Nitrite reduction and formation of corrosion coatings in zerovalent iron systems

Abstract

Batch tests were conducted to investigate nitrite reduction in a zerovalent iron (Fe 0) 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 Fe 0 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 Fe 0 grains with respect to nitrite reduction. Adding aqueous Fe 2+ significantly accelerated reduction of nitrite by Fe 0 to nitrogen gas with lepidocrocite as the main iron corrosion product. Substantially, though still substoichiometrically, 0.55 mol of Fe 2+ were concomitantly consumed per 1.0 mol nitrite reduction, indicating that Fe 0 was the main electron source. In the presence of Fe 2+, 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 Fe 2+, and the evolution of the iron corrosion coating.