Bioelectrochemical Conversion of Carbon Dioxide to Methane for Biogas Upgrading Christy M. Dykstra and Spyros G. Pavlostathis School of Civil & Environmental Engineering Georgia Institute of Technology Atlanta, GA 30332 ‐ 0512, USA College of Environmental and Chemical Engineering Nanchang Hangkong University Nanchang, Jiangxi Province, P.R. China 15 May 2018 1
Municipal Wastewater Treatment Opportunities for CO 2 reuse and energy recovery in municipal wastewater treatment plants (WWTP), now referred to as Water Resource Recovery Facilities (WRRF) 2
Anaerobic Digestion & Biogas Composition Biogas (CO 2 , CH 4 , trace gases) Complex organic compounds Gujer and Zehnder, 1983 Carbohydrates, Proteins, Fats Hydrolysis Simple organic compounds Sugars, Amino acids, Fatty acids Acidogenesis Organic acids and alcohols Acetogenesis H 2 , CO 2 Acetate Methanogenesis CO 2 , CH 4 OS = 4 ‐ 1.5 (COD/TOC) CH 4 (%) = 100 ‐ 12.5 (OS + 4) Liquid digestate, Biosolids 3
CO 2 Capture & Conversion Chemical (MEA, caustic, etc.) Absorption Physical (Selexol, Rectisol, etc.) Adsorption Alumina Carbon Capture Zeolite Sequestration Cryogenics & Activated C Conversion Gas separation (Polyphenyleneoxide, Polydimethylsiloxane) Membranes Gas absorption (Polypropelene) Phototrophic bacteria, Microbial/Algal Ceramic based systems systems algae BES CO 2 CH 4 4
Bioelectrochemical Systems (BES) Microbial Fuel Cell (MFC) Produces electrical current Reduction Oxidation Microbial Electrolysis Cell (MEC) Produces hydrogen (H 2 ) Microbial Electromethanogenesis Produces methane (CH 4 ) Microbial Electrosynthesis (MES) Produces 2+ carbon compounds A, Resistor (MFC) or applied potential (MEC) (e.g., acetate, methanol, etc.) B, Proton exchange membrane R1, Reactant in the anode (oxidation half reaction) 2H + + 2e ‐ → H 2 P1, Product in the anode E H °' = ‐ 0.414 V R2, Reactant in the cathode (reduction half reaction) P2: Product in the cathode CO 2 + 8H + + 8e ‐ → CH 4 + 2H 2 O E H °' = ‐ 0.244 V CO 2 + 4H 2 → CH 4 + 2H 2 O Δ E°' = 0.170 V 5 At 25 °C, 1 atm, pH 7.
Electron Transfer Mechanisms in BES e ‐ H + CO 2 Non ‐ H 2 ‐ Mediated CH 4 Electron Transfer Direct Electron CO 2 Transfer (DET) CH 4 M ox H 2 O M red H 2 H 2 ‐ Mediated CH 4 CO 2 CO 2 CH 4 Electron Transfer (MET) M red H 2 M ox H 2 O 6
Overall Objective To develop and test bioelectrochemical systems (BESs) to directly convert CO 2 to CH 4 for anaerobic digester biogas upgrading 7
Materials & Methods Batch ‐ fed systems at 22±2 o C Hydraulic retention time, 7 days ANODE CATHODE • Carbon felt • Carbon felt electrode/SS collector electrode/SS collector • Acetate ‐ fed (1.5 g • CO 2 ‐ fed (1.6 atm, COD/L) absolute) • N 2 ‐ flushed headspace • CO 2 ‐ flushed headspace PEM CH 4 CO 2 • Potential allowed to • Potential fixed at ‐ 0.8 V Acetate CO 2 fluctuate; measured (vs. SHE) using an against an adjacent adjacent Ag/AgCl Ag/AgCl reference reference electrode electrode • 300 mL total volume • 300 mL total volume • 250 mL liquid catholyte • 250 mL liquid anolyte (phosphate buffer, pH (phosphate buffer, pH 7.0; trace minerals; 7.0; trace minerals; vitamins) vitamins) • Inoculated with a • Inoculated with suspended ‐ growth, biofilm ‐ attached enriched carbon felt from an hydrogenotrophic active MFC culture 8
Materials & Methods Gases Pressure transducer GC ‐ TCD for gas composition Liquids GC ‐ FID for acetate measurement Dissolved CO 2 measured by sample acidification (6 N H 2 SO 4 ) followed by composition analysis of evolved gas (conditional calibration) Solids and Biomass TSS/VSS for suspended biomass Protein analysis of biofilm and suspended biomass Molecular Analysis DNA extraction using UltraClean Soil DNA Kit and PowerSoil DNA Isolation Kit (Mo Bio Laboratories, Carlsbad, CA) 16S rRNA gene sequencing (Illumina MiSeq) Phylogenetic analysis using Mega 7.0 software Diversity analyses performed with QIIME 1.9.0 and R 9
Results Biocathode performance with respect to: Methanogenic inoculum Hydrogen sulfide (H 2 S) gas feed contaminant Anaerobic digester biogas feed (upgrading) 10
Biocathode Performance – Effect of Inoculum Biocathode methanogenic inocula: MM, mixed; EHM, pre ‐ enriched hydrogenotrophic 50 MM-B A MM-biocathode 4 EHM-B EHM-biocathode 40 (mg COD/L) ACETATE 3 CURRENT (mA) 30 2 20 1 10 0 CURRENT DENSITY MM-B 20 B 0 EHM-B 15 -10 (A/m 2 ) 10 -20 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 5 VOLTAGE (V) 0 Final Biofilm Mean CH 4 Production MM-B C HEADSPACE CH 4 3 Biomass (mmol CH 4 /mg biomass ‐ EHM-B Biocathode (mg) day) (mmol) 2 MM ‐ 0.54 ± 0.07 0.15 ± 0.01 inoculated 1 EHM ‐ 0.64 ± 0.19 0.59 ± 0.03 0 inoculated 0 5 10 15 20 TIME (d) Dykstra, C.M.; Pavlostathis, S.G. 2017. Methanogenic biocathode microbial community development and the role of Bacteria. Environ. Sci. Technol. 51(9) 5306 ‐ 5316. 11
Biocathode Performance – Effect of Inoculum Biocathode methanogenic inocula: MM, mixed; EHM, pre ‐ enriched hydrogenotrophic Archaea Bacteria 100 100 RELATIVE ABUNDANCE (%) RELATIVE ABUNDANCE (%) 80 80 60 60 Bacteroidetes Protoebacteria Actinobacteria 40 Firmicutes 40 Spirochaetes Synergistia Thermotogae 20 Anaerolineales 20 Acidobacteria Deferribacteres Chloroflexi 0 Unclassified MM MM-biocathode EHM EHM-biocathode 0 MM MM-biocathode EHM EHM-biocathode Methanobrevibacter spp. Methanogens Methanobrevibacter arboriphilus Cell lysis Methanoculleus spp. • MM ‐ biocathode enriched in Methanolinea spp. products CH 4 Spirochaetes and other non ‐ Methanomethylovorans spp. Methanosaeta spp. exoelectrogenic, fermentative Methanobacterium spp. Exoelectrogen Cathode • Biocathode archaeal communities Bacteria converged on the same phylotypes, • EHM ‐ biocathode enriched in Methanobrevibacter arboriphilus e ‐ , H + , Proteobacteria , exoelectrogens CO 2 • Inoculum pre ‐ enrichment with H 2 /CO 2 and putative producers of CO 2 , selects for methanogens that are also electron shuttle mediators Oxidized selected for by biocathode conditions carbon (faster biocathode start ‐ up) Dykstra, C.M.; Pavlostathis, S.G. 2017. Methanogenic biocathode microbial community 12 development and the role of Bacteria. Environ. Sci. Technol. 51(9) 5306 ‐ 5316.
Biocathode Performance – Effect of Inoculum • The bacterial community of a biocathode has a significant effect on archaeal CH 4 production • Increased biocathode CH 4 production occurs with a bacterial community enriched in: • Putative producers of electron shuttles/mediators • Proteobacteria • Exoelectrogens Role of Bacteria CO 2 CH 4 M red Acetate cathode CO 2 CH 4 H 2 M ox Cell lysis debris Produce electron shuttles Recycle lysed cells Dykstra, C.M.; Pavlostathis, S.G. 2017. Methanogenic biocathode microbial community 13 development and the role of Bacteria. Environ. Sci. Technol. 51(9) 5306 ‐ 5316.
Biocathode Performance – Effect of H 2 S Biocathode Headspace H 2 S (1% v/v) 80 INITIAL 3-DAY CH4 PRODUCTION RATE (mmol/d) A 1.2 CURRENT DENSITY BES1 (Control) Headspace CE CCE 60 BES2 (H 2 S amended) H 2 S (% v/v) (%) (%) n = 3 (A/m2) 1.0 n = 3 0 11 100 40 4 19 99 n = 3 0.8 20 5 58 13 n = 1 0 0.6 6 58 15 0 5 10 15 20 8 B CE, Coulombic efficiency METHANE (mmol) 0.4 n = 7 6 CCE, cathode capture efficiency n = 1 n = 1 4 0.2 2 0.0 0 1 2 3 4 5 6 0 INITIAL CATHODE HEADSPACE H2S (%) 0 5 10 15 20 TIME (d) Two competing effects: • Depression of CH 4 production ( ≥ 4% H 2 S): Inhibition of methanogens? • Enhancement of CH 4 production ( ≤ 3% H 2 S): What is/are the process(es) involved? 14
Biocathode Performance – Effect of H 2 S Gas transport between biocathode and bioanode e ‐ Potentiostat e ‐ N 2 N 2 N 2 CH 4 CO 2 CO 2 H 2 S CO 2 Ag/AgCl reference electrode N 2 , CO 2 , CH 4 CO 2 CH 4 H + H + H + H + H 2 S H 2 S CO 2 Acetate Anode Cathode PEM Dykstra, C., Pavlostathis, S.G. (2017), “Evaluation of gas and carbon transport in a methanogenic bioelectrochemical system (BES)”, Biotechnology & Bioengineering , 114(5), 961-969. 15
Biocathode Performance – Effect of H 2 S Cathode e ‐ CH 4 CO 2 20% H 2 S H 2 S is the 80% CO 2 most toxic of H 2 S HS ‐ S 2 ‐ Neutral pH CH 4 the sulfide e ‐ species High local pH H + CO 2 H 2 S The methanogenic biocathode is protected from sulfide inhibition by biofilm formation and a local high pH at the cathode surface. 16
Biocathode Performance – Effect of H 2 S Anode e ‐ Potential anode H 2 S oxidation products CO 2 N 2 S 0 2 ‐ 2 ‐ S x S 4 O 6 S 2 O 3 2 ‐ SO 4 2 ‐ Sun et al., 2009. ES&T N 2 H 2 S CO 2 2 ‐ SO 4 CO 2 Acetate SRB Acetate • Low H 2 S → more electrons donated to the anode → higher biocathode CH 4 production • High H 2 S → s � mulate sulfur cycling → divert acetate eeq from the anode → lower biocathode CH 4 production 17
Biocathode Performance – Effect of H 2 S 0.16 • H 2 S stimulated total biomass growth in both anode and cathode Suspended 0.14 • H 2 S stimulated SRB growth in the anode Biofilm biofilm 0.12 CARBON (mmol) 0.10 0.08 BES1 BES2 cathode cathode 0.06 0.04 0.02 0.00 BES1 BES2 BES2 BES1 Cathode Anode Cathode Anode Control H 2 S ‐ amended 18
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