Enhanced Biogas and Biohydrogen Production from Cotton Plant

Dec 6, 2016 - Maximum biogas production values of 246 and 219 mL/g volatile solids (VS) ... literature on biogas production from cotton stalks, and in...
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Enhanced Biogas and Biohydrogen Production from Cotton Plant Wastes Using Alkaline Pretreatment Mohsen Ghasemian, Hamid Zilouei,* and Ahmad Asadinezhad Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran ABSTRACT: Enhancement of biogas and biohydrogen production from cotton plant wastes (stalk and boll) was investigated in the current work. This was achieved using alkaline pretreatment by sodium hydroxide solution of 8% (w/w) concentration at 0 and 100 °C for 10, 30, and 60 min, together with ammonia solution of 4 and 8% (w/w) concentrations at 40 and 80 °C for 6 and 12 h. Maximum biogas production values of 246 and 219 mL/g volatile solids (VS) were achieved respectively from boll pretreated by sodium hydroxide 8% (w/w) at 100 °C for 10 min and stalk pretreated by ammonia solution 4% (w/w) at 80 °C for 12 h. Sodium hydroxide-pretreated boll and ammonia solution-pretreated stalk, under conditions similar to those leading to the maximum methane production, gave maximal hydrogen production values of 17 and 15.2 mL/g VS, respectively. The results of hydrogen production in this study showed satisfactory agreement with the Gompertz equation. Structural analysis of the samples using scanning electron microscopy and Fourier transform infrared spectroscopy established that the reduction of crystallinity, lignin removal, and increase in porosity of the pretreated samples were the main reasons behind these observations.

1. INTRODUCTION The exponential depletion of the current non-renewable energy resources together with increasing concerns about greenhouse gas emissions from fossil fuel combustion has stimulated serious attention to new sustainable alternatives. Biogas and biohydrogen, as clean, versatile, and renewable fuels, are considered two suitable alternatives to replace fossil fuels in power and heat production as well as gas vehicle fuel.1,2 At the present time, biogas production at an industrial scale mainly uses sugar-based materials. However, long-term use of these substances results in food versus fuel conflict in many cases.3 Lignocellulosic materials such as agricultural residues are cheap and plentiful, and they can be considered as alternative feedstocks for bioenergy production.4,5 Cotton is a soft, fluffy staple fiber that grows in a boll and, as an agricultural product, plays a key role in economic, political, and social aspects of human life. It is the most widely used natural fiber, comprising approximately 40% of the entire global fiber production.6 The annual production of cotton in the world is 33 million tons (reported by FAO in 2012), and therefore, a large amount of lignocellulosic residue, so-called cotton stalk and boll (stem and branches), is produced as a byproduct.6,7 Because of critical disposal issues and the subsequent adverse effects on farming, much of the cotton waste world-wide is incinerated on the ground, resulting in potentially serious environmental issues.6,8 Cotton stalk and boll, like other lignocellulosic materials, contain considerable amounts of carbohydrate polymers (cellulose and hemicellulose), which, as a major source of pentose and hexose sugars, gives them capacity to be used as a feedstock for biofuels.9 However, there is yet a limited extent of available literature on biogas production from cotton stalks, and indeed, no effort has been focused on biohydrogen production from cotton stalk and boll thus far. The conversion of lignocellulosic materials to bioenergy is very challenging. Due to the highly crystalline cellulosic structure being quite resistant to bacterial hydrolysis, an © XXXX American Chemical Society

effective pretreatment, as a key step in making the lignocellulosic materials feasible for biogas and biohydrogen production, is necessary prior to the anaerobic digestion or dark fermentation steps.5,10 Lignin, as a physical barrier, plays a central role in the recalcitrance of lignocellulosic biomass through limiting the bacterial accessibility and hence degradability of cellulose.11,12 Among different pretreatment methods, alkaline pretreatment alleviates this problem by removing a substantial part of the lignin from lignocellulosic materials.13,14 Alkaline pretreatment with ammonia has been tested for the pretreatment of various lignocelluloses and reported to be effective for low-lignin raw materials such as agriculture residues.15−17 Ammonia is a non-polluting, non-corrosive, and also inexpensive chemical, and, as an effective swelling reagent, it has high selectivity to react with lignin rather than carbohydrates. Furthermore, high volatility, easy recovery, and reusability make it an ideal solvent.18 The remaining ammonia may also be utilized as a source of nitrogen to compensate for high C/N ratio in agricultural biomass.9 However, it is important to consider that ammonia concentrations higher than 0.1 g/L can inhibit the anaerobic digestion process. Similarly, sodium hydroxide can simultaneously disrupt the compact lignin−carbohydrate structure, affect the lignin barriers, reduce cellulose crystallinity as well as polymerization degree, and increase the swelling capacity and internal surface area, which significantly enhance the porosity of the biomass.19,20 To the best of our knowledge, there has been published no attempt dedicated to the alkaline pretreatment of cotton stalk and bolls with ammonia and sodium hydroxide prior to biogas and biohydrogen production so far. In the present work, the potential of cotton plant wastes (cotton stalk and boll) for biogas and biohydrogen production Received: August 12, 2016 Revised: October 31, 2016

A

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Energy & Fuels Table 1. Chemical Composition (%) of the Untreated and Pretreated Cotton Stalk and Bolla stalk pretreatment conditions

glucan

boll

xylan

arabinan

lignin

ash

glucan

xylan

arabinan

lignin

ash

untreated

39(2.1)

20.2(1.1)

3.8(0.9)

25(3.1)

3.8(2)

41(0.8)

21.8(3)

4.2(0.28)

22(2.1)

4.7(2.1)

NaOH-treated at 100 °C 10 min 30 min 60 min

41.1(3.1) 44.3(2.3) 44.8(0.5)

17(0.6) 15.1(1.2) 14.1(0.6)

3.1(0.6) 2.9(0.5) 2.1(1.2)

21.2(2.3) 19.1(0.8) 18.8(0.7)

3.2(2.1) 1.6(0.6) 1.1(2.3)

49.9(1.2) 45.8(2.3) 43.5(3.3)

20.6(2.1) 18.3(1.3) 17.1(0.9)

3.3(2.5) 3(1.8) 2.7(1.2)

15.2(2.2) 18.1(2) 19.2(0.3)

2.5(3.2) 2.5(1.5) 2.7(1.7)

NaOH-treated at 0 °C 10 min 30 min 60 min

39.8(1.4) 40.3(3.2) 42(0.3)

19.9(3.3) 20.1(0.4) 19.5(1)

3.5(3.3) 3.6(2) 3.7(3.1)

23.3(0.5) 22.1(0.3) 21.3(1.9)

2.3(1.3) 2.3(0.5) 2.2(1.4)

44.1(2.6) 41.8(3.2) 40.2(1.9)

21.3(0.6) 20.9(3.5) 20.1(3.4)

4.1(3.6) 4.1(3.9) 4(0.8)

19.2(0.8) 19.8(1.1) 22.3(2.5)

4.7(0.9) 5.2(1.9) 5.1(0.8)

NH3-treated, 4% (w/w) 40 °C, 6 h 40 °C, 12 h 80 °C, 6 h 80 °C, 12 h

39.1(3.2) 39.2(1.4) 42.1(2.9) 43(0.9)

19.2(2.2) 18.8(3.1) 18.6(2.8) 18.6(1.5)

3.7(0.9) 3.6(0.5) 3.5(1.5) 3.5(1.8)

23.1(0.5) 22.5(0.4) 22(0.9) 20(1.8)

1.7(0.6) 2.5(3.2) 1.3(3.1) 2.7(0.4)

41.6(2.1) 42.3(3.2) 42.4(0.9) 44(1)

21.7(0.9) 21.6(1.3) 21.4(0.3) 21.5(1.4)

4.1(3.2) 3.9(0.6) 3.5(2.7) 3.5(2.8)

20(2.2) 19.1(2.4) 19(3.6) 18.4(1.4)

2.5(0.5) 3.3(3.1) 2.2(1.3) 3.4(2.9)

NH3-treated, 8% (w/w) 40 °C, 6 h 40 °C, 12 h 80 °C, 6 h 80 °C, 12 h

39(1.1) 38.9(3.3) 37.8(0.8) 36.1(1.8)

21(3.5) 18.9(3.4) 16.7(3.8) 15.2(2.6)

3.4(2) 2.8(3.2) 2.6(0.9) 2.1(0.9)

22(0.5) 20.1(2.8) 19.7(2.3) 18.3(3.2)

3(1.5) 2.4(2.1) 1.3(0.6) 1.7(0.8)

41.2(3.1) 41.3(2.5) 43.5(0.8) 43.6(0.4)

20.5(0.6) 19.1(1.4) 18.9(1.9) 18(2.9)

3.7(1.5) 3.4(0.9) 2.3(1.9) 1.3(0.8)

21(0.8) 19.1(2.6) 17.4(3.8) 17.3(0.8)

2.8(1.3) 3(0.8) 2.4(0.9) 2.5(0.3)

a

Percentages were calculated from values on a dry-weight basis. Corresponding results are given as the average of two measurements. The values in parentheses represent standard deviations. 2.4. Biohydrogen Production. Biohydrogen production was performed through anaerobic dark fermentation process at mesophilic conditions (37 °C). Mixed culture (obtained from Isfahan North Wastewater Treatment sludge) was heat-shocked at 85 °C for 45 min prior to use for biohydrogen production. Anaerobic batch dark fermentation was done similar to the biogas production procedure, except that the gas production in each bottle was measured every 12 h for the first 2-day period and then daily for the following 7-day period. All experiments were carried out in triplicate. 2.5. Analytical Methods. The composition of gas produced in bioreactors was measured on a gas chromatograph (Sp-3420A, TCD detector, Beijing Beifen Ruili Analytical Instrument Co.) equipped with a packed column (Porapack Q column, Chrompack). Nitrogen was used as a carrier gas at 45 mL/min flow rate. Temperatures of the column, injector, and detector were 40, 100, and 150 °C, respectively. A high-performance liquid chromatograph (HPLC), equipped with UV/vis and refractive index detectors (Jasco International Co., Tokyo, Japan), was used to determine sugars and volatile fatty acids (VFAs). Concentrations of VFAs such as acetic, butyric, and propionic acids were analyzed by an Aminex HPX-87H column at 60 °C and using 0.005 M sulfuric acid as an eluent at the flow rate of 0.6 mL/min. Moreover, sugars were determined by an Aminex HPX-87P column at 85 °C using deionized water as eluent with a flow rate of 0.6 mL/min. The carbohydrate, lignin, ash, total solids (TS), and volatile solids (VS) contents of the treated and untreated cotton stalk and boll were determined according to the appropriate methods.21,22 The microscopic structure morphology of treated and untreated cotton stalk and boll was examined by scanning electron microscopy (SEM). The samples were coated with a thin layer of gold and then analyzed by the microscope (KYKY-EM3200) at 26 kV. The crystallinity and molecular structure of samples were examined using Fourier transform infrared (FTIR) spectrometer (Bruker Tensor 27 FTIR). Their spectra were obtained after averaging 60 scans at 2 cm−1 resolution within the wavenumber range 600−4000 cm−1.

is explored. The effects of alkaline pretreatment by ammonia and sodium hydroxide solutions at high and low temperatures over different retention times on the yield of methane and hydrogen production are studied. In addition, the structural changes of the wastes as a result of the pretreatments are investigated.

2. MATERIALS AND METHODS 2.1. Raw Materials. Cotton plant (Gossypium hirsutum) was obtained from Isfahan south fields. Cotton stalks and boll were dried in sunlight for 3 days. They were milled and screened in order for the characteristic size to be less than 1 mm (20 mesh). Dry weight content of the cotton stalk and boll were measured after drying at 105 °C after there was no change in its residual weight. 2.2. Pretreatment Methods. The pretreatments were performed by using 8% (w/w) sodium hydroxide solution at 0 and 100 °C for 10, 30, and 60 min, and also by using 4 and 8% (w/w) ammonia solution at 40 and 80 °C for 6 and 12 h. In both pretreatment methods, 5 g of milled substrate and 50 g of pretreatment solution were mixed. After the pretreated solid and liquid were separated, the solids were washed by distillated water until pH 7 was achieved. The pretreated solids were stored at 5 °C before further use. 2.3. Biogas Production. Anaerobic batch digestion process was performed with mixed culture obtained from Isfahan North Wastewater Treatment sludge at mesophilic conditions (37 °C). Glass bottles of 118 mL volume were used as anaerobic digester reactor, inside which 20 mL of inoculum, 5 mL of deionized water, and 0.25 g of treated or untreated substrate were added. The bottles were then sealed with butyl rubber and aluminum caps. In addition, inoculum and deionized water were used as a blank to monitor gas production of the inoculum. The glass bottles were purged with pure nitrogen gas for 3 min to maintain anaerobic conditions and held at 37 °C in a shaker incubator. Samples from the gas phase of each bottle were taken every 3 days for the first 15-day period and then every 5 days for the next 40day period. All experiments were carried out in duplicate. B

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Figure 1. SEM images of (A) untreated cotton stalk, (B) 8% (w/w) sodium hydroxide treated cotton stalk at 0 °C for 60 min, (C) 4% (w/w) ammonia treated cotton stalk at 80 °C for 12 h, (D) untreated boll, (E) 8% (w/w) sodium hydroxide treated boll at 100 °C for 10 min, and (F) 4% (w/w) ammonia treated boll at 80 °C for 12 h.

3. RESULTS AND DISCUSSION 3.1. Characterization of Cotton Stalk and Boll as Feedstocks. The compositions of cotton stalk and boll used in this study are presented in Table 1. Glucan is regarded as the cellulose fraction, while the sum of xylan and arabinan percents signifies the hemicellulose portion. The cotton stalk used in this study consists of 39% glucan, 20.2% xylan, 3.8% arabinan, 25% lignin, and 3.8% ash. The cotton boll contains 41% glucan, 21.8% xylan, 4.2% arabinan, 22% lignin, and 4.7% ash. These results reveal the feasibility of biogas production from cotton stalk and boll due to their high cellulose content. Moreover, there is relatively lower content of ash in cotton stalk and boll compared with the other non-wood fiber resources. However, cotton stalk and boll have higher lignin than other agricultural raw materials, such as rice straw sweet sorghum bagasse, as well as corn stover, as reported by some researchers.23,24 Therefore, a pretreatment step is necessary to remove or break down lignin to improve anaerobic digestion of 63% and 67% holocellulose in cotton stalk and boll, respectively, and also to facilitate their conversion to biogas. 3.2. Effects of Pretreatment on Cotton Stalk and Boll Composition. The chemical composition of the pretreated materials is summarized in Table 1. Glucan is the dominant carbohydrate in all materials consisting 39−49.9% of total composition, and xylan stands in the second place consisting 18.6−21.8% of the whole formulation. The other carbohydrate is arabinan with fraction of 3.3−4.2%, depending on the pretreatment method. According to Table 1, the highest lignin

removal which amounts to 30.9% is achieved after pretreating boll at 100 °C for 10 min by 8% (w/w) sodium hydroxide, which also results in the highest increase in glucan content (21.7%) and the highest decrease in xylan and arabinan contents (5.5% and 21.4%, respectively) compared to the untreated boll. On the other hand, pretreatment of boll by 4% ammonia for 12 h at 80 °C increases glucan content by 7.3% while decreasing that of xylan, arabinan, and lignin by 1.4%, 16.7%, and 16.4%, respectively, in comparison with the untreated sample (Table 1). In the case of cotton stalk, the highest increase in glucan content (10.2%), the highest decrease in xylan and arabinan (7.9% and 7.8%, respectively) and the highest lignin removal (20%) are attained after pretreatment of cotton stalk at 80 °C for 12 h by 4% (w/w) ammonia. Cellulose, hemicellulose, and lignin contents, 47.1%, 24.1%, and 22.0% respectively, have been reported after pretreating cotton stalk by 2% (w/w) ammonia solution in an aqueous medium at 100 °C for 15 min.9 The pretreatment of cotton stalk with 8% NaOH for 60 min at 0 °C increases glucan fraction by 7.9%, decreases xylan, arabinan, and lignin by 3.5%, 2.6%, and 14.8%, respectively, compared to the control sample. Silverstein et al.25 reported the glucan, xylan, and lignin contents of 35.5−50.3%, 7.9−13.0%, and 17.6−25.2%, respectively, for the sodium hydroxidepretreated cotton stalk under different conditions. As the concentration of ammonia and sodium hydroxide has been much lower than the conventional alkaline pretreatment methods for biofuel production, their effect on lignin content reduction has found to be insignificant accordingly.26,27 C

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Figure 2. FTIR spectra of untreated (A), sodium hydroxide-treated (B), and ammonia-treated (C) cotton stalk and boll.

structure of the fibers and destroy lignin seal. Moreover, the structure of the cotton stalk and boll is open becoming more sponge-like after pretreatment. This can provide higher surface area for subsequent bacterial reactions.30 Similar structural changes have been reported for cotton stalk pretreated by ammonia and ionic liquids including 2-hydroxyethylammonium formate.9,31 3.4. Effects of Pretreatment on Cellulose Crystallinity. FTIR spectra are investigated to study the effects of alkaline pretreatment on chemical structure of the cotton stalk and boll. The respective results are summarized in Figure 2 for the samples associated with the most methane production. The crystallinity index (CI) has been calculated using the absorbance at 1430 and 898 cm−1, respectively signifying cellulose I and cellulose II.32 Researchers have shown that the crystallinity index of biomass increases significantly at low pH pretreatments, while the crystallinity index of high pH pretreated biomass decreased in some cases.33,34 In the current study, the same effect was observed using sodium hydroxide and ammonia. According to the obtained results, the untreated boll has more crystalline cellulose than the untreated stalk. However, a different trend after pretreatment was observed, and the portion of amorphous cellulose in the boll was more than the pretreated stalk. As it is seen, the crystallinity index decreases after pretreatment by ammonia or sodium hydroxide. Crystallinity indices are 0.85, 0.80, and 0.82 for untreated, ammonia-treated, and sodium hydroxide-treated cotton stalk, respectively. As for boll, the corresponding values are 0.96, 0.78,

The percentage of total solid (TS) and volatile solid (VS) of the untreated and treated cotton stalk and boll are also analyzed whose data are not shown. The ratio of VS to TS increases in all treated samples compared to the untreated ones. The greatest value of VS-to-TS ratio for cotton stalk is 0.99 which is observed in the sample pretreated with sodium hydroxide solution at 100 °C for 60 min. The highest value of VS-to-TS ratio for boll is 0.98, which is seen for the sample pretreated with 4% (w/w) ammonia solution at 80 °C for 6 h. It is worth mentioning that the hydrolysate phase was not used for biogas production. As it is shown in the literature, the most important effect of alkaline pretreatment is lignin removal from biomass. Hence, there are inhibitor components of lignin and phenolic in hydrolysate that caused a decrease in biogas production rate.15,25,28,29 On the other hand, the concentrations of bases that are used in the research were almost high (ammonia 4 and 8% w/w, sodium hydroxide 8% w/w). Therefore, the hydrolysate with its basal medium needed a neutralization process by high amounts of acid which enhance the salinity of liquid for furthered fermentation. 3.3. Effects of Pretreatment on Morphological Structure. Significant changes in morphology are identified after examining the samples with SEM illustrated in Figure 1. As it can be seen, the untreated cotton stalk and boll are represented by a flat, smooth, and continuous surface which shows compact rigid particle structure, while pretreated samples have sparse and rough shape with some missing parts on the surface. Both ammonia and sodium hydroxide disrupt the D

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Figure 3. Cumulative methane production yields of sodium hydroxide- (A) and ammonia-pretreated (B) cotton stalk and sodium hydroxide- (C) and ammonia-pretreated (D) boll during 40 days of anaerobic digestion.

The methane yield in produced biogas at final stages of 40day period of anaerobic digestion is presented in Figure 4. The range of methane content for cotton stalk and boll varies from 48.0% to 58.0% and from 50.0% to 61.3%, respectively, depending on the pretreatment type and conditions (data not shown). The highest amount of methane (246.4 mL/g VS, 61.3% volumetric methane) is produced from the boll pretreated by 8% (w/w) sodium hydroxide solution for 10 min at 100 °C. This corresponds to an improvement of the yield by around 178% when compared with the untreated. These results of methane production can be compared to those obtained from the cotton stalk (449 mL/g VSadded38), antibiotic mycelial residue (231 mL/g VS39), co-digestion of cattle manure with corn stover (194 mL/g VS40), and finally giant reed (217 mL/g VS41). In addition, sodium hydroxide pretreatment of boll at 100 °C for 30 and 60 min resulted in 195.1 and 170.9 mL/g VS of methane production (58.2% and 58.0% volumetric methane), respectively (Figure 4). In other words, increasing the pretreatment time reduces the methane production. The decomposition of dissolved carbohydrates and formation of alkali-stable end-groups, which negatively

and 0.74, respectively. It shows that the portion of amorphous cellulose in the pretreated boll was more than that in the pretreated stalk. The similar trend has been reported in the literature.35−37 A reduction of crystallinity of cellulose, as one of the main consequences of the pretreatment can be attributed to swelling of cellulose upon diffusion of the alkali solution into the amorphous domains of cellulose microfibrils. This process enhances the cellulose chain mobility and diminishes the crystalline parts of cellulose in terms of the characteristic size. Ultimately, cellulose I is rearranged into cellulose II having lower crystallinity index. 3.5. Biogas production. Batch anaerobic digestion for biogas production was performed using mixed anaerobic culture. The time course of cumulative methane production during 40 days anaerobic digestion is presented in Figure 3. It is observed that the most methane production yields are attained within 10−30 days of digestion in all experiments. Furthermore, a decrease in methane production is observed after 35 days of digestion, and the methane production is completely stopped after 40 days of digestion in all tests. E

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Figure 4. Yield of methane production from sodium hydroxide- (A) and ammonia-pretreated (B) cotton stalk and sodium hydroxide- (C) and ammonia-pretreated (D) boll after 40 days anaerobic digestion.

pretreatment time and temperature are similar in trend to those of the pretreatment by sodium hydroxide due to the same rationale. In the case of cotton stalk, pretreatment by 4% (w/w) ammonia solution at 80 °C for 12 h gives rise to the maximum methane production yield 218.9 mL/g VS, while the yield from the untreated cotton stalk is almost 80.8 mL/g VS (Figure 4). This means an enhancement by 171% of the methane yield with respect to the untreated cotton stalk. According to Figure 4, at 4% (w/w) ammonia concentration, biogas yields of 148.5 and 218.9 mL/g VS are obtained from the cotton stalk pretreated at 80 °C for 6 and 12 h, respectively, whereas the pretreatment at 40 °C for 6 and 12 h, respectively, leads to 114.2 and 125.4 mL/g VS biogas yields. Even though increasing temperature and prolonging the pretreatment at the ammonia concentration of 4% (w/w) enhances the biogas production yield from cotton stalk, it reduces the quantity of methane production at higher ammonia concentration (that is, 8% (w/ w)). Further details on the effects of the ammonia concentration upon biogas formation and remedial options are available elsewhere.44,45

influences the bioconversion yield, could be the possible reasons behind this behavior.12 The pretreatment by sodium hydroxide at higher temperature is caused to break the ester bonds cross-linking between hemicelluloses, cellulose, and lignin of the biomass. Hence, the porosity of biomass is increased after further modification or removal of lignin.12,36 The biogas production results from NaOH-treated cotton stalk reveal more biodegradability in comparison to the untreated ones. Similar results have been reported in literature.42,43 Ammonia-pretreated boll is converted to methane in the range of 86.6−228.9 mL/g VS (50.0−59.2% volumetric methane) depending on the treatment conditions. As shown in Figure 4, the best result by ammonia pretreatment of boll is obtained at 4% (w/w) concentration of ammonia and 80 °C for the duration 12 h. At fixed pretreatment temperature and time, increasing the ammonia concentration from 4% to 8% w/w results in lower amounts of methane produced. This is likely due to the fact that a larger amount of fermentable sugars is dissolved in liquid phase because of the higher concentration of ammonia, and thus the available amount of these materials for the bacteria is reduced.12,44 The effects of the ammonia F

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Energy & Fuels Pretreatment using sodium hydroxide elevates the methane production yield from cotton stalk to 95.4−205.4 mL/g VS depending on the pretreatment conditions (Figure 4). As shown in Figure 4, increasing the retention time at both temperatures exerts positive impact on biogas yield during pretreatment using sodium hydroxide, especially when the pretreatment is performed at 0 °C. Sodium hydroxide at low temperatures (e.g., 0 °C) is more effective for decreasing the crystallinity index in comparison to high temperatures, e.g., 100 °C. Indeed, it is possible for Na+ and OH− ions to bind with water at low temperatures. Thus, the sodium hydroxide solution can breakdown the hydrogen bonds within cellulosic structure and convert cellulose I to cellulose II.46,47 On the other hand, the breakdown of cross-linking bonds between cellulose, hemicellulose, and lignin is the predominant mechanism of the sodium hydroxide pretreatment at high temperatures. Therefore, the delignification tends to occur as well as increasing porosity of the biomass.12 According to this phenomenon, it can be interpreted durable pretreatment creates sufficient opportunity for crystalline structure of breakdown cellulose which enhances biogas production at low and high temperatures (0 and 100 °C). Additionally, more biogas is produced by the disintegration of ester bonds between lignin, hemicellulose, and cellulose. Similar results have been reported by Nieves et al.37 In contrast, methane production is reduced in amount after increasing treatment temperature from 0 to 100 °C at the fixed treatment durations. Similar results have been reported in experiments using different feedstock by Salehian et al.36 and Kayhanian.44 3.6. Biohydrogen Production. Pretreated samples with the highest methane production together with untreated samples were used as the substrate to produce hydrogen by anaerobic dark fermentation. Figure 5 illustrates the time course of cumulative hydrogen production during 168 h anaerobic dark fermentation of cotton stalk and boll. During the first 20-h period, the amount of hydrogen production is marginal due to adaptation and concentration of the hydrogenproducing bacteria. However, hydrogen production increases exponentially over following 20−100 h interval of the fermentation so that 71.3% and 79.5% of theoretical biohydrogen production yield from sodium hydroxide- and ammonia-pretreated boll, and 60.9% and 69.1% of theoretical biohydrogen production yield from sodium hydroxide- and ammonia-pretreated cotton stalk, are obtained within this period of fermentation. For all untreated and pretreated substrates, the biohydrogen production is completely stopped after 7 days of fermentation. According to Figure 6, which displays the effects of pretreatments on the cumulative hydrogen volumes, higher amount of hydrogen is produced from pretreated samples compared to the untreated ones. After 168 h of anaerobic dark fermentation, hydrogen is produced with the maximum yield 17.1 mL/g VS from pretreated boll by 8% (w/w) sodium hydroxide at 100 °C for 10 min, while the yield from untreated boll is only 3.4 mL/g VS. Furthermore, the anaerobic dark fermentation of 4% (w/w) ammonia-pretreated boll results in hydrogen production yield 15.1 mL/g VS. Regarding the cotton stalk, hydrogen yield 15.2 mL/g VS is obtained from the stalk pretreated at 80 °C for 12 h by 4% (w/w) ammonia solution and 13.3 mL/g VS from the stalk pretreated at 0 °C for 60 min by 8% (w/w) sodium hydroxide solution, while it is only 2.4 mL/g VS for the untreated cotton stalk (Figure 6). These

Figure 5. Biohydrogen production profile from cotton stalks and boll during 168 h anaerobic fermentation. Data are shown for untreated samples (dashed line), samples pretreated by 4% (w/w) NH3 at 80 °C for 12 h (□), and samples pretreated by 8% (w/w) NaOH at 0 °C for 60 min (○).

results suggest that the improved hydrogen yield correlates well with an increase in the soluble sugar and lignin removal. In other words, the pretreatment of the cotton stalk and boll plays an important role in conversion of these substrates into biohydrogen under anaerobic dark fermentation. The hydrogen yields in this study are in accordance with the findings of other researchers where hydrogen production from other lignocellulosic materials in batch dark fermentation has been studied.48,49 The modified Gompertz equation (eq 1) was used to explain the progress of the cumulative hydrogen production from batch experiments in terms of hydrogen production potential, maximum hydrogen production rate, and lag-phase time.49−51 ⎧ ⎡R e ⎤⎫ H(t ) = Hmax exp⎨−exp⎢ max (λ − t ) + 1⎥⎬ ⎣ Hmax ⎦⎭ ⎩ ⎪

⎪

⎪

⎪

(1)

where H(t) is cumulative hydrogen production (mL/g VS), Hmax is the maximum hydrogen production (mL/g VS), Rmax is the maximum rate of hydrogen production (mL/g VS.h), e is 2.71828, t is the digestion time (h), and λ is the lag phase duration (h). To further investigate the effects of pretreatments on hydrogen production, the data in Figure 6 are fitted on the G

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Figure 6. Cumulative hydrogen volumes from cotton stalk (A) and boll (B) after 72 h (gray bars) and 168 h (white bar) anaerobic dark fermentation.

Table 2. Hydrogen Production Characteristics Based on Modified Gompertz Equation Together with Residual VFAs Concentrations in Samples after 168 h Dark Fermentation material

pretreatment condition

Rmaxa (mL/h)

λ (h)b

R2c

acetic acid (mg/L)

butyric acid (mg/L)

propionic acid (mg/L)

stalk

untreated 4% (w/w) NH3 pretreated at 80 °C for 12 h NaOH pretreated at 0 °C for 60 min

0.03 0.17 0.13

21.2 23.2 26.1

0.975 0.973 0.981

50 244 231

30 105 103

23 61 50

boll

untreated 4% (w/w) NH3 pretreated at 80 °C for 12 h NaOH pretreated at 100 °C for 10 min

0.05 0.29 0.26

19.6 17.3 12.0

0.974 0.974 0.985

64 241 253

37 102 109

25 62 68

a

Maximum hydrogen production rate. bLag-phase time. cCorrelation coefficient.

under appropriate conditions. The pretreatment resulted in a significant decrease of the lignin content and cellulose crystallinity, which could be responsible for the improvements in biogas and biohydrogen production. Alkaline pretreatment of cotton boll and stalk led to yields of 246.4 and 218.9 mL of methane/g of volatile solids, which correspond to enhancements by 178% and 171%, respectively, when compared to the untreated feedstocks.

basis of eq 1, and the hydrogen production characteristics are presented in Table 2. The specific hydrogen production rate from cotton stalk and boll is increased subsequent to both pretreatments. The maximum specific hydrogen production rates, 0.29 and 0.17 mL/h, are observed upon treating cotton stalk and boll, respectively, by 4% (w/w) ammonia. The shortest lag phase durations, 21.2 and 12.0 h, are demonstrated by the untreated stalk and sodium hydroxide-pretreated boll, respectively. The final VFAs concentrations values of the untreated and pretreated samples after fermentation are presented in Table 2. The main metabolites remained after fermentation are acetic acid, butyric acid, and propionic acid. Acetic acid is the dominant VFA in all materials in the range of 50−253 mg/L, butyric acid stands second with the range 30−109 mg/L, and propionic acid is ranked the last VFA having the range 23−68 mg/L. In previous studies, it has been observed that at slightly acidic pH (between 4.5 and 6), acetic acid and butyric acid are the dominant VFAs, while at neutral and higher pH values, ethanol and propionic acid are the main byproducts in hydrogen production using anaerobic dark fermentation by mixed cultures.49 In the current study, pH has been adjusted to 5.5, and as expected, acetic acid and butyric acid comprise 77− 85% of the total remaining VFAs.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +98 311 3915632. Fax: +98 311 3912677. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION In this work, the yield of biogas production from cotton stalk and boll as well as the yield of hydrogen production has been considerably enhanced after an alkaline pretreatment step H

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