Dissolved organic matter (DOM), ubiquitous in natural waters, can both accelerate and inhibit the transformation of organic contaminants under the action of sunlight. In this project we relate the inhibition effect of DOM to its antioxidant capacity.
Triplet-induced oxidation of several organic contaminants is likely to play a significant role for their fate in the aquatic environment. DOM carries not only photosensitizer moieties, able to form excited triplet states with oxidizing character, but also moieties that may be considered as antioxidants. Thus, DOM can act as a light-activated oxidant as well as a reductant. In a preceding project, we employed model aromatic ketones as the photosensitizers to disentangle this dual chemical nature and study the possible inhibition of triplet-induced oxidation. We could show that DOM is able to inhibit (or, in other words; slow down the rate of) triplet-induced oxidation of several organic contaminants, but especially those containing aniline functionalities. Various phenolic compounds, used as model antioxidants to mimic the corresponding moieties of DOM, were also shown to inhibit triplet-induced oxidation. The basic hypothesis, still valid at the present state of knowledge, is that oxidation intermediates of the target contaminant interact with model antioxidants or antioxidant moieties of the DOM, leading to reduction of the oxidation intermediates with consequent regeneration of the parent compound. However, the reason why certain contaminants undergo inhibition while others don’t is still open.
Objectives and methods
We now intend to improve our understanding of the mechanisms that govern triplet-induced oxidation of contaminants and its inhibition by DOM. The two main objectives and the corresponding experimental methods are:
1. Detection, by nanosecond laser flash photolysis, of the oxidation intermediates of a few selected aniline derivatives and investigation of their decay kinetics in the presence of DOM and model antioxidants.
2. Study of the relationships between extent of inhibition of triplet-induced oxidation and electron donating capacity of different types of DOM to narrow down the suite of possible antioxidant moieties in DOM responsible for the observed inhibition of oxidation. Electrochemical measurements and selective oxidation of DOM by various chemical oxidants (ozone, chlorine, chlorine dioxide) will be used as additional experimental tools.
The results are anticipated to provide quantitative methods to assess the photoinduced degradation of a wide class of organic contaminants containing aniline moieties, including sulfonamide antibiotics, in natural waters.