Study On The Synthesis Of Medium And Long Chain Fatty Acids From Lignocellulosic Organic Wastes

As a renewable resource,lignocellulosic organic waste has unique advantages in terms of energy and chemical utilization.Anaerobic biotransformation is the main way for the utilization.However,traditional anaerobic fermentation technologies,such as biogas fermentation and cellulosic ethanol,face some common problems like high costs and low added-value products.On the contrary,the synthesis of medium and long chain fatty acid technology using carbon chain extension of organic waste is a promising resource utilization technology,in which hexanoic acid has the characteristics of high energy density,strong hydrophobicity,and potential to be converted into a variety of high-value chemicals and products.In recent years,it has received a lot of attention from researchers in various countries.This study uses lignocellulosic organic waste,maize straw,as the raw material for the synthesis of hexanoic acid without the addition of exogenous ethanol,including improving the raw material utilization,enhancing the acidification efficiency through biological pretreatment,and regulating the structure and ratio in the liquid product.（1）Aerobic biological pretreatment technology was used to enhance the hydrolytic acidification capacity of maize straw.The effect of pretreatment time and humidity on the pretreatment and the mechanism were studied and analyzed.The results of the study showed that the maxium acid production from anaerobic fermentation was achieved by an aerobic biological pretreatment for 2 d.The specific yield of volatile fatty acids increased by 46.73% as compared to the control group.The aerobic biological pretreatment for 3 d only increased the yield by 7.49%.At the aerobic pretreatment of 5 d,the yield was reduced by 18.66%.For the aim of energy and resource utilization,the use of aerobic biological treatment with complex bacterial strains as a pretreatment method for anaerobic fermentation of maize straw should be strictly controlled in terms of pretreatment time to avoid the yield of reduced product causing excessive degradation of cellulose and hemicellulose.Although it is generally acknowledged that high humidity is beneficial to the aerobic treatment of straw,this study found that low humidity was beneficial in promoting lignin degradation during aerobic biological treatment and avoiding excessive degradation of cellulose and hemicellulose,thus improving the efficiency of subsequent straw hydrolysis and acidification.Lignin removal in the pretreatment was 28.25%at 40% moisture for 2 d,and the specific volatile organic acid in the fermentation broth increased by 213.71% as compared to the untreated maize straw.The microbial community analysis showed that Actinomyces,Bacillus,Aspergillus and Corynebacterium were the dominant populations in the microbial community as time increasing,but the microbial community structure was different between at low and high humidity,and different functional microorganisms worked together to degrade lignocellulose.By absolute quantitative analysis,the number of bacteria was much higher than the number of fungi during the whole pretreatment process,and the bacteria contributed the most to the degradation of lignocellulose.（2）The highest accumulation of hexanoic acid was determined by water distribution tests at alcohol and acid in an ratio of 3:1,followed by the ratio of 1:1.Unreasonable ratios of alcohol and acid can affect the production of hexanoic acid and cause excessive accumulation of acetic and butyric acids.The maximum values of ethanol and acetic acid in the pH = 6 group were1491.8 mg/L and 1924.83 mg/L,respectively,by controlling the pH and the liquid components.The addition of YUAN-3 flora increased the proportion of ethanol in the liquid phase,but the ethanol yield was reduced as compared to the absence of YUAN-3 flora,when pH 6 was chosen as the optimum pH condition.（3）In the ethanol-based fermentation coupled with carbon chain extension technology for the synthesis of hexanoic acid,pH is an essential factor to improve the fermentation efficiency of the hexanoic acid produce.At an optimum pH of 5.5,hexanoic acid production could reach1804.68 mg/L,representing approximately 80.84% of the total liquid SCOD.Low pH environment severely inhibited the microbial activity,allowing the oxidation of ethanol to acetic acid.The conversion rate of acetic acid is significantly reduced,resulting in the accumulation of acetic acid.While,the neutral pH conditions cause inadequate carbon chain lengthening,resulting in a large accumulation of butyric acid,which cannot be used for the synthesis of hexanoic acid.In contrast,alkaline conditions severely inhibit the biological activity of hexanoic acid in producing bacteria.The analysis of the microbial community showed that pH was one of the main factors,contributing to the variation in the microbial community for hexanoic acid synthesis.This study shows that lignocellulosic organic waste,maize stover,can be converted into medium and long chain fatty acids（MCFAs）and that high caproic acid yields can be obtained by regulating ethanol-based fermentation coupled with carbon chain extension technology without exogenous ethanol addition.This research also presents an interesting investigation of the degradation mechanism of maize straw under low humidity aerobic biological pretreatment with enhanced anaerobic acidification capacity.The results could be used as a reference for the synthesis of medium and long chain fatty acids from lignocellulosic feedstocks,and are of academic significance and practical value for the development of lignocellulosic biomass resource utilization technology.