Microbial electrosynthesis of butyrate from carbon dioxide: Production and extraction

To date acetate is the main product of microbial electrosynthesis (MES) from carbon dioxide (CO 2 ). 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 − 0.8 V 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 (p H2 ). During the experiment, CO 2 supply was limited to build up p H2 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 59.7 mMC versus 20.3 mMC of acetate, but limitation on CO 2 supply resulted in low product titers. CO 2 limitation was relaxed in test 2 to increase the bioelectrochemical activity but increase p H2 and promote the production of butyrate, what resulted in the production of 87.5 mMC of butyrate and 34.7 mMC 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 252.4 mMC 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 MESThis research was financially supported by the Spanish Ministry of Economy and Competitiveness (CTQ 2014-53718-R) and the University of Girona (MPCUdG2016/137). P. B-V gratefully acknowledges the Catalan Government for the pre-doctoral grant received (2015FI-B2 00076). R.G. gratefully acknowledges support from Ghent University BOF postdoctoral fellowship (BOF15/PDO/068). LEQUIA and EcoAqua have been recognised as consolidated research group by the Catalan Government (Refs. 2014-SGR-1168 and 2014-SGR-484, respectively