Extracellular electron transfer modes and rate-limiting steps in denitrifying biocathodes

Area of Science:

• Environmental Science
• Electrochemistry
• Microbiology

Background:

• Denitrifying bioelectrochemical systems (DBES) offer a sustainable approach for wastewater nitrogen removal and energy generation.
• The efficiency of DBES is significantly influenced by extracellular electron transfer (EET) mechanisms and rate-limiting steps within denitrifying biocathodes.

Purpose of the Study:

• To investigate the EET modes and rate-limiting steps governing nitrate and nitrite reduction in denitrifying biocathodes.
• To elucidate the electrochemical behavior of denitrifiers during wastewater treatment.

Main Methods:

• Cyclic voltammetry was employed to analyze the electrochemical reactions occurring at the denitrifying biocathode.
• Cathode potential was swept to identify characteristic peaks associated with different reduction and oxidation processes.

Main Results:

• Three distinct peak potentials (E1, E2, E3) were identified, corresponding to nitrate reduction, nitrite oxidation, and nitrite reduction, respectively.
• Nitrate reduction primarily occurs via direct electron transfer, with intracellular catalytic reaction as the rate-limiting step.
• Nitrite reduction involves mediated electron transfer, and its rate-limiting step is influenced by nitrate supply, involving both mediator transfer and intracellular catalysis under sufficient nitrate conditions.

Conclusions:

• The findings clarify the distinct EET pathways for nitrate and nitrite reduction in DBES.
• Understanding these rate-limiting steps is crucial for enhancing the performance of denitrifying biocathodes in wastewater treatment and energy recovery applications.