Mackinawite (FeS) chemodenitrification of nitrate (NO₃‾) under acidic to neutral pH conditions and its stable N and O isotope dynamics

Abstract

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 δ¹⁵N or δ¹⁸O 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 δ¹⁵N-NO₃‾ and δ¹⁸O-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‾.