Construction of bidirectional electron transfer biofilms via periodic polarity reversal

Abstract

Bioelectrochemical denitrification has been widely applied to remove nitrate in water. Thereinto, bidirectional electron transfer biofilm removes nitrate effectively without nitrite accumulation. However, it is critical to maintain bidirectional electron transfer capability of biofilm. In this study, periodic polarity reversal strategy was applied to cultivate bidirectional electron transfer biofilm. The nitrate removal was observed at 93 ± 5.9 % in bidirectional electron transfer system (R1) cultivated by periodic polarity reversal strategy. High bio-community similarity was observed in anode and R1 biofilm with Geobacter sp. as dominant bacteria, indicating that the periodic polarity reversal strategy sustained anodic respiring bacteria. The exoelectrogens (Geobacter pickeringii) and denitrifiers (Chryseobacterium sp., Acidovorax sp. and Stenotrophomonas sp.) exhibited a positive correlation, implying interspecies mutualistic growth. Electroactive sites in R1 and anode biofilm derived from turnover CVs and DPVs were within the same range, indicating that the sustained anodic respiring bacteria also played essential role in cathodic electron transfer. Periodic polarity reversal utilized the “Bio-pseudocapacitance” feature of electroactive biofilm, making it possible to donate stored electrons (at anode period) for nitrate reduction in cathode period. The detailed contributions of direct electron transfer (DET) and mediated electron transfer (MET) were investigated to reveal the electron transfer mechanism at the bacteria-electrode interface. Different electron transfer pathway was suggested for electron donating and extracting process in this bidirectional electron transfer biofilm. Generally, periodic polarity reversal strategy provides an effective way to regulate bidirectional electron transfer biofilm and utilize the “Bio-pseudocapacitance” feature of biofilm to facilitate bioelectrochemical nitrate reduction.