Continuous long-term electricity-driven bioproduction of carboxylates and isopropanol from CO2 with a mixed microbial community

Abstract

Electricity-driven bioproduction processes such as microbial electrosynthesis enable converting CO2 and organic feedstocks into target chemicals with minimal addition of external chemicals. Bioelectrochemical CO2 conversion to (mainly) acetate has mostly been demonstrated in batch processes. Continuous reactor operation and the operational parameters associated with it have received limited attention. Here, we demonstrate that improving bioelectrochemical reactor design to a higher cathode surface to volume ratio results in an enhanced acetate titer; 5.7±0.74gL−1 (11.5±6.6gm−2d−1) in galvanostastically controlled (−5Am−2cathode) batch reactors with a mixed microbial community. A long-term and stable bioproduction process could be established in which hydraulic residence time (HRT) affected the product patterns as well as the acetate production rate, up to 21gm−2d−1 for an HRT of 3.3d (63% coulombic efficiency) was achieved; the highest reported thus far in a continuous process. The specific energy input per kilogram of acetic acid produced during batch and continuous processes (HRT: 3.3d) was 29±0.7 and 16±1.3kWhelkg−1, respectively. Butyrate and isopropanol were the other major biochemicals produced at maximum rates of 3.7 and 3.3gm−2d−1 (18.6% and 21.8% of the electrons, respectively) leading to titers of 0.67 and 0.82gL−1 during the continuous process. This is the first report on the production of a secondary alcohol (isopropanol), using a mixed culture, in CO2 fed systems. The product ratios between these organics could be steered based on operational pH and HRTs. Operating reactors at an HRT of 5 d at pH 5 led to stable production of butyrate (1.9±0.6gm−2d−1) and isopropanol (1.17±0.34gm−2d−1). Cyclic voltammetry suggested an “ennoblement” of the cathode over time, shifting the onset for reductive current by more than 150mV. Microbial community analysis revealed Acetobacterium as the main bacterial group involved in CO2 reduction to acetate, and the presence of diverse bacterial groups in response to different operational conditions.