Nitrite reduction by Fe(II) associated with kaolinite

Interactions of iron (Fe) with the nitrogen (N) cycle have emerged and contain elements of abiotic and biological reactions. One such abiotic reaction which has received little study is the reactivity of NO₂ ⁻ and Fe(II) associated with a major clay mineral, kaolinite. The main objective of this study was to evaluate the reactivity of NO₂ ⁻ with Fe(II) added to kaolinite under anoxic conditions. Stirred batch reactivity experiments were carried out with 10 g L⁻¹ kaolinite spiked with 25 and 100 µM Fe(II) at pH 6.45 in an anaerobic chamber. Approximately 500 µM NO₂ ⁻ was added to initiate the reaction with Fe(II)-loaded kaolinite. The rate of nitrite removal from solution was 2.4-fold slower in the high Fe(II) treatment when compared with the low Fe(II) treatment. A large portion of the NO₂ ⁻ removed from solution was confirmed to be reduced to N₂O_(g) in the Fe(II)-kaolinite slurries. However, NO₂ ⁻ reduction was also noticed in the presence of kaolinite-alone and to somewhat lesser extent in the presence of dithionite-citrate-bicarbonate (DCB)-treated kaolinite. Chemical extractions coupled with infrared spectroscopy suggest that Fe(III) oxide mineral impurities and structural Fe(III) in kaolinite may participate in NO₂ ⁻ removal from solution. Furthermore, a magnetite mineral was identified based on X-ray diffraction analysis of untreated kaolinite and DCB-treated kaolinite. Our findings reveal a novel pathway of NO₂ ⁻ transformation in the environment in the presence of Fe(II) associated (sorbed and impurity) with kaolinite.