Enhanced Anaerobic Digestion by Graphene-stimulated Direct Interspecies Electron Transfer

Transcription

1 Enhanced Anaerobic Digestion by Graphene-stimulated Direct Interspecies Electron Transfer Richen Lin 1, Jun Cheng 2, Jerry Murphy 1 1 University College Cork, Ireland 2 Zhejiang University, China Friday May 8 th, 2017 CONTENTS 1. Anaerobic Digestion: Interspecies Electron Transfer (IET) 2. Promoting Direct IET with Conductive Graphene 3. Future Research 2 1

2 1. Introduction of Anaerobic Digestion Seaweed, grass, food waste, animal slurry Anaerobic digestion H 2 Needs of alternative energy! Exhaustion of fossil fuel Global warming Biohydrogen Biomethane Biohythane Clean energy Carbon neutral Renewable 3 1. Interspecies Electron Transfer in Anaerobic Digestion bacteria Electron Acetate Free electrons in water are not thermodynamically permissible Acetoclastic + Anaerobic digester Simplified Model for Conversion of to Methane 2

3 1. Interspecies Electron Transfer in Anaerobic Digestion Interspecies Hydrogen Transfer (IHT) Direct Interspecies Electron Transfer (DIET) Fermentable Fermentable 2H + + 2e - H 2 bacteria bacteria + H 2 Acetate + H + + e - Acetate Hydrogenotrophic Acetoclastic? Acetoclastic + + H 2 CO2 e - e - e- e - e - CO2 Bacteria Hydrogenotrophic Bacteria Interspecies Hydrogen Transfer (IHT) between Bacteria and Hydrogenotrophic Direct Interspecies Electron Transfer (DIET) between Bacteria and Interspecies Hydrogen Transfer and Direct Interspecies Electron Transfer Table Reactions and thermodynamics of Ethanol Degradation via IHT and DIET under standard conditions. Process Reaction ΔG 0 (kj/mol) IHT CH 3 CH 2 OH + H 2 O CH 3 COO - + H + + 2H DIET CH 3 CH 2 OH + 1/2 CH 3 COO - + H + + 1/ Positive value of ΔG 0 indicates the reaction is thermodynamically unfavorable. Negative value of ΔG 0 indicates the reaction is thermodynamically favorable. IHT can only happen at very low hydrogen concentration due to the thermodynamic limit. DIET is thermodynamically more favorable to produce methane. 3

4 2. Promoting Direct IET with Conductive Graphene Hypothesis: Highly conductive nano-graphene promotes Direct Interspecies Electron Transfer? High surface area, Microbes electron mobility thermal conductivity. Tempera ture ph Graphene Anaerobic Digestion Research objectives: Assess of AD performance: biomethane yield, production rate, and other kinetic parameters Reveal the bacterial and archaeal communities responsible for DIET? Calculate the theoretical maximum electron transfer flux of IHT and graphene-based DIET? 2. Promoting Direct IET with Conductive Graphene Table Experimental design Batch Biomethane Potential System Group Inoculum and Substrate Graphene Concentration ml sludge ml Ethanol No ml sludge ml Ethanol 0.5 g/l graphene ml sludge ml Ethanol 1.0 g/l graphene ml sludge ml Ethanol 2.0 g/l graphene 4

5 Biomethane production kinetics yield production rate Graphene concentration H m (ml/g) Kinetic model parameters λ R m (ml/g/d) (d) T m (d) No g/l graphene g/l graphene Optimal graphene concentration (1 g/l): Biomethane yield +25%; Peak production rate +20% Lag-phase time reduced by 48%. 2.0 g/l graphene Ethanol degradation and microbial morphology Ethanol degradation Degradation rate Acetate degradation Electron recovery In the presence of graphene: Ethanol degradation was more rapid, Acetate generation and consumption were much faster. 5

6 Structures of bacterial and archaeal community Others 90 unclassified Bellilinea 80 Desulfovibrio Syntrophomonas 70 Meniscus 60 Longilinea Victivallis 50 Smithella Synergistes Aminobacterium Pseudomonas Thermovirga 20 Clostridium 10 Geobacter Levilinea 0 Inoculum No addition 1.0 g/l graphene Inoculum No addition 1.0 g/l graphene Bacterial and archaeal communities after AD in the presence and absence of graphene With the addition of graphene in AD, Geobacter in bacteria community increased from 6% to 10%. (electrogenic bacteria) Methanobacterium in archaea group greatly increased from 24% to 35%. Others unclassified Methanospirillum Methanolinea Methanobacterium Methanosaeta Calculation of Theoretical Maximum Electron Transfer of IHT and DIET H 2 CO2 e - e - e- e - e - CO2 Bacteria Hydrogenotrophic Bacteria Interspecies Hydrogen Transfer (IHT) between Bacteria and Hydrogenotrophic Direct Interspecies Electron Transfer (DIET) between Bacteria and Interspecies Hydrogen Transfer Or Graphene-stimulated Direct Electron Transfer? 12 6

7 Calculation of Theoretical Maximum Electron Transfer of IHT and DIET 13 Calculation of Theoretical Maximum Electron Transfer of IHT and DIET The highest H 2 concentration : ΔG = ΔG 0 + RTln Acetate ph 2 2 Ethanol In a similar way, the lowest H 2 concentration: ΔG = ΔG 0 + RTln pch 1Τ2 4 1Τ p 2 2 ph 2 The maximum driving force for direct electron transfer is given by the redox potential (ΔE = ΔE met - ΔE ace ) of the overall reaction (CH 3 CH 2 OH + 1/2 1/2 + CH 3 COO - + H +, ΔG 0 = kj/mol). Graphene-based DIET sustains much higher electron transfer flux (up to 6 orders of magnitude) than conventional IHT. 14 7

8 3. Future Research Highly-conductive graphene is capable of promoting DIET, and enhancing AD (up to 25.0%). Future work: To develop continuous DIET-based anaerobic digester. To assess the renewable feedstocks (such as seaweed, grass) for DIET-AD. To reutilize graphene to make the process economically more viable. (Conductive Biochar) Thanks for your attention! Bioenergy & Biofuels research group 8