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 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)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 ± 10σ), consistentwith previous resultsforvarious mineral-bound reductants. ¹⁵N fractionation for reduction of 1,2-dinitrobenzene and 2,4,6-trinitrotoluene by AHQDS^- and that of4-chloronitrobenzene by Fe(II)L₂^6-, 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 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.