Compound-specific isotope analysis is a useful approach to track transformations of many organic soil and water pollutants. Applications of CSIA to characterize photochemical processes, however, have hardly been explored. In this work, we systematically studied C and N isotope fractionation associated with the direct photolysis of 4-Cl-aniline used as a model compound for organic micropollutants that are known to degrade via photochemical processes. Laboratory experiments were carried out at an irradiation wavelength of 254 nm over the pH range 2.0 to 9.0 as well as in the presence of Cs(+) as a quencher of excited singlet 4-Cl-aniline at pH 7.0 and 9.0. We observed considerable variation of C and N isotope enrichment factors, ϵC and ϵN, between -1.2 ± 0.2‰ to -2.7 ± 0.2‰ for C and -0.6 ± 0.2‰ to -9.1 ± 1.6‰ for N, respectively, which could not be explained by the speciation of 4-Cl-aniline alone. In the presence of 1 M Cs(+), we found a marked increase of apparent (13)C-kinetic isotope effects ((13)C-AKIE) and decrease of 4-Cl-aniline fluorescence lifetimes. Our data suggest that variations of C and N isotope fractionation originate from heterolytic dechlorination of excited triplet and singlet states of 4-Cl-aniline. Linear correlations of (13)C-AKIE vs (15)N-AKIE were distinctly different for these two reaction pathways and may be explored further for the identification of photolytic aromatic dechlorination reactions.

Isotope fractionation associated with the photochemical transformation of organic contaminants is not well understood and can arise not only from bond cleavage reactions but also from photophysical processes. In this work, we investigated the photolytic dechlorination of 2-Cl- and 3-Cl-aniline to aminophenols to obtain insights into the impact of the substituent position on the apparent (13)C and (15)N kinetic isotope effects (AKIEs). Laboratory experiments were performed in aerated aqueous solutions at an irradiation wavelength of 254 nm over the pH range 2.0 to 7.0 in the absence and presence of Cs(+) used as an excited singlet state quencher. Photolysis of 2-Cl-anilinium cations exhibits normal C and inverse N isotope fractionation, while neutral 2-Cl-aniline species shows inverse C and normal N isotope fractionation. In contrast, the photolysis of 3-Cl-aniline was almost insensitive to C isotope composition and the moderate N isotope fractionation points to rate-limiting photophysical processes. (13)C- and (15)N-AKIE-values of 2-Cl-aniline decreased in the presence of Cs(+), whereas those for 3-Cl-aniline were not systematically affected by Cs(+). Our current and previous work illustrates that photolytic dechlorinations of 2-Cl-, 3-Cl-, and 4-Cl-aniline isomers are each accompanied by distinctly different and highly variable C and N isotope fractionation due to spin selective isotope effects.

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.