Multi-element isotope fractionation analysis to investigate the photosensitized reactions of humic substance with 3-chloroaniline

Dissolved organic matter including humic-like substances (HS), acting as photosensitizers participating in electron transfer reactions, can generate a variety of reactive species, such as OH radicals and excited triplet state HS (³HS*) and related, which promote the degradation of organic contaminants such as 3-chloroaniline (3-CA). Multi-element-compound-specific stable isotope analysis (ME-CSIA) was applied to characterize photosensitized mechanisms employing 3-CA as a probe. HS were irradiated with artificial sunlight for elucidation of the reaction mechanisms by studying the kinetic isotope effect (²H, ¹³C, ¹⁵N and ³⁷Cl) to characterize the first irreversible bond change reaction. Unique enrichment factors (-1.0 ± 0.3 ‰ for ¹³C, 7.4 ± 1.7 ‰ for ²H and 2.3 ± 0.7 ‰ for ³⁷Cl) have been detected in HS/UV experiments (OH radical and triplet state), which indicate complex reaction mechanisms. Triplet state reference experiments with the artificial photosensitizers 4,5,6,7-tetrachloro-2',4',5',7'-tetraiodofluorescein in the presence of O₂ (Rose Bengal-O₂) or absence of O₂ (Rose Bengal-O₂ free) yielded characteristic enrichment factors (-0.3 ± 0.2 ‰ and -1.2 ± 0.2 ‰ for ¹³C, 2.7 ± 0.5 ‰ and 4.8 ± 1.0 ‰ for ¹⁵N and 8.4 ± 3.3 ‰ and 11.2 ± 6.8 ‰ for ³⁷Cl), allowing interpretation of reaction mechanisms of triplet state with 3-CA. The correlation of ²H vs ¹³C, ¹⁵N vs ¹³C and ³⁷N vs ¹³C fractions could be used diagnostically to determine photosensitized reactions in the environment and to differentiate between biodegradation, hydrolysis and photosensitized HS reaction.