Studies On The Pretreatment Of Herb Residues And Coupling Production Of Hydrogen And Methane By Biochar Amendment

With the rapid growth of urbanization in China,it is urgent to develop clean,green,and efficient technology of solid waste treatment to meet the needs of sustainable development.As a kind of the large-scale solid wastes in China,herb residue has typical lignocellulosic structure and promising application in the biorefinery.Therefore,the bioenergy technology based on herb residue is worthy of further exploration.In this study,a pretreatment technology using ionic liquid（IL）assisted by aromatic acid（p-toluenesulfonic acid,Ts OH）was developed.The microstructures and physico-chemical properties of the recovered two solid products,cellulose-rich material（CRM）and lignin-rich material（LRM）,were analyzed comprehensively.The recyclability of IL was evaluated by reusing in pretreatment process,and the dissolved impurities were analyzed qualitatively and quantitatively.Subsquently,the feasibility of adding biochar to enhance the hydrogen production in dark fermentation with hydrolysate of herb residues as substrate was investigated.The effects of adding different types of biochar on hydrogen production,metabolites,and microbial growth were compared.The relationship between the physico-chemical properties of biochar and the hydrogen production potential was analyzed,and the promotion mechanism was clarifed by analyzing the influence of biochar on the microbial community distribution.Finally,the feasibility of biochar to enhance biohythane production in semi-continuous mode was studied.The hydrogen and methane prduction efficiencies of different periods were investigated.The effects of adding biochar on microbial growth and metabolic activity were explored and the carbon flux in different periods was analyzed.The effect of biochar on the succession of microbial community during semi-continous fermentation was clarified.The energy input and output during the whole process were calculated to provide a preliminary techno-economic analysis of bioenergy conversion technologies based on herb residue substrate.The result showed that the addition of low concentration of Ts OH into IL could effectively deconstruct the recalcitrant intercellular structure of herb residues and rapidly extract the lignin fraction.Under the optimum conditions（79%[Bmim]Cl,1.0%Ts OH,and20%H₂O at 130℃and 2 h）,the highest cellulose recovery of 96.4%,the lignin removal rate of 79.9%and ideal saccharification efficiency of 98.9%were achieved.According to results of Scanning electron microscopy（SEM）,Fourier transform infrared spectroscopy（FTIR）and X-ray diffraction（XRD）,CRM showed a loose and porous microstructure,and the bands of amorphous components（hemicellulose,lignin）were significantly weakened or disappeared,and the relative crystallinity of cellulose fraction was significantly decreased.Moreover,the main byproduct lignin exhibited comparable yield,high purity,and thermal stability,suggesting the potential application in the conversion of bio-based chemicals and materials.Furthermore,the[Bmim]Cl was successfully recycled three times and the recovery rate was at least 95%each time.High saccharification efficiency of the recovered CRM and lignin recovery were achieved during all cycles.The results analyzed by high performance liquid chromatography（HPLC）and heteronuclear singular quantum correlation nuclear magnetic resonance（HSQC NMR）showed the hemicellulose-derived saccharides were the main impurities detected in the recycled IL.The physico-chemical properties of biochar obtained from different sources and different preparation temperatures were significantly different.Additionally,hydrogen production from hydrolysate of herb residues and microbial growth and metabolism were significantly affected by the addition of different types of biochar.The biochar yield decreased with increasing preparation temperature,while the values of surface alkalinity,electrical conductivity and specific surface area increased gradually.The highest biochar yield of 40.4%was obtained from Ficus microcarpa leaf at 500℃（FML5）,and the highest specific surface area of 288.50m²/g was obtained from Ficus microcarpa branch at 700℃（FMB7）.In the batch fermentation experiment,the shortest lag time of hydrogen production of 4.44 h was achieved by the FML6 amended group,which was 24.5%shorter than that of control group.Besides,the maximum hydrogen production potential of 190.5 m L/g was achieved by the FMB7amended group,which was 75.6%higher than that of the control group.Linear correlation analysis and pearson analysis showed that the conductivity of biochar had the highest correlation with hydrogen production.FMB7 could significantly increase the consumption rate of saccharides,increase the accumulation of organic acids related to hydrogen-producing pathway,promote cell growth,maintain the oxidation-reduction potential（ORP）,increase the hydrogenase activity and the relative concentration of NAD（H/⁺）.Moreover,the addition of FMB7 significantly increased the relative abundance of Clostridiaceae and decreased the relative abundance of Peptostreptococcaceae,resulting in an advantage of metabolic flux of hydrogen-producing pathway.The FMB7 amendment could run well for at least 20 cycle times in semi-continous fermentation mode.The hydrogen production in FMB7 amendment group was higher than100 m M in each group,and the maximum value was 204.9 m M at the 3^rdcycle.By contrast,the maximum hydrogen production was only 75.9 m M at the 3^rdcycle in control group,and the hydrogen production was almost negligible after the 9^thcycle.These resluts suggested that the addition of FMB7 could remarkablely increase the fermentation stability and hydrogen yield under semi-continous mode.Additionally,FMB7 addition significantly increased the saccharides consumption（9～15 mmol）,inhibited the lactate accumulation（0～2 mmol）,maintained the organic acids production related to hydrogen-producing pathway（5～8 mmol）,and maintained the suitable ORP value of broth（-300 m V）.Furthermore,FMB7 addition could inhibit the growth of Lactococcus and enrich the relative abundance of hydrogen-producing bacteria such as Clostridium.spp and Caproiciproducens.Compared with the effluent of control group after hydrogen production process,the effluent of FMB7amendment group could be used more effectively for biomethane production in semi-continous mode.The methane production of FMB7 effluent increased from 69.4 m M in the strat-up period to 107.9 m M in the 3^rdcycle and remained stable,while the methane production in the control group was always less than 20 m M in each cycle.The mass balance of the whole process showed that 140 L of hydrogen and 104 L of methane could be obtained per Kg of herb residue in the FMB7 amendment group,which were 16.56 times and 8.43times higher than those in the control group,respectively.Besides,the average energy recovery and the total chemical oxygen demand（COD）removal rate of FMB7 amendment group were 5.49 MJ/Kg herb residue and 85.2%,which were 6.95 times and 1.67 times higher than those of the control group,respectively.