Electrocatalytic metals and microorganisms can be combined for CO2 conversion in microbial electrosynthesis (MES). However, a systematic investigation on the nature of interactions between metals and MES is still lacking. To investigate this nature, we integrated a copper electrocatalyst, converting CO2 to formate, with microorganisms, converting CO2 to acetate. A co-catalytic (i. e. metabolic) relationship was evident, as up to 140 mg L-1 of formate was produced solely by copper oxide, while formate was also evidently produced by copper and consumed by microorganisms producing acetate. Due to non-metabolic interactions, current density decreased by over 4 times, though acetate yield increased by 3.3 times. Despite the antimicrobial role of copper, biofilm formation was possible on a pure copper surface. Overall, we show for the first time that a CO2 -reducing copper electrocatalyst can be combined with MES under biological conditions, resulting in metabolic and non-metabolic interactions.

Carbon dioxide (CO₂) can be converted to valuable products using different catalysts, including metal or biological catalysts (e. g. microorganisms). Some products formed by metal electrocatalysts can be further utilized by microorganisms, and therefore catalytic cooperation can be envisioned. To prevent cumbersome separations, it is beneficial when both catalyst work under the same conditions, or at least in the same reaction medium. Here, we will show that a formate-producing copper electrocatalyst can function in a biological medium. Furthermore, we will show that the effluent of the copper-containing reactor can be used without purification as the sole medium for a bio-reactor, inoculated with a mixed culture of microorganisms. In that second reactor, formate, H₂ and CO₂ are consumed by the microorganisms, forming acetate and methane. Compared to simple buffer electrolyte, catalytic activity of copper was improved in the presence of microbial growth medium, likely due to EDTA (Ethylenediaminetetraacetic acid) present in the latter.

In the biotechnological desulfurization process under haloalkaline conditions, dihydrogen sulfide (H ₂ S) is removed from sour gas and oxidized to elemental sulfur (S ₈ ) by sulfide-oxidizing bacteria. Besides S ₈ , the byproducts sulfate (SO ₄ ^2- ) and thiosulfate (S ₂ O ₃ ^2- ) are formed, which consume caustic and form a waste stream. The aim of this study was to increase selectivity toward S ₈ by a new process line-up for biological gas desulfurization, applying two bioreactors with different substrate conditions (i.e., sulfidic and microaerophilic), instead of one (i.e., microaerophilic). A 111-day continuous test, mimicking full scale operation, demonstrated that S ₈ formation was 96.6% on a molar H ₂ S supply basis; selectivity for SO ₄ ^2- and S ₂ O ₃ ^2- were 1.4 and 2.0% respectively. The selectivity for S ₈ formation in a control experiment with the conventional 1-bioreactor line-up was 75.6 mol %. At start-up, the new process line-up immediately achieved lower SO ₄ ^2- and S ₂ O ₃ ^2- formations compared to the 1-bioreactor line-up. When the microbial community adapted over time, it was observed that SO ₄ ^2- formation further decreased. In addition, chemical formation of S ₂ O ₃ ^2- was reduced due to biologically mediated removal of sulfide from the process solution in the anaerobic bioreactor. The increased selectivity for S ₈ formation will result in 90% reduction in caustic consumption and waste stream formation compared to the 1-bioreactor line-up.