Nitrite Reactivity with Magnetite

Under Fe3+-reducing conditions, 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 magnetite, a mixed valence Fe2+/Fe3+ mineral found in many natural environments. Currently, little information exists on NO3 – and NO2 – reactivity with magnetite. This study investigates NO3 – and NO2 – reactivity with magnetite under anoxic conditions using batch kinetic experiments across a range of pH values (5.5–7.5) and in the presence of added dissolved Fe2+. Solid phase products were characterized using X-ray diffraction (XRD), Mössbauer spectroscopy, and scanning electron microscopy (SEM). Nitrate removal by magnetite was much slower when compared with NO2 –. There was a pH-dependence in the reduction of NO2 – by magnetite; the initial rate of NO2 – removal was two times faster at pH 5.5 than at pH 7.5. The influence of pH was explained by the binding of NO2 – to positively charged sites on magnetite (S–OH2 +) and to neutral sites (S–OH0). As NO2 – was removed from solution, nitric oxide (NO) and nitrous oxide (N2O) were identified as products confirming that nitrite was reduced. Structural Fe2+ in magnetite was determined to be the reductant of NO2 – based on the lack of measurable dissolved Fe2+ release to solution coupled with Mössbauer spectra and XRD analysis of solid phase products. Addition of dissolved Fe2+ to magnetite slurries resulted in adsorption and an acceleration in the rate of nitrite reduction at a given pH value. In summary, findings reported in this study demonstrate that if magnetite is present in Fe3+-reducing soil and NO2 – is available, it can remove NO2 – from solution and reduce a portion of it abiotically to NO and subsequently to N2O by a heterogeneous electron transfer process.