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Kinetic Isotope Effects in Production of Nitrite-Nitrogen and Dinitrogen Gas during In Situ Denitrification

Water Sciences Laboratory, Univ. of Nebraska, Lincoln, NE 68583-0844.

^* Corresponding author (rspaldin{at}unlinfo.unl.edu).

ABSTRACT

In multistep redox reactions, stable isotopes of the product species can provide insight into the complexities of the reaction. A method was developed that chromatographically separated nitrite (NO^–₂) from nitrate (NO^–₃) in quantities that permitted isotopic analysis both N species. Only a 3% peak-to-peak crossover contamination was measured in the column eluates during chromatographic separation using ¹⁵N-enriched NO^–₂. During in situ microcosm-amended denitrification, low ¹⁵N values of NO^–₂ ranging from –2 to 4 verified that NO^–₂ produced early persisted throughout much of the reaction. Significant isotopic depletion of the residual NO^–₃ plus NO^–₂ fraction resulted. This depletion effect was reflected in lower apparent isotopic-enrichment factors (flatter slopes) in plots of ¹⁵N vs. In [NO^–₃ + NO^–₂] than those vs. In [NO^–₃] alone. Dinitrogen concentrations produced during the reaction were determined by measuring Ar/N₂ values and showed that significant N₂ losses occurred in the latter part of the reaction due to supersaturated conditions. The ¹⁵N values for the N₂ produced during denitrification ranged from –5.2 to higher values of +17 when the reaction was 70% complete. A slope reversal in ¹⁵N of produced N₂ followed when NO^–₂ reduction became more dominant, as isotope dilution of product N₂ occurred. At the culmination of the reaction, the ¹⁵N value of N₂ approached but did not return to the original ¹⁵N value of the NO^–₃. The results caution the blind use of Ar/N₂ ratios in groundwaters that initially contain high NO^–₃ levels and emphasize the importance of NO^–₂ in environmental sampling.

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