Nitrate Reduction in the Presence of Wüstite
2005
Rakshit, Sudipta | Matocha, Christopher J. | Haszler, Gerald R.
Recent strategies to reduce elevated nitrate NO₃ ⁻ 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 [NO3 − ] / d t = k [ FeO ]0.57[H + ]0.22[NO3 − ]1.12 where k = 3.46 × 10⁻³ ± 0.38 × 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.
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