Abstract
The utilization of propionic, n-butyric, and isobutyric acids in microbial electrolysis cells (MECs) was examined by monitoring individual short-chain fatty acid concentration and using electrochemical techniques, such as linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). When n-butyric or isobutyric acid was provided as a single substrate, acetic acid was consistently observed in experiments, indicating that acetic acid was produced as a byproduct and utilized by exoelectrogenic bacteria as an additional substrate in MECs. When isobutyric acid was given as a sole substrate, the applied potential governed the electric current (i.e., rate of substrate utilization). In addition, the coulombic efficiency was substantially high (90%), indicating direct utilization of isobutyric acid by exoelectrogenic bacteria. However, the coulombic efficiency was relatively low (30-60%) when n-butyric acid was provided as a sole substrate. In another experiment, the magnitude of electric current was more dependent on the concentration of acetic acid than that of other short-chain fatty acids. In the EIS analysis, the exchange current was found to be a more reliable indicator of substrate favorability than the charge transfer resistance.
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