Substituent effects on nitrogen isotope fractionation during abiotic reduction of nitroaromatic compounds.
Compound-specific analysis of nitrogen isotope fractionation is an important tool for assessing transformation pathways of N-containing organic contaminants. We investigated 15N-fractionation during the abiotic reduction of a series of nitroaromatic compounds (NACs) with intrinsic reactivities covering almost 6 orders of magnitude to evaluate substituent effects on 15N kinetic isotope effects, KIEN. Insights into reaction mechanisms and isotopic elementary reactions of NAC reduction were obtained from comparison of experimental results to density-functional theory (DFT) calculations of intrinsic KIEN. Apparent KIEN values for reduction of NACs by structural Fe(II) in octahedral layers of an iron-rich clay mineral were substantial (average +la of 1.038 +/- 0.003), independent of the NACs' reactivity and ring substituent, and larger than reported previously for reduction by Fe(II) species bound to Fe(III)(oxy)hydroxides and mercaptojuglone species (1.031 +/- 0.002). DFT-calculations accounting for semiclassical contributions and quantum-mechanical tunneling yielded a KIEN for N-O bond cleavage between 1.031 and 1.041, showed no substituent effect, and thus agreed well with experimental observations. Calculated transition-state structures of NAC reduction intermediates were consistent with H2O elimination from substituted N,N-dihydroxyanilines as the predominant 15N-fractionating elementary reaction. The absence of substituent effects on the apparent KIEN of NAC reduction may simplify the practical application of 15N-fractionation data for the quantification of contaminant transformation in the environment.