Enhancing biocathode denitrification performance with nano-Fe₃O₄ under polarity period reversal

The presence of excessive concentrations of nitrate poses a threat to both the environment and human health, and the bioelectrochemical systems (BESs) are attractive green technologies for nitrate removal. However, the denitrification efficiency in the BESs is still limited by slow biofilm formation and nitrate removal. In this work, we demonstrate the efficacy of novel combination of magnetite nanoparticles (nano-Fe₃O₄) with the anode-cathode polarity period reversal (PPR-Fe₃O₄) for improving the performance of BESs. After only two-week cultivation, the highest cathodic current density (7.71 ± 1.01 A m^-2) and NO₃^--N removal rate (8.19 ± 0.97 g m^-2 d^-1) reported to date were obtained in the PPR-Fe₃O₄ process (i.e., polarity period reversal with nano-Fe₃O₄ added) at applied working voltage of -0.2 and -0.5 V (vs Ag/AgCl) under bioanodic and biocathodic conditions, respectively. Compared with the polarity reversal once only process, the PPR process (i.e., polarity period reversal in the absence of nano-Fe₃O₄) enhanced bioelectroactivity through increasing biofilm biomass and altering microbial community structure. Nano-Fe₃O₄ could enhance extracellular electron transfer as a result of promoting the formation of extracellular polymers containing Fe₃O₄ and reducing charge transfer resistance of bioelectrodes. This work develops a novel biocathode denitrification strategy to achieve efficient nitrate removal after rapid cultivation.