Microbial electrosynthesis of butyrate from carbon dioxide: Production and extraction.

To date acetate is the main product of microbial electrosynthesis (MES) from carbon dioxide (CO2). In this work a tubular bioelectrochemical system was used to carry out MES and enhance butyrate production over the other organic products. Batch tests were performed at a fixed cathode potential of -0.8V vs SHE. The reproducibility of the results according to previous experiments was validated in a preliminary test. According to the literature butyrate production could take place by chain elongation reactions at low pH and high hydrogen partial pressure (pH2). During the experiment, CO2 supply was limited to build up pH2 and trigger the production of compounds with a higher degree of reduction. In test 1 butyrate became the predominant end-product, with a concentration of 59.7mMC versus 20.3mMC of acetate, but limitation on CO2 supply resulted in low product titers. CO2 limitation was relaxed in test 2 to increase the bioelectrochemical activity but increase pH2 and promote the production of butyrate, what resulted in the production of 87.5mMC of butyrate and 34.7mMC of acetate. The consumption of ethanol, and the presence of other products in the biocathode (i.e. caproate) suggested that butyrate production took place through chain elongation reactions, likely driven by Megasphaera sueciensis (>39% relative abundance). Extraction and concentration of butyrate was performed by liquid membrane extraction. A concentration phase with 252.4mMC of butyrate was obtained, increasing also butyrate/acetate ratio to 16.4. The results are promising for further research on expanding the product portfolio of MES.