The Enhancement Mechanisms On Biohydrogen Production By The Driving Forces From Anaerobic Respiration Of Dissimilatory Metal Reduction Bacteria

The use of fossil fuels has caused serious pollution to the environment.In order to protect our environment,people put forward the concept of carbon neutrality.In this context,the importance of biohydrogen production has been highlighted,but the problem was that the hydrogen yield was relatively low,which was difficult to achieve industrial applications.This study conducted a series of experiments to increase hydrogen production,firstly,the feasibility of promoting hydrogen production through the internal driving forces of bacteria was explored,and a series of mechanisms were provided;secondly,the influence of cultivation methods on hydrogen production was explored,and mediators were further introduced to explain the role of microorganisms on hydrogen production;Finally,the enhancement mechanisms on hydrogen production by biochar were further elaborated.The following are concrete researches:Previous studies mostly focused on using external forces to promote hydrogen production,but this study explored the enhancement mechanisms of internal driving forces on hydrogen production,that is,the respiration of dissimilatory metal reduction bacteria on iron oxides as the driving force to promote hydrogen production.The results showed that in the presence of 100 mg L^-1 Fe₃O₄,the hydrogen production was increased by 13.02%and 14.35%from xylose and glucose by Klebsiella,respectively,compared to the blank group.And the proportion of electron equivalent（e^-eq）in the xylose and glucose groups also increased compared to the control group.Moreover,the addition of 100 mg L^-1 Fe₃O₄ increased the NADH concentrations of the xylose and glucose groups by 8.84%and 8.37%,respectively.In addition,the formation of Fe（II）/Fe（III）and the increase of redox active substances in extracellular polymers（EPS）were beneficial for hydrogen production.Therefore,the respiration of Klebsiella on iron oxides as an internal driving force for biohydrogen production was verified.The reason for low hydrogen yield was that the metabolites production consumed e^-eq during hydrogen production,the cultivation method of bacteria was closely related to metabolites production.There are two methods for bacterial cultivation.The first one is to use the bacteria liquid after pre-culture bacteria（PCB）for hydrogen production;The second one is to directly add anaerobic granular sludge after heat treatment（HTAGS）for hydrogen production.The hydrogen production results of two cultivation methods were compared in this study,the results indicated that PCB had a positive effect on hydrogen production,with a hydrogen molar yield（HMY）about21%～35%higher than HTAGS.The reason was that the precultured process lost some ethanol producing bacteria（Enterobacteria）and retained many a large number of Clostridium,which has better hydrogen production ability than Enterobacter.Both Fe₃O₄ and biochar were used to further explain the importance of microorganisms.The results suggested that Fe₃O₄ had a positive effect on hydrogen production by HTAGS,but the promotion effect on hydrogen production by PCB was not obvious,because the anaerobic respiration of Enterobacter on iron oxide could as driving forces for hydrogen production,while Clostridium did not have the ability of extracellular respiration.As an electron shuttle,biochar has good effects on both of PCB and HTAGS.The study found that PCB had a better effect on hydrogen production,the enhancement mechanisms of biochar on hydrogen production by precultured bacteria from xylose were further explored.The results indicated that the actual hydrogen yield increased by approximately 17.92%with the 0.5 g L^-1 biochar,compared to the control group.The electron balance indicated that the metabolic pathway shifted from butyrate to acetate with 0.5 g L^-1 biochar,which was beneficial to hydrogen production.Moreover,the fluorescence intensity of soluble microbial byproducts in EPS decreased significantly compared to the control group.The addition of biochar increased the currents of peak 1 and 2 of the CV curves by 29.16%and 18.18%,respectively,which was conducive to EET.The proportion of Clostridium＿sensu＿stricto＿1 increased by approximately11.23%compared to the control group.This study proposed the concept of anaerobic respiration of Klebsiella as an internal driving force to promote hydrogen production,and explored the importance of different cultivation methods for biohydrogen production,and different mediators were introduced to elucidate the importance of microorganisms in hydrogen production.