Thursday, 9 October 2008: 10:30 AM
George R. Brown Convention Center, 381A
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 (FeCO3(s)) by nitrite.
Accordingly, the objective of this study was to investigate the reactivity of FeCO3(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 FeCO3(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, FeCO3(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 FeCO3(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 FeCO3(s) surface sites (>FeHCO30, >FeOH2+, and >CO3Fe+) and nitrous acid concentration ([HNO2]). These findings indicate that if FeCO3(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.