| Given the increasing depletion of non-renewable resources and the environmental pollution caused by the use of fossil fuels,it is crucial to find ways to utilize lignocellulosic biomass—such as agroforestry residues—to produce fuel ethanol.The key to producing fuel ethanol from this type of biomass is to reduce the cost of processing raw materials and increase the enzymatic saccharification efficiency,thereby maximizing the fermentable sugar yield.Using cassava residue（CR）from factory waste as a raw material,this study investigated the effects of two pretreatment methods—ultra-low acid（ULA）and deep eutectic solvent（DES）—on saccharification and fermentation.The conditions for pretreating CR were optimized for the enzymatic saccharification of cellulase with CR as the substrate.Furthermore,separate hydrolysis and fermentation processes were used to optimize the enzymatic hydrolysis conditions.In addition to bioethanol fermentation from CR,this study also explored the recycling of cellulase attached to the solid residue and the effect of the concentration of fermentation inhibitors on bioethanol production by fermentation.The results are as follows.（1）Response surface analysis showed that the main order of hydrolysis factors affecting the ULA pretreatment of CR was pretreatment temperature>pretreatment time>acid concentration.The optimal pretreatment conditions were a pretreatment temperature of 160°C,pretreatment time of 22 min,and acid concentration of 0.17%（w/w）.Under these conditions,the glucose concentration was 19.54 g·L-1.In addition,the concentrations of formic acid,acetic acid,5-HMF,and furfural were 0.281 g·L-1,0.101 g·L-1,0.032 g·L-1,and 0.048 g·L-1,respectively,in the prehydrolysate;all these values were lower than the reported limit of the inhibitors.（2）The ULA pretreatment of CR contributed to an improvement in the yield of enzymatic glucose.The total enzymatic glucose yield after pretreatment was80.64%,which was much higher than the 51.22%yield obtained from the untreated CR.In addition,bioethanol production levels were higher after pretreatment.Under the optimal pretreatment conditions,the glucose concentration in the fermentation broth was 47.22 g·L-1 and the maximum ethanol yield in 24 h was 21.67 g·L-1,or 90.20%of the theoretical bioethanol yield.In contrast,the glucose concentration in the CR that received no pretreatment was only 22.38 g·L-1,with a 24 h maximum bioethanol yield of 10.53 g·L-1.（3）The DES,ChCl:FA,was synthesized with choline chloride（ChCl）as a hydrogen bond acceptor and formic acid（FA）as a hydrogen bond donor for effective pretreatment of CR.The optimized ChCl:FA pretreatment conditions were a pretreatment temperature of 130°C,pretreatment time of 1.5 h,and solid-to-liquid ratio of 1:20（w/w）.The ChCl:FA effectively improved the enzymatic glucose yield of CR.Under the optimal conditions,the yield from the enzymatic hydrolysis of the pretreated residue was 82.21%,which was much higher than that of the untreated CR.After pretreatment,the saccharification and fermentation of CR resulted in a higher bioethanol yield.Furthermore,the glucose concentration in the fermentation broth was 40.93 g·L-1 and the 24 h maximum bioethanol yield was 19.24 g·L-1,which was 92.16%of the theoretical bioethanol yield.（4）Experiments with recycling cellulase attached to CR treated using the two pretreatment methods showed an increase in the concentration of fermentation inhibitors;however,both methods presented concentrations lower than the minimum concentration required to inhibit bioethanol production from fermentation.Therefore,the pretreatment likely only influenced the bioethanol fermentation rate during its early stages.In contrast,the bioethanol production rate in the fermentation broth with fresh cellulase was higher than that hydrolyzed with the recycled enzyme;however,the two had similar 24 h bioethanol production rates.These results indicated that it was feasible to recycle the cellulase attached to the enzymatic residue pretreated with ULA and ChCl:FA,which could effectively reduce the amount of fresh enzymes required for the process. |
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