Depolymerization Of Straw By The Combination Of Ultrasound And Fenton Reagent

The use of clean and renewable energy to replace fossil fuels is badly needed due to the increasingly serious environmental pollution and energy shortage problems.As a traditional agricultural country,China discharges a huge amount of agricultural by-products annually,leading to great waste of resources.Crop straw,the main agricultural by-product,is a valuable bioresource.In recent years,utilizing the crop straw to produce the environmentally friendly and renewable bioethanol has received great attention.The yield of crop straw in China can reach up to 600 million tons annually,and the conversion of some crop straw into bioethanol can produce significant social and economic benefits.The bioethanol production from crop straw needs the fermentable sugars resulting from enzyme hydrolysis process,while the pretreatment of crop straw is required to improve the efficiency of hydrolysis process.In this study,ultrasound,Fenton reagent,and the combination of Fenton reagent and ultrasound were respectively used as pretreatment to depolymerize microcrystalline cellulose(MCC)and rice straw to facilitate enzyme hydrolysis.The main contents and conclusions are as follows:(1)The capture and generation mechanism of hydroxyl radical brought about by Fenton reaction were investigated.hydroxyl radical can be captured by using salicylic acid and the obtained steady products can be characterized by UV-Vis spectrophotometry.The optimal dosage of ferrous sulfate heptahydrate is 0.05 g when 3 mL of hydrogen peroxide is added in 100 mL water.The generation of hydroxyl radicals in the first 10 min is very fast,then slows down,and the generation amount reaches the largest in about 20 min.The generation rate and amount of hydroxyl radicals can be increased with the assistance of ultrasound.(2)MCC was subjected to Fenton reagent,ultrasound,and the combination of Fenton reagent and ultrasound.Polarization microscope was employed to characterize the morphologies of MCC and the results show that both Fenton reagent and ultrasound can shorten the length and reduce the diameter of MCC,indicating that both Fenton reagentand ultrasound can depolymerize MCC.The length and diameter of MCC can be further reduced under the pretreatment of the combination of Fenton reagent and ultrasound.The degree of polymerization(DP)analyses show that the DP values are 197,166,137 and125 for raw MCC(R-MCC),Fenton reagent pretreated MCC(F-MCC),ultrasound pretreated MCC(U-MCC)and the combination of Fenton reagent and ultrasound pretreated MCC(U/F-MCC),respectively.The calculated crystallinity index(ICR)values are 88.0%,85.6%,87.2% and 84.9% for R-MCC,F-MCC,U-MCC and U/F-MCC,respectively.The enzymatic hydrolysis results show that the yields of reducing sugar released from R-MCC,F-MCC,U-MCC and U/F-MCC are 3.7,8.1,19.9,and 22.7 g/100 g,respectively,indicating that the hydrolysis efficiency of MCC can be greatly improved after pretreatment.(3)Rice straw of Hubei province was subjected to Fenton reagent,ultrasound,and the combination of Fenton reagent and ultrasound.The analysis of the contents of cellulose,hemicellulose and ligin shows that the content of cellulose in rice straw is increased after pretreatment and rice straw pretreat by the combination of Fenton reagent and ultrasound has the largest cellulose content.The strong oxidizing property of Fenton reagent can oxidize,destroy and dissolve hemicellulose,leading to the decrease of the content of hemicellulose as well as the slight decrease of the content of lignin.Ultrasonic cavitation and mechanical shear force resulting from ultrasound can destroy the structure of ligin,leading to the partial dissolution of lignin and the increase in relative contents of cellulose and hemicellulose.The calculated crystallinity index(ICR)values for raw rice straw(R-RS),Fenton reagent pretreated rice straw(F-RS),ultrasound pretreated rice straw(U-RS),and the combination of Fenton reagent and ultrasound pretreated rice straw(U/F-RS)are 37.71%?35.19%?36.94%? 32.55%,respectively.The enzymatic hydrolysis results show that the yields of reducing sugar released from R-RS,F-RS,U-RS and U/F-RS are 32.38,44.39,39.18,and 47.41 g/100 g,respectively,indicating that the hydrolysis efficiency of rice straw can be largely improved after pretreatment.