Kinetics And Thermodynamics Of Biochar Promoting Volatile Fatty Acid Syntrophic Methanogenesis

Anaerobic digestion technology has broad prospects in the field of organic wastewater and waste energy recovery because of its recyclable energy to alleviate environmental pollution and energy shortage.In the traditional anaerobic fermentation process,there are problems such as low conversion rate of organic matter and low productivity efficiency.The main reason is that the rate of hydrolysis acid production and syntrophic methanogenesis is unbalanced,resulting in excessive accumulation of volatile fatty acids(VFAs)in the system,which restricts the overall efficiency of the anaerobic fermentation process.The unfavorable environmental conditions(high temperature,high load)aggravate the imbalance of the rate,and accompanied by the production of high concentration of hydrogen,resulting in more obvious VFAs accumulation problems.In recent years,adding exogenous biochar to the anaerobic digestion system to accelerate the reaction and solve the problem of VFAs accumulation has become a research hotspot,but the mechanism of biochar promotion is still unclear.In addition,the effects of biochar on methanogenesis,microbial growth kinetics and thermodynamic properties,the effects of biochar on DIET and IHT,and the synergistic mechanism of kinetics and thermodynamics need to be further studied.Based on this,this study aims to effectively improve the efficiency of anaerobic fermentation and solve the problem of VFAs accumulation.Wood biochar was prepared from waste wood chips and added to a single VFAs anaerobic fermentation system to explore the effect of biochar on VFAs in the experimental process under the principle of " treating waste with waste." Based on the degradation kinetics,thermodynamics,interspecies electron transfer,microbial community structure and metabolic characteristics of VFAs,the mechanism of biochar promoting VFAs mutual oxidation and methanogenesis was further analyzed.The following results were obtained:(1)Biochar can promote methanogenesis and degradation processes to enhance VFAs syntrophic methanogenesis,which is manifested in accelerating the methanogenesis rate and degradation rate of 6.37 % ～ 71.87 % and 0.28 % ～ 91.84 %,and shortening the reaction delay period of 4.09 % ～ 51.14 %.At the same time,biochar also enhanced the growth adaptation of acetate and methanogenic microorganisms in the system,further accelerating the VFAs syntrophic methanogenesis.(2)Biochar enhanced the electron transfer in the system by reducing the hydrogen partial pressure,thereby reducing the actual reaction free energy by 4.51 %-29.62,further improving the environmental thermodynamics and breaking the thermodynamic barriers to strengthen the VFAs mutualism process.At the same time,the improvement of the thermodynamic environment can expand the thermodynamic window of the possible reaction by more than 200 %,further strengthening the possibility of continuing the reaction under unfavorable environmental conditions(unfavorable hydrogen partial pressure).(3)Biochar can increase the abundance of microorganisms related to syntrophic oxidation,and accelerate the degradation and metabolism of VFAs by triggering direct interspecies electron transfer(DIET)process and strengthening syntrophic oxidation between microorganisms.For microbial metabolism,biochar can regulate the transformation of metabolic pathways from acetic acid to hydrogen nutrition,and enhance the abundance of functional genes related to electron transfer in hydrogen nutrition methanogenesis pathway,and promote the efficient transfer of extracellular electrons by participating in DIET.In addition,biochar and extracellular polymer substances(EPS)have similar electron mediator effects,and biochar may alleviate microbial secretion of EPS through potential similar functional substitution.(4)Biochar can be used as a medium for electron transfer due to its own electronic mediator properties,which is conducive to strengthening electron transfer in the system and improving electron transfer efficiency.At the same time,its redox properties can promote methanogenesis by stimulating DIET between syntrophic microorganisms.Usually,biochar promotes the kinetics and thermodynamics of the reaction system at the same time.Under the ’ dual action ’ of kinetics and thermodynamics,VFAs in the system are rapidly metabolized.