Bioelectrochemical Conversion of Carbon Dioxide to Methane for Biogas - - PowerPoint PPT Presentation

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Bioelectrochemical Conversion of Carbon Dioxide to Methane for Biogas - - PowerPoint PPT Presentation

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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

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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

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 Opportunities for CO2 reuse and energy recovery in municipal wastewater treatment plants (WWTP), now referred to as Water Resource Recovery Facilities (WRRF)

Municipal Wastewater Treatment

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Biogas (CO2, CH4, trace gases)

Liquid digestate, Biosolids

Complex organic compounds

Carbohydrates, Proteins, Fats

Simple organic compounds

Sugars, Amino acids, Fatty acids

Organic acids and alcohols CO2, CH4 H2, CO2 Acetate

Hydrolysis Acidogenesis Acetogenesis Methanogenesis

Gujer and Zehnder, 1983

OS = 4 ‐ 1.5 (COD/TOC) CH4 (%) = 100 ‐ 12.5 (OS + 4)

Anaerobic Digestion & Biogas Composition

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Carbon Capture Sequestration & Conversion

Absorption Chemical (MEA, caustic, etc.) Physical (Selexol, Rectisol, etc.) Adsorption Alumina Zeolite Activated C Cryogenics Membranes Gas separation (Polyphenyleneoxide,

Polydimethylsiloxane)

Gas absorption (Polypropelene) Ceramic based systems Microbial/Algal systems

Phototrophic bacteria, algae CO2 CH4 BES

CO2 Capture & Conversion

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Reduction

Microbial Fuel Cell (MFC)

Produces electrical current

Microbial Electrolysis Cell (MEC)

Produces hydrogen (H2)

Microbial Electromethanogenesis

Produces methane (CH4)

Microbial Electrosynthesis (MES)

Produces 2+ carbon compounds (e.g., acetate, methanol, etc.) A, Resistor (MFC) or applied potential (MEC) B, Proton exchange membrane R1, Reactant in the anode (oxidation half reaction) P1, Product in the anode R2, Reactant in the cathode (reduction half reaction) P2: Product in the cathode

2H+ + 2e‐ → H2 EH°' = ‐0.414 V CO2 + 8H+ + 8e‐ → CH4 + 2H2O EH°' = ‐0.244 V CO2 + 4H2 → CH4 + 2H2O ΔE°' = 0.170 V

At 25 °C, 1 atm, pH 7.

Oxidation

Bioelectrochemical Systems (BES)

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e‐

Direct Electron Transfer (DET) CO2 CH4 CO2 CH4 H2O H2 H2 H2O CH4 H2‐Mediated Electron Transfer (MET) Mox Mred H+ CO2 Mred Mox CO2 CH4 Non‐H2‐Mediated Electron Transfer

Electron Transfer Mechanisms in BES

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To develop and test bioelectrochemical systems (BESs) to directly convert CO2 to CH4 for anaerobic digester biogas upgrading

Overall Objective

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Materials & Methods

ANODE

  • Carbon felt

electrode/SS collector

  • Acetate‐fed (1.5 g

COD/L)

  • N2‐flushed headspace
  • Potential allowed to

fluctuate; measured against an adjacent Ag/AgCl reference electrode

  • 300 mL total volume
  • 250 mL liquid anolyte

(phosphate buffer, pH 7.0; trace minerals; vitamins)

  • Inoculated with

biofilm‐attached carbon felt from an active MFC CATHODE

  • Carbon felt

electrode/SS collector

  • CO2‐fed (1.6 atm,

absolute)

  • CO2‐flushed headspace
  • Potential fixed at ‐0.8 V

(vs. SHE) using an adjacent Ag/AgCl reference electrode

  • 300 mL total volume
  • 250 mL liquid catholyte

(phosphate buffer, pH 7.0; trace minerals; vitamins)

  • Inoculated with a

suspended‐growth, enriched hydrogenotrophic culture CO2 CH4 Acetate CO2

PEM Batch‐fed systems at 22±2oC Hydraulic retention time, 7 days

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Materials & Methods

Gases

Pressure transducer GC‐TCD for gas composition

Liquids

GC‐FID for acetate measurement Dissolved CO2 measured by sample acidification (6 N H2SO4) 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

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Biocathode performance with respect to:  Methanogenic inoculum  Hydrogen sulfide (H2S) gas feed contaminant  Anaerobic digester biogas feed (upgrading) Results

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Biocathode Performance – Effect of Inoculum

ACETATE (mg COD/L)

1 2 3 4

CURRENT DENSITY (A/m2)

5 10 15 20 5 10 15 20

TIME (d) HEADSPACE CH4 (mmol)

1 2 3

A C B

MM-B EHM-B MM-B EHM-B MM-B EHM-B

Biocathode methanogenic inocula: MM, mixed; EHM, pre‐enriched hydrogenotrophic

Dykstra, C.M.; Pavlostathis, S.G. 2017. Methanogenic biocathode microbial community development and the role of Bacteria. Environ. Sci. Technol. 51(9) 5306‐5316.

VOLTAGE (V)

  • 1.0 -0.8 -0.6 -0.4 -0.2

0.0 0.2 CURRENT (mA)

  • 20
  • 10

10 20 30 40 50 MM-biocathode EHM-biocathode

Biocathode Final Biofilm Biomass (mg) Mean CH4 Production (mmol CH4/mg biomass‐ day) MM‐ inoculated 0.54 ± 0.07 0.15 ± 0.01 EHM‐ inoculated 0.64 ± 0.19 0.59 ± 0.03

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Biocathode Performance – Effect of Inoculum

Biocathode methanogenic inocula: MM, mixed; EHM, pre‐enriched hydrogenotrophic

Dykstra, C.M.; Pavlostathis, S.G. 2017. Methanogenic biocathode microbial community development and the role of Bacteria. Environ. Sci. Technol. 51(9) 5306‐5316.

MM MM-biocathode EHM EHM-biocathode

RELATIVE ABUNDANCE (%)

20 40 60 80 100 Methanobrevibacter spp. Methanobrevibacter arboriphilus Methanoculleus spp. Methanolinea spp. Methanomethylovorans spp. Methanosaeta spp. Methanobacterium spp.

  • Biocathode archaeal communities

converged on the same phylotypes, Methanobrevibacter arboriphilus

  • Inoculum pre‐enrichment with H2/CO2

selects for methanogens that are also selected for by biocathode conditions (faster biocathode start‐up)

MM MM-biocathode EHM EHM-biocathode

RELATIVE ABUNDANCE (%)

20 40 60 80 100 Bacteroidetes Protoebacteria Actinobacteria Firmicutes Spirochaetes Synergistia Thermotogae Anaerolineales Acidobacteria Deferribacteres Chloroflexi Unclassified

Cathode

Cell lysis products Methanogens CO2, Oxidized carbon Exoelectrogen e‐, H+, CO2 CH4

Archaea Bacteria

  • MM‐biocathode enriched in

Spirochaetes and other non‐ exoelectrogenic, fermentative Bacteria

  • EHM‐biocathode enriched in

Proteobacteria, exoelectrogens and putative producers of electron shuttle mediators

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Biocathode Performance – Effect of Inoculum

Dykstra, C.M.; Pavlostathis, S.G. 2017. Methanogenic biocathode microbial community development and the role of Bacteria. Environ. Sci. Technol. 51(9) 5306‐5316.

  • The bacterial community of a biocathode has a significant effect on archaeal CH4 production
  • Increased biocathode CH4 production occurs with a bacterial community enriched in:
  • Putative producers of electron shuttles/mediators
  • Proteobacteria
  • Exoelectrogens

Acetate CO2 H2 CH4 Cell lysis debris Recycle lysed cells Produce electron shuttles cathode Mox Mred CH4 CO2 Role of Bacteria

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Biocathode Performance – Effect of H2S

Biocathode Headspace H2S (1% v/v)

5 10 15 20

CURRENT DENSITY (A/m2)

20 40 60 80

TIME (d)

5 10 15 20

METHANE (mmol)

2 4 6 8

A B

BES1 (Control) BES2 (H2S amended) Headspace H2S (% v/v) CE (%) CCE (%) 11 100 4 19 99 5 58 13 6 58 15

INITIAL CATHODE HEADSPACE H2S (%)

1 2 3 4 5 6

INITIAL 3-DAY CH4 PRODUCTION RATE (mmol/d)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 n = 7 n = 3 n = 3 n = 3 n = 1 n = 1 n = 1

Two competing effects:

  • Depression of CH4 production (≥4% H2S):

Inhibition of methanogens?

  • Enhancement of CH4 production (≤3% H2S):

What is/are the process(es) involved?

CE, Coulombic efficiency CCE, cathode capture efficiency

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Biocathode Performance – Effect of H2S

Gas transport between biocathode and bioanode Anode Cathode

Potentiostat

PEM

CO2 H2S CO2 N2 N2 N2 CO2 CO2

Ag/AgCl reference electrode

CH4 N2, CO2, CH4

H+ H+ H+ H+ e‐ e‐

H2S H2S CO2 CH4 Acetate

Dykstra, C., Pavlostathis, S.G. (2017), “Evaluation of gas and carbon transport in a methanogenic bioelectrochemical system (BES)”, Biotechnology & Bioengineering, 114(5), 961-969.

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Biocathode Performance – Effect of H2S

CO2 CH4 CH4

e‐ e‐ H+

H2S is the most toxic of the sulfide species

80% 20% H2S

H2S HS‐ S2‐ CO2 CO2 H2S

High local pH Neutral pH

The methanogenic biocathode is protected from sulfide inhibition by biofilm formation and a local high pH at the cathode surface.

Cathode

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Biocathode Performance – Effect of H2S

Anode N2 CO2 CO2

Potential anode H2S

  • xidation products

S0 Sx

2‐

S4O6

2‐

S2O3

2‐

SO4

2‐

Sun et al., 2009. ES&T

N2

e‐

Acetate H2S SO4

2‐

Acetate CO2

SRB

  • Low H2S → more electrons donated to the anode → higher biocathode CH4

production

  • High H2S → smulate sulfur cycling → divert acetate eeq from the anode → lower

biocathode CH4 production

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Biocathode Performance – Effect of H2S

CARBON (mmol)

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 Suspended Biofilm BES1 Anode BES1 Cathode BES2 Anode BES2 Cathode

  • H2S stimulated total biomass growth in

both anode and cathode

  • H2S stimulated SRB growth in the anode

biofilm

BES1 cathode BES2 cathode

Control H2S‐amended

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Biocathode Performance – Effect of H2S

Bacteroidetes Proteobacteria Actinobacteria Firmicutes Spirochaetes Synergistia Unclassified

RELATIVE ABUNDANCE (%)

10 20 30 40 50 60 70 80 90 100

B BES1 BES2 Biofilm Susp. Biofilm Susp. RELATIVE ABUNDANCE (%)

10 20 30 40 50 60 70 80 90 100

A BES1 BES2 Biofilm Susp. Biofilm Susp.

Anode Cathode

  • Deltaproteobacteria were not detected in any anode or cathode biofilm or

suspended growth samples, except in the BES2 anode biofilm.

  • SRB phylotypes in the BES2 anode biofilm represented 32% of

Deltaproteobacteria and 1% of total Bacteria.

  • Identified SRB phylotypes include Desulfobulbus propionicus, Desulfovibrio sp.

and Syntrophobacterales spp.

Control H2S‐amended Control H2S‐amended

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Biocathode Performance – AD Biogas Feed

METHANE (mmol) 1 2 3 4 5 CARBON DIOXIDE (mmol) 2 4 6 HYDROGEN (mmol) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 TIME (d) 10 20 30 40 CURRENT DENSITY (A/m

2)

10 20 30 40

a b c d

  • Day 14: Switched from feeding the biocathode 100%

CO2 to feeding anaerobic digester biogas (53‐66% CH4 and 34‐47% CO2).

  • No CH4 production for the first 2 biogas feedings.

However, after the 2 days, the biocathode CH4 production rate increased substantially.

  • Although less total CO2 was removed during biogas‐

fed cycles than during CO2‐fed cycles, the remaining CO2 was consistently lower at the end of biogas‐fed cycles.

  • Hydrogen production occurred after switching to

biogas, likely due to the increase in current density and slowed methanogenesis.

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Biocathode Performance – AD Biogas Feed

CYCLE 5 10 15 20 25 MEAN CH4 PRODUCTION RATE (mmol/d) 0.0 0.5 1.0 1.5 2.0 INITIAL PARTIAL PRESSURE (atm) 0.0 0.5 1.0 1.5 2.0 CH4 rate Initial CO2 Initial CH4

  • Maximum CH4 production rate occurred during a biogas‐fed cycle (1.85 mmol/d), which was

350% higher than the maximum CH4 production rate during a CO2‐fed cycle (0.41 mmol/d).

  • No significant correlation between the CH4 production rate and the initial CO2 or CH4 partial

pressure.

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Biocathode Performance – AD Biogas Feed

APPLIED POTENTIAL (V vs. SHE)

  • 0.80 -0.75 -0.70 -0.65 -0.60 -0.55 -0.50

RATE (mmol CH4/d) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

  • At a more positive applied cathode potential, the cell

potential (driving force for electron transport) decreased and the anode potential decreased.

  • At lower anode potentials, the transfer of electrons

from a substrate to the anode is less energetically favorable.

  • However, anode acetate removal did not reflect the

biocathode CH4 production rate, likely due to microbial acetate uptake and storage.

Cathode Potential (V vs. SHE) Mean CH4 Production Ratea (mmol/d) Final Biocathode CH4

a (%)

Final Biocathode CO2

a (%)

Anode Acetate Removala (%) Anode Potential (V vs. Ag/AgCl) Cell Potential (V) ‐0.80 1.22 ± 0.07 79.9 ± 1.4 3.7 ± 0.1 21.2 ± 0.3 1.14‐1.17 2.28‐2.33 ‐0.75 0.98 ± 0.04 76.1 ± 0.9 4.1 ± 0.1 9.3 ± 1.1 1.09‐1.10 2.14‐2.17 ‐0.70 0.87 ± 0.12 78.3 ± 2.5 4.8 ± 0.1 NRb 1.04‐1.07 2.05‐2.08 ‐0.65 0.97 ± 0.05 78.4 ± 1.0 5.4 ± 0.1 9.1 ± 5.3 1.03‐1.04 1.98‐2.01 ‐0.60 0.74 ± 0.04 72.4 ± 0.9 6.2 ± 0.2 13.4 ± 0.4 1.02‐1.04 1.93‐1.94 ‐0.55 0.86 ± 0.14 70.7 ± 2.7 6.4 ± 0.1 NR 1.02‐1.05 1.88‐1.90 ‐0.50 0.53 ± 0.08 76.7 ± 1.7 8.3 ± 0.1 4.0 ± 0.1 0.16‐0.36 1.15‐1.34

a Mean ± standard deviation; n = 3; b No removal.

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Biocathode Performance – AD Biogas Feed

Do biogas components contribute to BES current?

  • Other biogas components (CH4, H2S and NH3)

may contribute to increasing current and CH4 production.

  • CO2, CH4, N2, H2 and H2S can be transported

as dissolved gases across a Nafion 117 proton exchange membrane when there is a concentration gradient.1

  • As reduced species, CH4, H2S and NH3 could

become oxidized at the bioanode, donating electrons to the electrode and contributing to current production.

CH4 CO2 H2S S0, SO4

2‐,

  • thers?

ANODE

e‐ NH4

+

NO3

‐,

N2

1Dykstra, C., Pavlostathis, S.G. (2017),

“Evaluation of gas and carbon transport in a methanogenic bioelectrochemical system (BES)”, Biotechnology & Bioengineering, 114(5), 961-969.

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Biocathode Performance – AD Biogas Feed

Do biogas components contribute to BES current?

  • At more negative applied cathode potentials, the maximum possible contribution of CH4, H2S and NH3

to the charge transferred during a 1‐d feeding cycle was minimal.

  • NH3 was not an important contributor to charge due to its low abundance in biogas.
  • At ‐0.80 V cathode potential, the difference in the amount of charge transferred in the biogas‐fed cycle

and the CO2‐fed cycle cannot be completely explained by the oxidation of biogas CH4, H2S and NH3 at the anode.

  • Changes in microbial community and/or gene expression? (Under investigation)

CHARGE TRANSFERRED (C) 50 100 150 200 APPLIED POTENTIAL (V vs. SHE)

  • 0.80
  • 0.75
  • 0.70
  • 0.65
  • 0.60
  • 0.55
  • 0.50

CH4 H2S NH3 Biogas-fed CO2-fed

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Conclusions

  • The biocathode bacterial community significantly affects archaeal CH4 production.

Exoelectrogenic heterotrophs may promote biocathode CH4 production within a biofilm fed only with CO2.

  • H2S, a common biogas contaminant, is transported across the proton exchange

membrane from cathode to anode, where it is oxidized and contributes electrons to the anode.

  • At low H2S concentrations (≤ 3%, v/v), H2S increases biocathode CH4 production due

to increased current. At high H2S concentrations (≥ 4%, v/v), H2S may be inhibitory to the methanogenic biofilm and reduce overall biocathode CH4 production.

  • Anaerobic digester biogas was successfully upgraded using a methanogenic

bioelectrochemical system.

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Acknowledgement

For further information contact: S. G. Pavlostathis E-mail: spyros.pavlostathis@ce.gatech.edu CEE Website: http://www.ce.gatech.edu/people/faculty/961/overview

This material is based in part upon work supported by the US National Science Foundation Graduate Research Fellowship under Grant No. DGE‐1148903.

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Extra Slides

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MATERIALS AND METHODS 28

Efficiency Calculations

CE, Coulombic Efficiency: The ratio of total Coulombs actually transferred to the anode from the substrate, to maximum possible Coulombs if all substrate removal produced current. [1] CCE, Cathode Capture Efficiency: The ratio of total Coulombs actually transferred to the CH4 from the cathode, to maximum possible Coulombs if all current produced CH4. [2]

Electrochemical Analyses

Ohm’s Law: I = V/R where I is the current, V is the voltage between the anode and cathode and R is the sum of all resistances within the circuit. Cyclic Voltammetry: [3]

[1] Logan et al., 2006. ES&T; [2] Villano et al., 2013. Bioresource Technol.; [3] http://www.ceb.cam.ac.uk/research/groups/rg‐eme/teaching‐ notes/linear‐sweep‐and‐cyclic‐voltametry‐the‐principles

Reversible rxn Non‐reversible rxn Biofilm development on the cathode may catalyze the cathodic reaction and reduce the total resistance in the circuit, thereby increasing the current at a particular applied voltage.

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EFFECT OF CATHODE H2S ON BES PERFORMANCE 29

  • Corrosive, toxic (NIOSH, IDLH = 100 ppm)
  • Produced by sulfate‐reducing bacteria during

anaerobic digestion and by the breakdown of HS‐ containing compounds (e.g., cysteine)

  • Inhibitory to methanogenesis during anaerobic

digestion [1]

  • Feedstock C:S ratio predicts biogas H2S [2]

Desulfovibrio vulgaris

[1] Chen, Y., et al., 2008. Biores.

  • Technol. 99(10), 4044‐4064.

[2] Peu, P., et al., 2012. Bioresource

  • Technol. 121, 419‐424.

Feedstock C/S (g/g) Theoretical Biogas H2S (%, range) Grease trap waste 798 0.0 – 0.1 Biological sludge 59 0.6 – 1.9 Industrial WW biological sludge 46 0.8 – 2.0 Pig bristles 19 2.0 – 4.9 Harvested green seaweed 7 5.5 – 17.7

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SLIDE 30

TIME (d)

2 4 6 8 10

METHANE (mmol)

1 2 3

R2 = 0.997 R2 = 0.999

EFFECT OF CATHODE H2S ON BES PERFORMANCE 30 7 cycles 3 cycles 3 cycles 3 cycles 1 cycle 1 cycle 1 cycle

100% CO2, no H2S 99% CO2, 1% H2S 98% CO2, 2% H2S 97% CO2, 3% H2S 96% CO2, 4% H2S 95% CO2, 5% H2S 94% CO2, 6% H2S

Linear biocathode CH4 production during the first 3 days of a feeding cycle

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EFFECT OF CATHODE H2S ON BES PERFORMANCE 31 Abiotic H2S Transport within a BES H2S Transport Across the Membrane

  • Abiotic BES with Pt‐coated carbon cloth anode and

carbon felt cathode

  • Open circuit conditions
  • Magnetically mixed
  • 3% H2S added to the headspace of the cathode
  • Cyclic voltammetry from ‐1.2 V to 0.2 V (vs. Ag/AgCl)

at 100 mV/s

  • Sodium sulfide calibration curve constructed to

convert current at 0.2 V vs. Ag/AgCl to sulfide ions

H2S Dissolution in the Cathode

  • Abiotic BES with Pt‐coated carbon cloth cathode and

carbon felt anode

  • Open circuit conditions
  • Magnetically mixed
  • 3% H2S added to the headspace of the cathode
  • Cyclic voltammetry from ‐1.2 V to 0.2 V (vs. Ag/AgCl)

at 100 mV/s; Measured current at 0.2 V.

TIME (min)

20 40 60 80

TOTAL SULFIDE IONS (mM)

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

TIME (min)

5 10 15 20 25 30

CURRENT (mA)

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18

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EFFECT OF CATHODE H2S ON BES PERFORMANCE 32

‐0,03 ‐0,02 ‐0,01 0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 ‐1,2 ‐1,0 ‐0,8 ‐0,6 ‐0,4 ‐0,2 0,0 Current (mA) Voltage (V vs. Ag/AgCl) 0% H2S 4% H2S 5% H2S 6% H2S

H2S (%) CE (%) CCE (%)

11 100 4 19 99 5 58 13 6 58 15

2 2 2 2

Above 4% H2S v/v, the cathode biofilm is significantly inhibited.