Study Of The Pretreatment And Enzymatic Hydrolysis Of Rice Straw

The depletion of fossil fuels and the deterioration of environment are two global challenges.Therefore,the development of renewable energy that is environmentally friendly is of great significance.Bioethanol as a renewable bioenergy has attracted great attention in recent years,largely because it is a readily available,clean fuel for combustion engines made from plant-based feedstocks.Currently,the food-based feedstock such as corn is mainly used for bioethanol production,which is not sustainable and economical in the long run.Use the non-food feedstock lignocellulose such as agricultural straw as the feedstock for bioethanol production has attracted great attention in recent years.The conversion of lignocellulose to bioethanol mainly involves two processes:the hydrolysis of cellulose in lignocellulose to fermentable sugars and the subsequent fermentation of these sugars to bioethanol.Currently,the cost of bioethanol produced from lignocellulose is relatively high,largely resulting from the recalcitrant structure of lignocellulose which prevents its efficient hydrolysis into fermentable sugar.Pretreatment of lignocellulose is highly required with the aim to disrupt the recalcitrant structure of lignocellulose and convert the recalcitrant lignocellulose to reactive cellulosic intermediates.In this work,several pretreatment processes of rice straw were developed,and the following results were obtained.?1?Fenton's reagent,ultrasound,and the combination of Fenton's reagent and ultrasound were used to pretreat rice straw.Composition analysis showed that Fenton's reagent could preferentially degrade hemicellulose and lignin in rice straw while ultrasound could degrade cellulose,hemicelluloseandligninsimultaneously.N₂adsorption/desorption analysis showed that both ultrasound and Fenton's reagent could increase the specific surface area and pore volume of rice straw.The degree of polymerization?DP?analysis showed that Fenton's reagent could greatly decrease the DP value of rice straw while ultrasound had negligible effect on DP.The enzymatic hydrolysis results showed that the rice straw pretreated by ultrasound-assisted Fenton's reagent?U/F-RS?had the highest enzymatic activity,and the concentration of reducing sugar released from U/F-RS at 48 h of enzymatic saccharification is about 4 times as large as that from raw rice straw and 1.5 times as large as that from Fenton's reagent pretreated rice straw.Ultrasonic-assisted Fenton method is an effective pretreatment method for rice straw.?2?Alkaline hydrogen peroxide produced by the electrochemical reduction of oxygen was used to pretreat rice straw.First,alkaline hydrogen peroxide was produced by the electrochemical reduction of oxygen in alkaline solution with a self-made gas diffusion electrode?GDE?,and then the alkaline hydrogen peroxide was diluted and used to pretreat rice straw.The compositional analysis of rice straw after pretreatment showed that alkaline hydrogen peroxide had excellent lignin and hemicellulose removal capabilities,and the relative content of cellulose in ricestrawafterpretreatmentwasgreatlyincreased.N₂adsorption/desorption analysis showed that the specific surface area and the pore volume of rice straw were greatly increased after alkaline hydrogen peroxide pretreatment.XRD crystallinity analysis showed that the crystallinity of rice straw after pretreatment was increased significantly.The enzymatic hydrolysis results showed that the pretreatment with alkaline hydrogen peroxide prepared by electrochemical reduction of oxygen with self-made GDE?AEH?had an effect comparable to the alkaline hydrogen peroxide prepared by chemical reagents?AH?.The concentrations of reducing sugar released from AH-and AEH-pretreated rice straw samples were 2.02 and 1.99 g/L,respectively,after 48 h of enzymatic hydrolysis,which were about 1.7 and 1.68 times as large as that from the untreated rice straw sample?1.18 g/L?,respectively.The alkaline hydrogen peroxide produced from the electroreduction of oxygen using self-made GDE was proved to be an efficient pretreatment method.?3?Rice straw samples were exposed to ultrasound-assisted alkaline?NaOH?pretreatment by using the heat energy dissipated from ultrasonication to increase their enzymatic digestibility for saccharification.The characterization showed that the pretreatment could selectively remove lignin and hemicellulose without degrading cellulose,and increase porosity and surface area of rice straw.The porosity,surface area and cellulose content of rice straw increased with the increasing concentration of NaOH used.The rice straw sample pretreated by using the heat energy dissipated from ultrasonication had a higher surface area and a lower crystallinity index value than that pretreated by using the external source of heating,and the amount of reducing sugar released from the former sample at 48 h of enzymatic saccharification,which was about 3.5 times as large as that from the untreated rice straw sample?2.91 vs.0.85 g/L?,is slightly larger than that from the latter sample?2.91 vs.2.73 g/L?.The ultrasound-assisted alkaline pretreatment by using the heat energy dissipated from ultrasonication was proved to be a reliable and effective method for rice straw pretreatment.