Substituent Effects on Nitrogen Isotope Fractionation During Abiotic Reduction of Nitroaromatic Compounds

摘要

Compound-specific analysis of nitrogen isotope fractionation is an important tool for assessing transformation pathways of N-containing organic contaminants. We investigated ~(15)N-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 ~(15)N 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 KIE_N. Apparent KIEn values for reduction of NACs by structural Fe(Ⅱ) in octahedral layers of an iron-rich clay mineral were substantial (average ±1σ 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-0 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 H_2O elimination from substituted N,N-dihydroxyanilines as the predominant ~(15)N-fractionating elementary reaction. The absence of substituent effects on the apparent KIEn of NAC reduction may simplify the practical application of ~(15)N-fractionation data for the quantification of contaminant transformation in the environment.