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Project Progress

A possibly catalytic process was discovered on pyrite surfaces involving nitrite (NO₂^-) in aqueous media that can be considered an extension of earlier work in the Schoonen laboratory. In the earlier study a redox conversion of freshly precipitated FeS showed improved ammonia formation in water from dissolved hydrogen sulfide (H₂S_(g)) and dinitrogen (N_2(g)) at hydrothermal temperatures (T ~ 120°C) relative to the background ammonia signal in the absence of FeS. Our current 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), and the latter species becomes further reduced in concomitant steps to ammonia. A systematic experimental effort was completed to map the parameter space of temperature, iron-sulfur particle loading, and presence of dinitrogen/nitrite/nitrate substrates. Surface complexes were indentified using attenuated total reflection Fourier Transform infrared spectrophotometer (ATR-FTIR), which eliminates the intense water absorbances found in traditional FTIR measurements and allows for detection of signals for species adsorbed on the surface in low concentrations.

Room temperature (~22 °C) experiments using FeS particles and either NO₃- or NO₂- substrates did not show any ammonia formation. However, increasing the temperature toward hydrothermal conditions (~ 70 °C) showed ammonia formation (85 M concentration within a day), but only when NO₂- is used as the substrate. Nitrate did not show any reactivity. Kinetic measurements revealed that the amount of ammonia formed is dependent on the initial nitrite concentration. In contrast to FeS, a more modest (~10 M per day), but unambiguous ammonia formation was detected on pyrite (FeS₂) for both nitrate and nitrite at elevated temperatures. This reaction could further be enhanced by elevating the reaction temperature into the true hydrothermal regime of 120 °C.