Effect of Solution pH on the Dual Role of Dissolved Organic Matter in Sensitized Pollutant Photooxidation

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.