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

Jannis Wenk,Michael Sander,Silvio Canonica,Cornelia Graf,Michael Aeschbacher

doi:10.1021/acs.est.1c03301

Abstract

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.

Highlights

Dissolved organic matter (DOM) is a chemically complex and structurally diverse component of natural water bodies that plays critical roles in a wide range of environmentally relevant processes.[6−8] In clear surface waters, DOM is the main absorber of sunlight in the upper water layer
Photon absorption by DOM can trigger both indirect phototransformation of contaminants and photoinactivation of pathogens and result in DOM photobleaching.[9−18] Photochemical processes in surface waters involve various short-lived reactive species and different reaction pathways,[19−23] with DOM and its excited triplet states (3DOM*) as key participants.[24−29] DOM plays a dual role in photochemical transformation reactions by acting both as a photosensitizer, enhancing photochemical transformations, and as an antioxidant, slowing down photochemical transformations by quenching reactive intermediates
According to eq 1, 3DOM* may withdraw an electron from an oxidizable organic target compound (TC) to form a DOM radical anion DOM− and a one-electron oxidized contaminant radical cation target compounds (TCs)+. The latter are short-lived intermediates that may further react to stable oxidation products TCox

Summary

Introduction

Direct and indirect photochemical reactions are important transformation pathways of biomolecules and contaminants in the aquatic environment,[1−3] leading, for example, to detoxification of halogenated disinfection byproducts.[4,5] Dissolved organic matter (DOM) is a chemically complex and structurally diverse component of natural water bodies that plays critical roles in a wide range of environmentally relevant processes.[6−8] In clear surface waters, DOM is the main absorber of sunlight in the upper water layer. Excited triplet states, including 3DOM*, are generally known to undergo electron-transfer reactions.[30] According to eq 1, The latter are short-lived intermediates that may further react to stable oxidation products TCox (eq 2). TC+ may itself abstract an electron from an electron-rich antioxidant (AO) moiety, a reaction that reconstitutes the parent TC

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Methods

Results