Nitrite reduction and formation of corrosion coatings in zerovalent iron systems

Batch tests were conducted to investigate nitrite reduction in a zerovalent iron (Fe⁰) system under various conditions. Nitrite at 1.4 mM initial concentration was slowly reduced to nitrogen gas in the first stage (days 1-6), which was mediated by an amorphous, Fe(II)-rich iron oxide coating. The second stage (days 7-14) featured a rapid reduction of nitrite to both ammonia and nitrogen gas and the formation of a more crystalline, magnetite form iron oxide coating. Water reduction by Fe⁰ occurred concurrently with nitrite reduction from the beginning and contributed significantly to the overall iron corrosion. Nitrite at 14 mM was found to passivate the surface of Fe⁰ grains with respect to nitrite reduction. Adding aqueous Fe²⁺ significantly accelerated reduction of nitrite by Fe⁰ to nitrogen gas with lepidocrocite as the main iron corrosion product. Substantially, though still substoichiometrically, 0.55 mol of Fe²⁺ were concomitantly consumed per 1.0 mol nitrite reduction, indicating that Fe⁰ was the main electron source. In the presence of Fe²⁺, nitrite reduction out-competed water reduction in terms of contributing to the overall iron corrosion. Results of this study help understand complicated interactions between water reduction and nitrite reduction, the roles of surface-bound Fe²⁺, and the evolution of the iron corrosion coating.