Compound-specific isotope analysis (CSIA) is an increasingly popular approach to track transformations of many organic soil and water pollutants. Depending on the chemical bonds broken in a degradation process, the stable isotope compositions of a contaminant molecule change in a characteristic manner due to kinetic and equilibrium isotope effects. Many examples have shown how the combination of isotope fractionation for the different chemical elements in organic pollutants can be indicative for ongoing transformation processes and, in many instances, also enable the quantification of the extent of degradation. Unfortunately, applications of CSIA to characterize photochemical processes have hardly been explored. In this work, we systematically studied C and N isotope fractionation associated with the direct and indirect photolysis of chloroanilines used as a model compound for organic micropollutants that are known to degrade via photochemical processes. In laboratory experiments, we observed that subtle differences in photolytic dechlorination mechanisms of 2-Cl-, 3-Cl-, and 4-Cl-aniline were accompanied by distinctly different C and N isotope fractionation trends. Depending on the position of Cl substitution, isotope effects were both normal (i.e., molecules containing light isotopes, 12C and 14N, reacted faster than molecules with heavy isotopes, 13C and 15N) and inverse. The isotopic sensitivity during direct photolysis was very pH-dependent and originated from spin-selective isotope effects for dechlorinations of excited triplet and singlet states. Oxidation of substituted anilines through indirect photolysis by model photosensitizers, on the other hand, revealed 15N-kinetic isotope effects associated with the one-electron oxidation of the N atom. Correlations of C and N isotope fractionation for indirect and direct photolysis of chloroanilines show distinctly different patterns and offer new opportunities to distinguish between the various degradation pathways of such pollutants in aquatic environments.

Stable isotope fractionation associated with the photochemical transformation of chloroanilines

251st National Meeting (2016)

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