Improvement Of Bioethanol Production Using A Waste Biomass-based Adsorbent From Enzymatic Hydrolysis Of Rice Straw And Its Mechanisms

Environmental pollution and energy crisis related to fossil fuels consumption have stimulated the development of clean and renewable energy.Among these,straw based bioethanol has attracted the great interest due to its extensive raw materials and low cost.Rice straw is one of the most widely used raw materials for straw based bioethanol production.The degradation of lignocellulose structure using alkali pretreatment is an indispensable rate limiting step in rice straw based bioethanol production process.However,alkaline pretreatment process will produce various inhibitory compounds,mainly including ferulic acid,which will inhibit the subsequent fermentation processes.This study innovatively proposed that the efficient detoxication agent of ferulic acid was prepared by using the enzymatic hydrolysis residue of rice straw obtained from the bioethanol hydrolysis process.Meanwhile,this detoxication agent was added in situ to the bioethanol production process for building a cleaner bioethanol production system,thus strengthening the bioethanol production process and revealing its macro response changes and micro stress mechanisms.These obtained results will provide a theoretical and methodological guidance for efficiently removing fermentation inhibitors,developing the modified adsorbents and engineering strains,as well as exploring the important metabolic pathways,thus enhancing the scale-up production of bioethanol.The main conclusions are as follows:(1)The basic properties,adsorption effect and adsorption behaviors of a waste biomass-based adsorbent(AEPA₂₅₀)obtained from enzymatic hydrolysis of rice straw were studied by TG/XRD/BET method,equilibrium adsorption experiment and adsorption model.The results showed that AEPA₂₅₀prepared under specific conditions(gas environment:air,temperature:250?)had strong stability and combustion characteristics.AEPA₂₅₀had a larger specific surface area and microporous volume.Meanwhile,it had a higher adsorption efficiency of ferulic acid(70.330%).AEPA₂₅₀could be regenerated after ferulic acid adsorption using heat treatment at 100?.The first and second regeneration efficiency reached 56.965%and 52.035%,respectively.Moreover,a rate limiting step of micropore diffusion was determined between ferulic acid and AEPA₂₅₀adsorption by the analysis of Webber-Morris and Bangham's model.Their adsorption behavior was also studied by adsorption kinetics,adsorption isotherm and adsorption thermodynamics.It was found that the fitting results of the pseudo second-order(PSO)(0.997?R~2?0.999)and Liu models(0.931?R~2?0.997)were more perfect.Meanwhile,the adsorption process presented that the enthalpy change was greater than 0 while the entropy change and gibbs free energy change was less than 0(?H^o>0,?S^o<0).The above results showed that the adsorption process between AEPA₂₅₀and ferulic acid was mainly chemical and saturable adsorption,as well as the adsorption process was spontaneous and endothermic.(2)The detoxification mechanism between AEPA₂₅₀and ferulic acid was obtained using experimental characterizations(SEM-EDX,XRD,FTIR and XPS,etc),as well as microscopic simulation calculation(DFT theoretical simulation).The characterization results showed that the contents of Mg,Al,Ca,K,P and C elements on the adsorbent surface were changed after ferulic acid detoxification,and the SEM image was relatively smooth;Some functional groups like O-H,N-H,C=C,-C=O,C-O also were different,especially intermolecular force(like O-H bonding).Mineral components such as Na₂Ca(CO₃)₂,Mg Si O3 and Ca₃Si₂O₇presented variation.DFT theoretical simulation results showed that AEPA₂₅₀model functionalized by-OH,-COOH and-NH₂groups had higher binding energy(45.667,-27.046 and-11.008kcal mol^-1,respectively),electron cloud overlap and shorter bond distance(1.015,1.010 and 2.094(?),respectively).Additionally,under acidic conditions,AEPA₂₅₀model functionalized by-OH,-COOH and NH₂possessed the higher energy gap values of Eg₁than that of Eg₂.Compared with s orbital,the partial density of states(PDOS)of p orbital was closer to Fermi level(0 Ha).The above characterization and simulation results showed that the adsorption process between AEPA₂₅₀and ferulic acid was related to the types of functional groups.Meanwhile,their detoxification mechanisms mainly included ion exchange,mineral coprecipitation,?-?*EDA interaction and intermolecular force(such as hydrogen bond)together.The protonated AEPA₂₅₀had stronger adsorption capacity,and p-orbital electrons dominated the ferulic acid detoxification process.(3)The simulated fermentation broth containing ferulic acid was used to explore the detoxification effect of AEPA₂₅₀on ferulic acid for bioethanol production,Meanwhile,the macroscopic response changes and microscopic stress mechanisms of S.cerevisiae for ferulic acid detoxification using AEPA₂₅₀were determined by growth kinetics simulation,metabonomics,transcriptomics and their combined analysis of S.cerevisiae.By simulating the fermentation broth that containing ferulic acid,three groups of fermentation systems were established,including ODF system(the optimized detoxified fermentation systems using AEPA₂₅₀with adsorbent filtration),ODFA system(the optimized detoxified fermentation systems using AEPA₂₅₀containing adsorbent)and NDF system(non-detoxified system).Fermentation process parameters of conductivity,redox potential(ORP)and p H were monitored.The results showed that the fermentation capacity of ODF system was better than ODFA system followed by NDF system.The evaluation of bioethanol production before 12 h showed that ODF system had the largest ethanol concentration(7.206 g L^-1).Meanwhile,the S.cerevisiae cells in this system grew promisingly with a specific growth rate of 0.221 h^-1higher than NDF system.The results of metabolomics and transcriptomics analysis showed that some differentially expressed metabolites and functional genes of S.cerevisiae were regulated after ferulic acid detoxification using AEPA₂₅₀,which were mainly involved in amino acid and purine metabolism.The joint analysis of metabolomics and transcriptomics data showed that the genes of YDR316W-B and YPR137C-B encoding gag pol fusion protein in S.cerevisiae were related to ferulic acid metabolic pathway.(4)A cleaner bioethanol production system was established by using the actual alkali-pretreated rice straw hydrolysate.The mass balance and bioethanol production potential of this system were studied,and its mechanism of high effective performance was thoroughly explored by proteomics technology.The mass balance from alkali pretreatment to enzymatic saccharification processes in this system was studied.The results indicated that AEPA₂₅₀had higher ferulic acid detoxification efficiency of 94.393%,while the lower glucose and xylose loss of 2.532%and8.219%,respectively.The evaluation of bioethanol production before 12 h showed that this system had a larger bioethanol concentration after ferulic acid detoxification using AEPA₂₅₀,which was close to 1.7 times higher than that of the non-detoxified fermentation system.Proteomic analysis showed that some differentially expressed proteins of S.cerevisiae in this system were regulated after AEPA₂₅₀detoxification.They were mainly involved in the ribosome,TCA cycle,amino acid metabolism and biosynthesis.Protein-protein interaction(PPI)network analysis indicated that the differentially expressed genes(DEGs)coded by corresponding proteins,which mainly included ACO1,MRP2,RPL24B,MRPL33,RPL32,RPL39,RPS17B and RPS19A,were related to the high bioethanol production of S.cerevisiae in this system.