Chemodenitrification by Fe(II) and nitrite: Effects of temperature and dual N[sbnd]O isotope fractionation

Abstract

Chemodenitrification between Fe(II) and nitrite, an important nitrogen (N) cycling pathway, varies depending on the season, indicating the impact of ambient temperatures. However, the underlying mechanism for this occurrence has not been explained at a molecular scale. As an effective approach for studying N cycling, dual NO isotope fractionation was analyzed to reveal the reactions of temperature-dependent chemodenitrification. The kinetics of chemodenitrification under different temperatures showed that nitrite reduction rates and N2O production increased with temperature (5–35 °C). Lepidocrocite was identified as the secondary mineral of Fe(II) oxidation at 5, 15, and 25 °C, and goethite was observed at 35 °C. Further, the value of the O isotope enrichment factor (18ε) decreased with increasing temperature while values of N isotope fractionation enrichment factor (15ε) were less sensitive to temperature. The isotope fractionation ratios of O to N (18ε:15ε) were 1.00 ± 0.25, 0.75 ± 0.05, 0.54 ± 0.04, and 0.46 ± 0.06 at 5, 15, 25, and 35 °C, respectively, indicating that the temperature effect should be considered when dual NO isotope data are applied to distinguish chemodenitrification with biodenitrification . A kinetic model was established to correlate the rate constants of the elementary reactions during chemodenitrification with NO isotope fractionation. The intrinsic isotope fractionation of NO bond breakage during chemodenitrification was obtained. This study provides both qualitative and quantitative outcomes on temperature-dependent chemodenitrification. This study provides inputs for a better understanding of the role of chemodenitrification in effecting seasonal changes to the environment.