Elsevier

Bioresource Technology

Volume 276, March 2019, Pages 343-348
Bioresource Technology

Lignin monomer in steam explosion assist chemical treated cotton stalk affects sugar release

https://doi.org/10.1016/j.biortech.2019.01.008Get rights and content

Highlights

  • SE assist chemistry method could positively affect biomass digestibility.

  • SE-2.4 MPa-5%NH3·H2O followed by EH could give 73.22% of biomass digestibility for CS.

  • CS rich in G- and S-monomers showed relatively high biomass saccharification.

  • High content of H-monomer exhibited a certain inhibitory effect in sugar release.

Abstract

In this study, the fermentable sugar released from cotton stalk (CS), which were pretreated by instant catapult steam explosion (SE) combined with different concentrations of strong monobasic acid (HCl), weak monobasic acid (CH3COOH), strong monobasic alkali (NaOH) and weak monobasic alkali (NH3·H2O), followed by hydrolysis in cellulase/xylanase mixed enzyme solutions, were comparably investigated. The highest yield of 73.22% of fermentable sugar yield was obtained in SE-2.4 MPa-5%NH3·H2O treated CS substrates, which was 5.14 times higher than that from enzymatic hydrolysis (EH) of raw CS. Furthermore, evaluation of monolignins content (H, G, S) in different CS samples suggested that substrates rich in guaiacyl (G) and syringyl (S) would generate a higher efficiency of enzymatic saccharification. Therefore, the slight genetic modification of monolignins for cotton stalk might be a potential way to enhance biomass degradation and transformation.

Introduction

Cotton is one of the most important crops in the world, accompanied with which an abundance of cotton stalk resources is produced, on the basis of a large planting area of cotton every year. However, some cotton stalks were piled up in the field casually, other parts were burnt by farmers, which resulted in the waste of natural resources and environment polluted (Kaur et al., 2012). In fact, cotton stalk is a valuable biomass resources rich in lignocellulose, which can be utilized for bioconversion and energy reproduction, and is considered to be a main resource for biofuels (Sun et al., 2014).

As like other biomass materials, cotton stalk is mainly composed of cellulose, hemicellulose and lignin. Lignin is linked to cellulose and hemicellulose, which plays a protective role on the cell wall of plants. Lignin has a three-dimensional network structure which composed of three phenylpropane units linked together by ether and carbon–carbon bonds. According to the molecular structure and source, lignin can be divided into three major phenylpropane units: p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S). Given the structural diversity and chemical heterogeneity of lignin, evaluation of lignin effect on biomass digestibility could be difficult (Fu et al., 2011, Xie and Peng, 2011). Furthermore, due to the high content of lignin, it is extremely difficult for cotton stalk to achieve direct saccharification or biotransformation. Therefore, pretreatment should be firstly carried out to destroy the structure of lignocellulose, increase the porosity of biomass, raising the contact specific surface area and the accessibility of enzymes to cellulose, thereby improving the conversion efficiency of cellulose and hemicellulose. Advanced technologies should be implemented as far as possible to realize the following goals: (1) reduce crystallization and increase the surface area of cellulose used for enzymatic digestion; (2) avoid carbohydrate degradation to inhibit the subsequent hydrolysis and fermentation process; (3) avoid using chemical reagents that would require high quality of reactor materials and produce serious environmental pollution; and (4) require minimum heat and energy so as to reduce production costs (Singh et al., 2017).

The pretreatment methods of biomass stalk can be divided roughly into physical pretreatment, chemical pretreatment, physical-chemical process and biological pretreatment at present, which have been explored by multiple previous studies described as follows. For example, Silverstein et al. (2007) reported the conversion efficiency of cotton stalk to ethanol in sulfuric acid, sodium hydroxide, hydrogen peroxide and ozone solutions. Binod et al. (2012) removed lignin effectively by high temperature assisted alkali pretreatment, which greatly improved the total process economy of ethanol production from cotton waste. Du et al. (2013) explored high-pressure assist alkali pretreatment of cotton stalks, and found that high pressure and high NaOH concentration were conducive to lignin dissolution, which resulted in the cellulose content and conversion yield increased. Haykir et al. (2013) carried out a study related to the pretreatment of cotton stalk by different kinds of ionic liquids. Gaur et al. (2016) optimized dilute acid hydrolysis of cotton stalk in laboratory scale by using central composite design (CCD) of response surface methodology (RSM). Singh et al. (2017) investigated microwave-assisted FeCl3 pretreatment of cotton stalk.

Steam explosion (SE) is a perfect pretreatment method which has the advantages, such as dissolve hemicellulose, increase substrate spaces, enlarge the contact area of cellulose and/or hemicellulose to enzymes, and environmental friendliness etc. (Kim, 2018). Although steam explosion can effectively remove hemicellulose, lignin in pretreated biomass substrates may block the pathways of polysaccharides and make the access of enzymes to cellulose ineffective (Kellock et al., 2017). Fortunately, the negative effects of lignin can be modified by introducing chemical additives, which in turn highlights the necessity of combination of steam explosion with chemical treatment.

The study was performed aiming at screening out a method of extracting lignin and/or hemicellulose effectively, and seeking out the optimal combination of pretreatment and enzymatic hydrolysis, so as to provide a basis for commercialization of cotton stalk conversion and utilization. In this study, reducing sugar yields released from cotton stalk, which were pretreated by steam explosion combined with different concentrations of strong monobasic acid (HCl), weak monobasic acid (CH3COOH), strong monobasic alkali (NaOH) and weak monobasic alkali (NH3·H2O), were comparable investigated in detail. Furthermore, H-monomer, G-monomer and S-monomer levels were detected so as to evaluate how did monolignins affect fermentable sugar released from cotton stalk.

Section snippets

Materials

Cotton stalk samples were obtained from experimental farmland in Xinjiang, China, and the collected stalk was air-dried at room temperature to a moisture content of less than 10% and stored in a dry container until further use. Cellulase (originated from Penicillium sp.), and xylanase (originated from Aspergillus niger) were bought from Heshibi Biological Technology Ltd. (Yinchuan, Ningxia Province, China). The standard samples used for HPLC quantification were from Sigma and of chromatographic

Steam explosion effectively removes hemicellulose

The raw materials of cotton stalk were pretreated by steam explosion at different pressures (1 MPa, 2.4 MPa). As shown in Table 1, cellulose and hemicellulose accounted for 42.30% and 24.56% of dry weight in untreated raw materials of cotton stalk, respectively, with total carbohydrate content accounting for over 60%, which made cotton stalk be a cheap biomass material with great potential for renewable energy. However, after steam explosion, the hemicellulose content decreased from 24.56% to

Conclusions

A new strategy was developed in this study, SE-2.4 MPa-5%NH3·H2O followed by enzymatic hydrolysis could release 73.22% of reducing sugar from cotton stalk samples. Evaluation of monolignins (H, G, S) in cotton stalk substrates indicates that there was a positive correlation between substrates rich in G and S-monomer and sugar release, while H monomer seemed to exhibit a certain inhibitory effect. Therefore, modification of lignin monomer in plant cell walls might benefit degradation and

Acknowledgements

We gratefully acknowledge financial support provided by National Natural Science Foundation of China (Grant No. 21464011) and the Scientific Research Program of Shihezi University (RCZX201208).

References (28)

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