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 [FeCO3(s)] by NO2-. Accordingly, the objective of this study was to investigate the reactivity of FeCO3(s) with NO2-. Stirred-batch reactions were performed in an anoxic chamber across a range of pH values (5.5, 6, 6.5, and 7.9), initial FeCO3(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, FeCO3(s) was oxidized to lepidocrocite [gamma-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 NO2- concentration and FeCO3(s), with a second-order rate coefficient (k) of 0.55 +/- 0.05 M-1 h(-1) at pH 5.5 and 25 degrees 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 terms of the availability of FeCO3(s) surface sites (>FeHCO30, >FeOH2+, and >CO3Fe+) and HNO2 concentration. These findings indicate that if FeCO3(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 NO3- and Fe(III) -reducing zones overlap or across aerobic-anaerobic interfaces.