Carbon-based cathode as an electron donor driving direct bioelectrochemical denitrification in biofilm-electrode reactors: Role of oxygen functional groups

Abstract

Carbon-based cathodes are widely used in the biofilm-electrode reactor (BER) favoring a direct bioelectrochemical denitrification. However, little is known about how the surface chemistry of carbon affects the formation of bacterial biofilms on the electrode surface, which is fundamental to the functioning of BER for the nitrate removal. In the present study, graphite granules used in the three-dimension BER were oxidized by different methods, such as treatment with HCl, H2O2, HNO3 and HNO3+heat, providing the surface oxygen content as 1.98, 4.10, 7.70 and 11.54 at %, respectively. Likewise, the denitrifying BER equipped with the HNO3+heat-treated cathode exhibited a reaction rate of 27.36 mg NO3−-N L−1 d−1, exceeding 19.96, 14.82 and 13.93 mg NO3−-N L−1 d−1 obtained from the BERs with HNO3, H2O2- and HCl-treated cathodes, respectively. It was revealed that the oxidation treatment led to the increase in the amounts of oxygen functional groups (largely in the form of carboxyl), which were beneficial for the growth of metabolically active bacteria and thus the increased bioelectrochemical reaction kinetics. Bacterial analysis showed that the cathode-driven autotrophic denitrifiers were dominantly represented by Thiobacillus and Sulfurimonas, which displayed the highest abundance on the cathode pretreated with a hot and refluxed HNO3 solution.