Study On The Mechanism Of Chain Elongation Using Lactic Acid As Electron Donor

Anaerobic fermentation is an effective way which can recover energy from organic waste However,as the product of traditional anaerobic fermentation,biogas is a fuel which has a low calorific value.Instead of converting organic waste to the biogas,the anaerobic fermentation can stop at production of volatile fatty acids(VFA)and then convert VFA to valued commodity chemicals,such as medium-chain fatty acids(MCFA).MCFA,mainly caproic acid,can be produced by elongating VFA through chain elongation process.As a promising technology,chain elongation is a possible approach to further achieve sustainable biofuel development,particularly from organic waste.The effects of different lactic acid/butyric acid ratio and concentration on the caproic acid production and products structure in chain elongation process were investigated.The results showed that lower lactic acid/butyric acid ratio is beneficial to the production of caproic acid at lower lactic acid concentration.Low p H and low lactic acid concentration effectively decreased propionic acid production by restraining acrylate pathway,resulting in higher electron efficiency of caproic acid production.When the initial butyric acid concentration was 80 m M the optimum mole ratio of lactic acid to butyric acid was 1:4,resulting the highest caproic acid electron efficiency.In addition,high butyric acid concentration suppressed the formation of odd-carbon-number carboxylates and promoted the formation of caproic acid.Compared with batch fermentation,the production of caproic acid in se-continuous fermentation was enhanced by 3.45 times to 90.91 m M.According to the optimum ratio of lactate to butyrate and the operation mode,the influence of biochar on the caproate production and microbial community in the CCE was explored.The results showed that high biochar concentration can promote the caproate production.When the biochar concentration reached 15g/L,the caproate production reached78.91 m M,the electron efficiency of caproate was 63.13%,and the utilization rate of butyrate was 58.18%.Low biochar concentration(5g/L)did not significantly promote the caproate production,and the caproate production was only increased by 7.78 m M compared to the blank group.As the biochar concentration increased,the concentration of extracellular polymeric substances(EPS)in the serum bottle increased,the highest concentration reached214.17mg/L,and the main component is protein.EPS can resist toxic effects from the environment.In addition,when the p H value was 5,biochar has a strong adsorption effect on caproate,and 15g/L biochar can adsorb 26.75 m M caproate.The high concentration of biochar caused a decrease in the concentration of caproate in the mixed solution and weakens the toxicity of undissociated caproate to microorganisms.Caproiciproducens is the main carbon chain elongation bacteria in this experiment,and its relative abundance was positively correlated with the biochar concentration.