Reduced iron induced nitric oxide and nitrous oxide emission

Formation of the greenhouse gas nitrous oxide in water treatment systems is predominantly studied as a biological phenomenon. There are indications that also chemical processes contribute to these emissions. Here we studied the formation of nitric oxide (NO) and nitrous oxide (N₂O) due to chemical nitrite reduction by ferrous iron (Fe(II)). Reduction of nitrite and NO coupled to Fe(II) oxidation was studied in laboratory-scale chemical experiments at different pH, nitrite and iron concentrations. The continuous measurement of both NO and N₂O emission showed that nitrite reduction and NO reduction have different kinetics. Nitrite reduction shows a linear dependency on the nitrite concentration, implying first order kinetics in nitrite. The nitrite reduction seems to be an equilibrium based reaction, leading to a constant NO concentration in the liquid. The NO reduction rate is suggested to be most dependent on reactive surface availability and the sorption of Fe(II) to the reactive surface. The importance of emission of NO and N₂O coupled to iron oxidation is exemplified by iron reduction experiments and several examples of environments where this pathway can play a role.