The Effect of Bismuth and Tin on Methane and Acetate Production in a Microbial Electrosynthesis Cell Fed with Carbon Dioxide.

This study investigates the impacts of bismuth and tin on the production of CH 4 and volatile fatty acids in a microbial electrosynthesis cell with a continuous CO 2 supply. First, the impact of several transition metal ions (Ni 2+ , Fe 2+ , Cu 2+ , Sn 2+ , Mn 2+ , MoO 4 2- , and Bi 3+ ) on hydrogenotrophic and acetoclastic methanogenic microbial activity was evaluated in a series of batch bottle tests incubated with anaerobic sludge and a pre-defined concentration of dissolved transition metals. While Cu is considered a promising catalyst for the electrocatalytic conversion of CO 2 to short chain fatty acids such as acetate, its presence as a Cu 2+ ion was demonstrated to significantly inhibit the microbial production of CH 4 and acetate. At the same time, CH 4 production increased in the presence of Bi 3+ (0.1 g L -1 ) and remained unchanged at the same concentration of Sn 2+ . Since Sn is of interest due to its catalytic properties in the electrochemical CO 2 conversion, Bi and Sn were added to the cathode compartment of a laboratory-scale microbial electrosynthesis cell (MESC) to achieve an initial concentration of 0.1 g L -1 . While an initial increase in CH 4 (and acetate for Sn 2+ ) production was observed after the first injection of the metal ions, after the second injection, CH 4 production declined. Acetate accumulation was indicative of the reduced activity of acetoclastic methanogens, likely due to the high partial pressure of H 2 . The modification of a carbon-felt electrode by the electrodeposition of Sn metal on its surface prior to cathode inoculation with anaerobic sludge showed a doubling of CH 4 production in the MESC and a lower concentration of acetate, while the electrodeposition of Bi resulted in a decreased CH 4 production.