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

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 N-15 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)L-2(6-)) 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 +/- 1 sigma), consistent with previous results for various mineral-bound reductants. N-15 fractionation for reduction of 1,2-dinitrobenzene and 2,4,6-trinitrotoluene by AHQDS(-) and that of 4-chloronitrobenzene by Fe(II)L-2(6-), 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 N-15 fractionation upon N-0 bond cleavage) to protonation or reduction of nitroaromatic radical anions (small N-15 isotope effect upon electron transfer) consistent with imply that a quantitative assessment of NAC reduction using compound-specific isotope analysis (CSIA) might need to account for homogeneous and heterogeneous reactions separately.