Pretreatment of rice straw by ultrasound-assisted Fenton process

Accepted Manuscript Short Communication Pretreatment of rice straw by ultrasound-assisted Fenton process Zi-Yao Xiong, Yuan-Hang Qin, Jia-Yu Ma, Li Yang, Zai-Kun Wu, Tie-Lin Wang, Wei-Guo Wang, Cun-Wen Wang PII: DOI: Reference:

S0960-8524(16)31783-7 http://dx.doi.org/10.1016/j.biortech.2016.12.105 BITE 17481

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

13 November 2016 27 December 2016 28 December 2016

Please cite this article as: Xiong, Z-Y., Qin, Y-H., Ma, J-Y., Yang, L., Wu, Z-K., Wang, T-L., Wang, W-G., Wang, C-W., Pretreatment of rice straw by ultrasound-assisted Fenton process, Bioresource Technology (2016), doi: http:// dx.doi.org/10.1016/j.biortech.2016.12.105

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Pretreatment of rice straw by ultrasound-assisted Fenton process Zi-Yao Xiong a, Yuan-Hang Qin a,*, Jia-Yu Ma a, Li Yang b, Zai-Kun Wu b, Tie-Lin Wang b, Wei-Guo Wang b, Cun-Wen Wang b a

Key Laboratory of Green Chemical Process of Ministry of Education, School of

Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430205, China b

Key Laboratory of Novel Reactor and Green Chemical Technology of Hubei

Province, School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430205, China *E-mail: [email protected] (Y.-H. Qin); Tel.: +86 27 87194882; Fax: +86 27 87194882. Abstract: Fenton’s reagent, ultrasound, and the combination of Fenton’s reagent and ultrasound were used to pretreat rice straw (RS) to increase its enzymatic digestibility for saccharification. The characterization shows that compared with ultrasound, Fenton’s reagent pretreatment was more efficient in increasing the specific surface area and decreasing the degree of polymerization (DP) of RS. The enzymatic hydrolysis results showed that the RS pretreated by ultrasound-assisted Fenton’s reagent (U/F-RS), which exhibited the largest specific surface area and the lowest DP value, had the highest enzymatic activity, and the amount of reducing sugar released from U/F-RS at 48 h of enzymatic saccharification is about 4 times as large as that from raw RS and 1.5 times as large as that from Fenton’s reagent pretreated RS. The ultrasound-assisted Fenton process provides a reliable and effective method for RS pretreatment. Keywords: rice straw; Fenton reaction; ultrasound; pretreatment 1. Introduction Bioethanol produced from lignocellulosic biomass is one of the most promising

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alternatives for fossil fuels. Currently, the cost of bioethanol production from lignocellulosic biomass is relatively high, largely due to the low yield and high cost of the enzymatic hydrolysis process resulting from the recalcitrant structure of lignocellulose (Zhu et al., 2015). Various pretreatment technologies have been developed to facilitate the enzyme hydrolysis of lignocellulose by converting the recalcitrant lignocellulose to reactive cellulosic intermediates (Rabemanolontsoa & Saka, 2016; Sun et al., 2016). Among the various pretreatment technologies, biological pretreatment is probably the most economical one. Researches have confirmed that in biological pretreatment microorganisms such as brown- and soft-rot fungi could produce cellobiose dehydrogenase to generate OH radicals through Fenton reaction (Fe2+ + H2O2 → Fe3+ + ·OH + OH-) for lignocellulose degradation (Arantes et al., 2012). Although Fenton-based biological pretreatment bears the advantages of mild reaction conditions and low energy consumption, it is low efficient and time consuming, largely due to the low concentration of OH radicals generated during the pretreatment process. Therefore, intensification of Fenton-based pretreatment of lignocellulose deserve further investigation. The pretreatment of lignocellulose by directly using Fenton’s reagent has received great attention recently because it is considered to be an environmentally benign process that does not require high temperature, high pressure and high concentrations of chemicals (He et al., 2015; Jung et al., 2015; Kato et al., 2014). Recently, the ultrasonic pretreatment of lignocellulose has attracted considerable attention (Zhang et al., 2013). Ultrasound produces its effects mainly through cavitation, and the main chemical effects of cavitation include the decomposition of water molecules into extremely reactive radicals such as ·OH and ·H, which can aid

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in cleaving lignocellulose, resulting in decreased crystallinity of lignocellulose and increased surface area (Velmurugan & Muthukumar, 2012). Cavitation can also introduce physical effects such as intense shear forces, shock waves and microjets, which can also facilitate cleavage of lignocellulose (Bussemaker et al., 2013). Combinational lignocellulose pretreatment process presents a promising approach to overcome the drawbacks demonstrated by various individual pretreatment processes, by increasing efficiency of sugar production, decreasing formation of inhibitors and/or shortening process time (Zhang et al., 2016). Given that the lignocellulose pretreatment can be implemented by both Fenton’s reagent and ultrasound, ultrasound-assistant Fenton process may provide a promising alternative pretreatment process for lignocellulose pretreatment because the process can provide a large amount of hydroxyl radicals in situ and at the same time enhance the contact between the substantial radicals and hydrocarbons in lignocellulose, thereby increasing the pretreatment efficiency. In this work, rice straw (RS), the most attractive low cost feedstock for bioethanol production, was subjected to three pretreatment procedures: Fenton’s reagent, ultrasound, and ultrasound-assisted Fenton’s reagent. The changes in morphology, composition and structure of RS after pretreatment were characterized and the effect of the pretreatment was evaluated by performing the enzymatic saccharification test. 2. Methods 2.1. Materials RS from Hubei province, China was grinded to pass through a 120-mesh sieve, washed thoroughly with deionized water and then air dried at 105 °C until a constant weight. H2O2, FeSO4·7H2O, H2SO4, NaOH, HCl, tetracycline, and citric acid monohydrate of analytical grade were purchased from Sinopharm Chemical Reagent

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Co., Ltd. All solutions were prepared in deionized water. Commercial cellulase (≥15000 U/g, Sinopharm Chemical Reagent Co., Ltd.) was used for enzymatic hydrolysis. 2.2. Pretreatment of rice straw Fenton’s reagent pretreatment: 50 mL of 88 mmol of H2O2 solution and 50 mL of 2 mmol of FeSO4·7H2O solution both with a pH value of 2.5 adjusted by H2SO4 were prepared separately. 5 g of RS was added to a magnetically stirred jacket beaker (200 mL) kept at 25 °C with circulating water, and then 50 mL of H2O2 solution was added to the beaker, to which 50 mL of FeSO4·7H2O solution was added dropwise during about 1 h. Ultrasound pretreatment: a RS suspension (5 g in 100 mL water) in a magnetically stirred jacket beaker (200 mL) was ultrasonicated with a horn-type ultrasonic processor (VOSHIN-501D, Wuxi Voshin Instruments Co., Ltd.) operated at a frequency of 22 kHz. The ultrasonication with an on-time of 2 s and an off-time of 4 s was transferred through a titanium cylindrical horn submerged in the RS suspension. The temperature of the suspension was kept at 25 °C with circulation of water bath. Ultrasound-assisted Fenton’s reagent pretreatment: 5 g of RS in a magnetically stirred jacket beaker (200 mL) was simultaneously subjected to Fenton’s reagent and ultrasound. To investigate the effect of ultrasonication power on the ultrasoundassisted Fenton’s reagent pretreatment, different ultrasonication power (200, 300, 400, 500, and 600 W) was applied to the Fenton’s reagent pretreatment. In each case, after 3 h of pretreatment, the resultant suspension was filtrated and the obtained residual (pretreated RS) was washed sequentially with 5% oxalic acid solution and deionized water, and then dried at 105 °C for 48 h. Raw RS, Fenton’s reagent pretreated RS, ultrasound pretreated RS, and ultrasound-assisted Fenton’s

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reagent pretreated RS were denoted as R-RS, F-RS, U-RS, and U/F-RS, respectively. 2.3. Enzyme hydrolysis 0.0275 g of cellulase and 0.02 g tetracycline were added to an Erlenmeyer flask containing 50 mL of citric acid-NaOH buffer with a pH value of 4.8, then 1 g of the substrate (R-RS, F-RS, U-RS, or U/F-RS) was added to the above solution, which was then incubated in a shaker bath at 50 ºC and 150 rpm for 48 h. During the incubation period, 1 mL of the medium was taken every 12 h by using a syringe with a 0.22-µm membrane for reducing sugar analysis. 2.4. Analytical methods After pretreatment, the yield of the obtained residual (pretreated RS), defined as the mass ratio of the pretreated RS to the raw RS, was determined gravimetrically. The contents of cellulose, hemicellulose and acid-insoluble lignin in the raw and pretreated RS samples were determined according to the Van Soest method (Liu et al., 2014). The morphologies of RS samples were characterized by a JEOL JSM-5510LV Scanning Electron Microscope (SEM). The specific surface area and total pore volume of RS samples were determined by a Micromeritics ASAP 2460 surface area and porosimetry analyzer. The average degree of polymerization (DP) of RS was determined (25 °C) by copper ethylenediamine (CED) solution method (ISO 5351: 2010), and triplicate runs were carried out. X-ray diffraction (XRD) patterns of RS were recorded on a Bruker D8 Advance X-ray diffractometer using Cu Ka radiation generated at 40 kV and 40 mA. The crystallinity index of RS was calculated according to the conventional peak intensity method (Zhang et al., 2016). The amount of reducing sugar released by enzymatic hydrolysis was determined by the dinitrosalicylic

acid

(DNS)

method

(El-Zawawy

et

al.,

2011)

spectrophotometer (UV-9000S, Shanghai Metash Instruments Co., Ltd.).

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with

a

3. Results and discussion 3.1. Morphology of rice straw The SEM image (Supplementary Fig. S1) of the intact fibers of R-RS shows that the structure of which is highly crystalline and rigid. Compared with R-RS which has a relatively smooth surface, the F-RS exhibits a relatively rough surface containing voids and cracks, which can be attributed to the action of the hydroxyl radicals produced from Fenton reaction. It is recognized that hydroxyl radicals can attack the lignocellulosic cell wall constituents at close proximity, causing disruption of the lignocellulose matrix with internal cleavage of cellulose chains and lignin modification (Arantes et al., 2012). The crystalline structure of U-RS (pretreated at 400 W) fibers is disrupted and microfibrils are visible. The disrupted structure of URS can be attributed to the cavitation effect. Cavitation can generate high temperature and pressure, which may provide a supercritical/subcritical environment as well as extremely reactive OH radicals for RS pretreatment. In addition, cavitation can generate intense shear forces, shock waves and microjets, which can cleave RS and modify the surface structure of RS. The image of U/F-RS (pretreated at 400 W) shows that some of the covering materials on the surface of RS are removed, resulting in more open structures with visible exposure of microfibrils. The exposed surface of U/F-RS is expected to enhance the adsorption of cellulase and allow a better access of cellulase to RS. 3.2. Composition of rice straw Compositional changes of RS after pretreatment were determined and the results are shown in Table 1. It can be seen that the total content of cellulose, hemicellulose and acid-insoluble lignin of R-RS is 86.0%, and all the pretreated RS samples have a yield close to 93%, indicating that the investigated pretreatment processes cannot

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severely degrade RS. The F-RS and U/F-RS have low hemicellulose and lignin contents as compared with R-RS, while the composition of U-RS is similar to that of R-RS. We can reasonably conclude that the ultrasound pretreatment may degrade RS in a non-selective manner while the Fenton’s reagent may degrade RS in a selective way, that is, the Fenton’s reagent may preferentially degrade hemicellulose and lignin in RS while the ultrasound can degrade cellulose, hemicellulose and lignin simultaneously. Therefore, F-RS has lower hemicellulose and lignin contents while the contents of cellulose, hemicellulose and lignin in U-RS are close to those in R-RS. It is reported that hemicellulose and lignin can impede the enzymatic hydrolysis by blocking the access of cellulase as well as unproductive binding with the cellulase, therefore the removal of hemicellulose and lignin could increase the pore size and accessible surface of RS and enhance the cellulase accessibility to cellulose, resulting in increased saccharification rate (Karimi & Taherzadeh, 2016; Sindhu et al., 2016). 3.3. Specific surface area and pore volume of rice straw It can also be seen from Table 1 that both the specific surface area and pore volume show increase for pretreated RS samples. This increase is well correlated with the partial breakdown of the microstructure of the RS, which leads to the formation of internal voids. The existence of such an internal porosity would be desirable because it would allow better access of the enzymes to the inner of the RS particles. In addition, the higher surface area of the pretreated samples would enhance interfacial interactions between RS and cellulase, leading to improved saccharification (Nitsos et al., 2013). The largest specific surface area and pore volume values demonstrated by U/F-RS can be attributed to the combinational effect of ultrasound and Fenton’s reagent. Ultrasound in the heterogeneous RS suspension can degrade RS as well as increase the mass transfer within the suspension, which can enhance the accessibility

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of Fenton’s reagent to RS and improve the pretreatment efficiency of Fenton process. 3.4. XRD and FT-IR analyses of rice straw XRD analyses show that the raw and the pretreated RS samples have similar XRD patterns (Supplementary Fig. S2), indicating that the structure of RS after pretreatment was not disrupted significantly. The calculated crystallinity index (ICR) values are 36.6%, 41.5%, 38.2% and 38.5% for R-RS, F-RS, U-RS, and U/F-RS, respectively. Based on the above composition analysis it can be concluded that the change in the crystallinity index is primarily due to removal of lignin and hemicellulose in the RS, rather than to structural change in the cellulose fibers. The U-RS presents the smallest crystallinity index among the pretreated RS samples, which may result from the non-selective degradation of RS under ultrasound, which is consistent with the above composition analysis. Based on the observed small change of crystallinity index we can reasonably conclude that in the present case both ultrasonication and OH radicals can only disrupt the micro-fibrous structure of RS but cannot disrupt the supramolecular structure of RS (Zhang et al., 2013). FT-IR analysis also demonstrates that raw and pretreated samples have similar spectra (Supplementary Fig. S3), indicating that the composition of RS after pretreatment was not significantly changed, which is consistent with the XRD and composition analyses. 3.5. DP of rice straw The DP values of R-RS, F-RS, U-RS and U/F-RS determined by the CED method are 231±1.53, 107±0.57, 219±1.52 and 83±0.57, respectively. The decreased DP of FRS can be attributed to the effect of hydroxyl radical, which could abstract a proton and generate a carbon-centered radical at either the C-3 or C-4 carbon atom, and this unstable radical species can undertake a variety of pathways, such as ring expansion, fragmentation, ring opening or even complete oxidation to glyoxal (Jain &

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Vigneshwaran, 2012). The further decrease of DP observed on U/F-RS can be attributed to the combined effect of ultrasound and Fenton’s reagent, which could provide a large amount of hydroxyl radicals in situ and at the same time enhance the contact between the radicals and hydrocarbons in RS, thereby increasing the pretreatment efficiency. In addition, ultrasound could improve the recycling of Fe2+ by dissociating the intermediate complex Fe-OOH (Fe-OOH2+))) → Fe2+ + ·O2H), and the recycled Fe2+ can in turn react with H2O2 to generate more hydroxyl radicals for RS degradation (Pradhan & Gogate, 2010). 3.6. Enzymatic hydrolysis It can be seen from Fig. 1 that the concentration of reducing sugar released from U-RS is close to that from R-RS, although U-RS has a higher specific surface area and lower DP value. The much higher concentration of reducing sugar released from F-RS can be mainly attributed to the higher specific surface area and lower DP of FRS, and the lower hemicellulose and lignin content may also contribute to the enhanced saccharification. The reducing sugar released from U/F-RS, as expected, has the highest concentration over the entire enzymatic time investigated. The concentration of reducing sugar released from U/F-RS at 48 h of enzymatic saccharification is about 4 times as large as that from R-RS (6.93 vs. 1.78 g L-1) and 1.5 times as large as that from F-RS (6.93 vs. 4.53 g L-1). Since the hemicellulose and lignin contents in U/F-RS are close to those in F-RS, the high enzymatic activity observed on U/F-RS can be attributed to the higher specific surface area and lower DP of U/F-RS. Compared with the conventional Fenton’s reagent process which usually takes dozens of hours (Jung et al., 2015), the ultrasound-assisted Fenton process only takes 3 h to obtain the similar results of enzymatic saccharification. It is worth noting that the ultrasound was pulsed in a 1:3 duty cycle, so it is energy efficient since the

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actual ultrasonication time is only 1 h. To investigate the effect of ultrasonic power on the Fenton’s reagent pretreatment of RS, different ultrasonication power was applied (Supplementary Fig. S4). The results that the RS pretreated by ultrasonication-assisted Fenton process operated at 400 W has the largest reducing sugar concentration. Ultrasonication operated at lower power on the one hand may have limited degradation effect on RS, and on the other hand may have limited promotion effect on Fenton’s reagent. Although ultrasonication operated at higher power could effectively degrade RS, the strong cavitation effect could also decompose H2O2, leading to decreased pretreatment efficiency of Fenton’s reagent. 4. Conclusions RS samples were exposed to Fenton’s reagent, ultrasound, and ultrasoundassisted Fenton’s reagent. The characterization shows that Fenton’s reagent pretreatment was more efficient in increasing the specific surface area and decreasing the DP of RS than ultrasound. With the combination of ultrasound and Fenton’s reagent, the enzymatic hydrolysis of RS was improved in comparison with ultrasound alone and Fenton’s reagent alone and thus the combined ultrasound and Fenton’s reagent process was proved to be a reliable and effective method for RS pretreatment. Acknowledgements The present study was supported by the National Natural Science Foundation of China (21306144). References [1] Arantes, V., Jellison, J., Goodell, B. 2012. Peculiarities of brown-rot fungi and biochemical Fenton reaction with regard to their potential as a model for bioprocessing biomass. Appl. Microbiol. Biot. 94(2), 323-338.

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[2] Bussemaker, M.J., Xu, F., Zhang, D. 2013. Manipulation of ultrasonic effects on lignocellulose by varying the frequency, particle size, loading and stirring. Bioresour. Technol. 148(0), 15-23. [3] El-Zawawy, W.K., Ibrahim, M.M., Abdel-Fattah, Y.R., Soliman, N.A., Mahmoud, M.M. 2011. Acid and enzyme hydrolysis to convert pretreated lignocellulosic materials into glucose for ethanol production. Carbohyd. Polym. 84(3), 865-871. [4] He, Y.-C., Ding, Y., Xue, Y.-F., Yang, B., Liu, F., Wang, C., Zhu, Z.-Z., Qing, Q., Wu, H., Zhu, C., Tao, Z.-C., Zhang, D.-P. 2015. Enhancement of enzymatic saccharification of corn stover with sequential Fenton pretreatment and dilute NaOH extraction. Bioresour. Technol. 193, 324-330. [5] Jain, P., Vigneshwaran, N. 2012. Effect of Fenton’s pretreatment on cotton cellulosic substrates to enhance its enzymatic hydrolysis response. Bioresour. Technol. 103(1), 219-226. [6] Jung, Y.H., Kim, H.K., Park, H.M., Park, Y.-C., Park, K., Seo, J.-H., Kim, K.H. 2015. Mimicking the Fenton reaction-induced wood decay by fungi for pretreatment of lignocellulose. Bioresour. Technol. 179(0), 467-472. [7] Karimi, K., Taherzadeh, M.J. 2016. A critical review on analysis in pretreatment of lignocelluloses:

Degree

of

polymerization,

adsorption/desorption,

and

accessibility. Bioresour. Technol. 203, 348-56. [8] Kato, D.M., Elía, N., Flythe, M., Lynn, B.C. 2014. Pretreatment of lignocellulosic biomass using Fenton chemistry. Bioresour. Technol. 162(0), 273-278. [9] Liu, L., Ju, M., Li, W., Jiang, Y. 2014. Cellulose extraction from Zoysia japonica pretreated by alumina-doped MgO in AMIMCl. Carbohyd. Polym. 113, 1-8. [10] Nitsos, C.K., Matis, K.A., Triantafyllidis, K.S. 2013. Optimization of hydrothermal pretreatment of lignocellulosic biomass in the bioethanol

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production process. ChemSusChem 6(1), 110-122. [11] Pradhan, A.A., Gogate, P.R. 2010. Degradation of p-nitrophenol using acoustic cavitation and Fenton chemistry. J. Hazard. Mater. 173(1), 517-522. [12] Rabemanolontsoa, H., Saka, S. 2016. Various pretreatments of lignocellulosics. Bioresour. Technol. 199, 83-91. [13] Sindhu, R., Binod, P., Pandey, A. 2016. Biological pretreatment of lignocellulosic biomass – An overview. Bioresour. Technol. 199, 76-82. [14] Sun, S., Sun, S., Cao, X., Sun, R. 2016. The role of pretreatment in improving the enzymatic hydrolysis of lignocellulosic materials. Bioresour. Technol. 199, 49-58. [15] Velmurugan,

R.,

Muthukumar, K. 2012.

pretreatment of sugarcane

bagasse

for

Ultrasound-assisted

fermentable

sugar

alkaline

production:

Optimization through response surface methodology. Bioresour. Technol. 112(0), 293-299. [16] Zhang, M.-F., Qin, Y.-H., Ma, J.-Y., Yang, L., Wu, Z.-K., Wang, T.-L., Wang, W.G., Wang, C.-W. 2016. Depolymerization of microcrystalline cellulose by the combination of ultrasound and Fenton reagent. Ultrason. Sonochem. 31, 404-408. [17] Zhang, Q., Benoit, M., De Oliveira Vigier, K., Barrault, J., Jegou, G., Philippe, M., Jerome, F. 2013. Pretreatment of microcrystalline cellulose by ultrasounds: effect of particle size in the heterogeneously-catalyzed hydrolysis of cellulose to glucose. Green Chem. 15(4), 963-969. [18] Zhu, S., Huang, W., Huang, W., Wang, K., Chen, Q., Wu, Y. 2015. Pretreatment of rice straw for ethanol production by a two-step process using dilute sulfuric acid and sulfomethylation reagent. Appl. Energ. 154, 190-196. Figure captions Fig. 1 Reducing sugars released from raw and pretreated rice straw samples.

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Table 1 Yield, composition and textural characteristics of rice straw samples Sample

R-RS

Yield Cellulose Hemicellulose Acid-insoluble Surface area a Pore volume b (%) (%) (%) lignin (%) (m2 g-1) (cm3 g-1) 100 46.4±0.53 31.4±0.60 8.2±0.06 1.1631 ± 0.003647

0.0429 2.1177 ± 0.0204 U-RS 93.3 45.4±0.40 29.4±0.15 7.6±0.20 1.3659 ± 0.0457 U/F-RS 92.7 49.4±0.40 21.6±0.35 6.6±0.10 2.3854 ± 0.0252 a Measured by using the multipoint Brunauer-Emmett-Teller method. b Measured at P/P0=0.95. F-RS

93.2

49.2±0.15

22.0±0.21

7.2±0.35

13

0.006920 0.005078 0.008645

14


Fenton’s reagent can preferentially degrade hemicellulose and lignin.
Ultrasound can degrade cellulose, hemicellulose and lignin simultaneously.
Both ultrasound and Fenton’s reagent can increase the surface area of rice straw.
The highest concentration of reducing sugar was obtained from the U/F-RS.

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