Processes and Electron Flow in a Microbial Electrolysis Cell Fed with Furanic and Phenolic Compounds

Transcription

1 Processes and Electron Flow in a Microbial Electrolysis Cell Fed with Furanic and Phenolic Compounds Xiaofei (Sophie) Zeng 1, Abhijeet P. Borole 2 and Spyros G. Pavlostathis 1 1 School of Civil & Environmental Engineering Georgia Institute of Technology, Atlanta, GA, USA 2 Biosciences Division, Oak Ridge National Laboratory Oak Ridge, TN, USA ATHENS th International Conference on Sustainable Solid Waste Management, June 2017, Athens, Greece

2 Introduction Overall Objective Produce H 2 through the biotransformation of specific furanic and phenolic compounds using MEC technology Pyrolysis Hydrogenation Bio-oil Biofuel Lignocellulosic biomass Aqueous phase Sugars Acetic acid, etc. Waste disposal X Furans Phenols MEC Hydrogen Gas (H 2 ) X Natural Gas (CH 4 ) funded by 2

3 Introduction Selected Furanic and Phenolic Compounds Furanic Compounds Phenolic Compounds (FF) (HMF) (SA) (VA) (HBA) Lignocellulosic biomass Cellulose Sugar monomers Hemicellulose Lignin Phenolic compounds Furanic compounds Commonly found in biomass-derived streams (hydrolysates and pyrolysates) Inhibitory components in hydrolysates for dark fermentation FF and HMF are predominant furan derivatives from biomass carbohydrates SA, VA and HBA represent major lignin units (syringyl, guaiacyl, p-hydroxyphenyl) Klinke et al., Appl. Microbiol. Biotechnol., 2004; Monlau et al., Biotechnol. Adv., 2014; Liu et al., Bioresour. Technol.,

4 Introduction Microbial Electrolysis Cell (MEC) Exoelectrogenesis e Organic substrate OH COO 2 H + H + H 2 e.g., Geobacter spp., Shewanella spp., Desulfovibrio desulfuricans etc. Anode Cathode Water Electrolysis ( 1.7 V or higher) Microbial Electrolysis ( V) Substrates used in exoelectrogenesis: Acetate (mainly), H 2, lactate, formate, and propionate (possible) > 0 Applied voltage > V Fermentable, complex organic compounds: Fermentation (full rxn) required prior to exoelectrogenesis (half rxn) Logan and Rabaey, Science, 2012; Nealson and Rowe, Microb. Biotechnol., 2016; Torres et al., FEMS Microbiol. Rev.,

5 Introduction Specific Objective Elucidate the processes and electron equivalents flow during the conversion of the two furanic (FF, HMF) and three phenolic (SA, VA, and HBA) compounds in a MEC bioanode Assessment of abiotic processes Quantification of electron equivalents flow 5

6 Possible Processes in Bioanode 1. Abiotic anode 3. Active bioanode 2. Non active bioanode Non active Adsorption Electrochemical reaction Electrochemical reaction Biofilm Parent compounds Transformation products Active Active Microbial reaction (electrodedependent) Microbial reaction (electrodeindependent) 6

7 Possible Processes in Bioanode 1. Abiotic anode Adsorption (abiotic electrode) No, verified in serum bottle Yes (demonstrated in the abiotic control) ~ 20% of phenolic, more for furanic compounds Negligible current p 2. Non active bioanode Non active Negligible (abiotic electrode) Current (ma) 20 Cyclic A voltammetry (CV) Potential (V, vs. Ag/AgCl) Non active bioanode, (i.e., not fed), w/o compounds Non active bioanode, immediately upon compounds addition Active bioanode, after 1 d of incubation with the compounds 7

8 Possible Processes in Bioanode 3. Active bioanode Active (microbial, electrodeindependent) Active (microbial, electrodedependent) Active Parent compounds Products Concentration (mg/l) Open circuit Parent compounds Acetate Closed circuit Current Time (d) Current (ma) Transformation of the furanic and phenolic compounds was primarily due to microbial activity Two sub-steps: fermentation and exoelectrogenesis 8

9 Biotransformation Pathways Summary Compound Class Common reactions Aromatic ring cleavage Fermentative Transformation extent Number of O CH 3 Acetate production Exoelectrogenic capacity Possible ring cleavage pathway Syringyl Y High 2 High High Phloroglucinol Demethylation Guaiacyl ( O CH 3 ) Decarboxylation N Low 1 Low Low ( COOH) CoA addition p hydroxyl N Low 0 None detected Very low CoA addition The number and position of OH and O CH 3 substituents make a great difference in the extent of fermentative biotransformation of SA, VA and HBA 9

10 Electron Equivalents Flow Mass balance based on electron equivalents Parameters α: anode efficiency of the parent compound Y obs,1 / Y obs,2 : observed yield coefficient of fermentative/exoelectrogenic biomass, Fermentative biomass (X f ), 1, Exoelectrogenic biomass (X e ) Parent compound (S p ) 1, Exoelectrogenic substrates (S e ) Current/Cathodic H 2 (I) 1, 1, Non-exoelectrogenic end products (S ne ),, 10

11 Electron Equivalents Flow : experimentally measured for individual compounds Y obs,1, Y obs,2 : assumed; 0.1 and 0.15 g biomass-cod/g substrate-cod, respectively Variable (g COD) Fractionation factor Parent compound SA VA HBA FF HMF S p S e 1, S ne 1, 1, I X f, X e, 1, Low S e and I, but high S ne from VA and HBA; consistent with previous experimental results Estimated the quantity of fermentative and exoelectrogenic biomass, which cannot be easily measured experimentally Provided quantitative insights without the information on stoichiometric reactions 11

12 Conclusions Primary processes in bioanode: fermentation and exoelectrogenesis Negligible extent of electrochemical reactions Electron equivalents flow quantified based on two parameters: anode efficiency and observed biomass yield coefficients The findings of the present study advance our understanding of the processes and electron flow in MEC bioanode fed with complex, fermentable organic compounds resulting from the pretreatment of biomass The proposed mass-based framework of substrate utilization and electron flow can be used for the dynamic simulation of bioanode processes 12

13 Acknowledgement This work is supported by the U.S. Department of Energy, BioEnergy Technologies Office under the Carbon, Hydrogen and Separations Efficiency (CHASE) in Bio-Oil Conversion Pathways program, DE-FOA

14 Extra Slides 14

15 Physical and Chemical Properties Property Furfural (FF) 5-Hydroxymethyl furfural (HMF) Syringic acid (SA) Vanillic acid (VA) 4-Hydroxybenzoic acid (HBA) Molecular formula C 5 H 4 O 2 C 6 H 6 O 3 C 9 H 10 O 5 C 8 H 8 O 4 C 7 H 6 O 3 Molecular Weight logk ow Water solubility (g/l, 25 o C) pk a NA f (kj/mol) (V) ThOD (g O 2 /g) Electron equivalents (e- mol/mol)

16 Material and Methods Microbial Electrolysis Cell (MEC) H-type configuration (250 ml each chamber) Typical, commercially available electrode materials Room temperature (20-22 o C) Anolyte: mineral microbial growth medium (buffered, ph 7.0) Catholyte: 100 mm phosphate buffer ph 7.0 (Flushed with N 2 prior to use; N 2 headspace) Displacing solution: acid brine (10% NaCl w/v, 2% H 2 SO 4 v/v) 16

17 Material and Methods Microbial Electrolysis Cell (MEC) Potentiostat V W.E. C.E. & REF Bioanode Abiotic Cathode 17

18 Cyclic Voltammetry Non-active bioanode, w/o acetate Non-active bioanode, upon acetate addition Active bioanode, after 1 day incubation with acetate Rinsed, active bioanode in fresh anolyte B Current (ma) Potential (V, vs. Ag/AgCl)

19 Exoelectrogenic Capacity of Individual Compounds Batch-fed MEC, 1 mm initial concentration, 6-day duration Initial ph 7.0 (anode & cathode); end ph 6.8 ~ 7.0 (anode), 7.0 ~ 7.2 (cathode) Removal (%) A SA VA HBA FF HMF Compound Current (ma) B SA VA HBA FF HMF Time (d) Cumulative H 2 o C) C None detected SA VA HBA FF HMF Compound Compound Removal Complete removal of all parent compounds by 6 d Current Different current The highest current resulted from SA and FF H 2 Production Different H 2 production SA > FF > HMF >> VA, HBA Consistent with the cumulative current Zeng, X.; Collins, M.A.; Borole, A.B.; Pavlostathis, S.G., Water Research,

20 Exoelectrogenic Capacity of Individual Compounds Parameter Bioanode batch run with: SA VA HBA FF HMF e Total substrate input (e - mmol) Substrate eeq moles Substrate H + H 2 H + Electrons recovered as current (e - mmol) Cumulative current Anode Cathode Anode efficiency (%) substrate current (COD removal Coulombic efficiency) SA Electrons recovered as cathodic H 2 (e - mmol) Measured ND Cathode efficiency (%) current H NA VA HBA Zeng, X.; Collins, M.A.; Borole, A.B.; Pavlostathis, S.G., Water Research,

21 Summary Exoelectrogenesis Fermentation Acetate Parent Compounds Intermediates Current (H 2 ) H 2 CO 2 Homoacetogenesis Furanic and phenolic compounds utilized as the sole carbon and energy source Promising H 2 yield, Coulombic efficiency Transformation mainly due to microbial activity, i.e., fermentation followed by exoelectrogenesis Microbial species in different physiological groups 21

22 Biotransformation Pathways Syringic Acid (SA) MEC Bioanode Fermentative Culture Direct substrate for exoelectrogens Concentration (mm) SA DHMBA GA Pyrogallol Acetate Total carbon accounted as SA Concentration (mm) System MEC Bioanode Fermentative Culture Total Protein (mg/l) 352 ± ±1.5 Time (d) Time (d) 22

23 Biotransformation Pathways Vanillic Acid (VA) Possible Anaerobic Pathway of Ring Cleavage (Schink et al., Naturwissenschaften, 2000) Energy demanding for fermenters Lacking requisite microbial species Extremely slow rate Concentration (mm) Concentration (mm) Time (d) VA PA Catechol Acetate Total carbon accounted as VA 23

24 Biotransformation Pathways Syringic Acid (SA) MEC Bioanode Fermentative Culture Direct substrate for exoelectrogens Concentration (mm) SA DHMBA GA Pyrogallol Acetate Total carbon accounted as SA Concentration (mm) System MEC Bioanode Fermentative Culture Total Protein (mg/l) 352 ± ±1.5 Time (d) Time (d) 24