Effects Of Combined Ionic Liquid And Bacterial Pretreatment On Enzymatic Hydrolysis Of Rice Straw

The rapidly growing demand for energy,diminishing fossil fuel reserves and concerns about environmental impact by the use of fossil fuel have led to arose interest in renewable energy resources.Lignocellulose is the most abundant renewable biomass on the earth,efficient conversion of lignocellulose to fuel ethanol has been a hot topic in the field of renewable energy.The conversion of lignocellulose to fuel ethanol generally involves three main steps: pretreatment,enzymatic hydrolysis,and fermentation,among this,breaking the recalcitrant structures,releasing and degrading the locked polysaccharides and increacing exposure area of cellulose through pretreatment could improve the subsequent enzymatic hydrolysis and fermentation efficiency greatly.Efficient pretreatment is the key step of lignocellulose conversion,and the bottleneck of commercial application of cellulosic fuel.The typical pretreatment methods,such as acid,alkali,and steam explosion,have some disadvantages in the commercialization of lignocelluloses conversion,including inefficiency,special equipment requirements and a lot of energy consumption.To overcome these obstacles,in this study,the ionic liquid(IL)was combined with a high efficient lignin-degrading bacterial strain,make full use of the effects of ionic liquids on the destruction of lignocellulose molecules and the characteristics of high efficiency lignin removal from bacteria,a new type of lignocellulose pretreatment technology-green solvent and biological pretreatment was developed.This technique was applied to the pretreatment of rice straw(RS),the effect and mechanism of combination pretreatment were studied and the main research results and conclusions are as follows:(1)1-Ethyl-3-methylimidazolium acetate(EMIMAC)was used as solvent in pretreatment of RS.The crucial factor,such as the ratio of RS/EMIMAC,the pretreatment temperature and the pretreatment time were discussed to investigate the effects on the composition and enzymatic hydrolysis of RS and the optimal conditions for enzymatic hydrolysis such as cellulase dosage,enzymolysis temperature,enzymolysis time and buffer pH was investigated.The results showed that the pretreatment of ionic liquid could decrease the concentration of cellulase and the enzymolysis time,the optimum hydrolysis conditions were cellulose 12 FPU/g biomass,enzymolysis time of 48 h,hydrolysis temperature of 50 °C,buffer pH 4.8.The sugar yield increased with the decrease of solid to liquid ratio and the increase of temperature,pretreatment time had little effect on sugar yield.(2)Two strains of bacteria were screened by using kraft lignin medium and rice straw liquid medium.Cupriavidus basilensis B-8 is capable to reducing the RS lignin content from 13.7% to 7.1% after 3 days of incubation.The reducing sugar yield and glucose yield of RS raised from 189 mg and 155.3 mg per gram of dry RS to 254.6 mg and 200.6 mg per gram of dry RS.The removal efficiency of lignin by Cupriavidus basilensis B-8 is obvious,and it is suitable for pretreatment.(3)High biomass loading(25 wt.%)was adopted in IL pretreatment process,and then Cupriavidus basilensis B-8 was combined in pretreatment process.The results showed that the combination pretreatment decreased the lignin content of RS significantly while most cellulose and hemicellulose reserved,lignin may have been re-distributed during the IL pretreatment process and then improved the bacteria degradation of lignin.The lignin content of RS decreased by 5.5% ~ 7.2% from combined pretreatment to IL pretreatment.The combination pretreatment performed well at increasing enzymolysis efficiency of rice straw,for example,the reducing sugar yield and glucose yield of RS were 510.3 mg and 324.5 mg per gram of dry RS after 4 h,110 °C IL pretreatment,and the reducing sugar yield and glucose yield of RS rose to 713.2 mg and 444.8 mg per gram of dry RS after the combination pretreatment,which increased 1.40 and 1.37-fold respectively.(4)The mechanism of combined pretreatment of ionic liquid and bacteria was analyzed.SEM analysis showed that IL pretreatment destroy the structure of RS significantly,surface area of RS increased and the adsorption rate of rice straw to cellulase increased from 18.8% to 52.5%.TG analysis showed that IL destroyed the tight binding state of hydrogen bonds in cellulose,and the thermal stability of RS decreased.FTIR analysis showed that IL destroyed the structure of RS by cracking the acetyl and ester bond.XRD analysis showed that IL pretreatment can reduce the crystal structure of cellulose in RS,the crystallinity decreased from 0.58 to 0.44.Further utilization of Cupriavidus basilensis B-8 showed more serious damage to RS,the C=C bond and C=O bond in the benzene ring skeleton of lignin cracked,more lignin removed and the adsorption rate of RS to cellulase increased to 58.6%.The above results show that IL treatment can effectively destroy the stable structure of RS,reduced the crystallinity of cellulose significantly,but the lignin removal efficiency is low.The bacteria treatment can effectively remove the lignin of rice straw,two kinds of technology have good synergistic effect.This study shows that combined IL pretreatment and biological pretreatment technology can effectively destroy the structure of rice straw,reducing cellulose crystallinity,effectively remove lignin components,significantly enhance the realization of cellulose saccharification effect.This technology is green,simple,and is a potential pretreatment method of lignocellulose conversion.