Variability of Nitrogen Isotope Fractionation during the Reduction of Nitroaromatic Compounds with Dissolved Reductants

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• 作者：Akané ; E. Hartenbach ; Thomas B. Hofstetter ; Michael Aeschbacher ; Michael Sander ; Dongwook Kim ; Timothy J. Strathmann ; William A. Arnold ; Christopher J. Cramer ; René ; P. Schwarzenbach
• 刊名：Environmental Science & Technology
• 出版年：2008
• 出版时间：November 15, 2008
• 年：2008
• 卷：42
• 期：22
• 页码：8352-8359
• 全文大小：276K
• 年卷期：v.42,no.22(November 15, 2008)
• ISSN：1520-5851

文摘

Compound-specific nitrogen isotope analysis was shown to be a promising tool for the quantitative assessment of abiotic reduction of nitroaromatic contaminants (NACs) under anoxic conditions. To assess the magnitude and variability of ¹⁵N fractionation for reactions with dissolved reductants, we investigated the reduction of a series of NACs with a model quinone (anthrahydroquinone-2,6-disulfonate monophenolate; AHQDS⁻) and a Fe(II)-catechol complex (1:2 Fe(II)-tiron complex; Fe(II)L₂⁶⁻) over the pH range from 3 to 12 and variable reductant concentrations. Apparent kinetic isotope effects, AKIE_N, for the reduction of four mononitroaromatic compounds by AHQDS⁻ ranged from 1.039 ± 0.003 to 1.045 ± 0.002 (average ± 1σ), consistent with previous results for various mineral-bound reductants. ¹⁵N fractionation for reduction of 1,2-dinitrobenzene and 2,4,6-trinitrotoluene by AHQDS⁻ and that of 4-chloronitrobenzene by Fe(II)L₂⁶⁻, however, showed substantial variability in AKIE_N-values which decreased from 1.043 to 1.010 with increasing pH. We hypothesize that the isotope-sensitive and rate-limiting step of the overall NAC reduction can shift from the dehydration of substituted N,N-dihydroxyanilines (large ¹⁵N fractionation upon N−O bond cleavage) to protonation or reduction of nitroaromatic radical anions (small ¹⁵N isotope effect upon electron transfer) consistent with calculations of semiclassical ¹⁵N isotope effects. Our results imply that a quantitative assessment of NAC reduction using compound-specific isotope analysis (CSIA) might need to account for homogeneous and heterogeneous reactions separately.