In-situ membrane electrolysis enables high-rate production and electrochemical pH control in microbial electrosynthesis of acetic acid from carbon dioxide

Microbial electrosynthesis (MES) has emerged as a new bioreactor technology for the production of organics from CO2 and renewable current. In a previous report we presented a novel reactor configuration that can uniquely couple the production and recovery of acetic acid from CO2 through the integration of in situ membrane electrolysis; extraction of the produced acetate over an anion exchange membrane (AEM). This three-chambered reactor design allows the simultaneous production, extraction and concentration of the product as a single organic acid in a solid-free extraction liquid (Gildemyn et al, 2015). Although it has been shown that electricitydriven extraction of acetate over an AEM allows pure product recovery at high efficiency, the impact of the electrolytic operation on the production process still remains unknown. In this study we compared the performance of the three-compartment reactor with two conventional, two-compartment reactors that lack the intrinsic property to extract the charged products from the catholyte. Both a cation exchange membrane (CEM) and bipolar membrane (BPM) were tested as the separation barrier between the anode and the cathode in two-compartment reactors. Acetate production was cathodically driven by a mixed microbial culture at a fixed current density of 5 A m-2. During a 43 days batch run in the reactor with in situ extraction a production rate of 13.8 g/m² projected cathode surface/d was observed, an increase of 57% and 41% in comparison with the production rate in the reactors with CEM and BPM, respectively. This performance enhancement is obtained through the combination of a stable pH environment and a low cathodic VFA concentration, both preventing the inhibition caused by undissociated organic acids. The pH of the catholyte remained stable at 8.2 ± 0.2, in contrast to the two-compartment reactors where a pH decrease below 5.3 was observed when VFAs were produced and accumulated. Whereas a base addition up to 16 mmol OH-/g acetate produced was needed to correct the pH, membrane electrolysis fully replaced chemical pH control through the in situ electrochemical production of hydroxide ions. This study demonstrates that in addition to pure product recovery, membrane electrolysis generates a stable pH environment in the biocathode, resulting in a zero-chemical input process at higher production rate in comparison with MES reactors without extraction.

Publication year:2016