10% NaOH-treated rice straws reached, respectively, 51.4%, 55.7%, 74.3%, 67.1%, and 71.4% of the MPY of rice straw. It was evident that the biogas yields of NaOH-treated rice straws approached values more similar to the MPY value than that of the untreated one, which suggests that NaOH pretreatment is effective in enhancing the biogas production of rice straw. 3.2. Changes of Main Compositions. Lignin, cellulose, and hemicellulose (LCH) are the main compositions of rice straw, which provide the main carbon source for anaerobic microorganisms. The chemical compositions and structures, as well as physical characteristics of rice straw, were changed during alkali pretreatment, because of alkali chemical reactions. Some of those changes have been investigated and discussed in another paper.21 In this part, the main compositions were analyzed to investigate their changes during NaOH pretreatment and the effects of the changes on biodegradability and biogas production (see Table 1). After different doses of NaOH pretreatment, the total LCH, cellulose, hemicellulose, and lignin amounts were all decreased as the doses of NaOH increased. The total LCH, cellulose, hemicellulose, and lignin contents of the 10% NaOH-treated rice straw were lowest, which was reduced by 32.8%, 12.9%, 53.5%, and 43.2%, as compared to that of the untreated rice straw. The results implied that the greater the NaOH dose, the stronger the reaction of lignocellulose with NaOH. However, an overdose of NaOH applied did not help to increase biogas production, as indicated by the biogas yield with 10% NaOH doses in Figure 1. It was also found that the decomposition rates were different for different main compositions. The respective decomposition rates of hemicellulose, cellulose, and lignin for 4%, 6%, 8%, and 10% NaOH-treated rice straws were 35.2%-54.2%, 14.2%-16.4%, and 8.0%-44.5%. More hemicellulose was decomposed than lignin and cellulose. This might be due to the chemical reaction of more hemicellulose with NaOH. The decomposition of hemicellulose and lignin also helped to enhance the availability of cellulose. According to (26) Symons, G. E.; Bushwell, A. M. J. Am. Chem. Soc. 1933, 55, 2028– 2039.

0%

4%

6%

8%

10%

content (%) mass (g)a content change (%) mass change (%)

7.4 7.4

Lignin 7.2 6.8 -2.7 -8.1

5.7 5.3 -23.0 -28.4

4.9 4.7 -33.8 -36.5

4.2 4.1 -43.2 -44.6

content (%) mass (g) content change (%) mass change (%)

33.4 33.4

Cellulose 30.3 28.5 -9.3 -14.7

29.9 28.0 -10.5 -19.3

29.7 28.6 -11.1 -14.4

29.1 28.7 -12.9 -16.4

content (%) mass (g) content change (%) mass change (%)

28.2 28.2

Hemicellulose 19.5 19.1 18.3 17.9 -30.9 -32.3 -35.1 -36.5

13.7 13.2 -51.4 -53.2

13.1 12.9 -53.5 -54.3

content (%) mass (g) content change (%) mass change (%)

69.0 69.0

LCH 57.4 53.6 -16.8 -22.3

54.7 51.2 -20.7 -25.8

48.3 46.5 -30.0 -32.6

46.4 45.7 -32.8 -33.8

content (%) mass (g) content change (%) mass change (%)

12.8 12.8

Ash 18.2 17.1 +42.2 +33.6

20.8 19.5 +62.5 +52.3

24.7 23.8 +93.0 +85.9

27.2 26.8 +112.5 +109.4

a

Note: Based on an initial mass of 100 g.

Mansfield et al.,27 lignocellulosic materials in the original form are relatively resistant to microorganisms attack, but the removal of hemicellulose and lignin leads to extensive changes in the structure and accessibility of cellulose, thus making cellulose become more accessible to microorganisms. During the process of AD, carbohydrates are converted to biogas by anaerobic microorganisms, resulting in the reduction of main composition mass in digesters. The more biogas is produced, the greater the mass is reduced. Therefore, mass reduction could be used to reflect the availability of substrates and improvement of biodegradability. A mass balance was calculated for the untreated and 6% NaOH-treated rice straws. It was found that 88.0%, 75.0%, and 72.3% of cellulose, hemicellulose, and total lignocellulose, respectively, were reduced and converted to biogas for the 6% NaOHtreated rice straw, whereas 61.1%, 64.5%, and 55.6% of cellulose, hemicellulose, and total lignocellulose, respectively, were reduced and converted to biogas for the untreated one. It was apparent that more substrate was used with the NaOHtreated rice straw, which implies improved biodegradability and availability. 3.3. Changes of Extractives. 3.3.1. Changes of ExtractiVe Contents. Extractives are the materials extracted by water and some organic solvents such as ethanol, acetone, dichloromethane, chloroform, or a mixture of ethanol/benzene. Extractives usually consist of lipids, phenolic compounds, terpenoids, fatty acids, resin acids, steryl esters, sterol, waxes, etc. They also play an important role in enhancing the biogas production (27) Mansfield, S. D.; Mooney, C.; Saddler, J. N. Biotechnol. Prog. 1999, 15, 804–816. (28) Dorado, J.; Claassen, F. W.; van Beek, T. A.; Lenon, G.; Wijnberg, J. B.; Alvarez, R. S. J. Biotechnol. 2000, 80, 231–240. (29) Fernandez, M. P.; Watson, P. A.; Breuil, C. J. Chromatogr., A 2001, 922, 225–233. (30) Kallioinen, A.; Vaari, A.; Ratto, M.; Konn, J.; Siika-aho, M.; Viikari, L. J. Biotechnol. 2003, 103, 67–76.

Rice Straw Pretreated with NaOH To Produce Biogas

Energy & Fuels, Vol. 23, 2009 2223

Table 2. Changes of Different Extractives of the Untreated and NaOH-Treated Rice Straws for Various Doses Value 0%

6%

8%

10%

content (%)a mass (g)

Cold-Water Extractive 15.5 28.0 35.0 15.5 26.3 32.8

4%

36.7 35.4

42.5 41.9

content (%) mass (g)

17.8 17.8

Hot-Water Extractive 32.1 39.5 30.2 37.0

41.2 39.7

44.9 44.3

content (%) mass (g)

Benzene-Ethanol Extractive 10.4 7.2 5.2 10.4 6.8 4.9

5.1 4.9

5.0 4.9

a Note: Content was defined as the percentage of dry matter of extractives extracted from rice straw, on the basis of the dry matter of rice straw.

of lignocellulosic materials.28-30 Table 2 shows the cold-water, hot-water, and benzene-ethanol extractives from the untreated and NaOH-treated rice straws. It was found that, for the NaOH-treated rice straws, the contents of cold-water and hot-water extractives were all increased as the amount of NaOH used increased. Compared to the untreated rice straw, the 4%, 6%, 8%, and 10% NaOHtreated rice straws achieved, respectively, 80.3%, 125.1%, 135.9%, and 173.6% more cold-water extractives, and, respectively, 80.4%, 122.4%, 131.8%, and 152.8% more hot-water extractives. The cold-water and hot-water extractives were the substances generated mainly from the decomposition of hemicellulose and partially from the decomposition of lignin and cellulose, as indicated by the reductions of hemicellulose, lignin, and cellulose during NaOH pretreatment (see Table 1). They mainly consist of sugar, starch, pectin, tanning, cyclitol, and some inorganics.31 Generally speaking, the extractives have simpler chemical structures and smaller molecular weights and are more readily biodegradable. The increased amount of the cold-water and hot-water extractives would be generally favorable for the enhancement of biogas production. It was also determined that the content of benzene-ethanol extractives decreased as the amount of NaOH used increased. The 4%, 6%, 8%, and 10% NaOH-treated rice straws achieved, respectively, 30.9%, 50.9%, 51.5%, and 51.8% less benzeneethanol extractives than the untreated rice straw. The benzeneethanol extractives mainly contain resins, waxes, fattiness, tannins, and pigments, which are normally not readily biodegradable by anaerobic microorganisms.32 The decrease of benzene-ethanol extractive contents were conducive to the enhancement of biodegradability. The findings indicate that NaOH pretreatment could decompose rice straw to some biodegradable substances and, meanwhile, decrease some irresistible substances; both changes are favorable for improving biodegradability and biogas production. 3.3.2. Changes of Chemical Structures. Besides the changes of extractive contents, the chemical structures of extractives were also changed. Direct structure information and changes during various chemical treatments can be obtained through FTIR spectroscopy.33,34 Because the 6% NaOH-treated rice straw achieved better biogas production than other doses, the following research used 6% NaOH-treated rice straw as samples to explore the structure changes of the extractives. (31) Yang, S. H. Plant Fiber Chemistry; Chinese Light Industry Press: Beijing, China, 2001; pp 112-117. (32) Zhou, Q.; Lu, X. X.; Huang, L. H. China Wood Ind. 1999, 13 (1), 23–25 (in Chin.). (33) Alemdar, A.; Sain, M. Bioresour. Technol. 2008, 99, 1664–1671. (34) Liu, C. F.; Xu, F.; Sun, J. X.; Ren, J. L.; Curling, S.; Sun, R. C.; Fowler, P.; Baird, M. S. Carbohydr. Res. 2006, 341, 2677–2687.

Figure 2. FTIR spectra of hot-water extractives from the untreated rice straw (spectrum a) and NaOH-treated rice straw (spectrum b).

Figure 2 shows the FTIR spectra of the hot-water extractives from the untreated rice straw (spectrum a) and from the 6% NaOH-treated rice straw (spectrum b). The results showed that the two spectra had similar profiles but different intensities of the absorption bands. The differences indicated that the structures of hot-water extractives were changed after NaOH treatment. The changes could be classified into three types: (1) Decrease of functional group contents. The prominent peak at 3415 cm-1 is attributed to the OH stretching vibration in sterols, monoglycerides and diglycerides, or coextracted polysaccharides, or is due to water in the samples.35 The decreased content of such a band was due to the reaction of NaOH with rice straw. (2) Increase of functional group contents. The band at 2933 cm-1 corresponds to methylene and methyl stretching frequencies.36 The bands at 1634 and 1411 cm-1 are assigned to aromatic skeletal vibrations. The band at 1326 cm-1 can be ascribed to CH2-wagging vibrations in cellulose and hemicellulose.34 The contents of the functional groups previously mentioned were all increased after NaOH treatment, because of the decomposition of cellulose and hemicellulose during NaOH treatment. (3) Appearance of bands. Some bands, such as the band at 1120 cm-1, appeared after NaOH pretreatment. This band represents CO stretching for the syringyl type of lignin. Its appearance implied that the macromolecular lignin of rice straw was degraded to some small molecular substances that were soluble in water after NaOH pretreatment. This could explain the reason why the amount of hot-water extractives was increased after NaOH pretreatment, as discussed previously. The FTIR spectra of the benzene-ethanol extractives from the untreated and 6% NaOH-treated rice straws are illustrated in Figure 3. As can be seen, the spectral profiles of two bands were rather similar, but the relative intensities of the bands were different. The differences indicated that the structures of benzene-ethanol extractives were changed after NaOH treatment. The changes could be also classified into three types: (1) Disappearance of bands. The band at 1718 cm-1 is assigned to the carbonyl bonds in free fatty and resin acids. The CdC cis stretching (in unsaturated fatty acids and their esters or in sterols and steryl esters) gives an absorption band at 1653 cm-1. The band at 1461 cm-1 is due to the methylene bending vibration.37 The disappearance of such bands indicated (35) Che-Man, Y. B.; Setiowaty, G. Food Chem. 1999, 66, 109–114. (36) Gastaldi, G.; Capretti, G.; Focher, B.; Cosentino, C. Ind. Crops Prod. 1998, 8, 205–218. (37) Sun, R. S.; Sun, X. F. Ind. Crops Prod. 2001, 14, 51–64.

2224 Energy & Fuels, Vol. 23, 2009

He et al. Table 3. Soluble Sugar Contents in the Hot-Water Extractives of the Untreated and 6% NaOH-Treated Rice Straws Content (% of Dry Matter) sugar total sugars glucose a fructose a tetrasaccharide trisaccharide disaccharide polysaccharide

Figure 3. FTIR spectra of benzene-ethanol extractives from the untreated rice straw (spectrum a) and NaOH-treated rice straw (spectrum b).

that some esters and free fatty acids were removed from rice straw, because of the action of NaOH. (2) Decrease of bands. The prominent peak at 3415 cm-1 is attributed to the -OH stretching vibration in sterols, monoglycerides and diglycerides, or coextracted polysaccharides, or is due to water in the samples. Two strong bands at 2920 and 2851 cm-1 correspond to methylene and methyl stretching frequencies, respectively. A broad carbon single-bonded oxygen (C-O) stretching vibration occurs at 1267 cm-1. This band could be an interaction between carbon single-bonded oxygen stretching and in-plane carbon single-bonded hydroxyl bending in carboxylic acids.38 A prominent band at 1055 cm-1 is attributed to the symmetrical stretching of an ester bond (C-O-C) in coextracted polysaccharides. The contents of the functional groups previously mentioned were all decreased after NaOH treatment. (3) Appearance of bands. The occurrence of two small bands at 1591 and 1417 cm-1 after NaOH pretreatment is assigned to aromatic skeletal stretching vibrations.39 Their appearance was due to the degradation of some aromatic compounds in rice straw during NaOH treatment and their dissolution in benzeneethanol solution. The changes of chemical structures of benzeneethanol extractives were also responsible for the changes of extractive contents and had effects on biogas production. 3.3.4. Changes of Sugar in Hot-Water ExtractiVes. The substances in the hot-water extractives of the untreated and NaOH-treated rice straws are very complicated. Very little information is available on the substances in the hot-water extractives of NaOH-treated rice straw. The investigation on the identification of the substances has been conducted for a rather long time in our study. According to the research by Chen et al.,40 the predominant substances in the extractives of corn stover are sugars. In addition, sugars are also important substrates for anaerobic microorganisms. Therefore, sugars were first analyzed in this study. Only soluble sugars have been identified so far, and the results are presented in Table 3. It was observed that soluble sugar compositions and contents were different between the untreated and NaOH-treated rice straws. The total soluble sugar content in the NaOH-treated rice straw (38) Ajuong, E. M. A.; Breese, M. C. Holz Roh-Werkst. 1998, 56, 139– 142. (39) Xu, Z.; Wang, Q. H.; Jiang, Z. H.; Yang, X. X.; Ji, Y. J. Biomass Bioenergy 2006, 31, 162–167. (40) Chen, S.; Mowery, R. A.; Scarlata, C. J.; Chambliss, C. K. J. Agric. Food Chem. 2007, 55, 5912–5918.

b

untreated rice straw

NaOH-treated rice straw

6.1 1.3 0.4

13.2 0.42

0.9 3.5

0.12 0.5 12.1

a The results of Chen et al.40 showed that glucose and fructose were the predominant monosacharides components of water extracts, and the content of xylose, arabinose, and some other monosacharides was relatively low. Thus, only the content of glucose and fructose was analyzed when determining the content of monsaccharides. b Here, the polysaccharide refers to the pentasaccharides and the sugars with the molecular weight under 10000.

accounted for 13.2% of the dry matter, which is more than twice that in the untreated rice straw (6.1%). Soluble sugars are more readily biodegradable than lignocellulose. During the process of AD, lignocellulosic feedstock can be used by anaerobic microorganisms only when it has been degraded to small molecular substances by hydrolysis, so hydrolysis is the ratelimiting step in anaerobic biodegradation of rice straw.41 If more lignocellulosic feedstock is decomposed to soluble sugars through pretreatment before AD, the following hydrolysis rate would be fastened and the AD efficiency would be improved. 4. Conclusions Compared to the untreated rice straw, 3.2%-58.1% more biogas yields were obtained with 4%-10% NaOH-treated rice straws at the four loading rates, proving the effectiveness of NaOH pretreatment in enhancing the biogas production of rice straw. The changes of main compositions and extractives made important contributions to the enhancement. During NaOH pretreatment, hemicellulose, cellulose, and lignin were decomposed by 35.2%-54.2%, 14.2%-16.4%, and 8.0%-44.5%, respectively, for 4%, 6%, 8%, and 10% NaOH-treated rice straws. Considerable fractions of them were converted to relatively readily biodegradable substances, as indicated by increases of cold-water and hot-water extractives. Some irresistible substances were removed, as represented by decrease of benzene-ethanol extractives. The chemical structures of hotwater and benzene-ethanol extractives were also changed obviously, which were responsible for the changes of extractive contents. The substances in the extractives were very complicated; one of the findings was that the soluble sugar contents in the NaOH-treated rice straw were twice that of the untreated one. Acknowledgment. The authors are grateful for the financial support of this research from Hi-tech Research and Development Program of China (No. 2006AA10Z425), the Eleventh FiveYears Science and Technology Supporting Plan (No. 2008BADC4B13), and the Special Funding for Agricultural Technology Transfer from the Ministry of Science and Technology of China (No. 2008GB23600458). EF8007486 (41) Wang, Z. J.; Wang, W. EnViron. Sci. 2005, 26 (1), 68–71.