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Project Progress

Nitrate and nitrite, generated by dissolution of NO_x during lightning strikes, may have been an important source of reactive nitrogen in the Hadean. We evaluated the production of ammonia from nitrate and nitrite in aqueous solutions containing freshly precipitated FeS. Our studies suggest that a novel pathway may be operative where ammonia formation is driven by a disproportionation reaction of nitrite (NO₂-) to nitrate (NO₃-) and nitric oxide (NO), with the latter species becoming further reduced in concomitant steps to ammonia.

A systematic experimental effort in which we evaluated the production of ammonia from nitrate and nitrite in aqueous solutions containing freshly precipitated FeS was completed to determine the effect of temperature on the conversion kinetics. Surface complexes were indentified using attenuated total reflection Fourier Transform infrared spectrophotometer (ATR-FTIR), which eliminated the intense water absorbances found in traditional FTIR measurements and allowed detection of signals for species adsorbed on the surface in low concentrations.

Complementary small-scale batch experiments yielded kinetic measurements. Included in these experiments were a series of control experiments to determine the stability of the nitrite ion and nitrate in water as well as the release of ammonia from the FeS. Freshly precipitated FeS was synthesized by rapidly mixing a ferrous sulfate solution and a sodium sulfide solution. The FeS precipitate has an average crystallite size of several nm. All kinetic data were corrected for the rate of ammonia release in the control experiments.

The kinetic results show that a significant amount of ammonia forms in the nitrite/FeS system at 120°C. As shown in the figure below, within 40 hours close to 1.8 mmoles of ammonia are produced per kg of solution. This system remains reactive even at lower temperatures (see top panel figure). The nitrate/FeS system is less reactive, but does show ammonia formation at 120°C (see bottom panel of figure).

The results of this study provide a detailed insight into the mechanism of ammonia formation from nitrite and nitrate reduction. Furthermore, the kinetics suggest that some nitrite or nitrate that entered into an ocean with FeS in the water column would of been converted to ammonia, in particular if the ocean was warmer than it is now.