Stable Isotope Systematics of Abiotic Nitrite Reduction Coupled with Anaerobic Iron Oxidation: The Role of Reduced Clays and Fe-bearing Minerals

Abstract:

Under anaerobic conditions, it is widely assumed that nitrate (NO₃^-) and nitrite (NO₂^-) reduction is primarily the result of microbial respiration. However, it has also been shown that abiotic reduction of nitrate and nitrite by reduced iron (Fe(II)), whether mineral-bound or surface-associated, may also occur under certain environmentally relevant conditions. With a range of experimental conditions, we investigated the nitrogen and oxygen stable isotope systematics of abiotic nitrite reduction by Fe(II) in an effort to characterize biotic and abiotic processes in the environment. While homogenous reactions between NO₂^- and Fe(II) in artificial seawater showed little reduction, heterogeneous reactions involving Fe-containing minerals showed considerable nitrite loss. Specifically, rapid nitrite reduction was observed in experiments that included reduced clays (illite, Na-montmorillonite, and nontronite) and those that exhibited iron oxide formation (ferrihydrite, magnetite and/or green rust). While these iron oxides and clay minerals offer both a source of reduced iron in the mineral matrix as well as a surface for Fe(II) activation, control experiments with corundum as a non-Fe containing mineral surface showed little NO₂^- loss, implicating a more dominant role of structural Fe in the clays during nitrite reduction. The isotope effects for ¹⁵N and ¹⁸O (¹⁵ε and ¹⁸ε) ranged from 5 to 14‰ for ¹⁵ε and 5 to 17‰ for ¹⁸ε and were typically coupled such that ¹⁵ε ~ ¹⁸ε. Reactions below pH 7 were slower and the ¹⁸ε was affected by oxygen atom exchange with water. Although little data exist for comparison with the dual isotopes of microbial NO₂^- reduction, these data serve as a benchmark for evaluating the role of abiotic processes in N reduction, particularly in sediment systems low in organic carbon and high in iron.