Abiotic hydroxylamine nitrification involving manganese- and iron-bearing minerals

Abstract

Hydroxylamine (NH2OH) undergoes biotic and abiotic transformation processes in soil, producing nitrous oxide gas (N2O(g)). Little is known about the magnitude of the abiotic chemical processes in the global N cycle, and the role of abiotic nitrification is still neglected in most current nitrogen trace gas studies. The abiotic fate of NH2OH in soil systems is often focused on transition metals including manganese (Mn) and iron (Fe), and empirical correlations of nitrogen residual species including nitrite (NO2−), nitrate (NO3−), and N2O(g). In this study, abiotic NH2OH nitrification by well-characterized manganese (Mn)- and iron (Fe)-bearing minerals (pyrolusite, amorphous MnO2(s), goethite, amorphous FeOOH(s)) was investigated. A nitrogen mass balance analysis involving NH2OH, and the abiotic nitrification residuals, N2O(g), N2O(aq), NO2−, NO3−, was used, and specific reactions and mechanisms were investigated. Rapid and complete NH2OH nitrification occurred (4–5 h) in the presence of pyrolusite and amorphous MnO2(s), achieving a 95–96% mass balance of N byproducts. Conversely, NH2OH nitrification was considerably slower by amorphous FeOOH(s) (14.5%) and goethite (1.1%). Direct reactions between the Mn- and Fe-bearing mineral species and NO2− and NO3− were not detected. Brunauer–Emmett–Teller surface area and energy dispersive X-ray measurements for elemental composition were used to determine the specific concentrations of Mn and Fe. Despite similar specific concentrations of Mn and Fe in crystalline and amorphous minerals, the rate of NH2OH nitrification was much greater in the Mn-bearing minerals. Results underscore the intrinsically faster NH2OH nitrification by Mn minerals than Fe minerals.