The analysis of stable isotope ratios in organic water contaminants at natural abundance is a powerful diagnostic tool for the assessment, management, and mitigation of chemical pollution in environmental systems. While characteristic isotope signatures (e.g., δ¹³C, δ¹⁵N) can distinguish different sources of otherwise identical chemicals, the quantification of changes in isotope ratios, the so-called isotope fractionation, allows the unambiguous detection and specific interpretation of environmental (a)biotic transformation processes. However, the utility of compound-specific isotope analysis (CSIA) for studying contaminant removal during water treatment remains underexplored, despite its potential to improve remediation procedures by providing critical insights into underlying reaction mechanisms.
This study presents carbon and nitrogen isotope fractionation data, along with kinetic and quantum yield data, for (i) direct photolysis and (ii) TiO₂-catalyzed photolysis under UV-A irradiation of selected nitro- and amino-substituted aromatic compounds. Specifically, we focused on the light-induced transformation of the chemical intermediates 2- and 3-nitrotoluene (NT), 2,5- and 3,4-dichloronitrobenzene (DCNB), 2,5- and 3,4-dichloroaniline (DCA), and the agrochemical product propanil. These compounds are commonly found in high concentrations (µg/L to mg/L) at contaminated sites and pose a significant risk to both (aquatic) ecosystems and human health.
Direct photolysis of 2,5- and 3,4-DCA showed inverse carbon isotope fractionation, with enrichment factors (ε_C) of 4.8 ± 0.4 ‰ and 6.2 ± 0.2 ‰, respectively. In contrast, photocatalysis of 3,4-DCA followed a significantly smaller inverse isotope fractionation (ε_C = 0.9 ± 0.2 ‰), while 2,5-DCA showed a normal isotope effect (ε_C = -1.8 ± 0.1 ‰). These results highlight the potential of CSIA to track and disentangle transformation pathways that are typical of natural environments versus engineered systems. The data presented for all target analytes on both carbon and nitrogen CSIA, combined with transformation product identification by nontargeted high-resolution MS, will provide mechanistic insights to help monitor and optimize water remediation processes of substituted aromatic compounds.

Cassandra Chidiac

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