Exploring Trends of C and N Isotope Fractionation to Trace Transformation Reactions of Diclofenac in Natural and Engineered Systems

Michael P. Maier, Carsten Prasse, Sarah G. Pati, Sebastian Nitsche, Zhe Li, Michael Radke, Armin Meyer, Thomas B. Hofstetter, Thomas A. Ternes, Martin Elsner

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13 Scopus citations


Although diclofenac ranks among the most frequently detected pharmaceuticals in the urban water cycle, its environmental transformation reactions remain imperfectly understood. Biodegradation-induced changes in 15N/14N ratios (ϵN = -7.1‰ ± 0.4‰) have indicated that compound-specific isotope analysis (CSIA) may detect diclofenac degradation. This singular observation warrants exploration for further transformation reactions. The present study surveys carbon and nitrogen isotope fractionation in other environmental and engineered transformation reactions of diclofenac. While carbon isotope fractionation was generally small, observed nitrogen isotope fractionation in degradation by MnO2N = -7.3‰ ± 0.3‰), photolysis (ϵN = +1.9‰ ± 0.1‰), and ozonation (ϵN = +1.5‰ ± 0.2‰) revealed distinct trends for different oxidative transformation reactions. The small, secondary isotope effect associated with ozonation suggests an attack of O3 in a molecular position distant from the N atom. Model reactants for outer-sphere single electron transfer generated large inverse nitrogen isotope fractionation (ϵN = +5.7‰ ± 0.3‰), ruling out this mechanism for biodegradation and transformation by MnO2. In a river model, isotope fractionation-derived degradation estimates agreed well with concentration mass balances, providing a proof-of-principle validation for assessing micropollutant degradation in river sediment. Our study highlights the prospect of combining CSIA with transformation product analysis for a better assessment of transformation reactions within the environmental life of diclofenac.

Original languageEnglish (US)
Pages (from-to)10933-10942
Number of pages10
JournalEnvironmental Science and Technology
Issue number20
StatePublished - Oct 18 2016
Externally publishedYes

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© 2016 American Chemical Society.


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