Microbial electrosynthesis of acetate from CO2 under hypersaline conditions

Abstract

Microbial electrosynthesis (MES) enables the bioproduction of multicarbon compounds from CO₂ using electricity as the driver. Although high salinity can improve the energetic performance of bioelectrochemical systems, acetogenic processes under elevated salinity are poorly known. Here MES under 35-60 g L^-1 salinity was evaluated. Acetate production in two-chamber MES systems at 35 g L^-1 salinity (seawater composition) gradually decreased within 60 days, both under -1.2 V cathode potential (vs. Ag/AgCl) and -1.56 A m^-2 reductive current. Carbonate precipitation on cathodes (mostly CaCO₃) likely declined the production through inhibiting CO₂ supply, the direct electrode contact for acetogens and H₂ production. Upon decreasing Ca²⁺ and Mg²⁺ levels in three-chamber reactors, acetate was stably produced over 137 days along with a low cathode apparent resistance at 1.9 ± 0.6 mΩ m² and an average production rate at 3.80 ± 0.21 g m^-2 d^-1. Increasing the salinity step-wise from 35 to 60 g L^-1 gave the most efficient acetate production at 40 g L^-1 salinity with average rates of acetate production and CO₂ consumption at 4.56 ± 3.09 and 7.02 ± 4.75 g m^-2 d^-1, respectively. The instantaneous coulombic efficiency for VFA averaged 55.1 ± 31.4%. Acetate production dropped at higher salinity likely due to the inhibited CO₂ dissolution and acetogenic metabolism. Acetobacterium up to 78% was enriched on cathodes as the main acetogen at 35 g L^-1. Under high-salinity selection, 96.5% Acetobacterium dominated on the cathode along with 34.0% Sphaerochaeta in catholyte. This research provides a first proof of concept that MES starting from CO₂ reduction can be achieved at elevated salinity.

Keywords: Acetogenesis; Carbon capture and utilization; Carbonate precipitates; High salinity; Marine bacteria.

Graphical abstract

Fig. 1

Acetate production performance of S-MES with cathode poised at −1.2 V vs Ag/AgCl using simulated seawater as catholyte. a, VFA production; b, The chronoamperogram of the cathode; c, Coulombic efficiency for cumulative VFA production; d, Cathode CV scan results.

Fig. 2

a–b, SEM-EDX micrographs of the S-MES cathode after the experiment with precipitation on the electrode surface (a) and a new cathode before the experiment (b). c, The XRD patterns of the white precipitate and pure CaCO₃ and NaCl.

Fig. 3

Acetate production performance of B-MES with a reductive current density of −1.56 A m⁻² using simulated seawater as catholyte during the first three cycles but lower Ca²⁺ and Mg²⁺ concentrations in cycle 4. a, VFA production; b, Cathode CV scan results.

Fig. 4

Stable acetate production performance of 3C-B_c-MES at seawater salinity (35 g L⁻¹) with low Ca²⁺ and Mg²⁺ concentrations. a, VFA production; b, System voltage; c, Coulombic efficiency based on cumulative VFA production; d, Cathode CV scan results.

Fig. 5

Acetate production performance of 3C–B_i-MES as the salinity gradually increased from 35 to 60 g L⁻¹ in conjunction with low Ca²⁺ and Mg²⁺ concentrations. a, VFA production; b, System voltage; c, Coulombic efficiency for cumulative VFA production; d, Cathode CV scan results.

Fig. 6

SEM images of biofilms on cathodes from different systems and of the blank cathode. a, S-MES; b, B-MES; c, 3C-B_c-MES; d, 3C–B_i-MES systems; e, The blank cathode.

Fig. 7

Microbial community composition of the cathode and catholyte consortia from B-MES, 3C-B_c-MES, and 3C–B_i-MES systems in genus level over 1% based on 16S rRNA gene analysis.