Abstract
Current challenges for microbial electrosynthesis include the production of higher value chemicals than acetate, at high rates, using cheap electrode materials. We demonstrate here the continuous, biofilm-driven production of acetate (C2), n butyrate (nC4), and n-caproate (nC6) from sole CO2 on unmodified carbon felt electrodes. No other organics were detected. This is the first quantified continuous demonstration of n-caproate production from CO2 using an electrode as sole electron donor. During continuous nutrients supply mode a thick biofilm was developed covering the whole thickness of the felt (1.2 cm deep), which coincided with high current densities and organics production rates. Current density reached up to -14 kA m-3electrode (-175 A m-2). Maximum sustained production rates of 9.8 ± 0.65 g L-1 day-1 C2, 3.2 ± 0.1 g L-1 day-1 nC4, and 0.95 ± 0.05 g L-1 day-1 nC6 were achieved (averaged between duplicates), at electron recoveries of 60-100%. Scanning electron micrographs revealed a morphologically highly diverse biofilm with long filamentous microorganism assemblies (~400 µm). n-Caproate is a valuable chemical for various industrial application, e.g. it can be used as feed additives or serve as precursor for liquid biofuels production.
Highlights
The envisioned circular economy is depending on efficient raw materialsuse without any harmful emissions into the environment (Sharon and Kamp, 2016)
During the first 133 days, while the reactors were operated in fed-batch mode, biofilm development was visually observed only on the carbon felt surface facing the membrane (Figures S1A–C in Supplementary Material)
Within the first few days after the continuous nutrients supply started, biofilm growth could be observed with naked-eyes on the opposite side of the carbon felt as well as throughout the felt (Figures S1D–H in Supplementary Material)
Summary
The envisioned circular economy is depending on efficient raw materials (re)use without any harmful emissions into the environment (Sharon and Kamp, 2016). Water is already a scarce good while four billion people face severe water shortage at least 1 month per year (Mekonnen and Hoekstra, 2016). Clean recycling technologies must be developed that are efficient in usage of water, nutrients, and carbon. Carbon building blocks can be recycled from biomass residues, organic wastes, and CO2 sources. Microbial electrosynthesis is an emerging clean technology in which CO2 can be converted into biochemicals like acetate, with an electrode as sole direct electron source (Rabaey and Rozendal, 2010). MES uses minimal amounts of water and naturally occurring microorganisms as cheap, robust, and self-repairing catalysts
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