Nitrite reduction by siderite

Nitrate-dependent Fe(II) oxidation is an important process in the inhibition of soil Fe(III) reduction, yet the mechanisms are poorly understood. One proposed pathway includes chemical reoxidation of mineral forms of Fe(II) such as siderite [FeCO_3(s)] by NO₂^-. Accordingly, the objective of this study was to investigate the reactivity of FeCO_3(s) with NO₂^-. Stitted-batch reactions were performed in an anoxic chamber across a range of pH values (5.5, 6, 6.5, and 7.9), initial FeCO_3(s) concentrations (5, 10, and 15 g L ^-1) and initial NO₂^- concentrations (0.83-9.3 mmol L^-1) for kinetic and stoichiometric determinations. Solid-phase products were characterized using x-ray diffraction (XRD). Siderite abiotically reduced NO₂^- to N₂O. During the process, FeCO_3(s) was oxidized to lepidocrocite [γ-FeOOH_(s)] based on the appearance of XRD peaks located at 0.624, 0.329, and 0.247 nm. The rate of NO₂^- reduction was first order in total NO ₂^- concentration and FeCO_3(s), with a second-order rate coefficient (k) of 0.55 ± 0.05 M^-1 h ^-1 at pH 5.5 and 25°C. The reaction was proton assisted and k values increased threefold as pH decreased from 7.9 to 5.5. The influence of pH on NO₂^- reduction was rationalized in tetms of the availability of FeCO_3(s) surface sites (>FeHCO₃ ⁰, >FeOH₂⁺, and >CO₃Fe ⁺) and HNO₂ concentration. These findings indicate that if FeCO_3(s), is present in an Fe(III)-reducing soil where fertilizer NO₃^- is applied, it can participate in secondary chemical reactions with NO₂^- and lead to an inhibition in Fe(III) reduction. This process is relevant in soil environments where NO ₃^-- and Fe(III)-reducing zones overlap or across aerobic-anaerobic interfaces.