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Dissolved organic matter (DOM) has a dual role in indirect phototransformations of aquatic contaminants by acting both as a photosensitizer and an inhibitor. Herein, the pH dependence of the inhibitory effect of DOM and the underlying mechanisms were studied in more than 400 kinetic irradiation experiments over the pH range of 6-11. Experiments employed various combinations of one of three DOM isolates, one of two model photosensitizers, the model antioxidant phenol, and one of nine target compounds (TCs), comprising several aromatic amines, in particular anilines and sulfonamides, and 4-cyanophenol. Using model photosensitizers without antioxidants, the phototransformation of most TCs increased with increasing pH, even for TCs for which pH did not affect speciation. This trend was attributed to pH-dependent formation yields of TC-derived radicals and their re-formation to the parent TC. Analogous trends were observed with DOM as a photosensitizer. Comparison of model and DOM photosensitizer data sets showed increasing inhibitory effects of DOM on TC phototransformation kinetics with increasing pH. In systems with anilines as a TC and phenol as a model antioxidant, pH trends of the inhibitory effect could be rationalized based on the reduction potential difference (ΔEred) of phenoxyl/phenol and anilinyl/aniline couples. Our results indicate that the light-induced transformation of aromatic amines in the aquatic environment is governed by the pH-dependent inhibitory effects of antioxidant phenolic moieties of DOM and pH-dependent processes related to the formation of amine oxidation intermediates.

Original languageEnglish
Pages (from-to)15110-15122
Number of pages13
JournalEnvironmental Science and Technology
Issue number22
Early online date29 Oct 2021
Publication statusPublished - 16 Nov 2021

Bibliographical note

Funding Information:
This work was funded by the Swiss National Science Foundation (Projects 200021-117911 and 200020-134801). The authors thank Jeremy Schälchli and Alessandro Piazzoli for experimental assistance, Hans-Ulrich Laubscher and Lisa Salhi for technical support, and five anonymous reviewers who helped to improve this manuscript.


  • antibiotics
  • excited triplet states
  • humic substances
  • photolysis
  • radicals
  • reduction potential
  • speciation

ASJC Scopus subject areas

  • Chemistry(all)
  • Environmental Chemistry


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