Assessing Iron-Mediated Oxidation of Toluene and Reduction of Nitroaromatic Contaminants in Anoxic Environments Using Compound-Specific Isotope Analysis

Abstract

We evaluated compound-specific isotope analysis (CSIA) as a tool to assess the coupling of microbial toluene oxidation by Fe(III)-reducing bacteria and abiotic reduction of nitroaromatic contaminants by biogenic mineral-bound Fe(II) species. Examination of the two processes in isolated systems revealed a reproducible carbon isotope fractionation for toluene oxidation by Geobacter metal-lireducens with a solid Fe(III) phase as terminal electron acceptor. We found a carbon isotope enrichment factor, epsilonC, of -1.0 +/- 0.1 per thousand, which corresponds to an apparent kinetic isotope effect (AKIE(C)) of 1.0073 +/- 0.0009 for the oxidative cleavage of a C-H bond. Nitrogen isotope fractionation of the reduction of nitroaromatic compounds (NAC) by mineral-bound Fe(ll) species yielded a nitrogen isotope enrichment factor, epsilonN, of -39.7 +/- 3.4 per thousand for the reduction of an aromatic NO2-group (AKIE(N) = 1.0413 +/- 0.0037) that was constant for variable experimental conditions. Finally, AKIE values for C and N observed in coupled experiments, where reactive Fe(II) was generated through microbial activity, were identical to those obtained in the isolated experiments. This study provides new evidence on isotope fractionation behavior during contaminant transformation and promotes the use of CSIA for the elucidation of complex contaminant transformation pathways in the environment.