A dual nitrite isotopic investigation of chemodenitrification by mineral-associated Fe(II) and its production of nitrous oxide

Abstract Under anaerobic conditions, the environmental reduction of nitrate (NO 3 − ) and nitrite (NO 2 − ) 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 NO 2 − by mineral associated Fe(II) (chemodenitrification) and its production of the potent greenhouse gas nitrous oxide (N 2 O). By shedding light on factors controlling kinetics of the reaction and its corresponding dual isotopic expression in the reactant NO 2 − and product N 2 O, 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 NO 2 − and Fe(II) were kinetically slow, heterogeneous reactions involving Fe(II)-containing minerals often catalyzed considerable nitrite loss. In particular, rapid reduction of NO 2 − 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 NO 2 − 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 ( 15 e cDNF and 18 e cDNF ) 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 N 2 O (up to 31%), highlighting a strong potential for chemodenitrification to produce N 2 O – especially relative to its production by microbial pathways, which typically exhibit yields 15 N site preference (SP) of the linear N 2 O molecule (the difference in δ 15 N between the central and outer atoms), reflective of different production mechanisms, was also measured for N 2 O 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 N 2 O produced via bacterial denitrification (SP ∼ 0‰). Finally, the coupling of 18 e cDNF and 15 e cDNF 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 N 2 O, especially in iron-rich systems hosting dynamic redox oscillations, including hyporheic zones, estuarine sediments and groundwater aquifers.