Nitrite Reduction by Siderite.

Nitrate-dependent, iron(II) oxidation is an important process in the inhibition of soil iron(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 nitrite.  Accordingly, the objective of this study was to investigate the reactivity of FeCO_3(s) with nitrite.  Stirred-batch reactions were performed in an anoxic chamber over 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 mM) for kinetic and stoichiometric determinations.  Solid phase products were characterized using x-ray diffraction (XRD).  Siderite abiotically reduced NO₂^- to nitrous oxide (N₂O).  During the process, FeCO_3(s) was oxidized to lepidocrocite (γ-FeOOH_(s)) based on appearance of XRD peaks located at 0.624, 0.329, and 0.247 nm.  The rate of NO₂^- reduction was first order in total nitrite 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 25⁰ C.  The reaction was proton assisted and k values increased three-fold as pH decreased from 7.9 to 5.5.  The influence of pH on nitrite reduction was rationalized in terms of the availability of FeCO_3(s) surface sites (>FeHCO₃⁰, >FeOH₂⁺, and >CO₃Fe⁺) and nitrous acid concentration ([HNO₂]).  These findings indicate that if FeCO_3(s) is present in Fe(III)-reducing soil where fertilizer nitrate 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 NO₃^- and Fe(III)-reducing zones overlap or across aerobic-anaerobic interfaces.