Effects Of Hydrothermal Assisted Alkali Pretreatment On The Dissolution And Structure Of Hemicellulose From Cotton Stalks

Hemicellulose is the second most abundant carbohydrate in nature,and its reserves behind to cellulose.As one of the main components in plant cell walls,hemicellulose has been widely used in functional food and pharmaceutical fields due to its unique physical and chemical properties.Nevertheless,the complex three-dimensional network structure between hemicellulose,cellulose and lignin makes the extraction of hemicellulose particularly difficult.Therefore,how to efficiently separate hemicellulose from lignocellulosic materials has attracted the attention of researchers.As an agricultural country,China generates a huge amount of agricultural waste every year,most of which are directly discarded or burned,resulting in extremely low resource utilization.Separating hemicellulose from agricultural waste is of significance to environmental protection and social sustainable development.In this study,the hydrothermal assisted alkali pretreatment was used to extract hemicellulose from cotton stalks,the dissolution rules of hemicellulose under different conditions were discussed comprehensively.The experimental results showed that the maximum yield of xylan was 50.59%,when the hydrothermal pretreatment was carried out at170? for 60 min with the solid to liquor ratio(S/L)was 1:10.After hydrothermal pretreatment,the hemicellulose remaining in the solid fibers was further separated via diluted alkali treatment.And the xylan yield was 17.16%with 4%(w/v)sodium hydroxide at 65? for 3h,and the S/L ratio was 1:12.Combining hydrothermal and alkali treatment to separate hemicellulose from cotton stalks,the total yield of xylan reached to 67.75%.This method could effectively separate the hemicellulose from cotton stalks with less amount of chemical reagent and low environmental pollution.After pretreatment,the hemicellulose was precipitated from spent liquor.And the effect of acid precipitation on the purity and yield of hemicellulose was explored also.It was found that the purity of hemicellulose increased to 66.3%and the yield decreased slightly when the p H value decreased from 5 to 3.Meanwhile,the recovered hemicellulose was characterized,and the structural differences between water-soluble(H_water)and alkali-soluble(H_alkali)hemicelluloses were compared.The thermogravimetric analysis presented that H_waterhad higher thermal stability,and molecular weight indicated that H_alkalihad a larger molecular weight and lower polydispersity index.In addition,the infrared spectrum and nuclear magnetic resonance analysis indicated that the backbone of H_alkaliand H_waterhemicellulose were(1?4)-?-D-xylopyranose,linkage was?-glycosidic bond.and the branches of H_alkalihemicellelose were?-L-arabinose and 4-O-methyl-?-D-glucuronic acid.Compared with H_alkali,the difference was the absence of the?-L-arabinose side chain in H_waterhemicellulose.In the process of extracting hemicellulose by hydrothermal assisted alkali pretreatment,the 75%of lignin was separated from cotton stalks due to the break of the ester bond between lignin and hemicellulose.Hence,the application of lignin as by-product was preliminarily investigated as supplement.The characterization analysis found that alkali lignin had a higher molecular weight and specific surface area,which could be potentially used as an protein adsorbent.In general,the hydrothermal assisted alkali pretreatment as a clean and efficient technology is important for comprehensive utilization of the lignocellulosic materials.