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TECHNICAL REPORTS

Ground Water Quality

Nitrate Reduction in the Presence of Wüstite

Department of Agronomy, University of Kentucky, N-122G Agricultural Science Building-North, Lexington, KY 40546-0091

^* Corresponding author (sraks2{at}uky.edu)

Received for publication December 2, 2004. Recent strategies to reduce elevated nitrate concentrations employ metallic Fe⁰ as a reductant. Secondary products of Fe⁰ corrosion include magnetite (Fe₃O₄), green rust [Fe₆(OH)₁₂SO₄], and wüstite [FeO(s)]. To our knowledge, no studies have been reported on the reactivity of NO₃^– with FeO(s). This project was initiated to evaluate the reactivity of FeO(s) with NO₃^– under abiotic conditions. Stirred batch reactions were performed in an anaerobic chamber over a range of pH values (5.45, 6.45, and 7.45), initial FeO(s) concentrations (1, 5, and 10 g L^–1), initial NO₃^– concentrations (1, 10, and 15 mM), and temperatures (3, 21, 31, and 41°C) for kinetic and thermodynamic determinations. Suspensions were periodically removed and filtered to measure dissolved nitrogen and iron species. Solid phases were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Nitrate reduction by FeO was rapid and characterized by nearly stoichiometric conversion of NO₃^– to NH₄⁺. Transient NO₂^– formation also occurred. The XRD and SEM results indicated the formation of Fe₃O₄ as a reaction product of the heterogeneous redox reaction. Kinetics of NO₃^– reduction suggested a rate equation of the type: –d/dt = k^0.570.221.12 where k = 3.46 x 10^–3 ± 0.38 x 10^–3 M^–1 s^–1, at 25°C. Arrhenius and Eyring plots indicate that the reaction is surface chemical–controlled and proceeds by an associative mechanism involving a step where both NO₃^– and FeO(s) bind together in an intermediate complex.

Abbreviations: MES, 2-(N-morpholino)ethane sulfonic acid • SEM, scanning electron microscopy • XRD, X-ray diffraction

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