Photobleaching-induced changes in photosensitizing properties of dissolved organic matter

• Photosensitizing properties of dissolved organic matter.
• Photobleaching of dissolved organic matter.
• Photobleaching-induced changes in the photosensitization of DOM.
• Indirect photobleaching changes photosensitized contaminant degradation rate.
• Reactive contaminants in the matrix prevent the indirect photobleaching.

Photosensitizing properties of different dissolved organic matter (DOM) were investigated according to their performance in singlet oxygen (1O2), triplet state of DOM (3DOM*), and hydroxyl radical (·OH) productions. The photobleaching of DOM solutions after irradiation was characterized by fluorescence excitation-emission matrix and UV–Vis spectroscopy. The photosensitizing properties of pre-irradiated DOM solutions were changed in a sunlight simulator. The performance of DOMs in photosensitized degradation of several contaminants was investigated. For a 20 h exposure, the observed degradation rate constant (kobs) of some contaminants decreased as a function of exposure time, and highly depended on the properties of both DOM and contaminant. Degradation of contaminants with lower kobs was more susceptible to DOM photobleaching-induced decrease in kobs. Under the current experimental conditions, the photobleaching-induced decrease of DOM photo-reactivity in contaminant degradation was mainly attributed to indirect phototransformation of DOM caused by the interactions between photo-inductive DOM moieties and photochemically-produced reactive species. Reactive contaminants can inhibit DOM indirect photobleaching by scavenging reactive species, photosensitized degradation of these contaminants exhibited a stable kobs as a result. This is the first study to report DOM photobleaching-induced changes in the simultaneous DOM photosensitized degradation of contaminants and the inhibitory effect of reactive contaminants on DOM photobleaching.

Schematic pathways of contaminants and DOM interaction in photosensitized processes (α refers to the formation rate of 3DOM*; kRS refers to the reaction rate constant of DOM with reactive species: 3DOM*,
• OH, and 1O2; kq refers to the rate constant of contaminants with reactive species; ks refers to the solution quenching rate constants for reactive species; the red broken lines correspond to DOM photobleaching processes, and the blue solid lines correspond to DOM photosensitized contaminant degradation processes).Figure optionsDownload high-quality image (198 K)Download as PowerPoint slide