Methane
Production in Microbial Reverse-Electrodialysis
Methanogenesis Cells (MRMCs) Using Thermolytic Solutions
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Abstract
The
utilization of bioelectrochemical systems for methane production
has attracted increasing attention, but producing methane in these
systems requires additional voltage to overcome large cathode overpotentials.
To eliminate the need for electrical grid energy, we constructed a
microbial reverse-electrodialysis methanogenesis cell (MRMC) by placing
a reverse electrodialysis (RED) stack between an anode with exoelectrogenic
microorganisms and a methanogenic biocathode. In the MRMC, renewable
salinity gradient energy was converted to electrical energy, thus
providing the added potential needed for methane evolution from the
cathode. The feasibility of the MRMC was examined using three different
cathode materials (stainless steel mesh coated with platinum, SS/Pt;
carbon cloth coated with carbon black, CC/CB; or a plain graphite
fiber brush, GFB) and a thermolytic solution (ammonium bicarbonate)
in the RED stack. A maximum methane yield of 0.60 ± 0.01 mol-CH<sub>4</sub>/mol-acetate was obtained using the SS/Pt biocathode, with
a Coulombic recovery of 75 ± 2% and energy efficiency of 7.0
± 0.3%. The CC/CB biocathode MRMC had a lower methane yield of
0.55 ± 0.02 mol-CH<sub>4</sub>/mol-acetate, which was twice that
of the GFB biocathode MRMC. COD removals (89–91%) and Coulombic
efficiencies (74–81%) were similar for all cathode materials.
Linear sweep voltammetry and electrochemical impedance spectroscopy
tests demonstrated that cathodic microorganisms enhanced electron
transfer from the cathode compared to abiotic controls. These results
show that the MRMC has significant potential for production of nearly
pure methane using low-grade waste heat and a source of waste organic
matter at the anode