Shifts of the Rate-Limiting Step in Airlift Microbial Electrolysis Cells for Gaseous Propanethiol Degradation

The hydrophobicity and biotoxicity of volatile organic sulfur compounds hinder them from being efficiently biodegraded. Herein, combining the advantages of the airlift (AL) bioreactor and microbial electrolysis cell (MEC) on mass transfer and bioreaction enhancement, we proposed AL-MEC systems for gaseous propanethiol (PT) degradation for the first time, in which the electrodes were modified by polypyrrole (PPy) and polypyrrole/carbon nanotubes (PPy/CNTs) to further strengthen the biodegradation process. The resulting PPy-AL-MEC and PPy/CNTs-AL-MEC possessed the best PT removal efficiencies of 84.0 and 94.4%, respectively (PT, 50 mg m^–3; empty bed residence time (EBRT), 90 s; anode potential, 0.5 V vs. SHE), which were much higher than 68.8% in a control system with an unmodified graphite electrode (GS-AL-MEC). Moreover, the kinetics analysis reveals that the rate-limiting step shifted from biodegradation in GS-AL-MEC to mass transfer in PPy/CNTs-AL-MEC, while that in PPy-AL-MEC also shifted to mass transfer at an EBRT ≥ 45 s.