Simplified sodium chlorite pretreatment for carbohydrates retention and efficient enzymatic saccharification of silvergrass
Introduction
Currently, the production of bio-fuel from lignocellulosic biomass is the most popular way to generate the renewable energy. The second-generation bio-ethanol produced from biomass has attracted lots of attentions, since it would not cause any competition with food crops. Chinese Silvergrass, as a typical Miscanthus spp., is found widely in Asia. It is a kind of perennial herb, which has a high tolerance for salt and drought and are often found growing on infertile and poor land (Li et al., 2010). Thus, it is believed to have a great potential as an energy crop for the production of second-generation bio-ethanol and bio-chemicals.
However, due to the complex structure of lignin carbohydrate complexes, cellulosic biomass is highly recalcitrant to enzyme degradation (Lynd et al., 1991, Studer and Sederoff, 2011). Especially, the presence of lignin could increase the non-productive adsorption of enzyme, which would inhibit the efficiency of the enzymatic hydrolysis (Li et al., 2016). Therefore, the pretreatment is necessary to remove lignin, thus destroy the cross-linking of the carbohydrates and lignin to improve the enzymatic hydrolysis efficiency of lignocellulosic materials. Previous study has shown that there was a linear correlation between the lignin removal and cellulose conversion within a certain range of the lignin contents (Siqueira et al., 2013).
Biomass delignification can be achieved by a variety of pretreatment strategies. Alkaline peroxide pretreatment was an efficient delignification method, but their use was limited because of the extensive degradation of cellulose and hemicelluloses caused by the peroxide radical (Fang et al., 1999, Sun et al., 2004). Organosolv pretreatment and oxidative ionic liquid pretreatment could enhance lignin extraction with high selectivity at low carbohydrate loss (Perez et al., 2010, Pang et al., 2016). But these pretreatment methods are not cost-effective enough due to the high commercial price of the solvent and intensive power requirement. Acidic hydrogen peroxide is still a popular delignification in practice, but the disadvantage of this method is that the hydrogen peroxide is not stable and hard to keep (Yue et al., 2015).
Acid-chlorite pretreatment utilizing a mixed solution of acetic acid and sodium chlorite, which conducted at milder conditions, has been considered as one of the most well-established methods in lignin removal (Ahlgren and Goring, 1971, Hubbell and Ragauskas, 2010, Siqueira et al., 2013). Acidified sodium chlorite solution quickly dissociates into highly reactive chlorine anion (ClO2−) and chloride anion (Cl−) to selectively remove lignin (Abdel-Halim, 2014, Rabetafika et al., 2014). Acidified sodium chlorite primarily acts on lignin, but it can also affect the polysaccharides (Abdel-Halim, 2014). There are two possibilities for cellulose degradation during acid-chlorite pretreatment, including acidic cleavage of the glycosidic bonds and/or oxidative degradation of the polysaccharides. In addition to the effect of acidified sodium chlorite on lignocellulosic composition, several works have shown that removing lignin by chlorite-acetic acid pretreatment significantly enhanced the hydrolyzability of lignocellulosic materials. Li et al. (2014) reported that after sodium chlorite-acetic acid delignification, the cellulose to glucose conversion yield of two-year old bamboo substrate was 93.1%. Another study showed that the waste biomass was sufficiently delignified by sodium chlorite-acetic acid under the conditions of 0.4 g sodium chlorite and 0.2 mL acetic acid g−1 dry biomass at 80 °C, and the hydrolysis yield of whole rice waste biomass was 67.8% (Saratale and Oh, 2015). Generally, the sodium chlorite-acetic acid method is usually performed at 60–70 °C for 4–8 h with successive addition of fresh sodium chlorite and acetic acid (every hour or two) to produce holocellulose. However, the troublesome reagents addition procedure, the sugar loss in the acidic condition and corrosive hot acidified chlorite solutions, which make this method difficult to be scaled up to industrial application.
In the present study, a simplified sodium chlorite pretreatment process was employed to improve the hydrolysability of Chinese silvergrass. Unlike common used sodium chlorite/acetic acid (SCA) pretreatment, the sodium chlorite solution alone was used as pretreatment reagent (SC pretreatment). Furthermore, the comparison of SCA and SC pretreatment on changes of chemical compositions, especially lignin removal and sugar loss, and the physical structure of silvergrass was systematically investigated. The purpose of this work was to retain more carbohydrates in silvergrass by sodium chlorite pretreatment with simplified procedure and fewer chemicals.
Section snippets
Materials
Chinese silvergrass was supplied by Beijing Research and Development Center for Grass and Environment, Beijing Academy of Agriculture and Forestry Sciences Beijing, China. The moisture content of the material was 8.15%. Before pretreatment, silvergrass were milled and sieved through an 80-mesh screen scale.
Avicel PH-101, cellulose fiber (medium) and beechwood xylan were purchased from Sigma Chemical Co. (St. Louis, MO, USA). The commercial enzyme preparations Celluclast 1.5 L and Novozyme 188
Chemical compositions of solid silvergrass after pretreatment
The glucan, hemicellulose and lignin contents of raw silvergrass were 33.9%, 18.0% and 22.9%, respectively (Table 1). After SCA and SC pretreatment for 2 h, the lignin contents in silvergrass were 10.8% and 9.2%, and the lignin removal were 67.7% and 71.2%, respectively, which suggested that the lignin removal ability of SC pretreatment was slightly higher than SCA pretreatment. The contents of glucan increased from 33.9% to 47.6% and 44.4%, respectively. The percentage of xylan component
Conclusions
Compared to SCA pretreatment, SC pretreatment was found to be more effective in lignin removal and hemicelluloses retention in pretreated substrates. Simultaneously SC pretreatment seriously damaged the microscopic morphology of substrate, which was becoming more susceptible to enzyme, thus obtained higher efficiency in enzymatic saccharification. In addition, the pretreatment without acetic acid addition would make the process more cost-effective. The study suggests that the SC pretreatment is
Acknowledgements
This work was supported by the Natural Science Foundation of China, China (No. 31670598 and 31270622) and the Science Foundation for Distinguished Young Scholars of Northwest A and F University, China (No. 2452015098).
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