Highly efficient nitrate reduction driven by an electrocoagulation system: An electrochemical and molecular mechanism

Electrotrophic denitrification is suitable for nitrate removal in aqueous environments where bioavailable electron donors are limited such as in urban polluted water. Herein, a novel microbial denitrifying electrocoagulation cell (MDECC) with an Fe anode and an electrotrophic denitrifying biocathode was constructed. Nitrate reduction was verified relying solely on the electrons originated from the electrolysis process at the Fe anode. In situ generated coagulant at the anode was utilized to effectively flocculate and precipitate pollutants as well as naturally occurring components. Nitrate reduction by the biocathode showed pseudo-first-order kinetics with a maximum NO₃^--N removal rate of 67 ± 7 g m^-3 d^-1 and a total nitrogen (TN) removal rate of 39 ± 6 g m^-3 d^-1. Mechanistic research demonstrated that the system achieved highest current efficiency and denitrification enzyme activity at an initial NO₃^--N concentration (IC-N) of 100 mg L^-1. Further pyrosequencing evidenced that higher initial NO₃^--N concentration increased the abundance of denitrifiers on biocathode. Correlation analysis indicated that nitrate reductase (NAR) and nitrite reductase (NIR) activities were crucial for NO₃^--N and TN removal. The metal anode was a promising alternative for providing electrons for electrotrophic denitrification and pollutant elimination.