Effect of pH on selective butyrate production from CO2 and methanol in microbial electrosynthesis
Methanol assisted microbial electrosynthesis (MES) enables butyrate production from carbon dioxide and methanol by using external electricity. The effect of operational parameters on the butyrate production remains unclear. In the present study, the effect of pH on the methanol assisted MES process was investigated by comparing the performance at three pH values (5.5, 6 and 7). The most selective butyrate production was obtained at pH 6, with the highest butyrate selectivity of 87% (carbon based) and the highest butyrate production rate of 0.3 g L-1 d-1. At pH 7, the similar butyrate production rate was achieved at 0.3 g L-1 d-1, yet with the lower butyrate selectivity (70%). The butyrate production was hindered at pH 5.5 with the production rate of 0.1 g L-1 d-1, and 81% selectivity. The higher pH increased the methanol and CO2 consumption, resulting in a more negative cathodic potential and a more reduced redox potential, as well as affecting the thermodynamical feasibility of involved reactions. At all pH values, the microbial communities were dominated by the Eubacterium Callander, which assimilated methanol and CO2 via the Wood-Ljungdahl pathway and produced butyrate.