Nitrite Reactivity with Magnetite

Under Fe(3+)-reducing conditions, soil Fe(2+) 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 Fe(2+)/Fe(3+) 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 Fe(2+). 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-OH(0)). As NO2(-) was removed from solution, nitric oxide (NO) and nitrous oxide (N2O) were identified as products confirming that nitrite was reduced. Structural Fe(2+) in magnetite was determined to be the reductant of NO2(-) based on the lack of measurable dissolved Fe(2+) 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.