Article pubs.acs.org/IECR
Alkali Pretreatment for Improvement of Biogas and Ethanol Production from Different Waste Parts of Pine Tree Peyman Salehian† and Keikhosro Karimi†,‡,* †
Department of Chemical Engineering and ‡Institute of Biotechnology and Bioengineering, Isfahan University of Technology, Isfahan 84156-83111, Iran S Supporting Information *
ABSTRACT: Improvement of biogas, enzymatic hydrolysis, and ethanol production from different pine tree wastes, that is, needle leaves, branches, cones, and bark, were investigated using an alkali pretreatment. The pretreatment was performed with 8.0% w/w NaOH solution either at 0 °C for 60 min or 100 °C for 10 min. Different parts of the tree had dissimilar composition and properties, and effectiveness of the pretreatments was significantly different for each part. Among the untreated parts, the highest amount of biomethane production was obtained from needle leaves, which was 213 mL/(g·VS). The high temperature pretreatment was more effective on enzymatic hydrolysis, especially on needle leaves in which the yield was improved from 25.5 to 85%. Similar to enzymatic hydrolysis, ethanol yields from the wastes were improved more by the pretreatment at high temperature. Structural analyses indicated that crystallinity reduction and lignin removal were the main reasons for the observed improvements.
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INTRODUCTION Nowadays, worldwide concerns about greenhouse gas emissions and inevitable fossil fuels depletion encourage scientists and companies to develop renewable fuels such as biogas and bioethanol as alternatives.1 Today, ethanol and biogas are mainly produced in the industrial scale from easily degradable renewable biomass resources that result in food versus fuel conflict in many cases.2−4 On the other hand, the most plentiful renewable resources on the Earth are lignocellulosic materials, which are composed of high amounts of carbohydrates including cellulose and hemicelluloses.5 These inexpensive materials can be either anaerobically digested to biogas or hydrolyzed and then fermented to ethanol. However, they have a recalcitrant structure that is highly resistant to bacterial and enzymatic hydrolyses.6,7 Thus, a preliminary key stage, referred to as the pretreatment process, is added to the biogas and ethanol production processes to solve this problem. The pretreatment is aimed at opening up the compact lignocellulosic structure, reducing the cellulose crystallinity, and removing the physical barriers, that is, lignin and hemicellulose, for the microbial and enzyme accesses to cellulose surfaces.1,5,8,9 Several pretreatment processes including physical, physicochemical, chemical, and biological methods have been investigated to achieve these structural changes.1 Among the different pretreatment methods, chemical pretreatments, for example, alkali treatments, are the most effective processes.8 Alkali pretreatment with sodium hydroxide is one of the most effective and promising processes which are applied for the improvement of ethanol production and enzymatic hydrolysis of agricultural wastes and hardwoods.12 The pretreatment is classified into “high concentration” and “low concentration” NaOH processes. Low-NaOH concentration processes are typically performed at high temperature and pressure with low NaOH concentration (i.e., 0.5−4%)14 © 2012 American Chemical Society
without NaOH separation and reuse. Reactive destruction of lignocellulosic structure and disintegration of lignin and hemicellulose are the main mechanisms for the improvement of biodegradability. Alternatively, high-concentration NaOH pretreatments (6−20%) are typically performed at ambient pressure and low temperatures (e.g., at 0 °C).14 Dissolution and regeneration of cellulose, modification of lignin and hemicellulose structures, and increasing the channel sizes are the major mechanisms for the improvements. Since it is possible to reuse and recycle the NaOH solution, the high-concentration NaOH pretreatment processes are economically and environmentally preferable.12,14Recently, Mirahmadi et al.12 showed that the pretreatment with 7% NaOH for 2 h is promising for improvement of bioethanol production from hardwood birch, and Nieves et al.14 showed the potential of treatment with 8% NaOH for 60 min in the improvement of biogas production from oil palm empty fruit bunch which is a nonwooden lignocellulosic material. The NaOH pretreatment can break down the ester bonds cross-linking between hemicellulose, cellulose, and lignin10 which lead to modification or removal of lignin, thereby increasing the porosity of the biomass.11 However, the treatment process is very complicated involving several reactive and nonreactive phenomena. The process is not effective on softwoods at moderate conditions, whereas at severe conditions, it can result in formation of alkalistable end-groups (peeling-off reactions) and decomposition of dissolved polysaccharides, and negatively affect the bioconversions yield.12,13 Thus, it is important to optimize the pretreatment conditions, which highly depend on substrate properties.12 This Received: Revised: Accepted: Published: 972
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mL of the inoculum to which a certain amount of substrate was added to keep a VS (volatile solid) ratio as two parts VS from the inoculum and one part from the substrate. It was followed by the addition of deionized water to a final volume of 25 mL. In addition, biogas production from inoculum and deionized water was conducted as a blank. Anaerobic conditions were provided by purging the digesters with a gas containing 80% N2 and 20% CO2 for about 2 min. The digesters were manually shaken three times per day. Gas samples were taken every three days during the first 15 days and every five days until 45 days and analyzed for methane and carbon dioxide evolutions using gas chromatography. All these digesting experiments were run in triplicate. Enzymatic Hydrolysis. Two commercial enzymes were used for enzymatic hydrolysis: cellulase (Celluclast 1.5L, Novozyme, Denmark) and β-glucosidase (Novozym 188, Novozyme, Denmark). The activity of the cellulase was 70 FPU/mL (measured according to a standard method25), and the β-glucosidase activity was 230 IU/mL measured using the method presented by Ximenes et al.26 The hydrolysis was performed in 118 mL glass bottles at 45 °C and 100 rpm for 96 h. An amount of 0.3 g of substrate was immersed to 30 mL of 50 mM sodium citrate buffer (pH 4.8) and then autoclaved at 121 °C for 20 min. After cooling to room temperature, 20 FPU cellulase and 30 IU β-glucosidase per gram of substrate were added under sterile conditions. Afterward, 0.5 g/L NaN3 was added to each bottle as an antibacterial agent. Liquid samples were taken after every 24 h for monitoring the hydrolysis progress. All hydrolysis experiments were performed in duplicate. Simultaneous Saccharification and Fermentation. Treated and untreated parts of the pine tree were subjected to simultaneous saccharification and fermentation at 38 °C and 80 rpm for 96 h in 118 mL glass bottles with 30 mL working volume under anaerobic conditions. The fermenting microorganism which was used in the fermentation experiments was a flocculation strain of Saccharomyces cerevisiae (CCUG 53310, Culture Collection, University of Gothenburg, Sweden), isolated from an ethanol plant (Domsjö Fabriker AB, Ö rnsköldsvik, Sweden). The maintenance of microorganism was carried out according to Shafiei et al.27 Media containing the following (g/L) were used in the fermentation: yeast extract, 5; (NH4)2SO4, 7.5; K2HPO4, 3.5; MgSO4·7H2O, 0.75; CaCl2·2H2O, 1; pretreated or untreated samples, 50; and 0.05 M buffer citrate. The media pH was adjusted to 5 ± 0.1 by 1 M NaOH. They were then autoclaved at 121 °C for 20 min. After cooling to room temperature, 1 g/L microorganism and the hydrolytic enzymes (20 FPU cellulase and 30 IU β-glucosidase per each gram of substrate) were added to each bottle. Anaerobic conditions were provided by purging the bottles with pure nitrogen for about 2 min. Liquid samples were periodically withdrawn and stored in a freezer (at −18 °C) before analysis with HPLC. All fermentation experiments were run in duplicate. Analytical Methods. Total solids, volatile solids, and ash contents of the samples were determined according to the method presented by Sluiter et al.,28 which involves drying at 105 °C followed by heating at 575 °C to a constant weight. The treated and untreated samples were analyzed for carbohydrates and lignin contents according to NREL/TP510-42618 method.29 Methane and carbon dioxide produced in the anaerobic digestions were analyzed by a gas chromatograph (Sp-3420A,
optimization is performed for different pine tree wastes in this work. Pine trees are evergreen, coniferous, and fast-growing resinous trees which are widely grown in most temperate regions in relatively dense stands. Pines are among the most commercially important trees which are widely used in lots of industries. They generate acidic decaying needles that constitute a surface layer with open structure while conserving the soil moisture. Moreover, the needles are used as an agricultural commodity for mulching; however, the leaves together with barks comprise the major fraction of pine wood wastes when the tree is harvested.15 Similar to other wood wastes, bark, branches, leaves, and other parts of pine trees, are produced in the forest industries.16−18 Despite their potential for biofuel production, these residues are currently destined for landfills or burned to produce heat.19−21 Even though needle leaves, cones, bark, and branches are originated from the same tree, they have different compositions that may results in different manners in biofuel production and also in the pretreatments. According to their compositional analysis, the needles are similar to hardwoods and agricultural residue, while the wood is among the softwoods.22,13 To our knowledge, there is no report in the literature on biofuel production from different parts of pine tree wastes and their improvement by alkali pretreatments. Furthermore, effects of high concentrated NaOH pretreatment is not investigated for improvement of the tree wastes. Moreover, a comparison between effectiveness of the pretreatment on the enzymatic hydrolysis and ethanol and biogas productions is not presented in the literature. The potential of various waste parts of pine tree, that is, needle leaves, cones, bark, and branches, for biogas and bioethanol production was investigated. High-concentration NaOH pretreatment at high and low temperature was used to improve the biofuel productions. Effects of the pretreatment on the structure of the wastes were also studied.
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EXPERIMENTAL SECTION Raw Materials. Needle leaves, branches, cones, and bark were obtained from a pine tree cultivated in Isfahan University of Technology Forest (Isfahan, Iran). Different parts were milled and screened to achieve a size of less than 0.5 mm. Dry weight content of the samples was measured by a convection drying oven at 105 °C until constant weight. The materials were placed in resealable plastic bags and stored at room temperature until use. Pretreatment. An amount of 5 g sample was added to 95 g of NaOH solution (8% w/w) and incubated at 0 °C for 60 min or at 100 °C for 10 min. During the incubation period, the slurry was manually mixed every 10 min. The conditions were selected on the basis of the results of an optimization study.23 The mixtures were then centrifuged at 4500 rpm at room temperature for 10 min (10 000g) and washed with distilled water followed by vacuum filtration until pH 7 had been reached. The solids were kept at 4 °C until use. Biogas Production. The untreated and NaOH-treated samples were digested in 118 mL batch digesters which were closed with butyl rubber seals and aluminum caps. Biogas experiments were carried out at mesophilic conditions (37 °C) according to the method described by Hansen et al.24 The inoculum was obtained from a 6000 m3 anaerobic digester operating at mesophilic (37 °C) conditions (Isfahan Municipal Wastewater Treatment, Isfahan, Iran). Each setup contained 20 973
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Figure 1. FTIR spectra of NaOH treated needle leaves at conditions of 100 °C for 10 min (a) and of 0 °C for 60 min (b), compared to that of the untreated sample (c).
TCD detector, Beijing Beifen Ruili Analytical Instrument Co., China) equipped with a packed column (3 m length and 3 mm internal diameter, stainless steel, Porapak Q column, Chrompack, Germany). The carrier gas was helium operated with 20 mL/min flow rate. Temperature of the column, injector, and detector were 50, 90, and 140 °C, respectively. A pressure-tight syringe (VICI, Precision Sampling Inc., USA) with volume of 250 μL was used for gas sampling and injection, making it possible to take the gas samples at the actual pressure of the bioreactors. Avoiding the overpressure in the bottles, the excess gas was released through a needle after each gas sampling. The results of methane volumes are presented at standard conditions and pure methane was used as a standard gas. The glucose content in the enzymatic hydrolysis samples was measured by a glucose HK assay kit.30 Ethanol and sugar contents of fermentation and carbohydrate analysis samples were analyzed by high-performance liquid chromatography (HPLC) using RI detector (Jasco International Co., Tokyo, Japan). All sugars were analyzed on an ionexchange Aminex HPX-87P column (Bio-Rad, CA, USA) at 85 °C using deionized water as a mobile phase with a flow rate of 0.6 mL/min. Also, an Aminex HPX-87H column (Bio-Rad, CA, USA) at 60 °C with 0.6 mL/min eluent of 5 mM sulfuric acid was used for ethanol analysis. Fourier transform infrared (FTIR) spectrometer equipped with a universal ATR (attenuated total reflection) accessory and a DTGS detector (Tensor 27 FT-IR, Bruker, Germany) was used for structural analysis and to determine the crystallinity of the treated and untreated samples. The samples were dried at room temperature, and the spectra were obtained with an average of 60 scans from 600 to 4000 cm−1 with 2 cm−1 resolution.
Treatments, both at high and low temperatures, clearly affected the composition of the wastes. The treatments increased the glucan contents while reduced the lignin fractions, leading to more efficient enzymatic hydrolysis. Bark and leaves were more affected by the high temperature pretreatment, in which the glucan contents were increased to 40.5 and 39% while the amounts of lignin were decreased to 32.4 and 33.5%, respectively. On the other hand, lowtemperature treatment was more effective on cones and branches, in which the amount of glucan was increased to 39.5% and 29.7%, and the amount of lignin was reduced to 30.2 and 32.2%, respectively. Moreover, both treatments resulted in xylan degradation, especially at high temperature. The xylan percentage of needle leaves was decreased from 11.2 to 5.7%. Galactan content was also decreased except in the pretreatment of branches. Furthermore, mannan content was decreased in needle leaves and cones, while it was increased after the treatments of bark and branches compared to that of the untreated parts. The ratio of VS in TS increased in both high and low temperatures treated samples compared to the untreated ones. The greatest value of VS in TS was observed for needle leaves and cones in the low-temperature treatment and for branches and bark in the treatment at high temperature. Effects of Pretreatment on the Cellulose Crystallinity. The structural properties of the treated and untreated samples were analyzed by FTIR spectroscopy. Crystallinity index (CI) and total crystallinity index (TCI), which are the absorbance ratio of A1430/A896 and A1375/A2900, respectively,31 were calculated from the spectra. A sample of the spectra for the treated and untreated needle leaves are also demonstrated in Figure 1. As can be seen from Figure 1, the 898 cm−1 band which was denoted as cellulose II, was increased from 0.007 for the untreated sample to 0.020 and 0.012 for the high and low temperatures treated samples, respectively. The results indicated that the treatments, either at high or low temperatures, significantly decreased both CI and TCI. However, the treatment at the low temperature was more effective in crystallinity reductions for all parts than that at high temperature except for cones. Biogas Production. Methane yield for untreated and treated needle leaves, corn, branches, and bark of the pine tree are summarized in Figure 2, panels A, B, C, and D, respectively.
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RESULTS Effects of Pretreatment on Composition and Structure of the Pine Tree Wastes. The dominant carbohydrate in all wastes was glucan (20.7−26.7% of dry weight), and the lignin was mainly available in the form of acid insoluble with contribution of 34.2−38.7%. The needles contained higher xylan (11.2%), while higher mannan (14.7%) was detected in branches compared to the other parts. 974
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Figure 2. Yield of methane production from untreated and pretreated (A) needle leaves, (B) cones, (C) branches, and (D) bark after 8 days (light gray bars), 25 days (dark gray bar), and 45 days (black bars) anaerobic digestion.
improved the yield of hydrolysis, compared to the untreated samples. However, the pretreatment at the higher temperature resulted in higher yields. In the case of needle leaves, a drastic improvement on glucose yield was observed by the pretreatment. The glucose yield was only 27.2% after 96 h hydrolysis for the untreated leaves, while it was improved to 85.6 and 60.2% for the high and low temperature NaOH-treated samples, respectively. The hydrolysis of the cones was not as successful as the needle leaves. After four days hydrolysis, the glucose yield of the untreated cones was only 12.8%, and the treatments at the high and low temperatures enhanced the yields to 32.3 and 29.2%, respectively. The highest increase was observed in the case of hydrolysis of bark, where more than three times increase in the yield was obtained. The pretreatments showed the lowest effects on improvement of the yield of hydrolysis from the branches. Simultaneous Saccharification and Fermentation. The results of ethanol production from treated and untreated wastes (presented as theoretical ethanol yield) are reported in Table 1. The data indicated that the pretreatment at high temperature was more effective than low-temperature treatment. Theoretical ethanol yield from the untreated needle leaves was 26.4% which was increased to 81.3% by treatment at high temperature. However, the improvement was not that high for the branches and bark, in which the yield was increased from 24 and 12.6% for untreated branches and bark to 47.7 and 42.3% for the high temperature pretreated samples, respectively.
As shown in Figure 2, the pretreatments had no significant effects on the yields at the beginning of anaerobic digestion, while they showed their effects in the latter stages of digestion. The highest yield of methane was 213 mL/g·VS obtained from untreated needle leaves. In the case of needle leaves, the pretreatments showed negative effects. In contrast, the treatments increased the amount of methane production from cones, branches, and bark. Furthermore, the low-temperature treatment was more effective on the cones and branches, where the yields were increased from 25 and 36 mL/g·VS for untreated samples to 75 and 98 mL/g·VS, respectively. On the other hand, in the case of bark, methane production increased from 33 for the untreated sample to 107 and 74 mL/g·VS for the high and low-temperature treatments, respectively. Enzymatic Hydrolysis. Cellulose to glucose conversion yields were calculated according to eq 1 and presented in Figure 3. glucose yield (%) =
produced glucose (g/L) × 100 biomass (g/L) × 1.111 × F (1)
where “produced glucose” is concentration of glucose in the hydrolysate, “biomass” is concentration of the wastes (either treated or untreated) in the hydrolysis medium, F is cellulose fraction in the biomass, and 1.111 is a factor for hydration of cellulose to glucose. As shown in Figure 3, glucose yield after 96 h hydrolysis was less than 15.1% for bark and cones and less than 33.1% for branches and needle leaves. All pretreatments significantly 975
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Figure 3. Effect of NaOH pretreatment performed at high and low temperatures on enzymatic hydrolysis of (A) needle leaves, (B) cones, (C) branches, and (D) bark of pine tree. The symbols represent (○) untreated and pretreated samples at (□) 100 °C for 10 min and at (△) 0 °C for 60 min.
similar to softwoods (e.g., branches). On the other hand, the pretreatments have different effects on softwoods compared with hardwoods, in which generally the softwoods are more resistance to bioconversions and typically need more severe pretreatment conditions.12 Results of the current work showed that the pretreatments had a negative effect on biogas production yield from needle leaves, in spite of lignin removal and reduced cellulose crystallinity. One of the reasons can be degradation of hemicellulose (mainly xylan and mannan), since unlike cellulose, hemicellulose can be easily converted to biogas in anaerobic digestion; however, further investigation is necessary to find the other reasons. The theoretical yield of methane from carbohydrate is 415 mL/g·VS. The VS available in lignocelluloses comprises carbohydrates and lignin, in which carbohydrates can be fermented to biogas, but not lignin. In this work, the highest amount of methane production was 213 mL/g·VS (51.4% of theoretical value) obtained from the untreated leaves. On the other hand, the pretreatments significantly improved the yield of methane production from cones, branches, and bark of the tree. The best improvement for cones and branches was respectively 18 and 23.7% of theoretical methane yield, which was obtained after the low-temperature treatment (at 0 °C). On the other hand, the high-temperature treatment (at 100 °C) was the most effective treatment for improvement of methane production from bark, in which the methane yield was roughly 3.3 times more than that from the untreated bark. Therefore, different parts need different optimum conditions for efficient improvement of biogas production.
Table 1. Yield of Ethanol from Untreated and Pretreated Parts of Pine Tree material needle leaves
cones
branches
bark
pretreatment conditions untreated pretreated pretreated untreated pretreated pretreated untreated pretreated pretreated untreated pretreated pretreated
at 100 °C for 10 min at 0 °C for 60 min at 100 °C for 10 min at 0 °C for 60 min at 100 °C for 10 min at 0 °C for 60 min at 100 °C for 10 min at 0 °C for 60 min
ethanol yield (%)a 26.4 81.3 51.1 10.9 27.1 25.5 24.0 47.7 34.9 12.6 42.3 37.9
± ± ± ± ± ± ± ± ± ± ± ±
0.3 0.7 0.6 0.3 0.2 0.6 0.5 0.8 0.1 0.2 0.4 0.4
a
The yield was calculated as percentage of theoretical yield = [produced ethanol in the fermentation medium (g/L)]/[0.51 × 1.111× initial weight of biomass in the fermentation medium (g/L) × F] × 100, where F is cellulose fraction in biomass.
The lowest yield of ethanol production (only 10.9%) was obtained from cones, in which the best ethanol production yield (27.1%) was obtained by the high-temperature treatment.
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DISCUSSION Forest residues are mixtures of different parts of trees with different composition and properties. For instance, pine tree wastes contain a mixture of some parts that are more similar to hardwoods (e.g., needle leaves) while the other parts are more 976
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ACKNOWLEDGMENTS This work was financially supported by the Institute of Biotechnology and Bioengineering, Isfahan University of Technology.
Improvement of enzymatic hydrolysis of pine tree wastes was not similar to that of biogas production. The hydrolysis results of the treated and pretreated parts of the tree indicated that the treatment at high temperature (100 °C for 10 min) was more effective than that at low temperature (0 °C for 60 min). The highest improvement, approximately three times, was detected for the needle leaves. This is in agreement with other works findings which showed that the alkali pretreatments are more effective on improvement of hardwoods hydrolysis compared to that of softwoods.12 On the other hand, both pretreatments succeeded to improve the yield of ethanol production by SSF, whereas the high temperature pretreatment was more effective than the lowtemperature one. The best result obtained after the hightemperature treatment of the needle leaves (improved from 26.4 to 81.3%). A similar trend in the improvement was observed from enzymatic hydrolysis and ethanol production by SSF, indicating the hydrolysis step is the limiting stage of SSF. Decrease in crystallinity of cellulose, detected by FTIR analysis, could be one of the main effects of the pretreatments. The cellulose microfibrils contain both crystalline and amorphous regions. The alkali solution readily enters the amorphous regions existing at the interface regions between the crystallites, which results in swelling of the cellulose. Then, cellulose chain mobility is enhanced, and the crystalline parts of cellulose gradually diminish in sizes, and finally cellulose I is rearranged into cellulose II which has a less CI and TCI.32 Lignin and hemicellulose removal together with structural modifications can also be responsible for the improvements.
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CONCLUSIONS Alkali pretreatment can be used to improve methane and ethanol productions from different pine tree wastes, except for methane production from the needle leaves in which no pretreatment was necessary. However, the pretreatment was not equally successful for all parts, and also the improvement of different parts was not similar on biogas production compared to that on enzymatic hydrolysis and ethanol production. Hightemperature treatment was more efficient for improvement of methane production from cones and branches, while lowtemperature treatment was favored in biogas from bark. Similar to enzymatic hydrolysis, high-temperature treatment was more successful in improvement of different parts for ethanol production. ASSOCIATED CONTENT
S Supporting Information *
Table S1: Composition of untreated and NaOH pretreated pine tree wastes expressed as percentage of dry weight. Table S2: Total solids (TS) and volatile solid (VS) as well as their ratio to each other in the untreated and pretreated pine tree wastes. Table S3: Crystallinity of treated and untreated pine tree wastes obtained from FTIR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +98-311-3915623. Fax: +98-311-3912677. E-mail:
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The authors declare no competing financial interest. 977
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