An important constraint on the formation of the building blocks of life in the Hadean is the availability of small, activated compounds such as ammonia (NH(3)) relative to its inert dinitrogen source. Iron-sulfur particles and/or mineral surfaces have been implicated to provide the catalytic active sites for the reduction of dinitrogen. Here we provide a combined kinetic, spectroscopic, and computational modeling study for an alternative source of ammonia from water soluble nitrogen oxide ions. The adsorption of aqueous nitrite (NO(2)(-)) and nitrate (NO(3)(-)) on pyrite (FeS(2)) and subsequent reduction chemistry to ammonia was investigated at 22°C, 70°C, and 120°C. Batch geochemical and in situ Attenuated Total Reflection - Fourier Transform Infrared (ATR-FTIR) spectroscopy experiments were used to determine the reduction kinetics to NH(3) and to elucidate the identity of the surface complexes, respectively, during the reaction chemistry of NO(2)(-) and NO(3)(-). Density functional theory (DFT) calculations aided the interpretation of the vibrational data for a representative set of surface species. Under the experimental conditions used in this study, we detected the adsorption of nitric oxide (NO) intermediate on the pyrite surface. NH(3) production from NO(2)(-) occurred at 70 and 120°C and from NO(3)(-) occurred only at 120°C.

The reaction of nitrite (NO2(-)) and nitrate (NO3(-)) on nanometer-sized FeS particles was investigated in alkaline (initial pH = 10.3) solutions at reaction temperatures of 22, 70, and 120 °C using in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and fluorescence spectroscopy that allowed an analysis of adsorbate complexation on the FeS and reaction product in the aqueous phase, respectively. ATR-FTIR showed that NO was a surface-bound intermediate on FeS during its exposure to NO2(-) at all three reaction temperatures. Ammonia/ammonium (NH3/NH4(+)) product was also produced when FeS was exposed to NO2(-) at the 70 °C and 120 °C reaction temperatures. Activation of NO3(-) to form surface-bound NO was experimentally observed to occur at 120 °C on FeS, but not at the lower reaction temperatures. Furthermore, NH3/NH4(+) product in the aqueous phase was only present during the reaction of FeS with NO3(-) at the highest temperature used in this study.

The effect of simulated solar radiation on the oxidation of arsenite [As(III)] to arsenate [As(V)] on the layered manganese oxide, birnessite, was investigated. Experiments were conducted where birnessite suspensions, under both anoxic and oxic conditions, were irradiated with simulated solar radiation in the presence of As(III) at pH 5, 7, and 9. X-ray absorption spectroscopy (XAS) was used to determine the nature of the adsorbed product on the surface of the birnessite. The oxidation of As(III) in the presence of birnessite under simulated solar light irradiation occurred at a rate that was faster than in the absence of light at pH 5. At pH 7 and 9, As(V) production was significantly less than at pH 5 and the amount of As(V) production for a given reaction time was the same under dark and light conditions. The first order rate constant (kobs) for As(III) oxidation in the presence of light and in the dark at pH 5 were determined to be 0.07 and 0.04 h−1, respectively. The As(V) product was released into solution along with Mn(II), with the latter product resulting from the reduction of Mn(IV) and/or Mn(III) during the As(III) oxidation process. Post-reaction XAS analysis of As(III) exposed birnessite showed that arsenic was present on the surface as As(V). Experimental results also showed no evidence that reactive oxygen species played a role in the As(III) oxidation process.Electronic supplementary materialThe online version of this article (doi:10.1186/s12932-016-0037-5) contains supplementary material, which is available to authorized users.