Integrated production, extraction and concentration of acetic acid from CO2 through microbial electrosynthesis

Microbial electrosynthesis (MES) can uniquely couple production and recovery of products such as acetic acid from CO2. We demonstrate that an acetic acid concentration of up to 13.5 g L-1 (225 mM) can be achieved (Figure 1A), the highest reported for MES. In contrast to previous MES studies, a separated single acidic product was generated through in situ membrane electrolysis in a three-compartment reactor. A fixed current of 5 A m-2 was applied, driving acetate production by a mixed microbial culture in the catholyte. Acetate was extracted over an anion exchange membrane to the extraction compartment and protonated due to the low pH of the extractant, thus preventing acetate back diffusion into the catholyte. A cation exchange membrane (CEM) prevented chloride migration and undesired oxidation on the anode, while allowing proton migration from the anolyte to the extractant. This three-compartment design allows production of high concentrations of acetic acid as a single organic acid in an abiotic, solid-free extraction liquid, and the in situ extraction allows a stable production rate unhindered by product accumulation. During the 86 days of batch operation the pH of the catholyte remained stable at 8.4 ± 0.5, in contrast to two-compartment reactors with a CEM where a pH decrease below 5.5 is observed when VFAs are produced and accumulated. Acetate production rates over 86 days were 0.58 g d-1 L-1catholyte or 20.4 g d-1 m-2projected cathode surface, and extraction rates were 19.7 g d-1 m-2membrane surface. The three-compartment reactor outperformed batch and continuous two-compartment reactors with CEM (Figure 1B). This is the first study presenting MES from CO2 in the framework of a complete bioproduction pipeline. The possibility to operate the reactor under very stable circumstances while simultaneously extracting and concentrating the product as acetic acid are key factors in the development of this technology