Inhibition by free nitrous acid (FNA) and the electron competition of nitrite in nitrous oxide (N2O) reduction during hydrogenotrophic denitrification

Hydrogenotrophic denitrification is a promising technology for nitrate removal from organic-deficient wastewater or groundwater, and the attention of nitrous oxide (N2O) emission during this process is required. Both nitrite and free nitrous acid (FNA or HNO2) were reported to exert significant effects on N2O reduction in heterotrophic denitrification, whereas, little knowledge has been obtained in hydrogenotrophic denitrification. In this study, we conducted a series of batch tests to comprehensively investigate the effects of nitrite, pH and FNA on N2O production and reduction in a hydrogenotrophic denitrification process. The results showed that N2O reduction rate decreased under both conditions of low pH and presence of nitrite, which would exert synergetic inhibition on N2O reduction. The potential mechanisms that give rise to the results included electron competition and FNA inhibition. Electron competition between nitrite and N2O reductases occurred when both nitrite and N2O were added, which might contribute to the decrease in the N2O reduction rate. The electron supply, which was obtained from the uptake of molecular hydrogen, declined with increasing FNA concentration according to a logarithmic model (R2 = 0.9240). Additionally, the electron consumption rate of N2O reductase to nitrite reductase ratio was initially stable and then decreased with increasing FNA concentration. The inhibition of N2O reduction by FNA was determined to be reversible. The study suggested that both of the electron supply and N2O reduction in hydrogenotrophic denitrification could be inhibited by FNA.