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Kinetic Isotope Effects in Production of Nitrite‐Nitrogen and Dinitrogen Gas during In Situ Denitrification
Author(s) -
Bates H. K.,
Martin G. E.,
Spalding R. F.
Publication year - 1998
Publication title -
journal of environmental quality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 171
eISSN - 1537-2537
pISSN - 0047-2425
DOI - 10.2134/jeq1998.00472425002700010026x
Subject(s) - chemistry , denitrification , nitrite , isotopes of nitrogen , nitrate , nitrogen , isotope dilution , isotope analysis , isotope , dilution , chromatography , organic chemistry , mass spectrometry , ecology , physics , quantum mechanics , biology , thermodynamics
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 15 N‐enriched NO − 2 . During in situ microcosm‐amended denitrification, low δ 15 N 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 δ 15 N vs. In [NO − 3 + NO − 2 ] than those vs. In [NO − 3 ] alone. Dinitrogen concentrations produced during the reaction were determined by measuring δAr/N 2 values and showed that significant N 2 losses occurred in the latter part of the reaction due to supersaturated conditions. The δ 15 N values for the N 2 produced during denitrification ranged from −5.2‰ to higher values of +17‰ when the reaction was ∼70% complete. A slope reversal in δ 15 N of produced N 2 followed when NO − 2 reduction became more dominant, as isotope dilution of product N 2 occurred. At the culmination of the reaction, the δ 15 N value of N 2 approached but did not return to the original δ 15 N value of the NO − 3 . The results caution the blind use of Ar/N 2 ratios in groundwaters that initially contain high NO − 3 levels and emphasize the importance of NO − 2 in environmental sampling.