Mackinawite (FeS) Chemodenitrification of Nitrate (NO3-) under Acidic to Neutral pH Conditions and Its Stable N and O Isotope Dynamics

Although evidence has indicated that the biogeochemical coupling of Fe and N redox reactions has potentially important impacts on the cycling of N in the environment, the relevance of abiotic Fe and N redox transformations remain unclear, especially in acid sulfate soil (ASS) environments where biological denitrification processes are considered to be hindered under such acidic conditions. Considering natural conditions and one of the main Fe mineral components of ASS, we have evaluated the effect of low to circumneutral pH on NO3- chemodenitrification by mackinawite (FeS) at ambient temperature and investigated the isotopic fractionation of this N reduction pathway. At equal FeS concentrations, optimal NO3- reduction occurred at pH 3.5, where microbial denitrification is widely considered to be at its slowest and decreased as pH increased to 7. In all cases, NO3- chemodenitrification reactions had ceased within 24 h. At most, 10% of the NO3- was reduced to NH4+, and through a process of elimination, N2(g) was identified as the major reduced N product. Nitrate chemodenitrification resulted in FeS oxidation to greigite (Fe3S4), elemental sulfur (S0), thiosulfate (S2O32-), and SO42-, indicating oxidation of both Fe and S. As was the case for pH, an inverse relationship existed between the reduction potential and NO3- chemodenitrification, suggesting either the possible involvement of reduced aqueous sulfur species, such as H2S or S2O32-, in catalyzing NO3- reduction or mineral surface passivation inhibiting electron transfer processes at higher pH values. Nevertheless, in the presence of a large excess of FeS (82 mM), these kinetic differences were less pronounced. Stable N and O isotope measurements during NO3- reduction demonstrated that there were no kinetic isotope effects for either delta 15N or delta 18O in the residual NO3- pool. These results suggest that the kinetics of NO3- (re)population of reduction site(s) is exceedingly slow, compared to electron transfer, and this rate-limiting step prevents kinetic isotopic discrimination during reduction. This study demonstrates that NO3- chemodenitrification may be relevant in anoxic environments where FeS is abundant, like ASS that is found in subtropical/tropical climates. Furthermore, this NO3- chemodenitrification pathway does not contribute to the formation of the greenhouse gas N2O(g). However, the first step of NO3- reduction by FeS does not produce any significant effects on delta 15N-NO3- and delta 18O-NO3- and so is largely invisible to stable isotope analyses of the residual NO3- pool. As a result, its contribution to NO3- reduction, in the presence of other biotic/abiotic pathways, could be underestimated when using stable isotope measurements of NO3-.

Automated summary

This study evaluated the influence of low to nearly neutral pH on FeS in NO3- chemodenitrification, finding optimal reduction at pH 3.5 and no contribution to N2O(g) formation. Additionally, the first step of NO3- reduction by FeS does not affect stable isotope analysis of the residual NO3- pool significantly.