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Isotope Fractionation Associated with the Indirect Photolysis of Substituted Anilines in Aqueous Solution

Author:
Ratti, Marco, Canonica, Silvio, McNeill, Kristopher, Bolotin, Jakov, Hofstetter, Thomas B.
Source:
Environmental Science & Technology 2015 v.49 no.21 pp. 12766-12773
ISSN:
1520-5851
Subject:
aniline, aquatic environment, aqueous solutions, dissolved organic matter, isotope fractionation, methylene blue, models, nitrogen, oxidation, pH, photolysis, photosensitizing agents, pollutants, protons, stable isotopes, surface water, water treatment
Abstract:
Organic micropollutants containing aniline substructures are susceptible to different light-induced transformation processes in aquatic environments and water treatment operations. Here, we investigated the magnitude and variability of C and N isotope fractionation during the indirect phototransformation of four para-substituted anilines in aerated aqueous solutions. The model photosensitizers, namely 9,10-anthraquinone-1,5-disulfonate and methylene blue, were used as surrogates for dissolved organic matter chromophores generating excited triplet states in sunlit surface waters. The transformation of aniline, 4-CH₃-, 4-OCH₃-, and 4-Cl-aniline by excited triplet states of the photosensitizers was associated with inverse and normal N isotope fractionation, whereas C isotope fractionation was negligible. The apparent ¹⁵N kinetic isotope effects (AKIE) were almost identical for both photosensitizers, increased from 0.9958 ± 0.0013 for 4-OCH₃-aniline to 1.0035 ± 0.0006 for 4-Cl-aniline, and correlated well with the electron donating properties of the substituent. N isotope fractionation is pH-dependent in that H⁺ exchange reactions dominate below and N atom oxidation processes above the pKₐ value of the substituted aniline’s conjugate acid. Correlations of C and N isotope fractionation for indirect phototransformation were different from those determined previously for the direct photolysis of chloroanilines and offer new opportunities to distinguish between abiotic degradation pathways.
Agid:
5362325