Bioelectrochemical Conversion of CO2 to Chemicals: CO2 as Next Generation Feedstock for the Electricity-driven Bioproduction in Batch and Continuous mode
The recent concept of microbial electrosynthesis (MES) has evolved as an electricity-driven production technology of chemicals from low-value carbon dioxide (CO2) using micro-organisms as biocatalysts. MES from CO2 comprises bioelectrochemical reduction of CO2 to multi-carbon organic compounds using the reducing equivalents produced at the electrically-polarized cathode. The use of CO2 as a feedstock for chemicals is gaining much attraction, since CO2 is abundantly available and its use is independent of the food supply chain. MES based on CO2 reduction produces acetate as a primary product. In order to elucidate the performance of the bioelectrochemical CO2 reduction process at different operation modes (batch vs. continuous), an investigation was carried out with a MES system using a flow-through biocathode supplied with 20:80 (v/v) or 80:20 (v/v) CO2:N2 gas. Hightest acetate production rate of 149 mg L-1 d-1 was observed at 3.1 V applied cell-voltage under batch mode. While running in continuous mode, high acetate production were achieved with the maximun rate of 100 mg L-1 d-1. In the continuous mode, acetate production did not sustain for long-term operation likely due to insufficient microbial biocatalyst retention within the biocathode compartment (i.e. suspended micro-organisms where washed out of the system). Restarting batch mode operations resulted into the renewed production of acetate. This showed an apparent domination of suspended biocatalysts over the attached (biofilm forming) biocatalyst. The long term CO2 reduction at the biocathode resulted into the accumulation of acetate, and more reduced compounds like ethanol and butyrate were also formed. Improvements in production rates and different biomass retention strategies (e.g. selecting for biofilm forming micro-organisms) should be investigated to enable continous biochemical production from CO2 in MES. Certainly, other process optimizations will be required to establish MES as innovative sustainable technology for manufacturing biochemicals from CO2 as a next generation feedstock.