Under anaerobic conditions, the environmental reduction of nitrate (NO3–) and nitrite (NO2–) to more reduced forms is widely regarded as being microbially catalyzed. However, the chemical reduction of oxidized nitrogen species by reduced iron (Fe(II)), whether mineral-bound or surface-associated, may also occur under environmentally relevant conditions. Here we examine the nitrogen (N) and oxygen (O) stable isotope dynamics of the chemical reduction of NO2– by mineral associated Fe(II) (chemodenitrification) and its production of the potent greenhouse gas nitrous oxide (N2O). By shedding light on factors controlling kinetics of the reaction and its corresponding dual isotopic expression in the reactant NO2– and product N2O, this work contributes to a growing body of work aiming to improve our ability to identify chemodenitrification in the environment.
Consistent with previous studies, we find that while homogenous reactions between aqueous NO2– and Fe(II) were kinetically slow, heterogeneous reactions involving Fe(II)-containing minerals often catalyzed considerable nitrite loss. In particular, rapid reduction of NO2– was catalyzed by the Fe-rich smectite clay mineral nontronite as well as the mixed Fe(II)-Fe(III) oxyhydroxide phase green rust. These minerals serve as both a source of reduced iron within the mineral structure as well as a surface for promoting the reactivity of Fe(II). However, even in the presence of aqueous Fe(II), experiments with low-Fe and non-Fe containing minerals showed little to no NO2– loss, perhaps suggesting a more dominant role for structural iron during chemodenitrification. When catalyzed by nontronite and green rust, N and O isotope effects for chemodenitrification (15εcDNF and 18εcDNF) ranged from 2 to 11‰ and 4 to 10‰, respectively, with lower values generally observed at higher reaction rates. Higher reaction rates were also linked to higher molar yields of N2O (up to 31%), highlighting a strong potential for chemodenitrification to produce N2O – especially relative to its production by microbial pathways, which typically exhibit yields <1%. The intramolecular 15N site preference (SP) of the linear N2O molecule (the difference in δ15N between the central and outer atoms), reflective of different production mechanisms, was also measured for N2O produced during green rust catalyzed chemodenitrification. Relative to values measured in other recent studies of chemodenitrification, SP values were consistently high (+26.5‰ ± 0.8‰), especially relative to N2O produced via bacterial denitrification (SP ∼0‰). Finally, the coupling of 18εcDNF and 15εcDNF at a ratio of ∼1 during green rust catalyzed chemodenitrification contrasts distinctly with recently characterized bacterial nitrite reduction, potentially permitting disentangling of both processes under well-constrained conditions. This study contributes to the broader understanding of the potential relevance for mineral-derived Fe(II) to promote the reduction of nitrite and consequent production of N2O, especially in iron-rich systems hosting dynamic redox oscillations, including hyporheic zones, estuarine sediments and groundwater aquifers.
