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 NO ₃^- and nitrite (NO₂^-) with magnetite, a mixed valence Fe²⁺/Fe³⁺ mineral found in many natural environments. Currently, little information exists on NO ₃^- and NO₂^- reactivity with magnetite. This study investigates NO₃^- and NO ₂^- 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 (î -S-OH ₂⁺) and to neutral sites (î -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 Fe ²⁺ 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 Fe ³⁺-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.