Study On The Effect Of New Iron-based Additives On Hydrogen Production By Dark Fermentation

Global energy demand is growing exponentially,while fossil fuel reserves are plummeting.The burning of fossil fuels not only leads to more and more serious environmental pollution,but also greatly increases the emission of greenhouse gases.As a green and environmentally friendly energy carrier,hydrogen is regarded as one of the most important alternative energy sources to fossil fuels.Hydrogen production by dark fermentation has great development potential because it can use various industrial and agricultural wastes as raw materials and realize energy recycling.But the low efficiency of hydrogen production by dark fermentation makes it unable to be applied on a large scale.Adding nanoparticles or other catalysts in the process of hydrogen production by dark fermentation can effectively improve the hydrogen production rat.In this paper,zinc ferrate nanoparticles and MIL-53(Fe)were synthesized on the basis of zinc and iron elements,and the effects of them as exogenous additives on mesophilic and thermophilic dark fermentation system were studied.The main contents and conclusions are as follows:(1)ZnFe2O4 NPs and MIL-53(Fe)were prepared by high temperature decom-position synthesis and solvothermal method.The phase purity,crystal structure,specific surface area,magnetic properties,elemental content and elemental valence of the two Fe-based materials were characterized.It is proved that ZnFe2O4 NPs and MIL-53(Fe)have the characteristics of large specific surface area,high crystallinity and high porosity,which are suitable to be used as high-activity catalysts and have broad application prospects in the field of catalysis.(2)ZnFe2O4 NPs with different concentrations(0,5,10,25,50,100,200 mg/L)were added to the medium temperature(37℃)and high temperature(55℃)dark fermentation system with glucose as substrate and activated sludge as inoculum.The effects of different concentrations of ZnFe2O4 NPs on the medium and high temperature dark fermentation system were analyzed by hydrogen production,soluble metabolites(SMPs),concentration of major metal ions,microbial morphology,extracellular polymer(EPS)and microbial community structure.The results showed that proper amount of ZnFe2O4 NPs could promote hydrogen production in dark fermentation under different temperature conditions.Under moderate and high temperature conditions,the optimum adding concentration was 50 mg/L and 100 mg/L respectively,and the corresponding hydrogen production was 228.01 mL/g glucose and 149.12 mL/g glucose,which were 45.2%and 38.8%higher than those of the control group.Under the medium temperature condition,the contents of acetic acid and butyric acid in the soluble metabolites(SMPs)of fermentation broth were the highest.ZnFe2O4 NPs could increase the abundance of Firmicutes from 44.18%to 60.66%,which changed the fermentation route to butyric acid fermentation,and producing more hydrogen.Under high temperature,ethanol fermentation was the main fermentation route.During the experiment,ZnFe2O4 NPs was corroded by hydrogen producing bacteria(HPB)to release iron and zinc ions,and the released metal ions acted on HPB in turn to improve the activity of HPB.Due to the different tolerance of anaerobic bacteria to ZnFe2O4 NPs,different anaerobic bacteria showed different morphology in the face of ZnFe2O4 NPs.Some anaerobic bacteria was damaged cell wall,while some anaerobic bacteria still retained their structure.(3)MIL-53(Fe)was applied as an exogenous additive to the batch dark fermentation experiments at medium temperature(37℃)and high temperature(55℃)with glucose as substrate.The effects of different concentrations(0-200 mg/L)of MIL-53(Fe)on the hydrogen production system of medium and high temperature dark fermentation were explored,and the related mechanisms were analyzed.The conclusions were as follows:When the concentration of MIL-53(Fe)was 25 mg/L and 50 mg/L respectively,the hydrogen production was the highest,which were 223.94 mL/g and 163.95 mL/g,43.2%and 22.1%higher than that of the control group.The hydrogen production and soluble metabolite concentration were higher at 37℃ than at 55℃.The difference of H2 yield and production mechanism between medium and high temperature was related to the change of microbial community during dark fermentation.High-throughput sequencing showed that moderate amount of MIL-53(Fe)could increase the abundance of Clostridium sensu stricto 1 and promote hydrogen production at moderate temperature.Moderate amount of MIL-53(Fe)at high temperature could increase hydrogen production by increasing the abundance of Clostridium sensu stricto 10.The microbial diversity decreased with the increase of temperature.When the temperature increased from 37℃ to 55℃,the fermentation type changed from acetic acid and butyrate fermentation to ethanol fermentation.