Pretreatment Of Sisal Waste Based On UV Photo-Catalytic Hydrogen Peroxide Oxidation And Its Comprehensive Utilization

Sisal waste,the main by-product from fiber industry,consists of cellulose,hemicellulose,lignin and pectin,and thus has abundant fermentable sugars.The comprehensive utilization of sisal waste is of great significance in adding high value to the biomass resources and reducing environmental pollution as well.However,the intense structure of sisal lignocellulose has been a barrier for the conversion of sisal waste to fermentable sugars by enzymatic hydrolysis.Thus a pretreatment is necessary in order to promote the conversion by interrupting the dense lignocellulosic structure.In this paper,an advanced oxidation method based on UV photocatalysis and hydrogen peroxide pretreatment?UHP?was proposed to investigate the effect of UHP on lignin removal and the subsequent enzymatic conversion of the pretreated sisal waste,based on which,the potential mechanism involved in UHP pretreatment of sisal waste was speculated by analyzing the main components during UHP process.Meanwhile the main degraded products released into UHP pretreatment liquid were analyzed,and the antioxidant activity of the pretreated liquid was evaluated.Finally,UHP was combined with ionic liquid pretreatment?UHP+IL?in order to furtherly enhance the enzymatic conversion by solving the problem of the increased crystallinity aroused by UHP treatment.The main contents and results are as follows:?1?Development of UHP technique for sisal waste pretreatment.The single factor study showed that H₂O₂ loading and pretreatment time were the two key factors for UHP pretreatment.The UHP conditions were optimized to be H₂O₂ charge 0.1 g/g dried sisal waste,pH 10.0,and UV radiation for 6 h.Under these conditions,it resulted in the delignification rate of 76.6%,the conversion to reducing sugar at 71.2%,and the conversion to glucose at91.6%,respectively.Comparing to alkaline hydrogen peroxide pretreatment?AHP?,the delignification rate increased by 39.8%with UHP,the conversion rate of reducing sugar and glucose increased by 36.7%and 21.6%,respectively,and the loading of H₂O₂ decreased by nearly 80%,the pretreatment time was shortened by nearly 30%.The chemical composition and SEM analysis showed that the structure of UHP pretreated sisal waste was highly destroyed and the lignin-cellulose ratio was reduced greatly,both of which were beneficial for the subsequent enzymatic hydrolysis.Meanwhile,the crystallinity index of UHP pretreated sisal waste increased badly according to XRD analysis.?2?Isolation and structure characterization of the main components from UHP pretreated sisal waste.Based on the variation of the main components and their structure analysis,the mechanism of UHP pretreatment was suggested as follows.Hydroxyl radical,which was supposed to be the main oxidant in UHP process,was produced by H₂O₂ under UV irradiation and later a free radical chain reaction occurred,resulting in producing more hydroxyl radicals constantly.Through the advanced oxidation reaction by free radicals,lignin,which is wrapped in the outer layer,then pectin,and hemicellulose,cellulose in the inner layer were degraded in turn and a loosen lignocellulose structure was formed.The solid residue was recovered from UHP pretreated sisal waste by alkali treatment.According to structural analysis,the cellulose enrich component exhibited a typical cellulose I with high crystallinity.The hemicellulose component was identified to be 4-O-methyl-D-glucose xylose with?-L-furan arabinose and glucuronic acid as branches.The lignin component was guaiacyl-syringyl-hydroxyphenyl containing high concentration of ferulic acid and pcoumaric acid.?3?Analysis of the chemical components and the antioxidant activity of the UHP pretreatment liquid.The main components in UHP pretreatment liquid included oligosaccharides and monosaccharides such as glucose and xylose,which were mostly produced from hemicellulose and cellulose through the oxidation degradation,phenols and their derivatives which were resulted from the degradation of lignin.The content of the degraded components increased with the increasing of H₂O₂ loading or the pretreatment time.The antioxidant activity of the pretreatment liquid prepared with 0.5 g/g of H₂O₂ loading was relatively high.The EC₅₀ values of total reducing power,ABTS radical-scavenging and hydroxyl radical-scavenging were 2.02 mg/mL,0.45 mg/mL and 0.10 mg/mL,respectively.Pearson correlation analysis showed that the antioxidant ability was significantly correlated with the content of oligosaccharides and total phenols in the pretreatment liquid.?4?Enhancement of the enzymatic hydrolysis of UHP pretreated sisal residue by ionic liquid pretreatment.In order to promote the enzymatic hydrolysis of sisal waste furtherly,a process of ionic liquid pretreatment was proposed based on the UHP pretreated sisal residue,coded as UHP+IL.The optimum parameters of ionic liquid pretreatment were[TBA][OH]loading of 20 mL/g,60?,400 W ultrasound for 40 min.Under these conditions,the conversion rate to reducing sugar reached 88.8%.The condition of enzymatic hydrolysis was optimized to be cellulase amount of 10 FPU/g,and the enzymatic hydrolysis time of 36 h,which were 50%less than that of the UHP-treated sisal waste.The enzymatic kinetics analysis on UHP+IL-treated sisal waste hydrolysis indicated that the hydrolysis rate constant k was higher and the Michaelis constant K_M value was lower than that of UHP-treated sisal waste.XRD,FT-IR and SEM analysis demonstrated that the UHP+IL pretreatment was highly efficient in enhancing the enzymatic conversion of sisal waste can attribute to the results of the high removal of lignin and the decrystallization effect on cellulose.