Synergetic Degradation of Corn Cob with Inorganic Salt (or Hydrogen

Research Article pubs.acs.org/journal/ascecg

Synergetic Degradation of Corn Cob with Inorganic Salt (or Hydrogen Peroxide) and Electron Beam Irradiation Xiaoya Guo,*,† Sitao Shu,† Wei Zhang,† Enze Wang,† and Jinyu Hao‡ †

Department of Chemical Engineering, Shanghai University, Shanghai 200444, China SGS-CSTC Standards Technical Services (Shanghai) Co. Ltd., Shanghai 200233, China

‡

ABSTRACT: This study focused on the influence of inorganic salt (MnCl2, FeCl3, NaHCO3) or H2O2 combined with electron beam irradiation (EBI) at 90, 180, and 270 kGy on the content and structure of the three major components (cellulose, hemicellulose, and lignin) of corn cob in the pretreatment process. Acemonium cellulase (10 FPU/g of corn cob) were used to hydrolyze the pretreated samples for 96 h. The results indicated that the combined methods showed an obvious synergetic effect on the removal of hemicellulose and lignin and the reduction of degree of polymerization (DP) of cellulose, while the cellulose recovery decreased slightly during the treatment. Particularly, at the optimum conditions (2% NaHCO3 with EBI at 180 kGy), the highest 70.5% hemicellulose and 34.7% lignin removal were achieved with the DP of cellulose decreasing from 1081 of raw to 82. The results of Fourier-transform infrared spectra (FTIR) and scanning electron microscopy (SEM) analysis showed that the IR crystallinity index of cellulose decreased, and the structure was disrupted deeply after pretreatment due to the effective removal of the amorphous zone. After hydrolyzing the sample under optimum pretreatment conditions, the glucose yield was significantly higher than that of the controlled sample, which proved that an inorganic salt solution combined with EBI is an effective way to reduce the recalcitrance of lignocellulose biomass and improve the production of glucose. KEYWORDS: Lignocellulosic biomass, Electron beam irradiation, Inorganic salt, Cellulose, Degree of polymerization, Enzymatic hydrolysis, Glucose

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pollution, which limit their application.16−18 During the past years, EBI and ray irradiation technology have been well developed and applied to the degradation of lignocellulose as an environmentally friendly and convenient method. Irradiation leads to the cleavage of cellulose and reduction of its molecular weight and crystallinity, which makes it easier to be hydrolyzed and improve the sugar yield at high doses.14−23 According to Driscoll, the relative crystallinity of the microcrystalline cellulose was reduced from 87% to 45%, and the available surface area increased from 274 m2/g for the controlled sample (0 kGy) to 318 m2/g at a dose of 1000 kGy.24 Karthika once reported that 79% of the final reducing sugar was released within 48 h of hydrolysis at an enzyme loading rate of 30 FPU/ g of biomass, and significant improvements of glucose yields were observed in the hydrolysate of EBI pretreated biomass at 250 kGy compared to the controlled samples.15 Combining of different pretreatment methods was also put forward by some researchers because of their good synergetic effect and energy saving.25−28 In view of this, we focused on studying combining inorganic salt or H2O2 with EBI to pretreat corn cobs in order to obtain a good synergetic effect in this study, which tends to

INTRODUCTION In view of the increasing global energy demands and concern about the sustainability of the environment worldwide, many countries have been devoted to looking for a pollution-free, high-yield, and low-cost renewable resource. Lignocellulosic biomass has attracted more and more attention due to its high percentage (about 30−50%) of cellulose, which could be hydrolyzed to glucose and fermented to biofuels. However, hemicellulose and lignin connect with cellulose tightly by complex physical and morphological structures, forming complex cross-linking, which makes it difficult to be hydrolyzed.1 As a result, lignocellulosic biomass must be pretreated to reduce the recalcitrance of the complex structure and to improve the hydrolysis rate and efficiency. In recent years, a lot of researchers have focused on many pretreatment methods such as mechanical fragmentation,2 steam explosion,3 acid and alkali hydrolysis,4−6 oxidative delignification,7,8 biological pretreatment,9 ionic liquid pretreatment,10,11 hydrothermal pretreatment,12,13 etc. All of these methods tend to reduce the content of hemicellulose or lignin and decrease the degree of polymerization as well as the crystallinity of cellulose, resulting in the rise of sugar yields after enzymatic or acid hydrolysis.14,15 However, these pretreatment methods also have their demerits due to their operational conditions, such as high pressure, high energy, high demands for devices, and heavy © XXXX American Chemical Society

Received: September 27, 2015 Revised: January 25, 2016

A

DOI: 10.1021/acssuschemeng.5b01168 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering be more energy saving, economic, environmentally friendly, and less inhibitor produced compared with many other pretreatment methods.

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Glucose yield (% theoretical maximum) g of glucose released = × 100 g of glucan × 1.1

EXPERIMENTAL METHODS

where 1.1 is the conversion factor of glucan to glucose. Analytical Methods. The cellulose, hemicellulose, lignin content were measured by detergent extraction method according to Van Soest fiber analysis system.29 The glucose yield after the enzymatic hydrolysis was determined by Exformma 1600 HPLC (Shanghai Wufeng Scientific Instruments Company., Ltd. China) using the Aminex HPX-87P column (300 mm × 7.8 mm, Bio-Rad, America).31 The cellulose recovery, hemicellulose and lignin removal after pretreatment were calculated using the following equations:

Materials. Corn cob, purchased from Gaotang Technology Company., Ltd. (Shandong, China), was washed with distilled water to remove dust and dried at 105 °C for 2 h. Then it was crushed and sieved to collect 1 mm pieces. The sample compositions were composed of 43.5% cellulose, 37.9% hemicellulose, and 16.5% lignin (dry weight), according to Van Soest fiber analysis system.29 All of the chemical reagents such as MnCl2, FeCl3, NaHCO3, H2O2, benzene, ethanol, sodium hypochlorite, etc. used in experiment were analytical grade. Pretreatment. Prior to the EBI treatment, 5 g of corn cob (dry weight) were added to 50 g of 2% inorganic salt (MnCl2, FeCl3, NaHCO3) or H2O2 solution in an airtight glass bottle (150 mL) separately. The control sample was prepared by adding 5 g of corn cob to 50 g of deionized water to study the interference of water. After soaking at room temperature for 10 h, the samples were then treated by EBI at 90, 180, and 270 kGy. The irradiation was carried out by an electron accelerator (K-400, 400 kV, 1 mA, Ray Institute of Shanghai University) in open air in a stainless steel tray. After irradiation, the solids and liquor were separated by filtering. The solids were washed with deionized water to pH 7 and then dried at 105 °C for 2 h to obtain pretreated corn cobs. Cellulose Separation and analysis. The scheme for cellulose separated from corn cob is depicted in Figure 1. The separated

R recovery , x(%) =

Wcc,x Wr,x

R removal , x(%) = 1 −

Wcc,x Wr,x

where Wcc,x is the amount of cellulose (hemicellulose or lignin) in the pretreated corn cob, and Wr,x is the amount of cellulose (hemicellulose or lignin) in the raw corn cob. The changes in the functional groups of cellulose before and after the pretreatment were analyzed by Avatar370 FT-IR (Shanghai Thermo Fisher Scientific Company., Ltd.); 10 mg pretreated corn cobs were pelleted with KBr. IR spectra was recorded between 4000 and 400 cm−1 at 1 cm−1 resolution and 32 scans per sample. The IR crystallinity was calculated from the intensity of absorption of IR bands as A1372/A2900.32 The structural and surface morphology of raw and pretreated corn cobs were analyzed using SEM (Merlin Compact, Zeiss, Germany) at 10 kV accelerating voltage and a magnification of 1000. Before analysis, the samples were sputter-coated with a thin layer of gold.

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RESULTS AND DISCUSSION Influence of Irradiation Dose and Solution Type on Content of Major Components. To investigate the effect of irradiation dose and salt type on the content of major components, corn cobs were treated with 2% (w/w) inorganic salt (or hydrogen peroxide) solution at 0−270 kGy. Deionized water was used in the blank experiment instead of solution. Table 1 shows that hydrogen peroxide and all salts with a concentration of 2% increased the removal rate of hemicellulose and lignin, while cellulose recovery changed insignificantly. The hemicellulose and lignin removal increased with an increase in irradiation dose in FeCl3 and MnCl2 solution combined with EBI pretreatment, and the highest removal of hemicellulose and lignin were achieved at 270 kGy. FeCl3 gave the maximum hemicellulose removal of 26.1%, while MnCl2 gave the maximum lignin removal of 28.8%. The same general trend was found with NaHCO3 and H2O2 solution, but there were two exceptional cases that occurred at 180 kGy. NaHCO3 produced the largest hemicellulose removal (70.5%) and lignin removal (34.7%) at 180 kGy instead of 270 kGy, and the hemicellulose and lignin removals at 180 kGy were lower than that at 90 kGy when treated with EBI in the presence of H2O2, possibly owing to the crosslinking reaction generated during the irradiation. When lignocelluloses were exposed to the electron beam, similar to exposure to γ -ray, the radicals were produced by random chain scission.23,33 As for H2O2 solution, H2O2 decomposed to oxygen and hydroxyl radicals when it was exposed to the electron beam. Carbon radicals generated during irradiation are capable of interacting rapidly with oxygen molecules to produce peroxy radical intermediates. A graft reaction can be initiated by

Figure 1. Scheme for cellulose separation from corn cob.

cellulose was then dried at 105 °C for 2 h for further analysis. The intrinsic viscosity η of the cellulose was measured at 25 °C with a capillary tube viscometer using cuproethy lenediamine (CED) as solvent according to the the Martin equation (Scandinavian Standard SCAN-CM 15:99 and ISO 5351:2012), where DP was calculated as eq 1.30 DP0.905 = 0.75[η]

(1)

Enzymatic Hydrolysis. Ten FPU of acremonium cellulase was used to hydrolyze the pretreated corn cobs (1 g) in 50 mM citrate buffer (pH 5) at 10% (w/v) substrate consistency in triplicate following NREL standard procedures.31 The hydrolysis experiments were conducted at 50 °C on a rotary shaker at 200 rpm; 0.2 mL of hydrolysate was collected at certain time intervals (24, 48, 72, 96 h respectively) and centrifuged at 10,000 rpm for 5 min to obtain supernatant. The supernatant was then used for analysis of glucose content. The enzyme digestibility was expressed as the percentage of the theoretical maximum amount of glucose obtained from the corn cob as follow:32 B


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reinitialized to the protonation of the oxygen of ether bond between the sugar monomers.39 When treated with the combined method, after 10 h immersion in inorganic salt solution, parts of the cellulose, lignin, and hemicellulose were removed. For example, hemicellulose removal rate was 5.6%, and lignin removal rate was 8.3% after soaking in MnCl2 for 10 h. The removal of major components of lignocellulose indicated that the structure of lignocellulose may alter, and as shown in Figure 5, the tissue of corn cob is looser and more fragile compared with the untreated sample. The alteration made it more convenient for the electron beam to rupture the complex structure of lignocellulose and more directly scission the chain of lignin or hemicellulose. The maximum hemicellulose removal (70.5%) and lignin removal (34.7%) were achieved when treated with NaHCO3 combined with 180 kGy irradiation. When treated with EBI in the presence of H2O2, the hemicellulose and lignin removal reached the maximum value of 52.7% and 42.1% at 270 kGy, respectively. Different from the mechanism of inorganic salt treatment, the degradation of lignocellulose in the presence of H2O2 could be due to the OH radical produced by the radiolysis of H2O2 and water under the irradiation; the reactions might occur as follows:35

Table 1. Effect of Different Pretreatment Conditions on Contents of Major Componentsa pretreatment conditions inorganic salt (or H2O2) EBI (kGy) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

H2O

FeCl3

MnCl2

NaHCO3

H2O2

0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270

Rremoval,x (%) hemicellulose

lignin

3.7 (0.2) 4.5 (0.3) 14.8 (3.3) 12.5 (0.3) 2.2 (0.9) 17.1 (0.3) 22.6 (0.7) 26.1 (1.3) 5.6 (0.3) 5.7 (0.9) 14.9 (0.8) 18.1 (0.9) 8.3 (0.7) 6.9 (0.4) 70.5 (3.7) 30.5 (1.7) 1.0 (0.9) 27.0 (2.1) 16.3 (2.5) 52.7 (2.9)

0.9 (0.3) 22.6 (1.5) 13.6 (0.9) 24.4 (3.7) 3.6 (0.4) 19.4 (0.7) 16.2 (1.26) 21.1 (2.6) 8.3 (0.3) 26.0 (0.3) 17.5 (0.7) 28.8 (2.1) 0.8 (0.2) 12.2 (0.6) 34.7 (3.9) 28.6 (2.6) 12.9 (1.4) 23.5 (1.6) 20.7 (0.8) 42.1 (3.1)

Rrecovery (%) cellulose 98.4 99.2 97.6 95.4 95.1 94.9 93.7 95.5 99.9 94.0 98.0 98.1 91.9 97.0 95.1 91.5 99.1 98.7 94.8 99.2

(0.7) (0.5) (1.2) (0.9) (1.2) (3.2) (0.7) (2.2) (3.1) (0.8) (1.1) (0.9) (2.7) (2.9) (0.4) (0.2) (0.9) (0.5) (1.8) (1.4)

hv

− H 2O → H 2 , H 2O2 , eaq , H•, •OH, H3O+

a

H• + H 2O2 → H 2O + •OH

All samples were performed in triplicate (standard deviations are presented in parentheses).

− eaq + H 2O2 → •OH + OH−

peroxy radical intermediates in a suitable monomer environment.23,34 Khan et al. considered that some radicals are trapped in the crystalline or semicrystalline region of the cellulose structure, and these radicals can decay through recombination reactions, which lead to cross-linking.19,23 The chain scission and cross-linking existed simultaneously during radiation.23,34 If the chain scission was predominant over cross-linking, the degradation was expected to be enhanced, which led to an increase in lignin or hemicellulose removal. Otherwise, the degradation was expected to be inhibited, and the removal rate of lignin or hemicellulose decreased. From Table 1, we can see that higher removals of lignin and hemicellulose occurred when treated with a combined method than when treated with EBI or inorganic salt alone. It is obvious that the combination of irradiation and inorganic salt (or H2O2) was effective for degradation of hemicellulose and lignin. When samples are treated with EBI alone, the degradation may be ascribed to direct chain scission of lignocellulose by irradiation.35,36 However, when treated with inorganic salt, the reaction mechanism is more complicated, which may be similar to the mechanism of hydrolytic reactions.37,38 First, the inorganic salt MnCl2, FeCl3, and NaHCO3 dissociated into complex ions in aqueous solvent and diffused into lignocellulosic matrix, then protonated the oxygen of a heterocyclic ether bond between the sugar monomers and broke the ether bond, generating carbocation as an intermediate. Second, the water molecules directly reacted with carbocation through one of the two available lone pairs of electrons, resulting in a protonated hydroxide. The proton could then be abstracted by another water molecule to form a hydronium ion. Lastly, the carbocation was solvated with water, and the regeneration of the proton with cogeneration of the ether bond present in hemicellulose, cellulose or other oligomers occurred. The reaction products diffused in the liquid phase and then

hv

H 2O2 → 2•OH R + •OH → R• + H 2O R + •H → R• + H 2 R• → R1 + R 2

Hydroxyl radical, as a highly powerful oxidizing species, can attack phenols present in lignin, thus leading to an extensive degradation of unetherified phenolic units in lignin. Hydroxyl radicals also degrade cellulose or hemicellulose by attacking at the C-2 position of the anhydroglycose units in the polysaccharide chains by the pathway depicted in Figure 2.40 A ketone in the glucose unit formed along with the cleavage of the glycoside linkage in the polysaccharide by β-elimination. Combining H2O2 with other methods has also been used to degrade dye or other substance these years. Kang and coworkers previously reported a large synergetic degradation of chitosan with γ radiation and hydrogen peroxide.35

Figure 2. Oxidative cleavage of carbohydrates by hydroxyl radicals. C


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more energy to break the chain, and then the chain scission was dominant. During inorganic salt (or hydrogen peroxide) combined with EBI pretreatment, much of the lignin and hemicellulose were degraded, and the DP of cellulose was reduced, thus making the cellulase or other chemical reagents more effective and easier to hydrolyze lignocellulose. Characterization of Cellulose Extracted from Untreated and Treated Corn Cobs. Structure of the cellulose is another interest of this work in addition to the recovery and DP. Thus, the chemical structure was analyzed with infrared analyzer. FTIR spectroscopy, which potentially offers the assignment of absorbance bands to specific molecular structures, combined with other methods can be used for the analysis of cellulose. Thus, as is shown in Figure 4, the FTIR spectra analysis of cellulose in raw and pretreated corn cobs was carried out. It confirmed the occurrence of structural changes during pretreatment. The IR band at 1377 cm−1 was attributed to C−H bending vibration, and the band at 1059 cm−1 was assigned to the C−O stretching vibration in cellulose, hemicelluloses, and lignin or C−O−C stretching in cellulose and hemicelluloses.32 The sharp band at 1165 cm−1 indicated the presence of arobinosyl side chains.41 The absorption peak at 2896 cm−1 was attributed to CH stretching vibration.42 The absorbance band at 810 cm−1 stemmed from the C−H out of plane bending of syringyl content in lignin. The existence of these bands in the untreated sample indicated that a certain amount of lignin remains in the extracted cellulose; however, lignin was removed drastically in the pretreated sample. Disappearance of lignin in cellulose extracted from treated corn cobs showed that the combined pretreatment process scattered the lignocellulose structure and removed lignin effectively. Increase in the intensity was observed in the IR bands at 897 and 1429 cm−1, which can be attributed to the ßglucosidic linkage between the sugar units and −CH2, respectively.43 These two peaks showed the distinguishing feature of cellulose, and the increase indicated that the purity of cellulose was improved after pretreatment. A little enhancement was observed in the region of 3200−3400 cm−1, which was associated with the O−H stretching of the hydrogen bonds.42

Ghodbane found that the rate of dye degradation was increased 2.4 times for a concentration of 386 mg/L of H2O2 with 1700 kHz ultrasonic irradiation.27 Effects of Pretreatment Conditions on DP of Extracted Cellulose. Degree of polymerization calculated by eq 1 is depicted in Figure 3. It shows that the DP of cellulose

Figure 3. Effects of pretreatment conditions on DP of extracted cellulose.

decreased largely when treated with the combined methods. The general trend was that the DP of cellulose decreased rapidly with an increasing irradiation dose from 0 to 90 kGy and then changed slightly in the range of dose between 90 and 270 kGy. This phenomenon was interpreted as being due to the synergy of irradiation and solutions. NaHCO3 was an exception. The DP of cellulose still decreased rapidly when dose increased from 90 kGy to 180 kGy and then decreased slowly when dose increased to 270 kGy. The DP of cellulose reached a minimum value of 68 at 270 kGy in the presence of NaHCO3. As for MnCl2, FeCl3, and H2O2, the DP of cellulose increased slightly at 180 kGy compared to that of 90 kGy, which might be ascribable to an intensive cross-linking. The cross-linking may be predominant over the chain scission at 180 kGy. The DP of cellulose continued to decrease when the dose was increased to 270 kGy because the higher dose gave

Figure 4. FTIR spectra of cellulose extracted from untreated and pretreated corn cobs: (a) untreated, (b) NaHCO3 + 180 kGy, (c) FeCl3 + 180 kGy, (d) MnCl2 + 180 kGy, (e) H2O2 + 180 kGy, and (f) H2O2 + 270 kGy). D


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Table 2. IR Crystallinity and IR Band Positions for Cellulose Extracted from Untreated and Treated Corn Cobs IR band

untreated

NaHCO3 + 180 kGy

FeCl3 + 180 kGy

MnCl2 + 180 kGy

H2O2 + 180 kGy

H2O2 + 270 kGy

δCH2, C−O−C νC−O νC−O, δOH δCH, νCOO δCH2 H2O absorbed νCH IR crystallinity

897 1059 1165 1377 1429 1633 2897 0.88

895 1068 1163 1375 1421 1633 2893 0.79

897 1066 1165 1375 1431 1637 2897 0.79

897 1059 1163 1375 1425 1635 2897 0.78

897 1065 1163 1377 1429 1633 2897 0.75

895 1059 1163 1375 1429 1633 2899 0.65

Degradation of cellulose during pretreatment led to break of ßglucosidic linkage and then more −OH groups appeared. For the cellulose extracted from corn cob irradiated in H2O2, the peak at 3200−3400 cm−1 increased with an increase in dose, which indicated that increasing dose intensified the degradation of cellulose. Other shifts and changes observed from the FTIR spectrum are shown in Table 2. In general, there was no clear change in the structure of cellulose during the pretreatment process, which indicated no chemical reaction between the solution and the cellulose and no obvious destruction occurred during the pretreatment process. However, the removal of lignin and hemicellulose had positive effects on cellulose extraction, which increased the purity of the extracted cellulose. Apart from this, the IR crystallinity of cellulose extracted from the pretreated samples was much lower than that of the controlled sample. The decrease in cellulose crystallinity and DP was greatly beneficial for improving hydrolysis efficiency. SEM Analysis. The SEM was used to study the morphology and microstructure of raw and pretreated corn cobs. Figure 5 shows the surface morphology of native and pretreated corn cobs. The raw corn cobs had intact, regular, and compact surfaces, whereas the pretreated corn cobs were rough, contorted. The surface appeared looser and more fragile, compared with the native sample. The rigidity of the corn cob decreased, and the surface contorted due to the partial removal of lignin and hemicellulose. Fiber porosity, permeability, and specific surface area increased during the combining pretreatment process, which enhanced the accessibility of the cellulose to cellulase and made the corn cob more suitable for the hydrolytic process, hence improving the glucose yield. Enzymatic Hydrolysis of Corn Cobs. In order to produce ethanol at an economically viable scale from lignocellulose materials, the rate and ultimate glucose yield must be improved. In this study, the combination of NaHCO3 and EBI produced the maximum removal of hemicellulose and lignin. Thus, hydrolysis studies of corn cobs pretreated with EBI in the absence and presence of NaHCO3 were performed to assess the synergy effect of EBI and NaHCO3 on the glucose yield, and the results are presented in Figure 6. The amount of glucose increased with an increasing dose of EBI. It was shown that a significant improvement in glucose yield was observed in the presence of NaHCO3 compared to the controlled sample. The highest glucose yield (67.6%) was obtained after 96 h of enzymatic hydrolysis of corn cob pretreated with NaHCO3 combining with 180 kGy EBI, which was 1.9 times of that of NaHCO3 soaking alone and 1.4 times of irradiation alone. Hence, the combination pretreatment method significantly facilitated higher glucose production.

Figure 5. SEM of corn cobs: (A) native raw corn cob, (B) corn cob pretreated with NaHCO3 solution, and (C) corn cob pretreated with NaHCO3 combined with 180kGy EBI.

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CONCLUSIONS Inorganic salt combined with EBI had great synergetic effect on the removal of hemicellulose and lignin. The DP of cellulose decreased largely after pretreatment. The highest hemicellulose removal (70.5%) and lignin removal (34.7%) were achieved at the optimum condition (2% NaHCO3 with EBI at 180 kGy). Simultaneously, the DP of cellulose decreased from 1081 of raw to 82, which was beneficial for further hydrolysis. No clear change in the structure of cellulose happened during pretreatE


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domains from cellulases. Bioresour. Technol. 2011, 102 (3), 2910− 2915. (10) Haykir, N. I.; Bakir, U. Ionic liquid pretreatment allows utilization of high substrate loadings in enzymatic hydrolysis of biomass to produce ethanol from cotton stalks. Ind. Crops Prod. 2013, 51 (6), 408−414. (11) Lopes, A. M. D. C.; Joao, K. G.; Rubik, D. F.; Bogel-Lukasik, E.; Duarte, L. C.; Andreaus, J.; et al. Pre-treatment of lignocellulosic biomass using ionic liquids: wheat straw fractionation. Bioresour. Technol. 2013, 142 (8), 198−208. (12) Ewanick, S.; Bura, R. 1−Hydrothermal pretreatment of lignocellulosic biomass. Bioalcohol Production. 2010, 142 (3), 3−23. (13) Stephanidis, S.; Nitsos, C.; Kalogiannis, K.; et al. Catalytic upgrading of lignocellulosic biomass pyrolysis vapours: Effect of hydrothermal pre-treatment of biomass. Catal. Today 2011, 167 (1), 37−45. (14) Sundar, S.; Bergey, N. S.; Salamanca-Cardona, L.; et al. Electron beam pretreatment of switchgrass to enhance enzymatic hydrolysis to produce sugars for biofuels. Carbohydr. Polym. 2014, 100 (2), 195− 201. (15) Karthika, K.; Arun, A. B.; Rekha, P. D. Enzymatic hydrolysis and characterization of lignocellulosic biomass exposed to electron beam irradiation. Carbohydr. Polym. 2012, 90 (2), 1038−1045. (16) Mosier, N.; Wyman, C.; Dale, B.; Elander, R.; Lee, Y. Y.; Holtzapple, M.; Ladisch, M. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 2005, 96 (6), 673−686. (17) Eggeman, T.; Elander, R. T. Process and economic analysis of pretreatment technologies. Bioresour. Technol. 2005, 96 (18), 2019− 2025. (18) Tao, L.; Aden, A.; Elander, R. T.; et al. Process and technoeconomic analysis of leading pretreatment technologies for lignocellulosic ethanol production using switchgrass. Bioresour. Technol. 2011, 102 (24), 11105−11114. (19) Yang, C.; Shen, Z.; Yu, G.; et al. Effect and aftereffect of gamma radiation pretreatment on enzymatic hydrolysis of wheat straw. Bioresour. Technol. 2008, 99 (14), 6240−6245. (20) Czayka, M.; Fisch, M. Effects of electron beam irradiation of cellulose acetate cigarette filters. Radiat. Phys. Chem. 2012, 81 (7), 874−878. (21) Takacs, E.; Wojnarovits, L.; Borsa, J.; et al. Effect of γ -irradiation on cotton-cellulose. Radiat. Phys. Chem. 1999, 55, 663− 666. (22) Jipa, I. M.; Stroescu, M.; Stoica-Guzun, A.; et al. Effect of gamma irradiation on biopolymer composite films of poly (vinyl alcohol) and bacterial cellulose. Nucl. Instrum. Methods Phys. Res., Sect. B 2012, 278 (4), 82−87. (23) Khan, F.; Ahmad, S. R.; Kronfli, E. Gamma-radiation induced changes in the physical and chemical properties of lignocellulose. Biomacromolecules 2006, 7 (8), 2303−2309. (24) Driscoll, M.; Winter, W.; Stipanovic, A.; Cheng, K.; Manning, M.; Spiese, J.; Galloway, R. A.; Cleland, M. R.; et al. Electron beam irradiation of cellulose. Radiat. Phys. Chem. 2009, 7-8, 539−542. (25) Banchorndhevakul, S. Effect of urea and urea-gamma treatments on cellulose degradation of thai rice straw and corn stalk. Radiat. Phys. Chem. 2002, 64 (5−6), 417−422. (26) Yuan, T.-Q.; You, T.-T.; Wang, W.; Xu, F.; Sun, R.-C. Synergistic benefits of ionic liquid and alkaline pretreatments of poplar wood. Part 2: Characterization of lignin and hemicelluloses. Bioresour. Technol. 2013, 136 (3), 345−350. (27) Ghodbane, H.; Hamdaoui, O. Degradation of Acid Blue 25 in aqueous media using 1700 kHz ultrasonic irradiation: ultrasound/ Fe(II) and ultrasound/H2O2 combinations. Ultrason. Sonochem. 2009, 16 (5), 593−598. (28) Peng, H.; Chen, H.; Qu, Y.; et al. Bioconversion of different sizes of microcrystalline cellulose pretreated by microwave irradiation with/without NaOH. Appl. Energy 2014, 117 (3), 142−148.

Figure 6. Effect of different pretreatment parameters on glucose yield of enzymatic hydrolysis of corn cob.

ment. The surface of the corn cob appeared looser and more fragile after pretreatment. The glucose yield of enzymatic hydrolysis of the corn cob pretreated by the combined method was much higher than that of irradiation or NaHCO3 alone. In conclusion, inorganic salt combined with EBI is an effective way to pretreat lignocellulose. In the future, EBI combined with mild chemicals at low concentration and low dose will be studied as an attractive biomass pretreatment method.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-21-66137491. Fax: 86-21-66137725. E-mail: gxy@ shu.edu.cn, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Program for Innovative Research Team of Shanghai University (No. IRT13078). REFERENCES

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Synergetic Degradation of Corn Cob with Inorganic Salt (or Hydrogen

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