Ex-situ electrochemical characterisation of fixed-bed denitrification biocathodes: A promising strategy to improve bioelectrochemical denitrification.

The worldwide issue of nitrate-contaminated groundwater requires practical solutions, and electro-bioremediation offers a promising and sustainable treatment. While it has shown potential benefits, there is room for improvement in treatment rates, which is crucial for its further and effective implementation. In this field, electrochemical characterisation is a valuable tool for providing the foundation for optimising bioelectrochemical reactors, but applying it in fixed-bed reactors is challenging due to its high intrinsic electrical resistance. To overcome these challenges, this study employed the easy and swift eClamp methodology to screen different process parameters and their influence on the performance of fixed-bed denitrifying biocathodes composed of granular graphite. Granules were extracted and studied ex-situ under controlled conditions while varying key operational parameters (such as pH, temperature, and nitrate concentration). In the studied biocathode, the extracellular electron transfer associated with denitrification was identified as the primary limiting step with a formal potential of -0.225 ± 0.007 V vs. Ag/AgCl sat. KCl at pH 7 and 25 °C. By varying the nitrate concentration, it was revealed that the biocathode exhibits a strong affinity for nitrate (K of 0.7 ± 0.2 mg N-NO L). The maximum denitrification rate was observed at a pH of 6 and a temperature of 35 °C. Furthermore, the findings highlight a 2e/1H transfer, which holds considerable implications for the energy metabolism of bioelectrochemical denitrifiers. These compiled results provide valuable insights into the understanding of denitrifying biocathodes and enable the improvement and prediction of their performance.