We explored the N isotope fractionation associated with the oxidation of substituted primary aromatic amines, which are often the position of initial attack in transformation processes of environmental contaminants. Apparent (15)N-kinetic isotope effects, AKIE(N), were determined for the oxidation of various substituted anilines in suspensions of manganese oxide (MnO(2)) and compared to reference experiments in homogeneous solutions and at electrode surfaces, as well as to density functional theory calculations of intrinsic KIE(N)for electron and hydrogen atom transfer reactions. Owing to the partial aromatic imine formation after one-electron oxidation and corresponding increase in C-N bond strength, AKIE(N)-values were inverse, substituent-dependent, and confined to the range between 0.992 and 0.999 in agreement with theory. However, AKIE(N)-values became normal once the fraction of cationic species prevailed owing to (15)N-equilibrium isotope effects, EIE(N), of 1.02 associated with N atom deprotonation. The observable AKIE(N)-values are substantially modulated by the acid/base pre-equilibria of the substituted anilines and isotope fractionation may even vanish under conditions where normal EIE(N) and inverse AKIE(N) cancel each other out. The pH-dependent trends of the AKIE(N)-values provide a new line of evidence for the identification of contaminant degradation processes via oxidation of primary aromatic amino groups.

Solid-phase microextraction (SPME) coupled to gas chromatography/isotope ratio mass spectrometry (GC/IRMS) was used to elucidate the effects of N-atom protonation on the analysis of N and C isotope signatures of selected aromatic amines. Precise and accurate isotope ratios were measured using polydimethylsiloxane/divinylbenzene (PDMS/DVB) as the SPME fiber material at solution pH-values that exceeded the pK(a) of the substituted aniline's conjugate acid by two pH-units. Deviations of δ(15)N and δ(13)C-values from reference measurements by elemental analyzer IRMS were small (<0.9‰) and within the typical uncertainties of isotope ratio measurements by SPME-GC/IRMS. Under these conditions, the detection limits for accurate isotope ratio measurements were between 0.64 and 2.1 mg L(-1) for δ(15)N and between 0.13 and 0.54 mg L(-1) for δ(13)C, respectively. Substantial inverse N isotope fractionation was observed by SPME-GC/IRMS as the fraction of protonated species increased with decreasing pH leading to deviations of -20‰ while the corresponding δ(13)C-values were largely invariant. From isotope ratio analysis at different solution pHs and theoretical calculations by density functional theory, we derived equilibrium isotope effects, EIEs, pertinent to aromatic amine protonation of 0.980 and 1.001 for N and C, respectively, which were very similar for all compounds investigated. Our work shows that N-atom protonation can compromise accurate compound-specific N isotope analysis of aromatic amines.

Ferrous iron associated with clay minerals can be important for the reductive transformation of organic contaminants in anoxic soils and groundwaters. We investigated the reactivity of structural Fe(II) in ferruginous smectite for the reduction of a series of polychlorinated alkanes (hexa-, penta-, 1,1,1,2-and 1,1,2,2-tetrachloroethane, and carbon tetrachloride (CCl4)) in laboratory batch reactors. Evaluation of reaction kinetics, product distribution, and C-isotope fractionation suggest that polychlorinated ethanes containing three alpha-Cl atoms reacted via reductive beta-elimination to the corresponding ethenes while CCl4-reduction leads predominantly to the formation of chloroform. Reduction kinetics followed a typical biphasic behavior characteristic of the presence of two types of Fe(II) species exhibiting different reactivity in the octahedral sheet of smectites, and reaction rate constants were pH-independent. Dehydrochlorination reactions of chloroethanes containing at least one beta-H atom were found to compete with or even dominate over the reduction reaction with increasing suspension pH. Reference experiments in homogeneous solution and with non-reduced smectite performed in the pH range of 5.5-8.5 suggest that the HCl-elimination is not catalyzed at mineral surfaces. From the observed slow transformation of chloroalkanes, we hypothesize that structural Fe(II) in smectites will be important mainly as a reductant in the subsurface once iron(hydr)oxides have been reductively dissolved.