Year of Publication

2013

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Plant and Soil Sciences

First Advisor

Dr. Christopher J. Matocha

Abstract

Under iron (Fe3+)-reducing conditions where aqueous Fe2+ and unreduced solid Fe3+-oxides commonly coexist, soil Fe2+ oxidation has been shown to be coupled with nitrate (NO3-) reduction. One possible secondary reaction is the involvement of NO3- and nitrite (NO2-) with Fe-oxide minerals found in many natural environments. Yet, spectroscopic measurements and kinetic data on reactivity of NO3- and NO2- with Fe-containing oxide minerals such as goethite (a-FeOOH), and magnetite (Fe34) are not found in the literature. The reactivity of goethite and magnetite with NO3- and NO2- was studied over a range of environmentally relevant pH conditions (5.5-7.5) with and without added Fe2+(aq) under anoxic conditions. Laboratory experiments were conducted using stirred batch experiments and reaction products were analyzed using ion chromatography (IC), gas chromatography (GC), ultraviolet visible near infrared spectroscopy (UV-VIS-NIR), x-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer, and Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. Nitrate removal by goethite and magnetite was much slower when compared with NO2-. There was a pH-dependence in the reduction of NO2-, and the initial rate of NO2- removal was nearly 2 and 8 times faster at pH 5.5 than at pH 7.5 by magnetite and goethite, respectively. Nitric oxide (NO) and nitrous oxide (N2O) were identified as products when NO2- has reacted with magnetite, whereas N2O is the major reaction product in the experiment with goethite. In comparison to experiments containing magnetite or goethite alone, addition of Fe2+ greatly accelerated the NO2- removal rate. Wet chemical experiments combined with the Mössbauer study reveals that NO2- reduction to NO and subsequently to N2O by magnetite occurs via a heterogeneous electron transfer process. ATR-FTIR and diffuse reflectance spectroscopy (DRS) results from the studies with goethite indicate that NO2- was removed from solution by adsorption in a surface complex involving the oxygen atoms, and a portion of the nitrite is reduced to NO and N2O.

This study suggests that under anaerobic conditions soil and sediments that contain goethite, magnetite, and other Fe3+-oxides can catalyze abiotic NO2- reduction and the kinetics data from this study can be used to predict the NO2- removal under such conditions.

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