Manganese (Mn) oxides oxidize hydroxylamine (NH2OH), a reactive, enzymatic nitrogenous intermediate during nitrification, to produce the potent greenhouse gas nitrous oxide (N2O). No previous study has characterized the kinetics of NH2OH oxidation by particulate Mn oxides in environmentally relevant conditions. Synthesized birnessite (hereafter referred to as MnO2) was selected as the oxidant because of its ubiquity in natural marine systems, and N2O microsensors were used to obtain real-time data in synthetic ocean water at relevant pH (6.3-8.2). Mass balance shows that between 85-100% of NH2OH was oxidized to N2O in the presence of MnO2. Results confirmed that NOx- or NH4+ were not major products and that NH2OH autoxidation was not a source of N2O during the reaction. The reaction is overall first order with respect to NH2OH with a rate constant of 8.7 ± 1.6×10-3 s-1. The reaction is fast, with complete NH2OH consumption occurring within 4 minutes. Addition of a HNO scavenger reduced N2O yields and production rates by 52-60%, indicating that HNO is an intermediate during NH2OH oxidation. The proposed mechanism begins with the initial transfer of an electron from NH2OH to a Mn(IV) octahedron center to form a NH2O● radical and Mn(III). It is currently unknown if the second electron transfer from NH2O● radical occurs on the same octahedron center or another Mn(IV) center, but nitroxyl (HNO) is the resulting product of each electron transfer. The two HNO produced from each electron transfer step then dimerize in a possible rate limiting step to produce N2O. A general decrease in N2O production and small increase in NH2OH consumption rates with increasing MnO2 concentrations suggest that HNO is also trapped by Mn(III), which could explain the slightly above zero order of the reaction with respect to MnO2. The kinetics of NH2OH oxidation by MnO2 reveals a complex, cryptic coupling between the nitrogen and Mn cycles, with interactions occurring so rapidly it is nearly impossible to measure in situ. Given the ubiquity of Mn oxides and nitrogen-metabolizing bacteria in marine environments, abiotic N2O production has the potential to contribute to oceanic N2O emissions.

Potential role of Mn(IV) oxides in abiotic nitrous oxide production

255th National Meeting (2018)

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