Reinforcement of denitrification in a biofilm electrode reactor with immobilized polypyrrole/anthraquinone-2,6-disulfonate composite cathode.

In biofilm electrode reactors (BER), good nitrate removal performance can be achieved through cooperation of heterotrophic and hydrogen autotrophic denitrification under low carbon/nitrogen conditions. In this study, we proposed a more multifunctional composite cathode, which combine immobilized anthraquinone-2,6-disulphonic disodium salt (AQDS) with polypyrrole (PPy) by electrochemical polymerization-doping method. The nitrate removal performance in BER with PPy/AQDS composite cathode was obviously improved, the nitrate removal rate (4.96 mg/L·h) was almost 2.0 times higher than the control BER system, and relatively stabled nitrate removal efficiency (≥90.0%) was also achieved even as the COD/N of 2.50. Compared with the bare graphite felt, PPy/AQDS coating cathode showed much better electrocatalytic activities, which was more advantageous for in situ production of H to support hydrogen autotrophic denitrification process. The PPy-bound AQDS could also act as electron intermediaries, which is beneficial to greatly promote indirect electron process between the denitrifiers and nitrate. Moreover, the PPy/AQDS composite layer formed many particles for improving the specific surface area and bio-attachment site for bacterial attachment, which was conducive for the proliferation of microorganisms and denitrification efficiency. The ratio of biofilm and electrode of PPy/AQDS biocathode was 0.32 ± 0.08, which was 2.46 times than bare electrode (0.13 ± 0.06). Furthermore, enrichment of specific denitrifiers and enhancement of denitrifying enzyme activity was obtained using PPy/AQDS treated electrode, the much higher relative abundance of Thauera of PPy/AQDS biocathode was 1.58 times to the application of bare graphite felt.