Study On The Improvement Of Caproate Production Of Clostridium Kluyveri Through Structural Optimization,Modification Of Cell Carrier And Zero Valent Iron

Caproate is an important industrial chemical for synthesis of food additive,antibiotic and hexanol.The chemical method to synthetize caproate performed a lot of disadvantages such as heavy pollution and high cost.To this end,the biological method for caproate synthesis has been developed.Although promising,the biological method exhibits low yield and costliness,which limit its wide application.In order to improve the caproate yield and reduce the cost,the high yield strategy was studied based on the biofilm of Clostidium kluyveri,where the wheat straw was used as the cell carrier.Firstly,the effect of particle size of wheat straw on caproate fermentation was studied,on the basis of which,the continuous fermentation with food waste as substrate was carried out to realize efficient continuous transformation from food waste to caproate.After that,wheat straw was modified by three different chemical methods,and the effects of different modification methods on straw structure and caproate fermentation were investigated,so as to improve the performance of wheat straw as immobilized carrier.In the end,the effect of zero-valent iron on caproate fermentation was studied focusing on the addition amount,particle size and substrate concentration of iron powder.The mechanism of the improvement of caproate production with fuction of zero valent iron was finnaly discussed.In the immobilized system,the particle size of the carrier has an important effect on the formation of biofilm and the yield of caproate.In order to determine the optimum particle size of wheat straw,the effect of different particle size on caproate fermentation was studied.Results showed that the optimal particle size of wheat straw was in the range of 0.5-1 cm,and the corresponding caproate production was 124.2 mmol/L.The analysis of FTIR,biomass distribution and mass transfer coefficient showed that the highest mass transfer coefficient was0.7375 with the optimum particle size,and the maximum biomass on the carrier was 7.4 g/L,which was helpful to improve the tolerance of cells to inhibitors.In the continuous fermentation,the optimal operating conditions were:480 mmol/L ethanol and 180 mmol/L acetate,10 d hydraulic retention time.The average yield and productivity of caproate were 157.5 mmol/L and15.8 mmol/L/d,respectively,when the food waste was used as substrates.The biofilm was difficult to form on the side wall of wheat straw due to the smooth surface.To remove the smooth surface,three different modifiers（HCl,Na OH and Fe Cl₃）were used to modify the wheat straw.Results showed that the smooth structure of the straw modified by Na OH was removed,leading to an increase of 34.4%on the specific surface area.In batch fermentation,the caproate yield reached to 138.6 mmol/L,which was 15%higher than that of the control.In the fed-batch fermentation,the yield of caproate was increased to 181.6 mmol/L.Results of scanning electron microscopy and thermogravimetric showed that the thick biofilm was formed on the modified wheat straw.Moreover,the results of FTIR and XRD showed that the cell carrier kept its structure stable during the fermentation process.In conclusion,the modification of wheat straw by Na OH solution is an effective method to promote the formation of biofilm and improve the yield of caproate.In order to further increase the yield of caproate,zero valent iron was employed in the cell immobilized system.The results showed that the optimal amount of iron powder was 4 g/L,and the corresponding caproate and hydrogen productions were 203.6 and 50.05 mmol/L,respectively.By comparing the effects of three kinds of zero valent iron with different particle sizes（100 mesh,100 nm and 50 nm）on caproate fermentation,it was found that iron 100 nm produced the highest production of caproate,which was 214.1 mmol/L.However,the fermentation failed in the group with iron 50 nm due to the fact that the particle size of nano zero valent iron was too small,which could enter cells and destroy cell structures,and it is easy to exchange with H⁺in the fermentation broth,resulting in the p H exceeding the optimal initial p H growth of C.kluyveri fermentation by 6.8-7.4.Compared with the control,the reason for the increase of caproate yield was that alleviation of p H,reducing the inhibitory effect of hydrion to C.kluyver.