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Hydration treatments increase the biodegradability of native wheat straw for hydrogen production by a microbial consortium
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Hydration treatments increase the biodegradability of native wheat straw for hydrogen production by a microbial consortium zquez a, Arturo Sa nchez b, Idania Valdez-Vazquez a,* Anibal R. Lara-Va Depto. de Ciencias Ambientales, DICIVA, Universidad de Guanajuato, Mexico Unidad de Ingenierı´a Avanzada, Centro de Investigacion y Estudios Avanzados (CINVESTAV), Av. del Bosque 1145, Zapopan 45019, Jalisco, Mexico
a
b
article info
abstract
Article history:
Enhancing substrate accessibility is one of the prerequisites for efficient pretreatment and
Received 1 December 2013
bioconversion of lignocellulosic biomass, for which reducing particle size and increasing
Received in revised form
porosity have been implemented. Biomass porosity, characterized as the water retention
30 August 2014
capacity (WRC), increases significantly with the use of a culture filtrate of an H2-producing
Accepted 26 September 2014
microbial consortium, causing native lignocellulosic fibers to hydrate and swell exten-
Available online xxx
sively. This study describes the effects of hydration treatment and particle size on the
Keywords:
microbial consortium. A culture filtrate and tap water were used as hydration media, and
Accessibility
particle sizes of 0.212 and 3.35 mm were tested. As a result, biodegradability, measured as
biodegradability of native wheat straw fibers with regard to direct H2 production by a
Particle size
H2 production, doubled when the lignocellulosic substrate was hydrated with the culture
Porosity
filtrate. Also, hydration had a significant impact on H2 yield, increasing it from 6.8 to
Swelling
86.5 mL H2/g total consumed sugars when the 0.212 mm fibers were first hydrated. H2
Water retention capacity
production was higher with coarser particles (3.35 > 0.212 mm), but hydrogen yield improved with finer particles (0.212 > 3.35 mm). Based on our results, particle size and hydration improve the biodegradability of native wheat straw for H2 production, although hydration with a culture filtrate increased the biomass porosity (measured as WRC), which influenced the biodegradability more than a reduction in particle size. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is an energy carrier that can help global energy requirements be met, due to its high combustion power (H2 >120 MJ/kg versus >45 MJ/kg for fossil fuels). Further,
hydrogen is considered to be a green fuel, because water is the sole product at the end of its theoretical combustion [1]. Hydrogen can be produced from biological processes, specifically, from the conversion of carbohydrates into hydrogen by fermentative microorganisms under anaerobic environments.
mica Juriquilla, Instituto de Ingenierı´a, Universidad Nacional Auto noma de * Corresponding author. Present address. Unidad Acade xico, Blvd. Juriquilla 3001, 76230 Quere taro, Me xico. Tel.: þ52 (442) 1926170; fax: þ52 (442) 1926185. Me E-mail address: [email protected] (I. Valdez-Vazquez). http://dx.doi.org/10.1016/j.ijhydene.2014.09.155 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
zquez AR, et al., Hydration treatments increase the biodegradability of native wheat Please cite this article in press as: Lara-Va straw for hydrogen production by a microbial consortium, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.09.155
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
First generation biofuels are obtained primarily from crops, such as sugarcane and corn, but one of their major drawbacks is that they are part of the human food chain. In contrast, agricultural byproducts can be used as second-generation biofuels, because they are abundant, cheap and a good source of carbohydrates, some of which are plentiful [2]. Annually, nearly 200 billion tons of agricultural byproducts are produced worldwide, energy equivalent to 60e80 billion tons of crude oil [3]. In Mexico, close to 150 million tons of agricultural byproducts are generated from 20 crops, including corn stover and cobs, sorghum straw, the tops and leaves of sugarcane, and wheat straw [4]. In particular, wheat straw is one of the main agricultural products in certain regions; thus, it can be collected with minimal effort and at low cost for use as feedstock in hydrogen production. Wheat straw is classified as lignocellulosic biomass. In its native form, this biomass is arranged 3-dimensionally as a network, in which the core comprises cellulose microfibrils: large linear chains of anhydroglucose residues that are oriented parallel to hydrogen bonds between successive and adjacent residues, yielding highly ordered and tightly packed crystalline regions that are largely impenetrable by water; and amorphous regions with free hydroxyl groups that interact with the aqueous environment. On the outside of the network, the lignin-hemicellulose matrix is highly branched, with many charged groups rendering it water-swollen [5,6]. The cellulose crystallinity, the association of the ligninhemicellulose matrix with the cellulose microfibrils, and the accessible surface area contribute to the low biodegradability of lignocellulosic biomass [7]. Thus, the hydrogen yield from direct fermentation of native wheat straw is low ranging from 1.0 to 11 mL H2 per g of total volatile solids (TVS) [8e11]. Most approaches improve the biodegradability of wheat straw by obtaining hydrolysates with diluted acids [8,11,12], hydrothermal pretreatment [13], and enzymatic hydrolysis [9], which raises the hydrogen yields to 20e68 mL H2 per g TVS. These pretreatment methods decrease the times that are required for biomass processing and fermentation. However, pretreatment byproducts are released, which strongly inhibit H2-producing dark fermentation [14]; thus, fermentation can only be performed with a low hydrolysate percentage or if the inhibitors are removed. These steps can increase the processing costs in a scaled-up plant, compromising the benefit of the pretreatments. Developing economical and simple pretreatment methods that enhance the biodegradability of lignocellulosic biomass remains a significant challenge in the production of lignocellulosic biofuels. All pretreatment methods aim to increase the accessible surface area [7,15]. Accessibility refers to the substrate that is available for hydrolysis or biodegradation and is governed by biomass porosity (interior surface area) and particle size (exterior surface area) [16]. For years, particle size reduction has been used as an efficient pretreatment method of increasing the exterior surface area to improve biomass biodegradability, the production of biogas and, recently, hydrogen [17e20]. Conversely, cell wall porosity limits the physical, chemical, and enzymatic access to the native lignocellulosic substrate, which impacts the biodegradability of substrate more than the external surface or particle size [21]. In this respect, the biomass porosity can be characterized
by water retention capacity (WRC), which reflects the interactions between the charged surface groups and adsorbed molecules in an aqueous environment [22,23]. Lignocellulosic biomass is considered a cross-linked gel that swells when water molecules are drawn into the pores by osmotic pressure and interact with ionizable groups on the biomass surface. As a result of swelling, the cell wall volume increases and accessibility improves, because the crystallinity is reduced when the bonds between the cross-linked gel are weakened [24]. The hydration medium has significant effects on the WRC: salt ions neutralize the charged groups on the cell wall surface, generally decreasing WRC values and swelling, whereas enzymatic treatment increases them [25,26]. A recent report demonstrated that the WRC and swelling of native wheat straw fibers rose significantly by hydration with a culture filtrate of an H2-producing microbial consortium [27]. Based on that result, the objective of this study was to determine the effects of hydration and particle size on the biodegradability of native wheat straw for direct H2 production by a microbial consortium.
Material and methods Substrate and hydration treatment Wheat straw (Triticum aestivum L.) was used as the substrate and prepared according to [27]; Table 1 shows its chemical composition. Particle size was reduced using a ball mill, and the sample was separated into various fractions using screens; particles that were retained in the 0.212 mm and 3.35 mm-sieves were used. The wheat straw was subjected to 2 separate hydration treatments: tap water for 10 h and a culture filtrate for 4 h. The substrate:medium ratio was 1:20, and the mixture was homogenized well and kept statically at room temperature (28 ± 5 C). At the end of the hydration, the soluble fraction of the native wheat straw was released, yielding a reducing sugar concentration of 1.1 ± 0.23 g/L with tap water and 1.9 ± 0.45 g/L with the culture filtrate. The culture filtrate was obtained from a 3-L New Brunswick fermentor under uncontrolled pH with an initial pH of 5.5 with native wheat straw as substrate (5% w/v), seeded with anaerobic digestate (20% v/v), and incubated for 5 days at 300 rpm and 37 C. The culture was passed through a gauze filter and used directly in the hydration experiments. The
Table 1 e Chemical composition of the native wheat straw. Component Total solids Total volatile solids Carbon Fiber content Protein Total Kjeldahl nitrogen Phosphorus Ash
Content (g/kg) 956 860 419 387 30.6 4.4 0.46 86
zquez AR, et al., Hydration treatments increase the biodegradability of native wheat Please cite this article in press as: Lara-Va straw for hydrogen production by a microbial consortium, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.09.155
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cellulase, endoglucanase, xylanase, and pectinase activities in the culture filtrate were previously reported [27].
500
Batch reactors for hydrogen production
400
Analytical methods Biogas volume was measured daily using lubricated syringes, and biogas content was analyzed using a gas chromatograph (Perkin Elmer Clarus 580) that was equipped with a thermal conductivity detector and an Elite GCeGS Molsieve capillary column. The injector and detector temperatures were 50 C and 200 C, respectively. The following temperature program was used for the column: initial temperature 150 C, increases of 7 C/min to a final temperature of 100 C. Argon was used as the carrier gas at 6 mL/min. At the end of the fermentation, volatile fatty acids (VFAs) content was analyzed after filtration (Pall Life Sciences, 0.45 mm GHP Membrane) using a gas chromatograph (Perkin Elmer Clarus 580) that was equipped with a flame ionization detector and an Elite-Q-Plot capillary column. The injector and detector temperatures were 230 C and 240 C, respectively. The temperature of the column was increased from 100 C to 235 C. A mixture of VFAs was used as a standard (SUPELCO, 46975-U). With regard to substrate consumption, the glucose and xylose contents in the controls and treatments were measured with a biochemistry analyzer (YSI select 2710, Lab Tech). For these measurements, the samples was dried, weighed, hydrolyzed with concentrated H2SO4 at 25 C for 3 h, and neutralized (modified from Ref. [28]).
Results and discussion Effect of hydration on H2 production Fig. 1 shows the effect of hydration and particle size on substrate biodegradability for direct H2 production by a microbial consortium. On average, hydration (with water or culture filtrate) had a significantly positive effect on H2 production and H2 production rates (p < 0.001 and p < 0.01, respectively). Maximum H2 production was achieved with the culture filtrate as hydration medium, followed by hydration with tap water and the control, independent of particle size. Also, H2 production rates rose significantly; for example, with the
300
200
Cumulative hydrogen production (mL/L)
Batch reactors for H2 production were performed in 100 mL serological bottles. Sludge from an anaerobic digester, previously heat-treated in a water bath for 30 min, was used as the inoculum (20% v/v). The hydrated wheat straw (0.212 or 3.35 mm) and hydration medium were loaded into the batch reactors, and the working volume was adjusted to 70 mL with mineral medium with a substrate content of 25 g TVS/L. The mineral medium composition was as follows (g/L): KH2PO4 4.8, K2HPO4,3H2O 6.98, MgCl2,6H2O 0.1, CaCl2 0.02, NH4Cl 6.0, MnSO4,6H2O 0.015, FeSO4,7H2O 0.025, CuSO4,5H2O 0.005, and CoCl2,5H2O 0.000125. The initial pH was adjusted to 5.5. Unhydrated wheat straw was used as the control treatment. The batch reactors were incubated statically for 20 days at 37 C. All experiments were performed in triplicate.
a
100
0 500 0
5
10
15
20
5
10 Time (d)
15
20
b 400
300
200
100
0 0
Fig. 1 e Effect of hydration and particle size on the biodegradability of native wheat straw for direct hydrogen production. Particle sizes of a) 0.212 mm, and b) 3.35 mm. Substrate hydration with tap water (-), an enzyme culture filtrate (C), and control (unhydrated substrate, A). Error bars indicate standard deviation.
0.212 mm wheat straw fibers, the hydrogen production rate climbed from 10 mL/L$d with the unhydrated substrate to 29 mL/L$d and 44 mL/L$d with substrate that was hydrated with culture filtrate and water, respectively (Fig. 1(a)). The adaptation phase was shortened in the waterhydrated treatments with respect to the control, possibly because small concentrations of soluble sugars that accumulated during the hydration supported rapid cell growth and synthesis of hydrolytic enzymes. In contrast, this phase was prolonged in the filtrate-hydrated treatments versus the control, due to repression or inhibition during the first several days of fermentation with culture filtrate-hydrated substrate. However, unlike the water-hydrated and control treatments, in which hydrogen production plateaued by day 8, the filtrate-
zquez AR, et al., Hydration treatments increase the biodegradability of native wheat Please cite this article in press as: Lara-Va straw for hydrogen production by a microbial consortium, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.09.155
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Table 2 e Glucose and xylose consumption and product yield ðYH2 =s Þ. Particle size (mm)
Hydration treatment
Sugar consumption (%) Glucose
0.212
3.35
Control Water Culture filtrate Control Water Culture filtrate
64.0 ± 52.7 ± 25.2 ± 51.1 ± 77.1 ± 45.2 ±
4.8 1.7 0.3 10.6 3.3 7.9
hydrated treatments continued producing hydrogen. Consequently, hydrogen production doubled when the substrate was first hydrated with culture filtrate. Lara et al. [27] demonstrated that the porosity of native wheat straw fibers (measured as WRC) rose using a culture filtrate of H2-producing microbial consortium with hydrolytic activity. In that study, hydration with water increased the volume of the wheat straw fibers due to swelling, achieving a WRC value of 5.2 g/g in 4 h. In addition to the interaction between the wheat straw cell wall surface with water molecules, the enzymatic activity in the culture filtrate elicited greater swelling and higher surface area, yielding a WRC value of 9.8 g/g in 4 h. Several types of hydrolytic activity were detected in the culture filtrate, such as cellulases, endoglucanases, xylanases, and pectinases, which acted together on the native lignocellulosic matrix. Thus, hydration with culture filtrate increased the substrate porosity through the activity of the aqueous environment and hydrolytic enzymes, which resulted in significant biodegradability and hydrogen production in this study. Other authors have examined the effects of swelling on the accessibility of cellulase enzymes to cellulose. For instance, Ju et al. [29] reported the effects of swelling on the enzymatic digestibility of fibers using a PFI refining mill process as pretreatment, entailing a combination of mechanical agitation and hydration. After 10,000 revolutions of refining, the enzymatic conversion of modified Kraft pulping increased by 31% due to fiber swelling. Also, Hu et al. [26] concluded that there xylanase and cellulase synergize to significantly improve cellulose accessibility by increasing fiber swelling and fiber porosity. In our study, we combined the benefits of hydration and swelling with mild enzymatic hydrolysis of native wheat straw fibers to produce hydrogen directly using a consolidated bioprocess approach.
Effect of particle size on H2 production As substrate, larger wheat straw fibers (3.35 mm) yielded more hydrogen than finer ones (0.212 mm); this pattern was evident with unhydrated fibers, for which the H2 production rate was 193 mL H2/L with the 3.35 mm fibers versus 66 mL H2/L with the 0.212 mm fibers (Fig. 1). These results seem contradict most studies, which have reported that substrate biodegradability rises with decreasing particle size [19,20,30,31,]. However, by confocal laser scanning microscopic analysis, the cell wall of finer fibers (2.00 mm) suffered from a shaving effect due to grinding, decreasing its porosity (measured as WRC), where coarse fibers of 3.35 mm had a higher WRC than fibers
H2 yield (mL/total consumed sugars)
Xylose 78.6 ± 34.2 ± 30.1 ± 62.0 ± 67.5 ± 68.2 ±
1.2 13.7 7.6 10.7 1.8 5.1
6.8 34.5 86.5 25.3 27.2 58.0
2.00 mm. Thus, high WRC values of coarse fibers are indicative of improved biodegradability and the potential for high hydrogen production.
Substrate consumption and H2 yields Regarding substrate consumption, glucose and xylose were consumed simultaneously (Table 2) as a result of high metabolic diversity in the microbial consortium, which had the capacity to consume 5-carbon and 6-carbon sugars for hydrogen production. The sum of consumed sugars was used to calculate the hydrogen yield, expressed in mL of H2 per g of total consumed sugars. Higher hydrogen yields tended to be obtained with hydrated substrate, independent of the particle size, primarily with the substrate that was hydrated with culture filtrate. This finding might be related to the energy consumption that is required to synthetize hydrolytic enzymes. In addition to increasing substrate porosity, hydration effects the release simple sugars that are already available for growth and enzyme synthesis (which was intensified with culture filtrate). In contrast, with unhydrated substrate, microorganisms must expend more energy (obtained from the substrate) to access the substrate. According to Zhang and Lynd [32], 20% of the ATP that is produced by a cell is allocated to cellulase synthesis in anaerobic fermentations, which can require more energy than cell growth. Thus, more of the energy that is obtained from the substrate in hydration treatments can be allocated to the growth of H2-producing microorganisms.
Accumulation of volatile fatty acids Anaerobic H2 production is accompanied by the accumulation of VFAs, which are produced by metabolic pathways, such as propionic acid fermentation, butyric acid fermentation, and ethanol fermentation. The route that is used is influenced by many factors, including pH, temperature, redox potential, and hydraulic retention time [33]. Fig. 2 shows the VFA profile at the end of fermentation for each treatment. The chief soluble metabolites that were produced were acetic, propionic, butyric, and formic acids. According to these results, acetic and butyric acids accounted for 40%e50% of the VFAs in the filtrate-hydrated treatment. In general, these metabolites are good indicators of H2 production. Also, a significant amount of propionic acid was generated at the end of fermentation, which theoretically does not contribute to H2 production [34,35]. Notably, in the batch reactors that were fed unhydrated substrate, we observed high accumulation of formic
zquez AR, et al., Hydration treatments increase the biodegradability of native wheat Please cite this article in press as: Lara-Va straw for hydrogen production by a microbial consortium, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.09.155
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0.212 mm fibers. The cell wall in finer fibers was damaged, affecting its biodegradability. The biodegradability of a native lignocellulosic substrate improves, primarily by increasing the porosity via hydration and swelling rather than decreasing particle size. This simple and economical treatment should be incorporated as a routine step in the production of lignocellulosic biofuels.
Acknowledgments Financial support was received from the CONACYT projects, Fondo Sectorial “CONACYT-SENER e Sustentabilidad Enertica” (grant no. 150001) and “Apoyo al Fortalecimiento y ge gica” Desarrollo de la Infraestructura Cientı´fica y Tecnolo (grant no. 188432).
references
Fig. 2 e Volatile fatty acid accumulation at the end of the direct fermentation of native wheat straw hydrated with water and an enzyme culture filtrate. Error bars indicate standard deviation.
acid (32%e50%). Formic acid is accumulated by cellulolytic microorganisms, such as Clostridium sp. and Enterobacter sp., wherein 1 mol H2 is produced per 1 mol formic acid that has accumulated [36,37]. Thus, formic acid production correlates with low H2 production.
Comparison with other studies Typically, hydrogen yield is expressed in mL H2 per g of TVSadded. Previous studies have reported an H2 yield of 1.0e11 mL H2/g TVSadded when unhydrated native wheat straw is used as substrate under mesophilic conditions [8e11]. In our study, when the substrate was first hydrated with an enzyme culture filtrate for 4 h, the H2 yield improved from 9 to 17 mL H2/g TVSadded. These results confirm the potential of hydration treatments to increase substrate accessibility and biodegradability when direct fermentation of native lignocellulosic substrate is desired by a consolidated bioprocess. This method can be extrapolated to many biological systems for the production of ethanol, enzymes, and other products.
Conclusions Prior hydration and swelling of native wheat straw fibers increased their porosity and significantly enhanced substrate biodegradability and direct hydrogen production by a microbial consortium. These results were intensified when a culture filtrate with hydrolytic enzymes was used to hydrate the substrate. Regarding biodegradability particle size, we found that hydrogen production was greater with 3.35 mm versus
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zquez AR, et al., Hydration treatments increase the biodegradability of native wheat Please cite this article in press as: Lara-Va straw for hydrogen production by a microbial consortium, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.09.155
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Hydration treatments increase the biodegradability of native wheat straw for hydrogen production by a microbial consortium zquez a, Arturo Sa nchez b, Idania Valdez-Vazquez a,* Anibal R. Lara-Va Depto. de Ciencias Ambientales, DICIVA, Universidad de Guanajuato, Mexico Unidad de Ingenierı´a Avanzada, Centro de Investigacion y Estudios Avanzados (CINVESTAV), Av. del Bosque 1145, Zapopan 45019, Jalisco, Mexico
a
b
article info
abstract
Article history:
Enhancing substrate accessibility is one of the prerequisites for efficient pretreatment and
Received 1 December 2013
bioconversion of lignocellulosic biomass, for which reducing particle size and increasing
Received in revised form
porosity have been implemented. Biomass porosity, characterized as the water retention
30 August 2014
capacity (WRC), increases significantly with the use of a culture filtrate of an H2-producing
Accepted 26 September 2014
microbial consortium, causing native lignocellulosic fibers to hydrate and swell exten-
Available online xxx
sively. This study describes the effects of hydration treatment and particle size on the
Keywords:
microbial consortium. A culture filtrate and tap water were used as hydration media, and
Accessibility
particle sizes of 0.212 and 3.35 mm were tested. As a result, biodegradability, measured as
biodegradability of native wheat straw fibers with regard to direct H2 production by a
Particle size
H2 production, doubled when the lignocellulosic substrate was hydrated with the culture
Porosity
filtrate. Also, hydration had a significant impact on H2 yield, increasing it from 6.8 to
Swelling
86.5 mL H2/g total consumed sugars when the 0.212 mm fibers were first hydrated. H2
Water retention capacity
production was higher with coarser particles (3.35 > 0.212 mm), but hydrogen yield improved with finer particles (0.212 > 3.35 mm). Based on our results, particle size and hydration improve the biodegradability of native wheat straw for H2 production, although hydration with a culture filtrate increased the biomass porosity (measured as WRC), which influenced the biodegradability more than a reduction in particle size. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is an energy carrier that can help global energy requirements be met, due to its high combustion power (H2 >120 MJ/kg versus >45 MJ/kg for fossil fuels). Further,
hydrogen is considered to be a green fuel, because water is the sole product at the end of its theoretical combustion [1]. Hydrogen can be produced from biological processes, specifically, from the conversion of carbohydrates into hydrogen by fermentative microorganisms under anaerobic environments.
mica Juriquilla, Instituto de Ingenierı´a, Universidad Nacional Auto noma de * Corresponding author. Present address. Unidad Acade xico, Blvd. Juriquilla 3001, 76230 Quere taro, Me xico. Tel.: þ52 (442) 1926170; fax: þ52 (442) 1926185. Me E-mail address: [email protected] (I. Valdez-Vazquez). http://dx.doi.org/10.1016/j.ijhydene.2014.09.155 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
zquez AR, et al., Hydration treatments increase the biodegradability of native wheat Please cite this article in press as: Lara-Va straw for hydrogen production by a microbial consortium, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.09.155
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First generation biofuels are obtained primarily from crops, such as sugarcane and corn, but one of their major drawbacks is that they are part of the human food chain. In contrast, agricultural byproducts can be used as second-generation biofuels, because they are abundant, cheap and a good source of carbohydrates, some of which are plentiful [2]. Annually, nearly 200 billion tons of agricultural byproducts are produced worldwide, energy equivalent to 60e80 billion tons of crude oil [3]. In Mexico, close to 150 million tons of agricultural byproducts are generated from 20 crops, including corn stover and cobs, sorghum straw, the tops and leaves of sugarcane, and wheat straw [4]. In particular, wheat straw is one of the main agricultural products in certain regions; thus, it can be collected with minimal effort and at low cost for use as feedstock in hydrogen production. Wheat straw is classified as lignocellulosic biomass. In its native form, this biomass is arranged 3-dimensionally as a network, in which the core comprises cellulose microfibrils: large linear chains of anhydroglucose residues that are oriented parallel to hydrogen bonds between successive and adjacent residues, yielding highly ordered and tightly packed crystalline regions that are largely impenetrable by water; and amorphous regions with free hydroxyl groups that interact with the aqueous environment. On the outside of the network, the lignin-hemicellulose matrix is highly branched, with many charged groups rendering it water-swollen [5,6]. The cellulose crystallinity, the association of the ligninhemicellulose matrix with the cellulose microfibrils, and the accessible surface area contribute to the low biodegradability of lignocellulosic biomass [7]. Thus, the hydrogen yield from direct fermentation of native wheat straw is low ranging from 1.0 to 11 mL H2 per g of total volatile solids (TVS) [8e11]. Most approaches improve the biodegradability of wheat straw by obtaining hydrolysates with diluted acids [8,11,12], hydrothermal pretreatment [13], and enzymatic hydrolysis [9], which raises the hydrogen yields to 20e68 mL H2 per g TVS. These pretreatment methods decrease the times that are required for biomass processing and fermentation. However, pretreatment byproducts are released, which strongly inhibit H2-producing dark fermentation [14]; thus, fermentation can only be performed with a low hydrolysate percentage or if the inhibitors are removed. These steps can increase the processing costs in a scaled-up plant, compromising the benefit of the pretreatments. Developing economical and simple pretreatment methods that enhance the biodegradability of lignocellulosic biomass remains a significant challenge in the production of lignocellulosic biofuels. All pretreatment methods aim to increase the accessible surface area [7,15]. Accessibility refers to the substrate that is available for hydrolysis or biodegradation and is governed by biomass porosity (interior surface area) and particle size (exterior surface area) [16]. For years, particle size reduction has been used as an efficient pretreatment method of increasing the exterior surface area to improve biomass biodegradability, the production of biogas and, recently, hydrogen [17e20]. Conversely, cell wall porosity limits the physical, chemical, and enzymatic access to the native lignocellulosic substrate, which impacts the biodegradability of substrate more than the external surface or particle size [21]. In this respect, the biomass porosity can be characterized
by water retention capacity (WRC), which reflects the interactions between the charged surface groups and adsorbed molecules in an aqueous environment [22,23]. Lignocellulosic biomass is considered a cross-linked gel that swells when water molecules are drawn into the pores by osmotic pressure and interact with ionizable groups on the biomass surface. As a result of swelling, the cell wall volume increases and accessibility improves, because the crystallinity is reduced when the bonds between the cross-linked gel are weakened [24]. The hydration medium has significant effects on the WRC: salt ions neutralize the charged groups on the cell wall surface, generally decreasing WRC values and swelling, whereas enzymatic treatment increases them [25,26]. A recent report demonstrated that the WRC and swelling of native wheat straw fibers rose significantly by hydration with a culture filtrate of an H2-producing microbial consortium [27]. Based on that result, the objective of this study was to determine the effects of hydration and particle size on the biodegradability of native wheat straw for direct H2 production by a microbial consortium.
Material and methods Substrate and hydration treatment Wheat straw (Triticum aestivum L.) was used as the substrate and prepared according to [27]; Table 1 shows its chemical composition. Particle size was reduced using a ball mill, and the sample was separated into various fractions using screens; particles that were retained in the 0.212 mm and 3.35 mm-sieves were used. The wheat straw was subjected to 2 separate hydration treatments: tap water for 10 h and a culture filtrate for 4 h. The substrate:medium ratio was 1:20, and the mixture was homogenized well and kept statically at room temperature (28 ± 5 C). At the end of the hydration, the soluble fraction of the native wheat straw was released, yielding a reducing sugar concentration of 1.1 ± 0.23 g/L with tap water and 1.9 ± 0.45 g/L with the culture filtrate. The culture filtrate was obtained from a 3-L New Brunswick fermentor under uncontrolled pH with an initial pH of 5.5 with native wheat straw as substrate (5% w/v), seeded with anaerobic digestate (20% v/v), and incubated for 5 days at 300 rpm and 37 C. The culture was passed through a gauze filter and used directly in the hydration experiments. The
Table 1 e Chemical composition of the native wheat straw. Component Total solids Total volatile solids Carbon Fiber content Protein Total Kjeldahl nitrogen Phosphorus Ash
Content (g/kg) 956 860 419 387 30.6 4.4 0.46 86
zquez AR, et al., Hydration treatments increase the biodegradability of native wheat Please cite this article in press as: Lara-Va straw for hydrogen production by a microbial consortium, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.09.155
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cellulase, endoglucanase, xylanase, and pectinase activities in the culture filtrate were previously reported [27].
500
Batch reactors for hydrogen production
400
Analytical methods Biogas volume was measured daily using lubricated syringes, and biogas content was analyzed using a gas chromatograph (Perkin Elmer Clarus 580) that was equipped with a thermal conductivity detector and an Elite GCeGS Molsieve capillary column. The injector and detector temperatures were 50 C and 200 C, respectively. The following temperature program was used for the column: initial temperature 150 C, increases of 7 C/min to a final temperature of 100 C. Argon was used as the carrier gas at 6 mL/min. At the end of the fermentation, volatile fatty acids (VFAs) content was analyzed after filtration (Pall Life Sciences, 0.45 mm GHP Membrane) using a gas chromatograph (Perkin Elmer Clarus 580) that was equipped with a flame ionization detector and an Elite-Q-Plot capillary column. The injector and detector temperatures were 230 C and 240 C, respectively. The temperature of the column was increased from 100 C to 235 C. A mixture of VFAs was used as a standard (SUPELCO, 46975-U). With regard to substrate consumption, the glucose and xylose contents in the controls and treatments were measured with a biochemistry analyzer (YSI select 2710, Lab Tech). For these measurements, the samples was dried, weighed, hydrolyzed with concentrated H2SO4 at 25 C for 3 h, and neutralized (modified from Ref. [28]).
Results and discussion Effect of hydration on H2 production Fig. 1 shows the effect of hydration and particle size on substrate biodegradability for direct H2 production by a microbial consortium. On average, hydration (with water or culture filtrate) had a significantly positive effect on H2 production and H2 production rates (p < 0.001 and p < 0.01, respectively). Maximum H2 production was achieved with the culture filtrate as hydration medium, followed by hydration with tap water and the control, independent of particle size. Also, H2 production rates rose significantly; for example, with the
300
200
Cumulative hydrogen production (mL/L)
Batch reactors for H2 production were performed in 100 mL serological bottles. Sludge from an anaerobic digester, previously heat-treated in a water bath for 30 min, was used as the inoculum (20% v/v). The hydrated wheat straw (0.212 or 3.35 mm) and hydration medium were loaded into the batch reactors, and the working volume was adjusted to 70 mL with mineral medium with a substrate content of 25 g TVS/L. The mineral medium composition was as follows (g/L): KH2PO4 4.8, K2HPO4,3H2O 6.98, MgCl2,6H2O 0.1, CaCl2 0.02, NH4Cl 6.0, MnSO4,6H2O 0.015, FeSO4,7H2O 0.025, CuSO4,5H2O 0.005, and CoCl2,5H2O 0.000125. The initial pH was adjusted to 5.5. Unhydrated wheat straw was used as the control treatment. The batch reactors were incubated statically for 20 days at 37 C. All experiments were performed in triplicate.
a
100
0 500 0
5
10
15
20
5
10 Time (d)
15
20
b 400
300
200
100
0 0
Fig. 1 e Effect of hydration and particle size on the biodegradability of native wheat straw for direct hydrogen production. Particle sizes of a) 0.212 mm, and b) 3.35 mm. Substrate hydration with tap water (-), an enzyme culture filtrate (C), and control (unhydrated substrate, A). Error bars indicate standard deviation.
0.212 mm wheat straw fibers, the hydrogen production rate climbed from 10 mL/L$d with the unhydrated substrate to 29 mL/L$d and 44 mL/L$d with substrate that was hydrated with culture filtrate and water, respectively (Fig. 1(a)). The adaptation phase was shortened in the waterhydrated treatments with respect to the control, possibly because small concentrations of soluble sugars that accumulated during the hydration supported rapid cell growth and synthesis of hydrolytic enzymes. In contrast, this phase was prolonged in the filtrate-hydrated treatments versus the control, due to repression or inhibition during the first several days of fermentation with culture filtrate-hydrated substrate. However, unlike the water-hydrated and control treatments, in which hydrogen production plateaued by day 8, the filtrate-
zquez AR, et al., Hydration treatments increase the biodegradability of native wheat Please cite this article in press as: Lara-Va straw for hydrogen production by a microbial consortium, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.09.155
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Table 2 e Glucose and xylose consumption and product yield ðYH2 =s Þ. Particle size (mm)
Hydration treatment
Sugar consumption (%) Glucose
0.212
3.35
Control Water Culture filtrate Control Water Culture filtrate
64.0 ± 52.7 ± 25.2 ± 51.1 ± 77.1 ± 45.2 ±
4.8 1.7 0.3 10.6 3.3 7.9
hydrated treatments continued producing hydrogen. Consequently, hydrogen production doubled when the substrate was first hydrated with culture filtrate. Lara et al. [27] demonstrated that the porosity of native wheat straw fibers (measured as WRC) rose using a culture filtrate of H2-producing microbial consortium with hydrolytic activity. In that study, hydration with water increased the volume of the wheat straw fibers due to swelling, achieving a WRC value of 5.2 g/g in 4 h. In addition to the interaction between the wheat straw cell wall surface with water molecules, the enzymatic activity in the culture filtrate elicited greater swelling and higher surface area, yielding a WRC value of 9.8 g/g in 4 h. Several types of hydrolytic activity were detected in the culture filtrate, such as cellulases, endoglucanases, xylanases, and pectinases, which acted together on the native lignocellulosic matrix. Thus, hydration with culture filtrate increased the substrate porosity through the activity of the aqueous environment and hydrolytic enzymes, which resulted in significant biodegradability and hydrogen production in this study. Other authors have examined the effects of swelling on the accessibility of cellulase enzymes to cellulose. For instance, Ju et al. [29] reported the effects of swelling on the enzymatic digestibility of fibers using a PFI refining mill process as pretreatment, entailing a combination of mechanical agitation and hydration. After 10,000 revolutions of refining, the enzymatic conversion of modified Kraft pulping increased by 31% due to fiber swelling. Also, Hu et al. [26] concluded that there xylanase and cellulase synergize to significantly improve cellulose accessibility by increasing fiber swelling and fiber porosity. In our study, we combined the benefits of hydration and swelling with mild enzymatic hydrolysis of native wheat straw fibers to produce hydrogen directly using a consolidated bioprocess approach.
Effect of particle size on H2 production As substrate, larger wheat straw fibers (3.35 mm) yielded more hydrogen than finer ones (0.212 mm); this pattern was evident with unhydrated fibers, for which the H2 production rate was 193 mL H2/L with the 3.35 mm fibers versus 66 mL H2/L with the 0.212 mm fibers (Fig. 1). These results seem contradict most studies, which have reported that substrate biodegradability rises with decreasing particle size [19,20,30,31,]. However, by confocal laser scanning microscopic analysis, the cell wall of finer fibers (2.00 mm) suffered from a shaving effect due to grinding, decreasing its porosity (measured as WRC), where coarse fibers of 3.35 mm had a higher WRC than fibers
H2 yield (mL/total consumed sugars)
Xylose 78.6 ± 34.2 ± 30.1 ± 62.0 ± 67.5 ± 68.2 ±
1.2 13.7 7.6 10.7 1.8 5.1
6.8 34.5 86.5 25.3 27.2 58.0
2.00 mm. Thus, high WRC values of coarse fibers are indicative of improved biodegradability and the potential for high hydrogen production.
Substrate consumption and H2 yields Regarding substrate consumption, glucose and xylose were consumed simultaneously (Table 2) as a result of high metabolic diversity in the microbial consortium, which had the capacity to consume 5-carbon and 6-carbon sugars for hydrogen production. The sum of consumed sugars was used to calculate the hydrogen yield, expressed in mL of H2 per g of total consumed sugars. Higher hydrogen yields tended to be obtained with hydrated substrate, independent of the particle size, primarily with the substrate that was hydrated with culture filtrate. This finding might be related to the energy consumption that is required to synthetize hydrolytic enzymes. In addition to increasing substrate porosity, hydration effects the release simple sugars that are already available for growth and enzyme synthesis (which was intensified with culture filtrate). In contrast, with unhydrated substrate, microorganisms must expend more energy (obtained from the substrate) to access the substrate. According to Zhang and Lynd [32], 20% of the ATP that is produced by a cell is allocated to cellulase synthesis in anaerobic fermentations, which can require more energy than cell growth. Thus, more of the energy that is obtained from the substrate in hydration treatments can be allocated to the growth of H2-producing microorganisms.
Accumulation of volatile fatty acids Anaerobic H2 production is accompanied by the accumulation of VFAs, which are produced by metabolic pathways, such as propionic acid fermentation, butyric acid fermentation, and ethanol fermentation. The route that is used is influenced by many factors, including pH, temperature, redox potential, and hydraulic retention time [33]. Fig. 2 shows the VFA profile at the end of fermentation for each treatment. The chief soluble metabolites that were produced were acetic, propionic, butyric, and formic acids. According to these results, acetic and butyric acids accounted for 40%e50% of the VFAs in the filtrate-hydrated treatment. In general, these metabolites are good indicators of H2 production. Also, a significant amount of propionic acid was generated at the end of fermentation, which theoretically does not contribute to H2 production [34,35]. Notably, in the batch reactors that were fed unhydrated substrate, we observed high accumulation of formic
zquez AR, et al., Hydration treatments increase the biodegradability of native wheat Please cite this article in press as: Lara-Va straw for hydrogen production by a microbial consortium, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.09.155
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5
0.212 mm fibers. The cell wall in finer fibers was damaged, affecting its biodegradability. The biodegradability of a native lignocellulosic substrate improves, primarily by increasing the porosity via hydration and swelling rather than decreasing particle size. This simple and economical treatment should be incorporated as a routine step in the production of lignocellulosic biofuels.
Acknowledgments Financial support was received from the CONACYT projects, Fondo Sectorial “CONACYT-SENER e Sustentabilidad Enertica” (grant no. 150001) and “Apoyo al Fortalecimiento y ge gica” Desarrollo de la Infraestructura Cientı´fica y Tecnolo (grant no. 188432).
references
Fig. 2 e Volatile fatty acid accumulation at the end of the direct fermentation of native wheat straw hydrated with water and an enzyme culture filtrate. Error bars indicate standard deviation.
acid (32%e50%). Formic acid is accumulated by cellulolytic microorganisms, such as Clostridium sp. and Enterobacter sp., wherein 1 mol H2 is produced per 1 mol formic acid that has accumulated [36,37]. Thus, formic acid production correlates with low H2 production.
Comparison with other studies Typically, hydrogen yield is expressed in mL H2 per g of TVSadded. Previous studies have reported an H2 yield of 1.0e11 mL H2/g TVSadded when unhydrated native wheat straw is used as substrate under mesophilic conditions [8e11]. In our study, when the substrate was first hydrated with an enzyme culture filtrate for 4 h, the H2 yield improved from 9 to 17 mL H2/g TVSadded. These results confirm the potential of hydration treatments to increase substrate accessibility and biodegradability when direct fermentation of native lignocellulosic substrate is desired by a consolidated bioprocess. This method can be extrapolated to many biological systems for the production of ethanol, enzymes, and other products.
Conclusions Prior hydration and swelling of native wheat straw fibers increased their porosity and significantly enhanced substrate biodegradability and direct hydrogen production by a microbial consortium. These results were intensified when a culture filtrate with hydrolytic enzymes was used to hydrate the substrate. Regarding biodegradability particle size, we found that hydrogen production was greater with 3.35 mm versus
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zquez AR, et al., Hydration treatments increase the biodegradability of native wheat Please cite this article in press as: Lara-Va straw for hydrogen production by a microbial consortium, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.09.155