The industrial adoption of microbial electrosynthesis (MES) is hindered by high overpotentials deriving from low electrolyte conductivity and inefficient cell designs. In this study, a mixed microbial consortium originating from an anaerobic digester operated under saline conditions (∼13 g L-1 NaCl) was adapted for acetate production from bicarbonate in galvanostatic (0.25 mA cm-2) H-type cells at 5, 10, 15, or 20 g L-1 NaCl concentration. The acetogenic communities were successfully enriched only at 5 and 10 g L-1 NaCl, revealing an inhibitory threshold of about 6 g L-1 Na+. The enriched planktonic communities were then used as inoculum for 3D printed, three-chamber cells equipped with a gas diffusion biocathode. The cells were fed with CO2 gas and operated galvanostatically (0.25 or 1.00 mA cm-2). The highest production rate of 55.4 g m-2 d-1 (0.89 g L-1 d-1), with 82.4% Coulombic efficiency, was obtained at 5 g L-1 NaCl concentration and 1 mA cm-2 applied current, achieving an average acetate production of 44.7 kg MWh-1. Scanning electron microscopy and 16S rRNA sequencing analysis confirmed the formation of a cathodic biofilm dominated by Acetobacterium sp. Finally, three 3D printed cells were hydraulically connected in series to simulate an MES stack, achieving three-fold production rates than with the single cell at 0.25 mA cm-2. This confirms that three-chamber MES cells are an efficient and scalable technology for CO2 bio-electro recycling to acetate and that moderate saline conditions (5 g L-1 NaCl) can help reduce their power demand while preserving the activity of acetogens.
Keywords: Acetobacterium; Bioelectrochemical system; Conductivity; Electrochemical cell design; Gas diffusion electrode.
© 2023 The Authors.